iron and steel making - budapest university of technology ......steel making plant foundry....
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Iron and steelmaking
Department of Materials Science and Engineering
Materials Engineering
Prof. Péter János SZABÓ
Department of Materials Science and Engineering
Metals: rarely exist in pure state → mostly in ores
Ore: Metallic and other compounds,
mostly oxides
Iron ores: 30-70% Fe
Copper ores: 0.1-0.8 % Cu
Molybdenum: 0.01-0.1% Mo
4 basic way to gain the metallic parts from ore:
Reduction by carbon
Electrolytic way
Metallotermical process
Dissociation
Metallic content:
costs
1) Reduction by carbon MeO + C → Me + CO
FeO + C → [Fe] + {CO}
gasmolten metals
2) Electrolytic way Al2O3 → Al23+ + 3O2-
on the cathode: Al3+ + 3e-→ Al
3) Metallothermical process
TilCl4 + 2Mg→ [Ti] + 2MgCl
4) Dissociation MeX → [Me] + [x]
only at high energy level
Iron and steel making
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Blast furnace
Foundry
Steel making
plant
Foundry
Production of molten steel
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Iron producing processes
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Purpose: Iron ore → Pig Iron
ore types: Fe3O4 magnetite ~70% Fe
Fe2O3 hematite ~70% Fe
FeCO3 siderite ~50% Fe
+ tailings: silicates, sand, other non ferrousMnO, Al2O3, P2O5, etc
Concentration of ore
cost
cost of blast furnace
cost of concent-rating
cost of pig iron
Fe % in ore 30 ~ 70 % magnetite
Blast Furnace Plant
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Blast Furnace Plant
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Dimension of the BF:
Diameter: 4-10 m
Height: 25-30 m
Volume: 300 – 5000 m3
Charge: Ore + Coke + Limestone
Tasks
1) Reduction of the ore
2) Extraction of tailings3) Melting → separation of the molten iron from the
molten tailings (spec. weight difference)
For 1000 t of iron:
2000 t ore +
800 t coke +
500 t limestone +
~ 4000t hot air
Processes in blast furnace
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Charge moves down (6-8 hours)
- Preheating by gas: coke burns more efficient
Formation of CO
CO reacts with iron ore
- Coke reduces CO2 in the gasC + CO2 → 2CO
- CO reduces the surface of the iron ore. Indirect reductionFeO + CO → Fe + CO2
- Slag producing by limestone.CaCO3 → CaO + CO2
MgCO3 → MgO + CO2
- In the bosh the coke burnsC + O2 → CO2 + Heat
- The coke reduces the molten ore. Direct reductionFeO + C → Fe + CO
- Molten limestone + other slag components produce eutectic slag
Slag floats over molten iron
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C+O2→CO2
• Gibbs free energy
• Reduction of FeO from
690 °C
Processes in blast furnace- thermodynamics
Processes in blast furnace
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Carbon reduces the oxides:
FeO + C → Fe + CO
MnO + C → Mn + CO
SiO2 + 2C → Si + 2CO
P2O5 + 5C → 2P + 5CO
SO2 + 2C → S + 2CO
gasin molten iron
alloying elements
impurities
In BF carbon can reduce S, P, Cr, Mn, Si 70-90%and Ti 10-20%
Sulfur and phosphorous are harmful in pig iron, and they must be removed.
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Processes in blast furnace
Desulfurization
FeS + CaO → FeO + CaS
in molten slag
Dephosphorization
P2O5 + 5FeO + 5C + 4 CaO → CaO4P2O5 + 5Fe + 5CO
in molten iron
in molten slagin molten iron
in molten iron
gas
Result: pig iron
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At the bottom of the BF:
Slag on the top
Molten iron on the bottom with~4% C
Near to eutectic composition
Taping at different heights:
different composition different
purpose
C% Mn% Si% S% P%
for casting 3 - 4 < 1 < 4 < 0.1 < 0.1
for steel with Bessemer method
3 - 4 0.4 – 1 ~ 3 < 0.1 < 0.1
for steel with Thomas method
3 - 4 0.4 – 1 ~ 2 < 0.1 < 0.1
for steel with Siemens-Martin method
3 - 4 0.4 – 1 ~ 1 < 0.1 < 0.1
Product of blast furnace
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http://www.youtube.com/watch?v=QBLRIEZZEsU
Taping:
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Product of blast furnace
Metallurgy
http://www.youtube.com/watch?v=kPH4dJUVOfc
Steel making
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Purpose Pig iron → steel by fire – refining treatments that
decrease the C content and impurities.
