metallurgy

1. Mechanical Properties of
Carbide Free Bainitic Steel
Xiaoxu Zhang
Supervisor: Dr. Zurob
Dr. Purdy
1

2. Environmental Issue +
Safety
Weight
Reduce
Higher
Strength
More Complicated
shape of part
Higher
Ductility
Motivation
2

3. 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 Steel
4
Microstructure
Caballero 2004

5. 5
Carbide Free Bainitic Steel
Heat Treatment Process Design
A3
Bainite
Fe-0.4%C-2.8%Mn-1.8%Si (mass%)
30% 80%

6. 300CX30mins 300CX60mins
300CX90mins 300CX120mins
Optical Microstructure
6

7. 7
Tensile Test Results

8. Strength correlation with carbon content
8
500
700
900
1100
1300
1500
1700
1900
2100
2300
2500
0 0.2 0.4 0.6 0.8 1
UTS
(MPa)
C wt%
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)

9. Comparison between CFB and DP steel
9
Bouaziz 2012
Caballero 2012
𝜎𝑦 = 𝜎𝑜 + 𝑘 𝑑−1/2
Scale Effect
DP
UTS and UEI of DP and CFB steel with same carbon content
CFB

10. Work Hardening
10
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

11. 0.00E+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
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Work
Hardening
Rate
True Stress (MPa)
11
Work-Hardening Behaviour
30 minutes
60 minutes
90 minutes
120 minutes
ϴII =E/50

12.  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

13. Elasto-Plastic Transition
13
dσ/dε = f ϴII + (1-f) E
0
0.2
0.4
0.6
0.8
1
1.2
0 500 1000 1500 2000
F
Stress (MPa)
the calculated fraction of the material which has
yielded (f) for specimen heat at 300C for 120mins
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0 500 1000 1500 2000
Probability
Density
Distribution
True stress (MPa)
Probability Density distribution of the yielded
material for specimen heat at 300C for 120mins
ϴII =E/50

14. 14
Bauschinger Test
200
300
400
500
600
700
800
900
0 0.02 0.04 0.06 0.08
Back
Stress,
MPa
Pre Strain with 0.01% offset
-2000
-1500
-1000
-500
0
500
1000
1500
2000
-0.02 0 0.02 0.04 0.06 0.08
Stress,
MPa
Strain
2
%
01
.
0
.
%
01
.
0
.
R
F
b





specimen heated at 300C for 120mins

15. 15
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.
-2
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5
volume
fraction
of
retained
austenite
(%)
cold rolling strain
volume fraction of retained austenite (%)

16. Macrostructure-banding
16
 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

17. 17
Summary
Work hardening
Good combination of strength and ductility
• Bainitic ferrite
lath
• Retained
austenite film
Micro-scale
structure
(below
1um)
Fracture
Flangeability
Reproducibility
• Banding
structure
Macro-scale
structure
(above
100um)

18. Next Step
18
•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

19.  Natural Science and Engineering Research Council of Canada
 ArcelorMittal
 Dr. Zurob
 Dr. Purdy
 Dr. Embury
 Dr. Brechet
 Dr. Olivier
 Xiang Wang
 Jim, Doug, Xiaogang
19
Acknowledgement

20. 20
Questions?

21. 21
Mn stabilize Austenite

22. Kocks Mecking Model
22
ϴ
σ
Stage II
Stage III
ϴII =E/50