<p>The conventional CaO–SiO₂-based mold fluxes are not suitable for high-Al steel casting because of the strong reaction between silica in the flux and aluminum in the steel strand. In the process of casting of high-Al steel, flux composition changes, with the decrease of the silica concentration and increase of alumina. Knowledge and understanding of the effect of the Al₂O₃/SiO₂ ratio on flux structure and properties are useful for flux design for the high-Al steel continuous casting. This paper investigated the effect of the Al₂O₃/SiO₂ ratio in the range from 0.7 to 10.8 on structure, viscosity, and heat transfer of CaO–Al₂O₃–SiO₂–B₂O₃–Na₂O–Li₂O–MgO–F fluxes. It was found that flux melting temperature increased with the increase in Al₂O₃/SiO₂ ratio. Viscosity of the flux melts increased significantly with the increase of the Al₂O₃/SiO₂ ratio from 0.7 to 1.2, reaching the maximum value, and then decreased with further increase of the Al₂O₃/SiO₂ ratio. Raman spectroscopy analysis revealed that the change of the Al₂O₃/SiO₂ ratio led to the change of aluminate and silicate structural units. The turning point for viscosity was attributed to the change in the degree of flux polymerization which was governed by the amphoteric nature of Al₂O₃. X-ray diffraction (XRD) analysis showed that increasing Al₂O₃/SiO₂ ratio increased crystallization tendency of the fluxes. Heat transfer measurement by infrared emitter technique (IET) revealed that increasing Al₂O₃/SiO₂ ratio led to the decrease in heat flux which is correlated well with the increased crystallinity of the flux. The results suggested that the fluxes with Al₂O₃/SiO₂ ratio 2.1–4.3 are the best candidates among the studied CaO–Al₂O₃-based mold fluxes for casting of high-Al steel.</p> Graphical Abstract <p></p>