In the process of manufacture, the required chemical composition obtained when the steel in the form of a liquid phase at high temperatures.
At the time of cooling from its melting temperature, the steel begins to change into the solid phase at a temperature of 13,500, in this phase is going on the micro structural changes. Micro structural changes can also be done by heat treatment.
When the cooling process is done slowly, it will be achieved each type of micro-balanced structure in accordance with the chemical composition and temperature of steel. Micro structural changes at various temperatures and levels of carbon can be seen in the phase diagram Balance (Equilibrium Phase Diagram).

Diagram explanation:
In the carbon content reaches 6.67% micro structure formed is called cementite Fe3C (can be seen on the far right vertical line).
Properties - properties cementitte: very hard and very brittle
On the left side of the diagram in which the carbon content is very low, at room temperature ferrite micro structure is formed.
In the steel with carbon content of 0.83%, micro structure is formed Perlit, conditions of temperature and carbon levels is called Eutectoid point.
In the steel with low carbon content up to the eutectoid point, the micro structure formed is a mixture of ferrite and pearlite.
In the steel containing eutectoid point to 6.67%, micro structure formed is a mixture of pearlite and cementite.
During cooling from melting temperature of steel with low carbon content, will form the micro structure of delta ferrite and austenite into micro structure.
In the steel with carbon content higher melting temperature falls with increasing levels of carbon, the transition from the liquid form directly into austenite.
From the diagram above we can see that in the cooling process of change - change in crystal structure and micro structure is very dependent on the chemical composition