Framework member

Abstract

The skeleton member (100) is provided with a closed cross-section part (10) and a stress concentration part (20). The closed cross-sectional portion (10) includes flat portions (41, 42, 311, 312) and ridge portions (313, 314, 32, 33, 43, 44, 45, 46). The flat portions (41, 42, 311, 312) have a width (W1, W2, W3, W4) of 70.0 mm or less. The stress concentration section (20) is provided on at least one of the flat sections (41, 42, 311, 312) and the ridge sections (313, 314, 32, 33, 43, 44, 45, 46). The Vickers hardness of the center portion of the flat portion (41, 42, 311, 312) in the plate thickness direction is 350 Hv or more. The standard deviation ratio obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer part of the flat part (41, 42, 311, 312) by the standard deviation of the frequency distribution of the Vickers hardness of the center part of the flat part (41, 42, 311, 312) in the plate thickness direction is greater than 1.0.

Description

Technical Field

[0001] The present disclosure relates to a bone structure member, and more particularly, to a bone structure member formed using a steel plate. Background Art

[0002] For example, a bone structure member formed using a steel plate is used for an automobile body or the like. Among bone structure members, there are bone structure members that deform upon impact and absorb energy.

[0003] For example, Patent Document 1 discloses an automotive side frame that bends due to an impact load and absorbs the load. The side frame of Patent Document 1 includes a plurality of ultra-high strength portions and a plurality of high strength portions. The ultra-high strength portions are arranged at intervals in the length direction of the side frame and each has a tensile strength exceeding 1400 MPa. The high strength portions are arranged between the ultra-high strength portions so as to connect the ultra-high strength portions to each other and each has a tensile strength of 500 MPa to 1000 MPa. A stress concentration portion is provided in each high strength portion. The plate thickness of each high strength portion is larger than the plate thickness of each ultra-high strength portion. According to Patent Document 1, when an impact load is input in the length direction (axial direction) of the side frame, stress is concentrated on the stress concentration portion, and the stress concentration portion becomes the starting point for bending the side frame. Further, according to Patent Document 1, by ensuring a relatively large plate thickness for the high strength portions, it is possible to ensure the elongation amount when bending the high strength portions and suppress the generation of cracks at the high strength portions.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2017-001601 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] As described in Patent Document 1 as well, for bone structure members used for an automobile body or the like, weight reduction is required. In order to achieve weight reduction of the bone structure member, the bone structure member is sometimes formed of a high-strength and thin-walled steel plate. However, when a high-strength steel plate is used for the bone structure member, the bone structure member may break during deformation due to the input load. In this case, the desired deformation behavior of the bone structure member cannot be obtained, and the energy absorption performance of the bone structure member deteriorates. As a result, it is difficult to use a high-strength steel plate for the bone structure member, and weight reduction of the bone structure member achieved by thinning the steel plate may become difficult.

[0009] An object of the present disclosure is to provide a lightweight bone structure member capable of exhibiting good energy absorption performance.

[0010] Means for Solving the Problems

[0011] The bone structure member of the present disclosure is formed using a steel plate. The bone structure member includes a closed cross-sectional portion and a stress concentration portion. The closed cross-sectional portion is a portion of the bone structure member where the cross-section perpendicular to the axial direction of the bone structure member is a closed cross-section. The closed cross-sectional portion includes a flat portion and a ridge line portion. The flat portion is linear when observed in a cross-section perpendicular to the axial direction of the bone structure member and has a width of 70.0 mm or less. The ridge line portion is continuous with the flat portion. The ridge line portion is curved when observed in a cross-section perpendicular to the axial direction of the bone structure member. The stress concentration portion is provided in at least one of the flat portion and the ridge line portion. The Vickers hardness at the central portion in the plate thickness direction of the flat portion is 350 Hv or more. The standard deviation ratio obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portion by the standard deviation of the frequency distribution of the Vickers hardness of the central portion in the plate thickness direction of the flat portion is greater than 1.0.

[0012] Effects of the Invention

[0013] The bone structure member of the present disclosure can exhibit good energy absorption performance. In addition, according to the present disclosure, the bone structure member can be made lightweight. Description of the Drawings

[0014] Figure 1 is a perspective view of the bone structure member of the first embodiment.

[0015] Figure 2 is Figure 1 a cross-sectional view of the bone structure member shown.

[0016] Figure 3 is a perspective view of the bone structure member of the second embodiment.

[0017] Figure 4 is Figure 3 a cross-sectional view of the bone structure member shown.

[0018] Figure 5 is a perspective view of the bone structure member of the third embodiment.

[0019] Figure 6 is Figure 5 a cross-sectional view of the bone structure member shown.

[0020] Figure 7 is a cross-sectional view of the bone structure member of the fourth embodiment.

[0021] Figure 8 is Figure 7 a perspective view of the bone structure member shown.

[0022] Figure 9 is a graph showing the results of the VDA bending test for cold-rolled steel sheets with a tensile strength of 1180 MPa grade.

[0023] Figure 10 It is a graph showing the results of the VDA bending test for cold-rolled steel sheets with a tensile strength of 1470 MPa.

[0024] Figure 11 It is a schematic diagram of the model used in CAE analysis.

[0025] Figure 12 It is a graph showing the results of CAE analysis. Detailed implementation mode

[0026] For example, in the bone structure members used in automobiles and the like, there are bone structure members that absorb energy by bending deformation when a compressive load in the axial direction is input. In order to ensure the energy absorption performance of the bone structure members, it is necessary to bend and deform the bone structure members at the target position when a compressive load is input. In addition, by compactly folding the bone structure members, the decrease in the load (yield strength) after bending deformation can be suppressed, and the energy absorption performance of the bone structure members can be improved.

[0027] On the other hand, in order to lighten the weight of the bone structure members, it is conceivable to use high-strength steel sheets when forming the bone structure members and make the bone structure members thin-walled. However, when a compressive load in the axial direction is input to the bone structure members and the bone structure members are compactly folded, deformation concentration occurs at the bending portions of the bone structure members. Therefore, depending on the fracture resistance characteristics of the high-strength steel sheets, there is a possibility that the bone structure members break during deformation and the desired deformation behavior cannot be obtained for the bone structure members. As a result, there are cases where thin-walled high-strength steel sheets cannot be used for the bone structure members and it is difficult to achieve the weight reduction of the bone structure members.

[0028] In order to lighten the weight of the bone structure members using high-strength steel sheets and obtain the desired deformation behavior for the bone structure members, the inventors have intensively studied the shape and material characteristics of the bone structure members. As a result, the inventors have completed the bone structure members of the implementation mode.

[0029] The bone structure members of the implementation mode are formed using steel sheets. The bone structure members include a closed cross-sectional portion and a stress concentration portion. The closed cross-sectional portion is a portion of the bone structure members where the cross-section perpendicular to the axial direction of the bone structure members is a closed cross-section. The closed cross-sectional portion includes a flat portion and a ridge line portion. The flat portion is linear when observed in a cross-section perpendicular to the axial direction of the bone structure members and has a width of 70.0 mm or less. The ridge line portion is continuous with the flat portion. The ridge line portion is curved when observed in a cross-section perpendicular to the axial direction of the bone structure members. The stress concentration portion is provided in at least one of the flat portion and the ridge line portion. The Vickers hardness at the central portion in the plate thickness direction of the flat portion is 350 Hv or more. The standard deviation ratio obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portion by the standard deviation of the frequency distribution of the Vickers hardness at the central portion in the plate thickness direction of the flat portion is greater than 1.0 (first structure).

