Rear floor panel and rear structural assembly for motor vehicle

Abstract

A rear floor panel (1) for a motor vehicle, the rear floor panel (1) being made from a single metal blank and comprising reinforced rear side portions (11L), (11R) and front side portions (12L), (12R) compared to a mid-rear portion (11C) and a mid-front portion (12C). The rear structural assembly (2) comprises a rear floor panel (1) as described above assembled to a rear underlying structure (3) which itself comprises a right side member (3R) and a left side member (3L) joined together 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 to a rear structure assembly for a motor vehicle. Background Art

[0002] Automobile manufacturers are facing increasingly demanding requirements. They are required to increase the passive safety of vehicles and at the same time reduce vehicle weight in order to minimize greenhouse gas emissions for internal combustion engines or to increase the driving range of vehicles for electric vehicles. In addition, vehicle production costs must remain low and productivity must remain high. Furthermore, automobile manufacturers want to simplify vehicle production by reducing the number of individual components.

[0003] The rear floor panel and rear structure components are critical structural elements that contribute to the safety of the occupants in the event of rear impact and side impact. In the case of an internal combustion engine, they also protect the fuel tank, which is typically located under the passenger seat. They may protect the rear electric engine in the case of an electric or hybrid vehicle. They also help protect the battery pack or hydrogen tank, which is typically located under the vehicle in the case of an electric or fuel cell vehicle.

[0004] They involve ensuring good safety performance of the vehicle, for example in the following standardized rear impact simulations:

[0005] - National Highway Traffic Safety Association (NHTSA) rear impact evaluation, where the vehicle is impacted by a deformable barrier weighing 1368kg, covering a 70% width offset and travelling at an initial speed of 80km / h.

[0006] - European New Car Assessment Programme (Euro-NCAP) and China New Car Assessment Programme (C-NCAP) rear impact tests, where the vehicle is impacted by a rigid barrier weighing 1100kg, covering a full 100% width offset and travelling at an initial speed of 50km / h.

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

[0008] - United States New Car Assessment Program (USNCAP) pole test, in which a vehicle with an initial lateral velocity of 32.2 km / h strikes a fixed pole on its side.

[0009] - The IIHS side movable deformable barrier (MDB) test, in which a vehicle is impacted on its side by a deformable barrier weighing 1500kg and travelling at 50km / h.

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

[0011] Both the rear floor panel and the rear structure assembly comprise a number of individual components. The manufacture of said components involves an expensive manufacturing process: multiple forming operations and assembly steps to obtain the finished structure. Summary of the invention

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

[0013] Compared to the reference, the design of the present invention can be produced and assembled with very few manufacturing steps. 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 overall CO2 emissions when manufacturing vehicles.

[0014] The object of the invention is achieved by providing a rear floor panel according to claim 1, optionally comprising the features of claims 2 to 6 taken alone or in any possible combination. Another object of the invention is achieved by providing a rear structure assembly according to claim 7, optionally comprising the features of claims 8 to 9 taken alone or in any possible combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Other aspects and advantages of the invention will emerge on reading the following description given by way of example and made with reference to the accompanying drawings, in which:

[0016] - Figure 1 is an overall perspective view of a vehicle including a rear floor panel according to the present invention.

[0017] - Figure 2 is a perspective view of an embodiment of a rear floor panel according to the present invention and its surrounding components.

[0018] - Figure 3 is a perspective view of a first embodiment of a rear floor panel according to the present invention;

[0019] - Figure 4 is a perspective view of a second embodiment of a rear floor panel according to the present invention;

[0020] - Figure 5 is an exploded perspective view of an embodiment of a rear structural assembly according to the present invention;

[0021] - Fig. 6Ais a perspective view of what a rear structure assembly according to the present invention will look like when assembled on a vehicle, and Figure 6B yes Fig. 6A The basis Fig. 6A The cross section shown on the cross-sectional plane AA. DETAILED DESCRIPTION

[0022] In the following description and appended claims, directional terms are defined according to the general orientation of the vehicle in which they are installed.

[0023] In particular, the terms "top", "up", "upper", "above", "bottom", "lower", "lower", "below", etc. are defined according to the height direction of the vehicle. The terms "front", "rear", "rear", "front", "forward", "rearward", etc. are defined according to the longitudinal direction of the vehicle, i.e. the direction in which the vehicle moves when following a straight line. The terms "left", "right", "lateral", etc. are defined according to an orientation parallel to the width of the vehicle. The terms "inside", "outside" should be understood according to the width direction of the vehicle: "inside" is closest to the central axis of the vehicle, i.e. closest to the inside of the vehicle, while "outside" is positioned further away from the central axis of the vehicle, in fact closer to the outside of the vehicle. The same applies to the terms "distal" and "central": the "distal" part is positioned closest to the outside of the vehicle, and the "central" part is positioned closest to the center of the vehicle. The term "horizontal" refers to the orientation of a plane including the longitudinal direction and the transverse direction. The term "vertical" refers to any orientation including the height direction.

