JP5114748B2 - Casting nozzle preheating method - Google Patents

Description

  The present invention relates to a method for preheating a casting nozzle used when pouring molten steel in a steel continuous casting facility.

  Casting nozzles in continuous casting equipment include a long nozzle used when pouring molten steel from a molten steel pan into a tundish, and an immersion nozzle used when pouring molten steel from a tundish into a mold.

The casting nozzle is used by preheating before the start of continuous casting for the purpose of preventing solidification of molten steel in the casting nozzle and mitigating thermal shock received by the casting nozzle.
By preheating the casting nozzle, it is possible to prevent the molten steel from being cooled and solidified when high-temperature molten steel comes into contact with the casting nozzle, and to prevent the casting nozzle from being blocked.
Further, by mitigating the thermal shock caused by the rapid change in temperature of the casting nozzle, it is possible to suppress the formation of cracks in the casting nozzle and to prevent the life of the casting nozzle from being shortened.
In order to obtain these effects, the preheating temperature is preferably closer to the molten steel temperature.

As a general preheating means, preheating by a gas burner is known. For example, in the method disclosed in Patent Document 1, a casting nozzle is inserted into a heat-resistant furnace, and a fuel gas such as coke oven gas (COG) is burned in the furnace with a gas burner to generate a high temperature. The casting nozzle is preheated by the combustion gas.
In preheating with a gas burner, the casting nozzle can be preheated to about 800 ° C to 1200 ° C. However, in the preheating method using a gas burner, since heating takes time, the refractory material of the immersion nozzle deteriorates during heating, and troubles such as spalling may occur during casting after preheating.

As a means for preheating the casting nozzle at a higher temperature and in a shorter time than the gas burner preheating, there is an induction heating method. The induction heating method can be applied as long as the object to be heated has conductivity. Since the casting nozzle is generally formed of alumina containing graphite, it has conductivity.
Since the preheating of the casting nozzle by the induction heating method is a preheating method in which the casting nozzle itself generates heat, heat loss is small and energy efficiency is high. Moreover, the controllability with respect to the preheating set temperature of the casting nozzle is also good.

  As what preheats by such an induction heating system, there exists a thing shown in patent document 2, for example. In the method disclosed in Patent Document 2, a coil orthogonal to the immersion nozzle is disposed on the outer peripheral surface of the immersion nozzle, and the entire immersion nozzle is made uniform by forming a coil shape along the shape of the outer peripheral surface of the immersion nozzle. Preheating to a high temperature has been achieved.

Japanese Patent Laid-Open No. 61-262455 JP 2007-185682 A

  Since the life of the immersion nozzle is determined by erosion by the hot water surface protecting material (hereinafter referred to as powder) in the mold, cracks in the contact portion (hereinafter referred to as powder line) with the powder outside the immersion nozzle. Preheating for the purpose of suppressing the formation of is important.

  As disclosed in Patent Document 2, it has been conventionally possible to preheat the casting nozzle uniformly by minimizing the temperature distribution generated in the casting nozzle during preheating. It has been considered advantageous for prevention.

However, even when preheated uniformly, fine cracks may occur on the surface of the immersion nozzle, and the life of the immersion nozzle has not necessarily been improved.
This invention is made | formed in view of this situation, and it aims at providing the preheating method which improves the lifetime of the nozzle for casting.

This invention solves the said subject by providing a difference in the preheating temperature of the inner side of a casting nozzle, and an outer side, The summary is as follows.
(1) In a method of preheating a casting nozzle in a continuous casting facility,
An induction heating device in which an induction heating coil is arranged so as to surround the nozzle is entirely or partially outside the casting nozzle, and the outside of the nozzle is heated by the induction heating device and at the same time by a heating means other than the induction heating device. A casting nozzle preheating method, wherein the inside of the casting nozzle is heated, and the outside preheating temperature is set higher than the inside preheating temperature.

(2) The casting nozzle preheating method according to (1), wherein the inside of the casting nozzle is preheated by a burner.
(3) The casting nozzle is an immersion nozzle, and the burner preheats the refractory disposed on the inner upper part and the upper part of the immersion nozzle with the immersion nozzle connected to the tundish (2) A method for preheating a casting nozzle according to claim 1.

(4) The casting nozzle according to (1), wherein the casting nozzle is an immersion nozzle, and exhaust gas when the tundish is heated by a burner is drawn into the immersion nozzle and the inside of the nozzle is preheated. Nozzle preheating method.
(5) The casting nozzle according to (1), wherein the casting nozzle is a long nozzle, and exhaust gas when the tundish is heated by a burner is drawn into the inner surface of the long nozzle, and the inside of the nozzle is preheated. Nozzle preheating method.

