JP2022020932A - Method for predicting shape change of press-molded product - Google Patents

Abstract

To provide a method for predicting a shape change of a press-molded product which predicts a shape change due to lapse of time of a press-molded product curved in a convex shape in a side view.SOLUTION: A method for predicting a shape change of a press-molded product predicts a shape change due to stress relaxation with lapse of time after springing back at the moment of mold release with respect to a press-molded product having a top plate part and a vertical wall part and including a shape curved in a convex shape on the top plate part side in a side view. The method includes: a process (S1) in which a shape and a residual stress of the press-molded product immediately after springing back is acquired by spring back analysis; a process (S3) in which a value of a residual stress, which is further relaxed and reduced than the residual stress immediately after springing back, is set to at least the top plate part of the press-molded product immediately after springing back; and a process (S5) in which a shape with the balanced moment of force is determined with respect to the press-molded product to which the value of the relaxed and reduced residual stress is set.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for predicting a shape change of a press-molded product, and more particularly to a press-molded product having a top plate portion and a vertical wall portion and including a shape that is convexly curved toward the top plate portion in a side view. The present invention relates to a method for predicting a shape change of a press-molded product that predicts a shape change that occurs with the passage of time after the mold is released from the mold and springed back.

Press molding is a manufacturing method capable of manufacturing metal parts at low cost and in a short time, and is used for manufacturing many automobile parts. In recent years, higher-strength metal plates have been used for press molding of automobile parts in order to achieve both collision safety of automobiles and weight reduction of automobile bodies.

One of the main problems in press forming a high-strength metal plate is a decrease in dimensional accuracy due to springback. The residual stress generated in the press-molded product when deforming the metal plate by press molding becomes the driving force, and the press-molded product released from the mold momentarily returns to the shape of the metal plate before press molding like a spring. The phenomenon of trying is called springback.

Since the residual stress generated during press forming increases as the strength of the metal plate (for example, high-strength steel plate) increases, the shape change due to springback also increases. Therefore, the higher the strength of the metal plate, the more difficult it is to keep the shape after springback within the specified dimensions. Therefore, a technique for accurately predicting the shape change of the press-molded product due to springback is important.

Press forming simulation by the finite element method is generally used to predict the shape change by springback. As a procedure in the press forming simulation, first, a press forming analysis in the process of press forming a metal plate to the forming bottom dead point is performed, and a first step of predicting a residual stress at the press forming bottom dead point (for example, Patent Document 1). ), Springback analysis is performed on the process in which the shape of the press-formed product released (taken out) from the die changes due to springback, and the moment of force and residual stress in the released press-formed product can be balanced. It is divided into a second stage (for example, Patent Document 2) of predicting the shape.

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

By performing a press forming simulation that integrates the above-mentioned first-stage press-forming analysis and second-stage springback analysis, the shape of the press-formed product that has been separated from the die and spring-backed has been predicted. It has been.
However, when the inventors compare the shape of the press-molded product predicted by the press-form simulation with the shape of the press-molded product actually press-molded, the shape prediction accuracy by the press-form simulation becomes low. I noticed that there is.

Therefore, as a result of investigating a press-formed product having a low shape prediction accuracy by a press-forming simulation and its cause, it has a top plate portion 3, a vertical wall portion 5, and a flange portion 7 as shown in FIG. 8 as an example. In the press-molded product 1 which has a cross-sectional hat shape and is curved convexly toward the top plate portion 3 in a side view, the end side in the longitudinal direction of the press-molded product 1 is formed several days after the mold is released. It was discovered that deformation (hereinafter, such deformation is referred to as "camberback") occurs on the top plate portion 3 side, and the shape of the press-molded product 1 is different immediately after press molding and after several days have passed.

The time-dependent change of such a press-molded product over time seems to be similar to a phenomenon in which a structural member that continues to receive a high load from the outside is gradually deformed (for example, Patent Document 3), such as a creep phenomenon. However, the change in shape that occurs in the press-molded product that is not subjected to external load has not been known so far.

