Continuous casting method of steel

Abstract

To provide a method for continuously casting a steel capable of suppressing an inclusion from adhering to an inner surface of a nozzle flow channel.SOLUTION: This invention relates to a method for continuously casting a steel in which a nozzle is used to pour molten steel from a tundish into a casting mold, wherein: the nozzle includes an upper nozzle, a sliding gate, and an immersion nozzle; the sliding gate is positioned between the upper nozzle and the immersion nozzle; an Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, a dew point of -65°C or less, and a nitrogen concentration of 10 ppm or less is caused to flow into a flow channel of the immersion nozzle; the flow rate of molten steel poured into the mold is adjusted using the sliding gate; and an inert gas is caused to flow into the outer peripheral part of the sliding surface of the sliding gate to reduce the oxygen concentration in the outer peripheral part to 5% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for continuous casting of steel using a nozzle for injecting molten steel from a tundish into a mold, and more particularly to a method for continuously casting a molten metal such as aluminum-killed steel in which the immersion nozzle is likely to be blocked by high melting point non-metallic inclusions.

Background Art

[0002] In the continuous casting of steel, the blockage of the inner surface of the immersion nozzle due to the adhesion of high melting point non-metallic inclusions typified by alumina is a problem that has a great influence on the operation and the quality of the cast slab. Conventionally, various countermeasure techniques have been disclosed for preventing the blockage of the immersion nozzle.

[0003] For example, Patent Document 1 discloses a spinel-periclase-graphite-based refractory that forms a dense inner surface by a chemical reaction at a high temperature during casting. Further, Patent Document 2 discloses a magnesia-graphite or spinel-graphite-based refractory that contains low melting point beryl and forms a semi-molten layer on the inner surface.

[0004] On the other hand, the present inventors have disclosed in Patent Document 3 an invention in which the adhesion of alumina inclusions is prevented by containing a small amount of CaO or the like in alumina graphite, and the effect is further improved by using energization in combination. The present inventors have further disclosed in Patent Document 4 an invention in which an alternating current pulse-like current is passed through the immersion nozzle to prevent the adhesion of non-metallic inclusions. Patent Documents 5 to 7 disclose a method for optimizing the method of blowing an inert gas into the immersion nozzle to achieve both prevention of blockage of the immersion nozzle and prevention of bubble defects.

[0005] It has been confirmed that these methods each exhibit a certain effect. However, there are also steel types such as REM (rare earth element)-added steel in which the blockage of the immersion nozzle by non-metallic inclusions cannot be sufficiently prevented by the conventional technology.

[0006] In addition, as a result of repeated research and development on factors affecting the clogging of the immersion nozzle, the inventors have found that, as disclosed in Patent Document 8, by increasing the purity of the Ar gas blown into the immersion nozzle to be higher than normal, the adhesion of high melting point inclusions such as alumina into the immersion nozzle can be effectively suppressed.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Summary of the Invention

Problems to be Solved by the Invention

[0008] As in the invention described in Patent Document 8, by increasing the purity of the Ar gas blown into the immersion nozzle to be higher than normal, the adhesion of high melting point inclusions such as alumina into the immersion nozzle can be effectively suppressed. However, there is still room for improvement in order to stabilize the effect of suppressing the adhesion of inclusions to the inner surface of the immersion nozzle.

[0009] Therefore, an object of the present invention is to provide a method for continuous casting of steel that can suppress the adhesion of inclusions to the inner surface of the nozzle flow path.

Means for Solving the Problems

[0010] In order to solve the above problems, as a result of intensive studies by the present inventors, it has been found that in the invention described in Patent Document 8, outside air is sucked from gaps such as joints existing on the inner surface of the nozzle flow path, thereby reducing the purity of the Ar gas blown onto the inner surface of the flow path. As a result, there is a problem that the effect of suppressing the adhesion of inclusions to the inner surface of the nozzle flow path becomes unstable. Therefore, in order to solve the problem, the present inventors further found that the effect of suppressing the adhesion of inclusions to the inner surface of the nozzle flow path (nozzle clogging suppression effect) can be improved by suppressing the suction of outside air to the inner surface of the nozzle. Based on this finding, the present invention has been completed.

