WO2013157062A1 - Method for predicting press formation cracks and method for manufacturing pressed product - Google Patents

Method for predicting press formation cracks and method for manufacturing pressed product Download PDF

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Publication number
WO2013157062A1
WO2013157062A1 PCT/JP2012/060214 JP2012060214W WO2013157062A1 WO 2013157062 A1 WO2013157062 A1 WO 2013157062A1 JP 2012060214 W JP2012060214 W JP 2012060214W WO 2013157062 A1 WO2013157062 A1 WO 2013157062A1
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Prior art keywords
cracks
metal plate
crack
strain
press
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PCT/JP2012/060214
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French (fr)
Japanese (ja)
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新宮　豊久
祐輔 藤井
雄司 山▲崎▼
和彦 樋貝
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Ｊｆｅスチール株式会社
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Priority to PCT/JP2012/060214 priority Critical patent/WO2013157062A1/en
Publication of WO2013157062A1 publication Critical patent/WO2013157062A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/20—Design optimisation, verification or simulation
    • G06F30/23—Design optimisation, verification or simulation using finite element methods [FEM] or finite difference methods [FDM]
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02—Details not specific for a particular testing method
    • G01N2203/0202—Control of the test
    • G01N2203/0212—Theories, calculations
    • G01N2203/0216—Finite elements

Definitions

  • the present invention relates to a method for accurately determining cracks in press molding, and a method for producing a pressed part by press molding under the condition that cracks are not generated using the method.
  • Press forming is a typical metal processing method in which a metal plate is sandwiched and pressed between a pair of molds, and a metal plate such as a steel plate is formed to follow the shape of the die to obtain a part having a desired shape. It is used in a wide range of manufacturing fields such as automobile parts, machine parts, building components, and home appliances.
  • the element is divided into a two-dimensional (planar) mesh, but the element is often divided into a three-dimensional (three-dimensional) mesh.
  • the element to be divided may be a triangle (triangular prism) as in the part simulating the upper mold 1 and the lower mold 2 in FIG. In some cases, it may be a quadrangle (a rectangular parallelepiped), and although not shown, it may be a hexagon (a hexagonal column).
  • the thickness center of the metal plate is often divided into two-dimensional meshes.
  • the conventional prediction of the presence or absence of press cracking is a press forming simulation of a metal plate using the finite element method, and the maximum principal strain ⁇ 1 after forming at the plate thickness center of each element divided into meshes as described above and The minimum principal strain ⁇ 2 (each scalar) is obtained by calculation from the coordinate change of the contact point, and the maximum principal strain ⁇ 1 and the minimum principal strain ⁇ 2 are prepared separately as shown in FIG. Fig.
  • Figure FLD Forming Limit Diagram
  • Forming limit line FLC Forming Limit Curve
  • the typical FLD creation methods include the Nakajima method and the Marciniak method. These methods use several kinds of samples with various shapes (widths) marked with various patterns, such as scribed circles, ball head punches (Nakajima method) or round heads with a tip curvature radius of about 25 to 50 mm.
  • a punch Marciniak method
  • stretch molding until fracture or necking occurs stretch molding until fracture or necking occurs, and determine the maximum and minimum principal strains at the positions where fractures and necking occur from the change in marking after molding.
  • It is a method of displaying and obtaining a forming limit line (FLC).
  • This FLC shows the forming limit at the sheet thickness center, and predicts the occurrence of cracks by comparing with the press forming simulation calculated using the element at the sheet thickness center.
  • the present invention has been developed to solve the above-mentioned problems of the prior art, and its purpose is to use a crack prediction method capable of accurately predicting the occurrence of cracks in press molding and the prediction method.
  • the object is to propose a method of manufacturing a pressed part that is press-formed under the condition that no cracking occurs.
  • the inventors further studied a method for accurately predicting the occurrence of cracks including not only ductile-dominated cracks but also bendability-dominated cracks.
  • the maximum principal strain at the center of the thickness of the press-formed product, the minimum principal strain and the forming limit diagram are compared to predict the occurrence of cracks, and the occurrence of cracks on the surface of the press-formed product ( The occurrence of cracks based on the prediction of the more severe of the results for the cracks.
  • it was conceived that the occurrence of cracks can be predicted with high accuracy, and the present invention has been completed.
  • a limit surface strain at which a crack is generated on the surface of a metal plate to be press-formed is obtained in advance, and then a simulation for press-forming the metal plate into a desired part shape using a finite element method is performed.
  • the crack prediction method in press molding of the present invention is characterized in that the limit surface strain is obtained using a bending test.
  • the crack prediction method in the press forming of the present invention is to perform a bending test of a metal plate to be press formed in advance to obtain a limit bending radius at which the crack is generated, and then simulate the bending test using a finite element method.
  • the maximum principal strain on the surface of the metal plate corresponding to the limit bending radius is obtained, this is set as the limit surface strain, and then the metal plate is subjected to a simulation of press forming into a desired part shape using the finite element method, A first crack prediction for predicting that a crack will occur when the maximum principal strain on the surface of any element is equal to or greater than the limit surface strain of the metal plate is performed.
  • the present invention proposes a method for manufacturing a pressed part characterized by predicting the presence or absence of cracks in press forming by any of the methods described above and press forming a metal plate under conditions that do not cause cracks. To do.
  • occurrence of cracks in press molding can be accurately predicted
  • occurrence of cracks during press molding of various parts such as automobile panel parts and structural / framework parts can be accurately predicted.
  • FIG. 1A and 1B are diagrams for explaining a press forming simulation using a finite element method.
  • FIG. 1A shows a relationship between a mold and a material to be formed (metal plate), and
  • FIG. 1B shows an example of mesh division.
  • FIG. 2 is a diagram for explaining a forming limit diagram (FLD).
  • 3A and 3B are diagrams for explaining the form of cracks.
  • FIG. 3A shows a crack in which ductility is controlled
  • FIG. 3B shows a crack in which bendability is controlled.
  • FIG. 4 is a flowchart illustrating a method for predicting the occurrence of cracks according to the present invention.
  • FIG. 5 is a diagram for explaining a punch that can be used for measuring the limit surface strain.
  • FIG. 5 is a diagram for explaining a punch that can be used for measuring the limit surface strain.
  • FIG. 6 is a flowchart for explaining another method for predicting the occurrence of cracks according to the present invention.
  • FIG. 7 is a diagram illustrating shell elements used for press molding simulation.
  • FIG. 8 is a diagram illustrating a 90-degree bending test of the example.
  • FIG. 9 is a diagram showing the relationship between (limit bending radius / plate thickness) of the steel plate used in the example and the bending tip outer surface strain at the time of occurrence of cracking.
  • FIG. 10 is a diagram for explaining the shape and dimensions of a press-molded part in the example.
  • FIG. 11 is a diagram showing the forming limit diagram FLD of the steel plates A to C in Table 1.
  • FIG. 12 is a diagram showing a comparison between the mold position where a crack occurred in the steel sheet when the steel sheets A to C in Table 1 were press-formed and the value predicted by the two types of crack prediction methods at the same position. is there.
  • the inventors have further studied the cause of the above-mentioned unique crack that is likely to occur in a high-strength steel sheet, and as a result, have found that bendability is greatly involved in the crack.
  • factors affecting cracking include ductility and bendability.
  • ductility dominates cracks in press forming, so necking in which the sheet thickness decreases locally.
  • the cracks are generated when the ductility reaches the limit (hereinafter, this crack is also referred to as “ductile-dominated crack”).
  • this crack is also referred to as “ductile-dominated crack”.
  • the bendability of the steel sheet decreases, and cracks in press forming also shift from ductility control to bendability control.
  • this crack is also referred to as “bending-dominated crack”). Called).
  • the inventors further studied on accurately predicting the occurrence of cracks in press forming simulations using the finite element method, including not only ductility-controlled cracks but also bendability-controlled cracks. Piled up.
  • the maximum principal strain at the center of the thickness of the press-formed product, the minimum principal strain, and the crack limit prediction for predicting the occurrence of cracks by comparing the forming limit diagram, the strain on the surface of the press-formed product Two types of crack predictions are performed: bending-dominant crack generation prediction, which predicts the occurrence of cracks by comparing the maximum principal strain) and the limit surface strain, and based on the prediction that is severer of those cracks
  • the inventors have conceived that the occurrence of cracks can be accurately predicted by predicting the occurrence of cracks. This invention is made
  • FIG. 4 shows a flow of one embodiment of the crack prediction method of the present invention.
  • it is necessary to predict the occurrence of cracks controlled by bendability.
  • it is necessary to obtain in advance a limit surface strain at which a crack occurs on the surface of the steel sheet due to bending deformation by some test.
