U.S. patent application number 13/716039 was filed with the patent office on 2013-05-02 for press working method.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD. The applicant listed for this patent is SUMITOMO ELECTRIC INDUSTRIES, LTD. Invention is credited to Ryuichi Inoue, Nozomu Kawabe, Takahiko Kitamura, Koji Mori, Nobuyuki Mori, Masatada Numano, Yukihiro Oishi, Nobuyuki Okuda.
Application Number | 20130104619 13/716039 |
Document ID | / |
Family ID | 42039387 |
Filed Date | 2013-05-02 |
United States Patent
Application |
20130104619 |
Kind Code |
A1 |
Inoue; Ryuichi ; et
al. |
May 2, 2013 |
PRESS WORKING METHOD
Abstract
The invention offers a pressed product (F) produced by
press-forming a metal plate (1). The pressed product (F) has a
peripheral surface that has a corner portion (12) connecting two
surfaces in the peripheral surface. The corner portion (12) has an
outside corner radius R that is equal to or smaller than the
thickness "t" of the metal plate (1). That is, the pressed product
(F) has the sharp corner portion (12).
Inventors: |
Inoue; Ryuichi; (Osaka-shi,
JP) ; Kawabe; Nozomu; (Osaka-shi, JP) ; Oishi;
Yukihiro; (Osaka-shi, JP) ; Okuda; Nobuyuki;
(Osaka-shi, JP) ; Mori; Nobuyuki; (Osaka-shi,
JP) ; Numano; Masatada; (Osaka-shi, JP) ;
Kitamura; Takahiko; (Osaka-shi, JP) ; Mori; Koji;
(Itami-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMITOMO ELECTRIC INDUSTRIES, LTD,; |
Osaka-shi |
|
JP |
|
|
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD,
Osaka-shi
JP
|
Family ID: |
42039387 |
Appl. No.: |
13/716039 |
Filed: |
December 14, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13063657 |
Mar 11, 2011 |
|
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PCT/JP2009/062855 |
Jul 16, 2009 |
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13716039 |
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Current U.S.
Class: |
72/379.2 |
Current CPC
Class: |
B21D 22/02 20130101;
B21D 5/00 20130101; B21D 51/52 20130101; Y10T 428/12382 20150115;
B21D 22/20 20130101 |
Class at
Publication: |
72/379.2 |
International
Class: |
B21D 5/00 20060101
B21D005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 18, 2008 |
JP |
2008-239712 |
Claims
1-9. (canceled)
10. A method for press working a metal plate, the method
comprising: a first step of forming a pressed material having a
side wall portion and an inside corner portion by pressing a first
punch on the metal plate, the first punch including a shoulder
portion having a corner radius Rp of 0.3 mm or less; and a second
step of forming a pressed product by pressing with a second punch,
the inside corner portion and an end face of the side wall portion
of the pressed material.
11. The method according to claim 10, wherein the second step of
forming the pressed product includes: forming a thickness of a
top-plate portion of the pressed product and a thickness of the
side-wall portion of the pressed product substantially uniform.
12. The method according to claim 10, wherein the first and second
steps are performed under temperature of 200.degree. C. or more and
300.degree. C. or less.
13. The method according to claim 10, wherein the metal plate
comprises: 8.3 mass % or more and 9.5 mass % or less Al; 0.5 mass %
or more and 1.5 mass % or less Zn, and the remainder that comprises
Mg and impurities.
14. The method according to claim 10, further comprising the step
of: heat treating the pressed product at a temperature of
100.degree. C. to 450.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to a pressed product to be
used, for example, for the housing of a portable electronic device,
particularly to a pressed product having a sharp corner
portion.
BACKGROUND ART
[0002] Metal such as aluminum or its alloy is used as the material
for the housing of a portable electronic device or the like, such
as a cellular phone and a note-book-type personal computer.
Generally, in comparison with resin, metal has a higher strength
and is more resistant to an impact.
[0003] As the foregoing material for the housing, magnesium alloys
have been used which are formed by adding various elements to
magnesium. Although a magnesium alloy has excellent specific
strength and specific rigidity, it has poor plastic workability at
ordinary temperatures because it has a hexagonal crystalline
structure (hexagonal close-packed structure). Consequently, the
housing and the like are mainly formed by using a cast product
produced by a die-casting process or a thixomold process. In recent
years, engineers have been studying the performing of press working
on a magnesium alloy (Patent Literatures 1 and 2).
