EP4663778A1

EP4663778A1 - Method for producing refractory for gas-blowing nozzle, refractory for gas-blowing nozzle, and gas-blowing nozzle

Info

Publication number: EP4663778A1
Application number: EP24779488.6A
Authority: EP
Inventors: Seiji Hosohara; Yoshiyuki Nakamura; Atsuhisa Iida; Hiroki Yoshioka
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-03-27
Filing date: 2024-03-13
Publication date: 2025-12-17
Also published as: CN120898006A; KR20250140568A; WO2024203359A1; JPWO2024203359A1; TWI899907B; JP7793783B2; TW202442878A

Abstract

A method of producing a gas injection nozzle refractory in which one or more small metal tubes for gas injection are buried in a carbon-containing refractory, comprises performing a series of processes a plurality of times, the series of processes involving subjecting the carbon-containing refractory with the small metal tubes buried therein to non-oxidative firing and thereafter performing an impregnation treatment in which the carbon-containing refractory is impregnated with an organic substance having a residual carbon ratio of 30 mass% or more. As a result of performing non-oxidative firing and organic substance impregnation a plurality of times, the fracture energy of the carbon-containing refractory with the small metal tubes buried therein is increased, and, during use of a gas injection nozzle, the propagation of cracks initiated due to sharp temperature gradients near the working surface of the nozzle is suppressed. This significantly improves the life of the gas injection nozzle.

Description

TECHNICAL FIELD

The present disclosure is intended to improve refining efficiency and alloy yield rate in converters, electric furnaces, etc. The present disclosure relates to a method of producing a refractory for a gas injection nozzle for injecting a gas into a melt from a furnace bottom or the like, the refractory for a gas injection nozzle (gas injection nozzle refractory) having one or more small metal tubes for gas injection buried in a carbon-containing refractory. The present disclosure also relates to a gas injection nozzle refractory and a gas injection nozzle.

BACKGROUND

In converters, electric furnaces, etc., bottom blowing is performed in which a stirring gas (typically an inert gas such as nitrogen or Ar) or a refining gas is injected into a melt from a furnace bottom in order to improve refining efficiency and alloy yield rate. Examples of bottom blowing methods include the following (1) to (3): (1) A double tube method of injecting oxygen for decarbonization from an inner tube and injecting a hydrocarbon gas (propane, etc.) for cooling a molten steel contact portion from an outer tube. (2) A method of injecting an inert gas through a slit-like opening formed between a metal tube and a brick (a slit method). (3) A method of burying a plurality of (several to hundreds of) small metal tubes in a carbon-containing brick, supplying an inert gas to the small metal tubes from the bottom of the brick through a gas introduction tube and a gas reservoir, and injecting the inert gas through the small metal tubes.
In methods (1) and (2), typically, a tuyere brick is produced in advance by a usual method, then the installation portion of a double tube or a metal tube forming a slit is processed or the tuyere brick is divided into two or four parts to form a space for installing a metal tube, and, during installation, a metal tube for gas injection is set in advance and the tuyere brick is placed around the metal tube.
A gas injection plug (nozzle) used in method (3) is referred to as a multiple hole plug (hereinafter referred to as "MHP"). For example, Patent Literature (PTL) 1 discloses that in such an MHP, the gas flow rate can be controlled in the range of 1 time to 20 times (0.01 Nm³/min to 0.20 Nm³/min). MHPs are therefore easier to be employed than the double tube method or the slit method.
MHPs have a structure in which a plurality of small metal tubes coupled to a gas reservoir are buried in a carbon-containing refractory such as a magnesia-carbon brick. Unlike nozzles in the double tube method or the slit method, MHPs are produced by the following method.
Raw materials including an aggregate such as a magnesia raw material, a carbon source such as flake graphite, and a binder such as pitch, metal species, or phenolic resin are kneaded by a kneading means with high dispersion performance, such as a high-speed mixer. A kneaded material for forming a carbon-containing refractory in which small metal tubes are to be buried is thus obtained. Subsequently, in one example, after small metal tubes are laid on and buried in the kneaded material in a layered form, the kneaded material is shaped with a pressing machine at a predetermined pressure and is then subjected to predetermined drying, and then the small metal tubes are joined by welding to a gas reservoir member to produce a MHP. In another example, after small metal tubes are joined by welding to a gas reservoir member in advance, the kneaded material is supplied to the periphery of the small metal tubes, is then shaped with a pressing machine at a predetermined pressure, and is then subjected to predetermined drying to produce a MHP.
Bottom blowing nozzles are more greatly damaged (worn) than refractories such as furnace walls, and are important members that influence the furnace life. Thus, various proposals have conventionally been made to suppress damage. The following improvements have also been proposed for MHPs.
PTL 2 proposes integrating a gas injection nozzle portion of a MHP and a peripheral tuyere to reduce premature erosion and abrasion from a joint portion. However, damage to a MHP also occurs in the part where the small metal tubes are buried. Hence, this technique cannot be an effective measure.
One of the causes of MHP damage is the lowering of the melting point of the small metal tubes buried in the refractory due to carburization (premature damage of the small metal tubes). The following measures against this have been proposed.
PTL 3 proposes forming, by thermal spraying, an oxide layer on the surface of small metal tubes made of stainless steel and buried in a carbon-containing refractory such as magnesia-carbon, to suppress carburization of the small metal tubes. In refining furnaces used for extended periods (for example, for two to six months), such as converters, however, there is a problem in that the oxide layer does not have a sufficient thickness and its carburization suppression effect is limited.
PTL 4 proposes placing a refractory sintered body between a small metal tube and a carbon-containing refractory to suppress carburization of the small metal tube. This technique has an effect of suppressing carburization. In nozzles with a large number of small metal tubes buried therein, however, it is difficult to place a refractory sintered body in the narrow space between the small metal tubes. It is therefore difficult to put this technique to practical use.

CITATION LIST

Patent Literature

PTL 1: JP S59-31810 A
PTL 2: JP S63-24008 A
PTL 3: JP 2000-212634 A
PTL 4: JP 2003-231912 A
PTL 5: JP S58-15072 A
PTL 6: JP 3201678 B2
PTL 7: JP 2017-144460 A

SUMMARY

(Technical Problem)

As mentioned above, to improve the durability of gas injection nozzles (MHPs, etc.) with small metal tubes buried in carbon-containing refractories, various studies have been made on the material or structure of the refractories, but sufficient improvement effects have not been obtained.
It could therefore be helpful to provide a method of producing a gas injection nozzle refractory with one or more small metal tubes for gas injection buried in a carbon-containing refractory, capable of improving the durability of the gas injection nozzle.

