The relevant standard is BS 970 ‘Wrought Steels’. The standard is in six parts:
Part 1 Carbon and carbon manganese steels including free-cutting steels
Part 2 Direct hardening alloy steels
Part 3 Steels for Case Hardening
Part 4 Stainless, heat resisting and spring steels
Part 5 Carbon and alloy spring steels
Part 6 SI metric values (for use with Parts 1 to 5)
Each steel is designated by six symbols:
First three digits
000-199: Carbon and carbon-manganese steels. Digits represent 100 times the percentage of manganese.
200-240: Free cutting steels. Second and third digits represent 100 times the percentage of sulphur.
250: Silicon-manganese steel
300-449 : Heat-resistant, stainless and valve steels
500-999: Alloy steels
Letter
The letters A, M, H and S indicate if the steel is supplied to - chemical analysis, mechanical properties, hardenability requirements, or is stainless, respectively.
Last two digits
These give 100 times the percentage of carbon, except for stainless steels.
Example
070M20: A plain carbon steel with 0.2% carbon and 0.7% manganese. The mechanical properties, i.e. tensile strength, yield strength, elongation and hardness, are given in the standard.
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