JPH06212368A - Low alloy sintered steel excellent in fatigue strength and its production - Google Patents

Abstract

PURPOSE:To produce low alloy sintered steel excellent in fatigue strength by treating powdery iron by a dry type mill to execute the introduction of dislocations and the pulverization of non-metallic inclusions, subjecting it to softening and the regulation of C content and thereafter executing cold forming, sintering and heating treatment. CONSTITUTION:Iron powder or iron alloy powder or the raw material powder of low alloy steel contg. the same is, if required, mixed with the grains of oxides, nitrides and carbides of <=0.5mum and/or Nb, V, Ti, W and Al by 0.05 to 3wt.%, and after that, this mixture is treated by a dry type mill. In this way, dislocations are introduced into the raw material powder, and non-metallic inclusions are pulverized into <=50mum by the maximum size. Next, the treated powder is subjected to softening to regulate the C content into 0.15 to <0.8% by the addition of carbon powder and, if required, to regulate the B content into 10 to 300ppm by the addition of ferroboron powder. After that, this mixed powder is subjected to cold rolling and is subjected to sintering or hot plastic working to densify it into >=96% of the theoretical density. Furthermore, it is subjected to heating treatment to form the matrix into gamma crystal tempered martensite having <=15mum average grain size.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low alloy sintered steel used for mechanical structural parts such as gears and races of bearings which require high fatigue strength, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Sintered machine parts made of low alloy sintered steel produced by powder metallurgy are widely used, for example, for automobile parts, office equipment, home electric appliances, agricultural machinery, etc. because of their excellent economical efficiency. , Its demand is increasing year by year.
With this increase in demand, the properties required for low alloy sintered steel have become increasingly severe.

Various studies have been carried out to meet this requirement. For example, in the composition of low alloy sintered steel, Fe-N has long been used.
Fe-Ni as well as i-C and Fe-Cu-C
-Mo-C system, Fe-Ni-Cu-Mo-C system, Fe-
Low alloy sintered steels such as Cr-Mn-Mo-C series have been developed. In terms of raw material powder, atomized powder, which is easy to obtain high density from conventional reduced powder, is becoming the mainstream.

With the development of these technologies, the strength of low alloy sintered steel has been greatly improved, and the static characteristics represented by the tensile strength thereof are comparable to those of general mechanical structural steel produced by the melting / forging method. I have reached the level to do. However, the dynamic properties represented by toughness and fatigue strength are not sufficient, and it is essential to improve and improve the dynamic properties in order to meet the demand for property improvement and further expand the applications.

The most effective way to improve the dynamic properties of low alloy sintered steels produced by powder metallurgy is to increase their density and reduce the voids that remain inside.
The powder forging method has long been known as one method of increasing the density of sintered steel. However, the low alloy steel produced by the powder forging method could not exceed the level of general mechanical structural steel produced by the melting / forging method having the same composition.

[0006] The cause is that the oxide film on the surface of the steel powder hinders the progress of sintering or causes segregation of alloying elements, non-metallic inclusions are present, and non-metallic inclusions are caused by the difference in steelmaking technology. There are differences in level, and holes are left on the surface of the forged body. Japanese Patent Publication No. 57-8841 describes a method of peeling and removing the oxide coating on the surface of the steel powder by mechanical pulverization. However, this method only removes the oxide coating on the surface of the steel powder. It has not been possible to remove non-metallic inclusions existing inside the particles.

[0007]

SUMMARY OF THE INVENTION In view of such conventional circumstances, the present invention is a low alloy sintered steel in which, in addition to excellent static properties, dynamic properties mainly including fatigue strength are greatly improved. ,
And its manufacturing method.

[0008]

To achieve the above object, the low alloy sintered steel excellent in fatigue strength of the present invention is 0.1
In a low alloy sintered steel containing carbon in an amount of 5% by weight or more and less than 0.8% by weight, the matrix is tempered martensite having an average grain size of old γ crystal grains of 15 μm or less, and pores contained in the matrix. Alternatively, the maximum diameter of the non-metallic inclusion is 50 μm or less, and the density is 96% or more of the theoretical density.

