Method for producing a component assembly with a press-hardened sheet metal component

Abstract

Method for producing a component assembly (10) for a motor vehicle, comprising the steps of: - producing at least one first press-hardened sheet metal component, namely a press-hardened B-pillar (50); and - joining the press-hardened B-pillar (50) with at least one second component (30) by means of at least one welded joint; wherein, during the production of the press-hardened B-pillar (50), a lower strength and / or hardness is formed locally in the area of ​​at least one such welded joint than in the other areas of the press-hardened B-pillar (50), characterized in that the press-hardened B-pillar (50) has a joining flange (54) which is formed with a lower strength and / or hardness only section by section over its entire width, namely in a section (55).which extends from a lower door hinge connection (53) to the end of a component curvature at the transition between a longitudinal profile section (51) and a transverse profile section (52), wherein this section (55) of the joining flange (54) is already formed with a lower strength and / or hardness from the press hardening process than the other areas of the press-hardened B-pillar (50), and that the at least one welded joint is a weld seam in this section (55) of the joining flange (54), as the area with the lower strength and / or hardness, which corresponds to a surface area extending longitudinally to the weld seam path, wherein the width of this area corresponds to at least 2.5 times the weld seam width and the weld seam runs substantially in the middle of this surface area, and wherein the area with the lower strength and / or hardness also surrounds the endpoints of the weld seam,to prevent crack initiation in the weld area of ​​the component assembly (10) under mechanical stress.

Description

According to the preamble of claim 1, the invention relates to a method for producing a component assembly with a press-hardened sheet metal component, namely a press-hardened B-pillar, for a motor vehicle. Press-hardened sheet metal components, as well as methods and equipment for their production, are known from the prior art. Reference is made, by way of example, to DE 10 2007 040 013 A1 and DE 10 2007 040 028 A1 of the same applicant, as well as to DE 200 14 361 U1. In press hardening, the strength and / or hardness of a sheet metal material is increased by heat treatment and subsequent cooling in a forming tool. The increase in strength and / or hardness (hardening) of the sheet metal material is essentially based on a microstructural transformation. In addition to their high strength and / or hardness, press-hardened sheet metal components also exhibit a high energy absorption capacity until failure (fracture), which is why such press-hardened sheet metal components are preferably used in automotive engineering. The closest relevant patent, DE 103 23 746 A1, describes a press-hardened component, in particular a B-pillar, in which flanges with lower hardness are provided. A flange with reduced hardness offers the possibility of providing areas on the component with good weldability. Such a flange can therefore be used, in particular, as a spot-welded flange. In automotive manufacturing, press-hardened sheet metal components are typically joined to other components or integrated into a higher-level structure, such as a body structure, using welded joints. However, in practice, a problem arises: under heavy mechanical stress, and especially under highly dynamic stress during a crash, cracks can form in the press-hardened sheet metal component originating from such a welded joint, leading to premature failure of the component. DE 10 2006 047 582 A1 describes a composite consisting of two sheets, comprising a first sheet and a second sheet, in which the sheets form a common flange where they are welded together, the second sheet being made of hardened steel. The hardened steel exhibits a lower hardness, known as a minimum hardness, in the heat-affected zone of a weld spot than in the weld spot or the base material. Due to this minimum hardness, tensile stress in the heat-affected zone will cause yielding and the formation of a crack, which then propagates through the base material. To prevent this cracking, the second sheet is designed in such a way that its edge in the flange area projects further outwards at the weld points than between the weld points. This reduces the stress at the weld points and thus promotes yielding of the base material between the weld points. The object of the present invention is to demonstrate an alternative way of avoiding such crack formation. This problem is solved by the method according to the invention with the features of claim 1. The dependent claims relate to advantageous and preferred embodiments. To solve the problem, a method for manufacturing a component assembly for a motor vehicle is proposed, comprising the steps of: manufacturing at least one first press-hardened sheet metal component, namely a press-hardened B-pillar (hereinafter also referred to simply as a press-hardened sheet metal component), and joining this press-hardened sheet metal component to at least one second component by means of at least one welded joint. According to the invention, it is provided that during the manufacturing of the press-hardened sheet metal component, i.e., the press-hardened B-pillar, a lower strength and / or hardness is formed locally in the area of ​​at least one such (subsequently arranged) welded joint than in other areas of this press-hardened sheet