Automotive structural components

Abstract

This automotive frame member is a hat-shaped member comprising a top plate portion, a vertical wall portion, and a flange portion, the average Vickers hardness of this member being 300 Hv or more and 800 Hv or less, the vertical wall portion having at least one bead extending from the flange portion side to the top plate portion side, and a portion of the connecting part between the vertical wall portion and the top plate portion being a thickened portion with a greater thickness than the vertical wall portion and the top plate portion.

Description

Technical Field

[0001] The present invention relates to an automotive skeletal member. More specifically, the present invention relates to an automotive skeletal member having excellent shock absorption characteristics against impacts from the vehicle traveling direction. This application claims priority based on Japanese Patent Application No. 2024-173129 filed in Japan on October 2, 2024, and incorporates its content herein.

Background Art

[0002] An automotive skeletal member is a member that forms the skeleton of an automobile and is mainly made of steel plates. In recent years, in this automotive skeletal member, further improvement in safety has been demanded from the viewpoint of passenger protection, and further weight reduction has also been demanded from the viewpoints of fuel consumption regulations and achievement of carbon neutrality.

[0003] Automotive skeletal members include deformation restraining members arranged for the purpose of protecting the passenger space during a collision, and shock absorbing members arranged for the purpose of absorbing energy during a collision and reducing the impact on the passengers. For example, in an automotive skeletal member for shock absorption, in order to enhance the shock absorption characteristics, it is required to exhibit a sufficient amount of plastic work during a collision. That is, in an automotive skeletal member for shock absorption, during a collision, the deformation stress of the steel plate becomes high, and a larger plastic strain is required to occur over a wide area. The amount of plastic work (for example, when the displacement is A, the amount of plastic work) corresponds to the area enclosed by the load (Y)-displacement (X) curve, the curve and the X-axis, and X = A. Therefore, the higher the deformation stress and the larger the plastic strain, the larger the amount of plastic work.

[0004] In order for an automotive skeletal member for shock absorption to exhibit a sufficient amount of plastic work during a collision, generally the following two countermeasures are taken. The first is to increase the strength of the steel plate applied to the automotive skeletal member, and the second is to control the deformation mode during a collision as an automotive skeletal member.

[0005] One of the technical challenges in increasing the strength of the materials mentioned above is the development of forming methods to suppress cracking and dimensional inaccuracies that become apparent during forming due to increased strength. Another technical challenge in controlling the deformation form during impact is the addition of beads to the material to intentionally generate buckling deformation during impact. Through these measures, efforts have been made to improve the amount of plastic work performed.

[0006] Patent Document 1 discloses an automobile frame part manufactured by a hot press forming method that can suppress dimensional inaccuracies caused by shape changes such as springback that occur when high-strength steel sheets are press-formed. For example, Patent Document 1 specifies the shape of the shoulder portion of the die of the mold.

[0007] Patent Document 2 discloses a hot press forming apparatus and method for metal sheet materials that heat the metal sheet material and rapidly and uniformly cool the workpiece and mold during and / or after hot press forming. For example, Patent Document 2 specifies the shape of the shoulder portion of the punch of the mold. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2015-000431 [Patent Document 2] Japanese Patent Application Publication No. 2005-169394 [Overview of the project] [Problems that the invention aims to solve]

[0009] As described above, attempts have been made to increase the amount of plastic work done during a collision by increasing the strength of the steel plates used in automotive frame components and by adding buckling beads to the shape of automotive frame components.

[0010] However, excessively increasing the strength of steel sheets used in automotive structural components reduces their ductility, making them more susceptible to fracture during collisions. Furthermore, excessive buckling beads in automotive structural components reduce their yield strength, making it difficult to obtain sufficient plastic work during a collision. Currently, there is a need to further increase the plastic work performed during collisions in automotive structural components.

[0011] The technology disclosed in Patent Document 1 specifies the shape of the shoulder portion of the die of the mold, but does not consider the shape of the shoulder portion of the punch of the mold. The technology disclosed in Patent Document 2 also specifies the shape of the shoulder portion of the punch of the mold, but does not consider the application of a bead to the member. With these prior arts, it is difficult to sufficiently increase the amount of plastic work during a collision as an automotive frame member.

[0012] The present invention has been made in view of the above problems. The object of the present invention is to provide an automobile frame member that has excellent shock absorption characteristics against impacts from the direction of travel of the vehicle, and in particular an automobile frame member in which the shock absorption characteristics are preferably enhanced while minimizing the increase in the mass of the automobile frame member. [Means for solving the problem]

[0013] The gist of this invention is as follows: (1) An automobile frame member according to one aspect of the present invention is A hat-shaped member, when viewed in a cross-section with the longitudinal direction as the normal, comprising a top plate portion located in the center in the width direction, a pair of vertical wall portions located at both ends of the top plate portion, and flange portions located at the ends of the vertical wall portions, When the Vickers hardness of the member is measured at 100 or more measurement points at equal intervals along the longitudinal direction, the average value of the Vickers hardness is 300 Hv or more and 800 Hv or less. The vertical wall portion has at least one bead extending from the flange portion side to the top plate portion side in a direction perpendicular to the longitudinal direction, In at least a portion of the longitudinal direction, the connecting portion between the vertical wall portion and the top plate portion is a thickened portion that is thicker than the vertical wall portion and the top plate portion. (2) In the automotive frame member described in (1) above, the thickened portion may include the connecting portion in the bead in the connecting portion. (3) In the automotive frame member described in (1) or (2) above, the thickened portion may be present only in the connecting portion of the bead in the connecting portion. (4) In the automotive frame member described in any one of (1) to (3) above, the thickened portion may be provided by laser welding, spot welding, brazing, or thickening. (5) In the automotive frame member described in any one of (1) to (4) above, the thickened portion may have a VDA bending angle of 90 degrees or more as defined in VDA-238-100. (6) In the case of an automobile frame member described in any one of (1) to (5) above, when the tensile strength is measured using a test piece taken so that the longitudinal direction of the member is the tensile direction, the average value of the tensile strength may be 950 MPa or more and 2512 MPa or less. [Effects of the Invention]

