JP7173246B1 - Press forming limit line acquisition method - Google Patents

Abstract

A press forming limit line obtaining method for obtaining a forming limit line for determining whether or not cracking occurs at a portion where a deformation path changes from compressive deformation to tensile deformation in press forming of a thin metal plate is provided. A method for obtaining a press-forming limit line according to the present invention includes a step of performing a basic forming test for obtaining whether or not cracks occur in a portion of a thin metal sheet where a deformation path changes from compressive deformation to tensile deformation (S1); The step (S3) of calculating the thickness change amount in the basic forming test of the thin metal sheet by FEM analysis, and from the calculated thickness change amount, the maximum thickness increase amount of the metal sheet in compressive deformation and the tension after compressive deformation A step (S5) of determining the relative plate thickness reduction amount of the thin metal plate due to deformation as a crack determination parameter, a step of plotting on two-dimensional coordinates in association with the presence or absence of crack generation and the crack determination parameter (S7); and a step (S9) of creating a forming limit line for classifying the presence or absence of crack generation based on the plotted distribution of the presence or absence of crack generation. [Selection diagram] Fig. 1

Description

The present invention relates to a press forming limit line acquisition method for obtaining a forming limit line for determining the presence or absence of cracking at a portion where the deformation path of a thin metal plate changes from compressive deformation to tensile deformation in press forming of a metal sheet.

In recent years, as a response to energy and global environmental issues, there has been an increasing demand for vehicle weight reduction and collision safety in order to improve the fuel efficiency of automobiles. In order to meet these demands, the application of high-strength steel sheets is expanding for the purpose of reducing the weight of automobile bodies. In order to achieve both crash performance and vehicle weight reduction, there is a growing demand for the development of techniques for forming high-strength steel sheets into automobile parts of various shapes. In addition, advanced countries have set goals for the abolition of gasoline vehicles, and the shift to power is progressing rapidly, and the shift to electric vehicles is particularly noticeable. Since electric vehicles require batteries to be installed in the vehicle body, there is a possibility that demand for automotive parts such as battery cases, which are deep-drawn thin metal plates, will increase sharply in the future. Development is urgent.

One of the biggest problems in press forming is cracking that occurs during the press forming process.
In general, press forming forms that cause cracks during press forming can be classified into four types: bending deformation, stretch flanging deformation, drawing deformation, and bulging deformation. In addition, several techniques have been proposed for determining in advance whether or not cracks have occurred in these forms of press molding.
For example, as a method of judging the occurrence of cracks in bending deformation, a method of judging bending cracks from the bending outer surface strain amount at the time of cracking in a V-bending test is disclosed (Patent Document 1).
Further, as a method for determining the occurrence of cracks in stretch flange deformation, a method of calculating the forming limit of plate edge cracks in the stretch flange portion from the strain gradient in the vicinity of the shear edge after the hole expansion test is disclosed (Patent Document 2. ).

Furthermore, a forming limit diagram (FLD) is widely used to determine the occurrence of cracks in drawing deformation and bulging deformation (Non-Patent Document 1). FLD can be obtained by a simple molding test, and in the press molding process of press molding products using thin metal plates (blanks) printed with scribed circles and various dot patterns, the printed shapes such as scribed circles and dot patterns can be obtained. By measuring the strain distribution in the thin metal sheet based on the change in , it can be easily applied to determine the presence or absence of cracks in the actual press-formed product. In addition, many commercial CAE (Computer Aided Engineering) solvers are equipped with a function to determine whether or not cracks have occurred by FLD using the results obtained from press forming simulations.

JP 2013-128956 A JP 2009-204427 A

ISO 12004-2:2008, "Metallic materials - Sheet and strip - Determination of forming-limit curves - Part 2: Determination of forming-limit curves in the laboratory", 2008.

