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BY

DR. BIAN JIAN* Jian.bian@niobiumtech.com

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

Park, Colo, USA, 2011 7. M. Nagal and P. Hoefel, “HD” high ductility, low alloying, fine-grain structure steels,

ThyssenKrupp techforum 1/ 2012 8. Mohrbacher, H. Microstructure optimisation for multiphase steels with improved formability

and damage resistance. Conference: Advanced high strength automobile steel of ISAS2013, Anshan China

9. Bian, J. et al, Application potential of high performance steels for weight reduction and efficiency increase in commercial vehicles, Advances in Manufacturing: Volume 3, Issue 1 (2015), Page 27-36