Method for acquiring material properties and method for manufacturing press-molded products

Abstract

The present invention provides a technology that enables easy and accurate acquisition of material properties that affect the dimensional accuracy of presses, even within a press line.The method for acquiring material properties of a metal plate for press working involves performing a stretching process on the metal plate, measuring the forming load and die stroke during the stretching process, and measuring the thickness of the metal plate, and then determining the yield stress of the metal plate from the measured forming load, die stroke, and thickness.

Description

[Technical Field]

[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. [Background technology]

[0002] From the perspective of improving collision safety and fuel economy, the performance required of automotive parts includes light weight, high strength, high rigidity, etc. To improve these performances, there is a need to accommodate the increasing complexity of part shapes and to increase the strength of the materials used in parts. As part shapes become more complex and the strength of the materials used increases, 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. This requires high dimensional accuracy in press-formed parts. Press-formed parts 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. Furthermore, Patent Document 2 discloses a metal processing method in which a predetermined processing is performed on a portion that will not be a product, and a material property test is performed. In Patent Document 2, the portion that will not be a product is a portion of the material that will ultimately become a product within the press line. Furthermore, in Patent Document 3, the weight of scrap is measured after forming and the weight is compared with a threshold value. Patent Document 3 discloses a technique for evaluating the quality of a press-formed product from the comparison. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 186337 [Patent Document 2] Japanese Patent Application Laid-Open No. 2024-14479 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-202178 Summary of the Invention [Problem to be solved by the invention]

[0006] As mentioned above, the fluctuation range of material properties of steel materials increases as the strength of the material increases. The greater the fluctuation range of 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 the press conditions. This appears to avoid the problems associated with these methods. Here, feedback of material properties corresponds to adjustment of press conditions. However, this feedback requires correlation between the material properties and the press conditions and press forming results. One possible method for achieving this is to perform press forming using a material with known material properties and then determine the correlation from the results. Another possible method is to link various data acquired on the press line with the forming results in a big data manner. However, when various numerical values ​​are acquired on the press line and feedback is applied to the press conditions based on these values, the following problems arise. Specifically, the former requires precise prediction of material properties from the numerical values ​​acquired 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 have found that one of the problems that may arise when accurately predicting material properties is fluctuations in plate thickness. Similar to the variations in material strength mentioned earlier, variations in plate thickness also occur within and between coils. To normalize this variation in plate thickness, typical tensile tests convert the load into stress by dividing it by the cross-sectional area, and output this converted value as a material property. The cross-sectional area is the product of the width and thickness of the test piece. Even for materials with the same yield stress, if the plate thickness is thicker, the load at break will be higher. Conversely, if the plate thickness is thinner, the load at break will be lower. Similarly, even if the load at break is the same, if the plate thickness is different, the yield stress of the material will be different.

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

[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. [Means for solving the problem]

[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. [Effects of the Invention]

[0011] According to this aspect of the present invention, by correcting for the 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 as a material property that affects the dimensional accuracy of press forming. Furthermore, in this embodiment of the present invention, the measured values ​​for determining the material properties are obtained from a metal plate (blank) for pressing, so the accuracy of the obtained material properties is high. As a result, it becomes 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 becomes possible to suppress deterioration of dimensional accuracy and the occurrence of forming defects. [Brief explanation of the drawings]

