Friction coefficient identification method, press-forming simulation method and press-formed product manufacturing method

Abstract

To provide a technique for accurately identifying a friction coefficient between a metal plate and a press forming die.SOLUTION: A method for identifying a friction coefficient between a metal sheet and a press forming die in press forming of a metal sheet, includes: a strain distribution acquisition step of performing a stretch forming test of the metal sheet and measuring measured strain distribution of a stretched portion of the metal sheet stretched by the stretch forming test by a digital image correlation method; a strain distribution analysis step of performing a forming simulation simulating the stretch forming test under three or more conditions of friction coefficient and calculating an analytical strain distribution of the stretched portion under each friction condition; and a determination step of calculating the friction coefficient from a relation between each friction condition and a difference between the analytical strain distribution and the measured strain distribution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for identifying the coefficient of friction of a metal plate, particularly a steel plate, a press-forming simulation method using the friction coefficient, and a method for manufacturing a press-formed product. [Background technology]

[0002] Forming simulations using the finite element method (FEM) are commonly performed in advance when manufacturing automotive parts. These FEM simulations evaluate the risk of steel plate cracking and consider manufacturing methods to ensure dimensional accuracy. For exterior panel parts, predictions are also made in advance to ensure that thresholds, such as surface accuracy, are not exceeded. These efforts have been successful in reducing the time required for part manufacturing. This work relies on high FEM prediction accuracy. Based on the analysis results, dies are manufactured and actual presses are performed to confirm the difference between the predictions and the actual results. Key factors affecting FEM prediction accuracy include material models and die elastic deformation. Additionally, it is generally known that the friction coefficient also has a significant impact, making it necessary to set a reliable and accurate friction coefficient in the FEM. Traditionally, plate sliding tests have been widely used to measure the friction coefficient in press forming. However, plate sliding tests are limited in their measurement conditions, such as surface pressure and sliding speed. Therefore, although this is a simple and suitable method for evaluating the quality of sliding properties between steel types, there is a concern that it may not be accurate enough to be used as a friction coefficient to set in a press forming simulation.

[0003] Against this background, Patent Document 1 discloses a method for determining an apparent friction coefficient to be applied to a forming simulation method. In this method, the correlation between the contact pressure and the friction coefficient is measured in advance. First, the contact pressure of the drawbead portion is calculated using a die that simulates the drawbead portion of a forming die. Then, the previously measured friction coefficient is applied to calculate the pull-out force required to pass the bead. After that, the apparent friction coefficient is calculated from the contact pressure and pull-out force of the drawbead portion, and is applied to the forming simulation.

[0004] Patent Document 2 discloses a method for calculating the friction coefficient of metal materials and a forming simulation method. This method calculates the friction coefficient through polynomial approximation using one or more of the following: surface pressure, friction work, forming speed, relative hardness difference with the tool, roughness, lubricant viscosity, plastic strain, temperature, and diameter of wear debris. The calculated friction coefficient is then used to perform a forming simulation.

[0005] Patent Document 3 discloses a method for acquiring a friction coefficient for use in forming simulation. In this method, a forming simulation is first performed in which a predetermined friction coefficient is set. Then, the surface pressure and relative velocity, for example, the sliding velocity, at a predetermined location on the steel material are calculated. After that, a sliding test is performed under conditions that satisfy the surface pressure and sliding velocity, and a forming simulation is performed again using the obtained friction coefficient to estimate the formed state of the steel material.

[0006] Patent Document 4 discloses a method for determining the coefficient of friction during the compression of a cylindrical sample. This method involves determining the relationship between the sample shape after compression and the coefficient of friction through a molding simulation in which a cylindrical sample is compressed, and then determining the coefficient of friction based on the sample shape and strain actually obtained in a compression test.

[0007] Patent Document 5 discloses a method for measuring the coefficient of friction between a sample and a member. Specifically, like Patent Document 4, this is a method for determining the coefficient of friction based on a compression test of a cylindrical sample and a forming simulation, but this technology has been expanded to include warm forming. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-311338 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-207774 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-002926 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-196758 [Patent Document 5] Japanese Patent Publication No. 2023-005956 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the above-mentioned conventional techniques have the following problems to be solved. That is, the method described in Patent Document 1 does not take into consideration changes in contact pressure and sliding speed that occur in actual press forming. In addition, since it is presumed that the drawbead portion falls under conditions of high contact pressure and relatively fast sliding speed, it is considered that the method is unsuitable for predicting conditions of low contact pressure and relatively slow sliding speed when predicting surface precision of outer panel parts.

