JP7435895B2 - Method for improving delayed fracture characteristics of metal plate, method for producing blank material, and method for producing press-formed product - Google Patents

Description

The present invention relates to a technique for improving the delayed fracture characteristics of a metal plate as a blank material used when manufacturing a molded product by press forming. In particular, the present invention is a technique for improving delayed fracture characteristics at a sheared end face. The present invention also relates to a technique for press-forming a metal plate made of a high-strength steel plate to produce a molded product with good delayed fracture characteristics.
Here, in this specification, an end surface obtained by shearing a metal plate is referred to as a sheared end surface. Further, in this specification, a steel plate having a tensile strength of 1470 MPa or more is referred to as an ultra-high strength steel plate. The present invention is suitable for high-strength steel plates having a tensile strength of 980 MPa or more.

Currently, automobiles are required to improve fuel efficiency and collision safety through weight reduction. High-strength steel plates are used in the vehicle body to reduce weight and protect passengers in the event of a collision. Particularly in recent years, ultra-high strength steel plates with a tensile strength of 1470 MPa or more have been used for vehicle bodies. One of the issues when using high-strength steel plates, especially ultra-high-strength steel plates, for car bodies is delayed fracture. For high-strength steel plates with a tensile strength of 980 MPa or more, countermeasures against delayed fracture and stretch flange cracking that occur from the sheared end face, which is the end face after shearing, have become an important issue.

Here, it is known that a large tensile stress remains on the sheared end face. In press parts using metal plates having sheared end surfaces, there is a concern that delayed fracture may occur at the sheared end surfaces. This concern is particularly noticeable with ultra-high strength steel sheets. In order to suppress the above-mentioned destruction at the sheared end face, it is necessary to reduce the tensile residual stress at the sheared end face.

Here, as a simple method for reducing the tensile residual stress on the sheared end surface, for example, there is a method of shearing using a stepped upper blade during punching (Non-Patent Document 1). Moreover, as another method, there is a method of dividing the shearing process into two steps and reducing the cutting margin of the second shearing step (Non-Patent Document 2). However, with such shearing methods, the higher the strength of the material, such as ultra-high-strength steel plates, the more problems arise with wear of the shearing blades and management of shearing conditions. That is, these methods have practical difficulties.

Further, as a method for reducing the tensile residual stress at the sheared end face by plastic working after shearing, there is a method described in Patent Document 1. In this method, the sheared scrap is pushed up by a punch opposing the punch, and the sheared end faces are spread apart. However, such a plastic working method requires a special equipment configuration such as opposed punches, and the lead time of the shearing process also increases. Therefore, this method was not necessarily easy to apply.
Conventionally, in molded products using high-strength steel plates, especially ultra-high-strength steel plates, there has been concern about delayed fracture occurring from the sheared end surfaces of the plates.

Yuzo Takahashi et al.: Improving the expandability of punched holes in high-strength thin steel plates by punching under tension using a punch with protrusions, Plasticity and Processing, 54-627 (2013), 343-347 Takeo Nakagawa, Seita Yoshida: Cutting method - Measures to improve the elongation deformability of the sheared surface -, Plasticity and Processing, 10-104 (1969), 665-671

Patent No. 6562070

The present invention has been made with attention to the above points, and an object of the present invention is to suppress delayed fracture from a sheared end surface after forming by a simple method. For this reason, an object of the present invention is to improve the delayed fracture characteristics of a metal plate made of a high-strength steel plate, thereby making it possible to provide a molded product with good delayed fracture characteristics.

The present disclosure is a technology for improving the delayed fracture characteristics of metal plates by plastic working after shearing, which is easy to apply, even for high-strength steel plates such as ultra-high strength steel plates.
That is, in order to solve the problem, one aspect of the present invention is a method for improving delayed fracture characteristics of a metal plate made of a high-strength steel plate and having a sheared end face on at least a part of the plate end. The gist of the present invention is to apply plastic deformation to at least a portion of the sheared end surface of the metal plate.
The above-mentioned plastic deformation may be applied to at least the sheared end face, for example, the end portion including the sheared end face.
Moreover, the above-mentioned plastic deformation does not necessarily need to be applied to the entire sheared end face. In the present disclosure, for example, the above-described plastic deformation may be applied to a portion of the sheared end face where it is estimated that at least a predetermined amount of delayed fracture will occur.

