Significance of epitaxial ferrite formation on phase transformation kinetics in quenching and partitioning steels: modeling and experiment

Experimental and simulation methods were applied to analyze the transformation kinetics of epitaxial ferrite (EF) formed during a continuous cooling process and relevant carbon content heterogeneity in a low-carbon quenching and partitioning steel, with an emphasis on the influence of austenite carbon heterogeneity issued from EF formation on subsequent martensite and bainite transformation behaviors. It revealed that EF transformation possessed a kinetic curve with sigmoid shape and accelerated with decreasing cooling rate. With EF/ γ interface under negligible partition local equilibrium condition in Dictra simulation, the simulation EF transformation kinetics can reproduce the experimental results well and the partial inheritance of Mn and Si from austenite into EF was also predicted. Furthermore, martensite transformation behaviors (one-stage or two-stage transformation) significantly depended on cooling rate and were explained by the austenite carbon heterogeneity issued from EF formation. The increase in initial martensite fraction with elevating cooling rate accelerated the subsequent bainite transformation and was favorable to austenite retention as well.