JP6112278B1 - Method for producing alloy steel powder for powder metallurgy - Google Patents

Abstract

This is a method for producing alloy steel powder for powder metallurgy using a moving bed furnace, and heat treatment is performed on iron-based powder containing Cr and Mn without requiring gas analysis that requires complicated maintenance and management. Provided is a method for producing alloy steel powder for powder metallurgy that can stably reduce the amount and O content. An atomized iron-based powder having a specific component composition is prepared, and the atomized iron-based powder is mixed with a carbon component as necessary, and then in a moving bed furnace so as to form a packed bed having a thickness d (mm). The hydrogen-containing gas is supplied into the moving bed furnace so as to have an average gas flow velocity v (mm / s), and the atomized iron-based powder is reduced by heat treatment in the moving bed furnace, A method for producing an alloy steel powder for powder metallurgy, wherein the d and v satisfy the following formula: d / √v ≦ 3.7 (mm1 / 2 · s1 / 2)

Description

  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.

  The powder metallurgy technique can manufacture a component having a complicated shape in a shape very close to a 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.

  Moreover, in manufacture of the iron-base powder for powder metallurgy, heat processing is performed in order to reduce C content and O content in the iron-base powder as a raw material. The heat treatment is generally performed continuously using a moving bed furnace, and the iron-based powder may be a crude iron-based powder that has been atomized, or a coarsely 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.

  As an apparatus for performing the heat treatment, for example, an apparatus described in Patent Document 4 is known. 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 the atmospheric gas countercurrently, that is, in a direction opposite to the traveling 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 Special Table 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, heat treatment as described in Patent Document 4 in order to reduce the C content and O content. When the method was used, there were the following problems. That is, Cr and Mn are elements that are more easily oxidized than Fe (hereinafter referred to as “easily oxidizable elements”). 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 when manufacturing 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 of the present invention is as follows.
1. % By mass
C: 0.8% or less,
O: 1.0% or less,
Mn: 0.08% or less,
Cr: 0.3 to 3.5%,
Mo: 0.1 to 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);
In the moving bed furnace, an inert gas is supplied 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,
Said d and v satisfy | fill following (1) Formula, C content [C] (mass%) of the said atomized iron base powder, and O content [O] (mass%) of the said atomized iron base powder, The following equation (2) is satisfied,
In the supply of the atomized iron-based powder to the moving bed furnace, when the C content [C] and the O content [O] of the atomized iron-based powder satisfy the following formula (3), the atomized iron-based powder Is supplied to the moving bed furnace as it is, and if the following formula (3) is not satisfied, the atomized iron-based powder is further mixed with a carbon component so as to satisfy the following formula (4), and then the moving bed furnace A method for producing alloy steel powder for powder metallurgy, which is supplied to the company.
D / √v ≦ 3.7 (mm ^1/2 · s ^1/2 ) (1)
[O] ≧ 4/3 [C] −2/15 (2)
4/3 [C] + 0.28 ≧ [O] (3)
4/3 ([C] + [MXC]) + 0.28 ≧ [O] (4)
(Here, [MXC] is (mass of carbon component mixed with the atomized powder / mass of the atomized iron-based powder) × 100 (mass%))

2. 2. The method for producing alloy steel powder for powder metallurgy according to 1 above, wherein the reduced atomized iron-based powder is cooled using hydrogen gas or a hydrogen-containing gas.

3. 3. The method for producing alloy steel powder for powder metallurgy according to 1 or 2 above, wherein a dew point of the inert gas is 5 ° C. or less.

4). The alloy steel for powder metallurgy according to any one of the above 1 to 3, wherein deoxidation is performed in the heat treatment under conditions of an atmospheric temperature T: 1000 ° C. or higher and a holding time t: 10 ^{4−0.0037 · T} hours or longer. Powder manufacturing 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 an inert gas in 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, the thickness d of the packed bed and the average gas flow velocity v must satisfy the above-described conditions. In addition, when supplying the atomized iron-based powder into the moving bed furnace, the atomized iron-based powder is further mixed with a carbon component according to the C content and O content of the atomized iron-based powder, and then the moving bed Supply to the furnace.

