Impact absorbing member and vehicle body

Abstract

To provide an impact absorption member having lighter weight and capable of achieving high collision resistance performance, and a vehicle body including the impact absorption member.SOLUTION: An impact absorption member 4 is provided in a vehicle body 1, and includes a closed cross section part formed along a predetermined longitudinal direction and having a cross section orthogonal to the longitudinal direction being a closed cross-sectional shape. The impact absorption member includes: a low strength part 51; a high strength part 53 aligned with the low strength part and having a plate thickness direction center part having a higher Vickers hardness than a Vickers hardness of a plate thickness direction center part of the low strength part; and a bonding part 52 bonding the low strength part and the high strength part. The maximum bending angle of the high strength part is set according to the maximum bending angle of the low strength part.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an impact absorbing member and a vehicle body. [Background technology]

[0002] While automobiles are required to reduce CO2 emissions, collision safety standards are becoming stricter. Therefore, automotive components, such as impact-absorbing components, are required to be lighter and have improved collision safety, making material selection and structural design of impact-absorbing components important. Center pillars, which are important components for protecting occupants in side collisions, require more advanced material selection and structural design expertise. The upper part of the center pillar requires high strength to prevent deformation from the perspective of occupant protection, so the use of high-strength materials is effective. Meanwhile, the lower part of the center pillar is required to absorb energy, so a material with a good balance of strength and fracture resistance is required. Therefore, from the perspective of weight reduction, tailor-welded blanks (TWBs), which allow two different materials to be combined into a single blank, are useful as center pillar materials. Patent Document 1 (Patent Document 1) is an example of a TWB material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-55526 Summary of the Invention [Problem to be solved by the invention]

[0004] With the aim of reducing CO2 emissions, automakers are rapidly developing electric vehicles. In vehicles that transmit the rotational output of an internal combustion engine to the wheels, such as gasoline and diesel engine vehicles, the structure of the side sill means that in the event of a side collision, the side sill connected to the bottom of the center pillar tends to deform significantly and rotate, while the center pillar also tends to deform. For this reason, the deformation of the center pillar is localized.

[0005] On the other hand, in electric vehicles, the battery is located to the side of the side sill. To protect this battery, the side sill is reinforced with an impact-absorbing member or the like installed inside the side sill, making the side sill less likely to deform. In this way, in electric vehicles, where the side sill tends to be less likely to deform due to the battery protection structure, the amount of deformation of the side sill is relatively small during a side collision, but the amount of deformation of the center pillar tends to increase due to impact absorption. In particular, in center pillar outers formed from TWBs and having higher tensile strength in the upper part than in the lower part, the lower end of the high-strength upper part (the portion near the weld line with the lower part) may also deform significantly due to the deformation of the lower part, which has a low strength, potentially resulting in base material fracture. Since the possibility of base material fracture is preferably avoided from the perspective of ensuring impact absorption performance, it is preferable to suppress such possibility. Increasing the thickness of the center pillar outer is one possible way to suppress the possibility of base material fracture, but increasing the thickness of the center pillar outer is undesirable because it increases the weight of the center pillar outer. In order to reduce the weight of the center pillar outer while improving its impact absorption performance in the event of a side collision, it is preferable to make the steel plate that makes up the center pillar outer thin and strong, and also to reduce the possibility of the component breaking in the event of a side collision.

[0006] An object of the present invention is to provide an impact absorbing member that is lighter in weight and can achieve high crash resistance, and a vehicle body equipped with the same. [Means for solving the problem]

[0007] The present invention relates to the following impact absorbing member and vehicle body.

[0008] (1) An impact absorbing member provided on a vehicle body, formed along a predetermined longitudinal direction, including a portion having a closed cross-sectional shape in a cross section perpendicular to the longitudinal direction, A low strength portion; a high-strength portion that is aligned with the low-strength portion in the longitudinal direction and has a central portion in the plate thickness direction having a Vickers hardness higher than the Vickers hardness of a central portion in the plate thickness direction of the low-strength portion; a joining portion joining the low strength portion and the high strength portion; Equipped with An impact absorbing member, wherein the maximum bending angle of the high strength portion is set according to the maximum bending angle of the low strength portion.

[0009] According to this configuration, for example, by increasing the maximum bending angle of the material of the high-strength portion, the difference between the maximum bending angle of the material of the high-strength portion and the maximum bending angle of the material of the low-strength portion can be reduced. By setting such a bending angle, the high-strength portion material can exhibit high bending performance due to its high Vickers hardness. Therefore, even when the high-strength portion undergoes relatively large plastic deformation due to the influence of plastic deformation of the low-strength portion during a side collision, the deformability of the high-strength portion can be increased, thereby reducing the possibility of fracture of the high-strength portion near the joint. Furthermore, the high-strength portion can be applied to locations requiring high strength from the perspective of occupant protection. Here, if a material with low Vickers hardness (low strength) is used to ensure high bendability, the plate thickness must be increased to reduce the amount of deformation toward the occupant, resulting in an increase in mass. On the other hand, according to the above configuration, the high-strength portion has high strength due to its high Vickers hardness, thereby improving impact absorption performance during a collision such as a side collision, while the high-strength portion can be made thinner, thereby achieving a lightweight impact-absorbing component. In this way, by using a material that combines Vickers hardness (strength) and bendability as the high-strength portion, it is possible to realize an impact-absorbing component that is lightweight and has high impact absorption performance by reducing the possibility of breakage.

[0010] (2) The impact absorbing member according to (1), wherein the difference between the maximum bending angle of the high strength portion and the maximum bending angle of the low strength portion is 100 degrees or less.

[0011] This configuration more reliably suppresses the possibility of fracture of the high-strength portion that follows the bending deformation of the low-strength portion during a side collision of the vehicle, thereby realizing an impact absorbing member with high impact absorption performance through suppression of the possibility of fracture.

[0012] (3) The impact absorbing member according to (1) or (2), wherein the Vickers hardness of the central portion in the thickness direction of the high strength portion is 500 HV or more.

[0013] With this configuration, the difference in strength (strength ratio) between the high-strength portion and the low-strength portion is large, and the difference in bendability between the material of the high-strength portion and the material of the low-strength portion tends to be large. That is, when the Vickers hardness of the high-strength portion is 500 HV or higher, the high-strength portion is likely to be prone to fracture during a side collision due to the bending deformation of the high-strength portion that follows the bending deformation of the low-strength portion. Thus, when the Vickers hardness of the high-strength portion is 500 HV or higher, the problem of fracture in the high-strength portion becomes more pronounced. Even in such cases, the possibility of fracture in the high-strength portion during a side collision can be reduced by setting the maximum bending angle of the high-strength portion according to the maximum bending angle of the low-strength portion.

[0014] (4) The impact absorbing member according to any one of (1) to (3), wherein the Vickers hardness of the central portion in the thickness direction of the low strength portion is 150 HV or more.

[0015] With this configuration, during a side collision, the impact energy absorption effect due to plastic deformation of the low-strength parts can be increased while excessive plastic deformation of the high-strength parts can be suppressed, thereby improving impact absorption performance while reducing the amount of penetration of the high-strength parts into the cabin side of the vehicle body.

[0016] (5) An impact absorbing member according to any one of (1) to (4), wherein the ratio HV1 / HV2 of the Vickers hardness HV1 of the center portion in the thickness direction of the high strength portion to the Vickers hardness HV2 of the center portion in the thickness direction of the low strength portion is 1.3 or more.

[0017] This configuration tends to result in a large difference in strength (strength ratio) between the high-strength portion and the low-strength portion, which in turn tends to increase the difference in bendability between the material of the high-strength portion and the material of the low-strength portion. That is, when HV1 / HV2 is 1.3 or greater, the high-strength portion, which has a high Vickers hardness, is more likely to fracture during a side collision due to the bending deformation of the high-strength portion that follows the bending deformation of the low-strength portion. Thus, when HV1 / HV2 is 1.3 or greater, the need to reduce the likelihood of fracture in the high-strength portion becomes more pronounced. Even in such cases, the maximum bending angle of the high-strength portion is set according to the maximum bending angle of the low-strength portion, thereby reducing the likelihood of fracture in the high-strength portion during a side collision.

[0018] (6) The impact absorbing member according to any one of (1) to (5), wherein the difference between the maximum bending angle of the low strength portion and the maximum bending angle of the high strength portion is 30 degrees or less.

[0019] With this configuration, the high strength portion can bend and deform sufficiently to follow the bending deformation of the low strength portion during a side collision, thereby reducing the possibility of breakage of the high strength portion during a side collision.

[0020] (7) The impact absorbing member is a center pillar that includes an inner pillar and an outer pillar and is disposed along the vertical direction of the vehicle body, the pillar outer includes the low strength portion, the joint portion, and the high strength portion, The impact absorbing member according to any one of (1) to (6), wherein the high strength portion is disposed above the low strength portion.

[0021] With this configuration, in the event of a side collision, the outer pillar portion of the center pillar receives most of the impact load, and the high-strength portion located above the low-strength portion has high bendability, which more reliably prevents the high-strength portion from breaking, thereby more reliably protecting the occupant from damage.

[0022] (8) The center pillar is provided with a pair of upper and lower brackets for supporting a door installed behind the center pillar, The impact absorbing member according to (7) above, wherein the joint is positioned at a position lower than the height position of the lower end of the upper bracket.

[0023] This structure allows the high-strength portion to withstand the impact load that is input to the center pillar from the upper bracket (the input point of the impact load) located farther from the side sill, thereby reducing the amount of deformation of the center pillar toward the occupant in a side collision.

[0024] (9) The impact absorbing member according to (8), wherein the joint is positioned at a height higher than the height position of the upper end of the lower bracket.

[0025] This configuration ensures a sufficient area for the low-strength portion, which allows for both impact absorption by the low-strength portion in cooperation with the side sill in the early stages of a side collision and suppression of deformation of the center pillar toward the occupant in the later stages of a side collision, due to the high-strength portion receiving the impact load from the upper bracket.

[0026] (10) The impact absorbing member according to (8), wherein the joint is positioned at a position lower than the height position of the lower end of the lower bracket.

[0027] During a side collision, the side sill may undergo torsional deformation around an axis along the fore-and-aft direction. In cases where the side sill undergoes torsional deformation, positioning the joint at a height lower than the lower end of the lower bracket allows the high-strength portion to be installed close to the side sill, which experiences the greatest amount of deformation during a side collision. Therefore, during a side collision, the high-strength portion can suppress deformation of the center pillar, which follows the side sill's intrusion into the cabin due to torsional deformation of the side sill. As a result, the amount of intrusion of the center pillar into the cabin can be reduced, further suppressing torsional deformation of the side sill.

[0028] (11) The vehicle body further includes a side sill joined to a lower portion of the center pillar, arranged along the longitudinal direction of the vehicle body, and having a closed cross-sectional shape in a cross section perpendicular to the longitudinal direction, The side sill includes an outer wall disposed on the outer side of the side sill in a width direction of the vehicle body, The lower part of the center pillar has an overlapping portion arranged to cover the side sill at a connection point with the side sill, The impact absorbing member according to any one of (7) to (9), wherein the overlapping portion is arranged along a portion below half the height of the outer wall of the side sill in the up-down direction.

[0029] This configuration increases the joint area between the center pillar and the side sill, thereby reducing the stress acting between the side sill and the center pillar during a side collision. Furthermore, the load transferred from the center pillar to the side sill during a side collision is primarily received by the vehicle body as a torsional moment. This reduces deformation of the closed cross-sectional shape of the side sill, particularly the outer side sill. Even in a side collision, the original closed cross-sectional shape of the side sill before the collision is unlikely to change, so the side sill has high torsional rigidity and can reduce the twisting angle of the side sill during a side collision. This reduces the amount of intrusion of the center pillar and side sill into the cabin. Meanwhile, for example, by positioning the center pillar so that it does not reach the bottom of the side sill, the center pillar can be kept from being unnecessarily heavy, achieving a high level of balance between the enhanced side sill reinforcement effect provided by the lower shape of the center pillar and the reduction in the weight of the center pillar.

[0030] (12) An impact absorbing member according to any one of (1) to (11), wherein the Vickers hardness at the surface in the thickness direction of the high strength portion is at least 100 HV lower than the Vickers hardness at the central portion in the thickness direction of the high strength portion.

[0031] This configuration allows the high-strength portion to have a larger maximum bending angle, which allows the high-strength portion to more reliably follow the bending deformation of the low-strength portion during a side collision, thereby reducing the possibility of breakage in the high-strength portion and improving impact absorption performance.

[0032] (13) The high strength portion has a high strength portion softened layer provided from the surface in the plate thickness direction, The Vickers hardness of the central portion in the plate thickness direction in the portion where the high strength portion softened layer is provided in the high strength portion is 500 HV or more, The thickness of the high-strength portion softened layer is 80 μm or more, and is 5% to 20% of the plate thickness at the portion where the high-strength portion softened layer is provided, The Vickers hardness of the high-strength portion softened layer on the surface is 0.5 times or more and less than 0.9 times the Vickers hardness of the central portion in the plate thickness direction in the portion where the high-strength portion softened layer is provided, the high-strength portion softened layer has, in the plate thickness direction, a first hardness change region that is a region from the surface to 40% of the thickness of the high-strength portion softened layer, and a second hardness change region that is a region of the high-strength portion softened layer that is not the first hardness change region, The impact absorbing member according to (12), wherein the absolute value ΔHV1 of the hardness change in the thickness direction in the first hardness change region is greater than the absolute value ΔHV2 of the hardness change in the thickness direction in the second hardness change region.