The method for producing a low alloy sintered steel excellent in fatigue strength according to the present invention is a method in which an iron powder produced by an atomizing method, an iron alloy powder, or a raw material powder of a low alloy steel containing these is used in an inert gas atmosphere. Alternatively, by treating with a dry mill in the air, dislocations are introduced into the raw material powder and the non-metallic inclusions are pulverized to a maximum diameter of 50 μm or less, and then the treated powder is softened and annealed so that carbon in the final composition is 0.
After carbon powder is added and mixed so as to be 15% by weight or more and less than 0.8% by weight, the mixed powder is cold-formed and densified to 96% or more of the theoretical density by sintering or hot plastic working. It is characterized in that the matrix is heat treated to be tempered martensite.

Here, the low alloy sintered steel is a conventionally known carbon steel composed of iron and carbon and a low alloy steel composed of iron, carbon and other alloying elements, and a raw material powder is sintered. Means that obtained by. The old γ crystal grains mean those in the low alloy sintered steel and the γ crystal grains in the austenite region during quenching and heating. It is performed by comparing the grain sizes of crystal grains.

[0011]

In general, although the fatigue strength of steel improves with an increase in hardness, when the hardness reaches a certain level, the fatigue strength tends to become almost constant or decrease. The cause of this is the presence of non-metallic inclusions, and as the hardness improves, small inclusions that could not be the starting point at low hardness become the starting points of fatigue cracks, so it is thought that the fatigue strength will not improve. Has been. Therefore, it is expected that if a material without inclusions can be obtained, the fatigue strength thereof can be greatly improved, but it is difficult to industrially obtain such a material.

It is known that the fatigue life depends on the crack growth rate at a high hardness level where improvement in fatigue strength reaches a ceiling. However, the crack growth rate is significantly reduced near the grain boundaries. Therefore, it is considered that improvement in fatigue strength can be achieved if a structure having a large number of crystal grain boundaries is achieved. From this point of view, the present invention reduces the number of vacancies to increase the density and at the same time increases the grain boundaries by using the matrix as a particularly fine tempered martensite to further increase the size of vacancies and non-metallic inclusions. By making it smaller, the fatigue strength and other dynamic characteristics of the low alloy sintered steel are improved and improved.

In the present invention, the matrix of the low alloy sintered steel is fine martensite having an average grain size of the former γ crystal grains of 15 μm or less. In order to obtain such particularly fine martensite, quenching heating is performed. It is necessary to refine the austenite in time. As the method, there is a work heat treatment method such as ausforming in a general low alloy steel by a melting / forging method, but in the present invention by a powder metallurgy method, a raw material powder is ball milled in an inert gas atmosphere or in the air, a vibration mill, It has been found that the most effective means is to preliminarily introduce a large amount of dislocations into the raw material powder by treating for a long time with a dry mill such as an attritor. That is, by using the raw material powder in which a large amount of dislocations are introduced in this way, austenite during quenching and heating becomes fine, and as a result, fine tempered martensite is obtained.

By processing the raw material powder using this dry mill, a large amount of dislocations are introduced into the raw material powder, and at the same time,
It is possible to reduce non-metallic inclusions contained in the raw material powder by pulverizing them to reduce large inclusions harmful to fatigue strength. That is, it was found that the influence on the fatigue strength of the low alloy sintered steel can be significantly reduced by setting the maximum diameter of the non-metallic inclusions to 50 μm or less, preferably 25 μm or less.

Since the hardness of the treated powder processed by the dry mill increases due to the introduction of a large amount of dislocations, it is difficult to perform cold forming as it is. Therefore, it is softened by strain relief annealing to form cold formability. Need to be increased. Annealing is preferably performed at a temperature of 600 to 1000 ° C. in a vacuum or a non-oxidizing atmosphere. If the annealing temperature is less than 600 ° C, the removal of work strain is insufficient, the softening of the powder is small and the formability cannot be improved, and if it exceeds 1000 ° C, the sintering progresses between the powders and the crushing treatment is performed again. Otherwise, cold forming will not be possible.

The treated powder thus obtained is mixed with a carbon powder such as graphite to adjust the amount of carbon. Carbon is an important alloying element that controls the properties of steel, and as the amount of carbon increases, the strength and hardness increase, but the ductility decreases. If the amount of carbon is less than 0.15% by weight, good hardness cannot be obtained after heat treatment, and if the amount of carbon is more than 0.8% by weight, the effect of improving the hardness is small, rather the increase of residual γ crystals and quenching In the present invention, the carbon content is set to be 0.15% by weight or more and less than 0.8% by weight because the fatigue strength is lowered due to the remarkable deterioration of the properties.

