In recent years, high strength linepipe needs high longitudinal deformability to meet the requirements of strain-based design pipeline employed when passing through geologic hazard-prone regions, such as seismic areas, landslide zones, permafrost zones, etc. The geologic hazard section of the natural gas pipeline from Myanmar to China is going to utilize a strain-based design. Linepipes with high deformability are required in these areas.
The microstructures with different phases and the effect of the microstructure on deformability of pipeline steel were studied. Based on a dual phase of ferrite and banite microstructure design, the X70 grade UOE linepipe with low yield-tensile strength ratio, good uniform elongation and high stress ratio were developed. Two dimensions of UOE linepipe for the strain-based design area of Myanmar-China pipeline project, which were 1016mm O.D. and 17.5mm W.T., 1016 mm O.D. and 21.0mm W.T., were industrially trial-produced.
The transverse properties of the trial produced linepipes meet the requirements of X70 steel grade of API Specification 5L. And also the linepipe has good toughness, the Charpy impact energy at −5°C is more than 200J, and the shear fracture area of DWTT test is more than 85% at 0°C. The longitudinal tensile properties of the trial produced linepipe exhibit good deformability. The stress-strain curve shows a typical round-house shape, the uniform elongation u-EL% is more than 7%, the yield-tensile strength ratio is lower than 0.80, the stress ratios of Rt1.5/Rt0.5, Rt2.0/Rt1.0 and Rt5.0/Rt1.0 are more than 1.100, 1.040 and 1.088 respectively. The properties of the linepipe after aging at 200°C for 5min to simulate the coating process of the pipe were also investigated. The results show that the linepipe trial-produced has good strain-aging resistance.