Further, the second stage (springback analysis) in the conventional press molding simulation predicts the shape of the press molded product immediately after springback that occurs at the moment of taking out from the die. Therefore, for the spring-backed press-molded product, which is the object of the present application, for example, the prediction of the shape change after several days has not been studied.
Moreover, the shape change of the spring-backed press-molded product over time occurs without receiving an external load, as described above. Therefore, even if an attempt was made to predict the shape change of the press-molded product over time, it was not possible to apply an analysis method that handles the shape change due to the creep phenomenon.

The present invention has been made to solve the above-mentioned problems, and the time unit elapsed after springing back at the moment of releasing the mold from the press-molded product which is curved in a convex shape in a side view. It is an object of the present invention to propose a method for predicting a shape change of a press-molded product for predicting a shape change of the press-molded product.

(1) 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, and is curved convexly toward the top plate portion in a side view. For press-molded products including shapes, it predicts shape changes due to stress relaxation over time after springing back at the moment of mold release from the mold.
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 residual stress that is relaxed and reduced from the residual stress immediately after springback with respect to at least the top plate portion of the press-formed product immediately after springback.
It is characterized by including 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 value of the residual stress is relaxed and reduced.

(2) The method for predicting a shape change of a press-molded product according to the present invention has a top plate portion, a vertical wall portion continuous from the top plate portion, and a flange portion continuous from the vertical wall portion, and is described in a side view. For press-molded products that include a shape that is convexly curved toward the top plate, shape changes due to stress relaxation over time after springing back at the moment of mold release are predicted.
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 of setting a value of residual stress that is relaxed and reduced from the residual stress immediately after springback with respect to at least the top plate portion and / or the flange portion of the press-formed product immediately after springback. ,
It is characterized by including 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 value of the residual stress is relaxed and reduced.

(3) In the above-mentioned (1) or (2),
In the residual stress relaxation reduction setting step, it is characterized in that the value of the residual stress whose relaxation is reduced by 5% or more from the residual stress immediately after springback is set.

(4) In any of the above (1) to (3),
The blank used for press molding of the press-molded product is characterized by being a metal plate having a tensile strength of 150 MPa class or more and 2000 MPa class or less.

In the present invention, the press-molded product has a top plate portion and a vertical wall portion continuous from the top plate portion, and includes a shape that is convexly curved toward the top plate portion in a side view. The shape / residual stress acquisition step immediately after springback to acquire the shape and residual stress of the press-molded product immediately after springback, and at least the above-mentioned heaven of the acquired press-molded product immediately after springback by the springback analysis of. The moment of force for the residual stress relaxation reduction setting step in which the residual stress value relaxed and reduced compared to the residual stress immediately after springback is set for the plate portion, and the press-molded product in which the residual stress value is relaxed and reduced. By including a residual stress relaxation shape analysis step for determining a shape in which the stress is balanced, it is possible to accurately predict the shape change of the press-molded product with the passage of time after the mold is separated from the mold and springed back. As a result, in the manufacturing process of automobile parts, car bodies, and the like, a press-molded product having higher dimensional accuracy than before can be obtained, and the manufacturing efficiency can be significantly improved.

It is a flow chart which shows the flow of the process of the shape change prediction method of the press-molded article which concerns on embodiment of this invention. FIG. 3 is a perspective view showing a press-molded product having a hat-shaped cross-sectional shape that is convexly curved in a side view, which is the object of the embodiment of the present invention and the first embodiment. It is a figure explaining the stress relaxation decrease that the stress relaxes and decreases with the passage of time in the state of holding it constant after applying strain. It is a figure explaining the shape change by stress relaxation in the top plate part and the flange part of the press-molded product of the hat-shaped cross-sectional shape curved convexly in the side view ((a) the bottom dead center of molding immediately after press molding, (b). ) Immediately after springback, (c) after time has passed). FIG. 3 is a perspective view illustrating a press-molded product having a U-shaped cross-sectional shape, which is the target of the second embodiment, and a shape change (camberback) that occurs after the press-molded product is press-molded and spring-backed. It is a perspective view explaining the press-molded product of the Z-shaped cross-sectional shape targeted in Example 3, and the shape change (camber back) that occurs after the press-molded product is press-molded and spring-backed. It is a figure which shows the press-molded article which was targeted in Example 4 ((a) top view, (b) perspective view). It is a figure explaining the shape change (camberback) which occurs after press-molding and spring-backing the press-molded product of the hat type cross-sectional shape which was curved convexly in the side view.

