WO2022024462A1

WO2022024462A1 - Shape change prediction method for press-molded article

Info

Publication number: WO2022024462A1
Application number: PCT/JP2021/014542
Authority: WO
Inventors: 徹簑手; 祐輔藤井; 正樹卜部
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2020-07-30
Filing date: 2021-04-05
Publication date: 2022-02-03
Also published as: JP6973580B1; JP2022025865A

Abstract

The present invention provides a shape change prediction method for a press-molded article, by which, for a press-molded article 1 having a top plate part 3 and a vertical wall part 7 that is continuous from the top plate 3 via a punch shoulder R part 5 and having a curved shape in plan view, a shape change caused by stress relaxation with the elapse of time after springback has occurred at the moment of release from a mold is predicted. The shape change prediction method includes: a step (S1) for acquiring the shape of the press-molded article 1 and the residual stress immediately after the springback by springback analysis; a step (S3) for setting, for the entire press-molded article 1 or a portion thereof immediately after the springback, a value of stress relaxed and reduced as compared with the residual stress immediately after the springback; and a step (S5) for determining a shape that achieves balance of the moment of force for the press-molded article 1 for which the value of relaxed and reduced stress has been set.

Description

Method for predicting shape change of press-molded products

The present invention relates to a method for predicting a shape change (prediction method for shape variation) of a press-formed part, and in particular, has a top plate portion (web portion) and a vertical wall portion (side wall portion) and is flat. The shape of a press-molded product that is visually curved and predicts the shape change that occurs over time after the die is released from the mold (tool of press forming) and springback is performed. Regarding the change prediction method.

Press molding is a manufacturing method that can manufacture metal parts at low cost and in a short time, and is used for manufacturing many automobile parts. In recent years, higher-strength metal sheets have been used for automobile parts in order to improve the crashworthiness of automobiles and reduce the weight of the automobile body. ing.

One of the main issues when press-molding a high-strength metal plate is a decrease in dimensional accuracy of the press-molded product due to springback. The residual stress generated when the metal plate is deformed using the mold by press molding becomes the driving force, and the press-molded product released from the mold is the shape of the metal sheet before press molding. The phenomenon of returning to the moment like a spring is called springback.

Since the residual stress of the press-formed product generated by press molding increases as the metal plate has higher strength (for example, high-tensile strength steel sheet), the shape variation of the press-formed product due to springback. Will also grow. Therefore, the press-molded product using a high-strength metal plate is more difficult to keep the shape immediately after springback within the specified dimensions, and the dimensional accuracy is lowered. Therefore, a technique for accurately predicting the shape change of a press-molded product due to springback is important.

In general, press forming simulation by the finite element method is used to predict the shape change of the press-molded product by springback. The procedure in the press molding simulation is divided into a first stage (for example, Patent Document 1) and a second stage (for example, Patent Document 2). In the first stage, a press forming analysis is performed in the process of press forming a metal plate to the bottom dead center of forming using a die, and residual stress generated in the press molded product is performed. Predict. In the second stage, springback analysis is performed on the process in which the shape of the press-molded product taken out from the die changes due to springback, and the shape of the press-molded product that balances the moment of force and the residual stress is predicted. do.

Japanese Patent No. 5795151 Japanese Patent No. 5866892 Japanese Unexamined Patent Publication No. 2013-11344

By performing a press molding simulation that integrates the above-mentioned first-stage press-molding analysis and second-stage springback analysis, the shape of the press-molded product immediately after being separated from the die and springed back can be obtained. It has been predicted. However, when the inventors compare the shape of the press-molded product predicted by the press-molding simulation with the shape of the press-molded product actually press-molded, the shape prediction accuracy by the press-molding simulation becomes low. I noticed that there was an item.

Therefore, as a result of investigating the press-molded products whose shape prediction accuracy is low by the press-molding simulation and the causes thereof, the press-molded products having a top plate portion and a vertical wall portion continuous from the top plate portion and curved in a plan view. In, it was found that the shape was different immediately after press molding (immediately after the mold was removed from the mold and springed back) and after several days had passed.

Such a shape change with the passage of time of a press-molded product is a phenomenon in which a structural member (structural part) that continues to receive a high load from the outside, such as a creep phenomenon (creep phenomenon), is gradually deformed (for example, Patent Document 3). Seems to be similar. However, the phenomenon that the shape of the press-molded product changes with the passage of time without receiving a load from the outside has not been known so far.

