Method of producing atomized metal powder

Abstract

A water-atomized metal powder is produced by dividing a molten metal stream into a metal powder by making injection water having a liquid temperature of 10 C. or less and an injection pressure of 5 MPa or more impinge on the molten metal stream and cooling the metal powder. Cooling with injection water having a liquid temperature of 10 C. or less and an injection pressure of 5 MPa or more enables can be performed not in the film boiling region but in the transition boiling region from the beginning of cooling. A gas-atomized metal powder may also be produced by dividing a molten metal stream into a metal powder by making an inert gas impinge on the molten metal stream and cooling the metal powder with injection water having a liquid temperature of 10 C. or less and an injection pressure of 5 MPa or more.

Claims

1. A method of producing an atomized metal powder comprising dividing a molten metal stream into a metal powder by making a fluid impinge on the molten metal stream; and cooling the metal powder, the fluid being injection water having a temperature of 10 C. or less and an injection pressure of 5 MPa to 20 MPa, the fluid being used to divide the molten metal stream and cool the metal powder.

2. The method according to claim 1, wherein the molten metal stream includes a FeB alloy or a FeSiB alloy, and the atomized metal powder is an amorphous metal powder.

3. A method of producing an atomized metal powder comprising dividing a molten metal stream into a metal powder by making a fluid impinge on the molten metal stream; and cooling the metal powder, the fluid being an inert gas, the fluid being used to divide the molten metal stream, cooling of the metal powder being performed with injection water having a temperature of 10 C. or less and an injection pressure of 5 MPa or more.

4. The method according to claim 3, wherein the impinging the injection water is performed after a temperature of the metal powder has reached 1000 C. or less.

5. The method according to claim 4, wherein the molten metal stream includes a FeB alloy or a FeSiB alloy, and the atomized metal powder is an amorphous metal powder.

6. The method according to claim 3, wherein the molten metal stream includes a FeB alloy or a FeSiB alloy, and the atomized metal powder is an amorphous metal powder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph illustrating the impact of the water temperature and injection pressure of cooling water on the MHF point.

(2) FIG. 2 is a schematic diagram illustrating the structure of a water-atomized metal powder production device.

(3) FIG. 3 is a schematic diagram illustrating the structure of a gas-atomized metal powder production device.

(4) FIG. 4 is a schematic diagram illustrating a boiling curve.

REFERENCE SIGNS LIST

(5) 1 MOLTEN METAL 2 MELTING FURNACE 3 TUNDISH 4 MOLTEN-METAL-GUIDE NOZZLE 5 NOZZLE HEADER 6 NOZZLES (WATER INJECTION NOZZLES) 7 FLUID (INJECTION WATER) 8 MOLTEN METAL STREAM 8a METAL POWDER 9 CHAMBER 10 HOPPER 11 INERT GAS VALVE 12 OVERFLOW VALVE 13 METAL POWDER COLLECTION VALVE 14 WATER-ATOMIZED METAL POWDER PRODUCTION DEVICE 15 COOLING-WATER TANK 16 CHILLER (LOW-TEMPERATURE COOLING WATER PRODUCTION DEVICE) 17 HIGH-PRESSURE PUMP 18 COOLING-WATER PIPE 19 GAS-ATOMIZED METAL POWDER PRODUCTION DEVICE 21 NOZZLE HEADER (GAS NOZZLE HEADER) 22 GAS NOZZLES 24 HEADER VALVE 25 COOLING-WATER INJECTION NOZZLES 25a INJECTION WATER 26 COOLING-WATER VALVE 27 GAS BOMB FOR GAS ATOMIZATION 28 HIGH-PRESSURE GAS PIPE

DETAILED DESCRIPTION

(6) We conducted extensive studies of various factors that may affect the MHF point in water-injection cooling and, as a result, found that the temperature and injection pressure of cooling water greatly affect the MHF point. Our results are described below.

(7) As a material, a SUS304 stainless steel sheet (size: 20 mm thick150 mm wide150 mm long) was used. A thermocouple was inserted into the material from the rear surface such that the temperature of the material at a position (at the center in the width and longitudinal directions) 1 mm below the front surface can be measured. The material was charged into a heating furnace purged with an oxygen-free atmosphere and heated to 1200 C. or more. Immediately after the heated material was removed from the heating furnace, cooling water was made to impinge on the material through cooling nozzles for atomization at various water temperatures and various injection pressures. The changes in the temperature of the material at a position 1 mm below the front surface were measured. The cooling capacities of the cooling water during cooling of the material were estimated by a calculation based on the measured temperature data. A boiling curve was prepared on the basis of the estimated cooling capacities. The MHF point was determined by considering the point at which the cooling capacity was sharply increased as a point at which a transition was made from the film boiling state to the transition boiling state.