[0024] In the following figures, orientation and spatial references are all made using X, Y, Z coordinate references, where Z is the height direction of the vehicle, X is the longitudinal direction of the vehicle, and Y is the lateral direction of the vehicle. References are indicated in each figure. When the figures are 2D plan views, following established convention, an axis outside the figure is indicated by a dot in a circle when it is pointing towards the reader, and by a cross in a circle when it is facing away from the reader.

[0025] The term “substantially parallel” or “substantially perpendicular” means that the direction may deviate from the parallel direction or the perpendicular direction by no more than 15°.

[0026] Steel plate refers to a flat steel plate. The steel plate has a top face and a bottom face, which are also referred to as top side and bottom side or as top surface and bottom surface. The distance between the faces is designated as the thickness of the plate. The thickness can be measured, for example, using a micrometer, the spindle and anvil of which are placed on the top and bottom faces. In a similar manner, the thickness can also be measured on formed parts.

[0027] By average thickness of a component or average thickness of a portion of a component is meant the overall average thickness of the material constituting the component after it has been formed from an initially flat sheet into a three-dimensional component.

[0028] Custom welded blanks are made by assembling together, for example by laser welding, several steel plates or cut steel blanks, called sub-blanks, to optimize the performance of the component in different areas thereof, thereby reducing overall component weight, overall component cost, and material waste. The sub-blanks that form the custom welded blank can be assembled with or without overlap, for example, the sub-blanks can be laser butt welded (without overlap), or the sub-blanks can be spot welded to each other (with overlap).

[0029] In contrast to a custom welded blank, a monolithic blank refers to a blank comprising one single sub-blank, rather than a blank comprising several sub-blanks combined together.

[0030] The customized rolled blank is a blank having multiple plate thicknesses obtained by differential rolling during the steel plate production process.

[0031] The ultimate tensile strength, yield strength and elongation are measured according to ISO standard ISO 6892-1 published in October 2009. The tensile test specimens are cut from a flat area. If necessary, use undersized tensile test specimens to fit the entire available flat area on the component.

[0032] The bending angle is measured according to the VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For simplicity, the bending angle values ​​of the present invention refer to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle values ​​need to be normalized to 1.5 mm by the following calculation, where α1.5 is the bending angle normalized to 1.5 mm, t is the thickness, and αt is the bending angle for thickness t:

[0033] α1.5=(αt×√t) / √1.5

[0034] Cold stamping is a forming technique for metals that involves forming a sheet of metal into a formed part by pressing it between an upper die and a lower die, called a cold stamping tool. For example, a cold stamping tool has a blank holder that allows the sheet of metal to be held on its side. For example, a cold stamping tool includes several steps, each step involving an upper die and a lower die to produce a complex shape and / or perform further operations, such as punching holes in the part or trimming the sides of the part. There are other cold forming techniques, such as, for example: roll forming, which involves bending a continuous sheet between a continuous set of rollers; simple bending, which involves simply bending a steel sheet using a press and an upper bending tool and a lower bending tool, etc.

[0035] Hot stamping is a forming technology for steel which involves heating a steel blank or a preformed part made from a steel blank until the microstructure of the steel is at least partially transformed into austenite, forming the blank or preformed part at high temperature by stamping it and simultaneously quenching the formed part to obtain a microstructure with very high strength, possibly with an additional partitioning or tempering step during the heat treatment.

[0036] A multi-step hot stamping process is a specific type of hot stamping process that includes at least one stamping step and includes at least two process steps performed at a high temperature greater than 300°C. For example, a multi-step process may involve a first stamping operation and a subsequent hot trimming operation, so that the finished component at the outlet of the hot stamping process does not require further trimming. For example, a multi-step process may involve several consecutive stamping steps in order to manufacture a component with a more complex shape than can be achieved using a single stamping operation. For example, in a multi-step process, the component is automatically transferred from one operation to another, such as using a transfer press. For example, the component stays in the same tool, which is a multifunctional tool that can perform different operations, such as a first stamping operation and a subsequent in-tool trimming operation.

