Development of an efficient process route for producing K65 (X80) steel for pipes with high strain capacity

Abstract

Laboratory rolling tests and industrial trials on Severstal Mill 5000 were performed to produce various microstructures in K65 (X80) steel using modified accelerated-cooling schedules. The resulting microstructures were examined, and the temperature ranges for forming the main phases and constituents (ferrite, bainite, martensite, and the M/A constituent) were identified. The results show that promoting secondary constituents containing M/A, rather than cementite-containing constituents in the ferritic matrix, increases strength, modifies the stress–strain response, and reduces the YS/TS ratio without loss of ductility or low-temperature toughness. A low-temperature plate-cooling route was developed to obtain a microstructure consisting of a ferritic matrix with carbide-free bainite and an M/A constituent. Manufacturing 1420-mm-diameter pipes confirmed the favorable effect of this microstructure on the mechanical properties of the pipe metal after forming and expansion. Overall, the study enabled an industrial process for producing plates from K65 (X80) steel for pipes with a diameter of 1420 mm and a wall thickness of 27.7–33.4 mm, providing high uniform elongation, a smooth stress–strain curve, and high strain capacity.