JP7354996B2 - Iron-based alloy sintered body and its manufacturing method - Google Patents

Description

The present invention relates to an iron-based alloy sintered body and a method for manufacturing the same.

Powder metallurgy technology makes it possible to manufacture parts with complex shapes that are extremely close to the product shape (so-called near-net shape) and with high dimensional accuracy, significantly reducing cutting costs when manufacturing parts. can be achieved. Therefore, powder metallurgy products are used in a wide variety of fields as parts for various machines. Furthermore, demands for powder metallurgy technology are increasing to meet the demands for smaller, lighter, and more complex parts.

Against this background, techniques for manufacturing high-strength sintered parts have been developed.
For example, Patent Document 1 proposes a method of rapidly cooling at a cooling rate of 2° C./sec or more in quenching after sintering in order to increase the strength. Furthermore, this document states that the structure obtained by rapid cooling is a quenched structure, and that high strength can be obtained by tempering after rapid cooling.
Patent Documents 2 and 3 propose a method of performing heat treatment such as carburizing, quenching, and tempering after sintering in order to improve the mechanical properties of the sintered body. These documents indicate that additional heat treatment such as carburizing, quenching, and tempering after sintering results in higher strength than as-sintered material.
Patent Document 4 proposes a method that makes it possible to increase the strength of a sintered body by using alloyed steel powder alloyed with hardenable elements.

Japanese Patent Application Publication No. 2013-204112 Japanese Patent Application Publication No. 2016-145418 Japanese Patent Application Publication No. 2002-155303 Special Publication No. 2010-529302

However, the conventional techniques described in Patent Documents 1 to 4 have the following problems.
Patent Document 1 has the problem that it requires incidental equipment for rapid cooling, which is costly, and the quenched structure obtained by rapid cooling is hard and brittle, so tempering is required in order to increase the strength.
Patent Documents 2 and 3 require additional heat treatment such as carburizing, quenching, and tempering in order to increase the strength, which requires separate heat treatment equipment and increases the number of manufacturing processes. Ta.
Since Patent Document 4 requires Ni, it has the disadvantages of high cost, as well as supply instability and large price fluctuations.

The present invention has been made to solve the above problems, and is an iron-based alloy that has high strength and high toughness as sintered without the conventional carburizing, quenching, and tempering processes. The purpose is to provide a sintered body along with a method for manufacturing the same. Here, as-sintered refers to a state in which no further heat treatment such as carburizing, quenching, or tempering is performed after sintering.

The gist of the present invention is as follows.
[1] The component composition is
Mo: 0.5% by mass or more and 2.0% by mass or less,
Cu: 1.8% by mass or more and 10.2% by mass or less, and C: 0.2% by mass or more and 1.2% by mass or less,
The remainder consists of Fe and unavoidable impurities,
The microstructure consists of lower bainite, lower bainite and martensite, lower bainite and upper bainite, or lower bainite, upper bainite and martensite,
In the relative frequency distribution of Vickers hardness in which the microstructure is graded from 0 to 100Hv in increments of 1000Hv, the microstructure has a hardness of 400Hv or more and less than 500Hv, 500Hv or more and less than 600Hv, 600Hv or more and less than 700Hv, 700Hv or more and less than 800Hv, or 800Hv or more and less than 900Hv. An iron-based alloy sintered body that has the largest relative frequency in any class, and the largest relative frequency is more than 0.50.
[2] A method for producing an iron-based alloy sintered body, comprising a step of molding and sintering an iron-based mixed powder for powder metallurgy, and then cooling it,
The iron-based mixed powder for powder metallurgy includes alloy steel powder alloyed with Mo and Cu, Cu powder, and graphite powder,
The component composition of the iron-based mixed powder for powder metallurgy is
Mo: 0.5% by mass or more and 2.0% by mass or less,
Cu: 1.8% by mass or more and 10.2% by mass or less, and C: 0.2% by mass or more and 1.2% by mass or less, the remainder consisting of Fe and inevitable impurities,
The amount of Cu powder in the iron-based mixed powder for powder metallurgy is 0.3% by mass or more,
The cooling step includes cooling from the sintering temperature to a temperature of 200°C or more and 350°C or less at an average cooling rate of 10°C/min or more and 40°C/min or less, and holding at the temperature for 30 minutes or more and 120 minutes or less. include,
A method for producing an iron-based alloy sintered body.
[3] The method for producing an iron-based alloy sintered body according to [2], wherein the iron-based mixed powder for powder metallurgy further contains a lubricant.