  The details of each process and the reasons for limiting the above conditions will be 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.

  Further, 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: 0.08% or less Mn is an element having an action of improving the strength of the sintered body by improving hardenability, solid solution strengthening, and the like. However, if the Mn content is higher than 0.08%, the amount of Mn oxide produced 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 set to 0.08% or less. The Mn content is preferably 0.07% or less. On the other hand, the lower limit of the Mn content is not particularly limited and may be 0%, but from the viewpoint of improving the strength of the sintered body, it is preferably 0.01% or more, 0.05% More preferably.

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 to 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 0.08% 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 better because the solid solution S decreases. 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 0.08% 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 remaining Fe, and inevitable impurities.

Furthermore, in the present invention, the C content [C] (mass%) of the atomized iron base powder and the O content [O] (mass%) of the atomized iron base powder satisfy the following formula (2). There is a need.
[O] ≧ 4/3 [C] −2/15 (2)

 C and O contained in the atomized iron-based powder are removed from the iron-based powder as carbon monoxide gas mainly by the reaction of C + O = CO. At that time, if the C content [C] (mass%) of the atomized iron-based powder and the O content [O] (mass%) of the atomized iron-based powder satisfy the following formula (2): By the heat treatment, the amount of C in the powder can be reduced to a sufficiently low amount, for example, 0.1% by mass or less.

  Furthermore, in the present invention, as described below, a carbon component is further mixed with the atomized iron-based powder as necessary.

When the formula (3) is satisfied When the C content [C] and the O content [O] of the atomized iron base powder satisfy the following formula (3), the atomized iron base powder is used as it is in the moving bed furnace. To supply. “Supply as it is” means that only the atomized iron-based powder is supplied to the moving bed furnace without being mixed with other components such as a carbon component.
4/3 [C] + 0.28 ≧ [O] (3)

  When [C] and [O] of the atomized iron-based powder satisfy the condition of the formula (3), a sufficient amount of carbon is present with respect to oxygen, so the amount of O in the powder by heat treatment Can be reduced to a sufficiently low amount, for example, 0.28 mass% or less.

-When the formula (3) is not satisfied On the other hand, when [C] and [O] of the atomized iron-based powder do not satisfy the condition of the formula (3), the amount of O in the powder is sufficiently reduced by heat treatment. I can't. Therefore, the atomized iron-based powder is further mixed with a carbon component so as to satisfy the following expression (4), and then supplied to the moving bed furnace.
4/3 ([C] + [MXC]) + 0.28 ≧ [O] (4)
(Here, [MXC] is (mass of carbon component mixed with the atomized powder / mass of the atomized iron-based powder) × 100 (mass%))

  By mixing the carbon components in this way, the shortage of carbon contained in the atomized iron-based powder can be compensated. As a result, the amount of O in the powder can be reduced to a sufficiently low amount such as 0.28% by mass or less by heat treatment.

In addition, when adding a carbon component, it is more preferable that the following formula (5) is further satisfied.
[O] ≧ 4/3 ([C] + [MXC]) − 2/15 (5)

  In addition, the method of adjusting C content and O content of atomized iron-based powder is not specifically limited, Arbitrary methods can be used. For example, what is necessary is just to adjust the component composition of the molten steel used for manufacture of an atomized iron base powder so that C content and O content of atomized iron base powder may satisfy the said conditions. Adjustment of the composition of the molten steel can be performed by a steel refining and steelmaking technique using a general converter.

