Abstract: In order to improve the cleanliness and castability of low carbon Al-killed steel for CSP production line of WISCO, the changes of composition and inclusions of molten steel with different calcium treatment were studied by industry trial. And the effects of calcium treatment were analyzed by thermodynamics. The results show that most inclusions are CaS-Al2O3-CaO with high mass fraction of CaS after calcium treatment. Then the inclusions become liquid Al2O3-CaO and CaS-Al2O3-CaO with low mass fraction of CaS in subsequent process. Thermodynamic study reveals that inclusions modification can be realized by low mass fraction of Ca, and the mass fraction of Al and S are the major factors influencing calcium treatment effects. So the mass fraction of Ca, Al and S should be considered together to realize inclusions modification during calcium treatment. And the inclusions in tundish should be liquid after calcium treatment.

Key words: low-carbon aluminium killed steel, calcium treatment, inclusions modification, CSP, thermodynamics