Method for the conductive heating of a flat metallic component

Abstract

Method for the conductive heating of a planar metallic component (100) for hot forming, wherein the method comprises at least the following steps: a) Contacting two side faces of opposite narrow sides (10, 11, 12, 13) of the metallic component (100) with each electrode (14, 15), such that the first electrode (14) contacts only a section of the first side face with respect to a thickness (16) of the metallic component (100), and that the second electrode (15) contacts only a section of the second side face with respect to the thickness (16) of the metallic component (100), wherein the two electrodes (14, 15) are positioned against the two side faces such that a contact surface (21, 22) is formed between each electrode (14, 15) and side face, each contact surface (21, 22) extending over less than 75% of the thickness (16) of the metallic component (100); and b) Applying an electrical voltage (17) to the two electrodes (14, 15) such that a current flow (18) through the metallic component (100) heats it in a first region (19) to a first temperature (T1), wherein the first temperature (T1) is higher than a material-specific first predetermined temperature threshold (Ts1) above which a material of the metallic component (100) is completely austenitized, and that the metallic component (100) is heated in a second region (20) to a second temperature (T2), wherein the second temperature (T2) is lower than the material-specific first predetermined temperature threshold (Ts1).

Description

[0001] The invention relates to a method for conductive heating of a flat metallic component for hot forming or as a precursor to hot forming.

[0002] Furthermore, the invention relates to a conductively heated, flat metallic component and a device for conductively heating a flat metallic component.

[0003] Typically, flat metallic components, such as sheet metal, are formed using hot forming or press hardening. For this purpose, the components are heated to a specified forming temperature and then formed into the desired shape, for example, using a press. In this type of hot forming of flat components, the forming temperature prior to forming is a key criterion for the maximum achievable degree of forming and the required forming force. At temperatures that are too low, sophisticated or complex forming geometries are often impossible to produce, as cracking can occur due to reduced formability. In addition to the forming temperature, the cooling rate of the component during forming is crucial for achieving a martensitic material structure. The holding time or process duration is determined by the forming temperature and the thickness of the component.If the cooling rate is too low or too slow, the required strength and hardness values ​​will not be achieved. If the cooling rate is too fast, however, the component may become too brittle and prone to cracking.

[0004] In the automotive industry, the aforementioned hot forming processes are primarily based on roller-hearth furnaces or multi-layer chamber furnaces. The temperature change in the component is primarily due to heat radiation from the furnace, convection, and, to a lesser extent, heat conduction within the component material. Due to the specific material properties, different temperature-time curves must be set in the furnaces for each component thickness and coating in order to achieve homogeneous component properties.

[0005] When using thicker sheet metal, such technologies reach their limits and are very costly to implement. This is due to the fact that the heating time would have to be significantly increased for thicker components. Furthermore, the furnaces already used for thinner components cannot be used for heavier components. State of the art

[0006] In the industrial sector, several methods are known for rapidly heating metallic materials. In particular, inductive and conductive processes are well known. However, these processes are usually only intended for relatively thin sheet metal components, whereby the components are fully austenitized. The geometry of the component is often a limiting factor, as heating is sometimes made more difficult by locally occurring hotspots. Active regulation of the thermal expansion of the component or the heat distribution within the component is not achieved with known state-of-the-art processes. The adjustment of graded component properties usually takes place in separate process steps with adapted elements, for example between temperature control stations or absorber masses that remove heat from the component.

[0007] DE 102 12 819 A1 describes a method for producing a metallic component by heating, subsequent hardening, and rapid cooling, leaving unhardened areas in the component. The method involves heating the metallic component by means of electrical resistance heating, while selectively cooling or electrically or thermally bridging specific areas to keep these areas below the austenitizing temperature.

[0008] DE 10 2008 062 270 A1 describes a device and method for partially hardening a metallic workpiece. For this purpose, a heating unit with a discontinuously operated electric heating coil is proposed, the shape of which is adapted to the cross-section of the profile produced by the roller in such a way that only specific areas along the length of the profile can be heated.

[0009] US 2008 / 0 302 775 A1 describes an apparatus for forming a metallic article and a method for forming metallic articles using conductive heating. For this purpose, a pair of electrode clamps are applied to opposite narrow sides of a workpiece, with these electrodes connected to a generator. An electric current is applied to the workpiece, which is heated by a current flow and deformed by mechanical forces. The temperature can be controlled via a control unit. A second pair of electrodes can be used to separately heat a specific area of ​​the workpiece.

