Effect of pre-existed austenite on austenite reversion and mechanical behavior of an Fe-0.2C-8Mn-2Al medium Mn steel

Abstract

A Quenching-Austenite Reversion Treatment (Q-ART) is used to process an Fe-0.2C-8Mn-2Al (all in wt.%) medium Mn steel to investigate the effect of pre-existed austenite on austenite reversion and mechanical properties. The steel is firstly quenched to a certain temperature between martensite start and finish temperature to form the mixture of primary martensite matrix + pre-existed austenite, and then it is reheated to the intercritical region for the austenite reversion. It is interestingly found that the kinetics of austenite reversion is accelerated with a small amount of pre-existed austenite but decelerated when the fraction of pre-existed austenite is higher than 10%, and the fraction of austenite after ART in the cases containing pre-existed austenite could be even higher than the equilibrium value. Different from the conventional ART sample, fresh martensite as well as three kinds of retained austenite formed at different locations is present in the Q-ART processed samples. Such fresh martensite and heterogeneous retained austenite are found to play a significant role in the work hardening behavior and mechanical properties of the Q-ART processed medium Mn steels. Compared with the conventional ART, the ultimate tensile strength of the Q-ART sample with a small amount of fresh martensite is increased by around 200 MPa with a marginal ductility loss, as its initial work hardening is promoted due to the fast strain induced martensitic transformation kinetics. However, excessive fresh martensite significantly deteriorates the ductility.