Low-frequency and pulse combined electromagnetic continuous casting method

Abstract

The invention discloses a low-frequency and pulse combined electromagnetic continuous casting method, which belongs to the technical field of metal material preparation and processing, in the processes of semi-continuous casting, horizontal continuous casting and vertical continuous casting, n magnet exciting coils are placed in a crystallizer jacket in the direction parallel to the central axis of a cast ingot, n is greater than or equal to 2, and the n magnet exciting coils are divided into two groups; two groups of different current and voltage are respectively output by using power supply control systems of two sets of control circuits and are applied to two groups of excitation coils; one set of circuit system outputs low-frequency alternating-current voltage and periodic change current to generate a low-frequency alternating-current electromagnetic signal, and the other set of circuit system outputs a group of square-wave pulse voltage and zigzag pulse current to generate a pulse oscillation magnetic field; different magnetic field combinations generated by the two sets of magnet exciting coils act on a pasty area of metal melt and metal. For different kinds of metal or alloy, in the casting process, 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 technical field of metal material preparation and processing, and relates to an electromagnetic continuous casting method combining a pulse magnetic field and a low-frequency magnetic field. Background Art

[0002] Vertical and horizontal continuous casting technology and semi-continuous casting technology are the main methods for preparing metal and alloy ingots, which have the advantages of efficient production, low cost, and simple operation. However, during the material casting process, the cooling rates of the molten metal inside and outside the ingot are different, and the external molten metal solidifies earlier than the internal one, which leads to loose structure in the core of the ingot. In addition, for large-sized ingots and ingots with a large width-to-thickness ratio, the temperature difference between the inside and outside during solidification is large, which can easily make the grains in the core of the ingot coarse, produce serious macro- and micro-segregation, and easily produce defects such as cracks, which seriously affect the mechanical properties and deformation capacity of the material. Therefore, in the process of preparing metal billets, it is crucial to change the solidification temperature field of the ingot, reduce the temperature difference between the inside and outside of the ingot during the solidification process, make the solidification structure refined and uniform, and reduce the generation of stress during the solidification process for the preparation of high-performance ingots.

[0003] 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 such as electromagnetic fields and ultrasonic fields to control the microstructure and properties of ingots during the metal solidification process has become a hot topic of research. Related studies have shown that the addition of external fields can significantly affect the solidification behavior of metals and alloys, achieving the purpose of 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.

[0004] In the semi-continuous casting process, the existing ultrasonic melt treatment process has not yet been applied in large-scale industry due to the complex spatial and temporal matching of the existing ultrasonic melt treatment process and the serious attenuation of ultrasound in the metal melt. 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, the research on electromagnetic field to improve the metal solidification structure has been continuously developing and improving.

[0005] The pulse electromagnetic field is a magnetic field generated by a non-steady-state current generated by an energy storage capacitor passing through the crystallizer excitation coil. Its essence is a sawtooth pulse oscillating current generated by a single-phase rectangular pulse voltage as a control signal. The direction of the current remains unchanged but the magnitude changes continuously. The low-frequency electromagnetic field is a magnetic field generated by an alternating current passing through the crystallizer excitation coil. Its magnitude and direction change according to a certain rule over time. Patents CN1425520A, CN101020229A and CN104057046A 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.

[0006] In vertical and 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 electromagnetic, 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. As for pulse electromagnetic, it has the characteristics of large energy storage and strong instantaneous excitation effect. In addition, due to the pulse voltage, the electromagnetic force changes direction instantaneously, which can play a reciprocating oscillation role on the melt, change the state of atomic aggregation, and increase the nucleation rate. At present, in the casting process, the existing electromagnetic field application types are relatively single, and the electromagnetic conditions at different parts cannot be adjusted. It is impossible to control the temperature field and current field at different positions for a certain metal or alloy, so as to improve the ingot quality according to its solidification characteristics. Summary of the invention

[0007] Aiming at the problems existing in the current vertical and horizontal continuous casting technologies 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 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 pulse magnetic fields can be applied to the melts at different heights in the crystallization sleeve to improve the electromagnetic utilization rate, refine the grains, and improve the ingot quality.

