What Is Rimmed Steel?
Rimmed steel is a type of steel that undergoes minimal or no deoxidization during the ingot formation process. In contrast, steel ingots that are fully deoxidized are referred to as kiln-dried steel.
Rimmed steel, being insufficiently deoxidized, generates bubbles during the casting and solidification of the ingot. These bubbles form a tubular layer within the outer shell of the ingot, known as the rim layer. Originating from CO gas, these bubbles do not lead to defects in the steel as they collapse during the hot rolling process.
Uses of Rimmed Steel
Rimmed steel, due to its lack of deoxidization and the absence of oxygen scavengers, offers a high yield at a relatively low cost. However, its minimal deoxidization leads to significant internal segregation and inconsistent quality, marked by varying levels of impurities like carbon and phosphorus. This inconsistency makes rimmed steel less suitable for machine parts requiring high reliability. It is typically used for products where segregation is not a concern, such as wire rods, plates, bolts, nuts, and tin plates.
Characteristics of Rimmed Steel
Rimmed steel, characterized by its high dissolved oxygen content, releases sparks and gases during casting and solidification. This process forms a rimmed layer at the periphery, with a tubular bubble layer inside, leading to significant segregation. The quality of the steel varies greatly between the periphery, which is closer to pure iron, and the interior, which contains higher levels of impurities like phosphorus and carbon. When cut, the tubular bubbles in rimmed steel may manifest as surface imperfections, making it unsuitable for applications demanding high reliability.
Despite these characteristics, rimmed steel offers cost advantages due to the minimal use of oxygen absorbers and a high yield rate, with no cavities or shrinkage as seen in kiln-dried steels. The surface, being composed of pure iron, has good paint adherence, making rimmed steel suitable for applications like rolled steel where surface quality is a priority.