U.S. patent application number 15/309947 was filed with the patent office on 2017-09-21 for mixed powder for powder metallurgy.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). Invention is credited to Nobuaki AKAGI, Mitsuhiro SATO, Hironori SUZUKI.
Application Number | 20170266723 15/309947 |
Document ID | / |
Family ID | 54698735 |
Filed Date | 2017-09-21 |
United States Patent
Application |
20170266723 |
Kind Code |
A1 |
SATO; Mitsuhiro ; et
al. |
September 21, 2017 |
MIXED POWDER FOR POWDER METALLURGY
Abstract
An objective of the present invention is to provide a mixed
powder for powder metallurgy that makes it possible to improve
mold-filling ability and reduce spread in weight of molded bodies.
The mixed powder for powder metallurgy according to the present
invention is obtained by mixing a graphite powder with an average
particle diameter D50 of 1.0 .mu.m or more to 3.0 .mu.m or less and
D90 of 10 .mu.m or less, without adding a binder, with an
iron-based powder, while applying a sheer force. The thus obtained
mixed powder for powder metallurgy according to the present
invention is characterized by including the iron-based powder and
the graphite powder present so as to be collected in concave
portions of the iron-based powder.
Inventors: |
SATO; Mitsuhiro;
(Takasago-shi, JP) ; AKAGI; Nobuaki;
(Takasago-shi, JP) ; SUZUKI; Hironori;
(Takasago-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi
JP
|
Family ID: |
54698735 |
Appl. No.: |
15/309947 |
Filed: |
May 14, 2015 |
PCT Filed: |
May 14, 2015 |
PCT NO: |
PCT/JP2015/063889 |
371 Date: |
November 9, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 2302/40 20130101;
B22F 2304/10 20130101; B22F 2998/10 20130101; C22C 33/02 20130101;
B22F 2301/35 20130101; B22F 2303/01 20130101; B22F 1/02 20130101;
C22C 33/0207 20130101; B22F 2301/10 20130101; B22F 1/0059 20130101;
C22C 38/02 20130101; B22F 1/0003 20130101; C22C 38/002 20130101;
B22F 2003/023 20130101; C22C 38/00 20130101; C22C 38/04 20130101;
B22F 3/10 20130101; B22F 3/02 20130101; B22F 2998/10 20130101; B22F
1/0059 20130101; B22F 1/0014 20130101 |
International
Class: |
B22F 1/00 20060101
B22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 2014 |
JP |
2014-111418 |
Claims
1. A mixed powder, obtained by a process comprising mixing a
graphite powder with an average particle diameter D50 of 1.0 .mu.m
or more to 3.0 .mu.m or less and D90 of 10 .mu.m or less and an
iron-based powder without adding a binder while applying a sheer
force.
2. A mixed powder, comprising an iron-based powder and a graphite
powder present so as to be collected in concave portions of the
iron-based powder.
3. The mixed powder according to claim 1, wherein the average
particle diameter D50 of the graphite powder is 1.6 .mu.m or more
to 2.7 .mu.m or less.
4. The mixed powder according to claim 1, wherein the iron-based
powder is an atomized iron powder or reduced iron powder.
5. The mixed powder according to claim 2, wherein the iron-based
powder is an atomized iron powder or reduced iron powder.
6. The mixed powder according to claim 3, wherein the iron-based
powder is an atomized iron powder or reduced iron powder.
Description
TECHNICAL FIELD
[0001] The present invention relates to a powder metallurgy
technique for manufacturing a sintered body by molding and
sintering a mixed powder for which an iron-based powder is a main
starting material, and more particularly to a mixed powder for
powder metallurgy with which packing ability to pack a mold with
the mixed powder can be improved and spread in weight of the
obtained molded bodies can be reduced.
BACKGROUND ART
[0002] In powder metallurgy in which a sintered body is
manufactured using an iron powder or a copper powder as a main
starting material, a mixed powder is usually used which includes a
powder serving as the main starting material, a graphite powder for
improving physical properties of a sintered body, an auxiliary
starting material powder such as an alloying component, a
lubricant, and the like. In particular, a carbon-supplying
component, that is, a carbon source, such as graphite, is added,
molding is performed, and the carbon source is then caused to
diffuse and penetrate into an iron powder in the heating and
sintering step in order to improve mechanical properties, such as
strength and hardness, of the sintered body.
