JP6977825B1 - Method for predicting shape change of press-molded products - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for predicting a shape change of a press-molded product, which predicts a shape change in which the radius of curvature of the concave curve becomes smaller with the passage of time. SOLUTION: The method for predicting a shape change of a press-molded product according to the present invention includes a shape having a top plate portion, a punch shoulder portion and a vertical wall portion, and the top plate portion side is curved in a concave shape in a side view. It predicts the shape change of the product due to stress relaxation with the passage of time after it is separated from the mold and springed back, and the shape and residual stress of the press-molded product immediately after springback can be determined by springback analysis. The step of acquiring (S1) and the step of setting at least the punch shoulder portion and / or the vertical wall portion of the press-formed product immediately after springback a residual stress relaxed and reduced from the residual stress immediately after springback (S3). The step (S5) of obtaining a shape in which the moments of force are balanced for the press-molded product in which the relaxed and reduced residual stress is set is included. [Selection diagram] Fig. 1

Description

The present invention relates to a method for predicting a shape change of a press-molded product, and particularly for a press-molded product having a top plate portion and a vertical wall portion and including a shape in which the top plate portion side is curved in a concave shape 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 a 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-molded product released (taken out) from the die changes due to springback, and the moment of force and residual stress in the released press-molded 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 molding simulation that integrates the above-mentioned first-stage press-molding analysis and second-stage springback analysis, the shape of the press-molded product that has been separated from the die and springed back can be predicted. It has been.
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 is.

Therefore, as a result of investigating a press-molded product having a low shape prediction accuracy by a press-molding simulation and its cause, it has a top plate portion 3, a vertical wall portion 7, and a flange portion 11 as shown in FIG. 11 as an example. In the press-molded product 1 having a hat-shaped cross-sectional shape and the top plate portion 3 side is curved in a concave shape in a side view, both ends in the longitudinal direction of the press-molded product 1 are lifted after several days have passed since the mold was released. It was discovered that a deformation called camber go, in which the radius of curvature of the concave curve is small, occurs, 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 is press-molded having a top plate portion, a punch shoulder portion, and a vertical wall portion, and the top plate portion side is curved in a concave 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 that predicts a shape change of the press-molded product due to the passage of time unit after springing back at the moment of releasing the product from the die.

(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 via a punch shoulder portion, and the top plate is viewed from the side. For press-molded products containing a shape with a concave curve on the part side, the shape change is predicted so that the radius of curvature of the concave curve becomes smaller due to stress relaxation with the passage of time after springing back at the moment of removal from the die. hand,
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.
Residual stress relaxation reduction setting step for setting at least the punch shoulder portion and / or the vertical wall portion of the press-formed product immediately after springback a value of residual stress relaxed and reduced from the residual stress immediately after springback. When,
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 includes a top plate portion, a vertical wall portion continuous via a punch shoulder portion continuous from the top plate portion, and a die shoulder portion from the vertical wall portion. With the passage of time after springing back at the moment when the press-molded product has a flange portion continuous with the mold and has a shape in which the top plate portion side is curved in a concave shape in a side view. It predicts a shape change in which the radius of curvature of the concave curve becomes smaller due to stress relaxation.
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.
Residual stress relaxation reduction setting step for setting at least the punch shoulder portion and / or the vertical wall portion of the press-formed product immediately after springback a value of residual stress relaxed and reduced from the residual stress immediately after springback. When,
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, a press-molded product having a top plate portion and a vertical wall portion continuous from the top plate portion via a punch shoulder portion, and including a shape in which the top plate portion side is concavely curved 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 by the springback analysis of the press-molded product, and the press-molded product immediately after springback. The residual stress relaxation reduction setting step of setting the value of the residual stress relaxed and reduced from the residual stress immediately after springback and the relaxation reduction of the residual stress value for at least the punch shoulder portion and / or the vertical wall portion of the above. By including a residual stress relaxation shape analysis step for obtaining a shape in which the moments of force are balanced for the set press-molded product, the press molding by stress relaxation with the lapse of time after the mold is released from the mold and springed back. It is possible to accurately predict changes in the shape of the product. As a result, in the manufacturing process of automobile parts, car bodies, etc., 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. It is a figure which shows the press-molded article of the hat type cross-sectional shape which was curved concavely in the side view which was the object of Embodiment 1 of this invention ((a) perspective view, (b) side view). 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 (angle change) by relaxation of the residual stress in the punch shoulder part of the press-molded article which was curved concavely in the side view ((a) the bottom dead center of molding immediately after press molding, (b) spring back. Immediately after (c) after the lapse of time). It is a figure explaining the wall opening of a vertical wall part with an increase of a bending angle by stress relaxation in a punch shoulder part of a press-molded article which was curved in a concave shape in the side view. It is a figure explaining the shape change (wall warpage) by relaxation of the residual stress in the vertical wall part of the press-molded article which was curved concavely in the side view ((a) the bottom dead center of molding immediately after press molding, (b) spring back. Immediately after (c) after the lapse of time). It is a figure explaining the wall opening of the vertical wall part with the increase of the wall warp by stress relaxation in the vertical wall part of the press-molded article which was curved in a concave shape in the side view. An example of a press-molded product to which the present invention can be applied is a press-molded product having a U-shaped cross-sectional shape curved in a concave shape in a side view, and a shape change (camber go) that occurs after the press-molded product is press-molded and springed back. It is a perspective view explaining. An example of a press-molded product to which the present invention can be applied is a press-molded product having a Z-shaped cross-sectional shape curved in a concave shape in a side view, and a shape change (camber go) that occurs after the press-molded product is press-molded and springed back. It is a perspective view explaining. It is a figure which shows the press-molded article which is the floor cloth of the automobile which was curved concavely in the side view which was targeted in Example 2 ((a) top view, (b) perspective view). It is a figure explaining the shape change (camber go) which occurs after press molding and springing back of the press-molded product of the hat type cross-sectional shape which is curved in a concave shape in the side view.