  In the present invention, if the atomized iron-based powder is as-atomized and satisfies the conditions of the above formulas (2) and (3), the atomized iron-based powder can be directly subjected to heat treatment. This is preferable because it is possible. However, if the C content in the atomized iron-based powder becomes excessive and the condition of the formula (2) is not satisfied, the adjustment before the heat treatment cannot be performed to satisfy the formula (2). Therefore, when producing atomized iron-based powder, it is necessary to satisfy the formula (2), and oxygen is allowed to be excessive for that purpose. This is because even if oxygen is excessive and the formula (3) is not satisfied, the amount of C and O in the final alloy steel powder can be reduced by adding the carbon component as described above.

[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.

  As the moving bed furnace, a moving bed furnace as described in Patent Document 4 can be used. Therefore, for reference, the moving bed furnace in Patent Document 4 will be described below. However, the atomized iron-based powder and the atmospheric gas used in the present invention are different from the processing in Patent Document 4 described below. The atomized iron-based powder and atmospheric gas used in the present invention will be described later.

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. Moreover, in description of patent document 4, each process process of decarburization, deoxidation, and denitrification is made independent using the divided | segmented space of a moving bed furnace, 600-1100 degreeC in a decarburization process, deoxidation process In the denitrification step, 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, FIG. 1 shows a heat treatment apparatus of the same type as the continuous moving bed furnace described in Patent Document 4. 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 the steam (H ₂ O gas) introduced from the steam 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 atmosphere 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 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, the typical example of the heat processing temperature pattern by the belt furnace type heat processing 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.

[Inert gas]
In the present invention, the inert gas is not particularly limited, and any inert gas can be used. Examples of the inert gas that can be suitably used include argon (Ar) gas, nitrogen (N ₂ ) gas, and a mixed gas thereof.

  The inert 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 inert 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 inert 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 atmosphere gas discharge port at one end, and a discharge port and an inert gas supply port for treated powder (alloy steel powder) at the other end.

[Heat treatment]
The alloy steel powder for powder metallurgy can be obtained by heat-treating the atomized iron-based powder in the moving bed furnace with the inert gas supplied as described above. 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 ≦ 3.7
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 ≦ 3.7 (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 Fe, Cr, and Mn oxides and carbon contained in the atomized iron-based powder is represented by the following formulas (a) to (c).
FeO (s) + C = Fe (s) + CO (g) (a)
Cr ₂ O ₃ (s) + 3C = 2Cr (in Fe) + 3CO (g) (b)
MnO (s) + C = Mn (in Fe) + CO (g) (c)

  Since CO gas is generated in the above reaction, in order to advance the deoxidation reaction efficiently, it is necessary to keep the CO partial pressure in the furnace atmosphere low.

  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 CO partial pressure by removing the CO gas generated by the reaction or diluting with an inert gas introduced into the moving bed furnace. Therefore, in the present invention, the thickness d of the packed bed and the average gas flow velocity v in the furnace when the inert gas is introduced into the furnace are controlled so as to satisfy the above formula (1).

  By controlling the packed bed thickness d and the average gas flow velocity v so as to satisfy the above formula (1), the reason why deoxidation proceeds efficiently 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 the inert gas flowing 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. Further, since the diffusion rate of CO 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, it is considered that the concentration of CO on the surface of the packed layer is halved. Then, even if the thickness of the packed layer is doubled, It is estimated that the CO concentration in the lowermost layer 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. Therefore, as in the present invention, in the heat treatment, if the packed layer thickness and the gas flow rate are adjusted so that the condition of d / √v ≦ 3.7 is satisfied, a gas analyzer that requires complicated maintenance management is required. Even if it is not used, a state in which the CO partial pressure in the furnace atmosphere is lower than the equilibrium CO partial pressure determined by the reactions of the above formulas (a) to (c) is maintained.

From the viewpoint of further reducing the O content in the alloy steel powder for powder metallurgy, d / √v ≦ 3.2 (mm ^1/2 · s ^1/2 ) is more preferable. 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 above, and it is more preferable to set it as 0.3 or more.

[cooling]
In one embodiment of the present invention, it is preferable that the atomized iron-based powder reduced by the heat treatment is further cooled using hydrogen gas or a hydrogen-containing gas. The reason is as follows.

