Impact of intercritical annealing temperature and strain state on mechanical stability of retained austenite in medium Mn steel

Proper mechanical stability in retained austenite is a crucial factor for medium Mn steel to attain high strength and excellent ductility. In this paper, intercritical annealing and quenching and partitioning (IA & Q&P) processes are applied to medium Mn steel, to obtain retained austenite with proper mechanical stability. The effect of intercritical annealing temperature on mechanical stability of retained austenite is studied using interrupted tests. Moreover, from the perspective of usage of medium Mn steel, the mechanical stability of retained austenite at uniaxial tension and plane strain states is further investigated, to understand the transformation of retained austenite. Results indicate that high intercritical annealing temperature (680 °C) caused the austenite to transform immediately, due to the limited resistance of austenite to martensite transformation, caused by low carbon (C) content. This low C content also led to low stacking-fault energy, which may be another way to promote the transformation of austenite. Additionally, due to the high Schmid factor, caused by enhanced grain rotation and strain state, the retained austenite has relatively low stability at the early stage of the plane strain state. With increasing martensite surrounding the austenite, the retained austenite achieves high mechanical stability at the later stage of the plane strain state.