Research on the Symmetrical Three-roller Setting Round Process

In order to reveal the mechanism of the symmetrical three-roller setting round process and optimize the setting round strategy,the theoretical analysis and experimental research of the three-roller setting round process are carried out.The springback equation of small curvature plane pure bending about curve beam is a base,the bilinear simple kinematic hardening model is used,and the springback curvature equation and the unified springback curvature equation of reciprocating bending are established by the mathematical induction method.Theoretical analysis results show that the difference of initial curvature can be eliminated and the curvature values are unified through every micro-pipe-wall of the pipe with different initial curvature experiences multiple times of reciprocating bending.The quantitative relationship between the upper roller reduction,bending radius and elastic area ratio is established by the geometric relation,nominal curvature,wall thickness,and mechanical properties.The mechanism of the symmetrical three-roller setting round process is verified by three-roller setting round experiments.The experiment results show that with the decrease of the elastic area ratio the residual ovality decreases gradually.When the elastic area ratio is close to 40%,the residual ovality can be close to 0.3%,which is far less than the requirement of API 5L.It is verified that there is little relationship between residual ovality and initial ovality in the small deformation range.