All molten steels contain a certain amount of trapped gas, chiefly oxygen. As the molten mass solidifies, some of the gas escapes, but some doesn't and remains as "voids" within the ingot. This is called "Rimmed or rimming steel". It is perfectly useful for a number of processes that require severe rolling, during which the internal voids are squeezed and welded shut. Many sheet types and wire gauges are typical usages of rimmed steel
