Microstructure and texture evolution in warm rolled API 5L X70 pipeline steel for sour service application

The evolution of microstructure and texture of API 5L X70 pipeline steel after warm rolling treatments has been studied using X-ray diffraction and electron backscatter diffraction (EBSD). Our investigations revealed that the microstructure after the rough rolling stage consist of coarse grained ferrite and bainite clusters, while the specimens finished rolled at 600 degrees C and 700 degrees C consist mainly of elongated ferrites and a well recrystallized polygonal shaped ferrite grains. EBSD investigations confirmed that dynamic recovery was dominant during the rough rolling stage; whereas a partial recrystallization and a well recrystallized structure were observed during finish rolling at 600 degrees C and 700 degrees C respectively. X-ray macro-texture measurement showed that the intensity of the desired gamma-fiber texture is dependent on the finish rolling temperature. The finish rolling treatments lowered the fractions of the Goss, Brass, S and Copper texture components. However, higher fractions of the Goss {011} < 100 >, brass ({110}< 112 >), S ({123}< 634 >) and Copper ({112}< 111 >) were observed in steels rolled at 600 degrees C finish rolling temperature. Strong texture inhomogeneity during warm rolling was observed. The gamma-fiber is seen to be better formed at the mid thickness compared to the surface. Most grains at the mid thickness were oriented in the {111}parallel to ND and {001}parallel to IND at both finish rolling temperatures. The finish rolling temperature played a key role in the development of texture such that the recrystallization of grains in steel finish rolled at 700 degrees C favoured the development of the gamma-fiber texture.