JP6152841B2 - Press molding method and manufacturing method of press mold - Google Patents

Description

  The present invention relates to a press molding method and a press molding die manufacturing method, and more particularly, to a press molding method and a press molding die manufacturing method for suppressing springback that occurs in a press molded product curved in the longitudinal direction. Is.

In recent years, the application of high-strength steel sheets to automobile parts has been expanding due to demands for improving collision safety and reducing the weight of automobile bodies. However, in the production of automobile parts, there is a problem of defective shape freezing due to springback that occurs after press forming of a high-strength steel plate. In particular, the shape freezing failure is remarkable in a super high tensile material of 980 MPa class or higher.
In order to cope with this problem, Patent Documents 1 to 3 disclose a press molding method excellent in shape freezing property that suppresses springback after press molding. Patent Document 4 discloses a press molding method in which the entire vertical wall is molded while the clearance between the die and the punch is narrowed for the purpose of preventing warpage of the vertical wall of the hat-shaped part.

JP 2009-255117 A JP 2009-202189 A Japanese Patent Laid-Open No. 2004-174551 JP 2009-23000 A

  The press molding method disclosed in Patent Document 1 produces a press-molded product by two steps. First, in the first step, an intermediate product is formed in which a plurality of embossments are disposed in a portion to be an elongated flange portion, and a plurality of surplus beads are disposed in a portion to be a contracted flange portion. In the subsequent second step, the embossing is crushed to give a compressive stress to the stretched flange part to level the residual stress in the stretched flange part, and the residual stress is leveled by generating a tensile stress in the contracted flange part. Turn into. Thus, although the press molding method of patent document 1 reduces a springback by equalizing the residual stress of a press-molded product, it is a clear indicator about the shape of the embossed or surplus bead and the place to place it. There is no. Further, depending on the press-formed product, the curvature of the contracted flange portion changes and the compressive stress is not necessarily uniform, and therefore the press-forming method cannot be applied. In addition, in order to give compressive stress to the stretch flange part, the second step for crushing a plurality of embosses formed in the first step is indispensable, so the productivity is poor, and further, the crushing cannot be done in the second step. There is a problem that wrinkles due to embossing remain in the press-formed product.

  In the press forming method of the cross-sectional hat-shaped member disclosed in Patent Document 2, the cross-sectional hat is formed by providing an elliptical fillet portion at the joint between the ceiling portion and the vertical wall portion of the cross-sectional hat-shaped member. This is to reduce warpage and twist caused by springback after the mold-shaped member is released. However, in the press molding method, it is necessary to change the shape of the press-molded product, and if the shape change of the press-molded product is impossible due to design requirements, it is not an effective measure.

  In the thin steel plate press forming method disclosed in Patent Document 3, the in-plane stress of the vertical wall is adjusted immediately before the end of the press forming process by providing a bead on the flange portion of the hat-shaped cross-sectional component, and the spring back is applied. Although the bead provided on the flange portion is a projection unnecessary for the product, it is necessary to remove the bead by a trim process performed after the press molding process, resulting in poor yield and productivity. There is a problem.

  In the press molding method disclosed in Patent Document 4, it is possible to prevent warpage that occurs in the vertical wall portion of the hat-shaped cross-sectional shape part, but it is possible to prevent warpage of the entire hat-shaped cross-sectional shape part that is curved in the longitudinal direction in a side view. It is difficult to prevent.

  As described above, any of those disclosed in Patent Documents 1 to 4 has some problem. The present invention has been made to solve the above-described problems, and by effectively suppressing the spring back generated in the press-formed product curved in the longitudinal direction, it is possible to prevent the shape failure of the entire press-formed product. An object of the present invention is to provide a press molding method and a press molding die manufacturing method that are possible and that do not change the shape of the press molded product.

(1) A press molding method according to the present invention is a press molding method for producing a press-molded product having a cross-sectional hat shape that is curved in the longitudinal direction,
A press molding analysis step for performing a press molding analysis on the press molded product, and calculating a residual tensile stress distribution of the press molded product at a molding bottom dead center in the press molding analysis;
A region specifying step of performing a springback analysis based on the residual tensile stress distribution calculated in the press molding analysis step, and specifying a region where residual tensile stress that causes a shape freezing failure due to the springback exists,
A clearance changing step of changing the clearance of the mold of the part that comes into contact with the region specified in the region specifying step;
A press molding step of performing press molding using a mold in which the clearance of the portion that abuts the region in the clearance changing step is changed , and
In the clearance changing step, the relationship between the longitudinal tensile stress to be relaxed and the necessary compressive stress in the plate thickness direction is obtained in advance, and the necessary plate thickness direction compressive stress and the plate thickness before forced tensile displacement are used as a reference. force tensile thickness reduction rate after displacement to previously obtain a relation between alpha _2, press molding method, wherein that you calculate the amount of change in the clearance (3) is.
(Clearance change amount) = (Clear clearance of the part before change) − (Target plate thickness) (3)
However, target plate thickness = plate thickness * {1- (plate thickness reduction rate α ₁ due to longitudinal tensile stress + plate thickness reduction rate α ₂ due to plate direction compressive stress )}
Here, the plate thickness reduction rate α ₁ due to the forced tensile displacement is obtained by the following equation (1).
α ₁ [%] = (t ₀ −t ₁ ) / t ₀ * 100 (1)
Further, the thickness reduction rate α ₂ due to the application of the compressive stress in the thickness direction after the forced tensile displacement is obtained by the following formula (2) using the thickness t ₀ before the forced tensile displacement as a reference.
α ₂ [%] = (t ₁ −t ₂ ) / t ₀ * 100 (2)
The plate thickness before the forced tensile displacement is t ₀ , the plate thickness after the forced tensile displacement is t _1, and the plate thickness after applying the plate thickness direction compressive stress is t ₂ .