[0030] According to the research results of the present inventor, in order to bend and deform the bone structure member at a target position and fold it compactly, it is advantageous to reduce the width of the flat portion on the bending compression side in the closed cross-sectional portion of the bone structure member. Therefore, in the bone structure member of the first structure, the width of the flat portion provided in the closed cross-sectional portion is 70.0 mm or less. Thus, when a compressive load is input to the bone structure member in the axial direction, elastic buckling at the flat portion is not likely to occur, and even if the bone structure member is thin-walled, the load (yield strength) of the flat portion can be ensured. Thereby, the bone structure member can be bent starting from a stress concentration portion provided in at least one of the flat portion and the ridge line portion continuous with the flat portion. In addition, by setting the width of the flat portion to 70.0 mm or less, the bone structure member can be folded compactly starting from the stress concentration portion of the flat portion and / or the ridge line portion. More specifically, when a compressive load is input to the bone structure member in the axial direction, the bone structure member bends with the flat portion as the bending compression side and starting from the stress concentration portion, and the flat portion is folded steeply. Therefore, adjacent portions can be brought into contact with each other in the folded flat portion, and the load after bending deformation can be increased. As a result, the bone structure member can exhibit good energy absorption performance.

[0031] In the bone structure member of the first structure, the standard deviation ratio obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portion by the standard deviation of the frequency distribution of the Vickers hardness of the central portion in the plate thickness direction of the flat portion is greater than 1.0. That is, the metallographic structure of the surface layer portion of the flat portion is different from the metallographic structure of the central portion of the flat portion. Thereby, the bending performance of the flat portion can be improved. Thus, when the bone structure member is bent and deformed with the flat portion as the bending inner side, fracture of the flat portion is not likely to occur during the deformation, and the bone structure member can generate a desired bending deformation starting from the stress concentration portion. Therefore, the energy absorption performance of the bone structure member can be improved. In particular, when the width of the flat portion is 70.0 mm or less, the energy absorption efficiency can be improved. In addition, since the flat portion is formed of a high-strength material having a Vickers hardness of 350 Hv or more at the central portion, at least the flat portion can be made thin-walled. Therefore, a lightweight bone structure member can be obtained.

[0032] Alternatively, the stress concentration portion may be a concave portion or a convex portion formed in at least one of the flat portion and the ridge line portion (second structure).

[0033] Alternatively, the flat portion may include a bent portion that is bent when viewed from the ridge line portion side or along the plate thickness direction. In this case, the stress concentration portion is the bent portion (third structure).

[0034] Alternatively, at least one of the flat portion and the ridge portion includes a plurality of high-strength portions and a low-strength portion. The plurality of high-strength portions are arranged at intervals in the axial direction of the skeleton member. The low-strength portion is arranged between the high-strength portions in the axial direction of the skeleton member. The low-strength portion has a lower strength than the high-strength portion. In this case, the stress concentration portion is the low-strength portion (the fourth structure).

[0035] Alternatively, the skeleton member further includes a plurality of reinforcing members. The plurality of reinforcing members are arranged at intervals in the axial direction of the skeleton member. The plurality of reinforcing members are arranged in at least one of the flat portion and the ridge portion. In this case, the high-strength portion is a portion of the skeleton member where the reinforcing member is arranged. The low-strength portion is a portion of the skeleton member located between the reinforcing members in the axial direction thereof (the fifth structure).

[0036] Alternatively, the flat portion includes a plurality of ribs. The plurality of ribs are arranged at intervals in the axial direction of the skeleton member. Each rib extends in the axial direction of the skeleton member. In this case, the high-strength portion is a portion of the flat portion where the ribs are provided. The low-strength portion is a portion of the flat portion located between the ribs in the axial direction of the skeleton member (Sixth Structure).

[0037] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In each of the drawings, the same reference numerals are given to the same or corresponding structures, and the same description will not be repeated.

[0038] <First embodiment>

[0039] Figure 1 It is a perspective view of the framework member 100 according to the first embodiment. Figure 2 yes Figure 1 The cross-sectional view of the skeleton member 100 shown in the figure. The cross section of the skeleton member 100 refers to the cross section when the skeleton member 100 is cut by a plane perpendicular to the axial direction thereof. The skeleton member 100 is used, for example, for the body of an automobile. The skeleton member 100 is, for example, a member that assumes that a collision load (compression load) is input in the axial direction when the automobile collides. The skeleton member 100 may be a member extending in the front-rear direction of the automobile. Although not particularly limited, the skeleton member 100 is, for example, a longitudinal beam of an automobile. The skeleton member 100 may be a front longitudinal beam or a rear longitudinal beam.

[0040] Reference Figure 1 The frame member 100 can be formed using one or more steel plates. The frame member 100 is, for example, a cylindrical member. The frame member 100 includes a closed cross-section portion 10 and a stress concentration portion 20 .

[0041] The closed cross-sectional portion 10 is a portion of the bone structure member 100 that has a closed cross-section when cut by a plane perpendicular to its axis. In the example of the present embodiment, the closed cross-sectional portion 10 is provided over the entire length of the bone structure member 100 in the axial direction. That is, the bone structure member 100 has a cylindrical shape as a whole. However, the closed cross-sectional portion 10 may also be provided locally in the axial direction of the bone structure member 100. The closed cross-sectional portion 10 is preferably provided over 50% or more of the entire length of the bone structure member 100 in the axial direction, and more preferably provided over 80% or more of the entire length.

[0042] The bone structure member 100 includes a first member 30 and a second member 40. The first member 30 and the second member 40 are each formed of a steel plate. The first member 30 and the second member 40 are formed, for example, by cold press forming a cold rolled steel plate of 1180 MPa or more. The first member 30 and the second member 40 may also be formed by cold press forming a cold rolled steel plate of 1470 MPa or more. The bone structure member 100 is a cold press formed product.

[0043] Refer to Figure 2 , in the cross-sectional view of the bone structure member 100, the first member 30 and the second member 40 are included in the closed cross-sectional portion 10.

[0044] The first member 30 substantially has a cap shape in the cross-sectional view of the bone structure member 100. The first member 30 includes a top plate 31, ridge lines 32, 33, side walls 34, 35, and flanges 36, 37.

[0045] The ridge lines 32, 33 are continuously provided on both sides of the top plate 31. In the cross-sectional view of the bone structure member 100, the ridge lines 32, 33 have a curved shape protruding outward from the bone structure member 100. The ridge lines 32, 33 may also have an arc shape in the cross-sectional view of the bone structure member 100. The ridge lines 32, 33 are the corners between the top plate 31 and the side walls 34, 35. One side wall 34 is connected to the top plate 31 via the ridge line 32. The other side wall 35 is connected to the top plate 31 via the ridge line 33. The top plate 31, the ridge lines 32, 33, and the side walls 34, 35 extend over the entire length of the closed cross-sectional portion 10 in the axial direction of the bone structure member 100.

[0046] The flanges 36, 37 are connected to the side walls 34, 35 on the opposite side of the top plate 31. One flange 36 protrudes outward from the side wall 34 to the outside of the bone structure member 100. The other flange 37 protrudes outward from the side wall 35 to the outside of the bone structure member 100. The flanges 36, 37 extend along the side walls 34, 35 in the axial direction of the bone structure member 100.

[0047] In the closed cross-sectional portion 10, the second member 40 is arranged so as to substantially close the openings on the flange 36, 37 sides of the first member 30. The second member 40 includes flat portions 41, 42, ridge line portions 43, 44, 45, 46, groove portion 47, and flanges 48, 49.

[0048] The flat portions 41, 42 are arranged to face the top plate 31 of the first member 30. Ridge line portions 43, 44 are continuously provided on both sides of one flat portion 41. Ridge line portions 45, 46 are continuously provided on both sides of the other flat portion 42. In the cross-sectional view of the bone structure member 100, the ridge line portions 43, 44, 45, 46 have a curved shape protruding outward from the bone structure member 100. The ridge line portions 43, 44, 45, 46 may also have an arc shape in the cross-sectional view of the bone structure member 100. The groove portion 47 is arranged between the flat portions 41, 42. The groove portion 47 is formed in the second member 40 so as to be recessed inward with respect to the flat portions 41, 42 toward the bone structure member 100. The flat portions 41, 42, ridge line portions 43, 44, 45, 46, and groove portion 47 extend over the entire length of the closed cross-sectional portion 10 in the axial direction of the bone structure member 100.