(Solution to Problem)

It has conventionally been assumed that the main cause of damage to MHPs used in converters or electric furnaces is erosion and abrasion due to molten steel flow near the working surface of the nozzle, resulting from the vigorous gas injection from the small metal tubes. The countermeasure proposed in PTL 2 is based on this assumption. It has also been assumed that premature consumption of the small metal tubes due to carburization or the like leads to increased damage. In view of this, countermeasures such as those described in PTL 3 and PTL 4 have been employed to prevent carburization of the small metal tubes. On the other hand, it has been suggested that during blowing, the refractory is cooled by the vigorous injection of inert gas, and the temperature difference between the blowing and non-blowing periods causes spalling damage. Furthermore, it has also been suggested that cracks form on the working surface at around 600 °C, where the strength of carbon-containing refractories is minimal, resulting in damage. Thus, there have been various theories, and no conclusion has been reached. As a result, sufficient countermeasures have not been implemented, and satisfactory durability has not been achieved as mentioned above.
To identify the true cause of MHP damage, we conducted a detailed investigation of the refractory microstructure near the working surface of the nozzle by collecting used MHPs which had been used in actual furnaces. As a result, it was found that significant temperature changes of 500 °C to 600 °C occur inside the refractory at a depth of about 10 mm to 20 mm from the working surface. Furthermore, cracks parallel to the working surface were observed in this region. Through repeated detailed examinations of the vicinity of the working surface of these MHPs used in actual furnaces, we concluded that the main damage mode of MHPs is not damage caused by erosion or abrasion but damage caused by thermal shock resulting from sharp temperature gradients occurring near the working surface.
In terms of thermal shock resistance, efforts have so far been mainly focused on preventing initiation of cracks in the carbon-containing refractory itself, which is the base material of the nozzle, by improving material properties such as lowering the elastic modulus, reducing thermal expansion, and increasing strength. However, under conditions where sharp temperature changes occur in a very narrow region of the working surface as mentioned above, preventing crack initiation is difficult. We accordingly proceeded with the investigation of methods to suppress crack propagation even if cracks occur, and focused on the fracture energy of the carbon-containing refractory.
The fracture energy of a refractory is defined as the energy required for creating new surfaces during crack propagation. Given that thermal stress is applied to the refractory to store a certain amount of elastic energy and cracks are generated by this energy, crack propagation is less likely to occur when fracture energy is greater.
Various methods of improving the fracture energy of refractories have been studied so far. For example, it is known that adding long carbon fibers can improve fracture energy. However, this method is currently not in practical use because the addition of long carbon fibers deteriorates the packing property of carbon-containing refractories.
Conventionally, techniques for non-oxidative firing and organic substance impregnation of refractories have been known primarily for improving the corrosion resistance, thermal spalling resistance, etc. of refractories used as furnace linings. For example, PTL 5 discloses that magnesia-carbon bricks containing metallic Al powder are subjected to firing heat treatment under a non-oxidizing atmosphere at 500 °C to 1000 °C and then to impregnation with an organic substance having a carbon yield of 25 % or more to fill the brick pores, thereby improving hot strength and corrosion resistance. PTL 6 discloses that, by firing magnesia-carbon bricks containing 0.5 wt% to 10 wt% of calcined anthracite under a reducing atmosphere at 600 °C to 1500 °C, slag erosion resistance is improved and thermal spalling resistance is improved through reduction in elastic modulus. In PTL 6, the elastic modulus after reduction firing at 1400 °C is used as an index for spalling resistance, and it is stated that an elastic modulus of 1.2 × 10⁴ MPa or less is important. It is also described that tar impregnation after reduction firing may be performed to seal pores, increase strength, and improve durability although no examples of such impregnation are provided.
As described above, conventional techniques for non-oxidative firing and organic substance impregnation of refractories have mainly been aimed at improving the corrosion resistance or thermal spalling resistance of furnace lining refractories. In contrast, PTL 7 discloses that a method of firing a refractory under non-oxidative conditions followed by organic substance impregnation (non-oxidative firing and organic substance impregnation) is effective in improving fracture energy.
The reason why fracture energy increases when a carbon-containing refractory is subjected to non-oxidative firing and organic substance impregnation is not entirely clear but is considered as follows.
Carbon-containing refractories (bricks) are typically produced using phenolic resin or the like as a binder. Phenolic resin undergoes thermal decomposition at high temperatures, leaving residual carbon and functioning as a bonding material in the carbon-containing refractory. However, the degree of bonding is high. Moreover, cracks propagate easily once initiated, resulting in low fracture energy. In contrast, in the case where the refractory is subjected to organic substance impregnation after non-oxidative firing, the organic substance diffuses and penetrates uniformly into the interior of the refractory, entering the matrix portion or between the layers of flake graphite in the refractory. This organic substance decomposes upon heating during nozzle use, forming carbon bonds. As a result, loose bonds are formed between the carbon materials such as flake graphite and the refractory aggregates, enhancing the degree of bonding. Consequently, even when cracks occur, they do not propagate easily. Additionally, because of the formation of such loose bonds, the carbon bonds derived from the organic substance are peeled off under appropriate stress, functioning as cross-links between brick microstructures, as in the case where long carbon fibers are added. Thus, pull-out resistance is improved, leading to an increase in fracture energy.
However, upon further investigation, we found that the method disclosed in PTL 7 has limitations in increasing fracture energy and the associated durability improvement. We then discovered that performing non-oxidative firing and organic substance impregnation a plurality of times significantly increases fracture energy.
Meanwhile, a problem with MHPs is the carburization of small metal tubes that occurs when gas is blown through the small metal tubes, as indicated in PTL 3 and PTL 4. The carburization of the small metal tubes occurs as a result of the carbon source contained in the refractory (bricks) infiltrating into the small metal tubes at high temperatures during actual operation. It is known that this carburization lowers the melting point of the small metal tubes, thereby increasing nozzle damage. In the present disclosure, we discovered that, in the production of a gas injection nozzle refractory, subjecting a carbon-containing refractory in which small metal tubes are buried to non-oxidative firing and organic substance impregnation a plurality of times can significantly increase fracture energy. However, even in this non-oxidative firing, depending on the heat treatment conditions, carbon components derived from the carbon-containing refractory may infiltrate into the small metal tubes, causing lowering of the melting point of the small metal tubes due to carburization. It is thus necessary to prevent lowering of the melting point of the small metal tubes due to carburization when the carbon-containing refractory with the small metal tubes buried therein is subjected to non-oxidative firing a plurality of times. To achieve this, we conducted detailed investigations regarding the non-oxidative firing conditions (firing temperature and firing time), the carbon content of the small metal tubes after non-oxidative firing, and so on, and discovered practically applicable optimal conditions that can suppress the lowering of the melting point of the small metal tubes.
As described above, by subjecting a carbon-containing refractory with small metal tubes for MHPs buried therein to non-oxidative firing and organic substance impregnation a plurality of times, it is possible to significantly improve the fracture energy of the refractory surrounding the small metal tubes. This can suppress the propagation of cracks initiated near the working surface of the MHP and significantly improve the life of the MHP. We also discovered that the MHP life can be further improved by optimizing the non-oxidative firing conditions and the like and suppressing carburization of the small metal tubes during the production process of the MHP.
The present disclosure is based on these discoveries. We thus provide the following.