Furthermore, the second stage (springback analysis) in the conventional press molding simulation predicts the shape of the press molded product immediately after springback at the moment of taking out from the mold. For this reason, no studies have been made so far on predicting the shape change of the spring-backed press-molded product after several days have passed. Moreover, the shape change of the spring-backed press-molded product over time occurs without receiving an external load, as described above. Therefore, it has not been possible to apply an analysis method for dealing with the shape change due to the creep phenomenon to predict the shape change of the press-molded product with the passage of time.

The present invention has been made in view of the above problems, and an object thereof is to have a top plate portion and a vertical wall portion continuous from the top plate portion in a plan view (when viewed from the top plate portion). It is an object of the present invention to provide a method for predicting a shape change of a press-molded product which predicts a shape change with the passage of time after the press-molded product curved in () is separated from the mold and springed back.

The method for predicting a shape change of a press-molded product according to the present invention has a top plate portion and a vertical wall portion continuous from the top plate portion via a punch shoulder R portion and is curved in a plan view. For a press-molded product with a shape, it predicts the shape change of the swing in the width direction of the top plate due to stress relaxation with the passage of time immediately after springing back at the moment of removing from the mold. Then, by the springback analysis of the press-molded product, the shape / residual stress acquisition step immediately after the springback for acquiring the shape and residual stress of the press-molded product immediately after springback, and the said immediately after the acquired springback. Residual stress relaxation reduction setting process that sets the value of stress that is relaxed less than the residual stress immediately after springback for all or part of the press-molded product, and the relaxation reduction. The present invention includes a residual stress relaxation shape analysis step of obtaining a shape in which the moments of force are balanced for the press-molded product in which the stress value is set.

It is preferable that a part of the portion for setting the value of the stress reduced by relaxation in the residual stress relaxation reduction setting step is only the punch shoulder R portion of the press-molded product or the punch shoulder R portion and its vicinity.

The press-molded product has a flange portion continuous from the vertical wall portion via the die shoulder R portion, and is a part of setting the value of the stress reduced by relaxation in the residual stress relaxation reduction setting step. The portion may be only the flange portion of the press-molded product or the flange portion and its vicinity.

The blank used for press molding of the press-molded product is preferably a metal plate having a tensile strength of 150 MPa class or more and 2000 MPa class or less.

According to the method for predicting the shape change of a press-molded product according to the present invention, it is possible to accurately predict the shape change of the press-molded product with the passage of time after the die is removed from the mold and springed back. As a result, in the manufacturing process of automobile parts, car bodies, etc., a press-molded product having higher dimensional accuracy than the conventional one can be obtained, and the manufacturing efficiency can be significantly improved.

FIG. 1 is a flow chart showing a processing flow of a method for predicting a shape change of a press-molded product according to an embodiment of the present invention. FIG. 2 is a perspective view showing a press-molded product curved in a plan view, which is the object of the embodiment and the embodiment of the present invention. FIG. 3 is a plan view of a press-molded product curved in a plan view, which is the object of the embodiment and the embodiment of the present invention (dashed-dotted line: the width direction of the top plate portion in the plan view (XY plane in the figure). Center line). FIG. 4 is a diagram illustrating a stress relaxation phenomenon in which stress decreases with the passage of time while the strain is kept constant. FIG. 5 is a diagram illustrating a shape change due to stress relaxation in a punch shoulder R portion and a flange portion of a press-molded product having a hat-shaped cross-sectional shape curved in a plan view (formation bottom dead center immediately after press molding). FIG. 6 is a diagram illustrating a shape change due to stress relaxation in a punch shoulder R portion and a flange portion of a press-molded product having a hat-shaped cross-sectional shape curved in a plan view (immediately after springback). FIG. 7 is a diagram illustrating a shape change due to stress relaxation in a top plate portion and a flange portion of a press-molded product having a hat-shaped cross-sectional shape curved in a plan view (after a lapse of time). FIG. 8 shows a swing (sway), which is a shape change of the press-molded product with the passage of time, and a shape change amount (swing amount DA) of the press-molded product due to the swing in the embodiments and examples of the present invention. ). FIG. 9 is a diagram showing a constrained position and an evaluation position of the shape change for evaluating the shape change of the press-molded product subjected to the springback analysis and the residual stress relaxation shape analysis in the embodiment (hatched). Circle mark: restraint position, point A: evaluation position of shape change). FIG. 10 is a diagram showing a restraint portion and an evaluation position of the shape change in the press-molded product for evaluating the shape change of the press-molded product actually press-molded with the passage of time in the embodiment (hatching region: Restraint site, point A: Evaluation position of shape change).

