The steel industry began falling into step yesterday with its acknowledged leader, U.S. Steel Corp., which announced a 6 per cent price increase Monday.

National Steel Corp., the nation's third largest producer, matched the 6 per cent level and gave credence to the widely held belief that the figure shortly will become the industry standard.

Youngstown Sheet & Tube Co., a division of Lykes Corp. of New Orleans, rolled back its scheduled 8 per cent increase to 6 per cent to meet the latest industry standard.

Lykes' president had told newsman, following National's price announcement, that it was "very likely" the firm would reduce the size of its own increase, which had been announced Friday.

A spokesman for Republic Steel Corp., which had set an 8.8 per cent increase on flat-rolled products, said the company would study the U.S. Steel action. He added that he could not say immediately whether Republic would roll back its prices.

The holding action maintained by the second largest producer, Bethlehem Steel, continued to provide an element of uncertainty to the rate-raising announcements. Some industry analysts said most of the second-level producers are waiting to see if Bethlehem will break away from U.S. Steel in a two-tier pricing attempt.

Most analysts believe the 6 per cent figure will hold, however, because it won administration approval and it comes at a time when the steel makers are lobbying the government for relief from foreign steel imports.

The price hikes cover flat-rolled sheet steel used in auto bodies and major appliances, and bar products, which are used in machinery and tools. The 6 per cent increase will add $32 to the cost of manufacturing a car.

Financial analysts said auto makers probably will raise the price of 1978 model cars $300 to $350 because of the steel price increase and other inflationary costs.

"The prices won't go up before the new models are introduced," said Arvid Jouppi, Detroit-based analyst for Colin Hochstin Co.