Microstructure Control And Toughness Study Of X80 Pipeline Steel In High Coiling Temperature

X80 pipeline steel with high toughness can effectively increase the pressure of oil and gas pipelines,improves efficiency of transport and provide guarantee for social and economic development.In current industrial production,producers tend to use low temperature coiling process(coiling temperature<400?)in order to obtain excellent mechanics performance.However,low-temperature coil process increases the difficulty of industrial production,resulting in a series of manufacturing operations and equipment problems,which limits the production of X80 pipeline steel.Due to expanding the scope of oil and gas pipeline project,sometimes projects inevitably way to some extremely cold areas,and the low temperature toughness of pipeline steel are also put forward higher requirements in order to ensure safety of operation.Based on the above background,the purpose of this paper is improvement of coiling temperature to higher temperature range(coiling temperature:500-600?)on the premise of guarantee the low temperature toughness of X80 pipeline steel.The paper is based on the design of the alloy system and rolling process,the laboratory hot rolling experiment,the mechanical performance examination and the characterization of microstructure,and a series of systematic studies have been carried out on the microstructure of X80 pipeline steel in high and low temperature rolling process,the influence of rolling parameters on microstructure and mechanical properties,which lays the theoretical foundation for actual industrial production.The main results of the paper are as follows:(1)The design of the alloy system and rolling process is tested through the laboratory hot rolling experiment.The main results are as follows:expect steel3,the four kinds of alloy systems are designed to meet the mechanical performance requirements of X80 pipeline steel in high coiling temperature:Steel2(high Mo)has the highest alloy elements with the best toughness;Steel1,Steel5 and Steel4 are both low cost schemes with superior mechanical properties;Steel3'(low Ni)yield strength is less than 555MPa,which does not meet the X80 pipeline requirements.(2)The microstructure of high and low temperature basic rolling process was analyzed by using characterization techniques such as SEM,EBSD,etc.The main results are as follows:Compared with the traditional rolling process,the grain size gradient of experimental steels in the direction of thickness is decreased significantly,and the microstructure is very uniform.The microstructure of the low-temperature base rolling scheme is multi phase microstructure of granular bainite and bainite ferrite.The micro-structure is small and uniform,and the grain is flat.The number of MA constituent is small and the content of retained austenite is high.Thus it has excellent low temperature toughness;In high-temperature base rolling scheme,microstructure is mainly granular bainite,and the amount of bainite ferrite decreases.MA constituents are coarse and the content increases.Beside,the retained austenite is less abundant.As a result,low temperature toughness reduces slightly.(3)Through laboratory trial rolling and micro-structure characterization,the influence of finish rolling temperature and coiling temperature on the microstructure and mechanical properties of X80 pipeline steel is studied in detail.The main results are as follows:the low temperature toughness of experimental steel improved as finish rolling temperature and coiling temperature are decreased.But coiling temperature should be improved on the promise of absorbed energy is qualified.On one hand,it reduces the requirement for cooling capacity and reduces the difficulty of industrial production.On the other hand,raising coiling temperature can reduce the grain size gradient in the direction of thickness and improve the uniformity of microstructure.(4)The influence of Cr and Mo alloy elements on the characteristics of microstructure of experimental steel and low temperature toughness was characterized by laboratory trial rolling and microstructure characterization.The main results are as follows:compared with Steel2(0Cr-0.2 Mo),0.3Cr-0.2 Mo alloy system contains more granular bainite and the grain size is even smaller,and the grain size gradient in the direction of the thickness decreases significantly.The effective grain size is also more small,and MA constituents are small dispersion distribution adn content of retained austenite increases.