EP4620593A1

EP4620593A1 - Structure around through-holes in metal sheet, and blank and automotive part

Info

Publication number: EP4620593A1
Application number: EP23891659.7A
Authority: EP
Inventors: Ryo URUSHIBATA
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-11-16
Filing date: 2023-11-16
Publication date: 2025-09-24
Also published as: EP4620593A4; CN120187541A; WO2024106512A1; JPWO2024106512A1; MX2025005580A; JP7506354B1

Abstract

A metal sheet 200 includes a flat portion 102, and a processed portion 204 in which a through-hole 104a is formed in a center portion. The processed portion 204 includes a flat surface 406 formed on one side in a thickness direction A of a flat surface 102a of the flat portion 102, and an inclined surface 408 extending toward the flat surface 102a from the flat surface 406, and satisfies "1.0 ≤ W ≤ -3.8314 × (R/t) + 11.47 ≤ 5.0" and "0.1 ≤ H ≤ -0.6604 × (R/t) + 2.0489 ≤ 1.0." Note that, in the formulas, W is a length (mm) of the inclined surface 408 in a direction orthogonal to the thickness direction A, R is a curvature radius (mm) of the through-hole 104a when viewed from the thickness direction A, t is a thickness (mm) of the metal sheet 200, and H is a distance (mm) between the flat surface 102a and the flat surface 406 in the thickness direction A.

Description

TECHNICAL FIELD

The present invention relates to a structure around a through-hole in a metal sheet, a blank, and an automotive part.

BACKGROUND ART

In recent years, with the tightening of carbon dioxide emission regulations, there has been a demand for reducing the weight of automotive bodies. In connection with this, metal sheets used for automotive parts are becoming stronger and thinner. On the other hand, automotive parts are required to have excellent fatigue durability.
A metal sheet utilized as a starting material of automotive parts may be subjected to punching. When an automotive part made of such a metal sheet is deformed, stress concentration occurs around a hole-punched portion (hole edge). Accordingly, cracks may occur from the hole-punched portion, and fatigue fracture may occur. Therefore, in order to improve the fatigue durability of automotive parts, it is important to prevent fatigue cracks from occurring in hole-punched portions.
Conventionally, a method for preventing occurrence of cracks in hole-punched portions has been proposed. For example, Patent Document 1 discloses a punching method that improves the stretch flangeability of a punch-processed end face. A die used in the punching method disclosed in Patent Document 1 is provided with a blade edge at a position below a flat portion of an upper surface of the die by a predetermined distance in a punching direction, and a blade edge portion is chamfered.
In the metal sheet punched with the method of Patent Document 1, at the time when the punching is completed, a work material will be bent in the punching direction. Therefore, when the flange up processing is performed in the same direction as the punching direction after the punching, the punched end face is already flanged up to some extent at the initial stage of the flange up processing. In this case, when performing the flange up processing, the plastic deformation amount of the end face due to stretch flange deformation can be reduced. Accordingly, the stretch flangeability of the punch-processed end face can be improved, and occurrence of cracks in the punched end face can be prevented.

LIST OF PRIOR ART DOCUMENT

PATENT DOCUMENT

Patent Document 1: JP2009-255167A

SUMMARY OF INVENTION

TECHNICAL PROBLEM

In Patent Document 1, as described above, improvement of stretch flangeability has been studied. However, study has not been conducted for the stress concentration that occurs around a hole-punched portion when an entire metal sheet is deformed. That is, study has not been conducted for the fatigue durability of a metal sheet including a hole-punched portion.
Therefore, an objective of the present invention is to improve the fatigue durability of a metal sheet in which a through-hole is formed by punching.

SOLUTION TO PROBLEM

The gist of the present invention is a structure around a through-hole in a metal sheet, a blank, and an automotive part as follows.

(1) A structure around a through-hole formed in a metal sheet, the structure including
- a flat portion, and a processed portion rising from the flat portion to one side in a thickness direction of the flat portion and having the through-hole formed in a center,
- wherein the flat portion includes a first flat surface surrounding periphery of the processed portion on the one side of the flat portion,
- the processed portion includes a wall surface extending in the thickness direction and constitutes an inner wall of the through-hole, an annular second flat surface extending in a direction intersecting the thickness direction from an edge on the one side of the wall surface at an inner side of the first flat surface when viewed from the thickness direction and on the one side of the first flat surface in the thickness direction, and an inclined surface connecting an outer edge of the second flat surface and an inner edge of the first flat surface, and inclined with respect to the thickness direction,
- when viewed from the thickness direction, the through-hole includes a curved portion curved in an arc so as to be convex toward an outside of the through-hole,
- in a cross section of the processed portion that is parallel to the thickness direction and passes through a center of the through-hole and the curved portion, following formulas (i) and (ii) are satisfied: $1.0 \leq W \leq - 3.8314 \times (R / t) + 11.47 \leq 5.0$ $0.1 \leq H \leq - 0.6604 \times (R / t) + 2.0489 \leq 1.0$
- where, in the formulas (i) and (ii), W is a length (mm) of the inclined surface in a direction orthogonal to the thickness direction, R is a curvature radius (mm) of a portion corresponding to the cross section of the curved portion, t is a thickness (mm) of the metal sheet, and H is a distance (mm) between the first flat surface and the second flat surface in the thickness direction.
(2) The structure around the through-hole in the metal sheet according to (1) described above, wherein the second flat surface extends in a direction orthogonal to the thickness direction.
(3) The structure around the through-hole in the metal sheet according to (1) or (2) described above, wherein the wall surface includes a sheared surface extending in a direction orthogonal to the first flat surface.
(4) The structure around the through-hole in the metal sheet according to (3) described above, wherein, in the cross section, a portion of the wall surface between the sheared surface and the second flat surface is provided to be located outside the sheared surface in a direction orthogonal to a central axis of the through-hole, and the inclined surface is provided to be located outside the outer edge of the second flat surface in the orthogonal direction.
(5) The structure around the through-hole in the metal sheet according to any one of (1) to (4) described above, wherein, in the cross section, a following formula (iii) is satisfied: $H \leq 0.0975 \times (R / t) + 0.3009$ where, in the formula (iii), H is the distance (mm) between the first flat surface and the second flat surface in the thickness direction, R is the curvature radius (mm) of the portion corresponding to the cross section of the curved portion, and t is the thickness (mm) of the metal sheet.
(6) The structure around the through-hole in the metal sheet according to (5) described above, wherein, in the cross section, a following formula (iv) is satisfied: $H \leq 0.0975 \times (R / t) + 0.2381$ where, in the formula (iv), H is the distance (mm) between the first flat surface and the second flat surface in the thickness direction, R is the curvature radius (mm) of the portion corresponding to the cross section of the curved portion, and t is the thickness (mm) of the metal sheet.
(7) The structure around the through-hole in the metal sheet according to any one of (1) to (6) described above, wherein the through-hole has a circular shape when viewed from the thickness direction.
(8) A blank including the structure around the through-hole in the metal sheet according to any one of (1) to (7) described above.
(9) An automotive part including the structure around the through-hole in the metal sheet according to any one of (1) to (7) described above.