The mixed powder obtained by adding and mixing carbon powder to the treated powder can be used for uniaxial pressing using a die or cold isostatic pressing (CI).
After cold forming by P) or the like, a theoretical density of 96 is obtained by sintering at a temperature of A3 transformation or higher or hot plastic working such as hot forging, hot extrusion, hot isostatic pressing (HIP).
It is densified to more than%. It is desirable to perform re-pressurization, hot forging, hot extrusion, HIP, or the like after the sintering in order to eliminate the remaining pores, if necessary. Voids have a large effect on fatigue strength and can be regarded as equivalent to non-metallic inclusions, so their maximum diameter must be 50 μm or less, preferably 25 μm or less, and the remaining amount is 4% by volume.
The following is desirable.

The treated powder in which a large amount of dislocations have been introduced by a dry mill is apt to undergo coarsening of crystal grains when exposed to high temperature in a densification step such as sintering or hot plastic working. In order to prevent this, a method of uniformly dispersing fine particles and pinning the movement of grain boundaries is effective. As such dispersed particles, oxides, nitrides or carbides that do not form a solid solution or have a low solid solubility in the matrix in the high temperature austenite region, such as alumina and aluminum nitride, are effective.

The average particle size of these dispersed particles should be less than 5 μm, preferably 0.5 μm or less, and more preferably 0.1 μm or less. Average particle size of dispersed particle size is 0.5
If it exceeds μm, the fatigue strength will decrease, especially if the average particle size is 5μ.
This is because dispersed particles act as defects when m or more.
Further, if the addition amount of these dispersed particles is increased, the ductility is lowered, so about 0.5 to 5% by volume relative to the treated powder is suitable.

The thus obtained sintered body or hot-plastic worked body is strengthened by a usual heat treatment through transformation from austenite to martensite, and the fine alloy tempered martensite of the present invention is low alloy fired. Becomes steel. As a specific heat treatment, heating and holding to an austenite region, oil quenching, and then tempering are carried out, but carburizing treatment, carbonitriding treatment, nitriding treatment, surface hardening by induction heating, etc. combined with these heat treatments are also effective. .

The composition of the low alloy sintered steel of the present invention may be the composition of known carbon steel or low alloy steel as described above. However, in the present invention by the powder metallurgy method, a preferable alloy composition exists due to the process restriction. As an example, F
e-Ni-Mo-C system, and Fe-Cr-Mo-Mn-
There are C series etc. Fe-Cr-Mo-Mn-C-based low alloy sintered steel is characterized by being superior in hardenability than Fe-Ni-Mo-C-based steel, but contains easily oxidizable metallic elements such as Cr and Mn. Therefore, it is sensitive to the heating atmosphere during sintering or forging. Therefore, it is important to select and control the atmosphere during powder processing and heating.

In the above alloy composition, Ni greatly improves the hardenability, but if it is less than 0.5% by weight, its effect is not exhibited, and if it exceeds 3% by weight, cold forming of powder becomes difficult, and The residual γ crystals after heat treatment increase, which is harmful to the fatigue strength characteristics. Mo improves the hardenability by adding a small amount, and forms carbon and carbide to improve wear resistance and heat resistance. However, if Mo is less than 0.1% by weight, the effect is not obtained, and if it exceeds 1.5% by weight, solid solution strengthening deteriorates moldability.

Further, Cr has the functions of significantly improving the hardenability and increasing the temper softening resistance. However, if Cr is less than 0.5% by weight, the effect of improving hardenability is not provided, and if it exceeds 3% by weight, formability is deteriorated due to solid solution strengthening. Mn improves the hardenability and also acts as a deoxidizing agent and a desulfurizing agent during powder production to reduce the oxygen content of the powder. However, if it is less than 0.2% by weight, there is no improvement effect on the hardenability, and if it exceeds 1.6% by weight, the formability deteriorates due to solid solution strengthening.

As described above, the pinning effect of dispersed particles is effective in suppressing grain growth at high temperatures, but similarly, alloy powders made of raw materials containing elements that form carbides and nitrides with low solid solubility in the austenite region It is also effective to add to. According to this method, the growth of crystal grains during austenitization can be suppressed, so that the grain size of old γ crystal grains can be further refined. As this kind of element, niobium, vanadium, titanium, tungsten, or aluminum is promising, and the total addition amount thereof is preferably 0.05 to 3.0% by weight. The reason is that if it is less than 0.05% by weight, there is no effect of suppressing the growth of crystal grains, and if it is added in excess of 3.0% by weight, the crystal grains will no longer become finer, but rather the solid solution hardening will make the powder formable. Is reduced.

