Skeleton member, vehicle body, and method for manufacturing skeleton member

Abstract

A closed cross section part 10 of a skeleton member 1 includes a first flat part 21, a second flat part 22, and a ridge line part 23 in which the first flat part 21 and the second flat part 22 are connected and the radius of curvature R is smaller than both the radius of curvature of the first flat part 21 and the radius of curvature of the second flat part 22. The axial cross section of the closed cross section part 10 has a shape that causes a deformation in which the first flat part 21 and the second flat part 22 become convex in the same phase with respect to the phase toward the inside of the closed cross section part 10 and the phase toward the outside of the closed cross section part 10 when the skeleton member 1 undergoes bellows deformation due to a compressive load along the longitudinal direction Z.

Description

Frame member, vehicle body, and method for manufacturing frame member

[0001] The present invention relates to a frame member, a vehicle body, and a method for manufacturing a frame member.

[0002] The rear portion of a passenger car body, which is one type of vehicle, is equipped with a rear frame. The rear frame is equipped with skeletal components such as rear side members as impact-absorbing components (see, for example, Patent Documents 1 and 2). In a rear-end collision in which another vehicle or the like rear-ends the passenger car, the rear frame axially collapses and absorbs the impact load, thereby protecting the occupants in the cabin, the gas tank, and the battery. For a vehicle rear structure with such functions, there is a need for a way to better protect occupants in a rear-end collision.

[0003] JP 2008-32227 A JP 2005-121144 A

[0004] Generally, a steel rear frame included in the lower part of a vehicle rear structure is produced by press-forming a plate with uniform strength in the thickness direction, and therefore has uniform strength in the thickness direction. During a rear collision of a passenger vehicle equipped with such a rear frame, an axial load is applied to the rear frame, compressing the rear side members in the axial direction and causing the rear side members to deform like bellows. There is a demand for a lower part of a vehicle rear structure including such rear side members to exhibit improved impact energy absorption efficiency. Similar challenges exist with other impact-absorbing components, such as front side members, particularly axially crushable components.

[0005] Patent Document 2 discloses an invention of an impact absorbing member (energy absorbing member for automobile frames) made of aluminum alloy. Specifically, the impact absorbing member is made of a tempered, heat-treated aluminum alloy hollow shape, and the hollow shape has a square or rectangular cross section with one hollow portion, a wall thickness of more than 2 mm, and a cross-sectional area excluding the hollow portion of 350 mm. 2The present invention discloses an aluminum alloy energy absorbing member for automobile frames, characterized in that, when the average length of each side in the cross section of the hollow extrusion is a, the radius of the corner is R, and the average thickness of each side is t, the relationships 0<R / a≦0.2, t / a≦0.2, and R / a<5×(t / a)−0.37 are satisfied. It is disclosed that when an impact load is applied in the axial direction, all sides in the same cross section perpendicular to the axis bend in the same direction.

[0006] However, the invention of Patent Document 2 is limited to aluminum alloys. In addition, a specific embodiment is envisioned as an aluminum extrusion material, and from the viewpoint of strength, a configuration with a large wall thickness and a large cross-sectional area is aimed for.

[0007] One object of the present disclosure is to provide a frame member, a vehicle body, and a method for manufacturing a frame member that are not limited by the material and can exhibit superior impact energy absorption efficiency.

[0008] The present invention provides the following frame member, vehicle body, and method for manufacturing a frame member.

[0009] (1) A skeletal member having a closed cross-section portion whose cross section perpendicular to the longitudinal direction is a closed cross-section, the closed cross-section portion comprising a first flat portion, a second flat portion, and a ridge portion connecting the first flat portion and the second flat portion and having a radius of curvature smaller than both the radius of curvature of the first flat portion and the radius of curvature of the second flat portion, the cross-section of the closed cross-section portion having a shape that causes deformation in which the first flat portion and the second flat portion are convex in the same phase with respect to the phase in the direction toward the inside of the closed cross-section portion and the phase in the direction toward the outside of the closed cross-section portion when the skeletal member undergoes bellows deformation due to a compressive load along the longitudinal direction.

[0010] (2) The skeletal member according to (1), wherein the radius of curvature of the ridge line portion in the cross section is 15 mm or more.

[0011] (3) The skeletal member according to (1) or (2), wherein R / bf satisfies the following formula (1), where E is the Young's modulus of the skeletal member, t is the plate thickness of the closed cross-sectional portion, R is the radius of curvature of the ridge line portion, and bf is the length of at least one of the flat portions in the cross section. R / bf≧(1.49×10 －７ ×Et ３ ) + 0.153 ... (1)

[0012] (4) The skeletal member according to any one of (1) to (3), wherein the skeletal member is a steel plate member, and R / bf satisfies the following formula (2), where t is the plate thickness of the skeletal member, R is the radius of curvature of the ridge line portion, and bf is the length of at least one of the flat portions in the cross section: R / bf≧0.1616t+0.0235 (2)

[0013] (5) The skeletal member according to (4), wherein the skeletal member is formed by joining a first half portion and a second half portion together, the first half portion being hat-shaped in cross section, the first half portion including the first flat portion, a part of the second flat portion, a first ridge portion as the ridge portion, a flange joined to the second half portion, and a second ridge portion connecting the part of the second flat portion and the flange, wherein R2 / bf satisfies the following formula (3) when the radius of curvature of the second ridge portion is R2: R2 / bf<0.1616t+0.0235 (3)

[0014] (6) The skeletal member according to any one of (1) to (5), wherein, when the radius of curvature of the ridge line portion is R and the length of at least one of the flat portions in the cross section is bf, 0.20≦R / bf≦0.60.

[0015] (7) The skeletal member according to any one of (1) to (6), wherein, when the relatively smaller length of the length of the closed cross-section portion in the width direction of the closed cross-section portion in the cross-section and the length of the closed cross-section portion in the height direction of the closed cross-section portion in the cross-section is b, b≧75 mm; when the skeletal member is a steel plate member, the plate thickness of the closed cross-section portion in the cross-section is 0.8 mm to 2.6 mm; and when the skeletal member is an aluminum alloy plate member, the plate thickness of the closed cross-section portion in the cross-section is 1.1 mm to 3.7 mm.

[0016] (8) The skeletal member according to any one of (1) to (7), wherein bf is 75 mm or more, where bf is the length of at least one of the flat portions in the cross section.

[0017] (9) The skeletal member according to any one of (1) to (8), wherein the Vickers hardness of the central part in the thickness direction of the closed cross-section portion is 300 HV or more.

[0018] (10) A skeletal member according to any one of (1) to (9), wherein in the cross section, the Vickers hardness at the surface in the thickness direction of the closed cross section portion is at least 100 HV lower than the Vickers hardness at the center in the thickness direction of the closed cross section portion.

[0019] (11) In the cross section, the closed cross section portion is provided with a softened layer from the surface in the thickness direction, the Vickers hardness of the central portion in the thickness direction of the closed cross section portion is 300 HV or more, the thickness of the softened layer is 80 μm or more and is 5% to 20% of the thickness of the portion where the softened layer is provided, the Vickers hardness of the softened layer at the surface is 0.5 to less than 0.9 times the Vickers hardness of the central portion in the thickness direction of the portion where the softened layer is provided, and the softened layer has, in the thickness direction, a first hardness change region that is a region from the surface to 40% of the thickness of the softened layer, and a second hardness change region that is a region of the softened layer that is not the first hardness change region, The skeletal member according to (10), wherein an absolute value ΔHV1 of the hardness change in the plate thickness direction in the first hardness change region is greater than an absolute value ΔHV2 of the hardness change in the plate thickness direction in the second hardness change region.

[0020] (12) A vehicle body comprising the frame member according to any one of (1) to (11).

[0021] (13) A method for manufacturing a skeletal member having a closed cross-sectional portion whose cross section perpendicular to the longitudinal direction is a closed cross-section, the closed cross-sectional portion comprising a first flat portion, a second flat portion, and a ridge portion connecting the first flat portion and the second flat portion and having a radius of curvature smaller than both the radius of curvature of the first flat portion and the radius of curvature of the second flat portion, the method comprising: setting a shape of the closed cross-sectional portion so that, when the skeletal member undergoes bellows deformation due to a compressive load along the longitudinal direction, the first flat portion and the second flat portion undergo deformation in which they become convex in phase in the cross-section with respect to the phases toward the inside of the closed cross-sectional portion and the phases toward the outside of the closed cross-sectional portion; and manufacturing the closed cross-sectional portion so as to have the set shape of the closed cross-sectional portion.

[0022] (14) In the method for manufacturing a skeletal member according to (13), in setting the shape of the closed cross-sectional portion, R / bf is set to a value that satisfies the following formula (1), where E is the Young's modulus of the skeletal member, t is the plate thickness of the closed cross-sectional portion, R is the radius of curvature of the ridge line portion, and bf is the length of at least one of the flat portions in the cross section. R / bf≧(1.49×10 －７ ×Et ３ ) + 0.153 ... (1)

[0023] (15) The method for manufacturing a skeletal member according to (13) or (14), wherein, in setting the shape of the closed cross-sectional portion, when the radius of curvature of the ridge line portion is R and the length of at least one of the flat portions in the cross-section is bf, the values ​​are set to 0.20≦R / bf≦0.60.

[0024] (16) The method for manufacturing a skeletal member according to any one of (13) to (15), wherein, in setting the shape of the closed cross-sectional portion, when the relatively smaller length of the length of the closed cross-sectional portion in the width direction of the closed cross-sectional portion in the cross-section and the length of the closed cross-sectional portion in the height direction of the closed cross-sectional portion in the cross-section is b, b≧75 mm; when the skeletal member is a steel plate member, the plate thickness of the closed cross-sectional portion in the cross-section is 0.8 mm to 2.6 mm; and when the skeletal member is an aluminum alloy plate member, the plate thickness of the closed cross-sectional portion in the cross-section is 1.1 mm to 3.7 mm.