[0014] According to the above-described embodiment of the present invention, it is possible to provide an automobile frame member that has excellent shock absorption characteristics against impacts from the direction of travel of the vehicle. In particular, according to the above-described embodiment of the present invention, it is possible to provide an automobile frame member in which shock absorption characteristics are preferably enhanced while minimizing the increase in mass of the automobile frame member. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram of an automotive frame component relating to one embodiment of the present invention. [Figure 2] This is a schematic diagram of an automobile frame member according to another embodiment of the present invention. [Figure 3] This is a schematic diagram of an automobile frame member according to another embodiment of the present invention. [Figure 4]It is a schematic diagram of an automobile skeleton member according to another embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0016] Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the configurations disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention. In addition, the drawings used in the following description may clearly show the features of this embodiment, and the dimensional ratios of each component are not necessarily the same as the actual ones. Also, the lower limit value and the upper limit value are included in the numerical limitation range described below. The numerical values indicated by "more than" or "less than" are not included in the numerical range. "%" regarding the content of each element means "mass%".

[0017] The inventors of the present invention have intensively studied the deformation state when an automobile skeleton member receives an axial compression load for the purpose of improving the shock absorption characteristics against impacts from the traveling direction of the vehicle. As a result, even when the strength of the steel plate is generally increased as an automobile skeleton member, if the automobile skeleton member has at least one bead in the vertical wall portion and at least a part of the connecting portion between the vertical wall portion and the top plate portion is a thickened portion, it has been found that a large amount of plastic work can be obtained when the automobile skeleton member receives an axial compression load while suppressing an increase in the mass of the member.

[0018] Specifically, the automobile skeleton member according to this embodiment is A hat-shaped member including a top plate portion arranged at the central portion in the width direction, a pair of vertical wall portions arranged at both ends of the top plate portion, and a flange portion arranged at the tips of these vertical wall portions when viewed in a cross section with the longitudinal direction as the normal line, When the Vickers hardness is measured at 100 or more measurement points at equal intervals along the longitudinal direction of the member, the average value of this Vickers hardness is 300 Hv or more and 800 Hv or less, The vertical wall portion has at least one bead extending from the flange portion side to the top plate portion side in a direction orthogonal to the longitudinal direction, In at least a portion of the longitudinal direction, the connecting portion between the vertical wall portion and the top plate portion is a thickened portion that is thicker than the vertical wall portion and the top plate portion.

[0019] Figure 1 shows a schematic diagram of an automobile frame member according to this embodiment. As shown in Figure 1, the automobile frame member 1 according to this embodiment has a hat shape when viewed in a cross section normal to the longitudinal direction, having a top plate portion 11 located in the center in the width direction, a pair of vertical wall portions 12 located at both ends of the top plate portion 11, and flange portions 13 located at the ends of the vertical wall portions 12. These vertical wall portions 12 have at least one bead 12a, which is an uneven portion extending from the flange portion 13 side to the top plate portion 11 side in a direction perpendicular to the longitudinal direction. In addition, in at least a part of the longitudinal direction, the connecting portion (punch shoulder portion 15) between the vertical wall portion 12 and the top plate portion 11 is a thickened portion 14 that is thicker than the vertical wall portion 12 and the top plate portion 11. In Figure 1, the vertical wall portion 12 is connected to the top plate portion 11 by the thickened portion 14 in the entire longitudinal direction.

[0020] In this embodiment, when the automotive frame member 1 is viewed in a cross-section with the longitudinal direction as the normal, the top plate portion 11, the vertical wall portion 12, and the flange portion 13 correspond to the straight portions in the above cross-section and to portions that do not include curved portions (R portions). For example, when viewed in the above cross-section, the curved portion (R portion) between the top plate portion 11 and the vertical wall portion 12 corresponds to the punch shoulder portion 15 (the connecting portion 15 between the vertical wall portion 12 and the top plate portion 11). Also, when viewed in the above cross-section, the thickened portion 14 includes the connecting portion 15 between the vertical wall portion 12 and the top plate portion 11. In addition, when viewed in the above cross-section, the thickened portion 14 may include the connecting portion 15 between the vertical wall portion 12 and the top plate portion 11 and also include the straight portions of the vertical wall portion 12 and the top plate portion 11.

[0021] When subjected to an axial compressive load, in the automotive frame member 1 according to this embodiment, plastic deformation begins starting from the bead 12a of the vertical wall portion 12, and the rigidity of the punch shoulder portion 15, which is the connecting portion between the vertical wall portion 12 and the top plate portion 11, is increased by the presence of the thickened portion 14, resulting in a large amount of plastic work. For example, in the automotive frame member 1 according to this embodiment, only the thickened portion 14 is thickened, so the impact absorption characteristics are preferably improved while minimizing the increase in mass of the automotive frame member.

[0022] The automotive frame members according to this embodiment will be described in detail below.

[0023] <Beads on vertical walls> In the automotive frame member according to this embodiment, the vertical wall portion has at least one bead, which is an uneven portion that extends from the flange portion side to the top plate portion side in a direction perpendicular (approximately perpendicular) to the longitudinal direction.

[0024] The presence of at least one bead in the vertical wall ensures that even when the input load is dynamic or contains moment components, and therefore includes various disturbances, buckling deformation occurs starting from the bead when subjected to axial compressive load, resulting in highly reproducible and stable plastic work.

