Investigation into the stress-strain curves of deformation-induced ferrite transformation in a low
Baosteel Technical Research Volume 6，Number 2，June 2012，Page 49
Corresponding author ：ZHAO Shengsheng ；E-mail ：zhaoheshan@baosteel．com
Investigation into the stress-strain curves of deformation-induced ferrite transformation in a low carbon steel
ZHAO Shengsheng 1）and ZHAO Heshan 2）
1）Shenzhen Polytechnic ，Shenzhen 518055，China
2）Silicon Steel Department ，Baoshan Iron ＆Steel Co．，Ltd．，Shanghai 200941，China
Abstract ：A series of tests of deformation-induced ferrite transformation （DIFT ）in a low carbon steel were carried out by
the Gleeble-3500hot simulation machine at a temperature range of Ae 3-Ar 3．The overall stress-strain curves during DIFT
can be divided into three typical types
：“double-humped ”，“single-humped ”and “transitional ”．The peaks exhibited in the curve are involved with deformation-induced transformation which happened in grains or at the grain boundaries．
According to the stress-time curve and strain-time curve ，
strain capacity dramatically postponed the strain-induced transformation ，which leads to the start of the transformation right ahead of the finish of deformation and the majority of the ferrite transformation process mainly happened after the deformation．Deformation-induced transformation is a
metadynamic transformation process with dynamic nucleation．
Key words ：low carbon steel ；deformation-induced ferrite transformation ；stress-strain curve ；metadynamic transformation doi ：10．3969/j．issn．1674－3458．2012．02．010
1Introduction
The stress-strain curve can well reflect the metal ’s
structural change during the stress deformation．Medina ，et al ．had an intensive study on low alloy austenitic high-temperature deformation．He gave a detailed summary of morphological features of the dynamic recrystallization of the stress strain curve．Najafi-Zadeh ，
et al ．who had studied the dynamic recrystallization of ferrite morphology ，also summarized the corresponding stress-strain curve．Later on ，researchers studying strain-induced phase transition ［1-2］
judged whether a ferrite-austenite recrystallization occurred or not based on the degree of similarity between the stress-strain curves and
the dynamic recrystallization characteristic curve．Yang Ping ，et al ．［2］thought that the steel material deformed in the ferrite temperature range ，where ferrite could be strain-induced and precipitated．When at a high temperature ，it mainly takes the form of the austenite dynamic recrystallization and DIFT as the supplement．When at a low temperature ，it mainly takes the form of the induced precipitation of ferrite and austenite /ferrite dynamic recrystallization as the supplement．
Actually ，as various structural changes have the similar situation in terms of the curves reflecting the law of such changes ，it is difficult to judge what kind of changes will happen according to the curve alone．Therefore ，based on the microstructure of the DIFT ，the author studied the obtained stress-strain curves further．
2Test methods
The low-carbon Q235steel billet was used as the test
material．It was normalized at 950?for 1h ，and then
machined to a cylindrical sample （ 8mm ?15mm ）．Its chemical composition in the mass fraction ：w C =0．13%，w Si =0．19%，w Mn =0．49%，w P =0．012%，w S =0．013%．The deformation-induced ferrite of the cylindrical sample
was tested by the Gleeble-3500thermal simulation testing
machine．The test process is shown in Fig．1．The sample was heated to 950?and kept for 5min．After it turned to austenite ，the sample was cooled to 800?（Ar 3≈760?，Ae 3=848?）at the cooling rate of 15?/s ，and then quenched immediately as soon as it was compressed at the strain rate of 20s －1．The deformation structure was kept at room temperature for further observation and analysis．The deformation process parameters can be recorded by the Gleeble-3500．The deformation-induced ferrite sample
would eventually be cut open at the mid of the sample along the direction perpendicular to the compression axis ，and was observed under the optical microscope after the sample was polished and corroded with 4%alcohol
solution．
Fig．1Schematic diagraph of hot deformation and quenching process
Baosteel Technical Research ，Vol．6，No．2，Jun．2012
3Results and discussion
3．1The stress-strain curve
Previous researchers made different analysis on the structural changes ，which reflected in the stress-strain curve corresponding to the DIFT．They considered that the DIFT occurred while the dynamic recrystallization of austenite happened．Because of this ，the deformation-induced phase transformation was significantly delayed．Thus the stress-strain curve had two peaks．Yang Ping ，
et al ．［1］
investigated the low-carbon steel ’s strain-induced
phase transformation in a relatively wide temperature range．The result showed that the stress-strain curve shape
of the austenite dynamic recrystalli-zation was different
from that of its deformation-induced phase transformation．
It was found that even if the induced phase transformation test was carried out at a temperature much lower than the dynamic recrystalli-zation temperature of the austenite
