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POLITECNICO DI MILANO
School of Industrial and Information Engineering
Master of Science in Mechanical Engineering
Chemical composition design of a boron deep drawable
steel to avoid rolling defects
A dissertation submitted for the Master of Science degree in Mechanical Engineering
Author:
Francesco Buffo 860720
Thesis Supervisor: Professor Carlo Mapelli
Thesis Foreign Supervisor: Eng. Giuseppe Frustaci
Academic Year 2017/2018
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Sommario
Figures index .............................................................................................. 6
Tables index ............................................................................................... 9
Graphs index ............................................................................................ 10
Extended summary ................................................................................... 11
Introduction ............................................................................................ 15
Chapter 1 ................................................................................................. 17
Ilva’s Novi Ligure ................................................................................... 17
1.1 Introduction..................................................................................... 17
1.2 Production route ............................................................................. 18
1.2.1 Static Annealing .................................................................... 18
1.2.2 Continuous annealing ............................................................... 19
1.2.3 Hot deep coating lines: ZIN 3, ZIN 4 ................................... 19
1.2.4 Electrogalvanizing line.......................................................... 20
1.3. Continuous annealing production line – (CAPL) ........................... 22
1.3.1 Process metallurgy ................................................................ 22
1.3.2 CAPL plant description ......................................................... 25
1.3.3 Plant layout ............................................................................ 28
Chapter 2 ................................................................................................. 43
State of the art ........................................................................................ 43
2.1 Boron and its importance for steelmakers ...................................... 43
2.1.1 Further effects of boron addition .......................................... 45
2.1.2 Boron nitrade (BN) formation and consequences ................. 48
2.1.3 Mn/S Ratio ............................................................................ 50
2.2 Boron steel in ILVA: CH3N ........................................................... 52
2.3 High defectiveness issue ................................................................. 54
Chapter 3 ................................................................................................. 57
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Experimental procedure ........................................................................ 57
3.1 Introduction..................................................................................... 57
3.2 Data collection and Parsytec investigation ..................................... 58
3.2.1 Parsytec- Surface Quality Yield Management ...................... 59
3.3 Metallographic characterization of defects .................................... 65
3.3.1 Surface analysis ..................................................................... 66
3.3.2 Cross section analysis ........................................................... 67
3.4 Metallographic texture .................................................................... 69
3.4.1 Grain dimension .................................................................... 71
3.4.2 EBSD-proof description ........................................................ 73
3.5 Tensile test ...................................................................................... 76
Chapter 4 ................................................................................................. 77
Results and discussion ........................................................................... 77
4.1 Defect distribution .......................................................................... 77
4.2 Parsytec considerations................................................................... 78
4.3 Grains analysis and EBSD .............................................................. 80
4.4 Tensile test result ............................................................................ 86
Chapter 5 ................................................................................................. 87
Continuous casting proposal ................................................................. 87
5.1 Introduction..................................................................................... 87
5.2 B, Al, N, Ti effects ......................................................................... 88
5.3 Effect of Manganese and Sulfur and their relationship Mn / S ...... 89
5.4 New casting design proposal .......................................................... 90
5.5 The new cast ................................................................................... 92
Chapter 6 ................................................................................................. 94
New cast coils analysis ........................................................................... 94
6.1 Data collection and Parsytec analysis ............................................. 94
6.2 SEM inclusions detection and study............................................... 95
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6.3 Comparisons: CH3N vs CH3S ....................................................... 97
6.4 Coils characterization – Mechanical properties ............................. 98
6.4.1 Grain analysis ........................................................................ 99
6.4.2 Aging Test ........................................................................... 100
6.5 Conclusions................................................................................... 102
Conclusion ............................................................................................. 103
Bibliography ......................................................................................... 105
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Figures index
Figure 1 Ilva's Novi Ligure plant ................................................................ 17
Figure 2 Plant scheme ................................................................................. 21
Figure 3 Equiaxed grains microstructure of a hot rolled coil (C.R.R. = 0%)
and stretched grains microstructure along rolling direction of a cold rolled
coil (C.R.R. = 50% and 75%). .................................................................... 23
Figure 4 Evolution of mechanical characteristics of a cold rolled steel,
during an annealing treatment, as a function of the microstructural
evolution. ..................................................................................................... 24
Figure 5 Continuous annealing processing line (CAPL) ............................ 25
Figure 6 Plant overview .............................................................................. 28
Figure 7 Entry overview .............................................................................. 29
Figure 8 Unwinding coiler .......................................................................... 30
Figure 9 Cutter ............................................................................................ 30
Figure 10 Pre-narrow-lap welder (left); notcher (right) .............................. 32
Figure 11 Heating and maintentance furnaces overview ............................ 34
Figure 12 Accelerated cooling - overaging - final cooling overview ......... 36
Figure 13 Pickling and treatment overview ................................................ 38
Figure 14 HCl pot ........................................................................................ 38
Figure 15 Temper and delivery overview ................................................... 39
Figure 16 Temper ........................................................................................ 40
Figure 17 Example of the nucleation decrease caused by boron addition
(Fundamentals of steel microalligation) ..................................................... 47
Figure 18 EDX-EELS mapping of a BN precipitate in a steel a) image of
the investigated area, b)-g) Fe, Cr, V, Ta, B, N elemental map (TEM study
of boron effect on the microstructure of 9Cr…). ........................................ 49
Figure 19 Direct thermal cycle (left); continuous casting simulation cycle
(right) ........................................................................................................... 49
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Figure 20 Jan-Apr 2018 coils data collection ............................................. 58
Figure 21 Cameras equipment .................................................................... 59
Figure 22 Online Parsytec ........................................................................... 60
Figure 23 Offline Parsytec .......................................................................... 60
Figure 24 Multiple band defect a); single line defect b) ............................. 61
Figure 25 Strip scheme ................................................................................ 62
Figure 26 Example of the mechanism of class selection ............................ 63
Figure 27 Nodular oxide ............................................................................. 65
Figure 28 a) 15x 15 mm sample; b) sample at 10x magnification; c) sample
at 25x magnification .................................................................................... 66
Figure 29 a) Defect at 1000 x magnification; b) chemical analysis ........... 67
Figure 30 Sample cross sections embedded and polished .......................... 67
Figure 31 Cross section SEM analysis of cold rolled and annealed CH3N
samples ........................................................................................................ 68
Figure 32 Cross section SEM analysis of full hard CH3N samples ........... 68
Figure 33 BN investigation at SEM microscope ........................................ 69
Figure 34 15 x 15 mm samples of: A) Point free of defect in a defective
strip edge zone; B) Defective point in a defective strip edge zone; C) Point
free of defect in a defective strip centre zone; D) Defective point in a
defective strip centre zone; E) Point free of defect at strip edge in a free of
defect strip zone; F) Point free of defect at strip centre in a free of defect
strip zone. .................................................................................................... 71
Figure 35 Grain dimension test samples ..................................................... 72
Figure 36 Grain dimension test example .................................................... 72
Figure 37 Kikuchi bands ............................................................................. 73
Figure 38 EBSD schematic representation ................................................. 74
Figure 39 Specimen holding system and EBSD hardware ......................... 74
Figure 40 Tensile test samples of: A) Point free of defect in a defective
strip edge zone; B) Defective point in a defective strip edge zone; C) Point
free of defect in a defective strip centre zone; D) Defective point in a
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defective strip centre zone; E) Point free of defect at strip edge in a free of
defect strip zone; F) Point free of defect at strip centre in a free of defect
strip zone. .................................................................................................... 76
Figure 41 Defects distribution on strip surface as a function of quality
classes .......................................................................................................... 78
Figure 42 Grains dimension test micrographs of samples A,B,C,D,E,F .... 82
Figure 43 EBSD sample A: texture map along RD, IPF and ODF ............ 83
Figure 44 EBSD sample B: texture map along RD, IPF and ODF ............. 83
Figure 45 EBSD sample C: texture map along RD, IPF and ODF ............. 84
Figure 46 EBSD sample D: texture map along RD, IPF and ODF ............ 84
Figure 47 EBSD sample E: texture map along RD, IPF and ODF ............. 85
Figure 48 EBSD sample F: texture map along RD, IPF and ODF ............. 85
Figure 49 Parsytec analysis of the new coils .............................................. 93
Figure 50 Superficial inclusions on one of the 13 coils .............................. 93
Figure 51 Defect Free CH3S coil ................................................................ 95
Figure 52 Inclusion caught by Parsytec monitoring system ....................... 96
Figure 53 SEM inlcusional defect characterization .................................... 96
Figure 54 Defectiveness distribution on CH3N vs CH3S steels (for the
legend see scheme in Fig.) .......................................................................... 97
Figure 55 Grain analysis ............................................................................. 99
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Tables index
Table 1 Material characteristics .................................................................. 26
Table 2 Continuous annealing line specification ........................................ 27
Table 3 Cleaning section characteristics ..................................................... 33
Table 4 Furnace compositions .................................................................... 34
Table 5 Pickling and treatment data ............................................................ 39
Table 6 Temper technical data .................................................................... 40
Table 7 Boron properties ............................................................................. 43
Table 8 Qualitative data .............................................................................. 55
Table 9 Example of pivot table related to B content vs class ..................... 64
Table 10 Nodular oxide data sheet (23). ..................................................... 65
Table 11 Average grain size G (adimensional) and real grain size d (µm) 82
Table 12 Tensile tests results ...................................................................... 86
Table 13 CH3N vs CH3S steel .................................................................... 92
Table 14 ID number, dimensional properties and mechanical properties .. 94
Table 15 Chemical composition .................................................................. 95
Table 16 CH3N Mechanical properties acceptance ranges [] ..................... 98
Table 17 Mechanical properties means of CH3N vs CH3S ....................... 98
Table 18 Sample characteristics ................................................................ 100
Table 19 Tensile test before and after Aging treatment ............................ 101
Table 20 Aging indexes............................................................................. 101
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Graphs index
Graph 1 An example of a continuous annealing thermal cycle .................. 37
Graph 2 The critical ratio (%Mn/S)c as a function of S content (20). ........ 51
Graph 3 Pivot graph related to Boron pivot table ....................................... 64
Graph 4 Mn/S pivot table ............................................................................ 79
Graph 5 B/N pivot table .............................................................................. 79
Graph 6 Mn/S ratio as a function of defectiveness ..................................... 80
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Extended summary
Chapter 1 gives a brief description of the Ilva di Novi plant and of the CAPL continuous
annealing line. The Novi plant, one of the most modern and technologically advanced in
Europe, equipped with modern production lines with high productivity, has a production
capacity of over two million tons of cold rolled and hot galvanized & hot dip aluminized
or electrogalvanized. Its products are destined for important sectors of use, among which
the automotive and appliance sectors stand out, for which the quality and service
standards are very high. In the same chapter there is a schematic description of the
continuous annealing plant, which with its 440 meters of length is one of the most
important high speed and productivity lines in Europe.
The main factors influencing the hot ductility of a drawable low carbon steel are boron,
manganese and sulfur, and in particular the Mn/S ratio. For this reason, in Chapter 2,
through the analysis of the state of the art, the effect of boron (more specifically of Boron
Nitride) and the Mn/S ratio on the hot fragility of the steels examined was investigated,
with the aim of identifying possible actions useful for the improvement of steel chemical
composition and process parameters in order to reduce the occurrence of defects from the
slab casting and hot rolling phases.
In Chapter 3 the experimental procedure adopted has been described, that is the complex
of the in-line inspection activities for the clear classification of the defects, of the possible
correlations between defects and chemical composition, of metallurgical, metallographic
and microstructural investigations carried out with the aid of microscopes electronic
devices made available by the Ilva Company and the Polytechnic University of Milan.
In Chapter 4 are presented the results of all the investigations and analysis carried out
according to the experimental procedure defined in the previous Chapter 3. Significantly
relevant were both the behavior of boron and other elements such as nitrogen, both the
activities that allowed to determine the critical value of the Mn/S ratio above which
progressively increases the percentage of finished products free from superficial defects.
In Chapter 5 an operative proposal for the partial modification of the chemical
composition of the steel in question is formalized on the basis of the technical
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considerations made on the individual chemical elements. Furthermore, some process
parameters of the casting production phase have been evaluated and modified. On the
basis of the indications emerged from this thesis work and from the technical literature,
the Ilva Steelworks of Taranto has produced a new experimental casting.
Chapter 6 shows the results of the investigations and inspections on cold rolled products
obtained by the new experimental casting. After a careful analysis at the inspection desk
and subsequently at the PCs of the automatic monitoring system (Parsytec), the absence
of the defect under examination and therefore the surface integrity of the coils was
checked. Furthermore, the mechanical properties of the coils have been verified in
compliance with the imposed corporate standards. The steel has therefore been approved
for its sector of use.
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Abstract
In the modern continuous annealing plants, the use of low-carbon aluminum killed steel
containing small boron additions is widespread. As a matter of fact, boron enhances the
possibility to keep the coiling temperature (during hot rolling) and the annealing
temperature low, compared to an aluminum killed steel without boron, thus allowing a
high process speed and therefore a high productivity of the production line. But the
addition of boron and critical values of the Mn/S ratio have negative effects on the hot
ductility of the steel causing a high surface defectiveness on the cold rolled coils which
makes the product no longer suitable for use in the most important drawing sectors, such
as automotive and household appliances. These defects compromise the uniformity of a
high-quality enameling or varnishing or a thin layer of metallic or organic coating. In
this thesis work carried out at the Ilva plant in Novi Ligure (AL), an experimental
procedure was defined for the classification of defects and for the research of possible
correlations with the content of some chemical elements of the steel composition. Then,
with the help of automatic surface inspection systems and other investigations conducted
using electronic microscope, the causes of surface defects were analyzed and
investigated. Starting from the metallurgical and metallographic observations of the
defective areas, it was possible to formalize an operative improvement proposal to modify
some chemical elements in the composition of the steel. On the basis of these results, the
Ilva Taranto Steel Plant produced a cast (about 300 tons) according to the new chemical
analysis proposed. The coils obtained from this cast, cold rolled and annealed in the Novi
Ligure plant, no longer showed surface defectiveness. Further structural and mechanical
investigations, always carried out applying the original experimental procedure, have
also confirmed that the new chemical composition of the steel also guarantees the
characteristics of resistance and drawability within the limits imposed by the applicable
international product standards. The new experimental casting design can therefore be
considered homologated. The next step will be that of a wider industrial experimentation.
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Abstract
Nei moderni impianti di ricottura continua “veloce” di un laminato a freddo è diffuso
l’utilizzo di acciai low carbon calmati all’alluminio e contenenti piccole aggiunte di boro.
Il boro, infatti, permette di abbassare, rispetto ad un acciaio calmato al solo alluminio,
la temperatura di avvolgimento nella laminazione a caldo e la temperatura di ricottura,
consentendo un’elevata velocità di processo e quindi un’elevata produttività della linea
di produzione. Ma l’aggiunta di boro e valori critici del rapporto Mn/S hanno effetti
negativi sulla duttilità a caldo dell’acciaio in quanto provocano un’elevata difettosità
superficiale sul laminato a freddo che rende il prodotto non più idoneo all’utilizzo nei
più importanti settori dello stampaggio, quali quello automobilistico e
dell’elettrodomestico. In questo lavoro di tesi svolto presso lo stabilimento Ilva di Novi
Ligure (AL) è stata dapprima definita una procedura sperimentale per la classificazione
dei difetti e per la ricerca di possibili correlazioni con il tenore di alcuni elementi chimici
della composizione dell’acciaio, poi successivamente, con l’ausilio di sistemi automatici
di ispezione della superficie e di indagini metallografiche, sono state analizzate ed
indagate le cause della difettosità superficiale. Partendo dalle osservazioni metallurgiche
e metallografiche delle aree difettose è stato possibile formalizzare una proposta
operativa di miglioramento per modificare alcuni elementi chimici nella composizione
dell’acciaio. Sulla base di tali indicazioni l’Acciaieria Ilva di Taranto ha prodotto una
colata (circa 300 tonnellate) secondo la nuova analisi chimica proposta. I rotoli ottenuti
da questa colata, laminati a freddo e ricotti nell’impianto di Novi Ligure, non hanno più
presentato difettosità superficiali. Le ulteriori indagini strutturali e meccaniche, sempre
condotte applicando l’originaria procedura sperimentale, hanno inoltre confermato che
la nuova composizione chimica dell’acciaio garantisce anche le caratteristiche di
resistenza e di stampabilità nei limiti imposti dalle applicabili normative di prodotto. La
nuova composizione chimica sperimentale può quindi considerarsi omologata. Il
passaggio successivo sarà quello di una più ampia sperimentazione di tipo industriale.
