Sulfur segregation and inclusion modification in steel using magnesium addition

Abstract

In order to investigate the effect of magnesium treatment on the distribution and segregation of sulfides in carbon high-sulfur free-cutting steel in industrial tests, scanning electron microscope, energy dispersive spectrometer, electrolysis device, and other characterization methods are used to examine the sulfide morphology in the steel continuous casting billet. The sulfide type, spatial distribution and three-dimensional morphology of the slab in the casting slab are analyzed, and the law of sulfur segregation is calculated and summarized using the diffusion growth model. The results of the study showed that the area of cluster-like sulfides before and after magnesium modification decreased by 6.4%, the area of chain-like sulfides reduced by 6.6%, and the area of single-type sulfides increased by 12.7%. In the magnesium modified steel, a fine composite sulfide with MgO·Al2O3 as the core is formed, which provides a heterogeneous nucleation site for the later precipitated MnS, enhancing the shape and size of the sulfide. The sulfide segregation index was reduced from 1.30 to 1.04 before and after magnesium modification, and the sulfur concentration was 1.56 times the original precipitation when the middle position of the slab was entirely cemented. The macroscopic shrinkage cavity on the surface of the cast slab is minimized after magnesium treatment, which enhances sulfide dispersion and segregation dramatically.