The Carter administration announced yesterday it will aim its previously unveiled $100 million loan-guarantee program for the steel industry primarily at large and medium-sized producers who can't obtain loans on their own.

The Commerce Department's Economic Development Administration said underwritting would be limited to firms with production capacities of 250,000 tons or more, where authorities believed modernizing would keep a plant afloat.

Robert T. Hall, the assistant Secretary of Commerce who is heading the program, said the restrictions were intended to channel the loan guarantees to companies where the impact on employment would be greatest.

The $100 million in loan guarantees is expected to generate loans totaling $550 million or more. The money could be used by the steel companies for either plant modernization or purchase of pollution-control equipment.

The loan-guarantee program is part of a massive rescue plan for the steel industry worked out by an interagency task force headed by Anthony M. Solomon, undersecretary of the Treasury for monetary affairs.

Among the task force's recommendations was the recently begun system of minimum prices for imported steel products, under which anti-dumping action is begun automatically when foreigners violate its terms.

In addition to the loan-guarantee plan for the steel industry, the Economic Development Administration also is working on a program to provide $20 million in aid to communities hard-hit by steel-plant closings.

The steel industry has been in financial difficulty in recent years, in part because of competition from imports and in part because U.S. firms have lagged behind in plant modernization.