Dissolution of TiO2 and TiN inclusions in CaO–SiO2–B2O3-based fluorine-free mold flux

Mold flux serves the crucial metallurgical function of absorbing inclusions, directly impacting the smoothness of the casting process as well as the cast slab quality. In this study, the dissolution behavior and mechanism of TiO2 and TiN inclusions in molten CaO–SiO2–B2O3-based fluorine-free mold flux were explored by in situ single hot thermocouple technology combined with X-ray photoelectron spectroscopy. The results showed that TiO2 inclusions are effectively dissolved by the molten slag within 76 s, during which the original octahedral [TiO6]8− structures are destroyed and convert to the networker tetrahedral [TiO4]4− structures. However, the dissolution rate is much lower for TiN inclusions than for TiO2 inclusions. This can be attributed to the fact that the TiN particles need to be oxidized and then dissolved in the molten slag to form tetrahedral [TiO4]4− and octahedral [TiO6]8− structures during the TiN inclusion dissolution process, which is accompanied by the generation of a large amount of N2 gas. Moreover, CaTiO3 crystals tend to nucleate and grow on bubble surfaces with sufficient octahedral [TiO6]8− structures and Ca2+ ions, eventually resulting in the molten slag being in a solid–liquid mixed state.