  • a bending test as shown in FIG. 5 (a), a ball head punch as shown in FIG. 5 (b), and an overhang test using an elliptical ball head punch (not shown).
  • the bending test may use any bending angle such as 90 degree bending (V bending) or 180 degree bending (U bending).
  • the “material” may be yield stress, hardness, bendability, etc. in addition to tensile strength. “Depending on the material” means that, for example, the limit surface strain is 0.50 when the tensile strength is 980 MPa class, and the limit surface strain is 0.22 when the tensile strength is 1180 MPa class. means. By doing so, the limit surface strain at which cracking can occur can be expressed as a function of tensile strength, yield stress, etc., so that other materials can be used as an index of cracking instead of the above limit surface strain. It is.
  • a forming limit diagram representing a forming limit (maximum principal strain and minimum principal strain) at the center of the plate thickness when cracking occurs in the steel sheet.
  • a method for obtaining the FLD there are the Nakajima method using a ball head punch, the Marciniak method using a circular head punch, and the like, and any of them may be used.
  • the maximum principal strain and the minimum principal strain at the center of the plate thickness of each element are obtained by simulation using a finite element method in which the steel sheet is press-formed into a desired part shape, and the above-mentioned FLD forming limit line FLC is compared. It is predicted that when the plot of the maximum principal strain and the minimum principal strain at the center of the thickness of any element is equal to or greater than the above FLC, that is, when it exists in the crack generation region, ductile-dominated cracking occurs.
  • the results of the prediction of the occurrence of cracks controlled by bendability and the results of the prediction of the generation of cracks controlled by ductility are compared, and the presence or absence of occurrence of cracks in press forming is determined based on the prediction that is severer to crack Predict.
  • both the ductility-predicted crack prediction and the bendability-dominated crack prediction are considered, and the prediction that is disadvantageous to the crack is adopted.
  • the critical bending radius at which cracking occurs (the maximum bending radius at which cracking occurs) is determined, and then the bending test is simulated using the finite element method to determine the maximum principal strain at the limiting bending radius. Strain may be used instead of the above limit surface strain. For example, as shown in FIG. 6, a 90-degree bending test is performed in advance on a steel sheet that is a material to be formed, and a limit bending radius at which cracking occurs is obtained.
  • the maximum principal strain of the surface at the position where the crack is generated in the 90-degree bending test is obtained by simulation using the finite element method, and this is defined as the limit surface strain.
  • a simulation of press forming the steel sheet into a desired part shape using the finite element method is performed, the maximum principal strain of the surface of each divided element is obtained, the maximum principal strain of the surface of each element and the limit surface strain of the steel sheet It is predicted that when the maximum principal strain on the surface of any element is equal to or greater than the above-mentioned limit surface strain, a bending-dominated crack will occur. Thereafter, the ductility-dominated crack prediction described above is performed, and the occurrence of the crack is predicted using the prediction that is more severe with respect to the crack among the bendability-dominated crack prediction and the ductility-dominated crack prediction.
  • the maximum principal strain of the surface obtained by the press forming simulation using the finite element method and the limit surface strain of the steel sheet do not need to be performed for all elements of the molded part, and there is a concern that cracks may occur from experience.
  • the maximum principal strain of the surface and the limit surface strain may be compared only with respect to the element of the portion.
  • a simulation model is often created with a shape. For example, as shown in FIG. 7, a point (integration point) for calculating stress and strain is set in each element, and an integration point is set in the plate thickness equal position in the plate thickness direction. The maximum principal strain and the minimum principal strain are calculated at each integration point.
  • the surface strain in the present invention is the average value of the maximum principal strain and the average value of the minimum principal strain calculated at the integration point on the surface of the metal plate. To explain with the middle symbols, it is the average value of d s 1 to d s 4. Note that the number and position of the integration points are determined by the type of elements used in the simulation.
  • the method for manufacturing a pressed part according to the present invention predicts cracking of a metal plate in press molding by the crack prediction method described above, and if it is predicted that cracking will occur, the condition (metal plate In this method, parts are manufactured by stopping the press molding in the mold) and changing to a condition that does not cause cracking. Specifically, the mold shape used for press molding is changed, the molding conditions such as press speed, temperature and lubrication are changed, or the metal plate material (ductility, bendability, etc.) is changed. Thus, press molding can be performed under conditions that do not cause cracks.
  • the steel plates A to C shown in Table 1 were press-molded into a part shape (see FIG. 10) simulating the roof rail connection portion at the upper part of the center pillar of an automobile, using a model die consisting of an upper die and a lower die.
  • the mold position (distance from the bottom dead center) when the crack occurred was investigated.
  • the mold positions where the cracks of the steel plates A to C occurred were 12 mm for the steel plate A, 11 mm for the steel plate B, and 9 mm for the steel plate C.
  • the press forming is subjected to a three-dimensional simulation, and cracks in the steel plate during the forming process until the die reaches the bottom dead center.
  • the maximum principal strain on the steel sheet surface of the element corresponding to the generation position, the maximum principal strain ⁇ 1 and the minimum principal strain ⁇ 2 at the center of the plate thickness were determined.
  • the die position (the distance from the bottom dead center) obtained by the simulation, in which the maximum principal strain on the surface of the steel plate where cracking occurs is equal to or greater than the limit surface strain corresponding to the limit bending radius described above. )
  • the mold positions where cracks occurred were 11 mm for steel plate A, 9 mm for steel plate B, and 0 mm for steel plate C (no cracks).
  • the steel sheet A will be described.
  • the limit surface strain is 0.22
  • the maximum principal strain on the steel plate surface at a position where the mold position is 12 mm from the bottom dead center is 0.211 and the position at a position 11 mm from the bottom dead center. Since the maximum principal strain on the steel sheet surface was 0.224, it was predicted that a crack would occur at a position 11 mm from the bottom dead center.
  • a forming limit diagram FLD was obtained by using an overhang test (Nakajima method) using a ball head punch having a tip curvature radius of 25 mm, and the result is shown in FIG. It was.
  • the maximum principal strain ⁇ 1 and the minimum principal strain ⁇ 2 at the center of the plate thickness at the position where cracking occurs in the steel plate by the above-described press forming simulation are shown in FIG. It showed how it would change until it reached.
  • the steel plate A is 5 mm and the steel plate B was 10 mm, and the steel plate C was 8 mm.
  • the crack prediction by FLD it is predicted that the crack will occur when the mold is lowered from the bottom dead center to a position of 5 mm.
  • the crack prediction it is predicted that a crack will occur when the die is lowered to a position 11 mm from the bottom dead center, and the crack prediction using the limit surface strain is actually when the crack occurs in the press molding.
  • the mold position (12 mm from the bottom dead center) is accurately predicted.
  • the steel plate B having better bendability than the steel plate A as shown in FIG. 12B, in the prediction of cracking by FLD, it is predicted that cracking will occur when the mold is lowered from the bottom dead center to a position of 10 mm.
  • the crack prediction using the FLD is predicted. Actually predicts the mold position (11 mm from bottom dead center) at the time of occurrence of cracks in press molding with higher accuracy. Further, in the steel plate C having better bendability, as shown in FIG. 12 (c), in the crack prediction by FLD, it is predicted that the crack will occur when the die is lowered from the bottom dead center to the position of 8mm. On the other hand, in the crack prediction due to the limit surface strain, it is predicted that no crack will occur even if the mold is lowered to the bottom dead center. Again, contrary to the steel plate A, the crack prediction using FLD is actually more effective. Furthermore, the mold position (9 mm from the bottom dead center) at the time of occurrence of cracks in press molding is accurately predicted.
  • the present invention is not limited to the contents described above.
  • a steel plate having a tensile strength of 980 MPa class or more (1180 MPa class steel plate) is shown. Is preferably applied to press forming of such a high-strength steel plate, but can also be applied to a steel plate having a tensile strength of less than 980 MPa or a metal plate other than a steel plate.

Abstract

A first crack prediction is executed, whereby the limit surface strain at which cracks occur in the surface of a metal plate which is to be press-formed is calculated in advance, and a simulation is performed using a finite element method, wherein the metal plate is press-formed into a desired product shape, and when the surface strain of any of the elements is equal to or greater than the limit surface strain of the metal plate, it is predicted that cracks will occur. A second crack prediction is executed, whereby the forming limit diagram of the metal plate which is to be processed is determined in advance, and a simulation is performed using a finite element method, wherein the metal plate is press-formed into a desired product shape, and when the plot of the maximum main strain and the minimum main strain at the plate thickness center of any of the elements is equal to or greater than the forming limit curve of the forming limit diagram, it is predicted that cracks will occur. Then, the occurrence of cracks is predicted on the basis of whichever of the two predictions is stricter with respect to cracking, thereby improving the precision with which the occurrence of cracks during press formation can be predicted.