<Examination leading up to the invention>
The inventors have established a method for predicting a shape change of the press-molded product 1 shown as an example in FIGS. 2 and 3 with the passage of time after springing back at the moment of mold release. As a preliminary step, the cause of the change in the shape of the press-molded product 1 with the passage of time was investigated.

As shown in FIGS. 2 and 3, the press-molded product 1 to be examined includes a top plate portion 3, a vertical wall portion 7 continuous from the top plate portion 3 via the punch shoulder R portion 5, and a vertical wall. It is a hat-shaped cross section having a flange portion 11 continuous from the portion 7 via a die shoulder R portion (a die shoulder) 9. Further, the press-molded product 1 has a curved portion 13 having a curved shape in a plan view, and a side portion (linear portion) 15 extending from both ends of the curved portion 13.

As a result of the above study on such a press-formed product 1, the inventors have applied the strain and then kept the strain constant as shown in the stress-strain curve of FIG. We focused on the stress relaxation phenomenon (stress relaxation phenomenon) in which the stress gradually relaxes with the passage of time. Then, it was found that even in the press-molded product 1 after springback, the shape that balances with the moment of the force of the press-molded product 1 is changed by gradually relaxing the residual stress with the passage of time.

The shape change due to the relaxation of the residual stress with the passage of time of the press-molded product 1 will be described with reference to the schematic views shown in FIGS. 5 to 7.

In the press molding of the press-molded product 1, the metal plate (blank) is bent into a curved shape in a plan view as shown in FIGS. 2 and 3. Therefore, at the bottom dead point of molding, the curved outer side is as shown in FIG. Tensile stress is generated in the vicinity of the punch shoulder R portion 5a, and compressive stress is generated in the vicinity of the punch shoulder R portion 5b inside the curve. Further, compressive stress is generated in the flange portion 11a on the outer side of the curve due to shrinkage flangeing, and tensile stress is generated in the flange portion 11b on the inner side of the curve due to stretch flangeing.

Subsequently, when the press-molded product 1 is released from the mold, springback occurs in the press-molded product 1 using the residual stress at the bottom dead center of molding as a driving force. At that time, the press-molded product 1 tries to be deformed into a shape in which the radius of curvature (curvature radius) of the curve becomes large, so that the tip of the side portion 15 extending from both ends of the curve is curved in the width direction of the top plate portion 3. Deformation that moves outward (hereinafter, such deformation is referred to as "swinging") occurs. Then, as shown in FIG. 6, the released press-molded product 1 has a shape in which the moments are balanced in a state where the residual stress in the direction opposite to the residual stress at the bottom dead center of molding (FIG. 5) is generated.

After that, as shown in FIGS. 6 to 7, the residual stress is relaxed with the passage of time, the residual stress immediately after the end of springback is weakened, the press-molded product 1 further swings, and the shape deviates from the bottom dead center shape of molding. Changes.

That is, when the press-molded product that has been press-molded to the bottom dead point of molding is momentarily springed back from the mold, residual stress is generated in the press-molded product 1 at that time, but with the passage of time. The residual stress is relaxed and reduced, and the difference between the stress outside the curve and the stress inside the curve is also reduced. As a result, the press-molded product 1 is deformed into a balanced shape with a smaller stress than the shape immediately after the springback.

As described above, in the press-molded product 1, due to the relaxation of the residual stress with the passage of time, a swing occurs in which both ends of the curve move to the outside of the curve in the width direction of the top plate. It was found that the shape changes from the shape at the bottom dead center of molding to a shape further deviated from the shape.

Therefore, based on the above-mentioned new findings, the inventors predict, for example, a shape change due to stress relaxation with the passage of time after springback in the press-molded article 1 as shown in FIGS. 2 and 3. I recommended to consider. As a result, the residual stress of all or part of the press-molded product 1 immediately after springback obtained in the second stage (springback analysis) of the press-molding simulation described above is relaxed and reduced, and the force of the press-molded product 1 is reduced. It was found that the shape change (swing) with the passage of time of the press-molded product 1 as described above can be predicted by further performing the analysis of the third stage to obtain the shape that balances with the moment of.

