skeleton member

Abstract

The skeleton member is formed by spot welding a first steel plate member and a second steel plate member at a spot weld joint. The skeleton member has a cross-sectional region with a closed cross-section perpendicular to its length direction. The first steel plate member has a tensile strength of 1900 MPa or more. The spot weld joint has a molten metal portion formed by the spot weld and a heat-affected zone adjacent to the outer side of the molten metal portion. The average Vickers hardness Hv at the measurement location corresponding to the first region on the imaginary straight line is... Ave The lowest Vickers hardness Hv at the measurement location corresponding to the third region on the hypothetical straight line described above. Min Satisfy Hv Ave -Hv Min ≤100.

Description

Technical Field

[0001] This invention relates to a skeleton component that can exert excellent energy absorption performance commensurate with its high strength by suppressing fractures originating from spot welds during impact.

[0002] This application claims priority based on Japanese Patent Application No. 2021-072691 filed on April 22, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] In the automotive industry, the development of body structures that can reduce the impact of collisions is underway. For the skeletal components that form the body structure, it is required to absorb collision energy; for example, the following structure is used: after multiple steel plates are formed into a specified shape through pressing or other methods, a closed section is created by spot welding.

[0004] In such a structure, it is important to ensure the following strength: the spot welds will not easily break even if the component deforms due to impact input, and the closed section of the component can be maintained.

[0005] Generally speaking, increasing the strength of steel plates can improve the energy absorption per unit mass of the frame components during a collision. Therefore, increasing the strength of steel plates is often used as a means of reducing the weight of automobile bodies.

[0006] On the other hand, it is known that the strength of spot welds decreases due to the increase in the strength of steel plates. Therefore, when using high-strength steel plates to achieve a closed cross-section through spot welding, it is necessary to ensure that the spot welds do not break when impact input is applied to the component. This is because if fracture occurs originating from the spot welds, the closed cross-section cannot be maintained, and energy absorption performance commensurate with the increase in strength cannot be obtained.

[0007] Based on this practical situation, a framework component was proposed with the aim of achieving energy absorption performance commensurate with high strength.

[0008] For example, Patent Document 1 discloses an automobile frame component comprising a first steel plate, a second steel plate, and a first welded metal portion, wherein the minimum Vickers hardness of the area within 4 mm around the first welded metal portion of the second steel plate is more than 80% of the hardness of the outer side of the area of ​​the second steel plate.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: International Publication No. 2020 / 090916 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] According to the technology in Patent Document 1, it is believed that it can simultaneously improve the overall strength of the component, including the welded parts, and improve its impact absorption characteristics.

[0014] However, the problem that Patent Document 1 aims to solve is about suppressing the decrease in hardness caused by HAZ softening. However, the reason for the decrease in joint strength when the steel plate is made stronger is not limited to the decrease in hardness caused by HAZ softening. Even if the technology of Patent Document 1 is adopted, depending on the steel used, the desired energy absorption performance may not be achieved due to the fracture originating from the spot weld. There is room for further improvement in energy absorption performance.

[0015] The present invention was made in view of the above-mentioned problems, and the object of the present invention is to provide a skeleton member that can exert excellent energy absorption performance commensurate with high strength by suppressing the breakage at the spot weld during a collision.

[0016] Methods for solving problems

[0017] To address the aforementioned issues, the present invention employs the following configuration.

