Study of austenite grain growth and recrystallization behavior in pipeline steels containing niobium

Abstract

The austenite grain growth and recrystallization behaviors of three pipeline steels with different Nb contents were investigated through reheating and thermal simulation compression experiments. The initiation conditions for dynamic and sub-dynamic recrystallization of austenite were analyzed, and sub-dynamic recrystallization equations in Avrami form were established. The influences of Nb content and deformation conditions on the evolution of grain size during austenite recrystallization was examined. The findings indicate that the austenite grain size of the three steels increases gradually with higher reheating temperatures, while the average grain size decreases with increasing Nb content. Sub-dynamic recrystallization initiation temperatures for the B150-steel, B145-steel, and 73-steel were found to be 920 °C for 10 s, 940 °C for 30 s, and 960 °C for 30 s, respectively. During high-temperature deformation, Nb in solid solution hindered recrystallization by impeding grain boundary and dislocation movement. At lower deformation temperatures, Nb(C, N) precipitation pinned grain boundaries and dislocations and consumed substantial free energy, thus competing with recrystallization. As Nb content increased, strain-induced precipitation became more pronounced, resulting in more effective inhibition of recrystallized grain growth.

Full text

Mater. Res. Express 11 (2024)106513 https://doi.org/10.1088/2053-1591/ad8328
PAPER
Study of austenite grain growth and recrystallization behavior in
pipeline steels containing niobium
Fengliang Tan
1
, Jinbiao Cui
1
, Ning Liu
2
, Li Wang
1
, Jiansheng Chen
1,3
, Shiwei Tian
4,∗
and Yantao Li
1,3
1
Hunan University of Humanities, Science and Technology, Hunan, 417000, People’s Republic of China
2
Hunan Valin Lianyuan Iron and Steel Co Ltd, Hunan, 417000, People’s Republic of China
3
Loudi City Zhiyuan Emerging Industry Research Institute, 417000, People’s Republic of China
4
University of Science and Technology Beijing, Beijing, 100083, People’s Republic of China
∗
Author to whom any correspondence should be addressed.
E-mail: tswustb@sina.com
Keywords: Austenitising, strain-induced precipitation, Nb element, sub-dynamic recrystallization
Abstract
The austenite grain growth and recrystallization behaviors of three pipeline steels with different Nb
contents were investigated through reheating and thermal simulation compression experiments. The
initiation conditions for dynamic and sub-dynamic recrystallization of austenite were analyzed, and
sub-dynamic recrystallization equations in Avrami form were established. The influences of Nb
content and deformation conditions on the evolution of grain size during austenite recrystallization
was examined. The findings indicate that the austenite grain size of the three steels increases gradually
with higher reheating temperatures, while the average grain size decreases with increasing Nb content.
Sub-dynamic recrystallization initiation temperatures for the B150-steel, B145-steel, and 73-steel
were found to be 920 °C for 10 s, 940 °C for 30 s, and 960 °C for 30 s, respectively. During high-
temperature deformation, Nb in solid solution hindered recrystallization by impeding grain boundary
and dislocation movement. At lower deformation temperatures, Nb(C, N)precipitation pinned grain
boundaries and dislocations and consumed substantial free energy, thus competing with recrystalliza-
tion. As Nb content increased, strain-induced precipitation became more pronounced, resulting in
more effective inhibition of recrystallized grain growth.
1. Introduction
To reduce pipeline construction costs, enhance oil and gas transmission capacity, and withstand severe climatic
conditions, there is growing interest in developing high-strength, high-toughness, weldable, and corrosion-
resistant pipeline steel materials [1–3]. A key research focus is the production of ferrite-bainite dual-phase high
deformation resistance pipeline steel [4,5]. This steel is achieved through thermos-mechanical controlling
processing (TMCP), employing a low-carbon, Mo-free C-Mn-Si-Nb alloy system. The microstructure is
characterized by alternating distribution of polygonal ferrite and lath bainite (containing a small amount of
acicular ferrite).
Fine grain strengthening is among several common strengthening mechanisms that enhance both strength
and ductility [6,7]. Grain refinement is pivotal for improving the strength and toughness of pipeline steel. The
austenitizing temperature and holding time directly influence the initial size of austenite grains, the extent of
homogenization, and the dissolution of nanoscale carbon and nitride precipitation phases within the steel [8,9].
The hot rolling process of pipeline steel typically involves multiple rolling deformations where austenitic
recrystallization occurs during the intervals between passes. Pipeline steel benefits from repeated dynamic and
static recrystallization to refine its grain structure [10,11]. Factors influencing recrystallization during thermal
deformation include deformation conditions (temperature, strain, strain rate), inter-pass delay times, and the
number of passes [12–14]. Austenite recrystallization is a critical stage in austenite grain refinement, governing
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27 July 2024
REVISED
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ACCEPTED FOR PUBLICATION
3 October 2024
PUBLISHED
15 October 2024
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the microstructure evolution during hot rolling. The size of austenite grains directly determines the size and
