Rear structure for electric vehicles

Abstract

To provide a design that optimizes energy absorption potential of a rear rail of an electric vehicle.SOLUTION: A rear part structure 1 for an electric vehicle has a rear rail 3 including a rear part, a front part, and a transition region. Both the rear part and the transition region can be deformed at the time of a rear collision so as to maximize an energy absorption amount. The product of an average thickness of the transition region and the limit tensile strength is included in a range of 1 to 1.5 times of the product of an average thickness of the rear part and the limit tensile strength. The transition region is produced from a material having a fracture strain of 0.6 and a critical bending angle of 75°.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a rear structure for a motor vehicle having an electric powertrain, hereinafter referred to as an electric vehicle. The present invention further relates to a method for manufacturing such a rear structure. [Background technology]

[0002] Environmental concerns and regulations related to increasing atmospheric carbon dioxide levels and local air pollution levels are driving the rise of electric vehicles. Compared to traditional internal combustion engine vehicles, electric vehicles have smaller engines and no fuel tanks or exhaust systems. On the other hand, electric vehicles have large battery packs that are not present in internal combustion engines.

[0003] The rear structure of a vehicle is designed to withstand a rear impact collision. One example of a rear impact test used in evaluating the safety of a vehicle is Federal Motor Vehicle Safety Standard 301 (FMVSS 301), in which a vehicle is impacted by a moving deformable barrier having a mass of 1361 kg, traveling at a speed of 80 km / h, and impacting the rear of the vehicle with an overlap of 70%.

[0004] The rear rail is part of the rear structure of a vehicle that extends longitudinally from the rear end of the vehicle to under the rear floor panel. a rear portion extending substantially longitudinally at the same height as the rear bumper assembly and attached at its rear end to the rear bumper assembly; a front portion extending substantially longitudinally at a lower height than the rear portion and attached to the vehicle lateral reinforcement structure; a transition region connecting the rear section and the front section, the transition region including at least an upper bend and a lower bend; Equipped with.

[0005] Current concepts of rear rails take into account the need to absorb energy in the event of a rear collision while protecting the fuel tank, which is generally located under the rear passenger seats. Indeed, the integrity of the fuel tank is essential for passenger safety. A damaged fuel tank can lead to fuel leaks and fire hazards.

[0006] The current design of the rear rail is as follows: - the rear section absorbs energy during a rear collision, for example by controlled buckling, -The front part and the transition area act as anti-intrusion elements to protect the fuel tank.

[0007] This rear rail design has some limitations. In fact, in the case of a rear impact with an energy higher than the energy absorption capacity of the rear structure, the transition area and the rear section will transfer the remaining collision energy in the form of kinetic energy to the vehicle body and occupants. In such a case, the vehicle may be pushed forward or crushed against the obstacle in front, affecting the safety of the occupants. Summary of the Invention [Problem to be solved by the invention]

[0008] One of the objectives of the present invention is to overcome these limitations in the case of electric vehicles, taking into account the absence of a fuel tank, by proposing a design that optimizes the energy absorption potential of the rear rail. [Means for solving the problem]

[0009] To this end, the invention relates to a rear structure for an electric vehicle, wherein the product of the average thickness and the ultimate tensile strength of the transition region is comprised between 1 and 1.5 times the product of the average thickness and the ultimate tensile strength of the rear portion, and the transition region is made from a material having a fracture strain of at least 0.6 and a critical bend angle of at least 75°.

[0010] By applying the above-mentioned invention, it is possible to absorb energy during a rear collision not only by a controlled buckling deformation of the rear section, but also by double bending deformation of the transition regions at its upper and lower bends.

[0011] According to other optional features of the rear structure according to the invention, taken alone or according to any possible technical combination, there is provided: The material from which the rear rail is manufactured has an ultimate tensile strength in part of at least 700 MPa.

[0012] The rear portion includes a geometric variation that locally varies its cross section.

[0013] At least a portion of the rear rail is made by hot stamping a material having a tensile strength of at least 1000 MPa after hot stamping.

[0014] At least a portion of the rear rail is fabricated by hot stamping press hardened steel, the press hardened steel comprising, in weight percent: 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 resulting from processing.

[0015] At least a portion of the rear rail is made by cold stamping a material having a tensile strength of at least 950 MPa.

[0016] At least a part of the rear rail is produced by cold stamping a material, the material having a chemical composition containing, by weight %, 0.13% < C < 0.25%, 2.0% < Mn < 3.0%, 1.2% < Si < 2.5%, 0.02% < Al < 1.0%, 1.22% < Si + Al < 2.5%, Nb < 0.05%, Cr < 0.5%, Mo < 0.5%, Ti < 0.05%, the balance being Fe and unavoidable impurities, having a microstructure containing 8% - 15% retained austenite, the balance being ferrite, martensite and bainite, and the sum of the martensite fraction and the bainite fraction being included between 70% and 92%.

