Method for acquiring material property, method for manufacturing press-formed article, stretching device, and press-formed article manufacturing equipment

Abstract

Provided is technology that makes it possible to easily and accurately acquire a material property that affects the dimensional accuracy of a press, even within a press line. This method for acquiring a material property, for obtaining a material property of a metal plate for press working, involves: stretching the metal plate; measuring the forming load and die stroke during stretching, and measuring the plate thickness of the metal plate; and obtaining the yield stress of the metal plate from the measured forming load, die stroke and plate thickness.

Description

Method for acquiring material properties, method for manufacturing press-molded products, stretching processing device, and manufacturing equipment for press-molded products

[0001] The present invention relates to a technique for obtaining material properties of a metal plate for press working from measurements taken during stretching. In this invention, the material properties are material properties related to yield stress. The present invention also relates to a stretching device for measuring the material properties, and a method and equipment for manufacturing a press-formed product equipped with the device.

[0002] From the viewpoint of improving collision safety and fuel economy, performance requirements for automotive parts include light weight, high strength, and high rigidity. To improve these performance requirements, it is necessary to address the increasing complexity of part shapes and to increase the strength of the materials used in the parts. As part shapes become more complex and the materials used become stronger, problems arise. One of these problems is the dimensional accuracy of the part shape. For example, as part shapes become more complex, more precise combinations with other parts are required. For this reason, high dimensional accuracy is required for press-formed products. Press-formed products are, for example, products or parts that are processed into products.

[0003] Furthermore, with steel materials, the range of variation in material properties increases as the strength of the material increases. This variation in material properties can occur even within the same coil. Therefore, even when press forming is performed on the same lot under the same press conditions, the amount of springback may vary. This is one of the causes of the variation in material properties. As a result, the dimensions of the press-formed product may also vary, resulting in a deterioration in accuracy. Furthermore, in some cases, the variation in material properties may cause forming defects such as cracks in the press-formed product.

[0004] To address these issues, methods for measuring material properties within a press line have been proposed. For example, Patent Document 1 describes measuring material properties within a press line. Patent Document 1 also describes feeding back the measured material properties to the press conditions. Patent Document 2 discloses a metal processing method in which a predetermined process is performed on a portion of the material that will not be used in the final product, followed by a material property test. In Patent Document 2, the portion that will not be used in the final product refers to a portion of the material that will not be used in the final product in the press line. Patent Document 3 also discloses a method for measuring the weight of scrap after forming and comparing the weight with a threshold value. Patent Document 3 also discloses a technique for evaluating the quality of a press-formed product based on this comparison.

[0005] International Publication No. 2022 / 186337 Japanese Patent Application Laid-Open No. 2024-14479 Japanese Patent Application Laid-Open No. 2009-202178

[0006] As mentioned above, the range of variation in material properties of steel materials increases as the material strength increases. Furthermore, the greater the range of variation in material properties, the greater the impact on press forming results. The methods described in Patent Documents 1 and 2 provide feedback of material properties to press conditions. This is believed to enable these methods to avoid the problems. Here, feedback of material properties corresponds to adjustment of press conditions. However, this feedback requires correlation between material properties and press conditions and press forming results. To achieve this, a method can be considered: press forming using a material with known material properties and then determining the correlation from the results. Alternatively, a method can be considered: linking various data acquired on the press line with the forming results in a big data manner. However, when considering acquiring various numerical values ​​on the press line and providing feedback to press conditions based on them, the following issues arise. Specifically, the former requires precise prediction of material properties from numerical values ​​obtained on the press line. Meanwhile, the latter requires control of the press line until mass production has progressed to a certain extent and sufficient data has been accumulated to enable predictions using big data.

[0007] The inventors discovered that one of the challenges in accurately predicting material properties is thickness variation. Similar to the aforementioned variation in material strength, thickness also varies within and between coils. To normalize this thickness variation, typical tensile tests convert the load into stress by dividing it by the cross-sectional area, and output this converted value as the material property. The cross-sectional area is the product of the test piece's width and thickness. Even for materials with the same yield stress, a thicker plate will require a higher load to break. Similarly, a thinner plate will require a lower load to break. Similarly, even if the load at break is the same, different plate thicknesses will result in different yield stresses.

[0008] In Patent Documents 1 and 2, the values ​​obtained by measurement are the molding reaction force and deformation amount during a specified molding process. In other words, Patent Documents 1 and 2 do not directly obtain material properties. Therefore, to obtain material property values ​​as numerical values, some kind of conversion is required. However, Patent Documents 1 and 2 do not describe such a method. Furthermore, according to the inventor's study, it is preferable to use plate thickness as a parameter. However, Patent Documents 1 and 2 do not calculate plate thickness. Therefore, Patent Documents 1 and 2 may not be able to accurately predict material properties. Furthermore, the technology described in Patent Document 3 allows for evaluation of molded products within the production line. However, when a defect is detected, it is not possible to identify the cause of the defect.

[0009] The present invention has been made in light of the above points, and one of its objects is to provide a technology that makes it possible to easily and accurately obtain material properties that affect the dimensional accuracy of a press even within a press line.

[0010] In order to solve the problem, one aspect of the present invention is a method for obtaining material properties of a metal plate for press processing, which method comprises performing a stretching process on the metal plate, measuring the forming load and the stroke of a die during the stretching process, and measuring the thickness of the metal plate, and obtaining the yield stress of the metal plate from the measured forming load, stroke of the die, and thickness.

[0011] According to this aspect of the present invention, by correcting for plate thickness, it is possible to easily and accurately determine the yield stress YS even within a press line. That is, according to this aspect of the present invention, it is possible to easily and accurately obtain the yield stress, which is a material property that affects the dimensional accuracy of press forming. Furthermore, in this aspect of the present invention, measurements for determining the material properties are obtained from a metal plate (blank) for pressing. Therefore, the accuracy of the obtained material properties is also high. As a result, it is possible to adjust the press conditions for forming a press-formed product using the measured material properties. Furthermore, by using this aspect of the present invention, it is possible to suppress deterioration of dimensional accuracy and the occurrence of forming defects.

