MS&T’15_Mechanical Properties Plasticity Enhancement Q&P Medium Mn Steel_OCT 4-8 2015

MATERIALS DESIGN LABORATORY

Tensile Behavior and Plasticity-enhancement by

Quenching and Partitioning (Q&P) Processing

Bruno C. DE COOMAN, Eun Jung SEO, Lawrence CHO

Graduate Institute of Ferrous Technology

Pohang University of Science and Technology

Pohang, South Korea

Yuri Estrin

Monash University

Australia

MS&T’15, October 4-8, 2015, Columbus, OH, USA

Phase Stability, Diffusion Kinetics, and their Applications

Session honoring Dr.J.G.Speer, J.W.Gibbs Phase Equilibria Award Recipient


Introduction

Quench and Partitioning Processing (Q&P)

is a fundamentally new way to produce martensitic

steel containing a considerable volume fraction of

retained austenite …

1. J.G. Speer, A.M. Streicher, D.K. Matlock, F. Rizzo, G. Krauss,

2003 Symposium on Austenite Formation and

Decomposition, ed. E.B. Damm and M.J. Merwin

TMS, Warrendale, PA, 2003, pp. 505–522

2. J.G. Speer, D.K. Matlock, B.C. De Cooman, J.G. Schroth,

Acta Materialia, 51, 2003, pp. 2611–2622

Dr. J. G. Speer

J.W.Gibbs Phase Equilibria Award Recipient


Introduction

1. Microstructure of Q&P processed steel

2. Q&P processed Fe-4%Mn-0.2%C-Si-Cr steel

3. Microstructure evolution during straining

4. Model Mechanical Properties of Q&P Steel

5. Conclusions


Tem

pera

ture

Time

Annealing

Temperature, AT

C-Partitioning

Temperature, PTMs

Mf

Quenching

Temperature, QT

g

M’s

RT

Introduction


Stationary

g /a’ interface

Stationary

a/g interface

C partitioning

g

(a) Fe C Mn Si Cr

20 nm

(b) ga'p a'p

2.0

4.0

6.0

Mn

, w

t. %

0 10 20 30 40 50 60

Distance, nm

1.5

2.0

2.5

1.0

Si,

wt.

%

0.0

1.0

2.0

1.5

0.0

0.5

1.0

Cr,

wt.

%C

, w

t. %


20 nm

q

g

0 10 20 30 40

0

2

4

6

8

Co

nce

ntr

ati

on

, w

t. %

Distance, nm

CMn

Si

Cr

(a) (b)

3D atom map for C in Q&P processed 4%Mn steel

Austenitized, quenched to 170 °C, and partitioned at 450 °C for 300 s

Mn, Cr, Si, and C concentration profiles along the solid arrow

Carbide precipitation


~ 200°C ~300°C ~ 400°C

Optimal Quench Temperature

Fraction gTQFraction a’TQ

Fraction gFinal

C gFinal

Fraction a’Final Quench

Temperature

f gTQ

f a’TQ

C gFinal

f gFinal

1. C partitioning

2. Stationary a’/g interface

3. No carbide precipitation

4. No isothermal transformation at T<Ms; no bainite transformation

Introduction


Materials Selection

Fe-0.2%C-4.0%Mn-1.6%Si-1.0%Cr

Si, Cr: increase gret volume fraction

Ms:273ᵒC

No isothermal transformation

No bainite transformation

No carbide precipitation

TQ

, C

0 200 400 600

TQ: 270 ºC

TQ: 210 ºC

TQ: 230 ºC

TQ: 250 ºC

Ms TP

270

250

230

210

190

Temperature, ºC

0 200 400 600RT

(a)

(b)

(c)

a'p

g

Low TQHigh TQ

a'p

a's

g

a'p

g

Partitioning

Final quenching

C

a'p

g

C

- Cg : High TQ < Low TQ

- gstability : High TQ < Low TQ

-150

0

150

300

Rel

ati

ve

len

gth

ch

an

ge,

mm

/cm

Secondary Ms

temperature (Ms)


Materials Selection

0 50 100 150 200 250 300 3500.0

0.2

0.4

0.6

0.8

1.0

Quench temperature, ºC

Ph

ase

fra

ctio

n

Austenite fraction

after initial quenching

Secondary martensite fraction

Retained austenite fraction

Secondary martensite


Ph

ase

fra

ctio

n

Retained austenite

120 150 180 210 240 270 300

0.0

0.2

0.4

0.6

0.8

(a)

(b)

