JP7015524B2 - Preliminary prediction method for the presence or absence of cracks during molding of a press-molded body - Google Patents

Description

The present invention relates to a method for predicting in advance whether or not cracks will occur in a press-molded body by a press-molding simulation in a stage before the press-molded plate is press-molded to obtain a press-molded body.

Conventionally, when designing a press-molded body used for automobiles, home appliances, etc., a press using a computer is used in order to minimize rework during the design and manufacture of the press die for forming the press-molded body. The work of predicting in advance whether or not cracks will occur during the molding of the press-molded body is performed by the molding simulation.

For example, in Patent Document 1, model data consisting of a plurality of finite elements modeling a plate to be pressed is created, and numerical analysis by press molding simulation is performed using the model data on a computer, and then each finite element is obtained. The computer determines whether the maximum and minimum principal strains of the elements are located above the molding limit line in the molding limit line diagram, and the press-formed body corresponding to each finite element located above the molding limit line. It is designed to predict in advance that a crack will occur at the position.

Japanese Unexamined Patent Publication No. 2006-167766

By the way, as a plate to be pressed before being formed into a press-molded body, a coil formed by winding a steel strip is generally used and cut to a predetermined length in the winding direction, and a press die is generally used. The set state (winding direction of the coil) of the plate to be pressed before pressing is naturally determined by material removal for the purpose of improving the material yield. Then, in the press-molded body, rather than bending the bend-molded portion so that the bend ridge line intersects the coil winding direction to form the press-molded body, the bend-molded portion so that the bend ridge line follows the coil winding direction. It is generally known that cracking is more likely to occur during press molding when the press-molded body is formed by bending.

However, in the preliminary evaluation by the conventional press forming simulation as in Patent Document 1, the set state (coil winding direction) of the pressed plate with respect to the press die before pressing is not taken into consideration, and the pressed plate is pressed. There is a risk of development rework, such as variations in the evaluation of crack occurrence due to differences in the posture of the plate before pressing (coil winding direction), and the need to make design changes that were not originally necessary. there were. Therefore, when making a preliminary prediction, there is a request to greatly contribute to shortening the lead time of development by reflecting the difference in the set state (coil winding direction) of the pressed plate with respect to the press die before pressing. rice field.

The present invention has been made in view of these points, and an object thereof is to shorten the lead time of development by predicting the occurrence of cracks in consideration of the winding direction of the coil of the pressed plate. It is to make a big contribution.

In order to achieve the above object, the present invention is characterized in that the present invention is devised so that the pre-evaluation can be performed in consideration of the set state of the press plate to be pressed with respect to the press die, which is considered to be difficult to form. do.

Specifically, in the pre-stage of press-molding the plate to be pressed to obtain a press-molded body, model data consisting of a large number of finite elements modeling the plate to be pressed is created and press-molding using the model data. Numerical analysis by simulation is performed on a computer to derive the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element in the model data, and then the maximum principal strain ε ₁ and the minimum principal strain ε 1 of each finite element. The computer determines whether or not ε ₂ is located above the molding limit line, and predicts that cracks will occur at the position of the press-molded body corresponding to each finite element located above the molding limit line. The following solutions were taken for the method of predicting the presence or absence of cracks during molding of the molded body.

That is, in the first invention, in a simple bending test of a test plate obtained by unwinding a steel strip from a coil, the bending ridge line is bent so as to intersect the winding direction of the coil. The computer calculates the modified forming limit line by shifting the entire forming limit line downward by the difference in the limit strain when the bending ridge line is bent along the winding direction of the coil. It is determined whether the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of the finite element are located above the modified forming limit line, and the press corresponding to each finite element located above the modified forming limit line. It is characterized by predicting that cracks will occur at the position of the molded product.

Further, in the second invention, in the first invention, when the pressed plate is 980 MPa class and the plate thickness is 1 mm, the difference of the limit strain derived by the experiment is -0.011 only for the computer. Is characterized in that the modified molding limit line is calculated by shifting the entire molding limit line downward.

