JP6133965B1 - Evaluation method of stretch flangeability - Google Patents

Abstract

The present invention provides an evaluation method capable of simply and efficiently evaluating stretch flangeability in secondary press molding. The present invention is a step of primary press-molding the target material by a press workability evaluation apparatus having a specific structure to form a primary press-molded body having a flange portion. A primary deformation process for obtaining a deformed primary press-molded body including a flange region by performing a primary press-molding step for obtaining a plurality of different primary press-molded bodies and a process for removing a part of the area from the primary press-molded body. Using the press molding process and the press workability evaluation apparatus, secondary press molding is performed to flange-up the flange region of the deformed primary press molded body to obtain a plurality of secondary press molded bodies. It is an evaluation method of stretch flangeability including a process and a deformation amount measurement step of measuring a plate thickness change amount of a portion corresponding to the stretch flange region in the secondary press-formed body. [Selection] Figure 1

Description

  The present invention relates to a method for evaluating stretch flangeability in a molded material such as a thin metal plate.

  A press-formed product using a steel material or a non-ferrous material has a desired shape by causing a predetermined plastic deformation in each part.

Processing elements for plastic deformation in press molding are roughly classified into four types according to combinations of deformation in the tensile direction, deformation in the compression direction, and no deformation using the maximum principal strain and the minimum principal strain. The three orthogonal axes that maintain orthogonality after deformation are called the main axes of distortion. The vertical distortion in the main axis direction is the main distortion, and the maximum and minimum values are called the maximum main distortion and the minimum main distortion. ing. FIG. 8 illustrates these processing elements. The maximum principal strain is represented by ε ₁ , the minimum principal strain is represented by ε ₂ , and the plate thickness direction strain orthogonal to both the maximum principal strain ε ₁ and the minimum principal strain ε ₂ is represented by ε _t . A processing element in which the maximum principal strain ε ₁ is tensile deformation and the minimum principal strain ε ₂ is not deformed is a plane strain tensile deformation (A in FIG. 8), and the maximum principal strain ε ₁ and the minimum principal strain ε ₂ are both. The working element which is tensile deformation is biaxial tensile deformation (C in FIG. 8), the working element whose maximum principal strain ε ₁ is tensile deformation and the minimum principal strain ε ₂ is compression deformation, and shrinks the flange deformation (FIG. 8). D) A working element in which the maximum principal strain ε ₁ is tensile deformation, the minimum principal strain ε ₂ is compression deformation, and the compression is a strain equivalent to ½ of tension is called uniaxial tensile deformation or stretch flange deformation. (B in FIG. 8).

Stretch flange cracking is a molding defect when manufacturing automobile parts by press molding of steel or non-ferrous materials. Stretch flange cracking is a molding failure that causes breakage when an edge of a flange portion of a press-molded product is subjected to stretch flange deformation that is one mode of uniaxial tensile deformation.
There are many methods for evaluating the workability of steel or non-ferrous materials. However, most of them are evaluation methods by single molding with one processing element. For example, a method for evaluating workability by single molding such as a hole expansion test and a saddle type test is used. In addition, as a method for evaluating stretch flange formability of a metal plate, a metal blank having a corner cut into a V shape is press-molded using a punch, a die and a pad, and the stretch flange formability of the metal plate is improved. A test method for evaluation is known (see, for example, Patent Document 1).

  By the way, the press molding of the actual part is not completed only by the primary press molding in which the target material is press molded. A desired shape can be obtained by repeatedly performing press molding. In this specification, press molding performed after primary press molding is referred to as “secondary press molding”. Moreover, the deformation | transformation provided to the target material before secondary press molding is called "predeformation."

  The evaluation result related to stretch flangeability in an article provided with pre-deformation does not necessarily match the evaluation result related to stretch flangeability of a raw material not subjected to pre-deformation. Moreover, it is normal that a difference arises in the evaluation result of stretch flangeability also by the difference in the amount of pre-deformation.

  When designing a press molding process for actual parts, an optimal molding process that does not cause stretch flange cracking is selected. Therefore, it is necessary to grasp the stretch flangeability at the time of secondary press molding. Therefore, there has been a demand for an evaluation test method and an evaluation result that can accurately and accurately grasp the influence of the amount of pre-deformation on stretch flangeability.

  Conventionally, as an evaluation test for grasping the effect of the amount of pre-deformation on secondary press formability, a hole expansion test using a plate-shaped target material to which pre-deformation has been applied is known (for example, non-deformation). (See Patent Document 1). In this hole expansion test, first, predeformation by various single deformations is given to the plate-like material. For example, a uniaxial tensile deformation is added using an expander, or a biaxial tensile deformation is added using a flat bottom punch. Thereafter, a hole expansion test by secondary press molding is performed on the target material to which the pre-deformation is applied, using a conical punch or a flat bottom punch. By evaluating the degree of hole expansion, it is possible to evaluate stretch flangeability in secondary press molding.

JP 2008-264829 A

Michio Takida, Takeo Nakagawa, Kiyota Yoshida, "Penetration test of pre-deformed cold-rolled thin steel sheet", RIKEN report, Vol. 46, No. 1, January 1970

  However, in actual press working, it is often processed by composite molding including a plurality of processing elements in one molded product. For example, as shown in FIG. 19, a press-formed part having a structure in which a tube branches from one to a plurality of pipes has four typical processing elements (plane strain tensile deformation, biaxial tension) for the target material. Deformation, shrinking flange deformation, and uniaxial tensile deformation). Further, in the press-molded part shown in FIG. 19, in the primary press-molded product, after removing a part of the multi-branch-shaped area or a part of the planar area, leaving a flange area of a predetermined width, On the other hand, secondary press molding including stretch flange deformation, which is an aspect of uniaxial tensile deformation, may be performed.

  As described above, in the case where the above-described composite molding is included, in order to evaluate stability relating to workability (hereinafter, also referred to as “machining stability”), various types of primary press molding capable of imparting various machining elements are used. It is necessary to prepare a plurality of primary press-molded bodies to which the above pre-deformation is applied, and then perform secondary press molding on the primary press-molded bodies to evaluate stretch flangeability.

  However, in the conventional stretch flangeability evaluation method, various pre-strains are imparted to the target material by single molding as shown in Non-Patent Document 1. Composite molding is greatly affected by separate processing elements adjacent to each other in one molded product. Therefore, when secondary press molding is added to a primary press molded product that has been pre-deformed by a single deformation. In some cases, a flaw occurs between the evaluation result and the press molding result of the actual part. For this reason, for the design of a process for press-molding an actual part, the evaluation result by the conventional evaluation test is insufficient as an evaluation result for selecting an optimal process that does not cause stretch flange cracking.

  Further, as a method for applying composite molding to the primary press-molded product, it is conceivable to mold using a mold having a desired shape. However, since the form of deformation due to molding differs depending on the type of press-molded part, it is very complicated and expensive to prepare different dies (dies, punches) for each press-molded part.

  The present invention has been made to solve the above-described problems. An object of the present invention is to provide an evaluation method that can be easily and efficiently evaluated for evaluating the stretch flangeability in the secondary press molding with respect to the secondary press molding using the composite-molded primary press molded body. It is in.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, it has been found that the above-described problems can be solved by using an apparatus including a punch and a die having a specific structure, and the present invention has been completed. Specifically, the present invention provides the following.

  (1) The present invention provides a target material to be subjected to press molding between a punch having a multi-branch-shaped convex part, a die having a concave part and a flat part that can be fitted to the convex part, and the flat part. A press workability evaluation apparatus including a plate presser for pressing the target material is subjected to primary press molding of the target material to form a primary press molded body having a flange portion, and the plurality of the first deformations having different pre-deformation amounts. A primary press molding step for obtaining a next press molded body, a modified primary press molding step for obtaining a deformed primary press molded body including a flange region by performing a process of removing a partial region from the primary press molded body, A secondary press molding step of obtaining a plurality of secondary press molded bodies by performing secondary press molding for flange-up of the flange region of the deformed primary press molded body using the press workability evaluation apparatus; Next In press forming body includes a deformation amount measurement step of measuring a thickness change amount of the portion corresponding to the stretch flange region, a stretch flangeability evaluation method.

  (2) In the present invention, the deformed primary molding step has a predetermined width in at least one of a multi-branched region in the primary press-molded body or another region different from the multi-branched region. The stretch flangeability evaluation method according to (1), wherein a process for removing the flange region is performed.

