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Kinetics of both slag/metal reactions and metal/inclusion reactions were investigated experimentally using 20 kg vacuum induction furnace in order to clarify the mechanism of the formation of MgAl 2 O 4 spinel inclusions in aluminum killed ferritic stainless steel (SUS430). The results obtained are summarized as follows : 1) By reducing CaO/SiO 2 and CaO/Al 2 O 3 ratio of top slag, MgO contents in Al 2 O 3 based inclusions decreased.2) The two film theory was employed to analyze the rate determining step of slag/metal reaction (reduction of MgO in top slag). By this model, it was found that the rate determining step of the reaction was the mass transfer of Mg through the film in molten steel. The increase rate of Mg in molten steel is determined by the activities of soluble oxygen and MgO at the slag/metal interface, and hence by slag composition.3) The unreacted core model was employed to analyze the rate determining step of metal/inclusions reaction.The analysis showed that the rate determining step of the reaction in the case of 20 kg vacuum induction furnace was the diffusion of Mg in molten steel.

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