[0025] When a member is subjected to an axial compressive load, if the member buckles and deforms in a way that is uneven in the direction normal to the vertical wall, this deformation can be said to be a stable deformation mode. In order to induce such a stable deformation mode, it is preferable that the beads placed on the vertical wall of the member have an uneven shape in the direction normal to the vertical wall. For example, as shown in Figure 1, it is preferable that the beads placed on the vertical wall of the member have a concave shape that is recessed from the outside to the inside of a hat shape.

[0026] The number of beads extending from the flange side to the top plate side should be at least one along the longitudinal direction of the member, but two or more may be provided. For example, if the length of the longitudinal direction of an automobile frame member is L in mm, the number of beads extending from the flange side to the top plate side may be L / 200 or more along the longitudinal direction of the member. On the other hand, beads can become weak points when subjected to axial compressive load, and if there are too many beads, the yield strength of the member may decrease, and sufficient plastic work may not be obtained. For this reason, the number of beads extending from the flange side to the top plate side should be L / 50 or less along the longitudinal direction of the member. More preferably, it should be L / 80 or less.

[0027] Furthermore, the shape of the bead extending from the flange side to the top plate side is not particularly limited, but it is acceptable as long as it is U-shaped, V-shaped, W-shaped, etc., when viewed in a cross-section normalized to the longitudinal direction of the bead. Also, the width of the bead extending from the flange side to the top plate side (length in the longitudinal direction of the automotive frame member) is not particularly limited, but it is acceptable if it is 20 mm or more, 30 mm or more, 40 mm or more, and 70 mm or less, 60 mm or less, or 50 mm or less. Also, the depth of the bead extending from the flange side to the top plate side is not particularly limited, but it is acceptable if it is 5 mm or more, 10 mm or more, 20 mm or more, 30 mm or more, and 70 mm or less, 60 mm or less, or 50 mm or less.

[0028] Furthermore, the bead extending from the flange side to the top plate side may be placed at any position relative to the longitudinal direction of the member, but it is preferable that it be placed in a location where deformation of the member does not significantly affect the occupant space. Also, the bead extending from the flange side to the top plate side may be placed at least once on at least one of the vertical walls, but it may also be placed at least once on each of the pair of vertical walls facing each other on either side of the top plate. When beads are placed on each of the pair of vertical walls facing each other on either side of the top plate, it is preferable that the bead included in one vertical wall and the bead included in the other vertical wall are placed facing each other between the pair of vertical walls. Furthermore, it is preferable that the beads placed facing each other between the pair of vertical walls have the same shape, width, and depth.

[0029] <Thickened section> In the automotive frame member according to this embodiment, in at least a portion in the longitudinal direction, the connecting portion (punch shoulder portion) between the vertical wall portion and the top plate portion is a thickened portion that is thicker than the vertical wall portion and the top plate portion.

[0030] In this embodiment, the automotive frame member has beads arranged on the vertical wall portion as described above. While these beads improve the plastic work by controlling the deformation shape, they also become weak points when the member is subjected to axial compressive load, which can reduce the amount of plastic work. As a result of detailed studies by the inventors, it has been found that even if beads are arranged on the vertical wall portion, if at least a part of the connecting portion between the vertical wall portion and the top plate portion is a thickened portion, the rigidity of the connecting portion (punch shoulder portion) between the vertical wall portion and the top plate portion is increased, and the decrease in the amount of plastic work can be suppressed.

[0031] If a thickened section is placed on at least a part of the punch shoulder, which is the connecting portion between the vertical wall and the top plate, the axial deformation resistance will increase when the member is subjected to an axial compressive load, resulting in a larger plastic work rate. For example, if the length of the longitudinal direction of the automotive frame member is L in mm, the longitudinal lengths of each thickened section should be L × 0.002 mm or more, L × 0.003 mm or more, L × 0.004 mm or more, and L × 1 mm or less, L × 0.1 mm or less, and L × 0.01 mm or less.

[0032] Furthermore, when viewing a cross-section of the automotive frame member with the longitudinal direction as the normal, and assuming the average thickness of the top plate portion is t in mm, the thickness of the thickened portion when viewed in the above cross-section should be t × 1.10 mm or more, t × 1.14 mm or more, t × 1.20 mm or more, t × 1.28 mm or more, and t × 1.30 mm or more. On the other hand, since automotive frame members are required to be lightweight, the thickness of the thickened portion when viewed in the above cross-section should be t × 1.70 mm or less, t × 1.60 mm or less, and t × 1.50 mm or less.

[0033] Furthermore, the thickened portion only needs to have a length of 5 to 80 mm in the direction perpendicular to the longitudinal direction of the automobile frame member. Specifically, the thickened portion should have a length of 5 to 40 mm on the vertical wall side and 5 to 40 mm on the top plate side, with the center of the punch shoulder portion as the reference point on the hat-shaped outer surface as shown in Figure 1. As described above, the thickened portion may include the connecting portion between the vertical wall portion and the top plate portion, as well as the straight portions of the vertical wall portion and the top plate portion, when viewed in a cross-section normalized to the longitudinal direction of the automobile frame member.

[0034] The average thickness of the vertical wall and top plate can be determined by measuring the thickness of the vertical wall and top plate at 100 or more equally spaced points along the longitudinal direction of the automobile frame member, at the center of each section (the center of the vertical wall and top plate when viewed in a cross-section perpendicular to the longitudinal direction of the automobile frame member), and then calculating the average value of each. The thickness of the thickened portion of the punch shoulder can be determined by measuring the thickness of the thickened portion at the center of the punch shoulder of the automobile frame member (the center of the punch shoulder when viewed in a cross-section perpendicular to the longitudinal direction of the automobile frame member).

[0035] Furthermore, in the automotive frame member according to this embodiment, it is preferable that the thickened portion includes the connecting portion (punch shoulder portion) of the bead in the above-mentioned connecting portion.

