Rear floor panel and structural assembly for automotive vehicles

Abstract

A rear floor panel (1) for a motor vehicle, made from a single metal blank and having rear and front side portions (11L), (11R), (12L), (12R) reinforced relative to a central rear and central front portion (11C), (12C), and a rear structure assembly (2) comprising the aforementioned rear floor panel (1) assembled on a rear underfloor structure (3), which itself comprises left and right side members (3L), (3R) connected to each other by at least one cross member (3C).

Description

[Technical Field]

[0001] The present invention relates to a rear floor panel for a motor vehicle and a rear structural assembly for a motor vehicle. [Background technology]

[0002] Automotive manufacturers are under increasing pressure to reduce vehicle weight while increasing the passive safety of their vehicles in order to minimize greenhouse gas emissions from internal combustion engines or to extend the driving range of electric vehicles. Furthermore, vehicle production costs must be low and production rates must be high, and automakers are seeking to simplify vehicle production by reducing the number of separate, individual parts.

[0003] Rear floor panels and rear structure assemblies are important structural elements that contribute to occupant safety in rear and side collisions. They also protect the gas tank, which is often located under the passenger seats in combustion-engine vehicles. They can also protect the rear electric engine in electric or hybrid vehicles. They also help protect the battery pack or hydrogen tank, which is typically located under the vehicle in electric or fuel cell vehicles.

[0004] They are involved in ensuring good safety performance of the vehicle, for example in the following standardized rear crash simulations.

[0005] -A rear impact assessment by the US Department of Transportation's National Highway Traffic Safety Administration (NHTSA) in which a vehicle is struck by a deformable barrier weighing 1,368 kg, covering a 70% offset width and travelling at an initial speed of 80 km / h.

[0006] -European New Car Assessment Programme (Euro-NCAP) and China New Car Assessment Programme (C-NCAP) rear impact test in which the vehicle is impacted by a rigid barrier weighing 1,100 kg, covering 100% of the full width offset and travelling at an initial speed of 50 km / h.

[0007] They are involved in ensuring good safety performance of the vehicle, for example in the following standardized side impact simulations.

[0008] -The US New Car Assessment Program (USNCAP) pole test involves a vehicle with an initial lateral speed of 32.2 km / h colliding with a fixed pole on its side.

[0009] -IIHS Lateral Moving Deformable Barrier (MDB) test, in which a vehicle is impacted on the side by a deformable barrier weighing 1500 kg and traveling at a speed of 50 km / h.

[0010] These standardized tests are periodically updated to take into account ever more severe crash conditions, for example by increasing the weight of the barrier, the impact speed, and the criteria required to pass the test.

[0011] Both the rear floor panel and rear structure assembly are made up of numerous individual parts, the manufacturing of which involves an expensive manufacturing process with multiple forming and assembly steps to obtain the finished structure. Summary of the Invention [Problem to be solved by the invention]

[0012] The object of the present invention is to address the combined challenges of safety, lightweighting, and high manufacturability by providing a rear floor panel and rear structure assembly with a reduced number of parts, excellent safety performance, and optimized overall weight.

[0013] The design of the present invention can be produced and assembled in significantly fewer manufacturing steps compared to references. In addition to simplifying production, reducing costs, and increasing productivity, reducing the number of production steps also reduces the environmental footprint of the production process and reduces the overall CO2 emissions when manufacturing a vehicle. [Means for solving the problem]

[0014] The object of the present invention is achieved by providing a rear floor panel according to claim 1, optionally comprising the features of claims 2 to 6, used individually or in any possible combination. A further object of the present invention is achieved by providing a rear structure assembly according to claim 7, optionally comprising the features of claims 8 to 9, used individually or in any possible combination.

[0015] Other aspects and advantages of the present 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 explanation of the drawings]

[0016] [Figure 1] 1 is an overall perspective view of a vehicle equipped with a rear floor panel according to the present invention; [Figure 2] 1 is a perspective view of an embodiment of a rear floor panel and its surrounding components according to the present invention. FIG. [Figure 3] 1 is a perspective view of a first embodiment of a rear floor panel according to the present invention. [Figure 4] FIG. 2 is a perspective view of a second embodiment of a rear floor panel according to the present invention. [Figure 5] FIG. 1 is a perspective exploded view of one embodiment of a rear structure assembly according to the present invention. [Figure 6A] 1 is a perspective view of a rear structure assembly according to the present invention when assembled to a vehicle; [Figure 6B] 6B is a cross-sectional view of FIG. 6A taken along the cross-sectional plane AA shown in FIG. 6A. DETAILED DESCRIPTION OF THE INVENTION

[0017] In the following description and claims, directional terms are defined according to the normal directions of the on-board vehicle.

