Jones & Laughlin Steel Corp., the nation's seventh largest steel maker, was given $111.1 million yesterday in federal loan guarantees, the largest such guarantee under a special loan program for the industry.

The 90 percent guarantee, provided through the Commerce Department's Economic Development Administration, will go toward plant modernization, and is expected to protect the jobs of more than 28,000 workers, an EDA spokesman said.

It is the fifth multimillion-dollar loan guarantee granted a steel maker within the past 15 months, and it comes on the heels of a $1.2 billion request for a government loan guarantee by the ailing Chrysler Corp., which Treasury Secretary G. William Miller has rejected as too large a sum.

At a press conference yesterday in Pittsburgh, Assistant Commerce Secretary Robert T. Hall said the action would help strengthen the firm's ability to compete with foreign steel companies and assure continued operation of its 16 steel-making plants in eight states. J&L is a subsidiary of LTV Corp. of Dallas, a diversified firm ranked 42 on the Fortune 500.

The assistance was granted under President Carter's special steel loan guarantee program started in May 1978. It is intended to assist the steel industry in overcoming the problems of foreign competition and help them invest in expensive modernization and pollution-control equipment, the EDA said.

Part of the reason the company needs the federal guarantees is because another federal agency, the Environmental Protection Agency, is requiring it to conform to stricter pollution standards, an EDA spokesman said.

Since the program began, those receiving 90 percent loan guarantees include: Korf Industries of Beaumont, Tex., $21.5 million; Phoenix Steel of Phoenixville, Pa., $42.6 million; Wisconsin Steel, $90 million and Wheeling-Pittsburgh, $100 million. As a result of those five loan guarantees, EDA estimates that 52,000 jobs in the steel industry have been saved.

In addition to the federally guaranteed loans, the Steel maker must contribute $52.8 million of its own to complete the financing package.