Abstract:
A self-piercing rivet for connecting high-strength steels. The rivet having a head which has a head diameter (DH); a shank which has a shank diameter (DS); and, located at the foot end of the shank opposite the head, the shank has radially outward a flat surface section and radially inward an axial recess defining an axial depth (LB). The ratio of axial depth (LB) of the recess to the shank diameter (DS) is smaller than 0.3.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of international application PCT/EP2014/072452, filed Oct. 20, 2014 which claims priority from German Patent Application No. DE102013020504.2 filed Dec. 11, 2013, the disclosures of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a self-piercing rivet for connecting high-strength steels, with a head which has a head diameter and with a shank which has a shank diameter, wherein, at the foot end opposite the head, the shank has an axial recess which has an axial depth, and wherein, at the foot end the shank has a flat surface section. 
         [0003]    Furthermore, the present invention relates to a self-piercing riveted joint with at least one upper workpiece and one lower workpiece, of which at least one is formed from a high-strength steel, and with a deformed self-piercing rivet of the type referred to above, the head of which bears against the upper workpiece. 
         [0004]    Finally, the present invention relates to a method for producing such a self-piercing riveted joint, with the steps of providing a workpiece arrangement which has at least one upper and one lower workpiece, and of driving a self-piercing rivet of the type referred to above into the workpiece arrangement with a punching force. 
         [0005]    A self-piercing rivet of the form referred to above is known from EP 1 229 254 A2. In this document, it is proposed to provide a self-piercing riveted joint with at least two joining parts made of a high-strength steel, which are connected to each other by means of a semi-tubular self-piercing rivet which is formed from steel and which has a rivet head and an adjoining rivet shank with a rivet foot on the end side, wherein the rivet foot is of truncated design in the initial state before the joining operation. The shape of the self-piercing rivet is in this case intended to be identical to the shape of a self-piercing rivet as is known also for joining light metal workpieces, specifically from EP 0 833 063 A1. The truncated design of the rivet foot is intended to achieve a favourable deformation behaviour of the semi-tubular self-piercing rivet during the joining of the high-strength joining parts, wherein the endeavour of the rivet shank to expand is reduced in comparison to a pointed rivet foot. The expansion here is intended to take place only when the rivet shank pierces with the rivet foot into the lower joining part. The joining parts are intended to have a tensile strength of greater than 500 N/mm 2  to up to 1500 N/mm 2 . The tensile strength of the semi-tubular self-piercing rivet used is intended to lie within a range of between 1200 and 1400 N/mm 2 , but can even reach values of up to 2000 N/mm 2 . 
         [0006]    In order to ensure a suitable expansion behaviour, the quotient from the axial depth of the shank cavity and the outside diameter of the rivet foot is to lie between 0.3 and 0.7. At too small an axial depth of the shank cavity, the rivet shank will not expand sufficiently after perforating the upper steel sheet. 
         [0007]    A further semi-tubular self-piercing rivet is known from WO 2007/132194 A1. The shank here is to be provided with a central blind hole bore, wherein the ratio of a difference between an outside diameter and an inside diameter of the shank in the region of the bore to the outside diameter of the shank is intended to lie within the range of 0.47 to 0.52. 
         [0008]    Furthermore, EP 2 314 890 A2 discloses a semi-tubular self-piercing rivet for connecting high-strength and super-high-strength steels, wherein a head diameter is generally smaller than or equal to 1.3 times the shank diameter. 
         [0009]    However, as before, prior-art self-piercing riveted joints, with which high-strength or super-high-strength steels are connected, may have diverse problems. Firstly, the extent of the expansion may not be symmetrical with respect to a rivet axis. Furthermore, the shank may be compressed and twisted. In some cases, it is not even possible to press the rivet into the workpiece arrangement, with it even being possible for the self-piercing rivet to be fractured. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Against this background, it is an object of the invention to specify an improved self-piercing rivet, an improved self-piercing riveted joint and an improved self-piercing riveting method, which are suitable for connecting high-strength and super-high-strength steels. 
         [0011]    This object is achieved in the case of the self-piercing rivet referred to at the beginning in that the ratio of axial depth of the recess to shank diameter is smaller than 0.3, in particular smaller than 0.28, and particularly preferably smaller than 0.25, or even smaller than 0.2. 