(3) A method for producing a press-molding die according to the present invention is a method for producing a press-molding die for press-molding a press-formed product having a cross-sectional hat shape that is curved in the longitudinal direction,
A press molding analysis step for performing a press molding analysis on the press molded product, and calculating a residual tensile stress distribution of the press molded product at a molding bottom dead center in the press molding analysis;
A region specifying step of performing a springback analysis based on the residual tensile stress distribution calculated in the press molding analysis step, and specifying a region where residual tensile stress that causes a shape freezing failure due to the springback exists,
A mold manufacturing process for manufacturing a mold provided with a convex portion at a portion that contacts the area specified in the area specifying process in the die and / or punch ,
The amount of change in the clearance of the mold to be changed by providing the convex portion is obtained in advance by obtaining the relationship between the longitudinal tensile stress to be relaxed and the necessary thickness direction compressive stress, and the necessary thickness direction compressive stress. leave the plate thickness before compulsory tensile displacement obtained relation between sheet thickness reduction rate alpha ₂ from after forced tensile displacement to the reference, characterized that you calculate the amount of change in the clearance (3) A method for manufacturing a press mold.
(Clearance change amount) = (Clear clearance of the part before change) − (Target plate thickness) (3)
However, target plate thickness = plate thickness * {1- (plate thickness reduction rate α ₁ due to longitudinal tensile stress + plate thickness reduction rate α ₂ due to plate direction compressive stress )}
Here, the plate thickness reduction rate α ₁ due to the forced tensile displacement is obtained by the following equation (1).
α ₁ [%] = (t ₀ −t ₁ ) / t ₀ * 100 (1)
Further, the thickness reduction rate α ₂ due to the application of the compressive stress in the thickness direction after the forced tensile displacement is obtained by the following formula (2) using the thickness t ₀ before the forced tensile displacement as a reference.
α ₂ [%] = (t ₁ −t ₂ ) / t ₀ * 100 (2)
The plate thickness before the forced tensile displacement is t ₀ , the plate thickness after the forced tensile displacement is t _1, and the plate thickness after applying the plate thickness direction compressive stress is t ₂ .

  In the present invention, a press molding analysis is performed on a press-molded product having a cross-sectional hat shape that is curved in the longitudinal direction, and the residual tensile stress distribution of the press-molded product at the molding bottom dead center is calculated in the press molding analysis. The springback analysis is performed based on the calculated residual tensile stress distribution, the region where the residual tensile stress that causes the shape freezing failure due to the springback exists is identified, and the gold of the portion that comes into contact with the identified region is identified. Residuals that cause shape defects due to springback without changing the clearance of the mold and performing press molding using the mold with the changed clearance, without requiring a special press mechanism or significant product shape change Tensile stress can be relieved, three-dimensional shape defects such as warpage in the longitudinal direction of press-formed products are suppressed, and dimensional accuracy is excellent It is possible to obtain a press-molded article.

It is explanatory drawing of the press molding method of the press molded product which concerns on embodiment of this invention. It is a figure explaining the shape of the press-formed product of a cross-sectional hat shape. It is a figure explaining the residual stress which exists in the press-formed product of a cross-sectional hat shape. It is a figure explaining the shape defect by the springback of the press-formed product of a cross-sectional hat shape. It is explanatory drawing of the model material made into the analysis object by the numerical analysis in the process which led to this invention. It is a graph showing the result obtained by the numerical analysis in the process which led to this invention (the 1). It is a graph showing the result obtained by the numerical analysis in the process which led to this invention (the 2). It is a graph showing the result obtained by the numerical analysis in the process which led to this invention (the 3). It is explanatory drawing of the press molded product made into the analysis object of numerical analysis in Example 1. FIG. It is a figure explaining the area | region which applies a compressive stress in Example 1 (the 1). It is a figure explaining the area | region which applies a compressive stress in Example 1 (the 2). It is explanatory drawing of the press molded product made into the analysis object in the numerical analysis of Example 2. FIG. It is a figure showing the convex part provided in the punch in the numerical analysis of Example 2. FIG.

  Before explaining the details of the press molding method and the manufacturing method of the press molding die according to the embodiment of the present invention, the background to the present invention will be described first.

<Background to the Present Invention>
In the process of producing a press-formed product 1 (see FIG. 2), which is curved in the longitudinal direction and is convex in the upper part in the center as viewed from the side, by press molding, as shown in FIG. A strong tensile stress is generated in a part of the region 7 of the top plate part 3 in contact with and a compressive stress is generated in a part of the region 9 of the flange part 5. Thereafter, when the press-formed product 1 is released from the mold, the press-formed product 1 is warped and twisted in the longitudinal direction as shown in FIG.

