Patent Application
20230113628
The present invention is a technology related to the manufacture of a pressed component having a component shape with concern about the occurrence of a delayed fracture in press forming.
The present invention is a technology particularly suitable for the manufacture of a pressed component using metal sheets containing high-strength steel sheets having a tensile strength of 980 MPa or more.
At present, automobiles have been required to improve fuel consumption by a reduction in weight and collision safety. For the purpose of achieving both the reduction in weight of a vehicle body and the protection of passengers in the event of a collision, high-strength steel sheets tend to be used for structural components for automobiles. Particularly in recent years, as the high-strength steel sheets, ultrahigh-strength steel sheets having higher strength, i.e., having a tensile strength of 980 MPa or more, have been applied to the vehicle body.
A delayed fracture is one of the problems when the high-strength steel sheets are applied to the vehicle body. In particular, a delayed fracture occurring from the end surface after shearing work (hereinafter also referred to as sheared end surface) is a serious problem in high-strength steel sheets having a tensile strength of 1180 MPa or more among the high-strength steel sheets.
Herein, it is known that a large tensile stress remains in the sheared end surface. The remaining of the tensile stress causes concern about the occurrence of the delayed fracture in the sheared end surface with time in a product after pressing (pressed component). To suppress the delayed fracture in the sheared end surface, it is required to reduce the tensile residual stress in the sheared end surface.
As a method for reducing the tensile residual stress of the sheared end surface, a method for raising the steel sheet temperature in shearing work (NPLS 1, 2), a method using a stepped punch in punching work (NPL 3), and a method using shaving (NPL 4, PTL 1) are mentioned, for example.
However, the method for raising the steel sheet temperature in the shearing work requires time to heat the steel sheet. Therefore, this method is not suitable for a mass production step of automobiles and the like. The method using a stepped punch has a problem that an effect of improving a delayed fracture resistance property is low. The method using shaving has a problem of difficulty to manage the clearance in a shaving step.
NPL 5 describes a cut-off punching method by double punching. However, the method of NPL 5 is a punching work technology, and thus cannot be applied to an outer peripheral portion of a product.
The present invention has been made focusing on the above-described points. It is an object of the present invention to provide a technology capable of suppressing a delayed fracture occurring with time while suppressing the occurrence of restrictions on the target pressed component shape.
To solve the problems, one aspect of the present invention is a method for manufacturing a pressed component including manufacturing a pressed component through one or two or more times of press forming, and the method includes: when it is estimated that at least one press forming of the one or two or more times of press forming causes concern about a delayed fracture in an end of a material to be pressed, double cutting processing including performing cutting processing of the end containing at least a place where the delayed fracture is concerned twice as preprocessing for the press forming causing concern about end cracking due to the delayed fracture, in which the double cutting processing includes performing, in first cutting, cutting to form a partial beam-shaped projection portion at a position containing the place where the delayed fracture is concerned, and cutting the projection portion in second cutting.
Another aspect of the present invention is a method for manufacturing a blank material to be formed into a pressed component through one or two or more times of press forming, and the method includes: when it is estimated that at least one press forming of the one or two or more times of press forming causes concern about end cracking due to a delayed fracture in an end of a material to be pressed, double cutting processing including performing cutting processing of the end containing at least a place where the delayed fracture is concerned twice, in which the double cutting processing includes performing, in first cutting, cutting to form a partial beam-shaped projection portion at a position containing the place where the delayed fracture is concerned, and cutting the projection portion in second cutting.
According to the aspects of the present invention, it is possible to suppress the delayed fracture after the press forming while suppressing the occurrence of restrictions on the target pressed component shape.
Embodiments of the present invention will now be described with reference to the drawings.
A method for manufacturing a pressed component of this embodiment is a method for manufacturing a pressed component including manufacturing a target pressed component through one or two or more times of press forming. The press forming in each press forming is performed by stamping or drawing, for example. The method for manufacturing a pressed component of this embodiment is a technology in which a delayed fracture occurs, in at least one press forming, along the sheet edge after the press forming.
