Materials Science and Engineering A 438 440 (2006) 300 305
Microstructures and stability of retained austenite in TRIP steels
a," a a
X.D. Wang , B.X. Huang , Y.H. Rong , L. Wanga,b
a
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China
b
Baosteel Research and Development Technology Center, Shanghai 201900, China
Received 27 June 2005; received in revised form 13 January 2006; accepted 11 February 2006
Abstract
Transformation induced plasticity (TRIP) steels exhibit a combination of high strength and ductility due to their multi-phase microstructure,
including ferrite, bainite and retained austenite which transforms to martensite under the external stress. The characterization of microstructures
is necessary for understanding the relationship between microstructure and property. In the present work, an effort to determine the volume
fraction of various phases was made for the conventional TRIP steel containing silicon. The microstructures in the TRIP steel were characterized
by optical microscopy, scanning electron microscopy and transmission electron microscopy, especially, an effective method was developed to
identify multi-phase microstructures by atomic force microscopy based on the height difference. Furthermore, the stability of retained austenite
determining TRIP effect was evaluated by electrical resistance tests and tensile tests. The results show that retained austenite does not generate
ć%
martensitic transformation at -80 C and exhibits a good thermodynamic stability, and the transition temperature from stress-induced martensitic
ć%
transformation to strain-induced martensitic transformation is determined as about -5 C, and thus strain-induced martensitic transformation over
ć%
-5 C (somewhat lower than room temperature) is favorable for the application of TRIP steels in the automobile industry.
© 2006 Elsevier B.V. All rights reserved.
Ã
Keywords: TRIP steel; Microstructure; Retained austenite; Ms temperature; Mechanical properties
1. Introduction stabilities. The Ms temperature is used to characterize the ther-
modynamic stability of retained austenite against transformation
Ã
Low-carbon multi-phase sheet steels, which were developed under cooling, whereas the Ms temperature is used to evalu-
for automotive applications, have attracted a growing interest in ate mechanical stability against stress-assisted transformation.
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recent years due to their high strength and ductility combination. At the Ms temperature, the stress reaches the yield stress for
These excellent mechanical properties mainly arise from a com- slip in the austenite phase [4 6]. In the present paper, a devel-
bination of a ferrite matrix, for ductility, bainite, for strength, and oped method, measuring the height of various phases by means
retained austenite, for uniform elongation produced by marten- of atomic-force microscopy (AFM) after etching, is built eas-
site transformation from austenite under the influence of external ily and fast to identify various phases in a TRIP steel, and the
tensile stress [1 3]. The multi-phase microstructures are formed validity of the method is verified by comprehensive analysis by
after an intercritical annealing followed by austempering at the use of optical microscopy (OM), scanning electron microscopy
bainite transformation temperature. During the above heat treat- (SEM) and transmission electron microscopy (TEM). Moreover,
ment, the austenite phase is stabilized by the enrichment of the volume fraction of various phases is determined by combina-
carbon due to the presence of alloying elements such as sili- tion of thermodynamic calculation, metallographic examination
con or aluminum. From this viewpoint, the volume fraction, the and X-ray diffraction (XRD). The stability of retained austenite
distribution of various phases, especially the stability of retained is further investigated by means of the measurement of Ms and
Ã
austenite is of great importance to the mechanical properties of Ms temperature, and the different mechanical behaviors in the
transformation induced plasticity (TRIP) steels. The stability TRIP steel are explained.
of retained austenite includes thermodynamic and mechanical
2. Experimental procedure
"
The chemical composition of the TRIP steel investigated in
Corresponding author. Tel.: [phone].
