The explosion happened at around 5.40 a.m. at the Nam Kim Steel plant at an industrial park in Binh Duong Province, 60km (37 miles) from Ho Chi Minh City, state-run Vietnam News Agency
The molten metal explosion occurred while workers were handling a smelting pot filled with zinc and aluminum.
The accident injured eight workers, four of whom suffered extensive burns to more than 70% of their bodies.
The victims were rushed to Binh Duong Hospital for treatment.
“Six people with severe burns have been transferred to Cho Ray Hospital,” a doctor later said.
The cause of the explosion is under investigation.
Nam Kim Steel, based in Thuan An Commune, is a top steel producer with three plants in Vietnam.
The company controlled 15.2% of Vietnam’s galvanized sheet steel market and 5.14% of its galvanized steel pipe market last year, data from the Vietnam Steel Association showed.