Main steps
1) Charging
2) Oxidation decreasing C content
3) Increasing temp. with decreasing C% the Tmelt increases !
4) Deoxidation decrease FeO and O in molten steel
5) Alloying
6) Casting, solidification
casting of ingots or
continuous casting of bars and billets
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Steel making
Processes
▪ Siemens Martin (open hearth furnace)
▪ Bessemer converter process
▪ Thomas converter process
▪ Oxygen converter process (Linz-Donawitz process - LD)
▪ Electric arc steel furnace
Siemens Martin process (1864)
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Charging
pig iron+scrap
pig iron + ore
Capacity
10-900 t
6-12 h
Too expensive
carbon
0,3 %/hour
burns out
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Siemens Martin process (1864)
Bessemer process (1856)
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Charging
molten iron
1210-1250ºC
~3% Si
Capacity
5-60 t
15-20 min
First converter method
No external heat
Acidic lining (slag
react.)
Si + O2 → SiO2
from the air
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Bessemer process (1856)
During the blow C, Si, Mn % decreases.
%
C
blowing time
15 min.
ºC 1700ºC
1250ºC
4%3%
1%
Si
Mn
O
N
Thomas converter process (1878)
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Charging
molten iron
1210-1250ºC
~2% P
No external heat
4 P +5 O2 → 2 P2O5
from the air
Similar to Bessemer, but basic lining for slag reaction.
Oxygen-converter process (LD)
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Charging
molten iron
~3% C
~0.5% Mn
~1% Si
~0.1% P, S
Capacity
15-400 t
No external heat
To avoid overheating when blowing iron ore or scrap are changed.Limestone is changed for desulfurization & dephosphorization.
Variants of Oxygen-converter process
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OLP process
oxygen – limestone – powder
oxygen & CaO powder is blown through the lance
AOD process
argon – oxygen – decarbonizing
oxygen & argon is blown through the lance
Mixed gas system → decreased partial pressure of oxygen
→ C% decreases
up to 0.002% C e.g. for stainless steels
Electric Arc Steel Furnace
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Charging
scrap + solid pig iron
~3% C
~0.5% Mn
~1% Si
~0.1% P, S
Capacity
5-200 t
For high grade steels
T > 2500ºC intensive reaction, N2 dissociation
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Electric Arc Steel Furnace
Charging scrap
http://www.youtube.com/watch?v=nolpiat6Sk0
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during work
http://www.youtube.com/watch?v=G6Uxh-xtU-g
Electric Arc Steel Furnace
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Electric Arc Steel Furnace
Electrode in the furnace
http://www.youtube.com/watch?v=3gg9_zTlg4M
Steel making - oxidization
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Purpose decrease C% and oxidize the impurities (S, P)
In open hearth and electric arc furnace
In air or oxygene blowing converters
C + FeO → Fe + CO
from scrapor iron ore
turbulence in the charge
2C + O2 → 2CO
from blowing air
The dissolved oxygen contentincreases
Hamilton’s law
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At the given temperature [C][O]=constant
O%
C%0.01
p=1bar
0.1 1
0.0001
0.001
0.1
0.01
p=1mbar
Stainless steels oxidization
requires vacuum
C < 0.02%O < 0.01%
The law of distribution and massaction
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At a given temperature the ratio of the amount of a given compoundIn the molten iron and in the molten slag is constant.
𝐿(𝑇) =𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔
𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛
The law of distribution
The law of mass action
Determines the direction of the reaction
mA + nB pABv1
v2
v1=k1(CAB)p V2=k2(CA)m(CB)
n
At equilibrium v1=v2
Effect of nonmetallic elements (S, P, O, N)
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Effect of sulfur
S does not dissolve, forms FeS eutectic with iron.
T
FeS %
~80% 100%
1000
1600
0%
Crystallization at the grain
boundaries.
