The United States began to develop D6AC in the early 1960s. It is improved from AISI 4340 steel and is widely used in the manufacture of tactical and strategic missile engine shells and aircraft structural parts. By the mid-1970s, D6AC gradually replaced other alloy structural steels and became a special steel grade for manufacturing solid rocket motor casings. The new American surface-to-air missile "Patriot", small missile "red eyes", large and medium-sized missiles "Minuteman", "Pershing", "Polaris", "Hercules", etc., the φ3.7m booster shell of the US space shuttle The body is also made of D6AC steel. D6AC was also used to manufacture the landing gear and wing shafts of F-111 aircraft.