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

Abstract

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 °C and 700 °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 °C and 700 °C respectively. X-ray macro-texture measurement showed that the intensity of the desired γ-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 {001}〈100〉, brass ({110}〈112〉), S ({123}〈634〉) and Copper ({112}〈111〉) were observed in steels rolled at 600 °C finish rolling temperature. Strong texture inhomogeneity during warm rolling was observed. The γ-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}‖ND and {001}‖ND 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 °C favoured the development of the γ-fiber texture.