Stamping method and method for evaluating the shape of a stamped product

Abstract

The stamping method of the present invention suppresses the shape change over time of a stamped product (1) after demolding from a mold and springback, and includes: a stamping process (S1) of stamping a metal plate into a stamped product (1) using a mold; a demolding process (S3) of demolding the stamped product (1) that has been stamped from the mold; and a post-demolding mold holding process (S5) of holding the demolded stamped product (1) in the forming bottom dead center shape for 30 minutes or more using the mold.

Description

Technical Field

[0001] The present invention relates to a press forming method and a shape evaluation method of a press formed part, and particularly relates to suppressing the shape change over time of a press formed part starting from just after springback after die release, and further, a press forming method and a shape evaluation method of a press formed part for taking countermeasures against the shape change over time of the press formed part provided to a subsequent process. Background Art

[0002] Press forming is a manufacturing method capable of manufacturing metal parts at low cost and in a short time, and is used in the manufacture of many automotive parts. In recent years, in order to simultaneously achieve an improvement in the collision safety of an automobile and a weight reduction of the automotive body, a metal sheet of higher strength has been used for the press forming of automotive parts.

[0003] One of the main problems when press forming a high-strength metal sheet is the deterioration of the dimensional accuracy of the press formed part caused by springback. Residual stress generated in the press formed part when deforming the metal sheet using a die during press forming becomes a driving force, and the phenomenon that the press formed part released from the die instantaneously returns to the shape of the metal sheet before press forming like a spring is called springback.

[0004] The higher the strength of the metal sheet (for example, a high-tensile steel sheet), the greater the residual stress of the press formed part generated by press forming, and thus the greater the shape change of the press formed part caused by springback. Therefore, the higher the strength of the metal sheet, the more difficult it is to converge the shape of the press formed part after springback within a specified dimension. Therefore, a technique for accurately predicting the shape change of the press formed part caused by springback is crucial.

[0005] In predicting the shape change of a stamping formed product caused by springback, stamping forming simulation based on the finite element method is usually used. As the sequence in this stamping forming simulation, it is divided into: first, a stamping forming analysis is performed to predict the residual stress generated in the stamping formed product by using a die to stamp a metal sheet to the bottom dead center in the first stage (for example, Patent Document 1); a springback analysis is performed on the stamping formed product removed from the die to predict the shape of the stamping formed product by balancing the moment of force and the residual stress due to the shape change caused by springback in the second stage (for example, Patent Document 2).

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent Publication No. 5795151

[0009] Patent Document 2: Japanese Patent Publication No. 5866892

[0010] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2013 - 113144 Summary of the Invention

[0011] Outline of the Invention

[0012] Problems to be Solved by the Invention

[0013] So far, by performing a stamping forming simulation that combines the above-mentioned first-stage stamping forming analysis and the second-stage springback analysis, it has been possible to predict the shape of the stamping formed product just after springback when removed from the die. However, when the inventors compared the shape of the stamping formed product predicted by the stamping forming simulation with the shape of the actually stamped stamping formed product, they found that there were cases where the shape prediction accuracy based on the stamping forming simulation was reduced for the stamping formed product.

[0014] Therefore, when studying the stamping formed products with reduced shape prediction accuracy in the stamping forming simulation and their causes, it was found that, for example, in a stamping formed product having a bent portion where the metal sheet is bent, the shape is different just after stamping (just after springback when removed from the die) and after several days.

[0015] Figure 11 Shows the measurement of Figure 10 An example of the shape change over time of a stamping formed product 21 having a U-shaped cross-sectional shape as shown. As Figure 11As can be seen, if the opening amount of the press-formed product 21 immediately after springback from the mold is set as the reference (0), a shape change occurs in which the opening amount of the side wall portion 25 gradually increases with the passage of time thereafter.

[0016] Such a shape change of the press-formed product over time is considered to be similar to the phenomenon in which a structural member that continuously bears a high press load from the outside, such as the creep phenomenon (for example, Patent Document 3), gradually deforms. However, the phenomenon that occurs in a press-formed product in a state where no load is applied from the outside as described above has not been known so far.

[0017] Therefore, if only the method of designing a mold using the shape change caused by predicted springback or the method of determining the portion contributing to springback and taking countermeasures to reduce the springback are used, the further shape change of the press-formed product after springback cannot be reduced. Thus, it is known that it is necessary to suppress the shape change of the press-formed product over time after springback.