In the present invention, the computer calculates the lower limit of the molding limit line assuming the most difficult case of molding, such as bending each bending and forming portion so that the bending ridge line follows the winding direction of the coil to obtain a press-molded body. Since it is predicted in advance whether or not cracks will occur in the press-molded body, if it is predicted that cracks will occur, it will be understood that it is absolutely necessary to change the design of the mold, etc., but cracks will occur. If it is predicted that it will not occur, it will be possible to freely study material removal regardless of the coil winding direction for the purpose of improving the yield. Therefore, the time spent on changing the design of the mold and examining the material removal is reduced, and the number of rework is reduced, which can greatly contribute to shortening the lead time of development.

It is a flowchart which shows the procedure of the prediction method which concerns on embodiment of this invention. It is a schematic perspective view which shows the press-molded body which predicts in advance whether or not cracks occur at the time of molding using the prediction method of this invention, and the pressed plate before molding into the press-molded body, respectively. It is a schematic perspective view which shows the model data of the pressed plate in FIG. 2 which shows the state displayed on a computer, and the model data after deformation by a press forming simulation, respectively. It is a molding limit diagram used when predicting the presence or absence of cracking at the time of molding of the press-molded article of FIG. 2, and the maximum principal strain and the minimum of each finite element in the model data after deformation by the press-molding simulation of FIG. It is the figure which plotted each main strain. It is a perspective view of the model data after deformation by the press forming simulation which showed the position of each finite element corresponding to the point cloud located in the E2 part of FIG. It is general data of the limit surface strain at the time of simple bending deformation.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It should be noted that the following description of the preferred embodiment is essentially merely an example.

FIG. 1 is a flowchart 1 showing a work procedure of a prediction method according to an embodiment of the present invention. As shown in FIGS. 2 and 3, the prediction method of the present invention press-molds the pressed plate 2 into a press-molded body 3 having a cross-section hat shape, which is a target shape. By creating model data D1 consisting of a large number of modeled finite elements An ( _n is a natural number) and performing numerical analysis by press molding simulation using the model data D1 on a computer, the press-molded body is formed at the time of molding. It predicts in advance whether or not cracks will occur in 3, and whether or not cracks will occur during molding of the press-molded body 3 by going through the six steps S1 to S6 (see FIG. 1) in order. It is designed to be known in advance.

As shown in FIG. 2, the press-molded body 3 has four bending-molded portions 3a extending along the longitudinal direction thereof and having a cross section bent at about 90 °.

The plate 2 to be pressed is obtained by winding a steel strip having a plate thickness t from a coil and cutting it at a predetermined interval. For a part having a cross-section hat shape in which each bending die 3a extends in the same direction, such as a press die 3, each bending die 3a is bent so that the bending ridge line intersects the winding direction of the coil to obtain the press molding 3. (Mold with the pressed plate 2 set in the die so that the coil winding direction is the X1 direction in FIG. 2), or each bending and forming portion 3a is bent so that the ridge line is in the coil winding direction. It is common to obtain a press-molded body 3 by bending along the same direction (molding with the pressed plate 2 set in a die so that the winding direction of the coil is the X2 direction of FIG. 2).

That is, in the case of the press-molded body 3 as shown in FIG. 2, the pressed plate 2 is used as a die so that the winding direction of the coil intersects the longitudinal direction of the press-molded body 3 (so as to be in the X1 direction). Molding is performed in the set state, or the pressed plate 2 is set in the die so that the winding direction of the coil is along the longitudinal direction of the press-molded body 3 (so as to be in the X2 direction). It is designed to do.

A molding limit diagram G as shown in FIG. 4 is used for predicting in advance whether or not cracks will occur during molding of the press-molded body 3. In this molding limit line diagram G, a molding limit line L1 having a substantially V shape and a modified molding limit line L2 located below the molding limit line L1 and having the same shape as the molding limit line L1 are provided. Each is displayed.

