U.S. patent application number 15/738739 was filed with the patent office on 2018-07-12 for method of producing mixed powder for powder metallurgy, method of producing sintered body, and sintered body.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Akio KOBAYASHI, Toshio MAETANI, Naomichi NAKAMURA, Itsuya SATO, Akio SONOBE.
Application Number | 20180193911 15/738739 |
Document ID | / |
Family ID | 58240660 |
Filed Date | 2018-07-12 |
United States Patent
Application |
20180193911 |
Kind Code |
A1 |
KOBAYASHI; Akio ; et
al. |
July 12, 2018 |
METHOD OF PRODUCING MIXED POWDER FOR POWDER METALLURGY, METHOD OF
PRODUCING SINTERED BODY, AND SINTERED BODY
Abstract
A method of producing a mixed powder for powder metallurgy
comprises: mixing an iron-based powder with a Mo-containing powder
and a Cu-containing powder, to obtain a raw material mixed powder;
heat-treating the raw material mixed powder to cause Mo and Cu to
diffusionally adhere to a surface of the iron-based powder, to
obtain a partially diffusion-alloyed steel powder; and mixing the
partially diffusion-alloyed steel powder with a graphite powder, to
obtain a mixed powder for powder metallurgy, wherein the iron-based
powder has an average particle size of 30 .mu.m to 120 .mu.m, a
cuprous oxide powder is used as the Cu-containing powder, and the
mixed powder for powder metallurgy has a chemical composition
containing Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0 mass
%, and C: 0.1 mass % to 1.0 mass %, with a balance being Fe and
incidental impurities.
Inventors: |
KOBAYASHI; Akio;
(Chiyoda-ku, Tokyo, JP) ; NAKAMURA; Naomichi;
(Chiyoda-ku, Tokyo, JP) ; MAETANI; Toshio;
(Chiyoda-ku, Tokyo, JP) ; SONOBE; Akio;
(Chiyoda-ku, Tokyo, JP) ; SATO; Itsuya;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku, Tokyo
JP
|
Family ID: |
58240660 |
Appl. No.: |
15/738739 |
Filed: |
September 12, 2016 |
PCT Filed: |
September 12, 2016 |
PCT NO: |
PCT/JP2016/004150 |
371 Date: |
December 21, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/12 20130101;
C22C 33/02 20130101; B22F 1/0059 20130101; B22F 2998/10 20130101;
C22C 38/16 20130101; C22C 1/05 20130101; C22C 33/0264 20130101;
C22C 38/00 20130101 |
International
Class: |
B22F 1/00 20060101
B22F001/00; C22C 33/02 20060101 C22C033/02; C22C 38/16 20060101
C22C038/16; C22C 38/12 20060101 C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 11, 2015 |
JP |
2015-179868 |
Claims
1. A method of producing a mixed powder for powder metallurgy, the
method comprising: mixing an iron-based powder with a Mo-containing
powder and a Cu-containing powder, to obtain a raw material mixed
powder; heat-treating the raw material mixed powder to cause Mo and
Cu to diffusionally adhere to a surface of the iron-based powder,
to obtain a partially diffusion-alloyed steel powder; and mixing
the partially diffusion-alloyed steel powder with a graphite
powder, to obtain a mixed powder for powder metallurgy, wherein the
iron-based powder has an average particle size of 30 .mu.m to 120
.mu.m, a cuprous oxide powder is used as the Cu-containing powder,
and the mixed powder for powder metallurgy has a chemical
composition containing Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass %
to 4.0 mass %, and C: 0.1 mass % to 1.0 mass %, with a balance
being Fe and incidental impurities.
2. The method of producing a mixed powder for powder metallurgy
according to claim 1, wherein the Cu-containing powder has an
average particle size of 5 .mu.m or less.
3. A method of producing a sintered body, the method comprising:
forming a mixed powder for powder metallurgy obtained by the method
of producing a mixed powder for powder metallurgy according to
claim 1; and sintering the formed mixed powder.
4. A sintered body obtained by the method of producing a sintered
body according to claim 3.
5. A method of producing a sintered body, the method comprising:
forming a mixed powder for powder metallurgy obtained by the method
of producing a mixed powder for powder metallurgy according to
claim 2; and sintering the formed mixed powder.
6. A sintered body obtained by the method of producing a sintered
body according to claim 5.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a method of producing a mixed
powder for powder metallurgy, and particularly relates to a method
of producing a mixed powder for powder metallurgy that, despite not
containing Ni, has properties suitable for the production of high
strength sintered parts for vehicles and the like. This disclosure
also relates to a method of producing a sintered body, and a
sintered body obtained by the production method.
BACKGROUND
[0002] Powder metallurgical techniques enable producing parts
having complicated shapes in shapes (i.e. near net shapes)
extremely close to product shapes, with high dimensional accuracy.
The use of powder metallurgical techniques in producing parts thus
contributes to significantly lower machining costs. For this
reason, powder metallurgical products obtained by powder
metallurgical techniques have been used as various mechanical parts
in many fields.
[0003] Powder metallurgical techniques mainly use iron-based
powders. Iron-based powders are categorized into iron powder (e.g.
pure iron powder), alloyed steel powder, and the like, depending on
the components. Iron-based powders are also categorized into
atomized iron powder, reduced iron powder, and the like, based on
the production method. In the case of using the categories by the
production method, the term "iron powder" has a broad meaning
encompassing not only pure iron powder but also alloyed steel
powder.
[0004] With powder metallurgical techniques, an iron-based powder
mentioned above is used to produce a green compact, and the green
compact is sintered to produce a sintered body. The green compact
is typically produced by mixing an iron-based powder with alloying
powders such as a Cu powder and a graphite powder and a lubricant
such as stearic acid or lithium stearate to obtain a mixed powder,
and then charging the mixed powder into a die and pressing it.
[0005] The density of a green compact obtained by a typical powder
metallurgical process is about 6.6 Mg/m.sup.3 to 7.1 Mg/m.sup.3.