According to the present invention, an iron-based alloy sintered body that has high strength and high toughness as sintered without performing carburizing, quenching, and tempering, which have been conventionally applied, and a method for manufacturing the same are provided. .
The manufacturing method of the present invention is advantageous because a high-strength and high-toughness iron-based alloy sintered body can be obtained without using incidental equipment for rapid cooling and without performing additional heat treatment after sintering. be. Further, since the iron-based alloy sintered body of the present invention does not contain Ni and has high strength and toughness, it also makes a large economic contribution.

<Method for producing iron-based sintered body>
A method for producing an iron-based alloy sintered body, which is one embodiment of the present invention, includes the steps of molding and sintering an iron-based mixed powder for powder metallurgy, and then cooling it.

[Iron-based mixed powder for powder metallurgy]
The iron-based mixed powder for powder metallurgy (hereinafter also referred to as "mixed powder") used in the production method of the present invention includes alloy steel powder alloyed with Mo and Cu, Cu powder, and graphite powder,
The composition of the iron-based mixed powder for powder metallurgy is
Mo: 0.5% by mass or more and 2.0% by mass or less,
Cu: 1.8% by mass or more and 10.2% by mass or less, and C: 0.2% by mass or more and 1.2% by mass or less, the remainder consisting of Fe and inevitable impurities,
The amount of Cu powder in the iron-based mixed powder for powder metallurgy is 0.3% by mass or more.
Here, "iron-based" means containing 50% by mass or more of Fe.

(Alloy steel powder alloyed with Mo and Cu)
Alloy steel powder obtained by alloying Mo and Cu (hereinafter also referred to as "alloy steel powder") is made of an iron-based alloy containing Mo and Cu as essential components. Hereinafter, the composition of the alloy steel powder is based on 100% by mass of the alloy steel powder.

The reason why Mo and Cu were selected as the alloying elements of the alloyed steel powder will be explained.
In order to have sufficient tensile strength as sintered, alloying with elements that improve hardenability is necessary.
The hardenability-improving effects of the hardenability-improving elements are, in descending order of magnitude, Mn>Mo>P>Cr>Si>Ni>Cu>S.
Furthermore, in the production of general alloy steel powder, the atomization method is often adopted, and the powder produced by the atomization method is subjected to heat treatment (finish reduction). Among the above hardenability-improving elements, the ease of reduction at 950° C. and H ₂ atmosphere, which is the general condition for finish reduction, is in descending order of Mo>Cu>S>Ni.
Mn and Cr cannot be reduced at 950° C. and H ₂ atmosphere, which are the general conditions for final reduction.
From these points, Mo and Cu were selected as alloying elements.

The alloy steel powder alloyed with Mo and Cu contains Mo: 0.5% by mass or more and 2.0% by mass or less, Cu: 1.5% by mass or more and 10.0% by mass or less, and the balance is Fe and Preferably, alloyed steel powder is an unavoidable impurity.

Mo: 0.5% by mass or more and 2.0% by mass or less Mo is an element that improves hardenability, and in order to exhibit the effect of improving hardenability, the Mo content can be 0.5% by mass or more, Preferably it is 0.7% by mass or more, more preferably 1.0% by mass or more. On the other hand, if the Mo content becomes too large, the compressibility of the alloyed steel powder during pressing will decrease due to high alloying, and the density of the compact will decrease. As a result, the strength of the sintered body decreases. In order to avoid such a situation, the Mo content can be set to 2.0% by mass or less, preferably 1.5% by mass or less.

Cu: 1.5% by mass or more and 10.0% by mass or less Cu is an element that improves hardenability, and in order to fully exhibit the effect of improving hardenability, the Cu content can be 1.5% by mass or more. , preferably 2.0% by mass or more, more preferably 3.0% by mass or more. On the other hand, if the Cu content becomes too large, the compressibility of the alloyed steel powder during pressing will decrease due to high alloying, and the density of the compact will decrease. As a result, the strength of the sintered body decreases. In order to avoid such a situation, the Cu content can be set to 10.0% by mass or less, preferably 8.0% by mass or less, and more preferably 4.0% by mass or less.

