Study On Springback And Quality Control Method In L-bending Of High-strength Steel

The phenomenon of springback is much more obvious in the bending process of high strength steel. Many traditional methods for controlling springback can not be used in progressive dies and step dies, because of the limits of material feeding direction and arrangement of pre/post processes. In this article, research and improvement have been made on the two popular springback controlling methods for L-bending: over-pressure method and negative angle method. The two methods mentioned above have their problems when used in engineering. The mechanism of over-pressure method in controlling springback still needs further research and it is difficult to determine the amount of over-pressure in the engineering application. For the negative angle method, a phenomenon of sidewall curl is often occured after bending and up to present there is no efficient method to control the side wall curl. This paper is focused on solving the above-mentioned problems by using the method of analysis, experiment and FEM. The main contents are as follows:Firstly,the mechanical parameters of Jac270 C and DP590 were tested using DIC method, and the measurement of anisotropic coefficient r is emphatically discussed. Considering the difference of r values under different strains, an average anisotropy coefficient value is proposed to replace the r value under designated strain.Secondly, the over-pressure method’s mechanism of controlling springback is invested by using theory and FEM analysis, and the influence of process parameters, which included amount of over-pressure,die radicus and anisotropy coefficient, on springback is also studied based on the experiment and FEM results. Besides, an empirical formula for calculating springback angle is fitted using the FEM bending moment results.Thirdly, this paper made an explaination of the negative springback occured in the integral over-pressure bending. It is generally believed that the decrease of springback is caused by the reduced stress difference between the inner and outer layers in the over-pressure deformation. However, it is found that the movement of the material in the bottom of corner and subsequent un-bending process are the main reasons that cause negative springback.Lastly, mechanism of sidewall curl caused by negative clearence is invested by using analytical and FEM method. The relationship of negative clearence and sidewall curl is also investigated. A method using a clamping plate to control the sidewall curl is proposed and verified by desgined experiments. The results show that this method has significant effect on suppressing the sidewall curl in negative clearance forming.