The incident occurred at the plant in the country's port city of Dangjin, south of Seoul, Yonhap News reported.
The victims were found dead early Friday and authorities said argon gas had leaked from the furnace as they were removing the scaffolding from inside the furnace after scheduled repair work, the report said.
Yonhap quoted experts that while argon is non-toxic, it is denser than air which would make it dangerous in closed areas. The gas is used to check the piping system of the furnace.
Police, along with local labor administration and the Korea Gas Safety Corp., were investigating the incident, the report said.