JP2002316215A - Method of manufacturing bent article with excellent fatigue property, and bent article - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the fatigue property of a bent part by reducing residual stress without depending on a special residual stress reducing process such as return and heating even when performing a bending work of a steel plate in which the ratio of curvature radius R to sheet thickness t is >=2. SOLUTION: By performing the almost L-shaped bending of the steel plate having the thickness t of 7-10 mm and the tensile strength of >=490 MPa, to >=70 deg. using a die and a punch the radius R of curvature in a sholder part of which is >=2 times the thickness t, the bent article is manufactured. The clearance c between the die and the punch is wider than the thickness t and narrower than 1.02×thickness t. In the bent article manufactured in this manner, the peak of the residual stress in the bending ending region (B) is not more than the maximum value of the residual stress in the bending begining region (A).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a bent product (bent product of a steel plate) having excellent fatigue characteristics useful for underbody parts of automobiles and the like, and a bent product.

[0002]

2. Description of the Related Art Bent products of steel sheets are used in various fields. However, when they are used for undercarriage parts of automobiles and the like, stress is repeatedly applied thereto, so that high fatigue properties are required. However, when bending a steel sheet, tensile stress in the surface direction (bending direction) inevitably remains in the bent portion (hereinafter referred to as residual stress or residual tensile stress). Fatigue strength decreases.

As a method for preventing a decrease in fatigue strength, for example, Japanese Patent Application Laid-Open No. HEI 8-174080 discloses that a bend is made 5 to 10% smaller than a predetermined bend radius and then 5 to 10% smaller than a predetermined bend radius. A method of largely reverse bending is disclosed. Japanese Patent Application Laid-Open No. H11-61243 discloses a method of reducing residual stress by locally heating an edge portion (bent portion). However, these methods require a special step (reverse bending step, heating step, etc.) for reducing the residual stress in addition to the bending step, which increases the cost. Therefore, when an automobile part (such as an underbody part) is actually pressed, the above-described residual stress reduction method is rarely adopted.

In order to secure desired fatigue characteristics without performing reverse bending or heating, a processed product is often manufactured by appropriately selecting a steel type and processing conditions. In this case, the clearance c between the punch and the die is a plate thickness t + 0.1 to 0.3 mm.
(Hereinafter, referred to as a conventional method). However, even if the steel type and the processing conditions are appropriately selected, the tensile residual stress may not be sufficiently reduced, and the fatigue strength of the bent portion may not be sufficiently increased. In particular, when bending a high-tensile steel sheet having a thickness equal to or more than a predetermined amount, tensile stress tends to remain, and it is difficult to increase the fatigue strength of the bent portion.

[0005]

SUMMARY OF THE INVENTION Accordingly, the present inventors
Detailed investigation was conducted to find out the cause of the insufficient reduction of the tensile residual stress.

First, an L-shaped bending (L-bending) according to the conventional method is used.
The behavior of the stress in the case of performing was investigated. That is, a rolled steel sheet having a thickness of 3.0 mm and a tensile strength of 780 MPa was L-shaped bent at an angle of 90 ° using a die and a punch. Immediately after the end of processing (the state in which predetermined bending is completed and the steel sheet is subjected to a force from a die and a punch; the same applies hereinafter) and the residual stress after springback after processing. The state (residual stress) was analyzed. In addition, since fatigue fracture is likely to occur in the shape of the part in the direction of 45 ° with respect to the rolling direction, as residual stress,
The tensile residual stress (σ ₄₅ ) in the direction of 45 ° with respect to the rolling direction was analyzed.