[0025] According to the present invention, it is possible to exhibit a superior impact energy absorption efficiency without being limited by the material.

[0026] FIG. 1 is a perspective view showing a state in which a conventional skeletal member has been axially crushed and deformed, with color shading according to the amount of strain. FIG. 2 is a perspective view showing a state in which a skeletal member according to an example of the present disclosure has been axially crushed and deformed, with color shading according to the amount of strain. FIG. 3 is a schematic perspective view showing the state of the skeletal member shown in FIG. 2 before being axially crushed and deformed. FIG. 4 is a view showing the skeletal member in an axial cross section perpendicular to the longitudinal direction of the skeletal member. FIGS. 5A and 5B are schematic diagrams for explaining a bending test, with FIG. 5A being a plan view of a test specimen before the test and FIG. 5B showing the test specimen, punch, and roll. FIG. 6 is an axial cross section of a main portion of Modification 1, with the back side of the cross section not shown. FIG. 7 is an image diagram for explaining an example of a method for measuring the boundary between the softened layer and the central portion. FIG. 8 is an image diagram for explaining an example of a change in Vickers hardness in the softened layer. FIG. 9 is an axial cross section of Modification 2, in which the skeletal member is composed of a first half portion and a second half portion. FIG. 10A is a graph showing the amount of energy absorption when R / bf is changed for Test Examples 2 to 6. FIG. 10B is a graph showing the amount of energy absorption per kg of the frame member (energy absorption efficiency) when R / bf is changed for Test Examples 2 to 6. FIG. 11A is a graph showing the relationship between R / bf and buckling stress for Test Examples 7 to 11, and is displayed so that either antiphase deformation or inphase deformation can be seen. FIG. 11B is a graph showing the relationship between plate thickness t and R / bf when deformation changes from antiphase to inphase for Test Examples 7 to 11. FIG. 12 is a graph plotting the minimum R / bf at which antiphase deformation occurs for Test Examples 6 to 11.

[0027] [Background to the Invention] The inventors of the present invention focused on a skeletal member having a closed cross-sectional portion as an axially crushable part that undergoes plastic deformation and collapse in the axial (longitudinal) direction when absorbing an impact. The closed cross-sectional portion is formed, for example, by combining a pair of hat-shaped members in a cross section (axially perpendicular cross section) perpendicular to the longitudinal direction (axial direction) of the skeletal member. FIG. 1 is a perspective view showing a conventional skeletal member 1x in an axially crushed state, with color shading according to the amount of strain. The skeletal member 1x has two hat-shaped portions 11x and 12x. The flanges 25ax, 25bx, 25cx, and 25dx of each hat-shaped portion 11x and 12x are joined together by welding or the like to form the skeletal member 1x. The skeletal member 1x has a closed cross-sectional portion 10x whose cross section perpendicular to the longitudinal direction of the skeletal member 1x is a closed cross section. The closed cross-sectional portion 10x is formed in an approximately rectangular shape in a cross section perpendicular to the axis, and has two first flat portions 21ax and 21bx on the top and bottom, two second flat portions 22ax and 22bx on the left and right, and four ridge portions 23x (corner portions).

[0028] The inventors of the present application focused on the relationship between the width (length) of the flat portion (the first flat portion 21ax, which is the shorter flat portion) in the cross section perpendicular to the axis of the closed cross-sectional portion 10x and the radius of curvature of the ridge portion 23x. More specifically, when the skeletal member 1x is subjected to an impact load along the longitudinal direction, each flat portion 21ax, 21bx, 22ax, 22bx is compressed while repeatedly deforming to become convex outward and inward in the cross section perpendicular to the axis of the closed cross-sectional portion 10x. FIG. 1 shows an outwardly convex deformed portion 51x and an inwardly convex deformed portion 52x. This deformation behavior is generally referred to as axial crushing deformation or bellows deformation. When the radius of curvature of the ridgeline portion 23x in the bellows-shaped member is small relative to the width of the first flat portions 21ax, 21bx, the ridgeline portion 23x serves as a "node" that traces the convex deformation of the first flat portions 21ax, 21bx and the second flat portions 22ax, 22bx. Therefore, adjacent portions 21ax, 22ax, or portions 21ax, 22bx, or adjacent portions 21bx, 22ax, or portions 21bx, 22bx, exhibit anti-phase deformation behavior in which the adjacent portions deform convexly in opposite directions relative to the inside and outside of the closed cross-sectional portion 10x. That is, both an outwardly convex deformation portion 51 and an inwardly convex deformation portion 52 appear in a single cross-section perpendicular to the axis. Hereinafter, the anti-phase deformation in which adjacent flat portions deform convexly in opposite directions relative to the inside and outside of the closed cross-sectional portion 10x in a cross-section perpendicular to the axis is also referred to simply as anti-phase deformation.

[0029] On the other hand, as shown in FIG. 2 , when the radius R of the ridge portion 23 is a certain ratio or greater to the width bf of the first flat portion 21 (21a), the ridge portion 23 serves to rigidly separate adjacent first and second flat portions 21a, 22a or 21a, 22b from each other or adjacent first and second flat portions 21b, 22a or 21b, 22b from each other in a cross section perpendicular to the axis (i.e., serves to prevent the ridge portion 23 from rotating around the central axis of the skeletal member 1). FIG. 2 is a perspective view showing a state in which the skeletal member 1 according to an example of the present disclosure has been subjected to axial crushing deformation, and color shading is applied according to the amount of strain. In this case, for example, in-phase deformation behavior occurs in which adjacent flat portions 21a, 22a (21b, 22b) deform convexly toward the same side inward and outward directions of the closed cross-sectional portion 10. That is, in one cross section perpendicular to the axis, either an outwardly convex deformed portion 51 or an inwardly convex deformed portion 52 appears. Note that hereinafter, the same-phase deformation in which adjacent flat portions in the cross section perpendicular to the axis deform convexly to the same side in the inward and outward directions of the closed cross-sectional portion 10 is also simply referred to as same-phase deformation.

[0030] 1, the strain introduced into the ridge line portion 23x in the antiphase deformation behavior is relatively small, so there is little risk of base material fracture occurring in the skeletal member 1x. However, since the ridge line portion 23x does not withstand the impact load very well during the axial crushing deformation of the skeletal member 1x, the average load acting on the skeletal member 1x is small, and the amount of energy absorption tends to be small.

[0031] In contrast to the conventional configuration described above, in the skeletal member 1x according to the example of the present disclosure, the flat portions 21, 22 adjacent to each other via the ridge line portion 23 deform in the same phase on the inside and outside of the closed cross-sectional portion 10 during axial collapse, resulting in large strain being introduced into the ridge line portion 23. As a result, the ridge line portion 23 braces itself against the impact load during axial collapse of the skeletal member 1, which tends to increase the average load acting on the skeletal member 1 and the amount of energy absorption. Based on this knowledge, the present inventors have arrived at the present invention.

[0032] [Configuration of Frame Member] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this embodiment, a frame member applied to a vehicle body will be described.

[0033] Fig. 3 is a schematic perspective view showing the state of the skeleton member 1 shown in Fig. 2 before axial crushing deformation. Fig. 4 is a view showing the skeleton member 1 in an axial cross section perpendicular to the longitudinal direction Z of the skeleton member 1.

[0034] 3 and 4 , the skeletal member 1 is an elongated member, and the longitudinal direction (axial direction) of the skeletal member 1 is the longitudinal direction Z. The skeletal member 1 constitutes part of the structural members of the body of a vehicle such as a passenger car, and is a member that absorbs impact energy by deforming like an accordion in the longitudinal direction Z and collapsing during a vehicle collision. Examples of such skeletal members 1 include front side members, rear side members, side sills, A-pillars, B-pillars, roof rails, floor cross members, roof cross members, and under-reinforcements.

[0035] The skeleton member 1 has a uniform shape in a cross section perpendicular to the longitudinal direction Z, which is a cross section perpendicular to the longitudinal direction Z, at least in a part of the longitudinal direction Z. Note that through holes such as service holes may be formed in the skeleton member 1. The skeleton member 1 has a closed cross section portion 10 that is closed around the entire circumference in the cross section perpendicular to the longitudinal direction Z. The closed cross section portion 10 is a cross section perpendicular to the longitudinal direction Z. In the cross section perpendicular to the longitudinal direction Z, directions that are perpendicular to each other are defined as the width direction X and the height direction Y.

[0036] The skeleton member 1 including the closed cross-sectional portion 10 is formed by a first half portion 11 and a second half portion 12 each having a hat shape.

[0037] The first half 11 and the second half 12 of the skeletal member 1 may each be formed into a thin plate shape by pressing a steel plate, or may each be formed into a thin plate shape by pressing or extruding an aluminum alloy plate, or may each be formed into a thin plate shape using a resin material other than metal, etc. In other words, the first half 11 and the second half 12 of the skeletal member 1 may each be a member obtained by processing and joining a steel plate, or may each be a member obtained by processing and joining an aluminum alloy plate, or may each be a member obtained by processing and joining a resin material other than metal, etc.

[0038] When the skeletal member 1 is formed of a steel plate, the steel plate is preferably a high-strength steel plate, and the tensile strength of this steel plate is preferably 590 MPa or more, and preferably 780 MPa or more. The steel plate is more preferably an ultra-high-strength steel plate, and in this case, the tensile strength is preferably 980 MPa or more, more preferably 1180 MPa or more, and even more preferably 1470 MPa or more. As a method for measuring the tensile strength of the skeletal member 1, for example, a part of the first flat portion 21 or a part of the second flat portion 22 described below is taken as a test piece, and a tensile test is performed on this test piece using a method in accordance with JIS Z 2241:2022.

