Method for producing water-atomized metal powder
10589356 ยท 2020-03-17
Assignee
Inventors
Cpc classification
B22F9/008
PERFORMING OPERATIONS; TRANSPORTING
B22F9/08
PERFORMING OPERATIONS; TRANSPORTING
C22C38/002
CHEMISTRY; METALLURGY
B22F2009/0832
PERFORMING OPERATIONS; TRANSPORTING
B22F2999/00
PERFORMING OPERATIONS; TRANSPORTING
International classification
B22F9/08
PERFORMING OPERATIONS; TRANSPORTING
B22F9/00
PERFORMING OPERATIONS; TRANSPORTING
Abstract
A method for producing a water-atomized metal powder, comprising applying water to a molten metal stream, dividing the molten metal stream into a metal powder, and cooling the metal powder, wherein the metal powder is further subjected to secondary cooling with cooling capacity having a minimum heat flux point (MHF point) higher than the surface temperature of the metal powder in addition to the cooling and the secondary cooling is performed from a temperature range where the temperature of the metal powder after the cooling is not lower than the cooling start temperature necessary for amorphization nor higher than the minimum heat flux point (MHF point).
Claims
1. A method for producing a water-atomized metal powder, comprising applying water to a molten metal stream, dividing the molten metal stream into a metal powder, and cooling the metal powder, wherein the metal powder is further subjected to secondary cooling with cooling capacity having a minimum heat flux point (MHF point) higher than the surface temperature of the metal powder in addition to the cooling and the secondary cooling is performed from a temperature range where the temperature of the metal powder after the cooling is not lower than the cooling start temperature necessary for amorphization nor higher than the minimum heat flux point (MHF point), wherein the secondary cooling is cooling in which water ejection is performed using water different from water used to divide the molten metal stream.
2. The method for producing the water-atomized metal powder according to claim 1, wherein the cooling in which water ejection is performed is cooling in which jet water with a temperature of 10 C. or lower and an ejection pressure of 5 MPa or higher is used.
3. The method for producing the water-atomized metal powder according to claim 1, wherein the molten metal is composed of an FeB alloy or an FeSiB alloy and the water-atomized metal powder is powder containing 90% or more of an amorphous metal powder.
4. The method for producing the water-atomized metal powder according to claim 2, wherein the molten metal is composed of an FeB alloy or an FeSiB alloy and the water-atomized metal powder is powder containing 90% or more of an amorphous metal powder.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
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DETAILED DESCRIPTION OF THE INVENTION
(8) In embodiments of the present invention, first, a metal material that is a raw material is melted into molten metal. The metal material, which is used as a raw material, may be any of pure metals, alloys, pig iron, and the like conventionally used in the form of powder. The following materials can be exemplified: for example, pure iron; low-alloy steels; iron-based alloys such as stainless steel; non-ferrous metals such as Ni and Cr; non-ferrous alloys; and amorphous alloys (non-crystalline alloys) such as FeB alloys, FeSiB alloys, and FeNiB alloys. Needless to say, the above-mentioned alloys may possibly contain an element other than the above-mentioned elements in the form of an impurity.
(9) A method for melting the metal material need not be particularly limited and any of melting means, such as an electric furnace and a vacuum melting furnace, in common use can be used.
(10) The molten metal is transferred to a container such as a tundish from a melting furnace and is then processed into a water-atomized metal powder in a water-atomized metal powder production apparatus.
(11) An embodiment of the present invention, which uses a water atomization method, is described with reference to
(12) Molten metal 1 is dropped into a chamber 9 from a container such as a tundish 3 through a molten metal guide nozzle 4 in the form of a molten metal stream 8. Needless to say, an inert gas valve 11 is opened such that the chamber 9 has an inert gas atmosphere. A nitrogen gas and an argon gas can be exemplified as the inert gas.
