Coated steel is a type of deoxidized steel with a thick layer of high iron, low gas porosity on top. It offers excellent surface quality and is ideal for cold working processes. The capping process during production results in a homogeneous composition and reduced impurity segregation.
Coated steel is one of four types of steel that is deoxidized during production to impart various physical properties to the finished product. These properties include chemical uniformity and lack of gas porosity, particularly at the surface. The steel capping process consists of physically closing or capping the ingot mold after casting. This causes the molten steel surface 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 coated steel make it ideal for sheet and strip production.
The controlled removal of oxygen from the steel during the manufacturing process offers several useful variations on the base product. These include steels featuring excellent chemical homogeneity, controlled segregation of impurities and the exclusion of gas porosity resulting in improvements in surface quality. The four basic forms of deoxidized steel in descending order of oxygen removal are killed, semi-killed, coated, and rimmed steels. The oxygen removal process involves adding deoxidizing agents such as aluminum, manganese and ferrosilicon 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 coat with an excellent and uniform finish. The capping process begins as a standard deoxidation with a small amount of fluxing agents added to the melt during casting. This starts the deoxidation process and starts the segregation of the elements. This segregation process sees the carbon, sulfur and phosphorus migrate to the center of the ingot with the formation of a relatively high iron content ‘rim’ on the outside of the ingot.
Before the molten metal solidifies completely, a cap or lid is placed on the die which accelerates the solidification of the molten adjacent to the cap. The result is an ingot of steel with a high iron, low gas porosity rim on the sides and bottom and with a thick layer of similar characteristics on top. With the exception of the capping and reduced fluxing agent, this is the same basic process used to make hooped steel. The final product also differs in a reduced edge thickness and a significantly increased thickness and quality of the top layer.
Coated steel is effective in processes that require high quality surfaces. The excellent surface of the coated steel is of particular value in cold working processes where outgassing imperfections on the steel surface are difficult to eliminate. These processes include the cold drawing of sheet and steel strip. Coated steel is generally less hard than semi-killed or killed steels and is not as pure; this doesn’t have much to do with produced items, however.
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