EP3259377B1 - Method for conductively heating sheet metal and heating device therefor - Google Patents

Description

The invention relates to a method for conductive heating of a sheet for preparation of the sheet on a subsequent shaping press hardening process, wherein the sheet or at least one conductively heated portion of the sheet has an outer contour which is rectangular or non-rectangular, wherein to carry out the conductive heating at least one electrode having a contact region is brought into electrical contact with the metal sheet, and the electric current for conducting the conductive heating is fed into the metal sheet via the contact region and / or is discharged from the metal sheet. The invention also relates 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 to each other 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 warm-up 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 scale protection coatings are required and tool wear is reduced becomes. A conductive heating process of metal sheets is e.g. B. in the DE 10 2006 037 637 A1 and in the EP 2 489 747 A1 described.

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 in motor vehicles, but do not have this 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 points or the current discharge points, i. 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 to provide a method and a heating device for conductive heating of a sheet, with which the mentioned problems can be solved.

This object is achieved by a method according to claim 1. 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, in many cases can be dispensed with an additional manufacturing step for separating the edge region of the sheet.

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 ones Resistors drops more voltage 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 on the entire surface of 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.

For this purpose, an electrode is suitable 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 area with a plurality of spaced-apart contact surfaces, through which a plurality of individual, spaced-apart contact points between the Electrode and the sheet can be formed

The above-mentioned object is further achieved by a conductive heating device according to claim 6.

The electrode with the uneven, structured contact region can be used as the current supply electrode, current discharge electrode and / or transfer electrode. A power supply electrode serves to supply power, i. for feeding electric current into the sheet for conducting the conductive heating. A Stromableitungselektrode serves to dissipate the electric current from the sheet. A transfer electrode serves to transfer the electric current from one metal sheet to another metal sheet, e.g. be heated in pairs in trapezoidal surface sections. Furthermore, transition electrodes can be used on a metal sheet in order not to heat specific areas.

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, the pieces separated from each other along the outer Contour are arranged and are connected to each other from electrically separate 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 Stromableitungselektroden. 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 portions or at least substantially rectangular surface portions are divided and created for each surface section a customized power supply and Stromableitungselektrode with which exactly this area is acted upon by 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. As a result, the cost of descaling the components and the tool wear during the subsequent press-hardening of the heated sheets are 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 divert the current from the sheet back to the electrical energy source. 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. electrically connect two power supply electrodes to each other or electrically connect two Stromableitungselektroden together. 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 further advantageous if between a pair of associated Stromableitungs- and power supply electrodes, no other electrode is arranged, via which the sheet is supplied by another electric power source current or derived from the sheet of electricity to another electrical energy source. 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 region to be heated conductively into rectangular and / or trapezoidal 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 one sheet with respect to the other sheet can be created in this way with respect. The two connected by means of the transition electrodes trapezoidal surface sections turn a total rectangular surface section, connected to the one side at least one power supply electrode and at the other side at least one current dissipation electrode can be. 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 only connected at one end to an electrical lead 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 drain electrode must be traversed 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: $$\mathrm{total\; resistance}=\mathrm{internal\; resistance}\mathrm{of\; the}\mathrm{power\; source}+\mathrm{resistors}\mathrm{leads}+\mathrm{electrodes}+\mathrm{sheet},$$

According to an advantageous embodiment of the invention, one, several or all of the power supply, Stromableitungs- and / or transmission electrodes are 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. cutting areas in the subsequent further processing of the component, ie the sheet after forming, can be defined, which are not cured. 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 favorable to have non-hardened edge regions. Curing can be achieved by a subsequent press hardening process respectively.

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 embodiment of the invention, the power supply to a power supply electrode for power transmission from or to a transfer electrode and / or for current discharge from a Stromableitungselektrode a parallel conductor connected to this electrode, which is electrically insulated over the sheet over a portion of the sheet to be heated parallel to the flow lines flowing therein in relation to 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, unwanted current flow through the area of the sheet which is not to be heated can be avoided by means of current displacement effects (repulsion of conductors arranged in parallel current-carrying). 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 metal sheet, wherein the metal sheet or at least one region of the metal sheet to be conductively heated has an outer contour that is not rectangular is, wherein the conductive heating means comprises a matched to the outer contour arrangement of power supply and Stromableitungselektroden which are arranged piecewise separated from each other along the outer contour and are connected or connectable via separate electrical leads with mutually electrically isolated electrical energy sources, the electrical energy sources are dimensioned such that the same current densities are generated in the sheet between all pairs of associated power supply and Stromableitungselektroden. 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 apparatus may in particular be adapted to move certain current supply, current discharge 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 means, then electrodes of the conductive heating means 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 CuBe ₂ . 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.