According to the aspect of the present invention, it is not necessarily necessary to control blade wear and shear conditions even when using a high-strength steel plate. Further, according to the aspect of the present invention, it is possible to reduce the tensile residual stress on the sheared end surface of the steel plate that occurs during shearing by a simple method. As a result, according to the aspect of the present invention, delayed fracture characteristics can be improved when high-strength steel plates are applied to various parts such as panel parts and structural/skeletal parts of automobiles.

It is a figure showing an example of a manufacturing process of a molded article concerning an embodiment based on the present invention. It is a schematic diagram of a sheared end face, (a) is a sectional view, and (b) is a plan view seen from the end face direction. It is a figure which shows the example of stress distribution in the edge part which has a sheared end surface. FIG. 3 is a diagram illustrating a stress relaxation mechanism near a sheared end surface due to processing. It is a figure which shows the bending and unbending process by press forming. It is a figure which shows the bending unbending process by leveling using a leveler. It is a figure which shows the angle (theta) (angle of bending) which the outline of a sheared end surface (extending direction of an end surface) and the bending direction of bending and unbending make. It is a figure which shows the example of the residual stress which arises in the inside and outside of bending after the final bending process in bending and unbending process. (a) is a diagram showing a state in which bending has been performed. (b) is a diagram showing a state in which the metal plate is released from the mold (springback occurs).

Next, aspects of the present invention will be described with reference to the drawings.
(composition)
As shown in FIG. 1, the method for manufacturing a molded product of this embodiment includes a blank manufacturing process 1 and a press molding process 2.
The present invention is suitable when the target metal plate is a high-strength steel plate, particularly a high-strength steel plate having a tensile strength of 980 MPa or more.
(Blank material manufacturing process 1)
The blank material manufacturing process 1 is a process of manufacturing a blank material (metal plate) used in the press forming process 2, which is press-molded into the shape of a molded product. The blank manufacturing process 1 includes a shearing process 1A and an end surface improvement process 1B.

<Shearing process 1A>
The shearing step 1A is a step of cutting a metal plate into a blank shape suitable for manufacturing a molded product.
<End face improvement process 1B>
The end surface improvement step 1B is a step of applying plastic deformation to at least a portion of the sheared end surface of the metal plate after the shearing step 1A. Plastic deformation is defined as deformation in which distortion is input along the extending direction of the end face.

At this time, plastic deformation may be applied only to a region including a portion of the end face where it is estimated that a preset residual stress will occur due to shearing, for example, by structural analysis such as CAE.
Furthermore, the plastic deformation described above applies a plastic strain greater than 0 in the direction along the extending direction of the end face. Although there is no regulation regarding the upper limit of the plastic strain to be applied, the plastic deformation should be applied to an extent that does not cause cracking.

It is preferable to apply plastic deformation by bending and unbending.
At this time, during each bending and unbending, it is preferable to set the bending angle at each end face position where plastic deformation is applied to be less than 90 degrees. "Bending angle less than 90 degrees" will be explained with reference to FIG. 7. This bending angle refers to the angle formed by a straight line (tangential direction) along the extending direction of the sheared end surface and each bending direction of bending and unbending at a location on the sheared end surface where plastic deformation is applied. However, the above-mentioned bending is based on the premise that a plastic strain larger than 0 is applied in the direction along the extending direction of the end face.

The bending and unbending process is performed, for example, by bending process by press forming (see FIG. 5). Further, the bending and unbending process is performed, for example, by leveling process using a leveler having a plurality of rolls lined up in the conveyance direction of the plate (see FIG. 6). Leveling processing is a processing method used when flattening a plate.

