Effects of coiling temperature and pipe-forming strain on yield strength variation after ERW pipe forming of API X70 and X80 linepipe steels

Since pipes are undergone repeated tension and compression strains during pipe-forming, and flattening, flattened sheets often show too higher or lower yield strength than hot-rolled coils, which poses to difficulties in satisfying yield strength standards. In this study, effects of microstructure and pipe-forming strain (thickness/diameter (t/D)) on yield strength variation were investigated in X70 (483 MPa) and X80 (552 MPa) linepipe steels fabricated by controlling Mo content and coiling temperature, and their yield strength, strain hardening exponent, and Bauschinger stress parameter were measured by tension-compression tests with varying tensile-pre-strain. In the X80 steels whose Mo content was higher than that of the X70 steels, the higher Mo content promoted the formation of low-temperature transformed microstructures such as acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent, which played a role in decreasing Bauschinger effect. The reduction in yield strength was smaller in the X80 steel than in the X70 steel. As the coiling temperature decreased, the volume fractions of AF, BF, and pearlite increased, while those of QPF, GB, and MA decreased, and led to the increase in yield strength by about 30 MPa. The yield strength slightly increased after the pipe forming at higher coiling temperature, while it was largely reduced at lower coiling temperature. When the steels having different t/D were compared, the yield strength after the pipe forming increased largely by 65 MPa under the higher t/D as the strain hardening effect overrode the Bauschinger effect. In order to prevent or minimize the large reduction in yield strength after the pipe forming, low-temperature transformation microstructures, coarse grain size, and high t/D were desirable.