Method and apparatus for evaluating fracture limit

Abstract

To provide a method capable of efficiently evaluating a fracture limit (a limit value of an allowable plate thickness) of a bead passing part which cannot be evaluated by a uniaxial tensile test or the like.SOLUTION: A fracture limit evaluation method of the present invention includes a pulling step of pulling a test piece (T) made of a strip-shaped plate material having a width decreasing from a front side to a rear side relatively forward with respect to a dummy die (1) in a state where the test piece is sandwiched by the dummy die simulating a draw bead of a press forming die to fracture the test piece, and a measurement step of measuring a limit thickness which is a thickness of the test piece in the vicinity of a fracture portion (uniformly stretched portion). By using the limit thickness thus obtained, the maximum sheet thickness reduction rate obtained from the limit thickness and the initial sheet thickness, or the like, it is possible to accurately evaluate whether or not a forming defect (crack or the like) occurs in the sheet material that has passed through the draw bead (bead passing portion).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating a fracture limit in a bead portion of a press-formed plate material. [Background technology]

[0002] Press-formed products (also simply called "formed products"), which are often used in automobiles, home appliances, and other products, are obtained by applying pressure to a metal plate (usually a flat plate) using a forming mold (usually a metal die) to cause plastic deformation. When designing a forming mold, not only the forming portion (cavity) but also the drawbeads disposed around it are important to prevent molding defects (cracks, wrinkles, scoring (scratches), etc.). By appropriately setting the shape, gap, pressing force, etc. of the drawbead, it is possible to control the flow of the plate material being formed and adjust the tension acting on the plate material.

[0003] When designing a forming die or developing a forming process, forming simulations are generally performed in advance using CAE (Computer Aided Engineering). In this case, the characteristics of the sheet material (mechanical properties, fracture limit, etc.) are required to determine whether the sheet material can be formed without cracks or other defects. The following documents contain information related to the evaluation of such sheet material characteristics: [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2024-17821 [Non-patent literature]

[0005] [Non-Patent Document 1] "Over five-times improved elongation-tofractureof dual-phase 1180 steel by continuous-bendingunder-tension",Materials & Design Volume 161, 5 January 2019,Pages 95-105 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 evaluates the forming limit (the point at which local necking occurs in the thickness direction) at the top of the stretch-bent part.

[0007] Non-Patent Document 1 reports that the breaking elongation of a dual-phase stainless steel sheet can be improved by repeatedly stretching and bending it back and forth between three rollers.

[0008] None of the documents focuses on the characteristics (fracture limit) of the plate material passing through the drawbead, and no specific evaluation or suggestion related thereto is made.

[0009] When actually press-forming, the plate thickness at which cracks or other defects occur varies depending on the forming location, forming process (processing history), etc. For this reason, accurate evaluation based on actual forming cannot be performed in forming simulations using only the uniform plate material properties obtained from uniaxial tensile tests.

[0010] The present invention has been made in consideration of the above circumstances, and aims to provide a new method etc. that can efficiently evaluate the characteristics (fracture limit) of a plate material passing through a drawbead. [Means for solving the problem]

[0011] As a result of intensive research, the inventors have newly discovered that by clamping a plate material whose width has been continuously reduced (monotonically reduced) in a mold simulating a drawbead while pulling it out, it is possible to efficiently and accurately evaluate the limit value (minimum value / fracture limit) of the plate thickness allowable for the plate material that has passed through the drawbead (bead passing portion).By expanding on this result, the present invention, which will be described below, has been completed.

[0012] <<Fracture limit evaluation method>> (1) The present invention is a method for evaluating the fracture limit of a plate material passing through a drawbead, the method comprising: a tensioning step in which a test piece made of a strip-shaped plate material whose width decreases from the front to the rear is clamped between a mock mold that imitates a drawbead of a press forming die, and the test piece is pulled forward relative to the mock mold to fracture the test piece; and a measurement step in which a critical thickness, which is the thickness of the test piece near the fractured portion, is measured, and the fracture limit of the plate material passing through the drawbead is evaluated based on the critical thickness.

[0013] According to the fracture limit evaluation method of the present invention (simply referred to as the "evaluation method"), it is possible to efficiently evaluate the limit value of the plate thickness (or the maximum plate thickness reduction rate / fracture limit) that is permissible for a plate material that has passed through a drawbead (also simply referred to as a "bead"). This makes it possible to appropriately design forming dies, develop forming processes, and perform forming simulations in advance in accordance with actual press forming. The mechanism by which such effects are obtained is thought to be as follows.

