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The present work analyzes the application of quenching and partitioning processing to medium Mn steel to obtain a new type of ultra-high-strength multiphase medium 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.
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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.
[35]
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.
[79]
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
... Third-generation AHSS (Type A): TRIP-aided bainitic ferrite (TBF) steel [3,[8][9][10], onestep and two-step quenching and partitioning (Q&P) steels [3,[11][12][13], carbide-free bainitic (CFB) steel [14][15][16], and duplex-type, laminate-type, and bainitic ferrite-type medium manganese (D-MMn [17][18][19][20][21][22], L-MMn [23][24][25], and BF-MMn [26]) steels, 4. ...
... The third-generation AHSSs are classified into two types, Type A and Type B, by their kinds of matrix structures and tensile strengths (TS) [4]. The matrix structures and the TSs of Type A are bainitic ferrite (BF) and bainitic ferrite/martensite (BF/M), and higher than 1.0 GPa, respectively, except for D-MMn and L-MMn steels with an annealed martensite [17][18][19][20][21][22][23][24][25] and/or δ-ferrite [24] matrix structure. On the other hand, the main matrix structure of Type B is martensite and its TS is higher than 1.5 GPa [27][28][29][30][31][32][33]. ...
... The M-MMn steel contains a larger amount of retained austenite than the TM steel [30][31][32][33], although its amount is much less than those of the D-MMn and L-MMn steels [17][18][19][20][21][22][23][24][25]. Resultantly, the M-MMn steel has an ultra-high tensile strength and shows a large elongation by the TRIP effect of the retained austenite, although the impact toughness is inferior to that of TM steel [30]. ...
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... Subsequently, C partitioning occurs during isothermal holding along with tempering of the initially-formed martensite, and other phase transformations are ideally suppressed. Table 1 shows representative chemical compositions of Q&P steels that have been considered in several recent studies (Li et al., 2010;Santofimia et al., 2010Santofimia et al., , 2011De Moor et al., 2011Jirková et al., 2012;Paravicini Bagliania et al., 2013;De Knijf et al., 2014;Toji et al., 2014Toji et al., , 2016Cheng et al., 2014;Seo et al., 2015Seo et al., , 2016aArlazarov et al., 2015;Kim et al., 2016;Kähkönen et al., 2015;Johnson et al., 2013;Streicher et al., 2004;Pierce et al., 2018). Similar to TBF steel compositions, Q&P steels have C, Mn, and Si as major alloying elements, and may include additions of Ni, Cr, and/or Mo. ...
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Quench and Partitioning (Q&P) steels are produced by implementing a unique thermal history designed to produce microstructures that contain martensite, and potentially ferrite, along with significant amounts of retained austenite stabilized by high carbon contents. Carbon-stabilized austenite is obtained by carbon transfer from martensite into austenite after a controlled amount of martensite is introduced by judicious selection of a so-called quench temperature at which quenching below the martensite start temperature is interrupted. Following the quench interruption, during the partitioning step, the steel is either held at the quench temperature or brought to a higher temperature and held for a specific time, to stimulate carbon transfer from martensite to austenite, to decrease the carbon supersaturation in martensite and correspondingly stabilize the austenite by carbon enrichment. A final quench to room temperature may be associated with the transformation of a certain fraction of austenite into secondary or “fresh” martensite, which is usually undesirable. A review of the Q&P process is presented, including prediction of austenite retention, alloying effects on its stabilization, a mechanical properties survey, including tensile and local formability relevant to sheet steels for automotive applications, along with perspectives on reactions competing with carbon partitioning that may operate during partitioning.
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Microstructure heterogeneity has been regarded as being detrimental in obtaining reliable mechanical performance of steels. However, in the present study, we demonstrated that a proactive control of microstructure heterogeneity could deliver unprecedented tensile properties that was hardly achieved by using chemically homogeneous initial microstructure. The heterogeneity of Mn distribution generated by utilizing its solubility difference between ferrite, austenite and cementite at intercritical annealing, promoted the retention of austenite in the final microstructure subjected to the room quenching and partitioning process. The enhancement of fraction as well as the stability of austenite contributed to the simultaneous improvement of tensile strength and ductility which have been regarded as mutually exclusive properties. Furthermore, even in steel with lean Mn composition, the room temperature quenching and partitioning process combined with the chemically heterogeneous initial microstructure presented a tensile properties comparable to those expected in steels with much higher Mn content, which exhibited the potential of heterogeneity-driven microstructure control for the development of advanced steel products.
