STUDIES FOR THE IMPROVEMENT OF THE SECONDARY COOLING SYSTEM OF CSP THIN SLAB CASTERS

摘要

Recent investigations of the heat transfer rate occurring during the interaction of air-mists with hot steel surfaces under conditions relevant to thin slab continuous casting have revealed that the cooling process takes place mostly in the transition-boiling regime. This cooling conditions contrast with the gentler ones characterizing the stable film-boiling regime, which prevails in conventional continuous casting. However, current operating practice is not intensive enough to guarantee that the slab leaves the supported length fully solidified at casting speeds above 5 m/min in the event of a system upset, e.g., an increase in the steel tundish temperature. Laboratory results have shown that for similar impact water fluxes the heat extraction rates can be substantially increased by increasing the typical air-nozzle pressure of 200 kPa to 250 kPa, while further increase ceases to have any effect. The implementation of these new cooling conditions in a mathematical model for thin slab solidification indicates that it is possible to reach casting velocities above 5.5 m/min without the risk that the strand liquid core length exceeds the relatively short supported length of the machine of 6.42 m. These findings have been validated by plant trials using secondary cooling strategies based on augmenting the air pressure with the water flow rate in the necessary proportion to maintain the air-to-water flow rate ratio, AAV, at a value equal to 10. The results of the trials indicate that appropriately controlled intensified cooling conditions may offer a flexible and more practical alternative for increasing the productivity of certain steel grades than enlarging the supported length of the machine.