Effects of granular bainite and polygonal ferrite on yield point phenomenon in API X65 line-pipe steels

Yield strength and yield-point (YP) phenomenon are regarded as crucial properties to be assured in practical applications such as line-pipe forming. In this work, the impacts of microstructural constituents on YP and yield ratio were explored in four API X65 line-pipe steel plates fabricated by altering the start or finish rolling temperature and plate thickness. The four steel plates were predominantly made up of granular bainite (GB) and polygonal ferrite (PF), along with the greater volume proportion of PF in the case of the lower start and finish rolling temperatures and the thicker plate thickness. According to the skin-pass rolling to reduce the YP elongation, the increased amount of mobile dislocations was greater in the PF than in the GB, indicating that the PF was more responsible for the YP alleviation than the GB because the more mobile dislocations were formed in the PF than in the GB. Thus, the decreasing rate of YP elongation was faster for the thicker plate and the low start or finish rolling temperature. On the other hand, the lower yield ratio was preferred for the thicker plates as they exhibited the shorter YP elongation and the higher strain-hardening rate, thereby leading to the rapid enhancement of yield strength in the YP range and consequently to the continuous increase in yield ratio. Thus, this work would be favorably utilized for designing optimal microstructural and TMCP parameters for securing the yield-strength guarantee in line-pipe steels.