Automobile front structure

Abstract

A front member assembly (1) for an automobile comprising an upper shell (11) and a lower shell (12), the upper and lower shells (11, 12) having right, left, and transverse portions, respectively, the upper and lower shells being assembled together by attaching the upper and lower shells along at least upper and lower contours (1121, 1122, 1221, 1222, 1131, 1132, 1231, 1232) of the right and left portions.

Description

[Technical field]

[0001] Automotive manufacturers are faced with ever more demanding requirements to increase the passive safety of vehicles, reduce vehicle weight to minimize greenhouse gas emissions in the case of internal combustion engines, and increase vehicle range in the case of electric vehicles, all while keeping manufacturing costs low and production rates high. [Background technology]

[0002] The front member and dash panel reinforcement form important structural components of the automobile that contribute to the safety of the occupants in the case of a frontal crash. The front member acts to take the crash energy transmitted by the bumper beam through the front crash box and absorb a portion of said energy to protect the occupants at the front of the vehicle and to transmit the energy to the rest of the vehicle structure. The dash panel cross member acts to further prevent intrusion into the passenger compartment and distribute the energy to the two sides of the vehicle structure, thus the dash panel cross member cooperates efficiently. The front member and dash panel cross member are particularly important when dealing with offset frontal crashes, i.e. frontal crashes where only a part of the front of the vehicle is impacted. In fact, the front member is located at the side of the vehicle, which makes it highly demanding in the case of offset impacts. Such impacts are known to occur frequently in real crashes and are the subject of several safety evaluation tests, such as the Insurance Institute for Highway Safety (IIHS) Small Overlap Rigid Barrier (SORB) crash, where the vehicle is impacted with an overlap of only 25% of the width of a rigid barrier moving at 64.4 km / h.

[0003] Furthermore, the front energy absorption and anti-intrusion structure of the vehicle is particularly important when designing motor vehicles that do not have a front combustion engine. This is the case, for example, for electric vehicles, fuel cell vehicles, or vehicles with an engine at the rear. Indeed, the front combustion engine absorbs part of the energy in the case of a frontal impact, but also needs to be removed from the load path to protect the occupants. The absence of a front combustion engine calls for new ways of designing frontal crash management systems. Summary of the Invention [Problem to be solved by the invention]

[0004] It is an object of the present invention to provide a design and assembly method for a front member assembly having a very high crash management efficiency. Such a design can be advantageously used in vehicles that do not have a front combustion engine. It is also an object of the present invention to provide a vehicle having a front member assembly according to the present invention.

[0005] It is also an object of the present invention to provide a front member assembly that is lighter than current designs, thereby saving fuel in the case of combustion engines and increasing range in the case of electric engine powered vehicles.

[0006] Furthermore, the object of the present invention is to address the challenges of increasing productivity, reducing complexity, and reducing costs in vehicle manufacturing. Indeed, the present invention provides a front member assembly with fewer parts than the reference design. The design of the present invention can be manufactured and assembled in very few manufacturing steps compared to the reference. In addition to simplifying manufacturing, reducing the number of manufacturing steps also reduces the environmental footprint of the manufacturing process and reduces the overall CO2 emissions when manufacturing a vehicle. [Means for solving the problem]

[0007] The object of the present invention is achieved by providing a front member assembly according to claim 1, optionally including the features of claims 2 to 4 taken individually or according to any possible combination. A further object of the present invention is achieved by providing a motor vehicle according to claim 5, optionally including the features of claims 6 and 7.

[0008] In the following description and claims, directional terms are defined according to normal directions of the vehicle in which they are mounted.

[0009] In particular, terms such as "top", "upper", "upper", "bottom", "lower", "lower" and the like are established according to the elevation direction of the vehicle. Terms such as "front", "rear", "rear", "front", "forward", "rearward" and the like are established according to the longitudinal direction of the vehicle, i.e. the direction in which the vehicle moves when following a straight line. Terms such as "left side", "right side", "transverse" and the like are established according to an orientation parallel to the width of the vehicle. The terms "inner" and "outer" are to be understood according to the width direction of the vehicle, with "inner" being closest to the central axis of the vehicle, i.e. closest to the interior of the vehicle, while "outer" is positioned further away from said central axis of the vehicle, and is in fact closer to the exterior of the vehicle. The term "horizontal" refers to an orientation in a plane including the longitudinal and transverse directions. The term "vertical" refers to any orientation including the height direction.