(8) FIG. 1 summarizes the results.

(9) As illustrated in FIG. 1, when cooling water having a water temperature of 30 C., which has been commonly used in an ordinary water atomization process, is made to impinge on a material to be cooled at an injection pressure of 1 MPa, the MHF point is about 700 C. while the cooling water is made to impinge on the material to be cooled. When cooling water having a water temperature of 10 C. or less and 2 C. or more is made to impinge on a material to be cooled at an injection pressure of 5 MPa or more and 20 MPa or less, the MHF point is about 1000 C. or more while the cooling water is made to impinge on the material to be cooled. Thus, we found that reducing the temperature (water temperature) of the cooling water to 10 C. or less and increasing the injection pressure to 5 MPa or more increases the MHF point, that is, the temperature at which a transition is made from the film boiling state to the transition boiling state.

(10) In general, a metal powder has a surface temperature of about 1000 C. to 1300 C. immediately after the metal powder has been produced by atomization of a molten metal. The temperature range in which cooling needs to be performed to prevent crystallization from occurring is from about 1000 C. to the first crystallization temperature or less. If water-injection cooling is started such that the temperature at which the metal powder starts being cooled is higher than the MHF point, cooling is performed in the film boiling region, in which the cooling capacity of the cooling water is low, at the beginning of cooling. Therefore, when water-injection cooling is performed such that the MHF point is equal to or higher than the temperature range in which cooling needs to be performed, it become possible to start cooling the metal powder at least from the transition boiling region, in which cooling of the metal powder is facilitated compared with the film boiling region. As a result, the rate at which the metal powder is cooled can be markedly increased. We found if the metal power is cooled in the above-described manner with a high cooling capacity, a rapid cooling in the crystallization temperature range, which is essential to produce an amorphous metal powder, can be readily achieved.

(11) At first, a metal material used as a raw material is melted to form a molten metal. Examples of the metal material that can be used as a raw material include pure metals, alloys, and pig iron, which have been commonly used in powder form. Specific examples thereof include pure iron, iron-base alloys such as low-alloy steel and stainless steel, nonferrous metals such as Ni and Cr, nonferrous alloys, and amorphous alloys such as FeB alloys, FeSiB alloys, and FeNiB alloys. Needless to say, the above alloys may contain impurities other than the above-described elements.

(12) It is not necessary to limit a method of melting the metal material. Common melting means such as an electric furnace, a vacuum melting furnace, and a high-frequency melting furnace may be used.

(13) The molten metal is transferred from the melting furnace to a container such as a tundish and formed into an atomized metal powder inside an atomized metal powder production device. FIG. 2 illustrates a preferable example of a water-atomized metal powder production device.

(14) An example in which a water atomization process is employed is described below with reference to FIG. 2.

(15) A molten metal 1 is passed downward from a container such as a tundish 3 into a chamber 9 through a molten-metal-guide nozzle 4 in the form of a molten metal stream 8. The inside of the chamber 9 is purged with an inert gas (e.g., a nitrogen gas or an argon gas) atmosphere by opening an inert gas valve 11.

(16) A fluid 7 is made to impinge on the molten metal stream 8 through nozzles 6 disposed on a nozzle header 5 to divide the molten metal stream 8 into a metal powder 8a. When a water atomization process is used, injection water (water jet) is used as a fluid 7.

(17) Injection water (water jet) is used as a fluid 7. The injection water (water jet) used has a liquid temperature of 10 C. or less and an injection pressure of 5 MPa or more.

(18) If the liquid temperature (water temperature) of the injection water is higher than 10 C., it becomes impossible to perform water-injection cooling such that the desired MHF point of about 1000 C. or more is achieved and, as a result, the desired cooling rate may fail to be achieved. Accordingly, the liquid temperature (water temperature) of the injection water is limited to be 10 C. or less and is preferably set to 7 C. or less. The term desired cooling rate used herein refers to the minimum cooling rate at which an amorphous metal powder can be produced, that is, a cooling rate of about 10.sup.5 to 10.sup.6 K/s on average at which the temperature is reduced from the temperature at which solidification has terminated to the first crystallization temperature (e.g., about 400 C. to 600 C.) on average.

