Method and apparatus for producing outer panel having character line

In a method for producing an outer panel having a character line, a punch includes a projecting portion for forming a bent surface of the character line at a punch top portion, and a punch shoulder R portion. A contact pressure applied to a blank by at least one of the projecting portion and the punch shoulder R portion is higher than a contact pressure applied to the blank by a remaining portion of the punch which excludes the projecting portion and the punch shoulder R portion. A static friction coefficient of a first portion of the punch which includes at least one of the projecting portion and the punch shoulder R portion with respect to the blank is larger than a static friction coefficient of a second portion of the punch which excludes the first portion with respect to the blank.

TECHNICAL FIELD

The present invention relates to method and apparatus for producing an outer panel having a character line.

BACKGROUND ART

For example, a door outer panel, a fender panel, a side panel, a hood outer panel, a back door outer panel and the like are known as outer panels of a vehicle body of an automobile. In recent years, providing a sharp character line to the outer panel, particularly to the outer panel for the side surface of the vehicle body, has been adopted in many cases as one of the means for realizing exterior design having high design property.

The character line is a projecting ridge line (bent surface) formed between at least two surfaces of the outer panel. Further, the sharpness of the character line means that the radius of curvature of the bent surface is small, that is, 10 mm or less, for example, or an included angle, which is a smaller angle of two angles formed by extension surfaces of the two surfaces, is small, that is, 150° or less, for example.

It is known that providing a character line to the outer panel by performing press working by draw forming or stretch forming causes line displacement, which is quality defect on the outer surface.

FIG.14AandFIG.14Bare cross-sectional views schematically and chronologically describing the situation where line displacement occurs.FIG.14Ashows an initial stage of forming, andFIG.14Bshows a state where the forming is completed. Two black circles inFIG.14Bindicate two ends of R portion of a character line4a, which is a bent surface. InFIG.14AandFIG.14B, a blank3indicates a center position in the sheet thickness direction of the blank3.

As shown inFIG.14AandFIG.14B, an outer panel4having the character line4ais produced by performing press working on the blank3by draw forming or stretch forming using a punch1, a blank holder (not shown in the drawing), and a die2.

As shown inFIG.14A, at the initial stage of forming, an initial bent portion (also referred to as “bending tendency”)3a, which is an extremely small local concavity, is formed at an initial striking portion on the blank where a character line forming projecting portion1aprovided to the punch1initially comes into contact with the blank3. This initial bent portion3ais a portion to be formed into the bent surface of the character line when the forming is completed.

In stretch forming or draw forming, the amount of inflow of the blank into the punch1and the die2differs depending on a portion of the blank3. Accordingly, a difference is inevitably generated in tension which acts on each of at least two surfaces which form the character line. Due to this difference in tension, the initial bent portion3ais moved and displaced from the projecting portion1awith the progress of the press working. Therefore, when the forming is completed, the initial bent portion3aremains as a concavity4a-2substantially extending along a portion in the vicinity of the character line4a-1actually formed on an outer panel4.

As described above, line displacement is a phenomenon where the initial bent portion3awhich is formed at the initial stage of forming is moved and displaced from a position where the initial bent portion3ashould be originally positioned in the forming process, and remains also at the time when the forming is completed. When the degree of line displacement, that is, the amount of movement of the initial bent portion3awhich is formed in the forming process is large, highlight fold is found at a portion where line displacement occurs in a highlight test of the outer panel of a completed vehicle so that the outer surface is assumed as quality defect.

Line displacement is particularly liable to occur when a character line is sharp as described above. In recent years, exterior design with sharp character lines formed on the entire body has been used in many cases. Accordingly, there is a situation where line displacement is liable to occur.

Further, there is a tendency that line displacement is not liable to occur on a blank made of high strength steel having tensile strength of 440 MPa or more, whereas line displacement is liable to occur on a blank made of soft steel having tensile strength of approximately 270, 340 MPa.

For example, in stretch forming performed on a door outer panel having a character line, the height, distance and the like of material inflow preventing beads provided to a press tooling are changed according to the region of the press tooling to take a countermeasure for line displacement. However, an effect of suppressing line displacement is not sufficiently obtained.

Further, line displacement is sensuously evaluated such that an inspector visually observes a completed vehicle. Accordingly, errors are inevitably included in the inspection for line displacement. In view of the above, the applicant of the present invention has proposed a method for quantitatively evaluating line displacement in Patent Document 1.

In this method, first, distribution of curvature (second order derivative of the cross-sectional profile) is obtained based on the cross-sectional profile of the outer panel, and then the second order derivative (fourth order derivative of the cross-sectional profile) of the curvature is obtained. Then, a value H [mm−3] at a peak of the second order derivative of the curvature which appears at a portion where a line displacement occurs, and a displacement width L [mm] between a position which corresponds to the peak and a position of an end of R portion in a designed value for the character line are obtained, and the amount of line displacement is quantitatively obtained as the evaluation parameter S=L×|H|1/3. The results of the quantitative evaluation on the amount of line displacement using this evaluation parameter S shows a high correlation with the results of sensuous evaluation performed by an inspector.