[0012] FIG. 1 is a diagram illustrating a processing process according to an embodiment of the present invention. FIG. 2 is a schematic diagram illustrating the configuration of a bulging processing device according to an embodiment of the present invention. FIG. 3 is a schematic diagram illustrating an example in which a bulging processing device according to an embodiment of the present invention is integrally provided in a blanking die of a blanking device. FIG. 4 is a diagram illustrating differences in punching load history due to differences in metal plate thickness. FIG. 5 is a diagram illustrating the results of a bulging test in an example. FIG. 6 is a diagram illustrating a model of forming analysis in an example. FIG. 7 is a diagram illustrating a material model used in forming analysis. FIG. 8 is a diagram illustrating a load-time curve. FIG. 9 is a diagram illustrating extraction of the timing at which a load difference with respect to a perfect elastic body occurs. FIG. 10 is a diagram illustrating the relationship between yield stress YS and the timing at which a load difference occurs. FIG. 11 is a diagram illustrating the accuracy of yield stress prediction using a calibration curve not corrected by plate thickness. FIG. 12 is a diagram illustrating the accuracy of yield stress prediction using a calibration curve corrected by plate thickness. FIG. 13 is a diagram illustrating test results of a tensile test. FIG. 14 is an enlarged view of the initial stage of test results of a tensile test. FIG. 15 is a diagram illustrating the loading rate K (slope K). FIG. 16 is a diagram illustrating the relationship between the loading rate K and yield stress. FIG. 17 is a diagram illustrating the relationship between the loading rate corrected by plate thickness and yield stress.

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. In this embodiment, the present invention is applied to a press line for press manufacturing. Therefore, in this embodiment, the metal plate portion near the area that will become the press-formed part (product) is the target of the stretching process for measurement. In this embodiment, this improves press accuracy. However, the present invention can also be used simply to measure the yield stress of a material. As will be described later, one of the features of the present invention is that the plate thickness is also taken into account as a parameter when calculating the yield stress.

[0014] (Inventor's Observations) The inventors have obtained the following findings. In order to establish a simple method for measuring yield stress in a press line, forming tests and FEM analyses were conducted using various materials. First, a truncated cone bulging test was conducted as a forming test. During the test, the punch stroke during forming and the forming load applied to the punch during bulging were measured using time as a variable. For example, historical information on the forming load versus displacement was obtained. The punch stroke is the amount of displacement of the punch (upper die). FEM analysis was also conducted under the same conditions as the forming test. Then, the influence of material properties on the forming load during the bulging test was investigated. Here, in FEM analysis, material properties can be set arbitrarily. FEM analysis is a forming analysis using a computer. For this reason, multiple forming analyses were conducted using material properties that differed only in yield stress, with all other material properties remaining the same. Based on the analysis, the influence of yield stress on the forming load was investigated.

[0015] Typically, when steel materials deform, they first undergo elastic deformation, then reach a yield stress before undergoing plastic deformation. While only elastic deformation is occurring, the relationship between elongation and stress in steel materials is independent of various material properties. In other words, the relationship between elongation and stress is nearly constant. In a tensile test, the phenomenon from the start of load application until the yield stress is reached is as follows: The region where stress steadily increases with the applied strain is the elastic deformation region. The slope of stress versus strain in this elastic deformation region is called Young's modulus. For steel materials, Young's modulus is generally approximately 210 GPa. Then, the load required for deformation decreases for materials that undergo plastic deformation. Therefore, if the load that causes this plastic deformation can be observed, it is believed possible to determine the yield stress for each material. Therefore, when tensile tests are performed on various materials, the stress-strain curves for all materials are nearly linear until the yield stress is reached. Once the material reaches its yield stress, it gradually moves away from this straight line.

[0016] From the above, it is believed that the same tendency as in tensile tests is observed in stretching tests. That is, the inventors assumed that materials with different strengths exhibit the same load-displacement relationship in the elastic deformation region in stretching tests. Furthermore, they assumed that materials gradually deviate from this "same load-displacement relationship" once they reach their yield point, i.e., once plastic deformation begins. The "same load-displacement relationship" described above applies to a perfectly elastic body. In this specification, this relationship is also referred to as a "constant relationship." The load-displacement curve is also referred to as a "reference curve." Based on the above assumption, FEM analysis of stretching tests was performed on materials with various yield stresses, and the results were investigated. The following findings were obtained. Specifically, the load-displacement curves clearly show that the initial load-displacement relationship is the same for all materials, even when the material properties differ. Furthermore, materials with lower yield stresses gradually deviate from this constant relationship (see Figure 8). Furthermore, when the relationship between load and yield stress deviates from this constant relationship (reference curve), it was found to be linear.

[0017] Next, we investigated whether the relationship obtained in the FEM analysis also applied to the experimental results. The experimental results were the results of actual forming tests. The results were as follows: The initial load-displacement relationship in the experimental results was nearly identical. However, it was found that the relationship did not match as in the analysis, and variations occurred. Furthermore, the relationship between load and yield stress when deviating from a constant relationship is not linear as in the FEM analysis results. Note that a constant relationship is expressed by a reference curve. One possible reason for this is noise or measurement error. Another possible reason is variations in plate thickness. In the analysis, the plate thickness of the material is constant and given as a numerical value. However, in actual materials, the plate thickness may not be constant within the coil. In other words, thick and thin areas may occur within the coil.

[0018] If the plate thickness is thin, the load required for deformation is relatively low. Furthermore, if the plate thickness is thick, the load required for deformation is high. However, after various studies, the inventors discovered that a high correlation with the yield stress can be obtained by correcting the experimental load for the plate thickness. Specifically, a simple truncated cone bulging test was conducted in a press line. The inventors then discovered that the yield stress of a material can be accurately predicted from the load-displacement relationship and plate thickness measured during the test. Although the plate thickness is not constant within a coil, it is not usually abrupt. In other words, the plate thickness can be considered to be approximately constant within a single metal plate for pressing cut from a coil.

[0019] Here, we consider the noise and measurement errors mentioned above. In an actual press line, there are vibrations from the press and measuring equipment. There are also minute fluctuations in the current and voltage supplied to the measuring equipment. Furthermore, there are measurement errors inherent in the measuring equipment itself. In other words, there are unavoidable measurement errors and noise. To determine the yield stress using the above approach, precise measurement of the forming load-displacement relationship is required. However, this is difficult in reality.

