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

Mold flux serves a crucial metallurgical function of absorbing inclusions, which has a direct impact on the smooth of casting process and the casted 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 the rutile TiO₂ inclusion was effectively dissolved by the molten slag within 76 s, which the original [TiO₆] octahedral structure was destroyed and converted to the network former [TiO₄] tetrahedral. However, the dissolution rate of TiN inclusion was significantly lower than that of the TiO₂ inclusion. This was primarily due to that, during the dissolution process of TiN inclusion, the TiN particle needed to be first oxidized and then dissolved in the molten slag into [TiO₄] tetrahedral and [TiO₆] octahedral structures, accompanied by the generation of a large amount of N₂ gas. Besides, CaTiO₃ crystals tended to nucleate and grow on the surface of the bubbles with sufficient [TiO₆] octahedral and Ca²⁺ ions, resulting in the molten slag to be in a solid-liquid mixed state eventually.