Patent Document 2 discloses a method for forming a rear side outer panel having a character line by press forming using a punch, a split die including a first die and a second die, and a blank holder.

In this method, press forming is performed on the portion of a blank which includes the character line by the first die and the punch. Next, press forming is performed on remaining portions of the blank by the second die and the punch while maintaining a state where the formed character line is constrained by the first die and the punch. As described above, the character line is formed while line displacement is suppressed.

Patent Document 3 discloses a method for forming an outer panel having a sharp character line by press forming from a blank using a press tooling which includes a punch, a die, and a blank holder.

In this method, a lining layer formed of an elastic member is provided to a valley line portion disposed at the deepest side of a character line forming recessed portion of the die which opposes a character line forming projecting portion of the punch. A blank is subjected to forming while the lining layer is elastically deformed at a forming bottom dead center where the punch approaches closest to the die. With such a configuration, distribution of bending moment on the blank at the forming bottom dead center is reduced so that the occurrence of line displacement is suppressed.

LIST OF PRIOR ART DOCUMENTS

Patent Document

SUMMARY OF INVENTION

Technical Problem

In the invention disclosed in Patent Document 2, when a force which constrains the character line using the second die is increased, the movement of the blank is suppressed in forming a portion other than the character line. Accordingly, deflection or insufficient inflow occurs on the blank at the portion other than the character line so that forming defects are liable to occur. The forming defects are liable to occur particularly in forming a character line having a shape which is three-dimensionally curved.

In the invention disclosed in Patent Document 2, it is necessary to use the split die which includes the first die and the second die, thus increasing the cost of press tooling.

Further, in the invention disclosed in Patent Document 2, there is a possibility that a split line (split surface) between the first die and the second die appears on the surface of the rear side outer panel subjected to forming. Accordingly, there is a concern of the occurrence of quality defects on the outer surface different from quality defects on the outer surface attributable to line displacement.

In the invention disclosed in Patent Document 3, the lining layer provided at the valley line portion disposed at the deepest side of the character line forming recessed portion of the die is elastically deformed at the forming bottom dead center to suppress the occurrence of line displacement. Accordingly, the movement of the initial bent portion at the forming bottom dead center and a portion in the vicinity of the forming bottom dead center can actually be suppressed. However, it is impossible to suppress the movement of the initial bent portion at a portion other than the forming bottom dead center and the portion in the vicinity of the forming bottom dead center, that is, the movement of the initial bent portion in the forming process. Accordingly, an effect of suppressing line displacement cannot be sufficiently obtained.

An objective of the present invention, which has been made in view of such problems of the prior art, is to suppress or substantially eliminate line displacement without a significant increase in cost of press tooling and forming defects in producing an outer panel having a character line by press working.

Solution to Problem

To suppress line displacement, it is effective (i) to eliminate the generated initial bent portion, and (ii) to prevent the generated initial bent portion from moving in the forming process. The inventors of the present invention have made extensive studies in view of such points and, as a result, obtained the findings A to D enumerated below with respect to (ii), leading to the achievement of the present invention.

(A) The punch used for draw forming or stretch forming includes a high contact pressure generating portion which applies a high contact pressure to the blank during a period from the start of the forming to the completion of the forming, and a low contact pressure generating portion, which applies, to the blank during the period from the start of the forming to the completion of the forming, a contact pressure lower than the contact pressure applied to the blank by the high contact pressure generating portion.

For example, the character line forming projecting portion and the punch shoulder R portion are the high contact pressure generating portions, while the punch top portion and a punch vertical wall portion, excluding the character line forming projecting portion and the punch shoulder R portion, are the low contact pressure generating portions.

By partially increasing a static friction coefficient of the high contact pressure generating portion of the punch and a region around the high contact pressure generating portion with respect to the blank, a frictional force between such a region of the punch and the blank increases during press forming. Accordingly, the initial bent portion is prevented from easily moving in the forming process of the character line.

For example, increasing a static friction coefficient of the character line forming projecting portion of the punch with respect to the blank can suppress the movement of a portion in the vicinity of the initial bent portion. Further, increasing a static friction coefficient of the punch shoulder R portion with respect to the blank can suppress the movement of the entire blank including the initial bent portion.

Accordingly, the occurrence of line displacement on the outer panel having the character line can be suppressed or substantially eliminated.

(B) As a means for increasing the static friction coefficient of the high contact pressure generating portion of the punch and the region around the high contact pressure generating portion with respect to the blank, the following (B1) and (B2) are exemplified.

(B1) To provide particles having Vickers hardness of 700 HV or more in the high contact pressure generating portion of the punch and the region around the high contact pressure generating portion.

(B2) To provide protrusions by an appropriate means in the high contact pressure generating portion of the punch and the region around the high contact pressure generating portion.

(C) The means exemplified in (B1) and (B2) are performed on the punch which comes into contact with the inner surface of the outer panel which is a non-design surface to prevent impairing of the design property (aesthetics) of the outer surface of the outer panel subjected to forming.

