Apparatus for heating molten steel in tundish in continuous casting of steel

Abstract

To provide an apparatus for heating molten steel in a tundish in continuous casting of steel.SOLUTION: In an airtight tundish, a resistance heating element made of graphite is arranged above the molten steel surface so as to closely face the molten steel surface in an inert atmosphere. A spiral coil is formed on the disc-shaped heating element. The temperature of the heating element is raised to about 2000°C by AC energization to radiate and heat the space in the tundish. On the other hand, induction heating and induction stirring occur immediately below the molten steel surface below the coil. The power loss due to the self-inductance contributes to heat generation, and a power efficiency of 90% is obtained. A ceramic coating is applied to the upper surface of the heating element to suppress upward radiation and to improve the durability of the inner wall of the refractory. The trouble that slag and molten steel are stuck to the inner wall is eliminated by suitable upward radiation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an apparatus for heating molten steel in a tundish in the continuous casting of steel. [Background technology]

[0002] In continuous casting of steel, molten steel in a ladle is continuously poured into a mold through a tundish, which is an intermediate vessel that controls the flow rate and pouring temperature. In this case, high-end models are equipped with a device that reheats the molten steel in the tundish at the appropriate time, and precise temperature control not only stabilizes operation (prevents accidents due to low temperatures) and quality (reduces segregation and non-metallic inclusions), but also provides solid benefits such as reducing the heat load on the upstream melting and refining processes.

[0003] The method and apparatus for heating molten steel in a tundish will be described below. Patent Document 1 discloses a method for controlling the pouring temperature using high-frequency induction heating, which is the original method of "tundish heating." In single-strand continuous casting, the crucible-shaped tundish allows for the application of high-frequency solenoid coils. This method was used effectively for a long time as a global pioneer in tundish heating. The temperature of the poured stream from the ladle decreases over time. In the latter half of the pouring time, the input is gradually increased, heating is performed as if the poured stream temperature was constant, and the pouring temperature is made constant. Induction stirring also occurs, resulting in excellent heat uniformity. Compared to heating using what is commonly known as an LF furnace, which reheats molten steel in a ladle, the equipment capacity (kW) is less than one-fifth and the power consumption is also a fraction of that. The problem is that in the case of multi-strand tundishes, the shape is usually an odd polygon, making it difficult to design and operate the solenoid coils properly.

[0004] Subsequently, a device was developed for induction heating in special-shaped tundishes used for multi-strand applications. The tundish is divided into a steel receiving section and a casting section, and a low-frequency induction heating device is installed as an induction circuit via two or three connecting sections. This principle can be considered to be the same as that of the long-standing channel-type low-frequency induction melting furnace, but installed inside the tundish. It has the same effects as the previous example and is in practical use. Its heat uniformity is also not poor. The problem is that, as in the prior art, the inductor is integrated with the tundish body, and it is necessary to attach an inductor to every tundish available, which results in expensive equipment. Furthermore, the vessel and flow passages have complex shapes, which poses problems in terms of durability and repair of the refractory material. As with the previous method, this is induction heating, so the heating efficiency itself is not very good, at less than 50%.

[0005] Patent Document 2 discloses a method in which an ultra-high temperature (approximately 10,000°C) plasma torch is installed in a tundish cover, a heating chamber is provided above the molten steel in the tundish, and the chamber is heated. This method also achieves the same effects as the two previous examples. It also has a large output. One of the problems is that the liquid surface is heated by an ultra-high temperature jet, which results in extremely poor heat uniformity and large refractory melting damage. Another problem is that the equipment is expensive and requires high costs such as Ar gas consumption. In addition, as detailed in Non-Patent Document 1, it is complex and requires a high level of skill, expense, and management for use and repair (high maintenance costs). The power consumption rate is also not good.

