Lateral structure for a motor vehicle

Abstract

A side structure (1) for a motor vehicle (3) comprises an inner and outer frame (11, 13) each forming a closed loop and having two openings for front and rear doors (8, 10), wherein said inner and outer frames (11, 13) are each formed by hot stamping of an inner frame piece and an outer frame piece (111, 113), respectively, each being a custom welded piece made of steel, and wherein said inner and outer frames (11, 13) are assembled to form a hollow volume (7) therebetween.

Description

Lateral structure for a motor vehicle The present invention relates to a side structure for a motor vehicle. Automakers are addressing the growing requirements of increasing passive vehicle safety, reducing vehicle weight to minimize greenhouse gas emissions in the case of internal combustion engines, or increasing vehicle driving range in the case of electric vehicles, while maintaining production, low cost, and high efficiency rates. The side structure of a motor vehicle can be seen as the side wall that separates the vehicle occupants from the outside and protects them from any intrusion in the event of a crash. It is also one of the main structural elements that connects the front and rear crash management systems and is essential for the good transmission and absorption of the forces resulting from said systems. As such, the lateral structure of a motor vehicle is a key structural element of the vehicle and contributes to the safety of the occupants in the event of side impacts, frontal and rear impacts, as well as in the event of a rollover, whereby the vehicle rolls onto its side and / or roof due to an accident or a loss of control on a roadside. Furthermore, in the case of an electric vehicle with a battery pack under the vehicle floor panel, the lateral structure also plays a role in protecting the battery pack from side impacts. The side structure, consisting of several separate parts, is a significant mass of the car body. It also involves costly manufacturing processes: multiple forming operations and assembly steps to obtain the final structure. An example of such a side structure in which the structural components are made just below the outer shell of the vehicle using a single custom welded piece is disclosed in JP5764667B2. The present invention aims to address the combined challenges of safety, weight reduction, and high efficiency by providing a side structure with a reduced number of parts, excellent safety performance, and an optimized overall weight. To this end, the present invention relates to the following: A side structure for a motor vehicle comprising an inner frame and an outer frame, wherein said inner and outer frames each respectively comprise: - a roof rail section, corresponding to the upper part of the side structure that borders a vehicle roof, - a rocker panel section, corresponding to the lower part of the side structure that borders a vehicle floor panel, - an upper part of the A-pillar extending from the front end of the said roof rail section and corresponding to the part of the side structure bordering a vehicle windscreen, - a lower portion of the A-pillar extending from the upper portion of said A-pillar downwards to the portion of said rocker panel, - an upper portion of the B-pillar extending in a height direction from the roof rail portion between the front and rear doors down to the level of the windows of the front and rear doors, - a lower portion of the B-pillar extending from the upper portion of said B-pillar downwards to said rocker panel portion, - a lower section of the C-pillar extending from the rear end of the said rocker panel section behind the tailgate and up to the level of the tailgate window in the height direction, - an upper portion of the C-pillar extending from the lower portion of said C-pillar to the roof rail portion, wherein said inner and outer frames each form a closed loop having two openings corresponding to the front and rear doors, wherein said inner and outer frames are each hot stamped to form an inner frame piece and an outer frame piece, respectively, said inner and outer frame pieces each being a single piece made of steel, where the inner and outer frame parts are custom welded parts consisting of n inner sub-parts and m outer sub-parts, respectively, where n and m are exactly integers greater than 1, wherein at least two internal sub-pieces have a different thickness before hot stamping and at least two internal sub-pieces have a different tensile strength after hot stamping, wherein the at least two outer sub-pieces have a different thickness before hot stamping and the at least two outer sub-pieces have a different tensile strength after hot stamping, And in that the aforementioned inner and outer frames are assembled to form a hollow volume between them. �With regard to other optional features, the side structure according to the invention is considered, either alone or in any possible technical combination: - The inner and outer frame parts each include at least one inner and outer sub-part, respectively, covered with an aluminum-based metal coating. - The outer frame part comprises at least one outer sub-part covered with an aluminum-based metal coating comprising from 2.0 to 24.0 wt.% zinc, from 1.1 to 12.0 wt.% silicon, optionally from 0 to 8.0 wt.% magnesium and optionally additional elements selected from Pb, Ni, Zr or Hf, the weight content of each additional element being less than 0.3 wt.%, the remainder of aluminum and unavoidable impurities being optional. - The inner frame piece is composed of a series of n inner sub-pieces, where each inner sub-piece has a thickness before hot stamping ti and an ultimate tensile strength after hot stamping TSi, where the product Pi = ti*TSi is calculated for each inner sub-piece, said inner frame piece including: - A minimum internal sub-segment resistance has a product Pmin which is the minimum of all products Pi of said n internal sub-segments, - A maximum internal sub-component resistance has a product Pmax which is the maximum of all products Pi of said n internal sub-components, and where Pmax > 2*Pmin. - The outer frame piece is formed from a series of outer sub-pieces m, where each outer sub-piece has a thickness before hot stamping ti and an ultimate tensile strength after hot stamping TSi, where the product Pi = ti*TSi is calculated for each outer sub-piece, said outer frame piece (113) comprising: - A minimum outer sub-part resistance has a product Pmin that is the minimum of all products Pi of the m outer sub-parts. - A maximum outer sub-segment resistance has a product Pmax which is the maximum of all products Pi of the m outer sub-segments. and where Pmax > 2*Pmin. - The inner frame piece includes at least one inner sub-piece that includes an emissivity-enhancing top layer on at least one side. - The outer frame piece includes at least one outer sub-piece that includes an emissivity-enhancing top layer on at least one side. - The inner frame part includes at least one inner sub-part made of a pressure-hardened steel that has an ultimate tensile strength after hot stamping above 1800 MPa. - The outer frame part includes at least one outer sub-part made of a pressure-hardened steel having an ultimate tensile strength after hot stamping of above 1800 MPa. - An inner frame member includes at least one inner sub-member made of a strain hardening steel having a yield strength after hot forming between 700 and 950 MPa, an ultimate tensile strength after hot forming between 950 MPa and 1200 