Rear underlying structure for motor vehicle

Abstract

A rear underlying structure (2) for a motor vehicle (1) comprises first and second side members (4) and at least one cross member (5) joining the first and second side members (4), where the rear underlying structure (2) is made by stamping a single tailor-welded blank (26) comprising at least two sub-blanks.

Description

[0001] This invention patent application is a divisional application of the invention patent application with application date of November 4, 2021, application number 202180006968.3, and invention name “Rear underbody structure for motor vehicles”. Technical Field

[0002] The present invention relates to a rear underbody structure for a motor vehicle. Background Art

[0003] Automobile manufacturers are faced with increasingly demanding requirements to improve the passive safety of vehicles, to reduce the weight of vehicles in order to minimize greenhouse gas emissions in the case of internal combustion engines or to increase the vehicle's driving range in the case of electric vehicles, while keeping production costs low and productivity high.

[0004] The rear understructure of a motor vehicle is a critical structural element of the vehicle and contributes to the safety of the occupants in the event of a rear impact. In the case of an internal combustion engine, the rear understructure protects the fuel tank, which is usually located under the passenger seat. In the case of an electric or hybrid vehicle, the rear understructure protects the rear electric engine.

[0005] The rear underbody structure consists of many individual parts and constitutes the main mass of the vehicle body. The rear underbody structure also involves an expensive manufacturing process: multiple molding operations and assembly steps to obtain the finished structure. Summary of the invention

[0006] The object of the present invention is to solve the combined challenges of safety, weight reduction and high productivity by providing a rear underbody structure with reduced number of parts, excellent safety performance and optimized overall weight.

[0007] To this end, the present invention relates to:

[0008] A rear underbody structure for a motor vehicle, comprising first and second side members and at least one cross member connecting the first and second side members, wherein the rear underbody structure is produced by stamping a single tailor-welded blank comprising at least two sub-blanks.

[0009] Other optional features of the rear understructure according to the invention, considered individually or in any technically possible combination:

[0010] - The rear underbody structure is produced by hot stamping.

[0011] - The rear understructure comprises at least two cross members.

[0012] - The rear underbody structure has two side members, each of which comprises:

[0013] - inner wall and horizontal wall connected by inner radius,

[0014] - at least one cross-member attachment area corresponding to a portion of the side member to which a respective cross-member is attached, wherein the inner radius extends along at least a portion of the cross-member attachment area.

[0015] The tailor-welded blank for producing the rear underbody structure comprises at least one sub-blank coated with an aluminum-based metal coating.

[0016] -The tailor-welded blank for manufacturing the rear underbody structure comprises at least one sub-blank coated with an aluminum-based metal coating, which comprises, by weight percentage: 2.0% to 24.0% zinc, 1.1% to 12.0% silicon, optionally 0% to 8.0% magnesium, and optionally an additional element selected from Pb, Ni, Zr or Hf, the content of each additional element by weight percentage being less than 0.3%, the balance being aluminum and optionally unavoidable impurities.

[0017] The tailor-welded blank for producing the rear underlay structure comprises at least one sub-blank comprising an emissivity-increasing top layer on at least one side.

[0018] The tailor-welded blank for producing the rear underbody structure comprises at least one sub-blank made of press-hardened steel having an ultimate tensile strength of more than 1800 MPa after hot stamping.

[0019] - The tailor-welded blank for manufacturing the rear underbody structure comprises at least one sub-blank made of a press-hardened steel having: a yield strength after hot forming comprised between 700 MPa and 950 MPa; an ultimate tensile strength after hot forming comprised between 950 MPa and 1200 MPa; and a bending angle after hot forming comprised between 75° and above.

[0020] The tailor-welded blank for producing the rear understructure comprises at least one sub-blank including at least one metal patch.

[0021] The tailor-welded blank for producing a rear underlay structure comprises at least one sub-blank comprising at least one metal patch, the at least one metal patch comprising an emissivity-increasing top layer.

[0022] The tailor-welded blank for producing a rear understructure comprises at least one sub-blank comprising at least one weld reinforcement patch, wherein the weld reinforcement patch is applied to a region comprising a weld.

[0023] The tailor-welded blank for manufacturing the rear understructure comprises at least one sub-blank including at least one weld reinforcement patch, the at least one weld reinforcement patch itself comprising an emissivity-increasing top layer.

[0024] - The thickness of the interdiffusion layer in the aluminum-based metal coating region of the rear underlying structure is comprised between 3 and 15 micrometers.

[0025] - The thickness of the interdiffusion layer in the aluminum-based metal coating region of the rear underlying structure is comprised between 3 and 10 micrometers.

