Method for stirring metal materials in an electric furnace and corresponding apparatus

Abstract

The present invention relates to a method for stirring a metal material (M) in an electric melting furnace, heating furnace, and / or refining furnace (100), comprising supplying a three-phase AC system voltage (Ur) and system current (Ir) having a predetermined system frequency (fr), converting the system voltage (Ur) and system current (Ir) into a selectively configurable AC secondary voltage (Ua) and secondary current (Ia), rectifying the secondary voltage (Ua) and secondary current (Ia) into a DC intermediate voltage (Ui) and intermediate current (Ui), converting the DC intermediate voltage (Ui) and intermediate current (Ui) into a selectively configurable AC supply voltage (Ua) and supply current (Ia), and supplying the supply voltage (Ua) and supply current (Ia) to a plurality of electrodes (102) of the furnace (100). The present invention also relates to an apparatus (10) for stirring the metal material (M).

Description

Technical Field

[0006]

[0001] The present invention relates to a method for stirring a metallic material in a three-phase AC electric furnace and a corresponding device.

[0002] The present invention is applicable to the steel industry and the steelmaking field, and is also applicable to other metal processing fields equipped with electric furnaces, such as melting furnaces, heating furnaces, refining furnaces or furnaces similar thereto.

Background Art

[0003] Plants for melting, heating and / or refining metallic materials are known, and this type of plant includes an electric furnace and one or more power supply devices connected to a power supply network, and can supply materials derived from ores in addition to materials derived from metal scrap.

[0004] One of the problems in the process of melting metallic materials is that generally, the melting does not proceed uniformly, resulting in unmelted residues and requiring a long time for the melting. Further, when an alloy is added to the starting metallic material, it takes time for the alloy components to be homogenized, and as a result, the productivity of the furnace decreases.

[0005] In an alternating current (AC) electric furnace in which electrodes are arranged above a container for containing a molten material and an arc is closed between the electrodes by passing through the molten material to be melted, the stirring effect is usually obtained by blowing an inert gas using a porous plug provided at the furnace bottom, or by applying a magnetic field by an electromagnetic stirring device arranged below the molten material container.

[0006] In a direct current (DC) electric furnace, there is an upper electrode and at least one lower electrode necessary to close the electrical circuit with the upper electrode, and this lower electrode is located at the bottom of the container holding the molten material. The stirring effect can be obtained by an additional device located below the lower electrode, but the efficiency of the magnetic field and, consequently, the mixing effect is low. However, in a DC electric furnace, the magnetic field generated by the additional device interacts with the magnetic field generated by the electric arc, and as a whole, it generates an additional force that mixes the flow of steel more effectively than in an AC electric furnace.

[0007] U.S. Patent No. 5,115,447 relates to a system for positioning electrode heights according to a process, and specifies adjusting the voltage supplied to one or more other electrodes in the event of a condition that may cause a short circuit in one electrode or the extinction of the electric arc in that electrode. Specifically, if a short circuit occurs in any electrode, the voltage supplied to the adjacent electrode is increased to increase the arc length, while if the electric arc is extinct, the voltage supplied to the other two electrodes is decreased to shorten the length of each electric arc. U.S. Patent No. 5,115,447 does not disclose a solution using a circuit that modulates the power of each electrode and alternately changes its intensity to produce a rotational effect on a metallic material.

[0008] U.S. Patent No. 3,409,726 relates to a DC electric arc furnace that uses an anode placed at the bottom of the furnace to close the electrical circuit, and therefore there is no closed circuit between the electrodes. To stir the molten metal, it is necessary to supply a DC current to the electrodes and to use a DC electromagnetic stirring device located at the bottom of the furnace, which works in cooperation with magnetic members located on the side walls of the furnace to form magnetic poles. Unlike methods using three-phase circuits operating on AC, this process does not allow for changes in the amplitude of the voltage and current supplied to each electrode to obtain stirring of the metal.

[0009] Spanish invention patent specification 2018840 also describes a DC furnace in which the arc generated between the cathode and anode is deflected by a conductor placed around or below the furnace chamber.

[0010] Therefore, there is a need to improve the method and corresponding apparatus for stirring metal materials in electric furnaces, and a solution is required that is simpler than the aforementioned known solutions, yet at least equally effective in improving the stirring effect of molten materials.

[0011] To achieve this, it is necessary to solve the technical challenge of defining a technology that can achieve an efficient stirring process in a three-phase AC electric furnace by using equipment already included in the melting, heating, and / or refining plant as an alternative for stirring metal materials. Therefore, it is a method that does not require any special additional elements.

[0012] In particular, one of the objectives of the present invention is to complete a method that enables an efficient stirring process and to provide an apparatus that corresponds thereto.

[0013] Another objective of the present invention is to avoid the use of additional equipment specifically for stirring metallic materials, thereby reducing the cost and overall size of the plant.

[0014] Another object of the present invention is to reduce the time required to achieve homogeneous dissolution and / or homogeneous mixing of metallic materials, thereby improving the productivity of melting, heating and / or refining plants.

[0015] By inventing, testing, and embodying the present invention, the applicant has overcome the shortcomings of the prior art and achieved these and other objectives and advantages. [Overview of the project]

[0016] The present invention is defined and characterized by the independent claims. The dependent claims describe other features of the present invention and variations of the main inventive concept.

[0017] In accordance with the above objectives, and in order to solve the aforementioned technical problems in a novel and unique way, and to achieve significant advantages compared to the prior art, the method for stirring a metal material in an electric furnace according to the present invention provides supplying a supply voltage and supply current to multiple electrodes of the furnace. In particular, each electrode is energized by one phase of the three-phase supply voltage and current.

[0018] The supply voltage and supply current are obtained by rectifying a three-phase AC system voltage and current having a predetermined system frequency using multiple rectifiers, and then converting them using multiple converters.

[0019] The supply voltage and supply current can be selectively set by a control and command unit connected to the converter. In particular, both the amplitude and frequency of each phase of the supply voltage and current can be selectively set.

[0020] According to one aspect of the present invention, the method provides that, in at least a portion of the furnace's operating cycle, the adjustment device of the control and command unit is adjusted to sequentially adjust the supply voltage and supply current so that, in each of a plurality of consecutive predetermined time intervals and over the entire duration of each of the intervals, the amplitude of one of the supply voltages and supply currents of an electrical phase energized to one electrode is smaller than the predetermined amplitudes of the supply voltages and supply currents of the other two phases energized to the other two electrodes.