[0018] In particular, terms such as "top," "up," "upper," "above," "bottom," "low," "lower," and "below" are defined according to the elevation direction of the vehicle. Terms such as "front," "back," "rear," "front," "forward," and "backward" are defined according to the longitudinal direction of the vehicle, i.e., the direction the vehicle moves when traveling in a straight line. Terms such as "left," "right," and "side" are defined according to an orientation parallel to the width of the vehicle. The terms "inner" and "outer" should be understood according to the width direction of the vehicle, with "inner" being closest to the vehicle's central axis, i.e., closest to the interior of the vehicle, while "outer" is located further away from said central axis of the vehicle and, in effect, closer to the exterior of the vehicle. The same applies to the terms "distal" and "center," with the "distal" portion being closest to the exterior of the vehicle and the "center" portion being closest to the center of the vehicle. The term "horizontal" refers to an orientation in a plane that includes the longitudinal and lateral directions. The term "upright" refers to any orientation, including elevation.

[0019] In the following figures, all orientations and spatial references are established using an X, Y, Z coordinate reference system, where Z is the vehicle's elevation direction, X is the vehicle's longitudinal direction, and Y is the vehicle's lateral direction. The reference system is indicated on each figure. When the figure is a two-dimensional flat representation, an axis that is outside the figure is represented by a dot in a circle when it faces towards the reader, and by a cross in a circle when it faces away from the reader, following accepted convention.

[0020] By "substantially parallel" or "substantially perpendicular" is meant a direction that may deviate from the parallel or perpendicular direction by no more than 15°.

[0021] A steel plate is a flat steel plate. It has a top and bottom surface, also called the top and bottom sides or top and bottom surfaces. The distance between these surfaces is called the plate thickness. The thickness can be measured, for example, using a micrometer, the axis and base of which are located on the top and bottom surfaces. Similarly, the thickness can also be measured on a molded part.

[0022] The average thickness of a part or portion of a part means 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.

[0023] Tailor welded blanks are manufactured by assembling several plates or cut-out blanks of steel, known as sub-blanks, together, for example by laser welding, to optimize part performance in different areas, reduce overall part weight, reduce overall part cost, and reduce material waste. The sub-blanks that form the tailor welded blank 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).

[0024] In contrast to a tailor-welded blank, a monolithic blank refers to a blank consisting of a single sub-blank, without combining several sub-blanks together.

[0025] A tailor rolled blank is a blank with multiple thicknesses obtained by rolling with a pressure difference during the steel plate manufacturing process.

[0026] Ultimate tensile strength, yield strength and elongation are measured in accordance with ISO standard ISO 6892-1, published in October 2009. The tensile test specimen is cut from a flat area. If necessary, smaller tensile test samples are taken to cover the entire available flat area on the part.

[0027] The bend angle is measured according to the VDA-238 bending standard. For the same material, the bend angle depends on the thickness. For simplicity, the bend angle values ​​in this specification refer to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bend angle value must be normalized to 1.5 mm using the following calculation: where α1.5 is the bend angle normalized to 1.5 mm, t is the thickness, and αt is the bend angle relative to the thickness t.

[0028] α1.5=(αt×√t) / √1.5 Cold stamping is a metal forming technique that involves forming a metal sheet into a shaped part by pressing it between an upper and lower die, called a cold stamping tool. For example, a cold stamping tool has a blank holder that allows the metal sheet to be held by its edges. For example, a cold stamping tool may consist of several devices, each containing an upper and lower die for producing complex shapes and / or for performing further operations such as drilling holes in the part or trimming its edges. Other cold forming techniques exist, such as roll forming, which involves bending a continuous sheet between successive sets of rolls, and simple bending, which involves simply bending a steel sheet using a press and upper and lower bending tools.

[0029] Hot stamping is a steel forming technique that involves heating a steel blank, or a preform 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 preform 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 heat treatment.

[0030] A multi-step hot stamping process is a specific type of hot stamping process consisting of at least two processing steps, including at least one stamping step, performed at temperatures above 300°C. For example, a multi-step process can include a first stamping operation and a subsequent hot trimming operation, so that the finished part does not require further trimming at the end of the hot stamping process. For example, a multi-step process can include several consecutive stamping operations to produce parts with more complex shapes than can be achieved using a single stamping operation. For example, parts can be automatically transferred from one operation to another during a multi-step process, using, for example, a transfer press. For example, parts can remain in the same tool, which is a multi-purpose tool that can perform different operations, such as the initial stamping and subsequent in-tool trimming operations.