[0036] Figure 2 shows a schematic diagram of an automobile frame member in which the thickened portion includes the connecting portion in the bead. As shown in Figure 2, it is preferable that the thickened portion 14 includes the connecting portion (punch shoulder) in the bead 12a of the vertical wall portion 12 in the connecting portion 15. That is, as shown in Figure 2, in a part of the area where the bead 12a is not provided, the connecting portion between the vertical wall portion 12 and the top plate portion 11 may be the punch shoulder portion rather than the thickened portion 14.

[0037] In addition, in the automotive frame member according to this embodiment, it is even more preferable that the thickened portion exists only in the connecting portion (punch shoulder portion) of the bead in the above-mentioned connecting portion.

[0038] Figure 3 shows a schematic diagram of an automobile frame member in which the thickened portion exists only at the connecting portion of the bead. As shown in Figure 3, it is preferable that the thickened portion 14 exists only at the connecting portion (punch shoulder) of the bead 12a of the vertical wall portion 12 in the connecting portion 15. That is, as shown in Figure 3, in all areas where the bead 12a is not present, the connecting portion between the vertical wall portion 12 and the top plate portion 11 may be the punch shoulder portion rather than the thickened portion 14. In other words, it is preferable that the thickened portion exists only at the connecting portion in the region where a bead exists in the connecting portion between the vertical wall portion and the top plate portion, and furthermore, that the length of the thickened portion in the longitudinal direction of the member is shorter than the width of the bead (length in the longitudinal direction of the member).

[0039] As a result of detailed investigations by the inventors, it was found that when the thickened portion is present only at the connecting portion of the bead in the above-mentioned connecting portion, when the member is subjected to an axial compressive load, plastic deformation begins at the bead of the vertical wall portion, but the deformation does not concentrate at the bead, and the deformation is easily distributed to the vertical wall portion surrounding the bead. In this case, while ensuring the strength required for an automobile frame member, premature fracture is suppressed and sufficient plastic work can be obtained. In addition, since the thickened portion is present only at the connecting portion of the bead in the above-mentioned connecting portion, the impact absorption characteristics can be favorably enhanced while minimizing the increase in mass of the automobile frame member.

[0040] Furthermore, if an automobile frame member contains two or more beads, it is preferable that thickened sections be provided for all of the beads. Similarly, if beads are arranged opposite each other on a pair of vertical wall sections facing each other across a top plate, it is preferable that thickened sections be provided for all of the beads.

[0041] In the automotive frame member according to this embodiment, the thickened portion can be provided by any one of the following methods: laser welding, spot welding, brazing, or material thickening. In particular, in the automotive frame member according to this embodiment, it is preferable that the thickened portion is provided by material thickening.

[0042] For example, a thickened portion of an automobile frame member may be added to the automobile frame member by changing the shape of the mold used to form the automobile frame member so that the thickness of the punch shoulder portion, which is the connecting portion between the vertical wall portion and the top plate portion, is increased. Alternatively, a thickened portion of the automobile frame member may be added to the automobile frame member after it has been formed by attaching a reinforcing member to the punch shoulder portion, which is the connecting portion between the vertical wall portion and the top plate portion, by laser welding, spot welding, brazing, or the like. As an example, Figure 4 shows a schematic diagram of an automobile frame member to which a thickened portion has been added by spot welding.

[0043] In the automotive frame member according to this embodiment, it is preferable that the thickened portion has a VDA bending angle of 90 degrees or more as defined in VDA-238-100.

[0044] In axial compression members, excessively increasing the material strength makes them prone to fracture at the buckling deformation area. Furthermore, detailed studies by the inventors revealed that fracture at the buckling deformation area is caused by bending deformation. In the case of bending deformation, thicker materials are more prone to fracture compared to thinner materials because the distance from the neutral axis is greater. Therefore, fracture due to bending deformation is more likely to occur in the thickened sections, which are thicker than the vertical wall sections and top plate sections. For this reason, by controlling the thickened sections to have excellent bending resistance (bendability), fracture when the member is subjected to an axial compression load can be effectively suppressed.

[0045] For example, it is preferable that the thickened portion has a VDA bending angle of 90 degrees or more as defined in VDA-238-100. In this case, when the member is subjected to an axial compressive load, fracture is less likely to occur in the buckling deformation portion, and as a result, a large amount of plastic work can be easily obtained. There is no particular upper limit to the VDA bending angle of the thickened portion, but for example, it may be 130 degrees or 120 degrees.

[0046] Methods for controlling the characteristics of the thickened section will be described later, but for example, when the thickened section is added by laser welding, spot welding, or brazing, it is sufficient to attach a reinforcing member with excellent bendability to the high-strength base material so that the VDA bending angle of the thickened section as a whole is 90 degrees or more. Also, when the thickened section is added by increasing the thickness during press forming, the press forming conditions should be controlled so that the VDA bending angle of the thickened section is 90 degrees or more.

[0047] <Automotive structural components> Furthermore, since the automotive frame member according to this embodiment is assumed to have high tensile strength (Vickers hardness), when the Vickers hardness is measured at 100 or more measurement points at equal intervals along the longitudinal direction of the automotive frame member, the average value HVave of the Vickers hardness at all measurement points should be between 300 Hv and 800 Hv. It is even more preferable that this HVave be between 300 Hv and 500 Hv.

[0048] Similarly, since the automotive frame member according to this embodiment is assumed to have high tensile strength (Vickers hardness), when the tensile strength is measured using a test piece taken so that the longitudinal direction of the automotive frame member is the tensile direction, the average value TSave of the tensile strength should be 950 MPa or more and 2512 MPa or less. It is even more preferable that this TSave be 950 MPa or more and 1705 MPa or less.

[0049] Furthermore, the structural composition of the steel in the automotive frame member according to this embodiment is not particularly limited. The steel structure in the automotive frame member according to this embodiment may include martensite, tempered martensite, bainite, retained austenite, ferrite, pearlite, and the like.

