Effects of intercritical annealing time on microstructure, tensile properties, and cryogenic toughness of newly designed ultra-low C low Ni Medium-Mn steel

Abstract

An ultra-low carbon and low nickel medium manganese heavy steel plates was fabricated by quenching and intercritical annealing process. The effect of annealing time on the tensile properties and impact toughness were studied in relation to the formation of reversed transformation. The results show that excellent mechanical properties with yield strength of 680 MPa, tensile strength of 871 MPa, total elongation of 38.2% and high impact energy of 187 J at −60 °C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite (RA) together with a small amount of martensite after intercritical annealing. Increasing the intercritical annealing time resulted in coarser matrix microstructures, larger and more voluminous retained austenite, but with notably reduced stability. Highly stable RA enhances the work hardening capacity of the steel via sustained deformation-induced transformation (TRIP effect), significantly contributing to the toughening of the multiphase system. In addition, Austenite stability, rather than large-angle grain boundaries and the amount of austenite transformation, is the primary factor determining the low-temperature toughness of medium manganese steel.