Fastening components and design methods for fastening components

Abstract

To provide a fastening component made of two-phase stainless steel which hardly has its head part ruptured, and a design method for the fastening component.SOLUTION: There is employed a T-shaped fastening component that comprises a shaft part having a screw part and a head part connected to one end of the shaft part and extending orthogonally to the extension direction of the shaft part, and the fastening component is made of ferrite austenite two-phase stainless steel which has a 0.2% yield strength of 500 MPa or larger and a work cure index of 0.25 or smaller, wherein an S value found from S=(2×B×0.6) / A (1) exceeds 1.20. In the expression (1), A represents the cross sectional area of a cross section which is orthogonal to the length direction of the shaft part and has minimum cross sectional area, and B represents the cross sectional area of a cross section of the head part which is orthogonal to the length direction of the head part and also at an end point position of a corner part R where the one end of the shaft part and the head part are in contact from the corner part to a length-directional end part of the head part.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to fastening components and a method for designing fastening components.

Background Art

[0002] For example, T-head bolts and nuts are used as fastening components for the pipe joints of ductile cast iron pipes used in water pipes. The T-head bolt is composed of a shaft portion having a threaded portion and a head portion at one end of the shaft portion. The head portion extends in a direction orthogonal to the longitudinal direction of the shaft portion. And the T-head bolt has a T-shaped shape in appearance by the shaft portion and the head portion.

[0003] Also, as a fastening component other than the T-head bolt, an L-shaped fastening component is also known. The L-shaped fastening component is composed of a shaft portion having a threaded portion and a head portion at one end of the shaft portion. The head portion extends in a direction orthogonal to the longitudinal direction of the shaft portion. The L-shaped fastening component has an L-shaped shape in appearance by the shaft portion and the head portion.

[0004] As a simple specific example of a pipe joint using a T-head bolt, flange portions are provided at the ends of one pipe and the ends of the other pipe, respectively. In a state where the flange portions of both pipes are abutted, the T-head bolt is inserted into the through holes provided in the flange portions and fastened with nuts. The flange portion in the pipe joint is fastened in a state of being sandwiched between the head portion of the T-head bolt and the nut. For this reason, tensile stress is applied to the shaft portion of the T-head bolt in the pipe joint in the longitudinal direction of the shaft portion, and shear stress is applied to the head portion in a direction orthogonal to the longitudinal direction of the head portion.

[0005] The fastening components described above are generally manufactured by forging a steel bar into a predetermined shape, performing heat treatment to remove distortion, and further forming a threaded portion on the shaft. Conventionally, ductile cast iron, carbon steel, austenitic stainless steel such as SUS304, and martensitic stainless steel such as SUS403 have been used as materials for fastening components such as T-head bolts, but recently, austenitic stainless steel such as SUS304, which has excellent corrosion resistance, has been used as the material (for example, Patent Document 1).

[0006] However, while austenitic stainless steel offers excellent corrosion resistance, it contains expensive metals such as nickel, resulting in cost issues. Therefore, ferritic-austenitic duplex stainless steel has recently been attracting attention as an alternative to austenitic stainless steel. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2006-274295 [Overview of the project] [Problems that the invention aims to solve]

[0008] Ferritic-austenitic duplex stainless steel (hereinafter sometimes referred to as duplex stainless steel) has a low content of expensive metals such as nickel and its corrosion resistance is equivalent to that of austenitic stainless steel. However, when tensile fracture tests were performed on T-head bolts, it was found that T-head bolts made of austenitic stainless steel underwent ductile fracture at the shaft, while T-head bolts made of duplex stainless steel underwent shear fracture at the head. In the case of austenitic stainless steel, the fracture point is the shaft, which is located inside the through-hole of the flange of the pipe fitting, so even if ductile fracture occurs, the impact on the surroundings is small. However, in the case of duplex stainless steel, the fracture point is the head, which is exposed to the outside of the pipe fitting, so there is a risk that fragments of the head will scatter and damage surrounding objects due to shear fracture of the head. This can occur not only with T-head bolts but also with L-shaped fastening components.

