IUMRS-ICAM 2015_Deformation Mechansims in IA Medium Mn Steel_OCT 25-29_2015

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


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/lIssue #3:

Fuel efficiency

Introduction


Properties

Processing

Microstructure

PerformanceProperties

Processing

Microstructure

Performance

COST

Regulations

Steel

Unibody

Aluminum

Space frame

CFRC

Skeleton

Introduction

Al-alloy body

Steel frame


Improvements

to the internal

combustion engine

Increased use of advanced high strength

and ultra-high strength steel grades

Introduction


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


0 5 10 15 20 250

500

1000

1500

2000

2500

En

gin

ee

rin

g S

tres

s, M

Pa

Engineering Strain, %

Al

Al+HPF

Al+HPF+BH

0 5 10 15 20 250

500

1000

1500

2000

2500

En

gin

ee

rin

g S

tres

s, M

Pa

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


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


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

ρkρb

k

Dbd

dρ

0

21

1)(

Constitutive Model for UFG Ferritic Steel

50µm

ρ(ε)

Dislocation storage

accumulation

Dislocation annihilation

Dynamic recovery


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

Dis

locati

on

den

sit

y,

m-2

0

200

400

600

800

1000

0.0 0.1 0.2 0.3 0.4 0.5 0.6

True strainT

rue s

tress,

MP

a

Str

ain

hard

en

ing

, M

Pa



20,000MPa%

30,000MPa%

TARGET 1

1200MPa - 30%

TARGET 2

1500MPa - 25%

500 nm

200 nm

40,000MPa%

Un

ifo

rm e

lon

gati

on

, %


DOE TARGET

35% lightweighting

Max $3.18 per lb lightweighting


)()(

)0

bG

(εbGαMσσ(ε)

o

D

k

df

X

T

y

precprecprec

is

p

o

),(

)(

),(

parameterPetch Hall:

constanton Annihilati:

constanton Accumulati :

parameter Grainsize :

sizet size/PackeGrain :

ingstrengthensolution Solid :

ingstrengthenion Precipitat :

stress Peierls :

eTemperatur:

modulusShear :

vectorBurgers:

strainratestrain,: ,

densityn dislocatio:)(

2

1

y

s

prec

P

k

k

k

P

D

T

G

b

Ferrite

Martensite

Austenite

Twins

0D

111 D

ρkρb

k

bd

dρ

2

11)(

Model for a Medium Mn TWIP+TRIP Steel


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 0.6P

hase f

racti

on

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

Tru

e s

tress,

MP

a

Str

ain

hard

en

ing

rate

, M

Pa

UFG Ferrite

UFG Austenite

%C %Mn %Al %Si

Ferrite - 3.69 3.52 1.45

Austenite 0.56 8.07 2.54 1.55



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 0.6P

hase f

racti

on

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

Tru

e s

tress,

MP

a

Str

ain

hard

en

ing

rate

, M

Pa

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



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 0.6P

hase f

racti

on

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

Tru

e s

tress,

MP

a

Str

ain

hard

en

ing

rate

, M

Pa

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



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 0.6P

hase f

racti

on

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

Tru

e s

tress,

MP

a

Str

ain

hard

en

ing

rate

, M

Pa

)(

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



FEM Analysis: Strain Partitioning InversionAt 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.


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

’

’

’

Tem

pe

ratu

re

Ms

RT

’

CR: Fully

martensitic

+

dd

d

C/Mn partitioning

TimeMs

VC



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

γα

γ

Medium Mn TWIP+TRIP Steel

Austeno-Ferritic Microstructure

Ultra-fine grained

Mechanical twinning + Strain-induced Transformation


q

q

800C

Fe-6%Mn-x%C Fe-6%Mn-3%Al-1.5%Si-x%C

q

q

800C

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

Tem

pera

ture

, C


Fe-0.30%C-10%Mn-3%Al-2%Si750°C650°C

’ matrix

Martensite aging:

carbide precipitation

Reverse Transformation and Partitioning

Austenite nucleation:

carbide dissolution


Effect of Al on austenite fraction after IA

600 700 800 9000.0

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

Fra

ctio

n o

f a

uste

nite

at

RT

Intercritical Annealing T, degree C

600 700 800 9000.0

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

Fra

ctio

n o

f a

uste

nite

at

RT


Grain size: 1mm

600 700 800 9000

10

20

30

40 0.3C 1Al 1.5Si 6Mn

0.3C 2Al 1.5Si 6Mn

0.3C 3Al 1.5Si 6Mn

SF

E (

mJ/m

2)



0.000 0.004 0.008

500

600

700

800

900

1000

Tem

pera

ture

(oC

)

C mass fraction

-400 -200 0 200

500

600

700

800

900

1000

Ms (oC)

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/m2)

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

+

Tem

pera

ture

, ºC

C mass-%

1.000.750.500.250.00

500

600

700

800

900

1000

Wide stability

rangeLarge volume fraction of retained austenite

Optimum SFE

for TWIP+TRIP

Fe-6%Mn-3%Al-1.5%Si-x%C Design Concept


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


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


Fe-8%Mn-3%Al-2%Si-0.4%C-0.2%V

Intercritically Annealed @ 750C, 600 s

VC-precipitate

Ferrite

Austenite

Ferrite

VC-precipitate

Ferrite

Austenite

FerritePrecipitate

formation

Partitioning


0.2 μm

2 1/nm2 1/nm

0.2 μm

Fe-8%Mn-0.4%C-3%Al-1%Si steel intercritically annealed @ 750 °C


Fe-5%Mn-0.3%C-3%Al-1.5%Si steel intercritically annealed @ 800 °C

6


As-annealed

57% Austenite-43% Ferrite

As-deformed

Fe-8%Mn-0.4%C-3%Al-1%Si steel intercritically annealed @ 750 °C


1 μm

γ

α

γα


0.0 0.1 0.2 0.3 0.4

0

1000

2000

3000

4000

5000

780

800

840

True

str

ess,

str

ain

hard

enin

g ra

te, M

Pa

True Strain

840℃

820℃

800℃

780℃

820

TWIP

TRIPFe-0.3%C-6%Mn-1.5%Si-3%Al



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

8Mn0.4C3Al2Si

12Mn0.3C3Al18Mn0.6C1.5Al

En

g.

str

ess (

MP

a)

Eng. strain (%)

DP980 DP980

10Mn0.3C3Al2Si

En

g.

str

ess (

MP

a)

Eng. strain (%)

En

g.

str

ess (

MP

a)

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


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


Thank you

The support of the POSCO Technical Research Laboratories is gratefully acknowledged.

IUMRS-ICAM 2015_Deformation Mechansims in IA Medium Mn Steel_OCT 25-29_2015

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