JP7188457B2 - METHOD FOR SHEARING METAL PLATE AND METHOD FOR MANUFACTURING PRESS PARTS - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for shearing a metal plate made of a high-strength steel plate having a tensile strength of 590 MPa or more, and a method for manufacturing a pressed part.

Currently, automobiles are required to improve fuel efficiency and collision safety by reducing weight. High-strength steel sheets tend to be used for automobile structural parts for the purpose of achieving both weight reduction of the vehicle body and protection of passengers in the event of a collision. Particularly in recent years, as high-strength steel sheets, ultra-high-strength steel sheets having a tensile strength of 980 MPa or more have been applied to vehicle bodies.
Delayed fracture is one of the problems in applying high-strength steel sheets to vehicle bodies. In particular, among high-strength steel sheets, in high-strength steel sheets having a tensile strength of 1180 MPa or more, delayed fracture occurring from an end face after shearing (hereinafter also referred to as a sheared end face) is an important issue.

Here, it is known that a large tensile stress remains on the sheared end face. Due to this residual tensile stress, there is concern that delayed fracture may occur at the sheared end face over time in the product after pressing (pressed part). In order to suppress the delayed fracture at the sheared edge, it is necessary to reduce the tensile residual stress at the sheared edge.
As a method for reducing the tensile residual stress on the sheared end face, for example, a method of increasing the temperature of the steel sheet during shearing (Non-Patent Documents 1 and 2) and a method of using a stepped punch during hole punching (Non-Patent Document 3 ), and a method by shaving (Non-Patent Document 4, Patent Document 1).

However, the method of increasing the temperature of the steel sheet during shearing requires time to heat the steel sheet. Therefore, this method is not suitable for mass production of automobiles and the like. Moreover, the method using a stepped punch has a problem that the effect of improving the delayed fracture resistance is small. Furthermore, the shaving method has the problem that it is difficult to manage the clearance in the shaving process.
As described above, conventionally, there has been a demand for the development of a shearing method for metal sheets that is easy to apply to mass production and provides sufficient delayed fracture resistance.

Kenichiro Mori et al.: Plasticity and Processing，52-609(2011), 1114-1118 Kenichiro Mori et al.: Plasticity and Processing, 51-588 (2010), 55-59 The 326th Plastic Working Symposium "Forefront of Shearing", 21-28 M. Murakawa, M. Suzuki, T. Shinome, F. Komuro, A. Harai, A. Matsumoto, N. Koga: Precision piercing and blanking of ultrahigh-strength steel sheets, Procedia Engineering, 81(2014), pp.1114 -1120

JP-A-2004-174542

High-strength steel sheets are concerned about delayed fracture occurring from sheared edges after press forming.
The present invention has been devised to solve the above-described problems, and it is an object of the present invention to provide a pressed part that has excellent delayed fracture resistance even if a metal plate made of high-strength steel is used. there is

One aspect of the present invention provides a metal plate shearing technique that reduces the tensile residual stress on the sheared end face of the metal plate to prevent delayed fracture on the sheared end face and has good delayed fracture resistance of the sheared end face. do.
Applicant found that by shearing the metal plate twice and appropriately setting the cutting allowance for the second time, it is easy to apply to mass production and the residual stress on the sheared end face after shearing can be reduced. . Then, it was found that by reducing the residual stress on the sheared end face after shearing, the occurrence of delayed fracture is suppressed in press-formed pressed parts.

That is, one aspect of the present invention is a method for shearing a metal plate made of a high-strength steel plate, wherein shearing is performed twice on at least a part of the edge of the metal plate, and the shearing is performed twice. The gist of the invention is that the cutting margin of the second shearing process is 1.2 times or more and less than 20 times the plate thickness of the metal plate.
Further, a method for manufacturing a pressed part according to another aspect of the present invention is a method for manufacturing a pressed part by subjecting a metal plate to one or more press moldings, wherein the metal plate is the shearing method described above. The gist is to use a metal plate sheared at
Here, the high-strength steel sheet in this specification refers to a steel sheet having a tensile strength of 590 MPa or more.

ADVANTAGE OF THE INVENTION According to the aspect of this invention, the tensile residual stress of the sheared edge surface of the metal plate which consists of a high strength steel plate which generate|occur|produces at the time of shearing can be reduced. As a result, according to the aspect of the present invention, it is possible to improve the delayed fracture resistance when applying the high-strength steel sheet to various parts such as automobile panel parts and structural/skeletal parts.

