EP4446032A1

EP4446032A1 - Iron-based mixed powder for powder metallurgy, and iron-based sintered body

Info

Publication number: EP4446032A1
Application number: EP22927280.2A
Authority: EP
Inventors: Kohsuke ASHIZUKA; Shigeru Unami
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2022-02-18
Filing date: 2022-10-27
Publication date: 2024-10-16
Also published as: WO2023157386A1; KR20240095297A; JP2023121011A; CN118450954A

Abstract

Provided is an iron-based mixed powder for powder metallurgy that has excellent compressibility and, even in a typical production process with a compacting pressure of less than 600 MPa, enables production of a sintered body having a tensile strength of 1200 MPa or more and excellent impact resistance. An iron-based mixed powder for powder metallurgy comsists of: a partially diffusion-alloyed steel powder comprising an iron-based powder and Mo diffusionally adhered to a surface of the iron-based powder; and an alloying metal powder. The iron-based powder has a certain chemical composition, the partially diffusion-alloyed steel powder has a Mo content of 0.20 mass% or more and 1.5 mass% or less, and an apparent density of 2.8 g/cm<sup>3</sup> or more and 3.6 g/cm<sup>3</sup> or less, and the alloying metal powder contains at least one of a Cu powder and a Ni powder with a certain apparent density in a certain amount.

Description

TECHNICAL FIELD

The present disclosure relates to an iron-based mixed powder for powder metallurgy, and particularly to an iron-based mixed powder for powder metallurgy that has excellent compressibility as powder and enables production of a sintered body having excellent strength and impact resistance. The present disclosure also relates to an iron-based sintered body produced using the iron-based mixed powder for powder metallurgy.

BACKGROUND

Powder metallurgical products are typically produced by mixing raw material powder, charging the mixed powder into a die, then pressing the powder to obtain a green compact, and then sintering the green compact. The sintered body obtained as a result of sintering is optionally further subjected to sizing or machining (cutting work). For example, in the case of producing an iron-based powder metallurgical product, a mixed powder obtained by adding, to an iron-based powder, alloying powders such as Cu powder and graphite powder and lubricants such as stearic acid and lithium stearate is usually used as raw material powder.
With such powder metallurgy technique, parts having complex shapes can be produced in shapes (i.e. near net shape) extremely close to the product shapes with high dimensional accuracy, so that machining costs can be reduced significantly. Powder metallurgical products are thus used in many fields. In particular, iron-based powder metallurgical products are widely used as various mechanical and structural parts such as automotive parts, for their excellent strength.
In recent years, however, iron-based powder metallurgical products are required to have even higher strength in order to make parts smaller and lighter.
In order to produce an iron-based sintered body having a tensile strength of more than 1000 MPa, carburizing heat treatment or bright heat treatment needs to be performed after sintering. Especially, in order to produce an iron-based sintered body having a tensile strength of more than 1200 MPa, not only an alloy powder containing as much as 4 mass% of Ni, which is an expensive alloying element, needs to be used but also a compacting pressure exceeding 600 MPa is required. This severely wears the die and leads to higher production costs as well as higher material costs. In the case of producing such high-strength sintered parts at a compacting pressure of less than 600 MPa, sintering needs to be performed at a high temperature exceeding 1200 °C. This similarly leads to higher production costs.
In a typical production process for powder metallurgical products, sintering is performed using a continuous sintering furnace called a belt furnace. In the belt furnace, sintering is continuously performed while conveying parts on a mesh belt. This has the advantages of excellent productivity and low running costs. However, since the sintering temperature in the belt furnace is about 1150 °C at a maximum, a tray pusher furnace which is inferior in productivity needs to be used in order to perform sintering at a high temperature exceeding 1200 °C as mentioned above. Besides, in the case where sintering is performed at such high temperature, the furnace body wears severely and the running costs increase.
These circumstances have stimulated various studies to obtain high-strength sintered parts by an inexpensive process. In particular, many methods have been proposed that improve the strength of sintered bodies by adding alloying elements for improving hardenability to iron-based powder.
For example, JP H01-142002 A (PTL 1) proposes an alloyed steel powder pre-alloyed with at least one of Mo: 1.5 mass% to 2.0 mass% and W: 3.0 mass% to 20 mass%.
JP S61-295302 A (PTL 2) proposes an alloyed steel powder pre-alloyed with Mo: 0.2 % to 1.5 % and Mn: 0.05 % to 0.25 % in weight ratio.
JP S59-215401 A (PTL 3) proposes an alloyed steel powder obtained by diffusion-bonding Cu and Ni in powder form to the surface of an iron powder pre-alloyed with Mo: 0.1 mass% to 1.0 mass%.
WO 2020/202805 A1 (PTL 4) proposes an iron-based mixed powder for powder metallurgy containing: an alloyed steel powder containing 0.2 mass% to 1.5 mass% of Mo; and a copper powder having an average particle size of 25 µm or less and a specific surface area of 0.30 m²/g or more.
JP 2018-123412 A (PTL 5) proposes an iron-based mixed powder for powder metallurgy containing: a pre-alloyed steel powder alloyed with Mo, Cu, and Ni in advance; and a graphite powder.

CITATION LIST

Patent Literature

PTL 1: JP H01-142002 A
PTL 2: JP S61-295302 A
PTL 3: JP S59-215401 A
PTL 4: WO 2020/202805 A1
PTL 5: JP 2018-123412 A

SUMMARY

(Technical Problem)