[0020] Therefore, the inventors devised a method for predicting yield stress using a measurable method. As mentioned above, although noise is present, the relationship between the load-displacement curve in the elastic region is roughly the same for all materials. Materials that yield gradually deviate from this relationship (see Figure 14). The slope of the forming load-displacement curve after this deviation is nearly constant for each material. Furthermore, as mentioned above, it was found that the slope is smaller for materials with lower yield stress. In other words, it is believed that yield stress can be predicted by determining the slope of the load-displacement curve at the early stage of forming. Furthermore, the inventors discovered that by determining the average slope using multiple measurement points, it is possible to predict yield stress while reducing the effects of noise and measurement error.

[0021] From the above, we considered finding a feature quantity that defines the load-displacement curve found from the measured values, and then finding the yield stress from that feature quantity. The feature quantity is a quantity that characterizes the load-displacement curve in relation to the yield stress. Then, for example, we considered finding the correlation between the feature quantity and the yield stress in advance. Then, we considered finding the yield stress by referring to that correlation. Furthermore, we considered correcting the load-displacement curve or the found feature quantity with the measured plate thickness. In this way, we considered improving the accuracy of the found yield stress.

[0022] The following first embodiment is an example in which the degree of deviation of the load when it deviates from a certain relationship (reference curve) in a load-displacement curve is used as a feature quantity. The load for specifying the degree of deviation is, for example, the load when the amount of deviation from the reference curve reaches a predetermined amount. Furthermore, the second embodiment is an example in which the initial slope K of the load-displacement curve is used as a feature quantity of the load-displacement curve. The initial slope K is the slope of a line passing through the load value taken in the elastic region and the load value taken in the plastic region. This slope K is also one of the indicators (feature quantities) of the degree of deviation of the load when it deviates from a certain relationship (reference curve) in a load-displacement curve.

[0023] First Embodiment First, the first embodiment will be described. (Configuration) In a manufacturing facility (manufacturing method) for press-formed products according to this embodiment, a press line includes a material property acquisition process 30 and a main forming process 32, as shown in FIG. 1 . The main forming process 32 is a process in which a metal plate to be pressed is press-formed into a target part shape (product shape) using a press device (press die). Furthermore, a press condition adjustment process 31 in FIG. 1 is a process in which the material properties acquired in the material property acquisition process 30 are fed back to the press conditions in the main forming process 32. The press condition adjustment process 31 is a process in which the press conditions in the main forming process 32 are adjusted using a known method. Note that the processing of each process, such as the press condition adjustment process 31, is controlled by a control unit (not shown).

[0024] <Material property acquisition process 30> As shown in Fig. 1, the material property acquisition process 30 includes a stretching process 30A and a material property calculation process 30B. The processing of the material property acquisition process 30 is controlled by a control unit (not shown). That is, the device is configured so that the processing of each process is controlled by a control unit (not shown).

[0025] [Expansion Process 30A] In the expansion process 30A, expansion processing is performed on the metal plate using an expansion processing device. In expansion processing, as described below, an expansion punch is stroked (displaced) to perform expansion forming on the metal plate. In this embodiment, the forming load and punch stroke for the expansion processing are measured during the expansion processing. In addition, a process for measuring the plate thickness of the metal plate is executed. The forming load is the load applied to the punch. Information on the expansion processing forming load, punch stroke (displacement), and plate thickness is measured, for example, by a punching processing device described below.

[0026] In this embodiment, the bulging process is preferably performed on the metal plate portion in the non-pressing region. The non-pressing region is a region of the metal plate to be pressed other than the region that will become the press-formed part (product). However, the non-pressing region is a region close to the pressing region that will become the press-formed part (product). In this embodiment, the region to be bulged is selected from the metal plate (blank) to be used in this press, where the punching process will be performed. For example, it is desirable to perform the bulging process for measurement in the blanking process. The blanking process is a process in which the metal plate is blanked (trimmed) and shaped before the main forming process 32.

[0027] For measurement purposes, bulging processing is performed on the material of the portion discarded by blanking. This makes it possible to measure the bulging processing load without deteriorating the yield. The portion discarded by blanking is a non-press area. Furthermore, the blanking process usually occurs before the press forming process. Therefore, the measured load can also be used to adjust the press conditions of subsequent processes. However, the bulging processing process 30A performed for measurement is not limited to the blanking process. If there is a process that includes a portion that will not remain in the final product, it is possible to perform the measurement of this embodiment using that portion that will not remain in the final product.

[0028] <Bulging Device 1> The bulging device 1 will be described with reference to Figure 2. The bulging device 1 is a device for punching a metal plate used in the bulging process 30A. As shown in Figure 2, the bulging device 1 of this embodiment includes a lower mold 2 and an upper mold 3 arranged opposite each other in the bulging direction, a load measuring unit 5, a plate thickness measuring device 7, and a punch displacement measuring unit. The lower mold 2 has an installation surface 2a on its upper surface. The installation surface 2a is a flat surface on which the metal plate is placed. The lower mold 2 has an opening 2A for bulging, which is a through hole. The opening 2A opens to the installation surface 2a. The opening 2A may be a blind hole instead of a through hole.

[0029] The metal plate 20 placed on the installation surface 2a is restrained by a plate holder (not shown). For ease of understanding, the plate holder is omitted from Figure 2 and other figures. The plate holder is intended to hold the material so that it does not move during bulging. Therefore, any means of plate holding is acceptable as long as the purpose can be achieved. For example, the bulging processing device 1 of this embodiment may be incorporated into the blanking die 42 in the blanking process. In this case (see Figure 3), the plate holder of the bulging processing device 1 and the plate holder of the blanking die may be configured as an integrated unit.

[0030] The upper die 3 is disposed opposite the lower die 2 in the bulging processing direction. The upper die 3 is equipped with a punch 3A for bulging processing. The punch 3A is disposed coaxially with the opening 2A and is capable of advancing and retreating toward the opening 2A. The upper end of the punch 3A is connected to an upper die body 3B. A load measuring unit 5 is interposed between the upper end of the punch 3A and the upper die body 3B. Reference numeral 4 denotes a retainer. The retainer 4 adjusts the gap between the upper end of the punch 3A and the upper die body 3B. In this example, the retainer 4 is disposed between the load measuring unit 5 and the upper die body 3B. Reference numeral 3C denotes a pilot pin.

