Effect of Mg addition on TiN inclusions in GCr15 bearing steel

Compared with the original GCr15 bearing steel, TiN inclusions are greatly reduced by the effect of Mg addition, and many different types of non-metallic Mg-containing inclusions were observed in Mg-treated GCr15 bearing steel which includes MgO, MgS·MnS, MgO–MgS·MnS, MgAl2O4–MgS·MnS, MgO–TiN, MgS·MnS–TiN, and MgO–MgS·MnS–TiN. The inclusion size distribution based on automatic inclusion analysis software shows that the number of inclusion with the size ranging from 1 to 3 μm increases obviously because a large amount of MgO, MgS·MnS, TiN inclusions containing Mg with smaller sizes are massively generated. In situ observation on the experimental steel by high-temperature confocal laser scanning microscopy confirms that MgO can provide nucleation sites for TiN. In addition, the MgS·MnS and MgO–MgS·MnS inclusions can also provide positions for the nucleation of TiN. Thermodynamic calculations indicate that after Mg addition to liquid steel, a large number of fine MgO inclusions are generated in the liquid steel because of the strong reactivity of Mg and O. At the same time, MgS precedes TiN precipitates in the solid–liquid two-phase region; thus, MgO and MgS·MnS can provide sites for TiN nucleation. At last, two possible formation pathways for the above various TiN inclusions containing Mg are discussed.