The forming limit line L1 is, for example, derived experimentally or theoretically when the plate thickness of the plate 2 to be pressed is t, and it is determined on a computer whether or not cracks occur during press forming. It is used as a general threshold value when making predictions.

For example, in the case of obtaining the press-formed body 3, the maximum principal strain _{ε 1} and the minimum principal strain ε ₂ of each finite element An in the model data D1 of FIG. 3 are derived on a computer, and then each finite element _An is derived. The computer determines whether or not the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of are located above the molding limit line L1, and press molding corresponding to each finite element _Ann located above the molding limit line L1. It is predicted that a crack will occur at the position of the body 3.

Further, the maximum principal strain _{ε 1} and the minimum principal strain ε ₂ of each finite element An in the model data D1 of FIG. 3 are compared with the forming limit line L1, and the maximum principal strain _{ε 1} and the minimum of all the finite elements An are compared. When the main strain ε ₂ is located below the molding limit line L1, it is predicted that the press molded body 3 will not be cracked.

On the other hand, in the prediction method of the present invention, the modified molding limit line L2 is cracked at the time of molding assuming that the bending ridge line is bent along the winding direction of the coil to obtain the press-molded body 3. It is used as a threshold when predicting whether or not it occurs, and if the molding limit line L1 is defined as y = f (x) (y: maximum principal strain, x: minimum principal strain), y = f (x) by a computer. ) -Α × t (α: predetermined coefficient).

The α × t at the modified forming limit line L2 is such that the bending ridge line intersects the winding direction of the coil in the simple bending test of the test plate obtained by unwinding the steel strip having the plate thickness t from the coil. This is the difference in the limit strain when the bending ridge line is bent along the winding direction of the coil as compared with the case of bending and molding.

For example, when the bend forming portion 3a is bent so that the bending ridge line follows the winding direction of the coil to obtain the press-formed body 3, the maximum principal strain _{ε 1} and the minimum of each finite element An in the model data D1 of FIG. 3 are obtained. After the main strain ε ₂ is derived by the computer, it is determined whether the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element _Ann are located above the modified forming limit line L2, and the modified forming limit is determined. It is predicted that cracks will occur at the position of the press-formed body 3 corresponding to each finite element _An located above the line L2.

Further, the maximum principal strain _{ε 1} and the minimum principal strain ε ₂ of each finite element An in the model data D1 of FIG. 3 are compared with the modified molding limit line L2, and the maximum principal strain ε ₁ and the maximum principal strain _ε 1 of all the finite elements An are compared. When the minimum principal strain ε ₂ is located below the modified forming limit line L2, it is predicted that the press-formed body 3 will not be cracked.

As described above, in the prediction method of the present invention, the bent ridge line intersects the winding direction of the coil when the press-molded body 3 is formed by calculating the lower limit value of the assumed forming limit line on the computer. Consider not only the case where the bend-molded portion 3a is bent to form the press-molded body 3 but also the case where the bend-molded portion 3a is bent so that the bending ridge line is along the winding direction of the coil to obtain the press-molded body 3. It is possible to make an evaluation.

The modified molding limit line L2 was derived as follows.

In the simple bending test, assuming that the bending radius when a predetermined test plate (980 MPa class) is simply bent is R and the plate thickness is t, the relationship between R / t and the amount of strain on the outer surface is shown in the curve C1 in FIG. It is generally known that Then, in the simple bending test, when the bending ridge line is bent so as to intersect the coil winding direction, when the bending ridge line is bent along the coil winding direction, the curve C1 is R / t. It is also known that the value of the curve C2 is deviated by about -1 in the axial direction.

