Controlled casting of large steel ingots in molds using induction variable power and variable frequency.

Abstract

Large steel ingots are cast using overhead induction heaters powered by variable power and frequency. The overhead induction heaters provide induction heating and stirring, gradually solidifying the metal from bottom to top, with the molten metal above the feeder section compensating for the shrinkage of the solidified metal below the main section. Variable electromagnetic stirring forces selectively remove inclusions from the molten metal, reducing the formation of casting defects.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No. 63 / 116,455, filed November 20, 2020, which is incorporated herein by reference in its entirety.

[0002] The present invention relates generally to the casting of large volumes of metal ingots, and more particularly to the casting of large-volume steel ingots using a pancake-shaped coil or similar induction heating device suspended above the molten metal to supply induction heating power and electromagnetic stirring force to the molten material in a mold. The induction heating and electromagnetic stirring process retards solidification above the solidification line of the mold, causing the molten metal at the top to cool and solidify more slowly than the molten metal at the bottom. The general objective is to gradually solidify the metal from bottom to top, allowing the molten metal at the top to compensate for the shrinkage of the solidified metal at the bottom, and selectively migrate defects using variable power and variable frequency stirring force supplied to the induction heating device. The advantage of this technique is reduced casting defects and improved quality of the final ingot product. [Background technology]

[0003] Casting large steel ingots requires a fairly long time due to the large volume of molten material in the mold. Correspondingly, the ingot may experience significant shrinkage during the solidification process, which results in significant structural defects and ultimately reduces the quality of the cast product. Therefore, a well-designed casting method is desirable, primarily to optimize the solidification process.

[0004] The typical casting process for large steel ingots involves pouring molten material into a mold and cooling the molten metal until it completely solidifies. This process periodically requires the addition of feeder spaces, which are extra voids in the mold that are filled with molten material to compensate for shrinkage during solidification. As steel is poured into an ingot mold, it gradually solidifies, beginning at the mold's walls and base and moving inward toward the mold's thermal center or axis. Liquid steel shrinks in volume during and after solidification, and if the molten material in the feeder spaces solidifies faster or at the same pace as the material in the main part of the mold, there will be insufficient solid metal to fill the initially formed shell. This results in a body of steel with cavitation, especially in the region of the metal that solidifies last. Such cavitation is also associated with impurities concentrating by segregation, creating an undesirable distribution of undesirable elements in the final product.

[0005] In the past, various approaches have been applied to maintain the material in the feeder in a molten state until the metal in the main part of the ingot mold has completely solidified, based on the recognition that the gradual solidification of the molten metal from bottom to top is important to the quality of the final product.

[0006] Korean Patent Publication No. 10-2123914 (hereinafter referred to as Korean Patent '914) discloses a steel ingot casting technology using a pancake-shaped electric induction heater with an internal coil arranged in a spiral and grouted with refractory material attached and suspended above the mold. The ingot casting procedure using this technology consists of four steps: first, pouring molten steel into the mold, for example, through a bottom inlet; second, adding heat-generating and insulating materials to the top surface of the molten steel in the mold; third, lowering the induction heater to the required position adjacent to the top surface of the molten metal; and fourth, passing an alternating current of a specific frequency through the coil to heat the metal until it completely solidifies. The advantage of this technology is that the top of the metal remains molten, and solidification proceeds gradually from bottom to top. Therefore, the molten metal at the top compensates for the shrinkage of the solidified metal at the bottom, reducing structural defects.

[0007] In Korean Patent '914, an overhanging induction heating device along with the provided heating and insulating materials is intended to maintain the metal in a molten state at the top of the mold, which is important for eliminating internal defects in the final product. Structural defects in the final product include macrosegregation, shrinkage porosity, and inclusions. In terms of eliminating inclusions, the advantages of induction heating are the control of solidification direction and overall solidification time.

