High strength, slender parts with excellent energy absorption and penetration resistance properties

Abstract

The present invention encompasses highly slender structural parts made from materials that have excellent crashworthiness and energy absorption in both bending and compressive modes, an ultimate tensile strength greater than 1300 MPa, a ratio of the material's yield strength YS to its ultimate tensile strength UTS strictly less than 0.85, a bend angle greater than 70°, and a slenderness ratio of 10 or greater.

Description

[Technical Field]

[0001] The present invention relates to high strength structural parts having excellent energy absorption properties in the event of side and longitudinal impacts, and in particular to structural parts for use in motor vehicles. [Background technology]

[0002] High strength, highly slender structural components play an important role in the crashworthiness of vehicles. They are elongated assemblies containing hollow cavities.

[0003] In the event of a collision, such a component may be impacted on its side, i.e., generally transverse to its length, or may be impacted generally longitudinally.

[0004] In the event of a side impact, this type of structural component typically flexes under the impact load. The component's bending behavior plays a key role in absorbing the impact energy and preventing the impactor from penetrating into the vehicle. Sufficient energy absorption and prevention of intrusion are critical to minimizing the impact's effects on the vehicle occupants and the rest of the vehicle structure. For electric, hybrid, or hydrogen-fueled vehicles, intrusion prevention is also critical to ensuring the integrity of the battery pack and / or hydrogen tank, and also plays a key role in ensuring the safety of the vehicle occupants.

[0005] High strength and highly slender structural parts therefore play a fundamental role in improving the safety of vehicle occupants in the event of a side impact.

[0006] The side impact resistance of a vehicle is considered a major safety issue and is measured by several standardized tests, for example:

[0007] - The United States New Car Assessment Program (USNCAP) pole impact test, in which a vehicle with an initial lateral velocity of 32.2 km / h is impacted on the side by a fixed pole.

[0008] -IIHS side moveable deformable barrier (MDB) test, in which a vehicle is impacted from the side with a deformable barrier weighing 1500 kg and moving at a speed of 50 km / h.

[0009] These standardized tests are periodically updated to take into account ever more severe crash conditions, for example by increasing the weight of the barrier, raising the impact speed, and increasing the criteria required to pass the test.

[0010] When impacted longitudinally, the part experiences a compressive force. To absorb the maximum amount of energy, it is important that the highly slender part bends as far as possible onto itself while minimizing the occurrence of cracks.

[0011] For example, a longitudinal impact on a front member, which is generally a highly elongated part, is simulated, for example, by the following standardized crash test.

[0012] -The Insurance Institute for Highway Safety (IIHS) Small Overlap Rigid Barrier (SORB) crash test, in which a rigid barrier moving at 64.4 km / h is impacted with an overlap of only 25% of the vehicle's width.

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

[0014] It is an object of the present invention to provide a high strength, highly slender part that has excellent energy absorption and anti-penetration behavior characteristics in both lateral and longitudinal impact configurations.

[0015] The object of the present invention is achieved by providing a highly slender part according to claim 1, optionally including the features of claims 2-8.

[0016] The invention will now be described in detail with reference to the accompanying drawings, illustrated by way of example without introducing any limitations. [Brief explanation of the drawings]

[0017] [Figure 1] 1A-1C are schematic diagrams of a highly elongated part according to one embodiment of the present invention, with FIG. 1A being an inset detailing the definition of the different angles defined in the detailed description. [Figure 2] FIG. 1 is a schematic diagram of a three-point bending test performed in Examples 1 and 2 of the detailed description below. [Figure 3] 10 illustrates the end of a three-point bending simulation of Example 1 for part I1w, according to one embodiment of the present invention. [Figure 4] 10 illustrates the end of a three-point bending simulation of Example 2 for part I1w, in accordance with one embodiment of the present invention. [Figure 5] 1 illustrates the end of a compression test simulation for Example 3 for part I1 according to one embodiment of the present invention (left of the figure) and part R4 not according to the present invention (right of the figure). DETAILED DESCRIPTION OF THE INVENTION

[0018] The slenderness ratio, commonly used in Leonhard-Euler bending theory, is defined by the following formula, where L is the length of the part (expressed in mm) and S is the area of ​​its straight cross section (in mm 2 ) and Imin is the minimum second moment of area in the cross section considered.