However, the conventional techniques proposed in PTL 1 to PTL 5 have been found to have the following problems.
For the alloyed steel powder proposed in PTL 1, at least one of Mo and W, which are ferrite-stabilizing elements, is added to form α single phase with a high self-diffusion rate of Fe, as a result of which sintering can be accelerated. With this method, however, the addition amount of Mo is relatively large, so that the compressibility of the alloyed steel powder is low and high forming density cannot be achieved.
A decrease in density tends to cause a decrease in the impact resistance of the sintered body. Given that powder metallurgical products are used in various mechanical and structural parts such as automotive parts as mentioned above, excellent impact resistance is also required.
For the alloyed steel powder proposed in PTL 2, the Mo content is 1.5 mass% or less, so that α single phase is not formed. Sintering between particles is therefore not accelerated. Hence, sufficient strength of the sintered neck part cannot be obtained with sintering temperatures (1120 °C to 1140 °C) of mesh belt furnaces commonly used. Moreover, while Mn is added as a pre-alloying element in the alloyed steel powder proposed in PTL 2, if the addition amount of Mn is increased in order to improve hardenability, the compressibility of the powder decreases, making it impossible to achieve a sufficient strength improving effect.
For the alloyed steel powder proposed in PTL 3, pre-alloying with Mo and diffusion bonding of Cu and Ni achieve both compressibility during green compacting and the strength of the member after sintering. However, since the iron powder pre-alloyed with Mo does not have good sinterability as in the case of the alloyed steel powder in PTL 2, improvement in tensile strength and fatigue strength is limited.
The mixed powder proposed in PTL 4 enables obtaining high-strength sintered parts through sintering using an ordinary belt furnace and carburizing, quenching, and tempering. However, since the compressibility of the mixed powder is roughly equal to that of a typical iron-based alloy powder, compacting at a high pressure of 688 MPa is needed in order to achieve high tensile strength. This severely wears the die.
The mixed powder proposed in PTL 5 enables obtaining a sintered body having a tensile strength of 1300 MPa or more. However, since pressing at a high pressure of 690 MPa is needed, the die wears severely as in PTL 4.
It could therefore be helpful to provide an iron-based mixed powder for powder metallurgy that has excellent compressibility and, even in a typical production process with a compacting pressure of less than 600 MPa, enables production of a sintered body having a tensile strength of 1200 MPa or more and excellent impact resistance with an impact value of 13 J/cm² or more. The term "compressibility" herein means how easy the mixed powder can be compressed when it is charged into a die and pressed. As an index of compressibility, the density of a green compact obtained as a result of compacting at a certain pressure can be used, where higher density indicates better compressibility.
It could also be helpful to provide an iron-based sintered body produced using the iron-based mixed powder for powder metallurgy.

(Solution to Problem)

We thus provide the following.

1. An iron-based mixed powder for powder metallurgy, consisting of: a partially diffusion-alloyed steel powder comprising an iron-based powder and Mo diffusionally adhered to a surface of the iron-based powder; and an alloying metal powder, wherein the iron-based powder has a chemical composition containing (consisting of) Mn: 0.04 mass% or more and 0.15 mass% or less and Si: 0.01 mass% or more and 0.10 mass% or less with a balance consisting of Fe and inevitable impurities, the partially diffusion-alloyed steel powder has a Mo content of 0.20 mass% or more and 1.5 mass% or less, and an apparent density of 2.8 g/cm³ or more and 3.6 g/cm³ or less, the alloying metal powder contains one or both of a Cu powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³ and a Ni powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³, and an addition amount of the Cu powder is 0 mass% to 3.0 mass%, an addition amount of the Ni powder is 0 mass% to 3.0 mass%, and a total addition amount of the Cu powder and the Ni powder is 0.5 mass% or more, with respect to a total mass of the partially diffusion-alloyed steel powder and the alloying metal powder.
2. The iron-based mixed powder for powder metallurgy according to 1., wherein a mass ratio of the Ni powder to a total mass of the Cu powder and the Ni powder is 0.8 or less.
3. An iron-based sintered body obtainable by carburizing, quenching, and tempering a sintered body produced using the iron-based mixed powder for powder metallurgy according to 1. or 2.

(Advantageous Effect)

The iron-based mixed powder for powder metallurgy according to the present disclosure has excellent compressibility, and thus can be used to produce a high-density sintered body. Moreover, the iron-based mixed powder for powder metallurgy according to the present disclosure enables production of a sintered body having high tensile strength and excellent impact resistance even in a typical production process with a compacting pressure of less than 600 MPa. The iron-based mixed powder for powder metallurgy according to the present disclosure has these excellent properties while being inexpensive because it does not contain Ni or, even if it contains Ni, the Ni content is 3.0 mass% or less.

DETAILED DESCRIPTION

An embodiment of the present disclosure will be described in detail below. The following description shows a preferred embodiment of the present disclosure, and the present disclosure is not limited to such. The unit "%" regarding the chemical composition represents "mass%" unless otherwise specified.
An iron-based mixed powder for powder metallurgy (hereafter also simply referred to as "mixed powder") in one embodiment of the present disclosure comprises a partially diffusion-alloyed steel powder and an alloying metal powder. The term "iron-based mixed powder" herein refers to a mixed powder in which the mass proportion of Fe contained is 50 % or more with respect to the total mass of the partially diffusion-alloyed steel powder and the alloying metal powder.
Each of the partially diffusion-alloyed steel powder and the alloying metal powder will be described below.

[Partially diffusion-alloyed steel powder]

As the partially diffusion-alloyed steel powder (hereafter also referred to as "alloyed steel powder"), a partially diffusion-alloyed steel powder comprising an iron-based powder and Mo diffusionally adhered to a surface of the iron-based powder is used. The "partially diffusion-alloyed steel powder" is a technical term commonly used in this technical field, and typically refers to a powder that consists of an iron-based powder as a core and the particles of at least one alloying element adhering to the surface of the iron-based powder and in which the iron-based powder and the alloying element particles are diffusion-bonded. The "iron-based powder" refers to a powder in which the mass proportion of Fe contained is 50 % or more.
As the iron-based powder, an iron-based powder having a chemical composition containing Mn: 0.04 % or more and 0.15 % or less and Si: 0.01 % or more and 0.10 % or less with the balance consisting of Fe and inevitable impurities is used. The reasons for this limitation will be explained below.

Mn: 0.04 % to 0.15 %

Mn is an element contained as an inevitable impurity in iron-based powder. If the Mn content is more than 0.15 %, a large amount of Mn oxide forms. Mn oxide not only lowers the compressibility of the iron-based mixed powder for powder metallurgy, but also serves as a fracture origin inside the sintered body and causes a decrease in the strength of the sintered body. The Mn content is therefore 0.15 % or less, and preferably 0.10 % or less. Although low Mn content is desirable from the viewpoint of improving compressibility, excessive reduction leads to a longer time required for Mo removal treatment and resulting higher production costs. The Mn content is therefore 0.04 % or more.

