JP2011212720A - Air-permeable refractory and nozzle for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air-permeable refractory which prevents increase of danger on operation, such as breakout of an immersion nozzle caused by expansion of a gas bubble diameter with a lapse of time in the continuous casting operation.SOLUTION: The air-permeable refractory contains 15-30 mass% of free carbon ("free carbon" means carbon other than the compound), 2 to <12 mass% of SiOwhose particle size is ≤1 μm, and 53-83 mass% oxide(including inevitable impurities in manufacturing) which is a mineral composed of AlOor of a component of AlO, wherein the SiOlike particle of ≤1 μm is dispersed in the carbon structure of refractory matrix, and the mass ratio (SiO/carbon) of the SiOlike particle of ≤1 μm to carbon in matrix, is in the range of 0.5-2.0, and the total content of metal and any one or more compounds excluding oxide is 1-5 mass%.

Description

  The present invention relates to a breathable refractory used for a nozzle for discharging molten steel from a container in continuous casting of steel and a nozzle for continuous casting using the breathable refractory.

  When continuously casting molten steel, there are many methods of blowing an inert gas from the nozzle into the molten steel in order to clean the molten steel and suppress adhesion of non-metallic inclusions such as alumina to the inner surface of the nozzle. .

  A nozzle having a function of blowing an inert gas into such molten steel has a breathable refractory disposed on a part or all of the inner surface of the inner surface in contact with the molten steel, and has a gas flow on the outer peripheral side thereof. The structure is provided with a hollow chamber (also referred to as a gas pool in the present invention) for the purpose of uniformizing the path and gas pressure, etc., and gas is supplied to the hollow chamber and is not introduced into the molten steel through a breathable refractory. Many methods of blowing active gas are employed.

  However, in the continuous casting operation, there is a phenomenon in which the gas bubble diameter increases with the passage of time from the start of steel passing. Such expansion of the bubble diameter of the gas has led to a reduction in the quality of the steel, such as nozzle hole obstruction, bubble system defects, and inclusion of non-metallic inclusions due to the reduced effect of suppressing the adhesion of non-metallic inclusions such as alumina. In addition, in the case of an immersion nozzle for pouring molten metal into the mold, irregular fluctuations and turbulences in the molten steel surface in the mold increase, which may increase operational risks such as breakout.

  Such phenomena are often attributed to breathable refractory components, particularly silica.

  The breathable refractory is generally composed of refractory aggregate particles mainly composed of alumina or a compound thereof, carbon and amorphous silica (fused silica) aggregate particles, and the like. This amorphous silica aggregate grain is adopted because of its extremely low thermal expansion, and has the function of ensuring the thermal shock resistance that is essential for breathable refractories.

  However, this amorphous silica aggregate particle volatilizes and disappears during operation and creates voids in the breathable refractory structure. In addition, the amorphous silica aggregate particles are often used in a relatively large particle size in order to enhance the thermal shock resistance effect of the refractory, and the voids in the breathable refractory generated after the disappearance are large. Become. When a large number of such large voids are distributed in the breathable refractory structure, the bubble diameter that passes through the breathable refractory and blows out from the inner hole surface thereof also increases. In addition, a large reduction in gas back pressure is also observed with this phenomenon.

  There are many patent documents concerning breathable refractories that contain little or no silica as countermeasures.

For example, Patent Document 1 states that “in an alumina graphite immersion nozzle for continuous casting of steel, the inner wall surface below the inner hole body including at least the discharge hole with the SiO2 content of the inner hole body corresponding to the gas blowing portion being 5 wt% or less. , A continuous casting immersion nozzle composed of a refractory having a Na ₂ O content of 1 to 10 wt% and a SiO ₂ content of 5 wt% or less has been proposed. Is said to have been reduced.

  However, in the case of an inner hole body made of a breathable refractory whose SiO2 content is reduced to 5 wt% or less, as the inventor of Patent Document 1 described in Patent Document 2 thereafter, “indispensable for an immersion nozzle” The inventors of the present application have confirmed that there is a drawback that the wear resistance against molten steel is remarkably weakened in addition to “no spalling resistance can be obtained”.

  As a result of this, gas intensively leaks from the through cracks of the breathable refractory destroyed by thermal shock, or local or extensive damage to the breathable refractory during operation. It may also cause thinning of objects, penetration of the gas pool and the inner hole, and the like, and the stable securing of air permeability during operation may be impaired. At that time, it is impossible to control the amount of gas leaked and the bubble diameter.

  Further, Patent Document 2 states that "inner nozzle body that is a gas blowing portion in a gas blowing type alumina graphite continuous casting immersion nozzle containing 5 to 12 wt% of silica and having a silica particle size of 50 µm or less. It has been proposed that a continuous casting nozzle characterized by the formation of the above can be stably blown by using this nozzle. The detailed particle size configuration of 50 μm or less is not described, but 0.02 mm, 0.04 mm, and 0.05 mm are shown as specific examples.

