The word "Corten" is a registered brand name of the U.S. Steel Corp. As R. Johnson says, its main purpose is to resist corrosion far better then straight carbon steel.
25 years ago, the piling for an elevated highway in downtown Rio de Janeiro was made nailing down Corten steel structural shapes directly into the earth, with no surface protection whatsoever. For those who know Rio, is the highway going from Praca XV to Benfica. Before doing that, they made a careful study and tests to estimate the degree of corrosion the steel was going to support. They concluded that, after eating up the first 3 millimeters of steel, corrosion was going to stop. 25 years have passed and the highway is still there and in use. If you still have any doubt on Corten steel, get in touch with U.S. Steel who knows it better than anybody.
Giovanni S. Crisi
Sao Paulo - Brazil