skeleton member

Abstract

The present invention provides a skeleton member formed by hot stamping a steel sheet. The skeleton member has a closed cross section portion having a cross section perpendicular to a length direction as a closed cross section, the closed cross section portion having at least two flat portion which is a portion having a radius of curvature larger than a maximum outer dimension of the cross section, and a concave bead portion formed between the two flat portions. The concave bead portion has a pair of wall portions having a radius of curvature of 50 mm or more, which protrude toward an inner side of the closed cross section portion from mutually opposite ends of the two flat portions via a pair of curved portions curved toward an inside of the closed cross section. A Vickers hardness of a sheet thickness center portion in the wall portion is 520 Hv or more, a width of the wall portion is 0.5 times or more to 2.5 times or less of an effective width (W e ) of the closed cross section portion, and a standard deviation ratio obtained by dividing a standard deviation of a hardness frequency distribution of a surface layer portion of the wall portion by a standard deviation of a hardness frequency distribution of the sheet thickness center portion of the wall portion is less than 1.0.

Description

Technical Field

[0001] This invention relates to a skeletal component with excellent energy absorption efficiency.

[0002] This application is based on and claims priority to Japan Patent Application No. 2021-078463 filed on May 6, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] Traditionally, hollow components made by machining steel sheets into a specified closed cross-sectional shape were used as the skeletal components of automobiles. These skeletal components were required to achieve lightweighting while exhibiting sufficient yield strength and energy absorption performance under bending loads applied during a collision.

[0004] One primary means of achieving weight reduction is to increase the yield strength and energy absorption capacity of steel plates by increasing their strength, thereby making the components thinner and lighter. Therefore, in recent years, steel plates capable of tensile strengths exceeding 1.8 GPa have sometimes been used as materials for skeletal components.

[0005] Patent Document 1 discloses a vehicle impact-resistant reinforcement member, which is made of a molded thin plate for the purpose of improving pressure resistance and bending resistance. It has at least a main body and a pair of side wall portions integrated with the main body via a bending portion. A concave reinforcing rib is provided in the main body, extending along its length direction to the center of the width direction of the main body. The concave reinforcing rib is configured such that the distance between the concave reinforcing rib and the bending portion is an effective width c' and satisfies a specific range.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2009-286351 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] According to the technology in Patent Document 1, by considering the effective width when setting the reinforcing ribs, elastic buckling can be suppressed and yield strength can be improved. However, in order to achieve lightweighting through further thinning, it is necessary to study how to further improve the energy absorption per unit cross-sectional area of ​​the skeleton components, i.e., the energy absorption efficiency.

[0011] The present invention was made in view of the above-mentioned problems, and the object of the present invention is to provide a skeleton component with excellent energy absorption efficiency.

[0012] Methods for solving problems

[0013] The specific solution of the present invention is as follows.

[0014] (1) The first aspect of the present invention is a skeleton component formed by hot stamping of a steel plate, wherein the skeleton component has a closed section portion with a cross-section perpendicular to the length direction, the closed section portion having: at least two flat portions, which are portions with radii of curvature greater than the maximum external dimension of the cross-section; and a concave reinforcing rib portion formed between the two flat portions, the concave reinforcing rib portion having a pair of wall portions, the pair of wall portions having radii of curvature of 50 mm or more, protruding from the opposing ends of the two flat portions toward the inside of the closed section portion via a pair of curved portions bending toward the inside of the closed section portion, the Vickers hardness of the center portion of the plate thickness of the wall portion being 520 Hv or more, and the width of the wall portion being the effective width (W) calculated according to the Kalman effective width formula. e The standard deviation ratio obtained by dividing the standard deviation of the hardness frequency distribution of the surface layer of the aforementioned wall portion by the standard deviation of the hardness frequency distribution of the center layer of the aforementioned wall portion is less than 1.0.

[0015] (2) In the skeleton component described in (1) above, the closed section portion may be present in more than 50% of the total length of the skeleton component in the length direction above.

[0016] (3) In the skeleton component described in (1) or (2) above, the closed section portion may also be formed by joining multiple components.

[0017] (4) In any of the skeleton components described in (1) to (3) above, the plate thickness of the concave reinforcing rib portion may be 1.2 mm or less.

[0018] (5) In any of the skeleton components described in (1) to (4) above, there may be two or more of the above-mentioned concave reinforcing ribs.

[0019] (6) In any of the skeleton components described in (1) to (5) above, the standard deviation ratio is less than 0.8.

[0020] The effects of the invention

[0021] According to the above scheme, by controlling the width and hardness standard deviation ratio of the wall portion of the concave reinforcing rib within an appropriate range, elastic buckling and bending fracture can be suppressed. Therefore, even when using high-strength thin-walled components, high energy absorption performance can be achieved, resulting in excellent energy absorption efficiency. Attached Figure Description

[0022] Figure 1 This is a perspective view of a skeleton component according to one embodiment of the present invention.

[0023] Figure 2 yes Figure 1 The cross-sectional view of the cut line A1-A1.

[0024] Figure 3 yes Figure 2 A magnified view of the region enclosed by A.

