JPH07138602A - Low alloy steel powder for powder metallurgy - Google Patents

Abstract

PURPOSE:To produce a low alloy steel powder for powder metallurgy, capable of giving a sintered compact of high fatigue strength, by incorporating specific amounts of Ni, Cr, Mo, and Al into a base material of low alloy powder for powder metallurgy, providing a high level compressibility at the time of compacting, and minimizing the pores after sintering. CONSTITUTION:Ni is incorporated by 0.2-1%, by weight, into a base material of low alloy for powder metallurgy, and also Cr and Mo are incorporated so that respective inequalities of [Cr]+[Mo]<=3.1 and [Cr]+1.8[Mo]>=1.8 (where [Cr] and [Mo] represent the contents of Cr and Mo in the low alloy steel by weight percentage) are satisfied, and further, Al is controlled to <=0.01wt.%. By this method, the low alloy steel powder for powder metallugy, showing >=6.7g/cm<3> green compact density under 5t/cm<2> compacting pressure and, further, forming inclusions of a size of <=140mum by sintering and giving a sintered compact of a high fatigue 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 more particularly to powder metallurgy modified by heat treatment after sintering so as to obtain a product with excellent fatigue strength at the level of molten steel. It relates to low alloy steel powder.

[0002]

2. Description of the Related Art As powder metallurgy technology and sintering technology have advanced, the strength of sintered products made from low alloy steel powder has been remarkably increased. In particular, the tensile strength has the same level as that of ordinary ingot steel. It is becoming possible to achieve. However, the fatigue strength is still insufficient and the fatigue limit ratio is considerably smaller than that of the ingot steel. Therefore, the sintered steel material has been mainly used as a component material which is not so heavily loaded.

The main reason why the sintered body lacks fatigue strength is
It is believed that this is caused by the voids that inevitably remain in the sintered body, and as a means for compensating for this lack of fatigue strength, a method of increasing the density of the green compact before sintering, alloy element powder In addition, a method of strengthening by alloying,
Alternatively, a method of re-pressurizing and heating after sintering has been carried out, and in particular, an alloying method (diffusion and deposition method of alloy elements and use of prealloy type steel powder) has been widely put to practical use as an effective method. However, these attempt to increase the fatigue strength by modifying the alloy elements in order to improve the compressibility of the steel powder and the sintered metal structure and to improve the fatigue strength by satisfying the recent severe requirements for fatigue properties. Has not reached.

That is, a conventional sintered product using a low alloy steel powder for powder metallurgy can attain a level of 50 kgf / mm ² in rotating bending fatigue strength, and is not suitable as a mission component for automobiles or a gear material. Insufficient fatigue strength, especially 60 kgf / m at minimum rotational bending fatigue strength required for weight reduction of automobiles and fuel consumption and exhaust gas reduction in recent years.
m ² or more ”cannot be satisfied.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its purpose is to exhibit a high level of compressibility at the time of compacting and to obtain a material after sintering. Porosity can be reduced as much as possible, and it exhibits excellent hardenability and softening resistance during the sintering and subsequent heat treatment steps.
An object of the present invention is to provide a low alloy steel powder for powder metallurgy capable of producing a sintered body having a rotational bending fatigue strength of at least a level of 1 mm ² / mm ² .

[0006]

The constitution of the present invention which has been able to solve the above-mentioned problems is to provide a low alloy steel substrate for powder metallurgy,
Ni: 0.2 to 1% and Cr and Mo satisfying the requirements of the following formulas (1) and (2) are contained, and Al is suppressed to 0.01% or less. ] ≦ 3.1 (1) [Cr] +1.8 [Mo] ≧ 1.8 (2) (where [Cr] and [Mo] are the contents of Cr and Mo in the low alloy steel. Rate: represents weight%) 5 t / cm ²
Shows a green compact density of 6.7 g / cm ³ or more under the molding pressure of
Further, it has the gist of producing inclusions having a size of 140 μm or less by sintering. still,
As the low alloy steel substrate for powder metallurgy described above, C:
0.02% or less, Si: 0.10% or less and Mn:
Those containing 0.5% or less are used.

