Analysis and estimation of the yield strength of API X70 and X80 linepipe steels by double-cycle simulation tests

In this study, the spiral piping and electric resistance welding piping was conducted on API X70 and X80 linepipe steel sheets having different microstructures, and the yield strengths of the flattened sheets were measured. A double-cycle simulation test with tension-compression-tension or compression-tension-tension for the piping and flattening processes was conducted to estimate the yield strength. The simulation test results indicated that the yield strengths of the outer or inner wall of the pipe could be estimated by combination of Swift's equation and the Bauschinger stress parameter, and that these estimated yield strengths were well matched within a small error range with the measured yield strengths. Thus, the variations in yield strength before and after the piping could be effectively estimated using the tension/compression properties of the leveled sheets because the strength differential effect was small and the reverse flow curves were expressed by a single curve. These findings suggested that the present estimation method played an important role in controlling microstructural and manufacturing process parameters to minimize the reduction in yield strength of the linepipe steel sheets.