morphology of ferrite grains after phase transition.
High-grade pipeline steels are often microalloyed with niobium (Nb), and its influence on the ferrite phase
transition during controlled rolling is quite complex. Niobium’s role in microalloyed steels is significant and
varies depending on its form within the steel matrix. Niobium can either dissolve solidly in the iron matrix or
form stable carbides, nitrides, or composite carbides with carbon and nitrogen [15,16].
Research by Jiang [17]indicates that the addition of niobium promotes the nucleation and growth of acicular
bainite while inhibiting the formation of granular bainite, thereby improving austenite stability. However,
excessive niobium content can adversely affect the low-temperature impact properties of the steel. Studies by Li
[18]using 3D atom probe microscopy have shown that niobium tends to segregate at grain boundaries in
pipeline steels. The interaction between the 4d orbitals of niobium and the 3d orbitals of iron promotes a
redistribution of the 3d valence electron state density of iron at grain boundaries to a lower-energy state, thereby
enhancing the strength and plasticity of the steels. Aminorroaya [19]employed Thermal-Calc software to
determine the concentrations of carbon (C), nitrogen (N), niobium (Nb), and titanium (Ti)in the steel. Their
analysis also explored the relationship between these elements and the precipitation of niobium carbonitrides in
pipeline steels, revealing that higher niobium concentrations elevate the precipitation temperature of these
carbonitrides.
Currently, Nb microalloying combined with ‘controlled rolling-accelerated cooling’represents the most
effective method for producing large deformation-resistant pipeline steels [20]. During production, meticulous
control of phase transformation and microstructure refinement is imperative. The chemical composition of
high-grade ferrite-bainite dual-phase large deformation-resistant pipeline steel differs from that of conventional
high-grade pipeline steel, with Nb employed to simultaneously control austenite grain size and ferrite phase
transformation. The effects of Nb precipitation and solid solution behavior on the microstructure during
austenitization and subsequent hot rolling deformation of pipeline steels are complex, with relatively little
research conducted to date. Therefore, there is a critical need for systematic investigation into the influence of
different Nb additions on the austenitization process of pipeline steels, as well as the dynamic and sub-dynamic
recrystallization behaviors affecting original austenite grain growth and deformation during rolling. Such
studies are essential to elucidate the mechanisms by which Nb operates in pipeline steels, which is pivotal for
controlling phase transformations, refining microstructures, and enhancing mechanical properties in modern
TMCP pipeline steels.
2. Materials and methods
Three pipeline steels (B150-steel, B145-steel, and 73-steel)with varying Nb contents were utilized to investigate
their austenitization and recrystallization behaviors. The experimental steels were produced in a 50 kg vacuum
induction furnace, and their specific chemical compositions were Fe-0.065C-0.293Si-1.823Mn-0.041V-
0.009Ti-0.182Cu-0.265Ni-0.184Cr-xNb (wt%), the Nb contents of B150-steel, B145-steel, and 73-steel were
0.032%, 0.051%, and 0.080%, respectively. The ingots were heated to 1200 °C in the furnace, held for 3 h, and
forged into square billets measuring 60 mm ×80 mm ×Length. The final forging temperature reached 850 °C.
Subsequently, the experimental steels were shaped into square specimens measuring 20 ×20 ×20 mm and
cylindrical specimens of j8×15 mm for reheating and compression deformation recrystallization experiments,
respectively.
A box-type resistance furnace was employed to heat 20 ×20 ×20 mm specimens to various temperatures to
investigate the impact of reheating temperature and holding duration on austenite grain size. The specimens
were heated in the furnace to temperatures of 1050 °C, 1100 °C, 1150 °C, 1180 °C, 1200 °C, 1220 °C, and
1250 °C, respectively, and held for 1 to 7 h. Subsequently, they were quenched in ice brine to preserve the
austenite microstructure. To measure grain size after heating and austenitizing, the Intersection Point method is
employed. This involves drawing several random straight-line segments and counting their intersections with
grain boundaries to determine the average grain size, as depicted in figure 2(a).
To investigate the sub-dynamic recrystallization of experimental steels and the influence of Nb content on
recrystallization, single-pass compression experiments were conducted on B-150, B-145, and 73-steel using a
Gleeble-3500 thermal simulation tester. The specific procedure is illustrated in figure 1. The specimens were
heated to 1180 °C at a rate of 20 °C/s and held for 5 min. Subsequently, they were cooled to 850∼1000 °Cat
5°C/s and subjected to single-pass compression with a strain rate of 1 s
−1
until achieving 25% deformation
strain. After deformation, the specimens were held for 0 to 120 s and finally quenched in water to room
temperature. After completing the thermal compression deformation, a cut was made along the specimen’s axial
direction to create an observation surface for metallographic analysis. The grain size was measured in both the
transverse (X-direction)and longitudinal (Y-direction)directions using the Intersection Point method. For each
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Mater. Res. Express 11 (2024)106513 F Tan et al

