Induction heating apparatus

Abstract

To improve the heating efficiency of an induction heating apparatus for long metal objects traveling in a straight line.SOLUTION: A device that inductively heats a material moving in a straight line to approximately 1000°C by surrounding it. The structure of a solenoid-type induction coil has been changed from a conventional copper tube to a CC composite tube or graphite tube with a spiral groove to form a coil, eliminating water cooling. Radiation heating from a resistance heating coil is added to the induction heating of the material to be heated, improving heating capacity and heating efficiency. The coil's conductor cross-sectional area and coil pitch are gradually increased from the inlet side to the outlet side, distributing heating capacity predominantly to the upstream side and expanding the effective length of the coil. Radiation heating becomes effective by increasing the temperature of the coil (2000°C) and increasing the heating time (more than double). Abnormal temperature rise (2600°C) on the coil outlet side is avoided, and the refractory is durable.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an induction heating device suitable for saving energy in continuous induction heating of a long metal object moving in a straight line. [Background technology]

[0002] Induction heating is often used to heat metals, especially steel, due to its excellent metallurgical characteristics and convenience. The greatest feature of induction heating is its high energy density and the rapid heating that results from it. Rapid heating can provide metallurgical effects such as hardening the steel's surface, strengthening it by refining the crystal grains, and non-decarburizing treatment. In addition, by matching the shape of the non-heated material with the induction coil, partial heating and localized heat treatment can be easily performed, which is a special application. On the other hand, one problem is that expensive electricity is used, but the heating efficiency (= required heat amount J / power consumption J) is not very good. Heating efficiency has not been a particular issue for high-end goods, small items, or small-lot, high-mix production, but energy conservation is an urgent issue regardless of the size of the product or equipment.

[0003] The theoretical factors related to heating efficiency have been largely clarified. Therefore, when designing the device, factors such as coil shape, frequency, magnetomotive force (ampere turns), distance between the coil and the heated material, and coil protection are optimized for the heated object. When thick steel material is heated below its Curie point, a heating efficiency of nearly 80% can be achieved, but when thin steel wire is heated to approximately 1000°C, various factors, particularly the reduction in the coil diameter ratio (= material diameter / coil diameter), result in low induction efficiency, and the heating efficiency is often less than 50%.

[0004] Induction heating is an application of the skin effect of alternating current. The skin effect is manifested not only in the material to be heated but also in the primary conductor. Increasing the skin effect to improve induction efficiency increases the skin effect on the primary side, resulting in increased heat generation from the coil. There is some energy loss on the material to be heated side, but the loss is greater in the entire circuit, including the induction coil and power supply.

[0005] As shown in the example in Patent Document 1, induction coils are usually made of copper tubing, formed into a coil shape, and have cooling water channels built in. It is not the water cooling of the coil that is inconvenient, but the heat generation that is inconvenient. If even a portion of the heat generation can be utilized, it will contribute to energy conservation.

[0006] The general problems with induction heating have been presented above, but we will now consider the problems that arise when induction heating long steel materials (steel bars, wire rods, bar strips, flat steel, plate materials, etc.) continuously in a straight line. In the case of a flame furnace or radiant electric heating furnace, the heat transfer from the heat source to the heated object is not great, so the furnace length is long and the furnace wall temperature drops at the inlet due to the intrusion of coolant. The furnace wall and steel materials reach their highest temperatures at the outlet. Although the heating capacity at the inlet is strengthened, this tendency is not eliminated. Furthermore, the expansion of the furnace length and furnace body naturally leads to increased heat radiation loss, resulting in a decrease in heating efficiency. In contrast, induction heating has extremely high heating capacity and energy density, and because it is direct heating, the furnace length (coil length) is significantly shortened, the heating capacity is almost uniform in each part of the coil, and heat loss is small. However, there is nothing that can be done about the heat loss of the coil (a few tens of percent of the input power, which is wasted by water cooling).

[0007] Patent Document 2 discloses an induction heating coil that recovers the heat loss of the coil. According to this, the coil material is made of graphite, eliminating the conventional water-cooled copper tube, and in parallel with the induction heating, radiant heating is added to the non-heated material from the coil that is red-hot by Joule heat, thereby improving the heating efficiency. To prevent the graphite from being worn down by air oxidation, alumina powder is sprayed onto the material.

[0008] We consider the problems with this method. First, whether it is a batch type where the objects to be heated are heated one by one, or a continuous type where a long object is heated by passing through it, the heating speed of induction heating is usually 10 to several tens of times faster than that of flame or radiation heating. Otherwise, induction heating is meaningless. A high heating speed means that the heating time is extremely short. Even if radiation heating is added, it is only for a short time, so it follows that its contribution to heating intensification is small. In order to make the above hybrid heating successful, the problem of insufficient heating time must be solved. One possible solution is to radically strengthen radiation through ultra-high temperature radiation, but thorough quantitative consideration is essential, and unfortunately this is not stated or implied.

