Quasi-Static and Dynamic Torsional Deformation Behavior of API X70 and X80 Linepipe Steels

This study aimed at investigating quasi-static and dynamic torsional deformation behavior of three API X70 and X80 linepipe steels. Quasi-static and dynamic torsional tests were conducted on these steels, having different grain sizes and volume fractions of acicular ferrite and polygonal ferrite, using a torsional Kolsky bar. The test data were then compared via microstructures and adiabatic shear band formation. The dynamic torsional test results indicated that the steels rolled in the single phase region had higher maximum shear stress than the steel rolled in the two phase region, because the microstructures of the steel rolled in the single phase region were composed mainly of acicular ferrites. In the X80 steel rolled in the single phase region, the increased dynamic torsional properties could be explained by a decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. The possibility of adiabatic shear band formation was analyzed from the energy required for void initiation and variation in effective grain size.