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Coating Materials Under Oxygen-Free Silane Atmosphere for Hot Stamping
Abstract
In the hot-stamping process, a sheet blank, usually a manganese boron steel like 22MnB5, is heated up to the austenitising temperature in a roller hearth furnace and then formed in a cooled forming tool. This leads to tensile strengths of about 1500 MPa in the formed parts. The components produced in this process are used in particular in the automotive body construction. The roller hearth furnaces used require high investment costs, much space, are maintenance-intensive and have a low efficiency. As an alternative to roller hearth furnaces, resistance heating offers significantly higher heating rates (>100 K/s) and consequently an energy-efficient, cost-effective process for heating electrically conductive materials. The sheet material 22MnB5 is conventionally coated with an AlSi layer, specially developed for hot stamping and the purpose of scale protection. Therefore, the coated blanks must be heated for several minutes to achieve a sufficient intermetallic phase. Within the framework of SFB 1368, an experimental chamber was developed in which an uncoated 22MnB5 sheet metal is simultaneously heated and coated without scale in a nitrogen-silane atmosphere by means of resistance heating. In the process, nitrogen displaces the regular atmosphere in the test chamber. Subsequently, silane reacts with the residual oxygen, resulting in an oxygen-free atmosphere. The Ni700 coating material used is nickel-based and is specially designed for the high heating rates of resistance heating. The investigations prove that for the production of hot-stamped components, coating in an oxygen-free silane atmosphere during resistance heating is possible.
... Furthermore, it was possible to reduce the deviation to 5 HV using a holding time of 3 seconds at the austenitizing temperature. Behrens [10] and Albracht et al. [11] developed a new coating and heating system for uncoated sheets of 22MnB5. In this process, uncoated sheets could be simultaneously heated and coated with a specially developed nickel-based coating. ...
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 CO 2 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 CO 2 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.
Hot stamping is a process for the production of ultra-high-strength components used in the automotive industry for passenger protection. In this process, cut blanks are heated in a roller hearth furnace, which is operated with gas, to approx. 950 °C in 8 to 10 min and then formed and quenched in a water-cooled tool. This extends the yield strength of the components up to 1500 MPa. The long heating time is caused by the coating of aluminium and silicon (AlSi) with a thickness of 70 μm. During the 8 to 10 min, an intermetallic phase forms and ensures that the blank does not scale. Due to the long heating time and the energy carrier gas, the process is very energy inefficient and can only be controlled slowly. Resistance heating provides an energy-efficient alternative. Due to the direct current flow through the sheet, up to 68% of the energy can be saved compared to roller hearth furnace heating. The process is a high-speed heating process that can heat sheets to 950 °C in less than 10 seconds. The heating time is not sufficient for conventional coatings to bond sufficiently with the base material, which means that there are currently no suitable coatings for resistance rapid heating. A new approach is to suppress scale formation by reducing the oxygen during heating. By lowering the oxygen partial pressure to the XHV level, scale formation is not possible. At the same time, this absence of oxygen is an ideal condition for applying coatings. Due to the developed resistance heating device, sheets can be heated within a very short time without scale and without the need for fossil fuels.
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.
In this study, the microstructure and mechanical properties of the 22MnB5 steel sheet were investigated in different heat treatment conditions. The steel sheet heated to 700 °C, 800 °C, and 900 °C was subjected to air cooling, water quenching, and water quenching + tempering procedures. Microstructural properties post-heat treatment were analyzed under an optical microscope. To determine the mechanical properties, uniaxial tensile tests, and hardness measurements were performed. The highest mechanical properties were achieved with the water quenching from 800 °C and 900 °C. The elongation value was ranged of 2-4% with the water quenching. Also, the tempering process slightly increased the elongation value.
