JP4954919B2 - Flange-up molding test method - Google Patents

Description

  The present invention relates to a test method for selecting an optimal material in order to avoid stretch flange breakage during press molding.

  In recent years, in the automobile industry, the development of a vehicle body structure that can reduce injury to passengers during a collision has become an urgent issue. On the other hand, reducing the weight of the vehicle body is also important for improving fuel efficiency. In order to solve these problems, application of high-strength steel sheets is being studied for higher-strength materials, particularly steel materials. However, it is generally considered that an increase in strength causes deterioration of moldability, and it is important to improve the moldability, particularly to avoid breakage of the stretch flange formed portion, in order to expand the application.

  In order to solve such a problem, development of a material excellent in stretch flangeability is underway. Patent Document 1 discloses a material having improved stretch flangeability by controlling a microstructure such as ferrite and bainite. Patent Document 2 discloses an aluminum alloy plate having excellent stretch flangeability by defining plastic anisotropy and uniform elongation in a tensile test in a specific direction.

  However, it has been pointed out that stretch flangeability in actual parts is not directly correlated with the material properties evaluated in the usual tensile tests. The reason is considered to be because the fracture occurs from the end face of the flange, unlike the tensile test in which the fracture occurs due to necking at the center of the test piece. In order to simulate this situation, a hole expansion test is performed as shown in Non-Patent Document 1. Further, as shown in Patent Document 3, stretch flange formability is evaluated by performing a tensile test after adding an arc-shaped notch to the center of a test piece. However, in the evaluation by these test methods, the deformation state did not necessarily coincide with the actual deformation state of the stretch flange formed part.

JP 2002-60898 A JP 2006-257506 A JP 2005-98720 A “Press Forming Difficulty Handbook 2nd Edition” p. 470

  The present invention provides a test method for accurately evaluating the stretch flangeability of a material among the problems during press molding.

  The present inventors have observed in detail the part where the actual stretch flange breakage occurs, the deformation at that part occurs out of the plane of the material plate surface, and the deformation gradient occurs in the direction along the end face at the breakage risk part. I found out. The present inventors have found that such a deformation gradient affects the stretch flangeability of the material, and devised a test method for accurately evaluating the stretch flangeability by conducting a test simulating this in advance. It is. The gist of the present invention is as follows.

(1) Using a test piece having an arc-shaped shear end face, subjecting the test piece to out-of-plane deformation in the normal direction, and the forming height at which the shear end face breaks, A flange-up molding test method characterized by performing an evaluation.

  (2) The flange-up molding test method according to (1), wherein a test piece in which an arc-shaped shear end face is formed using a circular punch is used.

  (3) The flange-up molding test method as described in (1) or (2) above, wherein an out-of-plane deformation is applied by a punch, die, or wrinkle presser.

  (4) The flange-up molding test method according to (3), wherein the punch is a cylindrical punch.

  By performing the flange-up test method based on the present invention, stretch flange formability can be tested with high accuracy in a situation close to that of an actual part. As a result, the production cost is reduced through the improvement of productivity by providing the optimum material.

  The present inventors first investigated in detail the deformation state at the stretch flange fracture site in the actual part. As a result, it was found that the plate thickness reduction rate and the strain amount had a peak along the end face where cracking actually occurred, and those values decreased around the peak. That is, in a part, it is considered that after deformation concentrates in a certain area, the deformation is further localized in that area and eventually breaks.

  Until now, the results of the hole expansion molding test have been used as an index of stretch flangeability. A blank with a hole is spread with a conical or cylindrical punch, and the ratio of the initial hole diameter to the hole diameter when fracture occurs (called the λ value) is calculated to determine whether the stretch flangeability is good or bad. ing. In the case of the hole expansion molding test, the amount of deformation generated at the hole end is substantially uniform in the circumferential direction. Therefore, the deformation gradient in the direction along the hole edge surface is very small.

  As described above, the deformation of the end face of the material is not uniform in the stretch flange forming part of the actual part. In the case of a relatively soft material with sufficient local deformability such as mild steel sheet, there is a relatively good correspondence between the evaluation result of the hole expansion test and the stretch flangeability of the actual part. There is a problem that the correlation between the two is small in the high-strength steel plate which is being applied.

