It is obtained by coexistence theory calculation and thermodynamics analysis that the slag-steel phosphorus partition ratio Lp of compound dephosphorizing slag CaO-MgO-FeO-Li

2

O-Na

2

O-K2O-SiO

2

-P

2

O

5

is obviously higher than that of basic dephosphorizing slag series CaO-MgO-FeO-SiO

2

-P

2

O

5

and defined that the optimum adding lepidolite period is the early stage of BOF blowing. The results of 10 heats 120 t BOF smelting ultra-low phosphorus steel show that as compared with the heats non-adding lepidolite

the semisteel [P

]

of heats by adding lepidolite decreases quickly to less than 0. 044%

the semi-steel dephosph

orization ratio and phosphorus partition ratio Lp respectively increase from 40. 62% and 13.13 to 67.71% and 36. 89

and the BOF average end dephosphorization ratio and phosphorus partition increase respectively from 92.35% and 130.7 to 94. 85% and 191.9

and the BOF end [P

]

can be controlled in 0.005% ~0. 007% with [P

]

percentage of hits 100%

so the compound slag series meet the requirement of ultra-low phosphorus steel production.