Method for producing hardened steel components

Abstract

The invention relates to a method for the indirect PHS process for, in particular, galvanised hardened steel components. In order to improve the dimensional accuracy of the hardened components, in particular the dimensional accuracy of the outer trim edges and the hole pattern, at least one of the conditions a) to c) is fulfilled: a) the component blanks of a batch, each component blank taken individually and all component blanks in relation to each other, have a maximum temperature difference of △∂ until the time of completion of the closing of the hot stamping press, wherein the following applies: △∂ = φ / Lmax, where φ is a shape constant in K*m and Lmax is the largest dimension of the largest component blank in the batch in m, and the following values apply for the shape constant φ depending on Lmax: from Lmax greater than 1 m:φ = 117.4 K*m, in particular φ = 65.2 K*m, preferably φ = 43.5 K*m and particularly preferably φ = 21.7 K*m, from Lmax greater than 0.5 m:φ = 65.2 K*m, preferably φ = 43.5 K*m and particularly preferably φ = 21.7 K*m, b) the component blanks of a batch, containing parts of a first type and parts of a second type, are arranged at least in the furnace in such a way that in each case a part of the first type is located directly next to at least one second part of the first type, and a part of the second type is located directly next to at least one second part of the second type, c) in the case of the component blanks of a batch, containing parts of a first type and parts of a second type, the parts of the first type and the parts of the second type are cold-formed with different sizes or three-dimensional shapes due to the different thermal expansion and / or the parts of at least one type within the parts are cold-formed with different sizes or three-dimensional shapes in such a way that differences in size are substantially compensated for at the time of completion of the closing of the hot-stamping press.

Description

[0001] Process for producing hardened steel components

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

[0003] In particular, the invention relates to a method for producing hardened sheet steel components using the so-called indirect method, for which the applicant has also coined the term "die hardening".

[0004] It is known to produce high-strength sheet steel components, for example for automotive construction, by hardening. In particular, such components are produced by quench hardening or transformation hardening from the austenitic state.

[0005] Quench hardening means that an object made of a hardenable steel material is brought to a temperature at which the structure is largely or entirely in the form of austenite or gamma iron—i.e., to a temperature above the so-called Ac3 point of the respective steel material. Subsequently, by cooling above the so-called critical cooling rate, the austenite is converted to martensite, thereby achieving high tensile strength.

[0006] The steel material used in this case is a steel material that can be hardened by quench hardening, such as a boron-manganese steel of the alloy 22MnB5 or 34MnB5.

[0007] To produce such a hardenable steel sheet material, a steel strip is produced by melting a hardenable steel alloy, casting it into slabs, usually using a continuous casting process, hot rolling the resulting slabs in a hot strip mill, and then cold rolling them in a cold rolling mill. The steel strip is typically several hundred to over a thousand meters long, a few millimeters or less than one millimeter thick, and wound into coils or bundles.

[0008] During hot and cold rolling, the slab's thickness is significantly reduced, with a corresponding increase in length. However, the width is largely maintained through appropriate measures. Thickness reduction does not constitute forming within the meaning of this disclosure.

[0009] It is common practice to galvanize such cold-rolled strip in a galvanizing plant. For this purpose, the strip is unwound from the coil and rewound at the end. Galvanizing can be carried out by hot-dip galvanizing, electrolytic coating, or PVD coating. Galvanized or alloy-galvanized steel strip or steel sheet within the meaning of the invention is that steel strip or steel sheet which has a coating with zinc or a zinc-based alloy. Zinc-based means that zinc is the largest alloying component of the coating and, in particular, accounts for more than 50% by weight of the coating. Such cold-rolled strip can also be provided with an aluminum-silicon coating or further processed without a coating.

[0010] To produce sheet steel components, the steel strip is unwound from the coil and cut into pieces, called blanks. These blanks are thus flat steel sheets with a defined contour. To produce hardened sheet steel components, these blanks can be further processed in two ways.

[0011] In the so-called direct process, also known as press hardening, the blanks are austenitized by heating them up, usually in a furnace, and then hot-formed into a sheet metal component in a relatively colder tool (press-hardening tool), usually in a single step. Once the hot-forming has taken place and the tool is completely closed, the tool surfaces are in contact with the sheet metal component. As a result, the heat is dissipated into the usually cooled tool at a rate that is above the critical hardening rate, whereby the hardening outlined above is achieved by conversion to a martensitic structure. In press hardening, steel strips with an aluminum-silicon coating, galvanized steel strips, or uncoated steel strips are usually used. Since in press hardening only a single tool is available for forming and hardening, the final trimming (i.e.The final trimming (e.g., the final trimming of the outer and hole edges) only takes place after the formed and hardened components have been removed from the tool. This occurs in an additional process step, usually using laser cutting. While this essentially decouples the dimensional accuracy of the final trimming from the dimensional accuracy of the hardened geometry, it is not without its problems regarding the difficulty of correctly and uniformly accommodating the component for laser cutting. This is particularly time-consuming and expensive in the case of long trimming lengths, numerous holes, short cycle times or high output, multiple production runs, and large quantities.

[0012] In the so-called indirect process, also known as hot stamping or hot stamping (FH), the blank is formed into a component blank in a cold state in a usually multi-stage forming and trimming process, usually mainly through a combination of deep drawing, trimming and / or post-forming, collectively referred to here as cold forming (KU) in a cold forming tool set which comprises, for example, three to five individual tools, collectively referred to here as cold forming tool. This component blank is then usually austenitized in a furnace and, while still warm, placed in a usually cooled tool (hot stamping tool), usually on a storage device, also called a lifter, arranged in the hot stamping tool. This is usually spring-loaded and displaced by the upper part of the press in order to prevent uncontrolled cooling of the component blank from the insertion until the hot stamping tool is closed.The hot stamping tool usually no longer performs any actual forming during closing, although this is possible to a limited extent with spring-loaded component blanks and / or final forming. Once the tool is fully closed, the surface of the tool contacts the surface of the component blank and conducts heat away from it at a rate that is faster than the critical hardening rate, so that the component blank is hardened into the component. Hot stamping usually involves the use of galvanized steel strips, but in principle uncoated steel strips can also be used. However, steel strips with an aluminum-silicon coating cannot be hot stamped because the aluminum-silicon layer is very brittle at room temperature and the coating can spall off during cold forming.In the case of hot stamping, (laser) trimming in the hardened state is usually no longer necessary, since the final trimming is usually already completely carried out in the cold forming process, although (laser) trimming in the hardened state would of course also be possible to a small extent, e.g. on individual holes with particularly high accuracy requirements, with the corresponding additional effort.