ADVANTAGEOUS EFFECT OF INVENTION

According to the present invention, the fatigue durability of a metal sheet in which a through-hole is formed can be improved.

BRIEF DESCRIPTION OF DRAWINGS

[Figure 1]
Figure 1 is a diagram illustrating an example of a metal sheet in which a through-hole is formed by punching.
[Figure 2]
Figure 2 is a diagram illustrating another example of a metal sheet in which a through-hole is formed by punching.
[Figure 3]
Figure 3 is a diagram illustrating an analysis model of a metal sheet used in a simulation.
[Figure 4]
Figure 4 is a diagram illustrating another example of the analysis model of the metal sheet.
[Figure 5]
Figure 5 is a diagram illustrating an analysis model of a metal sheet for comparison.
[Figure 6]
Figure 6 is a diagram illustrating an analysis result.
[Figure 7]
Figure 7 is a diagram illustrating an analysis result.
[Figure 8]
Figure 8 is a schematic diagram illustrating an example of a press tooling for performing punching.
[Figure 9]
Figure 9 is a diagram illustrating a state of the press tooling and a blank immediately before the blank is cut.
[Figure 10]
Figure 10 is a diagram for describing a situation where burrs are formed when the blank is cut.
[Figure 11]
Figure 11 is a diagram illustrating an analysis result.
[Figure 12]
Figure 12 is a diagram illustrating other examples of metal sheets in which through-holes are formed by punching.
[Figure 13]
Figure 13 is a diagram illustrating variations of the die.
[Figure 14]
Figure 14 is a diagram illustrating variations of the die.

DESCRIPTION OF EMBODIMENTS

(Study by present inventor)

Figure 1 is a diagram illustrating an example of a metal sheet in which a through-hole is formed by punching, in which (a) is a perspective view illustrating the metal sheet, and (b) is a cross-sectional view of a portion surrounded by a dashed line in (a). A metal sheet 100 illustrated in Figure 1 includes a flat portion 102 and a processed portion 104. Note that, in Figure 1, an arrow A indicates a thickness direction of the flat portion 102. Hereinafter, the thickness direction of the flat portion 102 is also simply referred to as a thickness direction A.
The processed portion 104 is formed by punching so as to rise from the flat portion 102 to one side in the thickness direction A. A through-hole 104a is formed in the center of the processed portion 104. When viewed from the thickness direction A, the through-hole 104a has a circular shape.
The flat portion 102 includes a flat surface 102a provided on one side in the thickness direction A, and a flat surface 102b provided on the other side in the thickness direction A. The flat surfaces 102a and 102b are each provided so as to surround the periphery the processed portion 104.
As illustrated in Figure 1 (b), the processed portion 104 includes a wall surface 400, an inclined surface 402, and an inclined surface 404. The wall surface 400 extends in the thickness direction A, and constitutes an inner wall of the through-hole 104a. The wall surface 400 includes a sheared surface 400a and a fractured surface 400b. The inclined surface 402 is formed into an annular shape when viewed from the thickness direction A, so as to connect an edge on one side in the thickness direction A of the wall surface 400 and an inner edge of the flat surface 102a. The inclined surface 404 is a droop, and is formed into an annular shape when viewed from the thickness direction A, so as to connect an edge on the other side in the thickness direction A of the wall surface 400 and an inner edge of the flat surface 102b.
In an automotive part or the like using the metal sheet 100 as described above as a starting material, bending deformation as indicated by an arrow B may be applied around the processed portion 104. As a result of the study by the present inventor, it has been found that when such bending deformation is applied to the metal sheet 100, stress concentration occurs at an edge portion of the through-hole 104a.
Therefore, the present inventor has conducted study for suppressing the occurrence of such stress concentration. Specifically, the relationship between the shape of an end portion of a processed portion and the stress generated at an edge portion of a through-hole has been investigated.
Figure 2 is a diagram illustrating another example of a metal sheet in which a through-hole is formed by punching, in which (a) is a perspective view illustrating the metal sheet, and (b) is a cross-sectional view of a portion surrounded by a dashed line in (a). Note that a producing method of the metal sheet illustrated in Figure 2 will be described later.
A metal sheet 200 illustrated in Figure 2 is different from the metal sheet 100 illustrated in Figure 1 in that a processed portion 204 is formed instead of the processed portion 104. In addition, the processed portion 204 is different from the processed portion 104 in that the processed portion 204 includes a flat surface 406 and an inclined surface 408, instead of the inclined surface 402. The flat surface 406 and the inclined surface 408 are each formed into an annular shape when viewed from the thickness direction A. The flat surface 406 is formed so as to extend in a direction that intersects the thickness direction A from an edge on the one side in the thickness direction A of the wall surface 400. The inclined surface 408 connects an outer edge of the flat surface 406 and an inner edge of the flat surface 102a, and is inclined with respect to the thickness direction A.

