JP5578111B2 - Induction heating temperature raising method for molten metal - Google Patents

Description

The present invention relates to an induction heating temperature raising method for molten metal using a channel type induction heating device installed in an induction furnace.

Molten metal discharged from the blast furnace is supplied to a converter (smelting furnace) through a topped car (mixing car), a hot metal ladle, or the like. At that time, the molten metal supply conditions to the converter vary depending on the amount of molten metal discharged from the blast furnace (hereinafter sometimes referred to as “molten metal”) and the amount of molten steel produced in the converter. The molten metal is temporarily stored in the induction furnace. This induction furnace is provided with an induction heating device that performs heat retention and heating of the stored molten metal.

A loop-shaped molten metal flow path is formed in the induction heating device. An iron core around which an induction heating coil is wound is disposed at the center of a region surrounded by a loop-shaped molten metal flow path (hereinafter also referred to as “runner”). The molten metal is heated by configuring the transformer circuit with the induction heating coil as the primary circuit and the molten metal flowing through the runner as the secondary circuit.

In a channel type (groove type) induction heating device in which a channel type (groove type) runway is formed, in order to increase power efficiency, the thickness of the refractory facing the runway is made the minimum necessary thickness, and the induction heating coil And the runway as close as possible. As a result, when molten metal leaks due to melting or cracking of the refractory facing the runner, the outer case or induction heating coil covering the induction heating device is damaged, and cracks grow and are attached to the induction heating device. When reaching the water-cooled pipe, there is a risk that the cooling water and the molten metal react to induce a steam explosion.
Therefore, a leak detector antenna is embedded in the refractory facing the runway, a voltage is applied between the leak detector antenna and the molten metal, and the molten metal and the leak detector antenna are connected via the refractory. By monitoring the current flowing between them, detection of hot water leakage (leakage of molten metal) is performed (see, for example, Patent Documents 1 to 3).

In the case of an electric circuit of a hot water leak detection system, since electric resistance is large in normal times, almost no current flows. However, when a hot water leak occurs, the electric current is caused by the short circuit between the hot water leak detection antenna and the runway and the circuit closing. Flows and a leak is detected. For this reason, for example, when a lead abnormality extending from the hot water leak detection antenna breaks and cuts and a facility abnormality occurs in the hot water leak detection system, there is a high risk of not being aware of this. Therefore, in Patent Document 4, by drawing out two or more lead wires for one molten metal leak detection antenna, current is conducted between these lead wires to determine whether or not the lead wires are disconnected. Yes.

Japanese Utility Model Publication No. 53-48231 JP-A-9-133601 JP 2008-267758 A JP 2008-170062 A

As described above, the molten metal detection system flows between the molten metal and the molten metal detection antenna via the refractory by applying a voltage between the molten metal detection antenna embedded in the refractory and the molten metal. The current is monitored, and a leak is determined by the behavior of the current value. That is, when the electric resistance value between the molten metal in the induction furnace and the molten metal leak detection antenna falls below a certain reference value, it is determined that the molten metal leak has occurred.
However, the cause of the decrease in the electrical resistance value is not only the leakage of hot water, but the electrical resistance value may be decreased due to other factors, and it may be mistakenly recognized that a leakage of molten metal has occurred (water leakage error detection). Specifically, the applied electric power of the channel type induction heating device is in a low output state (heat insulation operation), that is, only the minimum electric power for maintaining the molten metal without melting the cold iron source such as scraps and molds. When a transition is made from the applied state to the high output state (heating operation), that is, the state where the maximum cold iron source is melted using the rated output of the induction heating device, erroneous detection of hot water leakage will occur temporarily. There is.

The present invention has been made in view of such circumstances, and in a channel-type induction heating apparatus, molten metal that can prevent erroneous detection of molten metal leakage that temporarily occurs when shifting from a heat retaining operation to a heating operation. It aims at providing the induction heating temperature rising method.

