A high-pressure tank exploded at a Portage steel mill, killing three workers and hurling metal siding and other debris a quarter-mile away. Nine workers were injured, one seriously. The cause of the blast at the Beta Steel Corp. plant wasn't clear. Workers said a supervisor had been warned Monday that the tank had been leaking. Kevin Myers, one of the three killed, had complained about the tank in a written report to a supervisor Monday, said a worker who asked not to be identified. Someone welded the tank Monday to keep it from leaking, workers said. The company also had a history of safety violations, according to state records.