Steel grades are classified by their technical properties and/or by their intended use.
Steel refers to all metallic alloys whose main component is iron and whose mass fraction of carbon C is less than 2 %. The two main groups of steels are known as carbon steel and stainless steel. Depending on the combination of alloying elements, the characteristics of a given steel can be very different: very soft and malleable, for example, or rather extremely hard and brittle.
Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten. Carbon and other elements act as hardening agents, preventing dislocations in the iron atom crystal lattice from sliding past one another. Altering the number of alloying elements and the form of their presence in the steel (solute elements, precipitated phase) affects qualities such as hardness, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but such steel is also less ductile than iron.