Formation Mechanism of Inclusions Containing MgO · Al₂O₃ Spinel in Type 304 Stainless Steel

Abstract

Inclusions in the commercial SUS304 slabs that differed in the aluminum content were observed to clarify the formation mechanism of spinel, MgO · Al₂O₃, in the stainless steels. In addition, the formation of spinel in the synthetic CaO-SiO₂-Al₂O₃-MgO inclusions was studied. In the experiment on the synthetic inclusion, its melt was cooled at the same cooling rate at which the molten steel was cooled in CC. It was found that the spinel could precipitate from the CaO-SiO₂-Al₂O₃-MgO melt when the composition of the synthetic inclusion was in a primary phase region of spinel. In the observation of inclusions in SUS304 slabs, the formation of spinel was influenced by the content of aluminum in the slabs. In the slab that contained 19 ppm of Al, the spinel inclusions were formed by the reaction of Al, Mg and O that were dissolved in the molten stainless steel. In the slab with 8 ppm of Al, the spinel inclusions were formed by the precipitation from the molten CaO-SiO₂-Al₂O₃-MgO inclusions. In the slab with 6 ppm of Al, the spinel inclusion was not formed. These results revealed that spinel formed not only when the composition of steel was in the region of spinel formation in the phase stability diagram of the system of Al, Mg, Ca, Si and O in molten steel, but also when the composition of CaO-SiO₂-Al₂O₃-MgO inclusion was in the area of primary phase of spinel in the phase equilibrium diagram of the system of CaO, SiO₂, Al₂O₃ and MgO.