High strength and high length components having excellent energy

Abstract

The present invention encompasses a high elongate structural component having excellent impact resistance and energy absorption in both flexural and compressive modes, and made of a material having an ultimate tensile strength greater than 1300 MPa, a ratio between the yield strength YS and the ultimate tensile strength UTS of the material strictly lower than 0.85, a bending angle greater than 70 DEG and a slenderness ratio equal to or greater than 10.

Description

Technical Field

[0001] The present invention relates to a high-strength structural component having excellent energy absorption characteristics in the case of side impact and longitudinal impact. In particular, the present invention relates to a structural component for use in a motor vehicle. Background Art

[0002] High-strength and high-aspect-ratio structural components play an important role in the anti-collision performance of vehicles. The structural components are elongated assemblies including a hollow cavity.

[0003] In the case of a collision, such a component may be impacted on its side, i.e., in a direction substantially transverse to the length direction, or may be impacted in a substantially longitudinal direction.

[0004] When a side impact occurs on this type of structural component, this type of structural component typically bends under the impact load. The bending behavior of the component plays a crucial role in absorbing impact energy and resisting impacter intrusion into the vehicle. Good energy absorption and anti-intrusion are very important for minimizing the impact on vehicle occupants and other parts of the vehicle structure. In the case of electric vehicles, hybrid vehicles or hydrogen fuel vehicles, anti-intrusion is also very important for ensuring the integrity of the battery pack and / or hydrogen tank, which in turn plays an important role in ensuring the safety of vehicle occupants.

[0005] Therefore, high-strength and high-aspect-ratio structural components play an important role in improving the safety of vehicle occupants in the case of side impact.

[0006] Resistance to side impact of a vehicle is considered a major safety issue and is measured by a number of standardized tests such as, for example:

[0007] - The pole impact test of the US New Car Assessment Program (USNCAP), in which a vehicle with an initial lateral speed of 32.2 km / h impacts a fixed pole on its side.

[0008] - The IIHS's Moving Deformable Barrier (MDB) side test, in which a vehicle is impacted on its side by a deformable barrier weighing 1500 kg and traveling at a speed of 50 km / h.

[0009] These standardized tests are regularly updated to account for even more severe collision conditions, such as by increasing the weight of the barrier, the speed of the impact, and the standards required for the tests are regularly updated.

[0010] When subjected to a longitudinal impact, the component is subjected to a compressive force. In order to absorb the maximum amount of energy, it is important that the high-aspect-ratio component collapses as much as possible on itself while minimizing the occurrence of cracks.

[0011] The longitudinal impact on a front member, which is typically a high aspect ratio member, for example, is simulated by the following standardized collision tests:

[0012] - The small overlap rigid barrier (SORB) collision of the Insurance Institute for Highway Safety (IIHS), in which the vehicle is impacted by a rigid barrier moving at 64.4 km / h with an overlap of only 25% in width.

[0013] - The frontal overlap deformable barrier (ODB) of the IIHS, in which the vehicle is impacted by a rigid barrier moving at 64.4 km / h with an overlap of only 40% in width. Summary of the Invention

[0014] An object of the present invention is to provide a high-strength and high aspect ratio member having excellent energy absorption and anti-intrusion performance in both configurations of lateral impact and longitudinal impact.

[0015] The object of the present invention is achieved by providing a high aspect ratio member according to claim 1, which optionally includes the features of claims 2 to 8. Brief Description of the Drawings

[0016] The present invention will now be described and illustrated in detail by way of example with reference to the accompanying drawings without introducing limitations:

[0017] - Figure 1 is a schematic view of a high aspect ratio member according to an embodiment of the present invention, wherein Figure 1 a is an illustration detailing the definition of different angles defined in the specification,

[0018] - Figure 2 is a schematic view of a three-point bending test performed in Examples 1 and 2 described below.

[0019] - Figure 3 is a graphical reproduction at the end of the 3-point bending simulation of Example 1 in the case of the member I1w according to an embodiment of the present invention.

[0020] - Figure 4 is a graphical reproduction at the end of the 3-point bending simulation of Example 2 in the case of the member I1w according to an embodiment of the present invention.

