Abstract
In the second report of this series, the effect of the secondary cooling on a cast billet was studied and it was revealed that too-intensive spray cooling and too-high casting temperature induced inner cracks of billets (ib., Tetsu-to-Hagane, vol.46, 1960, No.7, p.753). In this report the mechanism of the evolution of inner cracks of cast billets was investigated.
The mechanism of the evolution of cracks of cast billets is essentially the same as that in normal ingot casting. In continuous casting, however, one must not neglect the effect of relatively intensive spray cooling and pinch rolls on the evolution of cracks.
At the time of passing through the pinch rolls, internal stress which consists of thermal stress originated from intensive spray cooling and mechanical stress originated from pinch rolls may become to be greater than the strength of billet. That is, the inner cracks may occur. And this inner cracks will be terminated by molten steel which is still remained in the core of billet.
Calculation revealed that internal stress of a billet had the possibility to be greater than the strength and experiments revealed that the end point, the time of evolution, the selective direction, macroscopic and microscopic characteristics of inner cracks substantiated this mechanism of the evolution of inner cracks.
And the experimental results in the second report can be explained by this mechanism of the evolution of cracks.
