High-plasticity and low-yield-ratio straight seam welding steel round pipe with yield strength larger than or equal to 355MPa and manufacturing method of high-plasticity and low-yield-ratio straight seam welding steel round pipe

The invention belongs to the technical field of steel round pipe preparation, and particularly relates to a high-plasticity low-yield-ratio straight seam welding steel round pipe with the yield strength larger than or equal to 355 MPa and a manufacturing method of the high-plasticity low-yield-ratio straight seam welding steel round pipe. The high-plasticity low-yield-ratio straight seam welding steel round pipe comprises, by weight, 0.16%-0.20% of C, 0.12%-0.25% of Si, 0.95%-1.15% of Mn, smaller than or equal to 0.020% of P, smaller than or equal to 0.010% of S and the balance Fe and inevitable impurities. The manufacturing method comprises a steel smelting process, a refining process, a slab continuous casting process, a rolling process and a pipe manufacturing process. The yield strength of the round pipe is low, more plastic deformation can be borne after the round pipe is yielded and before the round pipe is fractured, the stress space of steel is increased, the time from deformation to fracture is prolo