[0014] As the test piece passes through the dummy mold, it elongates and thins due to (tensile) bending back deformation. Here, the test piece according to the present invention has a reduced width (length perpendicular to the drawing direction and thickness direction) along the direction from the widening side (front side) to the narrowing side (rear side) (referred to as the "drawing direction"). When this test piece is moved from the widening side to the narrowing side relative to the dummy mold (i.e., pulled out), the (tensile) stress acting on the test piece increases, and the thickness of the test piece also decreases accordingly, until the test piece breaks (tension process). The thickness near the fracture point (widening side / front side) is the limit value of the plate thickness allowable during forming (limit thickness: t).

[0015] The critical thickness may be the fracture limit, or the (maximum) thickness reduction amount (Δt=t0-t) or the (maximum) thickness reduction rate (Δt / t0 or its percentage) based on the initial thickness (t0) of the test piece may be the fracture limit (evaluation step). In this specification, the maximum thickness reduction rate is appropriately referred to as the fracture limit.

[0016] Incidentally, a test piece usually breaks after passing through the simulated mold due to diffuse necking (reduced widthwise) or localized necking (reduced thicknesswise). The critical thickness is preferably measured at a location where there is no such necking and where a substantially uniform elongation occurs (called a uniform elongation region). The uniform elongation region is in a plane strain state (a state where there is almost no strain in the direction perpendicular to the tensile direction within the sheet surface) and reflects the bead passage region of an actually formed sheet material.

[0017] Thus, according to the present invention, by appropriately selecting and adjusting the simulation mold (draw bead shape, gap, pressing force, etc.) and test piece (material, plate thickness, plate width change, length, tension, etc.) in accordance with the actual press forming that is envisioned, the fracture limit (limit thickness, maximum plate thickness reduction rate, etc.) of the plate material passing through the draw bead can be easily and efficiently evaluated simply by pulling the test piece out once in one direction relative to the simulation mold.

[0018] Destruction limit evaluation device The present invention can also be understood as an apparatus (evaluation apparatus, test apparatus) for carrying out the above-described evaluation method. For example, the present invention may be an evaluation apparatus including a simulation mold simulating a drawbead of a press-forming mold, a clamping mechanism that clamps a test piece made of a strip-shaped plate material whose width decreases from the front to the rear between the simulation mold, and a tensioning mechanism that moves the test piece forward relative to the simulation mold.

[0019] "others" (1) In the present invention, "fracture" includes not only a state in which the test piece is completely separated, but also a state in which defects such as cracks occur, and a state in which signs of fracture (divergent necking, local necking, etc.) appear. As long as a meaningful critical thickness is measured, it is not necessarily necessary to continue evaluation (testing) until the test piece is completely separated.

[0020] (2) In this specification, the direction of movement of the test piece (plate material) relative to the dummy mold is referred to as the front-rear direction, the direction in which the test piece is clamped (held) by the dummy mold or the thickness direction is referred to as the up-down direction, and the width direction of the test piece is referred to as the left-right direction. In addition, the forward direction in which the test piece moves relative to the dummy mold is referred to as the withdrawal direction.

[0021] (3) Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower or upper limit value to create a new range such as "a to b." [Brief explanation of the drawings]

[0022] [Figure 1] 1A and 1B are a front view and a cross-sectional view schematically showing a test device and a test piece for evaluation of a fracture limit. [Figure 2] 1 is a flowchart and a schematic diagram showing the evaluation process. [Figure 3] FIG. 1 is a schematic diagram showing the displacement of a test piece and the change in its plate thickness. DETAILED DESCRIPTION OF THE INVENTION

[0023] The contents described in this specification may apply not only to the evaluation method, but also to the evaluation device and test piece. One or more components arbitrarily selected from this specification may be added to the components of the present invention. Components related to the method may also be components related to the product. Which embodiment is best depends on the target, required performance, etc.

[0024] Test piece The test piece is made of a strip-shaped plate material and has a width-reducing portion (test area) where the plate width decreases from one side to the other. The width-reducing portion may be provided along the entire length of the test piece, or may be provided only at the center or one end of the test piece. In other words, the end of the test piece to be gripped does not necessarily have to have a reduced width.