[0010] In the figures below, all orientation and spatial references are done using X,Y,Z coordinate references, where Z is the elevation direction of the vehicle, X is the longitudinal direction of the vehicle, and Y is the transverse direction of the vehicle. The references are indicated on each figure. If the figure is a 2D flat representation, the other axis of the figure is represented by a dot in a circle when it is pointing towards the reader, and by a cross in a circle when it is pointing away from the reader, following established convention.

[0011] By "substantially parallel" or "substantially perpendicular" is meant a direction that may deviate from the parallel or perpendicular directions by no more than 15°.

[0012] A steel plate refers to a flat sheet of steel. It has a top and bottom surface, also called the top and bottom sides or top and bottom surfaces. The distance between said surfaces is specified as the thickness of the sheet. The thickness can be measured, for example, using a micrometer, whose spindle and anvil are placed on the top and bottom surfaces. Similarly, the thickness can also be measured on the formed part.

[0013] By average thickness of a part or portion of a part, we mean the overall average thickness of the material that makes up the part after it has been initially formed from a flat sheet into a three-dimensional part.

[0014] Tailor welded blanks are made by assembling multiple sheets or cut blanks of steel, known as sub-blanks, together, for example by laser welding together, to optimize the performance of the part in its different areas, reduce overall part weight, and reduce overall part cost. The sub-blanks that form the tailored welded blanks can be assembled with or without overlap, for example, they can be laser butt welded (no overlap) or spot welded to each other (with overlap).

[0015] In contrast to a tailored welded blank, a monolithic blank refers to a blank that consists of one single sub-blank, rather than multiple sub-blanks joined together.

[0016] A tailor rolled blank is a blank with multiple plate thicknesses obtained by differential rolling during the steel plate manufacturing process.

[0017] Ultimate tensile strength, yield strength, and elongation are measured according to ISO standard ISO 6892-1, published October 2009. Tensile specimens are cut from the flat area. If necessary, smaller sized tensile test samples are taken to cover the entire available flat area on the part.

[0018] 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 value in this invention refers to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle value needs to be normalized to 1.5 mm by the following calculation, where α1.5 is the bending angle normalized at 1.5 mm, t is the thickness, and αt is the bending angle for thickness t. α1.5=(αt×√t) / √1.5

[0019] Hot stamping is a steel forming technique that involves heating a steel blank, or a preformed part made from a steel blank, to a temperature at which the microstructure of the steel is at least partially transformed to austenite, forming the blank or preformed part by stamping at high temperature, and simultaneously quenching the formed part to obtain a microstructure with very high strength, possibly with an additional division or tempering step in the heat treatment.

[0020] A multi-step hot stamping process is a specific type of hot stamping process consisting of at least two process steps, including at least one stamping step and carried out at high temperatures above 300 °C. For example, a multi-step process can include a first stamping operation and a subsequent hot trimming operation, so that at the end of the hot stamping process, the finished part does not need to be further trimmed. For example, a multi-step process can include several successive stamping steps to produce parts with more complex shapes than can be realized using a single stamping operation. For example, the part is automatically transferred from one operation to another in a multi-step process, for example using a transfer press. For example, the part remains in the same tool, which is a multi-purpose tool that can perform various operations, such as the first stamping and the subsequent in-tool trimming operations.

[0021] The term "bottling" refers to a mode of deformation of a part, typically a highly slender part, subjected to a compressive load, in which the part progressively 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 less than the initial length of the part in said direction after deformation. In other words, when the part reacts to a compressive load by controlled buckling, it folds over on itself in the same way as a plastic bottle on which a compressive load is applied between the top and bottom of the bottle. [Brief description of the drawings]

[0022] [Figure 1] 1 is a perspective view of a motor vehicle having a front member assembly according to the present invention; [Diagram 2] 1 is a perspective view of a front portion of a motor vehicle according to the present invention; [Diagram 3] FIG. 2 is an exploded perspective view of a front member assembly according to a reference design. [Figure 4] FIG. 2 is an exploded perspective view of a front member assembly according to one embodiment of the present invention. [Diagram 5] FIG. 2 is a perspective view of an assembled front member assembly according to the present invention; [Figure 6] FIG. 2 is a top view of a lower shell in accordance with a specific embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] First, a general concept of a fully assembled and mounted front member assembly unrelated to the present invention is described. Next, conventional or reference designs of the front member assembly and of the individual parts that make up said assembly are described. Finally, the innovative design according to the present invention is disclosed.

