skeleton member

Abstract

The skeleton member is a skeleton member formed by cold press forming a steel sheet, and has a closed section portion having a cross section perpendicular to a length direction that is a closed section, the closed section portion having at least one flat portion that is a portion having a curvature radius larger than a maximum outer dimension of the cross section, a Vickers hardness of a sheet thickness center portion of the reference flat portion being 300 Hv or more, a width of the reference flat portion being 2.0 times or less of an effective width, and a standard deviation ratio obtained by dividing a standard deviation of a hardness frequency distribution of a surface layer portion of the reference flat portion by a standard deviation of a hardness frequency distribution of the sheet thickness center portion of the reference flat portion being greater than 1.0.

Description

Technical Field

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

[0002] This application claims priority based on Japanese Patent Application No. 2021-078462, filed in Japan 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 skeleton components of automobiles. Such skeleton components were required to be lightweight and to exhibit sufficient strength and energy absorption performance when subjected to axial input loads due to collisions.

[0004] As a primary means of achieving lightweighting, methods include increasing the strength of steel plates to improve their durability and energy absorption performance, and correspondingly reducing the thickness of component walls to achieve lightweighting. Therefore, in recent years, there have been instances of using cold-rolled steel plates with tensile strengths of 980 MPa or higher as materials for frame components.

[0005] Patent Document 1 discloses a vehicle impact-resistant reinforcement for improving buckling resistance. The vehicle impact-resistant reinforcement is made of a thin sheet that has been formed and processed. It has at least a main body and a pair of side wall portions integrated with the main body via a folded portion. A concave rib is provided in the main body, extending along its length direction at the center of the width direction of the main body. The distance between the concave rib and the folded portion is an effective width c', and the concave rib is provided in such a way that it meets 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 ribs, elastic buckling can be suppressed, thereby improving the endurance. However, in order to further achieve the weight reduction brought about by thinner walls, it is necessary 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 used to solve problems

[0013] The specific form of the present invention is as described below.

[0014] (1) The first technical solution of the present invention is a skeleton component formed by cold pressing a steel plate. The skeleton component is characterized in that it has a closed section portion with a cross section perpendicular to the length direction. The closed section portion has at least one flat portion with a radius of curvature larger than the maximum external dimension of the cross section. When the flat portion with the largest width relative to the effective width calculated according to the Karman effective width formula is defined as the reference flat portion, the Vickers hardness of the plate thickness center portion of the reference flat portion is 300 Hv or more, the width of the reference flat portion is less than 2.0 times the effective width, and the standard deviation ratio obtained by dividing the standard deviation of the hardness frequency distribution of the surface portion of the reference flat portion by the standard deviation of the hardness frequency distribution of the plate thickness center portion of the reference flat portion is greater 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 skeleton component may include a first skeleton component extending in the length direction and a second skeleton component extending in the length direction and engaging with the first skeleton component, and the closed section includes the first skeleton component and the second skeleton component.

[0017] (4) In any of the skeleton components described in (1) to (3) above, the standard deviation ratio may be greater than 1.20.

[0018] Invention Effects

[0019] According to the above technical solution, by controlling the width and hardness standard deviation ratio within an appropriate range in the reference flat area, it is possible to suppress elastic buckling while preventing fracture during wrinkling deformation caused by axial load. Therefore, high energy absorption performance can be achieved even when using high-strength thin-walled components. Consequently, excellent energy absorption efficiency can be achieved. Attached Figure Description

[0020] Figure 1 It is a diagram used to illustrate the amount of energy absorbed.

[0021] Figure 2 This is a perspective view showing the skeleton component 10 according to one embodiment of the present invention.

[0022] Figure 3 yes Figure 2A cross-sectional view at the cut line A1-A1.

[0023] Figure 4 This is a graph showing the relationship between the standard deviation of hardness and the VDA bending angle ratio in the VDA bending test for cold-rolled steel sheets with a tensile strength of 980 MPa or higher.

[0024] Figure 5 This is a perspective view of the skeleton component 20 in the modified example.