deformation condition, three metallographic images were selected, and measurements were taken along three
horizontal and three vertical lines. The average grain size in the X- and Y-directions for each deformation
condition was then calculated based on these measurements, as can be seen in figure 4(a).
The specimen was longitudinally split using a wire cutting machine, followed by grinding and polishing.
Metallographic preparation involved corrosion with 3% nitric acid in alcohol, followed by etching in a heated
1:1 mixture of saturated picric acid and detergent at 60 °C to reveal austenite grains. Microstructure
photographs were captured using an Olympus GX51 metallographic optical microscope. The GES and POLY
modules in Thermal-Calc software was employed to calculate the precipitation and solid solution of second-
phase particles in pipeline steels.
3. Results
3.1. Study of the austenitising behavior of different steels
Figure 2depicts the austenitic microstructure of the experimental steels with varying Nb contents after heating
to 1180 °C and holding for 2 h. It is evident that the austenite grains in B-150 and B-145 steels exhibit larger and
more uniform distribution, whereas those in 73-steel are predominantly smaller and less evenly dispersed.
The average austenite grain size during reheating of the experimental steels with varying Nb contents was
analyzed statistically, as depicted in figure 3. Figure 3(a)illustrates that the austenite grain size of the three
experimental steels increases gradually with rising reheating temperature. Conversely, the average austenite
grain size decreases with increasing Nb content, following the sequence ¯
D
73(0.080%Nb)<¯
D
B145(0.051%
Nb)<¯
D
B150(0.032%Nb). Particularly notable is the significant refinement of austenite grains when Nb
content reaches 0.08%, with grain sizes controlled to within 100 μm. In figure 3(b), it is observed that the
austenite grain size of the three experimental steels shows minimal sensitivity to holding time, especially evident
in 73-steel with 0.08% Nb content, where little austenite grain growth occurs after a 2 h hold.
Observation of figure 3(a)reveals the impact of increasing Nb content in experimental steel on austenite
grain size when reheated to 1050 °C–1250 °C. Two scenarios are identified: In the temperature range of
1050 °C–1100 °C, austenite grain sizes of steel with Nb content at 0.051% and 0.080% show minimal difference
but are finer compared to steel with 0.032% Nb. Thus, at lower temperatures, Nb content 0.051% effectively
inhibits austenite growth. Upon reheating to 1180 °C–1250 °C, increasing Nb content to 0.051% does not refine
austenite grains significantly. However, at 0.080% Nb, austenite grains undergo rapid refinement. For instance,
after holding at 1180 °C for 2 h, steel with 0.032% Nb exhibits an austenite grain size of approximately 128 μm,
whereas with Nb increased to 0.051%, the grain size measures about 124 μm, showing no significant difference.
Yet, with Nb content raised to 0.080%, the grain size notably reduces to 80 μm, considerably smaller than the
previous cases. These findings illustrate varying mechanisms by which Nb influences austenite grain size in
experimental steel across different temperature intervals, closely tied to the solid solution and precipitation
behaviors of Nb, Ti, and other elements [21].
Figure 1. Thermal compression process route.
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Mater. Res. Express 11 (2024)106513 F Tan et al

3.2. Study of the recrystallization behavior of high-temperature deformed austenite
To investigate the austenite recrystallization behavior following high-temperature deformation, metallographic
observations and analyses were conducted on three experimental steels subjected to deformation at various
temperatures and subsequent holding durations. For deformations where temperatures were 880 °C and
holding times were 120 s, no sub-dynamic recrystallization was observed in any of the experimental steels. The
influence of deformation temperature on recrystallization was substantial. Higher temperatures led to more
pronounced recrystallization. Additionally, increasing the interval between rolling passes promoted static
recrystallization [22]. Figures 4(a)–(c)depict the austenite microstructure of the three experimental steels
following deformation at 920 °C and subsequent 10-second holding. In B150-steel, fine equiaxed grains
appeared near the grain boundaries, indicating the onset of sub-dynamic recrystallization. However, B145-steel
and 73-steel maintained their deformed structures without sub-dynamic recrystallization. Figures 4(d)–(f)
illustrate the austenite microstructure of the three experimental steels after deformation at 940 °C and
subsequent 30-second holding. B150-steel exhibited an equiaxed grain structure, indicating complete sub-
dynamic recrystallization. In B145-steel (figure 4(e)), some fine grains appear at boundaries, suggesting the
initial stages of sub-dynamic recrystallization. Meanwhile, 73-steel (figure 4(f)) did not exhibit sub-dynamic
recrystallization.
In contrast, the 73-steel, which contains 0.080% Nb, exhibits recrystallization starting at a deformation
temperature of 960 °C and a holding time of 30 s, as illustrated in figure 5.
The presence of small, ‘necklace’-shaped equiaxed grains around the original deformed grains indicates
recrystallization [23]. As the process progresses, the microstructure evolves into a fully equiaxed structure,
signaling the completion of recrystallization. The recrystallization fraction is determined by measuring the
percentage of the metallographic microstructure that retains its deformed morphology, alongside the newly
formed fine equiaxed grain regions near the grain boundaries.
The extent of sub-dynamic recrystallization following deformation and prolonged holding at elevated
temperatures increases proportionally with the duration of the holding period. Given sufficient holding time,
sub-dynamic recrystallization eventually reaches completion. To analyze the occurrence of sub-dynamic
recrystallization under the current experimental conditions, a metallographic etching method was employed to
Figure 3. Average grain size of austenite (a)experimental steels reheated to different temperatures held for 2 h and then quenched (b)
experimental steel reheated to 1180 °C and held for different times and then quenched.
Figure 2. Microstructures of experimental steels after reheating to 1180 °C and holding for 2 h (a)B150-steel; (b)B145-steel;
(c)73-steel.
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Mater. Res. Express 11 (2024)106513 F Tan et al

