Atrasteel was an unbeliavably strong and versatile metal alloy, created from carbon, vanadium and iridium among other minerals. It was capable of withstanding extreme temperatures and unimaginable physical stress, even when very thin. Because of those properties, it was used for Warmonger-class Dreadnoughts' hull, the purpose it was actually invented for.
Behind the scenes
The word atrasteel is a portmanteau of atra (Latin word for black) and steel, to emphasize its characteristic, jet black color.