Molten metal temperature control method

Abstract

A molten metal temperature control method includes: with respect to relations among a spheroidization distance traveled by a molten metal of an alloy from a nozzle tip to a position where the molten metal turns into droplets, the temperature of the molten metal inside the crucible, and a pressure acting on the molten metal inside the crucible, obtaining a relation between the temperature and the spheroidization distance at a predetermined pressure, and setting a predetermined temperature range of the temperature; measuring a spheroidization distance when discharging the molten metal from the crucible at the predetermined pressure, and specifying a temperature corresponding to the measured spheroidization distance; and comparing the specified temperature and the predetermined temperature range, and when the specified temperature is outside the predetermined temperature range, controlling the specified temperature so as to be within the predetermined temperature range by adjusting the temperature inside the crucible.

Claims

1. A molten metal temperature control method comprising: (1) with respect to relations among (a) a spheroidization distance traveled by a molten metal of an alloy discharged from a nozzle of a crucible, with a predetermined nozzle diameter, from a nozzle tip to a position where the molten metal turns into droplets, (b) a temperature of the molten metal inside the crucible, and (c) a pressure acting on the molten metal inside the crucible, obtaining in advance a relation between the temperature of the molten metal inside the crucible and the spheroidization distance at a predetermined pressure that is the pressure acting on the molten metal inside the crucible, and setting a predetermined temperature range of the temperature of the molten metal inside the crucible; (2) measuring the spheroidization distance when discharging the molten metal from the crucible at the predetermined pressure, and specifying a temperature corresponding to the measured spheroidization distance; and (3) comparing the specified temperature and the predetermined temperature range, and controlling the specified temperature so as to be within the predetermined temperature range by adjusting the temperature of the molten metal inside the crucible.

2. The molten metal temperature control method according to claim 1, wherein the spheroidization distance is a distance traveled by the molten metal before droplets based on the Plateau-Rayleigh instability theory are formed.

3. The molten metal temperature control method according to claim 1, wherein the molten metal is an alloy used for forming a quenched ribbon that is a material for a rare-earth magnet.

4. The molten metal temperature control method according to claim 3, wherein the quenched ribbon includes an RE-FeB-based main phase, where RE is at least one of Nd and Pr, and a grain boundary phase of an RE-X alloy, where X is a metal element containing no heavy rare-earth element, present around the main phase.

5. The molten metal temperature control method according to claim 4, wherein the RE-X alloy constituting the grain boundary phase is any one type of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa, is or a mixture of at least two of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa.

6. The molten metal temperature control method according to claim 1, wherein the spheroidization distance is measured by an imaging device.

7. The molten metal temperature control method according to claim 6, wherein the imaging device is a charge-coupled device (CCD) camera.

8. The molten metal temperature control method according to claim 1, wherein the adjusting the temperature inside the crucible comprises controlling a high-frequency coil that heats the molten metal in the crucible by induction heating.

9. The molten metal temperature control method according to claim 8, wherein, when the specified temperature is above an upper limit of the predetermined temperature range, the heating with the high-frequency coil is stopped to lower the temperature of the molten metal inside the crucible.

10. The molten metal temperature control method according to claim 9, wherein, after the heating with the high-frequency coil is stopped, the molten metal is discharged to re-measure the spheroidization distance.

11. The molten metal temperature control method according to claim 8, wherein, when the specified temperature is below an upper limit of the predetermined temperature range, the temperature of the molten metal inside the crucible is raised with the high-frequency coil.

12. The molten metal temperature control method according to claim 11, wherein, after the temperature of the molten metal inside the crucible is raised with the high-frequency coil, the molten metal is discharged to re-measure the spheroidization distance.

13. The molten metal temperature control method according to claim 1, wherein a determination unit compares the specified temperature and the predetermined temperature range and determines whether the molten metal temperature is within the predetermined temperature range.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features, advantages, and technical and industrial significance of exemplary embodiments of the present disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

(2) FIG. 1 is a schematic view illustrating a molten metal temperature control method according to one aspect of the present disclosure;

(3) FIG. 2 is a view showing a pressure-versus-spheroidization distance correlation graph;

(4) FIG. 3 is a view showing a temperature-versus-spheroidization distance correlation graph;

(5) FIG. 4 is a view illustrating the molten metal temperature control method based on the temperature-versus-spheroidization distance correlation graph; and

(6) FIG. 5 is a flowchart illustrating the molten metal temperature control method according to one aspect of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) In the following, an embodiment of a molten metal temperature control method of the present disclosure will be described with reference to the drawings.

