Hook evolution and inclusion entrapment of ultralow-carbon steel slabs

Ultralow-carbon (ULC) steel slabs are usually used for manufacturing high surface quality products such as automobile panel. Severe hooks in the subsurfaces of ultralow-carbon steel slabs usually degrade the surface quality of slab because of inclusions entrapment, which results in unacceptable sliver and blister defects on the surface of the final cold-rolled strip products. The hook formation and evolution process during the initial solidification of a continuous casting slab were studied through numerical modelling. A physical model based on the numerical simulation results was constructed to simulate the process of inclusion entrapment near the hook region, and the forces of inclusions in different positions of the solidified hook region were analyzed. The results demonstrate that following formation, the hook is not immediately buried in the shell; it sustained several stages such as melting, coarsening, growing, and burying. It is predicted that the final hook depth, as buried in the shell, is 2.5 mm when the casting speed is 1.3 m·min^-1, which is basically the same as the actual size of the hooked shell observed by a metallographic experiment. The calculated shape of the shell inner face with hooks is similar to morphologies of the slab surface region and breakout shell. The results of physical simulation and force analysis show that inclusions are most likely to be caught by the lower face of the solidified hooks, but they are more difficult to be entrapped by the upper face of the hook, especially for large-size inclusions. However, when overflow occurs, the inclusions near the me-niscus may be wrapped by the rapidly cooled molten steel above the primary hook. In the vertical shell between the two adjacent hooks, small-size inclusions (less than 100 μm) may be wrapped by the solidified front, but large-size inclusions are difficult to be wrapped.