SUMMARY OF INVENTION
Technical Problem
[0004] A typical shape of the housing is a box type provided with a
rectangular top plate and four side walls formed from the edges of
the top plate. For the box-type shaped body, the market desires to
obtain a housing having sharpness both at the corner portion
connecting the top plate and the side wall and at the corner
portion connecting two side walls. It is likely that a housing
having sharp corner portions can be formed when the injection
molding of resin or casting is used. Nevertheless, a resinous
product and a cast product generally have a lower strength than
that of a pressed product of metal.
[0005] When the box-type shaped body falls and collides with the
ground or the like, the impact at the time of the collision is
applied to the above-described corner portions in many cases. As a
result, in a resinous product and a cast product both having low
strength, the corner portion is deformed (broken) or otherwise
damaged, so that it is difficult for the corner portion to maintain
the sharp condition.
Solution to Problem
[0006] The present invention is made in view of the foregoing
circumstances and offers a shaped body having a sharp corner
portion and having high strength. More specifically, the present
invention offers a pressed product produced by press-forming a
metal plate. The pressed product has a peripheral surface that has
a corner portion connecting two surfaces in the peripheral surface.
The corner portion has an outside corner radius R satisfying the
condition that R is equal to or smaller than (2/3).times.t, where
"t" is the thickness of the metal plate. The pressed product is
formed by using a material including 8.3 mass % or more and 9.5
mass % or less Al, 0.5 mass % or more and 1.5 mass % or less Zn,
and the remainder that includes Mg and impurities. In the pressed
product:
[0007] (a) the outside corner radius R is 0.2 mm or more and 0.4 mm
or less,
[0008] (b) the thickness "t" is 0.4 mm or more and 0.8 mm or less,
and
[0009] (c) the corner portion has a hardness of 90 Hv or more."
[0010] According to the above structure, by performing the press
forming on a metal plate, the hardness of the corner portion is
increased by the work hardening owing to the plastic working.
Consequently, even when an impact is applied to the corner portion
and the like, deformation is less likely to occur, so that the
sharp corner portion can be maintained for a long time. Moreover,
because the pressed product of the present invention is formed by
the press forming of a metal plate, in addition to the strength of
the material itself, the strength can be increased by the plastic
working, so that the entire pressed product has high strength.
Furthermore, because the pressed product of the present invention
has the foregoing sharp corner portion, it can give an impression
of stylishness with a refined design. As a result, it is expected
that the pressed product has excellent appearance as a commodity
and therefore has an enhanced commercial value.
[0011] The above-described pressed product of the present invention
having a sharp corner portion can be produced, for example, by
performing the below-described multistage press working on a blank
plate made of metal. More specifically, the production method is to
produce a pressed product having a corner portion by performing
press working on a metal plate and has the steps described
below.
[0012] A step of preparing a blank plate: this step prepares a
blank plate made of metal.
[0013] A first pressing step: this step produces a pressed material
that has at least one corner portion connecting two surfaces in the
peripheral surface under the condition that the blank plate is
heated at a temperature of 200.degree. C. or more and 300.degree.
C. or less. In particular, the first press working is performed so
that at least one corner portion can have an inside corner radius
"r" that is practically 0 mm by using a punch having a shoulder
portion with a corner radius Rp that is practically 0 mm.
[0014] A second pressing step: this step produces a pressed product
that has at least one corner portion having an outside corner
radius R that is equal to or smaller than the thickness "t" of the
metal plate by performing the second press working under the
condition that the foregoing pressed material is heated at a
temperature of 200.degree. C. or more and 300.degree. C. or less.
In particular, the second press working is performed so that the
above-described corner portion, which has an inside corner radius
"r" that is practically 0 mm, can have an outside corner radius R
that is equal to or smaller than the foregoing thickness "t" by
using a step-shaped punch for pressing both the end face of the
pressed material and the corner portion, which has been formed on
the inside surface in the first pressing step and which has "r"
that is practically 0 mm.
[0015] A sharp corner portion having an outside corner radius R
that is equal to or smaller than the thickness "t" of the metal
plate can become easily formed when the blank plate to be pressed
has the thinnest possible thickness. Nevertheless, when the blank
plate itself is excessively thin, the strength of the pressed
product is decreased, so that it cannot satisfy the strength and
rigidity required for the housing of a portable electronic device.
On the other hand, to increase through the work hardening the
hardness of the corner portion of a pressed product, the corner
portion being likely to suffer from an impact at the time of the
falling, it can be conceived to form the corner portion at a high
working ratio. However, when the bending or deep drawing is
performed at a high working ratio, the corner portion-formed place
in the blank plate is partially elongated reducing its thickness.
This thickness decrease leads to the reduction in strength.
