Method for manufacturing a metal plate structure

Abstract

To provide a method for manufacturing a metal plate structure, which manufactures a structure using a metal plate by improving the fatigue strength of a part causing concern about fatigue fracture in the structure.SOLUTION: The method for manufacturing the metal plate structure is for manufacturing a structure using a metal plate 11 by improving the fatigue strength of a part causing concern about fatigue fracture in the structure, and includes: a fatigue fracture risk part identification step S1 of identifying a shear fracture end surface 13a of the metal plate 11 causing concern about fatigue fracture due to stress concentration during use of a structure 1 as a fatigue fracture risk part 17; and a compression residual stress application step S3 of applying a load to the metal plate 11 or the structure so as to concentrate tensile plastic strain in the fatigue fracture risk part 17, and removing the load after the occurrence of the tensile plastic strain in the fatigue fracture risk part 17, thereby applying compression residual stress to the fatigue fracture risk part 17.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a metal plate structure, which manufactures a structure using a metal plate by improving the fatigue strength of the said structure at a portion where fatigue failure is a concern. [Background technology]

[0002] In metal plate structures such as automobile parts, the fatigue strength of the sheared edge of the metal plate is often an issue. In particular, in electric vehicles, the weight of the battery is added, so the vehicle weight is larger than that of gasoline vehicles, and the fatigue strength required for automobile parts is also higher. In view of this, several techniques have been proposed to increase the fatigue strength of the sheared end faces of metal plates in structures that use metal plates, such as automobile parts.

[0003] For example, Patent Document 1 discloses a method for improving the fatigue strength of a mechanical component made of steel and having a notch by striking the surface of the notch (corresponding to the shear end face) with an ultrasonic vibrator to impart compressive residual stress. Furthermore, Patent Document 2 discloses a method for improving the fatigue strength of a sheared end surface by forming a circular punched hole in a metal plate using a punch, and then rotating the punch in a circumferential direction while inserted into the punched hole, thereby polishing the sheared end surface of the punched hole. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2006-104551 A [Patent Document 2] Patent Publication No. 2022-42631 Summary of the Invention [Problem to be solved by the invention]

[0005] In the method disclosed in Patent Document 1, there are problems such as the high device cost of the ultrasonic vibrator that strikes the shearing end face of the notch, and the difficulty of accurately striking the shearing end faces of a plurality of notches. Further, although the method disclosed in Patent Document 2 can surely improve the fatigue strength because it forms a circular punching hole and polishes the shearing end face after the formation, there is a problem of high device cost because a special mold is used.

[0006] The present invention has been made to solve the above problems, and an object thereof is to provide a method for manufacturing a metal plate structure that can surely improve the fatigue strength of the shearing end face of a metal plate without requiring a large device cost.

Means for Solving the Problems

[0007] (1) The method for manufacturing a metal plate structure according to the present invention manufactures the structure by improving the fatigue strength of a portion where fatigue failure is a concern in a structure using a metal plate, a fatigue failure risk site specifying step of specifying, as a fatigue failure risk site, a portion on the shearing end face of the metal plate where stress concentration occurs during use of the structure and fatigue failure is a concern; a compressive residual stress applying step of applying a compressive residual stress to the fatigue failure risk site by loading a load on the metal plate or the structure so that tensile plastic strain concentrates on the specified fatigue failure risk site, generating tensile plastic strain in the fatigue failure risk site, and then unloading; and is characterized by including this.

[0008] (2) In the above (1), the fatigue failure risk site specifying step includes a fatigue strength specifying step of specifying the fatigue strength of the metal plate; a stress analysis step of performing a stress analysis for calculating the stress of the shearing end face of the metal plate during use of the structure; Of the shear end faces of the metal plate in the structure for which the stress has been calculated, a fatigue fracture risk site specifying step of specifying, as a fatigue fracture risk site where fatigue fracture is a concern, a site that exceeds the fatigue strength specified in the fatigue strength specifying step.

[0009] (3) In the case of the one described in the above (1) or (2), When a notch is provided at the end of the metal plate at a flat part of the structure, and the fatigue fracture risk site specified in the fatigue fracture risk site specifying step is the shear end face of the notch, the compressive residual stress applying step applies a tensile load in a direction parallel to the tangent of the fatigue fracture risk site in the notch, applies a load that causes bending deformation such that the fatigue fracture risk site becomes the bending outer side within the plane of the metal plate, or applies a load that causes bending deformation in the out-of-plane direction to a site including the fatigue fracture risk site along the edge of the notch, either alone or in combination, to generate tensile plastic strain in the fatigue fracture risk site.

[0010] (4) In the case of the one described in the above (1) or (2), When a circular or elliptical hole is provided in the metal plate at a flat part of the structure, and the fatigue fracture risk site specified in the fatigue fracture risk site specifying step is the shear end face of the hole, the compressive residual stress applying step applies a tensile load in a direction parallel to the tangent of the fatigue fracture risk site in the hole, or applies a load that causes bending deformation in the out-of-plane direction to a site including the fatigue fracture risk site along the edge of the hole, either alone or in combination, to generate the tensile plastic strain in the fatigue fracture risk site.

