Rear underfloor structure for a motor vehicle

Abstract

Rear underfloor structure (2) for a motor vehicle (1) manufactured by assembling together a left and right structure (6), wherein said left and right structures (6) each comprise respectively a left and right side member (4) extending in a substantially longitudinal direction and at least one left and right half cross member (7) extending inwards in a substantially transverse direction from said side member (4), wherein said left and right structures (6) are each manufactured by stamping a single metallic blank (8) and wherein said left and right structures (6) are joined together along distal ends (71) of said at least one half cross members (7).

Description

[0001] Rear underfloor structure for a motor vehicle

[0002]

[0001] The present invention relates to a rear underfloor structure for a motor vehicle.

[0003]

[0002] Car makers are submitted to the ever more demanding requirements of increasing the passive safety of vehicles, lowering the weight of the vehicle to minimize greenhouse gas emissions in case of internal combustion engines or increase the vehicle’s driving range in case of electric vehicles, while keeping production costs low and productivity rates high.

[0004]

[0003] The rear underfloor structure is defined as being the assembly consisting of two rear longitudinal members and the cross members which link said rear members.

[0005]

[0004] The rear underfloor structure is a key structural element of the vehicle and contributes to the safety of the occupants in case of a rear crash. It protects the gas tank, often located below the passenger seats, in case of a combustion engine. It protects the rear electric engine in case of an electric or hybrid vehicle.

[0006]

[0005] The rear underfloor structure, consisting of numerous individual parts, makes up a significant mass of the vehicle’s body. It also involves costly manufacturing processes: multiple forming operations and assembly steps to obtain the finished structure.

[0007]

[0006] Patent application PCT / IB2020 / 060465, published as WO 2022 / 096921A1, discloses a rear underfloor structure made by stamping a single tailor welded blank comprising at least two sub-blanks. This design allows to reduce the structure to only one large part to be stamped. The associated rationalization in terms of number of parts, number of stamping operations, logistics, assembling operations etc. leads to significant increases in productivity and reductions in CO2 emissions as well as production costs. However, one identified draw-back of such a massive single part is that it is difficult to integrate with the rest of the body in white, taking into account the inevitable variations in dimensional tolerances of such a large part. The rear underfloor structure is joined to numerous parts, such as the rocker assemblies, the rear bumper assembly and rear floor panel.Deviations in geometric tolerances in the rear underfloor structure geometry can create industrial difficulties when joining it to all these elements.

[0008]

[0007] The current invention purports to solve the assembly issue of a massive single part design while keeping significant rationalization and associated productivity gains and CO2 emission reduction.

[0009]

[0008] The object of the present invention is achieved by providing a rear underfloor structure according to claim 1, optionally comprising the features of claims 2 to 6 taken individually or according to any possible combination. A further object of the present invention is a manufacturing method according to claim 7 or claim 8. A further object of the present invention is a motor vehicle according to claim 9.

[0010]

[0009] Other aspects and advantages of the invention will appear upon reading the following description, given by way of example, and made in reference to the appended drawings, which are in no way limitative, wherein:

[0011]

[0010] -Figure 1 is an overall perspective view of a motor vehicle highlighting the location of the rear underfloor structure,

[0012]

[0011] -Figure 2 represents a rear underfloor structure according to an embodiment of the present invention,

[0013]

[0012] -Figure 3 represents a left and right underfloor structure according to an embodiment of the present invention,

[0014]

[0013] -Figure 4 depicts a specific embodiment of tailor welded blanks used to manufacture a rear underfloor structure according to an embodiment of the present invention.

[0015]

[0014] In the following descriptions and claims, the directional terms are defined according to the usual directions of a mounted vehicle.

[0016]

[0015] In particular, the terms “top”, “up”, “upper”, “above”, “bottom”, “low”, “lower”, “below” etc. are defined according to the elevation direction of a vehicle. The terms “front”, “back”, “rear”, “front”, “forward”, backward” etc. are defined according to the longitudinal direction of a vehicle, i.e. the direction in which the vehicle moves forward when following a straight line. The terms “left”, “right”, “transverse”, etc. are defined according to the orientation parallel to the width of the vehicle. The terms “steering side” and “passenger side” referred to the general transversal direction respectively of where thedriver and the passenger sit in a vehicle. Such directions will naturally be different according to whether the vehicle is a left-hand or right-hand drive vehicle. The terms “inner”, “outer” are to be understood according to the width direction of the vehicle: the “inner” is closest to the central axis of the vehicle, i.e. closest to the inside of the vehicle, whereas the “outer” is located further away from said central axis of the vehicle, in effect closer to the outside of the vehicle. The same applies to the terms “distal” and “central”: the “distal” part is located closest to the outside of the vehicle and the “central” part closest to the center of the vehicle. The term “horizontal” refers to the orientation of the plane comprising the longitudinal and the transverse directions. The term “vertical” refers to any orientation comprising the elevation direction.