[0010] DE 11 2011 105 617 T5 discloses an electric heating device capable of uniformly applying electric current to a workpiece without causing abrasion to the electrodes. An electric heating device includes a pair of electrodes for applying electric current to a workpiece and a pair of clamps for securing the workpiece. The electrode extends along the surface of the workpiece in a direction perpendicular to a direction of the electric current, with the width in the direction of the electric current remaining constant, and the workpiece is sandwiched therebetween. The clamp is mounted near the electrode and sandwiches a part of the workpiece other than the part to be heated. A pressure at which the clamp sandwiches the workpiece therein is higher than a pressure at which the electrode sandwiches the workpiece therein. Description of the invention: task, solution, advantages

[0011] The object of the present invention is to propose a method for heating a flat metallic component for subsequent hot forming of the component, wherein the method can also be carried out in cost-effective series production with components of greater component thickness.

[0012] According to the invention, a method for conductively heating a flat metallic component for hot forming is proposed. The method according to the invention comprises at least the following steps: a) Contacting two side surfaces of opposing narrow sides of the metallic component with one electrode each. The first electrode is only partially contacted with the first side surface with respect to a thickness of the metallic component or along the thickness of the metallic component. The second electrode is also only partially contacted with the second side surface with respect to the thickness of the metallic component or along the thickness of the metallic component; and b) Applying an electrical voltage to the two electrodes such that a current flow is initiated through the metallic component, which heats the metallic component in a first region to a first temperature. The first temperature is higher than a material-specific first predetermined temperature threshold, above which a material of the metallic component completely austenitizes. Furthermore, the metallic component is heated in a second region to a second temperature, wherein the second temperature is lower than the material-specific first predetermined temperature threshold.

[0013] The method according to the invention can be considered a type of precursor for a hot forming process. The metallic component comprises or consists of a metallic material. The metallic component is flat and preferably of constant thickness. Flat design means that the metallic component has a greater length and width than its thickness. For example, the metallic component can be formed as a sheet metal. Constant thickness means that the metallic component has an identical thickness in every area. The metallic component can be coated or uncoated.

[0014] According to the invention, the metallic component is heated by means of conductive heating. Conductive heating is understood to mean direct resistance heating, wherein an electrical voltage is applied to the component or to two opposite sides of the component by means of the two electrodes and the electrical component itself serves as an electrical conductor. The current thus flows from a first electrode through the metallic component to the second electrode. The metallic component is heated due to the current flow. For this purpose, the two electrodes are applied to the metallic component in such a way that the metallic component is heated in one area (first area) to above its austenitizing temperature, wherein the metallic component in a second area is only heated to a temperature which is below the austenitizing temperature of the metallic component.

[0015] The two electrodes are arranged on the side surfaces of two opposite narrow sides of the metal component. One narrow side of the metal component has a width corresponding to the thickness of the metal component. The length of the narrow side corresponds, for example, to the length or width of the metal component.

[0016] Because the electrodes are only applied to the respective side surface in sections relative to the thickness of the electrical component, contact is not made across the entire thickness of the metallic component, nor is contact made across the entire width of the contacted narrow side. The resulting contact area between the electrode and the side surface is thus smaller than the side surface itself and, in particular, has a smaller width compared to the thickness of the component.

[0017] The first temperature threshold at which the metal component fully austenizes is a material-specific constant. This temperature threshold can vary depending on the material. For example, the first specified temperature threshold is 750° Celsius.

[0018] With the method according to the invention, the temperature distribution in the metallic component can be controlled across its cross-section and by the current flow. The current flow is directed along the direct connection between the electrodes and thus in the longitudinal and / or transverse direction through the metallic component. The current flow is thus directed horizontally through the metallic component. The region along the direct connection between the electrodes defines the first region, which is heated to a first temperature. Due to the arrangement of the electrodes, this first region extends over only a portion of the thickness of the metallic component.

[0019] The second area, which is only heated to a second temperature, is heated partly by the current flow, but especially by the heat transfer within the component.

[0020] The method according to the invention thus proposes direct resistance heating across the thickness of the metallic component.

