Two employees were taken to a hospital with serious injuries. Two others were less seriously injured. All were employees and all were men, Lundy said.
The explosion at the plant, located at 6500 S. Boundary Rd., appeared to be a steam explosion. There was no fire, the Northwest Indiana Times reported.
A Nov. 14, 2009 explosion at the same location injured several employees.
Indiana Occupational Safety and Health Administrator Sean Keefer said the agency no longer is investigating the November incident.
Keefer said at the time that IOSHA was satisfied "that Beta Steel had the proper safety protocols in place, and one of the employees veered away from these protocols during their shift, leading to the accident," according to the Times.
A tank at the mill exploded on March 27, 1996, killing three employees and injuring nine more.
Beta Steel is a steel mini-mill specializing in the production and sale of hot rolled coil, according to the company's Web site. The company operates an electric arc furnace melt shop that produces cast steel slabs and a hot strip rolling mill for processing of slabs into flat rolled steel products.