Description

プレス成形における割れ予測方法およびプレス部品の製造方法Method for predicting cracks in press molding and method for manufacturing pressed parts
　本発明は、プレス成形における割れを精度よく判定する方法と、その方法を用いた割れ発生のない条件でプレス成形することにより、プレス部品を製造する方法に関するものである。 The present invention relates to a method for accurately determining cracks in press molding, and a method for producing a pressed part by press molding under the condition that cracks are not generated using the method.
　プレス成形は、一対の金型の間に金属板を挟んで挟圧し、鋼板等の金属板を型の形状に倣うよう成形して所望の形状の部品を得ようとする代表的な金属加工方法の一つであり、自動車部品、機械部品、建築部材、家電製品等、幅広い製造分野で用いられている。 Press forming is a typical metal processing method in which a metal plate is sandwiched and pressed between a pair of molds, and a metal plate such as a steel plate is formed to follow the shape of the die to obtain a part having a desired shape. It is used in a wide range of manufacturing fields such as automobile parts, machine parts, building components, and home appliances.
　プレス成形する際、成形途中で被成形材である金属板に割れが生じる現象がたびたび問題視されており、その解決策として、金属板自体の成形性を高めることの他に、プレス成形における割れを精度よく予測しようとする努力がなされている。例えば、近年よく用いられている方法としては、有限要素法を用いたプレス成形シミュレーションにおいて、金型形状や金属板の機械的特性（材質）の他、金型や金属板の温度、金型を閉塞させる速度（プレス速度）、潤滑条件などの各種プレス条件を変化させて、割れが生じない成形条件を探ろうとする方法がある。 During press molding, the phenomenon that cracks occur in the metal plate, which is the material being molded, is often regarded as a problem. As a solution to this, in addition to improving the formability of the metal plate itself, Efforts are being made to accurately predict this. For example, as a method often used in recent years, in the press molding simulation using the finite element method, in addition to the mold shape and the mechanical characteristics (material) of the metal plate, the temperature of the mold and the metal plate, the mold There is a method in which various pressing conditions such as a closing speed (pressing speed) and lubrication conditions are changed to search for molding conditions that do not cause cracking.
　有限要素法は、例えば、特許文献１より抜粋した、図１（ａ）に示すようなハット形部材をプレス成形する場合、図１（ｂ）に示すように、上型１、下型２および金属板３を模擬した部分を、仮想的なメッシュから構成される要素群に分解した上で、プレス成形における各段階で、各要素にどれだけの応力とひずみがどの方向に作用するかを解析（シミュレーション）する方法である。 In the finite element method, for example, when a hat-shaped member as shown in FIG. 1A extracted from Patent Document 1 is press-molded, as shown in FIG. The part simulating the metal plate 3 is disassembled into elements composed of virtual meshes, and then analysis is performed on how much stress and strain are applied to each element in each stage of press forming. (Simulation) method.
　ここで、図１（ｂ）の例では、二次元的（平面的）なメッシュに要素分割しているが、三次元的（立体的）なメッシュに要素分割する場合も多い。また、分割する要素は、図１（ｂ）の上型１、下型２を模擬した部分のように、三角形（三角柱）とする場合もあれば、金属板３を模擬した部分のように、四角形（直方体）とする場合もあり、さらに、図示していないが、六角形（六角柱）とする場合もある。なお、金属板のプレス成形シミュレーションでは、金属板の板厚中心を二次元的なメッシュに要素分割する場合が多い。 Here, in the example of FIG. 1B, the element is divided into a two-dimensional (planar) mesh, but the element is often divided into a three-dimensional (three-dimensional) mesh. In addition, the element to be divided may be a triangle (triangular prism) as in the part simulating the upper mold 1 and the lower mold 2 in FIG. In some cases, it may be a quadrangle (a rectangular parallelepiped), and although not shown, it may be a hexagon (a hexagonal column). In the press forming simulation of a metal plate, the thickness center of the metal plate is often divided into two-dimensional meshes.
　ところで、従来のプレス割れ発生有無の予測は、有限要素法を用いた金属板のプレス成形シミュレーションで、上記のようなメッシュに分割した各要素の板厚中心における成形後の最大主ひずみε１および最小主ひずみε２（各スカラー）を接点の座標変化から計算により求め、上記最大主ひずみε１および最小主ひずみε２が、別途作成しておいた、図２に示したような成形限界線図ＦＬＤ（Ｆｏｒｍｉｎｇ　Ｌｉｍｉｔ　Ｄｉａｇｒａｍ）の成形限界線ＦＬＣ（Ｆｏｒｍｉｎｇ　Ｌｉｍｉｔ　Ｃｕｒｖｅ）を挟んで割れ発生領域、割れなしの領域のいずれの側に存在するかを確認し、割れ発生領域に存在する場合に、割れが発生すると予測していた。 By the way, the conventional prediction of the presence or absence of press cracking is a press forming simulation of a metal plate using the finite element method, and the maximum principal strain ε 1 after forming at the plate thickness center of each element divided into meshes as described above and The minimum principal strain ε 2 (each scalar) is obtained by calculation from the coordinate change of the contact point, and the maximum principal strain ε 1 and the minimum principal strain ε 2 are prepared separately as shown in FIG. Fig. Figure FLD (Forming Limit Diagram) Forming limit line FLC (Forming Limit Curve) is sandwiched between the crack generation area and the non-cracking area to confirm whether it exists in the crack generation area. Was expected to occur.
　上記ＦＬＤの作成方法には、代表的なものとして、中島法やＭａｒｃｉｎｉａｋ法がある。これらの方法は、スクライブドサークル等、各種のパターンをマーキングした、形状（幅）の異なる数種のサンプルを使用し、先端曲率半径が２５～５０ｍｍ程度の球頭パンチ（中島法）あるいは円頭パンチ（Ｍａｒｃｉｎｉａｋ法）を用いて、破断もしくはネッキングが発生するまで張出し成形し、成形後のマーキングの変化から、破断やネッキング発生位置の最大主ひずみ、最小主ひずみを求め、その測定結果を二次元表示して成形限界線（ＦＬＣ）を得る方法である。このＦＬＣは、板厚中心の成形限界を示しており、前述した板厚中心の要素を用いて計算したプレス成形シミュレーションと照らし合わせることで、割れの発生を予測するものである。 The typical FLD creation methods include the Nakajima method and the Marciniak method. These methods use several kinds of samples with various shapes (widths) marked with various patterns, such as scribed circles, ball head punches (Nakajima method) or round heads with a tip curvature radius of about 25 to 50 mm. Using a punch (Marciniak method), stretch molding until fracture or necking occurs, and determine the maximum and minimum principal strains at the positions where fractures and necking occur from the change in marking after molding. It is a method of displaying and obtaining a forming limit line (FLC). This FLC shows the forming limit at the sheet thickness center, and predicts the occurrence of cracks by comparing with the press forming simulation calculated using the element at the sheet thickness center.
特開２００８－５５４８８号公報JP 2008-55488 A
　しかしながら、発明者らの研究によれば、上記従来の割れ予測方法を用いた場合、割れ発生なしと予測されたプレス成形条件でも、実際の金属板のプレス成形、特に高強度鋼板のプレス成形で割れが発生し、予測した結果と現実とが大きく乖離している事例が多々発生していた。 However, according to the research by the inventors, when the above conventional crack prediction method is used, even in the press forming conditions where it is predicted that there is no occurrence of cracks, the actual metal plate press forming, particularly the press forming of a high-strength steel plate. There were many cases in which cracks occurred and the predicted results and the reality were significantly different.
　本発明は、従来技術が抱える上記問題点を解決すべく開発したものであり、その目的は、プレス成形における割れの発生を精度よく予測することができる割れ予測方法と、その予測方法を用いた割れ発生のない条件でプレス成形するプレス部品の製造方法を提案することにある。 The present invention has been developed to solve the above-mentioned problems of the prior art, and its purpose is to use a crack prediction method capable of accurately predicting the occurrence of cracks in press molding and the prediction method. The object is to propose a method of manufacturing a pressed part that is press-formed under the condition that no cracking occurs.
　発明者らは、特に高強度鋼板において、従来の方法による割れ発生の予測とプレス成形における割れ発生の結果とが乖離している原因について、鋭意検討を重ねた。その結果、鋼板に発生する割れには、主に低強度、軟質材において起こる延性が支配する割れと、主に高強度、硬質材において起こる曲げ性が支配する割れとがあり、従来の割れ予測方法では、後者の曲げ性支配の割れについては一切考慮していなかったためであることが明らかとなった。 The inventors have made extensive studies on the cause of the difference between the prediction of cracking by conventional methods and the result of cracking in press forming, particularly in high-strength steel sheets. As a result, cracks that occur in steel sheets are mainly cracks that are governed by ductility that occurs in low strength and soft materials, and cracks that are mainly governed by bendability that occurs in high strength and hard materials. In the method, it became clear that this was because no consideration was given to the latter bending-dominated cracking.
