阅读说明：本技术 冲压部件的制造方法 (Method for manufacturing stamped member ) 是由 藤井健斗 新宫丰久 山崎雄司 于 2020-03-13 设计创作，主要内容包括：提供为了防止剪切端面发生延迟破坏而使在冲压成型后的金属板的剪切端面产生的拉伸残余应力减小的冲压加工技术。制造冲压部件的制造方法对具有剪切端面的金属板进行冲压成型,冲压部件的制造方法的特征在于,包括第1冲压成型工序,其被推定为在脱模后在上述金属板的剪切端面的一部分在沿着剪切边缘的方向上产生拉伸残余应力,作为上述第1冲压成型工序的后续工序具有拉伸残余应力缓和工序(5),在该拉伸残余应力缓和工序(5)中,对至少包含被推定为产生上述拉伸残余应力的、剪切端面的部位在内的区域在板厚方向上进行胀形成型。(Provided is a press working technique for reducing tensile residual stress generated at a sheared end face of a press-formed metal plate in order to prevent delayed fracture of the sheared end face. A method for manufacturing a stamped member by stamping a metal plate having a sheared end face, the method for manufacturing a stamped member comprising a 1 st stamping step of estimating that tensile residual stress is generated in a part of the sheared end face of the metal plate in a direction along a sheared edge after die release, and a tensile residual stress relaxation step (5) of performing bulging forming in a plate thickness direction on a region including at least a portion of the sheared end face estimated to generate the tensile residual stress in the tensile residual stress relaxation step (5), as a step subsequent to the 1 st stamping step.)

1. A method of manufacturing a stamped member by press-forming a metal plate having a sheared end face, the method being characterized in that,

includes a 1 st press forming step of forming a tensile residual stress in a direction along a shearing edge in a part of a shearing end face of the metal plate after the metal plate is removed from the die,

the step subsequent to the 1 st press forming step includes a tensile residual stress relaxation step of performing bulging in the plate thickness direction on a region including at least a portion of the sheared edge face which is estimated to generate the tensile residual stress.

2. The method of manufacturing a stamped member according to claim 1, wherein the bulging shape formed by the bulging forming in the tensile residual stress relaxing step is set such that a bulging height becomes smaller as being farther from the sheared end face.

3. The method of manufacturing a stamped member according to claim 1 or 2, wherein a molding analysis is performed on the metal plate, and a portion estimated to generate the tensile residual stress is identified from a result of the molding analysis after the die is removed.

4. The method of manufacturing a stamped member according to any one of claims 1 to 3, wherein the shear end face is formed into a bulging shape having a bulging height of 10mm or more and a radius of curvature in a direction along the shear edge at a bulging apex portion of 5mm or more by bulging in the tensile residual stress relaxation step.

5. The method of manufacturing a stamped member according to any one of claims 1 to 4, wherein the bulging in the tensile residual stress relaxation step satisfies the following expression (1) when a length of a portion bulging along the shear edge before bulging is X0 and a length of a portion bulging along the shear edge after bulging is X1,

X1＞1.03·X0(1)。

6. the method of manufacturing a stamped member according to any one of claims 1 to 5, wherein the tensile strength of the metal plate is 980MPa or more.

Technical Field

The present invention relates to a technique for suppressing delayed fracture from a sheared end face of a stamped member formed of a metal plate after stamping.

Background

Currently, improvement in combustion efficiency and improvement in collision safety due to weight reduction are required for automobiles. Therefore, in automobiles, high-strength steel sheets are used for vehicle bodies in order to achieve both weight reduction of the vehicle bodies and passenger protection at the time of collision. In particular, in recent years, ultrahigh strength steel sheets having a tensile strength of 980MPa or more tend to be applied to vehicle bodies. One of the problems in applying the ultra-high strength steel sheet to a vehicle body is delayed fracture that occurs with time due to use. In particular, delayed fracture that occurs from an end face after shearing (hereinafter also referred to as a sheared end face) is an important problem in press working of a steel sheet having a tensile strength of 1470MPa or more.

In press working involving compression flanging deformation by press forming, it is known that tensile residual stress is applied to a shear end face by rebound after mold release.