However, the presence or absence of cracks can be determined by FLD only for cracks that occur along a certain deformation path during draw forming and stretch forming, and the deformation path changes from compression deformation to tensile deformation during press forming. In this case, since the forming limit is different from that in the case of deformation along a constant deformation path, there is a problem that it is not possible to determine whether or not cracks have occurred by FLD.
In addition, when the deformation path changes from the primary path to the secondary path in the press molding of an actual press-formed product, the combination of the deformation patterns (compression deformation, tensile deformation) of the primary path and the secondary path, or the deformation from the primary path Due to the difference in the strain distribution ratio that changes to the secondary path, an infinite number of deformation paths can be considered. had its limits.
Furthermore, if wrinkles occur during compressive deformation of a thin metal plate in press forming, the stress around the place where the wrinkles occur changes. could not take into account such effects.

In particular, when the deformation path changes from compressive deformation to tensile deformation in press forming by drawing of press forming of a complicated shape such as an actual automobile body part, if the amount of compressive deformation at the time of compressive deformation in the primary path is large, Even when the amount of tensile deformation in the subsequent secondary path is small, cracks are likely to occur, and there were cases where it was not possible to appropriately determine the presence or absence of cracks using FLD.

The present invention has been made to solve the above-mentioned problems. In the process of press forming a thin metal sheet, the presence or absence of cracking at a portion where the deformation path of the thin metal sheet changes from compressive deformation to tensile deformation is checked. It is an object of the present invention to provide a press forming limit line acquisition method for obtaining a forming limit line for judgment.

(1) A method for obtaining a press-forming limit line according to the present invention is a method for determining whether or not a crack occurs at a portion where a deformation path of the thin metal plate changes from compressive deformation to tensile deformation in the press-forming process of the thin metal plate. Seeking a limit line,
A basic forming test in which the metal sheet is deformed along the deformation path is performed under various forming conditions, and whether or not cracks occur in the portion of the metal sheet where the deformation path changes from compressive deformation to tensile deformation for the various forming conditions. A basic molding test process to obtain
an FEM analysis step of performing an FEM analysis with the basic forming test of the thin metal plate as an analysis target for the various forming conditions, and calculating a change in the thickness of the thin metal plate;
Based on the change in the thickness of the thin metal plate calculated in the FEM analysis process, the amount of change in the thickness of the thin metal plate until the thin metal plate reaches the maximum thickness in the compression deformation of the deformation path for the various forming conditions A certain maximum plate thickness increase amount, and a relative plate thickness decrease amount that is the plate thickness change amount from the maximum plate thickness to the minimum plate thickness of the thin metal plate as the deformation path changes from compressive deformation to tensile deformation. A crack determination parameter acquisition step of obtaining , as a crack determination parameter;
By associating the presence or absence of cracks obtained for the various forming conditions in the basic forming test step with the crack determination parameters obtained for the various forming conditions in the crack determination parameter acquisition step, the maximum plate thickness increase a crack determination parameter plotting step of plotting the amount and the relative plate thickness reduction amount on two-dimensional coordinates with each axis;
a forming limit line creating step of creating a forming limit line that distinguishes whether or not a crack occurs in a portion where the thin metal plate is deformed along the deformation path, based on the distribution of presence or absence of crack occurrence plotted on the two-dimensional coordinates; It is characterized by including

(2) In the above (1),
The basic forming test step is characterized in that the thin metal plate is deformed along the deformation path by drawing the thin metal plate.

(3) In the above (2),
In the basic forming test step, the various forming conditions are set by changing the shape of the thin metal plate or the wrinkle holding force applied to the thin metal plate in the drawing of the thin metal plate. It is something to do.

