The amount and distribution of inclusions in casting slab of ultra-low carbon steel ( ≤0. 01% C) processing in 210 t ladle with and without adding modifying agent (/% : 38 ~43Al

20~30Al

2

O

3

27 ~ 31 CaO

≤6SiO

2

≤6MgO) to ladle top slag have been examined and analyzed

and the effect of modifying agent on oxygen equilibrium between steel and slag and inclusions in casting slab is studied by thermodynamic analysis. Results show that the ingredient of refining slag sample for ladle top slag with

out adding modifying agent is (/% ) 25. 55 ~ 39. 68CaO

8. 51 ~15. 14SiO

2

6. 34 ~ 27.09MgO

5. 92 ~6. 54Al

2

O

3

17. 32 ~22

24FeO

3. 86 ~7. 35MnO

and ingredient of refining slag sample for ladle top slag with adding modifying agent is (/% ) 34.36 ~ 40. 43CaO

7. 69 ~11.47SiO

2

6. 42 ~7. 31MgO

8.31 ~ 25. 54Al

2

O

3

11.94 ~ 20. 78FeO

2. 17 ~ 2. 63MnO； with using ladle top slag modification treatment

the actual activity coefficient a

(FeO)

in slag is less than that in equilibrium with dissolved oxygen in steel led to oxygen diffusion from steel to slag through the interface of metal-slag

thus to lower the activity of oxygen in liquid

obviously improve the cleanliness of liquid and decrease the amount and size of inclusions in casting slab

and markedly reduce the nozzle clogging occurrence. Besides

although the decrease of a

(FeO)

in slag is small

whereas the activity of oxygen in liquid decreases significantly.