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

[0009] During semi-continuous casting, horizontal continuous casting and vertical continuous casting, n excitation coils are placed inside the crystallizer jacket parallel to the central axis of the ingot, where n is an integer greater than or equal to 2, and the n excitation coils are divided into two groups. Using two sets of control circuit power control systems, two different current and voltage groups are output respectively and applied to the two sets of excitation coils. One circuit system outputs a low-frequency AC voltage and a periodically changing current to generate a low-frequency AC electromagnetic signal, and the other circuit system outputs a set of square wave pulse voltage and a sawtooth pulse current to generate a pulse oscillating magnetic field. The different magnetic fields generated by the two sets of excitation coils act in combination on the molten metal and the mushy area of ​​the metal.

[0010] In the method, the power control system generates and acts on the two sets of excitation coils with a current intensity of I=1-350A, an electromagnetic frequency f=2-50Hz, a square wave pulse circuit duty cycle D=5%-80%, and a target magnetic field intensity B=20-300mT is obtained in the metal melt and metal paste area.

[0011] In the method, two groups of excitation coils are stacked up and down and placed inside the crystallizer jacket by 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.

[0012] In the method, a group of excitation coils located at the lower position receives square wave pulse voltage and sawtooth pulse current to generate a pulse oscillating magnetic field, and another group of excitation coils located at the upper position receives low frequency AC voltage and periodically changing current to generate a low frequency AC electromagnetic signal.

[0013] In the method, two different types of magnetic fields are mainly generated by a power 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, 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 20 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 pad is fixed in the crystallizer together with the excitation coil.

[0017] In the method, n excitation coils are divided into two groups, A and B, n is an integer greater than or equal to 2, and the number of excitation coils in any single group is greater than 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 intensity of a specific type of magnetic field is adjusted by controlling the number of excitation coils connected to a power control system.

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

[0021] In the method, for vertical and 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 present invention is:

[0023] With regard to 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 by independently controlling two groups of solenoid coils and applying different types of currents and voltages to generate different types of magnetic fields, and by adjusting the number of excitation coils, the intensity, type, and action area of ​​the magnetic field are controlled; by applying different currents and voltages to the two groups of excitation coils, the electromagnetic action area can be adjusted for different metals or alloys while considering the solidification characteristics. The low-frequency magnetic field can force the melt to convect, play a homogenizing role, and significantly improve the segregation of the ingot; the pulsed magnetic field can have a significant oscillation and stirring effect on the molten metal, change the atomic aggregation state, break up dendrites, and increase the nucleation rate. The combined action of the two magnetic fields can produce alloy ingots with smaller grains and lower segregation.

[0024] The present invention can achieve the following positive effects through the above technical features:

[0025] (1) By adjusting the number, size and placement of the excitation coils, it can adapt to the preparation process of ingots of various types and sizes.

[0026] (2) In the horizontal, vertical continuous casting or semi-continuous casting process, the solidification characteristics of ingots of different types and sizes can be considered, and combined magnetic field conditions 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. Pulsed magnetic field can 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 magnetic field does not come into direct contact with the molten metal, and the production process is safe, clean and highly automated.

[0029] (5) This method is applicable to aluminum alloys, magnesium alloys, copper alloys and steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of an oscillating electromagnetic power control system for generating a combined magnetic field.

[0031] Figure 2 (a) is a typical current and voltage diagram for generating a pulsed oscillating magnetic field. max is the voltage peak, U min is the minimum voltage value, f1 is the sawtooth current frequency, D1 is the duty cycle, I max is the peak current, I min is the minimum current value. Figure 2(b) is a schematic diagram of the periodic current waveform that generates a low-frequency AC magnetic field. max is the current peak value, and f is the low-frequency AC current frequency.

[0032] Figure 3 The present invention is a schematic diagram of the structure of a low-frequency and pulse combined electromagnetic continuous casting method and 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, excitation coil, 3, fixing rod, 4, outer shell of crystallizer, 5, fixing bolt I, 6, pad, 7, molten metal, 8, metal paste area, 9, inner shell of crystallizer, 10, fixing bolt II, 11, fixing bolt III, 12, pad, 13, cooling water inlet, 14, secondary cooling water outlet, 15, solid phase metal, 16, ingot inlet.

