Lokale Wärmebehandlung mit Laserstrahlung zur Verbesserung der Umform- und Funktionseigenschaften von hochfesten Stählen

Lightweight construction is an effective method of reducing fuel consumption and CO2 emissions in the automotive industry. At the same time, crash safety specifications are constantly being tightened. High-strength steels meet both requirements. However, these steels are more difficult to form than deep drawing steels since their higher strength compromises formability, especially when using tensile strength above 1000 MPa. The cold formability of high-strength steel blanks can be improved by locally changing the microstructure in critical areas with laser radiation prior to cold forming. The laser heat treated areas exhibit up to five times increased elongation in a tensile test. Thanks to the locally modified mechanical properties of the blanks, parts can be formed without necking or cracks in critical areas. Another way to combine high strength with a good formability is hot-stamping. In the case of the widely used Manganese Boron Steel 22MnB5, the tensile strength is 1600 MPa after hot stamping. But such high strength is not desired in the whole part. Deformation zones for better crash performance and zones for joining require a more ductile material, what can be achieved by local heat treating these zones with laser radiation. In this work, material and process characteristics of laser heat treatment of four different high strength steels are investigated: CP-W®800, Docol®1200 and MS-W®1200 are used for cold forming, 22MnB5 for hot-stamping. A fiber-coupled 10 kW high-power diode laser and optics with rectangular, homogeneous intensity distribution of up to 90 mm width are used. Process parameters, temperature, microstructure, hardness and mechanical properties are correlated for all investigated steels. Corrosion and tool friction are investigated for different coatings, to ensure automotive applicability. The laser heat treatment process is demonstrated for different parts. The properties of these parts (e.g. tensile strength, crash properties, geometry and distortion) are analyzed. B-pillars can be formed from 50 % stronger blanks than before, when using laser heat treated blanks. These B-pillars can resist higher forces in a side-impact-test. The laser heat treatment of hot-stamped parts results in an increase of the breaking elongation from below 5 % to more than 18 %. With 10 kW laser power, up to 15 cm²/s can be heat treated, depending on the coating. The energy cost are less than 0,10 Euro per part (2012)