HIGH FREQUENCY INDUCTION HEATING METHOD

Abstract

A high frequency induction heating method includes: providing a film containing a component, which melts at a preset heating temperature, on a surface of a workpiece before heating the workpiece by high frequency induction heating using a high-frequency coil; and heating the workpiece by high frequency induction heating.

Claims

1. A high frequency induction heating method comprising: providing a film containing a component, which melts at a preset heating temperature, on a surface of a workpiece before heating the workpiece by high frequency induction heating using a high-frequency coil; and heating the workpiece by high frequency induction heating.

2. The high frequency induction heating method according to claim 1, wherein the workpiece is a sintered compact which is a rare earth magnet precursor, and the sintered compact is heated by high frequency induction heating while performing hot working on the sintered compact.

3. The high frequency induction heating method according to claim 1, wherein the film is formed of a graphite lubricating liquid and a melting component which is contained in the film.

4. The high frequency induction heating method according to claim 3, wherein the film is formed by applying a solution, which is obtained by adding the melting component to the graphite lubricating liquid, to the surface of the workpiece and drying the solution.

5. The high frequency induction heating method according to claim 1, wherein the workpiece has a NdFeB-based main phase with a nanocrystalline structure and a grain boundary phase of a NdX alloy, where X: metal element, the grain boundary phase being present around the main phase.

6. The high frequency induction heating method according to claim 5, wherein the NdX alloy constituting the grain boundary phase is any one of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa, or is a mixture of at least two of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa; and the NdX alloy is in a Nd rich state.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

[0034] FIG. 1 is a diagram showing a state where a film is provided on a surface of a sintered compact which is a workpiece;

[0035] FIG. 2 is a diagram showing a state where a sintered compact is about to be inserted into a high-frequency coil;

[0036] FIG. 3 is a diagram showing a state where the heated sintered compact is about to be transferred to a forming die;

[0037] FIG. 4A is a diagram showing the results of Comparative Example in an experiment for verifying an effect of a film containing a component, which melts at a preset heating temperature, suppressing an excessive increase in the temperature of a workpiece; and

[0038] FIG. 4B is a diagram showing the results of Example in the experiment for verifying an effect of a film containing a component, which melts at a preset heating temperature, suppressing an excessive increase in the temperature of a workpiece.

DETAILED DESCRIPTION OF EMBODIMENTS

[0039] Hereinafter, an embodiment of a high frequency induction heating method according to the invention will be described with reference to the drawings. A workpiece in an example shown in the drawing is a sintered compact which is a rare earth magnet precursor. However, of course, the workpiece is not limited to a sintered compact.

[0040] (Embodiment of High Frequency Induction Heating Method)

[0041] FIG. 1 is a diagram showing a state where a film is provided on a surface of a sintered compact which is a workpiece, and FIG. 2 is a state where a sintered compact is about to be inserted into a high-frequency coil. FIG. 3 is a diagram showing a state where a heated sintered compact is transferred to a forming die.

[0042] First, as shown in FIG. 1, a film 2 is formed on a surface of a sintered compact 1 (workpiece) which is a rare earth magnet precursor.

[0043] Here, the sintered compact 1 is manufactured by press-forming magnet powder in a forming die (not shown) in a high-temperature atmosphere of about 700 C. In a method of preparing the magnet powder, in a furnace (not shown) in which the pressure is reduced to 50 kPa or lower, an alloy ingot is melted by high-frequency induction heating using a single-roll melt spinning method, and molten metal having a composition of a rare earth magnet is injected to a copper roll to prepare a rapidly-quenched ribbon. Next, this rapidly-quenched ribbon is crushed to prepare a magnet powder. The grain size range of the magnet powder is adjusted to be in a range of 75 m to 300 m.

[0044] The sintered compact 1 has: a NdFeB-based main phase with a nanocrystalline structure (having an average grain size of about 300 nm or less, for example, a grain size of about 50 nm to 200 nm); and a grain boundary phase of a NdX alloy (X: metal element) present around the main phase. The NdX alloy constituting the grain boundary phase is an alloy of Nd and at least one of Co, Fe, Ga, and the like and is in a Nd-rich state. For example, any one of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa, or a mixture of at least two of NdCo, NdFe, NdGa, NdCoFe, and NdCoFeGa may be used.

[0045] The film 2 formed on the surface of the sintered compact 1 is formed of a graphite lubricating liquid and a melting component which is contained in the film 2.

[0046] This melting component refers to a component which melts at a preset heating temperature for the surface of the sintered compact 1 when the sintered compact 1 is heated by high frequency induction heating using a high-frequency coil Co shown in FIG. 2, that is, before the high frequency induction heating.

[0047] For example, when a rare earth magnet is manufactured by performing hot working on the sintered compact 1, the heating temperature before the hot working is set to be about 600 C. to 900 C. Accordingly, in a case where the preset temperature for the surface of the sintered compact 1 is 670 C., iron (II) chloride tetrahydrate (FeCl.sub.24H.sub.2O) having a melting point of 670 C. may be used as the melting component. In a case where the preset temperature is 770 C., potassium chloride (KCl) having a melting point of 770 C. may be used as the melting component.

[0048] As shown in FIG. 2, the sintered compact 1 having a surface on which the film 2 is provided is put into the high-frequency coil Co (X1 direction) and is heated by high frequency induction heating for a predetermined amount of time.