[0010] The method described in Patent Document 2 is a suitable method for determining the friction coefficient because it can take into account changes in contact pressure and plastic strain that occur during actual press forming. However, if the surface treatment state of the steel sheet or the lubricant used differs, the polynomial for calculating the friction coefficient will naturally differ. Therefore, there is a concern that the workload for deriving them will be extremely high. In addition, the friction coefficient calculated as a state function is the kinetic friction coefficient at any contact pressure and sliding speed. However, it is thought that sufficient simulation results cannot be obtained for forming at low contact pressures and sliding speeds, where the static friction coefficient is dominant, or for overhanging parts that are subjected to sliding.

[0011] The method described in Patent Document 3 involves conducting a sliding test that is considered suitable for low contact pressure and low sliding speed, and setting a static friction coefficient for the protruding portion. However, to determine the friction coefficient, a forming simulation is performed once with a predetermined friction coefficient to determine the contact pressure and sliding speed, a sliding test that satisfies these values, and a forming simulation set with the obtained friction coefficient is performed again. The contact pressure and sliding speed obtained in the repeated forming simulation using this method are expected to be different from the conditions used in the sliding test, and several repetitions are likely required to determine an accurate friction coefficient. This results in a high workload and raises concerns about the lack of accuracy in determining the friction coefficient.

[0012] The methods described in Patent Documents 4 and 5 determine the coefficient of friction through compression tests on cylindrical samples. Therefore, these techniques are based on compression processing such as forging, and do not target plastic deformation such as the stretching or bending of steel sheets, such as in pressed automobile parts. Therefore, there is a concern that the method may lack accuracy in determining the coefficient of friction in plastic deformation resulting from the elongation of steel sheets.

[0013] The present invention has been made to solve the above-mentioned problems, and its object is to provide a technology for accurately identifying the coefficient of friction between a metal sheet and a press-forming die in press-forming of a metal sheet, and further to propose a press-forming simulation method and a method for manufacturing a press-formed product using the friction coefficient. [Means for solving the problem]

[0014] The gist of the present invention is as follows. [1] A method for identifying the coefficient of friction between a metal plate and a press molding die in press molding of a metal plate, comprising: a strain distribution acquisition step of conducting a stretch forming test of the metal plate and measuring the measured strain distribution of the stretched portion of the metal plate stretched by the stretch forming test using a digital image correlation method; a strain distribution analysis step of conducting a forming simulation simulating the stretch forming test under three or more conditions of friction coefficient and calculating an analytical strain distribution for the stretched portion under each friction condition; and a determination step of calculating the friction coefficient from the relationship between each of the friction conditions and the difference between the analytical strain distribution and the measured strain distribution. [2] In the above [1], the measured strain distribution and the analytical strain distribution are strain distributions on a line on the surface of the metal plate including the punch shoulder contact portion and the punch bottom contact portion including the center of the punch in the stretch forming test, and in the determination step, the friction coefficient is calculated by comparing the integrated value of the analytical strain on the line with the integrated value of the measured strain. [3] In the above [1], the measured strain distribution and the analytical strain distribution are strain distributions on a line on the surface of the metal plate including the punch shoulder contact area and the punch bottom contact area including the center of the punch in the stretch forming test, and in the judgment step, the friction coefficient is calculated from three points: (1) a comparison of the integrated value of the analytical strain on the line with the integrated value of the measured strain, (2) a comparison of the analytical strain value at the punch shoulder contact area with the measured strain value, and (3) a comparison of the analytical strain value at the punch bottom contact area with the measured strain value. [4] A method for identifying a friction coefficient in any one of the above [1] to [3], wherein the friction coefficient is calculated under a plurality of overhang height conditions with different levels, and the friction coefficient is determined as the friction coefficient for each strain level. [5] A press-forming simulation method, which determines a friction coefficient by the method for identifying a friction coefficient according to any one of the above [1] to [4], and performs press-forming analysis using the determined friction coefficient. [6] A method for manufacturing a press-molded product, comprising the steps of determining a friction coefficient by the method for identifying a friction coefficient according to any one of [1] to [4] above, and performing press-molding analysis using the determined friction coefficient. [Effects of the Invention]