[0011] (5) In the case of the one described in the above (1) or (2), The compressive residual stress application step generates the tensile plastic strain in the fatigue fracture risk site by forming a plurality of beads so as to sandwich the fatigue fracture risk site in a direction parallel to the tangent line of the fatigue fracture risk site.

[0012] (6) In the above (1) to (5), The compressive residual stress application step is such that, with respect to the plate thickness t (mm) of the metal plate, the average gradient of the tensile plastic strain in the range from the fatigue fracture risk site to 2t (mm) in the direction orthogonal to the tangent line of the fatigue fracture risk site is 0.004 / 2t (mm -1 ) or more, and the tensile plastic strain is generated in the fatigue fracture risk site.

[0013] (7) In the above (1) to (6), The compressive residual stress application step is characterized in that the tensile plastic strain is generated in the fatigue fracture risk site so that the absolute value of the compressive residual stress applied to the fatigue fracture risk site is 40% or more of the tensile strength of the metal plate.

[0014] (8) In the above (1) to (7), The compressive residual stress application step is characterized in that the tensile plastic strain is generated in the fatigue fracture risk site so that the plate thickness reduction rate at the fatigue fracture risk site is 10% or less.

[0015] (9) In the above (1) to (8), The metal plate is characterized in that it has a tensile strength of 780 MPa or more.

[0016] (10) In the above (9), The metal plate is characterized in that it has a work hardening coefficient of 0.2 or less in the plastic strain range of 4% or more and 6% or less.

Advantages of the Invention

[0017] In the present invention, a part on the shearing end face of a metal plate, where stress concentration occurs during the use of a structure using the metal plate and fatigue failure is a concern, is specified as a fatigue failure risk part. Then, a load is applied to the specified fatigue failure risk part so that plastic tensile strain concentrates thereon, and after generating plastic tensile strain, the load is removed to impart compressive residual stress to the fatigue failure risk part. Thereby, even when stress concentrates on the fatigue failure risk part during the use of a structure using the metal plate, the progress of cracks at the fatigue failure risk part can be suppressed, so that a structure with improved fatigue strength can be manufactured. Further, according to the present invention, since compressive residual stress is imparted to the fatigue failure risk part, the delayed fracture characteristics of the fatigue failure risk part in the structure can also be improved. Furthermore, according to the present invention, since plastic tensile strain can be generated in the fatigue failure risk part to cause work hardening, the yield strength of the fatigue failure risk part increases, so that the deformation strength when a single load is input to the structure can also be improved.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

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Figure 8-1

Figure 8-2

Figure 8-3

Figure 8-4

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Figure 12

Modes for Carrying Out the Invention

[0019] <Background of the Invention> As exemplified in FIG. 2, at the shearing end face 13a of the notch bottom in the notch 13 formed by shearing the end of the metal plate 11, tensile residual stress is generated in the tangential direction in the plane of the metal plate 11, so it is known that the fatigue strength decreases. Therefore, when a load is applied during the use of a structure using such a metal plate 11 and stress concentrates at the notch bottom of the notch 13, there is concern that a crack 15 will progress in a direction perpendicular to the tangent of the shearing end face 13a of the notch bottom in the notch 13 and lead to fatigue failure.

[0020] Therefore, in order to improve the fatigue strength of a structure using a metal plate, it is considered necessary to improve the fatigue strength of the shear end face of the metal plate where fatigue failure is a concern. Thus, the inventor examined a method for improving the fatigue strength of the shear end face of the metal plate. As a result, the inventor considered that it might be possible to eliminate the tensile residual stress at the shear end face of the metal plate and change it to compressive residual stress, and further examined a specific method therefor.

[0021] As a result of repeated examinations in this way, the inventor came up with the idea of applying a load so that plastic tensile strain concentrates in a direction perpendicular to the direction (left direction in the drawing plane) in which the crack 15 propagates, to the shear end face 13a at the notch bottom of the metal plate 11 where fatigue failure is a concern, as shown in FIG. 2. Then, it was found that when unloading after locally generating plastic tensile strain in the shear end face 13a at the notch bottom, compressive residual stress is applied to the shear end face 13a at the notch bottom and the fatigue strength is improved. The present invention has been made based on the above examinations, and specific configurations will be described below.

[0022] <Method for manufacturing a metal plate structure> The method for manufacturing a metal plate structure according to an embodiment of the present invention manufactures the structure by improving the fatigue strength of a portion where fatigue failure is a concern in a structure using a metal plate. And the method for manufacturing a metal plate structure according to the present embodiment includes a fatigue failure risk site identification step S1 and a compressive residual stress application step S3, as shown in FIG. 1. Hereinafter, as an example, with reference to FIG. 3, each of the above steps will be described for a structure using a flat metal plate 11 in which a notch 13 is formed at the end of the metal plate by shearing.

[0023] ≪Fatigue failure risk site identification step≫ The fatigue failure risk site identification step S1 is a step of identifying the shear end face 13a of the metal plate 11 where stress concentration occurs during use of the structure and fatigue failure is a concern as the fatigue failure risk site 17.

[0024] The fatigue fracture risk area 17 can be specified by, for example, as shown in FIG. 4, a fatigue strength specifying step S11, a stress analysis step S13, and a fatigue fracture risk area specifying step S15.

