Stretcher strain mark evaluation method, stretcher strain mark measurement device, and press-formed article manufacturing method

Abstract

To provide a technique capable of accurately and quantitatively evaluating a stretcher strain mark.SOLUTION: Provided is a method for evaluating a stretcher strain mark on the surface of a metal plate. In a first process 1, a surface region to be evaluated in a metal plate to be evaluated is divided into a plurality of sections, and the surface inclination of the surface distortion of the surface of the metal plate in a preset direction is determined for each section. In a second process 2, the rate of change of the surface inclination in the set direction is obtained for each section based on the surface inclination obtained for each section. In a third process 3, a section in which the absolute value of the change rate of the surface inclination is equal to or larger than a preset threshold is selected as a selection section. In a fourth step 4, the stretcher strain mark of the surface region is evaluated on the basis of the selected group of selected sections.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a technique for evaluating stretcher strain marks on the surface of a metal sheet, and a method for manufacturing a press-formed product using the technique. [Background technology]

[0002] When sheet metal is press-formed into automobile parts and other products, surface defects, which are fine linear distortion patterns, may occur locally. These distortion patterns are commonly called stretcher strain marks. The presence of stretcher strain marks results in poor appearance, reducing commercial value and also contributing to a decrease in press-forming yield. The cause of stretcher strain marks is thought to be the yield point elongation that occurs in sheet metal. Since stretcher strain marks originate from the yield point elongation, they are known to appear early in plastic processing and then disappear as the plastic processing progresses. Therefore, the occurrence of stretcher strain marks has been prevented by reducing the yield point elongation to almost zero in advance through methods such as skin-pass rolling and leveling.

[0003] However, for automotive exterior panel parts, bake-hardenable sheet metal is often used to meet the required performance of the parts. When bake-hardenable sheet metal undergoes temper rolling during manufacturing, it ages over time, and the yield point elongation reappears. This can lead to unintended stretcher strain marks on the surface of press-formed parts.

[0004] Stretcher strain marks appear as minute surface irregularities. Diagnosis of stretcher strain marks has traditionally been performed by inspectors on automobile production lines who visually evaluate the marks by grinding them with a grinding stone or by applying oil to the surface of a thin metal plate and observing the flickering reflection of light from a light source. Such evaluation methods inevitably rely on the subjective judgment of the inspector, resulting in variations in the evaluation results. Therefore, a quantitative evaluation method is needed to determine whether stretcher strain marks have occurred and how large any existing stretcher strain marks are.

[0005] Methods for evaluating stretcher strain marks include those described in Patent Documents 1 to 3, for example. In Patent Document 1, the surface of a plastically deformed metal sheet is measured using a three-dimensional roughness meter. Furthermore, any microscopic surface roughness or warpage originally present in the sheet is removed and corrected, and the resulting stripe pattern is displayed as a contour line or an iso-tilt line. Based on this stripe pattern, Patent Document 1 then determines whether or not there are stretcher strain marks.

[0006] In Patent Document 2, the two-dimensional roughness distribution of the surface of a metal sheet is measured and processed by Fourier transform, power spectrum intensity, etc. Then, in Patent Document 2, surface properties such as ridging marks and stretcher strain marks are evaluated from the difference in power spectrum intensity.

[0007] In Patent Document 3, an object is divided into a plurality of regions and images are taken, and a feature image is generated for each image using the evaluation method disclosed in Patent Document 2. Patent Document 3 then discloses connecting the feature images and synthesizing them to the size of the original object. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 9-78169 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-181389 [Patent Document 3] Japanese Patent Application Publication No. 2017-219511 [Patent Document 4] Japanese Patent Application Laid-Open No. 2007-147587 Summary of the Invention [Problem to be solved by the invention]

[0009] In Cited Document 1, measurements are made using a three-dimensional roughness meter. However, it is estimated that such measurements are limited to a very localized area, such as a few mm x a few mm, or a few mm x a few tens of mm, per measurement. Determining the presence or absence of stretcher strain marks using such a limited area as a representative value poses the problem of low reliability. To solve this problem, it is conceivable to measure multiple areas individually. However, measuring multiple areas individually requires an enormous amount of time and is not practical. Furthermore, Cited Document 1 does not go so far as to quantitatively evaluate stretcher strain marks.

[0010] The methods described in the cited documents 2 and 3 are considered to be suitable evaluation methods in that they can easily evaluate a wider area than the method described in the cited document 1. However, they have not yet been able to quantitatively evaluate the length of stretcher strain marks or the proportion of stretcher strain marks on the surface of the thin metal sheet.

[0011] Furthermore, although it is not a method for measuring stretcher strain marks, there is also a method for measuring surface distortion, such as the method described in Patent Document 4. However, compared to the surface distortion detected by the measurement method described in Patent Document 4, stretcher strain marks consist of extremely fine irregularities. The method described in Patent Document 4 may also be able to measure stretcher strain marks by using a high-precision camera with a shorter focal length than when measuring surface distortion, and by capturing images at a closer distance. However, it has not yet reached the point where it is possible to quantitatively evaluate the strength of stretcher strain marks.