FIG. 1 is a schematic sectional view for explaining this working method, and FIG. 2 is a schematic sectional view showing a bent portion after working. As shown in FIG. 1, in this example, the steel plate 1 sandwiched between the die 2 and the pad 4 was bent L-shaped to one side by a punch 3. The radius of curvature R of the bending shoulder of the die 2 is 1
0 mm (curvature radius R / plate thickness t ≒ 3.3)
The clearance c between the punch 3 and the plate 3 is a plate thickness t (3.0).
mm) +0.2 mm = 3.2 mm (that is, clearance c ≒ 1.07 × plate thickness t). In this method, the bending of the steel plate 1 starts from the shoulder (curved surface) of the die 2 before bending.
(Hereinafter, referred to as a bending start portion) a and finally ends with a portion (hereinafter, referred to as a bending end portion) b which contacts the shoulder. And the bending start portion a
Is the origin (0), and the bending start portion a to the bending end portion b
The behavior of the stress on the inner peripheral side 5 of the bent portion was analyzed along the axis I (the scale of the bending end portion b is set to 10). A position corresponding to the bending end portion b on the inner circumferential surface of the bend is defined as an origin (0), and along an axis T in a thickness direction toward the outer circumferential surface (an intersection between the axis T and the outer circumferential surface is defined as a scale 10). Then, the behavior of the stress was analyzed. FIG. 3 shows the in-plane residual stress (σ ₄₅ ) in the direction of the axis I (bending direction) of the inner peripheral side 5 of the bent portion after springback, and FIG. The analyzed in-plane stress and the in-plane residual stress analyzed along the axis T direction (plate thickness direction) after springback are shown.

As shown in FIG. 3, the present invention shows that the in-plane residual stress on the inner peripheral side 5 of the bending portion is substantially constant in the bending start region A, but shows a remarkably high peak in the bending end region B. Found them. Moreover, the peak value is larger than the residual stress in the bending start region A. Therefore, the fatigue characteristics of the bent product are determined by the residual stress in the bending end region B. In the conventional method,
The bending method is determined so that the fatigue strength of the bent portion becomes a desired value, and the bending stress in the bending start region A is controlled to the desired value, but the bending end region B is controlled. It was found that, because of the remarkably large residual stress, a crack was generated in that region, and the fatigue properties of the bent product were greatly deteriorated.

The reason why the peak of the in-plane residual stress occurs in the bending end region B can be explained with reference to FIG. FIGS. 5 to 6 are views showing the state of generation of stress when a general bending process (such as a bending process in which the clearance c is sufficiently wider than the case of FIG. 4) is performed. FIG. 5 shows the axis T immediately after the end of machining.
FIG. 6 is a conceptual diagram showing the relationship between the stress (in the thickness direction) and the stress, and FIG. 6 is a conceptual diagram showing the relationship between the axis T (in the thickness direction) after springback and the residual stress in the surface direction. 7 and 8
FIG. 7 is a conceptual diagram for explaining the analysis result of FIG. 4. More specifically, FIG.
FIG. 8 is a conceptual diagram showing the relationship between the stress (in the thickness direction) and stress, and FIG. 8 is a conceptual diagram showing the relationship between the axis T (in the thickness direction) after springback and the residual stress in the surface direction.

As shown in FIG. 5, when a general bending process is performed, a compressive stress (compressive stress in the surface direction) is generated on the inner peripheral side of the bent portion immediately after the completion of the bending, and a tensile stress is generated on the outer peripheral portion of the bent portion. (Tensile stress in the plane direction) has occurred. After processing, when the bent product is released from the mold, springback is performed so as to cancel each stress generated immediately after the processing is completed. Then, as shown in FIG. 6, a tensile stress (tensile stress in a surface direction) remains on the inner peripheral side of the bent portion.

In the case of performing the bending process (the process of clearance c = 1.07 × plate thickness t) according to the above-mentioned conventional method, as shown in FIGS. The stress is even greater. That is, in FIGS. 7 and 8, the thick line indicates the conventional method (clearance c =
1.07 × bending thickness t), which corresponds to the analysis result of FIG. The broken line shows the compressive stress in the thickness direction (compressive stress in the thickness direction acting as a reaction force received from the mold) in the conventional method. The thin line corresponds to the general bending (bending with sufficiently large clearance c), and corresponds to the graphs shown in FIGS.

The condition of the conventional method (clearance c = 1.07)
When the sheet is bent in accordance with the sheet thickness t), as shown by the broken line in FIG. 7, a compressive stress in the thickness direction is generated near the inner peripheral portion of the steel sheet, and therefore, the compressive stress in the surface direction near the inner peripheral portion of the steel sheet is generated. Also increase. And this increased compressive stress in the plane direction is
After the springback, it remains as a local tensile stress. That is, when the compressive stress in the thickness direction is generated only in the vicinity of the inner peripheral portion, the increased compressive stress in the planar direction does not contribute as the compressive stress in the planar direction after the spring back (ie, the surface after the spring back). Rather than acting as a force that lowers the tensile stress in the direction), it acts as a moment and contributes as a force that increases the tensile stress after springback. Therefore, after the springback, as shown by the thick line in FIG. 8, a tensile stress remains in the inner peripheral portion of the bend. Direction compressive stress increases.