In the present invention, the presence or absence of cracks in the thin metal plate obtained by a basic forming test in which the thin metal plate is deformed along a deformation path that changes from compressive deformation to tensile deformation, and the crack determination parameter by FEM analysis for the basic forming test. The maximum plate thickness increase amount of the thin metal plate in compressive deformation and the relative plate thickness decrease amount in tensile deformation obtained as are plotted on a two-dimensional coordinate, and crack generation is based on the plotted distribution of the presence or absence of crack generation. By creating a forming limit line that distinguishes the presence or absence of, it is possible to obtain a forming limit line that determines the presence or absence of cracking at a portion where the deformation path of the thin metal sheet changes from compressive deformation to tensile deformation.

FIG. 4 is a flow chart explaining the flow of processing in the press-forming limit line acquisition method according to the embodiment of the present invention; In the press forming limit line acquisition method according to the embodiment of the present invention, a change in thickness of the thin metal plate at a portion where the deformation path of the thin metal plate changes from compressive deformation to tensile deformation, and a crack determination parameter for determining the presence or absence of cracking It is a graph explaining the maximum plate|board thickness increase amount and the relative plate|board thickness reduction amount which are calculated|required as. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining an example of a mold for drawing a thin metal plate into a rectangular tube-shaped container with a bottom as a basic molding test in the embodiment and examples of the present invention ((a) perspective view, (b) molding (c) cross-sectional view perpendicular to the molding direction). FIG. 5 is a diagram for explaining a portion where a deformation path changes from compressive deformation to tensile deformation when a thin metal plate is drawn into a prismatic container with a bottom. It is a figure explaining the shape and size of the metal thin plate used in the embodiment and example of the present invention. In the examples, the presence or absence of cracking determined by a basic forming test in which a thin metal plate is drawn into a square tube shape is determined by FEM analysis of the basic forming test as a crack determination parameter. The maximum plate thickness increase and relative plate thickness decrease. and a forming limit line created based on the plotted distribution of presence or absence of cracks. In the example, as a basic molding test, it is a diagram illustrating an example of a mold for drawing a thin metal plate into a cylindrical container with a bottom ((a) perspective view, (b) cross-sectional view parallel to the molding direction, ( c) Cross-sectional view perpendicular to the forming direction). In the examples, the presence or absence of cracks determined by a basic forming test for drawing a thin metal plate into a cylindrical shape, and the forming limit obtained by a basic forming test and FEM analysis for drawing a thin metal plate into a rectangular tube in the embodiment. 1 is a graph showing lines and

<Circumstances leading to the invention>
Prior to explaining the press forming limit line acquisition method according to the embodiment of the present invention, the circumstances leading to the idea of the present invention will be explained.

The inventors diligently studied the reason why the forming limit is different when the deformation path of the thin metal sheet changes from compressive deformation to tensile deformation during press forming compared to when deformation follows a constant deformation path. Then, focusing on the fact that if the amount of deformation due to compressive deformation in the primary path is large, even if the amount of deformation due to tensile deformation in the subsequent secondary path is small, material cracking (rupture) is likely to occur. The mechanism is presumed that the deformation characteristics such as ductility of the material are reduced (damaged) due to the work hardening of the material, and the subsequent tensile deformation in the secondary path causes cracks easily.
Therefore, it was conceived that the amount of compressive deformation due to compressive deformation in the primary path and the amount of tensile deformation due to tensile deformation in the secondary path are each represented by the true strain (plate thickness change amount) in the plate thickness direction of the thin metal plate. did.

FIG. 2 is a diagram schematically showing changes in plate thickness when a thin metal plate is deformed along a deformation path that changes from compressive deformation to tensile deformation.
In Fig. 2, the vertical axis represents the reduction in thickness given by the true strain (-ln (thickness after deformation/thickness before deformation)) in the thickness direction of the thin metal sheet on the deformation path that changes from compressive deformation to tensile deformation. rate, and the horizontal axis is the elapsed time from the start of deformation of the metal sheet.
At this time, (i) the true strain ε _compression in the thickness direction due to compressive deformation in the primary path and (ii) the true strain ε _{tension after compression} in the thickness direction due to tensile deformation in the secondary path are given by the formula ( 1) and equation (2).