         [0012]    The ratio of axial depth of the recess to shank diameter is preferably greater than 0.05, and preferably greater than 0.1, and in particular greater than 0.12. 
         [0013]    Furthermore, the above object is achieved by a self-piercing riveted joint with an upper workpiece and with a lower workpiece, of which at least one is formed from a high-strength steel, and with a deformed self-piercing rivet, the head of which bears against the upper workpiece, wherein the self-piercing rivet is in particular a self-piercing rivet according to the invention. 
         [0014]    Finally, the above object is achieved by a method for producing a self-piercing riveted joint, in particular a self-piercing riveted joint of the type referred to above, with the steps of providing a workpiece arrangement which has at least one upper and one lower workpiece, and of driving a self-piercing rivet of the type according to the invention into the workpiece arrangement with a punching force. 
         [0015]    By means of the configuration according to the invention of the self-piercing rivet, a deformation of the self-piercing rivet that is less focused on an expansion of the rivet shank is produced during the self-piercing riveting method. Rather, the effect achieved by the relatively short axial depth of the recess is that the connection is formed by an upsetting operation of the rivet, said upsetting operation being caused in particular by the counter pressure of the high-strength steel of the workpiece arrangement. The undercut which is thereby formed can be relatively small in this case. However, owing to the high-strength materials, even a relatively small undercut is sufficient in order to realize the required connection strength. 
         [0016]    In addition, the effect achieved by the relatively small axial depth of the recess is that the self-piercing rivet obtains a significantly greater stability which makes it possible to pierce through even high-strength and super-high-strength steels. 
         [0017]    A contribution to the new manner of producing self-piercing riveted joints is provided by the flat surface section at the foot end. In other words, it is preferred if the generally annular end side of the foot end, which end side is also referred to as a cutting edge, is of at least proportionally flat design, specifically is preferably oriented perpendicularly to a longitudinal axis of the self-piercing rivet. 
         [0018]    The upper workpiece of the workpiece arrangement is preferably produced from steel and has a tensile strength which is preferably greater than 800 N/mm 2 , in particular greater than 1000 N/mm 2 . The tensile strengths at least of the upper workpiece can be up to 1500 N/mm 2  and beyond. 
         [0019]    The tensile strength of the lower workpiece—without heating—is preferably limited to approximately 600 N/mm 2 . 
         [0020]    In other words, even forming steels, such as are known under the name “Usibor®”, in which, before a heat treatment, the microstructure consists in particular of a ferritic—pearlitic structure, can be joined with the required connection strength by the self-piercing rivet according to the invention. 
         [0021]    It goes without saying that the strength or hardness of the self-piercing rivet is correspondingly adapted. Furthermore, it goes without saying that the self-piercing rivet is a semi-tubular self-piercing rivet which is produced in particular rotationally symmetrically and/or as a single piece from steel. 
         [0022]    A small radius of 0.5 mm or less is preferably provided at the transition from the shank to the head in order to keep the setting forces as small as possible. 
         [0023]    The minimum rivet length is preferably the thickness of the upper workpiece plus a length which is preferably greater than 2 mm and in particular equal to 3 mm. The maximum rivet length preferably lies within the range of the thickness of the workpiece arrangement. 
         [0024]    The object is therefore completely achieved. 
         [0025]    It is particularly preferred if the recess is frustoconical in longitudinal section. 
         [0026]    In this alternative, the diameter of the recess in the region of the foot end is preferably greater than the diameter in the region of a base of the recess. In this embodiment, the base of the recess is preferably flat, but can also be curved concavely or convexly. 
         [0027]    According to a further preferred embodiment, the recess is arch-shaped in longitudinal section. 
         [0028]    The arch shape here can be produced by a single radius, and therefore the recess is in the shape of an arc of a circle in longitudinal section. 
         [0029]    However, it is particularly preferred if the recess is in the shape of a pointed arch or a gothic arch in longitudinal section. 
         [0030]    Such an arch shape is produced by two arcs constructed from circles and having a point. 