Figur 1

ein Blech-Rohteil zur Herstellung einer B-Säule eines Kraftfahrzeugs und

Figur 2

zwei Blech-Rohteile gemäß Figur 1 und

Figur 3

eine konduktive Erwärmungseinrichtung sowie die zwei Blech-Rohteile gemäß Figur 2 und

Figur 4

eine weitere konduktive Erwärmungseinrichtung sowie die zwei Blech-Rohteile gemäß Figur 2 und

Figur 5

die Anordnung eines Parallelleiters auf einem Blech in Querschnittsdarstellung und

Figur 6

eine weitere konduktive Erwärmungseinrichtung sowie die zwei Blech-Rohteile gemäß Figur 2 und

Figur 7

eine weitere konduktive Erwärmungseinrichtung sowie die zwei Blech-Rohteile gemäß Figur 2 und

Figur 8

eine Ausführungsform einer Elektrode in perspektivischer Darstellung.

Show it

FIG. 1

a sheet blank for the production of a B-pillar of a motor vehicle and

FIG. 2

two sheet metal blanks according to FIG. 1 and

FIG. 3

a conductive heating device and the two sheet blanks according to FIG. 2 and

FIG. 4

another conductive heating device and the two sheet blanks according to FIG. 2 and

FIG. 5

the arrangement of a parallel conductor on a sheet in cross-sectional view and

FIG. 6

another conductive heating device and the two sheet blanks according to FIG. 2 and

FIG. 7

another conductive heating device and the two sheet blanks according to FIG. 2 and

FIG. 8

an embodiment of an electrode in perspective view.

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

The FIG. 1 shows in plan view a cut into a specific shape sheet 1, the z. 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.

For further optimization of the production 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 heated simultaneously in the conductive heating device. For this, the two sheets 1 are first as in FIG. 2 shown arranged and created the required electrodes accordingly.

The FIG. 3 shows the in FIG. 2 The conductive heating device 10 comprises the aforementioned custom-made electrode assembly, the power supply electrodes 11, 12, 13, 14, 15, current dissipation electrodes 16, 17th , 18, 19, 20 and transfer electrodes 31. At the rectangular surface portions 2, 4 of the sheets 1 are respective pairs of each of a power supply electrode and a power dissipation electrode, as in FIG FIG. 3 shown, connected and each with its own electrical Power source 21, 22, 23, 24, 25, z. As a transformer with a rectifier connected to provide DC, connected or via a in the FIG. 3 not shown electrical switch connectable. 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 connected to each other in the middle between the sheets 1 with a plurality of transmission electrodes 31. 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 should be like sheet 1 according to FIG. 1 not be hardened at all points by heating, but it should, for. B. to achieve a desired deformation characteristic in Crash case of a vehicle certain areas can not be hardened. Also for this purpose, the present invention is suitable. The FIG. 4 shows a further conductive heating device 10, which is adapted to not heat the respective surface portions 4 of the 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 can be cooled, which has the further advantage that the cooling is also transferred to the sheet 1 and thus unwanted 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 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 are cooled. Also, the transfer electrodes 31 may be formed as cooled electrodes. The 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 extends 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, ie there is no electrical contact with the sheet 1. In the remaining electrodes 11, 12, 13, 14, 15, 16, 17, 18, 19 , 20, 31 of course, the electrical contact with the sheet 1 is necessary. The FIG. 5 shows by means of two parallel conductor 27 by way of example a paired arrangement above and below the sheet 1. The arranged below the sheet 1 parallel conductor 27 is based on an abutment 32 against a contact force F exerted on the above the sheet 1 arranged parallel conductor 27.

The FIG. 6 shows a further embodiment of a conductive heating device 10, which, except for the differences explained below of the device 10 according to FIG FIG. 4 equivalent. According to FIG. 6 are the non-heating surface areas of the sheets 1 relative to the FIG. 4 In addition to the surface sections 4, edge regions of the surface sections 3 still adjoining them also grasp and capture. Accordingly, the conductive heating device is adapted with regard to the formation of the parallel conductors 26, 27 and the transmission electrodes 21 by forming these not as linearly extending electrodes, but according to the desired ones , are angled to be heated surface areas. The current supply electrode 12 and the current discharge electrode 19 are accordingly compared to FIG. 4 something shortened trained.

The sheets 1 can also be different than before in the FIGS. 2 to 6 be arranged represented, for. B. as in the FIG. 7 specified. 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 there electrically isolated from each other, for. 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 portions 4 are connected in series with 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.