(D) During the press forming, the character line forming portion of the blank is not firmly constrained by the punch and the die. For this reason, the occurrence of forming defects, such as deflection of blank or insufficient inflow, are prevented at a portion other than the character line.

The present invention is as enumerated below.

(1) A method for producing an outer panel having a character line, which is a bent surface between two surfaces, by using a punch and a blank holder, and a die which opposes the punch and the blank holder, to perform press working on a blank disposed between the punch and the blank holder on one side and the die on the other side by draw forming or stretch forming, wherein

the punch includes a projecting portion for forming the bent surface and a punch shoulder R portion at a punch top portion,

a contact pressure applied to the blank by at least one of the projecting portion and the punch shoulder R portion during the press working is higher than a contact pressure applied to the blank by a remaining portion of the punch which excludes the projecting portion and the punch shoulder R portion during the press working, and

a static friction coefficient of a first portion of the punch which includes at least one of the projecting portion and the punch shoulder R portion with respect to the blank is larger than a static friction coefficient of a second portion of the punch which excludes the first portion with respect to the blank.

(2) The method for producing an outer panel having a character line according to 1, wherein a difference between the static friction coefficient of the first portion with respect to the blank and the static friction coefficient of the second portion with respect to the blank is 0.05 or more.

(3) The method for producing an outer panel having a character line according to 1 or 2, wherein the static friction coefficient is a static friction coefficient in a direction along which a concavity generated on the blank due to an initial strike of the projecting portion on the blank moves in a forming process.

(4) The method for producing an outer panel having a character line according to any one of 1 to 3, wherein at least one of the projecting portion and the punch shoulder R portion includes particles having Vickers hardness of 700 HV or more.

(5) The method for producing an outer panel having a character line according to 4, wherein the punch includes a particle fixing portion having a holding layer which holds the particles, and the particle fixing portion is provided to at least one of the projecting portion and the punch shoulder R portion.

(6) The method for producing an outer panel having a character line according to 4 or 5, wherein an embedding ratio of the particles is 50 to 80%.

Wherein the embedding ratio is calculated as (a height of the holding layer/an average particle size of the particles)×100(%).

(7) The method for producing an outer panel having a character line according to 6, wherein the average particle size of the particles is 5 to 600 μm.

(8) The method for producing an outer panel having a character line according to 6 or 7, wherein a particle ratio of the particles fixed is 5 to 20%.

Wherein the particle ratio is calculated as (a total volume of the particles/a volume of the particle fixing portion)×100(%).

(9) The method for producing an outer panel having a character line according to any one of 5 to 8, wherein the particle fixing portion is exchanged with another particle fixing portion according to an operating condition.

(10) The method for producing an outer panel having a character line according to any one of 4 to 9, wherein the particles come into contact with a non-design surface of the blank which is an inner surface of the outer panel.

(11) The method for producing an outer panel having a character line according to any one of 1 to 3, wherein at least one of the projecting portion and the punch shoulder R portion includes a protrusion.

(12) The method for producing an outer panel having a character line according to 11, wherein a height of the protrusion is 0.02 to 0.10 mm.

(13) The method for producing an outer panel having a character line according to 11 or 12, wherein the protrusion comes into contact with a non-design surface of the blank which is an inner surface of the outer panel.

(14) An apparatus for producing an outer panel having a character line, which is a bent surface between at least two surfaces, the apparatus including: a punch and a blank holder; and a die which opposes the punch and the blank holder,

the outer panel being produced by performing press working on a blank disposed between the punch and the blank holder on one side and the die on the other side by draw forming or stretch forming, wherein

the punch includes a projecting portion for forming the bent surface and a punch shoulder R portion at a punch top portion,

a contact pressure applied to the blank by at least one of the projecting portion and the punch shoulder R portion during the press working is higher than a contact pressure applied to the blank by a remaining portion of the punch which excludes the projecting portion and the punch shoulder R portion during the press working, and

a static friction coefficient of a first portion of the punch which includes at least one of the projecting portion and the punch shoulder R portion with respect to the blank is larger than a static friction coefficient of a second portion of the punch which excludes the first portion with respect to the blank.

Advantageous Effects of Invention

According to the present invention, in producing an outer panel having a character line, particularly a sharp character line, by press working, line displacement can be suppressed or substantially eliminated while the occurrence of forming defects is suppressed without a significant increase in cost of press tooling.

DESCRIPTION OF EMBODIMENTS

The present invention will be described. In the description made hereinafter, the case where an outer panel is a door outer panel is taken as an example. The present invention is not limited to the door outer panel, and is also similarly applicable to outer panels, such as a fender panel, a side panel, a hood outer panel, or a back door outer panel, for example.

FIG.1is an exploded perspective view showing one example of a production apparatus1before forming is started.

As shown inFIG.1, the production apparatus1includes a punch2, a blank holder4, and a die3.

The die3is disposed to oppose the punch2and the blank holder4. The punch2, the blank holder4, and the die3perform press working on a blank (not shown in the drawing) disposed between the punch2and the blank holder4on one side and the die3on the other side by draw forming or stretch forming, thus producing a door outer panel (not shown in the drawing).

