JP6728530B2 - Sintered body manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a sintered body.

BACKGROUND ART A sintered body (sintered alloy) obtained by press-molding iron-based powder such as iron powder and sintering it is used for automobile parts, machine parts and the like. Examples of parts (sintered parts) using a sintered body include gears, cams, and sprockets. Generally, a sintered body is manufactured by press-molding an iron-based powder to prepare a molded body and sintering the molded body (see, for example, Patent Document 1).

A continuous sintering furnace called a roller hearth furnace is used for manufacturing the sintered body (see, for example, Patent Document 2). The roller hearth furnace is a continuous sintering furnace that sinters a molded body while continuously conveying the molded body by rollers in the furnace. Patent Document 2 describes a continuous sintering furnace in which a degassing chamber, a preheating chamber, and a sintering chamber are sequentially provided in sequence.

JP, 2013-204112, A JP, 2005-14041, A

There is an increasing demand for higher fatigue strength of sintered bodies. The sintered body is formed by binding particles of iron-based powder to each other by sintering. Since the sintered body has pores (powder triple point), the fatigue strength is higher than that of the ingot material. There is a problem of being low. Therefore, it is desired to improve the fatigue strength of the sintered body.

Then, it aims at providing the manufacturing method of the sintered compact which improves the fatigue strength of a sintered compact.

The manufacturing method of the sintered body according to the present disclosure,
A molding step in which an iron-based raw material powder added with Cu powder is pressure-molded to produce a molded body;
A preheating step of preheating the molded body at a preheating temperature of 800° C. or higher and 1000° C. or lower;
And a sintering step of heating the preheated compact at a temperature rising rate of 80° C./min or more from the preheating temperature and sintering at a sintering temperature of 1120° C. or more and 1300° C. or less.

The method for producing a sintered body described above can improve the fatigue strength of the sintered body.

It is a schematic diagram which shows the hole which exists in the sintered compact manufactured by the manufacturing method of the sintered compact which concerns on embodiment.

The present inventor has earnestly studied the fatigue characteristics of the sintered body, and has obtained the following findings.

When compressive stress is applied to the sintered body, it is generally considered that the existence of pores has a small effect on fatigue strength. On the other hand, when tensile-compressive stress, in which tensile stress and compressive stress are alternately repeated, is applied to the sintered body, the internal voids are likely to be at the maximum stress position, and the presence of voids has a large effect on fatigue strength. Therefore, it is highly possible that the tensile-compressive fatigue strength of the sintered body is low.

The present inventor has paid attention to the shape of pores (three points of powder) in the sintered body. As a result, it was found that in the conventional sintered body, at the powder triple point, the corners of the pores formed at the grain interfaces were sharply pointed. Therefore, when a tensile-compressive stress is applied to the sintered body, the acute-angled portion of this hole becomes the maximum stress position, and there is a high possibility that it will become a stress-concentrated portion. As a result, in tensile-compressive stress, it is considered that the acute-angled portion of the hole becomes the starting point of fracture due to stress concentration.

Therefore, the present inventor has conducted research on a method for improving fatigue strength (tensile-compressive fatigue strength) by rounding the corners of pores in a sintered body to reduce stress concentration. As a result of earnest research by the present inventor, sintering was performed by using an iron-based raw material powder to which Cu powder was added and mixed as a raw material of a sintered body and increasing the temperature rising rate from the preheating temperature to the sintering temperature. It has been found that the fatigue strength of the body is improved.

The present inventor adds a Cu powder having a melting point lower than the sintering temperature to an iron-based raw material powder, which is a raw material of a sintered body, so that a liquid phase of Cu appears during sintering, and a liquid phase of Cu appears. It was considered that the corners of the holes in the sintered body were rounded by entering into the sharp corners of the holes. However, in the conventional manufacturing method, the rate of temperature rise during sintering is not taken into consideration so much, and it was at most 40° C./min. When the heating rate is slow, Cu is solid-dissolved in Fe and the melting point is high, so that the liquid phase of Cu does not appear, or even if the liquid phase appears, the amount becomes small, so that the liquid phase of Cu becomes The phenomenon that the corners of the holes are not rounded sufficiently does not easily enter the sharp corners of the holes. When the rate of temperature rise is increased, a liquid phase of Cu is generated in the initial stage of temperature rise, the liquid phase of Cu wets and spreads, and enters the acute-angled portion of the hole. The liquid phase of Cu that has entered the sharp corners of the pores gradually dissolves in Fe to increase its melting point and solidify at the sintering temperature, thereby rounding the corners of the pores during sintering. ..

