EP3259377A1

EP3259377A1 - Method for conductively heating sheet metal, electrode, and heating device therefor

Info

Publication number: EP3259377A1
Application number: EP15787138.5A
Authority: EP
Inventors: Sven HÜBNER; Bernd-Arno Behrens
Original assignee: Leibniz Universitaet Hannover
Current assignee: Leibniz Universitaet Hannover
Priority date: 2015-02-18
Filing date: 2015-08-14
Publication date: 2017-12-27
Anticipated expiration: 2035-08-14
Also published as: WO2016131501A1; EP3259377B1

Abstract

The invention relates to a method for conductively heating sheet metal, wherein the sheet metal or at least a region of the sheet metal to be conductively heated has an outer contour which is rectangular or non-rectangular, wherein, in order to carry out the conductive heating, at least one electrode having a contact region is brought into electrical contact with the sheet metal and the electrical current is fed into and/or discharged out of the sheet metal in order to carry out the conductive heating, wherein the electrode has an uneven, structured contact region having a plurality of contact surfaces spaced apart from each other, via which a plurality of individual contact points are formed, spaced apart from each other between the electrode and the sheet metal. The invention further relates to an electrode for conductively heating a plate and a conductive heating apparatus for carrying out a method for conductively heating sheet metal.

Description

Method for conductive heating of a sheet, electrode and heating device therefor

The invention relates to a method for conductive heating of a sheet, wherein the sheet or at least one conductively heated portion of the sheet has an outer contour which is rectangular or not rechteckformig, wherein for performing the conductive heating at least one electrode having a contact region in electrical contact is brought to the sheet and fed via the contact area of the electric current to carry out the conductive heating in the sheet and / or is derived from the sheet. The invention also relates to an electrode for conductive heating of a sheet and to a conductive heating device for carrying out a method for conductive heating of a sheet.

In general, the invention relates to the field of metalworking, in particular the production of parts made of sheet metal, such as. B. vehicle body panels. The production of such sheet metal parts is z. B. on production lines, such. B. extrusion lines. Such production lines usually have forming facilities and Beschnitteinrichtungen and optionally facilities for performing other methods, such. B. shape property changes, coating, press hardening, etc., which are connected in terms of process technology. In many cases, components made of high-strength, ultra-high strength or ultra-high-strength material should be produced.

In press hardening, a sheet is heated to a temperature of about 930 ° C and cooled during molding. By targeted cooling (hardening) during press hardening via cooled pressing tools, a martensitic structure can be created that leads to the desired material properties, eg. B. to a tensile strength of more than 1 500 MPa strains in the range of> 5%. A disadvantage of such heating processes are the relatively long heating times, which in the conventional heating z. B. occur in roller hearth furnaces. As a result of the long heating times, a scale formation on the material (burnup of material), which is also disadvantageous. In order to counteract this, coatings are applied to the component surfaces in accordance with the prior art, which diffuse into the component during oven heating. The production of coatings is associated with additional effort and costs. Uncoated material is also press-hardened industrially. For this purpose, a scale removal takes place on the components after pressing. A disadvantage is the increased tool wear by the hard scale, which acts abrasive on the tool surface.

An alternative to conventional, relatively long-lasting heating is the heating of a sheet by a conductive heating process. The sheet is heated by applying an electric current through the resulting current heat. With correspondingly large electrical currents, a typical heating process can be carried out in less than 10 seconds, which has the advantage that only very thin scale layers can form in the short time, which in turn has the advantage that no anti-scale coatings are required and the Tool wear is reduced. A conductive heating process of metal sheets is e.g. As described in DE 1 0 2006 037 637 A1.

The proposals from the prior art have hitherto been applicable in practice only to a very limited extent, because they are only suitable for heating processes of nearly rectangular components or rectangular partial regions of components. Many components to be manufactured in practice, eg. B. body parts for motor vehicles, but do not have the same ideal rectangular shape of the sheet to be heated or a portion of the sheet to be heated on. Rather, many components are irregular shaped. When applying the proposals of the prior art, uneven heating of the component would result, which in turn does not lead to the desired manufacturing results. But also the conductive heating of simple shaped sheets, ie. rectangular sheets or sheets with rectangular portions which are to be heated conductively, is associated with difficulties.

Particularly in the case of selective current introduction during conductive heating, as is necessary, for example, for printed circuit boards, the problem of so-called hotspots has not yet been satisfactorily solved. Hotspots are points of contact that arise during conductive heating with excessive heating compared to the environment. Physically, the prerequisite for homogeneous heating in a conductive heating process is the generation of approximately identical current densities within the entire sheet metal profile. Especially at the feed-in points or the current discharge points, ie. Where the electrodes are in contact with the sheet, this is difficult to realize.

