Electromagnetic continuous casting method with combination of low-frequency pulses and differential phase pulses

Abstract

The invention discloses a low-frequency and differential-phase pulse combined electromagnetic continuous casting method, which belongs to the field of metal material preparation and processing, in the processes of semi-continuous casting, horizontal continuous casting and vertical continuous casting, A + 2B excitation coils are placed in a crystallizer jacket in the direction parallel to the central axis of a cast ingot, and A and B are integers greater than or equal to 1; the 2B excitation coils are divided into a group B1 and a group B2; a coils form a group; a power supply control system with a plurality of sets of built-in control circuits is utilized to respectively output two different currents and voltages which are applied to three groups of magnet exciting coils; one set of circuit system outputs low-frequency alternating-current voltage and sinusoidal current and generates a low-frequency alternating-current electromagnetic signal; the other set of circuit system outputs two groups of square-wave pulse voltage and zigzag pulse current; and a low-frequency electromagnetic field and a differential-phase pulsed magnetic field are applied to melt combinations with different heights in the crystallization sleeve, so that the electromagnetic utilization rate is increased, grains are refined, and the ingot casting quality is improved.

Description

Technical Field

[0001] The invention belongs to the field of metal material preparation and processing, and in particular relates to an electromagnetic continuous casting method combining a differential phase pulse magnetic field and a low-frequency magnetic field. Background Art

[0002] Ingots produced by traditional vertical continuous casting, horizontal continuous casting and semi-continuous casting have problems such as coarse grains, uneven structure, macro-segregation and micro-segregation, and serious oxide inclusions. How to improve the metallurgical quality of ingots through technical improvements has always been the goal of people's efforts. It is well known that the microstructure and properties of ingots are closely related to the solidification behavior of the melt. In recent years, the technology of applying external fields (electromagnetic fields, ultrasonic fields, etc.) during the metal solidification process to regulate the microstructure and properties of ingots has become a hot topic of current research. Related studies have shown that the addition of external fields can significantly affect the solidification behavior of metals and alloys, thereby reducing casting stress, refining grains, and improving macro-segregation and micro-segregation. In addition, there is no direct contact between the external field and the metal melt, and the production process is safe and pollution-free.

[0003] Due to the complex spatial and temporal matching of the existing ultrasonic melt treatment process, the serious attenuation of ultrasound in the metal melt, and other problems, the ultrasonic melt treatment process has not yet achieved large-scale industrial application. As an advanced metal processing technology, electromagnetic casting technology has developed rapidly since the mid-20th century and has become one of the important technologies in the field of metal material preparation. Its core principle is based on electromagnetic induction, which uses the effect of the magnetic field to control the solidification process of molten metal, thereby achieving precise control of the casting process of metal materials. The electromagnetic field has the advantages of being clean and pollution-free, not changing the melt composition, significantly controlling the solidification structure, and significantly improving the surface quality of the ingot. Therefore, related research on electromagnetic field to improve the solidification structure of metal has been continuously developing and improving. The emergence of electromagnetic casting technology is a major breakthrough in traditional metal casting technology, which solves many problems that are difficult to overcome with traditional casting technology.

[0004] The low-frequency electromagnetic casting technology developed by Northeastern University (Chinese patents CN1425520A, CN101020229A), the pulse electromagnetic casting method developed by Baosteel Group (Chinese patent CN104057046A), and the differential phase pulse magnetic field electromagnetic continuous casting method developed by Northeastern University (Chinese patent CN 108465792A) all show that the electromagnetic field has a significant effect on improving the solidification structure of the ingot and improving macro-segregation, and there are significant differences in the action mechanisms of different electromagnetic fields.

[0005] In vertical continuous casting, horizontal continuous casting and semi-continuous casting, the electromagnetic field mainly affects the temperature distribution and flow of the melt in the liquid phase and paste phase. For low-frequency electromagnetics, the low-frequency magnetic field generates an induced current inside the metal melt. The interaction between the magnetic field and the induced current causes the melt to be acted upon by force, and then generates forced convection driven by the force in the melt, which plays a homogenizing role.

[0006] The phase-difference pulse electromagnetic field divides multiple groups of excitation coils into two groups, and applies pulse currents with a phase angle difference of 90° to each group. The melt under the action of the phase-difference pulse electromagnetic field will produce oscillations and convection, which will break up dendrites and increase the nucleation rate on the one hand; on the other hand, it will promote the formation of crystal rain in the melt, thereby refining the grains. However, relevant studies have shown that low-frequency electromagnetic casting technology (LFEC, LFEVC) has limited refining effects on ingots with a diameter greater than 300 mm. The phase-difference pulse magnetic field is only effective at a certain frequency, and has certain requirements for current intensity, and the process range is relatively narrow. Summary of the invention

[0007] In view of the problems existing in the current vertical continuous casting, horizontal continuous casting and semi-continuous casting technologies, such as the electromagnetic conditions in different parts cannot be changed, the electromagnetic action range is limited and the utilization rate is low. The present invention provides a low-frequency and phase-difference pulse combined electromagnetic continuous casting method, which adopts an electromagnetic oscillation system with multiple sets of built-in control circuits to independently control multiple sets of solenoid coils to generate different magnetic fields. For different types of metals or alloys, their solidification characteristics can be considered during the casting process, and low-frequency electromagnetic fields and phase-difference pulse magnetic fields can be applied to the melts at different heights in the crystallization sleeve to improve electromagnetic utilization, refine grains, and improve ingot quality.