[1] A method of producing a gas injection nozzle refractory in which one or more small metal tubes for gas injection are buried in a carbon-containing refractory, the method comprising performing a series of processes a plurality of times, the series of processes involving subjecting the carbon-containing refractory with the one or more small metal tubes buried therein to non-oxidative firing and thereafter performing an impregnation treatment in which the carbon-containing refractory is impregnated with an organic substance having a residual carbon ratio of 30 mass% or more.
[2] The method of producing a gas injection nozzle refractory according to [1], wherein the non-oxidative firing is performed at a firing temperature of 400 °C to 1100 °C for a firing time of 1 hour to 20 hours.
[3] The method of producing a gas injection nozzle refractory according to [1], wherein the non-oxidative firing is performed at a firing temperature of 800 °C to 1100 °C for a firing time of 3 hours to 20 hours.
[4] The method of producing a gas injection nozzle refractory according to any one of [1] to [3], wherein the series of processes involving the non-oxidative firing and the impregnation treatment with the organic substance is performed two or three times.
[5] The method of producing a gas injection nozzle refractory according to any one of [1] to [4], wherein conditions of the non-oxidative firing are set so that a total firing carburization index N in the non-oxidative firing performed the plurality of times will be less than or equal to a threshold.
[6] The method of producing a gas injection nozzle refractory according to any one of [1] to [5], wherein the carbon-containing refractory included in the gas injection nozzle refractory produced has a fracture energy of 175 J/m² or more.
[7] The method of producing a gas injection nozzle refractory according to any one of [1] to [6], wherein the carbon-containing refractory included in the gas injection nozzle refractory produced has a porosity of 3 % or less.
[8] The method of producing a gas injection nozzle refractory according to any one of [1] to [7], wherein a carbon content of the one or more small metal tubes after the non-oxidative firing is performed a last time is 2.0 mass% or less.
[9] The method of producing a gas injection nozzle refractory according to any one of [1] to [7], wherein a carbon content of the one or more small metal tubes after the non-oxidative firing is performed a last time is 1.3 mass% or less.
[10] The method of producing a gas injection nozzle refractory according to any one of [1] to [9], wherein the organic substance with which the carbon-containing refractory is impregnated in the impregnation treatment is one or more selected from coal tar pitch, phenolic resin, and furan resin.
[11] A gas injection nozzle refractory comprising: a carbon-containing refractory; and one or more small metal tubes for gas injection buried in the carbon-containing refractory, wherein the carbon-containing refractory has a fracture energy of 175 J/m² or more.
[12] The gas injection nozzle refractory according to [11], wherein the carbon-containing refractory has a porosity of 3 % or less.
[13] The gas injection nozzle refractory according to [11] or [12], wherein a carbon content of the one or more small metal tubes is 2.0 mass% or less.
[14] The gas injection nozzle refractory according to [11] or [12], wherein a carbon content of the one or more small metal tubes is 1.3 mass% or less.
[15] A gas injection nozzle comprising the gas injection nozzle refractory according to any one of [11] to [14].

(Advantageous Effect)

It is thus possible to produce a gas injection nozzle refractory in which a carbon-containing refractory with small metal tubes buried therein has high fracture energy and the propagation of cracks initiated due to sharp temperature gradients near the working surface of the nozzle is suppressed. Using this gas injection nozzle refractory can greatly improve the life of the gas injection nozzle.
Moreover, by optimizing the non-oxidative firing conditions (firing temperature and firing time), the carbon content of the small metal tubes after non-oxidative firing, and so on, it is possible to suppress carburization of the small metal tubes and thus prevent a decrease in the melting point of the small metal tubes, so that the life of the gas injection nozzle can be further improved.