The reduced atomized iron-based powder may contain N as an impurity. In particular, when the above reduction is performed in an N ₂ -containing atmosphere, the N content of the finally obtained alloy steel powder for powder metallurgy may become a high concentration such that it exceeds 0.005% by mass. . When the alloy steel powder contains N, the compressibility is lowered. Therefore, it is preferable to reduce the N content of the alloy steel powder as much as possible. Therefore, if the reduced atomized iron-based powder is cooled using hydrogen gas or a hydrogen-containing gas, N contained in the powder can be removed by the reaction of the following formula (d). In addition, it is preferable that N content of the alloy steel powder for powder metallurgy finally obtained shall be 0.005 mass% or less.
2N (in Fe) + 3H ₂ (g) = 2NH ₃ (g) (4)

  From the viewpoint of effectively advancing the reaction of the formula (d), it is more preferable to perform the cooling using hydrogen gas.

  The gas used for the cooling may be supplied by any method. For example, the inert gas in the moving bed furnace is exhausted at the position where the reduction of the iron-based powder in the moving bed furnace is completed, or at the downstream side in the transport direction from the position, and the cooling gas is transferred to the moving bed furnace. It can be introduced into the furnace. The timing (exhaust and introduction position in the moving bed furnace) does not have to be strictly the position where cooling starts, and there is no problem as long as it is after the position where the reduction of iron powder is completed even during soaking. . On the other hand, after the cooling (powder temperature decrease) starts, the atmosphere gas can be replaced. However, from the viewpoint of reaction efficiency, since it is desirable to proceed the reaction of the above formula (d) in a temperature range of 450 to 750 ° C., before the temperature of the powder becomes less than 450 ° C., in other words, the temperature of the powder When the temperature is 450 ° C. or higher, it is preferable to exhaust the inert gas and introduce a cooling gas.

(Dew point)
Inert gas dew point: 5 ° C. or less It is preferable that the dew point of the inert gas introduced into the furnace is 5 ° C. or less. If the dew point of the inert gas is too high, the deoxidation reaction (reduction reaction) is difficult to proceed. Originally, in the reaction of the above formulas (b) and (c), C that should be used for reduction of Cr ₂ O ₃ and MnO reacts with water vapor in the atmosphere and is consumed, or once reduced Cr and Mn are in the atmosphere This is because it is reoxidized by water vapor. In order to efficiently advance the reduction reaction in the heat treatment, it is necessary to suppress such wasteful consumption of C and reoxidation of Cr and Mn. From the above viewpoint, as a result of investigations by the present inventors, when the packed layer thickness d and the average gas flow velocity v of the inert gas satisfy the above conditions, the dew point of the inert gas is set to 5 ° C. or less. Thus, it has been found that the reduction reaction can proceed efficiently.

  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 Patent Document 4. However, since an iron-based powder containing Cr or Mn is used in the present invention, it is preferable that the dew point of the inert gas is 5 ° C. or less as described above.

  The lower the dew point of the inert gas, the better the deoxidation reaction proceeds. However, a gas having a low dew point is expensive, and the use of a gas having 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 the penetration of the gas outside the furnace into the furnace and the leakage of the gas inside the furnace outside 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)
Further, in the heat treatment, it is preferable to perform deoxidation under conditions of an atmospheric temperature T: 1000 ° C. or higher and a holding time t: 10 ^{4−0.0037 · 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: 1000 ° C. or more and a holding time t: 10 ^{4−0.0037 · T} hours or more. The reason will be described below.

-Atmospheric temperature T: 1000 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 the inert gas to be introduced was set to 40 ° C. or lower 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 the iron-based powder containing Cr and Mn, it is preferable that the atmospheric temperature is 1000 ° C. or higher. 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 It is preferable that the holding time t is 10 4 ^{-0.0037 · T} hours or more according to the ambient temperature T (° C.) because O can be further reduced. . 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.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to the following examples at all.