[0008] The low-frequency and differential-phase pulse combined electromagnetic continuous casting method of the present invention is:

[0009] In the process of vertical continuous casting, horizontal continuous casting and semi-continuous casting, A+2B excitation coils are placed inside the crystallizer jacket parallel to the central axis of the ingot, A and B are both integers greater than or equal to 1, and the 2B excitation coils are divided into two groups, each with B coils, called coil B 1 , B 2 ; A coil is set as a group, called coil A. Using a power control system with multiple sets of control circuits, two different currents and voltages are output respectively and applied to three sets of excitation coils. One circuit system outputs low-frequency AC voltage and sinusoidal current applied to coil A to generate low-frequency AC electromagnetic signals; another circuit system outputs two sets of square wave pulse voltage and sawtooth pulse current applied to two sets of coils B 1 , B 2The two sets of coils generate a phase difference of 60°-120° out-of-phase pulse oscillating magnetic field. The three sets of excitation coils generate a low-frequency AC magnetic field-phase difference pulse magnetic field combination to act on the metal melt and the mushy area.

[0010] In the method, the power control system generates a current intensity I=1-350A, an electromagnetic frequency f=2-50Hz, a duty cycle D=5%-80%, and obtains a target magnetic field intensity B=20-300mT in the metal melt and metal paste region.

[0011] In the method, three groups of excitation coils are stacked up and down and placed inside the crystallizer jacket by means of a fixing device, an insulating pad is arranged in the middle for isolation, and the coils and the solidifying metal are cooled by circulating water.

[0012] In the method, the A group of excitation coils is located at the upper position, and a low-frequency AC voltage and a sinusoidal current are applied to generate a low-frequency AC electromagnetic signal, which mainly acts on the metal liquid phase region. 1 , B 2 The two sets of coils are located at the lower position, and square wave pulse voltage and sawtooth pulse current with a phase difference of 60°-120° are applied to generate a phase difference pulse oscillating magnetic field, which mainly acts on the metal mushy area.

[0013] In the method, two different types of magnetic fields are mainly generated by a power supply control system, which is mainly composed of a transformer, a thyristor controller and corresponding components, a power amplifier, an inverter, a signal generator, a non-polar capacitor, an electrolytic capacitor, an IGBT controller and components, and a PLC controller.

[0014] In the method, the excitation coil is in a rectangular or annular shape, surrounds the metal melt and is placed in the crystallizer. The two excitation coil shapes correspond to the casting processes of flat ingots and cylindrical ingots respectively.

[0015] In the method, the excitation coil is wound by a flat copper wire coated with an insulating material, the number of turns of the excitation coil is 5 to 100 turns, and the number of superimposed layers is 5 to 20;

[0016] In the method, the excitation coil is 5 to 50 mm away from the bottom of the inner sleeve of the crystallizer and 5 to 40 mm away from the top of the inner sleeve of the crystallizer. A pad or a pad made of insulating material is used to achieve physical isolation. The pad or the pad is fixed together with the excitation coil.

[0017] In the method, A+2B excitation coils are divided into A, B 1 , B 2 Three groups, the number of coils in a single group is greater than or equal to 1.

[0018] In the method, the independent coils can be connected in series or in parallel through different wiring modes.

[0019] In the method, the current intensity, electromagnetic frequency and duty cycle can be adjusted.

[0020] In the method, the intensity, type and area of ​​the magnetic field acting on the metal melt and the mushy area are adjusted by controlling the number of excitation coils connected to the power supply control system.

[0021] In the method, for vertical continuous casting, horizontal continuous casting and semi-continuous casting processes, in order to ensure the action area of ​​the magnetic field, the height of the upper excitation coil needs to be 0 to 50 mm higher than the metal melt, and the lower excitation coil needs to be 0 to 30 mm lower than the metal paste area.

[0022] The main idea of ​​the invention is:

[0023] In view of the existing electromagnetic casting technology, the type of magnetic field applied is single, and it is impossible to apply a combined electromagnetic field in a targeted manner according to the solidification characteristics of a specific metal or alloy. The present invention improves the power supply control system, and generates different types of magnetic fields by independently controlling multiple groups of solenoid coils and applying different types of currents and voltages. The number of excitation coils can be adjusted to control the intensity, type, and action area of ​​the magnetic field; the low-frequency magnetic field generates an induced current inside the metal melt, and the interaction between the magnetic field and the induced current causes the melt to be acted upon by a force, and then generates forced convection driven by the force in the melt, which plays a homogenizing role; the melt under the action of the phase-difference pulse electromagnetic field will produce oscillations and convection, which will break up dendrites and increase the nucleation rate on the one hand; on the other hand, it will promote the formation of crystal rain in the melt, thereby refining the grains. For different metals or alloys, the combined electromagnetic field action area and intensity can be adjusted while considering the solidification characteristics, so that alloy ingots with smaller grains and lower segregation can be prepared.

