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Billet defects:

1. Corner crack. After serious wear of the lower mouth of the mould fittings, a large air gap is formed between the copper tube of the mould and the billet, which results in too weak cooling and thinning of the shell. The cooling gradient is too large and the stress is concentrated.

2. Intermediate cracks. When the billet passes through the secondary cooling zone, the cooling is not uniform, and the thermal stress produced by the excessive temperature rise exceeds the maximum stress between the columnar crystals, and the development of the columnar crystals increases the probability of forming the intermediate cracks.

3. Crackunder pressure. The pressure of the roller is too high, which makes the solid-liquid two-phase zone of the solidified billet produce the crack under pressure. For the cast billet with cylindrical crystal developed and intermediate crack, a large crack under pressure is often formed along the direction of the billet.

4. The center is loose. The columnar crystal on both sides of the billet grows to the center, which results in “bypass “, which prevents the molten steel above the bridge from solidification and shrinkage of the molten steel under the bridge. When the molten steel under the bridge solidifies, many small pores are left.