Method and device for producing hardened sheet-steel components

Abstract

The invention relates to a method for heating a sheet-steel blank or a pre-shaped sheet-steel component having a zinc coating or a zinc-alloy coating, in which method: the sheet-steel component or the sheet-steel blank is guided through a furnace or deposited or positioned in a furnace and is heated in the furnace in particular above the austenitization temperature; the sheet-steel component or the sheet-steel blank at least temporarily rests on a plurality of supporting faces of at least one carrier; a carrier is used in which, with regard to the supporting faces for a sheet-steel blank or a sheet-steel component, either a) the supporting faces each amount to a maximum of 200 mm², preferably a maximum of 113 mm², and / or b) the supporting faces consist of porous and / or rough oxide ceramics or carbide ceramics or a high-temperature-resistant cast steel, so that even in the region of the supporting face, oxygen entering the surface of the steel sheet or steel component is ensured. The invention also relates to a device for carrying out the method.

Description

[0001] Method and device for producing hardened sheet steel components

[0002] The invention relates to a method and a device for producing hardened steel components.

[0003] Hardened steel components have the advantage, particularly in the body construction of motor vehicles, that their outstanding mechanical properties make it possible to create a particularly stable passenger cell without having to use components that are much more massive and therefore heavier at normal strengths.

[0004] To produce such hardened steel components, steel grades that can be hardened by quench hardening are used. Examples of such steel grades include boron-alloyed manganese-carbon steels, the most commonly used being 22MnB5. However, other boron-alloyed manganese-carbon steels are also used for this purpose.

[0005] To produce hardened components from these steel grades, the steel material must be heated to the austenitizing temperature (> Ac3) and then waited until the steel material is austenitized. Depending on the desired degree of hardness, partial or full austenitization can be achieved.

[0006] If such a steel material is cooled after austenitization at a rate above the critical hardening rate, the austenitic structure transforms into a martensitic, very hard structure. In this way, tensile strengths R m up to over 1500 MPa achievable.

[0007] There are currently two common processes for producing steel components.

[0008] In so-called hot stamping, known from EP1651789B1, a steel sheet blank is separated from a steel strip, for example, cut or punched, and then formed into the finished component in a conventional, for example, four-stage deep drawing, trimming, and post-forming process. This finished component is dimensioned somewhat smaller to compensate for subsequent thermal expansion during austenitizing.

[0009] The resulting component is then austenitized, at least in part, and then placed in a hot-forming tool, where it is pressed but not, or only slightly, deformed. The pressing process dissipates the heat from the component into the hot-forming tool at a rate that exceeds the critical hardening rate.

[0010] The second common process is press hardening, also known as the direct process. In press hardening, a sheet steel component is produced by heating a flat blank made of hardenable steel to a temperature above the austenitizing temperature, so that at least some of the steel structure is in the high-temperature modification, namely austenite. This flat blank is then formed into a desired shape in a forming tool, usually with a single forming stroke. The contact with the forming tool halves extracts the heat from the steel material so quickly that martensitic hardening occurs, in which the austenite is essentially converted into martensite. The rate of heat dissipation for this to happen must be above the so-called critical hardening rate, which is usually above 20 Kelvin per second.

[0011] In both processes, blanks coated with metallic corrosion protection layers, particularly with zinc or a zinc-based alloy, can be used. Stamp hardening is also referred to as an indirect process (e.g. phs-ultraform) and press hardening as a direct process (e.g. phs-direct form). In the direct process, metallic corrosion protection layers made of aluminum-silicon with an aluminum content of at least 80 wt. % are also common. Since the aluminum-silicon layer is very brittle at room temperature, the coating can spall during cold forming, which is why this layer variant is not usually used for the indirect process.

[0012] The advantage of the indirect process is that, thanks to the upstream multi-stage cold forming, more complex component geometries can be realized, and trimming in the hardened state is eliminated. The advantage of the direct process is that only one tool, namely the press-hardening tool, is required. However, the achievable component complexity is lower, and trimming in the hardened state to the final contour is necessary.

[0013] During hot stamping, fully formed and usually also fully trimmed and punched components are passed through a furnace or placed in a furnace and heated to austenitizing temperature. For transport or storage, these components are placed on furnace supports. These furnace supports usually have to provide sufficient support points to prevent creep and / or sagging and / or twisting and / or tipping while in the furnace. To ensure precise removal after heating and subsequent precise insertion into the hot stamping tool, they can also have stops and / or mandrels that engage in corresponding holes in the components.Advantageously, the furnace supports have a component-specific contour that follows the component geometry in order to support the component in its entirety against creep and / or sagging and / or twisting and / or tilting. For simpler components that are not at risk of tipping, the components can also be placed on universal transport elements or universal support rails in the furnace and positioned during or after removal from the furnace and before insertion into the hot stamping tool.