[0017]

[0016] In the following figures, the orientations and spatial references are all made using an X, Y, Z coordinates referential, wherein Z is the elevation direction of the vehicle, X is the longitudinal direction of the vehicle and Y is the transverse direction of the vehicle. The X axis is oriented such that the X coordinates increase in the front to rear direction, i.e. a position located further back in the vehicle will have a higher X coordinate than a position located further in the front of the vehicle. The referential is represented in each figure. When the figure is a 2D flat representation, the axis which is outside of the figure is represented by a dot in a circle when it is pointing towards the reader and by a cross in a circle when it is pointing away from the reader, following established conventions.

[0018]

[0017] By “substantially parallel” or “substantially perpendicular” it is meant a direction which can deviate from the parallel or perpendicular direction by no more than 15°.

[0019]

[0018] A steel sheet refers to a flat sheet of steel. It has a top and bottom face, which are also referred to as a top and bottom side or as a top and bottom surface. The distance between said faces is designated as the thickness of the sheet. 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 way, the thickness can also be measured on a formed part.

[0019] By average thickness of a part, or of a portion of a part, it is meant the overall average thickness of the material making up the part after it has been formed into a 3-dimensional part from an initially flat sheet.

[0020]

[0020] Tailor welded blanks are made by assembling together by any known assembly technique, several sheets or cut-out blanks of steel, known as subblanks, in order to optimize the performance of the part in its different areas, to reduce overall part weight, to reduce overall part cost and material scrap and to simplify the production process.

[0021]

[0021] A tailor rolled blank is a blank having multiple sheet thicknesses obtained by differential rolling during the steel sheet production process.

[0022]

[0022] The use of tailor welded blanks for automotive parts is known in the industry, as illustrated for example by patent EP2056979 or for example by patent EP2736672. The use of tailor rolled blanks for automotive parts is known in the industry, as illustrated for example by patent EP2025771.

[0023]

[0023] A flexible blank is a type of tailor welded blank including regions wherein at least part of the connection between the different sub-blanks is not rigid, allowing the sub-blanks to move in different directions during the forming operation in the corresponding regions.

[0024]

[0024] By opposition to a tailor welded blank or a flexible blank, a monolithic blank refers to a blank which consists of one single sub-blank, without several sub-blanks being combined together.

[0025]

[0025] A patched blank is a blank comprising a main blank to which is attached at least one further reinforcing blank, known as a patch, designed to locally increase the thickness and mechanical resistance of said blank. Said patch can be fixed to the main blank by spot welding, laser stitch welding, adhesive bonding, clinching or any other known assembly technique. The main blank can be a tailor welded blank, a tailor rolled blank or a combination of both. In the case of tailor welded blanks in which the subblanks are assembled together by butt to butt welding, a patch can be advantageously applied to the welded area in order to increase the mechanical resistance of the welded area.

[0026]

[0026] The ultimate tensile strength, the yield strength and the elongation are measured according to ISO standard ISO 6892-1 , published in October 2009.The tensile test specimens are cut-out from flat areas. If necessary, small size tensile test samples are taken to accommodate for the total available flat area on the part.

[0027]

[0027] The bending angle is measured according to the VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For the sake of simplicity, the bending angle values of the current invention refer to a thickness of 1.5mm. If the thickness is different than 1.5mm, the bending angle value needs to be normalized to 1.5mm by the following calculation where a1.5 is the bending angle normalized at 1.5mm, t is the thickness, and at is the bending angle for thickness t:

[0028]

[0028] a1.5 = (at x t) / 1.5

[0029]

[0029] Cold stamping is a forming technology for metals which involves shaping a metallic sheet into a formed part by pressing it between an upper and lower die, called the cold stamping tool. For example, the cold stamping tool has a blank holder which allows to hold the metallic sheet on its sides. For example, the cold stamping tool consists of several steps, each involving an upper and lower die to produce complex shapes and I or to perform further operations such as punching holes in the part or trimming its sides. Other cold forming technologies exist such as for example roll forming, which involves bending a continuous sheet between a successive set of rolls, simple bending which involves simply bending a sheet of steel using a press and an upper and lower bending tool etc.

