Comparative interplay of C and Mn on austenite stabilization and low temperature impact toughness of low C medium Mn steels

To expand the use of medium Mn steels for the production of automotive and structural components intended for cryogenic applications, the present study focuses on attaining an enhanced low temperature impact toughness in two inter-critically annealed medium Mn steels with varying C and Mn contents without sacrificing the strength. In both 'low-C, high-Mn' and 'high-C, low-Mn' steels, the key role in regulating the impact toughness is mainly attributed to the volume fraction of stable retained austenite. Mn is found to stabilize austenite more effectively than C while allowing the attainment of near-equilibrium composition. Except for the 'low-C, high-Mn' steel, inter-critically annealed at lower temperature, Mn redistribution occurs in other samples, leading to a reduction of stable austenite fraction. The filmy shaped Mn enriched retained austenite is more thermally and mechanically stable in 'low-C, high-Mn' steel sample on inter-critical annealing at lower temperature than the other samples. The presence of fine dispersion of the filmy austenite at various substructural boundaries of 'low-C, high-Mn' steel after low temperature inter-critical annealing results in significant structural (Bain width) refinement and thereby, remarkably improves its impact toughness.