JPS63203701A - Alloy steel powder for powder metallurgy and its production - Google Patents

Abstract

PURPOSE:To permit production of prealloyed Ni steel powder which has high compressibility and is suppressed in the content of N by subjecting water atom ized alloy steel powder contg. Ni at a specific ratio to primary annealing and disintegrating after dehydrating and drying, then subjecting the powder to secon dary annealing and disintegrating. CONSTITUTION:The water atomized steel powder contg. <=4wt.% Ni is dehydrat ed and dried and is then subjected to the primary annealing by heating to 1,000-1,300 deg.C; thereafter, the powder is subjected to the ordinary disintegrating. The powder is then subjected to the secondary annealing at 600-800 deg.C to relieve the disintegration stress of the primary annealing and to control the content of N to <=0.0015wt.%. The powder is thereafter subjected to the ordinary disinte grating. The alloy steel powder for powder metallurgy which has <=6,400pieces/ mm<2> crystal grains and has >=7.10g/cm<2> compacting density under 7t/cm<2> mold ing pressure when mixed with 1% solid lubricant.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an alloy steel powder for powder metallurgy used for manufacturing various sintered parts and a method for manufacturing the same.

(Conventional technology) Sintered parts using pure iron powder as the main raw material are well known.
Since this type of sintered part has a low strength level and its applications are limited, techniques have been developed to use alloyed copper powder instead of pure iron powder.

Among these, alloyed steel powder prealloyed with Ni has excellent mechanical properties because the sintered structure is made uniform by the prealloying in addition to the metallurgical action of Ni.

On the other hand, when applying the alloy steel powder to powder metallurgy, it is important to improve the compressibility of the alloy steel powder in order to improve the mechanical properties of sintered parts.

Regarding alloy steel powder, Japanese Patent Publication No. 45-9649 discloses a method of diffusing and adhering alloy components to the surface of iron powder. It is difficult to uniformly form a solid solution in the body, and the resulting sintered structure inevitably becomes non-uniform, resulting in insufficient mechanical properties of the sintered parts. Therefore, a method of pre-alloying all the alloy components into steel powder may be considered, but in principle, pre-alloying promotes solid solution hardening due to the alloy components as the amount of alloy components increases, and the compressibility of the copper powder decreases. It will drop.

Furthermore, when Ni is prealloyed, Ni makes the crystal grains of the copper powder finer, so there is a problem that the compressibility is further reduced.

On the other hand, Japanese Patent Publication No. 48-34509 discloses an iron powder whose powder characteristics are improved by a manufacturing method that separates the recrystallization process and the reduction process of copper powder. However, no consideration is given when processing Ni-containing copper powder or the compressibility of iron powder and alloy steel powder.

(Problems to be Solved by the Invention) A problem with alloy steel powder prealloyed with Ni is that its compressibility is low, and the factors that affect compressibility are also unknown.

An object of the present invention is to provide Ni prealloyed steel powder with high compressibility and a method for producing the same.

(Means for Solving the Problems) The inventors studied the compressibility of Nk-based prealloyed steel powder and found the following facts.

In other words, Ni has a large effect on refining the crystal grains of steel powder, and pure iron powder has a 950
In the temperature range above ℃, the amount of N in pure iron powder after primary annealing decreases as the temperature increases, whereas the amount of N in Ni prealloyed steel powder does not differ at any temperature and is higher than pure iron powder. factors that reduce the compressibility, and the fact that the crushed bundle of pure iron powder and Ni prealloyed steel powder that are primarily annealed in a 1200'C11 atmosphere is larger in the case of Ni prealloyed steel powder are factors that reduce compressibility. It is.

The inventors have discovered that eliminating the factors that reduce the compressibility described above is advantageous in increasing the compressibility of Ni prealloyed steel powder, and have completed the present invention.

That is, this invention is an alloy steel powder for powder metallurgy produced by a water atomization method, which has a composition containing 4 wt% or less of Ni and suppressed N to 0.0015 wt% or less, and has a crystal grain number of 6400. /m" or less, and when 1 wt.chi. of solid lubricant is mixed, a molding pressure of 1 t/cva" is 7. l.

