There are a variety of numbering systems for materials such as steel. The unified numbering system (UNS) uses a letter to designate a material category followed by five numbers constructed from the existing materials designations to coordinate all of these numbering systems.
The unified numbering system (UNS) is described in Wikipedia at the link:
http://en.wikipedia.org/wiki/Unified_numbering_system
Some of the category letters for steel are:
- G for AISI/SAE carbon and alloy steels;
- S for stainless steels; and
- T for tool steels.
The complete listing of categories can be found in the Engineer’s Handbook:
http://www.engineershandbook.com/Tables/uns.htm
Generally, the standard designation is inserted after the letter defining the material category. For example, AISI/SAE 1018 carbon steel has the UNS designation G10180.
The AISI-SAE steel alloy designation system is listed in the Engineer’s Handbook table:
http://www.engineershandbook.com/Tables/carbonsteelalloys.htm
Also see Wikipedia at the link:
http://en.wikipedia.org/wiki/SAE_steel_grades