JPH07138694A - Production of low alloy steel powder for powder metallurgy and ferrous sintered parts with high dimensional accuracy - Google Patents

Abstract

PURPOSE:To obtain a low alloy steel powder for powder metallurgy, capable of producing a sintered compact of high dimensional accuracy, by allowing an Nb oxide of specific grain size to exist at least on the surface of a powder having a specific composition consisting of Nb, Ni, Cr, Mo, and iron. CONSTITUTION:An Nb oxide of <=40mum grain size is allowed to exist at least on the surface of a powder having a composition consisting of, by weight, 0.02-0.15% Nb, at least one kind among 0.1-2.0% Ni, 0.4-2.0% Cr, and 0.1-3.0% Mo, and the balance iron with inevitable impurities. This Nb oxide can be easily formed because Nb on the surface of the steel powder is brought into contact with oxygen in the above Nb-containing low alloy steel powder manufacturing process and oxidized. By using this low alloy steel powder in a powder metallugical method, dimensional change at the time of sintering can be inhibited and a sintered compact having high dimensional accuracy can be obtained. Further, by mixing graphite with the above low alloy steel powder for powder metallurgy, compacting the resulting mixture, and then sintering the resulting green compact under a nonoxidizing atmosphere, carbon is allowed to enter into solid solution and Nb carbide is formed, by which the sintered compact having high dimensional accuracy and high strength can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low alloy steel powder for powder metallurgy, and in particular, by containing an appropriate amount of Nb, the dimensional change at the time of sintering can be reduced as much as possible, and the sinterability with high dimensional accuracy can be obtained. The present invention relates to a low alloy steel powder for powder metallurgy modified so as to obtain a shaped body, and a method for producing a sintered component with high dimensional accuracy using the steel powder.

[0002]

2. Description of the Related Art In producing a sintered part using steel powder for powder metallurgy, graphite powder and a lubricant are mixed with steel powder, and further alloy powder is mixed if necessary, and then powder compacting is performed. A method in which this molded body is heated to 1100 to 1250 ° C. and sintered is generally adopted. However, depending on the type of low alloy steel powder, it is often experienced that dimensional changes occur in the sintering process after compacting, and the prescribed dimensional accuracy cannot be obtained. Machining, re-pressing, etc. must be performed, which increases the processing cost.

The influence on the dimensional change during sintering is as follows.
The chemical composition and particle size distribution of the steel powder, the density of the green compact, the sintering temperature, the heat pattern, the sintering atmosphere and the like are mentioned, and it is considered that these are related to each other and cause a dimensional change. For example, it has been reported that the amount of oxygen and the amount of fine powder in steel powder have a strong influence on dimensional changes, and methods have been attempted to reduce the content thereof in order to improve dimensional accuracy.
Further, it has been confirmed that when Cu powder is added as an alloy component, it tends to expand at the time of sintering, while when Ni powder is added, it tends to shrink, and by utilizing such a phenomenon, Cu powder and Ni powder are mixed at an appropriate ratio. A method of increasing the dimensional accuracy by including it is also practiced.

However, in the production of sintered parts, not only dimensional accuracy but also various physical properties required for a sintered product, such as tensile strength, hardness, fatigue strength, impact strength, and abrasion resistance, are present. It is not easy to satisfy both characteristics and dimensional accuracy at the same time. In addition, it is pointed out that the amount of dimensional change varies depending on the density of the powder compact when the alloy powder is added.

On the other hand, for example, in the automobile industry as well, the demand for sintered parts with high dimensional accuracy is increasing with the progress of higher quality and lower noise, and the powder compacting technology such as multi-stage press has been improved to meet such demand. However, even if it has a complicated shape, it is not easy to obtain a green compact having a uniform density. As a general tendency when sintering a green compact, there is a tendency that although the green compact has a low density, the amount of shrinkage during sintering is large, and conversely, the density is high, but the amount of shrinkage during sintering is small. Therefore, even if the same green compact has a difference in density inside, the difference in shrinkage amount partially causes the dimensional accuracy to deteriorate and the shape may be distorted. Machining and reprocessing after binding are required.