[0013] In both cases, i.e. in the direct and indirect process, the product is a hardened, finished trimmed sheet steel component, whereby in the direct process the finished trimming takes place after hot forming and hardening, i.e. on the hardened component, and in the indirect process during the cold forming of the component blank and thus before hardening.

[0014] In both cases, the steel material undergoes several changes in volume or length in all spatial directions from its state before heating up to insertion into the respective tool, this fact being particularly important for component blanks used in the indirect process. These changes in length can in principle be described using a dilatometer curve, i.e. a temperature-length change curve of a sample of the steel material used, as shown in Figure 1: In the furnace, thermal expansion initially occurs until austenitization begins (a). During microstructure transformation to austenite, shrinkage occurs (b). Upon further heating up to leaving the furnace, thermal expansion occurs again (c). After leaving the furnace, i.e. during transfer from the furnace to the hot stamping tool, a corresponding shrinkage occurs during cooling to the respective insertion temperature (d).The insertion temperature within the meaning of the invention in the indirect process is the average temperature of the component blank, weighted according to its surface area, after its insertion into the tool and shortly before reaching the bottom dead center of the tool or press. The transfer between the furnace and the tool usually takes place in ambient air; cooling to the respective insertion temperature depends in particular on radiant heat losses as well as heat losses due to forced and natural convection. These heat losses, in turn, depend in particular on the transfer time, the sheet thickness, the emissivity of the surface, the geometry (due to self-irradiation effects), and the air flows during the transfer movement.Based on the ratio between the length at insertion temperature and the length before heating begins, it is possible to determine at least approximately in the indirect process by how much the cold forming tools for producing the component blanks must be scaled down, i.e. made smaller in all spatial directions, so that the component blanks produced in the cold forming tools fit onto the form hardening tool after heating in the furnace and subsequent cooling during transfer from the furnace to the form hardening tool at press closure.

[0015] Since, as previously described, no final trimming takes place after hardening in the indirect process, it is necessary to ensure dimensionally accurate components in the indirect process that the austenitized component blanks are placed in the hot stamping tool at the correct temperature and therefore in the correct size, i.e. they fit, or that they are hardened at press closure. If the insertion temperature is too high and therefore the inserted component blank is too large, the outer trim on the hardened component is usually too large and the hole pattern is correspondingly distorted. If the insertion temperature is too low and therefore the inserted component blank is too small, the outer trim on the hardened component is usually too small and the hole pattern is correspondingly distorted. In addition, inserting a component blank that is too large or too small into the hot stamping tool can lead to further problems.If, for example, the component blank becomes jammed on the centering pin or if vulnerable areas of the component blank, usually in particular frame areas, are undesirably reshaped, this can lead to so-called component displacement with subsequent production interruptions or even tool breakage.

[0016] In the case of steel sheet with zinc coating, if the component blanks are too large or, in particular, too small when placed into the hot stamping tool, undesirable microcracks in the steel substrate can occur due to so-called liquid metal embrittlement (LME) due to the resulting post-forming processes when closing the hot stamping tool while the sheet is still warm.

[0017] Furthermore, when inserting component blanks with an unsuitable insertion temperature, further dimensional deviations, particularly in frame angles, but also due to twisting, may occur due to changes in local cooling conditions in the tool, particularly in areas of positive and negative radii.

[0018] For economic reasons, the hot-melt parts in the relatively large hot-melt presses are not usually hardened individually, but rather a number of parts are hot-melt hardened together as a so-called "batch" within the framework of so-called multiple production in order to produce as many parts as possible per stroke and thus per time and to utilize the press table size as best as possible. Figure 2 shows a highly simplified version of the process and how, for example, a batch consisting of four KU parts is heated to above Ac3 in a continuous furnace (DLO), here with a part arrangement next to each other in the direction of travel (DLR), and is placed as a batch of austenitized parts together into the hot-melt hardening tool arranged in the hot-melt press by means of a transfer device, where it is then hot-melt hardened together.

[0019] To explain the terms next to each other in the direction of flow and one behind the other in the direction of flow of a continuous furnace, it should be mentioned that these terms only refer to the arrangement of the parts within a batch. Next to each other in the direction of flow also means that there are no parts behind each other in the direction of flow within the batch. A part arrangement next to each other in the direction of flow is usually found for parts with a longitudinal axis essentially parallel to the direction of flow, i.e. the longest axis of a component, which defines its orientation, is essentially parallel to the direction of flow. For furnaces that do not have a direction of flow, for example multi-layer chamber furnaces (MLK), the information on the part arrangement relates accordingly to the conveying direction, i.e. the direction of insertion and removal.

[0020] For multiple production in the die hardening tool, there are different variants with some significant differences, which are shown schematically in Figure 3.

[0021] In the case of multiple production, there is, on the one hand, a significant difference in whether or not a so-called identical part production takes place in the hot-melt tool, which is particularly relevant in vehicle construction due to the quality requirements that usually result from this.

[0022] Identical part production in the form hardening tool occurs when there is more than one form hardening tool nest for the same hardened component, i.e. two or more identical parts are hardened in the form hardening tool per stroke.

[0023] If there is only one hot stamping tool nest for each hardened component, for example for a first-type component and for a second-type component such as a B-pillar for the left and a B-pillar for the right side of the vehicle, then in this example there is indeed multiple production (i.e. double production). With only one part of the first type, in this case a left B-pillar (FH-L), and one part of the second type, in this case a right B-pillar (FH-R), this does indeed constitute multiple production but does not constitute identical part production. This usually occurs due to space constraints with relatively large, bulky parts such as B-pillars for the left and right sides of the vehicle.

