PITTSBURGH, JAN. 31 -- USX Corp. said today it will issue a new class of stock for its steel unit, paving the way for the nation's biggest steelmaker to quit the business after 90 years and concentrate on more profitable energy operations.

The Pittsburgh industrial giant announced the move -- which is expected to boost the company's stock -- after coming under heavy pressure from corporate raider Carl Icahn.

The company also said it has entered into a "standstill" agreement with Icahn, the largest holder of USX stock.

USX has been under pressure from Icahn to restructure or sell off its steel division, which he believes has weighed down the company's extensive energy business.

"Investors have been clamoring for a long time for USX to split its assets," said David Fleischer of Prudential-Bache Securities Inc.

The company already earns more money from energy than from steel. With the nation in a recession and oil prices high, steel is not expected to turn around, while energy should remain a good bet.

"I think everyone potentially can really win here. There's no losers," said Andrew Gray of Pershing Securities. Gray said he values the separated company at $48 per share, including its $5.3 billion of debt.

The move was also expected to throw further doubt into USX's troubled labor situation. The company had faced a midnight strike by 18,000 members of the United Steelworkers of America, but the union granted a 24-hour contract extension shortly before the deadline.

Union officials said negotiators for the two sides would try to reach a deal overnight. They said the union is prepared to strike if no new agreement is reached by midnight Friday, when the current contract will expire.

One of the union's key demands is that wages and benefits be protected in case of any sale or restructuring of the steel division. The union also wants a pact as generous as those reached previously with USX competitors such as Bethlehem Steel Corp.

USX Chairman Charles Corry said the stock plan is intended to "allow USX shareholders to have separate securities for its two major businesses without restricting USX's future restructuring options."

The company said the move did not represent a spin-off of the steel division, which Icahn has pushed for. It said the new shares would be similar to special-class shares issued by General Motors Corp. in its Hughes Aircraft Co. and Electronic Data Systems Corp. subsidiaries.

But it left the door open for a future spin-off. It said that upon any sale of the steel business, it will either pay a special dividend to stockholders or execute a stock swap.

In October, the USX board approved the transfer of the steel division to a wholly owned subsidiary, a move analysts said might pave the way for its withdrawal from the business.

USX stock was up sharply today, even though the news came out after the close of trading. It rose $2.37 1/2 at $31.25 on the New York Stock Exchange.

USX said it recently held talks with Icahn regarding the proposal and other corporate matters and that Icahn, who holds 13.4 percent of the company, supported the move.

As part of the standstill agreement, Icahn will agree not to buy any more of USX for seven years, will not try to overthrow the board and will support the proposed spin-off at the 1991 annual stockholders' meeting.

USX officials told financial analysts at a meeting in New York today that they expect the company's steel division to post a loss for the first quarter of 1991, whether there is a strike or not, a company spokesman said.

USX said its fourth-quarter earnings rose 9.4 percent to $267 million.