Copperweld Corp., a specialty steelmaker, said 445 workers at a plant in Shelby, Ohio, began a strike after the United Steelworkers Local 3057 rejected a new contract on Tuesday.
The previous contract between Copperweld's and the union expired at midnight Tuesday.
The union vote to reject the proposed pact was 230-215.
Copperweld said it does n't expect a protracted strike.
It said it has taken measures to continue shipments during the work stoppage.