NEGOTIATORS FOR THE United Steelworkers union and USX Corp. worked Friday to reach a settlement on a new contract for about 20,000 workers in six states before a one-day extension expired at midnight. . . . SPAIN ON FRIDAY said it would sign an agreement with France to construct a 5.2-mile road tunnel linking the two countries. . . . BRITAIN'S MARGARET THATCHER is about to return to the public eye - in the United States - where she will help old friend Ronald Reagan celebrate his 80th birthday. . . . COSTA RICA PLANS to deport all 75,000 of its illegal aliens within the next few weeks, including a number of U.S. citizens and Central Americans, an official said Friday.