Method for obtaining the forming limit of a metal sheet, method for creating a forming limit diagram of a metal sheet, and method for manufacturing a press-formed product.

Abstract

We will obtain the forming limit strain and create a forming limit diagram for metal sheet forming tests using a spherical punch, correcting for changes in the strain path. Based on the created forming limit diagram, we will accurately determine cracks in press-formed products. [Solution] The present invention provides a method for obtaining the forming limit of a metal sheet, which involves determining the forming limit of a metal sheet by forming a test using a spherical punch, forming a test piece of metal sheet using a spherical punch, and determining the equivalent plastic strain ε at the forming limit in the forming test. eq The first step S1 involves obtaining the strain increment ratio β, and the obtained equivalent plastic strain ε eq The method includes a second step S3 in which, using the strain increment ratio β, the maximum principal strain ε1 and minimum principal strain ε2 at the molding limit when the test specimen is molded with a constant strain increment ratio β from the start of molding to the molding limit.

Description

[Technical Field]

[0001] The present invention relates to a method for obtaining the forming limit of a metal sheet, and a method for creating a forming limit diagram of a metal sheet based on the obtained forming limit. Furthermore, the present invention relates to a method for manufacturing press-formed products, which involves determining whether or not cracks occur in a press-formed product using a forming limit diagram of a metal sheet, and then adjusting the press-forming conditions to suppress crack occurrence based on the determination result. [Background technology]

[0002] Most metal sheets (e.g., thin steel sheets) used as materials for automobile bodies are processed into body parts by press forming. The press formability of body parts varies depending on the shape of the part, and is also greatly influenced by the material properties of the metal sheet, including its ductility. In recent years, in response to the demand for lighter automobile bodies, there has been a trend towards increasing the strength of the metal sheets used in body parts. However, as the strength of the metal sheets increases, their ductility decreases, making them more prone to cracking during press forming and thus reducing their press formability.

[0003] Therefore, in order to avoid problems such as cracking during the manufacturing of car body parts by press forming, it is important to design molds and adjust press forming conditions based on prior prediction of press formability using CAE (Computer-Aided Engineering). In CAE-based prediction of the press formability of metal sheets, the Forming Limit Diagram (FLD) is typically used. The Forming Limit Diagram is created through laboratory-scale forming tests to show the forming limits for various deformation modes of metal sheets during press forming, such as equibiaxial deformation, unequal biaxial deformation, plane strain deformation, and uniaxial deformation. In such forming tests, the width of the metal sheet specimen is changed to several levels, and the deformation ratio in the longitudinal and transverse directions of the specimen is altered. The strain in the longitudinal direction (maximum principal strain) and the strain in the transverse direction (minimum principal strain) at the time of fracture are then measured as the forming limit strain.

[0004] Predicting press formability is performed by plotting the strain in a press-formed product, obtained through press forming analysis (e.g., finite element method analysis) that replicates the actual press forming process, onto a forming limit diagram, and determining whether or not cracks will occur. Therefore, accurately creating a forming limit diagram for a metal sheet is becoming increasingly important when designing molds and adjusting press forming conditions based on pre-predicted press formability.

[0005] Non-patent document 1 standardizes a method for creating a forming limit curve. In this method, the strain distribution around the fracture point of a test specimen formed to fracture is measured, and this strain distribution is approximated by a curve represented by the following equation. Then, the maximum value of the strain is calculated from the approximated strain distribution, and this maximum value is defined as the forming limit strain.

number

[0006] However, the method described in Non-Patent Document 1 does not directly measure the forming limit strain, and therefore cannot adequately approximate the strain at the fracture site, making it impossible to determine the forming limit strain in some cases. Therefore, as an improvement to the method described in Non-Patent Document 1, Non-Patent Document 2 discloses a method for continuously measuring the strain generated in a test piece during molding and determining the molding limit strain from the time change of the strain at the fracture initiation site. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] ISO 12004-2:2008, Metallic materials - Sheet and strip - Determination of forming-limit curves, 2008. [Non-Patent Document 2] W Hotz, M Merklein et al., “Time Dependent FLC Determination Comparison of Different Algorithms to Detect the Onset of Unstable Necking before Fracture”, Key Engineering Materials, Vol 549, pp.397-404(2013). [Overview of the project] [Problems that the invention aims to solve]

[0008] Non-patent document 1 describes two forming test methods for shaping metal plate test pieces: the Nakajima method, which uses a spherical punch with a spherical tip, and the Marciniac method, which uses a flat punch with a planar tip. The Nakajima method determines the molding limit of a test specimen while it has adapted to the shape of the spherical punch. Therefore, the test specimen undergoes bending deformation to adapt to the shape of the spherical punch in the initial stages of molding, and then undergoes deformation along a predetermined strain path. On the other hand, in the Marciniac method, the test specimen undergoes deformation along a nearly constant strain path from the start of molding to the molding limit.