[0019]

number

[0020] In general, the minimum area second moment lmin (mm 4 ) is defined by the following formula:

[0021]

number

[0022] For example, the minimum area second moment Imin of a hollow rectangular cross section having outer dimensions b and h and inner dimensions b1 and h1 is calculated using the following formula:

[0023]

number

[0024] For example, the minimum area moment of inertia lmin of a hollow annular cross section having an outer radius R and an inner radius R1 is calculated using the following equation:

[0025]

number

[0026] A part can be considered to have high slenderness if its slenderness ratio is greater than 10, preferably if the slenderness ratio is greater than 15, and even more preferably if the slenderness ratio is greater than 20.

[0027] The bend angle is measured according to the VDA-238-100 bending standard. In the present invention, the bend angle is measured after springback. For the same material, the bend angle depends on the thickness. For simplicity, the bend angle value in the present invention refers to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bend angle value should 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 relative to the thickness t.

[0028] α 1.5 =(α t ×√t) / √1.5

[0029] The bend angle of a part represents the part's ability to withstand deformation without crack formation.

[0030] In the present invention, the bend angle was measured in the rolling direction, i.e., the direction in which the steel sheet moved during the hot rolling process. The bend angle was measured using a laser measuring device. When bend tests are performed on hot-stamped parts, samples are cut from the flat area of ​​the part. If necessary, small-sized samples are obtained to fit the entire available flat area on the part. If the rolling direction on a hot-stamped part is unknown, it can be determined using electron backscatter diffraction (EBSD) analysis on the cross section of the sample in a scanning electron microscope (SEM). The rolling direction was determined according to the intensity of the orientation density function (ODF) representing the main fiber at φ2 = 45°, where φ2 is the Euler angle defined in "H.-J. Bunge: Texture Analysis in Materials Science - Mathematical Methods. 1st English Edition, Butterworth Co. (Publ.) 1982" (see Figures 2.2 and 2.3 for the definition of φ2).

[0031] Ultimate tensile strength, yield strength, and elongation are measured according to ISO standard ISO 6892-1, published in October 2009. Tensile specimens are cut from flat areas. If necessary, smaller tensile test specimens are obtained to fit the entire available flat area on the part.

[0032] 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, Number 6, 2017. The fracture strain is the equivalent strain in the material at the deformation point when the critical bending angle is reached. The critical bending angle defines the angle at which the first crack is detected on the front side (extrados) of a specimen deformed according to the standardized VDA-238-100 standard.

[0033] The term "bottling" refers to a deformation mode of a part, typically a highly elongated part, subjected to a compressive load, in which the part gradually absorbs the mechanical energy of the compressive load by forming a series of successive waves caused by successive localized bending deformations. As a result, the length of the part measured in the direction of the compressive load is shorter than the initial length of the part in this direction after deformation. In other words, by controlled bending, when the part responds to the compressive load, it folds over on itself, similar to a plastic bottle when a compressive load is applied between the top and bottom of the bottle.

[0034] Hot stamping is a steel forming technique that involves heating a blank to a temperature at which the steel microstructure at least partially transforms to austenite, stamping the blank to form the blank at a high temperature, and quenching the formed part to obtain a microstructure with very high strength, possibly with an additional partial quenching or tempering step in the heat treatment. Hot stamping makes it possible to obtain very high-strength parts with complex shapes and offers many technical advantages. It should be understood that the heat treatment that the part undergoes includes not only the thermal cycle of the hot stamping process itself described above, but also other subsequent heat treatment cycles, such as a paint bake step, which may be performed after the part has been painted to bake the paint. The mechanical properties of the hot stamped parts described below are measured after the entire thermal cycle, possibly including, for example, the paint bake step if a paint bake step is actually performed, or an optional post-tempering step.

[0035] A blank refers to a flat sheet cut into any shape suitable for its use. A blank has a top and bottom surface, also called the upper and bottom sides or top and bottom surfaces. The distance between the surfaces is designated as the thickness of the blank. Thickness can be measured, for example, using a micrometer, the spindle and anvil of which are placed on the top and bottom surfaces. In a similar manner, thickness can be measured on a molded part.

