PITTSBURGH— U.S. Steel Corp. is alleging that Chinese government hackers stole proprietary methods for making lightweight steel on behalf of Chinese steel producers seeking to supply a bigger share of the U.S. auto-making market.

Pittsburgh-based U.S. Steel, in a complaint filed on Tuesday with the International Trade Commission, said a computer belonging to a Pittsburgh researcher was hacked in 2011, and that plans for developing new steel technology were stolen…