[0025] Figure 6 yes Figure 5 A sectional view at the cut line A2-A2.

[0026] Figure 7 This is a perspective view of a car frame 100, which is an example of the application of structural components.

[0027] Figure 8 This is a schematic diagram illustrating the cross-sectional shape of the square tube used in the embodiments.

[0028] Figure 9 It is to plot a curve showing the relationship between the effective width ratio and energy absorption efficiency in the experimental example. Detailed Implementation

[0029] The inventors conducted specialized research on the structure of the skeletal components that can achieve excellent energy absorption efficiency.

[0030] First, to achieve excellent energy absorption efficiency, a certain or higher level of withstand capability is crucial. When an axial input load is applied due to a collision, elastic buckling can occur in the flat areas during the initial stage of deformation. If elastic buckling occurs, the required withstand capability cannot be obtained, and excellent energy absorption efficiency may not be achieved.

[0031] Furthermore, to achieve excellent energy absorption efficiency, it is equally important that the skeleton components fold and deform in the desired mode immediately after being subjected to an axial input load due to a collision, thereby effectively absorbing impact energy. In particular, if fracture occurs midway through the folding deformation due to the axial load (fracture at the fold), excellent energy absorption efficiency may not be achieved.

[0032] Therefore, it can be said that as long as the cross-section design is made in the flat part to prevent elastic buckling and to give it high bending performance that is not easy to break, it can achieve excellent energy absorption efficiency.

[0033] Here, when components are made stronger and thinner as a means of achieving lightweighting, the following problems occur.

[0034] • Because thin-walled components are prone to elastic buckling at flat areas, it is difficult to achieve the required strength.

[0035] • Due to the reduced bending performance of high-strength steel plates, fractures are more likely to occur at the folds after deformation begins, making it difficult to effectively absorb impact energy.

[0036] The inventors noticed that the above-mentioned problems were the reason that hindered the further strengthening and thinning of high-strength steel plates.

[0037] Through further research, the inventors discovered that by controlling the ratio of width to hardness standard deviation within an appropriate range in the flat section, elastic buckling can be suppressed while preventing fracture during wrinkling deformation caused by axial loads. This control eliminates the aforementioned problems that are concerning when using high-strength steel plates, enabling excellent energy absorption efficiency, thus completing this invention.

[0038] Hereinafter, the skeleton component 10 of the first embodiment of the present invention, based on the above understanding, will be described.

[0039] In addition, in this specification and the accompanying drawings, the same reference numerals are assigned to constituent elements with substantially the same functional structure, and repeated descriptions are omitted.

[0040] First, let's explain the statements in this instruction manual.

[0041] "Length direction" refers to the material axis direction of the skeleton component, that is, the direction in which the axis extends.

[0042] 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 maximizes the distance between the ends of any two points in the cross-section.

[0043] "Corner section" refers to a non-linear section in a cross-section perpendicular to the length of the skeleton component, excluding flat sections.

[0044] "Width" refers to the length of the line along the circumference of the closed section, while "width of the flat section" refers to the length of the line between one end and the other end of the flat section.

[0045] The "effective width" is the effective width W calculated based on the following equation (1) (i.e., the Karman effective width formula) according to Karman's effective width theory. e .

[0046] [Formula 1]

[0047]

[0048] here,

[0049] σ y Yield stress (MPa) in the flat region

[0050] E: Young's modulus (MPa) of the flat region

[0051] t: Thickness of the plate in the flat section (mm)

[0052] ν: Poisson's ratio for flat areas.

[0053] Furthermore, in steel plates, the Young's modulus and Poisson's ratio of the aforementioned flat portions can be obtained using only general physical property values. Moreover, by replacing the yield stress of the flat portions with the Vickers hardness at the center of the plate thickness, the effective width W can be increased. e It can also be based on W e Find the expression = 577t / √h.

[0054] here,

[0055] t: Thickness of the plate in the flat section (mm)

[0056] h: Vickers hardness (Hv) at the center of the plate thickness in the flat section.

