The Collaborative International Dictionary of English v.0.48:

Alloy steel \Al"loy steel\
   Any steel containing a notable quantity of some other metal
   alloyed with the iron, usually chromium, nickel, manganese,
   tungsten, or vanadium.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

alloy steel
    n 1: steel who characteristics are determined by the addition of
         other elements in addition to carbon