As described in detail above, according to the present inventors' first investigation into the cause, the conventional method shows that the bending end region B
Is the maximum value of the in-plane residual stress in the bending start area A (hereinafter simply referred to as “residual stress in the bending end area B”) (hereinafter simply referred to as “the bending start area A”). In some cases, it may be referred to as “residual stress” in some cases). Therefore, secondly, a study was performed on when the residual stress in the bending end region B becomes higher than the residual stress in the bending start region A.

Except that the thickness t is changed to 1.2 to 6.2 mm, the same conditions as those in the first study (namely, tensile strength of the steel plate = 780 MPa, radius of curvature R of the shoulder portion of the die = 10)
mm, clearance c = plate thickness t + 0.2 mm;
(The radius of curvature R / plate thickness t = 1.6 to 8.3, and clearance c = 1.03 × plate thickness t to 1.17 × plate thickness t)
, And the in-plane residual stress on the inner peripheral side 5 of the bent portion after springback was analyzed. As a result, we have:
It has been found that the residual stress in the bending end region B becomes higher than the residual stress in the bending start region A when the radius of curvature R / the plate thickness t is 2 or more.

FIG. 9 is a graph showing the results of the analysis, and shows the relationship between the difference between the residual stress in the bending end region B and the residual stress in the bending start region A and the radius of curvature R / plate thickness t. . As is clear from this graph, when the radius of curvature R / the plate thickness t is 2 or more, the residual stress in the bending end region B becomes higher than the residual stress in the bending start region A, and as a result, the fatigue of the bent product We found that the characteristics deteriorated.

In other words, the conventional method (clearance c =
(1.03 × plate thickness t to about 1.17 × plate thickness t), the fatigue characteristics deteriorate when the radius of curvature R / the plate thickness t is 2 or more.

The present invention has been made in view of the above-mentioned circumstances, and the object is to achieve a curvature radius R / plate thickness t of 2
Even when the steel sheet is bent as described above, a method for manufacturing a bent product that can reduce the residual stress and improve the fatigue properties of the bent portion without using a special residual stress reduction step such as bending back and heating, and It is to provide a bent product.

Japanese Patent Application Laid-Open Nos. 56-117831 and 7-158443 disclose a clearance between a die and a punch having a thickness of 1 mm in order to prevent external warpage after U-shaped bending. 0 to 1.4 times (JP-A-56-1178)
No. 31) or 85 to 100% (JP-A-7-155)
No. 843), however, these publications do not disclose or suggest fatigue characteristics, and furthermore, a method for preventing a warp peculiar to U-shaped bending. This is an invention unrelated to L-shaped bending, which has no possibility of occurrence.

[0019]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems. That is, as described in the investigation of the cause, when compressive stress is generated only in the vicinity of the inner peripheral portion of the plate in the thickness direction, the tensile residual stress after springback is smaller than when no compressive stress is generated in the thickness direction. Is further increased (see FIGS. 7 and 8). However, as a result of further study, when compressive stress in the plate thickness direction is generated deeply, for example, when the compressive stress is generated substantially at the center of the plate (substantially at the center in the plate thickness direction), the tensile residual stress after springback is generated. (Tensile residual stress in the bending end region B on the inner peripheral side of the bent portion) is significantly reduced, and it has been found that fatigue characteristics can be significantly improved. Then, in order to generate a compressive stress in the thickness direction up to a substantially central portion of the plate (substantially the center portion in the thickness direction), the clearance c between the die and the punch is determined by the conventional method (1.03 × thickness). t-1.17 x thickness t)
The inventors of the present invention have found that it is necessary to make the size smaller than the above, and completed the present invention.