E-mail address: [e-mail] (X.D. Wang). the present work is listed in Table 1, together with Ac1, Ac3
0921-5093/$  see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2006.02.149
X.D. Wang et al. / Materials Science and Engineering A 438 440 (2006) 300 305 301
Table 1
Chemical composition of the TRIP steel (mass%)
C Si Mn PSAl NAc1 (ć%C) Ac3 (ć%C) T50 (ć%C)
0.11 1.19 1.67 0.013 0.006 0.038 0.003 683 861 803
temperatures and T50 temperature standing for the volume frac- 3. Results and discussion
tions of ferrite and austenite in the intercritical temperature are
50%, respectively, which is calculated by thermo-calc software 3.1. Determination of volume fractions of various phases
system. It was suggested that slow cooling from an intercritical
annealing temperature near Ac3 to a temperature close to Ac1 Microstructure observed with SEM on specimen treated as a
before rapid cooling could lead to a better balance of strength marked  2 line in Fig. 1 is shown in Fig. 2. The volume frac-
and ductility [7]. Therefore, the intercritical annealing, as shown tion of ferrite (fF) is determined by quantitative metallographic
ć%
a marked  1 line in Fig. 1, was carried out at 800 C for 70 s, examination as about 56.1%, which is consistent with thermo-
ć%
then slow cooling to 690 C with a cooling rate of 10 s-1, isother- dynamic calculation. By XRD analysis, the volume fraction of
ć%
mally held at 400 C for 4 min followed by water quenching. retained austenite (fAr) in undeformed samples is determined as

In order to check the volume fraction of ferrite phase calcu- about 16%, and about 7% (fAr ) in deformed samples. Therefore,
lated by thermo-calc software, a heat treatment was designed the volume fraction of bainite (fB) can be calculated, namely
as follows. A sample was treated by intercritical annealing at fB =1- fF - fAr = 27.9%, while the volume fraction of marten-
ć% ć%
800 C for 70 s, then slow cooling to 690 C followed by water site (fM) in deformed samples due to martensitic transformation

quenching to room temperature, as shown a marked  2 line in from austenite can be determined as 9% (fM = fAr - fAr ). It indi-
Fig. 1, and the volume fraction of ferrite in the sample was deter- cates that about 7% of retained austenite does not transform to
mined by quantitative metallographic examination. The volume martensite during tensile process.
fraction of retained austenite in undeformed specimen or ten-
sile specimen was measured with XRD. The specimens for OM
3.2. Phase identification by AFM
and SEM were prepared according to the traditional procedure,
that is, mechanical grinding and polishing up with 1 m dia- OM or SEM observation on specimens etched by nital cannot
mond paste, followed by etching with 2% nital for 15 s. Three
be used to distinguish bainite, retained austenite and marten-
specimens for AFM were prepared. One of them was prepared
site by their contrast, while ferrite matrix is clearly observable,
as, same as that of OM, the others were polished up with 1 m
as black contrast in Fig. 2, and white contrast in Fig. 3. This
diamond paste, then were deformed up to 3% strain, of which
is because the etching solution of nital attacks ferrite grains
one specimen was further etched with 2% nital for 15 s. Both
and grain boundaries while leaving austenite and martensite
deformed and undeformed specimens were observed by TEM.
The thermodynamic stability of retained austenite was deter-
mined by measuring the Ms temperature using electrical resis-
tance test. In order to characterize the mechanical stability of
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retained austenite, the Ms temperature of the TRIP steel was
determined by measuring its yield strength. The measurement
method described by Barbe et al. [6], Berrahmoune et al. [8] was
conducted, and tensile tests were carried out at a strain rate of
5 × 10-3 in the rolling direction in temperature range from 20
ć%
to -40 C.
Fig. 1. Heat treatment process. Fig. 2. SEM images treated as line 2 in Fig. 1.
302 X.D. Wang et al. / Materials Science and Engineering A 438 440 (2006) 300 305
Fig. 3. Optical micrographs of the TRIP steel.
intact [9], which results in different height of various phases.
The image contrast in OM is different from that in SEM since
their imaging principles are different. In recent years, although
a modified color etching method [9,10] was used to distinguish
bainite from ferrite and tempering martensite from austenite in
TRIP steels, this technique is somewhat complicate since the
strain-induced martensite must undergo tempering treatment.