[0025] Figure 4 This is a graph showing the relationship between the standard deviation of hardness and the bending angle ratio in the VDA bending test for 2.0 GPa grade materials.

[0026] Figure 5 This is a three-dimensional view showing the skeleton component of a modified example.

[0027] Figure 6 yes Figure 5 The cross-sectional view of the cut line A2-A2.

[0028] Figure 7 yes Figure 6 A magnified view of the region enclosed by B.

[0029] Figure 8 This is a cross-sectional view showing a modified example of the skeleton component.

[0030] Figure 9 This is a schematic diagram showing a deformation example of the concave reinforcing rib.

[0031] Figure 10 This is a schematic diagram showing other variations of the concave reinforcing rib.

[0032] Figure 11 This is a perspective view of a car frame as an example of an applied skeletal component.

[0033] Figure 12 This is a schematic diagram illustrating the cross-sectional shape of the square tube component used in the first embodiment.

[0034] Figure 13 This is a graph depicting the relationship between the effective width ratio and energy absorption efficiency for the first experimental example.

[0035] Figure 14 This is a schematic diagram illustrating the cross-sectional shape of the square tube component used in the second embodiment. Detailed Implementation

[0036] The inventors have conducted in-depth research on the structure of skeletal components that can achieve excellent energy absorption efficiency.

[0037] First, to achieve excellent energy absorption efficiency, it is important to have a certain flexural yield strength. By forming stiffeners along the length of the component, the flexural yield strength can be improved when an input load is applied in the bending direction due to an impact. However, when elastic buckling (deflection) occurs on the stiffener wall surface in the early stage of deformation, the required flexural yield strength is sometimes not obtained, and excellent energy absorption efficiency cannot be achieved.

[0038] Furthermore, to achieve excellent energy absorption efficiency, it is also important to effectively absorb impact energy by deforming the skeleton components in the desired deformation mode after a bending load is applied due to a collision. In particular, when fracture occurs at the bending section after deformation has begun, excellent energy absorption efficiency may not be achieved.

[0039] Therefore, if a cross-sectional design is made that makes it difficult for elastic buckling to occur in the surface with reinforcing ribs, and it is endowed with high bending performance that is difficult to break, it can be said that it can achieve excellent energy absorption efficiency.

[0040] Here, when the component is made thinner by increasing its strength as a means of achieving lightweighting, the following problems arise.

[0041] • Due to its thin walls, it is prone to elastic buckling, making it difficult to obtain the required yield strength.

[0042] • Due to the increased strength, the bending performance of the steel plate is reduced, and it is prone to fracture at the bending point after deformation begins, making it difficult to effectively absorb impact energy.

[0043] The inventors have focused on the aforementioned problems that hinder the further increase in strength and thinning of high-strength steel plates.

[0044] Through further research, the inventors discovered that by controlling the width of the concave reinforcing rib's wall portion and the standard deviation ratio of its hardness within an appropriate range, elastic buckling can be suppressed and bending fracture prevented. This control addresses the aforementioned concerns when using high-strength steel plates, enabling excellent energy absorption efficiency, thus completing this invention.

[0045] (First Embodiment)

[0046] Hereinafter, the skeleton component 1 of the first embodiment of the present invention will be described.

[0047] First, let's explain the statements in this instruction manual.

[0048] "Length direction" refers to the axial direction of the material of the skeleton component, that is, the direction in which the axis extends.

[0049] "Imaginary bending compression surface" refers to a part of a skeleton component that is imagined to generate compressive stress in the length direction when the skeleton component is subjected to bending loads due to collisions, etc.

[0050] A "flat section" refers to a straight section in a cross-section perpendicular to the length of the skeleton component; specifically, it refers to a section whose radius of curvature is larger than the maximum external dimension of the cross-section. The maximum external dimension is the length of the straight line that makes the distance between the ends of any two points in the cross-section the greatest. Straight sections within the concave reinforcing ribs are not considered flat sections.

[0051] "Concave reinforcing rib" refers to a part in a cross section perpendicular to the length direction of the skeleton component that protrudes from the imaginary bending and compression surface toward the inside of the closed cross section.

[0052] "Corner section" refers to a non-linear section in a cross-section perpendicular to the length of the skeleton component, excluding flat sections and concave reinforcing rib sections.

[0053] "Width" refers to the length of a line along the circumference of a closed section. For example, "width of a wall" refers to the length of a line between one end and the other end of a wall.

[0054] The "effective width" is the effective width W calculated based on the following equation (1) of the Kalman effective width theory, i.e., the Kalman effective width formula. e .

[0055]

Formula 1

[0056]

[0057] Here,

[0058] σy: Yield stress of the wall (MPa)

[0059] E: Young's modulus of the wall (MPa)

[0060] t: Plate thickness of the wall section (mm)

[0061] ν: Poisson's ratio of the wall

[0062] Furthermore, in steel plates, the Young's modulus and Poisson's ratio of the wall portion can be obtained using general physical parameters. By further replacing the yield stress of the wall portion with the Vickers hardness of the center of the plate thickness, it is also possible to determine the specific values ​​based on W. e The effective width W can be calculated using the formula =577t / √h. e .

