Resistance Heating by Means of Direct Current for Resource-Saving CO₂-Neutral Hot Stamping

Abstract

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.

Direct Resistance Heating of Aluminum Sheets for Rapid Superplastic Forming

“…It allows heating sheets up to 350 × 200 × 1 mm (Figure 1), about 1/60th the area of 2000 × 2000 × 1 mm aluminum sheets used in production, up to 4000 × 2000 × 2.3 mm. density uniformity, which changes with the contact uniformity of the electrodes and shee [4]. Furthermore, the process demonstrates an efficiency between 75% and 95% when it i used to heat a steel sheet in a hot stamping process [6].…”

“…Direct resistance heating involves passing an electrical current through the material to be formed, heating it through joule effect. In most cases, copper or aluminum electrodes linked to a generator are placed against the sheet to allow current flow [3,4]. This process is already promising, with studies showing it allows heating steel sheets over 800 • C in about 2 s with good temperature uniformity [5].…”

“…It is a crucial point in this process to allow uniform ductility over the entire sheet. Along the width, it mainly depends on the current density uniformity, which changes with the contact uniformity of the electrodes 2 of 7 and sheet [4]. Furthermore, the process demonstrates an efficiency between 75% and 95% when it is used to heat a steel sheet in a hot stamping process [6].…”

Oxygen-free resistance heating with nitrogen and silane as an energy-efficient heating process for hot stamping

“…Cooling of at least 27 K/s creates a component with a complete martensitic microstructure. [2] Due to the forming process in combination with rapid cooling, cracks are formed in the AlSi coating, which can also extend to the base material. Since these components have active aluminum coursed corrosion protection and are also sealed by cathodic dip painting, such cracks are considered negligible [3].…”