[0015] According to the present invention, it is possible to accurately identify the friction coefficient taking into account changes in contact pressure, sliding speed, and plastic deformation that occur during press forming of automobile parts. In this invention, by combining the strain distribution obtained from a stretch forming test with a forming simulation, it is possible to measure the friction coefficient in accordance with the press forming state, taking into account changes in contact pressure, sliding speed, and plastic deformation. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1( a ) is a schematic diagram showing a stretch forming tester used in a method for identifying a friction coefficient according to one embodiment of the present invention, and FIG. 1( b ) is a schematic diagram illustrating the stretch forming test. [Figure 2] 1A and 1B are diagrams comparing the strain distribution measured in the stretch forming test according to the embodiment with the strain distribution obtained by stretch forming analysis, in which (a) is a grayscale contour diagram of the strain distribution, (b) is a graph comparing the strain distribution of the central cross section, and (c) is a schematic perspective view showing an FEM analysis model. [Figure 3] 10 is a graph illustrating details of the strain distribution obtained in the stretch forming test according to the embodiment. [Figure 4] 10 is a graph showing an example of the relationship between the overall coincidence rate obtained from the analysis results for each friction coefficient and the set friction coefficient. [Figure 5] 1 is a graph showing the results of a stretch forming test in which strain distribution was obtained at the same location at three levels: 0.3, 0.6, and 0.9 times the fracture height. [Figure 6] 1 is a graph showing the relationship between the overall agreement rate determined from the analysis results at each friction coefficient when the breaking height is 0.3 times the breaking height in Example 1 and the set friction coefficient. [Figure 7] 1 is a graph showing the relationship between the overall agreement rate obtained from the analysis results at each friction coefficient when the breaking height is 0.6 times in Example 1 and the set friction coefficient. [Figure 8]1 is a graph showing the relationship between the overall agreement rate obtained from the analysis results at each friction coefficient when the breaking height is 0.9 times in Example 1 and the set friction coefficient. [Figure 9] 1(a) is a photographic image showing the appearance of the actual scale part in which a fracture occurred in Example 2, and FIG. 1(b) is a partially enlarged photographic image of the fractured portion A. FIG. [Figure 10] 10 is a graph comparing the breaking loads obtained by a forming simulation using the friction coefficients obtained in an actual press, a conventional example, and an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following describes in detail the embodiments of the present invention. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to the following. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0018] 1 is a schematic diagram showing a stretch forming tester 100 used in a method for identifying a friction coefficient according to one embodiment of the present invention. In this embodiment, the strain distribution of a metal plate during the forming process is measured using a stretch forming test die and DIC (digital image correlation). While the DIC method is suitable for measuring strain, other strain measurement methods include the strain gauge method, in which multiple strain gauges are attached to the metal plate to measure strain, and the moiré method, in which strain is determined from moiré fringes of a grid pattern drawn on the surface of the metal plate.

[0019] In this embodiment, a steel sheet S will be described as an example of a metal sheet. This embodiment is directed to stretch forming. In the example of FIG. 1, the steel sheet S is sandwiched between an upper die 2 and a lower die 3 for stretch forming testing, and stretched from the bottom surface of the steel sheet S toward the top surface by a flat-head punch 4 (FIG. 1(b)). At this time, the steel sheet S is restrained so that material does not flow from the periphery of the die into the part being formed.

[0020] The upper surface of the steel sheet S is equipped with multiple cameras 1 for capturing images of the area being formed by the punch 4. A random pattern is applied to the upper surface of the steel sheet S for calculating strain using DIC processing. Instead of a random pattern, grid points or Kagome lines may be applied. Images continuously taken during the test are loaded into an analysis device 5, which analyzes the strain distribution and its changes on the steel sheet surface. The analysis device 5 includes an analysis unit for performing DIC analysis. The analysis device 5 is preferably configured with a personal computer (PC). While a flat-headed punch is shown in the example in Figure 1, the punch shape is assumed to be a spherical shape, simulating press forming, and is not limited to a flat-headed punch. Since this test simulates press forming, the obtained strain distribution includes the effects of surface pressure and sliding speed. It is preferable to reproduce lubrication conditions similar to those in an actual press. In particular, it is preferable to use the same lubricant as in an actual press between the die and the steel sheet.

