Steel

Steel was a type of metal alloy made primarily of iron and a much smaller percentage of carbon.

Uses
On Earth, during the 13th century, Humans were still perfecting the mechanical clock and using steel crossbows in battle at the time the Promellians were building interstellar warships. 

The emergency baffles used in the ventaliation system at the facility on the trellium mining planet were steel plates. These plates would be locked into place during maintenance cycles. 

During the ecological attack caused by the Whale Probe that visited Earth in 2286, steel plates were utilized to reinforce the windows at Starfleet Command. 

In explaining the appeal of the game of darts to Quark, Miles O'Brien stated that because it involved "No lasers, no holograms; just steel tips and feathers."

The level of metallurgic technology achieved by the species who inhabited the class L planet, in the Delta Quadrant, that B'Elanna Torres crashed on in 2376 were not yet able to produce steel, as they were still at the bronze level of development. 

Figurative uses
Ira Graves described himself as "an incredible man, possessing an iron will and nerves of steel."

Dukat complimented Kira Nerys by stating that she was "the embodiment of the new Bajor-- a Bajoran born out of the ashes of the occupation. A Bajoran tempered with Cardassian steel."

A portion of the scripture spoken at the wedding of Worf and Jadzia Dax, explained that "with fire and steel did the gods forge the Klingon heart."

Also see

 * Bolian crystalsteel
 * Carbon steel
 * Chrome steel
 * Parsteel
 * Steel fortress
 * Steelplast