deformation ，there would still be something similar to the double-humped austenite dynamic recrystallization
phenomenon．For this phenomenon ，the stress-strain curves of deformation-induced phase transformation were
studied further．
There are only three cases of the common stress-strain
curve of the DIFT ：one has a similar “double-humped ”
feature with dynamic recrystallization ，as shown in Fig．2（a ）；the second is the “single-humped ”type ，as shown
in Fig．2（b ）；the third “transitional ”type is between the first and the second as shown in Fig．2（c ）
．
Fig．2Three kinds of stress-strain curves during the deformation of the tested steel
The Gleeble-3500thermal simulation machine has a
detailed record of various deformation parameters ，which changed over time in the process of DIFT．Since the stress-time curve and the strain-time curve are more intuitive than the stress-strain curve ，
and the covered information is more comprehensive ，
these two curves are listed in this article instead of the stress-strain curve
（each peak in the stress-strain curve is corresponding
with each peak in the stress-time curve ）．The stress-time and strain-time curves corresponding to the above
mentioned three typical stress-strain curves are shown
in Figs．3，
5and 7．The key mechanism of DIFT lies in the storage energy．One of the most effective parameters affecting the storage energy is deformation．Therefore ，with fixed deformation parameters （the deformation temperature ：800?and the deformation rate ：20s －1），and by changing the strain capacity ，the structural change ，which can be seen in the stress-strain curve during the
deformation process ，has been studied．Each structural change of the deformed austenite during the deformation process will be reflected in the curve．That is to say that a curvature change of the curve will
appear．To be the same ，
each curvature change of the curve ，corresponding to the platform or the apparent
peak on the curve ，
will also correspond to a structural change ：the dynamic recrystallization or the deformation-induced phase transformation．The curve
can be analyzed in combination with the information of crystal phase structures．
3．2The stress-time curve
First ，a typical “double-humped ”curve should be
observed carefully．As shown in Fig．2（a ），the basic
parameters are set as follows ：the deformation temperature is 800?；the deformation rate is 20s －1and the deformation amount is 80%（corresponding to strain
ε=1．61）．There are two peaks in the stress-strain curve ，corresponding to the stress plateau and peak in
the stress-time curve．It can be seen from the stress-time
curve that when the sample is compressed and deformed ，the stress will first increase rapidly with time．When the stress reaches a certain level ，the sample ’s hardening rate will be slowed down and the first stress plateau will emerge．The stress plateau here is due to the precipitation of deformation-induced ferrite at austenite
grain boundaries．Since the ferrite phase is softer than that of austenite ，the ferrite precipitation ’s softening effect will offset part of the strain hardening effect ，and a stress plateau will emerge in the overall stress-time
curve．As the deformation continues and the nucleating site reduces ，the hardening effect of austenite gradually dominates ，and the stress continues to increase．When the strain reaches a certain level ，and the storage energy in the sample reaches a critical value ，the nucleation will begin to occur in large quantity in austenite grains ，resulting in the rapid softening of the overall sample．The above speculation can be simply described as follows ：the deformation storage energy Q is a parameter increasing with the deformation （the deformation temperature and deformation rate are fixed ），the required driving force of the DIFT at the austenite grain boundary is Q inter ，and the driving force in the austenite grains is Q intra ．When Q ＞Q inter ，DIFT will first occur at
the grain boundary ，
and when Q ＞Q intra ，DIFT will occur 0
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ZHAO Shengsheng ，et al．Investigation into the stress-strain curves of deformation-induced ferrite transformation ……
in the inner grains．If the speculation is correct ，there must be a storage energy Q ，which is between Q intra and Q inter ，i．e．Q intra ＞Q ＞Q inter ．In this case ，DIFT will occur at the grain boundary instead of within the grains．
Therefore ，
by changing amount of deformation ，and in combination with the final crystal structure ，it can be determined whether this speculation is correct or not．As shown in Fig．3（a ）and Fig．4（a ），the parameters of the deformation process are as follows．The deformation temperature is 800?；the deformation rate is 20s －1；deformation amount is 80%（corresponding to the true strain ε=0．22）and the amount of deformation is 50%（corresponding to the true strain ε=0．69）．As there is only one peak reflected in the stress-time curve ，
according to the speculation ，if the deformation-induced phase transformation happens ，it belongs to the pattern ：Q intra ＞Q ＞Q inter ，and the final metallic phase
