JP2020171937A - Method for preheating continuous-casting nozzle - Google Patents

Abstract

To provide a method for preheating a continuous-casting nozzle capable of suppressing the variation of gas blow by an increase of a pore diameter of a gas-permeable refractory and stably producing high cleanliness steel.SOLUTION: A method for preheating a continuous-casting nozzle includes, in preheating continuous-casting nozzles 11, 12 in which gas-permeable refractories 15, 16 blended with SiO2 and C is arranged on an inner surface of a bore, providing a preheating inert gas flow rate [L/min] that is an inert-gas flow rate to pass through the gas-permeable refractories 15, 16 under preheating at a value satisfying the condition in the following expression. Cold gas-flow rate [L/min at 0.098 MPa]×preheating inert gas flow rate [NL/min/cm2] per unit area of the gas-permeable refractories 15, 16 exposed in a bore inner surface×preheating time [min]≥6.8.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for preheating a continuously cast nozzle in which a breathable refractory is arranged on an inner hole surface.

In order to highly clean the molten steel and suppress the adhesion of non-metal inclusions to the inner hole surface of the continuous casting nozzle, it is widely practiced to blow an inert gas into the molten steel from the continuous casting nozzle. In a continuous casting nozzle equipped with a function of blowing an inert gas into the molten steel, a breathable refractory is placed on a part or all of the inner hole surface in contact with the molten steel, and the inert gas is placed on the back side of the breathable refractory. The structure is provided with a hollow chamber (also called a gas pool) for the purpose of equalizing the flow path and gas pressure, supplying an inert gas to the hollow chamber, and inactive in molten steel via a breathable refractory. Many methods of blowing gas are adopted.

However, in the case of a continuous casting nozzle in which a breathable refractory is arranged on the inner hole surface, it has been confirmed that in the continuous casting operation, a phenomenon occurs in which the bubble diameter of the inert gas increases with the lapse of time from the start of casting. There is. The expansion of the bubble diameter of the inert gas not only causes the occurrence of bubble-based defects when gas bubbles remain in the slab, but also reduces the effect of suppressing the adhesion of non-metal inclusions such as alumina, resulting in a nozzle inner hole. It will also lead to blockage.

Therefore, for example, Patent Document 1 discloses a preheating method focusing on moisture during preheating for the purpose of preventing damage to refractories that hinder stable gas injection. In this method, the nozzle preheating process is divided into a first preheating and a second preheating, and the moisture absorbed in the nozzle is exhausted to the outside of the nozzle system without generating a positive pressure load inside the slit by the first preheating. Before an excessive heat load is applied to the gas supply pipe, the supply pipe is gas-cooled by the second preheating, and the fireproof material is heated to a specified temperature.
Further, in the nozzle for continuous casting described in Patent Document 2, by using a breathable refractory containing SiO ₂ having a particle size of 1 μm or less, the effect of not increasing the pore diameter is obtained and the breathability is obtained. The heat and impact resistance of refractories has been improved to stabilize gas injection.

Japanese Unexamined Patent Publication No. 2000-317626 Japanese Unexamined Patent Publication No. 2011-212720

According to the technique described in Patent Document 1, a corresponding effect can be obtained with respect to stabilization of gas injection. However, the present inventors have discovered that gas injection is not stable even if there is no damage to the refractory, and high-clean steel may not be stably produced.
Further, the technique described in Patent Document 2 also has a corresponding effect on the stabilization of gas injection, but the gas injection may not be stable, and further improvement is made in order to stably produce high-clean steel. Turned out to be necessary.

In addition, the fact that the gas injection is not stable (the ventilation characteristics of the inert gas become unstable) means that the cross-sectional area of the gas flow path (the pore diameter on the surface of the breathable fireproof material and the diameter of the gas flow path) with the operation When the flow rate of the inert gas is made constant, the gas supply pressure (back pressure) decreases with the passage of time, and when the back pressure is made constant, the flow rate of the inert gas increases with the passage of time. It means to increase.

The present invention has been made in view of such circumstances, and is a method for preheating a continuously cast nozzle capable of stably producing highly clean steel by suppressing fluctuations in gas injection due to an expansion of the cross-sectional area of the gas flow path. The purpose is to provide.

