The incident, which occurred yesterday at the blast furnace of the plant in the central town of Zenica, left "six workers slightly injured," police spokesman Senahid Hasic told national radio.
A preliminary investigation showed the blast happened when "a large amount of melted steel" fell into a pool of water, he added.
The blast furnace had been restarted less than three weeks ago after being dormant for 17 years, a step the steelworks described as "an important milestone in the resumption of the integrated route production."
The steelworks was closed at the beginning of Bosnia's devastating 1992-95 war.
Arcelor-Mittal is the world's biggest steelmaker. In 2004, the company bought a majority stake in the Zenica plant, which currently produces 220,000 tonnes of steel per year.