In the case of the low alloy sintered steel of the present invention, the crystal grain boundaries are increased by making the crystal grains very small, so that the core of the transformation is increased and the transformation of γ → α + Fe ₃ C occurs. However, there is a problem that the hardenability is likely to deteriorate as a result. However, boron segregates at the austenite grain boundaries and lowers the energy of the crystal grain boundaries, so that the function of the crystal grain boundaries as a nucleation site can be reduced, so the addition of a trace amount has the effect of improving hardenability. is there. To obtain this effect, boron is used in a weight fraction of 10 p
It is necessary to add more than pm, but even if it exceeds 300 ppm, not only the improvement of the hardenability is no longer observed, but also the strength of the crystal grain boundary becomes weak and it becomes brittle, which is not preferable.

[0026]

Example 1 A commercially available iron alloy powder having an AISI 4600 composition (Fe-1.8 wt% Ni-0.5 wt% Mo) produced by a water atomizing method was prepared using a high energy dry mill. In Ar atmosphere, the processing time is 2, 3, 4, 20, 40,
It was treated for 80 hours. Each of the obtained treated powders was annealed and softened by heating at 800 ° C for 1 hour in a nitrogen atmosphere, and graphite powder was added and mixed so that the carbon content in the final composition would be 0.25% by weight. Each was cold-molded by a die press so as to have a density of 6.9 g / cm ³ (density ratio 0.878). Each of the obtained compacts was sintered in nitrogen at 1150 ° C. for 1 hour and further forged at that temperature,
A forged body having a density ratio of 0.99 or more was obtained.

Each forged body was carburized at 910 ° C. so as to have an effective carburizing depth of 1 mm, then kept at 850 ° C., then quenched in oil and tempered at 200 ° C. for 90 minutes. For each low alloy sintered steel thus obtained,
The average particle size of the old γ crystal grains, the maximum diameter of the pores and the largest non-metallic inclusions in 400 mm ² , the bending strength, and the fatigue strength were measured, and the results in Table 1 were obtained. The fatigue strength was 4 × 9 as determined by a rotary bending fatigue test using a smooth test piece.
It was determined using a smooth test piece of × 45 mm.

[0028]

[Table 1] Treatment time Old γ grain size Maximum inclusion size Bending strength Fatigue strength Sample (hr) (μm) (μm) (kg / mm ² ) (kg / mm ² ) 1 2 18.7 186 220 80 2 3 15.0 48 230 85 3 4 12.6 47 244 90 4 20 10.4 33 261 95 5 40 8.8 25 277 100 6 80 8.2 27 273 100

From the results shown in Table 1, the average grain size of the old γ crystal grains and the size of the maximum inclusions become smaller as the treatment time of the raw material powder in the dry mill increases, and accordingly, the low alloy sintered steel It can be seen that the characteristics are improved, and especially the fatigue strength is greatly increased.

[0030]Example 2  AISI 4600 set manufactured by water atomization method
Composition (Fe-1.8 wt% Ni-0.5 wt% Mo)
Using a high energy dry mill, a commercially available iron alloy powder
It processed in Ar atmosphere for 40 hours. The resulting treated powder
Is annealed and softened by heating at 800 ° C for 1 hour in a nitrogen atmosphere.
After carbonization, the final composition has a carbon content of 0.25% by weight.
Graphite was added and mixed in this manner. Form this mixed powder
Shape density 6.9 g / cm³(Density ratio 0.878)
Cold-molded in a die press to 115 in nitrogen
Sinter at 0 ° C for 1 hour, forge at that temperature, and then forge
Change the density ratio of each in the range of 0.92 to 0.99.
It was

Each of the obtained forged bodies having different density ratios was heat treated in the same manner as in Example 1, and the obtained low alloy sintered steels had the same average grain size of the former γ crystal grains, bending strength, impact value, and The fatigue strength was measured and the results shown in Table 2 were obtained. The maximum diameters of pores and maximum inclusions present in each low alloy sintered steel were 50 μm or less. For comparison, a low alloy sintered steel was produced in the same manner as above except that the raw material powder was not treated with a dry mill, and the same evaluation was performed on this comparative material. The results are also shown in Table 2. It was