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Introduction
This thesis work was carried out at the Ilva plant in Novi Ligure and in particular at the
continuous annealing plant called CAPL (Continuous Annealing Process Line). Because
of the high production volumes and the equally high productivity of the line, the treatment
temperatures and cycle times must be as low as possible. The peculiarity of this
continuous and fast process of annealing of a cold rolled steel imposes a revision of the
metallurgical mechanisms, above all due to the reduced recrystallization times. For this
reason, low-carbon steels with small addition of boron are widely used in continuous
high-production annealing lines. As a matter of fact, compared to a low-carbon steel,
which is only aluminum-based, boron makes it possible to work with lower coiling
temperatures during hot rolling and with lower annealing temperatures, making it easier
to recrystallize the grains after cold rolling. However, the addition of boron can be very
detrimental for the hot ductility of the steel, favoring the formation of facial and
intergranular cracks during the casting phase of the slab and the subsequent heating before
hot rolling.
At the final inspection of the continuous annealing line, low-carbon aluminum killed with
small boron additions coils shows a high surface and subsurface defectiveness. These
surface defects, substantially consisting of elongated discontinuities with the presence of
oxides, make part of the finished product no longer suitable for use on important drawing
sectors such as the sheet metal sector for automotive use, for the household appliance and
for electrical equipment. Such defects can in fact compromise the uniform appearance of
a quality varnishing or glazing or a thin metallic coating, such as zinc deposited
electrolytically or a thin coating with organic film.
So, due to this kind of surface imperfections, a high percentage of finished product is
downgraded or destined for other uses less demanding and with lower added value,
therefore with a lower margin of contribution. This situation generates a marked
deterioration in the qualitative performance of the production line and an equally
important increase in production costs. But above all it generates delays in the preparation
of the finished product and consequently delays on deliveries to customers in a market
that heavily relies on the respect of timetables (promises) also to reduce the costs of stock
immobilization.
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On the basis of the indications provided by the technical literature taken into
consideration, the results of the inspection and laboratory investigations conducted in
ILVA, these elongated defects show the presence of iron oxides, iron sulphides and, in
some cases, boron precipitates, proving that the defect is caused by an integranular
embrittlement and so by a poor hot ductility that produces cracks on the surface of the
slab. From the technical literature it is also known that another important element for the
study of the surface defects of this kind of steels is the ratio between the Manganese and
Sulfur (Mn / S) which has a strong influence on the hot fragility of the steel.
The use of an alternative low-carbon steel, aluminum killed but without the addition of
boron, could guarantee a lower surface defectiveness of the cold rolled coils, but would
lead to an increase in costs and a reduction in line productivity by requiring higher coiling
temperature and higher re-heating temperatures during hot rolling phase. Moreover, due
to higher speeds implied during continuous annealing which restrict the time necessary
for grains re-crystallization, a simple aluminum killed steel will be higher affected by
aging phenomena with negative effects on products drawability.
A first objective of this work will therefore be to analyze and investigate the causes of
which brings these defects.
A second and more important objective of this thesis work will be that, starting from the
analysis of the defects and the results of metallurgical and metallographic investigations
carried out on defective products, to identify possible actions to improve and modify the
chemical composition of the steel and process parameters to guarantee, if not the total
elimination of defects, at least their net reductions. If this operative proposal will be
considered valid both from a metallurgical and above all an industrial point of view, the
ILVA TA Steelwork will produce a new casting with the new proposed chemical
composition. On this new production, the same inspections and investigations already
carried out on the defective coils will be carried out, in compliance with the same defined
experimental procedure. In case of a positive result, the new chemical composition can
be approved allowing the transition to a wider experimentation at an industrial level.
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Chapter 1
Ilva’s Novi Ligure
Figure 1 Ilva's Novi Ligure plant
1.1 Introduction
In 1912 “Ferriere”, the joint-stock company from Novi Ligure, began production of small
and medium size steel sections since then Ilva has continued to expand becoming a giant
in its field. In 1995 it was acquired by the Riva Group which in doing so accomplish one
of the most important privatizations in the steelmaking industry as part of its company
policy for expansion and quests for new resources.
Today, Ilva S.p.a. of Novi Ligure, has a production facility sided on 1 million square
meters with an infrastructure of 26 km each of road rail. It has an annual production
capacity of 2.300.000 tons of finished product cold rolled, galvanized, electrogalvanized
and aluminized steel bound for a multitude of sectors among which the historic customer
of the automotive industry. In order to guarantee a constant high degree of quality, Ilva
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has applied very specific methods to the running of its production processes. These follow
a quality management system which was developed according to the principles set out in
ISO/TS 16949 regulations. This has allowed to secure various types of qualification and
approval which guarantee the quality and reliability of the enrolled steel. The essential
characteristics for these qualifications are: uniform physiochemical properties, restricted
dimensional tolerance, moldability and drawability to a high degree, excellent surface
finish, corrosion resistance.
1.2 Production route
The hot rolled coils of steel strip from Ilva’s Taranto production plant, which are known
as “black coils”, are stored in the unworked coils warehouse until ready to begin their
transformation cycle. The first step of this long journey is Cold Rolling; the strip is taken
from the warehouse and fed via the entry end, with its laser welder, into the “Decatreno”.
This is a 380 mt. long single machine which is responsible for the pickling and rolling of
the strip in order to ensure and improve the level of quality in terms of planarity,
drawability and guarantee the final thickness. After pickling, the strip arrives at the rolling
mill train known as the “Tandem Train”; with its five stands the tandem train reduces the
strip to the thickness required. At the end of this process, the strip is rewound onto coils
to facilitate movement and storage. During the cold rolling process, the steel work-
hardens because of the reduction in thickness and it must therefore be re-softened or rather
rendered ductile, according to the degree of drawability required, by a process known as
“Annealing”. This process can be carried out either by Static Annealing or by Continuous
Annealing combined with the hot deep coating lines.
1.2.1 Static Annealing
In Static Annealing the coils are piled one on top of the other and covered with a steel
hood or a bell in which an inert gas atmosphere is created; carefully controlled mechanical
characteristics and excellent strips surface quality are ensured by using a hydrogen bell
annealing. In order to complete the mechanical characteristics of the strip and to guarantee
planarity and correct level of roughness the cold-rolled and annealed coils undergo a
further superficial rolling on the “Temper 80’” mill.
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1.2.2 Continuous annealing
Continuous annealing, on the other hand, is characterized by a 340 mt. long machine
called CAPL, one of the most important, of its kind, in Europe. CAPL can guarantee both
high production levels with reduced cycle times and constant and elevated standard of
quality. The plant involves a series of furnaces for the appropriate heat treatment and a
final 6-cylinder mill for skin rolling which is equipped with a high precision strip rolling
control system and can guarantee excellent planarity.
1.2.3 Hot deep coating lines: ZIN 3, ZIN 4
In order to satisfy the highest demands for quality and resistance to corrosion, since 1993
Ilva has produced thin zinc and aluminum coated strips in two hot deep coating
continuous production plants: the “ZIN 3” line, with a production capacity of 200.000
tons of aluminized steel and 200.000 tons of galvanized steel per year and the ZIN 4 plant
which went into production in November 2010 exclusively for the automotive sector with
a production capacity of 700.000 tons per year. From ductile steel for sheet metal drawing
to the most modern highly resistant steel, material is processed by precisely following the
most suitable operational parameters so as to offer the best possible products to the
customer. Productive capacity and quality flexibility combined with innovative technical
solutions, place the ZIN 4 at the vanguard of hot zinc deep coating worldwide while still
in full compliance with the environmental and workplace safety concerns. Each coil is
welded to the next with laser technology, cleaned and dried in an efficient degreasing
section and then annealed and cooled. Then, the zinc coating is applied while
continuously monitoring and recording the outcomes of the high precision process. Three
vertical accumulation loopers connect the sections of the plant and guarantee continuity
of production; before being levelled, passivated, inspected, oiled, trimmed and rewound,
the coated strip passes through the Skin-pass which confer the final characteristics of
mechanical resistance and superficial appearance. Advanced control systems allow
continuous monitoring of the product process parameters.
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1.2.4 Electrogalvanizing line
The static and continuous annealing plants also feed a modern radial electrogalvanizing
cell system capable of coating one or both sides of strips from few microns thick (for use
in white goods) until 8 microns. Further processing of the cold galvanized and
electrogalvanized products is carried out on the five finishing lines; here the material may
be trimmed, levelled and inspected. Finally, the coils are packed and stored ready for
dispatch. Ever aware of market demands, Ilva carries out feasibility tests, material
sampling and resource in close collaboration with customers. Global quality for Ilva also
means investing in ecology; all the facilities are in fact equipped with sophisticated waste
water purification and hydrochloric acid regeneration systems.
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Figure 2 Plant scheme
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1.3. Continuous annealing production line – (CAPL)
1.3.1 Process metallurgy
Before being packed and shipped, steel coils must undergo a cold rolling process. The
necessity of cold rolling derives from the fact that hot rolled coil, even if it can be
considered a finished product, shows some limitation which do not allow its
implementation in almost all the sectors of metal mechanics industry (automotive,
appliance, healthcare, etc.). Some of these limitations are:
• Thickness: hot rolled coil can be obtain with a minimum thickness of 1.5 mm, too
much high for many purposes;
• Mechanical characteristics: the hot rolled coil is not suitable for deep drawing
process and nowadays this is one of the most required peculiarity that steel must
have;
• Superficial characteristics: the surface quality level obtained by a cold rolled coil,
in terms of superficial aspects and tolerances, is fully better than that of a hot
rolled one.
From a metallurgical point of view, cold rolling implies a work hardening state inside the
material as a function of cold rolling reduction rate (C.R.R) which is defined as:
𝐶. 𝑅. 𝑅. =ℎ𝑜𝑡 𝑟𝑜𝑙𝑙𝑒𝑑 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 − 𝑐𝑜𝑙𝑑 𝑟𝑜𝑙𝑙𝑒𝑑 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠
ℎ𝑜𝑡 𝑟𝑜𝑙𝑙𝑒𝑑 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠× 100
Ferrite grains (Fe allotropic form stable at ambient temperature) undergo a deformation
along rolling direction and the equiaxed and rounded grains, typical of hot rolled coils,
tend to strongly stretch as the cold rolling reduction rate increases. In this condition, grain
boundaries become not distinguishable and, in some cases, when the reduction rate is
higher than 70%, deformation bands parallel to rolling direction can be observed. A
product made in this way is called full-hard coil (Fig.3).
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Figure 3 Equiaxed grains microstructure of a hot rolled coil (C.R.R. = 0%) and stretched grains microstructure along rolling direction of a cold rolled coil (C.R.R. = 50% and 75%).
The mechanical properties of full-hard coils depend on the density of dislocation
generated within the material do to the effect of applied plastic deformation; this density
can reach up to 1012 cm/cm3 (in case of hot rolling it reaches 106 cm/cm3). The cold rolled
material, in these conditions, will have increased values of Yield Strength and Ultimate
Strength and, consequently, a significant ductility reduction. The strong brittleness
increase and the plasticity reduction do not allow the direct implementation of the product
in almost all cases. So, it is necessary to progressively heat up the piece at increasing
temperatures to confer structural improvements of its properties. This operation is the so-
called annealing. As said in previous paragraph, steels annealing can be static o
continuous; in the latter case metal strip rapidly moves through a sequence of furnaces.
During annealing, steel significantly recovers ductility and progressively decreases its
hardness through the annihilation of most dislocations generated from the cold rolling
process. Then, the rearrangement of ferritic grain through recrystallization and their
coarsening take place.
The metallurgical phenomena involved during annealing are respectively:
• Recover
• Recrystallization
• Coarsening
The main structural modification taking place during recovery step is related to the
redistribution of dislocations. The dislocations with opposite sign tend to compensate
each other determining a reduction of their density, the dislocation with the same sign
tend to align and group forming subgrain contours (polygonalisation) leading to a
decrease of the energy stored within crystals (residual stresses). During recovery phase,
24
the material changes some of its physical properties like electrical resistivity but remains
approximately the same in terms of mechanical characteristics. In the recrystallization
step, the nucleation of ductile grains can be observed (they are strain free with small
dislocations density) and their coarsening inside the plastically deformed matrix. Once
new crystals formation take place in the matrix, further annealing causes only grains
coarsening. These 3 mechanisms can also act simultaneously; for example, after the
formation of a small crystal free of residual stresses (so when the recrystallization is
started), this crystal starts to coarse while the recovery phase is still occurring.
At the end of recrystallization stage, the structure consists of a polycrystalline aggregate
of equiaxed grains1; grains coarsening reduces the energy stored in the system because
the grain boundaries area is reduced and so the energy associated with them. The final
grains dimension depends on work hardening initial rate and so on rolling; the higher is
reduction rate the smaller will be grains size being annealing process equal. Grain
dimension and orientation are the most important parameters for the mechanical
characteristics of the final product. The next picture (Fig. 4) shows the evolution of
mechanical characteristics of a cold rolled steel during an annealing treatment, as a
function of the microstructural evolution.
Figure 4 Evolution of mechanical characteristics of a cold rolled steel, during an annealing treatment, as a function of the microstructural evolution.
1 Metallic grain can be considered like a crystal; all crystals contains atoms which are orderly arranged on planes, the
so called crystallographic plane. Nevertheless, in all crystals which undergo deformation, some defects are present;
more precisely there are zones in which the crystallographic planes are distorted due to the lack of atoms or due to
relative sliding between planes. These imperfections are defined as dislocations and are the main cause which influences
plastic deformation, recrystallization (dimension, form and crystallographic orientation) and consequently mechanical
characteristics of the material
25
1.3.2 CAPL plant description
Figure 5 ILVA Continuous annealing processing line (CAPL)
During the Seventies, the Continuous Annealing Processing Line project was developed
by Nippon Steel in Japan where the Hirohata and Nagoya lines were both commissioned
in 1982. A third line was then commissioned in the United States at the South Bend plant
(Chicago). ILVA’s continuous annealing line at Novi Ligure (Fig. 5) is the first of its kind
in Europe. Compared to static annealing, this line makes it possible to achieve significant
results. First of all, a marked decrease in processing time, followed by improved and more
consistent quality since the coil is unwound completely and no longer annealed in wound
state. This also disposes of handling problems by virtue of the elimination of intermediate
warehouse between Annealing and Temper. The substance improves surface flatness,
cleanliness and drawability, thus enabling to guarantee a standard high-quality product.
The strong points of the line include the high level of sophistication of the Temper and
the possibility of applying a thin layer of Nichel to the strip. The line produces cold-rolled
products capable of satisfying the quality required by the larger markets like automotive
and household appliance industries.
26
Processing cycle
The hot rolled steel coil from ILVA’s Taranto production units are to be pickled prior to
cold rolling in order to remove the surface oxidation layers formed during the high
temperature processing. After pickling, the thickness of the strip is reduced on the
Tandem mill. The steel is subject to work hardening in the course of the cold rolling
process which constrains a subsequent softening by means of appropriate heat treatment.
The steel is brought to a given temperatures in a protected environment and is then
allowed to cool while constantly monitored. With annealing, the steel strip acquires the
properties required for drawing, however it still lacks the special surface finish which is
attained by “skin passing” on the Temper mills, a process which provides it with the
required surface features and hardness.
Continuous annealing processing line features
Cold-rolled coil attributes
Material processed: cold rolled coils coming from Decatreno.