[0030]

[0030] Hot stamping is a forming technology for steel which involves heating a blank of steel, or a preformed part made from a blank of steel, up to a temperature at which the microstructure of the steel has at least partially transformed to austenite, forming the blank or preformed part at high temperature by stamping it and simultaneously quenching the formed part to obtain a microstructure having a very high strength, possibly with an additional partitioning or tempering step in the heat treatment.

[0031]

[0031] A complex hot stamping process is a particular type of hot stamping process including at least one stamping step and consisting of at least two process steps performed at high temperature, above 300°C. For example, a complex process can involve a first stamping operation and a subsequent hottrimming operation, so that the finished part, at the exit of the hot stamping process, does not need to be further trimmed. For example, a complex process can involve several successive stamping steps in order to manufacture parts having more complex shapes than what can be realized using a single stamping operation. For example, the parts are automatically transferred from one operation to another by using for example a transfer press. For example, the parts stay in the same tool, which is a multipurpose tool that can perform the different operations, such as a first stamping and a subsequent in-tool trimming operation.

[0032]

[0032] A partial hardening hot stamping process is a hot stamping process in which the heat profile to which the blank is submitted is purposely 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 to produce hot stamped parts using a single metallic blank made of a single material which will have different levels of hardness and elongation in different areas of the final part. For example, this allows to produce parts having soft zones and hard zones, said soft zones being able to deform under an impact load in order to absorb energy, whereas said hard zones will resist intrusion by resisting deformation. There are several different technologies to implement partial hardening. For example, the material can be heated at different temperatures in different areas of the blank, the higher temperature areas will be fully austenitic at the exit of the austenitizing furnace resulting in a very hard microstructure after hot stamping, whereas the lower temperature areas will have an intercritical ferrite I austenite microstructure at the exit of the austenitizing furnace resulting in a lower hardness microstructure after hot stamping. For example, the material can be quenched at different quenching speeds in different areas of the blank during the hot stamping step itself, the areas quenched at a higher quenching speed will have a higher hardness than those quenched at a lower speed.

[0033]

[0033] Referring to figure 1 , a rear underfloor structure 2 for a motor vehicle 1 is described. The motor vehicle 1 can be any type of passenger vehicle comprising at least a front and a rear set of doors: compact, sedan, Sport Utility Vehicle etc. The described rear underfloor structure is essentially thesame whatever the category of vehicle. Furthermore, the powertrain of said motor vehicle can be a combustion engine, electrical motors, fuel cells or any type of hybrid system.

[0034]

[0034] The rear underfloor structure 2 extends substantially longitudinally from the rear of the vehicle up to below the floor panel of the passenger cabin. It comprises at least left and right side members 4, located on either side of the vehicle and extending in a substantially longitudinal direction and at least one cross member 5 linking said left and right side members 4.

[0035]

[0035] The following is a general description of a typical side member 4, it should be understood that the below description of a side member 4 is not limitative of the scope of the current invention, which can be applied to any type of design of side member:

[0036]

[0036] -it comprises a rear portion extending substantially in the longitudinal direction at the same elevation as a rear bumper assembly 11 and attached at its rear end to said rear bumper assembly 11 ,

[0037]

[0037] -a front portion extending substantially in the longitudinal direction at a lower elevation than the rear portion and attached to a vehicle lateral reinforcement structure 12,

[0038]

[0038] -a transition zone comprising at least an upper and a lower bend linking said front and rear portions. The presence of said upper and lower bends allows for the difference in elevation level of the front and lower portions.

[0039]

[0039] Referring to figure 2, the side members 4 have a general U-shape, comprising a horizontal wall 41, which extends in a substantially horizontal plane and an inner and outer wall 42, 43 extending in substantially vertical planes. Said inner and outer walls 42, 43 are linked to said horizontal wall 41 respectively by an inner and an outer radius 45, 46. By “radius”, it is meant a portion of the part which is generally curved and allows for the transition between different planar orientations in a part. In the present case the inner radius 45 allows for the transition between the substantially horizontal orientation of the horizontal wall 41 and the substantially vertical orientation of the inner wall 42. Similarly, the outer radius 46 allows for the transitionbetween the substantially horizontal orientation of the horizontal wall 41 and the substantially vertical orientation of the outer wall 43.

