It is also known as 4130 steel, and is composed of 0.28 to 0.33% carbon, 0.4 to 0.6% manganese, 0.8 to 1.1% chromium, 0.15 to 0.25% molybdenum, 0.04% phosphorus, 0.04% sulfur, and 0.2 to 0.35% silicon, in addition to iron.
ok im not that smart, i took it from here:
http://en.wikipedia.org/wiki/Chrome_moly