Difference in transformation behavior between ferrite and austenite formations in medium manganese steel

The difference in transformation behavior between the γ → α and α′ → γ transformations at the transition temperature T0 was investigated in 0.1%C–5%Mn steel. The γ → α transformation was very slow, and so little ferrite phase formed at T0, even after a lengthy annealing time of 50 h. However, the α′ → γ transformation was relatively fast and rapidly proceeded at T0. In the dual-phase structure formed by the α′ → γ transformation, thin film-shaped reversed austenite grains with sufficient thermal stability formed along the lath boundaries of the tempered martensite matrix. The simulation using diffusion-controlled transformations (DICTRA) software confirmed that the rapid austenite formation in medium manganese steel is attributed to: (1) the high density of austenite nucleation sites derived from the hierarchical lath martensitic structure; (2) the negligible-partitioning local equilibrium growth controlled by very fast carbon diffusion at the initial stage; and (3) the relatively higher diffusivity of Mn within the martensite matrix under the partitioning local equilibrium growth that occurred at a subsequent stage.