[0026] The invention also relates to a method for producing a rear underbody structure as described above and to a motor vehicle comprising a rear underbody structure as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Other aspects and advantages of the invention will emerge on reading the following description given by way of example and made with reference to the accompanying drawings, in which:

[0028] - Figure 1 is an overall perspective view of a vehicle according to the present invention;

[0029] - Figure 2 is a perspective view of an undermount rear structure according to known prior art in assembled form (top) and exploded view (bottom);

[0030] - Figure 3 is a top view of an embodiment of a bottom-mounted rear structure according to the present invention;

[0031] - Figure 4 is a top view of an embodiment of a bottom-mounted rear structure according to the present invention;

[0032] - Figure 5 is a perspective view of an embodiment of a bottom-mounted rear structure according to the present invention;

[0033] - Figure 6 According to the embodiment of the bottom rear structure of the present invention, Figure 3 The stereogram when observing the AA cross section shown;

[0034] - Figure 7 According to an embodiment of the bottom rear structure of the present invention, Figure 3 An enlarged stereogram of the box area marked as B in FIG.

[0035] - Figure 8 1 is a top view of a tailor-welded blank for manufacturing an underbody rear structure according to the present invention. DETAILED DESCRIPTION

[0036] In the following description, the terms "upper", "lower", "front", "rear", "lateral" and "longitudinal" are defined according to the general direction of the vehicle on which they are installed. More specifically, the terms "upper", "lower", "up", "lower", "bottom" and "top" are defined according to the height direction of the vehicle, the terms "front", "rear", "forward", "rearward" and "longitudinal" are defined according to the front / rear direction of the vehicle, and the term "lateral" is defined according to the width of the vehicle. The terms "inside", "outside" are defined according to the inside and outside of the vehicle - "inside" parts, parts, areas, etc. will be closer to the inside of the vehicle than corresponding "outside" parts, parts, areas, etc. The term "height" refers to the distance between two points, lines, surfaces or volumes measured in the horizontal direction.

[0037] For better understanding, in all drawings showing the undermounted rear structure, the rear and front directions of the vehicle are indicated using arrows marked “R” and “F”, respectively.

[0038] A steel blank is a flat steel sheet that is cut into any shape suitable for its use. The blank has a top and a bottom surface, which are also called top and bottom sides or top and bottom surfaces. The distance between the surfaces is designated as the thickness of the blank. This thickness can be measured, for example, using a micrometer, the spindle and anvil of which are placed on the top and bottom surfaces. In a similar way, the thickness can also be measured on the formed part.

[0039] Tailor-welded blanks are made by assembling together, for example by laser welding, several steel blanks (referred to as sub-blanks) to optimize the performance of the part in different areas thereof, to reduce the overall part weight and to reduce the overall part cost. The sub-blanks forming the tailor-welded blank can be assembled with or without overlap, for example, the sub-blanks can be laser butt welded (without overlap), or the sub-blanks can be spot welded to each other (with overlap).

[0040] Hot stamping is a forming technique that involves heating a blank to a temperature as high as that at which the microstructure of the steel is at least partially transformed into austenite, obtaining a microstructure with very high strength by stamping the steel to form the blank at high temperature and quenching the formed part. Hot stamping enables very high strength parts with complex shapes and without springback to be obtained. In order to produce the described benefits of hot stamping, the material used is called a press-hardening material, which has a chemical composition that allows it to form the desired hardened microstructure when subjected to the hot stamping process described above. It should be understood that the heat treatment to which the part is subjected includes not only the thermal cycle of the hot stamping process itself described above, but also a subsequent paint-baking step performed for paint-baking after the part is painted. The mechanical properties of the hot stamped parts below are the mechanical properties measured after the paint-baking step (if a paint-baking step is indeed performed).

[0041] Yield strength, ultimate tensile strength and uniform and total elongation are measured according to ISO standard ISO 6892-1 published in October 2009.

[0042] The bending angle is measured according to the VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For simplicity, the bending angle values ​​in this invention refer to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle value needs to be adjusted by the following calculation:

[0043]

[0044] Among them, α 1.5 is the bending angle at 1.5 mm, t is the thickness, and αt is the bending angle for thickness t.

[0045] The bend angle of a part is a measure of how well the part resists deformation without forming cracks.

[0046] Emissivity is the relative power with which a surface emits heat by radiation. Emissivity represents the ratio of the radiant energy emitted by a surface to the radiant energy emitted by a black body at the same temperature, and is a value comprised between 0 and 1. The higher the emissivity of a surface of a blank, the more heat the blank absorbs by radiation, and therefore it will be easier to heat the blank using a radiation furnace.