[0003] However, the specific gravity and particle diameter of
graphite are less than those of the iron powder, and the resulting
problem is that where graphite is simply mixed with the iron
powder, the two are significantly separated, the graphite
segregates, and uniform mixing is impossible. In powder metallurgy,
a mixed powder is usually stored in advance in a storage hopper to
enable mass production of sintered bodies. In the storage hopper,
graphite with a small specific gravity tends to segregate to the
upper layer portion in the hopper, and when the mixed powder is
discharged from the hopper, the concentration of graphite becomes
higher in the latest part of the discharge from the hopper, so a
cementite structure precipitates in the portion of the sintered
body with a high graphite concentration, resulting in degradation
in mechanical properties. Where spread in the amount of carbon in
the sintered body occurs due to graphite segregation, products with
stable quality are difficult to manufacture. Further, the
segregation of graphite causes dusting of the graphite powder in
the mixing step or molding step, and this worsens the working
environment and makes the mixed powder more difficult to handle.
This segregation occurs not only in graphite but also in a variety
of other powders which are mixed with iron powders, and there is a
demand to prevent such segregation.
[0004] Generally, the following three methods have been suggested
for preventing the segregation and dusting. In the first method, as
disclosed, for example, in Patent Literature 1 and 2, a liquid
additive such as tall oil is added to a mixed powder. The merit of
this method is that the mixed powder can be manufactured with
simple equipment, but a problem lies in that when the liquid
additive is added in an amount such that the segregation-preventing
effect can be confirmed, a liquid bridging force acts between the
iron powder particles and the flowing ability is greatly degraded.
In the second method, as disclosed, for example, in Patent
Literature 3 and 4, a solid binder such as a high-molecular polymer
is dissolved in a solvent and uniformly mixed, and the solvent is
thereafter evaporated to cause adhesion of graphite to the surface
of an iron powder. The merits of this method are that graphite can
be reliably attached to the iron powder and a wide variety of
lubricants can be selected for use, but the problem associated with
this method is that, although depending on the amount and type of
the mixed powder, the flowing ability thereof can be insufficient
or compressibility can be degraded. The third method, which is
disclosed, for example, in Patent Literature 5, is the so-called
hot-melt method in which a lubricant with a comparatively low
molecular weight, such as a fatty acid, is heated and melted during
mixing with an iron powder. In this method, in order to attach the
melted lubricant fixedly and uniformly to the iron powder surface,
the temperature has to be controlled throughout the mixing process
and the number of lubricants that can be selected for use is
limited, which constitutes a drawback. The third method is also
problematic in terms of productivity, because it is necessary to
wait until the lubricant cools down.
[0005] Patent Literature 6 filed by the applicant of the present
application discloses a technique in which, in contrast with the
three methods above, graphite with a controlled average particle
diameter is mixed, without adding a binder, with an iron-based
powder while applying a sheer force, thereby suppressing the
segregation of the graphite powder. In addition, it is indicated
that this technique also ensures excellent flowability of the mixed
powder. In powder metallurgy, flowing ability is one of important
characteristics of a mixed powder when the mixed powder is
discharged from a storage hopper and packed into a mold. While
Patent Literature 6 adopts fluidity of a mixed powder, stipulated
by JIS Z2502 etc., as the indicator of flowability, mold-filling
ability is also an important characteristic in addition to the
fluidity, which is characterized by a mixed powder being discharged
from a hopper through a hose and packed satisfactorily into a mold.
When this mold-filling ability is decreased, it results in spread
in weight of molded bodies.
CITATION LIST
Patent Literature
[0006] Patent Literature 1: Japanese Patent Application Publication
No. S60-502158 [0007] Patent Literature 2: Japanese Patent
Application Publication No. H6-49503 [0008] Patent Literature 3:
Japanese Patent Application Publication No. H5-86403 [0009] Patent
Literature 4: Japanese Patent Application Publication No. H7-173503
[0010] Patent Literature 5: Japanese Patent Application Publication
No. H1-219101 [0011] Patent Literature 6: Japanese Patent
Application Publication No. 2012-102355
SUMMARY OF INVENTION
[0012] It is an objective of the present invention to provide a
mixed powder for powder metallurgy which has improved mold-filling
ability and can reduce the spread in weight of the molded
bodies.
[0013] The mixed powder for powder metallurgy according to the
present invention that attains the objective is obtained by mixing
a graphite powder with an average particle diameter D50 of 1.0
.mu.m or more to 3.0 .mu.m or less and D90 of 10 .mu.m or less,
without adding a binder, with an iron-based powder, while applying
a sheer force. The mixed powder for powder metallurgy of the
present invention which is thus obtained is characterized by
including the iron-based powder and the graphite powder which is
present so as to be collected in concave portions of the iron-based
powder.
[0014] In the present invention, an average particle diameter D50
of the graphite powder is preferably 1.6 .mu.m or more to 2.7 .mu.m
or less, and the iron-based powder is preferably an atomized iron
powder or a reduced iron powder.
