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MATERIALS DESIGN LABORATORY
Alloy Design UHS Intercritically Annealed
6%-12%Mn TWIP+TRIP Steel
B. C. De Cooman
Materials Design Laboratory, Graduate Institute of Ferrous TechnologyPohang University of Science and Technology
Pohang, South Korea
CAMS 2014
MATERIALS AUSTRALIA
November 26th-28th, 2014
Sydney, NSW, AUSTRALIA
MATERIALS DESIGN LABORATORY
Pohang University of Science and TechnologyGraduate Institute of Ferrous Technology
Pohang
MATERIALS DESIGN LABORATORY
The world’s only fully
accredited Institute
in Steel Science and
Technology
• Research Areas:
Alternative Technology
Control & Automation
Computational Metallurgy
Clean Steel
Environmental Metallurgy
Microstructure Control
Materials Design
Material Mechanics
Surface Engineering
MATERIALS DESIGN LABORATORY
The world’s only fully
accredited Institute
in Steel Science and
Technology
• Research Areas:
Alternative Technology
Control & Automation
Computational Metallurgy
Clean Steel
Environmental Metallurgy
Microstructure Control
Materials Design
Material Mechanics
Surface Engineering
MATERIALS DESIGN LABORATORY
The world’s only fully
accredited Institute
in Steel Science and
Technology
• Research Areas:
Alternative Technology
Control & Automation
Computational Metallurgy
Clean Steel
Environmental Metallurgy
Microstructure Control
Materials Design
Material Mechanics
Surface Engineering
MATERIALS DESIGN LABORATORY
Global Trends Automotive Steel Grades
The increasing use of AHSS/UHSS use is driven by…
• The need for high volume vehicles at competitive prices.
• Stringent regulations and corporate goals for:
Passenger safety
Fuel economy
Lower greenhouse gas emissions
• Sustained efforts by the steel industry to innovate and create advanced steels, and original,
steel-based solutions and methods, which underline the large potential of steel.
Car makers test, utilize multi-materials designs, but steel remains dominant…
• Steel, the material of choice for BIW: 99% passenger cars have a steel BIW.
• 60-70% of the car weight consisting of steel or steel-based parts.
• Globalization requires world-wide availability and global procurement of standard materials.
• The automotive industry makes excursions in light materials applications but there is only a
slight actual increase in the use of Al, Mg and plastics…. but this may change!
MATERIALS DESIGN LABORATORY
Lightweighting: Mass “Containment”, Mass “Reduction”• Low gas mileage: 0.3l-0.6l/100km fuel use reduction for a 100kg weight reduction
• Less greenhouse gas emissions: 2020 target ~100gr/km
• NHTSA CAFE Standards for 2017
New mpg target: DOUBLE the average mpg for new cars, trucks
54.5 mpg will cut of gas emissions by HALF
Current situation
Best US highway mileage 2012: 42 mpg (Chevrolet CRUZE)
Other example: 32 mpg (VW Passat )
General situation: 25mpg in US, 45 mpg in EU, better in Japan
Passenger Safety:• Low peak deceleration, long crush length, long time duration of crash pulse
• High energy dissipation with minimum intrusion
• Higher impact strength for A and B Pillars
• Anti-Intrusion applications: front and rear crash, side intrusion
• Tougher collision and rollover safety test for the 5-star rating
Closure Applications:• Dent resistance
Coated Products:• Perforation and cosmetic corrosion resistance
• Surface quality, visual
Other Issues:• Noise and Vibrations
• Vehicle Handling, Stiffness and Torsional Rigidity
Global Trends Automotive Steel Grades