[0021] According to some embodiments, the regulating device controls the converter such that, at each time interval, the amplitude of one of the three phases (the smaller amplitude) is within 0% to 70% of a predetermined amplitude of the other two phases. This predetermined amplitude may correspond to the allowable maximum amplitude (approximately 100%), or in any case, it may be a predetermined nominal maximum amplitude. The voltage and current amplitudes of the remaining two phases can be substantially identical to each other.

[0022] "Substantially the same" means that the amplitudes of the voltages / currents of the other two phases are the same value or differ by at most a difference within the range of 0 to 10%.

[0023] The term "amplitude" as used herein is used to generally indicate the maximum absolute value that either the supply voltage or current can take, and these are closely correlated with each other. The amplitude is considered substantially based on the maximum value of a sine wave.

[0024] According to some embodiments, the smaller amplitude is included within the range of 0% to 50% of the predetermined amplitude of the supply voltage and current of the other two phases.

[0025] According to some embodiments, the smaller amplitude is zero, that is, its value is 0.

[0026] According to some variations, the regulating device controls the converter such that, in each of a plurality of consecutive predetermined time intervals and over the entire duration of each of said intervals, the amplitudes of the supply voltage and supply current have an alternating waveform where they become zero in one of the electrical phases and have a predetermined amplitude in the other two phases.

[0027] According to one aspect of the present invention, the method stipulates that in subsequent time intervals, the phase in which the amplitudes of the supply voltage and supply current are zero or smaller is different from the phase in the immediately preceding time interval.

[0028] In other words, the method sequentially activates and deactivates each phase according to a predetermined order, such that in each time interval, two phases with a predetermined amplitude are on, while the third phase is on with a smaller amplitude or off.

[0029] In other words, each phase is controlled sequentially and periodically, and at each predetermined time interval, a current / voltage having a predetermined amplitude is supplied to two electrodes, and correspondingly, each electric arc has a corresponding arc length, while a current / voltage having a smaller or, in some cases, zero amplitude is supplied to the third electrode, and correspondingly, each electric arc has a shorter arc length or, in some cases, is extinguished.

[0030] In this way, at each time interval, the electrodes supplied by two phases having a predetermined amplitude generate forces acting in opposite directions to each other, while the third phase generates a force of a smaller or zero magnitude. As a result, at least one advantage is obtained that the repulsive force generated by the electric arc formed between each electrode and the metal material causes effective stirring of the molten metal material.

[0031] The greater the difference between the predetermined amplitudes of the two phases and the smaller amplitude of the other one phase, the greater the fluctuation of the generated force, and as a result, the greater the movement effect generated on the metal material.

[0032] Also, according to different stages of the working process and / or the type of the metal material to be processed, it is also possible to change the amplitude of each phase and / or the difference between the predetermined maximum amplitude and the minimum amplitude.

[0033] According to another aspect of the present invention, the method can define sequentially activating the first phase and the second phase, the second phase and the third phase, and the first phase and the third phase at a predetermined amplitude, and activating the third phase, the first phase, and the second phase at a smaller amplitude or turning them off, respectively.

[0034] According to another aspect of the present invention, the method can alternatively define sequentially activating the first phase and the second phase, the first phase and the third phase, and the second phase and the third phase at a predetermined amplitude, and activating the third phase, the second phase, and the first phase at a smaller amplitude or turning them off, respectively.

[0035] According to another aspect of the present invention, the method can specify that in each electrode pair, the electric arcs formed between each electrode and the material, which are powered by a supply voltage and current having a predetermined amplitude, generate repulsive forces at angles of 120° to 180° relative to each other.

[0036] When two phases operate with balanced amplitudes, the angle between the forces is approximately 180°. On the other hand, when three phases operate with unbalanced amplitudes, the angles between the phases deviate from 120°, and the angle corresponding to the phase with the smallest amplitude tends to be smaller in proportion to the imbalance of energy passing through that phase.

[0037] By sequentially activating the first and second phases, the second and third phases, and the first and third phases with predetermined amplitudes, or in the reverse order, while simultaneously decreasing the amplitudes of the third, first, and second phases, and in some cases sequentially turning them off, the direction of the repulsive force changes clockwise or counterclockwise, resulting in a stirring force that mixes the metal material while rotating it.

[0038] Preferably, the change in the direction of the repulsive force generated by a phase having a predetermined amplitude (clockwise or counterclockwise) results in the generation of a stirring force that mixes the molten metal material while rotating it.

[0039] According to another aspect of the present invention, the method may be defined as positioning the electrodes at locations corresponding to the vertices of a triangle located on a plane substantially perpendicular to the longitudinal direction of the electrodes.

[0040] Preferably, if the triangle is substantially equilateral, the distribution of stirring force in the molten metal material becomes more uniform.

[0041] According to another aspect of the present invention, the method can be defined such that the length of each time interval corresponds to an integer multiple of the supply frequency of the supply voltage and current.

[0042] According to another aspect of the present invention, the method can be defined such that the length of each time interval is in the range of 0.5 seconds to 10 seconds, preferably in the range of 1.5 seconds to 5.5 seconds.

[0043] According to another aspect of the present invention, the method may be configured such that the adjustment device controls the supply frequency during the work cycle.

[0044] According to another aspect of the present invention, the method can be configured such that the supply electricity frequency is lower than the grid frequency at least in the initial stages of the work cycle.

[0045] According to another aspect of the present invention, the method can be configured such that, in the final stage of the work cycle, the supply electricity frequency is higher than the grid frequency.

[0046] According to another aspect of the present invention, the method allows the adjusting device to control the supply frequency so that its value is within the range of 20 Hz to 90 Hz.

[0047] The semiconductor elements of the rectifier 14 and / or converter 15 are controlled by the adjustment device 18 so that the supply frequency differs from the system frequency at one or more stages, or possibly all, of the work cycle.

[0048] According to another embodiment of the present invention, the adjustment device can control the supply frequency to be in the range of 60% to 80% of the system frequency during the initial stages of the work cycle.

[0049] According to another aspect of the present invention, the method may involve a regulating device controlling the supply frequency so that its value is in the range of 30% to 60% of the system frequency at the final stage of the work cycle.

[0050] According to another aspect of the present invention, the apparatus for stirring metal materials in an electric furnace is also configured to supply power to the electric furnace in a three-phase manner.

