Metal plate bending test method and bending test system and automobile body design method

Abstract

To provide a technique to evaluate an effect of improving the flexural strength of a metal plate for an automobile body through heat treatment.SOLUTION: A metal plate bending test method includes: a primary bending step of bending and deforming a test piece that is a flat metal plate into a V-shape; a heat treatment step of heating the test piece and holding the test piece at a predetermined temperature for a predetermined time; and a secondary bending step of supporting two support points of the test piece with a support member and bending and deforming an intermediate part of the two support points of the test piece in a direction in which the direction of a bending ridge intersects a bending ridge direction of the test piece primarily bent into the V-shape. In conducting a test in the order of the primary bending step, the heat treatment step, and the secondary bending step, the bending load and the bending stroke of the test piece are recorded in the secondary bending step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a bending test method and bending test system for metal plate materials for evaluating the effect of heat treatment on improving the bending strength of the metal plate materials, and an automobile body design method. [Background technology]

[0002] When assembling an automobile, the paint applied to each part is baked onto the steel sheet for the automobile body by heat treatment using a heating furnace. This heat treatment changes the properties of the steel sheet. For example, it has the effect of increasing the yield stress. It is also known that the crashworthiness of automotive steel sheets can be expressed in terms of yield stress, with higher yield stress resulting in better crashworthiness. Therefore, since the crashworthiness is improved by the effect of heat treatment on improving the yield stress of automotive steel sheets, it is important to evaluate the effect of heat treatment on improving yield stress.

[0003] Conventionally, crashworthiness evaluation tests have been conducted by processing metal sheet material for automobile bodies into M-section test pieces that resemble structural members for automobile bodies, and then heat-treating the M-section test pieces before conducting three-point bending tests, as shown in Figure 8. By conducting tests with and without heating, it was clear that heating the test pieces improved the bending strength during crashworthiness evaluation tests. However, there was no other effective method for evaluating the magnitude of this effect, and the magnitude of the improvement was only revealed after the above-mentioned crashworthiness evaluation tests were conducted.

[0004] A method for evaluating the crashworthiness of metal sheet materials for automobile bodies is disclosed in Patent Document 1. In the method of Patent Document 1, a flat test piece made of the metal sheet material to be tested is first bent into a V shape using a press processing device. Next, the bent test piece is subjected to a second bending deformation in a direction perpendicular to the direction of the first bending deformation using a bending test device. The bending load and bending stroke of the test piece during the second bending deformation are then recorded to evaluate the crashworthiness. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2020 / 129903 Summary of the Invention [Problem to be solved by the invention]

[0006] However, there is no simple method for evaluating the effect of heat treatment on improving the bending strength of metal sheet materials for automobile bodies, and there is a problem that a collision performance evaluation test must be carried out without knowing the magnitude of the improvement effect. The method described in Patent Document 1 is a test method for evaluating the collision performance of metal sheet materials, but it is not possible to evaluate the effect of heat treatment on improving the bending strength of metal sheet materials for automobile bodies as it is.

[0007] Therefore, the effect of heat treatment on improving the bending strength of metal sheet materials for automobile bodies was evaluated as follows: (1) Several metal sheet materials were prepared with different steel types, heat treatment conditions, and whether or not heat treatment was performed. (2) These metal sheet materials were used to create rod-shaped test specimens with an M-shaped cross section, as shown in Figure 8. (3) These test specimens were subjected to a three-point bending test, as shown in Figure 9, and the maximum load value was evaluated.

[0008] However, this method has the following problems: (1) it is extremely time-consuming, and (2) it is difficult to detect changes in maximum load value due to the type of steel or heat treatment conditions.

