Longitudinal cracks on the surface of continuous casting billets leave delamination defects on the rolled sheet, which can cause breakout and waste billets.The research points out that the longitudinal cracks originate from the unevenness of the thickness of the primary billet shell on the bending moon surface of mould. 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 shell leading to fracture and expansion in the secondary cooling zone after exiting the mould.
The causes of longitudinal cracking can be summarized as follows
The water nozzle and the mould are not aligned and there is bias flow to scour the solidification shell.
The melting performance of the protective slag is poor, liquid slag layer is too thick or too thin resulting in uneven thickness of the slag film, so that the local solidification shell is too thin. Liquid slag layer<10mm, longitudinal cracks increased significantly.
Fluctuation of liquid level of the mould. Liquid level fluctuation >10㎜, the chance of longitudinal cracking 30%.
High S or P content. If S content is more than 0.02%, P content is more than 0.017%, the high temperature strength and plasticity of steel obviously reduces, that will easily happen the longitudinal crack on the surface of billets.
In steel C content is 0.12 ~ 0.17%, it is easily to generate the longitudinal crack.
Measures to prevent the occurrence of longitudinal crack
Water nozzle and the mould should be centering.
The level fluctuations of the mould shall be stable at ± 10mm.
The immersion water spout has suitable insertion depth.
The taper of the mould is right.
The mould and the upper part of the secondary cooling area should be aligned with the arc.
The protection properties of the slag is correct.
The use of hot-top mould, that is, in the bending moon area 75mm copper plate embedded stainless steel and other materials with poor thermal conductivity, reducing the bending moon area heat flow 50-70%, delaying the shrinkage of the billet shell, thus the chance of longitudinal crack gets less.
