Press molding method

Provided is a press molding method in which a plate material is molded into a target molded body including a ridge section, said method including: a first step for molding an intermediate molded body; and a second step for forming the target molded body from the intermediate molded body. The target molded body and the intermediate molded body have coincident regions where the cross-sectional shapes coincide on both sides of an intermediate ridge section, and an intermediate region where the cross-sectional shapes do not coincide. The intermediate region includes: an outer region in which the intermediate molded body protrudes outward of an edge radius with respect to the target molded body; and an inner region in which the intermediate molded body is curved inward of the edge radius with respect to the target molded body.

TECHNICAL FIELD

The present invention relates to a press forming method (press molding method) for forming a plate member into a predetermined shape.

BACKGROUND ART

Outer panels such as automobile hoods, side panels, and door panels or the like are generally produced by press forming of a plate member made of metal. The outer panel is a component part that determines the design of an automobile. For example, a design having a ridgeline section with a small radius of curvature referred to as a character line is used. An advanced press forming technique is required in order to form outer panels of this type.

In DE 102011115219 A1, there is disclosed a press forming method for a plate member in which, using a first die, a ridgeline section (also referred to as an edge portion) is preliminarily formed and sections other than the ridgeline section are formed into a final shape, and next, using a second die, the ridgeline section is formed into a final shape. A radius of curvature (also referred to as an edge radius) of the ridgeline section formed by the first die is on the order of 2 to 10 times the size of that in the final shape, and is formed into a predetermined size by a deep drawing process in the second die.

In JP 5959702 B1, a method is disclosed for producing a formed product having a ridgeline section by a two stage pressing process. JP 5959702 B1 discloses a method of preventing line displacement, by setting an intermediate shape formed by a first stage pressing process so as to project more outwardly than the shape of a target formed body formed by a second stage pressing process.

SUMMARY OF INVENTION

In the press forming method according to the aforementioned DE 102011115219 A1, because the drawing process is performed in two stages, it is difficult for surface distortion to occur. However, the elongation of the ridgeline section becomes large, and if an attempt is made to form the ridgeline section with a small radius of curvature (edge radius), cracks may be disadvantageously generated in the ridgeline section. Further, in the press forming method of the above-described JP 5959702 B1, since the second stage forming is performed in a state in which a tensile force is relieved, a concern arises in that surface distortion may occur.

Therefore, an object of the present invention is to suppress both the occurrence of cracks and the occurrence of surface distortion, in a press forming method for forming a ridgeline section having a small radius of curvature by a two stage drawing process.

One aspect of the present invention is characterized by a press forming method for forming a plate member into a target formed body in which a ridgeline section is included, the press forming method comprising a first step of forming an intermediate formed body including an intermediate ridgeline section with a radius greater than an edge radius of the ridgeline section, and a second step of forming the target formed body from the intermediate formed body, wherein the target formed body and the intermediate formed body each include coincident regions in which cross-sectional shapes thereof coincide on both sides of the intermediate ridgeline section, and an intermediate region in which the cross-sectional shapes thereof do not coincide, and include, in the intermediate region, an outside region in which the intermediate formed body projects more outwardly of the edge radius than the target formed body, and an inside region in which the intermediate formed body is curved more inwardly of the edge radius than the target formed body.

According to the press forming method of the above-described aspect, even in the case that a ridgeline section having a small radius of curvature is formed, it is possible to suppress both the occurrence of cracks and surface distortion.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a preferred embodiment of the present invention will be presented and described in detail with reference to the accompanying drawings. Moreover, in the description given below, a top dead center side in a press stroke direction is referred to as an “upper side,” and a bottom dead center side is referred to as a “lower side.”

The press forming method according to the embodiment is applied, for example, to an automobile hood, side panel, door panel, or the like. In this instance, as shown inFIG.1A, a description will be given of an example of forming a rectangular plate member10. The plate member10is made, for example, from a thin plate metal such as steel or an aluminum alloy having a thickness of 0.3 mm to 3 mm. The outer peripheral portion of the plate member10is formed in a quadrangular shape constituted by a first side10a, a second side10bfacing toward the first side10a, a third side10cextending in a direction intersecting the first side10a, and a fourth side10dfacing toward the third side10c. As shown in a first step ofFIG.2and a second step ofFIG.3, a target formed body12is obtained by press forming the plate member10in two stages.