DETAILED DESCRIPTION

The present disclosure provides a method of producing a gas injection nozzle refractory in which one or more small metal tubes for gas injection are buried in a carbon-containing refractory. The method comprises performing non-oxidative firing and organic substance impregnation a plurality of times, the non-oxidative firing and organic substance impregnation being a series of processes involving subjecting the carbon-containing refractory with the small metal tubes buried therein to non-oxidative firing (non-oxidative firing process) and thereafter performing an impregnation treatment in which the carbon-containing refractory is impregnated with an organic substance having a residual carbon ratio of 30 mass% or more (impregnation treatment process).
In the following description, a gas injection nozzle in which several tens or more small metal tubes are buried in a carbon-containing refractory is also referred to as "MHP," for the sake of convenience.
The material (raw materials) and forming method of the carbon-containing refractory used in the production method according to the present disclosure, the material and number of the small metal tubes, the method of burying the small metal tubes in the carbon-containing refractory, etc. will be described in detail later.
In the present disclosure, the object to be subjected to non-oxidative firing and organic substance impregnation is a carbon-containing refractory in which one or more small metal tubes are buried. In the case where the gas injection nozzle is of a type having a gas reservoir, the object may be a carbon-containing refractory in which only one or more small metal tubes are buried. Alternatively, the object may be a carbon-containing refractory in which one or more small metal tubes are buried and the whole or part of a gas reservoir member is joined to the small metal tubes.
In the present disclosure, after the carbon-containing refractory is subjected to non-oxidative firing, the carbon-containing refractory is subjected to an organic substance impregnation treatment. Here, organic substance impregnation cannot be performed unless non-oxidative firing is performed. Basically, carbon-containing refractories (bricks) are unfired refractories obtained without going through a firing process, and have an extremely low porosity of just a few percent due to binder curing. Therefore, in the unfired state, it is difficult to impregnate the entire refractory with the organic substance. Hence, non-oxidative firing needs to be performed in advance to enable organic substance impregnation. In the non-oxidative firing, heat-treating the entire refractory causes the carbon components derived from the binder and the like to uniformly form as a bonding material. Thus, organic substance impregnation can be performed easily while obtaining a uniform refractory microstructure as compared with an unfired product whose refractory microstructure varies upon heat reception during actual operation.
In the present disclosure, fracture energy is significantly increased by performing non-oxidative firing and organic substance impregnation a plurality of times. The reason for this is considered as follows. The organic substance with which the carbon-containing refractory is impregnated in the organic substance impregnation contains volatile components (components such as alcohol that become gaseous when heated even in the absence of oxygen and dissipate out of the refractory) and residual carbon components (components such as carbon that do not become gaseous even when heated in the absence of oxygen and remain within the refractory). Among these, the volatile components dissipate out of the refractory in normal-temperature environments after the production of the refractory and in high-temperature environments during actual use, so that the effect of organic substance impregnation decreases. Repeatedly performing non-oxidative firing and organic substance impregnation causes the following to occur: dissipation of volatile components by non-oxidative firing → filling of pores with organic substance by impregnation → dissipation of volatile components by non-oxidative firing (with the pore volume reduced compared to the previous cycle due to residual carbon components) → filling of pores with organic substance by impregnation, and so on. Thus, the pore volume that remains even after exposure to high temperatures gradually decreases. For example, if an organic substance whose volume is reduced by half by non-oxidative firing is used, with one impregnation cycle, the pore volume is reduced by half even when the volatile components dissipate thereafter. With two impregnation cycles, the pore volume is further reduced by half (i.e. one-quarter of the original volume). With three impregnation cycles, the pore volume is further reduced by half (i.e. one-eighth of the original volume). Thus, as a result of non-oxidative firing and organic substance impregnation being performed a plurality of times, the pore volume is greatly reduced, resulting in a significant increase in fracture energy.
Moreover, when non-oxidative firing and organic substance impregnation are performed a plurality of times, the pores are filled with residual carbon at each non-oxidative firing and organic substance impregnation cycle, leading to an increase in thermal conductivity. As a result, temperature gradients can be alleviated and thermal shock can be reduced.
The firing temperature (heat treatment temperature) in the non-oxidative firing of the carbon-containing refractory is preferably 400 °C or more. The firing temperature is preferably 1100 °C or less. If the firing temperature is less than 400 °C, the thermal decomposition of the binder (typically a resin such as phenolic resin) is insufficient, causing insufficient organic substance impregnation during the impregnation treatment after the non-oxidative firing. This is likely to hinder sufficient improvement of fracture energy. If the firing temperature is more than 1100 °C, the melting point of the small metal tubes may decrease due to the infiltration of the carbon components derived from the carbon-containing refractory into the small metal tubes. Furthermore, since non-oxidative firing is performed a plurality of times in the present disclosure, if the firing temperature is more than 1100 °C, the buried small metal tubes may melt or become clogged, undermining the gas injection function as a gas injection nozzle.
For more effective organic substance impregnation in the impregnation treatment after the non-oxidative firing, the firing temperature is preferably 800 °C or more.
The firing time (holding time) in the non-oxidative firing is preferably 1 hour to 20 hours. If the firing time is less than 1 hour, the heat treatment of the entire nozzle tends to be insufficient. If the firing time is more than 20 hours, carburization of the small metal tubes may occur as in the case where the firing temperature is more than 1100 °C, potentially causing a decrease in the melting point of the small metal tubes. From this viewpoint, the firing time is more preferably 3 hours to 20 hours.
In particular, in the present disclosure, since the non-oxidative firing is performed a plurality of times, it is preferable to control the total amount of heat treatment applied over the plurality of cycles. Specifically, it is preferable to set the non-oxidative firing conditions so that the total firing carburization index N expressed by the following equation (1) will be less than or equal to a threshold. The non-oxidative firing conditions are described as including the number of firing cycles, the firing temperature, and the firing time, for example.
[Math. 1] $N = \int_{0}^{t} Ddt \times 10^{9}$
In equation (1), D is the diffusion coefficient of carbon and is expressed by the following equation (2). The total firing carburization index N is a value obtained by integrating the carbon diffusion coefficient D over the total firing time across all cycles and multiplying the result by 10⁹.
[Math. 2] $D = D_{0} \exp (- \frac{Q}{RT})$
In equation (2), D₀ is the frequency factor of carbon diffusion, Q is the activation energy required for carbon diffusion, R is the gas constant, and T is the firing time. The total firing carburization index N is an index indicating the extent to which carbon penetrates into the small metal tubes. The threshold is calculated in advance through repeated testing and is appropriately determined according to the material of the small metal tubes. In this embodiment, the threshold is 118 as one example. That is, in this embodiment, it is preferable to set the number of firing cycles, the firing temperature, and the firing time so that the total firing carburization index N when non-oxidative firing is performed a plurality of times will be 118 or less. If the total firing carburization index N is more than the threshold, the carbon content of the small metal tubes after non-oxidative firing exceeds 2.0 mass%, resulting in a decrease in the durability of the nozzle itself. From the viewpoint of carburization of the small metal tubes, there is no lower limit for the total firing carburization index N. Accordingly, the lower limit for the amount of heat treatment may be set from other viewpoints such as the thermal decomposition of the binder.
