Method and device for acquiring molding limit of metal plate

Abstract

A method for obtaining a molding limit of a metal plate according to the present invention is a method for obtaining a molding limit of a metal plate by performing bulging molding on a test piece 100 of a metal plate at various bending deformation degrees using a plurality of ball punches 11a having different curvatures of tip sections 11a1. The molding limit of the metal plate, which is represented by the relationship between the degree of bending deformation of the test piece (100) after the bulging molding and the maximum principal strain and the minimum principal strain of the test piece (100) after the bulging molding by the degree of bending deformation, is obtained.

Description

Technical Field

[0001] The present invention relates to a method and device for obtaining a forming limit of a metal sheet by taking into account the bending deformation degree of the metal sheet. Background Art

[0002] Most of the metal 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 body parts, but the material properties represented by the ductility of metal sheets such as steel sheets as the material also have a great influence. In addition, due to the demand for weight reduction of automotive bodies in recent years, the high strength of metal sheets such as steel sheets used for body parts is developing. However, as the strength of metal sheets increases, the ductility decreases, and thus cracks become more likely to occur during press forming, and the press formability decreases.

[0003] Therefore, in order to avoid the problem of cracking during mass production of body parts caused by press forming, die design based on advance prediction of press formability using CAE (Computer Aided Engineering) has become important. In addition, in order to accurately predict the occurrence of cracking during press forming, the importance of technology for accurately determining the forming limit of metal plates such as steel plates has increased.

[0004] The forming limit diagram (hereinafter referred to as "FLD") is usually used to determine the forming limit of a metal sheet. FLD is produced by measuring the forming limits of various deformation modes (equibiaxial deformation, non-equibiaxial deformation, plane strain deformation, uniaxial deformation) in press forming through laboratory-scale forming tests. In addition, in the production of the forming limit diagram, the width of the test piece is changed to several levels to change the deformation ratio of the long axis direction and the short axis direction of the test piece, thereby measuring the strains in the long axis direction and the short axis direction when the fracture of the test piece occurs.

[0005] Generally speaking, in press forming of metal sheets, after the process of uniform deformation of the metal sheet, the process progresses to a process in which strain is concentrated in a specific location. In this process, a reduction in plate thickness called necking occurs at the location where the strain of the metal sheet is concentrated, and fracture occurs after the plate thickness reduction progresses. In press forming, the occurrence of necking will lead to product defects, so in the FLD used for pre-prediction of product defects in press-formed products, it is necessary to define it using the strain amount before necking occurs. Moreover, in high-strength steel sheets (high-strength steel sheets) with a tensile strength of more than 980MPa, necking occurs at a low strain of about 10%, and fracture occurs immediately afterwards. Therefore, a method for determining the forming limit with high accuracy has been proposed.

[0006] ISO12004 (see Non-Patent Document 1) standardizes a method for determining the forming limit curve (hereinafter referred to as "FLC") of a metal sheet. In this method, first, the strain distribution near the fracture occurrence portion of a test piece subjected to bulge forming until fracture is measured, and the maximum value of the approximation curve of the measured strain distribution is determined as the forming limit strain.

[0007] However, in the method specified in ISO12004, the strain of the fracture generating part cannot be sufficiently approximated and the forming limit strain cannot be obtained. Therefore, in non-patent document 2, as a method to improve ISO12004, the following method is proposed: the strain generated in the test piece during forming is continuously measured, and the FLC is obtained based on the time change of the strain of the fracture generating part.

[0008] In addition, Patent Document 1 proposes the following method: using a punch with a minimum curvature radius of 3 to 10 mm at the front end to perform bulging forming, measuring the maximum principal strain and the minimum principal strain when cracks occur on the metal sheet surface at the front end of the punch, and making a forming limit curve diagram.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Publication No. 2012-166252

[0012] Non-patent literature

[0013] Non-patent document 1: ISO 12004-2:2008, Metallic materials-Sheet and strip-Determination of forming-limit curves, 2008.

[0014] 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. Summary of the invention

[0015] Problems to be solved by the invention

[0016] ISO12004 stipulates two methods, the Nakajima test and the Marciniak test, as test methods for obtaining the forming limit curve. Fig. 9 As shown in (a) of FIG. 1 , the Nakajima method is a method for obtaining a molding limit by performing a bulge test on a test piece 100 by using a molding die 41 having a hemispherical punch 43, an upper die 45, and a blank holder 47. Fig. 9 As shown in (b) of FIG. 1 , the Marciniak method uses a forming die 51 having a flat punch 53, an upper die 55, and an anti-wrinkle plate 57. In the Marciniak method, a forming limit is obtained by performing a bulging test on the test piece 100 by sandwiching a driver sheet 110 between the flat punch 53 and the test piece 100 to form a bulge.

[0017] The Nakajima method performs bulging forming in a state where the test piece 100 and the front end 43a of the ball punch 43 match each other in shape. Therefore, the obtained forming limit (maximum principal strain and minimum principal strain at fracture) is affected by the bending deformation of the test piece 100. In contrast, the Marciniak method performs bulging forming using a flat punch 53 whose front end 53a is a flat surface. Therefore, the test piece 100 does not bend and deform, and the obtained forming limit is not affected by the bending deformation of the test piece 100.

[0018] In general, the FLD obtained by the Nakajima method is better than that obtained by the Marciniak method. Fig. 9 As shown in (c) in the figure, the FLD obtained by the Nakajima method shows that the strain amount before necking of about 1 to 2% occurs becomes larger. In press forming of high-strength materials with low ductility (e.g., high-tensile steel sheets), there are cases where a difference in the occurrence of necking occurs due to a small difference in the strain amount. Therefore, the forming limit has been evaluated by comparing the FLD of the Nakajima method with that of the Marciniak method based on the strain amount measured from the actual press formed part and the strain amount obtained from the press forming analysis results using CAE.

