Not so many years ago a job in a steel mill was a ticket to prosperity.

A job in a steel mill still pays a lucrative wage, but steel jobs are disappearing.

NBC's "Marvelous Machines--Expendable People," at 10 tonight on Channel 4, chronicles the plight of unemployed steel workers and recounts the by now hackneyed theme that many, if not most, of them will never be as well off as they once were.

Correspondent Edwin Newman tries not to exaggerate the man vs. machine motif, despite the title of the show. He documents the near-obsolescence of much of the industry and the high costs of production at the big mills.

David Roderick, chairman of U.S. Steel, says that steel workers, as a result of raises built into recent labor agreements, earn 110 percent more than the average manufacturing worker. "We gave too much, they took too much," Roderick laments. The high wages that made the steel job so attractive to many Americans is a major reason they no longer have those jobs.

Newman recounts an old story well: the once-proud breadwinner reduced to unemployment lines and charity food banks; the decrepit old steel facility; the angry displaced worker, beer in hand, calling for a return to "the way it was."

It never will go back to the way it was, Newman tells us. But "Marvelous Machines--Expendable People" is as confused about the future as any displaced worker.

High technology may provide jobs, but they won't compare with a mill job. Nearly 75 percent of those employed in the computer industry, for example, make less than $12,000 a year. Only 30 percent of steel jobs pay that little. Despite the low wages, high-tech companies seem to be preparing to move plants to the Far East where wages are lower still.

Newman recounts glowingly the success of Lowell, Mass., where a concerted effort by the public and private sectors--Sen. Paul Tsongas (D-Mass.) calls it almost a "love feast" between the two--attracted many high-tech firms. Lowell's unemployment rate has declined from 14 percent in 1975 to 5.3 percent today. But newcomers, not displaced old residents, benefited most by Lowell's recovery.

And the show's title, calculated to touch off the Luddite in us all, is a misnomer. The problem in the steel industry is not that machines have replaced people--although the introduction of basic oxygen furnaces, continuous casting and computerized rolling mills have reduced the number of workers needed to make a ton of steel.

The key problem is that the giant companies, like U.S. Steel and Bethlehem, failed to keep their technological lead--far more jobs were lost to low-priced foreign steel than to modernized oxygen furnaces--and granted wage settlements that were far too high.

Steel workers have grudgingly given back some gains in the last year--as unemployment in the industry passed the 40 percent level--but wages are far higher than those received not only by workers in Japan and Taiwan, but by non-unionized workers in smaller steel plants in the United States. These so-called mini-mills are thriving at a time when the big steel companies are counting their losses in the billions of dollars.

The steel industry, along perhaps with the auto industry, is merely the most prominent to recede into what seems to be a long-term decline. But much of Smokestack America is in for the same jolt. Many of its workers will join the ranks of what economists call the structurally unemployed, those who have lost their jobs because their jobs are no longer there.

There may be no way displaced steel workers and other blue-collar workers will attain their former affluence. The country should brace for that.