Optimization of Inclusion and Microstructure of As-Cast Nonquenched and Tempered Steel F38MnVS with Trace Ce

Abstract

Various techniques, such as optical microscopy, high-temperature confocal laser scanning microscopy, scanning electron microscopy, electron backscatter diffraction, and thermodynamic calculations, are employed to analyze the optimization mechanism of Ce on the inclusions and microstructures in F38MnVS as-cast samples. The results show that the addition of Ce inhibits MnS precipitation at the grain boundaries and significantly reduces the diameters and aspect ratios of the inclusions. After Ce treatment, a new type of composite inclusion appears in the steel. The modification of the inclusions by Ce promotes the optimization of the as-cast microstructure during solidification, which mainly exhibits austenite grain refinement and polygonal ferrite formation in the grains. Based on the misfit degree theory, the refinement of the austenite grains is because of the nucleation of austenite grains facilitated by Ce₂O₂S and CeAlO₃. The increased nucleation of the intragranular polygonal ferrite is mainly attributed to Ce₂O₂S, Ce₂S₃, CeAlO₃, and VC, which can promote the nucleation of α-Fe. Simultaneously, the addition of Ce reduces the phase transition temperature, increases the driving force of the phase transition, and increases the nucleation rate. This study's results can provide a reference for the application of Ce treatment in the preparation of nonquenched tempered steel for automobiles.