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1 23
JOM
The Journal of The Minerals, Metals &
Materials Society (TMS)
ISSN 1047-4838
JOM
DOI 10.1007/s11837-014-0913-3
Dynamic Ferrite Transformation Behaviors
in 6Ni-0.1C Steel
Nokeun Park, Lijia Zhao, Akinobu
Shibata & Nobuhiro Tsuji
1 23
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Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel
NOKEUN PARK,
1
LIJIA ZHAO,
2
AKINOBU SHIBATA,
1,2
and NOBUHIRO TSUJI
1,2,3
1.—Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University, Yoshida
Honmachi, Sakyo-ku, Kyoto 606-8501, Japan. 2.—Department of Materials Science and Engi-
neering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan. 3.—e-mail:
nobuhiro-tsuji@mtl.kyoto-u.ac.jp
Phase transformation from austenite to ferrite is an important process to
control the microstructures of steels. To obtain finer ferrite grains for
enhancing its mechanical property, various thermomechanical processes fol-
lowed by static ferrite transformation have been carried out for austenite
phase. This article reviews the dynamic transformation (DT), in which ferrite
transforms during deformation of austenite, in a 6Ni-0.1C steel recently
studied by the authors. Softening of flow stress was caused by DT, and it was
interpreted through a true stress–true strain curve analysis. This analysis
predicted the formation of ferrite grains even above the Ae
3
temperature
(ortho-equilibrium transformation temperature between austenite and fer-
rite), where austenite is stable thermodynamically, under some deformation
conditions, and the occurrence of DT above Ae
3
was experimentally confirmed.
Moreover, the change in ferrite grain size in DT was determined by defor-
mation condition, i.e., deformation temperature and strain rate at a certain
strain, and ultrafine ferrite grains with a mean grain size of 1 lm were
obtained through DT with subsequent dynamic recrystallization of ferrite.
INTRODUCTION
One of the most effective ways to enhance
strength without losing toughness in metallic
materials is grain refinement, especially in steels.
The fundamental idea to obtain finer ferrite grains
in low-carbon steels is to use phase transformation
phenomenon from austenite (c: high-temperature
phase) to ferrite (a: low-temperature phase). Many
innovative techniques have been developed to
achieve fine ferrite grains, for example, thermome-
chanical controlled processing (TMCP)
1
. Figure 1a
is a schematic illustration showing a continuous-
cooling-transformation (CCT) diagram and the
change in microstructures by TMCP in low-C steels.
Coarse austenite grains are first deformed at
recrystallization temperatures of austenite, result-
ing in fine austenite grains through recrystalliza-
tion of austenite. As a result, the austenite grain
boundary area per unit volume, which is one of the
effective nucleation sites for ferrite transformation,
increases. Once austenite is deformed at a nonre-
crystallization region, a large amount of lattice
defects, such as dislocations, which can be potential
nucleation sites for ferrite transformation, are
introduced into the austenite. Finally, ferrite
transformation occurs at a relatively lower tem-
perature by accelerating the cooling process, where
the nucleation rate of ferrite is enhanced by a large
driving force (under large supercooling). Through
the process explained in Fig. 1a, fine ferrite with
mean grain size of around 5–8lm could be obtained
through the TMCP.
Recent developments of facilities’ capacity (such
as that in rolling mills) can make it possible to apply
a larger amount of one-pass deformation at a much
lower temperature. As a result, ultrafine ferrite
structures with a mean grain size of approximately
1lm have been achieved.
2–5
As shown in a sche-
matic illustration of Fig. 1b, ferrite transformation
could occur during the plastic deformation of aus-
tenite that is conducted in larger one-pass defor-
mation at lower temperatures because the kinetics
of ferrite transformation are greatly accelerated by
deformation. The ferrite transformation occurring
during deformation of austenite has been termed as
dynamic (ferrite) transformation (DT), strain-induced
ferrite transformation,
6–12
deformation-induced
JOM
DOI: 10.1007/s11837-014-0913-3
Ó2014 The Minerals, Metals & Materials Society
Author's personal copy
ferrite transformation,
13–17
and so on, and it has
received great attention as another mechanism of
ferrite grain refinement.
