Effect of TMCP rolling schedules on the microstructure and performance of X70 steel
By Roccisano, A.; Nafisi, S.; Stalheim, D.; Ghomashchi, R.
Published in Materials Characterization
2021
Abstract
Four laboratory rolling schedules were devised and produced from a thick-wall API X70 steel plate. The microstructure and crystallographic texture were characterized followed by an evaluation of the performance of the rolled steels by investigating the mechanical properties and SCC susceptibility. High pH Stress Corrosion Cracking (SCC) is a damaging form of environmental corrosion that can cause rapid failure of buried gas transmission pipelines. It is a key question to ascertain if the manufacturing procedure has any bearing on the resistance of line pipe steels to SCC. Finish rolling above the recrystallization lower temperature, RLT (Tnr), resulted in the formation of large, equiaxed grains of upper bainite and granular bainite. Despite resulting in the highest Yield Strength (YS) and Ultimate Tensile Strength (UTS), the steel manufactured through this process had the highest SCC susceptibility. Finish rolling in the two-phase region increased the polygonal ferrite formation and deteriorated mechanical properties of the rolled steels. Finish rolling above the Ar3, combined with severe rough rolling, produced the lowest SCC susceptibility, with a primarily granular bainite microstructure and a high intensity of {332}<113> texture. Rolling strategies that balanced strength and ductility were shown to positively affect SCC performance.