metal component, in order to prevent crack initiation in the component assembly in the area of ​​this welded joint under mechanical stress. Such an area with lower strength and / or hardness is a spatially and / or surface-wise essentially precisely defined area or section of the press-hardened sheet metal component, as indicated by the term "local". According to the invention, this area surrounds the welded joint. The "other areas" of the press-hardened sheet metal component are the remaining areas that are characterized by high strength and / or hardness as a result of the press hardening process. Preferably, these other areas of the press-hardened sheet metal component exhibit essentially uniform strength and / or hardness. For example, a common sheet metal material has a tensile strength of approximately 400 MPa before press hardening and a tensile strength of approximately 1200 to 1600 MPa after press hardening. The "second component" must be a metal component with regard to weldability. Preferably, this is a sheet metal component. This can also be a press-hardened sheet metal component, which then preferably also has areas with lower strength and / or hardness. Strength refers to a material property of sheet metal that describes the mechanical resistance the material offers to plastic deformation or separation. Hardness refers to a material property that describes the mechanical resistance the material offers to external mechanical forces. Strength and hardness can be correlated according to common material models. A "mechanical load" refers to a one-dimensional or multi-dimensional load condition to which the press-hardened sheet metal component is exposed, particularly in the area of ​​a welded joint. Highly dynamic stress describes load conditions that occur suddenly or change rapidly, especially with high magnitudes. The invention is based on the understanding that the cause of crack formation in the area of ​​a weld joint is the heat input during the welding process. Due to this heat input, a partial transformation and / or re-transformation of the hard microstructure resulting from press hardening occurs in a heat-affected zone around the weld joint, resulting in a structure with lower strength and / or hardness, which can be described as a "soft zone." At a greater distance from the weld joint, however, the hard microstructure of the sheet metal component, as obtained through press hardening, remains unaffected. At a weld point, this soft zone is, for example, annular in shape around the weld point, with a ring width of approximately 0.5 to 1.0 mm.In contrast, the weld point located centrally in this soft area exhibits high strength and / or hardness, often exceeding that of the press-hardened sheet metal. A similar principle applies to a weld seam. Thus, significant differences in strength and / or hardness exist within a relatively small area (relative to the top view). Particularly at the transitions to the softer areas, there is a marked drop in strength or hardness, which creates a notch effect under mechanical stress. Due to this notch effect, especially under highly dynamic loading of the press-hardened sheet metal component, stress peaks occur in the sheet metal material in the area of ​​the weld joint. These peaks cause locally high plastic strains in the sheet metal material and can therefore initiate cracks. An initiated crack can then propagate into adjacent areas of the sheet metal component with a solid or hard structure, ultimately leading to fracture of the press-hardened sheet metal component. The undesired crack formation thus occurs through the mechanism of crack initiation in the area of ​​the weld joint and subsequent crack propagation, which is prevented according to the invention. According to the invention, the strength and / or hardness in the area of ​​a subsequent weld joint is kept at a lower level compared to other areas of the press-hardened sheet metal component during the manufacturing process. This reduces the notch effect in the area of ​​a weld joint and avoids or prevents crack initiation in this area. In the case of highly dynamic loading, the energy absorbed by the press-hardened sheet metal component can then be converted into plastic deformation work without resulting in failure by fracture. According to a preferred embodiment, the production of the press-hardened sheet metal component, i.e., the press-hardened B-pillar, includes the step of press hardening in a press hardening die, wherein, in the area of ​​at least one such weld joint, an increase in the strength and / or hardness of the sheet metal material is essentially prevented by process engineering and / or tooling measures. Thus, the press-hardened sheet metal component in this area essentially retains the original strength and / or hardness of the sheet metal material before press hardening. This has the advantage, for example, that the heat input during the welding process has a less significant effect, since no re-transformation of a hard microstructure can occur. However, the strength and / or hardness achieved in the relevant area during press hardening can also exceed the original strength and / or hardness of the sheet metal material. According to a preferred further development, it is provided that the sheet metal component, i.e. the B-pillar, is cooled significantly more slowly in the area of ​​at least one such weld joint during press hardening than in other areas, in order to prevent a structural transformation towards a solid and / or hard structure in this area. According to a preferred embodiment, the slower