[0031] The load measuring unit 5 is a load meter that measures the load applied to the punch 3A during bulging. In this embodiment, the load measuring unit 5 is configured with a load cell. The load measuring unit 5 is capable of supplying measurement information to the calculation unit 10. The installation position, device configuration, and measurement means of the load measuring unit 5 are not important as long as the purpose can be achieved. The upper mold 3 has an extension portion 6 located to the side of the punch 3A. A thickness measuring device 7 is provided at the lower end of the extension portion 6. The thickness measuring device 7 is attached to the upper mold 3 so that the lower part of the thickness measuring device 7 is located above the insertion portion on the tip side of the punch 3A when the punch 3A is inserted into the opening 2A.

[0032] The plate thickness measuring device 7 is a device that measures the plate thickness of the metal plate 20 placed on the installation surface 2a of the lower mold 2. The plate thickness measuring device 7 of this embodiment consists of two displacement meters, a first displacement meter 7A and a second displacement meter 7B. The displacement meters are distance meters. The two displacement meters 7A and 7B are synchronized to measure a first distance L1 and a second distance L2, respectively. The first distance L1 is the distance to the metal plate 20. The second distance L2 is the distance to the installation surface 2a of the lower mold 2. The measurement direction is a direction perpendicular to the installation surface 2a. It is preferable that the two displacement meters 7A and 7B are set so that their distances to the installation surface 2a are equal to each other.

[0033] The first displacement meter 7A is positioned to face the area of ​​the installation surface 2a where the metal plate 20 is placed. The second displacement meter 7B is positioned to face the area of ​​the installation surface 2a other than the area where the metal plate 20 is placed. A plurality of first displacement meter 7A and a plurality of second displacement meter 7B may be provided. In this case, the average value of the plurality of measurements may be used for each of the first displacement meter 7A and the second displacement meter 7B. The distance between the upper mold 3 and the lower mold 2 and the distance between the upper mold 3 and the metal plate 20 are measured synchronously. The difference between these distances can then be calculated as the plate thickness. Each of the displacement meter 7A and 7B supplies the measured measurement information to the calculation unit 10. Here, the displacement meter 7A and 7B may be of either a contact type or a non-contact type. Note that if a single displacement meter can measure the first distance L1 and the second distance L2, a single displacement meter may be sufficient. For example, the distance L1 before the installation of the metal plate 20 and the distance L2 after the installation are measured, and the plate thickness can be calculated from the difference between the two distances L1 and L2. In addition, the plate thickness measuring device 7 may be separate from the upper mold 3.

[0034] In this embodiment, the second displacement meter 7B also serves as a punch displacement amount measuring unit. That is, the measurement information of the second displacement meter 7B is used as punch stroke information (displacement information from the initial value). The punch stroke information is, for example, displacement information from the initial value (standby position). A sensor for the punch displacement amount measuring unit may be provided separately from the second displacement meter 7B. Since a typical bulging processing device has a punch stroke amount measuring unit, information from this known measuring unit may be used. Here, the bulging processing device 1 of this embodiment is configured so that the upper mold body 3B moves toward the lower mold 2 by a known load application device (not shown). The bulging processing is performed by this movement.

[0035] The bulging shape is preferably a truncated cone shape. This is because a truncated cone shape is axially symmetrical and is thought to result in less load imbalance. Furthermore, with spherical head bulging, yielding occurs sequentially over a wide area, making it difficult to identify the yield point. To suppress load imbalance, the tip shape of the punch 3A is preferably a cylindrical shape with a rounded punch shoulder. The punch diameter and punch shoulder R are not particularly specified. However, if the punch shoulder R is too small, cracks may occur during bulging. Furthermore, if the punch shoulder R reaches more than half the punch diameter, bulging will result in spherical head bulging rather than truncated cone bulging. Therefore, the relationship between the metal plate thickness t, punch shoulder R, and punch diameter must satisfy the following formula: t < punch shoulder R < 1 / 2 × punch diameter. Preferably, the following formula is satisfied: 2t < punch shoulder R < 1 / 4 × punch diameter.

[0036] If the overhang amount is too low, forming will end before sufficient plastic deformation occurs. As a result, it is not possible to observe the yield stress. On the other hand, if the overhang amount is too high, there is no problem in terms of observing the yield stress. However, if the overhang amount is too high, cracks may occur. In this case, unnecessary scrap may be generated, and the forming load may become unstable due to cracks. This may lead to problems such as the need for a mechanism to repair the scrap. From the above perspectives, it is preferable that the overhang amount h, where t is the plate thickness, satisfy the following formula: 20t > h > 2t

[0037] Furthermore, the diameter of the opening 2A in the lower die 2 must be at least larger than the sum of the punch diameter and the plate thickness t. The diameter of the opening 2A is also called the die diameter. Furthermore, for the same reason as the punch shoulder R, if the opening shoulder R is too small, forming defects will occur. Therefore, the die diameter and the opening shoulder R in the lower die must satisfy the following formula: Die diameter > Punch diameter + Plate thickness t x 2 Opening shoulder R > t It is more preferable to satisfy the following formula: Die diameter > Punch diameter + t x 4 Opening shoulder R > 2t

[0038] FIG. 3 shows an example in which the bulging processing device 1 of this embodiment is integrated with a blanking die 42. In FIG. 3, reference numeral 40 denotes the lower die of the blanking die 42. Reference numeral 41 denotes the upper die of the blanking die 42. Reference numeral 41A denotes the upper blade of the blanking die 42. Reference numeral 43 denotes a common plate connecting the upper die 41 of the blanking die 42 and the upper die 3 of the bulging processing device 1 of this embodiment. Reference numeral 20A denotes a pressing region, and reference numeral 20B denotes a non-pressing region. The load measurement by the load measuring unit 5 is triggered to start when the distance between the upper die 3 and the lower die 2 falls below a first threshold value, for example. The measurement is terminated when the distance falls below a second threshold value that is smaller than the first threshold value. This enables measurement of the load during bulging processing for each shot.