  (3) In the present invention, the flange-up in the secondary press molding step is the flange height substantially equal to the sum of the flange height of the primary press molded body and the width of the flange region of the deformed primary press molded body. The stretch flangeability evaluation method according to (1) or (2), wherein the stretch flangeability is processed so as to form a thickness.

  (4) In the present invention, the press workability evaluation apparatus includes a punch having a multi-branched convex portion including three or more branches, a concave portion including three or more branches that can be fitted to the convex portion, and the A die having a die side plane part surrounding the recess and a plate presser side plane part substantially parallel to the die side plane part, and the target material can be sandwiched between the die side plane part and the plate presser side plane part It is an evaluation method of stretch flangeability given in the above (1)-(3) provided with a board retainer.

  (5) According to the present invention, the angles formed by the multi-branched branches are obtuse when the number of branches is three, are substantially perpendicular when the number of branches is four, and the number of branches is five. The stretch flangeability evaluation method according to any one of claims 1 to 4, wherein when there are two or more, the angles are mutually acute.

  (6) The present invention provides the stretch flangeability according to any one of the above (1) to (5), wherein the multi-branched branch has four branches, and the convex part and the concave part have a substantially cross shape. This is an evaluation method.

(7) In the present invention, when the die is viewed in plan, the corner portion of the recess is curved, the width of the branch of the die is Wd, and the radius of curvature at the corner portion of the branch of the die is When Rc, the ratio Wd / Rc of Wd to Rc is 2 or more and less than 15,
When the punch is viewed from the front, the shoulder at the top of the convex portion is curved, the radius of curvature at the shoulder is Rp, the thickness of the convex is Hp, and the die is cross-sectioned. When viewed, the bottom of the concave portion is curved, and Hp ≧ (Rp + Rd) / 2, where Rd is the radius of curvature of the bottom, according to any one of (1) to (6) above This is an evaluation method for stretch flangeability.

  (8) The method for evaluating stretch flangeability according to any one of the above (1) to (7), wherein the target material uses a blank having a quadrangular shape, an octagonal shape, an elliptical shape, or a circular shape. It is.

  According to the present invention, a plurality of primary press-molded bodies having different pre-deformation amounts are obtained by performing primary press-molding on a material to be press-molded under various conditions using a specific press workability evaluation apparatus. It is done. Therefore, it is possible to easily and efficiently produce a plurality of primary press-molded bodies having various amounts of pre-deformation without producing a plurality of molds.

  A secondary press in which each of the primary press molded bodies is trimmed to produce a deformed primary press molded body, and then the flange area of the deformed primary press molded body is flanged up by the press workability evaluation apparatus. By performing the molding, a plurality of secondary press-molded bodies are obtained. The workability of the secondary press-formed product can be evaluated by measuring the amount of change in the thickness of the flange-up region (elongated flange region) in the secondary press-formed product.

  By adjusting the flange-up conditions in the secondary press molding process and producing a plurality of secondary press molded bodies having substantially the same shape, by measuring the plate thickness change amount, the plurality of secondary press molded bodies provided with the primary press molded body It is also possible to evaluate by comparing the influence on the amount of pre-deformation. Therefore, it is possible to easily predict the influence on the workability due to the press molding conditions, the molding target material, and the like.

  For example, by comparing the maximum value of the plate thickness change rate in the stretch flange region of each secondary press-formed product, superiority or inferiority regarding the processing stability of each secondary press-formed product can be evaluated.

  The present invention provides a method for simply evaluating the stretch flangeability when performing secondary press molding when a plurality of primary press molded products having different pre-deformation amounts are obtained by primary press molding comprising composite molding. it can.

It is a schematic sectional drawing for demonstrating the press workability evaluation apparatus 1 suitable for using the stretch flangeability evaluation method concerning this invention. It is a figure for demonstrating the punch 13 which concerns on one Embodiment. (A) is a schematic perspective view showing an example of the punch 13, (b) is a plan view of the punch 13, and (c) is a front view of the punch 13. It is a figure for demonstrating the punch 13 which concerns on other embodiment. (A) is a plan view showing an example of a punch 13 ′ having three multi-branched branches, and (b) shows an example of a punch 13 ″ having five multi-branched branches. It is a top view. It is a figure for demonstrating the dice | dies 23. FIG. (A) is a schematic perspective view which shows an example of the die | dye 23, (b) is a top view of the die | dye 23, (c) is sectional drawing in the XX line of (a). It is a figure for demonstrating the board presser. It is a schematic explanatory drawing which shows an example of the primary press molding 40 obtained by a primary press molding process. (A) is an image, (b) is a perspective view schematically showing the shape of (a), (c) is a plan view, and (d) is (c). The front views from the direction marked Z are shown respectively. It is a schematic explanatory drawing which shows an example of a press molding process. It is a figure for demonstrating the processing element of the plastic deformation of press molding. It is a figure which shows the relationship between the minimum principal strain in each said process element, and the largest principal strain. It is a figure which shows the dimension of the die | dye and punch which were used in the test example. (A) is a plan view of the die 23 used, (b) is a cross-sectional view of the die 23 taken along line YY, and (c) is used. A front view of the punch 13 is shown. In Test Examples 1 and 3, it is a figure for demonstrating the deformation | transformation primary press molded object after primary press molding. In Test Examples 2 and 4, it is a figure for demonstrating the deformation | transformation primary press molded object after primary press molding. In Test Examples 1-4, it is a figure for demonstrating the secondary press molding after secondary press molding. It is a figure which shows the measurement location of the distribution of the distortion state in FIG. It is a figure which shows the distortion state of the secondary press molded object which concerns on the test examples 1 and 2. FIG. It is a figure which shows the plate | board thickness change rate of the secondary press molded object regarding Test Examples 1-4. It is a figure for demonstrating the measurement position of the plate | board thickness change rate in a cross prototype and an actual press prototype. It is a figure which shows the plate | board thickness change rate in the measurement position of FIG. It is a figure which shows an example of the press molding component which has a structure where a pipe | tube branches from one to several. It is a figure for demonstrating the state of a press molding regarding ratio Wd / Rc of the width Wd of the branch in the die | dye 23, and the curvature radius Rc of a corner | angular part. It is a figure which shows the strain distribution before and behind the process of the primary press molded object which concerns on the test examples 1 and 2, and a secondary press molded object. It is a figure which shows the distortion state of the primary press molded object which concerns on the test example 2, and a secondary press molded object. FIG. 22 is a diagram obtained by superimposing ad in FIG. 21 on FIG.

  Hereinafter, specific embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and can be implemented with appropriate modifications within the scope of the object of the present invention.

<Stretch flangeability evaluation method>
The stretch flangeability evaluation method according to the present invention uses a specific press workability evaluation apparatus to perform a primary press on a target material to be subjected to press molding (hereinafter, also abbreviated as “press molding target material”). Forming a primary press-molded body having a flange portion, the primary press-molding step for obtaining a plurality of primary press-molded bodies having different pre-deformation amounts; The flange region of the deformed primary press-molded body is obtained using a deformed primary press-molding step for obtaining a deformed primary press-molded body including a flange region by performing a process of removing the region of the portion, and the press workability evaluation apparatus. The secondary press molding process for obtaining a plurality of secondary press molded bodies by performing the secondary press molding for flange-up, and the thickness of the portion corresponding to the stretch flange region in the secondary press molded body Including the deformation amount measurement step of measuring the reduction amount.

[Primary press molding process]
The primary press molding step is a step of performing a primary press molding of a material to be press molded using the specific press workability evaluation apparatus 1 to obtain a plurality of primary press molded bodies 40 having different pre-deformation amounts.

[Press workability evaluation apparatus 1]
FIG. 1 is a schematic diagram for explaining a press workability evaluation apparatus 1. The press workability evaluation apparatus 1 includes a punch portion 10, a die portion 20 that can be fitted to the punch portion 10, and guide portions 30 </ b> A and 30 </ b> B provided on both sides of the punch portion 10 and the die portion 20.

  The punch unit 10 includes a bottom plate 11, a punch holder 12 provided on the surface of the bottom plate 11, and a punch 13 supported on the surface of the punch holder. The die portion 20 includes an upper plate 21, a die holder 22 provided on the bottom surface of the upper plate, and a die 23 supported on the bottom surface of the die holder 22. The guide portions 30A and 30B are provided on both sides of the die portion 20 and are provided on both sides of the punch pins 10 and guide pins 31A and 31B that determine the moving direction of the die portion 20, and sandwich the material to be pressed together with the die portion 20. Possible plate pressers 32A and 32B, and cushion pins 33A and 33B that are provided at the bottom of the plate pressers 32A and 32B and support the plate pressers 32A and 32B.