[0018] In addition, when assembling and fabricating the press-formed product with other components, if the press-formed product undergoes a shape change over time, it will cause obstacles in subsequent processes. Therefore, it is also known that some countermeasures need to be implemented.

[0019] The present invention is an invention made to solve the above problems, and its object is to provide a press-forming method that suppresses the shape change that occurs in a press-formed product over time after press-forming, or a countermeasure for the shape change over time of a press-formed product that is assembled and fabricated with other components in a subsequent process, and a method for evaluating the shape of a press-formed product that is assembled and fabricated with other components in a subsequent process.

[0020] Means for Solving the Problems

[0021] The press-forming method of the present invention suppresses the shape change over time of a press-formed product that rebounds instantaneously after being released from a mold, and includes: a press-forming step of press-forming a metal plate into a press-formed product using the mold; a demolding step of demolding the press-formed press-formed product from the mold; and a die holding process after release of holding the demolded press-formed product in the forming bottom dead center shape using the mold for 30 minutes or more.

[0022] The stamping method of the present invention suppresses the shape change over time of a stamped product that rebounds instantaneously when released from a mold. The method includes: a stamping process of stamping a metal plate into a stamped product using the mold; a demolding process of demolding the stamped product from the mold; and a jig holding process after release of holding all or a part of the demolded stamped product in a predetermined specified shape for 30 minutes or more using a jig capable of holding the shape, including another mold having the same shape as the mold.

[0023] The stamping method of the present invention suppresses the shape change over time of a stamped product that rebounds instantaneously when released from a mold. The method includes: a stamping process of stamping a metal plate into a stamped product using the mold; a die holding process before release of holding the stamped product in the forming bottom dead center without demolding it from the mold for 30 minutes or more; and a demolding process of demolding the stamped product from the mold after the die holding process before release.

[0024] The stamping method of the present invention stamps a stamped product that is assembled and processed with other components after stamping. The method includes: a stamping process of stamping a metal plate into a stamped product using a mold; a demolding process of demolding the stamped product from the mold; and a shape change process of allowing the shape of the demolded stamped product to change by placing it for 30 minutes or more after demolding before using it for assembly and processing.

[0025] The shape evaluation method of a stamped product of the present invention is a shape evaluation method of a stamped product that evaluates the shape of a stamped product that is assembled and processed with other components after stamping. The method includes: a stamping process of stamping a metal plate into a stamped product using a mold; a demolding process of demolding the stamped product from the mold; a shape measurement process of measuring the shape of the demolded stamped product after placing it for 30 minutes or more after demolding before using it for assembly and processing; and a shape determination process of determining that the stamped product is used for the assembly and processing if the measured shape of the stamped product is within a predetermined specified range.

[0026] Advantages of the Invention

[0027] In the present invention, a metal plate is stamped into a stamped product using a die. After the stamped product is demolded from the die, the residual stress in the stamped product is relieved and reduced by using the die to form the bottom dead center shape, or by using a fixture that can maintain a predetermined shape to hold all or part of the stamped product in that predetermined shape for 30 minutes or more, etc., and the shape change of the stamped product over time after springback from the die can be suppressed.

[0028] In addition, in the present invention, before being assembled and processed with other components through subsequent processes, the stamped product demolded from the die is left for 30 minutes or more to cause a shape change in the stamped product, or the shape of the stamped product is measured after leaving the stamped product for 30 minutes or more before being used in subsequent processes. If the shape of the measured stamped product is within a predetermined range set in advance, it is determined that the stamped product can be used in subsequent processes, thereby avoiding defective conditions in subsequent processes caused by the shape change of the stamped product over time. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a flowchart showing the process of the stamping method according to Embodiment 1 of the present invention.

[0030] Figure 2 is a view showing a stamped product having a hat-shaped cross section as an example of a forming object and a stamped product in the present invention.

[0031] Figure 3 is a view for explaining the reason why the shape change over time can be suppressed in the stamping method according to Embodiment 1 of the present invention.

[0032] Figure 4 is a flowchart showing the process of the stamping method according to another form of Embodiment 1 of the present invention.

[0033] Figure 5 is a flowchart showing the process of the stamping method according to Embodiment 2 of the present invention.

[0034] Figure 6 is a view for explaining the reason why the shape change over time can be suppressed in the stamping method according to Embodiment 2 of the present invention.

[0035] Figure 7 is a view showing the process of the stamping method according to Embodiment 3 of the present invention.

[0036] Figure 8 It is a diagram showing the process of the treatment in the method for evaluating the shape of a stamping formed product according to Embodiment 4 of the present invention.