[0012] [Figure 1] 1A to 1C are diagrams illustrating processing steps according to an embodiment of the present invention. [Figure 2] 1 is a schematic diagram illustrating a configuration of a bulging processing device according to an embodiment of the present invention. [Figure 3] 1 is a schematic diagram showing 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. [Figure 4] FIG. 10 is a diagram showing differences in punching load history due to differences in the thickness of the metal plate. [Figure 5] FIG. 10 is a diagram showing the results of a bulge test in an example. [Figure 6] FIG. 1 is a diagram illustrating a model for molding analysis in an embodiment. [Figure 7] FIG. 10 is a diagram showing a material model used in forming analysis. [Figure 8] FIG. 1 shows a load-time curve. [Figure 9] 10A and 10B are diagrams illustrating extraction of timing at which a load difference with respect to a completely elastic body occurs. [Figure 10] FIG. 10 is a diagram showing the relationship between yield stress YS and the timing at which a load difference occurs. [Figure 11] FIG. 10 is a diagram showing the accuracy of prediction of yield stress using a calibration curve not corrected by plate thickness. [Figure 12] FIG. 10 is a diagram showing the accuracy of prediction of yield stress using a calibration curve corrected for plate thickness. [Figure 13] FIG. 10 is a diagram showing test results of a tensile test. [Figure 14]FIG. 1 is an enlarged view of the initial test results of a tensile test. [Figure 15] FIG. 10 is a diagram illustrating the load velocity K (gradient K). [Figure 16] FIG. 1 is a graph showing the relationship between loading rate K and yield stress. [Figure 17] FIG. 1 is a diagram showing the relationship between loading rate and yield stress corrected by plate thickness. DETAILED DESCRIPTION OF THE INVENTION

[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 bulging processing 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 determining the yield stress.

[0014] (Inventor's Observations) The inventors have made the following findings. In order to establish a simple method for measuring the yield stress in a press line, forming tests and FEM analysis were carried out 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 as time variables. 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). In addition, FEM analysis was performed under the same conditions as the above forming test. Then, the effect of material properties on the forming load during the stretch 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 performed with material properties that differed only in yield stress, while all other material properties were kept the same. Then, based on this analysis, the effect of yield stress on the forming load was investigated.

[0015] Normally, when steel materials deform, they first undergo elastic deformation, and then plastic deformation occurs after reaching the yield stress. As long as only elastic deformation is occurring, the relationship between the elongation and stress of the material is independent of the various material properties, so long as it is a steel material. In other words, the relationship between the elongation and stress of the material is almost constant. The phenomenon that occurs in a tensile test from the start of load application until the yield stress is reached is as follows. That is, the region where stress steadily increases with the applied strain is the elastic deformation region. The slope of stress with respect to strain in this elastic deformation region is called Young's modulus. For steel materials, Young's modulus is generally around 210 GPa. Then, for materials that have undergone plastic deformation, the load required for deformation decreases. In other words, if we can observe the load that causes this plastic deformation, it is thought that it will be possible to determine the yield stress of each material. For this reason, when tensile tests are performed on various materials, the stress-strain curves for all materials are nearly identical straight lines until they reach the yield stress. Once the material reaches the yield stress, it gradually deviates from the straight line.

[0016] From the above, it is believed that the same tendency as in the tensile test is observed in the stretch test. That is, the inventors assumed that in the stretch test, materials of different strengths will exhibit the same load-displacement relationship in the elastic deformation region. 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 above "same load-displacement relationship" is the relationship in the case of a perfectly elastic body. In this specification, this relationship is also referred to as a "constant relationship." Furthermore, this load-displacement curve is referred to as a "reference curve." Based on the above assumptions, we carried out FEM analysis of stretch forming tests on materials with various yield stresses, and investigated the results. As a result, we found the following: In other words, when we look at the load-displacement curves, we can see that the initial load-displacement relationship is indeed the same for all materials, even if the material properties are different. Furthermore, we can see that starting with materials with low yield stress, the relationship gradually deviates from this constant relationship (see Figure 8). Furthermore, when we investigated the relationship between load and yield stress that deviates from this constant relationship (standard curve), we found that it becomes a linear relationship.