Steel qualities:
• CQ (commercial quality)
• DQ (drawing quality)
• DDQ (deep drawing quality)
• EDDQ (extra deep drawing quality)
• HSS (high strength steel)
• MAGNETIC (MGS electric use)
Thicknesses 0.4-2-0 mm
Width 700-1680 mm
Max Coil weight 40 ton
Outer diameter 2150 mm
Inner diameter 610 mm
Table 1 Material characteristics
27
Speed in 600 mpm
Furnace Speed 450 mpm
Speed out 750 mpm
Acceleration/deceleration 30 m/min/sec
Furnace productivity 220 ton/h
Line total length 340 m
Plant height 25 m
In looper capacity 960 m
Out looper capacity 830 m
Table 2 Continuous annealing line specification
The principal features of the plant are:
• High productivity (max 220 ton/h);
• Plant flexibility;
• Constantly optimal quality of the treated product;
• Short cycle times.
CHEMICAL
COMPOSITION
FINISHED PRODUCT
CHARACTERISTICS
COLD CYCLE
HOT ROLING
THERMOMECHANICAL
CYCLE
COLD
ROLLING
(Cold rolling
reduction rate)
CONTINUOUS
ANNEALING
C.A.P.L
28
1.3.3 Plant layout
A macro subdivision of the plant (Fig. 6) can be made by considering four different areas
inside the plant; each of these areas is constituted by a series of machines which are
capable of transforming the semi-finished product into sub-product for the next section.
These macro-areas are featured as follow:
1) Entry and Electrolytic cleaning;
2) Furnaces;
3) Pickling and treatment;
4) Temper and delivery.
Figure 6 Plant overview
ENTRY AND
ELECTROLYTIC
CLEANING
FURNACES TEMPER AND
DELIVERY
PICKILNG AND
TREATMENT
29
Equipment description and characteristics
Entry (Fig.7)
The cold-rolled coil taken from the warehouse is placed on a platform by the overhead
crane and transported subsequently to the downcoiler. The line is supplied through carts
called walking beams one for each unwrapping coiler, which have the function to position
coils on specific supports called “saddle”. An automatic identification system allows to
perfectly recognize the coil outer diameter so to ensure the best fitting in the coiler
housing. Moreover, the last saddle is equipped with rollers capable of rotating the coil for
circumferential strapping removal. Once the strapping is removed, the coil is inserted on
the coiler capable of unwrapping the steel strip.
Figure 7 Entry overview
Unwinding coiler (Fig. 8)
The unwinding coiler spindle is supported on the motor side by the workstation of the
machine itself, while on the operator side it is held by an automatic device called “third
support”. In addition, a mechanical system with centering cylinder ensures the exact
position of the coil on the reel spindle. In order to guarantee the cycle continuity, the plant
has two coilers, when one of them is working, the other one is in stand-by. Each coiler is
30
equipped with a magnetic conveyor pinch-roll. The coil is centered on the coiler with
PLC logic, while there is a centering roller equipped with a laser sensor, directly
positioned on the coiler. The end of the coil, trapped by the magnetic conveyor, is
transferred to a double shearing machine.
Figure 8 Unwinding coiler
Scrapers and cutters (Fig. 9)
Each coiler in equipped with a system which couples scrapers and cutters useful for strip
preparation, for scraps discharge and for the trimming of the out of thickness in head and
end of the strip. The two unwinding systems are linked by a conveyor belt. This section
ends with a further pinch-roll and a deflection roller which brings the strip at the correct
work height of the following section.
Figure 9 Cutter
31
Welder
In this section, the junction of the previous strip tail with the head of the successive one
takes place by means of a “pre-narrow-lap” welder. It is a crucial point of the continuous
annealing process because its optimal success has an important influence on the
performance of the whole process. Welding is performed through the overlapping of head
and end of the metal strips and with the passage of two welding disks made of copper.
The welding apparatus is in general equipped by a series of accessories for the other
auxiliary operations like cutting, centering, progressing and welding seam finishing; these
operations are very important for the resulting welding quality.
The welding phases are the following:
• Levelling off head and tail to align the strips centers;
• Positioning under clamps;
• Cutting of the final parts of the strips so to properly define welding edges;
• Matching and overlapping of the head on the tail;
• Cart movement implying the closure of welding disks and pressing rollers;
• Welding current introduction;
• Welding seam smoothing to reduce the over-thickness;
• Welding inspection.
Depending on steel quality and thickness, the system is capable to properly set welding
parameters which are:
• Welding current;
• Cart speed;
• Welding disk pressure;
• Overlapping.
At the end of the welding process, two other operations take place, the punching and the
notching (Fig.10). The punching is useful to enable the process computer to follow the
welding seam along the line through the piercing of a reference hole; in this way it is
possible to set line parameters like temperatures, speed, strip stretch etc. The notcher
provides an arc-shaped cut which is useful to better couple the joined strips which have
different widths in almost all cases; as a matter of fact, it prevent jamming and blockage
of the coil in the welding area.
32
Electrolytic cleaning
The cleaning section allows the removal of oily residues, carbon particles and iron
powders coming from cold rolling process.
The section can be subdivided in the following phases:
• Alkaline tank – consists on the passage of the strip in a pot full of caustic soda.
The degreasing solution is introduced into the pot through a pumping system; an
automatic recirculating system provides the temperature and conductivity control
of the solution. Upstream and downstream of the pot there are some wringing
rollers which prevent the solution to come out. In this zone, an initial degreasing
takes place;
• Electrolytic tank - the same operation of the previous phase is performed with
the help of four electrodes;
• Brushing – consists on mechanical brushing and hot water cleaning of the strip
to remove the residual alkaline substances coming from the previous phases. A
suitable pumping system, spray jet and the relative recirculating system, ensure
the brushing operation to work in humid atmosphere.
• Rinse tank – the alkaline solution is removed from the surface by means of hot
water and spraying.
• Dryers – A series of nozzles blow hot air coming from the furnace heat
recuperators; in this section, the complete drying of the strip takes place (all water
Figure 10 Pre-narrow-lap welder (left); notcher (right)
33
particles deposited are dried). At the end of the section there is a recirculating
system of the solution.
Each pot is connected to the extraction and abatement plant which removes work vapors
through polyurethane pipes. The abatement operates by means of droplet separator
equipped with a washing system of deminarilesed water; the vapor is then channeled at
stack.
Alkaline pot sode cocentration 2 - 4 %
Alkaline pot sode temperature 70°C
Electrolytic pot sode concentration 2 – 4 %
Electrolytic pot sode temperature 70°C
Sode conductivity 120 – 150 ms/cm
Sode solution volume (alkaline +
electrolytic) 60 mc
Rinsing water temperature 75 °C
Drying temperature 130°C
Electrolytic cell current 2000 – 5000 A
Brushing rollers current 20 – 25 A
Fresh water flow rate from WQ 15 m3/h
Table 3 Cleaning section characteristics
Furnace entry accumulator
The accumulator is a system which guarantees the continuity of the process during the
operations that need the deceleration of the line. This accumulator is called “entry looper”
and it is situated horizontally on three levels. Each level contains a trolley with two
driving gears for a total rated capacity of 960 meters. In normal working condition of the
line, the entry looper is full; when for example the welding phase is needed, the entry
looper empties by shifting ahead the trolley and the driving gears. A stretch station closes
this section and prepares the strip for the heat treatment zone.
34
Furnaces
In this section, which is the central element of the entire line, the coil is submitted to the
annealing process proper thereby regaining the mechanical properties and features that
had been lost during the cold rolling process. The section is divided into two main zones
connected by means of special junctions which prevent the strip from contacting the
outside environment.
Heating Furnace n° 1 (1H) 153 radiant tubes 13.25 x 104 Kcal /h per
burner
Heating Furnace n° 2 (2H) 293 radiant tubes 9.75 x 104 Kcal /h per burner
Maintenance Furnace (1S) Electrical resistance 1620 kW
Maintenance and Cooling Furnace
(2S)
Electrical resistance 756 Kw + Jet cooler
Accelerated Cooling Furnace AcC Water and gas (N2) Spray 1400 kW
Overaging Furnace 1 and 2 Electrical resistances 2160 Kw + 1440 kW
Final Cooling Furnace Jet cooler 184 kW
Water cooling WQ Demineralised water
Table 4 Furnace compositions
Zone 1 – heating and maintenance furnaces
Figure 11 Heating and maintentance furnaces overview
35
The thermal sections which characterize the cycle in this zone are:
HEATING FURNACE 1-2 – annealing, in this section the strip reaches the highest
temperature of the thermodynamic cycle (max. 810 degrees), by means of radiant tubes
and methane. Radiant tubes are disposed inside the furnaces in vertical row and are
subdivided in control areas depending on temperature and so on the combustion
regulation. The 1H furnace is divided into 4 areas, the 2H one in 7 areas. Flue gases are
channeled in two harvest chambers, one for each furnace, and are diluted with air to raise
the temperature at 400 °C; then they are sent to boilers in order to recover heat producing
vapor. The combustion air is guaranteed by two fans which, maintaining the harvest
chambers of flue gases in depression, allow the aspiration of air on each burner. The max.
methane flow rate is 4500 m3/h, the combustion is performed with excess air so as to gain
the complete combustion of methane. The max. temperature reachable from the furnace
is about 900°C. Each radiant tube has a guiding burner and a principal one. The guiding
burners are fed by premixed air and methane and have electronic ignition by means of
specific control unit. An automatic purge system guarantees the pipes and burners
cleaning. In the 1H furnace, the recovery (around 400° C) and polygonalisation (around
500°C) take place, while the recrystallization starts at around 700°C in 2H furnace. The
recrystallization temperature is a function of thickness reduction rate and of steel
chemical composition.
SOAKING FURNACE 1-2 – maintenance, the strip is maintained at annealing
temperature to allow the steel the correct recrystallization and normalization. The
temperature remains constant inside the soaking furnaces, the heat loss through radiation
is compensated by a series of wall-mounted electric resistors.
SLOW COOLING FURNACE - In the second furnace (2S) the temperature is firstly
maintained like in 1S but in the end is decreases to 675 °C thanks to the combined action
of electric resistor tubes and gas circulating jet coolers. This section is projected so to
cool the strip from annealing temperature to a specific value for an efficient rapid cooling
in the next section. The cooling phase is necessary to avoid the excessive grain growth.
Moreover, a first moderate cooling prevents the accumulation of residual tensions inside
the material due to large temperature gradients. The section is equipped with 4 modules,
36
each made by an air-water heat exchanger and a centrifugal fan. The fan sucks gas (HNx)
from the furnace, cools down this gas through the heat exchanger and blows it on the
strip.
Zone 2 - accelerated cooling-overaging-final cooling
Figure 12 Accelerated cooling - overaging - final cooling overview
ACCELERATED COOLING – rapid cooling, the strip is cooled down till a temperature
at around 400°C as specified by thermal cycle. It is done through 4 modules like in the
slow cooling zone, but the gas flow rate in this case is more consistent. For example, at
nominal speed, a strip of 0.8 mm thickness undergoes a cooling of 70°C/s.
OVERAGING 1-2 - the strip is kept at constant temperature for 2 or 3 minutes within the
two overaging furnaces, which are fitted with wall-mounted electric resistors.
COOLING 2 AND FINAL COOLING FURNACE – the strip is cooled by means of jet
coolers in the second, as well as in the last cooling furnace, by means of water sprays and
submersion into the so called “quench tank” and exits the furnaces at a temperature of
approximately 70 degrees.
The cooling systems (NSC patent) has the following features:
37
• High cooling speed (100/sec)
• Possibility of modifying the speed by controlling the water flow rate
• Possibility of starting the cooling process at high temperature (675°C)
• Possibility of selecting the ending temperature.
The furnaces are pressurized and there is a continuous exchange with atmosphere in order
to guarantee a humidity level rather slow (Dew point < -20°C) and a slow concentration
of oxygen. As the DP and oxygen concentration increase, the strip is subjected to
oxidation which is detrimental for steel quality. Due to superficial oxidation, the strip
emissivity can change; it is an important parameter which influences not only the
temperature reading of pyrometers (which are responsible for the definition of thermal
cycle temperature) but also the surface cleaning at the exit of the furnaces. Not well
cleaned surface can bring dirt in the next phases (Temper).
Graph 1 An example of a continuous annealing thermal cycle
Pickling and treatment
The layer of oxide which deposits on the coil during the first cooling phase (slow cooling
furnace) is removed by hydrochloric acid pickling. The coil is subsequently covered with
a very thin layer of nickel which protects it and also favors the phosphatizing process.
The pickling area consists of a horizontal tank, a vertical pass spray tank and two pairs of
Tem
per
atu
re (
°C)
Time (min.)
38
wringers. The tank contains a 5% solution of hydrochloric acid and the treatment is
performed at a temperature of about 70°C.
Figure 13 Pickling and treatment overview
The pickling process is completed by a water-rinse tank, once again provided with
sprinklers. The treatment area consists of a main tank where a thin layer of nickel is
applied electrolytically and a rinsing tank similar to the previous one. The nickel layer is
approximately 15 mg/mm2. The section ends with a series of hot air blowers that dry the
coil. The delivery looper storage system, situated horizontally over 5 levels underneath
the second part of the continuous annealing line, is positioned immediately after the
treatment area. Each level contains a trolley with one driving gear for a total rated capacity
of 830 meters.
Figure 14 HCl pot
39
HCl concentration 5 – 7 %
HCl temperature 70 – 75 °C
Water 1st rinsing PH >6
Nickel concentration Ni ++ = 2,5 g/l
Water 2nd rinsing PH >6
Rinsing water flow rate > 35 m3/h
Drier temperature 130 °C
Nickel cell current 3000 A max
Table 5 Pickling and treatment data
Temper and delivery
Figure 15 Temper and delivery overview
Temper (skinpassing)(Fig.16)
During the final phase of the process the coil is brought to desired width also attaining
excellent results in terms of shape and flatness. This is a 6-high 1 stand mill. The
intermediate cylinders can shift axially with a maximum travel distance of 600
millimeters, whereas the work rolls are characterized by the possibility of particularly
accurate and precise adjustments. Only the resting cylinder is motorized. Elongation
control (8% max.) is implemented by adjusting the speed of the bridles placed before and
after the stand driven by a single motor. This procedure achieves an extreme level of
precision.
40
Rollers diameter entry bridle 1000 mm
Roller diameter exit bridle 800 mm
Elongation 0,4 – 8,0 %
Stretch 12 ton max.
Rolling force 1200 ton max.
Rolling speed 750 mpm max.
Bender -20 +20 ton
Work cylinders diameter 425 – 475 mm
Intermediate cylinders diameter 480 – 530 mm
Support cylinders diameter 920 – 1000 mm
Strip roughness 0,20 – 2,5 µm
Table 6 Temper technical data
Trimming machine
Downline of the Temper, a trimming machine serves the purpose of removing the excess
in width; this excess can vary between 3 and 25 mm per side and the speed of the operation
is 750 mpm max. Trimming machine is constituted by two rotating heads which allow
the knives change during line run; it is managed by the operator who can modify width
and thickness parameters and can control its correct execution.
Hydraulic cylinder for load and levelling
Load cell
Locker plate
Tensiometer
Tensiometer
Bridle 5 Bridle 6
Figure 16 Temper
41
Surface inspection
It is very important to verify the complete absence of burrs, the absence of knives
chipping which causes serrated edges, and the absence of scratches on strip edges.
Moreover, it is important to monitor the rolls state so to prevent the presence of trimmed
pieces which can cause marks and dents on the strip compromising the final quality. Then,
the coil is measured and tested for compliance with the required specifications.
Electrostatic oiling and cutting
After trimming and inspection, an electrostatic oiling machine coats the coil with a layer
of protective oil. Cutting by means of a flying shear can be executed without lowering the
speed of the furnace section, even when driving a larger coil into several lighter ones.
Specimen cuts are sampled during this phase for mechanical tests and special inspections
to be performed in the laboratory.