In the present disclosure, the firing of the carbon-containing refractory is performed under non-oxidative conditions in order to prevent deterioration of properties inherent to the carbon-containing refractory, such as thermal spalling resistance and slag penetration resistance. In detail, if firing is performed under firing conditions where the carbon contained in the carbon-containing refractory is significantly reduced, for example, under conditions of high-temperature and long-duration heating in an oxidizing atmosphere, the carbon in the carbon-containing refractory undergoes oxidation loss. Consequently, properties inherent to the carbon-containing refractory, such as thermal spalling resistance and slag penetration resistance, would be lost. In order to prevent such properties from being lost, the carbon-containing refractory is fired under non-oxidative conditions.
Although the non-oxidative firing conditions have been described as including the number of firing cycles, the firing temperature, and the firing time, the conditions are not particularly limited as long as the carbon, such as flake graphite, contained in the carbon-containing refractory is not substantially lost. For example, the non-oxidative firing conditions may include reduction firing, firing under a reducing atmosphere, firing under a non-oxidizing atmosphere, or short-duration firing under an oxidizing atmosphere.
The method of performing the non-oxidative firing is not particularly limited and may be a conventional method. For example, a sheath formed by combining bricks or a metal container is placed on a carriage loaded into a firing furnace, and the carbon-containing refractory (in which one or more small metal tubes are buried) to be subjected to reduction firing is set inside. Subsequently, a carbon source such as coke is placed around the carbon-containing refractory, a lid is placed on top, and reduction firing (heat treatment) is performed with a predetermined temperature and time while shielding the interior from external air.
The firing atmosphere for the firing of the carbon-containing refractory may be a reducing atmosphere containing a combustible gas such as NX gas to perform reducing atmosphere firing, or an inert gas atmosphere such as nitrogen or argon or a non-oxidizing atmosphere to perform non-oxidizing atmosphere firing. In the case of reducing atmosphere firing or non-oxidizing atmosphere firing, the sheath or metal container may be omitted.
Even when the firing of the carbon-containing refractory is under an oxidizing atmosphere, short-duration firing may be employed, and after firing, the decarburized layer formed on the surface may be removed so that the non-decarburized part inside the refractory can be used. In this method, the surface of the carbon-containing refractory becomes oxidized. However, the oxidized surface acts as a protective layer, allowing the interior of the refractory to be fired under non-oxidative conditions. Hence, the interior of the refractory can be substantially regarded as having undergone non-oxidative firing. Furthermore, methods such as pre-coating the surface of the carbon-containing refractory with a glaze for oxidation prevention may be used for the firing of the carbon-containing refractory.
Among the foregoing methods, reduction firing, firing under a reducing atmosphere, and firing under a non-oxidizing atmosphere are more preferable. Firing under an oxidizing atmosphere is economically disadvantageous because it requires the removal of the decarburized layer on the surface.
The carbon content of the small metal tubes (the small metal tubes buried in the carbon-containing refractory) after the final non-oxidative firing is preferably 2.0 mass% or less. If the carbon content of the small metal tubes is more than 2.0 mass%, the melting point of the small metal tubes decreases, increasing the risk of the small metal tubes melting near the working surface of the nozzle tip. This leads to reduced durability of the nozzle itself. From this viewpoint, the carbon content of the small metal tubes is more preferably 1.3 mass% or less.
Examples of methods of causing the carbon content of the small metal tubes after non-oxidative firing to 2.0 mass% or less (preferably 1.3 mass% or less) include: (i) setting the non-oxidative firing temperature to a lower level and avoiding excessively long non-oxidative firing time, specifically, setting the non-oxidative firing temperature to 1000 °C or less and the non-oxidative firing time to 20 hours or less; and (ii) applying a gas-impermeable coating film on the surface of the small metal tubes to suppress carburization. Among these, method (i) is particularly effective.
After undergoing the above-described non-oxidative firing process, the carbon-containing refractory is subjected to an impregnation treatment with an organic substance.
In the organic substance impregnation treatment, the residual carbon ratio of the organic substance with which the carbon-containing refractory is impregnated is 30 mass% or more. The residual carbon ratio of the organic substance is measured based on the fixed carbon determination method described in JIS K6910 (Testing methods for phenolic resin). If the residual carbon ratio of the organic substance used for impregnation is less than 30 mass%, the effect of strengthening the refractory microstructure by the residual carbon is insufficient, which is undesirable. From this viewpoint, the residual carbon ratio is more preferably 35 mass% or more.
Examples of the organic substance used for impregnation include coal tar pitch (heat-melted product), phenolic resin (liquid resin), and furan resin (liquid resin). One or more of these may be used. Among these, coal tar pitch is particularly preferable because the carbon produced after thermal decomposition readily crystallizes, thereby contributing to improved fracture energy. Phenolic resin is less effective in improving fracture energy than coal tar pitch because the carbon produced after thermal decomposition is less likely to crystalize and tends to be glassy carbon.
The organic substance impregnation method is not particularly limited, but it is preferable to first depressurize the carbon-containing refractory to vacuum and then impregnate the carbon-containing refractory with the organic substance under pressure. For example, after being depressurized to a vacuum pressure of 100 Torr or less, the carbon-containing refractory is held at an applied pressure of 5 kgf/cm² or more for 2 hours or more to be impregnated with the organic substance. If the vacuum pressure is high, residual air bubbles in the refractory may prevent uniform impregnation with the organic substance throughout the interior of the refractory during pressurization. Therefore, the vacuum pressure in the case of depressurization (pressure reduction) is preferably 100 Torr or less and more preferably 60 Torr or less. If the pressure applied after depressurization is low or the pressurization holding time is short, the impregnation of the refractory with the organic substance may be insufficient. Therefore, the pressure applied after depressurization is preferably 5 kgf/cm² or more and more preferably 10 kgf/cm² or more. The pressurization holding time is preferably 2 hours or more and more preferably 4 hours or more. By satisfying these impregnation conditions, the organic substance can uniformly penetrate the interior of the carbon-containing refractory, and the effect of improving the fracture energy of the carbon-containing refractory based on the above-mentioned principle can be achieved particularly effectively.
As an apparatus that performs the organic substance impregnation treatment by holding the carbon-containing refractory at a predetermined applied pressure after depressurization to a predetermined vacuum pressure, a typical impregnation apparatus used when impregnating slide plates and the like with organic matter may be used. After impregnation, a drying treatment at approximately 200 °C may be carried out to remove volatile matter remaining inside the carbon-containing refractory.
In the present disclosure, fracture energy is increased significantly by performing non-oxidative firing and organic substance impregnation a plurality of times. After a certain number of cycles, however, the effect of increasing fracture energy is saturated. Accordingly, it is desirable to set the number of times non-oxidative firing and organic substance impregnation are performed to about two or three times, from the viewpoint of economic efficiency.
