During the casting process, powder or granular slag, called mold slag, should be continuously added to the molten steel surface of the mold. The role of mold slag has the following aspects: (1) heat insulation to prevent heat dissipation; (2) isolate the air to prevent the oxygen in the air from entering the molten steel for secondary oxidation, which affects the quality of the steel; (3) absorb and dissolve from the molten steel and float up Inclusions at the interface of the steel slag to purify the molten steel; (4) There is a layer of slag film between the mold wall and the solidification shell to lubricate, reduce the drawing resistance, and prevent the bonding between the solidification shell and the copper plate; (5) Fill the shell The air gap between the mold and the mold improves the heat transfer of the mold. A good mold slag should be able to give full play to the above five aspects, so as to improve the surface quality of the slab and ensure the smooth running of continuous casting.