[0033] Figure 4 It is a schematic diagram of the working mode of the excitation coil in the electromagnetic continuous casting method of the low-frequency and pulse combined magnetic field in the embodiment of the present invention; wherein, (a) is Example 1, (b) is Example 2, and (c) is Example 3. In Example 1, the 1#2# coils are connected in series, and a periodic changing current is passed through to generate a low-frequency AC magnetic field, and the 3#4# coils are connected in series, and a square wave pulse voltage and a sawtooth pulse current are passed through to generate a pulse oscillating magnetic field. In Example 2, the 1# coil is passed through a periodic changing current to generate a low-frequency AC magnetic field, and the 2#3#4# coils are connected in series, and a square wave pulse voltage and a sawtooth pulse current are passed through to generate a pulse oscillating magnetic field. In Example 3, the 1# coil is passed through a periodic changing current to generate a low-frequency AC magnetic field, and the 2# coil is connected in series, and a square wave pulse voltage and a sawtooth pulse current are passed through to generate a pulse oscillating magnetic field.

[0034] Figure 5 This is a schematic diagram of the intensity variation of the combined addition of low-frequency + pulse magnetic field (abbreviated as L+P) within a single current cycle in Example 1 of the present invention.

[0035] 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+P magnetic field casting (b) are compared.

[0036] 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 between the conventional DC casting without magnetic field and the combined L+P magnetic field casting process at point b in a stable state.

[0037] 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+P magnetic field.

[0038] Fig. 9These 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+P magnetic field.

[0039] 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+P magnetic field is

[0040] Fig.11 The grain structure of the AZ31 magnesium alloy prepared by the comparative test between Example 2 of the present invention and the conventional DC casting is shown in the figure, (a) conventional DC casting; (b) Example 1 of the present invention.

[0041] Fig.12 The 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

[0042] The method and device of the present invention are further explained below in conjunction with the accompanying drawings and embodiments.

[0043] In the embodiment of the present invention, the low-frequency and pulse combined electromagnetic continuous casting method and device structure suitable for vertical continuous casting or semi-continuous casting are as follows: Figure 3 As shown. Figure 3 As shown, the crystallizer outer sleeve 4, the pad 6 and the crystallizer inner sleeve 9 form an internal space, in which four excitation coils are placed, and the coils are separated by insulating pads 12. The pads 12 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 I 5.

[0044] In the embodiment of the present invention, when vertical continuous casting or semi-continuous casting is performed, the excitation coil 2, the cushion block 12, the pad 6 and the fixing bolts I 5 are located in the crystallizer outer shell 4, and the circulating cooling water enters the crystallizer outer shell 4 through the cooling water inlet 13, and cools the excitation coil 2 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 6 are fixed by fixing bolts II 10, and the pad 6 is fixed to the crystallizer outer shell 4 by fixing bolts III 11. The crystallizer inner shell 9, the pad 6 and the crystallizer outer shell 4 constitute the overall appearance of the crystallizer.

[0045] 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 2 are divided into two groups, A and B. The excitation coils of group A need to be consistent with or slightly higher than the vertical height of the metal melt 7. Specifically, the height of the upper excitation coil needs to be 0 to 50 mm higher than the metal melt. The excitation coils of group B need to be consistent with or slightly lower than the vertical height of the metal mushy zone 8. Specifically, the excitation coils of the lower layer need to be 0 to 30 mm lower than the metal mushy zone, to ensure that different magnetic fields correspond to different solidification states.

[0046] exist Figure 4 In (a), coils 1# and 2# are connected in series, and low-frequency AC voltage and periodic current are passed through to generate a low-frequency AC magnetic field. Coils 3# and 4# are connected in series, and square wave pulse voltage and sawtooth pulse current are passed through to generate a pulse oscillating magnetic field. Figure 4 In (b), low-frequency AC voltage and periodic current are applied to coil 1# to generate a low-frequency AC magnetic field. Coils 2#, 3#, and 4# are connected in series and square wave pulse voltage and sawtooth pulse current are applied to generate a pulse oscillating magnetic field. At this time, the pulse oscillating magnetic field has a greater intensity and a wider area of ​​action.

