Grain structure, texture, and precipitation behavior of a thick gauge microalloyed line pipe steel

Natural gas, oil, and in near future increasingly hydrogen and CO₂ are expected to be carried through steel pipelines at higher pressures for increased productivity, demanding the development of thick gauge high strength – high toughness line pipe steels. For pipelines that demand both high strength and high toughness, thermo-mechanically processed (TMP) microalloyed steels with refined microstructures, often consisting of acicular ferrite or quasi-polygonal ferrite, bainitic ferrite, retained austenite, cementite, and martensite, are used. In this work, microstructural characterization of Nb-microalloyed API Grade X70 line pipe steel has been conducted to understand the variation of microstructure through the thickness of a 22-mm thick hot-rolled steel samples from hot-band coils for spiral pipe. Microstructural features including the grain statistics, texture properties, and rotated cube fractions in different regions have been characterized using advanced microstructural characterization tools including electron backscattered diffraction to assess the uniformity of microstructural evolution through the thickness of the hot-rolled plate. For characterizing nanosized Nb containing precipitates, transmission electron microscope was used in this study. A detailed understanding of the microstructure of the steel microstructure will facilitate correlation and prediction of mechanical properties such as strength, ductility, and toughness.