Si: 0.01 % to 0.10 %

Si is an element contained as an inevitable impurity in iron-based powder. If the Si content is more than 0.10 %, a large amount of Si oxide forms. Si oxide not only lowers the compressibility of the iron-based mixed powder for powder metallurgy, but also serves as a fracture origin inside the sintered body and causes a decrease in the strength of the sintered body. The Si content is therefore 0.10 % or less, and preferably 0.05 % or less. Although low Si content is desirable from the viewpoint of improving compressibility, excessive reduction leads to a longer time required for Si removal treatment and resulting higher production costs. The Si content is therefore 0.01 % or more.
The iron-based powder is preferably an atomized powder, without being limited thereto. The atomized powder may be any of gas atomized powder and water atomized powder, but is more preferably water atomized powder. The atomized powder is preferably a powder that has been, after atomization, heat-treated by heating in a reducing atmosphere (for example, hydrogen atmosphere) to reduce C and O. Alternatively, the atomized powder may be an as-atomized iron-based powder not subjected to such heat treatment.
Mo is diffusion-bonded to the particle surface of the iron-based powder. In the present disclosure, the Mo content in the partially diffusion-alloyed steel powder is 0.20 % or more and 1.5 % or less. The reasons for this limitation will be explained below.

Mo: 0.20 % to 1.5 %

Mo is an element that has the effect of improving hardenability and thus improving the strength of the sintered body. Adding a small amount of Mo as compared with Ni can achieve a sufficient hardenability improving effect. If the Mo content in the partially diffusion-alloyed steel powder is less than 0.20 %, the strength improving effect by Mo is insufficient. The Mo content in the partially diffusion-alloyed steel powder is therefore 0.20 % or more, and preferably 0.40 % or more. If the Mo content is more than 1.5 %, the effect of improving the strength of the sintered body by Mo is saturated, and also the compressibility of the partially diffusion-alloyed steel powder decreases and the compacting die tends to wear. The Mo content in the partially diffusion-alloyed steel powder is therefore 1.5 % or less, and preferably 1.0 % or less.
In the present disclosure, the partially diffusion-alloyed steel powder comprising an iron-based powder having the above-described chemical composition and 0.20 % to 1.5 % of Mo diffusionally adhered to a surface of the iron-based powder is used. Thus, the partially diffusion-alloyed steel powder in one embodiment of the present disclosure has a chemical composition containing Mn derived from the iron-based powder, Si derived from the iron-based powder, and diffusion-bonded Mo, with the balance consisting of Fe and inevitable impurities.
The components contained as the inevitable impurities and their amounts are not limited, but it is desirable to reduce the amounts of the inevitable impurities as much as possible. For example, in the case where the inevitable impurities include Ni, the Ni content is preferably 0.1 % or less because Ni causes an increase in material costs. The contents of C, O, P, S, and N as the inevitable impurities are preferably in the following ranges:

C: 0.01 % or less,
O: 0.20 % or less,
P: 0.025 % or less,
S: 0.025 % or less, and
N: 0.05 % or less.

The O content herein includes the amount of oxygen contained in oxides that inevitably form in the alloyed steel powder. The total amount of elements contained as the inevitable impurities other than those listed above is preferably limited to 0.01 % or less.

Apparent density: 2.8 g/cm³ to 3.6 g/cm³

The apparent density of the partially diffusion-alloyed steel powder is a parameter that depends on, for example, the shape and particle size distribution of the particles forming the alloyed steel powder, and greatly influences the compressibility of the iron-based mixed powder for powder metallurgy. When the apparent density of the alloyed steel powder is lower, the volume of the iron-based mixed powder for powder metallurgy charged into the die is larger. This increases the work hardening of the particles of the alloyed steel powder during press forming and hinders plastic deformation of the particles, resulting in lower green density. This decrease in green density is particularly noticeable when the apparent density of the partially diffusion-alloyed steel powder is less than 2.8 g/cm³. The apparent density of the partially diffusion-alloyed steel powder is therefore 2.8 g/cm³ or more, and preferably 2.9 g/cm³ or more. If the apparent density of the partially diffusion-alloyed steel powder is more than 3.6 g/cm³, not only the compressibility improving effect is saturated but also the strength of the green compact decreases and as a result the green compact tends to crack when taken out of the die after press forming. Moreover, excessively increasing the apparent density of the partially diffusion-alloyed steel powder requires treatment for making the shape of the particles forming the partially diffusion-alloyed steel powder closer to a spherical shape or treatment for making the particle size distribution of the alloyed steel powder a bimodal distribution, thus leading to higher production costs. The apparent density of the alloyed steel powder is therefore 3.6 g/cm³ or less, and preferably 3.3 g/cm³ or less. The apparent density can be measured in accordance with JIS Z 2504: 2012.
The particle size of the partially diffusion-alloyed steel powder is not limited and may be any particle size. From the viewpoint of ease of production, the average particle size of the partially diffusion-alloyed steel powder is preferably 30 µm or more and 150 µm or less. The alloyed steel powder having such average particle size can be produced industrially at low cost by using the water atomizing method. The term "average particle size" herein refers to a mass-based median size (D50). The average particle size can be determined from the particle size distribution measured by the dry sieving method described in JIS-Z 2510. Specifically, a mass-based cumulative particle size distribution is calculated from the obtained particle size distribution, the particle size (D50) at which the cumulative proportion is 50 % in the cumulative particle size distribution is determined by interpolation, and the determined particle size (D50) is taken to be the average particle size.
The method of producing the partially diffusion-alloyed steel powder is not limited. Typically, the partially diffusion-alloyed steel powder can be produced by mixing the foregoing iron-based powder and Mo raw material powder and then holding the mixture at a high temperature to diffusion-bond Mo to the surface of the iron-based powder.
The Mo raw material powder is a powder that functions as a Mo source in the below-described diffusion bonding process. As the Mo raw material powder, any powder that contains Mo as an element can be used. Hence, the Mo raw material powder may be any of metallic Mo powder (powder consisting only of Mo), Mo alloy powder, and Mo compound powder. As the Mo alloy powder, for example, Fe-Mo (ferromolybdenum) powder is preferably used. As the Fe-Mo powder, atomized Fe-Mo powder containing 5 % or more of Mo is preferably used. The atomized powder may be any of gas atomized powder and water atomized powder. As the Mo compound powder, Mo oxide is preferably used for its easy availability and ease of reduction reaction. These Mo raw material powders may be used alone or in a mixture of two or more.
In the method of producing the partially diffusion-alloyed steel powder, first, the foregoing iron-based powder and Mo raw material powder are mixed. In the mixing, the blending amounts of the iron-based powder and Mo-containing powder are adjusted so that the Mo content in the resulting partially diffusion-alloyed steel powder will be in the foregoing range. The mixing method is not limited, and the mixing may be performed according to a conventional method using, for example, a Henschel mixer or a cone mixer. In the mixing, 0.1 mass% or less of machine oil or the like may be added in order to improve the bonding between the iron-based powder and the Mo raw material powder.
The obtained mixture is then heat-treated at 800 °C to 1000 °C in a reducing atmosphere such as a hydrogen atmosphere to obtain an alloyed steel powder to which Mo is diffusion-bonded as metallic Mo or a Mo-containing alloy. In the case where an as-atomized iron-based powder is used as the iron-based powder, a large amount of C and O contained in the iron-based powder can be reduced by the heat treatment. It is preferable to use an as-atomized iron powder as the iron-based powder because, during diffusion bonding treatment, C and O are reduced to make the surface of the iron-based powder active and thus facilitate bonding by diffusion of metallic Mo or Mo-containing alloy.
In the partially diffusion-alloyed steel powder obtained in the above-described manner, at the site of contact between the metallic Mo or Mo-containing alloy and the iron-based powder, part of Mo in the metallic Mo or Mo-containing alloy has diffused into the particles of the iron-based powder and bonded (hereafter also referred to as "diffusion-bonded") to the surface of the iron-based powder. In the case where Mo oxide powder is used as the Mo raw material powder, Mo oxide is reduced to the form of metallic Mo in the heat treatment. As a result, a state in which the Mo content is partially increased by diffusion bonding is obtained as in the case where metallic Mo powder or Mo-containing alloy powder is used as the Mo raw material powder.
Such heat treatment (including diffusion bonding treatment) normally causes the iron-based powder and the metallic Mo or Mo-containing alloy to be in a sintered state. The sintered material is then ground and classified into a desired particle size. Annealing may be optionally further performed.