<Examination leading up to the invention>
In order to solve the above-mentioned problems, the inventors have made a change in the shape of the press-molded product 1 shown as an example in FIG. In order to establish a prediction method, as a preliminary step, the cause of the change in the shape of the press-molded product 1 with the passage of time was examined.

As an example shown in FIG. 2, the press-molded product 1 to be examined has a hat-shaped cross-sectional shape having a top plate portion 3, a vertical wall portion 5, a vertical wall portion 5, and a flange portion 7. The top plate portion 3 and the vertical wall portion 5 are continuous via the punch shoulder portion 9, and the vertical wall portion 5 and the flange portion 7 are continuous via the die shoulder portion 11.

Then, as a result of examining the above for such a press-molded product 1, the inventors, as shown in the stress-strain diagram of FIG. 3, stress is applied with the passage of time while being kept constant after the strain is applied. We focused on the stress relaxation phenomenon that gradually relaxes. Even in the press-molded product 1 after springback, the residual stress of the top plate portion 3 and the flange portion 7 is gradually relaxed with the passage of time, so that the shape is balanced with the force moment of the press-molded product 1. I found out that it would change.

The shape change due to the relaxation of the residual stress in the press-molded product 1 will be described with reference to the schematic diagram shown in FIG.

In the press molding of the press-molded product 1, the metal plate (blank) is bent into a convexly curved shape in the side view as shown in FIG. 2, and therefore, the bottom dead center of molding is as shown in FIG. 4 (a). In the side view, tensile stress is generated in the top plate portion 3 which is the outer side of the bend, and compressive stress is generated in the flange portion 7 which is the inner side of the bend.

Next, when the press-molded product 1 is removed from the die, springback occurs using the residual stress generated during press molding as a driving force, and camberback as shown in FIG. 8 described above occurs. By this camber back, the press-molded product 1 is deformed into a shape having a large radius of curvature of the curve. Then, as shown in FIG. 4 (b), the released press-molded product 1 has a moment in a state where a residual stress in the opposite direction to the residual stress at the bottom dead center of molding (FIG. 4 (a)) is generated. The shape is balanced. The broken line in FIG. 4B is the shape of the press-molded product 1 at the bottom dead center of molding (FIG. 4A).

After that, as shown in FIGS. 4 (b) to 4 (c), the compressive stress in the top plate portion 3 and the tensile stress in the flange portion 7 relax and decrease with the passage of time without being forced from the outside. .. As a result, the shape that balances with the moment of force changes, so that the press-molded product 1 further causes camber back, and the deviation from the bottom dead center shape of molding further increases. The broken line in FIG. 4 (c) is the shape of the press-molded product 1 immediately after springback (FIG. 4 (b)).

That is, when the press-molded product 1 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 residual stress of the top plate portion 3 on the outer side of the curve and the residual stress of the flange portion 7 on the inner side of the curve also decreases. As a result, the press-molded product 1 is deformed into a balanced shape with a smaller residual stress than the shape immediately after springback.

As described above, in the press-molded product 1 having a shape that is convexly curved toward the top plate portion 3 in the side view, the residual stress is relaxed with the lapse of further time after the press molding. It was found that camber back occurs in which both ends of the curve move to the outside of the curve, resulting in a shape further deviated from the bottom dead center of molding.

Therefore, based on the above-mentioned new findings, the inventors have studied, for example, a method for predicting a shape change due to stress relaxation with the passage of time after springback of the press-molded product 1 as shown in FIG. I recommended it. As a result, at least the residual stress of the top plate portion 3 and / or the flange portion 7 of the press-formed product 1 immediately after springback obtained in the second stage (springback analysis) of the press-forming simulation described above is relaxed and press-formed. It was discovered that the shape change (camberback) with the passage of time of the press-formed product 1 as described above can be predicted by further performing the third-stage analysis for determining the shape that balances the force moment of the product 1.

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 FIG. 2 but also a side surface having a top plate portion and a vertical wall portion continuous from the top plate portion. It was found that camberback can be predicted due to the relaxation and reduction of the residual stress of the top plate with the passage of time after springback, even for press-molded products containing a shape that is visually curved toward the top plate. rice field.