[0033] According to this configuration, the Vickers hardness of the high-strength portion at the center in the thickness direction is 500 HV or more, thereby significantly improving the deformability of the softened layer in the high-strength portion. Furthermore, if the thickness of the high-strength portion softened layer is 20% or less of the plate thickness, the proportion of the high-strength portion softened layer in the steel plate used for the high-strength portion is small, thereby maintaining the load-bearing capacity required for the high-strength portion. On the other hand, if the thickness of the high-strength portion softened layer is 80 μm or more and 5% or more of the plate thickness, the deformability of the high-strength portion softened layer can be fully exhibited. Furthermore, if the Vickers hardness of the surface of the high-strength portion is 0.5 times or more the Vickers hardness of the center in the thickness direction, the load-bearing capacity during a collision, particularly in the later stage of the stroke, can be improved. On the other hand, if the Vickers hardness of the surface of the high-strength portion is less than 0.9 times the Vickers hardness of the center in the thickness direction, the deformability can be sufficiently improved. Furthermore, if ΔHV1 is greater than ΔHV2, sufficient load-bearing characteristics can be obtained.

[0034] (14) An impact absorbing member according to any one of (1) to (13), wherein the Vickers hardness at the surface in the thickness direction of the low strength portion is at least 100 HV lower than the Vickers hardness at the central portion in the thickness direction of the low strength portion.

[0035] This configuration allows the maximum bending angle of the low-strength portion to be increased. Therefore, cracking in the low-strength portion during a side collision can be suppressed, improving impact absorption performance. As a result, the high-strength portion has a high Vickers hardness and high strength, so the amount of penetration into the vehicle interior can be suppressed, improving occupant protection. The low-strength portion can be further thinned to reduce the vehicle body weight while ensuring the absorption of impact energy. Furthermore, even if the low-strength portion is formed from a high-strength material with a higher Vickers hardness, the possibility of fracture in the low-strength portion can be suppressed, so the amount of penetration of the low-strength portion into the vehicle body during a side collision can be further reduced.

[0036] (15) The low-strength portion has a low-strength portion softened layer provided from the surface in the plate thickness direction, The Vickers hardness of the central portion in the plate thickness direction in the portion where the low-strength portion softened layer is provided in the low-strength portion is 150 HV or more, The thickness of the low-strength portion softened layer is 80 μm or more, and is 5% to 20% of the plate thickness at the portion where the low-strength portion softened layer is provided, The Vickers hardness of the low-strength portion softened layer on the surface is 0.5 times or more and less than 0.9 times the Vickers hardness of the central portion in the plate thickness direction in the portion where the low-strength portion softened layer is provided, the low-strength portion softened layer has, in the plate thickness direction, a first hardness change region that is a region from the surface to 40% of the thickness of the low-strength portion softened layer, and a second hardness change region that is a region of the low-strength portion softened layer that is not the first hardness change region, The impact absorbing member according to (14), wherein the absolute value ΔHV1′ of the hardness change in the thickness direction in the first hardness change region is greater than the absolute value ΔHV2′ of the hardness change in the thickness direction in the second hardness change region.

[0037] According to this configuration, the Vickers hardness of the low-strength portion at the center in the thickness direction is 150 HV or more, thereby significantly improving the deformability of the low-strength portion due to the low-strength portion softened layer. Furthermore, if the thickness of the low-strength portion softened layer is 20% or less of the plate thickness, the low-strength portion softened layer accounts for a small proportion of the steel plate used to make the low-strength portion, thereby maintaining the load-bearing capacity required for the low-strength portion. On the other hand, if the thickness of the low-strength portion softened layer is 80 μm or more and 5% or more of the plate thickness, the deformability of the low-strength portion softened layer can be fully exhibited. Furthermore, if the Vickers hardness of the surface of the low-strength portion is 0.5 times or more the Vickers hardness of the center in the thickness direction, the load-bearing capacity during a collision, particularly in the later stage of the stroke, can be improved. On the other hand, if the Vickers hardness of the surface of the low-strength portion is less than 0.9 times the Vickers hardness of the center in the thickness direction, the deformability can be sufficiently improved. Furthermore, if ΔHV1′ is greater than ΔHV2′, sufficient load-bearing characteristics can be obtained.

[0038] (16) A vehicle comprising a center pillar and a side sill joined to a lower portion of the center pillar, A vehicle body, wherein at least one of the center pillar and the side sill is the impact absorbing member according to any one of (1) to (15).

[0039] This configuration makes it possible to realize a vehicle body that is lighter in weight and has high crashworthiness. [Effects of the Invention]

[0040] According to the present invention, it is possible to realize a shock absorbing member that is lighter in weight and can achieve high crash resistance. [Brief explanation of the drawings]

[0041] [Figure 1] FIG. 1 is a schematic left side view showing a main part of a vehicle body to which an impact absorbing member according to one embodiment of the present invention is applied. [Figure 2]FIG. 2 is a cross-sectional view of the closed cross-section portion of the center pillar taken along line II-II in FIG. 1, perpendicular to the up-down direction, and does not show the back side of the cross-section. [Figure 3] FIG. 3 is a schematic cross-sectional view of the main parts taken along line III-III in FIG. 1 to explain the connection state between the side sill outer and the pillar outer of the center pillar, and shows a cross section perpendicular to the fore-and-aft direction of the vehicle body. [Figure 4] 4A and 4B are schematic diagrams for explaining the bending test, where FIG. 4A is a plan view of the test piece before the test, and FIG. 4B shows the test piece, punch, and roll. [Figure 5] FIG. 5 is a vertical cross-sectional view of the outer pillar near the joint in the first modification, taken along the plate thickness direction and the up-down direction, with the back side of the cross section not shown. [Figure 6] FIG. 6 is an image diagram for explaining an example of a method for measuring the boundary between the high-strength portion softened layer and the central portion. [Figure 7] FIG. 7 is an image diagram for explaining an example of a change in Vickers hardness in the softened layer. [Figure 8] FIG. 8 is a vertical cross-sectional view of the outer pillar near the joint in the thickness direction and the up-down direction in the second modified example, and the back side of the cross section is not shown. [Figure 9] FIG. 9 is a schematic left side view illustrating a third modification in which no reinforcing member is provided inside the side sill. [Figure 10] FIG. 10 is a schematic cross-sectional view of the main part of a fourth modification relating to a modification of the overlapping portion of the outer pillar, showing a cross section perpendicular to the front-rear direction. DETAILED DESCRIPTION OF THE INVENTION

[0042] An embodiment of the present invention will be described below with reference to the drawings. In this embodiment, a shock absorbing member applied to an automobile will be described.

[0043] Fig. 1 is a schematic left side view showing a main part of a vehicle body 1 to which an impact absorbing member according to one embodiment of the present invention is applied. Fig. 2 is a cross-sectional view of the closed cross-sectional portion 46 of the center pillar 4 taken along line II-II in Fig. 1, perpendicular to the up-down direction Z, with the back side of the cross-section omitted. Fig. 3 is a schematic cross-sectional view of the main part taken along line III-III in Fig. 1 to explain the connection state between the side sill outer 15 and the pillar outer 41 of the center pillar 4, showing a cross-section perpendicular to the front-rear direction X of the vehicle body 1. Unless otherwise specified, the following description will be made with reference to Figs. 1 to 3 as appropriate.

[0044] The vehicle body 1 is a part of a vehicle, and an example of the vehicle is an automobile. An example of an automobile is a passenger car. Examples of the passenger car include a sedan-type passenger car, a coupe-type passenger car, a hatchback-type passenger car, a minivan-type passenger car, and an SUV (Sport Utility Vehicle)-type passenger car. In addition, in this embodiment, a configuration in which the vehicle is a BEV (Battery Electric Vehicle) will be described as an example.

[0045] In this embodiment, the vehicle body 1 is formed from a material including a steel plate. An example of the steel plate is an aluminum-plated steel plate, but other steel types such as a zinc-plated steel plate may also be used. The plating on the plated steel plate is not particularly limited, but examples include hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, Zn-Ni plating (electroalloy zinc plating), Sn plating, Al-Si plating, alloyed electrogalvanizing, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, hot-dip zinc-aluminum-magnesium-Si alloy plating, and zinc-vapor-deposited Al plating. The plating process can be performed by passing the plate through a continuous line.

[0046] The vehicle body 1 includes a front pillar 2, a roof rail 3, a center pillar 4, a side sill 5, and a side sill rear 6.

[0047] The front pillar 2 has an endless closed cross-sectional shape (closed cross-sectional shape) in a cross section perpendicular to the longitudinal direction of the front pillar 2. The front pillar 2 has a front pillar upper 11 that extends upward and is connected to the roof rail 3 toward the rear, and a front pillar lower 12 that is disposed below the front pillar upper 11. The front pillar lower 12 is formed, for example, in an L-shape in a side view. A lower rear edge 12a of the front pillar lower 12 opens toward the rear.

[0048] The roof rail 3 is disposed on the roof of the vehicle body 1 and extends rearward from the front pillar upper 11.

[0049] The side sill 5 is provided at the lower part of the outer portion of the vehicle body 1 in the width direction Y. The side sill 5 is joined to the lower part (lower overlapping part 47) of the center pillar 4 and is arranged along the front-rear direction X of the vehicle body 1. The side sill 5 has an endless closed cross-sectional shape in a cross section perpendicular to the front-rear direction X.

[0050] The side sill 5 has a side sill outer 15 and a side sill inner 16 arranged in the width direction Y.

[0051] The outer side sill 15 and the inner side sill 16 each have a hat-shaped cross section perpendicular to the front-to-rear direction X, and together they form the closed cross section of the side sill 5. The outer side sill 15 is disposed outward in the width direction Y relative to the inner side sill 16.

[0052] The outer side sill 15 and the inner side sill 16 are formed of, for example, steel plates. There are no particular limitations on the tensile strength of the outer side sill 15 and the inner side sill 16, but an example of such a strength is 980 MPa or more.

[0053] The side sill outer 15 has an upper flange 151, an upper wall 152 extending outward in the width direction Y from the upper flange 151, an outer wall 153 extending downward from the upper wall 152 and positioned at the outermost position in the width direction Y on the side sill 5, a lower wall 154 extending inward in the width direction Y from the outer wall 153, and a lower flange 155 extending downward from the lower wall 154.

[0054] The side sill inner 16 has an upper flange 161, an upper wall 162 extending inward in the width direction Y from the upper flange 161, an inner wall 163 extending downward from the upper wall 162, a lower wall 164 extending outward in the width direction Y from the inner wall 163, and a lower flange 165 extending downward from the lower wall 164.

[0055] The upper flanges 151 and 161 are joined to each other by welding, adhesive, etc. Similarly, the lower flanges 155 and 165 are joined to each other by welding, adhesive, etc.

[0056] A front portion 5a of the side sill 5 is fitted into a front pillar lower section 12 of the front pillar 2 and joined to the front pillar lower section 12 by welding, adhesive, or the like. The side sill 5 extends rearward from a lower rear edge portion 12a of the front pillar lower section 12. A rear portion 5b of the side sill 5 is fitted into a side sill rear section 6 and joined to the side sill rear section 6 by welding, adhesive, or the like.

[0057] The side sill rear 6 is disposed behind the side sill 5. The side sill rear 6 is a hollow member extending along the front-to-rear direction X. The cross section of the side sill rear 6 perpendicular to the longitudinal direction of the side sill rear 6 is formed into a closed shape (closed cross-sectional shape) such as a rectangle. The side sill 5 extends forward from the front end edge 6a of the side sill rear 6.

[0058] A battery unit 18 is installed inward from the side sill 5 in the width direction Y (to the side of the side sill 5). The battery unit 18 is disposed below the floor panel 7, which extends inward from the side sill 5 in the width direction Y. The battery unit 18 includes a battery case 18a and a battery 18b housed in the battery case 18a. The battery 18b supplies power to a wheel-driving electric motor (not shown) via a cable or the like. The battery case 18a is fixed to components of the vehicle body 1, such as the floor panel 7, with fixing members (not shown), such as bolts. The battery unit 18 is also installed near the center pillar 4 and is aligned in the width direction Y with a lower overlapping portion 47 (described later) of the center pillar 4. The battery unit 18 is disposed adjacent to the side sill 5 in the width direction Y (for example, at a short distance of less than 10 cm). The battery case 18a protects the battery 18b in cooperation with the side sill 5 and the center pillar 4 during a side collision.

[0059] A reinforcing member 20 is disposed within the side sill 5. The reinforcing member 20 is provided to suppress transmission of an impact to the battery unit 18 during a side collision. The specific shape of the reinforcing member 20 is not limited. When an impact load of a certain level or greater acts from the side sill outer 15 toward the inside in the width direction Y (toward the cabin 10), the reinforcing member 20 absorbs the impact by plastically deforming and collapsing toward the inside in the width direction Y. In this embodiment, the reinforcing member 20 is disposed so as to be aligned with the battery unit 18 in the width direction Y, and its cross-sectional shape perpendicular to the front-rear direction X is a shape obtained by arranging multiple rectangles in the width direction Y. The reinforcing member 20 is fixed, for example, to the inner wall 163 of the side sill inner 16. The reinforcing member 20 is disposed, for example, on at least a portion of the side sill 5 in the front-rear direction X, and may be disposed over the entire area of the side sill 5.

[0060] Additionally, a floor cross member (not shown) is disposed around the center pillar 4 and on the side of the side sill 5 .

[0061] During a side collision at the side sill 5, particularly at the lower overlapping portion 47 between the center pillar 4 and the side sill 5, the side sill 5 receives an impact load from an object with the lower rear edge 12a of the front pillar 2 in the front fastening position, the lower overlapping portion 47 of the center pillar 4 in the middle fastening position, the front edge 6a of the side sill rear 6 in the rear fastening position, and the location where the floor cross member is located in the inner fastening position. When the impact load exceeds a certain level, the side sill 5 absorbs the impact load by plastically deforming inward in the width direction Y.