[0009] This invention has been made in view of the above circumstances, and aims to provide a fastening component made of duplex stainless steel that is less prone to fracture at the head, and a method for designing such a fastening component. [Means for solving the problem]

[0010] To solve the above problems, the present invention adopts the following configuration. [1] A T-shaped fastening component comprising a shaft having a threaded portion and a head connected to one end of the shaft and extending in a direction perpendicular to the extending direction of the shaft, It is made of ferritic-austenitic duplex stainless steel with a 0.2% yield strength of 500 MPa or higher and a work hardening index of 0.25 or less. A fastening component in which the S value of the following formula (1), calculated from the cross-sectional areas of the shaft and head, is greater than 1.20. S = (2 × B × 0.6) / A …(1) In equation (1), A is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the shaft portion at the position where the cross-sectional area is smallest, and B is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the head portion of the head portion at the end point of the corner R of the corner portion, extending from the corner portion where one end of the shaft portion and the head portion are in contact toward the longitudinal end of the head portion. [2] An L-shaped fastening component comprising a shaft having a threaded portion and a head connected to one end of the shaft and extending in a direction perpendicular to the extending direction of the shaft, It is made of ferritic-austenitic duplex stainless steel with a 0.2% yield strength of 500 MPa or higher and a work hardening index of 0.25 or less. A fastening component in which the S value calculated from the cross-sectional areas of the shaft and head, as shown in formula (2) below, is greater than 1.20. S = (B × 0.6) / A …(2) In equation (2), A is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the shaft portion at the position where the cross-sectional area is smallest, and B is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the head portion of the head portion at the end point of the corner R of the corner portion, extending from the corner portion where one end of the shaft portion and the head portion are in contact toward the longitudinal end of the head portion. [3] The fastening component according to [1] or [2], wherein the S value obtained by formula (1) or formula (2) is 1.50 or less.

[0011] [4] A method for designing a fastening component comprising a shaft having a threaded portion and a head connected to one end of the shaft and extending in a direction perpendicular to the extending direction of the shaft, As a prerequisite, the material of the fastening component is set to ferritic austenitic duplex stainless steel with a 0.2% yield strength of 500 MPa or more and a work hardening index of 0.25 or less. The first step is to set the shape of the fastening component having the shaft and the head as an initial setting, and then, based on the initial setting shape, predict the distribution of damage values ​​when tensile stress is applied to the shaft with the lower end of the head in the height direction fixed, by numerical simulation, and determine the maximum damage value at the corner where the shaft and the head meet just before the head breaks. When the maximum damage value at the corner exceeds the threshold value, the shape of the fastening component with the increased height of the head is reset, and the measurement of the damage value at the corner is repeated based on the reset shape of the fastening component until the maximum damage value becomes less than or equal to the threshold value in a second step. In the first step or the second step, when the maximum damage value at the corner is less than or equal to the threshold value, a third step of determining the shape of the fastening component having the height of the head in that case as the product shape is provided. A design method for a fastening component comprising the steps. [5] On the premise that the material of the fastening component is austenitic stainless steel, the damage value when a tensile stress is applied to the shaft portion with the lower end portion in the height direction of the head fixed is predicted by numerical simulation, and the maximum damage value at the corner where one end of the shaft portion and the head contact immediately before the head breaks is obtained, and a numerical value less than or equal to this maximum damage value is used as the threshold value. The design method of the fastening component according to [4]. [Effect of the Invention]

[0012] According to the present invention, it is possible to provide a fastening component made of duplex stainless steel in which breakage at the head is unlikely to occur and a design method for the fastening component. [Brief Description of the Drawings]

