From The Science and Engineering of Materials by Donald R. Askeland
AISI steel designations have 4 or 5 digits. The first two numbers refer to the major alloying elements present; the last two or three refer to the average percent carbon.
For example, a 1020 steel is a plain-carbon (10XX) with about .20% carbon (XX20). 4340 steel is an alloy steel with about .40% carbon (XX40) and other alloying elements (43XX). 52100 steel is an alloy steel (52XXX) that has an average of 1.00% carbon (XX100).
I do not have a good reference at this time that relates the first two digits with any specific alloy element and percentage of. If anyone does, please notify me so I can add it to this FAQ.