　そこで、発明者らは、延性支配の割れだけでなく、曲げ性支配の割れをも含めて、割れ発生の有無を精度良く予測する方法について、さらに検討を重ねた。その結果、プレス成形品の板厚中心における最大主ひずみ、最小主ひずみと成形限界線図とを対比して割れの発生を予測する延性支配の割れ発生予測と、プレス成形品の表面におけるひずみ（最大主ひずみ）と限界表面ひずみとを対比して割れの発生を予測する曲げ性支配の割れ発生予測とを実施し、そのうちの割れに対する結果の厳しい方の予測を基に、割れの発生の有無を予測することで、精度よく割れの発生を予測できることに想到し、本発明を完成するに至った。 Therefore, the inventors further studied a method for accurately predicting the occurrence of cracks including not only ductile-dominated cracks but also bendability-dominated cracks. As a result, the maximum principal strain at the center of the thickness of the press-formed product, the minimum principal strain and the forming limit diagram are compared to predict the occurrence of cracks, and the occurrence of cracks on the surface of the press-formed product ( The occurrence of cracks based on the prediction of the more severe of the results for the cracks. As a result, it was conceived that the occurrence of cracks can be predicted with high accuracy, and the present invention has been completed.
　すなわち、本発明は、予めプレス成形する金属板の表面に割れが発生する限界表面ひずみを求めておき、次いで、有限要素法を用いて上記金属板を所望の部品形状にプレス成形するシミュレーションを行い、いずれかの要素の表面におけるひずみ（最大主ひずみ）が上記金属板の限界表面ひずみ以上となるときに割れが発生すると予測する第１の割れ予測と、予めプレス成形する金属板の成形限界線図を求めておき、次いで、有限要素法を用いて上記金属板を所望の部品形状にプレス成形するシミュレーションを行い、いずれかの要素の板厚中心における最大主ひずみと最小主ひずみのプロットが上記成形限界線図の成形限界線以上となったときに割れが発生すると予測する第２の割れ予測とを実施し、上記２種類の割れ予測のうちの判定結果が厳しい方の予測を基に割れの発生を予測することを特徴とするプレス成形における割れ予測方法である。 That is, in the present invention, a limit surface strain at which a crack is generated on the surface of a metal plate to be press-formed is obtained in advance, and then a simulation for press-forming the metal plate into a desired part shape using a finite element method is performed. , A first crack prediction that predicts that a crack will occur when the strain (maximum principal strain) on the surface of any element is greater than or equal to the limit surface strain of the metal plate, and a forming limit line of the metal plate that is previously press-formed. Obtain a figure, then perform a simulation of press forming the metal plate into the desired part shape using the finite element method, and plot the maximum principal strain and the minimum principal strain at the center of the plate thickness of either element The second crack prediction that predicts that a crack will occur when the forming limit diagram is equal to or greater than the forming limit line is performed, and the determination between the two types of crack prediction is performed. A crack prediction method in the press molding, characterized in that predict the occurrence of cracks on the basis of the prediction result is more restrictive.
　本発明のプレス成形における割れ予測方法は、上記限界表面ひずみを、曲げ試験を用いて求めることを特徴とする。 The crack prediction method in press molding of the present invention is characterized in that the limit surface strain is obtained using a bending test.
　また、本発明のプレス成形における割れ予測方法は、予めプレス成形する金属板の曲げ試験を行って割れが発生する限界曲げ半径を求めておき、次いで、有限要素法を用いて上記曲げ試験をシミュレーションして限界曲げ半径に対応する金属板表面の最大主ひずみを求めて、これを限界表面ひずみとし、次いで、有限要素法を用いて上記金属板を所望の部品形状にプレス成形するシミュレーションを行い、いずれかの要素の表面の最大主ひずみが上記金属板の限界表面ひずみ以上となるときに割れが発生すると予測する第１の割れ予測を実施することを特徴とする。 Further, the crack prediction method in the press forming of the present invention is to perform a bending test of a metal plate to be press formed in advance to obtain a limit bending radius at which the crack is generated, and then simulate the bending test using a finite element method. The maximum principal strain on the surface of the metal plate corresponding to the limit bending radius is obtained, this is set as the limit surface strain, and then the metal plate is subjected to a simulation of press forming into a desired part shape using the finite element method, A first crack prediction for predicting that a crack will occur when the maximum principal strain on the surface of any element is equal to or greater than the limit surface strain of the metal plate is performed.
　また、本発明は、上記のいずれかに記載の方法でプレス成形における割れ発生の有無を予測し、割れが発生しない条件で金属板をプレス成形することを特徴とするプレス部品の製造方法を提案する。 In addition, the present invention proposes a method for manufacturing a pressed part characterized by predicting the presence or absence of cracks in press forming by any of the methods described above and press forming a metal plate under conditions that do not cause cracks. To do.
　本発明によれば、プレス成形における割れの発生有無を精度よく予測することができるので、自動車のパネル部品、構造・骨格部品等の各種部品をプレス成形する際の割れ発生を精度よく予測することが可能となり、プレス成形を安定して行うことができるとともに、プレス製品の不良率の低減にも大きく寄与することができる。 According to the present invention, since occurrence of cracks in press molding can be accurately predicted, occurrence of cracks during press molding of various parts such as automobile panel parts and structural / framework parts can be accurately predicted. As a result, it is possible to stably perform the press molding and greatly contribute to the reduction of the defective rate of the pressed product.
図１は、有限要素法を用いたプレス成形シミュレーションを説明する図であり、（ａ）は金型と被成形材（金属板）との関係を、（ｂ）はメッシュ分割例を示す。1A and 1B are diagrams for explaining a press forming simulation using a finite element method. FIG. 1A shows a relationship between a mold and a material to be formed (metal plate), and FIG. 1B shows an example of mesh division. 図２は、成形限界線図（ＦＬＤ）を説明する図である。FIG. 2 is a diagram for explaining a forming limit diagram (FLD). 図３は、割れの形態を説明する図であり、（ａ）は延性が支配している割れ、（ｂ）は曲げ性が支配している割れを示す。3A and 3B are diagrams for explaining the form of cracks. FIG. 3A shows a crack in which ductility is controlled, and FIG. 3B shows a crack in which bendability is controlled. 図４は、本発明の割れ発生を予測する方法を説明するフロー図である。FIG. 4 is a flowchart illustrating a method for predicting the occurrence of cracks according to the present invention. 図５は、限界表面ひずみの測定に用いることができるパンチを説明する図である。FIG. 5 is a diagram for explaining a punch that can be used for measuring the limit surface strain. 図６は、本発明の割れ発生を予測する他の方法を説明するフロー図である。FIG. 6 is a flowchart for explaining another method for predicting the occurrence of cracks according to the present invention. 図７は、プレス成形シミュレーションに用いるシェル要素を説明する図である。FIG. 7 is a diagram illustrating shell elements used for press molding simulation. 図８は、実施例の９０度曲げ試験を説明する図である。FIG. 8 is a diagram illustrating a 90-degree bending test of the example. 図９は、実施例に用いた鋼板の（限界曲げ半径／板厚）と、割れ発生時の曲げ先端外側表面歪との関係を示す図である。FIG. 9 is a diagram showing the relationship between (limit bending radius / plate thickness) of the steel plate used in the example and the bending tip outer surface strain at the time of occurrence of cracking. 図１０は、実施例においてプレス成形した部品形状、寸法を説明する図である。FIG. 10 is a diagram for explaining the shape and dimensions of a press-molded part in the example. 図１１は、表１の鋼板Ａ～Ｃの成形限界線図ＦＬＤを示す図である。FIG. 11 is a diagram showing the forming limit diagram FLD of the steel plates A to C in Table 1. 図１２は、表１の鋼板Ａ～Ｃをプレス成形したときに鋼板に割れが発生した金型位置と、同位置を２種類の割れ予測方法で予測した値とを対比して示した図である。FIG. 12 is a diagram showing a comparison between the mold position where a crack occurred in the steel sheet when the steel sheets A to C in Table 1 were press-formed and the value predicted by the two types of crack prediction methods at the same position. is there.
