Passivated magnesium-calcium composite ball and application of passivated magnesium-calcium composite ball in grading method for reducing sulfide inclusion of heavy rail steel

The invention belongs to the technical field of passivated magnesium-calcium composite balls, and particularly relates to a passivated magnesium-calcium composite ball and application of the passivated magnesium-calcium composite ball in a grading method for reducing sulfide inclusions of heavy rail steel. Each passivated magnesium-calcium composite ball sequentially comprises a core layer, a weight increasing layer and a fireproof layer from inside to outside and is wrapped layer by layer. The core layer is spherical metal particles of 2-5mm, and the metal particles comprise the following components in percentage by mass: 24.0%-28.0% of magnesium, 25.0%-30.0% of calcium, 5.0%-10.0% of silicon, 5.0%-10.0% of clay and the balance of iron and inevitable impurities. The composite ball is used in an RH vacuum refining process, the yield of magnesium is 18%-29%, the yield of calcium is 20%-32%, low-melting-point calcium aluminate formed by combining MgO and CaO with Al2O3 is generated and can serve as nucleation c