[0007]

As described above, according to the present invention, the low alloy steel base for powder metallurgy contains a proper amount of Ni, and in order to secure a high level of compressibility at the time of powder compacting and to increase the strength of the sintered body, Cr is added. Mo is contained in an appropriate mixing ratio, and the content of alloying elements is controlled so that the size of inclusions in the sintered body is 140 μm or less, thereby providing a sintered body with excellent fatigue strength. It succeeded in obtaining low alloy steel powder for powder metallurgy. Hereinafter, the reason why the type and content rate of the additive alloy element are defined in the present invention will be described in detail.

Ni: 0.2 to 1.0% When Ni is added to a low alloy steel substrate, it enhances the hardenability of the sintered product and contributes to the improvement of tensile strength, as well as toughness and fatigue strength. However, they are indispensable additional elements, and their effects are effectively exhibited by containing 0.2% or more. However, since Ni tends to harden the steel powder and deteriorate the compressibility at the time of powder compaction, the upper limit is 1.0% as an addition amount that does not substantially affect the compressibility. The more preferable content of Ni is in the range of 0.2 to 0.5%.

[Cr] + [Mo] ≦ 3.1 (1) [Cr] +1.8 [Mo] ≧ 1.8 (2)

When Cr and Mo are contained in the low alloy steel substrate in appropriate amounts, they have the effect of increasing the strength of the sintered body without adversely affecting the compressibility thereof and affecting the compressibility of the steel powder. The impacts are almost equivalent. On the other hand, regarding the effect of enhancing the strength of the sintered body by improving the hardenability, Cr is
1 has an influence of 1.8 for Mo. And, in order to effectively exert the strength improving effect of Cr and Mo, it is necessary to contain at least an amount satisfying the requirement of the above formula (2), but if it exceeds the upper limit defined by the formula (1), the compressibility becomes clear. This has an adverse effect and makes it difficult to sufficiently increase the density of the green compact, resulting in deterioration of fatigue properties.

Amounts of Ni and C that meet the above-mentioned requirements
Low alloy steel powder for powder metallurgy containing r and Mo has excellent compressibility, and even at a molding pressure of 5 t / cm ³ , it is 6.7 g / cm ^3.
It is possible to obtain the above green compact density, and by sintering it or by further heat treatment if necessary, it has excellent fatigue strength with few pore defects, and excellent tensile strength due to appropriate quench hardening. A strong sintered compact can be obtained.

By the way, when the present inventors investigated the fatigue properties of the low alloy steel powder to which the above alloying elements were added, the particles of non-metallic inclusions inevitably mixed in the sintered body were found. It was confirmed that the fatigue strength is significantly affected by the diameter as well, and that the inclusion size of 140 μm or less can stably exhibit excellent fatigue strength. The non-metallic inclusions are closely related to the content of Al mixed in the low alloy steel base in the melting stage, and the content of Al in the low alloy steel base is 0.01%. It was found that if the content is suppressed below, the fatigue strength of the finally obtained sintered body does not vary greatly, and a stable rotational bending fatigue strength of 60 kgf / mm ² level or more is guaranteed.

Incidentally, FIG. 1 shows the relationship between the Al content in the low alloy steel powder and the size of non-metallic inclusions in the sintered body, based on various experimental data including the examples, and FIG. It is a graph showing the relationship between the size of metal inclusions and the fatigue strength. When the Al content is suppressed to 0.01% or less, the size of non-metal inclusions in the sintered body can be suppressed to 140 μm or less. Further, it is understood that the fatigue strength of the sintered body having the inclusion size of 140 μm or less shows a high value of 60 kgf / mm ² level or more.

Therefore, as the low alloy steel base for powder metallurgy used in the present invention, it is necessary to use one having an Al content of 0.01% or less, and there are no particular restrictions on the other contained elements. Generally, C: 0.02% or less, S
i: 0.10% or less and Mn: 0.5% or less,
The balance is Fe and inevitable impurities.