Cold and hot brittleness
To reduce the effect: desulfurization
2) Increase S content1) Alloy with Mn
FeS + Mn → MnS + Fe generally S < 0.035%
MnS is formable at high temperature
grain
FeS at grain boundaries
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Effect of sulfur - desulfurization
To achieve low S%
• increase L : increase the temperature.
• increase CaO content in the slag
• decrease CaS content in the slag
• decrease FeO content in the slag
𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛 =𝐶𝑎𝑆𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔 ∙ 𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔
𝐾 ∙ 𝐶𝑎𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔∙ 𝐿
𝐾 =𝐶𝑎𝑆𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔 ∙ 𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛
𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛 ∙ 𝐶𝑎𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔=
𝐶𝑎𝑆𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔 ∙ 𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔
𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛 ∙ 𝐶𝑎𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔∙ 𝐿
𝐿 =𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔
𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛increases with temperature
The slag must be changed
Effect of nonmetallic elements (S, P, O, N)
37
Effect of nonmetallic elements (S, P, O, N)
Effect of nitrogen
nitride compounds precipitation
and/or the solidification of nitrogen in
interstitial solid solution.
▪ Increases strength decreases
toughness.
▪ Ageing
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Effect of nonmetallic elements (S, P, O, N)
Effect of phosphorous
Keep P content under 0.035%
(0.001%)
T
15%
1000
1.2%
αγ
Rm
Rp02
Z Rp02
P [%]
Rm
ZT
TTKV
Impact energy
TTKV
P% ↑
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Effect of nonmetallic elements (S, P, O, N)
Effect of phosphorous - dephosphorization
To achieve low P%
• decrease the temperature.
• increase CaO content in the slag
• decrease phosphate content in the slag
• increase FeO content in the slag
𝐾 =[𝐶𝑎𝑂4 · 𝑃2𝑂5] ∙ [𝐹𝑒] 5
𝑃 2 𝐹𝑒𝑂 5[𝐶𝑎𝑂]4
The slag must be changed
2 P + 5 FeO + 4 CaO → (CaO4 · P2O5) + Fe
in molten iron in molten slag Dissolves only in slag
in molten iron
[𝑃] =[𝐶𝑎𝑂4 · 𝑃2𝑂5] ∙ [𝐹𝑒]
5
𝐾 𝐹𝑒𝑂 5[𝐶𝑎𝑂]4
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Effect of nonmetallic elements (S, P, O, N)
Effect of oxygen
In form of O or FeO.
TTTKV
Impact energy
Brittle-to-ductile transition temperature
Work done till fracture
TTKV
O% ↑
strain
after long service period
after forming
initial state
stress
Ageing
To reduce the effect: deoxidation
Methods:
▪ Settling
▪ Diffusional deox.
▪ Synthetic slag
▪ Ladle metallurgy
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Effect of nonmetallic elements (S, P, O, N)
Effect of oxygen
Rm
Z
O [%]
Rm
Z
Deoxidation: settling
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Deoxidizing elements are loaded into the molten steel.
General reaction:
FeO + Me → MeO + Fe
The amount of deoxidizing elements are limited by their
disadvantageous effect on the properties:
Mn < 1% causes grain coarsening & brittleness
Si < 0.5 % it decreases the toughness.
V
Ti < 0.1 % they decreases the toughness.
Al
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Deoxidation: settling
Effect of deoxidizing element on the dissolved oxygen
O [%]
Si
Me [%]
0.01
CV
Mn
Ti
0.1 1
0.0001
0.001
0.01
0.1
Al
Zr
44
Deoxidation: settling
Rimmed steel Deoxidizing with Mn only
susceptible to ageing
Semi-killed steel Deoxidizing with Mn + Al
for continuous casting
Killed steel Deoxidizing with Mn + Si
lower TTKV than rimmed steel
Dead killed steel Deoxidizing with Mn + Si + Al/V/Ti/Zr
best quality from the point of brittleness
Deoxidation: diffusional and synthetic slag method
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Diffusional method
Deoxidizing element loaded on the top of the molten slag.
Diffusion of O to slag.
Diffusional synthetic slag method
The molten steel is poured on the top of prepared
FeO-free slag.
molten steel
FeO free slag
slag
molten steel
Deoxidation: ladle metallurgy
46
Ladle metallurgy
Powder injection
deoxidizing, desulfurizing, and dephosphorising
powder with Ar gas are blown into the molten
steel.
molten steel
This technology with the converter method is the most up-to-date steel
making process
- Inclusions are lifting to the slag.
- Almost isotropic
Vacuum handling
47
A process for deoxidation and degasification
The effect of vacuum on steel
1) Decreasing the partial pressure of the gas above the
molten steel
2) Decrease the content of oxygen.
3) Increase the vaporization rate of low melting point
metals (Zn, Pn, Sn, As)
4) Separates the compounds by dissociation.
Fe4N
CrN
FeO
AlN
TiN
SiO2
Al2O3
~10-6 bar ~10-9 bar 10-12-15 bar
Practically impossible
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Vacuum handling
Two type of process
Molten
steel
Vacuum chamber
Steel stream
Ladle
Degasificated
steel
Vacuum
Vacuum ladle degassing Vacuum stream degassing
Effect of dissolved gases on steel
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CO - in the rimmed steel produces gas bubbles
O2 - produces gas inclusions and oxide and silicate
inclusions
N2 - increase the ability to aging and nitride inclusions
H2 - flocking – H2 bubbles → cracking
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Effect of dissolved gases on steel
flocking – H2 bubbles
reason:
[H]
Temp
A3 A4Tmelting
[H]
Tmelting
A4
A3
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Effect of dissolved gases on steel
H2 – solution
let the gas atoms depart by diffusion
• slow cooling after casting (several days)
• forging by very soft deformation to make cohesion between
the surfaces of the cracks
+ for N make stable nitrides by mircoalloying elements
Al, V, Ti → AlN, VN …
Alloying, casting
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Alloying
Can only take place after a perfect deoxidation, otherwise
alloying elements would burn.
Casting
Two types: casting of ingots continuous casting
Casting of ingots
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• simple
• High productivity
• More homogeneous
• slow
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Casting of ingots
Solidification process for ingots
- Shrinking effect
- Crystallisation, grain-arrangement, mircostructure
- Segregation
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Casting of ingots
Shrinking effect
the top 12-15% of the total weight of killed
steel ingot must be cut off (rimmed steel only
3-5%)
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Casting of ingotsCrystallisation, grain-arrangement, mircostructure
RN N – number of crystal
nuclei
ΔT
R – rate of crystall growing
Supercooling –under theequilibrium
57
Casting of ingots
Segregation – normal segregation
During the solidification the liquid phase becomes enriched
with alloying elements and impurities.
T
Concentration of the liquid phase
B [%]
R – rate of crystall growing
P%
S%
C%
cross section of the ingot
The difference can be300% for S500-600% for P
58
Casting of ingots
Segregation – inverse segregation
Because shrinkage the alloying elements
and impurities can move inwards between
dendrites.
The impurity concentration is higher
between the dendrites’ arm.
Concentration %
dendrite
liquidphase
59
Casting of ingots
Microstricture and segragation in ingot
http://www.substech.com/
60
Casting of ingots
Continuous casting
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Continuous casting
• https://www.youtube.com/watch?v=d-72gc6I-_E
Steel refining methods
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All of these methods have a remelting and solidification period to:
- Decrease the dissolved gas content and the amount of inclusions
- Produce a homogeneous fine grained crystal structure
- Produce a homogeneous distribution of alloying elements
Used for
- tool steels
- high alloy steels
64
Steel refining methods
Vacuum arc remelting
process
▪ Removal of dissolved gases,
such as hydrogen, nitrogen
and CO;
▪ Reduction of undesired trace
elements with high vapor
pressure;
▪ Improvement of oxide
cleanliness;
▪ Achievement of directional
solidification of the ingot from
bottom to top, thus avoiding
macro-segregation and
reducing micro-segregation.
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Steel refining methods
Vacuum induction remelting process
▪ Removal of undesired trace elements with high vapor pressures
▪ Removal of dissolved gases (hydrogen and nitrogen)
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Steel refining methods
Electroslag remelting process
http://www.substech.com/
Similar technology:
electron beam
remelting process