[0024] If the hot stamping tool has two tool cavities for each hardened component, for example for a first-type component and for a second-type component such as a sill for the left and right sides of the vehicle, this results in quadruple production with two identical parts of the first type, in this case two sills on the left (FH-Ll and FH-L2), and two identical parts of the second type, in this case two sills on the right (FH-Rl and FH-R2). This typically occurs with relatively large but narrow, elongated parts such as sills, or with smaller parts of any suitable geometry for the left and right sides of the vehicle. For reasons of symmetry, force flow, and tool deflection, a left and a right part, which are usually largely mirror-symmetrical to one another, are arranged next to one another in the form Ll-Rl and L2-R2.For vehicle parts without a left-right variant, such as center tunnels, it would theoretically be possible to utilize the press table size, for example, by producing four identical parts (FH-1, FH-2, FH-3, and FH-4) or six identical parts, etc. However, this can lead to tolerance problems due to the high demands on the dimensional accuracy of identical parts.

[0025] Identical parts do not usually differ visually from one another and are generally not sorted by nest in the load carriers for subsequent vehicle construction, i.e. load carriers containing an unsorted mixture of identical parts arrive in vehicle construction for further processing.

[0026] Similar to cold-formed components, it is also important for hardened components for automotive construction, primarily for reasons of joining and dimensional accuracy, that the components not only lie within the specified dimensional tolerance, but also exhibit a certain degree of consistency within the specified tolerance. Ensuring this is more challenging for identical parts, since in addition to the deviations within the individual cavities, the maximum deviations between the cavities are also relevant.

[0027] This relationship can be seen in Figure 4 in the diagram, which shows the deviations of identical parts within the individual nests and all nests from each other.

[0028] In principle, the requirement for dimensional accuracy of the parts produced in multiple production increases with the number of identical parts, which results in additional tolerance restrictions and problems.

[0029] As mentioned at the beginning, with the indirect process it is imperative that the austenitized component blanks are inserted into the hot stamping tool at the correct temperature and thus in the correct size. If the temperatures are too high and thus the inserted component blanks of the batch are too large, this is usually corrected by extending the transfer time between the furnace and press and the associated reduction in the insertion temperatures. If the temperatures are too low and thus the inserted component blanks are too small, the transfer time between the furnace and press is usually shortened accordingly. It should be noted that extending or shortening the transfer time between the furnace and press always affects the entire batch.A change in the transfer time cannot therefore be used to correct the dimensional accuracy of only individual components in the respective batch, as all components in the batch are always affected. Another significant difference in multiple production is whether each cavity of the hot stamping tool obtains its component blanks from a specifically assigned cavity of the cold forming tool or whether several hot stamping tool cavities share a cavity of the cold forming tool, which is particularly relevant with regard to the effect of (trimming) corrections in cavities of the cold forming tool on hardened parts from cavities of the hot stamping tool. Some of these manufacturing combinations of KU and FH are shown as examples in Figure 3.

[0030] If each nest of the hot stamping tool obtains its component blanks from a specially assigned nest of the cold forming tool, for example by producing the component blanks of a left and a right B-pillar in each stroke in the cold forming process and by feeding a left and a right B-pillar as a batch through the furnace, transferring them to the hot stamping tool and hardening them together there, any dimensional deviation of individual hardened components of the respective batch, for example the left B-pillar, can be corrected by adjusting the specially assigned cold forming tool nest.

[0031] If several hot stamping tool nests share a nest of the cold forming tool, for example if the component blanks of a left and a right sill are produced in each stroke during cold forming, but two left and two right sills are fed through the furnace as a batch, transferred to the hot stamping tool and hardened there together, it must be noted that an adjustment of the cold forming tool nest affects all parts of the associated hot stamping nests, which means that a targeted correction of only individual hardened components of the respective batch is not possible.

[0032] Although it would theoretically be possible for each cavity of the hot stamping tool to obtain its component blanks from a specially assigned cavity of the cold forming tool in the case of identical part production, for logistical and cost reasons it is the rule for several hot stamping tool cavities to share one cavity of the cold forming tool in the case of identical part production in the hot stamping tool, i.e. multiple production with identical parts is usually realized with more than one hot stamping cavity per cold forming cavity for reasons of economy and confusion.

[0033] It has been determined that, in practice, temperature inhomogeneities within a batch at press closure, from furnace heating, from transfer between the furnace and press, or from other sources, cause dimensional deviations between the parts of a batch. Although economical multiple production would be possible for many parts given the available press bed size and press force, this is not implemented due to tolerance reasons, especially when producing identical parts.

[0034] The object of the invention is to provide a method for producing hardened steel components which improves the dimensional accuracy of the produced parts and enables more economical production.

[0035] The problem is solved by a method having the features of claim 1.

[0036] Advantageous further training is indicated in the dependent subclaims.

[0037] The inventors recognized that temperature inhomogeneities within a batch, as well as within a part, cause undesirable dimensional deviations. The inventors also recognized that, for the reasons mentioned above, this cannot be influenced by changing the transfer time between the furnace and the press for individual parts of the batch.

[0038] The inventors also recognized that the inhomogeneities are caused by the process itself, and that overlapping effects can occur. The invention also discovered that influencing the process by modifying the cold forming tools is not always advantageous, as this is the most expensive approach and is not always sufficient.

[0039] The solution according to the invention provides for mitigating or even eliminating the inhomogeneities within a batch, whereby this can be done in at least three identified areas of influence, namely firstly in the furnace, secondly during the transfer from the furnace to the die hardening tool and thirdly in the die hardening press from the insertion into the die hardening tool until the press or die hardening tool closes.

[0040] In this case, the inhomogeneities in the respective area of ​​influence can be influenced, as well as, in addition or alternatively, inhomogeneities that may arise in subsequent areas. For example, during furnace heating, temperatures of component blanks can be deliberately set higher locally to compensate for the local cooling that occurs during the subsequent transfer until the press closes. In the furnace, this can be influenced by the type of part arrangement, or, in addition or alternatively, by locally adapted heating, particularly in the last furnace zone of a continuous furnace or in the furnace chamber of a batch furnace.