[0040]

[0040] In a specific embodiment, the side members 4 also comprise flanges 47 extending along the top of the inner and outer walls 42, 43 in substantially vertical planes. Advantageously, said flanges allow to assemble the side members 4 with the rest of the vehicle structure, for example by spot welding or laser welding along said flanges 47.

[0041]

[0041] The rear underfloor structure 2 further comprises at least one cross member 5 linking said first and second side members 4. Said cross member 5 extends along a generally transverse direction. As with the side members 4, the cross member 5 comprises a horizontal wall 51, which extends in a substantially horizontal plane and a front and rear wall 52, 53 extending in substantially vertical planes. In a particular embodiment, the rear underfloor structure comprises 2 or more cross members 5. Advantageously, increasing the number of cross-members allows to increase the rigidity of the vehicle and its crash resistance.

[0042]

[0042] The side members 4 of the current invention each comprise at least one cross-member attachment zone 20, corresponding to the portion of the side member 4 on to which the corresponding cross-member 5 is attached. The cross-member attachment zone 20 is delimited by the dashed lines 21 on figure 2. It will be easily understood that there are as many cross-member attachment zones 20 per side member 4 as there are cross-members 5.

[0043]

[0043] Referring to figure 3, the rear underfloor structure 2 of the current invention is manufactured by assembling together a left and right structure 6. Said left and right structure 6 each comprise respectively a left and right side member 4 and at least one left and right half cross member 7 extending inwards in a substantially transverse direction from said side member 4. Said half cross members 7, each further comprise a distal end 71 , corresponding to said furthest inward end. Said left and right structures 6 are each manufactured by stamping a single metallic blank 8. Said left and right structures are joined together along said at least one left and right cross members.

[0044] The design and manufacturing process of the current invention allows to reduce the number of parts compared to a standard rear underfloor structure which is made by assembling together separately formed side members and cross members. It also allows manufacturing flexibility and robustness compared to a massive single part design in which the entire rear underfloor structure is made in one stamping operation. The current invention allows to absorb at least part of the geometrical deviations, in particular in the transverse direction.

[0044]

[0045] For example, said half cross members are joined together by overlapping said distal ends 71 and attaching them together in a thus formed overlapping region. For example they are then attached by spot welding them or remote laser welding them in said overlapping region.

[0045]

[0046] In a particular embodiment, said distal ends are joined together using an additional connecting part. For example, said additional connecting part has a cross section matching the cross sections of said distal ends so that said left and right distal ends can each be overlapped with said additional part and said distal ends can each be assembled to said additional part in the thus formed overlapping regions.

[0046]

[0047] In a particular embodiment said half cross members are joined together by butt welding their distal ends together using for example MIG, or MAG or TIG welding.

[0047]

[0048] A first embodiment of a method to manufacture the rear underfloor structure described previously and to attach it to a motor vehicle comprises the steps of:

[0048] • Providing a left and right structure 6 each comprising a side member 4 and at least one half cross member 7 extending from said side member 4,

[0049] • Assembling said left and right structures 6 together by joining them together along distal ends of said at least one half cross members to form a rear underfloor structure 2,

[0050] • Assembling said rear underfloor structure 2 to a left and right side reinforcement structure 12 and a rear bumper assembly 11.

[0049] The above described first manufacturing method embodiment allows for greater tolerance in the dimensional tolerance of said left and right structures 6 compared to a massive single part single stamping design, in particular in the transverse direction.

[0051]

[0050] A second embodiment of a method to manufacture the rear underfloor structure described previously to attach it to a motor vehicle comprises the steps of :

[0052] • Providing a left and right structure 6 each comprising a side member 4 and at least one half cross member 7 extending from said side member 4,

[0053] • Assembling said left and right structures 6 respectively to a left and right side reinforcement structure 12 and a rear bumper assembly 11 ,

[0054] • Assembling said left and right structures 6 together by joining them together along distal ends of said at least one half cross members.

[0055]

[0051] The above described second manufacturing method embodiment allows for greater tolerance ranges in the X, Y, Z positioning of the attachment points respectively between the side reinforcement structure 12 and the left and right structures 6 on the one hand and between the rear bumper assembly 11 and the left and right structures 6 on the other hand. Indeed, there can be a certain amount of mismatch between the location of the distal ends of the at least one left and right half cross members 6. This mismatch can then be compensated when attaching said half cross members 6 together.

