Sauerstofffreie konduktive Erwärmung/Hot stamping by resistance heating with coating application in oxygen-free atmosphere

  • Institute of Forming Technology and Machines
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In diesem Beitrag werden die Ergebnisse der sauerstofffreien konduktiven Erwärmung für das Formhärten, welches im Teilbereich A04 des Sonderforschungsbereiches 1368 behandelt wird, vorgestellt. Mit der entwickelten Prozesskammer können unbeschichtete Bleche sauerstofffrei, konduktiv erwärmt und beschichtet werden, die in einem miniaturisierten Stoßfänger-Werkzeug formgehärtet werden. Die Beschichtung wird durch Querschliffe untersucht, die Ergebnisse diskutiert und ein Ausblick dargestellt. This publication presents the results of oxygen-free resistance heating for hot stamping, dealt with in sub-project A04 of the Collaborative Research Centre 1368. The developed process chamber can be used for oxygen-free resistance heating and coating of uncoated sheet metal that is subsequently hot-stamped in a bumper tool. Cross-sections are used to investigate the coating, the results are discussed and an outlook is presented.

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... Copper alloys (e.g., CuBe2) are particularly suitable as materials for current transmission as well as bypass electrodes, since their electrical conductivity (σ CuBe2 ≈ 16 MS ⁄ m) is significantly higher than that of 22MnB5 (σ CuBe2 ≈ 1.4 MS ⁄ m). Furthermore, local heating can also be realized by applying the electrodes specifically to the area to be heated [27]. Thus, the area between the two electrodes is heated up by the current flow, while the rest of the material remains cold. ...
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The manganese-boron alloy 22MnB5 is particularly used for structural and safety-relevant parts in the automotive industry. Parts made from this alloy are usually produced using the hot forming process. Here, the sheet is heated to over 950 °C using an industrial roller hearth furnace. The heated sheet is then simultaneously formed and quenched in a cooled tool with a temperature gradient of more than 27 K/s. This leads to the formation of a martensitic microstructure with a hardness value of over 450 HV10 and an elongation at break of less than 6%. The small strain potential of such components makes them difficult to form after hot-stamping. Due to the high temperature gradients of resistance heating, a sheet can be heat-treated locally without a large temperature transition zone. This can be used to locally soften already hot-stamped components for subsequent operations such as bending. Within the scope of this paper, resistance heating is used to soften a hot-stamped 22MnB5+AlSi sheet stripe of 3 mm width. The sheet could consequently be bent over an angle of 90° without cracking the substrate.
As oxygen negatively affects most production processes in the metalworking industry, a truly oxygen‐free production environment appears attractive in terms of the resulting material and component properties. This overview summarizes research conducted within the Collaborative Research Centre (CRC) 1368. The objectives of this CRC are twofold. First, a fundamental understanding of the mechanisms that govern the interaction between a metal surface and the environment is established. Second, it is researched how this understanding can be exploited to improve current production processes and even develop completely new ones. Herein, data obtained within the first funding period, which already demonstrate that significant effects can be realized in processes such as thermal spraying, cold rolling, compound casting, laser brazing, milling or hot stamping to name just a few examples, are presented. In addition, key aspects such as initial deoxidation of the workpieces, their transport under conditions that prevent reoxidation, and the tools needed to establish and control an oxygen‐free process environment are given, and the ramifications with respect to actual applications are discussed.
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