The heat-treatable steel 22MnB5 is used in hot stamping processes to produce high-strength body-in-white components. In this process, sheet blanks are conventionally heated in roller hearth furnaces and then hot-stamped, whereby strengths of 1,500 MPa can be achieved. Disadvantages of this process are the low plastic deformation of the material in hardened state and the poor energy efficiency of roller hearth furnaces. In a new approach, these disadvantages are eliminated by combining edge decarburisation with resistance heating. Due to a diffusion-controlled removal of the carbon in the edge layer of the blanks heated in an oxygen-free atmosphere, the energy absorption in bending tests was improved by 61 % compared to customary hot-stamped 22MnB5. Furthermore, with a subsequent resistance heating in an oxygen-free silane atmosphere, the sheet can be heated and coated. A hermetically sealed heating chamber was developed which allows to heat the blanks up to 950 °C without scale formation. The coating during heating further improves the corrosion properties of the component. With this approach, hot-stamped components with improved properties and coated in an energy-efficient resistance-heated process can be manufactured.
Hot stamping is a well-established and frequently used manufacturing process in automotive body construction. The number of components manufactured in this way is continuously increasing. Hot stamping is used to produce components with a completely martensitic structure, resulting in high strength and hardness. These components are mainly used in safety-relevant areas of the passenger cell, such as the A-pillar, B-pillar, tunnel and sill. For hot-stamping processes, it is necessary to austenitize the blanks. Heating the sheet metal up to 930 °C in a furnace is very energy-intensive. In large-scale industrial applications, the sheets are generally heated in gas-fired roller hearth furnaces up to 60 m long. Apart from the poor energy balance and the high CO2 emissions of such furnaces, they are associated with high investment and maintenance costs, large space requirements and a long heating time. Rapid heating by means of the Joule effect and direct current instead of alternating current offer an energy-efficient and environmentally friendly alternative for sheet metal heating. Therefore, this technology can make a major contribution to environmental protection and resource saving. Within the scope of this work, parts were rapid-heated and subsequently hot-stamped by means of a novel heating system based on direct current with energy savings of up to 80 %. Using electricity guarantees a good CO2 balance. In addition, resistance heating with a new type of DC-heating system and an adapted process chain is compared with conventional furnace heating. In thermographic images and microstructural examinations of the hot-stamped parts, it can be demonstrated that this direct-current technique is well suited for achieving homogeneous hardness and strength in the whole sheet metal. Thus, this new heating system can enhance the efficiency of the hot-stamping technology.
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.
Due to their good mechanical properties and low structural weight, multi-material structures are a promising approach in the automotive industry to lightweight design, body construction and functionalization. Especially metal and plastic are mainly combined to achieve improved properties of the final component compared to mono-material structures. This paper describes the development of a manufacturing cell for the joint forming and heat-assisted press joining of steels and continuous fiber-reinforced thermoplastics in the form of unidirectional carbon-fiber tapes. In order to achieve shorter cycle times and to ensure economical production, a manufacturing cell, supplemented with automated handling by means of two robots and an isothermal, two forming stages tool concept was developed and tested. The composite components produced were tested with regard to their mechanical performance. The feasibility of the production was demonstrated. All composite components had a higher specific load capacity than a pure steel component. Cycle times of well under 60 seconds were achieved. An enormous reduction in process time compared to variothermal tool concepts could be achieved with the new manufacturing cell.
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