JP2006122922A - Induction heating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the heating cycle properties of a metal billet and to prevent the reduction in the mechanical properties of the stock by uniformly performing induction heating for the metal billet in a short time. <P>SOLUTION: In the induction heating method, at the time when a metal billet used for a semi-solid casting process is subjected to induction heating using a solenoid coil, the frequency of the electric current applied to the solenoid coil is changed in a heating stage. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for uniformly induction heating a metal billet used in a semi-molten and semi-solid cast method.

  The casting material is heated to prepare a semi-molten / semi-solid cast material in which the solid phase component and liquid phase component coexist, and then the semi-molten / semi-solid cast material is stirred through the nozzle of the container. There are known semi-molten and semi-solid casting methods in which a mold is cast or a mold cavity is pressurized and filled. This casting method is also called a thixocasting method or a rheocasting method. An alloy metal in a semi-molten and semi-solid state has a certain degree of fluidity, less entrainment of gas during molding, and uniform crystal grains, so that the mechanical properties of the cast product can be improved.

  In the semi-molten / semi-solid cast method, a cylindrical metal billet is accommodated in a solenoid coil and the metal billet is induction-heated. After heating to a predetermined temperature to obtain a semi-molten metal billet, the semi-molten metal billet is supplied to an injection sleeve of a die casting apparatus to form a molded product.

  By the way, the metal billet is heated by supplying a current to the solenoid coil via a high-frequency AC power supply in a state where the metal billet is disposed in the solenoid coil constituting the induction heater. In that case, in order to heat a metal billet to a desired solid-phase rate, considerable time is required and there exists a problem that cycle time becomes long.

  Therefore, it is considered to reduce the induction heating time of the metal billet by applying a high voltage to the induction coil. However, if the metal billet is heated rapidly, the heating balance is lost, and it becomes difficult to uniformly heat the entire metal billet.

In Patent Document 1 below, for the purpose of uniformly heating the entire metal billet in a short time, first, in the initial heating stage, the metal billet is heated by the first high-frequency voltage, and the metal billet reaches a predetermined heating state. After that, it is disclosed that the metal billet is heated by a second high frequency voltage lower than the first high frequency voltage.
JP 2002-146436 A

  In the heating method described in Patent Document 1, in which the temperature in the metal billet is made uniform in the later stage of heating by lowering the input voltage in the later stage from the initial stage of the heat treatment, the center part of the metal billet rises due to heat conduction from the outer peripheral part. Only warm can be expected. For this reason, when it is going to shorten heating time by raising input electric power, a metal billet outer peripheral part becomes high temperature compared with a center part, and time is required in order to equalize in the latter stage of heating. Moreover, even if the input power is lowered, the outer periphery of the metal billet is heated most, so the outer periphery of the metal billet is melted and eutectic composition elution occurs, the structure of the metal billet becomes uneven, It causes the mechanical properties of the material to deteriorate.

  Therefore, an object of the present invention is to perform induction heating of a metal billet uniformly and in a short time to prevent an improvement in heat cycleability and a decrease in mechanical properties of a material after molding.

  The present inventors have found that the penetration depth of the eddy current for induction heating of the metal billet varies with the frequency of the heating current, and reached the present invention.

  That is, the present invention is an invention of a method of induction heating a metal billet used in a semi-molten casting method using a solenoid coil, and is characterized in that the frequency of the current applied to the solenoid coil is changed in the heating stage.

As a specific method of changing the frequency of the current applied to the solenoid coil in the heating stage,
(1) Whether the current applied to the solenoid coil is set to a low frequency in the initial heating stage, and the metal billet is heated to the target temperature by switching to a high frequency after the central portion of the metal billet rises to a predetermined temperature. ,
(2) The current applied to the solenoid coil is switched at any time between a low frequency and a high frequency, and the central portion and the outer peripheral portion of the metal billet are simultaneously heated to a target temperature.
Is mentioned. Here, the low frequency is 500 Hz or less, and the high frequency is 1 kHz or more.

  In the induction heating method of the above (1), the first heating device having a low frequency applied to the solenoid coil and the second heating device having a high frequency applied to the solenoid coil are arranged in series. After raising the center part of a metal billet to predetermined temperature with 1 heating apparatus, heating the outer peripheral part of this metal billet to target temperature with a 2nd heating apparatus is preferable since productivity of metal billet heating is improved.

  As described above, in the present invention, since the frequency of the applied current is changed in the heating stage, the center part of the metal billet is heated in advance, and the outer peripheral part is heated in the final stage. Being heated, the temperature difference in the metal billet can be maintained in an optimum state. As a result, the heating time of the metal billet can be effectively shortened and the cycle time can be reliably shortened, and the entire metal billet can be reliably heated to the target heating state.

  The material of the metal billet to which the induction heating method of the present invention is applied is not particularly limited as long as it is a casting material used in the semi-molten / semi-solid casting method, and aluminum or an alloy thereof, or a magnesium alloy, a zinc alloy, copper or an alloy thereof. Metals such as alloys and iron-based alloys can be exemplified. Among these, an aluminum alloy or a magnesium alloy is preferably exemplified.

