Objective Long-distance pipelines face increasingly complex service conditions that complicate structural integrity assessments, with the realization of large-scale applications of Carbon Capture, Utilization, and Storage (CCUS), hydrogen energy, and cryogenic oil and gas transportation technologies. Rolling and manufacturing processes cause the evolution of grain textures in pipeline steel, leading to distinct mechanical anisotropy. Consequently, conventional isotropic constitutive models fail to accurately describe these direction-dependent plastic behaviors, falling short of engineering requirements for precise failure and fracture prediction.

Methods X52 seamless pipeline steel for CO₂ transportation is selected as the research material. Circumferential (0°), diagonal (45°), and axial (90°) tensile specimens are prepared via in-situ sampling to eliminate the extra pre-strain induced by conventional flattening-based sampling method. Combined with the Digital Image Correlation (DIC) technology, multi-orientation in-situ tensile tests are carried out to obtain material stress-strain curves, mechanical properties, and plastic strain ratio parameters.

Results X52 seamless pipeline steel exhibits significant strength anisotropy. As the specimen orientation shifts from circumferential to axial, the yield strength and ultimate tensile strength increase by 48％ and 14％, respectively, accompanied by a 36％ drop in uniform elongation. In contrast, the anisotropy of plastic flow is relatively mild. Circumferential specimens feature obvious thickness reduction, while plastic deformation of axial specimens mainly occurs along the width direction, and the two anisotropic features evolve asynchronously. Five common hardening models are compared in terms of fitting accuracy, and the Multi-Voce model is proven to accurately capture hardening evolution across the entire plastic regime. A non-associated flow anisotropic constitutive framework is established by combining the Hill48 yield function, independent plastic potential function and Multi-Voce hardening model, followed by full parameter calibration. A user material subroutine is developed in ABAQUS to implement numerical calculations for the proposed model. Both the proposed anisotropic constitutive framework and the classic Mises isotropic model yield satisfactory predictions of specimen load-displacement responses. Nevertheless, the new anisotropic framework characterizes the plastic behavior of specimens with different orientations using one unified set of parameters, featuring better physical consistency and higher parameter transferability.

Conclusion This study reveals the anisotropic plastic deformation behavior of X52 seamless pipeline steel. The developed framework provides a theoretical basis for plastic failure assessment, ductile fracture prediction, and structural integrity analysis of oil and gas pipelines. It also serves as a reference for the mechanical characterization and engineering applications of similar pipeline steels.