[0049] The flanges 48, 49 face the flanges 36, 37 of the first member 30, respectively. The flanges 48, 49 are joined to the flanges 36, 37 of the first member 30 by welding, for example. One flange 48 is connected to the ridge line portion 43 and protrudes outward from the bone structure member 100. The other flange 49 is connected to the ridge line portion 45 and protrudes outward from the bone structure member 100. The flanges 48, 49 extend in the axial direction of the bone structure member 100 along the ridge line portions 43, 45.

[0050] The flat portions 41, 42 are substantially linear in the cross-sectional view of the bone structure member 100, respectively. More specifically, the radius of curvature of each of the flat portions 41, 42 is larger than the maximum span length of the cross-section of the bone structure member 100. The maximum span length means the length of the straight line connecting any two points on the contour of the closed cross-sectional portion 10 and being the maximum distance between the two points when observed in a cross-section perpendicular to the axial direction of the bone structure member 100. The radius of curvature of each of the ridge line portions 43, 44 continuous with the flat portion 41 and the ridge line portions 45, 46 continuous with the flat portion 42 is equal to or less than the maximum span length of the cross-section of the bone structure member 100.

[0051] The flat portions 41 and 42 have widths W1 and W2 respectively in the cross-sectional view of the bone structure member 100. The width W1 of the flat portion 41 is the line length of the flat portion 41 when observing the bone structure member 100 in cross-section. For example, in the cross-sectional view of the bone structure member 100, the line length along the surface of the bone structure member 100 from the junction of the ridge line portion 43 and the flat portion 41 (the rounded end of the ridge line portion 43) to the junction of the flat portion 41 and the ridge line portion 44 (the rounded end of the ridge line portion 44) can also be set as the width W1 of the flat portion 41. Similarly, the width W2 of the flat portion 42 is the line length of the flat portion 42 when observing the bone structure member 100 in cross-section. For example, in the cross-sectional view of the bone structure member 100, the line length along the surface of the bone structure member 100 from the junction of the ridge line portion 45 and the flat portion 42 (the rounded end of the ridge line portion 45) to the junction of the flat portion 42 and the ridge line portion 46 (the rounded end of the ridge line portion 46) can also be set as the width W2 of the flat portion 42.

[0052] The width W1 is the width of the flat portion 41 measured within a range of 20.0 mm on both sides in the axial direction of the bone structure member 100 from the center of the stress concentration portion 20 ( Figure 1 ). Similarly, the width W2 is the width of the flat portion 42 measured within a range of 20.0 mm on both sides in the axial direction of the bone structure member 100 from the center of the stress concentration portion 20. The width W1 of the flat portion 41 and the width W2 of the flat portion 42 are each 70.0 mm or less. The widths W1 and W2 are preferably 60.0 mm or less. The widths W1 and W2 can also be 40.0 mm or less. The widths W1 and W2 are, for example, 5.0 mm or more. The width W2 of the flat portion 42 can be equal to or different from the width W1 of the flat portion 41.

[0053] The plate thickness of the flat portions 41 and 42 is, for example, 0.7 mm or more and 2.0 mm or less. In the present embodiment, the plate thicknesses of the flat portions 41 and 42 are the same. However, the plate thicknesses of the flat portions 41 and 42 can also be different.

[0054] The Vickers hardness of the central portions in the plate thickness direction of the flat portions 41 and 42 is 350 Hv or more. For example, when the second member 40 is formed by cold press forming a steel plate of 1180 MPa or more, the Vickers hardness of the central portions in the plate thickness direction of the flat portions 41 and 42 becomes 350 Hv or more. The Vickers hardness of the central portions in the plate thickness direction of the flat portions 41 and 42 is preferably 380 Hv or more. Although not particularly limited, the Vickers hardness of the central portions in the plate thickness direction of the flat portions 41 and 42 may also be 650 Hv or less. The central portion in the plate thickness direction of the flat portion 41 refers to the portion in the flat portion 41 where the distance (depth) in the plate thickness direction from the surface is 3 / 8 of the plate thickness. The central portion in the plate thickness direction of the flat portion 42 refers to the portion in the flat portion 42 where the distance (depth) in the plate thickness direction from the surface is 3 / 8 of the plate thickness.

[0055] The standard deviation ratios of the Vickers hardness of the flat portions 41 and 42 are each greater than 1.0. The standard deviation ratio of the Vickers hardness of the flat portion 41 is a value obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portion 41 by the standard deviation of the frequency distribution of the Vickers hardness of the central portion in the plate thickness direction of the flat portion 41. The standard deviation ratio of the Vickers hardness of the flat portion 42 is a value obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portion 42 by the standard deviation of the frequency distribution of the Vickers hardness of the central portion in the plate thickness direction of the flat portion 42. The surface layer portion of the flat portion 41 refers to the portion in the flat portion 41 where the distance (depth) in the plate thickness direction from the surface is 20.0 μm or more and 70.0 μm or less. The surface layer portion of the flat portion 42 refers to the portion in the flat portion 42 where the distance (depth) in the plate thickness direction from the surface is 20.0 μm or more and 70.0 μm or less. The standard deviation ratios of the Vickers hardness of the flat portions 41 and 42 are each preferably greater than 1.1, and more preferably greater than 1.2. As long as at least one of the surface layer portions on both sides in the plate thickness direction of the flat portions 41 and 42 satisfies the condition of such a standard deviation ratio of the Vickers hardness. However, preferably, the surface layer portions on both sides of the flat portions 41 and 42 satisfy the condition of the standard deviation ratio of the Vickers hardness.

[0056] The Vickers hardness at the central portion in the plate thickness direction of the pars planae 41 and 42 and the standard deviation of its frequency distribution can be measured by the Vickers hardness test specified in JIS Z 2244:2009. Specifically, the Vickers hardness at the central portion in the plate thickness direction of the pars planae 41 and 42 and the standard deviation of its frequency distribution are measured as follows. First, specimens having a cross-section perpendicular to the plate surface and including the pars plana 41 are collected from the bone structure member 100, and this cross-section is adjusted to a test surface in accordance with JIS Z 2244:2009. Similarly, specimens having a cross-section perpendicular to the plate surface and including the pars plana 42 are collected from the bone structure member 100, and this cross-section is adjusted to a test surface in accordance with JIS Z 2244:2009. More specifically, these test surfaces are polished with silicon carbide paper of #600 to #1500, and then, the test surfaces are finished to a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1.0 μm to 6.0 μm in a diluent such as ethanol or pure water. The size of the test surfaces of the pars planae 41 and 42 can be set to, for example, 10.0 mm × plate thickness. Then, for each test surface, using a micro Vickers hardness tester, the test force is set to 300 gf (2.9 N), and 30 points of Vickers hardness are measured at intervals of 3.0 times or more the indentation at a depth of 3 / 8 of the plate thickness, obtaining the frequency distribution of the Vickers hardness. The average value and standard deviation of the Vickers hardness are calculated from the obtained frequency distribution. The calculated average value is the Vickers hardness at the central portion in the plate thickness direction of the pars planae 41 and 42, and the calculated standard deviation is the standard deviation of the frequency distribution of the Vickers hardness at the central portion in the plate thickness direction of the pars planae 41 and 42.

