![a) T Q dependence of the volume fraction of retained austenite, V c , measured by the magnetic saturation method. (b) T Q dependence of the retained austenite C content, C c , based on the XRD austenite lattice parameter measurements. The dashed line is based on the Eq. [1]. (c) T Q dependence of the secondary M s temperature of the C-enriched retained austenite. The solid line is based on Eq. [2].](https://www.researchgate.net/profile/Eun-Jung-Seo/publication/269877772/figure/fig2/AS:669228482953217@1536567912132/a-T-Q-dependence-of-the-volume-fraction-of-retained-austenite-V-c-measured-by-the_Q320.jpg)
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Communication
Application of Quenching and
Partitioning Processing to Medium
Mn Steel
EUN JUNG SEO, LAWRENCE CHO,
and BRUNO C. DE COOMAN
The present work analyzes the application of quenching
and partitioning processing to medium Mn steel to ob-
tain a new type of ultra-high-strength multiphase med-
ium Mn steel. The selection of the quench temperature
makes it possible to vary the ultimate tensile strength
within a range of 500 MPa. The processing leads to low-
carbon lath martensite matrix with a controlled volume
fraction of retained austenite.
DOI: 10.1007/s11661-014-2657-7
The Minerals, Metals & Materials Society and ASM
International 2014
Increasing requirements related to passenger safety
and weight reduction in the automotive industry have
led to the development of a first generation of advanced
high strength steels (AHSS) such as dual phase and
transformation-induced plasticity (TRIP) steels. Second
generation austenitic high-Mn twinning-induced plas-
ticity (TWIP) steel with a Mn content in the range of 15
to 25 wt pct exhibits an excellent combination of tensile
strength (~1 GPa) and ductility (~60 pct) due to a
dynamic Hall–Petch effect resulting from the gradual
increase of the density of mechanical twins during
deformation.
[1,2]
The higher alloying costs and the lower
productivity associated with high-Mn TWIP steel are
currently the main drivers behind the development of
the intermediate and medium Mn steel. Adequate
combinations of mechanical properties have been
reported for intercritically annealed intermediate and
medium Mn steels. Their high work-hardening rate is
achieved by the TRIP effect or a combination of two
plasticity-enhancing mechanisms, the TWIP effect and
the TRIP effect.
[3–5]
The Mn content in these materials is
typically one third of the Mn content of the high-Mn
TWIP steels, but despite their lower Mn content, these
steels achieve excellent mechanical properties with a
strength-ductility balance in the range of 35,000 to
45,000 MPa pct.
Quenching and partitioning (Q&P) processing was
proposed by Speer et al.
[6]
as a new approach to produce
steel microstructures consisting of a low-C martensitic
matrix containing a considerable volume fraction of
retained austenite. Earlier studies have shown that the
Q&P processing of various AHSS enables the use of the
TRIP effect to achieve a pronounced improvement of
mechanical strength and ductility.
[7–9]
The Q&P processing consists of three stages: an initial
quenching stage, a partitioning stage, and a final
quenching stage. The austenitized steel is initially
quenched to a temperature, T
Q
, in the M
s
M
f
temperature range, and partially transformed to primary
martensite. It is then partition treated at the partitioning
temperature, T
P
. During the partitioning stage, C
diffuses from the supersaturated primary martensite
into the untransformed austenite. As a result, the M
s
temperature of the C-enriched austenite is lowered. This
leads to the stabilization of the untransformed austenite
upon cooling to room temperature. If C does not
partition enough to austenite, some of the austenite will
transform to secondary martensite in the final quenching
stage. The final microstructure consists of C-enriched
austenite islands in a low-C lath martensite matrix. The
martensite contributes a high strength level to the
material and the C-enriched austenite enhances the
elongation and the toughness. The compositions of Q&P
steels are lean, consisting mostly of C, Mn, and Si. C
and Mn are well known to enhance the austenite
stability. Addition of Si allows C partitioning to
austenite by suppressing the carbide precipitation during
the partitioning stage.
[10]
In the present work, the
application of the Q&P processing concept was analyzed
for medium Mn steel for the first time.
The chemical composition of the medium Mn steel
was Fe-0.21C-4.0Mn-1.6Si-1.0Cr (in wt pct). The M
s
temperature of the steel was 546 K (273 C). The
synergetic effect of the Si and Cr additions in increasing
the retained austenite volume fraction during Q&P
processing was reported previously.