[0063] Here,

[0064] t: Plate thickness of the wall section (mm)

[0065] h: Vickers hardness (Hv) at the center of the wall thickness.

[0066] The effective width W is difficult to determine using equation (1). e In the case of , it can be obtained using the above formula.

[0067] "Effective width ratio" is the ratio of the wall width H0 of the concave reinforcing rib to the effective width W. e The ratio is determined by H0 / W e The calculated value. It can be said that the smaller the effective width ratio, the more difficult it is to produce a cross-sectional shape with elastic buckling of the wall.

[0068] The “surface portion” refers to the area between a depth position where the separation distance from the surface of the steel plate in the thickness direction is 1% of the steel plate thickness and a depth position where the separation distance from the surface of the steel plate in the thickness direction is 5% of the steel plate thickness.

[0069] "Center of plate thickness" refers to the depth position where the separation distance from the surface of the steel plate in the plate thickness direction is 3 / 8 of the plate thickness.

[0070] The "surface of the steel plate" used as a reference for depth positioning refers to the surface of the base steel plate. For example, in cases where plating or painting has been performed, or where rust has formed, the surface of the steel plate after the plating, painting, and rust have been removed is used as the reference for depth positioning. Furthermore, when a surface coating such as plating, painting, or rust has formed on the surface of the base steel plate, the boundary between this surface coating and the surface of the base steel plate can be easily identified using various known methods.

[0071] "Energy absorption" refers to the energy absorption calculated based on the relationship between the impactor reaction force (load) and the stroke when a rigid planar impactor collides with a bending and compressing imaginary surface under conditions where both ends of the skeleton component are fully constrained.

[0072] "Energy absorption efficiency" refers to the amount of energy absorbed per unit cross-sectional area of ​​a skeleton component. When the skeleton component does not have a uniform cross-section along its length, it refers to the amount of energy absorbed per unit cross-sectional area of ​​the closed cross-section with the smallest cross-sectional area perpendicular to the component's length.

[0073] Figure 1 This is a three-dimensional view of skeleton component 1. Figure 2 yes Figure 1 The cross-sectional view of the cut line A1-A1, which is perpendicular to the length direction of the skeleton component 1. Figure 3 yes Figure 2 A magnified view of the region enclosed by A.

[0074] like Figure 1 as well as Figure 2As shown, the skeleton component 1 is composed of a hollow cylindrical body 10 extending along the length direction. That is, the skeleton component 1 is a component whose cross-section perpendicular to the length direction is formed as a whole into a closed cross-section.

[0075] The frame component 1 is configured such that the bending compression imaginary surface faces outwards from the vehicle body, thereby providing load-bearing capacity against compressive stress in the bending compression imaginary surface when subjected to a collision.

[0076] In the hypothetical curved compression surface, a concave reinforcing rib portion 100 is provided between two first flat portions 11, 11.

[0077] Assuming that the hypothetical bending compression surface consists only of flat parts, sometimes the load-bearing capacity cannot be fully utilized due to the deflection of the flat parts when subjected to compressive loads. By configuring the concave reinforcing rib 100 sandwiched between the two first flat parts 11, 11 as in the skeleton component 1, the load-bearing capacity can be improved.

[0078] First corner portions C1, C1 are formed at the outer ends of each of the first flat portions 11, 11. Furthermore, two second flat portions 13, 13 extend from the ends of the first corner portions C1, C1 opposite to the first flat portions 11, 11, facing each other.

[0079] Furthermore, at the ends of each of the second flat portions 13, 13 opposite to the first corner portions C1, C1, a second corner portion C2, C2 is formed that bends toward each other in a direction of approach. And, the ends of the second corner portions C2, C2 opposite to the second flat portion 13 are connected to each other by the third flat portion 15.

[0080] Therefore, the skeleton component 1 of this embodiment forms a closed cross section through the concave reinforcing rib portion 100, the first flat portion 11, 11, the second flat portion 13, 13, the third flat portion 15, the first corner portion C1, C1, and the second corner portion C2, C2.

[0081] like Figure 3 As shown, in a cross section perpendicular to the length direction of the skeleton component 1, the concave reinforcing rib portion 100 is composed of a first curved portion 121, 121, a wall portion 123, 123, a second curved portion 125, 125, and a bottom portion 127.

[0082] The first curved portions 121, 121 are portions that bend inward from the opposing ends of the two first flat portions 11, 11 toward the interior of the closed section. Since portions with a radius of curvature of 50 mm or more are considered part of the wall portion, the radius of curvature of the first curved portion 121 is less than 50 mm. For example, the radius of curvature of the first curved portion 121 only needs to be 3 mm to 5 mm.

[0083] The wall portions 123, 123 are portions that protrude inward toward the closed section via the first curved portions 121, 121. The wall portions 123, 123 are straight portions with a radius of curvature of 50 mm or more.