[0012] The present invention has been made in light of the above points, and one of its objects is to provide a technique that enables accurate quantitative evaluation of stretcher strain marks. [Means for solving the problem]

[0013] In order to solve the problem, one aspect of the present invention is a method for evaluating stretcher strain marks on the surface of a metal plate, which comprises dividing a surface area to be evaluated on the metal plate to be evaluated into a plurality of sections, measuring three-dimensional shape information of the metal plate surface, determining the surface tilt in a predetermined direction for each section from the measured three-dimensional shape information, determining the rate of change of the surface tilt in the predetermined direction for each section based on the surface tilt determined for each section, selecting sections whose absolute value of the rate of change of the surface tilt is equal to or greater than a predetermined threshold as selected sections, and evaluating the stretcher strain marks in the surface area based on the group of selected sections. [Effects of the Invention]

[0014] According to the aspects of the present invention, stretcher strain marks can be quantitatively evaluated with high accuracy. [Brief explanation of the drawings]

[0015] [Figure 1] 1A to 1C are diagrams illustrating the processing of a stretcher strain mark evaluation method according to an embodiment of the present invention. [Figure 2] 1A is a diagram showing an example of a matrix representing distribution information of surface inclination and distribution information of a rate of change of surface inclination. FIG. 1B is a diagram showing an example of division by pixel. FIG. 1C is a diagram showing distribution information of surface inclination. [Figure 3] FIG. 10 is a diagram illustrating how to obtain the rate of change of the surface inclination. [Figure 4] FIG. 1 is a diagram illustrating an example of a configuration for stretcher strain mark evaluation according to an embodiment of the present invention. [Figure 5] 1 is a process flow of a method for manufacturing a press-formed product according to an embodiment of the present invention. [Figure 6]FIG. 1 is a cross-sectional schematic diagram illustrating a die configuration and a blank (steel plate) for a stretch test. [Figure 7] FIG. 10 is a cross-sectional schematic diagram showing the process of forming in a stretching test. [Figure 8] FIG. 1 is a schematic diagram of a stretch-formed steel sheet (press-formed product). [Figure 9] 1A and 1B are diagrams showing an example of deleting all elements (sections) except for the selected section, using a color map. (a) shows the state before deletion, and (b) shows the state with only the selected section. [Figure 10] FIG. 10 is a diagram showing an example of distribution of differential values ​​of surface tilt in a state in which there are no stretcher strain marks. [Figure 11] FIG. 10 is a graph showing the relationship between each sample and the occupancy rate. [Figure 12] FIG. 10 is a diagram showing the arithmetic mean roughness of each sample measured by a laser microscope. DETAILED DESCRIPTION OF THE INVENTION

[0016] Next, an embodiment based on the present invention will be described with reference to the drawings. In this embodiment, the presence and severity of stretcher strain marks on the surface of a metal plate is evaluated.

[0017] (Evaluation method for stretcher strain marks) The stretcher strain mark evaluation method of this embodiment (hereinafter also simply referred to as the SS evaluation method) is a method for evaluating stretcher strain marks on the surface of a metal sheet. The material of the metal plate to be evaluated is not particularly limited, but examples thereof include steel, aluminum, and alloy materials. In this embodiment, the metal plate to be evaluated is a metal plate that has been formed into a press-formed product by press working. It is preferable that the surface of the metal plate to be evaluated in this embodiment is oiled before imaging, so that the surface of the metal plate is made mirror-finished or semi-mirror-finished.

[0018] The SS evaluation method of this embodiment includes four steps, a first step 1 to a fourth step 4, as shown in FIG.

[0019] <1st step 1> 1, the first process 1 conceptually includes the following processing steps: an imaging process 1A, a section setting process 1B, a three-dimensional shape information acquisition process 1C, and a surface inclination calculation process 1D. The processing of this first process 1 may be executed as a single processing step.

[0020] [Imaging process 1A] The imaging step 1A is a step of imaging a surface area to be evaluated on a metal plate to be evaluated with a camera (imaging device) to obtain an image of the metal plate surface in the surface area. In the three-dimensional shape information acquisition step 1C, when surface information is acquired by a method other than a camera, such as a laser rangefinder, the surface information is acquired by another acquisition method such as a laser rangefinder instead of a camera.

[0021] [Classification setting process 1B] The division setting step 1B is a step of dividing the surface area to be evaluated into a plurality of sections. In this example, one pixel of the camera corresponds to one section. In other words, sections are set to correspond to pixels. Then, after the camera is installed and before the actual image capture, a scale is captured and calculated to determine how many millimeters one pixel corresponds to, and the size of each section is determined.

[0022] The present invention is not limited to the case where one pixel of the camera corresponds to one section. The size of each section may be set regardless of the size of the pixel. Alternatively, multiple pixels may correspond to one section. However, it is simpler to correspond one pixel to one section.

[0023] [3D shape information acquisition process 1C] The three-dimensional shape information acquisition step 1C is performed by measuring three-dimensional shape information (information on the three-dimensional curved surface shape) of the surface area to be evaluated using a known three-dimensional curved surface shape measurement device. The three-dimensional shape information is, for example, position information of each part (each section) of the surface area to be evaluated and information on the surface inclination for each section.

[0024] In this embodiment, the three-dimensional shape information of the surface of the surface area to be evaluated is measured and acquired by referring to an image acquired by capturing an image of the surface area to be evaluated with a camera. In this embodiment, the three-dimensional shape information (information on the three-dimensional curved surface shape) of the surface area to be evaluated is acquired by a measurement method using a known means, for example, as described in Patent Document 4.