[0017] Next, we investigated whether the relationship obtained by FEM analysis also applies to the experimental results. The experimental results are the results of actual forming tests. The results of the investigation are as follows. In other words, the initial load-displacement relationship was almost identical in the experimental results. However, it was found that it did not match as well as in the analysis, and that there was some variation. Furthermore, when the relationship deviates from a certain value, the relationship between load and yield stress is not linear, as in the FEM analysis results. The certain value is expressed by a reference curve. One reason for this is thought to be noise or measurement error, and another reason is thought to be the variation in plate thickness. In the analysis, the thickness of the material is constant and given as a numerical value. However, in actual materials, the thickness may not be constant within the coil. In other words, there may be thick and thin areas 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 becomes high. However, after various studies, the inventors discovered that by correcting the load in the experimental results for the plate thickness, a high correlation with the yield stress can be obtained. Specifically, a simple truncated cone bulging test was conducted in a press line. The inventors then discovered that it is possible to accurately predict the yield stress of a material from the load-displacement relationship and plate thickness measured during the test. Although the thickness is not constant within the coil, it usually does not change sharply. In other words, within a single metal plate cut from a coil for pressing, the thickness can be considered to be almost constant.

[0019] Here, the above noise and measurement errors will be considered. 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-mentioned 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 in the early stages 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] Based on the above, we considered finding a feature quantity that defines the load-displacement curve obtained from the measured values, and then determining 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, and then determining the yield stress by referring to that correlation. Furthermore, the load-displacement curve or the obtained characteristic quantity is corrected by the measured plate thickness, thereby improving the accuracy of the obtained yield stress.

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

[0023] "First embodiment" First, the first embodiment will be described. (composition) In the manufacturing equipment (manufacturing method) for press-formed products of this embodiment, a press line is provided with a material property acquisition process 30 and a main forming process 32, as shown in FIG. The main forming step 32 is a step of press-forming a metal plate for pressing into a target part shape (product shape) using a press machine (press die). 1 is a process for feeding back the material properties acquired in the material property acquisition process 30 to the pressing conditions in the main forming process 32. The pressing condition adjustment process 31 is a process for adjusting the pressing conditions in the main forming process 32 by a known method. The processing of each process, such as the pressing condition adjustment process 31, is controlled by a control unit (not shown).

[0024] <Material property acquisition process 30> 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 such that the processing of each process is controlled by a control unit (not shown).

[0025] [Overhang processing process 30A] In the bulging process 30A, a bulging device bulges a metal plate. In the bulging process, a bulging punch is stroked (displaced) as described below to bulge the metal plate. In this embodiment, the forming load and the punch stroke of the bulging process are measured during the bulging process. At the same time, a process of measuring the thickness of the metal plate is performed. The forming load is the load applied to the punch. Information on the forming load, punch stroke (displacement), and plate thickness during bulging is measured, for example, by a punching device described below.

[0026] The bulging process in this embodiment 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-molded part (product). However, the non-pressing region is a region close to the pressing region that will become the press-molded 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, which is a process of blanking (trimming) the metal plate and shaping the metal plate before the main forming process 32.

