Abstract
The stability of retained austenite (RA) plays a critical role in governing the mechanical performance of medium-Mn advanced high-strength steels (AHSS) through the transformation-induced plasticity (TRIP) effect. In this study, the effect of IA temperature on the evolution, stability and deformation behavior of RA in a medium-Mn (Fe–0.18C-6.5Mn–1.5Al–1.2Si–0.05Nb) steel was systematically studied. Intercritical annealing of steel at three different temperatures, 630 °C, 650 °C and 670 °C, followed by water quenching, were employed and the resulting steel was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and mechanical testing. The higher the IA temperature, the more austenite reverted and the larger the amount of RA, but too high an annealing temperature resulted in coarser RA block and lower mechanical stability. The sample annealed at 650 °C had the most refined film-like RA morphology and most stable, resulting in the most favorable TRIP-assisted deformation initiation condition. The condition of 670 °C exhibited the highest fraction of RA, but the stability was compromised due to coarsening of the morphology and the loss of the geometrical constraint. These results illustrate that, optimizing the morphology and stability of RA instead of the volume fraction of RA is important to design the medium-Mn AHSS with better mechanical properties.