[0015] In accordance with the present invention, since the graphite
powder with D50 and D90 within the predetermined ranges is mixed,
without adding a binder, with the iron-based powder, while applying
a sheet force, the graphite powder is present so as to be collected
in concave portions of the iron-based powder, satisfactory
mold-filling ability can be ensured, and spread in weight of the
molded bodies can be reduced.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 are schematic diagrams illustrating a mold-filling
ability evaluation device used in the below-described examples,
where FIG. 1A is a front view, and FIGS. 1B to 1D are
cross-sectional views illustrating the states during operation.
[0017] FIG. 2 is a scanning electron microscope photograph obtained
when observing the mixed powder of the present invention in the
below-described examples.
[0018] FIG. 3 is a scanning electron microscope photograph obtained
when observing the mixed powder of the present invention in the
below-described examples.
DESCRIPTION OF EMBODIMENTS
[0019] On the basis of the technique disclosed in Patent Literature
6, the inventors have investigated the relationship between the
particle diameter of graphite and spread in weight of molded bodies
with the object of reducing the spread in weight of molded bodies
which was not addressed in Patent Literature 6, and have conducted
the following experiments. Commercial natural graphite
(manufactured by Nippon Kokuen Group, CPB, average particle
diameter 22.6 .mu.m) was pulverized in a dry jet mill to obtain an
average particle diameter D50 shown in Table 1. A mixed powder was
obtained by mixing the graphite powder obtained by the
pulverization, an iron powder (manufactured by Kobe Steel, Ltd.,
Atomel 300M, particle diameter: 180 .mu.m or less, average particle
diameter: 70 .mu.m), a copper powder (manufactured by Fukuda Metal
Foil & Powder Co., Ltd., CuAtw-250), and zinc stearate
(manufactured by Adeka Corporation, ZNS-730) as a lubricant. The
mixing ratio is 0.8 part by mass of the graphite powder, 2 parts by
mass of the copper powder, and 0.75 part by mass of the lubricant
per 97.2 parts by mass of the iron powder. The mixing was performed
for 4 min at 300 rpm by using a high-speed mixer having a stirring
blade.
[0020] A total of 300 testpieces with a target weight of 51 g were
continuously molded with a mechanical powder molding press by using
the obtained mixed powder, and the spread in weight of the obtained
molded bodies was evaluated. The spread in weight was evaluated by
the difference R (g) between the maximum weight and minimum weight
of 300 molded bodies. The results are shown in Table 1.
TABLE-US-00001 TABLE 1 Average particle diameter of Spread graphite
powder D50 (.mu.m) in weight R (g) Reference Example 1 Graphite is
not added 1.35 Reference Example 2 22.6 7.84 Reference Example 3
19.9 7.78 Reference Example 4 9.3 4.36 Reference Example 5 8.2 4.00
Reference Example 6 6.7 3.24 Reference Example 7 5.1 2.42 Reference
Example 8 5.0 2.46 Reference Example 9 3.1 2.33 Reference Example
10 2.7 2.45 Reference Example 11 1.8 2.43
[0021] The spread in weight in Reference Example 1 in which no
graphite powder was added during mixing was 1.35 g, and it is clear
that the spread in weight in Reference Examples 2 to 11 in which
the graphite powder was added was larger than that in Reference
Example 1. Further, although the spread in weight generally tends
to decrease with the decrease in the average particle diameter D50
of the graphite powder, it is impossible to conclude that the
spread in weight is simply affected only by the average particle
diameter D50 of the graphite powder, in particular, as shown by
Reference Examples 6 to 11. Further, the target of the spread being
about 4% or less with respect to the target weight, that is, about
2 g or less, cannot be attained by only controlling the average
particle diameter of the graphite powder.
[0022] Accordingly, the inventors have conceived of adjusting not
only the average diameter D50 of the graphite powder, as suggested
in Patent Literature 6, but also D90. As indicated by the
below-described examples, by mixing the graphite powder with
adjusted D50 and D90 with an iron-based powder, while applying a
sheer force, it is possible to rub the graphite powder into the
concave portions present on the surface of the iron-based powder,
increase the mold-filling ability, and reduce the spread in weight
of the molded bodies.
[0023] To obtain the sufficient rubbing effect into the concave
portions, the D50 of the graphite powder is set to 3.0 .mu.m or
less. The D50 is preferably 2.7 .mu.m or less, and more preferably
2.5 .mu.m or less. From the standpoint of the rubbing effect, a
smaller D50 of the graphite powder is preferred, but where the D50
becomes too small, although the spread in weight can be reduced,
the molded body density during press molding is greatly reduced and
the strength of the part obtained by sintering the molded body
cannot be ensured. Accordingly, the D50 of the graphite powder was
set to 1.0 .mu.m or more. The D50 of the graphite powder is
preferably 1.1 .mu.m or more, and more preferably 1.6 .mu.m or
more. From the standpoint of attaining all of the reduction in the
spread in weight of molded bodies, the improvement of mold-filling
ability, and the increase in the molded body density at a high
level, it is preferred that the D50 of the graphite powder be 1.6
.mu.m or more to 2.7 .mu.m or less. The decrease in molded body
density observed when the D50 of the graphite powder is less than
1.0 .mu.m can be explained by the collapse of the layered structure
of graphite and loss of lubricating ability of graphite due to
excessive pulverization.