Incidentally, C, together with N, is an interstitial element with respect to steel and has a function of hardening ferrite. When the steel powder is compression-molded, the softer the hardness of the ferrite matrix is, the higher the green compact density can be. Therefore, it is better to keep C low. In addition, increasing the green compact density improves the strength of the compact, which is also desirable in handling the compact. Therefore, C is preferably 0.02% or less.

Si has the effect of improving hardenability, but when atomizing molten steel, it forms an oxide on the surface of steel powder. Si has a strong bonding force with oxygen, and it is not only difficult to reduce this oxide in the reduction step, but also the effect of hardening ferrite is large and the compressibility of steel powder is impaired. Further, if excessively present in the molten steel before atomizing, it becomes a source of oxide-based inclusions, so the amount of Si should be kept low, preferably 0.01% or less.

Further, Mn is very effective in improving the hardenability and effectively acts in improving the heat treatment characteristics. However, it has a great effect of degrading the compressibility. Therefore, 0.5% or less is preferable as a range in which excellent mechanical properties can be secured without impairing the compressibility.

The unavoidable impurities include P, S, O and N.
However, since they all act as sources of non-metallic inclusions and adversely affect the fatigue strength, it is desirable to reduce them as much as possible.

The low alloy steel powder for powder metallurgy of the present invention is prepared by adding appropriate amounts of Ni, Cr, and Mo at the melting stage of the low alloy steel substrate having the above-described composition, and according to a conventional method, for example, a gas atomizing method, A method of obtaining finely powdered material that is completely alloyed by water atomization, or mixing additive elements with base steel powder as premix powder or alloy powder, and adhering to the surface to make a diffusion adhesion type low alloy steel powder You can also However, in order to obtain compressibility at the time of compacting and to obtain a uniform fatigue strength as a sintered body, it is preferable that the above-mentioned additional alloying elements are evenly distributed. From this point of view, a perfect alloy type powder is preferable. desirable. The particle size is not particularly limited, but those having an average particle size of 63 to 100 μm and a maximum particle size of 250 μm are preferable from the viewpoint of handleability and powder compacting property.

The production of a sintered compact using the above low alloy steel powder may be carried out by a conventional method. For example, the low alloy steel powder is mixed with carbon such as graphite powder, lubricant such as stearic acid, zinc and wax. After uniformly mixing, the powder is compacted into a predetermined product shape at about 5 to 9 t / cm ² and then heat-treated at about 1100 to 1250 ° C. in a non-oxidizing atmosphere such as ammonia decomposition gas to sinter. Just do it.

[0021]

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 having the chemical composition shown in Table 1 below (maximum particle size: 250 μm, average particle size: 63 to 100 μm)
Then, 0.6% of graphite powder was added and mixed uniformly, and a HIP treatment was performed to obtain a sintered body having a true density of 12.5 mm in diameter and 10 mm in height. The cross section of this sintered body was observed with an optical microscope to inspect inclusions. The inspection surface was a whole surface of 12.5 mm in diameter, and the inspection area was increased more than the JIS standard to improve the inspection accuracy and the maximum size of inclusions was measured. The results shown in Table 2 were obtained.

Further, 0.75% of zinc stearate was added to each of the low alloy steel powders and uniformly mixed, and the molding pressure was 5 t / cm ^3.
The density was measured for the powder compacted with, and the result is shown in Table 2.
Also described in.

[0024]

[Table 1]

[0025]

[Table 2]

As is clear from Tables 1 and 2, Nos. Which satisfy the requirements of the chemical composition defined in the present invention. Powder compacts using low alloy steel powders 1 to 5 are all 6.7 g / cm ³
It can be seen that the powder compact has the above density and is excellent in compressibility. On the other hand, in No. In the case of using the comparative steel powders of Nos. 7 and 11 to 13, the powder compact density of 6.7 g / cm ³ level was not obtained. Incidentally, No. For 6, 8, 10 and 14, (1)
Since the requirement of the formula is satisfied, the density of the green compact is 6.7 g /
Although cm ³ or more is obtained, the tensile strength and fatigue strength of the sintered body are insufficient as described later.

