Abstract: In order to explore the cause of the formation on breakout of the billets for low carbon steel, the characteristics of the breakout shell and the mold tube was investigated by optical microscope and scanning electron microscope. The result shows that the outer surface of the mold was attached with an unknown substance, what was worse, the inner surface plating was detached and the solidification shrink rate of the steel is high. Therefore, the heat transfer effect of the mold was poor, and as a result, the thickness of the shell was uneven and the internal cracks might generate. At last, when the weakest point of the billet shell couldn't withstand the external force, then the breakout would happen. By adjusting the appropriate continuous casting process parameters, and reducing the sulphate content of the mold cooling water, the breakout ratio of the low carbon steel decreased from 1.01 % to 0.30 %.

Key words: continuous casting, billet, breakout, mold tube, cooling water