(13) Jet water (water jet) 7 is applied to the falling molten metal stream 8 through nozzles 6 attached to a nozzle header 5 such that the molten metal stream 8 is divided, followed by cooling, whereby a metal powder 8a is obtained. A position A where the molten metal stream 8 and the jet water (water jet) 7 are brought into contact with each other is preferably a position apart from the molten metal guide nozzle 4 at an appropriate distance from the viewpoint that the molten metal stream 8 is cooled to near the melting point by heat radiation and the cooling action of the inert gas and the viewpoint that splashes of the jet water 7 are prevented from coming into contact with the molten metal guide nozzle 4.
(14) In embodiments of the present invention, the ejection pressure or temperature of the jet water (water jet) 7, which is used to divide the molten metal stream 8, is not particularly limited as far as the jet water (water jet) 7 may have an ejection pressure sufficient to divide the molten metal stream 8. The jet water (water jet) 7 preferably has a temperature of 30 C. or lower or has a temperature of 30 C. or lower and an ejection pressure of 5 MPa or higher. In particular, when the water temperature is higher than 20 C., the cooling rate of a metal powder is low and therefore the metal powder cannot be maintained in an amorphous state even when applying a secondary cooling. The water temperature is preferably 10 C. or lower and more preferably 5 C. or lower.
(15) In the production of the metal powder by water atomization in embodiments of the present invention, the jet water 7 is applied to the molten metal stream 8 at the position A as described above, whereby the molten metal stream is divided and the divided metal powder (including those in a molten state) 8a is cooled (primarily cooled). Furthermore, the metal powder (including those in a molten state) 8a is secondarily cooled at a position B apart from the position A at an appropriate distance.
(16) Secondary cooling is preferably performed in such a manner that cooling jet water 21 is ejected as shown in
(17) When the temperature (water temperature) of the cooling jet water 21 for secondary cooling is higher than 10 C., the MHF point is low and a desired cooling rate can hardly be ensured. Therefore, the temperature (water temperature) of the cooling jet water 21 for secondary cooling is preferably limited to 10 C. or lower. The temperature thereof is more preferably 8 C. or lower. When the ejection pressure of the cooling jet water 21 for secondary cooling is lower than 5 MPa, cooling cannot be performed such that the MHF point is a desired temperature, even if the temperature of cooling water is 10 C. or lower. Thus, a desired cooling rate can hardly be ensured. Therefore, the ejection pressure of the cooling jet water 21 is preferably limited to 5 MPa or higher. Even if the ejection pressure of the cooling jet water 21 is increased to higher than 10 MPa, the increase of the MHF point is saturated. Therefore, the ejection pressure thereof is preferably set to 10 MPa or lower.
(18) The term desired cooling rate as used herein refers to the minimum cooling rate at which amorphization can be achieved, that is, an average cooling rate of about 10.sup.5 K/s to 10.sup.6 K/s in a cooling temperature range necessary to prevent crystallization.
(19) The term cooling temperature range necessary to prevent crystallization as used herein refers to a range from the cooling start temperature necessary for amorphization to a first crystallization temperature (for example, 400 C. to 600 C.) that is a cooling stop temperature. The cooling start temperature necessary for amorphization varies depending on the composition of molten metal and may be, for example, 900 C. to 1,100 C.
(20) Secondary cooling is preferably performed from a temperature range where the temperature of the cooled (primarily cooled) metal powder is not lower than the cooling start temperature necessary for amorphization nor higher than the MHF point of secondary cooling. When the temperature of the cooled metal powder is higher than the MHF point of secondary cooling, secondary cooling cannot be set to cooling in a transition boiling state or cooling in a nucleate boiling state and therefore, the desired cooling rate can hardly be ensured. When the temperature of the cooled metal powder is lower than the cooling start temperature necessary for amorphization, the temperature of the metal powder is too low to ensure the desired cooling rate and crystallization is likely to proceed.
(21) It is preferable that cooling water used for the jet water 7 is cooled to low temperature in advance with a heat exchanger such as a chiller 16 which can cool cooling water to a low temperature and is stored in a cooling water tank 15 (heat-insulating structure) placed outside the water-atomized metal powder production apparatus 14. In a usual cooling water production apparatus, it is difficult to produce cooling water at 3 C. to lower than 4 C. because the inside of a heat exchanger is frozen. Therefore, a mechanism for supplying ice to the tank from an ice-making machine may be used. Needless to say, the cooling water tank 15 is further provided with a high-pressure pump 17 which is a pump for pressurizing and delivering the cooling water used for the jet water 7 and a pipe 18 for supplying the cooling water to the nozzle header 5 from the high-pressure pump.