[0025] (Fatigue strength specifying step) The fatigue strength specifying step S11 is a step of specifying the fatigue strength of the metal plate 11. The fatigue strength specifying step S11 can, for example, specify the fatigue strength by performing a fatigue test on a test piece manufactured by shearing the metal plate.

[0026] The fatigue test method using a test piece for specifying the fatigue strength is not particularly limited. For example, the method specified in JIS Z 2275 (a fatigue test using a test piece with a sheared end face and targeting a member that receives a bending load during use) can be applied.

[0027] Also, when the required fatigue life of the structure is known in advance, instead of performing a fatigue test using the above test piece, the time strength at the required fatigue life of the structure may be specified as the fatigue strength of the metal plate.

[0028] (Stress analysis step) The stress analysis step S13 is a step of performing a stress analysis to calculate the stress of the sheared end face 19 of the metal plate 11 when the structure is in use.

[0029] In the stress analysis step S13, a finite element model of the structure is created, and a stress analysis using the finite element method is performed for input conditions (for example, load conditions input to the structure) that simulate the use environment of the structure. Then, by the stress analysis, the stress for each element or each node corresponding to the sheared end face 19 of the metal plate 11 in the finite element model of the structure can be calculated. Note that, as the analysis method of the stress analysis, for example, elastic analysis or elastoplastic analysis using the static condensation method can be applied.

[0030] (Fatigue fracture risk area specifying step) The fatigue fracture risk part specifying step S15 is a step of specifying, as a fatigue fracture risk part 17 where fatigue fracture is a concern, a part that exceeds the fatigue strength specified in the fatigue strength specifying step S11 among the shear end faces 19 of the metal plate 11 for which stress was calculated in the stress analysis step S13. In the present embodiment, among the shear end faces 19 of the metal plate 11 for which stress was calculated in the stress analysis step S13, the stress of the shear end face 13a at the bottom of the notch in the notch part 13 exceeded the fatigue strength specified in the fatigue strength specifying step S11. Therefore, in the fatigue fracture risk part specifying step S15, the shear end face 13a at the bottom of the notch in the notch part 13 is specified as the fatigue fracture risk part 17.

[0031] ≪Compressive Residual Stress Applying Step≫ The compressive residual stress applying step S3 is a step of applying a load to the structure so that tensile plastic strain concentrates on the fatigue fracture risk part 17 specified in the fatigue fracture risk part specifying step S1, and generating tensile plastic strain in the fatigue fracture risk part 17. Further, the compressive residual stress applying step S3 is a step of applying compressive residual stress to the fatigue fracture risk part 17 by unloading after generating tensile plastic strain in the fatigue fracture risk part 17.

[0032] In the present embodiment, as shown in FIG. 3, by applying a tensile load in a direction parallel to the tangent of the shear end face 13a specified as the fatigue fracture risk part 17 to the metal plate 11, tensile plastic strain is generated so as to concentrate on the fatigue fracture risk part 17. Here, the direction parallel to the tangent of the fatigue fracture risk part 17 means the direction perpendicular to the direction in which the crack 15 propagates from the shear end face 13a at the bottom of the notch in the plane of the metal plate 11 (FIG. 2). When tensile plastic strain is generated in the fatigue fracture risk part 17 in this way (FIG. 3(a)), by unloading the tensile load, compressive residual stress in the direction opposite to the tensile plastic strain is applied in the direction parallel to the tangent of the fatigue fracture risk part 17 (FIG. 3(b)).

[0033] <Reasons for being able to improve fatigue strength> The reason why a structure with improved fatigue strength at a site where fatigue failure is a concern in a structure using a metal plate can be manufactured by the method for manufacturing a metal plate structure according to the present embodiment will be described below.

[0034] As described above, at the shearing end face 13a of the notch bottom in the notch portion 13 obtained by shearing the end portion of the metal plate 11, tensile residual stress is generated in the direction parallel to the tangent line of the shearing end face 13a. Therefore, in the present embodiment, in the compressive residual stress applying step S3, as shown in Fig. 3(a), a load is applied so that tensile plastic strain concentrates in the direction of the tangent line of the fatigue failure risk site 17. As a result, tensile plastic strain is locally generated at the fatigue failure risk site 17. Then, when unloading is performed after generating tensile plastic strain in the direction of the tangent line of the fatigue failure risk site 17, the periphery of the fatigue failure risk site 17 tries to return to its original shape. Therefore, the fatigue failure risk site 17 is pushed from the periphery and compressed in the direction opposite to the direction in which the tensile load is applied, as shown in Fig. 3(b). In this way, at the shearing end face 13a of the notch bottom of the notch portion 13 after generating tensile plastic strain and unloading, the tensile residual stress in the tangential direction generated by the shearing process of the notch portion 13 is eliminated, and compressive residual stress is applied in the tangential direction. Fig. 5 shows an example of measuring the magnitude of the tensile residual stress after shearing process and the compressive residual stress applied after generating tensile plastic strain and unloading, which are generated at the shearing end face 13a of the notch bottom in the notch portion 13, by the X-ray residual stress measurement method using the cosα method.