The green compact is then sintered to obtain a sintered body. The
sintered body is further subjected to optional sizing and machining
work, to obtain a powder metallurgical part (component). In the
case where higher strength is required, carburizing heat treatment
or bright heat treatment may be performed after sintering.
[0006] Increases in strength of powder metallurgical parts have
been strongly requested recently, for reductions in size and weight
of parts. There has been particularly strong demand for
strengthening iron-based powder parts (iron-based sintered bodies)
made from iron-based powders.
[0007] As iron-based powders, the following powders each obtained
by adding alloying elements to a raw material powder (pure iron
powder) are mainly known:
[0008] (1) a mixed powder obtained by adding each alloying element
powder to a pure iron powder;
[0009] (2) a pre-alloyed steel powder obtained by completely
alloying a pure iron powder with each alloying element; and
[0010] (3) a partially diffusion-alloyed steel powder (also
referred to as "composite alloyed steel powder") obtained by
causing each alloying element powder to partially diffusionally
adhere to the surface of a pure iron powder or a pre-alloyed steel
powder.
[0011] The mixed powder (1) is advantageous in that high
compressibility equivalent to that of a pure iron powder is
ensured. However, during sintering, each alloying element does not
sufficiently diffuse in Fe and the microstructure becomes
non-uniform, which can cause a decrease in the strength of the
resulting sintered body. Moreover, in the case where Mn, Cr, V, Si,
etc. which are more easily oxidizable than Fe are used as alloying
elements, they are oxidized during sintering, so that the strength
of the resulting sintered body decreases. To suppress such
oxidation and lower the oxygen content in the sintered body, it is
necessary to strictly control the sintering atmosphere and, in the
case of performing carburizing after sintering, the CO.sub.2
concentration and the dew point in the carburizing atmosphere.
Hence, the mixed powder (1) has not been used due to its failure to
meet the recent requests for strengthening.
[0012] With the pre-alloyed steel powder (2), the segregation of
alloying elements can be prevented completely, with it being
possible to achieve uniform microstructure of the sintered body.
This contributes to stable mechanical properties of the sintered
body. In addition, even in the case where Mn, Cr, V, Si, etc. are
used as alloying elements, lower oxygen content in the sintered
body can be achieved by limiting the types and amounts of such
alloying elements. However, since the pre-alloyed steel powder is
produced by atomizing molten steel, oxidation of the molten steel
in the atomizing process and solid solution hardening of the steel
powder due to complete alloying tend to occur. This hinders an
increase of green density in press forming.
[0013] The partially diffusion-alloyed steel powder (3) is produced
by adding a metal powder of each alloying element to a pure iron
powder or a pre-alloyed steel powder and heating the mixture in a
non-oxidizing or reducing atmosphere to partially diffusionally
bond the metal powder to the surface of the pure iron powder or
pre-alloyed steel powder. The use of the partially
diffusion-alloyed steel powder can thus provide the advantages of
the iron-based mixed powder (1) and the pre-alloyed steel powder
(2), while avoiding the problems of the iron-based mixed powder (1)
and the pre-alloyed steel powder (2).
[0014] In detail, the partially diffusion-alloyed steel powder
achieves both lower oxygen content and high compressibility
equivalent to that of a pure iron powder. Moreover, since the
microstructure of the sintered body can be a composite structure
made up of a complete alloy phase and a partially concentrated
phase, the strength of the sintered body can be further improved.
The partially diffusion-alloyed steel powder has therefore been
widely developed as it can meet the recent requests for
strengthening parts.
[0015] Basic alloy components typically used in the production of
the partially diffusion-alloyed steel powder include Ni and Mo.
[0016] Ni has an effect of improving the toughness of the sintered
body. By adding Ni, austenite is stabilized, as a result of which
more austenite remains as retained austenite without transforming
into martensite after quenching. Ni also has an effect of
strengthening the matrix of the sintered body by solid solution
strengthening.
[0017] Mo has an effect of improving hardenability. Thus, Mo
suppresses the formation of ferrite during quenching and
facilitates the formation of bainite or martensite, to
transformation-strengthen the matrix of the sintered body. Mo also
has both an effect of solid solution strengthening by dissolving in
the matrix and an effect of strengthening the matrix by
precipitation by forming fine carbides. Furthermore, Mo has good
gas carburizing property and is a non-grain boundary oxidizable
element, and so has an effect of strengthening the sintered body by
carburizing.
[0018] As an example of a mixed powder for high strength sintered
parts using a partially diffusion-alloyed steel powder containing
these alloy components, JP 3663929 B (PTL 1) discloses a mixed
powder for high strength sintered parts yielded by mixing an
alloyed steel powder obtained by partially alloying Ni: 0.5 mass %
to 4 mass % and Mo: 0.5 mass % to 5 mass %, with Ni: 1 mass % to 5
mass %, Cu: 0.5 mass % to 4 mass %, and a graphite powder: 0.2 mass
% to 0.9 mass %.
[0019] As an example of an iron-based sintered body having high
density and not containing Ni, JP H4-285141 A (PTL 2) discloses a
method of producing an iron-based sintered body by mixing an
iron-based powder of 1 .mu.m to 18 .mu.m in average particle size
with a Cu powder of 1 .mu.m to 18 .mu.m in average particle size at
a weight ratio of 100:(0.2 to 5) and forming and sintering the
mixed powder. The technique described in PTL 2 uses an iron-based
powder having an extremely smaller average particle size than a
typical iron-based powder, and thus achieves a high sintered body
density of 7.42 g/cm.sup.3 or more which is normally
impossible.
[0020] As an example of a mixed powder for high strength sintered
parts using a partially diffusion-alloyed steel powder, JP 4483595
B (PTL 3) discloses a mixed powder for high strength sintered parts
obtained by mixing an alloyed steel powder having diffusionally
adhering Ni and Mo, with a metal Cu powder and a graphite
powder.