[0024] The graphite powder of the present invention with the D50
adjusted to the ranges mentioned hereinabove can be obtained by
pulverizing the commercial natural graphite or artificial graphite,
and the usual pulverizer may be used for the pulverization. The
pulverization atmosphere is not particularly limited, and the
pulverization may be performed with a dry or wet system. The usual
pulverizer may be used, examples thereof including a roll crusher,
a cutter mill, a rotary crusher, a hammer crusher, a jet mill, a
vibration mill, a pin mill, a wing mill, a ball mill, and a
planetary mill.
[0025] In the pulverized graphite powder, the specific surface area
is increased and chemical forces are believed to be acting in
addition to physical forces such as electrostatic forces. Thus, a
large number of functional groups such as hydrogen groups are
apparently present on the pulverized surface of the finely
pulverized graphite, intermolecular forces act between the iron
powder and the graphite powder through the functional groups, and
the adhesion between the graphite powder and the iron powder
increases. The presence/absence of the functional groups and the
amount thereof can be determined, to a certain degree, by heating
the graphite powder in a nitrogen atmosphere and measuring the
weight variation rate from room temperature to 950.degree. C. The
temperature increase rate when the temperature is raised from the
room temperature to 950.degree. C. may be about 10.degree. C./min.
The type of gas generated from a graphite powder usually differs
depending on a heating temperature range, and the type of
functional groups removed in each temperature range can be
estimated from the type of the generated gas. It is generally known
that carboxyl groups (--COOH) or hydroxyl groups (--OH) are removed
at 150.degree. C. to 500.degree. C., oxo groups (.dbd.O) are
removed at 500.degree. C. to 900.degree. C., and hydrogen groups
(--H) are removed at 900.degree. C. or more. By examining the
weight loss from 150.degree. C. to 950.degree. C., it is possible
to eliminate the influence of weight loss of moisture that can be
removed at a temperature lower than 150.degree. C. and determine
the type and amount of functional groups contained in the graphite
powder.
[0026] In the present invention, it is important not only to adjust
the D50 of the graphite powder to the predetermined range, but also
to set the D90 to 10 .mu.m or less. By setting the D90 to 10 .mu.m
or less, it is possible to reduce the amount of the graphite powder
rubbed into the concave portions of the iron-based powder. The D90
of the graphite powder is preferably 9.5 .mu.m or less, more
preferably 9.0 .mu.m or less, and particularly preferably 8.5 .mu.m
or less. A smaller D90 of the graphite powder is preferred, but the
lower limit thereof is usually about 3.5 .mu.m. In order to set the
D90 of the graphite powder within such ranges, air-jet
classification may be performed after pulverization in the above
pulverizer.
[0027] The D50 and D90 of the graphite powder each can be measured
with a particle size distribution measuring device of a laser
diffraction type. The D50 means a cumulative particle diameter
corresponding to 50% (based on the volume standard), and D90 means
a cumulative particle diameter corresponding to 90% (based on the
volume standard).
[0028] The amount of the graphite powder is usually 0.1 part by
mass to 2.5 parts by mass per a total of 100 parts by mass of the
iron-based powder, graphite powder, and the below-described
strength enhancer. In application to parts for mechanical
structures, a compounding ratio of 0.2 part by mass to 1.2 parts by
mass is often used, and the graphite powder can be advantageously
used in this range.
[0029] In order to attain the satisfactory mold-filling ability, it
is important to mix the graphite powder and the iron-based powder
while applying a sheer force, without adding a binder. As a result
of applying a sheer force, it is possible to rub the graphite
powder into the concave portions of the iron-based powder. Further,
since no binder is added, the adhesion of the graphite powder
outside the concave portions, for example, to convex portions, of
the iron-based powder can be suppressed, and the graphite powder is
present so as to be collected in the concave portions of the
iron-based powder. Where a large amount of the graphite powder is
present outside the concave portions of the iron-based powder, the
flowability of the mixed powder is worsened and the mold-filling
ability is degraded. With a method in which a binder is added or a
method different from the below-described mixing method in which a
sheer force is applied, a large amount of the graphite powder is
also present outside the concave portions of the iron-based powder
and satisfactory mold-filling ability cannot be attained.
[0030] Another advantageous effect of not adding a binder is that
the density of the molded body molded under the same molding
pressure, and the density of the sintered body obtained by
sintering the molded body are higher than those obtained when a
binder is added, and a sintered body with a satisfactory strength
is obtained. Furthermore, with the mixed powder of the present
invention to which no binder is added, a step of binder removal
which is performed between a molding step and a sintering step can
be omitted or simplified, which makes a contribution to the
increase in productivity of sintered parts and improvement of
environment.