In the target formed body12, a surface thereof appearing inFIG.1Aserves as a design surface. Such a design surface corresponds to an upper surface shown inFIG.1B. As shown inFIG.1A, on the upper surface of the target formed body12, ridgeline sections14(also referred to as character lines) are formed in edge-like shapes in the vicinity of the third side10cand in the vicinity of the fourth side10d, and extend from the first side10atoward the second side10b. As shown inFIG.1B, a radius of curvature R (also referred to as an edge radius) of each of the ridgeline sections14in a cross-section perpendicular to the direction of the ridgeline section14is formed to be small, i.e., from 2.5 mm to 9 mm, and exhibits a sharp blade-like external appearance.

In the target formed body12, a first slope12ais formed on one side portion of the ridgeline section14, and a second slope12bis formed on the other side portion of the ridgeline section14. In this instance, the slope in closer proximity to a peripheral portion16is referred to as the first slope12a, whereas the slope farther away from the peripheral portion16is referred to as the second slope12b. The first slope12aand the second slope12bmay be positive surfaces that are convex when the design surface is viewed from the front, or may be negative surfaces that are concave when the design surface is viewed from the front. An angle θ (also referred to as a sandwiching angle) formed by the first slope12aand the second slope12bcan be appropriately set within a range of from 1200 to 175°.

The peripheral portion16to be finally cut out is formed in the vicinity of (a region B1of) the target formed body12. The peripheral portion16includes a first peripheral portion16aformed in a region B3retained by respective blank holders24and34(refer toFIGS.2and3), and a second peripheral portion16bthat is formed in a region B2supported by lower dies22and32.

In the first step shown inFIG.2, an intermediate formed body42is press formed from the plate member10using a first upper die26and a first lower die22. As shown in the drawing, a forming die20that is used in the first step is equipped with the blank holder24arranged in the region B3on the lower side of the first peripheral portion16aof the plate member10, the first lower die22arranged in the regions B1and B2on an inner side of the blank holder24, and the first upper die26arranged above the first lower die22and the blank holder24.

In the first step, at first, the plate member10is carried in between the first lower die22and the blank holder24, and the first upper die26. Thereafter, the first upper die26is lowered, and while a tensile force is generated by retaining the first peripheral portion16aby the blank holder24, the plate member10is pressed by the first lower die22and the first upper die26to thereby form the intermediate formed body42. It should be noted that the first step need not necessarily be performed by draw forming.

Thereafter, in the second step shown inFIG.3, the target formed body12is press formed from the intermediate formed body42. As shown in the drawing, a forming die30that is used in the second step is equipped with the blank holder34arranged in the region B3on the lower side of the first peripheral portion16a, a second lower die32arranged in the regions B1and B2on an inner side of the blank holder34, and a second upper die36arranged above the second lower die32and the blank holder34.

In the second step, the intermediate formed body42is carried in between the second lower die32and the blank holder34, and the second upper die36. Thereafter, the second upper die36is lowered, and while a tensile force is generated by retaining the first peripheral portion16aby the blank holder34, the intermediate formed body42is pressed by the second lower die32and the second upper die36to thereby form the target formed body12.

As shown inFIG.4, when the intermediate formed body42and the target formed body12are superimposed on each other, an intermediate region44in which the shapes of the intermediate formed body42and the target formed body12differ from each other is formed in the portion shown by the dashed line. Further, in the peripheral portion16of the intermediate formed body42and the peripheral portion16of the target formed body12as well, the shapes thereof also differ from each other. On the other hand, coincident regions46aand46bin which the shapes of the intermediate formed body42and the target formed body12coincide are formed on both side portions of an intermediate ridgeline section44cof the intermediate formed body42.

Focusing attention on the intermediate region44of the intermediate formed body42, the intermediate region44comprises an inside region44ain which the intermediate formed body42is curved on the lower side (inwardly of the edge radius) in the press stroke direction than the target formed body12, and an outside region44bin which the intermediate formed body42projects more upward (outwardly of the edge radius) in the press stroke direction than the target formed body12. The inside region44ais formed within a range starting from an inflection point48aof the second slope12buntil reaching an intersection48cwith the first slope12a. The outside region44bis formed within a range starting from the intersection48cwith the first slope12auntil reaching an inflection point48e. As shown in the drawing, the length of the outside region44bis formed to be longer than the length of the inside region44a.

A cross-sectional shape of the intermediate region44of the intermediate formed body42is made up from a plurality of arcuate regions. In the illustrated example, a range extending from the inflection point48ato a reference inflection point48bis formed of a first arcuate region having a radius of curvature Ra. Further, a range extending from the reference inflection point48bto an inflection point48dis formed of a second arcuate region having a radius of curvature Rb. Furthermore, a range extending from the inflection point48dto the inflection point48eis formed of a third arcuate region having a radius of curvature Rc and having the center thereof on the outer side. Moreover, the range extending from the inflection point48ato the reference inflection point48b, and the range extending from the reference inflection point48bto the inflection point48dmay be constituted by a plurality of arcuate regions having the same degree of curvature. Further, the third arcuate region need not necessarily be provided, and the second arcuate region may constitute a range extending from the reference inflection point48bto the inflection point48e.