  FIG. 1 shows a mode in which a plate material 50 as a material to be press-molded is arranged between the punch 10 and the die 23. The plate member 50 is drawn between the punch 13 and the die 23 during the press work, and the outer periphery thereof contracts. Therefore, the plate member 50 having a sufficiently larger width than the punch 13 is used.

[Punch 13]
FIG. 2A is a schematic perspective view showing an example of the punch 13 and is an example in the case where there are four multi-branched branches. The punch 13 has a convex portion 132. The convex part 132 has a multi-branch shape composed of four convex parts 132A, 132B, 132C, 132D.

  FIG. 2B is a plan view of the punch 13, and FIG. 2C is a front view of the punch 13.

  The convex portion 132 has a multi-branch shape including three or more branches 132A, 132B, 132C, and 132D. If the convex part 132 of the punch 13 is not configured in a multi-branch shape, when the press molding is performed, it is not possible to form an intersecting portion where adjacent branches of the press molded body intersect with each other. Since strain tensile deformation cannot be measured suitably, it is not preferable.

  The number of branches is not particularly limited. The shape of the press-molded body is relatively simple, and it is relatively easy to apply at least two types of deformation amount among plane strain tensile deformation, uniaxial tensile deformation, biaxial tensile deformation and shrinkage flange deformation of the press molded body. The number of branches is preferably 3 or more and 8 or less, more preferably 3 or more and 5 or less. Further, when examining the effect of the rolling direction of the material at the branch tip or branch intersection, it is 0 °, 45 °, 90 ° with respect to the rolling direction of the material to be press-formed, which is performed in a general tensile test. In order to collect accurate distortion amount data in each direction, the number of branches is particularly preferably four.

  A preferable angle formed between adjacent branches differs depending on the number of multi-branched branches. When there are three multi-branched branches, it is preferable that the angles formed by adjacent branches are obtuse. When there are four multi-branched branches, it is preferable that the angles formed by adjacent branches are substantially right angles. Further, when there are five or more multi-branched branches, it is preferable that the angles formed by adjacent branches are acute angles with each other. In addition, the angle | corner which adjacent branches make can also be made to include both an obtuse angle and an acute angle.

  3A is a plan view showing an example of a punch 13 ′ having three multi-branched branches, and FIG. 3B is a punch 13 having five multi-branched branches. It is a top view which shows an example of ''. As shown in FIG. 2 and FIG. 3, adjacent processing elements (plane strain tensile deformation, biaxial tensile deformation, contraction flange deformation, and uniaxial tensile deformation) can be given more accurately. It is more preferable that the preferable angles formed by the matching branches are substantially equal to each other. That is, the angle formed by adjacent branches is more preferably about (number of branches) / 360 °. Specifically, when there are three multi-branched branches, it is preferable that the angles formed by adjacent branches are approximately 120 ° with each other, and the convex portion 12 is formed in a substantially three-pointed shape. When there are three multi-branched branches, it is preferable that the angles formed by adjacent branches are substantially perpendicular to each other, and the convex portion 12 is formed in a substantially cross shape. When there are five multi-branched branches, the angle between adjacent branches is approximately 72 ° and is preferably substantially star-shaped.

As shown in FIG. 2B, when the punch 13 has a plan view of the convex portion 132 of the punch, the punch corner portion e ₁ of the convex portion 132 is curved inward in the radial direction of the punch 13. The corner f _{1 of} 132 is curved outward in the radial direction of the punch 13. In this specification, the corner portion e ₁ of the convex portion 132 refers to a portion of the peripheral edge (edge) of the convex portion 132 in which a branch protrudes to the periphery and is formed in a convex direction. ₁ means a portion formed in a concave direction formed by adjacent branches on the periphery (edge) of the convex portion 132.

Also, as shown in FIG. 2C, when the punch convex portion 132 is viewed from the front, the punch shoulder g _{1 at} the top of the branches 132A, 132B, 132C, 132D among the peripheral edges (edges) of the convex portion 132. is formed in a R (curve) for curving radius of curvature Rp radially inside of the punch 13, the outer shape extending from the punch shoulder g ₁ at the base of the protrusion 132 has a shape extending substantially longitudinally . In this specification, the top part of the convex part 132 refers to the side of the convex part 132 that comes into contact with the material to be pressed, and the base part of the convex part 132 is disposed on the punch holder 12 in the convex part 132. Say the side.

  In addition, when the thickness from the top to the base of the convex portion 132 of the punch is Hp, it is preferable that Hp ≧ (Rp + Rd) / 2. When Hp is (Rp + Rd) / 2 or more, the plane strain tensile deformation and uniaxial tensile deformation of a press-molded product obtained by press-molding a material to be press-molded can be suitably measured.

[Dice 23]
FIG. 4A is a schematic perspective view showing an example of the die 23, which is an example of the die used in combination with the punch 13 having four branches. 4B is a plan view of the die 23, and FIG. 4C is a cross-sectional view taken along line XX in FIG. 4A.

  As shown to (a) of FIG. 4, the die | dye 23 is formed so that fitting with the punch 13 is possible. The die 23 has four concave portions 231 that can be fitted to the convex portion 132, and a die side plane portion 232 that surrounds the concave portion 231.

As shown in FIG. 4B, when the dice 23 has a plan view of the recess 231, the dice corners e ₂ of the branches 231 A, 231 B, 231 C, and 231 D of the recess 231 are curved radially inward of the dice 23. and none of R (curve) for curving radius Rc, branches of the recess 132 231A, 231B, 231C, die corners _{f 2} of 231D causes the bending radius of curvature Rc in the radially outer side of the die 23 R (curve) I am doing. In this specification, the dice corners e ₂ of the branches 231A, 231B, 231C, and 231D of the recess 231 refer to portions formed in the convex direction on the periphery (edge) of the recess 231, and the branch 231A of the recess 231. The die corners f _{2 of} 231B, 231C, and 231D are portions formed in the concave direction among the peripheral edges (edges) of the concave portion 231.

Further, as shown in FIG. 4 (c), when the concave portion 231 of the die is viewed in a cross section of the concave portion (XX plane in FIG. 4 (a)), the side of the concave portion 231 on the side in contact with the material to be pressed. The bottom h ₂ forms an R (curve) that is curved with a radius of curvature Rd on the outside in the radial direction of the die 23. Contour extending from the die bottom part h ₂ at the base of the recess 231 has a shape extending substantially longitudinally. In this specification, the base of the recess 231 refers to the side of the recess 231 that is disposed on the die holder 22.

In order to be able to suitably give the biaxial tension and shrinkage flange of the press-molded body obtained by press-molding the target material, the width of the branches 231A, 231B, 231C, 231D of the die recess 231 is Wd, when branches 231A, 231B, 231C, the curvature radius of the die corner _{e 2} of the 231D and Rc, the ratio Wd / Rc for Rc of Wd, it is less than 2 to 15 and preferably less than 3 more than 12 Is more preferable.

As shown in (a) of FIG. 20, Wd / Rc is too small (if Rc is too large), the pressed bodies, stress at the boundary of the corner e ₃ branches are locally concentrated, pressed bodies Can cause cracking. As shown in (b) of FIG. 20, Wd / Rc is too large (when Rc is too small), the pressed bodies, plastic strain region is reduced in the circumferential direction of the corner portion e ₃ of the branch, corner e ₃ is not preferable because cracks may occur. In addition, in this specification, a curvature radius shall mean the value calculated | required with the contour shape measuring device (model: Contracer CV-2000, product made by Mitutoyo Corporation).

[Plate presser 32]
FIG. 5 is a view for explaining the plate presser 32. The plate retainer 32 has a plate retainer side planar portion 321 substantially parallel to the die side planar portion 232, and is configured such that a target material to be press-molded can be sandwiched between the die side planar portion 232 and the plate retainer side planar portion 321. Is done. When the press workability evaluation apparatus 1 does not include the plate presser 32, when a target material to be press-molded is press-molded, a substantially planar region surrounding the multi-branched region of the press-molded body cannot be formed. This is not preferable because the shrinkage flange deformation of the molded body cannot be suitably imparted.

(Modification)
1 to 5, the punch 13 has a convex multi-branch shape and the die 23 has a concave multi-branch shape. However, the present invention is not limited to this. Even when the punch 13 has a concave multi-branch shape and the die 23 has a convex multi-branch shape, the same effect can be obtained.