[0019] However, in the Nakajima method, even under press-forming conditions where no cracking is predicted based on the strain obtained by press-forming analysis, cracking may occur in actual press-formed products of metal plates, especially in actual press-formed products of high-strength steel plates of 980 MPa or more. In addition, in the Marciniak method, even under press-forming conditions where cracking is predicted, cracking may not occur in actual press-formed products. As such, there are many cases where there is a large deviation between the prediction result of whether cracking occurs based on FLD and whether cracking occurs in actual press-formed products, which is problematic.

[0020] In addition, the method proposed in Non-Patent Document 2 can obtain the forming limit with higher accuracy than ISO 12004. However, as in ISO 12004, the forming limit cannot be obtained by taking into account the degree of bending deformation. Therefore, the result of predicting the occurrence of cracks based on the forming limit curve obtained by the method proposed in Non-Patent Document 2 may not be consistent with the occurrence of cracks in the actual press-molded product.

[0021] Furthermore, the forming limit curve prepared by the method of Patent Document 1 is specific to the case where cracks are generated from the surface of the steel sheet without necking, resulting in cracking, as the tensile strength of the steel sheet reaches high strength and hardening such as 980 MPa or 1180 MPa. Therefore, the forming limit strain is higher than that of the Nakajima method, and in actual press forming, the ductility-dominated cracking that progresses from the generation of localized necking to the reduction of sheet thickness leading to cracking cannot be predicted.

[0022] As described above, although in actual press forming there are a mixture of parts that are subjected to bending deformation and parts that are not subjected to bending deformation, and the degree of bending deformation varies depending on the part, the degree of bending deformation is not taken into consideration in the conventional method of calculating FLD.

[0023] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method and apparatus for obtaining a forming limit of a metal sheet, which can obtain the forming limit of the metal sheet in consideration of the influence of bending deformation.

[0024] Means for solving problems

[0025] <Background of the Invention>

[0026] As described above, for the difference in FLD based on the Nakajima method and the Marciniak method, although the strain is as small as about 1 to 2%, in a high-tensile steel sheet with low ductility, a difference in the occurrence of fracture occurs due to a small difference in strain (forming amount). As one of the reasons for the difference in forming limits based on the Nakajima method and the Marciniak method, the inventors of the present application focused on the presence or absence of bending deformation of the test piece in the forming limit test.

[0027] As described above, in the Nakajima method, the test piece 100 is bulged by the ball punch 43, so the forming limit is affected by the bending deformation of the test piece 100 ( Fig. 9 On the other hand, in the Marciniak method, the test piece 100 is bulged by the flat punch 53 and is therefore not affected by the bending deformation of the test piece 100 .

[0028] Therefore, in order to study the influence of bending deformation of the test piece 100 in the forming limit test, the inventors of the present application conducted a test under the condition of plane strain (minimum principal strain = 0), as shown in FIG. Fig. 9 As shown, a bulging test was performed using a plurality of ball punches 43 and flat punches 53 having different curvatures of the tip portions 43a. Then, based on the results of the bulging test using the ball punches 43 and flat punches 53, the respective forming limits were determined.

[0029] Fig.10 The relationship between the curvature of the front end portion 43a of the ball punch 43 and the maximum principal strain indicating the forming limit is shown. Fig.10 The maximum principal strain when the curvature is 0 is given by Fig. 9 It is obtained by a bulging test using a flat punch 53 shown in (b) in FIG.

[0030] like Fig.10 As shown in FIG. 1 , it was found that the greater the curvature of the front end portion 43a, that is, in the case of bulging forming in which a sharp bending deformation is applied to the front end portion 43a of the ball punch 43, the greater the maximum principal strain as the forming limit. From this result, it can be seen that in order to predict whether cracks occur in actual press forming, it is necessary to consider the degree of bending deformation to determine the forming limit. The present invention is completed based on the above-mentioned findings, and specifically has the following configuration.

[0031] The method for obtaining the forming limit of a metal plate involved in the present invention uses a plurality of ball-head punches with different curvatures at the front end to bulge-form a test piece of the metal plate with various bending deformation degrees, and obtains the forming limit of the metal plate represented by the relationship between the bending deformation degree of the test piece after bulging and the maximum principal strain and the minimum principal strain of the test piece after bulging with the bending deformation degree.

[0032] The degree of bending deformation can be represented by the curvature of the front end portion of the ball punch.

[0033] The method for obtaining the forming limit of a metal plate according to the present invention obtains the forming limit of the metal plate by taking into account the bending deformation degree, and comprises: a forming test step, a forming limit analysis step, and a forming limit surface (boundary surface of forming limit) preparation step, wherein the aforementioned forming test step comprises: a test piece preparation step, preparing a test piece with a predetermined grid or strain analysis pattern on the surface of the aforementioned metal plate; a bulging forming step, photographing the surface of the aforementioned test piece with the aforementioned grid or strain analysis pattern, and bulging the aforementioned test piece with various bending deformation degrees using a plurality of ball-head punches with different curvatures at the front end; a strain measurement step, analyzing the image of the surface of the aforementioned test piece photographed in the bulging forming step, and measuring the strain generated in the aforementioned test piece for each bending deformation degree of the aforementioned test piece after the bulging forming; and a strain database construction step, storing the front end measured for each bending deformation degree of the aforementioned test piece after the bulging forming. The strain is stored in a time series from the start of forming to the fracture, and a strain database is constructed. The forming limit analysis step includes: a strain distribution extraction step, for each bending deformation of the test piece after bulging forming, extracting the strain distribution near the fracture occurrence part of the test piece from the strain database; and a forming limit acquisition step, for each bending deformation of the test piece, according to the extracted strain distribution, obtaining the forming limit strain represented by the maximum principal strain and the minimum principal strain at the fracture occurrence part of the test piece. The forming limit surface preparation step includes: a forming limit plotting step, for plotting the forming limit strain obtained for each bending deformation in the forming limit analysis step in a three-dimensional coordinate space with the maximum principal strain, the minimum principal strain and the bending deformation as three axes; and a forming limit surface preparation step, for each bending deformation of the test piece, preparing a forming limit surface represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation based on the forming limit strain for each bending deformation plotted in the three-dimensional coordinate space.

