WO2017056509A1

WO2017056509A1 - Production method for alloy steel powder for powder metallurgy

Info

Publication number: WO2017056509A1
Application number: PCT/JP2016/004438
Authority: WO
Inventors: 小林　聡雄; 中村　尚道; 伊都也佐藤
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2015-09-30
Filing date: 2016-09-30
Publication date: 2017-04-06
Also published as: JP6112280B1; KR102026767B1; JPWO2017056509A1; KR20180033567A

Abstract

Provided is a method for producing an alloy steel powder for powder metallurgy. The method uses a moving bed furnace and, without the need for gas analysis, which makes cumbersome maintenance necessary, makes it possible to heat treat an iron-based powder that contains Cr and Mn and stably reduce C content and O content. A production method for an alloy steel powder for powder metallurgy wherein an atomized iron-based powder that has a specific component composition is prepared, wherein the atomized iron-based powder is supplied to the inside of a moving bed furnace such that a filling layer that has a thickness of d (mm) is formed, wherein a hydrogen-containing gas is supplied to the inside of the moving bed furnace such that the hydrogen-containing gas has an average gas flow velocity of v (mm/s), and wherein the atomized iron-based powder is reduced by being heat treated inside the moving bed furnace and an alloy steel powder for powder metallurgy is produced, wherein d and v satisfy the expression d/√v ≤ 2.9 (mm^1/2∙s^1/2).

Description

Method for producing alloy steel powder for powder metallurgy

The present invention relates to a method for producing an alloy steel powder for powder metallurgy by reducing atomized iron base powder to produce an alloy steel powder for powder metallurgy, and in particular, the atomized iron base powder is an easily oxidizable element such as Cr and Mn The present invention relates to a method for producing alloy steel powder for powder metallurgy that can effectively reduce the C (carbon) content and O (oxygen) content in the alloy steel powder.

Powder metallurgy technology allows parts with complex shapes to be manufactured in a shape very close to the product shape (so-called near net shape) and with high dimensional accuracy. Therefore, if a part is produced using powder metallurgy technology, the cutting cost can be greatly reduced. For this reason, powder metallurgy products to which powder metallurgy technology is applied are used in various fields as various machine parts. Recently, there has been a strong demand for improving the strength of powder metallurgy products in order to reduce the size and weight of parts. In particular, there is a demand for higher strength in iron-based powder metallurgy products (iron-based sintered bodies). Is strong.

In order to meet this demand for higher strength, an alloy element is added to the iron-based powder used in powder metallurgy. As the alloy element, for example, Cr and Mn are used because they have a high effect of improving hardenability and are relatively inexpensive.

Examples of the alloy powder for powder metallurgy containing the above alloy elements include Cr—Mo alloy steel powder (Patent Document 1) and Cr—Mn—Mo alloy steel powder (Patent Document 2, Patent Document 3). Are known.

Also, in the production of iron-based powder for powder metallurgy, heat treatment is performed to reduce the C content and O content in the iron-based powder as a raw material. The heat treatment is generally carried out continuously using a moving bed furnace (moving bed furnace). Examples of the iron-based powder include a crude iron-based powder that has been atomized and a roughly reduced iron obtained by roughly reducing a mill scale. A crude iron-based powder such as a base powder is used. In the heat treatment, at least one treatment of decarburization, deoxidation, and denitrification is performed according to the use of the powder.

For example, an apparatus described in Patent Document 4 is known as an apparatus for performing the heat treatment. In the apparatus described in Patent Document 4, the space in the moving bed furnace is divided into a plurality of partitions by a partition wall provided so as to be perpendicular to the traveling direction of the raw material powder. And the flow path for flowing atmospheric gas is provided in the upper part of each divided | segmented space. The heat treatment is continuously performed while flowing an atmospheric gas through the channel in a direction opposite to the moving direction of the raw material powder.

Japanese Patent No. 3224417 Japanese Patent No. 5125158 Japanese Patent No. 5,389,577 Japanese Patent Publication No. 01-40881 JP-T-2002-501123 Japanese Patent No. 4225574

However, in the production of alloy powders for powder metallurgy containing alloy elements as described in Patent Documents 1 to 3, a heat treatment as described in Patent Document 4 is performed to reduce the C content and O content. When the method was used, there were the following problems. That is, it contains elements such as Cr and Mn (hereinafter referred to as “easily oxidizable elements”) that are more easily oxidized than Fe. Therefore, when an iron-based powder containing Cr or Mn is produced by an atomizing method (particularly, a water atomizing method), the obtained iron-based powder contains an oxide formed by oxidation of Cr or Mn during atomization. It will be. The oxide remains without being sufficiently reduced even in the heat treatment. In some cases, Cr and Mn are further oxidized during the heat treatment, and the amount of oxide is increased. In general, when the C content and the O content are large, the compressibility of the alloy steel powder at the time of pressure forming is lowered, so that a large amount of oxide remains is a problem.

Therefore, Patent Document 5 and Patent Document 6 propose a method that enables decarburization and deoxidation in the production of alloy steel powder containing easily oxidizable elements such as Cr and Mn.