Another problem of previous conductive heating processes is also that in the edge regions of the sheets, where the electrodes are brought into contact with the sheet, not the desired heating (austenitization) and the associated hardening of the sheet material can be achieved. Accordingly, the edge regions are not usable for the final component to be used or must be tolerated in an insufficiently hardened form. Accordingly, an additional manufacturing step is required to remove the edge areas, eg by laser cutting. Especially for small sheet metal parts, this leads to a relative unfavorable material utilization, since the proportion of the blend is relatively high.

The invention is therefore based on the object of specifying a method, an electrode and a heating device for conductive heating of a sheet, with which the mentioned problems can be solved.

This object is achieved according to claim 1 by a method for conductive heating of a sheet, wherein the sheet or at least one conductively heated region of the sheet has an outer contour which is rectangular or non-rectangular, wherein at least one electrode to carry out the conductive heating a contact region is brought into electrical contact with the sheet and fed via the contact region of the electric current for performing the conductive heating in the sheet and / or is derived from the sheet, the electrode having an uneven, structured contact area with a plurality of spaced contact surfaces , is formed by a plurality of individual, spaced-apart contact points between the electrode and the sheet. A square sheet is considered in the context of the invention as a special case of a rectangular sheet.

The invention has the advantage that by means of the electrode according to the invention, a relatively steep thermal gradient in the contact region between the electrode and the sheet can be produced, with the advantageous effect that the desired heat treatment of the sheet takes place in the region of the electrode. Especially for smaller components, the waste can be significantly reduced. In addition, can be dispensed with in many cases to an additional manufacturing step for separating the edge region of the sheet become.

In principle, in all conductive heating processes, in fact, the sheet remains cold in the contact area of the cooled electrode. In addition, an area of influence of the electrode forms, which is insufficiently heated. Here, no complete austenitization and subsequent hardening can take place. This concerns in particular the sheet edge. The physical reason is firstly the thermal cooling effect of the electrodes. On the other hand there is a consequence effect. The resistance in the sheet generally increases with increasing heating. If the edge area of the sheet is cold, the resistance does not increase so much here. The edge area of the sheet is in series with the rest of the sheet. For larger resistors more voltage drops and it is also more power implemented with the result of further heating. This creates an avalanche effect, which is detrimental to the heating of the sheet metal edge, ie the electrode influence area.

A further advantage of the invention is that the previous disadvantageous hotspots are not avoided, but are deliberately and specifically used by being generated homogeneously in the areas where there is normally insufficient heating, namely in the area of influence of the electrodes.

Therefore, many small hotspots are generated in the sheet directly in front of the electrode, the distance from each other in the sheet thickness range. Thus, they can advantageously not act as a hotspot in terms of uneven heating, but they act areal homogeneous in the edge region of the sheet, so the electrode influence range, which is not heated otherwise sufficient. In addition, the new electrode geometry is not completely on the sheet, so advantageously less heat is dissipated by the cooled electrodes. Furthermore, the heat can advantageously develop better in exposed areas in the electrode area. The consequences are a heating of the sheet to target temperature directly in front of the electrode and a heating in the direct electrode area.

The aforementioned object is further achieved by an electrode for conductively heating a metal sheet, having a contact surface for making electrical contact with the metal sheet, the electrode having an uneven, structured contact region with a multiplicity of spaced contact surfaces through which a plurality of individual, spaced apart contact points between the electrode and the sheet can be formed

The aforementioned object is further achieved by a conductive heating device for carrying out a method for conductive heating of a metal sheet, wherein the conductive heating device has at least one electrode of the previously described type as current supply electrode, current discharge electrode and / or transfer electrode.

The electrode according to the invention with the uneven, structured contact region can be used as a current supply electrode, current discharge electrode and / or transfer electrode. A current supply electrode serves to supply current, ie to feed, electrical current into the metal sheet for carrying out the conductive heating. A Stromableitungselektrode serves to dissipate the electric current from the sheet. A transfer electrode serves to transfer the electrical current from one metal sheet to another metal sheet, if these are heated in pairs, for example, in the case of trapezoidal surface sections. Furthermore, transition electrodes can be used on a metal sheet in order to rich not to warm.