[0037] A partially hardened hot stamping process is a hot stamping process in which the heat profile to which the blank is subjected is deliberately tailored to be different in different areas of the blank in order to obtain different material properties in these different areas at the end of the hot stamping process. For example, this allows the production of hot stamped parts using a single metal blank made of a single material, which will have different levels of hardness and elongation in different areas of the finished part. For example, this allows the production of parts with soft and hard zones, the soft zones being able to deform under impact loads in order to absorb energy, while the hard zones will resist intrusion by resisting deformation. There are several different techniques to achieve partial hardening. For example, the material can be heated at different temperatures in different areas of the blank, the higher temperature area will be fully austenitic at the exit of the austenitizing furnace, resulting in a very hard microstructure after hot stamping, while the lower temperature area will have a subcritical ferrite / austenite microstructure at the exit of the austenitizing furnace, resulting in a microstructure of lower hardness after hot stamping. For example, the material may be quenched at different quenching rates in different areas of the blank during the hot stamping step itself, and areas quenched at a higher quenching rate will have a higher hardness than areas quenched at a lower rate.

[0038] Reference Figure 1The motor vehicle 100 has a passenger compartment 101 in which the occupants of the vehicle are located during normal operation of the vehicle. The rear floor panel 1 generally closes the bottom rear portion of the passenger compartment and extends in the longitudinal direction to the rear end of the vehicle. If the vehicle is equipped with a luggage compartment, the rear floor panel 1 extends under the rear passenger seats (not shown in the drawings) and extends into the rear luggage compartment 102 of the vehicle.

[0039] Reference 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 seat and extends transversely between the left wheel housing 103 and the right wheel housing 103 and between the left rocker inner member 104 and the right rocker inner member 104. Figure 2 In the figure, for the sake of clarity, only a small part of the rocker inner part is shown. In fact, the rocker inner part extends longitudinally on most of the vehicle length.

[0040] Towards the rear end of the vehicle, the rear floor panel 1 is attached, for example, to a rear panel 105, which is itself attached to a crash box and a rear bumper, which are not shown in the figures. The rear floor panel is attached at its front part, for example, to a heel plate 106, which is a transverse part located at the feet of the rear passengers, hence the term "heel plate".

[0041] All the above-mentioned surrounding parts of the rear floor panel are given as examples of typical surrounding parts in a typical vehicle structure, but in no way limit the present invention. The surrounding parts are assembled to the rear floor panel, for example, by resistance spot welding, or by remote laser welding, or by mechanical assembly such as riveting, clinching, etc.

[0042] The rear suspension assembly of the vehicle is located below the rear portion of the rear floor panel and is not shown in the drawings. The presence of the suspension assembly and the fact that the passenger compartment 101 is generally designed to be as large as possible, especially as high as possible in terms of height, result in a height difference within the rear floor panel 1. The left rocker inner member 104 and the right rocker inner member 104 are located at a lower height than the rear portion of the rear floor panel, and since the sides of the rear floor panel are connected to the rocker inner members, the sides of the front portion of the rear floor panel are located at a lower height than the rear portion of the rear floor panel.

[0043] In order to better understand the concept of the rear floor panel 1 according to the present invention, the rear floor panel 1 will be divided into six parts: a rear part and a front part, each of which has a left part, a center part and a right part. Figure 3 , the parts have the following reference numerals:

[0044] a rear portion 11 which extends behind the rear passenger seats and is divided into a left rear portion 11L, a central rear portion 11C and a right rear portion 11R respectively - the left rear portion and the right rear portion will also be referred to as rear side portions in the remainder of the description and in the claims,

[0045] - a front part 12, which extends in the longitudinal direction at the level of the rear passenger seats, that is, it is located directly below the rear passenger seats and is part of the passenger compartment 101. The front part 12 is divided into a left front part 12L, a central front part 12C and a right front part 12R, which will also be referred to as front side parts in the rest of the description and in the claims,

[0046] The right front section 12R and the left front section 12L are assembled to the rocker inner component panel 104 and are therefore located at a lower height than the rear section 11. In the figures, the middle front section 12C includes a height transition between a maximum height and a minimum height close to the height of the rear section 11 on its right and left sides. Other designs for managing the height transition are also possible and constitute other embodiments of the invention that are not represented in the figures. For example, the height transition can be a substantially height transition across the entire width of the component forming a substantially inclined portion at the rear of the front section 12.

[0047] The rear floor panel 1 according to the invention is formed from a single metal blank. This has huge advantages in simplifying the overall vehicle manufacturing, saving the complexity, time and cost of forming several components separately and then assembling them together. It also ensures better performance of the components in terms of impact resistance, fatigue and rigidity, because there are no assembly points made, for example, by spot welding, which are usually weak points and tend to break under load or when subjected to repeated drawing. In addition, if, for example, a single blank is formed by butt welding of separate sub-blanks, no overlapping areas for assembling several separate components are required - limiting the number of overlapping areas reduces material use and saves CO2 emissions when manufacturing the materials required for manufacturing the rear floor panel, as well as reducing the overall weight of the rear floor panel, which in turn saves energy when the vehicle is in use.

[0048] The rear floor panel is involved in resisting intrusion and absorbing energy in the event of a side impact and in the event of a rear impact. When a vehicle is impacted from the rear, the impact energy initially imparted to the rear bumper is transferred to the rear crash box which is longitudinally aligned with the side portions of the rear floor panel.