[0024] The present invention has the following advantages and positive effects through the above technical features:

[0025] (1) The electromagnetic continuous casting method combined with low-frequency and differential-phase pulses can adapt to the preparation process of ingots of various types and sizes by adjusting the number, size, and placement of the excitation coils.

[0026] (2) In the process of vertical continuous casting, horizontal continuous casting or semi-continuous casting, the solidification characteristics of ingots of different types and sizes can be considered, and a better combined magnetic field condition can be applied to prepare high-quality ingots with smaller grains and lower segregation.

[0027] (3) Low-frequency magnetic field can cause forced convection in metal melt and reduce macro-segregation. Differential phase pulse magnetic field can cause oscillation and convection in the melt, change the atomic aggregation state in the mushy zone, and provide more nucleation centers. The combined application of the two can improve the utilization efficiency of the magnetic field and save energy.

[0028] (4) The combined magnetic field of low-frequency and phase-difference pulses improves electromagnetic utilization, further reduces the required current intensity and frequency, and achieves the purpose of saving energy and reducing costs.

[0029] (5) The magnetic field does not come into direct contact with the molten metal, and the production process is safe, clean and highly automated.

[0030] (6) This method is applicable to aluminum alloys, magnesium alloys, copper alloys and steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of an oscillating electromagnetic power system for generating a combined magnetic field.

[0032] Figure 2 (a) is a typical current and voltage diagram for generating a phase difference pulse oscillating magnetic field. max is the voltage peak value, f is the sawtooth current frequency, Dc is the duty cycle, I max is the current peak value, ΔФ is the phase angle difference; Figure 2 (b) is a schematic diagram of the sinusoidal current waveform that generates a low-frequency AC magnetic field. max is the current peak value, f is the low-frequency AC current frequency; Figure 2 Here 1 / f represents the period of current and voltage change.

[0033] Figure 3 The present invention is a schematic diagram of the structure of a low-frequency and phase-difference pulse combined electromagnetic continuous casting device for vertical continuous casting or semi-continuous casting according to an embodiment of the present invention. In the figure, 1, cooling water outlet, 2, insulating pad, 3, fixing rod, 4, outer shell of crystallizer, 5, pad, 6, fixing bolt I, 7, molten metal, 8, metal paste area, 9, inner shell of crystallizer, 10, fixing bolt II, 11, fixing bolt III, 12, cooling water inlet, 13, excitation coil, 14, solid metal, 15, ingot inlet, 16, secondary cooling water outlet.

[0034] Figure 4 Schematic diagram of the connection mode of the excitation coil in the low-frequency and differential phase pulse combined magnetic field electromagnetic continuous casting method in the embodiment of the present invention; wherein (a) is embodiment 1, and (b) is embodiment 2; in embodiment 1, coils 1# and 2# are connected in series to form coil group A, and sinusoidal current is passed through to generate a low-frequency AC magnetic field; coils 3# and 5# are connected in series to form coil B 1 , 4#6# coils are connected in series to form coil B 2 , two sets of coils B 1 and B 2 A square wave pulse voltage and a sawtooth pulse current with a phase difference of 90° are passed to generate a phase difference pulse oscillating magnetic field. In Example 2, coil 1# is idle, a sinusoidal current is passed through coil 2# to generate a low-frequency AC magnetic field, and coils 3# and 4# are connected in series to form coil B.1 , 5# and 6# coils are connected in series to form coil B 2 , two sets of coils B 1 , B 2 A square wave pulse voltage and a sawtooth pulse current with a phase difference of 90° are introduced to generate a phase difference pulse oscillating magnetic field; in the figure, 17, A group excitation coil, 18, magnetic lines of force, 19, metal melt, 20, coil B 1 , 21, coil B 2 .

[0035] Figure 5 This is a schematic diagram of the intensity variation of the combined addition of low-frequency + differential phase pulse magnetic field (abbreviated as L-DP) within a single current cycle in Example 1 of the present invention.

[0036] Figure 6 In Example 1 of the present invention, the streamline changes during the conventional DC casting without a magnetic field (a) and the combined addition of L-DP magnetic field casting (b) are compared.

[0037] Figure 7 In Example 1 of the present invention, (a) schematic diagram of the semi-continuous casting model and the location of relevant analysis points; (b) flow rate comparison during traditional DC casting without magnetic field and combined addition of L-DP magnetic field casting at point b in a stable state.

[0038] Figure 8 In Example 1 of the present invention, they are respectively the temperature field and liquid fraction distribution cloud diagrams in the semi-continuous casting stable state without magnetic field and in the combined addition of L-DP magnetic field.

[0039] Fig. 9 These are the total heat flux variation diagrams in the radial direction in Example 1 of the present invention, respectively, in the semi-continuous casting stable state without a magnetic field and in the combined addition of an L-DP magnetic field.