　先ず、本発明の基本的技術思想について説明する。
　発明者らは、高強度の薄鋼板を対象としたプレス成形の研究を行っていた際、鋼板の引張強さＴＳが９８０ＭＰａ級あるいは１１８０ＭＰａ級と高強度化、硬質化するのに伴い、プレス成形において、くびれ（ネッキング）を起こすことなく破断に至るケースが多く見られることに気付いた。この現象を、図３を用いて説明すると、例えば、球頭パンチを用いて低強度で軟質な鋼板を張出し成形する場合には、一般に、図３（ａ）に示したように局部的にくびれが発生し、そのくびれ部の板厚減少量が大きくなって割れに至るのが普通である。しかし、上述した引張強さが９８０ＭＰａ級や１１８０ＭＰａ級のように高強度化、硬質化した鋼板では、図３（ｂ）に示したように、鋼板表面からクラックが発生して、一気に割れに至ることがあることがわかった。 First, the basic technical idea of the present invention will be described.
When the inventors were conducting research on press forming for high strength thin steel sheets, press forming as the tensile strength TS of the steel sheets increased to 980 MPa class or 1180 MPa class and became harder. , It was noticed that there were many cases that led to fracture without causing necking. This phenomenon will be described with reference to FIG. 3. For example, when a low strength and soft steel sheet is stretched and formed using a ball head punch, generally, as shown in FIG. Usually, the amount of reduction in the thickness of the constricted portion becomes large, and cracks usually occur. However, in the steel plate with the above-described tensile strength of 980 MPa class or 1180 MPa class, which has been strengthened and hardened, as shown in FIG. I found out that there was something.
　発明者らは、高強度鋼板に発生し易い上記特異な割れの原因について、さらに研究を重ねた結果、上記割れには曲げ性が大きく関与していることを知見した。すなわち、割れに影響する因子としては延性と曲げ性があり、鋼板が低強度、軟質である場合には、延性がプレス成形における割れを支配しているため、板厚が局部的に減少するネッキングが進行し、延性が限界に達した段階で割れが発生する（以降、この割れを「延性支配の割れ」とも称する）。しかし、鋼板が高強度化、硬質化するにつれて、鋼板の曲げ性が低下するとともに、プレス成形における割れも、延性支配から曲げ性支配に移行する。その結果、鋼板の曲げ性が限界に達して鋼板表面に亀裂が発生すると同時に、その亀裂が一気に伝播して割れに至るものと考えられた（以降、この割れを「曲げ性支配の割れ」とも称する）。 The inventors have further studied the cause of the above-mentioned unique crack that is likely to occur in a high-strength steel sheet, and as a result, have found that bendability is greatly involved in the crack. In other words, factors affecting cracking include ductility and bendability. When steel sheets are low in strength and softness, ductility dominates cracks in press forming, so necking in which the sheet thickness decreases locally. The cracks are generated when the ductility reaches the limit (hereinafter, this crack is also referred to as “ductile-dominated crack”). However, as the steel sheet becomes stronger and harder, the bendability of the steel sheet decreases, and cracks in press forming also shift from ductility control to bendability control. As a result, it was considered that the bendability of the steel sheet reached the limit and cracks occurred on the surface of the steel sheet, and at the same time, the crack propagated all at once and led to cracking (hereinafter, this crack is also referred to as “bending-dominated crack”). Called).
　しかるに、従来の割れ発生の予測方法においては、低強度で軟質な鋼板の割れのように板厚減少から割れに至る延性支配の割れを想定し、板厚中心における最大主ひずみを用いて割れの予測を行っていた。そのため、高強度鋼板における割れのように、曲げ性が割れを支配し、表面亀裂から一気に割れに至るような場合には、精度よく割れを予測することができなかったのである。 However, in the conventional method for predicting the occurrence of cracks, it is assumed that the ductility-dominated cracks from the thickness reduction to the cracks, such as cracks in low-strength and soft steel plates, and the maximum principal strain at the center of the plate thickness is used to determine the cracks. I was making a prediction. Therefore, when the bendability dominates the crack, such as a crack in a high-strength steel sheet, and the cracks from the surface crack all at once, the crack could not be accurately predicted.
　そこで、発明者らは、延性支配の割れだけでなく、曲げ性支配の割れをも含めて、有限要素法を用いたプレス成形シミュレーションで、割れ発生の有無を精度良く予測することについてさらに検討を重ねた。その結果、プレス成形品の板厚中心の最大主ひずみ、最小主ひずみと成形限界線図とを対比して割れの発生を予測する延性支配の割れ発生予測と、プレス成形品の表面におけるひずみ（最大主ひずみ）と限界表面ひずみとを対比して割れの発生を予測する曲げ性支配の割れ発生予測との２種類の割れ予測を実施し、そのうちの割れに対して厳しい方の予測を基に、割れの発生を予測することで、割れの発生有無を精度よく予測できることに想到したものである。
　本発明は、上記の新規知見に基づきなされたものである。 Therefore, the inventors further studied on accurately predicting the occurrence of cracks in press forming simulations using the finite element method, including not only ductility-controlled cracks but also bendability-controlled cracks. Piled up. As a result, the maximum principal strain at the center of the thickness of the press-formed product, the minimum principal strain, and the crack limit prediction for predicting the occurrence of cracks by comparing the forming limit diagram, the strain on the surface of the press-formed product ( Two types of crack predictions are performed: bending-dominant crack generation prediction, which predicts the occurrence of cracks by comparing the maximum principal strain) and the limit surface strain, and based on the prediction that is severer of those cracks The inventors have conceived that the occurrence of cracks can be accurately predicted by predicting the occurrence of cracks.
This invention is made | formed based on said novel knowledge.
　次に、本発明の実施の形態について、被成形材（金属板）として鋼板を例にとって、具体的に説明する。
　図４に、本発明の割れ予測方法の一実施形態のフローを示す。
　まず、本発明においては、曲げ性支配の割れ発生予測を行う必要がある。
　そのためには、曲げ変形により鋼板の表面に割れが発生する限界表面ひずみを、予め何らかの試験により求めておくことが必要である。ここで、上記試験としては、図５（ａ）に示したような曲げ試験や、図５（ｂ）に示したような球頭パンチや、図示されていない楕円球頭パンチを用いた張出し試験などがあるが、その他の試験方法を用いてもよい。また、上記曲げ試験は、９０度曲げ（Ｖ曲げ）や１８０度曲げ（Ｕ曲げ）など、いずれの曲げ角度を用いてもよい。 Next, an embodiment of the present invention will be specifically described taking a steel plate as an example of a material to be formed (metal plate).
FIG. 4 shows a flow of one embodiment of the crack prediction method of the present invention.
First, in the present invention, it is necessary to predict the occurrence of cracks controlled by bendability.
For this purpose, it is necessary to obtain in advance a limit surface strain at which a crack occurs on the surface of the steel sheet due to bending deformation by some test. Here, as the above test, a bending test as shown in FIG. 5 (a), a ball head punch as shown in FIG. 5 (b), and an overhang test using an elliptical ball head punch (not shown). However, other test methods may be used. The bending test may use any bending angle such as 90 degree bending (V bending) or 180 degree bending (U bending).
　なお、上記限界表面ひずみは、鋼板の材質に応じて求めておくことが好ましい。ここで、上記「材質」とは、引張強さの他、降伏応力や硬さ、曲げ性等が考えられる。また、「材質に応じて」とは、例えば、引張強さが９８０ＭＰａ級では限界表面ひずみが０．５０、引張強さが１１８０ＭＰａ級では限界表面ひずみが０．２２というように求めておくことを意味する。こうすることで、割れが発生する限界表面ひずみを、引張強さや降伏応力等の関数として表すことができるので、上記限界表面ひずみに代えて、他の材質を割れの指標とすることができるからである。 In addition, it is preferable to obtain | require the said limit surface distortion according to the material of a steel plate. Here, the “material” may be yield stress, hardness, bendability, etc. in addition to tensile strength. “Depending on the material” means that, for example, the limit surface strain is 0.50 when the tensile strength is 980 MPa class, and the limit surface strain is 0.22 when the tensile strength is 1180 MPa class. means. By doing so, the limit surface strain at which cracking can occur can be expressed as a function of tensile strength, yield stress, etc., so that other materials can be used as an index of cracking instead of the above limit surface strain. It is.
　次いで、上記限界表面ひずみを求めた鋼板を、所望の部品形状にプレス成形するシミュレーションを、有限要素法を用いて行い、分割した各要素の表面におけるひずみ（最大主ひずみ）を求め、各要素の表面における最大主ひずみと鋼板が有する限界表面ひずみとを対比し、いずれかの要素の表面における最大主ひずみが上記限界表面ひずみ以上となるときに、曲げ性支配の割れが発生すると予測する。 Next, a simulation of press forming the steel sheet from which the above-mentioned limit surface strain is obtained into a desired part shape is performed using the finite element method, and the strain (maximum principal strain) on the surface of each divided element is obtained. The maximum principal strain on the surface is compared with the critical surface strain of the steel sheet, and when the maximum principal strain on the surface of any element is equal to or greater than the above-mentioned limit surface strain, it is predicted that cracking controlled by bendability will occur.
