Initial solidification in continuous casting

A number of defects associated with initial solidification are present in continuously cast steel. The defects range from surface flaws, to inclusions which are found inside of argon bubbles that are trapped while floating out of the melt. Sub-surface solidification structures known as "hooks" have been found in association with these defects, and are thought to be related to their occurrence. The hooks are thought to be extensions of the initial frozen shell which protrudes beneath the cast surface at the meniscus marks. Thus the initial solidification of continuously cast steel can be seen to be important in the understanding of these defects.; The origin of the hooks, and meniscus marks in general is not currently agreed upon. There are competing theories as to the nature of their formation. The hooks lengths vary between steel grades, and as yet there is no explanation for this phenomenon. The current work sought to understand the mechanism behind the formation of the meniscus marks, and their associated hooks. An explanation for the change in hook length with grade was of particular interest. The project involved analysis of industrial samples, numerical modeling, and development, and use of a laboratory scale continuous caster mold simulator.; X-ray analysis of surface samples from industrial slabs has found many of the bubbles in the region of the meniscus marks. Metallographic examination of the samples after the x-ray analysis enabled measurement of the bubble size and depth. Some of the bubble samples were etched successfully for solidification structure, and hooks were found associated with the bubbles. Hook lengths for various industrial grades were measured.; Numerical modeling was undertaken to simulate the solidification conditions using a commercial casting program called Procast{dollar}rmsp{lcub}TM{rcub}.{dollar} This was employed to analyze the conditions in the meniscus regions during the short periods of time associated with initial solidification ({dollar}<{dollar}0.5 seconds). The modeling yielded results which indicated that mold flux properties were of the greatest importance due to heat transfer considerations. Changes in steel grades were found to have a strong effect on the initially solidified shells. A remelting condition was developed and applied to the initially solidified meniscus, and it was found the extent of remelting was highly dependent on the size of the liquidus-solidus temperature gap.; A mold simulator was developed and employed to observe the effect of changes in casting parameters on initial cast structure. The simulator yielded cast structures similar to those found in industrial casters, and the thermal and solidification performance was comparable. Mold flux infiltration was also acceptable, though no slag rim was found after casting. Mold flux found in the mold/shell gap was found to be completely glassy after solidification. This indicates a high rate of cooling. Subsurface hooks were found in the simulator castings, and lengths were measured. Melt composition was varied to understand its effect on solidification structure.; Meniscus marks were found to be caused by solidification of a frozen meniscus which is overflowed or bent back. The variation in hook length was found to be a result of liquidus-solidus gap size changes on remelting behavior.