[0040] Fig.10 In Example 1 of the present invention, the temperature gradient in the radial direction of the melt in the semi-continuous casting stable state without magnetic field and in the combined addition of L-DP magnetic field is

[0041] Fig.11 In Example 1 of the present invention, they are the cooling rates in the radial direction of the melt in the semi-continuous casting stable state without a magnetic field and in the combined addition of an L-DP magnetic field.

[0042] Fig.12 The grain structure of the AZ31 magnesium alloy prepared by the comparative test between Example 1 of the present invention and conventional DC casting. In the figure, (a) conventional DC casting; (b) Example 1 of the present invention.

[0043] Fig.13The grain structure of the AZ80 magnesium alloy prepared by the comparative test between Example 2 of the present invention and conventional DC casting. In the figure, (a) conventional DC casting; (b) Example 2 of the present invention. DETAILED DESCRIPTION

[0044] The method and device of the present invention are further explained below in conjunction with the accompanying drawings and embodiments.

[0045] In the embodiment of the present invention, the structure of the low-frequency and differential-phase pulse combined electromagnetic continuous casting device for vertical continuous casting or semi-continuous casting is as follows: Figure 3 As shown. The crystallizer outer sleeve 4, the pad 5 and the crystallizer inner sleeve 9 constitute the internal space of the crystallizer. The pad 5 is fastened to the crystallizer outer sleeve 4 by the fixing bolts I 6, and is fastened to the crystallizer inner sleeve 9 by the fixing bolts II 10. In this internal space, 6 excitation coils are placed therein, and the coils are separated by insulating pads 2. The insulating pads 2 corresponding to each coil are processed with positioning holes of the same specifications, which are positioned in conjunction with the fixing rod 3 and fixed with fixing bolts III 11. During the production process, the molten metal enters the crystallizer inner sleeve through the upper opening of the crystallizer inner sleeve 9. The lower melt solidifies first to form solid metal 14, and the early bottoming is completed at this time. As the molten metal continues to be poured, the upper layer of the molten metal has not yet solidified to form a liquid phase zone, and the lower layer of the molten metal is incompletely solidified to form a paste zone 8. At this time, the excitation coil A generates a low-frequency AC magnetic field, causing the molten metal 7 to undergo forced convection, and the coil B 1 , B 2 A phase-difference pulse oscillating magnetic field is generated to break up the dendrites and provide more nucleation centers. As the casting process proceeds, the solidified part of the ingot is slowly pulled out through the ingot introduction port 15 until the casting process is completed.

[0046] In the embodiment of the present invention, when vertical continuous casting or semi-continuous casting is performed, the excitation coil 13, the insulating pad 2, the pad 5 and the fixing rod 3 are located in the crystallizer outer shell 4, and the circulating cooling water enters the crystallizer outer shell 4 through the cooling water inlet 12, and cools the excitation coil 13 and the metal in the crystallizer inner shell 9, and then the circulating cooling water is discharged from the cooling water outlet 1. The crystallizer inner shell 9 and the pad 5 are fixed by fixing bolts II 10, and the pad 5 is fixed to the crystallizer outer shell 4 by fixing bolts I 6, and the crystallizer outer shell 4, the crystallizer inner shell 9 and the pad 5 constitute the overall appearance of the crystallizer.

[0047] When the crystallizer is working, according to the solidification characteristics of the metal, taking into account the vertical height between the liquid phase and the solid phase, that is, the size of the mushy zone, all the excitation coils are divided into A and B 1 , B 2 There are three groups. The excitation coil of group A needs to be consistent with or slightly higher than the vertical height of the metal melt 7 (i.e. 0 to 50 mm higher than the metal melt). 1, B 2 The two sets of excitation coils need to be consistent with or slightly lower than the vertical height of the metal paste area 8 (i.e. 0 to 30 mm lower than the metal paste area) to ensure that different magnetic fields correspond to different solidification states, such as Figure 4 shown.

[0048] In the embodiment of the present invention, the secondary cooling water cools the excitation coil 13 and the metal in the solidification process through the secondary cooling water outlet 16 and is discharged.

[0049] In the embodiment of the present invention, the ingot starter head is embedded in the ingot starter port 15, and continuous production is performed by applying a pulling force, and the pulling force can be matched and adjusted according to the casting speed.

[0050] In the embodiment of the present invention, the excitation coil located in the outer sleeve of the crystallizer is spaced 5 to 20 mm from the inner sleeve of the crystallizer, and is spaced 5 to 40 mm from the outer sleeve of the crystallizer.

[0051] The melt processed in the embodiment of the present invention is aluminum alloy, magnesium alloy, copper alloy or steel.

[0052] Example 1

[0053] In the present invention, in the low-frequency and phase-difference pulse combined electromagnetic continuous casting method, the circuit principle of the electromagnetic oscillation system is as follows: Figure 1 As shown. Through two independent circuit systems, the same power supply system can output current and voltage with two different waveforms.