  However, according to the knowledge of the inventor of the present application, the expansion of the bubble diameter is not enough even if 5 to 12 wt% of silica of 50 μm or less, specifically 0.02 mm, 0.04 mm, 0.05 mm is contained. It turns out that it cannot be suppressed. Furthermore, it was found that the wear resistance against molten steel still cannot be improved sufficiently.

  On the other hand, removing or drastically reducing amorphous (fused) silica from a breathable refractory not only reduces the thermal shock resistance of the refractory, but also tends to significantly impair the wear resistance. The present inventors have found out.

  Thus, with respect to breathable refractories, there has been found a means for suppressing the reduction of thermal shock resistance and wear resistance of breathable refractories and preventing the expansion of the bubble diameter over time during the operation of the blown gas. It has not been.

Japanese Patent Laid-Open No. 6-179056 JP-A-4-270040

  The subject of this invention is suppressing the expansion of the bubble diameter with the passage of time during operation of the gas which blows off from a breathable refractory, without reducing the thermal shock resistance and the abrasion resistance with respect to molten steel.

The breathable refractory of the present invention comprises 15% by mass to 30% by mass of free carbon (“free carbon” means carbon other than the compound), and 2% by mass or more of SiO ₂ in particles of 1 μm or less. Less than 12% by mass, Al ₂ O ₃ or an oxide which is a mineral substance composed of Al ₂ O ₃ and SiO ₂ component is 53% by mass to 83% by mass (including impurities inevitable in production), The basic feature is that the particulate SiO 2 of 1 μm or less is dispersed in the carbon structure of the refractory matrix.

  The nozzle for continuous casting in the present invention refers to all nozzles used for discharging molten steel from a molten steel container such as a ladle or tundish in a continuous casting operation.

  Further, in the present invention, the “breathable refractory” means a refractory for allowing gas to pass through the structure of the refractory and blowing out gas from the inner surface of the refractory. “Refractory” may be simply referred to as “refractory”.

  Furthermore, the degree of “breathability” is a design matter according to the necessity in the intended operation, and is not familiar with defining as an absolute value. However, it can be used as one of the basic characteristics for evaluating air permeability and as a relative indicator.

In the present invention, it refers to a refractory having an air permeability of approximately 0.5 Nl · cm / (min · MPa · cm ² ) or more when compressed air is circulated inside a cylindrical sample. Further, a value of about 0.5 Nl · cm / (min · MPa · cm ² ) or more is a level that can be used as a breathable refractory in a general casting nozzle.

  This air permeability can be measured by the following exemplified method. That is, the refractory is a cylindrical sample (φ80-φ60 × L50) and shown in FIG.

  In the image experimental apparatus, the upper and lower sides of the sample are sandwiched using rubber packings, and compressed air is allowed to flow inside the sample.

  This air permeability can be obtained by the following equation.

Here, each symbol means the following.
K: Air permeability (Nl · cm / (min · MPa · cm ² ))
V: Inner hole air flow rate (Nl / min)
ri: inner radius of the sample (cm)
ro: Sample outer radius (cm)
H: Sample height (cm)
ΔP: Back pressure (MPa)

  The breathable refractory of the present invention is particularly effective when disposed in at least part of the inner hole of the continuous casting nozzle.

  It should be noted that the position and range of this arrangement are also matters to be optimized according to individual operating conditions and purposes (for example, whether or not to serve as adhesion prevention as well as gas dispersion in molten steel).

  In the present invention, “free carbon” refers to carbon excluding carbon that forms a compound with another element, such as carbide with silicon or boron. This “free carbon” includes crystalline graphite, amorphous, and a continuous structure that bonds between aggregate particles constituting the matrix of the refractory structure (in the present invention, referred to as matrix carbon). And carbon black.

  As mentioned above, removal of amorphous silica from breathable refractories, or if significantly reduced, not only reduces the thermal shock resistance of the refractories, but also significantly reduces the wear resistance. The refractory of the present invention includes amorphous silica.

  However, as shown in the above-mentioned background art, even if amorphous silica has a size of about 20 μm (a size exceeding 20 μm to 1 μm), the bubble diameter with the passage of time during the operation of the gas to be blown out The expansion of.

  Such continued expansion of the bubble diameter over a long period of operation destabilizes factors that adversely affect the quality of the slab (for example, increased inclusion entrainment due to molten metal level fluctuations, gas non-use). Increasing defect trapping due to dispersion in uniform molten steel). For this reason, even if the physical properties of the refractory change such that the gas outflow state fluctuates in a short time at the beginning of casting, it is required to stabilize the gas outflow state during the subsequent casting time.

