WO2025037353A1

WO2025037353A1 - Method for stirring metal material in electric furnaces and corresponding apparatus

Info

Publication number: WO2025037353A1
Application number: PCT/IT2024/050164
Authority: WO
Inventors: Antonello MORDEGLIA
Original assignee: Danieli Automation S.P.A.
Priority date: 2023-08-11
Filing date: 2024-08-06
Publication date: 2025-02-20

Abstract

The invention concerns a method for stirring metal material (M) in electric melting, heating and/or refining furnaces (100), which provides to supply three-phase alternating mains voltage (Ur) and mains current (Ir) having a predefined mains frequency (fr); transform said mains voltage (Ur) and mains current (Ir) into selectively settable alternating secondary voltage (Us) and secondary current (Is); rectify said secondary voltage (Us) and secondary current (Is) into direct intermediate voltage (Ui) and intermediate current (li); convert said direct intermediate voltage (Ui) and intermediate current (li) into selectively settable alternating supply voltage (Ua) and supply current (la); supply said supply voltage (Ua) and supply current (la) to a plurality of electrodes (102) of the furnace (100). The invention also concerns an apparatus (10) for stirring metal material (M).

Description

“METHOD FOR STIRRING METAL MATERIAL IN ELECTRIC FURNACES AND CORRESPONDING APPARATUS”

FIELD OF THE INVENTION The present invention concerns a method for stirring metal material in three- phase alternating current electric furnaces and a corresponding apparatus.

The present invention can be applied in the iron and steel industry and in the steel production sector, or also in sectors that work other metals in which electric furnaces are present, for example melting, heating, refining or similar furnaces. BACKGROUND OF THE INVENTION

Plants for melting, heating and/or refining metal material are known, comprising an electric furnace and one or more power supply apparatuses connected to a power supply network, which can be fed with metal material deriving from metal scrap but also from ore. One of the problems in the process for melting metal material is that this does not generally occur uniformly, and undissolved residues are generated which take a long time to melt. In addition, when an alloy is added to the starting metal material, it takes time to homogenize the alloy’s components, reducing the productivity of the furnace. In alternating current (AC) furnaces comprising electrodes positioned at the upper part of the container of the melting material and in which the arc closure occurs between one electrode and the other, by means of the passage of the arc in the material to be melted, the stirring effect is usually achieved with inert gas injections using porous plugs on the bottom of the furnace, or by applying magnetic fields using magnetic stirrers positioned under the container of the melting material.

In direct current (DC) furnaces there are an upper electrode and at least one lower electrode, necessary to close the electric circuit with the upper electrode, positioned on the bottom of the container of the melting material. The stirring effect can be achieved by means of additional devices disposed below the lower electrode: the efficiency of the magnetic field, and therefore of the mixing effect, is lower. However, in DC furnaces the magnetic field exerted by the additional devices collaborates with the magnetic field created by the electric arc and defines additional forces that globally lead to a more effective mixing of the steel flow than in AC furnaces.

US511447 concerns a system for positioning the electrodes in height as a function of the process, wherein when conditions occur that may lead to a short circuit of an electrode or respectively to a switching off of the respective electric arc, a regulation of the electric voltages supplied to one or more of the other electrodes is provided. In particular, in the event of a short circuit of one of the electrodes, the electric voltage supplied to the next electrode is increased in order to increase its arc length, while when the electric arc is switched off, the electric voltage supplied to both the other electrodes is lowered in order to reduce the length of the respective electric arcs. US511447 does not concern a solution in which a circuit is used to modulate the power of each of the electrodes, alternating it in intensity in order to generate a rotating effect in the metal material.

US3409726 concerns a DC electric arc furnace which uses an anode located on the bottom of the furnace to close the electric circuit, and therefore there is no closure between one electrode and the other. Stirring the molten metal requires the electrodes to be powered using direct current and also provides to use a DC electromagnetic stirrer positioned on the bottom of the furnace and able to cooperate with magnetic components positioned on the lateral walls of the furnace and defining respective magnetic poles. It is a different process from the one that uses a three-phase circuit that works in alternating current, and does not allow to vary the amplitude of the voltage and current fed to the different electrodes to obtain the stirring of the metal.

ES20 18840 also describes a DC furnace, in which the arc generated between cathode and anode is deflected by means of electric conductors disposed around or below the furnace tank.

There is therefore the need to perfect a method for stirring metal material in electric furnaces, and a corresponding apparatus, that can solve the problem of improving the stirring effect of the melting material in a simpler and at least as effective manner compared to the know solutions described above.

To do this, it is necessary to solve the technical problem of defining, for three- phase AC furnaces, a technology that allows to achieve an efficient stirring process by using, to mix the metal material, the apparatuses that are already comprised in the melting, heating and/or refining plant in an alternative manner, without therefore requiring any specific additional elements.

In particular, one purpose of the present invention is to perfect a method, and to provide a corresponding apparatus, that allows for an efficient stirring process. Another purpose of the present invention is to avoid using additional apparatuses dedicated only to stirring the metal material, with a consequent reduction in the cost and overall dimensions of the plant.

Another purpose of the present invention is to reduce the time required to achieve a homogeneous melting and/or a homogeneous mixing of the metal material, thus increasing the productivity of the melting, heating and/or refining plant.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages. SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims. The dependent claims describe other characteristics of the present invention or variants to the main inventive idea.

In accordance with the above purposes and to resolve the technical problem described above in a new and original way, also achieving considerable advantages compared to the state of the prior art, a method for stirring metal material in electric furnaces according to the present invention provides to feed, to a plurality of the furnace’s electrodes, a supply voltage and a supply current. In particular, each electrode is powered with one phase of a three-phase supply voltage and current. The supply voltage and current are obtained by rectifying, with a plurality of rectifiers, and converting, with a plurality of converters, a three-phase alternating mains voltage and mains current which have a predefined mains frequency.

The supply voltage and current can be selectively set by means of a control and command unit connected to the converters. In particular, both the amplitudes as well as the frequencies of the respective phases of the supply voltage and current can be selectively set.