That is, the manufacturing method of the bent product of the present invention which can achieve the above object is that the thickness t is 0.7 to 10 mm.
(Preferably 2 to 4 mm), a steel plate having a tensile strength of 490 MPa or more (preferably 490 to 1000 MPa), a curvature radius R of a shoulder portion is twice or more (preferably 2.2 or more, or more) the plate thickness t. Using a die having a diameter of 2.5 to 4 times and a punch, the angle is 70 ° or more (preferably 80 °).
A method of manufacturing a bent product by performing a substantially L-shaped bending process of about 100 ° to about 100 °, wherein a clearance c between a die and a punch is equal to or more than the plate thickness t (preferably, a plate thickness t + 10 μm or more). ), And has a gist in that it is 1.02 × plate thickness t or less. According to this manufacturing method, a bent product having excellent fatigue characteristics can be manufactured.

The bent product according to the present invention, which has achieved the above object, has a peak of residual stress in the bending end region (B),
The point is that the residual stress in the bending start area (A) is not more than the maximum value (preferably 20 to 100% of the maximum value).

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a plate thickness of 0.7 to 10 m is used.
A steel plate having a tensile strength of about 490 MPa or more is bent approximately L-shaped. The reason why a steel sheet having such properties (plate thickness and tensile strength) is intended is that when such a steel sheet is bent by a conventional method, tensile stress remains and fatigue properties are likely to deteriorate. In the present invention, even if such a steel plate is bent, the fatigue characteristics are not deteriorated.

The thickness of the sheet can be appropriately selected from the above range according to the application. For example, when the sheet is used as a structural member,
The plate thickness may be 1.0 mm or more, and when strength is further required, it may be 1.5 mm or more. It is recommended to apply the present invention to a steel plate having such a thickness. On the other hand, if the plate thickness is too thick, the radius of curvature R / the plate thickness t described below may be reduced. That is, when performing the bending process, the curvature radius R of the shoulder portion of the die has an upper limit to some extent. Therefore, when the plate thickness t is extremely increased, R / t becomes small (below the lower limit specified later), and The effect of the invention cannot be obtained. In addition, since the processed product becomes heavy, it is not desirable in some applications (such as automobile parts). The board thickness is 10 mm or less, preferably 5
mm or less, more preferably 4 mm or less.

The tensile strength is preferably at least 600 MPa, more preferably at least 780 MPa. Although the upper limit of the tensile strength is not particularly limited, it is usually 1000MP.
In many cases, a steel plate of a or less is used.

The type (component, structure, etc.) of the steel sheet is not particularly limited as long as it has the above-mentioned sheet thickness and tensile strength.
The steel sheet may be a rolled steel sheet or may not be rolled.

In the present invention, the steel plate is connected to the steel plate shown in FIGS.
1 and 2 (FIGS. 1 and 2).
Avoid duplicate explanations for the same parts as above.) In the present invention, the radius of curvature R is at least twice the thickness t (the radius of curvature R / the thickness t = 2 or more). Curvature radius R / plate thickness t = 2
The reason for the above (or 2.0 or more) is that when processing is performed according to the conventional method in this range, the tensile residual stress cannot be reduced. In the present invention, the tensile stress can be sufficiently reduced even when the radius of curvature R / the thickness t is in the above range. The ratio (R / t) of the radius of curvature R to the plate thickness t is preferably 2.2 or more, more preferably 2.5 or more (especially 2.6 or more), and preferably 7.8 or less, more preferably Is 5 or less (especially 4 or less).

In the present invention, the clearance c between the die 2 and the punch 3 is determined by the conventional method (clearance c).
= 1.03 x thickness t ~ 1.17 x thickness t)
More specifically, the sheet thickness t or more (1.00 × sheet thickness t or more),
1.02 x sheet thickness t or less (including 1.024 x sheet thickness t). FIG. 10 is a graph showing the relationship between the difference between the residual stress in the bending end region B and the residual stress in the bending start region A and the clearance c. As is clear from this graph, if the clearance c / plate thickness t is set to 1.02 or less (that is, the clearance c is set to 1.02 × plate thickness t or less), the residual stress in the bending end region B is reduced. It can be smaller than the residual stress, and the fatigue properties of the bent product can be improved.