[0057] The effective width W is difficult to determine using equation (1). e In the case of [condition], the above formula can be used to find the answer.

[0058] "Effective width ratio" is the width W of the flat portion relative to the effective width W0. e The ratio is determined by W / W e The calculated value. It can be said that the smaller the effective width ratio, the less likely the cross-sectional shape is to undergo elastic buckling.

[0059] "Reference flat section" refers to the flat section with the largest effective width ratio among the flat sections in a closed section at any position along the length direction.

[0060] The “surface portion” refers to the area between a depth position where the distance from the surface of the steel plate in the thickness direction is 1% of the thickness of the steel plate and a depth position where the distance from the surface of the steel plate in the thickness direction is 5% of the thickness of the steel plate.

[0061] "Center of plate thickness" refers to the depth position at a distance of 3 / 8 of the plate thickness from the surface of the steel plate in the thickness direction.

[0062] 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 applied, 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.

[0063] "Energy absorption" is calculated based on the relationship between the impact reaction force (load) and the stroke when the skeleton component wrinkles and deforms. The impact reaction force (load) and stroke are as follows: Figure 1 As shown, the skeleton components can be configured with the length direction as the vertical direction, and the rigid flat impactor is made to collide from the upper side towards the direction of the hollow arrow while the lower side is fully constrained, thus obtaining the desired result.

[0064] "Energy absorption efficiency" is the amount of energy absorbed per cross-sectional area (plate thickness × section length) of a skeleton component. In the case where the skeleton component does not have a uniform cross-section along its length, it is the amount of energy absorbed per unit cross-sectional area (plate thickness × section length) of the closed cross-section perpendicular to the component's length direction, where the cross-sectional area (plate thickness × section length) is minimized.

[0065] Figure 2 This is a perspective view of the skeleton component 10. The skeleton component 10 is a hollow cylindrical component extending in the longitudinal direction.

[0066] Figure 3 yes Figure 2 A cross-sectional view at the cut line A1-A1. As shown... Figure 3 As shown, the skeleton component 10 is formed by four flat portions 11 and four corner portions C, creating a generally rectangular closed cross-section.

[0067] Specifically, the closed section portion includes a first flat portion 11a, a second flat portion 11b connected to the first flat portion 11a via a corner portion C, a third flat portion 11c connected to the second flat portion 11b via a corner portion C, and a fourth flat portion 11d connected to the third flat portion 11c via a corner portion C. The fourth flat portion 11d is connected to the first flat portion via a corner portion C, thereby forming a closed section portion.

[0068] All four corner sections C have the same radius of curvature r. For example, if the maximum external dimension is 140 mm, the radius of curvature r only needs to be less than 140 mm. The radii of curvature of the four corner sections C do not need to be the same and can be different from each other. There is no specific upper limit for the radius of curvature, but sections with a radius of curvature larger than the maximum external dimension of the cross-section are not considered corner sections, but rather as separate flat sections or part of adjacent flat sections. Therefore, the upper limit for the radius of curvature of a corner section C can essentially be said to be "smaller than the maximum external dimension of the cross-section".

[0069] In this application, the reference flat portion is defined as the flat portion with the largest effective width ratio among the flat portions in the closed section.

[0070] The first flat region 11a, the second flat region 11b, the third flat region 11c, and the fourth flat region 11d all have the same yield stress σ. y Young's modulus E, plate thickness t, and Poisson's ratio ν.

[0071] Therefore, for each flat part 11, the width W / effective width W e The calculated effective width ratio depends only on the width W of each flat part 11.

[0072] Therefore, in this embodiment, the first flat portion 11a and the third flat portion 11c, which have the largest width W in the closed section, are set as reference flat portions.

[0073] In the reference flat region, when the skeleton member 10 is subjected to an axial compressive force, elastic buckling is most likely to occur in the initial stage of deformation. Therefore, if the width W of the reference flat region... S If the width is too large, the required tolerance cannot be obtained, making it difficult to achieve excellent energy absorption efficiency. Therefore, the width W of the reference flat section... S The upper limit is set to the effective width W. e Less than 2.0 times.