In the bending and unbending process, bending and unbending (reverse bending) are performed multiple times on the same sheared end surface in the plate thickness direction. At this time, it is preferable to set the final bend so that the outer side of the bend is on the burr side of the sheared end surface. The burr side is the side in the plate thickness direction where burrs are formed due to shearing.
Here, the bending and unbending process may be performed such that the plastic deformation described above is applied to the end portion including the target sheared end surface (for example, a range including a range of 1 mm from the end surface).
Further, it is preferable that the plastic deformation is applied so that the plate end after the plastic deformation is applied in the end face improvement step 1B becomes flat.

(Press molding process 2)
The press forming step 2 is a step of press forming the blank material made of the metal plate manufactured in the blank material manufacturing step 1 into the desired part shape. Press forming is performed in one press process or in multiple stages.
(Press molded product)
In the press-formed product (product) manufactured by the manufacturing method of the present embodiment, a plastic strain greater than 0 is applied to at least a portion of the sheared end face in a direction along the extending direction of the end face.
As a result, the press-formed product of this embodiment becomes a press-formed product with improved delayed fracture characteristics.

(Modified example)
The above embodiment is an example in which the present disclosure is applied to the manufacture of a blank material before the step of press-working a metal plate into a desired product shape. That is, in the above embodiment, a case is illustrated in which the method for improving delayed fracture characteristics of a metal plate (end face improvement step 1B) of the present disclosure is applied as a pretreatment for press working.
The end surface improvement step 1B of the present disclosure may be applied during or after press processing to form the desired product shape. Specifically, the end surface improvement step 1B of the present disclosure may be applied to a sheared end surface generated by shearing an end portion for shaping the outer periphery of a plate.
For example, when the end of the plate is sheared to shape the part after being molded into the desired product shape, the process of the end face improvement step 1B described above may be applied to the sheared end face.
However, the plastic deformation in the end face improvement step 1B is different from press forming for forming the plate into the desired product shape. Considering the influence on press molding for molding into a product shape, it is preferable to carry out the following procedure. That is, it is preferable to perform the process of applying plastic deformation in the end surface improvement step 1B only to the end portion having the sheared end surface (for example, only the flange portion).

(effect)
In this embodiment, the sheared end face is subjected to plastic deformation by plastic working. Preferably, the plastic working of the present disclosure is performed by bending and unbending. As a result, even if the metal plate (blank material) is a high-strength steel plate such as an ultra-high-strength steel plate, the residual stress at the sheared end surface can be reduced by a simple method. Moreover, in this embodiment, it is possible to obtain the above effects while maintaining the shape of the plate in the same flat state as after shearing.
Further, by reducing the residual stress at the sheared end face, the occurrence of delayed fracture is suppressed. That is, the delayed fracture characteristics at the sheared end face of the metal plate are improved.

Here, by setting the angle of each bend at each sheared end surface in the bending and unbending process to less than 90 degrees, it is possible to introduce sufficient plastic deformation to the sheared end surface.
Note that plastic deformation can be easily imparted to the end face of the plate by performing the bending and unbending process by bending deformation by press forming or leveling process to flatten the plate.
At this time, it is desirable that the outer side of the final bending process be on the burr side of the sheared end surface. Here, the burr side in the plate thickness direction is a site where delayed fracture is likely to occur due to the influence of burrs and rough surface texture. In this case, delayed fracture starting from the burr can be further suppressed.

(About action (mechanism) and others)
<About stress relaxation due to plastic deformation>
Hereinafter, relaxation of stress due to plastic deformation of the sheared end surface, which is generated by applying the present disclosure, will be explained.
FIG. 2 is a diagram showing the state of the end of the board when the end of the board is cut by moving the shearing blade from the upper side toward the lower side. In the case of FIG. 2, the burr side is downward.
In this case, in the sheared end surface 10A and the end portion including the sheared end surface 10A, the residual stress in the direction along the extending direction of the sheared end surface 10A is as shown in FIG. At this time, the direction along the extending direction of the sheared end surface 10A is the plate width direction (see FIG. 2(b)). FIG. 3 shows an example of stress distribution in a direction away from the sheared end surface 10A (direction perpendicular to the end surface 10A), as indicated by the arrow in FIG. 2(a).