[0062] Because the reinforcing member 20 is installed inside the side sill 5, a larger impact load is required to deform the reinforced side sill 5 during a side collision. Therefore, the amount of bending deformation and the amount of torsional deformation of the side sill 5 are both small. On the other hand, the center pillar 4 receives a correspondingly larger impact load, and therefore undergoes significant plastic deformation toward the cabin 10.

[0063] In this embodiment, the side sill rear 6 is described as the component that constitutes the rear fastening position. However, this is not necessarily the case. For example, another component, such as the lower end of the C-pillar connected to the rear end of the side sill 5, may constitute the rear fastening position. The rear fastening position may be any fastening point on the rear side when the side sill 5 is subjected to an impact load during a side collision and undergoes plastic deformation, and specific examples of components that constitute the rear fastening position are not limited. Similarly, specific examples of components that constitute the front fastening position are not limited for the front fastening position. If a component other than the side sill rear 6 constitutes the rear fastening position, the side sill rear 6 does not have to be present on the vehicle body 1.

[0064] The center pillar 4 is disposed behind the front pillar 2 and extends in the vertical direction Z (predetermined longitudinal direction) from the roof rail 3 to the side sill 5. The center pillar 4 is an impact absorbing member that absorbs impact when the vehicle body 1 is hit by a side collision.

[0065] The center pillar 4 has an upper overlapping portion 45 that is joined to the roof rail 3, a closed cross-section portion 46 that is located below the upper overlapping portion 45 and above the side sill 5, and a lower overlapping portion 47 that serves as a lower portion that is located below the closed cross-section portion 46 and is arranged to cover the outer surface of the side sill 5 at the connection point with the side sill 5.

[0066] The center pillar 4 also includes an outer pillar 41 and an inner pillar 42 aligned in the width direction Y, and a reinforcing patch 43 disposed between the outer pillar 41 and the inner pillar 42. The outer pillar 41 and the inner pillar 42 form an upper overlapping portion 45, a closed cross-sectional portion 46, and a lower overlapping portion 47. The patch 43 is a member formed by pressing a blank, and is joined to at least one of the outer pillar 41 and the inner pillar 42. The patch 43 is not necessary.

[0067] In the closed cross-sectional portion 46, the outer pillar 41 and the inner pillar 42 each have a hat-shaped cross-section perpendicular to the up-down direction Z. Because the outer pillar 41 and the inner pillar 42 are joined to each other, the closed cross-sectional portion 46 has a closed cross-section perpendicular to the up-down direction Z. The outer pillar 41 is disposed outward in the width direction Y from the inner pillar 42.

[0068] The pillar outer 41 in the closed cross-section portion 46 has a front flange 411, a front wall 412 extending outward from the front flange 411 in the width direction Y, a side wall 413 extending rearward from the front wall 412, a rear wall 414 extending inward from the side wall 413 in the width direction Y, and a rear flange 415 extending rearward from the rear wall 414.

[0069] The pillar inner 42 in the closed cross-sectional portion 46 has a front flange 421, a front wall 422 extending inward in the width direction Y from the front flange 421, a side wall 423 extending rearward from the front wall 422, a rear wall 424 extending outward in the width direction Y from the side wall 423, and a rear flange 425 extending rearward from the rear wall 424. Note that the pillar inner 42 may have a flat plate shape so that its cross-sectional shape perpendicular to the up-down direction Z is substantially straight in the front-rear direction X. In this case, the front wall 422 and the rear wall 424 are omitted, and the front flange 421, the side wall 423, and the rear flange 425 are aligned along the front-rear direction X.

[0070] The front flanges 411 and 421 are joined to each other by welding, adhesive, etc. Similarly, the rear flanges 415 and 425 are joined to each other by welding, adhesive, etc.

[0071] A more specific configuration of the closed cross-sectional portion 46 will be described later.

[0072] Next, a more detailed description will be given of the configuration of the lower overlap portion 47. The pillar inner 42 at the lower overlap portion 47 is joined to the side sill inner 16 by welding, adhesive, or the like.

[0073] In this embodiment, the pillar outer 41 at the lower overlapping portion 47 may be simply referred to as the overlapping portion 47.

[0074] The overlapping portion 47 is arranged so as to cover the outer surface of the side sill 5 from the outside in the width direction Y at the connection portion between the pillar outer 41 of the center pillar 4 and the side sill 5 .

[0075] The overlapping portion 47 has an upper flange 471, an upper wall 472 extending outward in the width direction Y from the upper flange 471, and an outer wall 473 extending downward from the upper wall 472.

[0076] The upper flange 471 is joined to the outer surface 151a of the upper flange 151 of the side sill outer 15 by welding, adhesive, or the like.

[0077] The upper wall 472 is preferably in contact with the upper surface 152a of the upper wall 152 of the side sill outer 15 along the length thereof, and may or may not be joined to this upper surface 152a by welding, adhesive, etc.

[0078] The outer wall 473 is preferably in contact with and extends along the outer surface 153a of the outer wall 153 of the side sill outer 15, and may or may not be joined to this outer surface 153a by welding, adhesive, or the like. In this embodiment, the lower end 473b of the outer wall 473 is the lower end of the overlapping portion 47. Note that the lower end 473b of the overlapping portion 47 may be aligned with the upper wall 152 rather than along the outer wall 153 of the side sill outer 15.

[0079] As shown by the two-dot chain line in FIG. 3 , it is preferable that the lower end 473b (overlapping portion 47) of the outer wall 473 extend along a portion below a height position 153b that is halfway in the up-down direction Z of the outer wall 153 of the side sill 5. The height position 153b is the center position of the length in the up-down direction Z at a portion of the outer wall 153 that overlaps with the central portion of the outer wall 473 in the front-to-rear direction X. The length of the outer wall 153 in the up-down direction Z is the length between an upper end 153c and a lower end 153d in the up-down direction Z at a portion of the outer wall 153 that overlaps with the central portion of the outer wall 473 in the front-to-rear direction X. The upper end 153c of the outer wall 153 is the starting point of the curved portion that curves from the outer wall 153 to the upper wall 152 of the side sill outer 15. Lower end 153d of outer wall 153 is the starting point of the curve of the curved portion of side sill outer 15 that extends from outer wall 153 to lower wall 154. When lower end 473b of outer wall 473 is aligned downward from height position 153b, lower end 473b may be positioned lower than a position that is 1 / 6 of the length of outer wall 153 in the up-down direction Z from height position 153b (a position 1 / 3 of the way from the bottom of outer wall 153), or may be positioned lower than a position that is 1 / 4 of the length of outer wall 153 in the up-down direction Z from height position 153b (a position 1 / 4 of the way from the bottom of outer wall 153), or may be positioned lower than a position that is 1 / 3 of the length of outer wall 153 in the up-down direction Z from height position 153b (a position 1 / 6 of the way from the bottom of outer wall 153).

[0080] The overlapping portion 47 thus positioned increases the joint area between the center pillar 4 and the side sill 5, thereby reducing the stress acting between the side sill 5 and the center pillar 4 during a side collision. Furthermore, the load transferred from the center pillar 4 to the side sill 5 during a side collision is primarily received by the vehicle body 1 as a torsional moment. This reduces deformation of the closed cross-sectional shape of the side sill 5, particularly the outer side sill 15. Even in the event of a side collision, the original closed cross-sectional shape of the side sill 5 before the collision is unlikely to change. This ensures high torsional rigidity, reducing the torsional angle of the side sill 5 during a side collision. This reduces the amount of intrusion of the center pillar 4 and the side sill 5 into the cabin 10. On the other hand, if the center pillar 4 is positioned so that it does not reach the lower end (lower flange 155) of the side sill 5, the center pillar 4 does not become unnecessarily heavy, achieving a good balance between the enhanced reinforcing effect of the lower portion of the center pillar 4 and the reduced weight of the center pillar 4.

[0081] The lower end 473b of the overlapping portion 47 is preferably a linear end portion along the fore-and-aft direction X. With this configuration, the vehicle body 1 can receive the load transmitted from the center pillar 4 to the side sill 5 during a side collision as a torsional moment over the entire area where the overlapping portion 47 is located in the fore-and-aft direction X. This more reliably prevents the closed cross-sectional shape of the side sill 5, and in particular the side sill outer 15, from being crushed.

[0082] The center pillar 4 is provided with a pair of upper and lower brackets 61 and 62 for supporting a door 8 installed behind the center pillar 4 .

[0083] Each bracket 61, 62 may be a plate-like member formed by pressing a steel plate, or may be a block-like member formed by cutting steel or the like. Each bracket 61, 62 constitutes part of a hinge mechanism that connects the center pillar 4 and the door 8. Door shafts 63, 64 are installed between each bracket 61, 62 and the door 8. In addition, a pair of upper and lower brackets 65, 66 corresponding to the brackets 61, 62 are installed on the door 8. The brackets 61, 62 and the corresponding brackets 65, 66 are connected via the corresponding door shafts 63, 64. The door 8 moves around the door shafts 63, 64 to open and close.

[0084] Each bracket 61, 62 is fixed to the pillar outer part 41 of the center pillar 4 by welding or with fixing members such as bolts. It is preferable that each bracket 61, 62 is fixed to the pillar outer part 41, particularly to the side wall 413. With this configuration, the impact absorption effect of the center pillar 4 can be further improved against an impact load input to the pillar outer part 41 through each bracket 61, 62, due to the provision of the high-strength portion 53 formed of a material with high hardness and high bending deformability. The lower bracket 62 is installed in the closed cross-sectional portion 46 near the overlapping portion 47 of the center pillar 4. The upper bracket 61 is installed above the lower bracket 62 and is spaced apart from the lower bracket 62. The upper bracket 61 is installed, for example, near the lower end 8a of the window opening of the door 8.

[0085] (More detailed structure of the pillar outer) The material of the pillar outer 41 is a tailor weld blank (TWB), which is made by joining two separately formed blanks together to form a single member. The pillar outer 41 is formed by pressing this material.

[0086] As described above, the pillar outer 41 of the center pillar 4 includes the low-strength portion 51, the joint portion 52, and the high-strength portion 53. The lower portion of the pillar outer 41 is made up of the low-strength portion 51, and the upper portion is made up of the high-strength portion 53. In this embodiment, in the pillar outer 41 that receives the majority of the impact load of the center pillar 4 during a side collision, the material of the high-strength portion 53 located above the low-strength portion 51 has high bendability, which more reliably prevents the high-strength portion 53 from breaking. This more reliably prevents the center pillar 4 from providing occupant protection.

[0087] As described above, the pillar outer 41 includes the low-strength portion 51, the high-strength portion 53 that is aligned with the low-strength portion 51 in the vertical direction Z and has a Vickers hardness HV1 higher than the Vickers hardness HV2 of the low-strength portion 51, and the joint portion 52 that joins the low-strength portion 51 and the high-strength portion 53. In this embodiment, the Vickers hardness HV2 of the low-strength portion 51 and the Vickers hardness HV1 of the high-strength portion 53 refer to the Vickers hardness of the center portion of the low-strength portion 51 in the sheet thickness direction and the Vickers hardness of the center portion of the high-strength portion 53 in the sheet thickness direction, respectively. In this embodiment, the "center portion" refers to a majority of the steel sheet portion when no coating layer is present, and refers to a majority of the steel sheet base material directly below the coating layer when a coating layer is present. In the case where an alloy layer is present between the coating layer and the steel sheet base material in addition to the coating layer, the "center portion" refers to a majority of the steel sheet base material directly below the alloy layer.

[0088] Here, a layer containing at least one of a plating layer and an alloy layer is referred to as a plating film. When a plating film is present, the thickness of the plating film is measured, and the position corresponding to the thickness of the plating film in the thickness direction from the surface of the steel sheet, including the plating film, is defined as the boundary position between the plating film and the base steel sheet. In this embodiment, the "thickness" refers to the thickness of the base steel sheet, calculated by subtracting the thickness of the plating film on the front side of the steel sheet and the thickness of the plating film on the back side of the steel sheet from the total thickness of the steel sheet, including the plating film. Unless otherwise specified, the surfaces of the low-strength portion 51 and the high-strength portion 53 refer to the surfaces of the base steel sheet. The thickness of the plating film is measured using a high-frequency glow discharge optical emission surface analyzer (GDS). A specific measurement method is described below.

[0089] Three arbitrary measurement positions are determined in the region having the plated coating in each of the low-strength portion 51 and the high-strength portion 53. The element concentrations of Fe, Mn, Zn, Si, Al, O, Cr, Ni, Mg, Cu, and Sn are measured at each measurement point while sputtering from the surface of the plated coating.

[0090] The content of each element was analyzed in the depth direction, and the depth at which the Fe concentration first reached 90% by mass or greater was determined. The depth values at each measurement point were then averaged, and this average was used as the plating film thickness of the low-strength portion 51 or the high-strength portion 53. If the Fe concentration did not reach 90% by mass or greater up to the depth that could be analyzed in a single GDS measurement, i.e., if the plating film thickness was greater than the measurable depth, a portion of the plating film equivalent to 80% to 90% of the previously measured depth was removed by polishing at an arbitrary position other than the previously measured position within the same area. The depth of the plating film removed by polishing was determined from the change in sheet thickness before and after polishing, and a new GDS analysis was then performed on the polished surface. The plating film thickness was then determined by combining the results of the first and subsequent measurements. Even if a structure other than the plating film is present on the surface of the steel sheet base material, the boundary between the steel sheet base material and other structures can be determined using the above-mentioned measurement method.