[0036] Hardness is a measure of the resistance to localized plastic deformation induced by mechanical indentation. It correlates well with the mechanical properties of a material and is a useful local measurement method that does not require cutting out a sample for tensile testing. In this invention, hardness measurements are performed using a Vickers indenter according to standard ISO 6507-1. Vickers hardness is expressed using the unit Hv.

[0037] The heat-affected zone is the area of ​​material surrounding the weld that is heated during the welding operation. It is well known that in high-strength materials, such as high-strength steels, the heat-affected zone exhibits reduced mechanical properties. In fact, the heat-affected zone undergoes a heat treatment similar to tempering, which can result in softening.

[0038] The cross tensile strength, also known as the alpha CTS value for spot weld resistance, reflects the strength of a spot weld under cross-tension type loading and is expressed as the ratio of the maximum cross tensile strength to the product of the weld nugget diameter by the average thickness of the steel sheets being joined. Dividing the strength by the product of the average metal sheet thickness and the weld nugget diameter yields a normalized value that remains valid and applicable to a wide range of industrial welded assembly configurations. It is widely used in the metal sheet and welding industries. The alpha CTS value is obtained by the following procedure:

[0039] -Providing a cross-welded assembly of two metal specimens having thicknesses t1 and t2, each measuring 100 mm x 50 mm, with a weld nugget having a diameter d, in accordance with the ISO 14272 standard published on March 1, 2016; measuring the cross tensile strength (CTS) (expressed in kN) of said assembly in accordance with the ISO 14272 standard published on March 1, 2016; The CTS (expressed in kN) is defined as the ratio of the product of the average thickness and the weld nugget diameter (both expressed in mm) to give Alpha CTS (kN / mm 2 (expressed as

[0040]

number

[0041] In the detailed description, drawings, and claims, all orientation and spatial references are made using L, T, Z coordinates as a reference, where L is the length direction of the part, i.e., the longitudinal direction parallel to the longest dimension of the part, T is the transverse direction in which the part extends perpendicular to said longitudinal direction, and Z is the upward direction perpendicular to the plane formed by the L and T directions. References are represented in each figure. When a figure is a 2D flat representation, an axis that is outside the figure is represented, according to established convention, by a dot in a circle if it faces the reader or by a cross in a circle if it faces away from the reader.

[0042] The directional terms "top", "up", "upper", "above", "bottom", "low", "lower", "below", etc. are defined according to the Z vertical direction. The directional terms "front" and "back" are defined according to the L direction. The "width" or "lateral" direction refers to the direction parallel to the T direction.

[0043] With reference to Figure 1, the elongated part 1 extends in a main longitudinal direction L between two ends E1 and E2 and in a transverse direction T. The elongated part 1 comprises a hollow volume 4 housed between a top part 3 and a bottom part 2.

[0044] The highly elongated part 1 is made by separately forming a top portion 3 and a bottom portion 2 and then joining them together. For example, the top portion 3 and the bottom portion 2 are joined by welding, for example by spot welding on a flange 6, to create a spot weld 5.

[0045] In certain embodiments, the top 3 has a generally omega shape and the bottom 2 is a flat, closed plate, as shown in Figure 1. In certain embodiments not shown in the figures, the top 3 has a generally omega shape and the bottom 2 also has a generally omega shape (this is the case, for example, for the part in Example 2, described in more detail below).

[0046] Highly elongated parts are common in vehicle structures, some examples are the front sections joining the front crash boxes to the rocker assemblies, the rear sections joining the rear crash boxes to the rocker assemblies, cross members that extend laterally within the vehicle, the rocker panels themselves, etc. In the case of electric or hybrid vehicles, the battery pack is typically surrounded by a set of highly elongated parts designed to protect the battery cells in the event of a collision.

[0047] The elongated part is generally attached to the rest of the vehicle structure at each of its ends E1 and E2. When the vehicle is involved in a collision, some of the energy of the collision may be transferred to the elongated part by the attached parts. In this case, the elongated part is generally subjected to a compressive load, which is applied between its ends E1 and E2 and results from a force F1 (shown in FIG. 1 ) transmitted by the surrounding element to which the part is attached, and a resistance force R1 resulting from resistance at the other end of the other element to which the part is attached. The compressive force F1 does not necessarily have to be strictly parallel to the longitudinal direction, but can form an angle β with the L axis, as shown in FIG. 1 a. As will be explained in more detail later in this example, this situation corresponds to a compression load test and related numerical simulation. In the following description, this is referred to as the compression mode.