[0037] Figure 9 In the embodiment, it is a diagram showing the cross-sectional shape of a stamping formed product having a hat-shaped cross-sectional shape to be formed and the position of an evaluation point for evaluating the amount of deviation from the shape at the bottom dead center of forming.

[0038] Figure 10 It is a diagram showing a stamping formed product having a U-shaped cross-sectional shape which is a measurement object of the shape change over time.

[0039] Figure 11 As an example of the shape change over time of a stamping formed product, it is a diagram showing the measurement result of the opening amount after the stamping formed product having a U-shaped cross-sectional shape is just sprung back after being removed from the mold.

[0040] Figure 12 It is a diagram for explaining the stress relaxation phenomenon in which the stress decreases over time while the strain is kept constant.

[0041] Figure 13 It is a diagram for explaining the shape change caused by stress relaxation of the punch shoulder of a stamping formed product having a hat-shaped cross-sectional shape ((a) bottom dead center of forming, (b) just after springback, (c) after passage of time). Detailed Embodiment

[0042] In order to solve the above-mentioned problems, the inventors aimed to establish a method for suppressing the shape change of a stamping formed product starting from just after it is sprung back from the mold. Taking the stamping formed product 1 having a hat-shaped cross-sectional shape as shown in Figure 2 as an example, various studies were conducted on the reasons for the shape change over time.

[0043] As a result, the inventors focused on Figure 12 the stress relaxation phenomenon in which the stress gradually relaxes and decreases over time in a state where the strain is constant in the stress-strain diagram as shown in. It was found that even in the stamping formed product 1 after springback, due to the residual stress in the punch shoulder 9, die shoulder 11, longitudinal wall portion 5, etc. that are bent by stamping, the stress gradually relaxes without being forced from the outside over time, resulting in a change in the shape that balances the moment of force of the stamping formed product 1.

[0044] As an example, regarding the shape change caused by the relaxation of the residual stress in the punch shoulder 9 and the die shoulder 11 of the stamping formed product 1, the following explanation is given using Figure 13 the schematic diagram shown. It should be noted that Figure 13 is a diagram showing an example of the cross-sectional shape and residual stress of the punch shoulder 9, but the same relaxation of residual stress and shape change also occur in the die shoulder 11.

[0045] First, during stamping forming, if a blank (such as a metal plate) is stamped to the forming bottom dead center using a die equipped with a punch and a die, as shown in Figure 13 (a), tensile stress is generated on the outer side of the bend of the punch shoulder 9, and compressive stress is generated on the inner side of the bend. It should be noted that the outer side of the bend is the side opposite to the center of curvature of the bend with respect to the center line of the plate thickness in the cross-section of the bent portion. In contrast, the inner side of the bend is the side on the same side as the center of curvature of the bend (the same hereinafter).

[0046] Next, if the stamping formed product 1 is removed (demolded) from the die, the springback of the stamping formed product 1 occurs instantaneously with the residual stress generated during stamping forming as the driving force. At this time, as shown in Figure 13 (b), a change in the bending angle of the punch shoulder 9 occurs such that it returns to the shape of the flat blank before stamping forming, and it deforms from the shape at the forming bottom dead center ( Figure 13 (the dotted line in (b)) to the shape with an increasing bending angle of the punch shoulder 9 ( Figure 13 (the solid line in (b)). However, since the punch shoulder 9 has rigidity, the force to return to the shape before stamping forming is hindered, and as shown in Figure 13 (b), compressive stress is generated on the outer side of the bend, and tensile stress is generated on the inner side of the bend.

[0047] Then, as shown in Figure 13 (c), as time passes, the force to return to the shape before stamping forming weakens, and a change in the bending angle with a further increase in bending occurs in the punch shoulder 9 so as to become a shape that balances the moment of the force in the stamping formed product 1 ( Figure 13 (the solid line in (c)).

[0048] That is, if springback occurs from the bottom dead center of forming after stamping, residual stress is generated in the stamped product at this time point. However, regarding the generated residual stress, the difference between the residual stress on the front side and the residual stress on the back side in the plate thickness direction of the stamped product is alleviated and reduced as time passes. As a result, it has been found that the processed part of the stamped product becomes a shape with further reduced residual stress compared to the shape immediately after springback.