(19) If the injection pressure of the injection water (water jet) is less than 5 MPa, it becomes impossible to perform water-injection cooling such that the MHF point is equal to or higher than the desired temperature even when the water temperature of the cooling water is 10 C. or less and, as a result, the desired rapid cooling treatment (desired cooling rate) may fail to be achieved. Accordingly, the injection pressure of the injection water is limited to 5 MPa or more. The injection pressure of the injection water is preferably 10 MPa or less because the MHF point stops increasing when the injection pressure is higher than 10 MPa.

(20) In the production of a metal powder in which water atomization is used, injection water having a water temperature and an injection pressure controlled to be specific values as described above is made to impinge on a molten metal stream to divide the molten metal stream into a metal powder and cool and solidify the metal powder (including a metal powder in a molten state) at the same time.

(21) The cooling water used as injection water is preferably stored in a cooling-water tank 15 (heat-insulated structure) disposed outside the water-atomized metal powder production device 14 after it has been cooled to a low temperature with a heat exchanger such as a chiller 16 capable of cooling the cooling water to a low temperature. Means for feeding ice from an ice-making machine into the tank may optionally be provided because it is difficult to make cooling water having a temperature of less than 3 C. to 4 C. with a common cooling-water-making machine due to freezing of the inside of the heat exchanger. It is preferable to make cooling water having a temperature of more than 0 C. since cooling water having a temperature of 0 C. or less is likely to freeze. Needless to say that the cooling-water tank 15 is provided with a high-pressure pump 17 that increases the pressure of the cooling water and feeds the cooling water to the nozzle header 5 and a pipe 18 through which the cooling water is fed from the high-pressure pump to the nozzle header 5.

(22) Division of the molten metal stream may be performed by a gas atomization process in which an inert gas 22a is used as a fluid 7. The resulting metal powder is further cooled with injection water. That is, in the production of a metal powder in which a gas atomization process is used, an inert gas is made to impinge on a molten metal stream to divide the molten metal stream into a metal powder, and the metal powder (including a metal powder in a molten state) is cooled with injection water having an injection pressure: 5 MPa or more and a water temperature of 10 C. or less. FIG. 3 illustrates a preferable example of a gas-atomized metal powder production device.

(23) An example in which a gas atomization process is used is described below with reference to FIG. 3.

(24) A molten metal 1 is transferred from a melting furnace 2 to a container such as a tundish 3 and passed downward from the container into a chamber 9 through a molten-metal-guide nozzle 4 of a gas-atomized metal powder production device 19 in the form of a molten metal stream 8. The inside of the chamber 9 is purged with an inert gas atmosphere by opening an inert gas valve 11.

(25) An inert gas 22a is made to impinge on the molten metal stream 8 through gas injection nozzles 22 disposed in a gas nozzle header 21 to divide the molten metal stream 8 into a metal powder 8a. Injection water 25a is made to impinge on the metal powder 8a at the position at which the temperature of the metal powder 8a is about 1000 C., at which the temperature range in which cooling needs to be performed is preferably achieved to cool the metal powder 8a. The injection water 25a has an injection pressure of 5 MPa or more and a water temperature of 10 C. or less.

(26) Performing cooling with injection water having an injection pressure of 5 MPa or more and a water temperature of 10 C. or less increases the MHF point to about 1000 C. Accordingly, a metal powder that preferably has a temperature of about 1000 C. or less is cooled with injection water having an injection pressure of 5 MPa or more and a water temperature of 10 C. or less. This enables cooling to be performed in the transition boiling region from the beginning of cooling and facilitates cooling the metal powder. As a result, the desired cooling rate may be readily achieved. The temperature of the metal powder can be controlled by changing the distance between the gas atomization point and the position at which the injection water is made to impinge on the metal powder.

(27) When the temperature of the metal powder 8a is as high as more than 1000 C. at the beginning of cooling with the injection water, cooling is performed in the film boiling state even when the water temperature of the injection water is less than 5 C. The cooling capacity of injection water is low compared with cooling performed in the transition boiling state, which occurs when cooling is started at 1000 C. or less, but high compared with an ordinary cooling treatment performed in the film boiling state at an injection pressure of less than 5 MPa and a water temperature of 10 C. or more. In addition, the amount of time during which cooling is performed in the film boiling state can be reduced. Furthermore, reducing the water temperature of the injection water and increasing the injection pressure of the injection water increases the MHF point and enhances the amorphous nature of the metal powder to be produced. For example, setting the water temperature of the injection water to 5 C. or less and the injection pressure of the injection water to 10 MPa or more increases the MHF point to about 1030 C. This enables a metal powder having a large particle diameter to be changed into an amorphous state.