[0031] The partial hardening hot stamping process is a hot stamping process in which the heat distribution of the blank is intentionally tailored to be different in different regions of the blank to achieve different material properties in different regions at the end of the hot stamping process. For example, this allows a single metal blank made of a single material to be used to produce a hot stamped part with different levels of hardness and elongation in different regions of the final part. For example, this allows for the production of a part with soft and hard regions, where the soft regions can deform under impact loads to absorb energy, while the hard regions resist deformation and therefore withstand penetration. There are several different techniques for implementing partial hardening. For example, a material can be heated to different temperatures in different regions of the blank, with the higher temperature regions becoming fully austenitic at the exit of the austenitizing furnace, resulting in a very hard microstructure after hot stamping, while the lower temperature regions have a dual-phase ferrite / austenite microstructure at the exit of the austenitizing furnace, resulting in a less hard microstructure after hot stamping. For example, the material may be quenched at different quench rates in different areas of the blank during the hot stamping process itself, with the areas quenched at a faster quench rate having a higher hardness than the areas quenched at a slower rate.

[0032] 1, a motor vehicle 100 has a passenger compartment 101 in which vehicle occupants are located during normal vehicle function. A rear floor panel 1 generally closes off the bottom rear of the passenger compartment and extends longitudinally to the rear end of the vehicle. The rear floor panel 1 extends below the rear passenger seats (not shown in the accompanying drawings) and into the rear trunk 102 of the vehicle, if the vehicle is equipped with a trunk.

[0033] Referring to Figure 2, the rear floor panel 1 is a large component that extends longitudinally from the rear end of the vehicle to the front end of the rear passenger seats, and extends laterally between left and right wheel houses 103 and left and right inner rockers 104. For clarity, only a small portion of the inner rockers is shown in Figure 2. In reality, the inner rockers extend longitudinally over most of the vehicle length.

[0034] Near the rear end of the vehicle, the rear floor panel 1 is attached to, for example, a back panel 105, which is itself attached to a crash box and rear bumper, not shown in the accompanying drawings. At its front, the rear floor panel is attached to, for example, a heel board 106, which is a transverse section positioned at the feet of rear passengers, hence the term "heel board".

[0035] All the above-mentioned peripheral components of the rear floor panel are given as examples of typical peripheral components in a typical vehicle structure, but are in no way limiting of the invention, and are assembled to the rear floor panel by, for example, resistance spot welding, or remote laser welding, or by mechanical assembly such as clinching, riveting, etc.

[0036] The vehicle's rear suspension assembly is located below the rear of the rear floor panel and is not shown in the accompanying drawings. The presence of said suspension assembly, and the fact that the passenger compartment 101 is generally designed to be as large as possible, and particularly as tall as possible, entails that there will be height differences within the rear floor panel 1. The left and right rocker inners 104 are located at a lower height than the rear of the rear floor panel, and because the sides of the rear floor panel are connected to the rocker inners, the front sides of the rear floor panel are located at a lower height than the rear of the rear floor panel.

[0037] To better understand the concept of the rear floor panel 1 according to the invention, it is divided into six parts, namely a rear and a front part, each having a left part, a center part and a right part. Turning to Figure 3, said parts have the following reference numerals:

[0038] A rear portion 11 extending behind the rear passenger seats and divided into left rear, center rear, and right rear portions 11L, 11C, and 11R, respectively. The left rear and right rear portions are also referred to in the remainder of this specification and in the claims as rear sides.

[0039] a front portion 12 extending longitudinally at the height of the rear passenger seats, i.e., the front portion 12 is located directly below the rear passenger seats and is part of the passenger compartment 101. The front portion 12 is divided into a left front portion, a center front portion, and a right front portion, 12L, 12C, and 12R, respectively, the left front portion and the right front portion also being referred to in the remainder of this specification and in the claims as the front sides.

[0040] The right and left front portions 12R, 12L are mounted to the rocker inner panel 104 and are therefore located at a lower height than the rear portion 11. In the accompanying drawings, the front center portion 12C has height transitions on its right and left sides between a maximum height and a minimum height that are close to the height of the rear portion 11. Other designs for managing the height transitions are possible and constitute further embodiments of the present invention that are not represented in the accompanying drawings. For example, the height transitions could be a general height transition that runs across the entire width of the part forming a general slope behind the front portion 12.

[0041] The rear floor panel 1 according to the present invention is formed from a single metal blank. This has significant advantages in simplifying overall vehicle manufacturing, eliminating the complexity, time, and cost of forming several components separately and then assembling them together. It also ensures better performance of the component in terms of crashworthiness, metal fatigue, and rigidity, since there are no assembly points, such as those created by spot welding, which are typically weak spots and prone to failure under load or repeated demands. Furthermore, when a single blank is formed by butt welding individual sub-blanks, for example, overlapping areas that are used to assemble several individual components are unnecessary. Limiting the number of overlapping areas reduces material usage, not only reducing CO2 emissions in manufacturing the materials needed to create the rear floor panel, but also reducing the overall weight of the rear floor panel, thereby saving energy during vehicle operation.