The inventors of the present invention have discovered that the erroneous detection of hot water leakage that temporarily occurs when shifting from the heat retaining operation to the heating operation is caused by moisture present in the induction heating apparatus.
In the induction heating apparatus, in order to increase power efficiency, a refractory having a minimum thickness is arranged facing the runner, and the induction heating coil and the runner are as close as possible. For this reason, as a refractory constituting the runner, a powdery amorphous refractory called a dry stamp material (also referred to as a dry ramming material) to which water is not added is often applied. When the dry stamp material is applied, the vicinity of the running surface of the runner exposed to high temperature is baked and solidified to become a sintered body, and the runner wall surface is formed. On the other hand, the back surface is maintained in a dry and unsintered powder compact. As a result, even if a crack occurs in the sintered body and the molten metal enters the crack (start of leakage), the powder compact on the back surface is sintered by the heat of the molten metal, and further leakage of molten metal is caused. Be blocked. As a result, hot water leakage that reaches the outer case is avoided.
However, even when a dry stamp material is applied, the refractory constituting the induction heating device contains a small amount of free water or crystal water. Moreover, the moisture contained in the atmosphere at the time of construction of the induction heating device is confined in the induction heating device. The present inventors evaporate the water at a high temperature portion near the runner and form a water-cooled outer case or bushing (a partition wall provided between the refractory forming the runner and the induction heating coil. It was discovered that the electrical resistance between the outer case and the leak detector antenna decreases when condensation occurs in the vicinity of the leak detector antenna in many cases.

Since the channel type induction heating device has a large output (applied power), the outer case and the bushing are often water-cooled to protect the outer case and the bushing from induction heat generation. On the other hand, the hot water leak detection antenna is often embedded in the vicinity of the back surface of the refractory, that is, in the vicinity of the exterior case and the bushing. Therefore, even when the channel type induction heating apparatus is in operation, a relatively low temperature is maintained in the vicinity of the hot water leak detection antenna. Therefore, water vapor desorbed from the vicinity of the operating surface of the refractory that constitutes the runway of the channel type induction heating device is condensed near the leak detector antenna, and the electric resistance value between the outer case and the leak detector antenna is determined. Reduce.

In general, there is a bare metal that adheres so as to bridge from the refractory operating surface of the molten metal outlet to the furnace shell, and there is electrical continuity between the outer case and the molten metal in the induction furnace by the bare metal There are many cases to do. For this reason, a decrease in electrical resistance value between the outer case and the molten metal detection antenna cannot be distinguished from a decrease in electrical resistance value between the molten metal and the molten metal detection antenna, and erroneous detection of molten metal leakage occurs.
In particular, in the transition period in which the induction furnace shifts from the standby state (heat retention operation state) to the scrap melting state (heating operation state), the applied power of the channel type induction heating device shifts from the low output state to the high output state, The temperature in the vicinity of the working surface of the refractory constituting the runner of the channel type induction heating device is greatly increased. Along with this, water vapor generated from a high-temperature portion near the runner is condensed in the vicinity of the hot water leak detection antenna, thereby causing false detection of hot water leak. However, when the channel type induction heating device is in a high output state, the temperature of the outer case also rises due to heat generated by the applied power, etc., so that condensation does not occur after about 1 to 6 hours, and erroneous detection of hot water leakage is eliminated.

In short, the cause of hot water leak detection that occurs temporarily when shifting from heat insulation operation to heating operation is that the channel temperature rises as the channel type induction heating device shifts from the low output state to the high output state. Accordingly, moisture evaporated from the vicinity of the running surface of the runner is condensed in the vicinity of the exterior case where the temperature has not risen yet. Further, when the high output state continues, the temperature of the exterior case also gradually increases, so that moisture condensed in the vicinity of the exterior case evaporates again, thereby eliminating erroneous detection of hot water leakage.

In order to prevent erroneous detection of hot water leakage due to the above mechanism, the present invention improves the induction heating temperature rising method of the molten metal using the channel type induction heating device, thereby temporarily Prevent false detection of hot water leaks.
That is, the present invention provides a refractory that forms a loop-shaped flow path through which molten metal flows , an exterior case that covers the refractory, a bushing that is lined on the inner peripheral surface of the refractory, the exterior case, A channel-type induction heating apparatus comprising: a water-cooled tube attached to the bushing; an induction heating coil disposed in the center of a cavity defined by the bushing; and a hot water leak detection antenna embedded in the refractory. An induction heating temperature rising method for molten metal using
For each induction heating device, after continuing an output of 20% or less of the maximum output of the induction heating device for 8 hours or more, an operation of increasing the output to 70 to 100% of the maximum output of the induction heating device. Further, the present invention is characterized in that the average value of the increase rate of the output in an arbitrary 20 minutes is 0.1 MW / min or less.