[0051] According to another aspect of the present invention, the apparatus comprises a plurality of rectifiers and a plurality of converters. The converters are connected to the rectifiers and a command and control unit, which is configured to control and command the operation of the converters and to adjust the supply voltage and current over time. Furthermore, the converters are configured to supply alternating current supply voltage and current to the electrodes of a furnace.

[0052] According to another aspect of the present invention, the control and command unit comprises an adjustment device configured to sequentially adjust the supply voltage and supply current for at least a portion of the furnace's operating cycle, such that in each of a plurality of consecutive predetermined time intervals, and throughout the entire duration of each interval, the amplitude of the supply voltage and supply current of one of the electrical phases energizing one of the electrodes is smaller than the predetermined amplitudes of the supply voltage and supply current of the other two phases energizing the other two electrodes, respectively.

[0053] According to another embodiment, the control unit is configured to control the supply voltage and supply current by an adjustment device so that, for each time interval, the amplitude of the supply voltage and supply current becomes zero in one of the electrical phases, and the other two phases exhibit alternating values ​​with predetermined amplitudes, i.e., alternating and / or sinusoidal changes.

[0054] The adjustment device is also configured to change the phase in which the amplitude of the supply voltage and supply current is smaller, or zero, in the subsequent time interval relative to the immediately preceding time interval.

[0055] According to another aspect of the present invention, the adjustment device may be configured to sequentially activate the first phase and the second phase, the second phase and the third phase, and the first phase and the third phase with a predetermined amplitude, or to sequentially activate them in the reverse order. In this case, the third phase, the first phase, and the second phase are each sequentially activated with an increasingly smaller amplitude or turned off.

[0056] According to another aspect of the present invention, the number of electrodes is 3 or a multiple of 3, and they can be positioned at locations corresponding to the vertices of a triangle located on a plane substantially perpendicular to the longitudinal direction of the electrodes. Preferably, the triangle formed is substantially equilateral.

[0057] According to another aspect of the present invention, the adjustment device can be configured to control the supply frequency of the supply voltage and supply current so that the supply frequency is lower than the system frequency at least in the initial stage of the work cycle and higher in the final stage.

[0058] According to another aspect of the present invention, the apparatus comprises a transformer connected to a power source for three-phase AC system voltage and system current having a predetermined system frequency. The transformer is configured to convert the AC system voltage and system current into selectively configurable AC secondary voltage and secondary current, respectively, the secondary frequencies of which are substantially the same as the system frequency.

[0059] According to another aspect of the present invention, a plurality of rectifiers are connected to a transformer and are configured to convert AC secondary voltage and secondary current into DC intermediate voltage and intermediate current.

[0060] According to another aspect of the present invention, a plurality of converters are configured to convert a DC intermediate voltage and intermediate current into an AC supply voltage and supply current.

[0061] These and other aspects, features, and advantages of the present invention will become apparent from the following description of some embodiments shown as non-limiting examples with reference to the accompanying drawings. [Brief explanation of the drawing]

[0062] [Figure 1] Figure 1 is a schematic diagram of an apparatus for stirring a metal material in an electric furnace according to the present invention. [Figure 2] Figure 2 is a schematic diagram of an apparatus for stirring a metal material in an electric furnace according to another embodiment. [Figure 3] Figure 3 shows the voltage changes of the electrodes in an apparatus for stirring metal materials in an electric furnace as shown in Figure 1 or Figure 2, in the operating mode of the prior art. [Figure 4] Figure 4 is a time diagram of the voltage in Figure 3. [Figure 5] Figure 5 shows the arc discharge of electrodes in an apparatus for stirring metal materials in an electric furnace shown in Figure 1 or Figure 2, operating in the operating mode of Figure 3. [Figure 6a] Figure 6a shows the voltage changes of the electrodes in an apparatus for stirring a metal material in an electric furnace shown in Figure 1 or Figure 2, operating in the first operating mode according to the present invention. [Figure 6b] Figure 6b shows the voltage transition of the electrodes in an apparatus for stirring a metal material in an electric furnace shown in Figure 1 or Figure 2, operating in the second operating mode according to the present invention. [Figure 7a] Figure 7a is a time diagram of the voltage in Figure 6a. [Figure 7b] Figure 7b is a time diagram of the voltage in Figure 6b. [Figures 8a-8c] Figures 8a, 8b, and 8c show the arc discharge of electrodes in an apparatus for stirring metallic materials in an electric furnace shown in Figure 1 or Figure 2, operating in the operating mode of Figure 6a. [Figures 9a-9c] Figures 9a, 9b, and 9c show the arc discharge of electrodes in an apparatus for supplying power and stirring metal materials in an electric furnace shown in Figure 1 or 2 according to one embodiment. [Figure 10-10a] Figure 10 is a graph showing the power consumption over several operating cycles of an electric furnace with an ECS charge system. Figure 10a is a detailed view of the graph in Figure 10. [Modes for carrying out the invention]

[0063] The terms and expressions used herein, as well as the illustrations in the accompanying drawings (including how they are depicted), are for the sole purpose of better illustrating and describing the present invention and providing non-limiting examples of the invention itself, for the scope of protection is defined by the claims.

[0064] For ease of understanding, identical or common elements in the drawings are given the same reference numeral whenever possible. It is understood that elements and features of one embodiment can be combined or incorporated into other embodiments as appropriate without requiring further explanation.

[0065] Referring to Figure 1, several embodiments of the present invention relate to a device 10 for stirring a metallic material M in at least one electric furnace 100 for melting, heating and / or refining the material M. According to the present invention, the device 10 also functions as a three-phase power supply device for the electric furnace 100.

[0066] According to some embodiments, the apparatus 10 can be powered by a three-phase power energy supply means. In this specification, a three-phase power grid 200 will be described with reference as a non-limiting example.

[0067] The system voltage Ur and system current Ir supplied by the network may have a predetermined system frequency fr.

[0068] According to a possible embodiment, the grid frequency fr has a value selected from the range of 50 Hz to 60 Hz, i.e., it is based on the frequency of the power grid in the area where the electric furnace 100 is installed.