[0054] Figure 2 Two typical current and voltage schematics generated by power control systems. Figure 2 (a) is a phase difference pulse signal, the voltage is a square wave, and the current is a sawtooth pulse wave. Figure 2 (b) is a low-frequency sinusoidal current. The current I, voltage V, duty cycle D and frequency f can be adjusted through the power control system. The adjustable range is: current intensity I = 1 ~ 350A, electromagnetic frequency f = 2 ~ 50Hz, duty cycle D = 5% ~ 80%.

[0055] In Example 1, the current of the excitation coil of group A is sinusoidal with a frequency of 50 Hz and a current range of 0 to 15 A. 1 , B 2 The current intensity of the two sets of excitation coils is 1 to 11.5A, and the voltage duty cycle of the two sets of excitation coils is 20%. 1 , B 2 There is a 90° phase difference between the pulse currents applied to the group excitation coils.

[0056] In this embodiment 1, the alloy grade is AZ31. Although the alloying degree of AZ31 is low, the two-phase region is narrow during solidification, resulting in a large solute viscosity in the mushy region. In traditional semi-continuous casting, it is not only very easy to cause a large macro-segregation, but also the solidification structure is uneven, which is very easy to cause a large casting stress, leading to cracking of the casting. The low-frequency magnetic field can enhance the forced convection effect of the solute, so the 1#2# magnetic field is connected in series, and a sinusoidal current is passed to generate a low-frequency AC magnetic field. Before the metal melt enters the mushy region, the low-frequency magnetic field can enhance the convection movement of different parts in the metal melt 7, achieve a homogenization effect, and reduce macro-segregation.

[0057] In this embodiment 1, a semi-continuous casting method is used to produce a cylindrical billet with a diameter of 95 mm, specifically, the alloy grade is AZ31. In this embodiment 1, 6 annular coils are built into the crystallizer, namely coils 1#, 2#, 3#, 4#, 5# and 6#. The number of turns of a single coil is 27, the inner diameter is 120 mm, and it is made into a circular ring shape using a coated copper wire with a rectangular cross section. The cross section of the copper wire is 8 mm long and 2.5 mm wide. Coils 1# and 2# are stacked and placed in series to form group A, and a sinusoidal current is passed through to generate a low-frequency AC magnetic field. Under the action of cooling water, the metal melt 7 with a greatly reduced degree of segregation is transformed into a mushy zone 8. The phase-difference pulse electromagnetic field has a large instantaneous energy, and the magnetic field force acts in different directions, which can cause the melt to oscillate and can effectively refine the grains. Therefore, coils 3# and 5# are connected in series to form coil B. 1 , 4# and 6# coils are connected in series to form coil B 2 , two sets of coils B 1 , B 2 A square wave pulse voltage and a sawtooth pulse current with a phase difference of 90° are introduced to generate a phase difference pulse oscillating magnetic field to act on the mushy area 8.

[0058] 6 sets of coils according to Figure 4 (a) is connected and divided into three groups, which are placed in the inner space of the crystallizer in an orderly manner, and the individual coils are physically isolated using insulating pads with a thickness of 2 mm, and are fastened using fixing screws. After fastening, the bottom layer of coils is 8 mm away from the bottom of the crystallizer, and the top layer of coils is 12 mm away from the cover plate. In this embodiment 1, the inner diameter of the inner sleeve of the crystallizer matches the size of the cylindrical ingot.

[0059] Before casting begins, a sinusoidal current is passed through the three sets of excitation coils in set A to generate a low-frequency AC magnetic field; 1 , B 2 A square wave pulse voltage is passed through the excitation coil to generate a sawtooth pulse current, which excites the pulse oscillating magnetic field. 1 The square wave pulse voltage passed into the excitation coil of the group is 2The square wave pulse voltage passed into the excitation coil has a phase difference of 90°. 1 , B 2 The group of excitation coils forms a pulse oscillating magnetic field with a phase difference of 90°.

[0060] During the casting process of Example 1, the voltage of the excitation coil of group A remains unchanged, and the current changes sinusoidally according to the frequency. The low-frequency current acting on the excitation coil of group A is 0-15A, and the electromagnetic frequency is 50Hz. 1 , B 2 The voltage on the excitation coil of the group is Figure 2 (a) is changed, acting on B 1 , B 2 The sawtooth current on the two sets of excitation coils has a current intensity of 1 to 11.5A, and the voltage duty cycle in the two sets of excitation coils is 20%. 1 , B 2 There is a 90° phase difference between the pulse currents applied to the group excitation coils.

[0061] Then, the mushy zone 8 is further cooled and transformed into a solid phase metal 14 , and finally, a traction device arranged at the ingot inlet 15 is used to complete the continuous production.

[0062] Figure 5 This is a schematic diagram of the intensity variation of the combined addition of low-frequency + differential phase pulse magnetic field (abbreviated as L-DP) within a single current cycle in Example 1 of the present invention.