[0011] That is, a first aspect of the present invention for solving the above problems is a continuous casting method of steel using a nozzle for injecting molten steel from a tundish into a mold, wherein the nozzle includes an upper nozzle and a submerged nozzle, and a stopper movable in the vertical direction is disposed above the upper nozzle in the tundish. An Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, a dew point of -65°C or less, and a nitrogen concentration of 10 ppm or less is flowed through the flow path of the submerged nozzle, and the flow rate of the molten steel injected into the mold is adjusted by adjusting the gap between the upper nozzle and the stopper. It is a continuous casting method of steel.

[0012] In the first aspect, it is preferable that the nozzle has no step at the joint on the inner surface of the flow path, or the nozzle is an integral type submerged nozzle having no joint on the inner surface of the flow path.

[0013] Further, a second aspect of the present invention for solving the above problems is a continuous casting method of steel using a nozzle for injecting molten steel from a tundish into a mold, the nozzle including an upper nozzle, a sliding gate, and a submerged nozzle, the sliding gate being disposed between the upper nozzle and the submerged nozzle, flowing Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, a dew point of -65°C or less, and a nitrogen concentration of 10 ppm or less in the flow path of the submerged nozzle, adjusting the flow rate of the molten steel injected into the mold using the sliding gate, and flowing an inert gas on the outer peripheral portion of the sliding surface of the sliding gate to make the oxygen concentration of the outer peripheral portion 5% or less. A continuous casting method of steel is characterized by this.

[0014] Furthermore, a third aspect of the present invention for solving the above problems is the first aspect or the second aspect described above, in which a zirconia graphite-based refractory containing 50 to 85 mass% of ZrO2, 1 to 25 mass% of CaO, and 8 to 40 mass% of C as a chemical composition is disposed on at least the inner surface of the flow path of the submerged nozzle among the inner surfaces of the flow paths of the nozzle, one electrode is connected to the nozzle, and the other electrode is immersed in the molten steel in the tundish to form an energization circuit between the nozzle and the molten steel passing through the inside thereof, and the absolute value of the average current density in the nozzle is 1 to 50 mA / cm 2 A continuous casting method of steel is characterized by casting while energizing so that the polarity of the nozzle becomes positive.

[0015] In the third aspect, energization is performed with an alternating current pulse-shaped current waveform in which the polarity of the nozzle alternates between positive and negative, the period of the current waveform is 0.5 ms to 20 ms, and the average current density × energization time during the period when the polarity of the nozzle is positive is larger than the average current density × energization time during the period when the polarity of the nozzle is negative. It is preferable to cast while energizing so that the polarity of the nozzle becomes positive.

Effects of the Invention

[0016] According to the present invention, it is possible to provide a continuous casting method of steel that can suppress the adhesion of inclusions to the inner surface of the flow path of the nozzle.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0018] The continuous casting method of steel of the present invention will be described below using each aspect.

[0019] [First Aspect] The first aspect of the present invention is a continuous casting method of steel using a nozzle for injecting molten steel from a tundish to a mold, the nozzle including an upper nozzle and a submerged nozzle, a stopper movable in the vertical direction being disposed above the upper nozzle in the tundish, Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, a dew point of -65°C or less, and a nitrogen concentration of 10 ppm or less being flowed through the flow path of the submerged nozzle, and the flow rate of the molten steel injected into the mold being adjusted by adjusting the gap between the upper nozzle and the stopper.