[0027] The material of the portion discarded by blanking is subjected to bulging for measurement. This makes it possible to measure the bulging load without deteriorating the yield. The portion discarded by blanking is the non-pressed area. Furthermore, since the blanking process usually precedes the press forming process, the measured load can be used to adjust the press conditions in the subsequent process. However, the bulging 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] <Overhang processing device 1> The bulging device 1 will be described with reference to Fig. 2. The bulging device 1 is a device for punching out the metal plate used in the bulging process 30A. As shown in FIG. 2, the bulging processing device 1 of this embodiment includes a lower mold 2 and an upper mold 3 arranged opposite to each other in the bulging processing direction, a load measuring unit 5, a plate thickness measuring device 7, and a punch displacement amount measuring unit. The lower mold 2 has a mounting surface 2a on its upper surface. The mounting surface 2a is a flat surface on which a metal plate is placed. The lower mold 2 has an opening 2A for bulging processing, which is a through hole. The opening 2A opens to the mounting 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 processing. 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 direction. The upper die 3 is equipped with a punch 3A for bulging. 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 the 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 die 3 also has an extension 6 at a position to the side of the punch 3A. A thickness measuring device 7 is provided at the lower end of the extension 6. The thickness measuring device 7 is attached to the upper die 3 so that the lower part of the thickness measuring device 7 is located above the inserted 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 for measuring the plate thickness of the metal plate 20 placed on the placement surface 2 a of the lower mold 2 . The plate thickness measuring device 7 of this embodiment consists of two displacement gauges, a first displacement gauge 7A and a second displacement gauge 7B. The displacement gauges are distance meters. The two displacement gauges 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 gauges 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 disposed so as to face the area of ​​the installation surface 2a where the metal plate 20 is installed. The second displacement meter 7B is disposed so as to face an area of ​​the installation surface 2a other than the area where the metal plate 20 is installed. There may be a plurality of first displacement gauges 7A and a plurality of second displacement gauges 7B. In this case, the average value of the plurality of measurements may be used for each of the first displacement gauges 7A and the second displacement gauges 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 meters 7A and 7B supplies the measurement information to the calculation unit 10. Here, the displacement gauges 7A and 7B may be of either a contact type or a non-contact type. Note that if one displacement meter can measure the first distance L1 and the second distance L2, one displacement meter may be sufficient. For example, by measuring the distance L1 before the installation of the metal plate 20 and the distance L2 after the installation, it is possible to obtain the plate thickness from the difference between the two distances L1 and L2. Furthermore, 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 measuring the punch displacement amount may be provided in addition to the second displacement meter 7B. Since a typical bulging processing device has a measuring unit for the punch stroke amount, information from this known measuring unit may be used. Here, the bulging processing device 1 of this embodiment is configured so that the upper die body 3B is moved toward the lower die 2 by a known load applying device (not shown). The bulging processing is performed by this movement.

[0035] The bulging shape is preferably a truncated cone shape because it is axially symmetric and is thought to cause less load imbalance. In addition, with a spherical bulging, yielding occurs sequentially over a wide area, making it difficult to identify the yield point. To prevent load imbalance, it is desirable that the tip shape of punch 3A be cylindrical with a rounded punch shoulder. There are no particular restrictions on the punch diameter or punch shoulder R. However, if the punch shoulder R is too small, cracks may occur during bulging. Furthermore, if the punch shoulder R is more than half the punch diameter, the bulging will be spherical rather than truncated cone. For this reason, 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 x punch diameter It is desirable to satisfy the following formula: 2t < punch shoulder R < 1 / 4 x punch diameter

[0036] If the extension amount is too low, forming will end before sufficient plastic deformation occurs. As a result, it will be impossible to observe the yield stress. On the other hand, if the extension amount is too high, there will be no problem in terms of observing the yield stress. However, if the extension amount is too high, cracks may occur. In this case, wasteful scrap will be generated and the cracks may cause the forming load to become unstable. This may result in problems such as the need for a mechanism to repair the scrap. From the above viewpoint, it is preferable that the overhang amount h satisfies the following formula, where t is the plate thickness. 20t > h > 2t

[0037] Additionally, 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. Also, 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 × 2 Opening shoulder R > t It is more preferable that the following formula be satisfied: Die diameter > punch diameter + t × 4 Opening shoulder R > 2t

[0038] FIG. 3 shows an example in which the bulging processing device 1 of this embodiment is integrally incorporated into a blanking die 42. In Fig. 3, reference numeral 40 denotes the lower mold of the blanking mold 42. Reference numeral 41 denotes the upper mold of the blanking mold 42. Reference numeral 41A denotes the upper blade of the blanking mold 42. Reference numeral 43 denotes a common plate that connects the upper mold 41 of the blanking mold 42 and the upper mold 3 of the bulging processing device 1 of this embodiment. Furthermore, reference numeral 20A denotes a pressing area, and reference numeral 20B denotes a non-pressing area. The load measurement by the load measuring unit 5 is triggered, for example, when the distance between the upper mold 3 and the lower mold 2 falls below a first threshold value. The measurement is also triggered to end when the distance falls below a second threshold value that is smaller than the first threshold value. This makes it possible to measure the load during bulging processing for each shot.