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

<Method for predicting shape change of press-molded products>
As an example, as shown in FIG. 2, the method for predicting a shape change of a press-molded product according to an embodiment of the present invention is continuous from a top plate portion 3 and a vertical wall portion 5 continuous from the top plate portion 3 and a vertical wall portion 5. The time after the press-molded product 1, which has a hat-shaped cross-sectional shape having the flange portion 7 and includes a shape that is convexly curved toward the top plate portion 3 in a side view, is springed back at the moment of being released from the mold. It predicts the shape change due to stress relaxation over time, and as shown in FIG. 1, the shape / residual stress acquisition step S1 immediately after springback, the residual stress relaxation reduction setting step S3, and the residual stress relaxation shape analysis. The process S5 and the like are provided. Hereinafter, each of the above steps will be described.

≪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, a die model that models the mold used for the press molding of the actual press-molded product 1 is used. A press molding analysis is performed in the process of press-molding a metal plate to the bottom dead point of molding, and the first step of obtaining the press-molded product 1 at the bottom dead point of molding and the press-molded product 1 at the obtained bottom dead point of molding are molded. A press forming simulation by a finite element method having a second step of performing a springback analysis for obtaining a shape and residual stress in which the moment of force of the press formed product 1 after being removed from the model can be balanced and a residual stress can be mentioned.

≪Residual stress relaxation reduction setting process≫
The residual stress relaxation reduction setting step S3 is performed on at least the top plate portion 3 and / or the flange portion 7 of the press-formed product 1 immediately after springback acquired in the shape / residual stress acquisition step S1 immediately after springback. This is the process of setting the value of the residual stress that has been relaxed and reduced.

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 value of the residual stress in 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 set. It means to relax and reduce the absolute value of).

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

The press-formed product 1 after the residual stress is relaxed and reduced by applying the same analysis method as the springback analysis in the shape / residual stress acquisition step S1 immediately after the springback to the analysis in the residual stress relaxation shape analysis step S5. Shape can be obtained.

As described above, according to the method for predicting the shape change of the press-formed product according to the present embodiment, at least the top plate portion and / or the flange portion of the press-formed product 1 immediately after springback obtained by the springback analysis. By setting the value of the residual stress that is relaxed and reduced from the residual stress, and performing the analysis to obtain the shape that balances the moment of force with respect to the press-formed product 1 for which the value of the residual stress is set to be relaxed and reduced, the actual press molding is performed. By simulating stress relaxation and shape change with the passage of time in the product 1, it is possible to predict the shape change (camberback) with the passage of time of the press-formed product 1 after the die is removed from the mold and springed back.

In the above description, in the residual stress relaxation reduction setting step S3, the residual stress value obtained by relaxing and reducing the residual stress of each portion with respect to at least the top plate portion 3 and / or the flange portion 7 in the press-formed product 1. Was to set.

However, according to the present invention, the residual stress may be relaxed and reduced with respect to parts other than the top plate portion 3 and the flange portion 7 in the press-molded product 1, or the entire press-molded product 1 may be subjected to the present invention. The residual stress may be relaxed and reduced.
Further, the ratio or value of relaxing and reducing the residual stress may be changed for each portion such as the top plate portion 3 and the flange portion 7.

Further, the above description was intended for the press-molded product 1 having a hat-shaped cross-sectional shape, but the present invention has a top plate portion and a vertical wall portion continuous from the top plate portion, and is viewed from the side. This can be applied to any press-molded product containing a shape that is convexly curved toward the top plate portion. Examples of such a press-molded product include a U-shaped cross-sectional shape press-molded product 21 and L having a top plate portion 23 and a vertical wall portion 25 continuous from the top plate portion 23 as illustrated in FIG. A press-molded product having a character-shaped cross section (not shown), a vertical wall portion 35 continuous from the top plate portion 33 and the top plate portion 33, and a flange portion 37 continuous from the vertical wall portion 35, as illustrated in FIG. Examples thereof include a press-molded product 31 having a Z-shaped cross-sectional shape.

The above description is intended for a press-molded product having a shape that is convexly curved in a side view over the entire length in the longitudinal direction, but the present invention is a portion having a shape that is convexly curved in a side view. The press-molded product may be any press-molded product having, for example, a curved portion and a side portion extending linearly from the curved end of the curved portion to both sides or one side in the longitudinal direction. Can be targeted.