[0032]

[Table 2] Old γ particle size Bending strength Impact value Fatigue strength Sample density ratio (μm) (kg / mm ² ) (kgf ・ m / cm ² ) (kg / mm ² ) 7 0.92 8.8 202 0.66 70 8 0.94 8.9 221 0.92 75 9 0.96 8.8 248 1.4 85 10 0.99 8.7 277 3.2 100 Comparative material 0.99 22.4 210 1.72 75

From the results shown in Table 2, it can be seen that the strength, toughness, and fatigue strength of the low alloy sintered steel also increase as the density ratio increases. However, in the comparative material in which the raw material powder is not processed by the dry mill, the average grain size of the old γ crystal grains is large, so that even if the density is high, the strength, toughness, and fatigue strength are inferior to those of the sample of the present invention. ing.

Example 3 A commercially available iron alloy powder having an AISI 4600 composition (Fe-1.8 wt% Ni-0.5 wt% Mo) produced by the water atomizing method was added to alumina having an average particle size of 0.05 μm. The powder has a volume fraction of 0.5%, 1.0%, 2.0%, 5.0.
%, And then mixed and then treated in an Ar atmosphere for 40 hours using a high energy dry mill. Each of the obtained treated powders was annealed and softened, graphite powder was mixed, cold compacted, sintered, and the density ratio was 0.99 under the same conditions as in Example 1.
The above forging and heat treatment were performed to manufacture a low alloy sintered steel.

With respect to each of the obtained low alloy sintered steels, bending strength, impact value and fatigue strength were measured together with the average grain size of the old γ crystal grains, and the results are shown in Table 3. The maximum diameters of pores and inclusions present in each low alloy sintered steel are 50
It was less than μm. From the results shown in Table 3 below, it can be seen that the old γ crystal grains are refined by the addition of alumina, but if the amount of addition exceeds 5% by volume, the effect of refining reaches a ceiling and the impact value tends to decrease. I see.

[0036]

[Table 3] Al ₂ O ₃ content Old γ grain size Bending strength Impact strength Fatigue strength Sample (vol%) (μm) (kg / mm ² ) (kgf ・ m / cm ² ) (kg / mm ² ) 11 0.5 7.8 279 3.4 100 12 1.0 4.8 330 6.1 120 13 2.0 2.9 338 6.7 120 14 5.0 2.5 342 5.9 120

Example 4 A commercially available iron alloy powder having an AISI 4600 composition (Fe-1.8 wt% Ni-0.5 wt% Mo) produced by a water atomizing method was used, and the average particle size was 0.05 μm. 0.1
Alumina powders of μm, 0.5 μm, 5 μm, 15 μm, and 24 μm were mixed and mixed so that the volume fraction was 1.0%, and then A using a high energy dry mill.
It was treated for 40 hours in an atmosphere of r. Each of the obtained treated powders was annealed and softened, graphite powder was mixed, cold compacted, sintered, and forged to have a density ratio of 0.99 or more, under the same conditions as in Example 1.
And heat treatment was performed to produce a low alloy sintered steel.

With respect to each of the obtained low alloy sintered steels, the bending strength, impact value and fatigue strength were measured together with the average grain size of the old γ crystal grains, and the results are shown in Table 4. From the results shown in Table 4 below, as the average particle size of the alumina powder to be added becomes smaller, the old γ crystal grains in the low alloy sintered steel become finer, but if the average particle size of the alumina powder is 5 μm or more, the alumina is rather defective. It can be seen that it acts as an alloy and reduces strength and toughness.

[0039]

[Table 4] Al ₂ O ₃ grain size Old γ grain size Bending strength Impact strength Fatigue strength Sample (μm) (μm) (kg / mm ² ) (kgf · m / cm ² ) (kg / mm ² ) 15 0.05 4.8 330 6.1 120 16 0.1 6.2 315 4.4 120 17 0.5 10.8 272 3.2 115 18 5 14.2 232 2.0 80 19 15 17.3 218 1.8 78 20 24 22.2 212 1.6 75

Example 5 Composition of AISI 4100 manufactured by water atomization method (Fe-0.8 wt% Mn-1.0 wt% Cr-0.2
A commercially available iron alloy powder having 5 wt% Mo), and ferroniobium powder, titanium powder, ferrovanadium powder, tungsten powder as Nb source, Ti source, V source, W source, and Al source, respectively, to the iron alloy powder. Ferroaluminum powder was added to the final composition, and the content of the above-mentioned additional elements was 0.5
Each powder added and mixed so as to have a weight percentage was treated for 40 hours in an Ar atmosphere using a high energy dry mill.