[0047] Reference Figure 1 , a rear underbody structure 2 for a motor vehicle 1 is described. The motor vehicle 1 may be any type of passenger vehicle including at least one set of front doors and one set of rear doors: a compact vehicle, a sedan, a sports utility vehicle, etc. Regardless of the class of vehicle, the rear underbody structure described is substantially the same. Furthermore, the power system of the motor vehicle may be an internal combustion engine, an electric motor, a fuel cell, or any type of hybrid system.

[0048] The rear floor structure 2 extends longitudinally from the rear of the vehicle to below the floor panel of the passenger compartment. The rear floor structure 2 includes at least first and second side members 4 on either side of the vehicle and at least one cross member 5 connecting the first and second side members 4 transversely.

[0049] The following is a general description of a typical side member 4. It should be understood that the following description of the side member 4 does not limit the scope of the present invention, and the present invention can be applied to the design of any type of side member 4:

[0050] the side member 4 comprises a rear portion which extends in the longitudinal direction substantially at the same height as the rear bumper assembly 11 and is attached to the rear bumper assembly 11 at its rear end,

[0051] a front portion which extends in the longitudinal direction substantially at a lower level than the rear portion and which is attached to the vehicle lateral reinforcement structure 12 ,

[0052] - A transition zone comprising at least an upper and a lower bend joining said front and rear parts. The presence of said upper and lower bends allows a difference in height level between the front and rear parts.

[0053] Reference Figure 5 , the side member 4 has a generally U-shaped shape, including a horizontal wall 41 extending in a substantially horizontal plane and an inner wall 42 and an outer wall 43 extending in a substantially vertical plane. The inner wall 42 and the outer wall 43 are connected to the horizontal wall 41 by an inner radius 45 and an outer radius 46, respectively. The so-called "radius" means a part of a part, which is usually curved and allows a transition between different plane orientations in the part. In the present case, the inner radius 45 allows a transition between a substantially horizontal orientation of the horizontal wall 41 and a substantially vertical orientation of the inner wall 42. Similarly, the outer radius 46 allows a transition between a substantially horizontal orientation of the horizontal wall 41 and a substantially vertical orientation of the outer wall 43.

[0054] In a particular embodiment, the side member 4 also comprises a flange 47 extending in a substantially vertical plane along the top of the inner wall 42 and the outer wall 43. Advantageously, said flange allows the side member 4 to be assembled with the rest of the vehicle structure, for example by spot welding or laser welding along said flange 47.

[0055] The rear understructure 2 further comprises at least one cross member 5 connecting the first and second side members 4. The cross member 5 extends in a substantially transverse direction. Like the side members 4, the cross member 5 comprises a horizontal wall 51 extending in a substantially horizontal plane and a front wall and a rear wall 52, 53 extending in a substantially vertical plane. Figure 3 , the rear bottom structure 2 may have a cross member 5. Figure 4 , the rear understructure 2 may have two crossmembers 5. And other embodiments according to the invention are also possible in which the rear understructure 2 comprises more than 2 crossmembers.

[0056] Reference Figure 5 and Figure 7 The side members 4 of the present invention each include at least one cross member attachment area 20, which corresponds to the portion of the side member 4 to which the corresponding cross member 5 is attached. The cross member attachment area 20 is composed of Figure 5 and Figure 7 It will be readily appreciated that there are as many cross-member attachment areas 20 per side member 4 as there are cross-members 5 .

[0057] Figure 2 The prior art rear undermount structure depicted in is made of several different parts that are assembled together after forming. Figure 2 In the example of the prior art depicted in , a structure with only one cross member is shown, with 3 different parts present.

[0058] Compared to the prior art, the rear underbody structure 2 of the present invention is manufactured by stamping a single welded blank comprising at least two sub-blanks. This makes it possible to optimize the performance of the part in its different regions, to reduce the overall part weight and to reduce the overall part cost and manufacturing complexity. An example of a welded blank 26 for manufacturing the rear underbody structure 2 according to the present invention is shown in FIG. Figure 8 Given in. Figure 8 The line on embodies the weld 25. This is an example of a tailor-welded blank configuration. The choice of weld location and the choice of grade and thickness distribution are determined by the specific requirements of the vehicle into which the part is integrated. For example, the rear portion of the side member 4 will be made of a material that can absorb the energy of a collision by deformation, while the front portion of the side member 4 will be made of an anti-intrusion material that will not deform during a collision to protect passengers sitting in the rear of the vehicle.

