Microstructure Control And Mechanical Properties Study Of X80 High Grade Pipeline Steel

As China's demand for oil and natural gas energy continues to increase,the resource-rich northwest region has gradually become a key area for development.In order to increase the pipeline transportation pressure of oil and gas and improve the transportation efficiency,China's construction of pipelines mostly use X80 pipeline steel with high strength and good low temperature toughness.As the scope of pipelines continues to expand,it will inevitably pass through some extremely cold regions.The strength of the currently produced X80 pipeline steel is easy to meet the requirements of use,but its low temperature toughness limits application in cold regions.In industrial production,a relatively dense microstructure is generally obtained by controlled cooling,such as lowering the final cooling temperature,increasing the cooling rate,thereby obtaining excellent low temperature toughness.However,the measure only regulates the phase transformation process,ignoring the effect of the austenite grain before phase transformation on toughness at low temperature.Based on the above background,this study aims to refine the austenite grain before phase transformation by controlled rolling technology under a reasonable controlled cooling process,thereby improve the low temperature toughness of X80 pipeline steel.Through thermal simulation,hot rolling at laboratory,microstructural analysis technology and mechanical property testing,the effects of rolling parameters and cooling parameters on microstructure and mechanical properties of X80 pipeline steel were studied,thus laying a theoretical foundation and controlling idea for practical industrial production.The main conclusion of the study were as follows:(1)The effect of controlling pass deformation on austenite was studied by thermal simulation test.The results were as follows:after basic four-pass compression deformation,the austenite grains were flattened and the average grain size was 55.9?m;the increase of the final pass reduction in recrystallization zone promoted dynamic recrystallization,and finally the average grain size of austenite was refined to 32.1?m,and it had a distinct flattend state.The austenite with fine grain size and obvious flattening state could not be obtained by increasing the total deformation in recrystallization zone,non-recrystallization zone and the final pass deformation in non-recrystallization zone.(2)The effect of increasing the final pass deformation of rough rolling and finishing rolling and reducing the temperature-holding thickness on the austenite were studied by hot rolling at laboratory.The results showed that the appropriate total deformation is ensured in the rough and finish rolling stages,the recrystallization was fully achieved by increasing the final pass deformation,and finally the fine and flattened austenite structure was obtained.However,the increase in the final pass deformation of finishing rolling had little effect on the austenite.Although the austenite grain size was refined by decreasing the temperature-holding thickness,the flattening state of austenite was not obvious.(3)The effect of the cooling rate during the interval between two passes and reverse phase transformation process of partial recrystallization zone on the austenite and the micro structure at room temperature were studied by thermal simulation.The main results were as follows:(a)during the basic four-pass compression deformation,the change of cooling rate in the recrystallization zone and the non-recrystallization zone had no obvious effect on the refinement of austenite grains;(b)in the partial recrystallization zone,the austenite gradually refined as the cooling rate increased,resulting in a denser microstructure at temperature.When the cooling rate was 3?/s,the average grain size of the austenite was 55.9?m.And when the cooling rate was 25?/s,the average grain size of the austenite was 38.7?m.The introduction of the reverse phase transformation process in the partial recrystallization zone could refine the microstructure significantly.(4)The dilatometer was used to study the continuous cooling phase transformation behavior of supercooled austenite,and the dynamic CCT curve was drawn.The results showed that the ferrite was obtained at a lower cooling rate.With the increase of cooling rate,the microstructure was refined gradually and all granular bainite was obtained When the cooling rate was 40?/s,all lath bainite was obtained.(5)The effects of final cooling temperature,cooling rate and coiling cooling rate on the micro structure at room temperature were studied by thermal simulation.The results showed that the decrease of the final cooling temperature and the increase of the cooling rate were favorable for obtaining a dense microstructure at room temperature.When the final cooling temperature was 500?,the change in the coiling cooling rate had no significant effect on the microstructure at room temperature.(6)The effect of rolling and cooling process parameters on the microstructure and mechanical properties at room temperature were studied by hot rolling at laboratory.The main results were as follows:the total deformation amount in the finishing rolling stage was unchanged and the reduction of the final pass of rough rolling was increased,the low temperature toughness was improved while maintaining strength;as the final cooling temperature decreased and the cooling rate increased,the yield strength and tensile strength of the test steel were improved,but the low temperature toughness decreased.