[0009] However, because the Marciniac method does not involve contact between the test specimen and the tool (flat-head punch), the friction conditions differ from those of actual press forming. Therefore, the Marciniac method generally tends to underestimate the forming limit. Furthermore, the Marciniac method often results in cracking occurring in areas other than the evaluation section where the forming limit is measured, such as the punch shoulder and die shoulder, making it more difficult to adjust the test conditions than the Nakajima method. For these reasons, the Nakajima method is often used for forming tests to create forming limit diagrams.

[0010] Figure 10 shows, as an example, the strain path of a test specimen in a forming test using the Nakajima method. Figure 10 is a graph plotting the minimum principal strain ε2 and the maximum principal strain ε1 generated in a test specimen, which is a hot-rolled steel sheet with a thickness of 2.6 mm and a tensile strength of 980 MPa, from the start of forming through the forming limit to fracture.

[0011] As forming begins, the specimen undergoes bending deformation to conform to the shape of the spherical punch, as described above. Until this conforming is complete, it deforms with a strain increment ratio β (=Δε2 / Δε1) in the equibiaxial stretching direction (O→A in Figure 10). After the specimen has conformed to the spherical punch, it deforms with a predetermined strain increment ratio β, corresponding to the width of the specimen, remaining almost constant until it reaches the forming limit (A→B in Figure 10). Beyond the forming limit, the specimen deforms in the plane strain direction due to rapid necking in the thickness direction (B→C in Figure 10).

[0012] Thus, the strain path from the start of molding to the molding limit in the molding test using the Nakajima method can be divided into two stages: the initial deformation stage (primary deformation) from the start of deformation until the specimen conforms to the shape of the tip of the spherical punch, and the later deformation stage (secondary deformation) from when the specimen conforms to the spherical punch until the molding limit.

[0013] Typically, the forming limit strain of a metal sheet is minimized in the case of plane strain deformation. Therefore, on a forming limit diagram with the horizontal axis representing the minimum principal strain and the vertical axis representing the maximum principal strain, the forming limit strain (maximum principal strain) is minimized on the vertical axis where the minimum principal strain is 0. However, when creating a forming limit diagram by the Nakajima method as shown in FIG. 11, since the strain path changes as described above, the minimum value of the forming limit strain shifts to the first quadrant (ε1>0, ε2>0) side.

[0014] The amount of shift of the minimum value of the forming limit strain is considered to be proportional to the bending strain, which is the amount of bending deformation in the initial stage of forming until the test piece conforms to the shape of the spherical punch. When the thickness of the test piece is t and the punch radius at the tip of the spherical punch is R, the bending strain deformation is represented by t / 2R. Therefore, the shift of the forming limit strain is larger in a metal plate with a larger thickness. Also, as the material strength (tensile strength, etc.) of the metal plate increases, the formable region becomes narrower, so the influence of the shift of the forming limit strain due to the bending deformation in the initial stage of forming becomes relatively larger.

[0015] Thus, when determining the presence or absence of cracking in a press-formed product using a forming limit diagram in which the minimum value of the forming limit strain has shifted to the first quadrant side, there is a risk that the occurrence of cracking at the site where plane strain, which is originally most likely to crack, occurs cannot be appropriately determined. However, conventionally, the cracking determination of press-formed products has been performed without correcting the shift of the forming limit strain in the forming limit diagram created by the Nakajima method.

[0016] The present invention has been made to solve the above problems, and when obtaining the forming limit of a metal plate by a forming test using the Nakajima method, it is an object to provide a forming limit acquisition method for obtaining the forming limit strain by correcting the change in the strain path in the test piece, and a method for creating a forming limit diagram of the metal plate. Furthermore, it is an object of the present invention to provide a method for manufacturing a press-formed product that determines the presence or absence of cracking in a press-formed product using the forming limit diagram created by the above method, adjusts the press-forming conditions to suppress cracking, and performs press-forming under the adjusted press-forming conditions.