  Further, when amorphous silica having such a particle size is used, a decrease in wear resistance cannot be sufficiently suppressed. The present inventors have found that this cause is related to the connective structure of the breathable refractory.

  The carbonaceous structure around the aggregate particles in the matrix part contributing to the bonded structure of breathable refractories has a thickness of about 2 μm, although the thickness is large and small. Covered with connective tissue of a certain thickness. (See Figure 10)

  The connective tissue configured in such a film shape is three-dimensionally arranged to express the strength, wear resistance, and the like of the refractory. Therefore, destroying this film-like connective structure impairs the strength and wear resistance of the refractory.

  On the other hand, the wear resistance of the breathable refractory can be improved by strengthening the connective structure formed in this film shape.

  As in the prior art, when amorphous silica having a size of at least 20 μm is included in the refractory matrix, the carbonaceous connective tissue is divided along with the disappearance and the particle size is reduced. It becomes a fragile connective tissue with many close voids.

On the other hand, the refractory according to the present invention constitutes a matrix portion with amorphous silica as SiO ₂ particles of 1 μm or less in the structure of the refractory before operation, that is, before being passed through molten steel. The state is dispersed in the carbon structure. (See Figure 12)

If SiO ₂ as smaller particles 1 [mu] m, there is no possibility to separate the carbon connective tissue of the minimum thickness of about 2μm existing around the many particles. That is, when the SiO ₂ particle size is approximately about ½ of the thickness of the film-like carbon structure, the film can be prevented from being broken or significantly weakened.

  It can also be dispersed over a wide range of refractory structures, contributing to increasing the homogeneity of refractories.

SiO ₂ as particles of 1 μm or less dispersed in the carbon of the matrix part is reacted by the following formulas 1 and 2 by preheating or passing through molten steel (at a high temperature exceeding about 1200 ° C.). This produces SiC.

  These reactions are accelerated by passing an inert gas.

C + SiO ₂ = CO (g) + SiO (g) ……… Formula 2
SiO (g) + 2C = CO (g) + SiC Equation 3
3C + SiO ₂ = 2CO (g) + SiC Equation 4

As a result, the portion where SiO ₂ exists becomes a void, and SiC is generated around it. The generated silica voids are extremely fine (less than 1 μm) compared to the voids generated by volatilization of the fused silica added as an aggregate having a size of about 0.2 mm, and the matrix carbon structure. It does not become coarse because it exists inside.

  Further, since the pore diameter of the breathable refractory is larger than 1 μm, the pore diameter of the refractory does not increase due to these reactions.

  For this reason, the magnitude | size of the gas discharge | released in molten steel, ie, a bubble diameter, can be made very small, It becomes possible to maintain the magnitude | size of the bubble during operation.

Moreover, SiO ₂ as a particle of 1 μm or less has extremely high reactivity due to its small diameter (proportional to the radius) and is surrounded by carbon, so the contact area with carbon is extremely large. , The reactivity with carbon becomes higher.

  For this reason, in the initial stage of operation (short time after receiving steel), a SiC shell having a fine space smaller than 1 μm in the center is formed. (The structural change is completed in the initial stage after the start of casting. See FIG. 13)

  Since this SiC is excellent in abrasion resistance, it becomes possible to improve the abrasion resistance of the carbon bond structure itself existing around the particles.

For this reason, the size of SiO ₂ present in the refractory according to the present invention is set to 1 μm or less, which is ½ of the thickness of the film-like carbon structure of about 2 μm.

  Due to the above-described action, the breathable refractory of the present invention can suppress the decrease in thermal shock resistance and wear resistance, and can prevent the bubble diameter from expanding over time during the operation of the blown-out gas. In addition, these properties can be maintained almost without deterioration during operation.

As described above, the breathable refractory of the present invention in which SiO ₂ as particles of 1 μm or less is dispersed in the carbon of the matrix has a high reaction rate such as SiC conversion of SiO _2. The accompanying structural changes and changes in ventilation characteristics (aeration volume, back pressure) are completed in the initial stage after the start of casting.

  Therefore, a stable state of gas flow and bubble diameter can be obtained within a short time from the start of casting to the stabilization of operation including other elements. These effects cannot be obtained by a conventional refractory using amorphous silica having a size larger than that of the amorphous silica in the present invention, a refractory having SiC previously present as raw material particles, or the like.

In addition, if SiO ₂ is made SiC in the manufacturing stage, that is, the state of the product to be operated, the refractory structure has a higher elastic modulus, and since SiC has a larger thermal expansion than amorphous SiO ₂ , Incurs high thermal expansion. As a result, such a refractory is easily destroyed by thermal shock at the start of casting.