As can be seen from FIG. 3, first to third regions ARA1, ARA2, and ARA3 exist inward from the sheared end surface 10A. The first region ARA1 is a region having strong tensile residual stress on the surface of the sheared end face. The second region ARA2 is a region having compressive residual stress to balance the tensile residual stress. The third area ARA3 is an area where no residual stress exists, which is inside the second area ARA2.
Among these three areas ARA1 to ARA3, plastic deformation is introduced by uniform tensile strain caused by bending and tensile working, with the burr side being the outside of the bend, centering on the first and second areas ARA1 and ARA2. After that, when the plastically deformed plate is uniformly spring-backed, it becomes as shown in FIGS. 4(a) to 4(c). That is, in the first region ARA1, the initially existing tensile residual stress is relaxed due to the stress-strain history. Then, the difference between the stress in the first region ARA1 on the front side and the stress in the second region ARA2 inside is reduced. This also applies when the plastic strain introduced by shearing is compressive strain.

From the above, it can be seen that if sufficient tensile or compressive plastic strain can be introduced into the sheared end face 10A, it is possible to relieve the residual stress on the surface of the sheared end face 10A.
In particular, if bending and unbending is employed as a method for introducing plastic strain, it is possible to relieve stress while maintaining the shape of various plates in the same flat state as after shearing.
However, the sheared end surface 10A that is considered in the present disclosure is, for example, a sheared end surface of a metal plate 10 of an arbitrary shape produced by shearing. In the present disclosure, the sheared end surface 10A is intended to be an end surface 10A of a punched portion or an end surface 10A forming a contour line defining the outer shape of the blank material.

Here, FIG. 3 shows a case where a sample piece made of a high-strength steel plate with a tensile strength of 980 MPa was used. In this case, the depth d from the end surface 10A to the boundary between the second area ARA2 and the third area ARA3 was 1 mm. Therefore, the region to which plastic deformation is applied may be a region within a depth d=1 mm from the surface of the end face 10A produced by shearing, where strain and stress due to shearing exist. That is, the bending and unbending process may be performed so as to apply shear deformation to the end portion at least in a region of 1 mm from the sheared end surface 10A. Note that the depth d of the first region ARA1 is, for example, 100 μm.

<About the method of imparting plastic deformation>
Here, let us consider the case where plastic strain is introduced by uniaxial tension or uniaxial compression. In this case, the thickness of the plate changes due to the introduction of plastic strain. Furthermore, in the case of a blank with a complicated shape, the strain is concentrated in the narrow portion in the direction perpendicular to the tension axis, so that the blank cannot be deformed uniformly. Furthermore, if plastic strain is introduced through simple bending, the entire blank will bend significantly after forming. Therefore, it is impossible for the metal plate 10 to maintain the same flat state as after shearing.

From these facts, it can be seen that it is preferable to apply plastic deformation by bending and unbending. Note that when the end surface 10A has a curved shape in which the contour shape in the extending direction changes in a direction perpendicular to the end surface 10A, the following procedure may be performed. That is, the bending and unbending process may be performed such that a depth of 1 mm or less from the surface of the end surface 10A is secured at the end of the sheared end surface 10A, which is the most concave portion.
It should be noted that although a simple bending process may be performed only once, bending and unbending processes are employed in consideration of returning to the original flat shape.

The bending and unbending process is performed by bending by press forming as shown in FIG. 5 or by leveling as shown in FIG. In this case, bending and unbending deformation is caused on the surface of the sheared end surface 10A to relieve residual stress on the sheared end surface 10A and suppress delayed fracture.
The die 20 and punch 21 for bending and the die 22 and punch 23 for reverse bending shown in FIG. 5 may have the same mold, or may use different molds. Good too.
Furthermore, the diameters of the leveler rolls 30 do not have to be the same.

Here, the bending and unbending process can also be performed by bending and deforming by press forming. However, between the blanking process and the subsequent molding process, at least two pressing processes and the addition of a mold are required.
On the other hand, by performing the bending and unbending process by leveling process, it can be performed relatively easily using only a leveler between the blanking process by shearing and the subsequent forming process. However, in the present disclosure, it is necessary to use a sufficiently strong leveler that can introduce plastic strain even into a steel plate with a tensile strength of 980 MPa or higher.