[0035] As a result, even if stress concentrates on the shearing end face 13a of the metal plate 11 specified as the fatigue failure risk site 17 during the use of the structure, the progress of cracks can be suppressed, and a structure with improved fatigue strength can be manufactured.

[0036] Further, according to the method for manufacturing a metal plate structure according to the present embodiment, since compressive residual stress is generated in the fatigue failure risk site 17, the delayed fracture characteristics at the fatigue failure risk site 17 can also be improved.

[0037] Furthermore, according to the method for manufacturing a metal plate structure according to the present embodiment, work hardening can be caused by generating tensile plastic strain in the fatigue fracture risk site 17, and the yield strength of the fatigue fracture risk site 17 can be increased. Thereby, the deformation strength of the fatigue fracture risk site 17 in the metal plate 11 when a single load is input to the structure can also be improved.

[0038] In the above description, in the fatigue fracture risk site identification step S1, the fatigue strength identification step S11 and the stress analysis step S13 are performed, and the shear end face 13a of the metal plate 11 where the stress obtained by the stress analysis exceeds the fatigue strength is identified as the fatigue fracture risk site 17.

[0039] However, in the present invention, when the site where fatigue fracture is a concern in the metal plate structure is known in advance, the fatigue fracture risk site identification step may identify the fatigue fracture risk site without performing the fatigue strength identification (S11) and the stress analysis (S13).

[0040] Also, in the above description, in the compressive residual stress application step S3, a uniform tensile load is applied to the metal plate 11 so that tensile deformation occurs in a direction parallel to the tangent of the fatigue fracture risk site 17 in the plane of the flat metal plate 11. This is because the notch portion 13 is originally a stress concentration portion, so simply applying a uniform load to the metal plate 11 can concentrate plastic strain on the shear end face 13a at the bottom of the notch in the notch portion 13 identified as the fatigue fracture risk site 17.

[0041] However, in the compressive residual stress application step S3, the load may be applied so that tensile plastic strain concentrates on the fatigue fracture risk site 17. For example, as shown in FIG. 6(a), a load may be applied to cause a bending deformation (in-plane bending deformation) in which the fatigue fracture risk site 17 becomes the outer side of the bend in the plane of the metal plate 11. Alternatively, as shown in Fig. 6(b), a load may be applied to bend the metal plate 11 in the out-of-plane direction (out-of-plane bending deformation, stretch flange processing) along the edge of the notch 13 so that the shear end face 13a, which is the fatigue fracture risk site 17, has a V shape. Furthermore, in the compressive residual stress application step, a load may be applied by combining tensile deformation (Fig. 3(a)), in-plane bending deformation (Fig. 6(a)), or out-of-plane bending deformation (Fig. 6(b)).

[0042] Also, in the compressive residual stress application step, as shown in Fig. 6(c), two beads 21 may be formed so as to sandwich the fatigue fracture risk site 17 in the tangential direction of the shear end face 13a, thereby generating tensile plastic strain in the fatigue fracture risk site 17.

[0043] Moreover, the above description was for the case of the shear end face 13a of the notch 13 formed at the end of the flat metal plate 11, but the present invention does not limit the shape of the shear end face in the metal plate. Therefore, even when a shear end face sheared in a substantially linear shape or a shear end face of a hole punched in a circular shape is specified as the fatigue fracture risk site, a load may be applied so as to concentrate tensile plastic strain on these shear end faces.

[0044] For example, when there is a risk of fatigue fracture occurring at the shear end face 33a in the hole 33 of the sheared flat metal plate 31 as shown in Fig. 7, a load is applied so that tensile plastic strain concentrates on the shear end face 33a of the hole 33 specified as the fatigue fracture risk site 35. Then, after generating tensile plastic strain in the fatigue fracture risk site 35, the load is removed to apply compressive residual stress. Thereby, in a structure using the flat metal plate 31 in which the hole 33 is formed, the fatigue strength of the shear end face 33a of the hole 33 where fatigue fracture is a concern can be improved, and the structure can be manufactured.

[0045] In order to apply a load so that tensile plastic strain concentrates on the shear end face 33a of the hole portion 33, as shown in Fig. 7(a), bending deformation (out-of-plane bending deformation, flanging process) may be performed in the out-of-plane direction along the edge of the hole portion 33. Alternatively, as shown in Fig. 7(b), two beads 37 may be formed so as to sandwich the hole portion 33 in a direction parallel to the tangent of the shear end face 33a.

[0046] Also, although the hole portion 33 shown in Fig. 7 is circular, the present invention is not limited to this, and an elliptical hole portion may be used.

[0047] In the present invention, in the compressive residual stress application step, plastic strain may be generated so that the absolute value of the compressive residual stress at the fatigue fracture risk site after tensile plastic strain is generated is 40% or more of the tensile strength of the metal plate (for example, 392 MPa or more in the case of a steel plate with a tensile strength of 980 MPa). Since tensile residual stress exists on the sheared shear end face, even if yielding is performed once to cause stress redistribution, there is an effect of suppressing the propagation of fatigue cracks. And this is because if the compressive residual stress is 40% or more of the tensile strength in absolute value, the effect of suppressing the propagation of fatigue cracks at the shear end face of the metal plate can be sufficiently obtained. This point will be specifically described in the examples described later.

