Abstract: To improve the internal quality of high carbon wire steel produced by a domestic steel factory, the fluid flow behavior in a 180 mm×240 mm billet continuous casting mould was investigated with 1∶1 water model, based on the similarity principle. The distribution of the molten steel, the fluctuation of the mold liquid surface and the state of the slag layer using singleport straight nozzle and fourport swirling nozzle were compared by the experiment. The results show that using swirling nozzle can effectively decrease the impact depth of fluid; the average fluctuation height of swirling nozzle is larger than the straight one at the same immersion depth and casting speed. In addition, casting speed and immersion depth affect the fluctuation of mold liquid surface of swirling nozzle more obviously, and the fluctuation of slag layer is more frequent for the swirling nozzle than for the straight one. Considering the fluctuation of the mold liquid surface, the state of the slag layer and cost, the swirling nozzle is more fit for casting the high quality billet in this section, and the optimum process is at the casting speed 1.0 m/min and the immersion depth 120 mm with the average wave height of free surface at 0.18 mm. In this case, the slag layer fluctuates properly, and no slag is entrapped and molten steel exposed. The billet of high carbon steel produced by using this nozzle combining with moldEMS and secondary cooling has stable quality; over 95 % of wire rods in diameter 5.5mm possess center carbon segregation level below 2.0 grade, and the porosity and shrinkage levels of all wire rods are below 0.5 grade.

Key words: billet mold, swirling nozzle, straight nozzle, surface fluctuation, slag entrapment