Manufacturing Method of Press Formed Part

The present invention relates to a manufacturing method of a press formed part, and more specifically relates to a manufacturing method of a press formed part having a top plate portion and vertical wall portions continuous with the top plate portion via ridge portions, and including an outward flange portion continuously formed at end portions of the top plate portion, the ridge portions, and the vertical wall portions.

As a vehicle body collision safety is being improved due to tightening of a vehicle collision safety standard, it is also necessary to reduce weight of the vehicle body in order to improve fuel consumption and to make a shift to electric vehicles according to carbon dioxide emission control. In order to achieve both the collision safety improvement and the weight reduction of the vehicle body, application of a high strength steel sheet (also referred to as high tensile strength steel material) having a tensile strength of 590 MPa-class or more to a vehicle body structural part is in progress.

For example, as illustrated in, as an automobile part, there is a press formed parthaving a top plate portion, vertical wall portions, and ridge portionsthat connect the top plate portionand the vertical wall portions, and including an outward flange portionformed continuously at end portions of the top plate portion, the ridge portions, and the vertical wall portions, such as a floor cross member.

In a case where such a press formed partis press formed, a portion of the outward flange portionwhich portion is continuous with the ridge portion(hereinafter, referred to as a “corner flange portion”) becomes a stretch flange deformation, and a fracture is likely to be generated at a tip portion of the corner flange portion

Conventionally, with respect to a manufacturing method of a press formed part having an outward flange portion continuously formed at the end portions of the vertical wall portions, Patent Literature 1 discloses a manufacturing method of a saddle-shaped press formed part. In Patent Literature 1, a top plate forming portion of a blank is curved, first force is applied from an inner surface side to an outer surface side of the blank, and combined force of second force in a direction facing each other and third force in a direction opposite to the first force is applied to each of the outer surface side of vertical wall forming portions, and press forming is performed. The method disclosed in Patent Literature 1 is a method of simultaneously forming a top plate portion, vertical wall portions, and an outward flange portion.

In addition, Patent Literature 2 discloses, as another press forming method, a press forming method of bending and raising an outward flange portion after forming a metal sheet into a U-shaped cross section. In the method disclosed in Patent Literature 2, when the outward flange portion is bent and raised, forming of the flange portion is started from a top plate portion and vertical wall portions, whereby it is assumed that a strain of the outward flange portion can be dispersed to a side of the top plate portion and sides of the vertical wall portions.

Patent Literature 3 discloses a press forming method of forming an outward flange by a shearing step of forming a step portion on a workpiece having a top plate, a vertical wall, and a flange, and a cutting step of cutting an unnecessary portion near the step portion.

In the method disclosed in Patent Literature 1, a structure of a die of press forming is considerably complicated, and a manufacturing cost of a die of press forming is high. In addition, since the combined force of the third force opposite to the first force is applied, it is difficult to perform press forming while pressing the blank sufficiently and uniformly, and it is also difficult to acquire a press formed part with high dimensional accuracy.

In this regard, as disclosed in Patent Literature 2, in the press forming method of bending and raising the outward flange portion after forming the metal sheet into the U-shaped cross section, a die of press forming is simple and dimensional accuracy can be improved. However, in press forming of a high tensile strength steel material having further increased strength, there is a problem that buckling (wrinkles and fold) is likely to be generated at a connection portion between a ridge portion and a corner flange portion of the outward flange portion (hereinafter, simply referred to as a “connecting portion of a corner flange portion”).

In addition, the method disclosed in Patent Literature 3 has a problem that a configuration of a die of press forming is complicated and the cutting step is essential.

The present invention has been made in view of the above problems, and an object thereof is to provide a manufacturing method of a press formed part which method is capable of stably manufacturing a press formed part with high dimensional accuracy without causing buckling in a connection portion of a corner flange portion without using a die of press forming having a complicated structure when manufacturing a press formed part having a top plate portion and vertical wall portions continuous to the top plate portion via ridge portions, and including an outward flange portion continuously formed at end portions of the top plate portion, the ridge portions, and the vertical wall portions.

In order to solve the above-described problems and achieve the object, (1) a manufacturing method of a press formed part, according to the present invention, is the method in which the press formed part includes at least a main body portion, which has a top plate portion and vertical wall portions formed via ridge portions, and an outward flange portion formed continuously with the top plate portion, the ridge portions, and the vertical wall portions at an end portion of the main body portion. The method includes: an intermediate forming step of forming an intermediate formed part having the top plate portion and the vertical wall portions formed via the ridge portions, and including a step-shaped portion bulging outward at a root portion of an outward flange corresponding portion which is to be formed into the outward flange portion; and a target shape forming step of forming the outward flange portion by bending and raising the outward flange corresponding portion of the intermediate formed part outward, and acquiring a target shape.

