Mechanism of Hook Formation in Ultra-low Carbon Steel based on Microscopy Analysis and Thermal-stress Modeling

Periodic transverse depressions called “oscillation marks” are routinely observed on the surface of steel slabs manufactured by continuous casting processes. Additionally, a distinctive sub-surface microstructural feature called a “hook” 4, 5) often accompanies deep oscillation marks in steels with low (< 0.1%) carbon contents . Figure 1 shows typical shapes and sizes of oscillation marks and hooks observed on specially-etched samples obtained from ultra-low (0.003% C) carbon steel slabs cast at POSCO Gwangyang Works, South Korea . Hooks can be classified as “curved” (Figure 1(a)) if they angle steeply inwards from the surface or “straight” (Figure 1(b)) if they are shallow and lie just beneath and parallel to the surface. Curved hooks reach deeper beneath the surface (1.42 mm vs. 0.48mm for straight hooks) and are accompanied by deeper oscillation marks (0.26 mm vs. 0.19 mm for straight hooks). Each hook has a central line, the “line of hook origin”, which distinguishes the region inside the hook microstructure from the portion that solidified after liquid steel overflow.