MPa, and a bending angle after hot forming above 75 degrees. - The outer frame member includes at least one outer sub-member made of a strain-hardened steel having a yield strength after hot forming of between 700 and 950 MPa, an ultimate tensile strength after hot forming of between 950 MPa and 1200 MPa, and a bending angle after hot forming of above 75 degrees. - The outer frame piece includes at least one metal patch. - At least one metal patch of the outer frame piece includes an emissivity enhancing top layer. - The outer frame piece includes at least one weld seam reinforcing patch, wherein said weld seam reinforcing patch is applied to an area that includes a weld seam. - At least one weld seam reinforcement patch of the outer frame piece includes an emissivity enhancing top layer. - The thickness of the cross-penetration layer in the aluminum-based metal-coated areas of the inner frame is between 3 microns and 15 microns. - The thickness of the cross-penetration layer in the aluminum-based metal-coated areas of the outer frame is between 3 and 15 microns. Other aspects and advantages of the invention will become apparent upon reading the following description, given by way of example and made with reference to the accompanying drawings, in which: - Figure 1 is a first general perspective view of a vehicle according to the invention - Figure 2 is a second overall perspective view of a vehicle according to the invention, in which the outer shell of the vehicle has been made transparent in order to see the underlying structural parts. - Figure 3 is an enlarged perspective view of a side structure according to the invention - Figure 4 is a top view of a part used to form an inner frame according to the invention. - Figure 5 is a top view of a part used to form an outer frame according to the invention. - Figure 6 is a top view of an interior frame according to the invention - Figure 7 is a top view of an outer frame according to the invention - Figures a8 and b8 are schematic examples of cross-sections based on any given plane perpendicular to the internal perimeter of the lateral structure according to the invention, said internal perimeter being marked by line 33 of Figure 2. In the following descriptions, the terms "top", "bottom", "front", "rear", "transverse" and "longitudinal" are defined with respect to the usual directions of a mounted vehicle. Specifically, the terms "top", "bottom", "top", "bottom", "bottom" and "top" are defined with respect to the height direction of the vehicle, the terms "front", "rear", "front", "rear" and "longitudinal" are defined with respect to the front / rear direction of the vehicle, and the term "transverse" is defined with respect to the width of the vehicle. The term "height" refers to the distance between two points, lines, surfaces or volumes measured in the horizontal direction. A steel part is a flat sheet of steel that has been cut into any shape suitable for use. A part has a top and a bottom face, also called a top and bottom side or a top and bottom surface. The distance between the said faces is designated as the thickness of the part. The thickness can be measured, for example, using a micrometer with the spindle and anvil located at the top and bottom faces. In a similar way, the thickness can also be measured on a formed part. Yield strength, ultimate tensile strength and uniform and total elongation were measured in accordance with the ISO standard, ISO 6892-1, published in October 2009. The bending angle is measured in accordance with the bending standard VDA-238. For a similar 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 value must be adjusted by the following calculation, where α1,5 is the bending angle at 1.5 mm, t is the thickness and αt is the bending angle for thickness t: The bend angle of a part is a way to measure the part's ability to resist deformation without cracking. Emissivity is the relative ability of a surface to radiate heat. It is the ratio of the radiant energy emitted by a surface to the energy emitted by a blackbody at the same temperature and is a value between 0 and 1. The higher the emissivity of a surface, the more heat it will absorb by radiation, and therefore the easier it will be to heat it using a radiant furnace. Referring to Figures 1 and 2, a side structure 1 of a motor vehicle 3 is described. The outer shell of the vehicle is made transparent for clarity in Figure 2, the side structure 1 being located under the shell. The motor vehicle 3 can be any type of passenger vehicle that includes at least one set of front and rear doors: compact, sedan, sport utility vehicle, etc. The side structure described is essentially the same in each vehicle class. Furthermore, the power transmission of said motor vehicle can be a combustion engine, electric motors, fuel cells or any type of hybrid system. Figure 3 is an enlarged view of the side structure 1 according to the present invention and a view of the outer side of the body 5. The outer side of the body 5 constitutes the outer shell of the vehicle and has essentially an aesthetic purpose, while the side structure 1 has a structural purpose ensuring crash resistance and overall rigidity of the body. Referring to Figures 1 and 2, the lateral structure 1 includes several parts, which are indicated by the lines in Figure 1, which will be explained below: - a roof rail section RR1 which corresponds to the upper part of the side structure 1 at the boundary of a roof 6. The roof rail section RR1 is connected to the roof cross beams 21 and plays an important role in the torsional rigidity of the vehicle as well as in the resistance of the vehicle structure to a rollover. -A rocker panel section RP1, corresponding to the lower part of the side structure 1, borders the floor panel 20 of the vehicle. Said rocker panel section RP1 is connected in the transverse direction to the floor cross members 23. It is connected longitudinally at its front end to a front cross member 15, possibly via intermediate parts. It is connected longitudinally at its rear end to a rear cross member 25, possibly via intermediate parts. The rocker panel section RP1 is effective in preventing penetration and absorbing energy in the event of side impacts to the passenger compartment. It is also very useful in preventing penetration and absorbing energy in the event of a front or rear collision, due to its connection to the front and rear side members 15, 25. This is especially important in a frontal or rear-end collision that only affects a portion of the vehicle's width, such as an Insurance Institute for Highway Safety (IIHS) Small Overlapping Rigid Barrier (SORB) crash in which a vehicle is struck by a rigid barrier traveling at 40 mph (64.4 km / h) with only 25 percent overlap across the vehicle's width.In such a configuration, only the front or rear crash management system portion plays a role in impact resistance. The side structure 1, which is connected to the front and rear components 15, 25 at its rocker panel portion RP1, will play an important role in enhancing the vehicle's resistance in such cases, absorbing part of the crash energy, resisting penetration, protecting the occupants and transmitting crash energy to other components of the vehicle structure. In the case of an electric or hybrid vehicle having a battery pack located under the floor panel 20 (the battery is not shown in the figures), the rocker panel portion RP1 also plays a role in protecting the battery pack both from penetration in the event of a side impact and from deformation