Abstract: Due to the geometric particularity of longitudinally submerged arc welding(LSAW) pipes, the multi-step three-point bending method is mainly used to correct its straightness in manufacturing. The theoretical straightening moment, which corresponds with the pipe’s initial deflection distribution, is calculated based on the springback equation of small curvature plane bending and the equivalent principle for over-bend straightening. The pipe can be completely straightened by applying this theoretical straightening moment. To facilitate industrial applications, the multi-point bending one-off straightening control strategy is proposed by discretizing and linearizing the theoretical moment curve. Besides, the method to determine the corresponding straightening parameters is established. After measuring the initial deflection distribution of a LSAW pipe, the finite element methods (FEM) simulation model of its multi-point straightening process is built. The reliability of this model is verified by physical simulation experiments of small size pipes. Meanwhile, the feasibility of the multi-point bending one-off straightening control strategy is verified by the simulation results. This new strategy provides a theoretical basis to the design and the application of the automatic straightening machine.

Key words: longitudinally submerged arc welding pipes;multi-point bending;pressure straightening;bending moments;control-strategies