U.S. Steel Chairman Degar B. Speer's surprise decision to leave the board of American Telephone & Telegraph Co. renews speculation that the ailing chief executive may soon step down from his post as head of the nation's biggest steel maker.

Speer, 62, has been ill off and on for the last few years and has not been at his desk regularly since last fall after undergoing surgery for what company spokesmen describe as stomach ulcers.

If Speer does step down, which could happen as early as the company's annula meeting May 7, it is likely that President David M. Roderick would succeed him. Roderick had been U.S. Steel's top financial executive until he was named president in the summer of 1975.

However, industry sources say, William R. Rosesch, executive vice president for steel and domestic raw materials, must be considered a dark horse candidate for the company's top job.

Roesch, 53, has been chairman and president of Jones & Laughlin Steel Corp. (a subsidiary of LTV Corp. which late last year merged with Lykes-youngstown to become probably the nation's third biggest producer) as well as chairman of Kaiser Industries Corp.

Roesch shocked the steel community in October, 1973, when he announced he was leaving the old line Jones & Laughlin to head to the West Coast. But Rosech apparently had a falling out with Kaiser Chairman Edgar F. Kaiser and left the company in 1977. Rosech joined U.S. Steel in early 1978.

Speer joined U.S. Steel in 1938 as a metallurgical observer and worker his way up through the company on the "operations" side. For two years in the late 1950s he was general superintendent of U.S. Steel's Fairless Works, probably the company's most important production facility, and in 1958 was named general manager of the company for all its steel operations.

He became chairman of the giant steel maker, which regularly ships more than 20 percent of all steel produced in the Unites States.

There have been rumors for more than a year that Speer's ill health may force him to leave the giant steel maker-which also is an important chemicals producer.

A company spokesman said U.S. Steel would have no comment on Speer's plans.

At last year's annual meeting Speer told shareholders that rumors about his ill-health were unfounded and that he planned to be around "for a long time." CAPTION: Picture, EDGAR B. SPEER . . . ill off and on