Rimmed steel is a type of low-carbon steel that has a clean surface and is easily bendable. When steel is normally made, it is deoxidized completely; with rimmed steel, the steel is only partially deoxidized, which allows a rim to form. This rim is almost completely free of other elements and is mostly iron. There is less carbon, so this steel is softer than other types, which makes it ideal for cold-bending and rolling applications. Of all the low-carbon steels, rimmed steel is one of the most common.
When steel is created, the metallurgist uses a deoxidizer to remove the oxygen from the iron and carbon to create steel. If the desired product is rimmed steel, then the deoxidization is only partial, which allows the ingot to remain liquid. During this time, a rim forms around the ingot, which contains fewer impurities than the central area. The impurities, such as carbon and manganese, are held in the center, while the rim is almost completely iron. This means the core has more impurities than most steels, while the outside has fewer impurities than most steels.
Rimmed steel makes use of less carbon than other steel types, so it is much softer than other steels. This means projects that require easily machinable or bendable metals make use of the rimmed metal. It is easily rolled and can be cold-bent and cold-formed. The surface finish also is much cleaner than that of other steels, so it is used for its aesthetic properties and low roughness. Using this steel in hot-bending and other hot processes is actually advised against, though, because the alloying materials and impurities are not uniform, which can cause problems.
While rimmed steel is known for its good surface quality, the quality is usually poor when the steel is being formed. The way in which the rim forms means blowholes will appear on the surface of the steel, because the materials are moving outward and deoxidizing. These holes are removed during the rolling process, to make the metal’s surface uniform and clean.
Rimmed steel is considered a low-carbon steel. Most steels use about 1 percent to 2 percent carbon alloyed with iron. The rimmed variety only uses about 0.15 percent to 0.25 percent carbon. Most steels that are made with 0.15 percent carbon are this type of steel, because it is valued more for being easy to work with than for its strength and durability.