Therefore, as another measure for reducing the problem caused by the dimensional change at the time of sintering, two or more kinds of steel powders having mutually positive and negative tendencies in the dimensional change behavior at an appropriate ratio. A method of mixing and using it to reduce the amount of dimensional change during sintering has also been implemented, but in this case as well,
It is difficult to simultaneously satisfy the above-mentioned physical properties and dimensional accuracy requirements for sintered products.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to reduce the amount of dimensional change during sintering after compaction molding and to compact powder compaction. Low alloy steel powder for powder metallurgy and a high dimensional accuracy sintering process using the low alloy steel powder that gives a sintered compact with high dimensional accuracy without shape distortion even if the density varies within the body. It is intended to provide a feature.

[0008]

The composition of the low alloy steel powder for powder metallurgy according to the present invention, which has been able to solve the above problems, is as follows.
The overall composition contains Nb: 0.02 to 0.15% by weight, Ni: 0.1 to 2.0%, Cr: 0.4.
.About.2.0% and Mo: 0.1 to 3.0%, and at least one element selected from the group consisting of, and the balance consisting of iron and unavoidable impurities.
It is characterized by the presence of Nb oxide of 0 μm or less.

Then, carbon is solid-dissolved in the iron matrix in a non-oxidizing atmosphere in a compact obtained by mixing graphite with the low alloy steel powder for powder metallurgy and pressing the mixture into a predetermined shape. Along with this, sintering at a temperature / time for forming Nb carbide of 40 μm or less makes it possible to obtain an iron-based sintered component with high dimensional accuracy.

[0010]

As described above, the present invention has the greatest feature in that a low alloy steel base for powder metallurgy contains a specific amount of Nb, which is bound to oxygen, nitrogen and carbon. It has high power and easily forms Nb compounds in the coexistence with them.

Therefore, when oxygen is exposed in the manufacturing process of the low alloy steel powder containing an appropriate amount of Nb, Nb on the surface of the low alloy steel powder changes into Nb oxide, which is usually burnt. Since it is not easily reduced in a reducing gas atmosphere due to binding, Nb oxide is present on the surface of the Nb-containing low alloy steel powder, and the presence of the Nb oxide reduces the sintered area, resulting in It is possible to obtain a sintered product with high dimensional accuracy without causing shape distortion even when the dimensional shrinkage is significantly suppressed and the density has a complicated shape and the density varies within the powder compact.

In compacting / sintering the low alloy steel powder, a carbon source such as graphite powder or a lubricant is mixed with the low alloy steel powder and compacted, and then an atmosphere of ammonia decomposition gas or the like is produced. Sintering is carried out under the following conditions. In the sintering step, Nb in the steel powder forms carbides and nitrides, and the dimensional shrinkage is suppressed by reducing the sintered area in the same manner as described above. Also has the effect of suppressing coarsening of crystal grains in the sintered body and increasing the mechanical strength.

In order to effectively exert such Nb addition effect, it is necessary to contain at least 0.02% or more of Nb in the low alloy steel base.
The effect of reducing the sintering dimensional change brought about by the formation of the compound b is not sufficiently exerted, and the purpose intended in the present invention cannot be achieved. However, if the amount of Nb is too large, a coarse Nb compound is formed and the physical properties of the sintered body, particularly the tensile strength, are adversely affected, so the content must be suppressed to 0.15% or less. A more preferable Nb content is 0.03 to 0.10 for effectively exhibiting the above Nb addition effect.
%, And more preferably 0.03 to 0.07%.

As described above, the present invention is characterized in that a specific amount of Nb is contained in the low alloy steel base for powder metallurgy, and in particular, the dimensional change during sintering is suppressed to improve the dimensional accuracy. However, in this case, the physical properties of the sintered body can be further improved by adding an appropriate amount of Ni, Cr or Mo as a component of the low alloy steel as shown below.

Ni: 0.1 to 2.0% Ni has the effect of increasing the toughness of the sintered body, and the effect is effectively exhibited by containing 0.1% or more, but if added in excess, steel Since the powder becomes hard and the compressibility at the time of powder compaction decreases, it must be suppressed to 2.0% or less.

