(1169) Effect of CaO on Phosphate Capacity of Deoxidation Slag for Effective Utilisation of Phosphorus in Steel

摘要

Recently, a growing attention has been paid to utilisation of impurities in steel as a countermeasure of increasing amount of steel scrap and degradation of iron ore quality. As one of typical impurities, phosphorus would be a promising candidate for the utilisation due to its refinement effect of as-cast austenite grain size of steel by its segregation reported in addition to its conventional strengthening effect of steel. Hence, it is important to preserve phosphorus or control phosphorus content in molten steel during its refining or deoxidation process. In the present study, the phosphorus partition has been investigated between the MnO-SiO_2-FetO-CaO-MgO-P_2O_5 slag and molten iron to know the phosphorus absorbing ability of deoxidation by-product slag having CaO. The phosphate capacities (CPO_4^3-) for the present slag having CaO content from 4.73 to 19.6 mass % have been determined to be from 2.22×10^15 to 1.25×10^17 at temperatures from 1843 to 1923 K. Focused on the slag basicity, a thermodynamic discussion gives the capacity as a functio of optical basicity(Λ) and temperature(T) as follows; logCPO_4^3- = (11 300Λ + 54 560)/T + 18.30Λ －27.43 On the basis of this function, the enthalpy change for the dissolution reaction of P_2O_5 in the present system has been obtained as functions of optical basicity as follows; deltaH = －216.0Λ － 277.0 [kJ/mol] Estimated values of phosphorus distribution ratio ranges from 0.003 to 0.08 between the present slags and molten iron in practical operations according to empirical phosphate capacities. Consequently, CaO is useful to improve phosphate capacity of this system to some extent.