  Specifically, the press workability evaluation apparatus includes a punch having a multi-branch-shaped concave portion including three or more branches at the tip, and a multi-branched convex portion including three or more branches that can be fitted to the concave portion, And a die having a die side plane part surrounding the convex part and a plate pressing side plane part substantially parallel to the die side plane part, and the target material to be press-molded is the die side plane part and the plate. It may be provided with a plate presser that can be sandwiched between the presser side flat portions.

  Even in the press workability evaluation apparatus according to this modified example, if the irregularities of the punch and the die are exchanged, the angle between adjacent branches and the parameters such as Wd, Rc, Hp, Rp, Rd, etc. are suitable. This range is the same as described above.

[Primary press-formed body 40]
FIG. 6 is a perspective view showing an example of a plurality of primary press-formed bodies 40 obtained by primary press-molding a target material to be press-molded using the press workability evaluation apparatus 1. 6A is an image, FIG. 6B is a perspective view schematically showing the shape of FIG. 6, and FIG. 6C is a plan view thereof. FIG. 6D shows a front view from the direction indicated by Z in FIG. 6C. The primary press-molded body 40 includes a convex portion 41 and a substantially flat plane portion 42 surrounding the convex portion 41. Moreover, the convex part 41 is exhibiting the multi-branch shape which consists of the branches 43A, 43B, 43C, and 43D.

Since the material to be press-molded is deformed by being processed between the branch 132 of the punch 13 and the concave portion 231 of the die 23, the convex portion 41 (branches 43A, 43B, etc.) of the press-formed body 40 is formed by the punch 132A, 132B, etc. The flat portion 42 is shaped so as to correspond to the top surface of the die 23 and the flat surface of the plate presser 32. Molded. That is, the convex part 41 of the press-molded body has a shape corresponding to the punch corner part e ₁ , the punch corner part f ₁ , and the punch shoulder part g ₁ of the punch 13 as shown in FIGS. As the shapes corresponding to the die corner part e ₂ , the die corner part f ₂ , and the die bottom part h ₂ of the die 23 as shown in FIGS. 4A to 4C, the corner part e ₃ , the corner part f ₃ , and the shoulder part. Sites such as g ₃ and bottom h ₃ are provided.

[Deformation primary press molding process]
The modified primary press molding process will be described. The modified primary press-molding step is a step of obtaining a deformed primary press-formed body 60 including a flange region by performing a process of removing a part of the region from the primary press-formed body 40. The primary press-molded body 40 has a flange portion. When a part of the region is removed from the primary press-formed body 40, trimming is performed so that the region including the flange portion remains. In the present specification, a region including the flange portion is referred to as a “flange region”. A flange having a predetermined width in at least one of a multi-branched region (for example, a flange portion) in the primary press-formed body or another region (for example, a flat portion around the flange portion) different from the multi-branched region. It is preferable to carry out the configuration in which the area is removed and removed.

[Secondary press molding process]
The secondary press molding process will be described. The secondary press molding step is a step of obtaining a plurality of secondary press molded bodies 70 by performing secondary press molding for flange-up the flange region of the deformed primary press molded body 60 using the press workability evaluation apparatus 1. is there.

  Flange-up in the secondary press molding process is substantially the same when processed so as to form a flange height substantially equal to the sum of the flange height of the primary press molded body and the width of the flange region of the deformed primary press molded body. Since a secondary press-molded body having a shape is obtained, it is preferable.

  FIG. 7 is a diagram for explaining specific modes of primary press molding, modified primary press molding, and secondary press molding. FIGS. 7A and 7B illustrate two types of processing modes 1 and 2 as specific processing modes. Processing form 1 in FIG. 7A and processing form 2 in FIG. 7B are primary press forming (abbreviated as “cross forming” in the figure) formed into a cross shape by the press workability evaluation apparatus. To obtain a primary press-molded body, and then a trimming process is performed on the primary press-molded body to obtain a deformed primary press-molded body. Thereafter, secondary press molding is performed on the deformed primary press-molded body. “Primary press molding” in FIG. 7 shows a shape before trimming, and “deformed primary press molding” shows a schematic cross-sectional view taken along line yy ′ in the shape after trimming. The “trim portion” described in FIG. 7 indicates a trimmed portion, and the “flange up portion” indicates a region to be flanged up by secondary press molding.

  As shown in the processing form 1 of FIG. 7A, the first specific mode is that after the primary press molding step, from the central portion of the convex portion 41 (multi-branched region) in the primary press molded body 40. The trim part is removed, leaving a flange region with a predetermined width, and a deformed primary press-formed body is obtained. Thereafter, in the secondary press molding step, the press workability evaluation apparatus 1 is used to flange up the flange region of the deformed primary press molded body, whereby a secondary press molded body is obtained. In this mode, as shown in “assumed shape” of the processing mode 1 in FIG. 7, in the primary press molding, a bulging process which is one mode of biaxial tensile deformation is performed, and in the secondary press molding, uniaxial tensile deformation is performed. This corresponds to the case of performing the stretch flange deformation which is one aspect of the above.

  In the second specific mode, as shown in the processing form 2 of FIG. 7B, the flat portion 42 in the primary press-formed body 40 is provided after the primary deformation amount measuring step and before the secondary press-forming step. From the other region different from the multi-branched region, the trim part is removed leaving a flange region having a predetermined width, and a deformed primary press-formed body is obtained. Thereafter, in the secondary press molding step, the press workability evaluation apparatus 1 is used to flange up the flange region of the deformed primary press molded body, whereby a secondary press molded body is obtained. In this mode, composite molding including plane strain tensile deformation and uniaxial tensile deformation is performed in the primary press molding as shown in “assumed shape” in FIG. 7B, and uniaxial tension is performed in the secondary press molding. This corresponds to the case of performing stretch flange deformation, which is one mode of deformation.

[Deformation measurement process]
Subsequently, the deformation amount measuring step will be described. The deformation amount measuring step is a step of measuring the maximum sheet thickness reduction rate in the stretch flange region of each secondary press-formed body.

  As a method for measuring the plate thickness change amount, there is a cheek method using an ultrasonic thickness meter (model: 38DL-PLUS, manufactured by Olympus Corporation).

[Crack prediction process]
Furthermore, it is preferable that this invention includes the process of estimating the stretch flange crack position of a secondary press molding by measuring the maximum principal strain and the maximum principal strain of a secondary press molding.
The relationship between the maximum principal strain and the minimum principal strain can be plotted with a plurality of marks, and cracks in the secondary press-formed product can be predicted from the results of the plot. This plot can be created as a shaping diagram, for example.

As a method for measuring the maximum principal strain and the minimum principal strain, a non-contact strain measuring device ARGUS (manufactured by GOM Corporation / Kobelco Research Institute) is used and the following procedures (1) to (3) are performed.
(1) Formed by electrolytic transfer of φ0.8 mm dot marks (1.5 mm intervals) as a plurality of marks on the surface of a press molding target material (test material).
(2) Photograph the formed dot marks from multiple directions using a CCD camera before and after processing.
(3) The distance between dots before and after processing is measured based on the photographed dot mark, and the maximum principal strain and the minimum principal strain are calculated from the amount of change.

  The stretch flangeability can be evaluated by calculating the maximum principal strain and the minimum principal strain. Here, processing elements for plastic deformation of press molding will be described with reference to FIG.

The plastic deformation processing elements are roughly classified into four types according to combinations of deformation in the tensile direction, deformation in the compression direction, and no deformation using the maximum principal strain and the minimum principal strain. FIG. 8 shows the machining elements. The maximum principal strain is ε ₁ , the minimum principal strain is ε ₂ , and the plate thickness direction strain orthogonal to both the maximum principal strain ε ₁ and the minimum principal strain ε ₂ is shown. It has been described as ε _t.

A machining element in which the maximum principal strain ε ₁ is tensile deformation and the minimum principal strain ε ₂ is not deformed is referred to as plane strain tensile deformation (A in FIG. 8). A working element in which the maximum principal strain ε ₁ and the minimum principal strain ε ₂ are both tensile deformations is referred to as biaxial tensile deformation (C in FIG. 8). A machining element in which the maximum principal strain ε ₁ is tensile deformation and the minimum principal strain ε ₂ is compression deformation is referred to as flange deformation (D in FIG. 8). A working element in which the maximum principal strain ε ₁ is tensile deformation, the minimum principal strain ε ₂ is compression deformation, and the compression is a strain equivalent to ½ of tension is called uniaxial tensile deformation or stretch flange deformation (see FIG. 8 B).