[Alloying metal powder]

Next, the alloying metal powder which is another component of the iron-based mixed powder for powder metallurgy according to the present disclosure will be described. The iron-based mixed powder for powder metallurgy in one embodiment of the present disclosure contains, as the alloying metal powder, one or both of a Cu powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³ and a Ni powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³. Herein, the expression "the alloying metal powder contains a Cu powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³" means that the apparent density of a Cu powder contained in the alloying metal powder is 0.5 g/cm³ to 2.0 g/cm³. The expression "the alloying metal powder contains a Ni powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³" means that the apparent density of a Ni powder contained in the alloying metal powder is 0.5 g/cm³ to 2.0 g/cm³.
The addition amounts of the Cu powder and the Ni powder need to satisfy the following conditions:

The addition amount of the Cu powder is 0 mass% to 3.0 mass%.
The addition amount of the Ni powder is 0 mass% to 3.0 mass%.
The total addition amount of the Cu powder and the Ni powder is 0.5 mass% or more.

Herein, the addition amount of the Cu powder is defined as the mass proportion of the Cu powder to the total mass of the partially diffusion-alloyed steel powder and the alloying metal powder. The addition amount of the Ni powder is defined as the mass proportion of the Ni powder to the total mass of the partially diffusion-alloyed steel powder and the alloying metal powder. The total addition amount of the Cu powder and the Ni powder is defined as the sum of the addition amount of the Cu powder and the addition amount of the Ni powder.
The reasons for limiting the addition amounts and apparent densities of the Cu powder and the Ni powder will be explained below.

Cu powder: 0 % to 3.0 %

Cu is an element that improves hardenability and is advantageous in that it is less expensive than Ni. However, while sintering is normally performed at about 1130 °C in the production of sintered bodies, Cu melts into liquid phase at 1083 °C, and the molten Cu expands the sintered body and causes a decrease in density after sintering. If the addition amount of the Cu powder is more than 3.0 %, the mechanical properties of the sintered body degrade noticeably due to this decrease in density. The addition amount of the Cu powder is therefore 3.0 % or less, and preferably 2.0 % or less. No lower limit is placed on the addition amount of the Cu powder and the lower limit may be 0 %. From the viewpoint of enhancing the hardenability improving effect by Cu, the addition amount of the Cu powder is preferably 0.5 % or more and more preferably 1.0 % or more.

Apparent density of Cu powder: 0.5 g/cm³ to 2.0 g/cm³

The apparent density of the Cu powder is a parameter that depends on, for example, the size and shape of the particles forming the Cu powder and the particle size distribution of the Cu powder, and influences the powder properties and sintering properties of the mixed powder. If the apparent density of the Cu powder is less than 0.5 g/cm³, the flowability of the mixed powder degrades. This not only increases the height of the mixed powder charged into the die but also hinders rearrangement of the particles of the alloyed steel powder during press forming, resulting in a decrease in the density of the green compact (green density). The apparent density of the Cu powder is therefore 0.5 g/cm³ or more, and preferably 1.0 g/cm³ or more. If the apparent density of the Cu powder is more than 2.0 g/cm³, sintering expansion during liquid phase sintering increases, as a result of which an achieving density decreases. The apparent density of the Cu powder is therefore 2.0 g/cm³ or less, and preferably 1.5 g/cm³ or less. The apparent density can be measured in accordance with JIS Z 2504: 2012.

Ni powder: 0 % to 3.0 %

The Ni powder has the effect of activating the sintering reaction of the alloyed steel powder and refining the pores of the sintered body to enhance the tensile strength and impact resistance of the sintered body. If the addition amount of the Ni powder is more than 3.0 %, however, retained austenite in the sintered body increases significantly and the strength of the sintered body decreases. Moreover, since Ni is an expensive element, if the addition amount of the Ni powder is more than 3.0 %, raw material costs increase noticeably. The addition amount of the Ni powder is therefore 3.0 % or less, and preferably 2.0 % or less. No lower limit is placed on the addition amount of the Ni powder and the lower limit may be 0 %. From the viewpoint of enhancing the effect of activating the sintering reaction by Ni, the addition amount of the Ni powder is preferably 0.5 % or more and more preferably 1.0 % or more.
The Ni powder is not limited and any Ni powder may be used. Examples of Ni powders suitable for use include a Ni powder produced by reducing Ni oxide and a carbonyl Ni powder produced by a thermal decomposition method.