[0050] However, since the automotive frame member according to this embodiment is assumed to have high tensile strength (Vickers hardness), it is preferable that the steel structure of the member be mainly composed of martensite. In addition, the automotive frame member according to this embodiment may also contain retained austenite, bainite, tempered martensite, etc., in addition to martensite. For example, since it is preferable that the VDA bending angle of the thickened portion is 90 degrees or more, the thickened portion only needs to be harder than the other parts, and may contain retained austenite, bainite, tempered martensite, ferrite, pearlite, etc.

[0051] Furthermore, in the automotive frame members according to this embodiment, members subjected to axial compressive loads are assumed, and in order to suppress material fracture, it is preferable to have high ultimate deformability. It is preferable that the number of minute voids that serve as fracture initiation points in the steel structure is small, as this increases the ultimate deformability. Also, in the automotive frame members according to this embodiment, it is preferable that the steel structure of the bead does not contain a large amount of ferrite or pearlite, as this suppresses the concentration of deformation in the bead and increases the ultimate deformability.

[0052] Furthermore, the chemical composition of the steel in the automotive frame member according to this embodiment is not particularly limited. For example, the steel composition of the automotive frame member according to this embodiment may be, in mass%, C: 0.05~0.70%, Si: 0.100~3.000%, Mn: 0.100~3.000%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.1000% or less, Al: 3.0000% or less, B: 0.0005~0.0200%, Nb: 0~0.100%, Ti: 0~0.200%, Cr: 0~1.00%, Mo: 0~1.00%, Co: 0~5.00%, Ni: 0~3.00%, Cu: 0~3.00%, V: 0-3.00%, W: 0~3.00%, Ca: 0~1.0000%, Mg: 0~1.0000%, REM: 0~1.0000%, Sb: 0~1.00%, Zr: 0~1.00%, Sn: 0~1.00%, As: 0~1.0000%, It should contain [the specified ingredient], with the remainder being Fe and impurities.

[0053] The above steel composition preferably has a carbon content of 0.06% or more by mass, more preferably 0.10% or more, and even more preferably 0.14% or more. Furthermore, the carbon content is preferably 0.50% or less, more preferably 0.25% or less, and even more preferably 0.16% or less.

[0054] Next, a method for measuring the characteristics of the automotive frame member according to this embodiment will be described.

[0055] The VDA bending angle described above should be measured in accordance with German Association of the Automotive Industry (Verband der Automobilindustrie, VDA) standard 238-100. The test specimen should be cut out so that its bent ridge falls within the area of ​​the thickened section. For example, when viewed in a cross-section normalized to the longitudinal direction of the automotive frame member, the test specimen should be taken from the thickened section on the top plate side, with the center of the punch shoulder as the reference point. The shape of the test specimen should conform to VDA standard 238-100. If the thickened section is small, a small test specimen may be taken from within the area of ​​the thickened section and the bending angle measured. For example, a test specimen with a length of 10 mm in the direction of the bent ridge and a length of 60 mm in the direction perpendicular to the direction of the bent ridge may be used. Alternatively, if the material of the thickened section is deemed to be the same as the material of the top plate, a test specimen may be taken from the top plate and the bending angle measured, and the measurement result may be adopted as the bending angle of the thickened section. In this case, the bending angle of the thickened portion can be converted based on the method disclosed in L. Durrenberger and P. Dietsch 2018 IOP Conf.Ser.:Mater.Sci.Eng.418 012076. Alternatively, a test specimen can be prepared using a flat plate of a material equivalent to the thickened portion (a flat plate with equivalent chemical composition, microstructure, mechanical properties, plate thickness, etc.), the bending angle can be measured, and the measurement result can be adopted as the bending angle of the thickened portion. Furthermore, although the bending ridge direction of the test specimen is not particularly specified, it may be taken so as to be parallel to the longitudinal direction of the member, for example.

[0056] The Vickers hardness mentioned above should be measured in accordance with JIS Z2244-1:2024. The measurement load can be set appropriately to produce the largest possible indentation for more accurate measurement. For example, the measurement load should be 9.8 × 10⁻⁶. -5 ~9.8×10 0 It should be set to N. For measuring Vickers hardness, an electron microscope such as a SEM may be used to obtain high accuracy.

[0057] Vickers hardness is measured along the longitudinal direction of the automotive frame member. For example, the longitudinal direction of the automotive frame member can be the long axis of the automotive frame member in the field of view where the automotive frame member appears longest when viewed from various directions. The position where the Vickers hardness is measured should be the center in the transverse direction (direction perpendicular to the longitudinal direction). For example, the Vickers hardness is measured along the longitudinal direction of the automotive frame member, but the measurement position should be the center of the top plate portion as described above.

[0058] Note that automotive frame members may have holes as part of their shape. When measuring Vickers hardness at regular intervals along the longitudinal direction of an automotive frame member (line analysis), it is preferable to set the measurement line for line analysis so as to avoid holes as much as possible. If the location to be measured when performing line analysis of Vickers hardness along the longitudinal direction is a hole, the Vickers hardness should be measured on a flat area at least 2 mm away from the hole in a direction perpendicular to the measurement line. Also, if the location to be measured when performing line analysis of Vickers hardness along the longitudinal direction is a processed area (bent area), the Vickers hardness should be measured on a flat area at least 2 mm away from the processed area in a direction perpendicular to the measurement line. Vickers hardness can be measured in sets of 100 or more measurement points at equal intervals along the longitudinal direction from any position on the automotive frame member, following the procedure described above.

[0059] The tensile strength mentioned above should be measured in accordance with JIS Z2241:2022 (AMENDMENT 1:2023). Test specimens should be taken from JIS No. 13B, with the longitudinal direction of the automobile frame member oriented in the tensile direction. The tensile test should be conducted at room temperature (25°C) with a strain rate of 0.001 s². -1 Under these conditions, the strain rate control method should be used.