[0031] A mixing method in which a sheer force is applied is a
method different from counterflow mixing methods represented by
methods implemented with a V-shaped mixer and a double-cone mixer,
and mixing methods using a vibration mill or a ball mill, such as a
vibro mill and an electromagnetic mill. Mixing in which a sheer
force is applied can be realized, for example, by using a mixer
equipped with a stirring blade. The stirring blade preferably moves
so as to cut the powder, and examples of the shape thereof include
paddle, turbine, ribbon, screw, multistage blade, anchor-type,
horseshoe-type, and gate-type shapes. The mixer container may be of
a fixed or rotating configuration, provided that it is equipped
with a stirring blade. Specific examples of the mixers equipped
with a stirring blade include a high-speed mixer, a plow-type
mixer, and a Nauta mixer. The mixing time varies depending on the
type of the mixer used and the amount of mixed powder, but is
generally 1 min to 20 min.
[0032] The graphite powder and iron-based powder may be mixed by a
dry or wet system. The mixing sequence of the graphite powder and
iron-based powder is not particularly limited, and the powders may
be simultaneously placed into the mixer, or initially one powder
may be loaded into the mixer and then the other powder may be
loaded. The graphite powder and iron-based powder may be mixed, for
example, at a normal temperature, rather than under heating to a
temperature equal to or higher than that at which a lubricant, or
the like, melts, as in the so-called hot-melt method.
[0033] The powder for powder metallurgy of the present invention
may include a lubricant and, for example, at least one physical
property improving agent from among a strength enhancer, an
abrasion resistance improving agent, and a machinability-improving
agent, in addition to the graphite powder and iron-based powder.
These components may be added when mixing the graphite powder and
iron-based powder. The addition sequence thereof is not
particularly limited and, for example, the lubricant and
machinability-improving agent may be added to the mixer
simultaneously with the graphite powder and iron-based powder, or
the graphite powder and iron-based powder may be mixed and then the
lubricant and machinability-improving agent may be added by one or
in a combination of two or more thereof, while mixing, e.g., by
actuating a stirring blade.
[0034] Examples of the lubricant include metallic soaps, alkylene
bis-fatty acid amides, and fatty acids, and these may be used
individually or in combinations of two or more thereof. A fatty
acid salt can be used as the metallic soap. For example, a fatty
acid salt with a carbon number of 12 or more, in particular, zinc
stearate, can be advantageously used. Specific examples of the
alkylene bis-fatty acid amides include C.sub.2-6 alkylene
bis-C.sub.12-24 carboxylic acid amides, and ethylene bis-stearyl
amide can be advantageously used. Compounds represented by
R.sub.1COOH, with R.sub.1 being, for example, a hydrocarbon group,
can be used as the fatty acid, carboxylic acids with a carbon
number of about 16-22 are preferred, and stearic acid and oleic
acid are particularly preferred. The amount of the lubricant is,
for example, 0.3 part by mass or more to 1.5 parts by mass or less,
preferably 0.5 part by mass or more to 1.0 part by mass or less per
a total of 100 parts by mass of the iron-based powder, graphite
powder, and strength enhancer.
[0035] Examples of strength enhancers include powders containing at
least one of copper, nickel, chromium, molybdenum, manganese, and
silicon, specific examples including a copper powder, a nickel
powder, a chromium-containing powder, a molybdenum powder, a
manganese-containing powder, and a silicon-containing powder. The
strength enhancers may be used individually or in combinations of
two or more thereof. The amount added of the strength enhancer is,
for example, 0.2 part by mass or more to 5 parts by mass or less,
more preferably 0.3 part by mass or more to 3 parts by mass or less
per a total of 100 parts by mass of the iron-based powder, graphite
powder, and strength enhancer.
[0036] Examples of abrasion resistance improving agents include
hard particles such as carbides, silicides, and nitrides which may
be used individually or in combinations of two or more thereof.
[0037] Examples of machinability-improving agents include manganese
sulfide, talc, and calcium fluoride which may be used individually
or in combinations of two or more thereof.
