The steel mill is a large complex of furnaces and channels of molten Steel. It produces the steel from Iron Ore which is delivered by a transport company.
The steelmaking process is sufficiently interesting that the mill can in fact become a tourist attraction if you place a passenger station close enough. That is, it will accept Passengers if enough of the industry is within the catchment area of the station.
Steel Mills are only available in the Temperate climate, since in other climates iron ore is not available.
You can find out more about Iron Ore, its extraction and usage at None.