All AR barrels are steel.
Colt barrels (except for a couple of models that use stainless steel) are Mil-Spec 4150 steel.
Colt chrome-lines the chambers of all of their barrels, but only chrome-lines the bores on their military-profile barrels. So, if the barrel in question is M4 profile or 20" A2 profile, it will be fully chrome-lined. Otherwise, no.
Colt used to fully chromeline all of their barrels, but stopped doing so in 1997.