Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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