Effects of Alloying Elements on Mechanical Properties of API X80 Linepipe Steels

Four kinds of steels were fabricated by varying Mo, Cr, and V additions, and their microstructures, tensile and Charpy impact properties, and effective grain size measurement were analyzed. Since the addition of Mo and V promoted to form fine acicular ferrite and granular bainite, while prohibiting the coarsening of granular bainite, it improved strengths and upper shelf energy, and decreased the energy transition temperature. The Cr addition promoted the formation of coarse granular bainite and hard secondary phases, which led to the increased effective grain size, energy transition temperature, and strength, and the decreased upper shelf energy. The steel containing 0.3 wt.% Mo and 0.06 wt.% V without Cr had the highest upper shelf energy and the lowest energy transition temperature because its microstructure was composed of fine acicular ferrite and granular bainite, while its tensile properties maintained excellent.