Then, the true strain in compressive deformation and tensile deformation is obtained as the amount of compressive deformation and the amount of tensile deformation, respectively. The idea was to determine the presence or absence of cracks in thin plates.

However, although it is possible to determine the presence or absence of cracks in a thin metal sheet by a forming test in which the thin metal sheet is deformed along a deformation path that changes from compressive deformation to tensile deformation, the maximum thickness of the thin metal sheet due to compressive deformation along the deformation path It is difficult to directly measure hc and the minimum plate thickness ht due to tensile deformation.

Therefore, FEM (Finite Element Method) analysis was performed to reproduce the deformation of the metal sheet along the deformation path that changes from compressive deformation to tensile deformation. A change in the plate thickness of the element is calculated, and the maximum plate thickness and the minimum plate thickness are obtained from the calculated plate thickness change.

The present invention has been made based on the above studies, and an embodiment of the present invention will be described below.

<Method for obtaining press forming limit line>
A press-forming limit line acquisition method according to an embodiment of the present invention is a method for determining whether or not a crack occurs at a portion where a deformation path in a thin metal plate changes from compressive deformation to tensile deformation in press-forming of a thin metal plate. A limit line is obtained, and as shown in FIG. 1, a basic forming test step S1, an FEM analysis step S3, a crack determination parameter acquisition step S5, a crack determination parameter plotting step S7, and a forming limit line creation step. and S9.
Each of the above steps will be described below.

≪Basic molding test process≫
In the basic forming test step S1, a basic forming test is performed in which the thin metal plate is deformed along a deformation path that changes from compression deformation to tensile deformation under various forming conditions. This is a step of obtaining whether or not cracks have occurred in the portion that changes to deformation.

In this embodiment, a basic forming test is performed in which a thin metal plate (not shown) is drawn using a mold 1 having a punch 3, a die 5, and a blank holder 7, as illustrated in FIG.

The drawing process involves pressing the punch 3 into the die hole 5a of the die 5 while holding the edge of the thin metal plate between the die 5 and the blank holder 7, thereby drawing the thin metal plate to the center and forming the corners from the flat thin metal plate. It refers to forming a cylindrical columnar container 11 with a bottom. In addition, FIG. 4 shows a 1/4 area around the corner portion 11a of the columnar container 11 with the bottom.

In the drawing process for forming the thin metal plate into the bottomed columnar container 11 using the die 1, the thin metal plate is first drawn toward the die hole portion 5a by the punch 3, so that the thin metal plate moves toward the die hole portion 5a. , and undergoes compression deformation along the circumferential direction of the outer edge of the die hole portion 5a.
The portion of the thin metal sheet that has undergone compressive deformation is subjected to tensile deformation when it is pushed into the die hole portion 5a in contact with the punch shoulder portion 3a, and is also subjected to tensile deformation after being drawn into the die hole portion 5a.
As a result, the corner portion 11a of the bottomed columnar container 11 and the vertical wall portion 15 around the corner portion 11a are formed along a deformation path that changes from compressive deformation to tensile deformation.

Various forming conditions in the basic forming test include the shape and size of the metal sheet, the wrinkle holding force of the metal sheet by the blank holder, the lubrication conditions of the part of the metal sheet sandwiched between the die and the blank holder (type of lubricating oil, viscosity, The supply amount, the addition of the extreme pressure additive, etc.), the shape of the beads applied to the thin metal plate, etc. may be changed and set as appropriate.

In particular, in the basic forming test in which drawing is performed using the die shown in FIG. 3, the forming conditions can be easily changed by changing the shape of the metal sheet or the wrinkle holding force applied to the metal sheet by the blank holder 7. can do.