[0042] The rear floor panel is responsible for resisting penetration and absorbing energy in the event of a side impact and a rear impact. When a vehicle is impacted from behind, the impact energy is initially applied to the rear bumper and is then transferred to the rear crash boxes, which are longitudinally aligned with the sides of the rear floor panel.

[0043] Therefore, the left and right sections 11L, 11R, 12L, and 12R are reinforced compared to the center sections 11C and 12C, which do not have a primary structural role in the event of a crash. The product of the ultimate tensile strength (UTS) in MPa and the average thickness (UTS) in mm is generally considered a good indicator of a material's strength. The higher this value, the stronger the material. In the case of the rear floor panel, the blank used to manufacture the part has material properties tailored to the corresponding section of the rear floor panel to take into account the structural role of the side sections. Each of the rear left and right sections 11L and 11R has at least one region with a product of the average thickness and ultimate tensile strength (UTS) that is at least twice, and preferably three times, the product of the average thickness and UTS of the rear center section 11C. Similarly, the front left and front right portions 12L, 12R each comprise at least one region having a product of average thickness and UTS that is at least two times, preferably three times, preferably four times, preferably five times, preferably six times the product of average thickness and UTS of the front center portion 12C.

[0044] The front portion 12 of the rear floor panel 1 is part of the passenger compartment 101 and therefore serves to protect the occupant from penetration during a rear or side collision. On the other hand, the rear portion 11 is not part of the passenger compartment 101 and therefore can deform during a rear or side collision to absorb collision energy and prevent it from injuring the occupant. Considering these different roles of the front and rear portions, in certain embodiments, the front left and front right portions 12L, 12R include at least one region having a product of average thickness and UTS that is, for example, at least 1.5 times greater, and preferably at least 2 times greater, than the maximum product of average thickness and UTS of the rear left and rear right portions 11L, 11R.

[0045] For example, the rear floor panel 1 is formed from a single blank by cold stamping. For example, the rear floor panel is formed from a single blank by hot stamping. For example, the rear floor panel is formed from a single blank by multi-stage hot stamping.

[0046] For example, the rear floor panel may be formed from a single tailor-welded blank by cold stamping or hot stamping. The thickness and material quality of each sub-blank may be selected such that the desired difference in the product thickness and UTS is achieved in the final part after the stamping operation. The sub-blanks may be assembled by laser welding. For example, the rear floor panel may be formed from a single tailor-welded blank, with some of the sub-blanks assembled together by laser butt welding and some of the sub-blanks assembled together by overlap welding. For example, the overlap welding operation may be resistance spot welding.

[0047] For example, a rear floor panel is formed from a single blank by partial cure hot stamping to obtain the above differences in the product of product thickness and UTS in different areas of the final part.

[0048] Generally speaking, differences in UTS due to tensile strength between the side and center sections result in material transition regions. Referring to Figure 3, these transition regions are designated 12CR, 12CL, 11CR, and 11CL, respectively, and refer to the transitions between the front center and front right, front center and front left, rear center and rear right, and rear center and rear left sections. These transition regions can be, for example, laser weld seams in the case of butt-welded laser-welded blanks, overlap spot welds in the case of tailor-welded blanks with overlap regions, or graduated material property transition regions in the case of hot stamping with partial hardening. These transition regions can also be a combination of the previously listed options, such as when performing a partial hardening hot stamping process on a laser-welded blank. Other strategies are possible to tailor the final material properties, such as using tailor-rolled blanks with thicker sides than the center, in which case the transition regions are thickness transition regions.

[0049] The inventors have discovered that the strength and thickness differences between the side and center portions, coupled with the height differences within the rear floor panel, typify formability challenges for manufacturing the part using a single metal blank. The sides behave differently than the center portion during deformation, and the height differences mean that more deformation is required to form the part. As a result, forming defects are more likely to occur in the transition regions 12CR, 12CL, 11CR, and 11CL described above.

[0050] In certain embodiments, the inventors have found that the formation of defects in transition regions 12CR, 12CL, 11CR, and 11CL can be significantly limited and suppressed by positioning the transition regions such that there is no height difference on either side of any cross-section of the transition region in the corresponding final molded part. A cross-section of a part within a given region refers to a cross-section taken along a plane perpendicular to the part within that region. In certain embodiments, the inventors have found that defects can be minimized and suppressed by ensuring that there is no height difference across any cross-section centered at the center of transition region 12CR, 12CL, 11CR, or 11CL and spanning a width W of at least two times the thickness of the thicker material on either side of the transition region. In certain embodiments, the inventors have found that to avoid defects, it is preferable for W to be at least three times the thickness of the thicker material, and more preferably for W to be at least four times the thickness of the thicker material.

