Without going into detail, because it is a complex subject that requires knowledge of chemistry and physical-chemistry, it is necessary to know that crystalline grains structure of the steel is closely related to the temperature at which the material is treated and with its cooling speed.
Commercial steels are never simple iron-carbon alloys, but often contain other elements, including:
Steel can be defined as an iron-carbon alloy with a carbon content less than 2.06%. In practice, however, other elements are always present in non-negligible quantities, so to constitute a complex alloy.