<About bending angle θ>
In order to improve the delayed fracture characteristics by relaxing the residual stress of the sheared end face 10A, bending and unbending deformation to the extent that sufficient plastic deformation occurs is preferable. In order to obtain this effect, it is necessary that the tensile or compressive plastic strain on the sheared end surface 10A be 0.003 or more. Preferably, if the plastic strain is 0.005 or more, residual stress at the sheared end surface 10A can be significantly alleviated.
Note that the processing step in which this plastic strain is introduced may be either bending or unbending deformation, and if sufficient plastic strain is introduced even once, the residual stress will be alleviated.

Here, as shown in FIG. 7, the bending angle θ formed by the contour line of the sheared end face 10A (extending direction of the end face 10A) and the bending direction of the bending and unbending is, for example, in the range of 0 degrees or more and 75 degrees or less. The bending angle θ is preferably in the range of 0 degrees or more and 45 degrees or less. This is because when the bending direction and the contour line of the sheared end surface 10A are close to 90 degrees, it becomes difficult to introduce strain into the surface of the sheared end surface 10A. The reason for this is that the surface of the sheared end surface 10A is open to the deformation direction of tension and compression in the direction along the end surface 10A due to bending. In FIG. 7, the outline of the sheared end face 10A before bending is shown as a straight line, but the outline of the end face 10A may be a curved line or a partially discontinuous line.

<Final bending direction>
Due to the final bending process, tensile strain is applied to the tensile part as shown in FIGS. 8(a) to 8(b). Note that FIG. 8 shows a case where the final bending is downward. For this reason, after springback when the press is released, the residual stress is due to the compression side. Therefore, it is desirable that the outer side of the final bending process (the lower side in FIG. 8) be on the burr side of the sheared end surface 10A. Note that the burr side is a site where delayed fracture is likely to occur due to the influence of burrs and rough surface texture. By making the burr side the outer side of the bend, the residual stress on the burr side of the sheared end surface 10A is reduced by the residual stress due to compression due to forming.

(others)
The present disclosure can also take the following configuration.
(1) A method for improving delayed fracture characteristics of a metal plate made of a high-strength steel plate and having a sheared end face on at least a part of the plate end, the method comprising: Apply plastic deformation to the part.
(2) In the plastic deformation, a plastic strain greater than 0 is applied to at least a portion of the sheared end face in a direction along the extending direction of the end face.
(3) The above-mentioned plastic deformation is performed by bending and unbending.
(4) The angle of each bend in the bending and unbending process is less than 90 degrees.
(5) The above bending and unbending process is performed by bending process by press forming.
(6) The bending and unbending process is performed by leveling process using a plurality of rolls.
(7) The final bend in the bending and unbending process was set so that the outside of the bend was on the burr side of the sheared end surface.

(8) The metal plate is a high-strength steel plate with a tensile strength of 980 MPa or more.
(9) A method for manufacturing a blank material for press forming, comprising a step of shearing a metal plate made of a high-strength steel plate, and a step after the shearing step, the present disclosure as described above. applying plastic deformation to the sheared end face by the method for improving delayed fracture characteristics.
(10) A method for manufacturing a molded product by press-forming a metal plate made of a high-strength steel plate, which includes a step of shearing the metal plate made of a high-strength steel plate, and a step after the shearing step. and a step of applying plastic deformation to the sheared end face by the method for improving delayed fracture characteristics of the present disclosure described above.
(11) A press-formed product formed by processing a metal plate made of a high-strength steel plate and having a sheared end face on at least a part of the plate end, in which at least a part of the sheared end face is formed in the direction in which the end face extends. A plastic strain greater than 0 is applied in the direction along.

Next, an example based on this embodiment will be described.
Here, an example will be described using a test material A using a steel plate having a thickness of 1.4 mm and a tensile strength of 1470 MPa. Note that the present disclosure is not limited to steel plates having a tensile strength of 1470 MPa. The present disclosure can be applied to metal materials such as steel plates with a tensile strength of 980 MPa or more, which cause delayed fracture at sheared end faces.
(About shearing)
In this example, first, sample material A was sheared to produce a linear sheared end face with a length of 500 mm to be evaluated. Note that the clearance during shearing was 12% of the plate thickness.