In the intermediate formed body42of the intermediate region44, the radius of curvature Rb of the second arcuate region is greater than the radius of curvature Ra of the first arcuate region. The radius of curvature Ra of the first arcuate region can be, for example, from 15 mm to 30 mm, the radius of curvature Rb of the second arcuate region can be, for example, from 40 mm to 60 mm, and the radius of curvature Rc of the third arcuate region can be greater than or equal to 40 mm. The length of the outside region44bcan be appropriately set depending on the size of the radius of curvature Rc of the third arcuate region.

Further, the reference inflection point48bbetween the first arcuate region and the second arcuate region is formed in the vicinity of the ridgeline section14of the target formed body12. In addition, the vicinity of the reference inflection point48bserves as the intermediate ridgeline section44cwhich projects maximally upward in the press stroke direction within the intermediate formed body42. As shown in the partially enlarged view, the reference character1defines a length between the inflection point48aand the reference inflection point48bin a direction along the cross section of the intermediate formed body42. Further, the reference character L defines a length between the reference inflection point48band the inflection point48ein the direction along the cross section of the intermediate formed body42. In the present embodiment, the intermediate formed body42is formed in a manner so that the inequality 1<L is satisfied. Although not particularly limited, for example, the length L can be less than or equal to 70 mm.

When a length along the cross section of the intermediate formed body42in the intermediate region44is defined by L0, and a length along the cross section of the target formed body12in the intermediate region44is defined by L1, it is preferable for the cross-sectional shape of the intermediate formed body42to be formed in a manner so that the rate of elongation ((L1−L0)/L0) becomes from 0 to 2%. Such a rate of elongation can be adjusted by the shape of the outside region44bof the intermediate formed body42.

Within the inside region44a, when a maximum deviation of a portion where the deviation between the target formed body12and the intermediate formed body42in the press stroke direction (the vertical direction in the figure) becomes maximal is defined by Ha, and within the outside region44b, when a maximum deviation of a portion where the deviation in the press stroke direction between the target formed body12and the intermediate formed body42becomes maximal is defined by Hb, the intermediate formed body42is formed in a manner so that the inequality Ha>Hb is satisfied. The maximum deviation Ha can be, for example, less than or equal to 3.1 mm, and the maximum deviation Hb is set to be less than or equal to the maximum deviation Ha.

On the other hand, focusing attention on the regions B2and B3in which the peripheral portions16are formed, the first peripheral portion16aof the intermediate formed body42is formed at a position which is higher, by Hd, in the press stroke direction, than the first peripheral portion16aof the target formed body12. The height deviation Hd is provided in order to prevent the intermediate formed body42in the intermediate region44from coming into contact with the second upper die36and being deformed, when the intermediate formed body42is retained by the blank holder34in the second step (refer toFIG.6). Accordingly, it is preferable for the intermediate formed body42to be formed in a manner so that the deviation Hd in the press stroke direction in the first peripheral portion16abecomes greater than the maximum deviation Hb in the outside region44b.

The second peripheral portion16bin the region B2is provided in order to absorb the deviation Hd in the press stroke direction between the first peripheral portion16aof the intermediate formed body42and the first peripheral portion16aof the target formed body12, within a range up to the coincident region46b. In the second step, in order so as not to pull on the intermediate region44, a length L1stof the second peripheral portion16bof the intermediate formed body42, and a length L2ndof the second peripheral portion16bof the target formed body12are set so as to be of approximately the same length. Moreover, in order to adjust the amount of pulling in the second step, the length L1stof the second peripheral portion16bof the intermediate formed body42may be longer than the length L2ndof the second peripheral portion16bof the target formed body12. The value of L2nd−L1stcan be, for example, on the order of 0 to 0.05 mm.

Hereinafter, press forming in the second step and operations of the intermediate formed body42will be described with reference toFIGS.5to8.

As shown inFIG.5, in an initial state, the blank holder34projects more upward in the press stroke direction by a predetermined height than the second lower die32. The blank holder34can be displaced so as to stop at the position of the lower end as indicated by the two-dot dashed line, by being pressed downward by the second upper die36.

As shown in the drawing, the intermediate formed body42is carried in between the second lower die32and the blank holder34, and the second upper die36. Then, the first peripheral portion16ais arranged and positioned on the blank holder34.