　また、本発明においては、上記曲げ性支配の割れ発生予測とは別に、延性支配の割れ発生予測を行う必要がある。
　そのためには、鋼板に割れが発生する際の板厚中心における成形限界（最大主ひずみと最小主ひずみ）を表した成形限界線図（ＦＬＤ）を予め求めておくことが必要である。このＦＬＤを求める方法には、球頭パンチを用いる中島法や、円頭パンチを用いるＭａｒｃｉｎｉａｋ法等があるが、いずれを用いてもよい。 Further, in the present invention, it is necessary to perform ductility-dominated crack occurrence prediction separately from the bendability-dominated crack occurrence prediction.
For this purpose, it is necessary to obtain in advance a forming limit diagram (FLD) representing a forming limit (maximum principal strain and minimum principal strain) at the center of the plate thickness when cracking occurs in the steel sheet. As a method for obtaining the FLD, there are the Nakajima method using a ball head punch, the Marciniak method using a circular head punch, and the like, and any of them may be used.
　次いで、鋼板を所望の部品形状にプレス成形する、有限要素法を用いたシミュレーションで、各要素の板厚中心の最大主ひずみと最小主ひずみを求め、上記ＦＬＤの成形限界線ＦＬＣとを対比し、いずれかの要素の板厚中心の最大主ひずみと最小主ひずみのプロットが、上記ＦＬＣ以上となる、すなわち、割れ発生領域に存在するときに、延性支配の割れが発生すると予測する。 Next, the maximum principal strain and the minimum principal strain at the center of the plate thickness of each element are obtained by simulation using a finite element method in which the steel sheet is press-formed into a desired part shape, and the above-mentioned FLD forming limit line FLC is compared. It is predicted that when the plot of the maximum principal strain and the minimum principal strain at the center of the thickness of any element is equal to or greater than the above FLC, that is, when it exists in the crack generation region, ductile-dominated cracking occurs.
　次いで、本発明では、上記曲げ性支配の割れ発生予測の結果と、延性支配の割れ発生予測の結果とを対比し、割れに対して厳しい方の予測に基づいて、プレス成形における割れの発生有無を予測する。 Next, in the present invention, the results of the prediction of the occurrence of cracks controlled by bendability and the results of the prediction of the generation of cracks controlled by ductility are compared, and the presence or absence of occurrence of cracks in press forming is determined based on the prediction that is severer to crack Predict.
　上記のように、本発明の割れ発生を予測する方法では、延性支配の割れ予測と、曲げ性支配の割れ予測の両方を考慮し、割れに対して不利な方の予測を採用しているので、従来の延性支配の割れ予測に基づく割れ発生予測より、格段に精度を向上することが可能となる。 As described above, in the method of predicting the occurrence of cracks according to the present invention, both the ductility-predicted crack prediction and the bendability-dominated crack prediction are considered, and the prediction that is disadvantageous to the crack is adopted. Thus, it is possible to significantly improve the accuracy compared to the conventional crack generation prediction based on the ductility-dominated crack prediction.
　なお、本発明の上記割れ予測方法では、曲げ性支配の割れを予測するために、予め、何らかの試験で鋼板に割れが発生する限界表面ひずみを求めておく必要があるが、予め、曲げ試験を行って割れが発生する限界曲げ半径（割れが発生する最大の曲げ半径）を求め、次いで、有限要素法を用いて曲げ試験をシミュレーションして上記限界曲げ半径における最大主ひずみを求め、この最大主ひずみを上記限界表面ひずみに代えて用いてもよい。
　例えば、図６に示したように、予め被成形材である鋼板の９０度曲げ試験を行い、割れが発生する限界曲げ半径を求める。次いで、有限要素法を用いたシミュレーションで、上記９０度曲げ試験で割れが発生した位置の表面の最大主ひずみを求め、これを限界表面ひずみとする。次いで、有限要素法を用いて鋼板を所望の部品形状にプレス成形するシミュレーションを行い、分割した各要素の表面の最大主ひずみを求め、各要素の表面の最大主ひずみと上記鋼板の限界表面ひずみとを対比し、いずれかの要素の表面の最大主ひずみが上記限界表面ひずみ以上となるときに、曲げ性支配の割れが発生すると予測する。
　その後、上述した延性支配の割れ予測を行い、曲げ性支配の割れ予測と延性支配の割れ予測のうちで、割れに対して厳しい方の予測を用いて、割れの発生を予測する。 In the above crack prediction method of the present invention, in order to predict the bending-dominated crack, it is necessary to obtain in advance a limit surface strain at which cracking occurs in the steel sheet in some test. The critical bending radius at which cracking occurs (the maximum bending radius at which cracking occurs) is determined, and then the bending test is simulated using the finite element method to determine the maximum principal strain at the limiting bending radius. Strain may be used instead of the above limit surface strain.
For example, as shown in FIG. 6, a 90-degree bending test is performed in advance on a steel sheet that is a material to be formed, and a limit bending radius at which cracking occurs is obtained. Next, the maximum principal strain of the surface at the position where the crack is generated in the 90-degree bending test is obtained by simulation using the finite element method, and this is defined as the limit surface strain. Next, a simulation of press forming the steel sheet into a desired part shape using the finite element method is performed, the maximum principal strain of the surface of each divided element is obtained, the maximum principal strain of the surface of each element and the limit surface strain of the steel sheet It is predicted that when the maximum principal strain on the surface of any element is equal to or greater than the above-mentioned limit surface strain, a bending-dominated crack will occur.
Thereafter, the ductility-dominated crack prediction described above is performed, and the occurrence of the crack is predicted using the prediction that is more severe with respect to the crack among the bendability-dominated crack prediction and the ductility-dominated crack prediction.
　なお、上記有限要素法を用いたプレス成形シミュレーションで得られる表面の最大主ひずみ、鋼板の限界表面ひずみとの対比は、成形部品すべての要素について行う必要はなく、経験上割れの発生が懸念される部位に絞り込み、その部位の要素についてのみ表面の最大主ひずみと限界表面ひずみとの対比を行ってもよい。 It should be noted that the maximum principal strain of the surface obtained by the press forming simulation using the finite element method and the limit surface strain of the steel sheet do not need to be performed for all elements of the molded part, and there is a concern that cracks may occur from experience. The maximum principal strain of the surface and the limit surface strain may be compared only with respect to the element of the portion.
　また、上記シミュレーションを行う場合のシェル要素は、薄鋼板の場合、一般に板厚方向の応力は無視できるため、板厚方向に要素を考えないシェル要素を用いることができ、通常、板厚中央位置形状でシミュレーション用モデルを作成することが多い。例えば、図７に示すように、各要素内に応力やひずみを計算するための点（積分点）を設定し、板厚方向にも板厚等分位置に積分点を設定する。最大主ひずみおよび最小主ひずみは、各積分点において計算するが、本発明における表面ひずみとは、金属板表面の積分点で計算された最大主ひずみの平均値および最小主ひずみの平均値、図中の記号で説明すると、ｄｓ１～ｄｓ４の平均値である。なお、上記積分点の数や位置は、シミュレーションに用いる要素の種類によって決まるものである。 In addition, in the case of performing the above simulation, in the case of a thin steel plate, since the stress in the thickness direction is generally negligible, a shell element that does not consider the element in the thickness direction can be used. A simulation model is often created with a shape. For example, as shown in FIG. 7, a point (integration point) for calculating stress and strain is set in each element, and an integration point is set in the plate thickness equal position in the plate thickness direction. The maximum principal strain and the minimum principal strain are calculated at each integration point. The surface strain in the present invention is the average value of the maximum principal strain and the average value of the minimum principal strain calculated at the integration point on the surface of the metal plate. To explain with the middle symbols, it is the average value of d s 1 to d s 4. Note that the number and position of the integration points are determined by the type of elements used in the simulation.
　本発明のプレス部品の製造方法は、上記に説明した割れ予測方法で、プレス成形における金属板の割れを予測し、もし、割れが発生することが予測された場合には、その条件（金属板、金型）でのプレス成形を止め、割れが発生しない条件に変更してプレス成形することにより、部品を製造する方法である。具体的に、プレス成形に用いる金型形状を変更したり、プレス速度や温度、潤滑状態等の成形条件を変更したり、あるいは、金属板の材質（延性、曲げ性等）を変更したりして、割れ発生のない条件にてプレス成形を実施することができる。 The method for manufacturing a pressed part according to the present invention predicts cracking of a metal plate in press molding by the crack prediction method described above, and if it is predicted that cracking will occur, the condition (metal plate In this method, parts are manufactured by stopping the press molding in the mold) and changing to a condition that does not cause cracking. Specifically, the mold shape used for press molding is changed, the molding conditions such as press speed, temperature and lubrication are changed, or the metal plate material (ductility, bendability, etc.) is changed. Thus, press molding can be performed under conditions that do not cause cracks.