In accordance with one aspect of the present invention, the method provides that, during at least part of a work cycle of the furnace, regulating devices of the control and command unit regulate the supply voltage and supply current sequentially, in such a way that, in each of a plurality of consecutive predefined time intervals, and for the entire duration of each of the intervals, the amplitude of the supply voltage and current of one of the electric phases that powers an electrode has a smaller amplitude than a determinate amplitude of the supply voltage and current of the other two phases that power the other electrodes.

According to some embodiments, the regulating devices command the converters in such a way that in each of the time intervals the smaller amplitude of one of the phases is comprised between 0% and 70% of the determinate amplitude of the other two phases, which can correspond to the maximum amplitude allowed, around 100%, or in any case to a predefined nominal maximum amplitude. The voltage and current amplitude of the other two phases can be substantially the same as each other.

“Substantially the same” is understood to mean that the voltage/current amplitudes of the other two phases have a same value, or at most differ by a quantity comprised between 0-10%.

The term amplitude is used here to generically indicate the maximum absolute value that can be assumed by one or the other of either the supply voltage or current, since they are closely correlated to each other. The amplitude is substantially considered with respect to the maximum value of the sinusoidal wave.

According to some embodiments, the smaller amplitude is comprised between 0% and 50% of the determinate amplitude of the supply voltage and current of the other two phases. According to some embodiments, the smaller amplitude is zero, that is, it has a value equal to zero.

According to some variants, the regulating devices command the converters so that, in each of a plurality of consecutive predefined time intervals, and for the entire duration of each of such intervals, the amplitude of the supply voltage and current is zero for one of the electric phases and has an alternating trend with a determinate amplitude on the other two phases.

In accordance with one aspect of the present invention, the method provides that, in a subsequent time interval, the phase for which the supply voltage and supply current have a zero or smaller amplitude is different from the phase of the immediately preceding time interval.

In other words, the method provides to sequentially activate and deactivate the phases so that in each time interval two phases with a determinate amplitude are switched on, while a third phase is switched on with a smaller amplitude, or possibly switched off, according to a predefined order.

In other words, the phases are controlled sequentially and cyclically so that in each predefined time interval two electrodes are powered with a current/voltage having the determinate amplitude, for which the respective electric arcs have a corresponding arc length, while a third electrode is powered with a current/voltage having a smaller or possibly zero amplitude, for which the respective electric arc has a shorter arc length or is possibly switched off.

In this way, in each time interval the electrodes powered with the two phases having the determinate amplitude will generate forces acting in opposing directions, while the third phase will generate a force with a smaller or zero entity. Doing so achieves at least the advantage that the repulsion forces generated by the electric arcs that are established between each electrode and the metal material cause an effective stirring of the molten metal material.

The greater the difference between the determinate amplitude of two of the phases and the smaller amplitude of another of the phases, the greater the variation of the forces generated and therefore the greater the movement effect created in the metal material.

It can also be provided that the amplitude of the phases and/or the difference between the determinate maximum amplitude and the minimum amplitude can be varied in relation to the different stages of the work process and/or the type of metal material being worked.

In accordance with another aspect of the present invention, the method can provide to activate in sequence with the determinate amplitude the first and the second phase, the second and the third phase and the first and the third phase, and to activate with the smaller amplitude, or switch off, the third phase, the first phase and the second phase, respectively.

In accordance with another aspect of the present invention, the method can provide, alternatively, to activate in sequence the first and the second phase, the first and the third phase and the second and the third phase with the determinate amplitude, and to activate with the smaller amplitude, or switch off, the third phase, the second phase and the first phase, respectively.

In accordance with another aspect of the present invention, the method can provide that, for each pair of electrodes, each powered by a phase in which the supply voltage and current have the determinate amplitude, the electric arcs that are established between the material and each electrode generate repulsion forces which repel each other with an angle comprised between 120° and 180°.

In the event two phases with balanced amplitudes are active, the angle between the forces can be of approximately 180°, while if there are three phases with unbalanced amplitudes, the angles between the phases will differ from 120°, and in particular the angle associated with the phase with the smallest amplitude tends to decrease proportionally to the imbalance of energy passing through that phase.

The sequential activation of the first and second phase, second and third phase and first and third phase with the determinate amplitude, or vice versa, at the same time as the reduction of the amplitude up to the possible sequential switching off of the third phase, first phase and second phase, generates a variation of the directions of the repulsion forces in a clockwise or counterclockwise sense, which leads to the generation of a mixing force which mixes, in rotation, the metal material.

Advantageously, the variation of the direction of the repulsion forces in a clockwise or counterclockwise sense given by the phases having the determinate amplitude leads to the generation of a mixing force which mixes, in rotation, the molten metal material. In accordance with another aspect of the present invention, the method can provide to position the electrodes in correspondence with the vertices of a triangle located on a plane which is substantially perpendicular to the longitudinal development of the electrodes.

Advantageously, if the triangle is substantially equilateral, the distribution of the mixing force in the molten metal material can be more homogeneous.

In accordance with another aspect of the present invention, the method can provide that each time interval has a duration corresponding to a multiple of the supply frequency of the supply voltage and current. In accordance with another aspect of the present invention, the method can provide that each time interval has a duration comprised between 0.5 and 10 seconds, preferably between 1.5 and 5.5 seconds.

In accordance with another aspect of the present invention, the method can provide that during the work cycle the regulating devices regulate the supply frequency.

In accordance with another aspect of the present invention, the method can provide that the supply electric frequency is lower than the mains frequency at least during initial stages of the work cycle. In accordance with another aspect of the present invention, the method can provide that the supply electric frequency is higher than the mains frequency during final stages of the work cycle.

In accordance with another aspect of the present invention, the method can provide that the regulating devices regulate the supply frequency in such a way that it assumes a value between 20 and 90 Hz.

The semiconductor devices of the rectifiers 14 and/or converters 15 can be commanded by the regulating devices 18 in such a way that the supply frequency is different from the mains frequency in one or more stages of the work cycle, and possibly in all stages. In accordance with other embodiments of the present invention, the regulating devices can regulate the supply frequency in such a way that it assumes values comprised between 60% and 80% of the mains frequency during the initial stages of the work cycle.