[0051] As previously mentioned, in certain embodiments, the front left and right sides 12L, 12R comprise at least one region having a product of average thickness and UTS that is, for example, at least 1.5 times greater, and preferably at least 2 times greater, than the largest product of average thickness and UTS of the rear left and right sides 11L, 11R, thereby providing a transition region 112R and 112L between the front and rear portions, as shown in FIG.

[0052] For the same reasons as above, in certain embodiments, there is no height difference between the front and rear sides of the transition regions 112R and 112L for any cross-section of the transition region. In certain embodiments, the inventors have found that defects can be minimized and suppressed by ensuring that there is no height difference across any cross-section through an area centered on the transition region 112R, 112L and spanning a width W of at least two times the thickness of the thicker material on either side of the transition region. In certain embodiments, the inventors have found that a value of W of at least three times the thickness of the thicker material, and more preferably a value of W of at least four times the thickness of the thicker material, is preferred to avoid defects.

[0053] Referring to Figures 2 and 4, in specific embodiment 4, the rear left and right sections 11L and 11R comprise inner sections 11LI and 11RI and outer sections 11LO and 11RO, and the product of the average thickness and UTS of each of the inner sections 11LI and 11RI is at least twice the product of the average thickness and UTS of each of the rear outer sections 11LO and 11RO. Advantageously, this allows for further integration into a single part made from a single metal blank, with the outer region of the body being white and closing the rear floor panel at its rear outer side. The rear left and right outer sections 11LO and 11RO do not play a critical structural role in the vehicle and have relatively complex shapes. Therefore, it is interesting to reduce the thickness and lower the quality of these parts, allowing for more complex shapes and avoiding the unnecessary use of high-strength materials in areas where they are not needed. For example, the product of the average thickness and UTS of each of the medial portions 11LI, 11RI is at least twice the product of the average thickness and UTS of each of the posterior lateral portions 11LO, 11RO.

[0054] 5, 6A and 6B, a further object of the present invention is a rear structure assembly 2 for an automobile, comprising at least the rear floor panel 1 as described above, and a rear underfloor structure 3 which itself comprises left and right side members 3L, 3R and at least one cross member 3C connecting the left and right side members 3L, 3R. When the rear structure assembly 2 is assembled to the vehicle, the left rear and left front portions 11L, 12L of the rear floor panel 1, together with the left side member 3L of the rear underfloor structure, form a closed compartment surrounding a left hollow space 20L; the right rear and right front portions 11R, 12R of the rear floor panel 1, together with the right member 3R of the rear underfloor structure, form a closed compartment surrounding a right hollow space 20R; The central rear and central front portions 11C, 12C of the rear floor panel 1, together with at least one cross member 3C of the rear underfloor structure, form a closed compartment enclosing a central hollow space 20C.

[0055] Advantageously, by combining the rear floor panel 1 and rear underfloor structure 3 into a rear structure assembly 2 having hollow spaces 20L, 20R, 20C using reinforced high-strength material that forms at least part of the walls surrounding the hollow spaces, it is possible to produce a white, extremely rigid, crash-resistant structure that can absorb collision energy in the event of a collision, resist penetration, and provide excellent rigidity to the rear of the vehicle body.

[0056] In certain embodiments, for the same reasons discussed above regarding the difference in the product of thickness and UTS between the front and rear sides of the rear floor panel, the left and right members 3L, 3R of the rear underfloor assembly 3 each include a rear portion 31L, 31R and a front portion 32L, 32R that substantially correspond to the positions in the assembled vehicle of the rear and front portions 11, 12 of the rear floor panel 1. For example, the product of the average thickness and UTS of the front portions 32L, 32R is at least 1.15 times, preferably 1.20 times, and preferably 1.25 times the product of the average thickness and UTS of the rear portions 31L, 31R.

[0057] In a particular embodiment, the rear underfloor structure 3 is made by forming, for example by hot stamping, a single metal blank. Advantageously, this allows the complete rear structure assembly 2 to be provided by assembling only two parts, each made from a single metal blank. This offers significant advantages in terms of productivity, logistics, cost, crashworthiness, reduction in spot welding, and an overall reduction in CO2 emissions in the manufacturing process.

[0058] In a further embodiment, the rear structure assembly 2 further comprises at least one upper cross member 4 assembled on top of the rear floor panel 1, extending longitudinally between the left and right parts of said rear floor panel 1 and positioned at a height above the at least one cross member 3C of the rear underfloor structure 3. Advantageously, this makes it possible to further reinforce the area corresponding to the crash-resistant and rigid central hollow space 20C.