[0048] In the case of a collision, the impact force may also have at least one component directed along a direction perpendicular to the longitudinal direction, e.g., upward. This is the case for force F2 shown in FIG. 2. In this case, the part is subjected to a three-point bending load, with force F2 applied to one side and a resisting force in the opposite direction coming from the resistance of other elements to which the part is attached at both ends E1, E2 (said forces are not shown in FIG. 1 for clarity). As will be described in more detail later in this example, this situation corresponds to a three-point bending test and associated numerical simulations. In the following description, this is referred to as the bending mode.

[0049] To provide effective protection in the event of a crash under any loading condition, the highly slender parts must absorb a large amount of crash energy without significantly cracking. Indeed, by absorbing a large amount of crash energy, the parts minimize the amount of energy transferred to the remainder of the vehicle structure and its occupants. Furthermore, it is important to prevent cracking to maintain the structural integrity of the vehicle and to prevent penetration into the passenger cell or battery cell compartments of the vehicle.

[0050] Since it is impossible to predict the direction of impact under real-life conditions, it is important that the energy is absorbed in both compression and bending modes without significant cracking, ensuring very robust vehicle behavior regardless of crash conditions.

[0051] The inventors have found that by providing a part with a high slenderness ratio, for example greater than 10, preferably greater than 15, even more preferably greater than 20, made from a material having a tensile strength of greater than 1300 MPa, preferably 1500 MPa, a longitudinal bend angle greater than 70°, and a ratio of yield strength to tensile strength strictly less than 0.85, preferably less than 0.82, even more preferably less than 0.80, it is possible to absorb large amounts of energy in both compression and bending modes while minimizing the initiation of cracks.

[0052] As detailed above, deformation of a part caused by an impact force requires a large amount of energy, and the use of high tensile strength materials allows for the absorption of large amounts of energy. However, under the impact of the impact force, cracks in highly slender parts can initiate and propagate within the part, resulting in part failure. In this case, the part is no longer structurally sound and is no longer effective at absorbing further energy and preventing penetration. The inventors have found that this can be improved by using materials with high bend angles. In fact, folds formed in the deformed region will not result in crack initiation unless the deformation angle measured within these folds exceeds the maximum bend angle of the material used to form the part.

[0053] Furthermore, the inventors have surprisingly found that it is interesting to maintain the yield strength to ultimate tensile strength ratio below a given maximum level. This may be due to the fact that a lower yield strength to ultimate tensile strength ratio results in a smoother shape in the deformed region due to the strain hardening properties of the material. Furthermore, a smoother shape means a larger bending radius in the deformed region, thus reducing strain localization and the likelihood of crack initiation.

[0054] In the case of highly slender parts made by spot welding the top 3 and bottom 2, the inventors have further found that by using a material with a high alpha CTS resistance in the spot welds, it is possible to provide highly slender parts with the desired properties of high energy absorption and low crack initiation in compression and bending modes. For example, a material with a high alpha CTS resistance of 70 kN / mm 2 By using materials with an alpha CTS resistance greater than 0.05 . In fact, by using materials with such high alpha CTS resistance, it is possible to minimize the risk of weld failure under significant loads of crash energy, which typically results in a much less efficient performing part that no longer functions as a single, highly rigid unit against crash forces.

[0055] In a particular embodiment, the material used to manufacture the entire highly elongated part is a steel sheet containing the following elements expressed in % by weight:

[0056] C: 0.15 to 0.25% Mn: 0.5 to 1.8% Si: 0.1 to 1.25% Al: 0.01 to 0.1% Cr: 0.1 to 1.0% Ti: 0.01 to 0.1% B: 0.001 to 0.004% P≦0.020% S≦0.010% N≦0.010%.

[0057] and optionally including, in weight percent, one or more of the following elements: Mo≦0.40% Nb≦0.08% Ca ≤ 0.1%.

[0058] The remainder of the composition is iron and unavoidable impurities resulting from smelting.