(6) An outer diameter of the casting nozzle varies with respect to the height direction, and the outer diameter is relative to the number of turns of the induction heating coil positioned in a relatively large outer diameter of the nozzle. The casting nozzle preheating method according to any one of (1) to (5) , wherein the number of turns of the coil located in a small range is smaller.
(7) The casting nozzle preheating method according to any one of (1) to (6) , wherein the outer preheating temperature is set higher by 50 to 400 ° C. than the inner preheating temperature.

  According to the present invention, the casting nozzle can be efficiently preheated using induction heating, and cracking can be suppressed by generating a compressive stress on the outer surface of the casting nozzle, thereby improving the life of the casting nozzle. it can. In particular, in the case of a submerged nozzle, cracks can be suppressed by effectively generating compressive stress along the powder line, and even in the case of a long nozzle, it generates compressive stress on the outer surface of the nozzle, Erosion due to molten slag can be improved, and the life of each nozzle can be improved.

  The inventors of the present invention focused on the damage on the outside of the casting nozzle described above, and examined the reason why the life of the nozzle was not improved when the nozzle was preheated uniformly. As a result, when uniform preheating is performed, the temperature of the nozzle decreases from the outside between the end of preheating of the nozzle and the start of casting, and in that case, a fine crack may occur on the outer surface of the nozzle. all right.

  Therefore, rather than aiming for uniform preheating, the heating temperature is controlled so that the outside of the casting nozzle is higher than the inside, and after cooling, a compressive stress is generated on the outside of the casting nozzle. It was found that the service life of the casting nozzle can be improved.

Therefore, in the case where the casting nozzle is preheated to a high temperature using an induction heating device, a refractory material that generates compressive stress in the damaged portion such as the powder line portion after preheating and is disposed above and above the casting nozzle. The arrangement of the heating means effective for heating the whole to a high temperature was studied, and the preheating method of the present invention was reached.
Hereinafter, embodiments of the present invention will be described in detail.

First, a first embodiment of the present invention will be described.
In the present invention, the outside of the casting nozzle in the continuous casting facility is heated by an induction heating device, and at the same time, the inside is also heated by a heating means other than induction heating. In the first embodiment, a burner that generates combustion gas toward the inside of the casting nozzle is used for the inside heating.

FIG. 1 shows an example of preheating an immersion nozzle among casting nozzles. An induction heating coil 3 is arranged so as to surround the periphery of the immersion nozzle 1 connected to the tundish 2, and an inner heating burner 5 is inserted into the nozzle through the discharge port 4 of the immersion nozzle 1 from the side of the coil. The
Similarly, when heating a long nozzle, an induction heating coil and a burner are arranged.

The induction heating coil 3 is a spiral or spiral conductor, and generates a magnetic flux therein by energizing a high-frequency current and generates an induction current in the immersion nozzle 1 for preheating.
The burner 5 preheats the refractory located above the coil 3 by burning a fuel such as COG inside the immersion nozzle to generate high-temperature combustion gas.
Using these heating devices, the immersion nozzle 1 is preheated simultaneously from the outside and the inside. At that time, the heating temperature is controlled so that the outside temperature is higher than the inside temperature.

  In the case of the immersion nozzle 1, it is possible to preheat it alone without being connected to the tundish 2. However, in this case, since it is necessary to connect the immersion nozzle to the tundish after preheating, it takes time to start casting after preheating, and there is a possibility of cooling during that time. Therefore, in the case of preheating the immersion nozzle, it is preferable in terms of efficiency that the preheating is performed while connected to the tundish.

  As a method of controlling the temperature outside the casting nozzle (immersion nozzle and long nozzle) higher than the inside, first, there is a method of adjusting the output of the high frequency current and the output of the burner. The output of each heating means at this time is determined by measuring the temperature difference between the outside and the inside using a thermocouple or a radiation thermometer in advance.

There is also a method of controlling the heating temperature by setting the frequency of the high-frequency current higher than the current output. This is because the induction heating has a characteristic that the higher the frequency of the high-frequency current, the more the heating is concentrated on the coil side, that is, the outside.
When adjusting by the frequency, it is preferably set to 30 kHz to 100 kHz, more preferably 50 kHz to 30 kHz. In addition, when the temperature control of the outer side and the inner side of the casting nozzle is not performed by the frequency, a frequency of 10 kHz to 30 kHz is also possible.