[0021] Since the first region in the metallic component is heated to a first temperature above the first temperature threshold at which the material of the metallic component is fully austenitized, the austenite undergoes martensitic transformation during subsequent cooling of the metallic component, resulting in high hardness. Preferably, the process duration, the electrical voltage, and the first predetermined temperature threshold are selected such that the hardness of the starting material in the first region is increased many times over. Particularly preferably, the process duration, the electrical voltage, and the first predetermined temperature threshold are selected such that the Vickers hardness after cooling of the metallic component in the first region is between 400 and 575 HV 10.

[0022] The second region, namely the non-austenitized region, remains softer than the first region and, after cooling, consists of a ferritic-pearlitic structure that approximately corresponds to the initial state of the component before heating. Preferably, the electrical voltage, the process time, and a second predetermined temperature threshold for the second temperature of the second region are selected such that the Vickers hardness is in the range of approximately 200 HV10.

[0023] The method according to the invention makes it possible, in particular, to heat flat, thicker metallic components, for example, with a thickness of over 3 mm, for subsequent hot transformation. Furthermore, partial heating, i.e., differential heating in at least two areas across the thickness of the metallic component, requires less energy, since the metallic component does not need to be fully austenitized. The defined, flat contact of the electrodes enables a defined energy input to establish a temperature gradient within the metallic component. Thus, a defined adjustment of mechanical properties or microstructures can be achieved through direct, local resistance heating.

[0024] According to the invention, in step a), the two electrodes are applied to the two side surfaces in such a way that a contact surface is formed between the electrode and the side surface. Preferably, the respective contact surface extends over at least 50% of a length or at least 50% of a width of the metallic component. Particularly preferably, the respective contact surface extends over at least 90% of a length or at least 90% of a width of the metallic component. For example, the respective contact surface could extend over the entire length or across the entire width of the metallic component. Preferably, the two opposing contact surfaces are identical and arranged at the same height or level.

[0025] Furthermore, the invention provides that each contact surface extends over less than 75% of the thickness of the metallic component. Preferably, each contact surface extends over less than 50% of the thickness of the metallic component.

[0026] It is also preferably provided that the metallic component is clamped between the two electrodes during steps a) and b) by means of a device designed as a clamping device. The device designed as a clamping device can comprise the two electrodes themselves.

[0027] The clamping device thus holds the metal component laterally or in the area of ​​its two opposite narrow sides, eliminating the need for a base or support on the top and / or bottom of the metal component. Such bases or supports can influence the current flow through the metal component and thus the associated heating of the metal component. The clamping device presses the two electrodes against the two opposite narrow sides in such a way that the entire metal component is held in place.

[0028] Preferably, in step a), the two electrodes are applied to the two side surfaces centrally with respect to the thickness of the metallic component. The current flow is thus preferably directed through the central region of the component or through the core of the component and thus horizontally and centrally through the interior of the metallic component. This defines the first region, which is heated in such a way that it completely austenites. The second region, namely the region in which the metallic component is not heated to such an extent that this region completely austenites, is thus formed in the edge regions or outer regions, i.e. in the region of the top and bottom of the metallic component. The two outer regions, which form the second region, extend horizontally through the metallic component, just like the first region.The two edge areas of the metallic component thus form the top and bottom of the metallic component.

[0029] Preferably, the method is controlled such that the second temperature is not only lower than the first predetermined temperature threshold, but also lower than a material-specific second predetermined temperature threshold, wherein the second predetermined temperature threshold is lower than the first predetermined temperature threshold. The second predetermined temperature threshold, like the first predetermined temperature threshold, is a material-specific constant. Preferably, the second predetermined temperature threshold is the temperature at which austenite formation (alpha-gamma transformation) of the material of the metallic component begins, but at which temperature the material has not yet fully austenitized. For example, the second predetermined temperature threshold could be between 700° Celsius and 750° Celsius.Particularly preferably, the second predetermined temperature threshold is at least 25° Celsius lower than the first predetermined temperature threshold.

[0030] Furthermore, it is preferably provided that the first temperature is maintained above the first predetermined temperature threshold for a predetermined period of time. The second temperature is preferably maintained below the second predetermined temperature threshold for the same predetermined period of time. Particularly preferably, the second temperature never rises above the second predetermined temperature threshold. This allows the temperature distribution to be controlled over the process time or a predetermined period of time.

