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Mechanical Properties of Carbide Free Bainitic Steel. Xiaoxu Zhang Supervisor: Dr. Zurob Dr. Purdy. Motivation. CFB. Carbide Free Bainitic Steel. Microstructure. Caballero 2004. Complex microstructure : bainitic ferrite + retained austenite + martensite Nano-scale microstructure - PowerPoint PPT Presentation
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1
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
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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
)
9
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
20
Questions?
21
Mn stabilize Austenite
22
Kocks Mecking Model
ϴ
σ
Stage II
Stage III
ϴII =E/50