CITATION LIST
Patent Literatures
[0021] PTL 1: JP 3663929 B
[0022] PTL 2: JP H4-285141 A
[0023] PTL 3: JP 4483595 B
SUMMARY
Technical Problem
[0024] However, we found out as a result of study that the sintered
materials produced using the respective mixed powders described in
PTL 1 and PTL 3 and the sintered material produced by the method
described in PTL 2 have the following problems.
[0025] The sintered material described in PTL 1 requires at least
1.5 mass % Ni, and substantially contains 3 mass % or more Ni as
can be understood from its Example. Thus, a large amount of Ni,
e.g. 3 mass % or more, is needed in order to achieve a high
strength of 800 MPa or more in the sintered material described in
PTL 1. A larger amount of Ni is likely to be needed in order to
obtain a sintered body having a strength of 1000 MPa or more after
carburizing, quenching, and tempering.
[0026] Ni, however, is an unfavorable element in terms of
environmental responsiveness and recyclability in recent years, and
it is desirable to avoid using Ni as much as possible. Adding a few
mass % Ni is also very disadvantageous in terms of cost. Besides,
the use of Ni as an alloying element requires prolonged sintering
in order to sufficiently diffuse Ni in the iron powder or alloyed
steel powder.
[0027] The sintered material described in PTL 2 contains no Ni, but
the average particle size of the iron-based powder used is 1 .mu.m
to 18 .mu.m which is smaller than normal. Such a small particle
size causes lower fluidity of the mixed powder, and decreases work
efficiency when filling the die with the powder in press
forming.
[0028] The sintered material described in PTL 3 contains a metal Cu
powder in the mixed powder. The metal Cu powder melts and permeates
between the particles of the iron powder during sintering, and thus
increases the distance between the particles of the iron powder.
This causes the size of the sintered body to be larger than the
size of the green compact. Consequently, the density of the
sintered body is lower than the density of the green compact. This
phenomenon is commonly known as Cu growth. A significant decrease
in density caused by the Cu growth leads to drawbacks such as lower
strength and toughness of the sintered body.
[0029] It could therefore be helpful to provide a method of
producing a mixed powder for powder metallurgy that, despite not
containing Ni (Ni-free), enables the production of a sintered body
having excellent properties (e.g. tensile strength and toughness
after carburizing, quenching, and tempering) at least as high as
those in the case of containing Ni. It could also be helpful to
provide a method of producing a sintered body using the mixed
powder for powder metallurgy, and a sintered body obtained by the
production method.
Solution to Problem
[0030] We conducted various studies on alloy components of a mixed
powder for powder metallurgy not containing Ni and means for adding
the alloy components. As a result, we made the following
discoveries (1) to (6).
[0031] (1) In some cases, by using a mixed powder for powder
metallurgy obtained by partially diffusing Mo and Cu in an
iron-based powder to yield a partially diffusion-alloyed steel
powder beforehand and mixing the partially diffusion-alloyed steel
powder with a graphite powder, a sintered body having properties at
least as high as those in the case of containing Ni is obtained,
despite not containing Ni.
[0032] (2) In the cases of (1), Mo functions as a
ferrite-stabilizing element during sintering. Hence, ferrite phase
forms in a portion having a large amount of Mo and its vicinity to
facilitate the sintering of the iron powder, as a result of which
the density of the sintered body increases.
[0033] (3) In the cases of (1), when carburizing and quenching the
sintered body, Cu shifts the martensite transformation start
temperature to the lower temperature side, thus strengthening the
sintered body.
[0034] (4) In the cases of (1), to obtain a sintered body having
excellent properties, it is necessary to limit the chemical
composition of the mixed powder for powder metallurgy to a specific
range, set the average particle size of the iron-based powder to 30
.mu.m to 120 .mu.m, and use not a metal Cu powder but a cuprous
oxide (Cu.sub.2O) powder as a Cu source when producing the
partially diffusion-alloyed steel powder.
[0035] (5) The use of a cuprous oxide powder can prevent Cu growth
which occurs in the case of using a metal Cu powder, and suppress a
decrease in the density of the sintered body.
[0036] (6) The use of an iron-based powder having an average
particle size of 30 .mu.m to 120 .mu.m can improve the fluidity of
the mixed powder for powder metallurgy.
[0037] This disclosure is based on the above-mentioned discoveries.
We thus provide the following.
[0038] 1. A method of producing a mixed powder for powder
metallurgy, the method comprising: mixing an iron-based powder with
a Mo-containing powder and a Cu-containing powder, to obtain a raw
material mixed powder; heat-treating the raw material mixed powder
to cause Mo and Cu to diffusionally adhere to a surface of the
iron-based powder, to obtain a partially diffusion-alloyed steel
powder; and mixing the partially diffusion-alloyed steel powder
with a graphite powder, to obtain a mixed powder for powder
metallurgy, wherein the iron-based powder has an average particle
size of 30 .mu.m to 120 .mu.m, a cuprous oxide powder is used as
the Cu-containing powder, and the mixed powder for powder
metallurgy has a chemical composition containing (consisting of)
Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0 mass %, and C:
0.1 mass % to 1.0 mass %, with a balance being Fe and incidental
impurities.
[0039] 2. The method of producing a mixed powder for powder
metallurgy according to 1., wherein the Cu-containing powder has an
average particle size of 5 .mu.m or less.
[0040] 3. A method of producing a sintered body, the method
comprising: forming a mixed powder for powder metallurgy obtained
by the method of producing a mixed powder for powder metallurgy
according to 1. or 2.; and sintering the formed mixed powder.
[0041] 4. A sintered body obtained by the method of producing a
sintered body according to 3.
Advantageous Effect
[0042] It is possible to obtain a mixed powder for powder
metallurgy that, despite not containing Ni, enables the production
of a sintered body having excellent properties at least as high as
those in the case of containing Ni. The mixed powder for powder
metallurgy has high fluidity, and so contributes to excellent work
efficiency when charging the mixed powder for powder metallurgy
into a die for press forming. Moreover, a sintered body having both
excellent strength and excellent toughness can be produced at low
cost, even with an ordinary sintering method.