Figure 4. Microstructure of austenite under different conditions (a)B150-steel, 920 °C-10 s (b)B145-steel, 920 °C-10 s (c)73-steel,
920 °C-10 s (d)B150-steel, 940 °C-30 s (e)B145-steel, 940 °C-30 s (f)73-steel, 940 °C-30 s.
Figure 5. Austenite microstructure of 73-steel after deformation at 960 °C and different holding times (a)10 s;(b)30 s.
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Mater. Res. Express 11 (2024)106513 F Tan et al

visually quantify the percentage of recrystallization during holding times (120 s)following 25% deformation at
temperatures ranging from 850 to 1000 °C at a strain rate of 1 s
−1
, as depicted in figure 6.
The curves in figure 6show that the lower the deformation temperature of the experimental steel, the longer
the time required for the occurrence or completion of sub-dynamic recrystallization, and the slower the rate of
growth of the recrystallization ratio with the extension of the holding time, i.e., increasing the deformation
temperature is conducive to accelerating the recrystallization behavior and shortening the time for complete
recrystallization, and when the deformation temperature is sufficiently high, dynamic recrystallization even
occurs, and figure 7shows that dynamic recrystallized grains were found in B150-steel immediately after
quenching at 980 °C with 25% deformation strain.
Figure 6. Recrystallization curves of experimental steels during holding at different temperatures (a)B150-steel; (b)B145-steel; (c)73-
steel.
Figure 7. Austenitic microstructure of B150-steel after immediate quenching of 25% deformation at 980 °C.
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Mater. Res. Express 11 (2024)106513 F Tan et al

Under the same deformation conditions, the recrystallization behavior characteristics of the experimental
steels varied with different Nb contents. For the deformation at 920 °C, B150-steel containing 0.032% Nb
had >5% sub-dynamic recrystallization when the holding time was extended to 10 s, and the fraction of
recrystallization increased markedly with the extension of the holding time, and about 40% of sub-dynamic
recrystallization occurred when the holding time was extended to 120 s. While the Nb content was increased to
0.051% (B145-steel), the percentage of sub-dynamic recrystallization remained <5% when the holding time
was extended to 120 s. When the Nb content was increased to 0.080%, no sub-dynamic recrystallization
occurred within 120 s.
At deformation temperatures 940 °C, sub-dynamic recrystallization in both B145 and B150-steels
increased rapidly with the increase in holding time, except that the former required a longer time for complete
sub-dynamic recrystallization than the latter. However, only about 10% or so sub-dynamic recrystallization
occurred in the 73-steel at 940 °C after deformation and holding time of 120 s. When the deformation
temperature is higher than 940 °C, the increase rate of its recrystallization percentage is significantly lower than
that of B150 and B145 experimental steels, i.e., the time required for the occurrence and completion of sub-
dynamic recrystallization is significantly longer. In addition, more than 10% dynamic recrystallization occurred
in B150-steel at a deformation temperature 980 °C, while no dynamic recrystallization occurred in either
B145 or 73 experimental steels. In summary, increasing the Nb content increases the time required for
recrystallization and delays the occurrence of recrystallization, and the higher the Nb content, the more obvious
the effect. The main reasons for this mainly include the dragging effect of solid solution Nb, the pinning effect of
Nb precipitates and the presence of energy consumption during the precipitation of the second phase which
reduces the driving force for recrystallization [24,25].
3.3. Analysis of recrystallization kinetics
It has been demonstrated that the sub-dynamic recrystallization kinetics of low carbon microalloyed steels
adhere to the Avrami equation [26,27], which is expressed as follows:
⎜⎟
⎡
⎣
⎢⎛
⎝⎞
⎠⎤
⎦
⎥()=- -Xt
t
1 exp 0.693 1
s
n
drx
0.5
⎛
⎝⎞
⎠()ee=tAD Q
RT
exp 2
pq
0.5 0
ssdrx

where X
sdrx
is the sub-dynamic recrystallization ratio; t
0.5
is the time required to complete 50% sub-dynamic
recrystallization; tis the holding time; n,A,p,q,sare constants related to the material itself; Q
sdrx
is the activation
energy of sub-dynamic recrystallization, kJ/mol; Ris the gas constant, J/(mol·K);
e
e,
are the true strain and
strain rate, respectively; D
0
is the size of the austenite after reheating, μm; Tis the deformation temperature, K.
Taking the logarithm of each side of equations (1)and (2), the resulting expressions are as follows:
⎜⎟
⎛
⎝⎞
⎠()
-
=+
Xnt
t
ln ln 1
1ln 0.693 ln 3
sdrx 0.5
()ee=+++ +tApqsDQRTln ln ln ln ln 4
0.5 0 sdrx