[0069] The electric furnace 100 of this type may be an electric furnace for melting, heating, and / or refining a material M by an electric arc. Thus, the electric furnace 100 may be an arc furnace, a submerged arc furnace, a ladle furnace, or a melting furnace, refining furnace, or heating furnace, or similar furnace, generally suitable for use in steelmaking or metalworking plants in steel mills. Preferably, the present invention applies to an electric arc furnace (EAF), a ladle furnace (LF), and / or a melting furnace, or a submerged arc furnace (SAF), which uses electrodes 102, 106 to transfer thermal energy to the material M to be processed.

[0070] In the case of an EAF-type electric furnace 100, it comprises a container 101, i.e., a furnace body, into which the metal material M to be melted is introduced. The EAF furnace also comprises a plurality of electrodes 102 configured to generate an electric arc in the material M and melt it.

[0071] In the case of a ladle furnace LF, it generally comprises a ladle 104 suitable for containing the liquid material M extruded from an EAF furnace, a ceiling portion 105 that closes the top of the ladle 104, and a plurality of electrodes 106 positioned to penetrate the ceiling portion 105.

[0072] According to some embodiments of the present invention, electrodes 102, 106 are attached to moving devices 103, 107 for selectively moving a plurality of electrodes 102, 106 closer to or further away from a material M. These moving devices may include, for example, mechanical, electrical, pneumatic, or hydraulic actuators, joint mechanisms, mechanical kinematics, similar components, or combinations thereof.

[0073] The following explanation will primarily use the EAF reactor as an example. However, the disclosed information is also applicable to the different types of electric furnaces 100 described above.

[0074] Preferably, the number of electrodes 102 is 3 as shown in the figure, but it may be a multiple of 3. According to a possible embodiment of the present invention, if there are 3 electrodes 102, each electrode is connected to a phase of the three-phase power supply of the device 10.

[0075] Preferably, the electrodes 102 are arranged to form a triangle with respect to each other. In particular, they are arranged to correspond to the vertices of a triangle located on a plane substantially perpendicular to the longitudinal direction of electrodes 102 and 106 (see Figures 3, 5, and 6). Preferably, this triangle is substantially equilateral, but it may be an isosceles or scalene triangle.

[0076] While it is known that EAF and LF reactors typically have one electrode for each phase, totaling three electrodes, it is also possible to have more electrodes, such as four or more, and this is also true for SAF reactors.

[0077] In the case of four electrodes, they can be arranged, for example, at the vertices of a trapezoid, or in a rectangular or rhombus shape. In this case, the system can be configured to keep three phases corresponding to three electrodes active and turn off or reduce the amplitude of the fourth electrode. Alternatively, two phases can be kept active and the remaining two phases can be turned off or their amplitudes reduced as appropriate to generate a repulsive force suitable for constant circulation of the molten material.

[0078] According to several embodiments, the apparatus 10 can receive energy supplied from the system 200 and convert it into a supply voltage and supply current having predetermined electrical parameters Ua, Ia, fa suitable for powering the electric furnace 100. The supply voltage Ua and supply current Ia may also have electrical parameters Ua, Ia suitable for generating an electric arc 110 capable of heating the material M.

[0079] The plant transformer 300 is positioned between the system 200 and the device 10, and can isolate the device 10 from the system 200. The plant transformer 300 can supply low voltage (LV) (Figure 1) or medium voltage (MV) (Figure 2) to the device 10. In the latter case, the device 10 is further equipped with another plant transformer 301 that can supply low voltage (LV) to the electric furnace 100.

[0080] According to some embodiments, the device 10 includes at least one transformer 11 connected to a power system 200 and configured to convert a primary AC voltage Up and primary current Ip into a secondary AC voltage Us and secondary current Is.

[0081] According to a possible embodiment, the transformer 11 may include a transformer primary side 12 that is magnetically coupled to at least one transformer secondary side 13.

[0082] This configuration makes it possible to reduce the effects of grid-side faults, specifically the harmonic components and reactive power exchanged with grid 200.

[0083] The secondary power supplied by the transformer 11 has a secondary voltage Us, a secondary current Is, and a secondary frequency fs, which are predetermined by the design characteristics of the transformer 11 itself.

[0084] According to some embodiments, the secondary frequency fs can be set to be substantially equal to or lower than the aforementioned system frequency fr. Generally, the same applies to the primary frequency fp of the current flowing through the primary side 12.

[0085] The secondary voltage Us and secondary current Is are correlated, respectively, with the system voltage Ur and system current Ir, or more generally, the primary voltage Up and primary current Ip of the primary side 12, and the transformation ratio of the transformer 11 itself.

[0086] The transformer 11 may be equipped with an adjustment device (not shown) for selectively adjusting the electrical transformation ratio according to specific requirements.

[0087] The apparatus 10 according to the present invention also includes a plurality of rectifiers 14 connected to the transformer 11 and configured to convert the AC secondary voltage Us and secondary current Is into a DC intermediate voltage Ui and intermediate current Ii.

[0088] The rectifier 14 may be selected from the group including a diode bridge, a thyristor bridge, or others.

[0089] According to some embodiments, the apparatus 10 includes a plurality of converters 15 connected to a rectifier 14 and configured to convert a DC voltage and a DC current into an AC supply voltage Ua and a supply current Ia supplied to an electrode 102.

[0090] According to a possible embodiment, the rectifier 14 can be connected to the converter 15 via at least one DC-operating intermediate circuit 16.

[0091] The intermediate circuit 16 can be configured to create isolation between the rectifier 14 and the converter 15, i.e., between the electric furnace 100 and the power system 200 located upstream of the intermediate circuit 16. In particular, rapid power fluctuations caused by the process are partially filtered by the intermediate circuit 16, thereby reducing their impact on the power system 200.

[0092] The intermediate circuit 16 can also be configured to store DC power. According to some embodiments, this intermediate circuit 16 is a DC link and comprises at least one capacitor.

[0093] According to some embodiments, the apparatus 10 includes a control and command unit 17 configured to control the converter 15 and to selectively set the parameters of the supply voltage Ua and supply current Ia generated by the converter 15 and supplied to the electrode 102.

[0094] In particular, according to the present invention, the control and command unit 17 is configured to control the supply voltage Ua and supply current Ia during a predetermined time interval in the work cycle of the electric furnace 100, and to adjust the amplitude of one phase F1, F2, F3 to an amplitude Amin that is smaller than the predetermined amplitude Amax of the supply voltage Ua and supply current Ia of the other two phases.

[0095] The control and command unit 17 is also configured to control the supply voltage Ua and supply current Ia so that, in the following time interval, the phases F1, F2, and F3 of the supply voltage Ua and supply current Ia with small amplitudes are adjusted to be different from the previous phases F1, F2, and F3.