In this embodiment, the punch shoulder radius of the punch shoulder portion 3a is R12 mm, the die shoulder radius of the die shoulder portion 5b is R5 mm, and the corner radius of the corner portion 11a is R25 mm. As the thin metal plate, a steel plate of 980 MPa class with a thickness of 1.4 mm was used as a test material, and a thin metal plate 21 having the shape and dimensions shown in Fig. 5 was used. Various molding conditions in the basic molding test were set by changing the range.

Table 1 shows the forming conditions (shape, size and wrinkle pressing force of the metal sheet) in the basic forming test for drawing the metal sheet 21 shown in FIG. show.

<<FEM analysis process>>
The FEM analysis step S3 is a step of performing FEM analysis for various forming conditions with the basic forming test of the metal thin plate as the analysis target, and calculating changes in the plate thickness of the metal thin plate.

In the present embodiment, the FEM analysis in the FEM analysis step S3 is for a basic forming test in which the metallic mold 1 shown in FIG. The molding conditions in the FEM analysis step S3 are the same molding conditions as the basic molding test in the basic molding test step S1.

FIG. 2 shows an example of calculation by FEM analysis of changes in plate thickness at a portion of a thin metal plate where compressive deformation changes to tensile deformation.
As shown in FIG. 2, the thickness of the thin metal plate increases in the process of compressive deformation and reaches the maximum thickness hc, and the thickness decreases in the process of tensile deformation after compressive deformation. Here, the minimum value of plate thickness in tensile deformation is expressed as minimum plate thickness ht.

In addition, the FEM analysis step S3 calculates a change in the plate thickness of the element corresponding to the portion of the thin metal plate for which the presence or absence of crack generation is obtained in the basic forming test step S1.

≪Crack determination parameter acquisition process≫
In the crack determination parameter acquisition step S5, based on the change in the thickness of the thin metal plate calculated in the FEM analysis step S3, the thickness of the thin metal plate until the thin metal plate reaches the maximum thickness hc in compressive deformation along the deformation path is obtained for various forming conditions. The maximum thickness increase amount, which is the amount of change in thickness, and the relative It is a step of obtaining the plate thickness reduction amount as a crack determination parameter.

When the change in thickness of the thin metal plate is calculated in the FEM analysis step S3 as shown in FIG. Using and, the true strain ε _compression in compressive deformation given by the above formula (1) is calculated as the maximum plate thickness increase, and using the maximum plate thickness hc and the minimum plate thickness ht in tensile deformation, The true strain ε _{tension after compression} in tensile deformation after compressive deformation given by the above equation (2) is calculated as a relative plate thickness reduction amount.
Then, the maximum plate thickness increase amount and the relative plate thickness decrease amount calculated in this manner are acquired as crack determination parameters.

Table 1 shows the maximum plate thickness increase calculated as a crack determination parameter for each forming condition by FEM analysis of a basic forming test in which the thin metal plate 21 shown in FIG. and the results of the amount of relative plate thickness reduction.

≪Crack determination parameter plotting process≫
As shown in FIG. 6 as an example, the crack determination parameter plotting step S7 includes the presence or absence of cracks obtained for various molding conditions in the basic molding test step S1, and the various molding conditions in the crack determination parameter acquisition step S5. and the crack determination parameter, and plotted on two-dimensional coordinates with the maximum plate thickness increase and the relative plate thickness decrease as each axis.

In FIG. 6, the ◯ mark plot indicates no crack generation in the basic forming test step S1, and the x mark plot indicates the presence of crack generation in the basic forming test step S1.

≪Forming limit line creation process≫
The forming limit line creation step S9 is based on the distribution of the presence or absence of crack generation plotted on the two-dimensional coordinates in the crack determination parameter plotting step S7, and the portion where the thin metal plate deforms along the deformation path that changes from compressive deformation to tensile deformation. This is a step of creating a forming limit line that distinguishes whether or not cracks occur.

FIG. 6 shows an example of a forming limit line created based on the distribution of crack determination parameters plotted on two-dimensional coordinates in the crack determination parameter plotting step S7.

