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

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

3. CFB

4. Carbide Free Bainitic Steel
Microstructure
Complex microstructure: bainitic ferrite + retained austenite + martensite
Nano-scale microstructure
Bainitic ferrite: 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
Caballero 2004

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

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

7. Tensile Test Results

8. Strength correlation with carbon content

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

10. Work Hardening Considere criterion: dσ/dε=σT CFB DP σ σT Necking point
σ-σY
dσT/dεT DP
CFB
Necking point σT εT
dσT/dεT
Necking point
UEI
UTS

11. Work-Hardening Behaviour
30 minutes
60 minutes
90 minutes
120 minutes
ϴII =E/50

12. Masing Model 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 14
element

13. Elasto-Plastic Transition
dσ/dε = f ϴII + (1-f) E
ϴII =E/50
the calculated fraction of the material which has yielded (f) for specimen heat at 300C for 120mins
Probability Density distribution of the yielded material for specimen heat at 300C for 120mins

14. Bauschinger Test
specimen heated at 300C for 120mins

15. Stability of Retained Austenite
TRIP effect does not play a main role in work hardening of carbide free bainitic steel.
Wang, FGM McMaster, 2010
TEM image for 90 minutes at 300oC and cold-rolled to an equivalent strain of 0.3.

16. Macrostructure-banding
Elements of Metallurgy and Engineering Alloys
Banding Structure
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

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

18. Mechanical Properties
Next Step
Mechanical Properties
UTS:
1500 MPa
Uniform Elongation: 15%
Good flangeability
Good weldability (C<0.3wt%)
Target microstructure
Mainly bainitic ferrite + austenite
Reduce banding structure
Decrease Mn content and adding other alloy elements (Ni, Cr, Mo, B) to maintain hardenability
Increase bainite transformation kinetics
Refine prior austenite grain size
Adding alloy element (Co, Al, V)

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

20. Questions?

21. Mn stabilize Austenite

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