In accordance with another aspect of the present invention, the method can provide that the regulating devices regulate the supply frequency in such a way that it assumes a value comprised between 30% and 60% of the mains frequency during the final stages of the work cycle.

In accordance with another aspect of the present invention, an apparatus for stirring metal material in electric furnaces is also configured to electrically power the furnaces in three-phase mode.

In accordance with 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 to a command and control unit, which is configured to control and command the operation of the converters and regulate the supply voltage and current over time. The converters are also configured to supply an alternating supply voltage and current to the furnace’s electrodes.

In accordance with another aspect of the present invention, the control and command unit is provided with regulating devices configured to regulate, in at least part of a work cycle of the furnace, the supply voltage and supply current sequentially, in such a way that, in each of a plurality of consecutive predefined time intervals and for the entire duration of each of the intervals, the amplitude of the supply voltage and current of one of the electric phases that power the electrodes is smaller than a determinate amplitude of the supply voltage and current of the other two phases.

According to other embodiments, the control unit is configured in such a way as to regulate the supply voltages and currents by means of the regulating devices so that for each time interval the amplitude of the supply voltage and current is zero for one of the electric phases and has alternating values, that is, it follows an alternating and/or sinusoidal trend with determinate amplitude on the other two phases.

The regulating devices are also configured to vary the phase for which the supply voltage and supply current have a smaller or respectively zero amplitude, in a subsequent time interval with respect to an immediately preceding time interval.

In accordance with another aspect of the present invention, the regulating devices can be configured to activate in sequence the first and the second phase, the second and the third phase and the first and the third phase with the determinate amplitude, or vice versa. In this case, the third phase, the first phase and the second phase will be respectively activated in sequence with a smaller amplitude, or switched off.

In accordance with another aspect of the present invention, the electrodes can be three or multiples of three. The electrodes can be positioned in correspondence with the vertices of a triangle located on a plane which is substantially perpendicular to their longitudinal development. Advantageously, the triangle formed is substantially equilateral.

In accordance with another aspect of the present invention, the regulating devices can be configured to regulate the supply frequency of the supply voltage and of the supply current, in such a way that the supply frequency is lower than the mains frequency at least during the initial stages of the work cycle and higher during the final stages. In accordance with another aspect of the present invention, the apparatus comprises a transformer connected to power supply means of a three-phase alternating mains voltage and mains current, which have a predefined mains frequency. The transformer is configured to transform the alternating mains voltage and mains current into an alternating secondary voltage and secondary current, respectively, which can be selectively set and have a secondary frequency substantially the same as the mains frequency.

In accordance with another aspect of the present invention, the plurality of rectifiers is connected to the transformer and is configured to transform the alternating secondary voltage and secondary current into a direct intermediate voltage and intermediate current.

In accordance with another aspect of the present invention, the plurality of converters is configured to convert the direct intermediate voltage and intermediate current into the alternating supply voltage and supply current.

DESCRIPTION OF THE DRAWINGS These and other aspects, characteristics and advantages of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is a schematic view of an apparatus for stirring metal material in electric furnaces, according to the present invention; - fig. 2 is a schematic view of an apparatus for stirring metal material in electric furnaces, according to another embodiment;

- fig. 3 is a representation of the trend of the voltages on the electrodes of the apparatus for stirring metal material in electric furnaces of fig. 1 or 2, operating in a mode according to the state of the art; - fig. 4 is a time diagram of the voltages of fig. 3;

- fig. 5 is a representation of the arc discharge on the electrodes of the apparatus for stirring metal material in electric furnaces of fig. 1 or 2, operating in the mode of fig. 3; - fig. 6a is a representation of the trend of the voltages on the electrodes of the apparatus for stirring metal material in electric furnaces of fig. 1 or 2, operating in a first mode according to the invention;

- fig. 6b is a representation of the trend of the voltages on the electrodes of the apparatus for stirring metal material in electric furnaces of fig. 1 or 2, operating in a second mode according to the invention;

- fig. 7a is a time diagram of the voltages of fig. 6a;

- fig. 7b is a time diagram of the voltages of fig. 6b;

- figs. 8a, 8b, 8c are a representation of the arc discharge on the electrodes of the apparatus for stirring metal material in electric furnaces of fig. 1 or 2, operating in the mode of fig. 6a;

- figs. 9a, 9b, 9c are a representation of the arc discharge on the electrodes of the apparatus for supplying power and stirring metal material in electric furnaces of fig. 1 or 2, according to one embodiment; - fig. 10 is a graph of the trend of the power in some work cycles of an electric furnace with ECS charging;

- fig. 10a is a detail of the graph in fig. 10.

We must clarify that the phraseology and terminology used in the present description, as well as the figures in the attached drawings also in relation as to how described, have the sole function of better illustrating and explaining the present invention, their purpose being to provide a non-limiting example of the invention itself, since the scope of protection is defined by the claims.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can be conveniently combined or incorporated into other embodiments without further clarifications. DESCRIPTION OF SOME EMBODIMENTS OF THE PRESENT INVENTION

With reference to fig. 1 , some embodiments of the present invention concern an apparatus 10 for stirring metal material M in at least one electric furnace 100 for melting, heating and/or refining the material M. According to the invention, the apparatus 10 also comprises the function of three-phase power supply apparatus of the furnace 100.

According to some embodiments, the apparatus 10 can be powered by three- phase electric energy supply means. In the present description, by way of a nonlimiting example we will refer to a three-phase mains network 200.

The mains voltage Ur and the mains current Ir supplied by the network can have a predefined mains frequency fr. In accordance with possible solutions, the mains frequency fr has a value chosen between 50Hz and 60Hz, that is, based on the frequency of the mains network of the country in which the furnace 100 is installed.

The furnace 100 of the type in question can be an electric furnace for melting, heating and/or refining material M by means of an electric arc. The furnace 100 can therefore be an arc furnace, a submerged arc furnace, a ladle furnace, or in general a melting or refining or heating furnace, or suchlike, of the type suitable for use in a steel mill for the production of steel or in plants for working metal. Preferably, the invention is applicable to electric arc furnaces (EAFs), ladle furnaces (LF) and/or smelters which use electrodes 102, 106 to transfer thermal energy to the material M to be treated, or submerged arc furnaces (SAFs).

