1. MATERIALS DESIGN LABORATORY
Deformation Mechanisms in
Intercritically Annealed
Medium Mn Steel
B.C. DE COOMAN, Sunmi SHIN, Seon Jong LEE, Dongyeol LEE (GIFT)
Hyoung Seop KIM, Marat LATYPOV (Materials Science and Engineering)
Pohang University of Science and Technology
Pohang, Republic of Korea
Oct 25-29, 2015, Jeju Island

2. MATERIALS DESIGN LABORATORY
Golf 2 (1983-1992) Golf 7 (2014)
Issue #2:
Greenhouse gas emissions
Issue #1:
Passenger safety
2015: 130 g CO2 /km → 2020: 95 g CO2 /km
2015: 17 km/l → 2020: 25 km/l
Issue #3:
Fuel efficiency
Introduction

3. MATERIALS DESIGN LABORATORY
Properties
Processing
Microstructure
Performance
Properties
Processing
Microstructure
Performance
COST
Regulations
Steel
Unibody
Aluminum
Space frame
CFRC
Skeleton
Introduction
Al-alloy body
Steel frame

4. MATERIALS DESIGN LABORATORY
Improvements
to the internal
combustion engine
Increased use of advanced high strength
and ultra-high strength steel grades
Introduction

5. MATERIALS DESIGN LABORATORY
6% HPF + 5% MA
23%
HSS
30%
DP / Multi Phase
34%
IF
+LC
+BH
Cadillac CTS
PHS VW:
6% (GOLF 6)
28% (GOLF 7)
Automotive Body Materials Evolution

6. MATERIALS DESIGN LABORATORY
0 5 10 15 20 25
0
500
1000
1500
2000
2500
EngineeringStress,MPa
Engineering Strain, %
Al
Al+HPF
Al+HPF+BH
0 5 10 15 20 25
0
500
1000
1500
2000
2500
EngineeringStress,MPa
Engineering Strain, %
Al
Al+HPF
Al+HPF+BH
Aluminized PHS, 2nd WeekAluminized PHS, 1st Week
2GPa Press Hardening Steel
F+P F+P
CA (F+P) CA+HPF CA+HPF+BH

7. MATERIALS DESIGN LABORATORY
40,000MPa%
20,000MPa%
30,000MPa%
TRIP-aided Bainitic Ferrite Steel
Press Hardening Steel
Q&P Processed Steel
Medium Mn Steel
Low Density Steel
Automotive Body Materials Evolution

8. MATERIALS DESIGN LABORATORY
Mecking-Kocks-Estrin model based on the dislocation density evolution, with dislocation accumulation at grain
boundaries, forest dislocations and dislocation annihilation by dynamic recovery.
ρ(ε)bGMασσ
ρkρ
b
k
Dbd
dρ





0
2
11)(


Constitutive Model for UFG Ferritic Steel
50µm
ρ(ε)
Dislocation storage
accumulation
Dislocation annihilation
Dynamic recovery

9. MATERIALS DESIGN LABORATORY
10+10
10+11
10+12
10+13
10+14
10+15
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Grain size 1mm
True strain
Dislocationdensity,m-2
0
200
400
600
800
1000
0.0 0.1 0.2 0.3 0.4 0.5 0.6
True strainTruestress,MPa
Strainhardening,MPa
Constitutive Model for UFG Ferritic Steel

10. MATERIALS DESIGN LABORATORY
20,000MPa%
30,000MPa%
TARGET 1
1200MPa - 30%
TARGET 2
1500MPa - 25%
500 nm
200 nm
40,000MPa%
Uniformelongation,%
Constitutive Model for UFG Ferritic Steel
DOE TARGET
35% lightweighting
Max $3.18 per lb lightweighting

11. MATERIALS DESIGN LABORATORY
)()(
)0




bG
(εbGαMσσ(ε)
o
D
k
df
X
T
y
precprecprec
is
p
o




),(
)(
),(





parameterPetchHall:
constantonAnnihilati:
constantonAccumulati:
parameterGrainsize:
sizetsize/PackeGrain:
ingstrengthensolutionSolid:
ingstrengthenionPrecipitat:
stressPeierls:
eTemperatur:
modulusShear:
vectorBurgers:
strainratestrain,:,
densityndislocatio:)(
2
1
y
s
prec
P
k
k
k
P
D
T
G
b






Ferrite
Martensite
Austenite
Twins
0D  
111
 D
ρkρ
b
k
bd
dρ


 2
11)(


Model for a Medium Mn TWIP+TRIP Steel

12. MATERIALS DESIGN LABORATORY
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Phasefraction
True strain
f
0
500
1000
1500
2000
2500
3000
3500
4000
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
True strain
Truestress,MPa
Strainhardeningrate,MPa
UFG Ferrite
UFG Austenite
%C %Mn %Al %Si
Ferrite - 3.69 3.52 1.45
Austenite 0.56 8.07 2.54 1.55
Model for a Medium Mn TWIP+TRIP Steel