cooling in the press hardening tool during press hardening is achieved by selectively controlling the cooling in the area of ​​at least one such weld joint. This can be realized, for example, by a suitable tool design in combination with a control unit, as explained in more detail below in connection with the figures. According to the invention, the joining of the press-hardened sheet metal component is carried out by means of a seam weld, wherein, during the production of the press-hardened sheet metal component, a lower strength and / or hardness was formed locally in the area of ​​this seam weld than in other areas of the press-hardened sheet metal component. A motor vehicle can comprise a component assembly manufactured according to the inventive method, wherein this component assembly is then an integral part of the body structure. The motor vehicle is, in particular, a passenger car. The term "body structure" refers to the load-bearing structure of a motor vehicle's body, typically composed of joined sheet metal components. Body appendages, such as the doors, trunk lid, and hood, are attached to this structure. Therefore, the vehicle body is the entirety of the body structure and its appendages. The term "chassis" refers to all the components that connect the vehicle body to the road surface. This includes, in particular, steering components and wheel suspension components. As already mentioned, the press-hardened sheet metal component is a side wall pillar, namely a B-pillar. According to a preferred embodiment, the press-hardened sheet metal component, i.e., the press-hardened B-pillar, is made of 22MnB5 steel. Alternatively, the press-hardened sheet metal component can also be made of 16MnB5 steel. The second component of the assembly can also be made of this steel material. According to the invention, the welded joint is a weld seam. This does not preclude the presence of one or more spot welds. Furthermore, according to the invention, the area with lower strength and / or hardness corresponds to a surface area extending longitudinally to the weld seam, wherein the width of this area preferably corresponds to at least 10 times, in particular at least 5 times, and according to the invention at least 2.5 times, the weld seam width, and wherein the weld seam runs substantially in the center of this surface area. This surface area is defined from the top view in the direction of the weld seam. If a weld seam has, for example, a width of 2 mm, the width of the surface area surrounding this weld seam with lower strength and / or hardness, measured transversely to the weld seam, is preferably at least 20 mm, in particular at least 10 mm, and according to the invention then at least 5 mm. Common weld seam widths in automotive engineering, and in particular in body construction, are 0.7 to 3 mm. Common weld seam lengths are in the range of 20 to 30 mm.According to the invention, the area with lower strength and / or hardness also surrounds the endpoints of the weld. According to the invention, the press-hardened sheet metal component, i.e., the press-hardened B-pillar, has at least one joining flange, and the weld joint is arranged in the area of ​​this joining flange. This will be explained in more detail below in connection with Fig. 4. Furthermore, according to the invention, this joining flange is designed with a lower strength and / or hardness only in sections, essentially over its entire width. This will also be explained in more detail below in connection with the figures. The invention is explained in more detail below by way of example with reference to the figures. These show: Fig. 1 a schematic overview of the method for producing a component assembly; Fig. 2 a prior art spot weld between a press-hardened sheet metal component and a second component, in a schematic top view and a sectional view; Fig. 3 a spot weld without notch effect between a press-hardened sheet metal component and a second component, in a schematic sectional view, corresponding to Fig. 2; Fig. 4 a press-hardened B-pillar of a motor vehicle body according to the invention, the joining flange of which has a lower strength and / or hardness in certain sections, in a perspective view; Fig. 5 a section of a press-hardening tool for producing the B-pillar from Fig. 4, in a schematic sectional view; and Fig. 6 an alternative embodiment of the press-hardening tool from Fig. 5. Fig. 1 shows a schematic overview of a method for producing a component assembly 10. In step I, a press-hardened sheet metal component 20 is produced, which has a joining flange 21. In step II, this press-hardened sheet metal component 20 is welded along its joining flange 21 to a second component 30 to form the component assembly 10, wherein the welding is carried out, according to the invention, by means of a plurality of spot welds 40. The press-hardened sheet metal component 20 and the second component 30 are each made of sheet metal. Fig. 2a shows a top view of a weld point 40, which connects the press-hardened sheet metal component 20 along its joining flange 21 to the second component 30, for which purpose the second component 30 also includes a joining flange 31. According to the prior art, the press-hardened sheet metal component 20 has a substantially constant strength and / or hardness in all areas, i.e. also in the joining flange 21, as is established during the press-hardening process in step I. During the welding process in step II, an annular region 22 forms around the weld point 40 at the joining flange 21 of the press-hardened sheet metal component 20 as a result of the heat