Three-Dimensional Distributions of Large-Sized Inclusions in the Surface Layer of IF Steel Slabs

The distribution of surface macro-inclusions is an important parameter that can directly influence the surface quality of IF steel sheets. In the present work, macro-inclusions > 100 μ m within a 20-mm zone from the slab surface across the whole slab width were characterized by step machining methods, and the total analyzed area was 3,300,000 mm 2 . Three kinds of macro-inclusions were detected: bubbles (including single and aggregated), alumina associated with bubbles and refractory-related alumina. The three-dimensional distribution of surface macro-inclusions across the whole slab width was reconstructed, which showed macro-inclusions along the thickness direction almost concentrated 8 to 20 mm from the slab surface, corresponding to the center of the upper roll zone and stagnant zone below the submerged entry nozzle bottom according to the full-scale water mold model simulation. An inclusion stability model was established that indicated that increasing the flow velocity sharply decreased the stability degree at the solidification front because of the washing effect. The calculated results by this model agree with the fact that macro-inclusions were mainly concentrated in the slab center because of the low flow velocity at this location. The present work indicates that increasing the flow velocity at the solidification front as well as eliminating the stagnant zone is a potential way to improve the surface quality of IF steel slabs.