[0009] The present invention has been made in view of the above, and aims to provide a technique for easily, clearly, and accurately evaluating the effect of heat treatment on improving the bending strength of metal sheet materials for automobile bodies, and also to propose an automobile body design method using the evaluation method. [Means for solving the problem]

[0010] The gist of the present invention, which advantageously solves the above problems, is as follows. [1] A bending test method for metal plate material, comprising: a primary bending step in which a test piece which is a flat metal plate material is bent into a V shape; a heat treatment step in which the test piece is heated and maintained at a predetermined temperature for a predetermined time; and a secondary bending step in which two support points of the test piece are supported by support members and an intermediate portion of the test piece between the two support points is bent in a direction such that the direction of the bending ridge line intersects the direction of the bending ridge line of the test piece which has been primarily bent into a V shape; wherein the test is performed in the order of the primary bending step, the heat treatment step, and the secondary bending step, and the bending load and bending stroke of the test piece are recorded in the secondary bending step. [2] In the above [1], a plurality of test pieces having the same characteristics and the same thickness are used as the test pieces, and the test including the heat treatment step is performed on one test piece, and the test not including the heat treatment step is performed on the other test piece, and the maximum values ​​of the bending load of both test pieces from the start of the test until the test piece that has been bent into a V-shape is flattened are compared among the bending loads and bending strokes of the test pieces recorded in the secondary bending step. [3] A bending test system for metal plate materials, comprising: a press processing device that applies a primary bending deformation to a test piece that is a flat metal plate material into a V-shape; a heat treatment device that heats the test piece and holds it at a predetermined temperature for a predetermined time; and a bending test device that supports two support points of the test piece with support members and applies a secondary bending deformation to an intermediate portion of the test piece between the two support points in a direction such that the direction of the bending ridge line intersects the direction of the bending ridge line of the test piece that has been primarily bent into a V-shape, and outputs the bending load and bending stroke of the test piece. [4] In the above [3], the bending test device is configured to compare the maximum value of the bending load of the test piece from the start of the test until the test piece that has been bent into a V shape is flattened, among the bending load and bending stroke of the test piece, from multiple test pieces with the same characteristics and the same plate thickness, with and without heat treatment. [5] A method for designing an automobile body, comprising the steps of: determining the strength of a metal sheet material after baking paint by the bending test method for metal sheet material described in [2] above when selecting a metal sheet material for an automobile body; and selecting either or both of the characteristics and thickness of the components to be subjected to baking paint treatment. [Effects of the Invention]

[0011] According to the present invention, the effect of heat treatment on improving the bending strength of a metal sheet material for an automobile body can be easily, clearly, and accurately evaluated. Therefore, an automobile body can be designed using this evaluation method, and a significant improvement in the collision performance of the automobile can be achieved. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a perspective view showing a bending test device constituting a bending test system for metal plate material according to an embodiment of the present invention, which is suitable for use in a bending test method for metal plate material according to an embodiment of the present invention. [Figure 2] FIG. 2 is a plan view showing an example of a test piece used in the bending test method for a metal plate material according to the embodiment. [Figure 3] FIG. 2 is a perspective view showing a primary bending step of the bending test method for metal plate material according to the embodiment, showing a state before a punch is pressed in. [Figure 4] FIG. 10 is a perspective view showing a primary bending step of the bending test method for a metal plate material according to the embodiment, showing a state in which a punch is pushed in. [Figure 5] 10 is a graph showing an example of the relationship between punch load and punch stroke measured during secondary bending deformation of a heated and unheated test piece measured using the bending test method for a metal plate material according to the embodiment. [Figure 6] FIG. 2 is a schematic perspective view showing a state in which a test piece is bent back flat in the bending test method for a metal plate material according to the embodiment. [Figure 7] FIG. 7 is a schematic perspective view showing a state in which the test piece is further pressed in by the bending teeth from the state shown in FIG. 6. [Figure 8]FIG. 1 is a cross-sectional view of a test piece used in a conventional collision performance evaluation test. [Figure 9] FIG. 1 is a conceptual diagram showing an outline of a three-point bending test used in a conventional crash performance evaluation test. [Figure 10] 1 is a graph showing an example of the relationship between punch load and punch stroke measured during secondary bending deformation of a heated and unheated test piece measured in a conventional crash performance evaluation test. [Figure 11] 1 is a graph comparing the increase in maximum load value measured in a collision performance evaluation test of examples of the present invention due to heat treatment. [Figure 12] 10 is a graph comparing the increase in maximum load value due to heat treatment measured using a bending test method for metal plate material according to a conventional embodiment. [Figure 13] FIG. 10 is a perspective view showing a bending test device constituting a bending test system for metal plate material according to another embodiment of the present invention, which is used in a bending test method for metal plate material according to another embodiment of the present invention. [Figure 14] FIG. 10 is a perspective view showing a bending test device constituting a bending test system for a metal plate material according to another embodiment of the present invention, which is used in a bending test method for a metal plate material according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the following embodiments are intended to exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to that described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims. Furthermore, the dimensions and processing conditions shown below are merely examples, and the present invention is not limited to these dimensions and conditions.

