Influence Mechanism of Crucible Materials on Cleanliness and Inclusion Characteristics of High-Nitrogen Stainless Bearing Steel During Vacuum Carbon Deoxidation

High cleanliness is an important guarantee for the long-service of bearing steel. In this study, the effect of crucible materials on cleanliness and inclusion characteristics of high-nitrogen stainless bearing steel (HNSBS) during vacuum carbon deoxidation was comprehensively investigated by microstructure characterization and thermodynamic analysis. The results showed that the ultimate O contents using MgO·Al2O3, MgO, and ZrO2 (MA, M, and Z) crucibles could be decreased from about 0.0060 to 0.0028, 0.0012, and 0.0022 wt pct, respectively. The ranking of crucible thermodynamic stability during vacuum carbon deoxidation was M < MA < Z, representing the decreasing of oxygen transfer rate due to crucible decomposition. The deoxidation rate of carbon–oxygen reaction increased with the decreasing of CO partial pressure, and the ranking of deoxidation rate using various crucibles was Z < MA < M. Meanwhile, the main oxide inclusions in steel using MA, M, and Z crucibles transformed from Al2O3 to MgO·Al2O3, MgO, and ZrO2, respectively. The area and average size of inclusions in steel using M crucible were smaller than the others owing to the decreasing of large-size Al2O3 inclusions and the increasing of percentage of low-density Mg-containing inclusions, while the existence of high-density ZrO2 inclusions in steel using Z crucible restricted the floating and removal of inclusions. Therefore, MgO crucible was more appropriate to melt high-cleanliness HNSBS with lower O content and fewer deleterious inclusions.