P 1188 – Qualification of a new Cu-St material system for use in chassis components
Highly stressed chassis components in the automotive industry are currently often made of multiphase steels when yield strengths above 600 MPa are required. These steels combine a high strength level with a high amount of ductility. Therefore, they are becoming increasingly interesting for the manufacturing of thin-walled lightweight structures with complex load requirements. The necessary steel sheets are usually made of cold-rolled strips. With respect to the use of multiphase steels a continuous annealing line is required to achieve the corresponding temperature-time cycles to generate the special microstructure. However, most of the existing cold rolling factories apply a batch (hood) annealing process, due to the high flexibility and low costs of the process. A possibility for a cost-effective production of highly stressed lightweight structures is the treatment of a new, low-alloy steel grade SP-800. Through a targeted combination of strain hardening and precipitation hardening, due to the use of copper within the chemical composition, a much comprehensive grading of the resulting mechanical properties becomes possible. A yield strength in the range of 600-1000 MPa combined with elongations between 10-20% can be adjusted. Within the FOSTA project “Qualification of a new Cu-St material system for use in chassis components” various tests were conducted. Furthermore, the reference material CPW800 was tested for comparison. The tests on various heat-treated specimens have shown that the material behavior can be adjusted via the heat treatment (strength/ ductility). A previously expected negative effect caused by cooper depositions on the fatigue behavior has not been proved. Although the fatigue behavior of the Cu-Steel was worse than the mass-produced CPW800, the analysis of the fracture surface has shown that crack growth does not start from internal defects. To sum up, it has been confirmed that the laboratory-produced material, on the basis of the performed material characterization, has shown competitive material behavior to the material CPW800, depending on the application. Further optimization potential seems to be possible through industrial production. Due to the good mechanical properties, the material basically shows potential for future chassis applications.
Prof. Dr. rer. nat. T. Tröster