(22) Cooling water used for the cooling jet water 21, as well as the cooling water used for the jet water 7, is preferably stored in the cooling water tank 15 (heat-insulating structure), which is placed outside the water-atomized metal powder production apparatus 14, in advance. Needless to say, the cooling water tank 15 is provided with a high-pressure pump 27 for pressurizing and delivering the cooling water used for the cooling jet water 21 separately from the cooling water used for the jet water 7 and a pipe 28 for supplying the cooling water to the nozzles 26 for secondary cooling from the high-pressure pump 27. Incidentally, a surge tank, a switching valve, or the like may be placed between pipes such that high-pressure water is readily ejected suddenly.
(23) Secondary cooling is preferably set such that the divided metal powder 8a can be cooled to a transition boiling state or a nucleate boiling state. Therefore, the start position of secondary cooling (the position B: the position of a nozzle for secondary cooling) is preferably set such that the surface temperature of the water-atomized metal powder 8a is not lower than the cooling start temperature necessary to prevent crystallization nor higher than the MHF point of secondary cooling. The surface temperature of the metal powder 8a can be adjusted by varying the distance between the atomization position A and the cooling start position of secondary cooling (the position B). Therefore, the nozzles 26 for secondary cooling are preferably arranged to be vertically movable.
(24) Secondary cooling is preferably cooling by using a container 41 placed downstream of the position A instead of cooling by the cooling jet water. An example of the water-atomized metal powder production apparatus in this case is shown in
(25) The container 41 is placed at the position B, which is in the fall path of cooling water (atomizing cooling water) used to divide the molten metal stream 8 and subsequently used to cool the metal powder, the divided molten metal, and the metal powder in cooling and which is downstream of the position A. The position B is a position where the surface temperature of the metal powder 8a is not lower than the cooling start temperature necessary to prevent crystallization nor higher than the MHF point, that is, a secondary cooling start position. Since the container 41 is placed at the position B (preferably such that the position of the bottom surface of the container corresponds to the position B), cooling water is stored in the container to form a water pool and is stirred in the container and a steam film on the surface of the metal powder is likely to be removed by a stream along the surface of the metal powder stored at the same time. It is conceivable that shock waves generated when water collides with the surface of the water pool formed in the container at high speed facilitate the transition from film boiling to transition boiling.
(26) The placed container 41 preferably has a size sufficient to store cooling water (atomizing cooling water used to divide the molten metal stream 8 and subsequently used to cool the metal powder, the divided molten metal, and/or the metal powder. When the container is too large, a shock wave is unlikely to be generated. When the flow rate of atomizing cooling water is about 200 L/min, a container having an inside diameter of about 50 mm to 150 mm and a depth of about 30 mm to 100 mm is enough. The container is preferably made of metal in terms of strength and may be made of ceramic.
(27) Secondary cooling may be cooling performed by placing a collision plate 42 instead of cooling performed by placing the container 41. An example of the water-atomized metal powder production apparatus in this case is shown in
(28) The collision plate 42, as well as the container 41, is placed at the secondary cooling start position (the position B), which is in the fall path of atomizing cooling water, the divided molten metal, and the metal powder and which is downstream of the position A. Since the collision plate 42 is placed at such a position, the metal powder is likely to be shifted from a film boiling state to a transition boiling state by shock waves generated when atomizing cooling water and the metal powder collide with the collision plate 42; hence, cooling with high cooling capacity is similarly achieved.
(29) The collision plate 42 has only to be capable of blocking the fall path of atomizing cooling water, the molten metal, and the metal powder in cooling. The shape thereof is not particularly limited and may probably be a disk shape, a conical shape, an inverted conical shape, or the like. Since a shape capable of forming a surface perpendicular to the fall path is effective in generating a shock wave, an inverted conical shape (
(30) The present invention is further described below with reference to examples.
