Effect of a sudden level fluctuation on hook formation during continuous casting of ultra-low carbon steel slabs

Initial solidification in the meniscus region during continuous casting of steel slabs forms periodic surface depressions called oscillation marks, which are often accompanied by subsurface microstructural features called "hooks". To understand their formation mechanism, a transient finite-element model, CON2D has been applied to compute temperature, stress, and distortion of a steel shell during the initial stages of solidification of ultra-low carbon steel. Furthermore, the effects of sudden metal level fluctuations are investigated. The model uses a creep-type elastic-viscoplastic constitutive equation for steel, which is integrated using a two-level algorithm alternating between solutions at the local node point and the global system of equations. Results show that thermal stress causes the exposed portion of the thin shell to bend away from the mold as the liquid level suddenly drops. The subsequent rise in liquid level increases the bending. The maximum distortion of the shell was found to be ∼0.46 mm for a large level drop of 16 mm for 0.4 s. If the deformed shell tip develops enough strength to resist being pushed back to the mold, it will remain embedded in the solidified steel as a hook after the meniscus overflows the deformed shell tip. Further solidification in the overflowed region then creates an oscillation mark.