America's steel industry, which has been under siege from imports for years, is getting sweet revenge -- at least a few tons of it.

USS, the steelmaking division of USX Corp., announced yesterday that it is reactivating its international subsidiary, which has been largely dormant since 1984.

The export unit, United States Steel International Inc., is being restarted "to sell USS products outside the United States now that currency ratios have become more advantageous for American export activity," according to Thomas C. Graham, president of USS.

USS, which always prided itself on being on international company, exported more than 2 million tons of steel a year from 1973 to 1975, or 10 percent of total shipments. The company had 300 employes stationed throughout the world and 50 agents drumming up foreign sales.

Currently, the company is exporting negligible amounts of steel. "We have to start from scratch," said Giuseppe Colombari, a longtime USX executive who has been elected president of the reactivated export subsidiary.

Colombari said the company hopes to export a full line of products to developing countries, as well as to reassert itself in Europe. Japan and China also will be targeted.

"Our costs on a parallel basis have started looking better and better," Colombari said. "We think this condition will prevail for the next few years at least."

An example of how the tables have turned on the cost front: According to PaineWebber's World Steel Dynamics, a metric ton of cold-rolled steel cost $750 a ton in Japan last week, while U.S. costs were only $525 a ton on a pretax basis.

"The U.S. is fully cost competitive with Japan and Western Europe here," said John Jacobson, director of basic industries with WEFA Group. "In many product lines, the advantage carries over to export markets."

Like many other domestic steel companies that have been receiving inquiries from foreign customers, export activity for USS may be limited by capacity restraints because domestic demand is strong now.

"We have had some activity, but current operating rates are quite high and our first priority is to satisfy domestic customers," said Mark Tomasch, a spokesman for LTV Steel.

Similarly, Bethlehem Steel Corp. said it has little steel available for export because of the strength of the domestic market.

Armco Inc., however, has gotten more aggressive on the export front and its specialty steel group filled an increased number of foreign orders this year.

"We have steel mill merchandising people in Europe and we hope to see some encouraging signs there," an Armco spokesman said.

Overall, steel exports fell below a million tons from 1984 to 1986, according to the American Iron and Steel Institute, from a peak of 7.1 million tons in 1970.

With improvements in quality and currency advantages, U.S. exports are expected to top a million tons this year, with most of the shipments being in higher-value steel products.