[0047] In the embodiment of the present invention, the secondary cooling water cools the excitation coil 2 and the metal in the solidification process through the secondary cooling water outlet 14 and discharges the cooling water; the ingot starter is embedded in the ingot starter port 16, and a certain pulling force is applied to perform continuous production.

[0048] In the embodiment of the present invention, the excitation coil located in the crystallizer outer sleeve 4 is spaced 20 mm from the crystallizer inner sleeve 9 and 20 mm from the crystallizer outer sleeve 4 .

[0049] Through different coil combinations, the electromagnetic semi-continuous casting device can generate a variety of magnetic field characteristics to meet a wide range of casting needs. For example:

[0050] 1. Upper low-frequency magnetic field + lower oscillating magnetic field, that is, the upper coils (such as 1# and 2#) are connected in series, and the lower coils (such as 3# and 4#) are connected in series, such as Figure 4 (a) is shown. The magnetic field distribution range can be adjusted according to the differences in melt properties of different metals and different components. For example, for materials with a wide two-phase region, a low-frequency current can be passed through coil 1 alone to generate a low-frequency magnetic field; square wave pulse voltage and sawtooth pulse current can be passed through coils 2#, 3# and 4# to generate a pulse oscillating magnetic field.

[0051] 2. The upper and lower layers of composite magnetic field (combined series and parallel connection) are connected in series. The upper coils (1#, 3#) are connected in series, and the lower coils (2#, 4#) are connected in series, or some coils are connected in series and some in parallel to form a composite magnetic field. At this time, there is a low-frequency magnetic field + an oscillating magnetic field in the two-phase region of the melt. This coil combination can provide comprehensive magnetic field control in the semi-continuous casting process.

[0052] 3. Local area control (selective activation of coils) i.e., selectively activate 2-4 coils, for example, only activate coils 1# and 3#. A local magnetic field is formed in a specific area, and a local low-frequency magnetic field or an oscillating magnetic field can be generated as needed. Or it can be used for local strengthening of specific parts, such as if the density of the tissue needs to be enhanced in a certain area of ​​the casting, a strong oscillating magnetic field can be generated in this area. Or the magnetic field position and intensity can be dynamically adjusted according to the real-time needs of the casting process to achieve precise process control.

[0053] The melt processed in the embodiment of the present invention is aluminum alloy, magnesium alloy, copper alloy or steel.

[0054] Example 1

[0055] In this embodiment, in the low-frequency and 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.

[0056] Figure 2 Two typical current and voltage schematics generated by power control systems. Figure 2 (a) is a pulse signal, the voltage is a square wave, and the current is a sawtooth pulse wave. Figure 2 (b) is a low-frequency periodic current. The current I, voltage U, duty cycle D and frequency 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%.

[0057] The target magnetic field strength B=20-45 mT is obtained in the metal melt and the metal mushy region.

[0058] In Example 1, a semi-continuous casting method is used to produce a cylindrical billet with a diameter of 90 mm, and the specific alloy grade is AZ31.

[0059] AZ31 has a low degree of alloying, a narrow two-phase zone, and a high solute viscosity. In traditional semi-continuous casting, it is very easy to cause large macro-segregation. Therefore, it is necessary to enhance the forced convection effect of the low-frequency magnetic field on the solute. In Example 1, the crystallizer has 4 built-in annular coils, namely coils 1#, 2#, 3# and 4#. A single coil has 40 turns and an inner diameter of 120 mm. It is made into a circular ring shape using a coated flat copper wire with a rectangular cross-section. Coils 1# and 2# are stacked and connected in series to form Group A, and a periodically changing current is passed through to generate a low-frequency AC magnetic field. Coils 3# and 4# are stacked and connected in series to form Group B. A square wave pulse voltage and a sawtooth pulse current are passed through to generate a pulsed oscillating magnetic field. Figure 4 (a) shown.

[0060] The two groups of excitation coils A and B are placed in the inner space of the crystallizer in order, with group A on top and group B on the bottom, and are physically isolated using an insulating pad with a thickness of 2 mm, and fastened using a fixing screw. After fastening, the distance between the bottom coil and the bottom of the crystallizer is 8 mm, and the distance between the top coil and the cover plate is 12 mm. In Example 1, the inner diameter of the inner sleeve of the crystallizer matches the size of the cylindrical ingot.