Fe-0.21%C-4.0%Mn-1.6%Si-1.0%Cr

Si, Cr: increase gret volume fraction

Ms:273ᵒC

0 50 100 150 200 250 300 3500.0

0.2

0.4

0.6

0.8

1.0


Ph

ase

fra

ctio

n

Austenite fraction






Ph

ase

fra

ctio

n

Retained austenite

120 150 180 210 240 270 300

0.0

0.2

0.4

0.6

0.8

(a)

(b)


Materials Selection0 50 100 150 200 250 300 350

0.0

0.2

0.4

0.6

0.8

1.0

Quench temperature, ºCP

ha

se f

ract

ion

Austenite fraction






Ph

ase

fra

ctio

n

Retained austenite

120 150 180 210 240 270 300

0.0

0.2

0.4

0.6

0.8

(a)

(b)

g : High-C Retained Austenite

Cg: ~1.0%

SFE: larger

RT>Ms (Strain-induced transformation)

aP: Tempered Low-C Primary Martensite

aSt: High-C Strain-induced Martensite

g : Low-C Retained Austenite

Cg: ~0.35%

SFE: smaller

Ms>RT>Mf (Stress-induced transformation)


aS: Low-C Secondary martensite

aSt: Low-C Strain-induced martensite


0.0 0.1 0.2 0.30

500

1000

1500

Speich and Warlimont

Fleischer

Rodriguez and Gutierrez

Cohen

wt. % C

0.2

% o

ffse

t y

ield

stre

ss,

MP

a

Ca‘p Ca‘s

Reference Equations

Speich-Warlimont

Fleischer

Cohen

Rodriguez-Gutierrez

%) (wt.C101.72413 MPa)(inσ α

3

ys

%) (at.C10

G MPa)(inσ αys

%) (wt.C101.31461 MPa)(inσ α

3

ys

161)%C (wt.3065 MPa)(in σ ssα

Mechanical Properties of MartensiteaP Tempered

Low-C

Primary

martensite

aSt High-C

Strain-induced

martensite

aS Low-C

Secondary

martensite

1.8-2.9GPa


Engineering strain, % (compression)

1011

0 40 806020 100

Pure a-Fe

Martensite

Dis

locati

on

den

sit

y,

m-2

1012

1013

1014

1015

1016

Carbon content, mass-%

Volume change, V/V1.5 2.0 2.5 3.0

1015

2.1015

3.1015

4.1015

0.003 0.2 0.4 0.6 0.8

0

Dis

locati

on

den

sit

y,

m-2

Martensite

(a) (b)

Mechanical Properties of Martensite


Mechanical Properties of Martensite

Carbide variants

Carbide-dislocation interaction0.5mm

As-quenched Quenched and tempered

1mm

Secondary

martensite

Tempered

primary

martensite


En

gin

eeri

ng

Str

ess,

MP

a

0

500

1000

1500

2000

0 5 10 15 20

Engineering Strain, %


formation

5 10 15 20

highest volume fraction

retained austenite

5 10 15 20

TQ: 150 °C

TQ: 210 °C

TQ: 250 °C

TQ: 230 °C

TQ: 270 °C

TQ: 170 °C

TQ: 190 °C

Mechanical Properties








YS↘

Initial strain

hardening↗

YS↗


0

5000

10000

15000

20000

25000

30000

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

TQ: 150 °C

TQ: 170 °C

TQ: 190 °C

Tru

e S

tres

s, M

Pa

True Strain, %

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

True Strain, %

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

True Strain, %

TQ: 210 °C TQ: 230 °C

TQ: 250 °C

TQ: 270 °C

Mechanical Properties








Initial strain

hardening↗

“Sustained”

strain hardening


(a)g

ap

5 µm

(b)

g

ap

(c)

g

as

ap

(d)

ap

as

g

5 µm

5 µm

5 µm

TQ: 170°C

TP: 450°C (300s)

TQ: 250°C

TP: 450°C (300s)


TQ: 170°C

TP: 450°C (300s)

100 nm100 nm

g[110]Austenite twin

(d) (e)

T TM M TM

2 n m2 n m

2 nm

(f)

(g)

200 nm

a[111]

(110)a

[-111]a // [-110] g

(111)gg[110]

(a) (b) (c)SFs

5 nm

a[111]

0%

8%


(d)

SFs

5 nm100 nm

a[111]

g[110](a) g[110](b)

100 nm

2 1/nm

2 1/nm

(110)a

[-111]a // [-110] g

(111)g

(c)

T

M

T

M

100 nm

a[111]

g[110]

100 nm

SFs + Austenite twin

2 nm

(e) (f) (g) (h)

1 nm

TQ: 250°C

TP: 450°C (300s)