[0017] At least a part of the rear rail is produced by cold stamping a material, the material having a chemical composition containing, by weight %, 0.15% < C < 0.25%, 1.4% < Mn < 2.6%, 0.6% < Si < 1.5%, 0.02% < Al < 1.0%, 1.0% < Si + Al < 2.4%, Nb < 0.05%, Cr < 0.5%, Mo < 0.5%, the balance being Fe and unavoidable impurities, having a microstructure containing 10% - 20% retained austenite, the balance being ferrite, martensite and bainite.

[0018] The rear rail is formed by stamping a tailored welded blank.

[0019] The rear rail is formed by stamping a tailored rolled blank.

[0020] The present invention further relates to a method for manufacturing the aforementioned rear structure 1, the method comprising: providing a blank; stamping the blank into the shape of the rear rail 3; attaching the rear rail 3 to the rear bumper assembly 5; attaching the rear rail 3 to the lateral reinforcement structure 11; and.

[0021] Other aspects and advantages of the invention will become apparent on reading the following description, given by way of example and made with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0022] [Figure 1] 1 is an overall perspective view of a vehicle according to the present invention; [Diagram 2] FIG. 2 is an overall perspective view of the rear structure according to the present invention. [Diagram 3] FIG. 2 is an individual perspective view of a left rear rail according to the present invention; [Figure 4A] 1 is a series of diagrams illustrating a rear crash test simulation of a vehicle in accordance with the present invention using the FMVSS 301 standardized crash described above, showing the situation before the crash occurs. [Figure 4B] 1 is a series of diagrams illustrating a rear crash test simulation of a vehicle according to the present invention using the FMVSS 301 standardized crash mentioned above, showing the situation 60 ms after the crash. [Figure 4C] 1 is a series of diagrams illustrating a rear crash test simulation of a vehicle according to the present invention using the FMVSS 301 standardized crash mentioned above, showing the situation 100 ms after the crash. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] In the following description, the terms "upper", "lower", "front", "rear", "lateral" and "longitudinal" are defined according to the normal directions of the mounted vehicle. More specifically, the terms "upper" and "lower" are defined according to the elevation direction of the vehicle, the terms "front", "rear" and "longitudinal" are defined according to the fore-aft direction of the vehicle, and the term "lateral" is defined according to the width of the vehicle. "Substantially parallel" or "substantially vertical" means a direction that may deviate from the parallel or vertical directions by no more than 15°.

[0024] More specifically, the terms "fracture strain" and "critical bend angle" refer to the fracture strain and critical bend angle criteria defined by Pascal Dietsch et al. in "Methodology to assess fracture during crash simulation: fracture strain criteria and their calibration" in Metallurgical Research Technology Volume 114, Number 6, 2017. The critical bend angle defines the angle at which the first crack is detected on the back of a sample deformed according to the standardized VDA-238-100 standard. The fracture strain is the associated equivalent strain in the material at the deformation point when the critical bend angle is reached.

[0025] Yield strength, ultimate tensile strength, and uniform and total elongation are measured in accordance with ISO standard ISO 6892-1, published in October 2009.

[0026] The average thickness of a part or portion of a part is the thickness of the corresponding area of ​​the sheet used to manufacture said part.

[0027] The term "controlled buckling" refers to a deformation mode of a part subjected to a compressive load, in which the part gradually absorbs the mechanical energy of the compressive load by forming a series of successive waves resulting from successive local buckling deformations of the part. As a result, the length of the part measured in the direction of the compressive load is smaller after deformation than the initial length of the part in said direction. In other words, when a part reacts to a compressive load by controlled buckling, it folds in on itself in the same way as a plastic bottle when a compressive load is applied between the top and bottom of the bottle.

[0028] A rear structure 1 of an electric vehicle 2 will be described with reference to Figures 1 and 2. The rear structure 1 comprises at least two rear rails 3 and a rear bumper assembly 5. The rear bumper assembly 5 comprises, for example, a rear bumper beam 7 and two rear crash boxes 9. The rear rails 3 are attached at their rear ends to the rear bumper assembly 5, for example by welding. The rear rails 3 are further attached at their front ends, on both sides of the vehicle, to vehicle lateral reinforcement structures 11.

[0029] In a particular embodiment, the rear structure 1 further comprises an anti-intrusion cross-member 13 extending between the two rear rails 3 along a substantially lateral direction.