Further, according to the shape change prediction method, not only the press-molded product 1 having a hat-shaped cross-sectional shape as shown in FIGS. 2 and 3, but also the top plate portion and the top plate portion continuously via the punch shoulder R portion. It was found that a press-molded product having a vertical wall portion and curved in a plan view can predict a shape change (swing) with the passage of time after springback.

The present invention has been made based on such studies and findings, and a specific configuration of the present invention will be described based on the embodiments of the present invention described below.

<Method for predicting shape change of press-molded products>
As shown in FIGS. 2 and 3 as an example, the method for predicting a shape change of a press-molded product according to an embodiment of the present invention is continuous from the top plate portion 3 and the top plate portion 3 via the punch shoulder R portion 5. A press-molded product 1 having a vertical wall portion 7 and a flange portion 11 continuous from the vertical wall portion 7 via a die shoulder R portion 9 and having a curved shape in a plan view was separated from the mold and springed back. Predict the shape change of the swing due to stress relaxation with the passage of time from immediately after. As shown in FIG. 1, the method for predicting a shape change of a press-formed product according to an embodiment of the present invention includes a shape / residual stress acquisition step S1 immediately after springback, a residual stress relaxation reduction setting step S3, and residual stress relaxation. The shape analysis step S5 is included. Hereinafter, each of the above steps will be described. The dimensions and other specific numerical values shown in the specification and drawings of the present application are merely specific examples for explaining the present invention, and do not limit the present invention.

≪Shape and residual stress acquisition process immediately after springback≫
The shape / residual stress acquisition step S1 immediately after springback is a step of acquiring the shape and residual stress of the press-molded product 1 immediately after springback by the springback analysis of the press-molded product 1.

As an example of a specific process for acquiring the shape and residual stress of the press-molded product 1 immediately after springback, there is a press-molding simulation by the finite element method. This press molding simulation has a first stage and a second stage. In the first stage, a press in the process of press-molding a metal plate to the bottom dead point of molding using a die model (tool model of press forming) that models the mold used for press molding of the actual press-molded product 1. Molding analysis is performed to obtain a press-molded product 1 at the bottom dead point of molding. In the second stage, a springback analysis is performed to obtain a shape and residual stress in which the moments of the forces of the press-molded product 1 after the press-molded product 1 at the obtained bottom dead center are separated from the mold model can be balanced. ..

≪Residual stress relaxation reduction setting process≫
In the residual stress relaxation reduction setting step S3, all or part of the press-formed product 1 immediately after springback acquired in the shape / residual stress acquisition step S1 immediately after springback is relaxed and reduced more than the residual stress. This is the process of setting the stress value.

The residual stress in the residual stress relaxation reduction setting step S3 means the tensile stress and the compressive stress remaining in the press-formed product 1 immediately after springback. Further, setting the stress value for which the residual stress is relaxed and reduced in the residual stress relaxation reduction setting step S3 means that the tensile stress (positive value) and the compressive stress (negative value) remaining in the press-formed product 1 immediately after springback are used. Value) means to relax and reduce the absolute value.

For some parts of the press-molded product 1 immediately after springback, the residual stress is relaxed and reduced, for example, as follows.

First, in the press-formed product 1 immediately after the springback obtained in the shape / residual stress acquisition step S1 immediately after the springback, the stress distribution in the circumferential direction (stress distribution) in the curved portion 13 schematically shown in FIG. As described above, tensile stress is applied to the curved outer flange portion 11a, compressive stress is applied to the curved outer punch shoulder R portion 5a, compressive stress is applied to the curved inner flange portion 11b, and tensile stress is applied to the curved inner punch shoulder R portion 5b. It remains.

Therefore, in the residual stress relaxation reduction setting step S3, it is desirable that the punch shoulder R portion 5 or the punch shoulder R portion 5 and its vicinity where the tensile stress or the compressive stress remains is desirable as a part of the portion where the residual stress is relaxed and reduced. .. Here, the vicinity of the punch shoulder R portion 5 means a portion of the top plate portion 3 and / or the vertical wall portion 7 continuous from the punch shoulder R portion 5 and close to the punch shoulder R portion 5. For example, it refers to a range of about 1/5 of the width of the top plate portion 3 and the height of the vertical wall portion 7.