(28) As described above, a molten metal stream is divided by a gas atomization process and subsequently cooled with injection water having an injection pressure of 5 MPa or more and a water temperature of 10 C. or less. Performing water-injection cooling under the above-described conditions when the temperature of the metal powder is the MHF point or less further increases the cooling rate.

(29) Similarly to the above-described case where a water atomization process used, the cooling water used as injection water is preferably stored in the cooling-water tank 15 (heat-insulated structure) disposed outside the gas-atomized metal powder production device 19 after it has been cooled to a low temperature with a heat exchanger such as a chiller 16 capable of cooling the cooling water to a low temperature. Means for feeding ice from an ice-making machine into the tank may optionally be provided. Needless to say that the gas nozzle header 21 connects to a gas bomb 27 with a pipe 28 and that the cooling-water tank 15 is provided with, similarly to the water-atomized metal powder production device, a high-pressure pump 17 that increases the pressure of the cooling water and feeds the cooling water to cooling-water injection nozzles 25 and a pipe 18 through which the cooling water is fed from the high-pressure pump to the cooling-water injection nozzles 25.

(30) To change a metal powder into an amorphous powder, it is necessary to rapidly cool the metal powder in the crystallization temperature range. The critical cooling rate required to produce an amorphous powder varies depending on the type of the alloy system. For example, the critical cooling rate of FeB alloys (Fe.sub.83B.sub.17) is 1.010.sup.6 K/s and the critical cooling rate of FeSiB alloys (Fe.sub.79Si.sub.10B.sub.11) is 1.810.sup.5 K/s (The Japan Society of Mechanical Engineers: Boiling Heat Transfer and Cooling, p. 208, 1989, Japan Industrial Publishing Co., Ltd.). The critical cooling rates required to produce an amorphous powder of typical amorphous alloys such as Fe-base alloys and Ni-base alloys are about 10.sup.5 to 10.sup.6 K/s. The method of producing a metal powder in which performing cooling in the film boiling region is prevented from the beginning of cooling and cooling is performed in the transition boiling region or the nucleate boiling region as in our method, enables the above-described cooling rate to be achieved.

EXAMPLES

Example 1

(31) A metal powder was produced using a water-atomized metal powder production device illustrated in FIG. 2.

(32) A raw material having a composition containing (with the balance being inevitable impurities), by at %, 79% Fe-10% Si-11% B (Fe.sub.79Si.sub.10B.sub.11) was prepared. The raw material was melted in a melting furnace 2 at about 1550 C. Thus, about 50 kgf of a molten metal was prepared. The molten metal was slowly cooled to 1350 C. in the melting furnace 2 and subsequently charged into a tundish 3. The inside of a chamber 9 was purged with a nitrogen gas atmosphere by opening an inert gas valve 11. Before the molten metal was charged into the tundish 3, a high-pressure pump 17 was brought into operation and cooling water stored in a cooling-water tank 15 (volume: 10 m.sup.3) was fed to a nozzle header 5. Thus, injection water (fluid) 7 started being ejected through water injection nozzles 6. The position at which the molten metal stream 8 was brought into contact with the injection water (fluid) 7 was set at a position 200 mm below the molten-metal-guide nozzle 4.

(33) The molten metal 1 charged in the tundish 3 was passed downward into the chamber 9 through the molten-metal-guide nozzle 4 in the form of a molten metal stream 8. The molten metal stream 8 was contacted with injection water (fluids) 7 having various water temperatures and injection pressures as described in Table 1, and was divided into a metal powder and cooled by being mixed with the cooling water. The metal powder was collected through a collection port including a metal powder collection valve 13.

(34) After dust other than the metal powder particles had been removed from the metal powders, a sample was taken from each of the metal powders and subjected to an X-ray diffraction measurement. The crystallization ratio of each sample was determined on the basis of the ratio between the integrated intensities of diffracted X-rays. The amorphous ratio (=1crystallization ratio) of each sample was calculated by subtracting the crystallization ratio from 1. Table 1 summarizes the results. A sample having an amorphous ratio of 90% or more was evaluated as passed. Although some of the metal powders contained compounds as impurities, the contents of the compounds contained as impurities in such metal powders were less than 1% by mass.

