Method for manufacturing metal plate structure

Abstract

The method for manufacturing a metal plate structure according to the present invention is a method for manufacturing a structure using a metal plate by improving the fatigue strength of a portion at which fatigue damage is worried, the method comprising: a fatigue damage risk portion determination step (S1); a step for determining, as a fatigue failure risk site, a sheared end face of the metal plate which causes stress concentration and is worried about fatigue failure when the structure is used; and a compressive residual stress imparting step (S3) in which a load is applied to the metal plate or the structure such that the tensile plastic strain is concentrated on the fatigue failure risk site, and after the tensile plastic strain is generated in the fatigue failure risk site, the load is removed, thereby imparting compressive residual stress to the fatigue failure risk site.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a metal sheet structure by improving the fatigue strength of a portion of a structure using a metal sheet where fatigue fracture is a concern. Background Art

[0002] In sheet metal structures, such as automotive parts, fatigue strength at the sheared edges of the metal sheets is often a challenge. Electric vehicles, in particular, are heavier than gasoline vehicles due to the weight of their batteries, and the fatigue strength requirements for automotive components are also higher. Consequently, several technologies have been proposed to improve the fatigue strength of the sheared edges of sheet metal structures, such as automotive parts.

[0003] For example, Patent Document 1 discloses a method for improving fatigue strength in a mechanical component made of steel and having a notch by applying compressive residual stress to the surface of the notch (equivalent to the sheared end face) using an ultrasonic vibrator. Furthermore, Patent Document 2 discloses a method for improving fatigue strength by forming a circular blanking hole in a metal plate using a punch. The punch, while inserted into the hole, is then rotated circumferentially to polish the sheared end face.

[0004] Prior art literature Patent Literature Patent Document 1: Japanese Patent Application Laid-Open No. 2006-104551 Patent Document 2: Japanese Patent Application Laid-Open No. 2022-42631 Summary of the Invention

[0005] Problems to be solved by the invention

[0006] The method disclosed in Patent Document 1 suffers from issues such as increased equipment cost for the ultrasonic transducer that impacts the sheared end surfaces of the cutouts and difficulty accurately impacting the sheared end surfaces of multiple cutouts. Furthermore, the method disclosed in Patent Document 2 reliably improves fatigue strength by polishing the sheared end surfaces after forming circular punched holes. However, the use of a special die increases equipment cost.

[0007] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for manufacturing a metal plate structure that can reliably improve the fatigue strength of the shear end surface of the metal plate without requiring a large equipment cost.

[0008] Means for solving problems

[0009] A method for manufacturing a metal plate structure according to the present invention is a method for manufacturing the structure by improving the fatigue strength of a portion of the structure using the metal plate where fatigue failure is a concern. The method includes: a determination step of determining a portion of the shear end surface of the metal plate where fatigue failure is a concern due to stress concentration during use of the structure; and an imparting step of applying a load to the metal plate or the structure so as to concentrate tensile plastic strain on the identified fatigue failure portion, and removing the load after the tensile plastic strain is generated in the fatigue failure portion, thereby imparting compressive residual stress to the fatigue failure portion.

[0010] The determining process may include: a first determining step of determining the fatigue strength of the metal plate; a stress analysis step of performing stress analysis to calculate the stress at the shear end surface of the metal plate when the structure is used; and a second determining step of determining a portion of the shear end surface of the metal plate in the structure, where the stress has been calculated, that exceeds the fatigue strength determined in the first determining step as a fatigue failure risk portion where fatigue failure is a concern.

[0011] In a case where a notch is provided at the end of the metal plate at the flat portion of the structure and the fatigue failure risk portion determined in the determining step is a shear end face of the notch, in the imparting step, any one of a tensile load, a load causing the fatigue failure risk portion to bend to the outside of the plane of the metal plate, and a load causing the portion along the edge of the notch to bend in an out-of-plane direction, or a combination thereof, is applied in a direction parallel to a tangent line of the fatigue failure risk portion of the notch, thereby generating tensile plastic strain at the fatigue failure risk portion.

[0012] In a case where a circular or elliptical hole is provided on the metal plate at a flat portion of the structure and the fatigue failure risk portion determined in the determining step is a shear end face of the hole portion, in the imparting step, either a tensile load is applied in a direction parallel to a tangent line of the fatigue failure risk portion of the hole portion or a load is applied along the edge of the hole portion to cause the portion including the fatigue failure risk portion to bend and deform in an out-of-plane direction, or a combination of these loads is applied to generate the tensile plastic strain at the fatigue failure risk portion.

[0013] In the imparting step, a plurality of beads are formed so as to sandwich the fatigue-fracture-at-risk portion in a direction parallel to a tangent line of the fatigue-fracture-at-risk portion, thereby generating the tensile plastic strain in the fatigue-fracture-at-risk portion.

[0014] In the imparting step, the sheet thickness t (mm) of the metal plate is such that the average gradient of the tensile plastic strain in the range from the fatigue failure risk portion to 2t (mm) in a direction perpendicular to the tangent line of the fatigue failure risk portion is 0.004 / 2t (mm). -1 ) in the above manner, the tensile plastic strain is generated in the fatigue failure risk area.

[0015] In the imparting step, the tensile plastic strain may be generated in the fatigue-fracture-prone portion so that an absolute value of the compressive residual stress imparted to the fatigue-fracture-prone portion becomes 40% or more of the tensile strength of the metal plate.

[0016] In the imparting step, the tensile plastic strain may be generated in the fatigue-fracture-prone portion so that a thickness reduction ratio of the fatigue-fracture-prone portion is 10% or less.

[0017] The metal plate may have a tensile strength of 780 MPa or more.

