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BY
DR. BIAN JIAN* [email protected]
SYNOPSIS
Steel properties are predominantly influenced by microstructural features. These microstructural
features like phase constitutes as well as their volume fraction, distribution and size can be
significantly improved through grain refinement during the manufacturing processes. It is well
known today that grain-refined steels demonstrate superior properties to conventional ones in the
aspect of strength, toughness, formability, weldability and durability in terms of fatigue and wear
resistance. The development history of Nb metallurgy has clearly demonstrated that thermo-
mechanically controlled process (TMCP) tailored with Nb microalloying is the most effective and
practical way to achieve grain refinement compared to microalloying of Ti and V. Therefore, Nb
micro-alloyed steels have been widely produced for different applications in the different segments
for several decades. Today the Nb metallurgy for high strength steels such as low carbon
microalloyed steels (HSLA) and advanced high strength steels for automotive application and for
oil and gas transportation are well established and routinely practiced.
This paper will explain the fundamentals of grain refinement through Nb microalloying and its
impact on microstructural evolution and resulted property and performance improvement of
commonly produced steel grades containing different microstructures regardless of ferrite, perlite,
bainite, martensite or even multi-phase microstructure of advanced high strength steels.
Keywords: Nb microalloying, TMCP, microstructure, phase constitutes, grain refinement, steel property.
* Managing director, Niobium Tech Asia in Singapore
Consultant for CBMM Brazil
Grain refinement- the powerful metallurgical solution
for high performance steels
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1. Introduction
It is well known that the performance of steel products depends on the relevant material properties
which are dominantly controlled by the steel microstructure developed during the entire
manufacturing processes. In this regard achieving an optimum microstructure is always the target
of any metallurgical process for any steel grade no matter for which application. Microstructural
refinement provides power metallurgical solution to achieve such microstructure. Over last
decades Nb microalloying in combination with TMCP has been applied to produce a wide range
of fine-grained steels for different applications compared to Ti and V, because Ti will form TiN at
high temperature causing toughness deterioration, especially at lower temperature, while V has
little influence on the recrystallization behavior of austenite in solute condition during rolling
process [1, 2]. Therefore, by using much lower Nb content the same strength level can be achieved
with much finer microstructure and improved steel properties. This paper will explain the
fundamentals of grain refinement through Nb microalloying during TMCP and its impact on the
performances of steel products for different applications.
2. Fundamentals of grain refinement through Nb microalloying
The fundamentals of grain refinement through Nb microalloying are principally based on three
important effects throughout TMCP process (Fig.1) and will be explained in detail.
Reducing austenite grain size during TMCP rolling
Retarding phase transformation to lower temperature during cooling
Preventing grain coarsening during coiling
Fig.1. Fundamental principles of grain refinement through Nb microalloying during TMCP
process.
2.1 Reducing austenite grain size during TMCP rolling
Reducing austenite grain size at the end of rolling process is essential to produce fine-grained steels
because the transformed new phases regardless of ferrite, perlite, bainite or martensite will nucleate
(start) at grain boundaries or at deformation bands within austenite grain. Smaller, especially pan-
caked, austenite grains provide more grain boundary areas and high density of deformation bands
for nucleation of new phases. For the conventional rolling process of plane carbon steels static or
dynamic recrystallization (RX) process will takes place leading to grain refinement of austenite to
some extent. Nevertheless, the recrystallized austenite grains have much lower density of
dislocation and deformation bands which provide nucleation sites for phase transformation as well.
For the Nb microalloyed steels rolled in TMCP process the RX process can be severely retarded
by Nb during rolling process either through NbC precipitates which can pin the austenite grain
boundary and supress RX process or through so called “solute-drag-effect” of Nb which remains
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in solid solution. Due to the synergy of both effects higher temperature and/or very large
deformation is required for completion of RX of Nb alloyed steels. Generally, in the temperature
range of finish rolling due to limited deformation, RX process can be completely supressed in Nb
alloyed steels leading to austenite pancaking, as shown in Fig. 2 [3].
Fig. 2. Effect of deformation temperature and initial grain size on critical amount deformation
required for completion of recrystallization in plain carbon and Nb steels
In comparison to recrystallized grains the pancaked austenite grains provide far more grain
boundary areas and higher density of dislocation and deformation bands as nucleation sites for
phase transformation as found by the independent researcher [4] (Fig. 3), thus at the same cooling
rate Nb microalloyed steels have much smaller ferrite grain size after transformation during
cooling process (Fig. 4).