[0056]

[0052] Concerning both described manufacturing method embodiments, said half cross members are for example joined together by overlapping said distal ends 71 and attaching them together in a thus formed overlapping region, for example by spot welding or remote laser welding. Said distal ends are for example joined together using an additional connecting part and said distal ends are each assembled to said additional part. Said half cross members are for example joined together by butt welding their distal ends together using for example MIG, or MAG or TIG welding.

[0057]

[0053] A further object of the present invention is a motor vehicle comprising a rear underfloor structure as described previously.

[0054] Figure 4 depicts a specific embodiment of the two metallic blanks 8 which are used to manufacture by stamping the left and right half cross members.

[0058]

[0055] In a specific example, as depicted on figure 4, said metallic blanks are tailor welded blanks. The weld seams are represented by straight lines on figure 4. In a specific embodiment, said metallic blanks are tailor rolled blanks.

[0059]

[0056] For example, the material making up the rear part of said metallic blanks has less resistance than the material making up the front part of said blanks. Advantageously, in the case of a rear impact, this allows to absorb energy by deformation in the rear portion of the part and to resist intrusion in the front portion of the part, which protects the passengers. For example, the material in the rear portion of the part has a lower product of Ultimate tensile strength by average thickness after stamping than the material in the front portion of the part.

[0060]

[0057] In a particular embodiment, the material making up said metallic blanks 8 has a tensile strength after forming equal to or greater than 950MPa, preferably 1500 MPa.

[0061]

[0058] In a specific embodiment, the left and right structures are manufactured by hot stamping.

[0062]

[0059] In a specific embodiment, the left and right structures are manufactured by hot stamping press hardenable steel blanks.

[0063]

[0060] In a specific embodiment said metallic blanks 8 are made of steel, for example are made of tailor welded blanks made of steel, having the following composition.

[0064]

[0061] For example, said metallic blanks 8 are made by cold stamping and the composition of the steel is:

[0065]

[0062] Steel having a chemical composition comprising in weight %: 0.13% < C < 0.25%, 2.0 % < Mn < 3.0%, 1.2% < Si < 2.5%, 0.02% < Al < 1 .0%, with 1.22% < Si+AI < 2.5%, Nb < 0.05%, Cr < 0.5%, Mo < 0.5%, Ti < 0.05 %, the remainder being Fe and unavoidable impurities and having a microstructure comprising from 8% to 15% of retained austenite, the remainder being ferrite, martensite and bainite, wherein the sum of martensite and bainite fractionsis comprised from 70% to 92%. With this composition, the steel blank has, as measured in the rolling direction, a yield strength comprised from 600MPa to 750MPa and an ultimate tensile strength comprised from 980MPa to 1300MPa while keeping a total elongation above 19%.

[0066]

[0063] Steel having a chemical composition comprising in weight %: %:

[0067] 0.15% < C < 0.25%, 1.4 % < Mn < 2.6%, 0.6% < Si < 1 .5%, 0.02% < Al < 1.0%, with 1.0% < Si+AI < 2.4%, Nb < 0.05%, Cr < 0.5%, Mo < 0.5%, the remainder being Fe and unavoidable impurities and having a microstructure comprising from 10% to 20% of retained austenite, the remainder being ferrite, martensite and bainite. With this composition, the steel blank has, as measured in the rolling direction, a yield strength comprised from 850MPa to 1060MPa and an ultimate tensile strength comprised from 1180MPa to 1330MPa while keeping a total elongation above 13%.

[0068]

[0064] Fully martensitic steel wherein the composition of the fully martensitic steel comprises in % weight: 0.15% < C < 0.5%.

[0069]

[0065] Dual phase steel having a microstructure comprising at least martensite and ferrite and having a UTS of at least 590MPa.

[0070]

[0066] Dual phase steel having a microstructure comprising at least martensite and ferrite and having a UTS of at least 780MPa.

[0071]

[0067] Dual phase steel having a microstructure comprising at least martensite and ferrite and having a UTS of at least 980MPa.

