The cracking of carbon steel and low alloy steel at low temperature is related to the strength and low temperature toughness of the steel, while the strength and low temperature toughness of the material are related to the chemical composition, microstructure and liquid quality of the material. Subject on the basis of LCC, optimization of the main alloying element, add 0 ~ 0.08% niobium microalloy element, design quenching + tempering, annealing, tempering, normalizing + tempering three kinds of heat treatment process, through the microstructure observation, SEM scanning and energy spectrum analysis, XRD analysis, the low temperature impact test and tensile test, niobium is studied and the influence of heat treatment process on the microstructure and mechanical properties of low temperature steel are studied. This topic in low temperature tempered well divided into C0.17% ~ 0.19%, Si0.5% ~ 0.6%, Mn1.1% ~ 1.2%, P 0.02% or less, S 0.02%, or less Cr0.5% ~ 0.6%, Ni0.98% ~ 1.00%, Mo0.5% ~ 0.6%, Nb0.03% ~ 0.05%. The cast microstructure of the low temperature steel was lamellar pearlite plus a small amount of block ferrite. With the increase of niobium content, the distance between the plates of pearlite decreased, the block ferrite decreased, and the microstructure refined. The microstructure of the samples after heat treatment is tempered Soxhlet, and the ferrite in quenched + tempered tempered Soxhlet is basically multilateral in shape and uniform in size. The ferrite in annealed and tempered sorbent is mostly strip, and the polygonal ferrite is less. Normalizing + quenching and tempering state is polygonal ferrite with a small amount of lath ferrite, the length of lath is shorter than annealed + quenching and tempering state. The quenched and quenched quenching and tempering low temperature impact energy of the steel first increased and then decreased with the increase of niobium content, and the degree of decline was faster at -40℃ than at -20℃. When niobium content was 0.039%, AKv at -20℃ and -40℃ reached the maximum value of 45.7J and 29.9J, respectively. The impact fracture morphology of the sample at -40℃ is dominated by cleavage, with a small amount of quasi-cleavage. The cleavage step is mainly fan-like pattern, and there are tearing ridges inside.
With the increase of niobium content, the length and number of tear ridge gradually increased, and the dimple band increased. When the niobium content exceeded 0.039%, river pattern appeared on the cleavage plane. The changes of low temperature impact work in the three heat treatment states of the low temperature steel are basically similar. The quenched and quenched quenching and quenched tempering state with the same niobium content is the highest, followed by normalizing quenching and quenched tempering state, and annealed and tempered state are lower. AKV of samples in three heat treatment states reached the highest value when niobium containing 0.039%, which was 45.7j, 42.3j and 44.4j at -20℃, and 29.9j, 27.6j and 28.4j at -40℃, respectively. The number of tearing ridge in quenching + quenched quenching and normalizing quenching + quenched quenching + tempered quenching + tempered quenching -40℃ impact fracture morphology is significantly greater than that in annealed + tempered quenching. The cleavage step of annealed + tempered fracture morphology is fan like pattern, and the cleavage plane is river like pattern. The low temperature impact energy and fracture morphology show that the quenching and quenching, annealing and normalizing quenching and tempering processes can meet the requirements of the microstructure and properties of low temperature steel when niobium containing 0.039%, and the normalizing and tempering processes can be used in production. The tensile strength, yield strength and elongation after fracture of the normalizing and tempering state of the low temperature steel increase first and then decrease with the increase of niobium content. The variation rules of tensile strength and yield strength are very similar. When the niobium content is 0.057%, the tensile strength and yield strength reach the maximum values of 1110MPa and 990MPa, and when the niobium content is 0.039%, the elongation after fracture reaches the maximum value of 17.0%. The macroscopic morphology of the tensile specimen is typical cup-cone fracture, and the microscopic morphology is equiaxial dimple, showing typical characteristics of ductile fracture. With the increase of niobium content, the number of thick dimples decreased first and then increased. The low-temperature steel containing 0.039% niobium was treated by normalizing and tempering process, and the strength, plasticity and low-temperature impact toughness were well matched. The impact energy at -20℃ and -40℃ were 44.4j and 28.4j, the tensile strength and yield strength were 995MPa and 885MPa respectively, and the elongation after fracture was 17.0%. The low temperature steel has good comprehensive performance, which can be applied to the low temperature valve parts in the oil and gas pipeline transportation system, and has a good development prospect.