[0013] [Figure 1] FIG.1 is an enlarged schematic view showing a T-head bolt which is an example of a fastening component of an embodiment of the present invention. [Figure 2] FIG.2 is a cross-sectional view taken along line A-A of FIG.1. [Figure 3A] FIG.3A is a schematic view showing a T-head bolt broken at the shaft portion. [Figure 3B] FIG.3B is a schematic view showing a T-head bolt broken at the head. [Figure 4] FIG.4 is a diagram showing the distribution of damage values predicted by numerical simulation, where (a) is a distribution diagram of damage values of a T-head bolt made of duplex stainless steel, and (b) is a distribution diagram of damage values of a T-head bolt made of austenitic stainless steel. [Figure 5] FIG. 5 is a flowchart for explaining the design method of the fastening component of the present embodiment. [Figure 6] FIG. 6 is a schematic diagram showing a model of a tensile test when predicting damage values by numerical simulation in the design method of the fastening component of the present embodiment. [Figure 7] FIG. 7 is a diagram showing the distribution of damage values predicted by numerical simulation for an M24 T-head bolt made of duplex stainless steel. (a) is a distribution diagram of the reference damage values, (b) is a distribution diagram of the damage values when the head height is increased by 0.5 mm with respect to the reference, and (c) is a distribution diagram of the damage values when the head height is increased by 1.0 mm with respect to the reference. [Figure 8] FIG. 8 is a diagram showing the distribution of damage values predicted by numerical simulation for an M20 T-head bolt made of duplex stainless steel. (a) is a distribution diagram of the reference damage values, and (b) is a distribution diagram of the damage values when the head height is increased by 1.5 mm with respect to the reference.

MODE FOR CARRYING OUT THE INVENTION

[0014] There is known a T-shaped or L-shaped fastening component having a shaft portion provided with a threaded portion and a head portion provided at one end of the shaft portion. As an example of such a fastening component, there is a T-head bolt as shown in FIGS. 1 and 2. The T-head bolt 1 shown in FIGS. 1 and 2 has a shaft portion 2 having a threaded portion (not shown) and a head portion 3 extending in a direction orthogonal to the longitudinal direction of the shaft portion 2. And the T-head bolt 1 has a T-shaped shape in appearance by the shaft portion 2 and the head portion 3.

[0015] When stress is applied to such a T-head bolt 1 in a direction parallel to the longitudinal direction of the shaft, the shaft 2 may undergo ductile fracture as shown in Figure 3A, or the head 3 may undergo shear fracture as shown in Figure 3B. If the cross-sectional area of ​​the shaft 2 and the head 3 are the same, the head 3 will undergo shear fracture before the shaft 2 undergoes ductile fracture. Since the head 3 of the T-head bolt 1 is exposed to the outside, such as in a pipe joint, if the head 3 breaks, there is a risk that fragments will fly around and cause damage. Therefore, when designing a T-head bolt 1, it is necessary to design the shape so that it breaks at the shaft 2 and not at the head 3. Accordingly, the inventors investigated a fastening component that breaks at the shaft 2 before it breaks at the head 3.

[0016] In their investigation, the inventors focused on the damage value. The damage value is the amount of strain considering the stress during deformation, and for example, the D value given by the Cockcroft & Latham ductile fracture condition equation (M) below is known.

[0017]

number

[0018] In equation (M), σ max σ is the maximum principal stress (MPa), σ is the equivalent stress (MPa), and dε is the equivalent strain increment.

[0019] An example of the damage value distribution is shown in Figure 4. Figure 4 shows the damage value distribution predicted by numerical simulation, where (a) is the damage value distribution for a T-head bolt made of duplex stainless steel, and (b) is the damage value distribution for a T-head bolt made of austenitic stainless steel. Damage values ​​can be predicted by numerical simulation.

