Microstructure, Mechanical Properties, and Strain-Hardening Behavior of V-N Microalloyed Pipeline Steels Consisted of Polygonal Ferrite and Acicular Ferrite

A novel V-N microalloying method is adopted to produce pipeline steels using a two-stage thermomechanical controlled processing involving relaxation process and accelerated cooling. The main objective of the current work is to study the novel practice in terms of microstructure, mechanical properties, and strain-hardening behavior. The microstructure of the experimental steels consists of polygonal ferrite (PF), quasi-polygonal ferrite (QF), acicular ferrite (AF), and granular bainite (GB). Mechanical properties including strength, ductility, and low-temperature toughness of V-N microalloyed steels meet the requirements of X80 pipeline steels. Yield strength, tensile strength, total elongation, and -20 degrees C impact energy are 639 MPa, 761 MPa, 21.0%, and 283 J when the volume fraction of PF is 9.1%, and 603 MPa, 724 MPa, 24.1%, and 214 J when the volume fraction of PF increases to 30.4%. The V-N microalloyed steels exhibit high strain-hardening ability, and the values of strain-hardening exponent are 0.12 and 0.11 based on Hollomon analysis.