Thereafter, as shown inFIG.6, when the second upper die36is subjected to a downward stroke, the second upper die36and the blank holder34come into contact with each other via the intermediate formed body42. Then, the first peripheral portion16aof the intermediate formed body42is sandwiched and retained by the blank holder34and the second upper die36. As described previously, the first peripheral portion16aof the intermediate formed body42is formed to be higher, by the deviation Hd, than the first peripheral portion16aof the target formed body12(refer toFIG.4). Therefore, even if the first peripheral portion16ais retained by the blank holder34and the second upper die36, the inside region44aand the outside region44bof the intermediate formed body42do not come into contact with the second upper die36. Accordingly, when the intermediate formed body42is retained by the blank holder34, it is possible to prevent the intermediate formed body42from moving while in contact with the second upper die36, and thereby causing scratches to be generated on the design surface.

Thereafter, as shown inFIG.7, the second upper die36is further lowered. Upon doing so, the inside region44acomes into contact with ridgeline sections32aof the second lower die32, and is gradually deformed into the shape of the ridgeline sections14of the target formed body12. Further, the outside region44bis gradually deformed along the second lower die32. Since the inside region44aof the intermediate formed body42is curved inwardly of the ridgeline sections14, the length thereof is insufficient to form the ridgeline sections14of the target formed body12. Such an insufficiency is compensated for by the intermediate formed body42being moved from the outside region44bto the inside region44a. Further, since portions of the intermediate formed body42other than the intermediate region44are retained in a state of being suspended between the second lower die32and the second upper die36, forming of the intermediate region44proceeds with precedence over that of the other portions. Consequently, it is possible to prevent excessive elongation from occurring in the vicinity of the ridgeline sections14. Further, when the intermediate formed body42is subjected to deformation, a gap is formed between the design surface of the intermediate formed body42and the second upper die36, and such a gap is maintained until just prior to the second upper die36reaching the bottom dead center.

Furthermore, as shown inFIG.8, when the second upper die36is lowered, the entire area of the intermediate formed body42is sandwiched between the second lower die32and the second upper die36, and forming of portions of the ridgeline sections14having a small radius of curvature is performed. According to the present embodiment, at a stage at which the inside region44aand the outside region44bare fully elongated, the entire region of the second lower die32and the second upper die36is closed. Therefore, when the ridgeline sections14with the small radius of curvature are formed, elongation of the intermediate formed body42is suppressed. As a result, the target formed body12can be formed while suppressing the occurrence of cracks in the vicinity of the ridgeline sections14. The length L1along the cross section of the target formed body12in the intermediate region44is slightly longer than the length L0along the cross section of the intermediate formed body42in the intermediate region44, and therefore, the intermediate region44of the intermediate formed body42is formed while being elongated at a predetermined rate of elongation.

The press forming method according to the present embodiment exhibits the following advantageous effects.

The press forming method according to the present invention is characterized by a press forming method for forming the plate member10into the target formed body12in which the ridgeline sections14are included, the press forming method comprising the first step of forming the intermediate formed body42including the intermediate ridgeline section44cwith a radius greater than an edge radius of the ridgeline sections14, and the second step of forming the target formed body12from the intermediate formed body42, wherein the target formed body12and the intermediate formed body42may each include the coincident regions46aand46bin which the cross-sectional shapes thereof coincide on both sides of the intermediate ridgeline section44c, and the intermediate region44in which the cross-sectional shapes thereof do not coincide, and include, in the intermediate region44, the outside region44bin which the intermediate formed body42projects more outwardly of the edge radius than the target formed body12, and the inside region44ain which the intermediate formed body42is curved more inwardly of the edge radius than the target formed body12. In the case of forming the target formed body12using the intermediate formed body42that is formed in this manner, when the ridgeline sections14having a small radius of curvature are formed, the intermediate formed body42, which compensates for the insufficient length, can be provided from the outside region44btoward the ridgeline sections14. Consequently, the ridgeline sections14having such a small radius of curvature can be formed without causing the occurrence of scratches or cracks.

In the above-described press forming method, in the second step, an insufficiency in length when the inside region44ais formed into the ridgeline sections14is compensated for by the intermediate formed body42in the outside region44bbeing moved toward the ridgeline sections14. In accordance with this feature, the ridgeline sections14can be formed without causing excessive elongation to occur. As a result, it is possible to prevent scratches and cracks from occurring in the ridgeline sections14, and a press-formed product equipped with sharp ridgeline sections14having a small radius of curvature can be formed without causing the occurrence of surface distortion.

