In order to investigate the effect of the refractive index difference between glass matrix and cuspidine on the heat transfer characteristics of the mould flux, a comparison was made between the SiO2-CaO-Al2O3-CaF2-Na2OFe2O3 system, which was based on the conventional flux composition, and samples in which Na2O in the base flux was replaced by Li2O or K2O. When Na2O was replaced by Li2O or K2O, oxide crystals were also precipitated as well as cuspidine, resulting in smaller average refractive index difference between glass and crystal and smaller the apparent reflectivity of the crystallised samples. This result lead increase of the radiative heat transfer for the crystallised flux. Thus, the conventional flux was found to have the lowest radiative heat flux, which is better for mild cooling for the steel casting. Furthermore, the apparent thermal conductivity of the mould flux was estimated for cases of crystals, bubbles and iron particles dispersion. The crystallised flux was found to have the largest apparent thermal conductivity because the thermal conductivity of crystallised sample is larger than that of glass. The smallest apparent thermal conductivity was obtained for the iron particle dispersed flux since both of the conductive and radiative heat flux was reduced by the glass matrix and the dispersed iron particles, respectively.

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