| The use of high-thickness pipeline steel helps to increase pipeline transportation pressure and improve service safety.However,the microstructure uniformity of high-thickness pipeline steel is poor,and the mechanical properties are unstable,which is likely to cause brittle fracture in low temperature environment.Thus it is of great significance to study structure controlling and the relationship between the microstructure and low temperature properties.In this paper,the 31.75mm-thick pipeline steel plate is used as the research material,and the effects of start cooling temperature,quenching + tempering process and direct tempering process at different thickness of the experimental steel are determined by low temperature tensile test,low temperature impact test and low temperature drop weight test.The microstructure was characterized by scanning electron microscope observation,transmission electron microscope observation and electron backscatter diffraction analysis and other characterization methods.The microstructure evolution of experimental steel was analyzed.The main conclusions are as follows:(1)The microstructure transformation behavior at different thickness of the experimental steel under different start cooling temperatures was studied.In the range of 720 °C to 760 °C,ferrite and bainite transformations mainly occur at different thickness of the experimental steel.As the start cooling temperature decreases,the propotion of polygonal ferrite increases,and the alloying elements segregate at the M/A component.From the 1/4 thickness to 1/2 thickness,the propotion of granular bainite increases and the effective grain size decreases.(2)The effect of start cooling temperature on low-temperature mechanical properties of the experimental steel was studied.When the start cooling temperature is 740°C,the series of low-temperature tensile curves are the type of “round house”.The yield strengths at different thickness of experimental steel are greater than 518 MPa.The tensile strengths are greater than 696 MPa.The elongation after fracture is greater than 21.2%.The low-temperature impact energy is greater than 273 J.The ductile-brittle transition temperature is lower than-83°C,and the drop weight shear area at-20°C is 89%,which meets the standard of low-temperature crack arrest for pipeline steels on the China-Russia eastern gas pipeline.(3)The effect of direct tempering process on the microstructure and low temperature mechanical properties of the experimental steel was studied.The microstructures of the experimental steel are polygonal ferrite,quasi-polygonal ferrite,roughened bainitic ferrite and precipitates with sharp corners.For the experimental steel at different thickness,the proportion of high-angle grain boundaries is greater than 33.9%.The yield strengths and tensile strengths at-60°C are greater than 539 MPa and622MPa,respectively.The impact energy is greater than 285 J,and the ductile-brittle transition temperature is lower than-103°C.(4)The effect of quenching + tempering process on the microstructure and low temperature mechanical properties of the experimental steel was studied.The microstructure of the experimental steel is bainite,M/A component and fine dispersed precipitates.The compositions of precipitates are carbonitride of Nb and Ti.C-rich and Mn-rich phases at grain boundaries are less in content.For the experimental steel at different thickness,the effective grain sizes are lower than 2.8μm.The proportion of high-angle grain boundaries is greater than 42.9%.The yield strengths and tensile strengths at-60°C are greater than 553 MPa and 618 MPa,respectively.The impact absorption energy is greater than 298 J,and the ductile-brittle transition temperature is lower than-108°C.Compared with the rolled steel,the microstructure uniformity of the experimental steel after quenching and tempering is improved,and it can be used as a reference for the heat treatment process of high thickness pipeline steel. |
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