Next, the area | region to which deformation | transformation of each said process element is provided is demonstrated.
In the plane strain tensile deformation, since the maximum principal strain ε ₁ is a tensile deformation and the minimum principal strain ε ₂ is a working element without deformation, in a press-formed product, a region having a strain state close to the working element is It can be determined that the region has been processed by strong plane strain tensile deformation. In the present embodiment, a press-molded body 40 as shown in FIG. 6 is formed by processing with the punch 13 shown in FIG. 2 and the die 23 shown in FIG. On the periphery of the convex portion 41 of the press-formed body 40, the region C is adjacent to the corner portion f ₃ having a shape corresponding to the punch corner portion f ₁ of the punch 13 and corresponding to the die corner portion f ₂ of the die 23. Is formed. The region C includes a bottom portion h ₃ having a curved shape formed corresponding to the radius of curvature Rd of the die bottom portion h ₂ of the die 23. The region C is a region that is strongly subjected to plane strain tensile deformation because the maximum principal strain ε ₁ is large and the minimum principal strain ε ₂ is measured with a strain amount close to zero. In the region C, the amount of deformation before and after pressing at a location where the maximum principal strain ε ₁ is the largest and the minimum principal strain ε ₂ is substantially undeformed is an amount mainly involving plane strain tensile deformation.

In the uniaxial tensile deformation, the maximum principal strain ε ₁ is tensile deformation, the minimum principal strain ε ₂ is compression deformation, and the strain amount due to compression is a processing element corresponding to ½ of the strain amount due to tension. In the present embodiment, a region S is formed on the side surfaces in the length direction of the branches 43A, 43B, 43C, and 43D of the press-formed body 40 shown in FIG. The area S corresponds to the shape extending in the substantially vertical direction from the punch base to punch shoulder g _1, which is formed corresponding to the shape extending in the substantially vertical direction from the die base to the die bottom h ₂ region Is included. The region S is a region where the maximum principal strain ε ₁ is large and the minimum principal strain ε ₂ is measured as compressive deformation, and thus is strongly subjected to uniaxial tensile deformation. In the region S, the amount of deformation before and after pressing at the place where the maximum principal strain ε ₁ is the largest and the minimum principal strain ε ₂ is the compressive deformation is the amount mainly involving uniaxial tensile deformation.

Biaxial tensile deformation is a processing element in which the maximum principal strain ε ₁ and the minimum principal strain ε ₂ are both tensile deformations. In the present embodiment, a region T which is a corner portion of the convex portion 41 of the press-formed body 40 shown in FIG. 6 and is a shoulder portion is formed. The region T includes regions formed corresponding to the shapes of the punch corner part e ₁ and the punch shoulder part g ₁ of the punch 13 and the die corner part e ₂ and the die bottom part h ₂ of the die 23. The region T is a region that is strongly subjected to biaxial tensile deformation because the maximum principal strain ε ₁ and the minimum principal strain ε ₂ are measured as large tensile strains. In the region T, the amount of deformation before and after pressing at the location where the maximum principal strain ε ₁ is the largest and the minimum principal strain ε ₂ is the tensile deformation is an amount mainly involving the biaxial tensile deformation.

Shrink flange deformation is a working element in which the maximum principal strain ε ₁ is tensile deformation and the minimum principal strain ε ₂ is compression deformation. In the present embodiment, in the flat portion 42 of the press-formed body 40 shown in FIG. 6, for example, the region F around the corner of the convex portion corresponds to the upper surface on the top side of the die 23 and the flat surface of the plate presser 32. It includes the formed region. The region F is a region that is strongly subjected to shrinkage flange deformation because the maximum principal strain ε ₁ is measured as a large tensile strain and the minimum principal strain ε ₂ is measured as a large compressive strain. In the region F, the maximum principal strain epsilon ₁ is the largest, the minimum principal strain epsilon ₂ amount of deformation before and after the press of the smallest portion, shrinkage flange deformation is the amount involved in principal.

Next, the crack prediction process will be described with reference to FIG. 9, the ordinate indicates the magnitude of the maximum principal strain epsilon _1, the horizontal axis indicates the magnitude of the minimum principal strain epsilon _2. In the case of plane strain tensile deformation, since the maximum principal strain ε ₁ is tensile deformation and the minimum principal strain ε ₂ is not deformed, the deformation form in the direction of the straight line L ₁ (ε ₂ = 0) corresponding to the vertical axis in FIG. Corresponds to plane strain tensile deformation.

In the case of biaxial tensile deformation, since the maximum principal strain ε ₁ and the minimum principal strain ε ₂ are both tensile deformations, the deformation form in the direction of the straight line L ₃ (ε ₁ = ε ₂ ) in FIG. Corresponds to deformation.

In the case of shrinkage flange deformation, since the maximum principal strain ε ₁ is tensile deformation and the minimum principal strain ε ₂ is compression deformation, the deformation form in the direction of the straight line L ₄ (ε ₁ = −ε ₂ ) in FIG. Corresponds to shrinkage flange deformation.

In the case of uniaxial tensile deformation (elongation flange deformation), since the strain amount due to compression is equivalent to ½ of the strain amount due to tension, the deformation form in the direction of the straight line L ₂ (ε ₁ = −2ε ₂ ) in FIG. Corresponds to uniaxial tensile deformation.

If the amount of strain at the time of press working becomes excessive, processing cracks are likely to occur. As shown in FIG. 9, as the maximum principal strain ε ₁ and the minimum principal strain ε ₂ increase, tensile cracks are more likely to occur. On the other hand, when the maximum principal strain ε ₁ is reduced and the minimum principal strain ε ₂ is shifted in the compressing direction (minus side), the occurrence of tensile cracks is suppressed.

Further, the processing elements indicated by L _{1 to} L ₄ have a decrease or increase in the plate thickness depending on the respective strain states. When this plate thickness change amount becomes excessive, the processing cracks occur, Depending on the thickness change, tensile cracks or compression cracks are likely to occur. Among these, in the deformation mode close to L _{1 to} L ₃ , the ratio of tensile deformation is large and the thickness is in the direction of decreasing, so the occurrence of tensile cracks is predicted. Variations in close proximity to L ₄ represents the proportion of compressive deformation is large, because the direction in which the plate thickness is increased, the occurrence of compression cracking is predicted.

Under such prediction measure the strain state after the press working (strain distribution), on the basis of the strain on the portion undergoing strong processing elements L ₁ ~L _4, machining cracks possible Can be predicted, and the press workability of the original plate can be evaluated. Specifically, the strain state (strain distribution) is measured in the deformed region subjected to press processing, plotted as a forming diagram, and from the plotted results, a location where the maximum main strain and the minimum main strain are large is specified, The plate thickness reduction rate can be calculated based on the strain state at this specific location, and the success or failure of the work crack can be predicted.

  According to the present invention, it is possible to evaluate stretch flangeability when further performing secondary press molding by each primary press molded body having different pre-strain amounts obtained by primary press molding composed of composite molding.

  Hereinafter, the present invention will be described specifically by way of examples. The present invention is not limited to these.

[Test Example 1]
A first SUS430 improved steel (ferritic stainless steel for high workability) having a plate thickness of 1.5 mm was used as a test material, and a plate material of a press-molding target material cut into a substantially square having a side of 300 mm was used. The composition of the sample material is% by mass: C: 0.009%, Si: 0.27%, Mn: 0.97%, Cr: 18.12%, Cu: 0.18%, Mo: 1 0.93%, N: 0.010%, balance Fe and inevitable impurities.

  The plate material was subjected to press molding under the primary press conditions shown below using the above press workability evaluation apparatus to obtain a primary press molded body having a primary processing height of 25 mm.

  FIG. 10A shows a plan view of the die 23 used, and FIG. 10B shows a cross-sectional view of the die 23 taken along line YY. FIG. 10C shows a front view of the used punch 13.

(Primary press conditions)
Apparatus: 2000 kN Servo press Dies, punch: As shown in FIG. 10 (projection thickness Hp of punch 13: 25 mm)
Plate holding force: 9.5 tons Speed: 5 spm
Lubrication condition: Surface protective film for processing SPV3633 (manufactured by Nitto Denko Corporation)
Target material direction: The rolling direction is parallel to the longitudinal direction of the mold. Processing height: 25 mm

  Subsequently, as shown in FIG. 11, trimming is performed by leaving a flange region (flange remaining portion 62) having a width of 7 mm from the plane portion (corresponding to another region different from the multi-branched region) in the primary press-formed body 40. The part 61 was removed, and a deformed primary press-formed body 60 was obtained.