[0020] The first aspect is a form in which high-purity Ar gas is flowed through the flow path of the immersion nozzle, and a stopper is used to adjust the flow rate of the molten steel supplied from the tundish. The second aspect described later is a form in which a sliding gate is used for flow rate adjustment, and this is superior in adjusting the flow rate of the molten steel. However, when a sliding gate is used for adjusting the flow rate of molten steel, outside air is sucked in from the joints between the upper nozzle and the sliding gate, the joints between the sliding surfaces of the sliding gate, the joints between the sliding gate and the immersion nozzle, etc., and it has been confirmed that the purity of the Ar gas blown onto the inner surface of the nozzle flow path is lowered by the outside air. In particular, it has been found that outside air suction from the joints is significantly generated when there is a change in the flow path cross-sectional area at these joints. From that viewpoint, the sliding gate can be said to be a major outside air suction location. Therefore, in the first aspect, instead of using a sliding gate for flow rate adjustment, the flow rate is adjusted using a stopper, thereby suppressing the suction of outside air into the inner surface of the flow path.

[0021] Hereinafter, the first aspect will be described in more detail. The description of the first aspect will mainly be made with reference to FIG. 1, but FIGS. 2 and 3 will be referred to as appropriate. Although the size of the nozzle is shown in the figure, this is a numerical value (unit is "mm") for explaining the size of the nozzle in the examples described later, and is merely an example. Therefore, the size of the nozzle is not limited thereto.

[0022] <Nozzle> FIG. 1 shows a nozzle 100 which is an example of a nozzle that can be used in the first aspect. As shown in FIG. 1, the nozzle 100 includes an upper nozzle 110 and an immersion nozzle 120. The upper nozzle 110 is a part that receives the molten steel supplied from the tundish, and the flow path cross-sectional area (or flow path diameter) on the upper side (tundish side) in the height direction of the upper nozzle 110 is formed to be larger than the flow path cross-sectional area on the lower side (mold side). The immersion nozzle 120 is a part that injects the molten steel supplied to the upper nozzle 110 into the mold, and is arranged so as to be immersed in the molten steel in the mold. Further, a hole 120a (nozzle discharge hole) for injecting molten steel is provided at the lower part (mold side) of the immersion nozzle 120.

[0023] As shown in FIG. 1, the nozzle 100 may include a collector nozzle 130 between the upper nozzle 110 and the immersion nozzle 120. The collector nozzle 130 serves to connect the upper nozzle 110 and the immersion nozzle 120.

[0024] Generally, like the nozzle 100, the step at the joint of each part is provided in the direction in which the flow path expands in the direction of the molten steel flow. This is to prevent the molten steel from leaking from the joint. However, as described above, there is a risk that the purity of the blown Ar gas may decrease due to the suction of outside air from the joint on the inner surface of the nozzle flow path.

[0025] Therefore, in the first aspect, it is preferable to use a nozzle having no step at the joint on the inner surface of the flow path. FIG. 2 shows a nozzle 200 which is an example of a nozzle having no step at the joint on the inner surface of the flow path. By adopting a configuration without a step at the joint on the inner surface of the flow path like the nozzle 200, it is possible to suppress the suction of outside air from the joint on the inner surface of the nozzle flow path.

[0026] Here, "having no step at the joint on the inner surface of the flow path" does not mean that there is completely no step at the joint on the inner surface of the flow path, that is, there is no difference at all in the diameter (flow path cross-sectional area) of the inner surface of the flow path between the portions facing the joint. Instead, manufacturing errors are tolerated. For example, if the step at the joint has an error within 1 mm (within 2 mm in diameter), it can be said that there is no step at the joint.

[0027] Further, in order to eliminate the suction of outside air from the joint on the inner surface of the nozzle flow path, in the first aspect, it is more preferable to use, as the nozzle, an integral immersion nozzle having no joint on the inner surface of the flow path. FIG. 3 shows a nozzle 300 which is an example of an integral immersion nozzle. As shown in FIG. 3, by using the nozzle 300 in which the portion functioning as the upper nozzle and the portion functioning as the immersion nozzle are integrated, there is no joint on the inner surface of the nozzle flow path, so that the suction of outside air from the joint can be completely eliminated.