[0074] In addition, the width W of the reference flat area S The lower limit is not specifically set, but if the width W of the reference flat area is... S If it is too small, the area of ​​the closed section of the skeleton component 10 will decrease, making it difficult to ensure the strength.

[0075] Therefore, the width W of the reference flat portion S The preferred effective width W e More than 0.1 times.

[0076] From a lightweight perspective, the thickness of the plate in the reference flat section is preferably less than 4.2 mm.

[0077] On the other hand, when the plate thickness of the reference flat section is less than 0.4 mm, elastic buckling is prone to occur at the reference flat section, so the width W of the reference flat section is... S The limitations on the setting range become greater. Therefore, the thickness of the plate in the reference flat area is preferably 0.4 mm or more.

[0078] The skeleton member 10 is formed by stamping a cold-rolled steel sheet with a tensile strength of 980 MPa or more into a specified shape, and then joining the end faces. The skeleton member 10 thus formed has a tensile strength of 980 MPa or more. Furthermore, by this formation, in a hardness test performed using 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 reference flat portion of the skeleton member 10 is 300 Hv or more.

[0079] In this application, since the deformation capacity is improved with the premise of high strength and excellent energy absorption efficiency is achieved, the hardness of the center of the plate thickness in the reference flat part is specified to be 300Hv or higher.

[0080] There is no specific upper limit for the hardness of the center of the plate thickness; the Vickers hardness can be below 900 Hv.

[0081] The method for measuring the hardness of the center of the plate thickness is as follows.

[0082] A sample with a cross-section perpendicular to the plate surface is taken from the skeleton component, and this cross-section is prepared as a measuring surface, which is then used for hardness testing.

[0083] The size of the measuring surface also depends on the measuring device, but it can also be around 10mm × 10mm.

[0084] The preparation method of the measuring surface was performed according to JIS Z 2244:2009. After grinding the measuring surface with silicon carbide paper of #600 to #1500, the measuring surface was polished to a mirror finish using a liquid made by dispersing diamond powder with a particle size of 1 μm to 6 μm in a diluted solution of alcohol or pure water. The hardness test was performed according to the method described in JIS Z 2244:2009. Using a miniature Vickers hardness tester, 30 points were measured at 3 / 8 of the thickness of the sample at intervals of at least 3 times the indentation length under a load of 300 gf, and the average value of these measurements was taken as the hardness of the center of the sample thickness.

[0085] As mentioned above, the width W in the reference flat area S It is the effective width W eAt strengths less than 2.0 times, elastic buckling can be suppressed. However, in high-strength components, such as cold-rolled steel sheets with tensile strengths above 980 MPa, even by controlling the effective width W... e It can suppress elastic buckling, but if the bending performance is insufficient, the axial load will cause fracture in the middle of the wrinkling deformation, thus failing to achieve good energy absorption efficiency.

[0086] If it is the existing technology, the standard deviation of the hardness frequency distribution in the center of the plate thickness of the reference flat part is roughly the same as the standard deviation of the hardness frequency distribution in the surface part, and the ratio of the hardness standard deviation is 1.0.

[0087] However, in the skeleton component 10 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 plate thickness of the reference flat portion to the standard deviation of the hardness frequency distribution on the surface portion.

[0088] Therefore, even when using high-strength components, it is possible to suppress breakage during the wrinkling deformation process, and it can significantly improve energy absorption efficiency compared with existing technologies.

[0089] Specifically, for the skeleton component 10 of this embodiment, control is performed so that the standard deviation of the hardness frequency distribution of the surface layer divided by the standard deviation of the hardness frequency distribution of the center layer of the plate thickness in the reference flat area, i.e., the hardness standard deviation ratio, is greater than 1.0.

[0090] The inventors discovered through experiments that when using cold-rolled steel sheets with a tensile strength of 980 MPa or higher, 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 when the standard deviation of hardness is greater than 1.0.

