Carbon Steel
A carbon steel is an interstitial alloy of only iron and carbon. The carbon has a limited solid state solubility in the iron, and above 25 atomic % carbon a compound of iron and carbon, Fe3C is formed.

The diagram shows the iron-rich end of the iron-carbon phase diagram. The verttical line at 6.7 weight % carbon is Fe3C, and for the field shown this material and iron behave as a binary system. The equilibrium phases are shown on the diagram. Ferrite (a) is bcc iron with carbon in interstitial solid solution. Austenite (g) is fcc iron with interstitial carbon, and d-iron is bcc iron with interstitial carbon in the high temperature range. Boundaries with two phase zones represents the solubility limit for carbon in these phases.

From:Newey and Weaver, "Materials in Action" Butterworth (1990)