In order to achieve the above object, the present invention is used in preheating a continuously cast nozzle in which a breathable refractory containing SiO ₂ and C is arranged on the inner hole surface.
The preheating inert gas flow rate [L / min], which is the flow rate of the inert gas aerated through the breathable refractory during preheating, is characterized in that the value satisfies the condition of the following equation.
Cold air flow rate [L / min at 0.098 MPa] x Preheating inert gas flow rate [NL / min / cm ² ] x Preheating time [min] per unit area of the breathable refractory exposed on the inner hole surface ≧ 6.8

The inert gas in the present invention shall contain nitrogen gas in addition to the inert gas in the periodic table such as Ar.
The cold aeration rate is represented by the aeration rate per unit time when an air supply pressure of 0.098 MPa is applied to a breathable refractory under normal pressure at room temperature (5 to 40 ° C.). There is no significant difference in cold ventilation in the room temperature range. The unit L is liter.
The preheating time is a residence time of 600 ° C. or higher.

When SiO ₂ contained in the breathable refractory is gasified, the cross-sectional area of the gas flow path is expanded, and the ventilation characteristics of the inert gas become unstable. On the other hand, when C contained in the breathable refractory reacts with the gasified SiO ₂ to form a SiC layer so as to cover the entire surface of the gas flow path, the gasification of the SiO ₂ is stopped.
Therefore, the present inventors have come up with the idea of almost completing the gasification reaction of SiO ₂ by promoting the gasification of SiO ₂ and the reaction with C at the time of preheating the continuous casting nozzle.

The present inventors consider the surface area of the gas flow path before preheating (the smaller the reaction is, the easier it is to complete the reaction) and the gas flow rate during the preheating (the larger the reaction, the more the reaction) as factors that promote the gasification of SiO ₂ and the reaction with C. We thought that the preheating time (the longer the reaction is promoted) is important. However, since it is difficult to actually measure the surface area of the gas flow path, in the present invention, the cold aeration amount, which is considered to have a trade-off relationship with the surface area of the gas flow path, is used as a substitute for the surface area of the gas flow path. To do.

As a result of diligent studies based on the above assumptions, the present inventors have obtained a constant value of the product of the cold air flow rate, the flow rate of the inert gas during preheating per unit area of the breathable refractory exposed on the inner hole surface, and the preheating time. It was found that the expansion of the cross-sectional area of the gas flow path was almost completed by the above (6.8), and high-clean steel could be stably produced.

Further, the preheating method of the continuous casting nozzle according to the present invention, the SiO ₂ of the part or particle size 1μm to all following SiO ₂ particles may be blended.

The present inventors have found that stable production of highly clean steel becomes difficult at the initial stage of casting when SiO ₂ having a particle size of 1 μm or less is used. For example, when the gas flow rate is constant, the difference between the back pressure at the start of casting and the back pressure after that is extremely large, and the breathable refractory may be damaged at the start of casting. However, according to the present invention, even when the particle diameter of 1μm or less of SiO ₂ particles with a gas-permeable fireproof material which is incorporated into part or all of the SiO _2, the breakage of the permeable refractory prevention However, the decrease in back pressure can be remarkably suppressed.

In the method for preheating the continuous casting nozzle according to the present invention, since the gasification reaction of SiO ₂ is almost completed during the preheating of the continuous casting nozzle, the fluctuation of gas injection due to the expansion of the cross-sectional area of the gas flow path is suppressed, and the highly clean steel is produced. It can be manufactured stably.

It is a vertical cross-sectional view of the upper nozzle and the immersion nozzle used in the method of preheating the continuous casting nozzle which concerns on one Embodiment of this invention. It is a schematic diagram which showed the preheating method of the immersion nozzle and the preheating temperature measurement method of a breathable refractory.

Subsequently, an embodiment embodying the present invention will be described with reference to the attached drawings, and the present invention will be understood.

FIG. 1 shows the upper nozzle 11 and the immersion nozzle 12 used in the method for preheating the continuously cast nozzle according to the embodiment of the present invention.
A sliding nozzle 13 for adjusting the flow rate of the molten steel discharged from the upper nozzle 11 is attached to the lower surface of the tundish 10. The sliding nozzle 13 is sandwiched between an upper plate 13a fixed to the lower surface of the upper nozzle 11, a lower plate 13c fixed to the upper end of the immersion nozzle 12 via the lower nozzle 14, and the upper plate 13a and the lower plate 13c. It is roughly composed of an intermediate plate 13b that slides in and an actuator (not shown) that slides the intermediate plate 13b.