[0041] Furthermore, during the transfer from the furnace to the hot stamping tool, devices mounted on the robot or transfer tool can be used, for example, by means of wind shields and / or local reflector plates, which prevent specific areas from cooling down, and / or devices for localized blowing to cool specific areas. A tempering station with devices for localized blowing, in front of which the batch is held during the transfer from the furnace to the hot stamping tool, is also conceivable. During the cooling measures, care must be taken to ensure that no undesirable foreign structures develop in the component blanks.

[0042] In the press, too, the material can still be influenced from the time it is inserted into the hardening tool until the press is closed, i.e. until the closing of the hardening press is complete, for example by local blowing.

[0043] The invention thus relates in particular to a method for producing hardened steel components, wherein flat steel sheet blanks are cut out from a flat steel strip made of a quench-hardenable steel alloy, the steel sheet blanks are formed into component blanks in a cold forming process and are completely or substantially completely trimmed, and then the component blanks thus obtained are heated in a heating device to a temperature at which the steel material is partially or completely converted into austenite, and the component blanks are then transferred from the heating device to a hot stamping press and placed in the hot stamping tool and cooled at a speed above the critical hardening speed and thereby hardened, wherein in order to improve the dimensional accuracy of the hardened components, in particular the dimensional accuracy of the outer trimmed edges and the hole pattern,at least one of the conditions a) to c) is met: a) the component blanks of a batch, each component blank individually and all component blanks to each other, have a maximum temperature difference of A3 until the closing of the hot stamping press is completed, where: A3 — (p / Lmax where q> is a shape constant in K*m and L, max is the largest dimension of the largest component blank in the batch in m, and the following values ​​apply to the shape constant q> depending on Lmax: from Lmax greater than 1 m: cp = 117.4 K*m, in particular (p = 65.2 K*m, preferably (p = 43.5 K*m and particularly preferably q> = 21.7 K*m, from Lmax greater than 0.5 m: (p = 65.2 K*m, preferably (p = 43.5 K*m and particularly preferably q> = 21.7 K*m, b) the component blanks of a batch, containing parts of a first type and parts of a second type, are arranged at least in the furnace such that in each case one part of the first type is located directly next to at least a second part of the first type and one part of the second type is located directly next to at least a second part of the second type, c) for the component blanks of a batch, containing parts of a first type and parts of a second type,the parts of the first type and the parts of the second type are cold-formed with different sizes or three-dimensional shapes due to the different thermal expansion and / or the parts of at least one type within the parts are cold-formed with different sizes or three-dimensional shapes such that size differences are substantially compensated at the time of completion of the closing of the hot stamping press.

[0044] The maximum temperature difference A3 from condition a) depends on the size of the component blanks and is therefore expressed as a formula. Depending on customer requirements, a smaller or larger tolerance range is permitted on the hardened component. This is taken into account by selecting the form constant q> depending on the largest dimension of the component blank. The form constant D only applies to component blanks with a largest dimension greater than 1000 mm. The other form constants are already used for component blanks with a largest dimension greater than 500 mm.

[0045] As an example, the following calculation is made: For a component blank whose largest dimension is 1100 mm, at (p = 117.4 K*m the maximum tolerated temperature difference or the maximum tolerated temperature delta is 106.7 K. At (p = 65.2 K*m the maximum temperature delta is 59.3 K. At (p = 43.5 K*m this corresponds to 39.5 K. At q> = 21.7 K*m, i.e. the tightest tolerance which leads to the best results, the maximum temperature delta is 19.8 K.

[0046] In general, it should be noted that the temperatures of the component blanks within a batch in condition a) refer both to the surface temperatures of a component blank across its entire surface and to the temperatures of the entire surfaces of all component blanks in a batch. The maximum temperature difference, i.e., the maximum delta of the temperatures of the component blanks within a batch, is the difference between the highest and lowest temperatures.

[0047] A further development provides that, in condition a), the component blanks or areas of component blanks whose temperature would be colder at the time the hot stamping press closes are additionally heated in the furnace. This can advantageously be achieved using locally adjustable heating elements, such as electric heaters. The temperature of the colder areas in the furnace can reach the temperature of the remaining areas or, if necessary, even exceed it, if compensation for inhomogeneities from the subsequent transfer is required. Advantageously, the adjustment of the additional heating can thus be carried out simply, flexibly, and repeatably.

[0048] A further development provides that, in condition a), the component blanks or areas of component blanks whose temperature would be warmer at the time the hot stamping press closes are actively cooled during the transfer. This advantageously allows for simple, flexible, and repeatable adjustment of the active cooling.

[0049] A further development provides that, in condition a), the component blanks or areas of component blanks whose temperature would be colder at the time of the closing of the hot stamping press are largely protected from cooling during the transfer, particularly through heat loss due to forced convection and / or thermal radiation, preferably through the use of wind shields and / or reflector plates. Advantageously, preventing cooling can thus be achieved simply and repeatably.

[0050] A further development provides that, in condition a), the component blanks or areas of component blanks whose temperature would be warmer at the time of completion of the closing of the hot stamping press are actively cooled in the press before the hot stamping press is closed. Advantageously, the adjustment of the active cooling can thus be carried out simply, flexibly, and repeatably.

[0051] A further development provides that in condition b) in a batch containing parts of a first type and parts of a second type, the parts of the first type and the parts of the second type are each arranged directly next to each other and one behind the other in the direction of flow or conveying of the furnace.

[0052] A further development provides that the component blanks of a batch are arranged next to each other in the furnace in the direction of flow or conveyance of the furnace.

[0053] A further development provides that in the heating device the component blanks or areas of component blanks that are arranged closer to an exit door are additionally heated.

[0054] A further development provides for component blanks from a cold forming cavity to be hardened in at least two different hot stamping tool cavities. This can advantageously increase the cost-effectiveness of the process and eliminate the risk of confusion when selecting component blanks.

