Special Treatment Steel (STS), also known as Protective Deck Plate. Originally developed by Carnegie Steel around 1910, it became the U.S. Navy Bureau of Construction and Repair (later Bureau of Ships) standard form of high-percentage nickel steel used on all portions of a warship needing homogeneous direct impact protection armor, except gun mounts and conning towers, where Bureau of Ordnance Class "B" armor was used. Somewhat more ductile than the average for any similar armor, even Krupp's post-World War I "Wotan weich" armor, STS could be used as structural steel, whereas traditional armor plate was entirely deadweight. STS was expensive, but the United States could afford to use it lavishly, and did so on virtually every class of warship constructed from 1930 through the World War II era, in thicknesses ranging from bulkheads to splinter protection to armored decks to lower armor belts.
STS contained approximately 1.75-2% by weight chromium, 3-3.5% nickel, and 0.35-0.4% carbon. Unlike some similar steels, such as Krupp Ww, STS did not use molybdenum.