MATERIALS DESIGN LABORATORY
Weight spiral:Safety
Space
Performance
Reliability
Quality
Comfort
1500
1400
1300
1200
1100
1000
900
800
700
600
5001970 1974 1978 1982 1986 1990 1994 1998 2002 2006 2010
Cu
rb w
eig
ht,
kg
88% increase
in weight
EU mid size vehicles
Year
CAFE
Fuel Economy
40 45 50 55 60
Wheelbase . track
Gas m
ileag
e,
mp
g
40
45
50
35
30
Year
20122014
2016
2018
2020
2022
Prius
Doubling
mileage
Regulations Influence on Materials Selection
MATERIALS DESIGN LABORATORY
Weight spiral:Safety
Space
Performance
Reliability
Quality
Comfort
1500
1400
1300
1200
1100
1000
900
800
700
600
5001970 1974 1978 1982 1986 1990 1994 1998 2002 2006 2010
Cu
rb w
eig
ht,
kg
88% increase
in weight
EU mid size vehicles
Year
Regulations Influence on Materials Selection
Passive
Passenger safety
MATERIALS DESIGN LABORATORY
Mechanical
Properties
Yield Strength
Tensile Strength
Uniform elongation
Total Elongation
Automotive Sheet Steel Products
MATERIALS DESIGN LABORATORY
RoughingReheating Finishing Cooling Coiling
Conventional HSM, CSM and CA/HDG Processing
Cold rolling Continuous
Annealing
Hot Dip Galvanizing
MATERIALS DESIGN LABORATORY
Mechanical
Properties
Yield Strength
Tensile Strength
Uniform elongation
Total Elongation
Bake-hardening
Springback
Normal Anisotropy
Planar Anisotropy
Deep Drawability
Stretch Formability
Crashworthiness
Geometrical
Properties
Dimensional
Width
Thickness
Shape
Edge Drop
Crown
Flatness
Technical
Properties
Weldability
Phosphatabilty
Roughness
Waviness
Friction
Corrosion resistance
Phosphatability
Paint adhesion
Visual appearance
Automotive Sheet Steel Products
MATERIALS DESIGN LABORATORY
Press-forming
Spring-back
Hole expansion
Bending
RSW
MATERIALS DESIGN LABORATORY
300 400 500 600 700 800 900 1000
Elo
ng
ati
on
A80,
%
Tensile Strength, MPa
0
20
40
60
80
100
120
140
1100
IF CMn HSLA
Conventional Automotive Steels
60.000MPa.%
50.000MPa.%
40.000MPa.%
30.000MPa.%
20.000MPa.%
10.000MPa.%
IFLC
CMn
HSLA
MATERIALS DESIGN LABORATORY
MA
300 400 500 600 700 800 900 1000
Elo
ng
ati
on
A80,
%
Tensile Strength, MPa
0
20
40
60
80
100
120
140
1100
60.000MPa.%
50.000MPa.%
40.000MPa.%
30.000MPa.%
20.000MPa.%
IFLC
CMn
HSLA
First Generation Advanced High Strength Steels
DP TRIP MACP
MATERIALS DESIGN LABORATORY
Automotive Body Materials Selection
6% HPF + 5% MA
23%
HSS
30%
DP / Multi Phase
GM AVEO
34%
IF
+LC
+BH
Cadillac CTS
PHS VW: 6% (GOLF 6) → 28% (GOLF 7)
MATERIALS DESIGN LABORATORY
Al
alloys
Polymers
Mg
alloys
Automotive
Steel grades
CFR-Composites
Ti
alloys
Automotive Body Materials Selection
MATERIALS DESIGN LABORATORY
Al
alloys
Polymers CFR-Composites
Automotive Body Materials Selection
Monocoque:5754 (Structural)
6111 (External parts)
Space frame:Multiple Al products integration
High strength die casting
Al-extrusion
Hydroform extrusion
Issues:Strain hardening: low
Strain rate sensitivity: negative
Cost: 4-6$/kg (1.3$/kg Steel)
MATERIALS DESIGN LABORATORY
MA
300 400 500 600 700 800 900 1000
Elo
ng
ati
on
A80,
%
Tensile Strength, MPa
0
20
40
60
80
100
120
140
1100
60.000MPa.%
50.000MPa.%
40.000MPa.%
30.000MPa.%
20.000MPa.%
IFLC
CMn
HSLA
Second Generation Advanced High Strength Steels
TWIP
Fe22Mn0.6C
Fe18Mn1.5Al0.6C
MATERIALS DESIGN LABORATORY
MA
300 400 500 600 900 1000
Elo
ng
ati
on
A80,
%
Tensile Strength, MPa
0
20
40
60
80
100
120
140
1100
60.000MPa.%
50.000MPa.%
40.000MPa.%
30.000MPa.%
20.000MPa.%
IFLC
CMn
HSLA
Third Generation Advanced High Strength Steels
3rd
Generation
0.