[0055] A further development stipulates that at least two, in particular all three, conditions a) to c) are met. Each of the three conditions a) to c) relates to improving the dimensional accuracy of the hardened components and each represents an improvement in its own right. However, the dimensional accuracy of the hardened components can be further improved if two of the three conditions, for example, condition a) and b) or a) and c) or b) and c), are met. The best dimensional accuracy and thus the smallest tolerance deviation of all hardened parts within a batch can be guaranteed if all three conditions are met.

[0056] A further development provides for the component blank to be placed on a storage device, in particular a lifting device in the hot-press hardening tool, to prevent uncontrolled cooling of the component blank from the time of insertion until the closing of the hot-press hardening tool before the hardening process. This can represent a simple, robust, and repeatable method for the most controlled and reduced cooling of the component blanks upon insertion into the tool. The advantageously spring-loaded lifting devices, which can then be retracted into the tool upon press closure, ensure both secure storage and minimal temperature loss.

[0057] A further development provides that the sheet steel plate used is a plate formed with a metallic corrosion protection layer, in particular with a metallic corrosion protection layer based on or made of zinc. This can advantageously ensure good protection against corrosion, in particular cathodic corrosion protection.

[0058] A further development provides that the steel strip is made of a hardenable steel alloy, in particular a boron-manganese steel.

[0059] A further development provides that a material is used as the steel material which is transformation-retarded with regard to the austenite-martensite transformation during cooling, whereby the strip is hot-rolled or cold-rolled.

[0060] A further development provides for the use of a steel strip with the following composition (all data in % by weight):

[0061] 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

[0062] Nickel up to 0.9, titanium up to 0.2, preferably 0.02 to 0.1

[0063] 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 to 0.005 Sulphur max. 0.01, preferably max. 0.008

[0064] Phosphorus max. 0.025, preferably max. 0.01

[0065] Residual iron and impurities

[0066] A further development provides that the device comprises a cold forming tool, a furnace, a transfer device for transferring the component blanks, and a hot stamping tool. A storage device, in particular a lifting device, is provided in the hot stamping tool to prevent uncontrolled cooling of the component blank from the time of insertion until the closing of the hot stamping tool before the hardening process. This can represent a simple, robust, and cost-effective device for the most controlled and reduced cooling of the component blanks upon insertion into the tool. The advantageously spring-loaded lifting devices, which can then be retracted into the tool upon press closure, can ensure both secure storage and minimal temperature loss.

[0067] The invention is explained by way of example with reference to a drawing. It shows:

[0068] Figure 1: a dilatometer curve of an exemplary steel material, which is

[0069] quench hardening is hardenable;

[0070] Figure 2: the mold hardening process in a highly schematic form, with a continuous furnace and a linear transfer in a quadruple production with four parts per batch, with the parts arranged next to each other in the direction of flow;

[0071] Figure 3: known procedures for multiple production, from cold forming through hot stamping to delivery or supply in vehicle construction and corresponding quality requirements for the parts;

[0072] Figure 4: a diagram showing the deviations within the individual cavities and between all four cavities of a component with identical part production;

[0073] Figure 5: Examples of inhomogeneities identified according to the invention from the furnace and

[0074] Transfer between oven and press;

[0075] Figure 6: Effects of inhomogeneities depending on part size and part

[0076] Arrangement;

[0077] Figure 7: the comparison of parts arrangements according to the prior art and a parts arrangement according to the invention with the respective effects on output and dimensional deviations within the identical parts;

[0078] Figure 8: a comparison of a parts arrangement according to the prior art and a parts arrangement according to the invention with the respective effect on the dimensional deviation of the identical parts and the deviation of the identical parts with adapted cold forming in diagram form;

[0079] Figure 9: highly schematic illustration of a heating system adapted according to the invention in a continuous furnace

[0080] (DLO) and multi-layer chamber furnace (MLK);

[0081] Figure 10: Measures according to the invention during the transfer;

[0082] Figure 11: a local blowing system in the press according to the invention; Figure 12: the arrangement and treatment of non-side-specific identical parts according to the invention;

[0083] Figure 13: the representation of the maximum temperature delta within a batch in

[0084] Dependence of the dimension of the largest component blank in the batch with four different form constants.

[0085] Figure 1 shows a dilatometer curve, i.e. the temperature-length change curve of a heated and subsequently cooled sample of a quench-hardenable steel material. To determine the dilatometer curve, the sample is heated inductively at heating rates as close to series production as possible and, if necessary, cooled by blowing a gas over the induction coil. The change in length caused by the heating and cooling of the sample is recorded using measuring pins and a displacement sensor. The temperature at the dilatometer is controlled by a thermocouple applied to the surface of the sample. The temperature-length change diagram recorded throughout the entire measurement, the dilatometer curve, is the result of the measurement. The annotated dilatometer curve shows which length and / or length changes occurred.Volume changes experienced by a cold-formed component blank in the furnace and during the subsequent transfer from the furnace to the hot-press hardening tool until the hot-press hardening tool is closed. In the furnace, thermal expansion initially occurs until austenitization begins (a), shrinkage occurs during the microstructure transformation to austenite (b), further heating until leaving the furnace is followed by further thermal expansion (c), and after leaving the furnace, i.e. during transfer from the furnace to the press-hardening or hot-press hardening tool, a corresponding shrinkage occurs during cooling to the respective insertion temperature (d). The insertion temperature within the meaning of the invention in the indirect process is the average temperature of the component blank, weighted according to its surface area, after its insertion into the tool and shortly before reaching the bottom dead center of the tool or press. Based on the ratio between the insertion length dh length at insertion temperature and the initial length dhThe length before heating can be used to determine how much the cold forming tools for producing the component blanks need to be scaled down so that they fit onto the hot stamping tool after heating in the furnace and subsequent cooling. The upper section of the image shows a highly schematic view of the hot stamping process. In this process, a flat steel strip is unwound from a coil, blanks are then cut out of this flat steel strip, and the flat blanks are then cold formed (KU) in a multi-stage forming and trimming process, with the outer trim and hole pattern being finished. The component blanks produced in this way are then placed in a furnace, removed from the furnace while hot, placed in a hot stamping tool, and there they are quickly cooled by applying an upper and lower tool to the formed part.

