Transformation kinetics and density models of quenching and partitioning (Q&P) steels

Thermal dilatation during an isothermal partitioning process below the martensite start temperature (Ms) was characterized and modeled for quenching and partitioning (Q&P) steel sheet. The dilatations were attributed to changes in the phase composition and phase density due to austenitic decomposition and carbon partitioning. To develop a prediction model for the dilatation during the Q&P process, conventional kinetics models for non-diffusional and diffusional phase transformations were improved by introducing the evolution of the transformation rate parameters during the austenitic decomposition. Furthermore, a kinetics equation for carbon partitioning was also proposed after considering the effect of constituent martensite on the partitioning rate. Finally, variations of the phase densities in the partitioned phases were modeled to describe the effect of carbon partitioning on the dilatation. The proposed models were implemented in a finite element code, and simulated dilatometry tests could accurately capture the experimentally measured dilation behaviors under the Q&P process with single and 2-stage partitioning conditions.