The Treasury Department announced yesterday it has decided formally that five Japanese steel companies have been selling carbon steel plate here at below fair value - pushing the domestic steel industry toward its first major victory on an anti-dumping case in recent times.

The decision, in response to a petition by the Gilmore Steel Co. of San Francisco, now goes to the U.S. International Trade Commission to determine whether domestic firms have been injured by the Japanese price-cutting.

If the trade commission finds that American firms have suffered injury, the government could impose offsetting duties to make up the difference. The Treasury findings yesterday estimated the Japanese firms had been selling at 5.4 per cent to 18.5 per cent below fair value.

Meanwhile, in a companion action, the Treasury disclosed it has made a tentative finding that four other Japanese steel producers have been selling welded stainless steel pipe and tubing in the U.S. at prices ranging between 1 per cent and 12 per cent below fair value.

In that case, the Treasury will have 90 days more to make a final ruling on the petition. If this second review results in a similar finding, the stainless case also will be sent to the trade commission to determine whether there has been injury to domestic firms.

The decision on the Gilmore case marked the first Treasury has made in recent years that supported the claims of a U.S. steel producer on a major dumping charge. Traditionally, Treasury has been reluctant to approve dumping petitions, as a gesture to free trade.

In an effort to relieve protectionist pressure at home, the Carter administration last autumn began encouraging domestic steel producers to file anti-dumping cases, and more than a dozen have responded over the past 3 1/2 months.

Earlier this month, the administration began a so-called "trigger pricing" system designed to discourage dumping of Japanese and European steel products here by speeding up anti-dumping procedures in cases where imports are sold below pre-set minimum prices.

The Gilmore case was filed last summer before the recent relief action began. The other anti-dumping case involves stainless steel products, which are not included in the trigger pricing plan.

It was not fully certain how the trade commission would rule on the Gilmore case. THe panel is an independent body, with Republican and Democratic commissioners who have few, f any, ties to the administration.

Nevertheless, industry sources were encouraged because the panel has a reputation of being more protectionist than the administration, and therefore likely to approve a finding of injury. The trade commission has 90 days to announce its decision, and therefore likely to approve a finding of injury. The trade commission has 90 days to announce its decision.

The Treasury's finding that the firms involved in the Gilmore case have been selling at 5.4 to 18.5 per cent less than fair value marks a comedown from its earlier estimate. In a preliminary decision in October, Treasury officials put the disparity at 32 per cent.

The Treasury said in its announcement yesterday that the lower figures stemmed from a more "extensive study of Japanese steel costs . . . in connection with the establishment of the steel trigger price mechanism," and called the computation "the best available."

However, industry sources insisted the estimates were unrealistically low.

In the case involving the stainless steel pipe sales, the decision requires importers of those products to begin posting bonds to cover any duties that ultimately may be imposed. The monies would be returned if the final Treasury decision - or ITC ruling - barred any duties.