[0039] In this way, by forming the frame member 1 as an impact absorbing member from high-strength steel plate, it is possible to increase the energy absorption capacity while reducing the plate thickness of the frame member 1.

[0040] The "energy absorption amount" is the amount of energy absorption calculated from the relationship between the impactor reaction force (load) and stroke when the skeletal member is deformed in an accordion-like manner. The impactor reaction force (load) and stroke can be measured, for example, by arranging the skeletal member 1 so that its longitudinal direction is the up-down direction, and impacting it with a rigid flat impactor in the longitudinal direction (vertical direction) from the upper end while fully restraining the lower end. The "energy absorption efficiency" is the amount of energy absorption per axial cross-sectional area (plate thickness × cross-sectional line length) of the skeletal member 1. If the skeletal member 1 does not have a uniform cross-section in the longitudinal direction Z, the energy absorption efficiency is the amount of energy absorption per cross-sectional area (plate thickness × cross-sectional line length) of the closed cross-section perpendicular to the longitudinal direction Z that has the smallest cross-sectional area (plate thickness × cross-sectional line length).

[0041] When the skeletal member 1 is formed of an aluminum alloy plate, the aluminum alloy is preferably composed of an Al-Mg alloy (5000 series aluminum alloy), an Al-Mg-Si alloy (6000 series aluminum alloy), an Al-Zn-Mg alloy (7000 series aluminum alloy), or the like.

[0042] The frame member 1 may include a material other than a steel plate or an aluminum alloy plate, and may be formed using a material other than a metal material, such as a resin material or a carbon material, for example.

[0043] The first half 11 and the second half 12 are each formed in a hat shape when viewed from the longitudinal direction Z (in a cross section perpendicular to the axis). The first half 11 and the second half 12 are integrated by flange connection to form the skeletal member 1. As shown in Modification Example 2 described below, either the first half 11 or the second half 12 may be formed in an unbent flat plate shape when viewed from the longitudinal direction X. Alternatively, the first half 11 and the second half 12 may be integrally formed from a tubular member formed in a ring shape (e.g., a polygonal ring shape such as a rectangular ring) when viewed from the longitudinal direction Z. Hereinafter, unless otherwise specified, the skeletal member 1 will be described based on the state when viewed from a cross section perpendicular to the longitudinal direction Z.

[0044] The first half portion 11 has a first flat portion 21 (21a), a pair of ridge portions 23, 23 (23a, 23b), a pair of flat half portions 24, 24 (24a, 24b) connected to the first flat portion 21 (21a) via the pair of ridge portions 23, 23 (23a, 23b, first ridge portion), and a pair of flanges 25, 25 (25a, 25b) protruding outward in the width direction X from the pair of flat half portions 24, 24 (24a, 24b). Each flat half portion 24, 24 (24a, 24b) and the corresponding flange 25, 25 (25a, 25b) are connected via second ridge portions 26, 26 (26a, 26b).

[0045] In this embodiment, the second half 12 is formed in a shape symmetrical to the first half 11 in the height direction Y.

[0046] The second half 12 has a first flat portion 21 (21b), a pair of ridge portions 23, 23 (23c, 23d, first ridge portion), a pair of flat half portions 24, 24 (24c, 24d) connected to the first flat portion 21 (21b) via the pair of ridge portions 23, 23 (23c, 23d), and a pair of flanges 25, 25 (25c, 25d) protruding outward in the width direction X from the pair of flat half portions 24, 24 (24c, 24d). Each flat half portion 24, 24 (24c, 24d) and the corresponding flange 25, 25 (25c, 25d) are connected via second ridge portions 26, 26 (26c, 26d).

[0047] It is preferable that the radii of curvature R of each ridge line portion 23 are the same, as this allows for more uniform deformation of each portion in the cross section perpendicular to the axis of the skeletal member 1 during axial crushing deformation. If the radii of curvature of each ridge line portion 23 are not uniform, the smallest value of the radii of curvature of each ridge line portion 23 is referred to as the radius of curvature R. The radius of curvature R refers to the radius of curvature at the center in the plate thickness direction. Note that if each ridge line portion 23 is not a smooth arc, the radius of curvature R is the radius of curvature when a smooth arc connects the first flat portion 21, which is a straight line, and the corresponding flat half portion 24.

[0048] The first half 11 and the second half 12 are fixed to each other by joining a pair of flanges 25, 25 (25a, 25b) and a pair of flanges 25, 25 (25c, 25d). These flanges 25 are arranged in a position that protrudes outward from the frame member 1 in a cross section perpendicular to the axis. Methods for joining the pair of flanges 25a, 25b and the pair of flanges 25c, 25d include spot, laser, arc, or other welding; mechanical joining such as riveting, caulking, and bolting; and bonding with an adhesive or the like. The joining locations (joints) of the pair of flanges 25a, 25b and the pair of flanges 25c, 25d may be formed at multiple locations spaced apart in the longitudinal direction Z, or may be formed continuously in the longitudinal direction Z with a structural adhesive or the like.

[0049] When the frame member 1 is a rear side member, it is preferable that the cross-sectional outer shape of the connection portion between the frame member 1 and a member arranged at the front end of the frame member 1 is the same as the cross-sectional outer shape of the connection portion between the frame member 1 and a member arranged at the rear end of the frame member 1. This allows the impact load to be transmitted to the frame member 1 efficiently, thereby increasing the energy absorption efficiency of the frame member 1.

[0050] In this embodiment, the first flat portions 21 (21a, 21b) form horizontal walls at both ends of the framework member 1 in the height direction Y. In Figures 3 and 4, the upper first flat portion 21a is formed by the first half portion 11, and the lower first flat portion 21b is formed by the second half portion 12. In addition, the second flat portions 22 (22a, 22b) form vertical walls at both ends of the framework member 1 in the width direction X. In Figure 4, the second flat portion 22a is formed by the flat half portion 24a of the first half portion 11 and the flat half portion 24c of the second half portion 12, and the second flat portion 22b is formed by the flat half portion 24b of the first half portion 11 and the flat half portion 24d of the second half portion 12. In this embodiment, the first half 11 and the second half 12 are firmly joined by welding or the like, and therefore the first half 11 and the second half 12 can be treated as a single member. Furthermore, because the flanges 25a, 25b and the flanges 25c, 25d are firmly joined, the radius of curvature R26 of each second ridge portion 26 (26a, 26b, 26c, 26d) may be smaller than the radius of curvature R of the ridge portion 23. The radius of curvature R26 refers to the radius of curvature at the center in the plate thickness direction. Note that if each second ridge portion 26 is not a smooth arc, the radius of curvature R26 is defined as the radius of curvature when the straight flat half portion 24 and the corresponding flange 25 are connected by a smooth arc.

[0051] In this embodiment, the cross section perpendicular to the axis of the closed cross section portion 10 has a shape that causes deformation in which the first flat portion 21 and the second flat portion 22 become convex in the same phase with respect to the phase in the direction toward the inside of the closed cross section portion 10 and the phase in the direction toward the outside of the closed cross section portion 10 when the skeleton member 1 undergoes bellows deformation due to a compressive load along the longitudinal direction Z. Such a shape will be described in more detail below.

[0052] As described above, the closed cross-section portion 10 composed of the first half portion 11 and the second half portion 12 comprises a first flat portion 21, a second flat portion 22, and a ridge portion 23 connecting the first flat portion 21 and the second flat portion 22 and having a radius of curvature R smaller than both the radius of curvature R21 of the first flat portion 21 and the radius of curvature R22 of the second flat portion 22.

[0053] The first flat portion 21 and the second flat portion 22 have a radius of curvature in a cross section perpendicular to the axis that is larger than the maximum outer dimension W of the skeletal member 1. The maximum outer dimension means the length of a straight line that forms the maximum distance between the ends of any two points in the cross section perpendicular to the axis of the skeletal member 1.

[0054] In this embodiment, the first flat portion 21 and the second flat portion 22 are perpendicular to each other, but this does not have to be the case, and the angle formed by the second flat portion 22 with respect to the first flat portion 21 may be other than 90°.

[0055] (Radius of curvature of ridge portion) The ridge portion 23 connects the adjacent first flat portion 21 and second flat portion 22, and the radius of curvature of each ridge portion 23 in a cross section perpendicular to the axis is radius of curvature R. The radius of curvature R is preferably 15 mm or greater. A radius of curvature R of 15 mm or greater can increase the torsional rigidity of the ridge portion 23. As a result, the amount of twisting of the ridge portion 23 in each portion in the longitudinal direction Z when subjected to an external force can be reduced. This allows the ridge portion 23 to be constructed as a more "rigid" portion. As a result, the ridge portion 23 can rigidly separate the adjacent first flat portion 21 and second flat portion 22 in a cross section perpendicular to the axis, and can suppress rotation of the flat portions 21, 22 around an axis along the longitudinal direction Z when an impact load is applied in the longitudinal direction Z. As a result, in the frame member 1, adjacent flat portions 21, 22 are deformed convexly in the same direction, resulting in deformation behavior in phase with each other, thereby enabling to further increase the impact load efficiency.

[0056] The shorter length of the first flat portion 21 or the second flat portion 22 in a cross section perpendicular to the axis is defined as length bf. In this embodiment, the length of the first flat portion 21 is length bf. The radius of curvature R refers to the radius of curvature of the ridge portion 23 at the center of the closed cross-sectional portion 10 in the plate thickness direction.

