The Environmental Protection Agency has reached an agreement with two major steel firms to bring six large steel plants into compliance with air and water pollution control laws at cost of up to $350 million.

EPA Administrator Douglas Costle said the pact will mean "significantly cleaner air" and "greatly improved" water quality around the Jones & Laughlin Steel Corp. and Youngstown Sheet & Tube plants in Pennsylvania, Indiana and Ohio when compliance is complete in the spring of 1983.

The complex agreement is actually three separate consent decrees filed in the three states and follows protracted negotiation among the companies, state and local governments, unions, the Department of Justice and the EPA. It is the ninth major steel company agreement to result from understandings between the industry and the Carter administration that deadlines for compliance will be extended but negotated on a case-by-case basis.

Jones & Laughlin has been assessed a $10 million civil penalty in the case for past violations but that will be offset by installation of pollution-control devices that do a better job that the law requires, Monday's announcement said.

Air pollution requirements must be met by Dec. 31, 1982, and water cleanup finished by April 30, 1983.