[0014] During press hardening, flat blanks have to be transported through the furnace and heated or placed in the furnace and then heated. For this reason, in principle, considerably more support points, i.e. support surfaces, are necessary for press hardening than for die hardening. Otherwise, due to a lack of stiffening, the flat blank can bend significantly during heating and this would make precise transport through the furnace or precise removal from the furnace and subsequent precise insertion into the press hardening tool more difficult. During press hardening, the blank is therefore in principle supported by considerably more support points than the component during die hardening. This means that for a comparable size of blank and component, the number of support points is greater during press hardening, but the load per support point due to the dead weight is greater during die hardening.

[0015] The furnace for both press hardening and die hardening can be designed either as a continuous furnace, for example as a step conveyor furnace or walking beam furnace or chain girder furnace or roller hearth furnace, or as a chamber furnace, for example as a multi-layer chamber furnace or so-called "Pi z zaofen".

[0016] Thus, for the purpose of austenitization, the material being heated is always in contact with a transport, holding, or heating device. These contact surfaces or points always influence the surface of galvanized sheets. Due to vibrations and / or layer reactions (e.g., exothermic reactions) during the furnace stay, the influence of a contact surface on the surface can be greater than the contact surface itself. With galvanized material, there is a risk that the layer will be removed or damaged, and that so-called efflorescence or even local burn-off can occur on the layer.For aluminum-silicon coatings, which cannot be used in the indirect but can be used in the direct process, other technological conditions arise. For example, no efflorescence or local burning at the contact surfaces is known, but cathodic corrosion protection is also not possible with these coatings.

[0017] This can affect the surface of galvanized material, which can, for example, negatively impact the corrosion protection and / or the paint adhesion of the components and / or be undesirable from an optical point of view.

[0018] The object of the invention is to create a method for producing hardened steel components in which efflorescence and damage to the galvanized surface on the contact surfaces are reduced or eliminated during heating in a furnace. This object is achieved by a method having the features of claim 1.

[0019] Advantageous further developments are identified in the dependent subclaims.

[0020] It is a further object to provide a device for carrying out the method with which the transport and / or the placement of the circuit boards or components in the furnace takes place more gently.

[0021] The object is achieved by a device having the features of claim 11.

[0022] Advantageous further developments are identified in the dependent subclaims.

[0023] It is known that a coating of zinc or a zinc-based alloy on a steel sheet substrate forms a very thin aluminum oxide layer on the surface when heated for the purpose of austenitizing, provided the zinc coating contains a small amount of aluminum. This oxide layer protects the underlying zinc, which becomes liquid on further heating. Without this aluminum oxide layer, the zinc layer oxidizes to the point of burning or evaporates in parts. With increasing residence time in the furnace, the liquid zinc beneath the protective aluminum oxide skin transforms into zinc ferrite at the interface to the iron and above it into a zinc-iron alloy, usually with rather heterogeneously distributed concentrations.

[0024] According to the invention, a steel sheet blank or a steel strip is used which is provided with a zinc coating or a zinc alloy coating. This zinc coating or a zinc alloy coating can have a layer thickness of 5 μm to 20 μm per side. This advantageously enables good corrosion protection. In particular, the coating can be a Z 120 or Z 140 or Z 180 according to DIN EN 10346.

[0025] Zinc coatings can have a comparatively high zinc content of 85% to 98% by weight and, in addition to unavoidable impurities, also contain aluminum in the range of 0.2 to 2% by weight. Furthermore, they may contain other oxygen-affine elements such as magnesium.

[0026] Particularly preferably, the zinc coating or zinc alloy coating can be applied by hot-dip plating. This can represent a simple and robust application method.

[0027] According to the invention, it was recognized that, particularly during the press-hardening process and thus during the transport of a heated sheet through the furnace, a support surface must be present that promotes the self-healing effect of the material or surface in the event of damage to the protective aluminum oxide layer. According to the invention, this is achieved by the aluminum oxide skin forming again when sufficient oxygen is present and the damage is minimal, thus continuing to protect the liquid zinc.

[0028] According to the invention, the contact surface between a support or transport medium and the supporting steel plate must be designed such that sufficient oxygen reaches the support surface. Furthermore, the contact surface is not damaging to the aluminum oxide layer when subjected to vibration during furnace transport or furnace stay. To prevent adhesion or deposits of the coating, the contact surface or the material forming the contact surface on the transport or support medium is as inert as possible or has little affinity with the zinc coating of the supporting steel plate.

