Cerium Addition Improves Cleanliness and Refines Microstructure of As‐Cast High‐Nitrogen Stainless Bearing Steel

High‐nitrogen stainless bearing steel (HNSBS) is the most representative third‐generation aviation bearing steel, and the cleanliness and microstructure characteristics of bearing steel are crucial to its high performance and long service life. In this work, the effect of cerium addition on the cleanliness and microstructure characteristics of as‐cast HNSBS was systematically studied by combining microstructural characterization and thermodynamic analysis methods. The results show that with increasing Ce content, the cleanliness of steel first improved and then decreased, and Ce 2 O 2 S, Ce 2 O 3 and CeN inclusions were generated successively. The Ce content should be controlled to ≈0.012 wt% to ensure that all Al 2 O 3 , MgO · Al 2 O 3 , and MnS inclusions were modified, and minimize the formation of large‐size Ce‐bearing inclusions. Furthermore, cerium addition refined the dendrite structure due to more nucleation sites for γ‐Fe and the enrichment of Ce in the liquid phase at solidifying front. The finer and more dispersed distribution in precipitates was ascribed to the reduction of growth space and Cr segregation, as well as the increase in effective nucleation sites. These results can offer theoretical guidance for inhibiting the formation of harmful inclusions in high‐nitrogen alloy systems and refining the microstructure of alloy systems containing M 23 (C, N) 6 and M 2 (C, N) precipitates.