[0059] Among other features, the fact that the part is stamped from one single tailored blank means that it is necessary to adapt the shape of the inner wall 42 to fit in at least one cross-member attachment area 20. In a particular embodiment, the inner wall 42 is not present in the cross-member attachment area 20. In a particular embodiment, the inner wall 42 does not have the same height in the cross-member attachment area 20 as outside of said cross-member attachment area 20.

[0060] In a particular embodiment, the rear understructure 2 is made by hot stamping. Advantageously, this enables very high mechanical strength to be achieved, with complex part shapes and very good geometric tolerances on the finished part due to the absence of springback. This last point is of particular interest in the case of a rear understructure 2 that is a large part and would be subject to high geometric inaccuracies and global shape defects (such as distortions or warping) if a large amount of springback occurs.

[0061] In a specific embodiment, at least one sub-blank of the blank 26 for manufacturing the rear bottom structure 2 is made of the following material: the material makes the yield strength of the corresponding area of ​​the rear bottom structure 2 after press hardening included between 700MPa and 950MPa, the tensile strength included between 950MPa and 1200MPa and the bending angle is above 75°.

[0062] In a specific embodiment, at least one sub-blank of the blank 26 for manufacturing the rear bottom structure 2 has a composition including the following in weight percentage: 0.06%≤C≤0.1%, 1%≤Mn≤2%, Si≤0.5%, Al≤0.1%, 0.02%≤Cr≤0.1%, 0.02%≤Nb≤0.1%, 0.0003%≤B≤0.01%, N≤0.01%, S≤0.003%, P≤0.020%, less than 0.1% of Cu, Ni and Mo, the remainder being iron and unavoidable impurities resulting from processing. Within this composition range, the yield strength of the corresponding area of ​​the rear bottom structure 2 after press hardening is included between 700MPa and 950MPa, the tensile strength is between 950MPa and 1200MPa and the bending angle is above 75°. For example, the sub-blank is composed of 1000 made.

[0063] In a specific embodiment, at least one sub-blank of the blank 26 for manufacturing the rear understructure 2 has: an ultimate tensile strength between 1300 MPa and 1650 MPa after press hardening in a region corresponding to the sub-blank; and a yield strength between 950 MPa and 1250 MPa.

[0064] In a specific embodiment, at least one sub-blank of the blank 26 for manufacturing the rear bottom structure 2 has: an ultimate tensile strength between 1300MPa and 1650MPa after press hardening in an area corresponding to the sub-blank; a yield strength between 950MPa and 1250MPa; and a bending angle of more than 75°.

[0065] In a specific embodiment, at least one sub-blank of the blank 26 for manufacturing the rear bottom structure 2 includes, for example, by weight percentage: 0.20% ≤ C ≤ 0.25%, 1.1% ≤ Mn ≤ 1.4%, 0.15% ≤ Si ≤ 0.35%, ≤ Cr ≤ 0.30%, 0.020% ≤ Ti ≤ 0.060%, 0.020% ≤ Al ≤ 0.060%, S ≤ 0.005%, P ≤ 0.025%, 0.002% ≤ B ≤ 0.004%, and the remainder is iron and unavoidable impurities generated by processing. Within this composition range, the ultimate tensile strength of the part in the area corresponding to the sub-blank after press hardening is included between 1300 MPa and 1650 MPa and the yield strength is included between 950 MPa and 1250 MPa. For example, the sub-blank is composed of 1500 made.

[0066] In a particular embodiment, at least one of the sub-blanks of the blank 26 for producing the rear understructure 2 is made of a material such that the tensile strength of the corresponding region of the rear understructure 2 after press hardening is higher than 1800 MPa.

[0067] In a specific embodiment, at least one sub-blank of the blank 26 for manufacturing the rear bottom structure 2 includes, by weight percentage: 0.24%≤C≤0.38%, 0.40%≤Mn≤3%, 0.10%≤Si≤0.70%, 0.015%≤Al≤0.070%, Cr≤2%, 0.25%≤Ni≤2%, 0.015%≤Ti≤0.10%, Nb≤0.060%, 0.0005%≤B≤0.0040%, 0.003%≤N≤0.010%, S≤0.005%, P≤0.025%, and the balance is iron and unavoidable impurities generated by processing. Within this composition range, the tensile strength of the corresponding area of ​​the rear bottom structure 2 after press hardening is higher than 1800MPa. For example, the sub-blank is made of Made in 2000.