[0029] These contact surfaces can advantageously be cleaned, with the contact surfaces being geometrically designed so that there are no elongated lines or excessively sharp points that could damage the surface. Small, approximately round, or approximately square, or approximately rectangular contact surfaces with dimensions of 7 to 200 mm have proven advantageous according to the invention. 2 Preferably, for approximately rectangular contact surfaces, the length-to-width ratio can be between 1:1 and 5:1.

[0030] A particularly preferred material is yttrium-stabilized zirconium dioxide. This material is temperature-resistant, very hard, and allows oxygen ions to diffuse above 600°C. Another suitable material is aluminum oxide, which has a low affinity for the steel coating materials and is also high-temperature resistant. Another suitable material is a chromium-nickel steel with a high silicon content, on which a continuous oxide layer forms at high temperatures, which has little affinity for the zinc coating.

[0031] However, other ceramic materials with sufficiently high roughness and / or porosity can also serve as oxygen storage devices, for example, silicon carbide (SiC) or other carbide ceramics. It is advantageous if the ceramic material has an open porosity of 20 to 60 vol.%, particularly 25 to 35 vol.%, and / or a roughness of Rz > 30 pm, particularly Rz > 100 pm.

[0032] Furthermore, honeycomb bodies are conceivable which have an advantageous ratio between oxygen and contact area, as well as open-pore materials such as metallic or ceramic sponge or foam structures, high-temperature-resistant fabric materials and the like.

[0033] Surprisingly, it has been found that, for the purposes of the invention, a very smoothly polished surface—which one would expect to reduce adhesion—is not advantageous, but rather a rough surface. The invention is explained by way of example with reference to a drawing.

[0034] Alternatively, the inventors have recognized that a significant reduction in the support surfaces of the furnace supports, particularly during hot stamping, i.e. during the indirect process with preformed components, can be sufficient to prevent surface damage such as zinc blooming. Instead of the usual support surface of approximately 400 mm 2 ( so about 023 mm or 20 mm square edge length ) this to a maximum of 200 mm 2 (i.e. a maximum of approximately 016 mm or 14 mm square edge length). Support surfaces of 7 mm each have proven particularly suitable. 2 up to 200 mm 2 particularly preferably 13 mm2 up to 113 mm 2 (i.e. approximately 03 mm to 016 mm or 3 mm to 14 mm square edge length, particularly preferably 04 mm to 012 mm or 4 mm to 11 mm square edge length).

[0035] The lower limit of 7 mm 2 The edge length should not be less than 3 mm, as this can lead to increased wear or destruction of the support surface due to the constant temperature treatment and heating and cooling steps in conjunction with vibrations. The upper limit of 200 mm 2 per contact surface should not be exceeded in order to prevent efflorescence or other surface damage, which can lead to, among other things, inadequate corrosion protection, in a particularly process-safe manner.

[0036] This means that the steel material previously used for the furnace supports can be reused, which represents a cost advantage.

[0037] Furthermore, the dimensional stability of the parts should be guaranteed, or the components can be better protected against twisting or other undesirable changes in shape.

[0038] The invention thus relates in particular to a method for heating a steel sheet patin or a preformed steel sheet component with a zinc coating or a zinc alloy coating, wherein the steel sheet component or the steel sheet blank is passed through a furnace or placed or positioned in a furnace and is heated in the furnace in particular above the austenitizing temperature, wherein the steel sheet component or the steel sheet blank rests at least temporarily on at least one carrier on several support surfaces, wherein a carrier is used in which on the support surfaces for a steel sheet blank or a steel sheet component either a) the support surfaces each have a maximum of 200 mm 2 preferably maximum 113 mm 2and / or b) the contact surfaces consist of a porous and / or rough oxide ceramic or carbide ceramic or a high-temperature-resistant cast steel, so that oxygen access to the surface of the steel sheet or the steel component is also guaranteed in the area of ​​the contact surface. One embodiment provides that the carrier is coated, covered or formed from an oxide ceramic or carbide ceramic or a high-temperature-resistant steel.

[0039] One embodiment provides that the support surfaces on the carrier are each at least 7 mm 2 preferably 13 mm 2 is set .

[0040] One embodiment provides that the oxide ceramic or carbide ceramic or the high-temperature cast steel has an open porosity of 20 to 60 vol . -% and / or a roughness of Rz > 30 gm, in particular Rz > 100 gm.

[0041] One embodiment provides that a carrier is used which has a plurality of adjacent support surfaces, wherein the support surfaces are formed from the oxide ceramic, wherein the support surfaces are spaced apart from one another.