[0084] The second bends 125 and 125 are portions of the wall portions 123 that bend in a direction facing each other from the ends opposite to the first bends 121 and 121. Since portions with a radius of curvature of 50 mm or more are considered part of the wall portion 123 or part of the bottom 127, the radius of curvature of the second bend 125 is less than 50 mm. For example, the radius of curvature of the second bend 125 only needs to be 3 mm to 5 mm.

[0085] The bottom 127 is the part that connects the ends of the second curved portions 125, 125 on the opposite side to the wall portions 123, 123 in a straight line.

[0086] If the width H0 of the wall portion 123 of the concave reinforcing rib portion 100 is too large, elastic buckling will easily occur in the early stage of deformation when the skeleton component 1 is subjected to bending load, making it impossible to obtain the required yield strength. Therefore, the skeleton component 1 will struggle to achieve excellent energy absorption efficiency. Consequently, the width H0 of the wall portion 123 is set to the effective width W. e Less than 2.5 times.

[0087] On the other hand, if the width H0 of the wall portion 123 is too small, the effect of increasing the flexural yield strength by incorporating concave reinforcing ribs will be diminished. Therefore, the width H0 is set to the effective width W. e More than 0.5 times that.

[0088] In addition, in order to obtain the required yield strength, the effective width W e The upper limit is preferably below 60mm.

[0089] From a lightweight perspective, the thickness of the plate in the concave reinforcing rib portion 100 is preferably less than 1.2 mm.

[0090] On the other hand, when the plate thickness of the concave reinforcing rib portion 100 is less than 0.4 mm, the wall portion 123 of the concave reinforcing rib portion 100 is prone to elastic buckling, thus increasing the limitation on the setting range of the width H0. Therefore, the plate thickness of the concave reinforcing rib portion 100 is preferably 0.4 mm or more.

[0091] A hot-stamping steel sheet is heated to the austenitic region and, while held at a specified temperature, is hot-stamped into a specified shape through a stamping die with a quenching mechanism while undergoing quenching. The end faces are then joined, thereby forming the skeleton component 1. The skeleton component 1 thus formed has a tensile strength exceeding 1.8 GPa. Furthermore, through this formation, in a hardness test performed according to the method described in JIS Z 2244:2009, with a test load of 300 gf (2.9 N), the Vickers hardness at the center of the plate thickness of the wall portion 123 of the concave reinforcing rib portion 100 in the skeleton component 1 reaches 520 Hv or higher.

[0092] In this application, to improve the deformation capacity and achieve excellent energy absorption efficiency with the premise of high strength, the hardness of the plate thickness center of the wall portion 123 of the concave reinforcing rib portion 100 is set to 520Hv or higher according to Vickers hardness.

[0093] There is no specific upper limit for the hardness of the center of the plate thickness, but it can be below 850 Hv according to Vickers hardness.

[0094] The method for measuring the hardness of the central portion of the plate thickness in the wall portion 123 of the concave reinforcing rib portion 100 is as follows.

[0095] A sample with a cross-section perpendicular to the plate surface is collected from the wall portion 123 of the concave reinforcing rib portion 100, the cross-section is prepared as the measuring surface, and the measuring surface is used for hardness testing.

[0096] Although it is also based on a measuring device, the size of the measuring surface can be about 10mm × 10mm.

[0097] The method for preparing the measuring surface was implemented based on JIS Z 2244:2009.

[0098] After grinding the measuring surface with silicon carbide paper of #600 to #1500, the measuring surface is then polished to a mirror finish using a liquid obtained by dispersing diamond powder with a particle size of 1μm to 6μm in a diluent such as an alcohol or pure water.

[0099] The hardness test was performed on the test surface that was thus finely machined into a mirror finish using the method described in JIS Z 2244:2009.

[0100] Using a micro Vickers hardness tester, 30 points were measured at 3 / 8 of the plate thickness of the sample, with a load of 300 gf and at intervals of more than 3 times the indentation. The average value of these measurements was taken as the hardness of the center of the plate thickness.

[0101] As described above, the width H0 of the wall portion 123 is the effective width W. eWhen the strength is less than 2.5 times that of the wall portion 123, elastic buckling can be suppressed. However, in high-strength materials, such as hot-stamped materials exceeding 1.8 GPa, even if the effective width W can be controlled... e However, if elastic buckling is suppressed and bending performance is insufficient, fracture will occur during deformation due to bending load, thus failing to achieve excellent energy absorption efficiency.

[0102] However, in the skeleton component 1 of this embodiment, the bending performance is improved by appropriately controlling the ratio of the standard deviation of the hardness frequency distribution at the center of the wall portion 123 in the concave reinforcing rib portion 100 to the standard deviation of the hardness frequency distribution on the surface portion.

[0103] Therefore, in the skeleton component 1 of this embodiment, even if a high-strength material is used, it is possible to suppress fracture during deformation, and it can achieve an exceptionally high energy absorption efficiency compared to the past.

[0104] Specifically, in the skeleton component 1 of this embodiment, the standard deviation of the hardness frequency distribution of the surface layer is divided by the standard deviation of the hardness frequency distribution of the center layer (3 / 8 of the depth of the plate thickness) in the wall portion 123 of the concave reinforcing rib portion 100, and the resulting value, i.e., the hardness standard deviation ratio, is controlled to be less than 1.0.