[0018] (1) The first aspect of the present invention is a skeleton member, which is a skeleton member joined by spot welding a first steel plate member and a second steel plate member at a spot weld portion. The skeleton member has a cross-sectional area with a closed cross-section perpendicular to the length direction of the skeleton member. The first steel plate member has a tensile strength of 1900 MPa or more. The spot weld portion has a molten metal portion formed by the spot welding and a heat-affected portion adjacent to the outside of the molten metal portion. In the cross-section perpendicular to the length direction, including the center point of the molten metal portion, the area corresponding to the molten metal portion is defined as a first region, the area corresponding to the heat-affected portion is defined as a second region, and the area formed by the region from the boundary between the first region and the second region to a distance of 100 μm towards the first region and the region from the boundary to a distance of 100 μm towards the second region is defined as a third region. When the Vickers hardness is measured at a distance of 15 μm with a load of 10 gf along an imaginary straight line extending from the center of the first region towards the second region, the average Vickers hardness Hv at the measurement location corresponding to the first region on the imaginary straight line is... Ave The lowest Vickers hardness Hv at the measurement location corresponding to the third region on the hypothetical straight line described above. Min Satisfy Hv Ave -Hv Min ≤100.

[0019] (2) According to the skeleton member described in (1) above, it may also have the following cross-sectional area: the ratio h1 / w of the height h1 of the first steel plate member in the thickness direction along the portion of the first steel plate member where the spot weld is formed in the cross-section perpendicular to the length direction of the skeleton member and the width w of the skeleton member in the direction perpendicular to the thickness direction of the portion of the first steel plate member where the spot weld is formed is 0.6 or less.

[0020] (3) According to the skeleton member described in (2) above, the cross-sectional area with a ratio h1 / w of 0.6 or less may also exist in more than 50% of the total length of the skeleton member in the above-mentioned length direction.

[0021] Invention Effects

[0022] According to the above scheme, by properly distributing the hardness near the boundary between the molten metal and the HAZ section, it is possible to suppress the fracture at the spot weld during impact, thereby enabling excellent energy absorption performance commensurate with the high strength. Attached Figure Description

[0023] Figure 1 This is a perspective view of a skeleton component representing one embodiment of the present invention.

[0024] Figure 2A This is a schematic cross-sectional view showing the vicinity of the spot weld portion of the skeleton component in this embodiment.

[0025] Figure 2B It means along Figure 2A The curve of hardness distribution of the imaginary straight line a.

[0026] Figure 3A This is a schematic cross-sectional view of the area near the spot weld of a skeleton component that uses steel plate components with a Mn content of 1.27% by mass.

[0027] Figure 3B It means along Figure 3A The curve of hardness distribution of the imaginary straight line a.

[0028] Figure 4 This is a schematic diagram illustrating the cross-sectional shape of the component used in the embodiment.

[0029] Figure 5 This is a schematic diagram used to illustrate the three-point bending test conditions of the experimental example.

[0030] Figure 6A This is a schematic diagram showing the state of spot fracture caused by a 3-point bending test, indicating that spot fracture occurred at 5 locations on one side.

[0031] Figure 6B This is a schematic diagram showing the state of solder joint fracture caused by a 3-point bending test, indicating the state of solder joint fracture occurring at one location on one side. Detailed Implementation

[0032] The inventors of this invention have conducted in-depth research on the construction of a skeletal component that can achieve excellent energy absorption performance commensurate with high strength.

[0033] In analyzing weld fractures that occur when hot-stamped materials with a strength exceeding 2.0 GPa are used as skeleton components, the inventors of this invention focused on the fact that even with the same strength, the frequency of weld fractures differs depending on the Mn content.

[0034] Then, the inventors of this invention conducted a detailed investigation on the hardness distribution near the spot weld of a skeleton component obtained by overlapping and spot welding two hot-stamped materials with a hardness exceeding 2.0 GPa. The results showed that in skeleton components prone to weld breakage, there was a tendency for the following: unlike the HAZ softened area, near the boundary between the molten metal and the HAZ, there were areas where the hardness was more than 100 Hv lower than the average hardness of the molten metal.

[0035] Furthermore, considering that this tendency is significant when using steel sheet materials with high Mn content, further research was conducted. As a result, the inventors of this invention discovered that the presence of an Mn-deficient layer generated near the boundary between the molten metal and the HAZ section is the cause of the aforementioned tendency.

