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Effect of crystal structure: BCC metals and their alloys exhibit low-temperature brittleness, while FCC metals and their alloys generally do not. Effect of chemical composition: The purity of steel is a key factor affecting its toughness; the presence of trace amounts of elements such as phosphorus, sulfur, arsenic, tin, lead, and antimony, as well as gases like nitrogen, oxygen, and hydrogen, has a detrimental effect on the toughness of steel. Carbon has a significant effect on low-temperature toughness; as the carbon content increases, the transition temperature between ductility and brittleness in steel rises sharply. Therefore, the carbon content (mass fraction) of low-temperature steel is mostly limited to below 0.2%. Manganese is an element that expands the austenite region; adding manganese to steel can significantly reduce brittleness. By reducing the carbon content and increasing the manganese-to-carbon ratio (Mn/C), the transition temperature between toughness and brittleness in steel decreases. Nickel is more effective than manganese in improving the low-temperature toughness of steel. For every 1% increase in nickel content (by mass) in steel, the ductile-brittle transition temperature decreases by approximately 10%. Steel containing 3.5% nickel retains good toughness at -100°C, while steel containing 9% nickel can withstand temperatures as low as -196°C. Effect of microstructure ① Grain size. Refining the grain structure can increase the toughness of the material. The ductile-brittle transition temperature is linearly related to the ferrite grain diameter d^0.5; the smaller the grain diameter d^0.5, the lower the ductile-brittle transition temperature. This is because impurities and brittle phases exist at the grain boundaries, which often serve as sources of cracks. Refining the grain structure reduces the amount of brittle phases per unit area, thereby increasing the surface energy; this lowers the probability of crack nucleation and propagation, and as a result enhances the steel’s resistance to low-temperature brittle fracture ； On the other hand, fine-grained steel has relatively uniform properties, which lowers the temperature at which toughness transitions to brittleness. ②Heat treatment and microstructure. At lower strength levels, for steels with the same strength but different microstructures, the impact absorption energy and the temperature at which ductility gives way to brittleness are best in the quenched and tempered structure, second best in the normalized structure, and worst in the hot-rolled and annealed states. Furthermore, spheroidizing treatment can improve the toughness of steel. The annealed structure of steel is coarser than that of normalized steel, and its low-temperature toughness is far inferior to that of steel with a normalized or quenched-and-treated structure; therefore, steels used for low-temperature pressure vessels are not subjected to annealing. For low-temperature pressure vessels and their pressure-bearing components that require post-weld heat treatment, the temperature of such heat treatment shall under no circumstances exceed the tempering temperature of the steel. Both quenching aging and strain aging raise the tough-to-brittle transition temperature of steel; therefore, boiling steel, which is sensitive to aging, is not suitable as a low-temperature steel. The effect of work hardening: Work hardening reduces the toughness of steel and simultaneously raises the temperature at which ductile-to-brittle transition occurs. This effect varies depending on the type of steel and the degree of work hardening. The ductile-to-brittle transition temperature of cold-pressed heads is higher than that of hot-pressed heads, and their impact toughness values are also lower. Deformations such as cold bending, cold pressing, and welding during the container manufacturing process can lead to embrittlement; therefore, pressurized components that have undergone cold deformation under certain conditions, as well as those that have been welded, should be subjected to heat treatment. Effect of stress state: When the container has cracks or defects, low-stress brittle fracture is likely to occur ； The sharper the notch, the larger the size of the pre-existing crack, and the more likely it is to cause low-stress brittle fracture. When cracks are present in the welded joint along with residual stresses, low-stress brittle fracture becomes more pronounced. Effect of loading rate: Increasing the loading rate is similar to lowering the temperature, which increases the brittleness of the material and raises the transition temperature between toughness and brittleness. The effect of loading rate on steel brittleness is related to the strength level of the steel. Generally, the ductile-brittle transition temperature of medium and low-strength steels is relatively sensitive to the loading rate, whereas that of high-strength and ultra-high-strength steels is less sensitive to the loading rate. Effect of specimen shape and size: The smaller the radius of curvature of the notch, the higher Tk is; therefore, Tk for V-notch specimens is higher than that for U-notch specimens. When the notch size is unchanged and only the specimen width (or thickness) is increased, Tk increases.