[0105] Through experiments, the inventors discovered that when using hot-stamped materials with a hardness exceeding 1.8 GPa, and when the hardness standard deviation ratio is set to a value less than 1.0, the maximum bending angle in the VDA bending test based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry can be significantly increased.

[0106] Figure 4 This graph shows the results of VDA bending tests using steel plates made of 1.4 mm thick, 2.0 GPa grade material. It shows that the smaller the standard deviation ratio of hardness is than 1.0, the higher the maximum bending angle (°) and the higher the bending angle ratio in the VDA bending test. That is, when the standard deviation ratio of hardness is less than 1.0, it is difficult for fracture to occur midway through deformation due to axial load, thus exhibiting excellent energy absorption efficiency. The standard deviation ratio is preferably less than 0.8.

[0107] Here, the hardness frequency distribution of the center of the plate and the surface layer are obtained through Vickers hardness testing.

[0108] First, a specimen with a cross-section perpendicular to the plate surface is cut from any position of the concave reinforcing rib portion 100, including the wall portion 123, and this cross-section is prepared as a measuring surface, which is then used for hardness testing.

[0109] Although it is also based on a measuring device, the size of the measuring surface can be about 10mm × 10mm.

[0110] The method for preparing the measuring surface was implemented based on JIS Z 2244:2009.

[0111] After grinding the measuring surface with silicon carbide paper of #600 to #1500, the measuring surface is polished to a mirror finish using a liquid obtained by dispersing diamond powder with a particle size of 1μm to 6μm in a diluent such as alcohol or pure water.

[0112] The hardness test was performed on the test surface, which was thus finished to a mirror finish, using the method described in JIS Z 2244:2009.

[0113] The surface hardness was determined using a micro Vickers hardness tester.

[0114] The hardness frequency distribution of the surface layer was determined by measuring 30 points at intervals of more than 3 times the indentation with a load of 300gf.

[0115] Similarly, at the center of the plate thickness (3 / 8 of the plate thickness), 30 points were measured at intervals of more than 3 times the indentation with a load of 300gf to determine the hardness frequency distribution at the center of the plate thickness.

[0116] In the hardness frequency distributions of the center and surface of the plate obtained as a result of the Vickers hardness test, known statistical methods are used to determine the standard deviation.

[0117] As before, when the metal structure of the center and surface of the hot-stamped steel sheet is the same, the hardness frequency distribution of the surface is the same as that of the center of the sheet, and the hardness standard deviation ratio is 1.0.

[0118] On the other hand, when only the surface layer and the surrounding metal structure are modified, the hardness standard deviation ratio becomes a value different from 1.0.

[0119] In the skeleton member 1 formed from the hot stamping steel sheet of this embodiment, by modifying only the metal structure of the surface layer and its vicinity, the distribution and deviation of the hardness of the surface layer can be suppressed, and the ratio of the standard deviation of the hardness of the surface layer to that of the center of the plate thickness can be less than 1.0.

[0120] Specifically, the hardness standard deviation ratio can be controlled by adjusting the maximum heating temperature and holding time during decarburization annealing of hot-stamping steel sheets using known techniques. The preferred decarburization annealing conditions are: in a humid atmosphere containing hydrogen, nitrogen, or oxygen, setting the decarburization annealing temperature (the maximum temperature reached by the steel sheet) to 700–950°C, and setting the dwell time in the 700–950°C temperature range to 5–1200 seconds.

[0121] Furthermore, by setting the annealing temperature to a higher range within this condition and limiting the dwell temperature to a longer time range, it is possible to achieve a hardness standard deviation ratio of less than 0.8.

[0122] Furthermore, it is sufficient if at least one surface portion of the wall portion 123 satisfies the aforementioned condition regarding the hardness standard deviation ratio. However, it is preferable that the surface portions on both sides of the wall portion 123 satisfy the aforementioned condition regarding the hardness standard deviation ratio.

[0123] Thus, according to the skeleton component 1 of this embodiment, elastic buckling can be suppressed by controlling the width H0 of the wall portion 123 of the concave reinforcing rib portion 100, and fracture during deformation can be suppressed by controlling the hardness standard deviation ratio.

[0124] Therefore, it can have sufficient hardness, such as a Vickers hardness of 520 Hv or higher, at the center of the wall portion 123 of the concave reinforcing rib portion 100, and at the same time, it can further improve energy absorption efficiency.

[0125] (Second Implementation)

[0126] Hereinafter, the skeleton component 2 of the second embodiment of the present invention will be described.

[0127] The difference between the skeleton component 2 of the second embodiment and the skeleton component 1 of the first embodiment is that the cross-section perpendicular to the length direction is formed by two components to form a closed cross-section. That is, in the skeleton component 2, the closed cross-section is composed of two components joined together.

[0128] For constituent elements having a functional configuration that is substantially the same as the functional configuration described in the first embodiment, repeated descriptions are omitted.

