Effects of Microstructure and Pre-strain on Bauschinger Effect in API X70 and X80 Linepipe Steels

Abstract

In this study, effects of microstructure and pre-strain on the Bauschinger effect were investigated in two API X70 and two API X80 linepipe steel sheets fabricated by controlling the cooling condition, and their yield strength and Bauschinger parameters were measured by the tension-compression test with varying tension pre-strain. The fast-cooled steels had the higher fraction of acicular ferrite, granular bainite and martensite-austenite (MA) constituents and smaller grain sizes. The reduction in yield stress (Delta YS) of the steels having a higher fraction of MA and smaller grain sizes was higher than that of the steels having a lower fraction of MA and larger grain sizes. The Delta YS was smallest at the pre-strain of 1%, reached the maximum at the pre-strain of 2%, and then decreased with increasing pre-strain. This result could be explained by the amounts of mobile dislocations and back stress, which affected the Bauschinger effect and strain hardening effect simultaneously. Since these two effects affected the yield strength on a competing basis, the Bauschinger stress and hardening parameter were used to separately analyze these effects. It could be confirmed that the Bauschinger effect and strain hardening effect were activated at pre-strains of 1-2% and 3-4%, respectively.