[0096] According to some embodiments, in each of the time intervals, the small amplitude Amin of one of the phases F1, F2, and F3 falls within the range of 0% to 70% of the predetermined amplitude Amax of the other two phases F1, F2, and F3.

[0097] The amplitudes of the supply voltage Ua and supply current Ia of the other two phases may be substantially the same as each other, or they may differ from each other by 0 to 10%.

[0098] According to some embodiments, the small amplitude Amin of the supply voltage Ua and supply current Ia in one phase F1, F2, F3 falls within the range of 0% to 50% of a predetermined amplitude Amax of the supply voltage Ua and supply current Ia of the other two phases.

[0099] According to another embodiment, in each time interval, the small amplitude Amin of the supply voltage Ua and supply current Ia in one phase becomes zero.

[0100] The time intervals during which the amplitudes of phases F1, F2, and F3 become small (Amin) or zero can be set to correspond to periods that are integer multiples of the supply frequency fa. For example, if the supply frequency is equal to the system frequency fr, this time interval can be an integer multiple of the system frequency fr. The length of this time interval can be set in the range of 0.5 seconds to 10 seconds, preferably in the range of 1.5 seconds to 5.5 seconds.

[0101] In the conventional three-phase AC power supply operation mode, voltages F1, F2, and F3 of each phase are normally applied to each electrode 102, exhibiting sinusoidal fluctuations with respect to time as shown in Figure 3 (see Figure 4). Only at the moment when the sign of the voltage changes does one electrode 102 turn off while the other two turn on, and at this time a high-frequency on / off operation occurs (for example, for a certain electrode 102, if the frequency fa is 50 Hz, it switches between off and on every 0.01 seconds).

[0102] According to the present invention, each phase F1, F2, and F3 can be sequentially and periodically activated in two consecutive time intervals with a supply voltage and supply current having a predetermined amplitude Amax, and controlled to have a small amplitude Amin in a third time interval. This makes it possible to change the length of the electric arcs generated at electrodes 102 and 106 connected to each phase F1, F2, and F3. The sequential change of the electric arcs in one phase F1, F2, and F3 causes a change in the repulsive force between the electric arcs of each electrode 102 and 106.

[0103] According to several embodiments, and as shown in Figure 5, the electric arcs 110 generated between each electrode 102 and the bath of molten metal M tend to repel each other, consequently penetrating the bath and generating a repulsive force FA with a repulsion angle α of approximately 120°.

[0104] In relation to the present invention, according to embodiments described with reference to Figures 6a and 7a, only the phases applied to two electrodes 102 of the three phases F1, F2, and F3 have non-zero values ​​for voltage Ua and current Ia with a predetermined amplitude Amax, while the remaining third phase has zero values. As a result, the two corresponding electric arcs 110 tend to enter the metal bath, generating repulsive forces FA in opposite directions and stably having a repulsion angle α of approximately 180° (see Figures 8a, 8b, and 8c).

[0105] For example, as shown in Figure 6a, if phases F1 and F2 are activated first, then phases F2 and F3, and then phases F1 and F3 are activated again, a repulsive force FA of 180° in different directions will be generated in the next time interval. "Activating" means that the supply voltage Ua and supply current Ia of the corresponding phases F1, F2, and F3 have a non-zero amplitude.

[0106] The change in the direction of the repulsive force FA generates a stirring force in the molten metal bath in a clockwise direction according to the sequence described above. Reversing the activation order of phases F1, F2, and F3 also reverses the direction of stirring.

[0107] According to the present invention, the electric arc 110 can generate a stirring force that agitates the material M contained in the furnace 100.

[0108] According to the embodiment described with reference to Figures 6b and 7b, all three phases F1, F2, and F3 are powered. However, while two phases are powered with a voltage Ua and current Ia having a predetermined amplitude Amax, the remaining third phase is powered with a small amplitude Amin.

[0109] In this case, the two electric arcs 110 generated by the electrode 102, which is powered with a voltage and current of a predetermined amplitude Amax, generate repulsive forces FA of approximately equal magnitude, at least partially in opposite directions (for example, at an angle of about 120°). On the other hand, the electric arc 110 generated by the electrode 102, which is powered with a smaller amplitude Amin, generates a smaller force.

[0110] Preferably, the method described above according to the present invention can be advantageously applied to the refining stage, which corresponds to the final stage of the furnace 100's work cycle, i.e., the stage in which all material M has completely melted (see Figures 9 and 9a). However, the method described above can also be advantageously applied to any stage of the melting process, such as when fed in a continuous feed ECS (see Figure 10) or a basket, enabling a faster and more efficient melting of material M.

[0111] According to some embodiments, the control and command unit 17 can also be configured to selectively control the supply voltage Ua and supply current Ia according to the required working power, and in the case of an EAF furnace, it is controlled according to the operating melting power.

[0112] According to some embodiments, the control and command unit 17 can also be configured to control the supply voltage Ua amplitude and supply current Ia of each phase F1, F2, F3, and / or the difference between a predetermined amplitude Amax and a smaller amplitude Amin, depending on each step of the work process.

[0113] Furthermore, according to some embodiments, the control and command unit 17 can also be connected to a moving device 103 and configured to adjust the position of the electrode 102 according to each stage of the work cycle. In particular, the electrode 102 is moved by the moving device 103, changing the length of the electric arc 110 to follow the position of the material.

[0114] In fact, during the melting phase, the power supplied to the electrode 102 can be increased compared to the drilling phase. This is because, at this point, the electric arc 110 is considered to be away from and covered by the ceiling (vault) of the furnace 100, thus avoiding the risk of damage to the ceiling. The reference values ​​of the supply voltage Ua and supply current Ia can be changed by the control and command unit 17, thereby making it possible to increase the active power. At this stage, the electric arc 110 is more stable because it is protected by scrap or slag.

[0115] Furthermore, the refining process is more stable and requires less electricity.

[0116] In this way, the control and command unit 17 can manage and control parameters such as at least the supply voltage Ua, supply current Ia, and the position of the electrode 102 according to each stage of the work cycle. By being able to highly control these various parameters, the following advantages can be achieved: realization of the mixing effect of the molten material M, optimization of energy transfer to the process, and at the same time, reduction of the impact on the power system 200 caused by rapid power fluctuations on the furnace 100 side.