One controversial issue about DT is whether the
DT occurs at temperature above Ae
3
(ortho-equi-
librium transformation temperature between aus-
tenite and ferrite), where ferrite is unstable so that
austenite single phase should be stable thermody-
namically. Choi et al.
9
denied the possibility of the
occurrence of DT at temperatures near or above Ae
3
because the critical strain for the occurrence of DT
was higher than that of dynamic recrystallization of
austenite so that dynamic recrystallization of aus-
tenite preferentially occurred compared to DT dur-
ing deformation of austenite. On the other hand,
Yada et al.
2
, who were pioneers of studies on DT,
proposed a strain-temperature-phase map and in-
sisted that DT took place even above Ae
3
when
plastic deformation of austenite was large enough.
Recently, we systematically studied a thermome-
chanical process under various deformation condi-
tions, i.e., various deformation temperatures and
strain rates, for clarifying the occurrence of DT in a
6Ni-0.1C steel, and it was revealed that DT could
take place even at temperatures higher than Ae
3
when the combination of strain rate and deforma-
tion temperature was appropriately selected.
18–20
Therefore, we discuss the deformation condition for
the occurrence of DT as the first topic in this article.
Another important subject of DT is the ferrite
grain refinement by DT depending on deformation
conditions, such as deformation temperature, strain
rate, austenite grain size, and so on. There have
been several reports discussing grain refinement
mechanisms by DT:
2,5,11,21–24
transformation from
dynamically recrystallized austenite, continuous
nucleation of ferrite during orientation rotation of
austenite, dynamic recrystallization of ferrite during
DT, etc. However, a comprehensive understanding
has not been clearly revealed yet because it is hard to
consider the effect of each contribution in DT inde-
pendently. In the second section of this report, we try
to discuss the effects of deformation parameters on
the grain size of dynamically transformed ferrite.
EXPERIMENTAL PROCEDURES
Material
The material used in this study was a 6Ni-0.1C
steel (weight percent), which was designed to widen
austenite temperature range and to have a long
incubation time for the onset of static ferrite
transformation at given temperatures. The detailed
composition of the material is shown in Table I.
The Ae
3
and Ap
3
(paraequilibrium transformation
temperature between austenite and ferrite) in the
6Ni-0.1C steel are calculated by Thermo-Calc soft-
ware (Thermo-Calc Software, Inc., McMurray, PA,
USA) to be 728°C and 684°C, respectively.
18
Thermomechanical Process
As-received plates were homogenized at 1100°C
for 24 h, followed by water quenching. Cylindrical
specimens 12 mm in height and 8 mm in diameter
were cut from the homogenized plate by electric
discharge machining. Physical simulations of a
thermomechanical process were carried out using
a thermomechanical processing simulator Therm-
acmastor-Z (Fuji Electronic Industrial Co., Ltd.,
Saitama, Japan). Here, two austenitization condi-
tions were chosen to have different austenite grain
sizes; 1200°C for 180 s for obtaining coarse aus-
tenite grains (mean grain size of 400 lm) and
800°C for 180 s for having fine austenite grains
(mean grain size of 15 lm). The austenitized
Fig. 1. Schematic illustrations showing CCT diagrams and change in microstructures for (a) static ferrite transformation from deformed austenite
and (b) dynamic ferrite transformation during deformation of austenite.
Park, Zhao, Shibata, and Tsuji
Author's personal copy
specimens were cooled to deformation temperature
by N
2
gas at a cooling rate of 30°Cs
1
and then
kept for 60 s to ensure uniform temperature dis-
tribution within the specimen. It has been con-
firmed that no static ferrite transformation
occurred during holding for 60 s at the whole
temperature range in the case of coarse austenite
grain size (see the time–temperature-transforma-
tion diagram of this alloy shown in Ref. 18). An
uniaxial compression was applied to the austeni-
tized specimens at four constant strain rates
ranging from 10
3
s
1
to 10
0
s
1
to a true strain of
0.96. After completing the deformation, the speci-
mens were immediately quenched by a precisely
controlled water-injection system.
