Component with locally limmited reinforcement regions and method for production thereof
The invention relates to a component consisting of a high-strength sheet, in particular a structural component, which is provided with a deformation structure in a locally limited stiffening region, this deformation structure considerably increasing the local stiffness of the component compared with conventional components. The stiffness-increasing deformation structure consists of a periodic grid of concave and convex bulges nested one inside the other. It is embossed in certain, locally limited regions of the sheet by means of a deep-drawing method. The regular periodic grid structure of the stiffening pattern permits a computational simulation of the stiffnesses to be achieved and, as a consequence thereof, a systematic optimization of the stiffening structure for the respective component. The bulge depths may be locally varied inside the stiffening structure, as a result of which local variations in the stiffness of the component are specifically achieved inside the stiffening region.
 The invention relates to a component consisting of a high-strength sheet which is provided with a stiffness-increasing deformation structure in a locally limited stiffening region and to a method of producing it.
 Many components in automobile construction, in particular structural components, must meet high demands with regard to both strength and stiffness. At the same time, there is considerable interest in realizing lightweight construction concepts in vehicle building and therefore in reducing the weight of these parts as much as possible. The desired strength, with simultaneous reduction in weight, can be achieved if thin sheets made of high-tensile steels are used as the starting material, these sheets having a strength comparable with thicker sheets made of conventional steels. However, these thinner sheets, in order to achieve the required stiffness, must be provided with stiffness-increasing structures, such as, for example, beads and/or studs. Since high stiffness is only required in selected regions of the components as a rule, whereas in other regions the stiffness is only of secondary importance, it is advantageous to provide these stiffness-increasing structures only in those regions of the components which are subjected to particular loads with regard to stiffness during operation.
 The local stiffness increase of components made of sheet metal has been disclosed by DE 297 12 622 U1 establishing the generic type. In this publication, it is proposed to provide the sheet-metal billets with wart-shaped studs in selected regions before the forming of the component geometries, these wart-shaped studs being incorporated in the sheet-metal billets by means of an embossing method. Since the stud structure is largely lost during the subsequent shaping of the component geometry, e.g. by means of a drawing method, this stud stiffening is suitable mainly for those regions which are not formed or are not formed to an appreciable extent in the following forming process.
 The production of a locally stiffness-increased component according to DE 297 12 622 U1 therefore consists of two process steps—namely the embossing of the sheet-metal billet with a stud structure followed by the forming of the sheet-metal billet to produce the component geometry—and is therefore relatively expensive. Furthermore, there is often the requirement to also provide in particular the forming regions on the component with stiffness-increasing structures, which is not possible with the method proposed in DE 297 12 622 U1. Finally, there is the requirement to achieve a greater local stiffness increase, in particular a greater increase in the flexural stiffness, which cannot be achieved with the stud structures shown in DE 297 12 622 U1.
 DE 196 34 244 discloses a method for the stiffness-increasing texturing of sheets, by means of which a sheet-metal billet is textured with bulges from both sides in several stages. In this case, periodic patterns of large bulges are produced, in the hollows of which bulges small bulges form from the opposite side. Although this surface structure ensures very good compressive and flexural stiffness, the bulging method proposed for producing it can only be applied to very thin sheets and is therefore not suitable for the stiffness increase of structural components, e.g. for vehicle building. Furthermore, DE 196 34 244 describes a bulging method by a continuous process in which the entire surface of a crude sheet is provided with bulges. Therefore, on the one hand, no specific local stiffness increase of the crude sheet is possible; on the other hand, the bulge structure and thus the stiffness increase achieved would be lost in a forming process following the bulging process.
 The object of the invention is therefore to produce sheet-metal components having specifically incorporated, spatially limited stiffening regions which have a considerable increase in the local stiffness compared with conventional components provided with local stiffening regions. The object of the invention is also to propose a simple method of achieving such a local stiffness increase on sheet-metal components.
 This object is achieved according to the invention by the features of claims 1 and 3.
 Accordingly, the surface of the component is provided in selected regions with a stiffening structure which consists of a periodic grid of concave and convex bulges nested one inside the other. Such a stiffening structure ensures a considerable stiffness increase compared with the studs and beads incorporated in a conventional manner. This relates both to flexural and compressive stiffness and to stiffness against twisting. Furthermore, the regular periodic grid structure of the stiffening pattern permits a computational simulation of the stiffnesses achieved in the process and, as a consequence thereof, a systematic optimization of the stiffening structure for the respective component. The stiffening structure can be characterized by means of a few parameters (bulge radii and depths, grid constant of the stiffening structure, and orientation of the grid direction relative to the component), so that the parameters required for a certain local stiffness can be determined at the preliminary stages of the component production by means of a simulation. Furthermore, the bulge depths inside the stiffening structure can be locally varied, as a result of which specifically local variations in the stiffness can be achieved inside the stiffening region.