When a simple bending test using a test plate having a plate thickness of 1 mm was performed, the difference between the strain amount when the bending radius R = 3 and the strain amount when the bending radius R = 4 was 0.011. That is, it was found that when the difference in bending radius R was 1, the difference in strain amount was 0.011. Therefore, when viewed in the forming limit diagram G, the position of the forming limit line L1 is shifted so that the value of the limit strain when the minimum principal strain ε ₂ is 0 is -0.011 × t downward. Considering that it is possible to evaluate the presence or absence of the most severe cracks if the lower limit is set, when the molding limit line L1: y = f (x) is used for the evaluation of the presence or absence of cracks in the press molded body 3, first of all, correction is made. It was decided to evaluate the presence or absence of cracking using the molding limit line L2: y = f (x) −0.011 × t. The strain amount 0.011 is a value that changes depending on the material of the pressed plate 2.

Next, the results of evaluating the presence or absence of cracks using the method of the present invention will be described in detail.

First, as shown in FIGS. 2 and 3, cracks do not occur when the bent ridge line is bent so that the bending ridge line intersects the winding direction of the coil to obtain the press-molded body 3, but the bent ridge line is formed. Model data D1 of the press-molded body 3 in which cracks occur when the bending-molded portion 3a is bent and molded along the winding direction of the coil is prepared. The E1 portion in FIG. 2 is the crack occurrence site.

Next, the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element _Ann in the model data D1 were obtained by numerical analysis by press molding simulation on a computer and plotted on the molding limit diagram G, respectively. Then, as shown in FIG. 4, there were no plots above the forming limit line L1, but a large number of plots were extracted in the region E2 between the forming limit line L1 and the modified forming limit line L2.

When each finite element An corresponding to a large number of plots located in the region E2 is displayed in the model data _D1 after molding (part E3), as shown in FIG. 5, the bending ridge line is along the winding direction of the coil. It coincided with the crack generation site (E1 part) when the press-molded body 3 was actually obtained by bending the bend-molded part 3a.

That is, as shown in FIGS. 4 and 5, when the bent ridge line is bent along the winding direction of the coil when the press-molded body 3 is formed, the critical strain predicted to cause cracking. Was found to be concentrated in the region between the molding limit line L1 and the modified molding limit line L2 in the molding limit diagram G.

Next, the procedure for predicting the presence or absence of cracks during molding of the press-molded body 3 using the prediction method of the present invention will be described in detail.

First, as shown in FIG. 1, in step S1, model data _D1 composed of a plurality of finite elements An that model the pressed plate 2 is created on a computer.

Next, in step S2, a forming limit diagram G when the plate thickness of the pressed plate 2 is t is obtained (see FIG. 4).

Next, in step S3, a press forming simulation is carried out on a computer using the model data D1. Then, the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element _Ann in the model data D1 which has become the target shape by the numerical analysis by the press forming simulation are obtained.

Then, in step S4, the computer determines whether or not the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element _Ann are located above the modified molding limit line L2 in the molding limit diagram G.

If the determination in step S4 is YES, that is, if there is a _finite element An located above the modified forming limit line L2, the process proceeds to step S5, and the maximum principal strain _{ε 1} and the minimum of each finite element An are reached. It is determined whether or not the main strain ε ₂ is located above the molding limit line L1 in the molding limit diagram G.

On the other hand, when the determination in step S4 is NO, that is, when there is no finite element An located above the modified molding limit line L2, it is determined that _cracking does not occur during molding regardless of the coil winding direction. Then, the preliminary prediction of the presence or absence of cracks during molding of the press molded body 3 is completed.

Then, when the determination in step S5 is YES, that is, when there is a finite element _An located above the molding limit line L1, the process proceeds to step S6 to reexamine the shapes of the press-molded body 3 and the die. ..

On the other hand, when the determination in step S5 is NO, that is, when there is no finite element _Ann located above the forming limit line L1, the bending forming portion 3a is formed so that the bending ridge line intersects the winding direction of the coil. It is determined that cracks do not occur during molding if the press-molded body 3 is bent to obtain the press-molded body 3, and the preliminary prediction of the presence or absence of cracks during molding of the press-molded body 3 ends.