EXAMPLES
Example 1
(31) Each metal powder was produced using a water-atomized metal powder production apparatus shown in
(32) Raw materials were blended (partly containing impurities is inevitable) such that an FeB alloy (Fe.sub.83B.sub.17) with a composition of 83% Fe-17% B and an FeSiB alloy (Fe.sub.79Si.sub.10B.sub.11) with a composition of 79% Fe-10% Si-11% B on an atomic basis were obtained, followed by melting the raw materials at about 1,550 C. in a melting furnace 2, whereby about 50 kgf of each molten metal was obtained. The obtained molten metal 1 was slowly cooled to 1,350 C. in the melting furnace 2 and was then poured into a tundish 3. An inert gas valve 11 was opened in advance such that a chamber 9 had a nitrogen gas atmosphere. Before the molten metal was poured into the tundish 3, cooling water was supplied to a nozzle header 5 from a cooling water tank 15 (a volume of 10 m.sup.3) by operating a high-pressure pump 17, whereby jet water (fluid) 7 was ejected from water ejection nozzles 6. Furthermore, cooling water was supplied to nozzles 26 for secondary cooling from the cooling water tank 15 (a volume of 10 m.sup.3) in such a manner that a high-pressure pump 27 for secondary cooling water was operated and valves 22 for secondary cooling water were opened, whereby cooling jet water 21 was ejected.
(33) A position A where a molten metal stream 8 was in contact with the jet water 7 was set to a position 80 mm apart from a molten metal guide nozzle 4. The nozzles 26 for secondary cooling were placed at a position B. The position B was set to each position 100 mm to 800 mm apart from the position A. The ejection pressure of the jet water 7 was set to 1 MPa or 5 MPa and the temperature thereof was set to 30 C. (2 C.) or 8 C. (2 C.). The ejection pressure of the cooling jet water 21 used for secondary cooling was set to 5 MPa and the temperature thereof was set to 20 C. (2 C.) or 8 C. (2 C.). The water temperature was adjusted with a chiller 16 placed outside the cooling water tank 15.
(34) The molten metal 1 poured into the tundish 3 was dropped into the chamber 9 through the molten metal guide nozzle 4 to form the molten metal stream 8, which was brought into contact with the jet water (fluid) 7 in such a manner that the temperature and ejection pressure of the jet water (fluid) 7 were varied as shown in Table 1, whereby the molten metal stream 8 was divided into a metal powder. The metal powder was cooled while being mixed with cooling water, was further secondarily cooled with the cooling jet water 21 ejected from the nozzles 26 for secondary cooling, and was collected from a collection port 13. Incidentally, an example in which no secondary cooling was performed was a comparative example. The surface temperature of the metal powder before secondary cooling was estimated from results of a separately performed primary cooling experiment. The MHF point of secondary cooling was estimated from a separately performed experiment and was listed in the table.
(35) After contaminants other than the obtained metal powder were removed, an amorphous halo peak and a crystalline diffraction peak of the metal powder were measured by X-ray diffractometry. The degree of crystallinity was determined from the ratio between the integrated intensity of a diffracted X-ray from the amorphous halo peak and that from the crystalline diffraction peak. The percentage of amorphousness (the degree of amorphousness: %) was calculated from (1the degree of crystallinity). The case where the degree of amorphousness (the degree of amorphization) was 90% or more was rated A and others were rated B.
(36) Obtained results are shown in Table 1.
