Ultrafine microstructure design of high strength pipeline steel for low temperature service: The significant impact on toughness

Abstract

An X80 (80 ksi, 555 MPa) pipeline steel was innovatively processed in the laboratory using the intercritical rolling process and excellent low temperature toughness was obtained. The Charpy v-notch impact energy was greater than 400 J at −80 °C and the shear fracture area in drop-weight tear test approached ~100% at −60 °C. In order to unravel the low temperature toughening mechanism, the crystallography of the microstructure was studied in detail by electron back scattered diffraction. The results indicated that the microstructure mainly consisted fine polygonal ferrite and granular bainite. The ferrite grains were refined to less than 3 μm and the ferrite grain boundaries predominantly had high misorientation angle. The granular bainite was also refined by some sub-boundaries, but these sub-boundaries were largely low angle grain boundaries. Therefore, the ultrafine ferrite contributed to the effective refinement of microstructure. Increasing the proportion of fine polygonal ferrite is the key to improve the low temperature toughness of pipeline steel.