Test equipment and test method

Abstract

To provide a testing device capable of stably measuring a forming limit diagram of a steel sheet in high-temperature press forming.SOLUTION: A testing device (1) measures the forming limit diagram of a steel sheet during high-temperature press forming. The testing device (1) includes a die (2) and a punch (3). The die (2) includes a die hole (21) having a circular cross section. The punch (3) has an axis (A) shared with the die hole (21) and is movable along the axis (A) toward the die (2). The punch (3) includes a punch body part (31) having the axis (A) as the central axis, and a ring-shaped punch tip part (32) positioned on the die (2) side of the punch body part (31) and having the axis (A) as the central axis.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a test device and a test method, and more particularly to a test device and a test method for measuring a forming limit diagram of a steel sheet during hot press forming. [Background technology]

[0002] Many automotive parts are formed by pressing steel sheets. During press forming, the steel sheets are bent and drawn. For this reason, forming analysis is performed in advance using numerical analysis such as the finite element method to determine the appropriate product shape and appropriate forming conditions. In forming analysis, it is necessary to accurately determine cracks that may occur during press forming, and a forming limit diagram (hereinafter sometimes referred to as FLD) is used as data for judgment.

[0003] Generally, the Nakajima method or the Marciniak method is used to measure FLD (see, for example, JP 2013-121607 A (Patent Document 1)). In the Nakajima method, a punch with a hemispherical tip is used. Hereinafter, this punch may be referred to as a ball-head punch. In the Marciniak method, a cylindrical punch with a flat tip surface is used. Hereinafter, this punch may be referred to as a cylindrical punch. In the Marciniak method, a driving plate is placed between the cylindrical punch and the test material (steel plate). A hole is formed in the driving plate at the position of the central axis of the cylindrical punch.

[0004] In recent years, as the strength of products has increased, high-strength steel sheets are often press-formed. When cold press-forming is performed, the increased forming load and reduced formability become issues. To solve these issues, a method of hot press-forming high-strength steel sheets, specifically hot stamping, is used. Even in high-temperature press-forming such as hot stamping, it is necessary to accurately determine cracks using forming analysis. For this reason, it is desirable to use FLD measured in a hot stretch forming test as the data for judgment. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-121607 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when a steel sheet as a test material is heated and a hot stretch forming test is performed according to the Nakajima method or the Marciniak method in order to measure the FLD of the steel sheet during hot press forming, the following problems arise.

[0007] In the Nakajima method, a spherical punch is used, and there is no driving plate between the spherical punch and the test piece. In this case, the forming load is relatively small. However, as the stretch forming progresses, the test piece comes into contact with the entire tip of the spherical punch, specifically the entire hemispherical surface. This wide contact between the test piece and the spherical punch causes a significant drop in the temperature of the test piece. This limits the accuracy and range of strain ratios in the measured FLD.

[0008] In the Marciniak method, a driving plate is placed between the cylindrical punch and the test piece, and a hole is formed in the driving plate. In this case, the test piece does not contact the cylindrical punch in the area of ​​the hole in the driving plate, but contacts the cylindrical punch through the driving plate around the hole in the driving plate. Because the test piece is stretched with a relatively small temperature drop, the accuracy and strain ratio range of the measured FLD are wider than in the Nakajima method. However, because the driving plate is stretched together with the test piece with extensive contact with the cylindrical punch around the hole in the driving plate, the forming load is larger than in the Nakajima method. In addition, the tip face of the cylindrical punch contacts the area around the hole in the driving plate over a wide area, and the temperature of the driving plate and the test piece decreases rapidly over this extensive contact area. As a result, the temperature of the test piece and the driving plate drops significantly in the area from the shoulder to the sidewall of the cylindrical punch, which can lead to cracking of the test piece or the driving plate. Ideally, the test material should crack on the central axis of the punch, but in the Marciniak method, the test material may crack in the area from the shoulder to the side wall of the cylindrical punch, making the measured FLD unstable.

[0009] An object of the present disclosure is to provide a test device and a test method that can stably measure the forming limit diagram of a steel sheet during high-temperature press forming. [Means for solving the problem]

[0010] The testing apparatus according to the present disclosure is a testing apparatus for measuring forming limit diagrams of steel sheets during high-temperature press forming. The testing apparatus includes a die and a punch. The die has a die hole with a circular cross section. The punch has an axis shared with the die hole and is movable along the axis toward the die. The punch includes a punch body having the axis as its central axis, and a ring-shaped punch tip located on the die side of the punch body and having the axis as its central axis.

