Ingot-cast rim steel surface purity

Why ingot-cast rim steel gave the best sheet properties — first stuff to solidify on the mold wall is the purest iron. Used to argue that the casting method has to be chosen for the product (sheet vs. plate).

In an ingot casting, the same initial nuclei form on the outside when you pour into the mold — fine grain. Then columnar grains, then dendrites may form in the center. Through all this you get fluid flow in the liquid as it cools. In a big casting this could take hours or days. The liquid gets enriched in one element. The first stuff that solidifies — in an ingot-cast rim steel — is very pure iron alloy on the surface. We got the best sheet properties from ingot-cast steel because the surface was very pure iron. The internal composition might be slightly different — for sheet you don't care, but for plate you did. So we have to worry about how we solidify.