The invention further makes it possible to provide a method for conductive heating of a sheet, wherein the sheet or at least one conductively to be heated portion of the sheet has an outer contour, which is not rectangular, yet a uniform heating of the area to be heated can be achieved , The sheet is called in the terminology of the expert also board and the non-rectangular board blank is called form board. Furthermore, a suitable for this purpose conductive heating device should be specified.

This is made possible by a method for conductively heating a sheet, wherein the sheet or at least one portion of the sheet to be conductively heated has an outer contour that is not rectangular, creating an outer contour matched arrangement of power supply and current dissipation electrodes, which are arranged piecewise separated from each other along the outer contour and are connected to electrically isolated electrical energy sources which are dimensioned such that substantially equal current densities are generated in the sheet between all pairs of associated power supply and current discharge electrodes. The invention has the advantage that a tailored to the outer contour of the sheet or the area to be heated sheet metal arrangement of power supply and current dissipation electrodes is provided, which are arranged piecewise separated along the outer contour and are acted upon by electrically separate electrical energy sources , So z. B. the outer contour of the area to be heated in individual turn rectangular surface sections or at least substantially rectangular surface sections are divided and for each surface section a be created adapted power supply and current dissipation electrode, which is applied to exactly this area with the desired current density. In a neighboring, likewise substantially rectangularly defined surface section, a further pair of power supply and current discharge electrodes can be arranged and applied via a second electrical energy source with a matched voltage or a matched current, so that the same current density is generated as in the adjacent, before specified area section. In this way, the entire area to be heated can be subdivided into substantially rectangular area sections, and the same current density can be generated in each area section. By generating the same current densities in all surface sections, transverse currents between the surface sections are avoided, which in turn prevents undefined heating results. Due to the resistance conditions, the current density inevitably sets in and basically can not be forced homogeneously. However, only a homogeneous current density obtains homogeneous heating, since always the same power per area is implemented. Here, the approach is followed to adjust the resistance ratios in the form of the board by the manner described so that homogeneous current densities result. In this way, a uniform conductive heating of even an irregularly shaped sheet or an irregularly shaped to be heated area of the sheet can be realized. By means of the invention, the conductive heating process with its advantages can thus be made universally usable for any shaped metal sheets. In order to produce virtually identical, homogeneous current densities, the sheet can be regarded as a resistor in which the current supplied via the electrodes flows.

With the invention sheets can be made scaled to the desired temperature. This reduces the effort required to ignite tion of the components and the tool wear during subsequent press hardening of the heated sheets is reduced, since the scale here abrasive grinding on the tool surface acts. The heating process can be performed in a period of 10 seconds or less. The heating time can be determined by the size of the power supply. Basically, the more current is passed through the sheet, the faster the heating can be performed.

As a sheet is every sheet of electrically conductive metal in question, such. B. steel, titanium, aluminum and magnesium sheets. The sheet has, in an advantageous development of the invention, a constant material thickness, at least before it is further processed after the conductive heating process and optionally deformed.

A current supply electrode serves to introduce electrical current from the electrical energy source into the metal sheet. A current dissipation electrode serves to drain the current from the sheet back to the source of electrical energy. According to an advantageous embodiment of the invention, the electrical energy sources are dimensioned such that the same current densities are introduced from the power supply electrodes in the sheet over all pairs of associated power supply and Stromableitungselektroden and derived from the sheet via the Stromableitungselektroden.

The number of current supply electrodes used may be equal to or different from the number of current discharge electrodes used. With the same number, it is advantageous if in each case one current supply electrode and one current discharge electrode form a pair of such electrodes, which are each connected to the same electrical energy source. It is also possible, for. B. two Stromzulei- electrically connect to each other or electrically connect two Stromableitungselektroden each other. The non-interconnected electrodes are then connected to different electrical energy sources with different voltage, so that in turn equal current densities can be generated in adjacent surface areas in the sheet.

The electrically separated electrical energy sources must be electrically separated from each other at least at one of their terminals. In an advantageous embodiment of the invention, the plurality of energy sources are not connected to each other and not grounded. As a result, the potential of the adjacent energy sources "floats" at the contact line, similar to a plurality of simultaneously operated spot welds on a vehicle body. The "swim" is a well-known term from the resistance welding technique.

To ensure the same current densities in the sheet, it is also advantageous if between a pair of associated Stromableitungs- and power supply electrodes, no other electrode is arranged, fed to the sheet of electricity from another electric power source or from the sheet of electricity to another electrical Energy source is derived. As a result, irregularities in the desired same current density in the sheet are avoided.

