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
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