Effects of microstructure and yield ratio on strain hardening and Bauschinger effect in two API X80 linepipe steels

In the present study, effects of microstructure and yield ratio on strain hardening and Bauschinger effect were investigated in two API X80 steel sheets fabricated by controlling the start cooling temperature. The steel whose start cooling temperature was lower had the higher fractions of granular bainite (GB) and martensite-austenite (MA) constituent and the lower fraction of acicular ferrite (AF), and showed the higher yield ratio. According to the results of the strain-reversal test composed of compressive and tensile tests at various compressive pre-strains, the reduction in yield strength of the steel having higher fractions of GB and MA was generally higher than that of the steel having lower fractions. This result could be explained by difference in density of mobile dislocations and by competing mechanisms between Bauschinger effect and strain hardening, which were susceptible to the minute change in pre-strain during the piping process. When the pre-strain was low, e.g., lower than 4%, the steels having low yield ratio and small Bauschinger effect were desirable to minimize the reduction in yield strength.