Winding coiler and delivery
After cutting, the strip comes back into its initial shape (coil) but with completely renewed
physical and mechanical properties. For this purpose, 2 coilers wind the strip like those
positioned at the entry. Having completed the coiling process, the coil removed from the
“coil-car” is carried to the warehouse by a 9-step conveyor. Then it is sent to finishing
line for further operations or to delivery.
Automatic control system
The continuous annealing line automatic control system can be illustrated at two different
levels: upper and lower. At the top there is the process computer (Toshiba Tosbac 7/70G)
the main function of which are primarily to interface with the plant Business Computer
(Unisys 1100), thus ensuring the exchange of production planning data, obtaining all the
dimensional and qualitative data for the coils and also issuing a technical process report
for each. Secondly, the process computer calculates and issues the required “presets” to
the high-level actuators also performing the necessary corrections to maintain established
“set points” in the presence of any disturbance. Furthermore, the system tracks the
welding along the line and updates the computation models for the preset values and
42
furnace temperatures resorting to adaptive control models and connects with the
automatic system (Centum Yokogawa) to issue the temperature “presets”. Finally, the
system is connected to the computer that controls the CAPL offload coil conveyor for the
transmission of the processed coils primary data. The lower level of the system contains
the actuators (Toshiba PLC PC 200) which serve the purpose of actuating and maintaining
the “presets” received from the upper level.
Features and use of continuous annealing products
Thanks to their features, continuous annealing products are suitable for all cold-rolled
products uses. Some special properties such as flatness, cleanliness and mechanical
feature uniformness are particularly suited for some applications. CAPL production is
able to satisfy different field of application: household appliances, automotive, drums,
metal furniture, electric motors, tube for furnitures and others.
43
Chapter 2
State of the art
2.1 Boron and its importance for steelmakers
Boron is the lightest non-metallic element on the periodic table after hydrogen and
helium; its properties are listed in Table 7.
It is a typical microalloying element and even very small additions have a great influence
on steel mechanical properties; moreover, it has long been used to replace other alloying
elements in heat treatable steels, especially when these elements were scarcely available.
One of the most important peculiarity of boron is its unique ability to improve
hardenability of the steel into which it is dissolved, when present in concentrations of
around 0,0015% to 0,0030. The available forms into which boron is supplied to
steelmakers are ferroboron or special boron master alloys; the choice between these two
resources depends on steelmaking practice, production mix and volume, operators
experience and price. Ferroboron has the lowest impact in terms of costs and its boron
content is relatively high: standard grades are sold to the steelmaker with incremental
boron levels between 12% and 14%. The classified impurities of this resource are carbon
(0,10-1,5%), silicon (0,30-4,0%) and aluminium (0,5-8,0%). Product can be supplied
either in lump form or in cored wire form. Since ferroboron does not contain appreciable
concentration of protective agents, it requires greater care with respect to boron master
alloy and for this purpose it is normally added after other oxygen/nitrogen removal
elements, such as ferrotitanium. Boron master alloys are more expensive than ferroboron
Main properties
Atomic number 5
Density, 20°C 2,34 g/cm3
Atomic weight 10,81
Melting point 2079 °C
Boiling point (sublimes) 2550 °C
Table 7 Boron properties
44
but are often preferred thanks to their greater efficiency, ease of application and better
results; like ferroboron, they also involve oxygen/nitrogen scavengers such as titanium,
aluminium, silicon and zirconium. Both two resources, to ensure the best effectiveness
for all boron steel, must be added after an adequate deoxidation. Boron combines
aggressively with oxygen and nitrogen dissolved in steel, so great care must be taken in
order to avoid these reactions, any error during the introduction of boron will cause an
irreversible loss of effectiveness. Steel should be fully killed before boron addition:
aluminium killing provides further protection against nitrogen thanks to its higher
affinity, but steelmakers prefer to keep aluminium levels as low as possible to prevent
nozzle clogging problem during continuous casting. Standard practice is to add boron to
the ladle after all other alloying addition have been performed and between the time the
ladle is ¼ to ¾ full (1). Thanks to the use of inert gas shrouded nozzles, synthetic slags,
etc. the re-oxidation phenomenon can be prevented, and it is highly recommended. To
make sure enough boron goes into solution, steelmakers use to provide much more boron
with respect to the small amount required even though this operation can cause some
breakage problems in the roughing stands or during hot forging. These problems can be
associated to the formation of a low melting point B-C-Fe eutectic (Fe2B/Fe3C/Fe), which
forms when the boron content overcomes the 0,007% value. Only soluble boron is
effective for hardenability, so the normal aim composition is between 0,0015 and
0,0030%. Another remarkable drawback is linked to the slightly greater danger of
overheating; as a matter of fact, boron’s diffusivity in steel is about the same as carbon’s,
and it is possible to deboronize steels in high temperature oxidizing atmospheres. To
prevent this unwanted event, furnace atmospheres should be kept low in nitrogen to
prevent the formation of boron nitrides. From the heat treatment point of view, boron can
suppress nucleation (but not growth) of proeutectoid ferrite on austenitic grain
boundaries. Among the numerous theories which have been advanced to explain why this
happens, the most suitable shows that the presence of boron on or near these boundaries
lowers strain- or interfacial energies, so reducing the driving force for ferrite nucleation.
Consequently, the TTT curve for a steel containing even a small amount of boron
(0,0005%) will be significantly shifted to the right compared to a boron-free steel. Carbon
content has a strong effect on boron’s hardenability factor FB, where
𝐹𝐵 = ℎ𝑎𝑟𝑑𝑒𝑛𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑜𝑓 (𝑏𝑎𝑠𝑒 𝑠𝑡𝑒𝑒𝑙 + 𝑏𝑜𝑟𝑜𝑛)
ℎ𝑎𝑟𝑑𝑒𝑛𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑜𝑓 𝑏𝑎𝑠𝑒 𝑠𝑡𝑒𝑒𝑙
45
The factor FB can also be evaluated with an empirical formula: 𝐹𝐵 = 1 ± 1,5(0,9 − %𝐶).
So, boron efficiency will be much more effective at low carbon levels showing that it falls
to zero as the eutectoid carbon content is approached. Medium carbon steels should have
boron factor within the range of 2,0-2,5. Boron has low significant effect on the Ae, Ae3
and Ar1 temperatures, while it is able to determine a reduction of Ar3 temperature.
Overheating should be avoided; boron is a grain coarsener and hardenability will not
improve if grain size raises. If overheating is performed, it is possible to recover
hardenability lost through slow cooling and reheating to proper quench temperature.
Boron has no effect on MS temperature; it will not change the fineness of pearlite, nor
will produce any solid solution strengthening in ferrite.
Carbon-manganese-boron steels are generally considered as replacement for alloy steels
due to costs, as a matter of fact they are far less expensive with respect to alloy steel with
equivalent hardenability. Boron is also used in deep drawing steels where it removes
interstitial nitrogen and allows lower hot rolling temperatures. Sometimes is used in non-
heat-treated steels; in that case, ferroboron is added as an intentional nitrogen scavenger
and the result is a steel with higher formability. This mechanism is used in the production
of steel for automotive applications. Aluminium is sometimes used for a similar duty, but
AlN is slower to precipitate so requiring higher annealing temperatures: the diffusion
velocity of Boron is rather higher than that of Al. Boron addition makes the steel more
formable and eliminates the need for strain age suppressing anneals (2).
2.1.1 Further effects of boron addition
As said before, the best known and most wide use of boron in steels is to improve the
hardenability of carbon and alloy steels but it has numerous other effect on steels
mechanical properties. For example, in stainless steel it markedly affects:
• Hot shortness;
• Creep resistance;
• Intergranular corrosion resistance;
• Neutron-absorption capacity.
Hot shortness - The low ductility of austenitic stainless steel and some superalloys at high
temperatures, is one of the causes of cracking or tearing in rolling and forging. Highly
alloyed grades are particularly subjected to this phenomenon, which is called hot
46
shortness. Armco Steel Corporation (3) established that small additions of boron
eliminate the problem by significantly increasing hot ductility and workability of these
steels. As a consequence, steel producers can achieve the maximum yield of mill products
avoiding expensive reheating cycles. The influence on ductility is verified in two ways:
1) modifying the inclusions, 2) affecting the dynamic recovery recrystallization process.
Creep- Small additions of boron have also beneficial aspect on creep resistance as they
markedly increase the creep-rupture life that is usually followed by an increase in creep
ductility (4).
Intergranular corrosion resistance - Boron has also the effect to retard the precipitation
of chromium carbides with significant beneficial effects on the intergranular corrosion of
sensitized stainless steels. This peculiarity verifies also for low concentrations (as low as
4 ppm) (5).
As regarding Iron-Carbon alloy, Henrique Silva Furtado et al. demonstrate that the boron
addition increases solid-liquid interface instability, causing faster secondary arms
formation and its dynamic coalescence. Moreover, boron seems to reduce the velocity
where dendrite-cellular transition can occur. Interdendritic carbon and boron segregation
and the equilibrium phase diagram, underline that remelting of already solidified
interdendrite region could occur in lower temperature range (around 1200°C), decreasing
hot ductility of the material. Directional solidification process simulation shows that the
boron is able to restrict the dendrite main arms increasing liquid fraction. These results
are a direct consequence of carbon and boron segregation influence on constitutional
undercoolings. The mechanism suspected to influence crack formation during continuous
casting are two: 1) deep dendritic primary arms space that would concentrate the tensile
stress, 2) remelting of this region at low temperature resulting in large localized ductility
loss. So, in order to reduce the sensitivity for crack formation, grain boundaries
segregation of these elements should be minimized or is necessary to add another element
that react with boron to form a third phase (6). In the investigation of low alloyed and
austenitic steels (7,8) it was demonstrated that boron segregations toward grain
boundaries depends on temperature and rate of cooling. At temperature greater than
980°C boron is present in solid solution in case of water cooling rates; while, at air cooling
rates, segregation process occurs. J. Lis et al. by following these considerations,
modelling with DICTRA, were able to verify that: 1) segregation phenomenon takes place
below the Ar3 temperature, 2) the concentration of boron in ferrite was four time greater
than austenite at given temperatures below Ar3, 3) the increase of the average grain
diameter lowers free energy and retards nucleation of phases (9). In a high-strength boron
47
steel, the influence of austenization and cooling on boron hardenability effect can be
appreciated. In particular, at higher austenization temperature this hardenability effect
decreases due to the precipitation of boron carbide along austenite grain boundaries.
While, an optimal control of the cooling rates by slow cooling after hot deformation
increases the B hardenability effect (10).
The strengthening mechanism produced by the boron introduced in the steel is related to
the generation of a particular metallurgical structure which is not thermodynamically
stable, i.e. bainite. The boron tends to concentrate on the grain austenite boundaries and
is responsible for the delaying of ferrite formation because it slows down the nucleation
of this phase (Fig. 17). As a consequence, the ferrite transformation is shifted in a lower
temperature range, but this decrease of the thermal range causes a not complete
arrangement of the transformed phases inducing the formation of acicular ferrite or even
the formation of bainite.
Figure 17 Example of the nucleation decrease caused by boron addition (11)
It is important to understand that only the soluble part of B present in metal matrix is
responsible for the formation of distorted structural constituent because only that content
can segregate to grain boundaries and then preform the delayed nucleation (11). It is well
known that the most common method to perform microstructure refinement for a hot
rolled steel are the refinement of initial austenite grain size and thermo-mechanical
processing (TMP). Various phenomena take place during high temperature deformation
process of austenitic steel: work hardening, dynamic recovery and dynamic
recrystallization. Yong-liang Gao et al. showed that the grain size in all the regions has
an increasing trend with austenitization temperature: the higher is boron content, larger is
48
the austenite grain size. This trend can be attributed to the fact that BN formation prevents
the formation of AlN. Increasing boron content has also the effect to decrease the flow
stress at lower strain rates due to the increase in self diffusivity of iron and expansion of
the austenite lattice; simultaneously boron addition leads to higher flow stress at higher
strain rates because of boron atoms blocking dislocation motion. Another result of Yong-
linag Gao et al., always related to boron increasing trend, remarks the reduction of the hot
deformation activation energy and the delay of the onset of dynamic recrystallization (12).
2.1.2 Boron nitrade (BN) formation and consequences
Considering a range of 0-100 ppm, the different forms into which boron could be present
in steels are: B precipitated M23(B,C)6, linked to N as BN, some remain soluble in steel as
an interstitial element and a small quantity linked to oxygen. While the B precipitated
M23(B,C)6 is the result of the soluble boron segregation and precipitation at grain
boundaries (13), boron linked to nitrogen (BN) comes from the insoluble part (14). BN
practically exists as inter or intra-granular depending on the history of specimen (15). In
industrial application, the control of hardenability of B is guaranteed by a good
deoxidation and a strong protection against N either by limitation of N content or by
addition of nitride forming elements such Ti, Zr amd Al (16). By considering a Fe-Al-N-
B system, it has been proved that a significant amount of N dissolved in the Al and B
containing melt exist in the form of AlN and BN molecules. The remaining part (not
bonded to N) of B and Al is bonded to Fe and only a minor amount stays in the atomic
form. So, in order to reduce BN formation, Al is fundamental as nitrogen scavenger (17).
The determination of BN precipitates is done by combining energy loss spectroscopy
(EELS) and energy-dispersive X-ray analysis (EDX) spectroscopic imaging (Fig.18). The
EELS spectroscopy is conducted in the transmission electron microscope (TEM) and it
represents a powerful tool not only for the detection of light elements like boron and
nitrogen but also for determining the chemical composition and crystals orientation. The
dimension of this kind of precipitates is in the order of 300-500 nm and they are
distributed randomly (18).
49
Figure 18 EDX-EELS mapping of a BN precipitate in a steel a) image of the investigated area, b)-g) Fe, Cr, V, Ta, B, N elemental map (TEM study of boron effect on the microstructure of 9Cr…).
BN generation is a very crucial aspect for the integrity and performance of a boron steel
because it strongly influences the hot ductility. Temperature cycling occurring in the slab
surface during secondary cooling of continuous casting (e.g. water spray on the surface)
imposes a rapid temperature drop and subsequently a rapid rise through recoalescence.
Kyung Chul CHO et al. investigated the influence of thermal cycling and N contents on
the hot ductility of B-bearing steels and proposed a possible solution to improve it. The
hot ductility was investigated by hot tensile test under two types of thermal cycles (direct
cooling to straining temperatures (A), under cooling and reheating to straining
temperature (B)) (Fig. 19).
In relation to the thermal cycles before mentioned, different N contents were investigated,
and the authors found out that:
1) Addition of B to Carbon steel is detrimental for the hot ductility when the steel is
subjected to the thermal cycle which introduced direct fast cooling to straining
Figure 19 Direct thermal cycle (left); continuous casting simulation cycle (right)
50
temperature because of the precipitation of BN in the lower temperature region of
Fe-γ phase;
2) The thermal cycle B highly deteriorated hot ductility of the B bearing steels
because the precipitation of BN was encouraged;
3) The hot ductility depends not only on BN formation, but also on their distribution
which is in turn determined by N content and thermal cycle. BN precipitates were
randomly distributed within the prior austenite and preferentially at austenite grain
boundaries. While, in case of low N content, the precipitates were rather sparsely
distributed inside the steel;
4) To improve hot ductility, a fast temperature drop below 1000° C in the secondary
cooling stage of the slab after solidification must be avoided and the amplitudes
of the under cooling and reheating in CC must be small.
The formation of BN precipitates at austenite grain boundary has a detrimental effect on
transverse facial cracks generation during continuous casting. K Yamamoto et al. were
able to simulate the embrittlement caused by BN precipitation through the hot tensile test
with Gleeble Tester. In order to avoid the facial cracks, the authors proposed two
methods: one is the reduction of N content to guarantee the condition 𝑤𝑡% 𝑁 −
0,2 × 𝑤𝑡% 𝑇𝑖 < 0,003 𝑤𝑡% and the other is to keep the cooling rate less than 0,5 °C/s
in the secondary cooling zone to allow the precipitation of coarse BN in steel matrix (19).
