Abstract: Bearing steel is mainly used for manufacturing the rolling element, rings and other components of bearings. Control of number and size of Ds type inclusions is an effective way to prolong bearing life. Effects of the slags on the total oxygen contents and the formation of Ds type inclusions in bearing steel were investigated by vacuum induction furnace experiments. The optimal middle basicity slags for VD treatment was selected in the range the CaO to SiO2 mass fraction ratio of 3－4, and the CaO to Al2O3 ratio of 1.6－1.8,resulting in lower than 10×10-6 total oxygen mass fraction and less Ds inclusions. Evolution of inclusions in bearing steel during the practical LFVD treatment was observed. It was found that more calcium aluminates Ds inclusions appeared after VD treatment due to the intensive mixing and reacting of steel and slag, resulting in the increase of calcium content in molten steel and enhancement of CaO content in inclusions. The Ds inclusions observed in the finished products were classified into three categories: those that derived from the endogenous inclusions without silica in the center, those that were wrapped by MgO from the tundish covering flux and those that came from ladle slag with silica. Combining the experiments results and the observation of inclusions, the Ds inclusions could be controlled less by controlling the refining slag, depressing the entrapment of inclusions aggregated under the covering agent and ladle slag.

Key words: bearing steel, Ds type inclusion, VD, slag