Means for Solving the Problems

[0017] (1) The method for obtaining the forming limit of a metal plate according to the present invention is to obtain the forming limit of the metal plate in a forming test of the metal plate using a spherical punch, and form a test piece of the metal plate using the spherical punch, and obtain the equivalent plastic strain ε eq and the strain increment ratio β at the forming limit in the forming test in the first step, from the obtained equivalent plastic strain ε eq and the strain increment ratio β at the forming limit, obtain the maximum principal strain ε1 and the minimum principal strain ε2 at the forming limit when the test piece is formed with a constant strain increment ratio β from the start of forming to the forming limit as the forming limit strain of the metal plate in the second step, and is characterized by including this.

[0018] (2) In the above (1), in the second step, the maximum principal strain ε1 and the minimum principal strain ε2 at the forming limit are calculated by the following formula, and is characterized by this.

Equation

[0019] (3) The method for creating a forming limit diagram of a metal plate according to the present invention is to obtain the forming limit of the metal plate in a forming test of the metal plate using a spherical punch and create a forming limit diagram, and for test pieces with different shapes of the metal plate, perform the method for obtaining the forming limit of the metal plate described in the above (1) or (2), and obtain the maximum principal strain and the minimum principal strain at the forming limit when the test piece is formed with a constant strain increment ratio from the start of forming to the forming limit in the first step, plot the maximum principal strain and the minimum principal strain at the forming limit obtained for each test piece, and create a forming limit diagram of the metal plate in the second step, and is characterized by including this.

[0020] (4) The method for manufacturing a press-formed product according to the present invention is to suppress cracking during press forming of a metal plate and manufacture a press-formed product, and A forming limit diagram acquisition process for acquiring the forming limit diagram of the metal plate created by the method for creating the forming limit diagram of the metal plate described in (3) above, A press-formed product crack determination process that acquires the maximum principal strain and minimum principal strain generated in the press-formed product as crack determination parameters, and determines whether or not cracks occur in the press-formed product based on the acquired forming limit diagram, If the press-formed product crack detection process determines that cracks have occurred, a press-forming condition adjustment process is performed to adjust the press-forming conditions to suppress crack occurrence. The present invention is characterized by including a press forming process for press forming the metal sheet using the press forming conditions adjusted in the press forming condition adjustment process. [Effects of the Invention]

[0021] According to the present invention, it is possible to appropriately determine the forming limit strain, corrected for the effect of strain path changes in forming tests using the Nakajima method, which involves forming metal plate test pieces with a spherical punch, and to create a forming limit diagram. Furthermore, according to the present invention, it is possible to accurately determine the occurrence of cracks in press-formed products and manufacture press-formed products while suppressing cracking. [Brief explanation of the drawing]

[0022] [Figure 1] This is a flowchart showing the processing flow in the method for obtaining the forming limit of a metal sheet according to Embodiment 1 of the present invention. [Figure 2] This figure illustrates a method for obtaining the forming limit of a metal sheet according to Embodiment 1 of the present invention, including the strain path in a forming test using a spherical punch and a method for calculating the forming limit strain when forming with a constant strain increment ratio from the start of forming to the forming limit. [Figure 3] This figure illustrates another method for calculating the forming limit strain when forming a metal sheet at a constant strain increment ratio from the start of forming to the forming limit, in the method for obtaining the forming limit of a metal sheet according to Embodiment 1 of the present invention. [Figure 4]This is a flowchart showing the processing flow in the method for creating a forming limit line diagram of a metal plate according to Embodiment 2 of the present invention. [Figure 5] This figure shows a forming limit diagram (after correction) created by the method for creating a forming limit diagram of a metal plate according to Embodiment 2 of the present invention, and a forming limit diagram (before correction) created by a conventional forming test using the Nakajima method. [Figure 6] This is a flowchart showing the process flow in the method for manufacturing a press-formed product according to Embodiment 3 of the present invention. [Figure 7] This flowchart shows a specific example of the process for manufacturing a press-formed product according to Embodiment 3 of the present invention. [Figure 8] The graph in the example shows a molding limit diagram for an inventive example prepared by the method according to the present invention, and a molding limit diagram for a comparative example prepared by a conventional molding test using the Nakajima method. [Figure 9] This figure shows the press-molded product that was molded in the example. [Figure 10] This figure shows an example of a strain path measured by a metal sheet forming test using the Nakajima method with a spherical punch. [Figure 11] This figure shows an example of a forming limit diagram created by a metal sheet forming test using the Nakajima method with a spherical punch. [Modes for carrying out the invention]

[0023] Before describing embodiments of the present invention, the background leading to the invention will be explained. The specific numerical values ​​shown in the specification and drawings of this application are merely illustrative to facilitate understanding of the present invention and do not limit it.