[0014] FIG. 1 is a perspective view showing a bending test device constituting a bending test system for metal plate material according to one embodiment of the present invention, which is suitable for use in a bending test method for metal plate material according to one embodiment of the present invention.

[0015] The bending test apparatus constituting the metal sheet bending test system according to this embodiment performs secondary bending tests on high-strength steel sheets, which are used to form parts for automobile bodies. This bending test apparatus conforms to the VDA Standard 238-100 bending test method, but uses test pieces that have been primarily bent into a V-shape rather than flat test pieces. As shown in FIG. 1 , the bending test apparatus includes two support rolls 3, each rotatably supported at its shaft end by two pairs of bearings 2 fixed opposite each other on a base 1. The two support rolls 3 are arranged horizontally and spaced apart from each other. The bending test apparatus includes a bending tooth 4 serving as a punch, whose tip 4a is supported at the center between the axes of the two support rolls 3 and extends parallel to the axes of the two support rolls 3. The bending test apparatus also includes a bending tooth holder 5 that supports the bending tooth 4 so that it extends vertically. The bending test device is provided with a load cell 6 that is attached to a bending tooth holder 5 and that detects and outputs the reaction force from the test piece T in response to the bending deformation load that is applied downward from the bending teeth 4 to the test piece T. The bending test device is also provided with a positioning jig (not shown) that abuts against the edge of the test piece T and positions the test piece T at a predetermined position on the two support rolls 3 so that the bent portion in the primary bending faces upward.

[0016] In this specification, the two positions in the direction of the bending ridge of the primary bending that support the test piece T after the primary bending are referred to as the two support points. In the example of Fig. 1, the portions of the test piece T that come into contact with the two support rolls 3 are the two support points of the test piece T.

[0017] This bending test device, which complies with the provisions of VDA Standard 238-100 bending test method, has a base 1 attached to the frame of a normal compression test machine or press machine (not shown), and a bending tooth holder 5 attached to a slide of the compression test machine, for example, driven by a hydraulic cylinder.By raising and lowering the slide relative to the frame, the bending tooth 4 can be raised and lowered between a standby position where the tip 4a is located above the two fulcrum rolls 3 and a lowered position where the tip 4a is located in the gap between the two fulcrum rolls 3, thereby performing secondary bending deformation of the test piece T.

[0018] That is, in the bending test method for metal sheets according to this embodiment, a primary bending step is performed in which a flat metal sheet, i.e., a test piece T, is bent into a V-shape. Next, a heat treatment step is performed in which the V-shaped test piece T is maintained at a predetermined temperature for a predetermined time. The heat-treated test piece T is then subjected to a secondary bending step in which the heat-treated test piece T is bent in a direction transverse to the bending direction of the primary bending, preferably perpendicular to the direction of bending, and in a direction opposite to the direction of bending. If necessary, the heat treatment step can be omitted. In the secondary bending step, the bending load of the test piece T during secondary bending is detected by the load cell 6 as a reaction force from the test piece T. In the secondary bending step, in addition to measuring the bending load, the bending stroke of the bending teeth 4 during the secondary bending is detected as the vertical movement of the slide by the compression testing machine. The bending load and bending stroke data are then recorded over time.

[0019] The bending test method for metal sheet material according to this embodiment will be described in detail below. First, prior to the test, a square test piece T with a thickness of t = 1.4 mm and sides L = 60 mm is prepared from a metal sheet material for automobile bodies. As shown in Figure 2, a dot pattern S for strain analysis is transferred to the center of the test piece T within an area of ​​sides L / 2 = 30 mm. The dot pattern S is used to analyze the strain state on the roller-side surface during the secondary bending test shown in Figure 1 using digital image correlation (DIC) to obtain information on the surface strain distribution, changes in that distribution, and the occurrence and progression of cracks. It is preferable to transfer the dot pattern S to the roller-side surface. If DIC analysis is not performed, it is not necessary to apply the dot pattern S.