Further, in the press-formed product 1 having a hat-shaped cross-sectional shape having a flange portion 11 continuous from the vertical wall portion 7 via the die shoulder R portion 9, some parts for setting the stress value in which the residual stress is relaxed and reduced are , The flange portion 11 or the portion in the vicinity of the flange portion 11 is preferable, and further, both the punch shoulder R portion 5 and the portion in the vicinity thereof, and the flange portion 11 and the portion in the vicinity thereof are preferable. Here, the portion in the vicinity of the flange portion 11 refers to the die shoulder R portion 9 that connects the flange portion 11 and the vertical wall portion 7.

The part where the stress value for relaxing and reducing the residual stress was set was specifically verified in the examples described later.

≪Residual stress relaxation shape analysis process≫
The residual stress relaxation shape analysis step S5 is a step of performing an analysis to obtain a shape in which the moments of force are balanced with respect to the press-formed product 1 in which the residual stress is relaxed and reduced in the residual stress relaxation reduction setting step S3.

For the analysis in the residual stress relaxation shape analysis step S5, the same analysis method as the springback analysis in the shape / residual stress acquisition step S1 immediately after springback is applied, and the residual stress is determined by the static implicit method. The shape of the press-molded product 1 after relaxation reduction can be obtained.

As a constraint condition in the analysis in the residual stress relaxation shape analysis step S5, for example, as shown in FIG. 9, each position of the top plate portion 3 of one side portion 15 (in the figure). The movement in the X, Y, and Z directions is constrained (dX = dY = dZ = 0) with respect to the node (the position of the hatched circle), and the movement in the X direction is not constrained and the movement in the Y and Z directions is not constrained. Is constrained (dY = dZ = 0), or movement only in the Z direction is constrained (dZ = 0). It is desirable that such a node constraint condition is the same as the node constraint condition of the springback analysis in the shape / residual stress acquisition step S1 immediately after the springback.

As described above, according to the method for predicting the shape change of the press-molded product according to the present embodiment, the residual portion of the press-molded product 1 immediately after springback, which is obtained by the springback analysis, is all or part of the portion. An analysis is performed in which the value of the stress that is relaxed and reduced rather than the stress is set, and the shape that balances the moment of force is obtained for the press-formed product 1 that has the value of the stress that is relaxed and reduced. As a result, stress relaxation and shape change with the passage of time in the actual press-molded product 1 are simulated, and the shape change (top plate width) with the passage of time after the press-molded product 1 is separated from the mold and springed back. Swing in the direction) can be predicted.

In the above description, the residual stress relaxation reduction setting step S3 relaxes and reduces the residual stress of each of the punch shoulder R portion 5 and its vicinity, and the flange portion 11 and its vicinity in the press molded product 1. The value of the stress was set. However, according to the present invention, in the residual stress relaxation reduction setting step, only a part of the press-molded product 1 such as the punch shoulder R portion 5, the punch shoulder R portion 5 and its vicinity, or the flange portion 11 only. The stress value obtained by relaxing and reducing the residual stress may be set. Further, the residual stress may be relaxed and reduced with respect to parts other than the punch shoulder R portion 5 and the flange portion 11 in the press-molded product 1, or the residual stress may be relaxed and reduced with respect to all of the press-molded product 1. May be set to a value that is relaxed and reduced.

Alternatively, when the residual stress is relaxed and reduced for a part of the press-molded product 1, the ratio and value for relaxing and reducing the residual stress may be changed for each part.

Further, the above description is intended for the press-molded product 1 having a hat-shaped cross-sectional shape curved in a plan view, but the present invention has a Z-shape having a top plate portion, a vertical wall portion, and a flange portion. A Z-shaped cross section, a U-shaped cross section having a top plate and a vertical wall, or an L-shaped cross section. It can also be applied to a press-molded product having a curved shape in a plan view.

In the case of a press-molded product having a U-shaped cross section or an L-shaped cross section, the punch shoulder R portion or the punch shoulder R portion and its vicinity (punch) are used as a portion for relaxing and reducing the residual stress in the residual stress relaxation reduction setting step. It is preferable to use a top plate portion and / or a vertical wall portion continuous from the shoulder R portion). Further, the residual stress may be relaxed and reduced for a portion other than the punch shoulder R portion or the entire press-molded product.