[0036] Based on the above findings, the inventors of this invention discovered that by appropriately distributing the hardness near the boundary between the molten metal and the HAZ section, weld point fracture can be suppressed even in skeleton components using hot-stamped materials exceeding 2.0 GPa, thereby achieving excellent energy absorption performance commensurate with high strength, thus completing this invention.

[0037] Hereinafter, a skeleton member 1 according to one embodiment of the present invention will be described.

[0038] It should be noted that in this specification and accompanying drawings, constituent elements that have essentially the same function are labeled with the same symbol to omit repeated descriptions.

[0039] First, let's explain the statements in this instruction manual.

[0040] "Length direction Z" refers to the axis direction of the frame member, that is, the direction in which the axis extends. "Width direction X" is the direction in which the joint surface of the two spot-welded steel plate members extends in a direction perpendicular to the length direction Z. "Height direction Y" is the direction perpendicular to both the length direction Z and the width direction X.

[0041] The "molten metal section" refers to the area where overlapping steel plate components melt and become a single unit through spot welding heat. The molten metal section is sometimes also called the weld nugget.

[0042] The "heat-affected zone" (HAZ) refers to the area formed adjacent to the outer side of the molten metal portion. It is a region with a microstructure different from the base metal portion due to the heat of spot welding. The HAZ is sometimes also called the Heat Affected Zone.

[0043] It should be noted that, typically, in the outer periphery of the heat-affected zone, there exists a HAZ softened zone that is more softened than the molten metal and the base metal due to the heat of spot welding.

[0044] Figure 1 This is a perspective view of the skeleton member 1. The skeleton member 1 is a hollow cylindrical strip extending along the length direction Z. The skeleton member 1 is constructed in such a way that the first steel plate member 10 and the second steel plate member 20 are joined by a plurality of spot welds 50.

[0045] The first steel plate component 10 is a component obtained by pressing a steel plate into a hat-shaped cross-section. The thickness of the first steel plate component 10 (i.e., the thickness of the steel plate before pressing) can be 0.4 mm to 4.2 mm.

[0046] like Figure 1 As shown, the first steel plate member 10 has: a top plate 11; a pair of sidewalls 13, 13 that bend and extend from the end edge of the top plate 11 in the width direction X; and a pair of flanges 15, 15 that bend from the end edge of one of the pair of sidewalls 13, 13 opposite to the top plate 11 and extend outward in the width direction X.

[0047] The first steel plate member 10 has a tensile strength of 1900 MPa or more. Because the first steel plate member 10 has a tensile strength of 1900 MPa or more, it can exhibit excellent energy absorption performance.

[0048] However, when the frame member 1 fractures at the spot weld 50 and the closed section breaks during impact deformation, the energy absorption capacity provided by the tensile strength of the first steel plate member 10 (over 1900 MPa) cannot be fully utilized. Therefore, in this application, as described below, the focus is on suppressing weld fracture while using a high-strength member by appropriately distributing the hardness near the spot weld 50.

[0049] The first steel plate component 10 can be manufactured by a construction method (hot stamping construction method) in which the steel plate is heated to above the austenitic phase transformation temperature and quenched while being formed by a water-cooled mold.

[0050] The second steel plate component 20 is a flat steel plate. The thickness of the second steel plate component 20 can be 0.4mm to 4.2mm.

[0051] The tensile strength of the second steel plate member 20 is not particularly limited, but it is preferred to have a tensile strength of 1900 MPa or above, which is the same as that of the first steel plate member 10, so that it can perform better energy absorption performance.

[0052] The spot weld 50 is formed by spot welding the second steel plate member 20 onto a pair of flanges 15, 15 of the first steel plate member 10.

[0053] The spot welded parts 50 are formed in multiples along the length direction Z of the skeleton member 1 at intervals of about 15mm to 50mm.

[0054] There are no particular limitations on the conditions for spot welding. For example, the following heat input conditions are acceptable: the diameter of the weld nugget (i.e., the diameter of the molten metal portion) is approximately 6√t (t is the thinner of the thickness of the first steel plate member 10 and the thickness of the second steel plate member 20).

