Eastmet Corp., the struggling Baltimore steel manufacturer, announced yesterday it has filed for bankruptcy after failing to negotiate a new line of credit with its lenders.

The bankruptcy filing Monday came after months of continuous red ink, layoffs and other financial difficulties for the company, which lost $14.5 million through the first nine months of 1985. Under protection of Chapter 11 of the federal bankruptcy code, the company will continue to operate while it negotiates a plan of reorganization with its creditors.

Two weeks ago, the company announced it would curtail production of stainless-steel sheet and strip to concentrate on its traditional stainless-plate line. As part of that move, the company said it would lay off 450 employes, almost half its work force, and take a $40 million write-down.

"We went back to the negotiating table to begin talking about our financial condition [with the company's lenders] as a plate-only operation," said William F. Dausch, the company's president. "It became clear that the debt we had incurred over the last five to seven years was too great to be supported by that business alone. The only option for all of us was to seek protection of the courts."

The lenders involved with providing short-term credit are headed by Irving Trust Co. of New York, and they include Continental Illinois National Bank & Trust Co., First National Bank of Maryland and Equitable Bank N.A. of Baltimore, according to E. Stephen Derby, a lawyer for Eastmet. Derby said the short-term debt totals about $10.4 million.

Eastmet lists in its bankruptcy filing more than $113 million in total liabilities, including $63 million in secured debt. It said its assets total $128 million, without allowing for the $40 million write-down taken in connection with the shut-down of sheet and strip operations.

Donald E. Phelps, a spokesman for Irving Trust, said that discontinuing the sheet and strip business represented a major change "in the character and nature of the company we had lent to." The lenders told Eastmet that, because of this change, "They did not believe they could continue funding past early this week," Phelps said.

Observers, however, described Eastmet's heavy investment in the sheet and strip business in the early 1980s as a heavy drain on resources. They said the opportunity to refocus on its plate business, which commands a big share of the market, could help restore the company to profitability over time.

"The problem was the decision to move into the sheet business," said R. Bently Offutt, a Baltimore researcher who follows the company. "I think it is possible to turn things around." He said the company even could show a profit by the spring, which would be its first since 1981.

"The company is losing money, but certainly the margins associated with plate are better than sheet and strip and ought to be able to support the business," Dausch said. He said the bankruptcy filing "gives us breathing room to accomplish this restructuring.