What are the causes of longitudinal cracks on the surface of continuous casting slabs and how to prevent them?

Longitudinal cracks on the surface of continuous casting slabs will affect the quality of rolled products. For example, a longitudinal crack with a length of 300mm and a depth of 2.5mm leaves a 1125mm delamination defect on the rolled sheet. When the longitudinal crack is serious, it will cause leakage and scrap. The study pointed out that the longitudinal crack originated from the inhomogeneity of the thickness of the primary shell of the mold meniscus. The tensile stress acting on the billet shell exceeds the allowable strength of the steel, resulting in stress concentration at the weak point of the billet shell, which leads to fracture, and expands in the secondary cooling zone after exiting the mold. The causes of longitudinal cracks can be summarized as:

1) The misalignment between the nozzle and the crystallizer produces a bias current to scour the solidification shell.

2) Poor melting performance of the mold slag, too thick or too thin liquid slag layer, resulting in uneven thickness of the slag film, making the local solidification shell too thin. The liquid slag layer is less than 10mm, and the longitudinal cracks increase significantly.

3) The liquid level of the crystallizer fluctuates. Liquid level fluctuation>10mm, the probability of longitudinal cracking is 30%.

4) S+P content in steel. When S>0.02% and P>0.017% in steel, the high-temperature strength and plasticity of the steel are obviously reduced, and the longitudinal crack tends to increase. 5) When C in the steel is 0.12-0.17%, the tendency of longitudinal cracking increases.​​

The measures to prevent longitudinal cracks are:

1) The nozzle and the crystallizer should be aligned.

2) The fluctuation of the liquid level of the crystallizer is stable at ±10mm.

3) Appropriate immersion nozzle insertion depth.

4) Appropriate mold taper.

5) The arc between the crystallizer and the upper part of the secondary cooling zone should be accurate.

6) Appropriate mold powder properties.

7) The use of a hot top crystallizer, that is, inserting materials with poor thermal conductivity such as stainless steel into the 75mm copper plate in the meniscus area, reduces the heat flow in the meniscus area by 50-70%, delays the shrinkage of the billet shell, and reduces the dent, so it is also Reduces the chance of vertical cracks.

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