Effect of Refining-Slag Basicity on Precipitation Behavior of Spinel Inclusions in Al-Killed Steel

Spinel is known to cause clogging of the submerged entry nozzle in continuous casting. The effect of basicity on the precipitation behavior of MgO·Al₂O₃ in Al-killed steel with w[Al] = 0.065% at 1550°C was investigated. The inclusions in steel are mainly MgO·Al₂O₃ spinel after slag-steel reaction, which is consistent with the thermodynamic results. The polygonal spinel transforms to nearly spherical or rod-like with the sizes smaller than 5 μm with the increase of refining-slag basicity. The number of inclusions shows little variation with basicity. In industrial production, it is often observed that spinel inclusions in molten steel are transformed into CaO-Al₂O₃-MgO inclusions. However, this phenomenon was not observed in this study. Two sets of experiments were conducted; therefore, the reasons behind the lack of transformation from spinel to CaO-Al₂O₃-MgO inclusions were elucidated. We determined that Mg in MgO·Al₂O₃ originated from the MgO crucible, and the [Ca] and [Mg] supplied through the steel slag reaction were insufficient to facilitate the modification of Al₂O₃.