The upper nozzle 11 (an example of a continuously cast nozzle) is composed of an inner hole 11a that serves as a flow passage for molten steel and a truncated cone-shaped nozzle body 11b made of a refractory material that surrounds the inner hole 11a. A breathable refractory 15 containing SiO ₂ and C is arranged on the inner hole surface. The breathable refractory 15 is supplied with an inert gas from outside the nozzle system toward the back surface of the breathable refractory 15.

The immersion nozzle 12 (an example of a continuous casting nozzle) is composed of a tubular body 12b having a bottom portion in which an upper end portion is an inflow port of molten steel and a flow path (inner hole) 12a extending downward from the inflow port is formed inside. Has been done. A pair of discharge holes 12c communicating with the inner holes 12a are formed to face each other on the lower side surface portion of the tube body 12b.
A breathable refractory 16 containing SiO ₂ and C is arranged on the inner hole surface, and a hollow chamber 17 communicating with the breathable refractory 16 is formed inside the pipe body 12b. The breathable refractory 16 is supplied with an inert gas from outside the nozzle system via the hollow chamber 17 to the back surface of the breathable refractory 16.

As described above, the instability of the ventilation characteristics of the inert gas occurs when the pore diameter and the diameter of the gas flow path on the surface of the breathable refractory increase with operation. In that case, if the flow rate of the inert gas is constant, the back pressure decreases with the passage of time, and if the back pressure is constant, the flow rate of the inert gas increases with the passage of time.
In the following description, the case where the flow rate of the inert gas is constant will be described, but the basic idea is the same when the back pressure is constant.

[About the technical idea of the present invention]
When the inert gas blowing technology from the continuous casting nozzle into the molten steel is used for the purpose of highly cleaning the molten steel and suppressing the adhesion of non-metallic inclusions to the inner hole surface of the continuous casting nozzle, from immediately after the start of casting to the initial stage ( Assuming that a new continuous casting nozzle is used at the start of casting, for example, immediately after the start of casting to 30 minutes), the ventilation characteristics of the inert gas become unstable. Specifically, when the inert gas is blown into the molten steel at a constant flow rate, the gas supply pressure (back pressure) decreases.

The decrease in back pressure is due to an increase in the cross-sectional area of the gas flow path (pore diameter and gas flow path diameter on the surface of the breathable refractory). Since the expansion of the cross-sectional area of the gas flow path is directly linked to the increase in the bubble diameter, it causes a bubble defect. Further, the increase in the bubble diameter causes a decrease in the number of bubbles, that is, a decrease in the total surface area of all the bubbles under a constant gas flow rate, so that the inclusion capture effect is lowered and a cause of inclusion defects is also caused.

On the other hand, if a breathable refractory whose porosity is adjusted in advance is used so that the back pressure becomes appropriate after the back pressure stabilizes (for example, 30 minutes after the start of casting), the casting starts (after the start of casting). Since the back pressure becomes high at 0 minutes), the refractory may be damaged. On the contrary, when the back pressure is lowered to reduce the gas flow rate in order to prevent the refractory from being damaged, the effect of capturing inclusions is lowered.
The above-mentioned problem discovered by the present inventors becomes a serious problem in the case of the technique described in Patent Document 2, that is, in the case of a breathable refractory using SiO ₂ of 1 μm or less, since the back pressure becomes extremely high.

Although SiO ₂ is blended in a general breathable refractory, when the breathable refractory is heated after the start of casting, SiO ₂ is gasified (SiO gas) and the cross-sectional area of the gas flow path is expanded. The present inventors have found. That is, the present inventors have found that the gasification of SiO ₂ contained in the breathable refractory causes the cross-sectional area of the gas flow path to expand and the ventilation characteristics of the inert gas to become unstable.

On the other hand, when C mixed in the breathable refractory reacts with gasified SiO ₂ (SiO gas) to form a SiC layer so as to cover the entire surface of the gas flow path, the gasification of SiO ₂ occurs. It stops and the expansion of the cross-sectional area of the gas flow path is suppressed.

Therefore, the present inventors have decided to almost complete the gasification reaction of SiO ₂ by promoting the gasification of SiO ₂ and the reaction with C at the time of preheating the continuous casting nozzle.
The present inventors considered that the surface area of the gas flow path before preheating, the gas flow rate during preheating, and the preheating time are important factors for promoting the gasification of SiO ₂ and the reaction with C.

