U.S. Steel is the latest steel maker to announce production cuts. They said yesterday they was laying off 675 union workers in the United States and Canada because of lower demand for steel amid the economic downturn. The layoffs, effective immediately, include 500 employees in the United States and 175 in Canada, said John Armstrong, a company spokesman. They affect workers at U.S. Steel's facilities in the Pittsburgh area, including 57 at the Irvin plant in West Mifflin and an undetermined number at the Edgar Thomson Works in Braddock; and at mills in Gary and Portage, Ind.; Fairfield, Ala.; Ecorse and River Rouge, Mich.; and Granite City, Ill.