Microstructural influence on the effects of forward and reverse mechanical deformation in HSLA X65 and X80 linepipe steels

摘要

Five API grade steels designed for linepipe applications produced using different processing routes and with varying microstructures were studied against differences in work hardening and work softening behaviour obtained from mechanical data. The rolling history and wt % additions of alloying elements will determine how the microstructures perform under reverse deformation schedules commonly seen during large diameter linepipe fabrication as steels can undergo work softening in the reverse direction of deformation, otherwise known as the Bauschinger effect. The Bauschinger effect is known to be dependent on the initial forward pre-strain, volume fraction (VF) of carbo-nitride particles and initial dislocation density. The effects of grain size and solid solution strengthening are a matter of debate in the literature and the combined effects of all five strengthening mechanisms have rarely been quantified. TEM investigations determined the dislocation densities to be between 2.2 x1014 m-2 - 5.8 x1014 m-2 in the as received condition. Observed trends presented and discussed in this body of work have given a greater insight into the influence microstructure has on the mechanical properties across a wide range of HSLA steels of similar strength grades, which are of important consideration for future development of low carbon steels designed for the petrochemical industry.