0%

5%


J.Nakano, Sci. Technol. Adv. Mater. 2013

Saeed-Akbari et al., Met. Mat. Trans. A 2013

0.00 0.05 0.10 0.15 0.20 0.25 0.30-20

-10

0

10

20

30

40

50

60

70

ISF

E (

mJ/m

2)

Mass fraction of Mn

Fe-Mn

Fe-Mn-0.6C

Fe-Mn-1.0C

Twinning

0.00 0.05 0.10 0.15 0.20 0.25 0.30-20

-10

0

10

20

30

40

50

60

70

ISF

E (

mJ/m

2)

Mass fraction of Mn

Fe-Mn

Fe-Mn-0.5C

Fe-Mn-1.0C

Twinning

Deformation-induced Twinning


0.00 0.02 0.04 0.06 0.08 0.10

True strain

Str

ain

-in

du

ced

ma

rten

site

fra

ctio

n

(a)

Experimental

Eq. (1)

0.00 0.02 0.04 0.06 0.08 0.10

True strainS

tra

in-i

nd

uce

d

ma

rten

site

fra

ctio

n

(b)

Experimental

Eq. (1)

0.00

0.05

0.10

0.15

0.20

0.25

0.00

0.05

0.10

0.15

0.20

0.25

}]αε))exp(β(1exp{[1ff n

γαst

Parameters TQ: 170 °C TQ: 250 °C

a 46 5.4

b 0.57 4.70

n 5.80 0.71


Cg: ~1.0%

SFE: larger

RT>Ms (Strain-induced transformation)

aSt: High-C Martensite


Cg: ~0.35%

SFE: smaller

Ms>RT>Mf (Stress-induced transformation)

aSt: Low-C Martensite


Constitutive Equation: Kocks-Mecking-Estrin Model

disgbssgggg

Cu) (wt.%17.5Ni) (wt.%5.7N) (wt.%877Ti) (wt.%118 Mo)(wt.%5.1

Cr) (wt.%0.1 Si)(wt.%23 Mn)(wt.%1.4C) (wt.%598 MPa)(in σ ss

γ

2

1

γγ

gb

γ dKσ

γγγγ

dis

γ ρbGMασ g

γγ

2

γ

γ

1

γγ

γγ dε)ρkρb

k

Λb

1(Mdρ g

gDislocation density evolution


(a)

Tru

e st

ress

,M

Pa

0.00 0.05 0.10 0.15

True strain

Experimental

Model

0

500

1000

1500

2000 (b)

Tru

e st

ress

,M

Pa

Experimental

Model

0

500

1000

1500

2000

(d)

d

/de,

MP

a

True strain

Experimental

Model

(c)

d

/de,

MP

a

0.00 0.05 0.10 0.15

True strain

Experimental

Model

0

5000

10000

15000

20000

25000

30000

0

5000

10000

15000

20000

25000

30000

0.00 0.05 0.10 0.15

0.00 0.05 0.10 0.15


0.00 0.05 0.10 0.150

1000

2000

3000

4000

5000

6000

0

500

1000

1500

2000

2500

3000

3500

0.00 0.05 0.10 0.15

0.00 0.05 0.10 0.15 0.00 0.05 0.10 0.15

Tru

e st

ress

, M

Pa

True strain True strain

Tru

e st

ress

, M

Pa

True strain True strain

Dis

loca

tio

n d

ensi

ty, m

-2

1x1012

1x1013

1x1014

1x1015

1x1016

1x1017

Dis

loca

tio

n d

ensi

ty, m

-2

1x1012

1x1013

1x1014

1x1015

1x1016

1x1017

Retained austenite

Primary martensite


Retained austenite

Primary martensite

Retained austenite

Primary martensite


Retained austenite

Primary martensite

Bulk

Bulk

Strain-induced martensite





Conclusions

1. The ultra-high strength Fe-4%Mn-0.2%C-Si-Cr Q&P steel has a near

perfect Q&P behavior without iso-thermal transformation below Ms,

Bainite transformation or carbide precipitation at low TQ.

2. Both the TWIP and TRIP effects are activated during deformation. This

TWIP+TRIP plasticity-enhancing mechanism occurs independently of the

selected TQ.

3. A physically-based model for the microstructure-mechanical properties

relationships of Q&P steel was developed.

5. The austenite transformation kinetics, the presence of secondary

martensite and the strength of the mechanically-induced martensite play

a key role in determining the mechanical properties.


Thank you

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

MS&T’15_Mechanical Properties Plasticity Enhancement Q&P Medium Mn Steel_OCT 4-8 2015

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