In order to produce clean stainless steel, the inclusion softening technology and the large inclusion removal technology are important. Then, to elucidate the origin of inclusion in slabs and billets, experiments were conducted, adding a tracer to AOD slag and tundish slag. To control the composition of inclusions, a thermodynamic calculation model was developed. And, to elucidate the characteristics of floating separation of large inclusions in the tundish, tests using the water model were conducted. The following results were obtained:
(1) The inclusions in stainless steel were caused by the AOD slag suspended in the molten steel which is not removed in the tundish but carried over into the cast shapes.
(2) The thermodynamic calculation model in which the suspended slag is considered was developed. The composition of inclusions could be predicted by this model. In the actual process, the precipitation of MgO-Al₂O₃ spinel in inclusion was prevented by reducing the MgO and Al₂O₃ contents of the AOD slag.
(3) For the removal of large inclusions in the tundish, an increase in residence time is particularly effective. In the actual casting, the quality of stainless steel products were improved by controlling the weight of steel and the residence time in the tundish.