[0031] Preferably, a process and holding time is selected such that the second temperature does not exceed the second predetermined temperature threshold, even due to heat conduction between the first region and the second region.

[0032] Furthermore, a metallic component is provided which has been conductively heated according to a previously described method.

[0033] The metallic component preferably has a substantially constant thickness of more than 2 mm, particularly preferably more than 3 mm, and most particularly preferably more than 4 mm. The data refer to the original state of the metallic component before heating and / or to the state after heating, but before further processing (e.g., forming).

[0034] Furthermore, the metallic component preferably has a substantially cuboid basic shape.

[0035] Furthermore, a device for conductive heating according to a method described above is provided.

[0036] The device preferably has two electrodes, wherein the electrodes each have a contact side for contacting the respective electrode with the respective side surface of the metallic component. A contact side of an electrode can be flat, in particular planar and parallel to the side surface of the metallic component. Alternatively, the contact side can be designed as a longitudinal edge, for example as an edge along the length of the side surface of the metallic component. A cross-section of such an electrode would thus have a pointed shape. A contact side designed as a longitudinal edge forms a narrow or thin contact surface along a line along the length of the side surface of the metallic component. Furthermore, the contact side of an electrode could be convex.

[0037] It is also preferably provided that the device is designed as a clamping device for clamping the metallic component between the two electrodes of the device. Short description of the drawings

[0038] The invention is explained below using preferred embodiments as examples.

[0039] They show schematically: Fig. 1: a perspective view of a metallic component with electrodes arranged on the side, Fig. 2a and Fig. 2b: a sectional view through the Fig. 1 shown arrangement, Fig. 3: a diagram showing the temperature curves in relation to the process time, Fig. 4: different designs of the electrodes or the contact sides of an electrode, and Fig. 5: a cross-sectional view of a metallic component after heating and after hot forming or cooling. Preferred embodiments of the invention

[0040] Fig. Figure 1 shows a perspective view of a metallic component 100 with electrodes 14, 15 arranged on two opposing narrow sides 10, 11. The contact sides 25 of the electrodes 14, 15 are arranged centrally with respect to the thickness 16 of the metallic component 100 over the entire length 23 of the metallic component 100. The resulting contact surfaces 21, 22 between the electrodes 14, 15 and the side surfaces of the opposing narrow sides 10, 11 are narrower or have a smaller width than the thickness 16 of the metallic component 100.

[0041] Because the electrodes 14, 15 are not arranged over the entire thickness 16 of the metallic component 100, but only centrally and in sections on the side surfaces 10, 11 of the metallic component 100, the current flow 18 resulting after application of an electrical voltage to the two electrodes 14, 15 is directed horizontally and centrally through the metallic component 100. Fig. 2a shows the current flow 18 in the horizontal direction centrally through the core of the metallic component 100.

[0042] Fig. Figure 2b shows the resulting heating of the metallic component 100. In the central region or core of the metallic component, a first temperature T1 above a first predetermined temperature threshold Ts1 is reached due to the current flow 18. This creates a first region 19 in which the material of the metallic component 100 completely austenitizes due to the first temperature T1 reached. In the edge regions or outer regions, a second temperature T2 is reached, which lies below a second predetermined temperature threshold Ts2. This ensures that the material in this second region 20 thus formed does not completely austenitize.

[0043] Fig. 3 shows a time diagram with the two predefined temperature thresholds Ts1 and Ts2. The first temperature T1 in the first region 19 exceeds the material-specific first predefined temperature threshold Ts1 for a predefined period of time, above which the material of the metallic component 100 fully austenitizes. The second temperature T2 remains below a material-specific second predefined temperature threshold Ts2 throughout the entire process duration. The material-specific second predefined temperature threshold Ts2 defines the temperature above which the material of the metallic component 100 begins to form austenite (alpha-gamma transformation) but is not yet fully austenitized.

[0044] Fig. Figure 4 shows different configurations of the electrodes 14, 15 and different configurations of the contact sides 25 of an electrode 14, 15. For example, the contact side 25 of an electrode 14, 15 can be narrow or thin and flat, or wide or thick and flat. Furthermore, the contact side could be tapered, i.e., form a longitudinal contact edge. Alternatively, the contact side of an electrode 14, 15 could be convex.