[0011] In the test method according to the present disclosure, the forming limit diagram of a steel sheet during high-temperature press forming is measured using the above-described test device. [Effects of the Invention]

[0012] According to the test device and test method of the present disclosure, it is possible to stably measure the forming limit diagram of a steel sheet during high-temperature press forming. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram of a test device according to the first embodiment. [Figure 2] FIG. 2 is a schematic diagram of a test device according to the second embodiment. [Figure 3] FIG. 3 is a schematic diagram of a test device according to the third embodiment. [Figure 4] FIG. 4 is a schematic diagram of a test device according to the fourth embodiment. [Figure 5] FIG. 5 is a schematic diagram of a test device according to the fifth embodiment. [Figure 6] FIG. 6 is a schematic diagram of a test device according to the sixth embodiment. [Figure 7] FIG. 7 is a schematic diagram of a test device according to the seventh embodiment. [Figure 8] FIG. 8 is a schematic diagram of a test device according to the eighth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] A testing apparatus according to an embodiment of the present disclosure is a testing apparatus for measuring forming limit diagrams of steel sheets during high-temperature press forming. The testing apparatus includes a die and a punch. The die has a die hole with a circular cross section. The punch has an axis shared with the die hole and is movable along the axis toward the die. The punch includes a punch body having the axis as its central axis, and a ring-shaped punch tip located on the die side of the punch body and having the axis as its central axis (first configuration).

[0015] In the first configuration of the testing device, a heated steel sheet, which is the test material, is placed between a die and a punch, and the punch moves toward the die. As the punch moves, the ring-shaped punch tip comes into contact with the steel sheet. As the punch moves further, the punch tip and the punch body enter the die hole, stretching the steel sheet.

[0016] During stretch forming using the first configuration of the test apparatus, the steel sheet comes into contact with the tip of a ring-shaped punch. In this case, the contact area between the punch and the steel sheet is much smaller than in the conventional Nakajima method. This results in a small temperature drop in the steel sheet. Furthermore, although the temperature drop of the steel sheet progresses in the area in contact with the tip of the punch, the contact area between the punch and the steel sheet is small. This suppresses the temperature drop in the area from the shoulder to the side wall of the punch, even compared to the conventional Marciniak method, thereby preventing cracking of the steel sheet. Furthermore, the area inside the tip of the punch does not come into contact with the punch, so the temperature drop is extremely small. This makes the steel sheet prone to cracking on or near the central axis of the punch. Therefore, the measured FLD is stable. This allows for stable measurement of the FLD of steel sheets during high-temperature press forming.

[0017] The above-mentioned testing apparatus preferably has the following configuration: The punch tip includes a leading edge located closest to the die, the leading edge being circular with the axis as its central axis, and an outer edge surface adjacent to the radially outer side of the leading edge, the outer edge surface being ring-shaped and convexly curved with the axis as its center (second configuration).

[0018] In the second configuration, during stretch forming, the steel sheet first comes into contact with the tip of the punch, and then gradually comes into contact with the outer edge of the punch. Because the outer edge is convexly curved, the steel sheet deforms smoothly along the outer edge. Therefore, the measured FLD is more stable.

[0019] In the above testing device, preferably, the cross section of the punch body is circular, and the difference between the diameter of the die hole and the diameter of the punch body is at least twice the thickness of the steel plate (third configuration).

[0020] In the third configuration, the difference between the diameter of the die hole and the diameter of the punch body is at least twice the thickness of the steel sheet, so the gap between the inner surface of the die hole and the outer surface of the punch body is at least one time the thickness of the steel sheet. In this case, contact between the steel sheet and the inner surface of the die hole and between the steel sheet and the outer surface of the punch body is suppressed during stretch forming. Therefore, it is possible to suppress an unexpected drop in the temperature of the steel sheet due to heat conduction and to suppress unexpected breakage of the steel sheet due to friction.

[0021] In the above-mentioned testing device, a lubricant film is preferably formed on at least one of the surface of the tip of the punch and the surface of the steel plate facing the punch (fourth configuration).

[0022] In the fourth configuration, the coefficient of friction between the contacting tip of the punch and the steel sheet is reduced by the lubricant coating, thereby enabling smooth stretch forming.

[0023] The testing device of the first configuration preferably further includes a drive plate. The drive plate is stacked on the surface of the steel plate on the punch side. The drive plate has a hole formed therein, the hole having the axis as its center (fifth configuration).

[0024] According to the fifth configuration of the testing device, a driving plate is stacked on the punch-side surface of a steel plate, which is the test material. The heated steel plate together with the driving plate is placed between the die and the punch, and the punch is moved toward the die. As the punch moves, the ring-shaped punch tip comes into contact with the driving plate. As the punch moves further, the punch tip and the punch body enter the die hole, causing the steel plate and driving plate to bulge.

[0025] During stretch forming using the fifth configuration of the test apparatus, the drive plate contacts the ring-shaped punch tip. In this case, the contact area between the punch and the drive plate is much smaller than in the conventional Marciniak method. Furthermore, the punch tip contacts the drive plate over a narrow area, and the temperature drop of the drive plate and the steel sheet progresses in this narrow contact area. Therefore, the temperature drop of the drive plate and the steel sheet is small. This suppresses the temperature drop of the steel sheet and the drive plate in the area from the punch shoulder to the side wall, even compared to the conventional Marciniak method, thereby suppressing cracking of the steel sheet or the drive plate. Furthermore, since the area inside the punch tip of the steel sheet does not contact the punch, the temperature drop is extremely small. As a result, the steel sheet is prone to cracking on or near the central axis of the punch. Therefore, the measured FLD is stable. Therefore, the FLD of steel sheets during high-temperature press forming can be measured stably.

