WAS CLEANING MATERIAL AWAY FROM HOLE IN CHUTE ON TOWER #2. OTHER EE'S WERE WORKING ON A LEVEL ABOVE CHUTE CUTTING A PIECE OF METAL FROM A PLATE.THE PIECE OF METAL AFTER BEING CUT FELL THRU AN OPENING ON THE DECK AND TRAVELED TO LOWER LEVEL WHERE INJURED WAS WORKING. METAL STRUCK INJURED IN THE BACK OF HIS LEFT LEG AT THE KNEE AREA.