Mechanism of mild cooling by crystallisation of mould flux for continuous casting of steel - A view from apparent thermal conductivity under steep temperature gradient

Effects of crystallisation on heat transfer across solid mould fluxes have been examined on the basis of apparent thermal conductivities including radiative contribution. The apparent thermal conductivities were measured on glassy and crystallised mould flux samples under steep temperature gradients using a parallel plate method improved in the present work. Both surfaces of the samples were coated with silver paste to reduce contact thermal resistance. Thermal resistance except the sample itself was experimentally determined to be 2.27 × 10^-4 m²KW^-1 based upon measurements on Inconel 600. To confirm the reasonableness of this value, the method was applied to fused silica. Apparent thermal conductivities were in good agreement with reported values. Apparent thermal conductivities of mould fluxes were measured up to 900°C at the high temperature side of the sample. The thermal conductivity of the glassy sample was 1.25 Wm^-1K^-1 below 300°C in the central temperature (T_c) of the sample, and was lower than those of the crystallised samples. With increasing degree of crystallinity, the thermal conductivities increased around room temperature. Samples with higher degrees of crystallinity showed negative temperature dependence more remarkably and resultantly were close to that of the glassy sample where T_c ~ 350–500°C. Where T_c > 500°C, the thermal conductivity of the glassy sample was 1.54 Wm^-1K^-1 and was greater than that of a crystallised sample, 1.32 Wm^-1K^-1, which would be due to the radiation. Apparent thermal conductivity at a practical temperature has also been estimated, which suggests that crystallisation enables radiative thermal conductivity to be reduced.