Local Laser Heat Treatment in Dual-Phase Steels
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TO combine fuel saving with increased safety of vehicles, the automotive industry was led to develop advanced high strength steels (AHSS). Their high yield and tensile strengths enable a decrease in sheet thickness (weight saving) and, at the same time, maintenance or even improvement of crash behavior (safety).[1–4] This is achieved by microalloying[5] and a thermomechanical treatment.[6] These AHSS steels, which are mostly multiphase steels, are characterized by good formability, high strength, and a bake hardening (BH) effect. Multiphase steels have proved to be a good compromise between strength and ductility.[7,8] They exhibit a continuous yielding behavior, low yield point, and high strain-hardening coefficient.[9,10] This has been attributed to an increase in the work hardening limits through forming mobile dislocations due to the martensite transformation during heat treatment and martensite twinning during forming.[11,12] The aging of special steel qualities is technically used in AHSS, where, e.g., the increase in strength is realized MEHDI ASADI, formerly with the IMET Institute of Metallurgy, Clausthal Univeristy of Technology, is Project Manager Roll Forming, Benteler Automobiltechnik GmbH, Paderborn, Germany. GEORG FROMMEYER, formerly Senior Manager, with the Max-PlanckInstitut fu¨r Eisenforschung (MPIE), Duesseldorf, Germany, is departed. ALI AGHAJANI, Assistant, is with the Institut fu¨r Werkstoffe, Ruhr Universita¨t Bochum, Bochum, Germany. ILANA TIMOKHINA, Researcher, is with the Center for Material and Fibre Innovation/Science and Technology, Deakin University, Geelong Campus, VIC 3217, Australia. HEINZ PALKOWSKI, Professor and Chair of Metal Forming and Processing, is with the IMET Intitute of Metallurgy, Clausthal University of Technology, Clausthal-Zellerfeld, Germany. Contact e-mail: [email protected] Manuscript submitted April 21, 2010. Article published online November 10, 2011 1244—VOLUME 43A, APRIL 2012
in the final heat treatment.[13,14] Our previous investigations showed that the aging effect of AHSS is much stronger than that for conventional BH steels.[15,16] This property provides new possibilities for designing structural components, which, e.g., are used in the automotive industry. By means of targeted local deformation, together with subsequent thermal treatment, it is possible to obtain a well-defined local behavior. On the other hand, graded strength behavior can be produced by a local, limited thermal treatment. The aging of steel is a process by which the mechanical properties of the material change with time. BH benefits from this effect and increases the strength of a finished part by means of a heat treatment. BH is a strengthening mechanism that exploits the controlled diffusion of interstitial atoms to pin dislocations, thereby raising the yield strength of the material.[17] Laser beams provide the controlled localized heat treatment in AHSS.[18,19] The main advantages of laser treatments over other alternative technologies essentially arise from the highly focused
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