To study the deformation behavior of ferrite at
elevated temperatures, one homogenized plate was
slowly cooled in the furnace to obtain a mixed
microstructure composed of ferrite and a small
amount of pearlite. The specimens composed of
ferrite and pearlite were heated up to the defor-
mation temperature ranging from 400°C to 700°C
(below Ae
3
), followed by an isothermal holding for
60 s, and then an uniaxial compression was applied
at a strain rate of 10
1
s
1
. True stress–true strain
data were obtained from the load–displacement
data taken during the hot deformation of both aus-
tenite and ferrite.
Microstructural Observation
Microstructures of the deformed specimens
were observed by optical microscopy (OM), elec-
tron backscattering diffraction (EBSD) analysis
in a field-emission scanning electron microscope
(FE-SEM; FEI XL30S FEG; FEI Company,
Hillsboro, OR, USA), and transmission electron
microscopy (TEM; Philips CM200 FEG; Philips,
Amsterdam, The Netherlands). The cross-sections
parallel to the compression direction of the deformed
specimens were mechanically polished and then
electropolished in a solution of 10% HClO
4
and
90% CH
3
COOH at 25°C. A 3% nital solution was
used to reveal microstructures for OM. The mean
grain sizes of ferrite were measured by the linear
interception method on the EBSD boundary
maps, which were similar to those presented in
our previous article.
18
Boundaries with misori-
entation above 2°were used for the measure-
ment. Thin foils for TEM observations were
prepared by twin-jet electropolishing using the
identical solution as that used for the EBSD
observations.
RESULTS AND DISCUSSION
Analysis of True Stress–True Strain Curves
for the Occurrence of DT
Because ferrite is softer than austenite at elevated
temperatures in low-carbon steels,
25
the occurrence of
DT influences the flow stress, which would decrease
with increasing the fraction of dynamically trans-
formed ferrite. Whether DT occurs or not can be de-
tected through the analysis of stress–strain behaviors
during hot deformation. We examined the change in
true stress–true strain behaviors of the specimens
deformed at various deformation temperatures and
strain rates. Figure 2a shows the true stress–true
strain curves of the specimens deformed at a strain
rate of 10
2
s
1
at various temperatures after au-
stenitization at 1200°C for 180 s. The mean grain size
of austenite was 400 lm. The shape of the stress–
strain curves can be classified into three different
types. At higher temperatures, the curves show the
shapes typical for the dynamic recrystallization of
austenite. With increasing the amount of strain, the
flow stress increases until the peak stress is reached.
After attaining the peak stress, the flow stress de-
creases and then keeps a constant value. At interme-
diate temperatures, the curves show the dynamic
recovery type where the flow stress initially increases
and then keeps a constant value without showing any
peak and stress drop. At lower temperatures, the
stress–strain curves exhibit a significant softening
again after the peak stress. The shape of these curves
is similar to that for dynamic recrystallization of
austenite at higher temperatures. It has been con-
firmed that the observed softening at lower tempera-
tures is attributed to the occurrence of DT.
9
Figure 2b
summarizesthe change in the maximum flow stress of
the specimens deformed at four different strain rates
as a function of deformation temperature. It should be
noted that the softening of the maximum flow stress
due to the occurrence of DT is significant when the
strain rate is slow, for example 10
3
or 10
2
s
1
,and
the deformation temperature is low.
The Zener–Hollomon (Z) parameter, described in
Eq. 1, is a well known parameter that unifies the
effects of deformation temperature and strain rate
in high-temperature deformations,
26
Z_
eexp Q=RTðÞ (1)
Table I. Chemical composition of the 6Ni-0.1C steel used in the current study (wt.%)
CNiMnSi Al P S Fe
0.112 5.97 0.003 0.008 0.037 0.002 0.0015 bal.
Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel
Author's personal copy
where _
eis the strain rate, Qis the activation energy
for high-temperature deformation of austenite, Ris
the gas constant, and Tis the absolute temperature.
The constitutive equation for the hot deformation of
metallic materials, which can cover wide range of Z
parameter, is described in Eq. 2.