  As a result of intensive studies, the present inventors have come to realize that the difference between work hardening ability and local deformation ability of the material contributes differently in the hole expansion test and the stretch flange molding of the actual part. In the stretch flange deformed part of the actual part, the distribution of the deformation along the end face of the material is non-uniform and has a peak in the fracture risk part. Even if the integrated amount of deformation along the end face is the same, it does not break when this peak height is small. It is the work hardening ability of the material that dominates the distribution of deformation along the end face, the characteristic value normally evaluated by the n value of the tensile test or the uniform elongation. In the case of a material with high work hardening ability, the deformation applied to the peak is propagated to the surroundings, and as a result, the peak height is reduced and breakage is avoided. On the other hand, in the case of the hole expansion test, there is no distribution of deformation in the direction along the hole edge. Therefore, changing the work hardening ability does not affect the deformation distribution in the direction along the hole edge. Therefore, in the case of the hole expansion test, the local deformability is evaluated.

  In general, when considering steel sheet materials, high-strength materials of 590 MPa or higher do not have both work hardening ability and local deformation ability (usually evaluated as local elongation in tensile tests) due to the nature of the method used to strengthen the material. It is known. As described above, the hole expansion test mainly evaluates local deformability, but in actual stretch flange molding, the balance between work hardening ability and local deformability is important.

  Therefore, the present inventors sought a test method in a state where the deformation along the end face is not uniform, and found a test method having a good correlation with stretch flange formability in an actual part. First, as a test piece, a shear end face is provided in the same manner as the stretch flange forming portion in actual press forming, and a deformation having a peak at the center of the end face is given to the end face. At this time, the test piece breaks corresponding to the molding ability of the material. The molding height at this time is used as an evaluation item in the test. The higher the molding height, the better the stretch flange formability. Although the molding height correlates with local deformability, it is affected by work hardening ability at the same time because it is non-uniform deformation along the end face. This state is close to the deformed state in actual molding. Giving a deformation having a peak at the center of the end face is achieved by applying an out-of-plane deformation in the normal direction of the specimen. In order to efficiently concentrate the deformation on the end face, it is preferable that the punch is moved in the normal direction of the test piece after the test piece is constrained by a die and a wrinkle presser. In consideration of conditions in actual molding, it is preferable to use a cylindrical punch, but when considering special molding, a punch shape such as a cone, a spherical head, or an elliptical cross section may be used.

  In addition, as a method of creating a shear end face on a specimen, a circular shear punch should be used considering the convenience of efficiently changing the clearance (gap between the shear punch and the die) that governs the properties of the shear surface. Is preferred. In the present invention, in order to give nonuniform deformation to the shear end face, the end face after shearing needs to have an open cross section, but when a hole is provided by using a circular punch, the base plate is made according to the test purpose. It can be set as the test piece of the test method of this invention by shearing. As will be described later, by changing the shear angle, nonuniform deformation along the end face can be controlled, and breakage other than at the end face can be avoided. In the present invention, a test piece having a sheared end surface by a mold widely used in actual processing is basically used, but the end surface is processed by cutting, laser, water jet, or the like according to actual conditions. May be.

  The maximum flange-up height Hmax during the test is the maximum molding height that can be given by the punch when it is assumed that there is no material breakage, but this can be set to various values by changing the shape of the tool or specimen. it can. Although it is necessary to select appropriately so that the difference in forming ability between the materials to be tested becomes clear, when the punch diameter is D, the range may be 0.10 ≦ Hmax / D ≦ 0.40. preferable. In the case of steel materials, it is preferable that the range of 0.20 ≦ Hmax / D ≦ 0.30 can be efficiently tested from the relationship with work hardening characteristics.

[Example 1]
The technical contents of the present invention will be described below with examples.

  Table 1 shows the materials (test pieces) used in this example. As materials, various materials (steel A to H) ranging from mild steel to high strength steel of 980 MPa class were used. The plate thickness is all 1.2 mm. This table also shows the hole expansion value (λ value). The λ value is the diameter of the hole when a hole of φ10 mm is formed in the center of the test piece by shearing and then widened using a 60 ° conical punch, and the end surface is ruptured (thickness penetration crack). It is a percentage obtained by measuring and dividing the difference from the initial hole diameter by the initial hole diameter. The λ value is usually used as an index of stretch flange formability, and it is considered that the higher the value, the better the flange flange formability.