(8) (Embodiment of Molten Metal Temperature Control Method)

(9) FIG. 1 is a schematic view illustrating the molten metal temperature control method of the present disclosure; FIG. 2 is a view showing a pressure-versus-spheroidization distance correlation graph; FIG. 3 is a view showing a temperature-versus-spheroidization distance correlation graph; and FIG. 4 is a view illustrating the molten metal temperature control method based on the temperature-versus-spheroidization distance correlation graph. FIG. 5 is a flowchart illustrating the molten metal temperature control method of the present disclosure.

(10) As shown in FIG. 1, a crucible 1 having a nozzle 1a with a predetermined diameter provided at the bottom, a high-frequency coil 2 disposed around the crucible 1, and a rotating roll 5 that is disposed under the nozzle 1a and quenches droplets of a molten metal falling thereon are disposed inside a chamber 10, and a quenched ribbon that is a material for a rare-earth magnet is manufactured inside the chamber 10 by the melt spinning method.

(11) As the high-frequency coil 2 is activated, an alloy used for forming a quenched ribbon that is a material for a rare-earth magnet is melted by high-frequency induction heating and the molten metal Y is generated inside the crucible 1. The inside of the chamber 10 is kept at a reduced pressure not higher than 50 Pa, for example, while the inside of the crucible 1 is placed in an Argas atmosphere. The molten metal Y is pressed with an Ar gas at a pressure P not higher than 100 kPa, for example, to discharge the molten metal Y downward (in the X-direction) through the nozzle 1a.

(12) The molten metal Y having been discharged downward from the nozzle 1a first stretches in the form of a stream over a predetermined spheroidization distance Lc, and turns into droplets down beyond the spheroidization distance Lc. These droplets fall on the top of the copper rotating roll 5 that is rotating (in the Z-direction), where the droplets are quenched and a quenched ribbon resulting from quenching is jetted in a direction tangential to the top of the rotating roll 5. The spheroidization distance Lc refers to a distance traveled by the molten metal before droplets, based on the Plateau-Rayleigh instability theory, are formed.

(13) Here, the quenched ribbon is composed of an RE-FeB-based main phase (RE: at least one of Nd and Pr) and an RE-X alloy (X: a metal element containing no heavy rare-earth element) present around the main phase, and in the case of a nanocrystal structure, for example, the quenched ribbon is composed of a main phase of crystal grains not larger than 200 nm.

(14) The NdX alloy constituting the grain boundary phase is an alloy composed of Nd and at least one of Co, Fe, Ga, Cu, Al, etc., and, for example, composed of any one kind of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa, or is a mixture of at least two of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa.

(15) Although not shown, a flow passage is disposed inside the chamber 10 in a direction in which the quenched ribbon is jetted, and the jetted quenched ribbon passes through the flow passage and is collected in a collection box.

(16) To measure the spheroidization distance Lc of the molten metal Y under the crucible 1, an imaging device 3, such as a charge-coupled device (CCD) camera, is disposed at a position obliquely under the crucible 1, and image data is transmitted to a computer 4 by wired or wireless transmission.

(17) Here, the present inventors have found that there is a linear correlation between the spheroidization distance Lc and a pressure acting on the molten metal Y inside the crucible 1 (pressure P in FIG. 1). FIG. 2 shows the pressure-versus-spheroidization distance correlation graph in three cases where the temperature of the molten metal Y is respectively 1300 C., 1400 C., and 1500 C. and the nozzle diameter is the predetermined diameter . On the creation of the graph, the pressure-versus-spheroidization distance correlation graph at each temperature is obtained on the following conditions: the vacuum degree inside the chamber 10 is not higher than 50 Pa; the Ar-gas pressure inside the crucible 1 is within the range of 0 to 100 kPa; the nozzle diameter is 0.6 to 1.0 mm; and the alloy weight is 4 kg.