[0059] The remainder of the steel's composition is iron and impurities resulting from the refining process. The level of impurities resulting from the refining process varies depending on the manufacturing route used. For example, if a blast furnace route is used, with a low proportion of steel scrap (recycled steel), the impurity level remains very low. On the other hand, if the steel is refined using an electric furnace with a very high proportion of recycled scrap steel, the impurity level increases significantly. For example, in the case of an electric furnace, the Cu level can be increased to 0.25%, Ni can be increased to 0.25%, Sn can be increased to 0.05%, As can be increased to 0.03%, Sb can be increased to 0.03%, and Pb can be increased to 0.03%.

[0060] The invention will now be illustrated by the following examples, which are in no way limiting. The examples compare the performance of highly elongated parts according to the invention with reference parts having the same geometry but different material properties. The parts according to the invention are shown to exhibit better energy absorption and less crack initiation than the reference parts. The behavior of the parts in compression and bending modes is evaluated.

[0061] The behavior of the part was simulated using LS-DYNA R 11.1.0, with a mesh size of 3 mm.

[0062] The behavior of spot welds under load was simulated by applying the method developed in the Fosta 806 project "P806-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 Research Association for Steel Application).

[0063] The failure behavior and the calculation of the associated deleted elements are simulated using material cards MAT123 and MAT_ADD_EROSION. Further description of the method can be found, for example, in "Simulation of Spot Welds and Weld Seams of Press-Hardened Steel (PHS) Assemblies", Stanislaw Klimek, International Automotive Body Congress 2008.

[0064] In principle, the number of removed elements is an assessment of the fractures that will occur during a crash. Since failure modeling does not consider crack propagation, the impact of fractures on the overall outcome is likely underestimated in the simulation. In a real physical crash test, it can be argued that a high number of removed elements, due to failure propagation and eventual overall failure of the part (e.g., the part shearing in two), would likely result in a lower energy absorption level. It should be noted that such catastrophic failure is not only a problem of energy absorption, but also of the overall behavior of the part in the vehicle's predicted crash situation. In practice, this means that the expected load paths are disrupted, and different vehicle parts, no longer connected to each other, move in uncontrolled directions. This lack of control leads to unpredictable and destructive vehicle behavior during a crash. [Example]

[0065] Example 1 In a first example, referring to Figure 1, a simulated highly slender part 1 is made by separately forming and then joining together a top part 3, which is a generally omega-shaped part, and a bottom part 2, which is a flat closed plate by spot welding on flanges 6 to produce spot welds 5. The joining is made with 20 spot welds on each side, spaced 30 mm apart along each flange. Each spot weld 5 has a 5.1 mm diameter nugget, and the heat affected zone is simulated by a 3 mm ring around each nugget.

[0066] The highly elongated part 1 has the following dimensions:

[0067] - an omega-shaped top 3 with a metal sheet thickness of 1.5 mm before forming; - a bottom part 2, which is a flat closed plate having a metal thickness of 1.0 mm before forming; -Length L600mm - a closure plate 2 with a total lateral width of 130 mm and two flanges 6 of 25 mm each. The width of the closure plate enclosing the hollow volume 4 is therefore 130-2*25=80 mm. - 4 hollow volumes with a height of 60 mm

[0068] Considering a mesh size of 3 mm, the above mentioned part consists of a total of 24331 elements.

[0069] For simplicity, the following slenderness factors are calculated for a perfect rectangular part with the same hollow volume 4 and the same sheet metal thickness. That is, the slenderness factors are calculated without taking into account the contribution of the flanges, which is considered to be very small.

[0070] In the following formula, coefficients b1 and b correspond to the inner width (i.e., 80 mm) and outer width (i.e., b = b1 + 2 * (top thickness) = b1 + 3 mm) of the rectangular section, respectively, and coefficients h1 and h correspond to the inner height (i.e., 60 mm) and outer height (i.e., h = h1 + (top thickness) + (bottom thickness) = h1 + 2.5 mm) of the rectangular section, respectively. The minimum second moment is obtained by the following formula:

[0071]

number

[0072] This is calculated as follows:

[0073]

number

[0074] The slenderness ratio is given by:

[0075]

number

[0076] Area of ​​the cross section S of the straight section = h*b-h1*b1

[0077] This is calculated as follows:

[0078]

number

[0079] The described shape therefore has a slenderness ratio of 23.7.

[0080] 2, Example 1 is a simulation of a three-point bending test that reflects the bending behavior of a part. The test conditions are as follows:

[0081] The part 1 is placed on two cylindrical support structures 9, each with a diameter of 50 mm.