Further, it is preferable that the present invention can satisfactorily predict the shape change after the lapse of time by reducing the residual stress by 5% or more from the residual stress immediately after springback in the residual stress relaxation reduction setting step.

The accuracy when the present invention is applied to press-molded products having various shapes and the shape change is predicted by changing the ratio of relaxing and reducing the residual stress is verified in Examples 1 to 4 described later. did.

Further, 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) to be used for press molding and the shape and type of the press-molded product are not particularly limited, but the residual stress of the press-molded product is high. It is more effective for automobile parts press-molded using the metal plate.

Specifically, the thickness of the blank is preferably 0.5 mm or more and 4.0 mm or more.
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 a press-molded product, there is little advantage in using the method for predicting a shape change of a press-molded product according to the present invention. The present invention has many advantages to apply the present invention because the outer plate parts of automobiles using metal plates with a tensile strength of 150 MPa class or higher and having low rigidity are susceptible to shape changes due to changes in residual stress. It 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 press-formed product 1 having a hat-shaped cross-sectional shape as shown in FIG. 2, the punch shoulder portion 9 and the die shoulder portion 11 are used. Cracks are likely to occur and press molding may not be possible.

Further, as the type of the press-molded product, it is preferable to target automobile parts such as skeleton parts such as front pillar uppers and roof rails, but the press-molded products include a shape that is convexly curved toward the top plate portion in a side view. The present invention can be widely used as long as it is an automobile part in which camber back occurs due to the passage of time after the skeleton.

The press method for the press-molded product, which is the subject of the present invention, is not particularly limited to bending molding, foam molding, draw molding, and the like.

<Press-molded product with a hat-shaped cross-sectional shape>
In the first embodiment, first, a steel plate A having the mechanical properties shown in Table 1 below is used as the metal plate, and the press-formed product 1 having a hat-shaped cross-sectional shape that is curved convexly in a side view as shown in FIG. Was press-molded. The shape of the bottom dead center of the press-molded product 1 is such that the radius of curvature of the curve is 170 mm and the height of the vertical wall portion 5 in the press-molding direction is 40 mm.

Then, the press-molded product 1 press-molded to the bottom dead center of molding was removed from the mold, and the shape of the press-molded product 1 after 2 days had passed was measured.

Next, an analysis was performed to predict the shape change of the press-molded product 1.
In the analysis, first, a press molding analysis in the process of press forming the steel plate A to the bottom dead point of molding is performed using a die model that models the mold used for press molding, and the press molded product 1 at the bottom dead point of molding is performed. Residual stress was calculated.

Subsequently, 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 at the bottom dead center of molding was released from the die model.

Further, the residual stress value obtained by reducing the absolute value of the residual stress by a predetermined ratio is set for the top plate portion 3 and / or the flange portion 7 of the press-formed product 1 immediately after springback, which is obtained by springback analysis. did.
Then, an analysis was performed to obtain a shape in which the moments of force were balanced for the press-molded product 1 having the reduced residual stress.

In Example 1, the residual stress immediately after springback is set to a predetermined ratio (relaxation rate of residual stress) to the top plate portion, the top plate portion 3 and the flange portion 7 of the press-formed product 1 acquired by springback analysis. ) Is set as the value of the residual stress lowered in ), and is designated as Invention Example 1 to Invention Example 3.

Further, as a comparison target, the press-molded product 1 was subjected to the press-molding analysis and the springback analysis in the same manner as in the inventions 1 to 3, and the analysis for obtaining the shape in which the force moments were balanced was not performed. Alternatively, a comparative example is obtained in which a springback analysis is performed and then an analysis is performed to obtain a shape in which the moments of force are balanced without reducing the residual stress for both the top plate portion 3 and the flange portion 7 of the press-molded product 1. It was set to 2.

For each of Invention Example 1 to Invention Example 3, Comparative Example 1 and Comparative Example 2, the shape of the press-molded product 1 at the bottom dead center of molding at the longitudinal tip (evaluation point a) of the top plate portion 3 of the press-molded product 1. The amount of deviation in the molding height direction from the above was calculated.
Table 2 summarizes the results of the relaxation rate of the residual stress and the amount of deviation of the evaluation point a in Invention Example 1 to Invention Example 3, Comparative Example 1, and Comparative Example 2.

