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

The quenching and partitioning (Q&P) heat treatment is a promising way to produce third generation advanced high strength sheet steels consisting of martensite and retained austenite. To extend the range of the mechanical properties, especially to increase the formability, ferrite can be introduced into the Q&P microstructure. This can be done by annealing in the intercritical temperature region, which, however, strongly depends on the initial microstructure in terms of recrystallization and transformation properties. Alternatively, ferrite can be introduced by complete austenitization followed by slow cooling. Since this has not been investigated thoroughly so far, the microstructure and properties of a partially ferritic Q&P steel obtained by slow cooling were analyzed in this study. For comparison, the steel was exposed to an intercritical Q&P heat treatment resulting in comparable amounts of ferrite, and to a standard heat treatment obtaining a non ferritic Q&P microstructure.
The mechanical properties were explored by tensile testing and nanoindentation. The microstructure was analyzed by light optical microscopy, scanning electron microscopy, electron backscatter diffraction and dilatometry. Furthermore, the distribution of alloying elements was investigated by energy dispersive X-ray spectroscopy.
The results demonstrate that introducing ferrite into the microstructure is accompanied by a pronounced increase in elongation and a decrease in strength for both partially ferritic heat treatments. In addition, it was found that due to the finer microstructure of the intercritically annealed steel, the local formability is higher than after slow cooling heat treatment. Overall, the intercritical heat treatment exhibits the best balance in formability and strength.