[0018] The metal plate may have a work hardening coefficient of 0.2 or less in a plastic strain region of 4% to 6%.

[0019] Effects of the Invention

[0020] The present invention identifies areas on the sheared end faces of metal plates where stress concentrates during use, potentially leading to fatigue failure, as fatigue-prone areas. Furthermore, the present invention applies a load to the identified fatigue-prone areas in a manner that concentrates tensile plastic strain. After the tensile plastic strain is generated, the load is removed, thereby imparting compressive residual stress to the fatigue-prone areas. This allows for the suppression of crack growth in these areas during use of the metal plate structure, thereby enabling the manufacture of structures with enhanced fatigue strength. Furthermore, the application of compressive residual stress to the fatigue-prone areas improves the delayed fracture properties of these areas within the structure. Furthermore, the induction of tensile plastic strain in the fatigue-prone areas induces work hardening, thereby increasing the yield strength of these areas and improving the deformation strength of the structure when a primary load is applied. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a flowchart showing a specific process flow of the method for manufacturing a metal plate structure according to an embodiment of the present invention. Figure 2 The diagram explains the tensile residual stress generated in a metal plate having a cutout portion after shearing, and the generation of cracks and fatigue failure caused by the tensile residual stress. Figure 3 This figure explains why fatigue strength can be improved by generating tensile plastic strain in a concentrated manner at the fatigue failure risk area in the method of the present invention ((a) generating tensile plastic strain in the notch portion, (b) applying compressive residual stress by removing the load after generating tensile plastic strain in the notch portion). Figure 4 This is a flowchart illustrating specific processing of the step of identifying a fatigue failure risk location that is at risk of fatigue failure in a structure using metal plates, in the method for manufacturing a metal plate structure according to an embodiment of the present invention. Figure 5 This is a graph showing a specific example of measuring the tensile residual stress generated in the notch portion after shearing and the compressive residual stress imparted after the load is removed due to tensile plastic strain generated in the notch portion in the method for manufacturing the metal plate structure of the present embodiment. Figure 6These figures illustrate how, in the method for manufacturing a metal plate structure according to this embodiment, load is applied so that tensile plastic strain is concentrated on the shear end surface of the notch portion, which is identified as a fatigue failure risk site ((a) in-plane bending deformation, (b) out-of-plane bending deformation, and (c) bead forming). Figure 7 This figure shows a specific example of a method for manufacturing a metal plate in the present embodiment in which a load is applied in such a manner that the tensile plastic strain is concentrated on the shear end face of a circular hole portion determined as a fatigue failure risk site ((a) out-of-plane bending deformation, (b) rib forming). Figure 8-1 This figure shows a test piece model (1 / 4 model) used in the FEM analysis in this embodiment. This FEM analysis is an analysis of the relationship between the tensile plastic strain and the residual compressive stress generated in the notch portion by applying a tensile load to the test piece at the notch portion. Figure 8-2 These are the results of FEM analysis of displacement caused by tensile deformation in a test specimen model with a notch (punching radius R = 10 mm) in this embodiment. The figures show (a) the distribution of tensile plastic strain and (b) the distribution of residual stress in a direction perpendicular to the tangent line of the shear end surface of the notch bottom, a fatigue failure risk area. Figure 8-3 The figure shows the results of FEM analysis of the displacement of tensile deformation applied to a test piece model with a notch (punching radius R = 10, 20, 30 mm) in this embodiment, and is a graph showing (a) the relationship between the compressive residual stress at the notch bottom of the notch and the displacement of tensile deformation, and (b) the relationship between the average gradient of tensile plastic strain and the displacement of tensile deformation. Figure 8-4 This figure shows the results of FEM analysis of displacement of a test piece model with a notch (punching radius R = 10, 20, 30 mm) subjected to tensile deformation in this embodiment, and is a graph showing the relationship between the compressive residual stress at the notch bottom and the maximum value of the average gradient of the plastic strain during the tensile deformation process. Figure 9 These are diagrams showing test pieces used in a fatigue test to investigate the improvement in fatigue strength of a metal plate having a notched portion formed therein by shearing in Example 1 ((a) after shearing, (b) application of compressive residual stress). Figure 10This is a diagram showing a structure using a metal plate having circular holes formed therein by shearing in Example 2 (Invention Example 2). Figure 11 This is a diagram illustrating a fatigue test method for a structure using a metal plate having circular holes formed therein by shearing in Example 2. Figure 12 This is a diagram showing a structure using a metal plate having circular holes formed therein by shearing in Example 2 (Comparative Example 2). DETAILED DESCRIPTION

[0022] <Process of Completion of the Invention>

[0023] Known as Figure 2 As illustrated, in the cutout portion 13 formed by shearing the end of the metal plate 11, tensile residual stress is generated in the sheared end surface 13a at the cutout bottom in the direction tangential to the plane of the metal plate 11, thereby reducing fatigue strength. Therefore, when a load is applied to a structure using such a metal plate 11 during use, and stress concentrates on the cutout bottom of the cutout portion 13, cracks 15 may propagate in a direction perpendicular to the tangent to the sheared end surface 13a at the cutout bottom of the cutout portion 13, potentially leading to fatigue failure.

[0024] Therefore, in order to improve the fatigue strength of structures using metal plates, it was considered necessary to increase the fatigue strength of the sheared end surfaces of the metal plates, which are prone to fatigue failure. Therefore, the inventors investigated methods for improving the fatigue strength of the sheared end surfaces of the metal plates. They concluded that eliminating the tensile residual stress in the sheared end surfaces of the metal plates and converting it to compressive residual stress would be sufficient, and further research was conducted on specific methods for this purpose.