(Simulation)

The present inventor conducted study about the stress generated at the edge portion of the through-hole 104a when bending deformation was applied to the metal sheet 100 and the metal sheet 200. Specifically, the present inventor performed a simulation by the FEM analysis using a computer, and investigated the stress generated at the edge portion of the through-hole 104a.
Figure 3 is a diagram illustrating an analysis model of the metal sheet 100 in Figure 1, and Figure 4 is a diagram illustrating an analysis model of the metal sheet 200 in Figure 2. In Figure 3 and Figure 4, (a) is a plan view of the analysis model, (b) is a front view of the analysis model, and (c) is a cross-sectional view of a portion surrounded by a dashed line in (a). Note that, in analysis models 300 and 310, portions having configurations corresponding to those of the metal sheets 100 and 200 are given the same numerals as those of the metal sheets 100 and 200. However, as illustrated in Figure 3 (c) and Figure 4 (c), the shapes of the processed portions 104 and 204 are simplified in the analysis models 300 and 310. A heat-rolled steel sheet having material properties with a tensile strength of 780 MPa class was set to the analysis models 300 and 310 and an analysis model 350, which will be described later.
In Figure 3 and Figure 4, W represents a length (mm) of the inclined surface 402, 408 in a direction orthogonal to the thickness direction A, R represents a curvature radius (mm) of the through-hole 104a when viewed from the thickness direction A, and t represents a thickness (mm) of the flat portion 102 (the metal sheet). In addition, in Figure 3, H represents a distance (mm) between the flat surface 102a and a front end of the processed portion 104 in the thickness direction A, and in Figure 4, H represents a distance (mm) between the flat surface 102a and the flat surface 406 in the thickness direction A. In addition, in Figure 4, F represents a length of the flat surface 406 in a direction orthogonal to the thickness direction A. In this simulation, the FEM analysis was performed by creating a plurality of analysis models 300 and 310 while changing the length W, a ratio R/t of the curvature radius R to the thickness t, and the distance H. The length and the width of the analysis models 300 and 310 were set to 90 mm and 30 mm, respectively. Note that, in the analysis model 300, the length W was set to 1.0 to 3.0 mm, the ratio R/t was set to 1.56 to 2.17, and the distance H was set to 0.1 to 1.0 mm. In the analysis model 310, the length W was set to 1.0 to 5.0 mm, the ratio R/t was set to 1.25 to 3.13 and the distance H was set to 0.1 to 1.0 mm. In addition, in the analysis model 310, the length F was 0.1 mm.
In addition, in this simulation, an analysis model for comparison object was created. Figure 5 is a diagram illustrating the analysis model for comparison, in which (a) is a plan view of the analysis model, and (b) is a cross-sectional view illustrating a b-b portion in (a). As illustrated in Figure 5, the analysis model 350 has a configuration in which a through-hole 350a is simply formed in a center portion of a metal sheet, and an inclined surface is not formed in the periphery of the through-hole 350a. That is, the analysis model 350 has a generally flat shape. Note that, similar to the analysis models 300 and 310, a plurality of analysis models 350 was created by changing the ratio R/t of the curvature radius R of the through-hole 350a to the thickness t. Similar to the analysis models 300 and 310, the length and the width of the analysis model 350 are 90 mm and 30 mm, respectively.
In the FEM analysis, a region 300a of 25 mm × 30 mm (hereinafter referred to as one end portion 300a) on one end side in the longitudinal direction of the analysis models 300, 310, and 350, and a region 300b of 25 mm × 30 mm (hereinafter referred to as the other end portion 300b) on the other end side were restrained. In addition, as illustrated by an arrow C in Figure 3 (b), Figure 4 (b), and Figure 5 (b), in a state where the position of the one end portion 300a is fixed, the position of the other end portion 300b was moved to apply bending deformation to the analysis models 300, 310, and 350. Then, the stress was investigated that is generated at edge portions of the through-holes 104a and 350a at an external surface side of the bending of the analysis models 300, 310, and 350 when the other end portion 300b was rotated 2 degrees with respect to the center of the analysis models 300, 310, and 350.

Specifically, first, the present inventor compared the stress (the maximum value of the maximum principal stress) generated at the edge portions of the through-holes 104a and 350a for the analysis models 310 and 350, the ratios R/t of which are equal to each other. Then, the length W and the distance H in a case where the stress generated at the edge portion of the through-hole 104a becomes lower than the stress generated at the edge portion of the through-hole 350a were clarified. Results of the investigation are illustrated in the following Table 1 and Table 2. Note that, in Table 1 and Table 2, "A" and "B" indicate that the stress generated at the edge portion of the through-hole 104a is lower than the stress generated at the edge portion of the through-hole 350a, and in particular, "A" indicates that the stress generated at the edge portion of the through-hole 104a is 90% or less of the stress generated at the edge portion of the through-hole 350a. In addition, "C" indicates that the stress generated at the edge portion of the through-hole 104a is not less than the stress generated at the edge portion of the through-hole 350a.