[0025] The rate of reduction (shape, trend) of the plate width in the width-reducing portion may be linear (linear function), curvilinear (power function, exponential function, etc.), or a combination of these. The length of the width-reducing portion and the rate of reduction of the plate width may be adjusted to cause the test specimen to break in the desired region. It is preferable that the plate width of the test specimen be reduced gently (gradually) in a continuous or monotonous manner. However, the plate width of the test specimen may be reduced in stages as long as it does not have a significant effect on the evaluation of the fracture limit (thickness limit).

[0026] The material and form (shape, size) of the test piece are appropriately selected depending on the intended press forming. The material of the test piece (plate material) is made of metal, for example, iron (alloy) such as steel, aluminum (alloy), magnesium (alloy), titanium (alloy), etc. The test piece is made of, for example, a rectangular flat plate (thin plate).

[0027] Depending on the type of press forming envisaged (cold forming, hot forming, hot stamping, etc.), the test specimens may be left cold or may be heated to a desired temperature and evaluated (tested).

[0028] 《Mock type》 The mock mold has convex portions (protrusions) and / or concave portions (grooves) that mimic the drawbeads provided on a press mold (e.g., a blank holder). The form (shape, size), number (number of threads), spacing, etc. of the convex portions and concave portions are adjusted appropriately depending on the assumed mold (molded product) and molding process.

[0029] The cross-sectional shape of the convex portions and concave portions provided on the simulation mold may be, for example, any of arc-shaped, oval (elliptical), polygonal (triangular, trapezoidal, etc.) shapes. The convex portions and concave portions may be one line (single bead), two lines (double bead), or three or more lines. The convex portions and concave portions may extend linearly in the plate width direction of the test piece, or may extend in a curved or broken line.

[0030] The dummy mold usually includes an upper mold and a lower mold that sandwich the upper surface (front surface) and the lower surface (back surface) of the test piece from above and below. The convex and concave portions simulating the drawbead may be provided in the upper and lower molds in a mating manner, or may be provided in only one of the upper and lower molds. For example, a convex portion provided in one mold may be opposed to a flat surface of the other mold.

[0031] "Device" The forward tensioning mechanism pulls the test specimen toward the front side (widening side) of the dummy mold. A rear tensioning mechanism may be provided to apply tension (rear tension) to the test specimen also on the rear side (contracting side) of the dummy mold. The forward tensioning mechanism and rear tensioning mechanism do not have to be arranged in a straight line. The driving source of the tensioning mechanism is, for example, hydraulic or electric power. The rear tensioning mechanism is not limited to one that generates a constant tension (resistance force), and may be a damping device such as a hydraulic damper.

[0032] The tension mechanism includes a mechanism for gripping the ends of the test piece (front gripping mechanism, rear gripping mechanism). The gripping mechanism may be a detachable chuck or screw, or may be replaced by a joint such as welding.

[0033] The clamping mechanism applies a pressing force or a holding force to the dummy mold (upper mold and / or lower mold) to clamp the test piece between the dummy molds. The control of the clamping mechanism may be position control, which adjusts the clearance between the dummy molds (upper mold and lower mold) and the test piece, or pressure control, which adjusts the pressure the dummy molds apply to the test piece. The driving source of the clamping mechanism is, for example, hydraulic or electric power. The clearance (gap) may be adjusted, for example, by disposing a distance block in the dummy mold.

[0034] "evaluation" In the tensile process, the test specimen is pulled out by moving it forward relative to the mock-up mold until it breaks. The speed of movement (pulling speed) of the test specimen or the tension of the test specimen may be constant or may be changed continuously or stepwise. The displacement (movement) of the test specimen and the load (tension) acting on the test specimen are measured, for example, using a laser displacement meter, a load cell, etc.

[0035] In the measurement process, the thickness of the broken test piece is measured near the break (critical thickness). The thickness measurement is preferably performed, for example, in a uniformly elongated portion where there is no diffuse or localized necking (more specifically, near the center in the width direction). The test piece usually breaks near the exit of the simulated mold. The thickness measurement is preferably performed in a uniformly elongated portion located forward of the break.

[0036] The thickness may be measured at one location or at multiple locations. When measurements are taken at multiple locations, the arithmetic mean value of the measured values ​​may be used as the critical thickness. The thickness is measured, for example, with a micrometer. [Example]

[0037] The present invention will be specifically described with reference to an example of evaluation of the fracture limit at the bead passage portion of a press-formed plate material.

[0038] [Device] The front view and cross-sectional view of the test device S (simply referred to as "device S" / evaluation device) used to evaluate the fracture limit are shown in Figure 1. For ease of explanation, the directions of the arrows shown in the figure are assumed to be the front-to-back direction, up-to-down direction, width direction, or pull-out direction.