[Description of Embodiments of the Present Invention]
Hereinafter, embodiments of the present invention will be listed and described.

(1) The method for manufacturing a sintered body according to one aspect of the present invention is
A molding step in which an iron-based raw material powder added with Cu powder is pressure-molded to produce a molded body;
A preheating step of preheating the molded body at a preheating temperature of 800° C. or higher and 1000° C. or lower;
And a sintering step of heating the preheated compact at a temperature rising rate of 80° C./min or more from the preheating temperature and sintering at a sintering temperature of 1120° C. or more and 1300° C. or less.

The Cu powder appears as a liquid phase in the early stage of temperature rise in the sintering process, and the liquid phase of Cu enters into the sharp corners of the pores (triangle of powder) to round the corners of the pores in the sintered body. And has an effect of improving the fatigue strength (tensile-compression fatigue strength) of the sintered body. According to the above method for producing a sintered body, by setting the rate of temperature increase from the preheating temperature to the sintering temperature to 80° C./min or more, the liquid phase of Cu appears and enters the sharp corners of the holes. , The corners of the holes in the sintered body are rounded. This can improve the fatigue strength of the sintered body.

By setting the preheating temperature to 800° C. or higher, the temperature rising time from the preheating temperature to the sintering temperature can be shortened and Cu can be prevented from forming a solid solution in Fe during the temperature rising. By setting the preheating temperature to 1000° C. or less, Cu can be prevented from forming a solid solution in Fe during preheating.

(2) As one aspect of the method for producing the sintered body, the content of the Cu powder in the iron-based raw material powder is 1.0% by mass or more and 3.0% by mass or less.

When the content of Cu powder is 1.0% by mass or more, the liquid phase of Cu sufficiently appears, the corners of the pores can be effectively rounded, and the effect of improving fatigue strength is high. Even if the content of Cu powder is 3.0 mass% or more, it does not contribute to the improvement of fatigue strength any more, so the upper limit of the content of Cu powder is 3.0 mass %. The content of Cu powder is preferably 1.5% by mass or more and 2.5% by mass or less.

(3) As one aspect of the method for producing the sintered body, the iron-based raw material powder contains Cr in an amount of 1.0% by mass to 3.5% by mass and Mo in an amount of 0.2% by mass to 1.0% by mass. Hereinafter, the content of C is 0.4% by mass or more and 1.0% by mass or less, and Ni is 0.5% by mass or more and 2.0% by mass or less.

Cr, Mo, and Ni improve the hardenability of the sintered body and contribute to the high strength of the sintered body. Further, C alloys with Fe and contributes to the strengthening of the sintered body. When the content of each element is within the above range, the effect can be sufficiently obtained. The Fe content is preferably 90% by mass or more.

[Details of the embodiment of the present invention]
A specific example of the method for manufacturing a sintered body according to the embodiment of the present invention will be described below. The present invention is not limited to these exemplifications, but is defined by the scope of the claims, and is intended to include meanings equivalent to the scope of the claims and all modifications within the scope.

<Sintered body manufacturing method>
The manufacturing method of the sintered body according to the embodiment, a molding step of press-molding the iron-based raw material powder to produce a molded body, a preheating step of preheating the molded body, and raising the temperature of the preheated molded body. And a sintering step of sintering. One of the characteristics of the method for manufacturing a sintered body according to the embodiment is that an iron-based raw material powder to which Cu powder is added and mixed is used as a raw material of the sintered body, and the temperature rising rate from the preheating temperature to the sintering temperature is 80° C. /Min or more. Hereinafter, each step will be described in detail.

(Iron-based raw material powder)
The iron-based raw material powder is a mixed powder containing iron-based powder such as pure iron powder and iron alloy powder (steel powder, etc.) containing Fe as a main component, and Cu powder added thereto. In addition to Cu powder, the iron-based raw material powder may contain additive element powder such as graphite powder, which is an alloy component. When the additive element powder is added and mixed, the additive element may diffuse into Fe during the sintering to form an alloy. As the pure iron powder and iron alloy powder, for example, water atomized powder, gas atomized powder, carbonyl powder, and reduced powder can be used.