[0025] The measurement method described in Patent Document 4 realizes two different methods, for example, the image encoding method and the binary code pattern projection method, in one system by switching between projection patterns consisting of stripes projected through a screen 9 (see Figure 4). Furthermore, the measurement method described in Patent Document 4 can obtain surface inclination distribution information consisting of information on the surface inclination in a predetermined direction for each section as three-dimensional shape information. When this method is adopted, a pattern projection means for projecting the projection pattern is provided.

[0026] The method for obtaining information on the three-dimensional curved surface shape is not limited to the above-described method. Other known three-dimensional curved surface shape measurement devices may also be used. Furthermore, a measurement device that obtains the three-dimensional curved surface shape using a laser distance meter or the like may also be used. However, three-dimensional curved surface shape measurement using image data obtained by a camera makes it possible to obtain information on the three-dimensional curved surface shape of a wide area in a short time. In other words, measurement methods that use image data obtained by a camera are more effective for detecting stretcher strain marks.

[0027] Here, the information on the 3D curved surface shape measured by the 3D curved surface shape measurement device is, for example, a collection of continuous digital data in two dimensions in a planar view, and its distribution can be expressed as matrix data. The area corresponding to each digital data in this collection can be made to correspond to each individual section.

[0028] [Surface inclination calculation process 1D] The surface inclination calculation process 1D performs a process of calculating the surface inclination in a predetermined direction for each section from the three-dimensional shape information of the surface region to be evaluated, and stores the surface inclination information for each section. In the measurement method described in Patent Document 4, the set direction coincides with the scanning direction of the striped pattern projected on the screen. For example, if the striped pattern extends in the width direction of the screen and is scanned in the vertical direction of the screen, the rolling direction of the metal plate to be evaluated is parallel to the width direction of the screen, and the scanning direction of the striped pattern is arranged in the direction perpendicular to the rolling direction, the set direction is the direction perpendicular to the rolling direction. In this example, an example will be described in which the rolling-perpendicular direction of the metal plate is the column direction when expressed as matrix data.

[0029] The surface inclination of the corresponding section can be determined by, for example, performing curve approximation on multiple adjacent sections in a set direction within the section and differentiating the curve with reference to the three-dimensional shape information. The three-dimensional shape information determined in the three-dimensional shape information acquisition step 1C may also be information on the surface inclination in the set direction for each pixel (each section). In this case, executing the three-dimensional shape information acquisition step 1C also results in the execution of the processing of the surface inclination calculation step 1D.

[0030] <Second step 2> In the second step 2, the rate of change of the surface inclination in the set direction is calculated for each section based on the surface inclination for each section calculated in the first step 1. The surface inclination for each section is expanded two-dimensionally to form surface inclination distribution data (matrix data). Based on this surface inclination distribution data, the rate of change of the surface inclination in the set direction can be found by differentiating each surface inclination for each section.

[0031] Here, as shown in Figure 2(b), the surface inclination distribution can be expressed using matrix data corresponding to the number of pixels in the image. In Figure 2(b), θxy represents the surface inclination information. For example, if the camera has 12 vertical pixels and 15 horizontal pixels, the matrix data will have 12 rows and 15 columns. Each element of the matrix stores the surface inclination [mrad (milliradians)].

[0032] In this embodiment, the setting direction is the column direction of the matrix. In this case, the surface inclination data of five consecutive points (equivalent to 1 mm) that are consecutive in the column direction and include the pixel position (section position) to be evaluated is curve-approximated by polynomial Li, as shown in FIG. 3. The pixel position X to be evaluated is set, for example, to a position on the central side of the five consecutive points. Note that the number of consecutive pixels is not limited to five. Here, when curve approximation is performed on five points, the approximation is performed using a cubic equation expressed as ax3+bx2+cx+d, for example.

[0033] Next, the calculated polynomial Li is differentiated to obtain the value at the pixel position to be evaluated and the rate of change of the surface inclination at that pixel (section). The rate of change of the surface inclination at multiple pixels may be obtained from the differentiation of one polynomial. The above processing can be performed by finite element analysis, with one pixel corresponding to one element (one section).

[0034] In this case, if the image is 12x15 pixels and the length of each pixel in the column direction is 0.25 mm, then curve approximation is performed using, for example, 5 pixels (2 pixels before and after the pixel to be differentiated). This is performed for 12 rows, and then the same process is performed for the remaining 14 columns.

[0035] In this embodiment, if the camera has 180 pixels, the number of elements (=divisions) is also 180. The element size (length of each side of the element) is converted to the same length as the length per pixel. For example, if the length per pixel is 0.25 mm x 0.3 mm, the size of one element is 0.075 mm. 2 This becomes:

[0036] Here, the unit of the rate of change of the tilt is [mrad / mm]. The tilt of the surface being measured is equal to the value obtained by first differentiating the height information of the three-dimensional curved surface shape of the object being measured with respect to the direction perpendicular to the height direction. Therefore, the rate of change of the tilt obtained by first differentiating the tilt of the surface is called the "second derivative value."

[0037] By performing this process for each section, we can obtain the distribution of the rate of change of the surface inclination as matrix data corresponding to the number of pixels in the image, as shown in Figure 2(c). θ′xy is information on the rate of change of the surface inclination.