[Table 1]

Table 1
W (mm)		1.0										2.0										3.0
H (mm)		0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0
R/t	3.13	B	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C
	2.50	A	A	A	B	C	C	C	C	C	C	B	B	B	B	C	C	C	C	C	C	C	B	B	C	C	C	C	C	C	C
	2.17	A	A	A	A	B	B	C	C	C	C	B	A	A	A	B	B	C	C	C	C	B	B	B	B	B	B	C	C	C	C
	1.92	A	A	A	A	A	B	B	B	C	C	A	A	A	A	A	A	B	B	B	B	B	B	A	A	B	B	B	B	B	B
	1.72	A	A	A	A	A	A	B	B	B	C	A	A	A	A	A	A	A	B	B	B	B	A	A	A	A	B	B	B	B	B
	1.56	A	A	A	A	A	A	A	B	B	B	A	A	A	A	A	A	A	A	A	A	B	A	A	A	A	A	A	A	A	A
	1.25	A	A	A	A	A	A	A	A	A	B	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A

[Table 2]

Table 2
W (mm)		4.0										5.0
H(mm)		0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0
R/t	3.13	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C
	2.50	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C
	2.17	B	B	B	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C	C
	1.92	B	B	B	B	B	B	B	B	B	C	C	C	B	B	B	C	C	C	C	C
	1.72	B	B	B	A	A	B	B	B	B	B	B	B	B	B	B	B	B	B	B	B
	1.56	B	B	A	A	A	A	B	B	B	B	B	B	B	A	A	B	B	B	B	B
	1.25	B	A	A	A	A	A	A	A	A	A	B	B	A	A	A	A	A	A	A	A

The present inventor determined, from the results illustrated in Table 1 and Table 2, upper limit values of the length W and the distance H for making the stress generated at the edge portion of the through-hole 104a of the analysis model 310 lower than the stress generated at the edge portion of the through-hole 350a of the analysis model 350. The following Table 3 illustrates the upper limit values of the length W and the distance H for each ratio R/t.

[Table 3]

Table 3

R/t	W (mm)	H (mm)
3.13	1.0	0.1
2.50	2.0	0.4
2.17	3.0	0.6
1.92	4.0	0.8
1.72	5.0	0.9
1.56	5.0	1.0
1.25	5.0	1.0

Note that, in each ratio R/t, when the length W and the distance H are each not more than a specific value, in Tables 1 and 2 described above, all the comparison results will be one of "A" and "B." Such a specific value is referred to as the upper limit value. For example, when describing the analysis model 310 with a ratio R/t of 1.92, as illustrated in Table 2, in a case where the length W was 4.0 mm, the comparison result was "B" in the range where the distance H was 0.1 to 0.9 mm. However, as illustrated in Table 1, the comparison result was "C" in a case where the length W was 1.0 mm and the distance H was 0.9 mm. Accordingly, the upper limit values of the length W and the distance H of the analysis model 310 with a ratio R/t of 1.92 are not 4.0 mm (the length W) and 0.9 mm (the distance H), but 4.0 mm (the length W) and 0.8 mm (the distance H).
In addition, the upper limit values of the length W and the distance H were determined so as to maximize the total number of the comparison results "A" and "B." For example, when the ratio R/t is 2.17, the combination of the upper limit value of the length W and the upper limit value of the distance H that maximizes the total number of the comparison results "A" and "B" is that the upper limit value of the length W is 3.0 mm, and the upper limit value of the distance H is 0.6 mm. Therefore, the upper limit values of the length W and the distance H in the case where the ratio R/t is 2.17 are 3.0 mm and 0.6 mm, respectively.
Figure 6 illustrates the relationship between the upper limit value of the length W and the ratio R/t, and Figure 7 illustrates the relationship between the upper limit value of the distance H and the ratio R/t. Figure 6 and Figure 7 illustrate approximation curves determined by the least squares method from a plurality of items of data (the length W and the distance H) obtained by the analysis. Note that, as described above, in the analysis using the analysis model 310, the length W was set to 1.0 to 5.0 mm, and the distance H was set to 0.1 to 1.0 mm. Here, in a case where the combination of the upper limit value of the length W and the upper limit value of the distance H is 5.0 mm (the maximum set value of the length W) and 1.0 mm (the maximum set value of the distance H) (that is, the combination in a case where R/t = 1.25 and R/t = 1.56), there is a possibility that the combination of the upper limit values of the length W and the distance H is not appropriately represented. Specifically, as illustrated in Table 1 and Table 2, when the ratio R/t was 1.25 and 1.56, all the comparison results were "B" or "A" in the range where the length W was 1.0 to 5.0 mm and the distance H was 0.1 to 1.0 mm. Considering