The blast on Wednesday night triggered a fire in the plant in the coastal city of Visakhapatnam. At least 16 other workers suffered serious burn injuries.
The blast took place when workers were checking a newly-installed oxygen unit in the plant, reports said.
The death toll is expected to rise as some of the injured are critical.
local media reported that at least three senior officials were among those killed.
Witnesses said the explosion was so powerful that cars parked outside the unit were flung in the air.
The 41-year-old Rashtriya Ispat Nigam Limited (RINL) plant, located 600km (370 miles) from the state capital, Hyderabad, is one of India’s biggest steel producers.
The plant has expanded over the past four years and added new units.