[0091] The GDS measurement device used is, for example, a Marcus-type high-frequency glow discharge optical emission spectrometer GD-Profiler2 (manufactured by HORIBA).In this case, the measurement is performed under the following conditions: discharge condition: 35 W, Ar pressure during measurement: 600 Pa, discharge range: diameter 4 mmφ, electrode distance: 0.15 mm to 0.25 mm, and measurement pitch in the plate thickness direction: 0.01 μm to 0.05 μm.

[0092] (Maximum bending angle of high strength and low strength parts) The maximum bending angle θ1 of the high-strength portion 53 is set according to the maximum bending angle θ2 of the low-strength portion 51. 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 4A and 4B are schematic diagrams illustrating the bending test. Figure 4A is a plan view of the test piece S before the test, and Figure 4B shows the test piece S, punch P, and roll R. As shown in Figures 4A and 4B, this VDA bending test involves deforming the test piece S into a V-shape by pressing the test piece S, placed on two rolls R, R, 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 10 mm plus the distance between the centers of the rolls R to 60 mm. The width of the test piece S ranges from 10 mm to 60 mm. The test specimen S is extracted from the high-strength portion 53 so that the direction in which the radius of curvature is 500 mm or more corresponds to the length direction of the test specimen S. As shown in Figure 4B, the test specimen S is deformed so that it has 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 increases 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 taken as the maximum bending angle of the test specimen S.

[0093] In this embodiment, a portion of the high-strength portion 53, for example, near the upper bracket 61, is cut out as the test piece S and subjected to a VDA bending test, thereby calculating the maximum bending angle θ1 of the high-strength portion 53. Also, a portion of the low-strength portion 51, for example, near the lower bracket 62, is cut out as the test piece S and subjected to a VDA bending test, thereby calculating the maximum bending angle θ2 of the low-strength portion 51. The number of tests performed on the high-strength portion 53 in the VDA bending test is three, and the average value of the measurement results of the three test pieces S is defined as the maximum bending angle θ1 of the high-strength portion 53. Similarly, the number of tests performed on the low-strength portion 51 in the VDA bending test is three, and the average value of the measurement results of the three test pieces S is defined as the maximum bending angle θ2 of the low-strength portion 51.

[0094] In the pillar outer 41, the low-strength portion 51 forms the overlapping portion 47 and a portion of the closed cross-sectional portion 46 on the overlapping portion 47 side (lower side). The joint portion 52 is a portion formed by joining the material of the low-strength portion 51 and the material of the high-strength portion 53. The joint portion 52 exists as a portion joining the material of the low-strength portion 51 and the material of the high-strength portion 53 even before the material of the low-strength portion 51 and the material of the high-strength portion 53 are press-molded to form the pillar outer 41. When the low-strength portion 51 and the high-strength portion 53 are joined by laser welding, the low-strength portion 51 and the high-strength portion 53 face each other in the longitudinal direction of the pillar outer 41 near the joint portion 52. On the other hand, when the low-strength portion 51 and the high-strength portion 53 are joined by spot welding, the low-strength portion 51 and the high-strength portion 53 face each other in the plate thickness direction of the pillar outer 41 near the joint portion 52. The joint 52 is preferably arranged straight in the front-rear direction X. This allows the strength distribution of the pillar outer 41 in the front-rear direction X to be more uniform around the joint 52. The joint 52 does not have to be straight in the front-rear direction X. The joint 52 only needs to join at least a portion of the low-strength portion 51 and the high-strength portion 53 in the front-rear direction X. The low-strength portion 51 is arranged below the joint 52, and the high-strength portion 53 is arranged above the joint 52. The high-strength portion 53 forms the upper overlapping portion 45 and a portion of the closed cross-sectional portion 46 of the pillar outer 41 on the upper overlapping portion 45 side.

[0095] In this embodiment, because the side sill 5 is reinforced with the reinforcing member 20, the deformation of the side sill 5 is small during a side collision, and bending deformation of the center pillar 4 is dominant rather than torsional deformation of the side sill 5. Because the amount of plastic deformation of the side sill 5 is small, the center pillar 4 undergoes large plastic deformation to absorb the impact. During this collision, the low-strength portion 51 undergoes large inward plastic deformation in the width direction Y to absorb the impact. Since the high-strength portion 53 has high strength to suppress a collision with an occupant, the amount of inward plastic deformation in the width direction Y tends to be smaller than the amount of plastic deformation of the low-strength portion 51. Meanwhile, near the joint 52, the amount of plastic deformation of the high-strength portion 53 also increases in response to the inward plastic deformation in the width direction Y of the low-strength portion 51.

[0096] Therefore, in this embodiment, the maximum bending angle θ1 of the high-strength portion 53 is set according to the maximum bending angle θ2 of the low-strength portion 51. Thus, for example, by increasing the maximum bending angle θ1 of the material of the high-strength portion 53, the difference between the maximum bending angle θ1 of the material of the high-strength portion 53 and the maximum bending angle θ2 of the material of the low-strength portion 51 is reduced. Setting the bending angle in this manner allows the material of the high-strength portion 53, which has high Vickers hardness, to exhibit high bending performance. Therefore, even when the high-strength portion 53 undergoes relatively large plastic deformation due to the influence of plastic deformation of the low-strength portion 51 of the pillar outer 41 during a side collision, the deformability of the high-strength portion 53 can be increased, thereby reducing the possibility of fracture of the pillar outer 41 (high-strength portion 53) near the joint 52. Furthermore, the upper portion of the center pillar 4 is required to have high strength from the perspective of occupant protection. If a material with low Vickers hardness (low strength) were used in the members above the joint 52 to ensure high bendability, it would be necessary to increase the plate thickness to reduce the amount of deformation toward the occupant, resulting in an increase in mass. On the other hand, in this embodiment, the high-strength portion 53 has a high Vickers hardness, so the high-strength portion 53 is strong, and the impact absorption performance in the event of a collision such as a side collision can be improved, while the weight of the pillar outer 41 can be reduced by making the high-strength portion 53 thinner. In this way, by using a material that combines Vickers hardness (strength) and bendability as the high-strength portion 53 in the upper part of the center pillar 4, a center pillar 4 can be realized that is lightweight and has high impact absorption performance by reducing the possibility of breakage.

[0097] The difference Δθ between the maximum bending angle θ1 of the high-strength portion 53 and the maximum bending angle θ2 of the low-strength portion 51 is preferably 100 degrees or less. By setting the bending angle difference Δθ to 100 degrees or less, it is possible to more reliably achieve bending deformation of the high-strength portion 53 that follows the bending deformation of the low-strength portion 51 during a side collision of the vehicle. This makes it possible to achieve a center pillar 4 with high impact absorption performance by reducing the possibility of breakage.

[0098] (Thickness of low strength and high strength parts) The plate thickness of the low-strength portion 51 and the high-strength portion 53 may be the same or different. The plate thickness of the low-strength portion 51 and the high-strength portion 53 can be, for example, 0.8 mm to 2.6 mm. By making the plate thickness 0.8 mm or more, the strength of the pillar outer 41 can be sufficiently increased, and the amount of deformation of the center pillar 4 toward the inside in the width direction Y during a side collision can be further reduced. By making the plate thickness 2.6 mm or less, the center pillar 4 can be prevented from becoming too heavy.

[0099] Examples of lower limits and upper limits of the thickness of each of the low-strength portion 51 and the high-strength portion 53 are 0.8 mm, 1.0 mm, and 1.2 mm, respectively. As is clear from the above, the thickness of each of the low-strength portion 51 and the high-strength portion 53 is preferably 1.0 mm to 2.5 mm, 1.2 mm to 2.3 mm, 1.2 mm to 2.2 mm, 1.2 mm to 2.0 mm, 1.2 mm to 1.8 mm, or 1.2 mm to 1.6 mm.

[0100] (Vickers hardness of low strength part) The Vickers hardness HV2 of the center portion of the low-strength portion 51 in the thickness direction may be 150 HV or more, 300 HV or more, 400 HV or more, or 500 HV or more to exhibit high impact absorption performance. While there is no particular upper limit to the Vickers hardness HV2 of the low-strength portion 51, examples of the Vickers hardness HV2 include 500 HV, 400 HV, and 300 HV from the viewpoint of more reliably suppressing the possibility of fracture during a side collision. Thus, an example of the Vickers hardness HV2 of the low-strength portion 51 is 150 HV to 500 HV.

[0101] (Vickers hardness of high strength part) The Vickers hardness HV1 of the central part of the high strength portion 53 in the thickness direction is preferably 300 HV or more, from the viewpoint of reducing the amount of displacement of the high strength portion 53 toward the cabin 10 (inside in the width direction Y) during a side collision, and may be 400 HV or more, 500 HV or more, 600 HV or more, 750 HV or more, 800 HV or more, 900 HV or more, 950 HV or more, 1000 HV or more, 1050 HV or more, or 1100 HV or more. Although there is no particular upper limit to the Vickers hardness HV1 of the high-strength portion 53, from the viewpoint of more reliably suppressing the possibility of breakage during a side collision, examples of the upper limit include 1100 HV, 1050 HV, 1000 HV, 950 HV, 900 HV, 800 HV, 750 HV, 650 HV, 600 HV, 550 HV, and 500 HV. Thus, a preferred example of the Vickers hardness HV1 of the high-strength portion 53 is 300 HV to 1100 HV, and a more preferred example is 500 HV to 1100 HV.

[0102] (Method for measuring Vickers hardness of low-strength and high-strength parts) In this embodiment, the Vickers hardness of the center portion of the low-strength portion 51 or the high-strength portion 53 in the thickness direction is measured as follows. A cross section perpendicular to the plate surface is taken from a sample cut from a portion of the low-strength portion 51, for example, near the lower bracket 62, or a portion of the high-strength portion 53, for example, near the upper bracket 61. The sample is then 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, 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 low strength portion 51 or the high strength portion 53.

[0103] Furthermore, by setting the Vickers hardness HV2 of the low-strength portion 51 to 150 HV or more and the Vickers hardness HV1 of the high-strength portion 53 to 500 HV or more, it is possible to increase the impact energy absorption effect due to plastic deformation of the low-strength portion 51 during a side collision while suppressing excessive plastic deformation of the high-strength portion 53, thereby improving impact absorption performance while reducing the amount by which the upper portion of the center pillar 4 penetrates into the cabin 10.

[0104] In particular, when the Vickers hardness HV1 of the high-strength portion 53 is 500 HV or higher, the difference in strength (strength ratio) between the high-strength portion 53 and the low-strength portion 51 is large, and the difference in bendability between the material of the high-strength portion 53 and the material of the low-strength portion 51 tends to be large. That is, when the Vickers hardness HV1 of the high-strength portion 53 is 500 HV or higher, the high-strength portion 53 is likely to fracture during a side collision due to bending deformation of the high-strength portion 53 that follows bending deformation of the low-strength portion 51. Thus, when the Vickers hardness HV1 of the high-strength portion 53 is 500 HV or higher, the problem of the possibility of fracture in the high-strength portion 53 becomes more pronounced. Even in such a case, the possibility of fracture of the high-strength portion 53 during a side collision can be reduced by setting the maximum bending angle θ1 of the high-strength portion 53 according to the maximum bending angle θ2 of the low-strength portion 51.

[0105] (An example of a combination of Vickers hardness in low strength areas and high strength areas) An example of a combination of the Vickers hardness HV2 of the low-strength portion 51 and the Vickers hardness HV1 of the high-strength portion 53 is a combination where the Vickers hardness HV1 of the high-strength portion 53 is 480 HV to 750 HV and the Vickers hardness HV2 of the low-strength portion 51 is 150 HV to 480 HV (less than the Vickers hardness HV1 of the high-strength portion 53). More specific preferred combinations include the following three examples. Example 1) Vickers hardness HV1 of high strength part 53 and Vickers hardness HV2 of low strength part 51: 600HV and 350HV Example 2) Vickers hardness HV1 of high strength part 53 and Vickers hardness HV2 of low strength part 51: 600HV and 420HV Example 3) Vickers hardness HV1 of high strength part 53 and Vickers hardness HV2 of low strength part 51: 720HV and 480HV

[0106] In all three of the above examples, the ratio HV1 / HV2 of the Vickers hardness HV1 of the high-strength portion 53 to the Vickers hardness HV2 of the low-strength portion 51 is 1.3 or greater. When HV1 / HV2 is 1.3 or greater, the difference in strength (strength ratio) between the high-strength portion 53 and the low-strength portion 51 is large, and the difference in bendability between the material of the high-strength portion 53 and the material of the low-strength portion 51 tends to be large. In other words, when HV1 / HV2 is 1.3 or greater, the high-strength portion 53, which has a high Vickers hardness, tends to be susceptible to fracture during a side collision due to bending deformation of the high-strength portion 53 following bending deformation of the low-strength portion 51. When HV1 / HV2 is 1.3 or greater, the need to suppress fracture in the high-strength portion 53 becomes apparent. Even in such a case, the possibility of fracture of high-strength portion 53 during a side collision can be reduced by setting the maximum bending angle θ1 of high-strength portion 53 according to the maximum bending angle θ2 of low-strength portion 51. HV1 / HV2 is preferably 1.4 or more, and more preferably 1.5 or more.

[0107] The higher the Vickers hardness HV1 of the high-strength portion 53, the smaller the plate thickness required to achieve the desired impact absorption performance can be, but cracks are more likely to occur during press working of the low-strength portion 51 and the high-strength portion 53 and in the event of a side collision. Therefore, by appropriately setting the Vickers hardness and plate thickness and further improving the maximum bending angle θ1 of the high-strength portion 53, it is possible to achieve a center pillar 4 that is lightweight and has excellent impact absorption performance.