this point, in the case where the ratio R/t is 1.25 and 1.56, even when the length W is more than 5.0 mm or the distance H is more than 1.0 mm, there is a possibility that the comparison results become "B" or "A." In addition, also when the combination of the upper limit value of the length W and the upper limit value of the distance H is 1.0 mm (the minimum set value of the length W) and 0.1 mm (the minimum set value of the distance H) (that is, the combination in a case where R/t = 3.13), there is a possibility that the combination of the upper limit values of the length W and the distance H is not appropriately represented. Specifically, there is a possibility that the combination of the upper limit values of the length W and the distance H is determined by values not more than the minimum set values of the length W and the distance H. Consideration these points, the approximation curves illustrated in Figure 6 and Figure 7 were determined by excluding the data in the cases where the ratio R/t was 1.25, 1.56, and 3.13.
From the results of this simulation, it has been found that the stress generated at the edge portion of the through-hole 104a can be reduced by satisfying the following formula (i) and formula (ii) in the metal sheet 200 illustrated in Figure 2. In this case, since it is possible to suppress the occurrence of stress concentration at the edge portion of the through-hole 104a even if bending deformation is applied to the metal sheet 200, it is possible to prevent cracks from occurring at the edge portion of the through-hole 104a. Accordingly, the fatigue durability of the metal sheet 200 can be improved. $1.0 \leq W \leq - 3.8314 \times (R / t) + 11.47 \leq 5.0$ $0.1 \leq H \leq - 0.6604 \times (R / t) + 2.0489 \leq 1.0$
Next, the present inventor compared the stress (the maximum value of the maximum principal stress) generated at the edge portions of the through-holes 104a and 350a for the analysis models 300 and 350, the ratios R/t of which are equal to each other. Then, similar to the case described above, the upper limit values of the length W and the distance H were obtained for making the stress generated at the edge portion of the through-hole 104a of the analysis model 300 lower than the stress generated at the edge portion of the through-hole 350a of the analysis model 350. As a result, the upper limit values of the length W and the distance H of the analysis model 300 in the case where the ratio R/t is 2.17 were 1.0 mm and 0.1 mm, respectively. In addition, the upper limit values of the length W and the distance H of the analysis model 300 in the case where the ratio R/t is 1.56 were 1.0 mm and 1.0 mm, respectively. Comparing these upper limit values with the upper limit values of the analysis model 310 illustrated in Table 3, it can be found that the range of the length W and the distance H capable of making the stress generated at the edge portion of the through-hole 104a lower than the stress generated at the edge portion of the through-hole 350a of the analysis model 350 is wider in the analysis model 310 than in the analysis model 300.
From this result, it can be found that the value of the stress generated at the edge portion of the through-hole 104a can be made smaller in the analysis model 310 (see Figure 4) in which the flat surface 406 was formed at the front end portion of the processed portion 204 than in the analysis model 300 (see Figure 3) in which the flat surface 406 was not formed.
Note that, as can be seen by comparing Figure 3 with Figure 4, an angle between the wall surface 400 and the inclined surface 402 of the analysis model 300 is significantly smaller than an angle between the wall surface 400 and the flat surface 406 of the analysis model 310. In other words, in the analysis model 300, the front end portion of the processed portion 104 has a sharper shape than in the analysis model 310. Accordingly, in the analysis model 300, it is conceivable that stress concentration easily occurred at the edge portion (the front end portion of the processed portion 104) of the through-hole 104a than in the analysis model 310. From the above result, it can be found that stress concentration at the edge portion of the through-hole 104a can be suppressed by providing the flat surface 406 at the front end portion of the processed portion 204 as in the metal sheet 200 in Figure 2.