[0049] This phenomenon is completely different from the behavior of springback that reduces residual stress in the past. In the past behavior of springback, regarding the residual stress generated at the bottom dead center of forming after stamping, if the value of the residual stress to be generated is forcibly reduced by a specific method or the difference between the residual stress on the front side and the back side of the stamped product to be generated is forcibly reduced, then as a result, the shape of the bottom dead center is suppressed from springing back and remains in the state after stamping.

[0050] On the other hand, in the behavior of stress relaxation targeted in the present invention, after springback starts from the bottom dead center of forming after stamping, the existing residual stress is relaxed without being forced from the outside, and thus it returns to a state without residual stress. As a result, the stamped product becomes a shape with an increased bending angle, curl, etc. compared to immediately after springback, and is further away from the target shape.

[0051] And, in such a stamped product 1 with a hat-shaped cross-section, even if sufficient countermeasures against springback generated at the moment of demolding are taken, as Figure 13 shown, due to stress relaxation associated with the passage of time on both the punch shoulder 9 and the die shoulder 11, a change in the bending angle occurs, and thus a deviation from the shape at the bottom dead center is generated in the flange portion 7 of the stamped product 1.

[0052] As a result, if a shape change associated with the passage of time occurs in the stamped product that has been stamped and springbacked before being used for assembly processing with other components in subsequent processes, problems may sometimes occur in the subsequent processes.

[0053] In response to such a problem, when the inventors studied countermeasures, before being used in subsequent processes, it was left for a specified time, thereby relaxing the residual stress of the stamped product to a state where almost no shape change occurs. Furthermore, in a state where no such shape change occurs, the shape of the stamped product was measured, and if the measured shape was within a specified range, it was determined that the stamped product was to be used in subsequent processes, and thus an insight was obtained that it was possible to avoid defects in subsequent processes caused by the shape change of the stamped product associated with the passage of time.

[0054] Hereinafter, regarding the stamping method and the shape evaluation method of the stamped product of the present invention, Embodiments 1 to 4 will be used for explanation. It should be noted that in Embodiments 1 to 4, the stamped product 1 is taken as an example for explanation, and the stamped product 1 has a hat-shaped cross-sectional shape having a top plate portion 3, a longitudinal wall portion 5, and a flange portion 7, and has a punch shoulder 9 connecting the top plate portion 3 and the longitudinal wall portion 5, and a die shoulder 11 connecting the longitudinal wall portion 5 and the flange portion 7 as a bend ridge portion. Figure 2 As shown, it has a hat-shaped cross-sectional shape with a top plate portion (top portion) 3, a longitudinal wall portion 5, and a flange portion 7, and has a punch shoulder 9 connecting the top plate portion 3 and the longitudinal wall portion 5, and a die shoulder 11 connecting the longitudinal wall portion 5 and the flange portion 7 as a bend ridge (bend ridge).

[0055] [Embodiment 1]

[0056] The stamping method of Embodiment 1 of the present invention suppresses the shape change over time of the stamped product 1 that rebounds at the moment of demolding from the mold, as Figure 1 shown, and includes a stamping process S1, a demolding process S3, and a post-demolding mold holding process S5.

[0057] The stamping process S1 is a process of stamping a metal plate into the stamped product 1 using a mold. The mold used in the stamping process S1, for example, includes a die and a punch, and as long as the die can be moved relative to the punch side to the forming bottom dead center to stamp and form the stamped product 1, there is no particular limitation.

[0058] The demolding process S3 is a process of demolding the stamped product 1 stamped in the stamping process S1 from the mold.

[0059] The post-demolding mold holding process S5 is a process of holding the stamped product 1 temporarily demolded in the demolding process S3 again using the mold used for stamping in the stamping process S1 for 30 minutes or more in the shape of the forming bottom dead center. Here, the shape of the forming bottom dead center refers to the shape of the stamped product 1 at the forming bottom dead center of the mold used in the stamping process S1 (the same applies hereinafter).

[0060] It should be noted that the reason for setting the time of holding the stamped product 1 using the mold to 30 minutes or more is that by holding for 30 minutes or more, the residual stress is sufficiently relaxed and reduced, and the shape change caused by stress relaxation after the held stamped product 1 is demolded from the mold again can be sufficiently suppressed.

[0061] Based on Figure 3 to explain the reason why the shape change over time of the stamped product 1 after the stamped product 1 rebounds from the mold can be suppressed by the stamping method of the present Embodiment 1. It should be noted that Figure 3Although the cross-sectional shape and residual stress of the punch shoulder 9 of the press-formed product 1 are shown in the figure, the same residual stress and shape change also occur in the die shoulder 11 of the press-formed product 1.