<Examination leading up to the invention>
In order to solve the above-mentioned problems, the inventors have a method of predicting a shape change of the press-molded product 1 shown as an example in FIG. 2 with the passage of time after springing back at the moment of removal from the mold. In order to establish the above, the cause of the change in the shape of the press-molded product 1 with the passage of time was investigated.

As shown in FIG. 2, the press-molded product 1 to be examined has a top plate portion 3, a vertical wall portion 7 continuous from the top plate portion 3 via the punch shoulder portion 5, and a vertical wall portion 7. It is a hat-shaped cross-sectional shape having a flange portion 11 continuous from the die shoulder portion 9 to the die, and includes a shape in which the top plate portion 3 side is concavely curved (concave curved) in a side view.

In such a press-molded product 1, a shape change such that both ends in the longitudinal direction of the curve are lifted due to the opening of the vertical wall portion 7 (wall opening) as shown in FIG. 11 described above with the passage of time. That is, camber go occurs. The relationship between the opening of the vertical wall portion 7 and the lifting of both ends of the curve is geometrically clear.

Therefore, the cause of the wall opening of the vertical wall portion 7 that causes camber go in the press-molded product 1 was investigated. As a result, the inventors focused on the stress relaxation phenomenon in which the stress is gradually relaxed with the passage of time while being kept constant after the strain is applied, as shown in the stress-strain diagram of FIG. In the press-formed product 1, the residual stress in the punch shoulder portion 5 and the vertical wall portion 7 is gradually relaxed with the passage of time, and the shape that balances with the moment of force changes, so that the wall of the vertical wall portion 7 is formed. It was found that the opening and the accompanying lifting of both ends of the curve occur.

The wall opening of the vertical wall portion 7 due to the relaxation of the residual stress in the punch shoulder portion 5 will be described with reference to the schematic views shown in FIGS. 4 and 5.
First, the punch shoulder portion 5 is formed as shown in FIG. 4A when a blank (metal plate or the like) is press-molded to the bottom dead point of molding using a die provided with a punch and a die during press molding. Therefore, tensile stress is generated on the outside of the bending of the punch shoulder portion 5, and compressive stress is generated on the inside of the bending. The outside of the bend is the side opposite to the center of curvature of the bend with respect to the line at the center of the plate thickness in the cross section of the bent portion, and the inside of the bend is the same side as the center of the curvature of the bend (the same applies hereinafter).