[0108] (Combination of maximum bending angle of low strength part and maximum bending angle of high strength part) The maximum bending angle θ2 of the low-strength portion 51 can be exemplified as 135 degrees to 50 degrees, with 135 degrees, 70 degrees, 60 degrees, and 50 degrees being examples within this range. The maximum bending angle θ1 of the high-strength portion 53 can be exemplified as 60 degrees to 40 degrees, with 60 degrees, 50 degrees, and 40 degrees being examples within this range. Therefore, the difference Δθ between the maximum bending angle θ1 of the high-strength portion 53 and the maximum bending angle θ2 of the low-strength portion 51 may be 95 degrees or less, 85 degrees or less, 75 degrees or less, 30 degrees or less, 20 degrees or less, 10 degrees or less, or 0 degrees. The minimum value of the difference Δθ is 0 degrees.

[0109] The ratio θ1 / θ2 of the maximum bending angle θ1 of the high-strength portion 53 to the maximum bending angle θ2 of the low-strength portion 51 may be 40 / 135=approximately 0.30 or more, 50 / 135=approximately 0.37 or more, 60 / 135=approximately 0.44 or more, 40 / 70=approximately 0.57 or more, 40 / 60=approximately 0.67 or more, 50 / 70=approximately 0.71 or more, 50 / 60=approximately 0.83 or more, or 60 / 70=approximately 0.86 or more. The upper limit of the ratio θ1 / θ2 is, for example, 1.00. The upper limit of the ratio θ1 / θ2 may be 0.95 or 0.90. As is clear from the above, the ratio θ1 / θ2 may be 0.30 to 1.00, 0.37 to 0.95, 0.44 to 0.90, 0.57 to 0.90, 0.67 to 0.90, 0.71 to 0.90, or 0.86 to 0.90.

[0110] (Combination of Vickers hardness and maximum bending angle) Regarding the above-mentioned examples 1) to 3) of the preferable combinations of the high-strength portion 53 and the low-strength portion 51, the difference Δθ between the maximum bending angle θ2 of the low-strength portion 51 and the maximum bending angle θ1 of the high-strength portion 53 will be described below. Example 1) When the Vickers hardness HV1 of the high strength portion 53 and the Vickers hardness HV2 of the low strength portion 51 are 600HV and 350HV, respectively, the difference Δθ between the maximum bending angle θ2 of the low strength portion 51 and the maximum bending angle θ1 of the high strength portion 53 is 30 degrees. Example 2) When the Vickers hardness HV1 of the high strength portion 53 and the Vickers hardness HV2 of the low strength portion 51 are 600HV and 420HV, the difference Δθ between the maximum bending angle θ2 of the low strength portion 51 and the maximum bending angle θ1 of the high strength portion 53 is 20 degrees. Example 3) When the Vickers hardness HV1 of the high strength portion 53 and the Vickers hardness HV2 of the low strength portion 51 are 720HV and 480HV, the difference Δθ between the maximum bending angle θ2 of the low strength portion 51 and the maximum bending angle θ1 of the high strength portion 53 is less than 15 degrees.

[0111] As is clear from the above description, it is preferable that the difference Δθ between the maximum bending angle θ2 of the low-strength portion 51 and the maximum bending angle θ1 of the high-strength portion 53 be 30 degrees or less. With this configuration, during a side collision, the high-strength portion 53 can bend and deform in a manner that sufficiently follows the bending deformation of the low-strength portion 51. As a result, the possibility of the high-strength portion 53 breaking during a side collision can be reduced.

[0112] Note that the difference Δθ between the maximum bending angle θ1 of the high-strength portion 53 and the maximum bending angle θ2 of the low-strength portion 51 is preferably set in accordance with the Vickers hardness HV1 of the high-strength portion 53 and the Vickers hardness HV2 of the low-strength portion 51. In this case, the difference Δθ in bending angles tends to increase as the difference between the Vickers hardness HV1 of the high-strength portion 53 and the Vickers hardness HV2 of the low-strength portion 51 increases, while the maximum bending angle θ1 of the high-strength portion 53 and the maximum bending angle θ2 of the low-strength portion 51 are set within the range of the difference Δθ.

[0113] For example, when the Vickers hardness HV2 of the low-strength portion 51 is 200 HV and the Vickers hardness HV1 of the high-strength portion 53 is 600 HV, the difference in bending angle Δθ may be 90 degrees or less. When the Vickers hardness HV2 of the low-strength portion 51 is 200 HV and the Vickers hardness HV1 of the high-strength portion 53 is 480 HV, the difference in bending angle Δθ may be 85 degrees or less.

[0114] When the Vickers hardness HV2 of the low-strength portion 51 is 350 HV and the Vickers hardness HV1 of the high-strength portion 53 is 600 HV, the difference in bending angle Δθ may be 30 degrees or less. When the Vickers hardness HV2 of the low-strength portion 51 is 350 HV and the Vickers hardness HV1 of the high-strength portion 53 is 480 HV, the difference in bending angle Δθ may be 20 degrees or less.

[0115] When the Vickers hardness HV2 of the low-strength portion 51 is 420 HV and the Vickers hardness HV1 of the high-strength portion 53 is 600 HV, the difference in bending angle Δθ may be 20 degrees or less. When the Vickers hardness HV2 of the low-strength portion 51 is 420 HV and the Vickers hardness HV1 of the high-strength portion 53 is 480 HV, the difference in bending angle Δθ may be 10 degrees or less.

[0116] When the Vickers hardness of the low-strength portion 51 is 350 HV to 470 HV and the Vickers hardness of the high-strength portion 53 is 430 HV to 530 HV, the difference in bending angle Δθ may be 25 degrees or less, 20 degrees or less, 15 degrees or less, or 10 degrees or less. This relationship between the Vickers hardness and the difference in bending angle Δθ of the low-strength portion 51 and the high-strength portion 53 allows the high-strength portion 53 to bend and deform sufficiently in response to the bending deformation of the low-strength portion 51 during a side collision. As a result, the possibility of fracture of the high-strength portion 53 during a side collision can be reduced. In this case, the lower limit of the difference in bending angle Δθ is 0 degrees.

[0117] (Height of joint) During a side collision, an impact load acts on the outer pillar 41 from the door 8 via the brackets 61, 62, 65, and 66. Therefore, the joint between the outer pillar 41 and the brackets 61 and 62 becomes the input point of the impact load during the side collision. More specifically, when a vehicle collides sideways with another vehicle, the input from the colliding vehicle to the underside of the vehicle tends to be large in the early stages of the collision due to the shape of the colliding vehicle (the front lower portion of the colliding vehicle protrudes forward relative to the front upper portion of the vehicle). For this reason, in the early stages of the collision, the impact must be absorbed not only by the side sill 5 but also by the portion of the center pillar 4 near the side sill 5, and it is desirable that the lower portion of the center pillar 4 be able to undergo large plastic deformation. Therefore, it is preferable that the input from the lower bracket 62 of the brackets 61 and 62 for the door 8, which receives more of the impact in the early stages of the collision, be absorbed by the low-strength portion 51 as in this embodiment. On the other hand, in the later stages of the collision, the upper portion of the colliding vehicle also collides with the vehicle, so that the impact is also input to the upper bracket 61 of the brackets 61 and 62. From the viewpoint of passenger protection, it is desirable to minimize plastic deformation of the center pillar 4 near the upper bracket 61. Therefore, it is preferable that the impact load from the upper bracket 61 be borne by the high strength portion 53 as in this embodiment.

[0118] In this embodiment, the joint portion 52 is disposed at a position lower than the height position of the lower end 61b of the upper bracket 61. This allows the high-strength portion 53 to withstand the impact load input to the center pillar 4 from the upper bracket 61 (the input point of the impact load) which is located far from the side sill 5. This makes it possible to further reduce the amount of deformation of the center pillar 4 toward the occupant due to a side collision.

[0119] Preferably, as in this embodiment, the joint portion 52 is disposed at a height position between the lower end 61b of the upper bracket 61 and the upper end 62a of the lower bracket 62 in the vertical direction Z of the vehicle body 1. This ensures a sufficient range for the low-strength portion 51. This makes it possible to achieve both impact absorption by the low-strength portion 51 in cooperation with the side sill 5 in the early stage of a side collision and suppression of deformation of the center pillar 4 toward the cabin 10 due to the high-strength portion 53 receiving the impact load from the upper bracket 61 in the later stage of the side collision.

[0120] 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.

[0121] <Variation 1> In the above-described embodiment, an example has been described in which the strength distribution in the thickness direction of the steel plate base material is substantially unchanged. However, this need not be the case. FIG. 5 is a longitudinal cross-sectional view of the pillar outer 41 in the thickness direction and the vertical direction Z near the joint 52 in Modification 1, with the back side of the cross section omitted. As shown in FIG. 5, a high-strength portion softened layer 55 may be present near at least one of the outer surface 53a and the inner surface 53b of the high-strength portion 53. In Modification 1, the high-strength portion softened layer 55 is present near both the outer surface 53a and the inner surface 53b, and a central portion 56 is present between the high-strength portion softened layers 55.

[0122] (Existence range of softened layer in high strength part) It is preferable that each high-strength portion softened layer 55 be directly connected to the entire joint 52, as this can enhance the effect of suppressing the possibility of fracture near the joint 52 during a side collision. Even if each high-strength portion softened layer 55 is not directly connected to the joint 52, it is preferable that it be present near the joint 52. Whether each high-strength portion softened layer 55 is directly connected to the joint 52 or not, it is preferable that each high-strength portion softened layer 55 extends from near the joint 52 to a position higher than the upper end 61a of the upper bracket 61. This increases the bending deformation ability of the high-strength portion 53 associated with bending deformation of the low-strength portion 51 during a side collision, thereby suppressing the possibility of fracture of the high-strength portion 53. In particular, if each high-strength portion softened layer 55 is connected to the joint 52 over the entire longitudinal direction (front-rear direction X) of the joint 52, the effect of suppressing the possibility of fracture of the high-strength portion 53 can be further enhanced. When each high-strength portion softened layer 55 is formed over the entire area of the high-strength portion 53 in the up-down direction Z, the effect of suppressing the possibility of breakage of the high-strength portion 53 can be further enhanced.

[0123] (Summary of thickness range of softened layer in high strength section) Each high-strength portion softened layer 55 is formed, for example, by reducing the carbon content in the vicinity of the portions that will become the outer surface 53a and the inner surface 53b (portions that will become the high-strength portion softened layer 55) of the blank, which is the material for the high-strength portions 53, compared to the portion that will become the central portion 56 of the blank. The method for forming the high-strength portion softened layer 55 is not particularly limited, and any method may be used. In this modified example, it is preferable that each high-strength portion softened layer 55 has a predetermined thickness from the corresponding surface 53a, 53b, for example, in order to increase the maximum bending angle θ1 of the high-strength portions 53 while ensuring sufficient strength of the high-strength portions 53. The lower limit of the predetermined thickness is, for example, 80 μm, and the upper limit is, for example, 200 μm.

[0124] (Summary of the Vickers hardness distribution in the softened layer of the high-strength section and summary of the Vickers hardness in the center section) In each high-strength portion softened layer 55, the Vickers hardness decreases with increasing distance from the 1 / 2 position of the plate thickness of the high-strength portion 53. In each high-strength portion softened layer 55, the portions that become the surfaces 53a and 53b have the lowest Vickers hardness in the high-strength portion 53, and are lower than the Vickers hardness of the central portion 56 by, for example, at least 100 HV. Examples of upper limits for the difference in Vickers hardness between the surfaces 53a and 53b and the central portion 56 of the high-strength portion 53 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 the high-strength portion 53," this refers to the Vickers hardness at the central portion 56.

[0125] In this variant example 1, in which each high-strength portion softening layer 55 is provided in the high-strength portion 53, the maximum bending angle θ1 of the high-strength portion 53 can be made higher, for example, by about 20 degrees, compared to when the high-strength portion softening layer 55 is not provided (embodiment).

[0126] In this way, the Vickers hardness of the thickness direction surfaces 53a, 53b of the high-strength portion 53 is set to be at least 100 HV lower than the Vickers hardness of the thickness direction central portion 56 of the high-strength portion 53. This makes it possible to further increase the maximum bending angle θ1 of the high-strength portion 53. Therefore, during a side collision, the high-strength portion 53 can more reliably follow the bending deformation of the low-strength portion 51, and the occurrence of cracks in the high-strength portion 53 can be suppressed, thereby improving impact absorption performance.

[0127] An example of the configuration of the high-strength portion 53 will be described more specifically.

[0128] (Vickers hardness at the center of the high-strength part) The Vickers hardness of the central portion 56 in the thickness direction of the plate in the portion where the high-strength portion softening layer 55 is provided is preferably 500 HV or more. When the Vickers hardness of the central portion 56 is 500 HV or more, the effect of improving the deformability of the high-strength portion 53 due to the high-strength portion softening layer 55 becomes significant. The Vickers hardness of the central portion 56 is preferably 600 HV or more, and more preferably 700 HV or more. There is no particular upper limit to the Vickers hardness of the central portion 56, but in consideration of formability, etc., it is preferably 900 HV or less, and more preferably 800 HV or less. Thus, examples of preferable lower limits for the Vickers hardness of the central portion 56 include 500 HV, 550 HV, 600 HV, 700 HV, 720 HV, and 750 HV. Furthermore, examples of preferable upper limits of the Vickers hardness of the central portion 56 include 1100 HV, 1050 HV, 1000 HV, 950 HV, 900 HV, 850 HV, and 800 HV. As is clear from the above, the Vickers hardness of the central portion 56 may be 500 HV to 1100 HV, 550 HV to 1050 HV, 600 HV to 1000 HV, 700 HV to 950 HV, 720 HV to 900 HV, 750 HV to 850 HV, or 750 HV to 800 HV. Furthermore, the Vickers hardness of the central portion 56 may be 550HV to 1000HV, 550HV to 950HV, 550HV to 900HV, 550HV to 850HV, or 550HV to 800HV.