[0021] Figure 2(a) shows a grayscale contour plot comparing the measured strain distribution S8 of the steel sheet measured by DIC with the analytical strain distribution S9 calculated by a forming simulation using a friction coefficient determined by a conventional flat-plate sliding test. The left half of Figure 2(a) shows the measured strain distribution S8, and the right half shows the analytical strain distribution S9. Figure 2(b) shows the strain distribution at the center cross section of the steel sheet, i.e., along line A-A' in Figure 2(a). In the test and analysis shown in Figure 2, the steel sheet S is a square with sides of 200 mm, so the graph shows the line 100 mm along a certain side. The strain peak in Figure 2(b) is near the position where the steel sheet S contacts the shoulder of the punch 4. The region between the two peaks, where the strain value is approximately constant, corresponds to the area where the steel sheet contacts the flat area of ​​the head of the flat-head punch 4. 2(c) shows an example of the appearance of the forming simulation. The friction coefficient between the steel plate 12 and the punch 11 is the object to be identified in this embodiment.

[0022] In this embodiment, forming simulation is performed multiple times by FEM analysis S10 to derive the optimal value of the friction coefficient. In order to identify the friction coefficient through a stretch forming test, the setting value of the friction coefficient between the FEM simulated punch 11 and the simulated steel plate 12 is treated as a design variable in the forming simulation. In this case, the set friction coefficient may be a constant value regardless of the location or the time of analysis.

[0023] The strain distribution obtained in the stretch forming test is explained in detail using the graph in Figure 3. The measured strain distribution S6 is integrated in the x-axis direction, i.e., across the central cross section of the steel sheet S, to obtain S20. The maximum strain at the position of the steel sheet S in contact with the shoulder of the punch 4 is S21. The average strain in the region of the steel sheet S in contact with the bottom (flat surface of the head) of the punch 4 is S22. The integration interval was the range measured by DIC, i.e., the range of the horizontal axis shown in Figure 2(b). Based on the analysis results of each friction coefficient set in the forming simulation, a cross section was taken of the steel sheet 12 at the same position as in the stretch forming test at the same forming volume. The strain distribution of this cross section was integrated, and the maximum strain at the shoulder of the punch 11 and the average strain at the bottom of the punch 11 were also extracted. The three criteria used are the integral strain value S20, the maximum strain value at the punch shoulder S21, and the average strain value at the punch bottom S22 obtained in the stretch forming test. The agreement rate with the forming simulation results is calculated for each criterion. The overall agreement rate is determined by weighting the areas where the strain distribution needs to be more closely aligned with the stretch forming test results. For example, if you want to more closely align the forming by the punch shoulder, you could use a weighting ratio of integral strain value: maximum strain value at the punch shoulder: average strain value at the punch bottom = 0.4:0.5:0.1. Alternatively, the weight may be a function of the X-axis direction, and the strain value at each position in the X-axis direction may be multiplied by the weight at that position and integrated, and the result may be used as the strain integral value S20 to find the match rate.

[0024] Figure 4 shows an example of the relationship between the overall agreement rate calculated from the analysis results for each friction coefficient and the set friction coefficient. As mentioned above, to calculate the overall agreement rate, it is first necessary to calculate the agreement rate between the analytically obtained value and the measured value for each of the integral strain value, the maximum strain value at the punch shoulder, and the average strain value at the punch bottom. This agreement rate is expressed as a percentage by dividing the analytically obtained value by the larger of the two values ​​and multiplying it by 100. The overall agreement rate is calculated by weighting the agreement rates for the integral strain value, the maximum strain value at the punch shoulder, and the average strain value at the punch bottom, where more strict alignment between the analytical and measured values ​​is desired. In the example shown in Figure 4, the weighting ratio for the integral strain value, the maximum strain value at the punch shoulder, and the average strain value at the punch bottom is 0.5:0.1:0.4. As shown in Figure 4, the correlation between the friction coefficient set in the analysis and the obtained overall match rate is such that in stretch forming, the overall match rate drops whether the friction coefficient is higher or lower than a certain friction coefficient. Therefore, three points are selected: the friction coefficient S30 that shows the highest overall match rate, and the friction coefficients S31 and S32 above and below that friction coefficient. These three points are then used to approximate the relationship between the friction coefficient setting and the overall match rate with a quadratic function, such as a parabola, and the extreme value of the resulting quadratic function is used as the friction coefficient identification result, thereby determining the optimal friction coefficient S33.