In the case of a furnace 100 of the EAF type, it comprises a container 101, or vat, into which metal material M to be melted is introduced. The EAF furnace is also provided with a plurality of electrodes 102 configured to ignite an electric arc through the material M and melt it. In the case of a ladle furnace LF, it generally comprises a ladle 104 suitable to contain the liquid material M tapped from the EAF furnace, a vault 105 which closes the ladle 104 at the top, and a plurality of electrodes 106 disposed passing through the vault 105.

According to some embodiments of the present invention, the electrodes 102, 106 are installed on movement devices 103, 107 configured to selectively move the electrodes 102, 106 toward or away from the material M, such as a mechanical, electrical, pneumatic, hydraulic actuator, an articulated mechanism, a mechanical kinematics, similar members or a possible combination thereof.

Hereafter in the description, we will refer primarily and by way of example to the EAF furnace; however, what is disclosed can be applied in a similar manner to the different types of furnaces 100 mentioned above.

There can be preferably three electrodes 102, as in the case shown, or they can be in multiples of three. In accordance with a possible solution of the present invention, if there are three electrodes 102, each one of them is connected to a respective phase of the three-phase power supply of the apparatus 10.

Preferably, the electrodes 102 are reciprocally positioned so as to form a triangular shape; in particular, they are positioned in correspondence with the vertices of a triangle located on a plane that is substantially perpendicular to the longitudinal development of the electrodes 102, 106 (figs. 3, 5 and 6). Preferably, this triangle is substantially equilateral, but it could also be isosceles or scalene.

Although it is known that in EAF or LF furnaces there are normally three electrodes, each powered with a respective phase, it is also possible to provide a larger number of electrodes, for example 4 or more, as provided in SAF furnaces.

In the case of four electrodes, they could for example be disposed at the vertices of a trapezoidal shape, or according to a rectangular or rhomboid conformation. In this case, it could be provided to keep the three phases associated with three electrodes active and switch off or decrease the amplitude of the fourth, or to keep two phases active and switch off or suitably decrease the other two, always in order to create repulsion forces suitable to generate a circulation of the molten material.

According to some embodiments, the apparatus 10 is able to receive energy supplied by the mains 200 and transform it into supply voltage and current having determinate electric parameters Ua, la, fa suitable to power the furnace 100. The supply voltage Ua and the supply current la can also assume electric parameters Ua, la which are also suitable to generate electric arcs 110, which are able to heat the material M.

A plant transformer 300 can be positioned between the mains 200 and the apparatus 10 in order to separate the apparatus 10 from the mains 200. The plant transformer 300 can supply low voltage LV (fig. 1) or medium voltage MV (fig.

2) to the apparatus. In the latter case, the apparatus 10 comprises another plant transformer 301 able to supply low voltage LV to the furnace 100.

According to some embodiments, the apparatus 10 comprises at least one transformer 11 connected to the mains 200 and configured to transform a primary alternating electric voltage Up and current Ip into a secondary alternating electric voltage Us and current Is.

In accordance with possible solutions, the transformer 11 can comprise a transformer primary 12 magnetically coupled to at least one transformer secondary This solution allows to reduce the impact of mains-side disturbances, that is, reduce the harmonic content and reactive power exchanged with the mains 200.

The secondary electric energy supplied by the transformer 11 has a secondary voltage Us, a secondary current Is, and a secondary frequency fs, which are predefined and set by the design characteristics of the transformer 11 itself.

According to some embodiments, the secondary frequency fs can be substantially equal to or lower than the mains frequency fr defined above or, in general, the primary frequency fp of the current circulating in the primary 12. The secondary voltage Us and the secondary current Is can be correlated, respectively, to the mains voltage Ur and to the mains current Ir or, in general, to the primary voltage Up and to the primary current Ip of the primary 12, by the transformation ratio of the transformer 11 itself.

The transformer 11 can be provided with regulating devices, not shown, provided to selectively regulate its electric transformation ratio in relation to specific requirements.

The apparatus 10 according to the present invention also comprises a plurality of rectifiers 14 connected to the transformer 11 and configured to transform the alternating secondary voltage Us and secondary current Is into direct intermediate voltage Ui and intermediate current li.

The rectifiers 14 can be chosen from a group comprising a diode bridge, a thyristor bridge, or other.

According to some embodiments, the apparatus 10 comprises a plurality of converters 15 connected to the rectifiers 14 and configured to convert the direct voltage and current into an alternating supply voltage Ua and supply current la for the electrodes 102.

In accordance with possible solutions, the rectifiers 14 can be connected to the converters 15 by means of at least one intermediate circuit 16 that works in direct current. The intermediate circuit 16 can be configured to generate a separation between the rectifiers 14 and the converters 15 and, therefore, with the mains 200 upstream of the intermediate circuit 16 with respect to the furnace 100. In particular, the rapid power fluctuations resulting from the process are partly filtered by means of the intermediate circuit 16, thus reducing their impact on the mains 200 side.

The intermediate circuit 16 can also be configured to store direct electric energy. According to some embodiments, this intermediate circuit 16 is a DC-link and comprises at least one capacitor. According to some embodiments, the apparatus 10 comprises a control and command unit 17 configured at least to control the converters 15 in such a way as to selectively set the parameters of the supply voltage Ua and supply current la generated by the converters 15 and supplied to the electrodes 102.

In particular and according to the invention, the control and command unit 17 is configured to regulate, during a predefined time interval in a work cycle of the furnace 100, the supply voltage Ua and current la in such a way that their amplitude on one of the phases Fl, F2, F3 has a smaller amplitude Amin compared to a determinate amplitude Amax of the alternating values of supply voltage Ua and current la on the other two phases. The control and command unit 17 is also configured to regulate the supply voltage Ua and current la in such a way that, in subsequent intervals, the phase Fl, F2, F3 of the supply voltage Ua and current la having smaller amplitude is different from the previous phase Fl, F2, F3.