[0025] As a result of repeated research, the inventor came up with a solution to the problem of worrying about Figure 2 The sheared end surface 13a at the notch bottom of the fatigue-failed metal plate 11 shown in the figure is loaded so that tensile plastic strain is concentrated in a direction perpendicular to the direction of propagation of the crack 15 (to the left in the drawing). Furthermore, it was discovered that if tensile plastic strain is locally generated at the sheared end surface 13a at the notch bottom and then the load is removed, compressive residual stress is imparted to the sheared end surface 13a at the notch bottom, thereby improving fatigue strength. The present invention was completed based on the above research, and the specific structure is described below.

[0026] <Method for Manufacturing Metal Plate Structure>

[0027] The method for manufacturing a metal plate structure according to an embodiment of the present invention is a method for manufacturing the structure by improving the fatigue strength of a portion of the structure using the metal plate where fatigue failure is a concern. Figure 1As shown, the manufacturing method of the metal plate structure of this embodiment includes a fatigue failure risk location determination step S1 and a compressive residual stress imparting step S3. The fatigue failure risk location determination step S1 corresponds to the determination step of the present invention, and the compressive residual stress imparting step S3 corresponds to the imparting step of the present invention. Figure 3 As shown in FIG. 1 , each of the above steps will be described with reference to a structure using a flat metal plate 11 having a cutout portion 13 formed at an end portion thereof by shearing.

[0028] Fatigue Failure Risk Location Identification Process

[0029] The fatigue-fracture-prone-site identifying step S1 is a step of identifying the sheared end surface 13 a of the metal plate 11 , which may cause stress concentration during use of the structure and may result in fatigue fracture, as a fatigue-fracture-prone site 17 .

[0030] Fatigue damage risk area 17, such as Figure 4 As shown, it can be determined through the fatigue strength determination step S11, the stress analysis step S13, and the fatigue failure risk location determination step S15. The fatigue strength determination step S11 corresponds to the first determination step of the present invention, and the fatigue failure risk location determination step S15 corresponds to the second determination step of the present invention.

[0031] (Fatigue strength determination steps)

[0032] The fatigue strength determination step S11 is a step of determining the fatigue strength of the metal plate 11. The fatigue strength determination step S11 can determine the fatigue strength by, for example, performing a fatigue test on a test piece produced by shearing the metal plate.

[0033] The fatigue test method for determining fatigue strength using test pieces is not particularly limited. For example, the method specified in JIS Z 2275 (fatigue test using test pieces with sheared end faces for members subjected to bending loads during use) can be applied.

[0034] Furthermore, when the fatigue life required for a structure is known in advance, the fatigue strength of the metal plate may be determined as the time intensity of the fatigue life required for the structure, without performing a fatigue test using the test piece described above.

[0035] (Stress analysis steps)

[0036] The stress analysis step S13 is a step of performing stress analysis to calculate the stress at the shear end surface 19 of the metal plate 11 when the structure is used.

[0037] In the stress analysis step S13, a finite element model (FEM) of the structure is created and stress analysis using the FEM is performed under input conditions that simulate the structure's operating environment (e.g., load conditions applied to the structure). This stress analysis calculates the stress at each element or node corresponding to the shear end surface 19 of the metal plate 11 in the FEM model of the structure. For example, elastic analysis or elastic-plastic analysis using the static implicit method can be applied as a stress analysis method.

[0038] (Steps for determining fatigue failure risk areas)

[0039] In the fatigue failure potential location identification step S15, a location on the sheared end surface 19 of the metal plate 11, for which stress was calculated in the stress analysis step S13, that exceeds the fatigue strength determined in the fatigue strength determination step S11 is identified as a fatigue failure potential location 17. In this embodiment, the stress on the sheared end surface 13a at the cutout bottom of the cutout portion 13 on the sheared end surface 19 of the metal plate 11, for which stress was calculated in the stress analysis step S13, exceeds the fatigue strength determined in the fatigue strength determination step S11. Therefore, the fatigue failure potential location identification step S15 identifies the sheared end surface 13a at the cutout bottom of the cutout portion 13 as the fatigue failure potential location 17.

[0040] ≪Compressive residual stress imparting process≫

[0041] The compressive residual stress imparting step S3 is a step of applying a load to the structure so that tensile plastic strain is concentrated on the fatigue-failure-risked portion 17 identified in the fatigue-failure-risked portion identifying step S1, thereby generating tensile plastic strain in the fatigue-failure-risked portion 17. Furthermore, the compressive residual stress imparting step S3 is a step of imparting compressive residual stress to the fatigue-failure-risked portion 17 by removing the load after generating tensile plastic strain in the fatigue-failure-risked portion 17.

[0042] In this embodiment, if Figure 3 As shown, by applying a tensile load in a direction parallel to the tangent line of the shear end face 13a identified as the fatigue failure risk portion 17 to the metal plate 11, tensile plastic strain is generated in a concentrated manner in the fatigue failure risk portion 17. The direction parallel to the tangent line of the fatigue failure risk portion 17 is the direction in which the crack 15 propagates from the shear end face 13a at the bottom of the cut in the plane of the metal plate 11 ( Figure 2) in the vertical direction. In this way, when the tensile plastic strain is generated in the fatigue damage risk area 17 ( Figure 3 (a) ), by removing the tensile load, a compressive residual stress in the direction opposite to the tensile plastic strain is applied in a direction parallel to the tangent line of the fatigue failure risk portion 17 ( Figure 3 (b)).

[0043] <Reasons for improving fatigue strength>

[0044] The reason why the method for manufacturing a metal plate structure according to the present embodiment enables manufacturing a structure using metal plates in which fatigue strength of a portion where fatigue failure is a concern is improved will be described below.

