JPH02263544A - Heater device for submerging into molten metal - Google Patents

Abstract

PURPOSE:To prevent runaway of heat generation and opening damage in an exothermic body by forming the exothermic body of the lower part thicker than the upper part. CONSTITUTION:A heater device 1 for submerging into molten metal is one submerged into the molten metal in order to heat the molten metal,. Then, the subject heater is constituted of the cylindrical exothermic body 2 having bottom wall part at the lower part made of the exothermic material as base material, and an electrode part 3 set in inner circumferential part of the exother mic body 2 and conducting electricity to the exothermic body 2, and the exother mic body 2 is made thicker the lower part than the upper part thereof. By this method, at the time of using, the runaway of heat generation and the open ing damage in the exothermic body can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a heater device that is immersed in a molten metal and used when heating the molten metal.

[Prior Art] A conventionally used heater device for immersion in molten metal will be explained by taking a continuous casting method as an example.

That is, in the continuous casting method, for example, 1400~
Molten steel at a temperature of about 1,600°C is first applied to a tundish, and the molten metal is poured into a water-cooled mold from the outlet of the tundish, where it is cooled and solidified. After being cooled by a cooling spray zone,
The cooled and solidified portion is stretched with pinch rolls and cut into predetermined lengths to produce slabs, bits, etc. In the continuous casting method described above, the quality of the steel products manufactured is improved compared to the blooming method, and the yield is also improved. However, in recent years, there has been a demand for even higher quality steel products, and continuous casting methods have been actively developed to improve the quality of steel products.

However, in the continuous casting method described above, the temperature of the molten steel tends to drop within the tundish because the molten steel is primarily received by the tundish. Particularly during continuous casting, at the end of casting, for example, 50 to 80 minutes have elapsed since the start of casting, the temperature of the molten metal drops by several to several tens of degrees Celsius, or even more in some cases. Here, since the tundish is the final container just before the molten metal solidifies, the temperature of the molten metal in the tundish has a great effect on the subsurface inclusion index and the central carbon segregation index of the steel product. ! This has a major impact on the high quality of 111 products.

Therefore, even if the molten metal in the tundish is lowered by at most several to several tens of degrees Celsius, this is not preferable in terms of quality control.

Therefore, in recent years, in order to adjust the temperature of the molten steel inside the tundish, a carbon electrode is immersed in the molten metal inside the tundish, and a current is passed directly through the molten metal within the tundish.
BACKGROUND ART A heater device for immersion in a molten metal is provided that generates heat in the molten metal itself using Joule heat generated in the molten metal. However, in this case, the electrical resistivity of the molten metal is low. Here, since the Joule heat generated by the molten metal is the product of the electrical resistance value of the molten metal and the square of the current value flowing through the molten metal, as mentioned above, if the electrical resistivity of the molten metal is small, the required Joule heat is In order to secure heat, there is a problem in that a considerably large amount of current is required to be passed through the molten metal, and furthermore, due to the large current, the size of the electrical equipment used is also increased.

In addition, as a heater device for heating the molten metal in the tundish can, an induction heating type device for inductively heating the molten metal in the tundish can has been conventionally provided. Furthermore, a plasma heating device is also provided in which a plasma torch is installed above the tundish dish and the molten metal in the tundish dish is heated by plasma.

[Problem to be Solved by the Invention] As a result of extensive research in order to reduce the current flowing through the molten metal, the present inventor has discovered that the present inventor has developed a method using a heat-generating material as a base material and a bottom wall portion at the bottom, as shown in FIG. A cylindrical heating element 100 with a bottom 104
and an electrode section 102 that is installed on the inner circumference of the heating element 100 and supplies electricity to the heating element 100. Then, using this heater device, the heating element 100 of the heater device is immersed in molten metal in an almost vertical state, and a voltage is applied between the heater device and the molten metal. We have developed a means to generate heat and heat molten metal.

However, as research progresses, in the heater device described above, runaway heat generation occurs near the bottom wall 104 of the heating element 100 during use, and as shown in FIG. 9, the area near the bottom wall 104 of the heating element 100 melts. It has been discovered that holes 100a tend to occur.

