Research On New Process Of Three-Roll Continuous Rounding For Large Thin-walled Pipes

With the increasing demand for steel pipes from land-based oil and gas pipeline renewal,coal mines and deep-sea natural gas extraction,more stringent requirements are placed on the quality and performance of pipe fittings.Ovality is one of the important criteria for evaluating the quality of fittings.According to the American Pipe Association API Spec 5L standard,the ovality of the fittings should not exceed 1.0% of their nominal outside diameter.Most of the currently widely used rounding techniques can only round the ends of the pipe fittings.Since the overall stress distribution of the pipe fittings is uneven,the ellipticity increases in the course of transportation and long-term placement.Therefore,it is necessary to round the entire pipe fittings.Based on the complex curvature theory of plane curvature and the unified theorem of reciprocating bending,a new type of roll bending and rounding method,three-roll continuous rounding process,is proposed.The process equipment does not need to turn over the cylinder device.When the rigidity of the roller shaft is insufficient,the support roller can be set for each roller axis,and is particularly suitable for a pipe with a large diameter and a relatively small diameter and a relatively small diameter.The continuous rounding process of ST12 and 304 stainless steel pipe fittings was numerically simulated by ABAQUS software.The distribution of stress and strain during continuous rounding process,the deformation characteristics of the section of the pipe and the variation of the rounding forming force of the rollers with time were analyzed.The ellipticity of the pipe after adjustment was investigated under different pressure,different relative thickness and different initial ellipticity.In the process of continuous rounding,there are three positive and negative bending regions alternately in the cross-section of the pipe.The stress in the forward and reverse bending areas of the pipe is declining from the inner and outer surfaces of the pipe to the neutral layer,decreasing from the center of the positive and negative bending area to both sides.The roll forming force is basically the same,with the growth,stability,and decline over time.The ellipticity of the tube after straightening decreases from the beginning to the end of the gradual trend as the amount of depression increases.With the increase of the relative thickness,the optimal downward pressure of the pipe tends to decrease.The initial ellipticity of the tube does not have a significant effect on the results after continuous rounding.According to the simulation output of the round forming force,the thrust of the tube and the rotation speed of the roller shaft,a rounding machine was designed.The structure of the rounding machine and the working principle of the rounding machine were introduced.