Microstructure, texture evolution and mechanical properties of X70 pipeline steel after different thermomechanical treatments

The evolution of microstructure, texture and mechanical properties of API X70 pipeline steel after different thermomechanical treatments has been studied using a combination of X-ray diffraction and electron backscatter diffraction (EBSD). Our investigations revealed that different microstructure consisting of polygonal ferrite, bainite, coarse and fine acicular ferrite grains was obtained with a centre line segregation traversing through and parallel to the rolling direction. EBSD investigations confirmed that both dynamic recovery and partial re crystallization occurred during hot rolling requiring further annealing for a more homogenous equiaxied grain structure. X-ray macro-texture measurement showed relatively a random texture components comprising of Cube, Goss, Brass, S, Copper, R cube and {331} < 1-10 >. Texture inhomogeneity during hot rolling were observed for the different processing parameters. The grains at the surface are oriented towards {110}parallel to ND while the mid thickness has grains mostly oriented in the {001}parallel to ND with a spread towards the {111}parallel to ND. We observed that after accelerated cooling, the fast cooling rate and low temperature interruption allows the formation of more bainite which in turn increased the tensile strength of the steel.