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High Mn twinning induced plasticity (TWIP) steel is a new type of structural steel, characterised by both high strength and superior formability. TWIP steel offers an extraordinary opportunity to adjust the mechanical properties of steel by modifying the strain hardening. The use of TWIP steel may therefore lead to a considerable lightweighting of steel components, a reduction of material use and an improved press forming behaviour. These key advantages will help implement current automotive vehicle design trends which emphasise a reduction of greenhouse gas emissions and lowering of fuel consumption. In addition, high strength TWIP steel will effectively contribute to weight containment in vehicles equipped with hybrid and electric motors, as these are considerably heavier than conventional motors. The present review addresses all aspects of the physical metallurgy of the high strength TWIP steel with a special emphasis on the properties and key advantages of TWIP sheet steel products relevant to automotive applications.
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Press hardening steel (PHS) has been increasingly used for the manufacture of structural automotive parts in recent years. One of the most critical characteristics of PHS is a low residual ductility related to a martensitic microstructure. The present work proposes the application of quenching and partitioning (Q&P) processing to improve the ductility of PHS. Q&P processing was applied to a Si- and Cr-added Q&P-compatible PHS, leading to a press hardened microstructure consisting of a tempered martensite matrix containing carbide-free bainite and retained austenite. The simultaneous addition of Si and Cr was used to increase the retained austenite fraction in the Q&P-compatible PHS. The Q&P processing of the PHS resulted in a high volume fraction of C-enriched retained austenite, and excellent mechanical properties. After a quench at 543 K (270 °C) and a partition treatment at 673 K (400 °C), the PHS microstructure contained a high volume fraction of retained austenite and a total elongation (TE) of 17 pct was achieved. The yield strength (YS) and the tensile strength were 1098 and 1320 MPa, respectively. The considerable improvement of the ductility of the Q&P-compatible PHS should lead to an improved in-service ductility beneficial to the passive safety of vehicle passengers.
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The exceptional elongation obtained during tensile testing of intercritically annealed 10 pct Mn steel, with a two phase ferrite–austenite microstructure at room temperature, was investigated. The austenite phase exhibited deformation-twinning and strain-induced transformation to martensite. These two plasticity-enhancing mechanisms occurred in succession, resulting in a high rate of work hardening and a total elongation of 65 pct for a tensile strength of 1443 MPa. A constitutive model for the tensile behavior of the 10 pct Mn steel was developed using the Kocks–Mecking hardening model.
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The current strong demand for vehicle lightening from the automobile sector requires flat carbon steel manufacturers to develop new advanced grades capable of fulfilling the increasingly stringent technical requirements of this market Two basic approaches are possible: - short term strategies for incremental improvements in the mechanical properties of existing products which can then be produced in thinner gauge strips with equivalent functional properties, - longer term solutions involving the development of breakthrough products such as ultra high strength ductile austenitic steels or low-density steels alloyed with light elements (Al, Mg, Si). Arcelor is actively pursuing both these avenues of research. Restricting the discussion to the second point, it is clear that there are many technical hurdles and conflicting requirements to be overcome in order to produce a marketable product Arcelor Research, in conjunction with TKS, has recently developed an ultra high strength Fe-Mn-C austenitic steel with excellent formability for automotive applications. The X-IP™1000 steel composition is optimised to provide the best compromise in ultimate tensile strength (>1000 MPa) and total elongation (>50%) at room temperature. These properties are achieved through the optimisation of the TWIP (TWinning Induced Plasticity) effect by careful control of the stacking fault energy (SFE) and the final microstructure. The austenite matrix is fine grained (grain size <10 μm for hot strips and <3 μm for cold strips), contains little or no cementite and is exempt from martensitic transformations under cold working. The steel can be processed on conventional industrial lines (continuous casting, hot and cold rolling and continuous annealing) in a wide range of formats. In this paper we present the factors that determine the choice of composition (phase stability diagrams and SFE modelling) and we describe the evolution of the microstructure at different stages in the production process. The relationship between the microstructure and the final mechanical properties is discussed. X-IP™ steel is the subject of a common research and development program launched by ARCELOR and TKS in February 2005.