[0091] Figure 4 This is a graph showing the results of VDA bending tests using cold-rolled steel sheets of 1470MPa, 1180MPa, and 980MPa grades with a thickness of 1.6mm. It shows that, for steel sheets of each strength grade, compared to steel sheets with a hardness standard deviation ratio of 1.0 in the prior art, steel sheets with a hardness standard deviation ratio greater than 1.0 exhibit a higher maximum bending angle (°) and a higher VDA angle ratio in the VDA bending test. That is, when the hardness standard deviation ratio is greater than 1.0, fracture due to axial load during wrinkling deformation is less likely to occur, thus exhibiting excellent energy absorption efficiency.

[0092] Therefore, the preferred hardness standard deviation is greater than 1.05, and more preferably greater than 1.20.

[0093] Even if the hardness standard deviation ratio is greater than 3.0, the effect of improving flexibility will saturate. Therefore, a hardness standard deviation ratio of 3.0 or less is preferred.

[0094] Here, the hardness frequency distribution at the center of the plate thickness and the hardness frequency distribution at the surface are obtained through Vickers hardness testing.

[0095] A sample with a cross-section perpendicular to the plate surface is taken from the skeleton component, and this cross-section is prepared as the measuring surface and used for hardness testing.

[0096] The size of the measuring surface also depends on the measuring device, but it can also be around 10mm × 10mm.

[0097] The method for preparing the measuring surface is in accordance with JIS Z 2244:2009. After grinding the measuring surface with silicon carbide paper of #600 to #1500, the measuring surface is finished to a mirror finish using a liquid made by dispersing diamond powder with a particle size of 1μm to 6μm in a diluent such as alcohol or pure water.

[0098] For such a mirror-finished measuring surface, a hardness test is performed using the method described in JIS Z 2244:2009.

[0099] The surface hardness was measured using a miniature Vickers hardness tester.

[0100] Under a load of 300 gf, 30 points were measured at intervals more than three times the indentation to determine the hardness frequency distribution of the surface layer.

[0101] Similarly, at a depth of 3 / 8 of the plate thickness, under a load of 300gf, 30 points were measured at intervals more than 3 times the indentation to determine the hardness frequency distribution at the center of the plate thickness.

[0102] Furthermore, in order to determine the standard deviation of the hardness frequency distributions at the center and surface of the plate obtained from the Vickers hardness test results mentioned above, known statistical methods were used.

[0103] In the prior art, when the metal structure of the center and surface of a cold-rolled steel sheet with a tensile strength of 980 MPa or above is the same, the hardness frequency distribution in the surface is the same as that in the center of the sheet, and the hardness standard deviation ratio is 1.0.

[0104] On the other hand, when only the surface layer and the surrounding metal structure are modified, the standard deviation of hardness is a value different from 1.0.

[0105] For the skeleton component 10 formed from cold-rolled steel sheet with a tensile strength of 980 MPa or more in this embodiment, by modifying only the metal structure of the surface layer and its vicinity, the metal structure of the surface layer becomes a structure close to that of a two-phase structure. Therefore, the hardness distribution deviation in the surface layer becomes larger, and the ratio of the hardness standard deviation between the surface layer and the center of the sheet thickness is greater than 1.0.

[0106] Specifically, the hardness standard deviation ratio can be controlled by adjusting the maximum heating temperature and holding time during decarburization annealing of the steel sheet, which is a known technique. The preferred decarburization annealing conditions are a humid gas environment containing hydrogen, nitrogen, or oxygen, with the decarburization annealing temperature (the maximum temperature reached by the steel sheet) at 700–950°C and the holding time in the temperature range of 700–950°C at 5–1200 seconds.

[0107] Furthermore, by setting the annealing temperature to a higher temperature range and reducing the dwell temperature to a longer time range within this condition, it is possible to achieve a hardness standard deviation ratio greater than 1.20.