Cr: 0.4 to 2.0% Cr is an element that enhances the hardenability and contributes to the improvement of the strength of the sintered body, and its effect is effectively exhibited by adding 0.4% or more. However, those effects are almost saturated at 2.0%,
If it is contained more than that amount, the amount of dimensional change at the time of sintering tends to increase, so it must be suppressed to 2.0% or less.

Mo: 0.1 to 3.0% Mo also acts effectively in enhancing the hardenability and the strength of the sintered body, and the effect is effectively exhibited by containing 1.0% or more. It But the effect is about 3.0
%, And if contained more than that, the amount of dimensional change during sintering tends to increase, so 3.0% is made the upper limit.

The unavoidable impurities contained in the steel powder according to the present invention include C, Si, Mn, P, S, O, N, etc.
The same elements as in ordinary steel materials can be mentioned, but if the C content is too high, the powder becomes hard and the compressibility during powder compaction deteriorates, so 0.02% or less, more preferably 0.01% or less. It is good to keep Si is added as a raw material for steel powder production and for deoxidizing molten metal, but if the content is high, the arrow-tight powder becomes hard, so 0.1% or less,
More preferably, it is desired to be 0.05% or less.

Similarly, Mn is also contained as an element for adjusting the properties of the molten metal, and usually 0.8% is contained in ordinary steel powder produced by the atomizing method. Further, Mn also has an effect of enhancing hardenability, but considering the compressibility of the powder, it is desirable to suppress it to 0.3% or less.
Other impurities, P, is preferably about 0.01% or less, and S is about 0.01% or less.

O and N are mainly mixed in during powder production, and these exist as metal oxides or metal nitrides of Nb, Cr, Fe and the like. Of these, O is 0.
It is contained in an amount of about 2% and forms the oxide of Nb on at least the surface portion of the powder to exhibit the above-described action. But,
When the O content is too high as in the case where the powder is oxidized, the dew point of the sintering gas is adversely affected, and the dimensional change in sintering may become unstable. Further, N is usually contained in an amount of about 0.03%, but if the N content is too large, it becomes hard and brittle, which is not preferable.

The low alloy steel powder for powder metallurgy of the present invention is a method of preparing a completely alloyed powder by an atomizing method by adjusting the molten metal so as to satisfy the requirements of the above component composition, or an additive element or its alloy powder and It can be manufactured by a method of mixing with iron powder and heating it to form diffusion-adhesion-type steel powder which is metallurgically adhered. However, in order to further reduce the dimensional change during sintering, it is preferable that Nb is evenly distributed. From this point of view, it is desirable to use a perfect alloy type powder. The particle size is not particularly limited, but from the viewpoint of yield, handleability, powder moldability, sinterability, etc., the average particle size is preferably 50 to 100 μm, more preferably 50 to 8 μm.
It is 0 μm and the maximum particle size is 250 μm or less.

The above-mentioned low alloy steel powder may be produced by a conventional method, for example, by using the low alloy steel powder as a carbon source such as graphite powder and a forming lubricant such as zinc stearate. After being uniformly mixed, the powder is compacted into a predetermined shape at about 400 to 800 MPa, and then heat-treated at about 1100 to 1250 ° C. in a non-oxidizing atmosphere such as an ammonia decomposition gas to sinter. At this time, when the low alloy steel powder according to the present invention is used, Nb oxides are present on the powder surface portion as described above, and Nb carbides or Nb nitrides are produced, and sintering is caused by the presence of these Nb compounds. Due to the area reduction effect, the amount of dimensional change is suppressed, and it is possible to obtain a sintered compact with high dimensional accuracy even if there is a slight density variation in the compact or in the individual compacts. It is possible to commercialize the product substantially as it is without performing mechanical processing or re-pressurizing processing for adjusting the post-dimension.

