Study On Theory And Computer Simulation Of JCO Forming For Large-Diamater Longitudinal Welded Pipe

With the more energy consumption such as global oil and natural gas, pipeline construction has been developed rapidly and the demand of large-diameter straight-welded welded pipe (LSAW) is increasing, which increases the production technology of LSAW and put forward higher requirements of product quality. JCO forming, as one of the production technique for LSAW, requires systematic and comprehensive investigation due to a long process line of JCO and too many parameters with improved steel levels and pipe quality, although the research on deformation characteristics of JCO forming process and comprehensive influence of multi-parameter related technologies have carried out.Firstly, according to forming characteristics of large-diameter longitudinal welded pipe, based on theory of plastic bending, the reasonable assumptions was considered and the forming model was simplified in the paper. The theory description with the mathematical methods is performed for pre-bending and JCO process of LSAW. According to geometric relations between pre-bending moulds and slab, the spacing from upper die to lower die, radius of curvature at any angle of pre-bending slab were deduced. Simultaneously regarding forming characteristics of wide-punch pure bending with large curvature radius, the slab shape curve equation after pre-bending springback, analytic expression at any angle of the slab, pre-bend forming force at any time, and the analytic equation of the residual stress distribution was obtained by the non-arc bending calculation model. Then, the two modes for the symmetric bending and asymmetric bending for JCO forming were analyzed theoretically. In theoretical analytical study on common air bending, mathematical model of JCO forming for the last bending was established. The analytic expression of bending moment, bending angle before and after springback, springback angle, forming force and punch die displacement was obtained, as well as forming force and punch die displacement of unsymmetrical bending.Then,in the base of the previous theoretical analysis, considering X80 pipe steel as the main study issue, the finite element simulation model on air-bending without bottoming was established and the deformation characteristics was investigated using orthogonal experimental design and numerical simulation. The effect of different finite element algorithm, different element type, and different meshing on spingback were discussed and the parameters of the finite element simulation were determined, which were suitable for air-bending without bottoming. Then simulation experiments were performed and the effect of punch and die radius, dies opening on springback angle, bending angle after spingback, forming force and maximum residual stress were discussed based on orthogonal design, combining response surface model technology and radial basis function neural network. Parameters prediction and optimization model was established combining multi-objective genetic algorithm and sequential quadratic programming. Simultaneously, the sensitivity of material property parameters and thickness variation on sheet springback has been analyzed based on orthogonal experimental design and FEM simulation, including initial yield stress, Young's modulus, hardening coefficient, hardening index and material thickness.Finally,the assumption of elastic-plastic material model and plane strain was adopted, continuous simulation model for pipe manufacturing of X80-Φ1219×22 (mm) as example has been established. The experimental investigation on pre-bending and JCO forming was performed to verify the simulation model effective. The characteristics of stress-strain field of pre-bending and JCO forming was analyzed and their deformation characteristics were revealed. By changing the pre-bending mold geometries and the assembly relationship, the variation of pre-bending force, lower die stroke and pre-bending shape was described. For the final symmetric bending and the other asymmetric bending during JCO forming, the relationship between punch insertion and bending angle after spingback was discussed. Prediction model of punch insertion was put forward to predict target punch insertion for required bending angle. The effect of mold structure parameters and punch die displacement on asymmetric forming was discussed and the analysis on the final forming shape of steel tube for different bending times was performed.Through the systematical research on pre-bending and JCO forming for large diameter longitudinal welded pipe, it provides important guidance for the choice of forming equipment, the design of dies and the establishment of forming process project, and has greater engineering application prospect.