[0028] Here, the refractory placed on the inner surface of the flow path of the nozzle 100 will be described. Regarding the position where the refractory is arranged on the inner surface of the flow path of the nozzle 100, there is no particular limitation, and it may be arranged on at least a part of the inner surface of the nozzle, but preferably it is arranged on the entire inner surface. This is usually because all of the inner surface of the nozzle 100 can come into contact with the molten steel. The refractory is not particularly limited, but an alumina graphite-based refractory (AG) or a zirconia graphite-based refractory (ZG) described later can be arranged. Preferably, it is a zirconia graphite-based refractory.

[0029] <Stopper> In the first aspect, a stopper 150 that can move vertically is arranged above the upper nozzle 110 in the tundish. The stopper 150 is a member for adjusting the flow rate of the molten steel supplied from the tundish to the nozzle. As shown in FIG. 1, by moving the stopper vertically, the gap between the stopper 150 and the upper nozzle 110 is adjusted, and the flow rate of the molten steel injected from the tundish into the mold is adjusted. In the first aspect, as described above, by using the stopper 150 for flow rate adjustment, compared with the case of using a sliding gate, the flow rate of the molten steel can be adjusted while suppressing the suction of outside air. As such a stopper 150, a known stopper can be used.

[0030] <Blowing of high-purity Ar gas> During normal continuous casting of steel, Ar gas flowing in the immersion nozzle is of a purity of about 99.9%. This is because from the perspective of preventing contamination of the molten steel by impurity components such as oxygen in the Ar gas, a purity of 99.9% is considered to be sufficiently high.

[0031] On the other hand, in the first aspect, Ar gas with a further increased purity is flowed. That is, in the first aspect, Ar gas (high-purity Ar gas) with a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, a dew point of -65°C or less, and a nitrogen concentration of 10 ppm or less is flowed through the flow path of the immersion nozzle 120. Thereby, the adhesion of inclusions to the inner surface of the flow path of the nozzle can be suppressed.

[0032] The inventors presume that such an effect is due to an increase in the surface tension of the molten steel (the interfacial tension between the Ar gas phase and the molten steel). As a result of a detailed investigation of the inner surface of the nozzle after casting, since an Ar gas film was stably formed between the molten steel and the inner surface of the immersion nozzle 120 by flowing high-purity Ar gas into the immersion nozzle 120, it is considered that the opportunity for the molten steel to contact the inner surface of the immersion nozzle 120 has decreased.

[0033] When the purity of the Ar gas is lower than the above-specified value, the oxygen concentration is higher than the above-specified value, the dew point is higher than the above-specified value, or the nitrogen concentration is higher than the above-specified value, the effect of preventing the adhesion of non-metallic inclusions by the Ar gas decreases. This is to lower the surface tension of the molten steel.

[0034] Here, the purity of the Ar gas used in the first aspect is preferably 99.999% or more. The oxygen concentration of the Ar gas is preferably 1 ppm or less. The dew point of the Ar gas is preferably -70°C or lower. The nitrogen concentration of the Ar gas is preferably 5 ppm or less. Regarding the surface tension of the molten steel, the influence of oxygen and moisture is large, so in the first aspect, the regulations on the concentration of these impurities are strict values. On the other hand, the influence of nitrogen on the surface tension of the molten steel is small compared to the influence of oxygen and moisture, so the regulations on the concentration of nitrogen are looser than the regulations on oxygen and the like. However, the influence of nitrogen cannot be completely ignored.

[0035] In the first aspect, high-purity Ar gas is flowed into the flow path of the immersion nozzle 120. The reason for blowing Ar gas into the flow path of the immersion nozzle 120 is that the immersion nozzle 120 has a larger contact area with the molten steel than other parts and a high frequency of adhesion of high-melting-point inclusions such as alumina. The method of flowing high-purity Ar gas into the flow path of the immersion nozzle 120 includes, for example, a method of directly flowing high-purity Ar gas into the flow path of the immersion nozzle 120, a method of flowing high-purity Ar gas into the flow path of the upper nozzle 110 and indirectly flowing high-purity Ar gas into the flow path of the immersion nozzle 120, or a method combining these. In FIG. 1, a method of flowing high-purity Ar gas into the flow paths of both the upper nozzle 110 and the immersion nozzle 120, which is a preferred form, is adopted.