[0025] In another embodiment of the press-forming simulation method of the present invention, a press-forming analysis is performed using the friction coefficient determined by the above-described method for identifying the friction coefficient. The press-forming analysis preferably includes stretch forming. In this case, the same friction coefficient may be used throughout the forming period, or different friction coefficients may be used depending on the amount of stretch forming.

[0026] A method for manufacturing a press-formed product according to another embodiment of the present invention includes a step of performing press-forming analysis using the friction coefficient determined by the above-described method for identifying the friction coefficient. Other steps may include a blanking step for a metal plate, a press die design step, a press-forming step, and a shearing step for an excess portion. The press-forming step preferably includes a stretch-forming step.

[0027] By using the friction coefficient determined by the above-described method for identifying the friction coefficient, it is possible to accurately simulate the press forming of actual products. For example, it is possible to accurately predict the occurrence of cracks and wrinkles during the press forming of automobile panel parts. Furthermore, by incorporating the results of this simulation into a step in the actual press manufacturing process, such as the design of the press die shape or the selection of the metal sheet material, it becomes possible to manufacture press-formed products with high yield and low cost. While the above description uses a steel sheet as an example, the method can also be applied to the forming of plated steel sheets with friction-related surfaces such as zinc, zinc alloys, and aluminum alloys, as well as aluminum alloy sheets and copper sheets. [Example]

[0028] Example 1 Table 1 shows the steel types evaluated in this example, the friction coefficients obtained from a conventional flat plate sliding test (conventional example), and the friction coefficients obtained using the technology of the present invention (inventive example). The test material, 440BH, is a steel type with a tensile strength of 440 MPa and is a baked hardened steel sheet. The sheet thickness is 0.50 mm. Bake hardened steel sheets are steel sheets that undergo strain aging due to the heat of the baked finish after press forming, resulting in a high yield point.

[0029] [Table 1]

[0030] Figure 5 shows the strain distribution obtained from the stretch forming test at the same location for three levels of fractured overhang height: 0.3, 0.6, and 0.9 times the fracture height. Five analyses were performed using FEM to simulate the formation, with the friction coefficient set in 0.1 increments from 0.1 to 0.5. Only for the 0.3-times overhang height condition was a friction coefficient of 0.6 added.

[0031] In the analysis steps where the extension heights were 0.3, 0.6, and 0.9 times the fracture height measured in the stretch forming test, strain distributions were obtained at the same locations as in the stretch forming test, and the overall agreement rate of the strain distributions was calculated for each of the three levels. For an extension height of 0.3 times the fracture height, three friction coefficients of 0.4, 0.5, and 0.6 were used. For an extension height of 0.6 times the fracture height, three friction coefficients of 0.3, 0.4, and 0.5 were used. For an extension height of 0.9 times the fracture height, three friction coefficients of 0.2, 0.3, and 0.4 were used. The relationship between the overall agreement rate and each friction coefficient was approximated by a quadratic function, and the optimal values ​​for each friction coefficient were determined. The results are shown in Tables 2-4 and Figures 6-8. In calculating the overall agreement rate, the weight of the integrated strain value: weight of the maximum strain value at the punch shoulder: weight of the average strain value at the punch bottom was set to a ratio of 0.5:0.1:0.4. While emphasis was placed on the prediction accuracy of the integrated value of the overall strain distribution, weight was also placed on the agreement rate of the average strain value at the punch bottom, because in the case of protruding parts, the part corresponding to the punch bottom becomes the product surface.

[0032] [Table 2]

[0033] [Table 3]

[0034] [Table 4]

[0035] The average values ​​of the optimum friction coefficients for each of the overhang heights shown in Tables 2 to 4 and Figures 6 to 8 are also shown in Table 1. When using the optimum value determined for one of the overhang heights, it is naturally unlikely that the strain distributions at other overhang heights will match with high precision. Therefore, in this example, the optimum values ​​for each of the three levels were averaged to ensure that the strain distributions match with as high precision as possible throughout the entire forming process.

[0036] On the other hand, the friction coefficient can be set as a function of the overhang height to achieve a more accurate forming simulation.