(Cu)
The Cu powder added to and mixed with the iron-based raw material powder becomes a liquid phase when the temperature rises, and has the function of rounding the pores in the sintered body, which has the effect of improving the fatigue strength (tensile-compression fatigue strength) of the sintered body. There is. The content of Cu powder is, for example, 1.0% by mass or more and 3.0% by mass or less, preferably 1.5% by mass or more and 2.5% by mass or less. By setting the content of Cu powder to 1.0% by mass or more, the liquid phase of Cu sufficiently appears, the corners of the holes can be effectively rounded, and the effect of improving fatigue strength is high. Even if the content of Cu powder is 3.0 mass% or more, it does not contribute to the improvement of fatigue strength any more, so the upper limit of the content of Cu powder is 3.0 mass %. Further, Cu diffuses in Fe during sintering and expands to act so as to cancel the contraction during sintering, thereby improving the dimensional accuracy of the sintered body.

(Cr, Mo, C, Ni)
Further, the iron-based raw material powder may contain Cr, Mo, C, Ni or the like as an alloy component. These elements are effective in improving the strength of the sintered body. The content of each element may be appropriately adjusted so as to obtain the desired mechanical properties. For example, the Cr content is 1.0% by mass or more and 3.5% by mass or less, and preferably 2.5% by mass or more. The content of Mo is 0.2 mass% or more and 1.0 mass% or less, preferably 0.4 mass% or more and 0.8 mass% or less. The content of C is 0.4 mass% or more and 1.0 mass% or less, preferably 0.8 mass% or less. The Ni content may be 0.5% by mass or more and 2.0% by mass or less, preferably 1.5% by mass or less. C and Ni may be added to and mixed with the iron-based raw material powder in the form of powder. The content of Fe in the iron-based raw material powder may be 90% by mass or more. When a steel powder in which C is prealloyed is used as the iron-based raw material powder (iron-based powder), it is hard to mold and it is difficult to increase the molding density of the molded body. Therefore, it is preferable to add C powder (for example, graphite powder) to the iron-based raw material powder without previously alloying C, which can increase the molding density of the molded body and increase the density of the sintered body. easy.

In the iron-based raw material powder, the average particle size of the iron-based powder such as pure iron powder or iron alloy powder is, for example, 20 μm or more and 200 μm or less, and further 50 μm or more and 150 μm or less. By setting the average particle size of the iron-based powder within the above range, it is easy to handle and press-mold easily. By setting the average particle size of the iron-based powder to 20 μm or more, it is easy to ensure the fluidity. By setting the average particle size of the iron-based powder to 200 μm or less, it is easy to obtain a sintered body having a dense structure. The average particle size of the additive element powder such as Cu powder, graphite powder, and Ni powder is preferably smaller than the average particle size of the iron-based powder, whereby the particles of the additive element powder are the particles of the iron-based powder. It becomes easy to disperse evenly between them, and alloying easily proceeds. The average particle diameter of the Cu powder, the Ni powder and the like is, for example, 1 μm or more and 20 μm or less, and further 10 μm or less. The average particle size of the graphite powder is, for example, 1 μm or more and 10 μm or less. The "average particle size" here is the particle size (D50) at which the cumulative volume in the volume particle size distribution measured by a laser diffraction particle size distribution measuring device is 50%.

(lubricant)
For the purpose of improving the moldability of the iron-based raw material powder, a lubricant may be added to and mixed with the iron-based raw material powder. A solid lubricant is preferably used as the lubricant. For example, at least one of stearic acid metal salts such as zinc stearate, fatty acid amides such as stearic acid amide, and alkylene bis fatty acid amides such as ethylene bis stearic acid amide is used. They can be used alone or in combination. A known lubricant can be used as the lubricant. The amount of the lubricant added to the iron-based raw material powder is, for example, 0.3 mass% or more and 1.0 mass% or less when the iron-based raw material powder is 100 mass %.

(Molding process)
The molding step is a step of pressure-molding the iron-based raw material powder to which Cu powder is added to produce a molded body. Specifically, it is possible to fill the mold with the iron-based raw material powder and press-mold it. For press-molding, a known press machine can be used. The higher the molding pressure at the time of pressure molding, the higher the density of the molded body and the higher the strength of the sintered body. The molding pressure is, for example, 600 MPa or more, and further 700 MPa or more. The upper limit of the molding pressure is, for example, 1000 MPa or less.