The forming limit line may be created, for example, by fitting a functional expression that approximates the boundary between occurrence of cracks and no occurrence of cracks in the crack determination parameters.

Cracking in a forming mode in which the deformation path changes from compression deformation to tensile deformation, which is the object of the present invention, has a low limit value for the relative plate thickness decrease when the maximum plate thickness increase is large, and conversely, the maximum plate thickness When the amount of increase in thickness is small, the limit value of the amount of relative thickness decrease is large. Therefore, the shaping limit line can be formulated as a high-order (eg, cubic) inverse function.

Specifically, among the crack determination parameters plotted in the crack determination parameter plotting step S7, a crack determination parameter indicating the occurrence of a crack is extracted, and a high-order inverse function is assumed as a forming limit line that smoothly connects the extracted crack determination parameters. Then, the forming limit line can be created by determining the coefficient of the inverse function so that the sum of squared errors of the assumed inverse function with the extracted crack determination parameter is minimized.

Here, all the crack determination parameters plotted in the area above the forming limit line must indicate that cracks have occurred. , extract the plot of the crack determination parameter for the minimum relative thickness reduction amount at each maximum thickness increase amount.

The forming limit line shown in FIG. 6 was created assuming a cubic inverse function shown in formula (3), and the values of each coefficient in formula (3) are a = 2.1 × 10 ^-10 , b =8.9×10 ⁻¹² , c=2.0, d=0, e=0.01.

As described above, according to the press-forming limit line acquisition method according to the present embodiment, it is possible to obtain a forming limit line for determining whether or not cracks occur in a thin metal plate that is deformed along a deformation path that changes from compressive deformation to tensile deformation. can.
Then, by using the forming limit line obtained by the press forming limit line acquisition method, cracks can be generated based on the change in plate thickness at the part where the deformation path that changes from compressive deformation to tensile deformation occurs in the press forming process. It is possible to determine the presence/absence and establish press molding conditions that enable stable press molding.

In the method for obtaining the press-forming limit line according to the present embodiment, drawing is performed using the die 1 shown in FIG. 3 as a basic forming test for the thin metal plate. During processing, compressive deformation occurs in the direction along the outer edge of the die hole portion 5a, and then tensile deformation occurs in the direction in which the thin metal plate is pushed into the die hole portion 5a. The direction of tensile deformation in the next path does not match.

Cracking in the deformation path from compressive deformation to tensile deformation in thin metal sheets is considered to be related to work hardening due to compressive deformation in the primary path and subsequent tensile deformation in the secondary path. In the basic forming test in the press forming limit line acquisition method according to the invention, it is not necessary to match the compression direction in the primary pass and the tensile direction in the secondary pass like the above-described drawing.

Therefore, the basic forming test in the basic forming test step S1 is not limited to the uniaxial compression/tension test in which the compression direction and the tension direction match, and as described in this embodiment, the drawing process in which the compression direction and the tensile direction do not match It's okay.

However, in a uniaxial compression-tension test in which compression deformation is reversed to tensile deformation in the plane of the metal sheet, buckling occurs during compression deformation of the metal sheet, so the amount of compressive deformation that can be imparted to the metal sheet remains in a narrow range. .
On the other hand, in the drawing process, a large amount of compressive deformation can be imparted in the primary path of compressive deformation of the thin metal sheet, and a large tensile deformation can be imparted in the subsequent secondary path. As a result, in the basic forming test by drawing, the maximum plate thickness increase in compressive deformation and the relative plate thickness decrease in tensile deformation after compressive deformation can be obtained in a wide range. The molding limit line can be created in a wide range, the accuracy of the molding limit line can be improved, and the applicable molding conditions can be widened.

In the basic forming test by drawing, the forming conditions can be changed by changing the inflow resistance of the material flowing toward the die hole portion 5a (see FIG. 3) of the thin metal plate. By changing the molding conditions, it is possible to change the maximum plate thickness increase amount due to compressive deformation and the relative plate thickness decrease amount due to tensile deformation.