[0086] As indicated in the lower section of the image in Figure 2, individual parts do not pass through the furnace one after the other. Instead, in order to increase throughput and operate the process as economically as possible, a number of component blanks - four parts in the example in Figure 2 - are fed into the furnace. They are then removed from the furnace simultaneously and placed in the hot stamping tool, preferably in the same arrangement, according to the direction of travel (DLR), and hardened. As already explained, the entire component contour, i.e. the three-dimensional shape, including the outer trim and hole pattern, must fit when the heated component blank is placed in the hot stamping tool, as there is no further possibility of correction afterwards.

[0087] Figure 3 shows which production or tool cavity combinations of cold forming (KU) and hot stamping (FH) are common, and the quality requirements that typically result from them. The requirement for dimensional accuracy of the produced parts increases with the number of identical parts in multiple production, resulting in additional tolerance restrictions.

[0088] In a first case, for example with relatively large, bulky parts such as two B-pillars for the left and right sides of the vehicle, there is only one tool cavity in the hot stamping tool for each hardened component. In this case, hot stamping involves dual production but no identical part production. Since there is no identical part production, standard quality requirements apply to the hardened components. Since in this case each cavity of the hot stamping tool draws its component blanks from a specially assigned cavity of the cold forming tool, any necessary corrections for the hardened finished part, for example concerning scaling and / or trimming, can be made in the respective plastic cavity. The output is comparatively low with two hardened parts per stroke.For further processing in vehicle construction, the hardened parts are usually sorted into right and left parts and delivered separately in load carriers.

[0089] Figure 3 above describes multiple production with one part each, each with a different side, i.e., a left- and right-side specific part. This could, for example, be a left and right B-pillar. One cavity is provided for each part in the cold forming process, and one cavity is provided for each part in the hot-forming process. Strictly speaking, cold-formed left-hand parts are then hot-formed in the corresponding, left-hand-specific shape; the same applies to parts with a right-specific shape.

[0090] In multiple production with identical parts (Figure 3 middle), one nest is provided for each part during cold forming; two or more nests can be provided for each part during hot stamping. In the example shown, for a sill for the left and right sides of the vehicle, for example, there are two or more tool nests each during hot stamping, i.e. there is at least quadruple production with at least two identical parts each. This usually occurs with relatively large but narrow, elongated parts such as sills or with smaller parts of any suitable geometry for the left and right sides of the vehicle. For further processing in vehicle construction, these hardened parts are also usually only sorted into right and left parts and thus delivered separately from one another in load carriers.The quality requirements are higher here, with the number of identical parts increasing. On the one hand, consistency must be ensured for each part on the left and right sides. On the other hand, this is more challenging with identical parts, since in addition to the deviations within the individual nests, the maximum deviation between the nests is also relevant. This can also be seen in Figure 4.

[0091] In multiple production with identical parts without a left-right variant (Figure 3 below), for example in the case of center tunnels, it would theoretically be possible to utilize the press table size by, for example, carrying out quadruple production with four identical parts or even six identical parts, etc. However, this can lead to tolerance problems due to the very high demands on the dimensional accuracy of identical parts.

[0092] The inventors have recognized that tolerance deviations in the processes discussed can be due to several causes (Figure 5). Regarding the furnace and the transfer (Figure 5), it was determined that the parts conveyed through the furnace and positioned toward the furnace door have a hotter side and a cooler side, with the temperature drop being noticeable toward the furnace door.

[0093] The same phenomenon was observed due to the movement from the furnace during transfer to the die hardening tool, i.e. due to the air movement (airstream) caused by the movement and the associated forced convection, the areas of the formed and heated parts which were previously closer to the furnace door and are conveyed out of the furnace first and are at the front in relation to the direction of passage are cooled even more, so that this effect is further enhanced.

[0094] In addition, a local cooling influence of the furnace support on which the parts rest can be observed (Figure 5 bottom left), which results from the diagram in Figure 5 bottom left.

[0095] These effects can therefore overlap and apply not only to the continuous furnace and the linear transfer shown, but also to multi-layer chamber furnaces and a non-linear robot transfer not shown.

[0096] In general, it has been found that the absolute length deviation due to temperature deviations is proportional to the part size, meaning that incorrect deviations in the insertion temperature have a more serious impact on large or long parts than on small, short parts. The arrangement within the furnace also has an influence.

[0097] Figure 6 above shows parts arranged side by side in the direction of travel and relatively short, which exhibit a temperature drop toward the furnace door. The absolute length deviation is small to medium, as the parts are only short to medium in length and, in this orientation, the parts exhibit identical temperature profiles and thus identical deviations. When such parts are manufactured in large series, it is even economically feasible to account for such length changes in the cold forming tool and scale the cold forming tool accordingly, eliminating the need for further post-processing at a later date.

[0098] The temperature profile is shown in Figure 6 above in the diagram.

[0099] The situation is fundamentally different if the parts pass through the furnace one after the other in the direction of travel, for example, because the parts are particularly long. The temperature profile, i.e., the temperatures that result differently for each part, is shown in Figure 6 below.

[0100] Since the parts vary in length due to the different temperatures and the associated expansion, the absolute length deviation within the part group is medium to high and increases with the length of the parts. The parts therefore generally have different temperatures and thus each have different deviations. Correcting these identical parts in the cold forming process is therefore fundamentally problematic.

[0101] According to the invention, the parts are influenced, particularly in the area of ​​the furnace, the transfer, and the hot-forming tool, in such a way that inhomogeneities within a batch are eliminated or mitigated and, in particular, the temperature deviation is reduced to below the respective maximum temperature delta A3. To this end, it is generally intended to influence the heating process through the type of part arrangement and, if necessary, locally adapted heating in the last furnace zone of a continuous furnace or the furnace chamber in a multi-layer chamber furnace.

[0102] During transfer, airflow can be shielded, local reflector plates can be used to keep heat in specific areas, or localized airflow can be used. Local reflector plates can also be adjusted, for example, by varying the spacing, perforations, production burrs, and the like.