Apparent density of Ni powder: 0.5 g/cm³ to 2.0 g/cm³

The apparent density of the Ni powder is a parameter that depends on, for example, the size and shape of the particles forming the Ni powder and the particle size distribution of the Ni powder, and influences the powder properties and sintering properties of the mixed powder. If the apparent density of the Ni powder is less than 0.5 g/cm³, the flowability of the mixed powder degrades. This not only significantly increases the volume of the mixed powder charged into the die but also hinders rearrangement of the particles of the alloyed steel powder during press forming, resulting in a decrease in the density of the green compact (green density). The apparent density of the Ni powder is therefore 0.5 g/cm³ or more, and preferably 1.0 g/cm³ or more. If the apparent density of the Ni powder is more than 2.0 g/cm³, the pores after sintering increase in size and as a result mechanical properties such as tensile strength and impact value degrade. The apparent density of the Ni powder is therefore 2.0 g/cm³ or less, and preferably 1.5 g/cm³ or less. The apparent density can be measured in accordance with JIS Z 2504: 2012.

Total addition amount of Cu powder and Ni powder: 0.5 % or more

Cu and Ni are each an element that has the effect of improving the strength of the sintered body, as mentioned above. In order to achieve the desired strength, the total addition amount of the Cu powder and the Ni powder needs to be 0.5 % or more. While no upper limit is placed on the total addition amount of the Cu powder and the Ni powder, given that the upper limit of the addition amount of the Cu powder is 3.0 % and the upper limit of the addition amount of the Ni powder is 3.0 % as mentioned above, the upper limit of the total addition amount is 6.0 %. The total addition amount is preferably 5.0 % or less and more preferably 4.0 % or less.

Mass ratio of Ni powder: 0.8 or less

Cu-Ni alloy is known as an all proportional solid solution, and the melting point of Cu-Ni alloy varies (increases) from 1083 °C in the case of 100 % Cu-0 % Ni to 1455 °C in the case of 0 % Cu-100 % Ni depending on the ratio of Ni. If the ratio of the mass (hereafter also referred to as "mass ratio") of the Ni powder to the total mass of the Cu powder and the Ni powder is 0.8 or less, an increase of the melting point is reduced, so that liquid phase sintering of the Cu powder is not hindered and the sintering promoting effect is enhanced. This further improves strength and impact resistance. The mass ratio of the Ni powder is therefore preferably 0.8 or less, and more preferably 0.5 or less. Since the Ni powder is not an essential component in the present disclosure, no lower limit is placed on the mass ratio of the Ni powder and the lower limit may be 0. From the viewpoint of achieving higher sintering density, the mass ratio of the Ni powder is preferably 0.2 or more.
In another embodiment of the present disclosure, the alloying metal powder may substantially consist of one or both of a Cu powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³ and a Ni powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³.
The iron-based mixed powder for powder metallurgy in another embodiment of the present disclosure may optionally further contain other components in addition to the above-described partially diffusion-alloyed steel powder and alloying metal powder. As such other components, for example, the iron-based mixed powder for powder metallurgy may contain at least one of a carbon powder, a lubricant, and a machinability improving powder.

- Carbon powder

Adding the carbon powder can further improve the strength of the sintered body. The carbon powder is not limited and any carbon powder may be used. As the carbon powder, for example, one or both of graphite powder and carbon black may be used. As the graphite powder, any of natural graphite powder and artificial graphite powder may be used. In the case of adding the carbon powder, the blending amount of the carbon powder is preferably 0.2 parts by mass or more with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder from the viewpoint of the strength improving effect. The blending amount of the carbon powder is preferably 1.2 parts by mass or less with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder.

- Lubricant

Adding the lubricant can ease taking the green compact out of the die. The lubricant is not limited and any lubricant may be used. As the lubricant, for example, one or both of metal soap and amide-based wax may be used. Examples of the metal soap include zinc stearate and lithium stearate. Examples of the amide-based wax include ethylenebisstearamide.
The lubricant is preferably powdery. In the case of using the lubricant, the addition amount of the lubricant is preferably 0.3 parts by mass or more with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder. The addition amount of the lubricant is preferably 1.0 part by mass or less with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder.

- Machinability improving powder

The machinability improving powder is not limited and any machinability improving powder may be used. As the machinability improving powder, for example, one or both of MnS powder and oxide powder may be used. In the case of using the machinability improving powder, the addition amount of the machinability improving powder is preferably 0.1 parts by mass or more with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder. The addition amount of the machinability improving powder is preferably 0.7 parts by mass or less with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder.

[Method of producing mixed powder]

The method of producing the iron-based mixed powder for powder metallurgy according to the present disclosure is not limited and any production method may be used. For example, the iron-based mixed powder for powder metallurgy can be produced by mixing the alloyed steel powder with the alloying metal powder in the foregoing addition amounts. Any method may be used for mixing. Examples of mixing methods include use of a V-shaped mixer, a double-cone mixer, a Henschel mixer, a Nauta mixer, etc. In the mixing, machine oil or the like may be added to prevent segregation of the Cu powder and the Ni powder. Alternatively, the alloyed steel powder and the alloying metal powder may be charged into a die for pressing in the foregoing addition amounts to form the mixed powder.

[Iron-based sintered body]

An iron-based sintered body in one embodiment of the present disclosure is an iron-based sintered body obtainable by carburizing, quenching, and tempering a sintered body produced using the iron-based mixed powder for powder metallurgy.
The iron-based sintered body in one embodiment of the present disclosure can be produced by pressing the iron-based mixed powder for powder metallurgy to obtain a green compact, sintering the green compact to obtain a sintered body, and further heat-treating the sintered body. Each of these processes will be described below.

(Pressing)

First, the iron-based mixed powder for powder metallurgy is pressed in a desired shape to obtain a green compact. An auxiliary raw material, a lubricant, a machinability improving powder, etc. may be optionally added to the iron-based mixed powder for powder metallurgy before the pressing. The pressing method is not limited and any method may be used. For example, the mixed powder may be charged into a die and pressed. A lubricant may be applied or adhered to the die. In this case, the amount of the lubricant is preferably 0.3 parts by mass or more with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder. The amount of the lubricant is preferably 1.0 part by mass or less with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder.
The pressure in the pressing may be 400 MPa or more and 1000 MPa or less. If the pressure is more than 600 MPa, however, the die wears considerably and production costs increase. The pressure is therefore preferably 400 MPa to 600 MPa. The iron-based mixed powder for powder metallurgy according to the present disclosure enables, for example, production of a green compact having a density of 7.10 g/cm³ or more at a compacting pressure of 588 MPa.