[0026] The testing apparatus of the fifth configuration preferably has the following configuration: The punch tip includes a leading edge closest to the die, the leading edge being circular with the axis as its central axis, and an outer edge surface adjacent to the radially outer side of the leading edge, the outer edge surface being ring-shaped and convexly curved with the axis as its center (sixth configuration).

[0027] In the sixth configuration, during stretch forming, the driving plate first contacts the very tip of the punch tip, and then gradually contacts the outer edge of the punch tip. Because the outer edge is convexly curved, the steel sheet deforms smoothly along the outer edge together with the driving plate. This results in a more stable FLD measurement.

[0028] In the testing device of the sixth configuration, the diameter of the hole in the driving plate is preferably smaller than the diameter of the most distal end of the punch tip (seventh configuration).

[0029] In the seventh configuration, the tip of the punch does not slip through the hole in the drive plate from the start to the end of stretch forming, and the tip of the punch can be prevented from unexpectedly coming into contact with the steel sheet.

[0030] In the testing device of any one of the fifth to seventh configurations, a ridge may be formed on the edge of the driving plate (eighth configuration).

[0031] In the eighth configuration, the drive plate has a ridge on its edge, which increases its edge rigidity. In this case, irregular deformation of the drive plate is suppressed during bulging. This makes the deformation of the drive plate and the steel plate uniform.

[0032] In the testing device of any one of the fifth to seventh configurations, the edge of the driving plate may be bent up (ninth configuration).

[0033] In the ninth configuration, the edge of the drive plate is bent and raised, and a bent portion is provided on the edge of the drive plate, thereby increasing the rigidity of the edge of the drive plate. In this case, irregular deformation of the drive plate is suppressed during bulging. This makes the deformation of the drive plate and the steel plate uniform.

[0034] In any one of the testing devices of the fifth to ninth configurations, preferably, the cross section of the punch body is circular, and the difference between the diameter of the die hole and the diameter of the punch body is at least twice the total thickness of the steel plate and the driving plate (tenth configuration).

[0035] In the tenth configuration, the difference between the diameter of the die hole and the diameter of the punch body is at least twice the combined thickness of the steel sheet and the drive plate, so that the gap between the inner surface of the die hole and the outer surface of the punch body is at least one time the combined thickness of the steel sheet and the drive plate. In this case, contact between the steel sheet and the inner surface of the die hole is suppressed during stretch forming, and contact between the drive plate and the outer surface of the punch body is also suppressed. Therefore, it is possible to suppress an unexpected decrease in the temperature of the steel sheet and the drive plate due to heat conduction, and to suppress unexpected breakage of the steel sheet and the drive plate due to friction.

[0036] In the testing device of any one of the fifth to tenth configurations, preferably, a lubricant film is formed on at least one of the surface of the tip of the punch and the surface of the driving plate facing the punch (eleventh configuration).

[0037] In the eleventh configuration, the coefficient of friction between the punch tip and the drive plate, which are in contact with each other, is reduced by the lubricant coating, thereby enabling smooth stretch forming.

[0038] A test method according to an embodiment of the present disclosure uses the above test apparatus to measure the forming limit diagram of a steel sheet during high-temperature press forming (twelfth configuration). According to the test method of the twelfth configuration, the above test apparatus is used, so that the FLD of a steel sheet during high-temperature press forming can be stably measured.

[0039] Hereinafter, each embodiment of the present disclosure will be described with reference to the drawings. In each drawing, the same or equivalent components are denoted by the same reference numerals, and redundant description will not be repeated.

[0040] First Embodiment [Test equipment] Fig. 1 is a diagram showing a schematic configuration of a test apparatus 1 according to a first embodiment. The test apparatus 1 has an axis A extending in the vertical direction. Fig. 1 shows a longitudinal cross section including the axis A. The test apparatus 1 is used to measure the FLD of a steel plate as a test material S during hot press forming.

[0041] 1, the test apparatus 1 includes a die 2 and a punch 3. The test apparatus 1 further includes a slide 4 and a heating device 5.

[0042] The die 2 is placed at the bottom of the test apparatus 1 and is fixed to a bed (not shown). The die 2 includes a die hole 21. The die hole 21 opens to the top surface of the die 2. The cross section of the die hole 21 is circular. In this specification, the cross section means a cross section perpendicular to the axis A. The central axis of the die hole 21 coincides with the axis A of the test apparatus 1.

[0043] The punch 3 is positioned on top of the test apparatus 1 and attached to the slide 4. The punch 3 extends along the axis A of the test apparatus 1. The punch 3 has an axis that coincides with the axis A of the test apparatus 1. In other words, the punch 3 has an axis that it shares with the die hole 21. This axis corresponds to the axis A of the test apparatus 1. As the slide 4 descends, the punch 3 descends along the axis A and enters the die hole 21. In other words, the punch 3 is movable along the axis A toward the die 2.