Example 1
An atomized iron-based powder having the component composition shown in Table 1 was produced by the water atomization method. Atomized iron-based powder symbols: A to H and K and M satisfy [O] ≧ 4/3 [C] −2/15, but 4/3 [C] + 0.28 ≧ [O] In order to adjust the C amount and O amount after the heat treatment to an appropriate range, the carbon component (ie, carbon powder (graphite powder, graphite)) must be mixed before the heat treatment. is necessary. For atomized iron-based powder symbol I, both [O] ≧ 4/3 [C] −2/15 and 4/3 [C] + 0.28 ≧ [O] are satisfied, and carbon powder before heat treatment No mixing is necessary. For atomized iron-based powder symbol J, [O] ≧ 4/3 [C] −2/15 is not satisfied and C is excessive. Atomized iron-based powder symbol: For L and M, the amount of C or O is outside the appropriate range for the atomized iron-based powder.

  These atomized iron-based powders were heat-treated using a moving bed furnace and crushed to obtain alloy steel powder for powder metallurgy. Tables 2 and 4 show the atomized iron-based powders used and the heat treatment conditions. In some examples, graphite powder as a carbon component was mixed with the atomized iron-based powder and then supplied to the moving bed furnace. The amount of the carbon component mixed is shown in Tables 2 and 4.

  In the heat treatment, the atomized iron-based powder is supplied into the moving bed furnace so as to have the packed bed thickness d shown in Tables 2 and 4, and the average gas flow velocity v shown in Tables 2 and 4 is obtained. Thus, the heat treatment was continuously performed while supplying the inert gas. The component composition of the obtained alloy steel powder for powder metallurgy was as shown in Tables 3 and 5. In addition,% display in the composition of the inert gas in each table | surface and the following description means vol%.

When Ar-containing gas is used Tables 2 and 3 are examples when Ar-containing gas is used as the inert gas. For powder symbols: A and K that are not mixed with carbon powder (powder symbols: A10, K10), the atomized iron-based powder has an excess of O. The amount of O after heat-treating is out of the proper value. Therefore, for the atomized iron-based powder symbols: A and J, an appropriate amount of carbon powder that satisfies 4/3 ([C] + [MXC]) + 0.28 ≧ [O] is required. The same applies to the atomized iron-based powder symbols: B to H because the atomized iron-based powder is O-excessive. About these, the example which mixed the graphite powder as shown in Table 2 so that 4/3 ([C] + [MXC]) + 0.28> = [O] may be performed is represented by powder symbols: A11 to A16, B11 to B13, C11 to C15, D11 to D14, E11 to E13, F11 to F13, G11, H11, K11, and M11.

  As is clear from the table, the relationship between the packed bed thickness d (mm) of the iron powder and the flow velocity v (mm / s) of the inert gas satisfies d / √v ≦ 3.7 (powder symbol: A11 -A15, B11-B13, C12-C15, D11-D14, E11-E13, F11-F13, G11, H11, I11, K11), the amount of O is 0.28% by mass or less, and conversely d For those not satisfying /√v≦3.7 (powder symbol: A16, C11), the O content exceeds 0.28 mass%. Further, the inert gas is 100% Ar and d / √v ≦ 3.2 (powder symbols: A11 to A14, B11, C13 to C15, D11 to D14, E11 to E13, F11 to F13, G11, H11, With respect to I11, K11), the C content is 0.1% by mass or less, the O content is 0.2% by mass or less, and the O content is further reduced, so d / √v ≦ 3.2. It is preferable.

In addition, regarding the powder symbols B11 to B13, the introduction gas was changed from 100% Ar to 50% Ar-50% N ₂ , and in each case, the amount of O was 0.28% by mass or less.

Furthermore, regarding the powder symbols: D11 to D14, the dew point is 5 ° C. or less (powder symbols: D12 to D14), and the amount of O is 0.15% by mass or less, which is a much better value. For powder symbols E11 to E13 and F11 to F13, soaking temperature is 1000 ° C. or higher and t ≧ 10 ^{4−0.0037 · T} is satisfied (powder symbol: E13, F13) and O content is 0.15% by mass or less A much better value is obtained.

