U.S. Steel Corp., the nation's largest steelmaker, yesterday said it will freeze for the rest of the year the published prices on all of its sheet steel products, which have been badly battered by low demand and high levels of imports.

A company spokesman called the action the first of its kind and said the steelmaker hoped it would help invigorate the nation's economy. The company, which recently filed massive unfair trade practices complaints against foreign steelmakers, said it hoped its action would offset the effect of imports on business.

"The devastating impact of unfairly dumped and subsidized imports has severely eroded our markets and seriously suppressed prices," a U.S. Steel spokesman said. "We hope our action will somewhat soften the adverse effects of such imports and will help stimulate an economic recovery in this nation."

The company also said it hoped its decision would "best permit our customers to plan their business activities on a firm price basis for the rest of the year."

The steel industry has been plagued with low demand partly because of the world-wide increases in steel production around the world as well as depressed sales in industries such as autos, homebuilding and appliances that use steel sheet. The sheet and strip products whose prices will be frozen constitute between 35 and 40 percent of U.S. Steel's annual shipments, the spokesman said. They are hot rolled, cold rolled, electrical and galvanized sheet and strip products.

The Commerce Department is investigating whether the steelmakers from nine countries have sold steel here at prices below their production costs or whether they were unfairly subsidized by their governments. Commerce is expected to make its decision on the cases covering 90 percent of steel imports this summer. The International Trade Commission may then decide whether the imports injured the domestic industry.

Imports last year accounted for 10.6 percent of hot rolled sheet, 9.9 percent of cold rolled sheet and 18.9 percent of galvanized sheet sold here, a steel industry analyst said. Import penetration of all steel mill products was 19 percent.

Although the percentage of import penetration in the sheet products was relatively small, that intrusion into the U.S. market was coupled with low demand, making it the most depressed part of the steel business, the analyst said. U.S. Steel and other steelmakers have in recent months been offering sizable discounts below published prices because of the weak demand, the analyst said.

The U.S. Steel spokesman wouldn't disclose what prices the company charged.

American steel mills have been running at about 60 percent of capacity in the last six months, far below the 80 percent level that is generally required to produce profits for the companies. Mills producing sheet are running at about 50 percent capacity, the analyst said.

U.S. Steel's action will tend to stabilize prices for prospective customers, the analyst said. Inventories of manufacturers who use steel have been low, the analyst said, because they tend "to buy hand to mouth" because of price fluctuations and the uncertainty of the economy.

The U.S. Steel spokesman said he didn't know whether the freeze would continue through 1983. "That's strictly a market influence situation," the spokesman said. The last price increase was last June.

The steelmaker hopes it won't lose profits by initiating the freeze.