[0039] Here, 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 comes into contact with the material is long. 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 that correlates with the thickness of the material. Therefore, it is possible to use this to correct for the thickness of the material. The relationship between time and punch load is shown schematically in Figure 4. In other words, the time it takes for the load to rise varies depending on the plate thickness. This time difference is a parameter that represents the variation in plate thickness. Therefore, it is possible to use this to correct the forming load for the plate thickness. However, to ensure the accuracy of the prediction, it is desirable to directly measure the distances L1 and L2.

[0040] [Material property calculation process 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 plate 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 between the distance information measured synchronously by the first displacement meter 7A and the second displacement meter 7B. The load curve calculation unit 10B obtains information on the load-displacement curve from the forming load and the stroke amount of the punch 3A, which are acquired in synchronization during the bulging process. The information on the load-displacement curve is a load history. The stroke amount of the punch 3A is represented by the amount of displacement 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 information on the load-displacement curve 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 can be performed 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. Then, one possible method is to perform tests on each and compare the results. However, this method is not limited to this.

[0044] All stretching tests for determining the above relationship are conducted after measuring the plate thickness. Any method for measuring the plate thickness can be used. For example, direct measurement with a micrometer or measurement with an ultrasonic plate thickness gauge can be considered. Alternatively, the same method as the measurement method in the press line described above can 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 when this constant relationship deviates can be determined, for example, using the following method. When 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. This 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 the actual test and that straight line (reference curve) is calculated. When this difference reaches a preset 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 or load difference at a preset reference displacement in a region larger than the displacement at the yield point may be used as the load P. In other words, the load P can 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 above tensile test is used as the objective variable. Furthermore, the load P and plate thickness t obtained in the above stretch forming test are used 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 tensile tests and stretch tests in which the material properties are known.

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

[0048] <Press condition adjustment process 31> Separately, the relationship between the yield stress, the pressing conditions, and the dimensional accuracy of the pressed product is determined. Then, in the press condition adjustment step 31, the above relationship is referenced and the obtained yield stress is fed back to the press conditions, and the press 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 press conditions to be adjusted include, for example, cushion pressure, pad pressure, die standby position, etc. However, the press conditions to be adjusted are not limited to these.

[0049] <Main molding process 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 etc.) When a coil of metal sheet is shipped, a material property test such as a tensile test is usually performed to obtain the material properties of the coil. However, the material property test is 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 relative 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. To measure the precise material properties of the material over the entire length of the coil, it is necessary to conduct material property tests such as tensile tests over the entire length of the coil. However, taking samples and conducting tests over the entire length of the coil is not realistic in terms of yield and cost.

[0051] Furthermore, when a representative value within a coil is used as the material property of the metal sheet 20 (blank) to be pressed, the following problem occurs: Since the precise material property is 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 property, and the greater the impact on the press-forming result. In contrast, in this embodiment, the yield stress as a material property is obtained with high accuracy from the material in the vicinity of the product (press-formed product) in the press line, which makes it possible to measure the yield stress YS of the metal plate 20 that will become the target press-formed product without conducting special sample or material tests. If the precise material properties are known, it is possible to obtain consistently high dimensional accuracy 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 that is not reflected in the current measurement method using stretching is plate thickness. Therefore, it is believed that by taking plate thickness into account, it will be possible to predict yield stress more precisely. In other words, it is believed that by correcting for load using plate thickness, it will be possible to predict yield stress more precisely. That is, in this embodiment, the load-related variables are corrected using the plate thickness for the correlation between the feature quantity defining the load-displacement curve and the yield stress of the material, thereby making it possible to obtain the yield stress of the material with higher accuracy. In addition, the plate thickness is determined from the material near the metal plate to be pressed to obtain the yield stress, which makes it possible to obtain with high accuracy the material properties (yield stress) of the metal plate that will become the press-formed product. Then, by adjusting the press conditions for forming the press-formed product using the measured material properties, it becomes 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. That is, 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 Figure 15). The load at the displacement set within the elastic range is considered to be the load in the elastic region. The above elastic range is the range that can be determined to be the elastic region for all materials that are the subject of press processing. The load at the displacement set within the plastic range is considered to be the load in the plastic region. The above plastic range is the range that can be determined to be the plastic region for all materials that are the subject of press processing.

