According to the practical experience, we conclude that rhomboudityof casting billet mainly originates from the mold and is affected by various factors affecting the uniform heat transfer on the cross section of the mold copper tube; The non-uniformity of secondary cooling water is the root cause of secondary rhomboudityafter the billet is discharged from the mold.

1.Conical degree

The reason of rhomboudity is that the taper of copper tube has been worn seriously. Because of the change of conical degree, the shape of the copper tube and the shell can not match well, and the uneven air gap is formed between the copper tube and the shell. The maximum thermal resistance of the mold comes from the air gap between the shell and the mold. The air gap accounts for more than 84% of the total thermal resistance, which destroys the uniformity of the heat transfer from the molten steel to the outside. The thin air gap has fast heat transfer, thick shell and large shrinkage.The thickness of air gap leads to slow heat transfer, and the thin shell leads to small shrinkage, which leads to rhomboudity.

2.Wall thickness

The general technical requirement for the wall thickness of copper pipe is that the limit deviation of copper pipe in any section is not more than 5%. Under normal conditions, it is the best condition that the wall thickness of copper pipe should be uniform on any section. It is precisely because of the serious uneven difference in wall thickness of copper pipe that the heat flow on different side walls is uneven, resulting in uneven cooling of steel in copper pipe and dislocation of cast billet.

3.Water quality

The influence of water includes water quality, water quantity, water temperature and so on. If the water quality can not meet the technical requirements, it will lead to scaling on the outer wall of the copper tube and intermittent boiling of the mold cooling water, which seriously affects the uniform cooling of the copper tube. At this time, the slab often changes the direction of square off in the length direction.