[0061] Before casting begins, a periodically changing current is passed through the two groups of excitation coils to generate a low-frequency AC magnetic field; a square wave pulse voltage and a sawtooth pulse current are passed through to generate a pulse oscillating magnetic field, and casting begins after the magnetic field stabilizes, and the power supply is turned off after casting is completed. In Example 1, the voltage applied to the control cabinet is 220V, the low-frequency current acting on the excitation coil of group A ranges from 0 to 15A, the electromagnetic frequency is 50Hz, the current intensity acting on the excitation coil of group B is 1 to 11.5A, and the duty cycle is 25%.

[0062] The low-frequency magnetic field generated by the A group of excitation coils mainly acts on the metal melt 7, which can cause forced convection of the metal melt 7, play a homogenizing role, and significantly improve the segregation of the ingot; under the action of cooling water, the metal melt 7 is transformed into a pasty zone 8, and the pulsed magnetic field can have a significant oscillation and stirring effect on the pasty zone 8, changing the atomic aggregation state, breaking up dendrites, and increasing the nucleation rate, and then continues to cool and transform into solid phase metal 15, and finally uses the traction device set at the ingot introduction port 16 to complete continuous production.

[0063] Figure 5 This is a schematic diagram of the intensity variation of the combined addition of low-frequency + pulse magnetic field (abbreviated as L+P) within a single current cycle in Example 1.

[0064] Figure 6(a) is a comparison of streamline changes during conventional DC casting without magnetic field and combined L+P magnetic field casting. Under the condition of no magnetic field, the flow direction and velocity of the fluid present regular streamlines, mainly moving in the longitudinal direction. Near the boundary and free surface, the fluid movement is restricted by the geometric shape and boundary effect, and the fluid forms vortices. Figure 6 In (b), under the action of the L+P magnetic field, the streamlines of the fluid have changed significantly. In particular, a larger vortex structure is formed on the upper outer side. The streamlines are more complex, the vortex area is significantly expanded, and the movement of the fluid is more intense near the wall. The color of the velocity change (from blue to red) shows that the magnetic field has caused a significant increase in the velocity in the local area. The action of the magnetic field seems to have led to the enhancement of the edge vortex, while suppressing the flow at 1 / 2R, causing the fluid to move in a concentrated manner at the outer boundary. This shows that the magnetic field may have an impact on the flow of the fluid in the lateral direction, resulting in enhanced local momentum transfer.

[0065] Figure 7 (b) shows Figure 7 In (a), the velocity of the fluid at point b changes with time with and without a magnetic field. The velocity is basically 0 m / s without a magnetic field, which means that in the absence of a magnetic field, the fluid has almost no velocity change at point b, proving that the point is in a stable relative static state and the melt flow rate is stable. After applying the L+P magnetic field, the velocity at point b shows periodic fluctuations, ranging from 0.0020 m / s to 0.0035 m / s. This fluctuation indicates that under the action of the magnetic field, the fluid at point b is affected by the magnetic field drive. Due to the effect of the magnetic field on the fluid, this point produces a relatively obvious cyclic flow. The periodic fluctuation of the velocity means that the pulse change of the magnetic field produces a periodic vibration effect on the fluid. The flow direction and intensity of the fluid can be adjusted, resulting in a change in the flow velocity. This is because the Lorentz force exerted by the magnetic field on the fluid causes the fluid near point b to periodically accelerate and decelerate, forming a vibration phenomenon.

