President Carter, ending six months of negotiations, today will unveil his long-awaited billion-dollar-a-year steel industry revitalization plan.

Key elements of the plan, announced five weeks before the presidential election, are expected to be:

A modified trigger-price mechanism lasting up to five years to give U.S. and European steelmakers a chance to retool and modernize. After the five-year limit, the U.S. will begin vigorous enforcement of anti-dumping laws. Trigger prices are also expected to rise by at least 10 percent and an antisurge mechanism will be created to prevent an influx of low-priced steel during the five-year period.

Relaxed environmental standards to give the industry approximatgely three more years to meet clean air and water deadlines, sources said.

Tax breaks including an increase in the amount of depreciation claimed for investment in plant and equipment and investment tax-credit benefits.

In addition, as a result of the plan of U.S. Steel Corp., which last March filed dumping charges against steel producers in seven European countries, is expected to drop its complaints today, ending, for the moment, a tense, six-month import dispute.

The plan is the result of an election-year push for revitalizing the nation's industries, in this case, steel. A steel tripartite committee was set up consisting of Commerce Secretary Philip M. Klutznick, Labor Secretary Ray Marshall, U.S., Trade Representative Reubin Askew, and union and industry leaders who have agreed that U.S. steelmakers will need to invest $4 billion to $5 billion a year for the next five years to catch up with leading steel producers in Japan, Canada and Western Europe.

What has been in dispute is how much should be done and what should the steel industry be required to do in return.

The plan is also a response to Republican presidential contender Ronald Reagan's own four-point steel revival plan which would allow companies to write off depreciation faster than the 12 years they now have, reinstate the trigger-price mechanism, extend pollution control deadlines, and allow the industry to make a higher "reasonable profit" to generate more investment and job-producing modernization and expansion.

Reagan has used the steel industry's plant closings and layoffs as an indictment of administration economic policies and Carter is said to be anxious to announce his answer to the steel industry's problems.

The steel industry's economic plight was high-lighted by the filing last March of the dumping complaints by U.S. Steel. They were brought after the administration refused to raise the trigger-prices during the second quarter this year. In return, administration officials suspended the trigger-prices, which are supposed to automatically initiate a government investigation of good sold here at prices below specified levels and result in stiff duties for importers.

The administration, after the complaints were filed, attempted to dissuade U.S. steel from filing its complaints against U.S. allies because they could lead to an increase in international tensions and a possible trade war. A month after the dumping cases were filed Klutznick siad that the administration hoped the complaints would be solved by means other than dumping complaints.

The complaints were filed against France, Belgium, Luxembourg, Italy, the United Kingdom, the Netherlands and West Germany.

Domestic steel producers have been dissatisfied with the trigger-prices, claiming that they are too low and place domestic producers at a competitive disadvantage. However, industry official have said they need a modified trigger-price mechanism to prevent dumping.

As recently as last week administration officials were seeking assurances from the European Economic Community that the five years under the trigger-prices would be used by the Europeans to become more efficient steelproducers so that dumping wouldn't be necessary. In the past the Europeans have been urged by the EEC to modernize their plants voluntarily, but they have resisted those efforts because it would be too expensive.

The EEC has approved of the new trigger-price plan, administration officials said. CAPTION: Picture, Steel from Japan: after six months of negotiations, a $1 billion U.S. plan. By James M. Thresher -- The Washington Post