[0044] The punch 3 includes a punch body 31 and a punch tip 32. The punch body 31 has a central axis that is the axis A of the testing apparatus 1. The central axis of the punch body 31 substantially coincides with the central axis of the die hole 21. The cross section of the punch body 31 is circular. The punch tip 32 is connected to the lower end of the punch body 31. In other words, the punch tip 32 is located on the die 2 side of the punch body 31. The punch tip 32 is ring-shaped with the axis A of the testing apparatus 1 as its central axis. The central axis of the ring-shaped punch tip 32 substantially coincides with the central axis of the die hole 21.

[0045] In this embodiment, the punch tip 32 includes a leading edge 321 and an outer edge surface 322. The punch tip 32 further includes an inner edge surface 323.

[0046] In the punch tip 32, the leading edge 321 is located closest to the die 2 and is circular and centered on the axis A. The center of the circular leading edge 321 is substantially on the central axis of the die hole 21. The outer edge surface 322 is adjacent to the radially outer side of the leading edge 321. The outer edge surface 322 is ring-shaped and centered on the axis A, and is convexly curved. The central axis of the ring-shaped outer edge surface 322 substantially coincides with the central axis of the die hole 21. The outer edge surface 322 is smoothly connected to the outer peripheral surface of the punch body 31. The inner edge surface 323 is adjacent to the radially inner side of the leading edge 321. The inner edge surface 323 is ring-shaped and centered on the axis A, and is convexly curved. The central axis of the ring-shaped inner edge surface 323 substantially coincides with the central axis of the die hole 21. The inner edge surface 323 is connected to the tip surface of the punch body 31.

[0047] More specifically, in a vertical cross-sectional view of the punch tip 32, the outer edge surface 322 is an arc of a quadrant. Similarly, the inner edge surface 323 is an arc of a quadrant. The radius of the arc of the inner edge surface 323 is the same as the radius of the arc of the outer edge surface 322. However, the radius of the arc of the inner edge surface 323 may be different from the radius of the arc of the outer edge surface 322 as long as the inner edge surface 323 smoothly connects to the outer edge surface 322 via the tip end 321. Furthermore, the outer edge surface 322 is not limited to being an arc and may be another curve (e.g., an elliptical arc). The inner edge surface 323 is not limited to being an arc and may be another curve (e.g., an elliptical arc), or may be a combination of an arc or a curve and a straight line.

[0048] The heating device 5 is disposed in the vertical center of the test apparatus 1. The heating device 5 is an electric current heating device that heats the test material S. The test material S is a steel plate used in hot press forming, such as a steel plate for hot stamping. The planar shape of the test material S is rectangular. A regular pattern (circular pattern, grid pattern, etc.) for strain measurement is drawn on the surface of the test material S.

[0049] The heating device 5 includes two electrodes 51, 52 and a power supply (not shown) connected to each of the electrodes 51, 52. The two electrodes 51, 52 are arranged at a distance from each other so as to sandwich the axis A of the testing device 1. The electrodes 51, 52 hold the side edges of the test material S. When a current is applied between the electrodes 51, 52, the test material S is heated.

[0050] In this embodiment, the difference between the diameter of the die hole 21 and the diameter of the punch body 31 is at least twice the thickness of the test material S. In other words, the gap between the inner surface of the die hole 21 and the outer surface of the punch body 31 is at least once the thickness of the test material S. There is no particular upper limit to the difference between the diameter of the die hole 21 and the diameter of the punch body 31. However, if the testing equipment has low capabilities in terms of load and stroke, the upper limit of this difference is limited according to that capability. In this case, a practical upper limit of this difference is, for example, 4.2 times the thickness of the test material S.

[0051] [Test method] In the testing method using the testing device 1 described above, the test material S is placed between the die 2 and the punch 3. The test material S is held by the electrodes 51, 52 of the heating device 5 and heated to a predetermined temperature (e.g., 800°C). After the test material S reaches the predetermined temperature, it is released from being held by the electrodes 51, 52.

[0052] After the electrodes 51, 52 release their grip, the slide 4 descends. As the slide 4 descends, the punch 3 descends along the axis A toward the die 2. As the punch 3 descends, the ring-shaped punch tip 32 comes into contact with the heated test material S. Specifically, the leading edge 321 of the punch tip 32 comes into contact with the test material S. As the punch 3 continues to descend, the punch tip 32 enters the die hole 21, and the punch body 31 then enters the die hole 21. The test material S is stretch-formed by the die hole 21 and the punch 3 that has entered the die hole 21. At this time, the leading edge 321 and the outer edge surface 322 of the punch tip 32 come into contact with the test material S. The test material S flows along the outer edge surface 322 of the punch tip 32 and is smoothly deformed.

[0053] Then, in the stretch-formed test piece S, the maximum principal strain and minimum principal strain at which a crack occurs are measured based on the deformed strain measurement pattern. Such measurements are performed on test pieces S of various sizes. The FLD is obtained from the measurement results.