[0021] - Figure 5 is a graphical reproduction at the end of the compression test simulation of Example 3 in the case of the member I1 (left side of the figure) according to an embodiment of the present invention and the member R4 (right side of the figure) not according to the present invention. Detailed Description of the Invention

[0022] In Leonhard Euler's buckling theory, the slenderness ratio commonly used is defined by the following formula, where L is the length of the component in mm, S is the area of the straight cross-section of the component in mm 2 represented, and I 最小 is the minimum second moment of the area of the cross-section under consideration.

[0023]

[0024] Generally, in a set of Cartesian coordinates (x, y), the minimum second moment I of the area on the cross-section A in mm 4 represented is 最小 defined by the following formula:

[0025] I 最小 = min(∫∫ A y 2 dxdy; 2 ∫∫ A x 2 dxdy)

[0026] For example, for a hollow rectangular cross-section with external dimensions b and h and internal dimensions b1 and h1, the minimum second moment I of the area is 最小 calculated using the following formula:

[0027]

[0028] For example, for a hollow annular cross-section with an outer radius R and an inner radius R1, the minimum second moment I of the area is 最小 calculated using the following formula:

[0029]

[0030] When the slenderness ratio of the component is greater than 10, preferably when the slenderness ratio is greater than 15, and even more preferably when the slenderness ratio is greater than 20, the component can be considered to have a high slenderness.

[0031] The bending angle is measured according to the VDA-238-100 bending standard. In the present invention, the bending angle is measured after springback. For the same material, the bending angle depends on the thickness. For simplicity, the bending angle value in the present invention refers to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle value needs to be normalized to an equivalent 1.5 mm thickness by the following calculation, where α 1.5 is the bending angle normalized to 1.5 mm, t is the thickness, and α t is the bending angle for the thickness t:

[0032] α 1.5 = (α t×√t) / √1.5

[0033] The bending angle of the component represents the ability of the component to resist deformation without forming cracks.

[0034] In the present invention, the bending angle is measured along the rolling direction, i.e., the direction in which the steel sheet travels during the hot rolling step. The bending angle is measured using a laser measuring device. When performing a bending test on a hot stamping component, a sample is cut from the flat area of the component. If necessary, small-sized samples are taken to fit the entire available flat area on the component. If the rolling direction on the hot stamping component is not known, it can be determined using electron backscatter diffraction (EBSD) analysis across the sample cross-section in a scanning electron microscope (SEM). The rolling direction is determined according to the intensity of the orientation density function (ODF) representing the main fiber under the Euler angles as defined in the first English edition (publication) of "H.-J. Bunge: Texture Analysis in Materials Science - Mathematical Methods" by Butterworth Co in 1982 (for the definition, see Figure 2 .2 and Figure 2 .3).

[0035] The ultimate tensile strength, yield strength, and elongation are measured according to the ISO standard ISO 6892-1 published in October 2009. The tensile test specimens are cut from the flat area. If necessary, small-sized tensile test samples are taken to fit the entire available flat area on the component.

[0036] The term "fracture strain" refers to the fracture strain criterion defined by Pascal Dietsch et al. in "Methodology to assess fracture during crash simulation: fracture strain criteria and their calibration" in Metallurgical Research Technology, Volume 114, Issue 6, 2017. The fracture strain is the equivalent strain at the internal deformation point of the material when the critical bending angle has been reached. The critical bending angle defines the angle at which the first crack is detected on the arch of a sample that has been deformed according to the standardized VDA-238-100 standard.

[0037] The term "bottling" refers to the deformation mode of a component subjected to a compressive load, typically a component with a high length-to-thickness ratio, where the component gradually absorbs the mechanical energy of the compressive load by forming a series of continuous waves due to continuous local buckling deformation. Thus, the length of the component measured in the direction of the compressive load after deformation is less than the initial length of the component in that direction. In other words, when the component responds to the compressive load by controlled buckling, the component folds onto itself in the same way as a plastic bottle with a compressive load applied between the top and bottom of the bottle.