[0030] In a particular embodiment, the electric vehicle 2 comprises an electric engine 15 arranged towards the rear of the vehicle behind the anti-intrusion cross member 13 .

[0031] The electric vehicle 2 further comprises a battery pack 17 arranged under a floor panel 19. Said battery pack 17 is used to store electrical energy that is used to power the vehicle by the electric engine or engines.

[0032] With reference to Figures 1 and 3, each rear rail 3 comprises at least:

[0033] A rear section 20 that extends substantially longitudinally at the same height as the rear bumper assembly 5 and is attached at its rear end to the rear bumper assembly 5. In a particular embodiment, the rear section 20 is provided with a geometric variation 25 designed to locally change the cross section of the rear section 20, thereby acting as a mechanical trigger in case of a compressive load of the rear section 20. The geometric variation 25 has a different cross section from the rest of the rear section 20, so that if the geometric variation has a smaller cross section than the body of the rear section 20, it deforms by buckling in front of the body of the rear section 20, or if the geometric variation has a larger cross section than the body of the rear section 20, it causes a buckling deformation between the two geometric variations. Advantageously, by arranging several such geometric variations 25 along the length of the rear section 20, it is possible to promote a controlled buckling mode of deformation in case of a collision and to control the exact behavior of the controlled buckling of the rear section 20. It should be noted that, by extension, such a geometric change may consist of a region in the rear portion 20 where material has been removed along a portion of the cross section. The presence of such a hole in the cross section acts as a preferential area for buckling to occur due to the localized reduction in cross section.

[0034] A front portion 24 which extends substantially longitudinally at a lower height than the rear portion 20 and is attached to the vehicle lateral reinforcement structure 11 . A transition area 22 comprising at least an upper bend 21 and a lower bend 23. Said upper bend 21 is a bend area connecting the front of the rear section 20 to the remainder of the rear rail 3, and said lower bend 23 is a bend area connecting the rear of the front section 24 to the remainder of the rear rail 3. The transition area 22 is defined by the upper bend 21 at its rear end and the lower bend 23 at its front end. The presence of said bends 21, 23 allows the rear rail 3 to comprise two different horizontal parts, the rear section 20 and the front section 24, extending longitudinally at different heights.

[0035] The front portion 24 is attached, for example, by spot welding, to the lateral reinforcing structure 11. For example, the rear portion 24 is attached to a side sill or a C-pillar lower, which are both part of the lateral reinforcing structure 11. The attachment can be done, for example, by spot welding to one of the following flat surfaces of the rear portion 24: the flange 26, the vertical wall 28, or the bottom wall 30.

[0036] The material from which the transition region 22 is made has a failure strain of at least 0.6 and a critical bend angle of at least 75°, which ensures that the upper flexure 21 and lower flexure 23 will bend and deform during a crash, ensuring maximum energy absorption without fracture, as described below.

[0037] The material from which the transition region 22 is made is such that the product of the average thickness and the ultimate tensile strength of the transition region 22 is comprised between 1 and 1.5 times the product of the average thickness and the ultimate tensile strength of the rear portion 20. The product of the average thickness and the ultimate tensile strength of a material is a measure of the tendency of this material to deform under a given load. The higher this product, the less likely the material is to deform. The design of the present invention ensures that, as will be explained in more detail below, once the rear portion 20 is fully deformed, the transition region 22 takes over the role of energy absorption in the rear rail 3.

[0038] Looking more closely at the sequence of events in a rear impact scenario, such as the FMVSS 301 standardized crash, as shown in Figures 4A, 4B, and 4C, Figure 4A shows the rear structure before it is struck by the movable deformable barrier. The impact force F exerted by the movable deformable barrier is shown in the figure. It has a longitudinal direction and acts on 70% of the width of the vehicle, with the left side of the vehicle being completely covered by the barrier and the right side 30% of the vehicle not affected by the barrier.

[0039] As shown in Figure 4B, the barrier first impacts the rear bumper beam 7, which transmits the impact force F to the crash box 9, which deforms by controlled buckling and itself transmits the impact force F to the rear rail 3. Due to the 70% overlap towards the left side of the vehicle, the left rear rail 3 is more involved in the crash scenario than the right rear rail 3. The important role of the rear rail 3 in case of a rear collision is clear at this stage. The impact force F generated by the impact is first transmitted to the rear part 20, which deforms by controlled buckling and thereby absorbed by the mechanical deformation part of the impact energy.

