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What is Capped Steel?

By Paul Scott
Updated May 17, 2024
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Capped steel is one of four types of steel deoxidized during production to impart various physical properties in the finished product. These properties include chemical uniformity and lack of gas porosity, particularly in the surface. The process of capping steel consists of physically closing or capping the ingot mold after casting. This causes the surface of the molten steel to solidify first into a layer with a relatively high iron content and a minimum of gas porosity defects. The good surface and mechanical properties of capped steel make it ideal for the production of sheet and strip products.

The controlled removal of oxygen from steel during the production process delivers several useful variations on the basic product. These include steels featuring excellent chemical homogeneity, controlled segregation of impurities, and the exclusion of gas porosity with resultant improvements in surface quality. The four basic forms of deoxidized steel in descending order of oxygen removal are killed, semi-killed, capped, and rimmed steels. The process of oxygen removal involves the addition of deoxidizing agents such as aluminum, manganese, and ferrosilicone during the casting process.

This steel variant features moderate oxygen removal. This results in a finished ingot with a homogeneous composition, reduced segregation of impurities, and a thick cap or top layer with an excellent, uniform finish. The capping process begins as standard deoxidization with a reduced amount of deoxidizing agents added to the melt during casting. This starts the process of deoxidization and initiates the segregation of elements. This segregation process sees carbon, sulfur, and phosphorus migrating to the center of the ingot with the formation of a “rim” of relatively high iron content on the outside of the ingot.

Before the molten metal solidifies completely, a cap or cover is placed on the mold which accelerates the solidification of the melt adjacent to the cap. The result is a steel ingot with a rim of high iron content and low gas porosity around the sides and bottom and with a thick layer of like characteristics on top. With the exception of the capping and reduced deoxidization agent, this is the same basic process used to produce rimmed steel. The end product also differs in a reduced rim thickness and greatly increased top layer thickness and quality.

Capped steel is effective in processes requiring high quality surfaces. The excellent surface of capped steel is of particular value in cold working processes where gas egress imperfections on the surface of the steel are hard to eliminate. These processes include the cold drawing of sheet and strip steel. Capped steel is generally less hard as semi-killed or killed steels and is not as pure; this does not have much bearing on the items produced, however.

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