Analyzing equations (3)and (4), it is observed that
⎜⎟
⎛
⎝⎞
⎠
-
~
X
t
t
l
nln 1
1ln
sdrx 0.5
forms a linear relationship,
as does ~tT
l
n1.
0.5 By substituting the experimental values into equations (3)and (4)for linear regression,
the parameters nand Q
sdrx
for the three experimental steels can be determined, as presented in table 1. Hence,
the kinetics of austenite sub-dynamic recrystallization for the experimental steels can be expressed by
equations (5)–(7).
⎜⎟
⎡
⎣
⎢⎛
⎝⎞
⎠
⎤
⎦
⎥()=- -
-
Xt
t
1 exp 0.693 5
sBdrx 150
0.5
1.52
Table 1. The index nand sub-dynamic
recrystallization activation energy Q
sdrx
of
test steels.
Steels n Qsdrx/kJ·mol
−1
B150-steel 1.52 315.865
B145-steel 1.26 328.381
73-steel 1.07 452.196
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Mater. Res. Express 11 (2024)106513 F Tan et al

⎜⎟
⎡
⎣
⎢⎛
⎝⎞
⎠
⎤
⎦
⎥()
=- -
-
Xt
t
1 exp 0.693 6
sB
drx 145
0.5
1.26
⎜⎟
⎡
⎣
⎢⎛
⎝⎞
⎠
⎤
⎦
⎥()
=- -
-
Xt
t
1 exp 0.693 7
sdrx 73
0.5
1.07
3.4. Evolution of grain size during recrystallization
The grain size of austenite during recrystallization comprises three components [28,29]:(1)the size of the
dynamically recrystallized grains grown, d
DG
,(2)the size of the sub-dynamically recrystallized grains grown,
d
SG
, and (3)the size of the unrecrystallized austenite, d
N
. The average grain size of the austenite during
recrystallization is described by equation (8).
() ()=++--ddXdXd XX18
ADGD SGSN S D
During the recrystallization process, X
D
and Xs represent the volume fractions of dynamic and sub-dynamic
recrystallization, respectively. The evolution of austenite grains progresses through stages of elongated
morphology, partial recrystallization, complete recrystallization, and recrystallization grain growth, as depicted
by the trend in average grain size shown in figure 8(a). As the proportion of sub-dynamic recrystallization
increases, the grain size decreases. Upon completion of sub-dynamic recrystallization, the grain size reaches a
minimum. With continued prolonged heat preservation, there is a tendency for grain growth.
Additionally, the occurrence of dynamic recrystallization following austenite deformation must be
considered. If dynamic recrystallization (or significant recovery)does not occur, the grains remain flattened,
whereas recrystallized grains become equiaxed. Consequently, changes in the X (horizontal)and Y (thickness)
dimensions of austenite with respect to insulation time can be categorized into three cases, as depicted in
figure 8(b).
The first case involves the absence of dynamic recrystallization, where initially, there is a noticeable
difference in size between the X and Y directions (reflected in the larger separation of the two curves). This size
difference shows minimal change with extended insulation time. After complete sub-dynamic recrystallization,
the X and Y dimensions exhibit the smallest disparity, with a subsequent increase over prolonged time periods.
In the second scenario, partial dynamic recrystallization occurs during deformation, resulting in a smaller initial
difference between the X and Y curves compared to the first case. The evolution of X and Y dimensions follows a
similar pattern to the first case as the insulation time increases. The third case involves complete dynamic
recrystallization during the deformation process, where the X and Y curves are nearly parallel. As the holding
time increases, there is a gradual increase in grain size in both dimensions.
Based on metallographic images of the experimental steels, the dimensions of austenite grains in the X and Y
directions were measured under various experimental conditions, represented as (xμm, yμm). Figure 9
illustrates the curves depicting changes in grain dimensions along the X and Y directions for three experimental
steels over different heat retention times, with deformation temperatures ranging from 880 to 1000 °C.
In figure 9(a), for deformation temperatures 880 °C and holding times less than 120 s, the austenite grain
sizes in the X and Y directions of the experimental steels show minimal change with increasing holding time. No
Figure 8. Schematic diagram of the variation rule of grain size during sub-dynamic recrystallization (a): average size; (b): size in X and
Y directions.
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Mater. Res. Express 11 (2024)106513 F Tan et al

