Effect of rare earth and magnesium complex treatment on inclusions in GCr15 bearing steel

Bearing steel has very strict requirements on the size, shape, and quantity of non-metallic inclusions. Even if the total oxygen content in steel is kept at very low levels, large inclusions are not completely removed. These large inclusions have a decisive effect on the fatigue life of bearing steel. To remove the large inclusions in the bearing steel as much as possible, the effect of rare earth and magnesium duplex treatment on inclusions in GCr15 bearing steel was investigated by adding moderate rare earth and magnesium to liquid steel under experimental conditions. The size, composition, and morphology of the inclusions were observed by combining Aspex inclusion automatic analysis technology and scanning electron microscope. The experimental results show that the inclusions in steel before modification are mainly composed of MnS-Al₂O₃, MnS, and Al₂O₃, and the inclusions are modified to be composed of a large number of compound inclusions containing sulfur and magnesium and a small amount of Al₂O₃ and magnesia alumina spinel after adding trace magnesium to steel. After complex treatment by rare earth and magnesium, the inclusions are mainly composed of Re-O-S. Al₂O₃, MnS, and magnesia alumina spinel vanish gradually. The inclusions are spherically distributed, and most of them have diameter less than 5 μm. Inclusions with diameters greater than 10 μm are greatly reduced. Thus, the inclusions in GCr15 bearing steel are obviously refined after rare earth and magnesium complex treatment. When the magnesium content in the steel remains unchanged, the proportion of large particle inclusions decreases with increasing content of rare earth. When the content of rare earth is similar, increasing the magnesium content in steel is beneficial to the removal of large particle inclusions. The interaction of rare earth and magnesium further promotes the refinement of inclusions.