  The cause of the warp is due to the tensile stress remaining in the region 7 of the top plate portion 3 and the compressive stress remaining in the region 9 of the flange portion 5. The cause of the twist is that the top plate portion 3 of the press-formed product 1 is deformed so that the longitudinal line length is shortened by the spring back, and the flange portion 5 is deformed so that the longitudinal line length is long, This is due to a slight line length difference occurring in the top plate portion 3 and the flange portion 5 or the left and right flange portions 5.

  Therefore, if at least the tensile stress generated in the top plate portion 3 with which the punch bottom abuts can be relaxed at the molding bottom dead center in the press molding process, the tensile stress remaining in the press-molded product 1 after release is relaxed, As a result, it is considered that both warpage and twisting are reduced.

  The inventors diligently studied a solution for relaxing the tensile stress generated in the top plate portion 3 of the press-formed product 1 at the bottom dead center of the press-forming process. As a result, the tensile stress of the top plate portion 3 is relieved by compressing the top plate portion 3 where the tensile stress is generated with a sufficient load in the plate thickness direction and causing plastic deformation in the plate thickness direction. I found it.

  However, since a large load is required to reduce the plate thickness by plastic deformation due to compression in the plate thickness direction, it is extremely difficult to sufficiently compress the entire press-formed product 1. Even when the region to be compressed is limited, in addition to the load required for normal press forming, a large load of several hundred kN to several thousand kN is required for compression in the plate thickness direction.

  Therefore, it is necessary to reduce the thickness of only the portion effective for the relaxation of the tensile stress, and for that purpose, there is a region in which residual tensile stress exists that causes a shape freeze defect in the press-formed product 1 after press forming. It is considered effective to relieve the residual tensile stress by specifying the above and applying compressive stress to the region. Furthermore, it is important to correctly grasp the relationship between the residual tensile stress and the compressive stress required for relaxation of the residual tensile stress.

  The present inventors have further studied and developed a method for obtaining a plate thickness direction compressive stress necessary for the relaxation of the tensile stress and a plate thickness reduction amount necessary for generating the plate thickness direction compressive stress by numerical analysis. I found it. Hereinafter, details of the method will be described.

  In the above method, for example, a 1180 MPa class steel plate having a plate width of 5 mm, a length of 10 mm, and a plate thickness of 1.5 mm shown in FIG.

  First, a forced tensile displacement of 0.5 mm is given as an example in the longitudinal direction of the model material 11. Assuming that the tensile strain of 5% is uniformly applied to the model material 11 in the longitudinal direction by the forced tensile displacement, the tensile stress at this time corresponds to about 1250 MPa. In this state, the longitudinal displacement of the model material 11 is constrained, and numerical analysis is performed when a compressive load is applied in the thickness direction. In the numerical analysis, the compression load in the plate thickness direction was changed from 10 kN to 100 kN, and the relationship between the compression load and the longitudinal tensile stress, the plate thickness direction compression stress, and the plate thickness reduction rate in the model material 11 was examined.

  FIG. 6 shows the relationship between the compressive load in the plate thickness direction and the longitudinal tensile stress, and the relationship between the compressive load in the plate thickness direction and the compressive stress in the plate thickness direction. As the compressive load in the plate thickness direction increases, the compressive stress in the plate thickness direction increases, while the tensile stress in the longitudinal direction decreases. When the compressive load in the plate thickness direction is about 72 kN, the longitudinal tensile stress becomes 0 MPa. When the compressive load in the plate thickness direction is further increased, the longitudinal tensile stress has a negative value, that is, a compressive stress is generated in the longitudinal direction.

  This result shows that the longitudinal tensile stress is relaxed by applying a compressive load in the plate thickness direction. Further, the compressive stress in the plate thickness direction when the longitudinal tensile stress was 0 MPa was about 1400 MPa. This indicates that in order to set the longitudinal tensile stress to 0 MPa, it is necessary to apply a thickness direction compressive stress having a value slightly larger than about 1250 MPa, which is the longitudinal tensile stress to be relaxed.

FIG. 7 shows the relationship between the longitudinal tensile stress to be relaxed and the necessary compressive stress in the thickness direction. FIG. 8 shows the relationship between the necessary plate thickness direction compressive stress and the plate thickness reduction rate α ₂ after forced tensile displacement based on the plate thickness before forced tensile displacement. That is, assuming that the thickness of the model material 11 before the forced tensile displacement is t ₀ , the thickness after the forced tensile displacement is t ₁ , and the thickness after applying the compressive stress in the thickness direction is t ₂ , the forced tensile displacement is caused. The plate thickness reduction rate α ₁ is obtained by the following formula (1).
α ₁ [%] = (t ₀ −t ₁ ) / t ₀ * 100 (1)
Further, the thickness reduction rate α ₂ due to the application of the compressive stress in the thickness direction after the forced tensile displacement is obtained by the following formula (2) with the thickness t ₀ before the forced tensile displacement as a reference.
α ₂ [%] = (t ₁ −t ₂ ) / t ₀ * 100 (2)