[0063] Depend on Figure 6 It can be seen that in the absence of a magnetic field, the flow of the molten metal proceeds in a relatively regular manner, and the overall flow field is relatively uniform. After the melt enters from the top, it flows downward along the mold wall. There is a more obvious eddy current area in the upper free surface area. This is because the melt flows under the action of gravity and generates backflow after hitting the mold wall. After the L-DP magnetic field is applied, the flow field changes significantly. At this time, the flow of the melt becomes more complicated. The most significant change is the increase in the upper eddy current area and the greater fluctuations in the streamlines. This shows that the L-DP magnetic field will significantly affect the flow of the melt. After the magnetic field is applied, the velocity distribution becomes more uneven, especially in the eddy current area, where the velocity increases significantly, indicating that the magnetic field accelerates the flow of the melt in this area. Compared with the condition without a magnetic field, the eddy current area under the action of the magnetic field is more obvious and concentrated. And in. Figure 7In the semi-continuous casting stable stage, taking point b as an example, after applying the L-DP magnetic field, the melt flow velocity showed obvious oscillation in the time dimension and showed periodic changes. In the absence of a magnetic field, the velocity hardly fluctuated, indicating that the magnetic field plays a significant role in controlling the flow and solidification process. Combining the theories of fluid mechanics and electromagnetism, the flow behavior of the melt under the action of a magnetic field can be better described. In the absence of a magnetic field, under the action of gravity and wall resistance, the melt flows downward in a relatively regular form. In the top area, due to inertia and fluid impact on the mold wall, a reflow area (eddy current) is formed, but the overall velocity is small and the velocity distribution is uniform.

[0064] This process can be described by the classic fluid dynamics equation Navier-Stokes equation:

[0065]

[0066] Where: ρ-fluid density, u-flow velocity vector, p-pressure, μ-dynamic viscosity coefficient, g-gravitational acceleration.

[0067] In the absence of a magnetic field, the movement of the melt is mainly controlled by gravity and viscous forces, the flow state is relatively stable, a certain eddy structure is formed, and the speed is relatively uniform. After the L-DP magnetic field is added, the flow of the melt changes significantly. The eddy current is more obvious, especially in the top area, indicating that the electromagnetic force introduced by the magnetic field causes the flow of the fluid to intensify. At this time, the movement of the melt is not only affected by gravity and viscous forces, but also the effect of electromagnetic force.

[0068] Magnetohydrodynamics (MHD) describes the effect of magnetic fields on fluids by adding the Lorentz force to the Navier-Stokes equations:

[0069]

[0070] Where: J×B is the Lorentz force, which describes the effect of the magnetic field on the fluid; J: current density B: magnetic induction intensity;

[0071] J×B: Lorentz force, which indicates the force generated by the interaction between electric current and magnetic field. The Lorentz force exerts additional force on the conductive fluid, causing the fluid's velocity and flow pattern to change.

[0072] Figure 8 It shows that the temperature distribution at the edge, 1 / 2R and center positions changes with the distance from the liquid surface during the semi-continuous casting process under no magnetic field and applied L-DP magnetic field.

[0073] The L-DP magnetic field improves heat conduction and reduces temperature gradients by enhancing the mixing effect of the fluid, especially in the central area. In the absence of a magnetic field, the temperature distribution of liquid metal is mainly controlled by conduction heat dissipation, while the action of the magnetic field introduces a more complex convection mechanism: the agitation of the fluid under the action of the Lorentz force promotes the uniform distribution of heat inside the melt. At the edge, heat conduction is still the dominant mechanism, so the influence of the magnetic field is not obvious, while at the 1 / 2R and center positions, the magnetic field enhances convection and reduces the temperature gradient, especially in the solidification zone, where the temperature change is smoother, which is conducive to the formation of a uniform solidification structure.

[0074] In the absence of a magnetic field, the temperature field of liquid metal during solidification is mainly determined by heat conduction, and the temperature gradient is large, especially at the center. After applying the L-DP magnetic field, the temperature distribution of the melt becomes more uniform and the temperature gradient decreases, especially at the center and 1 / 2R positions. The magnetic field significantly enhances the convection effect of the fluid, which contributes to a more uniform cooling and solidification process.

[0075] Fig. 9 The temperature field and liquid fraction of AZ31 magnesium alloy under no magnetic field and L-DP magnetic field conditions are compared. The changes in temperature field and solidification area under different magnetic field conditions can be seen. Under no magnetic field conditions: the liquid cavity depth is 134mm, and after applying the L-DP magnetic field, the liquid cavity depth is reduced to 104mm. This is because the magnetic field enhances the convection effect of the fluid, the heat is more evenly distributed, and the solidification process is accelerated. The fluid movement caused by the Lorentz force makes the temperature gradient more uniform, resulting in faster cooling of the casting.

[0076] Without a magnetic field, the temperature field and liquid fraction distribution are relatively smooth, and the liquid cavity is deeper, indicating that the solidification rate of the metal is slow. After the L-DP magnetic field is applied, the convection of the fluid is significantly enhanced, resulting in a more uniform temperature field, faster heat transfer, and reduced liquid cavity depth. This shows that the application of a magnetic field helps to accelerate the solidification process, improve the quality of castings, and reduce defects caused by uneven cooling.