For example, in a basic forming test for drawing a thin metal plate using the die 1, the size of the thin metal plate is increased in order to set the forming condition to increase the inflow resistance of the material flow toward the die hole portion 5a of the thin metal plate. , increasing the wrinkle pressing force of the blank holder 7, lubricating the thin metal plate with a high coefficient of friction between the die 5 and the blank holder 7, giving the thin metal plate a bead shape, and the like.

Under molding conditions that increase the inflow resistance of the material flow, the flow of the thin metal plate toward the die hole portion 5a is suppressed, so that the compressive deformation of the thin metal plate in the circumferential direction of the outer edge of the die hole portion 5a is alleviated. , the maximum plate thickness increase becomes smaller. Furthermore, since the material flow drawn into the die hole portion 5a is reduced, the tensile deformation of the thin metal plate due to the drawing process is increased, and the relative plate thickness reduction amount is increased.

In the drawing process of the thin metal plate, if wrinkles are generated in the flange portion, the generated wrinkles may induce an excessive drawing force and cause breakage (cracking) of the thin metal plate. Since such cracks in the thin metal plate are different from cracks at a portion where the deformation path changes from compressive deformation to tensile deformation, which is the object of the present invention, it is impossible to properly determine whether or not cracks have occurred. Therefore, when a basic forming test is performed by drawing a thin metal plate, it is preferable to use a blank holder 7 to prevent the generation of wrinkles, as shown in FIG.

Further, when drawing is performed using the die 1 as shown in FIG. 3, if the die shoulder radius of the die shoulder portion 5b is smaller than the plate thickness of the thin metal plate, the plate thickness is rapidly reduced by tensile deformation after compressive deformation. Therefore, it is not possible to properly determine whether or not cracks have occurred. Therefore, the die shoulder radius of the die 5 is preferably several times or more the thickness of the thin metal plate.

Note that the drawing test is not limited to forming a rectangular container using the mold 1 shown in FIG. may be

In the above description, as an example, the crack determination parameter obtaining step S5 obtains the true strain ε _compression in compressive deformation given by the formula (1) as the maximum plate thickness increase amount, and the true strain given by the formula (2). The strain ε _{tension after compression} was obtained as a relative reduction in plate thickness.
However, in the present invention, the maximum plate thickness increase amount and the relative plate thickness decrease amount obtained as crack determination parameters are, for example, the nominal strain converted from the true strain in compressive deformation and tensile deformation after compressive deformation, and the nominal strain in compressive deformation. The true strain in the plate thickness direction may be positive, and the true strain in the plate thickness direction in tensile deformation may be calculated as negative.

The above explanation is the result when the 980 MPa class steel plate was used as the test material for the thin metal plate. However, other metal materials may also be used.

Experiments and analyzes were conducted to verify the effects of the method for obtaining the press forming limit line according to the present invention, which will be described below.

In this example, a metal sheet was drawn into a cylindrical columnar container with a bottom (not shown) using a mold 31 shown in FIG. Determination was made using the forming limit line (Fig. 6) obtained by the basic forming test in which drawing was performed into a rectangular tube shape described in the mode.

A steel plate having a tensile strength of 980 MPa and a thickness of 1.4 mm was used as a test material for the thin metal plate, and the thin metal plate 21 having the shape and dimensions shown in FIG. 5 was used.

The die 31 includes a punch 33, a die 35, and a blank holder 37. The punch shoulder portion 33a has a punch shoulder radius of R12 mm, and the die shoulder portion 35b has a die shoulder radius of R5 mm. Also, the wrinkle pressing force by the blank holder 37 was set to 5 tonf.

First, the presence or absence of cracks in a cylindrical container with a bottom formed by drawing the thin metal plate 21 using the metal mold 31 was determined for each shape and size of the thin metal plate 21 shown in FIG.

