Low-basicity fluorine-containing mold flux, which forms a porous slag film with NaF pores, is expected to solve the contradiction between heat transfer and lubrication in the continuous casting process of peritectic steel. Herein, slag films were obtained by slag film heat flow simulator, and the thermal control of NaF pores in the slag film was evaluated by COMSOL. Based on the influence of fluorides on NaF gas generation, mold fluxes consisting of NaF, CaF 2 , and Na 3 AlF 6 were investigated separately at a basicity of 0.9 and a fluorine content of 6 wt%. The comprehensive heat control ability of these slag films ranked in the order: Na 3 AlF 6 > CaF 2 > NaF. Firstly, the slag film using NaF raw material exhibited larger NaF pores (30–50 μm), resulting in higher radiative transmittance and reduced thermal resistance. Its heat control is primarily dominated by the thickness of the film. Further, using CaF 2 raw material, the heat control ability and pore size were between NaF and Na 3 AlF 6 . Its slag film exhibited the highest surface roughness (27.32 μm), with interfacial thermal resistance playing a dominant role. With Na 3 AlF 6 as the fluorine source, the slag film exhibited finer and more uniformly distributed pores (10–30 μm), which enhanced reflectivity and reduced radiative heat transfer. Therefore, this study highlights that Na 3 AlF 6 as a raw material is conducive to low-basicity mold fluxes achieving thermal control through finely dispersed NaF pores.
The reverse gradient distributions of temperature and oxygen concentration in the three-layers structure of mold flux at the upper part of the continuous casting mold are easy to cause the insufficient burning of carbonaceous materials of mold flux, resulting in the generation of carbon-rich layer and too slow melting of mold flux. Through a melting behavior experiment, the addition of a small amount of iron oxide into mold flux was found to promote the melting of mold flux significantly. The calculations of the comprehensive combustion index and combustion kinetic based on thermogravimetric experiments proved that iron oxide facilitated the burning of carbonaceous materials and the melting of mold flux by decreasing the activation energy of the combustion reaction. The catalytic effect of iron oxide on the combustion of carbonaceous materials of mold flux in the high-temperature and oxygen-poor zone of the slag layer was realized through the oxidation–reduction cycle reactions of iron oxides with different valence states. The above findings provide a brand new approach to solve the carbon-rich predicament of mold flux caused by the incomplete burning of carbonaceous materials.
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 TiO 2 and TiN inclusions in molten CaO–SiO 2– B 2 O 3 -based fluorine-free mold flux were explored by in situ single hot thermocouple technology combined with X-ray photoelectron spectroscopy. The results showed that TiO 2 inclusions are effectively dissolved by the molten slag within 76 s, during which the original octahedral [TiO 6 ] 8− structures are destroyed and convert to the networker tetrahedral [TiO 4 ] 4− structures. However, the dissolution rate is much lower for TiN inclusions than for TiO 2 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 [TiO 4 ] 4− and octahedral [TiO 6 ] 8− structures during the TiN inclusion dissolution process, which is accompanied by the generation of a large amount of N 2 gas. Moreover, CaTiO 3 crystals tend to nucleate and grow on bubble surfaces with sufficient octahedral [TiO 6 ] 8− structures and Ca 2+ ions, eventually resulting in the molten slag being in a solid–liquid mixed state.
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