[0049] During this high frequency induction heating, on the surface of the sintered compact 1, a temperature distribution is likely to be generated depending on the position of the sintered compact 1 in the high-frequency coil Co, and a region having a higher temperature than in the other portions of the sintered compact 1 may be generated. Even in this case, once the temperature of the high-temperature region reaches a preset heating temperature, a temperature increase is suppressed for a predetermined amount of time by the melting of the component in the film 2 (melting component). As a result, the entire region on the surface of the sintered compact 1 can be uniformly heated, the surface temperature in the entire region can be controlled to be a preset heating temperature, and an excessive increase in the temperature of the entire or a partial region of the sintered compact 1 is suppressed.

[0050] Once the entire region of the sintered compact 1 is uniformly heated to a preset temperature by a predetermined amount of time of high frequency induction heating, the heated sintered compact 1 is transferred to a cavity C of the forming die M.

[0051] The forming die M includes: a die D; and an upper punch Pu and a lower punch Ps that slide on the inside of the die D. The cavity C is formed by the die D, the upper punch Pu, and the lower punch Ps.

[0052] The heated sintered compact 1 is put into the cavity C (X2 direction), and the sintered compact 1 is forged by being compressed by the upper punch Pu and the lower punch Ps. As a result, magnetic anisotropy is imparted to the sintered compact 1, and a rare earth magnet (not shown) is manufactured.

[0053] The entire region of the sintered compact 1 before the hot working is uniformly heated to a preset heating temperature by the high frequency induction heating method shown in the drawing, and an excessively heated portion is not present. Therefore, a rare earth magnet having satisfactory magnetic characteristics such as residual magnetization or coercive force is manufactured.

[0054] (Experiment for Verifying Effect of Film Containing Component, which Melts at Preset Heating Temperature, Suppressing Excessive Increase in Temperature of Workpiece, and Results Thereof)

[0055] The present inventors performed an experiment for verifying an effect of a film containing a component, which melts at a preset heating temperature, suppressing an excessive increase in the temperature of a workpiece.

[0056] First, a sintered compact was used as the workpiece, and a preset heating temperature of a surface of the sintered compact was set as 800 C. Next, NaCl (1.0 g; melting point: 800 C.) was mixed with 0.1 g of a graphite lubricating liquid (PROHYTE 15FU, manufactured by Nippon Graphite Industries, Ltd.), and this mixture was applied to a surface of the sintered compact and was sufficiently dried. As a result, a film having a thickness of 50 m to 100 m was formed. Heat absorption of 483 kJ can be expected from 1 g of melted NaCl.

[0057] The sintered compact having a surface on which the film was formed was heated using a high frequency induction heating device. At this time, a thermocouple was provided by welding on the surface of the sintered compact to measure the temperature (hereinabove, Example).

[0058] Here, the specific heat of the sintered compact was 410 J/kg.Math.K, the size of the sintered compact was 7.2 mm28.2 mm18.9 mm, the density of the sintered compact was 7.6 g/cm.sup.3, the amount of heat required to increase the temperature of the sintered compact by 1 C. was 11.96 J, and when all of 1 g of NaCl melts, an effect of suppressing an excessive temperature increase at 40.38 C. (483 J/11.96 (J/K)) can be expected.

[0059] On the other hand, as Comparative Example, a sintered compact having a surface on which a film consisting of only a graphite lubricating liquid without adding NaCl was formed was prepared. At this time, as in the case of Example, a thermocouple was provided by welding on the surface of the sintered compact to measure the temperature.

[0060] TYPE 3 (manufactured by Yutaka Electronics Industry Co., Ltd.) was used as a heating device, and heating conditions were 10 kHz, 50 A, and 75 seconds.

[0061] Regarding the measurement results, FIG. 4A shows the measurement results of Comparative Example, and FIG. 4B shows the measurement results of Example.

[0062] It was found from FIG. 4A that, in the sintered compact of Comparative Example, the surface temperature increased over time and was excessively increased to exceed 800 C. which was a preset heating temperature.

[0063] On the other hand, it was found from FIG. 4B that, in the sintered compact of Example, the surface temperature increased over time; however, a temperature increase was stopped at 800 C. which was the melting point of NaCl, the temperature was stable at 800 C. for about nine seconds, and thus an excessive temperature increase was suppressed. This result was derived from an endothermic reaction of NaCl, and the effect of the film containing a component, which was melted at a preset heating temperature, was able to be verified. That is, as shown in FIG. 4B, it was found that, in a time period from 54 seconds to 63 seconds, heat was absorbed by the melting of NaCl, and the temperature was stable. The period of time in which a temperature increase can be suppressed depends on the high frequency output (temperature increase rate).

[0064] Next, Table 1 below shows the list of components to be contained in a film and melting point thereof.

TABLE-US-00001 TABLE 1 Melting Compound Name Formula Point ( C.) Tin (II) Chloride SnCl.sub.2 246 Sodium Nitrite NaNO.sub.2 271 Zinc Chloride ZnCl.sub.2 293 Zirconium (III) Chloride ZrCl.sub.3 330 Potassium Nitrite KNO.sub.2 350 Potassium Nitrate KNO.sub.3 400 Ammonium Chloride NH.sub.4Cl 520 Iron (II) Chloride Tetrahydrate FeCl.sub.24H.sub.2O 670 Potassium Chloride KCl 770 Calcium Chloride CaCl.sub.2 782 Sodium Carbonate Na.sub.2CO.sub.3 851

[0065] A component to be contained in a film can be appropriately selected from the list of Table 1 according to the preset heating temperature for the surface of the workpiece. By selecting two or more components from the list of Table 1 and forming a film containing the components, the temperature can be controlled in various ways.

[0066] Hereinabove, the embodiments of the present invention have been described with reference to the drawings. However, a specific configuration is not limited to the embodiments, and design changes and the like which are made within a range not departing from the scope of the invention are included in the present invention.