[0024] With reference to FIG. 1, the present invention focuses on a front member assembly 1 of a motor vehicle 100. The front member assembly 1 comprises right and left front members 2, 3 and one or more dash panel cross members 4. With reference to FIG. 2, the front members 2, 3 are connected at their fronts directly or indirectly (i.e., via intermediate connecting parts) to a front bumper assembly 5. The front bumper assembly 5 comprises, for example, a front bumper beam 51 and right and left crash boxes 52. The front bumper assembly 5 is the first element of the vehicle to hit the impactor in the case of a frontal collision. The front member assembly 1 is a key element of the front load path since it acts to immediately capture the crash energy from the front bumper assembly 5.

[0025] The dash panel cross member 4 is connected to the dash panel 6. The front members 2, 3 are further connected at their rear to a vehicle longitudinal crash management system, not shown in the figures. For example, the front members 2, 3 are connected to a rear front member, which typically has a double bent shape in its front portion, allowing its rear portion to extend at a lower height to fit under the floor panel 8 of the vehicle.

[0026] During a frontal collision, the front member assembly 1 will act to partially absorb the collision energy through plastic deformation and will also act to transfer the collision energy to the remainder of the vehicle's structural elements while avoiding intrusion of the collision energy into the passenger compartment.

[0027] For example, the front members 2, 3 will partially absorb the crash energy, for example by bottling, for example by plastic deformation on at least a portion of the front of said front members. For example, rear portions of the front members 2, 3 and the dash panel cross member 4 will resist deformation and intrusion to protect the vehicle occupants and transfer the remaining crash energy to the remaining parts of the vehicle's structural elements.

[0028] Further elements of the vehicle can be connected to the front member assembly. For example, it is normal practice in vehicle architecture to connect the front suspension elements 7 to the front members 2, 3. The connection of the front suspension to the vehicle's body-in-white is also an important structural connection and a critical connection for safety, further reinforcing the important structural role of the front member assembly 1.

[0029] An example of a reference design of the front assembly module 1 and the different sub-parts making up said front member assembly is now described with reference to Figure 3. By reference design, we mean a design according to current design practices, as opposed to the design of the present invention which will be described later.

[0030] The right and left front members 2, 3 are made by assembling the inner 201, 301 and outer 202, 302 front members along vertical flanges oriented along planes substantially parallel to the elevation and longitudinal directions. The flanges are labeled for clarity only in the case of the right front member 2 in FIG. 3, with the upper inner flange 2011 assembled to the upper outer flange 2021 and the lower inner flange 2012 assembled to the lower outer flange 2022.

[0031] The reference front member assembly further comprises upper and lower dash panel cross members 401, 402 designed to be attached to the dash panel 6 to reinforce the dash panel 6 in the event of a collision.

[0032] The reference front member assembly further comprises left and right side front member extensions 203, 303, which are intermediate parts between the front members 2, 3 and the remainder of the vehicle structure rearward of the front members.

[0033] The above-mentioned reference design for the front member assembly presents a major drawback in that it consists of eight different parts, each of which must be fabricated independently with specific tooling and then assembled together. The assembly of the different sub-parts of the reference design is accomplished by attaching the sub-parts along planes that are oriented in very different directions. The flanges of the inner and outer front members are oriented longitudinally and substantially vertically, the upper and lower dash panel cross-members are assembled to the front member along a substantially transverse direction, and the front rail extensions are assembled along an inclined direction between the horizontal and vertical transverse directions. These diverging assembly planes make it impossible to simplify the front member assembly design and reduce the number of parts.

[0034] Besides the drawbacks of manufacturing complexity and high running costs, the use of many different parts to fabricate the front member assembly can also be a source of structural weakness. Indeed, sub-parts need to be assembled together, for example by means of spot welding. Assembly points can be structural weak points due to the effects of the assembly process itself (for example, in the case of welding there is a heat affected zone, which can induce metallurgical deformations and make the area in or around the weld softer and more brittle). Assembly points can also be a source of weakness when the assembly process is not performed correctly, which can happen in industrial conditions due to inevitable variations in the process, maintenance issues, etc. Assembly points can therefore be areas where cracks and failures occur during the running life of the vehicle or in case of an accident.