[0045] In Fig. Figure 5 shows a sectional view through the metallic component 100. The left-hand area shows the temperatures T1, T2, T3 reached at various points within the metallic component 100. The highest temperature, namely the first temperature T1, is reached in the core of the metallic component 100, at which temperature the material of the metallic component 100 completely austenitizes in this area. In the edge areas, i.e., in the second area 20, only a second temperature T2 is reached, at which no austenitization occurs and a ferritic-pearlitic structure is formed. In the intermediate area 27, a third temperature T3 is reached, which lies between the first temperature T1 and the second temperature T2.

[0046] In the right area of ​​the Fig.5 shows an enlarged view of the different regions 19, 20, and 27. The first region 19 is formed in the core and is fully austenitized, which undergoes martensitic transformation after cooling. The Vickers hardness is between 400 and 500 HV10. In the edge regions, i.e., the second region 20, no austenitization has occurred. The ferritic-pearlitic structure formed after cooling has a Vickers hardness of approximately 200 HV10, which roughly corresponds to the initial state of the metallic component 100. The intermediate region 27 is partially austenitized, resulting in a mixed structure, resulting in a Vickers hardness between approximately 200 and 400 HV10. List of reference symbols 100 Metallic component 200 device 10 First narrow side 11 Second narrow side 12 Third narrow side 13 Fourth narrow side 14 First electrode 15 Second electrode 16 Thickness of the metallic component 17 Electrical voltage 18 Current flow 19 First area 20 Second area 21 First contact surface 22 Second contact surface 23 Length of the metallic component 24 Width of the metallic component 25 Contact side of the electrode 26 Period 27 Intermediate area T1 First temperature T2 Second temperature T3 Third Temperature Ts1 First specified temperature threshold Ts2 Second specified temperature threshold

Claims

[1] Method for conductive heating of a flat metallic component (100) for hot forming, the method comprising at least the following steps: a) contacting two side surfaces of opposing narrow sides (10, 11, 12, 13) of the metallic component (100), each with an electrode (14, 15), such that the first electrode (14) contacts only a portion of the first side surface with respect to a thickness (16) of the metallic component (100), and that the second electrode (15) contacts only a portion of the second side surface with respect to the thickness (16) of the metallic component (100), wherein the two electrodes (14, 15) are applied to the two side surfaces in such a way that a contact surface (21, 22) is formed between the electrode (14, 15) and the side surface, wherein each contact surface (21, 22) extends over less than 75% of the thickness (16) of the metallic component (100); and b) applying an electrical voltage (17) to the two electrodes (14, 15) such that a current flow (18) through the metallic component (100) heats it in a first region (19) to a first temperature (T1), wherein the first temperature (T1) is higher than a material-specific first predetermined temperature threshold (Ts1) above which a material of the metallic component (100) completely austenitizes, and such that the metallic component (100) is heated in a second region (20) to a second temperature (T2), wherein the second temperature (T2) is lower than the material-specific first predetermined temperature threshold (Ts1). [2] Method according to claim 1, characterized byin that in step a) the two electrodes (14, 15) are applied to the two side surfaces in such a way that a contact surface (21, 22) is formed between the electrode (14, 15) and the side surface, wherein each contact surface (21, 22) extends over at least 50% of a length (23) or over at least 50% of a width (24) of the metallic component (100). [3] Method according to claim 1 or 2, characterized by that the electrodes (14, 15) to be applied according to step a) each have a contact side (25) for contacting the electrode (14, 15) with the respective side surface of the metallic component (100), wherein the contact side (25) is flat or formed as an elongated edge or convex. [4] Method according to one of the preceding claims, characterized by that the metallic component (100) is clamped between the two electrodes (14, 15) in step a) by means of a device (200) designed as a clamping device. [5] Method according to one of the preceding claims, characterized by that in step a) the two electrodes (14, 15) are applied to the two side surfaces centrally with respect to the thickness (16) of the metallic component. [6] Method according to one of the preceding claims, characterized by that the second temperature (T2) is lower than a material-specific second predetermined temperature threshold (Ts2), wherein the second predetermined temperature threshold (Ts2) is lower than the first predetermined temperature threshold (Ts1). [7] Method for hot forming a flat metallic component (100), wherein the flat metallic component (100) is conductively heated by means of a method according to one of claims 1 to 6 and is then formed and cooled.

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