The formation and transformation of inclusion in aluminium killed steel during refining have been studied with extra low oxygen steel steelmaking by 140 t LD - LF - RH - CC flow sheet. A sufficient of aluminium was added in test steel during tapping in order to as soon as decrease solution oxygen in liquid steel

and CaO-Al

2

O

3

refining slag series containing 3.00% ~ 8.42% SiO

2

were selected for Al

2

0

3

transforming to calcium aluminate inclusion. Test results showed that the transformation trend of inclusion in liquid steel during secondary refining was pure Al

2

O

3→

spinel→CaO - Al

2

O

3

-MgO complex inclusions

the tra

nsformation of inclusion in heat with slag containing 8. 42% SiO

2

was slower than that in heat with slag containing 3. 00% SiO

2

and as CaO/Al

2

O

3

of refining slag was 1. 60

most inclusions in steel transformed to low melting point CaO-Al

2

O

3

-MgO complex inclusions. The available ingredient of ladle refining slag is 55% - 60% CaO

35% -40% Al

2

O

3

5% ~10% MgO.