The case is exemplified where the blank is a blank made of metal, and is a steel sheet. The blank is not limited to a steel sheet, and may be an industrial pure aluminum sheet, an aluminum alloy sheet, an industrial pure titanium sheet, a titanium alloy sheet or other sheets.

The door outer panel includes a character line which is a bent surface (ridge line) between at least two surfaces. The door outer panel may include a plurality of character lines.

As the character line, exemplified is a sharp character line having the radius of curvature of the ridge line of 10 mm or less in a cross section orthogonal to the character line, or a sharp character line where an included angle, which is the smaller angle of two angles formed by two surfaces obtained by extending two surfaces forming the ridge line in a cross section orthogonal to the character line, is 150° or less. In the case of these sharp character lines, line displacement is liable to occur during press forming and hence, the effect of the present invention can be effectively obtained.

The punch2includes a high contact pressure generating portion. The high contact pressure generating portion applies a high contact pressure to the blank during a period from the start of the forming of the blank to the completion of the forming of the blank. For example, at least one of a character line forming projecting portion2band a punch shoulder R portion2cis the high contact pressure generating portion. The punch2may include a plurality of projecting portions2b.

The punch2includes low contact pressure generating portions. The low contact pressure generating portions are portions of the punch2excluding the high contact pressure generating portion, and are a remaining portion2dof a punch top portion2awhich excludes the projecting portion2band the punch shoulder R portion2c, and a punch vertical wall portion2e. The low contact pressure generating portions apply, to the blank, a contact pressure lower than the contact pressure which the high contact pressure generating portion10applies to the blank during a period from the start of the forming of the blank to the completion of the forming of the blank.

In the production apparatus1, the static friction coefficient μ1of a first portion of the punch2which includes at least one of the projecting portion2band the punch shoulder R portion2cwith respect to the blank is larger than the static friction coefficient μ2of a second portion of the punch2which excludes the first portion with respect to the blank.

Such a configuration increases a frictional force which is generated between the first portion and the blank. Accordingly, the first portion can suppress that an initial bent portion, which is generated on the blank due to initial striking of the punch2, is moved and displaced from the position where the initial bent portion should be originally positioned in the forming process and hence, line displacement on the door outer panel can be suppressed or substantially eliminated.

Specifically, in the case where a static friction coefficient of the projecting portion2bis increased, an effect of suppressing line displacement appears not only a point of time when the punch reaches the forming bottom dead center, but also during a period from the initial striking to the reaching to the forming bottom dead center. Further, a contact pressure applied to the punch shoulder R portion2cis higher than a contact pressure on the character line forming projecting portion2b. Accordingly, when the static friction coefficient of the punch shoulder R portion2cis increased, at the point of time when the punch reaches the forming bottom dead center, a greater effect of suppressing line displacement can be obtained than the prior art.

Accordingly, although it is sufficient to increase at least the static friction coefficient μ1of at least one of the projecting portion2band the punch shoulder R portion2c, the static friction coefficient μ1may be increased for an additional region in the vicinity of at least one of the projecting portion2band the punch shoulder R portion2c.

For example, in the case of (a) the projecting portion2b, the static friction coefficient μ1may be increased for a region within 100 mm inclusive from the character line or a region where a contact pressure during the forming becomes 2 MPa or more. In the case of (b) the punch shoulder R portion2c, the static friction coefficient μ1may be increased for a region where a contact pressure during the forming becomes 2 MPa or more.

A difference between the static friction coefficient μ1of the first portion with respect to the blank and the static friction coefficient μ2of the second portion with respect to the blank is preferably set to 0.05 or more, more preferably set to 0.10 or more, and further preferably set to 0.15 or more. With such a configuration, it is possible to suppress line displacement on an outer panel having a character line with certainty.

When the static friction coefficients μ1, μ2of the first portion and the second portion are static friction coefficients in a direction along which the initial bent portion (concavity), generated on the blank due to the initial strike of the projecting portion2bto blank, moves in a forming process, it is possible to reduce a region where the static friction coefficient μ1is required to be increased, thus suppressing an increase in manufacturing cost of the punch2. Accordingly, such a configuration is preferable.

Next, means 1, 2 for increasing the static friction coefficient μ1of the first portion with respect to the blank will be described.

FIG.2is a cross-sectional view showing the configuration of the production apparatus1. In the description made hereinafter with respect to the means 1, the case where particles are provided to the projecting portion2bof the punch2is taken as an example. However, the same situation arises also in the case where particles are provided to the punch shoulder R portion2cof the punch2.

The production apparatus1includes the punch2as an upper die, and includes the die3as a lower die which opposes the punch2. The production apparatus1includes the blank holder4, which press a blank S, on the side of the punch2and above the die3. The punch2is elevatably supported by an elevating mechanism not shown in the drawing.

FIG.3is an enlarged view of a particle fixing portion5provided to the punch top portion2a. InFIG.3, the size of the particle fixing portion5is exaggerated with respect to the punch2to facilitate the understanding of the function of the particle fixing portion5.

