Morphology Evolution of MnS Inclusions in Low‐Density Steel During Isothermal Heat Treatment and Its Influence on Microstructure and Tensile Properties

This study systematically investigates the morphology evolution of MnS inclusions in Fe–Mn–Al–C low‐density steel during isothermal heat treatment and its influence on microstructure and tensile properties. The evolution of MnS and AlN–MnS complex inclusions follows a sequential pathway of edge rounding, necking, splitting, and spheroidization. Elevated temperatures significantly accelerate this process, while the presence of AlN cores notably retards the evolution of external MnS. The final morphology is governed by the competition between splitting and Ostwald ripening, which is controlled by the initial aspect ratio of inclusions and heating parameters. Microstructurally, recrystallization nucleation is enhanced by the high dislocation density induced by MnS inclusions. Meanwhile, grain refinement is achieved through strong grain boundary pinning exerted by elongated inclusions, with maximum effectiveness observed along their short axes. This synergy between enhanced nucleation and selective pinning effectively refines the grain structure, resulting in superior strength of the steel. However, elongated MnS facilitates crack initiation, impairing ductility. After isothermal holding, their pinning effect is weakened due to the splitting and spheroidization of MnS inclusions, thereby promoting grain coarsening. Concurrently, the transition to a spherical morphology reduces stress concentration sites, which helps restore the elongation of the steel.