Low-temperature steels generally refer to steels applied below 0℃. According to the crystal lattice type, low temperature steel can be generally divided into ferrite low-temperature steel and Austenite low-temperature steel. Ferrite low temperature steel generally has obvious toughness, that is, brittle transition temperature. When the temperature drops to a certain critical value (or range), the toughness will suddenly decrease. The shock value conversion temperature of carbon steel with carbon content of 0.2% is about -20℃. Therefore, ferrite steel should not be used below its transition temperature. The addition of Mn, Ni and other alloying elements can reduce interstitial impurities, refine grain, control the size, shape and distribution of the second phase, etc., so as to reduce the Ductile – Brittle transition temperature of ferrite steel. Alloying elements in low temperature steel mainly affect the low temperature toughness of steel. Here today, we will specifically introduce it to you:
The brittle transition temperature of steel increases rapidly with the increase of carbon content, but the welding property decreases. Therefore, the carbon content of low-temperature steel should be limited below 0.2%.
Manganese can obviously improve the toughness of steel at low temperature. Manganese exists in steel mainly in the form of a solid solution and has the function of solid solution strengthening. In addition, manganese is an element that extends the Austenite zone and reduces the phase transition temperature (A1 and A3) to produce fine and ductile ferrite and Pearlite grains, thus increasing the maximum impact energy and reducing the brittle transition temperature. Therefore, the manganese-carbon ratio should be at least 3, which not only reduces the brittle transition temperature of steel but also compensates for the decrease in mechanical properties caused by the decrease in carbon content due to the increase in manganese content.
Nickel can slow down the brittle transition tendency and temperature of steel. The low-temperature toughness of steel increased by Nickel is 5 times as much as that of manganese, and the brittle transition temperature decreased by about 10℃ for every 1% increase in nickel content, which is mainly due to the fact that nickel was not reacted with carbon and all dissolved into the solid solution to strengthen it.
Nickel also causes the eutectoid point of steel to move to the lower left, reducing the carbon content and phase transition temperature (A1 and A2) of the eutectoid point. Compared with carbon steel with the same carbon content, the ferrite quantity is reduced and refined, and the Pearlite quantity is increased (The Pearlite also has lower carbon content than carbon steel). The experimental results show that the main reason for improving the toughness of nickel at low temperatures is that there are many movable dislocations in nickel steel at low temperature and cross slip is easy to be carried out.
P,S,Ti, As,Sb, Pb
Phosphorus, sulfur, arsenic, tin, lead, antimony and other elements have adverse effects on the toughness of steel at low temperature. They produce segregation in steel and reduce grain boundary resistance, which causes brittle cracks to originate from grain boundary and extend along grain boundary until complete fracture. Phosphorus can improve the strength of steel, but increase the brittleness, especially the low temperature brittleness, and obviously increase the brittle transition temperature. So their content should be strictly limited.
H, O, N
These elements will increase the brittle transition temperature of steel. The low temperature toughness of the steel can be improved by using silicon and aluminum deoxidized killed steel, but silicon will increase the brittle transition temperature of the steel, so the aluminum killed steel can obtain a lower brittle transition temperature than silicon killed steel.