Local heat treatment of ultra high strength steels to improve formability

Ultra high strength steels are of great interest especially in the automotive industry due to their potential in realising lightweight structures and improved safety. However the poor formability of these steels limits their application for many parts in the car body. A solution approach to this limitation can be a local heat treatment to soften the material where a high formability is needed. This paper will report on the first results of local heat treatment of ultra high strength steels using induction and laser technology carried out in an European project. Laser treatment was performed using high power Nd:YAG-lasers. The induction heat treatment was carried out with a 6 kW generator and multi-turn induction coils of pancake shape. In both processes the output power was temperature controlled to achieve a constant temperature level during the heat treatment. The materials under investigation are dual phase steels with a tensile strength of 1000 MPa and 1400 MPa. The thickness of the sheets are 1.2, 1.5 and 2 mm. It will be shown that induction and laser heating have a significant effect on microstructure and mechanical properties. The initial martensite/ferrite- or martensite-structure is tempered or annealed depending on the maximum temperature and the time-temperature characteristic achieved during heat treatment. In general the characteristic features of the microstructure formed within the local heat-treated area are tempering and lowering of the amount of martensite. However, the amount, size and distribution of martensite can vary to some extent. Both tempering and annealing result in a significant hardness reduction. The hardness is reduced about 30 - 50 %. Tensile tests reveal a significant decrease in technical yield strength and increase in ultimate strain, for example for DP 1400 from 3-4 % up to 13-16 %. Limiting Dome Height tests show a significant improvement in dome height for induction treated sheets.