Martensitic microstructural transformations from the hot stamping, quenching and partitioning process

Hot stamping, which combines forming and quenching in one process, produces high strength steels with limited ductility because the quenching is uncontrolled. A new processing technique has been proposed in which the hot stamping step is followed by a controlled quenching and partitioning process, producing a microstructure containing retained austenite and martensite. To investigate this microstructure, specimens were heated at a rate of 10 °C/s to the austenitizing temperature of 900 °C, held for 5 min to eliminate thermal gradients, and cooled at a rate of 50 °C/s to a quenching temperature of 300 °C, which is between the martensite start temperature and the martensite finish temperatures. The resulting microstructure was examined using optical microscope, scanning electron microscopy and transmission electron microscopy. The material produced contains irregular, fragmented martensite plates, a result of the improved strength of the austenite phase and the constraints imposed by a high dislocation density.

Research Highlights
► A novel heat treatment of advanced high strength steels is proposed.
► The processing technique is hot stamping plus quenching and partitioning process.
► The material produced contains irregular, fragmented martensite plates.
► The reason is strength of austenite phase and constraint of dislocation density.