B. C De Cooman - High Mn TWIP Steels for automotive applications.pdf

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High Mn TWIP Steels for
Automotive Applications
B. C. De Cooman
1
, Kwang-geun Chin
2
and Jinkyung Kim
1
Ferrous Technology
Pohang University of Science and Technology, Pohang
2
POSCO Technical Research Laboratories, Gwangyang
South Korea
1. Introduction
Modern car design puts an increasing emphasis on the notion that a material used in
building the body-in-white (BIW) should be selected on the basis of how well it helps
achieving specific engineering targets such as low vehicle weight, high passive safety,
stability, stiffness, comfort, acoustics, corrosion, and recycling. Steel is at present still the
material of choice for car bodies, with 99% of the passenger cars having a steel body, and 60-
70% of the car weight consisting of steel or steel-based parts. The automotive industry is
however continuously making excursions in the area of light materials applications. At
present, most car makers are routinely testing multi-materials concepts, which are not
limited to the obvious use of light materials for closures, e.g. the use of Al for the front lid or
thermosetting resins for trunk lids. The steel industry has made a sustained effort to
innovate and create advanced steels and original steel-based solutions and methods in close
collaboration with the manufacturers by an early involvement in automotive projects, but
also by involving automakers in their own developments. Carmakers have increasingly built
passenger cars with body designs which emphasize passenger safety in the event of a
collision, and most passenger cars currently achieve high ratings in standardized crash
simulations such as the EURO NCAP or the North American NHST tests. The safety issue
directly related to the BIW materials is passive safety. High impact energy absorption is
required for frontal crash and rear collision, and anti-intrusion properties are required in
situations when passenger injury must be avoided, i.e. during a side impact and in case of a
roll over, with its associated roof crush. Increased consumer expectations have resulted in
cars which have steadily gained in weight as illustrated in figure 1. This weight spiral is a
direct result of improvements in vehicle safety, increased space, performance, reliability,
passenger comfort and overall vehicle quality. This trend has actually resulted in an
increased use of steel in car body manufacturing in absolute terms, and this increase may in
certain cases be as high as 25%. The weight issue is therefore high on the agenda of BIW
design, as it is directly related to environmental concerns, i.e. emissions of CO
2
, and the
economics of the gas mileage. Reports on weight saving resulting from the use of Advanced
High Strength Steels (AHSS) are difficult to evaluate as these tend to focus on the use of
advanced steels and improved designs for a single part, rather than the entire car body. The
use of Dual Phase (DP) and Transformation-Induced Plasticity (TRIP) steels has been
1
Graduate Institute of
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102
New Trends and Developments in Automotive System Engineering
reported to result in a weight saving in the range of 10-25%. Similar weight reductions of
about 25% are reported for the use of stainless steels. The potential for weight reductions
become very important when very high strength steels are considered.
1500
1400
1300
1200
1100
1000
900
Fig. 1. Midsize passenger car weight increase in the EU. The weight increase is close to 90%
for the period of 1970 to 2010.
Weight reductions of about 30-40% are typically reported for 1300-1500MPa steels. A 36%
weight reduction can be expected when a body part used as anti-intrusion barrier is made of
press hardening 22Mn5B steel. Most industry experts agree that, as illustrated in figure 2,
steel based parts designs using advanced high strength steels offer both the potential for
vehicle mass containment and lower production cost. Hence, when material-specific
properties are considered, there is an increasingly important interest in very high strength
materials. This has been the driving force behind most of the current automotive steel
research efforts. This is obvious when one considers the need for the increased strength for
parts related to passenger safety, such as the B-pillar, an essential element for passenger
protection in side impact collisions.
DP and TRIP steels are now well established as AHSS, with major applications in BIW parts
related to crash energy management. In addition to a high strength, a high stiffness and only
very low levels of deformations, typically less than 5%, may be allowed for these parts.
Strength levels as high as 1800MPa have been mentioned as future requirements for anti-
intrusion parts. Whereas press-formable CMnB grades are receiving attention for the B-
pillar and front-rear reinforcements, there is still considerable interest in TRIP and DP steels.
In the case of DP grades the emphasis is on front end applications and exterior panels.
Having said this, standard high strength micro-alloyed steels continue to be still being
widely used. Two decades ago most BIW designs were based on steels with Ultimate Tensile
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Curb weight, kg
800
700
600
500
1970 1974 1978 1982 1986 1990 1994 1998 2002 2006 2010
Year
High Mn TWIP Steels for Automotive Applications
103
Strength (UTS) values in the 200-300MPa range. Recent BIW designs tend to use much more
high strength steels. Whereas less low strength steels (YS<180MPa) are being used, mainly
for outer body parts, there is a pronounced increase in the use of High Strength Steels (HSS),
with a yield strength (YS) >300MPa, Very High Strength Steels (VHSS) with a YS >500MPa,
and Ultra High Strength Steels (UHSS) with UTS values up to 1500MPa. This increased use
of high strength steel grades has resulted in a moderate relative decrease of steel mass per
car body.
High
high
mass
-20%
+40%
Low costs
Production cost
-10%
-10%
+40%
Conventional
low
Steel Unibody
AHSS
Steel
Hybrid
-20%
-20%
solutions
Future
Steel
Designs
-40%
-$1.6/kg
-40%
-60%
-60%
low
+$6.5/kg
Body weight
Low
mass
Carbody
Mass
+80%
+80%
Production costs
High costs
high
Fig. 2. Comparison of the production cost and vehicle mass containment for designs based
on different material selections.
