On the determining role of acicular ferrite and polygonal ferrite in V–N microalloyed X70 pipeline steel in optimising the strength-toughness combination

In this study, controlled rolling involving a relaxation process followed by accelerating cooling was applied for producing a V–N microalloyed pipeline steel during the industrial production process. The main objective of the research is to study the influence of the relaxation process on the microstructure and mechanical properties. The microstructure consisted of polygonal ferrite (PF), acicular ferrite (AF) and granular bainite in the experimental steels, while the volume fraction of PF and precipitates increased with the increasing air-cooling relaxation time. The yield strength, tensile strength, yield ratio, −20°C average impact energy and ductile–brittle transition temperatures are 535 ± 9 MPa, 602 ± 11 MPa, 0.89 ± 0.01, 263 ± 8 J and −81°C when the volume fraction of PF is 15.1 ± 0.3%, and 503 ± 8 MPa, 590 ± 9 MPa, 0.85 ± 0.02, 221 ± 10 J and −70°C when the volume fraction of PF increases to 30.4 ± 0.6%. The mechanical properties including strength and low-temperature toughness of V–N microalloyed steels meet X70 pipeline steel standards. The main strengthening mechanisms are grain boundary strengthening, dislocation strengthening and precipitation strengthening in this steel. In the process of plastic deformation, the non-uniformly distributed strain concentration regions could cause uneven distribution of deformation, which forms the nucleation of microcracks. AF and high-angle grain boundaries played an important role in improving the strength and toughness.