(Producing method of metal sheet)

Next, a producing method of the metal sheet 200 will be described. Figure 8 is a schematic diagram illustrating an example of a press tooling for producing the metal sheet 200. As illustrated in Figure 8, a press tooling 10 includes a punch 12, a die 14, and a blank holder 16. The punch 12 is formed into a columnar shape (in the present embodiment, a cylindrical shape), and is provided to be movable forward and backward in an up-down direction (pressing direction).
The die 14 has a hollow shape so that the punch 12 can be inserted therein. In the present embodiment, the die 14 includes a flat and annular supporting surface 40 that supports a blank (metal sheet) 18, a tubular (in the present embodiment, cylindrical) inner circumferential surface 42 extending downward from an inner circumferential edge of the supporting surface 40, a flat and annular (in the present embodiment, ring-shaped) flange surface 44 extending toward the inner side of the inner circumferential surface 42 from a lower edge of the inner circumferential surface 42, and a tubular (in the present embodiment, cylindrical) inner circumferential surface 46 extending downward from an inner circumferential edge of the flange surface 44. The blank holder 16 is formed into a hollow shape so that the punch 12 can be inserted therein, and includes an annular bottom surface 60 facing the supporting surface 40 of the die 14. In the present embodiment, the inner circumferential edge of the supporting surface 40 and the inner circumferential edge of the flange surface 44 each have a circular shape.
As illustrated in Figure 8, when performing punching on the blank 18, first, the blank 18 is placed on the supporting surface 40 of the die 14. Then, in a state where the blank 18 is pressed down with the blank holder 16, the punch 12 is moved downward, and a predetermined region of the blank 18 is cut (sheared) by the punch 12 and the die 14. Accordingly, as illustrated in Figure 2, the metal sheet 200 having the through-hole 104a is obtained.
Figure 9 is a diagram illustrating a state of the press tooling 10 and the blank 18 immediately before the blank 18 is cut. As illustrated in Figure 9, when cutting the predetermined region of the blank 18 by the press tooling 10, the blank 18 is cut by using each of shoulder portions of the punch 12 and the inner circumferential edge of the flange surface 44 of the die 14 as a cutting blade.
Here, in the present embodiment, a step is provided between the supporting surface 40 and the flange surface 44. Therefore, a portion of the blank 18 located inside the inner circumferential edge of the supporting surface 40 is pushed into the flange surface 44 side. Accordingly, as illustrated in Figure 2, the processed portion 204 is formed so as to rise from the flat portion 102 in the thickness direction A.
In addition, as illustrated in Figure 9, when the blank 18 is cut, a portion of the blank 18 is pressed against the flange surface 44 to be plastically deformed. Accordingly, as illustrated in Figure 2, the flat surface 406 can be formed in the processed portion 204.
Note that, in the metal sheet 200, the distance H (see Figure 2) between the flat surface 102a and the flat surface 406 in the thickness direction A becomes substantially equal to a distance h (see Figure 8) between the supporting surface 40 and the flange surface 44 in the pressing direction (up-down direction) of the press tooling 10. Accordingly, the distance H between the flat surface 102a and the flat surface 406 in the thickness direction A can be adjusted by adjusting the distance h between the supporting surface 40 and the flange surface 44. In addition, in the metal sheet 200, the length W (see Figure 2) of the inclined surface 408 in a direction orthogonal to the thickness direction A can be adjusted by adjusting a length w (see Figure 8) of the flange surface 44 in a direction orthogonal to the pressing direction of the press tooling 10. Note that, a clearance CL (see Figure 9) between the punch 12 and the die 14 (the inner circumferential edge of the flange surface 44) may be set appropriately according to the thickness of the blank 18, and the clearance CL is set to, for example, 18% or less of the thickness of the blank 18, and is preferably set to 15% or less of the thickness of the blank 18.
As the present inventor has further studied on the producing method of the metal sheet 200, it has been found that, when the distance h between the supporting surface 40 and the flange surface 44 in the pressing direction becomes large in the press tooling 10, burrs are more likely to occur. Figure 10 is a diagram for describing a situation where burrs are formed when the blank 18 is cut.
As illustrated in Figure 10, as a result of the study by the present inventor, it has been found that, when the blank 18 is cut, an inflection point P may occur on a bottom surface of the blank 18 between the supporting surface 40 and the flange surface 44, and a portion of the blank 18 between the inflection point P and the flange surface 44 may be deformed to bulge outward. In this case, it has been found that there is a possibility that a fracture occurs between the inflection point P and the shoulder portion of the punch 12 (a portion indicated by a dashed line), and the flat surface 406 (see Figure 2) cannot be appropriately formed, or excessive burrs occur.
Therefore, the present inventor investigated a condition for occurrence of the inflection point P by creating an axisymmetric model of two-dimensional solid elements for the press tooling 10 and the blank 18, and performing the FEM analysis. Specifically, a plurality of analysis models was created by changing a ratio R1/t1 of a curvature radius R1 (not illustrated) of an outer peripheral surface of the punch 12 to a thickness t1 of the blank 18, and the FEM analysis was performed. Then, a distance d between the inflection point P and the supporting surface 40 in the pressing direction was investigated in each of the analysis models. Note that the ratio R1/t1 was set to 1.25 to 6.25, the distance h was set to 1.0 mm, and the length w of the flange surface 44 in a direction orthogonal to the pressing direction was set to 1.0 mm. A ratio CL/t1 of the clearance CL (see Figure 9) between the punch 12 and the die 14 to the thickness t1 of the blank 18 was set to 12.5%. In addition, the material properties of the blank 18 were set similar to those of the analysis model 300.
Figure 11 illustrates the relationship between the ratio R1/t1 and the distance d. Note that, in Figure 11, an approximation curve determined by the least squares method from the distance d obtained by the analysis is illustrated by a dotted line. Furthermore, in Figure 11, a straight line that is parallel to the approximated curve, and passes through a point corresponding to the distance d in the case where the ratio R1/t1 is 1.25 is illustrated by a dashed line.
In order to prevent the inflection point P from occurring during the punching, it is conceivable that the distance h between the supporting surface 40 and the flange surface 44 in the pressing direction may be set to not more than the distance d between the inflection point P and the supporting surface 40 determined in the FEM analysis. Regarding this point, as described above, in the metal sheet 200, the distance H between the flat surface 102a and the flat surface 406 in the thickness direction A becomes substantially equal to the distance h. Accordingly, by performing the punching so that the distance H between the flat surface 102a and the flat surface 406 in the thickness direction A becomes not more than the distance d determined by the FEM analysis, the flat surface 406 (see Figure 2) can be more appropriately formed, and the occurrence of excessive burrs can be sufficiently suppressed. Note that, when the through-hole 104a in the metal sheet 200 is formed with the press tooling 10, the curvature radius R of the through-hole 104a becomes substantially equal to the curvature radius R1 of the outer peripheral surface of the punch 12. In addition, the thickness t of the flat portion 102 of the metal sheet 200 is substantially equal to the thickness t1 of the blank 18. Accordingly, by forming the through-hole 104a so as to satisfy the following formula (iii), the flat surface 406 (see Figure 2) can be more appropriately formed, and the occurrence of excessive burrs can be sufficiently suppressed. Note that it is more preferable to form the through-hole 104a so as to satisfy the following formula (iv). In this case, the area of the flat surface 406 can be sufficiently increased. $H \leq 0.0975 \times (R / t) + 0.3009$ $H \leq 0.0975 \times (R / t) + 0.2381$
Note that, in the formulas, H represents the distance (mm) between the flat surface 102a and the flat surface 406 in the thickness direction A, R represents the curvature radius (mm) of the through-hole 104a when viewed from the thickness direction A, and t represents the thickness (mm) of the flat portion 102 (the metal sheet).

