Simple route to produce excellent mechanical properties of 9Mn steel with heterostructure induced by pre-existing austenite with sub-rapid solidification

Abstract

Medium-Mn steels have garnered significant interest due to their unique combination of high strength and exceptional ductility. However, a high Mn content inevitably causes the macro-segregation of Mn, which can significantly compromise the mechanical properties. This study effectively prevented the formation of Mn macro-segregation by employing the sub-rapid solidification. It resulted in the distribution of Mn micro-segregation bands within dendritic gaps in Fe-9Mn-2Si-0.5Al-0.1C steel. Mn micro-segregation bands results in a high proportion of pre-existing austenite (γpre) with small size and suitable stability in the as-cast strip, forming a lamellar structure with alternating distribution of lath martensite and austenite on the micrometer scale. The as-cast strip necessitates only basic simple hot rolling control without the need for any tempering treatment to achieve excellent mechanical properties comparable to conventional lengthy processes (ultimate tensile strength (UTS) of 1762 MPa, total elongation (TE) of 20 %, and product of strength and elongation (PSE) up to 35 GPa%). Utilizing this structure as a foundation, a simple austenite reverse tempering (ART) treatment can lead to additional enhancements in comprehensive mechanical characteristics with the UTS of 1529 MPa, TE of 25 % and PSE of 38 GPa%.