recrystallization behavior is observed in the three experimental steels with varying Nb contents. At a
deformation temperature of 920 °C, the grain size of B150-steel decreased rapidly to (55 μm, 38 μm)after a
30-second holding period, as shown in figure 9(b). Partial sub-dynamic recrystallization was observed in the
B150-steel. Comparing the curves with those from deformation at 880 °C, the grain size of B150-steel
immediately after deformation at 920 °C remained unchanged from that at 880 °C, indicating the absence of
dynamic recrystallization at the conclusion of deformation at 920 °C. Similarly, no dynamic recrystallization
occurred after deformation for B145-steel and 73-steel.
Comparing figures 9(a)to (c), at a deformation temperature of 940 °C, none of the three experimental steels
exhibited dynamic recrystallization. In the case of B150-steel, the deformation behavior during the holding
process resembled that shown in figure 8(b). Initially, after a 5-second holding period, the grain size sharply
decreased to (55 μm, 45 μm), indicating noticeable sub-dynamic recrystallization. As the holding time increased,
the percentage of sub-dynamic recrystallization also increased, reaching completion at approximately 30 s with a
grain size of about (45 μm, 42 μm). Similarly, B145-steel exhibited deformation characteristics over time similar
to those of B150-steel, albeit with a noticeable rightward shift in the curve. Regarding the 73 -steel with 0.080%
Nb content, holding times exceeding 60 s resulted in slight reduction in grain size and some observable sub-
dynamic recrystallization behavior. Figure 9(d)illustrates that at a deformation temperature of 960 °C, dynamic
recrystallization occurred in B150-steel, whereas B145-steel did not exhibit dynamic recrystallization. Sub-
dynamic recrystallization in B145-steel was essentially complete after a 30-second holding period, resulting in
Figure 9. Curve of austenite average size with holding time under different deformation conditions (a)880 °C, (b)920 °C, (c)940 °C,
(d)960 °C, (e)980 °C, (f)1000 °C.
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Mater. Res. Express 11 (2024)106513 F Tan et al

grain sizes of (35 μm, 29 μm). Similarly, 73-steel did not experience dynamic recrystallization during
deformation, but the incidence of sub-dynamic recrystallization gradually increased during the holding period,
reaching incomplete sub-dynamic recrystallization at 120 s, with grain sizes of (50 μm, 32 μm).
At a higher deformation temperature of 980 °C(figure 9(e)), both B150 and B145 steels exhibited partial
dynamic recrystallization, with B150-steel showing a more complete process than B145-steel. A small amount of
dynamic recrystallization also occurred in 73-steel. When the deformation temperature was increased to
1000 °C, dynamic recrystallization in B150 and B145 steels became evident. After completing recrystallization,
the austenite grains in B150 and B145 steels grew noticeably. However, the grains in 73-steel did not exhibit
significant growth over time and remained at (30 μm, 25 μm).
To analyze the effect of deformation temperature on recrystallization of experimental steel and the
refinement of austenite grains in Nb-containing steels, curves of deformation temperature versus average grain
size under different holding times were plotted, as shown in figure 10. The influence of deformation temperature
on the austenite grain size is significant. With increasing deformation temperature, the average grain size of
austenite decreases when the holding time after deformation is short, whereas with longer holding times, the
grain size decreases initially and then increases with temperature. During the holding process after deformation,
the grain size of austenite in experimental steel remains essentially unchanged after complete sub-dynamic
recrystallization, with grain sizes of 35–40 μm for B150-steel, 30–35 μm for B145-steel, and 25–30 μm for 73-
steel. Based on the above results, for experimental steels with Nb content 0.08%, at high deformation
temperatures, austenite undergoes sub-dynamic recrystallization. The grain size of austenite remains essentially
unchanged after complete sub-dynamic recrystallization and is not significantly affected by deformation
temperature and holding time. However, influenced by Nb content, an increase in Nb content can reduce
grain size.
In summary, during the isothermal holding process at 880 °C–1000 °C following deformation of 25% at a
strain rate of 1 s
−1
, higher deformation temperatures promote dynamic recrystallization. The time required for
initiation and completion of sub-dynamic recrystallization decreases with increasing deformation temperature
and prolonged holding time, resulting in a higher percentage of sub-dynamic recrystallization. If holding
continues after completion of recrystallization, grain growth occurs. With increasing Nb content, the times
required for initiation and completion of sub-dynamic recrystallization (t
B150
<t
B145
<t
73
)increase. Notably, at
Figure 10. Effect of deformation temperature and holding time on the average grain size of austenite in experimental steels (a)B150-
steel; (b)B145-steel; (c)73-steel.
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Mater. Res. Express 11 (2024)106513 F Tan et al