As shown inFIG.3, the particle fixing portion5includes particles5aand a holding layer5bwhich causes the particles5ato be fixed to the punch2. As shown inFIG.2, the particle fixing portion5has a projecting shape which conforms to the shape of the punch top portion2ain the state of being provided to the punch2.

FIG.4is an explanatory view showing the situation of the punch2as viewed from below.

As shown inFIG.4, the punch2has a shape which extends along the X direction inFIGS.2,4. The particle fixing portion5also has a shape which extends along the X direction inFIGS.2,4in conformity with the shape of the punch top portion2a.

The particle fixing portion5is provided to form the projecting portion2bprovided to the punch top portion2a. The position of the projecting portion2bagrees with a position where a character line is formed on a door outer panel, which is a product.

Meanwhile, a die recessed portion3awhich conforms to the shape of the punch top portion2aof the punch2is provided on the upper surface of the die3. The position of the lowermost end of the die recessed portion3aagrees with the position where the character line is formed on the door outer panel, which is the product.

When the punch top portion2acomes into contact with the blank S, an initial bent portion is formed. Then, the punch top portion2aand the die recessed portion3aform the character line on the door outer panel.

The shapes of the punch2and the die3are not limited to the shapes shown inFIGS.1to4, and may be changed as needed according to the shape of a door outer panel to be produced. For example, the production apparatus1shown inFIGS.1to4forms one character line. However, in the case where a plurality of character lines are formed, a plurality of character line forming projecting portions2bof the punch top portion2aand a plurality of lowermost ends of the die recessed portion3aare provided.

FIGS.5to7are explanatory views chronologically showing the action of the production apparatus1of the embodiment. InFIGS.5to7, the particle fixing portion5is omitted.

With respect to the punch2, a part of the punch2which corresponds to a portion provided with the particle fixing portion5(hereinafter, referred to as “punch detachable portion6”) is detachably mounted on a punch body7.

The particle fixing portion5is provided to the surface of the punch detachable portion6which is detachable with respect to the punch body7. Accordingly, the particle fixing portion5is detachably disposed on the punch2by attaching/detaching of the punch detachable portion6.

The punch2is formed of the punch detachable portion6and the punch body7. Accordingly, depending on operating conditions, such as the wear state of the particles5aon the particle fixing portion5, hardness of the blank S and the shape of the character line, the punch detachable portion6can be easily exchanged with another punch detachable portion6provided with the particle fixing portion5or another punch detachable portion6including a particle fixing portion with different kind of particles, for example.

With such a configuration, a time for exchange can be significantly shortened compared with the case where the punch2per se is exchanged and hence, productivity can be enhanced. Note that the configuration where the particle fixing portion5is detachable with respect to the punch2is not limited to the above-described mode.

It is sufficient for the particles5aon the particle fixing portion5to have Vickers hardness of 700 HV or more, and the particles5aare not particularly limited. With the provision of the particle fixing portion5to the punch2, it is possible to increase a frictional force generated between the punch2and the blank S during press forming compared with the case where press forming is performed using a punch provided with no particle fixing portion5.

With the increase in frictional force, it is possible to suppress the movement of a material at a character line forming portion on the blank S. Accordingly, the amount of movement of an initial bent portion formed at the initial stage of press forming reduces so that line displacement of the character line can be suppressed or substantially eliminated.

When Vickers hardness of the particles5ais less than 700 HV, the particles5acannot easily bite into the blank S during press forming so that a frictional force between the punch2and the blank S is insufficient. For this reason, Vickers hardness of the particles5ais set to 700 HV or more, preferably set to 1500 HV or more, and further preferably set to 2000 HV or more.

The particles5aare appropriately selected according to hardness of the blank S. For example, fused alumina (2100 HV), silicon carbide (25000 HV), boron carbide (2750 HV), ceramic (2800 HV), diamond (9000 HV) or the like is used as the particles5a.

The particles5aare fixed to the surface of the punch detachable portion6via the holding layer5b. A method for fixing the particles5ais not particularly limited, and may be electrodeposition coating, welding or the like. In performing electrodeposition coating, the punch detachable portion6is immersed into a liquid layer containing the particles5a, and a voltage is applied to the immersed portion of the punch detachable portion6.

In the case where the particles5aare welded, metal plating is applied to the surface of the particles5a, and the particles5aare directly brazed to the surface of the punch detachable portion6. In this case, filler metal plays a role of the holding layer5b.

The width of the particle fixing portion5(width in the Y direction inFIGS.2,4) may be appropriately determined according to the kind or hardness of the particle, an embedding ratio, the kind of steel or hardness of the blank S, the shape of the character line and the like such that line displacement is suppressed by a frictional force between the punch2and the blank S.

The embedding ratio of the particles5ain the particle fixing portion5is preferably set to 50 to 80%. This embedding ratio is a ratio of the height H of the holding layer5bto the average particle size D of the particles5a, and is calculated as (height H of holding layer5b/average particle size D of particles5a)×100(%). The embedding ratio is an index indicating the height of the portion of the particles5aprotruding from the holding layer5b.

