Apparatus for the induction heating of ferromagnetic metals

Abstract

An apparatus for induction heating of ferromagnetic materials in which a single electromagnet is used for simultaneous generation of AC and DC fields.

Description

[0001] DESCRIPTION

[0002] Title

[0003] APPARATUS FOR THE INDUCTION HEATING OF FERROMAGNETIC METALS

[0004] Technical field

[0005]

[0001] The present invention relates to the field of devices for industrial heating of ferromagnetic semi-finished or finished products that are below the Curie temperature.

[0006]

[0002] The invention includes an apparatus and method for induction heating.

[0007] Present status of the art

[0008]

[0003] Industrial heating of large steel manufactured objects by the method of magnetic induction is well known; in this technology it is necessary to ensure high temperature uniformity throughout the entire mass of the object, which is difficult to achieve due to the high magnetic permeability of the material.

[0009]

[0004] In fact, the depth of heating of the object depends on the so-called penetration depth, that is, the depth, starting from the surface of the material, in which the intensity of the induced current is reduced to 1 / e of the intensity of the current on the surface of the material.

[0010]

[0005] This penetration depth can be expressed by the formula: 8 where 6 is the penetration depth, p is the resistivity of the material, f is the frequency of the induced current, po is the magnetic permeability of the air and prthe relative magnetic permeability of the material to be heated.

[0011]

[0006] Typically, the penetration depth 6 is controlled by varying the frequency f; for example, when only the surface of the artifact is to be heated, a high frequency is adopted, while a lower frequency is used to heat even the innermost portions of the object. However, with too low a frequency, the coupling between the electromagnet and the object to be heated gets worse, and very high magnetic fields are required, which in turn necessitates high currents; for this reason, if the material to be heated has a high relative magnetic permeability, it is difficult to conciliate deep heating of the object with acceptable electromagnetic coupling.

[0012]

[0007] One consequence of this is that induction heating often cannot be used for materials whose magnetic permeability is high, e.g., above 1; therefore, for the treatment of magnetic steel semi-finished products of industrial interest, i.e., thick slabs or blooms with minimum dimensions in the range of tens of centimeters, heating by gas or resistance ovens is the commonly adopted solution. These heating systems often take advantage of nonrenewable energy sources, e.g., fuel gas, and have certain disadvantages in turn, e.g., related to the fact that the thermal energy is transmitted to the object necessarily through the latter's surface, or the large amount of carbon dioxide produced by combustion.

[0013]

[0008] It is known in the industry to combine the effects produced by a first alternating current (AC) magnetic field with those produced by a second direct current (DC) magnetic field. The AC magnetic field generates induced currents responsible for thermal power, while the DC magnetic field modulates the magnetic properties of the material. Controlling the magnetic properties increases the depth of penetration of the induced currents, allowing deeper and more uniform heating. More specifically, the application of a DC magnetic field superimposed on the AC magnetic field results in the material operating in a condition of magnetic saturation (or close to it), reducing its magnetic permeability coefficient, and consequently, increasing the penetration depth of the AC field-induced currents and the material's ability to absorb thermal power in depth.

[0014]

[0009] It is known that when a magnetic material is at saturation, the value of magnetic induction in the material remains essentially constant even if the magnetic field to which the material is subjected increases. Since the magnetic permeability of the material is given by the ratio of magnetic induction to the magnetic field, under saturation condition an increase in the magnetic field corresponds to a decrease in the magnetic permeability of the material, moreover, the control of the magnetic properties during heat treatment allows for changes in its microstructural properties and macroscopic physical characteristics.

[0015]

[0010] WO2014088423 describes an apparatus comprising an AC coil, made of copper, powered with alternating current, arranged around the object to be heated and a DC coil made of superconducting material, powered with direct current and arranged in turn around the AC coil. The AC coil generates an alternating magnetic field that induces currents in the object suitable for heating it, while the DC coil generates a continuous magnetic field such that the magnetic material of the object is saturated, so as to reduce its magnetic permeability and increase the penetration depth, facilitating deep heating.

[0016] [Oil] In the apparatus according to WO2014088423, the electromagnetic coupling inevitably present between the superconducting material winding and the copper winding induces considerable voltages on the superconducting material winding, thus making its use in direct current complex. An additional disadvantage is the currents induced by the AC field in the metal structure that serves to house and cool the superconducting winding. To reduce the above coupling, it is known to interpose a metal shield between the copper winding and the superconducting material winding. However, even the use of a metal shield is not without disadvantages because it causes significant power losses and makes the heating of the object less effective.

