Effect of Coiling Temperature on Microstructure and Mechanical Properties of X70 Pipeline Steel

Microstructures and mechanical properties of the X70 pipeline steel with added composite micro-alloying elements of Nb-V-Cr at different coiling temperatures are analyzed by metallurgical microscope using the electron backscatter diffraction(EBSD).The results show that the microstructure of the X70 pipeline steel is transformed into bainitic ferrite from granular bainite with the coiling temperature decreasing under the same cooling condition and the martensite-austenite(M-A) constituent by volume also decreases as the coiling temperature goes down.However,the microstructure of the X70 pipeline steel is transformed into polygonal ferrite and degenerated pearlite at the relaxation condition after finishing rolling.So the max yield strength of the pipeline steel is up to 680 MPa when the coiling temperature is 470 ℃ while the max impact energy is 287 J as the coiling temperature goes down to 380 ℃.