2.2 Retarding phase transformation to lower temperature after rolling
After finish rolling the grain size of austenite is fixed, it means that the influence of austenite
structure on the phase transformation is also fixed. However, the thereafter cooling process on the
run-out table will also have important influence on the phase transformation, Fig. 5. Generally,
with increasing cooling rate the transformation temperature of ferrite, perlite and bainite except
martensite will be suppressed to lower temperature. Due to higher supercooling of austenite, more
nucleation sites will be available and the transformation takes place kinetically much faster than
the transformation at higher temperature which leads to further grain refinement. Fig. 6 shows the
significant difference in microstructure of the same steel with different ferrite transformation
temperature 700 and 6200C which led to the average ferrite grain size by 4.6 and 2.5µm after
transformation respectively. It is obvious that due to gran refinement the phase distribution of
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ferrite and perlite is very homogeneous (right picture in Fig. 6) and there are no big clusters of
perlite. The benefits of such microstructure will be explained later. During cooling process, Nb
which remains in solid solution after rolling process (investigations show that this is particularly
the case for steels containing low and medium carbon content) has the similar effects to retard the
phase transformation to lower temperature because Nb can stabilize austenite due to the strong
lattice distortion caused by solute Nb. Bian, et al. found that even in the medium carbon steel
containing 0.25%C and 0.05%Nb a large amount of Nb remained in solid solution after rolling
process because a high density of NbC carbide was identified in the range between 5 and 25
nanometer in the final microstructure. Such fine NbC carbide can only develop at lower
temperature during cooling and/or coiling process [5]. It is also reported that even fine-grained
austenite will not necessarily have lower hardability than coarse-grained austenite during cooling
process. It is hypothesized that austenite grains refined through Nb microalloying may facilitate
segregation of key solutes to austenite grain boundaries such as Mn, Cr and even Nb, which can
retard ferrite nucleation on grain boundaries and contribute to hardenability [6]. This hardability
effect of Nb is particularly beneficial to produce high strength steels with reduced cooling rate for
the improved band shape and flatness.
Fig. 5. Impact of cooling rate and Nb in solid solution on the transformation temperature of
different phase after hot rolling
Fig. 6 Impact of transformation temperature on the grain size of ferrite after transformation from
the same austenite conditions (0.07C-1.5Mn-0.023%Nb).
2.3 Preventing grain coarsening during coiling
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The final step of TMCP is to coil the steel for the strip production. In the most cases the set up
coiling temperature is also the transformation temperature of the targeted microstructural phases
and simultaneously Nb remained in the solid solution by then will precipitate as NbC during the
coiling process depending on the coiling temperature. The precipitated NbC carbide will help to
pin the grain boundaries of the transformed phases and prevent grain coarsening (Fig. 7). Another
important contribution of NbC carbide is precipitation hardening. The optimum coiling
temperature is around 6500C to get fine or even nano-sized particles for maximum strengthening.
Nb precipitation process could be suppressed if coiling temperature is too low (below 5000C). If
coiling temperature is beyond 7000C precipitated particles will grow due to Ostwald ripening effect
leading to weakening of pining effect on one hand and on the other hand drop of the strength. It
was found that for steels based on strengthening via grain refinement and precipitation hardening
the coiling temperature around 6000C will cause large scattering in mechanical properties along
the coil because precipitation hardening will be suppressed to some extent due to faster cooling
rate in inner and outside of the coil. It can be concluded that Nb microalloying makes important
contributions to the grain refinement throughout the entire hot strip production from rolling to final
coiling. In the following part of the paper examples will be given to demonstrate how Nb
metallurgy has been adopted to produce steel grades based on the different microstructures and the
impact of grain refinement on the property improvement.
Fig.7. Explanation of grain boundary pining through precipitated NbC carbide during coiling
process (schematically).
3. Benefits of grain refinement to steel performances
The contribution of grain refinement to the improvement of steel performances will be explained
based on some examples of different steel grades for different applications in the following
aspects:
3.1 Increasing the strength and toughness simultaneously through grain refinement
HSLA steels are microalloyed low carbon steels which have been widely used in mechanical
engineering, automotive segment, pipe line and other applications in both hot and cold rolled
conditions [7]. Due to the low carbon content (generally less than 0.08%) the strength of HSLA
steels is dominantly attributed to grain refinement and precipitation hardening in the typical
ferritic-perlitic or bainitic microstructure provided by Nb or Nb-Ti microalloying. By reducing the
grain size of ferrite and perlite or bainite and precipitation hardening the yield strength can be
significantly increased up to 700MPa and at the same time the DBTT temperature drastically
decreased. Among all strengthening mechanisms in the steel, grain refinement is the only one to
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increase strength and toughness simultaneously, as shown in Fig. 8. Fig. 9 shows one example of
commercially produced HSLA steel with ultra-fined microstructure containing bainitic ferrite and
little perlite. Through microalloying Nb/Ti and TMCP process the excellent mechanical properties
have been achieved (Fig. 10).