[0062] First, if the press-formed product 1 is press-formed and demolded from the mold, springback occurs with the residual stress generated during press-forming (tensile stress on the outer side of the bend of the punch shoulder 9 and compressive stress on the inner side of the bend) as the driving force. At this time, as Figure 3 (a) shows, the punch shoulder 9 changes in angle from the shape before the forming bottom dead center ( Figure 3 (the dotted line in (a)) in a manner that returns to the state of the flat metal plate before press-forming, and changes to a shape with an increased bending angle of the punch shoulder 9 ( Figure 3 (the solid line in (a)). However, since the punch shoulder 9 where the metal plate is bent has rigidity, the force to return to the shape before press-forming is hindered, and as Figure 3 (a) shows, compressive stress is generated on the outer side of the bend of the punch shoulder 9 and tensile stress is generated on the inner side of the bend.

[0063] Next, if the press-formed product 1 that has springback is held using the mold used for this press-forming, then as Figure 3 (b) shows, it deforms from the shape immediately after springback ( Figure 3 (the dotted line in (b)) to the forming bottom dead center shape ( Figure 3 (the solid line in (b)), and on the punch shoulder 9, tensile stress is generated on the outer side of the bend and compressive stress is generated on the inner side of the bend.

[0064] And, if the press-formed product is held in the forming bottom dead center shape using the mold for 30 minutes or more, then on the punch shoulder 9, as Figure 3 (c) shows, the punch shoulder 9 remains in the forming bottom dead center shape ( Figure 3 (the solid line in (c)), and its residual stress gradually relaxes (decreases). As a result, compared with immediately after being held by the mold or fixture (the forming bottom dead center in this embodiment), the residual stress relaxes and decreases, so the shape change over time after being demolded from the mold and springing back is significantly reduced.

[0065] It should be noted that in the above description, the post-demolding mold holding process S5 is a process of holding the entire press-formed product 1 in the forming bottom dead center shape using the mold used in the press-forming of the press-formed product 1. However, other forms of the press-forming method of the first embodiment may also include a post-demolding fixture holding process S7 instead of the post-demolding mold holding process S5 as Figure 4 shown.

[0066] The post-demolding jig holding step S7 is a step of holding all or a part of the press-formed product 1 in a predetermined specified shape for 30 minutes or more using a jig that can hold all or a part of the press-formed product 1 in the predetermined specified shape.

[0067] Here, the predetermined specified shape may be set to, for example, the bottom dead center shape or the target shape (the shape specified as the product) of the press-formed product 1, or an intermediate shape between the bottom dead center shape and the target shape (intermediate shape). Also, holding a part of the press-formed product 1 in a specified shape using a jig may be holding the entire press-formed product 1 in a specified shape, or holding a part of the press-formed product 1 using a jig that can hold it in a specified shape, such as only the punch shoulder 9. In addition, the reason for setting the time of holding the press-formed product 1 using the jig to 30 minutes or more is the same as in the case of holding using the mold described above.

[0068] In this way, after the press-formed product 1 is demolded from the mold and rebounds, all or a part of the press-formed product 1 is held in a predetermined specified shape in the post-demolding jig holding step S7, whereby the residual stress in the portion of the press-formed product 1 held by the jig can be alleviated and reduced, and the residual stress in the held portion of the press-formed product 1 after being disassembled from the jig is also reduced, and the shape change of the press-formed product 1 over time can be reduced.

[0069] It should be noted that in the post-demolding jig holding step S7, when holding the entire press-formed product 1, another mold having the same shape as the aforementioned mold may be used, and the portion where a part of the press-formed product 1 is held by the jig may be set to, for example, the bending ridge line portion of the press-formed product 1, i.e., the punch shoulder 9 or the die shoulder 11. However, the portion where the press-formed product 1 is held by the jig is not limited to the bending ridge line portions such as the punch shoulder 9 or the die shoulder 11, and may be set to a portion such as the longitudinal wall portion 5 that is subjected to bending and unbending and has a large influence on the shape change due to stress relaxation over time.

[0070] [Embodiment 2]

[0071] The press-forming method according to Embodiment 2 of the present invention is a method for suppressing the shape change over time of the press-formed product 1 that rebounds at the moment of demolding from the mold ( Figure 2 ), as Figure 5As shown, it includes a stamping process S11, a pre-release holding process S13, and a die releasing process S15. It should be noted that the stamping process S11 is the same as the stamping process S1 in the aforementioned Embodiment 1. Therefore, the pre-release holding process S13 and the die releasing process S15 will be described below.