However, in the treatment method proposed in Patent Document 5, heat treatment is performed in an inert gas atmosphere using an airtight batch furnace. Since a batch furnace is used in the method, the productivity is low as compared with the case where continuous heat treatment is performed using a moving bed furnace including a belt furnace, and thus is not suitable for mass production.

On the other hand, the method proposed in Patent Document 6 is a method of performing heat treatment continuously using a belt furnace and is therefore suitable for mass production. However, in the above method, it is essential to continuously measure the CO or CO ₂ concentration or the oxygen potential (O ₂ concentration or H ₂ / H ₂ O concentration ratio) in the atmospheric gas during the heat treatment, Furthermore, it is necessary to adjust the amount of water vapor injected into the furnace so that these measured values become target values. When such a device for gas analysis is actually used continuously in a factory that manufactures iron powder, etc., the sensor part is soiled and the gas intake port is clogged, making it impossible to perform measurement normally. There's a problem. Therefore, when the method of Patent Document 6 is continuously performed, maintenance of the analyzer is a heavy burden.

The present invention has been made in view of the above circumstances, and is a method for producing alloy steel powder for powder metallurgy using a moving bed furnace, without requiring gas analysis that requires complicated maintenance management, It aims at providing the manufacturing method of the alloy steel powder for powder metallurgy which can heat-process the iron-base powder containing Cr and Mn, and can reduce C content and O content stably.

The gist configuration of the present invention is as follows.

1. % By mass
C: 0.8% or less,
O: 1.0% or less,
Mn: more than 0.08% and 0.3% or less,
Cr: 0.3 to 3.5%,
Mo: 0.1-2%
S: 0.01% or less, and P: 0.01% or less,
Prepare the remaining Fe and atomized iron-based powder which is an inevitable impurity,
Supplying the atomized iron-based powder into a moving bed furnace so as to form a packed bed of thickness d (mm);
A hydrogen-containing gas is supplied into the moving bed furnace so as to have an average gas flow velocity v (mm / s),
The atomized iron-based powder is reduced by heat treatment in the moving bed furnace to obtain an alloy steel powder for powder metallurgy, a method for producing an alloy steel powder for powder metallurgy,
The manufacturing method of the alloy steel powder for powder metallurgy in which said d and v satisfy | fill following (1) Formula.
D / √v ≦ 2.9 (mm ^1/2 · s ^1/2 ) (1)

2. 2. The method for producing alloy steel powder for powder metallurgy according to 1 above, wherein a dew point of the hydrogen-containing gas is 0 ° C. or lower.

3. ^3. Production of alloy steel powder for powder metallurgy according to 1 or 2 above, wherein deoxidation is performed in the heat treatment under conditions of an atmospheric temperature T: 1050 ° C. or higher and a holding time t: ^{104−0.0037 · T} hours or longer. Method.

According to the present invention, even alloy steel powder containing Cr and Mn, which are easily oxidizable elements, is heat-treated using a moving bed furnace without performing gas analysis that requires complicated maintenance and management. The content and the O content can be stably reduced. As a result, it is possible to produce alloy steel powder that is low in cost and excellent in compressibility during pressure forming. In addition, sintered parts produced using the alloy steel powder for powder metallurgy obtained by the production method of the present invention have excellent mechanical properties such as strength, toughness and fatigue properties. Applications of powders and sintered bodies can be expanded.

It is side sectional drawing which shows the example of the heat processing apparatus which can be used in one Embodiment of this invention. It is a figure which shows the example of the temperature pattern in the heat processing apparatus described in patent document 4. FIG.

Hereinafter, the present invention will be specifically described. In the present invention, alloy steel powder for powder metallurgy (hereinafter sometimes simply referred to as “alloy steel powder”) is produced by heat-treating atomized iron-based powder as a raw material using a moving bed furnace. Specifically, the production method of the present invention includes the following treatments;
(1) Prepare atomized iron-based powder.
(2) supplying the atomized iron-based powder into a moving bed furnace so as to form a packed bed having a thickness of d (mm);
(3) By supplying hydrogen-containing gas into the moving bed furnace so as to have an average gas flow velocity v (mm / s), and (4) by heat-treating the atomized iron-based powder in the moving bed furnace. Reduce to alloy steel powder for powder metallurgy.

Each of the above processes can be performed independently at an arbitrary timing, and a plurality of processes can be performed simultaneously.

Furthermore, in the present invention, it is important that the packed bed thickness d and the average gas flow velocity v satisfy the following expression (1) when performing the above treatment.
d / √v ≦ 2.9 (mm ^1/2 · s ^1/2 ) (1)

The details of each process and the reasons for limiting the above conditions are described below.

[Atomized iron-based powder]
In the present invention, atomized iron-based powder is used as a raw material. The manufacturing method of the atomized iron-based powder is not particularly limited, and can be manufactured according to a conventional method. “Atomized iron-based powder” means an iron-based powder produced by the atomizing method. The “iron-based powder” means a powder containing 50% by mass or more of Fe.

As the atomized iron-based powder, either a gas atomized iron-based powder obtained by a gas atomizing method or a water atomized iron-based powder obtained by a water atomizing method can be used. In the gas atomization method, it is preferable to use an inert gas such as nitrogen or argon. However, since gas is inferior in cooling capacity compared to water, it is necessary to use a large amount of gas when producing iron-based powder by the gas atomization method. Therefore, it is preferable to use the water atomization method from the viewpoint of mass productivity and manufacturing cost. In addition, since the water atomization method is normally performed in an atmosphere in which air is mixed, the iron-based powder is more easily oxidized in the production process than the gas atomization method. Therefore, the method of the present invention is particularly effective when a water atomized iron-based powder is used.

