The lexicon of the top500 ( The Encyclopedia for Classification Codes ) explains the codes and definitions in the world of economical classifications.
NAICS Code 3311 Iron and Steel Mills and Ferroalloy Manufacturing
The NAICS Code 3311 is described as
Iron and Steel Mills and Ferroalloy Manufacturing It belongs to the code group
NAICS Code 331 Primary Metal Manufacturing
To this Code "3311" belongs the sub codes
NAICS Code 33111: Iron and Steel Mills and Ferroalloy Manufacturing