[0038] Hot stamping is a forming technique for steel that involves heating a blank until the temperature at which the microstructure of the steel has at least partially transformed into austenite, thereby forming the blank at high temperature by stamping it, and quenching the formed component to obtain a microstructure with very high strength, where additional partitioning steps or tempering steps may be involved in the heat treatment. Hot stamping allows the obtaining of very high-strength components with complex shapes and presents many technical advantages. It should be understood that the heat treatment to which the component is subjected includes not only the heat cycle of the hot stamping process itself described above, but may also include other subsequent heat treatment cycles, such as for example a baking step for baking paint after the component has been painted. The mechanical properties of the following hot-stamped components are the mechanical properties measured after all heat cycles - optionally including for example a baking step if a baking step has indeed been carried out - or after any post-tempering step.

[0039] A blank refers to a flat plate that has been cut into any shape suitable for its use. The blank has a top surface and a bottom surface, which are also referred to as the top side and the bottom side or the top surface and the bottom surface. The distance between the said surfaces is designated as the thickness of the blank. This thickness can be measured, for example, using a micrometer, with the spindle and anvil of the micrometer placed on the top and bottom surfaces. In a similar manner, the thickness can also be measured on the formed component.

[0040] Hardness is a measure of the resistance to local plastic deformation caused by a mechanical indentation. Hardness is closely related to the mechanical properties of a material and is a useful local measurement method that does not require cutting a sample for tensile testing. In the present invention, hardness measurements are carried out using a Vickers indenter in accordance with standard ISO 6507-1. Vickers hardness is expressed using the unit Hv.

[0041] The heat-affected zone is the area around a weld in a material that has been heated during a welding operation. In the case of high-strength materials, such as high-strength steel, it is well known that the heat-affected zone may have weaker mechanical properties. In fact, the heat-affected zone undergoes a heat treatment similar to tempering, which may result in softening.

[0042] The transverse tensile strength, also known as the α-CTS value for the resistance of spot welding, reflects the strength of the spot weld under transverse tensile-type loading and is expressed as the ratio of the maximum transverse tensile strength to the product of the weld nugget diameter and the average thickness of the steel sheets to be joined. When the strength is divided by the product of the average metal sheet thickness and the weld nugget diameter, a normalized value is obtained that remains valid and applicable to various industrial welding component configurations. This value is widely used in the sheet metal and welding industries. The α-CTS value is obtained through the following protocol:

[0043] - Provide a transverse welding assembly of two metal samples with thicknesses t1 and t2, each measured as 100 mm * 50 mm, in accordance with the ISO 14272 standard issued on March 1, 2016, with the diameter of the weld nugget being d,

[0044] - Measure the transverse tensile strength (CTS) of the assembly in kN in accordance with the ISO 14272 standard issued on March 1, 2016,

[0045] - Calculate the α-CTS expressed in kN / mm 2 i.e., the ratio of the CTS in kN to the product of the average thickness and the weld nugget diameter, each expressed in mm:

[0046]

[0047] In the specification, drawings, and claims, orientation and spatial references are made using the L, T, Z coordinate reference system, where L is the longitudinal direction parallel to the length direction of the component, i.e., parallel to the longest dimension of the component, T is the transverse direction in which the component extends perpendicular to the longitudinal direction, and Z is the height direction perpendicular to the plane formed by the L and T directions. The reference is shown in each figure. When the figure is a 2D planar schematic, following the established convention, axes outside the drawing are represented by a dot in a circle when pointing towards the reader and by a cross in a circle when pointing away from the reader.

[0048] The directional terms "top", "upper", "upper part", "above", "bottom", "lower", "lower part", "below", etc. are defined according to the Z height direction. The directional terms "front" and "rear" are defined according to the L direction. The "width" or "transverse" direction refers to the orientation parallel to the T direction.

[0049] Reference Figure 1 , the high aspect ratio component 1 extends between two ends E1 and E2 along the main longitudinal direction L and extends along the transverse direction T. The high aspect ratio component 1 includes a hollow volume 4 enclosed between a top component 3 and a bottom component 2.

[0050] The high aspect ratio component 1 is made by separately forming a top component 3 and a bottom component 2 and then joining the top component 3 and the bottom component 2 together. For example, the top component 3 and the bottom component 2 are joined together by welding, such as by spot welding on the flange 6, which produces the spot weld portion 5.

