Abstract: In the current paper, the development of classical thermodynamic calculation for the accurate calcium addition during calcium treatment of molten steel were briefly summarized. The calculation of the accurate addition amount of calcium alloys was calculated using thermodynamic software of FactSage. The purpose of calcium treatment is to modify undeformable solid inclusions, especially  Al₂O₃ based ones into deformable liquid inclusions, especially liquid calciumaluminates. The steel composition, especially the content of the total oxygen, dissolved aluminum and sulfur, and the temperature have great influence on the addition amount of calcium. After addition of calcium into the molten steel, many kinds of inclusions are generated, such as CaO·6Al₂O₃, CaO·2Al₂O₃, 12CaO·7Al₂O₃, 3CaO·Al₂O₃, CaO and CaS. Only 12CaO·7Al₂O₃ and 3CaO·Al₂O₃ are liquid at the temperature of the molten steel. With the increasing addition of calcium, many liquid and solid inclusions may simultaneously generated. The addition of calcium must satisfy that all inclusions are liquid and no solid CaS inclusions generated. Hence, there is a "liquid window" with minimum and maximum addition of calcium alloys. The minimum addition is to satisfy the vanishing of the solid inclusions and the maximum addition is to assure no generation of pure solid CaS inclusions. In the current study, the accurate addition of CaAl alloy wires, with a 1∶1 mass ratio of  Ca∶Al, for the LF refining process of a steel plant was calculated. At 1 571 ℃, in order to assure that all inclusions are liquid and no pure CaS inclusions are generated, the calcium content in the molten steel should be (22.6－ 47.4)×10^-6. If the yield of the alloy is 30 %, the length of the added wire should be 197－334 m. In the current industrial production, the addition is 350 m, slightly longer than the calculated value. It revealed that inclusions in the steel samples contained a slightly high amount of CaO, which agree well with the calculation. The current model for the accurate calcium addition of calciumalloys can be used for the real production.

Key words: calcium treatment, thermodynamics, FactSage, modification of inclusions