In the term "residual stress in the bending start region A", the "bending start region A" is defined as a region near the bending start portion a on the inner peripheral side of the bending portion (for example, in the axis I shown in FIG. , I = about -3 to 4). The term “residual stress” refers to the tensile residual stress in the plane direction, and more precisely, in each plane direction (for example, 0 to the rolling direction when a rolled steel sheet is used).
180 °) in the direction in which the tensile stress is the largest. In the term “residual stress in the bending end region B”, “bending end region B” refers to a region near the bending end portion b on the inner peripheral side of the bending portion (for example, in the axis I shown in FIG. (Approximately 13 regions). “Residual stress” has the same meaning as in the “bending end area A”.

It is considered that the residual stress in the bending end region B can be reduced by setting the predetermined clearance c for the following reason. The reason will be described with reference to FIGS. FIG. 11 is a schematic cross-sectional view showing the relationship between the clearance c and the compressive stress in the sheet thickness direction immediately after the completion of the processing. FIG. FIG. 12 is a graph (concept) showing the relationship between the stress (in the thickness direction) and the stress, and FIG. 12 is a graph (concept) showing the relationship between the axis T (the plate thickness direction) after springback and the residual stress in the surface direction.

That is, when the clearance c between the die 2 and the punch 3 is large, the compressive stress 10 in the thickness direction is generated only near the surface of the steel sheet 1 as shown in FIG. When the clearance c between the die 2 and the punch 3 is small, the compressive stress 10 in the thickness direction is generated not only in the surface portion of the steel plate 1 but also in the inside thereof, as shown in FIG.

When the compressive stress 10 in the thickness direction extends to the inside of the steel sheet, as shown in FIGS. 12 and 13, the tensile residual stress on the inner peripheral side of the bent portion after springback can be significantly reduced. That is, in FIGS. 12 and 13, a thick line indicates a stress (plane stress, plane direction residual stress) in the case of machining with a predetermined clearance c, and a broken line indicates a compressive stress in the sheet thickness direction (from the mold) under this condition. The thin line indicates the surface stress (or the surface residual stress) when a general bending process without the above-mentioned thickness direction compressive stress is performed.

When the clearance c is made smaller than the conventional method (approximately 1.03.times.plate thickness t to 1.17.times.plate thickness t), as described above, the compressive stress in the plate thickness direction immediately after the end of the processing is reduced to the inside of the plate. (See the broken line in FIG. 12). When processing is performed in such a state, the point at which the tensile stress in the surface direction occurs is at the conventional level (see FIG. 7).
The bending moment is greatly reduced to the side closer to the bending inner peripheral portion, so that the bending moment is greatly reduced. As a result, unlike the conventional method in which the compressive stress in the thickness direction occurs only near the inner periphery,
The in-plane compressive stress contributes as the in-plane compressive stress after springback rather than acting as a moment to increase the post-springback tensile stress (ie, lowering the in-plane tensile stress after springback). Contribute as power). Therefore, as shown in FIG. 13, the compressive stress in the surface direction of the bending inner peripheral portion can be significantly reduced.

The clearance c is preferably 1.015
× plate thickness t or less, more preferably 1.01 × plate thickness t or less. As the clearance c is smaller, the residual stress in the bending end region B can be reduced. On the other hand, the clearance c is preferably 1.00 × plate thickness t + 10 μm or more, and may be 1.00 × plate thickness t + 20 μm or more depending on the plate thickness. As the clearance c is increased, the damage of the surface of the steel plate 1 by the punch 3 can be prevented.

The die and / or the punch may be formed with a taper (inclined surface) as required. By forming the taper, galling of the steel plate by the punch can be prevented, and the steel plate can be prevented from sticking to the die or the punch.

The clearance c when the taper is formed will be described with reference to FIGS. FIG. 14 is a schematic sectional view for explaining an example in which the taper 6 is formed in the punch 3, and FIG. 15 is a schematic sectional view for explaining an example in which the taper 7 is formed in the die 2. As described above, in the present invention, the tensile residual stress on the inner peripheral side of the bent portion can be reduced because of the clearance c
This is because the compression stress in the sheet thickness direction is generated up to the inside of the steel sheet 1 by reducing the diameter T. The compression stress in the sheet thickness direction is defined as the axis T immediately after processing (as shown in FIG. (Refers to the axis in the plate thickness direction at the bending end portion b as described above). Therefore, the clearance c when the taper is formed refers to the clearance at the bending end portion b immediately after the processing (FIG. 1).
4, see FIG. 15).