[0024] [Background leading to the present invention] The inventor diligently investigated a method to appropriately correct the deviation in the forming limit strain in the forming limit diagram of a metal sheet produced in a metal sheet forming test using the Nakajima method with a spherical punch. In this study, the inventor focused on the experimental fact that, for metal plate test specimens deformed through various strain paths, the equivalent plastic strain at the forming limit is the same regardless of the strain path, provided that the strain increment ratio at the forming limit is the same. From this, the inventor considered whether it might be possible to assume that the equivalent plastic strain at the forming limit in a forming test using the Nakajima method, where the strain path changes, is equal to the equivalent plastic strain at the forming limit when forming with a constant strain path.

[0025] Under this assumption, the inventor conceived of using the equivalent plastic strain and strain increment ratio at the forming limit obtained in a forming test using the Nakajima method, where the strain path changes, to determine the maximum and minimum principal strains at the forming limit under deformation with a constant strain path. The inventor then found that by determining the forming limit strain in this way, it is possible to determine a forming limit strain that appropriately corrects for the effects of strain path changes, which are unavoidable in forming tests using the Nakajima method.

[0026] This invention was completed based on the above ideas and knowledge, and its specific configuration is described below.

[0027] [Embodiment 1] The first embodiment of the present invention provides a method for obtaining the forming limit of a metal sheet, which involves obtaining the forming limit strain of a metal sheet in a forming test using a spherical punch. As shown in Figure 1, the method includes a first step S1 of obtaining the equivalent plastic strain and strain increment ratio at the forming limit in a forming test using a spherical punch, and a second step S3 of calculating the forming limit strain when the forming is performed with a constant strain increment ratio from the start of forming to the forming limit.

[0028] <Step 1> In the first step S1, a metal plate test specimen is first formed using a spherical punch, and the maximum principal strain ε1 and minimum principal strain ε2 generated in the specimen from the start of forming through the forming limit until fracture occurs are measured at predetermined time intervals. Then, the strain path is obtained by plotting the time-series data of the measured maximum and minimum principal strains as shown in Figure 2. In the first step S1, the strain of the test specimen can be measured, for example, by digital image correlation (DIC).

[0029] Next, in the first step S1, the acquired strain path is divided into a primary deformation (O→A in Figure 2) until the specimen conforms to the shape of the spherical punch, and a secondary deformation (A→B in Figure 2) which occurs after the specimen has conformed to the spherical punch, with a predetermined strain increment ratio according to the width of the specimen. Here, point A in Figure 2 is the inflection point where the specimen conforms to the shape of the spherical punch and the strain path changes to secondary deformation, and point B is the molding limit in the molding test using the spherical punch.

[0030] The method for obtaining the forming limit in the strain path is not particularly limited, but it is preferable to obtain it using the method described in the publicly available document "Japanese Patent Publication No. 2023-35533".

[0031] Next, in the first step, the equivalent plastic strain ε at the forming limit (point B in Figure 2), which is the end of the secondary deformation, is measured. eq And determine the strain increment ratio β.

[0032] Equivalent plastic strain ε eq This can be calculated using the following equation (1).

number

[0033] The strain increment ratio β at the forming limit is the slope of the strain path at the forming limit. The strain increment ratio β can be calculated, for example, by calculating the increments (Δε1, Δε2) from the maximum and minimum principal strains at the time step where the forming limit is determined and the time step immediately preceding it, in the time series data of the measured maximum and minimum principal strains.

[0034] <Step 2> In the second step, the equivalent plastic strain ε obtained in the first step S1 is used. eq Based on the strain increment ratio β, the maximum principal strain ε1 and minimum principal strain ε2 at the molding limit are determined as the molding limit strain when molding is performed with a constant strain increment ratio β from the start of molding to the molding limit, as shown in Figure 2 (O→D).