[0042] One embodiment provides that yttrium-stabilized zirconium oxide and / or aluminum oxide is used as the material for the contact surface between the carrier and the circuit board and / or the component.

[0043] One embodiment provides that honeycomb bodies, in particular ceramic honeycomb bodies and / or ceramic fibers and / or ceramic fabrics and / or open-pore materials such as metallic or ceramic sponge or foam structures are used as the carrier and / or support surface.

[0044] One embodiment provides that the sheet steel component or sheet steel blank is made of a hardenable steel alloy, in particular a boron-manganese steel. One embodiment provides that the sheet steel component or sheet steel blank is used with the following composition (all data in wt.%):

[0045] Carbon up to 0.4, preferably 0.15 to 0.3

[0046] Silicon up to 1.9, preferably 0.11 to 1.5

[0047] Manganese up to 3.0, preferably 0.8 to 2.5

[0048] Chromium up to 1.5, preferably 0.1 to 0.9

[0049] Molybdenum up to 0.9, preferably 0.1 to 0.5

[0050] Nickel up to 0.9,

[0051] Titanium up to 0.2 preferably 0.02 to 0.1

[0052] Vanadium up to 0.2

[0053] Tungsten up to 0.2,

[0054] Aluminum up to 0.2, preferably 0.02 to 0.07

[0055] Boron up to 0.01 preferably 0.0005 to 0.005

[0056] Sulphur max. 0.01, preferably max. 0.008

[0057] Phosphorus max. 0.025, preferably max. 0.01

[0058] Rest iron and impurities.

[0059] One embodiment provides that the sheet steel blank is heated for the purpose of austenitizing and then formed or is first cold formed into a sheet steel component and then heated for the purpose of austenitizing and, after austenitizing, the sheet steel blank or the formed sheet steel component is cooled at a rate above the critical cooling rate.

[0060] One embodiment provides that the zinc coating or zinc alloy coating has a layer thickness of 5 μm to 20 μm per side, in particular 7 μm to 15 μm per side. This can further increase the corrosion protection, in particular cathodic corrosion protection. A further aspect of the invention relates to a device for heating steel sheet blanks and / or steel sheet components with a zinc coating or a zinc alloy coating, wherein the device has at least one carrier with several support surfaces for at least temporarily supporting a steel sheet blank or a steel sheet component, wherein the carrier has support surfaces on the blank or component side for a steel sheet blank or a steel sheet component, in which a) the support surfaces each have a maximum of 200 mm 2 preferably maximum 113 mm 2and / or b) the contact surfaces consist of a porous and / or rough oxide ceramic or carbide ceramic or a high-temperature cast steel.

[0061] One embodiment provides that the carrier has a sequence of adjacent truncated pyramids, truncated cones, columns or punches, wherein the support surfaces for the plate or the workpiece are formed by the workpiece-side or plate-side surfaces of the truncated cones, truncated pyramids, columns or punches.

[0062] One embodiment provides that the supporting surfaces of the ceramic have a square, polygonal or round surface with a size of up to 200 mm each. 2 have .

[0063] One embodiment provides that the support surfaces on the carrier have a support surface of 7 mm each 2 up to 200 mm 2 especially 13 mm 2 up to 113 mm 2 have .

[0064] One embodiment provides that the truncated pyramids and / or truncated cones and / or columns and / or stamps are arranged on a support made of high-temperature-resistant steel, silicon carbide, oxide ceramic or other heat-resistant supports.

[0065] One embodiment provides that the truncated cones, truncated pyramids, columns or stamps are plasma-sprayed with a rough surface created by the plasma spraying.

[0066] One embodiment provides that the supports are made of a ceramic material such as yttrium-stabilized zirconium dioxide or aluminum oxide.

[0067] One embodiment provides that the oxide ceramic or carbide ceramic or the high-temperature cast steel has an open porosity of 20 to 60 vol . -% and / or a roughness of Rz > 30 pm, in particular Rz > 100 pm.

[0068] The invention is explained by way of example with reference to a drawing. It shows:

[0069] Figure 1 shows the indirect process (form hardening, phs-ultraform i.e. without trimming in the hardened state) schematically;

[0070] Figure 2 shows the difference between furnace supports (contour-following) and furnace support strips (not contour-following) using the example of a chamber furnace;

[0071] Figure 3 shows an example of the effect of creep.