[0057] (Lower Limit of R / bf) When the Young's modulus of the skeleton member 1 is E and the plate thickness of the closed cross-sectional portion 10 is t, it is preferable that R / bf satisfies the following formula (1): R / bf≧(1.49×10 －７ ×Et ３) + 0.153 ... (1) When the above formula (1) is not satisfied, the deformation mode when the skeletal member 1 undergoes axial crushing deformation tends to be antiphase deformation, whereas when the above formula (1) is satisfied, the deformation mode when the skeletal member 1 undergoes axial crushing deformation tends to be inphase deformation. In other words, once the right and left sides of formula (1) are equivalent, whether the deformation is inphase or antiphase deformation is more clearly determined depending on the value of R / bf. The bending rigidity of a flat plate material is expressed as Et ３ Therefore, the variable Et proportional to the bending stiffness ３ Based on this, it is possible to predict whether anti-phase deformation or in-phase deformation will occur in the skeletal member 1. ３ The finding that it is possible to predict whether anti-phase deformation or in-phase deformation will occur in the skeletal member 1 based on the above is a finding that the present inventors discovered as a result of extensive research. In this way, whether anti-phase deformation or in-phase deformation will occur in the skeletal member 1 can be predicted using the Young's modulus E and the plate thickness t of the skeletal member 1.

[0058] When the skeletal member 1 is made of a steel plate, the Young's modulus E is approximately 210 GPa. When the skeletal member 1 is made of an aluminum alloy plate, the Young's modulus E is approximately 70 GPa. The radius of curvature R26 of the second ridge portions 26 (26a, 26b, 26c, 26d) may be configured not to satisfy formula (1). That is, R26 / bf<(1.49×10 －７ ×Et ３ ) + 0.153. With this configuration, the second ridge portions 26 (26a, 26b, 26c, 26d) can be made compact. Therefore, the frame member 1 can be made more compact in the width direction X and the height direction Y while realizing high energy absorption efficiency of the frame member 1.

[0059] (Lower Limit of R / bf Particularly When the Skeleton Member is a Steel Plate Member) In particular, when the skeleton member 1 is a steel plate member, it is preferable that R / bf satisfy the following formula (2), where t is the plate thickness of the closed cross-sectional portion 10 and bf is the length of at least one of the flat portions (in this embodiment, the first flat portion 21) in a cross section perpendicular to the axis. R / bf ≥ 0.1616t + 0.0235 (2) When the above formula (2) is not satisfied, the deformation mode when the skeleton member 1 undergoes axial crushing deformation is likely to be antiphase deformation. On the other hand, when the above formula (2) is satisfied, the deformation mode when the skeleton member 1 undergoes axial crushing deformation is likely to be inphase deformation. In other words, whether the deformation is antiphase or inphase is more clearly determined depending on the value of R / bf, with the boundary being the point where the right and left sides of formula (2) are equivalent. In this way, by setting R / bf based on formula (2) specialized for when the skeleton member 1 is a steel plate member, the skeleton member 1 can be designed through simpler calculations. Note that the radius of curvature R26 (corresponding to the radius of curvature R2 in the present disclosure) of the second ridge portions 26 (26a, 26b, 26c, 26d) may be configured so as not to satisfy formula (2). That is, the following may be satisfied: R26 / bf<0.1616t+0.0235 (3). With this configuration, the second ridge portions 26 (26a, 26b, 26c, 26d) can be made compact. Therefore, the skeletal member 1 can be made more compact in the width direction X and height direction Y while achieving high energy absorption efficiency of the skeletal member 1.

[0060] Thus, it is preferable to set the lower limit of R / bf based on the above-described formulas (1) and (2). The lower limit of R / bf is preferably 0.20 (0.2≦R / bf). By setting R / bf equal to or greater than the preferred lower limit, the radius of curvature R of the ridgeline portion 23 can be increased. This allows the ridgeline portion 23 to rigidly separate the adjacent first flat portion 21 and second flat portion 22 (preventing the ridgeline portion 23 from rotating around the central axis of the skeletal member 1). As a result, in-phase deformation occurs when the skeletal member 1 undergoes axial crushing deformation, thereby increasing the impact energy absorption capacity. Examples of lower limits of R / bf include 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, and 0.30.

[0061] (Upper Limit of R / bf) The upper limit of R / bf is preferably 0.60 (R / bf≦0.60). If R / bf exceeds the preferred upper limit, axial crushing deformation (bellows deformation) tends to become unstable. To stably generate bellows deformation, it is necessary to generate appropriate deflection in the flat portions 21, 22, causing the skeletal member 1 to contract so as to undergo bellows deformation. In this case, if R / bf is too large, the width of the ridge portion 23 in the cross section perpendicular to the axis becomes too wide, making the closed cross-sectional portion 10 more likely to bend than necessary, and making the bellows deformation unstable. As a result, it is difficult to stabilize the amount of impact energy absorption.

[0062] (Summary of the upper and lower limits of R / bf) As explained above, it is preferable that 0.20≦R / bf≦0.60. The lower limit of R / bf can be any of the values ​​mentioned above. With this configuration, R / bf is a value outside the range of the invention in Patent Document 2.

[0063] (Thickness of the Skeleton Member 1) When the skeleton member 1 is a steel plate member, the thickness t of the closed cross-section portion 10 (first half portion 11 and second half portion 12) is preferably 0.8 mm to 2.6 mm. When the thickness t is equal to or greater than the above-mentioned lower limit, a sufficient impact load can be generated in the skeleton member 1 during a vehicle collision, and the amount of impact energy absorption can be increased. Furthermore, when the thickness t is equal to or less than the above-mentioned upper limit, bellows deformation can be smoothly generated when the skeleton member 1 undergoes axial crushing deformation due to the impact load, and in-phase deformation can be more reliably generated.

[0064] In addition, when the skeletal member 1 is an aluminum alloy member, for the same reasons as above, the plate thickness t of the closed cross-sectional portion 10 (first half portion 11 and second half portion 12) is preferably 1.1 mm to 3.7 mm, and may be 1.1 mm to 2.0 mm.

[0065] (Width bf of Flat Portion) The length bf of at least one of the flat portions 21, 22 (in this embodiment, the first flat portion 21) in a cross section perpendicular to the axis of the closed cross-sectional portion 10 is preferably 75 mm or greater. The reason bf ≥ 75 mm is preferable is because the preferred minimum value of the radius of curvature R is 15 mm and the preferred minimum value of R / bf is 0.2. In other words, the length bf when R / bf = 15 / 75 = 0.2 is 75 mm. By ensuring a sufficient length bf of the first flat portion 21, the flat portions 21, 22 bend with appropriate resistance when the skeletal member 1 deforms in a bellows-like manner due to an impact load, thereby stabilizing the bellows-like deformation. As a result, the energy absorption efficiency of the skeletal member 1 can be increased.

[0066] (The shorter length b of the width and height of the closed cross-sectional portion) In this embodiment, the shorter length of the closed cross-sectional portion 10 in the width direction X in a cross section perpendicular to the axis of the closed cross-sectional portion 10 and the length of the closed cross-sectional portion 10 in the height direction Y in the cross section is defined as b. In this embodiment, since the closed cross-sectional portion 10 has a vertically elongated shape, the length of the closed cross-sectional portion 10 in the width direction X is defined as length b. This length b is preferably b ≥ 75 mm. By satisfying b ≥ 75 mm, the length of the flat portion 21 in the cross section perpendicular to the axis can be sufficiently ensured while the radius of curvature R of the ridge portion 23 can be sufficiently ensured. As a result, when the skeletal member 1 is subjected to bellows deformation due to an impact load, both the flat portions 21 and 22 bend with appropriate resistance, thereby stabilizing the bellows deformation. The length b is preferably b ≤ 150 mm. By satisfying b ≤ 150 mm, the skeletal member 1 can be prevented from becoming too heavy, thereby making the skeletal member 1 lighter.

[0067] (t / b) As described above, the relatively smaller length b of the width and height of the closed cross-sectional portion 10 is preferably 75 mm to 150 mm. Furthermore, when the skeletal member 1 is a steel plate member, the plate thickness t of the closed cross-sectional portion 10 is preferably 0.8 mm to 2.6 mm, as described above. Therefore, t / b is at most 2.6 / 150≒0.035, which is a value outside the range of the invention in Patent Document 2. Furthermore, when the skeletal member 1 is an aluminum alloy member, the plate thickness t of the closed cross-sectional portion 10 is preferably 1.1 mm to 3.7 mm, as described above. Therefore, t / b is at most 3.7 / 75≒0.049, which is a value outside the range of the invention in Patent Document 2.

[0068] (Vickers hardness of closed cross-section portion) The Vickers hardness of the central portion in the thickness direction of the closed cross-section portion 10 is preferably 300 HV or more. By forming the central portion in the thickness direction of the closed cross-section portion 10 with such a high hardness, the impact energy absorption amount can be increased without increasing the plate thickness. The lower limit of the Vickers hardness of the central portion in the thickness direction of the closed cross-section portion 10 may be 300 HV, 350 HV, 400 HV, 500 HV, 550 HV, 600 HV, or 650 HV.

[0069] (Method for Measuring Vickers Hardness of Closed Cross-Section) In this embodiment, the Vickers hardness of the central portion in the thickness direction of the closed cross-section 10 is measured as follows. A cross section perpendicular to the plate surface of a sample cut from, for example, the first flat portion 21 or the second flat portion 22 of the closed cross-section 10 is taken, and the sample of the measurement surface is prepared and subjected to a hardness test. The measurement surface is prepared in accordance with JIS Z 2244:2020. The measurement surface is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror finish using a liquid in which diamond powder with a particle size of 1 μm to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The hardness test is performed in accordance with JIS Z 2244:2020. Using a micro Vickers hardness tester, measurements are taken at 10 points at 1 / 2 the thickness of the sample with a test force of 1 kgf so that the distance between the centers of the indentations is at least three times the average diagonal length of the indentations, and the average value is taken as the Vickers hardness of the closed cross-section portion 10.