According to some embodiments, in each of the time intervals the smaller amplitude Amin of the supply voltage Ua and current la of one of the phases Fl, F2, F3 is comprised between 0% and 70% of the determinate amplitude Amax of the other two phases F 1 , F2, F3.

The supply voltage Ua and current la amplitude of the other two phases can be substantially the same as each other, or differ from each other by a quantity comprised between 0-10%.

According to some embodiments, the smaller amplitude Amin of the supply voltage Ua and current la of one of the phases Fl, F2, F3 is comprised between 0% and 50% of the determinate amplitude Amax of the supply voltage Ua and current la of the other two phases. According to other embodiments, in each time interval the smaller amplitude Amin of the supply voltage Ua and current la of one of the phases is zero.

The intervals in which a phase Fl, F2, F3 has a smaller amplitude Amin, or the amplitude is zero, can have a duration corresponding to a multiple of the supply frequency fa, for example it can be a multiple of the mains frequency fr when the supply frequency corresponds to the mains frequency fr. The duration can be comprised between 0.5 and 10 seconds, preferably between 1.5 and 5.5 seconds.

With reference to a conventional mode of operation with a three-phase alternating power supply, a voltage of the respective phase Fl, F2, F3 is usually applied on each electrode 102, as shown in fig. 3, having a sinusoidal trend over time (fig. 4). Only at the instant of variation of the voltage sign is there a condition in which one electrode 102 is switched off while the other two are switched on, with a high-frequency switching on/off dynamic (for example, on one electrode 102, at a frequency fa of 50hz, there is a switching off and subsequent switching on every 0.01 seconds).

According to the invention, each phase Fl, F2, F3 can be sequentially and cyclically activated for two successive intervals with supply voltage and current having the determinate amplitude Amax, and for a third interval with the smaller amplitude Amin, so as to modify the length of the electric arc generated by the respective electrode 102, 106 to which the phase Fl, F2 and F3 is respectively connected. The sequential modification of the electric arc of one of the phases Fl, F2, F3 causes a modification of the repulsion forces that are generated between the respective electric arcs of the respective electrodes 102, 106. According to some embodiments, and as shown in fig. 5, the electric arcs 110 generated between each electrode 102 and the bath of molten material M tend to repel each other and therefore to enter the bath generating repulsion forces FA having a repulsion angle a of about 120° between each other.

With reference to the present invention, according to the embodiment described with reference to figs. 6a and 7a, only the phases Fl, F2, F3 applied to two of the electrodes 102 have non-zero values of voltage Ua and current la, with the determinate amplitude Amax, while the third phase has a zero value. The two corresponding arcs 110 therefore tend to enter the metal bath generating repulsion forces FA opposite each other, stably with a repulsion angle a of about 180° (figs. 8a, 8b and 8c).

By way of example and as shown in fig. 6a, if phases Fl and F2 are activated first, then phases F2 and F3 and then phases Fl and F3, repulsion forces FA at 180° are generated with different directions in the subsequent intervals. By the term activate, we mean that the supply voltage Ua and supply current la of the respective activated phases Fl, F2, F3 have non-zero amplitude.

The variation of the directions of the repulsion forces FA leads to the generation of a mixing force of the molten metal bath, in a clockwise sense with the order described above. By reversing the order of activation of the phases Fl, F2 and F3, the direction of the mixing is also reversed.

According to the invention, the arcs 110 are therefore able to create mixing forces that mix the material M contained in the furnace 100.

According to the embodiment described with reference to figs. 6b and 7b, all three phases F 1 , F2, F3 are powered; however, while two phases are powered with voltage Ua and current la values having the determinate amplitude Amax, the third phase is powered with the smaller amplitude Amin.

In this case, the two arcs 110 generated by the electrodes 102 powered with voltages and currents having the determinate amplitude Amax generate repulsion forces FA of substantially equal magnitude but in at least partly opposite directions, for example at an angle of about 120°, while the arc 110 generated by the electrode 102 powered with voltage and current with smaller amplitude Amin will generate a smaller force.

Preferably, the aforementioned method according to the invention can advantageously be applied during a refining stage corresponding to the last stage of the work cycle of the furnace 100, as shown in figs. 9 and 9a, in which all the material M is completely melted. However, the aforementioned method can be advantageously applied at any stage of the melting process, whether fed by endless charging ECS (fig. 10) or in baskets, allowing for a faster and more effective process for melting the material M.

According to some embodiments, the control and command unit 17 can also be configured to selectively regulate the supply voltage Ua and supply current la in relation to the required working powers, in the case of an EAF furnace in relation to the melting powers in play. According to some embodiments, the control and command unit 17 can also be configured to regulate the supply voltage Ua amplitude and the supply current la of the different phases Fl, F2, F3 and/or the difference between the determinate amplitude Amax and the smaller amplitude Amin, as a function of the different steps of the work process.

Furthermore, according to some embodiments, the control and command unit 17 can also be connected to the movement device 103 to allow to regulate the position of the electrodes 102 in relation to the different stages of the work cycle. In particular, the electrodes 102 are moved by the movement device 103 to follow the position of the material and thus change the length of the arc 110.

During a melting stage, in fact, the electric power supplied to the electrodes 102 can be increased compared to the perforation stage, since the arc 110 is by now presumed to be covered and distant from the vault of the furnace 100, and therefore the risk of damage to the latter is avoided. The references of the supply voltage Ua and supply current la can be changed through the control and command unit 17 in order to increase the active power. In this stage, the arc 110 is more stable, since it is protected by the scrap or slag.

In addition, during the refining stage, the process is much more stable and also requires less power.

In this way, the control and command unit 17 can manage and command, in relation to the specific stages of the work cycle, at least the following parameters: supply voltage Ua, supply current la and position of the electrodes 102. The high possibility of controlling the different parameters allows to achieve the aforementioned advantages chosen from: achieving a mixing effect of the molten material M, optimizing the transfer of energy to the process and, at the same time, reducing the effects on the mains 200 resulting from the rapid variations of power on the furnace 100 side.

Through the electric topology adopted for the converters 15 it is also possible to protect the mains 200 from disturbances caused by the melting process (flicker reduction, harmonics, Power Factor, etc.), while guaranteeing the stability of the arc 110 in the entire work cycle of the furnace 100.