Further, when the residual stress of the punch shoulder R portion or the flange portion is relaxed, the residual stress is not limited to the relaxation of the entire range thereof, and one of the punch shoulder R portion and the flange portion. The range of parts may be relaxed. In this case, a range for relaxing and reducing the residual stress may be appropriately set according to the value of the residual stress in the press-molded product immediately after springback.

Similarly, even when the residual stress is relaxed and reduced for the top plate portion and the vertical wall portion near the punch shoulder R portion, or the die shoulder R portion near the flange portion, the top plate portion, The residual stress is not limited to relaxation and reduction for the entire range of the vertical wall portion and the flange portion. Depending on the value of the residual stress in the press-molded product immediately after springback, the range for relaxing and reducing the residual stress in the top plate portion, the vertical wall portion, and the flange portion may be appropriately set.

The press-molded product 1 whose shape change is predicted in the above description has a curved portion 13 having a curved shape in a plan view and side portions 15 extending from both ends of the curvature in the curved portion 13. Met. However, the present invention may be a press-molded product having a curved shape in a plan view. For example, unlike the press-molded product 1, it does not have a linearly extending side portion 15, and is generally viewed in a plan view. It may be a press-molded product having a curved shape. Further, it may be a press-molded product having a curved shape having a side portion 15 extending from one side of the curve.

In the method for predicting the shape change of a press-molded product according to the present invention, the shape and type of the blank (metal plate) used for press-molding the press-molded product and the shape and type of the press-molded product are not particularly limited, but the press-molded product can be used. It is more effective for automobile parts press-molded using a metal plate with high residual stress.

Specifically, the thickness of the blank is preferably 0.5 mm or more and 4.0 mm or less. The tensile strength of the blank is preferably 150 MPa class or more and 2000 MPa class or less, and more preferably 440 MPa class or more and 1470 MPa class or less.

Since a metal plate having a tensile strength of less than 150 MPa class is rarely used for press-molded products, there is little advantage in using the method for predicting shape change of press-molded products according to the present invention. The present invention is applied to those with low stiffness such as automobile outer panel parts (automobile outer panel) using a metal plate with a tensile strength of 150 MPa class or higher because they are susceptible to shape changes due to changes in residual stress. Since the advantages are increased, the present invention can be suitably applied.

On the other hand, since a metal plate having a tensile strength exceeding 2000 MPa class has poor ductility, for example, in the press forming process of the hat-shaped cross-sectional shape press-formed product 1 as shown in FIG. 2, the punch shoulder R portion 5 and the die Fracture is likely to occur at the shoulder R portion 9, and press molding may not be possible.

Furthermore, as the type of press-molded product, it is preferable to target automobile parts such as roof rails and body frame parts such as cross members, but they are curved in a plan view. The present invention can be widely used as long as it is an automobile part in which the shape of the swing changes with the passage of time after press molding.

The press method for the press-molded product targeted by the present invention is not particularly limited to bend-forming, crash forming, deep drawing, and the like.

Specific experiments and analyzes have been performed on the action and effect of the method for predicting the shape change of the press-molded product according to the present invention, which will be described below. In the experiment, as an example of the metal plate, a steel plate having mechanical properties shown in Table 1 was used, and the press-formed product 1 having the hat-shaped cross-sectional shape shown in FIGS. 2 and 3 was press-formed. The shape of the bottom dead center of the press-molded product 1 is a molding height (height of the press-molded product 1 in the Z direction in FIG. 2) of 50 mm, a width of the top plate portion 3 of 40 mm, and a width of the flange portion 11 of 15 mm. The radius of curvature of the curve of the curved portion 13 was set to R80 mm, and the length of the side portion 15 was set to 80 mm.

Then, two days after the press-molded product 1 press-molded to the bottom dead center of molding was released from the mold, the shape change of the press-molded product 1 was measured. In the measurement of the shape change, as shown in FIG. 10, the top plate portion 3 (the portion shaded in FIG. 10) of one side portion 15 is restrained, and the other side is restrained as shown in FIG. The amount of movement of the evaluation point A at the tip of the portion 15 in the width direction of the top plate portion (in FIG. 8, the X-direction distance between the evaluation point A at the bottom dead point of molding and the evaluation point A'after the passage of time) is accompanied by the passage of time. It was measured as the measured value of the swing amount DA. The measured value of the swing amount DA measured in this way was 6.4 mm.

