American Iron & Steel Institute
AISI steel grades are usually a four digit number where the first two digits indicate the alloy and the second two the the carbon content. For example 10xx are plain carbon steel, 41xx Chromium Molybdenum Steel and 43xx Nickel Chromium Molybdenum Steel.
SAE Steel Grades
Society of Automotive Engineers - SAE
SAE also designates steel grades using four digit numbers which represent chemical composition standards for steel specifications. The AISI started a similar system which have over time used the same numbers to refer to the same alloy,
DIN – Deutsches Institut für Normung - define West German steel specifications. The specifications are preceded by the letters DIN followed an alphanumeric or numeric code. The numeric code is the Werkstoff number, this uses numbers only with a decimal point after the first digit. Werkstoff - material, the stuff something is made from.
Below is a table of some of the more popular steel grades along with the BS 970 1991, AISI/SAE and Werkstoff equivalents.