2.1.3 Mn/S Ratio
Another important element linked to the superficial defectiveness in low carbon boron
alloyed steels, is the ratio between Mn and S content (Mn/S). The high temperatures
brittleness and the susceptibility to cracks initiation in continuous casted steels partly
depend on the presence of liquid oxides-sulphides and iron sulphides deposited on grain
boundaries. When liquid steel cools down and solidifies, the Sulphur solubility decreases
and it detaches from iron sulphide solution (FeS) which forms an eutectic with adiacent
iron; this eutectic just formed segregates at grain boundaries with a rather low temperature
of 988°C. The eutectic Fe-FeS weakens grains bonds and determines a significant loss of
steel mechanical properties (brittleness) at hot forming temperatures (hot rolling in our
case). Steel embrittlement at hot forming temperatures, that is called hot shortness, it is
caused by the presence of these low melting iron sulphides inside the material structure.
So, in order to prevent the iron sulphides formation, a solution comes from the addition
51
of a certain Mn amount (not smaller than 0,2%). Manganese reacts strongly with iron
sulphides during steel solidification, transforming FeS into MnS (manganese sulphide)
following the reaction:
𝐹𝑒𝑆 + 𝑀𝑛 = 𝑀𝑛𝑆 + 𝐹𝑒
MnS melting temperature is relatively high (around 1610 °C) and so all steels containing
manganese can be hot worked avoiding the formation of liquid phases very detrimental
for their stability. Unfortunately, MnS non-metallic inclusions are detrimental for steel
mechanical properties because they can generate cracks which easily propagate on grain
boundaries and can cause drawability and pressability losses in low carbon steels.
Therefore, it is necessary to perfectly tailor Mn and S content and in particular their ration
Mn/S (20). Many studies of the literature survey state that the hot ductility decreases as
the ratio Mn/S goes below a specific critical value (21). The diagram in Graph 2, shows
the result of three different studies based on aluminum killed steels. Critical ratio (Mn/S)c
is correlated with S content in steel. When the ratio Mn/S is above the curve, MnS are
formed, while when the ratio is below the curve, FeS are formed and they increase as the
ratio Mn/S becomes smaller and smaller than the critical value.
Graph 2 The critical ratio (%Mn/S)c as a function of S content (20).
52
The determination of Mn/S critical ratio useful for hot shortness study, becomes more
accurate as the S content becomes lower and lower. For a low carbon drawable steel with
Mn content of 0,2-0,3%, Mn/S c is between 25 and 30 (20). This estimation has been
obtained just after steel continuous casting where the precipitation of MnS takes place in
the local equilibrium phase of the interdendritic liquid. When the solidification velocity
is too much high, as it verifies in continuous casting, the local equilibrium reaction can
be incorrect and some FeS can precipitate in the interdendritic liquid; as a consequence,
the hot ductility can decrease even if the Mn/S in steel is greater than the critical value.
2.2 Boron steel in ILVA: CH3N
When the Continuous annealing line for cold rolled coils was introduced in Ilva Novi
Ligure, there was the necessity of studying and better understanding the effects that the
continuous process would have had in terms of final product characteristics. The
peculiarity of this process imposed a review of all the metallurgical mechanisms involved
focusing on their effects on product final properties. An experimentation was performed
by considering chemical compositions and heat treatments; the products involved in the
experimentation were hot rolled and pickled coils and full hard coils evaluated in different
positions: head, center and tail. The vast majority of the work was focused on the steel
quality “CQ” (commercial quality) because steels belonging to that category would have
been the first implemented in the production process with a production volume higher
with respect to other steels. Because of the requested mechanical characteristics and the
imposed large production volume, the main aim of the experimentation was to guarantee
the maximum production capacity by involving low heat treatments temperatures with
short cycle times. The addition of boron to the steel, allows to set smaller coiling
temperatures of the hot rolled coil than those used for an Al-killed steel, because B has a
higher chemical affinity with N than that of Al. The higher chemical affinity of boron
causes the formation of BN at higher temperatures with respect to those of AlN formation;
in this condition, the AlN precipitation is already completed during hot rolling so avoiding
high coiling temperatures.
For this kind of CQ steel, NSC (Nippon Steel Corporation) specification considered:
• Small amount of B and N, respectively 15÷25 ppm and ≤30 ppm;
53
• Small amount of Al;
• Standard C amount obtained by a RH degassing treatment.
For the experimentation mentioned before, different steels were considered:
• A steel which follows NSC specification;
• A steel like that of NSC specification but with higher amount of Al (0,03%);
• A steel with higher amount of B and N (respectively 57 and 40 ppm) and Al
content similar to the previous one;
• A steel with small amount of B and N and with a high amount of C (550 ppm) in
order to verify the effect of a carbon steel which doesn’t need a RH degassing
treatment.
Based on these samples, some operations have been performed:
• Continuous annealing simulation cycles with different annealing temperatures:
710, 750, 780 °C;
• Cold rolling with different reduction rates (70,75,82%);
• Tensile tests and measures of average r and n;
• For what concerns Al-killed steels with B, the results have shown that;
• The differences of mechanical properties in head, center and tail of the coils stay
within the statistical data dispersion and are not relevant;
• The reduction rate in cold rolling seems to be not influent;
• The mechanical characteristics of CQ quality can be also obtained with an
annealing temperature of 710°C not only for the first experimented steel but also
for the second;
• For what concerning the proof with higher amount of B and N, an annealing
temperature of 750°C is recommended;
• For what concerning the proof with higher C content, the values of yield strength
and total elongation are outside CQ specification.
54
In conclusion, at the end of the experimentation for CQ steels, the selected chemical
composition was (22) :
C Mn Al N B
≤ 0,045 % 0,2÷0,3 % 0,03÷0,045 % ≤ 50 ppm ≤ 35 ppm
with the following temperatures:
• Hot rolling end temperature: ≥ 860°C;
• Winding temperature: 640÷680°C;
• Annealing temperature: 710°C.
2.3 High defectiveness issue
At the final inspection section of the Continuous Annealing Production Line, low carbon
boron alloyed aluminium killed steel (CH3N) coils show high superficial and sub-
superficial defectiveness characterized by elongated defects which are known as “knobby
oxides” (23). This kind of defects, when inspected at electronic microscope, always show
the presence of iron oxides or iron sulphide. This peculiarity confirms that the defect is
caused by intergranular embrittlement during hot process, so by a low hot ductility of the
steel which produces superficial cracks during continuous casting (at the straightening of
the slab) and most of all by the annealing phase after slab hot rolling.
These defects are considered injurious for a product destined for the most important
drawing sectors like house-appliance, electrical machinery and automotive most of all
due to their dimensions and position on steel surface. As a matter of fact, they can
compromise the uniformity of a painting or of a quality glazing or of a thin metallic
coating (as zinc) applied on the surface through electrolytical way.
Due to the before mentioned superficial defectiveness, a high percentage of coils is
downgraded or destined to alternative employment surely less demanding and obviously
less profitable. The downgrade of these boron steels determines a significant decrease of
qualitative performances and at the same time an important raise of production costs.
Moreover, it generates delays in the preparation of the final product and so delays in
delivery to customers. The worst-case scenario is the non-delivery in a market, like
55
household appliance, which strongly relies in on time delivery in order to reduce the
warehouse immobilisation.
Qualitative data
Considering mean qualitative performances of the last three years, low carbon boron
alloyed steels annealed in CAPL (CH3N steel), have shown a variable defectiveness
between 15 and 20 % and a diversion on alternative orders of about 5-7%. The 70-80%
of the total defectiveness is attributable to the so called nodular oxide.).
The total defectiveness of low carbon aluminium killed steels without boron produced in
CAPL is around 6-7% (so it is about 1/3 of that encountered in boron alloyed steels) and
the percentage linked to the superficial elongated defect (where iron oxide or iron
sulphide are present) is smaller than 2% (Tab. 8) (24).
Low carbon steels
with boron
Low carbon steels
without boron
% total downgraded coils 15 – 20 % 6 – 7 %
% downgraded coils due to knobby oxides 12 – 15 % < 2 %
% coils replaced on other orders 5- 7 % < 2%
Table 8 Qualitative data
This situation underlines not only production and qualitative problems, but also
commercial.
• The usage of a boron steel produces a significant increase of production costs
starting from the steel mill. As a matter of fact, the steel mill has to produce a
surplus of 20% of this steel quality, that is really demanding (for the low carbon
content) and critical for the oxygen converter), in order to cope with the scrap of
the CAPL.
• As said before, delays in delivery or non-delivery can generate dissatisfaction of
the customers and the risk of substantial market share losses.
• Last but not least, it is crucial for the quality assurance of final products. The
replacement of a boron steel with other not alloyed can surely reduce the
defectiveness but on the other hand can introduce ageing phenomena involving
56
loss of mechanical properties which are detrimental for material formability and
drawability.
From these considerations comes to light the necessity of investigating the causes
which carry out this superficial defectiveness on boron alloyed steels so to identify
possible actions and analytical and/or process parameters variations which can bring
the suppression or at least a strong reduction of these defects.
57
Chapter 3
Experimental procedure
3.1 Introduction
To investigate the causes of the defectiveness which affects cold rolled and annealed
boron alloyed steel coil, a series of analysis and tests have been performed in this thesis
work. The aim of these proofs has been to find out any possible correlation between
defectiveness and material characteristics and/or plant performances. The problem has
been faced by following two main branches which characterize the final product:
chemical composition of the steel and material microstructure before, during and after the
Annealing process which can be reconducted to process parameters.
The actions provided in the experimental procedure are:
• The analysis of CH3N coils processed in CAPL from January to April 2018
(chemical and mechanical properties analysis);
• Defectiveness investigation on these coils thanks to the automatic surface
inspection system called Parsytec;
• Quality classes definition as a function of defects seriousness and extension on
coils surface;
• Investigation on the possible correlation between chemical analysis and position
and/or defects seriousness;
• Micrographic and metallographic characterization of defects through optical
microscope and scanning electron microscope (SEM) equipped with a
microanalysis probe;
• Study of steel metallographic structure through grain dimension and orientation
analysis in order to verify the microstructure influence, and so the thermal cycles
to which steel is subjected, on defectiveness onset. Grain dimension test has been
performed thanks to Olympus optical microscope equipped with an analytical
software capable of counting grains in a defined area, while Electron
Backscattered Diffraction technique has been implied for what concerning grains
orientation.
58
• Finally, Tensile tests have been performed so to evaluate mechanical properties
giving significant relevance: anisotropy coefficient r which is strictly correlated
with EBSD analysis and strain hardening coefficient n which is correlated to
Boron content in steel.
A brief description of inspection and analysis equipment, implied in this thesis work, in
shown below.
3.2 Data collection and Parsytec investigation
The starting point of the work has been the data collection of about 120 low carbon boron
alloyed steel coils coming from continuous annealing processing line (CAPL); more in
detail, all coils of the quality CH3N starting from January to April 2018 have been
considered. These coils have been collected on an excel sheet by showing their
Identification number ID, their casting code, their dimensional features, their production
date, their mechanical properties and their chemical composition. An example of the data
collection method is exposed in the picture below (Fig. 20).
Figure 20 Jan-Apr 2018 coils data collection
After data collection, these coils have been analyzed through an automatic monitoring
system in order to understand how the defectiveness appears on strip surfaces and in
59
which intensity it compromises the whole quality. This automatic monitoring system is
called Parsytec – Surface Quality Yield Management and it can recognize, online and
offline, surface appearance and so all defects or surface variations which distinguish a
“dirty” coil from a “clean” one.
3.2.1 Parsytec- Surface Quality Yield Management
The initial step of the chain consists of a large number of defects pictures classified and
recognized by operators (Atlante dei difetti) [1] which are uploaded into monitoring
computer. The monitoring system, placed downstream the CAPL (after Temper), is able
to take many frames of the coil in the points in which it recognizes not perfect cleanliness.
These frames are taken thanks to 22 cameras, 11 are focused on the superior surface and
the other 11 on the inferior surface, which are disposed along coil width and are fixed
(Fig. 21) while the coil is moving during continuous annealing process.
The frames are transferred from monitoring system to monitoring computer and through
a matching with the defects background database it can understand which kind of defect
Figure 21 Cameras equipment
60
it is. As said before, this kind of monitoring allows not only to follow in real time the coil
(online configuration (Fig. 22)) but also to focus on all coils already processed in the past
few months (offline configuration (Fig. 23)). In this way, by taking advantage of Parsytec
monitoring, it is possible to obtain a macro analysis of the defectiveness understanding
how it affect the coil, in which intensity and positions. For this purpose, in this thesis
work, a qualitative subdivision into 4 different classes has been proposed and applied;
each of these classes considers respectively four gradual levels of defects seriousness.
More specifically, the classes are the result of a tradeoff between defects intensity and
their extension area on the whole coil surface.
Figure 22 Online Parsytec
Figure 23 Offline Parsytec
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Intensity
For what concerns the intensity, it has been evaluated considering the length of the single
defects, the presence of defects bands and their engraving through the thickness. An
example is reported in Fig. 24 where it is shown the comparison between multiple defect
bands and single defect line. Furthermore, another aspect which can be observed from the
Fig. 24a is the engraving condition: when the gravity of defect itself increases the width
becomes higher and the color becomes dark.
Extension area
To properly identify the extension area, a scheme of the cold rolled coil has been created
by dividing the surface into 15 areas: 5 rows and 3 columns (Fig. 25).
The columns are:
• MS (LM) → Machine side (Lato macchina);
• CE (CE) → Center (Centro);
• OS (LO) → Operator side (Lato operatore).
The rows are:
• HE (TE) → Head (Testa)
• HC (TC) → Head-Center (Testa-Centro)
• CE (CE) →Center (Centro)
• CT (CC) → Center-Tail (Centro-Coda)
Figure 24 Multiple band defect a); single line defect b)
a b
62
• TA (CO) → Tail (Coda)
Starting from columns, the MS extension is ¼ of the total strip width, the CE extension
is ½ of the total strip width and the OS extension is ¼ as MS. For rows, the extension of
HE is 1/6 of the whole strip length, the HC is also 1/6, CE is 1/3, CT and TA are 1/6. This
dimensional subdivision has been quite useful because it has allowed not only to
transduce the results given by monitoring but also to obtain an overview of the case
studied.
Figure 25 Strip scheme
Considering these two features, the before mentioned four classes have been set and
named:
• Class 0 – No defects
• Class 1 – Light defectiveness
63
• Class 2 – Medium defectiveness
• Class 3 – High defectiveness
Once classes have been specified and explained, the 120 coils have been judged and at
each of them has been assigned the corresponding class number and the distribution zones
of the defects on strip surfaces. An example is reported to understand the mechanism (Fig.
26).
Figure 26 Example of the mechanism of class selection
After class assignation, pivot tables have been structured in order to find out the first
correlations in terms of chemical properties of the CH3N analyzed coils. An example of
pivot table is reported for clarifications in Tab. 9 where boron content is correlated with
0,1,2,3 classes accounting the overall count (Conteggio di Rotolo).
Conteggio di ROTOLO Giudizio
B 0 1 2 3 Totale complessivo
0.0025 19 5 4 4 32
0.0026 6 5 1 1 13
0.0028 5 2 4 2 13
0.0029 12 1 13
0.0032 3 1 4
0.0033 1 1 2
0.0034 10 2 5 2 19
0.0035 9 7 2 2 20
Totale complessivo 65 22 17 12 116
Defects distribution on almost
all strip surface; high intensity
due to extended defect bands.