[0070] (Maximum bending angle of closed cross-sectional portion 10) When the skeletal member 1 is a steel plate member, the maximum bending angle of the steel plates (first half portion 11 and second half portion 12) that make up the skeletal member 1 is preferably 30 degrees to 130 degrees. By setting the maximum bending angle within the above range, high bending performance can be exhibited even in a skeletal member 1 that has high Vickers hardness and high strength. Therefore, even when the skeletal member 1 undergoes bellows deformation during axial crushing deformation, the deformability of the skeletal member 1 can be increased, and the possibility of the skeletal member 1 breaking can be reduced.

[0071] The maximum bending angle can be determined by the VDA bending test (VDA238-100:2017) standardized by the German Association of the Automotive Industry (VDA). Figures 5A and 5B are schematic diagrams illustrating the bending test. Figure 5A is a plan view of the test piece S before the test, and Figure 5B shows the test piece S, punch P, and roll R. As shown in Figures 5A and 5B, this VDA bending test involves deforming the test piece S into a V-shape by placing the test piece S on two rolls R, R and forcing it between the rolls R, R with a punch P having a tip radius of 0.4 mm. The length of the test piece S ranges from 60 mm to the distance between the centers of the rolls R, R plus 10 mm. The width of the test piece S ranges from 10 mm to 60 mm. The test piece S is collected from the closed cross-section 10 so that the direction in which the radius of curvature is 500 mm or more corresponds to the length direction of the test piece S. As shown in FIG. 5B , the test specimen S is deformed so as to have a V-shape when viewed along the width direction. The ridge line L created by the bending of the test specimen S at this time is a line along the width direction of the test specimen S. The load of the punch P and the stroke of the punch P are measured at this time. The fracture resistance of the test specimen S is evaluated by calculating the maximum bending angle from the stroke when the load of the punch P, which increased with the start of the test, decreases by 60 N from the maximum load due to the occurrence of fracture at the apex of the bend of the test specimen S. The formula for calculating the maximum bending angle from the stroke is the formula described in Annex D of the above-mentioned VDA238-100. Note that if the test specimen S does not fracture even with a stroke of 14 mm, the value obtained by the formula for calculating the maximum bending angle from the bending angle at a stroke of 14 mm is defined as the maximum bending angle of the test specimen S. In this embodiment, the number of tests performed on the test specimen S in the VDA bending test is three, and the average value of the measurement results of the three test specimens S is defined as the maximum bending angle of the closed cross-sectional portion 10.

[0072] The skeletal member 1 having the above-described general configuration has a shape of the closed cross-section portion 10 designed (set) so that it deforms in the same phase when it undergoes bellows deformation due to a compressive load along the longitudinal direction Z, and the closed cross-section portion 10 is manufactured to have the designed shape of the closed cross-section portion 10.

[0073] (Effects) As described above, according to this embodiment, as shown in Fig. 2, when the frame member 1 is axially crushed and deformed by an impact load, adjacent flat portions 21, 22 undergo in-phase deformation, in which they deform convexly in the same direction inward and outward directions of the closed cross-sectional portion 10. When such deformation occurs, the torsional rigidity of the ridge portion 23 is high, allowing the flat portions 21, 22 to withstand a greater impact load, thereby achieving superior impact energy absorption efficiency. By improving energy absorption efficiency during a collision, occupant protection performance can be improved by reducing the amount of objects that enter the vehicle interior, and the frame member can also be made lighter.

[0074] The above describes an embodiment of the present invention. However, the present invention is not limited to the above embodiment. Various modifications of the present invention are possible within the scope of the claims. Note that the following mainly describes configurations that differ from the above embodiment and modified examples, and similar configurations are designated by similar reference numerals and detailed description thereof is omitted.

[0075] [Variation 1] In the above-described embodiment, an example was described in which the steel plate base material of the skeletal member 1 is a steel plate, and the strength distribution in the thickness direction is substantially unchanged. However, this need not be the case. FIG. 6 is an axial cross-sectional view of a main portion of Variation 1, with the back side of the cross section omitted. As shown in FIG. 6 , a softened layer 33 may be present near at least one of the outer surface 31 and the inner surface 32 of the first half 11 and the second half 12 of the skeletal member 1. In Variation 1, the softened layer 33 is present near both the outer surface 31 and the inner surface 32, and a central portion 34 is present between the softened layers 33. It is preferable that the entire outer surface 31 be formed with the softened layer 33, and it is also preferable that the entire inner surface 32 be formed with the softened layer 33.

[0076] (Overview of Softened Layer Thickness Range) Each softened layer 33 is formed, for example, by reducing the carbon content in the areas of the blank that form the outer surface 31 and the inner surface 32 (the areas that form the softened layer 33) that are the material for each half portion 11, 12 compared to the area that forms the central portion 34 of the blank. The method for forming the softened layer 33 is not particularly limited, and any method may be used. In this first modification, it is preferable that each softened layer 33 be provided to a predetermined thickness from the corresponding side surface (surface) 31, 32, for example, in order to increase the maximum bending angle of each half portion 11, 12 while ensuring sufficient strength of each half portion 11, 12. The lower limit of the predetermined thickness is, for example, 80 μm, and the upper limit is, for example, 200 μm.

[0077] (Outline of Vickers hardness distribution of softened layer and outline of Vickers hardness of central portion) In each softened layer 33, the Vickers hardness decreases with increasing distance from the half-thickness position of each half portion 11, 12. In each softened layer 33, the portions that become the surfaces 31, 32 have the lowest Vickers hardness in each half portion 11, 12, and are lower than the Vickers hardness of the central portion 34 by, for example, at least 100 HV. In each half portion 11, 12, examples of the upper limit of the difference in Vickers hardness between the surfaces 31, 32 and the central portion 34 include 250 HV, 300 HV, 350 HV, 400 HV, 500 HV, 550 HV, 600 HV, and 650 HV. Note that when simply referring to the "Vickers hardness of each half portion 11, 12," this refers to the Vickers hardness in the central portion 34.

[0078] In this variant example 1, in which a softening layer 33 is provided on each half 11, 12, the maximum bending angle of each half 11, 12 can be increased, for example, by about 20 degrees, compared to when the softening layer 33 is not provided (embodiment).

[0079] In this way, the Vickers hardness of the surfaces 31, 32 in the thickness direction of each half portion 11, 12 is set to be at least 100 HV lower than the Vickers hardness of the central portion 34 in the thickness direction of each half portion 11, 12. This makes it possible to increase the maximum bending angle of each half portion 11, 12. Therefore, it is possible to suppress the occurrence of cracks in each half portion 11, 12 during bellows deformation, thereby improving the impact absorption performance.

[0080] An example of the configuration of each half portion 11, 12 will be described in more detail.

[0081] (Vickers hardness of the central portion of each half) The Vickers hardness of the central portion 34 in the plate thickness direction in the portion where the softened layer 33 is provided is preferably 300 HV or more. When the Vickers hardness of the central portion 34 is 300 HV or more, the effect of improving the deformability of each half portion 11, 12 due to the softened layer 33 becomes significant. The Vickers hardness of the central portion 34 is preferably 500 HV or more, more preferably 600 HV or more, and even more preferably 700 HV or more. There is no particular upper limit to the Vickers hardness of the central portion 34, but in consideration of formability, etc., it is preferably 900 HV or less, and even more preferably 800 HV or less. Thus, examples of preferable lower limits of the Vickers hardness of the central portion 34 include 300 HV, 350 HV, 400 HV, 450 HV, 500 HV, 550 HV, 600 HV, 700 HV, 720 HV, and 750 HV. Examples of preferable upper limits of the Vickers hardness of the central portion 34 include 1100 HV, 1050 HV, 1000 HV, 950 HV, 900 HV, 850 HV, and 800 HV.

[0082] (Details of the Thickness of Each Softened Layer) The thickness of each softened layer 33 in the thickness direction is preferably 80 μm or more and 5% to 20% of the plate thickness t at the portion where the softened layer 33 is provided. If the thickness of each softened layer 33 is 20% or less of the plate thickness t, the proportion of each softened layer 33 in the steel plate from which each half 11, 12 is made is small, so that the load-bearing capacity required for each half 11, 12 can be maintained. The thickness of each softened layer 33 is preferably 17% or less of the plate thickness, and more preferably 14% or less. On the other hand, when the softened layer 33 is provided over the entire surface of the steel plate from which each half 11, 12 is made, the deformability of the softened layer 33 can be fully exhibited if the thickness of each softened layer 33 is 80 μm or more and 5% or more of the plate thickness t. It is more preferable that the thickness of each softened layer 33 be 8% or more of the plate thickness.

[0083] (Method for Measuring the Thickness Position of the Boundary Between the Softened Layer and the Central Portion) Next, a method for measuring the boundary between the softened layer 33 and the central portion 34 will be described. FIG. 7 is an image diagram illustrating an example of a method for measuring the boundary between the softened layer 33 and the central portion 34. A cross section perpendicular to the plate surface of the sample taken from each half portion 11, 12 is taken, and the sample is subjected to hardness testing after preparation of the measurement surface. The measurement surface is prepared to minimize unevenness and prevent sagging near the surface in order to accurately measure the Vickers hardness near the surface of the sample. Here, the measurement surface is sputtered with an argon ion beam using a cross-section polisher manufactured by JEOL Ltd. In this process, to prevent streaky unevenness from occurring on the measurement surface, an argon ion beam is irradiated from 360 degrees on the measurement surface using a sample rotating holder manufactured by JEOL Ltd.

[0084] The Vickers hardness of the sample with the prepared measurement surface is measured using a micro Vickers hardness tester. The hardness is measured in a region from the surface of the sample corresponding to the softened layer of the sample in a direction perpendicular to the surface (thickness direction) with a test force of 50 gf.