Next, when the press-molded product 1 at the bottom dead point of molding is removed (demolded) from the mold, springback of the press-molded product 1 is generated by using the residual stress generated during press molding as a driving force, and the above-mentioned FIG. 11 Camber go as shown in is generated. At that time, as shown in FIG. 4B, in the punch shoulder portion 5, the bending angle increases so as to return to the shape of the flat blank before press forming, and compressive stress is generated on the outside of the bending to bend. Tensile stress is generated on the inside, and the shape is such that the moments of force are balanced.

After that, as shown in FIGS. 4 (b) to 4 (c), the residual stress of the punch shoulder portion 5 gradually relaxes and decreases 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 bending angle of the punch shoulder portion 5 is further increased. Then, as the bending angle of the punch shoulder portion 5 increases, the wall opening of the vertical wall portion 7 occurs as shown in FIG.

On the other hand, the wall opening of the vertical wall portion 7 due to the relaxation of the residual stress in the vertical wall portion 7 will be described with reference to the schematic views shown in FIGS. 6 and 7.
In the process of press-molding a blank into a press-molded product 1 using a die provided with a punch and a die, first, a portion of the blank corresponding to the vertical wall portion 7 is bent by the die shoulder of the die. Tensile stress is generated on the outside of the bend and compressive stress is generated on the inside of the bend. Then, when the die further moves relative to the punch side, the portion bent by the die shoulder is bent back flat by the punch and the die to become the vertical wall portion 7. Therefore, in the vertical wall portion 7 at the bottom dead center of molding, as shown in FIG. 6A, compressive stress is generated on the outside of bending and tensile stress is generated on the inside of bending.

Next, when the press-molded product 1 press-molded to the bottom dead center of molding is removed (detached) from the mold, springback is generated using the residual stress generated during press molding as a driving force. At that time, the vertical wall portion 7 is deformed so as to return to the bent shape in the press forming process, a curved wall warp occurs as shown in FIG. 6B, and tensile stress is generated on the outside of the bending. Compressive stress is generated inside the bend, resulting in a shape in which the moments of force are balanced.

After that, as shown in FIGS. 6 (b) to 6 (c), the tensile stress and the compressive stress in the vertical wall portion 7 gradually 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 curvature of the curvature of the vertical wall portion 7 increases, and the wall warp further increases. Then, as the wall warp of the vertical wall portion 7 increases, as shown in FIG. 7, the wall opening of the vertical wall portion 7 occurs.

As described above, in the press-molded product 1 having a shape in which the top plate portion 3 side is curved in a concave shape in the side view, the residual stress of the punch shoulder portion 5 and the vertical wall portion 7 increases with the lapse of further time after the press molding. Due to the relaxation, as shown in FIG. 11 described above, (i) a wall opening such that the vertical wall portion 7 opens occurs, and as a result, (ii) a camber go that both ends in the longitudinal direction are lifted occurs. , It was found that the shape is further deviated from the shape of the bottom dead center of molding.

Therefore, based on the above-mentioned new findings, the inventors predict, for example, a shape change (camber go) due to stress relaxation with the passage of time after springback of the press-molded product 1 as shown in FIG. I recommended the method. As a result, at least the residual stress of the punch shoulder portion 5 and / or the vertical wall portion 7 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 pressed. It was discovered that the shape change (camber go) with the passage of time of the press-molded 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 molded 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 camber go can be predicted by the relaxation and decrease of the residual stress of the top plate with the passage of time after springback, even for the press-molded product containing the shape that the top plate side is curved in a concave shape by visual inspection.

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 includes a top plate portion 3 and a vertical wall portion continuous from the top plate portion 3 via a punch shoulder 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 the die shoulder portion 9 and having a shape in which the top plate portion 3 side is curved in a concave shape in a side view is obtained from a mold. It predicts a shape change in which the radius of curvature of the concave curve becomes smaller due to stress relaxation over time after springback at the moment of mold release. As shown in FIG. 1, the shape and residual stress immediately after springback. It includes an acquisition step S1, a residual stress relaxation reduction setting step S3, and a residual stress relaxation shape analysis step S5. 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 punch shoulder portion 5 and / or the vertical wall 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 a step of setting a value of residual stress that is relaxed and reduced rather than stress.