[0045] As described above, tensile residual stress is generated in the sheared end surface 13a of the cut bottom of the cut portion 13 formed by shearing the end of the metal plate 11 in a direction parallel to the tangent line of the sheared end surface 13a. Therefore, in the present embodiment, in the compressive residual stress imparting step S3, as shown in FIG. Figure 3 As shown in (a), the load is applied in such a way that the tensile plastic strain is concentrated in the direction of the tangent line of the fatigue failure risk portion 17. As a result, tensile plastic strain is generated locally in the fatigue failure risk portion 17. Moreover, if the load is removed after the tensile plastic strain is generated in the direction of the tangent line of the fatigue failure risk portion 17, the area around the fatigue failure risk portion 17 will return to its original shape. Therefore, the fatigue failure risk portion 17 is pressed from the surrounding area, as shown in FIG. Figure 3 As shown in (b), the material is compressed in the direction opposite to the direction in which the tensile load is applied. Thus, after the load is removed by the generation of tensile plastic strain, the tensile residual stress in the tangential direction generated by the shearing of the cutout 13 is eliminated, and a compressive residual stress is applied in the tangential direction. Figure 5 1 shows an example of measuring the magnitude of the tensile residual stress generated on the sheared end surface 13 a of the notch bottom of the notch portion 13 after shearing and the compressive residual stress applied after the load is removed due to the generation of tensile plastic strain, using an X-ray residual stress measuring method based on the cosα method.

[0046] As a result, even if stress concentrates on the shear end surface 13a of the metal plate 11 identified as the fatigue fracture risk portion 17 when the structure is used, crack propagation can be suppressed, and a structure with improved fatigue strength can be manufactured.

[0047] Furthermore, according to the method for manufacturing a metal plate structure of the present embodiment, since compressive residual stress is generated in the fatigue-fracture-prone portion 17 , the delayed fracture characteristics of the fatigue-fracture-prone portion 17 can also be improved.

[0048] Furthermore, the method for manufacturing a metal plate structure according to this embodiment generates tensile plastic strain in the fatigue-fracture-prone portion 17, causing work hardening, thereby increasing the yield strength of the fatigue-fracture-prone portion 17. This also improves the deformation strength of the fatigue-fracture-prone portion 17 of the metal plate 11 when a primary load is applied to the structure.

[0049] In the above description, the fatigue fracture potential location identification step S1 implements the fatigue strength determination step S11 and the stress analysis step S13 , and identifies the shear end face 13 a of the metal plate 11 where the stress obtained by the stress analysis exceeds the fatigue strength as the fatigue fracture potential location 17 .

[0050] However, in the present invention, when the locations of the metal plate structure where fatigue failure is a concern are known in advance, the fatigue failure potential location identification step may identify the fatigue failure potential location without performing fatigue strength determination ( S11 ) and stress analysis ( S13 ).

[0051] In the above description, in the compressive residual stress imparting step S3, a uniform tensile load is applied to the metal plate 11 so as to cause tensile deformation in a direction parallel to the tangent line of the fatigue-fracture-prone portion 17 within the plane of the flat metal plate 11. This is because the notch 13 is originally a stress concentration area, and therefore, simply by applying a uniform load to the metal plate 11, plastic strain can be concentrated on the shear end surface 13a of the notch bottom of the notch 13, which is determined to be the fatigue-fracture-prone portion 17.

[0052] However, in the compressive residual stress imparting step S3, the load may be applied in such a manner that the tensile plastic strain is concentrated on the fatigue fracture risk portion 17. For example, Figure 6 As shown in (a), a load may be applied that causes the fatigue fracture risk portion 17 to bend outward within the plane of the metal plate 11 (in-plane bending deformation). Alternatively, as shown in Figure 6 As shown in (b), a load may be applied to bend the metal plate 11 outward along the edge of the cutout 13 (outward bending deformation, stretch flanging) so that the shear end face 13a, which is the fatigue failure risk portion 17, becomes a V-shape. Furthermore, the compressive residual stress imparting step may be combined with the stretching deformation ( Figure 3 (a)), in-plane bending deformation ( Figure 6 (a)) or out-of-plane bending deformation ( Figure 6 (b)) to carry the load.

[0053] In addition, in the compressive residual stress imparting process, as Figure 6 As shown in (c), two reinforcing ribs 21 may be formed in the tangential direction of the shear end surface 13 a so as to sandwich the fatigue failure potential portion 17 , thereby generating tensile plastic strain in the fatigue failure potential portion 17 .

[0054] While the above description focuses on the sheared end surface 13a of the notch 13 formed at the end of the flat metal plate 11, the present invention is not limited to the shape of the sheared end surface in the metal plate. Therefore, even if a sheared end surface sheared into a substantially straight line or a sheared end surface of a circular hole punched out is identified as a fatigue failure risk site, it is sufficient to apply a load such that tensile plastic strain is concentrated on these sheared end surfaces.

[0055] For example, in Figure 7 In the case where fatigue failure is a concern in the sheared end surface 33a of the hole 33 of the flat metal plate 31 after shearing, a load is applied to the sheared end surface 33a of the hole 33, identified as a fatigue failure risk site 35. After the tensile plastic strain is generated in the fatigue failure risk site 35, the load is removed, thereby imparting compressive residual stress. This allows for improved fatigue strength at the sheared end surface 33a of the hole 33, where fatigue failure is a concern, in a structure using the flat metal plate 31 having the hole 33 formed therein.