[0055] Figure 2A It is along Figure 1 A schematic diagram of the cross-section of line A1-A1. In other words, Figure 2A This is a schematic diagram showing a cross section perpendicular to the length direction Z, including the center point P of the spot weld section 50.

[0056] Such as Figure 2A As shown, the spot welding portion 50 is composed of a molten metal portion 51 and a heat-affected portion 53 formed adjacent to the outside of the molten metal portion 51.

[0057] Here, in a cross section perpendicular to the length direction Z, including the center point P of the spot weld portion 50, the region corresponding to the molten metal portion 51 is defined as the first region α, and the region corresponding to the heat-affected portion 53 is defined as the second region β.

[0058] Furthermore, the region consisting of the region from the boundary between the first region α and the second region β (i.e., the melt boundary) to a distance of 100 μm towards the first region α and a distance of 100 μm towards the second region β is defined as the third region γ.

[0059] It should be noted that the third region γ overlaps with a portion of the first region α and a portion of the second region β.

[0060] Figure 2B This is a graph showing the hardness distribution of the spot weld 50. In this graph, the horizontal axis corresponds to... Figure 2B The position of the imaginary straight line a is indicated by a double-dotted line, and the vertical axis corresponds to the Vickers hardness measured along the imaginary straight line a.

[0061] An imaginary line a extends from the center of the first region α towards the second region β. More specifically, the imaginary line a extends from the joint surface between the first steel plate member 10 and the second steel plate member 20. Figure 2A The single-dot dashed line in the figure extends 200 μm apart from the first steel plate member 10 and is parallel to the joint surface.

[0062] Points a2 are the points on the imaginary straight line a that intersect with the boundaries of the first region α and the second region β, i.e., the melting boundary.

[0063] Compared to a pair of a2 points, a pair of a1 points existing on the inside are the points in the imaginary line a that intersect the inner edge of the third region γ.

[0064] Compared to the pair of a2 points, the pair of a3 points existing on the outside are the points in the imaginary straight line a that intersect with the outer edge of the third region γ.

[0065] Compared to the pair of a3 points, the pair of a4 points, which exist further out, are the points in the imaginary line a that intersect with the outer edge of the second region β.

[0066] Therefore, in the imaginary line a, the line segment connecting a pair of points a2 corresponds to the first region α, the two line segments connecting points a2 and a4 correspond to the second region β, and the two line segments connecting points a1 and a3 correspond to the third region γ.

[0067] like Figure 2B As shown, in the skeleton member 1 of this embodiment, although the hardness is reduced due to the presence of HAZ softening portion in the outer region (between points a3 and a4) in the second region β, the hardness is not reduced in the third region γ.

[0068] Therefore, the average (arithmetic mean) Vickers hardness Hv in the first region α Ave The lowest Vickers hardness Hv in the third region γ Min Satisfying Hv Ave -Hv Min ≤100.

[0069] In this case, weld point fracture during the deformation process of the skeleton member 1 caused by localized hardness reduction in the third region γ can be suppressed, maintaining a closed cross-section. Thus, the skeleton member 1 can exhibit excellent energy absorption performance commensurate with its high strength.

[0070] here, Figure 3AThis is a schematic diagram showing a cross-section perpendicular to the length direction Z at the center point P of the spot weld portion 150 for the skeleton member 101. The skeleton member 101 uses a first steel plate member 110 with a Mn content of 1.27% by mass and a tensile strength of 1900 MPa or more, and a second steel plate member 120 with a Mn content of 1.27% by mass and a tensile strength of 1900 MPa or more, instead of the first steel plate member 10 and the second steel plate member 20 of the skeleton member 1 in this embodiment. For example... Figure 3A As shown, the spot welding section 150 is configured with a molten metal section 151 and a heat-affected section 153.