From the above, according to the embodiment of the present invention, the molding limit line assuming the most difficult case of molding, such as bending each bending die 3a so that the bending ridge line follows the winding direction of the coil to obtain the press-molded body 3. Since the computer calculates the modified molding limit line L2, which is the lower limit of the above, to predict in advance whether or not cracks will occur in the press-molded body 3, if cracks are predicted to occur, gold While it is clear that it is absolutely necessary to change the design of the mold, etc., if it is predicted that cracks will not occur, the material can be freely examined regardless of the coil winding direction for the purpose of improving the yield. You will be able to do it. Therefore, the time spent on changing the design of the mold and examining the material removal is reduced, and the number of rework is reduced, which can greatly contribute to shortening the lead time of development.

In the embodiment of the present invention, first, the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element _Ann are compared with the modified forming limit line L2 of the forming limit diagram G, and then the modified forming limit. When the finite element _Ann is located above the line L2, the maximum principal strain ε1 and the minimum principal strain _ε2 of _each finite element Ann _are compared with the molding limit line L1 in the molding limit diagram G. However, the present invention is not limited to this, for example, after comparing the maximum principal strain _{ε 1} and the minimum principal strain ε ₂ of each finite element An with the molding limit line L1 of the molding limit diagram G, the maximum of each finite element _An . The principal strain ε ₁ and the minimum principal strain ε ₂ may be compared with the modified molding limit line L2 in the molding limit diagram G, or the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element _Ann may be compared with the molding. After comparing with the modified molding limit line L2 of the limit diagram G, without comparing the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element _Ann with the molding limit line L1 of the molding limit diagram G. The preliminary prediction of the presence or absence of cracks may be terminated.

Further, in the embodiment of the present invention, the presence or absence of cracks in the press-molded body 3 having a cross-sectional hat shape is predicted by using the method of prior prediction of the present invention, but cracks occur in the press-molded bodies having other shapes. The presence or absence is also predictable. For example, in the embodiment of the present invention, all the bending ridges of each bending ridge 3a of the press-molded body 3 extend in the same direction, but all the bending ridges of each bending ridge 3a extend in the same direction. It is predictable even if it is not the body.

The present invention is suitable for a method of predicting in advance whether or not cracks will occur in a press-molded body by a press-molding simulation in a stage before the press-molded plate is press-molded to obtain a press-molded body.

1 Flow chart 2 Pressed plate 3 Press-molded body G Molding limit line diagram L1 Molding limit line L2 Modified molding limit line

Claims (2)

In the pre-stage of press-molding the plate to be pressed to obtain a press-formed body, model data consisting of a large number of finite elements modeling the plate to be pressed is created, and numerical analysis is performed by press-forming simulation using the model data. The maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each finite element in the above model data are derived on a computer, respectively, and then the maximum principal strain ε ₁ and the minimum principal strain ε ₂ of each of the above finite elements are the forming limits. At the time of molding the press-molded body, the computer determines whether or not it is located above the line, and it is predicted that cracks will occur at the position of the press-molded body corresponding to each finite element located above the molding limit line. It is a method of predicting the presence or absence of cracks in
In a simple bending test of a test plate obtained by unwinding a steel strip from a coil, the bending ridge is the coil winding when the bending ridge is bent so as to intersect the coil winding direction. The computer calculates the modified forming limit line by shifting the entire forming limit line downward by the difference in the limit strain when bending along the direction, and the maximum principal strain ε ₁ of each finite element. And whether or not the minimum principal strain ε ₂ is located above the modified molding limit line is determined, and cracks occur at the positions of the press-molded article corresponding to each finite element located above the modified molding limit line. A method for predicting the presence or absence of cracks during molding of a press-molded article, which comprises predicting that. In the method for predicting the presence or absence of cracks during molding of a press-molded product according to claim 1,
When the plate to be pressed is 980 MPa class and the plate thickness is 1 mm, the difference of the limit strain derived by the experiment is -0.011 and the computer shifts the entire forming limit line downward. A method for predicting the presence or absence of cracks during molding of a press-molded product, which comprises calculating a modified molding limit line.

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