[0129] Figure 5 This is a 3D view of skeleton component 2. Figure 6 yes Figure 5 The cross-sectional view of the cut line A2-A2 is a cross-sectional view that is perpendicular to the length direction of the skeleton component 2. Figure 7 yes Figure 6 A magnified view of the region enclosed by B.

[0130] like Figure 5 as well as Figure 6 As shown, the skeleton member 2 forms a closed cross-section by joining the first skeleton member 20 and the second skeleton member 30. That is, the closed cross-section is composed of the first skeleton member 20 and the second skeleton member 30.

[0131] The first skeleton component 20 is a component with a hat-shaped cross section, and its top plate surface functions as a bending compression imaginary surface.

[0132] In the hypothetical curved compression surface, the concave reinforcing rib portion 200 is sandwiched between two first flat portions 21, 21.

[0133] First corner portions C1, C1 are formed at the outer ends of each of the first flat portions 21, 21. Furthermore, two second flat portions 23, 23 extend from the ends of the first corner portions C1, C1 opposite to the first flat portions 21, 21, facing each other.

[0134] Furthermore, at the ends of each of the second flat portions 23, 23 opposite to the first corner portions C1, C1, a second corner portion C2, C2 is formed that bends toward a direction of separation. And, from the ends of the second corner portions C2, C2 opposite to the second flat portions 23, a third flat portion 25, 25 extends toward a direction of separation.

[0135] The second frame member 30 is a flat steel plate with a pair of jointing portions 31, 31 that are in contact with the third flat portions 25, 25 of the first frame member 20 and joined by spot welding or the like, and a flat portion 33 sandwiched between the pair of jointing portions 31, 31.

[0136] Therefore, the skeleton member 2 of this embodiment forms a closed cross section through the concave reinforcing rib portion 200, the first flat portion 21, 21, the second flat portion 23, 23, the first corner portion C1, C1, the second corner portion C2, C2 in the first skeleton member 20, and the flat portion 33 in the second skeleton member 30.

[0137] like Figure 7 As shown, in a cross section perpendicular to the length direction of the skeleton component 2, the concave reinforcing rib portion 200 is composed of a first curved portion 221, 221, a wall portion 223, 223, a second curved portion 225, 225, and a bottom portion 227.

[0138] The concave reinforcing rib portion 200 has the same configuration as the concave reinforcing rib portion 100 described in the first embodiment, so detailed description is omitted.

[0139] Thus, in the skeleton member 2, which is composed of two joined parts forming a closed cross-section, similarly to the skeleton member 1, elastic buckling can be suppressed by controlling the width H0 of the wall portion 223 of the concave reinforcing rib portion 200, and fracture during deformation can be suppressed by controlling the hardness standard deviation ratio. Furthermore, the closed cross-section can also be composed of two or more joined parts.

[0140] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to these examples.

[0141] Anyone with ordinary knowledge of the technical field to which this invention pertains will be able to readily conceive of various modifications or variations within the scope of the technical concept described in the patent claims, and these should be understood to fall within the technical scope of this invention as well.

[0142] For example, in the skeleton member 1 of the first embodiment, one concave reinforcing rib is provided on the bending compression imaginary surface, but two or more concave reinforcing ribs may also be formed on the bending compression imaginary surface. That is, it may be as follows: Figure 8 As shown in the first modified example of the skeleton component 1A, two concave reinforcing ribs 100A, 100A are formed on the imaginary surface of bending compression. In this case, the effective width W calculated according to Kalman's effective width formula is achieved by satisfying the width H0 of the wall portion 123A of the two concave reinforcing ribs 100A, 100A. e Under conditions that are 0.5 times to 2.5 times higher, it can achieve a more superior energy absorption efficiency.

[0143] Furthermore, there is no particular limit to the number of flat parts, as long as there are at least two flat parts connected to the curved part of the concave reinforcing rib.

[0144] Furthermore, for example, the aforementioned concave reinforcing rib portion 100 has a pair of wall portions 123, 123 extending towards each other, but it can also be as follows: Figure 9 Like the concave reinforcing rib portion 100B of the second modified example shown, it has a pair of wall portions 123B, 123B extending obliquely to each other.

[0145] More specifically, the concave reinforcing rib 100B of this modified example is composed of a first curved portion 121B, 121B that bends toward the interior of the closed section, wall portions 123B, 123B that protrude obliquely toward the interior of the closed section via the first curved portions 121B, 121B, a second curved portion 125B, 125B that bends toward each other from the ends of the wall portions 123B, 123B opposite to the first curved portions 121B, 121B, in a mutually facing direction, and a bottom 127B that connects the ends of the second curved portions 125B, 125B opposite to the wall portions 123B, 123B in a straight line.

[0146] Furthermore, for example, the aforementioned concave reinforcing rib portion 100 has a pair of second curved portions 125, 125 and a bottom 127, but it can also be as follows: Figure 10 As shown in the third modified example with the concave reinforcing rib portion 100C, a pair of wall portions 123C, 123C extending obliquely to each other are connected by a single second bend portion 125C.

