Much of what a steel mill needs for ancillary structures depends on the era. Some thoughts...
Most of us think of the furnaces and then the shops to forge the steel into ingots and then blooms and then finished products. Massive brick and stone buildings in the earlier day which morphed into various types of corrugated sheathed structures in the more modern days.
Mills need lots of electric power as well as natural gas for gas blowing engines. They also need raw materials of iron ore and scrap and coke for carbon in large amounts so plan on lots of neat ore pits and transfer bridges/equipment.
They also need a steady supply of things such as dolomite, in newer mills this is shipped in center flow hoppers. Not to forget the other "ingredients" of lesser quantities such as manganese or chromium or vanadium.
And getting the finished steel on the outbound car was of utmost importance because once it hit the car deck and the car moved it was considered sold and shipped for billing purposes. 
Two interesting in-depth reads are Nevin Yeakel's "Bethlehem Steel Railroading" as well as the March 1994 Railpace "Bethlehem Steel Railroads. The first is by a PB&NE trainmaster that worked the steel and the latter covers much of the mill before it's closure. Both have lots of good pictures as well as track maps, etc.
The Nevin Yeakel book can be found with some searching, the Railpace back issue is seldom seen.
Oh, and "steel mill" doesn't just mean someone that produces iron or steel to sell. Many businesses "rolled their own" in earlier days. For example, in the steam era Bucyrus Erie in South Milwaukee made their own steel for their heavy equipment from raw materials.