DETAILED DESCRIPTION
[0043] One of the disclosed embodiments is described in detail
below.
[0044] A method of producing a mixed powder for powder metallurgy
according to one of the disclosed embodiments includes the
following (1) to (3):
[0045] (1) a first mixing step of mixing an iron-based powder with
a Mo-containing powder and a Cu-containing powder, to obtain a raw
material mixed powder;
[0046] (2) a diffusional adhesion step of heat-treating the raw
material mixed powder to cause Mo and Cu to diffusionally adhere to
the surface of the iron-based powder, to obtain a partially
diffusion-alloyed steel powder; and
[0047] (3) a second mixing step of mixing the partially
diffusion-alloyed steel powder with a graphite powder, to obtain a
mixed powder for powder metallurgy.
[0048] As the iron-based powder, an iron-based powder having an
average particle size of 30 .mu.m to 120 .mu.m is used. As the
Cu-containing powder, a cuprous oxide powder is used. The mixed
powder for powder metallurgy has a chemical composition containing
Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0 mass %, and C:
0.1 mass % to 1.0 mass %, with the balance being Fe and incidental
impurities.
[0049] With the conventional methods, a partially diffusion-alloyed
steel powder having diffusionally adhering Mo is mixed with a metal
Cu powder and a graphite powder to produce a mixed powder for
powder metallurgy, as mentioned above. With the method according to
this disclosure, on the other hand, Cu is caused to diffusionally
adhere to an iron-based powder together with Mo beforehand, and a
cuprous oxide powder is used as a Cu source for diffusional
adhesion of Cu.
[0050] Each of the steps (1) to (3) is described below. In the
following description, "%" denotes mass % unless otherwise noted.
Mo content, Cu content, and graphite powder content denote the
contents of the respective components in the whole mixed powder for
powder metallurgy.
[0051] [First Mixing Step]
[0052] In the first mixing step, an iron-based powder is mixed with
a Mo-containing powder and a Cu-containing powder, to obtain a raw
material mixed powder. The mixing method used in the first mixing
step is not limited, and may be a conventional method using a
Henschel mixer, a cone mixer or the like. The mixing ratio of the
iron-based powder, the Mo-containing powder, and the Cu-containing
powder is adjusted so that the resulting mixed powder for powder
metallurgy has a chemical composition in the below-mentioned range.
In detail, the powders are mixed so that the Mo content is 0.2% to
1.5% and the Cu content is 0.5% to 4.0% with respect to the whole
mixed powder for powder metallurgy.
[0053] (Iron-Based Powder)
[0054] Average particle size: 30 .mu.m to 120 .mu.m
[0055] In this disclosure, the average particle size of the
iron-based powder is 30 .mu.m to 120 .mu.m. If the average particle
size of the iron-based powder is less than 30 .mu.m, the iron-based
powder itself or the raw material mixed powder obtained using the
iron-based powder decreases in fluidity, which causes a decrease in
work efficiency such as die filling. The average particle size of
the iron-based powder is therefore 30 .mu.m or more. The average
particle size is preferably 40 .mu.m or more, and more preferably
50 .mu.m or more. If the average particle size of the iron-based
powder is more than 120 .mu.m, the driving force for density
enhancement during sintering decreases, and coarse holes are formed
around coarse iron powder particles, causing a decrease in the
density of the sintered body. This leads to lower strength and
toughness of the sintered body. The average particle size of the
iron-based powder is therefore 120 .mu.m or less. The average
particle size is preferably 100 .mu.m or less, and more preferably
80 .mu.m or less. In this disclosure, the term "average particle
size" denotes volume-based median size (d.sub.50).
[0056] The term "iron-based powder" denotes a powder whose Fe
content is 50 mass % or more. Examples of the iron-based powder
include a pure iron powder and an alloyed steel powder. As the
iron-based powder, an iron powder (pure iron powder) is
preferable.
[0057] The method of producing the iron-based powder is not
limited, and may be any method. In terms of availability, an
iron-based powder produced by an atomizing method or a reduction
method is preferable. As an iron-based powder produced by an
atomizing method, any of an as-atomized powder and an atomized
powder may be used. The as-atomized powder is a powder obtained by
atomizing molten steel and optionally drying and classifying the
resulting powder without heat treatment for deoxidation
(reduction), decarburization, or the like. The atomized powder is a
powder obtained by reducing an as-atomized powder through treatment
in a reducing atmosphere. As an iron-based powder produced by a
reduction method, a reduced iron powder obtained by reducing mill
scale generated during production of steel materials or iron ore is
preferable.
[0058] The apparent density of the iron-based powder is not
limited, but is preferably 1.7 Mg/m.sup.3 to 3.5 Mg/m.sup.3. In the
case of using an iron-based powder produced by an atomizing method
as the iron-based powder, the apparent density of the iron-based
powder is preferably about 2.0 Mg/m.sup.3 to 3.5 Mg/m.sup.3, and
more preferably 2.5 Mg/m.sup.3 to 3.2 Mg/m.sup.3. In the case of
using an iron-based powder produced by a reduction method as the
iron-based powder, the apparent density of the iron-based powder is
preferably about 1.7 Mg/m.sup.3 to 3.0 Mg/m.sup.3, and more
preferably 2.2 Mg/m.sup.3 to 2.8 Mg/m.sup.3. The apparent density
mentioned here is measured by the test method of JIS Z 2504.
[0059] The specific surface area of the iron-based powder is not
limited, but is preferably 0.002 m.sup.2/g to 0.5 m.sup.2/g. In the
case of using an iron-based powder produced by an atomizing method
as the iron-based powder, the specific surface area of the
iron-based powder is preferably about 0.005 m.sup.2/g or more, and
more preferably 0.01 m.sup.2/g or more. The upper limit of the
specific surface area is preferably 0.1 m.sup.2/g. In the case of
using an iron-based powder produced by a reduction method as the
iron-based powder, the specific surface area of the iron-based
powder is preferably about 0.01 m.sup.2/g or more, and more
preferably 0.02 m.sup.2/g or more. The upper limit of the specific
surface area is preferably 0.3 m.sup.2/g.