[0071] also, Figure 3B This is a graph showing the hardness distribution of the spot weld 150. This graph also... Figure 2B Similarly, the horizontal axis corresponds to Figure 3A The position of the imaginary straight line a, represented by a double-dotted line, is shown in the figure. The vertical axis corresponds to the Vickers hardness measured along the imaginary straight line a.

[0072] like Figure 3B As shown, in the skeleton member 101, unlike the HAZ softened portion present in the outer region (between points a3 and a4) of the second region β, there is a region in the third region γ where the hardness decreases sharply.

[0073] Therefore, the average Vickers hardness Hv in the first region α Ave The lowest Vickers hardness Hv in the third region γ Min Become Hv Ave -Hv Min >100.

[0074] According to the findings of the inventors of this invention, it is presumed that this phenomenon is caused by a Mn-deficient layer produced when spot welding high-strength steel plates with high Mn content.

[0075] In this skeleton member 101, since there is a softened part in the third region γ, the closed section may not be maintained due to the weld point fracture during deformation, and thus the excellent energy absorption performance commensurate with the high strength cannot be achieved.

[0076] On the other hand, according to the skeleton member 1 of this embodiment, the average Vickers hardness Hv at the measurement location corresponding to the first region α in the imaginary straight line a is calculated. Ave The lowest Vickers hardness Hv at the measurement site corresponding to the third region γ Min Satisfy Hv Ave -Hv MinWith a hardness ≤100, weld point fracture during deformation caused by localized hardness reduction can be suppressed, maintaining a closed cross-section. Therefore, the skeleton member 1 can exhibit excellent energy absorption performance commensurate with its high strength.

[0077] It should be noted that, in order to more reliably prevent weld point breakage during deformation, Hv is preferred. Ave -Hv Min ≤50, further preferably Hv Ave -Hv Min ≤30.

[0078] As a means to obtain Hv Ave -Hv Min For example, a strategy to achieve a hardness distribution of ≤100 can be considered, such as using a steel plate with an Mn content of 1.0% by mass or less, preferably 0.50% by mass or less, as the material for the first steel plate member 10. In this way, by reducing the Mn content, the occurrence of Mn segregation in the third region γ can be suppressed, thus preventing the formation of localized softening regions in the third region γ. Furthermore, by adjusting the amount of alloying elements other than Mn, the formation of localized softening regions in the third region γ can also be prevented.

[0079] (Determination Method)

[0080] The average Vickers hardness Hv at the measurement location corresponding to the first region α in the imaginary straight line a Ave The lowest Vickers hardness Hv at the measurement site corresponding to the third region γ Min The determination can be performed as follows.

[0081] Vickers hardness was measured continuously along an imaginary straight line a at 15 μm intervals, according to JIS Z 2244, with a load of 10 gf.

[0082] From the Vickers hardness value obtained through such measurement, the average Vickers hardness Hv in the first region α can be calculated. Ave and the lowest Vickers hardness Hv in the third region γ Min .

[0083] It should be noted that in this application, the aim is to prevent solder joint fracture by suppressing the hardness reduction in the narrow third region γ of approximately 200 μm. Therefore, a measurement spacing of 15 μm, which is narrower than usual, was adopted.

[0084] In other words, if the measurement interval is too large, even if there is a local decrease in hardness near the third region γ, such a decrease in hardness cannot be detected.

[0085] It should be noted that the method shown here is to measure along an imaginary straight line a that is 200 μm apart and parallel to the joint surface. However, if the measurement by this method is difficult, the measurement can also be performed from the center of the molten part outwards, in a manner that crosses the molten boundary, at intervals of 15 μm.

[0086] The chemical composition of the first steel plate member 10 and the second steel plate member 20 is not particularly limited. However, for each of the first steel plate member 10 and the second steel plate member 20, if the Mn content is excessive, Mn segregation will easily occur. Therefore, the Mn content is preferably 1.0% by mass or less, and more preferably 0.5% by mass or less.