(37) TABLE-US-00001 TABLE 1 Dividing-cooling (primary cooling) Water injection Secondary cooling conditions Water injection Water Cooling conditions tem- start Installation Water Degree of Ejection pera- tem- position Ejection tem- MHF amorphization Powder pressure ture perature B*** pressure perature point Eval- No. Composition (MPa) ( C.) ( C.) Cooling means (mm) (MPa) ( C.) ( C.) (%) uation Remarks 1 Fe.sub.79Si.sub.10B.sub.11* 5 30 28 B Comparative example 2 5 8 36 B Comparative example 3 5 30 955 Water injection 300 5 20 960 93 A Inventive example 4 1 8 958 Water injection 300 5 8 1010 95 A Inventive example 5 Fe.sub.83B.sub.17** 5 8 984 Water injection 300 5 8 1010 96 A Inventive example 6 5 8 953 Water injection 300 5 20 960 90 A Inventive example 7 1 8 1005 Water injection 300 5 8 1010 91 A Inventive example 8 5 8 1008 Water injection 100 5 8 1010 90 A Inventive example 9 5 8 998 Water injection 200 5 8 1010 92 A Inventive example 10 5 8 973 Water injection 400 5 8 1010 93 A Inventive example 11 5 8 920 Water injection 800 5 8 1010 88 B Comparative example *The cooling rate necessary for amorphization is 1.8 10.sup.5 K/s and the cooling start temperature necessary for amorphization is 950 C. **The cooling rate necessary for amorphization is 1.0 10.sup.6 K/s and the cooling start temperature necessary for amorphization is 970 C. ***The distance from a water atomization position A (vertical direction).
(38) In every inventive example, the degree of amorphousness of a water-atomized metal powder is 90% or more. This shows that in embodiments of the present invention, a cooling rate of 1.810.sup.5 K/s to 1.010.sup.6 K/s or more, which is the critical cooling rate for amorphization, is obtained. However, in comparative examples (Powders No. 1 and No. 2) in which no secondary cooling was performed, the degree of amorphousness is less than 90%.
(39) In some of inventive examples, the degree of amorphousness is slightly low. In Powders No. 3 and No. 6, the temperature of cooling jet water for secondary cooling is high. In Powder No. 7 , the ejection pressure of jet water for dividing a molten metal stream is lower than a preferable scope. In Powders No. 8 and No. 9, the cooling start position of secondary cooling is close to the position A; hence, the cooling start temperature of secondary cooling is close to the MHF point and the degree of amorphousness is slightly low though the degree of amorphousness is 90% or more. In Powder No. 10, the cooling start position of secondary cooling is far apart from the position A; hence, the time until the start of secondary cooling is long, cooling is slow because the surface temperature of the powder is too low, and the degree of amorphousness is slightly low though the degree of amorphousness is 90% or more. In Powder No. 11, the secondary cooling start position (position B) is too far apart from the position A, the temperature of the metal powder is lower than a necessary cooling start temperature, and it is conceivable that crystallization proceeded.
Example 2
(40) Each metal powder was produced using a water-atomized metal powder production apparatus shown in
(41) Raw materials were blended (partly containing impurities is inevitable) such that an FeB alloy (Fe.sub.83B.sub.17) with a composition of 83% Fe-17% B and an FeSiB alloy (Fe.sub.79Si.sub.10B.sub.11) with a composition of 79% Fe-10% Si-11% B on an atomic basis were obtained, followed by melting the raw materials at about 1,550 C. in a melting furnace 2, whereby about 50 kgf of each molten metal was obtained. The obtained molten metal 1 was slowly cooled to 1,350 C. in the melting furnace 2 and was then poured into a tundish 3. An inert gas valve 11 was opened in advance such that a chamber 9 had a nitrogen gas atmosphere. Before the molten metal was poured into the tundish 3, cooling water was supplied to a nozzle header 5 from a cooling water tank 15 (a volume of 10 m.sup.3) by operating a high-pressure pump 17, whereby jet water (fluid) 7 was ejected from water ejection nozzles 6. A container 41 made of metal was placed on the fall path of cooling water and a metal powder, the fall path being downstream of a position A, such that cooling water and the divided metal powder were stored therein after water atomization. The container 41 made of metal had a size of 100 mm in outside diameter90 mm in inside diameter40 mm in depth.
(42) The position A where a molten metal stream 8 was in contact with the jet water 7 was set to a position 80 mm apart from a molten metal guide nozzle 4. The container 41 for secondary cooling was placed at a position B. The position B was set to each position (the position of the bottom of a container) 100 mm to 800 mm apart from the position A. The ejection pressure of the jet water 7 was set to 3 MPa or 5 MPa and the temperature thereof was set to 40 C. (2 C.) or 20 C. (2 C.). The water temperature was adjusted with a chiller 16 placed outside the cooling water tank 15.