(35) TABLE-US-00001 TABLE 1 Division and cooling Fluid injection conditions Amorphous ratio Powder Atomization Type of Injection Water : 90% or more No. method injection fluid pressure (MPa) temperature ( C.) x: Less than 90% Remarks A1 Water Water 5 30 x 74% Comparative atomization Example A2 Water 5 8 92% Our Example A3 Water 1 8 x 82% Comparative Example

(36) The metal powder prepared in our Example had a crystallization ratio of less than 10%. This confirms that the most part of the metal powder was amorphous. On the other hand, the metal powders prepared in the Comparative Examples which did not fall within our range each had a crystallization ratio of 10% or more. This confirms that the metal powders were not amorphous. Since we believe that the critical cooling rate required to change a metal powder having the same alloy composition (Fe.sub.79Si.sub.10B.sub.11) as that of the metal powder used in Example 1 into an amorphous metal powder is 1.810.sup.5 K/s, we believe that a cooling rate of 1.810.sup.5 K/s or more was achieved in our Example.

Example 2

(37) A metal powder was prepared using a gas-atomized metal powder production device illustrated in FIG. 3.

(38) A raw material having a composition containing (with the balance being inevitable impurities), by at %, 79% Fe-10% Si-11% B (Fe.sub.79Si.sub.10B.sub.11) was prepared. The raw material was melted in a melting furnace 2 at about 1550 C. Thus, about 10 kgf of a molten metal was prepared. The molten metal was slowly cooled to 1400 C. in the melting furnace and subsequently charged into a tundish 3. The inside of a chamber 9 was purged with a nitrogen gas atmosphere by opening an inert gas valve 11. Before the molten metal was charged into the tundish 3, a high-pressure pump 17 was brought into operation and cooling water stored in a cooling-water tank 15 (volume: 10 m.sup.3) fed to water injection nozzles 25. Thus, injection water (fluid) 25a started being ejected through the water injection nozzles 25.

(39) The molten metal 1 charged in the tundish 3 was passed downward into the chamber 9 through the molten-metal-guide nozzle 4 in the form of a molten metal stream 8, which was brought into contact with an argon gas (fluid) 22a ejected through gas nozzles 22 at an injection pressure of 5 MPa to be divided into a metal powder 8a. The metal powder was cooled and solidified due to thermal radiation and the action of the atmosphere gas. The metal powder was subsequently cooled with each of injection waters having various injection pressures and water temperatures as described in Table 2 at the time the metal powder had been cooled to about 1000 C., that is, at the position 350 mm (or, 250 mm) below the gas atomization point (the point at which the molten metal stream 8 was brought into contact with the argon gas 22a). The cooled metal powder was collected through a collection port including a metal powder collection valve 13.

(40) After dust other than the metal powder particles had been removed from the metal powders, a sample was taken from each of the metal powders and subjected to an X-ray diffraction measurement. The crystallization ratio of each sample was determined on the basis of the ratio between the integrated intensities of diffracted X-rays. The amorphous ratio (=1crystallization ratio) of each sample was calculated by subtracting the crystallization ratio from 1. Table 2 summarizes the results. A sample having an amorphous ratio of 90% or more was evaluated as passed. Although some of the metal powders contained compounds as impurities, the contents of the compounds contained as impurities in such metal powders were less than 1% by mass.

(41) TABLE-US-00002 TABLE 2 Division Injection Cooling condition Fluid injection conditions Average powder Water Type of Injection Type of Injection Water temperature at injection Amorphous ratio Powder Atomization injection pressure injection pressure temperature the beginning of position* : 90% or more No. method fluid (MPa) fluid (MPa) ( C.) cooling ( C.) (mm) x: Less than 90% Remarks B1 Gas Gas 5 Water 5 30 997 350 x 77% Comparative atomization Example B2 Water 5 8 995 350 92% Our Example B3 Water 1 8 996 350 x 73% Comparative Example B4 Water 20 4 1046 250 94% Our Example *Distance from the gas atomization point

(42) The metal powders prepared in our Examples had a crystallization ratio of less than 10%. This confirms that most parts of the metal powders were amorphous. We also confirmed that most of the powder No. B4, which had been cooled with injection water that fell within our range, was amorphous although the average temperature of the powder at the beginning of cooling was 1046 C. This is because, that the MHF point was increased to about 1050 C. by setting the injection pressure of the injection water to 20 MPa and the water temperature of the injection water to 4 C.