(Sintering)

The sintering method is not limited and any method may be used. The sintering temperature may be 1100 °C or more and is preferably 1120 °C or more, from the viewpoint of sufficient progress of sintering. Since a higher sintering temperature contributes to a more uniform distribution of Cu and Mo in the sintered body, no upper limit is placed on the sintering temperature. From the viewpoint of reducing production costs, however, the sintering temperature is preferably 1250 °C or less and more preferably 1180 °C or less.
The sintering time may be 15 minutes or more and 50 minutes or less. If the sintering time is in this range, insufficient sintering and resulting insufficient strength can be prevented, and production costs can be reduced. The cooling rate when cooling the sintered body after sintering may be 20 °C/min or more and 40 °C/min or less. If the cooling rate is less than 20 °C/min, quenching is insufficient, which can cause a decrease in tensile strength. If the cooling rate is more than 40 °C/min, equipment for increasing the cooling rate is needed, causing an increase in production costs.
In the case where the lubricant is used, a degreasing process of holding the green compact at a temperature of 400 °C or more and 700 °C or less for a certain time may be performed to decompose and remove the lubricant before sintering.
Other production conditions, equipment, and the like for the sintered body are not limited and any production conditions, equipment, and the like may be used.

(Heat treatment)

The obtained iron-based sintered body may be further subjected to heat treatment. The heat treatment can further enhance the strength of the sintered body. As the heat treatment, treatment involving rapid cooling is preferable. For example, strengthening treatment such as carburizing-quenching, bright quenching, induction hardening, and carbonitriding heat treatment may be performed. The sintered body after rapid cooling may be subjected to impact resistance recovery treatment such as tempering. The tempering temperature is preferably about 100 °C to 300 °C.
The iron-based sintered body in one embodiment of the present disclosure can be obtained by pressing the iron-based mixed powder for powder metallurgy in a desired shape to obtain a green compact, sintering the green compact to obtain a sintered body, and subjecting the sintered body to carburizing, quenching, and tempering sequentially.

EXAMPLES

The presently disclosed techniques will be described in more detail below by way of examples. The examples described below represent preferred examples of the present disclosure, and the present disclosure is not limited to such.
Iron-based mixed powders for powder metallurgy were produced in the following manner.
First, iron-based powders having the chemical compositions shown in Tables 1 to 3 were each produced by the water atomizing method. The amounts of P and S contained in the iron-based powder as inevitable impurities were as follows: P: less than 0.025 mass% and S: less than 0.025 mass%.
MoOs powder as Mo raw material powder was added to the obtained iron-based powder and mixed for 15 minutes in a V-type mixer. The mixture was then heat-treated in a hydrogen atmosphere to reduce the MoOs powder and diffusion-bond Mo to the particle surface of the iron-based powder. The heat treatment was performed at a temperature of 900 °C for 60 minutes.
After the heat treatment, the heat-treated body of particles in lump form as a result of sintering was ground using a hammer mill, classified using a sieve with an opening of 180 µm, and the undersize powder was collected to thus obtain a partially diffusion-alloyed steel powder. The amounts of C, O, and N contained in the partially diffusion-alloyed steel powder as impurities were as follows: C: less than 0.01 mass%, O: less than 0.20 mass%, and N: less than 0.05 mass%.
The alloying metal powder shown in Tables 1 to 3, graphite powder, and lubricant were added to the obtained partially diffusion-alloyed steel powder and mixed using a double-cone mixer to obtain an iron-based mixed powder for powder metallurgy. The addition amount of the graphite powder was 0.3 parts by mass with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder. Zinc stearate was used as the lubricant, and the addition amount of the lubricant was 0.5 parts by mass with respect to 100 parts by mass of the total of the partially diffusion-alloyed steel powder and the alloying metal powder.
The apparent density of each of the partially diffusion-alloyed steel powder, Cu powder, and Ni powder used was as shown in Tables 1 to 3. The apparent density was measured in accordance with JIS Z 2504: 2012.
Next, in order to evaluate the properties of each of the obtained iron-based mixed powders for powder metallurgy, a sintered body was produced using the iron-based mixed powder for powder metallurgy in the following manner.
The iron-based mixed powder for powder metallurgy was compacted at a compacting pressure of 588 MPa into a green compact of a 10 mm × 10 mm × 55 mm rectangular parallelepiped shape. The weight of the obtained green compact was measured, and the measured weight was divided by the volume of the green compact to yield the density of the green compact. The density of the green compact was as shown in Tables 1 to 3.
The obtained green compact was then sintered (holding temperature: 1130 °C, holding time: 20 minutes) in a RX atmosphere (N₂-32 vol% H₂-24 vol% CO-0.3 vol% CO₂) to obtain a sintered body. The obtained sintered body was subjected to gas carburizing (holding temperature: 870 °C, holding time: 60 minutes) at a carbon potential of 0.8 mass%, and then subjected to quenching (temperature: 60 °C, oil quenching) and tempering (holding temperature: 200 °C, holding time: 60 minutes). The carbon potential is an index indicating the carburizing ability of the atmosphere for heating steel, and is expressed by the carbon concentration on the surface of the steel upon reaching equilibrium with the gas atmosphere at the temperature.
The density of the obtained sintered body was measured in accordance with JIS Z 2501. In addition, the tensile strength and impact value were measured in order to evaluate the strength and impact resistance of the sintered body. The tensile strength was measured by the tensile test prescribed in JIS Z 2241. The tensile test was conducted at room temperature using a test piece taken from the sintered body and having a parallel portion diameter of 5 mm. The maximum stress before breaking measured in the tensile test was taken to be the tensile strength. The impact value was measured by measuring the absorbed energy at room temperature in accordance with JIS Z 2242 and dividing the absorbed energy by the cross-sectional area of the test piece. The measurement results were as shown in Tables 1 to 3.
As can be seen from the results shown in Tables 1 to 3, in each example using an iron-based mixed powder for powder metallurgy satisfying the conditions according to the present disclosure, the green compact and the sintered body had high density and the powder had excellent compressibility. Moreover, in each example using an iron-based mixed powder for powder metallurgy satisfying the conditions according to the present disclosure, the sintered body had high tensile strength and impact value. Specifically, the tensile strength was 1200 MPa or more and the impact value was 13 J/cm² or more. Thus, according to the present disclosure, a sintered body having excellent properties can be produced in a typical production process with a compacting pressure of less than 600 MPa. In each example using an iron-based mixed powder for powder metallurgy not satisfying the conditions according to the present disclosure, on the other hand, at least one of compressibility, strength, and impact resistance was inferior.

