Two leading steel firms said they are raising the price they charge for steel--a sign of their new power to increase profits as foreign competitors get squeezed by the dollar's decline. Industry leader USX Corp. and No. 3-ranked Bethlehem Steel Corp. said they have informed their customers that their prices on sheet steel--much of which is sold to automobile companies--were being increased 4% to 8%. Bethlehem said it will raise prices $30 per ton, effective April 3. USX said it informed its customers of a similar price increase in a letter dated Jan. 7.