(Preheating process)
The preheating step is a step of preheating the compact at a preheating temperature of 800° C. or higher and 1000° C. or lower. By setting the preheating temperature to 800° C. or higher, the temperature rising time from the preheating temperature to the sintering temperature can be shortened and Cu can be prevented from forming a solid solution in Fe during the temperature rising. By setting the preheating temperature to 1000° C. or less, Cu can be prevented from forming a solid solution in Fe during preheating. The preheating temperature is preferably 850° C. or higher and 950° C. or lower, for example. In the preheating step, it suffices that the entire compact can be heated to the preheating temperature, and the preheating time may be set appropriately according to the shape and size of the compact. However, if the preheating time is too long, the solid solution of Cu in Fe may be promoted, so the preheating time is, for example, 5 minutes or more and 30 minutes or less, and further 8 minutes or more and 20 minutes or less. It can be mentioned.

(Sintering process)
The sintering step is a step of raising the temperature of the preheated compact at a temperature rising rate of 80° C./min or more from the preheating temperature and sintering at a sintering temperature of 1120° C. or more and 1300° C. or less. Specifically, the sintering step is continuously performed after the preheating step, and the compact preheated in the preheating step is heated from the preheating temperature to the sintering temperature. By sintering the compact, it is possible to obtain a sintered body in which particles of the iron-based powder are in contact with each other and bonded. The sintering temperature is preferably 1200° C. or higher and 1300° C. or lower, for example. The sintering time is, for example, 15 minutes or more and 150 minutes or less, and further 20 minutes or more and 60 minutes or less.

(Rate of heating)
The rate of temperature increase from the preheating temperature to the sintering temperature is 80° C./min or more. As a result, the liquid phase of Cu appears in the initial stage of the temperature rise, the liquid phase of Cu spreads wet, and enters into the acute-angled portion of the pores (three points of powder) existing in the compact of the iron-based raw material powder. Then, the liquid phase of Cu that has entered the acute corners of the pores gradually dissolves in Fe and solidifies at the sintering temperature, so that the corners of the pores in the sintered body become round. Since the liquid phase of Cu is more likely to appear as the heating rate is faster, the heating rate is, for example, 100° C./min or more, further 120°/min or more, 150° C./min or more, 200° C./min or more, 300° C. /Minute or more. Further, by increasing the temperature rising rate, the temperature rising time from the preheating temperature to the sintering temperature is shortened, and Cu can be suppressed from forming a solid solution in Fe. The upper limit of the rate of temperature increase is not particularly limited, but for example, the rate of temperature increase is 400° C./min or less.

When a C powder (eg, graphite powder) is added to and mixed with the iron-based raw material powder, the sintering temperature is the Fe-C eutectic point (1153°C) or higher, and C partially reacts with Fe to form a eutectic reaction. Then, a liquid phase may appear at the contact interface of Fe-C.

《Action effect》
The method for manufacturing a sintered body according to the above-described embodiment has the following effects.

When the temperature rise rate from the preheating temperature to the sintering temperature is set to 80° C./min or more when sintering the formed body of the iron-based raw material powder to which Cu powder is added, the liquid phase of Cu appears and the voids are generated. By entering into the sharp corner of (powder 3 point), the corner of the void in the sintered body becomes round. Thereby, the fatigue strength (tensile-compression fatigue strength) of the sintered body can be improved. Specifically, as shown in FIG. 1, in the pores v existing at the triple points of the particles p of the iron-based powder forming the sintered body, the acute-angled portion of the pores v (downward to the right in the figure) at the time of sintering. The liquid phase of Cu enters into the hatched portion (1) and solidifies to form rounded corners of the holes v. Since the rate of temperature rise is high, Cu does not completely diffuse into Fe, and some Cu (including the case where Fe diffuses into Cu) remains at the corners of the holes v.

Since the sintered body manufactured by the manufacturing method of the embodiment has excellent tensile-compressive fatigue properties, it can be suitably used for parts to which tensile-compressive fatigue stress is applied.

[Test Example 1]
By mass ratio, Cr: 3.0%, Mo: 0.5%, C: 0.5%, Ni: 1.0%, Cu: 1.0% are contained, and the balance is Fe and inevitable impurities. An iron-based raw material powder having a composition (Fe-3.0Cr-0.5Mo-0.5C-1.0Ni-1.0Cu) was prepared. Here, an iron alloy powder of Fe-Cr-Mo containing Fe, Cr and Mo (average particle diameter (D50): 100 μm) is prepared, and Cu powder (D50:10 μm) and graphite powder (D50:10 μm) are prepared. ) And Ni powder (D50: 2.5 μm) were added and mixed, and a mixed powder adjusted to a predetermined composition was used as an iron-based raw material powder. 0.6 mass% of a solid lubricant is added to and mixed with the iron-based raw material powder, and the mixture is pressure-molded at a molding pressure of 700 MPa, and the length×width×length is 14.5 mm×14.5 mm×95 mm. A rectangular column shaped molded body of was produced. Zinc stearate (D50: 100 μm) was used as the solid lubricant. The compact was placed in a sintering furnace, preheated at a preheating temperature of 950° C. for 8 minutes to be preheated, heated, and then sintered at a sintering temperature of 1250° C. for 30 minutes to sinter the sintered body Was manufactured.