(Component composition)
Next, the reason for limiting the component composition of the atomized iron-based powder in the present invention as described above will be described. Unless otherwise specified, “%” in the following description means “mass%”.

In the present invention, C and O are elements to be reduced by a heat treatment described later. And from the viewpoint of improving the compressibility of the finally obtained alloy steel powder for powder metallurgy, it is desirable to reduce the C content and O content of the alloy steel powder as much as possible, specifically, C: 0.1% or less, O: 0.28% or less are preferable. In order to achieve these appropriate amounts of C and O, the amount that can be reduced by the heat treatment according to the present invention is anticipated, and the appropriate ranges of the C content and O content of the atomized iron-based powder are determined as follows.

C: 0.8% or less C is present in the atomized iron-based powder mainly as a precipitate such as cementite or in a solid solution state. When the C content in the atomized iron-based powder exceeds 0.8%, it becomes difficult to lower the C content to 0.1% or less in the heat treatment of the present invention, and an alloy powder having excellent compressibility is obtained. I can't. Therefore, the C content of the atomized iron-based powder is set to 0.8% or less. On the other hand, the lower the C content, the easier the reduction (decarburization) of the C content during heat treatment. Therefore, the lower limit of the C content is not particularly limited, and may be 0% or industrially greater than 0%.

O: 1.0% or less O is present on the surface of the iron-based powder mainly as Cr oxide or Fe oxide. If the O content in the atomized iron-based powder exceeds 1.0%, it becomes difficult to reduce the O content to 0.28% or less during heat treatment, and an alloy powder having excellent compressibility cannot be obtained. Therefore, the O content of the atomized iron-based powder is set to 1.0% or less. The O content is preferably 0.9% or less. On the other hand, the lower the O content, the easier the reduction (deoxidation) of the O content during heat treatment. Therefore, the lower limit of the O content is not particularly limited, but excessive reduction leads to an increase in manufacturing cost, so the O content is preferably 0.4% or more.

Also, the contents of Mn, Cr, Mo, S, and P are not changed by the heat treatment of the present invention. Therefore, these elements contained in the atomized iron-based powder remain as they are in the alloy steel powder for powder metallurgy after the heat treatment. Based on this, the contents of these elements in the atomized iron-based powder are respectively defined as follows.

Mn: more than 0.08% and not more than 0.3% Mn is an element having an action of improving the strength of the sintered body by improving hardenability and strengthening solid solution. In order to obtain the effect, the Mn content is more than 0.08%. The Mn content is preferably 0.10% or more. On the other hand, if the Mn content is higher than 0.3%, the amount of Mn oxide generated increases, and the compressibility of the alloy steel powder decreases. Further, the Mn oxide serves as a starting point for destruction inside the sintered body, and reduces fatigue strength and toughness. Therefore, the Mn content is 0.3% or less. The Mn content is preferably 0.28% or less, and more preferably 0.25% or less.

Cr: 0.3-3.5%
Cr is an element that has the effect of improving hardenability and improving the tensile strength and fatigue strength of the sintered body. Further, Cr has the effect of increasing the hardness after heat treatment such as quenching and tempering of the sintered body and improving the wear resistance. In order to obtain these effects, the Cr content is set to 0.3% or more. On the other hand, when the Cr content exceeds 3.5%, the amount of Cr oxide generated increases. Since the Cr oxide serves as a starting point for fatigue failure inside the sintered body, the fatigue strength of the sintered body is reduced. Therefore, the Cr content is 3.5% or less.

Mo: 0.1-2%
Mo is an element having an action of improving the strength of the sintered body by improving hardenability, solid solution strengthening, precipitation strengthening, and the like. In order to acquire the said effect, Mo content shall be 0.1% or more. On the other hand, when the content of Mo exceeds 2%, the toughness of the sintered body decreases. Therefore, the Mo content is 2% or less.

S: 0.01% or less In the present invention, the Mn content in the atomized iron-based powder is set to 0.3% or less. Therefore, among S contained in the atomized iron-based powder, the amount present as MnS decreases, and the amount present as solute S increases. If the S content of the finally obtained alloy steel powder exceeds 0.01%, the solid solution S increases and the grain boundary strength decreases. Therefore, the S content at the stage of atomized iron-based powder is set to 0.01% or less. On the other hand, the lower the S content, the more the solute S decreases, which is preferable. Therefore, the lower limit of the S content is not particularly limited, and may be 0%, but industrially it may be more than 0%. However, since excessive reduction leads to an increase in manufacturing cost, the S content is preferably 0.0005% or more.

P: 0.01% or less When the contents of Mn and S are large, the content of P does not affect the toughness, but the Mn content of the alloy steel powder is 0.3% or less and the S content is 0.00. When the content is 01% or less, the grain boundary strength is increased and the toughness is improved by setting the P content to 0.01% or less. Therefore, the P content at the stage of atomized iron-based powder is set to 0.01% or less. On the other hand, the lower the P content, the greater the grain boundary strength and the better the toughness. Therefore, the lower limit of the P content is not particularly limited and may be 0%, but industrially it may be more than 0%. However, excessive reduction leads to an increase in manufacturing cost, so the P content is preferably 0.0005% or more.