[0060] In measuring tensile strength, if only one JIS 13B test specimen can be taken from a single automotive frame member, the measurement result of that specimen may be used as the average value of the tensile strength. However, if multiple JIS 13B test specimens can be taken from a single automotive frame member, the measurement results of those specimens may be used as the average value of the tensile strength. Furthermore, the tensile test specimen should be taken from the center in the lateral direction (the direction perpendicular to the longitudinal direction of the automotive frame member). For example, the tensile test specimen should be taken from the center of the top plate section.

[0061] As mentioned above, automotive frame members may have holes as part of their shape. Tensile test specimens should be taken while avoiding holes. If there is a hole at the location where a tensile test specimen is to be taken, the specimen may be taken from a flat area at least 2 mm away from the hole in the lateral direction as described above. Also, if there is a processed part (bent formed part) at the location where a tensile test specimen is to be taken, the specimen may be taken from a flat area at least 2 mm away from the processed part in the lateral direction as described above. If it is difficult to take a JIS 13B test specimen under the above conditions, as long as the longitudinal direction of the automotive frame member is the tensile direction, a test specimen smaller than JIS 13B may be taken and its tensile strength measured.

[0062] Furthermore, the structural framework and voids of the steel described above can be observed by the following method. The observation surface is a cross-section perpendicular to the longitudinal direction, this observation surface is polished and immersed in an acetylacetone-based electrolyte to perform electrolytic etching, and then scanned at a magnification of 5000x (for example, at least 625 μm) using a field emission scanning electron microscope equipped with a secondary electron detector. 2 By observing the secondary electron image within the field of view, one can distinguish between ferrite and pearlite and structures other than ferrite and pearlite. Structures with massive crystal grains that do not contain understructures such as lath within the structure are considered ferrite. Structures in which plate-like ferrite and Fe-based carbides are layered are considered pearlite.

[0063] Furthermore, at the same observation site as described above, a magnification of 10,000 times (for example, at least 150 μm) was used. 2To distinguish between tempered martensite and bainite, observe the secondary electron image within the field of view.Tempered martensite is considered to be an aggregate of lath-like crystal grains containing Fe-based carbides with a major axis of 20 nm or more and elongated in different directions within the structure.Bainite is considered to be an aggregate of lath-like crystal grains that does not contain Fe-based carbides with a major axis of 20 nm or more within the structure, but in which Fe-based carbides are precipitated between the laths, and in which Fe-based carbides are precipitated inside the laths and elongated in the same direction.

[0064] Alternatively, at the same observation site as described above, the observation surface can be repolished to a mirror finish, the strain removed, and crystal orientation information obtained by electron backscatter diffraction at measurement intervals of 0.4 μm. Regions with an fcc crystal structure are considered retained austenite.

[0065] Furthermore, regions other than ferrite, pearlite, tempered martensite, bainite, and retained austenite obtained by the above method are considered to be martensite. Also, voids contained in the observation surface can be identified as voids by those skilled in the art based on their shape and contrast.

[0066] Furthermore, the chemical composition of the steel described above can be measured using general analytical methods for steel. For example, the chemical composition can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-nondispersive infrared absorption method.

[0067] Next, a method for manufacturing an automobile frame member according to this embodiment will be described.

[0068] The method for manufacturing the automotive frame member according to this embodiment is not particularly limited. For example, the automotive frame member according to this embodiment may be manufactured by the manufacturing method described below. The manufacturing method described below is one example for manufacturing the automotive frame member according to this embodiment, and is a preferred example of the manufacturing method for the automotive frame member according to this embodiment.

[0069] For example, the automotive frame member according to this embodiment may be manufactured by either method (I) or (II) described below. (I) Hot stamping molding process and shaping after the hot stamping molding process When manufacturing the automotive frame member according to this embodiment by hot stamping, a bead can be formed in the hot stamping process, and a thickened portion can be added after the hot stamping process.

[0070] For example, as a mold for hot stamping, a mold with a shape having a recessed portion extending from the flange side to the top plate side on the vertical wall can be used. By changing the shape of the mold, the number, position, and shape of the beads can be formed as desired. In addition, after the hot stamping process, a thickened portion can be added to the punch shoulder portion, which is the connecting part between the vertical wall portion and the top plate portion, by attaching a reinforcing member by laser welding, spot welding, or brazing. By changing the number, position, and shape of the reinforcing member, the number, position, and shape of the thickened portion can be added as desired.

[0071] Furthermore, the overall characteristics of the thickened section may be controlled by using reinforcing members made of a different material than the hot-stamped molded product. For example, a reinforcing member with excellent bendability may be attached to a high-strength base material to control the overall VDA bending angle of the thickened section to 90 degrees or more.

[0072] (II) Shaping during the hot stamping process When manufacturing the automotive frame member according to this embodiment by hot stamping, the bead and thickened portion can be formed in the hot stamping process.

[0073] For example, as a mold for hot stamping, one can use a mold with a shape in which the vertical wall portion has an uneven surface that extends from the flange portion to the top plate portion, and in which the thickness of a part of the punch shoulder portion in the longitudinal direction, which is the connecting part between the vertical wall portion and the top plate portion, is increased. By changing the shape of the mold, the number, position, and shape of the beads can be formed as desired, and the number, position, and shape of the thickened portion can also be formed as desired.

[0074] Furthermore, when the bead and the thickened portion are integrally molded by hot stamping, the characteristics of the thickened portion can be controlled by any of the following methods (A) to (C). For example, the VDA bending angle of the thickened portion can be controlled to be 90 degrees or more by any of the following methods (A) to (C).

[0075] (A) Heat control in the hot stamping process When manufacturing the automotive frame member according to this embodiment by hot stamping molding, heating control can be performed during the hot stamping molding process.