  The lower limit of the heating temperature outside the nozzle is preferably 1200 ° C., more preferably 1350 ° C., and the upper limit is about 1530 ° C. to 1600 ° C., which is the temperature of the molten steel. However, when an antioxidant is applied to the surface of the nozzle, the temperature at which the function of the antioxidant can be maintained is the upper limit, and is selected according to the performance of the antioxidant. 1400-1500 degreeC is illustrated.

  The temperature difference between the outside and inside of the casting nozzle is preferably 50 ° C. to 400 ° C., more preferably 50 ° C. to 200 ° C., although it depends on the nozzle material. As the lower limit of the temperature difference, the temperature at which the temperature variation during the preheating can be sufficiently tolerated and the compressive stress is constantly generated on the surface during the preheating is selected. As the upper limit of the temperature difference, a temperature at which a fine crack does not enter the refractory inside the casting nozzle is selected.

  The coil is preferably insulated in order to prevent short-circuiting of the conductor, and is further insulated from the viewpoint of preventing thermal deformation due to heat received from the preheated casting nozzle. The conductor is exemplified by a copper pipe, and cooling water is caused to flow inside the pipe.

  The inner diameter of the coil is designed according to the shape of the casting nozzle, and the outer surface of the casting nozzle and the inner surface of the coil are preferably spaced from 10 mm to 100 mm. The smaller the gap between the outer surface of the casting nozzle and the inner surface of the coil, the smaller the leakage of magnetic flux and the higher the heating efficiency. However, the heat received by the coil increases and the life of the coil is reduced. Since it becomes difficult to insert, it is necessary to decide in consideration of these.

The number of turns of the coil is determined by the capacity of the high frequency power source, the shape of the casting nozzle, and the heat capacity. The number of turns of the coil is the number of turns per unit length (height).
When the outer diameter of the casting nozzle varies with respect to the height direction, the number of turns of the coil located in the range where the outer diameter of the casting nozzle is relatively large is It is preferable to make it smaller than the number of turns of the coil located in the small range. By reducing the number of turns, the temperature distribution can be reduced in the height direction of the casting nozzle.

  The length (height) of the coil is designed according to the length (height) of the casting nozzle and surrounds all of the casting nozzle as much as possible. Since a mechanical device or the like for connecting to a dish or the like is disposed, the length of the coil is physically limited.

  The coil is formed with a hole (hereinafter referred to as a burner hole) 6 into which a burner 5 for introducing combustion gas is introduced inside the nozzle. The size and shape of the burner hole 6 may be circular or rectangular with a size sufficient to insert the burner, and is not particularly limited. The burner 5 is arrange | positioned according to the discharge port of the nozzle for casting. Therefore, when the discharge port 4 is on the side as shown in FIG. 1, the burner hole 6 is disposed on the side of the coil 3, and when the discharge port is on the bottom as in the case of a long nozzle, the burner hole is formed on the bottom of the coil. Be placed.

  The output of the high-frequency current to be used is 10 kW to 50 kW when preheating a casting nozzle of 25 kg to 40 kg, but is selected depending on the heat capacity and preheating time of the casting nozzle. When the preheating time is halved with the same casting nozzle, the output may be selected twice.

The main purpose of the burner is to preheat outside the range surrounded by the coil of the casting nozzle that is not induction heated. In the case of preheating by connecting the immersion nozzle to the tundish, it is also an object to preheat the lower nozzle, the sliding nozzle, and the upper nozzle disposed thereon.
Although the inside of the nozzle is preheated by the burner, the upper limit of the temperature is adjusted to 800 ° C. to 1150 ° C. so as to be 50 ° C. to 400 ° C. lower than the outside of the nozzle. The temperature is adjusted by the burner combustion amount. When preheating by connecting an immersion nozzle to a tundish, it is preferable to make the flame of the burner a long flame in order to preheat the refractory disposed thereon.

  The burner is arbitrarily designed to achieve its purpose, and its capacity and shape are not particularly limited, but the material is austenitic stainless steel (in terms of heat resistance and resistance to magnetic fields) For example, SUS304 and SUS310) are preferable.

Next, a second embodiment of the present invention will be described. This form is a mode suitable for preheating the tundish and the immersion nozzle at the same time with the immersion nozzle connected to the tundish.
In the continuous casting facility, the tundish is also preheated before pouring molten steel into the tundish 2. The preheating is performed by a burner 8 inserted into the through hole of the cover 7 as shown in FIG. In this embodiment, the exhaust gas from the burner is drawn into the immersion nozzle and the inside of the nozzle is heated.

