yeah, a bit of tube-snobbery going on here. Was a time when most bikes were hi-ten and the fancier ones were 4130 (CrMo steel.) The very high end were 531. Steel has now gone the way of high-tech, where if it's not some just-declassified material some folks will judge it as junk. Fact is that a well-built frame made from 'generic' 4130 will outperform a poorly-built frame made from 631.

Gas pipes are actually made from "black iron" steel, which is a very basic steel with a high carbon content. More recently, corrugated stainless tubing or copper is being used for gas pipe.