[0016] In consideration of the foregoing phenomenon, the
above-described production method carries out the press forming at
a high working ratio by dividing the process into multiple stages
as described above, not by a process of one stage. Consequently,
the corner portion is prevented from becoming extremely thin, so
that the reduction in strength resulting from the thickness
reduction can be suppressed. As a result, the above-described
production method can not only produce a pressed product having a
sharp corner portion but also maintain the sharp corner portion for
a long period.
[0017] In addition, the above-described production method performs
the press working under a heated condition. Consequently, even a
metal that has poor plastic workability and therefore develops
springback, cracking, or the like in the cold working, such as
magnesium alloy having an elongation of 20% or so at the most at
ordinary temperatures, can increase the elongation of the object to
be worked (the blank plate and pressed material) to 100% or more at
the time of the pressing. Furthermore, because the object to be
worked has a sufficient elongation, a pressed product that has a
corner portion having an extremely small outside corner radius R
can be produced with high precision.
Advantageous Effects of Invention
[0018] The pressed product of the present invention has a sharp
corner portion and high strength.
[0019] According to the above-described production method, a corner
portion formed on the peripheral surface, that is, the corner
portion forming the appearance, is sharp. In addition, a corner
portion formed on the inside surface is also sharp. Consequently,
the pressed product has an ample internal space. As a result, when
the pressed product obtained by the foregoing production method is
used for a housing, various components can be adequately housed in
the housing.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIG. 1 is a schematic cross-sectional view for explaining
the procedure of the press working in the process of producing a
pressed product having a sharp corner. The (A) portion of FIG. 1
shows a state in which a blank plate is placed in a die. The (B)
portion of FIG. 1 shows a state in which a pressed material is
formed by the first press working. The (C) portion of FIG. 1 shows
a state in which a punch is performing pressing in the second press
working. The (C') portion of FIG. 1 is a partially enlarged view of
the corner portion shown in the (C) portion. The (D) portion of
FIG. 1 shows a state in which a pressed product having a sharp
corner portion is formed by the second press working. The (E)
portion of FIG. 1 shows an obtained pressed product.
DESCRIPTION OF EMBODIMENTS
[0021] An explanation is given below to the embodiments of the
present invention. In the explanation of the drawings, the same
component bears the same sign to eliminate duplicated explanations.
The ratio of the dimensions in the drawing does not necessarily
coincide with that of the explanation.
Pressed Product
Composition
[0022] The pressed product of the present invention may be formed
by using various metals having excellent press formability at a
temperature range of 200.degree. C. or more and 300.degree. C. or
less. In particular, it is desirable to use magnesium or its alloy
as the material for forming, for example, the housing of a portable
electronic device required to have light weight, because magnesium
and its alloy are lightweight, have high strength, and have
excellent impact resistance.
[0023] It is possible to use magnesium alloys having various
compositions formed by adding various elements to Mg (the
remainder: Mg and impurities). The types of magnesium alloy include
Mg--Al-based alloy, Mg--Zn-based alloy, Mg-RE (rare-earth
element)-based alloy, and Y-added alloy. In particular,
Mg--Al-based alloy, which contains Al, has high corrosion
resistance. The types of Mg--Al-based alloy include, as specified
in the Standards of American Society for Testing and Materials
(ASTM), AZ-family alloy (Mg--Al--Zn-based alloy, Zn: 0.2 to 1.5
mass %), AM-family alloy (Mg--Al--Mn-based alloy, Mn: 0.15 to 0.5
mass %), AS-family alloy (Mg--Al--Si-based alloy, Si: 0.6 to 1.4
mass %), and Mg--Al-RE (rare-earth element)-based alloy. It is
desirable that the Al content be 1.0 to at most 11 mass %. In
particular, an Mg--Al-based alloy containing 8.3 to 9.5 mass % Al
and 0.5 to 1.5 mass % Zn, represented by AZ91 alloy, has excellent
corrosion resistance and mechanical properties such as strength and
resistance to plastic deformation in comparison with other
Mg--Al-based alloys such as AZ31 alloy. The types of Mg--Zn-based
alloy, containing Zn, include, as specified in the ASTM Standards,
ZK-family alloy (Mg--Zn--Zr-based alloy, Zr: 3.5 to 6.2 mass %, Zn:
0.45 mass % or more).