  Furthermore, for the powder symbol: I11, the amounts of C and O in the atomized iron-based powder satisfy [O] ≧ 4/3 [C] −2/15 and 4/3 [C] + 0.28 ≧ [O]. Since both are satisfied, even if carbon powder is not mixed before heat treatment, it is shown that appropriate values can be obtained for the C and O amounts after heat treatment by treating in an inert gas atmosphere using appropriate heat treatment conditions. ing.

  On the other hand, for the powder symbol: J11, the amount of C and O in the atomized iron-based powder does not satisfy [O] ≧ 4/3 [C] −2/15, and is C-excessive. This shows that the amount of C after the heat treatment is out of the proper range even if the treatment is performed under the conditions.

  In addition, regarding the powder symbols: L11 and M11, the C amount or O amount of the atomized iron-based powder was too high, so that the C amount or O amount could not be reduced to the specified amount even by heat treatment.

When N ₂ -containing gas is used Tables 4 and 5 are examples when heat treatment is performed in an N ₂ -containing atmosphere. Atomized iron-based powder symbol: A to H and K are O-excessive, so an appropriate amount of carbon powder that satisfies 4/3 ([C] + [MXC]) + 0.28 ≧ [O] is mixed. is necessary. Then, the example which mixed graphite powder as shown in Table 4, and was processed on various heat processing conditions is powder symbol: A21-A26, B21-B23, C21-C25, D21-D24, E21-E23, F21. ~ F23, G21, H21, K21.

As is apparent from the table, the relationship between the packed bed thickness d (mm) of the iron powder and the flow velocity v (mm / s) of the inert gas satisfies d / √v ≦ 3.7 (powder symbol: A21 -A25, B21-B23, C22-C25, D21-D24, E21-E23, F21-F23, G21, H21, I21, K21), the amount of O is 0.28% by mass or less, and conversely d For those not satisfying /√v≦3.7 (powder symbol: A26, C21), the O amount exceeds 0.28 mass%. Further, the introduced gas is 100% N ₂ and d / √v ≦ 3.2 (powder symbols: A21 to A24, B21, C23 to C25, D21 to D24, E21 to E23, F21 to F23, G21, H21, For I21, K21), the C content is 0.1% by mass or less and the O content is 0.2% by mass or less. Since the O content is further reduced, the filling layer thickness d (mm) is inactive. It can be seen that the relationship with the gas flow velocity v (mm / s) is preferably d / √v ≦ 3.2.

In addition, regarding the powder symbols: B21 to B23, the introduction gas was changed from 100% N ₂ to 90% N ₂ -10% He, and in each case, the amount of O was 0.28% by mass or less. .

Furthermore, regarding the powder symbols: D21 to D24, the dew point is 5 ° C. or less (powder symbols: D22 to D24), and the O amount is 0.15% by mass or less, which is a much better value. Powder symbols: E21 to E23 and F21 to F23 satisfying t ≧ 10 ^{4−0.0037 · T} when the soaking temperature is 1000 ° C. or more (powder symbols: E23, F23) and the amount of O is 0.15 mass% or less A much better value is obtained.

  In addition, for the powder symbol: I21, the amounts of C and O in the atomized iron-based powder satisfy [O] ≧ 4/3 [C] −2/15 and 4/3 [C] + 0.28 ≧ [O]. Since both are satisfied, it is shown that, even if the carbon powder is not mixed before the heat treatment, appropriate values can be obtained for the amount of C and O after the heat treatment by reducing the carbon powder under appropriate heat treatment conditions.

  On the other hand, for the powder symbol J21, the amount of C and O in the atomized iron-based powder does not satisfy [O] ≧ 4/3 [C] −2/15, and is C-excess. This shows that the amount of C after the heat treatment is out of the proper range even if the treatment is performed in an inert gas atmosphere using conditions.