In order to suppress delayed fracture of the sheared edge face, it is necessary to reduce tensile residual stress of the sheared edge face.

Conventionally, in order to reduce the tensile residual stress at the sheared edge face, methods for shearing have been widely developed, such as a method of raising the temperature of a steel sheet during shearing (non-patent documents 1 and 2), a method of using a stepped die during punching (non-patent document 3), and a method of shaving (shaving) (non-patent documents 4 and 1).

Patent document 2 describes the following technique: in order to reduce the spring back and improve the dimensional accuracy of the component, a plurality of thickening ribs are formed at the compression-flange forming portion to impart tensile stress, and a concavity and a convexity (element) are formed at the tension-flange forming portion to crush the concavity and the convexity to impart compressive stress.

Documents of the prior art

Non-patent document

Non-patent document 1: senjianyilang, etc.: plasticity and processing, 52-609(2011), 1114-: senjianyilang, etc.: plasticity and processing, 51-588(2010), 55-59 non-patent document 3: shear front of 326 th Plastic working workshop, 21-28

Non-patent document 4: m.murakawa, m.suzuki, t.shinome, f.komuro, a.harai, a.matsumoto, n.koga: precision piping and blanking of ultra-high-string steel sheets, Procedia Engineering, 81(2014), pp.1114-1120

Patent document

Patent document 1: japanese patent laid-open publication No. 2004-174542

Patent document 2: japanese laid-open patent publication No. 2009-255117

Disclosure of Invention

Problems to be solved by the invention

However, the methods described in non-patent documents and patent document 1 are techniques for coping with delayed fracture at the time of shearing, and are not techniques for reducing residual stress at the sheared end face generated in the step of press-forming the metal plate after shearing.

In addition, the method described in patent document 2 is a technique for reducing the bounce, not a delayed destruction countermeasure technique. The thickened bead described in patent document 2 is introduced to reduce the compressive stress at the compressive burring portion, and is not introduced to reduce the tensile residual stress at the shear end face which causes delayed fracture.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a press working technique for reducing tensile residual stress generated in a sheared end face of a press-formed metal plate in order to prevent delayed fracture of the sheared end face.

Means for solving the problems

The inventors of the present application have found that, in order to solve the above-described problems, when a tensile deformation is applied to a sheared end face by applying a bulging deformation to an end face of a compression-flange deformed portion formed by press forming so as to form a bead, a tensile residual stress of the sheared end face due to a rebound deformation after mold release can be reduced.

In order to solve the above-described problems, one aspect of the present invention is a method for manufacturing a stamped member by press-forming a metal plate having a sheared end face, the method including a 1 st press-forming step of, after die-removal, estimating that tensile residual stress is generated in a portion of the sheared end face of the metal plate in a direction along a sheared edge, and a tensile residual stress relaxing step of, as a step subsequent to the 1 st press-forming step, performing bulging forming in a plate thickness direction on a region including at least a portion of the sheared end face estimated to generate the tensile residual stress.

Effects of the invention

According to the aspect of the present invention, the tensile residual stress generated at the sheared end face of the press-formed metal plate can be reduced. As a result, according to the aspect of the present invention, it is possible to improve the delayed fracture resistance when the high-strength steel sheet is applied to various members such as a panel member and a structural/skeleton member of an automobile.

Drawings

Fig. 1 is a diagram showing an example of a process of a method for manufacturing a stamped member according to an embodiment of the present invention.

Fig. 2 is a diagram illustrating an example of bulging in the tensile residual stress relaxation step, where (a) is a plan view illustrating an end surface side of the example of bulging, and (b) is a side view illustrating a bulging shape as viewed from a direction facing the end surface.

Fig. 3 is a side view of another example of the bulging shape, as viewed from a direction facing the end face.

Fig. 4 is a diagram illustrating the 1 st press molding step in the embodiment.

Fig. 5 is a diagram showing a mold used in the tensile residual stress relaxing process in the example.

Fig. 6 is a view showing a rib shape of a die used in the tensile residual stress relaxing step in the example.

