Low-Load and High-Precision Forming Technology for Large-Scale Graded Doubly Curved Q890 High-Strength Steel Thick Plates

Large-scale asymmetric doubly curved thick shells made of high-strength steel are key components in deep-sea and nuclear pressure vessels. Integral pressing is an important manufacturing method for such components, but it still faces two major challenges: excessive clamping force and poor springback predictability. To address these issues, this study conducts a combined experimental and numerical investigation on a 16 mm thick Q890 high-strength steel plate. First, the through-thickness plastic gradient and cyclic stress–strain response of the material were characterized, and a mixed hardening model incorporating through-thickness gradient plasticity was established. To suppress the lateral force induced by the asymmetric geometry, a blank positioning strategy was proposed, reducing the lateral force to below 2 t (Reduced by 65%). More critically, a striking phenomenon is revealed: during the final 1 mm of the clamping stroke, the forming force surges abruptly from approximately 680 t to 5090 t. Detailed analysis identifies the root cause as the synergistic effects of a sharp increase in contact area, a drastic rise in frictional resistance, and the onset of localized upsetting in regions already in contact with the die. To suppress the load surge while maintaining forming accuracy, a normal-direction over-compensation strategy was proposed. By deliberately increasing the normal compensation, the blank retains a bending-dominant deformation mode at the target clamping position, thereby avoiding the critical contact expansion, frictional buildup, and localized upsetting that trigger the force surge. Through iterative simulations, the optimal die surface is determined, achieving a forming force below 1000 t with a simulated shape deviation within 0.96 mm. Experimental validation using a purpose-built die on a 1000 t press successfully produces the shell with a maximum profile deviation of 1.67 mm, meeting high-accuracy requirements. This work establishes a new paradigm for low-load, high-accuracy forming of thick high-strength steel shells by actively managing contact evolution and deformation mode via normal-direction over-compensation, offering a practical pathway to one-shot tryout success for critical pressure hulls.