Next, the above No. No. 1 steel powder of the present invention and No. 1 steel powder of the present invention.
Graphite powders (0.6%) and zinc stearate (0.75%) were added to 14 comparative steel powders and uniformly mixed, and the mixture was compression-molded to have a density of 7.4 g / cm ³ , a size of 12.5 ×. 90 x 1
A 2.5 mm powder compact was produced. This density is
The 2 press 2 sinter method was adopted because it is difficult to obtain it by only one compression molding.

The green compact thus obtained was pre-sintered in an ammonia decomposition gas (75% H ₂ + 25% N ₂ ) for 30 minutes and then sintered in a nitrogen atmosphere at 1250 ° C. for 60 minutes. The sintered body was machined to obtain a tensile test piece (JIS No. 14: diameter 5 mm) and a fatigue test piece (JIS No. 1: diameter 3 m).
m) was produced, followed by carburizing treatment at 920 ° C. for 3 hours, oil quenching at 870 ° C. for 1 hour, and tempering treatment at 200 ° C., and subjected to a tensile test and a rotary bending fatigue test. The results are shown in Table 3.

[0029]

[Table 3]

As is clear from Table 3, No. 1 and No. No. 14 has almost the same chemical composition, but No. No. 1 has achieved 60 kgf / mm ² or more, whereas No. 1 has achieved. 14 did not achieve it. this is,
No. In the case of using the comparative steel powder of No. 14, inclusions of 140 μm or more are included, and it is considered that this adversely affected the fatigue strength.

Further, in the above No. Steel powders of the present invention 1 to 5 and No. Tensile tests and rotary bending fatigue tests were carried out in the same manner as above using the comparative steel powders 6 to 13, and the results shown in Table 4 were obtained.

[0032]

[Table 4]

As is clear from Table 4, No. 1 to
In the case of using the steel powder of the present invention of No. 5, both are 60 kgf / mm
Achieved a fatigue strength of ² or more. On the other hand, No.
No fatigue strength of 60 kgf / mm ² level was obtained with the comparative steel powders of 7, 11, 12, and 13. From these, it is understood that the object of the present invention cannot be achieved even if the content of Cr or Mo is excessively increased. Further, it can be confirmed that both of the comparative steel powders used have lower tensile strength and fatigue strength than the steel powders of the present invention.

[0034]

EFFECTS OF THE INVENTION The present invention is configured as described above and provides a sintered body which is excellent in compressibility at the time of compacting and has a small size of impure inclusions and is excellent in both tensile strength and fatigue strength. It is now possible to provide a low alloy steel powder for powder metallurgy to be given.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the Al content in low alloy steel powder and the size of inclusions in a sintered body.

FIG. 2 is a graph showing a relationship between inclusion size in a sintered body and rotary bending fatigue strength.

Claims (2)

[Claims] 1. A low alloy steel substrate for powder metallurgy is provided with Ni: 0.
2-1% (meaning% by weight, the same applies hereinafter) and the following (1),
Cr and Mo satisfying the requirements of the formula (2) are contained, and Al is suppressed to 0.01% or less, and [Cr] + [Mo] ≦ 3.1 (1) [Cr] +1. 8 [Mo] ≧ 1.8 (2) (However, [Cr] and [Mo] represent Cr and Mo content in the low alloy steel:% by weight) 5 t / cm ²
Shows a green compact density of 6.7 g / cm ³ or more under the molding pressure of
Further, a low alloy steel powder for powder metallurgy which gives a sintered body of high fatigue strength, which is characterized by producing inclusions having a size of 140 μm or less by sintering. 2. A low alloy steel substrate for powder metallurgy is C: 0.0
2% or less, Si: 0.10% or less and Mn: 0.5%
The low alloy steel powder for powder metallurgy according to claim 1, which contains: