Correlation of microstructural factors with the yield ratio and uniform elongation in API X80 linepipe steels

In the present study, the correlation of microstructural factors with yield ratio and uniform elongation was investigated in API X80 linepipe steels with different microstructures by varying thermo-mechanical control process conditions. The grain size and microstructure fraction of the API X80 linepipe steels were quantitatively measured by EBSD analysis and then tensile tests were carried out on them. Although all the steels showed complex microstructures of polygonal ferrite (PF), and bainitic microstructures such as acicular ferrite (AF), granular bainite (GB), and bainitic ferrite (BF), they had different grain sizes and microstructure fractions. The D steel with the finest grain size showed the highest yield strength due to grain refinement strengthening even though it had a relatively high fraction of PF with lower yield strength. The yield ratio usually increases with decreasing grain size because grain refinement typically produces an increase in yield strength and a decrease in work hardening due to the accumulation of dislocations at grain boundaries. On the other hand, uniform elongation increases with increasing PF fraction because an increase in the PF fraction eases slip and thus reduces plastic instability caused by accumulated dislocations at grain boundaries. Consequently, the microstructural factors affecting the yield ratio, uniform elongation, and work hardening exponent of the API X80 linepipe steels are different, and the work hardening exponent is considered to be a parameter affected by stress and strain simultaneously. Therefore, it was confirmed that there is no general correlation between yield ratios, uniform elongations, and work hardening exponents in the API X80 linepipe steels investigated in this study, and previously reported API linepipe steels.