[0034] In the aforementioned forming limit surface preparation step, a polygonal surface formed by a plurality of triangular planes may be prepared as the aforementioned forming limit surface, wherein the aforementioned triangular plane is formed by connecting two adjacent plotted points in a forming limit strain group having the same bending deformation degree and a plotted point in a forming limit strain group having a bending deformation degree different from the two plotted points, the point having the smallest distance from a line segment connecting the aforementioned two plotted points.

[0035] In the aforementioned forming limit surface manufacturing process, a forming limit plane (boundary flat surface of forming limit) or a forming limit curved surface (boundary curved surface of forming limit) is assumed, and the sum of squares of vertical distances between the assumed forming limit plane or the forming limit curved surface and the plotted points of the forming limit strain of each bending deformation degree in the aforementioned three-dimensional coordinate space is calculated. The aforementioned forming limit surface is manufactured by determining the aforementioned forming limit plane or the aforementioned forming limit curved surface in which the calculated sum of squares becomes the minimum.

[0036] In the aforementioned forming limit surface production process, a forming limit plane or a forming limit curved surface can be assumed, and the weighted sum of squares of the vertical distances between the assumed forming limit plane or forming limit curved surface and the plotted points of the forming limit strain of each bending deformation degree in the aforementioned three-dimensional coordinate space can be calculated, and the aforementioned forming limit plane or forming limit curved surface can be determined in a manner that minimizes the calculated sum of squares.

[0037] In the forming limit surface preparing step, a plurality of forming limit planes and forming limit curved surfaces may be combined to prepare the forming limit surface.

[0038] The forming limit acquisition device of the metal plate of the present invention obtains the forming limit of the metal plate by taking into account the bending deformation degree, and comprises: a forming test section, a forming limit analysis section, and a forming limit surface preparation section, wherein the aforementioned forming test section comprises: a forming die having a plurality of ball-head punches with different curvatures of the front end portion, and using the plurality of ball-head punches to perform bulging forming on the aforementioned metal plate test piece having a predetermined grid or strain analysis pattern on the surface at various bending deformation degrees; a photographing device, which photographs the surface of the aforementioned test piece during the process of bulging forming the aforementioned test piece by using the aforementioned forming die; a strain measuring device, which analyzes the image of the surface of the aforementioned test piece photographed by the photographing device, and measures the strain generated in the aforementioned test piece for each aforementioned bending deformation degree; and a strain database construction device, which, for each bending deformation degree of the aforementioned test piece after bulging forming, sequentially generates the aforementioned measured strain in a time series from the start of forming to fracture. The forming limit analysis unit is provided with: a strain distribution extraction device, which extracts the strain distribution near the fracture occurrence part of the test piece from the strain database for each bending deformation degree of the test piece after bulging forming; and a forming limit acquisition device, which obtains the forming limit strain represented by the maximum principal strain and the minimum principal strain at the fracture occurrence part of the test piece for each bending deformation degree of the test piece based on the extracted strain distribution; the forming limit surface preparation unit is provided with: a forming limit plotting device, which plots the forming limit strain obtained by the forming limit analysis unit for each bending deformation degree in a three-dimensional coordinate space with the maximum principal strain, the minimum principal strain and the bending deformation degree as three axes; and a forming limit surface preparation device, which prepares the forming limit surface represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation degree based on the forming limit plotted in the three-dimensional coordinate space.

[0039] Effects of the Invention

[0040] According to the present invention, a test piece of a metal plate is bulged at various bending deformations, and the bending deformation is obtained as an index of the forming limit in addition to the maximum principal strain and the minimum principal strain, thereby being able to obtain the forming limit of the metal plate in consideration of the influence of the bending deformation. Furthermore, according to the present invention, a forming limit surface represented by the relationship between the bending deformation, the maximum principal strain, and the minimum principal strain can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] [ Figure 1 ] Figure 1 This is a flowchart showing the processing flow of the method for obtaining the forming limit of a metal sheet according to the embodiment of the present invention.

[0042] [ Figure 2 ] Figure 2This is a diagram showing an example of a forming die used for bulging forming of a test piece in the forming limit acquisition method of a metal plate and the forming limit apparatus according to the embodiment of the present invention.

[0043] [ Figure 3 ] Figure 3 It is a figure which shows the shape of the metal plate test piece used for the bulging test in this embodiment and Example.

[0044] [ Figure 4 ] Figure 4 It is a diagram showing the structure of a metal plate forming limit test apparatus according to an embodiment of the present invention.

[0045] [ Figure 5 ] Figure 5 It is a diagram showing a process flow for obtaining the forming limit of a metal sheet in the embodiment.

[0046] [ Figure 6 ] Figure 6 It is a figure which shows the shape of the ball head punch used for bulging forming in Example.

[0047] [ Figure 7 ] Figure 7 This is a graph showing the forming limit strain obtained for each bending deformation degree in the examples.

[0048] [ Figure 8 ] Figure 8 This is a graph showing the results of obtaining the forming limit surface represented by the relationship between the maximum principal strain, the minimum principal strain, and the bending deformation in the examples.

[0049] [ Fig. 9 ] Fig. 9 The figures show examples of forming dies and forming limit curves used in conventional forming limit tests ((a) Nakashima method, (b) Marciniak method, and (c) forming limit curves obtained by the Nakashima method and the Marciniak method, respectively).