[0035] The inventors have developed a completely redesigned front member assembly 1, which makes it possible to reduce the number of parts from eight sub-parts in the reference design to only two sub-parts, which reduces the manufacturing complexity and the risk of structural weaknesses at the assembly points.

[0036] 4 and 5, the front member assembly 1 according to the present invention comprises an upper shell 11 and a lower shell 12. The upper shell 11 comprises a right portion 112 and a left portion 113, each of which extends substantially in a longitudinal direction. The right and left portions 112, 113 are connected together by a transverse portion 114 which extends substantially in a transverse direction and is attached to the right and left portions at attachment points located at the rear of the portions. The lower shell 12 comprises a right portion 122 and a left portion 123, each of which extends substantially in a longitudinal direction. The right and left portions 122, 123 are connected together by a transverse portion 124 which extends substantially in a transverse direction and is attached to the right and left portions at attachment points located at the rear of the portions.

[0037] The upper and lower right portions 112, 122 together define a right hollow volume 20. The upper and lower left portions 113, 123 together define a left hollow volume 30. The right and left hollow volumes 20, 30 extend from the front of the right and left portions to their attachment points with their respective transverse portions 114, 124.

[0038] These hollow volumes 20, 30 enable the front member assembly to withstand frontal crash energy, partially absorb the frontal crash energy through plastic deformation, resist penetration, and transfer the energy to the remainder of the structure.

[0039] The upper shell right portion 112 includes lower inner and outer contours 1121 and 1122 that extend across the lower tip of the upper shell right portion on the inner and outer sides, respectively, of the right portion. The lower contour may be, for example, straight or curved. The lower contour may take the form of a flange, for example, as shown in Figures 4 and 5. Similarly, the upper shell left portion 113 includes lower inner and outer contours 1131 and 1132, the lower shell right portion 122 includes upper inner and outer contours 1221 and 1222, and the lower shell left portion 123 includes upper inner and outer contours 1231 and 1232.

[0040] The corresponding upper and lower shell contours have complementary shapes so that they fit together when the upper and lower shells 11, 12 are brought together when the front member assembly is assembled. More specifically, the upper shell right portion lower contour inner 1121 has a complementary shape to the lower shell right portion upper contour inner 1221, the upper shell right portion lower contour outer 1122 has a complementary shape to the lower shell right portion upper contour outer 1222, the upper shell left portion lower contour inner 1131 has a complementary shape to the lower shell left portion upper contour inner 1231, and the upper shell left portion lower contour outer 1132 has a complementary shape to the lower shell left portion upper contour outer 1232. The upper and lower shells are assembled by attaching the upper and lower shells together along at least a portion of the upper and lower left and right inner and outer contours.

[0041] Thanks to the above-mentioned design, it is possible to assemble the entire front member assembly using only two parts, the top and bottom shells 11, 12. This greatly simplifies fabrication and logistics. It also reduces the amount of assembly points limited to the upper and lower contours of the right and left side portions, thereby limiting the amount of assembly-induced structural weaknesses and thus making it possible to produce a more robust front member assembly 1.

[0042] Furthermore, the front member assembly 1 according to the invention is well adapted to serve as an independent assembly base for the entire front module of the vehicle. With reference to FIG. 2, for example, the bumper assembly 5 and the wheelhouse 9 can be attached to the front member assembly 1 to form an independent front module unit, which can then be transported directly to the rest of the body-in-white at the final vehicle assembly plant. Further elements that can be integrated into the front module are possibly suspension elements 7. The fabrication of such an independent front module can significantly simplify the assembly sequence at the final vehicle assembly plant. The front member assembly according to the invention is particularly adapted for this type of implementation, since it is a monolithic structure that is completely rigid both in the longitudinal and transverse directions. This completely rigid longitudinal and transverse structure makes it possible to bond together all the different parts that make up the front module. Furthermore, the design according to the invention also makes it possible to easily provide assembly areas with other parts within the front member assembly 1.

