No abstract

This study aims to investigate the correlation between the microstructures and the anisotropy of low-temperature toughness for two high-strength API X70 pipeline steels fabricated at different coiling temperatures. The microstructures are characterized using an optical microscope, a scanning electron microscope, and an electron backscattered diffraction analysis, with tensile and Charpy V-notch impact tests also conducted on the steel specimens in various directions relative to the rolling direction. Some pearlites formed by a higher coiling temperature increase the ductile-to-brittle transition temperature (DBTT) by 38 °C in the T-L (transverse-longitudinal) direction. On the other hand, the DBTT of the specimens with the T-L and L-T (longitudinal-transverse) directions (À106.6 and À109.3 °C, respectively) exhibits excellent low-temperature toughness, but the specimen with the D-D (diagonal-diagonal) direction shows the highest DBTT (À65.2 °C). The resulting anisotropy in the low-temperature toughness of the API X70 pipeline steel is discussed from the standpoint of an orientation distribution function analysis in this study. It is suggested the anisotropy of the low-temperature toughness is mainly attributed to the texture components of RD (rolling direction) fibers originating from deformed austenite.

This study aims to examine the correlation between microstructures and the mechanical properties of two highstrength API X70 linepipe steels with different specimen directions and Moaddition. The microstructure of the Mo-added steel has an irregularly shaped AF, GB matrix with pearlite because of the relatively large deformation in the non-recrystallization temperature region, while that of the Mo-free steel shows a PF matrix with bainitic microstructure. In the Mo-added steel, the M/A (martensite-austenite) in granular bainite (GB) and pearlite act as crack initiation sites with decreased upper shelf energy and an increased ductile to brittle transition temperature (DBTT). Regardless of Mo addition, all of the steels demonstrate higher strength and lower elongation in the T direction than in the L direction because of the short dislocation glide path and ease of pile-up at grain boundaries. In addition, the impact test specimens with T-L direction had a lower impact absorbed energy and higher DBTT than those with the L-T direction because the former exhibit shorter unit crack path compared to the latter.

No abstract