[0038] Here, stretcher strain marks are very minute irregularities. Therefore, if the length of the section curved to approximate a single approximation formula when calculating the second derivative (or the rate of change of the slope) is longer than a predetermined length, the irregularities caused by the stretcher strain marks may be blurred, resulting in a decrease in evaluation accuracy. Therefore, when evaluating stretcher strain marks, it is preferable that the differential pitch (= the length of the section curved to approximate) be 1 mm or less. From this perspective, it is also preferable that three or more sections exist within 1 mm.

[0039] <Third step 3> The third step 3 refers to the rate of change of the surface inclination for each section calculated in the second step 2, and performs a process of selecting sections whose absolute value of the rate of change of the surface inclination is equal to or greater than a preset threshold value as selected sections.

[0040] The above threshold value is set, for example, as follows. A surface area is set on the surface of the metal plate that is assumed to have no stretcher strain marks, and the absolute value of the rate of change of the surface inclination for each section of the surface area is calculated using the SS evaluation method of this embodiment.Then, the above threshold is set based on the range of the rate of change (absolute value) of the surface inclination for each section that has been calculated. This threshold value is estimated to have little dependence on the material of the metal plate, and therefore may be calculated once and used as a fixed value.

[0041] When steel sheets were examined, as described below, the upper limit of the absolute value of the rate of change of the plane inclination on a surface free of stretcher strain marks was found to be in the range of 0.0015 mrad / mm to 0.0025 mrad / mm. Therefore, it was found that the threshold should be set in the range of 0.0015 mrad / mm to 0.0025 mrad / mm. For example, the threshold is set to 0.0025 mrad / mm. The threshold does not need to be the upper limit of the absolute value of the rate of change of the plane inclination on a surface free of stretcher strain marks; it is preferable to set the threshold slightly lower than the upper limit of the absolute value.

[0042] <4th step 4> The fourth step 4 performs a process of evaluating stretcher strain marks in the surface area based on the group of selected sections determined in the third step 3. Examples of evaluation are listed below and explained below. Evaluation can also be performed by a combination of the following evaluations.

[0043] [First evaluation] In the first evaluation, first, the occupancy ratio, which is the ratio of the number of selected sections to the total number of sections in the surface area to be evaluated, is calculated. The occupancy ratio can be expressed, for example, as "(number of selected sections) / (total number of sections) × 100".

[0044] The surface area is then evaluated for stretcher strain marks based on the calculated occupancy ratio. For example, if the occupancy ratio is greater than a preset occupancy judgment value, it is determined that there are stretcher strain marks that require action. The occupancy judgment value may be calculated by experiment or other means. The occupancy judgment value may also be changed depending on the quality required for the press-formed product. Multiple occupancy judgment values ​​may also be set.

[0045] Here, the occupancy rate is the rate of the area estimated to be a stretcher strain mark in the surface area to be evaluated.

[0046] [Second evaluation] In the second evaluation, a set of adjacent and continuous selected sections from the group of selected sections is calculated as a continuum. Note that continuity is defined as, for example, when the sides of the sections are adjacent to each other. When sections are in contact with each other at an angle, it is possible to determine in advance whether they are to be judged as continuous or discontinuous. The length of the stretcher strain mark is then estimated from the calculated length of the continuum. The length of the continuum can be calculated from the number of sections included and the direction in which the continuum extends. In this embodiment, since the dimensions of the sections in the column direction and row direction are different, the length is set taking the extension direction into consideration.

[0047] If it is determined that the length of the stretcher strain mark is longer than a predetermined judgment length, it is determined that there is a stretcher strain mark that requires action. The predetermined judgment length may be determined by experiment or other means. The predetermined length may also be changed depending on the quality required for the press-formed product. Multiple predetermined lengths may also be set. There are multiple continua to be detected, and the maximum length, average length, or most common length of those continua can be used as a representative value for evaluation.

[0048] [Third Evaluation] In a third evaluation, based on a group of selected segments, the orientation of the stretcher strain mark is estimated from the sequential orientation of adjacent consecutive selected segments. In the third evaluation, a continuum is detected by the same process as in the second evaluation, and the continuation direction is obtained from the extension direction of the continuum.

[0049] [Fourth evaluation] In the fourth evaluation, based on the group of selected sections, the area where the selected sections are concentrated within the surface area to be evaluated is determined to be the area where stretcher strain marks occur. If the area is larger than a predetermined size, it is determined that there are stretcher strain marks that require action. The size to be determined can be determined by experiment or other means. The predetermined size can also be changed depending on the quality required for the press-formed product.

[0050] [others] Here, the above-described evaluation may be performed all at once for the entire surface area to be evaluated, or the surface area to be evaluated may be divided into multiple areas and the above-described evaluation may be performed for each divided area.

[0051] (Stretcher strain mark measuring device) The stretcher strain mark measuring device is a measuring device that performs the SS evaluation method described above. Hereinafter, the stretcher strain mark measuring device will also be simply referred to as the SS measuring device. As shown in FIG. 4, the SS device of this embodiment includes a three-dimensional shape information acquisition unit 10, a surface inclination acquisition unit 11, a surface inclination change rate acquisition unit 12, a selected section selection unit 13, and an evaluation unit .

[0052] <3D shape information acquisition unit 10> The three-dimensional shape information acquisition unit 10 includes an imaging device that images a surface area to be evaluated on the metal plate to be evaluated, and a shape calculation unit. As shown in FIG. 4, the three-dimensional shape information acquisition unit 10 of this embodiment calculates three-dimensional shape information of the surface of the surface region based on image data captured by the imaging device 5A, and stores the calculated information in a storage unit.