Fig.8. Impact of ferrite grain size on the yield strength and toughness of HSLA steels.
3.2 Improving the formability of multi-phase steels through grain refinement
With increasing strength, steel formability becomes a critical issue both technically and
economically for some applications such as automotive segment. For this reason, multi-phase
steels, such as dual-phase steels (DP), transformation induced plasticity steels (TRIP) and complex
phase steels (CP), have been developed with good formability to make safety components in the
car body structure for both passenger and commercial vehicles. Particularly, DP steels in the
strength range between 600 and 1000MPa have been widely used due to good formability and high
light-weighting potential over the last few years. However, DP steels generally have high
sensitivity to edge cracking which very often lead to the component failure in the press shop despite
high elongation and high work hardening. The major reason for this problem is the microstructural
inhomogeneity of soft ferrite and hard martensite phases. Intensive research work has been carried
out to improve the microstructure of DP steel with Nb microalloying [8]. The example shown in
Fig. 11 reveals the fundamental differences of microstructure between conventional DP780 based
on 0.15%C and fine-grained DP780 based on 0.07%C +0.025%Nb. The fine-grained DP780 shows
the significant improvement in microstructure in the following aspects:
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Much smaller grain size of ferrite (gray) and martensite (white)
Homogeneous microstructure without martensite clusters (bending structure)
Martensite islands are homogeneously distributed among ferrite
More ferrite and less martensite
Due to alloying modification and resulted microstructural improvement, the fine-grained DP780
performed much better in the welding tests and fatigue tests. Particularly, in the tensile tests high
elongation and high hole expansion ratio have been achieved simultaneously, Fig. 12. Fine-grained
DP780 is suitable for both conventional press-forming and roll-forming operations with reduced
sensitivity to edge cracking. Furthermore, grain refinement and Nb precipitation hardening provide
additional strengthening to DP steel so that the total carbon content of DP steel can be reduced for
a better weldability. Finally, grain refinement contributes to the better toughness and fatigue
behavior which have important impact on the component performance, especially at lower
temperature.
Fig.11. Optical micrograph of cold rolled DP780, left: conventional DP780 (C=0.15%), right: Nb
alloyed fine-grained DP780 (C=0.07%, Nb=0.02%)
Fig.12. Impact of grain refinement on the total elongation and hole expansion property of DP780.
3.3 Improving the weldability by optimizing the property of HAZ through grain refinement
With increasing strength weldability of steel need to be specifically considered. weldability is
directly related to vehicle manufacturing having considerable impact on processing feasibility.
Material failure occurring during welding will reduce the production efficiency and consequently
increase the production costs. Generally, it is important to reduce the carbon equivalent for good
weldability. Increased carbon content in combination with low heat input causes high hardness in
the heat-affected zone (HAZ) with the risk of cold cracking. Welding with large heat input reduces
the cooling rate in HAZ and causes toughness drop. Steel based on low carbon content and Nb
microalloying can effectively provide high strength and good weldability simultaneously. Both
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hardness and toughness in the HAZ depend much on the heat input and cooling rate of the applied
welding process. For a good combination of both properties it is important to define a process
window in terms of heat input and cooling rate in order to limit the maximum hardness below
350HV and the transition temperature (T27) below -40°C. A narrow process window indicates
poor weldability from the material side and difficult weld processing from manufacturing side.
Reducing the carbon content from 0.08 to 0.03% and increasing Nb content from 0.06 to 0.09% in
an innovative alloy concept for steel grade S500MC (fine-grained hot rolled steel with min.500YS
for structure and automotive application) the HAZ hardness could be significantly reduced over
the entire range of heat input experienced by typical assembly welding processes. Such an alloying
concept also allows a larger process window in terms of cooling rate after welding avoiding cold
cracking and generally providing good toughness in the HAZ, Fig. 13.
Fig. 13: Influence of alloying strategies and cooling rate (heat input) on the operating window in
terms of adjusting hardness and toughness in HAZ in the cooling process
3.4 Improving the wear resistance of martensitic steels through grain refinement
Martensitic steels provide the highest strength among all steel grades, thus one of the most
important applications for martensitic steels is wear resistance. Traditionally, low alloyed wear
resistance steels did not contain Nb. However recent research results found out that the wear
resistance of martensitic steels does not always increase with hardness due to deterioration of
impact toughness caused by increasing carbon content, particularly when the steel is under heavy
impact of high incidence angle. The key solution to solve the problem is to improve the impact
toughness by refining the martensite substructure with Nb microalloying. For example, the Nb
alloyed NM450 (abrasion resistant steel plate with 420-780HBW according to China GBT / 24186)
was developed in China. By adding 0.03%Nb in the traditional NM450 the austenite grain size can
be remarkably reduced under the same production conditions compared to conventional NM450.