in the event of a front or rear impact. - An upper section 1AU of the A-pillar is associated with the section of the side structure 1 adjacent to a windscreen 4. Said upper section 1AU of the A-pillar plays an important role in the resistance to absorbing and transmitting crash energy in the event of a side impact and is also important in ensuring the overall torsional rigidity of the vehicle. - A lower section 1AL of the A-pillar extending from said upper section 1AU of the A-pillar downwards to said rocker panel section RP1. In the longitudinal direction, the lower section 1AL of the A-pillar is connected to the front crash management system, such as a section commonly referred to as a shotgun 17. As such, it plays a key role in transmitting, absorbing and resisting crash energy in the event of a frontal crash, particularly in the event of a small overlap crash, as described above for the rocker panel section RP1. In the transverse direction, the lower section 1AL of the A-pillar is connected to transverse members, such as the dashboard panel 19, and is useful in resisting penetration into the passenger compartment in the event of side crashes and in transmitting and absorbing the forces generated by side crashes to the rest of the structure via said transverse members. - An upper portion 1BU of the B-pillar extending in a height direction from said roof rail portion RR1 between the front and rear doors 8, 10 to the level of the windows of said front and rear doors 8, 10 in a height direction. The upper portion 1BU of the B-pillar plays a key anti-intrusion role in the event of a side impact. In the height direction, it is generally located at the level of the occupants' vital organs (upper torso) and should therefore effectively prevent intrusion into the compartment to protect the occupants' lives. - A lower section 1BL of the B-pillar extending from the upper section 1BL of the B-pillar down to the said rocker panel section RP1. The lower section 1BL of the B-pillar plays a role in preventing penetration and absorbing energy in the event of side impacts affecting the middle and front ends of the passenger compartment. - A lower portion 1CL of the C-pillar extending from the rear end of the aforementioned rocker panel portion RP1 behind the rear door 10 and up to the level of the rear door window 10 in the height direction. The lower portion 1CL of the C-pillar plays a role in preventing penetration and absorbing energy in the event of side impacts affecting the rear of the passenger compartment. It is also very useful in dispersing and transferring crash energy to the rest of the vehicle structure in the event of a rear impact. - An upper section 1CU of the C-pillar extends from the lower section 1CL of the C-pillar to the roof rail section RR1. The upper section 1CU of the C-pillar plays a role in preventing penetration and absorbing energy in the event of side impacts affecting the rear of the passenger compartment. It is also very useful in spreading and transferring the crash energy to the rest of the vehicle structure in the event of a rear-end collision. The side structure 1 described above forms a closed loop around the side of the vehicle 3 with two openings corresponding to the front and rear doors 8, 10. Referring to Figure 3, the side structure 1 according to the present invention is formed by the assembly of an inner frame 11 and an outer frame 13. The inner frame 11 is located closest to the passenger compartment, the outer frame 13 is located closest to the exterior of the vehicle. Referring to Figures 6 and 7, each of the above-described side structure sections RR1, AU1, AL1, RP1, CL1, CU1, BL1, BU1 corresponds to a corresponding section of the inner and outer frames, which are indicated in Figures 6 and 7 by lines RR11, AU11, AL11, RP11, CL11, CU11, BL11, BU11 and RR13, AU13, AL13, RP13, CL13, CU13, BL13, BU13, respectively. The inner and outer frames 11, 13 each form a closed loop around the vehicle 3 with two openings corresponding to the front and rear doors 8, 10. Referring to Figures 4 and 5, the inner and outer frames 11, 13 are each formed by stamping a single steel piece, forming an inner and outer frame piece 111, 113, respectively. The use of a single steel piece to produce each piece provides several advantages from the point of view of construction, structural strength and weight reduction. In terms of construction, this means that there is only one forming step and no separate assembly steps of the sub-parts. This allows for increased productivity and increased geometric accuracy of the inner and outer frames 11, 13. In fact, the geometric tolerances on the individual parts are added to calculate the geometric tolerance of an assembly. In the present case, there is no addition of the geometric tolerances of the individual parts. Furthermore, there is no problem of assembly tolerance between the individual parts. The use of a single piece also allows for an increase in the strength of the piece because there is no risk of rupture in the assembly joints between the sub-pieces in the event of an impact force being applied to the inner and outer frames 11, 13.Also, when a load is applied to the inner and outer frames, there is excellent energy transfer and dissipation in the inner and outer frames, ensuring optimal crash energy management. Furthermore, the fact that the inner and outer frames 11, 13 are each made from a single piece means that there is no overlap area for assembly between the sub-parts in the inner and outer frames 11, 13 - this lack of overlap reduces the weight of the parts. Referring to FIGS. 4 and 5, the inner and outer frame members 111, 113 are custom welded steel members. Custom welded steel members are fabricated by assembling multiple steel members together, such as by laser welding together, known as sub-members, to optimize the performance of the member in various regions thereof, reduce the overall weight of the member, and reduce the overall cost of the member. The inner frame member 111 is fabricated by assembling n inner sub-members IS1, IS2, ... ISn ... ISi, where n is an integer greater than 1. Each inner and outer sub-member ISi, OSi has a thickness before hot stamping and an ultimate tensile strength after hot stamping. The series of inner sub-members ISi includes at least two sub-members with two different thicknesses. The series of inner sub-members ISi includes at least two sub-members having two different tensile strengths after hot stamping. The outer frame piece 113 is constructed by assembling m outer sub-pieces OS1, OS2, ... OSm ... OSi, where m is an integer strictly greater than 1.The OSi outer sub-part series includes at least two sub-parts that have two different thicknesses. The OSi outer sub-part series includes at least two sub-parts that have two different tensile strengths after hot stamping. The sub-parts are assembled together by welding along weld lines 30. Weld lines 30 are shown by black lines in FIGS. 4 and 5, which depict specific embodiments of inner and outer frame parts 111, 113. They are shown by white lines in FIGS. 6 and 7, which depict specific embodiments of inner and outer frames 11, 13. It should be understood that the location of the weld lines 30 does not necessarily correspond to the different sections of the inner and outer frames 11, 13 described above. In fact, the vehicle designer will place different sub-parts of different thicknesses and different steel grades with different material strengths in appropriate areas to optimize crashworthiness, stiffness