[0072] Sagging after the furnace due to insufficient support in a continuous furnace; Figure 4 shows an example of the effect of creep or

[0073] Sagging after the furnace due to insufficient support in a chamber furnace;

[0074] Figure 5 shows an example of a component surface at contact points in the prior art;

[0075] Figure 6 shows an exemplary component surface at contact points when implemented according to the invention;

[0076] Figure 7 shows an example of the reduced

[0077] On a flat surface;

[0078] Figure 8 shows an example of the support or bearing surface according to the state of the art;

[0079] Figure 9 shows an exemplary reduced support surface according to the invention;

[0080] Figure 10 furnace support according to the invention for the indirect phs (ultraform) process;

[0081] Figure 11 four different variants of an oven shelf;

[0082] Figure 12 shows a plasma-sprayed, surface-untreated yttrium-stabilized zirconium dioxide deposition surface;

[0083] Figure 13 a detailed enlargement of the storage area after

[0084] Figure 12 ;

[0085] Figure 14 shows a support surface according to Figure 13 made of aluminum oxide; Figure 15 shows a support surface made of heat-resistant cast steel;

[0086] Figure 16 a ) a storage honeycomb body made of solid ceramic;

[0087] Figure 16 b ) a honeycomb body made of ceramic fibers;

[0088] Figure 16 c ) the surface contour of a honeycomb body made of

[0089] all-ceramic;

[0090] Figure 16 d) the surface contour of a ceramic fabric;

[0091] Figure 16 e ) a ceramic foam structure ;

[0092] Figure 17 is a representation of a honeycomb body from Figure 16 a) and three heated sheet metal parts (according to the invention) lying thereon;

[0093] Figure 18 shows a representation of a ceramic fiber board made of

[0094] Figure 16 d) and three heated sheet metal parts (according to the invention) lying thereon;

[0095] The indirect process is shown schematically in Figure 1. It is clear that the component geometry, including trimming of the outer contour and hole pattern, is already finished after cold forming, i.e. before furnace heating, and that in the hardened state, i.e. after quench hardening in the hot stamping tool, no further trimming of the outer contour and hole pattern takes place. This means that the components must be placed in the hot stamping tool in the correct position after furnace heating, as otherwise non-correctable dimensional deviations of the outer contour and hole pattern as well as undesirable deformations in the hot stamping tool will occur. Therefore, using the example of the continuous furnace shown, exact positioning of the component is necessary, particularly when removing it from the furnace, in order to advantageously enable exact positioning in the hot stamping tool on the process side, for example using robots.This can also be applied analogously to the direct process (not shown) for undeformed or slightly preformed blanks.

[0096] Figure 2 shows, by way of example, the difference between component storage on furnace supports that follow the component contour and component storage on (non-contour-following, possibly also universal) furnace storage strips, using the example of a chamber furnace. The left-hand side shows the side view, with the right-hand block that is not connected to the rest symbolizing the furnace door, and the right-hand side shows the respective front view. The upper figures show a contour-following component storage, and the lower figures show a component storage on strips.

[0097] During heat treatment in a furnace, creep or sagging of the steel material can occur. This is illustrated in Figure 3 using the example of a continuous furnace, and in Figure 4 using the example of a chamber furnace. To counteract creep or sagging, and possibly also twisting and / or tilting, sufficient support of the steel material must be ensured.

[0098] However, a very high contact surface on coated steel material can lead to efflorescence or other surface impairments, which are shown in Figure 5.

[0099] In contrast, with a reduced contact surface according to the invention, such surface impairment does not occur at the contact points or contact surfaces or only occurs to a comparatively small extent, as shown in Figure 6.

[0100] According to the invention, a support is used as the carrier or support that has a contour on the workpiece or blank side. According to the invention, this can be a support strip, support strip, furnace support, component support, or the like. An example of such a contour is shown in Figure 7 (the dashed area represents the carrier). This carrier has several support surfaces, preferably at least three, to ensure a geometrically secure condition.

[0101] A cross-section of an entire support, i.e. an example of a furnace support for a component for the indirect process, can be found in Figure 9 (the dashed area represents the component) as well as in three-dimensional representation in Figure 10.

[0102] From the prior art (Figure 8), however, a linear contact or linear support surfaces are known.

[0103] The contour of a support can also, for example, be a sequence of adjacent truncated pyramids, with the actual support surfaces being the top surfaces of the truncated pyramids, while the base surfaces of the truncated pyramids are adjacent to each other. An example of such a strip-shaped furnace support surface with truncated pyramids arranged on it is shown in Figure 11 c).

[0104] The four variants shown in Figure 11 are a ceramic rod Fig. 11 a) ; aluminum oxide (AI2O3) coated truncated pyramids in ground state Fig. 11 b) upper area; zirconium oxide (ZrO) coated truncated pyramids in ground state Fig. 11 b) lower area; aluminum oxide coated truncated pyramids in rough state Fig. 11 c) upper area; zirconium oxide coated truncated pyramids in rough state Fig. 11 c) lower area and sol-gel coated truncated pyramids shown in Fig. 11 d).