A developed method was presented in this paper, in which
the height of various phases in undeformed samples after etching
was firstly measured by means of AFM with reference to, the
height of retained austenite due to not suffering etching, then the
relative height of martensite with retained austenite in deformed
sample was determined, finally the height of various phases of
the deformed sample was measured. Let the height of retained
austenite be zero, and then the height of martensite is positive
while the height of the ferrite or bainite is negative.
Fig. 4 is an AFM image of various phases in an undeformed
sample. In Fig. 4(a), the retained austenite was located in the
cross of three lines, as reference object for measuring the height
Fig. 4. AFM image of various phases in an undeformed sample: (a) two-
of various phases. The relative height of various phases is shown
dimensional image and (b) section analysis.
in AFM image of section analysis (Fig. 4(b)), such as the phase
with the relative height -201.7 nm is ferrite, -97.6 nm is bainite
of various phases is austenite, martensite, bainite and ferrite.
and +1.5 nm is austenite. Fig. 5(a and b) are, respectively, three-
Therefore, the multi-phase microstructures in TRIP steel can be
dimensional and two-dimensional images of martensite surface
easily identified by the above AFM method.
relief of a deformed sample, and its section analysis is demon-
strated in Fig. 5(c). The height of martensite surface relief is
3.3. Stability of retained austenite
about 50.3 nm.The statistical results of the relative height and
the surface roughness of various phases are respectively listed
The stability of retained austenite includes thermody-
in Tables 2 and 3 for undeformed and deformed samples. The
namic and mechanical stabilities. Fig. 6 depicts the variation
statistical results indicate that the increasing sequence of abso-
lute height of various phases is ferrite, bainite, austenite and
Table 3
martensite, and the increasing sequence of surface roughness
Statistical analysis of the surface roughness and height difference in the deformed
sample
Table 2
Phase Martensite Retained Bainite Ferrite
Statistical results of the surface roughness and height difference in the unde-
austenite
formed sample
Mean height difference +58.5 0 -79.7 -189.3
Phase Retained austenite Bainite Ferrite
(nm)
Mean height difference (nm) 0 -89.2 -196.4 Surface roughness 7 5 10 18
Surface roughness (nm) 5 9 18 (nm)
X.D. Wang et al. / Materials Science and Engineering A 438 440 (2006) 300 305 303
Fig. 5. AFM image of martensite surface relief on polished surface after deformation: (a) three-dimensional image, (b) two-dimensional image and (c) section
analysis.
ć%
of electrical resistance as a function of temperature dur- up to -5 C, then decreasing with decreasing temperature
ć% ć%
ing the thermal cycle from 20 to -80 C. In the electrical till -20 C, further, it increases with decreasing temperature.
ć%
resistance temperature curve, there exists no exothermal peak This abnormal phenomenon at -5 C was explained as the
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during cooling, which indicates that martensitic transformation transition point (called Ms ) from stress-induced martensitic
does not occur. This experiment demonstrates a good thermo- transformation to strain induced martensitic transformation [5].
dynamic stability of the retained austenite. Fig. 7 shows the The relationship between the yield strength and temperature can
yield and tensile strength temperature curve of the TRIP steel be reasonably explained by on modulus change of TRIP steel.
ć%
in temperatures ranging from 20 to -40 C. It is worthy to note The modulus of the TRIP steel reflects mean modulus of various
that the yield strength increases with decreasing temperature phases, and ferrite and bainite have great weight with mean
modulus. The TRIP steel has the majority of ferrite and bainite
(about 84%) and the minority of retained austenite (about 16%).
With the decrease of temperature the mean modulus of the TRIP
Fig. 6. Electrical resistance of the sample at different temperatures.
Fig. 7. Yield strength of the TRIP steel at different deformation temperatures.