27
Z¼Asinh arðÞ (2)
where Aand aare constants, and ris the maxi-
mum flow stress of the true stress–true strain
curve in this study. The maximum flow stress val-
ues at higher temperatures in Fig. 2b were plotted
as a function of temperature in Fig. 2c to obtain the
activation energy for the high-temperature defor-
mation of austenite, and the estimated value was
310 kJ mol
1
. The maximum flow stress values in
Fig. 2b are plotted as a function of Zparameter in
Fig. 2d.AsshowninEq.2, the maximum flow
stress in a form of hyperbolic sine function is
proportional to the Zparameter when the Z
parameter is low, i.e., at high temperatures or at
low strain rates, which is generally accepted as a
typical hot-deformation behavior of metallic mate-
rials.
27
However, the maximum flow stress in a
form of hyperbolic sine function gradually deviates
from the extrapolated line when the Zparameter
is higher than a critical value (1.7 910
14
s
1
).
Because the deviation of the maximum flow stress
from the extrapolated line above the critical Z
parameter was attributed to the occurrence DT, the
deformation conditions for the occurrence of DT
could be described by a combination of deformation
temperature and strain rate. Table II shows the
deformation parameters above the critical Z
parameter, indicating that DT could occur even
above Ae
3
(728°C) when the strain rate is high
enough.
It is also found from Fig. 2d that the deviation of
the maximum flow stress from the extrapolated line
Fig. 2. Procedures of the true stress–true strain analysis for confirming the occurrence of dynamic ferrite transformation.
18
(a) True stress–true
strain curves of the specimens deformed at a strain rate of 10
2
s
1
at various temperatures. (b) Change in the maximum flow stress at different
strain rates as a function of the deformation temperature. (c) Determination of the activation energy (310 kJ mol
1
) for calculating the Zener–
Hollomon parameter about austenite deformation. (d) Change in the maximum stress in a form of hyperbolic sine function as a function of the
Zener–Hollomon parameter. Z
C
indicates the critical Zener–Hollomon parameter for the occurrence of stress softening.
Park, Zhao, Shibata, and Tsuji
Author's personal copy
in the lower Zregion is more significant at lower
strain rates. In the case of a lower strain rate, the
deformation time itself, which is another important
parameter describing kinetics of DT, becomes much
longer so that a larger fraction of ferrite can form
during deformation, resulting in the significant
deviation of the maximum flow stress.
In order to confirm the occurrence of DT above
Ae
3
, we used the specimen having finer austenite
grains (mean grain size of 15 lm) to accelerate
kinetics of DT. The specimens austenitized at 800°C
and the specimens having a ferrite and pearlite
microstructure were deformed at various tempera-
tures at a strain rate of 10
1
s
1
. The true stress–
true strain curves of the austenitized specimens and
those of the ferrite specimens are shown in Fig. 3a
and b, respectively. The maximum flow stress val-
ues obtained from the stress–strain curves are
plotted as a function of the deformation tempera-
ture in Fig. 3c (square: the austenitized speci-
mens, circle: the ferrite specimens). Figure 3disan
enlarged part of Fig. 3c close to Ae
3
. The maximum
flow stress values at temperatures higher than
800°C were taken from the Fig. 2b because austenite
Table II. Deformation Conditions Above the Critical Zener–Hollomon Parameter
18
Parameters Conditions
Strain rate, e(s
1
)10
0
10
1
10
2
10
3
Temperature, T(°C) <865 <790 <725 <667
Fig. 3. (a) True stress–true strain curves of the specimens deformed in austenite states.
19
(b) True stress–true strain curves of the specimens
having a ferrite and pearlite microstructure. (c) Change in the maximum stress of the austenite specimens obtained from (a) and that of the ferrite
specimens obtained from (b), as a function of the deformation temperature. (d) An enlarged area in (c). Ap
3
and Ae
3
temperatures are marked as
dashed lines in (c) and (d).
Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel
Author's personal copy
grain size did not influence the maximum flow
stress but only accelerated kinetics of dynamic
softening. The dash-dotted line in Fig. 3c and d is
the extrapolation of the fitting curve derived from
Eq. 2, and the dotted lines in Fig. 3d indicate upper
and lower 95% confidence intervals. It is clearly
seen that the flow stress deviates from the extrap-
olated curve at temperatures above Ae
3
in Fig. 3d,
and it comes close to the maximum flow stress of the
ferrite with decreasing the deformation tempera-
ture in Fig. 3c.