  Therefore, the breathable refractory of the present invention does not have a structure in which SiC is formed by such a reaction in the product stage before being subjected to operation (casting) in order to avoid such a decrease in thermal shock resistance. , The structure in which the above reaction and structural change occur at an early stage after the start of casting.

As will be described later, when SiC having a larger thermal expansion than amorphous SiO ₂ is added to the refractory as raw material particles in advance for the purpose of preventing oxidation of the refractory, it is about 5% by mass or less. It is preferable to stop.

Furthermore, in the present invention, SiO ₂ with respect to the carbon component of the remaining portion (that is, carbon in a matrix portion excluding graphite particles as an aggregate) excluding the particles of 0.05 mm or more in the free carbon. The mass ratio (hereinafter also simply referred to as “SiO ₂ / carbon ratio”) is preferably in the range of 0.5 to 2.0.

As described above, SiO ₂ as particles of 1 μm or less reacts with carbon in the matrix portion. Therefore, the ratio of SiO ₂ as particles of 1 μm or less and carbon in the matrix portion affects the degree of SiC conversion.

When this SiO ₂ / carbon ratio is less than 0.5, the thermal shock resistance and air permeability characteristics by SiO ₂ that can solve the problems of the present invention can be obtained, but the SiC present in the matrix structure is reduced. Therefore, it is difficult to obtain sufficient wear resistance. When the SiO ₂ / carbon ratio exceeds 2.0, the reaction such as SiC conversion of SiO ₂ cannot be completed only at the initial stage of casting, and the structure change and the change of the air flow characteristics in the subsequent stable casting stage. (Expansion of bubble diameter, reduction of back pressure, etc.) continues and it is difficult to obtain a casting stabilization effect. In this case, depending on the amount of SiO _2, the air permeability tends to be excessive or the refractory structure is likely to be weakened.

In order to obtain higher wear resistance and thermal shock resistance, SiO ₂ as particles of 1 μm or less is preferably as fine as possible in order to obtain a faster reaction.

  In order to discriminate the remainder excluding the particulate matter of 0.05 mm or more, that is, the graphite particles as the aggregate and the carbon of the matrix portion, the test refractory is placed in an oxidizing atmosphere of 350 ° C. or higher and 450 ° C. or lower. The refractory after the heat treatment is classified with a sieve having a gap of 0.05 mm, and the remainder exceeding 0.05 mm can be regarded as “particulate carbon of 0.05 mm or more” in the present invention.

In order to confirm the state in which SiO ₂ as particles of 1 μm or less according to the present invention is dispersed in the carbon of the matrix, the structure of the fracture surface of the breathable refractory before heat treatment at high temperature may be observed.

The remaining oxide aggregate particles refer to a mineral composed of Al ₂ O ₃ or Al ₂ O ₃ and a SiO ₂ component. The remaining oxide aggregate particles are the main components constituting the skeleton of the breathable refractory of the present invention.

An oxide (including impurities inevitable in production) that is a mineral substance composed of Al ₂ O ₃ or Al ₂ O ₃ and a SiO ₂ component is a main component constituting the skeleton of the breathable refractory of the present invention.

As a main component constituting such a skeleton, Al ₂ O ₃ or a mineral composed of Al ₂ O ₃ and a SiO ₂ component is preferable on the basis of the above-described composition ratio of SiO ₂ and carbon.

The mineral composed of Al ₂ O ₃ component refers to corundum (α alumina), and the mineral composed of Al ₂ O ₃ and SiO ₂ component refers to the genus sillimanite, which is mullite in terms of thermal stability including expandability. Is preferred. This is because the mineral consisting of Al 2 _O3 or Al ₂ O ₃ and SiO ₂ component is because has excellent wear resistance and corrosion resistance and thermal shock resistance that is required for continuous casting. Here, a mineral composed of Al ₂ O ₃ or Al ₂ O ₃ and a SiO ₂ component has a thermal expansion property close to corundum up to the molten steel temperature (about 1500 ° C. to 1550 ° C.), and its coefficient of thermal expansion is Means smaller than corundum. However, these are excluded because kyanite has a large thermal expansibility, and clay minerals such as kaolinite produce cristobalite and adversely affect aeration characteristics.

In the case where the oxide aggregate particles which are minerals composed of Al ₂ O ₃ or Al ₂ O ₃ and a SiO ₂ component are less than 53% by mass, carbon, SiO ₂ , carbides or metals to be added are relative to each other. In a large amount. As a result, the wear resistance and the hot air flow characteristics are lowered.

  When the oxide aggregate particles exceed 83% by mass, thermal expansion becomes large and it becomes difficult to maintain thermal shock resistance.