[0068] In a particular embodiment, the blank 26 for manufacturing the rear underbody structure 2 comprises at least one sub-blank coated with an aluminum-based metal coating. By aluminum-based is meant a coating comprising at least 50% aluminum by weight. For example, the metal coating is an aluminum-based coating comprising 8% to 12% Si by weight. For example, the metal coating is applied by immersing the base material in a molten metal bath. Advantageously, applying the aluminum-based metal coating avoids the formation of surface oxide scale during the heating step of the hot stamping process, which in turn enables the production of parts by hot stamping without the need for subsequent sandblasting operations. In addition, the aluminum-based coating also provides corrosion protection to the parts when in use on the vehicle.

[0069] In a particular embodiment, the blank 26 for manufacturing the rear understructure 2 comprises at least one sub-blank coated with an aluminum-based metal coating, which comprises, by weight percentage: 2.0% to 24.0% zinc, 1.1% to 12.0% silicon, optionally 0% to 8.0% magnesium, and optionally an additional element selected from Pb, Ni, Zr or Hf, each additional element having a content by weight percentage lower than 0.3%, the remainder being aluminum and optionally unavoidable impurities. Advantageously, this type of metal coating provides very good corrosion protection to the part and a good surface appearance after hot stamping.

[0070] To produce the tailor-welded blank described above having an aluminum-based coating on at least one of its sub-blanks, laser welding can be used. Sub-blanks on which the edges to be welded are previously prepared by ablating part of the metal coating can be used. Advantageously, this removes part of the aluminum present in the coating, which would contaminate the weld and deteriorate its mechanical properties.

[0071] In a particular embodiment, the blank 26 for making the rear bottom structure 2 includes at least one sub-blank, which includes a top layer with increased emissivity on at least one side. The top layer with increased emissivity is applied to the outermost surface of the sub-blank. The top layer with increased emissivity makes the surface of the sub-blank have a higher emissivity compared to the same sub-blank not coated with the top layer with increased emissivity. The top layer with increased emissivity can be applied on the top side or the bottom side of the sub-blank. The top layer with increased emissivity can also be applied on both sides of the sub-blank.

[0072] If the sub-blank comprises a metal coating, as previously described, the emissivity increasing top layer is applied on top of the metal coating.In practice, in order for the emissivity increasing top layer to increase the emissivity of the surface, it needs to cover the outermost surface of the sub-blank.

[0073] Advantageously, the emissivity increasing top layer will enable an increase in the heating rate of the sub-blanks and thus increase the productivity of the heating step in the hot stamping process.

[0074] In a particular embodiment, the blank 26 includes n sub-blanks S1, S2, ..., Sn, which are arranged in order of increasing thickness. An emissivity-increasing top layer is applied on at least one side of the sub-blank having the greatest thickness. In a particular embodiment, an emissivity-increasing top layer is applied on at least one side of the sub-blank having the greatest thickness and the sub-blank having a thickness just below the greatest thickness. In a particular embodiment, an emissivity-increasing top layer is applied to the x sub-blanks having the highest thickness, x being an integer greater than or equal to 1. Advantageously, by applying an emissivity-increasing top layer to a group of sub-blanks of higher thickness, a more uniform heating rate can be achieved during the heating step of the hot stamping process between the sub-blanks of higher thickness and the sub-blanks of lower thickness. In fact, the sub-blanks of lower thickness will naturally heat faster than the sub-blanks of higher thickness because the sub-blanks of lower thickness are thinner and therefore require less energy to reach the same temperature. By targeting a higher thickness sub-blank with an emissivity-increasing top layer, the difference in heating rate between blanks of different thicknesses can be reduced, and thus a more uniform heating rate can be achieved between the higher thickness sub-blank and the lower thickness sub-blank. In addition, by targeting a higher thickness sub-blank with an emissivity-increasing top layer, the size of the process window of the heating step of the hot stamping process of the blank can also be increased. When hot stamping large parts with high thickness differences, one of the problems is that there will be large differences in the process windows required to achieve the desired microstructure and coating properties of different coatings (the process window also includes heating time and heating temperature, among other parameters). The process window required to achieve the desired performance on the entire blank is the intersection between the process windows of each sub-blank in the individual sub-blanks. By applying the emissivity-increasing top layer to the higher thickness sub-blank, the process windows of each sub-blank in the individual sub-blanks can be brought closer to each other, and thereby increase the size of the intersection between the process windows of all sub-blanks, that is, increase the process window of the overall welded blank.

[0075] In a specific embodiment, the emissivity increasing top layer has a thickness between 2 and 30 microns. In a specific embodiment, the emissivity increasing top layer consists of a silicon-free polymer containing more than 1% nitrogen by weight and containing carbon pigments in an amount between 3 and 30%.

