In the current chapter, the thermodynamic analysis for the formation of inclusions in the IF steel, the formation of hooks, and inclusions captured by hooks at the subsurface of the continuous casting slab of the IF steel were investigated. Thermodynamic stability diagrams between different elements and the dissolved oxygen at 1873 K for pure liquid iron and IF steel were calculated. Characteristics and formation mechanisms of Al2O3-TiOx inclusions in the steel after adding titanium alloy during RH refining process were analyzed. The solidified hook was a structure formed after the solidification of the meniscus in the mold. Due to the limited mushy zone during the solidification of ultralow carbon steels, it was easy to form a large-sized solidified hook structure in the subsurface of the continuous casting slab. Solidified hooks in an ultralow carbon steel continuous casting slab were investigated. To quantify the capture of inclusions by the solidified hook, inclusions captured by the solidified hook area were determined by comparing the number density of inclusions in the solidified hook area and the nonsolidified hook area. Finally, a two-dimensional multiphase model coupling fluid flow, heat transfer, and solidification was established to predict the effect of casting parameters on the solidified depth of the shell near the meniscus.