Effect of calcium in ferrosilicon alloys on inclusions in Al-killed steel

Large CaO‒Al₂O₃-type inclusions easily induce fatigue failure of bearing steels, so controlling large CaO‒Al₂O₃-type inclusions is the key to producing high-quality GCr15-bearing steel. Impurity elements in alloys added during refining, slag refining, and slag entrainment during the refining process are the main potential sources for forming large CaO‒Al₂O₃-type inclusions in Al-killed bearing steels. The ferrosilicon alloy is applied to improve the quenching and tempering softening resistance of Al-killed bearing steels. In this work, the effect of the calcium element in ferrosilicon alloys on inclusions in Al-killed bearing steel was studied through laboratory experiments, observations, and thermodynamic calculations. The ferrosilicon alloy mainly consists of the dark silicon and light ferrosilicon phases. The calcium element in ferrosilicon alloys exists as a metal compound at the interface between the silicon phase and the ferrosilicon phase. The total calcium (T.Ca) content in molten steel increases after adding the ferrosilicon alloy and modifying the Al₂O₃ and MgO·Al₂O₃ inclusions into CaO‒Al₂O₃-type ones. The size of inclusions in the molten steel decreases with an increase in the CaO content in the inclusions. Large CaO‒Al₂O₃-type inclusions are rarely generated. With the increase of the T.Ca in the molten steel, the T.O in the steel increases, whereas the average size of the inclusions decreases. It was observed that the calcium element in ferrosilicon alloys is not the direct cause for the formation of large size CaO‒Al₂O₃ inclusions in the steel. However, the generated small, solid CaO‒Al₂O₃-type inclusions clog the wall of the submerged entry nozzle, leading to dislodging of the big clogging lump into the molten steel. This lump is one of the reasons for large and irregularly shaped CaO‒Al₂O₃ type inclusions in the steel product.