It's a steel bar. Useful for crafting much stronger weapons, armour and more!
Removed: No Longer Available
Steel Bar was removed from the game files, and is no longer obtainable.
It was included as placeholder content during early access, and replaced before full release
Steel Bar was a crafting material refined from an Iron Bar and a Copper Bar.
It was the primary component in tier 2 weapons and armor. The crafting schematic for Steel Bar was immediately available when using an Iron Furnace.
It was removed when steel was replaced with tungsten in update Cheerful Giraffe.