Study of Innovative Heavy-Gauge Strain-Based 33.0 mm X80 Pipeline Steel Plate and Pipe Designed via Niobium Microalloying

Abstract In order to be subjected to large displacement, which results in corresponding plastic deformation, the conventional stress-based approach for pipeline design turned to be insufficient for displacement-controlled or partly displacement-controlled load scenarios. As a replacement for a conventional solution, the strain-based pipeline steels were developed to meet the requirements through large plastic deformation. In this study, two heavy-gauge 33 mm (thickest strain-based pipe in the world) X80 pipeline steel plates were designed via niobium microalloying (low carbon and 0.075 wt.% niobium) and selected rolling procedures. The key rolling procedures, including rough rolling start temperature, finish rolling start temperature, final rolling temperature, cooling start temperature and cooling stop temperature, aiming at balanced phases of granular bainite (GB) and polygonal ferrite (PF) as the microstructure of base material, are of crucial importance based on the theory of phase transition, and have been optimized in order to obtain qualified longitudinal performances before and after being aged at 200 °C for 5 mins. Further, the niobium microalloying contributes to the microstructural homogeneity and refined grain distribution through its effective control of solid solution and precipitation. It is verified through this study and comparison analysis that during the development of innovative heavy-gauge strain-based 33 mm X80 pipe steels, the uniform elongation along the longitudinal direction as well as fracture performance, especially drop weight tear test (DWTT), have been improved to a great extent through optimized rolling procedures.