AK Steel Holding Corporation, formerly known as Armco, is a major American steel company founded in 1900 as the American Rolling Mills Corporation. With 2005 revenues of US$5 billion, it is on the list of the 500 largest companies from Forbes. Today the company is situated in Middletown, Ohio.

The company has production facilities in a number of American cities including: Middletown, Ohio, Ashland, Kentucky, Zanesville, Ohio, Mansfield, Ohio and Rockport, Indiana as well having production facilities in Canada, Mexico and Western Europe.

AK Steel's main products are carbon, stainless and electrical steels, cold rolled and aluminium coated stainless steel for automakers.

The current CEO at AK Steel Holding is James L. Wainscott

On February 28, 2006, AK Steel began a lockout of Armco Employees Independent Federation (AEIF), an employee union, at their Middletown Works plant, in Middletown, Ohio. This lockout is already the longest labor stalemate in the 105-year history of the Middletown Works. The previous longest stalemate was a five-day company lockout in 1986.

The aforementioned lockout is still in effect. On June 29, 2006, AK Steel made what it has called its "final offer" on the remaining 21 out of 29 items of interest to the union. Reports also claim that AK Steel is soliciting new unions to employ in the place of the AEIF, but this has been denied by AK Steel.