Shape
[0024] The pressed product of the present invention is formed by
performing press working, such as bending and deep drawing, on a
metal plate. In the pressed product, the peripheral surface has at
least one corner portion that connects two surfaces in the
peripheral surface. A typical shape of the pressed product has a
top-plate portion (an undersurface portion) and a side-wall portion
formed from the edge of the top-plate portion. More specifically,
the types of the shape include a box-shaped body having a top-plate
portion with the shape of a rectangular plate and only one pair of
opposed side-wall portions, a box-shaped body having two pairs of
opposed side-wall portions, and a lidded hollow cylindrical body
having a top-plate portion with the shape of a circular plate and a
hollow cylindrical side-wall portion. The top-plate portion and the
side-wall portion each have an outside surface and an inside
surface that are practically parallel to each other. In the pressed
product of the present invention, the corner portion connecting two
outside surfaces is sharp.
[0025] The above-described top-plate portion and side-wall portion
are each typically formed with a flat surface; their shape and size
are not particularly limited. They may have a unitarily formed or
bonded boss or the like. They may have a through hole penetrating
from the outside surface to the inside surface or a groove recessed
in the thickness direction. They may have a stepped shape. They may
have a portion having a locally different thickness, the portion
being formed by plastic working or cutting. The portion other than
the corner portion has a nearly uniform thickness, except the
above-described portions having a boss, a recessed portion, and a
different thickness. Hence, the thickness "t" of the metal plate
forming the pressed product is defined as the average thickness of
the entire portion excluding the foregoing boss and the like and
the corner portion. More specifically, five or more measuring spots
are chosen from the portion other than the above-described portions
such as the boss to obtain the average value. When the top-plate
portion is formed of a flat surface, the foregoing thickness "t"
may be the average thickness of the entire top-plate portion. The
thickness "t" mostly depends on the thickness of the blank plate.
In the case where the plate is not subjected to the press working,
the thickness "t" is practically the same as that of the blank
plate.
[0026] When the above-described metal plate has a thickness "t" of
0.4 mm or more and 2.0 mm or less, a pressed product having
excellent strength is obtained. When the thickness is less than 0.4
mm, because the blank plate is thin, it is easy to form a pressed
product having a smaller outside corner radius R. As a result, a
pressed product having very excellent appearance, such as
stylishness, is obtained. When the metal plate has a thickness "t"
of 2.0 mm or less, particularly 1.5 mm or less, yet particularly
0.6 mm or less, it is expected that the pressed product of the
present invention can be suitably used, for example, for the
housing of a portable electronic device.
[0027] The most noticeable feature of the pressed product of the
present invention is that in the peripheral surface, at least one
corner portion connecting two surfaces in the peripheral surface
has an outside corner radius R that is equal to or smaller than
"t." The conventional pressed product made of a metal plate has had
an outside corner radius larger than the thickness of the metal
plate; conventionally, no pressed product has satisfied the
condition that R is equal to or smaller than "t." In contrast, the
pressed product of the present invention has at least one corner
portion satisfying the condition that R is equal to or smaller than
"t." When all corner portions existing in the pressed product of
the present invention satisfy the condition that R is equal to or
smaller than "t," it is expected that the stylishness can be
further enhanced.
[0028] In particular, when the outside corner radius R satisfies
the condition that R is equal to or smaller than (2/3).times.t, the
corner portion can easily have a hardness higher than that of the
portion other than the corner portion, such as the top-plate
portion, so that the corner portion can have improved impact
resistance. When the condition that R is equal to or smaller than
(1/2).times.t is satisfied, the impact resistance of the corner
portion and the stylishness can be further improved. More
specifically, it is desirable that the outside corner radius R be
0.1 to 0.3 mm. When the outside corner radius R is 0.1 mm or more,
the possibility of being cut or damaged by a sharp corner portion
can be reduced. When R is 0.3 mm or less, the corner portion can
have excellent impact resistance while obtaining good appearance.
The decreasing of the outside corner radius R can be achieved, for
example, by increasing the pressing pressure in the second pressing
step.
[0029] In the pressed product obtained by the above-described
production method, at the corner portion that satisfies the
condition that R is equal to or smaller than "t," the inside corner
radius "r" becomes practically 0 mm. In other words, in the inside
surface, the two surfaces positioned at the inside of the
above-described corner portion are practically perpendicular to
each other. Such a pressed product has an ampler internal space
than the pressed product having an inside corner radius "r" that is
larger than zero and therefore can be suitably used for a housing
to be used to house various components.