[0051] In a particular embodiment, as Figure 1 depicted in, the top component 3 has a generally omega shape, and the bottom component 2 is a flat closed plate. In a particular embodiment not shown in the figures, the top component 3 is of generally omega shape and the bottom component 2 also has a generally omega shape (for example, this is the case in the components of Example 2, which will be described in further detail below).

[0052] High aspect ratio components are ubiquitous in vehicle architectures. Some examples are the front components that join the front collision box to the rocker assembly, the rear components that join the rear collision box to the rocker assembly, the lateral components that extend laterally in the vehicle, the rocker panel itself, etc. In the case of electric or hybrid vehicles, the battery pack is typically composed of a set of high aspect ratio components designed to protect the battery cells in the event of an impact.

[0053] High aspect ratio components are typically attached to the rest of the vehicle structure at each of their ends E1 and E2. When a vehicle collides, part of the energy of the collision can be transferred to the high aspect ratio component through the components to which the high aspect ratio component is attached. In this case, the high aspect ratio component will be subject to the following approximate compressive load: the compressive load is applied between the ends E1 and E2 of the high aspect ratio component and is generated by Figure 1 the force F1 transmitted by the surrounding elements to which the component is attached, as depicted by, and the resistance R1 generated by the resistance from the other elements to which the component is attached at its other end. The compressive force F1 will not necessarily be strictly parallel to the longitudinal direction and can form an angle β with the L axis, as Figure 1 depicted in a. As will be described in detail in the examples later, this situation corresponds to the compressive load test and the associated numerical simulation. In the remainder of the description, this will be referred to as the compression mode.

[0054] In the case of a collision, the impact force can also have at least a component directed along a direction perpendicular to the longitudinal direction, for example along the height direction. This is the case for Figure 2 the force F2 shown. In this case, the component will be subject to the form of a three-point bending load, with the force F2 being applied on one side and the resistance in the opposite direction coming from the resistance of the other elements to which the component is attached at the two ends E1, E2 (for clarity, in Figure 1The force is not depicted therein). As will be described in detail in the examples later, this situation corresponds to a three-point bending test and the associated numerical simulation. In the remainder of the description, this will be referred to as the bending mode.

[0055] Regardless of the load conditions, in order to provide effective protection in the event of a collision, high slenderness ratio components need to absorb a large amount of collision energy without significant cracking. In fact, by absorbing a large amount of collision energy, the component will minimize the amount of energy transferred to the rest of the vehicle structure and the vehicle occupants. Additionally, it is important to prevent cracking from occurring to maintain the integrity of the vehicle structure and prevent intrusion into the vehicle passenger compartment or battery cell compartment.

[0056] Since it is not possible to predict the direction in which the impact will occur under actual life conditions, it is important to absorb energy without significant cracking in both the compression mode and the bending mode. This will ensure that the performance of the vehicle is very robust regardless of the possible collision conditions.

[0057] The inventors have found that by providing the following component, it is possible to absorb a large amount of energy while minimizing the occurrence of cracks in both the compression mode and the bending mode: the component has a high slenderness ratio, for example greater than 10, preferably greater than 15, even more preferably greater than 20, and is made of a material with a tensile strength greater than 1300 MPa, preferably greater than 1500 MPa, a bending angle in the longitudinal direction greater than 70°, and a ratio of yield strength to tensile strength strictly lower than 0.85, preferably lower than 0.82, even more preferably lower than 0.80.

[0058] By using a high tensile strength material as detailed above, a large amount of energy can be absorbed because the deformation of the component caused by the collision force requires a large amount of energy. However, the risk is that under the action of the collision force, the high slenderness ratio component starts to crack and the crack propagates in the component, resulting in component failure. In this case, the component is no longer structurally strong and is no longer effective in absorbing more energy and preventing intrusion. The inventors have found that this can be solved by using a material with a high bending angle. In fact, the folds formed in the deformation zone will not cause cracking as long as the deformation angle measured within these folds does not exceed the maximum bending angle of the material used to form the component.