[0060] (Mo-Containing Powder)
[0061] The Mo-containing powder is a powder that functions as a Mo
source in the below-mentioned diffusional adhesion step. The
Mo-containing powder may be any powder that contains Mo as an
element. Hence, any of a metal Mo powder (pure Mo powder), a Mo
alloyed powder, and a Mo compound powder may be used. As the Mo
alloyed powder, for example, Fe--Mo (ferromolybdenum) powder may be
used. As the Mo compound powder, a powder of a Mo compound such as
Mo oxide, Mo carbide, Mo sulfide, or Mo nitride may be used. These
Mo-containing powders may be used singly or in combination of two
or more.
[0062] (Cu-Containing Powder)
[0063] The Cu-containing powder is a powder that functions as a Cu
source in the below-mentioned diffusional adhesion step. In this
disclosure, it is important to use a cuprous oxide powder as the
Cu-containing powder. The cuprous oxide powder is reduced to metal
Cu in the diffusional adhesion step, so that a partially
diffusion-alloyed steel powder with Mo and Cu diffusionally
adhering to the surface of the iron-based powder can be
obtained.
[0064] By using the cuprous oxide powder as the Cu-containing
powder, Cu growth which occurs in the case of using a metal Cu
powder can be prevented to suppress a decrease in the density of
the sintered body. Moreover, cuprous oxide is chemically stable and
does not oxidize (rust) like metal Cu, and so can be handled
easily. Furthermore, cuprous oxide (Cu.sub.2O) is lower in
oxidation number than copper oxide (CuO), and so can be easily
reduced to metal Cu in the diffusional adhesion step. For example,
in the case of heat-treating the raw material mixed powder in a
hydrogen atmosphere in the diffusional adhesion step, the use of
cuprous oxide can not only decrease the amount of hydrogen
necessary for reduction but also lower the heating temperature, and
further shorten the treatment time.
[0065] The average particle size of the Cu-containing powder is not
limited, but is preferably 5 .mu.m or less. An average particle
size of 5 .mu.m or less can further enhance the effect of improving
strength and toughness by Cu. The average particle size is more
preferably 4.5 .mu.m or less. No lower limit is placed on the
average particle size of the Cu-containing powder, yet excessively
decreasing the average particle size causes an increase in the
production cost of the Cu-containing powder. Accordingly, the
average particle size of the Cu-containing powder is preferably 0.2
.mu.m or more, and more preferably 1.0 .mu.m or more.
[0066] In the case of conventionally used metal Cu powders, the
average particle size of a typical commercially-available metal Cu
powder is about 20 .mu.m to 40 .mu.m.
[0067] [Diffusional Adhesion Step]
[0068] The raw material mixed powder is then heat-treated. As a
result of the heat treatment, Mo in the Mo-containing powder and Cu
in the Cu-containing powder diffuse into the iron-based powder on
the contact surface of the iron-based powder and the Mo-containing
powder and the contact surface of the iron-based powder and the
Cu-containing powder. A partially diffusion-alloyed steel powder
with Mo and Cu diffusionally adhering to the surface of the
iron-based powder is thus obtained.
[0069] The heat treatment may be performed in any atmosphere, but a
reducing atmosphere is preferable, and a hydrogen-containing
atmosphere is more preferable. As the hydrogen-containing
atmosphere, a hydrogen gas atmosphere may be used. The heat
treatment may be performed at atmospheric pressure, under reduced
pressure, or under vacuum.
[0070] The temperature of the heat treatment is not limited, but is
preferably 800.degree. C. to 1000.degree. C.
[0071] [Grinding and Classification]
[0072] Iron-based powder particles contained in the partially
diffusion-alloyed steel powder obtained in this way are usually
agglomerated as a result of sintering. It is therefore preferable
to perform a grinding and classification step of grinding and
classifying the partially diffusion-alloyed steel powder, after the
diffusional adhesion step and before the subsequent second mixing
step. For example, after grinding the partially diffusion-alloyed
steel powder into a desired particle size, the partially
diffusion-alloyed steel powder is classified using a sieve with a
predetermined opening, thus removing a coarse powder. The maximum
particle size of the partially diffusion-alloyed steel powder is
preferably 180 .mu.m or less.
[0073] The partially diffusion-alloyed steel powder may be
optionally annealed, before the subsequent second mixing step.
[0074] The partially diffusion-alloyed steel powder preferably has
a chemical composition containing Mo and Cu with the balance being
Fe and incidental impurities. Examples of the incidental impurities
contained in the partially diffusion-alloyed steel powder include
C, O, N, and S. The contents of these elements are preferably C:
0.02% or less, 0: 0.3% or less, N: 0.004% or less, and S: 0.03% or
less with respect to the total mass of the partially alloyed steel
powder including the impurities. In particular, the 0 content is
more preferably 0.25% or less. If the contents of the incidental
impurities exceed these ranges, the mixed powder for powder
metallurgy obtained as a result of the second mixing step decreases
in compressibility, and is difficult to be compression molded into
a green compact having sufficient density.
[0075] [Second Mixing Step]
[0076] The partially diffusion-alloyed steel powder obtained in
this way is then mixed with a graphite powder, to yield a mixed
powder for powder metallurgy. C which is a main component of the
graphite powder has an effect of increasing the strength of the
sintered body through, for example, strengthening by precipitation
of carbides and hardenability enhancement. The addition of graphite
is therefore essential, particularly to achieve a high tensile
strength of 1000 MPa or more after carburizing, quenching, and
tempering the sintered body.
