Correlation of Microstructure with Anisotropy of Low-Temperature Toughness in Two API X70 Pipeline Steels

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 degrees 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 (-106.6 and -109.3 degrees C, respectively) exhibits excellent low-temperature toughness, but the specimen with the D-D (diagonal-diagonal) direction shows the highest DBTT (-65.2 degrees 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.