Influence of Casting Parameters on Hooks and Entrapped Inclusions at the Subsurface of Continuous Casting Slabs

In the current study, several layers beneath the surface of ultra-low carbon steel continuous casting slabs were inspected to investigate non-metallic inclusions, and then faces vertical to the surface of the slab were etched to reveal hooks. The influence of casting parameters on hooks and the entrapment of inclusions by hooks was investigated. Most > 100 μm inclusions were found to be located within 0.5 to 3.5 mm beneath the surface of the slab. Leaf-like hook, double-hook, and truncated hooks were observed, and bubbles and inclusions were found to be located below hooks. With the increase of the cooling water flow rate in copper plates of the continuous casting mold from 5300 to 5940 L/min, the average depth of hooks at the edge of narrow face increased from 1.37 to 1.53 mm, the number of > 20 μm inclusions increased from 1.6 to 2.6 per 100 mm², and the average size of > 20 μm inclusions increased from 25.1 to 31.8 μm. With the decrease of the casting temperature from 1573 °C (1846 K) to 1549 °C (1822 K), the average depth of hooks at the edge of narrow face increased from 1.24 to 2.01 mm, the number of > 20 μm inclusions increased from 2.0 to 6.0 per 100 mm², and the average size of > 20 μm inclusions increased from 26.0 to 27.0 μm. Hence, the surface defect of the rolled steel plate could be improved by lowering the depth of hooks at the subsurface of continuous casting slabs.