In the present invention, when shifting from the heat retaining operation to the heating operation, the rate of increase in the output (applied power) of the induction heating device is moderated so that the temperature near the outer case becomes higher than the dew point. Condensation in the vicinity of

Here, “continuing an output of 20% or less of the maximum output of the induction heating apparatus for 8 hours or more” means a minimum output operation (heat retention operation). In the present invention, since it is premised on the transition from the heat insulation operation to the heating operation, it is clarified what kind of operation is performed as the heat insulation operation.
On the other hand, “increasing the output to 70 to 100% of the maximum output” refers to shifting the induction heating device from the heat retaining operation to the heating operation. The reason why the maximum output is set to 70% or more is that in the actual operation, the heating operation is performed at 70% or more of the maximum output.

In addition, the reason that “the average value of the increase rate of the output in any 20 minutes is 0.1 MW / min or less” is that when the output of the induction heating device is increased, it is usually stepped. In order to increase the output (for example, increase 1 MW every 5 minutes), the average value is used.
Note that when the output is increased at a rate exceeding 0.1 MW / min, the electrical resistance value between the hot water leak detection antenna and the outer case is lowered, and hot water leak detection may occur. On the other hand, when the output was increased at a rate of 0.1 MW / min or less, no erroneous detection of hot water leak occurred. The lower limit of the output increase speed is theoretically good if it exceeds zero, but if it is too low, the operation efficiency is lowered.

In the induction heating temperature rising method of the molten metal according to the present invention, when shifting from the heat retention operation to the heating operation, the output of 20% or less of the maximum output is continued for 8 hours or more per induction heating device, Since the operation to increase the output to 70 to 100% of the maximum output is carried out so that the average value of the output increase rate for any 20 minutes is 0.1 MW / min or less, the occurrence of condensation near the exterior case Is suppressed, and erroneous detection of hot water leakage can be prevented.

It is a schematic diagram of an induction furnace. (A) is a cross-sectional plan view of a channel type induction heating apparatus to which the molten metal induction heating method according to an embodiment of the present invention is applied, and (B) is a cross-sectional view taken along the line AA. It is the graph which showed the temperature distribution of the refractory material temperature and dew point in the exterior case vicinity of an induction heating apparatus for every operating state.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.

As shown in FIG. 1, the induction furnace 10 for storing molten metal (molten metal) has a cylindrical shape with both end faces sealed, and is installed on a support base 26 so that the center axis of the furnace body is horizontal. ing. The induction furnace 10 is covered with an iron skin, and a refractory layer is formed inside the iron skin.
A plurality of channel-type induction heating devices 11 (hereinafter simply referred to as “induction heating devices”) are provided at lower portions on both sides of the induction furnace 10 at intervals in the central axis direction of the induction furnace 10 (this is simply referred to as “induction heating device” hereinafter). In the embodiment, three on one side, a total of six are installed.) In addition, a scrap charging port 12 for charging scrap is provided on one side surface of the induction furnace 10, and a receiving port (not shown) for charging molten metal and a molten metal are discharged on the other side surface. Is provided with an outlet (not shown).

Each induction heating device 11 has a flat prismatic shape (see FIG. 2B), and is installed so as to protrude outward from the side surface of the induction furnace 10. The induction heating device 11 includes a flow path 13 through which the molten metal flows, refractories 18 and 19 disposed facing the flow path 13, an iron outer case 17 that covers the induction heating apparatus 11, and heating the molten metal. An induction heating means (see FIG. 2A), and heat retention and heating of the molten metal stored in the induction furnace 10 is performed.

The channel 13 has a substantially “mountain” shape in plan view, and is formed on both sides of the central channel 14 formed along the central axis of the induction heating device 11 and the central channel 14. A pair of side flow paths 15 and an end flow path 16 formed at the distal end portion of the induction heating device 11 that communicate with the central flow path 14 and the pair of side flow paths 15 are configured. The molten metal stored in the induction furnace 10 flows into the induction heating device 11 from the central flow path 14, branches left and right in the end flow path 16, and then flows from the pair of side flow paths 15 toward the induction furnace 10. It is said to be a looped runner that flows out.