[0054] [effect] In the testing apparatus 1 according to the first embodiment, the punch 3 includes a ring-shaped punch tip 32. During stretch forming, the test material S does not contact the punch 3 in the region inside the punch tip 32, but contacts the ring-shaped punch tip 32. In this case, the contact area between the punch 3 and the test material S is much smaller than in the conventional Nakajima method. Therefore, the temperature drop of the test material S is small. Furthermore, although the temperature of the test material S decreases rapidly in the region contacting the punch tip 32, the contact area between the punch 3 and the test material S is small. As a result, the temperature drop of the test material S is suppressed in the range from the shoulder to the sidewall of the punch 3, even compared to the conventional Marciniak method, and cracking of the test material S can be suppressed. Furthermore, since the region inside the punch tip 32 of the test material S does not contact the punch 3, the temperature drop is extremely small. As a result, the test material S is prone to cracking on or near the central axis of the punch 3. Therefore, the measured FLD is stable. Therefore, the FLD of steel sheets during high-temperature press forming can be stably measured.

[0055] In the first embodiment, the punch tip 32 includes a circular leading end 321 and an outer edge surface 322 adjacent to the radially outer side of the leading end 321. In this case, during stretch forming, the test material S first comes into contact with the leading end 321 of the punch tip 32, and then gradually comes into contact with the outer edge surface 322 of the punch tip 32. Because the outer edge surface 322 is convexly curved, the test material S deforms smoothly along the outer edge surface 322. Therefore, the measured FLD becomes more stable.

[0056] In the first embodiment, the difference between the diameter of the die hole 21 and the diameter of the punch body 31 is at least twice the thickness of the test material S, so the gap between the inner surface of the die hole 21 and the outer surface of the punch body 31 is at least once the thickness of the test material S. In this case, during stretch forming, contact between the test material S and the inner surface of the die hole 21 is suppressed, and contact between the test material S and the outer surface of the punch body 31 is also suppressed. This essentially ensures non-contact between the test material S and the inner surface of the die hole 21, and essentially ensures non-contact between the test material S and the outer surface of the punch body 31 during stretch forming. Therefore, it is possible to prevent the temperature of the test material S from decreasing unexpectedly due to heat conduction, and it is also possible to prevent the test material S from breaking unexpectedly due to friction.

[0057] Second Embodiment 2 is a diagram showing a schematic configuration of a test apparatus 1A according to the second embodiment. The test apparatus 1A differs from the test apparatus 1 according to the first embodiment in that it includes a blank holder 6.

[0058] Referring to FIG. 2, the testing apparatus 1A further includes a blank holder 6. The blank holder 6 is disposed between the die 2 and the punch 3. The blank holder 6 is annular and centered on the axis A of the testing apparatus 1A. The center of the annular blank holder 6 is substantially on the central axis of the die hole 21. The diameter of the inner peripheral surface of the blank holder 6 is substantially the same as the diameter of the die hole 21. The outer peripheral shape of the blank holder 6 is circular in plan view. However, the outer peripheral shape of the blank holder 6 is not limited to being circular in plan view and may be, for example, rectangular.

[0059] The wrinkle holder 6 is suspended from the slide 4 by a plurality of guide rods 61. Each guide rod 61 is movable relative to the slide 4 in the direction in which the axis A extends. Therefore, the wrinkle holder 6 is movable integrally with the guide rods 61 relative to the slide 4 in the direction in which the axis A extends.

[0060] A plurality of compression coil springs 62 are provided between the blank holder 6 and the slide 4. Each compression coil spring 62 biases the blank holder 6 toward the die 2. Instead of the compression coil springs 62, a cushion such as a hydraulic cylinder may be provided.

[0061] According to the testing apparatus 1A of the second embodiment, during stretch forming, the test material S is sandwiched between the blank holder 6 and the die 2. In this case, irregular deformation of the test material S is suppressed. This makes the deformation of the test material S uniform.

[0062] Third Embodiment 3 is a diagram showing a schematic configuration of a test apparatus 1B according to a third embodiment. The test apparatus 1B differs from the test apparatus 1 according to the first embodiment in that it includes a coating 7 of lubricant.

[0063] Referring to Fig. 3, in the testing apparatus 1B, a lubricant coating 7 is formed on the surface of the punch tip 32. The coating 7 may be formed not only on the punch tip 32 but also on the entire surface of the punch 3. The coating 7 may be formed, for example, by applying a lubricant for hot stamping. Alternatively, the coating 7 may be formed by surface treatment such as zinc plating or zinc alloy plating.

[0064] According to the testing apparatus 1B of the third embodiment, the coefficient of friction between the punch tip 32 and the test material S, which come into contact with each other, is reduced by the lubricant coating 7. This allows smooth stretch forming.

[0065] In the testing apparatus 1B shown in FIG. 3, the lubricant coating 7 is formed on the surface of the punch tip 32. However, the lubricant coating 7 may also be formed on the surface of the test material S facing the punch 3. The coating 7 may also be formed on both the surface of the punch tip 32 and the surface of the test material S facing the punch 3. In either case, the coefficient of friction between the punch tip 32 and the test material S, which come into contact with each other, is reduced.

[0066] The lubricant coating 7 of this embodiment can also be applied to the testing device 1A according to the second embodiment.

