Contamination of the Metal of a Large Ingot by Oxide, Sulfide, and Oxysulfide Inclusions

Resumo

Results are presented from a study of the contamination of a 24.2-ton ingot of the steel 38KhN3MFA by nonmetallic inclusions over its height and cross-section. It is shown that the pattern of contamination is different in each of these directions. Nonmetallic inclusions are compound oxide inclusions of manganese, silicon, vanadium, chromium, and aluminum, as well as sulfide and oxysulfide inclusions. The ingot was contaminated more by sulfides and oxysulfides than by oxides. It was established that the formation and morphology of the sulfide inclusions are complex phenomena which depend on aspects of the steel’s crystallization, the chemical composition of the melt, and its oxygen content. To obtain high-quality steel by vacuum degassing, it is necessary to ensure that the sulfur content is reduced to the lowest possible values. An inversely proportional relationship was found to exist between the impact toughness of the steel and the content of sulfides across the ingot. This finding is attributable to the accumulation of low-melting impurities ahead of the crystallization front and their displacement toward the central region of the ingot during solidification, which decreases impact toughness in this region.