In order to examine the effects of calcium addition method on inclusion composition, the variations of the inclusion composition were measured during simultaneous addition of calcium alloy and CaO–Al₂O₃ flux to the molten steel by 180 kg scale experiments. Fifty four grams CaSi alloy mixed with 13.5 g CaO–Al₂O₃ flux was added 10 times at the interval of 1 min and the samples of molten steel were sampled during the experiments. The composition of the molten steel and the composition of the inclusions were analyzed. From the experiments and chemical reaction kinetic calculation, the effects of CaO–Al₂O₃ composition in flux on the changes in the inclusion composition were examined. The results obtained are summarized as follows;
(1) Calcium concentration in molten steel increased with increasing CaO concentration in CaO–Al₂O₃ flux.
(2) The CaO concentration in inclusion at which CaS concentration began to increase was changed by the composition of CaO–Al₂O₃ flux.
The results of the reaction rate calculation considering the reaction of calcium in molten steel and flux showed a good agreement with the experimental results.
(3) The reaction path of the variation of inclusion concentration was changed as a result of the reaction among calcium in molten steel, inclusion and flux.
(4) It was suggested that an inclusion composition could be optionally controlled by adding CaSi alloy and CaO–Al₂O₃ flux at the same time.