and weight of the part. This optimal location of the said weld lines 30 does not necessarily correspond to the above-described constraints between the sections of the inner and outer frames 11, 13. For example, as shown in Figure 7, the roof rail section of the outer frame RR13 includes materials from three different sub-parts. The use of custom welded parts allows the use of sub-parts with different material thicknesses and strengths, which allows for optimization of the performance of the part. By placing thicker, higher strength materials in areas requiring high strength, such as in the parts involved in the side structure of the B-pillar above BU1, and by placing thinner, lower strength materials in areas requiring less strength, a part can be designed with optimal strength while exhibiting an optimal overall weight. In addition, manufacturing waste can be reduced by using custom welded parts for the inner and outer frame parts 111, 113. When using a monolithic part, instead of a custom welded part, the large openings in the inner and outer frame parts 111, 113 correspond to the doors 8 and 10, which must be cut out of the part and discarded.By using custom welded parts, it is possible to use parts that are roughly rectangular in shape, which is optimal for minimizing waste, or parts that have complementary left and right shapes to provide good alignment when cut from a steel coil. Minimizing waste allows for the cost of the final part to be minimized while also improving the environmental footprint of the part’s production. The inner and outer frames 11, 13 are made by hot stamping of the inner and outer frame parts 111, 113. Hot stamping is a forming technology that involves heating a part to a temperature at which the microstructure of the steel is at least partially transformed into austenite, forming the part at high temperature by stamping it, and quenching the formed part to obtain a microstructure with a very high strength. Hot stamping makes it possible to obtain very high strength parts with complex shapes and without spring return. In order to obtain the described benefits of hot stamping, the material used is known as a pressure hardening material which has a chemical composition that allows it to form the desired hardened microstructure when submitted to the hot stamping process described above. It should be understood that the heat treatment to which a part is submitted includes not only the thermal cycle described above of the hot stamping process itself, but also a subsequent paint curing step, after the part has been painted to cure the paint.The mechanical properties of hot stamped parts below are those measured after the paint curing step, if a paint curing step is actually performed. The inner and outer frames 11 and 13 are large parts that cover the entire length and height of the vehicle and have complex shapes. If there is a springback problem after the parts are formed, there will be warping, distortion and generally poor geometric tolerances that make it difficult to assemble the parts together and to the rest of the vehicle. By using hot stamping, the inner and outer frames 11 and 13 can be made with high geometric accuracy and no springback problems or with very little springback problems. By using custom welded parts that have been hot stamped to form the inner and outer frames 11 and 13, the parts can be designed to have very significant differences in thickness and strength in different areas of the parts. It is generally considered that a good indication of penetration resistance and energy absorption capacity is given by the product of its ultimate tensile strength after hot stamping by its thickness before stamping. Given that the inner frame 111 is composed of a series of n inner sub-pieces IS1, IS2, ..., ISi, ... ISn, where each inner sub-piece ISi has a thickness before hot stamping ti and an ultimate tensile strength after hot stamping TSi, where the product Pi = ti*TSi is calculated for each inner sub-piece ISi, a minimum inner sub-piece strength ISmin can be selected that has a minimum product Pmin of all inner sub-pieces ISi and a maximum inner sub-piece strength ISmax that has a maximum product Pmax of all inner sub-pieces ISi. In a particular embodiment, the maximum product Pmax will be significantly different from the minimum product Pmin. Advantageously, this means that the part will have very different strength levels in different areas of the part and thus an optimal distribution of weight and strength with respect to the area of ​​the part. For example, Pmax is at least twice as large as Pmin (in other words, Pmax > 2* Pmin). Given that the outer frame piece 113 is composed of a series of m outer sub-pieces OS1, OS2, ..., OSi, ... OSm, where each outer sub-piece OSi has a thickness before hot stamping ti and an ultimate tensile strength after hot stamping TSi, where the product Pi = ti*TSi is calculated for each outer sub-piece OSi, a minimum outer sub-piece strength OSmin can be selected that has a minimum product Pmin of all outer sub-pieces ISi and a maximum outer sub-piece strength OSmax that has a maximum product Pmax of all outer sub-pieces OSi. In a particular embodiment, the maximum product Pmax will be significantly different from the minimum product Pmin. Advantageously, this means that the piece will have very different strength levels in different areas of the piece and thus an optimal weight and strength distribution with respect to the area of ​​the piece. For example, Pmax is at least twice as large as Pmin (in other words, Pmax > 2* Pmin). For example, the inner frame 111 or the outer frame 113 includes at least one sub-piece made of a pressure-hardened steel having an ultimate tensile strength of more than 1800 MPa after hot forming. For example, the steel composition of the said sub-part in terms of weight percentage includes: 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%, residual iron and unavoidable impurities resulting from preparation. With this composition range, the ultimate tensile strength of the part in the area corresponding to the said sub-part after pressure hardening is higher than 1800 MPa. For example, the aforementioned sub-part is made of Usibor® 2000. For example, the inner frame 111 or the outer frame 113 includes at least one sub-piece made of a pressure-hardened steel having a tensile strength of more than 1300 MPa after hot forming. For example, the steel composition of the said sub-part in terms of weight percentage includes: 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%, residual iron and unavoidable impurities resulting from preparation. With this composition range, the ultimate tensile strength of the part in the area corresponding to the said sub-part after pressure hardening is between 1300 MPa and 1650 MPa and the yield strength is between 950 MPa and 1250 MPa. For example, the aforementioned sub-part is made of Usibor® 1500. For example, the inner frame 111 or the outer frame 113 includes at least one sub-piece having a steel composition comprising, in weight percent: 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% Cu, Ni and Mo, residual iron and unavoidable impurities resulting from preparation. With this composition range, the yield strength of this part in the area of ​​the said sub-part after compression hardening is between 700 and 950 MPa, the ultimate tensile strength is between 950 MPa and 1200 MPa and the bending angle is above 75 degrees. For example, the said sub-part is made of Ductibor® 1000. For example, the inner