[0039] Assuming that the initial standby position of punch 3A is constant, the distance from punch 3A to the material surface is longer for thinner materials. Furthermore, the distance from punch 3A to the material surface is shorter for thicker materials. Therefore, for thinner materials, the time from when the measurement start trigger is triggered until punch 3A contacts the material is longer. In other words, the thinner the material, the longer it takes for a load to be generated on punch 3A. On the other hand, for thicker materials, this time is shorter. This time difference is a value correlated with the thickness. Therefore, it is possible to use this to correct for the thickness. The relationship between time and punch load is shown schematically in Figure 4. In other words, the time it takes for the load to build up varies depending on the thickness. This time difference is a parameter that represents the variation in thickness. Therefore, it is possible to use this to correct for the forming load. However, to ensure accurate prediction, it is desirable to directly measure the distances L1 and L2.

[0040] [Material property calculation step 30B] The material property calculation step 30B executes a process of calculating the yield stress of the metal sheet 20 in the vicinity of the press from information on the forming load measured in the stretching step 30A, information on the punch stroke, and information on the sheet thickness. The material property calculation step 30B is executed by the calculation unit 10. As shown in FIG. 2 , the calculation unit 10 includes a sheet thickness calculation unit 10A, a load curve calculation unit 10B, and a yield stress calculation unit 10C.

[0041] The plate thickness calculation unit 10A calculates the plate thickness from the difference in distance information measured synchronously by the first displacement meter 7A and the second displacement meter 7B. The load curve calculation unit 10B calculates load-displacement curve information from the forming load and the stroke amount of the punch 3A, which are acquired synchronously during stretch forming. The load-displacement curve information is a load history. The stroke amount of the punch 3A is represented by the displacement amount from the initial position of the punch 3A. The initial position of the punch 3A is, for example, a standby position. The yield stress calculation unit 10C performs processing to calculate the yield stress of the metal plate 20 to be evaluated from the plate thickness calculated by the plate thickness calculation unit 10A and the load-displacement curve information calculated by the load curve calculation unit 10B.

[0042] In this embodiment, a stretching test is performed on a metal plate 20 made of a material whose plate thickness and yield stress are known in advance. From the test, the load P below relative to the yield stress is obtained as a feature value. Statistical processing is then performed to obtain the correlation between the load P, plate thickness t, and the yield stress YS. The load P and plate thickness t are feature values ​​that define the load-displacement curve. In this example, the correlation is obtained by using equation (1) of the calibration curve described below. The yield stress calculation unit 10C then references the correlation to obtain the yield stress corresponding to the obtained forming load and plate thickness.

[0043] Here, measurements using the above-mentioned device are performed on materials whose yield stress is known through prior material testing. The relationship between the yield stress and the characteristic quantities of the forming load-displacement curve is then determined. The bulging test does not necessarily have to be performed within the press line. It is possible to perform the bulging test independently. A method for performing bulging on materials whose yield stress is known is, for example, as follows: Rectangular samples of approximately 200 mm x 200 mm are taken from multiple locations on multiple coils. Tensile test pieces and bulging test pieces are taken from the rectangular samples. The tensile test pieces and bulging test pieces taken from the same rectangular sample are considered to have the same material properties because they are taken from nearby regions within the coil. A possible method is to perform separate tests and compare the results. However, this method is not limited to this.

[0044] All stretch tests to determine the above relationship are conducted after measuring the plate thickness. Any method for measuring the plate thickness is acceptable. For example, direct measurement with a micrometer or measurement with an ultrasonic plate thickness gauge are possible. Alternatively, the same method as the measurement method used in the press line described above may be used. As mentioned above, there is a correlation between the load and yield stress when the forming load-displacement curve deviates from a constant relationship. The load at which this constant relationship deviates can be determined, for example, by the following method. As yielding occurs, the forming load-displacement curve gradually deviates from the constant relationship. This means that a perfectly elastic material that does not yield will continue to maintain a constant relationship. The constant relationship can be expressed, for example, as a reference curve.

[0045] Therefore, material conditions that do not cause yielding are applied to the FEM analysis, and a load-displacement curve (reference curve) for the non-yielding material is obtained. The difference between the load on the load-displacement curve obtained in an actual test and that straight line (reference curve) is calculated. When this difference reaches a predetermined value, it is defined that the forming load-displacement curve has deviated from the fixed relationship (reference curve). The load P at that time is then used as a feature quantity to estimate the yield stress. Note that the method for calculating the load P is not limited to this, and any method may be used. For example, the load P may be the load or load difference at a preset reference displacement in a region larger than the displacement at the yield point. In other words, the load P may be calculated as the degree of deviation (feature quantity) of the "load-displacement curve" from the fixed relationship (reference curve).

[0046] The method of calculating the yield stress using the load P can be carried out by formulating the following equation (1), for example. For example, the yield stress YS obtained in the tensile test is set as the objective variable. Furthermore, the load P and plate thickness t obtained in the stretching test are set as explanatory variables. Then, a calibration curve constituting the correlation is created. For example, the equation for the calibration curve is as shown below: YS = αP / t + β (1) Here, α and β are numerical values ​​(coefficients) specific to each material. The coefficients α and β can be calculated from the results of a tensile test and a stretching test, where the material properties are known.

[0047] By using the above calibration curve, it is possible to determine the yield stress of the material from the plate thickness t and load P measured using a mechanism installed in the press line. By using the above method, it is possible to determine the yield stress YS of the material from the plate thickness t measured using a mechanism installed in the press line and the forming load-displacement curve during stretch forming. Note that the correlation is not limited to the above calibration curve. By machine learning, a learning model is created in which the load P and plate thickness t are used as input data and information on the yield stress is used as output data. Then, the learning model may be used as the correlation. A known learning method may be applied to the machine learning.

[0048] <Pressing Condition Adjustment Step 31> The relationship between the yield stress, the pressing conditions, and the dimensional accuracy of the pressed product is determined separately. Then, in the pressing condition adjustment step 31, the relationship is referenced and the determined yield stress is fed back to the pressing conditions. Then, the pressing conditions used in the main forming step 32 are adjusted. This makes it possible to improve the dimensional accuracy of the press-formed product. Possible pressing conditions to be adjusted include, for example, cushion pressure, pad pressure, and die standby position. However, the pressing conditions to be adjusted are not limited to these.

[0049] <Main Forming Step 32 > In the main forming step 32, the metal plate 20 is press-formed into a target part shape using a press die under the press conditions adjusted in the press condition adjustment step 31.