[0050] [ Fig.10 ] Fig.10 This is a graph illustrating the influence of the degree of bending deformation of a test piece on the forming limit strain. DETAILED DESCRIPTION

[0051] <Method for obtaining the forming limit of metal sheet>

[0052] The forming limit obtaining method of the metal plate according to the present embodiment (hereinafter also referred to as "forming limit obtaining method") is as follows: Figure 2As illustrated, a plurality of ball-tipped punches 11a having different curvatures of the tip 11a1 are used to bulge a metal plate test piece 100 at various bending deformations. The forming limit obtaining method is a method for obtaining the forming limit of the metal plate represented by the relationship between the bending deformation of the test piece after bulging and the maximum principal strain and the minimum principal strain at the fracture occurrence portion of the test piece after bulging at the bending deformation.

[0053] like Figure 2 As shown in FIG. 1 , the ball-end punch 11a has a spherical surface with a curvature radius R, that is, a front end portion 11a1 with a curvature ρ (=1 / R) greater than 0. Of course, in the present invention, the ball-end punch 11a also includes Fig. 9 The flat punch 53 shown in (b) has a flat tip portion 53a (curvature radius R=∞, curvature ρ=0). Thus, the ball punch 11a according to the present invention has a tip portion 11a1 with a curvature ρ of 0 or more.

[0054] For the test piece 100, in order to obtain the forming limit under each deformation mode (equibiaxial deformation, unequal biaxial deformation, plane strain deformation, uniaxial deformation) in press forming, the Figure 3 The test pieces 101 and 103 are shaped as shown in the example. Figure 3 As shown in (a) in FIG. 1 , the test piece 101 is in the shape of a disk, and the forming limit of the equibiaxial deformation is obtained. Figure 3 As shown in (b) and (c) in the figure, the test piece 103 has a notch 103a in the shape of an arc notch at a position opposite to the diameter direction of the peripheral portion of the disk-shaped portion. In addition, for the test piece 103, the width W of the narrowest part of the central portion 103b is changed in various ways. As the width W becomes narrower, the equibiaxial deformation is changed to unequal biaxial deformation and plane strain deformation, gradually approaching uniaxial tension, and the forming limit under each deformation mode is obtained. In addition, the test piece 100 has a predetermined grid or strain analysis pattern on the surface of the metal plate.

[0055] The bending deformation degree, which is an index of the forming limit, indicates the degree of bending deformation at the portion of the test piece 100 after bulging. The curvature radius and the actual measured value of the curvature of the portion after bulging can be used as the bending deformation degree.

[0056] Of course, in the present embodiment, the bending deformation degree may be expressed by the curvature of the tip 11 a 1 of the ball punch 11 a used for the bulging of the test piece 100 , instead of actually measuring the curvature radius and curvature of the bulged portion.

[0057] In addition, the method of obtaining the maximum principal strain and the minimum principal strain as indicators of the forming limit is not particularly limited, and the forming limit can be obtained based on the same reference for the strains measured using a plurality of ball-tipped punches 11a having different curvatures of the tip portion 11a1. For example, in the vicinity of the fracture occurrence portion of the test piece 100 that is bulged until fracture occurs, the maximum principal strain and the minimum principal strain when fracture occurs can be obtained based on the shape of the mark on the surface of the test piece 100, or can be obtained by the method described below.

[0058] The specific method of obtaining the forming limit involved in this embodiment is described below. As a specific method of obtaining the forming limit involved in this embodiment, Figure 1 As shown, a method including a forming test step S10, a forming limit analysis step S20, and a forming limit surface preparation step S30 can be exemplified.

[0059] 《Molding test steps》

[0060] like Figure 1 As shown, the molding test step S10 includes a test piece preparation step S11, a bulging molding step S13, a strain measurement step S15, and a strain database construction step S17.

[0061] (Test piece preparation process)

[0062] The test piece preparation step S11 is a step of preparing a test piece 100 having a predetermined grid or strain analysis pattern on the surface of a metal plate. Figure 3 As shown, a circular test piece 101 and a test piece 103 having a notch 103a formed on the outer edge of a circle are prepared. Then, for the test piece 103 having the notch 103a, a test piece having a central portion 103b with a width W changed in a plurality of levels is prepared.

[0063] (Bulging process)

[0064] The bulging step S13 is a step of imaging the surface of the test piece 100 having the grid or strain analysis pattern and bulging the test piece 100 with various degrees of bending deformation using a plurality of ball-tipped punches 11 a having tip portions 11 a 1 with different curvatures.

[0065] (Strain measurement process)

[0066] The strain measurement step S15 is a step of analyzing the image of the surface of the test piece 100 photographed in the bulging step S13, and measuring the strain generated in the test piece 100 for each bending deformation. As a method for measuring strain, for example, Digital Image Correlation (hereinafter referred to as "DIC") is used. The surface of the test piece 100 during the bulging process is photographed at a predetermined time interval, and the image photographed at each time step is analyzed. Based on the deformation of the grid or the pattern for strain analysis, the maximum principal strain and the minimum principal strain are measured as the strain in two directions in the plane generated in the test piece 100.

[0067] (Strain database construction process)

[0068] The strain database construction step S17 is a step of constructing a strain database by storing the strain measured for each bending deformation degree in the strain measurement step S15 in a time series from the start of forming to fracture.

[0069] That is, in the molding test step S10, first, a test piece 100 with a predetermined grid (sample grid) is prepared in the test piece preparation step S11. Next, the test piece 100 is bulged in the bulging molding step S13, and the strain on the surface of the test piece 100 is measured using an image analysis device in the strain measurement step S15, and the maximum principal strain and the minimum principal strain generated in the test piece 100 are recorded in the strain database. Then, molding is gradually performed until the test piece 100 breaks, and the strain measurement and the recording of the time series change are repeated, thereby constructing a strain database from the start of molding to the break. The above operation is performed for each test piece 100 with a different shape and a different curvature of the front end 11a1 of the ball head punch 11a.

[0070] 《Forming Limit Analysis Steps》

[0071] like Figure 1 As shown, the forming limit analysis step S20 includes a strain distribution extraction step S21 and a forming limit acquisition step S23.