[0117] The electrical topology employed in the converter 15 makes it possible to protect the system 200 from disturbances caused by the melting process (such as flicker reduction, harmonics, and power factor), while simultaneously ensuring the stability of the electric arc 110 throughout the entire working cycle of the furnace 100.

[0118] The control and command unit 17 may include an adjustment device 18.

[0119] According to some embodiments of the present invention, the adjustment device 18 may include, for example, a hysteresis modulator, a PWM (pulse width modulation) modulator, or similar devices.

[0120] These types of modulators can be used to control the semiconductor devices of the rectifier 14 and / or converter 15. When properly controlled, they generate the values ​​of the voltage Ua or current Ia supplied to the furnace 100, and in this case to the electrode 102.

[0121] In particular, the modulator processes the values ​​of these voltages Ua and currents Ia and generates commands to drive at least the rectifier 14 and the converter 15. This ensures that the voltages Ua and currents Ia required by the control are obtained at the connection terminals to the electrodes 102. The voltages Ua and currents Ia actually applied are determined as a result of calculations performed by the control and command unit 17 based on various quantities obtained from the process and the process model.

[0122] According to the present invention, the adjustment device 18 is configured to control the values ​​of voltage Ua and current Ia in one of the phases F1, F2, and F3 of the power supply at predetermined time intervals during the working cycle of the furnace 100, such that the amplitude of the voltage Ua and current Ia in one of the phases F1, F2, and F3 becomes zero or a small amplitude Amin, and the other two phases F1, F2, and F3 become AC values ​​with a predetermined amplitude Amax.

[0123] Furthermore, these adjustment devices 18 are configured to control the subsequent time intervals such that the phases F1, F2, and F3 having a small amplitude Amin or zero amplitude of the supply voltage Ua and supply current Ia are different from the previous phases F1, F2, and F3.

[0124] In particular, the control to ensure that each phase F1, F2, and F3 has a predetermined amplitude Amax in two time intervals and a smaller amplitude Amin in a third time interval is performed sequentially and periodically.

[0125] According to some embodiments, the adjustment device 18 can be configured to control the supply electrical frequency fa of the supply voltage Ua and supply current Ia at each stage of the working cycle of the furnace 100.

[0126] The semiconductor devices of the rectifier 14 and / or converter 15 can be controlled by the adjustment device 18 so that the supply frequency fa is different from the system frequency fr at one or more stages of the work cycle, and even at all stages if necessary.

[0127] In particular, the semiconductor devices of the rectifier 14 and / or converter 15 can be controlled by the adjustment device 18 so that the supply frequency fa is higher or lower than the system frequency fr during the initial stage SI of the work cycle.

[0128] The semiconductor devices of the rectifier 14 and / or converter 15 can be controlled by the adjustment device 18 so that the supply frequency fa is lower than the system frequency fr, at least in the final stage SF of the work cycle.

[0129] In particular, the operation cycle of furnace 100 may include an initial stage SI, an intermediate stage, and a final stage SF (Figures 10 and 10a).

[0130] The initial stage SI may include the charging stage and the drilling stage of material M.

[0131] The intermediate step may include a dissolution step.

[0132] The final stage SF may include a refining step for material M. In the final stage SF, the proportion of liquid material M is 90% or more, preferably 100%, excluding the proportion of slag generated during the melting process.

[0133] In particular, during the initial SI stage, the electrode 102 is brought close to the solid material M into which it is charged, generating an electric arc to begin dissolving the material M. As the material M gradually dissolves, the electrode 102 penetrates into the still-solid parts, dissolving them step by step. When the electrode 102 reaches a predetermined position within the container 101, the full-scale dissolution of the material M remaining around the electrode 102 begins.

[0134] According to one possible embodiment, the drilling and melting steps may be repeated multiple times before the refining step, with steps in between for charging additional material M into the furnace 100.

[0135] According to some embodiments, the supply frequency fa supplied to the electrode can be varied within a range of 20 to 90 Hz depending on the stage of the work cycle. For example, when the system frequency fr is 50 Hz, the supply frequency fa can be varied within a range of -40% to +180%, and when the system frequency fr is 60 Hz, it can be varied within a range of -33% to +150%.

[0136] According to some embodiments, the supply frequency fa can be varied in the initial stage SI within a range of 60% to 80% of the system frequency fr.

[0137] For example, the supply frequency fa can be less than 20%, preferably less than 30%, and more preferably less than 40% of the system frequency fr.

[0138] The supply frequency fa can be varied within a range of 30% to 70% of the system frequency fr in the final stage SF.

[0139] According to some embodiments, in the refining stage of the work cycle, it is preferable that the supply frequency fa is adjusted to a value at least 30% lower than the system frequency fr, i.e., about 30 Hz or less.

[0140] According to other embodiments, the supply frequency fa can be varied in the final stage SF within a range of 30% to 60% of the system frequency fr.

[0141] However, in certain stages of the dissolution process, cases where the supply frequency fa exceeds the system frequency fr are not ruled out; for example, it can be in the range of 101% to 200% of the system frequency fr.

[0142] For example, the control and command unit 17 can advantageously control the supply frequency fa to be lower than the system frequency fr during the initial stage of melting in a basket-charging furnace 100, and can similarly control it during part of the initial stage of ECS charging in an ECS-charging furnace 100 (Figures 10 and 10a).

[0143] Preferably, by reducing the supply frequency fa, the entry angle β of the arc 110 in the bath of material M becomes smaller, and heat transfer is maximized (Figure 9a).

[0144] Conversely, in stages where most or almost all of the material M becomes liquid and temperature uniformity is required, the entry angle β of the arc 110 in the material M bath can be tilted by increasing the supply frequency fa (Figures 9b, 9c). In particular, when the entry angle β is tilted significantly (Figure 9c), for example, exceeding 45°, a strong thrust similar to that of a propeller acts on the material M, allowing it to be moved effectively.

[0145] Therefore, the control and command unit 17 can control the entry angle β to vary from approximately 0° to approximately 90°.

[0146] The change in supply frequency fa may be continuous or discrete. Figures 9a, 9b, and 9c show the penetration angle β at a first value lower than the system frequency fr, a second value substantially equal to the system frequency fr, and a third value higher than the system frequency fr, respectively.

[0147] According to possible embodiments, the rectifier 14 and the converter 15 are connected in a modular configuration to form a power supply module 19 as a whole.