[0043] Below is a description of particular embodiments of the invention as described above, which can be implemented individually or according to any possible combination.

[0044] In a particular embodiment, the upper and lower shells 11, 12 are attached by welding, for example by MIG or MAG welding, along the upper and lower contours of the upper and lower shells 11, 12, respectively.

[0045] In the particular embodiment shown in Figures 4 and 5, the upper and lower profiles of the right and left upper and lower parts consist of flat flanges. Advantageously, this allows them to be joined by resistance spot welding. It also allows assembly by laser welding, for example remote laser welding, for example by forming a laser stitch, which is a short laser weld with a shape well adapted to the mounting function and reproduced along the length of the flange.

[0046] In a particular embodiment, the front member assembly 1 further comprises bulkheads within the hollow volumes 20, 30 to further increase resistance in the event of a frontal collision. The bulkheads are generally flat plates extending in a substantially horizontal plane and assembled to the right and left sides, for example by spot welding or by MIG / MAG welding.

[0047] In the particular embodiment depicted in FIG. 6, the lower shell 12 further comprises left and right side extensions 1225, 1235 at its rear end, which extend rearwardly further rearward than the lower shell transverse portion 124. For example, as shown in FIG. 6, said rear extensions 1225, 1235 flare outwardly to serve as attachment points for longitudinal members located under the floor panel. Indeed, the inward slope from front to rear of the right and left side portions 122, 123 coupled with the outward flare of the rear extensions 1225, 1235 shown in FIG. 6 allows optimizing the width of the front member assembly 1 at the front and capturing the remainder of the longitudinal members of the vehicle structure at the rear while allowing sufficient space for the wheels and wheel houses 9 outboard of the front member assembly towards the transverse portion 4.

[0048] In a particular embodiment, the upper and lower shells 11, 12 are each made by forming a single metal sheet. Advantageously, this allows to further reduce the number of material handling and stamping operations. Using a single metal sheet to make the upper and lower shells also ensures optimal coordination between all parts of said shells and therefore optimal performance when enduring, for example, a frontal collision. In particular, it ensures optimal coordination between the left part and the transverse part, and between the right part and the transverse part. This type of coordination is particularly important in the case of a small overlap frontal impact, where one side will capture most of the collision energy and, thanks to the coordination with the transverse part, will partially transfer it to the other side and to the rest of the vehicle structure.

[0049] In a particular embodiment, the upper and lower shells are each made by forming a single metal plate made of a tailored welded blank, for example using steel plate sub-blanks for tailored welded blanks. Advantageously, this allows for an optimal distribution of material grades and material thicknesses in the different areas of the upper and lower shells according to the desired performance and behavior of each area. This also allows for optimizing and reducing scrap involved in the fabrication of the upper and lower shells. In fact, the upper and lower shells are U-shaped in design, which would entail cutting out a large amount of scrap inside the U-shape if the part were made using a monolithic blank. On the other hand, when using multiple sub-blanks combined into a tailored welded blank, it is possible to use approximately rectangular sub-blanks for the left and right sides and transverse parts, which means that very little scrap is generated. The inventors have also found that by using a single metal plate made from laser welded blanks for the upper and lower shells, improved cooperation between the different parts of the front member assembly and a significant reduction in the overlap area for assembling the individual parts together leads to a significant weight reduction of the order of 15% to 20%.

[0050] In certain embodiments, the blanks or sub-blanks used to manufacture the upper and lower shells comprise tailor rolled blanks.

[0051] In a particular embodiment, the upper and lower shells 11, 12 further comprise a deformable portion extending over the front portion of the right and left portions 112, 122, 113, 123 of said shells, and a non-deformable portion extending over the rear portion of the right and left portions and over at least a part of the transverse portion 114, 124 of said shells. The resistance to plastic deformation of said deformable portion is lower than the resistance to plastic deformation of said non-deformable portion. For example, the product of the tensile strength and the average thickness of said deformable portion is lower than the product of the tensile strength and the average thickness of said non-deformable portion. Advantageously, in case of a frontal impact, this makes it possible to manufacture a front member assembly 1 having a front portion corresponding to the deformable portion closest to the bumper assembly 5 and furthest from the passenger compartment, which will undergo plastic deformation, for example by bottling, and thus absorb part of the crash energy. At the same time, the non-deformable portion of the front member assembly, which is located at the rear and therefore closest to the passenger compartment, resists intrusion, thus protecting the occupants, and also transfers the remaining energy of the impact to the remainder of the vehicle structure.