[0087] Regarding the first steel plate component 10 and the second steel plate component 20 respectively, from the viewpoint of ensuring hardenability, the Mn content is preferably 0.1% by mass or more.

[0088] Furthermore, to ensure strength, the C (carbon) content can be 0.30 to 0.60% by mass when reducing the Mn content.

[0089] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the examples described. Obviously, those skilled in the art to which this invention pertains will be able to conceive of various modifications or alterations within the scope of the technical concept set forth in the claims, and such modifications or alterations are, of course, also understood to fall within the technical scope of this invention.

[0090] For example, the skeleton member 1 in the above embodiment is constructed using a first steel plate member 10 and a second steel plate member 20, but it may also be constructed using three or more steel plate members.

[0091] Furthermore, regarding the skeleton member 1 in the above embodiment, the first steel plate member 10 has a cap-shaped cross-sectional shape and the second steel plate member 20 has a flat cross-sectional shape, but the cross-sectional shape is not limited as long as it has a closed cross-section. For example, the first steel plate member 10 may have a flat cross-sectional shape and the second steel plate member 20 may have a cap-shaped cross-sectional shape, or both the first steel plate member 10 and the second steel plate member 20 may have a cap-shaped cross-sectional shape.

[0092] Furthermore, regarding the skeleton member 1 in the above embodiment, the heights of the pair of sidewalls 13, 13 are the same, but they may also be different from each other.

[0093] Preferably, the cross-sectional area has the following characteristics: the ratio h1 / w of the height h1 of the first steel plate member 10 in the thickness direction along the portion of the first steel plate member 10 where the spot weld 50 is formed in the cross-section perpendicular to the length direction Z of the skeleton member 1 is 0.6 or less.

[0094] Based on this configuration, by using steel plates with increased strength in the spot weld portion 50 to manufacture a component with an appropriate aspect ratio in its cross-section, breakage of the spot weld portion 50 can be suppressed. This results in superior energy absorption performance.

[0095] It should be noted that, in the case of a hat-shaped cross-section where the heights of a pair of sidewalls 13, 13 are different from each other, the average length of the heights of the two sidewalls 13, 13 is set as height h1.

[0096] In the skeleton member 1 of this embodiment, the second steel plate member 20 is flat, so its height h2 is 0 mm. When the second steel plate member 20 is not flat, it is preferable to have a cross-sectional area where the ratio h2 / w of its height h2 to the width w of the skeleton member 1 is 0.6 or less.

[0097] It should be noted that the skeleton member 1 in this embodiment has the same cross-sectional shape along its entire length, but it may not have the same cross-sectional shape along its entire length.

[0098] The cross-sectional region with a ratio h1 / w of 0.6 or less preferably exists in more than 50% of the total length of the skeleton member 1 in the longitudinal direction Z, and more preferably in more than 80%.

[0099] Furthermore, similarly, the cross-sectional region with a ratio h2 / w of 0.6 or less preferably exists in more than 50% of the total length of the skeleton member 1 in the longitudinal direction Z, and more preferably in more than 80%.

[0100] This configuration allows for more reliable suppression of fractures originating from the spot weld during impacts, resulting in superior energy absorption performance.

[0101] (Example)

[0102] The effects of one aspect of the present invention will be illustrated more specifically through embodiments. However, the conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this single example of conditions. Various conditions may be adopted by the present invention as long as they do not depart from the spirit and purpose of the invention.

[0103] Will Figure 5The three-point bending test shown was reproduced using a numerical analytical model to evaluate the number of weld fractures. The spot weld was measured with respect to the weld nugget diameter: The model was constructed using a t = 1.4 mm depth. For the strength of the spot weld, results from shear-type joint welding tests were used. For material properties, tensile test results were used.

[0104] First, the criteria for determining the steel plates used for evaluation are set as shown in Table 1.