It is a figure which shows the example of a process concerning embodiment based on this invention. It is a conceptual diagram which shows the metal plate of a double shearing process. It is a conceptual diagram explaining the shearing process of the 1st time. It is a conceptual diagram explaining the shearing process of the 2nd time. FIG. 4 is a conceptual diagram showing the cutting edge radius of a tool;

Next, embodiments of the present invention will be described with reference to the drawings.
In this embodiment, a metal plate used in press molding will be described.
The metal plate used in the present embodiment is a high-strength steel plate that may cause delayed fracture at the edge over time after press forming due to tensile residual stress at the sheared edge. The present invention is applicable to high-strength steel sheets having a tensile strength of 590 MPa or more, but is effective for high-strength steel sheets having a tensile strength of 980 MPa or more, which is particularly concerned about delayed fracture. It's an effective technique.

In this embodiment, as shown in FIG. 1, there are a trim process 1 and a press process 2 as pre-processes for press molding. The metal plate manufactured in this embodiment is suitable as a metal plate for press forming in which tensile residual stress is generated in the sheared end face.
In the trim process 1, the metal plate is cut into a contour shape corresponding to the shape of the pressed part.
During this cutting, the entire circumference of the metal plate is continuously sheared twice (double shearing 1A).

At this time, as shown in FIG. 2, the second shearing is set to cut into the desired contour shape, and the first cutting is performed as shown in FIG. 2(a)→FIG. 2(b). The target contour shape is set so as to be cut into a provisional contour shape that is larger by the cutting allowance ΔC in the second cutting process. The dashed line position in FIG. 2(b) indicates the position of the target contour shape. FIG. 2(c) is an example of the metal plate 10 after being cut for the second time.
In the present embodiment, the size of the cutting allowance ΔC during the second cutting is set to 1.2 times or more and less than 20 times the plate thickness t of the metal plate 10 .

In the pressing process, the metal plate 10 subjected to the double shearing process according to the present invention is press-molded using a die to obtain the desired pressed part. Note that press molding is, for example, foam molding or draw molding.
Here, in the above description, the case where the entire circumference of the metal plate 10 is sheared twice based on the present invention is exemplified. However, the invention is not so limited. For example, only one side of the metal plate 10 may be sheared twice according to the present invention. In this case, the edges where a predetermined or more tensile residual stress is generated by press forming are estimated by CAE analysis, and only the edges where it is estimated that a predetermined or more tensile residual stress is generated are sheared twice based on the present invention.

Further, when performing the double shearing process according to the present invention, it is not necessary to perform the double shearing process on all the edges of the outer periphery of the metal plate 10 at the same time. For example, after performing the double shearing process based on the present invention on the first side, the double shearing process based on the present invention may be separately performed on the second side.
Further, as the shape of the pressed part becomes more complicated, the pressed part is manufactured by multistage press molding. In this case, the double shearing according to the invention does not necessarily have to be carried out before the first pressing. For example, double shearing according to the present invention may be performed after any pressing except the final pressing method. Also, in the double shearing process according to the present invention, one or more press forming steps may be performed between the first shearing process and the second shearing process.

According to this embodiment, it is possible to reduce the tensile residual stress on the sheared end surface of the metal plate 10 made of a high-strength steel plate that is generated during shearing. As a result, according to the aspect of the present invention, it is possible to improve the delayed fracture resistance when applying the high-strength steel sheet to various parts such as automobile panel parts and structural/skeletal parts.
In the above description, the case where the metal plate 10 subjected to the double shearing process according to the present invention is press-molded to form the desired product, but the metal plate 10 used without press-molding is used. Even if there is, the shearing method of the present invention is applicable.
Also, other shearing processes may be performed prior to the double shearing process according to the present invention.

As described above, according to the present embodiment, when the metal plate 10 is sheared, the same end portion is sheared twice, and sheared with an appropriate cutting allowance ΔC in the second shearing process. Thereby, according to this embodiment, the tensile residual stress of the sheared end surface can be reduced, and the occurrence of delayed fracture from the sheared end surface can be suppressed.

Next, this principle will be explained.
Shearing is performed, for example, by moving the upper blade 13 in the plate thickness direction relative to the lower blade 11 while the metal plate 10 is restrained by the lower blade 11 and the plate retainer 12 as shown in FIG. done. The lower blade 11 and the upper blade 13 are punches and dies, for example.
For example, the first shearing is normal shearing as shown in FIG.
In the second shearing process, the metal plate 10 is restrained again by the lower blade 21 and the plate retainer 22 as shown in FIG. In addition, in a series of shearing processes in which the material is sheared twice, the same lower blade and the same plate retainer 12 may be used for the first time and the second time.