[0067] <Fourth embodiment> 4 is a diagram showing a schematic configuration of a test apparatus 1C according to a fourth embodiment. The test apparatus 1C differs from the test apparatus 1 according to the first embodiment in that it includes a drive plate 8.

[0068] Referring to FIG. 4, the testing apparatus 1C further includes a driving plate 8. The driving plate 8 is stacked on the test material S. That is, the driving plate 8 is stacked on the surface of the test material S facing the punch 3. The planar shape of the driving plate 8 is rectangular. A hole 81 is formed in the center of the driving plate 8. The cross section of the hole 81 is circular. The center of the hole 81 coincides with the axis A of the testing apparatus 1C. The center of the hole 81 is substantially on the central axis of the die hole 21. In this embodiment, the diameter of the hole 81 in the driving plate 8 is smaller than the diameter of the tip end 321 of the punch tip portion 32. However, the cross section of the hole 81 is not limited to being circular, and may be substantially circular.

[0069] The material of the drive plate 8 is not particularly limited and may be the same as or different from the material of the test material S. The tensile strength of the drive plate 8 is not particularly limited and may be the same as or different from the tensile strength of the test material S. The tensile strength of the drive plate 8 may be lower or higher than the tensile strength of the test material S. The plate thickness of the drive plate 8 is not particularly limited and may be the same as or different from the plate thickness of the test material. The plate thickness of the drive plate 8 may be smaller or larger than the plate thickness of the test material S. However, it is preferable that the test material S breaks earlier than the drive plate 8. This is because if the drive plate 8 begins to break earlier than the test material S, this may cause the test material S to crack in places other than the desired locations. Therefore, as long as the fracture limit of the drive plate 8 is higher than that of the test material S, the combination of the materials and plate thicknesses of the drive plate 8 and the test material S is arbitrary.

[0070] In this embodiment, the difference between the diameter of the die hole 21 and the diameter of the punch body 31 is at least twice the total thickness of the test material S and the drive plate 8. In other words, the gap between the inner surface of the die hole 21 and the outer surface of the punch body 31 is at least once the total thickness of the test material S and the drive plate 8. There is no particular upper limit to the difference between the diameter of the die hole 21 and the diameter of the punch body 31. However, if the testing equipment has low capabilities in terms of load and stroke, the upper limit of this difference is limited depending on that capability. In this case, a practical upper limit of this difference is, for example, 4.2 times the total thickness of the test material S and the drive plate 8.

[0071] In the testing method using the testing device 1C, the test material S is placed together with the driving plate 8 between the die 2 and the punch 3. The test material S is gripped by the electrodes 51, 52 of the heating device 5 and heated to a predetermined temperature (e.g., 800°C) together with the driving plate 8. After the test material S and the driving plate 8 reach the predetermined temperature, the electrodes 51, 52 release them from gripping.

[0072] After the electrodes 51, 52 release their grip, the slide 4 descends. The descending slide 4 causes the punch 3 to descend along the axis A toward the die 2. The descending punch 3 brings the ring-shaped punch tip 32 into contact with the heated driving plate 8. Specifically, the leading edge 321 of the punch tip 32 comes into contact with the driving plate 8. The punch 3 continues to descend, causing the punch tip 32 to enter the die hole 21, and the punch body 31 to enter the die hole 21. The die hole 21 and the punch 3 that has entered the die hole 21 bulge out the test material S and the driving plate 8. At this time, the leading edge 321 and the outer edge surface 322 of the punch tip 32 come into contact with the driving plate 8. The test material S flows along the outer edge surface 322 of the punch tip 32 together with the driving plate 8, and is smoothly deformed.

[0073] In the testing apparatus 1C according to the fourth embodiment, the punch 3 includes a ring-shaped punch tip 32. During stretch forming, the driving plate 8 does not contact the punch 3 in the region inside the punch tip 32, nor does it contact the punch 3 in the region of the hole 81; instead, it contacts the ring-shaped punch tip 32. In this case, the contact area between the punch 3 and the driving plate 8 is much smaller than in the conventional Marciniak method. Furthermore, the punch tip 32 contacts the driving plate 8 over a narrow area, and the temperature of the driving plate 8 and the test material S decreases in this narrow contact area. Therefore, the temperature decrease of the driving plate 8 and the test material S is small. This suppresses the temperature decrease of the test material S and the driving plate 8 in the region from the shoulder to the sidewall of the punch 3, even compared to the conventional Marciniak method, thereby suppressing cracking of the test material S or the driving plate 8. Furthermore, because the region inside the punch tip 32 of the test material S does not contact the punch 3, the temperature decrease is extremely small. As a result, the test material S is prone to cracking on or near the central axis of the punch 3. This stabilizes the measured FLD, making it possible to stably measure the FLD of a steel sheet during high-temperature press forming.

[0074] In the fourth embodiment, the punch tip 32 includes a circular leading edge 321 and an outer edge surface 322 adjacent to the radially outer side of the leading edge 321. In this case, during stretch forming, the driving plate 8 first comes into contact with the leading edge 321 of the punch tip 32, and then gradually comes into contact with the outer edge surface 322 of the punch tip 32. Because the outer edge surface 322 is convexly curved, the test material S deforms smoothly along the outer edge surface 322 together with the driving plate 8. Therefore, the measured FLD becomes more stable.