The control and command unit 17 can comprise regulating devices 18.

In accordance with possible solutions of the present invention, the regulating devices 18 can comprise, purely by way of example, a hysteresis modulator or a

PWM (Pulse- Width-Modulation) modulator or suchlike.

These types of modulators can be used to command the semiconductor devices of the rectifiers 14 and/or converters 15: suitably controlled, they generate voltage Ua or current la values to be supplied to the furnace 100, in this specific case to the electrodes 102.

In particular, the modulator processes these voltage Ua and current la values and produces commands for driving at least the rectifiers 14 and the converters 15, so that the voltage Ua and current la quantities required by the control are present at the terminals for connection to the electrodes 102. The voltages Ua and currents la to be actuated are the result of operations carried out by the control and command unit 17 on the basis of the quantities read from the process and on the basis of the process model. According to the invention, the regulating devices 18 are configured to regulate, during the work cycle of the furnace 100, the values of the voltage Ua and current la, for a time interval, in such a way that their amplitude is zero, or has smaller amplitude Amin on one of the phases Fl, F2, F3 of the power supply and alternating values with determinate amplitude Amax on the other two phases F 1 , F2, F3.

They are also configured to regulate the supply voltage Ua and current la in such a way that, in subsequent intervals, the phase Fl, F2, F3 of the supply voltage Ua and current la having smaller amplitude Amin or zero amplitude is different from the previous phase Fl, F2, F3. In particular, the regulation of the phases Fl, F2, F3 so that they have the determinate amplitude Amax for two intervals and the smaller amplitude Amin for a third interval takes place sequentially and cyclically.

According to some embodiments, the regulating devices 18 can be configured to regulate, at each stage of the work cycle of the furnace 100, the supply electric frequency fa of the supply voltage Ua and supply current la.

The semiconductor devices of the rectifiers 14 and/or converters 15 can be commanded by the regulating devices 18 in such a way that the supply frequency fa is different from the mains frequency fr in one or more stages of the work cycle, and possibly in all stages. In particular, the semiconductor devices of the rectifiers 14 and/or converters 15 can be commanded by the regulating devices 18 in such a way that the supply frequency fa is higher or lower than the mains frequency fr during an initial stage SI of the work cycle. The semiconductor devices of the rectifiers 14 and/or converters 15 can be commanded by the regulating devices 18 in such a way that the supply frequency fa is lower than the mains frequency fr at least during a final stage SF of the work cycle. In particular, the work cycle of a furnace 100 can comprise an initial stage SI, an intermediate stage and a final stage SF (figs. 10 and 10a).

The initial stage SI can comprise a stage of charging and a stage of perforating the material M.

The intermediate stage can comprise a melting stage. The final stage SF can comprise a stage of refining the material M. In the final stage SF, the percentage of liquid material M can be higher than 90%, preferably 100%, without considering the percentage share of slag that is generated during the melting process.

In particular, during the initial stage SI, the electrodes 102 are brought closer to the charged solid material M, to trigger the electric arc and initiate the melting of the material M. As the material M progressively melts, the electrodes 102 penetrate into the still solid part of the material M to progressively melt it. When the electrodes 102 reach a position inside the container 101, the actual melting of the remaining material M surrounding the electrodes 102 begins. In accordance with one possible solution, the perforating and melting stages can be repeated several times before the refining stage, and between them there is provided a stage of charging additional material M into the furnace 100.

According to some embodiments, the supply frequency fa supplied to the electrodes can vary in a range comprised between 20-90Hz as a function of the stage of the work cycle. By way of example, the supply frequency fa can vary between -40% and +180% in the case of a mains frequency fr equal to 50Hz, and between -33% and +150% in the case of a mains frequency fr equal to 60Hz.

According to some embodiments, the supply frequency fa can vary in a range comprised between 60% and 80% of the mains frequency fr during the initial stages SI.

For example, it can reach a value lower than at least 20%, preferably at least 30%, even more preferably 40%, of the mains frequency fr.

The supply frequency fa can vary in a range comprised between 30% and 70% of the mains frequency fr during the final stages SF.

According to some embodiments, in a refining stage of the work cycle, the supply frequency fa can preferably be regulated so that it assumes values at least 30% lower than the mains frequency fr, that is, around or lower than 30Hz. According to other embodiments, the supply frequency fa can vary in a range comprised between 30% and 60% of the mains frequency fr during the final stages SF.

However, it is not excluded that in certain steps of the melting process it may be greater than that of the mains, for example comprised between 101% and 200% of the mains frequency fr.

For example, the control and command unit 17 can advantageously command the lowering of the supply frequency fa below the mains frequency fr during the initial stages of the melting in a basket-charged furnace 100, but also in some initial stages of the ECS-charging in an ECS-charged furnace 100 (figs. 10 and 10a). Advantageously, by lowering the supply frequency fa, the penetration angle p of the arc 110 into the bath of material M is reduced, maximizing heat transmission (fig. 9a).

Conversely, in those stages in which most or substantially all of the material M is liquid, and the requirement becomes making it uniform in temperature, by increasing the supply frequency fa the penetration angle p of the arc 110 into the bath of material M can be inclined (figs. 9b, 9c). A particularly inclined penetration angle p (fig. 9c), for example greater than 45°, generates a high thrust force that acts on the material M, comparable to that of a propeller, moving it effectively.

Indicatively, the control and command unit 17 can therefore be configured to command the variation of the penetration angle p from about 0° to almost 90°.

The variation of the supply frequency fa can be continuous or with discrete values. Figs. 9a, 9b and 9c show the penetration angles p at a first value lower than, at a second value substantially equal to, and at a third value higher than the mains frequency fr. According to possible solutions, the rectifiers 14 and the converters 15 are connected according to a modular configuration, defining as a whole a power supply module 19.

According to some embodiments, the apparatus 10 comprises a plurality of power supply modules 19, each of which contains at least one rectifier 14 and one converter 15, and is capable of supplying power from a minimum of 1 MW to a maximum of 30MW.