[0129] (Combination of Vickers hardness at the center and maximum bending angle difference) The Vickers hardness of the low-strength portion 51 (the central portion of the low-strength portion 51) in Modification 1 can be 350 HV to 530 HV. When the Vickers hardness of the low-strength portion 51 is 350 HV to 530 HV and the Vickers hardness of the central portion 56 of the high-strength portion 53 is 550 HV to 1050 HV, the difference in bending angle Δθ may be 30 degrees or less, 25 degrees or less, 20 degrees or less, 15 degrees or less, 10 degrees or less, or 5 degrees or less. The lower limit of the difference in bending angle Δθ is preferably 0 degrees. Such a relationship between the Vickers hardness and the difference in bending angle Δθ at the respective central portions of the low-strength portion 51 and the high-strength portion 53 allows the high-strength portion 53 to bend and deform in a manner that sufficiently follows the bending deformation of the low-strength portion 51 during a side collision. As a result, the possibility of fracture of the high-strength portion 53 during a side collision can be reduced. The Vickers hardness of the central portion 56 of the high strength portion 53 may be in the range of 350 HV to 770 HV.

[0130] (Details of the thickness of the softened layer in each high-strength section) The thickness of each high-strength portion softened layer 55 in the thickness direction is preferably 80 μm or more and 5% to 20% of the plate thickness at the portion where the high-strength portion softened layer 55 is provided. If the thickness of each high-strength portion softened layer 55 is 20% or less of the plate thickness, the proportion of each high-strength portion softened layer 55 in the steel plate from which the high-strength portions 53 are made is small, so that the load-bearing capacity required of the high-strength portions 53 can be maintained. The thickness of each high-strength portion softened layer 55 is preferably 17% or less of the plate thickness, and more preferably 14% or less. On the other hand, when the high-strength portion softened layer 55 is provided over the entire surface of the steel plate from which the high-strength portions 53 are made, the deformability of the high-strength portion softened layer 55 can be fully exhibited if the thickness of each high-strength portion softened layer 55 is 80 μm or more and 5% or more of the plate thickness. The thickness of each high-strength portion softened layer 55 is more preferably 8% or more of the plate thickness. As is clear from the above, the thickness of each high-strength portion softened layer 55 may be 5% to 17%, 5% to 14%, 8% to 20%, 8% to 17%, or 8% to 14% of the plate thickness.

[0131] (Method for measuring the thickness position of the boundary between the softened layer of the high-strength portion and the center portion) Next, a method for measuring the boundary between the high-strength portion softened layer 55 and the central portion 56 will be described. Figure 6 is an illustration for explaining an example of a method for measuring the boundary between the high-strength portion softened layer 55 and the central portion 56. A cross section perpendicular to the plate surface of a sample taken from the high-strength portion 53 is taken, and the sample is prepared for hardness testing. The preparation of the measurement surface is carried out 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. During this process, to prevent streaky unevenness from occurring on the measurement surface, an argon ion beam is irradiated from 360 degrees using a sample rotating holder manufactured by JEOL.

[0132] 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 is measured in the direction perpendicular to the surface (thickness direction) with a test force of 50 gf.

[0133] The measurement position on the surface-most side of the sample is 20 μm thick from either of the two surfaces 53a, 53b (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 high-strength portion softened layer 55 and the central portion 56, 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 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 20 μm from the surface to a position halfway along the thickness direction.

[0134] (Method for calculating the gradient of Vickers hardness at each thickness position after measuring the boundary between the softened layer of the high strength portion and the center portion) The slope of the Vickers hardness at each thickness position after measuring the Vickers hardness at the boundary between the high-strength portion softened layer 55 and the central portion 56 is, for example, a slope Δbi obtained from the Vickers hardness at multiple consecutive points (three points). The slope Δbi is calculated using the following formula (1).

number

[0135] Of the three measurement points at which the gradient Δbi calculated by equation (1) from the surface side of the high strength portion 53 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 high strength portion softened layer 55 and the central portion 56.

[0136] (Relationship between Vickers hardness of the softened layer in the high strength area and that of the center area) The Vickers hardness of the high strength portion softened layer 55 on the surfaces 53a and 53b of the high strength portion 53 is preferably 0.5 to less than 0.9 times the Vickers hardness of the central portion 56 where the high strength portion softened layer 55 is provided.

[0137] (Method for measuring Vickers hardness at the center of a high-strength part) The Vickers hardness of the central portion 56 is measured as follows. A cross section perpendicular to the plate surface of the sample cut from the high-strength portion 53 is taken, and the test surface is prepared for hardness testing. The test surface is prepared in accordance with JIS Z 2244:2020. The test 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 half the plate thickness of the sample, with a test force of 1 kgf. The average value is taken as the Vickers hardness of the central portion 56.

[0138] (Method for measuring Vickers hardness of the surface of high-strength parts) The Vickers hardness of the surface of the high strength portion 53 is measured on a cross section obtained by cutting the high strength portion 53 along the plate thickness direction in accordance with the Vickers hardness test described in JIS Z 2244:2020.

[0139] After the above cross section is subjected to sample preparation for the measurement surface, it is submitted 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, a JEOL cross-section polisher is used to sputter the measurement surface with an argon ion beam. To prevent streaky unevenness from appearing on the measurement surface, a JEOL rotating sample holder is used to irradiate the measurement surface with an argon ion beam from 360 degrees.

[0140] 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 of the high-strength portion 53. If a plating layer is present in the high-strength portion 53, the measurement point is 20 μm thick from the surface of the steel sheet base material directly below the plating layer. If the high-strength portion 53 has an alloy layer between the plating layer and the steel sheet base material in addition to the plating layer, 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, and the average value is taken as the Vickers hardness of the surface of the high-strength portion 53.

[0141] If the Vickers hardness of the surface of the high-strength portion 53 is 0.5 times or more the Vickers hardness of the central portion 56, 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 high-strength portion softened layer 55 on the surface of the high-strength portion 53 is 0.6 times or more the Vickers hardness of the central portion 56. On the other hand, if the Vickers hardness of the surface of the high-strength portion 53 is less than 0.9 times the Vickers hardness of the central portion 56, the deformability can be sufficiently improved. It is more preferable that the Vickers hardness of the high-strength portion softened layer 55 on the surface of the high-strength portion 53 is less than 0.8 times the Vickers hardness of the central portion 56.

[0142] (Change in Vickers hardness in the softened layer of the high-strength part) 7 is an image diagram illustrating an example of a Vickers hardness change in the high-strength portion softened layer 55. As shown in FIG. 7, the high-strength portion softened layer 55 preferably has, in the thickness direction, a first hardness change region that is a region from the surfaces 53a, 53b to 40% of the thickness of the high-strength portion softened layer 55, and a second hardness change region that is a region of the high-strength portion softened layer 55 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.

[0143] 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 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 the high-strength portion 53 can be improved. Specifically, the load drop immediately after the load peak can be made gentler in the later stroke of 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.

[0144] (Method for measuring Vickers hardness of first and second hardness change regions of the high strength portion softened layer) Next, we will explain how to measure the hardness of the first hardness change region and the second hardness change region. A cross section perpendicular to the plate surface of the sample taken from the high-strength portion 53 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. During this process, to prevent streaky unevenness from occurring on the measurement surface, an argon ion beam is irradiated from 360 degrees using a sample rotation holder manufactured by JEOL.

[0145] The Vickers hardness of the prepared sample is measured using a micro Vickers hardness tester. The area from the surface of the sample corresponding to the softened layer is measured perpendicular to the surface (thickness direction) with a test force of 10 gf. The total number of measurement points varies depending on the thickness of the sample, but the number of measurement points for calculating ΔHV1 and ΔHV2 (described below) is set in accordance with JIS Z 2244:2020.

[0146] The thickness of the high-strength portion softened layer 55 is 80 μm or more, and is 5% to 20% of the plate thickness at the portion where the high-strength portion softened layer 55 is provided. The first hardness change region is the region from the surface of the high-strength portion softened layer 55 to 40% of the thickness, and in this embodiment, it exists at a thickness position from the surface of the high-strength portion 53 to a minimum of, for example, 40 μm and a maximum of, for example, 80 μm.

[0147] The measurement position on the surface-most 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 a different point from the measurement point used when measuring the Vickers hardness of the surface of the high-strength portion 53 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 row along the thickness direction for the first hardness change region. In this case, measurements are made at different positions in the thickness direction and at different positions in the direction 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 thickness direction of the plate and the distance between the centers of the depressions being at least three times the average diagonal length of the depressions.

[0148] The thickness of the high-strength portion softened layer 55 is 80 μm or more, and is 5% to 20% of the plate thickness at the portion where the high-strength portion softened layer 55 is provided. 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 high-strength portion softened layer 55 is 80 μm to 200 μm, the second hardness change region exists in 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 of the high-strength portion 53. 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 dimples, 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 made at different positions in the thickness direction and at different positions perpendicular to the thickness direction. This allows the Vickers hardness of the second hardness change region to be measured while satisfying the measurement conditions of equal intervals of 15 μm or less in the thickness direction and a distance between the centers of the dimples that is at least three times the average diagonal length of the dimples. 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 exists up to 200 μm from the surface of the steel sheet, 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 surface of the steel sheet.

[0149] In the case of a sample in which high-strength portion softened layers 55 are arranged on both sides of the central portion 56 of the high-strength portion 53, similar measurements are performed from the first surface side of the sample, and also from the second surface side opposite the first surface.

[0150] (Method of calculating absolute value ΔHV1 of hardness change after measurement of first hardness change region) ΔHV1 is calculated by the following procedure: from all measurement points included in the region (first hardness change region) from the surface of the sample cut out from the high strength portion 53 to 40% of the total thickness of the high strength portion softened layer 55, the hardness gradient Δa of the first hardness change region is calculated using equation (2). 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, c i is the Vickers hardness (HV) at ai, and n is the total for all measurement points included in the region (first hardness change region) from the surface to 40% of the total thickness of the softened layer.

[0151]

number

[0152] where: Δa: Gradient of hardness change in the thickness direction in the first hardness change region (HV / %) ai: The ratio (%) of the distance from the surface at the i-th measurement point to the total thickness of the softened layer ci: Average value of Vickers hardness (HV) at three different points at the i-th measurement thickness position n: the sum of all measurement points included in the first hardness change region on the first surface side is.

[0153] In the case of a sample in which high-strength portion softened layers 55 are arranged on both sides of center portion 56, Δa1 on the first surface side is calculated from equation (2) based on the results of measuring Vickers hardness from the first surface side, and Δa2 on the second surface side is calculated from equation (2) based on the results of measuring Vickers hardness from the second surface side. The arithmetic mean of Δa1 and Δa2 can be taken as Δa.

[0154] ΔHV1 can be obtained by multiplying Δa obtained by formula (2) by the ratio of the thickness of the first hardness change region in the thickness direction of the entire softened layer.

[0155] (Method of calculating absolute value ΔHV2 of hardness change after measuring second hardness change region) ΔHV2 is calculated using the following procedure: From all measurement points included in the region (second hardness change region) from 40% to 100% of the total thickness of the high-strength portion softened layer 55 on the surface side of the sample, the hardness gradient ΔA of the second hardness change region is calculated using equation (3). Here, Ai is the percentage (%) 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.

[0156]

number

[0157] where: ΔA: Gradient of hardness change in the thickness direction in the second hardness change region (HV / %) Ai: The ratio (%) of the distance from the surface at the i-th measurement point to the total thickness of the softened layer Ci: Average value of Vickers hardness (HV) at three different points at the i-th measurement thickness position N: The sum of all measurement points included in the second hardness change region on the first surface side is.

[0158] In the case of a sample in which high-strength portion softened layers 55 are arranged on both sides of center portion 56, ΔA1 on the first surface side is calculated from equation (3) based on the results of measuring Vickers hardness from the first surface side, and ΔA2 on the second surface side is calculated from equation (3) based on the results of measuring Vickers hardness from the second surface side. The arithmetic mean of ΔA1 and ΔA2 can be taken as ΔA.

[0159] ΔHV2 can be obtained by multiplying ΔA obtained by equation (3) by the ratio of the thickness of the second hardness change region in the thickness direction of the entire softened layer.

[0160] It should be noted that it is mainly the high-strength portion softened layer 55 on the outer surface 53a side that can exhibit the effect of preventing cracking due to the large maximum bending angle θ1 during a side collision. Therefore, the high-strength portion softened layer 55 does not need to be provided on the inner surface 53b side. In this case, the inner surface 53b side has the same Vickers hardness as the central portion 56, and the high-strength portion softened layer 55 is present only on the outer surface 53a side.

[0161] <Variation 2> In the above-described embodiment, a configuration in which no softened layer is formed in the low-strength portion 51 has been described as an example. However, this is not necessarily the case. FIG. 8 is a longitudinal cross-sectional view of the pillar outer 41 in the thickness direction and the up-down direction Z near the joint 52 in Modification 2, with the back side of the cross section omitted. As shown in FIG. 8, a low-strength portion softened layer 57 may be present in the low-strength portion 51. In this case, the low-strength portion softened layer 57 is present both near the outer surface 51a and near the inner surface 51b of the low-strength portion 51. In this case, the low-strength portion softened layer 57 is present on both sides of the low-strength portion 51 in the thickness direction, and a central portion 58 is present between the low-strength portion softened layers 57.

