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Combined cutting and local heat treatment with laser radiation of ultrahigh strength press hardened steels

 
: Vogt, S.; Schneider, F.; Weisheit, A.; Flaischerowitz, M.

Steinhoff, Kurt:
5th International Conference "Hot Sheet Metal Forming of High-Performance Steel", CHS 2015. Proceedings : 31.05.-03.06.2015, Toronto
Zwickau: Verlag Wissenschaftliche Scripten, 2015
ISBN: 978-3-95735-023-7
S.629-637
International Conference "Hot Sheet Metal Forming of High-Performance Steel" <5, 2015, Toronto>
Englisch
Konferenzbeitrag
Fraunhofer ILT ()

Abstract
Manufacturers are increasingly using high strength press hardened steels, which allow components such as B-pillars or side sills to be thinner and more lightweight and yet offer the same or better performance in the event of a crash [1]. But using these high-strength steels requires changes in processing after press hardening. High-strength steels are mainly cut using lasers because die cutting leads to rapid wear of the die [2]. Considering that the strength of new steels exceed the limit of 1500 MPa of steels in use towards 2000 MPa (e.g. MBW-K 1900 [3]) die cutting will even no longer be possible. However, laser cutting may lead to an undesired increase in hardness along the cut edge which can cause severe failure during a crash [4]. A way to overcome this problem is a local softening of the edge via laser heat treatment. The ability of a local laser softening has already been proved for press hardened steels up to 1500 MPa [5]. The main challenge here is to combine laser cutting with laser softening which is mandatory to achieve an economic process. To successfully implement both processes in one operation it is the aim to accomplish a burr-free cutting edge and a lateral softening zone of 2.5 nun with a hardness of 300 HV10. Both processes operate normally at different speeds (laser cutting typically around 10000 mm/min, laser softening typically around 1000 mm/min). To combine both processes as a hybrid treatment they have to operate at the same speed. In a first stage cutting and softening were investigated regarding a decrease and an increase in speed, respectively. Laser cutting was performed with a 4 kW fiber laser, whereas for laser softening a 12 kW diode laser was used. Cutting at lower speeds (around 5000 mm/min) shows an increase of the cutting burr, which can be reduced by an increase in gas pressure. Increasing the speed of laser heat treatment towards 5000 mm/min leads to an inhomogeneous temperature profile across the irradiated area as well as across the sheet thickness. When the material is heated above the alpha/gamma transus martensite is formed again due to high cooling rates. Therefore tempering is the best way to reduce the hardness. To improve the temperature profile beam shaping is under investigation. First results on the combined process will also be presented regarding the effect of the high cutting gas flow on the cooling during local laser heat treatment.

: http://publica.fraunhofer.de/dokumente/N-404673.html