(bending and unbending processing)
The produced sheared end surfaces were bent back by press forming as shown in FIG. 5 or leveling as shown in FIG. 6 so that the maximum plastic strain in each step was changed.
Here, the angle between the contour line of the sheared end surface defined in FIG. 7 and the bending direction of bending and unbending was also changed, and each sample after bending and unbending was fabricated.
Note that during leveling, a large strain was applied with the first roll, as is usually done. At this time, the pushing amount of each roll was adjusted so that the strain applied gradually decreased toward the last roll.

(evaluation)
After the sample was prepared, the residual stress of the sheared end surface after cutting using X-rays was measured. Furthermore, each sample was immersed in a hydrochloric acid bath with a pH of 1 for 96 hours, and the presence or absence of cracks in the sample and the time at which cracks occurred were confirmed. At this time, it was determined that delayed fracture had occurred since the surface crack penetrated through the plate thickness due to delayed fracture of the sheared end face. In addition, the measurement range for the X-ray measurement was set to 500 μm in diameter. Then, the stress at the center of the plate thickness was measured in the direction parallel to the sheared end surface after shearing.

(Example 1)
Sample forming conditions and evaluation results in Example 1 are shown in Tables 1 and 2, respectively. In the examples shown in Table 1, the bending and unbending process was performed by press forming.
Table 1 shows the results when the angle between the contour line of the sheared end face and the bending direction of bending and unbending in press molding was set to 0 degrees. Specifically, Table 1 shows the relationship between the maximum amount of plastic strain introduced during bending and unbending, the presence or absence of delayed fracture, the time at which delayed fracture occurs, and residual stress.

Further, in Table 2, the bending and unbending process was performed by press molding.
Table 2 shows the results when the angle between the contour line of the sheared end face and the bending direction of bending and unbending was set to 0 degrees in leveling. Specifically, Table 2 shows the relationship between the maximum amount of plastic strain introduced during bending and unbending, whether delayed fracture occurs, the time at which delayed fracture occurs, and residual stress.

Here, in both of the examples shown in Tables 1 and 2, the final bent outer side in the bending and unbending process was set to be the sagging side of the sheared end surface.
As can be seen from Tables 1 and 2, when the plastic strain was 0.003 or less, the time until delayed fracture occurred was extended. Further, when the plastic strain was 0.005 or more, delayed fracture no longer occurred. Furthermore, the time at which delayed fracture occurred or the presence or absence of delayed fracture was found to be correlated with residual stress.

(Example 2)
In Example 2, we investigated the relationship between the maximum amount of plastic strain introduced during bending and unbending, the presence or absence of delayed fracture, and the occurrence time of delayed fracture when each bending angle in bending and unbending was changed. This is an example of what we investigated.
Table 3 shows an example where the bending and unbending process is performed by leveling.
However, the final bent outer side in the bending and unbending process was made to be on the sagging side of the sheared end surface. Further, the maximum amount of plastic strain was set to 0.005.

As can be seen from Table 3, it was confirmed that the occurrence of delayed fracture is suppressed when the angle between the contour line of the sheared end face and the bending direction of bending and unbending is between 0 degrees and 85 degrees, compared to when the angle is 90 degrees. Ta. That is, it was confirmed that when the angle is smaller than 90 degrees, the occurrence of delayed fracture is suppressed compared to when the angle is 90 degrees. In particular, remarkable effects were obtained when the bending angle between the contour line of the sheared end face and the bending direction of bending and unbending was from 0 degrees to 75 degrees.
In addition, in the example shown in Table 3, in the case of leveling, the influence of the angle formed by the contour line of the sheared end face and the bending direction of bending and unbending was explained. However, the present disclosure is not limited thereto. Even in bending and unbending processing by press forming, good results can be obtained within the above angle range even when the amount of plastic strain is different from 0.005.

