Weathering steel, often called corten steel (or its trademarked name, COR-TEN) is a group of steel alloys that form a stable external layer of rust that eliminates the need for painting.
U.S. Steel (USS) holds the registered trademark on the name COR-TEN. The name COR-TEN refers to the two distinguishing properties of this type of steel: corrosion resistance and tensile strength. Although USS sold its discrete plate business to International Steel Group (now ArcelorMittal) in 2003, it makes COR-TEN branded material in strip mill plate and sheet forms.