[0077] The graphite powder may be mixed, for example, according to
a conventional method typically used for powder mixing. The mixing
ratio of the partially diffusion-alloyed steel powder and the
graphite powder is adjusted so that the resulting mixed powder for
powder metallurgy has a chemical composition in the below-mentioned
range. In detail, the mixing is performed so that the C content is
0.1% to 1.0% with respect to the whole mixed powder for powder
metallurgy.
[0078] (Graphite Powder)
[0079] The graphite powder is not limited, and may be any graphite
powder. The average particle size of the graphite powder is not
limited, but is preferably about 1 .mu.m to 50 .mu.m.
[0080] [Chemical Composition of Mixed Powder]
[0081] In this disclosure, the resulting mixed powder for powder
metallurgy has a chemical composition containing Mo: 0.2% to 1.5%,
Cu: 0.5% to 4.0%, and C: 0.1% to 1.0%, with the balance being Fe
and incidental impurities. Although additives such as a lubricant
may be added to the alloyed steel powder for powder metallurgy as
described later, the "chemical composition of the mixed powder for
powder metallurgy" mentioned here denotes the chemical composition
of the mixed powder except the additives, i.e. the part composed of
the partially diffusion-alloyed steel powder and the graphite
powder.
[0082] The reasons for limiting the chemical composition of the
mixed powder are given below.
[0083] Mo: 0.2% to 1.5%
[0084] If the Mo content is less than 0.2%, the effect of improving
hardenability and the effect of improving the strength of the
sintered body are insufficient. The Mo content is therefore 0.2% or
more. The Mo content is preferably 0.3% or more, and more
preferably 0.4% or more. If the Mo content is more than 1.5%, the
hardenability improving effect is saturated, and the non-uniformity
of the microstructure of the sintered body rather increases,
causing a decrease in the strength and toughness of the sintered
body. The Mo content is therefore 1.5% or less. The Mo content is
preferably 1.0% or less, and more preferably 0.8% or less.
[0085] Cu: 0.5% to 4.0%
[0086] If the Cu content is less than 0.5%, the effects of solid
solution strengthening and hardenability improvement by Cu are
insufficient, and the strength and toughness of the sintered part
decreases. The Cu content is therefore 0.5% or more. The Cu content
is preferably 1.0% or more, and more preferably 1.5% or more. If
the Cu content is more than 4.0%, the effect of improving the
strength of the sintered part is saturated. The Cu content is
therefore 4.0% or less. The Cu content is preferably 3.0% or less,
and more preferably 2.5% or less.
[0087] C: 0.1% to 1.0%
[0088] C is an element having an effect of improving the strength
and fatigue strength of the sintered body. To achieve this effect,
the C content is 0.1% or more. If the C content is more than 1.0%,
the material becomes hypereutectoid, and a lot of cementite
precipitates and causes a decrease in the strength of the sintered
body. The C content is therefore 1.0% or less.
[0089] A method of producing a sintered body according to one of
the disclosed embodiments is described below. In this disclosure, a
sintered body can be yielded by forming the above-mentioned mixed
powder for powder metallurgy and sintering the formed mixed
powder.
[0090] [Forming]
[0091] The forming is not limited, and may be performed by any
method that can form the mixed powder for powder metallurgy. A
typical forming method is a method of charging the mixed powder for
powder metallurgy into a die and pressing it. The pressing force in
the pressing is preferably 400 MPa to 1000 MPa. If the pressing
force is less than 400 MPa, the density of the obtained green
compact is low, and the properties of the sintered body decrease.
If the pressing force is more than 1000 MPa, the life of the die
shortens extremely, which is economically disadvantageous. The
temperature in the pressing is preferably room temperature (about
20.degree. C.) to about 160.degree. C.
[0092] In the case where the resulting sintered body needs to be
subjected to machining work or the like to shape the part, a powder
for improving machinability may be added to the mixed powder for
powder metallurgy before the forming. For example, MnS and the like
may be used as the powder for improving machinability. The powder
for improving machinability may be added according to a
conventional method.
[0093] A lubricant may be added to the mixed powder for powder
metallurgy, before the forming. As the lubricant, a lubricant
powder is preferable. Alternatively, the forming may be performed
with the lubricant being applied or adhered to the die. In either
case, any lubricant such as metal soap, e.g. zinc stearate or
lithium stearate, amide-based wax, e.g. ethylenebisstearamide, may
be used as the lubricant. In the case of mixing the lubricant into
the mixed powder for powder metallurgy, the amount of the lubricant
is preferably about 0.1 parts by mass to 1.2 parts by mass with
respect to 100 parts by mass the mixed powder for powder
metallurgy.
[0094] [Sintering]
[0095] The green compact obtained in this way is then sintered. The
sintering is preferably performed in a temperature range of
1100.degree. C. to 1300.degree. C. If the sintering temperature is
less than 1100.degree. C., the sintering does not progress
sufficiently, and a sintered body having excellent tensile strength
(1000 MPa or more) is hard to be obtained. If the sintering
temperature is more than 1300.degree. C., the life of the sintering
furnace shortens, which is economically disadvantageous. The
sintering time is preferably 10 minutes to 180 minutes.
[0096] A sintered body obtained using the mixed powder for powder
metallurgy according to this disclosure by the above-mentioned
procedure has excellent tensile strength and toughness, as compared
with a sintered body obtained by forming and sintering a
conventional powder under the same conditions.
[0097] The obtained sintered body may be optionally subjected to
strengthening treatment. Examples of the strengthening treatment
include carburizing-quenching, bright quenching, induction
hardening, and carburizing nitriding treatment. Even in the case
where such strengthening treatment is not performed, the sintered
body produced using the mixed powder for powder metallurgy
according to this disclosure has improved strength and toughness as
compared with a conventional sintered body not subjected to
strengthening treatment. Each strengthening treatment may be
performed according to a conventional method.
EXAMPLES
[0098] More detailed description is given below, based on examples.
This disclosure is, however, not limited to these examples.
[0099] Each mixed powder for powder metallurgy was produced by the
following procedure.