When the embedding ratio is less than 50%, the portion of the particles5aprotruding from the holding layer5bhave the large height so that a frictional force between the punch2and the blank S is enhanced. However, an input load to the individual particle5ais increased.

Accordingly, the particles5aare liable to drop off the holding layer5band, further, the particles5aare liable to wear. For this reason, to enhance wear resistance of the particle fixing portion5, an embedding ratio is preferably set to 50% or more, more preferably set to 55% or more, and further preferably set to 60% or more.

To the contrary, when the embedding ratio exceeds 80%, the number of particles5awhich are buried in the holding layer5bincreases and hence, a frictional force between the particle fixing portion5and the blank S cannot be sufficiently increased. For this reason, the embedding ratio is preferably set to 80% or less, more preferably set to 75% or less, and further preferably set to 70% or less.

A method for measuring the height H of the holding layer5bwill be described. With respect to the punch2as viewed in a plan view inFIG.4, the entire length of the projecting portion2bis assumed as “W”, the starting end position of the projecting portion2bis assumed as “0”, and the finishing end position of the projecting portion2bis assumed as “W”. Under such conditions, a range of ±2.0 mm from each of a 0.25 W position, a 0.50 W position and a 0.75 W position in the extending direction of the projecting portion (the X direction inFIG.4) is set as an observation position.

Optical microscope specimens of cross sections perpendicular to the projecting portion2bare prepared with respect to these observation positions, and observation is performed on the cross section of each observation position (for a range of ±2.0 mm in the extending direction of the projecting portion) at the 0.25 W position, the 0.50 W position, and the 0.75 W position. The height of the holding layer5b(the direction perpendicular to the surface of the punch) is measured at 20 points in each observation position, and the average value of 60 points in total is obtained to give the height of the holding layer5b.

Next, a method for measuring the average particle size of the particles5awill be described. With respect to the punch2as viewed in a plan view inFIG.4, the entire length of the projecting portion2bis assumed as “W”, the starting end position of the projecting portion2bis assumed as “0”, and the finishing end position of the projecting portion2bis assumed as “W”. Assuming the above, at positions −2 mm, −1 mm, 0 mm, 1 mm, and 2 mm from each of the 0.25 W position, the 0.50 W position and the 0.75 W position in the extending direction of the projecting portion (the X direction inFIG.4), optical microscope specimens of cross sections perpendicular to the projecting portion2bare prepared, and are observed using an optical microscope with 100 to 400 times field.

The average value of the particle sizes of the particles5aon cross sections at these 15 observation positions is assumed as the average particle size of the particles5a. With respect to the particle size of the individual particle5a, an average value of a long side and a short side of the particle5ais defined as a particle size. The particle size of the particle5awhich overlaps with the observation position is not measured, thus not included as a value.

FIG.4shows the case where the projecting portion2bhas a straight line as viewed in a plan view. However, the projecting portion2bmay have a three-dimensional curve.

The average particle size of the particles5ais preferably set to 5 to 600 μm. When the average particle size of the particles5ais less than 5 μm, there is a concern that the frictional force between the punch2and the blank S may be insufficient depending on the operating conditions, such as a difference in hardness between the particles5aand the blank S or the shape of a character line. For this reason, the average particle size of the particles5ais preferably set to 5 μm or more, more preferably set to 10 μm or more, and further preferably set to 50 μm or more.

To the contrary, when the average particle size of the particles5aexceeds 600 μm, there is a concern that, depending on operating conditions, such as a difference in hardness between the particles5aand the blank S and the shape of a character line, an input load to the respective particles5aon the particle fixing portion5becomes excessively large during press forming of the blank S so that the particles5aare liable to drop off the holding layer5b. For this reason, the average particle size of the particles5ais preferably set to 600 μm or less, more preferably set to 500 μm or less, and further preferably set to 400 μm or less.

Note that, in the punch2shown inFIG.4, the projecting portion2bextends along the X direction in a straight line manner so that cross sections on which particle sizes are measured extend parallel to each other. However, for example, when the projecting portion2bhas a curved shape or a shape which curves three-dimensionally, the cross section at the 0.25 W position, the cross section at the 0.50 W position, and the cross section at the 0.75 W position may intersect with each other.

The particle ratio in the particle fixing portion5is preferably set to 5 to 20%. The particle ratio is a ratio forming an index indicating the degree of density of particles in a fixed region, and is calculated as (total volume of particles5ain particle fixing portion5/volume of particle fixing portion5)×100(%).

When the particle ratio is less than 5%, the number of particles5ais excessively small and hence, there is a concern that a frictional force between the punch2and the blank S is insufficient depending on operating conditions, such as a difference in hardness between the particles5aand the blank S or the shape of a character line. For this reason, the particle ratio in the particle fixing portion5is preferably set to 5% or more, more preferably set to 8% or more, and further preferably set to 10% or more.