　表１に示した板厚や機械的特性が異なる３種類の高強度冷延鋼板を、図８に示すように曲げ半径を種々に変更して９０度曲げ試験を行い、限界曲げ半径（割れが発生する最大の曲げ半径）を求めた。次いで、有限要素法を用いて上記曲げ試験をシミュレーションし、曲げ先端外側の鋼板表面におけるひずみ（最大主ひずみ）を求めた。なお、上記曲げ試験に用いた試験片の寸法は、板厚×幅３０ｍｍ×長さ１００ｍｍとした。また、有限要素法の解析ソフトは、ＬＳ－ＤＹＮＡ　Ｖｅｒ９．７１（ＬＳＴＣ；Ｌｉｖｅｒｍｏｒｅ　Ｓｏｆｔｗａｒｅ　Ｔｅｃｈｎｏｌｏｇｙ　Ｃｏｒｐｏｒａｔｉｏｎ社製）を用い、板厚方向へのメッシュ分割は、１メッシュ（分割なし）とした。上記の結果を、（限界曲げ半径／板厚）と、曲げ先端外側の鋼板表面におけるひずみ（最大主ひずみ）との関係として、図９に示す。この結果から、表１に示したように、鋼板Ａ，ＢおよびＣの限界曲げ半径に対応する曲げ先端外側の鋼板表面におけるひずみ（最大主ひずみ）を限界表面ひずみとし、鋼板Ａ、Ｂ、Ｃの限界表面ひずみは、それぞれ０．２２、０．２６および０．３２であることがわかる。 Three types of high-strength cold-rolled steel sheets with different thicknesses and mechanical properties shown in Table 1 were subjected to a 90-degree bending test with various bending radii as shown in FIG. The maximum bend radius generated) was determined. Next, the bending test was simulated using a finite element method, and the strain (maximum principal strain) on the steel plate surface outside the bending tip was determined. In addition, the dimension of the test piece used for the said bending test was made into board thickness x width 30mm x length 100mm. The analysis software of the finite element method was LS-DYNA Ver9.71 (LSTC; manufactured by Livermore Technology Corporation), and mesh division in the plate thickness direction was 1 mesh (no division). The above results are shown in FIG. 9 as the relationship between (limit bending radius / plate thickness) and strain (maximum principal strain) on the steel plate surface outside the bending tip. From this result, as shown in Table 1, the strain (maximum principal strain) on the surface of the steel sheet outside the bending tip corresponding to the critical bending radius of the steel plates A, B, and C is defined as the critical surface strain, and the steel plates A, B, C It can be seen that the critical surface strains of are 0.22, 0.26 and 0.32, respectively.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
　次いで、表１に示した鋼板Ａ～Ｃを、上型と下型からなるモデル金型を用いて、自動車のセンターピラー上部のルーフレール接続部分を模擬した部品形状（図１０参照）にプレス成形し、割れが発生したときの金型位置（下死点からの距離）を調査した。その結果、上記鋼板Ａ～Ｃの割れが発生する金型位置は、鋼板Ａは１２ｍｍ、鋼板Ｂは１１ｍｍ、鋼板Ｃは９ｍｍであった。 Next, the steel plates A to C shown in Table 1 were press-molded into a part shape (see FIG. 10) simulating the roof rail connection portion at the upper part of the center pillar of an automobile, using a model die consisting of an upper die and a lower die. The mold position (distance from the bottom dead center) when the crack occurred was investigated. As a result, the mold positions where the cracks of the steel plates A to C occurred were 12 mm for the steel plate A, 11 mm for the steel plate B, and 9 mm for the steel plate C.
　また、上記プレス成形と並行して、９０度曲げ試験のシミュレーションと同じ解析ソフトを用いて、上記プレス成形を３次元シミュレーションし、金型が下死点に至るまでの成形過程における、鋼板の割れ発生位置に対応する要素の鋼板表面の最大主ひずみと、板厚中心の最大主ひずみε１および最小主ひずみε２を求めた。 In parallel with the press forming, using the same analysis software as the 90-degree bending test simulation, the press forming is subjected to a three-dimensional simulation, and cracks in the steel plate during the forming process until the die reaches the bottom dead center. The maximum principal strain on the steel sheet surface of the element corresponding to the generation position, the maximum principal strain ε 1 and the minimum principal strain ε 2 at the center of the plate thickness were determined.
　次いで、上記シミュレーションで得られた、割れが発生する位置の鋼板表面の最大主ひずみが、前述した限界曲げ半径に対応する限界表面ひずみ以上となるプレス成形の金型位置（下死点からの距離）を求めた。その結果、割れが発生する金型位置は、鋼板Ａは１１ｍｍ、鋼板Ｂは９ｍｍ、鋼板Ｃは０ｍｍ（割れなし）であった。例えば、鋼板Ａについて説明すると、限界表面ひずみは０．２２であるが、金型位置が下死点から１２ｍｍの位置における鋼板表面の最大主ひずみは０．２１１、下死点から１１ｍｍの位置における鋼板表面の最大主ひずみは０．２２４であるため、下死点から１１ｍｍの位置で割れが発生すると予測した。 Next, the die position (the distance from the bottom dead center) obtained by the simulation, in which the maximum principal strain on the surface of the steel plate where cracking occurs is equal to or greater than the limit surface strain corresponding to the limit bending radius described above. ) As a result, the mold positions where cracks occurred were 11 mm for steel plate A, 9 mm for steel plate B, and 0 mm for steel plate C (no cracks). For example, the steel sheet A will be described. Although the limit surface strain is 0.22, the maximum principal strain on the steel plate surface at a position where the mold position is 12 mm from the bottom dead center is 0.211 and the position at a position 11 mm from the bottom dead center. Since the maximum principal strain on the steel sheet surface was 0.224, it was predicted that a crack would occur at a position 11 mm from the bottom dead center.
　一方、表１に示したＡ～Ｃの鋼板について、先端曲率半径２５ｍｍの球頭パンチを用いた張出し試験（中島法）を用いて、成形限界線図ＦＬＤを求め、その結果を図１１に示した。また、これらの図中には、上記プレス成形シミュレーションによって、鋼板に割れが発生する位置の板厚中心における最大主ひずみε１および最小主ひずみε２が、プレス成形時に金型が下死点に到達するまで、どのように推移するかを示した。そして、これらの図から、上記ＦＬＤとプレス成形シミュレーションとから予測されるプレス成形で割れが発生する金型位置（下死点からの距離）の予測位置を読み取ると、鋼板Ａは５ｍｍ、鋼板Ｂは１０ｍｍ、鋼板Ｃは８ｍｍとなった。 On the other hand, with respect to the steel sheets A to C shown in Table 1, a forming limit diagram FLD was obtained by using an overhang test (Nakajima method) using a ball head punch having a tip curvature radius of 25 mm, and the result is shown in FIG. It was. In these figures, the maximum principal strain ε 1 and the minimum principal strain ε 2 at the center of the plate thickness at the position where cracking occurs in the steel plate by the above-described press forming simulation are shown in FIG. It showed how it would change until it reached. And from these figures, when the predicted position of the mold position (distance from bottom dead center) where cracking occurs in the press molding predicted from the FLD and the press molding simulation is read, the steel plate A is 5 mm and the steel plate B Was 10 mm, and the steel plate C was 8 mm.
　上記、鋼板表面の限界表面ひずみから予測される割れが発生する金型位置と、成形限界線図ＦＬＤから予測される割れが発生する金型位置と、プレス成形で割れが発生する金型位置とを対比して、図１２に示した。これらの図から、曲げ性が良好な鋼板ＢおよびＣは、成形限界線図ＦＬＤから得られる予測値と、プレスの結果とはよい一致を示しているが、曲げ性が劣る鋼板Ａではプレスの結果と大きく乖離しており、むしろ、鋼板表面の限界表面ひずみから得られる予測値とよい一致を示していることがわかる。 The mold position where cracks predicted from the limit surface strain on the steel sheet surface occur, the mold position where cracks predicted based on the forming limit diagram FLD occur, and the mold position where cracks occur during press forming These are shown in FIG. From these figures, steel plates B and C with good bendability show good agreement with the predicted values obtained from the forming limit diagram FLD and the press results, but with steel plate A with poor bendability, It can be seen that there is a great divergence from the results, rather, it is in good agreement with the predicted value obtained from the critical surface strain on the steel sheet surface.