  Thereafter, using the press workability evaluation apparatus, the deformed primary press-formed body 60 is subjected to press molding under the secondary press conditions shown below, and the flange remaining portion 62 of the deformed primary press-formed body 60 is obtained. As a result, the secondary press-molded body 70 as shown in FIG. 14 was obtained. The edge of the secondary press-formed body 70 has a shape that does not substantially have a flat region corresponding to the flange remaining portion 62.

(Secondary press conditions)
Equipment: 2000 kN Servo press Dies and punches: As shown in FIG. 10 Plate pressing force: 9.5 tons Speed: 5 spm
Lubrication condition: Surface protective film for processing SPV3633 (manufactured by Nitto Denko Corporation)
Target material direction: The rolling direction is parallel to the longitudinal direction of the mold. Processing height: 32 mm

[Test Example 2]
As shown in FIG. 12, the primary processing height at the time of primary press molding and the convex portion thickness Hp of the punch 13 are 30 mm, respectively, and a flange region having a width of 2 mm in the flat surface portion of the primary press molded body 40. A secondary press-formed body 60 was obtained by the same method as in Test Example 1 except that the remaining flange portion 62 was removed.

  FIG. 11 shows the shape of the deformed primary press-formed body 60 after being trimmed in Test Example 1. FIG. 12 shows the shape of the deformed primary press-formed body 60 after being trimmed in Test Example 2. FIG. 13 shows the shape of the secondary press-formed body 70 in Test Examples 1 and 2.

  As described above, in Test Examples 1 and 2, after producing two types of primary press-molded bodies 40 having different deformation amounts, secondary press-molded bodies having the same shape (working height) are obtained by secondary press molding. Produced. Therefore, according to the processing height of each primary press-molded body in Test Examples 1 and 2, the width of the flange region (flange remaining portion 62) to be left when producing the deformed primary press-molded body 60 is set, Secondary press molding was performed under the press conditions that can obtain the same processing height. The secondary press working height consisting of the sum of the primary press working height and the flange region width was set to be 32 mm, as in Test Example 1 (25 mm + 7 mm, Test Example 2 30 mm + 2 mm).

[Test Example 3]
The same method as in Test Example 1 except that the target material to be pressed is a second SUS430 modified steel (ferritic stainless steel for high workability) having a length of 300 mm square and a plate thickness of 1.5 mm. A secondary press-molded body was obtained. In addition, the said 2nd SUS430 improved steel has a composition different from the SUS430 improved steel of Test Example 1. The composition is mass%, C: 0.005%, Si: 0.10%, Mn: 0.17%, Cr: 17.09%, Cu: 1.39%, Mo: 0.04%, N: 0.019%, remaining Fe and inevitable impurities.

[Test Example 4]
The same method as in Test Example 2 except that the target material to be press-formed is a second SUS430 improved steel (ferritic stainless steel for high workability) having a length of 300 mm square and a plate thickness of 1.5 mm. A secondary press-molded body was obtained.

<Analysis>
[Measurement of deformation (distortion)]
As described above, the deformation amount (strain amount) was measured by the following method using a non-contact strain measuring device ARGUS (manufactured by GOM / Kobelco Research Institute).
(1) φ0.8 mm dot marks (1.5 mm intervals) were formed by electrolytic transfer on the surface of the target material (test material) before processing.
(2) The dot mark before press processing and the dot mark after press processing were photographed from multiple directions using a CCD camera.
(3) The distance between dots before and after processing was measured based on the photographed dot marks, and the maximum principal strain and the minimum principal strain were calculated from the amount of change.

[Crack position prediction process]
The secondary press-formed body of the example has a symmetrical shape that is substantially divided into four equal parts. Accordingly, the shaded area shown in FIG. 14 was selected, and the distribution of the maximum principal strain amount in the area was examined.

  FIG. 15 shows the maximum principal strain distribution that can be visualized by superimposing the maximum principal strain amount calculated in (3) of [Measurement of Deformation] on the secondary press-formed bodies related to Test Examples 1 and 2. This is schematically shown. A region surrounded by a broken line near the edge indicates a portion corresponding to the stretch flange region of the secondary press-formed body. In addition, the shaded area corresponds to the size of the maximum principal distortion, indicating that the maximum principal distortion amount is larger as the color is closer to light color, and that the maximum principal distortion amount is smaller as the color is closer to dark color. Show.

  In the stretch flange region, the portion surrounded by the solid line in both Test Example 1 and Test Example 2 has the lightest color and the largest amount of main strain. As shown in FIG. 9, in stretch flange deformation, which is an aspect of uniaxial tensile deformation, it is considered that stretch flange cracking is more likely to occur as the maximum principal strain amount increases. Therefore, it can be predicted that the portion surrounded by the solid line is a portion where stretch flange cracking occurs. For the same reason, it can be easily predicted that a portion where the rate of change in plate thickness is large, that is, a portion where the plate thickness change rate is maximum is within the portion surrounded by the solid line.

  Further, in FIG. 15, the color of the portion surrounded by the solid line is lighter in Test Example 1 than in Test Example 2 and has a larger maximum principal strain amount. As described above, in the stretch flange deformation, the greater the maximum principal strain amount, the easier the stretch flange cracks are generated. Therefore, it can be predicted that Test Example 1 is more likely to cause stretch flange cracks than Test Example 2. For the same reason, it can be easily predicted that the plate thickness change rate of Test Example 1 is larger than that of Test Example 2 as well.

[Measurement of plate thickness change rate]
For Test Example 1 to Test Example 4, the plate thickness in the stretch flange region was measured before and after the secondary press molding, and the plate thickness change amount by the secondary press molding was calculated. In the stretched flange region, 10 points were arbitrarily selected so as to be substantially equally spaced, and the plate thickness was measured using an ultrasonic thickness meter (model: 38DL-PLUS, manufactured by Olympus Corporation). The plate thickness change rate is calculated based on the measured plate thickness, and the maximum value is shown in FIG.

  Test example 1 and test example 2 are compared. As shown in FIG. 16, the secondary press-molded body molded in the process of Test Example 1 has a plate thickness change rate in the stretch flange region as compared with the secondary press-molded body molded in the process of Test Example 2. Large and thin. From this, compared with the process of Test Example 1, the process of Test Example 2 can reduce the change in the plate thickness accompanying the secondary press molding, so that it is predicted that the stretch flange crack of the secondary press molded product can be suppressed. The

  Next, with respect to the type of stainless steel used, each secondary press-formed body of Test Examples 1 to 4 is compared. Test Examples 1 and 2 use the first SUS430 improved steel A, and Test Examples 3 and 4 use the second SUS430 improved steel B, and the types of steel materials are different. Test Examples 1 and 3 show the results of molding in the same process except for the type of steel material, and Test Examples 2 and 4 perform molding in the same process except for the type of steel material. Shows the results.

  As shown in FIG. 16, Test Example 1 is larger than Test Example 3 and Test Example 2 is larger than the Test Example 4 in that the plate thickness change rate of the stretch flange region is larger and the plate thickness is thinner. ing. From this, stainless steel B of Test Examples 3 and 4 is predicted to be a steel material that can suppress elongation flange cracking of the secondary press-formed product as compared with Stainless Steel A of Test Examples 1 and 2.

[Verification with actual product]
[Manufacture of actual products]
Except that the shapes of the dies (dies, punches and plate pressers) are different, primary press molding and secondary press molding are performed by the same process and conditions as in Test Examples 1 and 2, and the secondary press having the shape shown in FIG. A molded body (actual press prototype) was obtained.

  Then, as shown in FIG. 17, the change rate of the plate thickness before and after the secondary press molding was measured at positions 1 to 3. “Cross prototype” in FIG. 17 means the specimens of Test Examples 1 and 2, and “actual press prototype” means the secondary press molded article in FIG. In the region of positions 1 to 3, ten locations were arbitrarily selected so as to be substantially equidistant, and the plate thickness was measured using an ultrasonic thickness meter (model: 38DL-PLUS, manufactured by Olympus Corporation). The plate thickness change rate is calculated based on the measured plate thickness, and the maximum value is shown in FIG.

  As shown in FIG. 18, in the actual press prototype having the shape of FIG. 19, the prototype using the test example 2 process has a plate thickness reduction rate compared to the prototype using the test example 1 process. Small and the plate thickness change was suppressed. The tendency of such a plate thickness change was the same as the test result of the cross-shaped secondary press-formed body shown in FIG.