[0103] In the press, too, local blowing or, if necessary, infrared radiators acting on the part can be used from the time of insertion until the press is closed.

[0104] Figure 7 shows the effect of the arrangement of parts in the prior art and according to the invention.

[0105] In the current state of the art, both in a double and a quadruple configuration, a left and a right part are typically produced in the cold forming tool. With a double conveying system, these parts are also conveyed from left to right through the furnace and then into the hot stamping tool. To correct deviations in the finished part from left to right, the cold forming tool can typically be scaled differently from left to right, for example, the right larger than the left. The entire process is comparatively undemanding in terms of tolerances, since there are no identical parts. The disadvantage, however, is that only two parts can be produced per stroke.In the quadruple pass according to the state of the art (Figure 7 middle), for example, two parts, namely left and right, are manufactured in the cold forming tool and then placed in the furnace according to the sequence left one, right one, left two, right two, passed through the furnace, transferred to the die hardening tool and then die hardened accordingly in the die hardening tool.

[0106] By providing four parts per stroke, the output is high, but the tolerances are very demanding, as there are two identical parts in each stroke. When the temperature drops toward the furnace door, the identical parts exhibit greater differences from one another, which often exceeds the acceptable tolerances. Figure 8 above shows the temperature distribution in the arrangement and the corresponding deviation in the identical parts. The general deviation between right and left can be adjusted through cold forming, although the differences in the identical parts cannot be adjusted here.

[0107] The quadruple production according to the invention (Figure 7, bottom) provides for producing left and right parts in the cold forming process. However, instead of arranging them alternately in the furnace, the identical parts are arranged next to one another in the furnace and also next to one another in the hot stamping tool. The advantage of this is that the output is high. If the temperature drops towards the furnace door, the identical parts also exhibit small differences from one another. Here, too, the tolerances are demanding because each part is made up of two identical parts. Overall, however, this process is easier to control because, as shown in Figure 8, bottom center, the relative differences between the identical parts are not so great and, as can be seen, the differences in the identical parts can be significantly reduced through adapted cold forming (Figure 8, bottom right). Thus, with this measure alone, a very high level of uniformity can be achieved according to the invention.

[0108] Figure 8 shows a diagrammatic comparison of a parts arrangement according to the prior art and a parts arrangement according to the invention, showing their respective effects on the dimensional deviation of the identical parts and the deviation of the identical parts with adapted cold forming. It can be seen that the length deviation is considerably greater when producing identical parts according to the prior art than with the method according to the invention. The length deviations can be reduced accordingly using the parts arrangement according to the invention, even with adapted cold forming (right-hand image). A further possibility according to the invention arises, as shown in Figure 9, in that parts inside the furnace, i.e. closer to the furnace door, are heated more intensively using appropriate additional heating elements. As an example, locally adapted heating in the last zone of a continuous furnace (DLO) orin the chamber of a multi-layer chamber furnace (MLK) in order to eliminate or mitigate inhomogeneities within a batch that have arisen in the furnace or during transfer until the press closes during mold hardening.

[0109] This explicitly does not mean that the parts should be heated to different temperatures in order to achieve different properties later on - this is well known - but rather it is about transferring the parts from the furnace with a uniform heat or heating the parts that cool down more quickly during transfer to a significantly higher temperature so that the parts are at the same temperature in the die hardening tool when the press closes.

[0110] This measure can, of course, be combined with the previously described measure of part arrangement in the furnaces. This can then make adapted cold forming obsolete, especially if the cold forming tools already exist and an adjustment is to be made during an ongoing process.

[0111] Alternatively or additionally, it is possible, as shown in Figure 10 above, to protect the parts, which are moved together from the furnace into the hot stamping tool, from forced convection by "headwind" during transfer by means of a corresponding partial or full-surface hood, also called a windshield. This can be useful, for example, if all parts emerge from the furnace at a very homogeneous temperature.

[0112] If the parts come out of the oven at different temperatures, as shown in Figure 10 middle, for example, the parts or areas that usually cool down more rapidly or come out of the oven colder can be kept warm by appropriate reflectors, which may also be adapted by perforation or different emissivities, while the parts that are subject to less cooling or come out of the oven at a higher temperature are allowed to cool down by convection.

[0113] In addition, it is also possible, as shown in Figure 10 below, to cool the more heated parts by means of blowing, whereby this can be adjusted by means of the amount of air, the distance and the like. Alternatively or additionally, as shown in Figure 11, the temperature of the parts can also be influenced in the hot stamping tool or the hot stamping press until the tool is finally closed. For example, the hotter parts or areas can be specifically cooled by blowing. In this case, the parts are still stored on the spring-loaded lifters until the press closes and can be blown relatively easily. Alternatively or additionally, it is also possible to keep parts or areas warm or at least allow them to cool down more slowly using infrared radiators (not shown), which may act on the parts from the outside.

[0114] Figure 12 summarizes the difference compared to the state of the art for parts that are not specified as right or left, but are identical from the outset, for example, center tunnels arranged one behind the other in the direction of flow. In these cases, the number of identical parts and thus the need for homogenization for cost-effective production with the appropriate quality is even higher.

[0115] When manufacturing four identical parts using state-of-the-art technology, the yield is high, and the tolerances are very demanding, since there are four identical parts and no possibility of correction during cold forming, for the reasons already mentioned. Such quadruple production is de facto impossible in the state of the art.

[0116] According to the invention, the process can be carried out in the furnace, the transfer mold, and the hot-forming tool, as described above. This makes it possible to work with a high output, namely four parts per stroke, although this can only be achieved using the measures described above. Here, too, the tolerances are very demanding, and correction in the cold-forming tool is impossible.

[0117] The invention therefore has the advantage that it is possible to produce many parts economically, both with positional specification and as identical parts without positional specification.

[0118] The advantage of this is that tolerances are kept low and therefore less waste is produced.