[0006] Patent Document 3 discloses a method of widely radiating heat over the top surface of molten steel using a high-temperature graphite heating element. According to this method, Joule heat is utilized, resulting in an electricity usage efficiency of nearly 90%. The top surface of the molten steel is heated by a portion of the radiation from the approximately 2000°C heating element directly, while the rest is reflected by the inner wall of the tundish and passes through the floating slag. The essential requirements of this method are as follows: 1) To enhance the radiation absorption capacity, slag (emissivity 0.8) is placed on top of the molten steel (emissivity approx. 0.3). To make something float. 2) A non-oxidizing atmosphere is required to ensure the durability of graphite. 3) Apply gas bubbling to promote heat transfer from the slag to the molten steel and to achieve uniform heat transfer. The main paths of heat transfer are as follows: Radiation from heating element ⇒ Radiation absorption from slag surface ⇒ Heat transfer within slag ⇒ Convection and conduction heat transfer at the interface between slag and molten steel ⇒ Heat uniformity within molten steel The feature of this method is that it completely solves the problem of molten steel and slag adhesion to the inner wall of the tundish, which is troublesome for quality and operation, because the refractory wall above the molten steel surface is maintained at a temperature higher than the molten steel temperature, preventing adhesion.

[0007] The problem with this method is that the bottleneck for heat transfer is between the slag and molten steel, and the rapid heating (short heating time delay) is inferior to induction heating. If the current is increased to accelerate heating, the heating element will exceed 2000°C more quickly than the heat transferred to the molten steel, which can easily damage the refractory material on the inner wall of the tundish. One possible measure would be to increase the surface area of ​​the heating element to suppress the temperature rise to some extent, but in that case the cost of the heating element, which is a consumable item, becomes an issue. In radiant heating, there is a trade-off between rapid heating and refractory durability, and achieving both simultaneously is a challenge.

[0008] Patent Document 4 discloses a method for induction heating of metals in which the coil is changed from a water-cooled copper tube to a resistance heating element, and the heat loss of the inductor that occurs during induction is utilized for heating. This is a useful method for saving energy. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Unexamined Patent Publication No. 55-161554 [Patent Document 2] Published Patent Publication No. Hei 11-291023 [Patent Document 3] Patent No. 5690015 [Patent Document 4] Patent Publication 1966-7142 [Non-patent literature]

[0010] [Non-Patent Document 1] Nippon Steel Technical Report No. 382 (2005), p.16 Nippon Steel Twin Torch Tundish Plasma Heating Equipment Summary of the Invention [Problem to be solved by the invention]

[0011] The method of applying a crucible-type high-frequency furnace to a tundish in Prior Art Example 1 has been used successfully, but there is a problem that it can only be applied to a single strand due to shape restrictions. Moreover, the aspect ratio (height / diameter) of the coil is small, so heating efficiency is significantly reduced.

[0012] The method of incorporating a low-frequency induction heating furnace in the center of the tundish in Prior Example 2 can also be applied to multiple strands. The problem is that it is necessary to form a molten steel loop in the tundish, which is equivalent to the secondary coil of a transformer, and there are significant operational difficulties in forming, arranging and maintaining the flow path using refractory materials. Both methods use induction heating, which is effective in terms of uniform heating, as it involves stirring the molten steel, but because it is direct heating into the molten steel and not above the surface of the molten steel, there remains the problem of molten steel and slag sticking to the wall, which leads to quality and maintenance problems.The heating efficiency is also not great.

[0013] The method of attaching a heating chamber consisting of a plasma torch and gas bubbling as in Prior Art Example 3 is as effective as an arc furnace, but there are problems with the refractory material melting due to the plasma at approximately 10,000°C. The equipment costs are excessive, the heating efficiency is not high, and there are significant cost issues such as the consumption of argon gas. There is also the problem of adhesion to the inner wall of the tundish outside the heating chamber.

[0014] In the case of radiation from the graphite high-temperature heating element to the molten steel surface in Prior Art Example 4, heat transfer is promoted by the absorption of radiation by the slag and the stirring effect of gas bubbling, and because it is Joule heat, circuit loss is small and heating efficiency is improved. Since the entire space above the molten steel in the tundish becomes hotter than the molten steel temperature, the adhesion problem is reliably solved. The problems are: 1) radiation is the same above and below, so if the heating of the molten steel is strengthened, the refractory above the molten steel will be overheated, and 2) the emissivity of molten steel is less than 0.3, so direct heat absorption is weak. Although slag is required, the heat transfer resistance increases and the rapid heating is not possible.

