| Hot continuous rolling 403 Nb steel is experiment object in this paper, though creep properties testing, high-temperature tensile properties testing, microstructure observation in order to research on microstructure feature, creep behavior, rules of creep fracture and high-temperature tensile fracture. Obtain conclusions as follows:Phases of hot continuous rolling 403Nb steel are composed of ferrite, NbC mainly distributed as band and Cr23C6. There occurs recrystallization and forms fine equiaxed grain during hot continuous rolling. From 480℃to 520℃, its activation energy is measured to be Q1=287.5 kJ/mol; from 240 Mpa to 260 Mpa, its stress index is measured to be n1= 8.5. Its deformation is formed by dislocation motion, proliferation, cross-slip and grain boundary sliding. Carbide play a part in hindering dislocation motion. Cracks form in grain boundary beside coarse carbide preferentially. Fracture mode of hot continuous rolling 403Nb steel is mixed intergranular fracture and transgranular fracture. The high-temperature strength of this steel is serious influenced by temperature. Its tensile strength is 398.49 Mpa in 480℃, 334.08 Mpa in 600℃. Fracture mode is intergranular fracture.After optimized process, creep life of this steel increases 5.6-7.5 times while raise 100℃than hot continuous rolling 403Nb steel. From 580℃to 620℃, its activation energy is measured to be Q2=564.9 kJ/mol; from 240 Mpa to 280 Mpa, its stress index is measured to be n2= 11.8. Its tensile strength is 646.73 Mpa in 600℃,616.17 Mpa in 620℃. Fracture mode is mixed fracture that give main place to intergranular fracture. Fine carbide is the most important reason of enhance strength. Phases are almost the same before and after heat treatment, composed of ferrite, NbC, Cr23C6. But carbides growth along boundary of lath martensite and concentrate in grain boundary. Little M7C3,M2C and MC type carbide appear at the same time. Its deformation mechanism is dislocation slipping, proliferation, concentrating into dislocation cell with the help of thermal activation. Dislocation cells further concentrate into sub grain structure. Its creep fracture mechanism is crack initiating in larger carbide and triple line boundary, fracture mode is mixed fracture that give main place to intergranular fracture. |