As mentioned, the area to be conductively heated can be divided into substantially rectangular area sections. The region to be heated conductively can also be divided into trapezoidal surface sections or substantially trapezoidal surface sections. A combination is also advantageous, ie a division of the area to be heated conductively into rectangular and / or trapezoidal regions. pezförmige surface sections. To achieve the same current densities in trapezoidal surface portions, a pair of sheets is electrically interconnected by a plurality of electrically mutually insulated, side by side along a transition region from one to the other sheet in their respective trapezoidal surface portions arranged transfer electrodes. By correspondingly opposing arrangements of a sheet with respect to the other sheet with respect to. the two trapezoidal surface sections connected by means of the transition electrodes are again provided with an overall rectangular surface section, to which at least one current supply electrode can be connected on one side and at least one current discharge electrode on the other side. In this way, the flexibility and applicability of the method according to the invention is further increased. It is advantageous to connect trapezoidal surface portions of the sheets with the same or mirror-symmetrical outer contour with each other via the transition electrodes.

According to an advantageous embodiment of the invention, trapezoidal surface portions of the same sheet can be electrically connected to each other in pairs via the transfer electrodes to electrically behave then like a rectangular surface section. For this purpose, the trapezoidal surface portions are suitable to divide, in particular with equal angles bevelled sides.

As a result, even with trapezoidal surface sections in this way homogeneous resistance ratios can be realized analogous to rectangular sheets. According to the teachings of the invention, a variety of complicated shaped sheets are thus attributed to a rectangular shape or a combination of rectangular shapes, so that the same current densities can be generated with little operational effort.

According to an advantageous development of the invention, one, several or all current supply and current discharge electrodes are each formed as elongated electrodes extending with their largest dimension over a portion of the outer contour of the conductive region to be heated, each of which is connected to an electrical lead only at one end are connected to the electrical energy source. This has the advantage that the electrical energy is fed or removed at a defined point of the electrode. This simplifies the calculation and design of the required electrodes.

According to an advantageous development of the invention, a pair of current supply and current discharge electrodes are connected at diagonally opposite ends to the electrical energy source. This has the advantage that the same total resistance results for each current line of the current flowing through the sheet, since in each case a more or less large portion of the current supply and current dissipation electrode must be flowed through and these shares for each streamline in sum always give the same value. This also allows the goal of a desired same current density in the sheet to be promoted. The following applies:

Total resistance = internal resistance of the current source + resistors

Supply lines + electrodes + sheet metal.

According to an advantageous development of the invention, one, several or all current supply, current derivation and / or transmission Supply electrodes formed as cooled with a cooling medium electrodes. So can be used as a cooling medium z. B. be passed through a hollow channel of the respective electrode cooling water. The cooling of the electrodes has the advantage that they do not heat undesirable and a heating-related change in resistance of the electrodes is avoided. A further advantage is that the adjacent sheet is cooled by the cooled electrodes, so that by appropriate arrangement of the electrodes to desired, not to be heated areas of the sheet heating and a concomitant hardening can be avoided. This in turn has the advantage that by the location and arrangement of the electrodes z. B.

Intersections in the subsequent further processing of the component, i. of the sheet after forming, can be defined, which are not hardened. In this way, edge trimming can be done with conventional tools, eg. B. by the very economical applicable shear cutting. A more elaborate Hartbeschneiden is not required. Also for joining the component in later

Welding processes, it is beneficial to have non-hardened edge areas. Curing can be done by a subsequent press hardening process.

According to an advantageous embodiment of the invention, the conductive heating is carried out by means of direct current. This has the advantage compared to alternating current that electrical losses and other adverse effects can be excluded by existing inductors and capacitances in the system. It also generates no reactive power. The existing electrical power can be used completely in the form of active power. By eliminating inductive losses line cross sections and electrical energy sources, eg. As transformers are smaller. In addition, energy is saved. The electrical energy sources can, for. B. three-phase can be supplied from the three-phase network. Also, the calculation and design of the entire system, in particular the electrodes and their arrangement is simplified, because you can work with the simpler, applicable to direct current electrical engineering laws.

By the invention can be dispensed with complicated controls for the generation of uniform current densities in the sheet. As a result, the conductive heating device can be realized comparatively easily and inexpensively.

According to an advantageous embodiment of the invention, one, several or all current supply, Stromableitungs- and / or transfer electrodes are moved away from each other during the conductive heating process in order to stretch the sheet. As a result, a heating-related expansion of the heated area of the sheet during the heating process can be compensated.

