Effect Of Nb Content On Microstructure And Properties Of Heat-affected Zone Of High Strength Pipeline Steel Pipe Welding

Pipeline transportation is one of the most important transportation methods for large-scale oil and natural gas.With the development of modern microalloying pipelines and the development and application of low cost high Nb pipeline steel,high Nb pipeline steel has become one of the most common alloying methods for different grades of pipeline steel.However,so far,there is still a lack of unified understanding of the impact of Nb on the performance of pipeline steel and steel pipe,and there are still great differences in the design and application of Nb content in different enterprises.Therefore,it is of great significance to systematically study the influence of Nb content on the microstructure and properties of pipeline steel and welded joint,which is of low cost and high Nb pipeline steel.In this paper,the structure and performance of X65 pipeline pipe produced by a large number of practical analysis of Nb content on the structure and performance of X65 pipeline pipe with different composition system;The effects of different Nb content on microstructure and properties of coarsely grained heat affected zone in submerged arc welding and ring welding of X80 pipeline steel were studied by thermal simulation testing machine.The results show that: Although the composition of X65 steel in different systems is slightly different,there is a strong correlation between the Nb content and the properties of the base metal and the WELD heat affected zone.With the increase of Nb content,the strength of the base metal and the weld heat affected zone increases slightly,while the impact toughness increases significantly.However,when the Nb content is greater than 0.06%,Nb content has a weaker effect on the strength and toughness of pipeline steel and heat-affected zone.X80 pipeline steel has good strength and toughness in pipe body and weld coarsegrain zone.With the increase of Nb content,the strength of steel changes little,but the toughness of steel increases significantly.Nb content not only affects the strength of coarse-grain heat affected zone,but also increases its plasticity.The toughness of coarse-grain zone can be significantly improved with the increase of Nb content,and the properties of the heat-affected zone of ring welding are higher than those of the heat-affected zone of straight seam welding.With the increase of Nb content,the amount of Nb C precipitation in steel increases.The naining effect of Nb C particles on proto-austenite grains inhibits the growth of proto-austenite grains,refines proto-austenite grains,improves the strength,plasticity and toughness of steel,and stabilizes the properties of steel pipe.Based on the analysis results of X65 and X80 pipeline steel and the performance of the weld heat affected zone,the Nb content of X65 high Nb pipeline steel is controlled within the range of 0.055～0.065%.And X80 steel should be controlled in the upper limit of about 0.086%.