microstructure should show that the induced phase transformation only occurs at the grain boundary．If there is any difference ，it is the difference in the size and amount of ferrite grains．The final microstructure obtained is as shown in Fig．3（b ）and Fig．4（b ）．In
Fig．3（b ）corresponding to a 20%of reduction ，
there is only a small amount of induced ferrite at the austenite grain boundaries．In Fig．4（b ）corresponding to a 50%reduction ，it can be found that the induced ferrite phase has grown up at the austenite grain boundaries．All these are in line with the deduction based on the speculation mentioned above．This situation corresponds to the second single-humped type stress-strain curve as shown
in Fig．2（b ）．The induced phase transformation is the first to appear at the grain boundary ，which corresponds to the peak reflected in the
curve．
As shown in Fig．5（a ）and Fig．6（a ），the basic parameters of the deformation process are as follows．The deformation temperature is 800?；the deformation rate is 20s －1；the deformation amount is 60%（corresponding to the true strain ε=0．92）and 70%（corresponding to the true strain ε=1．2）．There is only one peak reflected in the stress-time curve ，but there is
also a significant curvature change appearing after the peak ，indicating a structural change at this moment．Thus ，based on the above speculation ，storage energy increases with deformation．When the deformation stored
energy Q ＞Q intra ＞Q inter ，a large number of deformation-induced ferrite grains should be displayed in the final grain organization．The result shown in Fig．5（b ）and Fig．6（b ）confirmed this speculation ，indicating that the second curvature shift in the curve is due to the softening effect of the deformation induced phase transformation in grains ，and the softening effect surpassing the hardening effect．This situation corresponding to the stress-strain curve is the third type
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Baosteel Technical Research ，Vol．6，No．2，Jun．2012
as shown in Fig．2（c ）．The first peak corresponds to the structural change of the DIFT at the grain boundary ，and
the second curvature shift reflects the occurrence of the
DIFT in grains．
As shown in Fig．7（a ）and Fig．7（b ），the basic parameters of the deformation process are as follows．The deformation temperature is 800?；the deformation rate is 20s －1，the deformation amount is 75%（corresponding to the true strain ε=1．39）and 80%（corresponding to the true strain ε=1．61）．What was reflected in the stress-time curve is an unconspicuous
peak followed by an obvious peak．When the strain is 60%，the deformation storage energy will reach a level ，which can meet the driving force of the induced phase transformation occurring within the grain．Therefore ，when the strain increases to 75%and 80%，
that is ，Q?Q intra ，if the deformation-induced phase
transformation only appears within the grain ，according to the speculation prior to the inference ，the metallographic morphology will not be much different from that when the strain is 60%．From the metallographic analysis ，the crystal structure shown in Fig．7（b ）and Fig．8（b ）has no essential difference
compared with that in Fig．5（b ）and Fig．6（b ），
which has verified the inference that the induced ferrite transformation would appear within the grain at the situation of Q ＞Q intra ＞Q inter ．As the storage energy increases constantly ，Fig．7（b ）and Fig．8（b ）
should
Fig．7Strain /Stress-time curves and the microstructure （strain ε=1．39）
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ZHAO Shengsheng ，et al．Investigation into the stress-strain curves of deformation-induced ferrite transformation
……
Fig．8Strain /Stress-time curves and the microstructure （strain ε=1．61）
have the second stress peak which is more significant than those in Fig．5（a ）and Fig．6（a ）．This corresponds with the first double-humped type stress-strain curve ，as shown in Fig．2（a ）．The first peak corresponds to the first occurrence of the induced ferrite phase transformation at the grain boundary．The second peak corresponds to the induced ferrite phase transformation within the grain．In summary ，in the process of DIFT ，there are three cases according to the relationship between the storage energy and the required driving force ，namely ：Q ?Q intra ，Q ＞Q intra ＞Q inter and Q intra ＞Q ＞Q inter ．The situation corresponds to the three typical stress-strain
curves respectively．Based on their corresponding morphology analysis ，the peak in the curve is due to the softening effect caused by the deformation-induced
phase transformation occurring at the grain boundaries or within the grain．That is an inflexion point characterized by the equilibrium between strain-hardening and phase-change softening．3．3The strain-time curve
In combination with the strain-time curves ，
researchers closely observed the stress-time curves in
