Austenite Formation Kinetics of Dual-Phase Steels: Insights from a Mixed-Control Model Under Different Heating Conditions

A semi-analytical mixed-control model based on the Non-Partitioned Local Equilibrium (NPLE) assumption was developed to simulate the austenite phase transformation kinetics during heating and isothermal processes. The model was validated by comparing the simulation results with experimental data, showing excellent agreement. The effects of various model parameters and process conditions on the phase transformation kinetics was investigated. The results indicate that higher heating rates lead to an increase in the austenite volume fraction at the start of the isothermal hold, accelerating the transformation and resulting in a more complete phase transformation. The transformation during the isothermal stage was found to follow a mixed control mode at all investigated heating rates. Increasing the mobility coefficient enhances interface migration, thereby accelerating the transformation kinetics, while decreasing the grain size promotes nucleation, further accelerating the phase transformation. Modifying the diffusion coefficient had a minor effect on transformation kinetics. Additionally, raising the isothermal temperature increased both the austenite volume fraction at the beginning and end of the isothermal process and the interface migration velocity, suggesting that temperature dominates the phase transformation rather than time. The phase transformation mode under different process conditions was also investigated. For both 5 °C/s and 100 °C/s heating rates, the phase transformation during the isothermal process was predominantly interface-controlled, as indicated by the mixed-mode parameter approaching 1, with a rapid increase followed by a decrease.