 A stiffening structure which is especially simple to simulate and which at the same time ensures a high stiffness in all spatial directions is a pattern of bulges nested one inside the other on a hexagonal grid (see claim 2).
 The stiffening structure is produced on the component by means of a drawing method (see claim 3): when the press punch is lowered during the course of the drawing method, sufficiently high forces can be applied in order to also provide sheets of high-tensile steel which are several millimeters thick with the above-described complex stiffening structures in a controlled manner in terms of the process. The method can therefore be applied to any desired sheets, as long as the sheets are made of a material capable of being drawn.
 It is especially favorable with regard to the production costs of the component if the shaping of the component geometry and the incorporation of the stiffening structure is effected in a single operation which essentially corresponds to a deep-drawing operation (see claim 4). In this case, the stiffening structure, which, depending on the stiffening increase required, projects beyond the surrounding component region by 2 to 4 mm, is formed in the sheet-metal part at the final pressure of the deep-drawing process. The high pressures occurring in the process produce an additional crystalline change in the sheet-metal structure, a factor which additionally contributes to the stiffness increase of the component. Furthermore, this incorporation of the stiffening structure during the shaping of the component geometry permits the local stiffening of component surfaces having any desired curvature.
 The invention is explained in more detail below with reference to an exemplary embodiment shown in the drawings, in which:
 FIG. 1 shows a detail of a side sheet of a longitudinal member with a local stiffness-increasing deformation structure,
 FIG. 2 shows a lateral section through a side sheet along section line II-II in FIG. 1,
 FIG. 3 shows an alternative configuration of the deformation structure,
 FIG. 4 shows a schematic representation of a deep-drawing tool for producing a side sheet for the longitudinal member in FIG. 1.
 FIG. 1 shows a detail of a longitudinal member 1 which is made of steel sheet and forms part of a vehicle frame of a goods vehicle. The longitudinal member 1 consists of a plurality of individual parts 2 produced by means of a deep-drawing method and contains in particular a side sheet 2′ which is connected to further individual parts (not shown in FIG. 1) of the longitudinal member 1 by welding. The longitudinal member 1 must fulfill certain criteria with regard to strength and with regard to stiffness, but at the same time, in the interests of minimizing weight, is to have as small a sheet thickness as possible. The individual parts 2, 2′ are therefore made of a high-tensile steel which, even in the case of small sheet thicknesses, has a comparatively high strength combined with good formability. The stiffness losses in the longitudinal member 1 which occur as a result of the small sheet thickness are compensated for by local stiffness-increasing deformation structures 3 which are embossed in selected regions of the individual parts 2 by means of a deep-drawing method.
 Especially high stiffness requirements are imposed on the individual parts 2 at those regions 4 which are subjected to especially high compressive and torsional loads during operation, especially in the event of an accident. In the present example of the longitudinal longitudinal member 1, this concerns in particular the center region 5, in which the longitudinal member 1 is of S-shaped configuration and in which an attachment 2″ is fastened, this attachment 2″ serving for the fastening of a cross member (not shown in FIG. 1). As indicated by arrows in FIG. 1, the main loading direction lies along the longitudinal axes of the longitudinal-member regions 6 which adjoin the center region 5. On account of the S-structure, the center region 5 is especially susceptible to lateral buckling under such loads, this buckling directly resulting in displacement or twisting of the cross member.
 In order to suppress the occurrence of lateral buckling in the center region 5 of the longitudinal member 1, the side sheet 2′ is provided in the center region 5 with a stiffness-increasing deformation structure 3. The hexagonal structure 3′ used in this application consists of a hexagonal grid of concave bulges 7, the hollows of which are provided with convex opposing bulges 8, so that the structure 3′, as shown in FIG. 2 in a sectional view, is formed from a grid of concave and convex bulges 7, 8 nested one inside the other. The depth 9 of the bulges 7 and the height 10 of the opposing bulges 8 vary over the center region 5, so that the depth 9 of the bulges 7 and the height 10 of the opposing bulges 8 in the center 11 of the center region 5 are greater than in the marginal zones 12 of the center region 5. As a result, a greater stiffness increase is achieved in the center 11 (especially sensitive to buckling) of the center region 5 than in the marginal zones 12 (not so sensitive to buckling), so that the side sheet 2′ in the entire center region 5 sets up a compensating resistance to a deformation force which acts on the longitudinal member 1 from the outside. The stiffness-increasing deformation structure 3′ is oriented to the side sheet 2′ in such a way that the direction of the maximum compressive stiffness lies approximately perpendicularly to the buckling direction 13 to be expected.