Figs．3-8，and then found that a great amount of DIFTs occurred almost at the end of the deformation ，regardless of induced ferrite nucleation at the grain boundaries or within the grains．According to the curves shown in Figs．3-8．Although the required driving force was the same and the induced ferrite transformations were all in the grains ，the start time for the induced phase transformation in large quantities was different．As the amount of deformation increases ，the true strain corresponding to the inflexion point also increases．Summarizing the true strain corresponding to the starting point of the phase transition based on what was seen from Fig．3（a ）to Fig．8（a ），researchers found that the start time when a great amount of induced phase transformation occurred within the grains was postponed．It means that the deformation gives a significant delay effect on induced phase transformation ，leading to the result that deformation-induced phase
transformations begin almost at the end of the deformation．
Because the deformation time is short ，deformation-
induced nuclei which are not yet fully grown up will
continue to grow after the deformation process．Contrasted with the strain-time curve ，it can be seen that
most of the deformation-induced phase transfor-mation
process takes place after the deformation．The stress-time curve analysis according to Reference ［3］shows that the process of the induced ferrite transformation from start to end lasts only 0．08s．For example ，as shown in Fig．9，the deformation temper-ature is 800?；
the deformation rate is 20s －1
，the deformation amount is 80%（corresponding to the true strain ε=1．61）．It takes 0．02s from the start of the induced ferrite transformation to the end of the deformation．It takes 0．06s from the end of the deformation to the end of the transformation ，which means 75%of the transformation process occurs after the
deformation．
Fig．9Schematic diagraph of the DIFT process （strain ε=
1．61）
Steel α－γphase transition consists of three stages ：nucleation ，nuclei growth and the grain coarsening．In the past ，researchers thought that the DIFT was an ongoing process of nucleation．That is ，in the whole process of deformation-induced phase transformation ，
since the high-energy region is increasing with
deformation ，ferrite nucleation sites increase along with the number of induced ferrite nuclei．Continuous nucleation gives the transformation some dynamic characteristics．However ，according to the above analytical results ，strain capacity would dramatically postpone the strain-induced transformation ，which lead to the start of the transformation right ahead of the finish of deformation
3
5
Baosteel Technical Research，Vol．6，No．2，Jun．2012 and the majority of the ferrite transformation process
mainly happened after the deformation．The induced
ferrite nuclei that had not yet fully grown up would
continue to grow after the deformation process．
Similarly，Dong Han，et al．［4］presented a concept of
metadynamic phase transition，called the isothermal
phase change after the DIFT（dynamic phase
transition）metadynamic phase transition．As the nuclei
would grow after deformation and the deformation-
induced transformation process was closely related to
each other．Strictly speaking，the growth is still a part
of the deformation-induced phase transformation．
Thus，it is not accurate to simply give the DIFT a
dynamic phase transition nature，instead，it should be
defined as a metadynamic phase transition process with
dynamic nucleation．
4Conclusions
（1）All the usual stress-strain curve shape during the
process of DIFT can be categorized into three cases：one
is a“double-humped”type；the second is a“single-
humped”type and the third is a transition type between
the first and the second．
（2）The sample’s microstructure morphology and the
stress-time curve analysis show that the three kinds of
stress-strain curves correspond to the three cases：
Q?Q
intra ，Q＞Q
intra
＞Q
inter
and Q
intra
＞Q＞Q
inter
．All the
curve peaks shown in these three kinds of curves were due to the deformation-induced phase transformation，which leads to the softening effect inter-or intra-crystal grains．These shown in the curves corresponding to the inflexion point position are the equilibrium phase transition point due to the hardening and softening effects．
（3）The deformation can significantly postpone the start point of the deformation-induced phase transfor-mation．The deformation-induced phase transformation occurs almost at the end of the deformation．As the deformation time is short，the deformation-induced nucleation comes into being but the nuclei are not yet fully grown up．They will continue to grow after the end of the transformation process．Thus，the DIFT is a kind of sub-dynamic phase transition．Strictly speaking，the DIFT is a kind of transformation with dynamic nucleation and sub-dynamic phase transformation．
References
［1］Yang Ping and Sun Zuqing．Effect of deformation temperature on the strain-induced phase transformation on
Q235carbon steel［J］．Acta Metallurgica Sinica，2001，37：
609-616．
［2］Yang Ping and Sun Zuqing．Analysis of the stress-strain curve of strain-induced phase transition on Q235carbon
steel［J］．Acta Metallurgica Sinica，2001，37：601-610．［3］Zhao Heshan，Li Dianzhong，Liu Zhaoxia，et al．Study on the diffusion and stability of carbon atoms in the
deformation induced ferrite phase on the low-carbon steel
［J］．Acta Metallurgica Sinica，2007，43：286-293．
［4］Weng Yuqing．Ultrafine-grained steel［M］．Beijing：Metallurgical Industry Press，2003：116-
117．
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