Production Method
Preparation of the Blank Plate
[0030] When magnesium alloy is used to form the blank plate, it is
desirable to use a rolled sheet produced by rolling a plurality of
times a cast sheet produced by a continuous casting process such as
the twin-roll process, particularly the casting process stated in
WO/2006/003899. Because the continuous casting process enables the
rapid solidification, the creation of oxides and segregation can be
decreased, so that a cast sheet having excellent rolling
workability can be obtained. When a cast sheet is subjected to heat
treatment such as the solution treatment (heating temperature:
380.degree. C. to 420.degree. C., heating time: 60 to 600 minutes)
or the aging treatment, the composition can be homogenized. In
particular, in the case of a magnesium alloy having a high Al
content, it is desirable to perform the solution treatment for a
long time. The size of the cast sheet is not particularly limited.
Nevertheless, if the sheet is excessively thick, segregation tends
to be created. Hence, it is desirable that the thickness be 10 mm
or less, particularly 5 mm or less.
[0031] When a plurality of rolling operations are performed on the
above-described cast sheet, the desired sheet thickness can be
obtained and the average crystal grain size can be decreased. In
addition, for a magnesium alloy having a high Al content, such as
AZ91 alloy, defects such as coarse impurities in crystal and coarse
precipitated impurities can be eliminated, so that the press
workability can be increased. The rolling operation may be
performed by combining with a well-known condition, for example, in
the case of magnesium alloy, the controlled rolling disclosed in
Patent Literature 2 or the like. When during the course of the
rolling operation, an intermediate heat treatment (heating
temperature: 250.degree. C. to 350.degree. C., heating time: 20 to
60 minutes) is conducted to remove or decrease the strain, residual
stress, aggregated texture, and so on all introduced into the
object to be worked through the working before the intermediate
heat treatment, the subsequent rolling operation can be performed
more smoothly by preventing accidental cracking, strain, and
deformation. The obtained rolled sheet may undergo a heat treatment
at 300.degree. C. or more to remove the work strain resulting from
the rolling operation and to achieve complete recrystallization.
Alternatively, the obtained rolled sheet may acquire strain through
the use of a roller leveler or the like under a heated condition to
be recrystallized during the press working.
Press Working
[0032] It is desirable that the press working in multiple stages be
performed in a temperature range of 200.degree. C. to 300.degree.
C. in every stage in order to increase the plastic workability of
the object to be worked (the blank plate and pressed material).
[0033] In the first press working, the use of a punch having a
shoulder portion with a corner radius Rp nearly equal to zero
(desirably, the corner radius Rp is 0.3 mm or less) forms a pressed
material having a sharp corner portion (practically right-angled)
at the inside, that is, a pressed material having an inside surface
in which a corner portion is formed by two surfaces perpendicular
to each other. In the second press working, the end face of the
pressed material and the foregoing inside corner portion are
pressed with a step-shaped punch. Thus, constituting materials of
the blank plate are forcefully gathered at the corner portion of
the die, and the forcefully gathered constituting materials are
deformed in such a way that a sharp corner portion is formed at the
outside of the pressed material. At the time of the press working,
a suitable die is used, such as a movable die or a recessed
die.
[0034] After the press forming, heat treatment may be conducted in
order to remove the strain and residual stress introduced by the
press working and to improve the mechanical property. An example of
the heat treatment condition is as follows: heating temperature:
100.degree. C. to 450.degree. C., heating time: 5 minutes to 40
hours or so. When the pressed product obtained by the press working
is provided with a covering layer aiming at corrosion proofing,
protection, and ornamentation, the corrosion resistance, commercial
value, and so on can be further increased.
Test Example 1
[0035] A plurality of pressed products each made of magnesium alloy
and having corner portions were produced to examine the outside
corner radius R of the corner portions and the hardness of the
pressed products.
[0036] A plurality of cast sheets (thickness: 4 mm) were prepared
that were made of magnesium alloy having a composition equivalent
to that of AZ91 alloy (Mg, 9.0 mass % Al, and 1.0 mass % Zn) and
that were produced by the twin-roll continuous casting process. The
obtained cast sheets were subjected to a plurality of rolling
operations until the thickness was reduced to 0.6 mm under the
following rolling conditions: roller temperature: 150.degree. C. to
250.degree. C., sheet temperature: 200.degree. C. to 400.degree.
C., and rolling reduction per pass: 10% to 50%. The obtained sheet
materials were subjected to blanking to prepare blank plates for
the press forming.
[0037] The prepared blank plates underwent two stages of press
working. FIG. 1 is a schematic cross-sectional view for explaining
the procedure of the press working. FIG. 1 shows the blank plate by
emphasizing it.
