| Microalloyed steels have received considerable interest over many years and continue to gain wider industrial applications, because of low capital investment and high productivity. Microalloying can get combination of strength-toughness and increase strength greatly. The precipitation behavior of Nb carbonitride and its effect on properties of microalloy by addition of 0.123 wt-% Nb were studied in this work.The ageing-harding method was used to get the dynamics curve of aging. By means of Optical microscopy, TEM and other experiments, the influence of precipitation on the microstructure, mechanical properties and welding performance were studied. The results showed that the dynamics curve of aging have the peak value and valley value. The deformation boosts the precipitation of Nb carbonitride. At the same aging temperature, deformation makes the time of arriving the peak point and valley shorten. At 640℃ aging for 7200s, undeformed steels occurred partly recrystallization and microstructure were tempered sorbite and granular ferrite. Unsolution NbN offered nucleation site and accelerate recrystallization. At 640℃ aging for 300s, the microstructure of the deformed steel was tempered sorbite keeping exposure of the lath martensite without recrystallized granular ferrite. Fine and disperse precipitation of Nb carbonitride prohibited recrystallization of deformation lath martensite. TEM results showed that Nb carbonitride pattern had three kinds: square, globular and disc. Deformation makes pricipitation Nb (CN) became finer and disperser. These precipitate particles, former at grain boundary and dislocation are very small in size and result in large strengthening. The precipitation strengthening of undeformation steel is 83MPa, while deformation steel is 148MPa.The carbon equivalent of steel was 0.138%, crack-sensitivity index was 0.0795 and the highest hardness of heat-affected zone was 214, so the steel has good welding peformance and can be welded without preheating.
|
PDF Full Text Request