Fig. 7 is a view showing the shape of the bead at the position of the end face a-a' in fig. 6.

Fig. 8 is a diagram illustrating the width L1 of the expanded shape in the embodiment.

Fig. 9 is a diagram showing a linear length X1 after the bulging forming in the embodiment.

Detailed Description

Next, embodiments of the present invention will be described with reference to the drawings.

< Metal plate >

First, a metal plate to be press-molded is explained.

The metal plate exemplified in the present embodiment is formed of a high-strength steel plate, which may cause delayed fracture at the end portion over time after press forming due to tensile residual stress of a sheared end face that is present after press forming. The present invention is a technique suitable for a high-strength steel sheet having a tensile strength of 590MPa or more, which is particularly effective for a high-strength steel sheet having a strength of 980MPa or more, which is likely to cause delayed fracture, and more effective for a high-strength steel sheet having a strength of 1180MPa or more.

Here, the tensile residual stress of the sheared end face is also input at the time of shearing of the end portion.

As shown in fig. 1, the present embodiment includes a trimming step 2, a pressing step 3, and a tensile residual stress relaxing step 5, which are steps prior to press forming. In addition, the present embodiment has a tensile residual stress generation site specifying unit 6.

< dressing Process 2 >

In the trimming step 2, the metal plate 1 is cut into a profile shape corresponding to the part shape of the press part 4, for example.

< pressing Process 3 >

In the press step 3, the metal plate 1 after the trimming step 2 is press-formed using a press die having an upper die and a lower die, thereby manufacturing a press member 4 having a target member shape. The press forming is, for example, press forming (stamping) or drawing. The press step 3 constitutes the 1 st press forming step.

Here, the more complicated the shape of the stamped member 4 is, the more the stamped member 4 is manufactured by multi-stage press forming. In the case of producing the press member 4 by multi-stage press forming, the press forming which is estimated to generate tensile residual stress in a direction along the shear edge in a part of the sheared end face of the metal plate 1 after the die release does not need to be the final press forming. However, when the tensile residual stress is generated in the press forming other than the final press forming of the multistage press forming, the tensile residual stress remaining after the final press forming of the multistage press forming becomes the tensile residual stress relaxed in the tensile residual stress relaxing step 5. In this case, the first press forming step 1 is a process of performing a series of multi-stage press forming or a press forming which is estimated to generate tensile residual stress in a direction along the shear edge in a part of the sheared end face of the metal plate 1 after the die is removed from the die.

< tensile residual stress generating portion determining part 6 >

The tensile residual stress generation site specifying unit 6 performs a process of specifying a tensile residual stress generation site generated at a sheared end face of the metal sheet after completion of the press step 3.

The 1 st method for determining the generation site of the tensile residual stress is a method for actually press-forming the metal plate 1 after the shearing process and directly measuring the residual stress after the die release of the press-formed product. The 2 nd method for determining the site of occurrence of tensile residual stress is a method for estimating the site of occurrence of tensile residual stress after mold release by molding analysis.

The method 1 is carried out by a destructive test method or a nondestructive test method. Examples of the destructive test method include a cutting method and a piercing method. The cutting method does not achieve sufficient accuracy of the measured value in the measurement of the bending deformation imparting portion of the press-formed product. The perforation method has difficulty in measuring the residual stress of the sheared edge. As a nondestructive test method, there is a residual stress measurement method by X-ray. This method can measure the residual stress of the sheared edge with sufficient accuracy, but the measurement is not realistic because it takes a long time.

From such a viewpoint, in the present embodiment, the generation site of tensile residual stress is specified by the following method 2, i.e., a method of estimating the generation site by molding analysis.

As the method 2, a method of analyzing the molding and estimating the residual stress after the mold release, which is represented by a finite element method, is preferably performed.