  In addition, the refractory according to the present invention may contain impurities inevitable in production in addition to the above components.

  In addition, the refractory according to the present invention may contain, in addition to the above, one or more of the compounds excluding metals and oxides in a total amount of 1% by mass to 5% by mass.

  The compound excluding the metal and oxide is one or more of metals such as silicon and aluminum, silicon carbide, boron carbide, silicon nitride, boron nitride, zirconium boride, and aluminum nitride. It contributes to enhancing the antioxidant function of the breathable refractory during preheating of the previous immersion nozzle and in use.

  However, compounds other than these metals and oxides are not essential in the present invention. If any of these exceeds 5% by mass, it becomes difficult to ensure the initial strength of the refractory structure, or the initial strength of the refractory structure becomes too high. It is easy to invite a decrease in

According to the present invention,
1. Expansion of the bubble diameter with the passage of time during the operation of blowing out from the breathable refractory can be suppressed.
2. The thermal shock resistance can be ensured, and the destruction of the nozzle and casting trouble using the refractory at the start of casting and the refractory of the present invention can be avoided.
3. Abrasion resistance against molten steel can be improved and secured. As a result, it is possible to suppress thinning due to wear of the breathable refractory, intensive jet of gas from the local area, and expansion of the bubble diameter, and further, the nozzle breakage and casting trouble using the refractory of the present invention can be prevented. Can be avoided.

  By these effects, the casting operation can be stabilized and the quality of the slab can be improved.

It is a figure which shows the image of the experimental apparatus for measuring the air permeability of the breathable refractory used by this invention. It is a figure which shows the image of the experimental apparatus for investigating the relationship between the average pore diameter used by this invention, and a bubble diameter. It is a figure which shows the relationship between an average pore diameter and a bubble diameter. It is a figure which shows the image of the experimental apparatus of a ventilation test (back pressure change investigation etc.). It is a figure which shows the image of the change of the back pressure during a ventilation test. It is a figure which shows the image of the experimental apparatus of a thermal shock resistance. The image which installed the refractory sample (thickness 10mm, vertical direction length 100mm) processed into the ring shape of the abrasion resistance test on the immersion nozzle inner hole upper part used by actual operation is shown. It is an image figure of the immersion nozzle which installed the breathable refractory in the inner hole. It is a figure which shows the image of the change of the back pressure during operation with ventilation | gas_flowing. It is an enlarged photograph which shows the example of the form of the carbon structure of the matrix part of a breathable refractory. It is an enlarged photograph which shows the example of the form of the space | gap produced after silica disappearance after use by casting of a breathable refractory. It is a figure which shows the image when the silica particle of various particle sizes exists in the matrix part carbon layer around the particle | grains before heat processing. It is a figure which shows the image of the form which the space | gap of the silica of various particle sizes disperse | distributed to the matrix part carbon layer and SiC mixed layer around the particle | grains before heat processing. A circle indicates a space surrounded by the SiC shell.

The breathable refractory of the present invention can be manufactured by the following steps.
(1) Kneading the breathable refractory clay, the base material AG clay, and the ZG clay constituting the powder line part (2) 800 ° C. or less on the outer periphery of the molded body of the breathable refractory clay Step (3) of disposing an organic material layer that disappears at a temperature of (3) Filling the outer periphery and other parts of the organic material layer with AG clay and ZG clay, and hydrostatic forming (4) A step of forming a hollow chamber in the portion of the organic layer by reducing and firing at a temperature of 800 ° C. or higher and 1200 ° C. or lower.

The kneading in the step (1) is not limited to a kneading model as long as it is a kneader in which SiO ₂ particles of 1 μm or less are dispersed in a matrix. Phenol resin, tar, pitch, furan resin, epoxy resin, etc. can be used as a carbonaceous additive (hereinafter also referred to as “binder”) that fulfills the product binding function, but it has a high residual carbon ratio. Therefore, phenol resin and pitch are desirable. Further, it is preferable to pre-disperse SiO ₂ particles of 1 μm or less in the binder used for kneading.

  The organic layer disposed in the step (2) can be formed of wax such as paraffin, paper, plastic, or the like.

  As the molding method of step (3), CIP (cold isostatic press) generally used for molding of a casting nozzle can be used.

The purpose of setting the step (4) to a reduction firing temperature of 800 ° C. or more and 1200 ° C. or less is to completely carbonize the binder used for kneading and to prevent the reaction of SiO ₂ particles of 1 μm or less with carbon. is there.

  The basic characteristics of the ventilation function, such as the amount of ventilation, should be set according to the individual operating conditions. This adjustment can be made by optimizing the raw material particle size composition, molding pressure and the like of the soil.

  The characteristics of the breathable refractory of the present invention obtained by the above process were confirmed by the following examples.

