The contribution of retained martensite to the high yield strength and sustainable strain hardening of a hierarchical metastable austenitic stainless steel

A hierarchical metastable austenitic stainless steel (ASS) is obtained through severe asymmetric cold rolling (about 92% reduction) and reversion annealing. The microstructure evolution and tensile deformation behavior were studied to clarify the effect of hierarchical microstructure including reversed austenite with different grain size and retained martensite on the mechanical properties and strain hardening behavior. The results show that a hierarchical multiphase structure including austenite with a bimodal structure and retained martensite is achieved after annealing at 700 °C, and this sample has a good combination of desirable strength and ductility (the yield strength (YS), the ultimate tensile strength (UTS) and the total elongation (TE) were 823 MPa, 916 MPa and 43.82%, respectively). The introduction of retained martensite can not only enhance the YS, but also promote martensitic transformation of the surrounding nano grains (NGs) at large strains. Therefore, the hierarchical TRIP effect, which occurred firstly in the fine grains (FGs) in the early deformation stage and then in the NGs in the late deformation stage, contributes to the good ductility. Meanwhile, the multiplication of high dislocation density and the ultrafine ε martensite contribute to the high work hardening.