In a major victory for the administration's new program to restrain steel imports, the nation's largest steel producer yesterday withdrew its charges that Japanese steel makers are selling their products illegally --below cost -- in the United States.

U.S. Steel Corp. said, in response to questions, that it reserved the right to reinstitute anti-dumping charges if it felt the system of minimum, or trigger, prices failed to curb below-cost imports from Japan.

It was the first nod of approval from a major U.S. producer for the administration's intricate and complex trigger price system.

Several other large steel producers --program a chance -- criticized the Treasury for setting the trigger prices too low to help domestic producers.

Privately, however, steel makers say that the trigger prices are high enough to keep out foreign steel.

In the early stages of the program, they concede, U.S. steel consumers are placing more of their orders with American makers than they were last year.

The Treasury's trigger price mechanism is based.

The Treasury's trigger price mechanism is based on the cost of producing and shipping steel products to the United States from Japan, supposedly the world's most efficient maker. If a shipment of steel comes in below the trigger price, the Treasury launches an immediate investigation to see if the products are being sold below fair value.

Unlike the normal anti-dumping proceeding -- such as the type U.S. Steel withdrew yesterday -- which may take 12 to 15 months to complete a finding, the Treasury plans to complete its trigger-price proceedings within three months.

The new system went into effect Feb. 21 for products that account for nearly 75 percent of total imports. The remaining trigger prices should be ready within a couple of weeks, according to Treasury officials.

U.S. Steel said yesterday that it recognized that Treasury did not have enough personnel to adequately construct and monitor the vast trigger price mechanism as well as prosecute anti-dumping investigations of the magnitude of the Japanese case.

However, the company said in a statement, if it finds that the trigger price system is not reducing the in-flow of low-priced steel imports, it will not hesitate to re-file the case it has withdrawn or to file new ones if it thinks it necessary.

In the meantime, the steel produced said, it will continue investigations of steel imports to insure that the price conforms with U.S. fair trade laws.

U.S. Steel has also filed suit in U.S. Customs Court to try to force the Treasury to rule that the European Community violates U.S. countervailing duty laws when it rebates value-added taxes on steel that it exports.

The countervailing duty statutes prohibit government export subsidies. They also require that the U.S. government levy a duty equivalent to the bounty or grant on goods sold in the United States. The Treasury ruled that the tax rebate was not a bounty or grant under the terms of the law.

U.S. Steel said yesterday it would continue to press the countervailing duty case in the Customs Court. The Supreme Court is considering a similar case brought by Zenith Radio Corp. against a wide array of consumer electronic products from Japan. Zenith won in Customs Court, and the Treasury won on appeal.

Last month the Supreme Court agreed to hear the Zenith case.

Treasury officials were pleased with U.S. Steel's announcement yesterday. Anthony M. Solomon, Under Secretary of the Treasury, who headed the task force that devised the steel import program, has said many times that the agency could not run its accelerated anti-dumping procedures for steel and at the same time pursue the exhaustive anti-dumping cases filed by companies.

He said that if companies were unwilling to drop their pending anti-dumping cases, the Treasury might be forced to cancel its trigger price mechanism.

The case U.S. Steel dropped yesterday was filed last fall against six Japanese steel producers.