[0038] The first press working produces a pressed material P (a
box-shaped body having outside dimensions of 45.times.95.times.6
mm) having a flat top-plate portion 10 as shown in the (B) and (C)
portions of FIG. 1 and two pairs of flat side-wall portions 11
formed from the top-plate portion 10. More specifically, as shown
in the (A) portion of FIG. 1, a blank plate B having a thickness
"t" of 0.6 mm is placed on a plate 51 and a die plate 52, and on
the blank plate B, a punch 53 and a holding plate 54 are placed.
Then, under the condition that the blank plate B is sandwiched
between the plate 51 and the punch 53, the punch 53 is moved
downward in FIG. 1 to form the pressed material. The punch 53 has a
shoulder portion whose corner radius Rp is practically 0 mm, and
the two surfaces forming the shoulder portion are perpendicular to
each other. In the pressed material P produced by this press
working, the corner portion 12, which connects the outside surface
10o of the top-plate portion 10 and the outside surface 11o of the
side-wall portion 11, has an outside corner radius R.sub.0 that is
larger than the thickness t.sub.0 of the top-plate portion 10. In
addition, the inside surface 10i of the top-plate portion 10 and
the inside surface 11i of the side-wall portion 11 are
perpendicular to each other. In other words, the inside corner
radius r.sub.0 of the corner portion 12 is practically 0 mm. In the
above description, the plate 51, the die plate 52, the punch 53,
the holding plate 54, and the below-described stepped punch 55 and
die 56 all can be heated with a heating means, which is unshown. In
the first press working, the heating temperature was 200.degree. C.
or more.
[0039] The second press working uses, for example, as shown in the
(C) and (D) portions of FIG. 1, a protrusion-type stepped punch 55
and a die 56 having a recessed portion to press an end face 11e of
the side-wall portion 11 of the pressed material P. This pressing
operation produces a pressed product F that has a sharp corner
portion whose outside corner radius R is equal to or smaller than
the thickness "t" of a metal plate 1. The stepped punch 55 has an
end-portion-pressing face 55p and a shoulder portion 55s that, at
the inside surface of the pressed material P, is brought into
contact with an inside corner portion 12i, which is nearly
right-angled, to press the inside corner portion 12i. In the
recessed portion of the die 56, a bottom face 56b and a side face
56s are perpendicular to each other, so that the corner portion is
right-angled.
[0040] As shown in the (C) and (C') portions of FIG. 1, under the
condition that the pressed material P is placed in the die 56,
which has a recessed portion fitting to the outer contour of the
box-shaped pressed material P, the inside of the pressed material P
is pressed with the stepped punch 55. At this moment, first, the
end-portion-pressing face 55p of the stepped punch 55 presses the
end face 11e of the side-wall portion 11. Then, when the stepped
punch 55 continues the downward pressing, a main pressing face 55m
of the stepped punch 55 is brought into contact with the inside
surface 10i of the top-plate portion 10 to press it. When the
inside corner portion 12i of the pressed material P is pressed with
the shoulder portion 55s of the stepped punch 55, constituting
materials both in a part of the side-wall portion 11 and in a part
of the top-plate portion 10 are forcefully gathered at the corner
portion of the recessed portion of the die 56. As a result, a sharp
corner portion is formed in the pressed material P in accordance
with the corner portion of the recessed portion. In the above
description, in order to facilitate uniform pressing of the end
face 11e of the side-wall portion 11, part of the side-wall portion
of the pressed material obtained by the first press working is
worked by the side cutting, so that the pressed material has a
side-wall portion with a uniform height of 5.5 mm. Under this
condition, the second press working was carried out. In the second
press working, the magnitude of the outside corner radius R was
varied by varying the pressure at the time of the pressing.
[0041] Through the above-described process, as shown in the (E)
portion of FIG. 1, a pressed product F can be obtained in which a
corner portion 22 that connects an outside surface 20o of a
top-plate portion 20 and an outside surface 21o of a side-wall
portion 21 has an outside corner radius R that is equal to or
smaller than the thickness "t" of the top-plate portion 20. When
the thickness of the top-plate portion 20 and the side-wall portion
21 was measured using a pointed micrometer, the thickness was 0.6
mm, which is nearly equal to the thickness of the blank plate
B.
[0042] The obtained pressed product was subjected to the
measurements of the outside corner radius R, the hardness of the
corner portion, and the hardness of the top-plate portion. The
results are shown in Table I.
[0043] The outside corner radius R (mm) was measured by the
following method. First, a pressed product was cut in a direction
perpendicular to the ridge line between the outside surface of the
top-plate portion and the outside surface of the side-wall portion.
The cut surface was buff-polished (using diamond abrasive grain No.