[0057] Similarly, the standard deviation of the frequency distribution of the Vickers hardness of the surface layers of the pars planae 41 and 42 can be measured by the Vickers hardness test specified in JIS Z2244:2009. Specifically, the standard deviation of the frequency distribution of the Vickers hardness of the surface layers of the pars planae 41 and 42 is measured as follows. First, specimens having a cross-section perpendicular to the plate surface and including the pars plana 41 are collected from the bone structure member 100, and this cross-section is adjusted to a test surface in accordance with JIS Z 2244:2009. In addition, specimens having a cross-section perpendicular to the plate surface and including the pars plana 42 are collected from the bone structure member 100, and this cross-section is adjusted to a test surface in accordance with JIS Z 2244:2009. More specifically, these test surfaces are polished with silicon carbide paper of #600 to #1500, and then, the test surfaces are finished to a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1.0 μm to 6.0 μm in a diluent such as ethanol or pure water. The size of the test surfaces of the pars planae 41 and 42 can be set to, for example, 10.0 mm × plate thickness. Then, for each test surface, using a micro Vickers hardness tester, the test force is set to 300 gf (2.9 N), and 30 points of Vickers hardness are measured at arbitrary depth positions not less than 20.0 μm and not more than 70.0 μm from the surface at intervals of not less than 3.0 times the indentation to obtain the frequency distribution of the Vickers hardness. The standard deviation of the Vickers hardness is calculated from the obtained frequency distribution. The calculated standard deviation is the standard deviation of the frequency distribution of the Vickers hardness of the surface layers of the pars planae 41 and 42.

[0058] In the flats 41 and 42, when the metallographic structure is the same in the central part and the surface part in the plate thickness direction, the frequency distribution of the Vickers hardness in the surface part is the same as the frequency distribution of the Vickers hardness in the central part in the plate thickness direction, and the standard deviation ratio of the Vickers hardness of each of the flats 41 and 42 becomes 1.0. On the other hand, when the metallographic structure in the surface part and its vicinity in the flats 41 and 42 is modified, the standard deviation ratio of the Vickers hardness of each of the flats 41 and 42 becomes a value other than 1.0. In the present embodiment, for example, with respect to the second member 40 formed by cold press forming of a steel plate, the metallographic structure in the surface part and its vicinity of the steel plate is modified, whereby the metallographic structure in the surface part becomes a structure close to a duplex structure. Therefore, the deviation of the Vickers hardness at the surface part of the steel plate becomes large, and the standard deviation ratio of the Vickers hardness between the central part and the surface part in the plate thickness direction can exceed 1.0 in the flats 41 and 42. The standard deviation ratio can be controlled, for example, by adjusting the maximum heating temperature and the holding time (residence time) during the decarburizing annealing of the steel plate. The conditions for the decarburizing annealing are preferably as follows: in a humid atmosphere containing hydrogen, nitrogen, or oxygen, the decarburizing annealing temperature (the maximum temperature reached by the steel plate) is 700°C to 950°C, and the residence time in the temperature range of 700°C to 950°C is 5 seconds to 1200 seconds. If the annealing temperature is higher and the residence time is longer within this condition range, the standard deviation ratio can be made greater than 1.2.

[0059] Return to Figure 1 , a stress concentration portion 20 is provided at the ridge line portion 43 continuous with the flat portion 41. A stress concentration portion 20 is also provided at the ridge line portion 45 continuous with the flat portion 42. These stress concentration portions 20 are configured to generate stress concentration when a compressive load is input to the bone structure member 100 in the axial direction. The stress concentration portion 20 is configured such that when the bone structure member 100 is subjected to a compressive load in the axial direction, starting from the stress concentration portion 20, the bone structure member 100 bends with the flat portions 41 and 42 as the bending inner sides.

[0060] In the present embodiment, the stress concentration portion 20 corresponding to the flat portion 41 is a recess formed in the ridge line portion 43 on the flange 48 side. The recess as the stress concentration portion 20 may also be formed in the ridge line portion 44 on the groove portion 47 side. The concave stress concentration portion 20 is formed in at least one of the ridge line portions 43 and 44 continuous with the flat portion 41. A plurality of stress concentration portions 20 arranged in the axial direction of the bone structure member 100 may be provided in one of the ridge line portions 43 and 44 or in both of the ridge line portions 43 and 44.

[0061] In the present embodiment, the stress concentration portion 20 corresponding to the flat portion 42 is a concave portion formed in the ridge portion 45 on the flange 49 side. The concave portion as the stress concentration portion 20 may also be formed in the ridge portion 46 on the groove portion 47 side. The concave stress concentration portion 20 is formed in at least one of the ridge portions 45 and 46 continuous with the flat portion 42. A plurality of stress concentration portions 20 arranged along the axial direction of the bone structure member 100 may be provided in one or both of the ridge portions 45 and 46.

[0062] [Effect]

[0063] In the bone structure member 100 of the present embodiment, the widths W1 and W2 of the flat portions 41 and 42 provided in the closed cross-sectional portion 10 are set to 70.0 mm or less. The flat portions 41 and 42 have widths W1 and W2 of 70.0 mm or less at least in the range of ±20.0 mm from the center of the stress concentration portion 20 along the axial direction of the bone structure member 100. Thus, when a compressive load is input to the bone structure member 100 in the axial direction, elastic buckling at the flat portions 41 and 42 is not likely to occur at least near the stress concentration portion 20, and the load (yield strength) of the flat portions 41 and 42 can be ensured. In this case, the bone structure member 100 is likely to bend starting from the stress concentration portion 20, and a desired deformation behavior can be easily obtained for the bone structure member 100. In addition, since the widths W1 and W2 of the flat portions 41 and 42 are 70.0 mm or less, the bone structure member 100 can be folded compactly starting from the stress concentration portion 20. More specifically, when a compressive load is input to the bone structure member 100, the bone structure member 100 bends with the flat portions 41 and 42 as the inner sides of the bend, and the flat portions 41 and 42 are folded steeply. Thereby, adjacent portions in each of the folded flat portions 41 and 42 can be brought into contact with each other, and the load after the bending deformation can be increased. As a result, the bone structure member 100 can exhibit good energy absorption performance.

[0064] In the present embodiment, the stress concentration portion 20 is a concave portion formed in the ridge portions 43 and 45 continuous with the flat portions 41 and 42, respectively. For example, when a vehicle using the bone structure member 100 collides and an axial collision load is applied to the bone structure member 100, the bone structure member 100 can bend and deform starting from the concave stress concentration portion 20. The concave stress concentration portion 20 may also extend from the ridge portions 43 and 45 to the flat portions 41 and 42. For example, a concave portion as the stress concentration portion 20 may be provided from the ridge portion 43 on the flange 48 side to the ridge portion 44 on the groove portion 47 side so as to cross the flat portion 41. Similarly, a concave portion as the stress concentration portion 20 may be provided from the ridge portion 45 on the flange 49 side to the ridge portion 46 on the groove portion 47 side so as to cross the flat portion 42. The concave portion as the stress concentration portion 20 may also be provided only in the flat portions 41 and 42.

[0065] The stress concentration part 20 may also be a convex part provided on one or more of the ridge line parts 43, 44, 45, 46 continuous with the flat parts 41, 42. In this case, when a compressive load in the axial direction is input to the bone structure member 100, the bone structure member 100 can be bent and deformed starting from the root of the convex stress concentration part 20. The convex stress concentration part 20 can also extend to the flat parts 41, 42. For example, a convex part as the stress concentration part 20 may be provided from the ridge line part 43 on the flange 48 side to the ridge line part 44 on the groove part 47 side so as to cross the flat part 41. Similarly, a convex part as the stress concentration part 20 may be provided from the ridge line part 45 on the flange 49 side to the ridge line part 46 on the groove part 47 side so as to cross the flat part 42. The convex part as the stress concentration part 20 may also be provided only on the flat parts 41, 42.

[0066] In the present embodiment, the stress concentration part 20 is provided corresponding to both of the flat parts 41, 42. However, the stress concentration part 20 may also be provided corresponding to only one of the flat parts 41, 42. That is, it may be that a concave or convex stress concentration part 20 is provided on one or more of the flat part 41, the ridge line part 43, and the ridge line part 44, while a concave or convex stress concentration part 20 is not provided on the flat part 42, the ridge line part 45, and the ridge line part 46. Similarly, it may be that a concave or convex stress concentration part 20 is provided on one or more of the flat part 42, the ridge line part 45, and the ridge line part 46, while a concave or convex stress concentration part 20 is not provided on the flat part 41, the ridge line part 43, and the ridge line part 44.