[Example A]
Regarding the suppression or prevention of expansion of the bubble size (diameter), which is the first object of the present invention, first, the relationship between the average pore diameter of the refractory and the bubble diameter is investigated by a water model and contrasted with the knowledge of actual operation. Therefore, the target value of the average pore diameter was set.

  An image of an experimental apparatus for investigating the relationship between the average pore diameter and the bubble diameter is shown in FIG. The figure shows an apparatus for flowing 1.0 l / min of compressed air 4 from an air tank 2 to a cylindrical sample 1 with a bottom of φ30-φ10 × L100 through an air flow rate / back pressure measuring device 3.

  Using this apparatus, the diameter of bubbles in water when compressed air 4 of 1.0 l / min was passed through the cylindrical sample 1 with a bottom was measured. Thereafter, the sample for which the measurement of the bubble diameter was completed was cut and processed, and the average pore diameter was measured.

  In addition, the cylindrical sample with a bottom in this Example A is a thing after a 1500 degreeC heating and a hot-air ventilation characteristic test mentioned later. The average pore diameter can be obtained by a method for measuring the pore diameter distribution of a molded article by JIS R 1655 mercury intrusion method.

  The result is shown in FIG. FIG. 3 is a graph showing the relationship between the pore diameter and the underwater bubble diameter of the breathable refractory after heating (after evaluation of hot air permeability characteristics).

  The inventors have empirically found that when the bubble diameter in this experiment is about 1.2 mm or more, bubble defects in steel (slab) are likely to occur. Therefore, in the present invention, the target is a pore size of a refractory that has a bubble diameter of less than about 1.2 mm, that is, about 1.8 μm or less.

[Example B]
In Example B, the size of the SiO ₂ (fused silica) particles in the refractory is the average pore size after heat reception (under a reducing atmosphere at 1500 ° C.), the change in pore diameter before and after heat reception of the refractory, The effect on changes in back pressure was investigated.

Table 1 shows the particle size and components of the provided silica (SiO ₂ particles). Each silica raw material shown in Table 1 is amorphous. A to C are silicas applied to the prior art refractories, and D and E are silicas applied to the refractories of the present invention.

  Table 2 shows the contents and measurement results of each sample containing these silicas.

  As for the average pore diameter, Comparative Examples 1 to 3 using silica of the prior art all exceed 1.8 μm. On the other hand, all of Examples 1 to 3 are below 1.8 μm, and the above-mentioned target can be achieved.

  As a result of investigating the degree of change before and after heating the sample (under a reducing atmosphere at 1500 ° C.), Comparative Example 1 to Comparative Example 3 all showed a large positive value. This indicates an increase in pore diameter and large pores. It shows that the ratio has increased.

  On the other hand, all of the examples showed a negative value. Such a negative value indicates that the average pore diameter of the refractory is reduced, that is, the bubble diameter does not increase. This is because the space generated in the center of the SiC shell where silica was present becomes pores, but the size of the newly generated pores is smaller than the pore size of the refractory that existed from the beginning, Moreover, it is a change in the numerical value resulting from the large number, and at least this does not hinder the expansion of the bubble diameter in operation or the distribution of gas in operation. It has been confirmed by experiments in actual operation that it is not directly connected to back pressure.

  In this example, as the investigation of the change in the aeration characteristics, that is, as the simulation of the degree of reduction in the back pressure of the supply gas during operation (casting), the degree of reduction in the back pressure due to aeration in the molten steel was examined.

  An image of the experimental apparatus used is shown in FIG. In the figure, the sample 1 is arranged in an induction furnace 5.

Here, the change with respect to the base point after 30 minutes and 90 minutes after the immersion time in the molten steel and the gas supply start time point was observed.

  FIG. 5 shows an image of the change in back pressure in time series. In other words, this figure shows the changes in the refractory structure and ventilation characteristics at each stage in the time series of operation (casting).

  That is, from the beginning of gas supply to 30 minutes later, the initial casting stage, from 30 minutes to 90 minutes later the subsequent casting stage (the stage where no change or very little stability is required), and after 90 minutes, the change Can be regarded as a stage where almost disappeared.

  During this stage, the back pressure change rate after 30 minutes (A in the figure) and the back pressure change rate after 90 minutes (B in the figure) were used as reference points for evaluation with respect to the back pressure at the starting point, and B / It is the present invention that A (hereinafter also simply referred to as “B / A ratio”) does not exceed 100, that is, changes in the refractory structure and aeration characteristics are completed in the initial stage, and there is no change thereafter. It was set as the standard from which the effect of was acquired.