The conditions used for the molding analysis include various setting items, and may be any known method. However, if the accuracy of the molding analysis is not improved, the error of the calculation result of the residual stress becomes large. The main influence on this is to construct a model of the material behavior in the modeling analysis. It is known that the accuracy can be improved particularly when a follow-up hardening model is applied to the shape after the mold release, and from the viewpoint of the analysis accuracy, it is also preferable to perform the molding analysis using the follow-up hardening model. Examples of the follow-up hardening model include the linear follow-up hardening theory and the Yoshida-Uemori model. As the evaluation of the molding analysis result in the present embodiment, there are a method of displaying the stress distribution after demolding in a contour map (contour map) and a method of outputting and evaluating a stress value from a cell or a node corresponding to a portion of a shear edge, but any method may be used. It should be noted that, as for the direction of the stress, the stress in the direction along the shear edge to be evaluated is assumed. The reason for this is that in press forming, the shear edge is uniaxially stretched or bent or a composite of uniaxial stretching and bending, and the main stress direction thereof is along the direction of the shear edge.

As a method of determining a region including a shear end face where tensile residual stress occurs in a direction along a shear edge among shear end faces of the metal plate 1 demolded after press forming, for example, there are a method of setting a portion where tensile residual stress exceeds a predetermined stress value, a method of setting a portion where a means where tensile residual stress exceeds a predetermined stress value continues for 10mm or more along a shear edge, a method of setting a portion where a means where tensile residual stress exceeds a predetermined stress value continues for 3mm or more in a direction perpendicular to a shear edge, and the like, but any method may be used. The predetermined stress value is preferably determined based on the tensile strength, material, plate thickness, and the like of the metal plate 1. The predetermined stress value may be set by, for example, a method of multiplying the tensile strength of the metal plate 1 by a coefficient as a threshold value, a method of multiplying the yield stress of the metal plate 1 by an equivalent plastic strain and a coefficient, or the like. The predetermined stress value is, for example, 200 MPa. When the metal plate 1 is a high-strength steel plate having a strength of 1180MPa or more, the predetermined stress value is, for example, 100 MPa.

The tensile residual stress generation portion specifying unit 6 may simply specify the sheared end face of the portion subjected to the compression flanging in the press forming as the portion estimated to generate the tensile residual stress.

< tensile residual stress relaxation step 5 >

In the tensile residual stress relaxation step 5, the region ARA including the portion S of the sheared edge face estimated to generate the tensile residual stress determined by the tensile residual stress generation portion determining portion 6 is bulging-formed in the plate thickness direction with respect to the pressed member 4 press-formed into the target member shape in the pressing step 3 (see fig. 2). The bulging region ARA may be set so as to exceed a region including a portion S of the sheared edge where tensile residual stress is estimated to occur in a direction along the sheared edge.

The bulging region ARA is set so that the tensile deformation in the direction along the shear edge accompanying the bulging is set over the entire region of the portion of the shear end face where the tensile residual stress is estimated to occur.

When the portion S of the sheared edge that exceeds the length of the portion S of the sheared edge that is estimated to generate the tensile residual stress is expanded by bulging, the tensile deformation in the direction along the sheared edge that accompanies bulging reliably extends over the entire area of the portion S of the sheared edge that is estimated to generate the tensile residual stress.

The bulging shape formed by the bulging is, for example, an arc shape (rib shape having an arc-shaped cross section or the like) as viewed from the side opposite to the sheared end face as shown in fig. 2. The bulging shape may be formed by, for example, a rib shape as shown in fig. 3, or a wave shape extending in the direction of the shear edge and continuously formed by an arc shape.

Further, it is preferable that the bulge shape is formed into a bulge shape having a bulge height H of 10mm or more and a curvature radius R in a direction along the shear edge at a bulge apex portion of 5mm or more. The bulging height H is set to the height at the top of the bulging shape. The outer shape of the bulging shape is preferably: the curvature radius R is 5mm or more at any position along the edge direction.

The radius of curvature R may be 5mm or more, and the upper limit is not limited. The radius of curvature R is infinite indicating that the cross-section is flat.

The radius of curvature R may be a radius of curvature of either the convex side surface or the concave side surface in the bulging shape, and in the present embodiment, may be a radius of curvature of the convex side surface.