[0067] In the bone structure member 100 of the present embodiment, the standard deviation ratio of the frequency distribution of the Vickers hardness between the central portion and the surface layer portion in the plate thickness direction of the flat portions 41 and 42 is greater than 1.0. That is, in the flat portions 41 and 42, the metallographic structure of the surface layer portion is different from the metallographic structure of the central portion in the plate thickness direction. Thereby, the bending performance of the flat portions 41 and 42 can be improved. Thus, when the bone structure member 100 is bent and deformed with the flat portions 41 and 42 as the bending inner sides, it is not easy for the flat portions 41 and 42 to break during the deformation, and the bone structure member 100 can generate a desired bending deformation starting from the stress concentration portion 20. Therefore, the energy absorption performance of the bone structure member 100 can be improved. In particular, in the present embodiment, by setting the widths W1 and W2 of the flat portions 41 and 42 to 70.0 mm or less, elastic buckling at the flat portions 41 and 42 is suppressed, and on the other hand, the bending performance of the flat portions 41 and 42 themselves is improved to suppress fracture, so that the bone structure member 100 can exhibit excellent energy absorption efficiency. In addition, since the second member 40 including the flat portions 41 and 42 is formed of a high-strength material having a Vickers hardness of 350 Hv or more at the central portion, at least the second member 40 can be thinned. Therefore, a lightweight bone structure member 100 can be obtained.

[0068] In the bone structure member 100 of the present embodiment, the second member 40 includes flat portions 41 and 42 having widths W1 and W2 of 70.0 mm or less and a stress concentration portion 20. A material having a standard deviation ratio of the Vickers hardness between the central portion and the surface layer portion in the plate thickness direction exceeding 1.0 and a Vickers hardness of the central portion in the plate thickness direction of 350 Hv or more is used for the second member 40. On the other hand, since the first member 30 does not include a stress concentration portion 20, it is not necessarily required to be made of the same material as the second member 40. The material of the first member 30 may be the same as or different from the material of the second member 40.

[0069] In the present embodiment, in the second member 40, the bottom plate opposite to the top plate 31 of the first member 30 is divided by a groove portion 47, thereby forming flat portions 41 and 42 having widths W1 and W2 of 70.0 mm or less. However, for example, when the total width of the bottom plate of the second member 40 is 70.0 mm or less, the groove portion 47 may not be provided in the second member 40. On the other hand, when the width of the bottom plate of the second member 40 is larger, a plurality of groove portions 47 can also be provided in the second member 40.

[0070] <Second Embodiment>

[0071] Figure 3 is a perspective view of the bone structure member 100 of the second embodiment. Figure 4 is to Figure 3Cross-sectional view (transverse cross-sectional view) of the bone structure member 100 when cut in a plane perpendicular to its axis.

[0072] As Figure 3 shown, in the present embodiment, the bone structure member 100 also includes a closed cross-sectional portion 10 and a stress concentration portion 20. Similar to the first embodiment, the bone structure member 100 includes a first member 30 and a second member 40. Different from the first embodiment, the stress concentration portion 20 is provided not only on the second member 40 but also on the first member 30.

[0073] Referring Figure 4 , the second member 40 has a structure substantially the same as that of the second member 40 ( Figure 2 ) described in the first embodiment in the transverse cross-sectional view of the bone structure member 100. On the other hand, the first member 30 is mainly different from the first member 30 ( Figure 2 ) described in the first embodiment in the shape of the top plate 31. In the present embodiment, the top plate 31 of the first member 30 includes flat portions 311, 312, ridge line portions 313, 314, and a groove portion 315.

[0074] The flat portions 311, 312 are substantially linear in the transverse cross-sectional view of the bone structure member 100. The curvature radii of the flat portions 311, 312 are the same as those of the flat portions 41, 42 of the second member 40 and are larger than the maximum span length of the cross-section of the bone structure member 100. The ridge line portion 313 is continuously provided with the flat portion 311. The ridge line portion 314 is continuously provided with the flat portion 312. In the transverse cross-sectional view of the bone structure member 100, the ridge line portions 313, 314 have a curved shape protruding outward from the bone structure member 100. The ridge line portions 313, 314 may also have an arc shape in the transverse cross-sectional view of the bone structure member 100. The curvature radii of the ridge line portions 313, 314 are equal to or less than the maximum span length of the cross-section of the bone structure member 100. The groove portion 315 is disposed between the flat portions 311, 312. The groove portion 315 is formed in the first member 30 so as to be recessed inward with respect to the flat portions 311, 312. The flat portions 311, 312, the ridge line portions 313, 314, and the groove portion 47 extend over the entire length of the closed cross-sectional portion 10 in the axial direction of the bone structure member 100.

[0075] The flat portions 311 and 312 have widths W3 and W4 respectively in the cross-sectional view of the bone structure member 100. The width W3 of the flat portion 311 is the line length of the flat portion 311 when observing the bone structure member 100 in cross-section. For example, in the cross-sectional view of the bone structure member 100, the line length along the surface of the bone structure member 100 from the junction of the ridge line portion 32 and the flat portion 311 (the rounded end of the ridge line portion 32) to the junction of the flat portion 311 and the ridge line portion 313 (the rounded end of the ridge line portion 313) can be set as the width W3 of the flat portion 311. Similarly, the width W4 of the flat portion 312 is the line length of the flat portion 312 when observing the bone structure member 100 in cross-section. For example, in the cross-sectional view of the bone structure member 100, the line length along the surface of the bone structure member 100 from the junction of the ridge line portion 33 and the flat portion 312 (the rounded end of the ridge line portion 33) to the junction of the flat portion 312 and the ridge line portion 314 (the rounded end of the ridge line portion 314) can be set as the width W4 of the flat portion 312.

[0076] The width W3 is the width of the flat portion 311 measured within a range of 20.0 mm on both sides in the axial direction of the bone structure member 100 from the center of the stress concentration portion 20 of the first member 30. Similarly, the width W4 is the width of the flat portion 312 measured within a range of 20.0 mm on both sides in the axial direction of the bone structure member 100 from the center of the stress concentration portion 20 of the first member 30. The width W3 of the flat portion 311 and the width W4 of the flat portion 312 are each 70.0 mm or less. The widths W3 and W4 are preferably 60.0 mm or less. The widths W3 and W4 can also be 40.0 mm or less. The widths W3 and W4 are, for example, 5.0 mm or more. The widths W3 and W4 can be the same or different.

[0077] The Vickers hardness of the central portions in the plate thickness direction of the flat portions 311 and 312 is 350 Hv or more. For example, when the first member 30 is formed by cold pressing a steel plate of 1180 MPa or more, the Vickers hardness of the central portions in the plate thickness direction of the flat portions 311 and 312 becomes 350 Hv or more. The Vickers hardness of the central portions in the plate thickness direction of the flat portions 311 and 312 is preferably 380 Hv or more. Although not particularly limited, the Vickers hardness of the central portions in the plate thickness direction of the flat portions 311 and 312 can also be 650 Hv or less. The central portion in the plate thickness direction of the flat portion 311 refers to the portion of the flat portion 311 where the distance (depth) in the plate thickness direction from the surface is 3 / 8 of the plate thickness. The central portion in the plate thickness direction of the flat portion 312 refers to the portion of the flat portion 312 where the distance (depth) in the plate thickness direction from the surface is 3 / 8 of the plate thickness.