The present inventors have focused on the fact that the cold air flow rate of a breathable refractory is measured in a dip nozzle in which a breathable refractory is arranged, which is generally sold.
It is considered that there is a general trade-off between the cold air volume and the surface area of the gas flow path. That is, as the cold ventilation amount is small (there are many thin gas flow paths), the surface area of the gas flow path increases and the amount of SiO ₂ exposed to the gas flow path increases, and the SiO ₂ to be gasified increases. Is expected to increase. On the contrary, as the cold air flow rate increases (there are a small number of thick gas flow paths), the surface area of the gas flow path decreases and the amount of SiO ₂ exposed to the gas flow path decreases, and the SiO ₂ to be gasified decreases. ₂ is considered to decrease.

Breathable refractories are manufactured with a generally constant bulk density, and the volume of the gas flow path is approximately constant, but the surface area of the gas flow path can be evaluated by the cold air flow rate as described above. it is conceivable that.

Further, when the preheating time inert gas flow rate per unit area of the gas-permeable fireproof material exposed in the nozzle bore surface increases, to obtain the effect of removing the SiO ₂ gasified into immersion nozzle outside of SiO ₂ gasification The reaction is thought to proceed.
It is self-evident that the gasification reaction of SiO ₂ proceeds as the preheating time becomes longer.

If the preheating temperature is equal to or higher than the gasification temperature of SiO ₂ (significant gasification occurs at 600 ° C or higher), the gasification of SiO ₂ is within the range of the preheating temperature (up to about 1100 ° C) that can be industrially adopted. It is considered that there is no big difference in the progress of the reaction between C and C.
On the other hand, if the temperature is lower than 600 ° C. (for example, about 500 ° C.), there is a concern that the gasification of SiO ₂ and the reaction promotion with C will be insufficient, and the refractory will be damaged by spalling at the start of casting (start of hot water flow). Is also a concern.

[Structure of the present invention]
In the present invention, when preheating a continuously cast nozzle in which a breathable refractory containing SiO ₂ and C is arranged on the inner hole surface,
The flow rate of the inert gas during preheating [L / min], which is the flow rate of the inert gas to be ventilated to the breathable refractory during preheating, is set to a value satisfying the condition of equation (1).
Cold air flow rate [L / min at 0.098 MPa] x Preheating inert gas flow rate [NL / min / cm ² ] x Preheating time [min] ≥ 6 per unit area of breathable refractory exposed on the inner hole surface .8 (1)

The inert gas includes nitrogen gas in addition to the inert gas in the periodic table such as Ar.
The preheating time is a residence time of 600 ° C. or higher.

With respect to 100% by mass of the breathable refractory, SiO ₂ of a simple substance (excluding the case of a composite oxide) is about ₂ to 12% by mass, and C to be blended as an aggregate is about 7 to 30% by mass.
The upper limit of the product of the cold air flow rate, the flow rate of the inert gas during preheating per unit area of the breathable refractory exposed on the inner hole surface, and the preheating time is not specified, but practical preheating conditions (maximum preheating temperature). , Maximum gas flow rate, preheating time that can be used industrially), it seems that the upper limit is about 115.

SiO ₂ particles having a particle size of 1 μm or less may be blended in a part or all of SiO ₂ . The blending amount of the SiO ₂ particles having a particle size of 1 μm or less is about 2 to 12% by mass with respect to 100% by mass of the breathable refractory.

FIG. 2 is a schematic view showing a method of preheating the immersion nozzle 12 and a method of measuring the preheating temperature of the breathable refractory material 16.
When preheating the immersion nozzle 12, the burner 20 is inserted from the discharge hole 12c into the inner hole 12a to heat the inner hole surface, and the burner 20 is arranged directly under the bottom surface of the tube body to heat the bottom surface of the tube body.
When measuring the preheating temperature of the breathable refractory 16 arranged on the inner hole surface, the breathable refractory 16 is irradiated with infrared rays by pointing the radiation thermometer 21 through the discharge hole 12c to preheat the breathable refractory 16. Measure the temperature.

The value obtained by irradiating the bottom surface of the inner hole 12a with infrared rays through the discharge hole 12c and measuring the preheating temperature of the bottom surface of the inner hole 12a may be substituted. It is considered that the bottom surface of the inner hole 12a has the lowest preheating temperature because it comes into contact with the outside air during preheating, and the temperature of the breathable refractory 16 is equal to or higher than the measured temperature.
Alternatively, a method may be used in which the thermocouple is brought into direct contact with the breathable refractory material 16 for measurement.