[0066] Figure 8 The temperature field and liquid fraction distribution under no magnetic field and L+P magnetic field conditions are shown. The left side of the figure shows the temperature field, and the right side shows the liquid fraction. By comparing the no magnetic field and L+P conditions, the changes in temperature distribution, solid fraction, and liquid cavity depth can be analyzed. Figure 8It can be seen that the temperature field distribution in the melt shows a large liquid cavity depth of about 134mm, the temperature in the upper area is higher, and the red and yellow areas occupy a larger part, indicating that heat accumulates at the top. Under the L+P magnetic field conditions, the temperature distribution is more uniform, the liquid cavity depth is reduced to 109mm, the high temperature area (red and yellow areas) in the upper area is reduced, the heat is better conducted to the bottom through the action of the magnetic field, and the overall temperature gradient is more uniform. This is because under the condition of no magnetic field, due to the weak natural cooling, the heat at the top is not easy to transfer, resulting in a larger liquid cavity depth. Under the L+P magnetic field conditions, the low-frequency + pulsed magnetic field enhances the fluidity and convection effect inside the melt, promotes the downward transfer of heat, and thus makes the temperature distribution more uniform and the liquid cavity depth is reduced. As for the liquid phase fraction, under the condition of no magnetic field, the liquid phase fraction distribution is deeper, the liquid cavity is larger, and a higher liquid area is shown. The red and yellow areas with a liquid phase fraction close to 1 indicate that the upper area remains liquid, while the blue area indicates a large solid phase fraction and a high degree of solidification. Under L+P conditions (low-frequency pulsed magnetic field), the liquid fraction area is significantly reduced, indicating that the liquid cavity depth is reduced, the blue area (high solid fraction) area is increased, and the speed and degree of solidification are accelerated. This shows that under the condition of no magnetic field, the cooling efficiency of the melt is low, resulting in a larger liquid phase area and a slower solidification process. Under L+P conditions, the low-frequency pulsed magnetic field enhances the fluidity of the melt through the magnetohydrodynamic (MHD) effect, promotes solidification, reduces the liquid phase area, accelerates solidification, and reduces the depth of the liquid cavity. That is, under the condition of no magnetic field: due to the uneven temperature distribution, the temperature in the upper area is higher and the cooling efficiency is lower, so the liquid phase fraction is also larger and the liquid cavity depth is deeper. Under the condition of L+P magnetic field, the magnetic field significantly enhances the cooling and solidification effect of the melt, the temperature field distribution is more uniform, the liquid phase fraction is reduced, the solidification process is accelerated, and the depth of the liquid cavity is significantly reduced.

[0067] Fig. 9 The distribution of the total radial heat flux under no magnetic field and L+P magnetic field conditions is shown. Under no magnetic field conditions, the heat flux gradually increases with the increase of radial distance, but the heat flux growth at the edge is relatively gentle. Under L+P magnetic field conditions, the heat flux is not only slightly higher in the central area, but also increases more significantly in the middle and edge areas, especially in the edge area, where the heat flux growth curve shows a steeper upward trend.

[0068] This is because under the condition of no magnetic field: due to natural cooling alone, heat transfer mainly depends on thermal conduction and natural convection, so the heat flux is relatively low and the growth at the edge is limited; while under the condition of L+P, the magnetic field strengthens the convection inside the melt through the magnetohydrodynamic (MHD) effect, especially in the edge area, where heat transfer is significantly enhanced through the flow induced by the magnetic field. This leads to a significant increase in the heat flux in the edge area, thereby improving the overall cooling effect.

[0069] Fig.10 The following is a graph showing the radial temperature gradient as a function of distance under no magnetic field and L+P magnetic field conditions. Under no magnetic field conditions: the temperature gradient increases sharply with increasing radial distance, especially at the edge, where the temperature gradient is close to 4000K / m, indicating that the edge is cooled strongly, resulting in uneven temperature distribution. Under L+P magnetic field conditions: the temperature gradient is significantly reduced, especially in the edge area, where the temperature gradient is only 2130K / m. This shows that the magnetic field effectively weakens the effect of cooling on the temperature gradient and homogenizes the temperature distribution of the melt. It can be seen that under the action of the magnetic field, the temperature distribution is more uniform, significantly reducing the temperature gradient in the edge area, which is of great significance for improving the problem of uneven temperature distribution during melt cooling.

[0070] 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.

