They are steels with elements of alloy in content superior to 3%.
This type of steels usually present a low carbon content, since they are usually subjected to heat treatment, and with the alloying elements the required mechanical characteristics are achieved. In this way steels of high strength and toughness are achieved. Ni, CrNi and CrNiMo steels belong to this category.
Their most common applications are high responsibility parts and at low temperatures.