[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 stretch test to determine the correlation between the yield stress and the slope K. For example, a calibration curve formula is determined. The range for determining the slope K of the forming load-displacement curve is preferably as follows: 0 0.5t<h1> Here, the overhang amount at the start point for calculating the slope is set to h1, the overhang amount at the end point for calculating the slope is set to h2, and the plate thickness is set to t. Preferably, 0.5t It is 1 ton.<h1>

[0056] Beyond this range, the correlation between yield stress and slope tends to decrease. If the load gradient found within the above range is K, then the equation (prediction equation) for the yield stress calibration curve can be expressed, for example, as in the following equation (2). The load gradient K is the difference in load, so it can also be expressed as the loading rate K. YS=α×K / t+β (2) Here, 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 calculated from the results of tensile tests and stretch tests where the material properties are known. The correlation to be determined is not limited to the formula of the calibration curve.

[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, thereby improving dimensional accuracy. Possible press conditions to be adjusted include, but are not limited to, cushion pressure, pad pressure, and die standby position. Other functions and effects are the same as those of the first embodiment.

[0058] (others) The present disclosure may also have the following configuration. (1) Disclosure 1 is a method for obtaining material properties of a metal plate for press working, Stretching is performed on the metal plate, and the forming load and the stroke of the die during the stretching are measured, and the thickness of the metal plate is measured. The yield stress of the metal plate is calculated from the measured forming load, die stroke, and plate thickness. How to obtain material properties. (2) Disclosure 2 is a method for obtaining material properties of a metal plate by determining the yield stress of the metal plate when the metal plate is press-formed into a press-formed product, A region of the metal plate other than the region to be formed into the press-formed product is subjected to bulging processing, and the forming load and the stroke of the die during the bulging processing are measured, and the thickness of the metal plate is measured; The yield stress of the metal plate is calculated from the measured forming load, die stroke, and plate thickness. How to obtain material properties. (3) Disclosure 3 determines the load-displacement curve of a metal plate from the measured forming load and die stroke, and further determines the characteristic quantities that define the load-displacement curve. The yield stress is then determined from the determined characteristic quantities and the measured plate thickness. (4) Disclosure 4 determines the correlation between the feature quantity and plate thickness that define the load-displacement curve and the yield stress in advance, The yield stress of the metal plate is determined by referring to this correlation. (5) Disclosure 5 is based on the principle that a load-displacement curve during stretch forming of a material that does not undergo plastic deformation is determined in advance through forming analysis using a computer, The degree of deviation of the load of the obtained load-displacement curve from the reference curve in the plastic region is defined as the feature amount. (6) In Disclosure 6, when the load difference between the reference curve and the obtained load-displacement curve reaches a predetermined threshold, the load on the obtained load-displacement curve is set as the degree of deviation. (7) In disclosure 7, the initial slope of the obtained load-displacement curve, which is determined by the load in the elastic region and the load in the plastic region, is used as the characteristic quantity. (8) Disclosure 8 is a method for manufacturing a press-molded product by press-molding a metal plate, Before the press forming, the yield stress of the metal plate is obtained by the method for obtaining material properties of the present disclosure using a metal plate portion in a region other than the region to be formed into the press-formed product. Manufacturing method for press-molded products. (9) Disclosure 9 includes a blanking process before press molding, The above-mentioned bulging process is carried out in the blanking step. (10) Disclosure 10 is a method for manufacturing a press-molded product, in which the press conditions for the press molding are adjusted based on the calculated yield stress. (11) Disclosure 11 is a stretching processing device that performs stretching processing on a metal plate for press processing, a lower mold having an opening for protruding on an installation surface 2a on which a metal plate is placed; an upper die having a punch for bulging processing that can advance and retreat toward the opening; a load measuring unit for measuring a load applied to the punch; a punch displacement amount measuring unit for measuring the stroke of the punch; a plate thickness measuring device for measuring the plate thickness of the metal plate installed in the lower mold; A bulging processing device comprising: (12) Disclosure 12 discloses that the plate thickness measuring device is provided on the upper mold, measures a first distance to the surface of a 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) Disclosure 13 is a manufacturing facility for press-molded products, which includes a press device for press-molding a metal plate, The press-molded metal plate is provided with a bulging processing device according to the present disclosure, Manufacturing equipment for press-molded products. (14) In the disclosure 14, a non-pressed region of the metal plate other than the region that will become the press-formed product is set as a target for protrusion. (15) Disclosure 15 includes a blanking device that blanks the metal plate before press-forming, The bulging device is provided in the mold of the blanking device. [Example]