         [0031]    It is preferred here if the point is rounded by means of a radius in a suitable manner. 
         [0032]    Furthermore, it is preferred in the case of the pointed arch shape if the center points of the respective arcs, as the respectively assigned arch, each lie on different sides—as seen in longitudinal section—of a longitudinal center axis of the self-piercing rivet. 
         [0033]    In the two embodiments referred to above—frustoconical or arch-shaped, it is advantageous for the punching forces acting from the head end to be suitably introduced into the foot end. 
         [0034]    Overall, it is furthermore preferred if the recess does not have a cylindrical section. 
         [0035]    A cylindrical section in the recess can result in instability and possibly in fracturing at very high punching pressures. 
         [0036]    The stability of the self-piercing rivet can be increased overall by omitting a cylindrical section within the recess. 
         [0037]    According to a further embodiment which, in conjunction with the precharacterizing clause of claim  1 , constitutes a separate invention, the recess has a recess volume, wherein a ratio of recess volume to volume of the shank is smaller than 0.25, in particular smaller than 0.18 and/or is greater than 0.05, in particular greater than 0.1. 
         [0038]    The recess volume in this case is calculated starting from the foot end of the self-piercing rivet. The volume of the shank is that volume of the shank at which the shank has a uniform outside diameter, i.e. exclusively a possible transition section to a head of the self-piercing rivet, but including the recess volume which is consequently contained in the volume of the shank. 
         [0039]    The relatively small recess volume results, firstly, in great stability of the self-piercing rivet. Secondly, a punched-out piece detached from the upper workpiece is not received by the recess, but rather is pressed in front of the rivet by the rivet during the punching operation. The effect which can advantageously be achieved by this means is that greater deformation of material takes place within a die of a self-piercing riveting tool instead of deformation in the recess. 
         [0040]    According to a further preferred embodiment, the flat surface section is designed as an annular surface section and has a radial width in cross section, wherein the ratio of radial width of the annular surface section to shank diameter is greater than 0.05 and/or is smaller than 0.25. 
         [0041]    The self-piercing rivet is preferably produced from a steel with a hardness of at least 500 HV10 (1630 MPa), in particular with a hardness of at least 650 HV10, in particular with at least 700 HV10. The hardness is generally smaller than 800 HV10. 
         [0042]    In the self-piercing riveted joint according to the invention, it is preferred if the axial thickness of the upper workpiece is greater than or equal to the axial depth of the recess in the undeformed state. 
         [0043]    Furthermore, it is advantageous in the case of the self-piercing riveted connection according to the invention if a punched-out piece is detached from the upper workpiece, and if less than 50% of the volume of the punched-out piece is located within the deformed recess, in particular less than 30%, preferably less than 25% and particularly preferably less than 20%. 
         [0044]    This results in the self-piercing rivet being designed in such a manner that it is substantially upset, as a result of which the volume of the recess is reduced, and therefore the punched-out piece is substantially pushed in front of the rivet during the self-piercing riveting operation. 
         [0045]    Material of the lower workpiece can thereby be suitably displaced within the die by means of the punched-out piece, and therefore said material flows behind an undercut in the shank of the self-piercing rivet. 
         [0046]    All in all, it is furthermore advantageous if the shank of the deformed self-piercing rivet forms an undercut in relation to forces in the direction of the head, wherein the ratio of undercut to shank diameter is smaller than 0.1 and/or is greater than 0.01. 
         [0047]    This results in the extent of the undercut being comparatively small. However, such a small undercut is sufficient when connecting high-strength steels in order to realize the necessary connection strength. 
         [0048]    According to a further preferred embodiment of the self-piercing riveted joint, the ratio of axial length of the self-piercing rivet after deformation and of axial length of the self-piercing rivet before deformation is greater than 0.8 and/or is smaller than 0.95. 
         [0049]    This results in the self-piercing rivet only being upset to a comparatively small extent because of the predetermined hardness thereof, which likewise leads to a relatively small undercut in the radial direction. 
         [0050]    This also results in the minimum length of the self-piercing rivet preferably being produced from the thickness of the upper workpiece plus a value of preferably 3 or 3.5 mm, whereas the maximum length of the self-piercing rivet is preferably calculated by the overall thickness of the workpiece arrangement plus 1 mm or is equal to the overall thickness of the workpiece arrangement. 