  This is because even if there are changes in the refractory structure and ventilation characteristics, if it stops at the initial stage, there are few adverse effects on the operation (such as unstable gas ejection state, large change in bubble diameter, etc.), and the size of the bubbles ( This is because it is considered that an effect of suppressing or preventing the expansion of (diameter) is obtained.

  As for this B / A ratio, the comparative examples 1 to 3 all show values that greatly exceed 100, which means that the structural change of the refractory and the change of the ventilation characteristics cannot be completed only at the initial stage of casting. ing.

  On the other hand, all of Examples 1 to 3 are 100, indicating that the structural change of the refractory and the change of the aeration characteristics are completed only in the initial casting stage.

  In this example, thermal shock resistance and wear resistance were also evaluated by experiments.

  The thermal shock resistance was evaluated by a thermal shock resistance experiment in which a refractory sample was directly heated and rapidly cooled.

  Specifically, a cylindrical sample with a bottom of φ100−φ40 × L80 is immersed in molten steel at 1550 ° C. in the induction furnace shown in FIG. 6, and then heated for 30 minutes, and then cooled until the sample outer surface temperature reaches 1200 ° C. In stages, water cooling was performed from the outer periphery. The state of occurrence of cracks (including those that stopped in the cracks) was observed and evaluated by the presence or absence of cracks.

  This evaluation method includes both thermal shock evaluation during pre-heating of breathable refractories during operation (casting) and thermal shock evaluation at the stage when quality characteristics of the breathable refractories changed during operation. .

  In addition to this thermal shock resistance test, the bending strength S (MPa), elastic modulus E (GPa), and thermal expansion (at 1000 ° C.) α (%) after heat treatment of the sample were measured, and the thermal shock resistance index S / (E · α) was examined as a reference.

  For wear resistance, as shown in FIG. 7, the refractory sample 1 processed into a ring shape having a thickness of 10 mm and a longitudinal length of 100 mm is installed on the inner hole 11 of the immersion nozzle 10 used in actual operation, The wear amount of the sample after being subjected to casting for about 200 minutes was examined. In addition, 12 is a discharge hole of molten steel, 13 shows the highly corrosion-resistant refractory arrange | positioned in the contact part with mold powder.

These results are also shown in Table 2.
Examples 1 to 3 show good results in both thermal shock resistance and wear resistance.
In Comparative Examples 1 to 3, these tests were not carried out (because the average pore diameter has not reached the target value. The same applies to the examples in which the following average pore diameter has not reached the target value).

[Example C]
Example C is an experimental example in which the influence of the content of silica of 1 μm or less was investigated.
The test equipment and method of this example are the same as those of Example B.

Table 3 shows the results.
In Comparative Example 4 in which the content of silica of 1 μm or less was 1% by mass, cracking occurred in the thermal shock resistance test. In Comparative Example 5 in which the content of silica of 1 μm or less is 13% by mass, cracking occurred in the thermal shock resistance test. The former was less susceptible to fracture due to thermal expansion during rapid heating due to its low silica content of 1 μm or less, and the latter was increased in voids after prolonged heating and fragmented / fragile of the thin matrix carbon structure around the particles. This is thought to be due to the occurrence of crystallization.

  On the other hand, the test results of Examples 4 to 7 in which the content of silica of 1 μm or less was 2% by mass to 12% by mass were good.

[Example D]
Example D is the experimental result of investigating the influence of the SiO ₂ / carbon ratio.
The test equipment and method of this example are the same as those of Example B.

Table 4 shows the results.
In Examples 9 to 11 in which the SiO ₂ / carbon ratio is in the range of 0.5 to 2.0, the pore diameter change, the back pressure change, the thermal shock resistance, and the wear resistance are all good.

In Comparative Example 6 in which the SiO ₂ / carbon ratio is 0.4, the pore diameter change, the back pressure change, and the thermal shock resistance are all good results, but the wear resistance is compared with Examples 9 to 11. The result was somewhat inferior.

In Comparative Example 7 in which the content of silica of 1 μm or less is 13% by mass and the SiO ₂ / carbon ratio is 2.2, the average pore diameter does not reach the target value, the degree of change in back pressure is large, and the thermal shock resistance Cracked in the property test. This is probably because SiO2 is unreacted and vaporizes without forming a SiC shell, relatively large pores increase, and the structure becomes weak. On the other hand, when the SiO ₂ / carbon ratio exceeds 2.0, the back pressure after the initial stage tends to decrease greatly. This is thought to be due to the brittleness of the refractory structure due to, for example, excess SiO2 reacting with the graphite particles as the aggregate and the disappearance of the graphite.

[Example E]
Example E is the experimental result of investigating the influence when metal (Si), SiC, B4C, and Si3N4 are present in the refractory from the beginning, that is, when these are included as a refractory raw material.

  Also in this example, the experimental equipment and method are the same as in Example B.