Although the reason why the holes 100a occur near the bottom wall portion 104 of the heating element 100 is not necessarily certain, it is presumed to be as follows. That is, heat-generating materials used in the 1300°C to 1600°C range generally tend to have a lower specific resistance as the temperature rises, and furthermore, the heat of the heating element 100 tends to escape from the electrode section 102 to the outside of the heater device. This is due to the fact that In other words, the heating element 100 is vertically immersed in the molten metal.
When comparing the operating temperatures of the lower part 106 and the upper part 108, as shown by the characteristic line X in FIG. The electrical resistance value of the lower part 106 of the heating element 100 is smaller than that of the upper part 108, and as a result, the current flowing through the lower part 106 of the heating element 100 in the thickness direction thereof increases, and due to this, the lower part 106 in a high temperature state It is presumed that this is because the temperature increases, resulting in runaway heat generation.

The present invention was completed in view of the above-mentioned circumstances, and its purpose is to be able to heat molten metal by generating heat in a heating element, and to prevent runaway heat generation of the heating element and holes in the heating element during use. An object of the present invention is to provide a heater device that is advantageous in suppressing damage.

[Means for Solving the Problems] As a result of further intensive research on the above-described heater device, the inventor of the present invention found that if the thickness of the lower part of the heating element is made thicker than the thickness of the upper part of the heating element, the heater device can be used easily. We have discovered that runaway heat generation at the bottom of the heating element can be suppressed, and based on this knowledge, we have developed the heater device of the present invention.

That is, the heater device for immersion in molten metal according to the present invention is a heater device that is immersed in molten metal to heat the molten metal, and includes a cylindrical heating element made of a heat-generating material and having a bottom wall portion at the bottom. , and an electrode section that is installed on the inner peripheral portion of the heating element and supplies electricity to the heating element, and the heating element is characterized in that the lower part thereof is thicker than the upper part.

Now, the heating element of the present invention uses a heating material as a base material.

Here, using a heat-generating material as a base material means that the base material may be formed only of a heat-generating material or may contain a filler other than the heat-generating material.

The lower part of the heating element is thicker than the upper part.

In this case, the relationship between the lower wall thickness and upper wall thickness of the heating element is as follows:
The thickness of the upper part of the heating element can be, for example, 99 to 20%, particularly 60 to 40%, of the thickness of the lower part of the heating element.
is desirable.

To make the lower part thicker than the upper part, the diameter of the outer circumferential surface of the lower part may be set larger than the diameter of the outer circumferential surface of the upper part, or the diameter of the inner circumferential surface of the lower part may be set to be the diameter of the inner circumferential surface of the upper part. It may be set smaller than . In this case, the variation in wall thickness may be a long taper or an arcuate taper in cross section.

In the heater device according to the present invention, the heat generating material forming the heating element may be either non-metallic or metallic. As for the type of heat generating material or the mixing ratio thereof, one having a high electric resistance value can be used. In addition, in the heater device according to the present invention, the type of heat-generating material or its blending ratio takes into consideration various factors such as melting resistance, thermal shock resistance, oxidation resistance, corrosion resistance, aging resistance, etc. in addition to heat generation properties. You need to make a selection.

Further, as the above-mentioned non-metallic heating material forming the heating element, ceramics having conductivity in the operating temperature range among oxide-based, nitride-based, boride-based, etc. can be used. As conductive ceramics, if the object to be heated is molten steel, it is desirable to have a high specific resistance value, because the resistance of the molten steel is low, so the resistance of the heating element needs to be increased. In this case, it is desirable that the specific resistance value be 1 Ωcm or more at around 1500°C, especially 200°C.
It is desirable that the resistance is Ωcm or more, and for example, one having a specific resistance value of about 360 Ωcm can be used. Note that the specific resistance value of the heating element can be changed by blending non-conductive ceramics or poorly conductive ceramics with conductive ceramics and adjusting the blending ratio.