[0147] More specifically, the concave reinforcing rib 100C in this modified example is composed of first curved portions 121C, 121C bending toward the interior of the closed section, wall portions 123C, 123C protruding obliquely toward the interior of the closed section via the first curved portions 121C, 121C, and a second curved portion 125C connecting the ends of the wall portions 123C, 123C opposite to the first curved portions 121C, 121C to each other. That is, the concave reinforcing rib 100C does not have the straight bottom 27 structure shown in the first embodiment.

[0148] Furthermore, in the skeleton components 1 and 2 of the above embodiments, a uniform cross-sectional shape is provided throughout the entire length. However, it is not necessary for the cross-sectional shape to be uniform throughout the entire length; as long as the closed cross-sectional portion exists in a portion of the entire length in the longitudinal direction, it is acceptable. However, it is preferable that the closed cross-sectional portion exists in 50% or more, more preferably 80% or more of the entire length in the longitudinal direction.

[0149] In addition, the frame components 1 and 2 are components in the structural parts of the automobile body that are expected to be subjected to axial load compression input mainly during a collision. Figure 11 This is a diagram showing an automobile frame 300 as an example of the application of frame components 1 and 2.

[0150] Referring to the figure, the frame components 1 and 2 can be applied to the front side beam 301, rear side beam 303, lower longitudinal beam 305, A-pillar 307, B-pillar 309, roof longitudinal beam 311, floor crossbeam 313, roof crossbeam 315, and lower reinforcement 317 in the structural components of the automobile body.

[0151] The effects of the present invention will be described in more detail below based on embodiments. However, the conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. Various conditions may be adopted as long as they do not depart from the spirit of the present invention and achieve the purpose of the present invention.

[0152] (First Embodiment)

[0153] Prepare steel plate A and steel plate B, both with a thickness of 0.5mm.

[0154] Steel plate A and steel plate B are both hot stamping steel plates used for hot stamping.

[0155] In steel plate B, during decarburization annealing, the decarburization annealing temperature (the highest temperature reached by the steel plate) is set to 750°C in a humid atmosphere mixed with hydrogen and nitrogen, and the residence time in the temperature range of 700 to 750°C is set to 300 seconds, thereby modifying the metal structure only in the surface layer and its vicinity.

[0156] Steel plates A and B are heated to the austenitic region and maintained at a temperature of 900–950°C, then hot-stamped using a stamping die with a rapid cooling mechanism. The end faces of the hot-stamped parts are then welded together to obtain a square tube component with a length of 296 mm.

[0157] Figure 12 This is a schematic diagram illustrating the cross-sectional shape of the square tube component used in each experimental example. For example... Figure 12 As shown, in every experimental example, the basic design is a roughly square cross-section with one side measuring 74mm.

[0158] The square tube component of this basic design was used in Experiment 1A and Experiment 1B.

[0159] In Experimental Examples 2A to 7B, by changing the shape of the stamping die used in hot stamping, a concave reinforcing rib with a specified width H0 is given to one side of the basic design of the square tube component.

[0160] In addition, the radius of curvature of the four corner sections C is designed to be 5mm, and the radius of curvature of the curved parts in the concave reinforcing ribs is set to 3mm.

[0161] Table 1 shows the material properties of the flat sections of the hot-stamped square tube components.

[0162] Table 1

[0163]

[0164] In the square tube component using steel plate A, since the metal structure at the center of the plate thickness is the same as that at the surface, the standard deviation ratio of hardness in the flat portion is 1.0. That is, in experimental examples 2A, 3A, 4A, 5A, 6A, and 7A, where concave reinforcing ribs are provided, the standard deviation ratio of hardness in the wall portion of the concave reinforcing rib portion is 1.0.

[0165] On the other hand, in the square tube component using steel plate B, since the metal structure of the surface layer was modified without modifying the metal structure of the center of the plate thickness, the standard deviation ratio of hardness in the flat portion was 0.65. That is, in experimental examples 2B, 3B, 4B, 5B, 6B, and 7B, which were given concave reinforcing ribs, the standard deviation ratio of hardness of the wall portion of the concave reinforcing rib portion was 0.65.

[0166] For these cylindrical components, with both ends fully constrained along their length, a rigid planar impactor is driven at 80 km / h to collide with a hypothetical bending and compressing surface. The absorbed energy is calculated and compared based on the deformation state, fracture occurrence, impactor reaction force (load), and stroke.

[0167] Table 2 shows the setup conditions and results for each experimental case.

[0168] Table 2

[0169]

[0170] In experimental examples 1A, 2A, 3A, 4A, 5A, 6A, and 7A, due to the hardness standard deviation ratio of 1.0, good bending properties could not be obtained, and cracks were generated during deformation. As a result, the energy absorption efficiency was poor.

[0171] Experimental Example 1B is a comparative example where the hardness standard deviation ratio was appropriately controlled, but no concave reinforcing ribs were formed. In this comparative example, the yield strength improvement effect brought about by imparting concave reinforcing ribs could not be obtained, and the energy absorption efficiency was poor.

[0172] Furthermore, Experimental Example 2B is a comparative example where, although the hardness standard deviation ratio was appropriately controlled and a concave reinforcing rib was formed, the effective width ratio was relatively low. In this comparative example, early buckling occurred due to the low effective width ratio, resulting in lower energy absorption and poorer energy absorption efficiency.

