On the Low Temperature Strain Aging of Bainite in the TRIP Steel
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TRODUCTION
ADVANCED high strength steels (AHSS) with increased strength and equivalent or improved ductility are generally produced by introducing a hard phase such as martensite in the dual phase (DP) and bainite and retained austenite in the transformation-induced plasticity (TRIP) steel into polygonal ferrite matrix. The multiphase microstructure is usually generated from a standard cold rolling and intercritical annealing or thermomechanical processing (TMP). Typical compositions of steels are 0.08-0.12C, 0.13-1.07Si, 1.55-1.08Mn (wt pct) and 0.12-0.3C, 0.1-1.77Si, 1.3-1.7Mn (wt pct) for the DP and TRIP steel, respectively.[1–3] Both steels showed the continuous yielding behavior and high initial work-hardening rates due to the continuous transformation of the retained austenite to martensite in the TRIP steels and generation of the mobile dislocations in polygonal ferrite and martensite twinning in the DP steels during forming.[1–3] In industrial processing, these steels undergo a finishing treatment, where the paint coating of the automotive body is baked at the temperature of 423 K to 473 K (150 C to 200 C) for ILANA TIMOKHINA and HOSSEIN BELADI, Senior Research Academics, are with the Institute for Frontier Materials, GTP Research, Geelong Technology Precinct, Deakin University, Geelong Waurn Ponds Campus, Geelong, VIC 3217, Australia. Contact e-mail: [email protected] XIANG-YUAN XIONG, Senior Research Academic, is with the Centre for Electron Microscopy, Monash University, Building 81, Clayton, VIC 3800, Australia. PETER D. HODGSON, ARC Laureate Fellow, Alfred Deakin Professor, Director, is with the Institute for Technology Research and Innovation, Deakin University, Geelong, VIC 3217, Australia. Manuscript submitted February 14, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
20 to 30 minutes after forming.[4] The maximum increase of ~100 MPa has been achieved in the intercritically annealed TRIP and DP steels after prestraining to 10 pct in an interrupted tensile test to simulate the process of forming and paint baking of the car body.[3,5] The yield strength increase was also accompanied by a return of the yield point, slight increase in the tensile strength, and a decrease in elongation.[1–3,5] Several microstructural features have been assumed to be responsible for the observed bake-hardening response of intercritically annealed TRIP and DP multiphase steels: (i) formation of Cottrell atmospheres around the dislocations introduced in ferrite matrix, (ii) transformation of the retained austenite to martensite during pre-straining in the TRIP steels, (iii) formation of clusters/carbides in polygonal and bainitic ferrite during bake hardening due to the carbon diffusion in the TRIP steels, and (iv) tempering and/or twinning of martensite in the DP steels.[1–3,5] Our recent research on the thermomechanically processed 0.2C-1.5-1.6Mn-1.5-1.6Si-0.036Nb (wt pct) TRIP steel with ferrite, bainite, and retained austenite after pre-straining and bake hardening[6,7] showed the continuous yielding beh
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