2
µm
UFG
TRIP
Low Mn
TWIP
SBIP
MBIP
+
700 800
MATERIALS DESIGN LABORATORY
Strain Hardening Engineering
True strain
00
Tru
e s
tress
, str
ain
hard
en
ing
rate
, M
Pa
)(
u
d
d
MATERIALS DESIGN LABORATORY
Strain Hardening Engineering
True strain
00
Tru
e s
tress
, str
ain
hard
en
ing
rate
, M
Pa
)(
u
d
d
MATERIALS DESIGN LABORATORY
Strain Hardening Engineering
00
True strain
Tru
e s
tress
, str
ain
hard
en
ing
rate
, M
Pa
Gain in strength
and ductility !
u
)(
d
d
MATERIALS DESIGN LABORATORY
00
True strain
Tru
e s
tress
, str
ain
hard
en
ing
rate
, M
Pa
Gain in strength
and ductility !
Strain Hardening Engineering
u
)(
d
d
Dislocation
Accumulation
or Storage
(Stage II)
Dislocation
Annihilation
or Dynamic recovery
(Stage III)
ρkρkdε
dρ
dε
dρ
dε
dρ
21
MATERIALS DESIGN LABORATORY
d/d
00 0.1 0.2 0.3 0.4
1000
2000
3000
4000
5000
True strain
Tru
e s
tre
ss
, s
tra
in h
ard
en
ing
ra
te, M
Pa
TRIP
TWIP
HS IF
TRIP
Strain Hardening Engineering
MATERIALS DESIGN LABORATORY
What is Strain Hardening Engineering?
1. Strengthening mechanisms:
• Solid solution strengthening (Alloying)
• Grain size refining (Alloying and Processing)
• Precipitation strengthening (Alloying)
• Bake-hardening (Processing)
2. Plasticity-enhancing mechanisms:
• Multi-phase steels: austenite required
• TRIP effect: Strain-induced Transformation
• TWIP effect: Deformation Twinning
gstrain
g →a’
TRIP-effect
a’g
g →gT
TWIP-effect
g
MATERIALS DESIGN LABORATORY
High Mn TWIP Steel
TWIP: TWinning-Induced Plasticity
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
En
g.
Str
ess
(M
Pa)
Eng. Strain (%)
20 22 24 26 28 30700
750
800
850
900
10-4s-1
Fe18Mn0.6C1.5Al
g →gT
TWIP-effect
g
10-3s-1
MATERIALS DESIGN LABORATORY
Rolling directionTWIP 1000
High Mn TWIP Steel
MATERIALS DESIGN LABORATORY
HERDiffuse
necking
No diffuse
necking
IF steelTWIP
LMIE
HDF
Fe22Mn0.6C Fe15Mn2Al0.7C
Zn Zn
MATERIALS DESIGN LABORATORY
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
1400
1600
En
gin
ee
rin
g s
tre
ss
, M
Pa
Fe15Mn0.6C
Fe15Mn0.6C1.5Al
Fe15Mn0.6C2Al
Engineering strain, %
YS
MPa
UTS
MPa
Elongation
(total) %
SFE*
mJ/m2
SFE**
mJ/m2
Fe15Mn0.6C 509 1124 51 12 13
Fe15Mn0.6C1.5Al 480 976 58 26 21
Fe15Mn0.6C2.0Al 488 939 58 30 24
* Saeed-Akbari et al., Metall. Mater. Trans. A 2009
** Dumay et al., Mater. Sci. Eng. A 2008
YS
MPa
UTS
MPa
Elongation
(total) %
SFE*
mJ/m2
Fe15Mn0.6C2.0Al
10%
20%
30%
40%
50%
60%
712
989
1071
1122
1261
2394
991
1167
1319
1407
1590
1737
43
23
15
10
9
7
30
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
1400
1600
En
gin
ee
rin
g s
tre
ss
, M
Pa
Engineering strain, %
10%
20%
30%
40%
50%
60%
MATERIALS DESIGN LABORATORY
YS (MPa) UTS (MPa) Total Elongation (%) SFE* mJ/m2 SFE** mJ/m2
Fe12Mn0.6C
Fe12Mn0.6C1.5Al
Fe12Mn0.6C2.0Al
486
492
478
838
900
915
16
30
41
12
26
30
10
18
21
Fe12Mn0.9C1Si-0.0V
