The United Steelworkers yesterday endorsed as a "significant first step" the administration's still-evolving plan to help revive the steel industry by setting minimum price guidelines for imported steel.

But the union's Basic Steel Industry Conference warned that it will "demand congressional action establishing legislated quotas" if the pricing policy fails to curb the rising tide of imports, especially the "dumping" of below-cost foreign steel on the steel on the U.S. market.

At a press conference, USW President Lloyd McBride acknowledged that the action represented a partial retreat from the union's earlier call for immediate imposition of quotas on steel imports.

"We're looking for results," said McBride, whose 1.4-million-member union says it has lost 75,000 jobs this year along because of import-caused layoffs. "In this situation, when we're drowning, we're not looking at the brand-name of the life-saying device that's thrown to us . . . If it stops dumping, that's our objective," said McBride.

The action by the conference, comprised of about 500 union officials from the basic steel industry, followed reports by Labor Secretary Ray Marshall and Treasury Under Secretary Anthony M. Solomon on a recommended import-curbing plan that is now before President Carter.

Solomon confirmed that the plan includes a "triggering" system under which duties would be imposed on imports that fall below a government-established minimum, or reference price. That price would be pegged to production and transportation costs of the most efficient foreign producer, in most cases the Japanese.

But he declined to spell out details of this and other proposals, although the conference delegates already had been given one draft of the plan in a packet of union information passed out at the start of the meeting.

McBride said the union had obtained the Administration draft from a reporter. Solomon indicated that, while some changes had been made, he draft was substantially what his ask force had submitted to Carter, who is expected to make his own recommendations next week.

McBride said the union's satisfaction with what the administration recommends will hinge on whether the minimum price is sufficiently close to domestic steel prices to make it unprofitable for fabricators to buy steel from abroad.

"They must be prices which will enable us to recapture lost jobs and create more jobs in the future by increasing the output of domestic steel and generating profits which must, in turn, be plowed back into the modernization and expansion of steel facilities," the conference said in its resolution.

The draft plan, envisioning a reference price within 5 per cent of the most efficient production cost, contemplates reducing steel imports by about one-third; from the current 20 per cent of the American market to 14 per cent.

The draft says such an import reduction would add about 6 million tons to domestic steel production, raise industry earnings by $900 million a year, provide at least 25,000 domestic jobs and raise plant utilization levels to 85 per cent from 81 per cent.

In its preliminary form, the plan also called for modernization incentives - permitting industry to write off new equipment costs in 15 years instead of 18, and directing the release of $215 million for industrial loan guarantees.

It suggested that environmental regulations be reexamined to "ensure that they are economically efficient and do not present any unnecessary barriers to modernization." although it opposed any "relaxation of environmental goals."

It also proposed $20 million in aid for communities stricken by plant closures, consideration of plans for community or worker takeover of abandoned steel facilities, and a review of the adequacy of federal research and development assistance for the steel industry.