The development of such homogeneous steel resulted from testing which showed that face-hardened armor was less effective against high-obliquity glancing impacts.
This alloy-steel became known as "Special Treatment Steel (STS)"; 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.
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.
Low-carbon STS became the forerunner of HY-80, which eventually became the standard steel for submarine construction during the Cold War.
[2][3] Unlike some similar steels, such as Krupp Ww, STS did not use molybdenum.