[0053] As described above, the three-dimensional shape information acquisition unit 10 may use a known three-dimensional curved surface shape measurement device. However, in this embodiment, a measurement method is adopted in which the three-dimensional shape is measured by referring to an image of the target surface. By performing processing by referring to the image of the target surface, it becomes possible to evaluate a wide area with a single measurement.

[0054] In this embodiment, the three-dimensional shape information acquisition unit 10 is, for example, a device as described in Patent Document 4. In this case, the three-dimensional shape information acquisition unit 10 includes a pattern display means 6, an imaging means 5, and a calculation means 7.

[0055] The pattern display means 6 includes, for example, a projector 6A and a screen 9. The pattern display means 6 is configured to be able to project a light and dark pattern from the projector 6A onto the surface area to be evaluated via the screen 9. The pattern display means 6 switches between a plurality of light and dark patterns and displays them by projecting them onto the surface area to be evaluated. Reference numeral 6B denotes a control unit that controls the projector 6A. An example of the light and dark pattern displayed on the flat screen 9 is a stripe array pattern (striped pattern).

[0056] The imaging means 5 constitutes an imaging device. The imaging means 5 captures an image of the target surface with a camera 5A, thereby obtaining a mirror image of a plurality of light and dark patterns projected from the pattern display means 6, which is reflected on the surface of the surface area to be evaluated. Reference numeral 5B denotes a control unit that controls the camera 5A.

[0057] The calculation means 7 processes the mirror images of the captured light and dark patterns to calculate the surface gradient distribution of the surface region to be evaluated as 3D shape information linked to the 2D image. The surface gradient distribution is obtained, for example, as surface gradient information for each section. The processing by the pattern display means 6, the image capture means 5, and the calculation means 7 may be performed using the measurement method described in Patent Document 4. After extensive investigation, the present inventors have found that the measurement method described in Patent Document 4 quantitatively measures uneven surface distortion of the order of several microns to several tens of microns, and that it is difficult to accurately quantitatively measure stretcher strain marks consisting of an aggregate of distortions of the order of several microns using the measurement values ​​as they are.

[0058] <Surface inclination acquisition unit 11> In this embodiment, as described above, one pixel of the camera can be associated with one division. The surface inclination acquisition unit 11 obtains the surface inclination in a preset direction for each section from the three-dimensional shape information acquired by the three-dimensional shape information acquisition unit 10. Here, when the three-dimensional shape information is information on the surface inclination for each section (each pixel), the three-dimensional shape information acquiring unit 10 also serves as the surface inclination acquiring unit 11.

[0059] <Surface inclination change rate acquisition unit 12> The surface inclination change rate acquisition unit 12 obtains the change rate of the surface inclination in the set direction for each section based on the surface inclination obtained for each section.

[0060] <Selection Area Selection Division 13> The selected section selection unit 13 selects sections whose absolute value of the rate of change of the surface inclination is equal to or greater than a preset threshold value as selected sections. The threshold value may be determined and set as described above.

[0061] <Evaluation Section 14> Based on the group of selected sections, the surface area is evaluated for stretcher strain marks. The evaluation unit 14 calculates an occupancy ratio, which is the ratio of the number of selected sections to the total number of sections in the surface area to be evaluated.Then, the presence or absence of stretcher strain marks is determined based on the calculated occupancy ratio.Then, based on the occupancy ratio, it is determined whether or not the stretcher strain marks require action.

[0062] The evaluation unit 14 also detects a set of adjacent, consecutive selected sections as a continuum based on the group of selected sections, estimates the length of the continuum, and determines whether the estimated length of the stretcher strain mark requires action.

[0063] Furthermore, the evaluation unit 14 determines the locations where stretcher strain marks occur from the distribution of the selected sections based on the group of selected sections.

[0064] (Method of manufacturing press-molded products) The method for manufacturing a press-formed product according to this embodiment includes a press working step 50 and a quality assessment step 51, as shown in FIG.

[0065] The press forming process 50 is a process in which a metal plate is pressed into a target shape. Examples of forming methods include draw forming and form forming. In the case of draw forming and form forming, strain is introduced during forming, which may cause stretcher strain marks in the produced press-formed product.

[0066] The quality determination step 51 of this embodiment includes an SS evaluation step 51A. In the SS evaluation step 51A, stretcher strain marks on the surface of the produced press-formed product are evaluated by the evaluation method in the fourth step 4 described above. If it is determined that a predetermined number of stretcher strain marks or more have been formed, the stretcher strain marks are removed or the press-formed product is removed from the final product.

[0067] (Operation etc.) According to this embodiment, stretcher strain marks can be quantitatively evaluated with high accuracy.

[0068] Furthermore, according to this embodiment, if a three-dimensional shape measuring device that uses captured images is used to acquire three-dimensional shape information, a wide area can be evaluated in a single measurement, enabling accurate quantitative evaluation. That is, this embodiment makes it possible to evaluate a wide area of ​​the metal plate surface in a single measurement, and enables simple and rapid quantitative evaluation of the presence or absence of stretcher strain marks, their proportion, etc.

