The "standard steel" used to make rifle barrels is chromemoly steel. This is a steel alloy with chromium and molybdenum added to the alloy.
There are two types of chromemoly steel commonly used: 4140 and 4150. 4140 is less expensive and easier to machine, and is used in virtually all commercial rifles, and by most AR manufacturers. 4150 steel is more expensive, somewhat more difficult to machine and work, and is the Mil-Spec steel for most US military rifles, including M16s, M249 SAWs, M240 GPMGs, and others. The primary advantage of 4150 is that it retains its structural strength better at higher temperatures than 4140, which means both better accuracy and a lower likelihood of the barrel bursting when the barrel is very hot. Only Colt, Bushmaster, FN, and LMT make 4150-barreled ARs, and DPMS makes special runs of 4150 barrels for CMMG.
Chrome-lining a steel barrel with industrial-grade hard-chrome protects the barrel from corrosion and rust, allows easier extraction of fired brass from the chamber, and extends the life of the barrel. Contrary to popular belief, modern chroming will not affect barrel accuracy. That may have been true in the 60s, but materials science has come a long way since then.
The other common steel used for ARs and other rifles is stainless steel, usually 416 stainless. Chrome-lining can't be done cost-effectively to a stainless steel barrel due to the different process required to chrome-plate stainless steel. Most stainless barrels are target barrels anyway, and the increased resistance to corrosion of the steel itself is usually sufficient for most applications.