[0076] In a specific embodiment, the blank 26 for manufacturing the rear bottom structure 2 further includes: Figure 8 At least one metal patch 31 is depicted to locally increase the strength of the part. In a particular embodiment, the patch 31 is attached by spot welding. In a particular embodiment, the patch 31 is attached by laser welding. The patch 31 is applied, for example, in areas that need reinforcement because these areas are designed to be attached to the chassis structure or because of mechanical problems such as folding of the part detected during crash testing.

[0077] Generally speaking, the patch 31 has the advantage of providing very local reinforcement on a larger sized part, thereby further optimizing the strength and thickness distribution of the entire tailor-welded blank and keeping the overall weight and cost of the part low.

[0078] The patch 31 is made of, for example, press-hardened steel. The patch 31 is coated with, for example, an aluminum-based metal coating.

[0079] In a particular embodiment, patch 31 is coated with an emissivity increasing top layer to produce the advantages described above of increasing the heating rate and thereby reducing differences in heating rates in areas of patch 31 associated with excessive thickness.

[0080] In certain embodiments, a patch is applied over an area that includes a portion of the weld 25. This is referred to as a weld reinforcement patch 32. Figure 8 Depicted. Such a patch 32 has exactly the same features and optional features as described above. Such a patch 32 strengthens the weld 25. The weld 25 is an area where there is a discontinuity between the two sub-blanks, which causes local inertia changes and may cause a collapse of the plastic hinge type when subjected to high loads generated by a collision. In addition, due to the heat input of the welding process, the weld is surrounded by a heat-affected zone, and the weld itself has a specific chemical composition produced by its metal coating (if any), the filler wire used for welding (if any), and the melting and mixing of the sub-blanks. This complex thermal, chemical and metallurgical situation means that there may be strength variations around the weld 25, and the discontinuity problem described above can further lead to fractures in the weld 25 area when the weld 25 is subjected to excessive loads. Strengthening the weld 25 by a weld reinforcement patch 32 can prevent this plastic hinge phenomenon. The weld reinforcement patch 32 is attached, for example, by welding it to the blank 26. In certain embodiments, the attachment point between the weld reinforcement patch 32 and the blank 26 will not be in the area of ​​the weld 25 so as not to interfere with the mechanical properties of the weld 25 .

[0081] When using a press-hardened steel coated with an aluminum-based metal coating on the blank 26, the hot stamping process results in the formation of an interdiffusion layer between the steel and the metal coating on the hot-formed part. The interdiffusion layer is the result of lateral diffusion of iron from the steel towards the metal coating and aluminum from the coating towards the steel at high temperatures. It has been shown that the thickness of the interdiffusion layer is related to the further performance of the part, for example, the ability of the part to be successfully assembled to the rest of the body by spot welding. In particular, it has been shown that hot-formed parts with an interdiffusion layer thickness comprised between 3 microns and 15 microns have good performance in use. More preferably, it has been shown that hot-formed parts with an interdiffusion layer thickness comprised between 3 microns and 10 microns have excellent performance in use.

[0082] In a specific embodiment, the thickness of the interdiffusion layer in the aluminum-based metal coating region of the rear bottom structure 2 is comprised between 3 and 15 microns. In a specific embodiment, the thickness of the interdiffusion layer in the aluminum-based metal coating region of the rear bottom structure 2 is comprised between 3 and 10 microns.

[0083] The fact that the rear understructure 2 is manufactured as a single part integrating the side members 4 and at least one crossmember 5 means that the shape of the side members 4 needs to be adapted in the crossmember attachment area 20. In particular, as mentioned previously, the shape of the inner wall 42 needs to be adapted. It is not possible to have a continuous inner wall 42 in the attachment area 20, as in the case of the multi-part solutions of the prior art.

[0084] In the event of a rear impact, which will impose a compressive load on the side member 4, the inventors have found that it may be advantageous to maintain the inner radius 45 at least partially in the cross-member attachment area 20. In practice, without being bound by theory, it is believed that the compressive forces are transmitted along the side member 4 mainly through the inner radius 45 and the outer radius 46 of said side member. If there were a large discontinuity in the inner radius 45 on the cross-member attachment area 20, the compressive forces generated by the impact would not be properly transmitted to the rest of the vehicle structure and stress concentrations would occur in said cross-member attachment area 20. This in turn could lead to severe defects of the part in this area and poor safety performance of the vehicle, for example, which could lead to severe buckling in the attachment area 20.

