the pig iron contains some 4% of carbon and it would be too hard and too brittle to use. the extra carbon must first be burned away. the pig iron is refined into steel by decarburizing (burning the extra carbon off of) it. a modern method for decarburizing the pig iron and refining it into steel is basic oxygen furnace. historically there has been other methods as well, such as bessemer converter, open hearth furnace and puddling furnace.