What is the reason for the depression of the casting

Surface depression of continuous casting billet
Concavity on the surface of a continuous casting slab refers to irregular pits on the surface of a continuous casting slab or billet. Most are horizontal pits, but also longitudinal pits. The depressions are more austenitic stainless steel and low-carbon steel (carbon content of 0.10% ~ 0.15%) casting billet surface. Lateral depressions are easy to occur in the corners of the cast slab's wide face or any part of the wide face. There are cracks in the severe depressions, and the deep cracks have surface seepage scars, heavy skins and even leaks.

There are many reasons for the surface depression of the cast slab.
1. Due to the "overcooling" of the initial solidification stage of the blank shell, the local blank shell shrinks greatly, which will cause the uneven growth of the bad shell and cause the formation of depressions;
2. When the billet is drawn, the longitudinal shrinkage of the billet shell and the hydrostatic pressure of the steel cause the interaction surface between the rebending to produce a horizontal depression;
3. The longitudinal pits (grooves) near the obtuse angle of the billet with rhombohedral deformation and the longitudinal pits at both ends of the wide side of the slab are caused by uneven cooling in the crystal plug;
4. The occurrence of pits is also related to the composition of the steel. Austenitic stainless steel and low-carbon steel grow unevenly in the mold shell and are prone to pits;
5. The high thermal expansion performance and low thermal conductivity of stainless steel are prone to dents;
6. The volume shrinkage of low-carbon steel is large when it is solidified to form ferrite and then immediately transforms into austenite. The steel with this composition has the greatest tendency to form pits.
At the same time, the occurrence of pits is also affected by continuous casting conditions: factors such as improper performance of mold powder, large fluctuations in the liquid level in the mold, and large changes in pouring speed. Metal foreign bodies stuck to the pull-straightening rollers will cause periodic pits (jade marks) on the surface of the cast slab .