frame 111 or the outer frame 113 includes at least one sub-piece corresponding to an inner or outer frame end region 11, 13 that has an ultimate tensile strength between 1350 MPa and 1650 MPa, a yield strength between 1000 MPa and 1300 MPa, and a bending angle above 70 degrees. For example, the inner frame 111 or the outer frame 113 includes at least one sub-piece corresponding to an inner or outer frame end region 11, 13 that has an ultimate tensile strength between 1500 MPa and 1800 MPa, a yield strength between 1250 MPa and 1500 MPa, and a bending angle above 70 degrees. Thanks to the use of custom welded parts and hot stamping technology, it is possible to obtain very high-strength inner and outer frames 11, 13, which have optimum strength in the various areas of said parts and a very good geometric tolerance despite their large size and very high strength. The inner and outer frames 11, 13 are assembled together around their perimeter, including around the inner edges in the openings of the doors 8, 10. The assembly is carried out, for example, by spot welding. The inner and outer frames are designed with shapes that are formed between each other when the hollow volume 7 is assembled, as shown in Figures a8 and b8. Figures a8 and b8 show simplified sections of the inner and outer frame assembly 11, 13 along each plane extending perpendicular to the inner perimeter of the side structure, which is schematically indicated by the dashed line marked with the number 33 in Figure 2. Said hollow volume 7 provides excellent torsional rigidity to the side structure 1, which in turn contributes to increasing the overall rigidity of the vehicle body.This configuration also allows for effective resistance to side impacts. The inertia provided by the hollow volume 7 provides good resistance to the forces generated by a side impact. Furthermore, due to the isotropic nature of the steel, the side structure 1, with the hollow volume 7, will also have good resistance and good energy absorption capacities in the event of a frontal or rear collision that exerts forces on the side structure 1 in the longitudinal direction. In this case, the fact that both the inner and outer frames are made of a single piece means that there is no risk of rupture between the sub-parts forming the assembly of the inner and outer frame structures. In fact, the longitudinal force of the collision leads to the application of shear forces to the assembly points, which is a crucial configuration for the strength of the assemblies. Furthermore, the inner and outer frames 11, 13 are assembled together along their respective perimeters, i.e. along a very large area. This ensures a good connection between the two frames and therefore reduces the risk of breakage in the event of a frontal impact. The design described above involves the use of hot stamping of two custom welded parts to form inner and outer frames 11, 13 and their assembly to form a hollow volume 7 that surrounds the entire perimeter of the side structure 1, which has the further advantage of offering very good resistance to collisions with the upper and lower sections CU1, CL1 of the C-pillar. This allows for better protection of the rear seat occupants of the vehicle. It also anticipates the fact that safety regulations are constantly changing towards more stringent requirements. The strength of the C-pillar area is not currently a major focus of safety testing, but may become so in the near future. In a particular embodiment, in areas requiring greater rigidity or additional impact resistance, a reinforcement 29 is provided inside the hollow volume 7. Said reinforcement 29 can have different cross-sections, as shown in the schematic examples of Figures a8 and b8. For example, the reinforcement 29 has a general U-shape, the bottom of which is connected to the outermost wall of the outer frame 13 (Figure b8). In another example, the reinforcement 29 has a general omega-shape, the bottom part of which can be used as flanges to connect it to the outermost wall of the outer frame 13 (Figure a8). For example, the reinforcement 29 is connected to the outer frame 13 by spot welding. In a particular embodiment, the inner frame part 111 and / or the outer frame part 113 comprises at least one sub-part covered with an aluminum-based metal coating. By aluminum base is meant a coating comprising at least 50% by weight of aluminum. For example, the metal coating is an aluminum-based coating comprising 8 to 12% by weight of Si. For example, the metal coating is applied by immersing the base material in a molten metal bath. Advantageously, applying an aluminum-based metal coating to the inner frame 111 or the outer frame 113 prevents the formation of surface deposits during the heating step of the hot stamping process, which in turn allows the parts to be produced by hot stamping without a sandblasting operation. In addition, the aluminum-based coating also provides corrosion protection to the part during service on the vehicle. In a particular embodiment, the inner frame 111 and / or the outer frame 113 is formed of at least one sub-part which is coated with an aluminum-based metallic coating comprising from 2.0 to 24.0 wt.% zinc, from 1.1 to 12.0 wt.% silicon, optionally from 0 to 8.0 wt.% magnesium and optionally additional elements selected from Pb, Ni, Zr, or Hf, the weight content of each additional element being less than 0.3 wt.%, the remainder of aluminum and unavoidable impurities being optional. The advantage is that this type of metallic coating provides very good corrosion protection on the part as well as a good surface appearance after hot stamping. Laser welding can be used to manufacture the custom welded parts described above with an aluminum-based coating on at least one of the sub-parts. Sub-parts can be used where the welded edges are already prepared by removing a portion of the metal coating. To its advantage, this removes some of the aluminum in the coating, which would contaminate the weld seam and deteriorate its mechanical properties. In a particular embodiment, the inner frame 111 and / or the outer frame 113 includes at least one sub-piece that includes at least one side with an emissivity-enhancing top layer. Said emissivity-enhancing top layer is applied to the outermost surface of said sub-piece. Said emissivity-enhancing top layer allows said sub-piece surface to have a higher emissivity compared to a similar sub-piece not coated with said emissivity-enhancing top layer. Said emissivity-enhancing top layer can be applied to the top or bottom of a sub-piece. Said emissivity-enhancing top layer can also be applied to both sides of said sub-piece. If said sub-component comprises a metal coating, as previously described, the emissivity enhancing top layer is applied to said metal coating. In fact, for the emissivity enhancing top layer to increase the surface emissivity, it must cover the outermost surface of the sub-component. Advantageously, said emissivity enhancing top layer allows to increase the heating rate of said sub-part and consequently to increase the efficiency of the heating stage of the hot stamping process. In a particular embodiment, the inner and outer parts ISi, OSi are arranged in order of increasing thickness. The emissivity enhancing top layer is applied to at least one side of the sub-part with the maximum thickness. In a particular embodiment, the emissivity enhancing top layer is applied to at least one side of the sub-part with the maximum thickness and the sub-part having a thickness just less than