(18) FIG. 3 shows the temperature-versus-spheroidization distance correlation graph created on the basis of FIG. 2. From the temperature-versus-spheroidization distance correlation graph shown in FIG. 3 specified by the present inventors, it can be seen that the spheroidization distance increases with the increasing temperature, and that the correlation graph is a curved graph that reaches an inflection point at around 1400 C.

(19) It can also be seen that the spheroidization distance increases as the pressure acting on the molten metal Y inside the crucible 1 decreases.

(20) FIG. 4 is a view schematically showing the temperature-versus-spheroidization distance correlation graph of FIG. 3, and illustrating the molten metal temperature control method of the present disclosure.

(21) The molten metal temperature control method of the present disclosure involves measuring the spheroidization distance of the molten metal instead of the temperature and the viscosity of the molten metal that are difficult to directly measure, plotting the measured spheroidization distance on the temperature-versus-spheroidization distance correlation graph shown in FIG. 4, specifying the temperature corresponding to that spheroidization distance, and controlling the specified temperature so as to be within a preset proper temperature range (predetermined temperature range).

(22) The molten metal temperature control method further involves examining in advance various temperatures of the molten metal Y inside the crucible 1 (and the spheroidization distances Lc corresponding to the respective temperatures of the molten metal Y) and the quality of a quenched ribbon produced from the molten metal Y at the respective temperatures, and setting, as the proper temperature range, an optimal temperature range of the molten metal Y within which a quenched ribbon of desired quality is produced.

(23) In FIG. 4, the lower limit and the upper limit of the proper temperature range of the molten metal Y are Ta C. and Tb C., respectively, and the spheroidization distances corresponding to the temperatures Ta and Tb are La cm and Lb cm, respectively.

(24) If a temperature Tc corresponding to the measured spheroidization distance Lc is within the proper temperature range of Ta to Tb, it is regarded that a quenched ribbon of desired quality can be produced, and control is performed so as to maintain the temperature of the molten metal Y inside the crucible 1 as it is.

(25) On the other hand, if the temperature Tc corresponding to the measured spheroidization distance Lc is below the lower limit Ta C., control is executed so as to raise the temperature of the molten metal Y inside the crucible 1 by further heating the crucible 1 with the high-frequency coil 2, and control is executed such that the temperature Tc corresponding to the measured spheroidization distance Lc falls within the proper temperature range of Ta to Tb.

(26) Conversely, if the temperature Tc corresponding to the measured spheroidization distance Lc is above the upper limit Tb C., control is executed so as to lower the temperature of the molten metal Y inside the crucible 1 by stopping the heating of the crucible 1 with the high-frequency coil 2, or cooling the crucible 1 in addition to stopping the heating, and control is executed such that the temperature Tc corresponding to the measured spheroidization distance Lc falls within the proper temperature range of Ta to Tb.

(27) Inside the computer 4 shown in FIG. 1, the temperature-versus-spheroidization distance correlation graphs corresponding to various pressures are stored. Data on the spheroidization distance Lc imaged by the imaging device 3 is transmitted to the computer 4, and the spheroidization distance Lc is plotted on the temperature-versus-spheroidization distance correlation graph inside the computer 4.

(28) Then, the temperature Tc corresponding to that spheroidization distance Lc is specified, and it is determined whether or not the specified temperature Tc is within the proper temperature range of Ta to Tb.

(29) Here, the molten metal temperature control method of the present disclosure will be described with reference to the flowchart of FIG. 5.

(30) First, a temperature-versus-spheroidization distance correlation graph is created for each of various pressures that can be set inside the crucible 1 having the nozzle with the predetermined diameter , and a proper temperature range of the molten metal Y is set in each correlation graph (step S1) (the end of a first step of the molten metal temperature control method). Since the temperature-versus-spheroidization distance correlation graph varies with different nozzle diameters , if there are a plurality of crucibles 1 with different nozzle diameters, the temperature-versus-spheroidization distance correlation graphs for the respective pressures are created for each crucible 1.