[0008] When used in cast steel, nonmetallic inclusions are classified as intrinsic or extrinsic depending on their origin. Intrinsic inclusions are deoxidation products or precipitates formed during the cooling and solidification process. For example, they are nonmetallic materials trapped by dendritic growth. Extrinsic inclusions arise from unintended chemical and mechanical reactions between the molten steel and its environment, leading to ingot defects such as porosity and pinholes. Some nonmetallic inclusions tend to flow to the top of the molten metal and become trapped in the slag, which can eventually be disposed of by being cut off from the final product. Discarded cutoffs are unusable and typically account for 13–15% of the final product. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Registration No. 10-2123914 Summary of the Invention [Problem to be solved by the invention]

[0010] There is always a demand for clean steel, a defect-free material used in challenging applications such as automotive parts and components used in corrosive environments.

[0011] One object of the present invention is to further reduce defects in casting ingot products by coordinately varying the induction power and frequency supplied to an overhung induction heating device. [Means for solving the problem]

[0012] In one aspect, the present invention includes a system and method specifically for large ingot mold casting applications, in which an electric induction heating device is used to maintain the top of the molten metal and gradually drive the solidification process from bottom to top by inducing thermal energy and electromagnetic stirring forces into the casting metal via induction heating coils supplied with alternating current from a power supply having variable power and variable frequency output. The advantage of maintaining the top of the casting metal melt in a molten state and using electromagnetic stirring forces is that structural defects are significantly reduced and the quality of the final ingot product is improved.

[0013] In another aspect, the invention includes an induction heating apparatus mounted and suspended above a casting mold, and a power supply having variable power and variable frequency output for powering the induction heating apparatus to heat molten steel in the mold for producing ingots. The induction heating apparatus includes an induction coil, refractory material for securing the coil and embedding the induction coil therein to reduce heat dissipation from the molten metal, and an elevating device for raising and lowering the induction coil. The power supply is configured to provide alternating current at a range of frequencies and power rates that best match the heating energy and stirring power requirements from the solidification process to reduce casting defects.

[0014] In another aspect, the present invention provides a method for producing ingots using an induction heater suspended above a mold, with a variable power and variable frequency power supply supplying power to the induction heater. The ingot casting process employing this technology consists of four steps: first, pouring molten steel into the mold through a bottom inlet; second, adding heat generation and insulation material to the upper surface of the molten steel in the mold; third, lowering the induction heater to a position where the refractory of the induction heater is preheated to the refractory preheat temperature; and fourth, applying alternating current at a molten metal heating frequency from the variable power and variable frequency power supply to a coil to heat the casting metal, maintaining the upper part of the metal in a molten state as solidification begins gradually upward from the bottom, and controlling the variable frequency output from the variable power and variable frequency power supply to increase the electromagnetic stirring speed and accelerate inclusion migration.

[0015] In certain applications of this method, the ingot mold cavity can be divided into a main section and a feeder section. In the fourth step introduced above, the heating process can begin when the main section has completely solidified, but the metal in the feeder section is still molten.

[0016] In another aspect, the present invention includes an upper induction heater mounted and suspended above a casting mold, and a power supply having a variable power and variable frequency output for powering the induction heater. The upper induction heater and power supply are configured for metal ingot production. The upper induction heater is configured as a pancake-shaped refractory with an embedded induction coil arranged in a spiral pattern. The induction coil is grouted inside the pancake refractory, and the grouted coil is covered layer by layer from the inside to the outside using shellak wool, insulating material, and castable cement. These structures form a solid body of the induction heater that can be raised and lowered by a hoisting device.

[0017] The above-described embodiments of the present invention are particularly applicable to retrofitting existing ingot molds because the electric induction heating device is positioned above the molten metal surface, however, the present invention is not limited to retrofit applications.

[0018] These and other aspects of the present invention are set forth herein and in the appended claims.