In Table 2, the predicted value D _c is the amount of deviation of the evaluation points a in Invention Examples 1 to 3 and Comparative Examples 1 to 2, and the experimental value D _e is 2 of the press-molded product 1 actually pressed. It is the amount of deviation (= 25.4 mm) of the evaluation point a after the lapse of days. Further, the difference and the error of the predicted value with respect to the experimental value are calculated by the following formulas, respectively.
Difference between experimental value and predicted value (mm) = _{De-D c ...} (1)
Predicted value error (%) = (D _e －D _c ) ÷ D _e × 100 ・ ・ ・ (2)

The amount of deviation of the evaluation points a in Comparative Example 1 and Comparative Example 2 was the same, the difference between the experimental value and the predicted value was 3.5 mm, and the error of the predicted value was 13.8%.

In Invention Example 1, the residual stress value obtained by reducing the residual stress by 5% with respect to the top plate portion 3 is set, the difference between the experimental value and the predicted value is 1.5 mm, and the error of the predicted value is 5.9%. It was improved as compared with Comparative Example 1 and Comparative Example 2.
In Invention Example 2, the residual stress values of the top plate portion 3 and the vertical wall portion 5 are set by reducing their residual stresses by 10%, respectively, and the difference between the experimental value and the predicted value is 0.3 mm. The error of the predicted value was 1.2%, which was improved as compared with Comparative Example 1 and Comparative Example 2, and the result was better than that of Invention Example 1.
In the third invention example, the residual stress values obtained by reducing the residual stresses of the top plate portion 3 and the vertical wall portion 5 by 20% are set, and the difference between the experimental value and the predicted value is −0.4. The error between mm and the predicted value was -1.6%, both of which were negative values, but when compared by absolute value, it was improved as compared with Comparative Example 1 and Comparative Example 2.

<Press-molded product with U-shaped cross section>
In Example 2, first, the steel plate A having the mechanical properties shown in Table 1 was used in the same manner as in Example 1 described above, and the press-molded product 21 having a U-shaped cross-sectional shape shown in FIG. 5 was press-molded. The press-molded product 21 has a U-shaped cross-sectional shape having a top plate portion 23 and a vertical wall portion 25 continuous from the top plate portion 23, and is curved convexly toward the top plate portion 23 in a side view. ing. The press-molded product 21 has a radius of curvature of 170 mm at the bottom dead center of molding, and the vertical wall height of the vertical wall portion 25 in the press-molding direction is 35 mm.

Subsequently, the press-molded product 21 press-molded to the bottom dead center of molding was removed from the mold, and the shape of the press-molded product 21 after 2 days had passed was measured.

Next, a press molding analysis of the press molded product 21 and a subsequent springback analysis were performed, and further, the residual stress was relaxed and reduced at a predetermined ratio with respect to the top plate portion 23 of the press molded product 21 immediately after the springback. The cases in which the residual stress value was set and the shape of the press-molded product 21 in which the moments of force were balanced were analyzed were referred to as Invention Example 4 and Invention Example 5.

Further, as a comparison target, the press forming analysis and the springback analysis of the press molded product 21 were performed in the same manner as in the invention example 4 and the invention example 5, but the shape in which the moments of the forces are balanced by setting the value obtained by relaxing and reducing the residual stress. The case in which the analysis for obtaining the above was not performed was designated as Comparative Example 3.

Then, for each of Invention Example 4, Invention Example 5, and Comparative Example 3, from the shape of the press-molded product 21 at the bottom dead center of molding at the tip (evaluation point b) in the longitudinal direction of the top plate portion 23 of the press-molded product 21. The amount of deviation in the molding height direction was calculated.
Table 3 summarizes the results of the relaxation rate of the residual stress and the amount of deviation of the evaluation point b in Invention Example 4, Invention Example 5, and Comparative Example 3.

In Table 3, the predicted value D _c is the amount of deviation of the evaluation points b in Invention Example 4, Invention Example 5 and Comparative Example 4, and the experimental value D _e is after 2 days of the press-molded product 21 actually press-molded. It is the amount of deviation (= 26.4 mm) of the evaluation point b of. Further, the difference between the experimental value and the predicted value and the error of the predicted value with respect to the experimental value are calculated by the above-mentioned equations (1) and (2), respectively.