[0052] In a particular embodiment, the above-mentioned deformable portions are obtained by manufacturing the upper and lower shells using tailored welded blanks and by using a material in front of the left and right portions that is less resistant to plastic deformation than the material used for the transverse portions behind the right and left portions. For example, the product of the tensile strength and average thickness of the deformable portions is lower than the product of the tensile strength and average thickness of the non-deformable portions. Figure 4 shows an example of such an embodiment, where the weld lines that define the boundaries of the materials of the deformable and non-deformable portions are represented diagrammatically by dotted lines 112w, 113w, 122w, and 123w.

[0053] In a particular embodiment, the upper and lower shells are each made by forming a single metal sheet, possibly made of tailored welded blanks, and manufactured using hot stamping. Advantageously, this allows for the production of very high strength parts with complex shapes and without any springback issues. When using tailored welded blanks, this also allows for the elimination of the influence of the assembly process of the sub-blanks on the properties of the final part, since the assembly points, e.g. laser welds or spot welds, undergo metallurgical transformation during the hot stamping process.

[0054] In a particular embodiment, at least one of the upper and lower shells is made by multi-step hot stamping, which advantageously allows the production of even more complex shapes while ensuring very high mechanical properties for the part.

[0055] In a particular embodiment, at least one of the upper and lower shells is manufactured by hot stamping, and the blank used to manufacture it contains one of the following materials, either in the form of a monolithic blank or combined in the form of a tailored welded blank:

[0056] A steel having a composition containing 0.06 wt%≦C≦0.1 wt%, 1 wt%≦Mn≦2 wt%, Si≦0.5 wt%, AI≦0.1 wt%, 0.02 wt%≦Cr≦0.1 wt%, 0.02 wt%≦Nb≦0.1 wt%, 0.0003 wt%≦B≦0.01 wt%, N≦0.01 wt%, S≦0.003 wt%, P≦0.020 wt%, less than 0.1 wt% Cu, Ni, and Mo, the remainder being iron and unavoidable impurities resulting from refining. In this composition range, the yield strength of the corresponding region after hot stamping is in the range of 700-950 MPa, the tensile strength is in the range of 950 MPa-1200 MPa, and the bending angle is greater than 75°. For example, this material is used in the deformable parts of the front member assembly 1 because its high bending angle combined with high mechanical properties makes it possible to absorb large amounts of collision energy, for example by bottling.

[0057] Steels having an ultimate tensile strength after hot stamping in the range of 1300 MPa to 1650 MPa, and a yield strength in the range of 950 MPa to 1250 MPa.

[0058] Steels with ultimate tensile strength after hot stamping in the range of 1300MPa to 1650MPa, yield strength in the range of 950MPa to 1250MPa, and bending angles greater than 75°.

[0059] A steel having a composition containing 0.20 wt%≦C≦0.25 wt%, 1.1 wt%≦Mn≦1.4 wt%, 0.15 wt%≦Si≦0.35 wt%, Cr≦0.30 wt%, 0.020 wt%≦Ti≦0.060 wt%, 0.020 wt%≦Al≦0.060 wt%, S≦0.005 wt%, P≦0.025 wt%, 0.002 wt%≦B≦0.004 wt%, the remainder being iron and unavoidable impurities resulting from refining. In this composition range, the maximum tensile strength of the corresponding area of ​​the part after hot stamping is in the range of 1300 MPa to 1650 MPa, and the yield strength is in the range of 950 MPa to 1250 MPa. For example, this steel composition is used for the non-deformable parts of the front member assembly, since its high mechanical properties make it possible to withstand penetration.

[0060] Steel with a tensile strength higher than 1800 MPa after press hardening.

[0061] 0.24wt%≦C≦0.38wt%, 0.40wt%≦Mn≦3wt%, 0.10wt%≦Si≦0.70wt%, 0.015wt%≦Al≦0.070wt%, Cr≦2wt%, 0.25wt%≦Ni≦2wt%, 0.015wt%≦Ti≦0.10wt%, Nb≦0.060wt%, 0.0005wt%≦B≦0.0040wt%, 0.003wt%≦N≦0.010wt%, S≦0.005wt%, P≦0.025wt%, and the rest is iron and unavoidable impurities resulting from refining, steel. In this composition range, the tensile strength of the corresponding area of ​​the front member assembly after hot stamping is higher than 1800MPa. For example, this material is used for non-deformed areas.