[0019] The drawings briefly summarized below are provided for an illustrative understanding of the invention and are not intended to limit the invention as further described herein and in the appended claims. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a simplified schematic diagram of one embodiment of a molten steel ingot casting system of the present invention. [Figure 2(a)] FIG. 2(a) is a schematic diagram of a large mold filled with molten steel at the start of directional solidification. [Figure 2(b)] FIG. 2(b) is a schematic diagram of when half of the main part of the molten steel has solidified. [Figure 2(c)] Figure 2(c) is a diagrammatic representation of the state when all the molten steel has solidified. [Figure 3(a)] FIG. 3( a) is a parametric model of adjustably selectable ranges of voltage and current (power) and frequency values ​​as output from a variable power and variable frequency power supply to an electric induction heating coil in an example of the present invention for producing a large steel ingot of 100 metric tons with a nominal 250 kW induction power from the power supply. [Figure 3(b)] FIG. 3(b) is a parametric model of adjustably selectable ranges of voltage and current (power) and frequency values ​​as output from a variable power and variable frequency power supply to an electric induction heating coil in an example of the invention for producing a large steel ingot of 100 metric tons with a nominal 250 kW induction power from the power supply. [Figure 4(a)] FIG. 4( a) is a parametric model of adjustably selectable ranges of voltage and current (power) and frequency values ​​as output from a variable power and variable frequency power supply to an electric induction heating coil in an example of the present invention for producing a large steel ingot of 100 metric tons with a nominal 325 kW induction power from the power supply. [Figure 4(b)]FIG. 4(b) is a parametric model of the adjustably selectable ranges of voltage and current (power) and frequency values ​​as output from a variable power and variable frequency power supply to an electric induction heating coil in an example of the present invention for producing a large steel ingot of 100 metric tons with a nominal 325 kW induction power from the power supply. [Figure 5(a)] FIG. 5( a) is a parametric model of adjustably selectable ranges of voltage and current (power) and frequency values ​​as output from a variable power and variable frequency power supply to an electric induction heating coil in an example of the present invention for producing a large steel ingot of 150 metric tons with a nominal 250 kW induction power from the power supply. [Figure 5(b)] FIG. 5(b) is a parametric model of the adjustably selectable ranges of voltage and current (power) and frequency values ​​as output from a variable power and variable frequency power supply to an electric induction heating coil in an example of the invention producing a large steel ingot of 150 metric tons with a nominal 250 kW induction power from the power supply. [Figure 6(a)] FIG. 6( a) is a parametric model of adjustably selectable ranges of voltage and current (power) and frequency values ​​as output from a variable power and variable frequency power supply to an electric induction heating coil in an example of the present invention for producing a large steel ingot of 150 metric tons with a nominal 350 kW induction power from the power supply. [Figure 6(b)] FIG. 6(b) is a parametric model of the adjustably selectable ranges of voltage and current (power) and frequency values ​​as output from a variable power and variable frequency power supply to an electric induction heating coil in an example of the invention producing a large steel ingot of 150 metric tons with a nominal 350 kW induction power from the power supply. DETAILED DESCRIPTION OF THE INVENTION

[0021] A "large" steel casting ingot is defined herein as an ingot having a cross section with an area of ​​at least 0.10 square meters, and which may be round, square, rectangular, or other shape.

[0022] FIG. 1 illustrates one embodiment of a system 10 of the present invention for producing large steel ingots by solidifying molten steel in a mold. In FIG. 1, a mold 80 is filled with molten steel through a bottom mold inlet 80′, and an electric induction heating device 14 is located above the mold 80. Molten steel (also referred to as “molten metal”) is poured through the bottom mold inlet to a molten metal fill height 78. In another embodiment of the present invention, a top-pour casting mold is used. Once the mold 80 is filled with molten steel to the molten steel fill height 78, directional solidification of the molten steel begins to form the final steel ingot product. The molten steel 70 in the mold (from the mold bottom 80a to the molten metal fill height 78) is designated as a main portion 70a and a feeder portion 70b. In this schematic, the feeder portion begins at mold height 74 and ends at the molten metal fill height 78. The size of the feeder portion 70b can be determined by one skilled in the art, taking into account the size of the large steel ingot and solidification process parameters for a particular application.

[0023] The first step in the process is to determine the amount of power to be induced into the molten steel in the mold. In one embodiment of the present invention, the initial value of the induced power is selected as 10 kW per square meter of the upper surface area of ​​the molten steel in the mold.

[0024] In alternative embodiments of the present invention, the electric induction heating device 14 includes an electric induction heating coil; a spirally wound flat induction heating coil; or an electric induction heating coil at least partially encased in refractory. Korean Patent '914 discloses a pancake-shaped heating coil encased in refractory as an example of an electric induction heating device suitable for the present invention. Preferably, the coil or coils are configured to be initially positioned adjacent to the top surface region of the molten steel in the casting mold at the molten metal fill height 78.

