Research On Microstructure And Properties Of Nb、V Micro-alloyed The Coiled Tubing

In this paper,Gleeble-3500thermo-mechanical simulator and Lenton heat treatment furnace were used. Through testing the continuous cooling transformation (CCT) curves and quenching and tempering the experiment steels. And using the modern analytical methods. Mainly studied the effection of micro-alloying elements Nb,V on the steel used as coiled tubing of CCT curves and the microstructure properties. And studied the effection of micro-alloying elements Nb,V on the steel used as coiled tubing of quenching and Tempering about the yield strength, tensile strength, elongation rate. Finally we used the transmission electron microscopy (TEM) to analyze Nb (C, N) the law of precipitates.The main works involved as follows.1. The heat expansion method is taken to test the continuous cooling transformation (CCT) curves. With metallographic observation. The influence of cooling rate and the microstructure can be got and also get the CCT curve.The result shows that:austenite phase transition experienced by the district gradually move to the left with the cooling rate increasing, When reaching a certain cooling rate, the austenite will not occur pearlite phase transition, from ferrite transformation zone to bainitic transformation zone, and ultimately the microstructure of ferrite and bainitic is constituted. The tissue is composed of many different phases; it changes in respect to the cooling velocity.2. Through comparing CCT curves and metallurgical structure of three experimental steel. The result shows that Micro-alloyed elements Nb、V reduces ferrite and bainite transformation starting temperature0.5Cr0.2Ni0.13Mo-0.02Nb and0.5Cr0.2Ni0.13Mo-0.02Nb0.02V,the ferrite and bainite can phase transition at a lower temperature, Refined the ferrite and bainite grain.3. Three different components of tested steel were done with the same process of quenching and tempering. And then find the different of microstructure and mechanical properties after different heat treatment. Finally, Analyzeing of micro-alloying elements Nb、V affection to the tested steel. The tested steel after quenching and tempering with the different tempering temperature, decrea se the strength of steel, while the plasticity increases. tensile strength, and yield strength Added micro-alloyed Nb, V of the tested steel is better than without adding the micro-alloying elements Nb、V of the tested steels. But the elongation decreased slightly. During the tempering, we can find Nb、V(CN) particles precipitated, the precipitates can pin dislocations, pinning the grain boundaries and improve the mechanical properties.4. Based on the results of the thermal simulation test on on Gleeble-3500thermo-mechanical simulator, carbo-nitride precipitation laws are analyzed by observing microstructure. The result shows that the fastest temperature of Nb (C, N) precipitation is900℃. A lot of precipitation in this temperature can effectively prevent austeniten grain growth, when the transformation starts, it will be able to fully refine the ferrite grain. Therefore, In this temperature range can be deformed areas of Non-recrystallization.