[0036] As a method of directly flowing high-purity Ar gas into the flow path of the immersion nozzle 120, for example, as shown in FIG. 1, high-purity Ar gas is supplied from the Ar injection plug 121 to the Ar gas injection part 123 through the slit 122 (gas storage space, for example, with a thickness of 2 mm), and the method of directly blowing high-purity Ar gas into the flow path of the immersion nozzle 120 can be mentioned. Then, the high-purity Ar gas supplied into the flow path rides on the molten steel flow and is injected into the mold together with the molten steel.

[0037] The configuration of the Ar gas injection part 123 is not particularly limited as long as it can supply Ar gas into the immersion nozzle 120. In FIG. 1, a porous refractory is used for the Ar gas injection part 123, and Ar gas is infiltrated from the gaps of the porous refractory to blow Ar gas into the immersion nozzle 120. As the porous refractory, for example, alumina graphite or zirconia graphite with an increased porosity can be adopted. Also, the position where the Ar gas injection part 123 is arranged is not particularly limited. This is because the effect of flowing high-purity Ar gas can be enjoyed regardless of the position in the flow path of the immersion nozzle 120. Further, it is preferable that the Ar gas injection part 123 is provided over the entire circumferential direction of the immersion nozzle 120 as shown in FIG. 1.

[0038] Also, as a method of flowing high-purity Ar gas into the flow path of the upper nozzle 110 and indirectly flowing high-purity Ar gas into the flow path of the immersion nozzle 120, for example, as shown in FIG. 1, high-purity Ar gas is supplied from the Ar injection plug 111 to the Ar gas injection part 113 through the slit 112 (gas storage space, thickness 3 mm), and by blowing high-purity Ar gas into the flow path of the upper nozzle 110, a method of indirectly flowing high-purity Ar gas into the flow path of the immersion nozzle 120 can be cited.

[0039] The configuration of the Ar gas injection part 113 is not particularly limited, and the same configuration as that of the Ar gas injection part 123 can be adopted. Also, the position where the Ar gas injection part 113 is arranged is not particularly limited. This is because, no matter at which position in the flow path of the upper nozzle 110 it is arranged, the effect of flowing high-purity Ar gas can be enjoyed.

[0040] The amount of high-purity Ar gas blown from the Ar gas injection part 123 of the immersion nozzle 120 into the flow path of the nozzle 100 is not particularly limited, but it is preferably 1 NL / min or more and 5 NL / min or less. The amount of high-purity Ar gas blown from the Ar gas injection part 113 of the upper nozzle 110 into the flow path of the nozzle 100 is also not particularly limited, but it is preferably 2 NL / min or more and 50 NL / min or less. However, when flowing high-purity Ar gas into the flow paths of both the upper nozzle 110 and the immersion nozzle 120, the amount of high-purity Ar gas blown from the Ar gas injection part 113 is preferably 2 to 10 times the amount of high-purity Ar gas blown from the Ar gas injection part 123. This is because a part of the high-purity Ar gas blown from the Ar gas injection part 113 of the upper nozzle 110 into the flow path of the nozzle 100 floats in the tundish, and usually, a larger amount of gas is required than the amount of high-purity Ar gas blown from the Ar gas injection part 123 of the immersion nozzle 120.

[0041] When flowing high-purity Ar gas in the flow paths of both the upper nozzle 110 and the immersion nozzle 120, more preferably, the amount of high-purity Ar gas supplied from the Ar gas injection section 113 of the upper nozzle 110 is q1, the amount of high-purity Ar gas supplied from the Ar gas injection section 123 of the immersion nozzle 120 is q2, and when the total area of the inner surface of the nozzle 100 is A, it is required that {(q1 / 5)+q2} / A be between 3 and 25. When {(q1 / 5)+q2} / A is less than 3, it is difficult to exert the inclusion adhesion suppression effect, and when it is greater than 25, the supply of high-purity Ar gas is excessive, and there is a risk that bubble-like defects will occur in the cast slab to be cast. More specifically, it can be carried out by referring to Patent Document 8.