Table 5 shows the contents and measurement results of each sample.
If these amounts are each alone or 5% by mass or less in total, the pore diameter change, back pressure change, thermal shock resistance, and wear resistance are all good. However, when both exceed 5 mass%, wear resistance and thermal shock resistance are deteriorated.

In the case of SiC, weakening of the structure, and in the case of metal (Si), B ₄ C, and Si ₃ N ₄ , the elastic modulus is excessively increased, and the physical properties of the breathable refractory are balanced. The cause is thought to have been broken.

[Example F]
Example F, as an example of the oxide which is a main skeleton portion of the refractory, corundum as Al2O3 component, when using mullite as Al ₂ O ₃ and SiO ₂ component, and a free in a system using the oxide It is the experimental result which investigated the influence of the amount of carbon.
Table 6 shows the contents of each sample and the results of measurement.

As Al ₂ O ₃ component corundum, none of the case of using mullite as _Al 2 _{O 3} and SiO ₂ components, free carbon amount of any the of 15% by weight to 30% by weight Example 19 Example 23 The result is also good.

  On the other hand, in Comparative Examples 12 and 14 in which the amount of free carbon was less than 15% and the oxide particles exceeded 83% by mass, and in Comparative Example 13 in which only the amount of free carbon was less than 15%, the thermal shock resistance was The decline in sex is remarkable. In Comparative Example 15 in which the amount of free carbon exceeds 30% by mass and the oxide particles are less than 53% by mass, the wear resistance tends to decrease. As described above, it is considered that the balance of physical properties as a breathable refractory is broken depending on the amounts of free carbon and main oxide aggregate particles.

[Example G]
Example G shows the result of subjecting a conventional technique and an immersion nozzle to which the refractory of the present invention is applied to actual operation.
The refractory samples were Comparative Example 16 in which a prior art refractory using silica A as SiO ₂ was applied, Comparative Example 17 in which a prior art refractory not containing SiO ₂ was applied, and Example 24 of the present invention. did.

  An image of the test immersion nozzle is shown in FIG. In addition, 11 shows an inner hole, 12 shows the discharge hole of molten steel, 13 shows the highly corrosion-resistant refractory arrange | positioned in the contact part with mold powder. Further, 14 indicates an argon gas inlet, and 15 indicates a gas pool. And the breathable refractory 16 for a test is arrange | positioned over substantially the full length of an inner-hole inner surface.

  FIG. 9 shows a change in the back pressure of the blown argon gas with the lapse of casting time. In addition, about the back pressure fall rate, the value of 50 minutes after the casting start (C) and 200 minutes after (D) was investigated.

  Table 7 shows the contents of the refractories and the measurement results of each sample.

  As a result, in Example 25, D / C is 100, that is, does not change, and it can be seen that the improvement effect in this respect is great and the wear resistance is also good.

  On the other hand, it can be seen that Comparative Example 14 has a very large D / C, that is, there is a large problem in the change in the bubble diameter and the resulting stability during operation.

  In Comparative Example 15, D / C is 100, i.e., does not change, and it can be seen that the improvement effect in this respect is great. However, damage to the inner hole due to wear is significant. In this experiment, the change in the back pressure at the end of casting was small, but it is considered that a stable back pressure cannot be maintained for a longer period of use that is considered to be affected by thinning.

DESCRIPTION OF SYMBOLS 1 Cylindrical sample with bottom 2 Air tank 3 Ventilation amount and back pressure measuring device 4 Compressed air 10 Immersion nozzle 11 Inner hole 12 Molten steel discharge hole 13 High corrosion resistance refractory 14 Inert gas inlet 15 Gas pool 16 Breathable refractory

Claims (4)

Free carbon (“free carbon” means carbon other than the compound) is 15% by mass or more and 30% by mass or less, and SiO ₂ of particles of 1 μm or less is 2% by mass or more and less than 12% by mass, Al ₂ O ₃ or _Al 2 _{O 3} and made of SiO ₂ oxide is a mineral consisting of components 53 mass% or more 83 wt% or less (including impurities manufacturing inevitable), SiO ₂ of the above 1μm or less of the particulate is, A breathable refractory characterized by being dispersed in the carbon structure of the refractory matrix. _2. The breathable refractory according to claim 1, wherein a mass ratio (SiO ₂ / carbon ratio) of SiO ₂ of the particles of 1 μm or less to carbon of the matrix is in a range of 0.5 or more and 2.0 or less.   The breathable refractory according to claim 1 or 2, comprising a total of 1% by mass or more and 5% by mass or less of any one of a metal and a compound excluding an oxide.   A continuous casting nozzle in which the breathable refractory according to any one of claims 1 to 3 is disposed in at least a part of an inner hole of the continuous casting nozzle.