[0049] Thus, the left parts 11L, 12L and the right parts 11R, 12R are reinforced compared to the central parts 11C, 12C which do not have a major structural role in the event of a collision. The product of the ultimate tensile strength UTS expressed in MPa and the average thickness expressed in mm is generally considered to be a good indicator of the strength of the material. The higher it is, the stronger the material. In the case of the rear floor panel, the blank used to manufacture the component will have material properties customized to the corresponding part of the rear floor panel in order to take into account the structural role of the side parts. Each of the left rear part 11L and the right rear part 11R includes at least an area where the product of the ultimate tensile strength UTS and the average thickness is at least twice, preferably three times, the product of the UTS and the average thickness of the central rear part 11C. Similarly, each of the left front portion 12L and the right front portion 12R includes at least an area where the product of UTS and average thickness is at least twice, preferably three times, preferably four times, preferably five times, preferably six times the product of UTS and average thickness of the central front portion 12C.

[0050] Because the front portion 12 of the rear floor panel 1 is part of the passenger compartment 101, it plays a role in protecting passengers from intrusion during a rear collision or a side collision. On the other hand, the rear portion 11 is not part of the passenger compartment 101, and therefore can be deformed during a rear collision or a side collision in order to absorb the impact energy and prevent the impact energy from injuring the passengers. In view of these different roles of the front portion and the rear portion, in a specific embodiment, the front left portion 12L and the front right portion 12R include at least an area where the product of UTS and the average thickness is, for example, at least 1.5 times larger, preferably at least 2 times larger, than the maximum product of UTS and the average thickness of the rear left portion 11L and the rear right portion 11R.

[0051] 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-step hot stamping.

[0052] For example, the rear floor panel is formed from a single custom welded blank by cold stamping or hot stamping. For example, the rear floor panel is formed from a single custom welded blank, wherein the thickness and material grade of each sub-blank are selected so that after the stamping operation, the desired difference in the above-mentioned UTS multiplied by the thickness is obtained on the finished part. For example, the sub-blanks are assembled by laser welding. For example, the rear floor panel is formed from a single custom welded blank, wherein a portion of the sub-blanks have been assembled together by butt laser welding and a portion of the sub-blanks have been assembled together by lap welding. For example, the lap welding operation is a resistance spot welding operation.

[0053] For example, the rear floor panel is formed from a single blank by partially hardened hot stamping in order to obtain the different UTS and thickness products in different areas of the finished part.

[0054] Generally speaking, the UTS difference in tensile strength of the side portion and the central portion results in a material transition zone. Figure 3 , these material transition zones are referred to as 12CR, 12CL, 11CR and 11CL, respectively, to represent the transition between the center front part and the right front part, between the center front part and the left front part, between the center rear part and the right rear part, and between the center rear part and the left rear part. These transition zones can be, for example, laser welds in the case of butt laser welded blanks, or overlapping spot weld areas in the case of custom welded blanks with overlapping areas, or progressive material property transition zones in the case of hot stamping with partial hardening. These transition zones can also be a combination of the possibilities listed previously, such as, for example, when a partially hardened hot stamping process is performed on a laser welded blank. Other strategies can also customize the final material properties, such as, for example, using a custom rolled blank with a greater thickness on the sides than in the center - in this case, the transition zone is a thickness transition zone.

[0055] The inventors have found that the difference in strength and thickness between the side portions and the center portion, as well as the height difference within the rear floor panel, represent a formability challenge for manufacturing the component using a single metal blank. The side portions will behave differently from the center portion during deformation, and the height difference means that a large amount of deformation needs to be applied to form the component. As a result, cracks due to forming may occur in the above-mentioned transition regions 12CR, 12CL, 11CR and 11CL.

[0056] In a specific embodiment, the inventors have found that crack formation in the transition zones 12CR, 12CL, 11CR and 11CL is greatly limited and crack formation can be inhibited by positioning the transition zones so that in the corresponding final formed part they are positioned so that for any transverse cross section of the transition zone, there is no difference in height on either side of the material transition zone. A transverse cross section of a part in a given area refers to a cross section along a plane perpendicular to the part in the area. In a specific embodiment, the inventors have found that cracks can be minimized and inhibited by ensuring that there is no difference in height on any transverse cross section over a width W centered at the center of the transition zones 12CR, 12CL, 11CR and 11CL and spanning a width equal to or greater than twice the thickness of the thicker material on either side of the transition zone. In a specific embodiment, the inventors have found that preferably, the value of W is greater than or equal to 3 times the thickness of the thicker material, and more preferably, the value of W is greater than or equal to 4 times the thickness of the thicker material to avoid cracks.