[8,11]
The microstruc-
ture of the industrially cold-rolled sheet steel prior to
Q&P processing was a complex microstructure contain-
ing deformed pearlite and martensite.
Q&P processing was carried out in a Ba
¨hr 805
pushrod dilatometer either in vacuum or in a He
atmosphere. The specimens with dimensions of
10 9591.2 mm
3
were heated at a heating rate of
+10 C/s to 1123 K (850 C), fully austenitized for
240 seconds at 1123 K (850 C) and initially quenched
to a T
Q
temperature in the range of 423 K to 543 K
(150 C to 270 C) with 293 K (20 C) increments.
Subsequently, the specimens were reheated to the
partitioning temperature (T
P
) using a heating rate of
+20 C/s, held at the T
P
temperature for 300 seconds,
and finally quenched to room temperature. The initial
and final quenching was done using He gas to obtain a
cooling rate of 50 C/s.
The volume fraction of retained austenite was mea-
sured by a METIS magnetic saturation device. The C
content of retained austenite was measured by means of
XRD measurements, using a Bruker D8 Advance X-ray
diffractometer equipped with a Cu tube. The austenite
lattice parameter was determined by the Nelson–Riley
method.
[12]
EUN JUNG SEO and LAWRENCE CHO, Graduate Students,
and BRUNO C. DE COOMAN, Professor and Director, are with the
Materials Design Laboratory, Graduate Institute of Ferrous Technol-
ogy, Pohang University of Science and Technology, Pohang 790-784,
South Korea. Contact e-mail:decooman@postech.ac.kr
Manuscript submitted July 21, 2014.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The microstructure of the Q&P-processed medium
Mn steel was observed by means of scanning electron
microscopy and electron backscattering diffraction
(EBSD) conducted in a FEI Quanta 3D FEG. All the
specimens used for the microstructural analysis and the
XRD measurements were prepared by electro-chemical
polishing in a solution of 5 pct HCIO
4
+ 95 pct
CH
3
COOH in order to minimize possible errors origi-
nating from the mechanically induced transformation of
retained austenite during sample preparation. Cross-
sectional transmission electron microscopy (TEM) spec-
imens were also prepared by the Focused Ion Beam
technique in a FEI Quanta 3D FEG. The cross-sectional
TEM specimens were analyzed in a JEOL JEM-2100F
FE-TEM operating at 200 keV.
ASTM E8 standard sub-size tensile specimens with a
gage length of 25 mm were prepared as follows. Using a
box furnace, the specimens were fully austenitized at
1123 K (850 C) for 240 seconds and initially quenched
in an oil bath (T
Q
<240 C) or in a salt bath
(T
Q
>240 C), prior to reheating to 723 K (450 C) in
a salt bath for the partitioning stage. The specimens
were then water quenched to room temperature. The
mechanical properties of the specimens were measured
with a Zwick/Roell universal tensile testing machine.
Each specimen was cut into 5-mm segments at 10-mm
intervals from the fracture to measure the volume
fraction of deformation-transformed austenite after the
tensile test. The retained austenite fraction of these
specimens was measured by magnetic saturation.
The applicability of the Q&P concept to medium Mn
steel was investigated by dilatometry. Figure 1shows the
dilatometry traces for the Q&P-processed medium Mn
steel. For the specimens quenched to T
Q
in the range of
423 K to 483 K (150 Cto210C), there was no volume
expansion during the final quenching, i.e., no secondary
martensite was formed. This implies that the C enrich-
ment of austenite at the end of the partitioning stage was
large enough to suppress its transformation to martensite
in the final quenching stage. On the other hand, for the
specimens quenched to a T
Q
temperature in the range of
503 K to 543 K (230 Cto270C), a volume expansion
due to the transformation of austenite into a
¢
martensite
was clearly observed during the final quenching stage.
The fact that the volume expansion during the final
quenching increased with increasing T
Q
temperature,
implies that the stability of the austenite prior to the final
quenching decreased with increasing T
Q
.
Figure 2a shows the volume fraction of retained
austenite, V
c
, as a function of T
Q
.AsT
Q
increased, V
c
initially increased up to 0.33. For T
Q
temperatures
above 483 K (210 C), V
c
gradually decreased. The T
Q
temperature at which the maximum austenite fraction
was obtained, T
Q,max
, was 483 K (210 C). It is clearly
shown in Figure 1that the decrease of V
c
at higher
T
Q,max
results from the formation of secondary mar-
tensite. The stability of the austenite was lower when the
steel was quenched to a higher T
Q
because of an
insufficient C enrichment. This is due to the fact that V
c
increased with increasing T
Q
. The amount of C parti-
tioned per unit volume of austenite is lower for larger
values of V
c
. The result is a less stable retained austenite.