(2) Moreover, in the intermediate forming step according to above (1), the top plate portion, the ridge portions, the vertical wall portions, and the step-shaped portion may be press formed in one step.

(3) Moreover, in the intermediate forming step according to above (1), the step-shaped portion may be press formed after the top plate portion, the ridge portions, and the vertical wall portions are press formed.

A manufacturing method of a press formed part according to the present invention has an effect of being capable of stably manufacturing a press formed part with high dimensional accuracy without generating buckling at a connection portion of a corner flange portion without using a die of press forming having a complicated structure when manufacturing a press formed part having a top plate portion and vertical wall portions continuous to the top plate portion via ridge portions, and including an outward flange portion continuously formed at end portions of the top plate portion, the ridge portions, and the vertical wall portions.

Hereinafter, an embodiment of a manufacturing method of a press formed part according to the present invention will be described. Note that the present invention is not limited by the present embodiment.

Before a description of the embodiment of the present invention, a shape of a press formed part to be an object of the present invention will be described with reference toillustrating an example thereof. As illustrated in, the object of the present invention is a press formed parthaving an outward flange portion, and a main body portionhaving a top plate portion, vertical wall portions, and ridge portionsconnecting the top plate portionand the vertical wall portionsis included. At an end portion of the main body portion, the press formed parthas the outward flange portionformed continuously with the top plate portion, the ridge portions, and the vertical wall portions. In the example illustrated in, the press formed partincludes a horizontal flange portionat a lower end of each of the vertical wall portions. Note that in the following description, portions of the outward flange portionwhich portions are respectively continuous with the top plate portion, the ridge portions, and the vertical wall portionsare respectively referred to as a top plate flange portion, corner flange portions, and vertical wall flange portions

is a view illustrating a conventional forming method of a press formed part. Conventionally, as illustrated in, an intermediate formed partincluding a top plate portion, ridge portions, vertical wall portions, and a horizontal flange portionat a lower end of each of the vertical wall portionsis first formed. At an end portion of the intermediate formed part, the top plate portion, the ridge portions, and the vertical wall portionsare extended, and the extended portion serves as an outward flange portion(top plate flange portion, corner flange portions, and vertical wall flange portions) in a target shape. Thus, in the present embodiment, the extended portion is referred to as an outward flange corresponding portion. Each portion of the intermediate formed partmay be press formed simultaneously or may be press formed for each portion. Then, as illustrated in, the outward flange corresponding portionin the intermediate formed partis bent and raised outward, whereby the outward flange portionis formed and the press formed parthaving the target shape is acquired.

As a forming method of bending and raising the outward flange corresponding portion, for example, as disclosed in Patent Literature 2, first, bending force is input to a top plate flange corresponding portionand a vertical wall flange corresponding portion. Then, in a forming method of bending and raising the outward flange corresponding portionthereafter, bending force may be input to a corner flange corresponding portion

is a view illustrating a result of FEM analysis in a case where a steel sheet having a sheet thickness of 1.4 mm and tensile strength of 590 MPa-class is used, and a height of the outward flange portionis 18 mm and R at a boundary portion between the top plate portion, the ridge portions, and the vertical wall portionsand the outward flange portionis 3 mm as a target shape. In, a state of the intermediate formed partbefore forming of the outward flange portion() to a state of a bottom dead center of forming after bending and raising of the outward flange portion() are illustrated with intermediate stages.

Tension acts on a tip portion of the outward flange portion, and a sheet thickness decreases. On the other hand, large compressive force acts on a connection portionof the corner flange portion(see). Thus, the sheet thickness increases due to the compressive force, a maximum sheet thickness increase rate at the bottom dead center of forming becomes 47%, and wrinkles and folds are generated (see). Here, the sheet thickness increase rate is acquired by acquisition of a difference between a sheet thickness at a corresponding portion and a sheet thickness of a metal sheet as a blank and calculation of a ratio to the sheet thickness of the metal sheet as the blank.

is a view schematically illustrating a study result acquired by studying of a process in which the wrinkles and folds are generated in conventional press forming in more detail. In, states of a punch, a die, a lower pad, and a metal sheetfrom a middle of forming (38 mmup) () to a vicinity of the bottom dead center of forming (4 mmup) () are illustrated with respect to a cross section cut in a direction of an arrow A along the ridge portionof the press formed partillustrated in. Note that a numerical value such as [30 mmup] is a distance in a press forming direction in which distance a sheet thickness to the bottom dead center of forming of the dieat the position is considered. Thus, for example, the position of the dieat the time of “30 mmup” is +30 mm in an opposite direction of the press forming from a bottom dead center of pressing.