[0105] Surprisingly, it was found that the ceramics, especially AI2O3 and ZrO in rough condition caused significantly less surface damage than those ceramics which were ground.

[0106] The truncated pyramid surfaces have an approximately square surface with an edge length of approximately 4 to 12 mm, which corresponds to a contact surface of 13 mm 2 up to 113 mm 2 corresponds .

[0107] The pyramid truncated members can, for example, be arranged on a support made of heat-resistant steel, silicon carbide or similar heat-resistant supports, or the entire support can consist of these.

[0108] Preferably, the surfaces of the pyramid truncated pieces are, for example, plasma-sprayed and consist of a ceramic and in particular oxide-ceramic material.

[0109] A certain surface roughness is achieved by plasma spraying or similar or specially used processes.

[0110] Zirconium dioxide and aluminum oxide are particularly suitable as oxide-ceramic materials. Carbide-ceramic materials can also be used. Instead of the truncated pyramids shown in Figures 12 to 14, other geometric shapes, such as truncated cones or columns, are of course also conceivable.

[0111] In a further advantageous embodiment, the support surfaces are formed as honeycomb bodies made of solid ceramic (Figure 16a). In this case, the honeycomb bodies can be arranged individually and at a distance from one another on a support, whereby the individual honeycomb bodies can, for example, have edge lengths of 5 to 25 mm. The honeycomb bodies are preferably formed from a solid ceramic material, such as yttrium-stabilized zirconium dioxide or aluminum oxide. This can, for example, have a roughness of Rz = 23 pm.

[0112] In a further advantageous embodiment, the supporting surface of the carrier is designed entirely as a honeycomb body, which in this case is designed to be correspondingly elongated.

[0113] The honeycombs can have a square or polygonal, particularly hexagonal, cross-section.

[0114] In a further advantageous embodiment, the honeycomb body is formed from ceramic fibers (Figure 16b), wherein this ceramic fiber honeycomb body can be located over an entire carrier or also only in partial areas or at specific points on a carrier.

[0115] It can be particularly advantageous for the surface contour not to be flat and, in particular, not polished, but rather to have a roughness or microcontour. Such a surface contour can be formed by not undergoing any further post-processing of the surface after flame or plasma spraying, or by not undergoing any further post-processing of the surface in the case of a honeycomb body, so that a surface contour such as that shown in Figure 16 c) is obtained.

[0116] Usable ceramic fabrics also usually have surfaces with an existing contour or roughness, as shown in Figure 16 d).

[0117] Also suitable are foams or microfoams which also have a microstructure formed by the pores on their surfaces ( Figure 16 e ).

[0118] For this purpose, a heat treatment of three galvanized sheet metal parts was carried out using a honeycomb body from Figure 16 a ) and these showed no surface impairments ( see Figure 17 ) .

[0119] Three steel sheet parts were also placed on a ceramic fiberboard according to Figure 16 d). In this example, a high-temperature composite made of silica fabric with a predominantly Al2O2 matrix was chosen as the ceramic fiberboard. This also did not result in any surface damage to the galvanized steel sheet parts (shown in Figure 18). This, for example, has a roughness of Rz = 57 pm.

[0120] The invention is explained using experiments.

[0121] Attempt 1

[0122] A sheet metal plate coated with an aluminum-silicon coating is placed on coated support elements. The support elements consist of a longitudinal ceramic rod, truncated pyramids made of aluminum oxide with ground contact surfaces, and truncated pyramids constructed in the same way as the aforementioned, but made of yttrium-stabilized zirconium oxide.

[0123] Truncated pyramids made of plasma-sprayed aluminum oxide without surface treatment, similarly formed truncated pyramids made of yttrium-stabilized zirconium oxide and a support with truncated pyramids coated using a sol-gel process.

[0124] A test furnace is heated to a temperature which is so high in terms of heat transfer that the board is heated to about 930 ° C.

[0125] After removing the circuit board and allowing it to cool down, the circuit board shows clear changes to the metallic surface in the area of ​​the ceramic rod, which are not acceptable after visual inspection.

[0126] The parts of the plate that rest on the ground aluminum oxide and ground zirconium oxide pyramid stumps are optically borderline and also not in a condition that can be described as acceptable.

[0127] Similarly, the board appears to be defective in the area of ​​the sol-gel pads.

[0128] Only in the area of ​​the aluminum oxide and zirconium oxide support elements, which are unground, is the surface quality of the board satisfactory. Test 2

[0129] A blank made of zinc-coated sheet metal is conveyed in a furnace.