[0085] The measurement position on the surface-most side of the sample is 20 μm from either of the two surfaces 31, 32 (if a coating layer is present, this refers to the surface of the steel sheet base material directly below the coating layer; if an alloy layer is present between the coating layer and the steel sheet base material in addition to the coating layer, this refers to the surface of the steel sheet base material directly below the alloy layer). When measuring the boundary between the softened layer 33 and the central portion 34, the measurement points are spaced at equal intervals of 5 μm to 15 μm in the thickness direction, and the distance between the centers of the indentations is at least three times the average diagonal length of the indentations. Depending on the average diagonal length of the indentations, it may not be possible to ensure a center-to-center distance of at least three times the average diagonal length of the indentations in a row along the thickness direction. In this case, measurements are made at different positions in the thickness direction as well as in the direction perpendicular to the thickness direction. This makes it possible to meet the measurement conditions of dimples being spaced at equal intervals of 5 μm to 15 μm in the thickness direction and the distance between the centers of the dimples being at least three times the average diagonal length of the dimples. Measurements are taken from a thickness position of 20 μm from the surface to a position halfway along the thickness direction.

[0086] (Method for calculating the slope of Vickers hardness at each thickness position after measurement of the boundary between the softened layer and the central portion) The slope of the Vickers hardness at each thickness position after measuring the Vickers hardness when measuring the boundary between the softened layer 33 and the central portion 34 is defined as a slope Δbi obtained from the Vickers hardness at multiple consecutive points (three points), for example. The slope Δbi is calculated using the following formula (4): where, Δbi: gradient (HV / μm) calculated from three points i-th to i+2-th; xi: thickness position (μm) of the i-th measurement point from the surface of each half 11, 12; xk: thickness position (μm) from the surface of each half 11, 12; yk: average value (HV) of Vickers hardness at three different points at the thickness position xk.

[0087] Of the three measurement points at which the slope Δbi calculated by equation (4) from the surface side of each half portion 11, 12 first becomes 0.5 (HV / μm) or less, the thickness position of the measurement point closest to the steel plate surface is determined to be the thickness position of the boundary between the softened layer 33 and the central portion 34.

[0088] (Relationship between Vickers hardness of softened layer and Vickers hardness of central portion) It is preferable that the Vickers hardness of the softened layer 33 on the surfaces 31, 32 of each half portion 11, 12 is 0.5 times or more and less than 0.9 times the Vickers hardness of the central portion 34 where the softened layer 33 is provided.

[0089] (Method for Measuring Vickers Hardness of the Center of Each Half) The Vickers hardness of the center portion 34 is measured as follows. A cross section perpendicular to the plate surface of the sample cut from each half portion 11, 12 is taken, and the sample surface is prepared for hardness testing. The measurement surface is prepared in accordance with JIS Z 2244:2020. The measurement surface is polished using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 μm to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The hardness test is performed in accordance with JIS Z 2244:2020. Using a micro Vickers hardness tester, 10 indentations are measured at a center-to-center distance of at least three times the average diagonal length of the indentations at a position halfway through the plate thickness of the sample with a test force of 1 kgf, and the average value is taken as the Vickers hardness of the center portion 34.

[0090] (Method for measuring Vickers hardness of surface of each half portion) The Vickers hardness of the surfaces 31, 32 of each half portion 11, 12 is measured on a cross section obtained by cutting each half portion 11, 12 along the plate thickness direction in accordance with the Vickers hardness test described in JIS Z 2244:2020.

[0091] After the above cross section is subjected to sample preparation of the measurement surface, it is subjected to a hardness test. The measurement surface is prepared so that the surface has as little unevenness as possible and no sagging occurs near the surface in order to accurately measure the Vickers hardness near the surface of the sample. Here, the measurement surface is sputtered with an argon ion beam using a cross-section polisher manufactured by JEOL. At this time, in order to prevent streaky unevenness from occurring on the measurement surface, an argon ion beam is irradiated from 360 degrees on the measurement surface using a sample rotation holder manufactured by JEOL.

[0092] The Vickers hardness of the sample with the prepared measurement surface is measured using a micro Vickers hardness tester. The measurement point is 20 μm thick from the surface 31, 32 of each half 11, 12. If a plating layer is present on each half 11, 12, the measurement point is 20 μm thick from the surface of the steel sheet base material directly below the plating layer. If an alloy layer is present between the plating layer and the steel sheet base material in addition to the plating layer on each half 11, 12, the measurement point is 20 μm thick from the surface of the steel sheet base material directly below the alloy layer. Ten indentations are measured at the above-mentioned thickness positions from the surface of the sample in a direction perpendicular to the sheet surface (sheet thickness direction) with a test force of 10 gf, with a center-to-center distance of at least three times the average diagonal length of the indentations. The average value is taken as the Vickers hardness of the surface 31, 32 of each half 11, 12.

[0093] If the Vickers hardness of the surfaces 31, 32 of each half portion 11, 12 is 0.5 times or more the Vickers hardness of the central portion 34, the load-bearing capacity during a collision can be improved, particularly in the later stage of the stroke during a collision. It is more preferable that the Vickers hardness of the surfaces 31, 32 of each half portion 11, 12 is 0.6 times or more the Vickers hardness of the central portion 34. On the other hand, if the Vickers hardness of the surfaces 31, 32 of each half portion 11, 12 is less than 0.9 times the Vickers hardness of the central portion 34, the deformability can be sufficiently improved. It is more preferable that the Vickers hardness of the surfaces 31, 32 of each half portion 11, 12 is less than 0.8 times the Vickers hardness of the central portion 34.

[0094] (Vickers hardness change in softened layer) Figure 8 is an image diagram for explaining an example of Vickers hardness change in the softened layer 33. As shown in Figure 8, the softened layer 33 preferably has, in the thickness direction, a first hardness change region that is a region from the surfaces 31, 32 to 40% of the thickness of the softened layer 33, and a second hardness change region that is a region of the softened layer 33 that is not the first hardness change region. The absolute value ΔHV1 of the hardness change in the thickness direction in the first hardness change region is preferably larger than the absolute value ΔHV2 of the hardness change in the thickness direction in the second hardness change region. If ΔHV1 is larger than ΔHV2, sufficient load characteristics can be obtained.

[0095] The absolute value ΔHV1 of the hardness change in the first hardness change region is preferably 100 HV or more and less than 200 HV. When ΔHV1 is 100 HV or more, stress concentration during bending deformation during bellows deformation can be further alleviated, thereby further improving bending characteristics. Furthermore, when ΔHV1 is less than 200 HV, the effect of alleviating stress concentration during bending deformation is further enhanced, resulting in better bending characteristics. Therefore, when ΔHV1 is 100 HV or more and less than 200 HV, good bending characteristics can be obtained, and the deformability of each half portion 11, 12 can be improved. Specifically, the load drop immediately after the load peak can be made gentler in the later stage of the stroke during a collision. Therefore, as described above, the absolute value ΔHV1 of the hardness change in the first hardness change region is preferably 100 HV or more and less than 200 HV. The lower limit of ΔHV1 is preferably 100 HV, while the upper limit may be less than 200 HV, less than 300 HV, or less than 400 HV.

[0096] (Method for Measuring Vickers Hardness of First and Second Hardness Change Regions of the Softened Layer) Next, a method for measuring the hardness of the first and second hardness change regions will be described. A cross section perpendicular to the plate surface of each sample taken from each half 11, 12 is taken, and the sample is prepared for the measurement surface before being subjected to a hardness test. The measurement surface is prepared to minimize unevenness and prevent sagging near the surface in order to accurately measure the Vickers hardness near the surface of the sample. Here, the measurement surface is sputtered with an argon ion beam using a cross-section polisher manufactured by JEOL Ltd. At this time, to prevent streaky unevenness from occurring on the measurement surface, an argon ion beam is irradiated from 360 degrees on the measurement surface using a sample rotating holder manufactured by JEOL Ltd.

[0097] The Vickers hardness of the sample with the prepared measurement surface is measured using a micro Vickers hardness tester. The area from the surface of the sample corresponding to the softened layer of the sample is measured in a direction perpendicular to the plate surface (plate thickness direction) with a test force of 10 gf. In this case, the total number of measurement points varies depending on the plate thickness of the sample, but the number of measurement points for calculating ΔHV1 and ΔHV2 described below is set in accordance with the description of JIS Z 2244:2020.

[0098] The thickness of the softened layer 33 is 80 μm or more, and is 5% to 20% of the plate thickness at the portion where the softened layer 33 is provided. The first hardness change region is the region from the surface of the softened layer 33 to 40% of the thickness, and in this embodiment, it exists at a thickness position of, for example, 40 μm at the minimum and 80 μm at the maximum, starting from the surfaces 31, 32 of each half portion 11, 12.

[0099] The measurement position on the most superficial side of the sample is a position 20 μm thick from the surface (if a coating layer is present, this refers to the surface of the steel sheet base material directly below the coating layer; if an alloy layer is present between the coating layer and the steel sheet base material in addition to the coating layer, this refers to the surface of the steel sheet base material directly below the alloy layer). The measurement point at this 20 μm thick position is different from the measurement points used for measuring the Vickers hardness of the surfaces 31, 32 of each half portion 11, 12 described above. The first hardness change region is measured by measuring the indentations at at least two locations in the thickness direction, at equal intervals of 15 μm or less in the thickness direction and with a center-to-center distance of at least three times the average diagonal length of the indentations. Depending on the average diagonal length of the indentations, it may be possible to measure only one point in the first hardness change region in a row along the thickness direction. In this case, measurements are made at different positions in the thickness direction and at different positions perpendicular to the thickness direction. This allows the Vickers hardness to be measured at at least two points in the first hardness change region while satisfying the measurement conditions of being spaced at equal intervals of 15 μm or less in the plate thickness direction and the distance between the centers of the depressions being at least three times the average diagonal length of the depressions.