the maximum thickness. In a particular embodiment, the emissivity enhancing top layer is applied to the sub-parts x having the highest thickness, x being an integer greater than or equal to 1. Advantageously, by applying the emissivity enhancing top layer to the set of sub-parts with higher thickness, a more homogeneous heating rate can be achieved during the heating stage of the hot stamping process between the sub-parts with higher thickness and the sub-parts with lower thickness. In fact, thinner sub-parts naturally heat up faster than thicker sub-parts because they are thinner and therefore require less energy to reach the same temperature.By targeting thicker sub-parts with an emissivity-enhancing top layer, the heating rate difference between parts of different thicknesses can be reduced, and thus a more homogeneous heating rate between thicker and thinner sub-parts can be achieved. In addition, by targeting thicker sub-parts with an emissivity-enhancing top layer, the process opportunity size of the heating step of the hot stamping process can be increased. When hot stamping large parts with high thickness differences, one concern is that there will be large differences in the process opportunity (which includes heating time and heating temperature, among other parameters) required to achieve the desired microstructure and coating properties of the different sub-parts. The process opportunity required to achieve the desired properties across the entire part is the intersection of the process opportunities of each individual sub-part.By applying an emissivity-enhancing top layer to thicker sub-parts, it is possible to bring the process opportunities of each individual sub-part closer together, thereby increasing the size of the intersection between the process opportunities of all sub-parts, i.e. to increase the process opportunity of the entire welded custom part. In a particular embodiment, the emissivity enhancing top layer has a thickness of between 2 microns and 30 microns. In a particular embodiment, it is comprised of a polymer that does not contain silicone, contains no more than 1% by weight of nitrogen, and contains carbon pigments in an amount of between 3% and 30% by weight. In a particular embodiment, the outer frame member 113 includes at least one metal patch 31 as shown in FIG. 5 to locally increase the strength of the member. In a particular embodiment, the patch 31 is attached by spot welding. In a particular embodiment, the patch 31 is attached by laser welding. The patch 31 is used, for example, in areas that require reinforcement due to the presence of door hinges, or due to mechanical problems such as folding of the member identified during crash tests. In general, 31-inch patches have the advantage of providing highly localized reinforcements over larger-sized parts, thus further optimizing the strength distribution and overall thickness of the custom welded parts, keeping the overall weight and cost of the part down. The 31 patches are made of pressure-hardened steel, for example. The 31 patches are coated with an aluminum-based metal coating, for example. In a particular embodiment, a patch 31 is coated with an emissivity enhancing top layer to provide the above-described advantage of increasing the heating rate and thereby reducing the heating rate difference in the patch 31 region associated with the excessive thickness of said patch 31. In a particular embodiment, the patch is applied to an area that includes a weld seam portion 30. As shown in column B of FIG. 5, it will be referred to as a weld seam reinforcement patch 32. Such a patch 32 has exactly the same features as described above and optional features. Such a patch 32 reinforces the weld seam 30. Weld seams 30 are areas where there is a discontinuity between two sub-parts that results in a local inertia change and can lead to the collapse of a type of plastic hinge when subjected to high loads caused by a crash. Reinforcing a weld seam 30 with a weld seam reinforcement patch 32 can prevent such a plastic hinge phenomenon. A weld seam reinforcement patch 32 is attached to the outer frame 13, for example, by welding it. In a particular embodiment, the connection points between the weld seam reinforcement patch 32 and the outer frame 13 will not be in the weld seam region 30 , in order not to interfere with the mechanical properties of the weld seam 30 . When using pressure-hardened steel coated with an aluminum-based metal coating for the inner or outer frame member 111, 113, the hot stamping process causes the formation of a cross-diffusion layer between the steel and the metal coating on the hot formed part. The cross-diffusion layer is the result of high-temperature cross-diffusion of iron from the steel to the metal coating and Al from the coating to the steel. The thickness of said cross-diffusion layer has been shown to be related to most of the in-use properties of the part, such as the ability of the part to be successfully assembled to the rest of the body by spot welding. In particular, it has been shown that hot formed parts with a cross-diffusion layer thickness of between 3 microns and 15 microns have good in-use properties. More preferably, it has been shown that hot formed parts with a cross-diffusion layer thickness of between 3 microns and 10 microns have excellent in-use properties. In a particular embodiment, the thickness of the cross-penetration layer in the aluminum-based metal-coated areas of the inner frame 11 is between 3 microns and 15 microns. In a particular embodiment, the thickness of the cross-penetration layer in the aluminum-based metal-coated areas of the inner frame 11 is between 3 microns and 10 microns. In a particular embodiment, the thickness of the cross-penetration layer in the aluminum-based metal-coated areas of the outer frame 13 is between 3 microns and 15 microns. In a particular embodiment, the thickness of the cross-penetration layer in the aluminum-based metal-coated areas of the outer frame 13 is between 3 microns and 10 microns. The present invention also relates to a process for producing the side structure 1 described above and assembling it to the rest of the vehicle body. In a particular embodiment, the process includes the following steps (Steps A, B, C, and D are listed in no particular order): A / Providing an internal frame piece 111 B / Providing an outer frame piece 113 C / Hot stamping of the inner frame piece 111 to form an inner frame 11 D / Hot stamping of outer frame piece 113 to form an outer frame 13 E / Assembling the inner and outer frames 11, 13 to form a lateral structure 1 F / Connecting the side structure 1 to the vehicle body G / Connecting an outer side of the body 5 to the pre-assembled side structure 1 Optionally, the process further includes the step of attaching the reinforcements 29 to the outer frame 13 between steps D and E. In a particular embodiment, the process includes the following steps (Steps A, B, C, and D are listed in no particular order): A / Providing an internal frame piece 111 B / Providing an outer frame piece 113 C / Hot stamping of the inner frame piece 111 to form an inner frame 11 D / Hot stamping of outer frame piece 113 to form an outer frame 13 F / Connecting the inner frame 11 to the vehicle body E / Connecting the outer frame 13 to form a side structure 1 G / Connecting an outer side of the body 5 to the pre-assembled side structure 1 Optionally, the process further includes the step of attaching the reinforcements 29 to the outer frame 13 between steps D and E.

