Large-diameter longitudinal submerged arc welded (LSAW) pipes represent a critical component of long-distance oil and gas transmission pipelines. To enhance the forming efficiency of the JCO (J-shape to C-shape to O-shape) forming process for LSAW pipes, and to reduce residual straight segment in order to minimize the ovality of the formed pipes, an asymmetric four-point air bending (AFB) process was proposed. In this process, one end of the sheet contacts the dies with a straight segment, while the other end contacts a circular arc segment. The distribution of bending moments and mechanical model under different bending stages were analyzed, and analytical formulas for the main forming indexes before and after springback were derived. Experimental and finite element simulation verification were conducted for the AFB process. The results indicated that the error between the experimental and simulation results and the theoretical results was small, and the variation trends were consistent. Furthermore, the ellipticity of the pipes formed by the AFB process was less than 0.66%, which is obviously lower than that of the pipe formed by the symmetric four-point air bending (SFB) process. The forming quality and production efficiency of the pipe is improved, thereby proving the feasibility and reliability of the AFB process and promoting the development of LSAW pipe JCO forming processes.

The current progressive forming process for large-scale longitudinally submerged arc-welded (LSAW) pipes is continuously losing competitiveness in terms of manufacturing quality and production efficiency. Based on the asymmetric four-point bending (AFB) mechanical model and classical elastic–plastic bending theory, in this paper, we investigate sheet bending behavior during the four-point bending stage. Analytical expressions are derived for the main influencing factors during the forming and springback process. Finite element simulation and experimental research into the mechanical model are conducted to analyze the effects of various process parameters on the forming results. On this basis, an AFB progressive forming process parameter formulation strategy is established and programmed. Experiments and simulations were conducted using the process parameters formulated by this strategy. The results showed that pipes could complete the progressive forming process in fewer passes, thereby improving production efficiency. The ovality of most experimental pipes was less than 1.5% and consistently below 0.7% in simulations—significantly lower than the engineering requirement of 2%. These results demonstrate the feasibility and reliability of the strategy, highlight the significant improvement in pipe quality achieved through the AFB process, and lay a solid foundation for the development and intelligentization of future progressive forming processes for LSAW pipes.