Geometrical accuracy improvement in flexible roll forming process by means of local heating

Abstract

In the last decade the tendency to reduce vehicle weight due to rising fuel prices and environmental requirements, have pushed two main trends of research in the field of vehicle structure manufacturing. On the one hand, the development of new high strength steels allowed to reduce weight and increase the structural performance of vehicles. However, these materials exhibit low formability and high springback effect at room temperature, making dificult to form high strength steel accurate parts. For all these reasons it is necessary to develop exible and consumer-oriented manufacturing processes in order to allow the manufacturing of accurate parts in an economic way. In this context, the flexible roll forming process was developed, which combines all the advantages of conventional roll forming, allowing to manufacture continuously, profiles with variable cross section along its longitudinal direction. However, the flexible roll forming process is still under development and characteristic geometrical error of the process must be solved. Therefore, the current dissertation proposes and studies the reduction of geometrical errors created during the manufacture of flexible profiles using a thermo-mechanical process called heat-assisted flexible roll forming. Three have been the main fields of research conducted for this purpose. In the first section of the present dissertation, the quasi-static mechanical behavior of three ultra high strength steel sheets (MS1200, CP800 and DP600) and a soft steel sheets (DC01) are characterized by means of uniaxial tensile tests. These materials are used to validate the numerical models developed during the thesis. On the other hand, the CP800 material, selected to perform heat-assisted flexible roll forming experiments, is mechanically characterized at high temperatures. In order to determine the microstructural changes that take place and identify possible deformation mechanisms at high temperatures, the microstructure of the specimens tested at high temperature are analyzed by optical microscopy and X-ray diffraction methods. In the second section, numerical models of the conventional roll forming process and heat assisted flexible roll forming process are developed. Numerical models are used to develop and optimize the heat assisted flexible roll forming process. However, in order to check the reliability of the models, conventional roll forming trials results are compared with numerical results. In the last section, heat assisted flexible roll forming experiments are carried out using the flexible roll forming machine developed within the European Proform project. The results show a significant decrease of the web warping when the flexible profile is heat assisted.