200) and then observed under an optical microscope (400 power). The
observed image was used to measure the radius. Similarly, the
inside corner radius "r" (mm) of the corner portion was measured.
The result showed that the radius was practically 0 mm and the
inside surface of the top-plate portion and the inside surface of
the side-wall portion were practically perpendicular to each
other.
[0044] The hardness Hv of the corner portion was measured by the
following method. First, a pressed product was cut in a direction
perpendicular to the ridge line between the outside surface of the
top-plate portion and the outside surface of the side-wall portion.
The obtained cut piece was used to produce an embedded specimen.
The cut surface was mirror-polished. Three measuring spots were
chosen from the center portion of the plate in the thickness
direction in the cut surface of the pressed product. The hardness
of the individual measuring spots was measured using a
micro-Vickers hardness tester. The average value of the three
measurements is shown in Table I.
[0045] The hardness Hv of the top-plate portion was measured by the
following method. First, a part of the top-plate portion was cut
from the pressed product. The obtained cut piece was used to
produce an embedded specimen. The cut surface was mirror-polished.
Three measuring spots were chosen from the center portion of the
plate in the thickness direction in the cut surface of the
top-plate portion. The hardness of the individual measuring spots
was measured using a micro-Vickers hardness tester. The average
value of the three measurements is shown in Table I. Alternatively,
the hardness Hv may also be measured by producing an embedded
specimen including both the corner portion and top-plate
portion.
TABLE-US-00001 TABLE I Thickness of metal plate (mm) 0.6 Sample No.
1-1 1-2 1-3 1-4 1-5 Outside corner 0.6 0.5 0.4 0.3 0.2 radius R
(mm) Relation between R is R is R is R is R is R and t equal larger
equal equal smaller to t than to to than (2/3)t (2/3)t (1/2)t
(1/2)t Hardness of corner 85 86 90 95 97 portion (Hv) Hardness of
top-plate 85 86 85 85 86 portion (Hv)
[0046] As can be seen from Table I, the performing of the
multistage hot press working described above can produce with high
precision pressed products whose outside corner radius R satisfies
the condition that R is equal to or smaller than the thickness "t."
In particular, when the above-described press working is performed
on a metal plate, the corner portion and the top-plate portion have
a comparable hardness. Consequently, because these pressed products
have corner portions with high strength, it is expected that they
are less likely to be deformed when they undergo an impact such as
one due to the falling.
[0047] Table I also shows that when the outside corner radius R
satisfies the condition that R is equal to or smaller than
(2/3).times.t, the hardness of the corner portion is increased. The
reason for this is attributable to the work hardening resulting
from the multistage press working. Furthermore, when the outside
corner radius R satisfies the condition that R is equal to or
smaller than (1/2).times.t, the hardness of the corner portion is
significantly increased. It can be expected that these pressed
products have corner portions that are excellent in impact
resistance and that can accordingly maintain the sharp condition
for a long time.
[0048] In Test example 1 described above, an explanation is given
to the case where the corner portion connecting the top-plate
portion and the side-wall portion has an outside corner radius R
satisfying the condition that R is equal to or smaller than "t" in
a box-type shaped body having two pairs of side-wall portions.
Similarly, a pressed product can also be produced in which a corner
portion connecting the side-wall portions has an outside corner
radius R satisfying the condition that R is equal to or smaller
than "t." A pressed product having only one pair of side-wall
portions can also be produced. Furthermore, in Test example 1
described above, an explanation is given by referring to the
unitarily formed stepped punch. Nevertheless, a stepped punch
formed by combining divided pieces may also be used. For example, a
stepped punch may also be used that is provided with a divided
piece for pressing mainly the top-plate portion and another divided
piece for pressing the end face of the side-wall portion and part
of the top-plate portion.
Test Example 2
[0049] Blank plates having various thicknesses were prepared to
produce pressed products made of magnesium alloy. The produced
pressed products were subjected to examination of strength and
appearance.
[0050] Cast sheets (thickness: 4 mm) of the same type as prepared
in Test example 1 were prepared. The prepared cast sheets had a
composition equivalent to that of AZ91 alloy and were subjected to
a rolling operation by varying the number of times of rolling to
produce rolled sheets having various thicknesses (thicknesses: 0.3
to 0.8 mm). As with Test example 1, the obtained rolled sheets were
subjected to blanking to prepare blank plates. As with Test example
1, the individual blank plates underwent two stages of hot press
working (the heating temperature at the time of pressing was
selected as appropriate from the range of 200.degree. C. to
250.degree. C.). Thus, pressed products were produced each of which
had a flat top-plate portion and two pairs of flat side-wall
portions formed from the top-plate portion.