[0072] The pre-release holding process S13 is a process of holding the stamped product 1 at the forming bottom dead center for 30 minutes or more without releasing it from the die after stamping the stamped product 1 using the die in the stamping process S11.

[0073] The die releasing process S15 is a process of releasing the stamped product 1 held in the pre-release holding process S13 from the die.

[0074] Based on Figure 6 to explain the reason why the shape change over time in the stamped product 1 that rebounds when released from the die can be suppressed by the stamping method of the present Embodiment 2. It should be noted that Figure 6 As an example, the cross-sectional shape and residual stress of the punch shoulder 9 of the stamped product 1 are shown, but the relaxation of the same residual stress and shape change also occur in the die shoulder 11 of the stamped product 1.

[0075] First, when the stamped product 1 is stamped using the die, at the punch shoulder 9, as Figure 6 (a) shows, tensile stress is generated on the outer side of the bend, and compressive stress is generated on the inner side of the bend.

[0076] And if the stamped product 1 is not released from the die and is held at the forming bottom dead center for 30 minutes or more, then at the punch shoulder 9, as Figure 6 (b) shows, the residual stress gradually relaxes (decreases), and the residual stress is smaller compared to the punch shoulder 9 at the forming bottom dead center. Therefore, the shape change caused by the stress relaxation over time after the stamped product 1 is released and rebounds is significantly reduced compared to the stamped product 1 that is released without being held in the die.

[0077] In this way, according to the stamping method of the present Embodiment 2, the shape change over time in the stamped product 1 that rebounds when released from the die can be suppressed.

[0078] [Embodiment 3]

[0079] The stamping method of Embodiment 3 of the present invention is a method of stamping a stamped product 1 that is assembled and processed with other components after stamping, as Figure 7As shown, it includes a stamping process S21, a demolding process S23, and a shape change process S25.

[0080] The stamping process S21 and the demolding process S23 are the same as the stamping process S1 and the demolding process S3 of the aforementioned Embodiment 1 of the present invention. Therefore, their descriptions are omitted here. Hereinafter, the shape change process S25 will be described.

[0081] The shape change process S25 is a process of allowing the shape of the stamped product 1 to change by leaving it for 30 minutes or more after demolding before using the stamped product 1 demolded in the demolding process S23 for assembly processing.

[0082] It should be noted that the reason for setting the time for leaving the demolded stamped product 1 to allow its shape to change to 30 minutes or more is that when 30 minutes or more have passed after demolding, the residual stress in the stamped product 1 is sufficiently relieved and reduced, and the further shape change after springback is reduced (see Figure 11 ).

[0083] According to the stamping method of the present Embodiment 3, the stamped product obtained by stamping is left before assembly processing. Thus, after the shape change caused by the stress relaxation of the stamped product 1 over time occurs, the stamped product 1 is used for the assembly processing with other components in the subsequent process. Therefore, it is possible to prevent the occurrence of shape changes during the period from stamping to being used in the subsequent process and the occurrence of defects in the subsequent process.

[0084] [Embodiment 4]

[0085] The shape evaluation method of the stamped product according to Embodiment 4 of the present invention is a method for evaluating the shape of the stamped product 1 assembled and processed with other components after stamping. As Figure 8 shown, it includes a stamping process S31, a demolding process S33, a shape measurement process S35, and a shape determination process S37. It should be noted that the stamping process S31 and the demolding process S33 are the same as the stamping process S1 and the demolding process S3 of the aforementioned Embodiment 1. Therefore, the shape measurement process S35 and the shape determination process S37 will be described.

[0086] The shape measurement process S35 is a process of measuring the shape of the stamped product 1 after leaving it for 30 minutes or more after demolding before using the demolded stamped product 1 for assembly processing.

[0087] The reason for measuring the shape of the stamped product 1 after it is placed after demolding is to sufficiently relieve and reduce the residual stress in the demolded stamped product 1, and cause a shape change due to stress relief. In addition, the placement time is set to 30 minutes or more because when 30 minutes or more have elapsed since demolding, the residual stress in the stamped product 1 is sufficiently relieved and reduced, and the further shape change after springback is reduced (see Figure 11 ).

[0088] The shape determination step S37 is a step of determining that the stamped product 1 is used for assembly processing if the shape of the stamped product 1 measured by the shape measurement step S35 is within a preset specified range.