[0020] Figures 4(a) and 4(b) are both distribution diagrams of damage values ​​immediately before fracture. As shown in Figure 4(a), in the case of a T-head bolt made of duplex stainless steel, the damage value is maximum at the corner, which is the connection point between the shaft and the head, and this value (maximum damage value) is 0.92. On the other hand, as shown in Figure 4(b), in the case of a T-head bolt made of austenitic stainless steel, the maximum damage value at the corner is 0.35. A higher maximum damage value suggests that fracture is likely to occur at that location. In fact, in tensile tests, the head of a T-head bolt made of duplex stainless steel fractures, while the shaft of a T-head bolt made of austenitic stainless steel fractures. Therefore, the maximum damage value serves as an indicator to predict whether or not fracture will occur. The maximum damage value is influenced by the strength of the material and the cross-sectional area of ​​the head, but the inventors have found that, in addition to these, the work hardening index of the material also affects the critical damage value.

[0021] The following describes fastening components and design methods for fastening components, which are embodiments of the present invention. For convenience of explanation, the design method for fastening components will be described first, followed by the fastening components themselves.

[0022] (Design methods for fastening components) The fastening component design method of this embodiment, as illustrated in Figures 1 and 2, is a method for designing a fastening component comprising a shaft portion 2 having a threaded portion (not shown) and a head portion 3 connected to one end of the shaft portion 2 and extending in a direction perpendicular to the extending direction of the shaft portion 2, and consists of a first step, a second step, and a third step. Each step will be described below with reference to Figure 5.

[0023] In the first step, as a prerequisite, the material of the fastening component shall be ferritic-austenite duplex stainless steel with a 0.2% yield strength of 500 MPa or higher and a work hardening index of 0.25 or lower. In addition to the 0.2% yield strength and work hardening index, elongation and tensile strength may also be considered as mechanical properties of the material. In this case, the elongation may be 20% or higher and the tensile strength 750 MPa or higher.

[0024] Here, we will describe the work hardening index n value in detail. The fastening components of this embodiment focus on the work hardening index, which has not received much attention as a mechanical property of fastening component materials until now. As shown in Figure 4, we found that the large difference in the distribution location of the maximum damage value just before fracture between duplex stainless steel and austenitic stainless steel is due to the difference in work hardening index. When strain is applied to austenitic stainless steel, a hard martensitic transformation is likely to occur due to work induction. As this hard martensite is generated, the deformation is carried out by the surrounding soft austenite phase, and the deformation area expands. As a result, austenitic stainless steel has a relatively high work hardening index of about 0.43 and also high ductility. For this reason, when strain is applied to a T-head bolt made of austenitic stainless steel, a martensitic transformation occurs at the corner 5 (see Figure 1) due to work induction, and the corner 5 hardens more than the surrounding area. As further strain is applied, the hardened area expands, and the deformed area spreads even further around it, making it less likely for strain to concentrate at the corner 5. For this reason, T-head bolts made of austenitic stainless steel have a smaller maximum damage value at the corner 5, and shear failure at the head 3 is less likely to occur.

[0025] On the other hand, when strain is applied to a T-head bolt made of duplex stainless steel, the austenite phase in the structure of the duplex stainless steel transforms into martensite through work induction, causing hardening. However, because the softer ferrite phase deforms preferentially, the work hardening index is low, and the degree of hardening at the corner 5 is less than that of austenitic stainless steel. Therefore, the expansion of the deformation region with increasing strain does not spread as much as in austenitic stainless steel, and deformation progresses at the corner, causing strain to concentrate. As a result, the maximum damage value at the corner 5 of a T-head bolt made of duplex stainless steel becomes larger, making it easier to reach the critical damage value, and thus increasing the likelihood of shear failure at the head 3.

[0026] As described above, it has been found that the work hardening index affects shear failure of the head of a fastening component. The fastening component of this embodiment has a work hardening index of 0.25 or less, making it possible to design a fastening component that can prevent shear failure of the head even when using a material that is unfavorable to shear failure.

[0027] In addition to the above preconditions, in this embodiment, as shown in ST1 of Figure 5, the shape of the fastening component having a shaft and a head is initially set as an initial condition.