[0050] (Operation and Others) Here, when a coil of metal sheet is shipped, a material property test such as a tensile test is typically performed, and the material properties of the coil are obtained from the test. However, material property tests are not performed over the entire length of the coil, but rather samples are taken from representative locations. For this reason, conventionally, only representative material properties of the coil are known. However, in reality, the material properties within the coil vary to a certain extent compared to the representative material properties. Furthermore, there is a range of variation in the sheet thickness within the coil, and the sheet thickness is not uniform throughout the entire length of the coil. Here, in order to measure the precise material properties of the material over the entire length of the coil, it is necessary to perform a material property test such as a tensile test over the entire length of the coil. However, taking samples and conducting tests over the entire length of the coil is not practical in terms of yield and cost.

[0051] Furthermore, using a representative value within the coil as the material properties of the metal sheet 20 (blank) to be pressed poses the following problem. That is, because the precise material properties are unknown, there is a risk of variations in the quality of the press-formed product. In particular, the higher the material strength, the greater the fluctuation range of the material properties, and the greater the impact on the press-formed results. In contrast, in this embodiment, the yield stress as a material property is obtained with high accuracy from material near the product (press-formed product) within the press line. This makes it possible to measure the yield stress YS of the metal sheet 20 that will become the target press-formed product without performing special samples or material tests. Furthermore, if the precise material properties are known, it is believed that consistently high dimensional accuracy can be achieved by determining the relationship between the material properties, press conditions, and dimensional accuracy in advance and adjusting the press conditions according to the material properties.

[0052] From the above, it is believed that the variable not reflected in the current measurement method using stretching is the plate thickness. Therefore, it is believed that taking the plate thickness into account will enable more precise prediction of yield stress. In other words, it is believed that correcting the load using the plate thickness will enable more precise prediction of yield stress. That is, in this embodiment, the load-related variable is corrected using the plate thickness to determine the correlation between the feature quantity defining the load-displacement curve and the material's yield stress. This makes it possible to determine the material's yield stress with higher accuracy. Furthermore, the plate thickness is determined from the material near the metal plate to be pressed, and the yield stress is obtained. This makes it possible to obtain the material properties (yield stress) of the metal plate that will become the press-formed product with high accuracy. Furthermore, by adjusting the press conditions for forming the press-formed product using the measured material properties, it is possible to suppress deterioration of dimensional accuracy and the occurrence of forming defects.

[0053] Second Embodiment Next, a second embodiment will be described. The configuration and effects of this embodiment are basically the same as those of the first embodiment. However, the second embodiment differs in the processing of the load curve calculation unit 10B of the calculation unit 10. In other words, the feature quantities that define the load-displacement curve are different. The rest is the same as the first embodiment.

[0054] Next, the processing of the load curve calculation unit 10B of this embodiment will be described. Similar to the first embodiment, measurements using the above-described device are performed on materials whose yield stress is known through prior material testing. The relationship between the yield stress and the characteristic quantities defining the load-displacement curve is then determined in advance. In this example, the initial slope K of the load-displacement curve is used as the characteristic quantity. The initial slope K is the slope of the load in the plastic region relative to the load in the elastic region (see FIG. 15). The load at a displacement set within the elastic range is taken as the load in the elastic region. The above elastic range is a range that can be determined as the elastic region for all materials to be press-formed. The load at a displacement set within the plastic range is taken as the load in the plastic region. The above plastic range is a range that can be determined as the plastic region for all materials to be press-formed.

[0055] Then, the yield stress measured by the tensile test is matched with the slope K of the forming load-displacement curve measured by the extension test to determine the correlation between the yield stress and the slope K. For example, the equation of the calibration curve is determined. The range for determining the slope K of the forming load-displacement curve is preferably the following range: 00.5t. Here, the extension amount at the start point for calculating the slope is defined as h1. The extension amount at the end point for calculating the slope is defined as h2. The plate thickness is defined as t. Preferably, 0.5t<h11t.<h1>

[0056] Beyond this range, the correlation between yield stress and the slope tends to decrease. If the load slope found within the above range is K, then the equation (prediction equation) for the calibration curve of yield stress can be expressed, for example, as in equation (2) below. Since the load slope K is the load difference, it can also be expressed as the loading rate K. YS = α × K / t + β (2) where t is the plate thickness. α and β are numerical values ​​(coefficients) specific to each material. The coefficients α and β in equation (2) can be found from the results of tensile tests and stretch tests where the material properties are known. The correlation to be found is not limited to the above calibration curve equation.

[0057] By using this method, it is possible to determine the yield stress of the material from the plate thickness measured using a mechanism installed in the press line and the forming load-displacement curve during stretch forming. It is also advisable to separately determine the relationship between the yield stress, press conditions, and the dimensional accuracy of the pressed product. In this case, the determined yield stress is fed back to the press conditions to adjust them. This makes it possible to improve dimensional accuracy. Possible press conditions to be adjusted include, but are not limited to, cushion pressure, pad pressure, and die standby position. Other effects are the same as those of the first embodiment.