[0057] As mentioned above, in a specific embodiment, the left front side portion 12L and the right front side portion 12R include at least an area where the product of UTS and average thickness is, for example, at least 1.5 times larger, preferably at least 2 times larger, than the maximum product of UTS and average thickness of the left rear side portion 11L and the right rear side portion 11R. This in turn leads to transition areas 112R and 112L between the front side portion and the rear side portion, as shown in FIG. Figure 3 As shown above.

[0058] For the same reasons as above, in a specific embodiment, for any transverse cross section of the transition zone, there is no difference in height between the front and rear sides of the transition zones 112R and 112L. In a specific embodiment, the inventors have found that cracks can be minimized and inhibited by ensuring that there is no height difference in any transverse cross section over an area centered on the transition zones 112R and 112L and spanning a width W equal to or greater than twice the thickness of the thicker material on either side of the transition zone. In a specific embodiment, the inventors have found that, preferably, the value of W is greater than or equal to 3 times the thickness of the thicker material, and more preferably, the value of W is greater than or equal to 4 times the thickness of the thicker material to avoid cracks.

[0059] Reference Figure 2 and Figure 4 , in a specific embodiment, 4, the left rear part 11L and the right rear part 11R include inner parts 11LI, 11RI and outer parts 11LO, 11RO, and the product of the UTS and the average thickness of each of the inner parts 11LI, 11RI is at least twice the product of the UTS and the average thickness of each of the rear outer parts 11LO, 11RO. Advantageously, this allows the outer area of ​​the body-in-white to be further integrated into a single component made of a single metal blank, the outer area of ​​the body-in-white enclosing the rear floor panel at its rear outer side. The left rear outer part 11LO and the right rear outer part 11RO do not play a major structural role in the vehicle and have a relatively complex shape. Therefore, it is interesting that these parts have a lower thickness and grade to allow them to form more complex shapes and not to unnecessarily use high-strength materials in areas where they do not need to be used. For example, the product of the UTS and the average thickness of each of the inner portions 11LI, 11RI is at least twice the product of the UTS and the average thickness of each of the rear outer portions 11LO, 11RO.

[0060] Reference Figure 5 , Fig. 6A and Figure 6BAnother object of the present invention is a rear structural assembly 2 for a motor vehicle, the rear structural assembly 2 comprising at least the rear floor panel 1 and the rear understructure 3 as described above, the rear understructure 3 itself comprising a left side member 3L and a right side member 3R and at least one cross member 3C connecting the right side member 3R and the left side member 3L. When the rear structural assembly 2 is assembled in a vehicle:

[0061] - the left rear portion 11L and the left front portion 12L of the rear floor panel 1 together with the left side member 3L of the rear understructure form a closed portion surrounding the left hollow volume 20L,

[0062] the right rear portion 11R and the right front portion 12R of the rear floor panel 1 together with the right member 3R of the rear understructure form a closed portion surrounding the right hollow volume 20R,

[0063] The central rear portion 11C and the central front portion 12C of the rear floor panel 1 form, together with at least one cross member 3C of the rear understructure, a closed portion enclosing a central hollow volume 20C.

[0064] Advantageously, by combining the rear floor panel 1 and the rear underbody structure 3 into a rear structural assembly 2 having hollow volumes 20L, 20R, 20C, wherein reinforced high-strength material constitutes at least a portion of the walls surrounding the hollow volumes, an extremely rigid and impact-resistant structure can be produced, which is capable of absorbing impact energy and resisting intrusion in the event of an impact and also imparting excellent rigidity to the rear components of the white vehicle body.

[0065] In a specific embodiment, for the same reasons as those described above regarding the difference in the product of UTS and thickness between the front side portion and the rear side portion of the rear floor panel, the left member 3L and the right member 3R of the rear undermount assembly 3 each include a rear portion 31L, 31R and a front portion 32L, 32R substantially corresponding to the positions of the rear portion 11 and the front portion 12 of the rear floor panel 1 in the assembled vehicle, respectively. For example, the product of UTS and average thickness of the front portions 32L, 32R is at least 1.15 times, preferably 1.20 times, preferably 1.25 times, the product of UTS and average thickness of the rear portions 31L, 31R.

[0066] In a specific embodiment, the rear underfloor structure 3 is made by forming a single metal blank, 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 components, each of which is made from a single metal blank. This provides significant advantages in terms of productivity, logistics, cost, impact resistance, reduction of spot welding and overall reduction of CO2 emissions during the manufacturing process.

[0067] In another embodiment, the rear structure assembly 2 further includes at least one top transverse member 4, which is assembled on top of the rear floor panel 1 and extends longitudinally between the left and right portions of the rear floor panel 1 and is located at a height above at least one transverse member 3C of the rear underfloor structure 3. Advantageously, this allows the region corresponding to the impact-resistant and rigid central hollow volume 20C to be further strengthened.

