Durasteel and how to get it has changed a lot over the course of Starbound’s history, ranging from Perturbed Koala to now. If you’re trying to get some these days, there’s two primary methods depending on your version.
If you’re playing on the Unstable version (Cheerful Giraffe), you have access to Durasteel Ore. This can be found and mined primarily on Jungle planets of any difficulty. Two Durasteel Ore can be smelted into a single Durasteel bar. Simply view a planet from your ship, and look for the ore’s sprite in the preview. If you spot it, head on down and begin your search.
If you’re playing on Starbound’s Stable version, there is no available ore. You can make Durasteel Bars by mixing a Titanium Bar and Silver Bar in an Atomic Furnace. It’s likely ore will make its way over eventually, but for now, look for Titanium Ore on Moderate and Risky planets. Two Titanium Ore can be smelted into a Titanium Bar. Silver Ore can only be found on Moderate and above planets, most commonly on Risky ones, deep within the ground. You will also need two ore for a bar.
Previously in Pleased Giraffe, bars were made using Titanium and a Uranium Rod at a Magnetic Crucible, which was just renamed Atomic Furnace later on.
Durasteel is essential to crafting Tier 4 weapons and armor, including the Owl and Dark Knight sets. If you’re looking for some good avian equipment, start mining ASAP.