Of tested Ti-IF steel (/%：≤0.0035C

≤0.03Si

0.08~0.20Mn

≤0.025P

≤0.015S

0.05〜0.07Ti

0.030~0.055Als

≤0.0040N) the BOF end [C

]

is 0.03%〜0.06%

end [O

]

is 0.0030%~0.0060%

the lime and top slag modification agent containing aluminium are added during lapping process

and the ingredient of RH end slag is/%: 53.38CaO

7.05FeO

1.01MnO

31.4Al

2

O

3

5.7MgO

0.3P

2

O

5

0.022S. The analysis results by sampling in RH refining process show that by adding top slag modification agent to control 8% (FeO+MnO) and CaO/Al

2

O

3

=1.7

it is available to remove the inclusions in steel；the oxygen content in liquid d

ecreases continuously during refining process

in casting slab the oxygen content is 0.0014% and the nitrogen content is 0.0015%；at finishing decarburization the main inclusions in steel is MnO

at finishing deoxidation by aluminium the main inclusions in steel is Al

2

O

3

and after alloying modified to Al

2

O

3

and Al-O-Ti compound inclusions；and in slab casting the above-mentioned inclusions are removed effectively but the TiN inclusions precipitated in slab solidification process.