On the other hand, in the analysis, the shape change due to the relaxation of the residual stress of the press-molded product 1 after springback was predicted. First, using a die model that models the die used for press forming, press forming analysis is performed to press-mold a steel plate having the mechanical properties shown in Table 1 to the bottom dead center of forming, and then press at the bottom dead center of forming. The residual stress of the molded product 1 was determined.

Next, a springback analysis was performed to determine the shape and residual stress of the press-molded product 1 immediately after the press-molded product 1 was released from the mold model at the bottom dead center of molding.

Subsequently, the stress value obtained by relaxing and reducing the absolute value of the residual stress at a predetermined ratio was set for some parts of the press-molded product 1 immediately after the springback, which was obtained by the springback analysis.

Then, with respect to the press-molded product 1 in which the residual stress is relaxed and reduced, as shown in FIG. 9, a nodal constraint condition is applied to each position of one side portion 15 to form a shape in which the moments of the forces of the press-molded product 1 are balanced. The required analysis was performed. The meanings of the symbols and mathematical formulas in FIG. 9 are as described in the above-described first embodiment.

After that, also for the press-molded product 1 subjected to the shape change analysis, as shown in FIG. 8, the amount of movement of the evaluation point A at the tip of the side portion 15 in the width direction of the top plate portion (in FIG. 8, at the bottom dead point of molding). The predicted value was calculated by using the X-direction distance between the evaluation point A and the evaluation point A'after the residual stress relaxation shape analysis) as the swing amount DA with the passage of time.

In the embodiment, each of the punch shoulder R portion 5 on both the curved outer side and the curved inner side (vertical wall portion 7), the flange portion 11 and the die shoulder R portion 9 in the press-formed product 1 obtained by the springback analysis. On the other hand, the residual stress immediately after springback was relaxed and reduced at a rate of 10 to 30% (stress relaxation rate), and the stress values were set as Invention Examples 1 to 6. ..

Inventive Example 1 is a punch shoulder R portion 5 and a vertical wall portion 7 in the vicinity thereof (from the direction of the top plate to 1/5 of the vertical wall height), and Invention Examples 2 to 5 are punch shoulder R portions 5. In the invention example 6, the residual stress is relaxed with respect to each of the punch shoulder R portion 5 and its vicinity, the flange portion 11 and the die shoulder R portion 9 with respect to each of the punch shoulder R portion 5 and the vicinity thereof and the flange portion 11. The value of the reduced stress is set.

Further, as a comparison target, the press molding analysis and the springback analysis of the press molded product 1 are performed in the same manner as in the inventions 1 to 6, but the shape is such that the moments of the forces with the passage of time after the springback analysis are balanced. A comparative example was used in which the required analysis was not performed and the effect of stress relaxation was not considered.

Then, for each of Invention Examples 1 to 6 and Comparative Example, the amount of deviation from the shape of the press-molded product 1 at the bottom dead center of molding at the tip in the longitudinal direction (evaluation point A) at the side portion 15 of the press-molded product 1. The predicted value of the swing amount DA was obtained. Furthermore, the difference and error between the predicted value of the swing amount DA and the measured value were calculated by the following formula.
(Difference) (mm) = (Predicted value of DA-Actual value of DA)
(Error) (%) = 100 × (Predicted value of DA-Actual value of DA) / (Actual value of DA)

In Invention Examples 1 to 6 and Comparative Examples, the sites where the residual stress is relaxed and reduced, the stress relaxation reduction rate and the predicted value of the swing amount DA, the difference between the measured value and the error are summarized in Table 2 above. ..

The predicted value of the swing amount DA of the evaluation point A in the comparative example was 7.1 mm, and the difference and error from the measured value were 0.7 mm and 10.9%, respectively.

In the first invention example, the value of the stress in which the residual stress is relaxed and reduced by 10% with respect to the punch shoulder R portion 5 and its vicinity is set. The predicted value of the swing amount DA of the evaluation point A in the first invention example is 6.8 mm, and the difference and the error from the measured value are 0.4 mm and 6.3%, respectively, which are better results than the comparative example. rice field.