[0072] (Strain distribution extraction process)

[0073] The strain distribution extraction step S21 is a step of extracting the strain distribution near the fracture occurrence portion of the test piece 100 after expansion molding at various bending deformation degrees from the strain database constructed in the strain database construction step S17. In the present embodiment, the strain distribution extracted in the strain distribution extraction step S21 is the time series data of the strain stored in the strain database for each bending deformation degree.

[0074] (Forming limit acquisition process)

[0075] The forming limit acquisition step S23 is a step of obtaining the forming limit strain represented by the maximum principal strain and the minimum principal strain at the fracture occurrence portion of the test piece 100 for each bending deformation of the test piece 100 based on the strain distribution extracted in the strain distribution extraction step S21 .

[0076] In the forming limit analysis step S20, the forming limit strain can be obtained, for example, as follows. First, in the strain distribution extraction step S21, for each bending deformation, for each predetermined time step from the start of forming to fracture, the maximum principal strain and the minimum principal strain near the fracture occurrence part of the test piece 100 are extracted to create time series data.

[0077] Next, in the forming limit acquisition step S23, the bending point (bend point) where the test piece 100 shifts from uniform deformation to inhomogeneous deformation is determined in the time series data of the maximum principal strain and the minimum principal strain produced for each bending deformation. Then, assuming that necking occurs in the test piece 100 at the bending point of the strain, the maximum principal strain and the minimum principal strain at this time are obtained as the forming limit strain under the bending deformation. In this way, by obtaining the forming limit strain for each bending deformation, the forming limit represented by the relationship between the bending deformation, the maximum principal strain, and the minimum principal strain can be obtained.

[0078] Of course, the present invention may also obtain the maximum principal strain and the minimum principal strain at the time when the test piece 100 breaks from the time series data of the maximum principal strain and the minimum principal strain as the forming limit strain.

[0079] Furthermore, the forming limit strain may be determined by any method as long as the strain is determined based on the same criteria as measured by a bulging test using a plurality of ball-tipped punches 11 a having different curvatures of the tip portions 11 a 1 .

[0080] 《Forming limit surface production steps》

[0081] like Figure 1 As shown, the forming limit surface preparation step S30 includes a forming limit point drawing step S31 and a forming limit surface preparation step S33.

[0082] (Forming limit point drawing process)

[0083] The forming limit plotting step S31 is a step of plotting the forming limit strain obtained for each bending strain degree in the forming limit analysis step S20 in a three-dimensional coordinate space having the maximum principal strain, the minimum principal strain and the bending strain degree as three axes.

[0084] (Forming limit surface production process)

[0085] The forming limit surface preparation step S33 is a step of preparing a forming limit surface based on the forming limit of the points plotted in the three-dimensional coordinate space with the maximum principal strain, the minimum principal strain and the bending deformation as the coordinate axes in the forming limit plotting step S31. In addition, the forming limit surface prepared in the forming limit surface preparation step S33 is represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation. As a method of preparing the forming limit surface in the forming limit surface preparation step S33, there is, for example, the following method.

[0086] The first method is to first extract two forming limit strain plots with equal bending deformation. Next, select a forming limit strain plot that has the smallest distance from the line segment connecting the two forming limit strain plots in the forming limit strain group with different bending deformation of the two forming limit strains. Then, a triangular plane is formed by the two extracted forming limit strain plots and the selected forming limit strain plot. Such a triangular plane is created for the plots of all forming strain groups. Then, a polygon formed by combining the created triangular planes is used as a forming limit surface.

[0087] The second method is to first assume a forming limit plane or forming limit surface in a three-dimensional coordinate space, calculate the perpendicular distance between the assumed forming limit plane or forming limit surface and the plot point of the forming limit strain, and then determine the forming limit plane or forming limit surface in such a way that the sum of the squares of the calculated perpendicular distances becomes the minimum.

[0088] A third method is to determine the forming limit plane or forming limit surface in the above-mentioned second method so that the sum of squares of weighted perpendicular distances between the assumed forming limit plane or forming limit surface and the plotted points of the forming limit strain becomes minimum.

[0089] As a method of assigning weights, for example, the weight may be increased for the forming limit of the bending deformation degree that is likely to cause cracking in actual press forming, thereby making it possible to reduce the error between the determined forming limit plane or forming limit curved surface and the plotted point of the forming limit strain at the bending deformation degree that is likely to cause cracking in actual press forming.

[0090] For the second method or the third method described above, a plurality of forming limit planes and / or forming limit surfaces may be combined. For example, forming limit planes or forming limit surfaces may be assumed in the region where the minimum principal strain is negative and in the region where the minimum principal strain is positive, respectively, and for each region, the forming limit plane or forming limit surface may be determined in such a manner that the sum of the squares of the perpendicular distances to the plotted points of the forming limit strain is minimized. The same is true when the forming limit surface is prepared in such a manner that the sum of the squares of the perpendicular distances to the plotted points of the forming limit strain is minimized after weighting.

[0091] In addition, as a method other than the above, three adjacent points may be selected from the plotted points of the forming limit strain in the three-dimensional coordinate space, and a triangular plane may be generated by connecting the selected three points with a straight line. The polygon formed by the multiple triangular planes generated in this way may be used as the forming limit surface.

[0092] Alternatively, a forming limit curve may be prepared based on the forming limit strain for each bending deformation degree, and a forming limit surface including the prepared forming limit curve may be prepared.

[0093] <Metal sheet forming limit acquisition device>

[0094] The above description is a method for obtaining the forming limit surface of a metal plate represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation. Of course, the present invention can also be configured as a device for making the forming limit surface of a metal plate. That is, the forming limit obtaining device for a metal plate involved in this embodiment (hereinafter referred to as "forming limit obtaining device") obtains the forming limit of the metal plate by taking the bending deformation into consideration. And, as Figure 4 As shown, the forming limit acquisition device 1 includes a forming test section 10 , a forming limit analysis section 20 , and a forming limit surface creation section 30 .

