| Tundish nozzle blockage during continuous casting of the Al-killed medium carbon steel has been studied. Samples were taken from each sequential step in the processes of ladle furnace down to tundish and their chemical composition were analyzed. Blocked nuzzle were removed from the tundish and cut longitudinally to examine with unaided eye and under an optical microscope. The clogging materials were X-ray diffractometically examined. Safe window was evaluated by thermodynamic data, especially including e^(Ca)_(Ca)=-18, was in good agreement with actual steel samples. In addition, the blockage patterns were investigated from the viewpoint of fluid dynamics. The results are as follows: The ratio of Ca/Al has no thermodynamic significance as regards to conversion of Al₂O₃ into molten Ca-aluminates. The amount of calcium added to prevent nozzle blockage have to set on the basis of Al₂O₃ contents in molten steel. Since excess additions of calcium could form CaS and cause nozzle blockage, the amount of calcium added has to be fine-tuned. Post-mortem investigation of blocked nozzle showed invariably that the CaS accumulated at the bore of sliding plate, which was about the entrance to the diverging section in the nozzle. |
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