The component composition of the atomized iron-based powder in the present invention is composed of the above elements, the remainder Fe and inevitable impurities.

(Average particle size)
The average particle size of the atomized iron-based powder is not particularly limited, and any particle size can be used as long as it is an iron-based powder obtained by the atomization method. However, when the average particle size of the atomized iron-based powder is less than 30 μm, the fluidity of the atomized iron-based powder is lowered, and it may be difficult to supply to the moving bed furnace using a hopper or the like. In addition, if the average particle size of the atomized iron-based powder is less than 30 μm, the fluidity of the alloy steel powder after heat treatment also decreases, so the work efficiency of filling the mold when the alloy steel powder is press-formed decreases. There is a case. For this reason, the average particle size of the atomized iron-based powder is preferably 30 μm or more, more preferably 40 μm or more, and even more preferably 50 μm or more.

On the other hand, if the average particle size of the atomized iron-based powder is larger than 120 μm, coarse pores are generated in the sintered body obtained using the obtained alloy powder, the density of the sintered body is lowered, and the strength and toughness are reduced. There may be a shortage. Therefore, the average particle size of the atomized iron-based powder is preferably 120 μm or less, more preferably 100 μm or less, and even more preferably 90 μm or less. Here, the average particle diameter means a median diameter (so-called d50, volume basis).

(Apparent density)
The apparent density of the atomized iron-based powder is not particularly limited, but is preferably 2.0 to 3.5 Mg / m ³ , more preferably 2.4 to 3.2 Mg / m ³ .

[Moving floor furnace]
Atomized iron-based powder having the above component composition is supplied to a moving bed furnace, and a packed bed having a thickness d (mm) is formed on the moving bed of the moving bed furnace. As the moving bed furnace, any one can be used as long as it can heat treat the atomized iron-based powder, but a moving bed furnace (hereinafter referred to as a “belt type moving bed furnace” or “ It is preferable to use a belt furnace). When heat treatment is performed using a belt furnace, an atomized iron-based powder can be supplied onto the belt to form a packed bed. The atomized iron-based powder can be supplied by any method, but it is preferable to use a hopper. Moreover, the conveyance direction of the atomized iron-based powder in the moving bed furnace is not particularly limited, but it is generally conveyed linearly from the inlet side to the outlet side of the moving bed furnace. The thickness of the filling layer will be described later.

The heating system of the moving bed furnace is not particularly limited, and any system can be used as long as it can heat the atomized iron-based powder. However, from the viewpoint of atmosphere control, the indirect heating system is used. It is preferable to use heating using a radiant tube. A muffle furnace can also be suitably used as an indirect heating furnace.

[Hydrogen-containing gas]
The moving bed furnace is supplied with a hydrogen-containing gas. As the hydrogen-containing gas, any gas can be used as long as it contains hydrogen. Examples of the hydrogen-containing gas include pure H ₂ gas and a mixed gas of H ₂ gas and inert gas. As the mixed gas, a mixed gas of H ₂ gas and N ₂ gas is preferably used. A mixed gas (so-called AX gas) of H ₂ gas and N ₂ gas obtained by decomposing ammonia can also be used. From the viewpoint of efficiently promoting reduction in heat treatment, that is, removal of oxygen from the atomized iron-based powder, the H ₂ content of the hydrogen-containing gas is preferably 75 vol% or more, and 90 vol% or more. it is more preferable, and even more preferably to a 100 vol% _{(H 2} gas).

The hydrogen-containing gas is supplied into the moving bed furnace so as to have an average gas flow velocity v (mm / s) during the heat treatment of the atomized iron-based powder in the moving bed furnace. The hydrogen-containing gas is preferably flowed in the moving bed furnace in a direction opposite to the moving direction of the raw material powder. For example, when supplying atomized iron-based powder from one end (upstream side) of the moving bed furnace, and transporting the atomized iron-based powder to the other end (downstream side) of the moving bed furnace by a conveying means such as a belt, It is preferable that the hydrogen-containing gas is introduced from the other end (downstream side) and exhausted from the one end (upstream side). Therefore, it is preferable that the moving bed furnace is provided with an atomized iron-based powder supply port and an atmospheric gas discharge port at one end, and a discharge port for treated powder (alloy steel powder) and a hydrogen-containing gas supply port at the other end.

[Heat treatment]
With the hydrogen-containing gas supplied as described above, the atomized iron-based powder is heat-treated in the moving bed furnace, whereby an alloy steel powder for powder metallurgy can be obtained. By the heat treatment, C and O contained in the atomized iron-based powder are removed by a decarburization and deoxidation (reduction) reaction described later.

・ D / √v ≦ 2.9
In the present invention, during the heat treatment, both the thickness d (mm) of the packed bed and the average gas flow velocity v (mm / s) are controlled so as to satisfy the following expression (1).
d / √v ≦ 2.9 (mm ^1/2 · s ^1/2 ) (1)

By performing the heat treatment under the above-mentioned conditions, it is possible to stably reduce C and O contained in the atomized iron-based powder even though the atomized iron-based powder contains Cr and Mn which are easily oxidizable elements. . As a result, the C content and the O content in the alloy steel powder after the heat treatment can be set to extremely low values such as C ≦ 0.1% and O ≦ 0.28%. The reason will be described below.