The FIG. 8 shows an advantageous embodiment of an electrode 80, which can be used as Stromzuleitungs-, Stromableitungs- and / or transfer electrode. The electrode 80 in turn has a bore 28 which forms a cooling channel. The in the illustration according to FIG. 8 The upward-facing side of the electrode 80, which has the contact portion 83 of the electrode 80 to the sheet, is formed unevenly with a certain structure. The thus formed uneven, structured contact region 83 has a plurality of elevations on which above respective contact surfaces 81 are formed, which serve for the actual electrical contacting of the sheet 1, ie between these contact surfaces 81 and the sheet 1, the plurality of individual, spaced-apart contact points with the electrode 80th educated. 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. By such an uneven surface, which is brought into contact with the sheet to be heated, targeted sporadic hot spots can be avoided, i. Points where there is an above-average heating 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, for example in the form of the illustrated comb electrode, the thermal gradient between the generated hotspot line and the Cooling area precipitates very steep, with the consequence that takes place directly after electrode contact the full heat input. 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. With a positive temperature coefficient of the sheet also increases with the heating of the specific electrical resistance at such locations of the sheet, so that a kind of avalanche effect occurs because the hotspot areas heat comparatively quickly and come at the points of highest temperature to damage the sheet can (burn).

Accordingly, according to the new teaching described herein, an undefined or sporadic generation of hotspots is avoided. 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 region of the electrode can be realized, for example, by providing a plurality of angled, eg rectangular, electrode edges, as in US Pat FIG. 8 shown. Instead of the illustrated linear pattern, the uneven structured contact area also be formed by punctual elevations on the surface. The surface structure of the contact area may be a regular or irregular structure.

Claims (8)

1. Method for conductively heating a metal sheet (1) to prepare the metal sheet for a subsequent shaping press-hardening process, wherein the metal sheet (1) or at least an area of the metal sheet (1) that is to be conductively heated has an outer contour that is rectangular or nonrectangular, wherein the conductive heating is performed by virtue of at least one electrode (80) having a contact area (83) brought into electrical contact with the metal sheet (1) and the contact area (83) being used to supply the electric current for performing the conductive heating to the metal sheet (1) and/or to remove said electric current from the metal sheet (1), characterized in that the electrode (80) has an uneven, patterned contact area (83) having a multiplicity of contact surfaces (81), at intervals from one another, that form a multiplicity of individual contact points, at intervals from one another, between the electrode (80) and the metal sheet (1).
2. Method according to Claim 1, characterized in that the electrode (80) having the uneven, patterned contact area (83) is used as a current supply electrode, current removal electrode and/or transfer electrode.
3. Method according to either of the preceding claims, characterized in that the multiplicity of contact points produce a multiplicity of small hotspots in the metal sheet (1) during the conductive heating.
4. Method according to one of the preceding claims, characterized in that a nonrectangular outer contour of the area to be conductively heated is split into substantially rectangular and/or trapezoidal surface sections (2, 3, 4) and the split into the surface sections (2, 3, 4) is taken as a basis for providing an electrode arrangement matched thereto that has at least current supply and current removal electrodes (11-20) and, when at least one trapezoidal surface section is present, transfer electrodes (31) .
5. Method according to one of the preceding claims, characterized in that metal sheets (1) having in each case at least one trapezoidal surface section (3) for their area that is to be conductively heated are conductively heated in pairs, wherein a pair of metal sheets (1) is electrically connected to one another by multiple transfer electrodes (31) that are electrically insulated from one another and arranged beside one another along a transition area from one to the other metal sheet (1).
6. Conductive heating device (10) for performing a method for conductively heating a metal sheet (1), characterized in that the conductive heating device has at least one electrode (80) as a current supply electrode, current removal electrode and/or transfer electrode that has an uneven, patterned contact area (83) having a multiplicity of contact surfaces (81), at intervals from one another, that can form a multiplicity of individual contact points, at intervals from one another, between the electrode (80) and the metal sheet (1), wherein the conductive heating device (10) is configured for performing the method according to one of Claims 1 to 5.
7. Conductive heating device according to the preceding claim, characterized in that the uneven, patterned contact area (83) of the electrode (80) is formed entirely or partially by a multiplicity of angled-off edges arranged in comb-like fashion behind one another in a row or beside one another in multiple rows.
8. Conductive heating device according to either of Claims 6 and 7, characterized in that the uneven, patterned contact area (83) of the electrode (80) has elevations containing the contact surfaces (81), and groove-like interspaces (82) are formed between the elevations.

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