Mechanism of hook formation in ultralow-carbon steel based on microscopy analysis and thermal-stress modeling

Mechanism for the formation of hooks and oscillations marks during continuous casting of ultralow-carbon steel slabs has been analyzed with the microscopic analysis and thermal-stress modeling. Optical and scanning electron microscopy were performed to simultaneously distinguish the hook structure, the surrounding dendrite microstructure, the final grain structure, the grain orientations relative to the hook, and the local composition variations. It has been observed that formation of curved hooks is initiated by meniscus solidification, and the instantaneous shape of the meniscus at this time dictates the curvature of the line of hook origin. Grain orientation measurements revealed that dendritic growth above the line of hook origin occurs after meniscus overflow over the solidified hook, which entraps liquid mold flux and persists in the final microstructure.