Chemical reaction for reducing phosphorus content in molten steel. Phosphorus is one of the harmful impurities in steel. Steel containing more phosphorus is easy to be brittle when used at room temperature or lower, which is called "cold brittleness". The higher the carbon content in steel, the more serious the brittleness caused by phosphorus. Generally, the phosphorus content in ordinary steel does not exceed 0.045%, and high-quality steel requires less phosphorus. Phosphorus in pig iron mainly comes from phosphate in iron ore. The thermodynamic stability of phosphorus oxide and iron oxide is similar. Under the reduction condition of blast furnace, almost all phosphorus in the burden is reduced and dissolved in molten iron. If phosphorus compounds cannot be removed by mineral processing, dephosphorization can only be carried out outside the furnace or in the alkaline steelmaking furnace.

Understanding and solving the problem of dephosphorization in iron is of special significance in the history of iron and steel production development. The large-scale industrial production of steel began with the acid converter steelmaking method invented by H.Bessemer in 1856. But acid converter steelmaking can't dephosphorize; The lack of low-phosphorus iron ore has seriously hindered the development of steel production. 1879, S.Thomas invented the basic converter steelmaking process, which can treat high phosphorus molten iron. Then the principle of dephosphorization of alkaline slag is extended to open hearth steelmaking, so that a large number of phosphorus-containing iron ores can be used for steelmaking, which has made great contributions to the development of modern iron and steel industry.

The dephosphorization reaction of alkaline slag is carried out at the interface between slag and phosphorus-containing molten iron. The reaction between phosphorus P and oxygen O in molten steel is gaseous P2O5: tapping operation when the temperature and composition of molten steel meet the requirements of specified steelmaking varieties. Pay attention to prevent slag from flowing into ladle when tapping. Additives used to adjust the temperature, composition and deoxidation of molten steel are added to ladle or tapping flow when tapping, which is also called deoxidation alloying.