Springback compensation by stress superposition in forming of ultra high-strength steel sheets

今天，汽车行业面临着越来越多的环境法规以及对乘客安全标准要求的不断提高的挑战。为了达到汽车工业的这些要求，人们把重点放在了通过使用轻质材料和较小厚度的金属板材来减轻汽车车身重量上。这种趋势导致越来越多的人使用超高强度钢(UHSS)来制造汽车车身零部件。车身结构件通常采用拉深或拉深的方法制造。当零件在成形过程后从模腔中取出时，应力松弛恰好在零件中达到适当的应力平衡状态。这种应力松弛的结果是，成形零件和参考几何图形之间会出现回弹或某些偏差。在这种情况下可能会出现不同类型的回弹：角度改变、侧壁卷曲、半径改变以及扭转或扭转。当用UHSS成形车身结构件时，可以预期回弹量极高。在现有技术中，存在一些基于机械和几何的方法，这些方法已经显示出降低回弹的某些可能性。然而，超高速钢零件回弹补偿的方法直到现在才被开发出来，本研究的动机在于缺乏这样一种方法来补偿超高速钢零件成形时的回弹。为了成功地开发这样一种回弹补偿方法，首先必须定义成形后的作用应力之间的相关性，从而导致零件形状偏差或回弹。所提到的依赖关系应该用数学函数来描述。了解零件中产生的应力和由此产生的回弹之间的这种相关性后，将定义一种基于应力的回弹补偿方法。首先，要开发的补偿方法将基于在新的成形过程中使用交替的毛坯拉深的专用应力叠加，然后，如果必要，通过改变同一工具中某些零件区域的零件横截面尺寸。所需的零件横截面修改将通过相应的拉伸来提供，而不需要毛坯拉深，或者通过使用后成形来减小拉深。

Today, the automotive industry is challenged by increasing environmental regulations as well as the continuous enhancement of the standard requirements regarding passengers safety. In order to achieve these requirements in car industry, the focus has been put on the reduction of car body weight through the use of lightweight materials as well as sheet metals with smaller thicknesses. Such tendencies lead to the increased use of ultra-high-strength steels (UHSS) for manufacturing of car body components. The structural car body components usually are manufactured by stretching or deep drawing. When the part is taken out of the die cavity after the forming process, a stress relaxation happens to reach a state of appropriate stress balance in the part. As a result of this stress relaxation, the springback or certain deviation between the formed part and the reference geometry occurs. Different kinds of springback can arise in this case: angle change, sidewall curl, radii change, and torsion or twisting. When forming car body structural parts from UHSS, an extremely high amount of springback can be expected. In state of the art exist some mechanically and geometrically based approaches which have shown certain possibilities for springback reduction. However, a methodology for springback compensation by parts manufactured from UHSS has not been developed until now.The motivation of this research results from the lack of such a methodology to compensate the springback when forming the parts from UHSS. For a successful development of such a kind of methodology for springback compensation, firstly, a dependency between the acting stresses after the forming and consequently caused part shape deviations or springback must be defined. The mentioned dependency should be described by a mathematical function. With the knowledge of this dependency between the stresses caused in the part and, therefore, occurred springback, a methodology for stress based springback compensation will be defined. Firstly, the compensation methodology to be developed will be based on dedicated stress superposition during the novel forming process with an alternating blank draw-in, and afterwards, if necessary, by changing the part cross-section size in certain part areas in the same tool. The required modification of the parts cross-sections will be provided by corresponding stretching without blank draw-in or by reduction of the drawing depth using back forming.