The alloy steel powder consists of Fe and inevitable impurities other than the above-mentioned Mo and Cu. Unavoidable impurities are impurities that are unavoidably mixed in during manufacturing processes, etc., and can contain, for example, one or more selected from the group consisting of C, S, O, N, Mn, and Cr. The content of the above elements as unavoidable impurities is not particularly limited, but is preferably within the following ranges.
C: 0.02% by mass or less O: 0.3% by mass or less, more preferably 0.25% by mass or less N: 0.004% by mass or less S: 0.03% by mass or less Mn: 0.2% by mass or less , more preferably 0.15% by mass or less Cr: 0.2% by mass or less, more preferably 0.10% by mass or less

As the average particle size of the alloy steel powder becomes smaller, springback increases during molding and cracks occur in the compact, so the average particle size is preferably 30 μm or more, more preferably 50 μm or more. In addition, the larger the average particle size, the larger the pores of the sintered body and the lower the strength, so the average particle size is preferably 120 μm or less, more preferably 100 μm or less. In this specification, the average particle size refers to the median diameter D50 of cumulative weight distribution, and the particle size distribution is measured using a sieve specified in JIS Z 8801-1, and the cumulative particle size distribution is created from the obtained particle size distribution. This is the value obtained when determining the particle size at which the weight on the sieve and under the sieve becomes 50%. The maximum particle size of the alloy steel powder is preferably 120 μm.

The method for producing alloy steel powder is not particularly limited, and any method can be used to produce the alloy steel powder. For example, the alloy steel powder can be an atomized powder produced by an atomization method, and preferably a water atomized powder produced by a water atomization method, which has a low manufacturing cost and is easy to mass-produce. When producing alloy steel powder by the atomization method, for example, molten steel adjusted to have a predetermined composition may be atomized to powder, and if necessary, reduced and/or classified to obtain alloy steel powder. can.

(Cu powder)
The Cu powder has the effect of increasing the strength of the sintered body by improving the hardenability, and also has the effect of melting into a liquid phase during sintering and making the particles of the alloy steel powder stick to each other. From this point of view, the amount of Cu powder in the iron-based mixed powder for powder metallurgy is 0.3% by mass or more, preferably 0.7% by mass or more, and more preferably 1.0% by mass or more. On the other hand, when the amount of Cu powder exceeds 4.0% by mass, the tensile strength of the sintered body decreases due to a decrease in sintered density due to expansion of Cu. Therefore, the amount of Cu powder is 4.0% by mass or less, preferably 3.0% by mass or less, more preferably 2.0% by mass or less.

The average particle diameter of the Cu powder is preferably 50 μm or less, and 40 μm or less, from the viewpoint of avoiding a decrease in the density of the sintered body by melting the Cu powder with a large particle size during sintering and expanding the volume of the sintered body. is more preferable. Although there is no particular restriction on the lower limit of the average particle size of the Cu powder, it is preferably 0.5 μm or more in order to reduce manufacturing costs.

(graphite powder)
C, which constitutes the graphite powder, dissolves in Fe during sintering, and further improves the strength of the sintered body through solid solution strengthening and improved hardenability. From this point of view, the amount of graphite powder in the iron-based mixed powder for powder metallurgy is 0.2% by mass or more, preferably 0.4% by mass or more, and more preferably 0.5% by mass or more. On the other hand, if the amount of graphite powder exceeds 1.2% by mass, hypereutectoid formation occurs, and a large amount of cementite precipitates, which actually reduces the strength of the sintered body. Therefore, the amount of graphite powder is 1.2% by mass or less, preferably 1.0% by mass or less, more preferably 0.8% by mass or less.
The average particle diameter of the graphite powder is preferably 1 μm or more and 50 μm or less.