[0015] In view of the above problems, the present invention aims to provide a method for reheating molten steel in a tundish that can obtain the desired heating capacity, can be applied to vessels of any shape, has excellent heat uniformity, and is advantageous in terms of equipment and operating costs (electricity and refractories). Specifically, the present invention simultaneously solves the problems of Prior Art 4, namely, the lack of rapid heating and the refractory meltdown. [Means for solving the problem]

[0016] The first invention of the present application is an apparatus for heating molten steel in a tundish during continuous casting of steel, characterized in that a disk-shaped resistance heating element made of graphite and having a spiral coil structure is placed directly above the surface of the molten steel so as to face it, and an alternating current is passed through the heating element to heat it to a temperature of 1600°C or higher but lower than 2100°C by Joule heat, thereby radiating heat to the slag floating on the surface of the molten steel, and inductively heating and inductively stirring the area directly below the surface of the molten steel, while the upper surface of the heating element is coated with alumina or magnesia mortar to suppress upward radiation.

[0017] The second is a heating device for molten steel in a tundish according to the first invention, characterized in that two resistance heating elements are connected in series or three resistance heating elements are arranged in a triangular configuration to form a three-phase star connection. [Effects of the Invention]

[0018] The heating device for molten steel in a tundish of the present invention combines the functions of induction heating / induction stirring and radiation heating, the latter of which has optimized distribution of radiation between the top and bottom. The first effect of the above invention is that the tundish heating device of the present invention can precisely control the pouring temperature (temperature of molten steel in the tundish) by appropriately heating and stirring the molten steel in the tundish in response to the temperature drop of the pouring stream from the ladle. This precise control makes it easier to set the refining temperature in the upstream process at a low level, avoiding excessive temperature increases that were previously required. As a result, this not only contributes to stable quality and operation, but also to energy savings.

[0019] The second advantage is that, in the past, slag and metal adhesion was always observed above and below the molten steel level of a tundish after use. This caused contamination of the molten steel, and repair work was troublesome and costly. However, with the present invention, because heating is performed from above, the temperature of both the tundish wall and the inner surface of the lid is higher than the molten steel, so adhesion does not occur. Furthermore, a ceramic coating is applied to the upper surface of the heating element, which appropriately suppresses upward radiation and prevents excessive radiation to the inner wall of the refractory even when heating is intensified, solving the fatal weakness of Prior Art 4 (Patent Document 3).

[0020] The third advantage is that the heating element is a spiral coil, so that the molten steel directly below is induction heated and stirred by passing AC current through it, which compensates for the drawback of radiation heating, which tends to be delayed. The impedance loss of the coil due to induction contributes to raising the temperature of the heating element, so no power is wasted. [Brief explanation of the drawings]

[0021] [Figure 1] 1 shows a schematic structure of an apparatus for heating molten steel in a tundish according to the present invention. [Figure 2] 1 shows the structure of a heating element having an induction function according to the present invention. [Figure 3] 1 shows the structure of the twin-type heating element of the present invention, where A is a top view, B is a cross-sectional view, and C is an equivalent circuit. [Figure 4] The diagram shows induction and molten steel flow using twin heating elements. [Figure 5] This is a three-electrode heating element of the present invention, where A indicates the structure and B indicates the circuit. [Figure 6]This shows how to attach a three-pole heating element to a two-strand tundish. [Figure 7] It indicates the amount of radiative heat transfer from the heating element to the slag, and the parameter is the slag temperature. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, an embodiment will be described with reference to the drawings. Figure 1 shows an apparatus for heating molten steel in a tundish according to the present invention, where A is a top view of the tundish, B is a longitudinal cross-sectional view of the central portion, and C is a side view of the central portion. Reference numeral 1 denotes the steel shell of a single-strand box-shaped tundish, 2 denotes the inner wall refractory, 3 denotes the injection hole, 4 denotes the pouring hole, 5 denotes a heating element body with induction function, 6 denotes an electrode that conducts electricity to the heating element body 5, 7 denotes molten steel, 8 denotes a gas injection plug that stirs the molten steel 8, and 9 denotes an airtight flange. As shown in the figure, the heating element body 5 is disc-shaped and faces directly above and parallel to the surface of the molten steel. The heating element body 5 emits powerful radiation.