[0056] In order to apply the load so that the tensile plastic strain is concentrated on the shear end surface 33a of the hole portion 33, as shown in FIG. Figure 7 As shown in (a), the edge of the hole 33 may be bent outward (outward bending deformation, burring). Alternatively, Figure 7 As shown in (b), two ribs 37 may be formed so as to sandwich the hole 33 in a direction parallel to the tangent line of the sheared end surface 33 a.

[0057] in addition, Figure 7 The hole portion 33 shown is circular, but the present invention is not limited thereto, and an elliptical hole portion may also be used.

[0058] In the present invention, the compressive residual stress imparting step can generate plastic strain such that the absolute value of the compressive residual stress at the fatigue failure risk site after the tensile plastic strain is generated is at least 40% of the tensile strength of the metal sheet (for example, at least 392 MPa for a steel sheet with a tensile strength of 980 MPa). Because the tensile residual stress remains at the sheared end surface after shearing, even if stress redistribution occurs due to yielding, the growth of fatigue cracks is effectively suppressed. Furthermore, if the compressive residual stress is at least 40% of the tensile strength in absolute value, the effect of suppressing fatigue crack growth at the sheared end surface of the metal sheet is fully achieved. This will be described in detail in the Examples described below.

[0059] In the present invention, after locally generating tensile plastic strain at a fatigue-prone area, the load is removed. The driving force for the compressive residual stress generated in the fatigue-prone area is the difference between the tensile plastic strain at the fatigue-prone area and its surroundings. Therefore, even if local tensile plastic strain is generated at the fatigue-prone area, if the difference between the tensile plastic strain at the fatigue-prone area and its surroundings is small, sufficient compressive residual stress may not be imparted to the fatigue-prone area after the load is removed. Therefore, the conditions for generating tensile plastic strain at the fatigue-prone area are sufficient to obtain a difference between the tensile plastic strain at the fatigue-prone area and its surroundings that imparts sufficient compressive residual stress.

[0060] In this regard, the relationship between the compressive residual stress in the fatigue failure risk area after the load is removed due to the generation of tensile plastic strain and the average gradient of the tensile plastic strain is described, focusing on the average gradient of the tensile plastic strain in the direction perpendicular to the tangent line of the fatigue failure risk area. The average gradient of the plastic strain is the average gradient (mm) obtained by dividing the difference between the tensile plastic strain at the shear end face and the tensile plastic strain at a position 2t (mm) away from the shear end face in the direction perpendicular to the tangent line of the fatigue failure risk area by the distance 2t (mm). -1 ).

[0061] Through Figure 8-1 The FEM analysis performed on a test specimen model 71 (1 / 4 model) with a thickness of t = 3 mm and a semicircular notch 73 (blanking radius R) was performed. Tensile deformations ranging from 0.18 to 2.0 mm were applied to the test specimen model 71, and the average gradient of the tensile plastic strain was determined. Furthermore, the residual stress in the fatigue failure risk area 75 (the shear end surface 73a at the notch bottom) was determined after the load was removed.

[0062] Figure 8-2(a) Distribution of tensile plastic strain and (b) distribution of the direction perpendicular to the tangent line of the fatigue failure risk area 75 ( Figure 8-1 An example of residual stress distribution in the Y direction (punching radius R = 10mm). The horizontal axis represents the distance from the cut bottom (shear end face) in a direction perpendicular to its tangent. The distribution of plastic strain within a range of 2t (=6mm) from the cut bottom (distance = 0mm) reverses its slope at a certain displacement (1.64mm), with the plastic strain on the inside being greater than at the cut bottom (distance Y = 0mm). This is because the necking of the test piece accompanying tensile deformation varies depending on the stress state (cut bottom: plane stress state, inside the test piece: plane strain stress state). In addition, the compressive residual stress reaches its maximum value at the cut bottom (distance Y = 0mm).

[0063] In addition, Figure 8-3 In the figure, (a) shows the relationship between the compressive residual stress at the bottom of the cut and the displacement of the tensile deformation, and (b) shows the relationship between the average gradient of the plastic strain and the displacement of the tensile deformation. The compressive residual stress at the bottom of the cut increases with the displacement of the tensile deformation, but approaches a constant value when the displacement exceeds the specified value. On the other hand, as mentioned above, the average gradient of the plastic strain reverses its positive and negative value when the displacement exceeds the specified value. In addition, Figure 8-3 The dashed line in (b) indicates the maximum value of the average gradient of plastic strain during tensile deformation.

[0064] Figure 8-4 It represents the relationship between the compressive residual stress at the bottom of the notch and the maximum value of the average gradient of the plastic strain during the tensile deformation process. Figure 8-4 As shown in , there is a strong correlation between the two. This means that in the direction perpendicular to the tangent line of the notch bottom (the fatigue failure risk area) Figure 8-1 By using the average gradient of the tensile plastic strain in the range from the fatigue failure risk portion to 2t (mm) as an index, the compressive residual stress generated in the fatigue failure risk portion 75 can be adjusted.

[0065] As a specific condition for generating tensile plastic strain, the average gradient of tensile plastic strain in the range of 2t (mm) from the fatigue failure risk portion to the direction perpendicular to the tangent line of the fatigue failure risk portion is 0.004 / 2t (mm) with respect to the plate thickness t (mm) of the metal plate. -1) or more. The average gradient of plastic strain refers to the maximum value of the deformation in the average gradient given by the plastic strain of the fatigue failure risk area and the tensile plastic strain at a position 2t away from the fatigue failure risk area. For example, in the case of a steel plate with a thickness of t = 3mm and a tensile strength of 980MPa, in order to generate a compressive residual stress at the fatigue failure risk area with an absolute value of 40% (=392MPa) of the tensile strength or more, the average gradient of the tensile plastic strain is set to 0.004 / 2t = 6.7×10 -4 (mm -1 By generating tensile plastic strain in this way, sufficient compressive residual stress can be imparted by utilizing the difference in plastic strain between the fatigue fracture risk portion and its surroundings.