Each of the obtained treated powders was heated in a nitrogen atmosphere at 800 ° C. for 1 hour to be annealed and softened, and then graphite was added and mixed so that the final composition had a carbon content of 0.2% by weight. did. These mixed powders were cold-formed, sintered and forged in the same manner as in Example 1 to obtain a forged body having a density ratio of 0.99 or more after forging. Each forged body was gas carburized so that the effective carburizing depth was 1 mm, heated at 850 ° C., quenched in oil, and then tempered at 200 ° C. for 90 minutes.

With respect to each of the obtained low alloy sintered steels, the average grain size of the old γ crystal grains, the bending strength, the impact value and the fatigue strength were measured, and the results in Table 5 were obtained. The maximum diameters of pores and inclusions present in each steel were 50 μm or less. From the results in Table 5, it can be seen that the addition of Nb, Ti, V, W or Al promotes the refinement of the former γ crystal grains and further improves the strength and toughness as compared with the case where these elements are not added. .

[0043]

[Table 5] Old γ grain size Bending strength Impact value Fatigue strength Sample addition element (μm) (kg / mm ² ) (kgf ・ m / cm ² ) (kg / mm ² ) 21 None 8.8 277 3.2 100 22 Nb 6.2 321 4.9 120 23 Ti 7.7 318 4.9 115 24 V 6.5 310 4.8 120 25 W 7.2 316 5.1 120 26 Al 6.3 319 5.0 118

Example 6 A commercially available iron alloy powder having an AISI 4600 composition (Fe-1.8 wt% Ni-0.5 wt% Mo) produced by a water atomizing method was added to alumina having an average particle size of 0.05 μm. The powder was mixed in an amount of 1.0% by volume, and then treated in a high energy dry mill in an Ar atmosphere for 40 hours. After the obtained treated powder is annealed and softened under the same conditions as in Example 1,
0.25 wt% carbon and 30 pp boron in final composition
Graphite powder and ferroboron powder were mixed so as to obtain m. This mixed powder was cold-formed, sintered, and forged in the same manner as in Example 1 to obtain a forged body having a density ratio of 0.99 or more.

From the obtained forged body, a round bar having a diameter of 25.4 mm and a length of 101.6 mm (1 × 4 inches) was carved out,
A Jominy test was conducted to evaluate the hardenability. For comparison, the forged body produced in Example 3 containing 1.0 vol% alumina and containing no boron (Sample 12).
= B additive-free present invention material) and a SCM 420 steel material, a round bar having the same shape was machined, and the hardenability of these was also comparatively evaluated. The result is shown in FIG. From FIG. 1, it can be seen that in the B-free invention material (Sample 12), the hardenability is deteriorated due to the refinement of the old γ crystal grains, but in the B-added invention material of the present example in which boron is added, the hardenability is significantly increased. Improved to
It turns out that it is almost the same level as SCM 420 steel.

Example 7 A commercially available iron alloy powder having an AISI 4600 composition (Fe-1.8% by weight Ni-0.5% by weight) produced by a water atomizing method was added to alumina having an average particle size of 0.05 μm. The powder was mixed in an amount of 1.0% by volume, and then treated in a high energy dry mill in an Ar atmosphere for 40 hours. The treated powder thus obtained was annealed and softened at 1000 ° C. for 1 hour in vacuum, and then graphite powder was mixed so that the final composition of carbon was 0.25% by weight, followed by cold forming in the same manner as in Example 1. Then, a molded product having a component shape of the one-way clutch was produced.

The molded body was sintered, forged, and heat-treated in the same manner as in Example 1, and then subjected to processing such as polishing to give an outer ring and a slug of a one-way clutch used for an automatic transmission for automobiles. For comparison, the same outer ring and slugs were made from similarly heat treated SCM 420. A stroking durability test was conducted by assembling a clutch using the outer ring and the sprags of the present invention material and the comparative material. The test conditions are as shown in Table 6 below.