[0095] Molding Test Department

[0096] The molding test section 10 includes a molding die 11 , an imaging device 13 , a strain measuring device 15 , and a strain database constructing device 17 .

[0097] The molding die 11 performs bulging molding on the test piece with various bending deformation degrees, such as Figure 2 As shown, it has a plurality of ball-head punches 11a with different curvatures of the front end portions 11a1, an upper die 11b, and an anti-wrinkle pressing plate 11c.

[0098] The imaging device 13 captures the deformation of the grid or the pattern for strain analysis on the surface of the test piece 100 during the expansion molding of the test piece 100 with the grid or the pattern for strain analysis by the molding die 11. The imaging device 13 is composed of, for example, two cameras, and performs stereo photography of the surface of the test piece 100.

[0099] The strain measuring device 15 analyzes the image of the surface of the test piece 100 photographed by the photographing device 13, and measures the strain generated in the test piece 100 for each bending deformation degree of the test piece 100 after the bulging. For example, the strain measuring device 15 uses the digital image correlation method to measure the maximum principal strain and the minimum principal strain as the strains in two directions in the plane generated in the test piece 100 during the bulging process.

[0100] The strain database construction device 17 constructs a strain database that stores and retrieves the strain measured by the strain measuring device 15 for each bending deformation of the test piece after bulging in a time series from the start of forming to fracture.

[0101] 《Forming Limit Analysis Department》

[0102] like Figure 4 As shown, the forming limit analysis unit 20 includes a strain distribution extraction device 21 and a forming limit acquisition device 23 .

[0103] (Strain distribution extraction device)

[0104] The strain distribution extraction device 21 extracts the strain distribution near the fracture occurrence portion of the test piece 100 from the strain database constructed by the strain database construction device 17 for each bending deformation degree of the test piece 100 after bulging.

[0105] (Forming limit acquisition device)

[0106] The forming limit acquisition device 23 obtains the forming limit strain represented by the maximum principal strain and the minimum principal strain at the fracture occurrence portion of the test piece 100 for each bending deformation degree of the test piece 100 based on the strain distribution extracted by the strain distribution extraction device 21 .

[0107] 《Forming Limit Surface Production Department》

[0108] like Figure 4 As shown, the forming limit surface preparation section 30 includes a forming limit point plotting device 31 and a forming limit surface preparation device 33 .

[0109] (Forming limit plotting device)

[0110] The forming limit plotting device 31 plots the forming limit expressed by the relationship among the bending strain, the maximum principal strain and the minimum principal strain obtained by the forming limit analysis unit 20 in a three-dimensional coordinate space having the maximum principal strain, the minimum principal strain and the bending strain as three axes.

[0111] (Forming limit surface production device)

[0112] The forming limit surface preparation device prepares a forming limit surface represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation based on the forming limit plotted in the three-dimensional coordinate space by the forming limit plotting device 31.

[0113] As described above, according to the method for obtaining the forming limit of a metal plate involved in this embodiment, a test piece of a metal plate is bulged with various bending deformations, and the maximum principal strain and the minimum principal strain of the fracture occurrence portion are obtained for each bending deformation, thereby obtaining the forming limit taking into account the influence of the bending deformation.

[0114] In the previous forming limit test methods, such as Fig. 9 As shown, a forming limit strain group obtained from a test piece 100 after bulging using a specific ball punch 43 or a flat punch 53 and a forming limit curve (FLC) as an approximate curve thereof can be prepared.

[0115] In contrast, the forming limit acquisition method and forming limit device involved in the present embodiment use the curvature of the front end of the ball punch to represent the bending deformation, and use multiple ball punches 11a with different curvatures of the front end to perform a bulging test. Thus, a forming limit strain group is obtained only according to the bending deformation of the number of shapes of the ball punch 11a. Therefore, the bending deformation in each forming limit strain group can be used as a new evaluation axis, and the forming limit can be plotted in a three-dimensional coordinate space with the maximum principal strain, the minimum principal strain and the bending deformation as the coordinate axis. Thus, the forming limit surface represented by the maximum principal strain and the minimum principal strain, and the relationship with the bending deformation can be obtained. In addition, as the bending deformation of the press-formed product, the maximum principal strain, the minimum principal strain, and the curvature in the direction of the maximum principal strain (for example, the maximum principal curvature, etc.) are obtained as the bending deformation through the press-forming analysis of the press-formed product. Furthermore, based on the relationship between the obtained maximum principal strain, minimum principal strain, degree of bending deformation, and the forming limit surface, it is possible to accurately predict the occurrence of cracks during press forming in consideration of the influence of bending deformation.

[0116] Example

[0117] Since an experiment was conducted to obtain the forming limit surface of a metal sheet using the forming limit obtaining method of a metal sheet according to the present embodiment, the result will be described below.

[0118] In this embodiment, based on Figure 5 First, in the molding test step S10, use Figure 2 The test piece 100 is bulged by the molding die 11 including the ball punch 11 a , the upper die 11 b , and the wrinkle-preventing plate 11 c .

[0119] When performing bulging forming, first, the shape (curvature) of the front end portion 11a1 of the ball-tipped punch 11a is determined. Figure 6 As shown, the ball head punch 11a is prepared with the curvature radius of the front end 11a1 being R25mm, R50mm, R100m and R=∞ (equivalent to Fig. 9 There are four types of flat punches 53 shown in (b). The curvatures of the tip portion 11a1 are 0.04, 0.02, 0.01, and 0.00, respectively.

[0120] Next, as a metal plate, a test piece 100 of a high-strength steel plate with a tensile strength of 1470 MPa and a plate thickness of 1.4 mm was used as a test material, and the shape of the test piece 100 was determined. Figure 3 As shown, the shape of the test piece 100 is a diameter The circular test piece 101 and the test piece 103 with a shape cut out in the width direction based on the circular test piece 101 are provided. In addition, the test piece 103 is provided with five shapes with the width W of the central portion 103b ranging from 25 mm to 160 mm. Figure 3 (b) shows that the width W is 60 mm and the Figure 3 (c) shows a test piece 103 having a width W of 80 mm. A grid pattern (grid) for strain measurement is transferred to the surface of the test piece 100 having a fixed shape as described above.