[0035] The maximum principal strain ε1 and minimum principal strain ε2 at the forming limit are equal to the equivalent plastic strain ε obtained in the first step S1. eq The strain increment ratio β can be calculated by substituting it into the following equation (2).

number

[0036] The derivation method of equation (2) will be explained using Figure 2. Point D in Figure 2 represents the molding limit when the strain increment ratio β remains constant from the start of molding to the molding limit. To calculate the molding limit strain at point D, the equivalent plastic strain ε at the molding limit (point D) is used. eq This is the equivalent plastic strain ε at the forming limit (point B) in a forming test using the Nakajima method where the strain path changes. eq It is assumed that this is equal to the following. Furthermore, the strain increment ratio β at point D is assumed to be equal to the strain increment ratio β of the quadratic deformation between A and B shown in Figure 2. Here, since the strain increment ratio β is constant from O to D, the strain increment ratio β at point D can be expressed as the ratio ε2 / ε1 of the minimum principal strain ε2 to the maximum principal strain ε1 at point D. From this relationship and the constant volume, the intermediate strain ε3 at point D can be expressed as ε3 = -(1+β)ε1, and therefore the equivalent plastic strain ε at point D iseq is expressed by the following formula: [Number]

[0037] Then, from the relationship between Equation (3) and the strain increment ratio β at point D, the maximum principal strain ε1 and the minimum principal strain ε2 at the forming limit when formed until the equivalent plastic strain ε eq at point D becomes constant with respect to the strain increment ratio β from the start of forming are derived from the above Equation (2).

[0038] Thus, in the first embodiment, the equivalent plastic strain ε eq at the forming limit in the forming test using the spherical punch and the strain increment ratio β are used to obtain the maximum principal strain ε1 and the minimum principal strain ε2 at the forming limit when formed with a constant strain increment ratio β from the start of forming. As a result, the influence of the strain path change in the forming test by the Nakajima method using the spherical punch can be corrected to obtain the forming limit strain.

[0039] In particular, according to the first embodiment, even in the case where the strain region of the secondary deformation is narrow and the contribution of the primary deformation is relatively large, such as in a high-tensile steel sheet with high material strength and small elongation (for example, a tensile strength of 980 MPa or more), the forming limit in plane strain deformation can be appropriately obtained.

[0040] In the above description, in the first step, as shown in FIG. 2, based on the strain path (time-series data of the maximum principal strain and the minimum principal strain) obtained in the forming test by the Nakajima method, the equivalent plastic strain ε eq at the forming limit and the strain increment ratio β were obtained.

[0041] However, the present invention may also apply when the strain path has not been obtained in the forming test using the Nakajima method, and only the forming limit strain (maximum principal strain, minimum principal strain) is known. In this case, first, the primary deformation in the forming test is assumed to be equibiaxial stretch deformation (strain increment ratio = 1.0), and the bending strain at point A in Figure 3 is calculated as t / 2R using the plate thickness t of the test piece and the punch R of the spherical punch. Then, the strain increment ratio β in the secondary deformation between A and B is calculated from the difference between the maximum principal strain and the minimum principal strain at points A and B, respectively.

[0042] By determining the strain increment ratio β in this way, the maximum and minimum principal strains at the molding limit when molding is performed with a constant strain increment ratio β can be determined, similar to the procedure described above.

[0043] [Embodiment 2] The method for creating a forming limit diagram for a metal sheet according to Embodiment 2 of the present invention involves obtaining the forming limit of a metal sheet through a forming test of the metal sheet using a spherical punch, and then creating a forming limit diagram. The method according to Embodiment 2 includes a first step S11 and a second step S13, as shown in Figure 4.

[0044] <1st process> In the first step S11, the metal plate forming limit acquisition method according to Embodiment 1 described above is performed on test pieces of metal plates of different shapes, and the maximum principal strain and minimum principal strain at the forming limit are obtained when the test piece is formed with a constant strain increment ratio from the start of forming to the forming limit.

[0045] The shape of the test specimen should be determined appropriately so that the forming limits can be determined under various strain paths (equibiaxial deformation, unequal biaxial deformation, plane strain deformation, uniaxial deformation).

[0046] <Second process> In the second step S13, as shown in Figure 5, the maximum and minimum principal strains at the forming limit obtained for each test piece in the first step S11 are plotted in a two-dimensional coordinate space to create a forming limit diagram of the metal plate.

[0047] In Figure 5, the gray solid line is the forming limit line plotted with the forming limit strain obtained by the method according to Embodiment 2, and the black solid line is the forming limit line plotted with the maximum and minimum principal strains at the forming limit obtained in a forming test using the Nakajima method with a spherical punch.

