Abstract:
X90 pipeline steel pipe is a new type of high-strength and high-toughness pipe. With the increase in strength, the revealing and controlling of strain aging behavior of X90 pipeline steel has become the focus of research. The microstructure characteristics, fracture morphology, strain-aging sensitivity, tensile properties, impact toughness in low temperature, and strain aging sensitivity of X90 high-strength pipeline steel before and after strain aging treatment were investigated by OM, SEM, TEM, strain-aging sensitivity test, tensile test and Charpy impact experiments. The results indicate that the strain aging has no significant effect on the microstructure of X90 high-strength pipeline steel. The microstructure of X90 high-strength pipeline steel before and after aging is a complex structure of acicular/bulk ferrite, lath/granular bainite and M/A component. However, strain aging has a significant effect on the tensile properties, tensile curve morphology and low temperature impact toughness of X90 high-strength pipeline steel. When the aging temperature is 230 ℃ (the fusion temperature of polyethylene layer), the yield strength, tensile strength, yield ratio and strain-aging sensitivity coefficient of X90 steel gradually increase with the prolongation of aging time. At the same time, the uniform elongation and low temperature impact absorption energy of X90 steel gradually decrease with the prolongation of aging time. In addition, the X90 pipeline steel gradually loses continuous yielding and strengthening characteristics, and the tensile curve changes from the round-house-type before aging to the Lüders-type yield curve after aging with obvious peaks and yield platform. 15min is the inflection point of aging time for the transformation of the stress-strain curve of X90 steel. In order to reduce the adverse effect of strain aging on X90 high-strength pipeline steel, comprehensive regulation should be carried out from three aspects: reducing strain, reducing anti-corrosion preheating temperature and shortening high temperature residence time of anti-corrosion preheating. It is recommended to innovate and revise the pipe production and preparation process, such as shortening the high temperature preheating residence time or reducing the preheating temperature, and adopting the flexible leveling method to replace the rigid roll leveling method, and the multi-step incremental forming to replace the one-step spiral forming method. It is recommended to innovate the production and preparation process of pipes. For example, in the pipe forming process, the flexible leveling method could be used to replace the rigid roll leveling method, and the multi-step progressive molding method could be used to replace the one-step spiral molding method to effectively control the prestrain. Another example, in the pipe anticorrosion process, the residence time of high temperature preheating should be shortened or the preheating temperature should be reduced (when the preheating temperature ≥ 230 ℃, the high temperature residence time should be ≤ 5 min; if the preheating temperature < 230 ℃, the high temperature residence time should be < 15 min).