Steel is an alloy of iron and carbon. Most swords and armour in early period were not made of steel, but iron, owing to the difficulty of forging steel. Later period smelting techniques were able to produce good quality steel and in large quantities.
In period, Japanese swords are notable for the quality of their steel despite being produced from poor quality (and extremely scarce) iron sands. Damascus and Toledo blades were also renowned for their quality.
Both steel and iron will form iron oxide (rust), which weakens the metal as the rust is water soluble and washes the material away exposing more of the original metal to oxidize. For this reason, period armour was often painted or left covered in forgeblack, rather than being polished. If armour was polished, it needed frequent scouring and oiling to maintain it.
Plain steel is often called mild steel by modern metalworkers, and will rust quite quickly if left unoiled. Modern stainless steel is less prone to oxidization, but will rust if left in a pool or water or sweat. Modern galvanized steel turns dark grey or black as the coating oxidises, but doesn't lose any strength from this process. Do not hot-work galvanized steel as it produces toxic fumes at high temperatures. Galvanized steel was developed during the industrial revolution and not used in period nor was stainless steel.
See Also: live steel.