Simultaneous Determination of Transformation Plasticity Coefficients of Bainite and Martensite Transformations by Oil Quenching of Steel Sheet for Machine Structural Use and Accurate Prediction of Distortion by Oil Quenching

The dimensional accuracy of power train parts depends on the distortion caused by oil quenching. The parts are manufactured from steels for machine structural use, and the steel transforms from austenite to bainite and martensite during the quenching. For accurately predicting the distortion, it is important to know the accurate transformation plasticity coefficients of both bainite and martensite transformations. Transformation plasticity is a phenomenon caused by the stress due to transformation expansion, resulting in large plastic deformation of the steel. In this paper, a method was proposed to determine both the coefficients of bainite and martensite transformations by fitting the distortions in numerical analyses of the quenching process to oil quenching experiments. In this method, assuming that the transformation plasticity of both bainite and martensite transformations does not interact, two steel sheets with different thicknesses were used. In the previous study, by using this method for a thin sheet of Cr-Mo steel (consisting of martensite), the coefficient of martensite was determined as 21×10−5 MPa−1. Thereafter, the coefficient of bainite was determined as 11×10−5 MPa−1 by using a thick sheet (consisting of bainite and martensite). An experiment using a heat treatment oil with different cooling capacity was conducted to check the accuracy of the obtained coefficients. The numerical analysis results of cooling curve and microstructure were verified by comparing with the experimental results. The predicted deflection using the obtained coefficients was in good agreement with the experimental result. Thus, this method enabled to accurately predict the distortion caused by quenching.