Although the analysis in Fig. 3implies that DT is
likely to occur above Ae
3
, two other possibilities
must be checked: One is the static transformation to
ferrite from deformed austenite during the cooling
procedure, and the other is that the Ae
3
calculated
by Thermo-Calc software (728°C) might be lower
than the true Ae
3
of the 6Ni-0.1C steel. Figure 4a
and b are OM images of the austenitized specimens
that were deformed at 750°C, i.e., above the calcu-
lated Ae
3
(728°C), to strains of (Fig. 4a) e= 0.33 and
(Fig. 4b) e= 0.96 at a strain rate of 10
1
s
1
. The
measured fractions of ferrite in Fig. 4a and b are
15% and 55%, respectively. Let us assume here that
DT did not happen during the deformation, but the
ferrite grains observed in Fig. 4a and b were stati-
cally transformed from the deformed austenite
during water quenching. The flow stress of the
specimen deformed to e= 0.96 (166 MPa) is lower
than that at e= 0.33 (188 MPa), as shown in
Fig. 3a. The flow stress can be regarded as a func-
tion of the dislocation density in the austenite, so
that the density of lattice defects in the austenite
deformed to e= 0.33 is expected to be higher than
that in the austenite deformed to e= 0.96. Accord-
ingly, the fraction of statically transformed ferrite
from the austenite deformed to e= 0.33 should be
larger than that that from the austenite deformed to
e= 0.96 because the larger amount of lattice defects
would accelerate the kinetics of static ferrite
transformation. However, the result shown in
Fig. 4a and b is opposite; i.e., the fraction of ferrite
at e= 0.33 (15%) is smaller than that at e= 0.96
(55%), suggesting that the observed ferrite grains
were not statically transformed during postdefor-
mation cooling but formed during deformation, i.e.,
dynamically transformed ferrite. Therefore, the
softening of the flow stress can be interpreted as the
increase in the fraction of dynamically transformed
ferrite, i.e., kinetics of DT.
20
Figure 4c is an OM image of the specimen
deformed to a strain of 0.96 and subsequently held
for 600 s at the identical temperature (750°C) after
the deformation. The fraction of ferrite decreased
from 55% in Fig. 4b to less than 2% during sub-
sequent holding at 750°C. The result demonstrates
that the dynamically transformed ferrite reversely
transformed to austenite during statically keeping
at 750°C after the deformation, and the deformation
temperature (750°C) was surely above Ae
3
. There-
fore, it can be concluded that DT can occur even
above Ae
3
when the deformation conditions are
above the critical value of the Zparameter.
Lee et al.
28
qualitatively explained how ferrite
formed during deformation of austenite at temper-
atures above Ae
3
. Once deformation is applied to
austenite, the extra energy generated by deforma-
tion is added into the austenite phase so that the
free energy of austenite increases much more than
that of ferrite. Therefore, DT can occur even at
temperatures above Ae
3
because newly formed fer-
rite is relatively stable compared to the deformed
austenite. Figure 5shows a part of the phase dia-
gram of a Fe-6Ni-C system calculated by Thermo-
Calc software. The change in Ae
3
lines with differ-
ent additional energies on austenite phase is pre-
sented in Fig. 5a. The dashed line corresponds to
the chemical composition of carbon, 0.1 wt.%, used
in this study. For the occurrence of DT at 750°C, a
large amount of additional energy in austenite
phase (50 J mol
1
) is required in Fig. 5a. Here, it is
necessary to consider the contributing factors that
increase the free energy of austenite. One major
factor is a stored energy of dislocations. Figure 5b
exhibits the required dislocation density to vary the
Ae
3
lines corresponding to the additional energy in
Fig. 5a. Here, it was assumed that the stored energy
[E(J mol
1
)] by dislocations is E¼lb2qVm, where l
Fig. 4. Optical microscope images of the austenite specimens deformed to different strains:
19
(a) e= 0.33, (b) e= 0.96 at a strain rate of
10
1
s
1
at 750°C. (c) The specimen deformed to a strain of 0.96 at 750°C and subsequently held at 750°C for 600 s. Arrows indicate ferrite
grains, of which fractions are shown in the figures as well.