The necessary plate thickness direction compressive stress decreases with the increase in the longitudinal tensile stress to be relaxed, and the plate thickness reduction rate α ₂ after forced tensile displacement increases due to the increase in the required plate thickness direction compressive stress. From this result, the thickness reduction amount after forced tensile displacement necessary to obtain the thickness direction compressive stress corresponding to the target longitudinal tensile stress to be relaxed (= thickness t ₀ before forced displacement t * thickness It is possible to calculate the thickness reduction rate α ₂ ) due to the application of directional compressive stress.
For example, when it is desired to relax the longitudinal tensile stress from 1250 MPa to 250 MPa up to 1000 MPa, it can be read from FIG. 7 that the plate thickness direction compressive stress needs about 1150 MPa in order to relax the tensile stress by 1000 MPa.

Further, as shown in FIG. 8, in order to apply the compressive stress to the material, the compressive stress is applied to the model material 11 so that 3.4% is obtained as the plate thickness reduction rate α ₂ when the compressive stress of about 1150 MPa is applied. Good. Alternatively, the plate thickness reduction rate α ₁ = 2.3% due to the forced tensile displacement and the plate thickness reduction rate α ₂ = 3.4 due to the thickness direction compression stress with respect to the plate thickness 1.5 mm before the forced tensile displacement is applied to the model material 11. It is only necessary to apply a compressive stress that gives a plate thickness reduction rate of 5.7% plus%. To that end, the mold clearance is changed so that a plate thickness reduction amount of 1.5 mm × 0.057 = 0.0855 mm is obtained. A compressive stress may be applied to the model material 11.

  By the above method, the plate thickness reduction amount necessary for relieving the residual tensile stress inside the model material 11 was clarified. However, as described above, a very large compressive load is required to reduce the plate thickness by plastic deformation due to compression in the plate thickness direction. Therefore, by making the area where the compressive load is applied only to the portion where the residual tensile stress that causes the shape freezing failure due to the spring back exists, the press molding apparatus is not provided with a special mechanism for obtaining an extremely large compressive load. It is possible to reduce the thickness of the part.

  The press molding method and the method for manufacturing a press molding die according to the embodiment of the present invention have been completed by the above process, and the specific method will be described below.

[Embodiment 1]
The press molding method according to Embodiment 1 of the present invention occurs when press-molding a press-molded product 1 having a cross-sectional hat shape that is gently curved in the longitudinal direction shown in FIG. This is a press molding method that suppresses shape freezing failure due to springback.

The press molding method includes a press molding analysis step (S1) for obtaining a residual stress distribution before release of the press molded product by press molding analysis by a finite element method at a molding bottom dead center in the press molding process of the press molded product. And by changing the residual stress distribution in a certain region with respect to the press-molded product before mold release, and determining the amount of spring back generated in the press-molded product after mold release based on the changed residual stress distribution A region specifying step (S3) for specifying a region in which the residual tensile stress should be relieved, and a clearance change for changing a clearance of a portion of the mold that contacts the region specified in the region specifying step based on the residual tensile stress A step (S5) and a press forming step of performing press forming using the mold whose clearance has been changed in the clearance changing step ( 7) and in which with a.
Hereafter, each said process is demonstrated in detail based on FIG.

<Press molding analysis process>
The press forming analysis process is performed using an analysis system based on the finite element method (FEM), and a plate-like member model is installed on a press forming die model composed of a die, a punch and a blank holder, and under the given analysis conditions. A numerical analysis of the process of manufacturing the predetermined press-formed product 1 is performed (S1). By the press forming analysis step, the shape and residual stress distribution of the press-formed product 1 before release at the bottom dead center of the press forming process are calculated.

<Area identification process>
The region specifying step is a step of performing processing similar to the processing disclosed in, for example, Japanese Patent No. 4894294 (S3).
In this process, the residual stress distribution for an arbitrary region from the reference stress state calculated by the press molding analysis step is changed to 0, and the springback analysis is performed based on the data of the changed residual stress distribution. Calculate the shape of the press-molded product after mold release, the residual stress distribution and the strain distribution data, and based on the shape of the press-molded product after mold release and the shape of the press-molded product before mold release, Processing to calculate a defined amount related to springback after changing the residual stress distribution to 0, and how the defined amount related to the springback is before and after the change of the residual stress distribution in the region. And processing for calculating whether to change.

  By performing these treatments, it is possible to determine which region of the residual tensile stress of the press-formed product before release from the bottom dead center of the press molding has a large effect on the springback. It is possible to specify a region where there is a residual tensile stress that causes Here, as the certain defined amount, for example, an arbitrary height difference of the press-formed product before and after the spring back can be set as the amount of change.

<Clearance change process>
The clearance changing step is a step of changing the clearance of the part that contacts the region specified in the region specifying step in the mold (S5). The clearance is changed based on a clearance change amount, and the clearance change amount includes a residual tensile stress existing in the region, a compressive stress required to relieve the residual tensile stress, and a plate required for applying the compressive stress. It is obtained from the relationship of thickness reduction rate.