[Table 1]

Table 1

No.	Iron-based mixed powder for powder metallurgy						Green compact	Sintered body			Remarks
	Partially diffusion-alloyed steel powder				Alloying metal powder		Density (g/cm³)	Density (g/cm ³)	Tensile strength (MPa)	Impact value (J/cm²)
	Iron-based powder		Diffusion bonding	Apparent density (g/cm³)	Cu powder
	Chemical composition*¹ (mass%)		Mo*² (mass%)		Cu powder
	Mn	Si	Mo*² (mass%)		Addition amount *³ (mass%)	Apparent density (g/cm³)
1-1	0.07	0.02	0.20	2.9	1.0	1.0	7.14	7.13	1287	15.3	Example
1-2	0.07	0.02	0.40	2.9	1.0	1.0	7.14	7.13	1340	15.2	Example
1-3	0.07	0.02	1.00	2.9	1.0	1.0	7.13	7.12	1315	15.4	Example
1-4	0.07	0.02	1.50	2.9	1.0	1.0	7.10	7.06	1252	14.2	Example
1-5	0.04	0.02	0.40	2.9	1.0	1.0	7.15	7.14	1343	15.2	Example
1-6	0.15	0.02	0.40	2.9	1.0	1.0	7.11	7.09	1281	14.3	Example
1-7	0.07	0.01	0.40	2.9	1.0	1.0	7.15	7.14	1338	15.1	Example
1-8	0.07	0.10	0.40	2.9	1.0	1.0	7.10	7.09	1284	14.3	Example
1-9	0.07	0.02	0.40	2.8	1.0	1.0	7.12	7.11	1322	15.8	Example
1-10	0.07	0.02	0.40	3.6	1.0	1.0	7.13	7.12	1316	13.8	Example
1-11	0.07	0.02	0.40	2.9	0.5	1.0	7.14	7.05	1252	14.1	Example
1-12	0.07	0.02	0.40	2.9	1.5	1.0	7.15	7.12	1317	15.1	Example
1-13	0.07	0.02	0.40	2.9	2.0	1.0	7.15	7.10	1295	14.7	Example
1-14	0.07	0.02	0.40	2.9	3.0	1.0	7.15	7.06	1244	13.6	Example
1-15	0.07	0.02	0.40	2.9	1.0	0.5	7.14	7.08	1285	15.1	Example
1-16	0.07	0.02	0.40	2.9	1.0	1.5	7.15	7.13	1331	14.6	Example
1-17	0.07	0.02	0.40	2.9	1.0	2.0	7.15	7.07	1259	13.0	Example
1-18	0.07	0.02	0.10	2.9	1.0	1.0	7.15	7.14	1194	14.1	Comparative Example
1-19	0.07	0.02	1.60	2.9	1.0	1.0	7.07	7.06	1108	12.3	Comparative Example
1-20	0.20	0.02	0.40	2.9	1.0	1.0	7.06	7.05	1180	12.4	Comparative Example
1-21	0.07	0.15	0.40	2.9	1.0	1.0	7.06	7.05	1178	12.0	Comparative Example
1-22	0.07	0.02	0.40	2.7	1.0	1.0	7.10	7.09	1199	14.8	Comparative Example
1-23	0.07	0.02	0.40	3.7	1.0	1.0	7.09	7.07	1196	12.4	Comparative Example
1-24	0.07	0.02	0.40	2.9	-	-	7.15	7.16	1113	14.2	Comparative Example
1-25	0.07	0.02	0.40	2.9	0.4	1.0	7.16	7.03	1160	12.0	Comparative Example
1-26	0.07	0.02	0.40	2.9	3.1	1.0	7.15	7.03	1168	12.0	Comparative Example
1-27	0.07	0.02	0.40	2.9	1.0	0.4	7.15	7.08	1199	13.4	Comparative Example
1-28	0.07	0.02	0.40	2.9	1.0	2.1	7.14	7.04	1191	11.2	Comparative Example

*1 Content in iron-based powder (balance consisting of Fe and inevitable impurities)
*2 Content in partially diffusion-alloyed steel powder
*3 Proportion to total mass of partially diffusion-alloyed steel powder and alloying metal powder

[Table 1]

