PRESIDENT REAGAN has now taken another long step toward the cartelization of the world's steel industry. He has initiated the process of imposing on the specialty steels -- stainless steel and the other high-performance alloys -- the same kinds of import limits that already apply to carbon steel. The philosophy of the free market seems to be fading fast, and leaving, like the Cheshire Cat, only the smile behind.

Specialty steel imports have been under limits before. But the Nixon administration at least considered them temporary. The new arrangements look increasingly permanent. Barriers have been going up around the older industries in the middle range of technology -- first textiles, then cars, now steel -- and there are repeated hints that the Reagan administration will proceed next to protect the new industries in the highest technologies.

It's a strange trend, and takes a lot of explaining, for everybody knows that the enormous expansion of world trade over the past generation has contributed crucially to the rising wealth and standards of living in this country. This week, the International Monetary Fund published an assessment of the world's accelerating trend toward protectionism and concluded that, after three decades, the momentum toward trade liberalization seems to have dissipated. The temptation to resort to trade restraints is always strongest during recessions, but the current performance goes well beyond anything in postwar experience. Why?

The social structures, and the social ideas, of the industrial democracies are growing more rigid. The IMF's economists point out, accurately, that in the long cycles of high growth and low unemployment, the richest industrial countries developed systems of social benefits, and definitions of security and social equity, that resist adjustment to economic change.

Economic growth is, after all, a matter of values and choices. It is not automatic. Neither its rewards nor its costs are equally distributed. Some -- like the specialty steelmakers -- are hit harder than others by expanding trade. Rapid change is always deeply threatening to many people, and uncomfortable for most. If people want to slow the process down, there are ways to do it -- as the Reagan administration is demonstrating with its import restrictions.

But the penalties are measured in terms of the future growth on which it is counting to bring unemployment down. In a world in which trade is controlled and checked by cartels like the one being built in steel, growth rates will be much lower than those to which the world has been accustomed. New jobs will be scarcer, and pay raises less frequent. Fewer new plants will be built, and fewer old ones will be closed. All the rates of change will slow down, the welcome with the unwelcome. Mr. Reagan doesn't seem to have thought much about the implications of the steel cartel. But the time has come for Americans to think about them very carefully.