Park, Zhao, Shibata, and Tsuji
Author's personal copy
is the shear modulus, bis the magnitude of Burgers
vector, qis the dislocation density, and V
m
is the
molar volume. As shown in Fig. 5b, however, the
calculated dislocation density (2 910
15
m
2
)isnot
realistic (seems too high for high-temperature
deformation of austenite). Some possible contribut-
ing factors, such as the heterogeneity of dislocation
distributions within austenite grains, austenite
grain boundary energy, elastic energy under defor-
mation, etc., have been proposed, and further
quantitative understanding about driving force for
DT above Ae
3
is required.
29–34
Change in Grain Size of Dynamically Trans-
formed Ferrite
It has been reported that the grain size of dynam-
ically recrystallized ferrite grains is a function of Z
parameter,
35
and the ferrite grain size obtained by
thermomechanical process using DT is much finer
compared with that obtained through conventional
TMCP processes.
3,5
Figure 6shows the change in the
grain size of ferrite dynamically transformed from
the austenite with a mean grain size of either 15 lm
or 400 lm, which was deformed to a strain of 0.96 in
the 6Ni-0.1C steel, plotted as a function of the Z
parameter. The activation energy for determining
the Zparameter used in Fig. 6is 254 kJ mol
1
,
which is the energy for self-diffusion of iron atom in
body-centered cubic structure. The grain size of the
dynamically transformed ferrite decreased with
increasing the Zparameter (i.e., at a high strain rate
or low deformation temperature), of which Z-depen-
dence was similar to the previous reports about
dynamic recrystallization of ferrite.
35
The reason
why finer ferrite grains were obtained at higher Z
parameter could be explained in the following ways.
A higher strain rate introduces a larger number of
lattice defects in austenite, and such a large density
of defects in the deformed austenite acts as the
nucleation site for ferrite transformation. That is, the
nucleation density of dynamically transformed fer-
rite is high. In addition, when the deformation tem-
perature is lower, the growth rate of newly formed
ferrite becomes slower. Consequently, the mean
grain size of the dynamically transformed ferrite
reaches to 1 lm when the austenite grain size is fine
(15 lm) and the Zparameter is high. The result also
indicates the effect of the grain boundary density of
austenite, which plays a role of preferential nucle-
ation site for ferrite transformation. A large density
of austenite grain boundaries in the specimens hav-
ing the austenite grain size of 15 lm also enhances
the number of nucleated ferrite grains per unit
volume.
There have been several explanations about grain
refinement in DT. Cellular automaton calculation
revealed that the finer ferrite grain size in DT could
be obtained owing to continuous nucleation of fer-
rite around austenite/ferrite phase boundaries as
well as at austenite grain interiors because retained
austenite is continuously deformed.
11
Adachi et al.
22
proposed that a crystal rotation of austenite and DT
occurred simultaneously during deformation so that
the orientation of dynamically transformed ferrite
in the earlier stage of DT was different with that in
the later stage of DT. Although Hurley et al.
36
denied any possibility of dynamic recrystallization
of ferrite after DT, Matsumura and co-workers
2–4
insisted dynamic recrystallization of dynamically
transformed ferrite. In the current study, the rep-
resentative TEM image of dynamically transformed
Fig. 5. A part of the phase diagram in a Fe-6Ni-C system showing the change in Ae
3
lines with respect to (a) the additional energy to austenite
and (b) the corresponding dislocation density in austenite phase. The dashed line corresponds to the chemical composition of carbon, 0.1 wt.%,
used in this study.
Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel
Author's personal copy
ferrite shown in Fig. 7, for which the specimen was
deformed to a strain of 0.96 at a strain rate of
10
1
s
1
at 650°C and exhibited a mixed micro-
structure composed of fine and equiaxed ferrite
grains and ferrite grains elongated perpendicular to
the compression direction (double-headed arrow).