For simplicity of the description, this embodiment gives the description taking, as an example, a case where a pressed component 10 having the shape illustrated in
The component shape of the pressed component 10 illustrated in
It is supposed in this example that, when press forming to which the present invention is not applied is carried out (when a step of
The reference numeral 1A indicates a flange corresponding portion corresponding to a region formed into the flange portion 13 in a material to be pressed 1. Herein, this embodiment describes, as an example, a case where the position of the portion with concern about cracking 3 due to the delayed fracture is located on the end surface formed by the flange portion 13, but the present invention is not limited thereto. It is also assumed that the position of the portion with concern about cracking 3 due to the delayed fracture is located on a sheared surface other than the end surface of the flange portion.
Herein, it is known that a large tensile stress remains in the sheared end surface. The remaining of the tensile stress causes concern about the occurrence of the delayed fracture in the sheared end surface with time in a product after pressing (pressed component). In the end where a compression stress is input in the press forming, the tensile residual stress is generated after the pressing, causing concern about the occurrence of the delayed fracture with time in the product after the pressing (pressed component). Therefore, in the end, which is the sheared end surface and in which the compression stress is input in the pressing, the occurrence of the delayed fracture is particularly concerned.
The confirmation of the presence or absence of the portion with concern about cracking 3 due to the delayed fracture and the specification of the position of the portion with concern about cracking 3 are obtained by carrying out a simulation analysis, such as a CAE analysis, for example. It may be acceptable that the press forming is actually carried out, and then a component after each press forming is observed to confirm the presence or absence of the portion with concern about cracking 3 due to the delayed fracture and specify the position of the portion with concern about cracking 3.
As described above, in the case of the simulation analysis, the delayed fracture may be evaluated by calculating the tensile residual stress after die release. In the case of the actual pressing, a produced sample is measured for the tensile residual stress value of the sheared end surface by X-rays for the evaluation of the delayed fracture, for example. Alternatively, the produced sample is dipped in hydrochloric acid having a pH of 3 for 96 hours, and then the delayed fracture is evaluated based on the presence or absence of the end cracking and the size of cracking in the resultant sample.
This embodiment has, as preprocessing for performing the press forming, a trimming step of shearing the outer periphery of the blank material 1 as an example of the material to be pressed into a profile shape according to the component shape of the pressed component 10.
However, in this embodiment, this trimming step applies double cutting processing of carrying out double cutting based on the present invention as illustrated in
The end position where the end cracking due to the delayed fracture is concerned is a part having the tensile residual stress after the die release of the press forming.
Therefore, the CAE analysis or the like sets, for the target pressed component, a case where the tensile residual stress equal to or larger than a predetermined tensile residual stress is generated as a case where it is estimated that the end cracking due to the delayed fracture is concerned in the end and sets a place where the tensile residual stress equal to or larger than a predetermined tensile residual stress is generated as a place where the delayed fracture is concerned, for example. Further, a place where the delayed fracture has occurred when the present invention is not applied is set as a place where the delayed fracture is concerned, for example.
In this embodiment, the end of the flange corresponding portion 1A to which the double cutting processing is applied in the blank material 1 which is the material to be pressed is cut in first cutting, such that a partial beam-shaped projection portion 2 is formed at a position containing a place where the end cracking due to the delayed fracture is concerned as illustrated in
More specifically, in this embodiment, when the blank material 1 is cut into the target profile shape in the trimming step, the side (edge) of the flange corresponding portion 1A is cut once into a shape having the projection portion 2 partially projecting in a cantilevered beam shape at the position containing the portion with concern about cracking 3A. Subsequently, the projection portion 2 is cut in the second cutting, so that the target profile shape is achieved. As described above, the cutting processing in
The double cutting processing based on the present invention may be carried out independently from the trimming step. For example, it may be acceptable that a plurality of steps (not illustrated) is provided between the steps of
Herein, a width W (length along the edge of the material) of the projection portion 2 is preferably set to ⅓ or less of a length L along the edge of the flange portion 13 or 150 times or less the sheet thickness of the blank material 1.