[0048]

[Table 6]

As a result of the above durability test, the cumulative damage probability due to peeling etc. of the outer ring of the material of the present invention and the comparative material is shown in FIG.
Shown in. From this FIG. 2, it is understood that the material of the present invention has a longer life against peeling and the like than the conventional SCM 420 carburized steel material which is a comparative material. Further, in the above durability test, the wear amount of each outer ring and the slug at the time of rocking numbers of 10 ⁴ , 10 ⁵ , and 10 ⁶ was measured, and the results shown in FIG. 3 were obtained. This Figure 3
From the above, it is understood that the material of the present invention is superior in wear resistance to the conventional SCM 420 carburized steel material which is the comparative material.

[0050]

According to the present invention, it is possible to provide a low alloy sintered steel which is excellent not only in static characteristics such as bending strength but also in dynamic characteristics such as fatigue strength and toughness. Therefore, the low alloy sintered steel of the present invention is particularly useful as a mechanical structural part requiring high fatigue strength such as gears and clutch parts,
It is expected that it will be possible to develop applications in fields where this type of low alloy sintered steel has not been used in the past.

[Brief description of drawings]

FIG. 1 shows the hardness (Rockwell C hardness) corresponding to the distance from the water-cooled end obtained by the Jominy test for the present invention material containing boron, the present invention material not containing boron, and the SCM 420 steel material. It is a graph which showed change.

[Fig. 2] In the stroking durability test of the clutch,
It is a graph of the cumulative damage probability showing the life of the outer ring produced by the material of the present invention and the comparative material of SCM 420 steel with respect to peeling and the like.

[Fig. 3] In the stroking durability test of the clutch,
Outer ring and sprag made from the material of the present invention, and SCM 4
It is a graph which shows the abrasion resistance of the outer ring and the slug which were produced with the comparative material of 20 steel materials.

Claims (8)

[Claims] 1. A low alloy sintered steel containing carbon in an amount of 0.15% by weight or more and less than 0.8% by weight, the matrix of which is tempered martensite having an average grain size of old γ crystal grains of 15 μm or less, A low alloy sintered steel excellent in fatigue strength, characterized in that the pores or non-metallic inclusions contained in the matrix have a maximum diameter of 50 μm or less and a density of 96% or more of the theoretical density. 2. An average particle size of at least one selected from oxides, nitrides and carbides which do not form a solid solution in the austenite region or have a low solid solubility in the matrix.
The low alloy sintered steel excellent in fatigue strength according to claim 1, wherein particles of 5 μm or less are uniformly dispersed. 3. Boron in a weight fraction of 10 to 300 ppm
The low alloy sintered steel excellent in fatigue strength according to claim 1 or 2, characterized in that it is contained. 4. At least one selected from niobium, vanadium, titanium, tungsten and aluminum is contained in a total amount of 0.05 to 3% by weight, according to any one of claims 1 to 3. Low alloy sintered steel with excellent fatigue strength as described. 5. Iron powder or iron alloy powder produced by an atomizing method, or a raw material powder of low alloy steel containing the same is treated with a dry mill in an inert gas atmosphere or in the air to transfer to the raw material powder. And the non-metallic inclusions are crushed to a maximum diameter of 50 μm or less, and then the treated powder is softened and annealed so that carbon in the final composition is 0.15.
After carbon powder is added and mixed so as to be not less than 0.8% by weight, the mixed powder is cold-formed, densified to 96% or more of theoretical density by sintering or hot plastic working, and further heat treated. And a tempered martensite matrix is used to produce a low alloy sintered steel having excellent fatigue strength. 6. At least one selected from oxides, nitrides and carbides which do not form a solid solution or have a low solid solubility in iron powder or iron alloy powder or a raw material powder of a low alloy steel containing the same in the austenite region. Particles having an average particle size of 0.5 μm or less and / or at least one selected from niobium, vanadium, titanium, tungsten and aluminum were mixed so that the total amount was 0.05 to 3% by weight. After that, this is processed with a dry mill, The manufacturing method of the low alloy sintered steel excellent in the fatigue strength of Claim 5 characterized by the above-mentioned. 7. The softening annealing of the treated powder treated with a dry mill is performed in vacuum or in a non-oxidizing atmosphere at 600 to 1000.
The method for producing a low alloy sintered steel having excellent fatigue strength according to claim 5 or 6, wherein the method is performed at a temperature of ° C. 8. The ferroboron powder is added and mixed to the treated powder after softening and annealing so that the weight fraction of boron is 10 to 300 ppm.
A method for producing a low alloy sintered steel having excellent fatigue strength according to any one of 1.