The effect of CaO-Al

2

O

3

-SiO

2

slag ingredient on T[O

]

in 0. 42C-1.15Cr-0. 2Mo steel and sulphur partition ratio Ls- (S)/[S

]

at 1 873 K has been studied by experiment of MgO crucible in Si-Mo high temperature furnace. It was found that with increasing slag basicity- ( CaO + MgO)/SiO2 from 1 to 3

the T[O

]

dramatically decreased from 0.001 4% to 0. 001 0%

and with further increasing basicity

the change of T[O

]

was not obvious； as ( FeO + MnO) W0. 50%

the T[O

]

was WO. 001 0% and as (FeO + MnO) more than 0. 50%

the T[O

]

obviously increased to 0.001 4% ； the slag basicity

activity of Al

2

O

3

in slag and [ Al

]

had obvious influence on Ls

that was with increasing the activity of Al2O3 in slag

the Ls decreased and with increasing the slag basicity and [ Al

]

the Ls increased.