Effects of Stress on Martensite Transformation During Continuous Cooling and Mechanical Response of a Medium-Carbon High
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NTRODUCTION
MARTENSITIC transformation (MT), as characterized by its displacive shearlike nature, is one of the important means to strengthen steels.[1,2] In order to better understand the kinetics of MT, it is necessary to analyze the martensite start temperature (Ms).[3] The Ms is affected by many factors such as austenite grain size,[4–7] cooling rate,[8] and alloying element.[9,10] For example, Yang and Bhadeshia[4] established a suitable equation to describe the strongly dependent relationship between the Ms and austenite grain size. As austenite grain size decreases, both the Ms temperature and the amount of transformation product at room temperature decrease.[7] Nikravesh et al.[8] reported that the Ms decreases at lower cooling rate if only martensite transformation occurs. Capdevila et al.[10] revealed the influence of microalloying elements on the Ms and claimed that when the carbon concentration is lower (0.1 wt pct), the effect of microalloying elements on the Ms is small, while alloying elements have a greater MAN LIU, GUANG XU, GUANGHUI CHEN, and JUAN JIA are with The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, P.R. China. Contact email: [email protected] ZHENYE CHEN and ZILIU XIONG are with the HBIS Group Technology Research Institute, Hebei Iron and Steel Group Co., Ltd., Shijiazhuang 05000, P.R. China Manuscript submitted July 27, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
influence on the Ms if the carbon content is higher (0.8 wt pct). So far, many empirical relationships and models have been proposed to predict the Ms.[11–14] At the same time, the effects of deformation on MT were investigated by many researchers.[15–18] It is reported that the ausforming can decrease the Ms, refine martensitic microstructure, and slightly increase hardness of a medium-carbon Si-Al-rich alloy steel.[16] Other studies gave similar results. For example, Maalekian et al.[17] reported that the Ms of a high-carbon low-alloyed steel decreases by deformation of austenite. A decrease in the Ms is also observed by hot deformation of Cu-P-Cr-Ni-Mo weathering steels.[18] However, few studies focus on the effect of stress on MT during continuous cooling and the mechanical response of medium-carbon high-strength steels. It is certain that stress affects MT during continuous cooling.[19,20] Hence, the purpose of the present study is to provide the theoretical reference for the control of martensite transformation during continuous cooling and mechanical properties of medium-carbon high-strength steels.
II.
MATERIALS AND EXPERIMENTAL METHODS
The tested steel with a chemical composition of Fe-0.45C-2.03Si-2.81Mn (wt pct) was refined by a 50-kg vacuum furnace. High manganese was designed to stabilize austenite.[21] The refined ingots were hot
rolled on a 350-mm four-high mill to 12-mm plates. Cylindrical specimens of 6 mm in diameter and 86 mm in height for thermal simulation tests were prepared with hot-rolled plates. Thermal simulation experiments were cond
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