[0077] Fig.10The results show the change of the total radial heat flux with the distance from the center position (m) under the conditions of no magnetic field and L-DP magnetic field. Under the conditions of no magnetic field, the total radial heat flux gradually increases with the increase of the distance from the center position, and the heat flux value reaches the maximum at the edge position (about 0.045m). This shows that under the conditions of no magnetic field, the heat conduction of metal mainly relies on the natural heat conduction mechanism, and the heat flux gradually increases when the heat is transferred from the center area to the edge area. After applying the L-DP magnetic field, the total radial heat flux is significantly higher than that under the conditions of no magnetic field, and the heat flux increases significantly with the increase of the distance from the center position. Compared with the conditions of no magnetic field, the heat flux under the L-DP magnetic field increases faster, indicating that the magnetic field significantly enhances the radial heat transfer, especially in the edge area far from the center, where the heat flux is particularly increased. This is because the magnetic field greatly improves the heat transfer efficiency, shortens the solidification time, and helps to obtain a more uniform casting structure by enhancing the convection of liquid metal.

[0078] Fig.11 The change of radial temperature gradient under no magnetic field and L-DP magnetic field conditions is shown. The temperature gradient is relatively small when there is no magnetic field, especially near the center of the casting, the temperature gradient is very gentle, indicating that under no magnetic field conditions, heat mainly depends on heat conduction transfer, the temperature distribution inside the casting is uniform, and solidification is relatively slow. Under the L-DP magnetic field, the temperature gradient increases significantly, especially near the edge, the temperature gradient rises sharply, reaching 3984K / m, compared with 1748K / m when there is no magnetic field, the temperature gradient increases significantly. This shows that under the action of the magnetic field, by enhancing the convection effect and heat transfer, the temperature gradient in the solidification zone is greatly increased, and the cooling rate of the casting is accelerated.

[0079] The temperature gradient can be expressed by the following formula:

[0080]

[0081] Where: G is temperature gradient, ΔT is temperature difference, Δx is distance difference.

[0082] The higher temperature gradient under the L-DP magnetic field indicates that the temperature difference between the liquid and solid phases is greater during the solidification process, which means that the magnetic field promotes heat conduction and convection effects, significantly increasing the solidification rate of the casting. The increase in temperature gradient helps to improve the microstructure of the casting. During the solidification process, a larger temperature gradient helps to form fine and uniform grains, thereby improving the mechanical properties of the material. In addition, the introduction of the magnetic field can accelerate the cooling of the casting, shorten the production cycle, and improve production efficiency.

[0083] The quality of the ingot is affected by the grain morphology, especially the distribution of dendrites, which has a crucial influence on the performance of the ingot.

[0084] The process of dendrite growth is usually affected by temperature gradient, cooling rate and melt flow. In the absence of a magnetic field, the flow of liquid metal mainly depends on natural convection, and the dendrite growth process is relatively slow, which may lead to a large distance between dendrites and form a coarse grain structure. After adding a magnetic field, especially under the L-DP (low-frequency and differential magnetic field combination) field, the flow of liquid metal is enhanced, resulting in more uniform heat and solute transfer in the liquid phase. This has the following two inhibitory effects on dendrite growth:

[0085] 1. Enhanced symmetry of dendrite morphology: Due to the strong convection effect generated after the introduction of the magnetic field, the local supercooling and concentration field at the dendrite tip become more uniform, which inhibits the disordered growth of the lateral branches of the dendrite, making the dendrite growth more uniform and symmetrical, which is conducive to reducing the formation of coarse dendrites.

[0086] 2. Dendrite spacing decreases: the temperature gradient increases (from Fig.11 The change in radial temperature gradient in the middle shows that the heat conduction efficiency is improved, which reduces the temperature and solute gradients on which dendrite growth depends. This will cause new dendrites to appear in a shorter interval, thereby shortening the dendrite spacing, avoiding excessive growth of large dendrites, and reducing the formation of dendrite networks. In addition, the vibration of the magnetic field can not only affect grain growth by strengthening convection and improving the uniformity of the temperature field, but also break up dendrites through its vibration effect, providing more nucleation centers. This crushing effect helps to interrupt growing coarse dendrites and add new nuclei, thereby significantly refining the grains. The process of dendrite crushing and nucleation center increase induced by the magnetic field, combined with the enhanced temperature gradient and convection effect, makes the microstructure of the casting more uniform and the grains finer. This is also one of the important reasons why electromagnetic fields are widely used in actual metal casting.

[0087] Fig.12 (a) is the metallographic structure of the center of the AZ31 ingot produced by traditional DC. Fig.12 (b) is the metallographic structure of the center of the AZ31 ingot produced by the low-frequency-pulse combined magnetic field electromagnetic continuous casting. It can be found that the ingot produced by the low-frequency and differential magnetic field method has a more uniform structure and finer grains.

[0088] Example 2

[0089] The circuit principle and device structure are the same as those in Example 1, except that:

[0090] (1) In Example 2, the alloy grade is AZ80.

[0091] (2) The combination of the excitation coils in Example 2 is different from that in Example 1, such as Figure 4 (b) as shown.

