Study on the behaviour of DS-Class inclusions in advanced bearing steel

The source and generated mechanisms of DS-size inclusions in bearing steel were studied by sampling systematically and using ASPEX, SEM and EDS to analyse the morphology, composition and amount of non-metallic inclusions larger than 13 µm. Two kinds of typical DS-size inclusions were found in the refining process: type I is CaO-MgO-Al2O3-SiO2 distributed evenly and wrapped by CaS; type II is composite inclusion with MgO-Al2O3 as the core, CaO-Al2O3 and CaO-SiO2 as the inner layer, covered by CaS. Based on the FactSage and thermodynamic calculations, the DS inclusions formation mechanisms were drawn. There are two formation mechanisms of the type II inclusions: one is the solid phases will precipitate from the matrix of type I inclusions as the temperature drops; another is that when the reaction time is not sufficient, the MgO∙Al2O3 spinel core will not be transformed completely, and the evolution can be summarized as: Al2O3 → MgO-Al2O3 → CaO-MgO-Al2O3 → surrounded by CaO-Al2O3(SiO2) → covered by CaS.