In the gas injection nozzle refractory produced by the method according to the present disclosure, the fracture energy of the carbon-containing refractory is preferably 175 J/m² or more. If the fracture energy is less than 175 J/m², the difference from conventional once-impregnated refractories (i.e. refractories obtained by performing non-oxidative firing and organic substance impregnation only once) is small. Hence, the effect of improving the life of the gas injection nozzle is insignificant. If the fracture energy of the carbon-containing refractory is 175 J/m² or more, the propagation of cracks initiated due to sharp temperature gradients near the working surface of the nozzle can be suppressed particularly effectively, with it being possible to significantly improve the life of the gas injection nozzle.
Fracture energy is measured using the three-point bending test method. Specifically, a three-point bending test is performed on a 25 mm × 25 mm × 140 mm test piece of the gas injection nozzle refractory under an inert atmosphere at 800 °C with a span of 100 mm. A bending load is applied to the test piece at a speed of 0.1 mm/min to obtain a stress-strain curve, and the fracture energy is calculated from the area under this stress-strain curve.
Fracture energy was compared among samples (i) to (v) made from carbon-containing refractories of the same material and subjected to the following non-oxidative firing and organic substance impregnation after forming. Sample (i) was a sample subjected only up to typical drying treatment. Sample (ii) was a sample subjected to non-oxidative firing after the drying treatment. Sample (iii) was a sample subjected to non-oxidative firing and organic substance impregnation only once after the drying treatment. Sample (iv) was a sample subjected to non-oxidative firing and organic substance impregnation twice under the conditions according to the present disclosure after the drying treatment. Sample (v) was a sample subjected to non-oxidative firing and organic substance impregnation three times under the conditions according to the present disclosure after the drying treatment. The fracture energy was 85 J/m² for sample (i), 62 J/m² for sample (ii), 160 J/m² for sample (iii), 187 J/m² for sample (iv), and 193 J/m² for sample (v). Thus, the fracture energy increases effectively by performing non-oxidative firing and organic substance impregnation a plurality of times.
In the gas injection nozzle refractories obtained by the conventional methods (i.e. with a fracture energy of 160 J/m² or less), cracks were generated in the refractory at locations about 100 mm inward from the working surface of the nozzle. A phenomenon in which the thickness of the gas injection nozzle refractory instantly decreased by about 100 mm then occurred, and durability decreased by approximately 10 %. In the gas injection nozzle refractories with a fracture energy of 175 J/m² or more obtained by the method according to the present disclosure, on the other hand, such phenomenon did not occur.
One method for enhancing the fracture energy of refractories is the addition of carbon fibers (long carbon fibers), as mentioned above. However, while the addition of carbon fibers is effective in increasing fracture energy, the compatibility between carbon fibers and refractories is very poor, resulting in a porous microstructure with a very high porosity. Such material containing carbon fibers suffers significant deterioration in properties such as corrosion resistance, making practical application difficult. In contrast, non-oxidative firing and organic substance impregnation are preferable because they can enhance the fracture energy while maintaining the density of the refractory microstructure.
In the gas injection nozzle refractory produced by the method according to the present disclosure, the porosity of the carbon-containing refractory is preferably 3 % or less. This porosity serves as an index of the organic substance impregnation amount. In detail, a high porosity indicates a low impregnation amount, and a low porosity indicates a high impregnation amount. If the organic substance impregnation amount is low and the porosity of the carbon-containing refractory is more than 3 %, the effect by organic substance impregnation is insufficient, reducing the effect of strengthening the refractory microstructure and improving toughness and making it difficult to ensure a fracture energy of 175 J/m² or more. The porosity of the carbon-containing refractory is more preferably 1.5 % or less. An effective way of reducing the porosity of the carbon-containing refractory is to sufficiently impregnate the carbon-containing refractory with the organic substance.
Next, the material (raw materials) and forming method of the carbon-containing refractory used in the production method according to the present disclosure, the material and number of small metal tubes, the method of burying the small metal tubes in the carbon-containing refractory, etc. will be described.
The raw materials of the carbon-containing refractory typically consist of an aggregate, a carbon source, other additives, a binder, etc.
Examples of the aggregate include magnesia, alumina, dolomite, zirconia, chromia, and spinel (alumina-magnesia, chromia-magnesia). One or more of these materials may be used. Among these, magnesia is particularly preferable from the viewpoint of corrosion resistance against molten metal and molten slag.
The carbon source is not particularly limited, and commonly used materials such as flake graphite, earthy graphite, petroleum pitch, and carbon black can be applied. One or more of these may be used. Although the amount of the carbon source blended in the carbon-containing refractory is not particularly limited, a suitable amount is typically about 10 mass% to 25 mass%.
Non-limiting examples of other materials include metal species such as metallic Al, metallic Si, and Al-Mg alloys, and carbides such as SiC and B₄C.
As the binder, materials commonly applicable as binders for shaped refractories can be used, such as phenolic resin and liquid pitch.
The small metal tubes are usually metal tubes with an inner diameter of about 1 mm to 5 mm and a wall thickness of about 0.5 mm to 4 mm. The material of the small metal tubes is not particularly limited, but it is preferable to use a metal material having a melting point of 1300 °C or more. An example of the material of the small metal tubes is a metal material (metal or alloy) containing one or more of iron, chromium, cobalt, and nickel, with stainless steel (ferritic, martensitic, or austenitic) and common steel being particularly common.
The number of small metal tubes buried in the carbon-containing refractory is not particularly limited. The number of small metal tubes is one or more. The number of small metal tubes is determined based on the inner diameter of the small metal tubes used and the required gas injection volume. For typical MHPs for converters, about 60 to 250 small metal tubes are usually buried in the carbon-containing refractory. For nozzles that blow only a small amount of gas, on the other hand, the number of small metal tubes may be one to several. In such gas injection nozzles, too, vigorous gas injection induces cooling at the tuyere tip, and crack propagation due to thermal shock leads to damage. Hence, the present disclosure can also be applied to such gas injection nozzles.
The method of burying the small metal tubes in the carbon-containing refractory is not particularly limited. For example, the raw materials of the carbon-containing refractory as mentioned above are mixed and kneaded using a mixer. After small metal tubes are laid on and buried in the kneaded material in a layered form, the kneaded material is shaped (formed) with a pressing machine at a predetermined pressure. After forming, a drying treatment is performed at an appropriate temperature. The carbon-containing refractory with the small metal tubes buried therein is then subjected to non-oxidative firing and organic substance impregnation a plurality of times according to the presently disclosed method. Subsequently, a gas reservoir member necessary for the gas injection nozzle function is joined (welded) to the small metal tubes, thus obtaining a gas injection nozzle product.
Another method involves joining (welding) small metal tubes to a gas reservoir member (top plate) in advance, filling the surrounding area with the kneaded material, then shaping (forming) it with a pressing machine at a predetermined pressure, and, after forming, performing a drying treatment at an appropriate temperature. The carbon-containing refractory with the small metal tubes buried therein is then subjected to non-oxidative firing and organic substance impregnation a plurality of times according to the presently disclosed method, thus obtaining a gas injection nozzle product.
The method of kneading the raw materials of the carbon-containing refractory is not particularly limited, and kneading means used for kneading of shaped refractories, such as high-speed mixers, tire mixers (conical mixers), and Eirich mixers, may be used.
For shaping (forming) the kneaded material, typical pressing machines used for refractory forming, for example, uniaxial forming machines such as hydraulic presses and friction presses, or isostatic pressing (cold isostatic pressing (CIP)), may be used.
The shaped carbon-containing refractory may be dried at a drying temperature of 180 °C to 350 °C for a drying time of about 5 hours to 30 hours.