The refractory 19 disposed between the central flow path 14 and the side flow path 15 has a “B” shape in plan view, and a copper bushing 20 is lined on the inner peripheral surface of the refractory 19. (See FIG. 2A). In addition, an iron core 23 around which an induction heating coil 22 that is induction heating means for heating the molten metal is wound is disposed in the center of the hollow portion 21 defined by the bushing 20. The iron cores 23 arranged in the respective hollow portions 21 are connected by a horizontal member 24 to constitute a “B” -shaped yoke as viewed from above (see FIG. 2B).
By passing an alternating current through the induction heating coil 22, magnetic flux is generated in the iron core 23. When this magnetic flux is linked to a molten metal circuit made of molten metal flowing in the flow path 13, an induced current is generated in the molten metal circuit, and the molten metal is heated by Joule heat generated by the induced current. Generally, the induced current generated in the one-turn molten metal circuit is a large current.

In order to detect molten metal leakage (melted metal leakage) due to melting or cracking of the refractories 18, 19, the refractory 18 in contact with the outer case 17 is placed along the outer case 17 and the refractory 19 in contact with the bushing 20. A hot water leak detection antenna 25 is embedded along each bushing 20. The distance between the flow path 13 and the outer case 17 and the bushing 20, that is, the thickness of the refractories 18 and 19 is about 100 mm to 400 mm, respectively.
The exterior case 17 and the bushing 20 are provided with water cooling tubes (not shown) for cooling each. Since the water cooling facility attached to the induction heating device 11 is designed to exhibit a cooling capacity corresponding to the rated output of the induction heating device 11, it is overcooled in a low output state.

Next, an induction heating temperature raising method for molten metal according to an embodiment of the present invention will be described.
The refractories 18 and 19 are made of a powdered and irregular refractory material called dry stamp material or dry ramming material, and the refractories 18 and 19 contain a small amount of free water or crystal water. Further, since the stamp material is driven in by a rammer, the moisture contained in the atmosphere is confined in the induction heating device 11 as the induction heating device 11 is installed. Since the moisture contained in the atmosphere varies depending on the season of construction, the amount of moisture trapped in the induction heating device 11 also varies depending on the season of construction. It may be considered as the amount of water contained in the atmosphere at 35 ° C. and humidity 40-70%.

As described above, moisture is always present in the induction heating device 11, and this inevitably present moisture induces erroneous detection of hot water leakage. In other words, when shifting from the heat retention operation to the heating operation, moisture evaporated from the vicinity of the flow path 13 is condensed in the vicinity of the outer case 17 where the temperature has not risen yet. As a result, the electrical resistance value between the outer case 17 and the hot water leak detection antenna 25 decreases, and erroneous hot water leak detection occurs.

Therefore, in the induction heating temperature rising method for molten metal according to the present embodiment, for each induction heating device 11, an output of 20% or less of the maximum output of induction heating device 11 is continued for 8 hours or more, and then induction heating is performed. The operation of increasing the output to 70 to 100% of the maximum output of the apparatus 11 is performed so that the average value of the output increase rate for any 20 minutes is 0.1 MW / min or less.

FIG. 3 is a graph showing the refractory temperature and dew point in the vicinity of the outer case 17 of the induction heating device 11 with the position in the thickness direction of the refractory as a horizontal axis. The distribution of the refractory temperature and the dew point varies depending on the operating state. Indicates. In the figure, the line A represents the refractory temperature during the heat insulation operation, and the line B represents the refractory temperature during the heating operation. The lines C1 and C2 are refractory temperatures in the transition period from the heat insulation operation to the heating operation, and when C1 is an output increase rate exceeding 0.1 MW / min, C2 is an output increase rate. Represents 0.1 MW / min or less.
On the other hand, the A0 line represents the dew point during the heat retention operation, and B0 represents the dew point during the heating operation. Further, C0 represents a dew point in a transition period in which the heat retaining operation is shifted to the heating operation.

In the simulation, the thickness of the refractory was 100 to 400 mm, the molten metal temperature was 1300 to 1500 ° C., and the range actually used for operation was used. The amount of heat dissipated in the furnace body was constant regardless of the temperature.