[0075] In the fourth embodiment, the difference between the diameter of the die hole 21 and the diameter of the punch body 31 is more than twice the total thickness of the test material S and the drive plate 8. Therefore, the gap between the inner surface of the die hole 21 and the outer surface of the punch body 31 is more than one time the total thickness of the test material S and the drive plate 8. In this case, during stretch forming, contact between the test material S and the inner surface of the die hole 21 is suppressed, and contact between the drive plate 8 and the outer surface of the punch body 31 is suppressed. As a result, during stretch forming, non-contact between the test material S and the inner surface of the die hole 21 is essentially ensured, and non-contact between the drive plate 8 and the outer surface of the punch body 31 is essentially ensured. Therefore, it is possible to suppress an unexpected decrease in the temperature of the test material S and the drive plate 8 due to heat conduction, and it is possible to suppress unexpected fracture of the test material S and the drive plate 8 due to friction.

[0076] In the fourth embodiment, the diameter of the hole 81 in the drive plate 8 is smaller than the diameter of the leading edge 321 of the punch tip 32. In this case, even if the hole 81 in the drive plate 8 is deformed so as to expand from the start to the end of the stretch forming, the punch tip 32 will not slip through the hole 81 in the drive plate 8. This makes it possible to prevent the punch tip 32 from unexpectedly coming into contact with the test material S. The ratio of the diameter of the leading edge 321 of the punch tip 32 to the diameter of the hole 81 in the drive plate 8 is preferably 1.6 or more and 2.0 or less.

[0077] Fifth Embodiment 5 is a diagram showing a schematic configuration of a testing apparatus 1D according to a fifth embodiment. The testing apparatus 1D differs from the testing apparatus 1C according to the fourth embodiment in that it includes a blank holder 6A similar to that of the second embodiment.

[0078] 5, the testing apparatus 1D further includes a wrinkle holder 6A, as in the second embodiment. As in the second embodiment, the wrinkle holder 6A is suspended from the slide 4 by a plurality of guide rods 61A. As in the second embodiment, a plurality of compression coil springs 62A are provided between the wrinkle holder 6A and the slide 4.

[0079] According to the testing apparatus 1D of the fifth embodiment, during stretch forming, the test material S and the driving plate 8 are sandwiched between the blank holder 6A and the die 2. In this case, irregular deformation of the test material S and the driving plate 8 is suppressed. This makes the deformation of the test material S uniform.

[0080] Sixth Embodiment 6 is a diagram showing a schematic configuration of a testing apparatus 1E according to a sixth embodiment. The testing apparatus 1E differs from the testing apparatus 1C according to the fourth embodiment in that it includes a lubricant coating 7A similar to that of the third embodiment.

[0081] 6, in the testing apparatus 1E, a lubricant coating 7A is formed on the surface of the punch tip 32. The coating 7A may be formed not only on the punch tip 32 but also on the entire surface of the punch 3. The coating 7A is of the same type as the lubricant coating 7 used in the third embodiment.

[0082] According to the testing apparatus 1E of the sixth embodiment, the coefficient of friction between the punch tip 32 and the driving plate 8, which come into contact with each other, is reduced by the lubricant coating 7A, thereby enabling smooth stretch forming.

[0083] In the testing apparatus 1E shown in Fig. 6, the lubricant coating 7A is formed on the surface of the punch tip 32. However, the lubricant coating 7A may also be formed on the surface of the drive plate 8 facing the punch 3. The coating 7A may also be formed on both the surface of the punch tip 32 and the surface of the drive plate 8 facing the punch 3. In either case, the coefficient of friction between the punch tip 32 and the drive plate 8, which come into contact with each other, is reduced.

[0084] The lubricant coating 7A of this embodiment can also be applied to the testing device 1D according to the fifth embodiment.

[0085] Seventh Embodiment 7 is a diagram showing a schematic configuration of a test apparatus 1F according to the seventh embodiment. The test apparatus 1F differs from the test apparatus 1C according to the fourth embodiment in the configuration of a drive plate 8A.

[0086] 7, in the testing apparatus 1F, a protruding rib 82 is formed on the edge of the driving plate 8A. The protruding rib 82 is a portion of the driving plate 8 that protrudes toward the punch 3. The protruding rib 82 extends along the edge of the driving plate 8A.

[0087] When the planar shape of the drive plate 8A is rectangular, the ridges 82 may be provided on all four edges of the drive plate 8A, or on two opposing edges, but it is sufficient that the ridges 82 are provided on at least one edge of the drive plate 8A.

[0088] In the testing apparatus 1F according to the seventh embodiment, the ridges 82 are provided on the edge of the drive plate 8A, which increases the rigidity of the edge of the drive plate 8A. In this case, the shape of the edge of the drive plate 8A is maintained during bulging, and irregular deformation of the drive plate 8A is suppressed. This makes the deformation of the drive plate 8A and the test material S uniform.