(Description of embodiment of present invention)

The present invention has been made based on the findings described above. Specifically, a structure around a through-hole in a metal sheet according to one embodiment of the present invention is the structure around the through-hole 104a in the metal sheet 200 illustrated in Figure 2, in which a cross-sectional shape of the processed portion 204 that is parallel to the thickness direction A and passes through the center of the through-hole 104a satisfies the following formulas (i) and (ii). $1.0 \leq W \leq - 3.8314 \times (R / t) + 11.47 \leq 5.0$ $0.1 \leq H \leq - 0.6604 \times (R / t) + 2.0489 \leq 1.0$
Note that, in the formulas (i) and (ii), W is the length (mm) of the inclined surface 408 in a direction orthogonal to the thickness direction A, R is the curvature radius (mm) of the through-hole 104a, t is the thickness (mm) of the metal sheet 200, and H is the distance (mm) between the flat surface 102a and the flat surface 406 in the thickness direction A.
Note that the structure around a through-hole in a metal sheet according to the present embodiment is formed by using, for example, a starting material (a steel sheet, an aluminum sheet, or the like) having a thickness of 1.6 mm to 4.0 mm, and a tensile strength of not less than 590 MPa class. However, a starting material having a thickness less than 1.6 mm or more than 4.0 mm may be used, and a starting material having a tensile strength less than 590 MPa class may be used.
In the present embodiment, the flat surface 102a corresponds to a first flat surface, the flat surface 406 corresponds to a second flat surface, and the wall surface 400 of the through-hole 104a corresponds to a curved portion. In addition, in the present embodiment, the curvature radius of the through-hole 104a means the curvature radius of the sheared surface 400a when viewed from the thickness direction A. The curvature radius of the through-hole 104a can be measured by using a spherometer.
Note that, in the present embodiment, the flat surface 406 is formed so as to extend in the direction orthogonal to the thickness direction A. In this case, since the front end portion of the processed portion 204 does not have a sharp shape, it is possible to sufficiently prevent stress concentration from occurring at the edge portion of the through-hole 104a. In the cross section (the cross section illustrated in Figure 2) of the processed portion 204 that is parallel to the thickness direction A, and passes through the center of the through-hole 104a, it is preferable that the length (corresponding to the length F in Figure 4 (c)) of the flat surface 406 in the direction orthogonal to the thickness direction A is 0.1 mm or more. Note that, in the metal sheet 200 illustrated in Figure 2, although the flat surface 406 is provided to be orthogonal to the thickness direction A, the flat surface 406 may not be precisely orthogonal to the thickness direction A. In the present description, "a flat surface extends in a direction orthogonal to the thickness direction A" includes a case where the flat surface is inclined at an angle of 0 to 10 degrees with respect to the direction orthogonal to the thickness direction A. Also in this case, it is possible to sufficiently prevent stress concentration from occurring at the edge portion of the through-hole 104a. Note that, in the present embodiment, similar to the flat surface 406, the flat surface 102a is also formed so as to extend in the direction orthogonal to the thickness direction A.
In addition, in the present embodiment, the wall surface 400 includes the sheared surface 400a extending in a direction orthogonal to the flat surface 102a. Note that, in the metal sheet 200 illustrated in Figure 2, although the wall surface 400 is provided so as to extend in the direction orthogonal to the flat surface 102a, the wall surface 400 may not be precisely orthogonal to the normal direction of the flat surface 102a. In the present description, "a sheared surface extends in a direction orthogonal to a flat surface" includes a case where the sheared surfaces is inclined at an angle of 0 to 10 degrees with respect to the normal direction of the flat surface.
In addition, in the present embodiment, the portion (in the present embodiment, the fractured surface 400b) of the wall surface 400 between the sheared surface 400a and the flat surface 406 is provided to be located outside the sheared surface 400a in a radial direction D of the through-hole 104a. In other words, the portion of the wall surface 400 between the sheared surface 400a and the flat surface 406 does not protrude further toward the inner side of the through-hole 104a than the sheared surface 400a when viewed from the thickness direction A. Furthermore, the inclined surface 408 is provided to be located outside the outer edge of the flat surface 406 in the radial direction D of the through-hole 104a. In other words, the inclined surface 408 does not extend further toward the through-hole 104a side than the outer edge of the flat surface 406 when viewed from the thickness direction A. In this case, since the shape around the processed portion 204 becomes a smooth shape, it is possible to sufficiently prevent stress concentration from occurring around the through-hole 104a. Accordingly, the fatigue durability of the metal sheet 200 can be improved. Note that, as will be described in Figure 12, which will be described later, the shape of the through-hole when viewed from the thickness direction A is not limited to a circular shape. When the shape of the through-hole when viewed from the thickness direction A is not a circular shape, a radial direction of the through-hole means a direction orthogonal to the central axis of the through-hole.
Note that, in the present embodiment, it is preferable that a cross-sectional shape of the processed portion 204 that is parallel to the thickness direction A and passes through the center of the through-hole 104a satisfies the following formula (iii). In this case, the flat surface 406 (see Figure 2) can be more appropriately formed, and the occurrence of excessive burrs can be sufficiently suppressed. $H \leq 0.0975 \times (R / t) + 0.3009$
Note that, in the formula (iii), H is the distance (mm) between the flat surface 102a and the flat surface 406 in the thickness direction A, R is the curvature radius (mm) of the through-hole 104a, and t is the thickness (mm) of the metal sheet 200.
Note that, although it is preferable that the formulas (i) and (ii) are satisfied in all the areas in the periphery of the through-hole, the formulas (i) and (ii) may be satisfied only in a part of the periphery of the through-hole. The same applies to the formula (iii). Note that, as long as the formulas (i) and (ii) are satisfied, the formula (iii) may not be satisfied.
In addition, in the embodiment, although the description has been given of the case where the present invention is applied to the metal sheet 200 in which the through-hole 104a having the circular shape when viewed from the thickness direction A is formed, the shape of the through-hole 104a is not limited to the circular shape. The through-hole formed in the processed portion may include one or more (in the present embodiment, four) linear portions, and one or more curved portions when viewed from the thickness direction of the flat portion. For example, as illustrated in Figure 12 (a), the through-hole 104a may have an oval shape. Note that the through-hole 104a illustrated in Figure 12 (a) includes two linear portions 50 and two curved portions 52. In addition, as illustrated in Figure 12 (b) and Figure 12 (c), the through-hole 104a may have a polygonal shape provided with the curved portions 52 in corner portions. Note that the curved portions 52 are curved in an arc so as to be convex toward the outside of the through-hole 104a. In this case, in a cross section of the processed portion 204 that is parallel to the thickness direction A and passes through the center of the through-hole 104a and the curved portion 52, the formulas (i) and (ii) may be satisfied. The same applies to the formula (iii). In addition, in this case, R in the formulas (i), (ii), (iii), and (iv) represents a curvature radius (mm) of a portion corresponding to the cross section of the curved portions 52.
Note that the metal sheet according to the present invention includes not only a flat metal sheet, but also various shaped products (for example, automotive parts) that are produced by using the flat metal sheet as a blank. That is, the structure around a through-hole in a metal sheet according to the present invention can be utilized in a blank used as a starting material of various shaped products (automotive parts and the like), and in various shaped products. For example, as automotive parts according to the present invention, suspension parts (a lower arm, an upper arm, a trailing link, a subframe, and the like), a body part, and a ladder frame having the structure around a through-hole can be listed.