[0028] Next, in the first step, as shown in ST2 of Figure 5, the distribution of damage values ​​when tensile stress is applied to the shaft portion with the lower end of the head portion fixed in the height direction, under the above preconditions and the initial shape of the fastening component, is predicted by numerical simulation. Figure 6 shows a model diagram of when tensile stress is applied in the numerical simulation. With the lower end 3a of the head portion 3 in the height direction fixed by the jig 4, tensile stress is applied to the other end 2b of the shaft portion 2 along the longitudinal direction of the shaft portion 2. The jig 4 is positioned as close to the shaft portion 2 as possible. As a result, stress is applied to the shaft portion 2 in a direction parallel to the extension direction of the shaft portion 2. In addition, at the contact point with the jig 4, shear stress is applied to the head portion 3 in the direction perpendicular to the extension direction of the head portion 3, i.e., the longitudinal direction of the shaft portion 2.

[0029] Numerical simulation involves dividing the fastening component into multiple elements, performing FEM analysis to simulate the deformation of each element, and converting this deformation into strain. Then, based on the above equation (M), the damage value is calculated for each element from the start to the end of tensile stress application.

[0030] Then, the maximum damage value (maximum damage value) just before the head 3 breaks is determined. The maximum damage value (maximum damage value) is often obtained near the point where the end 4a on the shaft side of the jig 4 contacts the head 3, that is, at the corner 5 where one end 2a of the shaft 2 and the head 3 contact. However, depending on the shape of the head 3, the position where the maximum damage value (maximum damage value) is obtained may be slightly different.

[0031] Next, in ST3 of Figure 5, if the maximum damage value at corner 5 is below the threshold, the process proceeds to step 3; if the maximum damage value at corner 5 exceeds the threshold, the process proceeds to step 2.

[0032] The second step, as shown in ST3 and ST4 of Figure 5, is to reset the shape of the fastener with an increased head height 3 if the maximum damage value at the corner exceeds a threshold. The amount of increase in head height 3 is not particularly limited, but for example it may be 0.1 mm, 0.3 mm, 0.5 mm, or 1.0 mm. The fastener with increased head height has an increased cross-sectional area of ​​head 3 compared to the initial shape.

[0033] Next, as shown in ST2 to ST4 in Figure 5, the maximum damage value at the corner 5 is measured based on the reconfigured shape of the fastening component.

[0034] In the second step, the operation of resetting the shape of the fastening component with increased head height 3, and the operation of measuring the maximum damage value of the corner 5 based on the reset shape of the fastening component, are repeated until the maximum damage value falls below a threshold.

[0035] Specifically, based on the reconfigured shape of the fastening component in ST2 of Figure 5, the distribution of damage values ​​when tensile stress is applied to the shaft 2 with the lower end 3a in the height direction of the head 3 fixed in ST2 of Figure 5 is predicted by numerical simulation, and the maximum damage value at the corner 5 where one end 2a of the shaft 2 and the head 3 meet just before the head 3 breaks is determined. Then, in ST3, it is checked whether the maximum damage value is below a threshold. If the maximum damage value exceeds the threshold, the reconfiguration of the fastening component shape in ST4 and the prediction of damage values ​​in ST2 are repeated. On the other hand, if the maximum damage value is below the threshold, the process proceeds to the third step.

[0036] In the third step, as shown in ST5 of Figure 5, if the maximum damage value at the corner 5 in the first or second step falls below a threshold, the shape of the fastening component with the head height in that case is determined as the product shape.

[0037] The threshold value for damage in ST3 in Figure 5 may be determined by numerically simulating the damage value when tensile stress is applied to the shaft 2 with the lower end 3a in the height direction of the head 3 fixed, under the assumption that the material of the fastening component is austenitic stainless steel, as described earlier for ST1 and ST2. The maximum damage value at the corner 5 where one end 2a of the shaft 2 and the head 3 meet just before the head 3 breaks is determined, and a value less than or equal to this maximum damage value may be used as the threshold value.

[0038] As an example, Figure 4(b) shows a distribution of damage values ​​for a T-head bolt made of SUS304, an austenitic stainless steel. In Figure 4(b), the maximum damage value is 0.35, so the threshold can be set to 0.35 or less.

