Mould flux or powder is used while casting steel from tundish in the mould. Mould flux or casting powder is applied to maintain and protect the surface of the molten metal. An effective mould powder provides chemical protection to molten steel from oxidation; and it also provides thermal insulation preventing solidification of steel surface. It lubricates the strand to the mould to provide uniform heat transfer between the strand and the mould wall. It absorbs harmful inclusions in the molten slag pool.

The mould powders are essentially of CaO-Al₂O₃-SiO₂ with addition of fluxing agents, carbon, alkali and some other glass formers and the basic oxides. In particular, carbon content adjusts the melting properties providing the system with suitable melting rate. It also prevents excess sintering of the powder layer, as it increases the thermal conductivity of the material.   

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