First, the value of the plate thickness direction compressive stress required to relieve the tensile stress in the longitudinal direction of the press-formed product remaining in the region is obtained from FIG. 7, and further the plate based on the obtained compressive stress value is obtained based on FIG. Read the thickness reduction rate α ₂ . In determining the clearance change amount, it is necessary to consider the gap due to the contact condition of the mold in addition to the plate thickness reduction rate α ₁ due to the longitudinal tensile stress and the plate thickness reduction rate α _{2 due} to the application of the plate thickness direction compressive stress. There is.

From the above, the clearance change amount of the mold can be calculated by the following equation.
(Clearance change amount) = (Clearance of the part concerned) − (Target plate thickness) (3)
However, target plate thickness = plate thickness * {1- (plate thickness reduction rate α ₁ due to longitudinal tensile stress + plate thickness reduction rate α ₂ due to plate direction compressive stress)}
The position and range where the clearance of the mold is changed is a portion that contacts the area specified in the area specifying step in the mold.

  As a method for changing the clearance of the mold, (a) a plate-like member having a plate thickness equal to the clearance change amount is attached to a part of the die and / or punch that contacts the area specified in the area specifying step. (B) using a mold in which a portion that contacts the region specified in the region specifying step in the die and / or punch is movable, and (c) specified in the region specifying step in the die and / or punch. There is a method of newly manufacturing a mold provided with a convex portion at a site that contacts the region, and any method may be used.

<Press molding process>
This is a step of performing press molding using the mold whose clearance has been changed in the clearance changing step (S7).
By performing press molding using the mold with the changed clearance, a compressive stress is applied to a region where there is a residual tensile stress that causes springback, and the residual tensile stress is relaxed by the compressive stress. Is done. As a result, it is possible to manufacture a press-formed product in which defective shape freezing due to the springback is suppressed in the press forming step.

  In the first embodiment, the press-formed product 1 is produced by draw molding. However, the present invention is not limited to draw molding, but is also effective in foam molding.

  Further, in the first embodiment of the present invention, as described in the region specifying step, the stress of a part of the region is set to 0, and by comparing the press molded product shape after the spring back before and after the stress change, Although the region contributing to the shape freezing failure is specified, the region may be specified by the following methods (a) to (c).

(A) Stress that acts in the plate thickness direction when the plate model is pressed to the bottom dead center in the process of forming the plate model with the mold model by computer numerical analysis based on the elasto-plastic finite element method Calculate the distribution. Next, the calculated stress distribution is calculated by inverting the calculated stress distribution and assuming that the inverted stress acts on the plate material model. By comparing the pressed plate model shape before reversing the stress distribution with the plate model shape after reversing the stress distribution and restoring the elasticity, the region where residual stress that causes shape failure due to springback exists is identified To do.

(B) The residual stress distribution of the press-formed product before release is calculated by press forming analysis based on the finite element method. Based on the residual stress distribution, the spring-back analysis after the release of the press-formed product is performed, and the shape data and the residual stress distribution of the press-formed product after the release are calculated. Next, the difference between the residual stress distributions of the press-formed product before release and after release is obtained, and the difference is calculated as the springback effective stress distribution. A spring back effective stress distribution obtained by changing or removing the spring back effective stress distribution in an arbitrary region of the press molded product is given, and shape data of the press molded product is calculated based on the spring back effective stress distribution. By comparing the shape data after demolding with the shape data calculated based on the springback effective stress distribution, an area where residual stress that causes a shape defect due to springback exists is specified.

(C) The press molding analysis is performed under the press molding conditions before the springback countermeasure, and the shape of the press molded product and the residual stress distribution before the mold release are calculated. A springback analysis is performed based on the residual stress distribution, a residual stress distribution of the press-formed product after mold release is calculated, and a difference between the residual stress distribution before and after the mold release is obtained. Next, press molding analysis is performed under the press molding conditions after measures against springback, and the shape and residual stress distribution of the press molded product before mold release are calculated. A springback analysis is performed based on the residual stress distribution, a residual stress distribution of the press-formed product after mold release is calculated, and a difference between the residual stress distribution before and after the mold release is obtained. A change amount of the difference between the residual stress distributions before and after the springback countermeasure is calculated, and a region where residual stress that causes a shape defect due to the springback exists is specified from the change amount.

  As described above, in the first embodiment, a region in which a residual tensile stress that causes a shape defect due to springback in a press-formed product exists is specified, and the region is subjected to the residual tensile stress in order to relax the residual tensile stress. By applying compressive stress, it is possible to use a springback without the need for a special press device to apply a large load of several hundred kN to several thousand kN in addition to the press load required to produce a press-formed product. It is possible to obtain a press-molded product with excellent dimensional accuracy by suppressing three-dimensional shape freezing failure.

[Embodiment 2]
In the press molding method described in the first embodiment, for the purpose of relaxing the residual tensile stress by applying a compressive stress to a region where there is a residual tensile stress that becomes defective due to springback, Press molding is performed using a mold in which the clearance of a portion corresponding to the region in the mold is changed.