[0119] Figure 13 graphically shows the maximum temperature delta A3 within a batch with four different form constants as a function of the largest dimension of the respective component. The largest dimension of the largest component blank within a batch represents the calculation basis; this is 1800 mm, for example. Four different form constants are then possible for this, which result in different component tolerances. These maximum temperature deltas are dependent on the largest dimension of the largest component blank within a batch and are inversely proportional to the dimension. The function with the form constant D, shown in the diagram as ATHETA_D, is the most tolerant up to the function ATHETA_A with the insert form constant A, which is the narrowest form constant and leads to the most precise component geometries.The shape constant D only applies to component blanks with a maximum dimension greater than 1000 mm. The other shape constants also apply to component blanks with a maximum dimension greater than 500 mm.

[0120] In the example of the longitudinal dimension of 1800 mm, the maximum temperature delta at press closure for the respective form constants is as follows:

[0121] Shape constant A: A3_A_1800mm = 12.1 K

[0122] Shape constant B: A3_B_1800mm = 24.2 K

[0123] Shape constant C: A3_C_1800mm = 36.2 K

[0124] Shape constant D: A3_D_1800mm = 65.2 K

[0125] This means that the temperatures of the component blanks, both individually and all within a batch, must, according to the invention, lie within a temperature range of this width.

Claims

Claims 1. A method for producing hardened steel components, wherein flat steel sheet blanks are cut out from a flat steel strip made of a quench-hardenable steel alloy, the steel sheet blanks are formed into component blanks in a cold forming process and are completely or substantially completely trimmed and then the component blanks thus obtained are heated in a heating device to a temperature at which the steel material is partially or completely converted into austenite and the component blanks are then Heating device to a hot-melt press and placed in the hot-melt tool and cooled at a speed above the critical hardening speed and thereby hardened, characterized in that in order to improve the dimensional accuracy of the hardened components, in particular the dimensional accuracy of the outer trimmed edges and the hole pattern, at least one of the conditions a) to c) is met: a) the component blanks of a batch, each component blank individually and all component blanks to each other, have a maximum temperature difference of A3 until the closing of the hot-melt press is completed, where: A3 = (p / Lmax where q> is a shape constant in K*m and L max is the largest dimension of the largest component blank in the batch in m, and the following values apply to the shape constant cp depending on Lmax: from Lmax greater than 1 m: cp = 117.4 K*m, in particular cp = 65.2 K*m, preferably cp = 43.5 K*m and particularly preferably q> = 21.7 K*m, from Lmax greater than 0.5 m: cp = 65.2 K*m, preferably cp = 43.5 K*m and particularly preferably cp = 21.7 K*m, b) the component blanks of a batch, containing parts of a first type and parts of a second type, are arranged at least in the furnace in such a way that in each case a part of the first type is directly next to at least a second part of the first type and a part of a second type is located directly next to at least a second part of a second type, c) in the case of the component blanks of a batch containing parts of a first type and parts of a second type, the parts of the first type and the parts of the second type are cold-formed with different sizes or three-dimensional shapes due to the different thermal expansion and / or the parts of at least one type within the parts are cold-formed with different sizes or three-dimensional shapes in such a way that size differences are substantially compensated for at the time of completion of the closing of the hot stamping press.

2. Method according to claim 1, characterized in that in condition a) the component blanks or regions of component blanks whose temperature would be colder at the time of completion of the closing of the hot stamping press are additionally heated in the furnace.

3. Method according to claim 1 or 2, characterized in that in condition a) the component blanks or regions of component blanks whose temperature would be warmer at the time of completion of the closing of the hot stamping press are actively cooled during the transfer.

4. Method according to one of the preceding claims, characterized in that in condition a) the component blanks or regions of component blanks whose temperature would be colder at the time of completion of the closing of the hot stamping press are largely protected from cooling during the transfer, in particular by heat losses due to forced convection and / or thermal radiation, preferably by the use of wind shields and / or reflector plates.

5. Method according to claim 1, characterized in that in condition a) the component blanks or regions of component blanks whose temperature would be warmer at the time of completion of the closing of the hot stamping press are actively cooled in the press before completion of the closing of the hot stamping press.

6. A method according to claim 1, characterized in that in condition b) in a batch containing parts of a first type and parts of a second type, the parts of the first type and the parts of the second type are arranged directly next to each other and one behind the other in the direction of flow or conveying of the furnace.

7. Method according to claims 1 to 5, characterized in that the component blanks of a batch are arranged next to one another in the furnace in the direction of flow or conveyance of the furnace.

8. Method according to claim 2, characterized in that in the heating device the component blanks or regions of component blanks which are arranged closer to an exit door are additionally heated.

9. Method according to one of the preceding claims, characterized in that component blanks from a cold forming nest are hardened in at least two different form hardening tool nests.

10. Method according to one of the preceding claims, characterized in that at least two, in particular all three, conditions a) to c) are met.

11. Method according to one of the preceding claims, characterized in that the component blank is placed on a depositing device, in particular a lifting device in the form hardening tool, to avoid uncontrolled cooling of the component blank from the insertion until the closing of the form hardening tool before the hardening process.

12. Method according to one of the preceding claims, characterized in that the sheet steel plate used is a plate which is formed with a metallic corrosion protection layer, in particular with a metallic corrosion protection layer based on zinc or made of zinc.

13. Method according to one of the preceding claims, characterized in that the steel strip is formed from a hardenable steel alloy, in particular a boron-manganese steel.

14. The method according to claim 13, characterized in that the steel material used is a material which is transformation-retarded with respect to the austenite-martensite transformation during cooling, wherein the strip is hot-rolled or cold-rolled.

15. A method according to claim 13 or 14, characterized in that the steel strip used is a strip with the following composition (all data 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 to 0.005 Sulphur max. 0.01, preferably max. 0.008 Phosphorus max. 0.025, preferably max. 0.01 Residual iron and impurities 16. Device in particular for carrying out a method according to claims 1 to 15, wherein the device comprises a cold forming tool, a furnace, a transfer device for transferring the component blanks and a form hardening tool, characterized in that a storage device, in particular a lifting device, is provided in the form hardening tool to prevent uncontrolled cooling of the component blank from the insertion until the form hardening tool is closed before the hardening process.

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