13. MATERIALS DESIGN LABORATORY
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Phasefraction
True strain
f fTwin
f’
0
500
1000
1500
2000
2500
3000
3500
4000
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
True strain
Truestress,MPa
Strainhardeningrate,MPa
Martensite
UFG Ferrite
UFG Austenite
%C %Mn %Al %Si
Ferrite - 3.69 3.52 1.45
Austenite 0.56 8.07 2.54 1.55
Deformation Twinning
+
Strain-induced transformation
Model for a Medium Mn TWIP+TRIP Steel

14. MATERIALS DESIGN LABORATORY
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Phasefraction
True strain
f fTwin
f’
0
500
1000
1500
2000
2500
3000
3500
4000
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
True strain
Truestress,MPa
Strainhardeningrate,MPa
Martensite
UFG Ferrite
UFG Austenite
)(


d
d )(
%C %Mn %Al %Si
Ferrite - 3.69 3.52 1.45
Austenite 0.56 8.07 2.54 1.55
Model for a Medium Mn TWIP+TRIP Steel

15. MATERIALS DESIGN LABORATORY
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Phasefraction
True strain
f fTwin
f’
0
500
1000
1500
2000
2500
3000
3500
4000
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
True strain
Truestress,MPa
Strainhardeningrate,MPa
)(

d
d )(
%C %Mn %Al %Si
Ferrite - 3.69 3.52 1.45
Austenite 0.56 8.07 2.54 1.55
+0.1% V
Model for a Medium Mn TWIP+TRIP Steel

16. MATERIALS DESIGN LABORATORY
FEM Analysis: Strain Partitioning Inversion
At low strains, softer austenite accommodates the imposed deformation, causing strain
hardening, TWIP and TRIP effects in the austenite. At larger strains, ferrite, having a lower
rate of strain hardening, increasingly accommodates the imposed strain.

17. MATERIALS DESIGN LABORATORY
Revised UFG Medium Mn steel: d-ferrite by (a) addition of Al/Si
(b) reduction C content
Deformation
Dislocation plasticity
Twinning-induced plasticity
Transformation-induced plasticity
Ultra-fine grained
ferrite + austenite
coarse d-ferrite
“bi-modal” grain size
VC precipitates
d
d






d
d
 
’
’
’
Temperature
Ms
RT
’
CR: Fully
martensitic
+


dd


d
C/Mn partitioning
Time
Ms
VC
Model for a Medium Mn TWIP+TRIP Steel

18. MATERIALS DESIGN LABORATORY
TWIP Steel
Fully Austenitic Microstructure
Coarse grained
Mechanical twinning
From TWIP to TWIP+TRIP Steel
5μm
TWIP steel
γ
500nm
8Mn HR TWIP+TRIP steel
α
γ
α
γ
Medium Mn TWIP+TRIP Steel
Austeno-Ferritic Microstructure
Ultra-fine grained
Mechanical twinning + Strain-induced Transformation

19. MATERIALS DESIGN LABORATORY

q
q
800C
Fe-6%Mn-x%C Fe-6%Mn-3%Al-1.5%Si-x%C

q
q

 800C
Fe-Mn-Al-Si-C Medium Mn Steel Design
0.3C 6.0Mn 1.5Si 3.0Al
780C, 10min
d
UFG 
10μmMs
Mass-% C Mass-% C
Temperature,C

20. MATERIALS DESIGN LABORATORY
Fe-0.30%C-10%Mn-3%Al-2%Si
750°C650°C

’ matrix
Martensite aging:
carbide precipitation
Reverse Transformation and Partitioning
Austenite nucleation:
carbide dissolution

21. MATERIALS DESIGN LABORATORY
Effect of Al on austenite fraction after IA
600 700 800 900
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Grain size: 1um
0.3C 1Al 1.5Si 6Mn
0.3C 2Al 1.5Si 6Mn
0.3C 3Al 1.5Si 6Mn
FractionofausteniteatRT
Intercritical Annealing T, degree C
600 700 800 900
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.3C 1Al 1.5Si 6Mn
0.3C 2Al 1.5Si 6Mn
0.3C 3Al 1.5Si 6Mn
FractionofausteniteatRT
Intercritical Annealing T, degree C
Grain size: 1mm
600 700 800 900
0
10
20
30
40
0.3C 1Al 1.5Si 6Mn
0.3C 2Al 1.5Si 6Mn
0.3C 3Al 1.5Si 6Mn
SFE(mJ/m2)
Intercritical Annealing T, degree C

