National Labor Relations Board has filed a formal complaint accusing ATI of unfair labor practices. Company says it will vigorously defend itself.

The National Labor Relations Board has filed a formal complaint accusing Allegheny Technologies Inc. and four subsidiaries of unfair labor practices.

Pittsburgh-based Allegheny locked out 2,200 union workers at plants in six states in August, including 120 members of United Steelworkers Local 1046 in Louisville.

The 31-page complaint filed late Thursday says the lockout was unlawful. Allegheny also repeatedly bargained in bad faith and supervisors unlawfully interrogated employees and threatened them with reprisals, including firing, if they went on strike, according to the complaint.

The filing wasn't a surprise. In December, the NLRB told the company and the union that it would be filing a complaint. The case includes allegations made by the Steelworkers throughout 2015 and early 2016.

A NLRB administrative law judge is scheduled to hold a hearing on the complaint May 23 in Pittsburgh.

In a written statement, Allegheny said it would vigorously challenge the allegations.

The company said its employees have acted lawfully and in good faith and remain committed to successfully resolving the labor dispute.

"This it the beginning of a potentially lengthy legal process that, including appeals, could take years to resolve," the statement said.

The United Steelworkers International welcomed the NLRB's complaint. In a statement, the union said the company has tried to intimidate and manipulate workers into taking deep cuts in pay and benefits.

The company and Steelworkers began negotiating in April, but were unable to reach an agreement before the contract expired June 30. An extension kept workers on the job through July. The company gave union members a final offer Aug. 6 and asked for a decision by Aug. 10. When the union didn't meet the deadline, Allegheny announced the lockout.