Kinetic Model for the Phase Transformation of High-Strength Steel Under Arbitrary Cooling Conditions

To meet the demands of energy conservation and security improvement, high-strength steel (HSS) is widely used to produce safety-related automotive components. In addition to fully high-strength parts, HSS is also used to manufacture components with tailored properties. In this work, a computational model is presented to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths in HSS. First, a kinetic model for both diffusional and martensite transformations under isothermal or non-isothermal with constant cooling rate cooling conditions is proposed based on the well-known Johnson-Mehl-Avrami-Kolmogorov and Kamamoto models. The model is then modified for arbitrary cooling conditions through the introduction of the effects of the cooling rate, and the influence of diffusional transformations on martensite transformation is considered. Next, the detailed kinetics parameters are identified by fitting experimental data from BR1500HS steel. The model is further verified by several experiments conducted outside of the fit domain. The results obtained by calculation are found to be in good agreement with the corresponding experimental data, including the transformation histories, volume fraction microconstituents and Vickers hardness. Additionally, the model is also implemented as a subroutine in ABAQUS to simulate a tailored-strength hot stamping process of HSS, and the results are consistent with the test data. Thus, this computational model can be used as a guideline to design manufacturing processes that achieve the desired microstructure and material properties.