As shown in FIGS. 4(b) and 4(c), as the upper blade 23 descends, the edge of the metal plate 10 is bent and cut. This bending deformation promotes breakage of the sheared portion (sheared edge) produced by the first shearing. This promotion of fracture is suppressed by the second shear according to the present invention. That is, according to the double shearing process according to the present invention, the work-hardened region of the sheared edge is made smaller than that of the normal sheared edge. As a result, it is possible to reduce the residual stress on the sheared end face after the second shearing. Also, by adjusting the cutting allowance ΔC for the second shearing, it is possible to suppress burrs on the sheared end face.
The upper blade 23 used for the second shearing may be the same as the upper blade 13 used for the first shearing.

Here, as a similar processing method, there is shaving processing. However, in this case, as described in Patent Document 1, for example, the shaving process has a very small cutting allowance in the second shearing process, and aims to remove the plastically processed region during the first shearing process. It is characterized by The shaving process has a large effect of reducing the tensile residual stress on the sheared end face, but it is difficult to manage the cutting allowance during the second shearing process, and the problem of the workpiece coming into contact with the tool occurs. On the other hand, in the present embodiment, since the cutting allowance ΔC is large, the possibility of the problem of tool contact occurring is small, and the degree of freedom in the second shearing process is high.

Next, the cutting allowance ΔC in the second shearing process will be supplemented.
As a result of a detailed study by the present inventor, in a high-strength steel plate, especially a metal plate 10 having a tensile strength of 1180 MPa or more, the cutting allowance ΔC that gives bending deformation to the end of the metal plate 10 by the second shear is It was found that the thickness should be 1.2 times or more the thickness t. If the cutting allowance ΔC is less than 1.2 times the plate thickness t, the member may not be subjected to sufficient bending deformation, and breakage of the sheared portion may not be facilitated.

In addition, when the cutting allowance ΔC in the second shearing is set to 20 times or more of the plate thickness t, the rigidity of the workpiece on the drop-off side during shearing is high, and the bending deformation of the sheared portion cannot be promoted. There was a risk that a large bending deformation could not be applied to the sheared end face on the left side. The tendency becomes higher as the tensile strength of the metal plate 10 to be used increases, but even with the metal plate 10 having a tensile strength of 1180 MPa or more, the cutting allowance ΔC in the second shearing is less than 20 times the plate thickness t. By suppressing it, the tensile residual stress on the sheared end face can be reduced. As a result, according to the present embodiment, it is confirmed that the occurrence of delayed fracture from the sheared end face is suppressed, and the delayed fracture resistance is improved.

For the above reasons, in the present embodiment, the cutting allowance ΔC in the second shearing is 1.2 times or more and less than 20 times the plate thickness t of the metal plate 10 . More desirably, the cutting allowance ΔC in the second shearing is 1.2 times or more and less than 3 times.
In the second shearing process, the clearance (d/t), which is the ratio of the gap d between the upper blade 23 and the lower blade 21 used and the thickness t of the metal plate 10, is 5% or more and less than 30%. is desirable.
If the clearance (d/t) is less than 5%, a secondary sheared surface is generated during shearing, which may result in an unfavorable state of the sheared end surface. Furthermore, the tensile residual stress may increase.

On the other hand, when the clearance (d/t) is 30% or more, burrs exceeding a predetermined amount are generated on the sheared edge, which may greatly impair the formability of the sheared edge. Furthermore, since non-uniform deformation stress is applied to the processed surface by the end of the shearing process, the residual tensile stress after the end of the shearing process may increase.
On the other hand, by setting the clearance (d/t) to 5% or more and less than 30%, the secondary shear surface is suppressed during the shearing process, and the tensile residual stress does not increase. Furthermore, burrs exceeding a predetermined amount do not occur on the sheared end surface, the formability of the sheared end surface is not significantly impaired, and uneven deformation stress is prevented from being applied to the processed surface by the end of the shearing process. Tensile residual stress after completion of shearing does not increase.
A more preferable clearance (d/t) is 10% or more and less than 20%.

Also, as shown in FIG. 5, the smaller the cutting edge radii R1 and R2 of the tools that constitute the upper blade 23 and the lower blade 21 used for shearing, the better. When the cutting edge radii R1 and R2 are large, the tensile residual stress on the sheared end face after processing becomes large. Therefore, the cutting edge radii R1 and R2 of the tools used for shearing are preferably 1 mm or less, more preferably 0.1 mm or less.
For example, in the case of the metal plate 10 having a tensile strength of 1180 MPa or more, the thickness of the metal plate 10 is preferably 0.8 mm or more and 3.0 mm or less from the viewpoint of press formability. If the plate thickness is 0.8 mm or less, the metal plate 10 is easily broken during press molding. On the other hand, if the plate thickness is 3.0 mm or more, the forming load during press forming becomes large, and a very large facility capacity is required.