[0023] Figure 2 shows the structure of the heating element. The heating element consists of a main spiral coil 21, electrodes 22 connected to both ends of the spiral coil 21, and a ceramic coating 23 that covers the upper surface of the spiral coil 21. The spiral coil 21 is a graphite disk with spiral grooves 24 formed on it. The graphite spiral coil 21 generates high-temperature radiation (approximately 2000°C) and electromagnetic induction heating and stirring functions when AC is applied. Stirring is advantageous because it promotes passage through the slag, which is a bottleneck for radiative heat transfer. The ceramic coating 23 has the function of suppressing radiation from the spiral coil 21, preventing excessive radiation to the refractory wall of the tundish, and contributing to the durability of the refractory. To form the ceramic coating, it is sufficient to simply apply a mortar made by kneading high-purity alumina or magnesia powder with water to ensure heat resistance.

[0024] Figure 3 shows a twin-type heating element to enhance its capacity and make installation easier. A is a top view, B is a longitudinal cross-section of the center, and C shows the circuit. 31 is the spiral coil, 32 is the electrode, 33 is the connection terminal, 34 is the current, and 35 is the distribution of magnetic field lines. The current is connected in series so that the left and right directions are opposite. In the circuit diagram, 36 is the AC power source, 37 is a resistor, and 38 is the coil. When AC is passed through the spiral coil 31, self-inductance appears, increasing the impedance. A capacitor of appropriate capacity is inserted in the circuit (not shown). A series connection is convenient because it halves the number of electrodes. The surface area of ​​the heating element is doubled, so the heating capacity is also doubled.

[0025] Figure 4 shows the state of the induced current and stirring flow in a twin-type heating element. 41 is the electrode, 42 is the load current, 43 is the induced current in the molten steel, and 44 is the flow that occurs just below the surface of the molten steel due to the repulsion of the load current and the induced current. There is no electromagnetic effect on the slag floating on the surface of the molten steel.

[0026] Figure 5 is a diagram illustrating a three-pole heating element, where A shows the structure as seen from above and B shows the wiring method. To accommodate three-phase AC, three heating elements 51, 52, and 53 are arranged in a triangular configuration with a single central connection terminal 54, and are connected in a star-shaped circuit. The three-pole configuration not only increases output, but also produces new effects. This is because a rotating magnetic field is formed, and vortex currents appear just below the surface of the molten steel in response to the movement of the magnetic field. With commercial frequencies, the heating capacity is unlikely to increase, but the stirring force is greater.

[0027] Figure 6 shows the above three-pole heating element attached to a two-strand tundish. Compared to the system incorporating a low-frequency induction furnace, this system is simpler, cheaper, and has greater capacity. 61 is the steel shell, 62 is the refractory wall, 63 is the injection hole, 64 is the casting hole, and 65 is the heating element. All 1-pole, 2-pole, and 3-pole types are incorporated into the tundish cover, so a maximum of two units are sufficient, reducing equipment costs.

[0028] The relationship between the heating element and the refractory material will be described. Refractories have an emissivity of 0.7 to 0.8, giving them a high heat absorption capacity. However, their thermal conductivity is low, so their surface temperature easily rises when they receive strong radiation from the heating element. A temperature of 1550°C or higher is above the melting point of iron, which prevents molten steel and slag from adhering to the inner wall surface and eliminates the risk of molten steel contamination. Therefore, the inner wall temperature must be maintained at 1550°C or higher. The heating element must be at least 1600°C or higher. On the other hand, if the wall temperature exceeds 1700°C, the refractory will melt. The ceramic coating on the top surface of the heating element suppresses radiation and prevents the refractory wall from melting. Since the heating element is expected to reach a maximum temperature of 2000°C, high-purity alumina or magnesia with a high melting point is appropriate to prevent the coating from melting. It is desirable to find the optimum conditions by adjusting the area and thickness of the coating as appropriate. When the temperature of the heating element exceeds 2100°C, the insulating alumina mortar begins to melt, and the refractory wall inside the tundish also rises in temperature to over 1700°C and melts. The maximum allowable temperature of the heating element is considered to be 2100°C.

[0029] The required power output of the power supply will be explained. The temperature of the pouring stream itself drops by approximately 30 to 50°C during continuous casting of one charge for approximately one hour. Furthermore, the starting temperature differs for each charge. To maintain a constant pouring temperature, the power output must be gradually increased in response to the fluctuations of both. Automatic control can be easily achieved by feedback from continuous temperature measurement. The estimated required output to obtain a temperature rise capacity of 40℃ at an efficiency of 100t / h is as follows: Output P = specific heat x temperature x casting efficiency / heating efficiency P=200(kcal / t℃)×40(℃)×100(t / h) / 0.9≒1000kW The reason for setting the heating efficiency at 0.9 here is that in induction heating devices, power loss occurs in the coil, resulting in a maximum of 0.7 or less, but in the present invention, all of the wasted heat is diverted to radiation, resulting in an efficiency of approximately 0.9.

