The steel mill is a facility that produces 1x [[steel]] every 35 seconds. [[steel]] used mainly for Construction Material rather than production input. the List of Building that use [[steel]] can be seen below.
You need 1/7th of a [[Coal mine]] and 2/3 of an [[Iron mine]] to maintain this factory.
You need 3 [[Coal mine]] and 14 [[Iron mine]] with 21 [[Steel mill]] to produce without overdoing it.