[0048] In the forming limit diagram created by the method according to this embodiment 2, as shown in Figure 5, the maximum principal strain is minimized when the minimum principal strain is 0. Therefore, in the method according to this embodiment 2, the maximum principal strain is minimized in plane strain deformation where the minimum principal strain is 0. This makes it possible to create a forming limit diagram that appropriately corrects for deviations in the forming limit strain due to the influence of bending deformation in the initial stages of forming in forming tests using the Nakajima method.

[0049] In particular, according to this embodiment 2, it is possible to create a forming limit diagram that appropriately corrects for the effects of bending deformation strain in the initial stages of forming of thick metal sheets, and the effects of deviations in the forming limit strain, which are relatively large due to the narrow formable region, such as with high-strength metal sheets.

[0050] Furthermore, it is known that in actual press-formed products, the areas subjected to plane strain deformation (plane strain areas) are at the highest risk of cracking. By applying the forming limit diagram created by the method according to Embodiment 2 to such press-formed products, cracking in the plane strain areas can be accurately determined.

[0051] [Embodiment 3] The method for manufacturing a press-formed product according to Embodiment 3 of the present invention manufactures a press-formed product while suppressing the occurrence of cracks during press forming of a metal sheet. The method according to Embodiment 3 includes, as shown in Figure 6, a forming limit diagram acquisition process P1, a press-formed product crack determination process P3, a press forming condition adjustment process P5, and a press forming process P7.

[0052] <Process for obtaining molding limit diagrams> In the forming limit diagram acquisition process P1, the forming limit diagram of the metal plate created by the metal plate forming limit diagram creation method according to the present invention described above is acquired.

[0053] In this third embodiment, the molding limit diagram acquisition process P1 creates the molding limit diagram using the first step S11 and the second step S13 of this second embodiment described above, as shown in Figure 7.

[0054] Furthermore, in the first step S11, the metal plate test specimens should be taken from the metal material used as the blank for the press-formed product to be manufactured. By determining the forming limit of the metal plate actually used as the blank and creating a forming limit diagram, the accuracy of predicting crack occurrence in the press-formed product can be further improved.

[0055] <Press-molded product crack detection process> In the press-formed product cracking detection process P3, the maximum and minimum principal strains occurring in the press-formed product are obtained as cracking detection parameters, and the presence or absence of cracking in the press-formed product is determined based on the forming limit diagram obtained in the forming limit diagram acquisition process P1.

[0056] An example of a specific embodiment of the press-formed product crack detection process P3 is shown in Figure 7. First, a FEM (Finite Element Method) analysis is performed on the process of press-forming a metal sheet into a press-formed product (S31). In the FEM analysis of the press-formed product, provisional press-forming conditions are first set. Then, the FEM analysis of the press-forming process is performed under the set provisional press-forming conditions. This allows the strain generated in the press-formed product to be determined for each element and node used in the FEM analysis.

[0057] Next, based on the FEM analysis results of the press-formed product, the maximum principal strain and minimum principal strain in the press-formed product are calculated as crack detection parameters for the press-formed product (S33).

[0058] Next, based on the calculated crack detection parameters and the forming limit diagram obtained in the forming limit diagram acquisition process P1, it is determined whether or not cracks have occurred in the press-formed product (S35).

[0059] The presence or absence of cracks in press-formed products can be determined by the following procedure. First, the crack detection parameters are plotted on the forming limit diagram obtained in the forming limit diagram acquisition process P1. Then, it is determined whether the plot of the crack detection parameters is located below the molding limit line. If the crack detection parameter is located below the forming limit line, for example, if the maximum principal strain of the crack detection parameter is less than the maximum principal strain on the forming limit line corresponding to the minimum principal strain of the crack detection parameter, then it is determined that no crack has occurred. In contrast, if the plot of the crack detection parameter is not located below the forming limit surface, for example, if the maximum principal strain of the crack detection parameter is greater than or equal to the maximum principal strain on the forming limit line corresponding to the minimum principal strain of the crack detection parameter, then it is determined that a crack has occurred.

[0060] <Press forming condition adjustment process> In the press forming condition adjustment process P5, if the press forming product crack detection process P3 determines that cracks have occurred, the press forming conditions are adjusted to suppress crack occurrence.

[0061] In press forming condition adjustment process P5, first, if the pressable molded product crack detection process determines that a crack has occurred, the provisional press forming conditions for the press-formed product are changed.

[0062] Provisional changes to press forming conditions include, for example, when deep drawing press-formed parts using a press forming die consisting of a die, punch, and wrinkle holder (blank holder), changes to the blank size or shape, modifications to the die shape (die shoulder radius, punch shoulder radius), wrinkle holder force, and lubrication of the blank in contact with the die and wrinkle holder.