In the heater device according to the present invention, the conductive ceramics forming the heating element include magnesia (MgO) and zironia (Zr) when heating molten steel.
o2), alumina<AQ203), a mixture of magnesia and zirconia, and a mixture of magnesia, zirconia, and alumina can be used. Here, magnesia usually does not have electrical conductivity at around room temperature, but takes on the required electrical conductivity at around 1,500 to 1,650° C., which is the heating temperature range of molten steel. As described above, a mixture of magnesia and zirconia can be used as the conductive ceramic, and in this case, the blending ratio is as follows:
It is selected appropriately considering the required resistance value, etc., but for example, magnesia is 60 to 100% by weight, especially 85 to 100%.
95% is preferred, zirconia 0-40% especially 5-2
5% is preferred, alumina 0-40% especially 2.5-1
5% is preferable, and one or more materials whose main components are alumina, CaO, chromia, beryllia, doria, and serif can also be blended in a content of 30 to 60% or more. In order to reduce the amount of ceramics, carbon powder, graphite, etc. may be added to suitably contain 1 to 5% by weight of carbon. In addition, as conductive ceramics,
For example, zirconium boride, boron nitride, silicon carbide, etc. can be used as necessary.

Furthermore, depending on the melting point of the molten metal, silicon carbide (S) may be used as a conductive ceramic to form the heating element.
i C), lanthanum chromate (LaCrO3), beryllium oxide (Bed), thorium oxide (ThO2)
, molybdenum silicide (MoSi2), titanium nitride (TiN), titanium carbide (TiC), etc. as main components can also be used.

In addition, in the heater device according to the present invention, as the above-mentioned metal-based heat-generating material forming the heating element, for example, nickel-chromium alloy, iron-chromium-aluminum alloy, tungsten, molybdenum, tantalum, etc. are used as necessary. Can be hired.

For reference, the relationship between operating temperature and specific resistance is not shown in FIGS. 6 and 7. In the case of molten steel, as described above, the target value of the specific resistance value of the ceramic forming the heating element is preferably 200 ΩcmJJ or more in the operating temperature range.

When the heat generating material is conductive ceramics, the particle size of the conductive ceramics may affect the resistance value, so it is desirable that the maximum particle size is about 1 to 5 mm.
In particular, about 1.5 to 3 mm is desirable. The main reason for this is that if the particle size is too large, the current tends to become uneven. Note that it is also possible to adjust the heat generation characteristics of the lower part and the upper part by changing the particle size between the lower part and the upper part of the heating element.

In the heater device according to the present invention, the resistance value of the heat generating material forming the heat generating element does not change or increases even if the operating temperature changes, unless there are other problems. One that shows correctness can be used. In this case, when the resistance value of the heat-generating material increases as the temperature rises, when a high-temperature part is generated in the heat-generating element, the resistance value of the high-temperature part becomes high. Therefore, current flows more easily through the lower temperature portion than the high temperature portion, which is convenient for uniformly generating heat throughout the heating element. If the heat-generating material has a large negative property in which the resistance value decreases significantly when the temperature rises, when a high-temperature portion is formed in the heat-generating element, the resistance value of the high-temperature portion will decrease. Therefore, it becomes difficult for current to flow in areas where the temperature is lower than in the high temperature areas.
Current flows more easily to high temperature parts with low resistance. Therefore, the high temperature portion becomes increasingly hot, which tends to cause the heating element to overheat.

By the way, the total resistance R (Ω) of the heating element is influenced by the specific resistance value ρ (0 cm) of the heating material such as conductive ceramics, the wall thickness t (cm) of the heating element, and the area S (Cm2) of the heating element. Ru. At this time, it is necessary to select the resistance value of the heating element in consideration of the following matters. In other words, the larger the outer diameter of the heating element,
Although the heat dissipation area can be secured, cracks may form during molding.
Easily susceptible to thermal shock. On the other hand, the smaller the outer diameter of the heating element, the more effective it is against thermal shock, but the smaller the heat radiation area becomes. Also,
The larger the inner diameter of the heating element, the larger the diameter of the electrode portion, and the greater the heat transfer loss from the electrode portion. On the other hand, the smaller the inner diameter of the heating element, the smaller the diameter of the electrode portion, and the smaller the heat transfer loss from the electrode portion, but the more uneven heat generation tends to occur.