304 X.D. Wang et al. / Materials Science and Engineering A 438 440 (2006) 300 305
Fig. 8. The stress strain curves of the TRIP steel.
steel increases, which results in the increase of yield strength
with decreasing temperature. In the range of temperatures from
ć%
-5 to -20 C, the onset of the martensitic transformation
due to tensile stress within the elastic range leads to the local
 soft of modulus in retained austenite, then the drop of yield
ć%
strength, such effect continues below -20 C. However, it is
ć% Ã
worthy to point out that below -5 C(Ms ), the yield strength
rises with decreasing temperature as the same tendency as in
ć%
the range of temperatures from 20 to -5 C, and this effect
is attributed to the predominant role of ferrite and bainite in
Fig. 9. TEM micrographs of twin-typed martensite of the deformed sample: (a)
the mean modulus, which is different from single austenite
Å» Å» Å»
bright field and (b) diffraction pattern [1 31]m//[13 1]t.
phase in Fe 20Ni 0.5C steel [11]. The TRIP steel studied in
this work exhibits a good thermodynamic stability and about
ć% Ã
4. Conclusions
-5 C of Ms (somewhat lower than room temperature), and
thus this is favorable for the application of TRIP steels in
The microstructure of conventional TRIP steel with Si was
engineering.
investigated by means of OM, SEM, AFM and TEM. The modi-
The stress strain relationship at different temperatures
fied method presented in this work exhibits an effective approach
was investigated. Fig. 8 shows the stress strain curves at 0
ć% ć%
to distinguish the multi-phase microstructures. In addition, both
and -15 C. In the curve for -15 C, there is a yield-drop,
thermodynamic and mechanical stability of retained austenite
corresponding to the local  soft of modulus in retained
were evaluated by electrical resistance measurement and tensile
austenite. From Fig. 8, it can be found that the work hardening
ć% ć%
tests, respectively. Main conclusions can be drawn as follows.
rate at -15 C is greater than that at 0 C, which results in
the earlier generation of martensitic transformation due to the
decrease of stacking fault energy of austenite with the decrease (i) The volume fraction of various phases can be determined
of temperature [12]. Moreover, the formation of matensite by quantitative metallographic examination and XRD. The
under stress produces more effective  strain hardening than studied TRIP steel contains 56.1% ferrite, 27.9% bainite
does the ordinary slip mechanism in fcc austenite [11]. From the and 16% retained austenite while 9% of austenite trans-
above analysis, it is suggested that martensitic transformation forms to martensite during tensile stress.
in TRIP steel has  strain hardening effect and  uniform (ii) AFM topographic analysis (height and roughness) makes
elongation effect due to martensitic transformation causing the it possible to identify various phases. The statistical results
redistribution of stress, as a consequence, TRIP steels exhibit indicate that the increasing sequence of height of various
greater tensile strength and ductility than many commercial phases is ferrite, bainite, austenite and martensite.
steels currently used in the manufacture of automobiles. The (iii) The electrical resistance measurement indicates that when
ć% ć%
strain-induced martensite in -15 and 0 C deformation was the retained austenite is cooled to -80 C, martensitic
verified as twin-type one by TEM observation, as shown in transformation does not occur, which shows that the
Fig. 9. The martensitic transformation from retained austenite retained austenite has a good thermodynamic stability,
Ã
can relax the accumulated stress in ferrite and bainite around the but it is metastable under stress. The Ms temperature is
ć%
austenite. determined as about -5 C by yield stress temperature
X.D. Wang et al. / Materials Science and Engineering A 438 440 (2006) 300 305 305
Ã
curve, moreover, the abnormal drop point (Ms ) in yield [4] J.R. Patel, M. Cohen, Acta Metall. 1 (1953) 531.
[5] G.N. Haidemenopoulos, M. Grujicic, G.B. Olson, et al., Acta Metall. 37
stress temperature curve and the different change tendency
(1989) 1677.
in stress strain curve at different temperatures are reason-
[6] L. Barbe, M.D. Meyer, B.C.D. Cooman, in: Proceeding of Interna-
ably explained respectively based on the mean modulus of
tional Conference on TRIP-Aided High Strength Ferrous Alloys, B.C. De
TRIP steel and martensitic transformation in TRIP steel.
Cooman (Ed.), Ghent, Belgium, 2001, p. 65.
[7] O. Matsumura, Y. Sakuma, H. Takech, Trans. ISIJ 27 (1987) 570.
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