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Mould taper, heat transfer and spray cooling in high speed continuous casting Fu, Junlong

Abstract

Competition around the world and market expectations are driving the steel industry to improve billet quality and lower costs by increasing casting speed of continuous casters. The demand for high casting speeds can be achieved by increasing mould length and by making upgrades to the secondary cooling system. A plant trial was conducted at Co- Steel Lasco for the purpose of assessing the performance of a 1016 mm long mould having a parabolic taper on the high speed casting of 6 grades of steel billets. The mould was instrumented with 52 thermocouples. The billet section size was 178x127 mm, cast at speeds in the range of 2.2-2.9 m/min. Additionally, three linear variable displacement transducers were installed on the mould wall to monitor the mould oscillation. Billet samples were collected for several operating conditions. An inverse heat conduction model was used to calculate mould heat fluxes from measured mould wall temperatures. Existing mathematical models were employed to investigate mould-billet interaction and the adequacy of the mould taper. It was shown that steel carbon content and casting speed had significant effects on heat flux in the mould. Heat flux was highest for medium carbon steel followed by high carbon steel; low carbon steel had the least amount of heat transfer. Increases in casting speed consistently led to heat flux increases, although not proportionately. Off-corner internal cracks and narrow face concavity were noted on all billet samples. It was demonstrated that the taper was too tight for the low carbon grades which caused squeezing of the shell by the mould and was responsible for the off-corner internal cracks and the narrow face concavity of the billets. For the medium and high carbon billets the mould taper was inadequate especially in the lower part of the mould. Here, it was likely that bulging of the broad face and corner rotation gave rise to longitudinal depressions on the narrow face and off-corner internal cracks. Optimum tapers were recommended respectively for low carbon, medium carbon and high carbon steels. Spray cooling systems for three companies were also investigated. Billet surface temperature was calculated with a mathematical model; the effect of surface reheating between mould and spray cooling, between different spray cooling zones and between spray cooling and radiation as well as the billet liquid pool depth were examined It was shown that midway cracks can result from the billet surface reheating in the secondary cooling zone. Additional spray zones and a longer spray chamber were shown to decrease the occurrences of surface reheating and thereby mitigating midway cracking. Finally, recommendations were made for a spray cooling chamber appropriate for high speed casting.

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