[0078] The standard deviation ratios of the Vickers hardness of the flat portions 311 and 312 are each greater than 1.0. The standard deviation ratio of the Vickers hardness of the flat portion 311 is a value obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portion 311 by the standard deviation of the frequency distribution of the Vickers hardness of the central portion in the plate thickness direction of the flat portion 311. The standard deviation ratio of the Vickers hardness of the flat portion 312 is a value obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portion 312 by the standard deviation of the frequency distribution of the Vickers hardness of the central portion in the plate thickness direction of the flat portion 312. The surface layer portion of the flat portion 311 refers to the portion in the flat portion 311 where the distance (depth) in the plate thickness direction from the surface is 20.0 μm or more and 70.0 μm or less. The surface layer portion of the flat portion 312 refers to the portion in the flat portion 312 where the distance (depth) in the plate thickness direction from the surface is 20.0 μm or more and 70.0 μm or less. The standard deviation ratios of the Vickers hardness of the flat portions 311 and 312 are preferably each greater than 1.1, and more preferably greater than 1.2. It is sufficient that at least one of the surface layer portions on both sides in the plate thickness direction of the flat portions 311 and 312 satisfies the condition of such a standard deviation ratio of the Vickers hardness. However, preferably, the surface layer portions on both sides of the flat portions 311 and 312 satisfy the condition of the standard deviation ratio of the Vickers hardness.

[0079] For example, in the first member 30 formed by cold press forming of a steel plate, similar to the second member 40, by modifying the metallographic structure of the surface layer portion and its vicinity of the steel plate, the standard deviation ratios of the Vickers hardness of the flat portions 311 and 312 can exceed 1.0. That is, as described in the first embodiment, for example, by adjusting the maximum heating temperature and holding time during the decarburizing annealing of the steel plate, the standard deviation ratios of the Vickers hardness of the flat portions 311 and 312 can be controlled.

[0080] The Vickers hardness of the central portion in the plate thickness direction of the flat portions 311 and 312 and the standard deviation of its frequency distribution can be measured by the same method as that for the flat portions 41 and 42 of the second member 40. The standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portions 311 and 312 can also be measured by the same method as that for the flat portions 41 and 42 of the second member 40.

[0081] Return to Figure 3, a stress concentration portion 20 is provided on the flat portions 311 and 312 of the first member 30. More specifically, on the flat portions 311 and 312 of the first member 30, there are provided bending portions that are recessed and bent (bent) toward the second member 40 when viewed from the ridge line portion 32 side or the ridge line portion 33 side. This bending portion is the stress concentration portion 20 corresponding to the flat portions 311 and 312. When the bone structure member 100 is subjected to an axial compressive load, the stress concentration portion 20 as the bending portion serves as a starting point, enabling the bone structure member 100 to generate a bending deformation with the flat portions 311 and 312 as the bending inner sides. The ridge line portions 32, 313 continuous with the flat portion 311 and the ridge line portions 33, 314 continuous with the flat portion 312 are also locally bent along the flat portions 311 and 312.

[0082] A stress concentration portion 20 different from the first embodiment is provided on the flat portions 41 and 42 of the second member 40. On the flat portions 41 and 42 of the second member 40, there are provided bending portions that are recessed and bent (bent) toward the first member 30 when viewed from the ridge line portion 43 side or the ridge line portion 45 side. This bending portion is the stress concentration portion 20 corresponding to the flat portions 41 and 42. When the bone structure member 100 is subjected to an axial compressive load, the stress concentration portion 20 as the bending portion serves as a starting point, enabling the bone structure member 100 to generate a bending deformation with the flat portions 41 and 42 as the bending inner sides. The ridge line portions 43, 44 continuous with the flat portion 41 and the ridge line portions 45, 46 continuous with the flat portion 42 are also locally bent along the flat portions 41 and 42.

[0083] The bone structure member 100 of the present embodiment can also achieve the same effect as the first embodiment. That is, in the present embodiment, by setting the widths W3 and W4 of the flat portions 311 and 312 of the first member 30 to be 70.0 mm or less, when a compressive load is axially input to the bone structure member 100, the flat portions 311 and 312 can be compactly bent starting from the stress concentration portion 20. In addition, by setting the widths W1 and W2 of the flat portions 41 and 42 of the second member 40 to be 70.0 mm or less, when a compressive load is axially input to the bone structure member 100, the flat portions 41 and 42 can be compactly bent starting from the stress concentration portion 20. Therefore, the bone structure member 100 can exhibit good energy absorption performance.

[0084] In the present embodiment, in the flat portions 311 and 312 of the first member 30 and the flat portions 41 and 42 of the second member 40, the standard deviation ratio of the frequency distribution of the Vickers hardness between the central portion and the surface layer portion in the plate thickness direction is greater than 1.0. That is, the flat portions 311, 312, 41, and 42 each have high bending performance. Therefore, when the bone structure member 100 is bent and deformed with the flat portions 311 and 312 of the first member 30 as the bending inner sides, it is not easy for the flat portions 311 and 312 to break during the deformation, and the flat portions 311 and 312 can generate a desired bending deformation starting from the stress concentration portion 20. Similarly, when the bone structure member 100 is bent and deformed with the flat portions 41 and 42 of the second member 40 as the bending inner sides, it is not easy for the flat portions 41 and 42 to break during the deformation, and the flat portions 41 and 42 can generate a desired bending deformation starting from the stress concentration portion 20. Thus, the energy absorption performance of the bone structure member 100 can be improved. In addition, since the first member 30 and the second member 40 are formed of a high-strength material having a Vickers hardness of 350 Hv or more at the central portion, the first member 30 and the second member 40 can be thinned. Therefore, a lightweight bone structure member 100 can be obtained.

[0085] The stress concentration portion 20 of the bone structure member 100 of the present embodiment is a bent portion provided in the flat portions 311, 312, 41, and 42. The flat portions 311, 312, 41, and 42 are locally bent in the side view angle of the bone structure member 100. However, for example, the flat portions 311, 312, 41, and 42 may also have a bent portion that is bent when viewed along its plate thickness direction as the stress concentration portion 20. The flat portions 311, 312, 41, and 42 are each configured to be able to bend starting from the bent portion as the stress concentration portion 20 when a compressive load in the axial direction is input to the bone structure member 100.

[0086] In the present embodiment, in the first member 30, the top plate 31 is divided by the groove portion 315, thereby forming flat portions 311 and 312 having widths W3 and W4 of 70.0 mm or less. However, for example, when the overall width of the top plate 31 is 70.0 mm or less, the groove portion 315 may not be provided in the first member 30. On the other hand, when the width of the top plate 31 is larger, a plurality of groove portions 315 can also be provided in the first member 30. For the second member 40, similar to the first embodiment, the number of groove portions 47 can be adjusted according to the width of the bottom plate, for example.

[0087] <Third Embodiment>

[0088] Figure 5 is a perspective view of the bone structure member 100 of the third embodiment. Figure 6 is to Figure 5A cross-sectional view (transverse cross-sectional view) of the bone structure member 100 shown when cut by a plane perpendicular to its axis.

[0089] As Figure 5 shown, the bone structure member 100 of the present embodiment includes a plurality of reinforcing members 50 in addition to the first member 30 and the second member 40. The reinforcing members 50 are arranged at intervals in the axial direction of the bone structure member 100.

[0090] In the present embodiment, the first member 30 includes flat portions 311, 312. Each reinforcing member 50 is joined to the first member 30 by welding, for example. In the present embodiment, the reinforcing members 50 are arranged inside the hollow bone structure member 100. However, the reinforcing members 50 may also be arranged outside the bone structure member 100.

[0091] Referring to Figure 6 , the reinforcing member 50 is arranged in a manner along at least a part of the first member 30 in the transverse cross-sectional view of the bone structure member 100. The reinforcing member 50 is arranged in a region of the first member 30 including the flat portions 311, 312 and the ridge lines 313, 314, 32, 33. In the transverse cross-sectional view of the bone structure member 100, the reinforcing member 50 reinforces the flat portions 311, 312 and the ridge lines 313, 314, 32, 33 from the inside of the first member 30. In the transverse cross-sectional view of the bone structure member 100, the reinforcing member 50 extends from the side wall 34 on the flat portion 311 side to the side wall 35 on the flat portion 312 side. The reinforcing member 50 is formed of a steel plate, for example. The material and plate thickness of the reinforcing member 50 may be the same as or different from those of the first member 30 or the second member 40.

