3rd CAMS 2014_TWIP-TRIP Steels_FINAL_2014

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


Pohang University of Science and TechnologyGraduate Institute of Ferrous Technology

Pohang


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





Technology

• Research Areas:




Clean Steel



Materials Design

Material Mechanics

Surface Engineering





Technology

• Research Areas:




Clean Steel



Materials Design

Material Mechanics

Surface Engineering


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!


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


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


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


Mechanical

Properties

Yield Strength

Tensile Strength

Uniform elongation

Total Elongation

Automotive Sheet Steel Products


RoughingReheating Finishing Cooling Coiling

Conventional HSM, CSM and CA/HDG Processing

Cold rolling Continuous

Annealing

Hot Dip Galvanizing


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


Press-forming

Spring-back

Hole expansion

Bending

RSW


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


MA

300 400 500 600 700 800 900 1000

Elo

ng

ati

on

A80,

%


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


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)


Al

alloys

Polymers

Mg

alloys

Automotive

Steel grades

CFR-Composites

Ti

alloys



Al

alloys

Polymers CFR-Composites


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)


MA

300 400 500 600 700 800 900 1000

Elo

ng

ati

on

A80,

%


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


MA

300 400 500 600 900 1000

Elo

ng

ati

on

A80,

%


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


Strain Hardening Engineering

True strain

00

Tru

e s

tress

, str

ain

hard

en

ing

rate

, M

Pa

)(

u

d

d



True strain

00

Tru

e s

tress

, str

ain

hard

en

ing

rate

, M

Pa

)(

u

d

d



00

True strain

Tru

e s

tress

, str

ain

hard

en

ing

rate

, M

Pa

Gain in strength

and ductility !

u

)(

d

d


00

True strain

Tru

e s

tress

, str

ain

hard

en

ing

rate

, M

Pa

Gain in strength

and ductility !


u

)(

d

d

Dislocation

Accumulation

or Storage

(Stage II)

Dislocation

Annihilation

or Dynamic recovery

(Stage III)

ρkρkdε

dρ

dε

dρ

dε

dρ

21


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



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


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


Rolling directionTWIP 1000

High Mn TWIP Steel


HERDiffuse

necking

No diffuse

necking

IF steelTWIP

LMIE

HDF

Fe22Mn0.6C Fe15Mn2Al0.7C

Zn Zn


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


10%

20%

30%

40%

50%

60%


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


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


V-free

V-additions: increased YS and increased strain hardening


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


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


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


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


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


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


DSA

)(

d

d


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



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 2: 10%Mn0.3%C3.0%Al2.0%Si

Mechanical properties at reduced Mn


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





Mechanical properties at reduced Mn

DSA


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


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


0 10 20 30 40 50 60 700

200

400

600

800

1000

1200

Fe12Mn0.6C

Fe12Mn0.6C1.5Al

Fe12Mn0.6C2Al


)(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


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


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-%-

+


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


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


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


10mm

d

agd

ag

d

ag

200nm

200nm

50nm

50nm

Ferrite

Austenite

VC

SF

Strain Hardening Engineering UFG Steel


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


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


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


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


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


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)


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)


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)


Microstructure Medium Mn SteelExample: 10% Mn, nucleation UFGs on cementite particles

2 μm1 μm

Austenite

Cementite

C , Mn diffusionC , Mn diffusion

Sub-grain boundary


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


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


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


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



0.2 μm

2 1/nm2 1/nm

g

a

0.2 μm

g

a





As-annealed

57% Austenite-43% FerriteAs-deformed (~60%)

1 μm 1 μm

γ

α

γαα


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


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


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


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






TWIP

TRIP


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


Thank you for your attention.

Documents

3rd CAMS 2014_TWIP-TRIP Steels_FINAL_2014