[0039] The following shows a specific example of the design method of this embodiment. Figure 7 is a diagram showing the distribution of damage values ​​predicted by numerical simulation for an M24 T-head bolt made of duplex stainless steel. Figure 7(a) is a standard damage value distribution diagram, Figure 7(b) is a damage value distribution diagram when the head height is increased by 0.5 mm compared to the standard in Figure 7(a), and Figure 7(c) is a damage value distribution diagram when the head height is increased by 1.0 mm compared to the standard in Figure 7(a). As the duplex stainless steel, NSSC2120 duplex stainless steel (manufactured by Nippon Steel Stainless Steel Corporation) with a 0.2% yield strength of 601 MPa, tensile strength of 793 MPa, elongation of 29.9%, and n-value of 0.19 was used.

[0040] As shown in Figure 7(a), the initial shape of the fastening component showed a maximum damage value of 0.42, exceeding the threshold of 0.35. Therefore, when the maximum damage value was measured for a shape with a head height 0.5 mm higher than the initial shape, as shown in Figure 7(b), the maximum damage value was 0.36, slightly exceeding the threshold of 0.35. Furthermore, when the head height was similarly measured for a shape with a head height 1.0 mm higher than the initial shape, as shown in Figure 7(c), the maximum damage value was 0.28, below the threshold of 0.35. The cross-sectional area of ​​the head in Figure 7(a) was 317 mm². 2 In the case of Figure 7(b), the length is 329 mm. 2 In the case of Figure 7(c), it is 340 mm. 2 Therefore, the head height was increased by 1.0 mm, and the cross-sectional area of ​​the head was increased to 340 mm². 2 A T-head bolt is expected to break at the shaft before it breaks at the head.

[0041] Next, another specific example of the design method is shown. Figure 8 shows the distribution of damage values ​​predicted by numerical simulation for an M20 T-head bolt made of duplex stainless steel. Figure 8(a) is the distribution of damage values ​​for the standard, and Figure 8(b) is the distribution of damage values ​​when the head height is increased by 1.5 mm compared to the standard in Figure 8(a). NSSC2120 duplex stainless steel was used.

[0042] As shown in Figure 8(a), the initial shape of the fastening component showed a maximum damage value of 0.92, significantly exceeding the threshold of 0.35. Therefore, when the damage value was measured for a shape with a head height 1.5 mm higher than the initial shape, the maximum damage value was 0.26, below the threshold of 0.35. The cross-sectional area of ​​the head in Figure 8(a) was 218 mm². 2 In the case of Figure 8(b), the length is 247 mm. 2 Therefore, the head height was increased by 1.5 mm, and the cross-sectional area of ​​the head was increased to 247 mm². 2 A T-head bolt is expected to break at the shaft before it breaks at the head.

[0043] Although detailed data is not shown, it is also possible to reduce damage values ​​by increasing the radius of curvature at the corners. For example, in an M20 T-head bolt made of NSSC2120, as shown in Figure 8(a), the initial shape of the fastening component showed a critical damage value of 0.92, and the shape with the head height increased by 1.5 mm from the initial shape showed a maximum damage value of 0.26. However, when the radius of curvature at the corners was increased from the original R3 to R5, the maximum damage value was 0.41, indicating that increasing the radius of curvature at the corners is effective. When improving solely by increasing the radius of curvature at the corners, the value depends on the bolt size and the shape of the mounting part, but a radius of curvature of R7 or greater is preferable.

[0044] Next, the fastening components of this embodiment will be described. As suggested in the design methods for fastening components, increasing the head height and thus the cross-sectional area of ​​the head is considered effective in preventing head fracture. However, the suitable range of head cross-sectional area varies greatly depending on the bolt size, making it difficult to generally determine the optimal head cross-sectional area.