By forming the temporary beam-shaped projection portion 2 having the width W in the first cutting (shearing), a strain input by the shearing into the portion with concern about cracking 3 can be more certainly suppressed while gaining the cutting amount (punching margin) of the second cutting (shearing), as compared with a case of not temporarily forming the beam-shaped projection portion 2 (see
The lower limit of the width W of the projection portion 2 is not particularly limited insofar as the position where the generation of the portion with concern about cracking 3 is estimated and the shearing can be performed. The lower limit of the width W is equal to or larger than the amount of opening in the edge resulting from the end cracking due to the delayed fracture, for example. The width W of the projection portion 2 is preferably 20 mm or more considering the ease of cutting by shearing.
The projection amount H of the projection portion 2 (maximum value of the projection amount from the target profile position) is 10 times or less the sheet thickness of the blank material 1 or 5.0 mm or less.
By setting a second cut portion as the cantilevered beam-shaped projection portion 2, the strain input by the shearing into the portion with concern about cracking 3 can be more certainly suppressed while gaining the cutting amount (punching margin) of the second cutting (shearing).
The lower limit of the projection amount H of the projection portion 2 is not particularly limited insofar as the projection portion 2 projects by more than 0 mm and the shearing can be performed. The lower limit of the projection amount H is preferably 1 mm or more and more preferably 3 mm or more considering the ease of shearing.
Then, after the double cutting processing above, the target pressed component 10 is manufactured by the press forming.
By performing the double cutting processing above as the preprocessing for the press forming causing the concern about the end cracking, cracking in the portion with concern about cracking 3 due to the delayed fracture can be prevented using common press forming without restricting the component shape.
Herein, the description above is given taking the case where the above-described double cutting processing is carried out as the preprocessing for the press forming as an example. However, a configuration may be acceptable in which the press forming into the target component shape is performed (
The description above gives the case where the portion with concern about cracking 3 is present in one place as an example, but the present invention is applicable even when the portions with concern about cracking 3 due to the delayed fracture are present in two or more places. For each of the portions with concern about cracking 3, the above-described double cutting processing may be performed as the preprocessing for the press forming causing the concern about the end cracking. When the adjacent portions with concern about cracking 3 are close to each other, one projection portion 2 containing the adjacent portions with concern about cracking 3 may be formed in the first cutting.
Herein, the operations/effects of the double cutting processing including cutting the partial cantilevered beam-shaped projection portion, which is formed in the first cutting, in the second cutting are described.
In general, when shearing work is performed, a large tensile stress remains in the edge of the material to be pressed. Therefore, when press forming is carried out which causes the generation of the tensile residual stress in an end 13a of the flange portion 13 along the edge of the flange portion 13, a possibility that the end cracking occurs tends to increase.
To address the problem, by applying the double cutting processing based on the present invention to a part with concern about the occurrence of the end cracking due to the delayed fracture, the tensile residual stress in the sheared end surface decreases (see examples). As a result, this embodiment can prevent the end cracking due to the delayed fracture caused by the tensile residual stress while preventing the occurrence of restrictions on the component shape.
Herein, as illustrated in
To address the problem, as illustrated in
The delayed fracture is more likely to occur in materials having a higher tensile strength, and therefore the present invention is suitable for high-tensile steel sheets having a tensile strength of 590 MPa or more, for example. However, as the material of the blank material 1, the present invention is applicable not only to steel but to iron alloys, such as stainless steel, and further non-iron materials and non-metal materials. The pressed component 10 manufactured according to this embodiment is suitable as automobile components, for example, but the present invention is applicable to all types of work of press forming a sheet material without being limited to the automobile components.
The embodiment above describes the case where the target pressed component 10 is manufactured in one-stage press forming as an example. In general, the more complicated the component shape of the pressed component, the more likely it is to manufacture the target pressed component through two or more times of press forming (a plurality of times of pressing step). When the target pressed component is manufactured by the plurality of times of press forming, the press forming causing the delayed fracture is not necessarily the final step. The delayed fracture sometimes occurs individually in press forming having two or more stages.
For example, in the manufacture of the target pressed component through press forming having five stages, when a simulation, such as the CAE, estimates that the tensile stress equal to or larger than a predetermined tensile stress remains in the press forming in the fourth stage, causing the concern about the delayed fracture, the above-described double cutting processing may be carried out before the press forming in the fourth stage.