EXAMPLES

Tables 1 to 3 show the production conditions and properties of gas injection nozzle refractories produced in this example (Examples and Comparative Examples).
As the raw materials of the carbon-containing refractory in which small metal tubes were to be buried, the following were used: fused magnesia (purity 98.2 mass%) as the aggregate magnesia raw material; flake graphite (purity 98.4 mass%, average particle size 0.18 mm) as the carbon source; and phenolic resin with a residual carbon content of 46 mass% as the binder.
As the small metal tubes buried in the carbon-containing refractory, small metal tubes made of common steel and having an outer diameter of 3 mm and an inner diameter of 2 mm were used.
As the organic substance with which the carbon-containing refractory was impregnated, coal tar pitch or phenolic resin was used. In Tables 1 to 3, the material with a residual carbon ratio of 42 mass% is coal tar pitch, and the material with a residual carbon ratio of 15 mass% is phenolic resin. The residual carbon ratio was measured based on the fixed carbon determination method described in JIS K6910 (Testing methods for phenolic resin).
The raw materials of the carbon-containing refractory were mixed at the ratios shown in Tables 1 to 3 and kneaded using an Eirich mixer. After this, using a 230 mm × 200 mm press mold, the small metal tubes were laid on and buried in the kneaded material in a layered form, and then shaping (forming) was performed with a hydraulic press at a pressure of 2.5 tons/cm². The formed refractory was then cured and dried using a dryer at 250 °C for 10 hours, thereby producing a carbon-containing refractory with small metal tubes buried therein.
The carbon-containing refractory produced as described above was subjected to non-oxidative firing in coke breeze and then to an organic substance impregnation treatment under the conditions shown in Tables 1 to 3, to obtain a gas injection nozzle refractory. In Examples, the non-oxidative firing and the organic substance impregnation treatment were performed a plurality of times. In the organic substance impregnation treatment, the carbon-containing refractory was held at a predetermined pressure for 10 hours.
For the measurement of porosity and fracture energy, carbon-containing refractories without buried small metal tubes were produced using the same raw materials and methods as above.
Comparative Examples included those not subjected to non-oxidative firing and organic substance impregnation, those subjected only to non-oxidative firing and not to organic substance impregnation, and those subjected to non-oxidative firing and organic substance impregnation only once.
For the gas injection nozzle refractories obtained as described above, the carbon content of the small metal tubes was measured. In addition, for the refractories without buried small metal tubes, the porosity and fracture energy were measured. The results are shown in Tables 1 to 3.
The porosity of the refractory was measured in accordance with JIS R2205. In the measurement, the vacuum method was used, and white kerosene was used as the medium.
The fracture energy of the refractory was measured as follows. Test pieces of 25 mm × 25 mm × 140 mm in size were subjected to a three-point bending test with a span of 100 mm. The bending test was conducted in an inert atmosphere at 800 °C. The tester used was the "Autograph AG-X/R" produced by Shimadzu Corporation, with a crosshead speed of 0.1 mm/min. From the stress-strain curve obtained by the three-point bending test, it was confirmed that stable fracture occurred. The fracture energy was calculated by dividing the area under the stress-strain curve by twice the projected area (25 mm × 25 mm) of the cut surface. In all measurements, stable fracture was confirmed.
The carbon content of the small metal tubes was measured by polishing the cut surface of the test piece with the buried small metal tubes after non-oxidative firing and performing quantitative analysis by analytical electron microscopy. The measurement range was set to a field of view of 100 µm × 100 µm along the outer periphery of the small metal tubes. The analysis equipment used was the "JXA-8230" produced by JEOL Ltd.
Furthermore, the firing carburization index n per one non-oxidative firing cycle and the total firing carburization index N over a plurality of non-oxidative firing cycles (n × number of firing cycles) were calculated. The results are shown in Tables 1 to 3.
The total firing carburization index N was calculated under the following conditions. First, the frequency factor D₀ of carbon diffusion and the activation energy Q required for carbon diffusion in equation (1) are expressed by the following equations. $D_{0} = (4.725 - 5.374 Wc + 1.779 {Wc}^{2}) \times 10^{- 5}$ $Q = 154.5 - 21.04 Wc - 3.285 {Wc}^{2}$
Here, Wc is the saturation carbon concentration. Wc = 2.14 % in this example. Wc is also referred to as the carbon solubility limit. Any portion exceeding the carbon solubility limit precipitates as cementite. When Wc = 2.14 %, D₀ = 1.37E-05 (m²/s) and Q = 94.43041 (k J/mol).
Moreover, in this example, R = 8.314J/(K·mol) where R is the gas constant in equation (1).
As shown in Tables 1 and 2, all Examples exhibited low porosity and high fracture energy. In particular, in Examples 1 to 5, 7 to 11, and 13 to 16, the total firing carburization index N was 118 or less, and the carbon content of the small metal tubes after non-oxidative firing was 2.0 mass% or less.
As shown in Table 3, Comparative Example 1 used commonly used magnesia-carbon bricks. In Comparative Example 1, the fracture energy was low. Comparative Example 2 involved performing non-oxidative firing at 1400 °C on the sample of Comparative Example 1 (without organic substance impregnation). In Comparative Example 2, the fracture energy was low. Moreover, the total firing carburization index N was more than 118, and the carbon content of the small metal tubes was as high as 3.1 mass%. Comparative Example 3 used a low non-oxidative firing temperature of 300 °C. In Comparative Example 3, the thermal decomposition of the binder by non-oxidative firing was insufficient, making organic substance impregnation impossible, so that the fracture energy was low. Comparative Example 4 used an organic substance with a low residual carbon ratio of 15 mass% in the impregnation treatment. In Comparative Example 4, the increase in fracture energy was not at a satisfactory level. Comparative Example 5 involved performing non-oxidative firing and organic substance impregnation only once. In Comparative Example 5, the fracture energy increased, but was still low as compared with Examples.

[Table 1]

Table 1

Example No.		1	2	3	4	5	6	7	8	9	10	11	12
Refractory raw materials	Magnesia (mass%)	85	85	85	85	85	85	85	85	85	85	85	85
	Flake graphite (mass%)	15	15	15	15	15	15	15	15	15	15	15	15
	Phenolic resin (mass% (not included in total))	3	3	3	3	3	3	3	3	3	3	3	3
Number of small metal tubes (pcs)		90	90	90	90	90	90	90	90	90	90	90	90
Non-oxidative firing conditions	Firing temperature (°C)	1000	400	800	1100	1200	1400	1000	1000	1000	1000	1000	1000
	Firing time (h)	5	5	5	5	5	5	0.5	1	3	15	20	30
	Firing carburization index n (-)	16.01	0.00	2.87	31.54	56.87	151.11	7.77	8.69	12.35	34.33	43.49	61.81
Organic substance impregnation conditions	Type of organic substance	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch
	Residual carbon ratio of organic substance (mass%)	42	42	42	42	42	42	42	42	42	42	42	42
	Impregnation reduced pressure (torr)	10	10	10	10	10	10	10	10	10	10	10	10
	Impregnation pressure (kgf/cm²)	15	15	15	15	15	15	15	15	15	15	15	15
Number of times of non-oxidative firing and organic substance impregnation (cycles)		2	2	2	2	2	2	2	2	2	2	2	2
Total firing carburization index N (-)		32.03	0.01	5.75	63.08	113.74	302.22	15.54	17.37	24.70	68.66	86.98	123.61
Properties of gas injection refractory produced	Porosity (%)	0.6	0.7	0.6	0.6	0.6	0.7	0.6	0.6	0.7	0.6	0.6	0.7
Properties of gas injection refractory produced	Fracture energy (J/m²)	187	136	176	188	180	179	125	135	178	185	178	179
Carbon content of small metal tubes after non-oxidative firing (mass%)		0.5	0.2	0.3	0.9	1.9	3.1	0.2	0.3	0.4	0.9	1.3	2.1