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 value for which the residual stress is relaxed and reduced is set 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-molded product according to the present embodiment, the top plate portion 3 has the top plate portion 3, the punch shoulder portion 5, and the vertical wall portion 7, and the top plate portion 3 side is concave in side view. The residual stress of the press-molded product 1 curved to the above was relaxed and reduced with respect to at least the punch shoulder portion 5 and / or the vertical wall portion 7 of the press-molded product 1 immediately after springback, which was obtained by springback analysis. By performing an analysis to obtain a shape that balances the moment of force with respect to the press-molded product 1 in which the stress value is set and the residual stress value is relaxed and reduced, the stress relaxation and shape over time in the actual press-molded product 1 are performed. By simulating the change, it is possible to predict a shape change (camber go) in which the radius of curvature of the concave curve becomes smaller due to stress relaxation with the passage of time after the press-molded product 1 is separated from the die and springed back.

In the above description, in the residual stress relaxation reduction setting step S3, the residual stress of at least the punch shoulder portion 5 and / or the vertical wall portion 7 of the press-molded product 1 in which the residual stress of each portion is relaxed and reduced is reduced. It was to set the value.

However, in the present invention, a value in which the residual stress is relaxed and reduced may be set for a portion other than the punch shoulder portion 5 and the vertical wall portion 7 in the press-molded product 1, or the press-molded product 1 may be set. A value in which the residual stress is relaxed and reduced may be set for all of them.
Further, the ratio or value of relaxing and reducing the residual stress may be changed for each portion such as the punch shoulder portion 5 and the vertical wall portion 7.

Further, the above description is intended for a press-molded product 1 having a hat-shaped cross-sectional shape having a flange portion 11 continuous from the vertical wall portion 7 to the die shoulder portion 9 with the die shoulder portion 9. Is a press-molded product having a top plate portion and a vertical wall portion continuous from the top plate portion via a punch shoulder portion, and including a shape in which the top plate portion side is curved in a concave shape in a side view. Can be applied.

As such a press-molded product, a U-shape having a top plate portion 23 and a vertical wall portion 27 continuous from the top plate portion 23 via the punch shoulder portion 25, as illustrated in FIG. Examples thereof include a press-molded product 21 having a mold cross-sectional shape and a press-molded product (not shown) having an L-shaped cross-sectional shape.
Further, the press-molded product having a flange portion continuous from the vertical wall portion is not limited to the above-mentioned press-molded product 1 (FIG. 2) having a hat-shaped cross-sectional shape, and as illustrated in FIG. 9, the top plate portion 33 , A Z-shaped cross-sectional shape comprising a vertical wall portion 37 continuous from the top plate portion 33 via the punch shoulder portion 35, and a flange portion 41 continuous from the vertical wall portion 37 via the die shoulder portion 39. The press-molded product 31 of the above can be mentioned.

The above description is intended for a press-molded product having a concavely curved shape in a lateral view over the entire length in the longitudinal direction, but the present invention includes a portion having a concavely curved shape in a lateral view. It may be a press-molded product. For example, a press-molded product including a curved portion and a side portion extending linearly from the curved end of the curved portion to both sides or one side outward 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 a press-molded product having a hat-shaped cross-sectional shape and the shape change is predicted by changing the ratio of relaxing and reducing the residual stress is described in Examples 1 and 2 described later. I verified it.

Although the verification results when the present invention is applied to the press-molded product having a U-shaped cross section, the press-molded product having an L-shaped cross section, and the press-molded product having a Z-shaped cross section are omitted. It has been confirmed that by applying the present invention to these press-molded products, it is possible to satisfactorily predict the shape change due to stress relaxation with the lapse of further time after press-molding and springing back.

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 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 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 molding process of the press-molded product 1 having a hat-shaped cross-sectional shape as shown in FIG. 2, the punch shoulder portion 5 and the die shoulder portion 9 are used. Cracks are likely to occur and press molding may not be possible.

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

The press molding method of the press molded product targeted by 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, which is curved in a concave shape in a side view, is shown in FIG. Press molding was performed. The shape of the bottom dead center of the press-molded product 1 is such that the radius of curvature of the concave curve is 170 mm and the height of the vertical wall portion 7 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 determined.

