JP4575753B2 - Induction heating method - Google Patents

Description

  The present invention relates to a method for preventing breakage when induction heating a metal billet used in a semi-molten casting 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 casting 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-solid metal billet, the semi-solid metal billet is supplied to an injection sleeve of a die casting apparatus to form a molded product.

  In many semi-molten castings, the billet is heated by induction heating at a high frequency or low frequency, but the heating time is from several minutes to several tens of minutes. For example, 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 is in a predetermined heating state. It is disclosed that the metal billet is heated by the second high-frequency voltage lower than the first high-frequency voltage after reaching the above.

  Shortening the heating time and improving productivity is effective in reducing production costs. However, if the induction heating output is increased with only the billet placed in the conventional induction coil, it is affected by electromagnetic induction. The current generated in the billet, which is a heated object, increases, and the force acting between the induction coil and the billet increases accordingly.

  When this force is applied in a state where a part of the heated billet is melted, a phenomenon occurs in which a part of the billet in the middle of heating is broken and scattered by being divided at the melted crystal grain boundary. Such a phenomenon does not occur when the heating is performed slowly as in the conventional case because the force is small. In addition, this crushing and scattering phenomenon is likely to occur at a corner portion where magnetic flux is easily collected and is easily crushed by an external force. Suppressing this phenomenon is essential for shortening the heating time.

  FIG. 2 is a conceptual diagram for explaining a state in which the billet is heated using the induction heating coil. The billet 12 is disposed in the induction heating coil 11 and is induction heated by an alternating magnetic field 13 generated by the induction heating coil 11. FIG. 2 also shows the distribution of the surface temperature in the length direction of the billet 12 (coil axial direction).

  In the case of FIG. 2, the induction heating coil 11 is too long compared to the billet 12. In this case, the edge part of the end face of the billet 12 (the edge part of the cylindrical end part) is excessively heated due to a phenomenon called the edge effect. The edge effect means that Joule heating due to the skin effect is performed on both the outer surface of the billet (the outer surface of the cylinder) and the vicinity of the end surface. It is a phenomenon that increases.

  In addition, for the purpose of improving the quality of the cast product, a high-purity material is used as the material constituting the billet. However, when the purity of the billet constituent material is increased, in the billet manufacturing process, it is close to the surface of the cylindrical shape. Impurities are precipitated, and a phenomenon that the impurity concentration in the central portion is reduced appears. As a result, when the billet is viewed in cross section, the melting point of the outer peripheral portion is slightly lower than the melting point near the center. Due to the synergistic effect of this phenomenon and the above-described edge effect, the phenomenon that the edge portion of the billet end face is likely to be melted in advance becomes more prominent.

  Excessive heating of the end face edge portion of the billet 12 causes inconveniences such as leakage of molten metal due to the progress of melting of the portion and deformation of the billet. Moreover, partial melting also causes non-uniformity in the material of the cast product.

  Excessive heating of the edge portion of the billet end face, which is undesirable due to the edge effect described above, is particularly noticeable when the billet is heated in a short time such as several tens of seconds. When the rate of temperature rise is slow, the temperature rises basically in a thermal equilibrium state, so excessive heating at the edge portion of the billet end face is not so noticeable and does not matter so much.

  In order to prevent excessive heating of the edge portion of the billet end face described above, the length of the induction heating coil 11 with respect to the billet 12 is adjusted to an appropriate dimension, and the magnetic flux density of the alternating magnetic field near the end face of the billet 12 is set to an appropriate value. It needs to be adjusted. However, if the length of the induction heating coil becomes too short relative to the billet, the density of the alternating magnetic flux near the end surface of the billet 12 becomes too small, and conversely the heating at the edge portion of the end surface of the billet 12 is insufficient. There is a fear.

JP 2002-146436 A

  Therefore, the present invention suppresses the phenomenon in which the metal billet is crushed and scattered at the end of the metal billet when the metal billet is induction-heated in a short time, and heats the entire metal billet in a short time. And it aims at improving heat cycle nature.

  The inventors of the present invention have arrived at the present invention after obtaining a hint that the phenomenon that the metal billet is crushed and scattered during induction heating occurs at the end thereof.

  That is, the present invention is an invention of a method in which a metal billet used in a semi-molten casting method is induction-heated using a solenoid coil, and is characterized in that protective caps are placed on the outer peripheral portions of both end faces of the metal billet.