The reaction (deoxidation reaction) of the oxides of Fe, Cr, and Mn contained in the atomized iron-based powder with hydrogen in the atmosphere is expressed by the following equations (2) to (4).
FeO (s) + H ₂ (g) = Fe (s) + H ₂ O (g) (2)
Cr ₂ O ₃ (s) + 3H ₂ (g) = 2Cr (in Fe) + 3H ₂ O (g) (3)
MnO (s) + H ₂ (g) = Mn (in Fe) + H ₂ O (g) (4)

Since H ₂ O gas is generated in the above reaction, in order to advance the deoxidation reaction efficiently, the dew point of the atmosphere gas in the furnace is always set higher than the equilibrium dew point determined by the equilibrium reaction of the above equations (2) to (4). Need to keep low. Therefore, it is necessary to reduce the amount of generated H ₂ O gas so that the dew point of the atmospheric gas is not increased too much by the H ₂ O gas generated by the reaction.

For this purpose, it is conceivable to suppress the amount of iron-based powder charged into the moving bed furnace, that is, the packed bed thickness. It is also conceivable to reduce the H ₂ O gas concentration by removing the H ₂ O gas generated by the above reaction or diluting with a hydrogen-containing gas introduced into the moving bed furnace. Therefore, in the present invention, the packed bed thickness d and the average gas flow velocity v in the furnace when the hydrogen-containing gas is introduced into the furnace are controlled so as to satisfy the above equation (1).

The reason why the deoxidation proceeds efficiently by controlling the packed bed thickness d and the average gas flow velocity v so as to satisfy the above expression (1) is not necessarily clear, but is estimated as follows.

That is, during the heat treatment in the moving bed furnace, a velocity boundary layer of flowing hydrogen-containing gas is formed in the space above the surface of the packed bed. It is derived from the theory regarding the boundary layer that the thickness of the velocity boundary layer is inversely proportional to √v. In addition, since the diffusion rate of hydrogen before the reduction reaction and water vapor generated by the reduction reaction is considered to be constant regardless of the thickness of the velocity boundary layer, the diffusion time is proportional to the thickness of the velocity boundary layer. Therefore, if the velocity boundary layer thickness is halved and the same diffusion time is given, the hydrogen concentration at the packed bed surface will be doubled and the water vapor concentration at the packed bed surface will be halved, It is estimated that even if the thickness of the packed bed is doubled, the concentration of hydrogen and water vapor in the lowermost layer of the packed bed can be made the same. Therefore, assuming that the concentration is constant, the packed layer thickness and the velocity boundary layer thickness are inversely proportional, that is, it is estimated that the packed layer thickness and √v are in a proportional relationship.

As a result of the examination based on the above knowledge, if the packed layer thickness and the gas flow rate are adjusted so that the condition of d / √v ≦ 2.9 is satisfied in the heat treatment, a gas that requires complicated maintenance management is required. It has been found that the dew point of the atmospheric gas in the furnace is maintained lower than the equilibrium dew point for reducing Cr ₂ O ₃ or MnO without using an analyzer.

From the viewpoint of further reducing the O content in the powder metallurgical alloy steel powder, it is more preferable that the ^{d / √v ≦ 2.5 (mm 1/2} · s 1/2). On the other hand, the lower limit of d / √v is not particularly limited. The lower the better, the lower the better. However, if d is excessively decreased, the production efficiency decreases, and if v is excessively increased, the cost increases. It is preferable to set it as the above, and it is more preferable to set it as 0.3 or more.

(Average gas flow velocity v)
In the present invention, the average gas flow velocity v (mm / s) is obtained by changing the volume flow rate f of hydrogen-containing gas supplied to the moving bed furnace (volume of hydrogen-containing gas supplied per second). It is defined as dividing by the cross-sectional area S of the floor furnace. Here, the cross-sectional area refers to the area of the space inside the annealing furnace that is perpendicular to the conveying direction of the atomized iron-based powder (in the belt furnace, the belt traveling method). However, when the cross-sectional area of the annealing furnace differs depending on the position in the conveying direction, the cross-sectional area S is the cross-sectional area at the highest temperature in the annealing furnace. As will be described later, when a decarburization zone, a deoxidation zone, and a denitrification zone are provided in a moving bed furnace, the deoxidation zone is usually at the highest temperature. May be used. Further, the volume flow rate f is a volume flow rate at the measurement position of the cross-sectional area S. That is, considering the volume expansion of gas at a high temperature, the above flow rate is multiplied by the volume expansion coefficient obtained from the temperature at the position.

Here, the definition of the cross-sectional area S will be further described. In the calculation of the cross-sectional area S, it is not necessary to consider the area occupied by the structure such as the conveying means and the heating means existing in the furnace and the iron-based powder as the object to be processed. That is, the cross-sectional area of the internal space of the moving bed furnace is used as it is as the cross-sectional area S without subtracting the area of objects existing in the furnaces.

