Study On The Relationship Between Microstructure And Properties Of Economical X80 Pipeline Steel

With the development of the petroleum and gas industry, the pipeline industry makes a rapid advance as well. X80pipeline is widely used in the West-East Gas Pipeline Project for its high strength,toughness, good crack arrest property, low Bauschinger Effect and good welding property. Extra-low carbon, high Nb, Mn, Mo, Cu, Ni alloying technology is applied in traditional X80hot rolled strip production. Whether low cost X80pipeline could be produced with the decreased amount of the noble metallic elements content yet qualifying the same standard, is not only a highlight of metallurgical industry but also the focus point of pipeline researching nowadays.In this thesis,the CCT curve and the phase transformation rule of the low cost and traditional X80pipeline at different cooling condition, and the transformation rules of the microstructure and its substructure in X80pipeline with different alloy content during the TMCP process are discussed by using Gleeble thermo mechanical simulation,formastor-F II thermo dilatometer, optical metallographic, SEM, TEM and microhardness tester. In addition, the welding property of different alloy content is studied in this thesis using the thermal simulating technology. In this way, the possibility of reducing the cost of alloy is discussed.The results show that the CCT curve of low cost X80pipeline shifts to the left compared with the traditional one, but two curves almost overlap in the zone of the cooling rate during the actual industrial production. The four kinds of tested steel are mainly the mixture of quasi-polygonal ferrite,acicular ferrite and granular bainite after different TMCP processes, including different transformation temperature,cooling rate and final cooling temperature. Although the volume fraction of M-A and the microhardness of the tested types fluctuate with regularity, there is little difference in general. In welding process, the higher content of Mo, Cu and Ni results in better property of CGHAZ. The tested steel with lowest content of Mo has the worst impact toughness, while the one without Cu and Ni has a better performance than it.From this, low cost X80pipeline could have the appropriate microstructure and mechanical property, which meets the requirement of X80pipeline hot rolled steel strip, by adjusting TMCP process. However, there is still a gap between the low cost and traditional X80pipeline in the welding property. Therefore, it’s viable that producing the low cost X80pipeline with decreased alloy content, but it does not mean that the content of the alloying element could be reduced significantly blindly, especially the content of Mo.