[0063] When changing the blank size or shape, it is advisable to adjust the provisional press forming conditions to reduce the wrinkle-holding force, thereby optimizing the tension acting on the blank during press forming and adjusting the resistance to material flow in the blank.

[0064] Furthermore, in the die shoulder and punch shoulder areas where the blank is subjected to bending and unbending deformation while under tension, the reduction in plate thickness is rapidly accelerated, making it prone to cracking and affecting material flow. Therefore, it is advisable to adjust the die shoulder radius and punch shoulder radius by changing the shape of the press forming die. In addition, the drawing limit may be adjusted by lubricating the blank that comes into contact with the die and the wrinkle holder.

[0065] Then, after changing the provisional press forming conditions (S51), in the press forming crack determination process P3, a press forming analysis is performed with the changed provisional press forming conditions (S31), and crack determination parameters are calculated (S33), and the presence or absence of cracks is determined (S35).

[0066] In contrast, if the press-formed product crack detection process P3 determines that no cracks have occurred, the press-formed condition adjustment process P5 confirms the provisional press-formed conditions for which no cracks have occurred as the press-formed conditions (S53), and the adjustment of the press-formed conditions is completed (S55).

[0067] The adjustment of press forming conditions (change of provisional press forming conditions) in press forming condition adjustment process P5 should be continued until it is determined in press forming crack detection process P3 that no cracks have occurred in the entire area of ​​the press forming product.

[0068] <Press forming process> In the press forming process P7, the metal sheet is press-formed using the press forming conditions adjusted in the press forming condition adjustment process P5 so that no cracks are detected in the press forming crack detection process P3.

[0069] In the manufacturing method for press-formed products according to this third embodiment, the press forming conditions are adjusted to suppress crack formation based on a forming limit diagram corrected for the forming limit strain obtained by forming tests using a spherical punch. This makes it possible to manufacture press-formed products while suppressing crack formation, particularly in areas where plane strain deformation occurs with a small maximum principal strain at the forming limit. Furthermore, the method according to this third embodiment significantly reduces the time required to determine the actual press forming conditions for press-formed products through trial and error. [Examples]

[0070] Experiments and analyses were conducted to verify the effectiveness of the method for drawing the forming limit line of a metal plate according to the present invention, and these will be described below.

[0071] In this example, a metal sheet forming test was performed using a spherical punch to obtain the forming limit strain and create a forming limit diagram. Based on the created forming limit diagram, the presence or absence of cracks in the press-formed product formed by draw forming was determined. In this example, a high-strength steel sheet with a tensile strength of 1470 MPa and a thickness of 1.6 mm was used as the metal sheet.

[0072] First, as described in Embodiment 1, a metal plate test specimen was formed using a spherical punch, and the maximum principal strain ε1 and minimum principal strain ε2 generated in the test specimen from the start of forming to the forming limit were measured at predetermined time intervals. Then, from the time-series data of the measured maximum and minimum principal strains, the equivalent plastic strain ε at the forming limit in the forming test using a spherical punch was determined. eq The strain increment ratio β was also determined.

[0073] Equivalent plastic strain ε at the molding limit in molding tests eqThe maximum principal strain ε1 and minimum principal strain ε2 at the forming limit were obtained by substituting them into equation (1) described above. The strain increment ratio β at the forming limit was calculated by determining the increments (Δε1, Δε2) of the maximum and minimum principal strains at the time step where the forming limit was determined and the time step immediately preceding it in the time series data of the maximum and minimum principal strains.

[0074] Next, the equivalent plastic strain ε at the molding limit in the molding test. eq By substituting the strain increment ratio β into equation (2) described above, the maximum principal strain ε1 and minimum principal strain ε2 at the molding limit were determined when molding was performed with a constant strain increment ratio β from the start of molding to the molding limit.

[0075] For various test specimens with different shapes, the maximum principal strain ε1 and minimum principal strain ε2 at the molding limit were determined by the above procedure when the strain increment ratio was kept constant from the start of molding to the molding limit, and a molding limit diagram was created (Example of Invention).

[0076] In this embodiment, the maximum and minimum principal strains at the molding limit were determined from the time-series data of the maximum and minimum principal strains measured in a molding test using the Nakajima method with a spherical punch, and these were plotted directly to create a molding limit diagram (comparative example).