[0072]

[0068] For example, said metallic blanks 8 are made by hot stamping and the composition of the steel is:

[0073]

[0069] The composition of the steel depends on the desired mechanical properties for the part. But preferably, the steel has a composition comprising, by weight %:

[0074] 0.062% < C < 0.4%

[0075] 0.4% < Mn < 3.9%

[0076] 0.10% < Si < 1.5%

[0077] 0.005% < Al < 1.0%

[0078] 0.001% < Cr < 4.0%

[0079] 0.001% < Ti < 0.2%

[0080] 0.0005% < B < 0.010%Ni < 2%

[0081] V < 1 %

[0082] Nb < 0.1%

[0083] Mo < 0.65%

[0084] W< 0.30%

[0085] N < 0.010%

[0086] 0.0001% < S < 0.05%

[0087] 0.0001% < P < 0.1%

[0088] Ca < 0.005%

[0089]

[0070] the balance of the composition consisting of iron and unavoidable impurities resulting from elaboration.

[0090]

[0071] The level of impurities resulting from the elaboration process will depend on the production route used. For example, when using a Blast Furnace route with a low level of steel scrap (recycled steel), the level of impurities will remain very low. On the other hand, when elaborating the steel using an electric furnace, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased. In this latter case, for example, the level of Cu can go up to 0.25%, Ni can go up to 0.25%, Sn can go up to 0.05%, As can go up to 0.03%, Sb can go up to 0.03% and Pb can go up to 0.03%.

[0091]

[0072] Hence, in an embodiment, the steel comprises up to 0.25% Cu, up to 0.05% Sn, up to 0.03% As, up to 0.03% Sb and / or up to 0.03% Pb as unavoidable impurities.

[0092]

[0073] The above composition is favorable to the achievement of high mechanical properties, in particular a tensile strength TS in the range of 950 MPa to 2100 MPa.

[0093]

[0074] Steel having a composition comprising in % weight: 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 the elaboration. With this composition range, the yield strength of the corresponding area after hot stamping is comprised from 700 to950MPa, the tensile strength from 950MPa to 1200MPa and the bending angle is above 75°.

[0094]

[0075] Steel having an ultimate tensile strength after hot stamping which is comprised from 1300MPa to 1650MPa and a yield strength which is comprised from 950MPa to 1250MPa.

[0095]

[0076] Steel having an ultimate tensile strength after hot stamping which is comprised from 1300MPa to 1650MPa, a yield strength which is comprised from 950MPa to 1250MPa and a bending angle which is above 75°.

[0096]

[0077] Steel having a composition comprising in % weight: 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 the elaboration. With this composition range, the ultimate tensile strength of the corresponding area of the part after hot stamping is comprised from 1300MPa to 1650MPa and the yield strength is comprised from 950MPa to 1250MPa.

[0097]

[0078] Steel having a tensile strength after press-hardening higher than 1800 MPa.

[0098]

[0079] Steel having a composition which comprises in % 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 remainder being iron and unavoidable impurities resulting from the elaboration. With this composition range, the tensile strength of the corresponding area after hot stamping is higher than 1800 MPa.

[0099]

[0080] Steel having a composition which comprises in %weight : C : 0.15 - 0.25 %, Mn: 0.5 - 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 comprising optionally one or more of the following elements, by weight percent: Mo < 0.40 %, Nb < 0.08 %, Ca < 0.1 %, the remainder of the composition being iron and unavoidable impurities resulting from the smelting.

[0081] Steel having a composition which comprises in %weight : C : 0.26 - 0.40 %, Mn: 0.5 - 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 comprising optionally one or more of the following elements, by weight percent: Ni < 0.5 %, Mo < 0.40 %, Nb < 0.08 %, Ca < 0.1 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting. With this composition range, the tensile strength of the corresponding area after hot stamping is higher than 1350 MPa and the bending angle is higher than 70°.

[0100]

[0082] Steel having a composition which comprises in %weight : C : 0.2 - 0.34 %, Mn: 0.50 - 1.24 %, Si: 0.5 - 2 %, P < 0.020 %, S < 0.010 %, N < 0.010 %, and comprising optionally one or more of the following elements, by weight percent: Al: <0.2 %, Cr < 0.8 %, Nb < 0.06 %, Ti < 0.06 %, B < 0.005%, Mo < 0.35%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting. With this composition range, the tensile strength of the corresponding area after hot stamping is equal to or higher than 1000 MPa and the bending angle is higher than 55°.

[0101]

[0083] Steel having a composition which comprises in %weight : C : 0.13 - 0.4 %, Mn: 0.4 -4.2 %, Si : 0.1 -2.5%, Cr < 2 %, Mo < 0.65 %, Nb < 0.1 %, Al < 3.0 %, Ti < 0.1 %, B < 0.005 %, P < 0.025 %, S < 0.01 %, N < 0.01 %, Ni < 2.0%, Ca < 0.1 %, W < 0.30%, V < 0.1%, Cu < 0.2%, and verifying the following combination: 114 - 68*C - 18*Mn + 20*Si - 56*Cr - 60*Ni - 36*AI + 38*Mo + 79*Nb - 17691*B < 20, the remainder of the composition being iron and unavoidable impurities resulting from the smelting. For example, this composition is used when hot stamping the part using a complex hot stamping process.