[0009] Secondly, a large current is usually applied to the induction coil in accordance with the heating capacity, and the coil generates a large amount of heat. Furthermore, the heat transfer surface area of ​​the heated object facing the inner surface of the coil is small, so the amount of radiation is not large. Due to these two effects, the coil easily heats up. The temperature increase promotes radiative heat transfer, but it also increases the amount of radiative heat dissipated to the outside, inducing a decrease in efficiency. If an attempt is made to reduce heat loss to the outside by installing a fireproof insulation around the heating coil, problems with the fireproof material will occur due to abnormally high temperatures. In particular, in the case of continuous heating, the coil is cooled at the inlet by the intrusion of cold material, but the outlet is prone to abnormal temperature rise. Assuming an ultra-high temperature coil of 2600°C, it would be expected to increase the capacity equivalent to induction heating, but the problem of refractory durability would become unsolvable. The hybrid heating of induction and radiation as a measure to reduce power loss is a good idea, but in order to put it into practical use, a trade-off must be resolved between the ultra-high temperature radiation required to make the radiation effective and the durability of the refractory material. The aforementioned problem of the essential shortage of heating time caused by the output ratio of the two heat transfers being more than 10 times (the input ratio when viewed from the heated side) may be resolved to some extent by ultra-high temperature radiation, but this problem must also be solved. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Published Patent Publication No. 2022-082731 [Patent Document 2] Patent Publication 1966-7142 Summary of the Invention [Problem to be solved by the invention]

[0011] In continuous induction heating of metal materials traveling in a straight line, a significant portion of the input power is consumed to generate heat in the induction heating coil, and is wasted through water cooling. The present invention aims to improve the heating efficiency (= amount of heat stored in the heated material / amount of heat consumed), which is at most 60%, and as a means of improvement, the induction coil is formed from graphite to provide the function of a resistance heating element, and hybrid heating of induction and radiation is simultaneously applied.

[0012] Hybrid heating seems easy to implement, but the heating capacity of ordinary radiation heating (approximately 1000°C) is less than one-tenth that of induction heating, meaning that it does not provide any auxiliary heating capacity. Considering both measures of ultra-high temperature radiation heating and extending the heating time, it seems that there is a certain degree of effect, but as a means of suppressing heat loss to the outside, it is essential to install fireproof insulation. In that case, the durability of the fireproof material becomes an issue due to the ultra-high temperature. Especially in continuous heating, the temperature rise at the outlet of the induction coil is inevitably greater than that at the inlet due to the intrusion of cold material, and it easily reaches abnormally high temperatures in response to the added current. The issue to be resolved is the trade-off between the enhancement of radiant heating capacity by increasing the coil temperature and the durability of the refractory material. [Means for solving the problem]

[0013] The first invention is an induction heating device which inductively heats a moving bar, wire, strip, flat or plate-shaped steel material by passing it through a solenoid coil, in which the conductor material of the solenoid coil is changed from a water-cooled copper tube to graphite or CC composite, thereby simultaneously providing the function of a resistance heating element, and the coil length is extended beyond that of a copper tube, and the shape of the solenoid coil is such that the conductor cross-sectional area and coil pitch increase gradually from the inlet side to the outlet side, making the distribution of resistance heating capacity maximum at the inlet and minimum at the outlet.

[0014] The second invention is an induction heating device according to the first invention, characterized in that the structure of the furnace body housing the solenoid coil comprises an atmosphere control device that makes the inside of the furnace non-oxidizing, and a multi-layered fireproof insulation wall made of two or more of the following refractories: graphite, alumina, alumina fiber, and magnesia.

[0015] The third invention is an induction heating device described in the first or second invention, characterized in that when the target heating temperature of the material to be heated is 1000±200°C, the temperature of the solenoid coil is set to 1600°C or higher and 2500°C or lower. Effect of the Invention

[0016] In conventional induction coils made of water-cooled copper tubes, 30-60% of the input power is consumed by induction heating of the coil, and is wasted as water to cool the coil. In the present invention, there is no waste heat, and the coil temperature rises due to the installation of a fireproof insulation around the coil, and the coil exhibits not only induction heating but also radiation heating capabilities. When the coil temperature is close to the target heating temperature of about 1000°C, the radiation is too small compared to induction and no noticeable effect is obtained, but when it approaches 2000°C, the radiation increases in proportion to the fourth power of the absolute temperature, and auxiliary heating capacity is realized. The total heating capacity far exceeds the capacity of the original induction heating. At the same time, energy savings are achieved.

