Plasteel was a material used in the construction of Earth Starfleet starships. In 2156, Travis Mayweather reported that the only remnants of the Challenger were traces of plasteel, duranium and polyalloy. (ENT novel: Beneath the Raptor's Wing)
Soldiers of the Starfleet Marine Corps routinely used plasteel to construct their bases. (FASA RPG - The Orions module: Book of Common Knowledge)