[0059] In a particular embodiment, the rear floor panel 1 and / or the rear underfloor structure 3 according to the invention are manufactured by cold forming steel sheets, the blanks used to manufacture them comprising at least one of the following materials combined in the form of a monolithic blank, a tailor rolled blank or a tailor welded blank:

[0060] - It is steel, and in terms of weight percentage, it has a chemical composition containing 0.13% < C < 0.25%, 2.0% < Mn < 3.0%, 1.2% < Si < 2.5%, 0.02% < Al < 1.0% (with 1.22% < Si + Al < 2.5%), Nb < 0.05%, Cr < 0.5%, Mo < 0.5%, Ti < 0.05%. The balance is Fe and inevitable impurities. It has a microstructure containing 8% - 15% retained austenite, and the rest is ferrite, martensite, and bainite. The total proportion of martensite and bainite is included between 70% and 92%. With this composition, the steel sheet has a yield strength between 600 MPa and 750 MPa and an ultimate tensile strength between 980 MPa and 1300 MPa while maintaining an elongation over 19% measured in the rolling direction. This material is used, for example, in at least a part of the regions corresponding to the sides 11L, 11R, 12L, 12R of the rear floor panel 1.

[0061] - It is steel, and in terms of weight percentage, it has a chemical composition containing 0.15% < C < 0.25%, 1.4% < Mn < 2.6%, 0.6% < Si < 1.5%, 0.02% < Al < 1.0% (with 1.0% < Si + Al < 2.4%), Nb < 0.05%, Cr < 0.5%, Mo < 0.5%. The balance is Fe and inevitable impurities. It has a microstructure containing 10% - 20% retained austenite, and the rest is ferrite, martensite, and bainite. With this composition, the steel sheet has a yield strength between 850 MPa and 1060 MPa and an ultimate tensile strength between 1180 MPa and 1330 MPa while maintaining an elongation over 13% measured in the rolling direction. This material is used, for example, in at least a part of the regions corresponding to the sides 11L, 11R, 12L, 12R of the rear floor panel 1.

[0062] - It is fully martensitic steel, and the composition of this fully martensitic steel contains 0.15% ≤ C ≤ 0.5% in terms of weight percentage. This material is used, for example, in at least a part of the regions corresponding to the sides 11L, 11R, 12L, 12R of the rear floor panel 1.

[0063] - Dual phase steel, which has a microstructure containing at least martensite and ferrite and has a UTS of at least 590 MPa. This material is used, for example, in at least a portion of the areas corresponding to the sides 11L, 11R, 12L, and 12R of the rear floor panel 1.

[0064] - Dual phase steel, which has a microstructure containing at least martensite and ferrite and has a UTS of at least 780 MPa. This material is used, for example, in at least a portion of the areas corresponding to the sides 11L, 11R, 12L, and 12R of the rear floor panel 1.

[0065] - Dual phase steel, which has a microstructure containing at least martensite and ferrite and has a UTS of at least 980 MPa. This material is used, for example, in at least a portion of the areas corresponding to the sides 11L, 11R, 12L, and 12R of the rear floor panel 1.

[0066] In a particular embodiment, the rear floor panel 1 and / or the rear underfloor structure 3 according to the invention are manufactured by hot stamping steel sheets, the blanks used to manufacture them comprising at least one of the following materials combined in the form of a monolithic blank, a tailor rolled blank or a tailor welded blank:

[0067] This steel has a composition, by weight, of 0.06% C ≤ 0.1%, 1% ≤ Mn ≤ 2%, Si ≤ 0.5%, Al ≤ 0.1%, 0.02% ≤ 0.1%, 0.02% ≤ Nb ≤ 0.1%, 0.0003% ≤ B ≤ 0.01%, N ≤ 0.01%, S ≤ 0.003%, P ≤ 0.020%, and less than 0.1% Cu, Ni, and Mo. The remainder is iron and unavoidable impurities resulting from processing. Within this composition range, the yield strength of the corresponding region after hot stamping is between 700 and 950 MPa, the tensile strength is between 950 and 1200 MPa, and the bending angle is greater than 75°. This material is used because it absorbs energy by deformation without cracking, for example, in the rear regions 11L, 11R, 31L, and 31R.

[0068] - steel, having an ultimate tensile strength after hot stamping comprised between 1300 MPa and 1650 MPa and a yield strength comprised between 950 MPa and 1250 MPa.

[0069] - Steel having an ultimate tensile strength after hot stamping comprised between 1300 MPa and 1650 MPa, a yield strength comprised between 950 MPa and 1250 MPa, and a bending angle greater than 75°.