(Variations of press tooling)

In the die 14 of the press tooling 10 illustrated in Figure 8 to Figure 10, although the supporting surface 40 and the inner circumferential surface 42 are connected to be orthogonal to each other, and the inner circumferential surface 42 and the flange surface 44 are connected to be orthogonal to each other, the shape of the die 14 is not limited to the example described above.
For example, as illustrated in Figure 13 (a), a connecting part between the inner circumferential surface 42 and the flange surface 44 may be curved in an arc. By rounding the connecting part between the inner circumferential surface 42 and the flange surface 44 in this manner, the durability of the die 14 can be improved.
In addition, for example, as illustrated in Figure 13 (b), a connecting part between the supporting surface 40 and the inner circumferential surface 42 may be curved in an arc. By rounding a portion that forms the inclined surface 408 (see Figure 2) in this manner, the durability around a rising portion 408a (see Figure 2 (b)) of the inclined surface 408 can be improved in the produced metal sheet 200.
In addition, for example, as illustrated in Figure 13 (c), the connecting part between the supporting surface 40 and the inner circumferential surface 42, and the connecting part between the inner circumferential surface 42 and the flange surface 44 may be each curved in an arc. In this case, the durability of the metal sheet 200 and the die 14 can be improved.
In addition, for example, as illustrated in Figure 14 (a), the inner circumferential surface 42 may be inclined with respect to the supporting surface 40 and the flange surface 44. Also in this case, the durability of the metal sheet 200 and the die 14 can be improved. In addition, as illustrated in Figures 14 (b) and (d), by curving the connecting part between the inner circumferential surface 42 and the flange surface 44 in an arc, the durability of the die 14 can be further improved. In addition, as illustrated in Figures 14 (c) and (d), by curving the connecting part between the supporting surface 40 and the inner circumferential surface 42 in an arc, the durability of the metal sheet 200 (the rising portion of the inclined surface 408) can be further improved.
Note that as long as the metal sheet 200 (the processed portion 204) can satisfy the formulas (i) and (ii), the press tooling may not include the blank holder. In addition, the producing method of the metal sheet 200 is not limited to the method described above, and as long as the metal sheet 200 (the processed portion 204) can satisfy the formulas (i) and (ii), the metal sheet 200 may be produced by using a press tooling having a shape and dimensions different from those of the press tooling described above. Accordingly, the shape and dimensions of the press tooling can be appropriately changed according to the starting material, dimensions, and the like of the metal sheet 200, so that the metal sheet 200 (the processed portion 204) satisfies the formulas (i) and (ii).

INDUSTRIAL APPLICABILITY

According to the present invention, the fatigue durability of a metal sheet in which a through-hole is formed can be improved. Accordingly, the present invention can be preferably utilized in metal sheets used as starting materials of various automotive parts.

REFERENCE SIGNS LIST

102: flat portion
102a: flat surface (first flat surface)
104a: through-hole
200: metal sheet
204: processed portion
400: wall surface
400a: sheared surface
400b: fractured surface
406: flat surface (second flat surface)
408: inclined surface

Claims

A structure around a through-hole formed in a metal sheet, the structure comprising:
a flat portion; and a processed portion rising from the flat portion to one side in a thickness direction of the flat portion and having the through-hole formed in a center,

wherein the flat portion includes a first flat surface surrounding periphery of the processed portion on the one side of the flat portion,

the processed portion includes a wall surface extending in the thickness direction and constitutes an inner wall of the through-hole, an annular second flat surface extending in a direction intersecting the thickness direction from an edge on the one side of the wall surface at an inner side of the first flat surface when viewed from the thickness direction and on the one side of the first flat surface in the thickness direction, and an inclined surface connecting an outer edge of the second flat surface and an inner edge of the first flat surface, and inclined with respect to the thickness direction,

when viewed from the thickness direction, the through-hole includes a curved portion curved in an arc so as to be convex toward an outside of the through-hole,

in a cross section of the processed portion that is parallel to the thickness direction and passes through a center of the through-hole and the curved portion, following formulas (i) and (ii) are satisfied: $1.0 \leq W \leq - 3.8314 \times (R / t) + 11.47 \leq 5.0$ $0.1 \leq H \leq - 0.6604 \times (R / t) + 2.0489 \leq 1.0$

where, in the formulas (i) and (ii), W is a length (mm) of the inclined surface in a direction orthogonal to the thickness direction, R is a curvature radius (mm) of a portion corresponding to the cross section of the curved portion, t is a thickness (mm) of the metal sheet, and H is a distance (mm) between the first flat surface and the second flat surface in the thickness direction.
The structure around the through-hole in the metal sheet according to claim 1, wherein the second flat surface extends in a direction orthogonal to the thickness direction.
The structure around the through-hole in the metal sheet according to claim 1 or 2, wherein the wall surface includes a sheared surface extending in a direction orthogonal to the first flat surface.
The structure around the through-hole in the metal sheet according to claim 3, wherein, in the cross section, a portion of the wall surface between the sheared surface and the second flat surface is provided to be located outside the sheared surface in a direction orthogonal to a central axis of the through-hole, and the inclined surface is provided to be located outside the outer edge of the second flat surface in the orthogonal direction.
The structure around the through-hole in the metal sheet according to any one of claims 1 to 4, wherein, in the cross section, a following formula (iii) is satisfied: $H \leq 0.0975 \times (R / t) + 0.3009$ where, in the formula (iii), H is the distance (mm) between the first flat surface and the second flat surface in the thickness direction, R is the curvature radius (mm) of the portion corresponding to the cross section of the curved portion, and t is the thickness (mm) of the metal sheet.
The structure around the through-hole in the metal sheet according to claim 5, wherein, in the cross section, a following formula (iv) is satisfied: $H \leq 0.0975 \times (R / t) + 0.2381$ where, in the formula (iv), H is the distance (mm) between the first flat surface and the second flat surface in the thickness direction, R is the curvature radius (mm) of the portion corresponding to the cross section of the curved portion, and t is the thickness (mm) of the metal sheet.
The structure around the through-hole in the metal sheet according to any one of claims 1 to 6, wherein the through-hole has a circular shape when viewed from the thickness direction.
A blank comprising the structure around the through-hole in the metal sheet according to any one of claims 1 to 7.
An automotive part comprising the structure around the through-hole in the metal sheet according to any one of claims 1 to 7.