Low temperature ferrite steel generally has obvious ductile-brittle transition temperature, when the temperature drops to a certain critical value (or interval), the ductility suddenly drops.According to the appended drawings carbon 0.2% carbon steel impact value and the relationship between temperature, the transition temperature is about 20 ℃ -.Therefore, ferritic steel should not be used below its transition temperature. In general, it is necessary to add Mn, Ni and other alloy elements, reduce interstitial impurities, refine grains, control the size, shape and distribution of the second phase in the steel, and reduce the ductile-brittle transition temperature of ferritic steel (see metal strengthening).
Ferritic low temperature steels are classified into three types according to their components
Low temperature steel ending low carbon manganese steel
Low carbon manganese steel (C0.05 ~ 0.28%,Mn0.6 ~ 2%).Make Mn/C (10), reduce harmful impurities such as oxygen, nitrogen, sulfur and phosphorus, and add some elements such as aluminum, niobium, titanium and vanadium to refine grains.This kind of steel lowest temperature is about 60 ℃.
Low temperature steel spur low alloy steel
Low alloy steel.There are mainly low nickel steel (Ni2 ~ 4%), manganese nickel molybdenum steel (Mn0.6 ~ 1.5%,Ni0.2 ~ 1.0%, Mo0.4 ~ 0.6%, C < 0.25%), nickel chromium molybdenum steel (Ni0.7 ~ 3.0%, Cr0.4 ~ 2.0%, Mo0.2 ~ 0.6%, C < 0.25%).The steel strength higher than that of low carbon steel, the lowest temperature can reach to 110 ℃ or so.China has developed several kinds of low temperature low alloy steels for nickel reduction, such as 09Mn2V.
Low temperature steel medium (high) alloy steel
Medium (high) alloy steel.There are 6%Ni steel, 9%Ni steel, 36%Ni steel, of which 9%Ni steel is widely used for cryogenic purpose.The use of such high nickel steel temperature as low as - 196 ℃.