According to some embodiments, each power supply module 19 can comprise at least one rectifier 14, one converter 15 and one intermediate circuit 16 for two or more phases of a multi-phase electric network.

Preferably, all the power supply modules 19 can have a same size, that is, they can supply the same range of electric power.

Normally, the preferred sizing ranges of each of these power supply modules 19 range from 5 to 20MW.

In a preferred embodiment, all the power supply modules 19 are of a same size, for example they are all 10MW, all 20MW, etc.

According to some embodiments, each power supply module 19 also comprises a transformer 11. According to some embodiments, the apparatus 10 can be provided with a plurality of power supply modules 19, connected in parallel to each other, to the mains 200 and to the furnace 100.

The combination of several power supply modules 19 allows to achieve an apparatus 10 that is scalable in size in relation to the specific size of the furnace 100 to be powered.

In accordance with one possible solution, the control and command unit 17 is connected to all the power supply modules 19 in order to control at least the respective converters 15 so that each module 19 supplies the same values of supply voltage Ua, supply current la and supply frequency fa to the electrodes 102. In this way, it is possible to prevent malfunctions of the entire plant.

According to other variants, it can also be provided that the power supply modules 19 can be controlled in such a way as to supply respective different supply voltage Ua, supply current la and supply frequency fa values to each electrode 102, for example in order to vary the power distribution within the metal bath. The operation of the apparatus 10 for stirring metal material M in electric furnaces 100 described heretofore, which corresponds to the method according to the present invention, provides to:

- supply, by means of three-phase electric power supply means, an alternating mains voltage Ur and mains current Ir having a predefined mains frequency fr;

- transform, by means of a transformer 11 , the alternating mains voltage Ur and mains current Ir into a selectively settable alternating secondary voltage Us and secondary current Is which have a secondary frequency fs substantially equal to the mains frequency fr;

- rectify the secondary voltage US and secondary current Is with a plurality of rectifiers 14 to obtain a direct intermediate voltage Ui and intermediate current li;

- convert, with a plurality of converters 15, the direct intermediate voltage Ui and intermediate current li into alternating supply voltage Ua and supply current la which are selectively settable by means of a control and command unit 17 connected to the converters 15;

- power a plurality of electrodes 102, 106 of the furnace 100, each with a respective phase Fl, F2, F3 of the supply voltage Ua and supply current la.

The method provides that, during a sequence of successive predefined time intervals in a work cycle of the furnace 100, regulating devices 18 of the control and command unit 17 regulate the supply voltage Ua and the supply current la in such a way that, in one such time interval, one of the electric phases Fl, F2, F3 that power one of the electrodes 102, 106 has a smaller amplitude Amin, or possibly a zero amplitude, compared to a determinate amplitude Amax of the supply voltage and current supplied to the other two phases F 1 , F2, F3 that power respective other electrodes 102, 106. In subsequent time intervals, the phase Fl, F2, F3 for which the supply voltage Ua and supply current la have a smaller or possibly zero amplitude Amin is different from the phase Fl, F2, F3 of the previous time interval.

According to preferred embodiments, the method can provide to sequentially activate the phases F 1 and F2, the phases F2 and F3 and the phases F 1 and F3 with the determinate amplitude Amax, while respectively activating with smaller amplitude Amin, or switching off, the third phase F3, the first phase Fl and the second phase F2.

Alternatively, phases Fl and F2 can be activated in sequence, then phases Fl and F3 and then phases F2 and F3, while phases F3, F2 and Fl are powered with lower amplitude, or switched off.

In particular, to each predefined time interval there corresponds the activation of a pair of phases Fl and F2, F2 and F3 or Fl and F3 with the determinate amplitude Amax and the reduction of the amplitude or the switching off of the remaining phase F3, Fl, F2. The number of time intervals, and therefore the number of activations of the pairs of phases Fl and F2, F2 and F3 or Fl and F3, can depend on the length of the work stages of the furnace 100 in which the material M is to be kept under stirring.

According to some embodiments, the determinate amplitude Amax can correspond to the maximum nominal amplitude that can be supplied to the electrodes 102, or be a smaller value, as a function of the requirements and stages of the work process, or even of the type of metal material. The method can provide that the supply frequency fa is smaller than the mains frequency fr during an initial stage SI of the work cycle. Alternatively, the method can provide that the supply frequency fa is higher than the mains frequency fr during a final stage SF of the work cycle.

Advantageously, thanks to the regulation of the supply frequency fa, the melting time can be reduced. By way of example, in an EAF furnace, the reduction in the melting time can be approximately 5%.

It is clear that modifications and/or additions of parts and/or steps may be made to the method for stirring metal material M in electric furnaces 100 and to the corresponding apparatus 10 as described heretofore, without thereby departing from the field and scope of the present invention, as defined by the claims.

It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art will be able to achieve other equivalent forms of method for stirring metal material in electric furnaces and corresponding apparatus, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

In the following claims, the sole purpose of the references in brackets is to facilitate their reading and they must not be considered as restrictive factors with regard to the field of protection defined by the claims.

Claims

1. Method for stirring metal material (M) in an electric arc furnace (100) which provides to power a plurality of electrodes (102, 106), each with a respective phase (Fl, F2, F3) of a three-phase type alternating supply voltage (Ua) and supply current (la), which are obtained by rectifying, with a plurality of rectifiers (14), and by converting, with a plurality of converters (15), a three-phase alternating mains voltage (Ur) and mains current (Ir) which have a predefined mains frequency (fr), said supply voltage (Ua) and supply current (la) being able to be selectively set by means of a control and command unit (17) connected to said converters (15), said method being characterized in that, during at least part of a work cycle of said furnace (100), regulating devices (18) of said control and command unit (17) regulate said supply voltage (Ua) and supply current (la) sequentially, in such a way that, in each of a plurality of consecutive predefined time intervals, and for the entire duration of each of said intervals, the amplitude of said supply voltage and current (Ua, la) of one of the electric phases (Fl, F2, F3) that powers one of said electrodes (102, 106) has a smaller amplitude (Amin) than a determinate amplitude (Amax) of said supply voltage and current (Ua, la) of the other two phases (Fl, F2, F3) that power respective other electrodes (102, 106), and in that, in a subsequent time interval, the phase (Fl, F2, F3) for which said supply voltage (Ua) and supply current (la) have a smaller amplitude (Amin) is different from the phase (Fl, F2, F3) of the immediately preceding time interval, so as to determine the stirring of the molten material.