[0025] In Figure 1, d osThe offset distance, denoted as d, is the distance between the top surface area of ​​the molten steel and the bottom of the induction heating device 14. The electric induction heating device 14 can be movably mounted above the top surface area of ​​the molten steel via a suitable lifting device known in the art. Korean Patent '914 discloses a lifting device for lifting and lowering a pancake-type heating device as an example of a lifting device suitable for the present invention. In some embodiments of the present invention, the distance d os is varied by raising or lowering the induction heater during the directional solidification of molten metal to form large steel ingots. For a given power to the molten metal, bringing the bottom of the induction heater closer to the surface of the molten metal increases the electrical efficiency, and because the Lorentz force is proportional to the melting force, the electromagnetic repulsive force remains the same, but the voltage and current from the output of the variable power variable frequency (VP / VF) power supply PS decreases.

[0026] In some embodiments of the present invention, the casting mold has an upper mold section that covers a bottom mold section containing molten steel to form a completely enclosed interior mold volume, and an induction heating device is positioned relative to the upper mold section to provide induction heat during directional solidification of the large steel ingot.

[0027] Molten steel 70 in mold 80 is shown schematically at melt fill height 78 in Figure 1 at the start of the directional solidification process. Once directional solidification of all the molten metal is complete, a final solidified large steel ingot 90 is obtained, as shown in Figure 2(c).

[0028] Variable power amount and variable frequency are provided from the VP / VF power supply PS shown in Figure 1. In some embodiments of the present invention, variable power is provided by varying the voltage magnitude output from the VP / VF power supply PS, e.g., from a variable voltage output controller associated with the VP / VF power supply PS. In other embodiments of the present invention, variable power is provided by varying a combination of voltage magnitude and current magnitude output from the VP / VF power supply PS, e.g., from a variable voltage and current output controller associated with the VP / VF power supply PS.

[0029] The variable frequency, voltage, and amperage of the alternating current supplied by the VP / VF power supply PS to the induction heater 14 are determined by the duration of the particular directional solidification process, which in turn is determined by the size and shape of the ingot and the chemical composition of the cast material. The process control parameters are selected so that the molten metal in the feeder portion 70b does not begin to solidify until the directional solidification process of the main portion 70a is completely completed in forming the main ingot portion.

[0030] In some embodiments of the present invention, the variable frequency is selected to control the nominal depth of the current penetration into the molten steel so that the nominal depth of the current penetration remains within the feeder section 70b. By limiting the nominal current depth to the feeder section, the molten steel remains in the feeder section and heat generated in the feeder section is transferred to the main section by conduction and convection, ensuring directional solidification in the main section.

[0031] In some embodiments of the present invention, the variable frequency output from the power supply PS is selected to suppress the occurrence of casting defects in the directional solidification of the main portion 70a, and the frequency is selected to create a particular type of electromagnetic stirring pattern and speed depending on the type of casting defect.

[0032] In some embodiments of the present invention, a human machine interface can be utilized to store variable power output values ​​and variable frequency output values ​​of the power supply for future large ingot casting processes. In some embodiments of the present invention, the real-time ingot directional solidification process control by the VP / VF power supply PS to the induction coil in the induction heating device 14 for a given mold and the steel alloy casting formula used to build a parametric model of process power and frequency control for casting future ingots in the same mold using the same alloy casting formula can be stored in a memory unit of the human machine interface (HMI) for casting future ingots.

[0033] In some embodiments of the present invention, the control parameters of the directional solidification process for a given mold and steel alloy type may be stored for future use in a suitable electronic device, such as the solid-state memory component of the human machine interface (HMI) shown in Figure 1. The power rate and frequency and the established control parameters are fixed as formulas by the casting process software running on the system processor of the HMI, and the formulas are saved in the solid-state memory component of the HMI for further use.

[0034] 3(a)-6(b) show four example parametric models of the present invention for controlled casting by in-mold casting of large steel ingots using induction variable power and variable frequency applied to the top surface region of the molten metal in the model.

[0035] The top half of each of Figures 3(a) through 6(a) plots a typical parametric relationship between the magnitude of the power supply's variable output voltage (value on the right vertical axis) and the magnitude of the variable current (value on the left vertical axis) (the variable frequency value is referenced on the horizontal axis).