  If the temperature of the outer peripheral portion of the metal billet is excessively higher than that of the central portion, the specific heat changes rapidly, so that the aluminum alloy constituting the metal billet is eluted and the temperature of the entire metal billet cannot be made uniform. In the present invention, by changing the frequency of the applied current in the heating stage, the temperature of the entire metal billet can be made uniform and heated in a short time.

  When the frequency of induction heating is constant, the position of the generated eddy current is constant and is the outermost surface of the billet, and heat is transferred from the billet surface toward the center. For this reason, the temperature of the outer peripheral portion rises compared to the central portion, the temperature distribution in the billet becomes non-uniform, and it becomes easy to melt out.

  Further, the penetration depth t of eddy current can be expressed by the following equation, and changes with the heating frequency f. Here, ρ is the resistivity, μ is the relative magnetic constant, and both are determined by the object to be heated.

  In the present invention, focusing on the above formula, the frequency of the applied current is changed in the heating stage, thereby achieving temperature control of the central portion and the outer peripheral portion of the metal billet.

  FIG. 1 shows an embodiment of the present invention, in which an applied current to a solenoid coil is set to a low frequency in the initial heating stage, and the metal billet is switched to a high frequency after the central portion of the metal billet rises to a predetermined temperature. The temperature rise of the center part and outer peripheral part of a metal billet when heating up to target temperature is shown typically. In the low-frequency heating stage, the billet center is higher in temperature than the outer periphery, and both temperatures approach in the high-frequency heating stage.

  FIG. 2 shows another embodiment of the present invention, in which the current applied to the solenoid coil is switched between a low frequency and a high frequency as needed, and the center and outer periphery of the metal billet are heated to the target temperature at the same time. The temperature rise of a billet is typically shown. The central portion and the outer peripheral portion of the metal billet have substantially the same temperature rise. Although depending on the size of the metal billet, the low frequency heating time in this case is preferably 20 seconds or less, more preferably 10 seconds or less, and the high and low frequency heating time is preferably 30 seconds or more, more preferably 40 seconds or more.

Example 1
A cylindrical aluminum alloy having a diameter of 50 mm and a length of 400 mm (components: Si: 7%, FeO: 1%, Mg: 0.55%, remaining Al) was heated at a billet temperature target of 585 ° C. In the initial stage of heating, induction heating was performed at 100 Hz and 200 kW · second for 10 seconds, and then induction heating was performed at 5 kHz and 800 kW · second for 50 seconds. In total, the entire metal billet could be heated uniformly to a target temperature of 585 ± 2 ° C. in 60 seconds. In general, the molding cycle of the semi-molten casting method is about 60 seconds, and the heating time of 60 seconds is much shorter than the conventional heating time, and matches the molding cycle of the semi-molten casting method. As a result, the productivity of the semi-molten casting method can be improved.

(Example 2)
The metal billet was induction heated in the same manner as in Example 1 except that 5 kHz and 100 Hz were switched every 2 seconds and induction heating was performed for 60 seconds. The entire metal billet could be uniformly heated to the target temperature: 585 ± 2 ° C.

  By changing the frequency of the applied current in the heating stage, the metal billet can be heated uniformly in a short cycle. This contributes to more practical use of the semi-molten casting method.

The center of the metal billet when the current applied to the solenoid coil is set to a low frequency in the initial heating stage, and the metal billet is heated to the target temperature by switching to a high frequency after the center of the metal billet rises to a predetermined temperature. The figure which shows the temperature rise of a part and an outer peripheral part typically. The figure which shows typically the temperature rise of a metal billet when the applied current to a solenoid coil is switched to a low frequency and a high frequency at any time, and the center part and outer peripheral part of a metal billet are heated to target temperature simultaneously.

Claims (6)

  An induction heating method characterized in that when a metal billet used in a semi-molten casting method is induction heated using a solenoid coil, the frequency of an electric current applied to the solenoid coil is changed in a heating stage.   The current applied to the solenoid coil is set to a low frequency in the initial heating stage, and the outer peripheral portion of the metal billet is heated to a target temperature by switching to a high frequency after the central portion of the metal billet rises to a predetermined temperature. The induction heating method according to claim 1.   A first heating device having a low frequency applied current to the solenoid coil and a second heating device having a high frequency applied current to the solenoid coil are arranged in series, and the first heating device uses a metal billet. 3. The induction heating method according to claim 2, wherein after the central portion is raised to a predetermined temperature, the outer peripheral portion of the metal billet is heated to a target temperature by the second heating device.   2. The induction heating method according to claim 1, wherein the current applied to the solenoid coil is switched at any time between a low frequency and a high frequency, and the central portion and the outer peripheral portion of the metal billet are simultaneously heated to a target temperature.   The induction heating method according to any one of claims 2 to 4, wherein the low frequency is 500 Hz or less and the high frequency is 1 kHz or more.   The induction heating method according to claim 1, wherein the metal billet is made of an aluminum alloy or a magnesium alloy.

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