According to an advantageous development of the invention, a parallel conductor is connected to this electrode for supplying current to a current supply electrode, for transmitting current from or to a transfer electrode and / or for current discharge from a current discharge electrode, which runs parallel to the flow lines flowing therein over part of the sheet to be heated is conducted electrically isolated over the sheet metal against the sheet. The parallel conductor may in particular be guided on an edge region of a rectangular or trapezoidal surface section to be heated. This has the advantage that by appropriate arrangement of such a parallel conductor certain areas of the sheet can be excluded from heating and at the same time the streamlines can be performed in a direction adjacent to the parallel conductor to be heated surface portion in the desired direction. In particular, an undesired current flow through the not to be warming area of the sheet can be avoided by Stromverdrängungseffekte (repulsion of parallel current-carrying conductors). The field lines are also repelled. In this way, for. As so-called tailored tempered blanks are made, ie sheets that are cured only in certain desired areas and remain uncured in other areas. This is z. B. desired in vehicle body parts for generating a certain deformation behavior in the event of a crash. The parallel conductor must be electrically isolated, but not necessarily thermally. In this way, the parallel conductor can cause a desired cooling of the sheet.

According to an advantageous development of the invention, one, several or all parallel conductors are designed as conductors cooled with a cooling medium.

The invention further relates to a conductive heating device for carrying out a method for conductive heating of a sheet, wherein the sheet or at least one region of the sheet to be conductively heated has an outer contour which is not rectangular, wherein the conductive heating means to the outer contour adapted arrangement of power supply and current dissipation electrodes, which are arranged piecewise separated from each other along the outer contour and are connected or connectable via separate electrical leads to each other with electrically separate electrical energy sources, wherein the electrical energy sources are dimensioned such that between all pairs of mutually associated Stromzuleitungs- and Stromableitungselektroden same current densities are generated in the sheet. With the conductive heating device and the further embodiments of the conductive heating device mentioned below, the advantages mentioned above with regard to the method for conductive heating can likewise be realized. According to an advantageous development of the invention, the conductive heating device is set up to carry out a method of the type described above. So z. B. the electrical energy sources may be formed as DC sources.

According to an advantageous development of the invention, the conductive heating device has a stretching device which is set up for stretching the metal sheet at least in the conductively heated region during the heating process. The stretching device may in particular be adapted to move certain current supply, current dissipation and / or transfer electrodes away from one another during the conductive heating process.

According to an advantageous development of the invention, the electrode arrangement of the conductive heating device has transfer electrodes for current transfer between two sheets simultaneously heated in the conductive heating device.

Using the conductive heating device explained above, a method of conductively heating a sheet, e.g. Example, be arranged such that the sheet to be heated is placed in the conductive heating device, then electrodes of the conductive heating device are pressed onto the sheet and then the electrical current flow through the sheet is switched via the electrodes to perform the conductive heating, and after sufficient heating the electrodes are removed from the sheet again, it being advantageous to first turn off the flow of current. The sheet can then be further processed in the heated state, for. B. be brought by pressing in a desired shape. To provide the direct current, an electrical energy source z. B. have a welding rectifier. In this way, the required direct current in the desired height of several thousand amps can be provided in a simple and cost-effective manner.

The current supply, Stromableitungs- and / or transfer electrodes can, for. B. made of copper or alloyed copper, z. B. CuCoBe or CuBe2. Especially with the latter alloys, very hard, robust electrodes can be provided. The mentioned parallel conductors can be made of the same material or another material. For the electrical insulation of the parallel conductors, these z. B. have on the surface of a plasma-sprayed ceramic layer.

The conductive heating process can be carried out heat-encapsulated according to an advantageous embodiment of the invention. The sheets are characterized by an external heat encapsulation, for. B. a thermal insulation of the conductive heating device, thermally shielded from the environment. As a result, the heat radiation is reduced in the environment and thus saves costs and heating time. Experiments have shown that simple Wärmeabschirmplatten already offer good benefits. An insulation as in the furnace is even better. As a result, environmental influences for the heating process can be excluded.

The invention will be explained in more detail by means of embodiments using drawings.

Show it 1 shows a sheet blank for the production of a B-pillar of a motor vehicle and

Figure 2 shows two sheet-metal blanks according to Figure 1 and

3 shows a conductive heating device and the two

Sheet metal blanks according to Figure 2 and

4 shows another conductive heating device and the two sheet-metal blanks according to Figure 2 and

Figure 5 shows the arrangement of a parallel conductor on a sheet in

Cross-sectional view and

6 shows another conductive heating device and the two sheet-metal blanks according to Figure 2 and

7 shows a further conductive heating device and the two sheet-metal blanks according to Figure 2 and

Figure 8 shows an embodiment of an electrode in perspective

Presentation.