[0062] A steel having a composition comprising C: 0.15-0.25 wt%, Mn: 0.5-1.8 wt%, Si: 0.1-1.25 wt%, Al: 0.01-0.1 wt%, Cr: 0.1-1.0 wt%, Ti: 0.01-0.1 wt%, B: 0.001-0.004 wt%, P≦0.020 wt%, S≦0.010 wt%, N≦0.010 wt%, and optionally containing one or more of the following elements: Mo≦0.40 wt%, Nb≦0.08 wt%, Ca≦0.1 wt%, with the remainder of the composition being iron and unavoidable impurities resulting from refining. In this composition range, the tensile strength of the corresponding region of the front member assembly after hot stamping is higher than 1350 MPa, and the bending angle is greater than 70°.

[0063] A steel having a composition comprising C: 0.26-0.40 wt%, Mn: 0.5-1.8 wt%, Si: 0.1-1.25 wt%, Al: 0.01-0.1 wt%, Cr: 0.1-1.0 wt%, Ti: 0.01-0.1 wt%, B: 0.001-0.004 wt%, P≦0.020 wt%, S≦0.010 wt%, N≦0.010 wt%, and optionally containing one or more of the following elements: Ni≦0.5 wt%, Mo≦0.40 wt%, Nb≦0.08 wt%, Ca≦0.1 wt%, with the remainder of the composition being iron and unavoidable impurities resulting from refining. In this composition range, the tensile strength of the corresponding region of the front member assembly after hot stamping is higher than 1500 MPa, and the bending angle is greater than 70°.

[0064] A steel having a composition including C: 0.2-0.34 wt%, Mn: 0.50-1.24 wt%, Si: 0.5-2 wt%, P≦0.020 wt%, S≦0.010 wt%, N≦0.010 wt%, and optionally containing one or more of the following elements: Al: ≦0.2 wt%, Cr≦0.8 wt%, Nb≦0.06 wt%, Ti≦0.06 wt%, B≦0.005 wt%, Mo≦0.35 wt%, and the remainder of the composition is iron and inevitable impurities resulting from refining. In this composition range, the tensile strength of the corresponding region of the front member assembly after hot stamping is 1000 MPa or more, and the bending angle is greater than 55°.

[0065] A steel having a composition containing C: 0.13-0.4%, Mn: 0.4-4.2%, Si: 0.1-2.5%, Cr≦2%, Mo≦0.65%, Nb≦0.1%, Al≦3.0%, Ti≦0.1%, B≦0.005%, P≦0.025%, S≦0.01%, N≦0.01%, Ni≦2.0%, Ca≦0.1%, W≦0.30%, V≦0.1%, Cu≦0.2%, and verifying the following combination: 114-68*C-18*Mn+20*Si-56*Cr-60*Ni-36*Al+38*Mo+79*Nb-17691*B<20, with the remainder of the composition being iron and unavoidable impurities resulting from refining. For example, this composition is used when hot stamping parts using a multi-step process.

[0066] Steel coated with an aluminum-based metallic coating. By aluminum-based it is meant a coating containing at least 50% aluminum by weight. For example, the metallic coating is an aluminum-based coating containing 8-12% Si by weight. The metallic coating is applied, for example, by immersing the substrate in a molten metal bath. Advantageously, applying an aluminum-based metallic coating avoids the formation of surface scales during the heating step of the hot stamping process, which in turn makes it possible to fabricate the part by hot stamping without a subsequent sandblasting operation. Furthermore, the aluminum-based coating also provides corrosion protection for the part during operation in the vehicle.

[0067] % by weight of zinc, 1.1 to 12.0 by weight of silicon, optionally 0 to 8.0 by weight of magnesium, and optionally an additional element selected from Pb, Ni, Zr, or Hf, the weight content of each additional element being lower than 0.3 by weight, the remainder being aluminum and, optionally, unavoidable impurities. Advantageously, this type of metallic coating provides very good corrosion protection on the part, and good surface aspects after hot stamping.