[0173] On the other hand, in experimental examples 3B, 4B, 5B, 6B, and 7B, the standard deviation of hardness was appropriately controlled, and the effective width ratio was also appropriate. Therefore, even hot-stamped parts with a hardness of 1800 MPa did not fracture or buckle elastically, and could exhibit excellent energy absorption efficiency.

[0174] in addition, Figure 13 This is a graph comparing the energy absorption efficiency relative to the effective width ratio for the experimental results shown in Table 2. As the graph shows, it can be seen that by appropriately controlling the standard deviation ratio of hardness based on the reinforcing rib shape that is within an appropriate range of effective width ratio, the energy absorption efficiency is significantly improved.

[0175] (Second Embodiment)

[0176] Furthermore, as a second embodiment, using the same steel plates A and B as in the first embodiment, an experiment was conducted to verify that excellent energy absorption efficiency could be achieved by providing multiple concave reinforcing ribs.

[0177] In experimental examples 8A, 8B, 9A, and 9B, for the cross-section perpendicular to the length direction of the square tube component, such as Figure 14 As shown, the basic design is a cross-section of a roughly square section with one side of 74mm and three concave reinforcing ribs with a width of H0 on one side. The width H0 of the wall is changed in each experimental example.

[0178] The radius of curvature of the four corner sections C is designed to be 5mm, and the radius of curvature of the concave reinforcing ribs is set to 3mm.

[0179] For these cylindrical components, with both ends fully constrained along their length, a rigid planar impactor is driven at 80 km / h to collide with a hypothetical bending and compressing surface. The absorbed energy is calculated and compared based on the deformation state, fracture occurrence, impactor reaction force (load), and stroke.

[0180] Table 3 shows the setup conditions and results for each experimental case.

[0181] Table 3

[0182]

[0183] In Experiments 8A and 9A, due to the hardness standard deviation ratio of 1.0, good bending properties could not be obtained, and cracks were generated during deformation. As a result, the energy absorption efficiency was poor.

[0184] Furthermore, Experimental Example 8B is a comparative example where the standard deviation of hardness was appropriately controlled, but the effective width ratio was 0. In this comparative example, the energy absorption efficiency was poor due to low energy absorption caused by early buckling.

[0185] On the other hand, in Experimental Example 9B, the hardness standard deviation ratio was properly controlled and the effective width ratio was also appropriate. Therefore, even hot-stamped parts of the 1800MPa grade did not break or buckle elastically, and could achieve excellent energy absorption efficiency.

[0186] In particular, when compared with Experimental Example 4B in Table 2, which has one concave reinforcing rib, the energy absorption efficiency increased by 1.74 times in Experimental Example 9B in Table 3, which has three concave reinforcing ribs. That is, it can be said that excellent energy absorption efficiency can be achieved by providing multiple concave reinforcing ribs.

[0187] Industrial availability

[0188] According to the present invention, a skeletal component with excellent energy absorption efficiency can be provided.

[0189] Explanation of symbols

[0190] 1, 2 Frame components

[0191] 20 First skeleton component

[0192] 30 Second skeleton component

[0193] 100, 100A, 100B, 200 Recessed reinforcing rib areas

[0194] 123, 123A, 123B, 223 Wall section

Claims

1. A frame component, formed by hot stamping of a steel plate, characterized in that, The aforementioned skeleton component has a closed section portion with a closed cross-section perpendicular to the length direction. The aforementioned closed section portion has: At least two flat sections are those with radii of curvature greater than the maximum external dimension of the cross section; as well as The concave reinforcing rib is formed between the two flat sections mentioned above. The aforementioned concave reinforcing rib portion has a pair of wall portions with a radius of curvature of 50 mm or more, which protrude from the opposing ends of the two flat portions toward the inside of the closed section via a pair of curved portions that bend toward the inside of the closed section. The Vickers hardness of the central part of the aforementioned wall thickness is above 520 Hv. The width of the aforementioned wall portion is the effective width (W) calculated according to Kalman's effective width formula. e 0.5 times to 2.5 times that of ) The standard deviation obtained by dividing the standard deviation of the hardness frequency distribution of the surface layer of the aforementioned wall portion by the standard deviation of the hardness frequency distribution of the center layer of the aforementioned wall portion is less than 1.

0.

2. The skeleton component according to claim 1, characterized in that, The aforementioned closed cross-section portion exists for more than 50% of the total length of the aforementioned skeleton component in the aforementioned length direction.

3. The skeleton component according to claim 1 or 2, characterized in that, The aforementioned closed cross-section is formed by joining multiple components.

4. The skeleton component according to claim 1 or 2, characterized in that, The thickness of the plate at the aforementioned concave reinforcing rib area is less than 1.2 mm.

5. The skeleton component according to claim 1 or 2, characterized in that, The above-mentioned concave reinforcing ribs are present in two or more locations.

6. The skeleton component according to claim 1 or 2, characterized in that, The above standard deviation ratio is less than 0.8.

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