(43) The molten metal 1 poured into the tundish 3 was dropped into the chamber 9 through the molten metal guide nozzle 4 to form the molten metal stream 8, which was brought into contact with the jet water 7 in such a manner that the temperature and ejection pressure of the jet water (fluid) 7 were varied as shown in Table 2, whereby the molten metal stream 8 was divided into a metal powder. The divided metal powder was mixed with cooling water, fell while being cooled, was stored in the container 41, was stirred in the container 41 together with cooling water, was cooled, and was collected from a collection port 13. The metal powder stored in the container was exposed to shock waves generated when falling cooling water collided with the surface of a water pool in the container at high speed. Incidentally, an example in which no secondary cooling was performed was a comparative example. The surface temperature of the metal powder before secondary cooling and the MHF point of secondary cooling were estimated in substantially the same manner as that used in (Example 1) and were listed together in the table.
(44) After contaminants other than the obtained metal powder were removed, an amorphous halo peak and a crystalline diffraction peak of the metal powder were measured by X-ray diffractometry. The degree of crystallinity was determined from the ratio between the integrated intensity of a diffracted X-ray from the amorphous halo peak and that from the crystalline diffraction peak in substantially the same manner as that used in Example 1. The percentage of amorphousness (the degree of amorphousness: %) was calculated from (1the degree of crystallinity). The case where the degree of amorphousness was 90% or more was rated A and the case where the degree of amorphousness was less than 90% was rated B in substantially the same manner.
(45) Obtained results are shown in Table 2.
(46) TABLE-US-00002 TABLE 2 Dividing-cooling (primary cooling) Water injection conditions Secondary cooling Ejection Water Cooling start Installation Degree of Powder pressure temperature temperature Cooling position B*** MHF point amorphization No. Composition (MPa) ( C.) ( C.) means (mm) ( C.) (%) Evaluation Remarks 2-1 Fe.sub.79Si.sub.10B.sub.11* 3 20 56 B Comparative example 2-2 3 20 963 Container 300 970 96 A Inventive example 2-3 3 40 982 Container 300 780 85 B Comparative example 2-4 3 20 968 Container 100 970 92 A Inventive example 2-5 3 20 951 Container 400 970 91 A Inventive example 2-6 3 20 922 Container 800 970 83 B Comparative example 2-7 Fe.sub.83B.sub.17** 5 20 53 B Comparative example 2-8 5 20 983 Container 300 1003 94 A Inventive example 2-9 5 40 1025 Container 300 840 87 B Comparative example 2-10 5 20 998 Container 100 1003 93 A Inventive example 2-11 5 20 971 Container 400 1003 90 A Inventive example 2-12 5 20 911 Container 800 1003 84 B Comparative example *The cooing rate necessary for amorphization is 1.8 10.sup.5 K/s and the cooling start temperature necessary for amorphization is 950 C. **The cooling rate necessary for amorphization is 1.0 10.sup.6 K/s and the cooling start temperature necessary for amorphization is 970 C. ***The distance from a water atomization position A (vertical direction).
(47) In every inventive example, the degree of amorphousness of a water-atomized metal powder is 90% or more. However, in comparative examples (Powders No. 2-1 and No. 2-7 ) in which no secondary cooling was performed, the degree of amorphousness is less than 90%. Incidentally, in some of the inventive examples that are outside a preferable scope of embodiments of the present invention, the degree of amorphousness is slightly low.
(48) In Powders No. 2-3 and No. 2-9, the temperature of jet water (primary cooling water) for dividing a molten metal stream is higher than the preferable scope, the secondary cooling start temperature is high, the time of cooling in a film boiling region is long, and the degree of amorphousness is low, less than 90%.
(49) In Powders No. 2-4 and No. 2-10, the installation position of the container 41 is close to the position A, which is a position where a molten metal stream is divided, and therefore the cooling start temperature of secondary cooling is high; hence, the degree of amorphousness is slightly low though the degree of amorphousness is 90% or more.