[0048] In the present invention, after locally generating tensile plastic strain at the fatigue fracture risk site and then unloading, the driving force of the compressive residual stress generated at the fatigue fracture risk site is the difference in tensile plastic strain between the fatigue fracture risk site and its surroundings. For this reason, even if local tensile plastic strain is generated at the fatigue fracture risk site, if the difference in tensile plastic strain between the fatigue fracture risk site and its surroundings is small, there is a risk that sufficient compressive residual stress cannot be applied to the fatigue fracture risk site after unloading. Therefore, regarding the conditions for generating tensile plastic strain at the fatigue fracture risk site, it is sufficient that a difference from the tensile plastic strain of its surroundings can be obtained to such an extent that sufficient compressive residual stress is applied.

[0049] Regarding this point, attention is paid to the average gradient of the plastic strain in the direction orthogonal to the tangent of the fatigue fracture dangerous part, and the relationship between the compressive residual stress and the average gradient of the plastic strain in the fatigue fracture dangerous part after generating and unloading the plastic strain in tension is described. Here, the average gradient of the plastic strain is obtained by dividing the difference between the plastic strain in tension on the shear end face and the plastic strain in tension at a position 2t (mm) away from the shear end face in the direction orthogonal to its tangent by the distance 2t (mm) with respect to the plate thickness t (mm) of the metal plate. The average gradient (mm -1 ) is defined as such.

[0050] Through FEM analysis targeting the test piece model 71 (1 / 4 model) with a plate thickness t = 3 mm provided with a semi-circular notch 73 (punching radius R) shown in Fig. 8-1, a tensile deformation with a displacement amount of 0.18 to 2.0 mm was applied to the test piece model 71, and the average gradient of the plastic strain in tension was obtained. Furthermore, the residual stress at the fatigue fracture dangerous part 75 (shear end face 73a at the bottom of the notch) after unloading was obtained.

[0051] Fig. 8-2 shows (a) the distribution of the plastic strain in tension and (b) an example of the residual stress distribution in the direction orthogonal to the tangent of the fatigue fracture dangerous part 75 (Y direction in Fig. 8-1) (punching radius R = 10 mm). The horizontal axis in both cases indicates the distance in the direction orthogonal to the tangent from the bottom of the notch (shear end face). The distribution of the plastic strain in the range from the bottom of the notch (distance = 0 mm) to a distance 2t (= 6 mm) has the positive and negative slopes reversed at a displacement amount of 1.64 mm or more, and the plastic strain becomes larger inside than at the bottom of the notch (distance Y = 0 mm). This is caused by the necking of the test piece accompanying the tensile deformation being different depending on the stress state (bottom of the notch: plane stress state, inner part of the test piece: plane strain stress state). Also, the compressive residual stress was the largest value at the bottom of the notch (distance Y = 0 mm).

[0052] Also, Fig. 8-3 shows (a) the relationship between the compressive residual stress at the notch bottom and the displacement of tensile deformation, and (b) the relationship between the average gradient of plastic strain and the displacement of tensile deformation. The compressive residual stress at the notch bottom increases with the displacement of tensile deformation, but approaches a constant value when it exceeds a predetermined displacement. On the other hand, as described above, the average gradient of plastic strain reverses its sign when it exceeds a predetermined displacement. The dashed line in Fig. 8-3(b) indicates the maximum value of the average gradient of plastic strain during the tensile deformation process.

[0053] Fig. 8-4 shows the relationship between the compressive residual stress at the notch bottom and the maximum value of the average gradient of plastic strain during the tensile deformation process. As shown in Fig. 8-4, a strong correlation is observed between the two. This indicates that the compressive residual stress generated at the fatigue failure risk site 75 can be adjusted using the average gradient of tensile plastic strain in the range from the fatigue failure risk site to 2t (mm) in the direction perpendicular to the tangent of the notch bottom (fatigue failure risk site) (Y direction in Fig. 8-1) as an index.

[0054] Thus, as a specific condition for generating tensile plastic strain, with respect to the plate thickness t (mm) of the metal plate, the average gradient of tensile plastic strain in the range from the fatigue failure risk site to 2t (mm) in the direction perpendicular to the tangent of the fatigue failure risk site should be 0.004 / 2t (mm -1 ) or more. Here, the average gradient of plastic strain refers to the maximum value during deformation among the average gradients given by the plastic strain at the fatigue failure risk site and the tensile plastic strain at a position 2t away from the fatigue failure risk site. For example, in the case of a steel plate with a plate thickness t = 3 mm and a tensile strength of 980 MPa, to generate a compressive residual stress whose absolute value at the fatigue failure risk site is 40% (= 392 MPa) or more of the tensile strength, the average gradient of tensile plastic strain should be 0.004 / 2t = 6.7×10 -4 (mm -1 ) or more. By generating tensile plastic strain in this way, sufficient compressive residual stress can be imparted due to the difference in plastic strain between the fatigue failure risk site and its surroundings.

[0055] Furthermore, in the compressive residual stress application step, it is preferable to generate tensile plastic strain so that the plate thickness reduction rate at the fatigue fracture risk site after generating and then unloading tensile plastic strain is 10% or less. This is because if necking occurs with the plate thickness reduction rate exceeding 10% due to the generation of tensile plastic strain, cracks are likely to occur from the necking site, and the fatigue strength of the shear end face may conversely decrease.