[0059] Furthermore, the inventors have surprisingly found that it is meaningful to keep the ratio of yield strength to ultimate tensile strength below a given maximum level. This may be due to the fact that due to the strain hardening characteristics of the material, a lower ratio of yield strength to ultimate tensile strength results in a smoother shape in the deformation zone. Correspondingly, a smoother shape means a larger bending radius in the deformation zone and thus less strain localization and a lower likelihood of crack occurrence.

[0060] In the case of a high aspect ratio component made by spot welding a top component 3 and a bottom component 2, the inventors have further found that by using a material having a high α-CTS resistance in the spot weld portion, a high aspect ratio component can be provided that has the desired properties of high energy absorption and low crack occurrence in both the compression mode and the bending mode. For example, by using a material having an α-CTS resistance greater than 70 kN / mm 2 In fact, by using a material having such a high α-CTS resistance, the risk of weld failure under the impact energy of a large load can be minimized. Such weld failure typically results in a significant reduction in the effective performance of the component, which no longer acts as a single high-stiffness unit to resist impact forces.

[0061] In a particular embodiment, the material used to fabricate the entire high aspect ratio component is a steel sheet containing the following elements in weight percent:

[0062] C: 0.15% - 0.25%

[0063] Mn: 0.5% - 1.8%

[0064] Si: 0.1% - 1.25%

[0065] Al: 0.01% - 0.1%

[0066] Cr: 0.1% - 1.0%

[0067] Ti: 0.01% - 0.1%

[0068] B: 0.001% - 0.004%

[0069] P ≤ 0.020%

[0070] S ≤ 0.010%

[0071] N ≤ 0.010%

[0072] And the steel sheet optionally contains one or more of the following elements by weight percentage:

[0073] Mo ≤ 0.40%

[0074] Nb ≤ 0.08%

[0075] Ca ≤ 0.1%

[0076] The balance of the composition is iron and unavoidable impurities resulting from melting.

[0077] The remaining portion of the composition of the steel is iron and impurities generated by the refining process. The level of impurities generated by the refining process will depend on the production route used. For example, when using the 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 using an electric arc furnace with a very high ratio of recycled steel scrap to refine the steel, the level of impurities will increase significantly. For example, in the case of using an electric arc furnace to refine the steel, the level of Cu can rise to 0.25%, the level of Ni can rise to 0.25%, the level of Sn can rise to 0.05%, the level of As can rise to 0.03%, the level of Sb can rise to 0.03%, and the level of Pb can rise to 0.03%.

[0078] The present invention will now be illustrated by the following examples, which are in no way limiting. These examples will compare the performance of the high aspect ratio components according to the present invention with the performance of reference components having the same geometry but different material properties. It will be shown that, compared with the reference components, the components according to the present invention exhibit better energy absorption and fewer crack appearances. The performance of the components in the compression mode and the bending mode will be evaluated.

[0079] The performance of the components was simulated using LS-DYNA R11.1.0. The mesh size used was 3 mm.

[0080] The method developed in the Fosta 806 project "P.806 – Characterization and simplified modeling of the fracture behavior of spot welds from ultra-high strength steels for crash simulation with consideration of the effects of the joints on component behavior" (Fosta stands for "Forschungsvereinigung Stahlanwendung", i.e., the Steel Application Research Association) was applied to simulate the performance of the spot welds under load.

[0081] The failure behavior and the associated deleted element calculations are simulated using material cards MAT123 and MAT_ADD_EROSION. Further explanations regarding this method can be found, for example, in "Simulation of Spot Welds and Weld Seams of Press-Hardened Steel (PHS) Assemblies" by Stanislaw Klimek at the 2008 International Automotive Body Congress.

[0082] Generally, the number of deleted elements is an assessment of the amount of fracture that occurs during a collision. Since the failure model does not account for crack propagation, it can be said that the impact of fracture on the overall results may be underestimated in the simulation, and in an actual physical crash test, due to failure propagation and the eventual complete failure of the component (for example, the component being cut in half), when the number of deleted elements is high, the energy absorption level may be low. It should be noted that such catastrophic failures not only affect energy absorption but also the overall performance of the component in the predicted vehicle crash scenario. In fact, it disrupts the expected load path and means that different components of the vehicle will travel in uncontrolled directions since they are no longer joined together. This lack of control leads to unpredictable catastrophic behavior of the vehicle during a collision.