In addition, the bulging apex portion is set to be located within a portion S of the sheared end face in a direction along the sheared edge (see fig. 2). When 1 bulging apex portion is arranged in the portion S of the sheared end face, the bulging apex portion is preferably provided at the center portion of the portion S of the sheared end face in the direction along the shearing edge. The center portion is, for example, a midpoint division position in the case of dividing the portion S where the end face is cut into 3 equal divisions.

The upper limit of the bulging height H is 200 mm. If the amount exceeds this value, the strain generated at the sheared edge during press forming becomes large, and there is a possibility that stretch flanging may occur. In addition, wrinkles, which are one of molding failures, may be generated in the press-molded article. More preferably, the bulging height H is 100mm or less.

In the bulging, when the length of the portion bulging along the shear edge before the bulging is X0 and the length of the portion bulging along the shear edge after the bulging is X1, the difference in linear length between before and after the bulging preferably satisfies the following expression (a).

X1＞1.03·X0(A)

The upper limit of the linear length difference (X1-X2) before and after the bulging is naturally defined by the bulging height H and the radius of curvature R.

Here, the bulging shape is a shape of an end face of the metal plate 1, and the bulging shape of the other portion is not particularly limited. From the viewpoint of avoiding excessive deformation of the shape of the member produced in the press step 3, the bulging height H of the bulging shape may be set to be continuously smaller from the end face toward the inside, that is, along the surface of the metal plate 1 as it is farther from the end face. That is, only the vicinity of the end face may be subjected to bulging molding. The vicinity of the end face is, for example, within 10mm, preferably within 5mm from the end face. By limiting the range to this range, the influence on the part shape of the stamped part 4 manufactured in the stamping step 3 can be suppressed to a small extent.

< other constitutions >

Here, as a step subsequent to the tensile residual stress relaxation step 5, press forming may be performed to reduce the bulging height H of the bulging shape of the end portion formed in the tensile residual stress relaxation step 5.

Further, the shape of the member having the bulging shape formed in the tensile residual stress relaxation step 5 may be designed to the shape of the product 7, and the press member 4 produced in the press step 3 may be designed to be formed into a shape in which the bulging shape is flat.

The bulging in the tensile residual stress relaxation step 5 may be performed for all of the sheared end faces, without being limited to the sheared end faces that are estimated to generate tensile residual stress.

< action et al >

(means for generating tensile residual stress)

Here, a case where square tube drawing is performed in the pressing step 3 and tensile deformation occurs in the sheared end faces of the press-formed product will be described as an example.

When square tube drawing is performed on the central portion of the square metal plate 1 in the pressing step 3, the material inflow accompanying drawing occurs and the central portion of the metal plate 1 is deformed into a square tube shape. At this time, a portion of the shear edge in the flange portion of the outer periphery of the square tube is deformed in a direction along the shear edge with contraction, that is, compression burring deformation. In the square tube drawing, a compressive stress due to the compressive flange deformation is generated in the portion of the shear edge, and a tensile stress due to the inflow difference and the frictional resistance of the shear edge is also generated in the vicinity of the compressive flange deformation portion. Thus, a non-uniform stress distribution is generated along the shearing edge. In this manner, the press member 4 restrained by the die has an uneven stress distribution due to press forming. When the mold is released from this state to release the uneven stress distribution, the internal stress remains in the press member 4, which becomes a residual stress. The tensile stress among the residual stresses is one of the factors that cause delayed fracture of the stamped member 4 after stamping.

(tensile residual stress reducing method)

As a result of intensive studies, the inventors have found that the tensile residual stress can be reduced by applying bulging deformation to the end portion of the member in which the tensile residual stress is left after press forming. This will be explained below.

Tensile residual stress is generated at the sheared edge of the press-formed product, and as described above, generation of uneven stress distribution of tension and compression in the forming is a main cause. In the present embodiment, in order to solve this problem, a uniform deformation is applied to a portion where tensile residual stress occurs in the tensile residual stress relaxation step 5. Specifically, in the tensile residual stress relaxation step 5, the line length of the shear edge of the tensile residual stress generating portion is increased by the bulging shape formed by the bulging forming, and a tensile deformation not including a compression is imparted. This makes it possible to release the tensile stress during the forming after the die release of the bulging forming, and to reduce the tensile residual stress.