[0077] Figure 8 shows molding limit diagrams for the inventive example and comparative example. In the example invention, the maximum principal strain is minimized when the minimum principal strain is 0. In contrast, in the comparative example, the minimum value of the maximum principal strain is shifted towards the first quadrant. Therefore, in the comparative example, the forming limit strain (maximum principal strain) at which the minimum principal strain is 0 is slightly larger compared to the inventive example.

[0078] Next, we investigated the effect of differences in forming limit strain in the created forming limit diagram on crack detection in press-formed products. Figure 9 shows the press-formed product 1 that was subjected to crack detection. The press-formed product 1 is made by press forming using a high-strength steel plate blank with a tensile strength of 1470 MPa and a thickness of 1.6 mm, and has a plane strain portion 3 that is formed by plane strain tensile deformation in which material inflow in the short direction from the ridge line 5 is suppressed. In this type of press-formed product 1, cracks occurred in the planar strained area 3, but no cracks were observed in other areas.

[0079] Therefore, in this embodiment, the validity of the forming limit diagram was verified by using the forming limit diagram shown in Figure 8 to determine cracks in the planar strain portion 3 of the press-formed product 1.

[0080] First, a press forming analysis was performed on the draw forming process of press-formed product 1, and the maximum and minimum principal strains in the plane strain area were determined as cracking detection parameters. Then, the obtained crack detection parameters were plotted as shown by the circles in Figure 8, and crack detection was performed.

[0081] In the comparative example, the minimum value of the forming limit line shifted towards the first quadrant, resulting in a higher forming limit in the plane strain area. Consequently, the crack detection parameter was located below the forming limit line, and no cracks were detected. In contrast, in the example invention, the crack detection parameter was located slightly above the molding limit line, and it was determined that cracks had occurred.

[0082] In summary, the present invention demonstrates that the deviation of the forming limit strain in the forming limit diagram created by the Nakajima method using a spherical punch can be appropriately corrected, and that cracks in the press-formed portion subjected to plane strain tensile deformation can be accurately determined. [Explanation of Symbols]

[0083] 1 Press-formed product 3 Plane strain area 5 Ridge

Claims

1. A method for obtaining the forming limit of a metal sheet in a forming test of a metal sheet using a spherical punch, wherein the forming limit of the metal sheet is obtained, Using the aforementioned spherical punch, a test piece of the metal plate is formed, and the equivalent plastic strain ε at the forming limit in the forming test is determined. eq The first step is to obtain the strain increment ratio β, The equivalent plastic strain ε at the acquired molding limit eq From the strain increment ratio β, the maximum principal strain ε at the molding limit is obtained when the test piece is molded with a constant strain increment ratio β from the start of molding to the molding limit. 1 and minimum principal strain ε 2 A method for obtaining the forming limit of a metal sheet, characterized by comprising: a second step of determining the forming limit strain of the metal sheet.

2. The method for obtaining the forming limit of a metal sheet according to claim 1, characterized in that, in the second step, the maximum principal strain ε1 and the minimum principal strain ε2 at the forming limit are calculated by the following formula. [Math 1]

3. A method for creating a forming limit diagram for a metal sheet, which involves obtaining the forming limit of the metal sheet and creating a forming limit diagram in a forming test of the metal sheet using a spherical punch, A first step is to perform the method for obtaining the forming limit of a metal plate according to claim 1 or 2 for test pieces of metal plates of different shapes, and to obtain the maximum principal strain and minimum principal strain at the forming limit when the test piece is formed with a constant strain increment ratio from the start of forming to the forming limit, A method for creating a forming limit diagram for a metal sheet, comprising: a second step of plotting the maximum principal strain and minimum principal strain at the forming limit obtained for each of the aforementioned test pieces, and creating a forming limit diagram for the metal sheet.

4. A method for manufacturing press-formed products that suppresses crack occurrence during press forming of metal sheets, A process for obtaining a forming limit diagram of a metal plate, which is obtained by the method for creating a forming limit diagram of a metal plate described in claim 3, A press-formed product crack determination process that acquires the maximum principal strain and minimum principal strain generated in the press-formed product as crack determination parameters, and determines whether or not cracks occur in the press-formed product based on the acquired forming limit diagram, If the press-formed product crack detection process determines that cracks have occurred, a press-forming condition adjustment process is performed to adjust the press-forming conditions to suppress crack occurrence. A method for manufacturing a press-formed product, comprising a press-forming process for press-forming a metal sheet using the press-forming conditions adjusted in the press-forming condition adjustment process.

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