[0017] When induction heating is applied to a material moving in a straight line and radiation heating is also applied, the temperature of the heating element in an evenly heated coil rises in response to the difference between the heat generation capacity of the heating element itself and the heat absorption by the material being heated, and balances out at a certain temperature. The latter heat absorption is large at the inlet because it is a cold material, but small at the outlet because it is rising in temperature, resulting in a gradual temperature rise overall. If the set temperature is set to about 2000°C, where radiation heating capacity is at its full potential, the temperature rises excessively at the outlet, making it difficult for the refractory to last.

[0018] In the radiant heating by the heating element (graphite coil) of the present invention, the cross-sectional area of ​​the conductor increases gradually toward the outlet, and therefore the resistance decreases gradually, so that the heat generation is strengthened at the inlet side and suppressed at the outlet side, and abnormal temperature rise of the coil is eliminated and uniform heating is obtained. This makes it easier to solve refractory problems. This is a heating device convenient for radiant continuous heating of straight steel materials.

[0019] The electrical resistivity of graphite is about 400 times that of copper. The dimensions of the coil are inevitably large in the radial and axial directions. The coil length is expanded. This increases the heat transfer area and time from the coil to the heated material, alleviating the time shortage problem of radiation heating. Graphite conductors are subject to oxidation and wear in air at temperatures above 450°C, which can be costly, but this problem can be easily solved by controlling the atmosphere to be non-oxidizing.

[0020] In the induction heating of the present invention, the magnetomotive force (magnetic flux density), i.e., the heating capacity, is large at the inlet and small at the outlet due to the coil pitch. The heat generated by the coil itself due to induction is also large at the inlet and small at the outlet. This acts to uniformly heat the coil, similar to the resistance heating mentioned above, and contributes to efficient heating. The gradual increase in the coil pitch has another effect. The effective length of the coil increases, which, as mentioned above, alleviates the problem of insufficient heating time for radiation heating, and allows the radiation heating effect to be fully utilized, resulting in practical energy-saving hybrid heating. [Brief description of the drawings]

[0021] [Figure 1] 1 is a schematic diagram of a continuous induction heating device for a straight-traveling rod / wire / strip / flat / plate-shaped metal according to the present invention. [Diagram 2] FIG. 1 is a schematic diagram of a coil portion of a continuous induction heating device. [Diagram 3] The figure shows a qualitative comparison of the changes over time in the coil temperature and the temperature of the material to be heated in three types of heating devices: a conventional water-cooled uniform coil, a uniform heating coil, and the gradient heating coil of the present invention. [Figure 4] FIG. 1 is a diagram showing the relationship between temperature and heat flux in radiation from a cylindrical graphite heating element to a columnar steel material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The device for continuous induction heating of a straight-traveling bar, wire, strip, flat or plate shaped metal of the present invention will be described with reference to FIG. A steel wire 1 to be heated runs through a solenoid-type induction coil 2. The induction coil 2 is made of graphite or CC composite and is shaped by cutting a spiral groove of a fixed width into a tube. Plate-shaped electrodes 3 and 4 are fitted into both ends of the induction coil 2. Electricity is applied to the electrodes 3 and 4 from a high-frequency power source 5 via bus bars 6 and 7. The electrodes 3 and 4 are also made of graphite or CC composite.

[0023] The electrical resistivity of graphite is 400 to 600 times that of copper. It is essential to increase the cross-sectional area of ​​the conductor. The dimensions of the coil will inevitably become larger in the radial and axial directions. Increasing the coil length increases the heat transfer area and time from the coil to the heated material, alleviating the problem of insufficient time for radiative heating.

[0024] A fireproof heat-insulating pipe 8 is provided around the induction coil 2, surrounding it at a distance. The material of the fireproof heat-insulating pipe 8 is preferably a CC composite or graphite fiber-coated graphite. The former has thermal insulation properties with a thermal conductivity of about 1 / 15 that of graphite, while the latter has high thermal conductivity of graphite, but the thermal conductivity in the direction perpendicular to the axis of the graphite fiber is about 1 / 100 that of graphite, and shows the best performance for fireproof insulation. Even if the temperature of the induction coil 2 reaches 2600°C, no problem occurs in terms of fire resistance. The arrangement of the fireproof and thermally insulated pipe 8 reduces the thermal load on the ceramic refractory material constituting the back surface. In other words, even if the temperature of the heating element fluctuates undesirably, the fireproof and thermally insulated pipe functions as a buffer for the refractory material on the back surface.

