Enhanced ductility of as-quenched martensite by highly stable nano-sized austenite

Austenite plays a key role to improve tensile properties of advanced high strength steel (AHSS). In this study, nano-sized austenite particle was produced in as-quenched martensite by using chemically heterogeneous initial microstructure consisting of Mn-enriched cementite and ferritic matrix. Mn-enriched cementite transforms into nano-sized austenite during austenitzation and considerable amount of them retain after cooling to ambient temperature. The austenite particles have an exceptional stability, which cannot be fully interpreted with the Mn enrichment. Possible contributions from the compressive stress and the C partitioning into the austenite are considered. A persistent TRIP effect by highly stable austenite contributes to remarkable improvement of tensile ductility without compromising tensile strength even in the as-quenched martensite.