[0100] The thickness of the softened layer 33 is 80 μm or more and 5% to 20% of the plate thickness at the portion where the softened layer 33 is provided, and the Vickers hardness of the second hardness change region is measured within this thickness range, excluding the first hardness change region. When the thickness of the softened layer 33 is 80 μm to 200 μm, the second hardness change region exists within a thickness range of 32 μm to 80 μm at minimum (48 μm thickness range) and 80 μm to 200 μm at maximum (120 μm thickness range) from the surface 31, 32 of each half portion 11, 12. The second hardness change region is measured at at least two locations in the plate thickness direction, at equal intervals of 15 μm or less in the plate thickness direction and with a center-to-center distance of at least three times the average diagonal length of the indentations. Depending on the average diagonal length of the indentations, it may not be possible to measure two adjacent points at a fixed interval in a row along the thickness direction for the second hardness change region. In this case, measurements are performed at different positions in the thickness direction while varying positions perpendicular to the thickness direction. This allows the Vickers hardness of the second hardness change region to be measured while meeting the measurement conditions of equal intervals of 15 μm or less in the thickness direction and a distance between the centers of the indentations that is at least three times the average diagonal length of the indentations. The second hardness change region may be measured in the same row as the first hardness change region. For example, the second hardness change region may be measured at four points: one point near the boundary with the first hardness change region, one point near the boundary with the central portion 58, and two points between these two points. When the second hardness change region is present up to 200 μm from the steel sheet surface, the measurement points of the first hardness change region and the second hardness change region can be measured at thickness positions, for example, 20 μm, 35 μm, 50 μm, 65 μm, 80 μm, 95 μm, 110 μm, 125 μm, 140 μm, 155 μm, 170 μm, 185 μm, and 200 μm from the steel sheet surface.

[0101] In the case of a sample in which a softening layer 33 is disposed on both sides of the central portion 34 of each half portion 11, 12, similar measurements are made from the first surface side of the sample, and also from the second surface side opposite the first surface.

[0102] (Method for calculating absolute value ΔHV1 of hardness change after measurement of first hardness change region) ΔHV1 is calculated by the following procedure. That is, from all measurement points included in the region (first hardness change region) from the surfaces 31, 32 of the samples cut out from each half 11, 12 to 40% of the entire thickness of the softened layer 33, the hardness gradient Δa of the first hardness change region is calculated using equation (5). Here, a is the proportion (%) of the distance from the surface at the i-th measurement point to the entire thickness of the softened layer, c is the Vickers hardness (HV) at a, and n is the total for all measurement points included in the region (first hardness change region) from the surface to 40% of the entire thickness of the softened layer.

[0103]

[0104] where, Δa: gradient (HV / %) of hardness change in the plate thickness direction in the first hardness change region, ai: proportion (%) of the distance from the surface at the i-th measurement point to the total thickness of the softened layer, ci: average value (HV) of Vickers hardness at three different points at the i-th measurement thickness position, and n: total of all measurement points included in the first hardness change region on the first surface side.

[0105] In the case of a sample in which softened layers 33 are disposed on both sides of the central portion 34, Δa1 on the first surface side is calculated from equation (5) based on the results of measuring the Vickers hardness from the first surface side, and Δa2 on the second surface side is calculated from equation (5) based on the results of measuring the Vickers hardness from the second surface side. The arithmetic mean of Δa1 and Δa2 can be taken as Δa.

[0106] ΔHV1 can be obtained by multiplying Δa obtained by equation (5) by the ratio of the thickness of the first hardness change region in the thickness direction of the entire softened layer.

[0107] (Method of calculating absolute value ΔHV2 of hardness change after measurement of second hardness change region) ΔHV2 is calculated by the following procedure. That is, from all measurement points included in the region (second hardness change region) from 40% to 100% of the total thickness of the softened layer 33 on the surface side of the sample, the hardness gradient ΔA of the second hardness change region is calculated using equation (6). Here, Ai is the proportion (%) of the distance from the surface at the i-th measurement point to the total thickness of the softened layer, Ci is the Vickers hardness (HV) at Ai, and N is the total for all measurement points included in the region (second hardness change region) from 40% to 100% of the total thickness of the softened layer on the surface side.

[0108]

[0109] where, ΔA: gradient (HV / %) of hardness change in the plate thickness direction in the second hardness change region; Ai: proportion (%) of the distance from the surface at the i-th measurement point to the total thickness of the softened layer; Ci: average value (HV) of Vickers hardness at three different points at the i-th measurement thickness position; N: total of all measurement points included in the second hardness change region on the first surface side.

[0110] In the case of a sample in which softened layers 33 are disposed on both sides of the central portion 34, ΔA1 on the first surface side is calculated from the results of measuring the Vickers hardness from the first surface side using formula (6), and ΔA2 on the second surface side is calculated from the results of measuring the Vickers hardness from the second surface side using formula (6). The arithmetic mean of ΔA1 and ΔA2 can be taken as ΔA.

[0111] ΔHV2 can be obtained by multiplying ΔA obtained by equation (6) by the ratio of the thickness of the second hardness change region in the thickness direction of the entire softened layer.

[0112] (Effect) According to the configuration described in Modification 1, the possibility of fracture of the skeletal member 1 can be reduced by improving the bending deformability of the skeletal member 1 due to the softened layer 33. When the skeletal member 1 undergoes in-phase deformation, large strain is introduced into the ridge portion 23, but by improving the bending deformability of the skeletal member 1 during bellows deformation, the possibility of fracture of the base material of the skeletal member 1 due to strain can be further reduced. This makes it possible to more stabilize the impact energy absorption capacity of the skeletal member 1. In this way, a high energy absorption capacity can be achieved in the skeletal member 1 by combining a design that achieves in-phase deformation with the use of steel material with a modified surface layer.

[0113] [Variation 2] In the above-described embodiment and variation, both the first half 11 and the second half 12 of the skeletal member 1 have a hat-shaped cross section perpendicular to the axis. However, this is not necessarily the case. FIG. 9 is a cross-sectional view perpendicular to the axis of variation 2 in which the skeletal member 1 is composed of the first half 11 and the second half 12A. As shown in FIG. 9, the second half 12A is formed flat overall and does not have a ridge line 23. In this case, the flanges 25c and 25d of the second half 12A are joined to the flanges 25a and 25b of the first half 11, and these flanges 25c and 25d are continuous with the first flat portion 21b. In this skeletal member 1, the flat halves 24a and 24b of the first half 11 form the second flat portions 22A and 22A (22aA and 22bA). In this case, the frame member 1 may be a steel plate member, an aluminum alloy member, or a composite member made by combining a plurality of types of materials.

[0114] (Relationship between R / bf and energy absorption amount) The skeleton member 1 shown in Fig. 3 was modeled on a computer and produced as test examples 1 to 6. The specifications common to test examples 1 to 4 of test examples 1 to 6 were as follows.

[0115] Specifications common to Test Examples 1 to 4 Material: Steel plate Tensile strength: 1300 MPa Plate thickness t: 1.2 mm Length in longitudinal direction Z: 480 mm Radius of curvature R of ridge portion 23: 5, 10, 15, 20, 25, 30 mm Length of flat portion bf: 100, 90, 80, 70, 60, 50 mm

[0116] The maximum bending angle settings for each of Test Examples 1 to 4 were as follows: Test Example 1: 50 degrees Test Example 2: 65 degrees Test Example 3: 80 degrees Test Example 4: 110 degrees

[0117] Test Example 5 was the same as Test Examples 1 to 4, except that the tensile strength was set to 590 MPa and that settings were made so that bending fracture would not occur. Test Example 6 was the same as Test Examples 1 to 4, except that the material was a 6000 series aluminum alloy, the tensile strength was set to 425 MPa, the plate thickness was set to 2.4 mm, and settings were made so that bending fracture would not occur.

[0118] For each of Test Examples 1 to 6, the base end in the longitudinal direction Z was fixed, and an impact load was applied to the tip in the longitudinal direction Z under the following conditions. The energy absorption amount and energy absorption efficiency were calculated using the finite element method (FEM). More specifically, fracture prediction software NSafe (registered trademark)-MAT developed by Nippon Steel Corporation was used. The surface strain fracture function of this software was used to calculate the energy absorption amount. Impactor shape: flat rigid body Impactor impact speed: 14.4 km / h The energy absorption amount was calculated for each of Test Examples 1 to 6 by sequentially changing R / bf to 0.05, 0.11, 0.19, 0.29, 0.42, and 0.60.

[0119] The results are shown in Figures 10A and 10B. Figure 10A is a graph showing the amount of energy absorbed by the frame member 1 when R / bf was changed for Test Examples 2 to 6. Figure 10B is a graph showing the amount of energy absorbed per kg (energy absorption efficiency) by the frame member 1 when R / bf was changed for Test Examples 2 to 6.

[0120] 10A and 10B, for Test Example 1 (maximum bending angle 50 degrees), large base material fracture occurred when R / bf > 0.00, so the energy absorption amount could not be calculated and is not shown in the graph. For Test Example 2 (maximum bending angle 60 degrees), large base material fracture occurred when R / bf > 0.19, so the energy absorption amount up to R / bf = 0.19 is shown in the graph. On the other hand, for Test Examples 3 to 6, the energy absorption amount up to R / bf = 0.60 is shown in the graph.

[0121] 10A and 10B, in Test Examples 3 to 6, when R / bf was between 0.20 and 0.60, exceeding 0.19, the amount of absorbed energy increased significantly, demonstrating that 0.20≦R / bf≦0.60 is preferable. Note that Test Example 5, which had a tensile strength of 590 MPa, had a relatively low Vickers hardness and no significant difference in the way strain was introduced during in-phase deformation and anti-phase deformation, so the amount of absorbed energy did not change significantly regardless of the value of R / bf.

[0122] (Preferable lower limit of R / bf from the viewpoint that the material of the skeletal member is a steel plate) The skeletal member 1 shown in Fig. 3 was modeled on a computer and produced as Test Examples 7 to 11. The specifications common to Test Examples 7 to 11 were as follows.