[0092] Since AZ80 has a wider two-phase region, the viscosity is small in the paste region, the atoms are sparsely aggregated, and it is not easy to form nucleation centers. The nucleation rate is low during solidification, and the grains are relatively coarse. In Example 2, the way the excitation coil is connected to the power supply is changed. Coil 1# is much higher than the metal melt, and AZ80 has good fluidity, so no current is temporarily connected. Coil 2# is connected to a low-frequency sinusoidal current, and coils 3# and 4# are connected in series to form coil B. 1 , 5# and 6# coils are connected in series to form coil B 2 , two sets of coils B 1 , B 2 A square wave pulse voltage and a sawtooth pulse current with a phase difference of 90° are introduced to generate a phase-difference pulse oscillating magnetic field.

[0093] Fig.13 (a) is the metallographic structure of the center of the AZ80 ingot produced by traditional DC. Fig.13 (b) is the central metallographic structure diagram of the AZ80 ingot produced by electromagnetic continuous casting with a low-frequency and phase-difference pulse combined magnetic field according to the present invention. Fig.13 (b) It can be seen that the grains in the central equiaxed crystal zone of the AZ80 ingot produced by electromagnetic continuous casting with a low-frequency and phase-difference pulse combined magnetic field are finer.

[0094] The low frequency and phase difference pulse combined electromagnetic field in the present invention is not limited to the above A and B 1 , B 2 The combination of three sets of excitation coils can be adjusted according to the solidification characteristics of the corresponding metal or alloy.

Claims

1. An electromagnetic continuous casting method combining low frequency and phase difference pulses, characterized in that: During the semi-continuous casting, horizontal continuous casting and vertical continuous casting process, A+2B excitation coils are placed inside the crystallizer jacket in a direction parallel to the central axis of the ingot, where A and B are integers greater than or equal to 1, wherein the 2B excitation coils are divided into two groups, each group of B coils, called coils B1 and B2; A coils are set as a group separately, called coil A; A power control system with multiple built-in control circuits is used to output two different current voltages and apply them to three sets of excitation coils; one circuit system outputs low-frequency AC voltage and sinusoidal current to generate low-frequency AC electromagnetic signals; the other circuit system outputs two sets of square wave pulse voltages and sawtooth pulse currents; the three sets of excitation coils generate a low-frequency AC magnetic field-phase difference pulse magnetic field combination to act on the metal melt and the paste area.

2. The electromagnetic continuous casting method of claim 1, characterized in that: In the method, the power control system generates a current intensity I=1-350A, an electromagnetic frequency f=2-50Hz, a duty cycle D=5%-80%, and obtains a target magnetic field intensity B=20-300mT in the metal melt and metal paste region.

3. The electromagnetic continuous casting method of claim 1, characterized in that: In the method, three groups of excitation coils are stacked up and down and placed inside the crystallizer jacket using a fixing device, an insulating pad is arranged in the middle for isolation, and circulating water is used to cool the coils and the solidifying metal.

4. The electromagnetic continuous casting method of claim 1, characterized in that: In the method, the A group of excitation coils is located at the upper position, and a low-frequency AC voltage and a sinusoidal current are applied to generate a low-frequency AC electromagnetic signal, which mainly acts on the liquid phase area of ​​the metal; the B1 and B2 groups of coils are located at the lower position, and a square wave pulse voltage and a sawtooth pulse current with a phase difference of 60° to 120° are applied to generate a phase-difference pulse oscillating magnetic field, which mainly acts on the metal paste area.

5. The electromagnetic continuous casting method of claim 1, characterized in that: In the method, two different types of magnetic fields are mainly generated by a power supply control system, which is mainly composed of a transformer, a thyristor controller and corresponding components, a power amplifier, an inverter, a signal generator, a non-polar capacitor, an electrolytic capacitor, an IGBT controller and components, and a PLC controller; the shape of the excitation coil is rectangular or annular, surrounds the metal melt and is placed in the crystallizer, and the two shapes of the excitation coils correspond to the casting processes of flat ingots and cylindrical ingots, respectively.

6. The electromagnetic continuous casting method of claim 5, characterized in that: The excitation coil is wound by flat copper wire coated with insulating material, the number of turns of the excitation coil is 5 to 100, and the number of stacked layers is 5 to 20; the excitation coil is 5 to 50 mm away from the bottom of the inner sleeve of the crystallizer and 5 to 40 mm away from the top of the inner sleeve of the crystallizer, and physical isolation is achieved by using pads or pads made of insulating material, and the pads or pads are fixed together with the excitation coil.

7. The electromagnetic continuous casting method of claim 1, characterized in that: Different wiring methods are used between independent coils to achieve series and parallel functions.

8. The electromagnetic continuous casting method of claim 1, characterized in that: The intensity, type and area of ​​the magnetic field acting on the metal melt and the pasty area are adjusted by controlling the number of coils connected to the power control system.

9. The electromagnetic continuous casting method of claim 1, characterized in that: In the method, the height of the upper excitation coil is 0 to 50 mm higher than the metal melt, and the lower excitation coil is 0 to 30 mm lower than the metal paste area.

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