In order to investigate the stability of the retained
austenite, the retained austenite C content, C
c
, was
measured. Figure 2(b) shows the T
Q
dependence of C
c
.
C
c
was determined by means of the room temperature
measurement of the lattice parameter of the retained
austenite, a
c
, and the following relationship
[13,14]
be-
tween the lattice parameter and the composition:
acðnm) ¼0:3556 þ4:53 103Cþ9:5105Mn
2:0105Ni þ6:0105Cr
þ5:6104Al þ3:1104Mo þ1:8104V
½1
C
c
was in the range of 0.63 to 1.17 wt pct, which
clearly shows that the Cin the martensite partitioned
into austenite during the Q&P processing. As T
Q
in-
creased, C
c
decreased and this resulted in lower austen-
ite stability. The composition dependence of the M
s
temperature is given by the following empirical rela-
tionship
[15,16]
:
Ms¼539 423 C 30:4Mn12:1Cr17:7Ni
7:50 Mo þ10:0Co7:50 Si ½2
Here, the concentration of each alloy addition is
in wt pct. Using Eq. [2], it can be shown that for the
0 200 400 600
-150
0
150
300
Relative Length Change, µm/cm
Temperature, ºC
MSTP : 450°C
TQ: 270 ºC
TQ: 150 ºC
TQ: 190 ºC
TQ: 210 ºC
TQ: 230 ºC
TQ: 170 ºC
TQ: 250 ºC
Fig. 1—Relative length change of dilatometry specimens of 4 wt pct
medium Mn steel quenched (1. initial quenching) from 1123 K
(850 C) to different T
Q
in the range from 423 K to 543 K (150 C
to 270 C), and partition treated at a T
P
of 723 K (450 C) for 300 s
(2. Partitioning). The volume expansion resulting from the formation
of secondary martensite during the final quenching stage (3. final
quenching) is indicated by the departure from the dotted line.
METALLURGICAL AND MATERIALS TRANSACTIONS A
medium Mn steel used in the present study, the M
s
temperature was below room temperature when C
c
exceeded 0.87 wt pct, indicating that austenite can be
stabilized at room temperature. Figure 3(c) shows that
the M
s
temperature is below room temperature when
T
Q
<210 C. This clearly indicates that the formation
of secondary martensite can be suppressed when a
sufficiently high C
c
is obtained by Q&P processing with
aT
Q
temperature lower than 483 K (210 C).
Figure 3shows the microstructure of the Q&P-pro-
cessed medium Mn steel quenched to 483 K (210 C)
and partitioning treated at 723 K (450 C) for 300 sec-
onds. The microstructure of the Q&P-processed medium
Mn steel consisted of a lath martensite matrix with
retained austenite islands. The EBSD results in Fig-
ure 3(a) show that the austenite was present both as
film-type and as blocky-type austenite. Figures 3(b) and
3(c) shows TEM micrographs of the Q&P-processed
medium Mn steel. FCC and BCC diffraction patterns
were obtained from retained austenite and martensite,
respectively. The retained austenite could be clearly
identified by means of dark-field imaging, as shown in
Figure 3(c). The grain size of the retained austenite
varied from 50 nm to 1 lm.
Figure 4shows the mechanical properties of the Q&P-
processed medium Mn steel quenched to a T
Q
temper-
ature in the range of 423 K to 543 K (150 C to 270 C)
and partition treated for 300 seconds at 723 K (450 C).
The engineering stress–strain curves were continuous
and no strain localization was observed. The mechanical
properties of the Q&P-processed medium Mn steel
could be divided into three types of behaviors depending
on T
Q
. Type-1 behavior was observed for specimens
quenched to a low T
Q
temperature (<210 C). In this
group, the specimens exhibited a high yield strength
(YS) and total elongation (TE). The YS ranged from
1050 to 1160 MPa and the TE ranged from 15 to 18 pct.
With increasing T
Q
, the YS decreased slightly and the
TE increased. This is due to the fact that V
c
increased
for the Q&P-processed steel quenched at a higher T
Q
temperature. These specimens exhibited a low work
hardening, related to the stability of the retained
austenite. When T
Q
was equal to 423 K (150 C), V
c
was 0.07 after the tensile test. This indicates that the
stability of the retained austenite was high enough to
suppress the strain-induced martensitic transformation.