         [0051]    In the case of the method according to the invention it is advantageous if the workpiece arrangement is supported on a die with a die volume into which at least the lower workpiece is driven, wherein the ratio of die volume to a volume of the self-piercing rivet is greater than or equal to 1.0 and/or is smaller than or equal to 1.5. 
         [0052]    The die volume is the volume into which material at least of the lower workpiece flows during the self-piercing riveting operation, wherein the upper edge of the die recess provided for this purpose is substantially flush with a supporting surface. The shape of the die recess here is preferably frustoconical, with a relatively large diameter in the region of the supporting surface and a smaller diameter in the region of a base of the die volume. 
         [0053]    Overall, the following can furthermore be noted in addition. In the case of conventional self-piercing riveting, the formation of the undercut is a feature which is relevant to the quality of the connection strength. Owing to the high strength of the rivet according to the invention, this feature no longer holds true by itself. The rivet requires a relatively strong upper workpiece, the punched-out piece of which then upsets the rivet and presses it somewhat apart in the process. In contrast to rivets from the prior art, the undercut in the connection is not produced by conventional expansion, but rather is produced by an upsetting operation of the rivet, the upsetting operation being caused by the counter pressure of the high-strength steel. A further differentiation criterion with respect to conventional rivets is the range of use which, as a rule, begins only at a tensile strength of the upper workpiece of 800 N/mm 2 , in particular of 1000 N/mm 2 . Steels of this strength category have found use in vehicle manufacturing because of the increased use of lightweight structures made from super-high-strength sheets. The range of use of the self-piercing rivet according to the invention downwards is preferably limited by a minimum punching force—the force for perforating/piercing the high-strength workpiece arrangement—of 8 kN. Above said force, a sufficient upsetting (not primarily spreading) of the self-piercing rivet begins, and the required degree of upset, which is preferably at least 0.15 mm, is achieved. In order to assess the connection quality, the degree of upset should also be taken into consideration in addition to the formation of an undercut. The degree of upset is calculated from the axial length of the self-piercing rivet before deformation minus the axial length of the self-piercing rivet after deformation, i.e. in the settled state. 
         [0054]    It goes without saying that the features referred to above and those which have yet to be explained below are usable not only in the respectively stated combination, but also in different combinations or on their own without departing from the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0055]    Exemplary embodiments of the invention are illustrated in the drawing and are explained in more detail in the description below. In the drawing: 
           [0056]      FIG. 1  shows a longitudinal sectional view through an embodiment of a self-piercing rivet according to the invention; 
           [0057]      FIG. 2  shows a longitudinal sectional view through a further embodiment of a self-piercing rivet according to the invention; and 
           [0058]      FIG. 3  shows a longitudinal sectional view through a self-piercing riveted joint produced by means of the self-piercing rivet of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0059]    A rotationally symmetrical semi-tubular self-piercing rivet is illustrated schematically in longitudinal section and denoted in general by  10  in  FIG. 1 . 
         [0060]    The self-piercing rivet  10  is produced from a strong steel and preferably has a hardness of greater than 500 HV. The self-piercing rivet is in particular produced by pressure deformation. 
         [0061]    The self-piercing rivet  10  has a head  12  and a shank  14  adjoining the latter in the axial direction. The shank  14  merges into the head  12  via a transition section  16 . An end of the shank  14  that is opposite the head  12  is designed as a foot end  18  in  FIG. 1 . 
         [0062]    A flat surface section  20  is formed at the foot end  18 , said surface section being designed as an annular surface section, the outside diameter of which is limited by an outside diameter of the shank  14  and the inside diameter of which is limited by an edge of a recess  22  which extends from the foot end  18  in the direction of the head  12 . 
         [0063]    In  FIG. 1 , the recess  22  is of frustoconical design and, starting from the foot end  18 , has a conically extending recess transition section  24  and a recess base  26 . The recess base  26  can be of flat design, as illustrated, but may also be of concave or convex design. 