(50) In Powders No. 2-5 and No. 2-11, the installation position of the container 41 is far apart from the position A, which is a position where a molten metal stream is divided; hence, the time until the start of secondary cooling is long, the surface temperature of the metal powder is low, cooling is slow, and the degree of amorphousness is slightly low though the degree of amorphousness is 90% or more. In Powders No. 2-6 and No. 2-12, the secondary cooling start position (position B) is too far apart from the position A, the temperature of the metal powder is lower than a necessary cooling start temperature, crystallization proceeds, and the degree of amorphousness is less than 90%.
Example 3
(51) Each metal powder was produced using a water-atomized metal powder production apparatus shown in
(52) Raw materials were blended (partly containing impurities is inevitable) such that an FeB alloy (Fe.sub.83B.sub.17) with a composition of 83% Fe-17% B and an FeSiB alloy (Fe.sub.79Si.sub.10B.sub.11) with a composition of 79% Fe-10% Si-11% B on an atomic basis were obtained, followed by melting the raw materials at about 1,550 C. in a melting furnace 2, whereby about 50 kgf of each molten metal was obtained. The obtained molten metal 1 was slowly cooled to 1,350 C. in the melting furnace 2 and was then poured into a tundish 3. An inert gas valve 11 was opened in advance such that a chamber 9 had a nitrogen gas atmosphere. Before the molten metal was poured into the tundish 3, cooling water was supplied to a nozzle header 5 from a cooling water tank (a volume of 10 m.sup.3) by operating a high-pressure pump, whereby jet water (fluid) 7 was ejected from water ejection nozzles 6. A collision plate 42 made of metal was placed on the fall path of cooling water and a metal powder, the fall path being downstream of a position A, such that secondary cooling was performed in such a manner that falling cooling water after water atomization and the divided metal powder collided with the collision plate 42. After secondary cooling, the metal powder was collected from a collection port 13.
(53) The size of the collision plate 42 made of metal was such that a surface perpendicular to the falling direction of the metal powder had an area with a diameter of 100 mm. This size is sufficient to allow substantially the whole of the falling metal powder after water atomization to collide therewith.
(54) The shape of the collision plate 42 was one of an inverted conical shape (a), a disk shape (b), and a conical shape (c) as shown in
(55) A position A where a molten metal stream 8 was in contact with the jet water 7 was set to a position 80 mm apart from a molten metal guide nozzle 4. The collision plate 42 for secondary cooling was placed at a secondary cooling start position (position B). The position B was set to each position 100 mm to 800 mm apart from the position A. The ejection pressure of the jet water 7 was set to 3 MPa or 5 MPa and the temperature thereof was set to 40 C. (2 C.) or 20 C. (2 C.). The water temperature was adjusted with a chiller placed outside the cooling water tank. Incidentally, an example in which no collision plate 42 was placed (no secondary cooling was performed) was a comparative example. The surface temperature of the metal powder before secondary cooling and the MHF point of secondary cooling were estimated in substantially the same manner as that used in Example 1 and were listed together in a table.
(56) After contaminants other than the obtained metal powder were removed, an amorphous halo peak and a crystalline diffraction peak of the metal powder were measured by X-ray diffractometry. The percentage of amorphousness (the degree of amorphousness: %) was calculated from the ratio between the integrated intensity of a diffracted X-ray from the amorphous halo peak and that from the crystalline diffraction peak in substantially the same manner as that used in Example 1. The case where the degree of amorphousness was 90% or more was rated A and the case where the degree of amorphousness was less than 90% rated B in substantially the same manner.
(57) Obtained results are shown in Table 3.
