Steel Composition

Steel is an interstitial alloy of iron and carbon in which the
percentage of C is less than 2%. Some other common elements present in
steel are Manganese (less than 1%) and small quantities of phosphorus,
silicon & sulfur. Iron exhibits two allotropic forms: one is bcc
allotropy at temperatures below 912 deg C and from 1394 deg C to  1539
deg C and the other is fcc allotropy at a temperature between 912 deg C
and 1394 deg C. The bcc structure of iron in the low-temperature zone is
known as alpha ferrite whereas at the higher temperature zone it is
referred to as the delta iron. The fcc structure of iron is termed gamma iron or austenite. Few alloying elements which are also introduced
in steel to improve its mechanical and chemical properties are copper,
niobium, vanadium, boron, molybdenum, nickel, zirconium, titanium, etc.
When Carbon percentage exceeds 2%, it is referred to as cast iron.