[0056] In the present invention, there is no particular limitation on the type of metal plate used as the material of the structure. In particular, it can be preferably applied to structures using high-strength metal plates. This is because the tensile residual stress introduced into the metal plate by shearing is greater for higher-strength metal plates, and the effect of applying the present invention to eliminate the tensile residual stress generated on the shear end face of the metal plate after shearing becomes greater. Therefore, in the present invention, the metal plate preferably has a tensile strength of 780 MPa or higher.

[0057] Furthermore, when a load is applied to a structure using a metal plate and local plastic deformation starts at the fatigue fracture risk site, a metal plate with a smaller work hardening coefficient is more likely to have the fatigue fracture risk site hardened and induce plastic deformation in the surrounding area, resulting in plastic strain concentration and an increased likelihood of fatigue fracture. Therefore, for the metal plate of the structure to which the present invention is applied, the work hardening coefficient in the plastic strain range of 4% or more and 6% or less is preferably 0.2 or less.

[0058] In this embodiment, as shown in FIG. 6, the compressive residual stress application step S3 is performed on the shear end face 13a of the metal plate 11 having the fatigue fracture risk site 17 specified in the fatigue fracture risk site identification step S1 before assembling the structure.

[0059] However, from the perspective of the manufacturing cost for applying compressive residual stress to the shear end face of the metal plate, among the plurality of steps for manufacturing the structure, in the step of press-forming the metal plate into the shape of the components constituting the structure, it is advisable to apply compressive residual stress to the fatigue fracture risk site.

[0060] However, among the plurality of steps for manufacturing the metal plate structure, there may be a step (for example, bending of the metal plate, etc.) in which tensile plastic strain occurs on the shearing end face of the metal plate specified as the fatigue fracture risk site. If such a step is performed after the compressive residual stress application step according to the present invention, the stress state will be different from the compressive residual stress applied to the fatigue fracture risk site, and there is a risk that the fatigue strength of the fatigue fracture risk site cannot be improved.

[0061] Therefore, it is preferable that the compressive residual stress application step is carried out after the step in which tensile plastic strain occurs at the fatigue fracture risk site of the metal plate among the plurality of steps for manufacturing the structure.

[0062] Furthermore, in the present invention, the compressive residual stress application step may be to load the structure with a load so that tensile plastic strain concentrates on the fatigue fracture risk site of the metal plate after assembling the structure using the metal plate.

Example

[0063] An experiment was conducted to confirm the effects of the present invention, and this will be described below.

[0064] In Example 1, a fatigue test was performed on a test piece 41 having the shape shown in FIG. 9 as a test subject, and the fatigue strength was evaluated. The test piece 41 was made of a hot-rolled steel sheet with a tensile strength of 1000 MPa class, a plate thickness t = 3 mm, and a work hardening coefficient of 0.1 in the plastic strain range of 4% - 6%, which was processed into a rectangular shape of 120 mm × 30 mm as a test material. Then, semi-circular notches 43 with a punching radius R = 10, 20, or 30 mm were sheared at both ends of the central part of the test piece 41, and three types of test pieces with different punching radii R were produced. Here, the clearance during shearing was 10%.

[0065] Next, a fatigue test was conducted to specify the fatigue strength of the hot-rolled steel sheet used as the test material. In the fatigue test, a repeated load was applied to the test piece 41 in a fully reversed bending manner, and the repetition frequency was set to 20 Hz. Then, when the time strength at 300,000 repetitions of the load application was determined as the fatigue strength, it was 300 MPa.

[0066] Subsequently, the shear end face 43a at the notch portion 43 of the test piece 41 was defined as the fatigue fracture risk site 45, and a compressive residual stress was applied. Here, since stress concentration occurs at the notch portion 43 of the test piece 41 and fatigue fracture occurs, without performing the stress analysis step S13 and the fatigue fracture risk site identification step S15 described in the embodiment, the shear end face 43a of the notch portion 43 was identified as the fatigue fracture risk site 45.

[0067] In Example 1, as shown in FIG. 9, by applying various displacements shown in Table 1 to both ends in the longitudinal direction of the test piece 41, a load was applied so that tensile plastic strain concentrated on the shear end face 43a of the notch portion 43 identified as the fatigue fracture risk site 45. Then, after generating tensile plastic strain at the fatigue fracture risk site 45, unloading was performed to apply a compressive residual stress to the fatigue fracture risk site 45 (Inventive Examples 1 to 10).

[0068] For example, in Inventive Example 3, the equivalent plastic strain gradient in the region from the shear end face 43a of the notch bottom in the notch portion 43 to a distance of 2t (= 6 mm) was 0.00303 mm -1 (0.0188 / 2t), and the residual stress generated on the shear end face 43a was -653 MPa.

[0069] Also, in Example 1, as a comparison, test pieces 41 with the notch portion 43 formed as they were, without generating tensile plastic strain on the shear end face 43a of the notch portion 43 of the test piece 41, were used as Comparative Examples (Comparative Examples 1, 2, 4). Further, test pieces 41 under conditions where a load was applied so that tensile plastic strain concentrated on the shear end face 43a of the notch bottom in the notch portion 43 and no compressive residual stress was applied after unloading were used as Comparative Examples 3 and 5.

