Abstract: Aiming at the problems of nozzle clogging and surface quality of high carbon steel produced by CSP process, the change of the cleanliness of molten steel and the morphology, the size and the compositions of inclusions in 51CrV4 steel during refining were studied by industrial experiments in “BOF-LF-thin slab casting” process. The results showed that mass fraction of T.O and [N] of molten steel in tundish were about 20×10-6 and 45×10-6 respectively with high basicity refining slag and process of soft argon blowing. The density of inclusion was 16.62/mm2，and the proportion with less than 10 μm was more than 97 %. The solid inclusions of Al2O3-MgO、SiO2-CaO-Al2O3 were modified into liquid compound inclusions of CaO-SiO2/MgO-Al2O3、CaO-MgO/SiO2-Al2O3-CaS by Ca treatment. The inclusion modification conditions with Ca treatment were analyzed by theory. When the mass fraction of [Al] was about 0.025 %, the mass fraction of [Ca] and [S] should be 0.001 % and less than 0.001 % respectively. And the inclusion of Al2O3 can be modified into liquid calcium aluminates.

Key words: refining, cleanliness of molten steel, inclusions, Ca treatment