President Donald Trump says U.S. Steel will keep its headquarters in Pittsburgh as part of what he called a “partnership” between the American steelmaker and Japan-based Nippon Steel, which sought to buy it.

“This will be a planned partnership between United States Steel and Nippon Steel, which will create at least 70,000 jobs, and add $14 Billion Dollars to the U.S. Economy,” Mr. Trump wrote on social me...