Study on the relationship between microstructure and properties of heavy-wall X70 pipeline steel with different processing directions

The neglect of microstructure and anisotropy in the design of subsea pipeline steel can seriously affect the pipe service life and lead to fracture failure. In this work, experimental and representative volume element (RVE) model results were used to analyze the effect of differences in the microstructural characteristics of heavy-wall ferritic/bainitic dual-phase (DP) x70 pipeline steel in the transverse and rolling directions on the mechanical properties. The results showed that blocky bainite with an aspect ratio 1.9 in the transverse microstructure induced the ferrite to produce strain localization, decreasing the plasticity. Banded bainite with an aspect ratio of 4.4 in the rolling direction was more prone to activate the dislocation slip system, which enhanced deformation compatibility by limiting the ferrite deformation space through plastic deformation at low strain levels. The dislocation wall type of the rolling direction promoted the production of back stress (long-range internal stress) and reduced the strain gradient at the interface. The dislocation arrangement in the transverse direction was transformed from a mixed type to a dislocation wall type during cyclic loading, and the back stress was dynamically saturated by grain size. Higher forest hardening in the transverse direction increased strength and hardening rates, while higher back-stress hardening in the rolling direction promoted plasticity. The results reveal the relationship between microstructural characteristics difference and mechanical properties and back stress evolution to provide a theoretical basis for designing the heavy-wall x70 pipeline steel.