[0089] In this way, according to the shape evaluation method of the stamped product of the present Embodiment 4, the stamped product 1 formed by stamping is demolded and placed for 30 minutes or more, and its shape is measured after sufficiently causing a shape change due to stress relief over time. If the measured shape of the stamped product 1 is within a preset specified range, it is determined to be used for subsequent processes. Therefore, it is possible to prevent a shape change from occurring in the stamped product 1 during the period from after stamping to assembly processing and to prevent a defective condition from occurring in subsequent processes.

[0090] It should be noted that the preset specified range in the shape determination step S37 can be appropriately set as long as it is within a range that does not cause a defective condition in the assembly processing with other components.

[0091] The above-described stamping method and shape evaluation method of the stamped product of the present invention are not particularly limited to the metal plate used as a blank in the stamping of the stamped product, the shape and type of the stamped product, etc. It is more effective for automotive components formed by stamping a metal plate with an increased residual stress in the stamped product.

[0092] Specifically, regarding the blank, a metal plate with a tensile strength of 150 MPa grade or more and 2000 MPa grade or less, and a plate thickness of 0.5 mm or more and 4.0 mm or less is preferred.

[0093] Blanks (metal plates) with a tensile strength less than 150 MPa grade are rarely used for stamped products, so the advantages of applying the present invention are few. Regarding the case where the rigidity of outer panel components of an automobile using a blank with a tensile strength of 150 MPa grade or more is low, since it is easily affected by shape changes caused by changes in residual stress, the advantages of applying the present invention increase, and thus the present invention can be applied well.

[0094] On the other hand, blanks with a tensile strength exceeding 2000 MPa lack ductility (elongation). Therefore, for example, during the stamping process of a stamped product 1 with a hat-shaped cross-sectional shape as shown in Figure 2 cracks occur at the punch shoulder 9 or the die shoulder 11, and sometimes stamping cannot be performed.

[0095] In addition, regarding the shape of the stamped product, the present invention is not limited to an invention targeting the stamped product 1 with a hat-shaped cross-sectional shape as shown in Figure 2 Rather, the present invention is preferably applicable to stamped products having a shape with a portion where residual stress increases, such as a stamped product with a Z-shaped (Z-shape) cross-sectional shape, a U-shaped cross-sectional shape, or an L-shaped (L-shape) cross-sectional shape.

[0096] Therefore, as the type of the stamped product, the present invention is preferably applicable to automotive components such as outer panel components with low rigidity, such as doors, roofs, and hoods, and frame parts such as A-pillars (pillars), B-pillars, roof rails, longitudinal beams, front side beams, rear side beams, and cross beams, which use high-strength metal plates.

[0097] It should be noted that the present invention can be applied to stamped products formed by crash forming, bend forming, or deep drawing, and the press method of the stamped product is arbitrary.

[0098] Examples

[0099] An experiment for confirming the effects of the stamping method of the present invention was conducted, and the results are described below.

[0100] In the experiment, first, using a metal plate A having the mechanical properties shown in Table 1 below, stamping of the stamped product 1 with a hat-shaped cross-sectional shape as shown in Figure 2 by bend forming was performed. For the forming bottom dead center shape of the stamped product 1, the radius of curvature and the bending angle of the punch shoulder 9 were set to 5 mm and 95°, and the radius of curvature and the bending angle of the die shoulder 11 were set to 5 mm and 95°. It should be noted that the thickness of the metal plate A was 1.6 mm, the yield strength was 880 MPa, the tensile strength was 1210 MPa, and the elongation was 13%.

[0101] [Table 1]

[0102] (Table 1)

[0103]

[0104] Moreover, the stamped product 1 that was stamped to the bottom dead center of forming was demolded from the mold, and the shape change of the stamped product 1 after springback over time was measured (existing example). The results are as Figure 9 shown. Angle changes occur at the punch shoulder 9 and the die shoulder 11, deviating from the shape at the bottom dead center of forming of the stamped product 1. It should be noted that the deviation amount described below is the distance in the cross-section in the width direction parallel to the top plate portion 3 with the center in the length direction of the top plate portion 3 of the stamped product 1 made consistent.

[0105] In the stamped product 1, the part that deviates the most from the shape at the bottom dead center of forming is Figure 2 the edge portion of the stamped product 1 shown (the front end in the length direction of the flange portion. Hereinafter, referred to as "evaluation point a"). Therefore, when the deviation amount from the shape at the bottom dead center of forming at the evaluation point a was measured, it was 14.3 mm immediately after stamping (immediately after demolding and springback), whereas it was 16.0 mm after two days, and the deviation amount increased over time.