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 obtained by the springback analysis is obtained by relaxing and reducing the absolute value of the residual stress at a predetermined ratio with respect to the punch shoulder portion 5 and / or the vertical wall portion 7 of the press-formed product 1 immediately after springback. Set the value.
Then, an analysis was performed to obtain a shape in which the moments of force were balanced for the press-molded product 1 in which the residual stress was relaxed and reduced.

In Example 1, the residual stress immediately after springback is set to a predetermined ratio (residual stress) to only the punch shoulder portion 5 of the press-molded product 1 acquired by the springback analysis, or to the punch shoulder portion 5 and the vertical wall portion 7. The value of the residual stress for which the relaxation was reduced was set by the relaxation reduction rate), and the examples 1 to 4 were designated as Invention Examples 1 to 4.

Further, as a comparison target, in the same manner as in Invention Examples 1 to 4, the press molding analysis and the springback analysis of the press molded product 1 were performed, and the analysis for obtaining the shape in which the moments of force were balanced was not performed. Alternatively, after performing a springback analysis, an analysis was performed to obtain a shape in which the moments of force are balanced in the press-molded product 1 without relaxing and reducing the residual stress in both the punch shoulder portion 5 and the vertical wall portion 7. It was referred to as Comparative Example 2.

For each of Invention Examples 1 to 4, Comparative Example 1 and Comparative Example 2, press molding at the bottom dead center of molding at the tip in the longitudinal direction (evaluation point a, see FIG. 2) in the top plate portion 3 of the press-molded product 1. The amount of deviation from the shape of product 1 in the molding height direction was calculated.
Table 2 summarizes the results of the relaxation reduction rate of the residual stress and the deviation amount of the evaluation point a in Invention Examples 1 to 4, 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 4 and Comparative Examples 1 to 2, and the experimental value D _e is 2 of the press-molded product 1 actually press-molded. It is the amount of deviation (= 23.5 mm) of the evaluation point a after the lapse of days. 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 following equations, respectively.
The difference between the predicted and experimental values _{(mm) = D e -D c} ··· (1)
Predicted value error (%) = (D _e − D _c ) ÷ _De x 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 2.6 mm, and the error of the predicted value was 11.1%.

In the first invention example, the residual stress value is set by relaxing and reducing the residual stress by 5% only for the punch shoulder portion 5. The difference between the experimental value and the predicted value is 1.4 mm, and the error of the predicted value is 1. It was 6.0%, which was an improvement as compared with Comparative Example 1 and Comparative Example 2.
In Invention Example 2, the residual stress values are set for the punch shoulder portion 5 and the vertical wall portion 7 by relaxing and reducing the residual stresses by 10%, respectively, and the difference between the experimental value and the predicted value is 0.8. The error of mm and the predicted value was 3.4%, 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 Invention Example 3, the residual stress values are set for the punch shoulder portion 5 and the vertical wall portion 7 by relaxing and reducing the residual stresses by 20%, respectively, and the difference between the experimental value and the predicted value is 0.4. The error of mm and the predicted value was 1.7%, which was improved as compared with Comparative Example 1 and Comparative Example 2, and the result was better than that of Invention Example 2.
In the fourth invention, the residual stress values obtained by relaxing and reducing the residual stress by 30% and 20%, respectively, are set for the punch shoulder portion 5 and the vertical wall portion 7, and the difference between the experimental value and the predicted value is set. The error of -0.4 mm and the predicted value was -1.7%, both of which were negative values, but when compared by absolute value, they were improved as compared with Comparative Example 1 and Comparative Example 2, and were as good as Invention Example 3. ..

<Floor cloth>
In Example 2, first, the press-molded product 51, which is the floor cloth of the automobile shown in FIG. 10, 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 51 has a hat-shaped cross-sectional shape having a top plate portion 53, a punch shoulder portion 55, a vertical wall portion 57, a die shoulder portion 59, and a flange portion 61, and the top plate portion 53 side is viewed from the side. Includes a concave curved shape. The press-molded product 51 has a radius of curvature of the concave curve at the bottom dead center of molding of 1000 mm, and the vertical wall height of the vertical wall portion 57 in the press-molding direction is 45 mm.