The fine equiaxed grains in Fig. 7are probably the
ferrite grains dynamically recrystallized during
subsequent deformation after DT.
37
Accordingly, it
can be considered that the refinement of ferrite
grains through DT is attributed to several factors:
the continuous nucleation of ferrite nuclei, limited
grain growth of ferrite, and dynamic recrystalliza-
tion of transformed ferrite. Subsequent systematic
research on the grain refinement mechanisms of DT
has been carried out using a 10Ni-0.1C steel in the
authors’ group, from which the results will soon be
published elsewhere.
SUMMARY AND CONCLUSION
We summarized the recent studies about dynamic
ferrite transformation in a 6Ni-0.1C steel in this
article. The major results are listed as follows:
1. The occurrence of the dynamic transformation
could be confirmed from the stress–strain ana-
lysis. The deformation conditions for the occur-
rence of dynamic transformation were clearly
determined by Zener–Hollomon parameter.
When the Zener–Hollomon parameter at a given
deformation condition was higher than the crit-
ical value (1.7 910
14
s
1
), softening of the flow
stress occurred, resulting from dynamic trans-
formation.
2. Dynamic softening of the flow stress due to
dynamic ferrite transformation was observed
even at 750°C (above Ae
3
). The fraction of the
dynamically transformed ferrite increased with
increasing the strain. The ferrite grains were
reversely transformed to austenite during sub-
sequent and static annealing at 750°C after the
deformation, which confirmed the occurrence of
the dynamic transformation above Ae
3
.
3. The grain size of the dynamically transformed
ferrite decreased with increasing the Zener–
Hollomon parameter, i.e., at lower deformation
temperatures or higher strain rates. The effect of
austenite grain size was significant on the grain
size of the dynamically transformed ferrite, and
the finer austenite grain size resulted in the finer
ferrite. Finally, the mean grain size of ferrite
could be smaller than 1 lm, which was consid-
ered to be achieved through dynamic transfor-
mation and subsequent dynamic recrystallization
of ferrite.
ACKNOWLEDGEMENTS
The authors would like to gratefully thank Prof.
Hiroyuki Yasuda of Osaka University for his con-
siderable support in thermomechanical experi-
ments. This study was financially supported by the
Grant-in-Aid for Scientific Research on Innovative
Area, ‘‘Bulk Nanostructured Metals’’ (Area
No.2201), the Grant-in-Aid for Scientific Research
(A) (No.24246114), and the Elements Strategy Ini-
tiative for Structural Materials (ESISM), all
through the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan (Contact
No. 22102002). N.P. was supported also by the
Japan Society for Promotion of Science (JSPS) as a
JSPS postdoctoral fellow. All the support is grate-
fully appreciated.
REFERENCES
1. K. Nishioka and K. Ichikawa, Sci. Technol. Adv. Mater. 13,
023001 (2012).
2. H. Yada, Y. Matsumura, and K. Nakajima, U.S. Patent
4466842 (August 1984).
3. Y. Matsumura and H. Yada, Trans. ISIJ 27, 492 (1987).
Fig. 6. Change in the grain size of dynamically transformed ferrite as
a function of the Zener–Hollomon parameter. Two austenite grain
sizes, 400 lm (square) and 15 lm (circle), were used for the
experiments. The activation energy used for calculating the Zener–
Hollomon parameter was 254 kJ mol
1
.
Fig. 7. A TEM micrograph of ferrite grains formed through a ther-
momechanical process at 650°C. The austenite grain size was
15 lm, the strain rate was 10
1
s
1
, and the strain was 0.96. The
double-headed arrow indicates the compression direction.
Park, Zhao, Shibata, and Tsuji
Author's personal copy
4. H. Yada, T. Matsumura, and T. Senuma, Steels Other Met.
ISIJ 88, 200 (1988).
5. P. Hodgson, M.R. Hickson, and R.K. Gibbs, Scr. Mater. 40,
1179 (1999).
6. M.R. Hickson, P.J. Hurley, R.K. Gibbs, G.L. Kelly, and P.D.
Hodgson, Metall. Mater. Trans. 33, 1019 (2002).