  In order to draw the exhaust gas from the burner 8 into the immersion nozzle, an ejector may be arranged at the position of the burner used in the first embodiment instead of the burner. This ejector sucks and exhausts gas through an immersion nozzle. Due to the ejector effect at that time, tundish-heated exhaust gas is drawn into the immersion nozzle, and especially at the top and top of the immersion nozzle. The refractory to be placed can be preheated from the inside. In this aspect, it is excellent in energy efficiency by using the exhaust gas of tundish heating.

  Also in this aspect, the heating by the induction heating apparatus is performed in the same manner as in the first aspect. The inner heating temperature is adjusted by adjusting the amount of gas drawn by the ejector, or by adjusting the arrangement of the tundish heating burner and the amount of combustion, and controlled to a temperature lower than the outer temperature as in the first mode. To do.

Furthermore, a third embodiment of the present invention will be described. This form is a mode suitable for the case where the long nozzle is simultaneously preheated using tundish heating.
In this embodiment, as shown in FIG. 2, the outside of the long nozzle 9 is preheated by the induction heating coil 3, and the tundish heated exhaust gas is drawn into the inner surface of the long nozzle 9 to preheat the inside of the long nozzle. . As a method for drawing the exhaust gas into the long nozzle, there is a method in which an ejector (not shown) is arranged on the upper side of the long nozzle, as in the second embodiment.
In this embodiment, similarly to the second embodiment, the energy efficiency is excellent by using the exhaust gas of tundish heating.

As an example of the present invention, an induction heating coil and a burner were arranged as shown in FIG. 1, the immersion nozzle was preheated, and then casting was performed.
The shape of the immersion nozzle used here is roughly a cylindrical shape of 140 mm, an inner diameter of 80 mm, and a length of 780 mm, with the lower end closed and having a pair of discharge ports on the side surface at a position 50 mm from the lower end. is there.

In addition, as the refractory material of the immersion nozzle body, a composition having Al ₂ O ₃ of 50 to 60% by mass, SiO ₂ of 15 to 25% by mass, and C of 15 to 30% by mass is used. A refractory having ZrO ₂ of 80 to 90% by mass and C of 5 to 15% by mass was arranged. This is the composition of a general immersion nozzle.

The powder line of the immersion nozzle is arranged in the height direction from 230 mm to 380 mm from the bottom.
A ceramic fiber blanket having a thickness of 25 mm was wound on the side surface of the immersion nozzle. This is to prevent the immersion nozzle from being cooled by the outside air during preheating and during casting from preheating.

  The coil used had an inner diameter of 220 mm and a height of 580 mm. The inner diameter was determined in consideration of the workability of inserting the immersion nozzle within the allowable range of the heating efficiency. Further, the height was set to a position where there was no influence on the hardware for fixing the immersion nozzle to the tundish.

  As the coil, a copper pipe whose periphery was insulated with silicon rubber was formed into a spiral shape. After fixing it with a standard heat insulating material, the periphery was constructed with an amorphous heat insulating material. The heat insulating material protects the coil from heat.

  In addition, the winding method was such that the range of the main part of the immersion nozzle to be heated was 25 mm to 28 mm pitch, and the range of the powder line to be heated was 30 mm to 35 mm pitch. This is because the powder line refractory has a smaller C content and higher electrical resistance than the main body, so that when the same current is applied, the amount of heat generated increases, so the coil winding pitch ( This is because the number of turns) is adjusted so as to be preheated uniformly in the height direction. The smaller the coil winding pitch, the greater the number of turns and the greater the induced current, and the greater the winding pitch, the smaller the number of turns and the smaller the induced current.

The burner was a 12A pipe with its tip bent 10 mm upward and connected to COG piping. The material was SUS310.
A high frequency power supply having a maximum output of 20 kW and a frequency of 20 to 40 kHz was used. The high frequency power source is connected to the coil by a cable and supplies a high frequency current.

  These devices were placed in place and preheating was started. At this time, R type thermocouples were arranged on the outside and inside of the 300 mm position (powder line) from the lower end of the immersion nozzle, and the temperature was measured.

First, 1 m ³ N / hr COG was passed through the burner, ignited and placed at a predetermined position. The combustion air was naturally aspirated and was not specifically controlled.
Next, a high frequency current having an output of 20 kW and a frequency of 25 kHz was passed through the coil. After 10 minutes, the burner COG flow was increased to 5 m ³ N / hr. Further, after 40 minutes, the temperature outside the powder line of the immersion nozzle became 1450 ° C., so the output of the high frequency current was 15 kW, and the temperature of 1450 ° C. was maintained. At this time, the inner temperature was 1300 ° C.
This state was maintained for 40 minutes, preheating was completed in 90 minutes in total, and the immersion nozzle was removed from the coil. It took 7 minutes to start casting, and the immersion nozzle was cooled during that time, so the outside temperature dropped to 1380 ° C. and the inside temperature dropped to 1260 ° C.