[0071] The process of dendrite growth is usually affected by temperature gradient, cooling rate and melt flow. Under the condition of no magnetic field: due to the large temperature gradient, especially in the area near the edge of the liquid cavity, the temperature field is uneven and the liquid cavity depth is large (134mm). The uneven temperature field leads to different cooling rates at different positions, which makes the dendrites grow faster in some areas, and coarse and irregular dendrites may appear. At the same time, the melt flow is weak, and the dendrite growth direction is less affected by the flow field, which may cause the grown dendrites to grow along the temperature gradient direction and form coarse columnar grains. Under the condition of L+P magnetic field: the oscillation effect of the magnetic field promotes the convection of the melt, enhances the uniformity of temperature, and reduces the liquid cavity depth (109mm), which makes the cooling process more consistent and inhibits the formation of coarse dendrites. By enhancing the fluidity inside the melt, the pulsed magnetic field interferes with the growth direction and growth rate of the dendrite, destroys the single growth direction of the dendrite, hinders the process of dendrite growth, and may promote the fragmentation and refinement of the dendrite. That is, under the condition of no magnetic field, the uneven temperature distribution leads to the growth of coarse dendrites; while under the condition of L+P, the uniform temperature distribution and enhanced fluidity inhibit the formation of coarse dendrites and promote dendrite refinement. And under the condition of enhanced melt fluidity, the growth of dendrites is interrupted, and the broken dendrites lead to the formation of more crystal nuclei, thereby refining the grains.

[0072] Fig.11 (a) is the metallographic structure of the center of the AZ31 ingot produced by traditional DC. Fig.11 (b) is the central metallographic structure diagram of the AZ31 ingot produced by the L+P combined magnetic field electromagnetic continuous casting of the present invention. Through the application of the L+P magnetic field, the convection of the melt is enhanced, the temperature field is more uniform, the cooling rate is accelerated, the growth of dendrites is suppressed, and grain refinement is promoted. The ingot produced by this method has uniform structure, fine grains, and high product quality.

[0073] The low-frequency and pulse combined electromagnetic field generated by the two current and voltage signals in the present invention is not limited to the combination of the two groups of excitation coils A and B mentioned above. It can be adjusted according to the solidification characteristics of the corresponding metal or alloy, that is, n excitation coils are divided into two groups A and B, and the two groups of coils are connected to different current and voltage signals to generate a low-frequency and pulse combined electromagnetic effect, and the number of coils in a single group is greater than 1.

[0074] Example 2

[0075] The circuit principle and device structure are the same as those in Example 1, except that:

[0076] (1) In Example 2, the alloy grade is AZ80.

[0077] (2) The combination of the excitation coils in Example 2 is different from that in Example 1, such as Figure 4 (b) as shown.

[0078] Since AZ80 has a high alloying degree and a wider two-phase region, in Example 2, the way the excitation coil is connected to the power supply is changed. The 1# coil is connected to a periodically changing current to generate a low-frequency AC magnetic field, and the 2#, 3# and 4# coils are connected in series, and a square wave pulse voltage and a sawtooth pulse current are passed through to generate a pulse oscillating magnetic field, such as Figure 4 (b) as shown.

[0079] Fig.12 (a) is the metallographic structure of the center of the AZ80 ingot produced by traditional DC. Fig.12 (b) is the metallographic structure diagram of the center of the AZ80 ingot produced by electromagnetic continuous casting with a low-frequency and pulsed magnetic field combination according to the present invention. Fig.12 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 pulse combined magnetic field are finer.

[0080] Example 3

[0081] The circuit principle is the same as that of Embodiment 1, except that:

[0082] (1) In Example 3, the alloy grade is AZ80.

[0083] (2) The diameter of the AZ80 ingot produced in Example 3 is 60 mm.

[0084] (3) The combination of the excitation coils in Example 3 is different from that in Example 1, such as Figure 4 (c) as shown.

[0085] The diameter of the AZ80 cylindrical ingot produced in Example 3 is 60 mm. In order to save energy and provide as much operating space as possible on the casting platform, the crystallizer structure is appropriately changed as follows: Figure 4 (c) as shown.