[0106] Next, as inventive examples, regarding the case where the stamped product 1 that was demolded from the mold and springback was returned to the mold and maintained in the shape at the bottom dead center for a specified time (Inventive Example 1 and Inventive Example 2), and the case where the stamped product 1 that was stamped to the bottom dead center of forming was kept in the mold as it was for a specified time (Inventive Example 3 and Inventive Example 4), the shape change of the stamped product 1 over time after being demolded while being held by the mold was measured. Table 2 shows the results of measuring the deviation amount of the evaluation point a from the shape at the bottom dead center of forming immediately after stamping the stamped product 1 (immediately after demolding and springback) and after being demolded after being held by the mold for a specified time.

[0107] [Table 2]

[0108] (Table 2)

[0109]

[0110] *1 Temporarily taken out from the mold to measure the deviation amount and then returned to the mold and held

[0111] *2 Held by the mold until two days have passed

[0112] ※ Difference from the deviation amount immediately after stamping in the existing example (= 14.3 mm)

[0113] Inventive Example 1 is an example in which the springback stamping product is returned to the mold and held for 30 minutes. The deviation amount at evaluation point a is 14.6 mm immediately after demolding while being held by the mold, and then it is 14.8 mm two days after returning to the mold and demolding. The difference between the deviation amount two days after and the deviation amount immediately after stamping (demolding and just springback) (= 14.3 mm) is 0.5 mm, which is lower than the difference in deviation amount of the prior example (= 1.7 mm).

[0114] Inventive Example 2 is an example in which the springback stamping product is returned to the mold and continuously held for two days. The deviation amount at evaluation point a is 14.5 mm immediately after demolding while being held by the mold. Moreover, the difference between the deviation amount two days after and the deviation amount immediately after stamping (demolding and just springback) (= 14.3 mm) is 0.2 mm, which is further decreased compared with Inventive Example 1, and the shape change over time can be suppressed.

[0115] Inventive Example 3 is an example in which it is demolded after being held in the mold as it is for 30 minutes after stamping. The deviation amount at evaluation point a is 14.9 mm immediately after demolding while being held by the mold, and then it is 15.3 mm two days after returning to the mold and demolding. Also, since Inventive Example 3 is an example in which it is held in the mold as it is after stamping, the shape immediately after stamping cannot be measured. However, the difference from the deviation amount immediately after stamping (demolding and just springback) in Prior Example 1 is 1.0 mm, which is lower than the difference in deviation amount of the prior example, and the shape change over time can be suppressed.

[0116] Inventive Example 4 is an example in which the holding time in the mold after stamping is continuously increased to two days compared with Inventive Example 3. The deviation amount at evaluation point a is 14.4 mm. Moreover, the difference from the deviation amount immediately after stamping (demolding and just springback) in Prior Example 1 becomes 0.1 mm, which is further decreased compared with Inventive Example 3, and the shape change over time can be sufficiently suppressed.

[0117] Industrial Applicability

[0118] According to the present invention, there can be provided a stamping method for suppressing shape changes occurring in a stamping product over time after stamping, or for taking measures against shape changes over time of a stamping product to be assembled and processed with other components in subsequent processes, and a method for evaluating the shape of a stamping product to be assembled and processed with other components in subsequent processes.

[0119] Reference Numeral Explanation

[0120] 1 Stamping product

[0121] 3 Top plate part

[0122] 5 longitudinal wall part

[0123] 7 flange part

[0124] 9 punch shoulder

[0125] 11 die shoulder

[0126] 21 stamping formed product

[0127] 23 top plate part

[0128] 25 longitudinal wall part.

Claims

1. A stamping forming method for suppressing the shape change over time of the stamped formed product after springback at the moment of demolding the stamped formed product from the mold, wherein, Comprising: A stamping process of stamping a metal sheet into a stamped product using the die; A demolding process of demolding the stamped product from the die; And A post-demolding die holding process of holding the demolded stamped product in the shape of the bottom dead center of forming for 30 minutes or more using the die.

2. A stamping method for suppressing the shape change over time of the stamped product after springback at the moment of demolding the stamped product from the mold, wherein, Comprising: A stamping process of stamping a metal sheet into a stamped product using the die; A demolding process of demolding the stamped product from the die; And A post-demolding fixture holding process of holding all or part of the demolded stamped product in the specified shape for 30 minutes or more using a fixture capable of holding in a predetermined specified shape including another die having the same shape as the die.

Images