As illustrated in, the outward flange corresponding portionthat is an end portion of the metal sheetstarts coming into contact with the punchat [38 mmup], and the outward flange corresponding portionof the metal sheetis bent and deformed in a direction of the dieat [30 mmup]. Then, at [24 mmup], the outward flange corresponding portionis bent at a position away from the dieand folded in such a manner that the tip portion approaches a side of the die. Furthermore, as the forming proceeds, a fold is generated between the lower padand a forming surface of the punchat [10 mmup], the fold remains as it is even when the forming proceeds to [4 mmup], and wrinkles are generated or the fold remains at the bottom dead center of forming.

A reason for such a formed shape is that the corner flange corresponding portionthat is an extension of the ridge portionis not wound along the diesince the ridge portionpasses therethrough and rigidity against bending is high, and is folded at a position away from the die(see). In addition, in the present example, the vertical wall flange portion(see) is not perpendicular to a surface of the top plate portionof the press formed partand is inclined inward. Thus, a large gap is generated between a flange forming portion of the punchand a flange forming portion of the dieat an initial stage of the press forming.

In considering a measure for preventing the fold of the connection portionof the corner flange corresponding portion, the present inventor has paid attention to a deformation of the metal sheetin [30 mmup] to [24 mmup] of. Such a deformation is generated by a free deformation of the end portion of the metal sheetand generation of the fold. In order to prevent this deformation, it is conceivable to restrain the free deformation of the end portion of the metal sheet. However, since it is necessary to form the corner flange corresponding portioninto the corner flange portion, it is difficult to completely restrain the end portion of the metal sheetwith a die of press forming. Thus, the present inventors have reached an idea that the above-described fold can be prevented when a portion corresponding to a root of the outward flange portionof the metal sheetcan be wound around the diein an initial stage of the press forming. The present invention is based on such an idea, and is specifically described below.

As illustrated in an example in, a step-shaped portionthat is a step bulging outward is provided at a root portion of an outward flange corresponding portion(top plate flange corresponding portion, corner flange corresponding portion, and vertical wall flange corresponding portion) in an intermediate formed part. Then, in such an intermediate formed part, the outward flange corresponding portionis formed into the outward flange portion(top plate flange portion, corner flange portion, and vertical wall flange portion) and a target shape is acquired. In such an intermediate formed part, winding around the diebecomes easier in the initial stage of press forming and the above-described fold can be prevented by provision of the step-shaped portion.

toare views illustrating a result of an FEM analysis of the press forming process of the intermediate formed partto the target shape. In, the intermediate formed part(before flange forming) in which the step-shaped portionis formed is illustrated.toare views illustrating a forming process of the flange portion.

Tension acts on a tip portion of the outward flange portion, and a sheet thickness decreases. In addition, compressive force acts on a connection portionbetween the ridge portionand the corner flange portion(see), and the sheet thickness increases. On the other hand, the maximum sheet thickness increase rate at the bottom dead center of forming is 28%, and is significantly reduced as compared with the maximum sheet thickness increase rate of 47% inillustrating the conventional example, and it can be seen that wrinkles can be prevented. As one of the reasons why the maximum sheet thickness increase rate is reduced, in the present invention, the sheet thickness increase portion of a connection portionbetween the ridge portionand the corner flange corresponding portion(see) is dispersed in [10 mmup] illustrated inas compared with conventional [10 mmup] illustrated in.

is a view schematically illustrating a forming process of forming the intermediate formed partinto a target shape in the press forming of the present invention. In, states of a punch, a die, a lower pad, and a metal sheetin a process from [38 mmup] to [4 mmup] with respect to a cross section cut in a direction of an arrow A along the ridge portionas illustrated in.

As illustrated in, the connection portionbetween the ridge portionand the corner flange corresponding portionis wound around the diefrom [38 mmup] to [30 mmup] since the step-shaped portionis formed. Subsequently, the outward flange corresponding portionstarts coming into contact with the punchat [30 mmup], and the tip of the metal sheetis bent toward a side of the dieat [24 mmup]. At that time, since the step-shaped portionis formed, the tip of the outward flange corresponding portiondoes not approach the side of the die. Thus, the connection portionbetween the ridge portionand the corner flange corresponding portion(see) is not bent at an acute angle. In this regard, at [24 mmup] ofillustrating the conventional example, the tip of the outward flange corresponding portionapproaches the side of the die, and the connection portionbetween the ridge portionand the corner flange portion(see) is bent at an acute angle.

