The deal covers about 16,000 union-covered steelworkers and U.S. Steel's domestic flat-rolled and iron ore mining facilities, as well as tubular operations in Lorain, Ohio, and Fairfield, Ala., the company and union said in releases posted on their Web sites.
Details of the agreements, which must still be ratified by the union's rank-and-file, were not released.
"We are pleased that a tentative agreement was reached with the United Steelworkers on a competitive three-year contract," U.S. Steel Chairman and Chief Executive Officer John P. Surma said. "We believe that this agreement is in the best interests of our company, our employees and all of our stakeholders."
U.S. Steel also said its U.S. Steel Tubular Products Inc. subsidiary reached a tentative agreement with the union for about 1,000 workers at its facility in Lone Star, Texas.
The union's previous contract with the steelmaker had expired Saturday. USW officials said the ratification vote will take place after members review the agreement's details over the next few weeks.
"Throughout the summer, USW members worked tirelessly to achieve an agreement that we believe is fair to both sides," USW International President Leo W. Gerard said. "We are proud of the work we have done to ensure a stable future for our members, our retirees and the company."
Healthcare for retires was a major issue, he said.
"Both sides worked hard to come up with a solution that continues to provide our current and future retirees with access to high-quality, low-cost medical care," USW International Vice President Tom Conway, who led the negotiations for the union, said.
The union is still negotiating with ArcelorMittal USA on a new contract covering 14,000 workers at 15 U.S. locations.
"We have been exchanging proposals with the company and remain engaged in negotiations," Gerard said. "Our members continue to work under the terms and conditions of our 2008 agreements with ArcelorMittal while this long, frustrating process continues."
Gerard said ArcelorMittal had begun to restart blast furnaces it had idled in the run-up to the contract expiration at midnight Saturday.
"We take this as a sign that the company may now recognize that neither the USW negotiating committee nor the union membership as a whole will be intimidated into accepting unwarranted concessions by the implied threat of a lockout," said USW District 1 Director David McCall, who is heading up the USW's talks with ArcelorMittal.