After dehydrating and drying alloy steel powder for powder metallurgy characterized by having a green density of 1000 to 130 g/cm'' or more and water atomized alloy steel powder containing Ni: 4 ht% or less,
After heating to a temperature of 0"C and performing primary annealing, normal crushing is performed, then heating to a temperature of 600 to 800C and 2
This is a method for producing alloy steel powder for powder metallurgy, which is characterized by carrying out normal crushing after subsequent annealing.

(Function) In powder metallurgy, it is extremely important to improve the compressibility of copper powder in order to improve the mechanical properties of the sintered body and the productivity during production of the sintered body.

Among these, prealloyed steel powder containing Ni has excellent mechanical properties because the sintered steel structure becomes uniform due to the prealloying, in addition to the metallurgical action of Ni. Due to the grain refining effect, the compressibility of the steel powder is insufficient for use in powder metallurgy.

In order to improve the compressibility of this alloy steel powder, the following characteristics are particularly required.

That is, alloy steel powder produced by the water atomization method has very fine crystal grains because it is rapidly cooled with sprayed water during atomization, and coarsening of the crystal grains is required to improve compressibility.

As a result of examining the relationship between the compressibility of steel powder and the number of steel powder grains in the composition range of the present invention, it was found that when 1 wtx of solid lubricant was mixed and molded at a pressure of 7 t/cm'', high-strength sintering In order to obtain the green powder density of 7.10 g/cm3 or more required for the material, the number of steel powder crystal grains should be 6400 pieces/ffl11! or less, the content of Nil O,0015wtX or less, and the crushing after primary annealing. It became clear that it was necessary to remove the strain in the steel powder.

In order to reduce the number of copper powder crystal grains to 6,400 pieces/2 or less, the primary annealing temperature of the copper powder is important.If the temperature is lower than 1000℃, the coarsening of the crystal grains will be insufficient, so if the temperature is lower than 1000℃, the coarsening of the crystal grains will be insufficient. It is necessary to perform primary annealing. In addition, at temperatures higher than 1300°C, sintering of the copper powder progresses during annealing, and processing strain applied to the copper powder during crushing after the primary annealing increases, and the subsequent secondary annealing, especially the strain, cannot be removed sufficiently. This is not suitable because it reduces the compressibility of the steel powder. Therefore, the primary annealing must be carried out at a temperature of 1000 to 1300°C.

On the other hand, the removal of crushed bundles after primary annealing and the content of N were reduced to 0.0O.
The secondary annealing temperature is important in order to reduce the temperature to 15 s*t% or less, and a temperature lower than 600''C is not appropriate because the de-Ning of the steel powder and the above-mentioned crushing strain removal will be insufficient.

In addition, at temperatures higher than 800°C, de-N becomes difficult again.
This is not appropriate because the N content in the steel powder increases. Therefore, the secondary annealing temperature to remove the crushing strain after the primary annealing is 600 to 8
It is necessary to carry out the test at 00°C.

In addition, the Ni content in the alloyed steel powder is determined according to the required properties of the sintered body, but an increase in the amount of pre-alloyed Ni will lead to a decrease in the compressibility of the steel powder, especially if the content exceeds 4wtχ. The upper limit was set at 4 wt$ because the compressibility of the steel rapidly decreases. On the other hand, the lower limit of 0.1 wt% or less is not appropriate because the effect of improving the strength during heat treatment of the sintered body is lost.

Furthermore, by adding one or more alloy components selected from Mo, H, and Cu in the following composition range in addition to Ni, the properties of the sintered body can be improved without impairing the compressibility of the copper powder. It is possible to further increase this.

Mo: 0.1 to 1.05 wtχ Mo is effective in improving the hardenability of the sintered body, but excessive content is undesirable because it causes a decrease in the compressibility of the copper powder. That is, if Mo is less than 0.1 wtχ, it will not contribute to improving the hardness of the sintered body during heat treatment, and if it exceeds 1 wtχ, the compressibility of the copper powder will decrease, which is not appropriate.