[0066] Furthermore, the compressive residual stress imparting step preferably generates tensile plastic strain so that the thickness reduction rate at the fatigue fracture risk area after load removal is 10% or less. This is because if the thickness reduction rate caused by the tensile plastic strain exceeds 10%, causing necking, cracks are likely to form at the necking area, which may reduce the fatigue strength of the shear end surface.

[0067] The present invention is not particularly limited in the type of metal sheet used as the raw material for the structure. However, it is particularly suitable for structures using high-strength metal sheets. This is because the higher the strength of the metal sheet, the greater the tensile residual stress introduced into the metal sheet during shearing, and the greater the effect of the present invention in eliminating the tensile residual stress generated at the sheared end faces of the metal sheet after shearing. Therefore, in the present invention, the metal sheet preferably has a tensile strength of 780 MPa or higher.

[0068] Furthermore, when a metal plate with a low work hardening coefficient is loaded onto a structure using the metal plate and localized plastic deformation begins at a fatigue-prone area, the fatigue-prone area does not harden, inducing plastic deformation in the surrounding area. This concentration of plastic strain can easily lead to fatigue failure. Therefore, the work hardening coefficient of the metal plate in the plastic strain region of 4% to 6%, which is the target of application of the present invention, is preferably 0.2 or less.

[0069] In this embodiment, if Figure 6 As shown, before assembling the structure, a compressive residual stress imparting step S3 is performed on the sheared end surface 13a of the metal plate 11 having the fatigue failure potential portion 17 identified in the fatigue failure potential portion identifying step S1.

[0070] However, from the perspective of manufacturing costs for imparting compressive residual stress to the sheared end faces of the metal sheet, compressive residual stress can be imparted to fatigue-fracture-prone areas during the press forming of the metal sheet into the shape of a component constituting the structure, among the multiple steps in manufacturing the structure.

[0071] However, among the various steps in manufacturing metal plate structures, there are sometimes steps that generate tensile plastic strain in the sheared end surfaces of the metal plates identified as fatigue-fracture-prone areas (e.g., bending the metal plates). If such steps are performed after the compressive residual stress imparting step of the present invention, the resulting stress state differs from the compressive residual stress imparted to the fatigue-fracture-prone areas, potentially failing to improve the fatigue strength of the fatigue-fracture-prone areas.

[0072] Therefore, the compressive residual stress imparting step is preferably performed after the step of generating tensile plastic strain at the fatigue fracture risk portion of the metal plate among the multiple steps of manufacturing the structure.

[0073] Furthermore, in the present invention, the compressive residual stress imparting step may be performed by applying a load to the structure after assembling the structure using the metal plates so that tensile plastic strain is concentrated on fatigue fracture-prone portions of the metal plates.

[0074] Example 1

[0075] Since experiments were conducted to confirm the effects of the present invention, they will be described below.

[0076] In Example 1, Figure 9 Fatigue testing was conducted on a test piece 41 of the shape shown to evaluate fatigue strength. Test piece 41 was made from a hot-rolled steel sheet with a tensile strength of 1000 MPa, a thickness of 3 mm, and a work hardening coefficient of 0.1 in the 4% to 6% plastic strain range, processed into a 120 mm x 30 mm rectangular shape. Semicircular cutouts 43 with a punching radius R of 10 mm, 20 mm, or 30 mm were sheared at both ends of the central portion of the test piece 41. Three test pieces with different punching radii R were produced. A clearance of 10% was used during shearing.

[0077] Next, fatigue testing was conducted to determine the fatigue strength of the hot-rolled steel sheet used as the test material. During the fatigue test, a cyclic load was applied to the test piece 41 in a completely alternating manner, with a repetition frequency of 20 Hz. The fatigue strength was calculated after 300,000 repetitions of the load, resulting in a value of 300 MPa.

[0078] Next, compressive residual stress is applied to the sheared end surface 43a of the notch 43 of the test piece 41 as the fatigue failure potential site 45. Since stress concentrates on the notch 43 of the test piece 41 and causes fatigue failure, the stress analysis step S13 and fatigue failure potential site identification step S15 described in the embodiment are not performed, and the sheared end surface 43a of the notch 43 is identified as the fatigue failure potential site 45.

[0079] In Example 1, Figure 9 As shown in Table 1, various displacements shown in Table 1 were applied to both ends of the test piece 41 in the longitudinal direction, thereby applying a load so that tensile plastic strain was concentrated on the shear end surface 43a of the notch 43, which was identified as the fatigue-fracture-prone portion 45. After the tensile plastic strain was generated in the fatigue-fracture-prone portion 45, the load was removed, thereby imparting compressive residual stress to the fatigue-fracture-prone portion 45 (Inventive Examples 1 to 10).

[0080] For example, in Inventive Example 3, the equivalent plastic strain gradient in the region from the shear end surface 43a of the cutout bottom of the cutout portion 43 to a distance of 2t (=6 mm) is 0.00303 mm. -1 , is (0.0188 / 2t), and the residual stress generated on the shear end face 43a is -653 MPa.

[0081] In Example 1, as a comparison, test pieces 41 in which the cutout 43 was formed without generating tensile plastic strain on the shear end surface 43a of the cutout 43 of the test piece 41 were used as comparative examples (Comparative Examples 1, 2, and 4). Furthermore, test pieces 41 in which a load was applied so that tensile plastic strain was concentrated on the shear end surface 43a at the cutout bottom of the cutout 43 and no compressive residual stress was generated after the load was removed were used as Comparative Examples 3 and 5.