[0070] This steel has a composition, by weight, of 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%, and 0.002%≦B≦0.004%, with the remainder being iron and unavoidable impurities resulting from processing. In this composition range, the ultimate tensile strength of the corresponding region of the part after hot stamping is between 1300 MPa and 1650 MPa, and the yield strength is between 950 MPa and 1250 MPa. For example, this steel composition is used in the regions corresponding to the front 12L, 12R, 32L, or 32R sections. In fact, this steel grade has high anti-crazing properties.

[0071] - Steel with a tensile strength of more than 1800 MPa after press hardening.

[0072] This steel has a composition by weight of 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%, and P ≤ 0.025%, with the remainder being iron and unavoidable impurities resulting from processing. In this composition range, the tensile strength of the corresponding region after hot stamping is greater than 1800 MPa. For example, this material is used for front sections 12L, 12R, 32L, or 32R to take advantage of its high anti-puncture properties.

[0073] A steel having a composition, by weight, of 0.15-0.25% C, 0.5-1.8% Mn, 0.1-1.25% Si, 0.01-0.1% Al, 0.1-1.0% Cr, 0.01-0.1% Ti, 0.001-0.004% B, P≦0.020%, S≦0.010%, N≦0.010%, and optionally one or more of the following elements, by weight: Mo≦0.40%, Nb≦0.08%, Ca≦0.1%, with the remainder being iron and unavoidable impurities resulting from melting. With this composition range, the tensile strength of the corresponding region of the dash panel assembly after hot stamping is greater than 1350 MPa, and the bend angle is greater than 70°.

[0074] A steel having a composition by weight of 0.26-0.40% C, 0.5-1.8% Mn, 0.1-1.25% Si, 0.01-0.1% Al, 0.1-1.0% Cr, 0.01-0.1% Ti, 0.001-0.004% B, P≦0.020%, S≦0.010%, N≦0.010%, and optionally one or more of the following elements by weight: Ni≦0.5%, Mo≦0.40%, Nb≦0.08%, Ca≦0.1%, with the remainder being iron and unavoidable impurities resulting from melting. In this composition range, the tensile strength of the corresponding region after hot stamping is greater than 1350 MPa, and the bend angle is greater than 70°.

[0075] Steel containing, by weight, 0.2-0.34% C, 0.50-1.24% Mn, 0.5-2% Si, P≦0.020%, S≦0.010%, N≦0.010%, and optionally one or more of the following elements by weight: Al≦0.2%, Cr≦0.8%, Nb≦0.06%, Ti≦0.06%, B≦0.005%, Mo≦0.35%, with the remainder being iron and unavoidable impurities resulting from melting. In this composition range, the tensile strength of the corresponding steel after hot stamping is 1000 MPa or more, and the bend angle is greater than 55°.

[0076] - Steel having a composition by weight 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%, demonstrating the following formula: 114-68*C-18*Mn+20*Si-56*Cr-60*Ni-36*Al+38*Mo+79*Nb-17691*B<20, the remainder of the composition being iron and unavoidable impurities resulting from melting. For example, the composition is used when hot stamping parts using a multi-step process.

[0077] -steel coated with an aluminum-based metallic coating. Aluminum-based means a coating containing at least 50% aluminum by weight. For example, the metallic coating is an aluminum-based coating containing 8-12% silicon by weight. The metallic coating is applied, for example, by immersing the substrate in a molten metal bath. Advantageously, applying the aluminum-based metallic coating prevents the formation of surface scale during the heating step of the hot stamping process, thereby making it possible to manufacture parts by hot stamping without a subsequent sandblasting operation. Furthermore, the aluminum-based coating also provides corrosion protection to the part during use in a vehicle.

[0078] Steel coated with an aluminum-based metallic coating containing 2.0-24.0% (by weight) zinc, 1.1-12.0% (by weight) silicon, optionally 0-8.0% (by weight) magnesium, and optionally an additional element selected from Pb, Ni, Zr, or Hf, the weight content of each additional element being less 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, as well as a good surface appearance after hot stamping.

[0079] In a particular embodiment, the rear floor panel 1 and / or the rear underfloor structure 3 are produced by hot stamping a laser-welded blank comprising at least one sub-blank with an aluminum-based metal coating, said aluminum-coated sub-blank having been previously prepared by cutting away at least a portion of the metal coating on the edges to be welded, thereby advantageously removing a portion of the aluminum present in the coating that would contaminate the weld seam and reduce its mechanical properties.