[0148] According to some embodiments, the device 10 comprises a plurality of power modules 19, each power module 19 including at least one rectifier 14 and one converter 15, and capable of supplying power from a minimum of 1 MW to a maximum of 30 MW.

[0149] According to some embodiments, each power module 19 may include at least one rectifier 14, one converter 15, and one intermediate circuit 16 for two or more phases of a polyphase power grid.

[0150] Preferably, all power modules 19 can be configured to be the same size, i.e., to supply the same power range.

[0151] Typically, the preferred rating range for each of these power modules 19 is 5 to 20 MW.

[0152] In a preferred embodiment, all power modules 19 are the same size and can be, for example, all 10MW or all 20MW.

[0153] According to some embodiments, each power module 19 further comprises a transformer 11.

[0154] According to some embodiments, the apparatus 10 may include a plurality of power modules 19, which are connected in parallel to each other and also connected in parallel to the power system 200 and the furnace 100.

[0155] By combining multiple power modules 19, the device 10 can be configured in a scalable manner according to the scale of the furnace 100 to be supplied.

[0156] According to one possible embodiment, a control and command unit 17 is connected to all power modules 19 and is configured to control at least each converter 15 so that each module 19 supplies the same supply voltage Ua, supply current Ia, and supply frequency fa to the electrodes 102. In this way, plant-wide malfunctions can be prevented.

[0157] In other modifications, the power module 19 can be configured to be controlled to supply different supply voltages Ua, supply currents Ia, and supply frequencies fa to each electrode 102, for example, to change the power distribution in a metal bath. The operation of the device 10 for stirring the metal material M in the electric furnace 100 described above (corresponding to the method according to the present invention) is as follows: - A step of supplying an AC system voltage Ur and system current Ir having a predetermined system frequency fr using a three-phase power supply means, - Using a transformer 11, the steps include converting the AC system voltage Ur and system current Ir into an arbitrarily configurable AC secondary voltage Us and secondary current Is, where the secondary frequency fs is substantially equal to the system frequency fr, -The steps of using multiple rectifiers 14 to rectify the secondary voltage Us and secondary current Is to obtain a DC intermediate voltage Ui and intermediate current Ii, - A step of using multiple converters 15 to convert a DC intermediate voltage Ui and intermediate current Ii into an AC supply voltage Ua and supply current Ia, wherein these can be arbitrarily set by a control and command unit 17 connected to the converters 15. - A step of supplying supply voltage Ua and supply current Ia of corresponding phases F1, F2, and F3 to multiple electrodes 102 and 106 of the furnace 100, Includes.

[0158] This method is characterized in that, in a series of predetermined continuous time intervals in the working cycle of the furnace 100, the adjustment device 18 of the control and command unit 17 controls the supply voltage Ua and supply current Ia, and in a certain time interval, one of the electrical phases F1, F2, and F3 that supply power to each electrode 102, 106 is adjusted to have a smaller amplitude Amin, or in some cases zero amplitude, compared to a predetermined amplitude Amax of the supply voltage and supply current supplied to each of the other two phases F1, F2, and F3 that supply power to the other two electrodes 102, 106. Furthermore, in subsequent time intervals, the phases F1, F2, and F3 with smaller or zero amplitude Amin of supply voltage Ua and supply current Ia are different from the phases F1, F2, and F3 of the previous time interval.

[0159] According to a preferred embodiment, the method can provide sequential activation of phases F1 and F2, phases F2 and F3, and phases F1 and F3 at a predetermined amplitude Amax, while activating or turning off the corresponding third phase F3, first phase F1, and second phase F2 at a small amplitude Amin, respectively.

[0160] Alternatively, phases F1 and F2 can be activated sequentially, followed by phases F1 and F3, and then phases F2 and F3, while simultaneously supplying or turning off phases F3, F2, and F1 at low amplitudes.

[0161] In particular, corresponding to each predetermined time interval, a pair of phases F1 and F2, F2 and F3, or F1 and F3 is activated at a predetermined amplitude Amax, while the amplitudes of the remaining phases F3, F1, and F2 are reduced or turned off. The number of time intervals, i.e., the number of times a pair of phases F1 and F2, F2 and F3, or F1 and F3 is activated, is determined according to the length of the work stage in which the material M is held in a stirred state in the furnace 100.

[0162] According to some embodiments, the predetermined amplitude Amax may correspond to the maximum nominal amplitude that can be supplied to the electrode 102, or it may be a smaller value depending on the requirements and stages of the work process or the type of metal material.

[0163] In this method, the supply frequency fa can be set lower than the system frequency fr in the initial stage SI of the work cycle. Furthermore, the supply frequency fa can be set higher than the system frequency fr in the final stage SF of the work cycle.

[0164] Preferably, the dissolution time can be shortened by controlling the supply frequency fa. For example, in an EAF furnace (electric furnace), the dissolution time can be shortened by approximately 5%.

[0165] With respect to the method for stirring a metal material M in an electric furnace 100 and the corresponding apparatus 10, it is possible to make partial and / or step changes and / or additions to the above-described content, but it is clear that this will not depart from the technical field and scope defined in the claims of the present invention.

[0166] Furthermore, although the present invention has been described based on specific embodiments, it will also be apparent to those skilled in the art that other equivalent methods and corresponding apparatus for stirring metal materials in an electric furnace can be realized, having the features described in the claims and thereby falling within the scope of protection defined in the claims.

[0167] In the following claims, references in parentheses are for readability purposes only and should not be considered restrictive to the scope of protection defined in the claims.

Claims

1. A method for stirring a metal material (M) in an electric arc furnace (100), The electric arc furnace (100) energizes multiple electrodes (102, 106) with phases (F1, F2, F3) of the three-phase AC supply voltage (Ua) and supply current (Ia), The three-phase AC supply voltage (Ua) and supply current (Ia) are obtained by rectifying a three-phase AC system voltage (Ur) and system current (Ir) having a predetermined system frequency (fr) using a plurality of rectifiers (14), and then converting them using a plurality of converters (15). The supply voltage (Ua) and supply current (Ia) can be selectively set by a control and command unit (17) connected to the converter (15). The adjustment device (18) of the control and command unit (17) sequentially adjusts the supply voltage (Ua) and supply current (Ia) for at least a portion of the work cycle of the furnace (100) so that, in each of a plurality of consecutive predetermined time intervals and throughout the entire duration of each of the intervals, the amplitude of the supply voltage (Ua) and supply current (Ia) of one of the electrical phases (F1, F2, F3) that energizes one of the electrodes (102, 106) is smaller (Amin) than the predetermined amplitude (Amax) of the supply voltage (Ua) and supply current (Ia) of the other two phases (F1, F2, F3) that energize the other two electrodes (102, 106), respectively. In the following time interval, the phases (F1, F2, F3) in which the amplitude (Amin) of the supply voltage (Ua) and supply current (Ia) is small are different from the phases (F1, F2, F3) in the immediately preceding time interval, and the molten material is stirred.