  In order to evaluate the stretch flangeability of this material, a molding test was performed with a shape close to that of an actual part. The mold shape used in the experiment has two corner radii of 100 mm and 150 mm, and the mold (punch) shape is shown in FIG. Moreover, a blank shape is shown to Fig.2 (a), (b). FIG. 2A shows a blank shape having shear surfaces of R75 mm and R125 mm, and FIG. 2B shows a blank shape having shear surfaces of R100 mm and R150 mm. A molding experiment was conducted on these two types of blanks. The results are shown in Table 2. As a whole, it was found that the blank shape (a) had stricter molding conditions than (b). After conducting molding experiments on all the materials, the experimental results were scored. First, a score of 2 was given when molding was possible without any problem, a score of 1 was given when necking occurred but no fracture occurred, and a score of 0 was given when fracture occurred. The respective scores of the blank shapes (a) and (b) were added together to obtain the stretch flange formability index in actual molding of each material. As estimated from the λ value, the grade deteriorates as the strength of the material increases, but it was found that the correlation between the λ value and the score was not necessarily high in some steel types. As described above, this is considered to be caused by the difference that there is almost no deformation distribution in the direction along the end face in the measurement of the λ value.

  Therefore, the present inventors examined a test method for efficiently evaluating stretch flangeability in a deformed state close to an actual part. The problem with the hole expansion test method is that the deformation distribution in the direction along the end face is uniform. Therefore, as a method of making the deformation distribution non-uniform while taking advantage of the fact that it is possible to test without using a large-scale mold using a relatively small test piece, which is an advantage of the hole expansion test method, A test method was devised that uses test pieces that have been split after punching holes.

  An outline of the test is shown in FIGS. (A) is a longitudinal sectional view before the start of the flange-up molding test, (b) is a plan view before the start of the flange-up molding test, and (c) is a longitudinal sectional view after the end of the flange-up molding test. It is. In this test, a test is performed by utilizing a mold for a hole expansion test using a cylindrical punch which is usually performed. At that time, the blank is divided and flange-up molding is performed. In this embodiment, as shown in FIG. 3 (a), the die 1 has a shoulder R of 5 mm and 106φ, and after being restrained by the wrinkle presser 2, the shoulder R is shown in FIG. 3 (c). Molding was performed using a 10 mm 100 mmφ cylindrical flat bottom punch 3. The blank 4 used for the molding was obtained by punching a hole of φ60 mm in the center and then cutting a 180 mm square into 1/4 (FIG. 4). The clearance at the time of punching (the punching tool punch and die gap divided by the test material plate thickness) was 15%. In the following examples, the clearance is constant under all conditions. However, the test may be performed after punching by changing the clearance at the time of punching for the purpose of corresponding to the actual molding conditions.

  At the time of the test, the crack occurrence state of the end face was visually observed in the same manner as in the normal hole expansion test, and the punch was stopped when a through crack occurred in the thickness direction. Here, the punch stroke when cracks occurred was used as an evaluation index. Instead of using this punch stroke, the molding height of the test piece stopped when the crack is generated may be measured as the flange-up molding height 5. Table 3 shows the results of tests performed on steels C, D, E, and F, which are 590 MPa class steels. Steel C (No. 1), Steel D (No. 2), and Steel F (No. 4) showed almost the same λ value, but different forming heights when evaluated by the test method of the present invention. It was found to be evaluated as. This is thought to be because the deformation distribution in the direction along the end face produced by the test method of the present invention was relaxed because the work hardening ability of Steel D and Steel F was excellent. On the other hand, it was found that steel E (No. 3), which is excellent in both work hardening ability and λ value, shows excellent characteristics even in this test method.

  The correspondence between the results (molding height) evaluated by the test method of the present invention and the actual molding test results was investigated. FIG. 5 illustrates the relationship between the score in actual molding in Table 2 and the molding height (Table 3) evaluated by the test method of the present invention. As shown here, a material having a high score in actual molding also has a large flange-up molding height, and it can be seen that both correspond. On the other hand, FIG. 6 shows the relationship between the λ value (Table 1), which has been used as an index of stretch flange formability, and the molding test score. In some materials, the superiority or inferiority in actual molding does not correspond to the magnitude of λ value, λ value is not suitable as an index of actual stretch flange formability, and the evaluation value by the test method of the present invention is more I found it appropriate.