(mixed powder)
The mixed powder has a component composition of
Mo: 0.5% by mass or more and 2.0% by mass or less,
Cu: 1.8% by mass or more and 10.2% by mass or less, and C: 0.2% by mass or more and 1.2% by mass or less, with the remainder being Fe and inevitable impurities, including alloy steel powder and Cu powder. and graphite powder. However, the amount of Cu powder in the mixed powder is 0.3% by mass or more.
Cu in the component composition is preferably 3.5% by mass or more, and preferably 8.0% by mass or less. Cu can be derived from Cu powder or alloyed steel powder obtained by alloying Mo and Cu.
Mo in the component composition is preferably 0.7% by mass or more, more preferably 1.0% by mass or more, and more preferably 1.5% by mass or less.
C in the component composition is more preferably 0.4% by mass or more, and more preferably 1.0% by mass or less. C can be derived from graphite powder.
The inevitable impurities in the mixed powder can contain, for example, one or more selected from the group consisting of C, S, O, N, Mn, and Cr, and the content of the above elements as the inevitable impurities is Although not particularly limited, the following ranges are preferable.
C: 0.02% by mass or less O: 0.3% by mass or less, more preferably 0.25% by mass or less N: 0.004% by mass or less S: 0.03% by mass or less Mn: 0.2% by mass or less , more preferably 0.15% by mass or less Cr: 0.2% by mass or less, more preferably 0.10% by mass or less

(lubricant)
The iron-based mixed powder for powder metallurgy may further contain a lubricant. By containing a lubricant, the molded article can be easily extracted from the mold. The lubricant is not particularly limited. For example, an organic lubricant can be used, and one or more selected from the group consisting of fatty acids, fatty acid amides, fatty acid bisamides, and metal soaps can be used. Metal soaps (eg, lithium stearate, zinc stearate), amide lubricants (eg, ethylene bisstearamide), and the like are preferred.

The blending amount of the lubricant is preferably 0.1 parts by mass or more and 1.2 parts by mass or less with respect to 100 parts by mass of the iron-based mixed powder for powder metallurgy. If it is 0.1 parts by mass or more, the molded product can be pulled out from the mold with sufficient ease, while if it is 1.2 parts by mass or less, the proportion of nonmetal in the entire mixed powder will be reduced. It is possible to avoid a decrease in the tensile strength of the sintered body due to an increase in the number of particles. The mixed powder can contain known additives and the like within a range that does not impair the effects of the present invention.

[Molding process]
The method for producing an iron-based alloy sintered body of the present invention includes a step of molding an iron-based mixed powder for powder metallurgy. It is preferable to use a mixed powder containing a lubricant.

Molding can be performed by filling a mold with the mixed powder and applying pressure. The pressure during molding is preferably 400 MPa or more and 1000 MPa or less. If it is less than 400 MPa, the compacted density is low, and accordingly the density of the sintered body is reduced, and the tensile strength may be reduced. On the other hand, if it exceeds 1000 MPa, the load on the mold increases, the life of the mold becomes short, and the economic burden increases.
The temperature during molding is preferably at least room temperature (about 20°C) and at most 160°C. In order to achieve a temperature below room temperature, equipment for cooling to such a temperature is required, while the molding density increases as the temperature rises, so there is little merit in molding at a temperature below room temperature. When the temperature exceeds 160°C, additional equipment is required, which increases the economic burden.

[Sintering process]
The method for producing an iron-based alloy sintered body of the present invention includes the step of sintering the molded body obtained by the above-mentioned forming process. The sintering temperature during sintering is preferably 1100°C or more and 1300°C or less. If the temperature is less than 1100°C, sintering will not proceed sufficiently and the tensile strength may decrease. If the temperature exceeds 1300°C, the tensile strength of the sintered body increases, but this causes an increase in manufacturing cost.
The sintering time is preferably 15 minutes or more and 50 minutes or less. If the time is less than 15 minutes, sintering will not be performed sufficiently, resulting in insufficient sintering, and the tensile strength may decrease. If the time is 50 minutes or more, the manufacturing cost required for sintering will increase significantly.

[Cooling process]
In the method for producing an iron-based alloy sintered body of the present invention, the cooling step after sintering is important in obtaining a sintered body that has high strength and high toughness as sintered. The cooling step includes cooling from the sintering temperature to a temperature of 200° C. or more and 350° C. or less at an average cooling rate of 10° C./min or more and 40° C./min or less, and holding at the temperature for 30 minutes or more and 120 minutes or less.