This leads to finer martensite substructure with high density of large angle grain boundary to
impede the crack propagation. Consequently, the Charpy energy was improved significantly at -
400C through grain refinement (Fig. 14). The wear time index (wear time index = total wear time
of NM450 / total wear time of Q235) was improved by 14% compared to traditional NM450 (Fig.
15). Through Nb microalloying the average prior austenite grain size is smaller and the grain size
distribution is much more uniform. This will help to improve the band shape and flatness after the
quenching process as well.
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Fig.14. Impact of Nb microalloying on the Charpy energy of wear resistance steel H450.
Fig.15. Increment of wear time index of Nb alloyed NM450 compared to traditional NM450
(wear time index = total wear time of N450 / total wear time of Q235).
3.5 Improving the fatigue behaviour of medium carbon steels for leaf and coil spring through
grain refinement
Medium carbon steels like GB50CrVA (Chinese brand for 50CrV4) are used to make coil or leaf
springs through QT process for vehicles and other applications. For such application, the fatigue
performance of the steel plays a central role in the component design. The high fatigue
performance enables high durability in the service life and at same time implementing lightweight
design for springs. The traditional alloying design of 50CrVA contains only V for precipitation
hardening during tempering process. However, V does not make grain refinement during hot
forming and quenching processes. In order to improve the fatigue behavior of the traditional steel
for high performance it is necessary to implement grain refinement for this particular grade. It was
found out that with 0.03% Nb addition the fatigue endurance of 50CrVA was increased by 40%
under the same manufacturing and test conditions (Fig. 16). Currently the comprehensive
investigations are still going on.
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Fig.16. Improvement of fatigue behavior of spring steel 50CrVA through grain refinement.
4. Discussion
Grain refinement is the only strengthening mechanism in steel which improves the toughness,
especially at lower temperature simultaneously, regardless of the developed microstructure, such
as ferrite, perlite, bainite, martensite or multiphase microstructure. Grain refinement can increase
the yield strength up to 300MPa in the ferritic-perlitic or bainitic steels according to Hall-Petch
relationship, however its effect on martensitic steels is much less profound. This enormous
strengthening potential through grain refinement is very often applied to produce high strength
steels for automotive, structure and line pipe applications which generally require low carbon
equivalent for good weldability. The alloying concept of low carbon and high Nb (HSLA) can
avoid excessive hardness in the weld seam on one hand and ensure sufficient toughness in the
HAZ after welding on the other hand. Due to grain refinement, more high angle grain boundaries
will develop in the steel matrix which can effectively deflect propagation of micro cracks and
increase the critical fracture stress of the steel, shown in Fig 17. This improves the behaviour of
the steel in the Charpy impact and fatigue tests. Grain refinement also makes important
contribution to homogenization of the steel microstructure, especially for the multi-phases so that
the different microstructural constitutes will distribute homogenously. This will help to suppress
the banding structure which could develop during phase transformation after hot rolling. The
severe banding structure in the steel matrix facilitates the crack propagation along the banding and
deteriorates the forming behaviour, especially in the bending operation. It is obvious that grain
refinement not only increases the strength, but also improves steel performance in forming and
welding and finally in the Charpy impact and fatigue tests. Nb alloyed fine-grained steels are high
performance steels.
Fig.17. Explanation of grain refinement effect on crack propagation and fracture process of high
strength steels (schematically)
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5. Conclusions
Steel properties are predominantly influenced by the microstructural features which can be
significantly improved by Nb microalloying in terms of phase constitutes, grain size and
homogeneity. Grain refinement through Nb microalloying is principally attributed to three
important effects throughout TMCP process.
Reducing austenite grain size through TMCP rolling process
Retarding phase transformation to lower temperature through solute Nb in austenite during
cooling process
Preventing grain coarsening through NbC precipitates during coiling and post annealing
processes
Today niobium metallurgy has been widely applied not only to develop advanced high strength
steels for sophisticated applications but also adopted to upgrade many conventional steel grades
for advanced usages [9].
References
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135 4. International Symposium on Thin Slab Casting and Rolling (TSCR’ 2002), Guangzhou,
China, December 3-5, 2002, Chinese Society for Metals.
5. J. Bian et al., 5th International Conference on hot sheet metal forming of high-performance
steel, chs2 2015, Toronto Canada, page 65-74
6. J. G. Speer, International Symposium on the Recent Developments in Plate Steels, Winter
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