[0037] Example 2 We performed a press on a full-scale part, simulating the outer panel of an automobile door, as shown in Figure 9(a), and made predictions using a forming simulation. The steel types used are the same as those in Table 1. In the actual press, we performed presses multiple times while changing the cushion pressure conditions, which is the clamping force of the blank holder, and investigated the upper limit cushion pressure (breaking load) at which the panel broke, which was found to be 1050 kN. The location where the break occurred was the punch shoulder, shown by the dashed line at A in the enlarged view of Figure 9(b).

[0038] Next, a forming simulation was performed by setting the friction coefficient obtained in Example 1 (invention example) and the friction coefficient obtained from a conventional flat plate sliding test (conventional example) between the punch and the steel plate. The same conditions were set for the cushion pressure used in the actual press test in the forming simulation, and analysis was performed. Based on the previously obtained forming limit line of the material, the upper limit cushion pressure at which the fracture occurred at position A in the actual press was also determined to be fractured was investigated. Figure 10 shows the results of a comparison of the prediction accuracy of the friction coefficient obtained from the conventional flat plate sliding test and the friction coefficient obtained in Example 1, using 1050 kN, the upper limit cushion pressure (breaking load) of the actual press, as the reference. The prediction accuracy increased from 71% for the conventional example to 90% for the invention example, demonstrating the superiority of the technology of the present invention. [Explanation of symbols]

[0039] 100 Stretch forming test machine 1 camera 2 (for stretch forming test) Upper die 3. Lower die (for stretch forming test) 4 (flat head) punches 5 Analysis device S6 Measured strain distribution (central cross section) S7 Analytical strain distribution (central cross section) S8 Measured strain distribution (contour) S9 Analysis strain distribution (contour) S10 FEM analysis (model) 11 (FEM simulation) punch 12 (Simulation for FEM) Steel plate S20 Strain integral value S21 Maximum strain (at punch shoulder) S22 Average strain (at punch bottom) S30 (highest agreement among analysis results) friction coefficient S31 (lower side adjacent to S30) coefficient of friction S32 (lower side adjacent to S30) coefficient of friction S33 (identified) friction coefficient S steel plate (metal plate)

Claims

1. A method for identifying a friction coefficient between a metal plate and a press-forming die in press-forming of a metal plate, a strain distribution acquisition step of performing a stretch forming test on the metal plate and measuring an actual strain distribution on the stretched portion of the metal plate stretched by the stretch forming test; a strain distribution analysis step of performing a forming simulation simulating a stretch forming test under three or more conditions of friction coefficients and calculating an analytical strain distribution for the stretch portion under each friction condition; a determining step of calculating a friction coefficient from the relationship between each of the friction conditions and a difference between the analytical strain distribution and the measured strain distribution.

2. the measured strain distribution and the analyzed strain distribution are strain distributions on a line on the surface of the metal sheet including a punch shoulder contact portion and a punch bottom contact portion including a center portion of the punch in the stretch forming test, In the determination step, a friction coefficient is calculated by comparing an integrated value of analytical strain on the line with an integrated value of actually measured strain. The method for identifying a friction coefficient according to claim 1 .

3. the measured strain distribution and the analyzed strain distribution are strain distributions on a line on the surface of the metal sheet including a punch shoulder contact portion and a punch bottom contact portion including a center portion of the punch in the stretch forming test, In the determination step, (1) A comparison between the integrated value of analytical strain and the integrated value of measured strain on the line; (2) Comparison of analytical strain values ​​and measured strain values ​​at the punch shoulder contact area, and (3) Comparison of analytical strain values ​​and measured strain values ​​at the punch bottom contact area. The coefficient of friction is calculated from these three points. The method for identifying a friction coefficient according to claim 1 .

4. Calculate the friction coefficient under multiple overhang height conditions with different levels and use it as the friction coefficient for each strain level. The method for identifying a friction coefficient according to any one of claims 1 to 3.

5. A friction coefficient is determined by the method for identifying a friction coefficient according to any one of claims 1 to 3, and the determined friction coefficient is used to perform press forming analysis. Press forming simulation method.

6. The method includes a step of determining a friction coefficient by the method for identifying a friction coefficient according to any one of claims 1 to 3, and performing a press forming analysis using the determined friction coefficient. Manufacturing method for press-molded products.

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