Table 2

No.	Iron-based mixed powder for powder metallurgy						Green compact	Sintered body			Remarks
	Partially diffusion-alloyed steel powder				Alloying metal powder		Density (g/cm³)	Density (g/cm³)	Tensile strength (MPa)	Impact value (J/cm²)
	Iron-based powder		Diffusion bonding	Apparent density (g/cm³)	Ni powder
	Chemical composition*' (mass%)		Mo*² (mass%)		Ni powder
	Mn	Si	Mo*² (mass%)		Addition amount*³ (mass%)	Apparent density (g/cm³)
2-1	0.07	0.02	0.20	2.9	1.0	1.0	7.15	7.19	1361	17.7	Example
2-2	0.07	0.02	0.40	2.9	1.0	1.0	7.14	7.19	1415	17.4	Example
2-3	0.07	0.02	1.00	2.9	1.0	1.0	7.14	7.18	1395	17.2	Example
2-4	0.07	0.02	1.50	2.9	1.0	1.0	7.07	7.12	1316	16.3	Example
2-5	0.04	0.02	0.40	2.9	1.0	1.0	7.14	7.19	1422	17.3	Example
2-6	0.15	0.02	0.40	2.9	1.0	1.0	7.10	7.13	1355	16.6	Example
2-7	0.07	0.01	0.40	2.9	1.0	1.0	7.14	7.19	1415	17.5	Example
2-8	0.07	0.10	0.40	2.9	1.0	1.0	7.10	7.14	1354	16.8	Example
2-9	0.07	0.02	0.40	2.8	1.0	1.0	7.12	7.17	1394	17.9	Example
2-10	0.07	0.02	0.40	3.6	1.0	1.0	7.13	7.17	1392	16.0	Example
2-11	0.07	0.02	0.40	2.9	0.5	1.0	7.15	7.13	1327	16.2	Example
2-12	0.07	0.02	0.40	2.9	1.5	1.0	7.14	7.16	1395	17.0	Example
2-13	0.07	0.02	0.40	2.9	2.0	1.0	7.15	7.16	1372	17.0	Example
2-14	0.07	0.02	0.40	2.9	3.0	1.0	7.15	7.12	1318	16.1	Example
2-15	0.07	0.02	0.40	2.9	1.0	0.5	7.15	7.15	1357	17.0	Example
2-16	0.07	0.02	0.40	2.9	1.0	1.5	7.14	7.18	1409	16.5	Example
2-17	0.07	0.02	0.40	2.9	1.0	2.0	7.15	7.13	1332	15.3	Example
2-18	0.07	0.02	0.10	2.9	1.0	1.0	7.16	7.20	1266	12.4	Comparative Example
2-19	0.07	0.02	1.60	2.9	1.0	1.0	7.07	7.11	1172	14.3	Comparative Example
2-20	0.20	0.02	0.40	2.9	1.0	1.0	7.05	7.10	1250	12.2	Comparative Example
2-21	0.07	0.15	0.40	2.9	1.0	1.0	7.06	7.11	1192	14.5	Comparative Example
2-22	0.07	0.02	0.40	2.7	1.0	1.0	7.10	7.15	1274	12.1	Comparative Example
2-23	0.07	0.02	0.40	3.7	1.0	1.0	7.09	7.14	1178	12.5	Comparative Example
2-24	0.07	0.02	0.40	2.9	0.4	1.0	7.14	7.07	1227	12.3	Comparative Example
2-25	0.07	0.02	0.40	2.9	3.1	1.0	7.14	7.08	1188	13.2	Comparative Example
2-26	0.07	0.02	0.40	2.9	1.0	0.4	7.11	7.13	1192	13.3	Comparative Example
2-27	0.07	0.02	0.40	2.9	1.0	2.1	7.14	7.10	1257	12.2	Comparative Example

[Table 1]

Table 3

No.	Iron-based mixed powder for powder metallurgy									Green compact	Sintered body			Remarks
	Partially diffusion alloyed steel powder				Alloying metal powder					Density (g/cm³)	Density (g/cm³)	Tensile strength (MPa)	Impact value (J/cm²)
	Iron-based powder		Diffusion bending	Apparent density (g/cm³)	Cu powder		Ni powder		Mass ratio of Ni powder (-)
	Chemical composition *¹ (mass%)		Mo*² (mass%)		Cu powder		Ni powder
	Mn	Si	Mo*² (mass%)		Addition amount *³ (mass%)	Apparent density (g/cm³)	Addition amount^*3 (mass%)	Apparent density (g/cm³)
3-1	0.07	0.02	0.4	2.9	1.0	1.0	0.3	1.0	0.23	7.14	7.15	1359	15.2	Example
3-2	0.07	0.02	0.4	2.9	1.0	1.0	0.5	1.0	0.33	7.14	7.15	1361	16.1	Example
3-3	0.07	0.02	0.4	2.9	1.0	1.0	1.0	1.0	0.50	7.14	7.19	1420	17.3	Example
3-4	0.07	0.02	0.4	2.9	1.0	1.0	3.0	1.0	0.75	7.14	7.23	1411	18.4	Example
3-5	0.07	0.02	0.4	2.9	1.0	1.0	1.0	0.5	0.50	7.15	7.22	1461	17.1	Example
3-6	0.07	0.02	0.4	2.9	1.0	1.0	1.0	1.0	0.50	7.15	7.22	1464	180	Example
3-7	0.07	0.02	0.4	2.9	0.75	1.0	3.0	0.5	0.80	7.15	7.20	1436	17.0	Example
3-8	0.07	0.02	0.4	2.9	1.0	1.0	1.0	2.0	0.50	7.16	7.20	1416	17.6	Example
3-9	0.07	0.02	0.4	2.9	3.0	1.0	3.0	2.0	0.50	7.17	7.22	1389	19.2	Example
3-10	0.07	0.02	0.4	2.9	0.2	1.0	0.2	2.0	0.50	7.14	7.15	1207	12.2	Comparative Example
3-11	0.07	0.02	0.4	2.9	1.0	1.0	3.5	1.0	0.78	7.14	7.25	1192	15.6	Comparative Example
3-12	0.07	0.02	0.4	2.9	1.0	1.0	1.0	2.5	0.50	7.12	7.16	1361	12.5	Comparative Example
3-13	0.07	0.02	0.4	2.9	0.2	1.0	2.0	1.0	0.91	7.12	7.16	1294	13.1	Example

Claims

An iron-based mixed powder for powder metallurgy, consisting of:
a partially diffusion-alloyed steel powder comprising an iron-based powder and Mo diffusionally adhered to a surface of the iron-based powder; and

an alloying metal powder,

wherein the iron-based powder has a chemical composition containing Mn: 0.04 mass% or more and 0.15 mass% or less and Si: 0.01 mass% or more and 0.10 mass% or less with a balance consisting of Fe and inevitable impurities,

the partially diffusion-alloyed steel powder has a Mo content of 0.20 mass% or more and 1.5 mass% or less, and an apparent density of 2.8 g/cm³ or more and 3.6 g/cm³ or less,

the alloying metal powder contains one or both of a Cu powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³ and a Ni powder with an apparent density of 0.5 g/cm³ to 2.0 g/cm³, and

an addition amount of the Cu powder is 0 mass% to 3.0 mass%, an addition amount of the Ni powder is 0 mass% to 3.0 mass%, and a total addition amount of the Cu powder and the Ni powder is 0.5 mass% or more, with respect to a total mass of the partially diffusion-alloyed steel powder and the alloying metal powder.
The iron-based mixed powder for powder metallurgy according to claim 1, wherein a mass ratio of the Ni powder to a total mass of the Cu powder and the Ni powder is 0.8 or less.
An iron-based sintered body obtainable by carburizing, quenching, and tempering a sintered body produced using the iron-based mixed powder for powder metallurgy according to claim 1 or 2.