[0162] (Vickers hardness at the center of the low strength part) The low-strength portion 51 may be able to avoid fracture during a side collision even when the Vickers hardness of the central portion 58 exceeds 300 HV, for example, 400 HV. However, in a configuration in which the Vickers hardness of the central portion 58 exceeds 300 HV, the presence of the low-strength portion softened layer 57 significantly enhances the effect of suppressing fracture of the low-strength portion 51 during a side collision. Examples of preferable lower limits for the Vickers hardness of the central portion 58 include 300 HV, 350 HV, 400 HV, 420 HV, and 450 HV. Examples of preferable upper limits for the Vickers hardness of the central portion 58 include 530 HV, 500 HV, 480 HV, and 450 HV. As is clear from the above, the Vickers hardness of the central portion 58 may be 300HV to 530HV, 350HV to 500HV, 400HV to 480HV, or 420HV to 450HV.

[0163] (Combination of Vickers hardness at the center and maximum bending angle difference) When the Vickers hardness of the low-strength portion 51 is 420 HV to 530 HV and the Vickers hardness of the central portion 56 of the high-strength portion 53 is 550 HV to 1050 HV, the difference in bending angles Δθ may be 50 degrees or less, 45 degrees or less, 40 degrees or less, 35 degrees or less, 30 degrees or less, or 25 degrees or less. The lower limit of the difference in bending angles Δθ is preferably 0 degrees. This relationship between the Vickers hardness and the difference in bending angles Δθ at the respective central portions of the low-strength portion 51 and the high-strength portion 53 allows the high-strength portion 53 to bend and deform in a manner that sufficiently follows the bending deformation of the low-strength portion 51 during a side collision. As a result, the possibility of fracture of the high-strength portion 53 during a side collision can be reduced. The Vickers hardness of the central portion 56 of the high-strength portion 53 may be in the range of 550 HV to 770 HV.

[0164] (Existence range of softened layer in low strength area) When the joint 52 is located at a height between the lower end 61b of the upper bracket 61 and the upper end 62a of the lower bracket 62 (when located in the position shown in FIG. 1 ), it is preferable that each low-strength portion softened layer 57 on each surface 51a, 51b of the low-strength portion 51 exists from a position higher than the upper end 62a of the lower bracket 62 to a position lower than the lower end 62b of the lower bracket 62. This allows each low-strength portion softened layer 57 to be arranged so as to cover the lower bracket 62, which is the part of the vehicle body 1 that deforms the most during a side collision. The low-strength portion softened layer 57 improves the bending deformability of the low-strength portion 51, thereby reducing the possibility of breakage of the low-strength portion 51 and improving the impact energy absorption of the low-strength portion 51. The low-strength portion softened layer 57 may be formed over the entire low-strength portion 51 in the up-down direction Z. In particular, as will be described later, when the joint 52 is located lower than the lower end 62b of the lower bracket 62 (when located in the position shown in FIG. 9 ), it is preferable that the low-strength portion softened layer 57 be formed over the entire area of the low-strength portion 51 in the up-down direction Z. In this case, the range of the low-strength portion 51 is relatively narrow within the center pillar 4, and the entire area of the low-strength portion 51 tends to deform during a side collision. Therefore, the low-strength portion softened layer 57 improves the bending deformability of the low-strength portion 51, thereby reducing the possibility of breakage of the low-strength portion 51 and improving the amount of impact energy absorption by the low-strength portion 51.

[0165] (Summary of thickness range of softened layer in low strength area) Each low-strength portion softened layer 57 is formed, for example, by reducing the carbon content in the vicinity of the portions that will become the outer surface 51a and the inner surface 51b of a blank, which is the material for the low-strength portions 51 (portions that will become the low-strength portion softened layer 57), compared to the portion that will become the central portion 58 of the blank. The method for forming the low-strength portion softened layer 57 is not particularly limited, and any method may be used. In this modified example, it is preferable that each low-strength portion softened layer 57 has a predetermined thickness from the corresponding surface 51a, 51b, for example, in order to ensure sufficient strength of the low-strength portions 51 while increasing the maximum bending angle of the low-strength portions 51. The lower limit of the predetermined thickness is, for example, 80 μm, and the upper limit is, for example, 200 μm.

[0166] (Summary of the Vickers hardness distribution in the softened layer of the low strength area and summary of the Vickers hardness in the center area) In each low-strength portion softened layer 57, the Vickers hardness decreases with increasing distance from the 1 / 2 plate thickness position of the low-strength portion 51. In each low-strength portion softened layer 57, the portions that become surfaces 51a and 51b have the lowest Vickers hardness in the low-strength portion 51, and are lower by, for example, at least 100 HV to 250 HV than the Vickers hardness of the central portion 58. In this case, when referring to the Vickers hardness of the low-strength portion 51, the Vickers hardness refers to the Vickers hardness of the central portion 58.

[0167] In this variant example 2, in which each low-strength portion 51 is provided with a low-strength portion softening layer 57, the maximum bending angle of the low-strength portion 51 can be made higher, for example, by about 20 degrees, compared to when the low-strength portion softening layer 57 is not provided (embodiment).

[0168] In this way, the Vickers hardness of the low-strength portion 51 at the thickness-direction surfaces 51a, 51b is set to be at least 100 HV lower than the Vickers hardness of the low-strength portion 51 at the center 58 at the thickness-direction center. This allows the maximum bending angle of the low-strength portion 51 to be increased. Therefore, the low-strength portion 51 can more reliably follow the bending deformation of the center pillar 4 during a side collision, suppressing cracking in the low-strength portion 51 and improving impact absorption performance. As a result, the high-strength portion 53 has a high Vickers hardness and high strength, which suppresses its intrusion into the vehicle interior and enhances occupant protection. The low-strength portion 51 can further reduce its thickness while ensuring the absorption of impact energy, thereby reducing the weight of the vehicle body 1. Furthermore, even if the low-strength portion 51 is formed of a high-strength material with a higher Vickers hardness, the possibility of fracture of the low-strength portion 51 can be suppressed, thereby further reducing the amount of intrusion of the low-strength portion 51 into the vehicle body during a side collision.

[0169] An example of the configuration of the low-strength portion 51 will be described more specifically.

[0170] (Vickers hardness at the center of the low strength part) The Vickers hardness of the central portion 58 in the plate thickness direction in the portion where the low-strength portion softened layer 57 is provided is preferably 150 HV or more. If the Vickers hardness of the central portion 58 is 150 HV or more, sufficient strength can be ensured in the low-strength portion 51 during a side collision. The Vickers hardness of the central portion 58 may be greater than 200 HV, 250 HV or more, 300 HV or more, 400 HV or more, or 500 HV or more. There is no particular upper limit to the Vickers hardness of the central portion 58, but in consideration of the ability to follow deformation of the center pillar 4 during a side collision, it is preferably 500 HV or less.

[0171] (Details of the thickness range of the softened layer in each low strength area) The thickness of each low-strength portion softened layer 57 in the thickness direction is preferably 80 μm or more and 5% to 20% of the plate thickness at the portion where the low-strength portion softened layer 57 is provided. If the thickness of each low-strength portion softened layer 57 is 20% or less of the plate thickness, the proportion of each low-strength portion softened layer 57 in the steel plate from which the low-strength portions 51 are made is small, so that the load-bearing capacity required of the low-strength portions 51 can be maintained. The thickness of each low-strength portion softened layer 57 is preferably 17% or less of the plate thickness, and more preferably 14% or less. On the other hand, when the low-strength portion softened layer 57 is provided over the entire surface of the steel plate from which the low-strength portions 51 are made, the deformability of the low-strength portion softened layer 57 can be fully exhibited if the thickness of each low-strength portion softened layer 57 is 80 μm or more and 5% or more of the plate thickness. The thickness of each low-strength portion softened layer 57 is more preferably 8% or more of the plate thickness. As is clear from the above, the thickness of each low-strength portion softened layer 57 may be 5% to 17%, 5% to 14%, 8% to 20%, 8% to 17%, or 8% to 14% of the above plate thickness.

[0172] (Method for measuring the thickness position of the boundary between the softened layer in the low strength portion and the center portion) The method for measuring the boundary between the low-strength portion softened layer 57 and the central portion 58 is the same as the method for measuring the boundary between the high-strength portion softened layer 55 and the central portion 56 .

[0173] (Method for calculating the gradient of Vickers hardness at each thickness position after measuring the boundary between the softened layer of the low strength portion and the center portion) The method for calculating the slope of the Vickers hardness at each thickness position after measuring the Vickers hardness when measuring the boundary between the low-strength softened layer 57 and the central portion 58 is the same as the method for calculating the slope of the Vickers hardness at each thickness position after measuring the Vickers hardness when measuring the boundary between the high-strength softened layer 55 and the central portion 56.

[0174] (Relationship between Vickers hardness of the softened layer in the low strength area and that of the center area) The Vickers hardness of the low-strength portion softened layer 57 on the surfaces 51a and 51b of the low-strength portion 51 is preferably 0.5 to less than 0.9 times the Vickers hardness of the central portion 58 where the low-strength portion softened layer 57 is provided.

[0175] (Method for measuring Vickers hardness at the center of a low-strength part) The method for measuring the Vickers hardness of the central portion 58 of the low strength portion 51 is the same as the method for measuring the Vickers hardness of the central portion 56 of the high strength portion 53.

[0176] (Method for measuring Vickers hardness of the surface of a low-strength part) The method for measuring the Vickers hardness of the surface of the low strength portion 51 is the same as the method for measuring the Vickers hardness of the surface of the high strength portion 53.

[0177] If the Vickers hardness of the surface of the low-strength portion 51 is 0.5 times or more the Vickers hardness of the central portion 58, 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 low-strength portion softened layer 57 on the surface of the low-strength portion 51 is 0.6 times or more the Vickers hardness of the central portion 58. On the other hand, if the Vickers hardness of the surface of the low-strength portion 51 is less than 0.9 times the Vickers hardness of the central portion 58, the deformability can be sufficiently improved. It is more preferable that the Vickers hardness of the low-strength portion softened layer 57 on the surface of the low-strength portion 51 is less than 0.8 times the Vickers hardness of the central portion 58.

[0178] (Change in Vickers hardness in the softened layer of the low strength part) FIG. 7 is also an image diagram for explaining an example of a Vickers hardness change in the low-strength portion softened layer 57. As shown in FIG. 7, the low-strength portion softened layer 57 preferably has, in the thickness direction, a first hardness change region that is a region from the surfaces 51a, 51b to 40% of the thickness of the low-strength portion softened layer 57, and a second hardness change region that is a region of the low-strength portion softened layer 57 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.

[0179] 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 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 the low-strength portion 51 can be improved. Specifically, the load drop immediately after the load peak can be made gentler in the later stroke of 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. It is preferable that the upper limit of ΔHV1′ is less than 200 HV, while the lower limit may be 50 HV, more than 50 HV, 55 HV, 60 HV, or 100 HV.

[0180] (Method for measuring Vickers hardness of first and second hardness change regions of the softened layer in the low strength portion) The method for measuring the hardness of the first hardness change region and the second hardness change region in the low-strength portion 51 is the same as the method for measuring the hardness of the first hardness change region and the second hardness change region in the high-strength portion 53.

[0181] In the case of a sample in which low-strength portion softened layers 57 are arranged on both sides of the central portion 58 of the low-strength portion 51, similar measurements are performed from the first surface side of the sample, and also from the second surface side opposite the first surface.

[0182] (Method of calculating absolute values ΔHV1' and ΔHV2' of hardness change after measuring first and second hardness change regions) The method for calculating ΔHV1′ and ΔHV2′ in the low strength portion 51 is the same as the method for calculating ΔHV1 and ΔHV2 in the high strength portion 53.

[0183] It should be noted that it is mainly the low-strength portion softened layer 57 on the outer surface 51a side that can exhibit the effect of preventing cracking due to the large maximum bending angle during a side collision. Therefore, the low-strength portion softened layer 57 does not need to be provided on the inner surface 51b side. In this case, the inner surface 51b side has the same Vickers hardness as the central portion 58, and the low-strength portion softened layer 57 is present only on the outer surface 51a side.

[0184] In this modification 2, the high strength portion softened layer 55 does not have to be provided in the high strength portion 53. In this case, the majority of the steel sheet base material in the high strength portion 53 is formed by the central portion 56. If a plating layer is present in the high strength portion 53, the steel sheet base material begins immediately below the plating layer, and if an alloy layer is present between the plating layer and the steel sheet base material in addition to the plating layer in the high strength portion 53, the steel sheet base material begins immediately below the alloy layer.

[0185] <Variation 3> In the above-described embodiment, the vehicle is a BEV, and a reinforcing member 20 for protecting the battery 18b from a side collision is provided within the side sill 5. However, this is not necessarily the case. FIG. 9 is a schematic left side view illustrating Modification Example 3 in which the reinforcing member 20 is not provided within the side sill 5. As shown in FIG. 9, if the vehicle is equipped with an internal combustion engine such as a gasoline engine or a diesel engine and a battery is not provided on the side of the side sill 5 (inside in the width direction Y), the reinforcing member 20 may not be provided within the side sill 5. Note that even in vehicles equipped with an internal combustion engine, it is common to provide a reinforcing member within the side sill 5. The vehicle shown in FIG. 9 may be a PHEV (Plug-in Hybrid Electric Vehicle). If the vehicle is a PHEV, the drive battery that drives the wheels is fixed to the floor panel 7 or the like at a relatively large distance (for example, a distance greater than 10 cm) from the side sill 5 in the width direction Y.