[0083] Example 1

[0084] In the first example, referring to Figure 1 , the simulated high aspect ratio component 1 is made by forming the top component 3 and the bottom component 2 separately and then joining the top component 3 and the bottom component 2 together by spot welding on the flange 6, which results in the spot welds 5. The top component 3 is a generally omega-shaped component, and the bottom component 2 is a flat closed plate. This joining is performed by producing 20 spot welds at 30 mm intervals along each flange on each side. Each spot weld 5 has a nugget with a diameter of 5.1 mm, and the heat affected zone is simulated by a 3 mm annular region around each nugget.

[0085] The high aspect ratio component 1 has the following dimensions:

[0086] - The omega-shaped top component 3 has a sheet metal thickness of 1.5 mm before forming,

[0087] - The bottom component 2, i.e., the flat closed plate, has a sheet metal thickness of 1.0 mm before forming,

[0088] - The length L is 600 mm,

[0089] - The total width of the closure plate 2 in the transverse direction is 130 mm, including two flanges 6 each 25 mm. Thus, the width of the closure plate enclosing the hollow volume 4 is 130 - 2 * 25 = 80 mm.

[0090] - The height of the hollow volume 4 is 60 mm.

[0091] Given a mesh size of 3 mm, the above components consist of a total of 24331 elements.

[0092] For simplicity, the following slenderness ratio is calculated for a perfect rectangular component with the same hollow volume 4 and the same metal plate thickness. That is, the slenderness ratio is calculated without considering the influence of the flanges, and the influence of the flanges will be very small.

[0093] In the following formula, the coefficients b1 and b correspond to the internal width (i.e., 80 mm) and the external width (i.e., b = b1 + 2 * (thickness of the top component) = b1 + 3 mm) of the rectangular component respectively, and the coefficients h1 and h correspond to the internal height (i.e., 60 mm) and the external height (i.e., h = h1 + (thickness of the top component) + (thickness of the bottom component) = h1 + 2.5 mm) of the rectangular component respectively. The minimum second moment is given by the following formula:

[0094]

[0095] The minimum second moment is calculated as:

[0096]

[0097] I 最小 = min(418057.29; 248639.32)

[0098] I 最小 = 248639.32

[0099] The slenderness ratio is given by the following formula:

[0100]

[0101] where the area S of the straight cross-section = h * b - h1 * b1.

[0102] The slenderness ratio is calculated as:

[0103]

[0104] Slenderness ratio = 23.7

[0105] Therefore, the slenderness ratio obtained from the said shape is 23.7.

[0106] Reference Figure 2 , Example 1 is a simulation of a three-point bending test, which reflects the bending performance of the component. The test conditions are as follows:

[0107] - Component 1 is placed on two cylindrical support structures 9, each of which has a diameter of 50 mm,

[0108] - The impactor 7 - weighing 370 kg and having a rounded punch 8 with a diameter of 50 mm - applies a force F2 and travels at an initial velocity of 8 m / s.

[0109] In Table 1 below, the collision test results of Component I1 made of the material according to the present invention are compared with the collision test results of four different components R1 - R4 made of materials not according to the present invention. The material properties of R1 - R4 outside the present invention are emphasized. For each material, two sets of results are listed, which correspond to the simulation made considering the performance of the spot weld and the heat affected zone during the test and the simulation made without considering the performance of the spot weld and the heat affected zone. Columns I1, R1, R2, R3, and R4 are the results without considering the welding performance, while columns I1w, R1w, R2w, R3w, and R4w (w stands for "welding") consider the possible failed welds and heat affected zones using the method defined above. The case without considering the performance of the spot weld and the heat affected zone corresponds to the simplification of the welded assembly or an assembly made of only one piece, for example, by metal extrusion or tube forming.

[0110] The results are expressed in terms of the total energy absorption and the energy absorption before the start of failure, both measured in kJ as directly provided by the simulation software. The moment when the first crack appears in the test is expressed as the ratio of the penetration of the impactor at the time of the first crack appearance to the maximum penetration of the impactor at the end of the test (referred to as "%Crush" (crush percentage) in the table).

