Abstract: In order to explore the characteristics of the breakout shell and its evolution process for the thread steel, it was investigated by optical microscope and scanning electron microscope analysis. The results indicate that the intergranular segregation especially sulphur plays an important role in the high temperature mechanical property of the steel and the crack propagation path. In the mold, the inhomogeneous growth of shell would cause the occurrence of longitudinal depression and then induce the formation of internal crack. The weak positions of the billet would subject to stress concentration at the exit of mold, cracks tend to propagate and the breakout may happen in severe cases. By controlling the content of sulphur in steel, adjusting the appropriate process parameters, the breakout ratio of the thread steel decreased from 6.32% to 1.10%.

Key words: continuous casting, Billet, steel-leakage, intergranular segregation