Next, for each thin metal plate 21 having the shape and dimensions shown in FIG. The maximum plate thickness increase and the relative plate thickness decrease in the vertical wall portion of the cylindrical container with a bottom were obtained as crack determination parameters.

Table 2 shows the presence or absence of cracks obtained by drawing a cylindrical columnar container with a bottom, and the results of crack determination parameters (maximum plate thickness increase and relative plate thickness decrease) obtained by FEM analysis.

Then, the presence or absence of crack generation obtained by drawing is associated with the crack determination parameter obtained by FEM analysis, and as shown in FIG. plotted on the coordinate plane.

Furthermore, the forming limit line (FIG. 6) obtained in the basic forming test for drawing the thin metal plate described in the above-described embodiment into the square cylindrical columnar container 11 with the bottom, and the presence or absence of crack generation shown in FIG. It is displayed on the plotted two-dimensional coordinates.

As shown in FIG. 8, even if the forming limit line obtained in the basic forming test for drawing a thin metal plate into a rectangular cylindrical columnar container 11 with a bottom is used, there is no difference in the case of drawing into a cylindrical columnar container with a bottom. It was shown that the presence or absence of crack generation can be determined with high accuracy, proving the effectiveness of the present invention.

1 Die 3 Punch 3a Punch Shoulder 5 Die 5a Die Hole 5b Die Shoulder 7 Blank Holder 11 Columnar Container with Bottom 11a Corner 13 Bottom 15 Vertical Wall 17 Flange 21 Thin Metal Plate 31 Die 33 Punch 33a Punch Shoulder Part 35 Die 35a Die hole 35b Die shoulder 37 Blank holder

Claims (3)

A press forming limit line acquisition method for obtaining a forming limit line for determining the presence or absence of cracking at a portion where a deformation path of the thin metal plate changes from compressive deformation to tensile deformation in press forming of a metal sheet, comprising:
A basic forming test in which the metal sheet is deformed along the deformation path is performed under various forming conditions, and whether or not cracks occur in the portion of the metal sheet where the deformation path changes from compressive deformation to tensile deformation for the various forming conditions. A basic molding test process to obtain
an FEM analysis step of performing an FEM analysis with the basic forming test of the thin metal plate as an analysis target for the various forming conditions, and calculating a change in the thickness of the thin metal plate;
Based on the change in the thickness of the thin metal plate calculated in the FEM analysis process, the amount of change in the thickness of the thin metal plate until the thin metal plate reaches the maximum thickness in the compression deformation of the deformation path for the various forming conditions A certain maximum plate thickness increase amount, and a relative plate thickness decrease amount that is the plate thickness change amount from the maximum plate thickness to the minimum plate thickness of the thin metal plate as the deformation path changes from compressive deformation to tensile deformation. A crack determination parameter acquisition step of obtaining , as a crack determination parameter;
By associating the presence or absence of cracks obtained for the various forming conditions in the basic forming test step with the crack determination parameters obtained for the various forming conditions in the crack determination parameter acquisition step, the maximum plate thickness increase a crack determination parameter plotting step of plotting the amount and the relative plate thickness reduction amount on two-dimensional coordinates with each axis;
a forming limit line creating step of creating a forming limit line that distinguishes whether or not a crack occurs in a portion where the thin metal plate is deformed along the deformation path, based on the distribution of presence or absence of crack occurrence plotted on the two-dimensional coordinates; A method for obtaining a press forming limit line, comprising: 2. The press-forming limit line acquisition method according to claim 1, wherein said basic forming test step deforms said thin metal plate along said deformation path by drawing said thin metal plate. In the basic forming test step, the various forming conditions are set by changing the shape of the thin metal plate or the wrinkle holding force applied to the thin metal plate in the drawing of the thin metal plate. 3. The press forming limit line acquisition method according to claim 2.

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