[0111] The number of deleted elements is also shown because it gives a good indication of the fracture level in the component caused by the collision. The levels of the energy absorbed before and after the start of failure are detailed separately because it is generally considered that in a real-life collision, once a crack starts to appear, the crack is likely to spread throughout the component and greatly affect the performance of the component. As previously explained, crack propagation is not considered in the simulation software, and therefore, compared with the amount of energy obtained in an actual physical collision test, the amount of energy absorbed after the start of the collision is likely to be overestimated by the simulation software.

[0112] In the columns considering the performance of the spot weld and the heat affected zone, more information about the α-CTS value of the component is provided, as well as simulation results regarding the onset of failure (% Crush at the appearance of the first crack) and the degree of failure (as reflected by the number of deleted elements) in both the spot weld and the heat affected zone.

[0113]

[0114] Table 1 : Results of Example 1

[0115] Figure 3 is a graphical reproduction at the end of the test in the case of component I1w, which shows the total deformation of the component after the punch has traveled its path.

[0116] It is noteworthy that components made of the material according to the invention do not show failure in both cases with and without considering the welded part. In the case without welding, the total amount of absorbed energy is just below the total amount of absorbed energy of R2 and R3. However, the components made of R2 and R3 start to crack at 59% and 56% of the punch penetration, i.e., just over half of the test. If crack propagation is considered, the total amount of absorbed energy of R2 and R3 is likely to decrease. In any case, it will be safer to select component I1 as a safe component to withstand transverse bending loads, because component I1 will absorb a very large amount of energy and will be significantly less likely to fail due to crack propagation under the load. This reasoning holds both in the case of considering the performance of the spot weld and the heat affected zone and in the case of not considering the performance of the spot weld and the heat affected zone. The absence of cracks is also a key point in ensuring the anti-intrusion performance of the component.

[0117] Example 2

[0118] The high slenderness component of Example 2 is a double omega-shaped component, which means that both the top component 3 and the bottom component 2 have an omega shape. The top component 3 and the bottom component 2 are joined in the following way: the top component 3 and the bottom component 2 are spot welded to each other using the spot weld 5 applied to the flange 6. For the first two examples, this joining is carried out by producing 20 spot welds at intervals of 30 mm along each flange on each side. Each spot weld 5 has a nugget with a diameter of 6.1 mm, and the heat affected zone is simulated by a 3-mm annular portion around each nugget.

[0119] The geometry of the component is as follows:

[0120] - The metal sheet thickness of the omega-shaped top component 3 and bottom component 2 before forming is 1.5 mm,

[0121] - The length L is 600 mm

[0122] - The total width of the bottom part 2 in the transverse direction is 130 mm, including two flanges 6 each with a width of 25 mm. Therefore, the width of the enclosed hollow volume 4 of the closing plate is 130 - 2 * 25 = 80 mm.

[0123] - The height of the hollow volume 4 is 120 mm.

[0124] Given a mesh size of 3 mm, the above-mentioned part consists of a total of 25,650 elements.

[0125] Regarding the part of Example 1, when calculating the slenderness ratio without considering the influence of the flange, the influence of the flange will be very small.

[0126] In the following formula, the coefficients b1 and b correspond to the inner width (i.e., 80 mm) and outer width (i.e., b = b1 + 2 * (thickness of the top part) = b1 + 3 mm) of the rectangular part respectively, and the coefficients h1 and h correspond to the inner height (i.e., 60 mm) and outer height (i.e., h = h1 + (thickness of the top part) + (thickness of the bottom part) = h1 + 3 mm) of the rectangular part respectively. The minimum second moment is given by the following formula:

[0127]

[0128] The minimum second moment is calculated as:

[0129]

[0130] I 最小 = min(150996.75, 740816.75)

[0131] I 最小 = 740816.75

[0132] The slenderness ratio is given by the following formula:

[0133]

[0134] where the area S of the straight cross-section = h * b - h1 * b1.

[0135] The slenderness ratio is calculated as:

[0136]

[0137] Slenderness ratio = 17.2

[0138] Therefore, the double omega-shaped part of Example 2 has a slenderness ratio of 17.2.