In the heater device according to the present invention, the heating element can be manufactured, for example, as follows. That is, after the raw ceramic powder is thoroughly ground and mixed using a ball mill, a motion mill, etc. to adjust it to a predetermined composition, a slurry made by mixing the raw ceramic powder and water is poured into the cavity of a mold and molded into a predetermined shape. A molding step is carried out to obtain a molded body, and a sintering process is further carried out in which the molded body is heated to a predetermined temperature and sintered. Prior to the sintering process, a curing process and a drying process are performed if necessary.

Note that in the molding process, motion molding can be performed in which molding is performed while applying vibration to the mold. Then, the heating element and the electrode section are assembled into one piece.

In the heater device according to the present invention, the bottom wall portion of the heating element has a three-dimensional curved shape, such as a hemispherical shape or a shape approximating a hemispherical shape, so that there are no corners. is desirable. The reason for this is that corners tend to be uneven during molding, making it difficult to ensure thermal shock resistance. Further, since current tends to concentrate at the corners, the heat generation temperature at the corners becomes high, which is likely to be one of the causes of runaway heat generation. In addition, when the tip part is made into a hemispherical shape, the radius of the hemispherical tip part of the heating element is, for example, 30 to 1QQ mm, especially 40 to
It can be about 5Qmm.

In the heater device according to the present invention, the electrode portion is for passing electricity through the heating element, and is provided on the inner peripheral portion of the heating element. The material of the electrode part is selected in consideration of electrical conductivity, heat transfer coefficient, etc. In this case, the conductivity can be increased and the heat transfer coefficient can be decreased to reduce heat transfer loss. however,
In general, the higher the conductivity of a conductive material, the higher the heat transfer coefficient. Therefore, rather than forming the electrode part with a single material, it is better to use a material with high conductivity and a material with low heat transfer coefficient. By appropriately combining these, it is possible to reduce the apparent degree of heat transfer of the electrode part while ensuring the required conductivity of the electrode part.

Further, the electrode portion can also be formed of conductive ceramics with low electrical resistance. In order to reduce heat transfer loss from the electrode part, it is preferable that the upper part of the electrode part is thinner than the lower part. Furthermore, a hole may be formed in the center of the electrode portion extending from the upper end to the lower end. This hole can be used as a thermocouple insertion hole or as a hole through which light for temperature detection is passed.

[Example] A first example of the heater device according to the present invention will be described with reference to FIGS. 1 and 2.

A heater device 1 according to this embodiment is shown in FIGS. 1 and 2. This heater device 1 is composed of a heating element 2 having a substantially flask shape and having an electrode hole 2a, and a rod-shaped electrode part 3 inserted into the electrode hole 2a of the heating element 2. The heating element 2 is made of a mixed ceramic containing unavoidable impurities such as 90% magnesia, 5% zirconia, and 5% alumina by weight.

As shown in FIGS. 1 and 2, the heating element 2 has an upper half 20 in the height direction and a lower half 21 in the height direction. The upper portion 20 is formed with a base end portion 200 having a large diameter. A bottom wall portion 210 having a three-dimensional curved surface shape, that is, a hemispherical shape, is formed in the lower portion 21 . Here, in this embodiment, when the overall length Ll in the axial direction of the heating element 2 is about 850 mm and the outer diameter L2 of the proximal end part 200 is about 140 mm, the dimensional relationship is that A is about 42 mm and day is about 32 mm. degree, C is 2
9 mm, D is about 8 mm, and the outer circumferential diameter E of the heating element 2 is about 120 mm. At this time, the thickness L of the bottom wall portion 210
3 is approximately E/2, and the diameter L4 on the inner surface side of the bottom wall portion 210
is about D/2, L5 is about 100mm, and L6 is 1
00 mm, and Ll is approximately 100 mm. Note that the base end portion 200 formed on the upper portion 20 has a large diameter in order to be placed on a heater holder.