For example, in the case of a radiant tube type heat treatment furnace as shown in FIG. 1, a radiant tube, a belt, a roll (not shown) for feeding the belt, and iron-based powder laminated on the belt are contained in the furnace. Exists. However, it is not particularly necessary to subtract these cross-sectional areas from the cross-sectional area of the space portion in the furnace. In the cross section of the space part in the furnace, the gas flow rate is slower in the part where there is no radiant tube or roll, but it is particularly important to control the flow rate in this slow part. It was because it was found from. In addition, the cross-sectional area of the belt or iron-base powder packed layer thickness portion is negligible with respect to the entire cross-sectional area of the furnace, and thus need not be considered.

In the case of a muffle-type heat treatment furnace, there is no radiant tube or roll installed in the furnace, so there is no need to consider these cross-sectional areas, and the cross-sectional area of the belt or iron-base powder packed layer thickness is The fact that the entire cross-sectional area of the furnace is negligible and need not be considered is the same as in the case of the radiant tube type heat treatment furnace.

And from the test results of the inventors, if the average gas flow rate of the gas introduced into the moving bed furnace is controlled based on the above definition, the C content and O content of the alloy steel powder after the heat treatment are sufficiently It has been found that it can be stably reduced.

(Dew point)
-Dew point of hydrogen-containing gas: 0 ° C or less The dew point of the hydrogen-containing gas introduced into the furnace is preferably 0 ° C or less. As described above, in order to efficiently advance the reduction reaction in the heat treatment, it is necessary to keep the dew point of the atmospheric gas lower than the equilibrium dew point determined from the equilibrium reaction represented by the above equations (2) to (4). There is. Therefore, it is preferable to lower the dew point of the introduced hydrogen-containing gas. Specifically, the temperature is preferably 0 ° C. or lower. Furthermore, the dew point is more preferably −10 ° C. or lower.

For example, when flowing a hydrogen-containing gas in the direction opposite to the iron-based powder conveyance direction, the hydrogen-containing gas flowing upstream in the iron-based powder conveyance direction contains water vapor generated by the reaction. The dew point is higher than the hydrogen-containing gas at the time of supply. Considering this, the dew point of the introduced hydrogen-containing gas is kept low at 0 ° C. or less. Thereby, even if a dew point rises with progress of reaction, deoxidation reaction can fully be advanced.

Here, if the atmospheric temperature (the temperature at which the deoxidation reaction is carried out) is increased, the equilibrium dew point increases, so it seems that the dew point of the hydrogen-containing gas may be increased at first glance. However, when the furnace temperature rises, the reaction rate of the deoxidation reaction (reduction reaction) increases accordingly, and the generation rate of H ₂ O also increases. Then, the dew point of the in-furnace gas also tends to increase. Therefore, it is preferable to control the dew point of the hydrogen-containing gas introduced into the moving bed furnace as described above.

Note that, as in the prior art, there is no problem if the dew point is set to 40 ° C. or less in the iron-based powder that does not contain an easily oxidizable element such as Cr and Mn as in the prior art. However, for iron-based powders containing a predetermined amount of Cr or Mn, it is desirable to further lower the dew point in order to proceed with the deoxidation reaction represented by the above formulas (3) and (4). Is preferably 0 ° C. or lower.

On the other hand, the lower the dew point of the hydrogen-containing gas, the better the deoxidation reaction proceeds. However, a gas with a low dew point is expensive, and the use of a gas with an excessively low dew point causes an increase in production cost. Therefore, it is usually preferable to set the dew point to −40 ° C. or higher.

In order to achieve the low dew point as described above, it is preferable to block intrusion of out-of-furnace gas into the furnace and leakage of out-of-furnace gas to the outside of the furnace. Therefore, it is preferable that the moving bed furnace includes a sealing unit for preventing gas leakage and intrusion. As the sealing means, for example, a water-sealed tank (15 in FIG. 1) as described in Patent Document 4 can be used, but it is more preferable to use a system that does not use water such as a seal roll. . The sealing means is preferably provided at both ends on the upstream side and the downstream side in the transport direction.

(Atmosphere temperature, holding time)
Furthermore, in the above heat treatment, it is preferable to perform deoxidation under conditions of an atmospheric temperature T: 1050 ° C. or higher and a holding time t: ^{104−0.0037 · T} hours or longer. In other words, in the heat treatment, it is preferable to provide a time for holding at an atmospheric temperature T: 1050 ° C. or higher and a holding time t: ^{104−0.0037 · T} hours or more. The reason will be described below.

-Atmosphere temperature T: 1050 degreeC or more As usual, when reducing the iron-based powder which does not contain easily oxidizable elements, such as Cr and Mn, the oxide which should be reduced is only FeO. Therefore, as described in Patent Document 4, when the atmospheric temperature in the deoxidation zone is set to 700 ° C. or higher, the equilibrium dew point determined from the equilibrium reaction of the above formula (2) is as high as 70 ° C. or higher. At this time, if the dew point of H _{2 to be} introduced was set to 40 ° C. or less as described in Patent Document 4, no problem occurred because the deoxidation reaction (reduction reaction) proceeded at a sufficient rate.