[0045] The tensile stress σ in the shaft can be expressed by the following equation (3). Here, F is the tensile load in the longitudinal direction of the rod-shaped material, and A is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the shaft, at the position where the cross-sectional area is minimized. Incidentally, the head of a T-head bolt fractures due to shear. On the other hand, it is known that the shear fracture stress in the direction perpendicular to the longitudinal direction of a rod-shaped material is 0.6 times the tensile fracture stress when the rod-shaped material is pulled in the longitudinal direction. From the above relationship and equation (3), the shear area of ​​the head that receives a stress equivalent to the tensile stress of the shaft can be expressed as 0.6 times the cross-sectional area of ​​the head, as shown in equation (4). In order to avoid shear fracture of the head, the shear stress of the head must exceed the tensile stress of the shaft. On the other hand, the fracture strength of the shaft increases as the diameter of the shaft increases. Based on these findings, the inventors have discovered that, based on the relationship between the cross-sectional areas of the shaft and the head, fracture of the head can be prevented when the S value obtained by formula (1) below is greater than 1.20 for T-shaped fastening components, and when the S value obtained by formula (2) below is greater than 1.20 for L-shaped fastening components. Equations (1) and (2) are derived from the stress calculation formula and the relationship between tensile stress and shear stress.

[0046] S = (2 × B × 0.6) / A …(1) S = (B × 0.6) / A …(2) σ = F / A …(3) A = B × 0.6 …(4)

[0047] In other words, the fastening component in this embodiment may be a T-shaped fastening component or an L-shaped fastening component.

[0048] The T-shaped fastening component comprises a shaft having a threaded portion and a head connected to one end of the shaft and extending in a direction perpendicular to the extending direction of the shaft. The T-shaped fastening component is made of ferritic austenitic duplex stainless steel with a 0.2% yield strength of 500 MPa or more and a work hardening index of 0.25 or less, and must satisfy an S value greater than 1.20 as determined by the above formula (1).

[0049] In equation (1), A is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the shaft portion at the position where the cross-sectional area is minimized, and B is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the head portion at the end point of the corner radius R of the corner, extending from the corner where one end of the shaft portion and the head portion meet toward the longitudinal end of the head portion. Corner radius R refers to a region that has been machined with a predetermined radius of curvature, and the end point of corner radius R refers to the position of the head-side end of the curved region. Note that the cross-sectional areas A and B and corner radius R can be calculated, for example, by calculating the data from product CAD or by measuring the shape of the actual product using a 3D shape measuring machine.

[0050] The endpoint of the corner radius R is the position corresponding to the inner diameter of the installation hole when the fastening component is inserted into the installation hole. The radius of curvature of the corner radius R may be R3 or greater, R5 or greater, or R7 or greater. The larger the radius of curvature of the corner radius R, the closer the endpoint of the corner radius R will be to the longitudinal end of the head. If the endpoint of the corner radius R is unclear, the endpoint of the corner radius R may be set at a position 2 mm away from the corner in the longitudinal direction of the head.

[0051] Furthermore, the L-shaped fastening component comprises a shaft having a threaded portion and a head connected to one end of the shaft and extending in a direction perpendicular to the extending direction of the shaft. The L-shaped fastening component is made of ferritic austenitic duplex stainless steel with a 0.2% yield strength of 500 MPa or more and a work hardening index of 0.25 or less, and the S value obtained by formula (2) above must satisfy a value greater than 1.20.

[0052] In equation (2), A is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the shaft portion at the position where the cross-sectional area is minimized, and B is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the head portion at the end point of the corner R of the corner, extending from the corner where one end of the shaft portion and the head portion meet toward the longitudinal end of the head portion. The details of the definition of B in equation (2) may be the same as the details of the definition of B in equation (1).

[0053] The S value obtained by formula (1) or formula (2) is more preferably 1.25 or higher, even more preferably 1.30 or higher, and even more preferably 1.35 or higher. The higher the S value, the higher the probability that the fracture will occur in the shaft.