Fe12Mn0.9C1Si-0.2V
Fe12Mn0.9C1Si-0.5V
Fe12Mn0.9C1Si-0.7V
434
614
722
741
1166
1324
1276
1260
45
38
25
22
26 -
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
1400
1600
Fe12Mn0.6C
Fe12Mn0.6C1.5Al
Fe12Mn0.6C2Al
En
gin
ee
rin
g s
tre
ss
, M
Pa
Engineering strain, %
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
1400
1600
0.2%V0.7%V 0.5%V
En
gin
ee
rin
g s
tre
ss
, M
Pa
Engineering strain, %
V-free
V-additions: increased YS and increased strain hardening
MATERIALS DESIGN LABORATORY
YS
MPa
UTS
MPa
Elongation
(total) %
SFE*
mJ/m2
Fe15Mn0.6C2.0Al
10%
20%
30%
40%
50%
60%
712
989
1071
1122
1261
2394
991
1167
1319
1407
1590
1737
43
23
15
10
9
7
30
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
1400
1600
En
gin
ee
rin
g s
tre
ss
, M
Pa
Engineering strain, %
10%
20%
30%
40%
50%
60%
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
1400
1600
0.2%V0.7%V 0.5%V
En
gin
ee
rin
g s
tre
ss
, M
Pa
Engineering strain, %
V-free
YS
MPa
UTS
MPa
Elongation
(total) %
SFE*
mJ/m2
Fe12Mn0.9C1Si-0.0V
Fe12Mn0.9C1Si-0.2V
Fe12Mn0.9C1Si-0.5V
Fe12Mn0.9C1Si-0.7V
434
614
722
741
1166
1324
1276
1260
45
38
25
22
26
MATERIALS DESIGN LABORATORY
Classification of the Mn UHSS Steels
High Mn Medium Mn%Mn >25 22 - 15 12 - 6 7 - 4
Processing Conventional annealing Intercritical annealing Q & P
Cold rolled Shear bandsDeformed g Deformed a’
After annealingg UFG
g a
Plasticity SBIP TWIP TWIP+TRIP TRIP
g-ISFE (mJ/m2)>75 >20 >20 <10
g-stability g-composition g-composition and size
Role of Mn g-stability / SFE g-stability / SFE / Hardenability
MATERIALS DESIGN LABORATORY
0.0 0.1 0.2 0.3 0.4 0.50
500
1000
1500
2000
2500
3000
3500
4000
DP980
Tru
e s
tre
ss
, M
Pa
Wo
rk h
ard
en
ing
ra
te,
MP
a
True strain
TiIF
Ti-IF: standard highly formable steel
DP 980: standard 1st generation AHSS
Mechanical properties at reduced Mn alloying
MATERIALS DESIGN LABORATORY
0.0 0.1 0.2 0.3 0.4 0.50
500
1000
1500
2000
2500
3000
3500
4000
TWIP1000
DP980
TiIF
Tru
e s
tre
ss
, M
Pa
Wo
rk h
ard
en
ing
ra
te,
MP
a
True strain
Ti-IF: standard highly formable steel
DP 980: standard 1st generation AHSS
Mechanical properties at reduced Mn alloying
DSA
)(
d
d
MATERIALS DESIGN LABORATORY
TWIP 1000: 18%Mn0.6%C1.5%Al
Medium Mn 1: 12%Mn0.3%C3.0%Al
0.0 0.1 0.2 0.3 0.4 0.50
500
1000
1500
2000
2500
3000
3500
4000
12Mn TWIP1000
Tru
e s
tress,
MP
a
Wo
rk h
ard
en
ing
rate
, M
Pa
True strain
Mechanical properties at reduced Mn alloying
MATERIALS DESIGN LABORATORY
0.0 0.1 0.2 0.3 0.4 0.50
500
1000
1500
2000
2500
3000
3500
4000
10Mn
Tru
e s
tre
ss
, M
Pa
Wo
rk h
ard
en
ing
ra
te,
MP
a
True strain
TWIP1000
TWIP 1000: 18%Mn0.6%C1.5%Al
Medium Mn 1: 12%Mn0.3%C3.0%Al
Medium Mn 2: 10%Mn0.3%C3.0%Al2.0%Si
Mechanical properties at reduced Mn
MATERIALS DESIGN LABORATORY
0.0 0.1 0.2 0.3 0.4 0.50
500
1000
1500
2000
2500
3000
3500
