Ultrafine lamellar microstructure with greatly enhanced strength and ductility of martensite steel via heavily warm rolling of metastable austenite

Abstract

Ultrafine lamellar-structured (UFLS) martensite microstructure was fabricated by heavily warm rolling (HWR) of a metastable austenite steel with 93% deformation reduction and subsequent quenching. Microstructure characterization demonstrates that the martensite microstructure after HWR is mainly characterized by ultrafine lamellar comprising of only aligned laths with reduced martensite variants, which is intrinsically different from the hierarchical martensite microstructure in conventional hot-rolling (HR) martensite steels where the basic unit of martensite consists of packets, blocks and laths with 24 crystallographically equivalent variants. Three kinds of typical martensite morphologies were observed. Type I: UFLS martensite with lamellar size of less than 200 nm, which is composed of the blocky martensitic lath unit with only one variant; Type II: UFLS martensite with lamellar size between 200 and 500 nm, which is composed of several irregular martensite lath units with mainly two variants; Type III: UFLS martensite with lamellar size more than 500 nm, which is composed of traditional martensite blocks and laths with six variants. Such martensite microstructures and crystallographic characteristics were well explained by the nucleation-growth mechanism controlled by the elastic strain energy of single variant martensite transformation. Compared to the HR martensite steel, the current UFLS martensitic steel exhibits an excellent strength-ductility combination. This is primarily attributed to the refinement of martensitic microstructures inherited from the refined prior lamellar metastable austenite by HWR.