The effect of ingredient of LF refining slag (/% : 37. 5 ~54. 8CaO

9. 8 ~ 18. 2SiO

2

20 ~ 30Al

2

O

3

4 ~ 10MgO

3 ~ 10CaF

2

) with basicity 3 ~5 on viscosity has been tested and studied. Results show that the effect of CaF

2

and A1

2

O

3

on viscosity of refining slag is great and the effect of basicity and MgO on viscosity of refining slag is rather minor. With increasing CaF

2

the viscosity of refining slag decreases first and then increases

with increasing A1

2

O

3

content in slag the viscosity of slag

gradually decreases ； with Al

2

0

3

content in slag being 20% and CaF

2

≥ 6% or A1

2

O

3

content being 25% and CaF

2 ≥

3%

the viscosity value of slag at 1 500 °C is less than 0. 5 Pa • s; and it is obtained by test that better basicity R and ingredient of refining slag for viscosity is 4 ~ 5R

25% ~30% Al

2

O

3

6% 〜10% MgO

3% 〜6% CaF

2

. The results of production pilot of 100 t LF refining steel TC80 show that after refining slag optimization the sulfur content in liquid decreases to less than 0. 005% from initial 0. 020%

the desulphurization ratio increases to 84% from before optimization 72% ； and the average T [O

]

at end refining is 14 x 10

-6

.