They are the most important for the industry because among them are the derivatives of Steel and Iron. In steels, which always contain carbon as an alloying element, two classes are distinguished: carbon steels and alloy steels. In the former, a minimum content of aluminum, chromium, cobalt, niobium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium or any other element added to obtain a given effect is not specified, and, instead, maximums are set for the manganese (1.65%), silicon (0.60%) and copper (0.60%). On the other hand, in alloyed steels, the manganese, silicon or copper contents are higher than those mentioned or a minimum defined for any other element is prescribed, within the limits accepted commercially for these steels (3.99% for the aluminum and chromium, 5.25% for cobalt, niobium, molybdenum and nickel).

Alloyed steels become increasingly important as metallurgy develops more accurate methods to give them valuable properties by adding small amounts of alloying elements. A very notable example is the use of chromium in the production of stainless steels, resistant to corrosion, heat and wear, with which cutting tools, armored plates, rails, apparatuses for the chemical industry and automotive parts are manufactured. subjected to severe work such as valves, bearings and crankshafts.

An iron alloy of growing interest is ductile graphite iron, also called nodular cast iron, which is obtained by adding small amounts of magnesium to molten iron, because it combines the iron price, typical of cast iron, with mechanical strength, ductility and strength. to heat and wear of steel. In 1949 the first licenses were granted for its manufacture and at five years it was already produced in more than 600 factories to build crankshafts, gears, bearings, plowshares, valves and many other articles.