If the average cooling rate in cooling after sintering is less than 10°C/min, sufficient quenching cannot be performed and the tensile strength may decrease, and if the cooling rate exceeds 40°C/min, the cooling rate will be accelerated. Additional equipment is required, which increases manufacturing costs. Therefore, the average cooling rate in cooling after sintering is 10° C./min or more and 40° C./min or less. The average cooling rate is preferably 20°C/min or more and 35°C/min or less.

In the cooling step, the material is cooled from the sintering temperature to a temperature of 200° C. or more and 350° C. or less, and held at that temperature for 30 minutes or more and 120 minutes or less. If the holding temperature is less than 200°C or more than 350°C, a predetermined microstructure described below cannot be obtained, and it becomes difficult to achieve both high strength and high toughness. Furthermore, when measuring the Vickers hardness of an iron-based sintered alloy body, it becomes difficult to obtain a predetermined relative frequency distribution, which will be described later. Therefore, the holding temperature is 200°C or more and 350°C or less, preferably 200°C or more and 250°C or less.

If the holding time is less than 30 minutes, transformation to a predetermined microstructure will be insufficient, and it will be difficult to achieve both high strength and high toughness. On the other hand, if the time exceeds 120 minutes, the transformation to a predetermined microstructure has been completed, so there is little point in holding it for any longer than this. Therefore, the holding time is 30 minutes or more and 120 minutes or less, preferably 60 minutes or more and 120 minutes or less.

Cooling after holding is not particularly limited, and can be cooled to room temperature by air cooling, for example.

In the sintering and subsequent cooling steps, the atmosphere is not particularly limited, but preferably an RX gas (propane modified gas) atmosphere, a nitrogen gas atmosphere containing hydrogen, an ammonia decomposition gas atmosphere, or a vacuum.

According to the manufacturing method of the present invention, an iron-based alloy sintered body having high strength and high toughness can be obtained as-sintered. Treatments such as carburizing, quenching, and tempering may be performed, but are preferably not performed from the viewpoint of reducing manufacturing costs.

[Iron-based alloy sintered body]
The iron-based alloy sintered body, which is another embodiment of the present invention, is
The ingredient composition is
Mo: 0.5% by mass or more and 2.0% by mass or less,
Cu: 1.8% by mass or more and 10.2% by mass or less, and C: 0.2% by mass or more and 1.2% by mass or less,
The remainder consists of Fe and unavoidable impurities,
The microstructure consists of lower bainite, lower bainite and upper bainite, lower bainite and martensite, or lower bainite, upper bainite and martensite,
In the relative frequency distribution of Vickers hardness with a grade from 0 to 100 in increments of 100, the microstructure is 400 Hv or more and less than 500 Hv, 500 Hv or more and less than 600 Hv, 600 Hv or more and less than 700 Hv, 700 Hv or more and less than 800 Hv, or 800 Hv or more and less than 900 Hv. has the largest relative frequency in that class, and the largest relative frequency is greater than 0.50. Such an iron-based alloy sintered body has high strength and high toughness, and can be obtained by the method for manufacturing an iron-based alloy sintered body of the present invention.

(microstructure)
The microstructure of the iron-based alloy sintered body of the present invention consists of lower bainite, lower bainite and upper bainite, lower bainite and martensite, or lower bainite, upper bainite and martensite. Such a microstructure is called a "predetermined microstructure." By creating a structure in which lower bainite is essential and optionally includes one or both of upper bainite and martensite, both strength and toughness can be improved.

The microstructure of the iron-based sintered body can be evaluated by observation using an optical microscope.