[0038] The iron-based powder used in the present invention may be a
pure iron powder or an iron alloy powder. The iron alloy powder may
be a partially alloyed powder in which an alloying powder, for
example, of copper, nickel, chromium, and molybdenum is diffusion
adhered to the surface of an iron-based powder, or a pre-alloyed
powder obtained from molten iron or molten steel including an
alloying component similar to the alloying powder. The iron-based
powder may be an atomized iron powder obtained by atomizing molten
iron or steel, or may be a reduced iron powder obtained by reducing
an iron ore or mill scale. An iron powder which is usually used for
machinery parts can be used as the iron-based powder. More
specifically, an iron-based powder with an average particle
diameter D50 of 70 .mu.m to 100 .mu.m and a maximum particle
diameter of 250 .mu.m or less, preferably 180 .mu.m or less, is
preferred. The average particle diameter of the iron-based powder
means a cumulative 50% pass particle diameter when the particle
size distribution is measured according to Japan Powder Metallurgy
Association Standard JPMA P02-1992 (Sieve Analysis and Test Methods
for Metal Powders).
[0039] In accordance with the present invention, the mold-filling
ability can be increased and the spread in weight of molded bodies
can be reduced. As a result of using the mixed powder of the
present invention, the spread in weight estimated by the maximum
value and minimum value of the molded body weight when a plurality
of molded bodies is molded can be made 4% or less with respect to
the target weight.
[0040] The present application claims priority based on Japanese
Patent Application No. 2014-111418 filed on May 29, 2014. The
entire contents of Japanese Patent Application No. 2014-111418
filed on May 29, 2014 are incorporated herein by reference.
EXAMPLES
[0041] The present invention will be explained hereinbelow in
greater detail with reference to examples. The present invention is
not intended to be limited to the below-described examples and
obviously can be implemented with appropriate modifications within
the scope adaptable to the essence described hereinbefore and
hereinafter, and all those modifications are included in the
technical scope of the present invention.
[0042] Commercial natural graphite (manufactured by Nippon Kokuen
Group, CPB, average particle diameter 22.6 .mu.m) was pulverized in
a dry jet mill such as to obtain an average particle diameter D50
shown in Tables 4 to 6 below, and the D90 was that of the
as-pulverized powder or was adjusted by air jet classification. A
mixed powder was obtained by mixing the graphite powder obtained by
the pulverization, an iron powder (manufactured by Kobe Steel,
Ltd., Atomel 300M, 300NH, or 250M), a copper powder (manufactured
by Fukuda Metal Foil & Powder Co., Ltd., CuAtw-250), and zinc
stearate (manufactured by Adeka Corporation, ZNS-730) as a
lubricant. The mixing ratio was 0.8 part by mass of the graphite
powder, 2 parts by mass of the copper powder, and 0.75 part by mass
of the lubricant per 97.2 parts by mass of the iron powder. The
mixing was performed for 4 min at 300 rpm by using a high-speed
mixer having a stirring blade. The following evaluations (1) to (3)
were performed using the obtained mixed powder.
[0043] (1) Measurement of Spread in Weight of Molded Bodies
[0044] A total of 300 of ring-shaped testpieces with a target
weight of 51 g, an outer diameter of 30 mm, and an inner diameter
of 10 mm were molded by a mechanical powder molding press, and the
spread in weight of the obtained molded bodies was evaluated. The
spread in weight was evaluated by the difference R (g) between the
maximum weight and minimum weight of the 300 molded bodies.
[0045] (2) Measurement of Mold-Filling Ability
[0046] The mold-filling ability was evaluated using the evaluation
device depicted in FIGS. 1. FIGS. 1 depict a device for evaluating
the mold-filling ability of a powder, the device being configured
of a base 1 accommodating a cavity container 3, an air cylinder 5
fixedly provided on the base on the side other than that of the
cavity container 3, and a powder supply box 2 mounted on the distal
end of a rod 4 of the air cylinder 5. The powder supply box 2 is a
bottomless box that, by means of an operation of the air cylinder
5, moves on an upper surface of the base 1 in a substantially
air-tight manner such as to reciprocate over the cavity container
3. The cavity container 3 has a slit-shaped cavity with a width of
several millimeters that is formed to extend in the direction
perpendicular to the reciprocating movement direction of the powder
supply box 2. FIG. 1A is a front view of the evaluation device, and
FIGS. 1B to 1D are cross-sectional views illustrating the state of
the powder supply box as it moves.
[0047] The measurement sequence is described below. Initially, as
depicted in FIG. 1B, a predetermined amount of the powder is loaded
into the powder supply box 2 in a state in which the rod 4 of the
air cylinder 5 is extended. Then, the rod 4 of the air cylinder 5
is contracted, and the powder supply box 2 is allowed to pass at a
predetermined rate above the slit-shaped cavity of the cavity
container 3. As a result of such passing, the powder contained in
the powder supply box 2 falls down into the cavity container 3, as
depicted in FIG. 1C. After the powder supply box 2 passes, as
depicted in FIG. 1D, the interior of the cavity container 3 is
packed with the powder. The size of the powder supply box 2 is
80.times.80.times.70 mm, the size of the cavity container 3 is
80.times.60.times.55 mm, the size of the slit is 2.times.60 mm, and
the shoe rate, that is, the passage rate of the powder supply box
2, is 100 mm/s. Three tests were performed for each Experiment No.,
the amount (mg) of the powder packed in each test was divided by
120 mm.sup.2 which was the surface area of the slit-shaped cavity,
and the average value of the obtained values was taken as the
mold-filling ability (mg/mm.sup.2) of each No.
[0048] (3) Measurement of Molded Body Density
[0049] The obtained mixed powder was loaded in a predetermined mold
and molded under a press pressure of 490 MPa and 686 MPa to
fabricate tablet-shaped testpiece with .phi.11.28 mm. The density
of the resultant molded body was then measured.
[0050] Properties of Atomel 300M, 300NH, and 250M, which were the
iron-based powders used, are presented in Tables 2 and 3. The
apparent density presented in Tables 2 and 3 was measured by the
method according to JIS Z2504 (Metal Powders--Apparent Density Test
Methods), and the fluidity was measured by the method according to
J1S Z2502 (Fluidity Test Methods for Metal Powders). The apparent
density of Atomel 300NH is large and the apparent density of 250M
is small, with 300M being taken as a reference. Thus, it can be
said that in 300NH, the unevenness of the iron powder surface is
small and the degree of difference in shape is low, and in 250M,
the unevenness is large and the degree of difference in shape is
high, with 300M being taken as a reference. The apparent density of
250M is substantially the same as the apparent density of the
reduced iron powder.
[0051] The results of (1) and (3) above are presented in Tables 4
to 6. Atomel 300M (average particle diameter: about 70 .mu.m),
Atomel 300NH (apparent density 3.10 g/cm.sup.3, average particle
diameter: about 90 .mu.m), and Atomel 250M (apparent density 2.42
g/cm.sup.3, average particle diameter: about 85 .mu.m) were used as
the iron-based powder in Tables 4, 5, and 6, respectively.
TABLE-US-00002 TABLE 2 Powder properties Apparent density Fluidity
Chemical component composition (mass %) Steel grade (g/cm.sup.3)
(s/50 g) C Si Mn P S O 300M 2.85 to 3.05 .ltoreq.30 .ltoreq.0.02
.ltoreq.0.05 0.10 to 0.30 .ltoreq.0.020 .ltoreq.0.020 .ltoreq.0.25
300NH 2.95 to 3.10 .ltoreq.30 .ltoreq.0.01 .ltoreq.0.03
.ltoreq.0.10 .ltoreq.0.010 .ltoreq.0.010 .ltoreq.0.20 250M 2.40 to
2.60 20 to 30 .ltoreq.0.02 .ltoreq.0.05 0.10 to 0.30 .ltoreq.0.020
.ltoreq.0.020 .ltoreq.0.25
TABLE-US-00003 TABLE 3 Particle size distribution (%) Steel 250
.mu.m or 180 .mu.m to 150 .mu.m to 106 .mu.m to grade more 250
.mu.m 180 .mu.m 150 .mu.m 75 .mu.m to 106 .mu.m 63 .mu.m to 75
.mu.m 45 .mu.m to 63 .mu.m 45 .mu.m or less 300M -- .ltoreq.1
.ltoreq.10 10 to 25 15 to 30 5 to 20 8 to 23 20 to 40 300NH
.ltoreq.1 .ltoreq.15 .ltoreq.15 10 to 30 15 to 30 5 to 20 8 to 23
10 to 30 250M .ltoreq.1 .ltoreq.10 .ltoreq.10 10 to 25 20 to 35 5
to 20 8 to 23 15 to 35
TABLE-US-00004 TABLE 4 Particle Molded diameter of body density
graphite powder (g/cm.sup.3) Mold-filling Experiment D50 490
ability No. (.mu.m) D90 (.mu.m) R (g) MPa 686 MPa (mg/mm.sup.2) 1-1
22.6 49.8 7.84 6.81 7.15 28.5 1-2 19.9 44.6 7.78 6.81 7.15 28.9 1-3
9.3 18.3 4.36 6.82 7.16 29.9 1-4 8.2 16.4 4.00 6.82 7.16 30.6 1-5
6.7 11.8 3.24 6.83 7.16 31.5 1-6 5.1 8.9 2.42 6.84 7.17 32.6 1-7
5.0 12.1 2.46 6.84 7.17 32.0 1-8 3.1 12.3 2.33 6.83 7.17 33.8 1-9
3.0 9.8 2.00 6.83 7.16 34.5 1-10 2.7 11.8 2.45 6.82 7.16 33.5 1-11
2.7 7.8 1.99 6.82 7.16 36.3 1-12 2.5 4.7 1.97 6.81 7.16 36.6 1-13
1.8 11.7 2.43 6.81 7.16 35.9 1-14 1.7 7.0 1.98 6.81 7.16 37.2 1-15
1.6 4.2 1.96 6.81 7.14 37.0 1-16 1.1 3.9 1.95 6.80 7.11 35.5 1-17
0.8 3.7 1.90 6.77 7.08 33.7 *Atomel 300M was used as the iron-based
powder
TABLE-US-00005 TABLE 5 Particle Molded diameter of body density
graphite powder (g/cm.sup.3) Mold-filling Experiment D50 490
ability No. (.mu.m) D90 (.mu.m) R (g) MPa 686 MPa (mg/mm.sup.2) 2-1
19.9 44.6 7.70 6.93 7.22 29.8 2-2 5.0 12.1 2.36 6.96 7.25 33.1 2-3
2.7 7.8 1.88 6.94 7.23 37.5 2-4 1.7 7.0 1.90 6.93 7.23 37.4 2-5 0.8