(Example 2)
Of the atomized iron-based powders in Table 1, A and I were reduced by heat treatment using N ₂ as an inert gas. Table 6 shows the processing conditions. At that time, the atomized iron-based powder A was mixed with the amount of graphite powder shown in Table 6 and then supplied to the moving bed furnace. On the other hand, the atomized iron-based powder I was supplied to the moving bed furnace without mixing with the graphite powder (only the atomized iron-based powder I).

Further, after the reduction, the inert gas in the moving bed furnace was exhausted and H ₂ gas was supplied, and the reduced powder was cooled in the H ₂ gas atmosphere (A31, I31). After cooling, the obtained powder was crushed to obtain alloy steel powder for powder metallurgy. The component composition of the obtained alloy steel powder is shown in Table 7. For comparison, the heat treatment conditions of A23 and I21 in Example 1 and the component composition of the alloy steel powder are listed in Tables 6 and 7. In A23 and I21, the cooling after the reduction is performed in an N ₂ atmosphere.

In A31 and I31 cooled in an H ₂ atmosphere, the amounts of C and O can be reduced to 0.1 mass% or less and 0.28 mass% or less, respectively, and the N content is 0.003 mass% (30 Mass ppm) or less. On the other hand, in A23 and I21 cooled in an N ₂ atmosphere, the N amount was around 0.007 mass% (70 mass ppm).

  From the above results, it can be seen that N as an impurity contained in the alloy steel powder can be further reduced by cooling the reduced atomized iron-based powder using hydrogen gas or a hydrogen-containing gas.

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 atmosphere gas)
7 Crude iron-based powder 8 Hopper 9 Belt
10 wheel
11 Radiant tube
12 Steam blowing pipe
13 Product powder
14 Product tank
15 Water-sealed tank
20 Product powder crusher
21 Cooler
22 Circulation fan
30 Furnace (heating furnace)
100 Heat treatment equipment

Claims (4)

% By mass
C: 0.8% or less,
O: 1.0% or less,
Mn: 0.08% or less,
Cr: 0.3 to 3.5%,
Mo: 0.1 to 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);
In the moving bed furnace, an inert gas is supplied 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,
Said d and v satisfy | fill following (1) Formula, C content [C] (mass%) of the said atomized iron base powder, and O content [O] (mass%) of the said atomized iron base powder, The following equation (2) is satisfied,
In the supply of the atomized iron-based powder to the moving bed furnace, when the C content [C] and the O content [O] of the atomized iron-based powder satisfy the following formula (3), the atomized iron-based powder Is supplied to the moving bed furnace as it is, and if the following formula (3) is not satisfied, the atomized iron-based powder is further mixed with a carbon component so as to satisfy the following formula (4), and then the moving bed furnace A method for producing alloy steel powder for powder metallurgy, which is supplied to the company.
D / √v ≦ 3.7 (mm ^1/2 · s ^1/2 ) (1)
[O] ≧ 4/3 [C] −2/15 (2)
4/3 [C] + 0.28 ≧ [O] (3)
4/3 ([C] + [MXC]) + 0.28 ≧ [O] (4)
(Here, [MXC] is (mass of carbon component mixed with the atomized powder / mass of the atomized iron-based powder) × 100 (mass%))   The method for producing alloy steel powder for powder metallurgy according to claim 1, wherein the reduced atomized iron-based powder is cooled using hydrogen gas or a hydrogen-containing gas.   The manufacturing method of the alloy steel powder for powder metallurgy of Claim 1 or 2 which makes the dew point of the said inert gas 5 degrees C or less. The alloy for powder metallurgy according to any one of claims 1 to 3, wherein in the heat treatment, deoxidation is performed under conditions of an atmospheric temperature T: 1000 ° C or more and a holding time t: 10 4 ^{-0.0037 · T} hours or more. Manufacturing method of steel powder.

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