[0042] [Second Aspect] The second aspect of the present invention is a method for continuous casting of steel using a nozzle for injecting molten steel from a tundish into a mold. The nozzle includes an upper nozzle, a sliding gate, and an immersion nozzle. The sliding gate is disposed between the upper nozzle and the immersion nozzle. An Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, a dew point of -65°C or less, and a nitrogen concentration of 10 ppm or less is flowed in the flow path of the immersion nozzle. The flow rate of the molten steel injected into the mold is adjusted using the sliding gate, and an inert gas is flowed through the outer peripheral portion of the sliding surface of the sliding gate to make the oxygen concentration of the outer peripheral portion 5% or less. This is a method for continuous casting of steel.

[0043] Unlike the first aspect, the second aspect is a form in which the flow rate of molten steel is adjusted by a sliding gate instead of a stopper. As described above, the sliding gate has better accuracy in adjusting the flow rate than the stopper. On the other hand, the sliding gate is likely to suck outside air from the sliding surface and the joint of the sliding surface. Therefore, in the second aspect, an inert gas is flowed through the outer peripheral portion of the sliding surface of the sliding gate to make the oxygen concentration of the outer peripheral portion of the sliding surface of the sliding gate 5% or less, thereby suppressing the influence caused by the suction of outside air.

[0044] Hereinafter, the second aspect will be described in more detail. The description of the second aspect will mainly be made with reference to FIG. 4. In the description of the second embodiment, the description of the parts overlapping with the first embodiment will be omitted. Also, although the size of the nozzle is shown in the figure, this is a numerical value (unit: "mm") for explaining the size of the nozzle in the examples described later, and is merely an example. Therefore, the size of the nozzle is not limited thereto.

[0045] <Nozzle> FIG. 4 shows an example of a nozzle 400 that can be used in the second aspect. The left side of FIG. 4 is a cross-sectional view of the nozzle 400, and the right side is a view of the sliding gate 430 observed from the right side of the paper surface of the left side figure. The straight line l is the center line of the nozzle 400.

[0046] As shown in FIG. 4, the nozzle 400 includes an upper nozzle 410, a sliding gate 430, and a dipping nozzle 420. Since the upper nozzle 410 and the dipping nozzle 420 have the same functions as those in the first aspect, the description thereof will be omitted here.

[0047] The sliding gate 430 is a member for adjusting the flow rate of the molten steel supplied to the dipping nozzle 420. The sliding gate 430 has a three-layer structure as shown in FIG. 4, and includes an upper solid plate 431, a lower fixed plate 432, and an intermediate plate 433 between them. The intermediate plate 433 is disposed slidably between them. The intermediate plate 433 is also called a slide plate, and the structure is such that the flow path is narrowed by the sliding of the intermediate plate 433. Due to this structure, a step is generated on the sliding surface during operation, and outside air is easily sucked from the step.

[0048] <Blowing of inert gas> Therefore, in the second aspect, an inert gas is flowed through the outer peripheral portion of the sliding surface of the sliding gate 430 to make the oxygen concentration of the outer peripheral portion 5% or less. Thereby, even in the nozzle 400 using the sliding gate 430, it is possible to suppress the adverse influence due to the suction of outside air from the sliding surface.

[0049] Here, the "sliding surface of the sliding gate 430" is a concept that includes the sliding surface (sliding surface A) between the upper solid plate 431 and the intermediate plate 433, and the sliding surface (sliding surface B) between the lower fixed plate 432 and the intermediate plate 433. The "outer peripheral portion of the sliding surface" is a predetermined range outside the outer periphery of the sliding surface, specifically, a position within 5 mm from the outer periphery of the sliding surface. The "inert gas" is Ar gas, nitrogen gas, or a mixed gas thereof. The Ar gas or nitrogen gas may be a gas of generally used purity. Preferably, it is Ar gas.