To the contrary, when the particle ratio exceeds 20%, the number of particles5ain the particle fixing portion5is excessively large and hence, there is a concern that clogging is liable to occur between the particles5adepending on operating conditions, such as a difference in hardness between the particles5aand the blank S or the shape of a character line. When clogging occurs between the respective particles5a, a frictional force between the punch2and the blank S becomes insufficient. For this reason, the particle ratio in the particle fixing portion5is preferably set to 20% or less, more preferably set to 15% or less, and further preferably set to 10% or less.

The production apparatus1is formed as described above. Next, a method for producing an outer panel having a character line using the production apparatus1will be described.

First, the blank S is disposed between the die3and a blank holder4. At this point of operation, the blank S is disposed such that, of the surface of the blank S forming the external appearance (design surface) and the surface of the blank S not forming the external appearance (non-design surface), the non-design surface side is directed to the punch2side.

Next, the punch2is lowered. At this point of operation, the projecting portion2bof the punch2comes into contact with the blank S first so that an initial bent portion which becomes the cause of line displacement is generated on the blank S.FIG.5shows a state where the projecting portion2bcomes into contact with the blank S first.

Thereafter, as shown inFIG.6, the punch2is further lowered. With such an operation, the punch2is lowered while maintaining a state where the particle fixing portion5, which is provided to cover the projecting portion2b, is in contact with the blank S. Accordingly, the particles5aon the particle fixing portion5bite into the blank S.

Accordingly, a frictional force between the particle fixing portion5and the blank S becomes higher than a frictional force obtained in the case of using a conventional punch having no particle fixing portion5. The punch2is lowered in such a state.

Therefore, the movement of a material at the character line forming portion on the blank S is suppressed. As a result, the initial bent portion formed at the initial stage of press forming cannot easily move and hence, the line displacement of the character line is suppressed.

The punch2is moved to the forming bottom dead center as shown inFIG.7in such a state to form a character line on the blank S. The particle fixing portion5of the punch2comes into contact with the non-design surface of the blank S and hence, eaves formed due to the contact with the particles5ado not appear on the external appearance of a product.

As described above, according to the production apparatus1, compared with the prior art, it is possible to suppress line displacement which is determined as quality defect on the outer surface of the door outer panel in a highlight test for a completed vehicle.

Further, the particles5aare fixed only on the punch2, and the particles5aare not provided to the die3. Accordingly, a large constraining force which completely constrains the character line forming portion on the blank S is not generated during press forming.

With such a configuration, forming defect, such as deflection of the blank or insufficient inflow, is not liable to occur during press forming for a portion other than the character line forming portion. Accordingly, it is also possible to produce an outer panel which includes a character line having a shape which curves three-dimensionally, such as a door outer panel.

FIG.8is a cross-sectional view schematically showing the configuration of the production apparatus1of another mode.

In the description made heretofore, the mode is exemplified where the punch2is formed of the punch detachable portion6and the punch body7. However, as shown inFIG.8, the punch detachable portion6and the punch body7may be formed as an integral body. In the production apparatus1, for example, the particle fixing portion5may be formed such that the holding layer5bis attached by adhesion to, or is applied by coating on the surface of the punch top portion2a, thus causing the particles5ato be fixed to the holding layer5bin a dispersed manner.

Next, the means 2 will be described. A point which makes the means 2 different from the means 1 is that protrusions8are used in place of the particles5aof the means 1 as the means for increasing a static friction coefficient μ1of the first portion with respect to the blank.

The plurality of protrusions8are provided to the first portion of the punch2.FIG.9Ais an explanatory view showing one example of the arrangement of the plurality of protrusions8provided, andFIG.9Bis an explanatory view showing one example of the dimension of the protrusion8. Note that a unit of the dimension inFIG.9AandFIG.9Bis “mm”.

The plurality of protrusions8may be formed by any means. However, to suppress an increase in manufacturing cost of the punch, for example, it is desirable to form the plurality of protrusions8by etching in a state where a normal punch is masked.

As shown inFIG.9A, the plurality of protrusions8are provided at the first portion in a grid pattern with a center distance of 0.80 mm. Further, as shown inFIG.9B, a chromium plating layer9having a target thickness of 20 μm is formed on the protrusions8formed at the first portion. The target value of the height of the protrusion8is 0.06 mm, and the target value of the diameter of the protrusion8is 0.11 mm. The chromium plating layer9increases hardness of the protrusion8.

These protrusions8are provided to the first portion of the punch2such that the protrusions8come into contact with the inner surface of the blank S which is the non-design surface of the door outer panel. With such a configuration, line displacement on the door outer panel can be suppressed while the lowering of quality of the external appearance of the outer surface, which is the design surface of the door outer panel, is prevented.

As exemplified inFIG.9AandFIG.9B, it is desirable that the protrusion8satisfies a height of 0.02 to 0.10 mm, a diameter of 0.05 to 0.15 mm, a density of 5×105to 5×106pieces/m2, and a center distance to adjacent protrusion8of 0.5 to 1.0 mm so as to suppress line displacement on the outer panel with certainty while the lowering of quality of the external appearance of the outer surface, which is the design surface of the door outer panel, is prevented.

A sliding test press tooling which includes extremely small protrusions8was prepared and a flat plate sliding test was performed to measure a static friction coefficient. Conditions of the sliding test are collectively shown below.