[0130] The circuit board has surface changes, including zinc efflorescence, which constitute an unacceptable surface impairment.

[0131] Attempt 3

[0132] A circuit board as in experiment 2 , with the supports again corresponding to the supports according to experiment 1 .

[0133] The circuit board shows such severe surface changes in the area of ​​the ceramic rod support that a ceramic rod support with a narrow, linear support surface cannot be used.

[0134] When applied, surface changes are also observed in the area of ​​the sol-gel coated layer, which are unacceptable.

[0135] Surface changes can also be observed on the ground aluminum oxide coating and the ground zirconium coating, although these are significantly less pronounced than those on the ceramic rod or the sol-gel coating.

[0136] No negative changes can be detected in an aluminum oxide coating, a zirconium oxide coating, or a silicon carbide coating, which exhibit roughness or porosity, respectively. For example, the silicon carbide coating can exhibit a roughness of Rz = 49 pm.

[0137] Attempt 4

[0138] This experiment is similar to experiment 3, except that the board is raised and lowered during heating.

[0139] Significant negative surface changes are evident in the area of ​​the ceramic rod coating and the sol-gel coating. Changes are also observed in the area of ​​the ground aluminum oxide coating and the unground aluminum oxide coating.

[0140] The zirconium oxide overlays show little to no changes, with the zirconium oxide overlay without surface grinding achieving the best result.

[0141] Overall, it can be stated that zirconium oxide and aluminum oxide in unground condition are the best supports.

[0142] Attempt 5

[0143] A preformed alloy-galvanized component is guided through a continuous furnace with a furnace support and austenitized.

[0144] The support surfaces each have an area between 210 mm 2 and 400 mm 2 on .

[0145] After hardening and cooling the component from the hot stamping tool, the hardened component exhibits significant changes to the metallic surface in the area of ​​the contact surfaces, which, according to visual inspection, are not acceptable. In particular, the zinc blooms shown in Figure 5 cannot be accepted, and the component is therefore rejected.

[0146] Attempt 6

[0147] As test 5, but the contact surfaces were further reduced and have an area of ​​13 mm each 2 up to 100 mm 2 on .

[0148] Surprisingly, it was found that after hardening and cooling the component, after removal from the hot stamping tool, the surface damage was significantly reduced. Despite no adjustment of the furnace support material, slight efflorescence was visually detectable, but this did not represent any material damage to the component. Therefore, this material was acceptable.

[0149] According to the invention it was found that in high temperature processes for austenitizing steel sheet blanks or steel sheet components with zinc coating or zinc alloy coating the carrier must be selected so that the existing zinc coating or zinc alloy coating of the steel sheet blanks or steel sheet components is not damaged or can perform its self-healing functions in the event of damage.

[0150] According to the invention, it was found that ceramic coatings on the supports or ceramic supports are suitable for this purpose if they do not have smooth, polished surfaces, but rather rough and / or porous surfaces. It has been shown that ceramic coatings made of oxide ceramics or carbide ceramics, and in particular of aluminum oxide and zirconium oxide, in particular yttrium-stabilized zirconium oxide, but also rough cast steel, achieve the desired effect. According to the invention, it was found that, alternatively or in addition to the ceramic coatings, a comparatively high reduction of the contact surfaces for furnace supports to less than 200 mm 2 This ensures that the existing zinc coating or zinc alloy coating is not damaged or can perform its self-healing functions in the event of damage. Within the scope of the invention, it is also possible to provide a support which has some contact surfaces below the 200 mm mentioned.2 and additionally some contact surfaces which are porous and / or rough, i.e. a mixture of both variants.

Claims

Patent claims 1. A method for heating a steel sheet blank or a preformed steel sheet component with a zinc coating or a zinc alloy coating, wherein the steel sheet component or the steel sheet blank is passed through an oven or placed or positioned in an oven and is heated in the oven, in particular above the austenitizing temperature, wherein the steel sheet component or the steel sheet blank rests at least temporarily on at least one support on several bearing surfaces, wherein a support is used in which the bearing surfaces for a steel sheet blank or a steel sheet component are either a) each bearing surface a maximum of 200 mm 2 preferably a maximum of 113 mm 2a) the bearing surfaces are made of a porous and / or rough oxide ceramic or carbide ceramic or a high-temperature resistant cast steel, so that oxygen can also reach the surface of the steel sheet or steel component in the area of ​​the bearing surface.

