Effect of Fluoride Ions in Slag on the Dynamic Change of the Interfacial Tension between Liquid Iron and Molten Slag

The interfacial tension between the liquid steel and molten slag is one of the key properties to control the entrapment of mold flux in molten steel in the continuous casting process. A dynamic change of the interfacial tension is observed when deoxidized iron and silicate slag are in contact, which can be explained by the oxygen absorption and desorption at the iron/slag interface. However, the dynamic change of the interfacial tension is influenced by other surfactant components of the molten iron and slag. Fluoride ions are fundamental component of mold flux, and recognized as the surface active component of molten slag. The effect of fluoride ions in slag on the interfacial tension has not been critically evaluated. Here, the effect of fluoride ions in slag on the interfacial tension between molten iron and molten silicate slag was evaluated at 1823 K, where the fluoride-containing slag compositions were designed to exhibit the same SiO₂ activity and slag viscosity as those of the fluoride-free slag. Compared with the case of molten iron and fluoride-free slag, the interfacial tension between the molten iron and fluoride-containing slag was initially lower. Except the effect of oxygen adsorption, fluoride ion was considered to directly decrease the interfacial tension. However, as the fluoride content in slag was higher, the interfacial tension tended to show the higher value at the final state. This behavior was attributed mainly to fluoride vaporization as SiF₄, which reduce the SiO₂ activity in slag and thus equivalent oxygen content at the iron/slag interface.