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Mechanical Properties of Carbide Free Bainitic Steel

Xiaoxu Zhang

Supervisor: Dr. ZurobDr. Purdy

2

Environmental Issue + Safety

Weight Reduce

Higher Strength

More Complicated shape of part

Higher Ductility

Motivation

CFB

3

4

Complex microstructure: bainitic ferrite + retained austenite + martensite

Nano-scale microstructure Bainitic ferrite: 200-400 nm thick Retained austenite: 20–40 nm thick

Retained austenite: carbon partitioning to austenite; austenite film trapped in between bainitic ferrite and stabilized at room temperature

Silicon (~1.5%) suppress carbide formation

Carbide Free Bainitic SteelMicrostructure

Caballero 2004

5

Carbide Free Bainitic SteelHeat Treatment Process Design

A3

Bainite

Fe-0.4%C-2.8%Mn-1.8%Si (mass%)

30% 80%

300CX30mins 300CX60mins

300CX90mins 300CX120mins

Optical Microstructure

6

7

Tensile Test Results

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Strength correlation with carbon content

0.15 0.2 0.25 0.3 0.35 0.4 0.45500

700

900

1100

1300

1500

1700

1900

2100

2300

HELL 2010 Wang 2011

sugimoto 2007 Hojo 2008

Caballero 2000 caballero 2009 (2)

caballero 2009 caballero 2012

garcia-mateo 2005 gomez 2008

hell 2010(2) putatundaa 2009

sugimoto 2002 sugimoto 2004

sugimoto 2000 sugimoto 2000 (2)

sugimoto 2006 sugimoto 2010

guang data caballero 2008(3)

C wt%

UTS

(MPa

)

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Comparison between CFB and DP steel

Bouaziz 2012Caballero 2012

𝜎 𝑦=𝜎𝑜+𝑘𝑑−1 /2Scale Effect

DP

UTS and UEI of DP and CFB steel with same carbon content

CFB

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Work Hardening

σT

εT

dσT/dεT σT

dσT/dεT

Necking point

UEI

UTS

Considere criterion: dσ/dε=σT

σ

σ-σY

dσT/dεT

dσT/dεT

σ

dσT/dεT

DP

CFB

Necking point

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0 200 400 600 800 1000 1200 1400 1600 1800 20000.00E+00

2.00E+04

4.00E+04

6.00E+04

8.00E+04

1.00E+05

1.20E+05

1.40E+05

1.60E+05

1.80E+05

2.00E+05

True Stress (MPa)

Wor

k Ha

rden

ing

Rate

Work-Hardening Behaviour

30 minutes 60 minutes 90 minutes 120 minutes

ϴII =E/50

Masing Model: elements yield at different stresses

Complex microstructure: mixture of elements with wide range of yield strength

Elasto-plastic transition Different stage of deformation of

each element Internal stress developed during

unloading and reversed loading

σy

n

element 14

Masing Model

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Elasto-Plastic Transition

dσ/dε = f ϴII + (1-f) E

0200

400600

8001000

12001400

16001800

20000

0.2

0.4

0.6

0.8

1

1.2

Stress (MPa)

F

the calculated fraction of the material which has yielded (f) for specimen heat at 300C for 120mins

0200

400600

8001000

12001400

16001800

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

True stress (MPa)

Prob

abili

ty D

ensi

ty D

istr

ibuti

on

Probability Density distribution of the yielded material for specimen heat at 300C for 120mins

ϴII =E/50

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Bauschinger Test

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07200

300

400

500

600

700

800

900

Pre Strain with 0.01% offset

Back

Str

ess,

MPa-0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

-2000

-1500

-1000

-500

0

500

1000

1500

2000

Strain

Stre

ss, M

Pa

2%01.0.%01.0.

RF

b

specimen heated at 300C for 120mins

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Stability of Retained Austenite

TEM image for 90 minutes at 300oC and cold-rolled to an equivalent strain of 0.3.

Wang, FGM McMaster, 2010 TRIP effect does not play a main role in work hardening of carbide free bainitic steel.

0 0.5 1 1.5 2 2.5-2

0

2

4

6

8

10volume fraction of retained austenite (%)

cold rolling strain

volu

me

frac

tion

of re

tain

ed a

uste

n-ite

(%)

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Macrostructure-banding

Banding structure due to Mn segregation during casting Bands of martensite with band width of 200um Increase hardenability (decrease potential of pearlite formation) Affect reproducibility of mechanical properties and transformation kinetics Homogenization procedure is not applicable to industrial production

Banding Structure Elements of Metallurgy and Engineering Alloys

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Summary

Work hardening

Good combination of strength and ductility

Micro-scale

structure (below 1um)

•Bainitic ferrite lath•Retained austenite film

Fracture

Flangeability

Reproducibility

Macro-scale

structure (above 100um)

•Banding structure

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Next Step

• Mainly bainitic ferrite + austenite

Target microstructure

• Decrease Mn content and adding other alloy elements (Ni, Cr, Mo, B) to maintain hardenability

Reduce banding structure

• Refine prior austenite grain size

• Adding alloy element (Co, Al, V)

Increase bainite

transformation kinetics

UTS: 1500 MPa

Uniform Elongation: 15%

Good flangeability

Good weldability (C<0.3wt%)

Mechanical

Properties

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Natural Science and Engineering Research Council of Canada ArcelorMittal Dr. Zurob Dr. Purdy Dr. Embury Dr. Brechet Dr. Olivier Xiang Wang Jim, Doug, Xiaogang

Acknowledgement

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Questions?

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Mn stabilize Austenite

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Kocks Mecking Model

ϴ

σ

Stage II

Stage III

ϴII =E/50