Low temperature steel is steel usually applied below 0℃. Based on the crystal structure, low-temperature steels can generally be divided into ferrite low-temperature steels and austenite low-temperature steels. Ferritic low-temperature steels generally have significant toughness, that is, brittle transition temperature. When the temperature drops to a certain critical value (or range), the toughness will suddenly decrease. The impact value conversion temperature of 0.2% carbon steel is about -20°C. Therefore, ferritic steels should not be used below their transition temperature. The addition of alloying elements such as Mn and Ni can reduce interstitial impurities, refine grains, control the size, shape and distribution of the second phase, thereby reducing the toughness-brittleness transition temperature of ferritic steel. The alloying elements in low-temperature steel mainly affect the low-temperature toughness of the steel. Today, we will give you a detailed introduction:
C
The brittle transition temperature of steel increases rapidly with the increase of carbon content, but the welding performance decreases. Therefore, the carbon content of low-temperature steel should be limited to approximately 0.2%.
manganese
Manganese can significantly improve the toughness of low-temperature steel. Manganese mainly exists in the form of 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 transformation temperature (A1 and A3) to produce fine and ductile ferrite and pearlite grains, thereby increasing the maximum impact energy and reducing the brittle transition temperature. Therefore, the manganese-to-carbon ratio should be at least 3, which not only reduces the brittle transition temperature of the steel, but also compensates for the mechanical properties caused by the reduced carbon content due to the increased manganese content.
Ni
Nickel can reduce the brittle transition tendency and temperature of steel. The low-temperature toughness of steel increases by 5 times that of nickel-manganese, while the brittle transition temperature decreases by about 10°C for every 1% increase in nickel content. This is mainly because nickel does not react with carbon and dissolves into solid solution for strengthening.
Nickel also causes the eutectic point of the steel to move to the lower left corner, reducing the carbon content and phase transition temperature of the eutectic point (A1 and A2). Compared with carbon steel with the same carbon content, the amount of ferrite is reduced and refined, and the amount of perlite is increased (the earliest carbon steel has a lower carbon content than carbon steel). Experimental results show that the main reason for improving nickel toughness at low temperatures is that there are many movable dislocations in nickel steel at low temperatures and they are prone to cross-slip.
P,S,Ti,AS,SB,PB
Elements such as phosphorus, sulfur, arsenic, tin, lead, and antimony have adverse effects on the toughness of low-temperature steels. They produce segregation in the steel and reduce the intergranular resistance, which results in brittle cracks originating at the grain boundaries and extending along them to complete fractures. Phosphorus can increase the strength of steel, but it also increases the brittleness, especially the low-temperature brittleness, and significantly increases the brittle transition temperature. So their content should be strictly limited.
H,O,N
These elements will increase the brittle transition temperature of the steel. The low-temperature toughness of steel can be improved by using silicon and aluminum to deoxidize and kill steel, but silicon will increase the brittle transition temperature of steel, so aluminum-killed steel can obtain a lower brittle transition temperature than silicon-killed steel.