Embodiment 2 according to the present invention is a method of manufacturing a press molding die for manufacturing a die having a convex portion provided at a portion in contact with the region in the die in order to change the clearance, and A press forming analysis process for performing a press forming analysis on a press-formed product having a hat-shaped cross section that curves in a direction, and calculating a residual tensile stress distribution of the press-formed product at the bottom dead center in the press forming analysis; and A region specifying step of performing a springback analysis based on the residual tensile stress distribution calculated in the press forming analysis step and specifying a region in which the residual tensile stress that causes a shape freezing failure due to the springback exists; Or a mold manufacturing process for manufacturing a mold in which a convex portion is provided at a portion that contacts the area specified in the area specifying process in the punch.
Since the press molding analysis process and the region specifying process are the same as those described in the first embodiment, the mold manufacturing process will be described below.

<Mold manufacturing process>
In the mold manufacturing process, a mold provided with a convex portion at a portion that comes into contact with an area where residual tensile stress exists that causes the shape freezing failure due to the spring back specified in the area specifying process in the die and / or punch is provided. It is a manufacturing process. The height of the convex portion is obtained from the relationship between the residual tensile stress existing in the region, the thickness direction compressive stress required to relieve the residual tensile stress, and the thickness reduction rate required to apply the compressive stress. It is done.

First, the value of the plate thickness direction compressive stress required to relieve the longitudinal tensile stress remaining in the region is obtained from FIG. 7, and the plate thickness reduction rate α is obtained from the obtained compressive stress value based on FIG. Read ₂ . In determining the height of the protrusions provided on the die and / or punch of the mold, in addition to the plate thickness reduction rate α ₁ due to the longitudinal tensile stress and the plate thickness reduction rate α _{2 due} to the application of the plate thickness direction compressive stress, It is necessary to consider the gap due to the contact condition of the mold.

  From the above, the height of the convex portion of the mold at the portion that is in contact with the region is calculated by Equation (3). Furthermore, the position where the convex portion is provided is a portion that contacts the die and / or the area specified in the area specifying step.

  By manufacturing a press molding die based on the height and position data of the convex portion thus determined and the shape data of the mold before providing the convex portion, the press molding die It becomes possible to suppress the shape freezing failure due to the spring back of the press-formed product manufactured using the mold.

  In Embodiment 2 of the present invention, in order to change the clearance of the portion that contacts the region causing the shape freezing failure due to the spring back specified in the region specifying step in the die, the die and / or punch is used. It is assumed that a convex part is provided. However, when press molding is performed using a mold having a convex part on the punch side, a concave part is formed on the lower surface (inner surface) of the top plate part of the press-formed product having a hat-shaped cross section. Since it is formed, it is more preferable from the viewpoint of the appearance of the press-formed product.

  As Example 1, the present invention is applied to press molding for manufacturing a hat-shaped press molded product 21 curved in the longitudinal direction shown in FIG. The effect of the present invention was verified by numerical analysis.

  The material used is a 1180 MPa class steel plate with a thickness of 1.2 mm, press forming analysis for manufacturing the press-formed product 21, spring back analysis based on the shape and stress distribution of the press-formed product 21 at the bottom dead center, and spring The back amount was calculated. As the evaluation value of the spring back amount, the center in the longitudinal direction of the press-formed product in FIG. 9 is fixed, and the displacement in the z direction before and after the spring back at the two evaluation points of the flange portion indicated by ● is measured. The average value was used.

  The molding method was draw molding, and the press load under the condition that the mold clearance was not changed was 800 kN. In order to consider the stress in the thickness direction, a solid model was used for the blank. The material model was an isotropic hardening model.

As a result of numerical analysis under the above conditions, the longitudinal tensile stress in the press-formed product remaining in a partial region 25 of the top plate portion 23 shown in FIG. A result of 600 MPa was obtained. The plate thickness direction compressive stress required to relieve the longitudinal tensile stress is about 800 MPa from FIG. 7, and the plate thickness reduction rate α ₂ for obtaining the necessary plate direction compressive stress is as shown in FIG. The sheet thickness reduction rate α ₁ = 2.3% due to the longitudinal tensile stress during press forming was added, and the total sheet thickness reduction rate was 4.4%. The mold clearance at the bottom dead center was 1.243 mm.

The above values were substituted into formula (3), and an appropriate mold clearance change amount was calculated as follows.
1.243mm- (1.2mm × (1-0.044)) = 0.096mm (4)
Using the value of 0.096mm or more obtained by Equation (4) as the amount of change in the die clearance, press molding analysis was performed using the mold with the changed clearance, and further the spring back analysis was performed to determine the amount of spring back after mold release. Asked.

  Table 1 shows the position and change amount of the mold clearance change, the press load when press molding is performed with the mold subjected to the clearance change, and the spring back amount obtained by the spring back analysis.

  Condition 1 is a conventional example in which the mold clearance is not changed. The amount of spring back generated after the release of the press-formed product 21 is 5.6 mm. The spring back amount is used as a reference for comparison, and the effect of reducing the spring back depending on the position and amount of change in the mold clearance will be examined below.

  Condition 2 is that the clearance of the part contacting the region 25 (FIG. 10) specified in the region specifying step in the mold is changed, but the clearance change amount of the part is given by the value 0.096 given by the equation (4). This is a comparative example with a value smaller than mm. In condition 2, the springback amount is 4.3 mm, and a sufficient reduction in the springback amount is not obtained.