[0102]

[0084] Steel having a composition which comprises in %weight :

[0103] 0.26 < C < 0.33

[0104] 0.15 < Si < 0.40

[0105] 1.0 < Mn < 1.8

[0106] 0.10 < Cr < 0.40

[0107] 0.02 < Al < 0.10

[0108] 0.01 < Ti < 0.060.0010 < B < 0.0050

[0109] 0 < S < 0.005

[0110] 0 < P < 0.04

[0111] 0 < N < 0.010

[0112] 0 < Ni < 0.5

[0113] 0 < Mo < 0.3

[0114] 0 < Nb < 0.1

[0115] 0 < V < 0.3

[0116] 0 < Cu < 0.4

[0117] Cr + Mo + Nb + V < 0.5%,

[0118]

[0085] The above described microstructure description applies to the majority of the steel substrate, which means that this microstructure is present in at least 95% of the volume of the steel substrate.

[0119]

[0086] The microstructure is determined through the following method: a specimen is cut from the hot-stamped coated steel part, polished as detailed below, and etched with Nital 2% (10s), to reveal the microstructure. The section is afterwards examined through optical microscope with a 500x magnification and, if it is required to distinguish martensite from bainite, with a Scanning Electron Microscope (SEM) (Back Scattered Electron mode, Magnification 500x, EHT (Electron High Tension Voltage) = 15.00 kV, Scale 10 micrometers). The determination of the volume fraction of each constituent (martensite, bainite, ferrite, austenite) is performed with image analysis through a method known per se.

[0120]

[0087] In an embodiment, the austenite fraction is of at most 5% by volume and / or the bainite fraction is of at most 10% by volume.

[0121]

[0088] In an embodiment, the microstructure consists of, by volume, at least 80% martensite, up to 10% of bainite, up to 5% austenite and up to 5% ferrite.

[0122]

[0089] In a preferred embodiment, the microstructure is essentially martensitic, i.e. consists of, by volume, at least 95% martensite and up to 5% of bainite and / or ferrite.

[0123]

[0090] Still preferably, the microstructure is fully martensitic.

[0091] The steel sheet used to manufacture the metallic blanks may be obtained, depending on its desired thickness, by hot rolling and / or by cold rolling followed by annealing, or by any other appropriate method.

[0124]

[0092] The metallic blanks typically have a thickness comprised from 0.5 mm to 5 mm.

[0125]

[0093] In a particular embodiment, the steel sheets used to manufacture the metallic blanks are obtained by hot-dip coating, i.e. by immersion of a steel substrate into a bath of molten metal. It comprises an intermetallic alloy layer in contact with said substrate and a metallic layer extending atop said intermetallic alloy layer.

[0126]

[0094] The intermetallic alloy layer is formed by reaction between the substrate and the molten metal of the bath. It comprises an intermetallic compound comprising at least one element from the metallic layer and at least one element from the substrate.

[0127]

[0095] The thickness of the intermetallic alloy layer is generally of the order of a few micrometers. In particular, its mean thickness is typically comprised from 2 and 7 micrometers.

[0128]

[0096] The metallic layer has a composition which is close to that of the molten metal in the bath. It is formed by the molten metal carried away by the strip as it travels through the molten metal bath during hot-dip coating.

[0129]

[0097] The metallic layer has, for example, a thickness comprised from 19 pm to 33 pm or from 10 pm to 20 pm.

[0130]

[0098] The metallic layer is a layer of aluminum, or a layer of aluminum alloy or a layer of aluminum-based alloy.

[0131]

[0099] In this context, an aluminum alloy refers to an alloy comprising more than 50% by weight of aluminum. An aluminum-based alloy is an alloy in which aluminum is the main element, by weight.

[0132]

[0100] The intermetallic alloy layer comprises intermetallic compounds of the Fex-Aly type, and more particularly Fe2AI5.

[0133]

[0101] The particular structure of the coating obtained by hot-dip coating is in particular disclosed in patent EP 2007545.

[0134]

[0102] According to one embodiment, the metallic layer is a layer of aluminum alloy further comprising silicon.

