The inclusion of the spinel, MgO · Al
2O
3, in type 304 stainless steel slab causes defect of surface, deterioration of mechanical property and so on. Thus, it is required to reduce the spinel inclusion. In this study, inclusions in molten type 304 stainless steel at each successive reactor, which were AOD, LF and tundish, were investigated in order to control the formation of the spinel in type 304 stainless steel slab. The formation of the spinel inclusions in the slab was discussed from the viewpoint of the effect of slag composition.
The Al
2O
3 content in inclusion increased in the course of production process from AOD to slab. Moreover, the Al
2O
3 content in inclusion in the slab increased with the increase of Al
2O
3 content in slag and %CaO/%SiO
2 ratio of slag. The following formation mechanism of inclusion is proposed based on these results. The origin of inclusion is formed by incorporation between the suspended AOD slag in molten steel and the deoxidation products that are SiO
2 and Al
2O
3. When the temperature of molten steel falls, the original inclusion works as a nucleation site, namely, Al
2O
3 as a secondary deoxidation product precipitates at the original inclusion. Accordingly, the Al
2O
3 content in the inclusion increases. Based on the formation mechanism of inclusion, the semi-empirical model was constructed. This model revealed that reduction of Al
2O
3 content in slag and %CaO/%SiO
2 ratio of slag was effective for eliminating the formation of harmful spinel inclusion.
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