W: 0.2-2. If Owtχ - is less than 0.2wtχ, it will not contribute to improving the hardness of the sintered body during heat treatment; Exceeding Owtχ is not preferable because the compressibility of the copper powder decreases, carbide formation is promoted during heat treatment of the sintered body, C in the matrix decreases, and hardness decreases.

Cu: 0.2 to 0.7 s*tχ If Cu is less than 0.2 wtχ, it does not contribute to improving the hardness of the sintered body during heat treatment, while if it exceeds 0.1 stχ, the compressibility of the steel powder is reduced.

The atmosphere during the primary annealing may be any atmosphere that does not hinder the coarsening of crystal grains, and may be a normal reducing, neutral, or inert atmosphere, or even a decarburizing atmosphere. In addition, the atmosphere during the secondary annealing may be any atmosphere that can eliminate the increase in the amount of N in the copper powder that may occur during the primary annealing and the processing strain due to crushing after the primary annealing.
Other atmospheres can also be used, provided they do not cause an increase in . On the other hand, reduction and decarburization of the alloy steel powder may be carried out by either primary annealing or secondary annealing, which may be appropriately selected depending on the process used.

(Example) 2 storm spot water atomized Ni prealloy steel (2-tχNi-0,
Atomized raw powder of 5wtχL0.5wtχMo) was heated to 950℃ in an ammonia decomposition gas atmosphere with a dew point of 25℃.
After primary annealing at 1,200°C for 25 minutes, the particles are sieved to a particle size of 60 mesh or less through normal crushing, and further dew point-
2 at 650℃ in an ammonia decomposition gas atmosphere at 50℃
After secondary annealing for 5 minutes, the same treatment as crushing and sieving after primary annealing was performed to obtain alloyed copper powder having the characteristics shown in Table 1. Note that the N content is 0.0011 to 0.0013.
wtX 2 suppression.

From the same table, Comparative Example 1 has low compressibility due to insufficient grain coarsening during primary annealing, while Example A has low compressibility.

Molding pressure 7 for B and C 7.10 at t/am snow
It can be seen that a high green density of g/cm3 or more was obtained.

In addition, in Comparative Example 2, the primary annealing temperature exceeds 1300°C, so the removal of crushing strain by secondary annealing is insufficient.
Compressibility is reduced again.

*Molding pressure of 1% zinc stearate 7t/cmz” actual water atomized Ni prealloy steel (,1wtχNi-0
．． After primary annealing, crushing, and sieving at a primary annealing temperature of 1100°C and the same conditions as in Example 1, the atomized raw powder of 3wtχCu-0.2sstχMo) was heated to 500°C.
Secondary annealing, crushing, and sieving were performed in the temperature range of 900° C. under the same conditions as in Example 1, and alloyed copper powder having the characteristics shown in Table 2 was obtained. Note that the number of crystal grains is 770/wm''.

From the same table, it can be seen that high compressibility could not be obtained in Comparative Example 3 due to insufficient removal of crushing strain after primary annealing, and in Comparative Example 4 due to poor denitrification during secondary annealing. It can be seen that in D-F, strain removal and denitrification were sufficiently performed and high compressibility was obtained.

The molding pressure of this 1% zinc stearate mixture is 7 t/c. This makes it possible to provide Ni pre-alloyed steel powder for powder metallurgy with properties.

Claims (1)

[Claims] 1. An alloy steel powder for powder metallurgy produced by a water atomization method, which contains Ni: 4 wt% or less and N: 0.001
The composition is suppressed to 5wt% or less, and the number of crystal grains is 640.
An alloy steel for powder metallurgy, characterized in that it has a powder density of 0 pieces/mm^2 or less and a green density of 7.10 g/cm^3 or more at a compacting pressure of 7 t/cm^2 when mixed with 1 wt% of solid lubricant. powder. 2. After dehydrating and drying the water atomized alloy steel powder containing 4 wt% or less of Ni, the powder is heated to a temperature of 1000 to 1300°C to undergo primary annealing, followed by normal crushing.
A method for producing alloy steel powder for powder metallurgy, which comprises heating the steel powder to 00 to 800°C, performing secondary annealing, and then performing normal crushing.