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Steel is commonly considered carbon steel when no minimum content is specified for any alloying element (for example, aluminum, chromium, nickel, molybdenum, vanadium, etc.) or any other element added to obtain a desired alloy effect. The manganese does not exceed 1.65%, the specified minimum for copper in not below .40% or exceed .60% and silicon does not exceed .60%. 

Carbon levels are divided as follows: 
low carbon = .06% to .25% carbon content (mild steel)
medium carbon = .25% to .55% carbon content (medium steel)
high carbon = >.55% to 1.00% carbon content (hard steel) 

Carbon grades are available in the following grade series:
10XX = non-resulpherized carbon steel, with manganese 1.00% maximum (for example 1018, 1045 and 1050).
11XX = resulpherized carbon steel (for example 1117, 1141 and 1144).
12XX = rephosphorized and resulpherized carbon steel (for example 12L14 and 1215).


The second two numbers in AISI-SAE designations indicate the approximate carbon content for steel (for example, in 1018 the carbon range is .15% to .20%; .18% [shown as "18"] is the approximate carbon content).