Effects of different intercritical annealing processes on microstructure and cryogenic toughness of newly designed medium-Mn and low-Ni steel

Through the idea of replacing Ni with Mn, a shortened process of ultra-fast cooling and two-step intercritical annealing for medium-Mn and low-Ni steel after rolling was designed. Microstructures were characterized through optical microscopy, electron probe micro analyzer, scanning electron microscopy, electron backscattered diffraction, and X-ray diffractometer. Furthermore, the effects of different intercritical annealing temperatures on austenite including content, stability, and the impact toughness of experimental steel were investigated. The microstructures of the experimental steel comprised retained austenite (RA), fine-grained ferrite, and part of martensite. The results show that fine grain structure and austenite with high stability enriched by Mn and Ni provided excellent mechanical properties for experimental steel with yield strength of 645 MPa, tensile strength of 745 MPa, total elongation of 32%, and high impact energy of 133 J at −150 °C. As the intercritical annealing temperature increased, the microstructures of the matrix coarsened, and the volume fraction and size of RA increased, while the stability decreased significantly. RA with high stability provided long-lasting work hardening ability through continuous deformation-induced transformation (TRIP effect) during the deformation of steel, which played an important role in toughening the multiphase system.