Furthermore, as the press forming proceeds, the outward flange corresponding portionis formed without being folded at a gap between the dieand the punchat [10 mmup]. Then, the forming proceeds to [4 mmup], the outward flange portionis formed at the bottom dead center of forming, and the connection portionbetween the ridge portionand the corner flange portion(see) does not become wrinkles. As described above, in the present embodiment, since the step-shaped portionis provided, the connection portionbetween the ridge portionand the corner flange corresponding portionis wound around the diein the initial stage of the forming. As a result, the connection portion does not enter a gap between the lower padand the punchand is not folded, and generation of the wrinkles is prevented.

Note that the step-shaped portionformed in an intermediate forming step can be press formed simultaneously with the forming of the top plate portion, the vertical wall portions, and the ridge portions. Since forming is performed in one step, manufacturing becomes efficient. Alternatively, the step-shaped portionmay be press formed after the top plate portion, the vertical wall portions, and the ridge portionsare formed.

In addition, in a target shape forming step of forming the outward flange portion, as disclosed in Patent Literature 2, first, bending force is input to the top plate flange corresponding portionand the vertical wall flange corresponding portion. Then, the bending force is preferably input to the corner flange corresponding portion. In such a manner, it is possible to prevent a fracture from being generated at the tip of the corner flange portion

Note that in the above description, although the vertical wall portionsare provided on both sides of the top plate portion, the horizontal flange portionsare included at the lower ends of the vertical wall portions, and an orthogonal cross section of the ridge portionshas a hat shape, the present invention is not limited thereto. That is, the present invention may be applied to one which has no horizontal flange portionand in which an orthogonal cross section of the ridge portionshas a U-shape, or one having a Z-shape or an L-shape acquired by half-splitting of one having a hat-shape or a U-shape in a direction of the ridge portionsin the top plate portion. In addition, in order to form the step-shaped portion bulging outward in the present invention, bending forming or drawing forming is preferable. Since the step-shaped portion is bent and the outward flange portionis formed, the forming is easy even when a width of the outward flange portionis wide. On the other hand, when the step-shaped portion is formed by the shear forming described in Patent literature 3, there is a not preferable case since a width of the flange portion is restricted due to a breaking limit of a material.

In the present example, in order to confirm an effect of the present invention, a press forming analysis of the press formed partthat is illustrated inand is the object of the present invention, and press forming of a real material were performed. As the high tensile strength steel material, a 980 MPa-class steel sheet having a sheet thickness of 1.4 mm was used. The press formed parthas a hat shape in which the top plate portionhas a width of 84 mm, the vertical wall portionshave a height of 100 mm, and the ridge portionshave a length of 180 mm. For the outward flange portion, an outward flange width defined inwas set to 16 mm, and a sheet inner curvature radius R of the root portion was set to 3 mm in the cross section cut in the direction of the arrow A along the ridge portion.

In addition, in the present example, a press forming analysis of forming the outward flange portionwas performed as the conventional example and the invention example for the intermediate formed part(see) that is press formed by utilization of a blank model of the high tensile strength steel material. As the conventional example, as illustrated inand, a press forming analysis of forming the outward flange portionwas performed without provision of the step-shaped portionat the root portion of the corner flange corresponding portion. In addition, as the invention example, as illustrated in, a press forming analysis of forming the outward flange portionwas performed on the intermediate formed partwith the step-shaped portionbeing provided at the root portion of the corner flange corresponding portion. In addition, in the invention example, a height of the step-shaped portion, that is, a difference in height between the step-shaped portionand the top plate portionwas changed from 1.0 mm to 2.5 mm and the press forming analysis was performed, and an influence of the height of the step-shaped portionwas also compared. Note that the sheet thickness increase rate of the connection portionor the connection portionbetween the ridge portionand the corner flange portionwas acquired from the sheet thickness of the connection portion on the basis of a sheet thickness of the blank model. Results of the analysis are illustrated in Table 1.

In a case of No. 1 that is the conventional example in which the step-shaped portionis not provided, the maximum sheet thickness increase rate was as large as 47.6%. In the actual press material, buckling (wrinkles and fold) was generated at the connection portionbetween the ridge portionand the corner flange portion. On the other hand, in each of invention examples No. 2 to No. 5 in which the step-shaped portionwas provided, the maximum sheet thickness increase rate was greatly decreased as the analysis result. In addition, buckling was not generated in actual press materials of these. Note that in a case of No. 4 in which the height of the step-shaped portionwas 2.0 mm, the maximum sheet thickness increase rate was 25%, was decreased the most, and was almost halved compared to 47.6% of the conventional example. As described above, it has been demonstrated that buckling of the connection portionbetween the ridge portionand the corner flange portioncan be prevented by provision of the step-shaped portionin the intermediate formed partof the present invention.

The present invention can provide a manufacturing method of a press formed part which method is capable of stably producing a press formed part with high dimensional accuracy without using a die of press forming having a complicated structure and without generating buckling at a connection portion of a corner flange portion.