Engineering under protective atmospheres or in vacuum allows the production of materials and components, where the absence of oxygen is an essential requirement for a successful processing. Ideally, joining or coating of (and with) metallic materials needs oxide free material surfaces, in order to achieve durable joints or coatings. Using the established technology of brazing in controlled atmosphere, fundamental physical mechanisms for deoxidation of metal surfaces are presented and the role of oxygen and water residue in the process atmosphere is analyzed. Furthermore, the doping of gases with monosilane for generating virtually oxygen-free process atmospheres is introduced and its advantages for an oxygen-free production are discussed.
To produce ultra-high-strength steel parts for improving fuel consumption and collision safety of automobiles, a compact hot-stamping process having resistance heating was developed. In this process, a blanking operation was introduced immediately after heating to deal with non-rectangular blanks in resistance heating. By blanking, not only were the blank shapes optimised for stamping but the formability was also improved by removing both non-heating edges in contact with the electrode. A rectangular quenchable steel sheet was resistance-heated; the sheet was immediately blanked into the desired shape, and the blank was hot-stamped. In addition, reinforcements used for a body-in-white were hot-stamped using the proposed process to examine the applicability. The reduction in thickness of the hot-stamped reinforcement using a hot-blanked sheet was reduced compared with that using a rectangular blank. The formability was improved by blanking immediately after heating. Moreover, ultrasonic cleaning with diluted hydrochloric acid removed the thin oxide scale caused by resistance heating, and the spot-resistance weldability and electrodeposition paintability were improved. Blanking immediately after heating and ultrasonic cleaning are useful for practical hot-stamping processes using resistance heating.
Within hot stamping of quenchenable steels the blank is heated up to austenitization temperature, transferred to the tool, formed rapidly and quenched in the cooled tool. Essential for the complete process is the heating of the blank which is currently carried out with roller hearth furnaces. As a consequence of rising energy costs new and more time and energy effective heating systems are needed. The paper presents investigations on induction heating as an alternative heating technology. Results concerning the process windows as well as material characteristics and grain structure will be presented and discussed.
Since hot sheet metal forming has become a key technology to produce high strength steel parts for modern automotive construction, detailed knowledge of tribological behaviour of coated steel blanks at elevated temperatures is necessary. Therefore, hot dip aluminium–silicium and electro-plated zinc alloy coatings for hot forming applications have been evaluated according to their friction and wear behaviour. The coefficient of friction was revealed in hot strip drawing experiments. Moreover, mass loss of the workpiece has been measured to characterize the amount of wear. According to the different constitution of the Al–Si coating in comparison to the electro-plated Zn–Ni after heat treatment, the zinc alloy coating showed higher mass loss but lower coefficient of friction. Additionally, wear characteristics have been evaluated in hot forming tests. Herein, parts formed of Al–Si coated blanks showed as well a lower mass loss but more aggressive wear behaviour by means of adhesive wear to the die, resulting in metallic build-up onto the die radius. In contrast, the workpiece related mass loss was higher for Zn–Ni but considerable metallic build-up layers onto the die did not occur. This may be advantageous in production lines hence maintenance and reconditioning frequency of the forming tools could be reduced.
The production of high strength steel components with desired properties by hot stamping (also called press hardening) requires a profound knowledge and control of the forming procedures. In this way, the final part properties become predictable and adjustable on the basis of the different process parameters and their interaction. In addition to parameters of conventional cold forming, thermal and microstructural parameters complicate the description of mechanical phenomena during hot stamping, which are essential for the explanation of all physical phenomena of this forming method.In this article, the state of the art in the thermal, mechanical, microstructural, and technological fields of hot stamping are reviewed. The investigations of all process sequences, from heating of the blank to hot stamping and subsequent further processes, are described. The survey of existing works has revealed several gaps in the fields of forming-dependent phase transformation, continuous flow behavior during the whole process, correlation between mechanical and geometrical part properties, and industrial application of some advanced processes. The review aims at providing an insight into the forming procedure backgrounds and shows the great potential for further investigations and innovation in the field of hot sheet metal forming.
Conductive heating opens up various new opportunities in hot stamping
- B.-A Behrens
- S Hübner
- J Schrödter
- J Uhe