In a major victory for the U.S. steel industry, the Treasury Department ruled tentatively yesterday that five Japanese steel companies have been selling their products here at a substantial loss of 24 per cent below their actual costs.

As a result, importers of steel from the five companies were ordered to post, at once, bonds covering "dumping duties" that may later be imposed.

For years, the domestic steel industry has been claiming that Japan was "dumping" steel in this country as a way of making serious competitive in-roads in U.S. markets. The classic definition of dumping has been the sale of a product in export markets below the selling price for the same or similar products in home markets.

But yesterday's ruling, relating to carbon steel products, from Japan amounting to $174 million in 1976, was based on a new formula in the 1974 trade act.

Under this formula, the domestic selling price is ignored, and a so-called "constructive value" or fair market price is figured at actual cost of production, plus an 8 per cent profit margin. On that basis, the Treasury concluded that the Japanese steel was being dumped at 24 per cent below cost, and 32 per cent below the "constructive value". Thus bonds equal to the value of 32 per cent of the value of Japanese Carbon steel exports by these companies must now be posted by importers.

If a final Treasury decision within three months confirms the preliminary findings, it will be referred to the International Trade Commission. If the ITC concludes that the Japanese imports have injured or threaten to injure the domestic steel industry, then the Treasury will impose higher tariffs in addition to the normal 7.5 per cent rate.

These are known as "dumping duties," payment of which would be assured by the bonds.

Treasury General Counsel Robert Mundheim said that the ruling was "the biggest cost of production anti-dumping action" ever taken.

The steel industry has been asking the Carter Administration to take various actions to limit steel imports, which the industry cites as the major reason for declines in domestic steel sales and profitability. The administration, in response, has been urging the industry to pursue anti-dumping claims where they can be proved, but has been fending off pressure for import quotas.

But demands for protection have accelerated in recent weeks as major companies laid off 18,000 workers, and warned that more cutbacks lay ahead.

President Carter late last week appointed a special Treasury task force headed by Under Secretary of the Treasury Anthony Solomon to recommend solutions for the domestic industry's many problems.

Meanwhile, it was learned, the Council on Wage and Price Stability (COWPS) is putting the finishing touches on a lengthy report on steel industry conditions, for delivery to President Carter on Wednesday.

This report requested by the President last summer after the industry announced a series of price increases that were sharply criticized by the Administration. Officials calculated the steel price proposals as the equivalent of a 12.5 per cent annual boost, which they said was intolerable for the economy as a whole.

The COWPS report is expected to recommend against any kind of formal quota relief for the steel industry.

Special Trade Representative Robert Strauss, who said last week that Congressional anxiety over steel industry unemployment might force the Administration to reconsider its opposition to steel "orderly marketing agreements," (OMAs), cautioned yesterday that he did not think this the most probable decision.

In a telephone interview, Strauss acknowledged that a steel OMA, limiting European and Japanese steel imports, would be discussed, because "we are going to look at every single possible solution." But he said he thought a "monitoring mechanism," by which the U.S., the Common Market, and Japan would undertake a voluntary agreement on import levels is more likely.

At luncheon meeting with reporters. Belgian Foreign Minister Henri Simonet warned yesterday that a steel OMA "could touch off others", and put the whole process of trade negotiations at Geneva under a cloud, just as they promise to get started again after "dragging for four years."