[0036] The bottom half of each figure plots a typical parametric relationship between the electromagnetic repulsion force (values ​​on the right vertical axis) and the melting rate (values ​​on the left vertical axis) (variable frequency values ​​are referenced on the horizontal axis).

[0037] The term "pre-solid" in the graphs refers to parameters applied when the mold is filled with molten metal and all steel in the mold is in a molten state as shown diagrammatically in Figure 2(a) until half of the molten metal is solidified (90') as shown in Figure 2(b). The term "semi-solid" in the graphs refers to parameters applied after the half-solidified main ingot portion 90' has been formed until the complete main ingot portion and feeder ingot portion have been formed. [Example]

[0038] Example 1 Example 1 is illustrated graphically in Figures 3(a) and 3(b) for the casting of a 100 metric ton (mt) steel ingot, where the magnitude of the power induced from the energized induction coil of the electric induction heating device 14 to the top surface of the feeder molten metal portion 70b is maintained at a nominal 250 kW from the start of directional solidification of the main portion 70a after the molten steel fills the mold, until the main portion 70a and the feeder portion 70b have directionally solidified to form the final product ingot.

[0039] Graph G1.1 (power parameters) and graph G1.2 (electromagnetic stirring parameters) in FIG. 3(a) are applied to the ingot solidification process of Example 1 from the start of directional solidification of the main portion 70a after the molten steel is filled into the mold until the bottom half 90′ of the main portion 70a is solidified as shown in FIG. 2(b).

[0040] Graph G1.3 (power parameters) and graph G1.4 (electromagnetic stirring parameters) (semi-solidification process curves) in FIG. 3(b) are applied to the ingot solidification process of Example 1 from when half of the main portion 70a solidifies until the entire ingot solidifies.

[0041] In Example 1, dashed solidification process line A1 in Figure 3(a) was selected to vary the power and stirring parameters from the start of directional solidification of main portion 70a until half of the main portion was solidified. From process dashed line A1 in Figure 3(a), the VP / VF power supply PS outputs 7,001 amperes at 976 volts with a frequency of 1 kHz, resulting in electromagnetic stirring characteristics of a typical molten steel velocity of 0.192 meters per second (m / s) and a Lorentz repulsion force of 59.3 kilograms (kg).

[0042] In Example 1, the dashed solidification process line A2 in Figure 3(b) (semi-solidification process curve) was selected for variable power and stirring parameters from after half of the main portion 70a solidified until the remainder of the main portion 70a and the feeder portion 70b solidified. From the dashed process line A2 in Figure 3(b), the output of the VP / VF power supply PS was adjusted to 6164.1 amperes at 1.2792 kilovolts (kV), with an increased frequency of 1.5 kHz, resulting in electromagnetic stirring characteristics of a typical molten steel velocity of 0.142 meters per second (m / s) and a Lorentz repulsion force of 46 kilograms (kg).

[0043] Example 2 Example 2 is shown graphically in Figures 4(a) and 4(b) for the casting of a 100mt steel ingot, where the magnitude of the power induced from the energized induction coil of the electric induction heating device 14 to the top surface of the feeder section 70b is maintained at a nominal 325kW from the start of directional solidification of the main section 70a after the molten steel has filled the mold, until the main section 70a and the feeder section 70b have directionally solidified to form the final product ingot.

[0044] Graph G2.1 (power parameters) and graph G2.2 (electromagnetic stirring parameters) in FIG. 4(a) are applied to the ingot solidification process of Example 2 from the start of directional solidification of the main portion 70a after the molten steel is filled into the mold until the bottom half 90′ of the main portion 70a is solidified as shown in FIG. 2(b).

[0045] Graph G2.3 (power parameters) and graph G2.4 (electromagnetic stirring parameters) (semi-solidification process curves) in FIG. 4(b) are applied to the ingot solidification process of Example 2 from the time when half of the main portion 70a has solidified until the entire ingot has solidified.