an Nb content of 0.080%, recrystallization is significantly delayed, likely due to dynamic precipitation of Nb and
its precipitates inhibiting and delaying recrystallization.
4. Discussion
4.1. Mechanism of austenite grain growth
The pinning effect of carbonitrides of Nb and Ti (referred to as M(C, N)) is a key factor affecting the growth of
austenite grains in experimental steels [30]. During the austenitization process, reheating temperature affects the
solid solution of M(C, N). Due to the increasing solubility of Nb in austenite with rising temperature, the
percentage content of M(C, N)decreases gradually, leading to a gradual weakening or disappearance of the
pinning effect.
In microalloyed steels, the size of M(C, N)particles varies. With prolonged holding time, the concentration
of elements (Nb, Ti, C, N)around small M(C, N)particles becomes higher compared to that around larger
particles. During the homogenization process, Nb, Ti, C, and N elements continuously diffuse from around
small particles to around larger ones. This causes gradual dissolution, contraction, and eventual disappearance
of small particles, known as Ostwald ripening of M(C, N)[31,32]. This process involves three stages: 1)
dissolution of small particles, 2)diffusion of Nb, Ti, C, and N to the interface of large particles, and 3)
penetration of Nb, Ti, C, and N through the interface into large particles.
Since the solubility of M(C, N)is controlled by the heating temperature and is independent of
homogenization time, the solubility of M(C, N)in austenite remains unchanged during the holding process.
Consequently, during Ostwald ripening, the percentage of M(C, N)remains constant while the particle size
increases. After homogenization, the particle size stabilizes at a certain value. The pinning effect of M(C, N)on
austenite grain boundaries can be expressed by the following equation [33]:
() ()
p
=-Dc d
fz
632 2 9
Here, Dc represents the critical core size required to prevent grain coarsening, ddenotes the particle size, f
denotes the total volume fraction of particles, and zdenotes the particle inhomogeneity factor, which varies with
the material.
If the initial grain size Dof grains exceeds a critical size Dc, the pinning force exerted by second-phase
particles on grain boundaries surpasses the driving force for grain boundary movement, thereby effectively
pinning the boundaries. Conversely, if Dis smaller than Dc, the second-phase particles are unable to effectively
pin the grain boundaries. In essence, a larger critical size Dc correlates with an increased grain size D. According
to equation (9), it is evident that an increase in particle size and a decrease in total volume fraction both lead to an
increase in D. During subsequent heating processes, as temperature rises, M(C, N)continuously dissolves,
reducing the volume fraction f. Consequently, Dsignificantly increases due to coarsening. With prolonged
holding times, M(C, N)particles undergo Ostwald ripening, where their diameter dincreases slowly. Thus, D
increases gradually. In experimental steels, a higher Nb content results in a larger volume fraction of M(C, N)
and hence stronger pinning effects. This leads to smaller austenite grain sizes D, so that the austenite grain size of
73-steel <B145-steel <B150-steel.
4.2. Effect of Nb on recrystallization behavior and grain size
With increasing Nb content, the time required for sub-dynamic recrystallization significantly lengthens at the
same deformation temperature. Nb inhibits both the initiation and completion of recrystallization, as well as the
growth of recrystallized grains in the experimental steels. The strain-induced precipitation of Nb plays a crucial
role in the recrystallization behavior of deformed austenite during the isothermal process. On one hand, it pins
down austenite grain boundaries and dislocations, hindering their migration. On the other hand, Nb
precipitation demands considerable energy and competes with recrystallization, thereby impeding the process.
The PTT curves of the experimental steels were calculated and analyzed using Dutta and Sellars’model for
analysis start time [34], as shown in equation (10):
⎜⎟
⎛
⎝
⎞
⎠
[] () ()e=-- -
tANb Z RT
B
Tk
exp 270000 exp ln 10
ps
pass pass s
11 0.5
32
where constants A and B pertain to the material itself, specifically for low-carbon microalloyed steel, where
A=3×10
−6
,B=2.5 ×10
10
;[Nb]represents the amount of Nb in solid solution in austenite (wt%),εdenotes
the deformation strain, T
pass
represents the deformation temperature, Zis the Zener-Hollomon parameter, and
K
s
signifies the degree of oversaturation of Nb(C, N)in austenite. The calculation for K
s
follows the formula
specified in equation (11).
11
Mater. Res. Express 11 (2024)106513 F Tan et al