On the other hand, when reducing iron-based powder containing Cr and Mn, the atmospheric temperature is set to 1050 ° C. or higher in order to make the equilibrium dew point higher than the dew point raised by H ₂ O generated by the deoxidation reaction. It is preferable. On the other hand, the upper limit of the ambient temperature is not particularly limited, but is preferably about 1200 ° C. in consideration of the heat resistance performance of the apparatus, the manufacturing cost, and the like. Here, the “atmosphere temperature” is a temperature measured by a thermocouple at a position 20 mm immediately above the surface of the iron-based powder (packed bed) in the moving bed furnace.

^-Holding time t: 10 4 ^{-0.0037 · T} hours or more If the holding time t is 10 4 ^{-0.0037 · T} hours or more according to the ambient temperature T (° C), O can be further reduced. Is preferable. In addition, the relationship between the said t and T was determined from the result of having conducted the experiment which manufactures alloy steel powder with various T and t. Specifically, the O content of the obtained alloy steel powder was plotted on a Tt diagram, and a curve (contour line) connecting the same oxygen content was determined as an approximate expression. On the other hand, the upper limit of the retention time is not particularly limited, but the retention time is preferably 4 hours or less because the production cost only increases even if the retention time is longer than the time required for completion of the deoxidation reaction.

Next, the case where this invention is implemented using the moving bed furnace described in Patent Document 4 will be described in more detail. In the description of Patent Document 4, it is supposed that one or more kinds of processes of decarburization, deoxidation, or denitrification are continuously performed by using a continuous moving bed furnace to heat-treat the iron-based powder. Further, in the description of Patent Document 4, each of the decarburization, deoxidation, and denitrification treatment steps is made independent using the divided space of the moving bed furnace, and the decarburization step is performed at 600 to 1100 ° C. In the denitrification process, the iron-base powder is heat-treated by controlling the temperature independently at 450 to 750 ° C. Further, Patent Document 4, as the atmosphere gas, a reducing gas such as H ₂ or AX gas at decarburization zone or an inert gas such as N ₂ or Ar, reduction such as H ₂ or AX gas in deoxidation zone It is said that gas mainly composed of H ₂ is used in the denitrification zone.

Here, a heat treatment apparatus of the same type as the continuous moving bed furnace described in Patent Document 4 is shown in FIG. A heat treatment apparatus 100 shown in FIG. 1 is provided on a furnace body 30 divided into a plurality of zones by a partition wall 1, that is, a decarburization zone 2, a deoxidation zone 3, and a denitrification zone 4, and an entrance side of the furnace body 30. The hopper 8 is provided, a wheel 10 provided on the entrance / exit side of the furnace body 30, a belt 9 that continuously rotates by the wheel 10 and circulates in each zone in the furnace body 30, and a radiant tube 11. The crude iron-based powder 7 supplied from the hopper 8 on the belt 9 continuously moved by the continuous rotation of the wheel 10 with a predetermined packed bed thickness (thickness of the crude iron-based powder loaded on the belt), It is heat-treated while moving through the

zones

2, 3 and 4 heated to an appropriate temperature by the radiant tube 11, and decarburized, deoxidized and denitrified to obtain the product powder 13. The product powder 13 is stored in the product tank 14.

And in the technique described in Patent Document 4, the reaction in each zone is considered as follows. In the decarburization zone 2, the ambient temperature is controlled to 600 to 1100 ° C. by the radiant tube 11, and water vapor (H ₂ O gas) introduced from the water vapor inlet 12 provided on the downstream side of the decarburization zone 2 It is supposed that decarburization is performed from the crude iron-based powder while adjusting the atmospheric gas in the deoxidation zone 3 as the next zone to a dew point of 30 to 60 ° C.
At the upstream side of the decarburization zone 2, an atmospheric gas discharge port 6 is provided to discharge the atmospheric gas to the outside of the apparatus. The decarburization reaction formula is represented by the following formula (I).
C (in Fe) + H ₂ O (g) = CO (g) + H ₂ (g) (I)

In the deoxidation zone 3, the ambient temperature is controlled to 700 to 1100 ° C. by the radiant tube 11, and deoxidation is performed from the crude iron-based powder using the atmospheric gas from the denitrification zone 4 (dew point: hydrogen gas of 40 ° C. or less). Is going to do. The reaction formula for deoxidation is represented by the following formula (II).
FeO (s) + H ₂ (g) = Fe (s) + H ₂ O (g) (II)

In the denitrification zone 4, the ambient temperature is controlled to 450 to 750 ° C. by the radiant tube 11, and hydrogen gas (dew point: 40 ° C.) as a reaction gas is supplied from the atmosphere gas inlet 5 provided on the downstream side of the denitrification zone 4. The following is introduced to denitrify the crude iron-based powder. The denitrification reaction formula is represented by the following formula (III).
N (in Fe) + 3 / 2H ₂ (g) = NH ₃ (g) (III)
The water sealing tank 15 functions to block the mixing of the outside gas into the furnace gas and the leakage of the inside gas to the outside of the furnace.

Moreover, a typical example of a heat treatment temperature pattern by a belt furnace type heat treatment apparatus described in Patent Document 4 is shown in FIG. As shown in (a) or (b), the iron-based powder to be treated is first heated in the decarburization zone, then soaked in the deoxidation zone, and finally in the denitrification zone. To be cooled. The hydrogen gas introduced in the direction opposite to the flow of the iron-based powder first enters the denitrification zone, denitrifies the iron-based powder while being heated, and then enters the deoxidation zone and is kept at a constant temperature. The iron-base powder is deoxidized while finally entering the decarburization zone together with a predetermined amount of water vapor, and the iron-base powder is decarburized while being cooled.

