Sheet hot stamping has been established as a productive, resource-efficient and economical process for the manufacture of high-strength components in lightweight automotive construction. The classical 22MnB5 boron steel can result in high strength but limited total elongation. Large number of studies have tried to alleviate this weak point in order to achieve larger crashworthiness in the crumple zone though not successful under the industrial constrains. This research proposes stamping a new ductile Transformation-Induced Bainitic Ferrite (TBF) steel with a chemical composition close to the serial steel production in preheated dies above Ms temperature (> 380 °C). The expected carbide-free bainite with highly stabilized filmlike retained austenite (RA) should result in stronger TRIP effect  (Transormation Induced Plasticity) and consequently total elongation with a high tensile strength. Pre-cooling the blank before stamping (550-600 C) enables the refinement of bainitic ferrite, which will further refine the filmlike retained austenite and raise its stability. 

Dilatometer tests in tensile mode based on the schematic hot stamping cycle (Figure 1) are examined to define the process window. The pre-cooling temperature of the blank and the isothermal holding temperature are varied and the microstructure is characterized using light and electron microscopy. The mechanical properties are characterized using a novel indentation method in which comparative values for mechanical properties such as tensile strength and yield strength are determined.

The results show that the process window for hot stamping for bainitic-ferritic microstructure can be defined at 380-475 °C. UTS of > 1000 MPa are achieved. The yield strength RP0.2 can be determined at up to 780 MPa.