[0089] The driving plate 8A of this embodiment can also be applied to the testing apparatus 1D according to the fifth embodiment. In this case, a groove corresponding to the ridge 82 is formed on the lower surface of the blank holder 6A (specifically, the surface on the driving plate 8A side). During bulging, the groove engages with the ridge 82. This better maintains the shape of the edge of the driving plate 8A and further suppresses irregular deformation of the driving plate 8A.

[0090] Furthermore, the driving plate 8A of this embodiment can also be applied to the testing apparatus 1E of the sixth embodiment.

[0091] Eighth Embodiment 8 is a diagram showing a schematic configuration of a test apparatus 1G according to the eighth embodiment. The test apparatus 1G differs from the test apparatus 1C according to the fourth embodiment in the configuration of a drive plate 8B.

[0092] 8, in the testing apparatus 1G, the edge of the drive plate 8B is bent and raised. This bent and raised portion 83 is a portion of the drive plate 8B that is bent and raised toward the punch 3. The bent and raised portion 83 extends along the edge of the drive plate 8B.

[0093] When the planar shape of drive plate 8B is rectangular, bent-up portions 83 may be provided on all four edges of drive plate 8B, or on two opposing edges, as long as bent-up portions 83 are provided on at least one edge of drive plate 8B.

[0094] In the testing apparatus 1G according to the eighth embodiment, the edge of the drive plate 8B is bent up and has a bent-up portion 83, which increases the rigidity of the edge of the drive plate 8B. In this case, the shape of the edge of the drive plate 8B is maintained during bulging, and irregular deformation of the drive plate 8B is suppressed. This makes the deformation of the drive plate 8B and the test material S uniform.

[0095] The driving plate 8B of this embodiment can also be applied to the testing apparatus 1D according to the fifth embodiment. In this case, during bulging, the blank holder 6A comes into contact with the driving plate 8B, and then the bent-up portion 83 abuts against the outer circumferential surface of the blank holder 6A. This helps to maintain the shape of the edge of the driving plate 8B and further suppresses irregular deformation of the driving plate 8B.

[0096] Furthermore, the driving plate 8B of this embodiment can also be applied to the testing apparatus 1E according to the sixth embodiment.

[0097] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0098] In each of the above embodiments, an electric heating device is used as the heating device 5 for heating the test material S. However, the heating device 5 may be a heating device of another type. Also, the test material S may be heated outside the test device 1. [Explanation of symbols]

[0099] 1, 1A, 1B, 1C, 1D, 1E, 1F, 1G: Test equipment A: Axis S: Test material 2: Dice 21: Die hole 3: Punch 31: Punch body 32: Punch tip 321: Cutting edge 322: Outer surface 4: Slide 5:Heating device 51,52: Electrode 6,6A: Wrinkle holder 7, 7A: Lubricant coating 8, 8A, 8B: Drive plate 81: Hole

Claims

1. A test device for measuring the forming limit diagram of a steel sheet during high-temperature press forming, a die having a die hole with a circular cross section; a punch having an axis shared with the die hole and movable along the axis toward the die; a heating device that heats the steel plate before the punch starts to move, The punch is a punch body having the shaft as a central axis; a ring-shaped punch tip portion located on the die side of the punch body portion and having the axis as its central axis.

2. 2. The test device according to claim 1, The punch tip is a tip end closest to the die, the tip end being circular with the axis as a center axis; an outer edge surface adjacent to the radially outer side of the foremost tip, the outer edge surface being ring-shaped and convexly curved around the axis.

3. 3. The test device according to claim 1 or 2, The punch body has a circular cross section, a difference between the diameter of the die hole and the diameter of the punch body being at least twice the thickness of the steel plate.

4. The test device according to any one of claims 1 to 3, A testing device in which a lubricant coating is formed on at least one of the surface of the tip of the punch and the surface of the steel plate facing the punch.

5. 10. The test device according to claim 1, further comprising: a drive plate stacked on a surface of the steel plate facing the punch, the drive plate having a hole centered on the axis; The heating device heats the steel plate together with the driving plate.

6. 6. The test device according to claim 5, The punch tip is a tip end closest to the die, the tip end being circular with the axis as a center axis; an outer edge surface adjacent to the radially outer side of the foremost tip, the outer edge surface being ring-shaped and convexly curved around the axis.

7. 7. The test device according to claim 6, A testing device, wherein the diameter of the hole in the drive plate is smaller than the diameter of the leading edge of the punch tip.

8. The test device according to any one of claims 5 to 7, A testing device, wherein the driving plate has a ridge formed on the edge thereof.

9. The test device according to any one of claims 5 to 7, A test device wherein the edges of the drive plate are bent up.

10. The test device according to any one of claims 5 to 9, The punch body has a circular cross section, a difference between the diameter of the die hole and the diameter of the punch body is at least twice the total thickness of the steel plate and the drive plate.

11. The test device according to any one of claims 5 to 10, a lubricant coating is formed on at least one of the surface of the tip of the punch and the surface of the drive plate facing the punch;

12. A test method for measuring a forming limit diagram of a steel sheet during high-temperature press forming using the test device according to any one of claims 1 to 11.

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