[0105] [Table 1]

[0106] steel plate Plate thickness Mn content YP TS U.EI Steel Plate A 1.4mm 0.39% by mass 1436MPa 2010MPa 5％ Steel plate B 1.4mm 0.80% by mass 1458MPa 2040MPa 5％ Steel plate C 1.4mm 1.27% by mass 1497MPa 2082MPa 5％

[0107] In Experiments 1-3 and 5-7, a skeleton component was used, consisting of a cap-shaped steel plate component with a specified component height h1 using steel plate A, and a flat steel plate component (h2 = 0 mm) with the same properties as steel plate A, which was joined to the flange of the cap-shaped steel plate component by spot welding. The spot welding spacing was set to 40 mm.

[0108] In Experiment 4, a skeleton component was used, consisting of a cap-shaped steel plate component with a specified component height h1 using steel plate B, and a flat steel plate component (h2 = 0 mm) with the same properties as steel plate B, joined to the flange of the cap-shaped steel plate component by spot welding. The spot weld spacing was set to 40 mm. In Experiments 8-13, a skeleton component was used, consisting of a cap-shaped steel plate component with a specified component height h1 using steel plate C, and a flat steel plate component (h2 = 0 mm) with the same properties as steel plate C, joined to the flange of the cap-shaped steel plate component by spot welding. The spot weld spacing was set to 40 mm.

[0109] Therefore, as Figure 4 As shown, a skeleton component with a width w of 130 mm and a height h1 as shown in Table 2 below is manufactured. It should be noted that the length of the skeleton component is 800 mm, and a structure with a fixed cross-sectional shape throughout its entire length is adopted.

[0110] Next, as Figure 5 As shown, on a pair of dies (R50mm) spaced 700mm apart, the skeleton member is arranged such that the midpoint of these dies overlaps with the center of the skeleton member in the height direction along its length. Then, a rigid semi-circular (R50mm) impactor is driven into the center of the skeleton member in the length direction at a constant speed of 7.2 km / h, and the number of weld fractures is evaluated based on the deformation state at that moment.

[0111] Figure 6AThis is an example of a case where the number of weld point fractures reaches 10 (5 locations on one side). In this example, weld point fractures occur and the back plate lifts up, failing to maintain a closed cross-section.

[0112] Figure 6B This is an example of a case where the number of weld fractures becomes two (one location on one side). In this example, weld fractures occur and the back plate extends into the steel plate member side, but a closed section is maintained.

[0113] In this embodiment, a weld breakage count of 4 or fewer is considered acceptable. The evaluation results are shown in Table 2.

[0114] It should be noted that for the spot weld obtained by overlapping and spot welding two steel plates A, it is assumed that: along an imaginary straight line extending 200 μm from the joint surface of the steel plate members toward the side of the cap-shaped steel plate member and parallel to the joint surface, the Vickers hardness is determined according to JIS Z 2244 with a load of 10 gf and a spacing of 15 μm.

[0115] Given such assumed Vickers hardness values, the average Vickers hardness Hv in the first region α is determined according to... Ave The lowest Vickers hardness Hv in the third region γ Min The difference (Hv) Ave -Hv Min It can be set to 45.

[0116] The same procedure is performed on the spot welded section obtained by overlapping and spot welding two steel plates B, with Hv... Ave -Hv Min The value is set to 90.

[0117] The same procedure is performed on the spot weld section obtained by overlapping and spot welding two steel plates C, with Hv... Ave -HV Min The value is set to 140.

[0118] [Table 2]

