Effect of Lüders Bands by Strain Ageing on Strain Distribution, Microstructure and Texture Evolution of High-Strength Pipe Steel

In this study, the high-strength pipe steel, i.e. X80, was heated at 260 °C for half an hour followed by air cooling for thermal simulation of pipe coating process. During the uniaxial loading of the specified tension samples, the migration behaviour of Lüders bands and the re-distribution of stress after strain ageing heat treatment were investigated using digital image correlation. Digital image correlation was adopted to characterise the local axial and shear strain fields of samples that were extracted from different locations across the wall thickness of the high-strength pipe steel, and electron backscatter diffraction was used to compare the evolution of crystallographic texture among these locations. Other characterisation methods regarding microstructure and mechanical properties were conducted meanwhile, including optical microscopy, scanning electron microscopy and X-ray diffraction to accurately define the detailed performances that strain ageing, i.e. being heated to 260 °C for half an hour, has led to. The obtained results show that the Lüders band-related microstructure, dislocation density, lattice constant and texture vary at the different locations across the wall thickness and influence the final mechanical properties of the selected high-strength pipe steel.