[0020] Next, the test piece T is subjected to primary bending (primary bending process), which corresponds to the press forming of an automobile body part, as shown in the perspective views of Figures 3 and 4. In Figures 3 and 4, reference numeral 7 denotes a V-bending punch, 8 denotes a V-bending die, and T denotes a test piece. Figure 3 shows the flat test piece T placed in a primary bending apparatus, while Figure 4 shows the V-bending punch 7 being pressed into the V-bending die 8, causing the test piece T to be primarily bent. The bending radius can be adjusted to correspond to the actual automobile body part and is generally adjusted to a range of R5 to R10 (bending radius 5 mm to 10 mm). The bending angle is also preferably adjusted to match the shape of the actual automobile body part. It is generally set to a range of 60° to 90°. In the example shown in Figures 3 and 4, the bending radius is 5 mm (R5) and the bending angle is 90°. After primary bending, tensile strain is introduced on the outside of the bend and compressive strain is introduced on the inside of the bend in the circumferential direction of the bend radius of the test piece T.

[0021] Next, the test piece T bent into a V-shape is subjected to a heat treatment in which it is held at a predetermined temperature for a predetermined time (heat treatment step). For example, the heating conditions may be such that the test piece is held for 20 minutes after reaching 170°C in a heating furnace. These heating conditions simulate the baking finish applied to steel sheets for automobiles, and the heating temperature and holding time can be appropriately set according to the baking finish conditions.

[0022] Next, after the primary bending into a V shape, the heat-treated test piece T is placed in the apparatus shown in Figure 1 and subjected to secondary bending deformation (secondary bending process). For secondary bending, a bending test specified in VDA Standard 238-100 using the above-mentioned bending test apparatus is recommended, but other bending tests are also possible. In the illustrated example, the test piece T is placed on the support roll 3 so that the bend ridge formed in the primary bending process is perpendicular to the tip 4a of the bending tooth 4, and bending is performed from above the bent portion of the test piece T. Examples of conditions include a bending radius of the tip 4a of the bending tooth of 0.4 mm, a pressing speed of the bending tooth 4 of 5 mm / min, and a distance between the support rolls 3 of the sheet thickness × 2 + 0.5 mm.

[0023] During the secondary bending test, data showing the relationship between punch force and punch stroke is recorded in a recording device (not shown), such as a personal computer. The recording device outputs a force-stroke curve (FS curve) as shown in Figure 5. In Figure 5, symbols F1, F2, and F3 indicate points on the FS curve (solid line) for the case with heat treatment. The dotted line in Figure 5 also shows the load-stroke curve for the case without heat treatment, in which the primary bent test specimen was subjected to secondary bending. Symbol F3 indicates the point of maximum load within the stroke range of 0-17 mm for the FS curve for the case with heat treatment. Figure 6 shows an image of the test specimen at its minimum value, indicated by symbol F1 in the FS curve, just before the load begins to increase. This is the stage at which the primary bent portion of test specimen T becomes flat. Test specimen T is subjected to a bending back load until it becomes flat. Next, Figure 7 shows an image of test specimen T at its maximum bending load, indicated by symbol F2. The test piece T was V-bent in a direction perpendicular to and opposite to the strain direction of the primary bending, shown by hatching. As deformation progressed, strain concentrated in the center of the test piece T, leading to fracture at the strain-concentrated area. In this example, the heat-treated test piece (solid line) had a higher load at F3. The bending test method for metal sheet material according to this embodiment can evaluate the increase ΔF3 in the maximum bending load F3 with and without heat treatment over a stroke range of 0-17 mm.

[0024] FIG. 13 is a perspective view showing a bending test device constituting a bending test system for metal plate materials according to another embodiment of the present invention, which is used in a bending test method for metal plate materials according to another embodiment of the present invention.