• Class 3
Defect
presence
No defect
64
Table 9 Example of pivot table related to B content vs class
Graph 3 Pivot graph related to Boron pivot table
This operation has been performed for all chemical elements and/or for chemical elements
ratios which have been stated as the most relevant for our issue. The choice was based
principally on literature survey and on industrial practice because those elements are
considered a weakness for mechanical properties and surface integrity. For our purpose,
the selected elements have been Boron (B), Nitrogen (N), Manganese (Mn), Sulphur (S)
and the ratios considered have been Mn/S and B/N. Mn/S because Mn content is relevant
to avoid the formation of FeS that is very detrimental for steel integrity because it forms
a liquid phase at around 1000°C. Indeed, during hot deformation processes this liquid
phase can migrate toward grain boundaries inducing intergranular corrosion, adhesion
loss and consequently the sliding of grains which can be transduced into high temperature
brittleness.
B/N because B content i relevant for the formation of BN which are quite detrimental for
steel integrity as explained in Cap. 2.
Conteggio di ROTOLO Giudizio
B 0 1 2 3 Totale complessivo
0.0025 59.38% 15.63% 12.50% 12.50% 100.00%
0.0026 46.15% 38.46% 7.69% 7.69% 100.00%
0.0028 38.46% 15.38% 30.77% 15.38% 100.00%
0.0029 92.31% 0.00% 7.69% 0.00% 100.00%
0.0034 52.63% 10.53% 26.32% 10.53% 100.00%
0.0035 45.00% 35.00% 10.00% 10.00% 100.00%
Totale complessivo 55.45% 19.09% 15.45% 10.00% 100.00%
65
3.3 Metallographic characterization of defects
After a macro analysis of the situation, the focus of the work has been a detailed micro
analysis of the defect specifying first its technical features and provenience. The defect is
classified into ILVA Defect Volume and its data sheet is reported as follow (Fig. 27).
Nodular oxide Identification sheet n° FPA8_0
Figure 27 Nodular oxide
Italian denomination Ossidi nodosi TNA
English denomination Nodular oxide - Endogenous inclusion
Type Provenience
Family Superficial defect
Group Nodular oxides
Defect code 50 Nodular oxide
Description
It consists on a breakage of the coil skin frequently covered
by a thin metal strip elongated and in some cases with Iron
oxide entrapment. Sometimes a dark ring of elliptical shape
is present in the neighbourhood of the defect. (thermal spot).
Origin
The defect origins can be various:
• Multiple arms cracks
It is put in evidence during hot rolling
Detection method Visually detected in inspection station
Treatment Removal of defective areas where it is possible or
downgrade in the worst situation.
References
Identification sheet is the following:
Volume
TNA PTQ20PNA01 NFA27 Nodula oxides TNA
Table 10 Nodular oxide data sheet (23).
10 mm
Rolling direction
66
Then, a small sample of 15x15 mm of a cold rolled CH3N steel has been taken to analyze
the strip surface covered by defects (Fig. 28a) and an optical microscope has been used
to take a picture at small magnification (Fig. 28 b,c) before using a Scansion Electron
Microscope (SEM) for a detailed overview.
Figure 28 a) 15x 15 mm sample; b) sample at 10x magnification; c) sample at 25x magnification
The next step has been the analysis at SEM microscope where the defect sample has been
observed not only on surface but also in cross section.
3.3.1 Surface analysis
For surface analysis defect appearance is provided in Fig. 29 a) while in Fig. 29 b) is
exposed the spot chemical analysis which confirm the presence of Iron oxide strictly
correlated to the defect.
c)
b) a)
a)
67
Figure 29 a) Defect at 1000 x magnification; b) chemical analysis
The presence of the iron oxide and so the presence of our defect is justified by the spongy
area that is a typical structure of this kind of defect.
3.3.2 Cross section analysis
For what concerning cross section analysis, a group of 15x15 mm defect samples have
been cut perpendicularly to defect extension and then embedded in resin. The embedding
was needed to properly fix the samples and to avoid any possible overturning caused by
the small thickness and by the vacuum induced inside the microscope chamber. Metal
clips have been used to avoid overturning caused by resin pouring inside embedding
machine. The resin used was a phenolic black powder resin which had been heated up till
about 300°C and then let solidify with a controlled cooling mechanism thanks to a
mounting press. Once solidified, polishing was performed to remove redundant resin and
material so as to obtain the desired cross section and an optimal surface quality. Polishing
operation was done by using, in sequence, specific abrasive papers with decreasing
roughness: from a mesh of 600 to 1200. Then to further improve surface quality some
cloth disks were used starting from 6 µm to 1µm. The final result of our embedded
samples is shown in Fig. 30.
Figure 30 Sample cross sections embedded and polished
b)
68
The result of cross section analysis on a cold rolled and anneal coil sample of CH3N is
shown in Fig. 31.
Figure 31 Cross section SEM analysis of cold rolled and annealed CH3N samples
The same analysis has been performed on a hot rolled coil samples of CH3N coming from
ILVA Taranto and it is shown in Fig. 32.
Figure 32 Cross section SEM analysis of hot rolled CH3N samples
69
In addition, SEM cross section analysis on these sample revealed the presence of BN
precipitates largely treated in Chapter 2 and considered as very influent for defects origin.
Their observation has been performed at 1000x magnification and the proof result is
exposed In Fig. 33.
Figure 33 BN investigation at SEM microscope
3.4 Metallographic texture
In addition to chemical composition analysis oriented to find out some dependences
between defects and chemical elements alloyed in the CH3N steel, another way has been
investigated to discover a possible influence of material microstructure on defects
outbreak. Material microstructure is related to the heat treatments history which
characterize the CH3N steel; as a matter of fact, after cold rolling, grains structure is
modified. Ferrite grains undergo a deformation along rolling direction and the equiaxed
and rounded grains, typical of hot rolled coils, tend to strongly stretch as the cold rolling
reduction rate increases. In these conditions, the product cannot be directly shipped or
delivered to the market as finished product because it has increased values of Yield
Strength and Ultimate Strength and, consequently, a significant ductility reduction. So, it
70
is necessary to progressively heat up the piece at increasing temperatures to confer
structural improvements of its properties. This operation is the so-called annealing.
For this purpose, some tests have been performed for the analysis of grains dimension
and orientation (EBSD) and mechanical parameters as anisotropy and strain hardening
coefficients.
Firstly, 6 specific samples have been taken in 3 different and selected zones of the same
strip:
• Defective strip edge zone
• Defective strip centre zone
• Free of defect strip zone
The 6 15x15 mm samples considered are shown in Fig. 34 and have been used both for
grains dimension test and Electron Backscattered Diffraction analysis (EBSD).
Point free of defects –
Sample A
Defective point– Sample
B
Point free of defects –
Sample C
Defective point– Sample
D
Point free of defects at strip edge –
Sample E
Point free of defect at strip centre –
Sample F
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Figure 34 15 x 15 mm samples of: A) Point free of defect in a defective strip edge zone; B) Defective point in a
defective strip edge zone; C) Point free of defect in a defective strip centre zone; D) Defective point in a defective
strip centre zone; E) Point free of defect at strip edge in a free of defect strip zone; F) Point free of defect at strip
centre in a free of defect strip zone.
3.4.1 Grain dimension
For what concerning grains dimension evaluation, the before shown 6 samples have been
grouped and disposed in a way which allowed the analysis of cross section (Fig. 35). In
particular, in order to verify any possible dependence of hot/cold rolling on grains shape,
the samples cross sections have been taken so as to be parallel to rolling direction. Then,
these samples have been embedded in resin to properly fix them and to avoid any possible
overturning caused by the small thickness. Metal clips have been used to avoid
overturning caused by resin pouring inside embedding machine. The resin used was a
phenolic black powder resin which had been heated up till about 300°C and then let
solidify with a controlled cooling mechanism thanks to a mounting press. Once solidified,
polishing was performed to remove redundant resin and material so as to obtain the
desired cross section and an optimal surface quality. Polishing operation was done by
using, in sequence, specific abrasive papers with decreasing roughness: from a mesh of
600 to 1200. Then to further improve surface quality some cloth disks were used starting
from 6 µm to 1µm. After polishing operations, a chemical etching was needed to show
5 mm 5 mm 5 mm
5 mm 5 mm 5 mm
A B C
D E F
72
clearly the material microstructure. Some droplets of a solution of C2H6O (Ethanol) and
2% of HNO3 (Nitric acid) have been poured on the exposed cross sections for 10 seconds.
Figure 35 Grain dimension test samples
The test has been performed through an optical microscope associated to the software
“Analysis” supplied by Olympus following the standard ASTM E112-13. An example of
the results emphasized by chemical etching, is shown in Fig. 36.
Figure 36 Grain dimension test example
73
3.4.2 EBSD-proof description
Electron backscatter diffraction (EBSD) is a microstructural-crystallographic
characterization technique useful to study the structure, crystal orientation and phase of
materials in the Scanning Electron Microscope (SEM). Typically, it is used to explore
microstructures, revealing texture, defects, grain morphology and deformation. EBSD
provides the absolute crystal orientation with sub-micron resolution and it is also very
useful for phases determination. This technique operates by arranging a highly polished
specimen tilted at high angle (usually 70°) to the incident electron beam. By fitting
electron beam acceleration voltages between 10 and 30 kV and incident beam currents
between 1 an 15nA, the electron beam is diffracted by the crystal lattice of the specimen
at the incident beam point on the specimen surface. An Electron Backscattered Diffraction
pattern (EBSP) emanates spherically from this point in all directions; the electrons
scattering in the material causes electrons movement in all direction and only electrons
that satisfy the Bragg condition are channeled and show the Kikuchi bands (Fig, 37).
Figure 37 Kikuchi bands
The EBSP is detected by a digital CCD camera which is illuminated by a phosphor screen
that intersects the spherical diffraction pattern. In a certain way, the phosphor converts
the diffracted electron into light suitable for the CCD. The pattern is so analyzed and is
defined only by lattice parameters and by its orientation in space. The system is able to
match the identities and orientation of the crystal until the best fit is found so leading to
an indexed pattern. As the speed of pattern analysis increases, it is easier to scan over
multiple point on the specimen so to create an orientation map. A schematic
representation of the proof is shown in Fig. 38.
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Figure 38 EBSD schematic representation
Figure 39 Specimen holding system and EBSD hardware
The Oxford Instrument Forescatter Detector (FSD) is a system used which produces
orientation contrast pictures from a wide range of materials. It consists of multiple silicon
diodes placed around the EBSD phosphor screen to produce microstructural images.
An EBSD is a surface-sensitive technique, with the diffraction signal coming from the
top few nanometers (5-50nm) of the crystal lattice, it is highly necessary that its surface
remains free from damage and also free from contamination and oxidation layers. As a
75
consequence, specimen preparation absolutely covers an important role for good EBSD
data collection. As a matter of fact, the specimen must be tilted of 70° that means keeping
the surface topography to an absolute minimum to avoid shadowing problems. Moreover,
it should be ensured that the specimen is fixed properly to the holder to avoid drift caused
by the specimen movement (25).
Following these guidelines, the 6 samples of Fig. 34 have been used for EBSD test
purpose. No resin blocks have been used because in this case the analysis is performed
on sample surface and not on its cross section. Polishing operation was done by using, in
sequence, specific abrasive papers with decreasing roughness: from a mesh of 600 to
1200. Then to further improve surface quality some cloth disks were used starting from 6
µm to 1µm. Very coarse papers and high pressures should be avoided as this introduces
deep deformation. Once reached 1 µm precision, it is not enough for the success of the
proof because the surface is not polished as much as needed for diffraction and pattern
generation. So, a final polish has been carried out with colloidal silica (OP-S/OP-U)
allowing to reach 0,2 µm precision. The time needed for the final polishing step has been
of 15 mins for each sample because only in this way a satisfactory result could be
obtained.
Properly polished and oriented, samples were disposed into SEM and then analyzed;
different graphs and results were extrapolated:
• Electron image → investigated area;
• Orientation map along rolling direction, sample normal, transverse direction;
• Inverse polar figure (log scale) rolling direction, sample normal, transverse
direction;
• ODF 0°, 45°, 90°;
• Schmid and Taylor graphs;
• CSL plot;
• Misorientation plot.
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3.5 Tensile test
In addition to grain dimension test and EBSD, 18 samples have been taken so to perfom
tensile test for the evaluation of the most important mechanical parameters which
characterize the steel; strong importance has been given to anisotropy coeffiecent (r)
which is strictly correlated to the metallografical texture analyzed by EBSD test. These
18 samples (Fig. 40) were taken in the same zones as specified before; more precisely, 3
samples for each area corresponding to three different angular directions:
• 0° = parallel to rolling direction;
• 45°;
• 90° = perpendicular to rolling direction.
Figure 40 Tensile test samples of: A) Point free of defect in a defective strip edge zone; B) Defective point in a
defective strip edge zone; C) Point free of defect in a defective strip centre zone; D) Defective point in a defective
strip centre zone; E) Point free of defect at strip edge in a free of defect strip zone; F) Point free of defect at strip
centre in a free of defect strip zone.
C A
B F
E
D
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Chapter 4
Results and discussion
4.1 Defect distribution
After the macro analysis of all CH3N coils coming from CAPL, interesting results have
been obtained starting from defects position and chemical compositions related to
defectiveness classes assigned. In relation to defects position, the main consideration is
that the vast majority of defects is present on strip edges as it is shown in Fig. 41.
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A possible explanation of this behaviour can be found in the solidification history of the
steel during continuous casting. Solidification phase, which starts at the exit of the ingot
mould, can lead to central porosity and problem related to mechanical and thermal
stresses; when these stresses overcome the hot resistance value of the steel, cracks are
formed. These cracks can be superficial or internal and the former ones are rather
detrimental for steel integrity because, being subjected to oxidation, they cannot be
recovered during hot rolling. The majority of defects encountered in a metal matrix
originates when steel is in its doughy phase, that means when its ductility is almost 0. In
practice, the interval related to cracks formation is included between ZDT temperature
(Zero Ductility Temperature) where the metal is fully solid, and ZST (Zero Strength
Temperature), where there is not enough liquid metal to fill these cracks. In summary,
being the solidification rate so important, cracks formation can be correlated to fast
cooling rates to which the slab edges are subjected with respect to slab centre, causing the
generation of defects which, in most of the cases, cannot be recovered after continuous
casting (26).
4.2 Parsytec considerations
Although the major defects are present on strip edges, the worst CH3N coils analysed
(which belong to Class 3) show high defectiveness on all strip surface. So, other issues
must be found to explain this behaviour. For this purpose, a great help comes from pivot
table sketched out on excel; considering all pivot tables drawn, the ones related to Mn/S
and B/N ratios are the most relevant.
For what concerning Mn/S ratio, when this ratio increases (that means an increase of Mn
content and/or a decrease of S content), the number of coils belonging to 1st,2nd and 3rd
classes tends to zero (Graph 4). More in detail, when Mn/S is approximately equal to 23,
almost all coils are free of defects. From the other pivot table related to B/N ratio (Graph
5), when the ratio increases (that means an increase of B content and/or a decrease of N
content), the number of free of defect coils (class 0) is drastically reduced in the face of
an increase in class 2 coils (where the defect is averagely distributed).
X → CLASS 1 X→ CLASS 2 X→ CLASS 3
Figure 41 Defects distribution on strip surface as a function of quality classes
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Graph 4 Mn/S pivot table
Graph 5 B/N pivot table
So, from these two tables, what can be evinced is that some modification of chemical
composition must be provided in order to solve/attenuate the high defectiveness
encountered. In particular, Mn, S, B and N must be properly tailored by following the
results obtained.
Mn content should be tailored with respect to S content, more in detail it should be
reduced S one by implying a deep steel desulphurization (that means when S content
decreases because Mn cannot overcome 0,3% for drawability gain). This behaviour is
80
perfectly in line with literature and can be explained accounting the formation of low
melting FeS compounds which are rather dangerous for steel hot cycling. Mn works as
S scavenger preventing their formation.
In particular, coils are completely free of surface defects when Mn/S is around 23 as
shown below (Graph 6).
Graph 6 Mn/S ratio as a function of defectiveness percentage
On the other hand, the result obtained from the second pivot table can be explained
considering that: as the B/N ratio increases, more boron atoms tend to link with nitrogen
ones due to their reciprocal affinity determining the formation of nitrides which are very
detrimental for steel integrity (as largely treated in Chapter 2).