[0086] In Example 3, the number of coils between the outer sleeve and the inner sleeve of the crystallizer is reduced to two groups, A and B, each of which is provided with an excitation coil, namely, 1# and 2# coils. In actual production, a periodically changing current is connected to the 1# coil to generate a low-frequency AC magnetic field, which acts on the melt and forces the melt to convect; a square wave pulse voltage and a sawtooth pulse current are passed through the 2# coil to generate a pulse oscillating magnetic field, which has the effect of breaking dendrites and refining grains, such as Figure 4 (c) as shown.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A low-frequency and pulse combined electromagnetic continuous casting method, characterized in that: During the semi-continuous casting, horizontal continuous casting and vertical continuous casting processes, n excitation coils are placed inside the crystallizer jacket in a direction parallel to the central axis of the ingot, where n is an integer greater than or equal to 2, and the n excitation coils are divided into two groups; a power supply control system of two sets of control circuits is used to output two sets of different currents and voltages respectively, which are applied to the two sets of excitation coils; one set of circuit systems outputs a low-frequency AC voltage and a periodically varying current to generate a low-frequency AC electromagnetic signal, and the other set of circuit systems outputs a set of square wave pulse voltage and a sawtooth pulse current to generate a pulsed oscillating magnetic field; the different magnetic fields generated by the two sets of excitation coils act in combination on the metal melt and the metal paste area.

2. A low-frequency and pulse combined electromagnetic continuous casting method according to claim 1, characterized in that: In the method, the power control system generates and acts on the two sets of excitation coils with a current intensity of I=1-350A, an electromagnetic frequency f=2-50Hz, a square wave pulse circuit duty cycle D=5%-80%, and a target magnetic field intensity B=20-300mT is obtained in the metal melt and metal paste area.

3. A low-frequency and pulse combined electromagnetic continuous casting method according to claim 1, characterized in that: In the method, two sets of excitation coils are stacked up and down and placed inside the crystallizer jacket by a fixing device, with an insulating pad in the middle for isolation, and circulating water is used to cool the coils and the solidifying metal; A group of excitation coils located at the lower position receive square wave pulse voltage and sawtooth pulse current to generate a pulse oscillating magnetic field, and another group of excitation coils located at the upper position receive low frequency AC voltage and periodically changing current to generate a low frequency AC electromagnetic signal.

4. A low-frequency and pulse combined electromagnetic continuous casting method according to claim 1, characterized in that: In the method, the excitation coil is wound by flat copper wire, the number of turns of the excitation coil is 5 to 100, and the number of superimposed 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 20 mm away from the top of the inner sleeve of the crystallizer.

5. The low-frequency and pulse combined electromagnetic continuous casting method according to claim 1, characterized in that: In the method, different wiring modes are used between independent coils to realize series connection and parallel connection functions; the intensity, type and area of ​​the magnetic field acting on the metal melt and the metal paste area are adjusted by controlling the number of excitation coils connected to the power supply control system.

6. The low-frequency and pulse combined electromagnetic continuous casting method according to 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 height of the lower excitation coil is 0 to 30 mm lower than the metal paste area.

7. The low-frequency and pulse combined electromagnetic continuous casting method according to claim 1, characterized in that: Two excitation coils are placed inside the crystallizer jacket parallel to the central axis of the ingot, namely coils 1# and 2#. A periodically varying current is passed through coil 1# to generate a low-frequency AC magnetic field, and a square wave pulse voltage and a sawtooth pulse current are passed through coil 2# to generate a pulsed oscillating magnetic field.

8. The low-frequency and pulse combined electromagnetic continuous casting method according to claim 1, characterized in that: Four excitation coils are placed inside the crystallizer jacket parallel to the central axis of the ingot, namely 1#2#3#4# coils. Among them, 1#2# coils are stacked and connected in series, and a periodically changing current is passed through them to generate a low-frequency AC magnetic field. 3#4# coils are stacked and connected in series, and a square wave pulse voltage and a sawtooth pulse current are passed through them to generate a pulsed oscillating magnetic field.

9. The low-frequency and pulse combined electromagnetic continuous casting method according to claim 1, characterized in that: Four excitation coils are placed inside the crystallizer jacket parallel to the central axis of the ingot, namely coils 1#, 2#, 3#, and 4#. Among them, coil 1# is fed with a periodically varying current to generate a low-frequency AC magnetic field, and coils 2#, 3#, and 4# are stacked and connected in series, and square wave pulse voltage and sawtooth pulse current are passed through them to generate a pulsed oscillating magnetic field.

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