The bending angle is usually 70 ° or more (preferably 80 ° or more). The bending angle is often 120 ° or less (especially 100 ° or less).

The bent product obtained as described above is
The peak of the residual stress in the bending end region (B) is equal to or less than the maximum value of the residual stress in the bending start region (A) (preferably 80% or less, more preferably 60% or less of the maximum value). Therefore, the fatigue characteristics of the bent product are not determined by the residual stress in the bending end region B, and the fatigue characteristics of the bent product can be improved.

The peak of the residual stress in the bending end region (B) is usually 20% or more, and often 50% or more, with respect to the maximum value of the residual stress in the bending start region (A).

The bent product obtained by the present invention has excellent fatigue properties and is useful for structural members as well as products that are repeatedly subjected to stress (automobile parts, especially automobile underbody parts, etc.). is there.

[0040]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. Of course, the present invention can be embodied with modifications, and all of them are included in the technical scope of the present invention.

Examples 1 and 2 and Comparative Examples 1 and 2 A rolled steel plate having a thickness of 3 mm and a tensile strength of 780 MPa was prepared by setting the radius of curvature R of the shoulder to 10 mm (curvature radius R / thickness t ≒ 3.
Using the die 2 of 3), the compressive residual stress when performing the L-shaped bending at an angle of about 90 ° as shown in FIG. 1 was obtained by analysis.

The clearance c between the die 2 and the punch 3
3.00 mm (clearance c = 1.00 × plate thickness t; Example 1), 3.05 mm (clearance c ≒ 1.
017 × plate thickness t; Example 2), 3.10 mm (clearance c ≒ 1.033 × plate thickness t; Comparative Example 1), or 3.2
With 0 mm (clearance cｍｍ1.067 × plate thickness t; Comparative Example 2), the residual stress was calculated as described below.

For the calculation of the residual stress, analysis was performed using the elasto-plastic finite element method, and the internal stress value after processing and springback was read for the integration point of each element. The element size used was a 0.3 × 0.3 mm size, and was calculated using a plane strain element. The plate and the die and the plate and the punch assumed Coulomb friction, and each friction coefficient was 0.1.

The tensile residual stress on the inner peripheral side of the bent portion after springback (in this example, 45 ° with respect to the rolling direction of the steel sheet)
FIG. 16 shows the tensile residual stress in each direction). As is clear from FIG. 16, in Comparative Examples 1 and 2, the peak of the residual stress in the bending end area B is larger than the residual stress in the bending start area A, whereas in Examples 1 and 2, the bending end The peak of the residual stress in the region B is equal to or less than the maximum value of the residual stress in the bending start region A. Therefore, according to the first and second embodiments, the fatigue characteristics of the bent product can be improved.

Further, the in-plane stress analyzed along the axis T direction (sheet thickness direction) immediately after the end of the processing and the in-plane residual stress analyzed along the axis T direction (sheet thickness direction) after springback are calculated as described above. 4 (in the case of Comparative Example 2) and FIG.
(In the case of the first embodiment). As is clear from the comparison between FIG. 4 (Comparative Example 2; clearance c = 1.067 × plate thickness t) and FIG. 17 (Example 1; clearance c = 1.00 × plate thickness t), the clearance c is large. And the bending moment cannot be reduced, and the tensile residual stress at the axis T = 0 after springback increases (Comparative Example 2).
When the clearance c is small, the bending moment can be reduced, and the tensile residual stress at the axis T = 0 after springback can be reduced (Example 1).

FIG. 10 shows the relationship between the difference between the residual stress in the bending end region B and the residual stress in the bending start region A and the clearance c. As apparent from FIG. 10, in the comparative example, the clearance c is 1.03 × the plate thickness t or more, so that the residual stress in the bending end region B is larger than the residual stress in the bending start region A. In the example, since the clearance c is 1.024 × the plate thickness t or less, the residual stress in the bending end region B can be made equal to or less than the residual stress in the bending start region A,
The fatigue properties of the bent product can be improved.

[0047]

According to the present invention, since the clearance c between the die and the punch is made sufficiently small, the tensile residual stress after springback (the tensile residual stress in the bending end region B on the inner periphery of the bent portion). Can be significantly reduced, and the fatigue characteristics can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view for explaining a bending method according to the related art and the present invention.