()=
-+
-+
K10
10 11
S
T
T
6770 2.26
6770 2.26
RH
pass
/
/
The austenite reheating temperature (T
RH
)of experiment steel is 1180 °C. Thermo-Calc thermodynamic software
was utilized to determine the concentration of [Nb]in the experimental steel (see table 2). The resulting experimental
data were then incorporated into equations (10)and (11)to compute the PTT curve, illustrated in figure 11.
Figure 11 depicts the PTT curve analysis of the experimental steel, characterized by a C-curve shape. For
B150 and B145 steels, the nose point temperature is approximately 890 °C. In contrast, 73-steel shows a lower
nose point temperature of about 860 °C. Upon observing the curve in figure 11, it becomes evident that an
increase in Nb content significantly reduces the time required for precipitation initiation. Thus, under identical
experimental conditions, higher Nb content in the experimental steel results in greater precipitation, effectively
inhibiting and delaying sub-dynamic recrystallization behavior.
The analysis of austenite grain refinement in the experimental steels with varying Nb contents involves both
dynamic recrystallization during deformation and sub-dynamic recrystallization during the post-deformation
holding period. The most pronounced austenite grain refinement effect occurs when sub-dynamic
recrystallization is just completed.
When the Nb content is lower in the experimental steel, recrystallization occurs at a lower temperature. This
requires less critical deformation and results in shorter sub-dynamic recrystallization completion times. However,
prolonged thermal insulation at high temperatures after deformation leads to significant grain growth and
coarsening. In contrast, as the Nb content increases, recrystallization occurs at higher temperatures, requiring more
critical deformation and longer times for sub-dynamic recrystallization to complete. After sub-dynamic
recrystallization is complete, the grain size remains relatively unchanged even with extended isothermal times. At
higher temperatures during deformation, Nb in solid solution acts to inhibit recrystallization by impeding grain
boundary movement and dislocation motion. This effect becomes particularly pronounced with Nb content
exceeding 0.080%. Additionally, at lower deformation temperatures, Nb can precipitate due to strain-induced
precipitation. This precipitation both pins grain boundaries and dislocations and consumes significant free energy,
creating competition with recrystallization [35,36]. With increasing Nb content, the impact of strain-induced
precipitation becomes more substantial, leading to greater inhibition of recrystallized grain growth. Therefore, the
addition of Nb in the experimental steels plays a critical role in controlling grain size through its effects on both
recrystallization kinetics and strain-induced precipitation mechanisms.
Figure 11. PTT curves of Nb (C, N)of test steels.
Table 2. Nb solution of test steels heated to 1180 °C.
Steel B150-steel B145-steel 73-steel
Solution/% 0.0303 0.0483 0.0749
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Mater. Res. Express 11 (2024)106513 F Tan et al

5. Conclusions
In examining and comparing the austenitizing and high-temperature deformation behaviors of experimental
steels with varying Nb contents, several conclusions emerge.
(1)The austenite grain size of the three experimental steels increases gradually with higher holding
temperatures during austenite reheating. Conversely, the average austenite grain size decreases with
increasing Nb content: specifically, 73 (0.080% Nb)<B145 (0.051% Nb)<B150 (0.032% Nb). Notably,
the grain size of these steels shows minimal sensitivity to holding time. During extended holds, M(C, N)
particles undergo Ostwald ripening, enlarging their diameter. Higher Nb content steels exhibit a greater
volume fraction of M(C, N)particles, which enhances their pinning effect on grain boundaries and leads to
smaller austenite grain sizes.
(2)The initiation conditions for sub-dynamic recrystallization in the B150, B145, and 73 experimental steels
were found to be 920 °C-10 s, 940 °C-30 s, and 960 °C-30 s, respectively. The kinetic equations governing
sub-dynamic recrystallization for these steels were experimentally determined.
(3)The effect of deformation temperature on austenite grain size is very obvious. With the increase of
deformation temperature, when the holding time after deformation is short, the average grain size of
austenite decreases; when the holding time is long, the grain size first decreases and then increases with the
increase of temperature. During the holding time after deformation, the grain size of the experimental steel
austenite after complete sub-dynamic recrystallization basically does not change with the deformation
temperature, but will decrease with the increase of Nb content.
(4)At high temperature deformation, solid solution Nb will drag grain boundaries and dislocations and inhibit
the recrystallization behavior; at lower deformation temperatures, Nb(C, N)precipitation will nail grain
boundaries and dislocations on the one hand, and consume a large amount of free energy on the other hand,
which forms a competitive relationship with recrystallization. With the increase of Nb content, the effect of
strain-induced precipitation increases significantly, and the inhibition of recrystallized grain growth
becomes more obvious.
Acknowledgments
This work was supported by Hunan Province ‘Double First-class’Discipline Construction Project (Xiangtong
2018-469); Hunan High-tech Industry Science and Technology Innovation Leadership Programme (2021-2022)
Project (2021GK4047); Natural Science Foundation of Hunan Province (2023JJ50089); Teaching Reform
Research Project of General Undergraduate Colleges and Universities in Hunan Province in 2024
(202401001445); Loudi City ‘Material Valley’Science and Technology Major Special Project (Lou Ke Fa
[2022]29).
Data availability statement
The data cannot be made publicly available upon publication because no suitable repository exists for hosting
data in this field of study. The data that support the findings of this study are available upon reasonable request
from the authors.
Author contributions
Conceptualization, Li Wang and Shiwei Tian; Data curation, Fengliang Tan, Li Wang and Jiansheng Chen;
Formal analysis, Li Wang; Funding acquisition, Fengliang Tan and Yantao Li; Investigation, Fengliang Tan and
Yantao Li; Methodology, Jinbiao Cui and Ning Liu; Project administration, Yantao Li; Resources, Jinbiao Cui;
Software, Ning Liu and Li Wang; Supervision, Yantao Li; Validation, Ning Liu; Visualization, Ning Liu and
Jiansheng Chen; Writing—original draft, Fengliang Tan and Jinbiao Cui; Writing—review & editing, Jiansheng
Chen and Shiwei Tian.
Conflicts of interest
The authors declare that they have no conflict of interest.
13
Mater. Res. Express 11 (2024)106513 F Tan et al

ORCID iDs
Shiwei Tian https://orcid.org/0000-0002-4211-2890
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