Evolution of the microstructure and texture of X70 pipeline steel during cold-rolling and annealing treatments

Abstract In this study, we used a combination of X-ray diffraction and electron backscatter diffraction (EBSD) to track the evolution of both the microstructure and texture in samples of X70 pipeline steel subjected to cold-rolling up to 80% thickness reduction and subsequent annealing at 800 °C and 850 °C. Our investigation of the macro-texture, using an X-ray method, indicates that weak texture in the as-received-steel was changed from ζ-fiber to ɣ-fiber and Ɵ-fiber after cold rolling. EBSD reveals a local grain refinement as a result of the formation of a substructure with a low angle grain boundary within cold-rolled ferrite grains near the ɣ-fiber and Ɵ-fiber textures. Development of a later substructure depended on the orientation of deformed ferrite grains. We observed partial recrystallization within grains with ɣ-fiber texture and a grain size of about 1.5 ± 0.5 μm after cold rolling. Annealing at 800 °C for 90 s caused incomplete static recrystallization with an inhomogeneous bimodal structure. Recrystallized grains depicted both ɣ-fiber and Ɵ-fiber textures, identical to their deformation textures. We achieved a homogeneous equiaxied grain structure using a higher annealing temperature of 850 °C, for 300 s.