input. Within this region, the strength and / or hardness of the sheet metal material is lower than in other areas of the press-hardened sheet metal component 20. The heat input during the welding process causes at least a partial transformation and / or re-transformation of the microstructure hardened during the press-hardening process into a microstructure with less strong and / or less hard properties within this region 22. This region 22 can therefore also be described as a soft region. Only the press-hardened sheet metal component 20 is affected by this, provided that the sheet metal material of the second component 30 does not itself react to the heat input during the welding process. The annular, soft region 22 is also shown in the sectional view of Fig. 2b. The annular, soft region 22, when subjected to mechanical stress, causes a notch effect in the area of ​​the weld point 40 of the sheet metal component 20. This notch effect leads to stress concentrations in the sheet metal material, which can initiate cracks in this region 22, particularly at its edges. From this initial crack initiation, crack propagation can occur into the hard regions of the sheet metal component 20 adjacent to the soft region 22, ultimately leading to failure of the sheet metal component 20 by fracture. This is undesirable and particularly critical with regard to the safety-relevant use of such press-hardened sheet metal components. In step I of the production of the press-hardened sheet metal component 20, a lower strength and / or hardness can be formed locally in the area of ​​the weld point 40, i.e. in the area of ​​the joining flange 21, or a structural transformation in this area can be prevented during press hardening. This is shown in Fig. 3. The joining flange 21 of the press-hardened sheet metal component 20 is designed here with a lower strength and / or hardness across its entire width (as shown), as indicated by the hatching 23. The strength and / or hardness corresponds, for example, to the strength and / or hardness of the sheet metal material before press hardening or may even be higher. This has the effect, firstly, that the heat input during the welding process cannot cause a re-transformation of the microstructure in the area of ​​the weld point 40, and secondly, that a notch effect in the area of ​​the weld point 40 is prevented or at least significantly reduced under mechanical stress. Although the joining flange 21 has a lower overall strength and / or hardness compared to the other press-hardened areas, the load-bearing capacity of the press-hardened sheet metal component 20 increases, particularly under highly dynamic stress in the area of ​​the weld point 40, because crack initiation is prevented. This allows the applied mechanical energy (up to a certain maximum) to be completely converted into deformation work on the sheet metal component 20 without causing failure of the sheet metal component 20 by fracture. This is therefore particularly suitable for joining or welding press-hardened sheet metal components with safety-relevant characteristics, such as the side wall pillar of a vehicle body. Fig. 4 shows the base of a press-hardened B-pillar 50 according to the invention (more precisely, it is an internal reinforcement part for a B-pillar), which is integrated into a body structure for a motor vehicle. The B-pillar 50 comprises a longitudinal profile section 51 which transitions into a transverse profile section 52 in its lower region. Reference numeral 53 designates a lower door hinge connection. Reference numeral 54 designates a joining flange or mounting flange by means of which the B-pillar 50 can be integrated into the body structure, for which purpose at least one welded connection is arranged along this joining flange 54, in the form of at least one weld seam. Reference numeral 55 designates a section of the joining flange 54 that extends approximately from the lower door hinge connection 53 to the end of the component curvature at the transition between longitudinal profile section 51 and transverse profile section 52. It has been shown that under mechanical stress on the B-pillar 50, particularly under highly dynamic stress such as in a side impact, a welded joint in this section 55 is especially critical with regard to crack initiation. This is due, for example, to the fact that a bending of the longitudinal profile section 51 associated with a side impact results in a high tensile load in this section 55 of the joining flange 54. According to the invention, this critical section 55 of the joining flange 54 is therefore designed with a lower strength and / or hardness from the press-hardening process than the other areas of the press-hardened B-pillar 50.The following are preferred methods for manufacturing such a B-pillar. Fig. 5 shows a press hardening tool 60 for manufacturing the B-pillar 50 from Fig. 4 in a partial section encompassing the critical section 55 of the joining flange 54. The press hardening tool 60 is formed from a lower tool 61 and an upper tool 62. Alternatively, the press hardening tool 60 can also be constructed in multiple parts. Any cooling devices within the press tool 60 are not shown; reference is made to the prior art mentioned above. To prevent an increase in strength and / or hardness to the level of other component areas in the critical section 55 of the joining flange 54 of the B-pillar 50 during the press hardening process, an insert 63 is arranged in the upper tool 62. The insert 63 extends along the critical section 55 of the joining flange 54, which is shown hatched in Fig. 