[0070] Then, fatigue tests were conducted on each of the test pieces 41 of Invention Examples 1 to 10 and Comparative Examples 1 to 5. The fatigue test was performed as plane bending in an out-of-plane bending mode, and a repeated load was applied to the test piece 41 in a full reverse manner. Here, the repeated load was applied at a repetition frequency of 2 Hz, and the stress amplitude at the end of the test piece 41 was set to 400 MPa. Then, in the fatigue test, those that reached 500,000 times of repeated load application without breakage of the test piece 41 were judged as qualified, and the test was terminated at 1,000,000 times. Table 1 shows the fatigue test results.

[0071]

Table 1

[0072] As shown in Table 1, in any of Invention Examples 1 to 9, since the number of repetitions until breakage reached 500,000 times or more, it was judged as qualified. In particular, in Invention Examples 2 to 3 and 6, there was no breakage even at 1,000,000 times of repetition, and the results were good. In Invention Examples 2 to 3 and 6, the average gradient of the tensile plastic strain was 0.00076 mm -1 or more (0.0047 / 2t mm -1 ), the absolute value of the compressive residual stress was 407 MPa or more, and the ratio of the absolute value of the compressive residual stress to the tensile strength of the metal plate was 0.4 or more (40% or more). On the other hand, in Comparative Examples 1 to 5, in all cases, breakage occurred at the shear end face 43a of the notch portion 43 before reaching 500,000 times of repetition, and it was judged as unqualified.

[0073] Comparing Invention Examples 3 and 4 with a punching radius R = 10 mm, Invention Example 4 with a larger absolute value of the compressive residual stress had necking occur, a plate thickness reduction rate exceeding 10%, and broke after 800,000 cycles of repetition. On the other hand, in Invention Example 3, necking did not occur, the plate thickness reduction rate was also 10% or less, and it did not break even after 1,000,000 cycles of repetition, showing better results than Invention Example 4. Similarly, when comparing Invention Examples 6 and 7 with a punching radius R = 20 mm and Invention Examples 9 and 10 with a punching radius R = 30 mm respectively, Invention Examples 6 and 9 with a plate thickness reduction rate of 10% or less did not break even after 1,000,000 cycles of repetition, showing good results.

[0074] As described above, Example 1 targeted the metal plate before manufacturing the structure, but a simple structure in a state close to the metal plate is also assumed. Therefore, even in such a structure, from the results of Example 1, it was suggested that the fatigue strength could be improved to manufacture the structure of the metal plate.

Example

[0075] In Example 2, a fatigue test was conducted on the structure 51 shown in FIG. 10 as the test object, and the fatigue strength was evaluated.

[0076] The structure 51 is assembled in a square pipe shape using two metal plates 53 and 55 formed into parts with a U-shaped cross-sectional shape. The metal plates 53 and 55 used a hot-rolled steel sheet with a tensile strength of 780 MPa class, a plate thickness t of 3 mm, and a work hardening coefficient of 0.1 in the plastic strain range of 4% - 6% as the test material.

[0077] One of the metal plates 53 was sheared to form a φ10 mm hole 53b in the steel plate and formed into a part with a U-shaped cross-sectional shape. Here, the clearance during the shearing process for forming the hole 53b was set to 10%. In the structure 51 using the metal plate 53 with the hole 53b formed, since it is considered that stress concentrates on the shearing end face 53b1 of the hole 53b during its use, the shearing end face 53b1 was specified as the fatigue fracture risk site. Therefore, after forming it into a U-shaped cross-sectional shape, as shown in Fig. 10, two beads 53c were formed on the top plate portion 53a so as to sandwich the hole portion 53b, and tensile plastic strain was generated on the shearing end surface 53b1 of the hole portion 53b, and compressive residual stress was imparted by unloading.

[0078] Regarding the tensile plastic strain on the shearing end surface 53b1 after forming the bead 53c, the average plastic strain gradient up to a position 3t (= 9 mm) away from the shearing end surface 53b1 of the hole portion 53b was 0.15 mm. -1 It was. Also, regarding the residual stress of the shearing end surface 53b1, it is impossible to measure by the normal X-ray residual stress measurement method due to the angle problem. However, since the compressive residual stress on the surface of the metal plate 53 in the vicinity of the shearing end surface 53b1 specified as the fatigue fracture dangerous part was -500 MPa, it is considered that at least -100 MPa or less of compressive residual stress was imparted to the fatigue fracture dangerous part.

[0079] The other metal plate 55 was formed into a U-shaped cross-sectional shape component from a steel plate in the same manner as the metal plate 53. Then, the metal plate 53 formed into a U-shaped cross-sectional shape and the metal plate 55 were joined by arc welding into a square pipe shape, and the structure 51 was assembled (Inventive Example 2).

