IN AN ATTEMPT to control steel imports, the Carter administration last spring imposed a system known as trigger pricing. As usual, the remedy turns out to have unexpected effects. Since steel remains the largest and most difficult of the import issues, it's worth keeping an eye on the way things seem to be working out.

The American steel companies' central complaint has been that foreign producers are subsidized by governments that, to avoid unemployment, are prepared to sell at heavy losses. When several American mills closed down a year ago, the Carter administration reluctantly decided that it had to do something about the scale of foreign imports. The trick was to protect the American companies from cut-throat pricing, without protecting them from the salutary pressure of legitimate competition from aboard. The solution, rather elegant in concept, was the trigger pricing system. The Treasury was to calculate the true producers costs of steel made by the most efficient producers - that is, the Japanese - and publish the figures. Any foreign source offering steel below those prices would be deemed to be selling at a loss and would invite Treasury prosecution for dumping.

When the trigger prices went into effect last May, steel imports dropped. But then in midsummer they started rising again. That's the reason for the current rising volume of protest from the American industry. But the odd thing is that nobody seems to know where all of this steel is going. Despite the higher imports, the production and sales from American mills are holdings up very nicely. Imports plus domestic production add up to much more steel than the country is using. Evidently a tremendous buildup of inventories is taking place. What's going on?

A hint: Since the beginning of the year, the value of the yen has been rising rapidly. Since the trigger prices are based on Japanese costs, they follow the yen upward. That means, first of all, that the trigger price schedules haven't been as much of a restraint on inflation in American steel prices as the administration had hoped. But there's more to it. The trigger prices are recalculated every three months. Each, revision raises the value of steel inventory. Buying and holding steel has become, it seems, a safe and easy way to speculate on the decline of the dollar.

Eventually, of course, the speculators will sell these inventories. If it happens suddenly, it will have a drastic effect on American-production. The prospect makes the American companies deeply apprehensive. Trigger pricing is providing far less assurance of stabiltiy in the steel markets than its authors had hoped.

A better answer - although much more difficult to achieve - is international agreement on the types and sizes of subsidies that will not be allowed in world trade. That subject is prominent on the agenda of the trade talks that have been grinding along in Geneva since 1973 and that will presumably come to a conclusion next year. But the negotiations on steel are particularly difficult because the worldwide pattern of trade is rapidly changing. Most of the imports into the United States come from Japan and Western Europe. Because the Japanese are currently exercising restraint in steel shipments, the volumes from Europe are sharply up. But more important for the future, there has been an astonishing increase in imports from countries outside the traditional industrial world - from South Korea, for example, and from Latin America.

Trigger pricing is at best a temporary expedient. For steel, there's no real alternative to the process of negotiation that is centered on Geneva. The issue is not merely a transient surge of imports into one nation's markets. The world now has the capacity to produce much more steel than it needs, and governments are entering into an increasingly anxious competition to dispose of the surpluses.