Local laser heat treatment of ultra high strength steels to improve formability

Ultra high strength steels are of enormous interest especially in the automotive industry due to their potential in realising light weight structures and improving the crash behaviour. However the poor formability of these steels limits their application for many parts in the car body. A solution to this limitation can be a local heat treatment using a laser beam to soften the material where a high formability is needed. Laser treatment was performed using a Nd:YAG laser with 3 kW maximum power. The output power was temperature controlled to achieve a constant temperature level during the heat treatment. Large areas are treated by scanning the surface with the laser beam. The materials under investigation are dual phase (DP), retained austenite (RA), complex phase (CP) and martensitic (MS) steels with a tensile strength of 700-1400 MPa. The thickness of the sheets lies between 1.2 and 2 mm. Depending on material and process parameters tempering of martensite, formation of ferrite and transformation of retained austentite are observed as a result of the heat treatment. For DP and M steels this changes in microstructure are accompanied by a reduction in hardness and yield strength, whereas CP and RA steels show no significant effect on the mechanical properties. Forming tests of a mock-up geometry reveal a reduction in forming capacity