[0121] Next, Figure 2 The molding die 11 shown is set in a hydraulic deep drawing test machine to perform bulging of the test piece 100. During bulging, in order to suppress the inflow of the test piece 100, a blank holder pressure of 50 tons is applied by a blank holder, and the punch speed of the ball punch 11a is set to 5 mm / minute. Then, while bulging is performed, the surface of the test piece 100 is photographed by a photographing device (not shown) set on the upper part of the molding die 11, that is, an image analysis camera.

[0122] Then, the image of the surface of the test piece 100 taken is analyzed, and the strain amount (maximum principal strain and minimum principal strain) generated on the surface of the test piece 100 is measured. The measured strain amount is stored in the strain database. In addition, it is visually determined whether a fracture has occurred in the test piece 100. If it is determined that no fracture has occurred, bulging molding is performed, and the photographing of the surface of the test piece 100, the measurement of the strain amount based on image analysis, and the storage in the strain database are repeated. As described above, in this embodiment, from the start of molding until the fracture occurs, the surface of the test piece is photographed at an interval of 1 time / second, and the strain amount is measured for each image taken, and this continues until it is determined that a fracture has occurred.

[0123] If it is determined that fracture has occurred, it is determined whether all the shapes of the prepared test pieces 100 have been tested. If it is determined that all the shapes of the test pieces 100 have not been tested, bulging, measuring the strain amount, and storing in the strain database are repeated for test pieces 100 of other shapes.

[0124] When it is determined that the test has been performed on all the shapes of the test pieces 100, it is determined whether the test has been performed on all the shapes of the prepared ball-tipped punches 11a. Then, the above-mentioned bulging test is performed on all the shapes of the ball-tipped punches 11a.

[0125] After the forming test step S10 is completed, in the forming limit analysis step S20 , a forming limit analysis is performed using the strain database constructed in the forming test step S10 .

[0126] First, evaluation points for obtaining the forming limit strain are set near the fracture occurrence portion of the test piece 100. Next, time series data of the maximum principal strain and the minimum principal strain are extracted from the strain database for each of the set evaluation points.

[0127] Then, the maximum principal strain and the minimum principal strain at the bending point where the uniform deformation changes to the non-uniform deformation are obtained as the forming limit strain from the time series data of the maximum principal strain and the minimum principal strain obtained for each shape of the test piece 100. Then, this operation is performed for all ball-tipped punches 11a with different curvature radii of the tip portion 11a1 used for the bulge test, and the forming limit strain (maximum principal strain, minimum principal strain) is obtained for each bending deformation degree.

[0128] Table 1 shows the values ​​of the forming limit strain obtained for each bending deformation degree. Figure 7 A graph is shown in which the forming limit strain is plotted on a two-dimensional coordinate plane with the minimum principal strain and the maximum principal strain as coordinate axes.

[0129] [Table 1]

[0130]

[0131] After the forming limit analysis step S20 is completed, in the forming limit surface preparation step S30 , a forming limit surface is prepared based on the forming limit strain obtained for each bending deformation degree.

[0132] In the forming limit analysis step S20, first, the forming limit strain (forming limit data) obtained for each bending deformation in the forming limit analysis step is extracted and plotted in a three-dimensional coordinate space with the maximum principal strain, the minimum principal strain and the bending deformation as coordinate axes.

[0133] Next, a forming limit plane that approximates the forming limit data is obtained. In this embodiment, a forming limit plane represented by the following equation (1) is assumed for each of the negative and positive regions of the minimum principal strain.

[0134] [Mathematical formula 1]

[0135] ax+by+cz+d=0…(1)

[0136] Then, the coefficients in equation (1) are determined for the forming limit plane A in the positive region and the forming limit plane B in the negative region so that the sum of the squares of the perpendicular distances between the assumed forming limit surface and the plotted points of the forming limit strain group for each bending deformation degree becomes minimum. In this way, the forming limit plane A and the forming limit plane B for which the coefficients in equation (1) are determined are obtained as the forming limit surfaces.

[0137] Table 2 shows the coefficients in equation (1) determined for the forming limit plane A and the forming limit plane B. In addition, Figure 8 The forming limit plane created based on the determined coefficients and the plotted points of the forming limit strain for each bending deformation degree actually measured are shown.

[0138] [Table 2]

[0139]

[0140] Above, such as Figure 8 As shown, according to the method and apparatus of the present invention, the forming limit surface represented by the relationship between the bending deformation degree, the maximum principal strain and the minimum principal strain can be obtained.

[0141] Industrial Applicability

[0142] According to the present invention, it is possible to provide a method and apparatus for obtaining a forming limit of a metal sheet, which can obtain the forming limit of the metal sheet by taking the influence of bending deformation into consideration.

[0143] Description of Reference Numerals

[0144] 1 Forming limit acquisition device

[0145] 10 Molding test department

[0146] 11. Molding mold

[0147] 11a Ball punch

[0148] 11a1 Front end

[0149] 11b Upper die

[0150] 11c Anti-wrinkle pressure plate

[0151] 13. Filming Equipment

[0152] 15 Strain measurement device

[0153] 17 Strain database construction device

[0154] 20 Forming Limit Analysis Department

[0155] 21 Strain distribution extraction device

[0156] 23 Forming limit acquisition device

[0157] 30 Forming limit surface production department

[0158] 31 Forming limit point drawing device

[0159] 33 Forming limit surface production device

[0160] 41 Molding mold

[0161] 43 Ball Punch

[0162] 43a Front end

[0163] 45 upper mold

[0164] 47 Anti-wrinkle pressure plate

[0165] 51 Molding mold

[0166] 53 Flat Punch

[0167] 53a Front end

[0168] 55 upper mold

[0169] 57 Anti-wrinkle pressure plate

[0170] 100 test pieces

[0171] 101 Test piece

[0172] 103 Test piece

[0173] 103a Notch

[0174] 103b Central

[0175] 110 Driver Plate

Claims

1. A method for obtaining the forming limit of a metal plate, wherein a plurality of ball-tipped punches having different curvatures at the tip end are used to bulge a metal plate test piece at various bending deformations. The forming limit of the metal sheet is obtained, which is represented by the relationship between the bending deformation of the test piece after bulging and the maximum principal strain and the minimum principal strain of the test piece after bulging at the bending deformation.