The following can be seen from the graph of FIG.
(A) Since the outer case 17 is cooled, the temperature of the refractory becomes lower as it gets closer to the outer case 17, and the temperature of the outer case 17 (refractory thickness 0 mm) during the heat insulation operation is 30 ° C., heating operation The temperature of the outer case 17 at that time is 40 ° C. When shifting from the heat retaining operation to the heating operation, the temperature of the molten metal in the flow path 13 that is induction-heated increases, and the temperature of the outer case 17 also starts increasing with a time lag. When the increase rate of the output is 0.1 MW / min or less, the molten metal temperature in the flow path 13 gradually increases, so that the refractory temperature from the inner surface of the outer case 17 to about 14 mm gradually increases as a whole. On the other hand, when the rate of increase in output exceeds 0.1 MW / min, the molten metal temperature in the flow path 13 rises quickly, so the refractory temperature from the inner surface of the outer case 17 to about 5 mm does not vary substantially, and the outer case The refractory temperature in the region of 17 to 5 to 14 mm increases.
(B) In the region where the refractory temperature is 100 ° C. or higher, the adsorbed water in the refractory evaporates and the amount of water decreases. Conversely, the region where the refractory temperature is less than 100 ° C. has a large amount of moisture (high dew point). However, since the refractory has air permeability, it is considered that there is no large deviation in the moisture content.
(C) At the time of heating operation, since the area below 100 ° C. is reduced as compared with that at the time of heat insulation operation, the amount of water (dew point) in the area below 100 ° C. increases. In other words, the dew point increases in the vicinity of the outer case 17 with the transition from the heat retaining operation to the heating operation.

(D) If the refractory temperature is lower than the dew point, there is a high possibility of condensation. When dew condensation occurs, the electrical resistance value between the hot water leak detection antenna 25 and the outer case 17 is lowered, causing a hot water leak detection error. During the heat insulation operation, A> A0, and during the heating operation, B> B0, and since both the refractory temperature is higher than the dew point, no condensation occurs.
(E) In the case of C1 where the transition from heat insulation operation to heating operation (output increase) is steep (output increase rate exceeds 0.1 MW / min), the relationship of refractory temperature <dew point (C1 <C0) Is long (approximately 1 to 6 hours), and condensation is likely to occur. On the other hand, when the transition from the heat retention operation to the heating operation (output increase) is slow C2 (output increase rate is 0.1 MW / min or less), the refractory temperature <dew point (C2 <C0) Since the time for which the relationship is satisfied is very short as compared with C1, or there is no region where the relationship is satisfied, condensation does not occur.

From the above, it can be seen that the case of C2 where the transition from the heat retaining operation to the heating operation is gentle (the output increasing rate is 0.1 MW / min or less) is desirable.

Table 1 shows that when the output of 20% of the maximum output of the induction heating device 11 is continued for 8 hours or more and the output is increased to 70% of the maximum output of the induction heating device 11, the output increase speed and the hot water leakage error This shows the correlation with the number of detections. Here, the number of times of decrease in the electric resistance value is regarded as the number of occurrences of erroneous detection of hot water leak, and the number of times of erroneous detection of hot water leak when 100 times are performed for each rising speed is shown.

From the table, it can be seen that when the output increase rate is reduced, the number of times of erroneous detection of hot water leaks decreases, and especially when the output increase rate is 0.10 MW / min, no erroneous detection of hot water leaks has occurred. .
The same result as described above was obtained when the output of the induction heating device 11 was increased to more than 70% of the maximum output. On the other hand, when the output of the induction heating device 11 was increased to the 60% level of the maximum output, no hot water leak detection occurred regardless of the increase rate of the output.

Although one embodiment of the present invention has been described above, the present invention is not limited to the configuration described in the above-described embodiment, and is within the scope of matters described in the claims. Other possible embodiments and modifications are also included.

10: induction furnace, 11: channel type induction heating device (induction heating device), 12: scrap charging inlet, 13: flow channel, 14: central flow channel, 15: side flow channel, 16: end flow channel, 17 : Outer case, 18, 19: Refractory material, 20: Bushing, 21: Cavity, 22: Induction heating coil, 23: Iron core, 24: Horizontal member, 25: Hot water leak detection antenna, 26: Support base

Claims (1)

A refractory that forms a loop-shaped flow path through which molten metal flows , an outer case that covers the refractory, a bushing that is lined on the inner peripheral surface of the refractory, and an attachment to the outer case and the bushing Molten metal using a channel-type induction heating apparatus comprising a water-cooled tube , an induction heating coil disposed in the center of the cavity defined by the bushing, and a hot water leak detection antenna embedded in the refractory An induction heating temperature raising method,
For each induction heating device, after continuing an output of 20% or less of the maximum output of the induction heating device for 8 hours or more, an operation of increasing the output to 70 to 100% of the maximum output of the induction heating device. The molten metal induction heating temperature-raising method is carried out so that an average value of the rate of increase of the output over an arbitrary 20 minutes is 0.1 MW / min or less.

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