[0025] The bending test apparatus constituting the bending test system for metal sheets of this embodiment has a configuration similar to that of the bending test apparatus shown in FIG. 1. This bending test apparatus includes two support rolls 3 arranged horizontally and with a gap between them, with their shaft ends rotatably supported by two pairs of bearings 2 arranged and fixed opposite each other on a base 1. This bending test apparatus includes bending teeth 4 serving as punches supported so that their tip ends 4a extend parallel to the axes of the two support rolls 3 at a central position between the axes of the two support rolls 3. This bending test apparatus also includes bending tooth holders 5 that support the bending teeth 4 extending vertically. This bending test apparatus also includes load cells 6 attached to the bending tooth holders 5 that detect and output a reaction force from the test piece T in response to a bending deformation load applied downward from the bending teeth 4 to the test piece T. This bending test device is provided with a positioning jig (not shown) that abuts against the edge of the test piece T to position the test piece T at a predetermined position on two support rolls 3.

[0026] Furthermore, this bending test apparatus can be applied to a test piece T that has been primarily bent into a V-shape in the primary bending process, similar to the bending test method for metal sheets in the previous embodiment, after a heat treatment process is performed as necessary, followed by a secondary bending process. When applying secondary bending deformation in the secondary bending process, a through-hole is formed in the center of the base 1 to continuously capture images of the deformation state of the test piece T during the secondary bending deformation from between the two support rolls 3. A continuous-shooting camera, such as a digital video camera 9 for video recording, is disposed below the base 1 so that the imaging optical system is positioned on the axis of the through-hole. The video camera 9 captures and outputs image data of the test piece T during secondary bending deformation, and the image data is recorded in a recording device, such as a personal computer, as in the bending test apparatus shown in FIG. 1. The recording device synchronizes the image of the test piece T with the load-stroke curve of the test piece T during secondary bending deformation and outputs it on the screen of a display device (not shown), for example.

[0027] Therefore, according to the bending test method for metal plates of this embodiment using the bending test system for metal plates of this embodiment, by comparing graph data showing the relationship between punch load (Force) and punch stroke (Stroke) during the secondary bending deformation process of the test piece T with image data continuously captured by the video camera 9 of the state of the bending deformation portion, the fracture initiation process including elongation, necking, crack initiation and crack propagation of the material during the secondary bending deformation process of the test piece T can be analyzed in detail, and the fracture limit stroke can be evaluated with higher accuracy. In addition, the influence of concentration and dispersion of strain that contributes to improvement of yield strength depending on whether or not heat treatment is performed can be evaluated.

[0028] Fig. 14 is a perspective view showing a bending test device constituting a bending test system for metal sheet material according to another embodiment of the present invention, which is used in a bending test method for metal sheet material according to another embodiment of the present invention. In Fig. 14, the same parts as those in the previous embodiment are designated by the same reference numerals. In the example of Fig. 14, the portions of the primary bending ridge of the test piece T that come into contact with the two support rolls 3 are the two support points of the test piece T.

[0029] That is, the bending test device in this embodiment also comprises two fulcrum rolls 3 arranged horizontally and with a gap between them, with their respective shaft ends rotatably supported by two pairs of bearings 2 arranged and fixed opposite each other on a base 1; bending teeth 4 serving as punches supported so that their tip ends 4a extend parallel to the axes of the two fulcrum rolls 3 at the center between the axes of the two fulcrum rolls 3; bending tooth holders 5 that support the bending teeth 4 by extending them vertically; load cells 6 that are provided on the bending tooth holders 5 and that detect and output the reaction force from the test piece T in response to the bending deformation load applied downward from the bending teeth 4 to the test piece T; and a positioning jig (not shown) that abuts against the edge of the test piece T and positions the test piece T at a predetermined position on the two fulcrum rolls 3 so that the bent portion in the primary bending faces downward, unlike in the previous embodiment.

[0030] In the bending test apparatus of this embodiment, a test piece T made of a steel plate that has been first bent into a V-shape by pressing in a press machine from a flat state is placed on the support roll 3 of the bending test apparatus specified in VDA Standard 238-100, with the bent portion from the first bend facing downward. The bending test apparatus then performs a second bending deformation from the top of the bottom of the test piece T in a direction transverse to the bending direction of the first bend, preferably a direction perpendicular to the bending direction of the first bend, and, unlike the previous embodiment, in the same direction as the bending direction of the first bend, not the opposite direction. The bending load of the test piece T during the second bending deformation is detected by the load cell 6 as a reaction force from the test piece T, and the bending stroke of the bending teeth 4 during the second bending deformation is detected by the compression tester as the amount of up and down movement of the slide. The data on the bending load and bending stroke are recorded over time by a recording device (not shown), such as a personal computer, and output as a graph or the like for evaluation.

