The U.S. International Trade Commission ruled yesterday that American steel companies are being damaged by imports of carbon steel plate from Japan.

The decision paves the way for the Treasury Department to impose new tariffs on carbon steel plate, which is used in buildings, bridges and ships.

Meanwhile, in separate announcements, U.S. Steel Corp. said it will close its 12-inch bar mill at its Gary, Ind., works May 15 in an economic move. The shutdown will affect about 200 employees, most of whom will be transferred to other jobs at the works, the company said.

And Bethlehem Steel Corp. said it has hired a New York real estate consulting firm to study possibilities for subdividing and selling its 98-acre Pottstown, Pa., fabricating facility, which was closed in 1976. The complex includes more than 1.3 million square feet of shop area, 30,000 square feet of office space and five receiving and shipping yards.

The ITC decision presumably will result in the imposition of tariffs ranging from 5.4 percent to 18.5 percent of the cost of the imports, the range now used by Treasury in requiring importers to post bonds on the carbon plate they bring into this country.

Treasury ruled in a landmark decision last October that five Japanese companies were selling that product here below their production costs, injuring domestic manufacturers in the process.