In Comparative Example 3, the difference between the experimental value and the predicted value was 2.6 mm, and the error of the predicted value was 9.8%.
In Invention Example 4, the residual stress is reduced by 5% with respect to the top plate portion 23, the difference between the experimental value and the predicted value is 1.2 mm, and the error of the predicted value is 4.5%, which is the same as that of Comparative Example 4. Improved in comparison.
In Invention Example 5, the residual stress is reduced by 10% with respect to the top plate portion 23, the difference between the experimental value and the predicted value is 0.3 mm, and the error of the predicted value is 1.1%, which is the same as that of Comparative Example 4. It was improved in comparison with and was better than that of Invention Example 4.

<Press-molded product with Z-shaped cross section>
In Example 3, first, the press-molded product 31 shown in FIG. 6 was press-molded using the steel plate A having the mechanical properties shown in Table 1 in the same manner as in Example 1 described above.
The press-molded product 31 has a Z-shaped cross-sectional shape having a top plate portion 33, a vertical wall portion 35 continuous from the top plate portion 33, and a flange portion 37 continuous from the vertical wall portion 35, and has a side view. It is curved convexly toward the top plate 33 side. The press-molded product 31 has a radius of curvature of 170 mm at the bottom dead center of molding, and the vertical wall height of the vertical wall portion 35 in the press-molding direction is 40 mm.

Subsequently, the press-molded product 31 press-molded to the bottom dead center of molding was removed from the mold, and the shape change of the press-molded product 31 after 2 days had passed was measured.

Next, the press molding analysis of the press molded product 31 and the subsequent springback analysis are performed, and further, the residual stresses of the top plate portion 33 and the flange portion 37 of the press molded product 31 immediately after the springback are relaxed and reduced. Invention Example 6 and Invention Example 7 were obtained by setting the value of the residual stress and performing an analysis to obtain a shape in which the moments of force are balanced with respect to the press-molded product 31.
Further, as a comparison target, the press forming analysis and the springback analysis of the press molded product 31 were performed in the same manner as in the invention example 6 and the invention example 7, but the shape in which the moments of the forces are balanced by setting the value in which the residual stress is relaxed and reduced is set. The case in which the analysis for obtaining the above was not performed was designated as Comparative Example 4.

Then, for each of Invention Example 6, Invention Example 7, and Comparative Example 4, from the shape of the press-molded product 31 at the bottom dead center of molding at the tip (evaluation point c) in the longitudinal direction of the top plate portion 33 of the press-molded product 31. The amount of deviation in the molding height direction was calculated.
Table 4 summarizes the results of the relaxation rate of the residual stress and the amount of deviation of the evaluation point c in Invention Example 6, Invention Example 7, and Comparative Example 4.

In Table 4, the predicted value D _c is the amount of deviation of the evaluation points c in Invention Example 6, Invention Example 7, and Comparative Example 4, and the experimental value D _e is after 2 days of the press-molded product 31 actually press-molded. It is the amount of deviation (= 26.5 mm) of the evaluation point d of. Further, the difference and the error of the predicted value with respect to the experimental value are calculated by the above-mentioned equations (1) and (2), respectively.

In Comparative Example 4, the difference between the experimental value and the predicted value was 2.5 mm, and the error of the predicted value was 9.4%.
In Invention Example 6, the residual stress of the top plate portion 33 is reduced by 5%, the difference from the experimental value is 1.0 mm, and the error of the predicted value is 3.8%, which is compared with Comparative Example 4. It has improved.
In Invention Example 7, the residual stress of each of the top plate portion 33 and the flange portion 37 is reduced by 10%, the difference between the experimental value and the predicted value is −0.4 mm, and the error of the predicted value is It was -1.5%, both of which were negative values, but when compared by absolute value, it was improved as compared with Comparative Example 4 and better than Invention Example 6.

<Front pillar upper>
In Example 4, first, the press-molded product 41, which is the front pillar upper of the automobile shown in FIG. 7, was press-molded using the steel plate A having the mechanical properties shown in Table 1 as in Example 1 described above. The press-molded product 41 has a hat-shaped cross-sectional shape having a top plate portion 43, a vertical wall portion 45, and a flange portion 47, and includes a shape that is convexly curved toward the top plate portion 43 in a side view. The press-molded product 41 has a radius of curvature of the curve at the bottom dead center of molding of 1000 mm, and the height of the vertical wall portion 45 in the press-molding direction is 45 mm.