4.3 Grains analysis and EBSD
In parallel with Parsytec microanalysis investigation based on 120 CH3N coils, another
investigation has been performed in order to verify if and how much the metallographic
texture can have influence on defects outbreak. Starting from grains analysis of the 6
samples taken into account, they have resulted principally equiaxed: none pancake
structure has been detected (pancake structure is a typical grains configuration of cold
rolled and not annealed steels). So, what can be deduced firstly, is that the annealing
treatment have been properly performed conferring a correct recrystallisation of the
material. Moreover, it has been evaluated the adimensional G number (average grain size)
and so the real grain diameter (d) as explained in standard E112. The micrographs related
0
20
40
60
80
100
12 13 14 15 16 17 18 19 20 21 22 23 24 25
% coils without defects
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to A, B, C, D, E, F samples are represented in Fig. 42 and the resulting G and d values in
Tab. 11.
50 µm 20 µm
A
50 µm 20 µm
B
50 µm
20 µm
C
50 µm 20 µm
D
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Sample G d(µm)
A 12.31 4.97292
B 12.14 5.278686
C 12.16 5.241759
D 11.68 6.203646
E 12.53 4.603363
F 11.41 6.820325
Table 11 Average grain size G (adimensional) and real grain size d (µm)
From data shown in Tab. 11, it is possible to verify that there is not any correlation in
terms of G (and so d) between sample where the defect is exposed (B,D) and those which
are free of defects (A,C,E,F) so remarking and confirming the correct execution of the
annealing treatment.
In addition to these considerations, other results come from the EBSD of the 6 samples
taken into account; it can be observed a fair orientation of crystallographic grains along
rolling direction as shown in IPF (Inverse Polar Figures) related to each sample (Fig. 43,
50 µm
20 µm
E
50 µm
20 µm
F
Figure 42 Grains dimension test micrographs of samples A,B,C,D,E,F
83
44, 45, 46, 47, 48). This fact results in general not influent because mechanical properties
obtained from tensile tests are rather coherent for the standards of CH3N steel.
Figure 43 EBSD sample A: texture map along RD, IPF and ODF
Figure 44 EBSD sample B: texture map along RD, IPF and ODF
Rolling direction
84
Figure 45 EBSD sample C: texture map along RD, IPF and ODF
Figure 46 EBSD sample D: texture map along RD, IPF and ODF
Rolling direction
Rolling direction
85
Figure 47 EBSD sample E: texture map along RD, IPF and ODF
Figure 48 EBSD sample F: texture map along RD, IPF and ODF
Rolling direction
Rolling direction
86
4.4 Tensile test result
Finally, from tensile tests related to the 6 samples studied, the following results have been
obtained. Once r coefficient has been evaluated in each direction (0, 45, 90°), mean r has
been calculated using the formula 𝑀𝑒𝑎𝑛 𝑟 = 𝑟0+𝑟90+2×𝑟45
4 and compared between the
samples with defects and those without. The results of the proof are summarized in Tab.
12.
Table 12 Tensile tests results
Analysing the obtained values, we can appreciate that there is not any correlation between
mean r and defects because all samples (with and/or without defects) show similar values
which are perfectly fitted with standards mechanical characteristics of this kind of steel.
ID: 1029505 Rp xx Rm Ag A rm n Rs/Rm Reh Rel Thickness (a) Width (b)
@0.20% @5.00-18.00% @5.00-18.00% Mean r
(N/mm²) (N/mm²) (%) (%) (%) (%) (N/mm²) (N/mm²) (mm) (mm)
45A 251.43 372.17 19.62 32.78 1.124 0.18 0.676 0.488 19.976 A 1.1875
LA 247.64 361.79 20.29 32.54 1.186 0.188 0.684 254.43 247.54 0.497 19.849
TA 234.74 362.18 19.87 33.96 1.316 0.188 0.648 0.499 19.873
45B 244.31 360.45 20.42 33.04 0.973 0.19 0.678 0.501 19.849 B 1.08875
LB 248.55 358.46 20.48 34.27 1.112 0.194 0.693 252.78 248.55 0.506 19.851
TB 237.31 358.45 21.11 33.74 1.297 0.194 0.662 0.498 20.011
45C 250.17 368.29 19.53 31.93 1.004 0.181 0.679 0.5 19.87 C 1.118
LC 246.42 357.98 20.51 33.43 1.209 0.189 0.688 0.505 19.8
TC 236.7 363.34 21 34.38 1.255 0.189 0.651 0.502 19.858
45D 249.67 359.24 20.98 33.36 0.972 0.192 0.695 0.507 19.983 D 1.08725
LD 256.87 362.78 21.96 34.26 1.066 0.195 0.708 0.498 19.852
TD 239.06 356.92 21.06 34.04 1.339 0.198 0.67 0.5 19.87
45E 258.86 368.77 18.94 31.82 0.936 0.179 0.702 0.498 19.963 E 1.084
LE 257.54 361.19 20.08 34.17 1.122 0.185 0.713 259.65 256.88 0.501 19.837
TE 238.87 363.36 20.66 33.11 1.342 0.188 0.657 0.5 19.865
45F 246.43 364.95 19.18 32.16 1.003 0.182 0.675 247.63 246.18 0.504 19.875 F 1.114
LF 252.57 364.69 20.84 33.93 1.128 0.188 0.693 254.44 251.81 0.498 19.88
TF 237.96 358.73 20.6 35.25 1.322 0.189 0.663 0.506 19.891
87
Chapter 5
Continuous casting proposal
5.1 Introduction
In the previous chapter of this thesis work, the causes that generate surface defects on low
carbon steels destined for drawing applications have been analyzed and investigated. The
objective is now to identify, on the basis of the results obtained and the indications that it
has been possible to find, from the technical literature on the subject, possible actions to
improve and modify the chemical composition of the steel or the process parameters to
eliminate or at least drastically reduce the presence of defects. The low carbon steel coils,
aluminum killed with boron addition, cold rolled and annealed on a continuous annealing
line, show a high superficial and sub-superficial defectiveness represented by the so called
elongated defects. These defects are observed on both surfaces of the strip, especially
after cold rolling, but are present in a less evident manner also on the surfaces of the strip
after hot rolling. Examinations under optical microscopy and electron microscopy
showed, in the areas affected by the defect, both on the hot rolling surfaces and on those
from cold rolling, the existence of iron oxides and iron sulphides followed by the
presence, for the truth not excessive and almost never observed at the grain boundary, of
boron precipitates. Cold rolling, with its thickness reductions and with fibers stretching,
further stresses the pre-existing defect and lengthens it in the rolling direction, making it
more visible. Moreover, the cold rolling brings to the surface those oxides initially trapped
in the sub-superficial layer of the strip after hot rolling. The defects observed arise, during
the hot rolling phase, by an intergranular embrittlement and so by a poor hot ductility that
produces cracks in slab production phase (during continuous casting cooling phase) but
above all in the heating phase of the same slab just before hot rolling. The investigations
carried out have shown that to reduce the surface defects found on the annealed CH3N
boron steel coils, it is possible to carry out some modifications of the chemical
composition of the steel to reduce the hot fragility. In particular, the amount of
Manganese, Sulphur, Boron, Aluminum, Nitrogen and Titanium can be modified
according to the indications emerged both from the examination of the relevant technical
88
literature taken into consideration, and from the metallographic and microstructural
investigations conducted in this thesis work.
5.2 B, Al, N, Ti effects
In low carbon continuous annealed steels, boron is used as nitrogen tailoring element: as
a matter of fact, boron makes the steel more formable reducing the interstitial nitrogen
(27). Boron, which has a greater chemical affinity with nitrogen than aluminum has,
diffuses much faster as an interstitial element and is able to block aluminum even at low
coiling temperatures during hot rolling. In other words, the addition of boron to steel
allows to use lower average coiling temperatures (even up to 80 ° C) than those that must
be used for aluminum killed steel without boron which have to undergo continuous
annealing treatment. Boron, having a higher chemical affinity with Nitrogen than
aluminum, shows a higher tendency to BN boron nitrides formation at higher
temperatures than the formation of AlN aluminum nitrides. Under these conditions the
precipitation of aluminum nitrides AlN is already completed during hot rolling and
therefore it is not necessary to adopt high coiling temperatures of the strip after rolling.
AlN is slower to precipitate and requires higher annealing temperatures. Therefore, the
addition of boron makes the steel easier to recrystallize in a continuous annealing line at
high speed and productivity like that of Novi Ligure. The addition of boron is much more
effective in low carbon steels and the most suitable amount of boron to be added is
between 15 and 30 ppm. However, the addition of boron and consequently the generation
of boron nitrides is an important aspect for the integrity and performance of steels because
it strongly influences hot ductility, which depends not only on the formation of boron
nitrides, but also on their distribution, which is at its time determined by the thermal
cycles and above all by boron content. In the case of low nitrogen, the boron nitride
precipitates BN are poorly distributed, whereas if the nitrogen content is higher, the BN
precipitates are randomly distributed in the matrix and preferentially on the austenitic
grain boundaries. The presence of BN precipitates on austenitic grain boundaries has a
negative effect on the generation of facial cracks during continuous casting and during
the subsequent hot rolling.
In the investigation carried out during this thesis work, the nitrogen content in steel was
almost always low enough, around values of 50 ppm. And it is for this reason that the
89
presence of boron nitrides appeared almost always poorly distributed and almost never
on the grain boundaries. A further reduction of the nitrogen content in the steel may have
a positive effect on hot fragility.
Boron combines very aggressively with oxygen and nitrogen dissolved in steel and for
this reason, in manufacturing it is necessary to pay close attention to the practice of
addition to avoid the occurrence of reactions that could irremediably compromise the
effectiveness of boron. The steel must be fully killed before adding the boron. To reduce
the risk of "boron fading", i.e. its loss of effectiveness at high temperatures, it is also
possible to add a limited quantity of Titanium in order to protect the boron from the
reaction with nitrogen until the low temperatures are reached.
Aluminum, on the other hand, is an effective killing agent and provides a safe protection
against nitrogen, but it is advisable to keep aluminum levels as low as possible in steel to
prevent submerged entry nozzle clogging or excessive presence of exogenous alumina
inclusions in steel. A vacuum degassing treatment of steel can reduce oxygen levels
thereby reducing the need for deoxidizing agents. The best standard practice of adding
boron is therefore to add it to the end after all other alloys additions.
5.3 Effect of Manganese and Sulfur and their
relationship Mn / S
We have already seen that another important element for the study of the superficial
defectiveness is the ratio between the contents of Manganese and of Sulfur, due to the
effect that this Mn / S ratio has on the hot fragility of the steel at high temperatures. When
the ratio Mn / S is below a certain value called critical (and determinable for the steel
being studied), in cooling and solidification phase of the slab Sulphur forms a low-melting
FeS eutectic (at 988 ° C) with iron which segregates at iron grain boundaries weakening
the bonds and increasing the hot fragility of the steel. In other words, during hot rolling
of the slab, after heating it at about 1250 ° C in the reheating furnaces, there are liquid
islands (the FeS low-melting eutectic) which under the effect of the rolling forces give
rise to the generation of cracks and fissures that propagate preferentially along grains
boundaries. This determines structural discontinuities (on the surface of the laminate but
more generally in the whole iron matrix) and presence of oxides that in the subsequent
cold rolling will rise on surface level due to thickness reduction and will lengthen due to
the stretching so creating those superficial defects that make those cold-rolled and
90
annealed coils no longer suitable for use on important drawing tasks such as the household
appliance, the automotive sector and the most electric equipment in general. As a matter
of fact, these surface defects can compromise the appearance, not only structural but also
visual, of a uniform painting or of a quality glazing or of a thin layer of metallic coating
(for example zinc) applied on the surface by electrolytic means. Also in this case, the
experimental results obtained in this thesis work confirmed what reported in the technical
literature. From the experimental analysis of the CH3N coils and from the comparison
with the observed superficial defects, it emerged that the latter decreases drastically when
the Mn / S ratio is greater than 23. This is therefore the minimum critical value to be
guaranteed for a steel like the one used, to reduce the formation of the low melting FeS
eutectic and therefore to improve the hot fragility of the steel at high temperatures.
The higher is the Mn / S ratio, the higher is the possibility of MnS manganese sulphides
formation, during solidification, which have a higher melting temperature and are not as
damaging as FeS iron sulphides are formed. The minimum value of Manganese must be,
as mentioned, 0.2%, while the maximum value is determined here by the steel drawing
class and cannot exceed 0.3%. This means that the value of the Sulphur to be guaranteed
for a value of Mn / S above the critical one must be that typical of a desulfurized steel,
with Sulphur levels lower than 0.010%.
5.4 New casting design proposal
To complete this thesis work with a verification in the field of technical considerations
and results arising from the study and metallographic and microstructural analysis of over
one hundred cold-rolled CH3N coils, it was proposed to carry out a new casting of the
boron CH3N steel modifying the chemical compositions of some elements and some
manufacturing and control procedures as indicated below:
1) The boron must be at the minimum of the indicated range because it is a chemical
element suspected of giving rise to the onset of fractures and structural discontinuities
which then cause surface defects called "nodular oxides";
2) Great attention must be paid to the practice of boron addition to avoid the reduction or
even the elimination of its effectiveness. The boron should be added at the end, after all
the other alloys additions, at the lowest possible temperature.
91
3) To protect the boron from the reaction with nitrogen, then to limit the formation of
boron nitrides BN until low temperatures are reached, it is appropriate, before adding the
boron, to add Titanium in a concentration between 0.015 and 0.020%.
4) In order to reduce the oxygen and nitrogen levels as much as possible, it is advisable
to carry out a vacuum degassing treatment of the steel at the RH plant. Optimal nitrogen
values should be less than 40 - 50 ppm.
5) With these low levels of nitrogen, even aluminum should be at the minimum level,
between a value of 0.020% which is the minimum value prescribed by the most important
and widespread product standards to ensure the steel’s killing, and a level maximum of
0.030%. A higher level would have the effect of excessively dirtying the steel and making
it sensitive to oxidation phenomena.
6) The sulfur concentration should be kept at the technically possible minimum level,
obviously compatible with the steel manufacturing costs. Having set a maximum level of
Manganese in steel equal to 0.3% for the guarantee of the cold rolled molding class, the
concentration of Manganese must be between 0.020 and 0.030%. In order to guarantee
an Mn / S ratio higher than the determined critical value (critical Mn / S> 23), the steel to
be produced must have a sulfur content not higher than 0.010% so as to limit the effect
of sulfur on the formation of low-melting FeS eutectic that causes hot fragility. With this
maximum level fixed for sulfur, it will be necessary to guarantee the desulphurisation of
the cast iron before proceeding with the manufacture of the steel to the converter.
7) All other chemical elements will be required with the concentrations already planned
on CH3N steel.
8) Finally, it will be appropriate to provide a straightening on the continuous casting
machine at temperatures above 990 ° C. A value of 1010 ° C shall be set to avoid the
formation of small transversal cracks that can be generated on the upper surface of the
slab due to the tensile stress induced on the upper fibers during the straightening of the
slab when the formation of the eutectic low-melting FeS takes place.
The following Tab. 13 compares the chemical analysis of CH3N steel on which the
characterization has been made and the new chemical analysis proposed, modified
according to the indications given, now called CH3S.
92
CH3N steel CH3S steel
Heat nr. 831513
CH3S
% min % max % min % max value
% C 0,030 0,040 0,030 0,040 0,031
% Mn 0,20 0,30 0,20 0,30 0,29
% Si -- 0,030 -- 0,030 0,010
% S -- 0,020 -- 0,010 0,007
% P -- 0,020 -- 0,020 0,016
% Ti -- -- 0,015 0,025 0,016
% Al 0,025 0,050 0,020 0,030 0,025
ppm B 25 35 15 25 17
ppm N -- 50 -- 40 34
Mn/S 41
Pig iron desolfurization not required required applied
Steel vacuum degassing not required required applied
Table 13 CH3N vs CH3S steel
The following figure shows the steel manufacturing board as loaded, after the necessary
approvals, in the computer of the Ilva steelworks in Taranto.