[0108] Furthermore, the above-mentioned condition for the hardness standard deviation ratio only needs to be satisfied by the surface portion of at least one of the skeleton member 10. However, it is preferable that the surface portions of both sides of the skeleton member 10 satisfy the above-mentioned condition for the hardness standard deviation ratio.

[0109] Thus, according to the skeleton member 10 of this embodiment, by adjusting the width W of the reference flat portion at the reference flat portion... S By controlling the process, elastic buckling can be suppressed, and fracture during wrinkling deformation can be suppressed by controlling the standard deviation ratio of hardness.

[0110] Therefore, it is possible to achieve a Vickers hardness of over 300 Hv in the center of the plate thickness in the reference flat area, while significantly improving energy absorption efficiency.

[0111] 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.

[0112] Obviously, anyone with ordinary knowledge of the technical field to which this invention pertains can conceive of various modifications or alterations within the scope of the technical concept of this application, and should understand that these modifications or alterations naturally fall within the technical scope of this invention.

[0113] For example, the skeleton component 10 described above may be composed of a single component, but it may also be composed of multiple components. Figure 5 This is a perspective view showing the skeleton component 20 of the modified example. Figure 6 yes Figure 5 A sectional view at the cut line A2-A2.

[0114] The skeleton component 20 includes a first skeleton component 20A extending in the longitudinal direction and a second skeleton component 20B extending in the longitudinal direction and engaging with the first skeleton component 20A. Furthermore, a closed section is formed by the first skeleton component 20A and the second skeleton component 20B.

[0115] The first frame component 20A is a component with an open cross-section that is approximately hat-shaped and perpendicular to the length direction, formed by cold pressing a steel plate with a thickness of 1.2 mm.

[0116] like Figure 6 As shown, the cross-section perpendicular to the length direction of the first skeleton component 20A has five flat portions 21 and four corner portions C.

[0117] Specifically, the cross-section of the first skeleton member 20A perpendicular to the length direction includes a first flat portion 21a, a second flat portion 21b connected to the first flat portion 21a via a corner portion C, a third flat portion 21c connected to the second flat portion 21b via a corner portion C, a fourth flat portion 21d connected to the third flat portion 21c via a corner portion C, and a fifth flat portion 21e connected to the fourth flat portion 21d via a corner portion C.

[0118] The second frame component 20B is a component with an open cross-section that is approximately hat-shaped and perpendicular to the length direction, formed by cold pressing a steel plate with a thickness of 0.8 mm.

[0119] like Figure 6 As shown, the cross-section of the second skeleton component 20B perpendicular to the length direction has five flat portions 23 and four corner portions C.

[0120] Specifically, the cross-section of the second frame member 20B perpendicular to the length direction includes a first flat portion 23a, a second flat portion 23b connected to the first flat portion 23a via a corner portion C, a third flat portion 23c connected to the second flat portion 23b via a corner portion C, a fourth flat portion 23d connected to the third flat portion 23c via a corner portion C, and a fifth flat portion 23e connected to the fourth flat portion 23d via a corner portion C.

[0121] Furthermore, the first flat portion 21a and the fifth flat portion 21e of the first skeleton component 20A and the first flat portion 23a and the fifth flat portion 23e of the second skeleton component 20B are respectively joined by spot welding.

[0122] With this configuration, the skeleton component 20 has a closed section in the cross-section perpendicular to the length direction.

[0123] In this application, the reference flat portion is defined as the flat portion with the largest effective width ratio among the flat portions of the closed section.

[0124] The flat portion 21 of the first skeleton component 20A and the flat portion 23 of the second skeleton component 20B both have the same yield stress σ. y Young's modulus E and Poisson's ratio ν. Therefore, for each flat part 21, 23, the width W / effective width W is used. e The calculated effective width ratio depends on the width W and plate thickness t of each flat section 21, 23.