In the above-described press forming method, the length, in the cross-sectional direction, of the intermediate formed body42in the outside region44bis longer than the length, in the cross-sectional direction, of the intermediate formed body42in the inside region44a. In accordance with this feature, the intermediate formed body42having a sufficient length can be provided from the outside region44btoward the ridgeline sections14.

In the above-described press forming method, the intermediate region44of the intermediate formed body42is formed by a plurality of arcuate regions having different curvatures, and includes, in the vicinity of the position of the ridgeline sections14of the target formed body12, the reference inflection point48bthat defines a boundary of the arcuate regions, and the length L of the intermediate region44on the side of the outside region44bfrom the reference inflection point48bis longer than the length1of the intermediate region44on the side opposite to the outside region44bfrom the reference inflection point48b. In accordance with such features, in the outside region44b, which is in closer proximity to the peripheral portion16than the ridgeline sections32awhere elongation is likely to occur, the amount of material can be controlled, elongation in the ridgeline sections14of the target formed body12can be suppressed, and it is possible to prevent the occurrence of cracks in the ridgeline sections14, as well as to prevent the occurrence of surface distortion of the outside region44b.

In the above-described press forming method, the intermediate region44of the intermediate formed body42may include the first arcuate region that is curved inwardly of the target formed body12from one end of the intermediate region44, and the second arcuate region that is connected to the first arcuate region at the reference inflection point48band is curved so as to project outwardly of the target formed body12, wherein the radius of curvature Rb of the second arcuate region may be greater than the radius of curvature Ra of the first arcuate region.

In the above-described press forming method, the maximum deviation Ha in the press stroke direction between the intermediate formed body42and the target formed body12in the inside region44ais greater than the maximum deviation Hb in the press stroke direction between the intermediate formed body42and the target formed body12in the outside region44b. In accordance with this feature, in the second step, the second upper die36does not come into contact with the upper surface of the intermediate formed body42, and the intermediate formed body42and the second lower die32do not come into contact with each other during blank holding. Therefore, it is possible to suppress generation of scratches due to slippage in a state in which the second lower die32and the second upper die36are placed in contact with the intermediate formed body42, and the target formed body12which is devoid of scratches can be formed.

In the above-described press forming method, prior to pressing the ridgeline sections14, the first step and the second step are performed in the forming dies20and30respectively including the blank holders24and34that retain the peripheral portion16of the plate member10, and the outside region44bof the intermediate formed body42is formed in a portion in closer proximity to the blank holder34than the ridgeline sections14. In accordance with such features, on the side of the blank holder34where the tensile force is generated, elongation can be controlled, and surface distortion in closer proximity to the outer side of the ridgeline sections14can be prevented.

In the above-described press forming method, the first step and the second step are performed in the forming dies20and30respectively including the blank holders24and34that retain the peripheral portion16of the plate member10, prior to pressing the ridgeline sections14, and the height of the blank holder34in the second step is set to be higher than the height of the blank holder24in the first step, by at least the maximum deviation Hb in the press stroke direction between the intermediate formed body42and the target formed body12in the outside region44b. In accordance with such features, in the second step, the second upper die36does not come into contact with the upper surface of the intermediate formed body42, and the intermediate formed body42and the second lower die32do not come into contact with each other during blank holding. Therefore, it is possible to suppress generation of scratches due to slippage in a state in which the second lower die32and the second upper die36are placed in contact with the intermediate formed body42, and the target formed body12which is devoid of scratches can be formed.

In the above-described press forming method, the length L2ndof the peripheral portion (second peripheral portion16b) in the second step may be set to be longer than the length L1stof the peripheral portion (second peripheral portion16b) in the first step.

In the above-described press forming method, the outer peripheral portion of the plate member10may be formed in a quadrangular shape constituted by the first side10a, the second side10bthat faces toward the first side10a, the third side10cextending in a direction intersecting the first side10a, and the fourth side10dthat faces toward the third side10c, and the ridgeline sections14may be formed in the vicinity of the third side10cand in the vicinity of the fourth side10d, and extend from the first side10atoward the second side10b. In accordance with such a forming method, the sharp ridgeline sections14can be formed on a member such as the hood or the like of an automobile.

In the above-described press forming method, the inside region44aof the intermediate formed body42may be formed inwardly of the ridgeline sections14of the target formed body12, and the outside region44bof the intermediate formed body42may be formed outwardly of the ridgeline sections14of the target formed body12.

Although a description concerning the present invention has been given above with reference to a preferred embodiment, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made thereto without departing from the essence and gist of the present invention.