Lamination is caused by both pipe and stringer-type inclusions which do not weld in the subsequent hot-rolling operations. A. Stringer-type laminations result from inclusions, usually oxides and sulfides, which are more numerous in rimmed steel along the secondary blowhole zone in the bottom third of the ingot and in the center of the core zone near the top. In the case of killed steel ingots, the position of these stringer inclusions depends upon the kind of inclusion present, but they are usually more numerous at the top and center. B. Pipe-type laminations: as a result of the shrinkage of steel during freezing or solidification, a cavity is formed at the top of all ingots. In the case of rimmed steel ingots, this cavity or pipe, as it is called, is porous and usually does not extend below the 85% yield position. Semi-killed and killed ingots poured into open-top molds have an open, well-defined pipe which extends from the 85% down to as low as the 70% yield position in some cases. If the steel is properly made and well heated, the walls of the pipe cavity usually weld together when the ingot is rolled so that the pipe is not visible in the product. In the case of killed steel poured into hot-top molds, the shrinkage cavity should be entirely contained in the metal within the hot top.
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