Type-2 behavior was observed for specimens quenched
to the T
Q,max
(210 C) temperature. This specimen had a
relatively low YS and a high ultimate tensile strength
(UTS) as compared to type-1 specimens. The low YS as
compared to type-1 specimens is clearly due to a higher
volume fraction of retained austenite, which is a much
(a)
Vγ
0.1
0.2
0.3
0.4
T
Q,max
0.0
0.5
1.0
1.5
(b)
Cγ , wt.%
0.0
Quench Temperature, °C
100 150 200 250 300
Secondary Ms, ºC
RT
(c)
* Eq. (1)
()
00453.0
35604.0Ta
CQ−
=
γ
γ
* Eq. (2)
Si50.7Cr1.12Mn4.30C423539M S⋅−⋅−⋅−⋅−=
γ
M
f
M
s
-400
-200
0
200
Fig. 2—(a)T
Q
dependence of the volume fraction of retained austen-
ite, V
c
, measured by the magnetic saturation method. (b)T
Q
depen-
dence of the retained austenite C content, C
c
, based on the XRD
austenite lattice parameter measurements. The dashed line is based
on the Eq. [1]. (c)T
Q
dependence of the secondary M
s
temperature
of the C-enriched retained austenite. The solid line is based on
Eq. [2].
Fig. 3—Microstructure of Q&P-processed medium Mn steel obtained by austenitizing, quenching to 483 K (210 C), and partitioning at 723 K
(450 C) for 300 s. (a) EBSD phase map for BCC (green) and FCC (red) phases. (b) Bright-field TEM micrograph. (c) Corresponding dark-field
TEM micrograph of the retained austenite in (b) using the (002)
c
reflection. cand a
¢
represent retained austenite and martensite, respectively.
METALLURGICAL AND MATERIALS TRANSACTIONS A
softer phase than martensite. The specimen exhibited a
higher work-hardening rate resulting from the TRIP
effect. Figure 4b clearly shows that a large amount of
austenite transformed to martensite during tensile test-
ing. Type-3 behavior was observed for specimens
quenched to a high T
Q
temperature (>210 C). Second-
ary martensite formed during the processing of the
specimens belonging to this group. These specimens
exhibited an ultra-high UTS, but a low TE compared to
specimens of the other two groups. The reason for this is
clearly shown in Figure 4(a). In these specimen, most of
the retained austenite transformed to martensite in the
initial stage of the straining, resulting in high initial
work-hardening rate. Another reason for the low TE of
these specimens is the presence of the secondary
martensite. As this is a high C brittle martensite, its
presence resulted in a limited elongation. The mechan-
ical properties of the secondary martensite formed
during the final quenching are different from those of
the primary martensite formed during initial quenching.
The primary martensite contains a very low C content
due to the C partitioning into the austenite, while
secondary martensite is a highly C-enriched and
extremely hard phase.
In conclusion, Q&P processing can successfully be
applied to medium Mn steel to obtain ductile ultra-high-
strength microstructures. Q&P processing of medium
Mn steel resulted in a large volume fraction of C-
enriched retained austenite, and a value of 0.33 was
achieved. The microstructure of Q&P-processed med-
ium Mn steel consisted of a low-C martensite matrix
with C-enriched retained austenite islands. This micro-
structure had mechanical properties with an excellent
strength-ductility balance of approximately
25,400 MPa pct. In addition, the selection of the quench
temperature made it possible to vary the UTS over
approximately 500 MPa in a controlled manner.
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Engineering Stress, MPa
0
500
1000
1500
2000
0 5 10 15 20
Engineering Strain, %
5101520
No secondary
martensite formation
With secondary
martensite formation
Maximum volume fraction
of retained austenite
5101520
500 MPa
TQ: 150 °C
TQ: 210 °C
TQ: 250 °C
TQ: 230 °C
TQ: 270 °C
TQ: 170 °C
TQ: 190 °C
(b)
(a)
V
γ
0.2
0.3
0.1
0.0
TQ: 210 °CT
Q: 270 °CTQ: 150 °C
Fig. 4—(a) Volume fraction of retained austenite measured at 0 pct strain, at the onset of necking and at fracture. (b) Room temperature stress–
strain curves for Q&P-processed medium Mn steel partition treated at 723 K (450 C) for 300 s.
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METALLURGICAL AND MATERIALS TRANSACTIONS A