In Test Example 1, the heater of the sintering furnace was controlled to change the heating rate from the preheating temperature (950° C.) to the sintering temperature (1250° C.). Sintered bodies 1-1 to 1-6 were obtained.

Sample No. Tensile-compressive fatigue strength was evaluated for the sintered bodies 1-1 to 1-6. The results are shown in Table 1. Here, the sintered body of each sample was processed to produce a fatigue test piece (No. 1 test piece, gauge part: diameter 8 mm x length 30 mm) according to JIS Z 2274, and fatigue was applied to each test piece. The test was conducted. The tensile-compression fatigue strength was evaluated by performing a fatigue test with an Ono-type rotary bending fatigue tester according to JIS Z 2274, and measuring the number of repetitions until the test piece broke. The larger the number of times, the higher the tensile-compressive fatigue strength. The measurement conditions were a rotation speed of 3600 rpm and a stress ratio R=-1 (both swings), and a tensile-compressive stress of ±300 MPa was repeatedly applied to the test piece.

From the results of Table 1, sample No. with a temperature rising rate of 80° C./min or more. Sample Nos. 1-1 to 1-5 have tensile-compression fatigue strength of more than 1×10 ⁶ times and have a slow heating rate. It can be seen that the tensile-compressive fatigue strength is higher than those of 1-6 to 1-8. From this result, it is considered that the tensile-compression fatigue strength of the sintered body can be improved by increasing the temperature rising rate from the preheating temperature to the sintering temperature. This is because Cu powder was added to the iron-based raw material powder, and the temperature rising rate from the preheating temperature to the sintering temperature was set to 80° C./min or more, so that the liquid phase of Cu was vacant during the sintering (powder triple point). It is presumed that the stress concentration is relaxed and the fatigue strength is improved by entering into the acute angle portion of) and rounding the corner portion of the void in the sintered body.

[Test Example 2]
In Test Example 2, an iron-based raw material powder having the same composition as in Test Example 1 was prepared except that the content of Cu powder was changed, and iron-based raw material powders having different Cu powder contents were used. To produce a sintered body. Here, the temperature rising rate was set to 120° C./min, and the sample No. A sintered body of 2-0 to 2-3 was obtained. Sample No. In No. 2-0, Cu powder was not added to and mixed with the iron-based raw material powder. Sample No. For the sintered bodies 2-0 to 2-3, the tensile-compression fatigue strength was evaluated in the same manner as in Test Example 1. The results are shown in Table 2.

From the result of Table 2, sample No. using the iron-based raw material powder to which Cu powder was added. 2-1 to 2-3 have a tensile-compressive fatigue strength of more than 1×10 ⁶ times, and it is understood that the tensile-compressive fatigue properties are excellent. On the other hand, the sample No. using the iron-based raw material powder to which Cu powder was not added was used. 2-0 is sample No. It can be seen that the tensile-compression fatigue strength is lower than 2-1 to 2-3 and is inferior. From this result, it is considered that the tensile-compression fatigue strength of the sintered body can be improved by adding Cu powder to the iron-based raw material powder.

p particle v hole

Claims (3)

A molding step in which an iron-based raw material powder added with Cu powder is pressure-molded to produce a molded body;
A preheating step of preheating the molded body at a preheating temperature of 800° C. or higher and 1000° C. or lower;
A sintering step of heating the preheated compact at a temperature rising rate of 80° C./min or more from the preheating temperature and sintering at a sintering temperature of 1120° C. or more and 1300° C. or less. Production method. The method for producing a sintered body according to claim 1, wherein the content of Cu powder in the iron-based raw material powder is 1.0% by mass or more and 3.0% by mass or less. The iron-based raw material powder contains 1.0 mass% or more and 3.5 mass% or less of Cr, 0.2 mass% or more and 1.0 mass% or less of Mo, and 0.4 mass% or more and 1.0 mass% of C. Hereinafter, the manufacturing method of the sintered compact according to claim 1 or 2, which contains 0.5% by mass or more and 2.0% by mass or less of Ni.

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