[0082] An impactor 7 having a mass of 370 kg and a round punch head 8 with a diameter of 50 mm applies a force F2 and moves with an initial speed of 8 m / s.

[0083] Table 1 below compares the crash test results of part I1, which uses a material according to the present invention, with the crash test results of four different parts R1 to R4, which use materials other than those according to the present invention. Material properties of R1 to R4 that are outside the scope of the present invention are underlined. For each material, two sets of results are listed, corresponding to simulations performed with and without considering the behavior of spot welds and heat-affected zones during testing. Columns I1, R1, R2, R3, and R4 are results without considering welding behavior, while columns I1w, R1w, R2w, R3w, and R4w (w for "weld") consider possible failures of the welds and heat-affected zones using the methodology defined above. When spot welds and heat-affected zone behavior are not considered, the results correspond to simplifications of welded assemblies or assemblies made solely from one-piece parts, for example, by metal extrusion or tube forming.

[0084] The results are expressed in terms of total energy absorption and energy absorption before failure initiation, both measured in kJ, as provided directly by the simulation software. The moment of the test at which the first crack occurred is given as the ratio between the penetration level of the impactor when the first crack occurred and the maximum penetration of the impactor at the end of the test (referred to in the tables as "% fragmentation").

[0085] The number of elements removed is also shown, which provides a good indication of the level of part destruction that occurs as a result of the impact. The levels of absorbed energy before and after failure initiation are detailed separately because it is generally believed that in a real crash, once a crack begins to appear, it will tend to propagate throughout the part, significantly affecting part performance. As mentioned previously, the simulation software does not take crack propagation into account, and therefore the amount of energy absorbed after impact initiation is likely to be overestimated by the simulation software compared to what would be obtained in an actual physical crash test.

[0086] The columns considering the behavior of the spot welds and heat affected zone provide further information on the alpha CTS values ​​of the assemblies as well as the simulation results regarding the initiation of failure (% fracture when the first crack appears) and the extent of failure (reflected by the amount of removed elements) in both the spot weld zone and the heat affected zone.

[0087] [Table 1] Table 1: Results of Example 1

[0088] FIG. 3 is a diagrammatic representation at the end of the test for part I1w, showing the total deformation of the part after the punch has run its course.

[0089] It is noteworthy that the parts made with the material according to the present invention do not fail, regardless of whether or not welding is considered. In the absence of welding, the total amount of absorbed energy is just below that of R2 and R3. However, the parts made with R2 and R3 crack at 59% and 56% of the punch penetration, respectively, i.e., just over halfway through the test. Considering crack propagation may lower the total amount of absorbed energy for R2 and R3. In either case, part I1 is much safer to select as a safety part subjected to transverse bending loads, since it absorbs a significantly larger amount of energy and is significantly less likely to fail due to crack propagation under load. This reasoning holds true regardless of whether or not spot welds and heat-affected zone behavior are considered. The absence of cracks is also a key factor in ensuring the anti-penetration behavior of the part.

[0090] Example 2 The highly slender part of Example 2 is a double omega-shaped part, meaning that both the top 3 and bottom 2 have an omega shape. The parts are joined by spot welding them together, with spot welds 5 applied on the flanges 6. For the two examples described above, the join is made with 20 spot welds on each side, spaced 30 mm apart along each flange. Each spot weld 5 has a 6.1 mm diameter nugget, and the heat-affected zone is simulated by a 3 mm ring around each nugget.

[0091] The geometry of the part is as follows:

[0092] - an omega-shaped top 3 and bottom 2 with a sheet metal thickness of 1.5 mm before forming; -Length L 600mm, a bottom 2 having a total lateral width of 130 mm, It has two flanges 6 of 25 mm each. The width of the closure plate surrounding the hollow volume 4 is therefore 130-2*25=80 mm. - Height of hollow volume 4: 120 mm

[0093] Considering a mesh size of 3 mm, the above mentioned part consists of a total of 25650 elements.

[0094] As with the part in Example 1, the slenderness factor is calculated without taking into account the contribution of the flange, which is considered to be very small.