[0039] The device S is installed on a base B, and includes a simulation mold 1, a clamping mechanism 2, a forward pulling mechanism 31, and a backward pulling mechanism 32 (together referred to as the "pulling mechanism 3").

[0040] The simulation mold 1 includes an upper mold 11 and a lower mold 12 that simulate drawbeads provided in an assumed press mold, and a distance block 13 .

[0041] The upper mold 11 has convex portions 111 and concave portions 112 formed alternately on the lower surface side of the substrate 110. The convex portions 111 have a substantially arc-shaped cross section and extend linearly in the width direction (the direction perpendicular to the paper surface of FIG. 1). The concave portions 112 are located on both sides of the convex portions 111 and have flat bottom surfaces that extend parallel to the convex portions 111.

[0042] The lower mold 12 has convex portions 121 and concave portions 122 formed alternately on the upper surface side of the substrate 120. The convex portions 121 and the concave portions 122 have the same shapes as the convex portions 111 and the concave portions 112, respectively.

[0043] The two convex portions 111 face two concave portions 122, respectively, and the three convex portions 121 face three concave portions 112, respectively. In this way, a fitting double drawbead is formed between the opposing upper mold 11 and lower mold 12.

[0044] The distance blocks 13 consist of four cylindrical bodies that stand upright from the four corners of the substrate 120. All of the distance blocks 13 are the same height. The four corners of the lower surface of the substrate 110 abut against the upper end surfaces of the distance blocks 13, and the distance (clearance) between the upper mold 11 and the lower mold 12 is regulated to a constant value.

[0045] The clamping mechanism 2 includes an actuator 21, a rod 22 connecting the actuator 21 and the upper die 11, and a load cell 23 disposed on the rod 22. The pressing force acting on the upper die 11 is measured by the load cell 23, and the actuator 21 is controlled based on the measurement value.

[0046] The forward tensioning mechanism 31 includes an actuator 311, a rod 312, a load cell 313, and a chuck 314. The chuck 314 (front gripping mechanism) detachably grips the front end of the test piece T. The rod 312 connects the actuator 311 and the chuck 314, and the load cell 313 is disposed on the rod 312. The load cell 313 measures the tension that pulls the test piece T forward, and the actuator 311 is controlled based on the measured value.

[0047] The rear tensioning mechanism 32 includes an actuator 321, a rod 322, a load cell 323, and a chuck 324. The rear end of the test piece T is detachably gripped by the chuck 324 (rear gripping mechanism). The rod 322 connects the actuator 321 and the chuck 324, and the load cell 323 is disposed on the rod 322. The rear tension applied to the test piece T is measured by the load cell 323, and the actuator 321 is controlled based on the measured value. In this example, the rod 312 of the front tensioning mechanism 31 and the rod 322 of the rear tensioning mechanism 32 are disposed in a substantially straight line.

[0048] Although hydraulic actuators are used for each actuator, electric (servo) actuators, screw drive actuators, electromechanical actuators, etc. may also be used.

[0049] [Test piece] As shown in Figure 1, the test piece T is a strip-shaped piece made from a flat metal plate (such as a mild steel plate) used in press forming. The width of the test piece T decreases gradually and symmetrically from the front end to the rear end. The thickness of the test piece T before evaluation (initial thickness: t0) is uniform throughout.

[0050] [Testing / Evaluation] Using the device S and the test piece T, a test to evaluate the fracture limit at the bead passage part of the plate material was carried out according to the flowchart shown in Figure 2.

[0051] In step S1, the front end and rear end of the test piece T disposed between the upper mold 11 and the lower mold 12 are gripped by the chuck 314 and the chuck 324, respectively (setting step).

[0052] In step S2, the actuator 21 is operated to press down the upper mold 11, and the front region of the test piece T is clamped between the upper mold 11 and the lower mold 12 on the base B. At this time, the distance between the upper mold 11 and the lower mold 12 is kept constant by the distance block 13. A double bead is formed on the test piece T by the convex portion 111 and the concave portion 112 of the upper mold 11 and the convex portion 121 and the concave portion 122 of the lower mold 12 (bead forming process).

[0053] In step S3, the actuator 311 is operated to pull the test piece T held between the upper mold 11 and the lower mold 12 forward (pulling direction) (pulling process / tension process). The pulling process continues until the test piece T breaks.