[0085] In certain embodiments, the inner radius 45 extends over at least a portion of the cross member attachment area 20, such as at Figure 6 For example, Figure 6 As shown, this can be achieved by introducing a step between the horizontal wall 41 of the side member 4 and the horizontal wall 51 of the cross member 5 in the cross member attachment area 20. In this step configuration, the inner radius 45 is followed by the cross member radius 55 to create a step. Another possibility to provide a continuation of the inner radius 45 over at least a portion of the transition zone 20 (not shown in the drawings) would be to introduce a bead in the transition zone 20 instead of the step described above. The bead can be formed, for example, by a shape similar to the step described above followed by a symmetrical step according to a symmetry plane perpendicular to the orientation of the cross member 5. This type of bead configuration makes it possible to keep the same height for the horizontal wall 41 and the horizontal wall 51 while maintaining the presence of the inner radius 45 over at least a portion of the transition zone 20.

[0086] In a particular embodiment, the rear bottom structure 2 comprises, for example, Figure 7The geometric variation 22 can be seen on the top of the figure. The geometric variation 22 makes it possible to concentrate the stress from the collision in a specific area of ​​the part, so as to protect other more vulnerable areas of the part. For example, the geometric variation can be placed in the weld 25 (composed of the weld seams 25) connecting the different sub-blanks of the tailor-welded blank for manufacturing the rear understructure 2. Figure 7 ) to protect the weld 25. Considering the sensitivity of the weld 25 under high load conditions described above, using the geometric change portion 22 as a trigger in the event of a collision can advantageously protect the weld 25 and ensure that the formation of a plastic hinge, for example, occurs away from the weld 25, thereby preventing crack formation in the part.

[0087] As mentioned above, the rear understructure 2 according to the present invention comprises at least one crossmember 5, but may also comprise more crossmembers 5. The rear understructure 2 may comprise two, three or more crossmembers 5, for example. Figure 4 is a schematic top view of a rear understructure 2 comprising two crossmembers 5. Integrating several crossmembers 5 in the same rear understructure 2 means increased benefits in terms of cost and simplicity of manufacture. It also means that the blank 26 used to manufacture the rear understructure 2 has better stiffness due to the presence of a plurality of crossmembers 5 connecting the side members 4. This is particularly advantageous for the handling of the blank 26 during manufacture and even more so in the case of hot stamping, which involves handling the blank during the step of transferring the blank heated at high temperature from an austenitizing furnace to a hot stamping press and therefore has a lower mechanical strength.

[0088] The invention also relates to a vehicle 1 comprising a rear understructure 2 as described above.

[0089] The invention also relates to a method for producing the rear understructure 2 described above and assembling it to the rest of the vehicle body.

[0090] The method comprises the following steps:

[0091] - providing a tailor-welded blank 26,

[0092] - stamping the tailor-welded blank 26, for example by hot stamping,

[0093] - Attaching the rear understructure 2 to the body of the vehicle 1 .

[0094] According to an embodiment of the present invention, the following additional notes are also disclosed:

[0095] Note 1. A rear underbody structure (2) for a motor vehicle (1), the rear underbody structure (2) comprising first and second side members (4) and at least one cross member (5) connecting the first and second side members (4), wherein the rear underbody structure (2) is made by stamping a single welded blank (26), the single welded blank (26) comprising at least two sub-blanks.

[0096] Note 2. The rear bottom structure (2) according to Note 1, wherein the rear bottom structure (2) is made by hot stamping.

[0097] Note 3. The rear bottom structure (2) according to Note 1 or 2, wherein the rear bottom structure (2) comprises at least two cross members (5).

[0098] Note 4. The rear bottom structure (2) according to any one of Notes 1 to 3, wherein each side member (4) comprises:

[0099] - an inner wall (42) and a horizontal wall (41) connected by an inner radius (45),

[0100] - at least one cross-member attachment area (20) corresponding to the portion of the side member (4) to which the respective cross-member (5) is attached,

[0101] Wherein, the inner radius (45) extends along at least a portion of the cross member attachment area (20).

[0102] Note 5. The rear bottom structure (2) according to any one of Notes 1 to 4, wherein the tailor-welded blank (26) comprises at least one sub-blank, and the at least one sub-blank is coated with an aluminum-based metal coating.

[0103] Note 6. A rear bottom structure (2) according to any one of Notes 1 to 5, wherein the welded blank (26) includes at least one sub-blank, and the at least one sub-blank is coated with an aluminum-based metal coating, and the aluminum-based metal coating includes, by weight percentage: 2.0% to 24.0% zinc, 1.1% to 12.0% silicon, optionally 0% to 8.0% magnesium, and optionally additional elements selected from Pb, Ni, Zr or Hf, the content of each additional element by weight percentage is less than 0.3%, and the remainder is aluminum and optionally unavoidable impurities.

[0104] Note 7. The rear bottom structure (2) according to any one of Notes 1 to 6, wherein the welded blank (26) includes at least one sub-blank, and the at least one sub-blank includes an emissivity-increasing top layer on at least one side.