[Table 2]

Table 2

Example No.		13	14	15	16
Refractory raw materials	Magnesia (mass%)	85	85	85	85
	Flake graphite (mass%)	15	15	15	15
	Phenolic resin (mass% (not included in total))	3	3	3	3
Number of small metal tubes (pes)		90	90	90	90
Non-oxidative firing conditions	Firing temperature (°C)	1000	1000	1000	1000
	Firing time (h)	5	5	5	5
	Firing carburization index n (-)	16.01	16.01	16.01	16.01
Organic substance impregnation conditions	Type of organic substance	Coal tar pitch	Coal tar pitch	Coal tar pitch	Coal tar pitch
	Residual carbon ratio of organic substance (mass%)	42	42	42	42
	Impregnation reduced pressure (torr)	100	60	10	10
	Impregnation pressure (kgf/cm²)	15	15	5	10
Number of times of non-oxidative firing and organic substance impregnation (cycles)		3	4	5	6
Total firing carburization index N (-)		48.04	64.05	80.07	96.08
Properties of gas injection refractory produced	Porosity (%)	0.6	0.5	0.5	0.5
Properties of gas injection refractory produced	Fracture energy (J/m²)	193	193	193	194
Carbon content of small metal tubes after non-oxidative firing (mass%)		0.5	0.5	0.5	0.5

[Table 3]

Table 3

Comparative Example No.		1	2	3	4	5
Refractory raw materials	Magnesia (mass%)	85	85	85	85	85
	Flake graphite (mass%)	15	15	15	15	15
	Phenolic resin (mass% (not included in total))	3	3	3	3	3
Number of small metal tubes (pes)		90	90	90	90	90
Non-oxidative firing conditions	Firing temperature (°C)	-	1400	300	1000	1000
	Firing time (h)	-	5	5	5	5
	Firing carburization index n (-)	0	151.11	0.00	16.01	16.01
Organic substance impregnation conditions	Type of organic substance	-	-	Coal tar pitch	Phenolic resin	Coal tar pitch
	Residual carbon ratio of organic substance (mass%)	-	-	42	15	35
	Impregnation reduced pressure (torr)	-	-	10	10	10
	Impregnation pressure (kgf/cm²)	-	-	15	15	15
Number of times of non-oxidative firing and organic substance impregnation (cycles)		0	0	0	0	I
Total firing carburization index N (-)		0	151.11	0.00	16.01	16.01
Properties of gas injection refractory produced	Porosity (%)	3.2	10.3	0.8	0.8	0.9
Properties of gas injection refractory produced	Fracture energy (J/m²)	85	62	92	109	160
Carbon content of small metal tubes after non-oxidative firing (mass%)		0.2	3.1	0.2	0.5	0.5

Claims

A method of producing a gas injection nozzle refractory in which one or more small metal tubes for gas injection are buried in a carbon-containing refractory, the method comprising
performing a series of processes a plurality of times, the series of processes involving subjecting the carbon-containing refractory with the one or more small metal tubes buried therein to non-oxidative firing and thereafter performing an impregnation treatment in which the carbon-containing refractory is impregnated with an organic substance having a residual carbon ratio of 30 mass% or more.
The method of producing a gas injection nozzle refractory according to claim 1, wherein the non-oxidative firing is performed at a firing temperature of 400 °C to 1100 °C for a firing time of 1 hour to 20 hours.
The method of producing a gas injection nozzle refractory according to claim 1, wherein the non-oxidative firing is performed at a firing temperature of 800 °C to 1100 °C for a firing time of 3 hours to 20 hours.
The method of producing a gas injection nozzle refractory according to any one of claims 1 to 3, wherein the series of processes involving the non-oxidative firing and the impregnation treatment with the organic substance is performed two or three times.
The method of producing a gas injection nozzle refractory according to any one of claims 1 to 4, wherein conditions of the non-oxidative firing are set so that a total firing carburization index N in the non-oxidative firing performed the plurality of times will be less than or equal to a threshold.
The method of producing a gas injection nozzle refractory according to any one of claims 1 to 5, wherein the carbon-containing refractory included in the gas injection nozzle refractory produced has a fracture energy of 175 J/m² or more.
The method of producing a gas injection nozzle refractory according to any one of claims 1 to 6, wherein the carbon-containing refractory included in the gas injection nozzle refractory produced has a porosity of 3 % or less.
The method of producing a gas injection nozzle refractory according to any one of claims 1 to 7, wherein a carbon content of the one or more small metal tubes after the non-oxidative firing is performed a last time is 2.0 mass% or less.
The method of producing a gas injection nozzle refractory according to any one of claims 1 to 7, wherein a carbon content of the one or more small metal tubes after the non-oxidative firing is performed a last time is 1.3 mass% or less.
The method of producing a gas injection nozzle refractory according to any one of claims 1 to 9, wherein the organic substance with which the carbon-containing refractory is impregnated in the impregnation treatment is one or more selected from coal tar pitch, phenolic resin, and furan resin.
A gas injection nozzle refractory comprising:
a carbon-containing refractory; and

one or more small metal tubes for gas injection buried in the carbon-containing refractory,

wherein the carbon-containing refractory has a fracture energy of 175 J/m² or more.
The gas injection nozzle refractory according to claim 11, wherein the carbon-containing refractory has a porosity of 3 % or less.
The gas injection nozzle refractory according to claim 11 or 12, wherein a carbon content of the one or more small metal tubes is 2.0 mass% or less.
The gas injection nozzle refractory according to claim 11 or 12, wherein a carbon content of the one or more small metal tubes is 1.3 mass% or less.
A gas injection nozzle comprising
the gas injection nozzle refractory according to any one of claims 11 to 14.