3.7 1.92 6.87 7.15 34.7 *Atomel 300NH was used as the iron-based
powder
TABLE-US-00006 TABLE 6 Particle Molded diameter of body density
graphite powder (g/cm.sup.3) Mold-filling Experiment D50 490
ability No. (.mu.m) D90 (.mu.m) R (g) MPa 686 MPa (mg/mm.sup.2) 3-1
19.9 44.6 6.50 6.80 7.14 30.1 3-2 5.0 12.1 2.43 6.83 7.16 32.5 3-3
2.7 7.8 1.95 6.82 7.14 36.8 3-4 1.7 7.0 1.88 6.80 7.13 35.9 3-5 0.8
3.7 1.94 6.75 7.05 34.0 *Atomel 250M was used as the iron-based
powder
[0052] Tables 4 to 6 indicate that in Experiment No. 1-9, 1-11,
1-12, 1-14 to 1-16, 2-3, 2-4, 3-3, and 3-4 of the present invention
in which the D50 of the graphite powder was 3.0 .mu.m or less and
the D90 was 10 .mu.m or less, the spread in weight R could be made
2.0 g or less, that is, 4% or less of the target weight, and the
molded body density was satisfactory. FIG. 2 is a scanning electron
microscope photograph obtained when observing Experiment No. 2-3.
It is clear from FIG. 2 that the graphite powder is present so as
to be collected in concave portions of the iron powder. FIG. 3 is a
scanning electron microscope photograph obtained when observing
Experiment No. 3-3. In FIG. 3, the state in which the graphite
powder is present so as to be collected in concave portions can be
also observed.
[0053] Comparing No. 1-9, 1-11, 1-12, 1-14 to 1-16 that are the
invention examples in Table 4 which used Atomel 300M with No. 2-3
and 2-4 that are the invention examples in Table 5 which used
Atomel 300NH, the spread in weight R in the invention examples in
Table 5 is further reduced. As mentioned hereinabove, in Atomel
300NH, the unevenness of the iron powder surface is less and the
degree of difference in shape is lower than those of Atomel 300M,
but since the ratio of particles with a large particle diameter was
high and the width of the concave portions was large, the graphite
powder could be sufficiently introduced therein.
[0054] Meanwhile, in Experiment No. 1-1 to 1-8, 1-10, and 1-13,
Experiment No. 2-1 and 2-2, and Experiment No. 3-1 and 3-2 in which
the D50 of the graphite powder was greater than 3.0 .mu.m or the
D90 was larger than 10 .mu.m, the spread in weight R increased.
Further, in Experiment No. 1-17, 2-5, and 3-5, the D50 was less
than 1.0 .mu.m. As a result, although the spread in weight R could
be made 2.0 g or less, the molded body density was less than that
of the experiment examples of the present invention. Since the
molded body density is also influenced by the shape of the
iron-based powder, it is appropriate to evaluate the molded body
density for each type of the iron-based powder. Thus, the
evaluation shows that the molded body density of No. 1-17 is lower
than that of No. 1-9, 1-11, 1-12, and 1-14 to 1-16, the molded body
density of No. 2-5 is lower than that of No. 2-3 and 2-4, and the
molded body density of No. 3-5 is lower than that of No. 3-3 and
3-4.
INDUSTRIAL APPLICABILITY
[0055] In the present invention, quality can be stabilized, for
example, dimensional changes caused by the refinement of graphite
powder can be minimized, and the reduction of energy consumption
and cost, such as decrease in sintering temperature and shortening
of sintering time, can be realized in the production of sintered
parts. The mixed powder of the present invention is suitable for
sintered parts for mechanical structures, in particular for thin
parts and parts of complex shape. Further, since weight reduction
is enabled, the mixed powder is also advantageous for high-strength
materials.
REFERENCE SIGNS LIST
[0056] 1 Base [0057] 2 Powder supply box [0058] 3 Cavity container
[0059] 4 Rod [0060] 5 Air cylinder
* * * * *