[0058] (Other) The present disclosure may also have the following configurations. (1) Disclosure 1 is a method for acquiring material properties of a metal plate for press working, comprising: performing bulging on the metal plate, measuring the forming load and die stroke during the bulging, and measuring the thickness of the metal plate; and determining the yield stress of the metal plate from the measured forming load, die stroke, and plate thickness. (2) Disclosure 2 is a method for acquiring material properties of a metal plate when press-forming the metal plate into a press-formed product, comprising: performing bulging on a region of the metal plate other than the region that will become the press-formed product, measuring the forming load and die stroke during the bulging, and measuring the thickness of the metal plate; and determining the yield stress of the metal plate from the measured forming load, die stroke, and plate thickness. (3) Disclosure 3 determines a load-displacement curve of a metal plate from the measured forming load and die stroke, and further determines a feature quantity defining the load-displacement curve. The yield stress is determined from the determined feature quantity and the measured plate thickness. (4) Disclosure 4 determines a correlation between the feature quantity defining the load-displacement curve, the plate thickness, and the yield stress in advance, and determines the yield stress of the metal plate by referring to the correlation. (5) Disclosure 5 determines a load-displacement curve during stretch forming of a material that does not undergo plastic deformation as a reference curve in advance through forming analysis using a computer, and defines the degree of deviation of the load from the reference curve in the plastic region of the determined load-displacement curve as the feature quantity. (6) Disclosure 6 sets the load on the determined load-displacement curve when the load difference between the reference curve and the determined load-displacement curve reaches a predetermined threshold as the degree of deviation. (7) Disclosure 7 defines, as the feature quantity, an initial slope of the determined load-displacement curve, which is defined by a load in an elastic region and a load in a plastic region. (8) Disclosure 8 defines, as a method for manufacturing a press-formed product, a press-formed product by press-forming a metal plate, wherein, before the press forming, a metal plate portion in a region other than a region to become the press-formed product is used to determine the yield stress of the metal plate by the method for obtaining material properties of the present disclosure.(9) Disclosure 9 provides a blanking process before press forming, and performs the bulging process in the blanking process. (10) Disclosure 10 provides a method for manufacturing a press-formed product, adjusting press conditions for the press forming based on the calculated yield stress. (11) Disclosure 11 provides a bulging processing apparatus for performing bulging processing on a metal plate for press forming, comprising: a lower mold having an opening for bulging processing on an installation surface 2a on which the metal plate is placed; an upper mold having a punch for bulging processing that can advance and retreat toward the opening; a load measuring unit that measures the load applied to the punch; a punch displacement measuring unit that measures the stroke of the punch; and a thickness measuring device that measures the thickness of the metal plate placed on the lower mold. (12) Disclosure 12 provides a thickness measuring device that is provided on the upper mold, measures a first distance to a surface of a metal plate placed on the installation surface, and measures a second distance to the installation surface, and calculates the thickness of the metal plate from the first and second distances. (13) Disclosure 13 provides a manufacturing facility for press-formed products, the manufacturing facility for press-formed products including a press device that press-forms a metal plate, and the manufacturing facility for press-formed products including a bulging processing device of the present disclosure that performs bulging processing on the metal plate before press-forming. (14) Disclosure 14 provides a manufacturing facility for press-formed products, the manufacturing facility for press-formed products including a non-pressed region of the metal plate other than the region that will become the press-formed product, which is set as the target for bulging. (15) Disclosure 15 provides a blanking device that blanks the metal plate before press-forming, and the bulging processing device is provided in a mold of the blanking device.

[0059] An example according to this embodiment will be described. In this example, a stretching test was performed using the stretching device 1 of this embodiment. The test materials were materials with the strength levels and surface treatments shown in Table 1. Eight samples for each material were cut out from various locations within the coil for tensile tests and stretching test samples. This represented the material variations within the coil. Samples cut out from the same location were assumed to have the same material properties. The tensile test results were then matched to the stretching test results. This allowed for tensile property predictions based on the stretching test. The above process is common to the first and second examples.

[0060]

[0061] (First Example) The first example is an example based on the first embodiment. FIG. 5 shows load data obtained by a stretching test. Then, in order to predict the yield stress from the results of measurement using the device of the embodiment, a calibration curve of equation (1) was created. The calibration curve can be created, for example, by the following method. First, an analytical model with the same dimensions as the stretching test is created as a model for FEM analysis (forming analysis), as shown in FIG. 6. Reference numeral 51 denotes an example of a punch model for stretching. Reference numeral 52 denotes an example of a lower mold model. Reference numeral 53 denotes an example of a blank model.

[0062] Next, a material model was created by creating a stress-strain curve for a material simulating the material used in the stretch test and a stress-strain curve with a hypothetical yield stress (see Figure 7). A material model simulating a perfectly elastic body was also created. Next, a forming analysis of the stretching process was performed using each of the created material models. This time, forming analyses were performed using material models with different yield stresses. Then, in the stretching test, we investigated how the load-displacement curve changed when the yield stress was changed. The punch stroke speed was a constant 10 mm / s. Figure 8 shows the relationship between the forming load (punch reaction force) and time obtained from the forming analysis. In Figure 8, the analysis data was organized by forming load and time. In this analysis, the punch was moved at a constant speed. Therefore, a similar analysis can be performed by using the punch stroke (displacement) as the horizontal axis.

[0063] As shown in Figure 8, models other than a perfect elastic body initially match the reference curve for a perfect elastic body. However, for models other than a perfect elastic body, the load gradually decreases relative to the reference curve for a perfect elastic body. In each curve, the section that matches the reference curve for a perfect elastic body is undergoing elastic deformation. Furthermore, the section that deviates from the reference curve for a perfect elastic body is undergoing plastic deformation. The point at which elastic deformation switches to plastic deformation is the yield point. The following method, for example, can be considered to determine the yield point for each material model. Taking the difference between each material model and a perfect elastic body at each time in the relationship shown in Figure 8, the result is shown in Figure 9. As can be seen from Figure 9, during elastic deformation, the load matches the perfect elastic body, so the difference is zero. After yielding, a difference in load occurs. In other words, the point at which the load difference is no longer zero is the yield point.

[0064] Figure 10 shows the results of determining the relationship between the time when a load difference occurred and the yield stress applied to each material model. As shown in Figure 10, there is an extremely high correlation between the time when a load difference occurred and the yield stress applied to each material model. In Figure 10, the horizontal axis is organized as time. However, a similar organization can be obtained even if the horizontal axis is organized as punch stroke or the load when a load difference occurs. In other words, it is possible to organize the data in a way that is easy to use in practice.

[0065] Using the above method, a calibration curve for Equation (1) was created. Then, using this calibration curve, the yield stress was predicted from the results shown in Figure 5 . The results are shown in Figure 11 . Furthermore, for the results shown in Figure 11 , the predicted yield stress was corrected by plate thickness. The results are shown in Figure 12 . Note that similar results are obtained even when the actual yield stress side is corrected by plate thickness. Here, Figure 12 shows the results when Equation (1) of the first embodiment is used as a calibration curve based on the present invention. On the other hand, Figure 11 shows the results when a calibration curve without correcting the load P by plate thickness t is used for the calibration curve for Equation (1) of the first embodiment. As shown in Figure 11 , the yield stress can be predicted with a certain degree of accuracy from the stretch test results. However, as shown in Figure 12 , it was found that correcting the load P by plate thickness enables the yield stress to be predicted with even greater accuracy.

[0066] Second Example The second example is an example based on the second embodiment. The load data obtained by the stretching test is the data shown in FIG. 5 above. Also, FIG. 13 shows an example of a load-stroke curve obtained by a tensile test. In the elastic region, all steel types have the same slope, and the point where the slope changes can be considered as the yield point.