[0100] (First Mixing Step)
[0101] An iron-based powder was mixed with a Mo-containing powder
and a Cu-containing powder, to obtain a raw material mixed powder.
As the iron-based powder, an as-atomized powder having the apparent
density listed in Table 1 was used. The specific surface area of
the iron-based powder was 0.39 m.sup.2/g. As the Mo-containing
powder, an oxidized Mo powder having an average particle size of 10
.mu.m was used. As the Cu-containing powder, a cuprous oxide powder
having the average particle size listed in Table 1 was used. The
mixing was performed for 15 minutes using a V-shaped mixer. The
amount of each powder was adjusted so that the Mo and Cu contents
in the resulting mixed powder for powder metallurgy were as listed
in Table 1.
[0102] (Diffusional Adhesion Step)
[0103] Next, the obtained raw material mixed powder was
heat-treated to cause Mo and Cu to diffusionally adhere to the
surface of the iron-based powder, thus yielding a partially
diffusion-alloyed steel powder. The heat treatment was performed at
a temperature of 880.degree. C. for 1 hour in a hydrogen atmosphere
with a dew point of 30.degree. C.
[0104] In some Comparative Examples (Nos. 1 and 3), instead of
adding the Cu-containing powder in the first mixing step, a metal
Cu powder was added in the second mixing step, and only the
Mo-containing powder was added in the first mixing step and then
the diffusional adhesion step was performed. For comparison, in No.
29, the iron-based powder was mixed with a metal Ni powder having
an average particle size of 8 .mu.m, a metal Cu powder (the same
metal Cu powder as in Comparative Examples Nos. 1 and 3) having an
average particle size of 28 .mu.m, and an oxidized Mo powder (the
same oxidized Mo powder as in Examples) having an average particle
size of 10 .mu.m in the first mixing step, and then subjected to
the diffusional adhesion step. The composition of the partially
diffusion-alloyed steel powder in No. 29 was 4% Ni--1.5% Cu--0.5
Mo--Fe.
[0105] (Grinding and Classification)
[0106] The obtained partially diffusion-alloyed steel powder was
ground and classified by the following procedure. First, since the
partially diffusion-alloyed steel powder tends to agglomerate due
to the heat treatment, the partially diffusion-alloyed steel powder
was ground three times using a hammer mill. Here, the grate opening
of the hammer mill was sequentially reduced as follows: 3 mm (first
time), 2 mm (second time), and 1 mm (third time). The ground powder
was then put through a vibrating sieve with an opening of 180
.mu.m, and only parts with a grain size of 180 .mu.m or less
passing the sieve were collected while removing/discarding a coarse
powder remaining on the sieve. The obtained powder was then
subjected to the subsequent second mixing step.
[0107] (Second Mixing Step)
[0108] Next, a graphite powder (average particle size: 5 .mu.m) of
the content listed in Table 1 was added to the partially
diffusion-alloyed steel powder. 0.6 parts by mass
ethylenebisstearamide was further added with respect to 100 parts
by mass the mixed powder for powder metallurgy, and the resulting
powder was mixed for 15 minutes in a V-shaped mixer.
[0109] The balance of the alloyed steel powder in the table is iron
and incidental impurities. The amount of incidental impurities in
the alloyed steel powder used in this disclosure was 0.2% or less
with respect to the partially diffusion-alloyed steel powder. In
Nos. 1 and 3, a metal Cu powder was mixed together with the
graphite powder so that the mixed powder for powder metallurgy had
the Cu content listed in Table 1.
[0110] (Forming)
[0111] After this, the mixed powder for powder metallurgy was
pressed to produce a rod-shaped green compact with a length of 55
mm, a width of 10 mm, and a thickness of 10 mm. Ten green compacts
were made from each mixed powder for powder metallurgy. The density
of the green compact was 7.0 Mg/m.sup.3.
[0112] (Sintering)
[0113] Each rod-shaped green compact was sintered to obtain a
rod-shaped sintered body. The sintering was performed in a propane
converted gas atmosphere which is a reducing atmosphere, at a
temperature of 1130.degree. C. for 20 minutes.
[0114] The properties of the mixed powder for powder metallurgy and
the rod-shaped sintered body obtained by the above-mentioned
procedure were then evaluated by the following methods. The results
are listed in Table 1.
[0115] (Fluidity of Mixed Powder for Powder Metallurgy)
[0116] 100 g of the test powder was collected from the mixed powder
for powder metallurgy, and passed through a nozzle of 5 mm.phi..
The case where the powder flew through the nozzle completely
without stopping was rated as "pass", and the case where a part or
whole of the powder stopped and did not flow through the nozzle was
rated as "fail".
[0117] (Tensile Strength)
[0118] A total of five tensile test pieces, i.e. one from each of
five rod-shaped sintered bodies out of the ten rod-shaped sintered
bodies, were cut out. Each tensile test piece had a parallel
portion diameter of 5 mm and a gauge length of 15 mm. Each obtained
tensile test piece was sequentially subjected to gas carburizing,
quenching, and tempering under the following conditions: [0119] gas
carburizing: carbon potential: 0.8 mass %, temperature: 870.degree.
C., time: 60 minutes, [0120] quenching: temperature: 60.degree. C.,
oil quenching, [0121] tempering: temperature: 180.degree. C., time:
60 minutes.
[0122] Using the tensile test piece obtained by this procedure, a
tensile test was conducted by the method defined in JIS Z 2241, to
measure the tensile strength. An average of the measurement values
of the five test pieces was set as the tensile strength. The case
where the measured tensile strength was 1000 MPa or more was rated
as "pass", and the case where the measured tensile strength was
less than 1000 MPa was rated as "fail".
[0123] (Toughness)
[0124] To evaluate the toughness of the sintered body, a Charpy
impact test was conducted. In the Charpy impact test, the remaining
five rod-shaped sintered bodies out of the ten rod-shaped sintered
bodies were used as test pieces without changing their shape, and
the impact value was measured by the method defined in JIS Z 2242.