　例えば、鋼板Ａでは、図１２（ａ）に示すように、ＦＬＤによる割れ予測では、下死点から５ｍｍの位置まで金型を下降させたときに割れが発生すると予測され、一方、限界表面ひずみによる割れ予測では、下死点から１１ｍｍの位置まで金型を下降させたときに割れが発生すると予測されており、限界表面ひずみを用いた割れ予測の方が、実際にプレス成形における割れ発生時の金型位置（下死点から１２ｍｍ）を精度よく予測している。
　また、鋼板Ａより曲げ性の良い鋼板Ｂでは、図１２（ｂ）に示すように、ＦＬＤによる割れ予測では、下死点から１０ｍｍの位置まで金型を下降させたときに割れが発生すると予測され、一方、限界表面ひずみによる割れ予測では、下死点から９ｍｍの位置まで金型を下降させたときに割れが発生すると予測されており、鋼板Ａとは逆に、ＦＬＤを用いた割れ予測の方が、実際にプレス成形における割れ発生時の金型位置（下死点から１１ｍｍ）を精度よく予測している。
　また、さらに曲げ性の良い鋼板Ｃでは、図１２（ｃ）に示すように、ＦＬＤによる割れ予測では、下死点から８ｍｍの位置まで金型を下降させたときに割れが発生すると予測され、一方、限界表面ひずみによる割れ予測では、下死点まで金型を下降させても割れが発生しないと予測されており、やはり鋼板Ａとは逆に、ＦＬＤを用いた割れ予測の方が、実際にプレス成形における割れ発生時の金型位置（下死点から９ｍｍ）を精度よく予測している。 For example, in the steel sheet A, as shown in FIG. 12A, in the crack prediction by FLD, it is predicted that the crack will occur when the mold is lowered from the bottom dead center to a position of 5 mm. According to the crack prediction, it is predicted that a crack will occur when the die is lowered to a position 11 mm from the bottom dead center, and the crack prediction using the limit surface strain is actually when the crack occurs in the press molding. The mold position (12 mm from the bottom dead center) is accurately predicted.
Moreover, in the steel plate B having better bendability than the steel plate A, as shown in FIG. 12B, in the prediction of cracking by FLD, it is predicted that cracking will occur when the mold is lowered from the bottom dead center to a position of 10 mm. On the other hand, in the crack prediction due to the limit surface strain, it is predicted that the crack will occur when the die is lowered from the bottom dead center to a position of 9 mm, and contrary to the steel plate A, the crack prediction using the FLD is predicted. Actually predicts the mold position (11 mm from bottom dead center) at the time of occurrence of cracks in press molding with higher accuracy.
Further, in the steel plate C having better bendability, as shown in FIG. 12 (c), in the crack prediction by FLD, it is predicted that the crack will occur when the die is lowered from the bottom dead center to the position of 8mm, On the other hand, in the crack prediction due to the limit surface strain, it is predicted that no crack will occur even if the mold is lowered to the bottom dead center. Again, contrary to the steel plate A, the crack prediction using FLD is actually more effective. Furthermore, the mold position (9 mm from the bottom dead center) at the time of occurrence of cracks in press molding is accurately predicted.
　上記のように、以上の結果から、プレス成形における割れを精度よく予測するには、ＦＬＤを用いた割れ予測方法（延性支配の割れ予測方法）と、限界表面ひずみを用いた割れ予測方法（曲げ性支配の割れ予測方法）の２つの方法で割れ予測を行い、割れに対して厳しい方の予測を採用することが有効であることがわかる。 As described above, in order to accurately predict cracks in press forming from the above results, a crack prediction method using FLD (ductility-dominated crack prediction method) and a crack prediction method using limit surface strain (bending) It can be seen that it is effective to perform crack prediction by the two methods of sex-dominated crack prediction method, and to adopt the prediction that is stricter against the crack.
　本発明は、上記に説明した内容に限られるものではなく、例えば、上記実施例では、引張強さが９８０ＭＰａ級以上の鋼板（１１８０ＭＰａ級の鋼板）に適用した例が示されており、本発明は、このような高強度鋼板のプレス成形に適用することが好ましいが、引張強さが９８０ＭＰａ級未満の鋼板や、鋼板以外の金属板に適用することもできる。 The present invention is not limited to the contents described above. For example, in the above-described embodiment, an example in which the tensile strength is applied to a steel plate having a tensile strength of 980 MPa class or more (1180 MPa class steel plate) is shown. Is preferably applied to press forming of such a high-strength steel plate, but can also be applied to a steel plate having a tensile strength of less than 980 MPa or a metal plate other than a steel plate.
　１：プレス金型の上型
　２：プレス金型の下型
　３：被成形材（金属板、鋼板）
　４：メッシュ分割
　５：要素
  1: Upper mold of press mold 2: Lower mold of press mold 3: Material to be molded (metal plate, steel plate)
4: Mesh division 5: Element

Claims (4)

  1. 予めプレス成形する金属板の表面に割れが発生する限界表面ひずみを求めておき、
    次いで、有限要素法を用いて上記金属板を所望の部品形状にプレス成形するシミュレーションを行い、いずれかの要素の表面ひずみが上記金属板の限界表面ひずみ以上となるときに割れが発生すると予測する第１の割れ予測と、
    予めプレス成形する金属板の成形限界線図を求めておき、
    次いで、有限要素法を用いて上記金属板を所望の部品形状にプレス成形するシミュレーションを行い、いずれかの要素の板厚中心における最大主ひずみと最小主ひずみのプロットが上記成形限界線図の成形限界線以上となったときに割れが発生すると予測する第２の割れ予測とを実施し、
    上記２種類の割れ予測のうちの判定結果が厳しい方の予測を基に割れの発生を予測することを特徴とするプレス成形における割れ予測方法。     Obtain the limit surface strain at which cracks occur on the surface of the metal plate to be press-formed in advance,
    Next, a simulation of press forming the metal plate into a desired part shape using a finite element method is performed, and it is predicted that a crack will occur when the surface strain of any element exceeds the limit surface strain of the metal plate. The first crack prediction;
    Obtain the forming limit diagram of the metal plate to be press-formed in advance,
    Next, a simulation of press forming the metal plate into a desired part shape using a finite element method is performed, and a plot of the maximum principal strain and the minimum principal strain at the center of the plate thickness of any element is formed into the forming limit diagram. Conduct a second crack prediction that predicts that cracks will occur when the limit line is exceeded,
    A crack prediction method in press forming, wherein the occurrence of a crack is predicted based on a prediction with a severer judgment result among the two types of crack prediction.
  2. 上記限界表面ひずみを、曲げ試験を用いて求めることを特徴とする請求項１に記載のプレス成形における割れ予測方法。 2. The crack prediction method in press forming according to claim 1, wherein the limit surface strain is obtained using a bending test.
  3. 予めプレス成形する金属板の曲げ試験を行って割れが発生する限界曲げ半径を求めておき、
    次いで、有限要素法を用いて上記曲げ試験をシミュレーションして限界曲げ半径に対応する金属板表面の最大主ひずみを求めて、これを限界表面ひずみとし、
    次いで、有限要素法を用いて上記金属板を所望の部品形状にプレス成形するシミュレーションを行い、いずれかの要素の表面の最大主ひずみが上記金属板の限界表面ひずみ以上となるときに割れが発生すると予測する第１の割れ予測を実施することを特徴とする請求項１または２に記載のプレス成形における割れ予測方法。     Perform a bending test on the metal plate to be press-formed in advance to determine the limit bending radius at which cracking occurs,
    Next, the bending test is simulated using the finite element method to obtain the maximum principal strain of the metal plate surface corresponding to the limit bending radius, and this is defined as the limit surface strain.
    Next, a simulation is performed to press-mold the metal plate into a desired part shape using the finite element method, and cracking occurs when the maximum principal strain on the surface of any element exceeds the limit surface strain of the metal plate. Then, the 1st crack prediction estimated is implemented, The crack prediction method in the press molding of Claim 1 or 2 characterized by the above-mentioned.
  4. 請求項１～３のいずれか１項に記載の方法でプレス成形における割れ発生の有無を予測し、割れが発生しない条件で金属板をプレス成形することを特徴とするプレス部品の製造方法。
          A method for producing a pressed part, wherein the method according to any one of claims 1 to 3 predicts the presence or absence of cracks in press forming and presses a metal plate under conditions that do not cause cracks.
PCT/JP2012/060214 2012-04-16 2012-04-16 Method for predicting press formation cracks and method for manufacturing pressed product WO2013157062A1 (en)

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WO2014208697A1 (en) * 2013-06-26 2014-12-31 新日鐵住金株式会社 Method for determining bending fracture in metal plate, program, and storage medium
CN105893708A (en) * 2016-05-03 2016-08-24 湖南大学 Method for use variable-thickness plate (VRB) three-dimensional thermoforming limit diagram
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