[0068] In a specific embodiment, the rear floor panel 1 and / or the rear underfloor structure 3 according to the present invention are made by cold forming of steel sheets and blanks, and the steel sheets and blanks for producing the rear floor panel 1 and / or the rear underfloor structure 3 include at least one of the following materials in the form of an integral blank, a custom rolled blank or a custom welded blank:

[0069] - Steel having a chemical composition including by weight: 0.13% < C < 0.25%, 2.0% < Mn < 3.0%, 1.2% < Si < 2.5%, 0.02% < Al < 1.0%, where 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, and having a microstructure including retained austenite between 8% and 15%, the balance being ferrite, martensite and bainite, where the sum of the martensite and bainite fractions is included between 70% and 92%. With this composition, as measured in the rolling direction, the steel sheet has a yield strength included between 600 MPa and 750 MPa and an ultimate tensile strength included between 980 MPa and 1300 MPa, while maintaining a total elongation greater than 19%. For example, this material is used at least for a part of the region corresponding to the side portions 11L, 11R, 12L, 12R of the rear floor panel 1.

[0070] - Steel with a chemical composition by weight percentage including the following: %: 0.15% < C < 0.25%, 1.4% < Mn < 2.6%, 0.6% < Si < 1.5%, 0.02% < Al < 1.0%, where 1.0% < Si + Al < 2.4%, Nb < 0.05%, Cr < 0.5%, Mo < 0.5%, the balance being Fe and unavoidable impurities, and having a microstructure including retained austenite between 10% and 20%, the rest being ferrite, martensite, and bainite. With this composition, as measured in the rolling direction, 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 a total elongation greater than 13%. For example, this material is used at least for a part of the area corresponding to the side portions 11L, 11R, 12L, 12R of the rear floor panel 1.

[0071] - Full martensitic steel, where the composition of the full martensitic steel by weight percentage includes: 0.15% ≤ C ≤ 0.5%. For example, this material is used at least for a part of the area corresponding to the side portions 11L, 11R, 12L, 12R of the rear floor panel 1.

[0072] - Dual-phase steel, having a microstructure including at least martensite and ferrite and having a UTS of at least 590 MPa. For example, this material is used at least for a part of the area corresponding to the side portions 11L, 11R, 12L, 12R of the rear floor panel 1.

[0073] - Dual-phase steel, having a microstructure including at least martensite and ferrite and having a UTS of at least 780 MPa. For example, this material is used at least for a part of the area corresponding to the side portions 11L, 11R, 12L, 12R of the rear floor panel 1.

[0074] - Dual-phase steel, having a microstructure including at least martensite and ferrite and having a UTS of at least 980 MPa. For example, this material is used at least for a part of the area corresponding to the side portions 11L, 11R, 12L, 12R of the rear floor panel 1.

[0075] In a specific embodiment, the rear floor panel 1 and / or the rear underbody structure 3 according to the present invention are made by hot stamping a steel sheet and a blank, and the steel sheet and the blank for producing the rear floor panel 1 and / or the rear underbody structure 3 include at least one of the following materials in the form of an integral blank, a customized rolled blank, or a customized welded blank:

[0076] - Steel having a composition including, by weight percentage, 0.06% ≤ C ≤ 0.1%, 1% ≤ Mn ≤ 2%, Si ≤ 0.5%, Al ≤ 0.1%, 0.02% ≤ Cr ≤ 0.1%, 0.02% ≤ Nb ≤ 0.1%, 0.0003% ≤ B ≤ 0.01%, N ≤ 0.01%, S ≤ 0.003%, P ≤ 0.020%, less than 0.1% of Cu, Ni and Mo, the remainder being iron and unavoidable impurities resulting from smelting. Within this composition range, the yield strength of the corresponding region after hot stamping is included between 700 MPa and 950 MPa, the tensile strength is between 950 MPa and 1200 MPa, and the bending angle is greater than 75°. For example, this material is used for the region corresponding to the rear portion 11L, 11R, 31L or 31R because it absorbs energy by deformation without cracking.

[0077] - Steels having an ultimate tensile strength after hot stamping comprised between 1300 and 1650 MPa and a yield strength comprised between 950 and 1250 MPa.

[0078] - Steels having an ultimate tensile strength after hot stamping comprised between 1300 and 1650 MPa, a yield strength comprised between 950 and 1250 MPa and a bending angle greater than 75°.

[0079] - Steel with a composition including, 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%, the remainder being iron and unavoidable impurities resulting from smelting. Within this composition range, the ultimate tensile strength of the corresponding area of ​​the component after hot stamping is comprised between 1300 MPa and 1650 MPa and the yield strength is comprised between 950 MPa and 1250 MPa. For example, this steel composition is used for the area corresponding to the front part 12L, 12R, 32L or 32R. In fact, this steel grade has high anti-intrusion properties.