  Conditions 3 to 5 change the clearance of the part that contacts the region 25 specified in the region specifying step in the mold, and the clearance change amount of the part is larger than the value 0.096 mm given by the equation (4). It is a set example of the present invention. In condition 3, the springback amount is reduced to 1.9 mm compared to the conventional example, and in conditions 4 and 5 where the clearance change amount is increased, the springback amount is further reduced.

  Condition 6 and condition 7, as shown in FIG. 11 (a), change the clearance of the part that contacts the region 27 narrower than the region specified in the region specifying step in the mold, and the mold clearance of the part This is a comparative example in which the amount of change is set to 0.10 mm and 0.15 mm which are equal to or greater than the value obtained by the equation (4). The spring back is reduced by changing the mold clearance in the region 27 and applying a compressive load. However, when the condition 3 and the condition 6, and the condition 4 and the condition 7 with the same clearance change amount are compared, the effect of reducing the springback in the condition 6 and the condition 7 is smaller than that in the condition 3 and the condition 4, which is an example of the present invention. .

  Conditions 8 and 9 are different from the areas specified in the area specifying step in the mold as shown in FIG. 11B, and are areas near the longitudinal ends of the top plate portion 23 of the press-formed product 21. 29 is a comparative example in which the clearance of the part abutting on 29 is changed, and the amount of change in the clearance of the mold in the part is set to 0.10 mm and 0.15 mm which are equal to or larger than the value obtained by Expression (4). Since the longitudinal tensile stress in region 29 is small compared to the longitudinal tensile stress in region 25, region 29 is not suitable for reducing springback by applying a compressive load. For this reason, both the conditions 8 and 9 have a slight effect of reducing the springback amount.

  Condition 10 is a comparative example in which the clearance of the entire surface of the top plate 23 is changed and the clearance change amount of the mold is set to 0.10 mm which is equal to or larger than the value obtained by the equation (4). Because it exceeded the specifications of the molding machine, press molding had to be stopped halfway.

  As described above, in a hat-shaped press-molded product that is press-molded by draw molding and curved in the longitudinal direction, a region where residual tensile stress that causes shape freezing failure due to springback exists is specified, and compressive stress is applied to the region. It was confirmed that the residual tensile stress can be relaxed by reducing the clearance of the mold for imparting, and it is possible to obtain a press-molded product in which warpage in the longitudinal direction of the press-molded product is suppressed.

  As Example 2, the present invention is applied to a press forming method for press-forming a press-formed product 31 simulating a front pillar upper that is used as a vehicle body part and has a complicated shape as shown in FIG. The effect of the method was verified.

  In order to shorten the calculation time required for the press forming analysis and the spring back analysis, the size of the press-formed product 31 is set to 1/3 of the actual product size. The press-formed product 31 is manufactured by foam molding. The material was a 1180 MPa class steel plate with a thickness of 1.4 mm, and the springback amount was calculated. The springback evaluation amount was the displacement in the z direction before and after the springback at the evaluation point indicated by the mark ● at the left end portion with the right end portion of the press-formed product of FIG. 12 fixed. In the press-formed product 31, the evaluation point is a position where the largest spring back is generated.

  Of the tensile stress in the longitudinal direction of the top plate portion 33 on the top surface of the press-formed product 31, a region that greatly affects the springback is identified by numerical analysis, and as shown in FIG. The height of the convex part 43 provided in the site | part which contact | connects, ie, the clearance change amount of the said site | part, was computed as follows.

First, from the press molding analysis, the tensile stress in the region was about 1000 MPa. Therefore, as shown in FIG. 7, the plate thickness direction compressive stress required to relieve the tensile stress is about 1150 MPa, and the plate thickness from the case where the convex portion 43 is not provided on the punch 41 required to obtain the compressive stress is shown. The reduction rate α ₂ is 3.4% from FIG. 8, and the total thickness reduction rate is estimated to be 5.7% by adding the thickness reduction rate α ₁ = 2.3% when the punch 41 is not provided with a convex portion. . Further, the clearance of the mold at the bottom dead center was 1.447 mm.

By substituting the above result into the equation (4), an appropriate mold clearance change amount was obtained by the following equation.
1.447mm-1.4mm × (1-0.057) = 0.127mm (5)
The height of the convex portion 43 provided on the punch 41 is determined by setting the value of 0.127 mm or more obtained by the equation (5) as the clearance change amount of the mold. Subsequently, press molding analysis was performed using a mold in which the protrusions 43 were provided on the punch 41, and the amount of spring back after release from the mold was determined.
The results of Example 2 are shown in Table 2.

Condition 20 is a conventional example in which the mold clearance is not changed, and the effect of reducing the spring back amount by the clearance change amount is examined on the basis of the spring back amount in this condition.
Condition 21 and condition 22 are comparative examples in which the clearance change amount is smaller than the value given by equation (5). Since the plate thickness direction compressive stress obtained by the clearance change amount is not sufficient, the longitudinal tensile stress is not relaxed, and the effect of reducing the spring back amount is slight.
Condition 23 is an example of the present invention in which the clearance change amount is larger than the value given by Expression (5). Since the clearance of the mold is changed based on the clearance change amount that can obtain a compressive stress in the plate thickness direction sufficient to relieve the longitudinal tensile stress, the amount of spring back can be greatly reduced.