5.5 The new cast
The new cast of CH3S steel, identified with the progressive number 831513, was
produced on May 14th 2018 obtaining 12 slabs which were then hot rolled on thickness
3,0 mm and width 1035 mm. In the third column of Tab.1 is reported the chemical
93
analysis of the produced cast. The inspections carried out on hot rolled coils with the help
of the automatic surface inspection system did not show surface defects classifiable as
"nodular oxide" (Fig. 49).
Figure 49 Parsytec analysis of the new coils
The same automatic inspection system has highlighted, on a single roll, the presence of
sporadic superficial inclusions, probably alumina, whose gravity and nature will be
investigated after cold rolling (Fig. 50).
Figure 50 Superficial inclusions on one of the 13 coils
94
Chapter 6
New cast coils analysis
6.1 Data collection and Parsytec analysis
After having been hot-rolled, the 12 rolls of CH3S obtained from the corrective cast
831513 were sent to the ILVA Novi plant where they underwent an initial pickling phase
and then the cold rolling to obtain thicknesses in line with the demands of the current
market. Subsequently, they were processed at the continuous annealing plant CAPL. In
the same way as done in chapter 3 relating to CH3N coils analysis, for the 12 rolls of
CH3S a similar path was followed in order to determine if the modification of the new
designed cast has led to the elimination or at least the drastic reduction of the nodular
oxides widely present in the standard CH3N produced in the plant. The 12 coils of CH3S
were catalogued on an excel sheet highlighting their ID (Identification number), the
casting number, their dimensional characteristics, the production date, the mechanical
properties and their chemical composition (Table 14; Table 15).
COILS IN
CAST THICKNESS (mm)
WIDTH (mm)
STEEL YIELD STR.
(MPa)
TENS. STR.
(MPa)
ELON. %
ANIS r
W.H. n
1122503 831513 0.50 1000 CH3S 251 357 36.1 1.307 0.219
1121004 831513 0.50 1000 CH3S 294 381 31.8 1.159 0.206
1121005 831513 0.50 1000 CH3S 273 365 35.8 1.205 0.218
1121008 831513 0.50 1000 CH3S 289 363 35 1,285 0,204
1121001 831513 0.50 1000 CH3S 260 359 35.9 1.261 0.217
1121002 831513 0.50 1000 CH3S 289 369 36.4 1.215 0.212
1121003 831513 0.50 1000 CH3S 241 352 36.5 1.388 0.221
1121006 831513 0.50 1000 CH3S 238 351 37.3 1.371 0.222
1121007 831513 0.50 1000 CH3S 251 353 38.1 1.328 0.223
1122501 831513 0.50 1000 CH3S 261 358 37 1.334 0.219
1122502 831513 0.50 1000 CH3S 255 355 37.6 1.264 0.219
1122504 831513 0.50 1000 CH3S 263 359 36 1.348 0.216
Table 14 ID number, dimensional properties and mechanical properties
95
C Si Mn P S Nb Al Ti B N Mn/S B/N
0.031 0.01 0.288 0.016 0.007 0.0002 0.025 0.016 0.0017 0.0034 41.1 0.5
Table 15 Chemical composition
Once cataloged, the 12 rolls were visually analyzed at the inspection desk of the CAPL
line and subsequently at the Parsytec automatic inspection system in order to obtain an
overall view coil per coil. From Parsytec analysis, coils were completely free of nodular
oxides (Fig. 51) and were therefore classified with judgment class 0. In the following
figure it is possible to observe how the surface of one of the 12 rolls of CH3S viewed at
Parsytec appears. The defect, widely diffused on the surface of most of the CH3N rolls,
is completely eliminated as evidenced by the new CH3S coils.
Figure 51 Defect Free CH3S coil
6.2 SEM inclusions detection and study
Further investigations were carried out on roll 1121008 which had persistent defects, in
the first half, but not clearly attributable to a specific class of defects. Specifically, on a
visual and qualitative level the defect is of an inclusive nature, above all because even
though it has a physiognomy very close to that of the nodular oxide defect, it is much
96
longer. For this reason, it cannot be confirmed only by a macroscopic analysis. Therefore,
defect samples were taken from the coil examined and their SEM characterization was
performed. In Fig. 52 the defect displayed at Parsytec is shown while in the Fig. 53 it is
possible to see a micrograph of the defect analyzed by SEM.
Figure 52 Inclusion caught by Parsytec monitoring system
Figure 53 SEM inlcusional defect characterization
In this regard, from the multiple samples with defect taken into account, the non-
correspondence with the classic nodular oxide defect and the hypothesis of an inclusive
97
nature of the defect was confirmed. From the micrographs it is possible to notice that
these inclusions are specifically Alumina inclusions (Al2O3) and that have nothing to do
with the problems of hot fragility treated in this thesis.
6.3 Comparisons: CH3N vs CH3S
It is therefore possible to state with complete certainty that the nodular oxide defect has
been completely avoided leading to the clear defectiveness comparison exposed in figure
54.
Figure 54 Defectiveness distribution on CH3N vs CH3S steels (for the legend see scheme in Fig. 41)
98
Considering the results obtained, it is possible to state that none of the 12 coils under
examination will undergo a derating due to nodular oxide defects (probably only a part
of 1 of the 12 will be downgraded due to the Al inclusional defects), constituting an
important improvement with respect to CH3N steels where, as previously seen, the total
production downgrade caused by nodular oxide reached the 16% of the whole production.
6.4 Coils characterization – Mechanical properties
If from the qualitative point of view the CH3S coils have been found to be optimal since
they are not affected by the defects widely observed previously with CH3N steels, in
order to check their efficiency and guarantee their performance it is necessary to verify
that the mechanical properties fall within the ranges established for this kind of steel. The
pre-established limits coming from EN10130 standards for grade DC01 (which is CH3N
steel) are shown in Tab. 16.
STEEL
QUALITY
STEEL
NAME
SNER Rp02
(MPa)
ROT Rm
(MPa)
T ALL %
min
ANIS r
min
INCR n
min
DC01 CH3N 140 - 320 270 - 410 24 / /
Table 16 CH3N Mechanical properties acceptance ranges (EN10130:2007)
Table 17 shows the average of the mechanical properties evaluated for CH3N steel (the
standard one used before the modification) and for CH3S steel (the steel obtained
following the new chemical composition).
SNER Rp02 (MPa) ROT Rm (MPa) T ALL % ANIS r INCR n
CH3N 228 346 37,4 1,391 0,226
CH3S 264 360 36,1 1,289 0,216
Table 17 Mechanical properties means of CH3N vs CH3S
99
From the data shown in Table 17 it is possible to notice an increase in terms of yield
strength of about 40 Mpa and breaking load of about 25 Mpa of CH3S compared to
CH3N, but it does not compromise the use of steel as it is perfectly within the range
established. This increase can be correlated mostly to the higher Manganese content in
CH3S steel (value close to the maximum value foreseen in the steel fabrication sheet) and
to a minimal extent to the introduction of Titanium in the casting process. As a matter of
fact, Titanium forms Fe3Ti intermetallic compounds which cause precipitation hardening
with a consequent reduction in ductility but, at the low percentages in which it is present
in this steel, the effect is rather negligible.
6.4.1 Grain analysis
Once verified that the mechanical properties are consonant with the limits imposed,
another important factor is constituted by crystalline grains, in particular their structure
and size. For this purpose, the test was performed through an optical microscope
associated with the "Analysis" software provided by Olympus following the ASTM
E112-13 standard. No pancake structure has been detected (the pancake structure is a
typical configuration of the grains of cold rolled and non-annealed steels). So, what can
be deduced first of all, is that the annealing treatment has been carried out correctly by
conferring a correct recrystallization of the material. In addition, the dimensionless G
number (medium granulometry) and therefore the real diameter of the grain (d) was
found, with values of 12.10 (G) and 5.20 μm (d) respectively. Figure 55 shows the grain
analysis on the CH3S steel sample under examination.
50 µm 20 µm
Figure 55 Grain analysis
100
6.4.2 Aging Test
One of the characteristics of low carbon boron steel is that it has a lower tendency to
aging than a steel, always low carbon, without boron. Therefore, in this thesis work some
laboratory tests were carried out to simulate the behavior of the new designed boron steel
(CH3S) compared to a common steel with no boron. The test is carried out by pre-
defroming the tensile test sample of 7% by means of a robotized island and then by
immersing the sample in an oil bath at a temperature of 100 ° C for about 1 hour. The
difference between the yield strength evaluated in the standard test tensile and that
evaluated after immersion at 100 ° C for 1 hour, gives the so-called Aging index (AI) (27)
. In this regard, 6 steel samples were taken, 3 related to a common low-carbon steel
without boron and 3 others related to the steel under examination (CH3S); to make the
test homogeneous and consistent, steels with thicknesses (̴ 0.5 mm) and widths (1000
mm) were selected. The characteristics of the samples taken into consideration are shown
below (Tab. 18).
ID 1122415 1125905 1125906
Thickness (mm) 0.55 0.55 0.55
Width (mm) 1008 1008 1008
Steel (L.C. no Boron) (L.C. no Boron) (L.C. no Boron)
Cast 821112 821119 821118
ID 1121004 1121005 1121008
Thickness (mm) 0.5 0.5 0.5
Width (mm) 1000 1000 1000
Steel CH3S (L.C. +
Boron) CH3S (L.C. + Boron)
CH3S (L.C. +
Boron)
Cast 831513 831513 831513
Table 18 Sample characteristics
101
Steel ID Thickness
(mm)
Rp02
(MPa)
Rm
(MPa)
A80 r n Rp/Rm
Tensile test
L.C
.
+Bo
ron
(CH
3S)
1121004 0.49 294 381 31.8 1.159 0.206 0.770
1121005 0.49 273 365 35.8 1.205 0.218 0.750
1121008 0.50 288 366 36.6 1,285 0.204 0.790
L. C
.
no
Bo
ron
1122415 0.54 270 376 33.3 1.347 0.187 0.720
1125905 0.54 275 384 29.7 1.308 0.177 0.720
1125906 0.55 266 363 35.6 1.292 0.210 0.730
Tensile test after 7% pre-elongation and baking at 100°C for 1 h
L.C
. +
Bo
ron
(CH
3S)
1121004 0.48 425 431 18.4 1.306 0.024 0.987
1121005 0.48 407 410 20.1 1.290 0.054 0.993
1121008 0.50 404 406 21.2 1.340 0.065 0.994
L. C
.
no
Bo
ron
1122415 0.53 438 450 11.2 - - 0.973
1125905 0.53 428 429 11.4 - - 0.999
1125906 0.54 385 391 27.1 1.369 0.095 0.988
Table 19 Tensile test before and after Aging treatment
ID AGING INDEX (AI) =Rp (after 7% el. and baking at 100°C
for 1 h)- Rp02 [MPa]
L.C. + Boron
(CH3S)
1121004 131
1121005 134
1121008 116
Mean 127
L. C.
no Boron
1122415 168
1125905 153
1125906 119
Mean 147
Table 20 Aging indexes
102
From the results obtained, it can be observed (Tab. 20) that in CH3S steels the Aging
index has a value of about 20 MPa less than the common low carbon steels without boron
and this confirms one of the most important peculiarities that characterize boron steels.
Therefore, CH3S steel is also approved from the point of view of resistance to aging.
6.5 Conclusions
The inspections, investigations and tests carried out showed that:
• The new chemical analysis has guaranteed a drastic reduction of the hot fragility of the
steel favoring a superficial quality, of the cold re-rolled coils, free from defects;
• The mechanical and metallographic characteristics of the new steel comply with the
limits imposed by the applicable product standards;
• The new CH3S steel has an aging resistance (AI) better than that of a corresponding
low-carbon steel without boron.
Considering the results obtained, the new CH3S steel can replace the old CH3N steel.
103
Conclusion
This thesis treats an experimental work carried out at the Ilva plant in Novi Ligure, in the
province of Alessandria, at the Continuous Annealing Production Line (CAPL). On this
"continuous and fast" high-performance annealing plant, low-carbon aluminum killed
steels with small addition of boron are widely used to produce cold-rolled coils for
medium and deep drawing. Boron, in fact, allowing to work at lower annealing
temperatures, makes it easier to recrystallize grains deformed by cold rolling.
Neverthless, the presence of boron worsens the steel's hot ductility, favoring an
intergranular embrittlement and the formation of facial and intergranular cracks right
from the casting and reheating phase of the slab before the subsequent hot rolling. During
hot rolling, under the effect of rolling forces, cracks and crevices are formed which
preferentially propagate along the edges of the grains creating on the surface of the
laminate, but more generally in the whole iron matrix, structural discontinuities and
presence of oxides which will emerge during and after cold rolling. As documented, the
defects are already present at the time of hot rolling, but they are even more evident after
the cold rolling, which, with its thickness reductions and the stretching of the fibers,
further stresses the pre-existing defect and lengthens in the direction of lamination. These
elongated defects, called "nodular oxides", when examined under an electron microscope,
show the presence of iron oxide, iron sulfides and in some cases boron nitrides. This kind
of defects stands far away from the acceptance windows of the most important drawing
sectors because it can compromise the surface appearance or even the integrity of the
finished product.
For this reason, at the final inspection on the continuous annealing line, a high percentage
of coils is downgraded or destined for other less demanding uses, thus generating a
deterioration in quality performance, an increase in production costs and delays in
delivering the materials to customers which are increasingly hard to please, both in terms
of product quality and compliance with delivery times.
So, the first objective of this work was to investigate and analyze the causes of high
surface defects. To this end, an experimental procedure was defined to characterize and
analyze all the steel coils examined in the first quarter of 2018, starting from the analysis
of the defects detected by a continuous automatic surface inspection system (Parsytec)
supplied to the Annealing line. The definition of a classification criterion of defects,
according to their gravity, extent and position, has been useful to establish the presence
of correlations with the chemical composition of the different castings involved.
104
Metallographic tests were also carried out to evaluate the crystallographic texture of the
steel and to look for any dependencies of the defect from the thermal cycle to which the
steel is subjected during annealing. The granulometric analyzes and EBSD tests carried
out using the electronic back-scatter diffraction technique showed the structure of a
classic cold-rolled and correctly annealed low-carbon steel, thus confirming the initial
hypothesis of the birth of the defect in the casting phases of the slab and subsequent hot
rolling.
The most important results of this work have emerged correlating the different classes of
defect with the content of some chemical elements such as boron, manganese,
phosphorus, nitrogen, aluminum and titanium. Particularly interesting were data referred
to the B / N and Mn / S ratios for the different casting designs involved. According to
what is reported in the technical literature of the sector, it has been seen, for example, that
the superficial defectiveness decreases drastically as the Mn / S ratio increases and that
there is a "critical" value below which the defectiveness can be high and above which it
is basically absent.
Starting from the analysis of the defects and the observations made, it was possible to
define an operative improvement proposal to modify some elements of the chemical
composition of the steel (among them boron, nitrogen, manganese, aluminum, sulfur and
titanium) and some parameters of process among which the modalities of addition of the
boron to keep its action effective and the casting speed to maintain the temperature of
straightening of the slabs in continuous casting above a certain minimum value.
The Ilva Company has considered promising the proposal made, both from a
metallurgical and an industrial point of view and has produced a new casting (about 300
tons) with the new proposed chemical composition design. The observations, surveys and
characterizations carried out, always respecting the experimental procedure initially
defined, confirmed that the coils are free from the defects that characterized the "old"
production. The new steel has also been characterized in terms of mechanical strength,
drawability characteristics and resistance to aging, and complies with the applicable
product standards.
In these conditions the new experimental chemical composition design can therefore be
considered homologated, at least relating to this thesis work and to the tested coils. The
next step will be that of a wider experimentation of an industrial type, extending the test
to a greater quantity with the aim of confirming the design of the new steel and
consolidating the operating procedures of steelmaking, casting and control.
105
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