From Halopedia, the Halo wiki
Vanadium steel is a type of alloy steel used by the United Nations Space Command in the construction of its starships. Vanadium steel cannot withstand the same stresses as Titanium-A alloy and so it is not used as armor plating, but rather in the construction of essential and unarmored components, likely laid down as the basic plating for a ship's hull, welded on over the TR steel superstructure of the ship.
Spy probes utilized by Covenant forces are designed specifically to blend in perfectly with vanadium steel components so that they may carry out their tasks undetected. One such probe was used to track a ship to the human colony of Reach, the foremost military stronghold of the UNSC, resulting in the planet's invasion.
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