[0067] Furthermore, Figure 14 shows an enlarged view of the load data obtained from the stretch test during the initial load rise period. As shown in Figure 14, the stretch test shows similar behavior to the tensile test. That is, while the material is initially undergoing elastic deformation, the load behaves similarly for all steel types. After that, plastic deformation occurs in order, starting with the material with the lowest yield point.

[0068] As shown in Figure 15, the starting point s1 is set within the region where all steel types are elastically deforming from the start of the load, and the end point s2 is set within the region where all steel types have undergone plastic deformation. Then, the slope K of the end point s2 relative to the starting point s1 is calculated for each material. The slope K is smaller for materials with lower yield points. This slope K is also referred to as the loading rate K. Note that if all stretch tests are performed at the same punch speed (punch stroke), the punch speed is a function of time. In this case, the loading rate K can be calculated by dividing the load by either the time or the stroke. However, if stretch tests are performed while changing the punch speed (punch stroke), the loading rate K is calculated by dividing the load by the punch stroke.

[0069] Figure 16 shows the relationship between the loading rate K measured using the above method and the yield stress obtained by a tensile test. As can be seen from Figure 16, there is a correlation between the loading rate K and the yield stress. However, for the purpose of the present invention, it is desirable to further improve the prediction accuracy. Therefore, in the present invention, the loading rate K is corrected by the plate thickness. Figure 17 shows the results of correcting the loading rate K by the plate thickness. As can be seen from Figure 17, it was found that correcting the loading rate K by the plate thickness t further improved the accuracy. Furthermore, it was found that by obtaining this relationship, the following becomes possible. In other words, even for unknown materials, it is possible to accurately predict the yield stress by performing a simple stretch test in the line.

[0070] The entire contents of Japanese Patent Application No. 2024-101955 (filed June 25, 2024), from which this application claims priority, are incorporated herein by reference. While the present invention has been described with reference to a limited number of embodiments, the scope of the invention is not limited thereto, and modifications of each embodiment based on the above disclosure will be obvious to those skilled in the art.

[0071] REFERENCE SIGNS LIST 1 Stretching device 2 Lower die 2A Opening 3 Upper die 3A Punch 3B Upper die body 5 Load measurement unit 7 Plate thickness measurement device 7A First displacement meter 10 Calculation unit 10A Plate thickness calculation unit 10B Load curve calculation unit 10C Yield stress calculation unit 20 Metal plate 30 Material property acquisition process 30A Stretching process 30B Material property calculation process 31 Press condition adjustment process 32 Main forming process 42 Blanking die K Load speed (inclination)

Claims

1. A method for obtaining material properties of a metal plate for press working, comprising the steps of: performing stretching on the metal plate; measuring the forming load and die stroke during the stretching; and measuring the thickness of the metal plate; and obtaining the yield stress of the metal plate from the measured forming load, die stroke, and thickness.

2. A method for obtaining material properties to determine the yield stress of a metal plate when the metal plate is press-formed into a press-formed product, comprising the steps of: performing bulging on an area of ​​the metal plate other than the area that will become the press-formed product; measuring the forming load and die stroke during the bulging; and measuring the thickness of the metal plate; and determining the yield stress of the metal plate from the measured forming load, die stroke, and thickness.

3. A method for obtaining material properties as described in claim 1 or claim 2, which comprises determining a load-displacement curve of a metal plate from the measured forming load and die stroke, determining a characteristic quantity that defines the load-displacement curve, and determining the yield stress from the determined characteristic quantity and the measured plate thickness.

4. A method for obtaining material properties as defined in claim 3, which involves determining the correlation between the feature quantity that defines the load-displacement curve, the plate thickness, and the yield stress, and then determining the yield stress of the metal plate by referring to that correlation.

5. A method for obtaining material properties as described in claim 4, in which a load-displacement curve during stretch forming of a material that does not undergo plastic deformation is obtained in advance as a reference curve through forming analysis using a computer, and the degree of deviation of the load from the reference curve in the plastic region of the load-displacement curve obtained in claim 3 is used as the feature quantity.

6. A method for obtaining material properties as described in claim 5, wherein the load on the load-displacement curve obtained in claim 3 when the load difference between the reference curve and the load-displacement curve obtained in claim 3 reaches a preset threshold is set as the degree of deviation.

7. A method for obtaining material properties as described in claim 4, wherein the initial slope of the load-displacement curve obtained in claim 3, which is defined by the load in the elastic region and the load in the plastic region, is obtained as the characteristic quantity.

8. A method for manufacturing a press-formed product in which a metal plate is press-formed to produce a press-formed product, wherein, prior to the press-forming, the yield stress of the metal plate is determined by the method for obtaining material properties set forth in any one of claims 1 to 7, using a metal plate portion in an area other than the area that will become the press-formed product.

9. A method for manufacturing a press-formed product according to claim 8, further comprising a blanking step prior to press forming, wherein the bulging step is carried out.

10. A method for manufacturing a press-molded product according to claim 8 or claim 9, wherein the press conditions for the press molding are adjusted according to the determined yield stress.

11. A stretching processing device for stretching a metal plate for press processing, comprising: a lower mold having an opening for stretching on a mounting surface on which the metal plate is mounted; an upper mold having a punch for stretching that can advance and retreat toward the opening; a load measuring unit that measures the load applied to the punch; a punch displacement measuring unit that measures the stroke of the punch; and a plate thickness measuring device that measures the plate thickness of the metal plate mounted on the lower mold.

12. The bulging processing device according to claim 11, wherein the plate thickness measuring device is provided on the upper die, measures a first distance to the surface of the metal plate placed on the installation surface, and measures a second distance to the installation surface, and calculates the plate thickness of the metal plate from the first distance and the second distance.

13. A manufacturing facility for press-formed products, comprising a press device for press-forming metal plates, and comprising the bulging processing device according to claim 11 or 12, which performs bulging processing on the metal plates before press-forming.

14. The press-formed product manufacturing equipment according to claim 13, wherein a non-pressed region of the metal plate other than the region that will become the press-formed product is set as the target of bulging.

15. The press-molded product manufacturing equipment according to claim 13, further comprising a blanking device for blanking the metal plate before press-molding, and the bulging device is provided in the mold of the blanking device.

Images