[0030] The frequency of the power supply is related to induction heating capacity and stirring. As the frequency increases, the heating capacity increases. Heating and stirring are roughly inversely proportional. If designed appropriately, the required heating capacity and stirring strength can be obtained even at commercial frequencies by combining it with radiant heating capacity.

[0031] The required surface area of ​​the heating element will be explained. If the surface area of ​​the heating element is relatively insufficient compared to the power output, the temperature of the heating element will rise and exceed the upper limit of 2100°C mentioned above. On the other hand, if it is too high, the amount of wear will increase. Keeping the inner wall temperature below 1700°C, preferably around 1600°C, is appropriate for refractory durability and for solving the problem of contamination caused by adhesion of molten steel and slag. Figure 7 shows the relationship between the temperature of the heating element and the temperature of the heat receiving element for the amount of heat generated by a unit area of ​​the heating element in radiant heat transfer. The required surface area can be roughly calculated from the diagram. The upper surface of the heating element is roughly estimated to be half the area, with no significant error. In the present invention, induction heating is added to radiant heating, so the required area does not exceed the approximate value calculated from the diagram.

[0032] The present invention is an improvement over Prior Art 4. Therefore, in the present invention, airtightness and an inert atmosphere to suppress oxidation wear of the heating element, and gas agitation using Ar to promote heat transfer are of course essential elements. The present invention aims to solve the weaknesses of the prior art, namely, excessive heating of the inner wall of the tundish and insufficient agitation power of gas bubbling. Increasing the flow rate of gas bubbling strengthens agitation, but splashing causes problems, so it must be suppressed. The agitation power is insufficient. In the present invention, excess heating is compensated for by the addition of a ceramic coating for thermal insulation, and insufficient stirring is compensated for by the addition of induction stirring. [Example]

[0033] The conditions for raising the temperature of molten steel by 40°C using the device of the present invention in continuous casting with a casting efficiency of 60 t / h are shown below. Productivity 60t / h Number of strands: 2 Output 600kVA (2000A x 300V) Heating element structure: 2-pole type Heating element outer diameter 600mm Heating element coating: 10mm thick alumina mortar The biggest cost factor is the discoid graphite. Graphite electrode rods for arc furnaces are the cheapest graphite material. Nowadays, diameters up to 800 mm are commercially available, so you can simply select the appropriate diameter and machine it. [Industrial Applicability]

[0034] The heating device for molten steel in a tundish according to the present invention can be easily applied to existing continuous casting machines, contributing to improvements in quality and productivity. [Explanation of symbols]

[0035] 1;Shell, 2;Inner wall refractory material, 3;Injection hole, 4;Casting hole, 5;Heater, 6;Electrode, 7;Molten steel, 8;Gas inlet plug, 21;Spiral coil, 22;Electrode, 23;Ceramic coating, 31;Spiral coil, 32;Electrode, 33;Connecting terminal, 34;Current, 35;Magnetic field lines, 36;AC power supply, 37;Resistance, 38;Coil, 41;Electrode, 42;Load current, 43;Induced current, 44;Flow, 51, 52, 53;Spiral coil, 54;Connecting terminal, 61;Shell, 62;Refractory wall, 63;Injection hole, 64;Casting hole, 65;Heater,

Claims

1. 1. A heating device for heating molten steel in a tundish in continuous casting of steel, comprising: a disk-shaped resistance heating element made of graphite and having a spiral coil structure, positioned directly above and facing the surface of the molten steel; an alternating current is passed through the heating element to heat it to a temperature of 1600°C or higher but lower than 2100°C by Joule heat, thereby radiating heat to slag suspended on the surface of the molten steel, and inductively heating and inductively stirring the area directly below the surface of the molten steel; and an alumina or magnesia mortar coating on the upper surface of the heating element to suppress upward radiation.

2. 2. The heating device for molten steel in a tundish according to claim 1, wherein two resistance heating elements are connected in series or three resistance heating elements are arranged in a triangular configuration to form a three-phase star connection.

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