steel

Used in armour chiefly during the 15th, 16th and 17th centuries, steel was occasionally found by accident and incorporated into earlier harnesses . Steel is made by adding carbon to iron , but the process was not applied to plate harness until the early 15th century, although this might prove too late as armouries were secretive in their techniques and there might have been houses, particularly the Missiglian or some of the better German houses, that might have known and used steel rather than iron prior to that.
The amount of carbon in steel from which armour was made varies from 0.1% (low carbon) to 0.6% (high carbon). Heterogeneous steels have an uneven distribution of carbon, while heterogeneous steels have a uniform distribution of carbon. Modern mild steels are similar in the amount of carbon present to the lower grade medieval metals, but they are homogeneous rather than heterogeneous. Modern stainless steels have trace amounts of chromium, nickel, silicon, tungsten and vanadium, contain no slag , and polish to a much whiter finish than medieval steels. See also Chronique: The Journal of Chivalry #13 .