Abstract: The equilibrium relationships of Ca-Al， Al-S and Ca-S were determined by thermodynamic calculation. The regional distribution of calcium aluminates was established on the basis of fixed sulfur content with the relationship of Ca-Al and Al-S as reference. The results indicate that liquid calcium aluminates can be generated with w_Ca▲T>0.001 7%~0.002 7% when w_Al▲T in X80 pipeline steel is 0.03%~0.06%， and Ca and S can form CaS by direct reaction when w_S>0.001 8%~0.002 8% correspondingly. The modification degree of Al₂O₃ inclusions has a great effect on the generation of CaS. With the increase of the S content in molten steel， the possibility of CaS formation at the edge of inclusion will increase. The agreement between thermodynamic calculation and actual production is proved by industrial test. Two kinds of CaS with different forming modes were found， and thus the rationality of predicted generating modes of CaS was proved.

Key words: thermodynamics, inclusion, calcium sulfide, pipeline steel, calcium treatment