2. Method according to one of the preceding claims, characterized in that the carrier is coated, covered or formed from an oxide ceramic or carbide ceramic or a high-temperature-resistant steel. Method according to one of the preceding claims, characterized in that the bearing surfaces of the carrier are each at least 7 mm 2 preferably 13 mm 2A method according to any of the preceding claims, characterized in that the oxide ceramic or carbide ceramic or the high-temperature-resistant cast steel has an open porosity of 20 to 60 vol. % and / or a roughness of Rz > 30 pm, in particular Rz > 100 pm. A method according to any of the preceding claims, characterized in that a carrier is used which has a plurality of adjacent bearing surfaces, wherein the bearing surfaces are formed from the oxide ceramic, and wherein the bearing surfaces are spaced apart from each other. A method according to any of the preceding claims, characterized in that yttrium-stabilized zirconium oxide and / or aluminum oxide is used as the material for the contact surface between the carrier and the circuit board and / or the component.A method according to any of the preceding claims, characterized in that honeycomb bodies, in particular ceramic honeycomb bodies and / or ceramic fibers and / or ceramic fabrics and / or open-pored materials such as metallic or ceramic sponge or foam structures, are used as the support and / or bearing surface. A method according to any of the preceding claims, characterized in that the sheet steel component or the steel... A sheet metal blank made of a hardenable steel alloy, in particular a boron-manganese steel. A method according to one of the preceding claims, characterized in that the sheet metal component or the sheet metal blank is used with the following composition (all values ​​in wt.%): Carbon up to 0.4, preferably 0.15 to 0.3 Silicon up to 1.9, preferably 0.11 to 1.5 Manganese up to 3.0, preferably 0.8 to 2.5 Chromium up to 1.5, preferably 0.1 to 0.9 Molybdenum up to 0.9, preferably 0.1 to 0.5 Nickel up to 0.9, Titanium up to 0.2, preferably 0.02 to 0.1 Vanadium up to 0.2 Tungsten up to 0.2, Aluminum up to 0.2, preferably 0.02 to 0.07 Boron up to 0.01, preferably 0.0005 0.005 Sulfur max. 0.01, preferably max. 0.008 Phosphorus max. 0.025, preferably max. 0.01 Residual iron and impurities. Method according to one of the preceding claims, characterized in that the steel sheet blank is heated for the purpose of austenitization and subsequently formed, or is first cold-formed into a steel sheet component and subsequently heated for the purpose of austenitization, and after austenitization the steel sheet blank or the formed steel sheet component is cooled at a rate above the critical cooling rate. A method according to one of the preceding claims, characterized in that the zinc coating or zinc alloy coating has a layer thickness of 5 pm to 20 pm per side, in particular 7 pm to 15 pm per side. A device for heating steel sheet blanks and / or steel sheet components with a zinc coating or a zinc alloy coating, wherein the device has at least one support with several bearing surfaces for at least temporarily placing a steel sheet blank or a steel sheet component, wherein the support has bearing surfaces on the blank or component side for a steel sheet blank or a steel sheet component, wherein a) the bearing surfaces each have a maximum diameter of 200 mm. 2 preferably a maximum of 113 mm 2a) the bearing surfaces consist of a porous and / or rough oxide ceramic or carbide ceramic or a high-temperature-resistant cast steel. Device according to claim 12, characterized in that the support has a sequence of adjacent truncated pyramids, truncated cones, columns, or punches, wherein the bearing surfaces for the circuit board or the workpiece are formed by the workpiece- or circuit board-side surfaces of the truncated cones, truncated pyramids, columns, or punches. Device according to claim 12 or 13, characterized in that the bearing surfaces of the ceramic are square, polygonal, or round surfaces, each with a size of up to 200 mm². 2 have . Device according to claims 12 to 14, characterized in that the bearing surfaces of the carrier each have a bearing surface of 7 mm. 2 up to 200 mm 2 especially 13 mm 2 up to 113 mm 2A device according to any one of claims 12 to 15, characterized in that the truncated pyramids and / or truncated cones and / or columns and / or punches are arranged on a support made of high-temperature-resistant steel, silicon carbide, oxide ceramic, or other heat-resistant supports. A device according to any one of claims 12 to 16, characterized in that the truncated cones, truncated pyramids, columns, or punches are plasma-sprayed with a rough surface resulting from the plasma spraying. A device according to any one of claims 12 to 17, characterized in that the supports are made of a ceramic material such as yttrium-stabilized zirconium dioxide or aluminum oxide. A device according to any one of claims 12 to 18, characterized in that the oxide ceramic, carbide ceramic, or high-temperature-resistant cast steel has an open porosity of 20 to 60 vol. -% and / or a roughness of Rz > 30 pm, especially Rz > 100 pm.

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