关键词: 超高强钢, 焊管, 精密弯曲, 回弹补偿, 冷热复合工艺

Abstract: Irregular sections and bending spatial axes are main characteristics of complex-shaped tubular parts. For tubes of ultra-high strength like DP steel and QP steel, multiple steps of springback compensation and die repairing are required to obtain high dimensional accuracy. An analytical model for springback and displacement is developed for thin-walled tubes with mutant curvature radius of axis. Mapping relation of springback angle in arc segments and relative displacement in different positions of the tube is analyzed. Influence of springback angle of arc segments on bending accuracy is clarified. Thereafter, a cold-hot integrated precision bending method for thin-walled tubes without springback compensation is proposed. By performing local hot pressing and in-die cooling on the pre-bent tube, precision deformation and homogeneous properties can be realized. Taking DP980 ERW tube as the object, springback angle, circumferential wall thickness of bend tubes, as well as the heating rate, temperature distribution and circumferential temperature difference during self-resistance heating are tested. Changing rules of springback angle, microstructure and properties of the bend tube before and after the cold-hot integrated precision bending is tested by hot pressing. Results showed that:(1) In self-resistance heating, the temperature increased gradually and tent to be stable from tube ends to the symmetrical section. However, the circumferential temperature difference gradually increased; the circumferential temperature difference was about 100℃ while the temperature of the central region reached 800℃; (2) Obvious springback was observed after cold pre-bending, and the larger the pre-bending angle, the larger the springback angle; after hot pressing and in-die cooling, the springback angle reduced to only 5% of that after pre-bending. (3) Although the strength and hardness of the tube can be improved by water cooling, the original microstructure of the DP980 steel would be destroyed. Meanwhile, local weakening appeared in the weld zone. Uniformity of the microstructure and properties in the bending section can be obtained by in-die cooling, with little sacrifice on the strength and hardness. The research results verified the feasibility of the cold-hot integrated precision bending method, which can be used for short-process precision bending of ultra-high strength steel welded tubes without springback compensation.

Key words: ultra-high strength steel, welded tube, precision bending, springback compensation, cold-hot integrated process