[0025] The refractory insulation pipe 8 is surrounded by a refractory insulation body 12 (usually referred to as the furnace body) made of multiple layers of refractory material 10 and insulation material 11 laminated inside the steel shell 9. On the high temperature side of the refractory insulation body 12, a good quality pure alumina refractory material is sufficient to withstand temperatures of 2500°C. Alumina fiber and magnesia refractory materials are also suitable, and silica and various other insulation materials can be used in slightly lower temperature areas. A gas inlet pipe 13 is provided to create a non-oxidizing atmosphere inside the refractory insulation body 12. The gas inlet pipe 13 is also used as a temperature measurement window for a radiation thermometer 14 that measures the temperature of the heating element. A temperature sensor 15 for the steel wire 1 is provided downstream of the induction coil 2. The steel shell 9 is designed to maintain the airtightness of the entire device.

[0026] When electricity is applied, induction heating of the material to be heated occurs in response to the magnetomotive force ampere turns, just like a conventional copper tube coil. At the same time, since induction coil 2 is not water-cooled, it generates heat and its temperature rises, and when it exceeds a certain limit, radiation assists the induction heating. The loss caused by water cooling is utilized.

[0027] The heat generated by the induction coil 2 can reach over 2000℃ in some cases, radiating heat to the steel wire 1. Some of the heat leaks out mainly through the electrodes. The structure of the electrodes is important in minimizing leakage. Graphite has extremely high thermal conductivity compared to oxide-based refractories. Structural ingenuity is required.

[0028] FIG. 2 shows the configuration of the induction coil of the present invention. The inner diameter Di and outer diameter Do of the induction coil 21 are constant. The coil pitch (P) 22 is gradually increased from the entry side to the exit side. Then, the conductor cross-sectional area (S) 23 of the induction coil 21 also increases proportionally from the entry side to the exit side. The resistance distribution of the coil gradually decreases from the entry side to the exit side. The magnetomotive force (ampere turns) is proportional to the coil pitch. When the pitch is expanded toward the downstream, the magnetomotive force is also distributed toward the downstream side with an inclination.

[0029] The reason for giving the coil a gradient cross-sectional area and gradient pitch will be explained below. The first reason is that it makes the heating element uniform, making it possible to induce a higher temperature. When radiating heat to a heated material moving in a straight line, the temperature of the heating element in an even heating element rises in response to the difference between the heat generated by the heating element itself and the heat absorbed by the heated material, and balances at a certain temperature. The latter heat absorption is large at the inlet because it is a cold material, but small at the outlet because it has risen in temperature. As a result, the temperature of the heating element is low at the inlet and becomes high toward the outlet. This causes insufficient heating capacity at the inlet and an excess at the outlet, which leads to abnormally high temperatures (over 2600°C) in the heating element, making the durability of the refractory impossible to resolve. As a result, the design output of the induction heater must be restricted.

[0030] In the case of radiation heating using the heating element (graphite coil) of the present invention, the cross-sectional area of ​​the conductor increases gradually, so the heating capacity is increased at the inlet side and decreased at the outlet side, preventing abnormal temperature rise and improving the heating uniformity of the coil. This eliminates problems with the passage of a specified current value. In the induction heating of the present invention, the magnetomotive force (magnetic flux density), i.e., the heating capacity, is large on the entry side and small on the exit side due to the coil pitch. The heat generated by the coil due to induction is also large on the entry side and small on the exit side. This acts to uniformly heat the coil, just like the resistance heating described above, and contributes to efficient heating.

[0031] The second reason is to reasonably expand the coil length to compensate for the lack of time for radiation heat transfer and to demonstrate the effect of radiation heating. Gradient increase in the coil pitch leads to an increase in the coil length. In order to effectively convert the heat loss of the coil in induction to radiation heating, it is necessary to somewhat reduce the induction heating rate, increase the coil length (heating length) accordingly, and extend the radiation heating time.

[0032] FIG. 3 is a graph qualitatively comparing the changes in coil temperature and temperature of the material to be heated over time in three types of heating devices: a conventional uniform water-cooled coil, a double-length uniform heating coil, and a double-length inclined heating coil of the present invention. Uniformly water-cooled coil A is made of conventional water-cooled copper tubes, and the heating capacity distribution is uniform along the length. The coil temperature is constant at room temperature due to water cooling, and the temperature of the heated material approaches the set temperature linearly. With the graphite uniform coil B, the heat transfer area needs to be increased to add radiant heat transfer, and it is desirable to enlarge the coil length to some extent (by doubling). Since the resistance heat is uniform, the coil temperature rises toward the outlet side, and in some cases may enter an abnormal range. The rate at which the temperature rises in the heated material is halved, but it will eventually approach the set temperature. If the abnormality is avoided, the temperature rise will decrease.