4000
6Mn
TWIP1000
Tru
e s
tre
ss
, M
Pa
Wo
rk h
ard
en
ing
ra
te,
MP
a
True strain
TWIP 1000: 18%Mn0.6%C1.5%Al
Medium Mn 1: 12%Mn0.3%C3.0%Al
Medium Mn 2: 10%Mn0.3%C3.0%Al2.0%Si
Medium Mn 3: 8%Mn0.4%C3.0%Al2.0%Si
Medium Mn 4: 6%Mn0.3%C3.0%Al1.5%Si
Mechanical properties at reduced Mn
DSA
MATERIALS DESIGN LABORATORY
Original concept: TWIP Steel
Deformation
g
g
Fully Austenitic
Low SFE
Dislocation plasticity
Twinning-induced plasticity
Low YS / High Strain Hardening
g
g
High Mn TWIP Steel Design Concept
Austenite:
e.g. 18% Mn 0.6% C +Al
MATERIALS DESIGN LABORATORY
YS (MPa) UTS (MPa) Total Elongation (%) SFE* mJ/m2 SFE** mJ/m2
Fe18Mn0.6C
Fe18Mn0.6C1.5Al
Fe18Mn0.6C3.0Al
484
498
499
1106
960
849
60
59
50
14
28
40
17
25
32
Fe15Mn0.6C
Fe15Mn0.6C1.5Al
Fe15Mn0.6C3.0Al
509
480
488
1124
977
939
51
58
58
12
26
30
13
21
24
Fe12Mn0.6C
Fe12Mn0.6C1.5Al
Fe12Mn0.6C2.0Al
486
492
478
838
900
915
16
30
41
12
26
30
10
18
21
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
Fe18Mn0.6C
Fe18Mn0.6C1.5Al
Fe18Mn0.6C3Al
E
ng
ine
eri
ng
str
es
s,
MP
a
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
Fe15Mn0.6C
Fe15Mn0.6C1.5Al
Fe15Mn0.6C2Al
Engineering strain, %
0 10 20 30 40 50 60 700
200
400
600
800
1000
1200
Fe12Mn0.6C
Fe12Mn0.6C1.5Al
Fe12Mn0.6C2Al
MATERIALS DESIGN LABORATORY
)(Gb)( o gag
Kocks-Mecking Model
[1] P. S. Follansbee, Metall. Mater. Trans. A, 41A (2010), pp. 3080-3090.
[2] T. Gladman, Mater. Sci. Tech-Lond, 15 (1999), pp. 30-36.
[3] J. G. Speer, B. C. De Cooman, Fundamentals of Steel Product Physical Metallurgy, AIST, 2011.
[4] S. Takaki, K. Takeda, N Nakada, T Tsuchiyama,, IAS 2008, Pohang, Korea, p. 107
[5] Y. Estrin, H. Mecking, Acta Metall., 32 (1984), pp. 57-70.
[6] O. Bouaziz, Y. Estrin, Y. Brechet, J.D. Embury, Scripta Mater., 63 (2010), pp. 477-479.
o )T,(p g [1]
)d,f( preprepre [2]
)X( is [3]
D
k y[4]
)(k)(b
k
b
P
d
d2
1 ggg
-
Ferrite Austenite
[5]
0D gg
111
a D
'
'0
F
F1c2
a
a-
p : Pierels stress
pre : Pierels stress
s : Solid solution strengtheing
ky : Hall-petch constant
D : Grain size
: Dislocation density
P : Grain size dependent constant [6]
K1: Constant
K2: Constant
G : Shear modulus
b : Burgers vector
MATERIALS DESIGN LABORATORY
Modeling result at room temperature
Exp.
Model
0.00 0.05 0.10 0.15 0.20 0.250
200
400
600
800
1000
1200
1400
Tru
e s
tre
ss
, M
Pa
True strain
Exp.
Model
0.00 0.05 0.10 0.15 0.20 0.250
1000
2000
3000
4000
5000
d
/d,
M
Pa
True strain
Model
Exp._Magnetic saturation
Exp._XRD
0.00 0.05 0.10 0.15 0.20 0.250.00
0.05
0.10
0.15
0.20
Ma
rten
sit
e v
olu
me
fra
cti
on
True strain
Coarse grained d
k1 : 0.01
k2 : 1.307
UFG a
UFG g
Martensite
k1 : 0.01
k2 : 1.012
k1 : 0.015
k2 : 1.005
k1 : 0.306
k2 : 39.1
Constitutive modeling of medium Mn steel
0.00 0.05 0.10 0.15 0.20 0.2524
26
28
30
32
34
36
38Te
mp
era
ture
, o
C
True strain
Exp.