2. Method as in claim 1, characterized in that it provides to activate in sequence a first and a second of said phases (F 1 , F2), a second and a third of said phases (F2, F3) and a first and a third of said phases (Fl, F3) with supply voltage and current having said determinate amplitude (Amax), or vice versa, while the remaining phases (F3), (Fl), (F2) are sequentially powered with supply voltage and current having said smaller amplitude (Amin).

3. Method as in claim 1 or 2, characterized in that each phase (Fl, F2, F3) is sequentially and cyclically activated for two subsequent intervals with said determinate amplitude (Amax) and for a third interval with said smaller amplitude (Amin) so as to modify the length of the electric arc generated by the respective electrode (102, 106) and consequently obtain variable repulsion forces between the electric arcs of the respective electrodes (102, 106) which cause the stirring of said molten material.

4. Method as in any claim from 1 to 3, characterized in that in each of said time intervals said smaller amplitude (Amin) of one of said phases (Fl, F2, F3) is comprised between 0% and 70% of said determinate amplitude (Amax) of the other two phases (Fl, F2, F3).

5. Method as in any claim hereinbefore, characterized in that said smaller amplitude (Amin) is zero.

6. Method as in any claim hereinbefore, characterized in that it provides that electric arcs (110), which are established between said material (M) and each of a pair of said electrodes (102, 106) powered by a phase (Fl, F2, F3) in which said supply voltage (Ua) and supply current (la) have a trend alternating with said determinate amplitude (Amax), generate repulsion forces (FA) which repel each other with a repulsion angle (a) comprised between 120° and 180° and in that the activation in sequence of a first and a second of said phases (F 1 , F2), a second and a third of said phases (F2, F3) and a first and a third of said phases (F 1 , F3), or vice versa, with said determinate amplitude (Amin) while the remaining phase (F3, Fl, F2) is sequentially activated with said smaller amplitude (Amin), generates a variation of the directions of the repulsion forces (FA) in a clockwise or counterclockwise sense, which leads to the generation of a mixing force which mixes, in rotation, said material (M).

7. Method as in any claim hereinbefore, characterized in that it provides to position said electrodes (102, 106) in correspondence with the vertices of a triangle located on a plane which is substantially perpendicular to the longitudinal development of said electrodes (102, 106) and in that said triangle is substantially equilateral.

8. Method as in any claim hereinbefore, characterized in that it provides that each of said time intervals has a duration corresponding to a multiple of the supply frequency (fa) of said supply voltage (Ua) and supply current (la).

9. Method as in any claim hereinbefore, characterized in that it provides that each of said time intervals has a duration comprised between 0.5 and 10 seconds, preferably between 1.5 and 5.5 seconds.

10. Method as in any claim hereinbefore, characterized in that it provides that said regulating devices (18) regulate, during said work cycle, the supply electric frequency (fa) of said supply voltage (Ua) and supply current (la) and in that said supply electric frequency (fa) is lower than said mains frequency (fr) at least during initial stages (SI) of said work cycle.

11. Method as in claim 10, characterized in that it provides that said regulating devices (18) regulate said supply frequency (fa) in such a way that it assumes a value comprised between 60% and 80% of said mains frequency (fr) during said initial stages (SI) and a value comprised between 30% and 60% of said mains frequency (fr) during said final stages (SF).

12. Apparatus (10) for stirring metal material (M) in an electric furnace (100) comprising a plurality of electrodes (102, 106) which are electrically powered, each with a respective phase (F 1 , F2, F3) of a three-phase type alternating supply voltage (Ua) and supply current (la), by said apparatus (10) which comprises a plurality of rectifiers (14) to which a plurality of converters (15) are connected which are controlled and commanded by a control and command unit (17), which is able to regulate a supply voltage (Ua) and a supply current (la) over time, said converters (15) being configured to supply said alternating supply voltage (Ua) and supply current (la) and power each of said electrodes (102, 106) of said furnace (100) with the respective phase (Fl, F2, F3), said apparatus (10) being characterized in that said control and command unit (17) is provided with regulating devices (18) configured to regulate, in at least part of a work cycle of said furnace (100), said supply voltage (Ua) and supply current (la) sequentially, in such a way that, in each of a plurality of consecutive predefined time intervals and for the entire duration of each of said intervals, the amplitude of said supply voltage and current (Ua, la) of one of the electric phases (Fl, F2, F3) that powers one of said electrodes (102, 106) has a smaller amplitude (Amin) than a determinate amplitude (Amax) of said supply voltage and current (Ua, la) of the other two phases (Fl, F2, F3) that power respective other electrodes (102, 106), and in that said regulating devices (18) are configured to sequentially vary the phase (F 1 , F2, F3) for which said supply voltage (Ua) and supply current (la) have a smaller amplitude (Amin), in a subsequent time interval with respect to an immediately preceding time interval.

13. Apparatus (10) as in claim 12, characterized in that said regulating devices (18) are configured to activate in sequence with supply voltage and current having said determinate amplitude (Amax) a first and a second of said phases (Fl, F2), a second and a third of said phases (F2, F3) and a first and a third of said phases (F 1 , F3) or vice versa, while the remaining phases (F3), (Fl), (F2) are sequentially powered with supply voltage and current having said smaller amplitude (Amin).

14. Apparatus (10) as in claim 12 or 13, characterized in that said electrodes (102, 106) are three or multiples of three, and they are positioned in correspondence with the vertices of a triangle located on a plane which is substantially perpendicular to the longitudinal development of said electrodes (102, 106).

15. Apparatus (10) as in any claim from 12 to 13, characterized in that said regulating devices (18) are configured to regulate the supply frequency (fa) of said supply voltage (Ua) and supply current (la), in such a way that said supply frequency (fa) is lower than said mains frequency (fr) at least during initial stages of said work cycle and that it is possibly higher than said mains frequency (fr) during final stages of said work cycle.