In the figures, like reference numerals are used for corresponding elements.

FIG. 1 shows a top view of a metal sheet 1 cut into a specific shape, which, for. B. is cut out of a steel coil. It is a shell component for a B-pillar of a motor vehicle before forming in a press. In order to conductively heat the metal sheet 1 with a uniform current density in the entire region, the metal sheet 1 is first subdivided into substantially rectangular surface sections 2, 4 and a substantially trapezoidal surface section 3. Corresponding to the subdivision into the surface sections 2, 3, 4, electrodes of an electrode arrangement of a conductive heating device are made tailor-made, which are then connected to the sheet 1 for carrying out the conductive heating process. In order to further optimize the manufacturing process in the case of non-rectangular area sections which result from the subdivision explained above, in this case the area section 3, in an advantageous embodiment of the invention two sheets 1 are simultaneously heated in the conductive heating device. For this purpose, the two sheets 1 are first arranged as shown in Figure 2 and created the required electrodes accordingly.

FIG. 3 shows the metal sheets 1 shown in FIG. 2, which are arranged somewhat closer to each other according to FIG. 3, in a conductive heating device 10. The conductive heating device 10 has the aforementioned custom-made electrode arrangement, the current supply electrodes 11, 12, 13 , 14, 15, current discharge electrodes 16, 17, 18, 19, 20 and transfer electrodes 31 has. At the rectangular surface portions 2, 4 of the sheets 1, respective pairs of each of a power supply electrode and a Stromableitungselektrode, as shown in Figure 3, connected and each with its own electrical energy source 21, 22, 23, 24, 25, z. As a transformer with a rectifier connected to provide DC power, connected or connectable via an electrical switch, not shown in the figure 3. The current flow between the respective electrodes is represented by the arrows shown on the respective sheet 1, which represent streamlines. In the central region of the sheets 1, namely in the two trapezoidal surface portions 3, a current supply electrode 12 and to the plate 1 shown on the left a Stromableitungselektrode 19 is connected to the plate 1 shown on the right. In order to ensure the same current density over the vertical extension of the trapezoidal surface sections 3, the trapezoidal surface sections 3 are interconnected in the middle between the metal sheets 1 with a plurality of transfer electrodes 31. prevented. The transfer electrodes 31 are electrically isolated from each other, e.g. B. by being arranged as metal blocks with a certain distance from each other. In this way, a uniform electrical rectangular area between the electrodes 12, 19 is created from the two trapezoidal surface portions 3.

The transfer electrodes 31 may, for. B. at a width of 20 mm at a distance of 5 mm from each other. In order to ensure a uniform spacing of the transfer electrodes from each other, they can, for. B. be mounted on an insulating plate and pressed as a one-piece transfer electrode assembly on the sheets 1. In an industrial implementation, e.g. all electrodes are mounted on a large base plate, with installation e.g. in a hydraulic press.

In many cases, sheets such as the sheet 1 according to Figure 1 should not be cured at all points by heating, but it should, for. B. certain areas are not hardened to achieve a desired deformation characteristics in the event of a crash of a vehicle. Also for this purpose, the present invention is suitable. FIG. 4 shows a further conductive heating device 10, which is designed not to heat the respective surface sections 4 of the metal sheets 1. For this purpose, the electrodes 14 and 17 are not connected directly to their respective electrical energy source 22 and 25, but over along the direction of current flow in the respective sheet guided parallel conductor 26, 27 by the parallel conductors 26, 27 can continue a parallel course of the streamlines in the adjacent surface portions 3 are ensured to the transfer electrodes 31, which would not be ensured without the parallel electrodes 26, 27. In addition, the parallel electrodes 26, 27 are cooled, which has the further advantage that the cooling also on the Sheet 1 transmits and thus undesirable heat transfer from the heated surface portions of the sheet can be prevented in the non-heated surface portions 4.