 The local stiffness increase, which is produced in a selected region 4 by the hexagonal structure 3′, depends on the depth 9 and the radius 14 of the bulges 7, on the height 10 and the radius 15 of the opposing bulges 8, and on the base length 16 of the hexagonal grid; furthermore, the local stiffness increase is not isotropic, but rather depends on the orientation of the grid relative to the direction of the introduction of force, which is identified by the arrows in the example of the longitudinal member 1 in FIG. 1. In order to obtain a stiffening deformation structure 3 optimized for a particular application, the abovementioned parameters have to be matched to this application. To this end, a simulation of the relevant individual part 2 (or of the component composed of the individual parts) and of the stiffening structure 3 is carried out, and the parameter setting is varied until the desired stiffness of selected regions 4 or the desired buckling behavior of the entire component is achieved.
 In principle, the stiffening structure 3 may have any desired grid structure and symmetry. However, in order to permit a quick and reliable simulation of the component stiffness thus achieved (and therefore to permit optimization of the component under load), it is favorable to select a grid which has translational and rotational symmetry and which can be characterized by a few parameters. In addition to the hexagonal grid shown in FIGS. 1 and 2, in particular quadrilateral and triangular structures are especially suitable for this purpose. Whereas no interplay between larger and smaller grid cells is possible when using hexagonal grids, grid cells 17 of different size can be combined in particular rectangular grid structures, as shown in FIG. 3, so that differentiated adaptation of the local stiffness of the relevant regions is possible in this case.
 The individual parts 2, the stiffness of which are to be locally increased in a specific manner by means of deformation structures 3, are often parts of structural components and therefore—depending on the function of the component—have sheet thicknesses up to several mm. In order to provide such thick sheets with the complex deformation structures 3′ shown in FIG. 1, a method which exerts high deformation forces on the sheet must be applied. To this end, it is especially favorable to incorporate the deformation structures 3 as part of a deep-drawing process, while the entire component geometry is shaped from a crude sheet 18. The production of the deformation structures 3 then requires no separate process step, but rather it is effected as part of the (single-stage or multi-stage) forming of the crude sheet 18.
 FIG. 4 shows a diagrammatic sketch of a deep-drawing tool 19 for the production of the side sheet 2′ in FIG. 1. The deep-drawing tool 19 comprises a punch 20 and a die 21, which are both provided with local surface structures 22, 23 which correspond to the deformation structure 3′ to be shaped on the crude sheet 18. When the punch 20 is lowered, first of all flanges 25, 25′ are bent on the crude sheet 18 by the action of edge regions 24, 24′ on punch 20 and die 21, additional sheets being welded at these flanges 25, 25′ to the side part 2′ in a later process step. When the punch 20 is lowered further, the deformation structure 3 is then also produced on the crude sheet 18; this is done at the final pressure of the deep-drawing punch 20. Suitable control of the pressure forces of the hold-downs 26 during the lowering of the punch 20 ensures that, during the shaping of both the flanges 25, 25′ and the deformation structure 3, sufficient material can flow out of the side regions 27 of the crude sheet 18 into the inner regions 28 to be shaped and thus cracks or folds of the crude sheet 18 cannot occur either in the region of the flanges 25, 25′ or on the stiffness-increasing deformation structures 3.
 From case to case, depending on the geometry of the individual part 2 to be shaped, those punch and die regions 22, 23 which shape the stiffness-increasing deformation structure 3 are subjected to higher wear during the deep-drawing process than the rest of the tool; it is therefore advisable from case to case to strengthen these regions 22, 23 of the punch 20 and the die 21, respectively, by tool inserts 29, 30 made of an especially hard or resistant material.
 The method according to the invention can be used for the local stiffness increase of plate-shaped workpieces 1 of different thickness which may be made from a wide range of different (deformable) materials. The stiffness-increasing structures 3 can be used for strengthening any regions which are subjected to particular compressive and/or torsional loads. Furthermore, weak points can be specifically provided in the component 1 by means of such a local deformation structure 3, at which weak points the component, in the event of a certain load, buckles or breaks. In addition to the above-described deformation structure 3 of concave bulges 7 and convex bulges 8 nested one inside the other, the grid cells 17 may also have a more complex convex/concave form if, for example, each concave bulge 7 is provided in its interior with a convex opposing bulge 8 which in turn has a convex bulge in its center.
1. A component consisting of a high-strength sheet which is provided with a stiffness-increasing deformation structure in a defined, locally limited stiffening region which is especially important for the dimensional stability of this component, characterized in that the deformation structure (3) consists of a periodic grid of adjacent cells (17), each grid cell (17) containing concave and convex bulges (7, 8) nested one inside the other.
2. The component as claimed in claim 1, characterized in that the deformation structure (3) has a hexagonal structure.
3. A method of producing a locally limited, stiffness-increasing deformation structure on a component whose geometry is shaped from a sheet-metal billet of high dimensional stability by means of a drawing method, characterized in that the deformation structure (3), consisting of a periodic grid of concave and convex bulges (7, 8) nested one inside the other, is produced on the component by means of a drawing method.
4. The method as claimed in claim 3, characterized in that the deformation structure (3) is shaped together with the component geometry in the same process step.
International Classification: B21D031/00;