Mechanistic role of Mn heterogeneity in austenite decomposition and stabilization in a commercial quenching and partitioning steel

Manipulating chemical heterogeneity, which could substantially alter austenite decomposition behavior upon cooling, has recently proven to be very effective in tailoring retained austenite and architecting novel microstructures in steels. However, the mechanistic role of chemical heterogeneity in austenite decomposition and stabilization has not been fully understood, and its potential in microstructural architecting in commercial advanced high strength steels (AHSSs) has not been explored. In this study, we investigate the potential effects of Mn heterogeneity, which originates from the starting microstructure of Mn-lean ferrite and Mn-rich cementite, on austenite decomposition and stabilization in a commercial Quenching and Partitioning (Q&P980) steel. It is interestingly found that there is a strong kinetic interaction between the Mn enriched region in austenite and the migrating austenite/ferrite interface upon cooling, and phase field model predicts that the austenite/ferrite interface could migrate through the Mn-rich region via either a cutting through or by-passing manner, depending on the absolute Mn content of the Mn-rich region and the degree of Mn heterogeneity. The by-passing interaction could substantially promote C partitioning into the Mn-rich region, which further enhances the region`s stability. The current study demonstrates austenite decomposition and C partitioning behavior can even be altered by a weak Mn heterogeneity, which needs to be carefully considered in the future design of AHSSs.