Effects of fluorides on NaF pore formation and heat control of mold slag films

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, CaF₂, and Na₃AlF₆ 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: Na₃AlF₆ > CaF₂ > 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 CaF₂ raw material, the heat control ability and pore size were between NaF and Na₃AlF₆. Its slag film exhibited the highest surface roughness (27.32 μm), with interfacial thermal resistance playing a dominant role. With Na₃AlF₆ 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 Na₃AlF₆ as a raw material is conducive to low-basicity mold fluxes achieving thermal control through finely dispersed NaF pores.