Enhancing austenite stability in a new medium-Mn steel by combining deep cryogenic treatment and intercritical annealing: An experimental and theoretical study

Abstract

We have systematically investigated the effect of deep cryogenic treatment (DCT) and intercritical annealing (IA) on the microstructural evolution and mechanical properties of a hot rolled medium-manganese transformation-induced plasticity (TRIP) steel. By intercritical annealing of DCT-treated steel in the fully martensitic state, ultra-fine-grained (UFG) austenite with different morphologies, namely lath-like and granular, was obtained. It is interesting that carbon segregation caused by DCT promoted the formation of carbide which can be additional nucleation sites for austenite besides the boundary of lath-martensite during austenite revision treatment (ART). Furthermore, the process of austenite growth was simulated by DICTRA and it revealed that austenite nucleated at the boundary of lath-martensite and carbide, and then grew into different morphologies, i.e. lath-like and globular, respectively. Moreover, the dissolution of cementite and finite diffusion distance of Mn limited austenite growth and partitioning of Mn, which resulted in refined grain size and high degree of Mn-enrichment. Consequently, enhanced mechanical-stability of austenite was obtained by DCT+IA, which ensured outstanding combination of mechanical properties, i.e. yield strength of 807 MPa, tensile strength of 1650 MPa and total elongation of 25.3%. The improved strength and ductility was attributed to stabilized austenite that enabled the moderate evolution of TRIP-related work hardening.