EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The concept of quenching and partitioning steels, with a high product of strength and strain, as well as good local formability, has arrived in the high-end commercial steel production and has found promising applications in the automotive industry. To this day, the research has focused on the development of uncoated Q&P grades. By using inline galvannealing lines, which includes beside the annealing- and skin passing, also a coating process, these steels could be produced more cost-effectively. As the chemical composition of the steel is decisive for its mechanical properties, the influence of the alloying elements Si, Al and Mn, was investigated studying three different chemical compositions.
First, the microstructure evolution during the Q&P-process and the influence of a galvannealing treatment was examined via dilatometry and SEM. A detailed look was given to the development of the retained austenite fraction. Afterwards tensile tests were carried out to describe the global formability. For a more in-depth understanding interrupted tensile tests were performed, determining the retained austenite mechanical stability. In order to describe the local formability, the reduction of thickness of the fractured tensile test samples was measured and the fracture areas of the tensile tests were investigated via SEM.
The investigations show a clear influence of the alloying elements Si, Al and Mn on the microstructural development and thus the global and local formability during the Q&P and the galvannealing process respectively. While after the Q&P process, the Si-alloyed grades in particular showed excellent properties, subsequent to galvannealing the Al-concept proved to be more suitable to fulfill the desired requirements.