Local laser softening of press-hardened steel at high feed rates

The development of hot stamped manganese–boron steels established new potentials for lightweight constructions for applications in the automotive industry. This type of high strength steel allows the usage of thin sheet thicknesses resulting in low specific weight retaining the optimized performance of passive safety. The parts have a high tensile strength of about 1500 MPa. However, this high strength is not favorable in all sections of a part, especially those which are relevant for safety or for joining. To ensure the requirements for the crash properties or the joining areas, a high ductility is required in these areas. A way to overcome this problem is a local softening of the crash and joining zones via laser heat treatment. By means of laser radiation, the brittle martensitic microstructure is either tempered or transformed into a ferrite/perlite dominated microstructure. A fiber-coupled diode laser with a maximum laser power of 12 kW and rectangular laser spot is used for the laser heat treatment. The temperature is controlled by a monochromic pyrometer integrated into the optical system. Generally, the necessary process feed rate of the heat treatment process is low (about 1000 mm/min), as a homogeneous result of the heat treatment is pursued. As the feed rates increase (up to 8000 mm/min), the temperature distribution across the thickness of the material becomes more inhomogeneous. With the topside being hotter than the backside, the material is tempered with a varying intensity resulting in a hardness distribution across the thickness. The influencing parameters for this effect are being characterized, and the mechanical properties of the anisotropic material are investigated.