[0082] Fatigue tests were then performed on the test pieces 41 of Inventive Examples 1 to 10 and Comparative Examples 1 to 5. The fatigue tests employed in-plane bending in an out-of-plane bending mode, with the test pieces 41 repeatedly loaded in a completely alternating manner. The repeated load had a repetition frequency of 2 Hz, and the stress amplitude at the ends of the test pieces 41 was 400 MPa. The test pieces 41 were deemed qualified if the load was repeated 500,000 times without breaking during the fatigue test, and the test was terminated at 1,000,000 times. Table 1 shows the fatigue test results.

[0083] [Table 1]

[0084] As shown in Table 1, in each of Invention Examples 1 to 9, the number of repetitions until rupture was 500,000 or more, and therefore the results were considered acceptable. In particular, in Invention Examples 2 to 3 and 6, no rupture occurred even after 1,000,000 repetitions, which is a good result. In Invention Examples 2 to 3 and 6, the average gradient of the plastic strain during tension was 0.00076 mm. -1 Above (0.0047 / 2tmm -1 ), the absolute value of the compressive residual stress was 407 MPa or greater, and the ratio of the absolute value of the compressive residual stress to the tensile strength of the metal plate was 0.4 or greater (40% or greater). In contrast, in Comparative Examples 1 to 5, cracks occurred on the sheared end surface 43a of the notch portion 43 when the number of repetitions was less than 500,000, and the samples were judged as unacceptable.

[0085] Comparing Invention Examples 3 and 4, which had a punching radius R = 10 mm, Invention Example 4, which had a larger absolute value of compressive residual stress, experienced necking, a plate thickness reduction exceeding 10%, and cracked after 800,000 repetitions. On the other hand, Invention Example 3 did not experience necking, had a plate thickness reduction of less than 10%, and did not crack even after 1,000,000 repetitions, a result superior to Invention Example 4. Similarly, comparing Invention Examples 6 and 7, which had a punching radius R = 20 mm, and Invention Examples 9 and 10, which had a punching radius R = 30 mm, Invention Examples 6 and 9, which had a plate thickness reduction of less than 10%, did not crack even after 1,000,000 repetitions, demonstrating excellent results.

[0086] As mentioned above, Example 1 is based on metal sheets before fabrication, but simple structures close to the metal sheet state can also be envisioned. Therefore, even in such structures, the results of Example 1 suggest that metal sheet structures can be fabricated with improved fatigue strength.

[0087] Example 2

[0088] In Example 2, Figure 10 The structure 51 shown was used as a test object for a fatigue test to evaluate fatigue strength.

[0089] Structure 51 is formed by assembling two metal plates 53 and 55, each formed into a U-shaped cross section, into a square tube. Plates 53 and 55 are made of hot-rolled steel sheets with a tensile strength of 780 MPa, a thickness of 3 mm, and a work hardening coefficient of 0.1 in the 4% to 6% plastic strain range.

[0090] The metal plate 53 on one side is formed into a U-shaped cross-section by shearing a hole 53b with a diameter of 10 mm on the steel plate. The clearance during the shearing process for forming the hole 53b is 10%. In the structure 51 using the metal plate 53 with the hole 53b formed therein, it is believed that stress is concentrated on the shear end face 53b1 of the hole 53b during use, and therefore the shear end face 53b1 is identified as a fatigue failure risk site. Therefore, after forming into a U-shaped cross-section, as shown in FIG. Figure 10 As shown, two reinforcing ribs 53c are formed on the top portion 53a so as to sandwich the hole portion 53b, so that tensile plastic strain is generated on the shear end surface 53b1 of the hole portion 53b, and compressive residual stress is applied by removing the load.

[0091] Regarding the plastic strain of the shear end surface 53b1 after the reinforcement rib 53c is formed, the average plastic strain gradient from the shear end surface 53b1 to the position 3t (=9mm) away from the hole 53b is 0.15mm. -1 The residual stress at the shear end face 53b1 cannot be measured using conventional X-ray residual stress measurement methods due to angle issues. However, since the compressive residual stress on the surface of the metal plate 53 near the shear end face 53b1, identified as a fatigue failure risk area, is -500 MPa, it is believed that a compressive residual stress of at least -100 MPa or less is applied to the fatigue failure risk area.

[0092] The other metal plate 55 is formed from a steel plate into a U-shaped cross-section, similarly to the metal plate 53. The U-shaped metal plates 53 and 55 are then joined together by arc welding to form a square tube, thereby assembling the structure 51 (Invention Example 2).

[0093] Then, if Figure 11As shown, a fatigue test was performed on the structure 51 of Inventive Example 2. The fatigue test was conducted by fixing the top plate 53a of a metal plate 53 having a hole 53b formed therein at both ends and applying a load to the center of the metal plate 55 using three-point bending. The test was conducted under pulsating conditions, with a repetition frequency of 1 Hz and a nominal stress of 300 MPa at the plate thickness surface. During the fatigue test, the plate 53 was considered broken when a crack was visually observed on the shear end surface 53b1 of the hole 53b. If the shear end surface 53b1 remained intact after 500,000 cycles, the test was considered acceptable.

[0094] In addition, in Example 2, Figure 12 As shown in FIG. 2, a metal plate 63 having a hole 63b formed in a top plate portion 63a was assembled into a square tube-shaped structure 61 by arc welding with a metal plate 55 without forming a rib. The structure 61 of the comparative example 2 was also subjected to a fatigue test under the same conditions as the structure 51 of the invention example 2 (see FIG. Figure 11 ), determine whether the shear end surface 63b1 of the hole portion 63b is cracked. Table 2 shows the fatigue test results.