[0069] The inventors have found that while it is possible to determine the presence or absence of stretcher strain marks using the measurements themselves of a three-dimensional shape measurement device that uses a camera, it is difficult to quantitatively evaluate their strength. In contrast, the present invention removes small noise components that are not stretcher strain marks before evaluation, making it possible to accurately quantitatively evaluate stretcher strain marks over a wide area.

[0070] It should be noted that the detection information may include irregularities larger than stretcher strain marks, but this is still effective in assessing whether or not the area requires attention.

[0071] (others) The present disclosure may also have the following configuration. (1) Disclosure 1 is a method for evaluating stretcher strain marks on a metal plate surface, Dividing a surface area to be evaluated in a metal plate to be evaluated into a plurality of sections, measuring three-dimensional shape information of the surface of the metal plate, and determining a surface inclination in a preset direction for each section from the measured three-dimensional shape information; Based on the surface inclination determined for each section, a rate of change of the surface inclination in the set direction is determined for each section; A section having an absolute value of a rate of change of the surface inclination equal to or greater than a predetermined threshold is selected as a selected section; assessing stretcher strain marks in the surface area based on the selected group of selected sections; Stretcher strain mark evaluation method. (2) Disclosure 2 calculates the occupancy rate, which is the ratio of the number of the selected sections to the number of all sections in the surface area to be evaluated; The stretcher strain marks in the surface area are evaluated based on the calculated occupancy rate. (3) Disclosure 3 detects a set of adjacently consecutive selected sections as a continuum based on the group of selected sections, and estimates the length of the stretcher strain mark from the length of the continuum. (4) Disclosure 4 estimates the orientation of the stretcher strain mark from the sequential direction of adjacent consecutive selected sections based on the group of selected sections. (5) Disclosure 5 calculates the absolute value of the rate of change of the surface inclination for each section in a surface area estimated to have no stretcher strain marks, and sets the threshold value based on the range of the calculated rate of change (absolute value) of the surface inclination for each section. (6) Disclosure 6 sets the threshold value to a value in the range of 0.0015 [mrad / mm] or more and 0.0025 [mrad / mm] or less. (7) Disclosure 7 is a measuring device for measuring stretcher strain marks on a metal plate surface, a three-dimensional shape information acquisition unit that acquires three-dimensional shape information of a surface of a surface area to be evaluated on a metal plate to be evaluated based on image data captured by an imaging device; a surface inclination acquisition unit that divides the surface area into a plurality of sections and obtains a surface inclination in a preset direction for each section from the acquired three-dimensional shape information; a surface inclination change rate acquisition unit that acquires, for each section, a change rate of the surface inclination in the set direction based on the surface inclination acquired for each section; a selected section selection unit that selects sections whose absolute values ​​of the rate of change of the surface inclination are equal to or greater than a preset threshold as selected sections; an evaluation unit for evaluating stretcher strain marks of the surface area based on the group of selected sections; A stretcher strain mark measuring device comprising: (8) Disclosure 8 states that the three-dimensional shape information acquisition unit divides the surface area to be evaluated into multiple sections and obtains the surface inclination in a predetermined direction for each section as three-dimensional shape information, thereby also serving as the surface inclination acquisition unit. (9) Disclosure 9 is that the three-dimensional shape information acquisition unit a pattern display means capable of switching between and displaying a plurality of light and dark patterns; an imaging means for imaging a mirror image of the plurality of light and dark patterns displayed on the pattern display means, the mirror image being reflected on the surface of the plane area; a calculation means for processing the captured mirror images of the plurality of light and dark patterns and calculating the inclination distribution of the surface of the measurement object as the three-dimensional shape information; Equipped with. (10) In disclosure 10, the evaluation unit calculates an occupancy ratio, which is the ratio of the number of the selected sections to the number of all sections in the surface area to be evaluated, and determines the presence or absence of stretcher strain marks based on the calculated occupancy ratio. (11) Disclosure 11 discloses that the evaluation unit detects a set of adjacently consecutive selected sections as a continuum based on the group of selected sections, and estimates the length of the stretcher strain mark from the length of the continuum. (12) Disclosure 12 sets the threshold value at 0.0015 [mrad / mm] or more and 0.0025 [mrad / mm] or less. (13) Disclosure 13 is a method for manufacturing a press-molded product by press-working a metal plate to manufacture a press-molded product, The stretcher strain mark evaluation method of the present disclosure includes a step of evaluating stretcher strain marks on the surface of a manufactured press-formed product, Manufacturing method for press-molded products. [Example]

[0072] Next, an example of this embodiment will be described. (1) Press processing First, a stretching test was carried out to simulate press forming. The metal plate used was a square blank made of steel plate with each side measuring 200 mm. The press forming was performed using the mold shown in Figure 6, and stretch forming was performed as shown in Figure 7. Specifically, a blank 15 was held by a die 20 and a plate holder 21, and as shown in Fig. 7, the blank 15 was pushed up by a punch 22 to perform stretch forming, thereby producing a press-formed product 16. Reference numeral 20A denotes a lock bead.

[0073] Here, the holding force of the die 20 and the plate holder 21 was set to 100 tons (≈980 kN). Since stretch forming is assumed, it is sufficient that the material outside the lock bead 20A does not flow into the mold. For this reason, the gripping force does not need to be 100 tons. A gripping force of around 50 tons will suffice.

