Through-Thickness Inhomogeneity of Texture, Microstructure, and Mechanical Properties After Rough and Finish Rolling Treatments in Hot-Rolled API 5L X70 Pipeline Steel

Abstract

The development of inhomogeneity in the microstructure, texture, and mechanical property in hot-rolled (API) 5L X70 pipeline steel after rough and finish thermomechanical control processing has been investigated by experimental measurements. Our investigations revealed that the rough rolling stage produced mostly large and coarse ferrite grains across the thickness, while the finish rolling stage led to a rather refined small-grained microstructure. EBSD investigations confirmed that after the rough rolling stage, recovered and deformed grains dominated the quarter and mid-thickness compared to the surface. Investigation of the finish rolled steel showed that in comparison with the quarter and mid-thickness sections of the plate, the surface section mostly consists of deformed grains. The texture components obtained from x-ray measurement showed that after rough and finish processing stages, the {001} , {110} , {123} , {112} , and {001} components were observed to be present. However, inhomogeneity of texture was observed at different depths of the rolled steel, such that in comparison with the surface, volume fractions of texture and intensities of the desired γ-fiber texture were seen to be highest at the mid- and quarter-thickness sections after both rolling stages. The room-temperature tensile stress–strain curves showed that the highest yield strength was observed for samples obtained from the near-surface section, while samples obtained from the mid-thickness section showed the highest elongation during tensile deformation. Overall, the final stage processing was observed to refine the final microstructure of the steel, thus improving the strength of the steel.