American holding company established in 1983 to facilitate the diversification of National Steel Corporation. Formerly headquartered in Pittsburgh, Pa., NII moved to Dallas, Texas, in 1991, and National Steel moved to Mishawaka, Ind., in 1992.

The steel company was formed in 1929 by Ernest T. Weir (1875–1957) through an amalgamation of Weirton Steel Company, Great Lakes Steel Corporation, and Hanna Iron Ore Company; the company controlled not only steel mills but also iron-ore mines and coalfields. National Steel was consistently one of the most profitable steel companies throughout the Great Depression of the 1930s and the only one to make a profit in 1932. A self-made rugged individualist, Weir was an outspoken opponent of many New Deal programs and battled successfully for many years to forestall unionization of his Weirton plant. He deftly undermined the appeal of the ironworkers, steelworkers, and tinworkers union among his men by unilaterally granting wage hikes as company profits rose.

National Steel diversified into aluminum manufacturing with the acquisition of Hastings Aluminum Products in 1968 and Pittsburgh Aluminum Alloys in 1970 and the creation of National Aluminum Corporation. In 1984 it formed a new computer-data and information-services subsidiary, GENIX. The company also engaged in the distribution of pharmaceuticals, petroleum products, and other goods.

National Intergroup sold 50 percent of its interest in National Steel to Tokyo-based Nippon Kōkan KK (NKK Corporation) in 1984 and another 20 percent to the same company in 1990.