(58) TABLE-US-00003 TABLE 3 Dividing-cooling (primary cooling) Water injection conditions Secondary cooling Ejection Water Cooling start Installation MHF Degree of Powder pressure temperature temperature Cooling position B*** point amorphization No. Composition (MPa) ( C.) ( C.) means**** (mm) ( C.) (%) Evaluation Remarks 3-1 Fe.sub.79Si.sub.10B.sub.11* 3 20 56 B Comparative example 3-2 3 20 962 Collision plate a 300 970 95 A Inventive example 3-3 3 40 1010 Collision plate a 300 780 83 B Comparative example 3-4 3 20 963 Collision plate b 300 970 94 A Inventive example 3-5 3 20 965 Collision plate c 300 Unclear 82 B Comparative example 3-6 3 20 965 Collision plate a 100 970 91 A Inventive example 3-7 3 20 951 Collision plate a 400 970 91 A Inventive example 3-8 3 20 930 Collision plate a 800 970 84 B Comparative example 3-9 Fe.sub.83B.sub.17** 5 20 53 B Comparative example 3-10 5 20 990 Collision plate a 300 1003 92 A Inventive example 3-11 5 40 1024 Collision plate a 300 840 89 B Comparative example 3-12 5 20 992 Collision plate b 300 1003 92 A Inventive example 3-13 5 20 988 Collision plate c 300 Unclear 72 B Comparative example 3-14 5 20 1002 Collision plate a 100 1003 91 A Inventive example 3-15 5 20 973 Collision plate a 400 1003 92 A Inventive example 3-16 5 20 942 Collision plate a 800 1003 88 B Comparative example *The critical cooling rate for amorphization is 1.8 10.sup.5 K/s and the cooling start temperature necessary for amorphization is 950 C. **The critical cooling rate for amorphization is 1.0 10.sup.6 K/s and the cooling start temperature necessary for amorphization is 970 C. ***The distance from a water atomization position A (vertical direction). ****For a collision plate a, refer to FIG. 5(a); for a collision plate b, refer to FIG. 5(b); and for a collision plate c, refer to FIG. 5(c).
(59) In every inventive example, the degree of amorphousness of a water-atomized metal powder is 90% or more. However, in comparative examples (Powders No. 3-1 and No. 3-9) in which no secondary cooling was performed, the degree of amorphousness is less than 90%. Incidentally, in some of the inventive examples that are outside a preferable scope of embodiments of the present invention, the degree of amorphousness is slightly low.
(60) In Powders No. 3-3 and No. 3-11, the temperature of jet water (primary cooling water) for dividing a molten metal stream is higher than the preferable scope, the secondary cooling start temperature is higher than the MHF point, the time of cooling in a film boiling region is long, and the degree of amorphousness is low, less than 90%.
(61) In Powders No. 3-5 and No. 3-13, the shape of the collision plate 42 is conical (
(62) In Powders No. 3-6 and No. 3-14, the installation position of the collision plate 42 is close to the position A, which is a position where a molten metal stream is divided; hence, the cooling start temperature of secondary cooling is high and the degree of amorphousness is slightly low though the degree of amorphousness is 90% or more.
(63) In Powders No. 3-7 and No. 3-15, the installation position of the collision plate 42 is far apart from the position A, which is a position where a molten metal stream is divided; hence, the time until the start of secondary cooling is long, the surface temperature of the metal powder is low, cooling is slow, and the degree of amorphousness is slightly low though the degree of amorphousness is 90% or more. In Powders No. 3-8 and No. 3-16, the cooling start temperature is lower than a necessary cooling start temperature and the degree of amorphousness is less than 90%.
REFERENCE SIGNS LIST
(64) 1 Molten metal (molten metal)
(65) 2 Melting furnace
(66) 3 Tundish
(67) 4 Molten metal guide nozzle
(68) 5 Nozzle header
(69) 6 Water ejection nozzles
(70) 7 Jet water
(71) 8 Molten metal stream
(72) 8a Metal powder
(73) 9 Chamber
(74) 10 Hopper
(75) 11 Inert gas valve
(76) 12 Overflow valve
(77) 13 Metal powder collection valve
(78) 14 Water-atomized metal powder production apparatus
(79) 15 Cooling water tank
(80) 16 Chiller (low-temperature cooling water production apparatus)
(81) 17 High-pressure pump
(82) 18 Cooling water pipe
(83) 21 Secondary cooling water (cooling jet water)
(84) 22 Valves for secondary cooling water
(85) 26 Secondary cooling water ejection nozzles
(86) 27 High-pressure pump for secondary cooling water
(87) 28 Cooling water pipe for secondary cooling water
(88) 41 Container
(89) 42 Collision plate