         [0064]    Furthermore, the following dimensions are shown in  FIG. 1 , wherein the preferred values for said dimension are in each case also plotted in the table below: 
         [0000]                                        Designation   Abbreviation   Preferred value   Note                   Axial length, self-piercing   LR       5 mm           rivet       Length, shank   LS   3.6 mm       Axial height, head   LH   0.5 mm       Axial depth, recess   LB       1 mm       Diameter, head   DH   7.75 mm        Outside diameter, shank   DS   5.5 mm       Recess diameter at the foot   DB   4.5 mm       end       Recess diameter at the base   DB′   ~2.5 mm        Radial width, annular   BF   0.5 mm       surface Section       Cone angle, recess   αB   ~40°   i.e. 25°-50°       Cone angle, transition   αH   ~27°   i.e. 20°-50°       section                    
In the case of the self-piercing rivet of  FIG. 1 , the ratio of axial depth LB of the recess  22  to the shank diameter DS is approximately 0.18.
 
         [0065]    The ratio of radial width BF to the shank diameter DS is approximately 0.09. 
         [0066]    Furthermore, the ratio of the recess volume to the volume of the shank is approximately 0.135, wherein the volume of the recess is approximately calculated at 
         [0000]        VB =( LB ·π)/3·[( DB/ 2) 2   +DB·DB ′+( DB′/ 2) 2 ],
 
         [0000]    and wherein the volume of the shank is calculated at 
         [0000]        VS =π·( DS/ 2) 2 ·LS.
 
         [0067]    The volume VS of the shank consequently includes the recess volume VB. 
         [0068]    The values, which are indicated in the table above, for the respective dimensions and angles can preferably each deviate within the scope of the invention upwards and downwards by at least 20%, preferably upwards and downwards by 10% in each case. 
         [0069]    A radius RB which is formed at the transition between the recess transition section  24  and the recess base  26  is furthermore shown in  FIG. 1 . The value of RB can be, for example, 0.35 mm. The value of DB′ is an approximate value which lies approximately in the center of the recess RB, as seen in the radial direction. 
         [0070]    Furthermore, a radius RH which forms the transition between the conical transition section  16  and the shank  14  is shown in  FIG. 1 . The value of RH can be, for example, 0.5 mm or less. 
         [0071]    An alternative embodiment of a self-piercing rivet according to the invention is illustrated in  FIG. 2  and is likewise generally denoted by  10 . The self-piercing rivet  10  of  FIG. 2  corresponds generally in respect of construction and function to the self-piercing rivet  10  of  FIG. 1 . Identical elements are therefore indicated by the same reference numbers. Essentially the differences are explained below. 
         [0072]    The recess  22  of the self-piercing rivet  10  of  FIG. 2  is not frustoconical, as in the case of the self-piercing rivet  10  of  FIG. 1 , but rather is of arch-shaped design. In more precise terms, the recess  22  in  FIG. 2  is in the shape of a pointed arch in longitudinal section, the pointed arch being assembled from two arcs of a circle which form a point on the longitudinal axis. The origins of the arcs of the circle lie in each case on that side of the longitudinal axis which is opposite the arc of the circle thereof. In the region of the point which is formed by the two arcs of the circle, the recess is rounded with a radius which can be, for example, 0.5 mm. This radius is indicated schematically in  FIG. 2  by R 1 . 
         [0073]    The radius of the two arcs of the circle is indicated schematically in  FIG. 2  by R 2  and can be, for example, approximately 4 mm. 
         [0074]    In the case of the self-piercing rivet  10  of  FIG. 2 , the maximum axial depth LB of the recess  22  is preferably approximately 1.5 mm, and therefore a ratio LB/DS of approximately 0.273 is produced. 
         [0075]    The shank diameter DS and the shank axial length LS and also other dimensions can be identical to those of the self-piercing rivet  10  of  FIG. 1 . 
         [0076]    A self-piercing riveted joint  30  produced by means of the self-piercing rivet  10  of  FIG. 1  is illustrated schematically in longitudinal section and is denoted in general by  30  in  FIG. 3 . 
         [0077]    The self-piercing rivet joint  30  connects a workpiece arrangement  32  which contains at least one upper workpiece  34  and one lower workpiece  36 , of which at least the upper workpiece can be produced in the form of steel sheet from high-strength or super-high-strength steels. 