[0076] For example, when heating the material to be molded before hot stamping, the area to be thickened can be covered with an insulating material, while the areas other than the thickened area can be painted black with heat-resistant paint. This method allows the heating temperature of the area covered with insulating material to be controlled to below or near Ac3, while the heating temperature of the area painted black with heat-resistant paint can be controlled to be above Ac3. Note that Ac3 is the temperature at which ferrite completes its transformation to austenite during heating. The material to be molded after the above heating control is then hot stamped.

[0077] In the automotive frame members subjected to the above heating control, martensite is mainly present in areas other than the thickened portion, while martensite, retained austenite, ferrite, etc., are present in the thickened portion.

[0078] (B) Cooling control in the hot stamping process When manufacturing the automotive frame members according to this embodiment by hot stamping molding, cooling control can be performed during the hot stamping molding process.

[0079] For example, when cooling the molded material after hot stamping, a material with low thermal conductivity can be used for the area of ​​the mold that you want to control to become thicker, while a material with high thermal conductivity can be used for the areas of the mold other than the thickened area. By doing this, the cooling rate of the area of ​​the mold using the low thermal conductivity material can be controlled to be less than Vc90 or close to Vc90, while the cooling rate of the area of ​​the mold using the high thermal conductivity material can be controlled to be Vc90 or higher. Vc90 refers to the critical cooling rate (°C / second) required to obtain a martensitic structure with a volume fraction of 90% or more. The molded material after hot stamping can be cooled by performing the above cooling control. Alternatively, the cooling rate can be controlled as described above by adding an uneven surface to the mold. For example, by adding small uneven surfaces to the surface of the mold that do not affect the shape of the molded material, the contact area for heat exchange between the mold and the molded material is reduced, and the cooling rate of the molded material can be controlled.

[0080] In the automotive frame members subjected to the above cooling control, martensite is mainly present in areas other than the thickened sections, while the thickened sections contain martensite, bainite, retained austenite, ferrite, pearlite, and other materials.

[0081] (C) Tempering control after the hot stamping process When manufacturing the automotive frame members according to this embodiment by hot stamping molding, tempering control should be performed after the hot stamping molding process.

[0082] For example, when tempering the molded material after the hot stamping process, the area that you want to control to become thicker can be locally tempered by controlling the laser irradiation output, while the areas other than the thickened part can not be tempered.

[0083] In automotive frame members subjected to the tempering control described above, martensite is mainly present in areas other than the thickened portion, while tempered martensite and other materials are present in the thickened portion. [Examples]

[0084] The effects of one aspect of the present invention will be further explained in detail by the examples, but the conditions in the examples are just one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention. The present invention will be specifically explained below with reference to examples and comparative examples.

[0085] The deformation state of an automotive frame member having a hat shape, comprising a top plate portion located in the center of the width direction when viewed in a cross-section normalized to the longitudinal direction, a pair of vertical wall portions located at both ends of the top plate portion, and flange portions located at the ends of the vertical wall portions, and having the chemical composition and characteristics shown in Tables 1 to 3, was analyzed by FEM (Finite Element Method) when subjected to an axial compressive load.

[0086] The automotive frame member used in the analysis, although not shown in the table, has a longitudinal length of 225 mm. When viewed in a cross-section with the longitudinal direction as the normal, the length of the top plate is 90 mm, the length of the vertical wall is 111 mm, and the length of the flange is 30 mm. Also, when viewed in the above cross-section, the angle between the top plate and the vertical wall is 95 degrees. Note that the lengths of the top plate, vertical wall, and flange mentioned above are the lengths of the straight sections when viewed in the above cross-section, and do not include the curved sections (R sections). When viewed in the above cross-section, the curved section (R section) between the top plate and the vertical wall corresponds to the punch shoulder (the connecting portion between the vertical wall and the top plate).

[0087] Furthermore, in the case of the automotive frame members used in the analysis, if the vertical wall portion had a bead, the bead was positioned opposite each other between a pair of vertical wall portions and had the same width and depth. The bead extended from the flange portion side to the top plate portion side in a direction perpendicular to the longitudinal direction of the automotive frame member, and when viewed in a cross-section normalized to the longitudinal direction of the bead, it had a U-shape. In addition, if there was a thickened portion, the thickened portion was positioned opposite each other between a pair of vertical wall portions and had the same length and thickness. In addition, if there was a thickened portion, the thickened portion was 15 mm on the vertical wall side and 15 mm on the top plate side, with the center of the punch shoulder portion (the center of the punch shoulder portion when viewed in a cross-section perpendicular to the longitudinal direction of the automotive frame member) as the reference point.

[0088] The axial compression load was applied under the condition that one end of the automotive frame member in the longitudinal direction was completely restrained, and the load was applied from the other end. Specifically, the axial compression load was applied by translating the member from the longitudinal direction at a constant speed of 5000 mm / second.

[0089] The solver for the FEM analysis was not particularly restricted, but LS-DYNA from Livermore Software Technology Corporation was used. In the FEM analysis, elements were set to 2 mm square shells, and in the R-shaped sections of the members, the elements were divided into three parts. For parts of the automotive frame members other than the thickened sections, the work hardening characteristics were set to the stress-strain characteristics obtained from tensile tests of steel sheets (martensite ratio of 99% or more) with the tensile strength shown in the table. For the thickened sections of the automotive frame members, the work hardening characteristics were set to the stress-strain characteristics obtained from tensile tests of steel sheets with the VDA bending angle shown in the table. In the FEM analysis, the yield function was set to the von Mises isotropic hardening function.

[0090] Furthermore, while there were no particular restrictions on the material fracture prediction software, we used NSafe-MAT from Nippon Steel Corporation. In the analysis, when it was determined that the material had fractured, the element at the fractured location was removed to simulate the fracture state of the material.