[0031] As with the previous embodiment, the method for evaluating the collision performance of metal sheet materials for automobile bodies of this embodiment, which uses the testing equipment for evaluating the collision performance of metal sheet materials for automobile bodies of this embodiment, can reproduce the complex phenomena that actually occur in a simple manner that takes into account the deformation history of both press forming and collision, and can perform a high-precision collision performance evaluation test with little variation in the results.

[0032] That is, according to the above embodiment, by subjecting a test piece that has been primarily bent into a V shape to secondary deformation in a direction that intersects the bending ridgeline and a reverse direction, it is possible to simulate the deformation of the bent portion when an M-shaped cross-section test piece is subjected to a three-point bending test. In other words, it is possible to simulate an automobile body part. Furthermore, it is possible to easily compare and evaluate the superiority or inferiority of the crashworthiness of M-shaped cross-section test pieces made from different materials, and the effect of heat treatment on improving crashworthiness. [Example]

[0033] <Example of invention> Table 1 shows the material properties of three types of 1470 MPa-class steel sheets manufactured under different manufacturing conditions. Test specimen T shown in Figure 2 was prepared using metal sheet materials A, B, and C for railcar bodies shown in Table 1. The size of test specimen T was a square with a side length of L = 60 mm and a thickness of t = 1.4 mm. In Table 1, YS represents yield strength (MPa), and TS represents tensile strength (MPa).

[0034] [Table 1]

[0035] For the primary bending process, the test piece T was formed into a V-shape using the V-bending machine shown in Figure 3. The bending angle at the bent section was 90°, and the bending radius was 5 mm (R5). Then, for the secondary bending process, the primary bent test piece was subjected to a secondary bending test in accordance with the VDA238-100 standard using the bending tester shown in Figure 1. During the secondary bending process, the load and stroke applied to the bent test piece were measured, and the maximum bending load F3 was obtained. The distance between the two support rolls 3 shown in Figure 1 was the sheet thickness t × 2 + 0.5 mm, the radius of the bending tooth tip 4a was 0.4 mm, and the pushing speed of the bending tooth 4 was 5 mm / min.

[0036] The V-shaped test piece T that had been bent once was subjected to a heat treatment process in a heating furnace, where it was held for 20 minutes after reaching 170°C. The secondary bending process was then carried out in the same manner as above. Table 2 shows the maximum bending load F3 for each material with and without heat treatment, the increase in the maximum bending load ΔF3 due to heat treatment, and the increase rate (%). Figure 11 shows the increase in the maximum bending load ΔF3 due to heat treatment for each material in a bar graph. As a result of the bending test, the increase in the maximum bending load F3 ΔF3 due to heat treatment increased in the order of material No. B, A, and C.

[0037] [Table 2]

[0038] <Conventional example> Figure 9 shows a three-point bending test apparatus 10 using an M-shaped cross-section test piece. Using this apparatus, three-point bending tests were performed on the metal sheet materials A, B, and C for automobile bodies with a thickness of t = 1.4 mm shown in Table 1. The cross-sectional shape of the test piece 13 is shown in Figure 8. A cold-rolled steel sheet with a thickness of t = 1.4 mm and a tensile strength of 590 MPa was welded as a backing plate 14 to connect the flange portions 13a, 13a of the M-shaped cross-section test piece 13. In Figure 9, the radius of the punch 11 was 100 mm, the radius of the two support points 12 was 25 mm, the distance between the support points 12 was 280 mm, and the length of the test piece 13 was 400 mm. The thrust speed of the punch 11 was 10 mm / min, and the thrust depth was 50 mm. During the three-point bending test, the stroke and load values ​​were measured, and the maximum bending load F3 was obtained. This maximum value F3 is the maximum value of the load during deformation until the convex and concave portions on the top side of the M-shaped cross section test piece 13 are flattened by the indentation of the punch in the three-point bending test.