(Relative frequency distribution of Vickers hardness)
The microstructure of the iron-based alloy sintered body of the present invention has a relative frequency distribution of 400 Hv or more, less than 500 Hv, 500 Hv or more and less than 600 Hv, 600 Hv, in a relative frequency distribution with a class from 0 to 1000 in increments of 100 for the measured value of micro Vickers hardness measurement. It has the largest relative frequency in any of the following classes: 700 Hv to less than 800 Hv, 800 Hv to less than 900 Hv, and the largest relative frequency is greater than 0.50. Such a relative frequency distribution is referred to as a "predetermined relative frequency distribution." Here, the classes from 0 to 1000 in the relative frequency distribution are less than 100Hv, 100Hv or more and less than 200Hv, 200Hv or more and less than 300Hv, 300Hv or more and less than 400Hv, 400Hv or more and less than 500Hv, 500Hv or more and less than 600Hv, 600Hv or more and less than 700Hv. , 700 Hv or more and less than 800 Hv, 800 Hv or more and less than 900 Hv, or 900 Hv or more and less than 1000 Hv, and the total frequency of all classes is 1.

When the relative frequency is the largest in any of the following classes: less than 100 Hv, 100 Hv or more and less than 200 Hv, 200 Hv or more and less than 300 Hv, or 300 Hv or more and less than 400 Hv, sufficient high strength cannot be obtained due to the soft microstructure.
On the other hand, when the relative frequency of 900 Hv or more and less than 1000 Hv is the largest, the strength is reduced due to the hard microstructure.

From the viewpoint of tensile strength, it is preferable that the class of 600 Hv or more and less than 700 Hv has the largest relative frequency.

Vickers hardness can be determined by micro Vickers hardness measurement as follows.
An indenter (diamond square pyramid with a facing angle of 136 degrees) is indented in the center of the cross section of the sintered body under an indentation load of 98 N and a holding time of 10 seconds. The dent should be at least 5 μm away from the pores, and if a pore exists at the expected dent position, the dent is not made and the next dent is made at the next planned dent position.

The relative frequency distribution of Vickers hardness can be obtained as follows.
As described above, the Vickers hardness is measured at 30 or more points, and the relative frequency distribution of the Vickers hardness is obtained based on the number of data belonging to the class from 0 to 1000 in increments of 100.

(composition)
The composition of the iron-based sintered body is
Mo: 0.5% by mass or more and 2.0% by mass or less,
Cu: 1.8% by mass or more and 10.2% by mass or less, and C: 0.2% by mass or more and 1.2% by mass or less,
The remainder is Fe and unavoidable impurities.
Cu in the component composition is preferably 3.5% by mass or more, and preferably 8.0% by mass or less. Cu can be derived from Cu powder or alloyed steel powder obtained by alloying Mo and Cu.
Mo in the component composition is preferably 0.7% by mass or more, more preferably 1.0% by mass or more, and more preferably 1.5% by mass or less.
C in the component composition is more preferably 0.4% by mass or more, and more preferably 1.0% by mass or less. C can be derived from graphite powder.
The inevitable impurities in the mixed powder can contain, for example, one or more selected from the group consisting of C, S, O, N, Mn, and Cr, and the content of the above elements as the inevitable impurities is Although not particularly limited, the following ranges are preferable.
C: 0.02% by mass or less O: 0.3% by mass or less, more preferably 0.25% by mass or less N: 0.004% by mass or less S: 0.03% by mass or less Mn: 0.2% by mass or less , more preferably 0.15% by mass or less Cr: 0.2% by mass or less, more preferably 0.10% by mass or less

Hereinafter, the present invention will be explained in more detail based on Examples. The following examples show preferred examples of the present invention, and the present invention is not limited to these examples in any way.

97.2 parts by mass of Fe-3Cu-1.3Mo alloy steel powder (average particle diameter 81 μm), 2.0 parts by mass Cu powder (average particle diameter approximately 25 μm), and 0.8 parts graphite powder (average particle diameter approximately 5 μm). Parts by mass were blended to form a mixed powder.
0.5 parts by mass of ethylene bisstearamide (EBS) was added as a lubricant to 100 parts by mass of the mixed powder to obtain a mixed powder for producing a molded body.
The mixed powder for producing a molded body was charged into a mold having a predetermined shape, and pressed under conditions to maintain a constant density of 7.0 Mg/m ³ to obtain a molded body.
The obtained molded body was sintered at 1130°C for 20 minutes in an RX gas (propane modified gas) atmosphere, and cooled under the cooling conditions shown in Table 1 to form a rod-shaped sintered body (length: 55 mm, A flat plate tensile test piece (width: 10 mm, thickness: 10 mm) specified in JIS Z 2550 was obtained.