The rod-shaped electrode part 3 is made of carbon, and its outer peripheral surface 3
0 has a shape very similar to the inner circumferential surface 2c that partitions the electrode hole 2a.

The heater device of this example was manufactured as follows.

That is, after adjusting the raw material ceramic powder for the heating element 2 at a predetermined blending ratio, water is added to form a slurry; a molding process of pouring the slurry into the cavity of a mold and molding; and a molded heating element 2. A drying step in which the molded body is removed from the mold, cured, and further dried at 150°C for 15 hours,
A sintering step of heating and sintering the dried molded body for heating element 2 at 1650° C. for 10 hours was performed in order to produce the product. Note that the maximum particle size of the raw material ceramic powder for the heating element 2 used in the adjustment step was about 3 mm. And heating element 2
The electrode part 3 is inserted into the electrode hole 2a defined by the inner peripheral surface 2c. At this time, as shown in FIG. 3, which is an enlarged view of the boundary area, there are particles with a maximum diameter of 1.5 m at the boundary area between the electrode part 3 and the heating element 2.
A carbon powder of about m is inserted as a spacer to form a conductive heating layer 20a.

If necessary, the heating element 2 and the electrode part 3 can be rotated relative to each other in the circumferential direction. This is because it is easy to charge carbon powder. At this time, since the carbon powder is rich in lubricity, the relative rotational movement is performed smoothly.

When using the heater device 1 manufactured as described above, for example, three heater devices 1 are used, and each heating element 2 is preheated by applying the flame of a burner. After preheating, the rod-shaped electrode portion 3 of each heater device 1 is heated as shown in FIG.
A conducting wire was fixed to the upper end of the container 4 with a band and connected to a power source, and the two heater devices 1 were immersed in the molten steel W in the container 4. In this state, O~440V is applied between the electrode part 3 and the molten metal W.
Apply a voltage of 0 to 800A with a frequency of 60H2.
Flow to a certain extent. Then, the heating element 2 of each heater device 1 generates heat, so that the molten metal W is heated.

(Effects of the Example) In this example, in order to heat the molten metal W using the calorific value of the heating element 2 of the heater device 1, the Joule heat generated in the molten metal itself is generated by passing an electric current directly through the molten metal itself, which is conventionally provided. Compared to the case where the molten metal is heated, the amount of current required is small, so it is easier to electrically control it, and the electrical equipment can be downsized.

By the way, since the heat on the side of the upper part 20 of the heating element 2 easily escapes to the outside of the heater device 1 via the electrode part 3,
The temperature of the first side becomes higher than that of the upper part 20, and the temperature rises. In this regard, in this embodiment, the wall thickness of the lower part 21 is set to be thicker than that of the upper part 20. The resistance of the lower part 21 becomes close to that of the lower part 21, and uniform heating of the heating element 2 can be achieved. Therefore, it is advantageous to suppress runaway heat generation of the heating element 2 and damage to the hole in the heating element 2 during use.

Furthermore, in this embodiment, as shown in FIG. 3, the conductive heating layer 20a made of carbon powder charged at the boundary between the heating element 2 and the electrode part 3 functions as a current diffusion layer and as a heat diffusion layer. As a result, the degree of electrical contact between the heating element 2 and the electrode part 3 is increased, as well as the degree of thermal contact, so localized heat generation of the heating element 2 is suppressed, and therefore, the heating element 2 prevents runaway heat generation. This is advantageous in that the heating element 2 generates uniform heat.

[Application Example] Next, an example in which the heater device 1 according to the embodiment shown in FIGS. 1 and 2 described above is applied to a continuous casting method will be described. First, a continuous casting apparatus used in the continuous casting method will be explained. As shown in FIG. 5, this continuous casting apparatus includes a tundish 50 as a container for holding molten steel, a water-cooled mold 51 disposed below the tundish 50, a cooling spray zone 52, and a vinyl roll 53. and a straightening roll 54. The tundish 50 has a capacity to hold about 5 ml of molten metal.