[0092] Returning to Figure 5 , the portion of the bone structure member 100 where the reinforcing member 50 is arranged is a high-strength portion having relatively high strength. The portion of the bone structure member 100 located between the reinforcing members 50 in the axial direction is a low-strength portion having lower strength than the high-strength portion. That is, the low-strength portion is the portion of the flat portions 311, 312 and the ridge lines 313, 314, 32, 33 that is not reinforced by the reinforcing member 50 and has relatively small strength compared to the high-strength portion reinforced by the reinforcing member 50. In the present embodiment, this low-strength portion becomes the stress concentration portion 20. When a compressive load in the axial direction is input to the bone structure member 100, it bends starting from the stress concentration portion 20 with relatively low strength. Even the bone structure member 100 of the present embodiment can achieve the same effects as the above-described embodiments.

[0093] In the present embodiment, a reinforcing member 50 is provided on the flat portion 311 and the ridge line portions 313 and 32 continuous therewith. However, the reinforcing member 50 may be provided only on the flat portion 311, or may be provided only on at least one of the ridge line portions 313 and 32. Similarly, the reinforcing member 50 does not necessarily need to be provided on both the flat portion 312 and the ridge line portions 314 and 33 continuous therewith. The reinforcing member 50 may be provided only on the flat portion 312, for example, or may be provided only on at least one of the ridge line portions 314 and 33.

[0094] The bone structure member 100 of the present embodiment includes two reinforcing members 50. However, the bone structure member 100 may also include three or more reinforcing members 50. That is, three or more reinforcing members 50 may be arranged at intervals in the axial direction of the bone structure member 100.

[0095] In the bone structure member 100 of the present embodiment, the first member 30 includes flat portions 311 and 312 having widths W3 and W4 of 70.0 mm or less and stress concentration portions 20. Therefore, a material having a standard deviation ratio of Vickers hardness between the central portion and the surface layer portion in the plate thickness direction exceeding 1.0 and a Vickers hardness of the central portion in the plate thickness direction of 350 Hv or more is used for the first member 30. On the other hand, the second member 40 does not include a stress concentration portion 20, and thus does not necessarily need to be made of the same material as the first member 30. The material of the second member 40 may be the same as or different from the material of the first member 30.

[0096] <Fourth Embodiment>

[0097] Figure 7 It is a cross-sectional view (transverse cross-sectional view) when the bone structure member 100 of the fourth embodiment is cut along a plane perpendicular to its axial direction. Figure 8 It is Figure 7 a perspective view of the bone structure member 100 shown.

[0098] Refer to Figure 7 In the bone structure member 100 of the present embodiment, the overall cross-sectional shape is different from that of other embodiments. In the bone structure member 100 of other embodiments, the closed cross-section divided by the first member 30 and the second member 40 has a substantially quadrilateral shape. On the other hand, in the bone structure member 100 of the present embodiment, the closed cross-section divided by the first member 30 and the second member 40 has a substantially octagonal shape. However, the bone structure member 100 may also have other polygonal shapes in the transverse cross-sectional view. In the present embodiment, a flat portion 311 having a width W3 of 70.0 mm or less is formed in the first member 30.

[0099] Refer to Figure 7 and Figure 8, the flat portion 311 of the first member 30 includes a plurality of rib portions 60. The plurality of rib portions 60 are arranged at intervals in the axial direction of the bone structure member 100. Each rib portion 60 extends in the axial direction of the bone structure member 100.

[0100] Each rib portion 60 has a shape that is recessed inwardly of the bone structure member 100. Each rib portion 60 may also have a shape that protrudes outwardly of the bone structure member 100. The portion of the flat portion 311 where the rib portions 60 are provided is a high-strength portion having a higher strength than other portions. The portion of the flat portion 311 that is located between the rib portions 60 in the axial direction of the bone structure member 100 is a low-strength portion having a relatively lower strength than the portion where the rib portions 60 are provided. In the present embodiment, this low-strength portion becomes the stress concentration portion 20. When a compressive load in the axial direction is input to the bone structure member 100, it bends starting from the stress concentration portion 20 having a relatively low strength. Even the bone structure member 100 of the present embodiment can achieve the same effects as the above-described embodiments.

[0101] In the bone structure member 100 of the present embodiment, two rib portions 60 are provided on the flat portion 311. However, three or more rib portions 60 may be provided on the flat portion 311. That is, three or more rib portions 60 may be arranged at intervals from each other in the axial direction of the bone structure member 100.

[0102] In the third embodiment, a plurality of high-strength portions and low-strength portions between the high-strength portions are respectively formed on the flat portions 311 and 312 by arranging a plurality of reinforcing members 50 at intervals in the axial direction of the bone structure member 100. In the fourth embodiment, a plurality of high-strength portions and low-strength portions between the high-strength portions are formed on the flat portion 311 by arranging a plurality of rib portions 60 at intervals in the axial direction of the bone structure member 100. However, the method of providing the high-strength portion and the low-strength portion in at least one of the flat portion and the ridge line portion continuous therewith is not limited to these methods.

[0103] For example, at least one of the flat portion included in the closed cross-sectional portion of the bone structure member and the ridge line portion continuous therewith can form a high-strength portion and a low-strength portion by changing the tensile strength or the plate thickness. That is, a plurality of high-strength portions made of a material having a relatively large tensile strength can be provided in the flat portion and / or the ridge line portion continuous therewith, and low-strength portions made of a material having a tensile strength smaller than that of the high-strength portions and disposed between the high-strength portions can be provided. Alternatively, a plurality of high-strength portions made of a material having a relatively large plate thickness can be provided in the flat portion and / or the ridge line portion continuous therewith, and low-strength portions made of a material having a plate thickness smaller than that of the high-strength portions and disposed between the high-strength portions can be provided. It may also be that, in the flat portion and / or the ridge line portion continuous therewith, both the tensile strength and the plate thickness of the low-strength portion are smaller than those of each high-strength portion. In these cases, a low-strength portion serving as a stress concentration portion can also be formed in the bone structure member.

[0104] In the bone structure member 100 of the present embodiment, similarly to the third embodiment, the first member 30 includes a flat portion 311 having a width W3 of 70.0 mm or less and a stress concentration portion 20. Therefore, a material having a standard deviation ratio of the Vickers hardness between the central portion and the surface layer portion in the plate thickness direction exceeding 1.0 and a Vickers hardness of the central portion in the plate thickness direction of 350 Hv or more is used for the first member 30. On the other hand, since the second member 40 does not include the stress concentration portion 20, it is not necessarily required to be made of the same material as the first member 30. The material of the second member 40 may be the same as or different from the material of the first member 30.

[0105] The embodiments of the present disclosure have been described above, but the present disclosure is not limited to the above embodiments, and various modifications can be made without departing from the gist thereof.

[0106] In the above first to third embodiments, in the closed cross-sectional portion of the bone structure member, a plurality of flat portions having a width of 70.0 mm or less in the cross-sectional view are provided. In the first to third embodiments, a stress concentration portion serving as a starting point of the bending deformation of the bone structure member is provided corresponding to each flat portion. However, in the case where the closed cross-sectional portion of the bone structure member includes a plurality of flat portions, it is not necessary to provide stress concentration portions for all the flat portions. The stress concentration portion only needs to be provided for one or more flat portions.

[0107] The skeletal members of the first to third embodiments described above have a cross-sectional shape that is substantially quadrilateral. On the other hand, the skeletal member of the fourth embodiment has a cross-sectional shape that is substantially octagonal. The cross-sectional shape of the skeletal member of the fourth embodiment can also be adopted in the first to third embodiments. Alternatively, the cross-sectional shape of the skeletal member of an embodiment other than the fourth embodiment can also be adopted in the first to third embodiments. Additionally, the cross-sectional shape of the skeletal member of any one of the first to third embodiments can be adopted in the fourth embodiment. The skeletal member can also have a cross-sectional shape other than the shapes described in the first to fourth embodiments. The skeletal member only needs to include at least one flat portion having a width of 70.0 mm or less in the closed cross-sectional portion.