  As described above, even in the case of a complex-shaped press-molded product that is press-molded by form molding and simulates an actual part of a vehicle body, as in the first embodiment, a spring is not required without requiring a special press mechanism or significant product shape change. To obtain a press-molded product with excellent dimensional accuracy that can relieve residual tensile stress, which causes shape freeze failure due to back, and suppresses three-dimensional shape freeze failure such as warpage in the longitudinal direction of the press-formed product. Is possible.

DESCRIPTION OF SYMBOLS 1 Press molded product 3 Top plate part 5 Flange part 7 Area | region where tensile stress remains 9 Area | region where compressive stress remains 11 Model material 21 Press molded article (Example 1)
23 Top plate (Example 1)
25 areas (conditions 2-5)
27 area (conditions 6 and 7)
29 regions (conditions 8 and 9)
31 Press-formed product (Example 2)
33 Top plate (Example 2)
41 Punch 43 Projection

Claims (2)

A press molding method for producing a press molded product having a cross-sectional hat shape that is curved in the longitudinal direction,
A press molding analysis step for performing a press molding analysis on the press molded product, and calculating a residual tensile stress distribution of the press molded product at a molding bottom dead center in the press molding analysis;
A region specifying step of performing a springback analysis based on the residual tensile stress distribution calculated in the press molding analysis step, and specifying a region where residual tensile stress that causes a shape freezing failure due to the springback exists,
A clearance changing step of changing the clearance of the mold of the part that comes into contact with the region specified in the region specifying step;
A press molding step of performing press molding using a mold in which the clearance of the portion that abuts the region in the clearance changing step is changed , and
In the clearance changing step, the relationship between the longitudinal tensile stress to be relaxed and the necessary compressive stress in the plate thickness direction is obtained in advance, and the necessary plate thickness direction compressive stress and the plate thickness before forced tensile displacement are used as a reference. force tensile thickness reduction rate after displacement to previously obtain a relation between alpha _2, press molding method, wherein that you calculate the amount of change in the clearance (3) is.
(Clearance change amount) = (Clear clearance of the part before change) − (Target plate thickness) (3)
However, target plate thickness = plate thickness * {1- (plate thickness reduction rate α ₁ due to longitudinal tensile stress + plate thickness reduction rate α ₂ due to plate direction compressive stress )}
Here, the plate thickness reduction rate α ₁ due to the forced tensile displacement is obtained by the following equation (1).
α ₁ [%] = (t ₀ −t ₁ ) / t ₀ * 100 (1)
Further, the thickness reduction rate α ₂ due to the application of the compressive stress in the thickness direction after the forced tensile displacement is obtained by the following formula (2) using the thickness t ₀ before the forced tensile displacement as a reference.
α ₂ [%] = (t ₁ −t ₂ ) / t ₀ * 100 (2)
The plate thickness before the forced tensile displacement is t ₀ , the plate thickness after the forced tensile displacement is t _1, and the plate thickness after applying the plate thickness direction compressive stress is t ₂ . A method for manufacturing a press molding die for press molding a press molded product having a cross-sectional hat shape that is curved in the longitudinal direction,
A press molding analysis step for performing a press molding analysis on the press molded product, and calculating a residual tensile stress distribution of the press molded product at a molding bottom dead center in the press molding analysis;
A region specifying step of performing a springback analysis based on the residual tensile stress distribution calculated in the press molding analysis step, and specifying a region where residual tensile stress that causes a shape freezing failure due to the springback exists,
A mold manufacturing process for manufacturing a mold provided with a convex portion at a portion that contacts the area specified in the area specifying process in the die and / or punch ,
The amount of change in the clearance of the mold to be changed by providing the convex portion is obtained in advance by obtaining the relationship between the longitudinal tensile stress to be relaxed and the necessary thickness direction compressive stress, and the necessary thickness direction compressive stress. leave the plate thickness before compulsory tensile displacement obtained relation between sheet thickness reduction rate alpha ₂ from after forced tensile displacement to the reference, characterized that you calculate the amount of change in the clearance (3) A method for manufacturing a press mold.
(Clearance change amount) = (Clear clearance of the part before change) − (Target plate thickness) (3)
However, target plate thickness = plate thickness * {1- (plate thickness reduction rate α ₁ due to longitudinal tensile stress + plate thickness reduction rate α ₂ due to plate direction compressive stress )}
Here, the plate thickness reduction rate α ₁ due to the forced tensile displacement is obtained by the following equation (1).
α ₁ [%] = (t ₀ −t ₁ ) / t ₀ * 100 (1)
Further, the thickness reduction rate α ₂ due to the application of the compressive stress in the thickness direction after the forced tensile displacement is obtained by the following formula (2) using the thickness t ₀ before the forced tensile displacement as a reference.
α ₂ [%] = (t ₁ −t ₂ ) / t ₀ * 100 (2)
The plate thickness before the forced tensile displacement is t ₀ , the plate thickness after the forced tensile displacement is t _1, and the plate thickness after applying the plate thickness direction compressive stress is t ₂ .