[0054] Furthermore, while a higher S value obtained by equation (1) or equation (2) is preferable, a higher S value increases the head height of the fastening component, i.e., the cross-sectional area of ​​the head, which increases the mass of the fastening component and thus the cost of the material. Therefore, an S value of 1.50 or less is preferable.

[0055] By satisfying the above conditions, it is possible to obtain a fastening component in which the shaft breaks before the head breaks.

[0056] Table 1 below shows an example of testing various shapes of T-head bolts as shown in Figure 1, and confirming the fracture location through tensile testing. The material of the T-head bolt is duplex stainless steel NSSC2120 (manufactured by Nippon Steel Stainless Steel Corporation). The bolt size is M24.

[0057] [Table 1]

[0058] As shown in Table 1, it was confirmed that fracture occurred in the shaft when the S value exceeded 1.20. [Explanation of symbols]

[0059] 1… Fastening component (T-head bolt) 2... Shaft 3...Head 4…Lower end of the head 5... Corner

Claims

1. A T-shaped fastening component comprising a shaft having a threaded portion, and a head connected to one end of the shaft and extending in a direction perpendicular to the extending direction of the shaft, It is made of ferritic-austenite duplex stainless steel with a 0.2% yield strength of 500 MPa or higher and a work hardening index of 0.25 or lower. Fastening components in which the S value calculated by the following formula (1) is greater than 1.

20. S=(2×B×0.6) / A…(1) In equation (1), A is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the shaft portion at the position where the cross-sectional area is smallest, and B is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the head portion of the head portion at the end point of the corner R of the corner portion, extending from the corner portion where one end of the shaft portion and the head portion are in contact toward the end of the head portion in the longitudinal direction.

2. An L-shaped fastening component comprising a shaft having a threaded portion, and a head connected to one end of the shaft and extending in a direction perpendicular to the extending direction of the shaft, It is made of ferritic-austenite duplex stainless steel with a 0.2% yield strength of 500 MPa or higher and a work hardening index of 0.25 or lower. Fastening components in which the S value calculated by the following formula (2) is greater than 1.

20. S=(B×0.6) / A…(2) In equation (2), A is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the shaft portion at the position where the cross-sectional area is smallest, and B is the cross-sectional area of ​​the section perpendicular to the longitudinal direction of the head portion of the head portion at the end point of the corner R of the corner portion, extending from the corner portion where one end of the shaft portion and the head portion are in contact toward the end of the head portion in the longitudinal direction.

3. The fastening component according to claim 1 or claim 2, wherein the S value obtained by formula (1) or formula (2) is 1.50 or less.

4. A method for designing a fastening component comprising a shaft having a threaded portion, and a head connected to one end of the shaft and extending in a direction perpendicular to the extending direction of the shaft, As a prerequisite, the material of the fastening component is set to ferrite-austenite duplex stainless steel with a 0.2% yield strength of 500 MPa or more and a work hardening index of 0.25 or less. The first step is to set the shape of the fastening component having the shaft and the head as an initial setting, and then, based on the initial setting shape, predict the distribution of damage values ​​when tensile stress is applied to the shaft with the lower end of the head in the height direction fixed, by numerical simulation, and determine the maximum damage value at the corner where the shaft and the head meet just before the head breaks. If the maximum damage value at the corner exceeds a threshold, the second step involves resetting the shape of the fastening component by increasing the height of the head, and repeating the measurement of the damage value at the corner based on the reset shape of the fastening component until the maximum damage value falls below the threshold. A method for designing a fastening component, comprising: a third step in which, if the maximum damage value at the corner is less than or equal to a threshold in the first or second step, the shape of the fastening component with the height of the head in that case is determined as the product shape.

5. The fastening component design method according to claim 4, wherein, under the premise that the material of the fastening component is austenitic stainless steel, the damage value when tensile stress is applied to the shaft portion while the lower end of the head in the height direction is fixed is predicted by numerical simulation, the maximum damage value at the corner portion where one end of the shaft portion and the head contact just before the head breaks is determined, and a value less than or equal to this maximum damage value is set as the threshold value.

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