[0033] In the graphite gradient coil C of the present invention, the coil length is extended in the same way as in the coil B. The heat generation of the coil is strengthened at the inlet and suppressed at the outlet, and if designed appropriately, the coil temperature can be maintained at about 2000°C over the entire length despite the intrusion of cold material. The uniform temperature is improved and abnormal temperature rise does not occur. The heating rate (℃ / s) of the material being heated is somewhat suppressed by the induction component and the radiation component close to the induction component, resulting in a heating capacity that is comparable to or exceeds that of a water-cooled copper tube. Power consumption is also reduced.

[0034] The radiation heating capacity will be explained. The heat flux Q that the heated material receives from the heating element is given by: Q(kcal / m2h)=e·k{(T1 / 100) 4 -(T2 / 100) 4} ---(1) e: Emissivity k: Boltzmann coefficient T1: Heating element temperature (K) T2: Heated object temperature (K) Figure 4 shows the above formula in a diagram. It can be seen that the radiant heat flux Q increases rapidly above 1500℃. The practical range was specified as 1600℃ or higher.

[0035] If the coil temperature T1 is 2300K (=2000℃), the average temperature of the heated object T2 is 800K (=500℃), and the emissivity of the heated object e is 0.7, Q≒1,000,000kcal / m2h ---(2) The heating rate R of a heated material with diameter D can be calculated using the following formula: R(℃ / s)=4Q / cρD / 3600 ---(3) c; Specific heat ρ; Density According to trial calculations, the heating rate when heating a steel wire with D=10 mm is approximately 100°C / s. This is the same order of magnitude as the induction heating rate (several hundred°C / s), supporting the practicality of hybrid heating of induction and radiation. If T1 = 2000K (1700℃), the heating rate R becomes approximately 60℃ / s, which is within the practical range. In summary, for hybrid heating to be effective, a heating element with a temperature of at least 1600° C. and a coil length sufficiently longer than that of a copper tube coil are required for heating to 1000° C.

[0036] The appropriate temperature of the heating element will be described. When hot working or heat treatment is performed on steel, it is usually heated to 1000±200℃. From Figure 4 and the above calculations, hybrid heating is effective if the heating element is 1600℃. On the other hand, even high-quality alumina will not last if it exceeds 2600℃. We have specified a temperature between 1600℃ and 2600℃ as the appropriate temperature. EXAMPLES

[0037] The conditions for applying the induction heating device of the present invention to the heat treatment of special steel wire are as follows: The dimensions are significantly thicker than conventional copper coils, but the insulation is excellent. Efficiency: 1000kg / h Wire diameter: 10mm Linear speed; 0.5m / s Heating temperature; 950℃ Power output: 300kVA Current; 1000A Voltage: 200~300V Coil resistance: 0.1Ω Coil inner diameter: 40mm Coil outer diameter: 100mm Coil pitch: 15~45mm Number of coil turns: 50 Furnace dimensions: 600mmφ×2500mm [Explanation of symbols]

[0038] 1;steel wire 2;induction coil 3,4;electrode 5;high frequency power source 6,7;bus bar 8;fireproof insulation pipe 9;iron shell 10;fireproof material 11;insulation material 12;fireproof insulation body 13;gas inlet pipe 14;radiation thermometer 15;temperature sensor 21;induction coil 22;coil pitch 23;coil conductor cross-sectional area 24;outer diameter 25;inner diameter

Claims

1. This is an induction heating device that inductively heats a moving rod, wire, strip, flat or plate-shaped steel material by passing it through a solenoid coil, in which the conductor material of the solenoid coil is changed from a water-cooled copper tube to graphite or CC composite, thereby simultaneously providing the function of a resistance heating element and extending the coil length, and the shape of the solenoid coil is such that the conductor cross-sectional area and coil pitch increase gradually from the inlet side to the outlet side, making the distribution of resistance heating capacity maximum at the inlet and minimum at the outlet.

2. 2. The induction heating device according to claim 1, characterized in that the structure of the furnace body housing the solenoid coil comprises an atmosphere control device that makes the inside of the furnace non-oxidizing, and a multi-layer fireproof insulation wall using two or more of the following refractories: graphite, alumina, alumina fiber, and magnesia.

3. 3. The induction heating device according to claim 1, wherein the temperature of the solenoid coil is set to 1600° C. or higher and 2600° C. or lower when the target heating temperature of the material to be heated is 1000±200° C.

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