[0051] The obtained individual pressed products were subjected to
the measurement of the outside corner radius R (mm) of the corner
portion by the same method as used in Test example 1. The results
are shown in Table II.
[0052] In addition, the strength of the pressed product was
measured as described below. The pressed product was placed such
that the top-plate portion of the pressed product pointed upward on
the side-wall portion used as a supporting member. Under this
condition, a cemented-carbide ball having a diameter of 38 mm is
pressed into the center of the top-plate portion at a load of 1 kgf
(9.8 N) to deform the pressed product permanently. The amount of
deformation (the dimensional difference between the most protruding
portion and the most recessed portion in the peripheral surface of
the top-plate portion) is measured with a contact profilometer. The
amount of deformation is evaluated as the strength of the pressed
product. When the amount of deformation is 1 mm or more, the
pressed product is considered to have an insufficient strength and
evaluated as "poor." When the amount of deformation is less than 1
mm, the pressed product is considered to have a sufficient strength
and evaluated as "good." When the amount of deformation is less
than 0.3 mm, the pressed product is considered to have an excellent
strength and evaluated as "excellent." The results are shown in
Table II.
[0053] The appearance is evaluated through a panel test conducted
by 10 panelist chosen randomly. When five or less panelists judged
that the pressed product has a sharp and distinctive outside corner
portion, stylishness, and excellent designability, the pressed
product is evaluated as "poor." When six to eight panelists judged
as described above, the pressed product is evaluated as "good."
When nine or more panelists judged as described above, the pressed
product is evaluated as "excellent." The results are shown in Table
II.
TABLE-US-00002 TABLE II Thickness of top-plate Outside Relation
Amount of Evaluation Evaluation Sample portion corner radius
between deformation of of Total No. t (mm) R (mm) R and t (mm)
strength appearance evaluation 2-1 0.3 0.2 R is equal to (2/3)t
1.30 Poor Excellent Poor 2-2 0.4 0.2 R is equal to (1/2)t 0.55 Good
Excellent Good 2-3 0.5 0.2 R is equal to or smaller 0.35 Good
Excellent Good than (1/2)t 2-4 0.6 0.2 R is equal to or smaller
0.25 Excellent Excellent Excellent than (1/2)t 2-5 0.6 0.3 R is
equal to (1/2)t 0.25 Excellent Excellent Excellent 2-6 0.6 0.4 R is
equal to (2/3)t 0.25 Excellent Good Good 2-7 0.6 0.5 R is larger
than (2/3)t 0.25 Excellent Poor Poor 2-8 0.8 0.2 R is equal to or
smaller 0.18 Excellent Excellent Excellent than (1/2)t Note:
Between the evaluation of strength and the evaluation of
appearance, whichever is worse in the evaluation is used as the
total evaluation.
[0054] As can be seen from Table II, as the thickness "t" of the
top-plate portion increases, the strength is increased. In
addition, when the outside corner radius R is equal to or smaller
than (1/2).times.t, the strength is high and the appearance is
excellent.
[0055] The above-described embodiments may be changed as
appropriate without deviating from the gist of the present
invention and not limited to the above-described constitutions. For
example, the material of the metal plate may be changed from
magnesium alloy to aluminum, its alloy, and other various
metals.
INDUSTRIAL APPLICABILITY
[0056] The pressed product of the present invention can be suitably
used for various electronic devices, particularly for the housing
of a portable electronic device or the like.
REFERENCE SIGNS LIST
[0057] 1: metal plate [0058] 10: top-plate portion; 10o: outside
surface of the top-plate portion; 10i: inside surface of the
top-plate portion [0059] 11: side-wall portion; 11o: outside
surface of the side-wall portion; 11i: inside surface of the
side-wall portion; 11e: end face of the side-wall portion [0060]
12: corner portion; 12i: inside corner portion [0061] 20: top-plate
portion; 20o: outside surface of the top-plate portion; 21:
side-wall portion; [0062] 21o: outside surface of the side-wall
portion; 22: corner portion [0063] 51: plate; 52: die plate; 53:
punch; 54: holding plate; 55: stepped punch; 55m: main pressing
face; 55p: end-portion-pressing face; 55s: shoulder portion; 56:
die; 56b: bottom face; 56s: side face [0064] B: blank plate; P:
pressed material; F: pressed product
CITATION LIST
Patent Literature
[0064] [0065] PTL 1: the published Japanese patent application
Tokukai 2002-239644 [0066] PTL 2: the published Japanese patent
application Tokukai 2007-098470
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