The Treasury announced yesterday that its minimum price plan for steel imports will go into effect Feb. 21, even though the agency has not yet computed the minimum prices for products that account for about a quarter of the nation's steel imports.

Even as the Treasury was making its announcement, the nation's second largest steel producer said the so-called trigger price mechanism - the centerpiece of an administration program to aid the financial troubled steel industry -- set the price too low.

If steel is imported at a price below the minimum, or trigger, price, the Treasury will launch an immediate, accelerated investigation to determine whether the steelis being sold at a price below "fair value."

The tigger prices are based on the cost of the world's most efficient steel producer, Japan, and include not only the costs of making steel in Japan but the costs of shipping it to the United States.

Steel makers have long contended that if foreign steel makers, including the Japanese sold their product at the true cost of production - rather than dumping it or selling it below cost - American steel companies could successfully compete with imported steel. In the final months of 1977 imports accounted for 20 percent of the United States steel market.

Dumping of imports is illegal under U.S. trade laws.

Bethelem Steel said yesterday that in its opinion, the Treasury has set most prices too low and do not "adequately reflect Japanese costs."

The Treasury "will not achieve its primary objective of reducing unfair import penetration resulting from dumping unless trigger prices are promptly and substantially revised to represent full and fair costs," it said.

In response, the Treasury said, "We have been and will continue carefully to review evidence which any person presents questioning the level of particular trigger prices and the relationships between trigger prices of various products. If our review indicates that adjustments are appropriate, we will make them."

The Treasury plans to review its trigger prices every quarter.

The Treasury is putting the trigger prices mechanism into effect about a week later than its Feb. 15 target. Officials said the plan conceivably could be put into effect next Wednesday, but said that steel importers need some time to digest the new forms they must fill out.

While American steel makers for the most part have made no comment on the trigger prices, many steel producers say privately that most of the prices are higher than they expected.

Some U.S. producers, including Bethlehem complain that the trigger price system will permit European steel makers, whose costs are higher than those of both Japan and the United States, to continue to dump their product in the U.S.

In response, Treasury officials say that it domestic producers are right that they can compete in the U.S. market with anyone, then trigger prices based on Japan's cost of production will not cause injury to U.S. steel makers.

One Treasury official reaffirmed yesterday that the success of the trigger price mechanism in restraining imports depends directly on how domestic steel makers price their products. If they raise prices substantially, the trigger prices will afford them less protection.

In another development, the President announced yesterday that he had rejected a recommendation from the International Trade Commission that he impose high import duties on industrial fasteners.

Special Trade Representative Robcome from Japan and a rapidly rising yen should put domestic producers in a "more competitive position."