Claims

Claims 1. A side structure (1) for a motor vehicle (3) comprising an inner frame (11) and an outer frame (13), wherein said inner and outer frames (11, 13) each respectively comprise: - a roof rail section (RR13, RR11), corresponding to an upper part of the side structure (1) which abuts a roof (6) of the vehicle, - a rocker panel section (RP13, RP11), corresponding to a lower part of the side structure (1) which abuts a floor panel (20) of the vehicle, - an upper part (AU13, AU11) of the A-pillar extending from the front end of said roof rail section (RR13, RR11) and corresponding to the part of the side structure (1) which abuts a windshield (4) of the vehicle, - a lower part (AL13, AL11) of the A-pillar extending from said upper part (AU13, AU11) of the A-pillar downwards to the rocker panel section (RP13,RP11) extends, - an upper portion (BU13,BU11) of the B-pillar extending in a height direction from the roof rail portion (RR13,RR11) between the front and rear doors (8, 10) to the window level of the front and rear doors (8, 10), - a lower portion (BL13,BL11) of the B-pillar extending from the upper portion (BU13,BU11) of the B-pillar to the rocker panel portion(RP13,RP11), - a lower portion (CL13,CL11) of the C-pillar extending from the rear end of the rocker panel portion (RP13,RP11) behind the rear door (10) and up to the surface of the rear door window (10) in the height direction, - an upper portion (CU13,CU11) of the C-pillar extending from the lower portion (CL13,CL11) of the C-pillar to the roof rail portion (RR13,RR11), wherein said inner and outer frames (11, 13) each form a closed loop having two openings corresponding to the front and rear doors (8, 10), wherein said inner and outer frames (11, 13) are each formed by hot stamping into an inner frame piece and an outer frame piece (111, 113), respectively, wherein said inner and outer frame pieces (111, 113) Each is a piece made of steel, where the inner and outer frame pieces (111, 113) are custom welded pieces consisting of n inner sub-pieces (IS1, IS2, ​​..., ISi,... ISn) and m outer sub-pieces (OS1, OS2, ..., OSi,… OSm), respectively, n and m are integers greater than 1, where the inner sub-pieces (IS1, IS2, ​​..., ISi,... ISn)comprising at least two inner sub-pieces with a different thickness before hot stamping and comprising at least two inner sub-pieces with a different tensile strength after hot stamping, wherein the outer sub-pieces (OS1, OS2, ..., OSi, OSm ...) comprise at least two outer sub-pieces with a different thickness before hot stamping and comprising at least two outer sub-pieces with a different tensile strength after hot stamping, and wherein said inner and outer frames (11, 13) are assembled to form a hollow volume (7) therebetween.