[0139]

[0140] Table 2 : Results of Example 2

[0141] Figure 4 is a graphical reproduction at the end of the test in the case of component I1w, which shows the total deformation of the component after the punch has traveled its path.

[0142] As in the first example, I1 does not crack under bending loads, and although I1 has a slightly lower level of energy absorption compared to R2 and R3 without welding, the fact that I1 does not crack at any time makes I1 a material of choice for a robust, safe, and reliable safety component.

[0143] On the other hand, when considering the performance of the welded joint, I1w has better performance in terms of energy absorption than all comparative examples.

[0144] Example 3

[0145] In the third example, the simulated high slenderness component 1 has the same geometric characteristics as those in the first example (a simple omega shape with a closed plate). On the other hand, the diameter of the weld nugget is 8.1 mm instead of 5.1 mm in Example 1. The heat affected zone is simulated by a 3 mm annular portion around each nugget.

[0146] This time, the component is subjected to a longitudinal impact to simulate a compression test. Component 1 is fixed at one end and is impacted at its other end by the following flat impactor 10: the flat impactor 10 travels at an angle β of 10° to the longitudinal direction and has an initial impact velocity of 16 m / s and a mass of 417 kg. Figure 4 is a graphical representation at the end of the simulation of the compression test of Example 3 on component I1 made of the material of the present invention and R4 made of a reference material.

[0147]

[0148]

[0149] Table 3 : Results of Example 3

[0150] Observing the relative energy absorption and failure rate of components made of the material according to the embodiments of the present invention under a transverse impact at an angle of 10°, the material according to the embodiments of the present invention seems to produce better results than all reference materials. In particular, the amount of energy absorbed is significantly higher both when considering the performance of the welded joint and when not considering the performance of the welded joint.

[0151] Reference Figure 5, it can be seen that component I1 absorbed a large amount of energy through buckling (as seen by the wrinkles formed on the impacted end of the component). On the other hand, although the tensile strength of component R4 was significantly higher, component R4 absorbed less impact energy due to the formation of a large number of cracks.

[0152] As a conclusion of these three examples, components made according to embodiments of the present invention perform better than comparative components in bending mode and compression mode. Therefore, components made according to embodiments of the present invention are most suitable for use as tall and slender structural components in a vehicle architecture.

Claims

1. A high slenderness component (1) extending along a main longitudinal direction L between two ends (E1) and (E2), the high slenderness component (1) comprising a hollow volume (4) enclosed between a top component (3) and a bottom component (2), the high slenderness component (1) being made by individually hot stamping a steel sheet and then joining the top component (3) and the bottom component (2) together, the steel sheet having an ultimate tensile strength greater than 1300 MPa after hot stamping, having a yield strength YS also expressed in MPa, having a length L expressed in mm, an area S of the straight cross-section of the high slenderness component (1) expressed in mm 2 and a minimum second moment I of the area of the straight cross-section expressed in mm 4 最小 ,​ Wherein: - The ratio between the yield strength YS and the ultimate tensile strength UTS of the material is strictly lower than 0.85, - The bending angle of the material normalized to a thickness of 1.5 mm in the rolling direction is greater than 70°, - The aspect ratio of the component, defined as aspect ratio = L / √(I 最小 / S), is equal to or greater than 10.

2. The high slenderness ratio component according to claim 1, wherein, The ratio between the yield strength YS and the ultimate tensile strength UTS of the steel sheet after hot stamping is strictly lower than 0.

82.

3. The high slenderness ratio component according to claim 1, wherein, The ratio between the yield strength YS and the ultimate tensile strength UTS of the steel sheet after hot stamping is strictly lower than 0.

80.

4. The high slenderness ratio component according to any one of claims 1 to 3, wherein, The slenderness ratio is equal to or greater than 15.

5. The high slenderness ratio component according to any one of claims 1 to 3, wherein, The slenderness ratio is equal to or greater than 20.

6. The high slenderness ratio component according to any one of claims 1 to 5, wherein, The top member 3 and the bottom member 2 are joined together by spot welding, and the α transverse tensile strength resistance of the spot welding portion is greater than 70 kN / mm 2 .

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