[0017]

[0012] To reduce the coupling between the AC winding made of copper and the DC winding made of superconducting material, it is known to use two DC windings made of superconducting material conformed in such a way that the magnetic field generated by the AC winding, supplied with alternating current, and the saturation DC magnetic field generated by the two DC windings made of superconducting material both supplied with direct current, are directed perpendicular to each other, so as to cancel or minimize the electromagnetic coupling between the AC winding and the DC windings, thus also minimizing the voltages that the magnetic field generated by the AC winding induces on the DC windings. However, this architecture reduces the efficiency in generating the DC magnetic field needed to achieve saturation because it is directed perpendicular to the longitudinal axis of the object to be heated and requires a lot of current to generate the saturation DC magnetic field.

[0018]

[0013] EP2441849 describes an apparatus comprising a DC magnetic field generator conformed as a coil of superconducting material, inside which an object to be treated is placed. The apparatus also includes an AC magnetic field generator by which the object is heated by induction only in a surface layer of predetermined thickness, the depth of which depends on the frequency chosen to feed the AC magnetic field generator. However, this apparatus is used only for surface treatments and does not allow inducing currents inside the entire object to heat it internally as well.

[0019]

[0014] WO2018172929 describes an apparatus for induction heating of two ferromagnetic products, comprising a first winding supplied with alternating current configured to generate a first alternating magnetic field that affects the first product; a winding powered with direct current, to generate a magnetic field capable of regulating the magnetic permeability of the two products. Specifically, the apparatus according to WO2018172929 also includes a second alternating current-fed winding configured to generate a second magnetic field that affects the second product; the two alternating current-fed windings are made and powered so that the respective alternating magnetic fields generated by them have the same direction, but opposite senses.

[0020] Objects and summary of the invention

[0021]

[0015] One object of the present invention is to improve apparatuses and methods of a known type for induction heating an object made of magnetic material, particularly a steel object, for temperatures below the Curie temperature, increasing the temperature even in depth, preparing the object for certain processing, such as hot plastic deformation processing, or influencing its magnetic properties during heat treatment.

[0022]

[0016] Another object is to provide an apparatus for induction heating of an object in which the electromagnetic coupling between AC and DC components is zero, or at least very low, particularly by simplifying the magnetic system and reducing the critical voltages induced on the DC winding.

[0023]

[0017] A further object is to provide an apparatus for induction heating an object, which is inexpensive, simple, compact, and efficient.

[0024]

[0018] These and other objects are achieved with an apparatus for induction heating of ferromagnetic materials in which a single electromagnet is used for simultaneous generation of AC and DC fields. This is achieved by controlling the current of the electromagnet, modulating its waveform to ensure a periodic pattern with non-zero mean value. This results in the creation of a DC component of the magnetic field, which controls the magnetic characteristics of the material, with a superimposed AC component that induces currents and, through them, injects thermal power deep into the material.

[0025]

[0019] In contrast to solutions belonging to the state of the art, which employ two separate electromagnets, a first electromagnet (typically of superconducting technology) for DC field generation and a second electromagnet for AC field generation, the present invention provides for the generation of both field components through a single electromagnet. This solution reduces the overall size of the magnetic system and eliminates the disadvantages associated with magnetic coupling between two separate magnets by eliminating unwanted voltages induced on the DC electromagnet.

[0026]

[0020] In accordance with this innovation, the single electromagnet required for generating both the DC and AC magnetic fields can be a resistive electromagnet made by conventional technology, such as by water-cooled copper conductors, or it can be an electromagnet made by superconducting technology.

[0027] Brief description of the drawings

[0028]

[0021] Figure 1 schematically depicts the configuration that is the subject of this patent, in which a steel billet (A) is placed inside a single electromagnet (1) that generates both AC and DC electromagnetic fields; two containment shields (6, 7) are also shown.

[0029]

[0022] Figure 2 shows a schematic view of the electrical circuit for powering the electromagnet (4) of the induction heater from the power source (2) by means of a power converter (3); it should be noted that in this schematic the equivalent resistance of the magnet is omitted for simplicity.

