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

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

CFB

Carbide Free Bainitic Steel Microstructure 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 Caballero 2004

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

Optical Microstructure 300CX30mins 300CX60mins 300CX90mins 300CX120mins

Tensile Test Results

Strength correlation with carbon content

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

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

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

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

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

Bauschinger Test specimen heated at 300C for 120mins

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.

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

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

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)

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

Questions?

Mn stabilize Austenite

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