Formation Mechanism of Oxide-Sulfide Complex Inclusions in High-Sulfur-Containing Steel Melts

The [S] content in resulfurized steel is controlled in the range of 200 to 800 ppm to ensure good machinability and workability. It is well known that “MgAl₂O₄(spinel)+CaS” complex inclusions are formed in molten steel during the ladle refining process, and these cause nozzle clogging during continuous casting. Thus, in the present study, the “Refractory-Slag-Metal-Inclusions (ReSMI)” multiphase reaction model was employed in conjunction with experiments to investigate the influence of slag composition and [S] content in the steel on the formation of oxide-sulfide complex inclusions. The critical [S] and [Al] contents necessary for the precipitation of CaS in the CaO-Al₂O₃-MgO-SiO₂ (CAMS) oxide inclusions were predicted from the composition of the liquid inclusions, as observed by scanning electron microscopy–electron dispersive spectrometry (SEM-EDS) and calculated using the ReSMI multiphase reaction model. The critical [S] content increases with increasing content of SiO₂ in the slag at a given [Al] content. Formation mechanisms for spinel+CaS and spinel+MnS complex inclusions were also proposed.