[0074] FIG. 8 is a sample image of a press-formed product 16 manufactured by stretch forming. In this example, the amount of strain at the punch bottom contact surface 16A, where the flat portion of the punch 22 comes into contact, may be such that stretcher strain marks appear. The appearance of stretcher strain marks is adjusted by the stroke amount (= overhang amount) of the punch 22. In this example, processing was stopped at the forming height where the equivalent strain at the punch bottom contact surface 16A was 2%. By setting the equivalent strain to 2%, the strain in each of the two orthogonal directions becomes 1%.

[0075] (2) SS measurement method (a) Next, the inclination of the surface of each section and the rate of change of the inclination of the surface were determined using the method described in the embodiment. The surface area was the punch bottom contact surface 16A (the bottom surface of the press-formed product 16). In this example, the distribution of plane inclination was determined by the measurement method described in Patent Document 4. At this time, after completing the setup and before taking the actual image, we took an image of the scale and calculated how many millimeters one pixel of the camera corresponds to.

[0076] In addition, a striped pattern was projected onto a screen using a projector, and the light projected through the screen and reflected from the surface of the metal plate was captured at fixed points using a camera (see Figure 4). At this time, the thickness and position of the striped pattern projected through the screen were changed frame by frame while each projection pattern was projected, and the reflected light from the surface area to be evaluated in each projection state was captured by the camera each time.

[0077] Then, for each captured image, we calculated where in the surface area the projected stripe pattern was projected. After that, we used image processing to calculate the distribution of surface tilt in the surface area from the light / dark history of each pixel, and measured the tilt for each pixel (each section). Note that the tilt was calculated with the direction perpendicular to the stripe direction of the stripe pattern as the set direction.

[0078] Furthermore, in this example, the slope of 14 consecutive pixels in the column direction is polynomial-approximated, and the polynomial is differentiated once to determine the rate of change of the surface slope at each pixel. In this embodiment, the measurement (camera field of view) range was set to a wide area of ​​about 50 mm×40 mm as the area to be evaluated.

[0079] (b) Next, the selection categories were selected. The distribution of the rate of change of the surface inclination in each section can be expressed as two-dimensional matrix information. From the rate of change of the surface inclination in each section, the section whose absolute value of the rate of change is equal to or greater than a threshold value is selected as the selected section. In this embodiment, the threshold value is set to 0.002 mrad / mm, as will be described later. For example, if the category in which the absolute value of the rate of change of the surface inclination is 0.002 mrad / mm or less is removed from all elements shown in FIG. 9(a), the distribution state shown in FIG. 9(b) will result.

[0080] (c) Next, the occupancy rate and the representative length of the stretcher strain mark were determined from the group of selected categories.

[0081] (3) Threshold For the metal sheet surfaces that were visually determined to have no stretcher strain marks, the rate of change in plane inclination in each section was determined using the SS measurement method described in (2) above. Figure 10 shows the distribution of the rate of change in plane inclination in the central cross section of the metal sheet as a representative example.

[0082] As can be seen from Fig. 10, for surfaces without stretcher strain marks, the range of change rate of plane tilt is generally within 0.0025 mrad / mm in absolute value. In this example, the threshold value was set to 0.002 mrad / mm.

[0083] Since stretcher strain marks are extremely minute irregularities, if the length of the section to be approximated by a single approximation formula when determining the rate of change of the second-order derivative slope is long, the irregularities will be blunted, resulting in a decrease in evaluation accuracy. For this reason, in this example, the differential pitch (= the length of the section to be approximated by a curve) was set to 1 mm.

[0084] Furthermore, since noise is introduced during data processing, the occupancy rate is not necessarily 0% even on surfaces without stretcher strain marks.

[0085] (4) Evaluation (a) In this example, the surface properties of various metal sheets after press forming in the press working described above in (1) were evaluated.

[0086] The metal plates used for the evaluation are shown in Table 1. Table 1 shows information about the four samples A, B, C, and D, including their plate thickness, yield point elongation, and whether or not they were ground after press forming. The grinding was performed on the back surface of the press-molded product 16 (the surface in contact with the punch 22). The tensile strength TS of each metal plate was 350 MPa. Samples B to D were intentionally aged by heat treatment to produce yield point elongation. [Table 1]

[0087] (b) Evaluation results The occupancy rate and the representative length of the bottom surface of each sample were determined using the SS measurement method described above in (2). The results are shown in Table 2. [Table 2]

[0088] Figure 11 is a graph showing the occupancy rate based on Table 2. Table 2 shows that stretcher strain marks can be quantitatively evaluated with high accuracy. Samples C and D also show that stretcher strain marks can be quantitatively evaluated with high accuracy, regardless of whether or not they are ground to make them easier to see.

[0089] Additionally, here, roughness measurements were carried out on samples A, B, and C using a laser microscope to simulate a conventional example. The area that could be measured at one time using the laser microscope was 1 mm x 80 mm. The measurement area was the position on sample B where stretcher strain marks were visually observed. The same locations on samples A and C as on sample B were also measured.

[0090] FIG. 12 shows the measured arithmetic mean roughness. As can be seen from Figure 12, a clear difference in roughness was observed in sample B, and it is believed that the stretcher strain marks were captured. On the other hand, no difference was observed between samples A and C, and it is believed that sample C had no stretcher strain marks within the measurement area.