[0080] - Steels having a tensile strength above 1800 MPa after press hardening.

[0081] - Steel having a composition including the following in percentage by weight: 0.24% ≤ C ≤ 0.38%, 0.40% ≤ Mn ≤ 3%, 0.10% ≤ Si ≤ 0.70%, 0.015% ≤ Al ≤ 0.070%, Cr ≤ 2%, 0.25% ≤ Ni ≤ 2%, 0.015% ≤ Ti ≤ 0.10%, Nb ≤ 0.060%, 0.0005% ≤ B ≤ 0.0040%, 0.003% ≤ N ≤ 0.010%, S ≤ 0,005%, P ≤ 0,025%, %, the rest being iron and unavoidable impurities resulting from smelting. Within this composition range, the tensile strength of the corresponding region after hot stamping is higher than 1800 MPa. For example, this material is used for the front part 12L, 12R, 32L or 32R to benefit from its high anti-intrusion performance.

[0082] - Steel having a composition including the following by weight percentage: C: 0.15% to 0.25%, Mn: 0.5% to 1.8%, Si: 0.1%-1.25%, Al: 0.01%-0.1%, Cr: 0.1%-1.0%, Ti: 0.01%-0.1%, B: 0.001%-0.004%, P≤0.020%, S≤0.010%, N≤0.010%, and the composition of the steel optionally contains one or more of the following elements by weight percentage: Mo≤0.40%, Nb≤0.08%, Ca≤0.1%, and the remaining components are iron and unavoidable impurities resulting from smelting. Within this composition range, the tensile strength of the corresponding area of ​​the instrument panel component after hot stamping is higher than 1350MPa, and the bending angle is greater than 70°.

[0083] - Steel having a composition including the following by weight percentage: C: 0.26% to 0.40%, Mn: 0.5% to 1.8%, Si: 0.1% to 1.25%, Al: 0.01% to 0.1%, Cr: 0.1% to 1.0%, Ti: 0.01% to 0.1%, B: 0.001% to 0.004%, P≤0.020%, S≤0.010%, N≤0.010%, and the composition of the steel optionally contains one or more of the following elements by weight percentage: Ni≤0.5%, Mo≤0.40%, Nb≤0.08%, Ca≤0.1%, and the remaining components are iron and unavoidable impurities resulting from smelting. Within this composition range, the tensile strength of the corresponding region after hot stamping is higher than 1350MPa, and the bending angle is greater than 70°.

[0084] - Steel having a composition including the following by weight percentage: C: 0.2% to 0.34%, Mn: 0.50% to 1.24%, Si: 0.5% to 2%, P≤0.020%, S≤0.010%, N≤0.010%, and the composition of the steel optionally contains one or more of the following elements by weight percentage: Al: ≤0.2%, Cr≤0.8%, Nb≤0.06%, Ti≤0.06%, B≤0.005%, Mo≤0.35%, and the remaining components are iron and unavoidable impurities generated by smelting. Within this composition range, the tensile strength of the corresponding region after hot stamping is equal to or higher than 1000 MPa, and the bending angle is greater than 55°.

[0085] - Steel having a composition including, in percentage by weight: C: 0.13% to 0.4%, Mn: 0.4% to 4.2%, Si: 0.1% to 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, the remainder being iron and unavoidable impurities resulting from smelting. For example, this composition is used when hot stamping parts using a multi-step process.

[0086] - Steel coated with an aluminium-based metallic coating. By aluminium-based is meant a coating comprising at least 50% aluminium by weight. For example, the metallic coating is an aluminium-based coating comprising 8 to 12% Si by weight. The metallic coating is applied, for example, by immersing the base material in a bath of molten metal. Advantageously, applying the aluminium-based metallic coating avoids the formation of surface scale during the heating step of the hot stamping process, which in turn allows the production of the component by hot stamping without subsequent sandblasting operations. Furthermore, the aluminium-based coating also provides corrosion protection to the component when used on vehicles.

[0087] - Steel coated with an aluminum-based metal coating comprising 2.0 to 24.0% 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 content of each additional element by weight being less than 0.3% by weight, the remainder being aluminum and optionally unavoidable impurities. Advantageously, this type of metal coating provides very good corrosion protection to the component and a good surface appearance after hot stamping.

[0088] In a specific embodiment, the rear floor panel 1 and / or the rear understructure 3 are manufactured by hot stamping a laser welding blank comprising at least one sub-blank having an aluminum-based metal coating, and the aluminum-coated sub-blank is previously prepared by ablation of at least a portion of the metal coating on the edge to be welded. Advantageously, this removes part of the aluminum present in the coating, which would contaminate the weld and deteriorate its mechanical properties.