[0105] Note 8. A rear bottom structure (2) according to any one of Notes 1 to 7, wherein the welded blank (26) includes at least one sub-blank, and the at least one sub-blank is made of pressed hardened steel having an ultimate tensile strength of more than 1800 MPa after hot stamping.

[0106] Note 9. A rear bottom structure (2) according to any one of Notes 1 to 8, wherein the welded blank (26) includes at least one sub-blank, and the at least one sub-blank is made of the following press-hardened steel, and the press-hardened steel has: a yield strength between 700MPa and 950MPa after hot forming; an ultimate tensile strength between 950MPa and 1200MPa after hot forming; and a bending angle of more than 75° after hot forming.

[0107] Note 10. The rear bottom structure (2) according to any one of Notes 1 to 9, wherein the tailor-welded blank (26) includes at least one metal patch (31).

[0108] Note 11. The rear bottom structure (2) according to Note 10, wherein at least one metal patch (31) includes an emissivity-increasing top layer.

[0109] Note 12. A rear bottom structure (2) according to any one of Notes 1 to 11, wherein the welded blank (26) includes at least one weld reinforcement patch (32), wherein the at least one weld reinforcement patch (32) is applied to an area including the weld (25).

[0110] Note 13. The rear bottom structure (2) according to Note 12, wherein at least one weld reinforcement patch (32) includes an emissivity-increasing top layer.

[0111] Note 14. The rear bottom structure (2) according to any one of Notes 5 to 13, wherein the thickness of the interdiffusion layer in the aluminum-based metal coating region of the rear bottom structure (2) is included between 3 micrometers and 15 micrometers.

[0112] Note 15. The rear bottom structure (2) according to the previous note, wherein the thickness of the interdiffusion layer in the aluminum-based metal coating area of ​​the rear bottom structure (2) is included between 3 microns and 10 microns.

[0113] Note 16. A motor vehicle (1) comprising a rear underbody structure (2) according to any one of the preceding notes.

Claims

1. A tailor-welded blank (26) for manufacturing a rear bottom structure (2), the tailor-welded blank (26) comprising at least one sub-blank, the rear bottom structure (2) being manufactured by stamping the tailor-welded blank (26), wherein The rear bottom structure (2) comprises a geometric change portion, which is placed near the weld connecting different sub-blanks.

2. The tailor-welded blank (26) according to claim 1, wherein: The at least one sub-blank is coated with an aluminum-based metal coating.

3. The tailor-welded blank (26) according to claim 1 or 2, wherein: The aluminum-based metal coating comprises, by weight percentage, 2.0% to 24.0% zinc, 1.1% to 12.0% silicon, and the remainder being aluminum and inevitable impurities.

4. The tailor-welded blank (26) according to claim 3, wherein: The aluminum-based metal coating further comprises, by weight percentage, 0% to 8.0% magnesium.

5. The tailor-welded blank (26) according to claim 4, wherein: The aluminum-based metal coating further comprises, by weight percentage, an additional element selected from Pb, Ni, Zr or Hf, and the content of each additional element by weight percentage is less than 0.3%.

6. The tailor-welded blank (26) according to claim 1 or 2, wherein: The at least one sub-blank comprises an emissivity increasing top layer on at least one side.

7. The tailor-welded blank (26) according to claim 1 or 2, wherein: The at least one sub-blank is made of a press-hardened steel having an ultimate tensile strength of above 1800 MPa after hot stamping.

8. The tailor-welded blank (26) according to claim 1 or 2, wherein: The at least one sub-blank is made of press-hardened steel having: a yield strength after hot forming comprised between 700 MPa and 950 MPa; an ultimate tensile strength after hot forming comprised between 950 MPa and 1200 MPa; and a bending angle after hot forming of more than 75°.

9. The tailor-welded blank (26) according to claim 1 or 2, wherein: The tailor-welded blank (26) includes at least one metal patch (31).

10. The tailor-welded blank (26) according to claim 9, wherein: The at least one metal patch (31) comprises an emissivity increasing top layer.

11. The tailor-welded blank (26) according to claim 1 or 2, wherein: The tailor-welded blank (26) comprises at least one weld reinforcement patch (32), wherein the at least one weld reinforcement patch (32) is applied to a region comprising a weld (25).

12. The tailor-welded blank (26) according to claim 11, wherein: The at least one weld reinforcement patch (32) includes an emissivity increasing top layer.

13. The tailor-welded blank (26) according to claim 1 or 2, wherein: The rear bottom structure (2) is manufactured by hot stamping the tailor-welded blank (26).

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