The double cutting processing of the present invention is applicable even to drawing as illustrated in
In this example, when the blank 1 is cut into the target sample shape, the beam-shaped projection portion 2 is formed at the position containing the place where the delayed fracture is concerned (
Thereafter, the drawing is applied to a central portion (
The description above is given taking the case where the above-described double cutting processing is carried out as the preprocessing for the drawing as an example. A configuration may be acceptable in which the drawing into the target component shape is performed (
Herein, the double cutting processing is not limited to the trimming step before the press forming described above, and the first cutting and the second cutting may be performed independently from the trimming step as the double cutting processing. When a plurality of press forming steps is provided between the first cutting and the second cutting in the double cutting processing, a configuration may be acceptable in which the double cutting processing is carried out before carrying out at least one press forming among the plurality of press forming steps.
A cutter used for the shearing is not particularly limited, and conventionally known equipment may be used. For example, a clearance C, which is a percentage of a ratio (d/t) of a gap d between an upper blade and a lower blade of the cutter to the sheet thickness t of the material to be pressed, is preferably 5.0% or more and 30.0% or less.
When the clearance C is smaller than 5.0%, a secondary sheared surface is generated in the shearing work, which is not preferable as the state of the sheared end surface. Further, there is a risk of an increase in tensile residual stress.
On the other hand, when the clearance C is larger than 30.0%, there is a risk that a burr equal to or larger than a predetermined burr is generated in the sheared end surface, which greatly impairs the formability of the sheared end surface. Further, non-uniform deformation stress is applied to the worked surface by the end of the shearing work, and therefore there is risk that the tensile residual stress after the end of the shearing work increases.
The clearance C is more preferably 10.0% or more and less than 20.0%.
Next, examples relating to this embodiment are described.
Herein, two types of test materials A, B containing a high-strength steel sheet having a sheet thickness of 1.4 mm are targeted. The dimension of the test materials A, B before the shearing is 100 mm×100 mm.
First, the test materials were cut into a dimension of 100 mm×50 mm in the first cutting. In the first cutting, a projection portion 20C was formed (
Next, after the first cutting work, second cutting to cut the projection portion 20C was carried out (
The cutting work above was carried out several times while changing the projection amount H of the projection portion 20C, thereby producing a plurality of samples.
After the production of the samples, the measurement of the residual stress of the sheared end surface after the cutting by X-rays in an end surface part where the projection portion 20C was cut was carried out. Further, the produced samples were dipped in hydrochloric acid having a pH of 3 for 96 hours, and then the presence or absence of the end cracking in the samples was confirmed, and the delayed fracture resistance property was evaluated.
The confirmation of cracking was performed by the measurement by X-rays, and the measurement range was set to a diameter of 300 μm. A stress at the center position was measured with respect to both directions of the sheet surface direction and the sheet thickness direction of the sheared end surface after the shearing work.
Table 1 shows the tensile strength, the projection amount H of the projection portion 20C (shown as a ratio to the sheet thickness t), the residual stress of the sheared end surface, and cracking determination results in the dipping test of the test materials.
In Table 1, the samples in which the column of the projection amount H of the projection portion 20C is “-” is an example in which the projection portion 20C was not provided and the second cutting was not carried out.
As is understood from Table 1, by providing the projection portion 20C in the first cutting work and cutting the cutting projection portion 20C in the second cutting work, the tensile residual stress of the sheared end surface decreases and the cracking determination results of the dipping test also correspond to the decrease.
However, when the cutting margin of the second cutting work was set to 20 times the sheet thickness, an effect of reducing the tensile residual stress is low. Thus, as is understood from Table 1, it was found that the delayed fracture resistance property was greatly improved by setting the projection amount H of the projection portion 20C to 1.2 times or more and less than 20 times the sheet thickness of a metal sheet 10.
Then, when based on the present invention, it was found that the end cracking due to the delayed fracture can be easily suppressed.
Herein, the entire contents of JP 2020-063178 A (filed Mar. 31, 2020), for which this application claims priority, form part of this disclosure by reference. Herein, the description is given with reference to a limited number of embodiments, but the scope of the invention is not limited thereto and modifications of each embodiment based on the disclosure above are obvious to those skilled in the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-063178 | Mar 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/011181 | 3/18/2021 | WO |