When carrying out the present invention using the heat treatment apparatus as described above, that is, when processing an iron-based powder containing oxidizable elements such as Cr and Mn, the iron-based powder not containing these elements is processed. Unlike the case, the following points should be noted. That is, normally, when hydrogen gas enters the final decarburization zone, a predetermined amount of water vapor is additionally introduced as described above. However, when processing iron-based powder containing easily oxidizable elements, water vapor is used. Do not introduce additional. This is because when water vapor is additionally introduced, the easily oxidizable element is further oxidized and deoxidation (reduction) may not be completed. Further, in the method of the present invention, as described above, the C content in the atomized iron-based powder is set to 0.8% or less so that the decarburization can be completed only with water vapor generated by the deoxidation reaction. Therefore, decarburization can be completed without additionally introducing water vapor.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples.

An atomized iron-based powder having the component composition shown in Table 1 was produced by the water atomization method. These atomized iron-based powders were heat-treated using a moving bed furnace and crushed to obtain alloy steel powder for powder metallurgy. Table 2 shows the atomized iron-based powder used and the heat treatment conditions. In the heat treatment, the atomized iron-based powder is supplied into the moving bed furnace so as to have a packed bed thickness d shown in Table 2, and contains hydrogen so that the average gas flow velocity v shown in Table 2 is obtained. The heat treatment was continuously performed while supplying the gas. The contents of C and O in the obtained alloy steel powder for powder metallurgy were as shown in Table 2. In addition,% display in the composition of the hydrogen-containing gas shown in Table 2 means vol%.

As can be seen from Table 2, in the examples in which the packed bed thickness d and the average gas flow velocity v satisfy the conditions of the present invention, the obtained alloy steel powder has a C content of 0.1% or less and an O content of 0. .28% or less. On the other hand, in comparative examples (A6, B1) in which d and v do not satisfy the conditions of the present invention, the O content exceeded 0.28%.

Also, in the example in which 100% H ₂ (pure hydrogen gas) is used as the hydrogen-containing gas and d / √v ≦ 2.5 (mm ^1/2 · s ^1/2 ), the obtained alloy steel The C content in the powder was 0.1% or less, the O content was 0.23% or less, and the O content was further reduced.

In addition, in powder symbols: A7 to A9, an O content of 0.28% or less was obtained, but a lower O content was obtained as the H ₂ concentration of the hydrogen-containing gas used was increased. Looking at the powder symbols: C1 to C4, in C2 to C4 where the dew point is 0 ° C. or less, the O content is 0.20% or less, and the lower the dew point, the better the results. I understand. Further, among the powder symbols: D1 to D4 and E1 to E3, those having an atmosphere temperature of 1050 ° C. or higher and satisfying the condition of t ≧ 10 4 ^{-0.0037 · T} (powder symbols: D3, D4, E3) An even better result was obtained with an O content of 0.20% or less.

On the other hand, as for I1 and J1, since the C content or O content of the atomized iron-based powder was too high, the C content or O content could not be reduced to the prescribed amount even by heat treatment.

1 Partition Wall 2 Decarburization Zone 3 Deoxidation Zone 4 Denitrification Zone 5 Atmosphere Gas Supply Port (Supply Atmosphere Gas)
6 Atmosphere gas outlet (exhaust gas)
7 Crude iron-based powder 8 Hopper 9 Belt 10 Wheel 11 Radiant tube 12 Steam blowing tube 13 Product powder 14 Product tank 15 Water seal tank 20 Product powder grinding device 21 Cooler 22 Circulating fan 30 Furnace (heating furnace)
100 Heat treatment equipment

Claims

% By mass
C: 0.8% or less,
O: 1.0% or less,
Mn: more than 0.08% and 0.3% or less,
Cr: 0.3 to 3.5%,
Mo: 0.1-2%
S: 0.01% or less, and P: 0.01% or less,
Prepare the remaining Fe and atomized iron-based powder which is an inevitable impurity,
Supplying the atomized iron-based powder into a moving bed furnace so as to form a packed bed of thickness d (mm);
A hydrogen-containing gas is supplied into the moving bed furnace so as to have an average gas flow velocity v (mm / s),
The atomized iron-based powder is reduced by heat treatment in the moving bed furnace to obtain an alloy steel powder for powder metallurgy, a method for producing an alloy steel powder for powder metallurgy,
The manufacturing method of the alloy steel powder for powder metallurgy in which said d and v satisfy | fill following (1) Formula.
D / √v ≦ 2.9 (mm 1/2 · s 1/2 ) (1)
The method for producing alloy steel powder for powder metallurgy according to claim 1, wherein the dew point of the hydrogen-containing gas is 0 ° C or lower.
3. The alloy steel powder for powder metallurgy according to claim 1, wherein deoxidation is performed in the heat treatment under conditions of an atmospheric temperature T: 1050 ° C. or more and a holding time t: 10 4−0.0037 · T hours or more. Production method.