The present contribution reviews the important development of ultra-ductile TWIP steel
for BIW applications. FeMn TWIP is a high-strength steel concept with superior
formability, which may be close to being produced industrially. High manganese TWIP
steels are highly ductile, high strength Mn austenitic steels characterized by a high rate of
work hardening resulting from the generation of deformation-nucleated twins (Grassel et
al., 1997; Grassel et al., 2000; Frommeyer, 2003; Prakash et al., 2008). Their Mn content is in
the range of 15-30 mass%. Alloying additions of C, Si and/or Al are needed to obtain the
high strength and the large uniform elongation associated with strain-induced twinning.
Depending on the alloy system, the carbon content is either low, i.e. less than 0.05 mass-%,
or high, typically in the range of 0.5-1.0 mass-%. Si and Al may be added to achieve a
stable fully austenitic microstructure with low stacking fault energy in the range of 15-
30mJ/m
2
. High Mn alloys characterized by strength ductility products 40.000-
60.000MPa% have reached the stage of large scale industrial testing and the industrial
focus is mainly on TWIP steels with the following compositional ranges: 15-25 mass-%Mn,
with 0-3%Si, 0-3% Al and 200-6000ppm C. The dominant deformation mode in TWIP steel
is dislocation glide, and the deformation-induced twins gradually reduce the effective
glide distance of dislocations which results in the “Dynamical Hall-Petch effect”
illustrated in the schematic of figure 3.
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104
New Trends and Developments in Automotive System Engineering
Dislocation source
Twin
Λ
Λ
Λ :
dislocation mean free path
Fig. 3. Illustration of the dynamical Hall-Petch effect. Mechanical twins are formed due to
the low stacking fault energy. They gradually reduce the effective glide distance of
dislocations, resulting in the very high strain hardening observed in TWIP steel.
The mechanical properties of typical TWIP steels are reviewed in figure 4. These steels have
received attention only recently, and the early work on high Mn ferrous alloys by Schuman
(Schuman, 1971) in Germany, Remy and Pineau (Remy & Pineau, 1977) in France and Kim
(Kim, 1993; Kim et al., 1993) in South Korea did not receive much attention originally. The
work of Frommeyer (Grassel et al., 1997; Grassel et al., 2000; Frommeyer, 2003) at the Max
Planck Institute in Dusseldorf, Germany, and the interest in advanced high strength steels
from the automotive industry renewed the interest in the properties of high Mn TWIP steels
and mainly three types of TWIP steel compositions have been extensively investigated: Fe-
22%Mn-0.6%C (Allain, 2004), Fe-18%Mn-0.6%C, Fe-18%Mn-0.6%C-1.5%Al (Kim et al., 2006)
and the low carbon Fe-25%-30%Mn-3%Si-%Al (Grassel et al., 2000). The high rate of strain
hardening associated with the deformation twinning phenomenon allows for the
combination of higher strengths and higher uniform elongations, as illustrated in figure 5
which compares the properties of conventional multi-phase TRIP steel with those of TWIP
steel.
1600
Yield strength, Tensile strength, MPa
Fe-18%Mn-C, Al
1400
1200
1000
800
600
400
200
0
0
20
40
60
80
100
120
Fe-25-31%Mn-Si, Al
Fe-22%Mn-C, N
YS
TS
YS
TS
YS
TS
140
Total elongation, %
Fig. 4. Typical ranges for the mechanical properties of TWIP steel.
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High Mn TWIP Steels for Automotive Applications
5000
105
True stress, strain hardening rate, MPa
4000
3000
2000
980TWIP
780TRIP
1000
0
0
0.1
0.2
0.3
0.4
True strain
Fig. 5. Comparison of the stress-strain curves and the strain hardening rate for TRIP and
TWIP steel. TWIP steel has a uniform elongation twice that of TRIP steel and a considerably
higher ultimate strength.
2. Thermodynamic properties of TWIP steel
The Fe-Mn equilibrium phase diagram has recently been revised (Witusiewicz et al., 2004).
On the Fe rich side of the diagram, the binary system would appear to be relatively simple
with an open
γ-loop.
The meta-stable Fe-Mn diagram (figure 6) however reveals much more
of the information which is required to understand the microstructures observed in practical
non-equilibrium conditions. Between 5 mass-% and 25 mass-% of Mn, the room temperature
multi-phase microstructure of Fe-Mn alloys is dominated by the presence of
α’
martensite,
at low Mn contents, and
ε-martensite,
at higher Mn content.
Small Mn additions have a pronounced hardenability effect, resulting in the formation of
cubic
α’
martensite. At higher Mn contents h.c.p.
ε-martensite
is formed. Both types of
martensite are also generated by stress and strain-induced transformations of the retained
austenite phase. Stabilizing the austenite at room temperature requires Mn contents in
excess of 27 mass-% in the binary Fe-Mn alloy system. In order to obtain a stable room
temperature austenite phase in alloys with less than 25 mass-% of Mn, the formation of
α’
and
ε
martensite must be suppressed. This can be done by carbon additions. Carbon
additions of approximately 0.6 mass-% make it possible to obtain uniform, carbide-free,
austenitic microstructures and avoid the formation of
ε-martensite
(Schumann, 1971).
Higher carbon additions result in M
3
C carbide formation.
Figure 7 illustrates the microstructure of a Fe-18%Mn-0.6%C TWIP steel. The structure is
single phase austenitic, with relatively coarse grains, which may contain wide
recrystallization twins. The XRD results also illustrate the fact that this TWIP steel does not
transform to martensite during straining.
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