Moscow Technical University of Communications and Informatics, Moscow, Russia
V. N. Shinkin, Dr. Phys.-Math., Prof., Dept. of Physics, e-mail: shinkin-korolev@yandex.ru

Bending the steel sheet’ edges is a key operation in the production of the large-diameter welded steel pipes, ensuring a high-quality gas assembly of the pipe billet’s edges before welding, hermeticity of the weld and compliance of the pipe geometry with the design parameters. In modern practice, two main technological schemes of pipes’ production are used. JCOE (SMS Meer) scheme: the edges are bent before the main part of pipe billet is formed on the presses; the process is step-by-step and symmetrical on both sides; ensures high accuracy of the edge geometry. The scheme with the three-roll rollers (by Haeusler): the edges are bent after rolling the sheet (required to eliminate non-deformable flat sections – the sheet splines); the specialized roller edge bending machine is used. Physico-mechanical fundamentals of the process: an elastic plastic deformation of the material occurs during bending. Key parameters: a radius of curvature of the neutral plane; a spring coefficient (which takes into account the elastic restoration of shape after removal of the load); resistance to deformation. The equipment and tools: the edge bending presses (characterized by hydraulic drive, step-by-step principle of operation, symmetrical edge deformation and the ability to adjust the bending radius of the edges); the rolls (the two-radius or three-radius profile of the surface of the sheet edge for smooth transition, a high hardness of the steel surface, cooling system to prevent overheating). The technological parameters: an angle of edges’ bending (set by the pipe design); the length of the bent edge; the bending force (depends on the thickness of the sheet, Young’s modulus, yield strength and strength of steel). Quality control: bending assessment criteria (compliance of the curvature’s radius with the design value, an absence of cracks and tears on the edges, a uniformity of deformation along the sheet’s length, bending angle within tolerance); the control methods (visual inspection, measurement by templates, ultrasonic flaw detection). Typical defects and ways to eliminate them: corrugation (the reason is exceeding the critical bending angle; the solution is reducing force, increasing the roll radius); uneven curvature (the reason is heterogeneity of steel properties; the solution is preannealing of the sheet); cracks on the edges (the reason is low ductility of the material; the solution is replacing steel with a grade with less carbon content). Bending edges on presses is a high-tech process: it requires an accurate calculation of deformation parameters; the use of specialized equipment; a quality control at all stages. Process optimization makes it possible to increase the weldsэ hermeticity, reduce energy consumption and increase the pipes’ service life. In this paper, the mathematical model is constructed for calculating the residual curvature of a steel sheet after its elastoplastic bending on an edge-bending press with a second-order matrix surface profile (an elliptical cylinder).

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