Material: soft steel sheet having a sheet thickness of 0.7 mm

Shape of protrusions8: diameter of 0.11 mm, height of 0.06 mm

Arrangement of protrusions8: in a grid pattern with center distance of 0.7 mm

Shape of press tooling: 40 mm×30 mm, drawing in 30 mm direction

Drawing condition: length of 300 mm

Test (nominal) contact pressure: contact pressure generated in the actual forming being set based on FEM analysis.

FIG.10is a graph showing the relationship between a nominal contact pressure and a static friction coefficient which changes depending on the presence or absence of the protrusions8.

As shown in the graph inFIG.10, a static friction coefficient μ2of a normal flat plate press tooling (no protrusion) is approximately 0.10 to 0.15. To the contrary, when the extremely small protrusions8are provided, a static friction coefficient μ1increases to approximately 0.30. When the height of the protrusions8is 5% or less with respect to the sheet thickness t of the blank, it is possible to prevent the lowering of quality of the external appearance of the outer surface, which is the design surface of the door outer panel.

In the description made heretofore, the punch2forming the upper die is movable and the die3forming the lower die is fixed. However, the punch2forming the upper die may be fixed, and the die3forming the lower die may be movable. Further, the die3may form the upper die, and the punch2may form the lower die.

That is, the production apparatus is not limited to the mode shown inFIGS.1to8. Provided that a portion having a static friction coefficient increased by the particles5aor the protrusions8is provided to the first portion of the punch2which includes at least one of the projecting portion2bof the punch top portion2aand the punch shoulder R portion2c, line displacement of the character line on the door outer panel can be suppressed while the occurrence of forming defect, such as deflection or insufficient inflow, is suppressed.

Next, the present invention will be more specifically described with reference to examples.

Under the following conditions of the press tooling and the conditions of the means 1 shown in Table 1, the punch2for evaluating line displacement shown inFIG.11AandFIG.11Bwas prepared, and press forming was performed on a specimen (TS270 MPa-class galvannealed steel sheet, a sheet thickness of 0.7 mm, a length of 400 mm, a width of 200 mm) using the production apparatus1which uses the means 1. Then, the amount of line displacement was quantitatively evaluated using evaluation parameter S=L×|H|1/3described in Patent Document 1.

[Conditions of Press Tooling]

With lubrication

First portion of punch: region A inFIG.11AandFIG.11B

TABLE 1InventiveInventiveExample ofExample ofConventionalpresentpresentSectionexampleinvention 1invention 2Presence or absence of particleAbsentPresentPresentfixing portion 5Kind of particles—CBN(Cubic boron nitrideabrasive grain)Grain size—#200#800Height of holding layer 5b [μm]—51.812.6Particle size of particles 5a [μm]—7418Particle ratio of particles 5a—12.512.5Embedding ratio of particles 5a—7070Vickers hardness of particles 5a—750750[HV]Static friction coefficient0.170.390.36Difference in static friction—0.220.19coefficient with respectto flat plate

The results are shown in the graph inFIG.12. An evaluation parameter S value is calculated using a displacement width L [mm] so that a small S value means that the amount of line displacement is small.

As shown in the graph inFIG.12, assuming an evaluation parameter S value of a flat plate in the conventional example as 1, an evaluation parameter S was improved to approximately 0.9 in Inventive Examples 1, 2 of the present invention.

Sensuous evaluation was also performed with respect to the Inventive Examples 1, 2 of the present invention by visual observation. Also in the sensuous evaluation, it was confirmed that line displacement is suppressed in the Inventive Examples 1, 2 of the present invention.

Under test conditions shown in Table 2 using a production apparatus1where the means 1 is replaced with the means 2, FEM analysis was performed on the above-mentioned press tooling for evaluating line displacement, and the amount of line displacement was quantitatively evaluated using the evaluation parameter S=L×|H|1/3described in Patent Document 1. As described in paragraph 0116, those skilled in the art can adjust a static friction coefficient by appropriately selecting a height, a diameter, a density, a center distance to an adjacent protrusion and the like.

In this FEM analysis, a friction coefficient of a portion in the vicinity of the projecting portion2bof the punch2and a friction coefficient of the punch shoulder R portion2cwere partially increased by forming the protrusions8. In such a state, the amount of movement (the amount of displacement) of an initial bent portion of the blank formed by the projecting portion2bwas obtained.

In this analysis, the reference of the static friction coefficient was set to 0.10.

FIG.13AandFIG.13Bare graphs showing the analysis results.FIG.13Ashows the evaluation results in the case where the protrusions8were provided to the projecting portion2b, andFIG.13Bshows the evaluation results in the case where the protrusions8were provided to the punch shoulder R portion2c.

As shown in the graphs inFIG.13AandFIG.13B, assuming an evaluation parameter S value of the reference flat plate (static friction coefficient of 0.10) as 1, the evaluation parameter S was significantly improved in Inventive Examples 1 to 3 of the present invention where the static friction coefficient of a high contact pressure portion was increased.

REFERENCE SIGNS LIST