Model
Max
Min
MATERIALS DESIGN LABORATORY
Medium Mn TWIP Steel Design Concept
Single phase Two Phase
Fe-18%Mn-0.6%C-1.5%Al → Fe-8%Mn-0.4%C-3.0%Al+Si
0
10000
20000
30000
40000
50000
60000
70000
80000
0 25 50 75 100
Ten
sil
e s
tren
gth
x T
ota
l elo
ng
ati
on
MP
a%
Volume percentage austenite, %
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14 16 18 20 22 24
Vo
lum
e p
erc
en
tag
e a
uste
nit
e,
%
Mn content, mass-%-
+
MATERIALS DESIGN LABORATORY
Deformation
g
gFully
austeniticg: Deformation-induced twinning
a: Dislocation glide
Ferrite/Austenite formation
C, Mn partitioning
Al, Si partitioning
Grain size refinement
SFE increase
Lowering Ms temperature
g
a
Cooling
Retained
g
a’Mainly
martensitic
g
a
C, MnAl, Si
Intercritical
annealing
Austenite
fg: 100%
8% Mn 0.3% C
Austenite
fg: 50%
16% Mn 0.6% C
Medium Mn TWIP Steel Design Concept
MATERIALS DESIGN LABORATORY
Deformation
g
gFully
austeniticg: Deformation-induced twinning
g: Transformation-induced plasticity
a: Dislocation glide
g
a
Cooling
g
a’Mainly
martensitic
g
aIntercritical
annealing
a’
C, MnAl, Si
Medium Mn TWIP+TRIP Steel
MATERIALS DESIGN LABORATORY
Strain Hardening Engineering of UFG Steel
Ultra Fine
Grain Size
a
Multi-phase
microstructure
g
Precipitates
VC
Bimodal
Grain size
Distribution
Larger grains
Martensite reversion +
intercritical annealing
MATERIALS DESIGN LABORATORY
10mm
d
agd
ag
d
ag
200nm
200nm
50nm
50nm
Ferrite
Austenite
VC
SF
Strain Hardening Engineering UFG Steel
MATERIALS DESIGN LABORATORY
Mn
Lowers Ms
Increases SFE
Increases hardenability
C Lowers Ms
Increases SFE
Increases hardenability
Al Increases Ms
Increases SFE
Required for d ferrite formation
Expands the two phase ag range
(higher austenite C content)
Si Lowers Ms
Austenite solid solution strengthening
Ferrite solid solution strengthening
Decreases SFE
Suppression cementite formation
g stabilizers a stabilizers
Composition Design TWIP+TRIP Quaternary Alloy
MATERIALS DESIGN LABORATORY
5% Mn
4% Mn
2% Mn
3% Mn
1% Mn
0% Mn
Tem
pera
ture
, °C
Time, s
0.1 1.0 10 100 10000.01
700
800
600
900
500
400
300
200
Ms
Fe-0.1%C-x%Mn
2μm
0.10C4Mn1Si
0.10C5Mn1Si
0.10C6Mn1Si
Role of Mn in Medium Mn Steel
MATERIALS DESIGN LABORATORY
0 200 400 600 800 10001200
Dilata
tio
n
Temperature
1200°C
650°C
700°C
750°C
800°C
850°C
900°C
0 200 400 600 800 10001200
Temperature
600°C
650°C
700°C
750°C
800°C
Dilata
tio
n
IAT IAT
1200°C
Ms
γ
γα
γ
Role of Mn in Medium Mn Steel
MATERIALS DESIGN LABORATORY
5
10
15
Mn
ma
ss
-%
10Mn0.3C3Al2Si (750°C)
5
10
15
Mn
ma
ss
-%
8Mn0.3C3Al1Si (750°C)
500 nm 200 nm
aa a
g gg g
a a
Partitioning of Mn in Medium Mn Steel