[0091] As such, it is believed that the accuracy of determining the presence or absence of stretcher strain marks across the entire sample is low based solely on the results of measurements taken within a limited area using a laser microscope. Therefore, it is preferable to evaluate a predetermined or larger area in one evaluation.

[0092] In contrast, with the method according to the present invention, the occupancy rate was 1.6% for the reference sample A and 22.2% for sample B, as shown in Table 2 and Figure 11, which is consistent with the qualitative trend determined by visual evaluation. Therefore, it was confirmed that stretcher strain marks can be quantitatively evaluated based on the present invention.

[0093] Furthermore, when comparing C and D, which are comparisons before and after grinding, the occupancy rates were almost the same: D (without grinding) was 15.2%, and C (with grinding) was 13.0%. This suggests that even stretcher strain marks, which are difficult to distinguish with the human eye without grinding, can be evaluated without grinding by using this invention.

[0094] Furthermore, from Table 2, it was found that accurate quantitative evaluation is possible even if evaluation is performed using the estimated length of the stretcher strain instead of the occupancy ratio. As described above, it has been found that the present invention can improve reliability and perform quantitative evaluation with high accuracy even when evaluating a wide area at once. [Explanation of symbols]

[0095] 1. First step 1A Imaging process 1B Classification process 1C 3D shape information acquisition process 1D surface inclination calculation process 2. Second step 3. Third step 4. Fourth step 5. Imaging Method 5A Imaging device (camera) 6 Pattern display method 6A Projector 7 Calculation means 10 3D shape information acquisition section 11 Surface tilt acquisition unit 12. Surface tilt change rate acquisition unit 13 Election District Election Division 14 Evaluation Section 15 Blank (metal plate) 16 Press-molded products (metal plates) 16A Punch bottom contact surface 50 Press processing process 51 Quality judgment process 51A SS evaluation process

Claims

1. A method for evaluating stretcher strain marks on a metal plate surface, comprising: Dividing a surface area to be evaluated in a metal plate to be evaluated into a plurality of sections, measuring three-dimensional shape information of the surface of the metal plate, and determining a surface inclination in a preset direction for each section from the measured three-dimensional shape information; Based on the surface inclination determined for each section, a rate of change of the surface inclination in the set direction is determined for each section; A section having an absolute value of a rate of change of the surface inclination equal to or greater than a predetermined threshold is selected as a selected section; assessing stretcher strain marks in the surface area based on the selected group of selected sections; Stretcher strain mark evaluation method.

2. Calculating an occupancy rate, which is the ratio of the number of the selected sections to the number of all sections in the surface area to be evaluated; evaluating the stretcher strain marks in the surface area based on the determined occupancy rate; 2. A stretcher strain mark evaluation method according to claim 1.

3. Based on the group of selected sections, a set of adjacently consecutive selected sections is detected as a continuum, and the length of the continuum is estimated from the length of the continuum.

2. A stretcher strain mark evaluation method according to claim 1.

4. estimating an orientation of a stretcher strain mark from a sequential direction of adjacent consecutive selected segments based on the group of selected segments; 2. A stretcher strain mark evaluation method according to claim 1.

5. calculating an absolute value of the rate of change of the surface inclination for each section in the surface region estimated to be free of stretcher strain marks, and setting the threshold value based on the range of the rate of change of the surface inclination for each section thus calculated; 2. A stretcher strain mark evaluation method according to claim 1.

6. The threshold value is set to a value in the range of 0.0015 [mrad / mm] or more and 0.0025 [mrad / mm] or less.

2. A stretcher strain mark evaluation method according to claim 1.

7. A measuring device for measuring stretcher strain marks on a surface of a metal plate, comprising: a three-dimensional shape information acquisition unit that acquires three-dimensional shape information of a surface of a surface area to be evaluated on a metal plate to be evaluated based on image data captured by an imaging device of the surface area; a surface inclination acquisition unit that divides the surface area into a plurality of sections and obtains a surface inclination in a preset direction for each section from the acquired three-dimensional shape information; a surface inclination change rate acquisition unit that acquires, for each section, a change rate of the surface inclination in the set direction based on the surface inclination acquired for each section; a selected section selection unit that selects sections whose absolute values ​​of the rate of change of the surface inclination are equal to or greater than a preset threshold as selected sections; an evaluation unit for evaluating stretcher strain marks of the surface area based on the group of selected sections; A stretcher strain mark measuring device comprising:

8. the three-dimensional shape information acquisition unit divides the surface area to be evaluated into a plurality of sections, and obtains a surface inclination in a preset direction for each section as three-dimensional shape information, thereby also functioning as the surface inclination acquisition unit; 8. The stretcher strain mark measuring device according to claim 7.

9. The three-dimensional shape information acquisition unit a pattern display means capable of switching between and displaying a plurality of light and dark patterns; an imaging means for imaging a mirror image of the plurality of light and dark patterns displayed on the pattern display means, the mirror image being reflected on the surface of the plane area; a calculation means for processing the captured mirror images of the plurality of light and dark patterns to calculate the inclination distribution of the surface of the measurement object as the three-dimensional shape information; Equipped with 8. The stretcher strain mark measuring device according to claim 7.

10. A method for manufacturing a press-molded product by press-working a metal plate to manufacture a press-molded product, The stretcher strain mark evaluation method according to any one of claims 1 to 6, comprising a step of evaluating stretcher strain marks on the surface of a manufactured press-formed product, Manufacturing method for press-molded products.

Images