2. A side structure (1) according to claim 1, wherein the inner and outer frame parts (111, 113) each comprise at least one inner and outer sub-part (ISi, OSi), respectively, covered with an aluminum-based metal coating.

3. The side structure (1) according to claim 1 or 2, wherein the outer frame piece (113) comprises at least one outer sub-piece (OSi) coated with an aluminum-based metal coating comprising from 2.0 to 24.0 wt.% zinc, from 1.1 to 12.0 wt.% silicon, optionally from 0 to 8.0 wt.% magnesium and optionally additional elements selected from Pb, Ni, Zr or Hf, the weight content of each additional element being less than 0.3 wt.%, the remainder of aluminum and unavoidable impurities being optional.

4. The side structure (1) according to any one of claims 1 to 3, wherein the inner frame piece (111) is formed from a series of n inner sub-pieces (IS1, IS2, ..., ISi, ... ISn), wherein each inner sub-piece (ISi) has a thickness before hot stamping ti and an ultimate tensile strength after hot stamping TSi, wherein the product Pi = ti*TSi is calculated for each inner sub-piece (Isi), said inner frame piece (111) comprising: - a minimum inner sub-piece strength (ISmin) having a product Pmin which is the minimum value of all the products Pi of said n inner sub-pieces, - a maximum inner sub-piece strength (ISmax) having a product Pmax which is the maximum value of all the products Pi of said n inner sub-pieces, and wherein Pmax > 2*Pmin.

5. The side structure (1) according to any one of claims 1 to 3, wherein the outer frame piece (113) is formed from a series of m outer sub-pieces (OS1, OS2, ..., OSi, OSm ...), wherein each outer sub-piece (OSi) has a thickness before hot stamping ti and an ultimate tensile strength after hot stamping TSi, wherein the product Pi = ti*TSi is calculated for each outer sub-piece (OSi), said outer frame piece (113) comprising: - a minimum outer sub-piece strength (OSmin) having a product Pmin which is the minimum value of all products Pi of said m outer sub-pieces, - a maximum outer sub-piece strength (OSmax) having a product Pmax which is the maximum value of all products Pi of said m outer sub-pieces, and wherein Pmax > 2*Pmin.

6. The side structure (1) according to any one of claims 1 to 5, wherein the inner frame piece (111) comprises at least one inner sub-piece (ISi) comprising an emissivity enhancing top layer on at least one side.

7. The side structure (1) according to any one of claims 1 to 5, wherein the outer frame piece (113) comprises at least one outer sub-piece (OSi) comprising an emissivity enhancing top layer on at least one side.

8. The side structure (1) according to any one of the preceding claims, wherein the inner frame piece (111) comprises at least one inner sub-piece (ISi) made of a pressure-hardening steel having an ultimate tensile strength after hot stamping of above 1800 MPa.

9. The side structure (1) according to any one of the preceding claims, wherein the outer frame piece (113) comprises at least one outer sub-piece (OSi) made of a pressure-hardened steel having an ultimate tensile strength after hot stamping above 1800 MPa.

10. The side structure (1) according to any one of the preceding claims, wherein the inner frame member (111) comprises at least one inner sub-member (ISi) made of a strain hardening steel having a yield strength after hot forming of between 700 and 950 MPa, an ultimate tensile strength after hot forming of between 950 MPa and 1200 MPa and a bending angle after hot forming of above 75 degrees.

11. The side structure (1) according to any one of the preceding claims, wherein the outer frame member (113) comprises at least one outer sub-member (OSi) made of a strain-hardened steel having a yield strength after hot forming of between 700 and 950 MPa, an ultimate tensile strength after hot forming of between 950 MPa and 1200 MPa and a bending angle after hot forming of greater than 75 degrees.

12. A side structure (1) according to any one of the preceding claims, wherein the outer frame piece (113) comprises at least one metal patch (31).

13. The side structure (1) according to claim 12, wherein at least one metal patch (31) of the outer frame piece (113) comprises an emissivity enhancing top layer.

14. A side structure (1) according to any one of the preceding claims, wherein the outer frame piece (113) comprises at least one weld seam reinforcement patch (32), wherein said weld seam reinforcement patch (32) is applied to an area comprising a weld seam (30).

15. The side structure (1) according to claim 14, wherein at least one weld seam reinforcement patch (32) of the outer frame piece (113) comprises an emissivity enhancing top layer.

16. A side structure (1) according to any one of the preceding claims wherein the thickness of the cross-penetration layer in the aluminum-based metal coating regions of the inner frame (11) is between 3 microns and 15 microns.

17. A side structure (1) according to any one of the preceding claims wherein the thickness of the cross-penetration layer in the aluminum-based metal coating regions of the outer frame (13) is between 3 microns and 15 microns.

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