[0108] In each of the above embodiments, an example in which the skeletal member has substantially the same cross-sectional shape over the entire length in its axial direction has been described. However, the cross-sectional shape of the skeletal member can also vary along the axial direction.

[0109] In each of the above embodiments, the skeletal member is formed by a first member and a second member. However, the skeletal member can also be formed by a single member. For example, the skeletal member can also be a member formed by cold-pressing a single steel plate and then joining the ends to each other to form a tubular shape.

[0110] Examples

[0111] Hereinafter, the present disclosure will be described in further detail using examples. However, the present disclosure is not limited to the following examples.

[0112] [First Example]

[0113] In order to confirm the difference in the bending performance of the steel plate caused by the standard deviation ratio of Vickers hardness, a plate bending test (VDA bending test) standardized by the German Association of the Automotive Industry (VDA) as VDA238-100 was conducted. In the VDA bending test, for cold-rolled steel plates with a thickness of 1.6 mm and a tensile strength of 1180 MPa grade and cold-rolled steel plates with a thickness of 1.6 mm and a tensile strength of 1470 MPa grade, the standard deviation ratio of Vickers hardness (standard deviation of the frequency distribution of Vickers hardness in the surface layer / standard deviation of the frequency distribution of Vickers hardness in the central part in the plate thickness direction) was changed, and the maximum bending angle (°) was confirmed. In Figure 9 and Figure 10 show the results of the VDA bending test.

[0114] Figure 9 are the test results of cold-rolled steel plates with a tensile strength of 1180 MPa grade, Figure 10 are the test results of cold-rolled steel plates with a tensile strength of 1470 MPa grade. In Figure 9 andFigure 10 In [it], the vertical axis is the ratio of the maximum bending angle to the maximum bending angle when the ratio of the maximum bending angle to the standard deviation is 1.0. From Figure 9 and Figure 10 it can be seen that among the steel plates of each strength grade, when the standard deviation ratio of the Vickers hardness is significantly greater than 1.0, the maximum bending angle of the VDA bending test becomes larger compared to the case where the standard deviation ratio is 1.0. That is, it can be said that when the standard deviation ratio of the Vickers hardness is greater than 1.0, the bending performance of the steel plate is improved. Therefore, it is not easy to break during the deformation of the bone structure member. For example, when the standard deviation ratio of the Vickers hardness exceeds 1.2, the bending performance of the steel plate is further improved.

[0115] [Second Embodiment]

[0116] In order to confirm the relationship between the standard deviation ratio of the Vickers hardness and the width of the flat part corresponding to the stress concentration part, a CAE analysis was performed using general-purpose structural analysis software (LS-DYNA, manufactured by LSTC Corporation). In Figure 11 it schematically shows the model used in this analysis.

[0117] Referring to Figure 11 , in this analysis, for the cylindrical member 70, with one end in the axial direction constrained, an impactor was made to collide from the other end side at a collision speed of 1.0 m / s, and an axial compressive load was applied. The member 70 includes a flat part 71 on the bending compression side. Concave rib parts (stress concentration parts) 72 are provided at the ridge lines continuous with both sides of the flat part 71 respectively. The material of the member 70 is a steel plate with a thickness of 1.6 mm and a tensile strength of 1470 MPa. The Vickers hardness at the center in the thickness direction of the member 70 is 350 Hv or more.

[0118] In this analysis, regarding both the case of the steel plate A with a standard deviation ratio of the Vickers hardness (standard deviation of the frequency distribution of the Vickers hardness in the surface layer part / standard deviation of the frequency distribution of the Vickers hardness at the center in the thickness direction) of 1.0 and the case of the steel plate B with a standard deviation ratio of the Vickers hardness greater than 1.0, the relationship between the width W of the flat part 71 and the energy absorption efficiency of the member 70 was investigated. In Figure 12 it shows the results of this analysis.

[0119] In Figure 12 the chart of, the horizontal axis is the width W of the flat part 71 on the bending compression side ( Figure 11) The vertical axis is the ratio of the energy absorption efficiency to the reference energy absorption efficiency. The reference energy absorption efficiency is the energy absorption efficiency when the material of the member 70 is steel plate A and the width W of the flat portion 71 is 80.0 mm. The energy absorption efficiency is the energy absorption amount per unit cross-sectional area (plate thickness × cross-sectional line length) of the member 70 in a specified displacement range.

[0120] It can be seen from Figure 12 that for both steel plate A and steel plate B, the smaller the width W of the flat portion 71 on the bending compression side, the higher the energy absorption efficiency. If the width W of the flat portion 71 is less than 80.0 mm, the energy absorption efficiency of steel plate B is significantly higher than that of steel plate A. Under the conditions that the width W of the flat portion 71 is 20.0 mm and 60.0 mm, in steel plate A with a standard deviation ratio of Vickers hardness of 1.0, fracture occurred during the bending deformation of the member 70. In contrast, in steel plate B with a standard deviation ratio of Vickers hardness greater than 1.0, fracture did not occur during the bending deformation of the member 70, and the member 70 was steeply bent starting from the rib portion 72. Therefore, when steel plate B is used, the energy absorption efficiency is higher.

[0121] Through this analysis, it can be confirmed that when the standard deviation ratio of the Vickers hardness of the steel plate constituting the flat portion is greater than 1.0, there is a tendency that the smaller the width of the flat portion on the bending compression side, the more the energy absorption performance is improved compared to the steel plate with a standard deviation ratio of 1.0. This tendency is the same even if the plate thickness of the steel plate changes. Based on the results of this analysis, it is considered that the width of the flat portion for enabling the skeletal member to exhibit good energy absorption performance is 70.0 mm or less.

[0122] Description of reference numerals

[0123] 100, skeletal member; 10, closed cross-sectional portion; 20, stress concentration portion; 311, 312, 41, 42, flat portion; 313, 314, 32, 33, 43, 44, 45, 46, rib line portion; 50, reinforcing member; 60, rib portion.

Claims

1. A bone structure member formed using a steel plate, the bone structure member comprising: A closed cross-sectional portion, the cross-section perpendicular to the axial direction of the bone structure member being a closed cross-section, including a flat portion that is linear when viewed in the cross-section and has a width of 70.0 mm or less, and a ridge line portion that is continuous with the flat portion and is curved when viewed in the cross-section; and A stress concentration portion provided in at least one of the flat portion and the ridge line portion, The Vickers hardness of the central portion in the plate thickness direction of the flat portion is 350 Hv or more, The standard deviation ratio obtained by dividing the standard deviation of the frequency distribution of the Vickers hardness of the surface layer portion of the flat portion by the standard deviation of the frequency distribution of the Vickers hardness of the central portion of the flat portion is greater than 1.

0.

2. The bone structure member according to claim 1, wherein, The stress concentration portion is a concave portion or a convex portion formed in at least one of the flat portion and the ridge line portion.

3. The bone structure member according to claim 1, wherein, The flat portion includes a bent portion that is bent when viewed from the ridge line portion side or along the plate thickness direction, The stress concentration portion is the bent portion.

4. The bone structure member according to claim 1, wherein, At least one of the flat portion and the ridge line portion includes a plurality of high-strength portions arranged at intervals in the axial direction, and low-strength portions arranged between the high-strength portions in the axial direction and having a lower strength than the high-strength portions, The stress concentration portion is the low-strength portion.

5. The bone structure member according to claim 4, wherein, The bone structure member includes a plurality of reinforcing members arranged at intervals in the axial direction and provided in at least one of the flat portion and the ridge line portion, The high-strength portion is the portion of the bone structure member where the reinforcing member is provided, The low-strength portion is the portion of the bone structure member located between the reinforcing members in the axial direction.

6. The bone structure member according to claim 4, wherein, The flat portion includes a plurality of rib portions arranged at intervals in the axial direction and each extending in the axial direction, The high-strength portion is the portion of the flat portion where the rib portion is provided, The low-strength portion is the portion of the flat portion located between the rib portions in the axial direction.

Images