(Example 3)
In Example 3, when the bending and unbending process is performed by press forming and leveling, the occurrence time of delayed fracture, the presence or absence of occurrence of delayed fracture, and the residual stress are shown, respectively. In Example 3, a case where the final outside of the bend in bending and unbending becomes the burr side, and a case where the final outside of the bend becomes the sag side are described. However, the maximum amount of plastic strain was set to 0.003. Further, the angle formed between the bending direction and the sheared end surface 10A was 0 degree.
The results are shown in Table 4.
Here, in Table 4, the measurement range for the X-ray measurement was set to 250 μm in diameter, and measurements were made at a position 0.25 mm from the plate surface on each of the burr side and the sag side of the plate thickness. The measurement was performed in a direction parallel to the sheared end surface 10A after shearing. The former was defined as the burr side residual stress, and the latter was defined as the sagging side residual stress.

As can be seen from Table 4, residual stress tends to increase on the inner side of the final bend after bending and unbending, and tends to move toward the tensile side. On the other hand, as can be seen from Table 4, there was a tendency for the residual stress to decrease on the outer side of the final bending and move toward the compression side.
The difference was greater in the case of bending and unbending by pressing than in the case of leveling. The reason for this is that in the leveling process, the amount of deformation due to bending and unbending gradually decreases from the start to the end of the process, so that the difference in stress in the plate thickness direction is equalized.

Furthermore, the time until delayed fracture occurred was longer as the residual stress on the burr side was lower. This is because the original residual stress is higher on the burr side, and delayed fracture is more likely to occur due to the influence of burrs and rough surface texture.
Therefore, it has been found that delayed fracture can be further suppressed by making the final outer side of the bending and unbending become the burr side of the sheared end face.

Here, the entire contents of Japanese Patent Application No. 2021-146245 (filed on September 8, 2021), to which this application claims priority, constitute a part of the present disclosure by reference. Although the description has been made here 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 Blank manufacturing process 1A Shearing process 1B End face improvement process (delayed fracture characteristic improvement method)
2 Press forming process 10 Metal plate 10A Sheared end surface θ Bending angle

Claims (9)

A delay in improving the delayed fracture characteristics of a metal plate used when manufacturing a molded product by press forming, which has a sheared end surface on at least a part of the plate edge and is made of a high-strength steel plate. A method for improving fracture characteristics,
Applying plastic deformation to at least a portion of the sheared end surface of the metal plate on which the sheared end surface has been formed by shearing processing so that plastic strain is input along the extending direction of the sheared end surface ,
The above-mentioned plastic deformation is performed by bending and unbending processing through multiple press forming steps.
A method for improving delayed fracture characteristics of a metal plate. In the plastic deformation, a plastic strain greater than 0 is applied to at least a portion of the sheared end face in a direction along the extending direction of the end face.
The method for improving delayed fracture characteristics of a metal plate according to claim 1. 2. The method for improving delayed fracture characteristics of a metal plate according to claim 1 , wherein the angle of each bend in the bending and unbending process is less than 90 degrees. 2. The method for improving delayed fracture characteristics of a metal plate according to claim 1 , wherein the bending and unbending process is performed by bending process by press forming. 2. The method for improving delayed fracture characteristics of a metal plate according to claim 1 , wherein the bending and unbending process is performed by leveling process using a plurality of rolls. 2. The method for improving delayed fracture characteristics of a metal plate according to claim 1 , wherein the last bend in the bending and unbending process is set so that the outer side of the bend is on the burr side of the sheared end surface. The method for improving delayed fracture characteristics of a metal plate according to claim 1, wherein the metal plate is a steel plate having a tensile strength of 980 MPa or more. A method for producing a blank material for press forming, the method comprising:
A process of shearing a metal plate,
A step after the step of applying the shearing process, a step of applying plastic deformation to the sheared end face by the method for improving delayed fracture characteristics according to any one of claims 1 to 7 ;
A method for producing a blank material comprising: A method of manufacturing a molded product by press-forming a metal plate made of high-strength steel plate, the method comprising:
A process of shearing a metal plate,
A step after the step of applying the shearing process, a step of applying plastic deformation to the sheared end face by the method for improving delayed fracture characteristics according to any one of claims 1 to 7 ;
A method for manufacturing a press-formed product comprising:

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