In an advantageous embodiment, some or all of the remaining electrodes 1 1, 12, 13, 14, 1 5, 16, 17, 18, 19, 20 are cooled. Also, the transfer electrodes 31 may be formed as cooled electrodes. FIG. 5 shows an example of a cooled electrode on the basis of the parallel conductor 27. In the longitudinal direction through the respective electrode runs a bore 28, which forms a cooling channel. Through the cooling channel can coolant, z. As cooling water, are passed. If the electrode is designed as a parallel conductor 26, 27, this is formed insulated on the outer surface, d. H. There is no electrical contact with the sheet 1. In the other electrodes 1 1, 1 2, 13, 14, 15, 16, 17, 1 8, 19, 20, 31 of course, the electrical contact with the sheet 1 is necessary. FIG. 5 shows, by way of example, a pairwise arrangement with two parallel conductors 27 above and below the metal sheet 1. The arranged below the plate 1 parallel conductor 27 is supported on a counter-bearing 32 against a contact force F, which is exerted on the arranged above the sheet 1 parallel conductor 27.

FIG. 6 shows a further embodiment of a conductive heating device 10 which, except for the differences explained below, corresponds to the device 10 according to FIG. According to FIG. 6, the area areas of the metal sheets 1 which are not to be heated are still slightly larger than those of FIG. 4 and, in addition to the area portions 4, still encompass edge regions of the surface portions 3 adjoining the same. Accordingly, the conductive heating device has the same shape as the parallel conductors 26, 27 and the transfer electrodes 21 adapted by these are not formed as linearly extending electrodes, but accordingly the desired, to be heated surface areas are angled. Accordingly, the current supply electrode 12 and the current discharge electrode 19 are somewhat shortened in comparison with FIG.

The sheets 1 can also be arranged differently than previously shown in Figures 2 to 6, for. B. as indicated in the figure 7. The rectangular surface sections 2 and 4 are connected as described above with the current supply electrodes and the current-carrying electrodes and the associated electrical energy sources. With respect to the mean, trapezoidal surface portions 3 results in comparison to the embodiments described above, the difference that to maintain the same current densities and thus the same heating behavior for connecting the surface portions 3 between the sheets 1 not juxtaposed transfer electrodes are provided, but as shown arranged crosswise Transfer electrodes 31, so that by the transfer electrodes 31 results in a spider-like construction. The transfer electrodes 31 are still electrically isolated from each other, z. B. by the electrodes are passed past each other in different height levels or are designed as electrically insulated cable. As a result, homogeneous resistance ratios can be realized analogous to rectangular sheets.

An advantage of this embodiment is the saving of two energy sources, since the rectangular surface sections 2, 4 of the board can be connected as a series circuit to a power source. Thus, the surface portions 2 are connected in series with the power source 22. The electrodes 14, 16 can be connected together or combined to form an electrode. The surface sections 4 are in series connected to the power source 24. The electrodes 15, 17 can be connected together or combined to form an electrode.

As mentioned, it is advantageous to connect a pair of power supply and drain electrodes at diagonally opposite ends to the source of electrical energy. As can also be seen in the figures, it is additionally advantageous to arrange the connection points of the connection lines of the energy sources to the electrodes on one side (left / right) of the sheet metal as far apart as possible so that the current introduction points are as far apart as possible and the current discharge points as possible further apart.

FIG. 8 shows an advantageous embodiment of an electrode 80 which can be used as a current supply, current discharge and / or transfer electrode. The electrode 80 in turn has a bore 28 which forms a cooling channel. The side of the electrode 80 pointing upwards in the illustration according to FIG. 8, which has the contact region 83 of the electrode 80 to the metal sheet, is formed uneven with a specific structure. The uneven, structured contact area 83 thus formed has a multiplicity of elevations on which respective contact surfaces 81 are formed at the top, which are used for the actual electrical contacting of the metal sheet 1, i. between these contact surfaces 81 and the sheet 1, the plurality of individual, spaced-apart contact points with the electrode 80 is formed. Between the individual elevations or their contact surfaces 81, intermediate spaces 82 are formed, which in particular may have a groove-like shape.

In the illustrated embodiment, a comb-like structure is created by 81 grooves 82 are formed between the elevations with the contact surfaces. Through such an uneven surface, the can be brought into contact with the sheet to be heated targeted isolated hotspots can be avoided, ie places where there is an above-average warming of the sheet. By generating a plurality of distributed small contact surfaces with the sheet, distributed over the surface or part of the surface of the sheet to be heated, can be achieved surprisingly homogenization of the heating, without causing damage through the contact surfaces or sporadic hot spots occur. In this way, adverse effects of known electrodes, which sometimes undesirable hot spots occur compensated. As a result, in particular a homogeneous heating of the sheet is promoted. This is favorable for carrying out a controlled, reproducible hardening process of the sheet.

