Stabilizing austenite in flash-annealed lean steel via laminate chemical heterogeneity

Retained austenite (RA) is especially valued in advanced high strength steels for its transformation-induced plasticity (TRIP) effect during deformation. However, the stabilization of RA in lean steels (C < 0.2, Mn < 2 wt%) is challenging due to the lack of austenite stabilization elements. In this study, a flash austenitization heat treatment approach was used to shorten the low-carbon TRIP steel processing time. This achieved a dual-phase microstructure composed of mass retained austenite and fine-grained ferrite in place of traditional bainite. Blocky, lamellar, and granular RAs were observed in the microstructure. Mn heterogeneity in the initial pearlitic cementite and ferrite was found to prevail in the retained austenite. Following the designed annealing route, phase-field simulations revealed that the ferrite transition kinetics accelerated in the Mn-depleted region which induced efficient carbon enrichment in austenite. The pronounced chemical heterogeneity of C and Mn co-resulted in the nonuniformity of the phase transition driving force from austenite to bainite and martensite, thus aiding the stabilization of austenite.

Automated summary

In this study, a flash austenitization heat treatment approach was used to achieve a dual-phase microstructure consisting of retained austenite and fine-grained ferrite in low-carbon TRIP steel. Phase-field simulations revealed the acceleration of ferrite transition kinetics in the Mn-depleted region and the influence of chemical heterogeneity of C and Mn on the stabilization of austenite.