MATERIALS DESIGN LABORATORY
1500
1250
1000
750
500
250
0
0.00 0.25 0.50 0.75 1.00
8Mn-XC-3Al-0.5Si
C content, mass-%
Te
mp
era
ture
, °C
Al/Mn=0.375
1500
1250
1000
750
500
250
0
0.00 0.25 0.50 0.75 1.00
10Mn-XC-3Al-0.5Si
agM5C2
C content, mass-%
Te
mp
era
ture
, °C
Al/Mn=0.3
d
g
ag
aM5C2
agq
agM5C2
g
ag
aM5C2
agq
d
Role of Al in Medium Mn Steel
MATERIALS DESIGN LABORATORY
γ
γ+α+θ
γ+α
γ+α+(Fe,Mn)5C2
1000
900
800
700
600
500
4000 0.1 0.2 0.3 0.4 0.5
Mass percent C
Tem
pera
ture
(°C
)
1000
900
800
700
600
500
4000 10 20 30
Mass percent
1000
900
800
700
600
500
4000 10 20 30
SFE* (mJ/m2)
γ, Cx10
γ, Mn γ, SFE
Microstructure Medium Mn Steel
(10%Mn)
MATERIALS DESIGN LABORATORY
γ
γ+α+θ
γ+α
γ+α+(Fe,Mn)5C2
1000
900
800
700
600
500
4000 0.1 0.2 0.3 0.4 0.5
Mass percent C
Tem
pera
ture
(°C
)
1 μm
1 μm
10 μm
γ+α΄
γ+α
α+ α΄+(Fe,Mn)5C2
Microstructure at room temperature
gaM23C6
Fe-C-10Mn-3Al-2Si
1500
1000
500
0
0.0 0.5 1.0
Te
mp
era
ture
, °C
Carbon content, mass-%
aM23C6
aM23C6
M5C2
aM5C2
g
Fe-0.3C-10Mn-3Al-2Si
T:900°C
3 mm 5 mm
g
a
a
a
g
g
a
a
a
g
Fe-0.3C-10Mn-3Al-2Si
T:750°C
gaM5C2
ga
900
750
g a q
(a) (b) (c)
Strain Hardening 10-12% Mn Steel
EBSD: Phase map
Microstructure Medium Mn Steel
(10%Mn)
MATERIALS DESIGN LABORATORY
Microstructure Medium Mn SteelExample: 12% Mn, nucleation UFGs on twin boundaries
After hot rolling After cold rolling
Hot rolled 12%Mn: Austenitic with 2% ferrite.
Cold rolled 12%Mn: Austenitic with martensite + twinning.
Ferrite
Austenite
Martensite
Twins
After intercritical annealing
UFG α+γ (1 < μm)
MATERIALS DESIGN LABORATORY
Microstructure Medium Mn SteelExample: 10% Mn, nucleation UFGs on cementite particles
2 μm1 μm
Austenite
Cementite
C , Mn diffusionC , Mn diffusion
Sub-grain boundary
MATERIALS DESIGN LABORATORY
Fe-10Mn-0.3C-3Al-2Si
S C M
-1 3
γn
/M =545 426X 30.4 60.5(V )X
400 500 600 700 800 9000
5
10
15
20
25
30
400 500 600 700 800 9000
5
10
15
20
25
30
400 500 600 700 800 900
-200
-100
0
100
200
Mn
C x 10
Mass-%
Temperature (C)
Sta
ckin
g f
au
lt e
nerg
y (
mJ/m
2)
Temperature (C)
Without grain size effect
With grain size effect
Ms t
em
pera
ture
(C
)
Temperature (C)
TWIP
SFE↑ Stability ↑
UHS 8%-10% Mn Steel: Composition and Processing
MATERIALS DESIGN LABORATORY
S C M
-1 3
γn
/M =545 426X 30.4 60.5(V )X
TWIP
400 500 600 700 800 9000
5
10
15
20
25
30
400 500 600 700 800 9000
5
10
15
20
25
30
400 500 600 700 800 900
-200
-100
0
100
200
Ma
ss
-%
Temperature (C)
Mn
C x 10
Sta
ck
ing
fa
ult
en
erg
y (
mJ
/m2)
Temperature (C)
Ms t
em
pe
ratu
re (
C)
Without grain size effect
With graiun size effect
Exp.