[0054] At this time, the actuator 321 may be operated to apply a rear tension to the test piece T. The output (driving force) or displacement (speed) of the actuators 21, 311, and 321 is controlled based on signals obtained from the load cells 23, 313, and 323, respectively.

[0055] In step S4, when a fracture (or a sign thereof) is observed in the pulled-out test piece T, the pulling-out of the test piece T is stopped.

[0056] In step S5, the thickness (limit thickness) of the test piece T is measured with a micrometer or the like in the uniform elongation portion (area where no diffuse necking or the like is observed) in the front region near the fracture portion of the test piece T (measurement process).

[0057] Figure 3 shows the state of test piece T before and during its extraction. Figure 3 also shows a schematic representation of the change in thickness of test piece T that occurs during its extraction. The extracted test piece T (initial thickness: t0) is bent and stretched as it passes between the upper die 11 and the lower die 12, and gradually becomes thinner up to the exit of the simulation die 1. The thickness (t) near the exit becomes smaller the further back the test piece T is.

[0058] [Experimental Example] The above-mentioned test was actually carried out under the following conditions. The maximum thickness reduction rates obtained as a result are shown in Table 1.

[0059] (1) Conditions A mild steel plate with an initial thickness of 0.65 mm (equivalent to a tensile strength of 270 MPa) was used for the test piece T. The test piece T had a total length of 700 mm, and its width decreased linearly from the front end (width 110 mm) to the rear end (width 30 mm).

[0060] The clearance between the upper mold 11 and the lower mold 12 (the distance between the apex of the convex portion and the bottom surface of the concave portion) was set to 0.70 mm or 0.85 mm.

[0061] The rear tension applied to the test piece T was set to 3.2 kN, 3.7 kN, or 4.2 kN. In each case, the chuck 314 holding the front end of the test piece T was moved forward by 200 mm at a pulling speed of 10 mm / s (pulling process).

[0062] The thickness (t) of test piece T was measured near the center of the area where no diffusion necking occurred in the bead passage (measurement process). The (maximum) thickness reduction rate (100 × (t0 - t) / t0) was calculated from the ratio of the thickness (t) to the initial thickness (t0) (evaluation process).

[0063] (2) Evaluation As can be seen from Table 1, the thickness reduction rate in the region that passed through the drawbead (referred to as the "bead passing zone") was much greater than the thickness reduction rate (approximately 22%) in the uniform elongation zone obtained in the uniaxial tensile test. In other words, it was confirmed that even if the bead passing zone became much thinner than other parts (portions that did not pass through the drawbead), no cracks or the like would occur.

[0064] As can be seen from Table 1, although there were cases where no fracture occurred within the above-mentioned travel distance (≦200 mm) due to differences in clearance and rear tension, the reduction in thickness in the bead passage area exceeded 30% in all cases. These results were consistent with the results of measuring the thickness of actual press-formed products.

[0065] From the above, it has been confirmed that the present invention can appropriately and efficiently evaluate the fracture limit of the bead passage portion (the maximum thickness reduction rate in the uniform elongation portion near the fracture location), which cannot be evaluated by uniaxial tensile tests, etc.

[0066] [Table 1] [Explanation of symbols]

[0067] S Test Equipment T test piece 1 Mock type 2 Clamping mechanism 3. Tension mechanism

Claims

1. a tensile process in which a test piece made of a band-shaped plate material whose width decreases from the front to the rear is sandwiched between a mock mold simulating a drawbead of a press forming die, and the test piece is pulled forward relative to the mock mold to break the test piece; a measuring step of measuring a critical thickness, which is a thickness of the test piece in the vicinity of the broken portion, A method for evaluating a fracture limit of a plate material passing through the drawbead based on the limit thickness.

2. An evaluation step of calculating a maximum thickness reduction rate allowable when forming the plate material with the press mold from the initial plate thickness and the limit thickness of the test piece, The fracture limit evaluation method according to claim 1, wherein the maximum thickness reduction rate is defined as the fracture limit.

3. The fracture limit evaluation method according to claim 1 , wherein the tension step is performed while applying a rear tension to the test piece.

4. a mock-up mold that mimics the draw bead of a press forming mold; a clamping mechanism that clamps a test piece made of a strip-shaped plate material whose width decreases from the front to the rear between the simulation molds; a tensioning mechanism for moving the test piece forward relative to the simulation mold; An apparatus used in the fracture limit evaluation method according to claim 1 or 3.

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