  From the above, when the stretch flangeability evaluation method according to the present invention is used, the processing stability when stretch flange molding is applied as secondary press molding to the primary press molded body that has been composite molded is as follows. Evaluation and prediction can be carried out easily.

  Further, by comparing the processing stability of various secondary press-formed bodies, it is possible to select an appropriate process that hardly causes stretch flange cracks.

  Further, by plotting the measurement results and obtaining the strain distribution of the molded body, the strain state can be unified and grasped. Therefore, the difference regarding the moldability and workability between the molding target materials can be easily compared.

  Moreover, the workability for each material to be molded can be evaluated by comparing the forming diagram and the plate thickness reduction rate.

  Furthermore, when performing a primary press, the shape of the die | dye and punch suitable for obtaining the primary press molded object of favorable property was examined.

<Optimization of die and punch shape>
[Test Example 5]
Thickness: 0.8 mm of the third SUS430 modified steel (ferritic stainless steel for high workability) as a test material, and the above press workability using a press molding target material cut into a substantially square with a side of 300 mm The primary press molding was performed by the evaluation device under the primary pressing conditions shown below to obtain a plurality of types of primary press molded bodies. In addition, the said test material is SUS430 improved steel which has a composition different from the 1st and 2nd SUS430 improved steel used in Test Examples 1-4. The composition of the sample material is% by mass: C: 0.011%, Si: 0.39%, Mn: 0.36%, Cr: 16.42%, Cu: 0.09%, Mo: 0 0.05%, N: 0.010, balance Fe and inevitable impurities.

(Primary press conditions)
The shapes of the die and punch are as shown in FIGS. The ratio Wd / Rc between the branch width Wd and the radius of curvature Rc of the corner of the branch is eight types of 1, 1.5, 2, 2.5, 5, 10, 15, and 15.5. The primary press conditions similar to those of Test Example 1 were used except that the processing height of the primary press molding and the protrusion thickness Hp of the punch 13 were each 32 mm.

About the obtained various primary press-molded bodies, the state of cracks in the branches was observed. The case where there is no branch crack is indicated as “◯”, and as shown in FIG. 20A, the case where a crack occurs at the boundary of the corner e ₃ is indicated as “× ₁ ”, and FIG. As shown, the case where a crack occurred at the corner e ₃ was defined as “× ₂ ”. The results are shown in Table 1.

As shown in Table 1, when Wd / Rc was 2 or more and 15 or less, no crack occurred in the branches of the press-formed product. On the other hand, when Wd / Rc is less than 2, as shown in (a) of FIG. 20, the primary press molded body, stress is locally concentrated at the boundary of the corner e _3, cracks in the pressed bodies There has occurred. Further, when Wd / Rc exceeds 15, as shown in (b) of FIG. 20, the primary press molded body, shrinking the circumferential direction of the plastic strain range of corners e ₃ branches, at the corner e ₃ Cracking occurred.

[Test Example 6]
The same high-working ferritic stainless steel as in Test Example 5 was used as a test material, and a plate material of a press-molding target material cut into a substantially square shape having a side of 300 mm was used. By the press workability evaluation apparatus, press molding was performed under the primary press conditions shown below to obtain a plurality of types of primary press molded bodies.

(Primary press conditions)
The shapes of the die and punch are as shown in FIGS. When the convex portion of the punch is viewed from the front, the thickness of the convex portion is Hp, the curvature radius of the punch shoulder g ₁ is Rp, and the curvature radius of the die bottom h ₃ is Rd, and Hp and (Rp + Rd) The ratio Hp / (Rp + Rd) of 10 of 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2 It is a kind. Other conditions are the same as in Test Example 5.

  The various primary press-formed bodies thus obtained were subjected to “measurement of deformation amount (strain amount)” and “creation of forming line” as follows, and four processing elements (plane strain tensile deformation and biaxial tensile deformation). , Shrinkage flange deformation and uniaxial tensile deformation) were confirmed.

[Measurement of deformation (distortion)]
As described in Test Example 1, the deformation amount (strain amount) was measured by the following method using a non-contact strain measuring device ARGUS (manufactured by GOM / Kobelco Research Institute).
(1) φ0.8 mm dot marks (1.5 mm intervals) were formed by electrolytic transfer on the surface of the target material (test material) before processing.
(2) The dot mark before press processing and the dot mark after press processing were photographed from multiple directions using a CCD camera.
(3) The distance between dots before and after processing was measured based on the photographed dot marks, and the maximum principal strain and the minimum principal strain were calculated from the amount of change.

[Create forming line]
From the above calculation results, for each dot mark, a plurality of coordinates can be obtained with the maximum principal strain as the vertical axis coordinate and the minimum main strain as the horizontal axis coordinate. In FIG. 21, “primary processing” is a strain distribution of the primary press-molded body obtained by plotting these plural coordinates. In FIG. 21, “secondary processing” is a strain distribution of the secondary press-formed body obtained by plotting these plural coordinates. In these strain distributions, the deformation in the direction of the straight line L ₁ (ε ₂ = 0) corresponds to plane strain tensile deformation, and the deformation in the direction of the straight line L ₂ (ε ₁ = −2ε ₂ ) corresponds to uniaxial tensile deformation. To do. The deformation in the direction of the straight line L ₃ (ε ₁ = ε ₂ ) corresponds to biaxial tensile deformation, and the deformation in the direction of the straight line L ₄ (ε ₁ = −ε ₂ ) corresponds to shrinkage flange deformation. The shaded area corresponds to the size of the maximum principal distortion, and indicates that the maximum principal distortion amount is larger as the color is lighter, and the maximum principal distortion amount is smaller as the color is closer to dark color.

Further, a forming line can be obtained by connecting the outermost lines of the strain distribution. For example, in the forming line shown in FIG. 21, a curved line having a shape protruding from a to d is drawn. These ad are located in a place where the maximum principal strain ε ₁ and the minimum principal strain ε ₂ are large in the region measured by the press-formed body, and the four processing elements A to D shown in FIG. This is a region that is strongly subjected to the deformation form D. These measurement points, of which, is a point closest to the straight line L _1, corresponds to a suitable location for placement of the plane strain tensile deformation, b that is closest to the straight line L ₂ is a uniaxial tensile deformation corresponds to a suitable location for placement, the c point closest to the straight line L _3, the biaxial tensile corresponds to a suitable location for placement of the deformation, d that is closest to the straight line L ₄ are, shrinkage measurements flange deformation It corresponds to a suitable location.

As shown in FIG. 21, since the amount of strain is very small in the primary processing, the secondary press-formed body according to Test Example 6 and the secondary press-formed body according to Test Example 5 have a large difference in strain distribution. Is not confirmed. In the primary processing, the convex portion of the primary press-formed body has a strain distribution close to plane strain deformation as indicated by point a. On the other hand, in the secondary processing, in both Test Example 5 and Test Example 6, a strain accompanied by uniaxial tensile deformation such as point a ′ is newly generated. When test example 5 and test example 6 are compared, the point a between the _two differs in the distance close to the uniaxial tensile deformation reference line L2, and the line segment aa ′ in test example 6 is the same as in test example 5. than the line a-a 'are inclined toward the reference line L ₂ of the uniaxial tensile deformation. As shown in FIG. 9, the uniaxial tensile deformation is a processing element in which the maximum principal strain is tensile and the minimum principal strain is compression. Therefore, a press-molded body to which uniaxial tensile deformation is applied has a large absolute value of the minimum principal strain. As shown, the decrease in thickness tends to be smaller due to the constant volume relationship. Therefore, the above line a-a 'is the minimum principal strain influence becomes stronger closer to the reference line L ₂ of the uniaxial tensile deformation, can visually recognize that the degree of thickness reduction decreases. And since excessive plate | board thickness reduction is related to a crack, the order between the steel types regarding the ease of a crack after secondary processing can be evaluated simply.

Test results shown in FIG. 21, line a-a of Test Example 6 'indicates that the closer to the reference line L ₂ of the uniaxial tensile deformation than the test example 1. For this reason, the stainless steel used in Test Example 6 has a smaller thickness reduction than that used in Test Example 5 and can be determined to be a steel type with good workability because cracking of the secondary press-formed product is suppressed. .

Table 2 summarizes the conditions under which machining cracks are likely to occur in the machining region. In addition to large maximum principal strain and minimum principal strain, work cracks are likely to occur depending on the degree to which a to d are close to or separated from L _{1 to} L ₄ . The reason will be described based on the fact that the plate material during press molding has a constant volume relationship.