Subsequently, the press-molded product 41 press-molded to the bottom dead center of molding was removed from the mold, and the shape change of the press-molded product 41 after 2 days had passed was measured.

Next, the press molding analysis of the press molded product 41 and the subsequent springback analysis are performed, and further, the residual stresses of the top plate portion 43 and the flange portion 47 of the press molded product 41 immediately after the springback are relaxed and reduced. Invention Example 8 and Invention Example 9 were analyzed by setting the value of the residual stress and determining the shape of the press-molded product 41 in which the moments of force are balanced.
Further, as a comparison target, although the press forming analysis and the springback analysis of the press molded product 41 were performed in the same manner as in the invention example 8 and the invention example 9, the shape in which the moment of force is balanced by setting the value in which the residual stress is relaxed and reduced is set. The case in which the analysis for obtaining the above was not performed was designated as Comparative Example 5.

Then, for each of Invention Example 8, Invention Example 9, and Comparative Example 5, the amount of deviation in the molding height direction from the shape of the bottom dead center of molding at the tip (evaluation point d) in the longitudinal direction of the top plate portion 43 of the press-molded product 41. Was calculated.
Table 5 summarizes the results of the relaxation rate of the residual stress and the amount of deviation of the evaluation point d in Invention Example 8, Invention Example 9, and Comparative Example 5.

In Table 5, the predicted value D _c is the amount of deviation of the evaluation points d in Invention Example 8 and Comparative Example 9, and the experimental value D _e is the evaluation point d after 2 days of the press-molded product 41 actually press-molded. The amount of deviation (= 22.5 mm). Further, the difference between the experimental value and the predicted value and the error of the predicted value with respect to the experimental value are calculated by the above-mentioned equations (1) and (2), respectively.

In Comparative Example 5, the difference between the experimental value and the predicted value was 2.2 mm, and the error of the predicted value was 9.8%.
In Invention Example 8, the residual stress of the top plate portion 43 is reduced by 5%, the difference between the experimental value and the predicted value is 1.0 mm, the error of the predicted value is 4.4%, and Comparative Example 5 Improved compared to.
In Invention Example 9, the residual stress is reduced by 10% for both the top plate portion 43 and the flange portion 47, the difference between the experimental value and the predicted value is 0.3 mm, and the error of the predicted value is 1.3%. It was improved as compared with Comparative Example 5 and better than Invention Example 8.

1 Press-molded product 3 Top plate part 5 Vertical wall part 7 Flange part 9 Punch shoulder part 11 Die shoulder part 21 Press-molded product 23 Top plate part 25 Vertical wall part 31 Press-molded product 33 Top plate part 35 Vertical wall part 37 Flange part 41 Press-molded product 43 Top plate part 45 Vertical wall part 47 Flange part

Claims (4)

A press-molded product having a top plate portion and a vertical wall portion continuous from the top plate portion and having a shape that is convexly curved toward the top plate portion in a side view is springd at the moment when the mold is released from the mold. It is a method for predicting the shape change of a press-molded product that predicts the shape change due to stress relaxation with the passage of time after backing.
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 residual stress that is relaxed and reduced from the residual stress immediately after springback with respect to at least the top plate portion of the press-formed 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 residual stress is relaxed and reduced. A press-molded product having a top plate portion, a vertical wall portion continuous from the top plate portion, and a flange portion continuous from the vertical wall portion, and including a shape that is convexly curved toward the top plate portion in a side view. This is a method for predicting the shape change of a press-molded product that predicts the shape change due to stress relaxation with the passage of time after springing back at the moment of mold release from the mold.
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 of setting a value of residual stress that is relaxed and reduced from the residual stress immediately after springback with respect to at least the top plate portion and / or the flange portion of the press-formed 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 residual stress is relaxed and reduced. The shape change of the press-molded product according to claim 1 or 2, wherein in the residual stress relaxation reduction setting step, a value of residual stress whose relaxation is reduced by 5% or more from the residual stress immediately after springback is set. Prediction method. The shape change of the 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. Prediction method.

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