Steel

Steel is a refined form of Iron added by both various mod. IndustrialCraft 2 adds Steel, but it is called Refined Iron. Making Steel requires either the Railcraft or IndustrialCraft Blast Furnace, the Bronze Plated Blast Furnace or the Industrial Blast Furnace. It is used for tools, armor and some higher tier machines.

Railcraft
In a Blast furnace some (undamaged) Iron tools and armor can be blasted into steel ingots as well. Undamaged Steel armor and tools can be blasted back into Steel, especially handy for the steel materials that can commonly found in the village blacksmith.

Blast Furnace (Railcraft):

Immersive Engineering
Steel is created in a Blast Furnace.