  After preheating, casting was performed. Casting was 180 minutes with 3 charges. During casting, powder is constantly supplied into the mold, and the powder is melted on the upper surface of the molten steel and is in contact with the powder line of the immersion nozzle. The basicity of the powder was 1.0.

As Comparative Example 1, preheating with a COG burner was performed using an immersion nozzle having the same shape. Similar to a general preheating method, COG is burned in a heat-resistant furnace to preheat the immersion nozzle. The COG flow rate was constant at 25 m ³ N / hr and preheated for 90 minutes. The temperature of the same part as the present invention was measured, and the temperature before casting was 1020 ° C. on the outside and 1180 ° C. on the inside. Casting was performed under the same conditions as in the present invention.

As Comparative Example 2, the same configuration as that of the present invention was used, and preheating was performed to make the temperature inside and outside the immersion nozzle uniform. First, after igniting the burner, COG of 10 m ³ N / hr was flowed and burned. The combustion air was naturally aspirated. Next, a high frequency current having an output of 20 kW and a frequency of 25 kHz was passed through the coil. After 40 minutes in that state, the outside became 1400 ° C., and the inside became 1450 ° C., so the output of the high-frequency current was 15 kW and held for 50 minutes. The temperature before casting was 1350 ° C. on the outside and 1380 ° C. on the inside. Casting was performed under the same conditions as in the present invention.

  The immersion nozzles after the completion of casting for each of the inventive example, comparative example 1 and comparative example 2 were collected, and the amount of erosion of the most eroded part in the powder line was measured. Further, the rate of erosion was determined by dividing the amount of erosion by the casting time. The smaller the melting rate, the better.

About the example of the present invention, comparative example 1, and comparative example 2, the temperature difference between the outside and the inside of the powder line of the immersion nozzle immediately before casting is plotted on the vertical axis (the higher the outside is the more positive). FIG. 3 shows the arrangement on the horizontal axis.
As can be seen from FIG. 3, in the example of the present invention, the erosion rate was improved by 15% compared to Comparative Example 1, and the erosion rate was improved by 7% compared to Comparative Example 2. As a result, the effect of the present invention was confirmed.

It is a figure which shows the preheating apparatus of the immersion nozzle which is the 1st form of this invention. It is a figure which shows the preheating apparatus of a long nozzle which is the 3rd form of this invention. It is a figure which shows the relationship between the erosion rate of the immersion nozzle at the time of applying the preheating of this invention of an Example, the comparative example 1, and the comparative example 2, and the temperature difference of an outer side and an inner side.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Immersion nozzle 2 Tundish 3 Induction heating coil 4 Immersion nozzle discharge port 5 Burner for heating inside nozzle 6 Burner hole 7 Tundish cover 8 Burner for tundish heating 9 Long nozzle

Claims (7)

In a method of preheating a casting nozzle in a continuous casting facility,
An induction heating device in which an induction heating coil is arranged so as to surround the nozzle is entirely or partially outside the casting nozzle, and the outside of the nozzle is heated by the induction heating device and at the same time by a heating means other than the induction heating device. A casting nozzle preheating method, wherein the inside of the casting nozzle is heated, and the outside preheating temperature is set higher than the inside preheating temperature.   2. The casting nozzle preheating method according to claim 1, wherein the inside of the casting nozzle is preheated by a burner.   The said nozzle for casting is a submerged nozzle, and the said burner preheats the refractory material arrange | positioned in the inner upper part and upper part of a submerged nozzle in the state which connected the submerged nozzle to the tundish. Preheating method for casting nozzle.   2. The preheating of the casting nozzle according to claim 1, wherein the casting nozzle is an immersion nozzle, and exhaust gas when the tundish is heated by a burner is drawn into the immersion nozzle and the inside of the nozzle is preheated. Method.   2. The preheating of the casting nozzle according to claim 1, wherein the casting nozzle is a long nozzle, and exhaust gas when the tundish is heated by a burner is drawn into the inner surface of the long nozzle to preheat the inside of the nozzle. Method. The outer diameter of the casting nozzle varies with respect to the height direction, and the number of turns of the induction heating coil located in a range where the outer diameter of the nozzle is relatively large is a range where the outer diameter is relatively small. The casting nozzle preheating method according to any one of claims 1 to 5 , wherein the number of windings of the coil located in the coil is smaller.
The casting nozzle preheating method according to any one of claims 1 to 6 , wherein the outer preheating temperature is set higher by 50 to 400 ° C than the inner preheating temperature.

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