In Invention Example 2, the stress value obtained by reducing the residual stress by 10% is set for both the punch shoulder R portion 5 and its vicinity and the flange portion 11. The predicted value of the swing amount DA of the evaluation point A in the second invention is 6.7 mm, and the difference and the error from the measured value are 0.3 mm and 4.7%, respectively, which are better results than the first invention. rice field.

In the third invention example, the stress values obtained by reducing the residual stress by 20% and 10% for the punch shoulder R portion 5 and its vicinity and the flange portion 11 respectively are set. The predicted value of the swing amount DA of the evaluation point A in the third invention is 6.5 mm, and the difference and the error from the measured value are 0.1 mm and 1.6%, respectively, which are better results than the second invention. there were.

In Invention Example 4, the stress value obtained by reducing the residual stress by 20% is set for both the punch shoulder R portion 5 and its vicinity and the flange portion 11. The predicted value of the swing amount DA of the evaluation point A in Invention Example 4 is 6.3 mm, and the difference and the error from the measured value are -0.1 mm and -1.6%, respectively, which are negative values, but are absolute. When compared by value, it was better than that of Comparative Example, and the result was equivalent to that of Invention Example 3.

In the fifth invention example, the stress values obtained by reducing the residual stress by 30% and 20% for the punch shoulder R portion 5 and its vicinity and the flange portion 11 respectively are set. The predicted value of the swing amount DA of the evaluation point A in the invention example 5 is 6.1 mm, and the difference and the error from the measured value are -0.3 mm and -4.7%, respectively, which are better results than the comparative example. However, the difference and the error from the measured value became larger than those of Invention Example 4.

In Invention Example 6, similarly to Invention Example 3, the stress values obtained by reducing the residual stress by 20% and 10% for the punch shoulder R portion 5 and its vicinity and the flange portion 11, respectively, are set, and further, the die shoulder is set. The value of the stress obtained by reducing the residual stress by 30% is set for the R portion 9. The predicted value of the swing amount DA in Invention Example 6 was in agreement with the measured value, and the result was even better than that of Invention Example 3.

According to the present invention, a press-molded product having a top plate portion and a vertical wall portion continuous from the top plate portion and curved in a plan view (when viewed from the top plate portion) is released from the mold. It is possible to provide a method for predicting a shape change of a press-molded product that predicts a shape change with the passage of time after springing back.

1 Press-molded product 3 Top plate part 5 Punch shoulder R part 5a Curved outer punch shoulder R part 5b Curved inner punch shoulder R part 7 Vertical wall part 9 Die shoulder R part 11 Flange part 11a Curved outer flange part 11b Curved inner side Flange part 13 Curved part 15 Side part

Claims

Immediately after a press-molded product having a top plate portion and a vertical wall portion continuous from the top plate portion via a punch shoulder R portion and having a curved shape in a plan view, springback is performed at the moment of removal from the die. It is a method for predicting the shape change of a press-molded product that predicts the shape change of the swing in the width direction of the top plate due to stress relaxation with the passage of time.
The shape / residual stress acquisition process immediately after springback to acquire the shape and residual stress of the press-molded product immediately after springback by the springback analysis of the press-molded product, and the process of acquiring the shape / residual stress immediately after springback.
A residual stress relaxation reduction setting step for setting a value of stress that is relaxed and reduced from the residual stress immediately after springback for all or a part of the obtained press-molded product immediately after springback.
A method for predicting a shape change of a press-molded product, which comprises a residual stress relaxation shape analysis step of obtaining a shape in which a force moment is balanced for the press-molded product in which a value of the relaxed reduced stress is set.
According to claim 1, a part of a portion for setting a value of stress reduced by relaxation in the residual stress relaxation reduction setting step is only the punch shoulder R portion of the press-molded product or the punch shoulder R portion and its vicinity. The method for predicting a shape change of a press-molded product according to the description.
The press-molded product has a flange portion continuous from the vertical wall portion via the die shoulder R portion.
The second aspect of claim 1 or 2, wherein a part of the portion for setting the value of the stress reduced by relaxation in the residual stress relaxation reduction setting step is only the flange portion of the press-molded product or the flange portion and its vicinity thereof. A method for predicting shape changes in press-molded products.
The method for predicting a shape change of a press-molded product according to any one of claims 1 to 3, wherein the blank to be subjected to press molding of the press-molded product is a metal plate having a tensile strength of 150 MPa class or more and 2000 MPa class or less. ..