The bulging shape preferably satisfies the following conditions (1) to (3).

(1) Imparting plastic deformation to a portion in the shear edge, which generates tensile residual stress, by bulging deformation;

(2) imparting tensile deformation to a region wider than a region where tensile residual stress is generated in the shear edge by bulging deformation;

(3) after applying a tensile stress to the sheared edge by bulging deformation, the tensile stress is sufficiently released at the time of mold release

If the condition (1) is not satisfied, the molded article returns to its original shape after being released from the mold, and therefore tensile stress remains as it is.

If the condition (2) is not satisfied, a region having a large tensile residual stress may remain at the shear edge, and the occurrence of delayed fracture may not be sufficiently suppressed.

If the condition (3) is not satisfied, a new portion that may be damaged with delay may be formed by bulging such as rib forming.

For the above reasons, there is a limitation on the bulging shape that sufficiently exerts the effects of the present embodiment.

As a result of extensive studies, the inventors have found that when the bulging height H is 10mm or more and the radius of curvature R of the apex portion of the bulging shape is 5mm or more, the above-described conditions (1) to (3) can be satisfied, and that plastic deformation can be imparted to the portion of the sheared edge where tensile residual stress occurs by bulging deformation, and the tensile residual stress of the sheared edge face after press forming can be reduced.

If the radius of curvature R of the apex portion of the bulging shape is less than 5mm, a shape accompanied by local large deformation may be formed at the apex portion by the bulging molding, and tensile stress may remain after the mold is removed, thereby causing delayed fracture.

If the bulging height H is less than 10mm, plastic deformation cannot be sufficiently imparted to the portion where tensile residual stress is generated at the sheared edge, and the delayed fracture suppression effect may not be expected. More preferably, the bulging height H is 20mm or more and the radius of curvature R of the apex of the bulging shape is 10mm or more.

As shown in the condition (2), it is required that, in the bulging shape, a tensile deformation is imparted to a region wider than a region where a tensile residual stress is generated in the shear edge by the bulging deformation. As a result of intensive studies, if the bulging shape satisfying "L1 > L0" is obtained by performing a forming analysis of the metal plate 1 with L1 as the width of the bulging shape of the metal plate 1 in the direction along the sheared edge and L0 as the length of the region where the tensile residual stress is generated at the sheared end face according to the result of the forming analysis after the die release, the condition of (2) can be reliably satisfied. The reason for this is that the bulge forming is performed along the shear edge in a region wider than the region where the tensile residual stress remains, and the tensile deformation is imparted. More preferably, the expanded shape has a size of "L1 > 1.1. L0".

The upper limit of L1 is naturally defined by the bulging height H and the radius of curvature R.

Further, as shown by the condition (1), it is required that plastic deformation can be imparted to the portion of the sheared edge where the tensile residual stress is generated by bulging deformation. As a result of intensive studies, it was confirmed that the above-described condition (1) can be satisfied when the length of the portion of the metal plate 1 subjected to bulging along the sheared edge before bulging is X0 and the length of the portion of the metal plate 1 subjected to bulging along the sheared edge after bulging is X1 in the bulging of the metal plate 1. Although the strain applied to the portion to which the bulging shape is applied is distributed, if the equation "X1 > 1.03 · X0" is satisfied, plastic deformation can be applied to the entire region of the shear edge in which the tensile residual stress is generated, and the tensile stress can be reduced in the entire region of the shear edge in which the tensile residual stress is generated. More preferably "X1 > 1.10. X0".

As described above, according to the present embodiment, the tensile residual stress at the sheared end surfaces of the press-formed metal plate can be reduced. As a result, according to the present embodiment, the delayed fracture resistance can be improved when the high-strength steel sheet is applied to various members such as a panel member and a structural/skeleton member of an automobile.

Examples

Next, an embodiment based on the present invention is explained.

Here, 1470MPa grade cold-rolled steel sheets having mechanical properties shown in table 1 will be described.