[0068] In a particular embodiment, at least the upper or lower shell is produced by hot stamping a laser-welded blank comprising at least one sub-blank with an aluminum-based metallic coating, said aluminum-coated sub-blank having been previously prepared by ablating at least part of the metallic coating on the edges to be welded, advantageously removing part of the aluminum present in the coating that could contaminate the weld seam and deteriorate its mechanical properties.

[0069] In a particular embodiment, at least the upper or lower shell is manufactured by hot stamping a laser-welded blank comprising at least one sub-blank, at least one side of which is covered with an emissivity-increasing top layer. The emissivity-increasing top layer is applied on the outermost surface of the sub-blank. The emissivity-increasing top layer allows the surface of the sub-blank to have a higher emissivity compared to the same sub-blank not coated with the emissivity-increasing top layer. The emissivity-increasing top layer can be applied either on the top or bottom surface of the sub-blank. The emissivity-increasing top layer can also be applied on both sides of the sub-blank. If the sub-blank comprises a metal coating as described above, the emissivity-increasing top layer is applied on top of the metal coating. In fact, the emissivity-increasing top layer needs to cover the outermost surface of the sub-blank in order to increase the emissivity of the surface. Advantageously, the emissivity-increasing top layer will allow to increase the heating rate of the sub-blank, thus increasing the productivity of the heating step of the hot stamping process. When using multiple sub-blanks of different thicknesses, said emissivity-increasing top layer is advantageously applied to the thickest sub-blank in order to reduce the difference in heating times between the different sub-blanks and thus increase productivity, widen the process window of hot stamping and make it possible to obtain a final part with globally uniform surface properties.

Claims

1. 1. A front member assembly (1) for an automobile comprising an upper shell (11) and a lower shell (12), each of the upper shell (11) and the lower shell (12) comprising a right side portion (112, 122) and a left side portion (113, 123) extending generally longitudinally, each of the right and left sides being connected to one another by a transverse portion (114, 124) extending generally transversely between attachment points located at rear portions of the left and right sides, the upper and lower right sides (112, 122) together defining a right side hollow volume (20), and the upper and lower left sides (113, 123) together defining a left side hollow volume (30), the right and left sides hollow volumes (20, 30) being defined by a right side hollow volume (20) and a left side hollow volume (30) extending generally transversely between the right side hollow volume (20) and the left side hollow volume (30) and ...). and extending from the front of the left side portion to attachment points of said right and left sides with their respective transverse portions (114, 124), said upper right and left sides (112, 113) being bounded by lower inner and outer contours (1121, 1122, 1131, 1132) and said lower right and left sides (122, 123) being bounded by upper inner and outer contours (1221, 1222, 1231, 1232), said upper and lower shells (11, 12) being assembled together by attaching said upper and lower shells (11, 12) along at least a portion of said corresponding lower and upper contours.

2. 2. A front member assembly (1) as described in claim 1, wherein the upper and lower profiles (1121, 1122, 1131, 1132, 1221, 1222, 1231, 1232) are flat flanges and the upper and lower shells (11, 12) are assembled to each other by attaching the upper and lower shells (11, 12) along the flanges.

3. 3. A front member assembly (1) as claimed in claim 1 or 2, wherein the upper and lower shells (11, 12) are each made by forming a single metal sheet.

4. A front member assembly (1) according to any one of claims 1 to 3, wherein the upper and lower shells (11, 12) are each made by forming a single tailored welded blank by hot stamping.

5. 5. A front member assembly (1) according to any one of claims 1 to 4, wherein the upper and lower shells (11, 12) respectively comprise deformable portions extending across a front portion of the right and left sides of the shells (112, 122, 113, 123) and non-deformable portions extending across a rear portion of the right and left sides of the shells (112, 122, 113, 123) and across at least a part of the transverse portions (114, 124) of the shells, wherein the resistance to plastic deformation of the deformable portions is lower than the resistance to plastic deformation of the non-deformable portions.

6. A motor vehicle comprising a front member assembly (1) according to any one of claims 1 to 5.

7. 7. The vehicle according to claim 6, wherein the front member assembly (1) is mounted to a front bumper assembly (5).

8. 8. The automobile according to claim 6 or 7, wherein the front member assembly (1) is mounted to a dash panel (6).

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