2. The first and second phases (F1, F2), the second and third phases (F2, F3), and the first and third phases (F1, F3) are sequentially driven with a supply voltage and supply current having a predetermined amplitude (Amax), while the remaining phases (F3, F1, F2) are sequentially driven with a supply voltage and supply current having a small amplitude (Amin). Alternatively, they are driven sequentially in the reverse order. The method according to claim 1.

3. Each of the phases (F1, F2, F3) is driven sequentially and periodically, with a predetermined amplitude (Amax) in two consecutive time intervals and with a smaller amplitude (Amin) in a third time interval, thereby changing the length of the electric arc generated by each of the electrodes (102, 106). As a result, a variable repulsive force is generated between the electric arcs of the electrodes (102, 106), and the molten material is stirred. The method according to claim 1 or 2.

4. In each of the aforementioned time intervals, the small amplitude (Amin) of one of the phases (F1, F2, F3) falls within the range of 0% to 70% of the predetermined amplitude (Amax) of the other two phases (F1, F2, F3). The method according to any one of claims 1 to 3.

5. The aforementioned small amplitude (Amin) is zero. The method according to any one of claims 1 to 4.

6. The electric arc (110) formed between each pair of electrodes (102, 106) and the material (M), which are powered by phases (F1, F2, F3) in which the supply voltage (Ua) and supply current (Ia) alternately fluctuate with a predetermined amplitude (Amax), generates a repulsive force (FA) that repels each other at a repulsion angle (α) of 120° to 180°. The first and second phases (F1, F2), the second and third phases (F2, F3), and the first and third phases (F1, F3) are sequentially driven with a predetermined amplitude (Amax), while the remaining phases (F3, F1, F2) are sequentially driven with a smaller amplitude (Amin), or in the reverse order, so that the direction of the repulsive force (FA) changes clockwise or counterclockwise, and as a result, a mixing force is generated that rotates and mixes the material (M). The method according to any one of claims 1 to 5.

7. The electrodes (102, 106) are positioned at locations corresponding to the vertices of a triangle located on a plane substantially perpendicular to the longitudinal direction of the electrodes (102, 106). The aforementioned triangle is substantially an equilateral triangle. The method according to any one of claims 1 to 6.

8. Each of the aforementioned time intervals is an integer multiple of the length of the period corresponding to the supply frequency (fa) of the supply voltage (Ua) and supply current (Ia). The method according to any one of claims 1 to 7.

9. Each of the aforementioned time intervals is in the range of 0.5 seconds to 10 seconds, preferably in the range of 1.5 seconds to 5.5 seconds. The method according to any one of claims 1 to 8.

10. The adjustment device (18) adjusts the supply electrical frequency (fa) of the supply voltage (Ua) and supply current (Ia) during the work cycle. The supply electricity frequency (fa) is lower than the system frequency (fr) at least in the initial stage (SI) of the work cycle. The method according to any one of claims 1 to 9.

11. The adjustment device (18) adjusts the supply frequency (fa) to a value in the range of 60% to 80% of the system frequency (fr) in the initial stage (SI), and to a value in the range of 30% to 60% of the system frequency (fr) in the final stage (SF). The method according to claim 10.

12. A device (10) for stirring a metal material (M) inside an electric arc furnace (100), The device (10) comprises a plurality of electrodes (102, 106) that are energized in the phases (F1, F2, F3) of the three-phase AC supply voltage (Ua) and supply current (Ia), respectively. The apparatus (10) comprises a plurality of rectifiers (14) connected to a plurality of converters (15), the plurality of converters (15) are controlled and commanded by a control and command unit (17), and the control and command unit (17) can adjust the supply voltage (Ua) and supply current (Ia) according to time. The converter (15) is configured to supply the AC supply voltage (Ua) and supply current (Ia), and to energize the corresponding phases (F1, F2, F3) to each of the electrodes (102, 106) of the furnace (100), The control and command unit (17) includes an adjustment device (18) configured to sequentially adjust the supply voltage (Ua) and supply current (Ia) for at least a portion of the work cycle of the furnace (100), such that in each of a plurality of consecutive predetermined time intervals, and throughout the entire duration of each interval, the amplitude of the supply voltage (Ua) and supply current (Ia) of one of the electrical phases (F1, F2, F3) energizing one of the electrodes (102, 106) is smaller (Amin) than the predetermined amplitude (Amax) of the supply voltage (Ua) and supply current (Ia) of the other two phases (F1, F2, F3) energizing the other two electrodes (102, 106), respectively. The adjustment device (18) is configured to change the phases (F1, F2, F3) in which the amplitude (Amin) of the supply voltage (Ua) and supply current (Ia) is small, in the next time interval, so that they are different from the previous time interval, in the next time interval, in the device (10).

13. The adjustment device (18) is configured to sequentially drive the first and second phases (F1, F2), the second and third phases (F2, F3), and the first and third phases (F1, F3) with a supply voltage and supply current having a predetermined amplitude (Amax), while sequentially driving the remaining phases (F3, F1, F2) with a supply voltage and supply current having a small amplitude (Amin). Alternatively, it may be configured to drive sequentially in the reverse order. The apparatus (10) according to claim 12.

14. The number of electrodes (102, 106) is 3 or a multiple of 3, and they are arranged at positions corresponding to the vertices of a triangle located on a plane substantially perpendicular to the longitudinal direction of the electrodes (102, 106). The apparatus (10) according to claim 12 or 13.

15. The adjustment device (18) is configured to adjust the supply frequency (fa) of the supply voltage (Ua) and supply current (Ia), wherein the supply frequency (fa) is lower than the system frequency (fr) at least in the initial stage of the work cycle and higher than the system frequency (fr) in the final stage of the work cycle. The apparatus (10) according to claim 12 or 13.

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