It's actually defined best in the A&E errata found on the website:
Steel: Steel is an alloy of iron and either carbon, manganese, chromium, vanadium, or tungsten. Steel is harder and more durable than iron but require different smelting techniques to create which may not be known in all cultures. The process for creating steel is reflected in its x4 Cost Multiplier.
Items made from steel gain +1 AP. There is no increase in ENC.