7. S.C. Hong, S.H. Lim, H.S. Hong, K.J.K.S. Lee, and D.H.
Shin, Mater. Sci. Eng. 355, 241 (2003).
8. S.C. Hong, S.H. Lim, K.J. Lee, and D.H. Shin, ISIJ Int. 43,
394 (2003).
9. J.K. Choi, D.H. Seo, J.S. Lee, K.K. Um, and W.Y. Choo, ISIJ
Int. 43, 746 (2003).
10. T.H. Ahn, K.K. Um, J.K. Choi, D.H. Kim, K.H. Oh, M. Kim,
and H.N. Han, Mater. Sci. Eng. 523, 173 (2009).
11. C. Zheng, N. Xiao, L. Hao, D. Li, and Y. Li, Acta Mater. 57,
2956 (2009).
12. M.H. Cai and H.D.Y.K. Lee, Mater. Trans. 52, 1722 (2011).
13. S.C. Hong and K.S. Lee, Mater. Sci. Eng. 323, 148 (2002).
14. W. Choo, Trends Met. Mater. Eng. 16, 93 (2003).
15. J.K. Park, K.H. Kim, J.H. Chung, and S.Y. Ok, Metall.
Mater. Trans. 39, 235 (2008).
16. Y.Q. Weng, X.J. Sun, and H. Dong, Symposium on Ultrafine
Grain Structures (Beijing: Metallurgical Industry Press,
2005), pp. 9–15.
17. Y. Weng, Ultra-Fine Grained Steels (New York: Springer, 2009).
18. N. Park, A. Shibata, D. Terada, and N. Tsuji, Acta Mater. 61,
163 (2013).
19. N. Park, S. Khamsuk, A. Shibata, and N. Tsuji, Scr. Mater.
68, 538 (2013).
20. N. Park, S. Khamsuk, A. Shibata, and N. Tsuji, Scr. Mater.
68, 611 (2013).
21. R.Z. Wang and T.C. Lei, Scr. Metall. Mater. 31, 1193 (1994).
22. Y. Adachi, P.G. Xu, and Y. Tomota, ISIJ Int. 48, 1056 (2008).
23. C. Zheng, D. Li, S. Lu, and Y. Li, Scr. Mater. 58, 838 (2008).
24. S.Y. Ok and J.K. Park, Scr. Mater. 52, 1111 (2005).
25. A.Z. Hanzaki, R. Pandi, P.D. Hodgson, and S. Yue, Metall.
Trans. 24, 2657 (1993).
26. T. Sakai and J. Jonas, Acta Metall. 32, 189 (1984).
27. A. Cingara and H.J. McQueen, J. Mater. Process. Technol.
36, 17 (1992).
28. S.W. Lee, D.H. Seo, and W.Y. Choo, Korean J. Met. Mater.
36, 1966 (1998).
29. M. Tong, D. Li, Y. Li, J. Ni, and Y. Zhang, Metall. Mater.
Trans. 35, 1565 (2004).
30. M. Tong, J. Ni, Y. Zhang, D. Li, and Y. Li, Scr. Mater. 50,
909 (2004).
31. C. Ghosh, V.V. Basabe, J.J. Jonas, Y.M. Kim, I.H. Jung, and
S. Yue, Acta Mater. 61, 2348 (2013).
32. Nokeun Park (Ph.D. Dissertation, Kyoto University, 2013).
33. J.J. Jonas and C. Ghosh, Acta Mater. 61, 6125 (2013).
34. H. Dong and X. Sun, Curr. Opin. Solid State Mater. Sci. 9,
269 (2005).
35. A. Ohmori, S. Torizuka, K. Nagai, N. Koseki, and Y. Kogo,
Metall. Mater. Trans. 45, 2224 (2004).
36. P.J. Hurley, P.D. Hodgson, and B.C. Muddle, Scr. Mater. 40,
433 (1999).
37. L.Zhao,N.Park,A.Shibata,andN.Tsuji,TMS 2014
Annual Meeting Supplemental Proceedings, 2014, p. 919.
Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel
Author's personal copy