Subsequently, the press-molded product 51 press-molded to the bottom dead center of molding was removed from the mold, and the shape change of the press-molded product 51 after 2 days had passed was measured.

Next, as in Example 1, an analysis was performed to predict the shape change of the press-molded product 51.
First, a press molding analysis of the press molded product 51 and a subsequent springback analysis were performed.
Further, the absolute value of the residual stress of only the punch shoulder portion 55 of the press-molded product 51 immediately after springback, or the punch shoulder portion 55 and the vertical wall portion 57, which is obtained by springback analysis, is set to a predetermined ratio. The value of the residual stress that was relaxed and reduced was set in, and the analysis was performed to find the shape in which the moment of force is balanced.

In Example 2, Invention Example 5 and Invention Example 6 were set with the value of the residual stress obtained by relaxing and reducing the residual stress immediately after springback at a predetermined ratio (relaxation reduction rate of the residual stress).
Further, as a comparison target, although the press molding analysis and the springback analysis of the press molded product 51 were performed in the same manner as in the invention example 5 and the invention example 6, 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 3.

Then, for each of Invention Example 5, Invention Example 6 and Comparative Example 3, the molding height direction from the bottom dead center shape at the tip (evaluation point b, FIG. 10) in the longitudinal direction of the top plate portion 53 of the press-molded product 51. The amount of deviation was calculated.
Table 3 summarizes the results of the relaxation reduction rate of the residual stress and the deviation amount of the evaluation point b in Invention Example 5, Invention Example 6 and Comparative Example 3.

In Table 3, the predicted value D _c is the amount of deviation of the evaluation points b in Comparative Example 3, Invention Example 5 and Comparative Example 6, and the experimental value D _e is after 2 days of the press-molded product 51 actually press-molded. It is the amount of deviation (= 28.7 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 3.0 mm, and the error of the predicted value was 10.5%.
In Invention Example 5, the residual stress of the punch shoulder portion 55 is relaxed and reduced by 5%, the difference between the experimental value and the predicted value is 1.7 mm, and the error of the predicted value is 5.9%, which is a comparative example. It improved compared to 3.
In Invention Example 6, the residual stress is relaxed and reduced by 10% with respect to the punch shoulder portion 55 and the vertical wall portion 57, respectively, the difference between the experimental value and the predicted value is 0.5 mm, and the error of the predicted value is 1.7%. It was improved as compared with Comparative Example 3 and better than Invention Example 5.

1 Press-molded product 3 Top plate part 5 Punch shoulder part 7 Vertical wall part 9 Die shoulder part 11 Flange part 21 Press-molded product 23 Top plate part 25 Punch shoulder part 27 Vertical wall part 31 Press-molded product 33 Top plate part 35 Punch shoulder Part 37 Vertical wall part 39 Die shoulder part 41 Flange part 51 Press molded product 53 Top plate part 55 Punch shoulder part 57 Vertical wall part 59 Die shoulder part 61 Flange part

Claims (4)

A press-molded product having a top plate portion and a vertical wall portion continuous from the top plate portion via a punch shoulder portion and having a shape in which the top plate portion side is curved in a concave shape in a side view is obtained from a mold. It is a method for predicting the shape change of a press-molded product that predicts the shape change in which the radius of curvature of the concave curve becomes smaller due to stress relaxation after springback at the moment of mold release.
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.
Residual stress relaxation reduction setting step for setting at least the punch shoulder portion and / or the vertical wall portion of the press-molded product immediately after springback a value of residual stress relaxed and reduced from the residual stress immediately after springback. When,
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. It has a top plate portion, a vertical wall portion continuous from the top plate portion via a punch shoulder portion, and a flange portion continuous from the vertical wall portion via a die shoulder portion. For press-molded products that include a shape with the top plate curved in a concave shape, a press that predicts a shape change in which the radius of curvature of the concave curve becomes smaller due to stress relaxation over time after springing back at the moment of removal from the die. It is a method for predicting the shape change of a molded product.
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.
Residual stress relaxation reduction setting step for setting at least the punch shoulder portion and / or the vertical wall portion of the press-formed product immediately after springback a value of residual stress relaxed and reduced from the residual stress immediately after springback. When,
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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