Officials said a large explosion and heavy black smoke could be seen from miles away Sunday morning at the plant on Camp Hollow Road.
Chuck Rice, the head of public affairs for U.S. Steel, said everyone who was in the area was safely evacuated and no injuries were reported.
The fire was extinguished by 2:30 p.m. by U.S. Steel personnel, including a hazmat team, with assistance from the West Mifflin Fire Department, Rice said.
Rice also said the plant is operating normally aside from the 84-ich pickle line where the fire occurred.
The cause of the fire is under investigation.