Further, regarding the heat-treated product of the sintered body using the low alloy steel powder, the existence form and size of Nb added to the steel powder, and the dimensional accuracy and physical properties at the time of sintering were examined,
Nb exists mainly in the form of carbides, and some are found in the state of oxides and trace amounts of nitrides. It was also confirmed that when the maximum particle size of the Nb compound was 40 μm or less, excellent dimensional accuracy and physical properties were exhibited. The maximum particle size of this Nb compound is closely related to the Nb content, and it is small when the Nb content is small, and becomes large as the Nb content increases, and the Nb content is the upper limit of 0.15 of the specified range. %, The maximum particle size of the Nb compound size in the sintered body is more than 40 μm, and the sintered body has a low tensile strength, which may defeat the purpose of the present invention. confirmed. Therefore, the sintered body of the present invention,
The content of Nb is 0.02 to 0.15%, and it is distinguished from the conventional sintered low alloy steel sintered compact in that the Nb compound of 40 μm or less exists on the surface or arbitrary cross section of the sintered compact. be able to.

The structure of the sintered body produced by the method according to the present invention varies depending on the amount of the carbon source added and the cooling rate during sintering, but exhibits a pearlite and ferrite or bainite structure. It usually has a martensite structure.

[0025]

EXAMPLES The constitution and effects of the present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples, and
Modifications can be made within a range that is compatible with the gist of the description below, and all of them are included in the technical scope of the present invention.

Example Low alloy steel powder for powder metallurgy (average particle size 50) having the composition shown in Table 1
To 70 μm), 0.6% of graphite powder and 0.75% of zinc stearate are added and uniformly mixed, and pressed to an outer diameter of 30 m.
m, an inner diameter of 20 mm, and a total length of 10 mm were obtained. At this time, the density of the molded body was 6.7 g / cm ³ , 6.9 g / c.
The molding pressure was adjusted so that m ^{3 was} 7.1 g / cm ³ .

Steel powder Nos. 1 to 6 are Ni, Cr, and M.
It is a low alloy steel powder containing O and having different Nb contents. Further, steel powder Nos. 7 to 9 are conventionally used for comparison, steel powder No. 7 is 3% Cr atomized alloy iron powder, and steel powder No. 8 is Ni-Mo type diffusion alloy iron. Powder, steel powder No.
9 is atomized iron powder.

After accurately measuring the outer diameter dimension of each of the obtained green compacts, the temperature was set to 125 at the atmosphere of ammonia decomposition gas.
Sintering was performed by heating at 0 ° C. for 60 minutes, and the outer diameter dimension after sintering was measured to obtain the dimensional change rate. The dimensional change rate is measured according to the calculation method of Japan Powder Metallurgy Association Standard JPMA P12-1992, and the dimensional change rate is Y and the green compact density is X.
Then, the value of A represented by [Y = AX + B] was obtained by the method of least squares, and the absolute value α (= | A |) of A, that is, the influence coefficient to the dimensional change was obtained. Also, after pressing each mixed powder in a mold in the same manner as described above to obtain a molded body having a density of 7.2 g / cm ³ , a size of 10 mm inner diameter, an outer diameter of 60 mm and a total length of 10 mm, the same conditions as above Sintered and machined into tensile test pieces specified by JPMA, then oil-hardened at 850 ° C, 1
A tensile test was performed on the material that was tempered at 80 ° C. The results are shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

As is clear from Tables 1 and 2, in the case of using the low alloy steel powder Nos. 1 to 4 satisfying the requirements of the present invention, the influence coefficient α is 0.4 or less. It can be seen that it has excellent dimensional accuracy. On the other hand, the comparative steel powder of No. 5 does not contain Nb, so that the influence coefficient α shows a high value, and it can be seen that the dimensional accuracy cannot be satisfied. Further, No. 6 is a comparative example in which Nb exceeds the specified range, and although the influence coefficient α has a very small value, the tensile strength is low. The relationship between the Nb content and the coefficient of influence α and the tensile strength in Nos. 1 to 6 is shown in FIGS. 1 and 2. As is apparent from this graph, Nb should be contained in an amount of 0.02% or more and the tensile strength of It can be seen that Nb should not exceed 0.15% to prevent the decrease.

On the other hand, the comparative Nos. 7 and 8 show tensile strength to some extent, but the influence coefficient α shows a high value, and the pure iron powder of No. 9 has an influence coefficient α. Is small and excellent in dimensional accuracy, but shows a very low value in tensile strength.