[0046] In Example 2, the dashed solidification process line B1 in Figure 4(a) was selected to vary the power and stirring parameters from the start of directional solidification of the main portion 70a until half of the main portion was solidified. From the dashed process line B1 in Figure 4(a), the output of the VP / VF power supply PS was adjusted to 6,460 amperes at 1.779 kV with a frequency of 2 kHz, resulting in electromagnetic stirring characteristics of a typical molten steel velocity of 0.16 meters per second (m / s) and a Lorentz repulsion force of 50.44 kilograms (kg).

[0047] In Example 2, the dashed solidification process line B2 in Figure 4(b) (semi-solidification process curve) was selected until the remainder of the main portion 70a and the feeder portion 70b solidified. From the process dashed line B2 in Figure 4(b), the output of the VP / VF power supply PS was adjusted to 6,093 amperes at 2.091 kV with a frequency of 2.5 kHz, resulting in electromagnetic stirring characteristics of a typical molten steel velocity of 0.131 meters per second (m / s) and a Lorentz repulsion force of 44.88 kilograms (kg).

[0048] Example 3 Example 3 is shown graphically in Figures 5(a) and 5(b) for the casting of a 150mt steel ingot, where the magnitude of the power induced from the energized induction coil of the electric induction heating device 14 to the top surface of the feeder section 70b is maintained at a nominal 250kW from the start of directional solidification of the main section 70a after the molten steel has filled the mold, until the main section 70a and the feeder section 70b have directionally solidified to form the final product ingot.

[0049] Graph G3.1 (power parameters) and graph G3.2 (electromagnetic stirring parameters) in FIG. 5(a) are applied to the ingot solidification process of Example 3 from the start of directional solidification of the main portion 70a after the molten steel is filled into the mold until the bottom half 90′ of the main portion 70a as shown in FIG. 2(b) solidifies.

[0050] Graph G3.3 (power parameters) and graph G3.4 (electromagnetic stirring parameters) (semi-solidification process curves) in FIG. 5(b) are applied to the ingot solidification process of Example 3 from when half of the main portion 70a solidifies until the entire ingot solidifies.

[0051] In Example 3, in Figure 5(a), the dashed solidification process line C1 was selected for variable power and stirring parameters from the start of directional solidification of the main portion 70a until the main portion 70a solidified. From the dashed process line C1 in Figure 5(a), the VP / VF power supply PS outputs 7,834.7 amperes at 1.1296 kV with a frequency of 1 kHz, resulting in electromagnetic stirring characteristics of a typical molten steel velocity of 0.216 meters per second (m / s) and a Lorentz repulsion force of 86.2 kilograms (kg).

[0052] In Example 3, the dashed solidification process line C2 in Figure 5(b) (semi-solidification process curve) was selected for variable power and stirring parameters from after half of the main portion 70a solidified until the remainder of the main portion 70a and the feeder portion 70b solidified. From the dashed process line C2 in Figure 5(b), the output of the VP / VF power supply PS was adjusted to 7,022.1 amperes at 1.5135 kV with a frequency of 1.5 kHz, resulting in electromagnetic stirring characteristics of a typical molten steel velocity of 0.16 m / s and a Lorentz repulsion force of 70 kilograms (kg).

[0053] Example 4 Example 4 is shown graphically in Figures 6(a) and 6(b) for the casting of a 150mt steel ingot, where the magnitude of the power induced from the energized induction coil of the electric induction heating device 14 to the top surface of the feeder section 70b is maintained at a nominal 325kW from the start of directional solidification of the main section 70a after the molten steel has filled the mold, until the main section 70a and the feeder section 70b have directionally solidified to form the final product ingot.

[0054] Graph G4.1 (power parameters) and graph G4.2 (electromagnetic stirring parameters) in FIG. 6(a) are applied to the ingot solidification process of Example 4 from the start of directional solidification of the main portion 70a after the molten steel is filled into the mold until half of the lower half 90′ of the main portion 70a is solidified as shown in FIG. 2(c).

[0055] Graph G4.3 (power parameters) and graph G4.4 (electromagnetic stirring parameters) (semi-solidification process curves) in FIG. 6(b) are applied to the ingot solidification process of Example 4 after half of the main portion 70a has solidified until the entire ingot has solidified.

