Hook-Like Microstructure at the Subsurface of Ultra-low Carbon Steel Slab

The characteristics of a hook-like microstructure in an ultra-low carbon (ULC) steel continuous casting slab under different casting conditions were investigated via metallographic experiments. The results revealed that the hook depth decreased with increasing pouring temperature and casting speed. Bikerman’s equations and the iron-carbon phase diagram were used to explain the hook formation mechanism. In addition, evolution of the hook microstructure during the initial solidification process of the ULC steel slab under different casting conditions was evaluated through numerical modeling. For the hook-less shell, the present model yielded a thickness that corresponds closely to the value obtained from traditional models, but the calculated inner shapes of the shells differed significantly. The calculated shape of the hook-containing shell was validated by metallographic observations of a common slab and a breakout shell. The simulation results showed that at a given “bury point,” the hook depth is equal to the shell thickness. Moreover, the distance from the bury point to the meniscus increased with increasing casting speed, whereas the distance between the bury point and the slab surface decreased. The distances from the bury point to the meniscus and the slab surface both decreased with increasing casting temperature.