[Example 2]
In Example 1, it tested using the test piece cut | disconnected by 1/4. As a result of the study by the present inventors, it has been found that the cutting angle of the test piece changes the deformation state in the direction along the end face. That is, when the cutting angle is small (90 ° for 1/4 cutting), the deformation distribution in the direction along the end surface tends to be smaller, and when this cutting angle increases, the direction along the end surface at the center of the arc-shaped portion. It has been found that the deformation gradient increases and breakage easily occurs at a low molding height. Also, when the cutting angle is small, especially when the ductility of the material is small, the concentration of deformation at the portion of the end of the test piece that wraps around the die shoulder (indicated as (A) in FIG. 4) increases, resulting in an arc-shaped end surface. Before the break occurred at the central part of the film, the break may occur at the (a) part. In this case, the test can be performed by partially cutting a test piece on the die shoulder from the wrinkle retainer to relieve the restraint caused by the wrinkle retainer and restrain the deformation concentration at the winding portion. In addition, the test can be performed more simply by increasing the cutting angle. Hereinafter, an example in which the cutting angle is 180 ° will be described (1/2 cutting).

  FIG. 7 shows a punched hole (R20 mm) having a diameter of 40 mm in the center of a 150 mm square test piece, which is cut in half after being processed by shearing (clearance 15%). This test piece was tested in the same manner as in Example 1 using a die having a shoulder R of 3 mm and an 85 mmφ and an 80φ cylindrical flat bottom punch having a shoulder R of 10 mm. The results are shown in Table 4. In this experiment, there was no breakage at the die shoulder as described above. Furthermore, the score in actual molding shown in Table 2 and the flange-up molding height shown in Table 4 were evaluated. As shown in FIG. 8, the evaluation results by the test method of the present invention corresponded well with the actual molding results.

  In order to correspond to the target actual molding conditions, the initial shear end face shape may be other than a circular arc, but it is preferable to make the shape smooth and reduce the change in curvature in order to avoid early breakage due to excessive deformation concentration.

[Example 3]
The test was performed using a blank that was divided into ¼ after a shear hole was provided in the center, as in Example 1. However, various punching diameters D0 and punching diameters D of the initial holes were combined. A test was conducted. At this time, the maximum flange-up height Hmax is Hmax = (D−D0) / 2 if the elongation deformation of the blank (test piece) is ignored. Tests were performed using steels C, D, and F shown in Table 1. Table 5 shows the test conditions and test results. In this test method, the molding height at the time when the end face broke was evaluated. However, even under the maximum deformation that could be given by the punch under some conditions, no break occurred. This is presumably because there was a margin in the molding ability of the material because the deformation that occurred at the center of the end face was small. However, if all the test materials do not break, the superiority or inferiority of the materials cannot be determined. When the maximum flange-up height was excessively large or excessively small, the maximum deformation was small, and all of the test materials of this time did not break, and the superiority or inferiority could not be determined. The range of the value of Hmax / D where the difference between the materials occurred was 0.10 ≦ Hmax / D ≦ 0.40. Further, it was found that when 0.20 ≦ Hmax / D ≦ 0.30, fracture occurred in all materials, and the superiority or inferiority of the materials could be evaluated as the molding height.

It is explanatory drawing of a metal mold | die (punch) shape. It is a figure which shows the blank shape ((a)-(b)) used for the experiment of Example 1. FIG. (A) is a longitudinal sectional view before the start of the flange-up molding test, (b) is a plan view before the start of the flange-up molding test, and (c) is a longitudinal sectional view after the end of the flange-up molding test. . It is explanatory drawing of the blank shape used for the shaping | molding experiment of Example 1. FIG. It is a figure which shows the relationship between the flange-up shaping | molding height evaluated in Example 1, and the stretch flange formability score. It is a figure which shows the relationship between the hole-expansion value ((lambda) value) of a steel plate, and a stretch flange formability score. It is explanatory drawing of the blank shape used for the shaping | molding experiment of Example 2. FIG. It is a figure which shows the relationship between the flange-up shaping | molding height evaluated in Example 2, and the stretch flange formability score.

Explanation of symbols

1 Die 2 Wrinkle presser 3 Punch 4 Work plate (blank)
5 Flange-up molding height

Claims (4)

Using a test piece having an arc-shaped shear end face, an out-of-plane deformation is applied in the normal direction of the test piece, and the stretch flange formability is evaluated by the forming height at which the shear end face breaks. A flange-up molding test method characterized by the above.   The flange-up molding test method according to claim 1, wherein a test piece in which an arc-shaped shear end face is formed using a circular punch is used.   The flange-up molding test method according to claim 1 or 2, wherein an out-of-plane deformation is applied by a punch, a die, or a wrinkle presser.   The flange-up molding test method according to claim 3, wherein the punch is a cylindrical punch.

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