[0186] In this configuration, the deformation of the side sill 5 tends to be large during a side collision, and the side sill 5 also undergoes torsional deformation. Because the plastic deformation of the side sill 5 is relatively large, the side sill 5 absorbs a large amount of impact energy, and the side sill 5 and the center pillar 4 share the impact absorption. When the impact energy absorption share of the center pillar 4 is small, in order to prevent contact between the vehicle body 1 and the occupant, it is preferable to narrow the range of the low-strength portion 51, which undergoes large plastic deformation inward in the width direction Y to absorb the impact, and to position the high-strength portion 53, which reduces the amount of plastic deformation of the vehicle body 1 inward in the width direction Y, close to the side sill 5.

[0187] Therefore, in this modification, the joint 52 is preferably disposed at a height lower than the upper end 62a of the lower bracket 62 in the vertical direction Z, and particularly preferably disposed at a height lower than the lower end 62b of the lower bracket 62 in the vertical direction Z. The joint 52 is disposed, for example, above the side sill 5, below the lower end 62b. In the vehicle shown in FIG. 9 , the side sill 5 undergoes torsional deformation around an axis along the fore-aft direction X during a side collision. In this case, when the side sill 5 undergoes torsional deformation, the above-described arrangement of the joint 52 allows the high-strength portion 53 to be located close to the side sill 5, which experiences a large amount of deformation during a side collision. Therefore, during a side collision, the high-strength portion 53 can suppress deformation of the outer pillar 41 that follows the intrusion of the side sill 5 into the cabin 10 due to torsional deformation of the side sill 5. As a result, the amount of intrusion of the center pillar 4 into the cabin 10 can be reduced, and torsional deformation of the side sill 5 can be further suppressed.

[0188] <Variation 4> In the above embodiment, the lower end 473b of the overlapping portion 47 of the pillar outer 41 is aligned with the outer wall 153 of the side sill outer 15. However, this does not have to be the case. Fig. 10 is a schematic cross-sectional view of the main part of Variation 4, which relates to a modified example of the overlapping portion 47 of the pillar outer 41, and shows a cross section perpendicular to the front-rear direction.

[0189] The overlapping portion 47 of the fourth modification has, in addition to an upper flange 471, an upper wall 472, and an outer wall 473, a lower wall 474 extending inward in the width direction Y from the outer wall 473.

[0190] The outer wall 473 is disposed up to the lower end of the outer surface 153 a of the outer wall 153 of the side sill outer 15 .

[0191] The lower wall 474 is preferably in contact with the lower surface 154a of the lower wall 154 of the side sill outer 15 along the length thereof, and is joined to this lower surface 154a by welding, adhesive, etc. In this embodiment, the lower end 474b (lower tip) of the lower wall 474 is the lower end of the overlapping portion 47.

[0192] In this way, the overlapping portion 47 extends to the lower wall 474 of the side sill outer 15, so that the center pillar 4 is configured to embrace the lower part of the side sill 5. This increases the joint area between the center pillar 4 and the side sill 5, thereby reducing the stress acting between the side sill 5 and the center pillar 4 during a side collision. Furthermore, the load transmitted from the center pillar 4 to the side sill 5 during a side collision is received by the vehicle body 1 as a torsional moment. This prevents deformation of the closed cross-sectional shape of the side sill 5, particularly the side sill outer 15. Even in the event of a side collision, the original closed cross-sectional shape of the side sill 5 before the collision is unlikely to change, so the side sill 5 has high torsional rigidity and can reduce the torsional angle of the side sill 5 during a side collision. This reduces the amount of intrusion of the center pillar 4 and the side sill 5 into the cabin 10. On the other hand, by devising a positioning arrangement in which the center pillar 4 does not reach the lower end (lower flange 155) of the side sill 5, the center pillar 4 does not become unnecessarily heavy, and an excellent balance can be achieved between improving the reinforcing effect of the side sill 5 due to the lower shape of the center pillar 4 and suppressing the weight increase of the center pillar 4.

[0193] The lower end 474b of the overlapping portion 47 is, for example, a linear end portion along the fore-and-aft direction X. With this configuration, the vehicle body 1 can receive the load transmitted from the center pillar 4 to the side sill 5 during a side collision as a torsional moment over the entire area where the overlapping portion 47 is located in the fore-and-aft direction X. This more reliably prevents the closed cross-sectional shape of the side sill 5, and in particular the side sill outer 15, from being crushed.

[0194] In the fourth modification, as described in the embodiment, the height position of the joint 52 is preferably between the height position of the lower end 61b of the upper bracket 61 and the height position of the upper end 62a of the lower bracket 62. By extending the overlapping portion 47 to the lower wall 474 of the side sill outer 15, the overlapping portion 47 acts in the same manner as the reinforcing member 20 during a side collision, thereby suppressing deformation of the side sill 5. In this case, by setting the height position of the joint 52 to the above-described position, the range of the low-strength portion 51 can be sufficiently secured. Therefore, it is possible to achieve both impact absorption by the low-strength portion 51 in cooperation with the side sill 5 in the early stage of a side collision and suppression of deformation of the center pillar 4 toward the cabin 10 due to the high-strength portion 53 receiving the impact load from the upper bracket 61 in the later stage of a side collision.

[0195] In the fourth modification, the height position of the joint 52 may be disposed at a position lower than the height position of the lower end 62b of the lower bracket 62. Although the reinforcing member 20 is illustrated in Fig. 10 showing the fourth modification, the reinforcing member 20 may be omitted.

[0196] <Other variations> In the above-described embodiment and modified example, a center pillar has been described as an example of the impact absorbing member of the present invention. However, this is not necessarily the case. A side sill may also be used as the impact absorbing member of the present invention. In this case, for example, the side sill outer 15 may have a low-strength portion 51 near the overlapping portion 47, and a high-strength portion 53 may be disposed on the front or rear fastening position side of the side sill 5. In this case, during a side collision, the side sill 5 near the overlapping portion 47 exhibits an impact absorbing effect, while the high-strength portion on the front or rear portion of the side sill 5 can more reliably prevent the side sill 5 from penetrating into the cabin 10. The impact absorbing member of the present invention may also be applied to components other than center pillars and side sills. [Example]

[0197] A model of the vehicle body 1 described in the embodiment (FIGS. 1 to 3) was created by computer. The configurations of the vehicle body 1 model in the example and comparative example are as follows. (Configuration of the Example) The Vickers hardness HV1 and thickness of the high strength portion 53 of the pillar outer 41 are 600 HV and 1.2 mm. The high strength portion 53 is made of the material described in the first modification, and includes a high strength portion softened layer 55. Vickers hardness HV2 and thickness of low strength portion 51 of pillar outer 41: 350HV, 1.4mm (Configuration of Comparative Example) The Vickers hardness HV1 and thickness of the high strength portion 53 of the pillar outer 41 are 1000 HV and 1.2 mm. The high strength portion 53 does not include the high strength portion softened layer 55 described in the first modification. Vickers hardness HV2 and thickness of low strength portion 51 of pillar outer 41: 150HV, 1.4mm (Configuration common to Examples and Comparative Examples) Vickers hardness and thickness of Pillar Inner 42: 350HV, 1.3mm Patch 43 Vickers hardness and thickness: 600HV, 1.2mm

[0198] The comparative example and the example differ in particular in the following respects. The difference Δθ between the maximum bending angle θ1 of the high strength portion 53 and the maximum bending angle θ2 of the low strength portion 51 in the comparative example: 105 degrees The difference Δθ between the maximum bending angle θ1 of the high strength portion 53 and the maximum bending angle θ2 of the low strength portion 51 in the example: 20 degrees

[0199] For the comparative example and the example, the presence or absence of cracks in the pillar outer 41 when a side collision occurs to the vehicle body 1 was measured by computer simulation. The side collision conditions were set in accordance with the Insurance Institute for Highway Safety (IIHS). Specifically, the test conditions were set under conditions simulating a full-car collision, in which a collision load was applied to the center pillar.

[0200] As a result, in the comparative example, cracks occurred in the front wall 412 and the rear wall 414 of the high strength portion 53 near the joint 52. On the other hand, no cracks occurred in the example. Thus, the example was thin and lightweight, but did not crack even during a side collision, and impact absorption performance was ensured by suppressing cracks. [Industrial Applicability]

[0201] The present invention can be widely applied as a shock absorbing member. [Explanation of symbols]

[0202] 1. Body 4 Center pillar (shock absorbing member) 5 Side sill 8 doors 20 Reinforcement member 41 Pillar outer 42 Pillar inner 47 Overlapping Section 51 Low strength section 52 Joint 53 High strength section 61 Bracket 61a Top edge of upper bracket 62 Bracket 153 Outside wall Z vertical direction (longitudinal direction) θ1 Maximum bending angle of high strength part θ2 Maximum bending angle of low strength part

Claims

1. An impact absorbing member provided on a vehicle body, formed along a predetermined longitudinal direction, and including a portion having a cross section perpendicular to the longitudinal direction that has a closed cross-sectional shape, A low strength portion; a high-strength portion that is aligned with the low-strength portion in the longitudinal direction and has a central portion in the plate thickness direction having a Vickers hardness higher than the Vickers hardness of a central portion in the plate thickness direction of the low-strength portion; a joining portion joining the low strength portion and the high strength portion; Equipped with a ratio HV 1 / HV 2 of the Vickers hardness HV 1 of the high strength portion at the center in the thickness direction to the Vickers hardness HV 2 of the low strength portion at the center in the thickness direction is 1.3 or more; An impact absorbing member, wherein the Vickers hardness at the surface of the high strength portion in the plate thickness direction is at least 100 HV lower than the Vickers hardness at the central portion of the high strength portion in the plate thickness direction.

2. 2. The impact absorbing member according to claim 1, wherein the Vickers hardness of the central portion in the thickness direction of the high strength portion is 500 HV or more.

3. The impact absorbing member according to claim 2 , wherein the Vickers hardness of the central portion in the thickness direction of the low strength portion is 150 HV or more.

4. the impact absorbing member is a center pillar that includes an inner pillar and an outer pillar and is disposed along the up-down direction of the vehicle body, the pillar outer includes the low strength portion, the joint portion, and the high strength portion, The impact absorbing member according to any one of claims 1 to 3, wherein the high strength portion is disposed above the low strength portion.

5. The center pillar is provided with a pair of upper and lower brackets for supporting a door installed behind the center pillar, The impact absorbing member according to claim 4 , wherein the joint portion is disposed at a position lower than a height position of a lower end of the upper bracket.

6. The impact absorbing member according to claim 5 , wherein the joint portion is disposed at a position higher than a height position of an upper end of the lower bracket.

7. The impact absorbing member according to claim 5 , wherein the joint portion is disposed at a position lower than a height position of a lower end of the lower bracket.

8. the vehicle body further includes a side sill joined to a lower portion of the center pillar, arranged along a longitudinal direction of the vehicle body, and having a closed cross-sectional shape in a cross section perpendicular to the longitudinal direction, The side sill includes an outer wall disposed on the outer side of the side sill in a width direction of the vehicle body, The lower part of the center pillar has an overlapping portion arranged to cover the side sill at a connection point with the side sill, The impact absorbing member according to claim 4 , wherein the overlapping portion extends along a portion below half the height of the outer wall of the side sill in the up-down direction.

9. The high strength portion has a high strength portion softened layer provided from the surface in the plate thickness direction, The Vickers hardness of the central portion in the plate thickness direction in the portion where the high strength portion softened layer is provided in the high strength portion is 500 HV or more, the thickness of the high-strength portion softened layer is 80 μm or more, and is 5% or more and 20% or less of the plate thickness at the portion where the high-strength portion softened layer is provided, the Vickers hardness of the high-strength portion softened layer on the surface is 0.5 times or more and less than 0.9 times the Vickers hardness of the central portion in the plate thickness direction in the portion where the high-strength portion softened layer is provided, the high-strength portion softened layer has, in the plate thickness direction, a first hardness change region that is a region from the surface to 40% of the thickness of the high-strength portion softened layer, and a second hardness change region that is a region of the high-strength portion softened layer that is not the first hardness change region, 2. The impact absorbing member according to claim 1, 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.

10. 2. The impact absorbing member according to claim 1, wherein the Vickers hardness at the surface in the thickness direction of the low strength portion is at least 100 HV lower than the Vickers hardness at the central portion in the thickness direction of the low strength portion.

11. The low-strength portion has a low-strength portion softened layer provided from the surface in the plate thickness direction, The Vickers hardness of the central portion in the plate thickness direction in the portion where the low strength portion softened layer is provided in the low strength portion is 150 HV or more, the thickness of the low-strength portion softened layer is 80 μm or more, and is 5% to 20% of the plate thickness at the portion where the low-strength portion softened layer is provided, the Vickers hardness of the low-strength portion softened layer on the surface is 0.5 times or more and less than 0.9 times the Vickers hardness of the central portion in the plate thickness direction in the portion where the low-strength portion softened layer is provided, The low-strength portion softened layer has, in the plate thickness direction, a first hardness change region that is a region from the surface to 40% of the thickness of the low-strength portion softened layer, and a second hardness change region that is a region of the low-strength portion softened layer that is not the first hardness change region, The impact absorbing member according to claim 10, wherein the absolute value ΔHV1′ of the hardness change in the thickness direction in the first hardness change region is greater than the absolute value ΔHV2′ of the hardness change in the thickness direction in the second hardness change region.

12. A center pillar and a side sill joined to a lower portion of the center pillar, A vehicle body, wherein at least one of the center pillar and the side sill is the impact absorbing member according to claim 1.

Images