Besides carbon, which determines hardness, steel contains a trace or more of manganese, silicon, phosphorus and sulphur, which came to it from the ore, or during the various stages of manufacture. Traces of other metals also come from ores, but these are not frequent. In additin to these impurities, almost all steels contains some slag and oxides acquired during the molten state in the furnace, and more or less occluded gas acquired in furnace reactions or during pouring into ladle and moulds.

Skill and modern equipment in the manufacture of steel have made it possible to control in a great degree the quantity of impurities in the steel, but there is no way to control the distribution of these impurities throughout the mass of the ingot while it is cooling in the ingot mould. As the ingot cools, these impurities tend to go to the hottest part, which is the center of the mass. Through this tendency the metal is not uniform in composition and hence not uniform in strength and quality. This concentration of the impurities at the center of the ingot is known as segregation.

After steel is poured into the mould, and becomes quiet, there is an effort of the gases, slag, oxides, and of some other impurities to rise to the top, as most of them are lighter than the metal. After the ingot has cooled or after it has been re-heated to be rolled, a quantity of the impurities is eliminated by cutting off the upper end of the ingot before it is used. This is called the discard or crop end, and is frequently as much as 30% of the whole ingot. A discard of from 3 to 6% is frequently made from the bottom end of an ingot. Steel for uses not particularly requiring strength may have little or no discard cut from the ingot.