Steel

Steel is a refined form of Iron added by various mods. 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, the Industrial Blast Furnace, or the High Oven. It is used for tools, armor and some higher tier machines and tools.

Immersive Engineering
Steel is created in a Blast Furnace.

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):

Tinkers' Steelworks
Anything which when melted in a Smeltery or High Oven produces Molten Steel can be used instead of the Iron Ingot. The output can be poured into a Casting Basin to produce a Block of Steel, or onto a Casting Table with the correct cast to produce Steel Nuggets, Steel Ingots, or steel tool parts.