Austenite stability and its effect on the toughness of a high strength ultra-low carbon medium manganese steel plate

Abstract

A novel two-step intercritical annealing process was designed for an ultra-low carbon medium manganese steel plate. Excellent mechanical properties with yield strength of 590MPa, tensile strength of 840MPa, total elongation of 28.5% and high impact energy of 106J at −80°C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite together with a small amount of martensite after the two-step intercritical annealing. Both lath-like and blocky retained austenite with volume fraction of ~25% and relatively poor stability were obtained. The submicron-sized lath-like retained austenite exhibited Nishiyama-Wassermann (N-W) orientation relationship with the neighboring martensitic ferrite lath. The fine grain size played a crucial role in stabilizing austenite during phase transformation by significantly lowering Ms temperature and increasing the elastic strain energy. The overall stability of retained austenite during deformation was considered to be mainly governed by the chemical composition of the studied steel. The mechanism of toughening was elucidated. The superior low-temperature toughness was associated with TRIP effect of metastable retained austenite, which relieved the local stress concentration, enhanced the ability to plastic deformation and delayed the initiation and propagation of microcracks.