[0103] According to one example, the metallic layer 12 comprises, by weight: - 6% < Si < 11 %,

[0135] - 2% < Fe < 10%,

[0136] the rest being aluminum and possible impurities.

[0137]

[0104] In a particular embodiment, the metallic layer 12 comprises, by weight:

[0138] - 3% < Si < 12%,

[0139] - 2% < Fe < 10%,

[0140] - 0.1% < Mg < 10%,

[0141] - 0% < Ca < 0.2%,

[0142] the rest being aluminum and possible impurities.

[0143]

[0105] In a particular embodiment, the metallic blanks 8 further comprises patches in areas in which it is necessary to increase the resistance or stiffness of the part.

[0144]

[0106] In a particular embodiment, the metallic blanks 8 further comprise an emissivity increasing top layer above the steel substrate or above the metallic coating. Emissivity is the relative power of a surface to emit heat by radiation. It represents the ratio of the radiant energy emitted by a surface to that emitted by a blackbody at the same temperature, and is a value comprised between 0 and 1. The higher the emissivity of the surface of a blank, the more it will absorb heat by radiation and therefore the easier it will be to heat it using a radiant furnace. Advantageously, by using an emissivity increasing top layer, it is possible to reduce the heating time in the austenitizing furnace and thus increase productivity.

[0145]

[0107] In a particular embodiment, the metallic blanks 8 further comprise patches in areas in which it is necessary to increase the resistance or stiffness of the part and said patches are coated with an emissivity increasing top layer, while the rest of the metallic blank is not coated with an emissivity increasing top layer. This allows to compensate locally for the over-thickness of the patched area in the austenitizing furnace, which will thus have a heating curve closer to that of the rest of the metallic blank.

[0146]

[0108] In a particular embodiment, the metallic blanks 8 are tailor welded blanks and further comprise at least one patch covering at least partially one of the weld seams of said tailor welded blanks. Advantageously, this allowsto strengthen the part in the weld seam, which can be a more fragile area of the part.

Claims

CLAIMS1) Rear underfloor structure (2) for a motor vehicle (1) manufactured by assembling together a left and right structure (6), wherein said left and right structures (6) each comprise respectively a left and right side member (4) extending in a substantially longitudinal direction and at least one left and right half cross member (7) extending inwards in a substantially transverse direction from said side member (4), wherein said left and right structures (6) are each manufactured by stamping a single metallic blank (8) and wherein said left and right structures (6) are joined together along distal ends (71) of said at least one half cross members (7).2) Rear underfloor structure according to claim 1 , wherein said left and right structures (6) are each manufactured by stamping a single tailor welded blank (8).3) Rear underfloor structure according to claim 1 , wherein said left and right structures (6) are each manufactured by stamping a single tailor rolled blank (8).4) Rear underfloor structure according to any one of claims 1 to 3, wherein said left and right structures (6) are joined together by overlapping said distal ends (71 ) of said at least one half cross members (7) and attaching them together in a thus formed overlapping region.5) Rear underfloor structure according to any one of claims 1 to 4, wherein at least one additional connecting part is used to assemble said left and right structures (6) along said distal ends (71) of at least one left and right halfcross members (7).6) Rear underfloor structure according to any one of claims 1 to 5, wherein said left and right structures (6) are manufactured by hot stamping.7) Method to manufacture a rear underfloor structure (2) according to any one of claims 1 to 6 and to attach it to a motor vehicle, comprising the steps of:• Providing a left and right structure (6) each comprising a side member (4) and at least one half cross member (7) extending from said side member (4),• Assembling said left and right structures (6) together by joining them together along distal ends (71 ) of said at least one half cross members (7) to form a rear underfloor structure (2),• Assembling said rear underfloor structure (2) to a left and right side reinforcement structure (12) and a rear bumper assembly (11).8) Method to manufacture a rear underfloor structure (2) according to any one of claims 1 to 6 and to attach it to a motor vehicle, comprising the steps of • Providing a left and right structure (6) each comprising a side member (4) and at least one half cross member (7) extending substantially inward from said side member (4),• Assembling said left and right structures (6) respectively to a left and right side reinforcement structure (12) and a rear bumper assembly (11),• Assembling said left and right structures (6) together by joining them together along distal ends (71 ) of said at least one half cross members (7).9) Motor vehicle (1 ) comprising a rear underfloor structure (2) according to any one of claims 1 to 6.

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