PROBLEM TO BE SOLVED: To provide a plastic-worked article formed by deforming a material having a cylindrical part from the outside to the inside of the cylindrical part, configured to improve fatigue strength against an external force, such as a torsional force to be applied in a circumferential direction of the cylindrical part, and to provide a method for manufacturing the plastic-worked article, a torsion beam manufactured by applying the method for manufacturing the plastic-worked article, a torsion beam Assy, a torsion beam suspension device, and a method for manufacturing a torsion beam.SOLUTION: The method for manufacturing a torsion beam for connecting right and left arms in a suspension device includes: adding deformation from the outside to the inside in the longitudinal direction of a material tube to form a substantially V shape or a substantially U shape; applying circumferential tensile stress to the substantially V-shaped or substantially U-shaped closed cross section; uniformly adjusting the circumferential stress distributed in the thickness direction of the material constituting the substantially V-shape or U-shape; and reducing residual stress after spring-back.
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