(31) Next, the heating conditions of the molten metal Y inside the crucible 1 are set (step S2). In this step of setting the heating conditions, it is preferable that the heating conditions are set such that the temperature of the molten metal Y discharged from the nozzle 1a falls within the set proper temperature range of Ta to Tb. However, the initial heating conditions do not have to be set exactly. This is because, as will be described later, if the specified temperature of the molten metal Y is not within the proper temperature range of Ta to Tb, measures are taken to bring the specified temperature into the proper temperature range of Ta to Tb by executing the control of raising or lowering the temperature of the molten metal Y inside the crucible 1.

(32) When the heating conditions have been set, the heating of the crucible 1 and the molten metal Y inside the crucible 1 with the high-frequency coil 2 is started (step S3).

(33) Prior to the start of heating, or after the start of heating, the inside of the chamber 10 is depressurized and the inside of the crucible 1 is placed in an Ar-gas atmosphere, and the pressure of the Ar gas, i.e., the pressure P acting on the molten metal Y (discharge pressure) is set (step S4). Then, the discharge of the molten metal Y from the nozzle 1a is started (step S6).

(34) A temperature-versus-spheroidization distance correlation graph corresponding to the set discharge pressure is selected (step S5), and the molten metal temperature is controlled on the basis of the selected temperature-versus-spheroidization distance correlation graph.

(35) After the discharge of the molten metal Y is started, the spheroidization distance Lc of the molten metal Y is measured (step S7). The measured spheroidization distance Lc is transmitted to the computer 4, and the spheroidization distance Lc is plotted on the temperature-versus-spheroidization distance correlation graph already selected inside the computer 4, and the molten metal temperature Tc corresponding to the spheroidization distance Lc is specified (step S8) (the end of a second step of the molten metal temperature control method).

(36) It is examined inside the computer 4 whether or not the specified molten metal temperature Tc is within the proper temperature range of Ta to Tb (step S9).

(37) Although not shown, a determination unit, a central processing unit (CPU) comprising a microprocessor or the like, a RAM, a ROM, a correlation graph storage unit, etc. are connected with one another through buses inside the computer 4, and the determination unit determines whether or not the molten metal temperature Tc is within the proper temperature range of Ta to Tb.

(38) If the molten metal temperature Tc is within the proper temperature range of Ta to Tb, no change is made to the conditions, such as the heating conditions and the pressure condition of the Ar gas (step S10), and the discharge of the molten metal Y onto the rotating roll 5 is continued with the current temperature of the molten metal Y maintained. Then, a quenched ribbon formed by the molten metal Y being quenched on the surface of the rotating roll 5 is selected as the material for the rare-earth magnet (step S11).

(39) On the other hand, if the molten metal temperature Tc is lower than the lower limit Ta of the proper temperature range of Ta to Tb (step S12), the molten metal temperature inside the crucible 1 is raised with the high-frequency coil 2 (step S13), and the molten metal Y is discharged to re-measure the spheroidization distance Lc (step S7).

(40) The above steps are repeated until the molten metal temperature Tc corresponding to the re-measured spheroidization distance Lc falls within the proper temperature range of Ta to Tb, and at a point when the temperature of the molten metal Y falls within the proper temperature range of Ta to Tb, the temperature of the molten metal Y is maintained and the discharge of the molten metal Y onto the rotating roll 5 is continued.

(41) If the molten metal temperature Tc is higher than the upper limit Tb of the proper temperature range of Ta to Tb (step S14), heating with the high-frequency coil 2 is stopped to lower the molten metal temperature inside the crucible 1 (step S15), and the molten metal Y is discharged to re-measure the spheroidization distance Lc (step S7).

(42) In this case, too, the above steps are repeated until the molten metal temperature Tc corresponding to the re-measured spheroidization distance Lc falls within the proper temperature range of Ta to Tb, and at a point when the temperature of the molten metal Y falls within the proper temperature range of Ta to Tb, the temperature of the molten metal Y is maintained and the discharge of the molten metal Y onto the rotating roll 5 is continued (the end of a third step of the molten metal temperature control method).

(43) According to the shown molten metal temperature control method, it is possible to accurately specify the temperature of the molten metal that is difficult to directly measure, and obtain a quenched ribbon of desired quality by controlling the specified temperature so as to be within the proper temperature range.

(44) While the embodiment of the present disclosure has been described in detail using the drawings, the specific configuration is not limited to that of the embodiment, and any design changes etc. made within the scope of the present disclosure shall be included in the disclosure.