[0059] An example according to this embodiment will be described. In this example, a bulging test was carried out using the bulging 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 from various locations within the coil for tensile testing and stretching testing. These samples were used to represent the material variations within the coil. Samples cut from the same location were assumed to have the same material properties. The tensile test results were then matched with the stretching test results. This allowed for the prediction of tensile properties from the stretching test. The above processing is common to the first and second embodiments.

[0060] [Table 1]

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

[0062] Next, a material model is created by creating a stress-strain curve for the material that mimics the material used in the stretch test, and a stress-strain curve that virtually varies the yield stress from that curve (see Figure 7).At that time, a material model that mimics a perfectly elastic body is also created. Next, a forming analysis of the bulging process was performed using each of the created material models. At this time, forming analysis was performed for each material model with different yield stresses. Then, in the bulging test, it was investigated what changes occurred in the load-displacement curve when the yield stress was changed. The punch stroke speed was kept constant at 10 mm / s. The relationship between the forming load (punch reaction force) and time obtained from the forming analysis is shown in Fig. 8. In Fig. 8, the analysis data is organized by forming load and time. In this analysis, the punch was moved at a constant speed. Therefore, it is possible to organize the data in the same way even if the horizontal axis is the punch stroke (displacement).

[0063] As shown in Figure 8, models other than the perfect elastic body initially match the reference curve for the perfect elastic body. However, for models other than the perfect elastic body, the load gradually decreases relative to the reference curve for the perfect elastic body. In each curve, the section that matches the reference curve for the perfect elastic body is undergoing elastic deformation. In addition, the section that deviates from the reference curve for the perfect elastic body is undergoing plastic deformation. The point where elastic deformation switches to plastic deformation is the yield point. The yield point for each material model can be determined, for example, by the following method. If we take the difference between each material model and a perfect elastic body at each time in the relationship shown in Figure 8, we get 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 the 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 the 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 by changing the horizontal axis to the punch stroke or the load when the 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 in Figure 5. The results are shown in Figure 11. Furthermore, the predicted yield stress from the results in Figure 11 was corrected by the plate thickness. The results are shown in Figure 12. Note that similar results can be obtained even if the actual yield stress is corrected by the plate thickness. Here, Fig. 12 shows the results when equation (1) of the first embodiment is used as the calibration curve based on the present invention, while Fig. 11 shows the results when a calibration curve in which the load P is not corrected by the plate thickness t is used in contrast to the calibration curve of equation (1) of the first embodiment. As shown in Fig. 11, the yield stress can be predicted with a certain degree of accuracy from the stretch test results. However, as shown in Fig. 12, it was found that by correcting the load P for the plate thickness, it is possible to predict the yield stress with even greater accuracy.