[0089] In a particular embodiment, the rear floor panel 1 and / or the rear understructure 3 are manufactured by hot stamping a laser welded blank comprising at least one sub-blank having at least one side portion having an emissivity increasing top layer on top. 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 on the top or bottom side 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, for the emissivity increasing top layer to increase the emissivity of the surface, it needs to cover the outermost surface of the sub-blank. Advantageously, the emissivity increasing top layer will allow increasing the heating rate of the sub-blank and thus increasing the productivity of the heating step of the hot stamping process. When using several sub-blanks of different thickness, the emissivity-increasing top layer is advantageously applied to the sub-blank with the greatest thickness in order to reduce the differences in heating times between the different sub-blanks and thus improve productivity, increase the hot stamping process window and generally allow obtaining a finished part with uniform surface properties.

Claims

1. A rear floor panel (1) for a motor vehicle (100), the rear floor panel (1) extending longitudinally from the rear end of the vehicle to the front end of a rear passenger seat and extending transversely between a left wheelhouse (103) and a right wheelhouse (103) and between a left rocker inner member (104) and a right rocker inner member (104), the rear floor panel (1) comprising a front portion (12) extending below the rear passenger seat and a rear portion (11) extending behind the rear passenger seat, the front portion (12) and the rear portion (11) each comprising a left portion (11L), (12L), a central portion (11C), (12C) and a right portion (11R), (12R), respectively, wherein - the rear floor panel (1) is made by forming a single metal sheet, - the right front part (12R) and the left front part (12L) are assembled to the right rocker inner panel (104) and the left rocker inner panel (104) and are located at a lower height than the rear part (11), - each of the left rear portion (11L) and the right rear portion (11R) comprises at least a region whose product of the ultimate tensile strength UTS expressed in MPa and the average thickness expressed in mm is at least twice the product of the UTS and the average thickness of the central rear portion (11C), - Each of the left front portion (12L) and the right front portion (12R) includes at least a region where the product of the ultimate tensile strength UTS and the average thickness is at least twice the product of the UTS and the average thickness of the central front portion (12C).

2. The rear floor panel (1) according to claim 1 further comprises a material transition zone (12CR) between the central front portion and the right front portion, a material transition zone (12CL) between the central front portion and the left front portion, a material transition zone (11CR) between the central rear portion and the right rear portion, and a material transition zone (11CL) between the central rear portion and the left rear portion, respectively, wherein: For any given transverse cross-section of the component, there is no height difference on either side of the material transition zone over a width centered on the center of the material transition zone and spanning at least twice the thickness of the thicker material on either side of the material transition zone (12CR), (12CL), (11CR), (11CL).

3. The rear floor panel (1) according to claim 2, wherein: The single metal blank used to produce the rear floor panel (1) is a custom-made welded blank comprising butt-welded sub-blanks, the welding operation using at least a laser source.

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

5. The rear floor panel (1) according to any one of claims 1 to 4, wherein: Each of the left rear portion (11L) and the right rear portion (11R) includes at least a region where a product of UTS and an average thickness is at least three times a product of UTS and an average thickness of the central rear portion (11C).

6. The rear floor panel (1) according to any one of claims 1 to 5, wherein: Each of the left front portion (12L) and the right front portion (12R) includes at least a region where the product of UTS and average thickness is at least four times the product of UTS and average thickness of the central front portion (12C).

7. A rear structural assembly (2) for a motor vehicle, the rear structural assembly (2) comprising at least a rear understructure (3) and a rear floor panel (1) according to any one of claims 1 to 6, the rear understructure (3) itself comprising a left side member (3L) and a right side member (3R) and at least one cross member (3C) connecting the right side member (3R) and the left side member (3L), wherein: When the rear structure assembly (2) is assembled in the vehicle (100): - the left rear portion (11L) and the left front portion (12L) of the rear floor panel (1) together with the left member (3L) of the rear understructure form a closed portion enclosing a left hollow volume (20L), - the right rear portion (11R) and the right front portion (12R) of the rear floor panel (1) together with the right member (3R) of the rear understructure form a closed portion surrounding a right hollow volume (20R), The central rear portion (11C) and the central front portion (12C) of the rear floor panel (1) together with at least one cross member (3C) of the rear understructure form a closed portion enclosing a central hollow volume (20C).

8. The rear structure assembly (2) according to claim 7, wherein: The rear underbody structure (3) is manufactured by forming a single metal blank.

9. The rear structure assembly (2) according to claim 7 or 8, further comprising at least one top cross member (4), which is assembled on the top of the rear floor panel (1), extends longitudinally between the left part (11L), (12L) and the right part (11R), (12R) of the rear floor panel (1), and is located at a height above the at least one cross member (3C) of the rear understructure (3).

Images