[0091] The above FEM analysis was performed to confirm the load-displacement curve of the member subjected to axial compressive load. Using this load-displacement curve, the absorbed energy E (in J) was calculated. Specifically, the absorbed energy was calculated using the formula E = ∫FdS, where F (in kN) is the load and S (in mm) is the displacement. In this case, the displacement S was set to a range of 0 to 80 mm. This absorbed energy E corresponds to the plastic work done. If this absorbed energy E is less than 5000 J, it can be considered unsuitable as an impact absorbing member.

[0092] Using the absorbed energy E described above, an index A was calculated, and the impact absorption characteristics of the automotive frame member were evaluated using this index A. The index A is given by A = (ΔE / E0) / (ΔM / M0). Here, E0 is the absorbed energy E of the automotive frame member without a thickened section, ΔE is the value obtained by subtracting E0 from the absorbed energy E calculated by performing FEM analysis, M0 is the mass (in kg) of the automotive frame member without a thickened section, and ΔM is the value obtained by subtracting M0 from the mass of the automotive frame member with a thickened section. When the value of this index A is greater than 0, it can be determined that the impact absorption characteristics are excellent while suppressing the increase in the mass of the member. Thus, in the automotive frame member according to the above embodiment, it is preferable that the index A, represented by A = (ΔE / E0) / (ΔM / M0), is greater than 0. Since a larger value of this index A is preferable, there is no particular upper limit to the index A, but for example, the upper limit may be considered to be 20.

[0093] Tables 1-5 show the analysis conditions and results. In the tables, the "width" (length in the longitudinal direction of the member) and "depth" of the "vertical wall bead" indicate the values ​​for one bead, and similarly, the "length" (length in the longitudinal direction of the member) and "thickness" of the "thickened section" indicate the values ​​for one thickened section. Furthermore, regarding the "location" of the "thickened section" shown in the table, "bead" indicates that the thickened section exists including the connecting portion between the bead and the top plate, while "other than bead" indicates that the thickened section exists without including the connecting portion between the bead and the top plate. In addition, if the "length" (length in the longitudinal direction of the member) of the "thickened section" is smaller than the "width" (length in the longitudinal direction of the member) of the "vertical wall bead" shown in the table, it indicates that the thickened section exists only in the connecting portion between the bead and the top plate. Furthermore, in the table, "spot welding," "laser welding," and "brazing" under "method of application" for "thickened section" indicate that the thickened section was formed by attaching a reinforcing member having the same material as the base material to the base material. Also, in the table, "thickening" under "method of application" for "thickened section" indicates that the thickened section was formed by thickening during press molding.

[0094] The chemical composition of the steel plate, the structural composition of the steel, the VDA bending angle of the thickened section, the Vickers hardness, and the tensile strength can be determined based on the method described above.

[0095] As can be seen from Tables 1 to 5, among Examples No. 1 to 38, the present invention example satisfies the plastic work requirement (absorbed energy E) as an impact absorbing member, and furthermore, exhibits excellent impact absorption characteristics while suppressing the increase in mass of the member. In contrast, among Examples No. 1 to 38, the comparative example either did not satisfy the plastic work requirement (absorbed energy E) as an impact absorbing member, or could not be said to exhibit excellent impact absorption characteristics while suppressing the increase in mass of the member.

[0096] [Table 1-1]

[0097] [Table 1-2]

[0098] [Table 2]

[0099] [Table 3]

[0100] [Table 4]

[0101] [Table 5] [Industrial applicability]

[0102] According to the above-described embodiment of the present invention, it is possible to provide an automotive frame member that exhibits excellent shock absorption characteristics against impacts from the longitudinal direction of the member. In particular, according to the above-described embodiment of the present invention, it is possible to provide an automotive frame member in which shock absorption characteristics are preferably enhanced while minimizing the increase in mass of the automotive frame member. Therefore, it has high potential for industrial application. [Explanation of symbols]

[0103] 1. Automotive frame components 11 Top panel 12 Vertical wall section 12a Bead 13 Flange section 14 Thickened section 15. Punch shoulder section (connecting section between the vertical wall section and the top panel section)

Claims

1. In a hat-shaped member, when viewed in a cross-section with the longitudinal direction as the normal, the member comprises a top plate portion located in the center in the width direction, a pair of vertical wall portions located at both ends of the top plate portion, and flange portions located at the ends of the vertical wall portions, When the Vickers hardness of the member is measured at 100 or more measurement points at equal intervals along the longitudinal direction, the average value of the Vickers hardness is 300 Hv or more and 800 Hv or less. The vertical wall portion has at least one bead extending from the flange portion side to the top plate portion side in a direction perpendicular to the longitudinal direction, In at least a portion of the longitudinal direction, the connecting portion between the vertical wall portion and the top plate portion is a thickened portion that is thicker than the vertical wall portion and the top plate portion. The thickened portion, in the connecting portion, includes the connecting portion in the bead. Automotive frame member characterized by the following features.

2. The automobile frame member according to claim 1, characterized in that the thickened portion is present only in the connecting portion of the bead in the connecting portion.

3. The automobile frame member according to claim 1 or 2, characterized in that the thickened portion is provided by one of the following: laser welding, spot welding, brazing, or thickening.

4. The automobile frame member according to claim 1 or 2, characterized in that the thickened portion has a VDA bending angle of 90 degrees or more as defined in VDA-238-100.

5. The automobile frame member according to claim 3, characterized in that the thickened portion has a VDA bending angle of 90 degrees or more as defined in VDA-238-100.

6. The automobile frame member according to claim 1, characterized in that when the tensile strength is measured using a test piece taken such that the longitudinal direction of the member is the tensile direction, the average value of the tensile strength is 950 MPa or more and 2512 MPa or less.

7. The thickened portion in the earlier stage is present only in the connecting portion of the bead, The aforementioned thickened portion is provided by increasing the thickness. The automotive frame member according to feature 1.

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