Abstract: In order to study the effects of metallurgical properties of slag at the first deslagging on the metallurgical effect of dephosphorization in the converter high-carbon smelting process，the single slag method was used to carry out the industrial test of high efficiency dephosphorization of single slag high draw carbon on a 50 t top-bottom blowing converter in a domestic steel plant. Through theoretical calculation analysis and combined with laboratory research, the relationship between the mineral phase structure, melting temperature, viscosity, composition and other metallurgical properties of the converter slag at the first deslagging and the converter dephosphorization effect was studied. The research results show that the mineral phase structure of Si, Ca and P enriched areas of slag at the first deslagging is mainly 2CaO·SiO₂ and 3CaO·P₂O₅, both of which are favorable to the dephosphorization reaction. The enriched areas of Fe, Mn and Mg elements mainly form RO phase and matrix phase, which are not favorable to the dephosphorization reaction. Reducing the melting temperature and viscosity of the single slagging high carbon tapping of slag at the first deslagging is conducive to the dephosphorization reaction. The melting temperature of slag at the first deslagging is controlled at 1 370-1 375 ℃, the viscosity of slag at the first deslagging is around 0.033 Pa·s, the dephosphorization effect is good, the lowest phosphorus mass fraction of molten steel at the first deslagging is 0.018 %, the phosphorus distribution ratio of slag at the first deslagging is 
greater than 60, the highest dephosphorization rate of the first deslagging is 84 %. Binary basicity R=2.60-3.00, w(FeO)=22.00 %-27.00 %，w(MgO)＜7.50 %，w(MnO)＜4.60 %,slag at the first deslagging has a good dephosphorization effect, the phosphorus distribution ratio of slag at the first deslagging is greater than 60, the dephosphorization rate of the first deslagging is no less than 75 %.

Key words: converter, single slagging, distribution ratio of phosphorous, metallurgical properties