Before the Charpy impact test, each of the rod-shaped sintered
bodies was subjected to gas carburizing, quenching, and tempering
under the same conditions as the above-mentioned tensile test
pieces. An average of the measurement values of the five test
pieces was set as the impact value. The case where the measured
impact value was 14.5 J/cm.sup.2 or more was rated as "pass", and
the case where the measured impact value was less than 14.5
J/cm.sup.2 was rated as "fail".
TABLE-US-00001 TABLE 1 Production conditions First mixing step
(partially diffusion-alloyed steel powder) Second mixing step
Iron-based powder Cuprous oxide powder Metal Cu powder Graphite
Average Average Content of alloying element Average powder Apparent
density particle size particle size Ni*.sup.1 Mo*.sup.1 Cu*.sup.1
particle size Cu*.sup.1 C*.sup.1 No. (Mg/m.sup.3) (.mu.m) (.mu.m)
(mass %) (mass %) (mass %) (.mu.m) (mass %) (mass %) 1 3.10 60 -- 0
0.4 0 28 2.0 0.3 2 3.10 60 4.2 0 0.4 2.0 -- 0 0.3 3 3.10 60 -- 0
0.8 0 28 2.0 0.3 4 3.10 60 2.5 0 0.8 2.0 -- 0 0.3 5 2.50 20 3.1 0
0.6 2.0 -- 0 0.3 6 2.80 40 3.1 0 0.6 2.0 -- 0 0.3 7 2.90 50 3.1 0
0.6 2.0 -- 0 0.3 8 2.95 80 3.1 0 0.6 2.0 -- 0 0.3 9 3.00 100 3.1 0
0.6 2.0 -- 0 0.3 10 3.00 150 3.1 0 0.6 2.0 -- 0 0.3 11 3.10 60 3.1
0 0.1 2.0 -- 0 0.3 12 3.10 60 3.1 0 0.2 2.0 -- 0 0.3 13 3.10 60 3.1
0 0.4 2.0 -- 0 0.3 14 3.10 60 3.1 0 0.6 2.0 -- 0 0.3 15 3.10 60 3.1
0 0.8 2.0 -- 0 0.3 16 3.10 60 3.1 0 1.5 2.0 -- 0 0.3 17 3.10 60 3.1
0 2.0 2.0 -- 0 0.3 18 3.10 60 3.1 0 0.6 0.2 -- 0 0.3 19 3.10 60 3.1
0 0.6 0.5 -- 0 0.3 20 3.10 60 3.1 0 0.6 1.5 -- 0 0.3 21 3.10 60 3.1
0 0.6 3.0 -- 0 0.3 22 3.10 60 3.1 0 0.6 4.0 -- 0 0.3 23 3.10 60 3.1
0 0.6 5.0 -- 0 0.3 24 3.10 60 3.1 0 0.6 2.0 -- 0 0.05 25 3.10 60
3.1 0 0.6 2.0 -- 0 0.2 26 3.10 60 3.1 0 0.6 2.0 -- 0 0.5 27 3.10 60
3.1 0 0.6 2.0 -- 0 1.0 28 3.10 60 3.1 0 0.6 2.0 -- 0 1.5 29 2.80 65
--*.sup.2 4 0.5 1.5 -- 0 0.3 30 3.10 60 0.2 0 0.4 2.0 -- 0 0.3 31
3.10 60 9 0 0.4 2.0 -- 0 0.3 Evaluation results Sintered body
Partially diffusion-alloyed Strength Toughness steel powder Tensile
strength Impact value No. Fluidity (MPa) Pass/fail (J/cm.sup.2)
Pass/fail Remarks 1 Pass 1050 Pass 14.2 Fail Comparative Example 2
Pass 1070 Pass 14.9 Pass Example 3 Pass 1038 Pass 13.9 Fail
Comparative Example 4 Pass 1065 Pass 14.6 Pass Example 5 Fail -- --
-- -- Comparative Example 6 Pass 1075 Pass 15.0 Pass Example 7 Pass
1070 Pass 14.9 Pass Example 8 Pass 1056 Pass 14.6 Pass Example 9
Pass 1041 Pass 14.5 Pass Example 10 Pass 1025 Pass 14.1 Fail
Comparative Example 11 Pass 995 Fail 13.5 Fail Comparative Example
12 Pass 1032 Pass 14.5 Pass Example 13 Pass 1058 Pass 15.1 Pass
Example 14 Pass 1068 Pass 14.7 Pass Example 15 Pass 1079 Pass 14.7
Pass Example 16 Pass 1052 Pass 14.5 Pass Example 17 Pass 998 Fail
14.0 Fail Comparative Example 18 Pass 1001 Pass 13.9 Fail
Comparative Example 19 Pass 1025 Pass 14.5 Pass Example 20 Pass
1044 Pass 14.6 Pass Example 21 Pass 1072 Pass 14.7 Pass Example 22
Pass 1085 Pass 14.6 Pass Example 23 Pass 1110 Pass 14.2 Fail
Comparative Example 24 Pass 992 Fail 14.8 Pass Comparative Example
25 Pass 1024 Pass 14.8 Pass Example 26 Pass 1075 Pass 14.7 Pass
Example 27 Pass 1089 Pass 14.5 Pass Example 28 Pass 985 Fail 13.8
Fail Comparative Example 29 Pass 998 Fail 13.3 Fail Comparative
Example 30 Pass 1060 Pass 15.5 Pass Example 31 Pass 1048 Pass 14.5
Pass Example *.sup.1Content with respect to whole mixed powder for
powder metallurgy, with balance being Fe and incidental impurities.
*.sup.2Metal Cu powder added instead of cuprous oxide powder.
[0125] As is clear from the results in Table 1, in Examples
satisfying the conditions according to this disclosure, a sintered
body having tensile strength and toughness at least as high as
those in Comparative Example (No. 29) using Ni was obtained,
despite not using Ni. Moreover, the alloyed steel powder for powder
metallurgy in these Examples had excellent fluidity.
* * * * *