超高强钢焊管无回弹补偿冷热复合精密弯曲

Cold-hot Integrated Precision Bending of Ultra-high Strength Steel Welded Tube without Springback Compensation

复杂异形金属管件常具有不规则截面和空间弯曲轴线。对于DP双相钢、QP淬火延性钢等超高强度金属管件,需要进行多道次回弹补偿和修模才可获得高尺寸精度。以轴线曲率半径分段突变的薄壁弯管为对象建立了弯管回弹与位移理论解析模型,分析了圆弧段回弹角与弯管不同位置处相对位移的映射关系,明晰了圆弧段回弹角对弯管精度的影响。基于此,提出薄壁管无回弹补偿冷热复合精密弯曲方法,通过对预弯曲管坯进行局部热态模压及模内约束冷却,实现弯曲段精确变形及环向组织性能均匀化。以DP980钢ERW焊管为对象,测试了不同弯曲角度弯管的回弹角和环向壁厚,以及预弯管自阻加热时升温速率、温度分布及环向温差;通过模压实验,测试了冷热复合弯曲前后弯管回弹角及组织性能的变化。结果表明:(1)弯管自阻加热时沿管端向对称截面处温度逐渐升高并趋于稳定,但同时环向温差逐渐增加,当管坯中心区温度为800℃时环向温差约100℃;(2)冷弯后弯管回弹明显且弯曲角度越大回弹角越大,但在热模压及模内冷却后,弯管的回弹角仅为预弯后回弹角的约5%;(3)模压后水冷虽可提高弯管的强度和硬度,但将破坏DP980钢的原始组织状态,同时在焊缝区出现局部弱化。采用模冷可实现弯曲段环向组织性能均匀化,且强度和硬度与原始管坯接近。研究结果验证了“冷热复合精密弯曲”的可行性,无需回弹补偿即可实现超高强钢焊管的短流程精密弯曲。

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.