[0095] [Table 2]

[0096] As shown in Table 2, structure 51 of Inventive Example 2 showed no cracking at sheared end surface 53b1 of hole 53b even after 500,000 repetitions, and was judged as acceptable. In contrast, structure 61 of Comparative Example 2 showed cracking at sheared end surface 53b1 of hole 63b after 300,000 repetitions, and was judged as unacceptable.

[0097] As described above, the results of Example 2 show that according to the present invention, in a structure using a metal plate, the fatigue strength of a portion where fatigue failure is a concern can be improved, and the structure can be manufactured.

[0098] Industrial Applicability

[0099] According to the present invention, it is possible to provide a method for manufacturing a metal plate structure that can reliably improve the fatigue strength of the shear end surface of the metal plate without requiring a large equipment cost.

[0100] Description of labels 1 Structure 11 Metal Plate 13 incision 13a Shear end face 15 Crack 17 Fatigue damage risk areas 19 Shear end face 21 reinforcement 31 Metal Plate 33 hole 33a Shear end face 35 Fatigue damage risk areas 37 reinforcement 41 test pieces 43 incision 43a Shear end face 45 Fatigue damage risk areas 51 structures 53 Metal Plate 53a Top plate 53b hole 53b1 Shear end face 53c reinforcement 55 Metal Plate 57 Fatigue damage risk areas 59 Strain concentration induced reinforcement 61 structures 63 Metal Plate 63a Top plate 63b hole 63b1 Shear end face 71 Test piece model 73 incision 73a Shear end face 75 Fatigue damage risk areas.

Claims

1. A method for manufacturing a metal plate structure, wherein the fatigue strength of a portion of a structure using a metal plate that is likely to be fatigue-fractured is increased to manufacture the structure, wherein: The manufacturing method of the metal plate structure comprises: a step of determining, as a fatigue failure risk portion, a portion of the shear end surface of the metal plate where stress concentration may occur during use of the structure and fatigue failure may be a concern; and The imparting step is to apply a load to the metal plate or the structure in such a manner that tensile plastic strain is concentrated on the determined fatigue failure risk portion, and remove the load after the tensile plastic strain is generated in the fatigue failure risk portion, thereby imparting compressive residual stress to the fatigue failure risk portion.

2. The method for manufacturing a metal plate structure according to claim 1, wherein: The determining process includes: A first determining step is to determine the fatigue strength of the metal plate; a stress analysis step of performing stress analysis to calculate stress on the shear end surface of the metal plate when the structure is used; and The second determining step is to determine a portion of the shear end surface of the metal plate in the structure where the stress is calculated, which portion exceeds the fatigue strength determined in the first determining step, as a fatigue failure risk portion where fatigue failure is a concern.

3. The method for manufacturing a metal plate structure according to claim 1 or 2, wherein: In a case where a notch is provided at the end of the metal plate at the flat portion of the structure and the fatigue failure risk portion determined in the determining step is a shear end face of the notch, in the imparting step, any one of a tensile load applied in a direction parallel to a tangent line of the fatigue failure risk portion of the notch, a load applied to cause the fatigue failure risk portion to bend outward within the plane of the metal plate, and a load applied to cause the portion including the fatigue failure risk portion to bend outward along the edge of the notch, or a combination thereof, is applied to generate tensile plastic strain at the fatigue failure risk portion.

4. The method for manufacturing a metal plate structure according to claim 1 or 2, wherein: In a case where a circular or elliptical hole is provided on the metal plate at a flat portion of the structure and the fatigue failure risk portion determined in the determining step is a shear end face of the hole portion, in the imparting step, either a tensile load is applied in a direction parallel to a tangent line of the fatigue failure risk portion of the hole portion or a load is applied along the edge of the hole portion to cause the portion including the fatigue failure risk portion to bend and deform in an out-of-plane direction, or a combination of these loads is applied to generate the tensile plastic strain at the fatigue failure risk portion.

5. The method for manufacturing a metal plate structure according to claim 1 or 2, wherein: In the imparting step, a plurality of reinforcing ribs are formed so as to sandwich the fatigue-fracture-at-risk portion in a direction parallel to a tangent line of the fatigue-fracture-at-risk portion, thereby generating the tensile plastic strain in the fatigue-fracture-at-risk portion.

6. The method for manufacturing a metal plate structure according to claim 1 or 2, wherein: In the imparting step, the thickness t (mm) of the metal plate is such that the average gradient of the tensile plastic strain in the range from the fatigue failure risk portion to 2t (mm) in a direction perpendicular to the tangent line of the fatigue failure risk portion is 0.004 / 2t (mm). -1 ) in the above manner, the tensile plastic strain is generated in the fatigue failure risk area.

7. The method for manufacturing a metal plate structure according to claim 1 or 2, wherein: In the imparting step, the tensile plastic strain is generated in the fatigue-fracture-prone portion so that the absolute value of the compressive residual stress imparted to the fatigue-fracture-prone portion becomes 40% or more of the tensile strength of the metal plate.

8. The method for manufacturing a metal plate structure according to claim 1 or 2, wherein: In the imparting step, the tensile plastic strain is generated in the fatigue-fracture-prone portion so that a thickness reduction rate of the fatigue-fracture-prone portion is 10% or less.

9. The method for manufacturing a metal plate structure according to claim 1 or 2, wherein: The metal plate has a tensile strength of 780 MPa or more.

10. The method for manufacturing a metal plate structure according to claim 9, wherein: The metal plate has a work hardening coefficient of 0.2 or less in a plastic strain region of 4% to 6%.

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