4, although this is not apparent in Fig. 5. This insert 63 can, for example, be made of a material that has a different heat capacity and / or thermal conductivity than the tool material, such as a ceramic material, thereby achieving slower cooling of the critical section 55 of the joining flange 54 compared to the other areas of the B-pillar 50 and, for example, preventing a structural transformation. The insert 63 is arranged here in such a way that the transition radius R to the critical section 55 of the joining flange 54 at the B-pillar 50 is still included in the press hardening.Alternatively and / or additionally, the insert 63 can also be arranged in the lower tool 61. The insert 63 does not have to extend to the outer edge of the press hardening tool 60, as shown. Alternatively, the insert 63 can also be an actively heated heating element, by means of which the cooling in the critical section 55 of the joining flange 54 can be directly influenced. For this purpose, this heating element can be coupled to a control unit. Instead of such a heating element, a heating device can also be installed directly in the upper tool 62 and / or lower tool 61. Fig. 6 shows an alternative embodiment of a press hardening tool 60a for manufacturing the B-pillar 50 from Fig. 4. The design of the press hardening tool 60a essentially corresponds to the design of the press hardening tool 60 from Fig. 5. However, instead of the insert 63, a recess or air gap 64a is provided in the upper tool 62a, which causes a slower cooling of the critical section 55 of the joining flange 54 during the press hardening process, compared to the other areas of the B-pillar 50 in contact with the lower tool 61a and the upper tool 62a. To prevent excessive distortion and / or excessive shrinkage of the critical section 55 of the joining flange 54, it is clamped firmly at its outer edge between the lower tool 61a and the upper tool 62a during press hardening. The air gap 64a can alternatively and / or additionally also be arranged in the lower tool 61a.Furthermore, the air gap 64a can also extend to the outer edge of the press hardening tool 60a, as shown in the insert 63 in Fig. 5. Alternatively, it is also possible to allow the critical section 55 of the joining flange 54 to protrude from the press hardening tool 60 or 60a during press hardening, thus excluding this section from the press hardening process. Reference symbol list 10 Part assembly 20 Press-hardened sheet metal component 21 Joining flange 22 Soft area or area with soft structure 23 Soft area or area with soft structure 30 Second component 31 Joining flange 40 Weld point 50 Press-hardened B-pillar of a motor vehicle body 51 Longitudinal profile section 52 Transverse profile section 53 Lower door hinge connection 54 Joining flange 55 Critical section of the joining flange R Transition radius 60, 60a Press hardening tool 61, 61a Lower tool 62, 62a Upper tool 63 Insert 64a Air gap

Claims

Method for producing a component assembly (10) for a motor vehicle, comprising the steps of: - producing at least one first press-hardened sheet metal component, namely a press-hardened B-pillar (50); and - joining the press-hardened B-pillar (50) with at least one second component (30) by means of at least one welded joint; wherein, during the production of the press-hardened B-pillar (50), a lower strength and / or hardness is formed locally in the area of ​​at least one such welded joint than in the other areas of the press-hardened B-pillar (50), characterized in that the press-hardened B-pillar (50) has a joining flange (54) which is formed with a lower strength and / or hardness only section by section over its entire width, namely in a section (55).which extends from a lower door hinge connection (53) to the end of a component curvature at the transition between a longitudinal profile section (51) and a transverse profile section (52), wherein this section (55) of the joining flange (54) is already formed with a lower strength and / or hardness from the press hardening process than the other areas of the press-hardened B-pillar (50), and that the at least one welded joint is a weld seam in this section (55) of the joining flange (54), as the area with the lower strength and / or hardness, which corresponds to a surface area extending longitudinally to the weld seam path, wherein the width of this area corresponds to at least 2.5 times the weld seam width and the weld seam runs substantially in the middle of this surface area, and wherein the area with the lower strength and / or hardness also surrounds the endpoints of the weld seam,to prevent crack initiation in the weld area of ​​the component assembly (10) under mechanical stress. Method according to claim 1, characterized in that the production of the press-hardened B-pillar (50) comprises the partial step of press hardening in a press hardening tool (60), wherein an increase in strength and / or hardness of the sheet material in the area of ​​the weld seam is substantially prevented by process engineering and / or tool engineering measures. Method according to claim 2, characterized in that the B-pillar (50) is cooled significantly more slowly in the area of ​​the weld seam during press hardening than in other areas in order to prevent a structural transformation to a solid and / or hard structure in this area. Method according to claim 3, characterized in that during press hardening the slower cooling in the press hardening tool (60) is achieved by targeted control of the cooling in the area of ​​the weld seam.

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