INJURED AND CO WORKER WERE WELDING TWO 30' JOINTS OF 6"" STEEL PIPE TOGETHER. AFTER WELDING TOP SIDE OF PIPE, BOTH MEN USED SLATE BARS TO ROLL PIPE OVER. WHILE ATTEMPTING TO LIFT THE SLATE BAR, INJURED FELT PAIN IN LOWER BACK. INJURED FINISHTHE SHIFT AND REPORTED FOR WORK ON MONDAY 2-25-02 BUT LEFT AT 100 PM WAS TREATED AND RELEASED FROM HOSPITAL ER.