In addition, this increases the usable area of the sheet, since in particular also the area of the sheet arranged below the electrode can be heat-treated in the desired manner. Experiments have shown that by using an electrode according to the invention, e.g. in the form of the illustrated comb electrode, the thermal gradient between the generated hotspot line and the cooling region is very steep, with the consequence that the full heat input takes place directly after electrode contact. Thus, exactly one heat treatment takes place on the hotspot line. Accordingly, the previously unusable transition zone must not be separated from the component, so that the waste is minimized.

So far, hotspots have been considered to be disadvantageous and attempts have been made to avoid them because they have the disadvantage that the sheet is heated uncontrollably in such places. In addition, with a positive temperature coefficient of the sheet, the heating increases the specific electrical resistance at those points of the sheet, so that a kind of avalanche effect occurs, since the Heat spot areas comparatively quickly and damage to the sheet can occur at the highest temperature points (burning through).

Accordingly, an undefined or sporadic generation of hotspots is avoided according to the new teaching described herein. Instead, many small contact points are created close to each other, which are then conducive to a controlled, reproducible heating process and thus to a hardening process of the sheet. By cooling the electrode, this effect can even be enhanced. Due to the large number of small contact points, a line of "mini-hotspots" can be generated, which compensates for the cooling caused by the cooling effect of the electrode.

The contact region 83 of the electrode is the region which is brought into direct electrical contact with the metal sheet. The uneven structured contact area of the electrode may be e.g. by providing a plurality of angled, e.g. perpendicular, electrode edges are realized, as shown in Figure 8. Instead of the illustrated linear pattern, the uneven structured contact area may also be formed by punctiform elevations on the surface. The surface structure of the contact area may be a regular or irregular structure.

Claims

claims:

1 . Method for conductively heating a sheet (1), wherein the sheet (1) or at least one region of the sheet (1) to be conductively heated has an outer contour which is rectangular or non-rectangular, at least one electrode (for carrying out the conductive heating) 80) is brought into electrical contact with the metal sheet (1) with a contact region (83) and via the contact region (83) the electric current for carrying out the conductive heating is fed into the metal sheet (1) and / or out of the metal sheet (1) characterized in that the electrode (80) comprises an uneven textured contact area (83) having a plurality of spaced-apart contact surfaces (81) through which a plurality of discrete, spaced-apart contact locations between the electrode (80) and the sheet (10). 1) is formed.

2. The method according to claim 1, characterized in that the electrode (80) with the uneven, structured contact region (83) is used as the current supply electrode, current discharge electrode and / or transfer electrode.

3. The method according to any one of the preceding claims, characterized in that a plurality of small hotspots in the sheet (1) are generated by the plurality of contact points during the conductive heating.

4. The method according to any one of the preceding claims, characterized in that a non-rectangular outer contour of the conductive area to be heated in substantially rectangular and / or trapezoidal surface portions (2, 3, 4) is divided and according to the division into the surface portions (2, 3, 4) provided thereon an electrode assembly is provided, the at least current supply and Stromableitungselektroden (1 1 -20) and in the presence of at least one trapezoidal surface portion transfer electrodes (31).

Method according to one of the preceding claims, characterized in that sheets (1) each having at least one trapezoidal surface portion (3) of its conductive area to be heated are heated in pairs conductive, wherein a pair of sheets (1) by a plurality of electrically isolated from each other, side by side along a transition region from one to the other sheet (1) arranged transmission electrodes (31) are electrically connected to each other.

An electrode (80) for conductively heating a sheet (1) having a contact surface for making electrical contact with the sheet (1), characterized in that the electrode (80) has a non-planar, structured contact region (83) with a plurality of spaced-apart ones Contact surfaces (81) through which a plurality of individual, spaced-apart contact points between the electrode (80) and the sheet (1) can be formed.

Electrode according to the preceding claim, characterized in that the uneven, structured contact region (83) of the electrode (80) is wholly or partly formed by a multiplicity of angled edges arranged in a comb-like manner next to one another in a row or in a plurality of rows.

8. Electrode according to one of claims 6 or 7, characterized characterized in that the uneven, structured contact area (83) of the electrode (80) has protrusions which include the contact surfaces (81) and groove-like spaces (82) are formed between the protrusions.

9. Conductive heating device (10) for carrying out a method for conductive heating of a sheet (1), characterized in that the conductive heating device at least one electrode (80) according to one of claims 6 to

8 as a current supply electrode, current discharge electrode and / or transfer electrode.

10. Conductive heating device according to the preceding claim, characterized in that the conductive heating device (10) is arranged for carrying out a method according to one of claims 1 to 5.