For example, if a is close to L _1, by the tensile strain is biased in one axial direction (maximum principal strain direction), becomes narrow strain range in the plane of the plate, it tends to occur a local thickness reduction . b is if leaving the maximum principal strain direction from L _2, the effect of the plate thickness increase due to compressive strain is reduced, leading to local thickness reduction. Further, when c is close to L ₃ , the strain is equalized in the biaxial direction, so that the in-plane strain is easily spread uniformly, whereas when c is away from L _{3 in the} maximum principal strain direction, The uniform strain area becomes narrow and local thickness reduction is likely to occur. If d is separated from L ₄ to the minimum principal strain direction, pulling elements of one axial (maximum principal strain direction) is reduced, since the compressive strain in the plane of the plate becomes excessive, the follow-up of the plate thickness increases It becomes difficult and cracking is likely to occur.

  FIG. 22 schematically shows the strain distribution visualized by superimposing the forming line shown in FIG. 21 on the primary press-formed body and the secondary press-formed body according to Test Example 6. A broken line shows the outline of the convex part of a press-molded body. In addition, the shaded area corresponds to the size of the maximum principal distortion, indicating that the maximum principal distortion amount is larger as the color is closer to light color, and that the maximum principal distortion amount is smaller as the color is closer to dark color. Show.

FIG. 23 is a detailed description of the maximum principal strain and the minimum principal strain at the locations corresponding to the points a to d shown in FIG. The part enclosed by the circle | round | yen of ad of the press-molding body of a primary process shows the location corresponded to the points ad. An arrow ε ₁ described in a circle indicates a direction in which the maximum principal strain is generated at the points a to d, and an arrow ε ₂ indicates a direction in which the minimum principal strain is generated at the points a to d. The direction of the arrow indicates the direction of deformation, the outward arrow indicates that the deformation is tensile deformation, and the inward arrow indicates that the deformation is compression deformation. Incidentally, the measurement of plane strain tensile deformation is suitable at the position of point a, the measurement of uniaxial tensile deformation is suitable at the position of point b, and the measurement of biaxial tensile deformation is suitable at the position of point c, At the position of the point d, it is preferable to measure the shrinkage flange deformation. The portion surrounded by the circle a ′ in the press-formed body of the secondary processing indicates a portion corresponding to the point a ′. The point a ′ is at a position corresponding to the point a in the first press-formed body, and the meanings of the arrows ε ₁ and ε ₂ are the same as described above. Hereinafter, the points a to d and the point a ′ may be referred to as the positions a to d and the position a ′.

As shown in the primary processed press-formed body of FIG. 23, (a) a location (including a position a) corresponding to a region including the bottom h ₃ adjacent to the corner f ₃ of the convex portion of the primary press-formed body, In a region corresponding to C in FIG. 6, it can be seen that plane strain tensile deformation occurs. In addition, (a) Uniaxial tensile deformation occurs in a portion corresponding to the region of the side surface in the length direction in the branch of the primary press-formed body (the region including the position b and corresponding to S in FIG. 6). I understand. In addition, (c) At the corner (e ₃ ) of the convex portion of the primary press-molded body and the portion corresponding to the region that is the shoulder g ₃ (the region that includes the position c and corresponds to T in FIG. 6). It can be seen that biaxial tensile deformation occurs. In addition, (d) it can be seen that shrinkage flange deformation has occurred in a portion corresponding to the region of the flat portion of the primary press-formed body (including the position d and corresponding to F in FIG. 6). That is, the press-formed body according to the test example includes all of four processing elements (plane strain tensile deformation, biaxial tensile deformation, shrinkage flange deformation, and uniaxial tensile deformation), and the position a is plane strain tensile deformation. For measurement, position b is suitable for measurement of uniaxial tensile deformation, position c is suitable for measurement of biaxial tensile deformation, and position d is suitable for measurement of uniaxial tensile deformation. Moreover, it turns out that the plane strain tensile deformation | transformation and the uniaxial tensile deformation have arisen in the area | region containing position a 'in a secondary press molding.

  For the plurality of types of primary press-molded bodies obtained in Test Example 6, the case where the deformation forms corresponding to the above-described processing elements A to D were revealed was “◯”, and the case where the deformation was not revealed was “×” " The results are shown in Table 3.

As shown in Table 3, in the primary press-molded product obtained by subjecting the target material to primary press molding, Hp / (Rp + Rd) is 0.5 or more, that is, Hp is 0.5 times or more of (Rp + Rd). In some cases, it was confirmed that all four processing elements (plane strain tensile deformation, biaxial tensile deformation, shrinkage flange deformation, and uniaxial tensile deformation) can be measured suitably. On the other hand, in a press-molded product having Hp / (Rp + Rd) of less than 0.5, that is, Hp of less than 0.5 times (Rp + Rd), only biaxial tensile deformation can be measured among the four working elements. Other processing elements (plane strain tensile deformation, shrinkage flange deformation, and uniaxial tensile deformation) could not be suitably measured.
When Hp ≧ (Rp + Rd) / 2, a plurality of processing elements appear in one primary press-formed body, which is preferable in that it can be used for evaluation of workability.

DESCRIPTION OF SYMBOLS 1 Press workability evaluation apparatus 10 Punch part 11 Bottom plate 12 Punch holder 13 Punch 132 Convex part 20 Die part 21 Upper plate 22 Die holder 23 Dice 231 Recess 232 Dice side plane part 30, 30A, 30B Guide part 31, 31A, 31B Guide Pin 32, 32A, 32B Plate retainer 321 Plate retainer side plane portion 33 Cushion pin 40 Primary press molded body 41 Press molded body convex portion 42 Planar portion 43 Branch 50 Plate material 60 Deformed primary press molded body 61 Trim portion 62 Flange remaining portion 70 Secondary press-formed body

Claims (8)

A punch having a multi-branch-shaped convex part, a die having a concave part and a flat part that can be fitted to the convex part, and a plate presser for pressing a target material to be press-molded between the flat part. A step of primary press molding the target material by a press workability evaluation device to form a primary press molded body having a flange portion, and obtaining a plurality of primary press molded bodies having different pre-deformation amounts 1 Next press molding process,
A modified primary press molding step of performing a process of removing a partial region from the primary press molded body to obtain a modified primary press molded body including a flange region;
Using the press workability evaluation device, a secondary press molding step of performing a secondary press molding for flange-up of the flange region of the deformed primary press molded body to obtain a plurality of secondary press molded bodies;
A method for evaluating stretch flangeability, comprising a deformation amount measuring step of measuring a thickness change amount of a portion corresponding to the stretch flange region in the secondary press-formed body.   The deformation primary forming step is a process of removing at least one of the multi-branched region in the primary press-formed body or another region different from the multi-branch region, leaving the flange region having a predetermined width. The method for evaluating stretch flangeability according to claim 1.   The flange-up in the secondary press molding step is processed so as to form a flange height substantially equal to the sum of the flange height of the primary press molded body and the width of the flange region of the deformed primary press molded body. The stretch flangeability evaluation method according to claim 1 or 2.   The press workability evaluation apparatus includes a punch having a multi-branch-shaped convex portion including three or more branches, a concave portion including three or more branches that can be fitted to the convex portion, and a die side plane portion surrounding the concave portion. And a plate presser having a plate presser side plane portion substantially parallel to the die side plane portion, and capable of sandwiching the target material between the die side plane portion and the plate presser side plane portion. The evaluation method of stretch flangeability of claim | item 1-3.   The angles formed by the multi-branched branches are obtuse when there are three branches, are substantially perpendicular when there are four branches, and when there are five or more branches. The evaluation method of stretch flangeability according to any one of claims 1 to 4, which are acute angles to each other.   The stretch flangeability evaluation method according to any one of claims 1 to 5, wherein the number of the multi-branched branches is four, and the convex portions and the concave portions are substantially cross-shaped. When the die is viewed in plan, the corners of the recesses are curved, the width of the branch of the die is Wd, and the radius of curvature at the corner of the branch of the die is Rc. The ratio Wd / Rc to Rc is 2 or more and less than 15,
When the punch is viewed from the front, the shoulder at the top of the convex portion is curved, the radius of curvature at the shoulder is Rp, the thickness of the convex is Hp, and the die is cross-sectioned. When viewed, the bottom of the recess is curved, and when the curvature radius of the bottom is Rd, Hp ≧ (Rp + Rd) / 2, and stretch flangeability according to claim 1 Evaluation method.   The stretched flange property evaluation method according to any one of claims 1 to 7, wherein a blank having a quadrangular shape, an octagonal shape, an elliptical shape, or a circular shape is used as the target material.