  When induction heating is performed by placing a metal billet on a pedestal, the protective cap is preferably covered only on the upper portion where the metal billet is not protected by the pedestal.

As the material of the protective cap, a non-permeable and heat-resistant material is used. Among these, a heat resistant ceramic is preferably exemplified. Examples of ceramics include nitride ceramics, oxide ceramics, and carbide ceramics. Examples of nitride ceramics include titanium nitride TiN, aluminum nitride AlN, chromium nitride CrN, silicon nitride Si ₃ N ₄ , iron nitride FeN, gallium nitride GaN, zirconium nitride ZrN, and the like. Examples include SiC, titanium carbide TiC, zirconium carbide ZrC, boron carbide B ₄ C, and tungsten carbide WC. Other protective cap materials include Al ₂ O ₃ , Al ₃ O ₄ , MgO, TiO, ZrO ₂ , such as yttrium stabilized ZrO ₂ , or glass, or refractory cement, or a mixture containing the above materials. . Furthermore, fiber reinforced ceramic materials or materials containing fiber reinforced ceramic materials are also used. The fiber of the fiber reinforced ceramic material is preferably exemplified by, for example, SiC, Al ₂ O ₃ , glass, or carbon.

  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 casting method, and aluminum or an alloy thereof, magnesium alloy, zinc alloy, copper or an alloy thereof, iron Metals such as alloys of the system can be exemplified. Among these, an aluminum alloy or a magnesium alloy is preferably exemplified.

  As described above, in the present invention, by covering the outer peripheral portions of both end faces of the metal billet with the protective cap, the end portion of the metal billet is prevented from being crushed or scattered in the induction heating stage, and the metal billet heating cycle is performed. The time can be reliably shortened and the entire metal billet can be reliably heated to the target heating state.

  Fig.1 (a) shows the perspective view of the metal billet and protective cap which is one embodiment of this invention. FIG. 1B is a sectional view thereof. In FIG. 1, the entire circumference of both ends of the metal billet is protected. However, when induction heating of the metal billet is performed on a pedestal of various shapes, the pedestal portion may not have a protective cap. .

(Example)
A heat resistant ceramic is attached to both ends of a metal billet made of a cylindrical aluminum alloy having a diameter of 50 mm and a length of 400 mm (components: Si: 7%, Fe: 0.1%, Mg: 0.55%, remaining Al). A protective cap having an outer diameter of 75 mm, an inner diameter of 50 mm, and a depth of 10 mm was put on and heated by induction. The heating temperature target of the metal billet was set to 585 ° C. The heating was induction heating at 5 kHz and 800 kW for 60 seconds. The entire metal billet was heated to a target temperature of 585 ± 2 ° C. in a short period of 60 seconds. Despite rapid heating for a short time of 60 seconds, there was no breakage or scattering at the end of the metal billet.

  By covering the both ends of the metal billet with protective caps, the end can be prevented from being broken or scattered, and heating can be performed in a short cycle. This contributes to more practical use of the semi-molten casting method.

Fig.1 (a) shows the perspective view of the metal billet and protective cap which is one embodiment of this invention. FIG. 1B is a sectional view thereof. It is a conceptual diagram for demonstrating the state which heats a billet using an induction heating coil.

Explanation of symbols

11: induction heating coil, 12: billet, 13: alternating magnetic field.

Claims (4)

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, a protective cap made of a heat resistant ceramic is placed on the outer peripheral portions of both end faces of the metal billet.   The induction heating method according to claim 1, wherein when the metal billet is placed on a pedestal, the protective cap is placed only on an upper portion where the metal billet is not protected by the pedestal. The induction heating method according to claim 1 or 2 , wherein the metal billet is made of an aluminum alloy or a magnesium alloy. The heat-resistant ceramic is TiN, AlN, CrN, Si ₃ N ₄ , FeN, GaN, ZrN, SiC, TiC, ZrC, B ₄ C, WC, Al ₂ O ₃ , Al ₃ O ₄ , MgO, TiO, ZrO ₂ , Yttrium stabilized ZrO ₂ , Glass or refractory cement or mixtures thereof, SiC, Al ₂ O ₃ 4. The induction heating method according to claim 1, wherein the induction heating method is selected from fiber reinforced ceramics reinforced with glass or carbon fibers.

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