Microstructure near corners of continuous-cast steel slabs showing three-dimensional frozen meniscus and hooks

Frozen meniscus features near the slab corners in continuous-cast ultra-low-carbon steel samples were investigated using special etching reagents and optical microscopy. The three-dimensional (3D) continuous shapes of hook defects along the oscillation marks around the slab corners were constructed from a set of micrographs taken at different vertical sections. The hook depth variation was traced around the slab perimeter. The maximum hook depth was observed at the corners and concave curvature was observed on a 45° vertical section from the corner. Horizontal cross-sections through the oscillation marks near the corners provide evidence that liquid steel overflow caused the oscillation mark. Shrinkage of the corner allows the overflowing steel to penetrate deeper into the larger corner gaps, giving rise to oscillation marks that point down in the casting direction at the corner. These results also explain the complex, 3D subsurface microstructures observed near the corners of the slab and quantify the hook depth and shape.