[0039] After preparing the M-shaped cross-section specimen 13 shown in Figure 8, a heat treatment process was performed in a heating furnace, where the specimen was heated to 170°C and held there for 20 minutes. A three-point bending test was then performed in the same manner as described above. Figure 10 shows an example of the load-stroke curve for Material A. The solid line represents the case with heat treatment, and the dotted line represents the case without heat treatment. Table 3 shows the maximum bending load F3, the increase in the maximum bending load ΔF3 due to heat treatment, and the increase rate (%) for each material with and without heat treatment. Figure 12 shows a bar graph of the increase in the maximum bending load ΔF3 due to heat treatment for each material. As a result of the three-point bending test, the increase in the maximum bending load ΔF3 due to heat treatment increased in the order of Material Nos. B, A, and C.

[0040] [Table 3]

[0041] <Summary> Materials Nos. A, B, and C shown in Tables 2 and 3 are classified as the same type, and the increase in maximum bending load ΔF3 showed similar trends in tests for the inventive and conventional examples. This allowed for a simple evaluation of the effect of heat treatment on improving the bending strength of metal sheet materials for automobile bodies. Furthermore, the increase rate in the maximum bending load for material No. C was 0.5% using the conventional method and 5.6% using the inventive method. In this way, the inventive method makes it easier to understand the effect of heat treatment on improving strength, allowing for a more accurate evaluation. [Industrial Applicability]

[0042] The bending test method and bending test system for metal sheet material according to the present invention are industrially useful because they make it possible to simply, clearly, and accurately evaluate the effect of heat treatment on improving the bending strength of metal sheet material for automobile bodies. [Explanation of symbols]

[0043] 1 base 2 bearings 3 Support Roll 4 curved teeth 4a Tip 5 Bent Tooth Holder 6 load cells 7 (V-bend) punch 8 (V-bend) die 9. Video Camera 10. Three-point bending test equipment 11 Punch 12 support points 13 (M-shaped cross section) test piece 13a Flange 14 Back plate T test piece S dot pattern F1 (minimum bending load during flattening) F2 (bending load at break) maximum value F3 Maximum bending load (from the start of secondary bending to the flattening of the specimen)

Claims

1. a primary bending process in which a test piece, which is a flat metal plate, is bent into a V-shape; a heat treatment step of heating the test piece and holding it at a predetermined temperature for a predetermined time; a secondary bending step of supporting the test piece at two support points with support members and bending and deforming an intermediate portion of the test piece between the two support points in a direction in which a bending ridge line intersects with a bending ridge line direction of the test piece that has been primarily bent into a V shape, When the test is performed in the order of the primary bending process, the heat treatment process, and the secondary bending process, In the secondary bending step, the bending load and bending stroke of the test piece are recorded.

2. As the test pieces, a plurality of test pieces having the same characteristics and the same plate thickness are used, One test piece is subjected to the test including a heat treatment step, Other test pieces were subjected to the above test without the heat treatment step.

2. The bending test method for metal plate materials according to claim 1, wherein the maximum values ​​of the bending loads of both test pieces from the start of the test until the test piece that has been primarily bent into a V shape is flattened are compared among the bending loads and bending strokes of the test piece recorded in the secondary bending process.

3. a press processing device that applies a primary bending deformation to a test piece that is a flat metal plate material into a V-shape; a heat treatment device for heating the test piece and maintaining it at a predetermined temperature for a predetermined time; a bending test device that supports two support points of the test piece with support members, performs secondary bending deformation on an intermediate portion of the test piece between the two support points in a direction in which the direction of the bending ridge line intersects with the direction of the bending ridge line of the test piece that has been primarily bent into a V shape, and outputs the bending load and bending stroke of the test piece; A bending test system for metal plates comprising:

4. The bending test device is 4. The bending test system for metal plate materials according to claim 3, wherein the maximum value of the bending load of the test piece from the start of the test until the test piece that has been bent into a V shape is flattened is compared from a plurality of test pieces having the same characteristics and the same plate thickness, with or without heat treatment.

5. When selecting metal sheet materials for automobile bodies, 3. A method for designing an automobile body, comprising a step of determining the yield strength of the metal plate material after baking paint by the bending test method for metal plate material according to claim 2, and selecting either or both of the characteristics and plate thickness of the member to be subjected to baking paint treatment.

Images