Next, continuous casting will be explained. First, using an appropriate number of heater devices 1 shown in FIGS. 1 and 2, the heating element 2 of each heater device 1 is heated with the flame of a burner to
Preheat to about 200℃.

With the heater device 1 preheated in this way, the ladle 55
14 transferred from and held in Tandaitsu 50
An appropriate number of heater devices 1 are immersed in molten steel at a high temperature of about 0.000 to 1600° C. from the bottom wall portion 210 in a substantially vertical state. The molten metal in the tundish 50 transferred from the ladle 55 flows toward the discharge port 50a shown in FIG. 7 and falls into the water-cooled mold 51.

If the heater device 1 is preheated before immersing the molten metal as described above, rapid heating of the heating element 2 can be prevented, and the occurrence of cracks in the heating element 2 due to rapid heating can be suppressed as much as possible.

Moreover, the above-mentioned preheating makes it possible to ensure the conductivity of the heating element 2, especially the heating element 2, which has magnesia as its main component and therefore becomes conductive only in a high temperature range.

In this application example, with the heater device 1 immersed in the molten metal in the tundish dish 50 as described above, an appropriate number of electrode portions 3
Connect the terminals to the AC power supply and apply 100 to 600V between the terminals
Apply a voltage of As a result, a current with a frequency of 60H2 is caused to flow between the heating elements 2 of the heater device 1 via the molten metal held in the tundish 50. The amount of current is O~800
It is about A. At this time, the heating element 2 generates heat to a high temperature. Therefore, the molten metal held in the tundish 50 is heated to, for example, rise in temperature by about 1 to 30° C., and the temperature is controlled.

The molten metal whose temperature has been adjusted in the tundish 50 in this way is discharged from the discharge port 50a of the tundish 50,
It is cooled and solidified in a water-cooled mold 51, and is roughly sprayed into a cooling spray zone 5.
The cooled and solidified material is pulled downward by pinch rolls 53. After that, it is cut into a predetermined length by a cutting machine.

As described above, in this application example, the heating element 2 of the heater device 1 did not suffer from the hole damage that conventionally occurs, and could be used satisfactorily for a long period of time. Furthermore, in this application example, since the temperature of the molten metal held in the tundish shell 150 can be adjusted by heating the molten metal held in the tundish shell 150 with the heater device 1, the temperature of the molten metal held in the tundish shell 50 can be maintained at an appropriate value. It is advantageous for improving the quality of manufactured products such as blooms and billets.

[Effects of the Invention] According to the heater device for immersion in molten metal according to the present invention, it is possible to heat the object to be heated, such as the molten metal, with the heating element, and therefore the temperature of the object to be heated, such as the molten metal, can be adjusted. I can do it. Moreover, since the lower part of the heating element is thicker than the upper part, it is advantageous in suppressing runaway heat generation and damage to holes in the heating element during use.

[Brief explanation of drawings]

1 to 4 show a first embodiment of the present invention, in which FIG. 1 is a sectional view of a heating element, and FIG. 2 is a sectional view of a heater device. FIG. 3 is an enlarged sectional view of the vicinity of the conductive heating layer, and FIG. 4 is a schematic sectional view of the state in which electricity is being applied between the heater device and the molten metal. FIG. 5 is a schematic cross-sectional view of an apparatus used in the continuous casting method. FIGS. 6 and 7 are graphs showing the relationship between the operating temperature and specific resistance of the heat generating material. 8 and 9 show the conventional method, and FIG. 8 is a graph showing the relationship between the position of the heating element and the temperature together with the heating element.
The figure is a sectional view showing the vicinity of the hole damage in the heating element. In the figure, 1 is a heater device, 2 is a heating element, 3 is an electrode part, 20
21 indicates the upper part and 21 indicates the lower part.

Claims (1)

[Claims] (1) A heater device that is immersed in the molten metal to heat the molten metal, and includes a cylindrical heating element made of a heat-generating material and having a bottom wall at the bottom, and equipped on the inner circumference of the heating element. and an electrode section that supplies current to the heating element, wherein the heating element has a lower part thicker than an upper part.