(43) On the other hand, the metal powders prepared in the Comparative Examples which did not fall within our range each had a crystallization ratio of 10% or more. This confirms that the metal powders were not amorphous. Since we believe that the critical cooling rate required to change a metal powder having the same alloy composition (Fe.sub.79Si.sub.10B.sub.11) as that of the metal powder used in Example 2 into an amorphous metal powder is 1.810.sup.5 K/s, we believe that a cooling rate of 1.810.sup.5 K/s or more was achieved in our Examples.

Example 3

(44) A metal powder was prepared using a gas-atomized metal powder production device illustrated in FIG. 3.

(45) A raw material having a composition containing (with the balance being inevitable impurities), by at %, 83% Fe-17% B (Fe.sub.83B.sub.17) was prepared. The raw material was melted in a melting furnace 2 at about 1550 C. Thus, about 10 kgf of a molten metal was prepared. The molten metal was slowly cooled to 1500 C. in the melting furnace and subsequently charged into a tundish 3. The inside of a chamber 9 was purged with a nitrogen gas atmosphere by opening an inert gas valve 11. Before the molten metal was charged into the tundish 3, a high-pressure pump 17 was brought into operation and cooling water stored in a cooling-water tank 15 (volume: 10 m.sup.3) fed to water injection nozzles 25. Thus, injection water (fluid) 25a began being ejected through the water injection nozzles 25.

(46) The molten metal 1 charged in the tundish 3 was passed downward into the chamber 9 through the molten-metal-guide nozzle 4 in the form of a molten metal stream 8, which was brought into contact with an argon gas (fluid) 22a ejected through gas nozzles 22 at an injection pressure of 5 MPa to be divided into a metal powder 8a. The metal powder was cooled and solidified due to thermal radiation and the action of the atmosphere gas. The metal powder was subsequently cooled with injection water having a specific injection pressure and a specific water temperature described in Table 3 at the time the metal powder had been cooled to about 1000 C., that is, at the position 450 mm (or, 250 mm) below the gas atomization point. The metal powder was collected through the metal powder collection valve 13. After dust other than the metal powder particles had been removed from the metal powders, a sample was taken from each of the metal powders and subjected to an X-ray diffraction measurement. The crystallization ratio of each sample was determined on the basis of the ratio between the integrated intensities of diffracted X-rays. The amorphous ratio (=1crystallization ratio) of each sample was calculated by subtracting the crystallization ratio from 1. Table 3 summarizes the results. A sample having an amorphous ratio of 90% or more was evaluated as passed. Although some of the metal powders contained compounds as impurities, the contents of the compounds contained as impurities in such metal powders were less than 1% by mass.

(47) TABLE-US-00003 TABLE 3 Division Injection Cooling condition Fluid injection conditions Average powder Water Type of Injection Type of Injection Water temperature at the injection Amorphous ratio Powder Atomization injection Pressure injection pressure temperature beginning of position* : 90% or more No. method fluid (MPa) fluid (MPa) ( C.) cooling ( C.) (mm) x: Less than 90% Remarks C1 Gas Gas 5 Water 5 30 995 450 x 87% Comparative atomization Example C2 Water 5 8 994 450 93% Our Example C3 Water 1 8 995 450 x 78% Comparative Example C4 Water 20 4 1047 250 95% Our Example *Distance from the gas atomization point

(48) The metal powders prepared in our Examples had a crystallization ratio of less than 10%. This confirms that the most parts of the metal powders were amorphous. We confirmed that most of the powder No. C4, which had been cooled with injection water that fell within our range, was amorphous although the average temperature of the powder at the beginning of cooling was 1047 C. This is because, while the metal powder was cooled, the MHF point was increased to about 1050 C. by setting the injection pressure of the injection water to 20 MPa and the water temperature of the injection water to 4 C.

(49) On the other hand, the metal powders prepared in the Comparative Examples which did not fall within our range each had a crystallization ratio of 10% or more. This confirms that the metal powders were not amorphous. Since we believe that the critical cooling rate required to change a metal powder having the same alloy composition (Fe.sub.83B.sub.17) as that of the metal powder used in Example 3 into an amorphous metal powder is 1.010.sup.6 K/s, we believe that a cooling rate of 1.010.sup.6 K/s or more was achieved in our Examples.