Temperature (C)
Fe- 8Mn-0.3C-3Al-1Si
SFE↑ Stability ↑
UHS 8%-10% Mn Steel: Composition and Processing
MATERIALS DESIGN LABORATORY
650 700 750 800 850 900400
500
600
700
800
900
1000
1100
1200
1300
1400
Fe-10Mn-0.3C-3Al-2Si
YS
UTS
UTS
Str
en
gth
(M
Pa)
Annealing temperature (C)
YS
Fe-8Mn-0.3C-3Al-1Si
650 700 750 800 850 90010
20
30
40
50
60
70
To
tal e
lon
ga
tio
n (
%)
Annealing temperature (C)
Fe10Mn0.3C3Al2Si
Fe8Mn0.3C3Al1Si
8%-10% Mn Steel: Mechanical Properties
MATERIALS DESIGN LABORATORY
As-annealed
57% Austenite-43% Ferrite
As-deformed (~60%)
Twin
1 μm 1 μm
γ
α
γα γα
Microstructure 8%Mn TWIP+TRIP Steel
Fe-8%Mn-0.4%C-3%Al-1%Si steel intercritically annealed @ 750 °C
MATERIALS DESIGN LABORATORY
Microstructure 8%Mn TWIP+TRIP Steel
0.2 μm
2 1/nm2 1/nm
g
a
0.2 μm
g
a
Fe-8%Mn-0.4%C-3%Al-1%Si steel intercritically annealed @ 750 °C
MATERIALS DESIGN LABORATORY
Microstructure 8%Mn TWIP+TRIP Steel
Fe-8%Mn-0.4%C-3%Al-1%Si steel intercritically annealed @ 750 °C
As-annealed
57% Austenite-43% FerriteAs-deformed (~60%)
1 μm 1 μm
γ
α
γαα
MATERIALS DESIGN LABORATORY
Medium 8% Mn TWIP+TRIP Steel Concept
IAT: 700°C IAT: 750°C
0 5 10 15 20 25 30 35 40 45 50
0
200
400
600
800
1000
1200
1400
0 5 10 15 20 25 30 35 40 45 50
0
200
400
600
800
1000
1200
1400
En
gin
ee
rin
g s
tre
ss
, M
Pa
Engineering strain, %
8Mn-0.4C-3Al-2Si-0V8Mn-0.4C-3Al-2Si-0.1V8Mn-0.4C-3Al-2Si-0.2V
En
gin
eeri
ng
str
ess,
MP
aEngineering strain, %
8Mn-0.4C-3Al-2Si-0V8Mn-0.4C-3Al-2Si-0.1V8Mn-0.4C-3Al-2Si-0.2V
Fe18Mn0.6C1.5Al
MATERIALS DESIGN LABORATORY
Single phase
TWIP steel
Multi-phase
TWIP-TRIP transition
Multi-phase
TRIP steel
Model for the mechanical properties
g →gT
TWIP-effect
g
g →a’
TRIP-effect
a’g
g →gT
TWIP-effect
g
g →a’
TRIP-effect
a’g+
0 10 20 30 40 50 600
200
400
600
800
1000
1200
1400
0 10 20 30 40 50 600
200
400
600
800
1000
1200
1400
0 10 20 30 40 50 600
200
400
600
800
1000
1200
1400
8Mn
12Mn18Mn
En
g.
str
es
s (
MP
a)
Eng. strain (%)
DP980 DP980
10Mn
En
g.
str
es
s (
MP
a)
Eng. strain (%)
En
g.
str
es
s (
MP
a)
Eng. strain (%)
DP9806Mn
MATERIALS DESIGN LABORATORY
0.0 0.1 0.2 0.3 0.4 0.50
500
1000
1500
2000
2500
3000
3500
4000
6Mn
TWIP1000
Tru
e s
tre
ss
, M
Pa
Wo
rk h
ard
en
ing
ra
te,
MP
a
True strain
TWIP 1000: 18%Mn0.6%C1.5%Al
Medium Mn 1: 12%Mn0.3%C3.0%Al
Medium Mn 2: 10%Mn0.3%C3.0%Al2.0%Si
Medium Mn 3: 8%Mn0.4%C3.0%Al2.0%Si
Medium Mn 4: 6%Mn0.3%C3.0%Al1.5%Si
Mechanical properties at reduced Mn alloying
TWIP
TRIP
MATERIALS DESIGN LABORATORY
Conclusions
New 1GPa UHSS grades for automotive applications:
High Mn, Austenitic MBIP-SBIP Steel
High Mn, Austenitic TWIP Steel
Medium Mn, multi-phase TWIP+TRIP Steel
Medium Mn, Multi-phase TRIP Steel
Press Hardening Steel
Quench and Partitioning Processed Steel
Some concepts are “out-of-the-box” in terms of cost, processing, application
performance, … and the alloy fundamentals are challenging.
Current research focus on selecting and optimizing best concepts:
Multi-phase TWIP+TRIP steel with 6-10% Mn
Multi-phase UFG TRIP steel with 5-7% Mn
Application properties receiving attention:
Delayed fracture
Hole expansion and stretch forming performance
Coatings
MATERIALS DESIGN LABORATORY
Thank you for your attention.