Effect of microstructural variations through the thickness on mechanical properties of an Nb-microalloyed thick gauge line pipe steel

Oil and gas are increasingly being transported at higher pressures through steel pipelines for higher efficiency, requiring the production of thick gauge line pipe steels with an excellent combination of strength, formability, and toughness. This work examined the effect of microstructural variations on mechanical properties across the thickness of a 22 mm hot-rolled Nb-microalloyed API Grade X70 line pipe steel. The evolution of mechanical properties has been investigated in different directions to understand anisotropic deformation behavior across the thickness of the plate. Tensile specimens were prepared from the surface, quarter, and center locations of a 22 mm plate and tested to analyze the type of fracture, deformation behavior, strength, and ductility. Microstructural characterization has been carried out with the help of an optical microscope, scanning electron microscope, energy dispersive spectroscopy, and electron backscattered diffraction to understand the microstructure variations across the thickness. Microstructural characteristics, including grain structure, grain statistics, texture properties, and precipitates, were analyzed to study the variations in microstructural properties through the thickness of the hot-rolled plate. An in-depth understanding of the microstructure of steel will facilitate understanding the reasons behind the variations of mechanical properties through the thickness of 22 mm thick gauge line pipe steel.