FIG. 2 is a schematic sectional view of a bent portion of a bent product according to the related art and the present invention.

FIG. 3 is a graph showing a surface residual stress (σ ₄₅ ) on the inner peripheral side of a bent portion of a conventional bent product.

FIG. 4 is a graph showing the relationship between stress and axis T in Comparative Example 2.

FIG. 5 is a graph showing the relationship (concept) between the stress immediately after processing and the axis T when no compressive stress is generated in the thickness direction.

FIG. 6 is a graph showing the relationship (concept) between the residual stress after springback and the axis T when no compressive stress is generated in the thickness direction.

FIG. 7 is a graph showing the relationship (concept) between the stress immediately after processing and the axis T in the conventional method.

FIG. 8 is a graph showing the relationship (concept) between the residual stress after springback and the axis T in the conventional method.

FIG. 9 is a graph corresponding to the conventional method and showing the relationship between the difference between the residual stress in the bending end region B and the residual stress in the bending start region A and the radius of curvature R / plate thickness t.

FIG. 10 is a graph showing the relationship between the difference between the residual stress in the bending end region B and the residual stress in the bending start region A and the clearance c / plate thickness t.

FIG. 11 is a schematic sectional view for explaining a manufacturing method of the present invention.

FIG. 12 is a graph showing the relationship (concept) between the stress immediately after processing and the axis T in the present invention.

FIG. 13 is a graph showing the relationship (concept) between the residual stress after springback and the axis T in the conventional method.

FIG. 14 is a schematic sectional view for explaining another example of the present invention.

FIG. 15 is a schematic cross-sectional view for explaining another example of the present invention.

FIG. 16 is a graph showing the in-plane residual stress (σ ₄₅ ) on the inner peripheral side of the bent portion of the bent products of the example and the comparative example.

FIG. 17 is a graph showing the relationship between stress and axis T in Example 1.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Steel plate 2 ... Die 3 ... Punch A ... Bending start area B ... Bending end area

Claims (6)

[Claims] 1. A steel plate having a thickness t of 0.7 to 10 mm and a tensile strength of 490 MPa or more is prepared by using a die having a radius of curvature R of a shoulder of at least twice the thickness t and a punch. A method of manufacturing a bent product by performing a substantially L-shaped bending process at an angle of 70 ° or more, wherein a clearance c between a die and a punch is not less than the plate thickness t and 1.02 ×
A method for producing a bent product having excellent fatigue characteristics with a plate thickness t or less. 2. The manufacturing method according to claim 1, wherein the radius of curvature R of the shoulder is at least 2.2 times the plate thickness. 3. The method according to claim 1, wherein the clearance c is equal to or more than a plate thickness t + 10 μm. 4. A plate thickness t of 2 to 4 mm and a tensile strength of 490
A steel plate of about 1000 MPa is subjected to a substantially L-shaped bending process at an angle of 80 ° to 100 ° using a die having a radius of curvature R of the shoulder of 2.5 to 4 times the plate thickness t and a punch. 2. The method according to claim 1, wherein the clearance c between the die and the punch is not less than 1.00 × the thickness t and not more than 1.02 × the thickness t. . 5. A bent product obtained by subjecting a steel plate having a thickness t of 0.7 to 10 mm and a tensile strength of 490 MPa or more to a substantially L-shape using a punch and a die, and having a bending angle of 70 °.
As described above, a bent product having a substantially L-shaped bent portion having a curvature radius R of 2.0 times or more the plate thickness t and having the following residual stress characteristics on the inner peripheral side of the bent portion. The peak of the residual stress in the bending end region (B) is equal to or less than the maximum value of the residual stress in the bending start region (A). 6. A sheet thickness t of 2 to 4 mm and a tensile strength of 490.
This is a bent product obtained by bending a steel sheet of about 1000 MPa in a substantially L-shape using a punch and a die, and has a bending angle of 80 °.
6. The bending according to claim 5, further comprising: a substantially L-shaped bent portion having a curvature radius R of 2.5 to 4 times the plate thickness t, and a residual stress characteristic on an inner peripheral side of the bent portion is as follows. Processed goods. The peak of the residual stress in the bending end region (B) is 20 to 100% with respect to the maximum value of the residual stress in the bending start region (A).