A WORD OF ADVICE on the steel cases: don't get entangled in the numbers. Some of them are more or less spurious, and some are already obsolete because of currency fluctuations. The issue here is not whether a certain mill in Belgium might be producing steel plate at a price that's 5.17 percent below "fair value," as defined in a notoriously quirky American statute. Nor is the issue whether the Europeans are entitled to the 5.754 percent of the American market that the secretary of commerce, Malcolm Baldrige, offered in a quota agreement that the industry rejects.

The first real issue is unemployment in the steel towns, and how it is to be divided among a number of countries in which unemployment is rapidly rising. The second is who's to carry the blame in the anguishing process of laying off labor, closing mills and writing off losses. It's always tempting to blame the foreigners with their nasty foreign ways.

The world's steel industry has built far more capacity than it can use. The present slack is not merely the result of the recession. It's quite true that the Europeans undertook a vigorous expansion of steel production, under government leadership, not long ago. The efficient new mills were supposed to replace the old ones but, for the usual reasons, governments have been slow to shut the old ones down. That's an example, incidentally, to keep in mind the next time someone tells you about the wonders that national industrial policies have accomplished for Europe, and how badly the United States needs one. Picking the winners among a future decade's industries is not quite so easy as it's cracked up to be.

The American steel industry is now engaged in a kind of guerrilla warfare through the hedges and brush of the American legal system. It knows that its fortunes do not depend on the small share of the market that the Europeans precariously hold. But it also knows that these tactics generate severe trouble for the Reagan administration in its foreign policy. What is it that the industry really wants? Perhaps it's after even tighter quotas on the Europeans. But even Secretary Baldridge's offer of 5.754 percent would have set a bad precedent. Perhaps the industry wants broader quotas on all imports, regardless of dumping and regardless of source. That would be even worse.

But the most harmful consequence of this campaign against foreign steel is purely domestic. It leaves the American companies' employees, their stockholders and the unions under the impression that somehow the government can restore them to their previous prosperity. That, unfortunately, is incorrect.