[0056] In Example 4, in FIG. 6(a), the main part 7 The dashed solidification process line D1 was selected for variable power and stirring parameters from the start of directional solidification at 0a until half of the main section solidified. From the process dashed line D1 in Figure 6(a), the VP / VF power supply PS outputs 7,428.2 amperes at 2.1229 kV with a frequency of 2 kHz, resulting in electromagnetic stirring characteristics of a typical molten steel velocity of 0.185 meters per second (m / s) and a Lorentz repulsive force of 78.3 kilograms (kg).

[0057] In Example 4, dashed solidification process line D2 in Figure 6(b) (semi-solidification process curve) was selected for variable power and stirring parameters from after half of main portion 70a solidified until the remainder of main portion 70a and feeder portion 70b solidified. From dashed process line D2 in Figure 6(b), the output of VP / VF power supply PS was adjusted to 7,010.0 amperes at 2.5041 kV with a frequency of 2.5 kHz, resulting in electromagnetic stirring characteristics of a typical molten steel velocity of 0.15 m / s and a Lorentz repulsion force of 70.3 kilograms (kg).

[0058] As shown in the example above, a 150mt ingot has a larger top surface area of ​​the molten metal than a 100mt ingot and utilizes a larger induction coil in the electric induction heater. However, the total power requirements for casting the larger 150mt ingot are less than for casting the 100mt ingot. This is due to the increased power efficiency over the larger top surface area.

[0059] In some embodiments of the present invention, when less agitation is preferred, for example, due to concerns about non-metallic inclusions migrating to the main part, a higher frequency is selected by coordinated variation of the voltage and current output from the VP / VF power supply PS. For a given melt power, the melt flow rate is inversely proportional to the square root of the frequency. As the frequency increases from 1000 Hz to 3000 Hz, the rate decreases by a factor of 1.732.

[0060] As illustrated in the above examples, in some embodiments of the present invention, the variable frequency provided by the VP / VF power supply PS increases with increasing voltage and decreasing current.

[0061] The induction power and electromagnetic stirring force are concentrated near the surface. They are not affected by the depth of the molten metal. However, the flow velocity and size of each vortex are affected by the depth of the molten metal.

[0062] Most of the induction heat output and electromagnetic stirring force are generated within twice the nominal depth, a value determined by the frequency of the AC current. The solidification line is not affected by the depth of the melt until it moves to a level twice the nominal depth. However, the velocity of the melt flow and the size of each vortex are significantly affected by the depth of the melt.

[0063] By applying the present invention, the cooling rate of the bottom end of the mold is faster and the cooling rate of the top end of the mold is slower, resulting in the production of high-quality steel ingot products with fewer solidification defects in the casting. Furthermore, by controlling the applied variable power and variable frequency in a coordinated manner, precise temperature control can be performed without introducing impurities into the ingot, allowing the production of high-quality ingot products.

[0064] Although preferred embodiments of the present invention have been disclosed for purposes of illustration, the present invention is not limited to the construction and operation of those embodiments. Moreover, those skilled in the art will recognize that various changes and modifications can be made without departing from the subject matter of the disclosure. Accordingly, such changes and modifications are within the spirit and scope of the present disclosure.

Claims

1. 1. A system for producing large steel ingots by solidifying molten steel in a mold, comprising: a mold for containing the molten steel, wherein the molten steel forms a feeder portion of the molten steel and a main portion of the molten steel located below the feeder portion; an electric induction heating coil disposed within the mold parallel to and adjacent to a top surface region of the molten steel within the mold, the electric induction heating coil suspended entirely at an offset distance from the top surface region of the molten steel; a power supply configured for continuously supplying a variable power output and a variable frequency output to the electric induction heating coil, the variable power output and the variable frequency output being simultaneously adjusted over a range of power and frequency to provide induction heating energy and stirring power to maintain the molten steel in the feeder section in a molten state until the molten steel in the main section solidifies, and the induction heating energy supplied by the power supply to the electric induction heating coil is consistently maintained at a nominal induction power; Including, the system.

2. 10. The system of claim 1, wherein the electric induction heating coil comprises a flat spiral wound induction coil, the turns of the flat spiral wound induction coil being coplanar and forming a pancake-shaped heating coil.