[0080] Then, regarding the structure 51 according to Inventive Example 2, a fatigue test was conducted as shown in Fig. 11. The fatigue test was performed by three-point bending in which both end sides of the top plate portion 53a in the metal plate 53 where the hole portion 53b was formed were fixed and a load was applied to the center of the metal plate 55, with single-sided vibration, a repetition frequency of 1 Hz for applying the load, and a nominal stress of 300 MPa on the plate thickness surface. In the fatigue test, when a visible crack was confirmed on the shearing end surface 53b1 of the hole portion 53b of the metal plate 53, it was determined as fracture, and when the number of repetitions reached 500,000 times without fracture on the shearing end surface 53b1, it was determined as qualified.

[0081] In Example 2, as shown in FIG. 12, a structure 61 was fabricated as Comparative Example 2 by arc welding a metal plate 55 to a metal plate 63 having a hole 63b formed in a top plate portion 63a without forming a bead, and assembling it into a square pipe shape. For the structure 61 according to Comparative Example 2, a fatigue test was also conducted under the same conditions as the structure 51 according to Invention Example 2 (see FIG. 11), and the presence or absence of fracture at the shearing end face 63b1 of the hole 63b was determined. Table 2 shows the fatigue test results.

[0082]

Table 2

[0083] As shown in Table 2, the structure 51 according to Invention Example 2 was determined to be qualified because no fracture occurred at the shearing end face 53b1 of the hole 53b even after 500,000 repetitions. In contrast, for the structure 61 according to Comparative Example 2, fracture occurred at the shearing end face 53b1 of the hole 63b after 300,000 repetitions, and it was determined to be unqualified.

[0084] As described above, from the results of Example 2, it was shown that according to the present invention, in a structure using a metal plate, the fatigue strength of a portion where fatigue fracture is a concern can be improved, and the structure can be manufactured.

Explanation of Reference Numerals

[0085] 1 Structure 11 Metal plate 13 Notch 13a Shearing end face 15 Crack 17 Fatigue fracture risk site 19 Shearing end face 21 Bead 31 Metal plate 33 Hole 33a Shearing end face 35 Fatigue fracture risk site 37 Bead 41 Test piece 43 Notch 43a Shearing end face 45 Fatigue fracture risk area 51 Structure 53 Metal plate 53a Top plate part 53b Hole part 53b1 Shearing end face 53c Bead 55 Metal plate 57 Fatigue fracture risk area 59 Strain concentration inducing bead 61 Structure 63 Metal plate 63a Top plate part 63b Hole part 63b1 Shearing end face 71 Specimen model 73 Notch part 73a Shearing end face 75 Fatigue fracture risk area

Claims

1. A method for manufacturing a metal plate structure for manufacturing a structure using a metal plate, which improves the fatigue strength of a part where fatigue failure is a concern in the structure, comprising: a fatigue failure risk site specifying step of specifying, as a fatigue failure risk site, a site on the shear end face of the metal plate where stress concentration occurs during use of the structure and fatigue failure is a concern; a compressive residual stress applying step of applying a tensile load to the metal plate or the structure so that tensile plastic strain concentrates on the specified fatigue failure risk site, generating tensile plastic strain in the fatigue failure risk site, and then unloading to apply compressive residual stress to the fatigue failure risk site; The compressive residual stress applying step generates the tensile plastic strain in the fatigue failure risk site by forming a plurality of beads so as to sandwich the fatigue failure risk site in a direction parallel to the tangent of the fatigue failure risk site. A method for manufacturing a metal plate structure, characterized by the above.

2. The fatigue failure risk site specifying step includes: a fatigue strength specifying step of specifying the fatigue strength of the metal plate; a stress analysis step of performing stress analysis to calculate the stress on the shear end face of the metal plate during use of the structure; a fatigue failure risk site specifying step of specifying, as a fatigue failure risk site where fatigue failure is a concern, a site on the shear end face of the metal plate in the structure where the stress has been calculated and that exceeds the fatigue strength specified in the fatigue strength specifying step. The method for manufacturing a metal plate structure according to Claim 1, characterized by having the above.

3. In the compressive residual stress imparting step, with respect to the plate thickness t (mm) of the metal plate, the average gradient of the tensile plastic strain in the range from the fatigue fracture risk site to 2t (mm) in the direction orthogonal to the tangent of the fatigue fracture risk site is 0.004 / 2t (mm -1 ), and the tensile plastic strain is generated in the fatigue fracture risk site so as to be equal to or more than the above value. A method for manufacturing a metal plate structure according to claim 1 or 2, characterized by this.

4. The compressive residual stress applying step generates the tensile plastic strain in the fatigue failure risk site so that the absolute value of the compressive residual stress applied to the fatigue failure risk site is 40% or more of the tensile strength of the metal plate. The method for manufacturing a metal plate structure according to Claim 1 or 2, characterized by the above.

5. The compressive residual stress applying step generates the tensile plastic strain in the fatigue failure risk site so that the plate thickness reduction rate at the fatigue failure risk site is 10% or less. The method for manufacturing a metal plate structure according to Claim 1 or 2, characterized by the above.

6. The metal plate has a tensile strength of 780 MPa or more. The method for manufacturing a metal plate structure according to Claim 1 or 2, characterized by the above.

7. The method for manufacturing a metal plate structure according to claim 6, wherein the metal plate has a work hardening coefficient in the plastic strain range of 4% or more and 6% or less of 0.2 or less.

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