Visualization of Hook and Oscillation Mark Formation Mechanism in Ultra-Low Carbon Steel Slabs During Continuous Casting

Oscillation marks accompanied by sub-surface hooks routinely appear on the surface of continuously cast steel slabs, and are especially severe in ultralow carbon steel. This paper presents a new detailed mechanism for their formation, which has been developed by combining existing theoretical modeling results, experimental observations, and analyses based on optical and scanning electron microscopy. Hooks form by solidification and dendritic growth at the liquid meniscus during the negative strip period. Oscillation marks are generated when molten steel partially overflows over the frozen meniscus shortly afterwards and incompletely fills in the gap before solidifying. The results are presented in the form of a graphical animation that allows easy visualization of the various events occurring near the meniscus that lead to the formation of these defects. BACKGROUND The surface quality of continuous-cast steel slabs is greatly affected by oscillation marks 2] and sub-surface hook formation, which lead to cracks and slivers in the final rolled product. Oscillation marks are transverse depressions (Figure 1(a)) running around the strand perimeter that form during each vertical oscillation of the mold. Periodic mold oscillation is needed to prevent sticking of the solidifying shell to the mold walls. Providing a “negative strip time” period in each oscillation cycle, when the mold moves downward faster than the casting speed also encourages infiltration of the mold slag into the gap between the mold wall and steel shell. A “hook” is a distinct sub-surface microstructural feature that often accompanies an oscillation mark, depending on the steel grade and casting conditions. Hooks increase in severity with decreasing carbon content, slow oscillation, and low casting speed. Hooks tend to entrap mold slag, floating inclusions and gas bubbles that often lead to surface defects such as slivers and blisters 9] during subsequent rolling processes. Examples of typical “curved hook” and “straight hook” shapes are shown (shaded in blue) in Figures 1(b) and JOMe, (Journal of Metals – electronic edition), December, 2006,