[ Table 1]

As a press step (hereinafter, also referred to as a 1 st step), square tube drawing was performed on a metal plate 1 sheared into 400mm × 400mm using a die shown in fig. 4. That is, the press forming is performed by moving the die 20 toward the die 21 while the outer periphery of the metal sheet 1 is restrained by the blank holder 22 and the die 21.

The die R was set to 25mm, and the molding depth was set to 25 mm.

Next, as a tensile residual stress relaxation step (hereinafter, also referred to as a 2 nd step), a test piece was manufactured by press molding a flange portion of the metal plate 1 subjected to square tube drawing using a press die formed of an upper die 30 and a lower die 31 having a wavy bead shape shown in fig. 5 to 7. The rib shapes of the upper die 30 and the lower die 31 are the same, and as shown in fig. 7, the rib shape having a height h and a bending radius R0 is transferred to the end portion of the metal plate by press forming. That is, bulging is performed in which a sheared end surface of a metal plate is given a bulging shape having a wave shape continuous along an end edge.

The rib shape is set so that the height becomes continuously smaller from the end portion toward the inner side.

At this time, as shown in table 2, the bulging height of the formed bulging shape and the radius of curvature of the bulging top portion were changed, and a plurality of test pieces were produced.

Next, in order to simulate delayed fracture for each of the manufactured test pieces, a dipping test was performed.

The chemical solution used in the immersion test was composed of 0.1% NH₄SCN solution and McILVAINE buffer solution, and making into medicinal liquid with pH of 5.6. The immersion time was set to 24 hours.

Then, the presence or absence of cracks generated from the cut end face after the dipping was confirmed, and the crack was determined as a delayed fracture in a simulated manner.

Further, a molding analysis based on square tube drawing and bulging was performed to calculate the stress generated at the shear edge. The molding analysis was made to 1/4 model in consideration of symmetry. The residual stress after the mold release was evaluated in terms of molding analysis using a Yoshida-Uemori model as a material model.

The results of the immersion test and the residual stress measurement of the test pieces obtained using the die having the bulging shape are shown in tables 2 to 4. Here, the expanded shape width L1 is the position shown in fig. 8. The molded line length X1 is the position shown in fig. 9.

[ Table 2]

[ Table 3]

[ Table 4]

As is clear from table 2, cracks caused by the immersion test occurred when the bulging height of the bulging shape was 5mm, and cracks caused by the immersion test could be avoided when the bulging height was 10mm to 40 mm.

In addition, a residual stress reducing effect of the sheared edge was also confirmed. Regarding the radius of curvature of the apex portion, when the bulging height H is 40mm and the radius of curvature of the apex portion is 5mm to 30mm, cracks caused by the immersion test can be avoided. On the other hand, when the radius of curvature of the apex portion is 3mm, cracks occur.

From this, it is found that it is preferable that the height of the bulging shape is 10mm or more and the radius of the apex of the bulging shape is 5mm or more.

It is also understood from table 3 that when the bulging height H is 20mm and the radius of the apex portion is 55mm, cracks due to the immersion test are not generated in the range where the ratio of L1 to L0 (L1/L0) is 1.1 or more and 1.4 or less, and cracks are generated when the ratio (L1/L0) is 1.0. From this, it can be said that L1 > L0 is preferable.

It is also understood from table 4 that when the bulging height is 10mm and the radius of curvature of the apex portion is 104mm, if the ratio of X1 to X0 (X1/X0) is 1.05 and 1.15, no cracks are generated by the immersion test, and when the ratio (X1/X0) is 1.02 and 1.03, cracks are generated. From this, it can be said that X1 > 1.03 · X0 is appropriate for the linear length difference between the length X0 before molding and the length X1 after molding in bulging.

The entire contents of japanese patent application 2019 and 047362 (filed 3/14 of 2019) to which this application claims priority are hereby incorporated by reference as part of the present disclosure. While the present invention has been described with reference to a limited number of embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments described above without departing from the scope of the invention.

Description of the reference numerals

1 Metal sheet

2 finishing Process

3 punching step (1 st Press Molding step)

4 stamping part

5 tensile residual stress relaxation step

6 tensile residual stress generating portion specifying portion

7 products of manufacture