The following evaluations were performed using a rod-shaped sintered body and a flat plate tensile test piece. The results are shown in Table 1.
The strength of the sintered body was evaluated by a tensile strength test based on JIS Z 2550 using a flat plate tensile test piece.
Using a rod-shaped sintered body, the toughness of the sintered body was evaluated by a Charpy impact test based on JIS Z 2242.
The Vickers hardness of the rod-shaped sintered body was evaluated by micro Vickers hardness measurement as follows.
The central part of the rod-shaped sintered body was cut and polished, and a micro Vickers hardness measurement was performed on a 6 mm x 6 mm area at the central part of the cross section as described above. The dents were made at regular intervals of 0.1 mm, but if the planned dent position was a pore, no dents were made and the next planned dent position was removed. The Vickers hardness was measured at a total of 30 points, and a relative frequency distribution of the Vickers hardness was obtained based on the number of data belonging to classes ranging from 0 to 1000 in increments of 100.
The central part of the rod-shaped sintered body was cut and polished, and the cross section was observed with an optical microscope (40x magnification) to evaluate the precipitated phase constituting the microstructure.

From Table 1, it can be seen that the invention examples have a predetermined microstructure and a predetermined relative frequency distribution, have high tensile strength and impact value, and have high strength and high toughness.
On the other hand, a comparative example in which the cooling conditions for manufacturing the sintered body are outside the scope of the present invention does not have the predetermined microstructure and/or the predetermined relative frequency distribution, and the tensile strength and impact value are lower than those of the invention. It wasn't as good as the example.

Claims (3)

The ingredient composition is
Mo: 0.5% by mass or more and 2.0% by mass or less,
Cu: 1.8% by mass or more and 10.2% by mass or less, and C: 0.2% by mass or more and 1.2% by mass or less,
The remainder consists of Fe and unavoidable impurities,
The microstructure consists of lower bainite, lower bainite and upper bainite, lower bainite and martensite, or lower bainite, upper bainite and martensite,
In the relative frequency distribution of Vickers hardness in which the microstructure is graded from 0 to 100Hv in increments of 1000Hv, the microstructure has a hardness of 400Hv or more and less than 500Hv, 500Hv or more and less than 600Hv, 600Hv or more and less than 700Hv, 700Hv or more and less than 800Hv, or 800Hv or more and less than 900Hv. An iron-based alloy sintered body that has the largest relative frequency in any class, and the largest relative frequency is more than 0.50. A method for producing an iron-based alloy sintered body, comprising a step of molding an iron-based mixed powder for powder metallurgy, sintering it, and then cooling it,
The iron-based mixed powder for powder metallurgy includes alloy steel powder alloyed with Mo and Cu, Cu powder, and graphite powder,
The composition of the iron-based mixed powder for powder metallurgy is
Mo: 0.5% by mass or more and 2.0% by mass or less,
Cu: 1.8% by mass or more and 10.2% by mass or less, and C: 0.2% by mass or more and 1.2% by mass or less, the remainder consisting of Fe and inevitable impurities,
The amount of Cu powder in the iron-based mixed powder for powder metallurgy is 0.3% by mass or more,
The cooling step includes cooling from the sintering temperature to a temperature of 200°C or more and 350°C or less at an average cooling rate of 10°C/min or more and 40°C/min or less, and holding at the temperature for 30 minutes or more and 120 minutes or less. including,
The microstructure of the iron-based alloy sintered body is composed of lower bainite, lower bainite and upper bainite, lower bainite and martensite, or lower bainite, upper bainite and martensite,
In the relative frequency distribution of Vickers hardness in which the microstructure is graded from 0 to 100Hv in increments of 1000Hv, the microstructure has a hardness of 400Hv or more and less than 500Hv, 500Hv or more and less than 600Hv, 600Hv or more and less than 700Hv, 700Hv or more and less than 800Hv, or 800Hv or more and less than 900Hv. has the largest relative frequency in any class, and the largest relative frequency is greater than 0.50;
A method for producing an iron-based alloy sintered body. The method for producing an iron-based alloy sintered body according to claim 2, wherein the iron-based mixed powder for powder metallurgy further contains a lubricant.