Next, Table 3 shows the steel powders of the present invention Nos. 1 to 4 and N.
3 shows the Nb compound size in the fracture surface after the tensile test of the heat-treated body manufactured using the comparative steel powders of o.5 and 6 and the crystal grain size as seen from the sectional structure. The Nb compound size was obtained by analyzing the EPMA surface of the fracture surface and determining the maximum grain size. As an example, the fracture surface SEM image of the sample using the steel powder of the present invention of No. 3 is shown in FIG. Figure 4 is a surface analysis photograph
Shown in. It is shown that Nb is present in the fracture surface and the maximum Nb particle size of this sample is 40 μm. The crystal grain size is the crystal grain size No. of the result of microscopic observation of the polished cross section of the heat-treated product, which was corroded with Gh liquid (picric acid + sodium dodecylbenzenesulfonate + water).

As is apparent from Table 3, the maximum grain size of the Nb compound increases as the Nb content increases, and the grain size becomes finer due to the Nb content, and the grain refining effect is 0.07. It is saturated when it exceeds%.

[0035]

[Table 3]

Next, a sintered body using the steel powder of the present invention No. 4 was electrolytically extracted at a voltage of 0 to 100 mV using a methanol solution of 1% acetylacetone and 1% tetramethylammonium chloride as an electrolytic solution. After that, the residue was separated with a 0.2 μm filter, and the residue was subjected to X-ray analysis. The results are shown in FIG. 5, the detected spectra are Nb and C, and it can be seen that Nb carbide is generated in this sintered body.

On the other hand, the steel powder of the present invention No. 4 was sieved with a sieve opening 1
The particle size was adjusted with 50 μm and 180 μm sieves, the powder that passed through the 180 μm sieve and became on the 150 μm sieve was collected, and the surface of this powder was analyzed by EPMA. The signal waveforms of Nb and oxygen were almost as shown in FIG. It is confirmed that the Nb oxide is generated on the surface of the steel powder.

[0038]

EFFECTS OF THE INVENTION The present invention is configured as described above, and by containing an appropriate amount of Nb in a low alloy steel base for powder metallurgy, it is possible to suppress dimensional change during sintering, Even if there is a variation in density within the molded body, it is possible to obtain a sintered component with high dimensional accuracy without causing distortion.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the Nb content and the influence coefficient α of dimensional change during sintering.

FIG. 2 is a graph showing the relationship between the Nb content and the tensile strength of heat-treated bodies.

FIG. 3 is a drawing-substitute micrograph showing a metal structure of a fracture surface of a heat-treated body obtained in an example.

FIG. 4 is a drawing-substituting photograph showing an EPMA surface analysis result of a fracture surface of a heat-treated body obtained in an example.

FIG. 5 is an X-ray diffraction chart of the residue obtained by electrolytically extracting the sintered body obtained in the example.

FIG. 6 is a photograph and an EPMA analysis chart showing an SEM image of the surface of the low alloy steel powder obtained in the example.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masaaki Sato Inventor Masaaki Sato 2-3-1, Niihama, Arai-cho, Takasago, Hyogo Pref., Takasago Works, Kobe Steel, Ltd. (72) Kei Ishii, 520 Minorita, Matsudo-shi, Chiba Hitachi Powder Inside the Metallurgical Co., Ltd. (72) Inventor, Yuiyuki Tsutsui, 520 Minorita, Matsudo City, Chiba Prefecture Inside Hitachi Powdered Metals Co., Ltd.

Claims (2)

[Claims] 1. The total composition is Nb: 0.02 by weight.
0.15% and Ni: 0.1-2.0
%, Cr: 0.4-2.0% and Mo: 0.1-3.
A low alloy for powder metallurgy, characterized in that at least the surface of a powder containing at least one element selected from the group consisting of 0% and the balance consisting of iron and unavoidable impurities has an Nb oxide of 40 μm or less. Steel powder. 2. A molded body obtained by mixing graphite with the low alloy steel powder for powder metallurgy according to claim 1, and pressing the mixture into a predetermined shape to obtain a carbon powder in a non-oxidizing atmosphere. A method for producing an iron-based sintered component having high dimensional accuracy, which comprises sintering at a temperature and for a time to form a solid solution in an iron matrix and form Nb carbide of 40 μm or less.