2. The method for obtaining the forming limit of a metal sheet according to claim 1, wherein: The bending deformation degree is represented by the curvature of the front end portion of the ball punch.

3. A method for obtaining the forming limit of a metal sheet, which considers the bending deformation to obtain the forming limit of the metal sheet. The method for obtaining the forming limit of the metal plate comprises: Forming test steps, forming limit analysis steps, and forming limit surface production steps, The molding test steps include: a test piece preparation step of preparing a test piece having a predetermined grid or pattern for strain analysis on the surface of the metal plate; A bulging step of photographing the surface of the test piece with the grid or the pattern for strain analysis, and bulging the test piece with various degrees of bending deformation using a plurality of ball-tipped punches having different curvatures at the front end; a strain measuring step of analyzing an image of the surface of the test piece photographed in the bulging step, and measuring the strain generated in the test piece for each bending deformation degree of the test piece after the bulging; and a strain database construction step of storing the strain measured for each bending deformation degree of the test piece after bulging in a time series from the start of forming to fracture to construct a strain database; The forming limit analysis step comprises: a strain distribution extraction step of extracting, from the strain database, a strain distribution near a fracture occurrence portion of the test piece for each bending deformation degree of the test piece after bulging; and a forming limit obtaining step of obtaining, based on the extracted strain distribution, a forming limit strain represented by a maximum principal strain and a minimum principal strain at a fracture occurrence portion of the test piece for each bending deformation degree of the test piece, The forming limit surface manufacturing step comprises: a forming limit plotting step of plotting the forming limit strain obtained for each bending deformation degree in the forming limit analysis step in a three-dimensional coordinate space with maximum principal strain, minimum principal strain and bending deformation degree as three axes; and The forming limit surface preparation step prepares a forming limit surface represented by a relationship among a maximum principal strain, a minimum principal strain and a bending deformation degree based on the forming limit strain for each bending deformation degree of a point plotted in the three-dimensional coordinate space.

4. The method for obtaining the forming limit of a metal sheet according to claim 3, wherein: In the forming limit surface preparation step, a polygonal surface formed by a plurality of triangular planes is prepared as the forming limit surface, wherein the triangular plane is formed by connecting two adjacent plot points in a forming limit strain group with the same bending deformation degree and a plot point in a forming limit strain group with a different bending deformation degree from the two plot points, the plot point having the smallest distance from a line segment connecting the two plot points.

5. The method for obtaining the forming limit of a metal sheet according to claim 3, wherein: In the forming limit surface production process, a forming limit plane or a forming limit curved surface is assumed, and the sum of the squares of the vertical distances between the assumed forming limit plane or the forming limit curved surface and the plotted points of the forming limit strain of each bending deformation degree in the three-dimensional coordinate space is calculated. The forming limit surface is produced by determining the forming limit plane or the forming limit curved surface in which the calculated sum of squares becomes the minimum.

6. The method for obtaining the forming limit of a metal sheet according to claim 3, wherein: In the forming limit surface preparation process, a forming limit plane or a forming limit curved surface is assumed, and the weighted sum of squares of the vertical distances between the assumed forming limit plane or the forming limit curved surface and the plotted points of the forming limit strain for each bending deformation degree in the three-dimensional coordinate space is calculated. The forming limit surface is prepared by determining the forming limit plane or the forming limit curved surface in such a way that the calculated sum of squares is minimized.

7. The method for obtaining the forming limit of a metal sheet according to claim 5 or 6, wherein: In the forming limit surface preparing step, a plurality of forming limit planes and forming limit curved surfaces are combined to prepare the forming limit surface.

8. A device for obtaining the forming limit of a metal sheet, which obtains the forming limit of the metal sheet by taking into account the bending deformation degree. The forming limit acquisition device for metal sheets comprises: a forming test section, a forming limit analysis section, and a forming limit surface production section. The molding test section has: A forming die having a plurality of ball-tipped punches with different curvatures at the front end, and using the plurality of ball-tipped punches to bulge the metal plate test piece having a predetermined grid or strain analysis pattern on the surface at various bending deformations; a photographing device for photographing a surface of the test piece during the process of expansion molding the test piece using the molding die; a strain measuring device for analyzing the image of the surface of the test piece photographed by the photographing device to measure the strain generated in the test piece for each degree of bending deformation; and A strain database construction device stores the measured strain in a time series from the start of forming to fracture for each bending deformation degree of the test piece after bulging forming, thereby constructing a strain database, The forming limit analysis unit has: a strain distribution extraction device for extracting, from the strain database, a strain distribution near a fracture occurrence portion of the test piece for each bending deformation degree of the test piece after bulging; and a forming limit obtaining device for obtaining, based on the extracted strain distribution, a forming limit strain represented by a maximum principal strain and a minimum principal strain at a fracture occurrence portion of the test piece for each bending deformation degree of the test piece, The forming limit surface production unit comprises: a forming limit plotting device for plotting the forming limit strain obtained by the forming limit analysis unit for each bending deformation degree in a three-dimensional coordinate space with maximum principal strain, minimum principal strain and bending deformation degree as three axes; and A forming limit surface preparation device prepares a forming limit surface represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation based on the forming limit of the points plotted in the three-dimensional coordinate space.

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