[0125] In this closed-section portion, the third flat portion 21c of the first frame member 20A and the third flat portion 23c of the second frame member 20B are both flat portions with the largest width among all flat portions. However, since the plate thickness of the third flat portion 23c of the second frame member 20B is smaller than that of the third flat portion 21c of the first frame member 20A, the effective width ratio of the third flat portion 23c of the second frame member 20B is the largest. Therefore, the third flat portion 23c of the second frame member 20B is the reference flat portion.

[0126] Therefore, in the skeleton member 20 of the relevant modified example, for the third flat portion 23c of the second skeleton member 20B, which serves as the reference flat portion, the Vickers hardness at the center of the plate thickness is controlled to be 300 Hv or higher, and the width W is controlled to be... s Controlled to effective width W e The standard deviation ratio is controlled to be greater than 1.0, which is less than 2.0 times the standard deviation ratio, thereby achieving excellent energy absorption efficiency.

[0127] In addition, the skeleton component 10 has a generally rectangular cross-sectional shape with opposite sides having the same width, but the four flat parts 11 may also have a generally square cross-sectional shape with the same width.

[0128] Furthermore, the number of flat portions 11 is not particularly limited, as long as there is at least one.

[0129] Furthermore, the skeleton member 10 of the relevant embodiment has a cross-sectional shape that is uniform across its entire length. However, it may not have a cross-sectional shape that is uniform across its entire length. Instead, the closed section with the smallest cross-sectional area (plate thickness × section line length) among the closed sections perpendicular to the length direction of the member can be the aforementioned closed section portion, as long as it exists in a portion of the entire length direction. However, it is preferable that the aforementioned closed section portion exists in 50% or more of the entire length direction, and more preferably 80% or more.

[0130] In addition, the frame components 10 and 20 are used in the structural components of the automobile body, in the components that are expected to be subjected to compression mainly in the axial direction during a collision. Figure 7 This is a diagram showing an automobile frame 100 as an example of the application of frame components 10 and 20.

[0131] Refer to this Figure 7 The frame components 10 and 20 can be used in the structural components of an automobile body, including the front side member 101, rear side member 103, side sill 105, A-pillar 107, B-pillar 109, roof rail 111, floor cross 113, roof cross 115, and under reinforcement 117.

[0132] (Example)

[0133] Prepare 1470MPa grade cold-rolled steel plates (plate A and plate B) with a thickness of 1.6mm, 1180MPa grade cold-rolled steel plates (plate C) with a thickness of 1.6mm, and 980MPa grade cold-rolled steel plates (plate D) with a thickness of 1.6mm.

[0134] For steel plates B, C, and D, during decarburization annealing, in a humid gas environment obtained by mixing hydrogen and nitrogen, the decarburization annealing temperature (the highest temperature reached by the steel plate) is set to 700–900°C, and the residence time in the temperature range of 700–900°C is set to 60–600 seconds, thereby modifying only the surface layer and the metal structure in the vicinity.

[0135] By cold-pressing these steel plates A, B, C, and D into shape and welding their end faces together, a 300mm high square tube component is obtained, consisting of the individual steel plates.

[0136] Since steel plate A has the same microstructure in its center and surface regions, the standard deviation of the hardness frequency distribution in the center of the reference flat region is equal to the standard deviation of the hardness frequency distribution in the surface region of the reference flat region, with a hardness standard deviation ratio of 1.0. On the other hand, steel plates B, C, and D modify the microstructure of their surface regions without modifying the microstructure in the center of the thickness, thus changing the hardness frequency distribution in the surface regions and adjusting their standard deviation. Therefore, the hardness standard deviation ratio of the surface region relative to the center of the thickness in the reference flat region of steel plate B is 2.37, the ratio for steel plate C is 1.25, and the ratio for steel plate D is 1.28.

[0137] Table 1 shows the material properties of the flat parts after stamping.

[0138] [Table 1]

[0139]

[0140] like Figure 8 As shown, the cross-section perpendicular to the length of the square tube component is a roughly square cross-section design with four flat sections of equal width. That is, in each square tube component, all four flat sections are reference flat sections with the largest effective width-to-width ratio. Based on this condition, the width W of the reference flat section is set for each experimental example. S In addition, the radius of curvature of the four corners C is designed to be 5mm.

