Martensitic Phase Transformation from Non-isothermally Deformed Austenite in High Strength Steel 22SiMn2TiB
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DUCTION
BORON-MANGANESE steels are of great interest for their potential in fabrication of high strength steel components for automobiles.[1–4] During the process of hot pressing (stamping), shape forming in the austenite region and simultaneous heat treatment are integrated into a single step, and the plastic deformation of austenite caused by shape forming is expected to have an effect on subsequent phase transformations and the final microstructure. In simulating studies of hot stamping, it has been reported that non-isothermal deformation in austenite of 22MnB5[1,2] and 22SiMn2TiB[5] steels promotes portion formation of ferrite or bainite prior to martensite under some specific circumstances with a lowered martensitic phase transformation temperature Ms. It is well understood that martensitic transformation is a diffusionless transformation, in which shape deformation is a major step to establish the crystal structure of martensite. Strengthening of austenite is anticipated to mechanically stabilize the austenite and decrease the Ms temperature.[6,7] The yield strength of austenite at the transformation temperature r0.2(Ms) is considered Z.M. SHI, Associate Professor, and JIAN LI, Professor, are with the School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luo Yu Road, Wuhan, Hubei 430074, P.R. China, and also with the College of Mechanical and Material Engineering, China Three Gorges University, Yichang 443002, P.R. China. Contact e-mail: [email protected], plumarrow@ 126.com B. CHI, Associate Professor, J. PU and Y.S. ZHANG, Professors, are with the School of Materials Science and Engineering, Huazhong University of Science and Technology. M.Q. WANG, J. SHI, and H. DONG, Senior Engineers, are with the National Engineering Research Center for Advanced Steel Technology, Central Iron and Steel Research Institute, Beijing 100081, P.R. China. Manuscript submitted November 12, 2012. Article published online May 8, 2013 4136—VOLUME 44A, SEPTEMBER 2013
the main resistance to martensitic phase transformation, acting as a part of the non-chemical free energy of the transformation for determination of the Ms temperature.[8] And, the driving force of martensitic transformation is derived to be proportional to the r0.2(Ms) of austenite, as reported in a study of Ni-Mn-Ga alloy.[9] However, the thermodynamics of martensitic phase transformation from pre-deformed austenite has not yet been fully understood. In the present study, a high strength 22SiMn2TiB was used to investigate the effect of non-isothermal compressive deformation of austenite on martensitic transformation, and the associated mechanism was explored thermodynamically by considering the deformation stored energy in the deformed austenite.
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EXPERIMENTAL
The 22SiMn2TiB steel contains 0.96 at. pct C, 1.71 at. pct Si, 1.64 at. pct Mn, 0.028 at. pct Ti, 0.0076 at. pct B, 0.0098 at. pct N, and Fe as the balance. Upon heating at 278 K (5 °C)/s, its Ac1 and Ac3 temperatures are 998 K and 1123 K (725 °C and 850°C), respect
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