[0119] steel plate <![CDATA[Hv Ave -HV M in]]> h1 W h1 / w Number of fractures Qualification or failure determination Remark Experimental Example 1 A 45 20mm 130mm 0.15 2 very good Invention Examples Experimental Example 2 A 45 40mm 130mm 0.31 2 very good Invention Examples Experimental Example 3 A 45 60mm 130mm 0.46 2 very good Invention Examples Experiment Example 4 B 90 60mm 130mm 0.46 4 good Invention Examples Experimental Example 5 A 45 80mm 130mm 0.62 4 good Invention Examples Experimental Example 6 A 45 100mm 130mm 0.77 4 good Invention Examples Experimental Example 7 A 45 120mm 130mm 0.92 4 good Invention Examples Experimental Example 8 C 140 20mm 130mm 0.15 8 NG Comparative example Experimental Example 9 C 140 40mm 130mm 0.31 10 NG Comparative example Experimental Example 10 C 140 60mm 130mm 0.46 12 NG Comparative example Experimental Example 11 C 140 80mm 130mm 0.62 14 NG Comparative example Experimental Example 12 C 140 100mm 130mm 0.77 14 NG Comparative example Experimental Example 13 C 140 120mm 130mm 0.92 14 NG Comparative example

[0120] In Experiments 1-3, 5-7, assuming the use of steel plate A with a Mn content of 0.39% by mass, and Experiment 4, assuming the use of steel plate B with a Mn content of 0.80% by mass, no localized decrease in hardness occurred, Hv Ave -Hv Min The value is below 100. Therefore, it is possible to suppress the number of weld point fractures during the deformation process to below 4.

[0121] On the other hand, in Experiments 8-13, which assumed the use of steel plate C with a Mn content of 1.27% by mass, a localized decrease in hardness and HV occurred. Ave -Hv Min The value exceeds 100. Therefore, the number of weld point fractures during the deformation process reaches more than 8.

[0122] Furthermore, a comparison of experimental examples 1-3 and 5-7 of the invention examples confirms that the lower the value of h1 / w is (below 0.6), the more effectively the number of solder joint breaks can be suppressed.

[0123] Industrial availability

[0124] According to the present invention, a skeleton component can be provided that exhibits excellent energy absorption performance commensurate with its high strength by suppressing breakage at the spot welds during impact.

[0125] Explanation of symbols

[0126] 1. Skeleton Components

[0127] 10 First steel plate component

[0128] 11 top plate

[0129] 13 Sidewalls

[0130] 15 Flanges

[0131] 20 Second steel plate component

[0132] 50 spot welds

[0133] 51 Molten Metal Section

[0134] 53 Heat-affected zone

[0135] α First Region

[0136] β second region

[0137] γ third region

Claims

1. A skeleton component, characterized in that, It is a frame component joined by spot welding the first steel plate component and the second steel plate component at the spot welding section. The skeleton member has a cross-sectional region with a closed cross-section perpendicular to the length direction of the skeleton member. The first steel plate component has a tensile strength of over 1900 MPa. The spot weld portion has a molten metal portion formed by the spot weld and a heat-affected portion adjacent to the outer side of the molten metal portion. In a cross-section perpendicular to the length direction, including the center point of the molten metal portion, the region corresponding to the molten metal portion is defined as a first region, the region corresponding to the heat-affected zone is defined as a second region, and the region formed by the region extending 100 μm from the boundary between the first and second regions toward the first region and the region extending 100 μm from the boundary toward the second region is defined as a third region. When measuring Vickers hardness at 15 μm intervals with a load of 10 gf along an imaginary straight line extending from the center of the first region toward the second region, the average Vickers hardness Hv at the measurement location corresponding to the first region on the imaginary straight line is... Ave The lowest Vickers hardness Hv at the measurement location corresponding to the third region on the imaginary straight line. Min Satisfy Hv Ave -Hv Min ≤100Hv.

2. The skeleton component according to claim 1, characterized in that, The cross-sectional region has the following characteristics: the ratio h1 / w of the height h1 of the first steel plate member along the thickness direction of the portion of the first steel plate member where the spot weld is formed in the cross-section perpendicular to the length direction of the skeleton member, and the width w of the skeleton member along the direction perpendicular to the thickness direction of the portion of the first steel plate member where the spot weld is formed, is 0.6 or less.

3. The skeleton component according to claim 2, characterized in that, The cross-sectional region with a ratio h1 / w of 0.6 or less exists in more than 50% of the total length of the skeleton member in the longitudinal direction.

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