[0123] Specifications common to Test Examples 7 to 11 Material: Steel plate Curvature radius R of ridge line portion 23: 5 to 30 mm

[0124] The plate thicknesses of Test Examples 7 to 11 were as follows: Test Example 7: 0.8 mm Test Example 8: 1.0 mm Test Example 9: 1.2 mm Test Example 10: 1.4 mm Test Example 11: 1.6 mm

[0125] The buckling stress was calculated by FSM (finite strip method) for each of Test Examples 7 to 11. The results are shown in Figures 11A and 11B.

[0126] Figure 11A is a graph showing the relationship between R / bf and buckling stress for Test Examples 7 to 11, and is displayed so that either antiphase deformation or inphase deformation can be identified. Figure 11B is a graph showing the relationship between plate thickness t and R / bf when deformation changes from antiphase deformation to inphase deformation for Test Examples 7 to 11.

[0127] In FIG. 11A, the horizontal axis represents R / bf and the vertical axis represents buckling stress (MPa). Also, in FIG. 11A, R / bf during antiphase deformation is represented by a triangle, and R / bf during in-phase deformation is represented by a circle. In each of Test Examples 7 to 11, consecutive triangles are connected by a line, and consecutive circles are connected by a line, with no line connecting the triangles and circles. In each of Test Examples 7 to 11, the points where the triangles and circles are not connected by a line represent the boundary between R / bf where antiphase deformation occurs and R / b where in-phase deformation occurs. In FIG. 11B, the horizontal axis represents plate thickness t (mm) and the vertical axis represents R / bf. For each of Test Examples 7 to 11, the R / bf when changing from antiphase deformation to in-phase deformation is shown.

[0128] 11B, when R / bf changes from antiphase deformation to inphase deformation for each of Test Examples 7 to 11, the regression line shows that R / bf = 0.1616t + 0.0235. In other words, it was demonstrated that when the skeleton member 1 is a steel plate member, it is preferable that R / bf ≧ 0.1616t + 0.0235.

[0129] (Preferable lower limit of R / bf from the viewpoint that the material of the skeletal member is a metal material) For Test Examples 7 to 11, from the above analysis, the minimum value of R / bf when in-phase deformation occurs during axial crushing deformation was calculated as shown in Table 1 below. Table 1 shows the plate thickness t, Young's modulus E, and Et ３ is stated.

[0130] Furthermore, for Test Example 6 made of an aluminum alloy material, the minimum value of R / bf when in-phase deformation occurs during axial crushing deformation was calculated by the same analysis as in Test Examples 7 to 11. This minimum value was calculated based on the plate thickness, Young's modulus E, and Et ３ These are also shown in Table 1.

[0131] Et in Test Examples 6 to 11 in Table 1 ３The graph shown in Figure 12 is obtained by plotting the R / bf values ​​in Table 1 with the horizontal axis representing R / bf and the vertical axis representing R / bf. Figure 12 is a graph plotting the minimum R / bf at which anti-phase deformation occurs for Test Examples 6 to 11. The trend line in Figure 12 is R / bf = (1.49 x 10 －７ ×Et ３ ) +0.153, and it has been demonstrated that when R / bf is equal to or greater than this value, anti-phase deformation occurs.

[0132] The present invention can be widely applied to frame members, vehicle bodies, and methods for manufacturing frame members.

[0133] REFERENCE SIGNS LIST 1 skeletal member 10 closed cross-section portion 11 first half portion 12, 12A second half portion 21 first flat portion 22 second flat portion 23 ridge portion (first ridge portion) 26 second ridge portion 24 flat half portion 25 flange 31 outer surface 32 inner surface 33 softened layer 34 central portion b length bf length R26 radius of curvature (radius of curvature R2) X width direction Y height direction Z longitudinal direction W maximum outer dimension

Claims

1. A skeletal member having a closed cross-section portion whose cross section perpendicular to the longitudinal direction is a closed cross-section, the closed cross-section portion comprising a first flat portion, a second flat portion, and a ridge portion connecting the first flat portion and the second flat portion and having a radius of curvature smaller than both the radius of curvature of the first flat portion and the radius of curvature of the second flat portion, the cross-section of the closed cross-section portion having a shape that causes deformation in which the first flat portion and the second flat portion become convex in the same phase in the direction toward the inside of the closed cross-section portion and the direction toward the outside of the closed cross-section portion when the skeletal member undergoes bellows deformation due to a compressive load along the longitudinal direction.

2. The framework member according to claim 1, wherein the radius of curvature of the ridge line portion in the cross section is 15 mm or more.

3. The skeletal member according to claim 1, wherein R / bf satisfies the following formula (1): R / bf≧(1.49×10) / ... －７ ×Et ３ ) + 0.153 ... (1) 4. The skeletal member according to claim 1, wherein the skeletal member is a steel plate member, and R / bf satisfies the following formula (2), where t is the plate thickness of the skeletal member, R is the radius of curvature of the ridge line portion, and bf is the length of at least one of the flat portions in the cross section: R / bf≧0.1616t+0.0235...(2) 5. The skeletal member according to claim 4, wherein the skeletal member is formed by joining a first half portion and a second half portion together, the first half portion being hat-shaped in cross section, the first half portion comprising the first flat portion, a portion of the second flat portion, a first ridge portion as the ridge portion, a flange joined to the second half portion, and a second ridge portion connecting the portion of the second flat portion and the flange, and wherein, when the radius of curvature of the second ridge portion is R2, R2 / bf satisfies the following formula (3): R2 / bf<0.1616t+0.0235...(3) 6. A skeletal member according to claim 1, wherein, when the radius of curvature of the ridge line portion is R and the length of at least one of the flat portions in the cross section is bf, 0.20≦R / bf≦0.

60.

7. A skeletal member according to claim 1, wherein, when the shorter of the length of said closed cross-section portion in the width direction at said cross-section and the length of said closed cross-section portion in the height direction at said cross-section is b, b≧75 mm; when said skeletal member is a steel plate member, the plate thickness of said closed cross-section portion at said cross-section is 0.8 mm to 2.6 mm; and when said skeletal member is an aluminum alloy plate member, the plate thickness of said closed cross-section portion at said cross-section is 1.1 mm to 3.7 mm.

8. The framework member according to claim 1, wherein bf is 75 mm or more, where bf is the length of at least one of the flat portions in the cross section.

9. A frame member according to claim 1, wherein the Vickers hardness of the central portion in the thickness direction of the closed cross-section portion is 300 HV or more.

10. A skeletal member according to claim 1, wherein in the cross section, the Vickers hardness at the surface in the thickness direction of the closed cross section portion is at least 100 HV lower than the Vickers hardness at the center in the thickness direction of the closed cross section portion.

11. In the cross section, the closed cross section portion has a softening layer formed from the surface in the thickness direction, the Vickers hardness of the central portion in the thickness direction of the closed cross section portion is 300 HV or more, the thickness of the softening layer is 80 μm or more and is 5% to 20% of the thickness of the portion where the softening layer is formed, the Vickers hardness of the softening layer at the surface is 0.5 to less than 0.9 times the Vickers hardness of the central portion in the thickness direction of the portion where the softening layer is formed, and the softening layer has a first hardness change region that is a region from the surface to 40% of the thickness of the softening layer in the thickness direction, and a second hardness change region that is a region of the softening layer that is not the first hardness change region, 11. The skeletal member according to claim 10, wherein an absolute value ΔHV1 of the hardness change in the thickness direction in the first hardness change region is greater than an absolute value ΔHV2 of the hardness change in the thickness direction in the second hardness change region.

12. A vehicle body comprising the frame member according to any one of claims 1 to 11.

13. A method for manufacturing a skeletal member having a closed cross-section portion whose cross section perpendicular to the longitudinal direction is a closed cross-section, the closed cross-section portion comprising a first flat portion, a second flat portion, and a ridge portion connecting the first flat portion and the second flat portion and having a radius of curvature smaller than both the radius of curvature of the first flat portion and the radius of curvature of the second flat portion, wherein a shape of the closed cross-section portion is set so that when the skeletal member undergoes bellows deformation due to a compressive load along the longitudinal direction, the first flat portion and the second flat portion are deformed to become convex in phase in the cross-section with respect to the phases toward the inside of the closed cross-section portion and the phases toward the outside of the closed cross-section portion, and the closed cross-section portion is manufactured to have the set shape of the closed cross-section portion.

14. A method for manufacturing a skeletal member according to claim 13, wherein in setting the shape of the closed cross-sectional portion, R / bf is set to a value that satisfies the following formula (1), where E is the Young's modulus of the skeletal member, t is the plate thickness of the closed cross-sectional portion, R is the radius of curvature of the ridge line portion, and bf is the length of at least one of the flat portions in the cross section. R / bf≧(1.49×10 －７ ×Et ３ ) + 0.153 ... (1) 15. A method for manufacturing a skeletal member as set forth in claim 13, wherein, in setting the shape of the closed cross-sectional portion, when the radius of curvature of the ridge line portion is R and the length of at least one of the flat portions in the cross-section is bf, the values ​​are set to 0.20≦R / bf≦0.

60.

16. A method for manufacturing a skeletal member according to claim 13, wherein, in setting the shape of said closed cross-sectional portion, when the relatively smaller length of the length of said closed cross-sectional portion in the width direction of said closed cross-sectional portion in said cross-section and the length of said closed cross-sectional portion in the height direction of said closed cross-sectional portion in said cross-section is b, b ≥ 75 mm; when said skeletal member is a steel plate member, the plate thickness of said closed cross-sectional portion in said cross-section is 0.8 mm to 2.6 mm; and when said skeletal member is an aluminum alloy plate member, the plate thickness of said closed cross-sectional portion in said cross-section is 1.1 mm to 3.7 mm.

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