[0040] As shown in FIG. 4C, when the rear portion 20 is completely crushed, the transition region 22 is subjected to the pressure of the impact force F. Since the front portion 24 is attached to the transverse reinforcement structure 11, the front portion 24 does not move during the rear collision. This generates a reaction force R against the impact force F. The transition region 22 is therefore subjected to the combined effect of the impact force F and the reaction force R, which act in opposite directions, approximately longitudinally, and at two different ridges. The impact force F follows the ridges of the rear portion 20, and the reaction force R follows the ridges of the front portion 24. Under the combined effect of F and R, the transition region 22 deforms in the areas with the highest stress concentration, namely the upper flexure 21 and the lower flexure 23. The combined effect of F and R creates bending moments in the upper flexure 21 and the lower flexure 23, which result in bending deformations in these areas, thereby absorbing a significant amount of the energy of the collision.

[0041] It should be noted that the front portion 24 does not deform significantly during a collision, since it is attached to an element of the transverse reinforcing structure 11. This ensures that the space located between the front portions 24 of each rear rail 3 is not invaded during a rear collision. It is therefore possible to have in this space sensitive elements, such as for example the rear end of the battery pack 17, which can extend up to the rear end of the rear floor panel. The larger the battery pack 17, the more energy can be stored and therefore the longer the range of the vehicle, which is an important point when designing an electric vehicle. By guaranteeing an intrusion-proof area between the front portions 24 of each rear rail 3, the invention makes it possible to design a long battery pack 17 that extends up to the rear end of the rear floor panel.

[0042] The transition region 22 has a minimum failure strain of 0.6 and a critical bend angle of at least 75° so that the upper flexure 21 and lower flexure 23 do not break but rather deform under the combined effects of F and R. If the minimum failure strain and critical bend angle of the transition region 22 were too low, cracks would rapidly develop in the upper and lower flexures followed by catastrophic failure of the components. This would result in a significant reduction in crash energy absorption, creating a significant safety hazard for passengers and potentially damaging the battery pack.

[0043] As previously mentioned, the product of the average thickness and ultimate tensile strength of the transition region 22 is at least equal to or greater than the product of the average thickness and ultimate tensile strength of the rear section 20. This allows for a controlled sequence of deformations within the rear rail 3, ensuring that the rear section 20 deforms first by controlled buckling before initiating the deformation sequence of the transition region 22. In certain embodiments where the rear section 20 further includes a geometric change 25, the rear section 20 has an even greater tendency to deform under compressive loads, adding robustness to the sequence of events during a crash.

[0044] As detailed above, it is important for vehicle designers to ensure that the behaviour of the rear rail 3 during a collision follows a predetermined sequence of deformations. In effect, the designers can predict what will happen next and plan accordingly to ensure the best protection of passengers and vital elements of the vehicle.

[0045] As mentioned above, the product of the average thickness and ultimate tensile strength of the transition region 22 is no more than 1.5 times the product of the average thickness and ultimate tensile strength of the rear portion 20. In fact, if the transition region 22 were too stiff to deform under the impact of a collision, the transition region 22 would act as an anti-intrusion region without significantly deforming. This would significantly reduce the energy absorbed by the rear rail 3, thereby increasing the amount of kinetic energy transferred from the collision, potentially endangering the vehicle and passengers in surrounding vehicles as well.

[0046] In a particular embodiment, the material from which the rear rail 3 is made has an ultimate tensile strength of at least 700 MPa. Advantageously, this ensures structural stability to the rear rail 3 and also ensures that the rear rail 3 absorbs a significant amount of energy when deforming during a crash.

[0047] In a particular embodiment, at least a part of the rear rail 3 is made by hot stamping a material that has a tensile strength of at least 1000 MPa after hot stamping. Advantageously, the use of hot stamping techniques allows to manufacture complex shapes with high resistance and no springback problems after forming. Furthermore, the use of high strength materials with mechanical resistance of more than 1000 MPa for the final parts ensures high energy absorption during a crash.

[0048] For example, the above-mentioned press-hardened steel contains, 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 balance is iron and inevitable impurities resulting from processing.

[0049] In a particular embodiment, at least a portion of the rear rail 3 is produced by cold stamping a material having a tensile strength of at least 950 MPa. Advantageously, by using a high-strength material having a mechanical resistance exceeding 950 MPa for the final part, high energy absorption during a collision is ensured. Furthermore, by using cold stamping instead of hot stamping as described in the previous embodiment, the manufacturing cost can be reduced.