[0141] For these cylindrical components, with the lower end fully constrained, a rigid flat impactor was used to collide with them from the upper end at a speed of 90 km / h. The absorbed energy was calculated and compared based on the deformation state, fracture occurrence, impact reaction force (load), and stroke. The setup conditions and results for each experimental example are shown in Table 2.

[0142] [Table 2]

[0143]

[0144] In the 1470MPa grade cold-rolled steel sheets (sheets A and B), for experiments No. 1A, 2A, and 3A, the standard deviation of hardness is 1.0, resulting in poor bending properties. Fracture occurs at the fold during the wrinkling process, preventing further deformation. Consequently, energy absorption efficiency is poor. Furthermore, for experiments No. 4A, 4B, 5A, and 5B, although fracture does not occur during wrinkling, the relatively high effective width leads to elastic buckling at the flat section, preventing the application of axial force and resulting in poor energy absorption efficiency.

[0145] For experiments No.1B, 2B, and 3B, due to the appropriate control of the standard deviation ratio of hardness and the appropriate effective width ratio, even when using 1470MPa grade cold-rolled steel sheets, fracture and elastic buckling do not occur during the wrinkling deformation, and excellent energy absorption efficiency can be achieved.

[0146] in addition, Figure 9 This is a graph comparing the energy absorption efficiency with respect to the effective width ratio based on the experimental results shown in Table 2. As the graph shows, simply reducing the effective width ratio does not result in an improvement in energy absorption efficiency. However, when the hardness standard deviation ratio is appropriately controlled as in this application, it is evident that reducing the effective width ratio significantly improves the energy absorption efficiency.

[0147] Furthermore, in Experiment No. 2C (1180MPa grade cold-rolled steel sheet) and Experiment No. 2D (980MPa grade cold-rolled steel sheet), the hardness standard deviation ratio was appropriately controlled, and the effective width ratio was also appropriate. In these Experiments No. 2C and No. 2D, the strength grade was lower compared to Experiment No. 2B (1470MPa grade cold-rolled steel sheet), so although the energy absorption efficiency was poor, no fracture or elastic buckling occurred. Therefore, compared to the comparative example No. 2A (1470MPa grade cold-rolled steel sheet), where the hardness standard deviation ratio was not properly controlled, a higher energy absorption efficiency was observed despite the lower strength grade.

[0148] Industrial applicability

[0149] According to the present invention, a skeletal component with excellent energy absorption efficiency can be provided.

[0150] Label Explanation

[0151] 10, 20 frame components; 20A first frame component; 20B second frame component; 100 automobile frame.

Claims

1. A skeleton component, formed by cold pressing a steel plate, characterized in that, The aforementioned skeleton component has a closed-section portion with a cross-section perpendicular to the length direction. The aforementioned closed-section portion has at least one flat portion, which is a part whose radius of curvature is larger than the maximum external dimension of the section. When the flat region with the largest width relative to the effective width calculated according to the Karman effective width formula is defined as the reference flat region, The Vickers hardness at the center of the plate thickness in the aforementioned flat section is above 300 Hv. The width of the aforementioned reference flat portion is less than 2.0 times the aforementioned effective width. The standard deviation obtained by dividing the standard deviation of the hardness frequency distribution of the surface layer of the aforementioned reference flat region by the standard deviation of the hardness frequency distribution of the center layer of the plate thickness of the aforementioned reference flat region is greater than 1.

0.

2. The skeleton component as described in claim 1, characterized in that, The aforementioned closed 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 as described in claim 1 or 2, characterized in that, The aforementioned skeleton component includes a first skeleton component extending in the aforementioned length direction, and a second skeleton component extending in the aforementioned length direction and engaging with the aforementioned first skeleton component. The aforementioned closed section includes the aforementioned first skeleton component and the aforementioned second skeleton component.

4. The skeleton component as described in claim 1 or 2, characterized in that, The above standard deviation ratio is greater than 1.20.

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