         [0078]    It is illustrated in  FIG. 3  that the self-piercing rivet  10 * has cut a punched-out piece  38  out of the upper workpiece  34  and has pressed said punched-out piece in front of itself during the self-piercing riveting operation. The remaining base thickness between the lower side of the punched-out piece and the lower side of the lower workpiece  36  is denoted by  40 . This may be, for example, greater than 0.5 mm. 
         [0079]    Furthermore, a radial undercut of the deformed shank  14 * is shown in  FIG. 3 . The self-piercing rivet  10 * has been upset, in particular in the region of the foot end, because of the relatively hard material of the upper workpiece  34 , and therefore the material of said self-piercing rivet has flowed somewhat outwards radially in the region of the foot end. Owing to the great hardness of the self-piercing rivet  10 * too, the undercut  42  is nevertheless very small and may be, for example, smaller than 0.5 mm, but is, as a rule, greater than 0.05 mm. Correspondingly, the ratio of undercut  42  to shank diameter DS is preferably within a range of 0.1 to 0.01. 
         [0080]    Finally,  FIG. 3  shows a projecting length  44  by which the head  12 * protrudes in relation to the upper side of the upper workpiece  34 . The projecting length  44  is preferably smaller than the axial height LH of the self-piercing rivet  10  in the undeformed state. 
         [0081]    Furthermore,  FIG. 3  shows the axial length LR* of the deformed self-piercing rivet  10 *. In the example illustrated, said length can be, for example, approximately 4.4 mm. The ratio of axial length LR* of the self-piercing rivet  10 * after deformation and axial length LR of the self-piercing rivet  10  before deformation is preferably greater than 0.8 and/or smaller than 0.95. 
         [0082]    As stated, the self-piercing rivet  10 * has been upset in the region of the foot end, and therefore the remaining volume of the remaining recess  22 * is relatively small. Accordingly, in the embodiment illustrated, at most a portion of 50%, in particular at most a portion of 25%, of the volume of the punched-out piece  38  is accommodated within the deformed recess  22 *. 
         [0083]    The axial thickness of the upper workpiece  34  is denoted by L 34 . Said thickness can be greater than or equal to the axial depth LB of the self-piercing rivet  10  in the undeformed state. The axial thickness of the lower workpiece  36  is denoted by L 36 . Said thickness is preferably greater than L 34 . The lower workpiece  36  is preferably softer than the upper workpiece  34 . 
         [0084]      FIG. 3  furthermore schematically illustrates a die  50  of a self-piercing riveting tool, by means of which an axial force (punching force)  52  is exerted on the upper side of the head  12  of the self-piercing rivet  10  during the self-piercing riveting operation. The recess of the die  50  is of approximately frustoconical design. The somewhat softer material of the second workpiece  34  is pressed away radially by the punched-out piece  38  and the die  50  and in this case flows behind the undercut  42  such that the self-piercing riveted joint  30  provides an interlocking connection between the workpieces  34 ,  36 . 
         [0085]    The volume of the die recess is preferably greater than or equal to the volume of the self-piercing rivet  10  in the undeformed state. In particular, the ratio of the die volume to the volume of the self-piercing rivet  10  is preferably greater than or equal to 1.0 and/or smaller than or equal to 1.5. 
         [0086]    The minimum punching force  52  is preferably 8 kN. 
         [0087]    The minimum length of the self-piercing rivet  10  in the undeformed state is produced form the thickness L 34  plus a value which can be, for example, 3 or 3.5 mm. The maximum length of the self-piercing rivet  10  in the undeformed state can be equal to the overall thickness L 34 +L 36 , or a value which is formed to be equal to the overall sheet thickness+a value of, for example, 1 mm. 
         [0088]    The upper workpiece  34  preferably has a tensile strength in the region of greater than 800 N/mm 2 , in particular greater than 1000 N/mm 2 . The lower workpiece  36  preferably has a tensile strength of smaller than 600 N/mm 2 . The self-piercing rivet  10  preferably has a (Vickers) hardness of more than 650 HV. 
         [0089]    Although exemplary embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.