3. The system of claim 2 , wherein the flat spirally wound induction coil is at least partially encased in refractory.

4. 3. The system of claim 2, wherein the flat spiral wound induction coil is completely encased in refractory.

5. The system of claim 1 , wherein the variable power output is provided by a variable voltage controller.

6. The system of claim 1 , wherein the variable power output is provided by a variable voltage and variable current controller.

7. 10. The system of claim 1, further comprising an elevating device configured to raise or lower the electric induction heating coil above the upper surface region of the molten steel to selectively increase or decrease, respectively, the offset distance.

8. 10. The system of claim 1, further comprising a human machine interface for storing variable power output values ​​and variable frequency output values ​​of the power supply for future large ingot casting processes.

9. 10. The system of claim 1, wherein the variable frequency output increases with increasing voltage and decreasing current.

10. 10. The system of claim 1, wherein the mold includes an upper mold section and a lower mold section, the upper mold section covering the lower mold section containing the molten steel to form a completely enclosed mold interior volume.

11. 1. A method for producing a large steel ingot by solidifying molten steel fed into a mold, the molten steel forming a riser portion of the molten steel above a main portion of the molten steel, the method comprising: positioning an electric induction heating coil within the mold above a top surface region of the molten steel; continuously supplying variable amounts of power and variable frequencies to the electric induction heating coil; selecting a directional solidification process according to the size of the large steel ingot, the shape of the large steel ingot, and the chemical composition of the molten steel, wherein the directional solidification process includes parameter ranges of solidification power magnitude and solidification frequency over a process time; and simultaneously adjusting the amount of variable power and the variable frequency supplied to the electric induction heating coil over the process time to match the solidification power magnitude and the solidification frequency parameters of the selected directional solidification process. Including, wherein the solidification power magnitude and solidification frequency parameters are configured to maintain induction power from the electric induction heating coil at a nominal induction power level from the time the molten steel in the main portion is in a molten state until at least the main portion has solidified, and to maintain the molten steel in the feeder portion in a molten state until the main portion below has solidified.

12. 12. The method of claim 11, further comprising adjusting a variable frequency supplied to the electric induction heating coil to move one or more inclusions from the main portion to the feeder portion.

13. 12. The method of claim 11 , wherein the variable frequency is adjusted to limit a defined depth of current penetration into the feeder portion.

14. 12. The method of claim 11, wherein the variable amount of power is controlled by adjusting the height of the electric induction heating coil above the top surface area of ​​the molten steel.

15. 12. The method of claim 11, wherein adjusting the amount of variable power comprises adjusting a supply voltage and a supply current to the electric induction heating coil on a scale of multiples of 10 kilowatts per square meter of the top surface area of ​​the molten steel.

16. The method of claim 11 , wherein adjusting the variable frequency comprises controlling an electromagnetic stirring force for a stirring pattern and rate of molten metal flow in the molten steel.

17. 12. The method of claim 11, further comprising adding heat generating and insulating materials to the upper surface region before placing the electric induction heating coil on the upper surface region.

18. 1. A method for producing a large steel ingot by solidifying molten steel fed into a mold, the molten steel forming an upper riser portion of the molten steel above a lower main portion of the molten steel, the method comprising: placing an electric induction heating coil above a top surface region of the molten steel; continuously supplying variable amounts of power and variable frequencies to the electric induction heating coil; First, adjusting the supply voltage and supply current to the electric induction heating coil on a scale of multiples of 10 kilowatts per square meter of the top surface area of ​​the molten steel; and simultaneously adjusting the variable power amount and variable frequency supplied to the electric induction heating coil so as to maintain induction power from the electric induction heating coil at a nominal induction power level from the time the molten steel in the main portion is in a molten state until at least the time the main portion solidifies, and so as to maintain the molten steel in the feeder portion in the molten state until the main portion below solidifies. A method comprising:

19. 20. The method of claim 18, further comprising controlling the amount of variable power by adjusting the height of the electric induction heating coil above the top surface region of the molten steel.

20. 20. The method of claim 18, further comprising building and storing a parametric model of variable power magnitude and variable frequency from the start of solidification to the end of solidification, the parametric model being associated with a particular alloy of the mold and the molten steel.

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