Next, examples relating to this embodiment will be described.
Here, two types of test materials A and B made of high-strength steel sheets with a thickness of 1.4 mm were used. The dimensions of test materials A and B before shearing are 100 mm×100 mm.
First, the test material was cut into a size of 100 mm×50 mm in the first cut.
For the samples for verifying the effect of the shearing method of the present invention, after the first cutting, the cutting allowance ΔC was changed and the second cutting was performed. The clearance at the time of cutting was set to 12.5% for both the first cutting and the second cutting.

After the sample preparation, the residual stress of the sheared end face after cutting by X-ray was measured. Further, the prepared sample was immersed in hydrochloric acid having a pH of 3 for 96 hours, and then the presence or absence of edge cracks in the sample was checked to evaluate the delayed fracture resistance.
The cracks were confirmed by X-ray measurement, and the measurement range was 300 μm in diameter. In addition, the stress at the central position with respect to both the plate surface and plate thickness directions of the sheared end face after shearing was measured.
Table 1 shows the tensile strength of the test material, the amount of the second cutting allowance of the sample, the residual stress of the sheared end face, and the crack judgment result of the immersion test.

As can be seen from Table 1, by cutting twice, the tensile residual stress at the sheared end face is reduced, and it can be seen that the result of crack determination in the immersion test also corresponds.
However, when the cutting allowance for the second cutting was set to 20 times the plate thickness, the tensile residual stress reduction effect did not appear. Thus, as can be seen from Table 1, by setting the cutting allowance ΔC of the second shearing process to 1.2 times or more and less than 20 times the thickness of the metal plate 10, the delayed fracture resistance is greatly improved. found to improve.

Table 2 shows the results of the residual stress and the immersion test on the sheared edge surface when the tip radius of the tool constituting the upper cutting edge 13 and the lower cutting edge 11 is changed when sheared twice.

As can be seen from Table 2, when the radius of the cutting edge of the tool is 2 mm, the residual stress on the sheared edge surface is large, and cracking occurs in the immersion test.
In addition, Table 3 shows the residual stress of the sheared end surface and the results of the immersion test when the clearance is changed when sheared twice.

As can be seen from Table 3, when the clearance conditions are changed, the residual stress at the sheared end faces increases under the clearance condition of 3%. This is presumed to be caused by an increase in residual stress due to the occurrence of secondary shear planes. Moreover, when the clearance was 30%, it was confirmed that excessive burrs were generated and the residual stress was increased.
From the above, it can be said that the shearing method according to this method is effective in reducing the tensile residual stress by setting appropriate cutting allowance, cutting edge radius, and clearance.

Here, the entire contents of Japanese Patent Application No. 2019-002194 (filed on January 9, 2019), to which this application claims priority, constitute a part of the present disclosure by reference. Although described herein with reference to a limited number of embodiments, the scope of rights is not limited thereto, and modifications of each embodiment based on the above disclosure will be obvious to those skilled in the art.

1 Trimming process 1A Double shearing process 2 Pressing process 10 Metal plate 11 Lower blade (for first shearing)
13 Upper blade (for first shearing)
21 Lower blade (for second shearing)
23 upper blade (for second shearing)
ΔC Cutting allowance for the second shear t Plate thickness

Claims (5)

A method for shearing a metal plate made of a high-strength steel plate,
Shearing is performed twice on at least a part of the edge of the metal plate,
A method of shearing a metal plate, wherein the cutting allowance of the second shearing of the double shearing is two times or more and less than 20 times the thickness of the metal plate. In the upper and lower blades used in the second shearing process, the clearance (d / t), which is the ratio of the gap d between the upper and lower blades to the thickness t of the metal plate, is set to 5% or more and less than 30%. 2. The method for shearing a metal plate according to claim 1, wherein the range is set. 3. The method for shearing a metal plate according to claim 1, wherein the metal plate has a tensile strength of 1180 MPa or more. When making a metal plate into a pressed part through one or more press moldings,
A method of manufacturing a pressed part, wherein a metal plate sheared by the shearing method according to any one of claims 1 to 3 is used as the metal plate. The first shearing process and the second shearing process in the double shearing process are performed individually until the last press forming of the one or more press formings. A method for manufacturing the stamped parts described.

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