Research on alloy concepts and process strategies for the development of safety-relevant lightweight car body parts with increased rest formability within the press hardened state

  Stress-strain-curves IEHK Figure 1: Stress-strain-curves of the novel alloying concepts in comparison to 22MnB5

The fundamental objective in the development of structural components for the car body is substantial weight reduction, while ensuring safety for the passenger in case of a crash. An often considered development trend is hot stamping. It is a method for hot forming metal sheets. It offers the opportunity to produce, almost free of spring back, components made of steel with high stability as well as complex geometries. The customarily used material, 22MnB5, experiences, due to martensite formation during hot stamping, an increase in strength to up to 1500 MPa while severely reducing ductility. The thoroughly martensitic structure of hot stamped boron-alloyed steels exhibits a high mechanical resistance to plastic deformation. If the elasticity limit is exceeded, the range of the homogenous deformation up to the uniform elongation is very little. This presents a crucial disadvantage to the safety systems. After the appearance of a crack within the martensitic matrix there is a lack in mechanisms to prevent crack propagation.

Goal of this project is the development of new alloy concepts and process strategies creating press hardened auto body parts with increased ductility. The first concept varies only little from 22MnB5 and is supposed to lead to grain refinement through micro-alloying. The other two concepts introduce a ductile second phase, for example retained austenite and ferrite, to increase ductility. In contrast to the concept of grain refinement, they need adaption of the time-temperature curves during hot stamping.

In the course of the project all the alloys are characterized in regard to the most important thermal and mechanical parameters. This is to achieve an optimal process control. Using tensile, shear and bending tests the ductility will be determined and validated. Afterwards the joinability is examined by laser and resistance spot welding and the coatability of AlSi and Zn investigated.

After project completion, the existing range of press-hardenable steels is to be expanded by the new alloy concepts and process strategies to improve product properties.