[0030]

[0023] Figure 3 shows two periods of a waveform composed of a DC and a sinusoidal AC component. In the case shown, the amplitude of the DC component is higher than that of the AC component, but it could also be the same or lower. In general, amplitude of the DC component and amplitude and frequency of the harmonic components may vary depending on the requirements of the part's processing. Specifically, the waveform shown in Figure 3 is achievable, for example, with multilevel DC / AC converters, usually called "Modular Multilevel converters (MMCs)" or "Cascaded H-bridge".

[0031]

[0024] Figure 4 shows three periods of a square wave with a nonzero mean value over the period, which then has DC component and various harmonics (thus various AC components at frequencies multiple of the fundamental). This current waveform is easier to generate because it does not require filtering stages or multilevel architectures. In fact, a conventional single-phase or "full bridge" inverter - controlled by varying the duty cycle - is sufficient.

[0032]

[0025] Figure 5 shows an alternative design configuration of the invention. While the current I is controlled similarly to what has already been described with reference to the system in Figure 1, the electromagnet (5) in which this current is placed produces a magnetic field (B) that passes through the steel plate or other ferromagnetic object (C) in a transverse direction and not axially as in Figure 1. This configuration may include one or more electromagnets facing the semi-finished product to be heated; in the embodiment depicted there are two electromagnets, one below and the other above the object, carrying the same current I.

[0033] Detailed description of an embodiment of the invention

[0034]

[0026] In the embodiment shown in Figure 1, the electromagnet is configured as one or more solenoidal inductors, coaxial with a billet, however, it should be noted that this embodiment is shown as an example only, this term being understood to include configurations with a winding, or a coil, or a solenoid, among others.

[0035]

[0027] In addition, electromagnets can generally be configured so that they surround the object to be heated, or so that the magnetic field they generate affects the object from one or more sides.

[0036]

[0028] Containment shields (6, 7) are generally provided around the electromagnet, which are of the cryogenic type in the case where the electromagnet (1) is of the superconducting type.

[0037]

[0029] In application of the same inventive concept it is possible to realize different configurations, which always use a combined DC and AC field produced by a single winding, the different configurations can be applied for heating parts of different shapes such as raw products such as slabs, blooms and slabs, or finished products such as bars, profiles, other structural shapes, rails, rods, plates, sheets, strips, tubes, rings, wires, coils, tapes, etc. With particular reference to flat geometries (sheets, thin plates), for example, embodiments of the invention are possible that include separate windings, e.g., top and bottom, for the application of a transverse field including DC and AC components.

[0038]

[0030] As shown in Figure 2, an AC / AC type converter, implementable through different architectures, receives input power from the grid (or from a different source) and acts internally to control the current I in the ways previously described.

[0039]

[0031] Periodic waveforms with nonzero mean value can also be obtained from DC / DC switching converters in which the output current has a high harmonic content (thus is not filtered), and the switching frequency corresponds to the fundamental AC component or its multiples. In general, usable switching converters can have very varied architectures. This allows design complexity to be limited and costs to be kept down considerably.

Claims

CLAIMS1. Apparatus for the induction heating of one or more products comprising at least one ferromagnetic material which can be heated through the application of a magnetic field produced by an electromagnet, characterized in that said electromagnet is powered by electric current which has both a direct component and an alternating component superimposed on it so that said electromagnet produces both the alternating current field and the direct current field.

2. Apparatus according to claim 1, characterized in that said electromagnet is of the resistive type.

3. Apparatus according to claim 1 characterized in that said electromagnet is made of superconducting material.

4. Apparatus according to one of the preceding claims characterized in that said electric current is obtained with a multilevel DC / AC converter.

5. Apparatus according to one of the preceding claims 1 to 3 characterized in that said electric current is obtained with a DC / DC switching converter in which the output current has a high harmonic content and the switching frequency corresponds to the fundamental AC component or its multiples.

6. Method for induction heating of one or more products comprising at least one ferromagnetic material which can be heated through the application of a magnetic field produced by an electromagnet included in an apparatus according to one of the preceding claims characterized in feeding said at least one electromagnet with an electric current comprising a direct component and an alternating component.

7. Method according to the preceding claim characterized in that said electric current is obtained with a DC / DC switching converter in which the output current has a high harmonic content and the switching frequency corresponds to the fundamental alternating current component or its multiples.

Images