Role of retained austenite in improving the mechanical properties of 1.5 GPa-grade high-strength martensitic steels

Abstract

This study investigated the role of retained austenite (γ) in improving the mechanical properties of 1.5 GPa-grade ultra-high strength (UHS) martensitic steels from in situ neutron diffraction experiments during tensile deformation. The UHS martensitic steel with retained γ of about 10% had a tensile strength (TS) of 1550 MPa and uniform elongation (U.El) of 6%. The 0.2% proof strength (YS) was smaller but better U.El was obtained with the same TS compared to the UHS martensite single microstructure steel. γ phase was judged from the phase strains to have higher strength than the tempered martensite of the matrix phase (α). However, from a detailed investigation using lattice strain, the yield behavior of γ and α phases were different due to the heat treatment at 573 K. The mechanical stability of γ varied widely and the carbon content of γ increased with increasing strain. Because the difference in phase stress between γ and deformation-induced martensite (α′) phases is small, the strength of α′ alone hardly affects the work hardening of UHS martensitic steels. The more γ transforms to α′, the more effective it is for high TS and large U.El as a transformation-induced plasticity (TRIP) effect.