A second American steel company has joined U.S. Steel Corp. in trying to import steel slabs for finishing in this country, union officials disclosed yesterday.

The United Steel Workers quoted Wheeling-Pittsburgh Steel Corp. as saying its chairman, Dennis J. Carney, is in Brazil negotiating to buy between 20,000 tons to 30,000 tons a month of unfinished steel slabs from Steelmaker Brazil for shipment to the United States. According to the union, which held a press conference in Pittsburgh, the steel slabs would undergo final rolling and finishing at the company's plant in Steubenville, Ohio.

As its part of the deal, Steelmaker Brazil would invest $400 million in financially troubled Wheeling-Pittsburgh Steel through the purchase of preferred stock.

Officials at the company, the nation's eighth largest steel maker, declined comment on the union announcement and refused to say whether Carney was in Brazil. Independent sources confirmed, however, that he was involved in the Brazilian negotiations yesterday.

Union officials, facing massive unemployment and layoffs in the industry, promised to fight the Wheeling-Pittsburgh purchase plan as they have been battling U.S. Steel's attempts to buy slab steel from Britain for finishing here.

"The only difference is that Brazil is in worse shape than Wheeling-Pittsburgh," said one union official. Brazil, which is becoming a major Third World steel producer, is in the midst of a massive debt crisis--owing $90 billion, more than any other nation.

The union is especially angry at the Wheeling-Pittsburgh action because it has made two wage concessions, totaling $132 million, to help keep the company solvent. In 1982, Wheeling-Pittsburgh had pre-tax operating losses of more than $75 million.

The steel workers' union, trying to galvanize public and political opinion against the purchase of the Brazilian steel, is organizing meetings next week with senators from the three states where Wheeling-Pittsburgh owns major facilities--Pennsylvania, West Virginia and Ohio.

The trend of American companies trying to buy unfinished foreign steel for final processing here started with U.S. Steel's move last March to buy slabs from British Steel for finishing at the Fairless Works outside of Philadelphia. Union officials, saying that move would throw thousands of American workers off the job, have protested vigorously. But U.S. Steel Chairman David Roderick said Tuesday that negotiations are continuing, though "no agreement appears imminent."

Steel union officials have feared that other companies would follow U.S. Steel's example, which one said "will shut down the front end of the industry" and leave the United States in the position of doing finishing work for the rest of the world's steel makers.