When measuring the preheating temperature of the upper nozzle 11, infrared rays are applied to the breathable refractory 15 arranged on the inner hole surface from above the inner hole 11a by directing the radiation thermometer 21 to the breathable refractory 15. Measure the preheating temperature.

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 the matters described in the claims. It also includes other possible embodiments and variations.

The verification test carried out for verifying the effect of the present invention will be described.
For the verification test, two types of alumina-graphite immersion nozzles in which a breathable refractory was arranged on the inner hole surface were used. Table 1 shows the composition and structure of the breathable refractories arranged in the two types of immersion nozzles A and B.

The breathable refractory contains Al ₂ O ₃ , SiO ₂ , SiC, and C blended in the aggregate, and the balance is an oxide such as CaO, C as a binder, ignition loss, and the like.
The breathable refractory of the immersion nozzle A contains 2% by mass of SiO ₂ particles having a particle size of 1 μm or less, and the breathable refractory of the immersion nozzle B does not contain SiO ₂ particles having a particle size of 1 μm or less.

A list of test results is shown in Table 2.
The preheating time was set to the time during which the heating was performed at 600 ° C. or higher, and the inert gas (nitrogen gas) was supplied to the breathable refractory during preheating with the flow rate of the inert gas during preheating as shown in Table 2 constant.

The test results were evaluated as follows.
When the supply pressure (back pressure) when supplying an inert gas (Ar gas) to the breathable refractory 0 minutes after the start of casting of the actual machine is 100% and the back pressure 30 minutes after the start of casting is 90% or more Since it can be applied to the actual machine, it is marked with ○.
Although it is possible to ventilate at the specified flow rate, if the back pressure at 0 minutes after the start of casting is 100% and the back pressure at 30 minutes after the start of casting is less than 90%, or if cracks occur in the refractory due to spalling, △ did. However, it cannot be applied to the actual machine.
Since the back pressure at the initial stage of casting is high and the refractory may be damaged, if the gas flow rate is reduced, it cannot be applied to the actual machine, so it was marked as x.

The findings from the verification test are listed below.
-All the examples were evaluated as ◯.
-In Comparative Examples 1 to 4 using a breathable refractory containing SiO ₂ particles having a particle size of 1 μm or less, the back pressure at the initial stage of casting is high and the refractory may be damaged, so that the gas flow rate can be reduced. I had no choice but to do it.
-In Comparative Examples 5 and 6, a breathable refractory containing no SiO ₂ particles having a particle size of 1 μm or less was used, but the value of Eq. (1) was less than 6.8, so the evaluation was Δ. Met.

-Stabilization of back pressure was confirmed by changing the cold air flow rate and setting the value of Eq. (1) to 6.8 or more (comparison between Example 1 and Comparative Example 1).
-Stabilization of back pressure was confirmed by changing the flow rate of the inert gas during preheating per unit area of the breathable refractory exposed on the inner hole surface and setting the value of Eq. (1) to 6.8 or more (). Comparison of Example 1 and Comparative Example 3).
・ Stabilization of back pressure was confirmed by changing the preheating time and setting the value of Eq. (1) to 6.8 or more (comparison between Example 1 and Comparative Example 2, and Example 5 and Comparative Example 5). Comparison).

10: Tandish, 11: Upper nozzle (example of continuous casting nozzle), 12: Immersion nozzle (example of continuous casting nozzle), 11a, 12a: Inner hole, 11b: Nozzle body, 12b: Tube body, 12c: Discharge hole , 13: Sliding nozzle, 13a: Upper plate, 13b: Intermediate plate, 13c: Lower plate, 14: Lower nozzle, 15, 16: Breathable fireproof material, 17: Hollow chamber, 20: Burner, 21: Radiation thermometer

Claims (2)

When preheating a continuously cast nozzle in which a breathable refractory containing SiO ₂ and C is arranged on the inner hole surface,
A continuously cast nozzle characterized in that the preheating inert gas flow rate [L / min], which is the flow rate of the inert gas aerated through the breathable refractory during preheating, is set to a value satisfying the conditions of the following equation. Preheating method.
Cold air flow rate [L / min at 0.098 MPa] x Preheating inert gas flow rate [NL / min / cm ² ] x Preheating time [min] per unit area of the breathable refractory exposed on the inner hole surface ≧ 6.8 In the preheating process of continuous casting nozzle according to claim 1 wherein, the preheating method of the continuous casting nozzle, characterized in that the SiO ₂ of part or all the particle size 1μm or less of SiO ₂ particles are blended.

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