Kinetic models of multiple-stage martensite transformation and subsequent isothermal bainite formation excluding ε-carbide precipitation in intercritical quenching and partitioning steels

The kinetic models of multiple-stage martensite transformation and subsequent isothermal bainite formation were investigated in intercritical quenching and partitioning steels. The modified K-M model that considering instantaneous kinetic characteristics was established and can well describe the kinetics of respective stage of martensite transformation, which was explained as a synergistic result of transformation stress relaxation and austenite plastic accommodation. The selection and combination of martensite variants played a vital role in stress relaxation of stage-2 martensite transformation. The precipitation of ε carbide inside initial martensite was initiated during isothermal partitioning process, while cementite formation was entirely suppressed. Meanwhile, the ε carbide was confirmed to be significantly Mn-enriched, while no obvious difference in the distribution of Si element. Moreover, a modified model based on the dilatometric results excluding ε carbide precipitation was applied to describe isothermal bainite transformation kinetics and revealed that the autocatalytic effect is negligible in samples evaluated here. The accelerated effect of initial martensite on bainite transformation was ascribed to the pre-existed defect originated from martensite formation and was enhanced with the increase in initial martensite fraction. The bainite transformation kinetics with identical annealing condition and partitioning temperature evaluated here can be precisely predicted, irrespective of initial martensite fraction.