The way steel will be cast in the third millenium promises to be radically different from current day practices, in the same way that the ingot casting practices of the first half of the 20th cnetury gave way to continuously cast slabs over the past thirty years. Given the general move towards process rationalization and near net shape casting, one can expect that a time will come when the incremental productivity improvements still being made in terms of stationary mould technologies will be surpassed by those promised with moving mould technologies. In all cases however, the rate of heat extraction from the freezing steel by a cold substrate is a fundamental design factor, common to all. Following a review of fixed and oscillating mould heat fluxes measured by many researchers, recent results obtained for instantaneous heat fluxes during twin roll casting and single belt casting are presented. Given the greatly reduced fluxes measured versus those expected for perfect metal/mould contact, it is shown that the early stages of apparent contact of molten metal on smooth rolls or on a horizontal moving mould, can be interpreted in terms of heat conduction through coherent insulating films of gas in the order of 10 to 200 microns thick. It is postulated that such coherent gas films can be stabilised by viscous flows of gas entrained between the moving mould and the freezing shells. Circumstantial evidence, such as the formation of powdery deposits on cooling surfaces reported by BHP and Carnegie Mellon researchers, can be interpreted as fume formation from manganese and silicon vapor emissions from the molten interfaces.
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