22. MATERIALS DESIGN LABORATORY
0.000 0.004 0.008
500
600
700
800
900
1000
Temperature(
o
C)
C mass fraction
-400 -200 0 200
500
600
700
800
900
1000
Ms (
o
C)
RT
0.0 0.2 0.4 0.6 0.8 1.0
500
600
700
800
900
1000
Austenite fraction
0 10 20 30 40
500
600
700
800
900
1000
SFE (mmJ/m
2
)
0.05 0.10 0.15 0.20
Mn mass fraction
Characteristics of austenite after IAPhase diagram
Grain size effect
Red: 1mm g.s.
Blue: 0.5mm g.s.
C, Mn partitioning
during IA

+q
 + 
Temperature,ºC
C mass-%
1.000.750.500.250.00
500
600
700
800
900
1000
Wide    stability
range
Large volume fraction of retained austenite
Optimum SFE
for TWIP+TRIP
Fe-6%Mn-3%Al-1.5%Si-x%C Design Concept

23. MATERIALS DESIGN LABORATORY
Fe-6%Mn-0.15%C-3%Al-1.5%Si steel intercritically annealed @ 840 °C
Medium Mn TWIP+TRIP Steel Concept
d - ferrite
  
0% strain
36% retained austenite
3% strain
d - ferrite
    ’
5.9% martensite
6% strain
d - ferrite
    ’
13.7% martensite
9% strain
d - ferrite
    ’
2 μm
17.3% martensite

24. MATERIALS DESIGN LABORATORY
10mm
d

d

d

200nm
200nm
50nm
50nm
Ferrite
Austenite
VC
SF
V-added UFG Medium Mn Steel
800ppmC 6%Mn 1.5%Si 2%Al

25. MATERIALS DESIGN LABORATORY
Fe-8%Mn-3%Al-2%Si-0.4%C-0.2%V
Intercritically Annealed @ 750C, 600 s
VC-precipitate
Ferrite
Austenite
Ferrite
VC-precipitate
Ferrite
Austenite
Ferrite
Precipitate
formation
Partitioning

26. MATERIALS DESIGN LABORATORY
0.2 μm
21/nm21/nm


0.2 μm


Fe-8%Mn-0.4%C-3%Al-1%Si steel intercritically annealed @ 750 °C
Medium Mn TWIP+TRIP Steel Concept
Fe-5%Mn-0.3%C-3%Al-1.5%Si steel intercritically annealed @ 800 °C
6





27. MATERIALS DESIGN LABORATORY
As-annealed
57% Austenite-43% Ferrite
As-deformed
Fe-8%Mn-0.4%C-3%Al-1%Si steel intercritically annealed @ 750 °C
Medium Mn TWIP+TRIP Steel Concept
1 μm
γ
α
γα

28. MATERIALS DESIGN LABORATORY
0.0 0.1 0.2 0.3 0.4
0
1000
2000
3000
4000
5000
780
800
840
Truestress,strainhardeningrate,MPa
True Strain
840℃
820℃
800℃
780℃
820
TWIP
TRIP
Fe-0.3%C-6%Mn-1.5%Si-3%Al
Medium Mn TWIP+TRIP Steel Concept

29. MATERIALS DESIGN LABORATORY
0 10 20 30 40 50 60
0
200
400
600
800
1000
1200
1400
0 10 20 30 40 50 60
0
200
400
600
800
1000
1200
1400
0 10 20 30 40 50 60
0
200
400
600
800
1000
1200
1400
8Mn0.4C3Al2Si
12Mn0.3C3Al18Mn0.6C1.5Al
Eng.stress(MPa)
Eng. strain (%)
DP980 DP980
10Mn0.3C3Al2Si
Eng.stress(MPa)
Eng. strain (%)
Eng.stress(MPa)
Eng. strain (%)
DP980
6Mn0.3C3Al1.5Si
Single phase
TWIP steel
 →T
TWIP-effect

 →T
TWIP-effect

 →’
TRIP-effect
’
+
Multi-phase
TWIP+TRIP steel
5Mn0.3C3Al1.5Si

30. MATERIALS DESIGN LABORATORY
Conclusions
An UFG intercritically-annealed UHSS (UTS>1GPa) medium Mn steel grades for
automotive applications are being developed.
The material does not deform by localized Lüders band propagation, a common
problem for UFG materials.
The steel is designed to have two plasticity-enhancing mechanisms, the TWIP and
TRIP effects, being activate in succession in the austenite.
The alloy is designed to be compatible with current processing of CR strip for
automotive applications in CA and HDG lines. Concept is also compatible
with current requirements in terms of cost, processing, and application
performance.
Current research is focusing on:
- Zn and Zn-alloy coating
- Development of a hot rolled variant
- Further reduction of the alloy content
- Further increase in strength and ductility

31. MATERIALS DESIGN LABORATORY
Thank you
The support of the POSCO Technical Research Laboratories is gratefully acknowledged.