[0050] For example, the rear rail 3 is produced by cold stamping a material having a chemical composition containing, by weight percentage, 0.13% < C < 0.25%, 2.0% < Mn < 3.0%, 1.2% < Si < 2.5%, 0.02% < Al < 1.0%, 1.22% < Si + Al < 2.5%, Nb < 0.05%, Cr < 0.5%, Mo < 0.5%, Ti < 0.05%, with the balance being Fe and inevitable impurities, and having a microstructure containing 8% to 15% retained austenite, the balance being ferrite, martensite and bainite, and the total of the martensite fraction and the bainite fraction being included between 70% and 92%.

[0051] In another example, the rear rail 3 is produced by cold stamping a material having a chemical composition containing, by weight percentage, 0.15% < C < 0.25%, 1.4% < Mn < 2.6%, 0.6% < Si < 1.5%, 0.02% < Al < 1.0%, 1.0% < Si + Al < 2.4%, Nb < 0.05%, Cr < 0.5%, Mo < 0.5%, with the balance being Fe and inevitable impurities, and having a microstructure containing 10% to 20% retained austenite, the balance being ferrite, martensite and bainite.

[0052] According to a particular embodiment, the rear rail 3 is made by stamping a tailored welded blank. The tailored welded blank can be made of materials with different thicknesses and strength levels for cold stamping or can be made of materials with different thicknesses and strength levels for hot stamping. Advantageously, using different grades and thicknesses allows the designer more flexibility to optimize the performance and weight of the part. Furthermore, it allows further control of the deformation sequence of the rear rail 3 by placing a material in the rear section 20 whose ultimate tensile strength by average thickness is less than that of the transition region 22. As explained above, this causes the rear section 20 to deform first in the case of a rear impact.

[0053] According to a particular embodiment, the rear rail 3 is made by stamping a tailor rolled blank, which offers similar advantages as the tailor welded blank discussed above. For example, the rear section 20 is made of a material having a lower average thickness than the transition region 22.

[0054] In a particular embodiment, the rear rail is made of a material having an average thickness comprised between 0.8 mm and 2.0 mm. For example, the rear rail 3 is made by hot stamping a tailored welded blank having a first portion corresponding to the front portion 24 and the transition region 22, made of a material having an average thickness of 1.1 mm and an ultimate tensile strength after hot stamping of more than 1000 MPa, and a second portion corresponding to the rear portion 20 of the rear rail 3, made of a material having an average thickness of 0.9 mm and an ultimate tensile strength after hot stamping of more than 1000 MPa.

[0055] Next, a method for manufacturing the above-mentioned rear structure will be described, which comprises: Providing a blank; stamping the blank into the shape of the rear rail 3; attaching the rear rail 3 to the rear bumper assembly 5; Attaching the rear rail 3 to the lateral reinforcing structure 11; Equipped with.

Claims

1. A rear structure (1) for an electric vehicle (2) comprising at least two rear rails (3), each rear rail (3) comprising at least a rear portion (20) extending substantially longitudinally at the same height as the rear bumper assembly (5) and attached at its rear end to said rear bumper assembly (5); a front portion (24) located forward of the rear portion, extending substantially in the longitudinal direction at a height lower than the rear portion (20) and attached to the vehicle lateral reinforcing structure (11); a transition region (22) located between the rear portion and the front portion in the longitudinal direction of the electric vehicle, the transition region (22) having at least an upper bend (21) and a lower bend (23) and connecting the rear portion (20) and the front portion (24); Equipped with the product of the average thickness and the ultimate tensile strength of the transition region (22) is comprised between 1 and 1.5 times the product of the average thickness and the ultimate tensile strength of the rear portion (20); A rear structure (1), wherein the transition region (22) is made from a material having a fracture strain of at least 0.6 and a critical bend angle of at least 75°, and the rear rail (3) is made by hot stamping a material having a tensile strength of at least 1000 MPa after hot stamping.

2. 2. The rear structure (1) according to claim 1, wherein the rear portion (20) comprises at least one geometric change (25) locally changing its cross section.

3. The composition of the press hardened steel is, by weight, 3. The rear structure (1) according to claim 1 or 2, wherein the contents of the alloy are 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 being iron and unavoidable impurities resulting from processing.

4. A rear structure (1) according to any one of claims 1 to 3, wherein the rear rail (3) is formed by stamping a tailored welded blank.

5. A rear structure (1) according to any one of claims 1 to 3, wherein the rear rail (3) is formed by stamping a tailor rolled blank.

6. A method for manufacturing a rear structure (1) according to any one of claims 1 to 5, comprising the steps of: Providing a blank; stamping the blank into the shape of the rear rail (3); attaching the rear rail (3) to a rear bumper assembly (5); Attaching the rear rail (3) to the lateral reinforcing structure (11); A method for providing the above.

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