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CMnSi steel grades with carbon contents ranging from 0.2 to 0.3 wt% and manganese contents of 3 and 5 wt% were Quenched and Partitioned (Q&P). Tensile properties were assessed and retained austenite fractions measured. Intercritically annealed and fully austenitized conditions were studied. The best combinations of tensile strength and total elongation obtained in the 0.2C-3Mn-1.6Si grade after intercritical annealing were associated with strength levels in the 1 000-1 200 MPa range and total elongations ranging from 14 to 20%. Optimum properties were obtained in the 0.3C-3Mn-1.6Si steel after full austenitization with tensile strength levels ranging from 1 450 to 1 700 MPa and total elongations ranging from 11 to 18%. The 0.2C-3Mn-1.6Si fully austenitized samples also exhibited remarkable strength/ductility combinations albeit at lower strength levels of 1 200-1 450 MPa UTS with 9-15% total elongation indicating the effectiveness of the manganese addition to develop novel property combinations.
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In this paper, four different steel compositions, centered on Mn as the main alloying element, are designated as candidates for Third Generation AHSS grades. The design of these steels is based on controlling the deformation behavior of the retained austenite. Thus, heat treatment process parameters are determined in order to obtain different amounts and morphologies of retained austenite. The evolution of the microstructure, during processing as well as deformation, is characterized by using optical, electron microscopy techniques and mechanical tests. The effect of alloy composition and processing parameters on the deformation mechanisms of these steels is discussed.
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The effect of different microstructures on the tensile and toughness properties of a low alloy medium carbon steel (0.28C–1.4Si–0.67Mn–1.49Cr–0.56Mo wt%) was investigated, comparing the properties obtained after the application of selected quenching and partitioning (Q&P) and quenching and tempering (Q&T) treatments. After Q&T the strength–toughness combination was the lowest, whereas the best combination was achieved by Q&P, as a result of the carbon depletion of the martensite and the high stabilization of the austenite. Nonetheless, the presence of islands of martensite/austenite (MA) constituents after Q&P treatments prevented the achievement of toughness levels comparable to the ones currently obtainable with other steels and heat treatments.
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Ultrafine grain refinement by intercritical annealing at 680°C and 640°C was investigated in a Fe–0.05%C–6.15%Mn–1.4%Si multiphase TRIP steel. A large volume fraction of retained austenite was obtained at room temperature in both cases. Whereas a pronounced localization of the deformation during tensile testing appeared in the steel annealed at 640°C, strain localization occurred only in the initial deformation stages in the steel annealed at 680°C. The retained austenite was transformed to strain-induced martensite during tensile testing in the sample annealed at 680°C. However, no martensitic transformation was observed in the sample annealed at 640°C. The activation volume showed a sharp decrease during the tensile test and saturated to the same value in both cases. Two different dislocation structures were observed in the ferrite grains of the samples annealed intercritically at 680°C after tensile deformation, but only the dislocation-free structure of ferrite was observed in the sample annealed at 640°C.
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The effects of Mn and Si addition on the growth rate of cementite in Fe–0.6 mass% C martensite have been studied by means of scanning electron microscopy, transmission electron microscopy and a three-dimensional atom probe. The growth rate of the cementite during tempering at 723 K decreases substantially with the addition of Si due to the redistribution of Si between the cementite and ferrite matrix. Mn retards cementite coarsening more effectively than Si at 923 K. In tempering at both 723 and 923 K, the Si concentration in the cementite starts to decrease from an early stage of precipitation, whereas the cementite develops initially without the redistribution of Mn, before the Mn gradually enriches into the cementite during tempering. Calculations of phase boundaries for stable equilibrium (partition local equilibrium) and metastable equilibria (para and negligible-partition local equilibriums) have revealed that there is a sufficient driving force for the formation of paracementite in the Mn-added alloys. On the other hand, paracementite is difficult to form in the Si-containing alloy because the cementite becomes unstable due to the dissolution of Si.