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

[0067] Furthermore, FIG. 14 shows an enlarged view of the initial stage of load rise in the load data obtained from the extension test. As shown in Figure 14, the stretch test shows similar behavior to the tensile test. That is, during the initial period when the material is undergoing elastic deformation, all steel types show similar load behavior. 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 undergoing elastic deformation 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, for each material, the slope K from the starting point s1 to the end point s2 is calculated. The slope K becomes smaller for materials with lower yield points. This slope K is also called the loading rate K. If all the extension tests are performed at the same punch speed (punch stroke), the punch speed is a function of time. In this case, when calculating the load speed K, the load can be divided by time or by the stroke. However, if the extension tests are performed while changing the punch speed (punch stroke), the load speed K is calculated by dividing the load by the punch stroke.

[0069] FIG. 16 shows the relationship between the loading rate K measured by the above method and the yield stress obtained by the tensile test. As can be seen from Fig. 16, there is a correlation between the loading rate K and the yield stress. However, for the purpose of the present invention, it is desired to further improve the prediction accuracy. Therefore, in the present invention, the loading rate K is corrected based on the plate thickness. The results of correcting the loading speed K with the plate thickness are shown in Figure 17. As can be seen from Figure 17, correcting the loading speed K with the plate thickness t further improved the accuracy. It was also found that by obtaining this relationship, the following becomes possible: Even for unknown materials, it becomes 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 application has described a limited number of embodiments, the scope of the invention is not limited thereto, and modifications of each embodiment based on the above disclosure would be obvious to one skilled in the art. [Explanation of symbols]

[0071] 1 Overhang processing device 2 Lower mold 2A aperture 3 Upper mold 3A Punch 3B Upper mold body 5 Load measurement section 7 Plate thickness measuring device 7A First Displacement Meter 10 Arithmetic section 10A Plate thickness calculation section 10B Load curve calculation section 10C Yield stress calculation section 20 metal plate 30 Material property acquisition process 30A overhang processing process 30B Material property calculation process 31 Press condition adjustment process 32 Main molding process 42 Blanking mold K Loading speed (inclination)

Claims

1. A method for obtaining material properties of a metal plate for press working, comprising: Stretching is performed on the metal plate, and the forming load and the stroke of the die during the stretching are measured, and the thickness of the metal plate is measured. The yield stress of the metal plate is calculated from the measured forming load, die stroke, and plate thickness. How to obtain material properties.

2. A method for obtaining material properties for determining the yield stress of a metal plate when the metal plate is press-formed into a press-formed product, comprising: A region of the metal plate other than the region to be formed into the press-formed product is subjected to bulging processing, and the forming load and the stroke of the die during the bulging processing are measured, and the thickness of the metal plate is measured; The yield stress of the metal plate is calculated from the measured forming load, die stroke, and plate thickness. How to obtain material properties.

3. The load-displacement curve of the metal plate is determined from the measured forming load and die stroke, and further, a characteristic quantity that defines the load-displacement curve is determined, and the yield stress is determined from the determined characteristic quantity and the measured plate thickness.

3. A method for obtaining material properties according to claim 1 or 2.

4. The correlation between the characteristic quantity and plate thickness that define the load-displacement curve and the yield stress is determined in advance, By referring to this correlation, the yield stress of the metal plate is calculated. A method for obtaining material properties according to claim 3.

5. A load-displacement curve for stretch forming of a material that does not undergo plastic deformation is determined in advance as a reference curve through forming analysis using a computer. The characteristic quantity is a degree of deviation of the load from the reference curve in a plastic region of the load-displacement curve obtained in claim 3. A method for obtaining material properties according to claim 4.

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

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

8. A method for manufacturing a press-molded product by press-molding a metal plate to manufacture a press-molded product, Before the press forming, the yield stress of the metal plate is obtained by the method for obtaining material properties according to claim 1 or 2, using a metal plate portion in a region other than the region to be formed into the press-formed product of the metal plate. Manufacturing method for press-molded products.

9. Before press forming, a blanking process is provided. The above-mentioned bulging process is performed in the above-mentioned blanking process. The method for producing a press-molded product according to claim 8.

10. The press conditions for the press molding are adjusted based on the obtained yield stress. The method for producing a press-molded product according to claim 8.