[0095] In the following formula, coefficients b1 and b correspond to the inner width (i.e., 80 mm) and outer width (i.e., b = b1 + 2 * (top thickness) = b1 + 3 mm) of the rectangular section, respectively, and coefficients h1 and h correspond to the inner height (i.e., 60 mm) and outer height (i.e., h = h1 + (top thickness) + (bottom thickness) = h1 + 3 mm) of the rectangular section, respectively. The minimum second moment is obtained by the following formula:

[0096]

number

[0097] This is calculated as follows:

[0098]

number

[0099] The slenderness ratio is given by:

[0100]

number

[0101] Area of ​​the cross section S of the straight section = h*b-h1*b1

[0102] This is calculated as follows:

[0103]

number

[0104] Thus, the double omega section of Example 2 has a slenderness ratio of 17.2.

[0105] [Table 2] Table 2: Results of Example 2

[0106] FIG. 4 is a diagrammatic representation at the end of the test for part I1w, showing the total deformation of the part after the punch has run its course.

[0107] Like the first example, I1 will not crack under bending loads and in the absence of a weld it has a slightly lower energy absorption level than R2 and R3, but the fact that I1 will not crack at any point makes I1 the material of choice for robust, safe and reliable safety components.

[0108] On the other hand, when the welding behavior is taken into consideration, I1w has better energy absorption performance than all the comparative examples.

[0109] Example 3 In the third example, the simulated highly slender part 1 has the same geometric characteristics as the first example (a simple omega shape with closed plates), but the diameter of the weld nuggets is 8.1 mm instead of 5.1 mm in Example 1. The heat affected zone is simulated by a 3 mm ring around each nugget.

[0110] This time, the part is impacted in the longitudinal direction, simulating a compression test. Part 1 is fixed at one end and is impacted at the other end by a flat impactor 10, moving at an angle β of 10° to the longitudinal direction, with an initial impact velocity of 16 m / s and a mass of 417 kg. Figure 4 is a graphical representation of the end of the simulation of the compression test of Example 3 on parts I1 made of the material of the invention and R4 made of the reference material.

[0111] [Table 3] Table 3: Results of Example 3

[0112] Looking at the comparative energy absorption and failure rates of parts made with a material according to an embodiment of the present invention and subjected to a 10° transverse impact, it appears to perform better than all of the reference materials. In particular, the amount of energy absorbed is significantly higher, whether or not the welding behavior is taken into account.

[0113] Referring to Figure 5, it can be seen that part I1 absorbs a large amount of energy through bottling (as seen by the folds that form on the impacted end of the part), while part R4 absorbs less impact energy despite having significantly higher tensile strength due to the higher amount of crack formation.

[0114] The conclusion of these three examples is that parts made according to an embodiment of the present invention behave better in bending and compression modes than the comparative parts, and therefore they are best suited for high slender structural parts in vehicle construction.

Claims

1. A highly elongated part (1) extending in a main longitudinal direction L between two ends (E1) and (E2), the highly elongated part comprising a hollow volume (4) accommodated between a top part (3) and a bottom part (2), which are produced by hot stamping the top part (3) and the bottom part (2) of a steel plate separately and then joining them together, said steel plate having an ultimate tensile strength of more than 1300 MPa, a yield strength YS (expressed in MPa), a length L (expressed in mm), an area of ​​a cross section S of a straight section (expressed in mm ２ ) and the minimum area moment of inertia lmin (mm ４ ) where: the ratio between the yield strength YS and the ultimate tensile strength UTS of the material is strictly less than 0.85, - the bending angle of the material in the rolling direction normalized to a thickness of 1.5 mm is greater than 70°, - Highly slender parts, where the slenderness ratio of the part, defined as slenderness ratio = L / √(Imin / S), is 10 or greater.

2. 2. The highly slender part 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 less than 0.

82.

3. 2. A highly slender part according to claim 1, wherein the ratio between the yield strength YS and the ultimate tensile strength UTS of said steel sheet after hot stamping is strictly less than 0.

80.

4. A highly slender part according to any one of claims 1 to 3, having a slenderness ratio of 15 or more.

5. A highly slender part according to any one of claims 1 to 3, having a slenderness ratio of 20 or more.

6. The top 3 and the bottom 2 are joined together by spot welding, and the alpha cross tensile strength resistance of the spot welding is 70 kN / mm ２ The highly elongated part according to any one of claims 1 to 5, wherein the highly elongated part exceeds

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