[0080] In certain embodiments, the rear floor panel 1 and / or rear underfloor structure 3 are fabricated by hot stamping a laser-welded blank comprising at least one sub-blank having at least one side surfaced with an emissivity-increasing top layer. The emissivity-increasing top layer is applied to 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 may be applied to either the top or bottom surface of the sub-blank. The emissivity-increasing top layer may also be applied to 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 must cover the outermost surface of the sub-blank to increase the emissivity of the surface. Advantageously, the emissivity-increasing top layer allows for an increased heating rate of the sub-blank, thus increasing the productivity of the heating step of the hot stamping process. When using several sub-blanks of different thicknesses, said emissivity-increasing top layer is advantageously applied to the sub-blank with the greatest thickness in order to reduce the difference in heating time between the different sub-blanks and therefore increase productivity, increase the process window for hot stamping and make it possible to obtain a final part with uniform surface properties overall.

Claims

1. A rear floor panel (1) for an automotive vehicle (100), which extends longitudinally from a rear end of the vehicle to a front end of a rear passenger seat and extends transversely between left and right wheel houses (103) and left and right rocker inners (104), has a front portion (12) extending under the rear passenger seat and a rear portion (11) extending behind the rear passenger seat, and the front and rear portions (11), (12) have left, center and right portions (11L), (11C), (11R), (12L), (12C), and (12R), respectively; The rear floor panel (1) is manufactured by molding a single metal plate, The right front portion (12R) and the left front portion (12L) are assembled to the right and left rocker inner panels (104) and are located at a lower height than the rear portion (11). each of said rear left and rear right portions (11L), (11R) comprises at least one region having a product of average thickness and UTS that is at least twice the product of the average thickness, expressed in mm, and ultimate tensile strength, UTS, expressed in MPa, of said rear central portion (11C); each of said front left and front right portions (12L), (12R) comprises at least one region having a product of average thickness and ultimate tensile strength UTS that is at least twice the product of average thickness and ultimate tensile strength UTS of said front central portion (12C); Rear floor panel (1).

2. 2. The rear floor panel (1) of claim 1, further comprising material transition areas between the front center and front right (12CR), between the front center and front left (12CL), between the rear center and rear right (11CR), and between the rear center and rear left (11CL), respectively, wherein for any given cross-section of the part, there is no height difference on either side of the material transition area over a width centered on the center of the material transition area and spanning at least twice the thickness of the thicker material on either side of the material transition area (12CR), (12CL), (11CR), (11CL).

3. 3. A rear floor panel (1) as described in claim 2, wherein the single metal blank used to manufacture the rear floor panel (1) is a tailor welded blank with sub-blanks butt-welded together, and the welding operation uses at least one laser source.

4. The rear left and rear right portions (11L), (11R) comprise inner portions (11LI), (11RI) and outer portions (11LO), (11RO), and the product of the average thickness and UTS of each of the inner portions (11LI), (11RI) is at least twice the product of the average thickness and UTS of each of the rear outer portions (11LO), (11RO). A rear floor panel (1) according to any one of claims 1 to 3.

5. A rear floor panel (1) according to any one of claims 1 to 4, wherein each of the rear left and rear right portions (11L), (11R) has at least one region having a product of average thickness and UTS that is at least three times the product of average thickness and UTS of the rear central portion (11C).

6. A rear floor panel (1) according to any one of claims 1 to 5, wherein each of the front left and front right portions (12L, 12R) has at least one region having a product of average thickness and UTS that is at least four times the product of average thickness and UTS of the front central portion (12C).

7. A rear structure assembly (2) for an automotive vehicle, comprising at least the rear floor panel (1) according to any one of claims 1 to 6, and a rear underfloor structure (3) which itself comprises left and right side members (3L), (3R), and at least one cross member (3C) connecting the left and right side members (3L), (3R), wherein when the rear structure assembly (2) is assembled to a vehicle (100), the left rear and left front portions (11L, 12L) of the rear floor panel (1) together with the left member (3L) of the rear underfloor structure form a closed compartment surrounding a left hollow space (20L); the right rear and right front portions (11R, 12R) of the rear floor panel (1) together with the right member (3R) of the rear underfloor structure form a closed compartment surrounding a right hollow space (20R); The central rear and central front portions (11C), (12C) of the rear floor panel (1) together with at least one cross member (3C) of the rear underfloor structure form a closed compartment surrounding a central hollow space (20C). Rear structure assembly (2).

8. 8. The rear structure assembly (2) according to claim 7, wherein the rear underfloor structure (3) is made by molding a single metal blank.

9. 9. A rear structure assembly (2) as described in claim 7 or 8, further comprising at least one upper cross member (4) assembled to the upper part of the rear floor panel (1), extending longitudinally between the left (11L), (12L) and right (11R), (12R) parts of the rear floor panel (1), and positioned at a predetermined height above at least one cross member (3C) of the rear underfloor structure (3).

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