[0050] Since the structures for flowing the inert gas to the outer peripheral portions of the sliding surfaces A and B of the sliding gate 430 are the same, here, the structure for flowing the inert gas to the outer peripheral portion of the sliding surface B will be described with reference to FIG. 5.

[0051] FIG. 5 is a three-view drawing of the lower fixed plate 432. The center is a plan view of the lower fixed plate 432, the left is a left side view of the lower fixed plate 432, and the bottom is a front view of the lower fixed plate 432.

[0052] As shown in FIG. 5, the lower fixed plate 432 is fixed inside the lower fixed plate fixture 432a, and a plurality of gas injection holes 432b (gas injection holes 431b in the upper fixed plate fixture 431a) for injecting an inert gas are arranged in the lower fixed plate fixture 432a so as to surround the outer periphery of the lower fixed plate 432. In FIG. 5, 48 inert gas injection holes 432b having a diameter of φ1.5 mm are arranged so as to surround the outer periphery of the lower fixed plate 432. The specific positions of the gas injection holes 432b are not particularly limited, and the gas injection holes 432b may be arranged so that the oxygen concentration in the outer peripheral portion of the lower fixed plate (sliding surface B) can be made 5% or less. Therefore, it is not always necessary to arrange the gas injection holes 432b at the outer peripheral portion of the lower fixed plate (sliding surface B).

[0053] The gas injection hole 432b supplies an inert gas to the outer periphery of the sliding surface through a gas channel 432c provided inside the lower fixing metal fitting 432a. The flow rate of the inert gas is not particularly limited, but it is preferably 10 to 30 Nl / min. The upper fixing plate 431 and the like also have a similar configuration.

[0054] FIG. 6 shows a cross-sectional view of the sliding gate 430 superimposed with each solid metal fitting (upper solid plate fixing metal fitting 431a, lower solid plate fixing metal fitting 432a, intermediate plate fixing metal fitting 433a). Note that the intermediate plate 433 is not completely fixed to the intermediate plate fixing metal fitting 433a, but is fixed so as to be slidable in the sliding direction. As shown in FIG. 6, between the upper solid plate fixing metal fitting 431a and the intermediate plate fixing metal fitting 433a, and between the lower solid plate fixing metal fitting 432a and the intermediate plate fixing metal fitting 433a, there are spaces (space C) into which an inert gas can be blown from the gas injection holes, respectively. The space C is arranged so as to include the outer periphery of the sliding surface. By arranging a seal member D such as refractory wool at a portion connecting the space C and the outside, the space C is kept in a loosely sealed state. Then, by blowing an inert gas into such a space C, the inside of the space C can always be maintained at a positive pressure, so that the intrusion of outside air into the space C is suppressed, and thereby the oxygen concentration at the outer periphery of the sliding surface of the sliding gate 430 can be made 5% or less, and the suction of outside air from the sliding surface can be effectively suppressed.

[0055] Here, the "oxygen concentration" can be calculated from the average value of the oxygen concentrations of the air sampled from, for example, any three locations on the outer periphery of the sliding surface in the space C.

[0056] <Injection of High-Purity Ar Gas> In the second aspect, a high-purity Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, a dew point of -65°C or less, and a nitrogen concentration of 10 ppm or less is flowed through the flow path of the immersion nozzle 420. A method of flowing high-purity Ar gas into the flow path of the immersion nozzle 420 includes, for example, a method of directly flowing high-purity Ar gas into the flow path of the immersion nozzle 420, a method of flowing high-purity Ar gas from the upper fixing plate 431 of the sliding gate 430 and indirectly flowing high-purity Ar gas into the flow path of the immersion nozzle 420, or a method combining these. In FIG. 4, a method of flowing high-purity Ar gas into the flow paths of both the upper fixing plate 431 and the immersion nozzle 420, which is a preferred form, is adopted.

[0057] The method of directly flowing high-purity Ar gas into the flow path of the immersion nozzle 420 is the same as the first aspect. That is, high-purity Ar gas is supplied from the Ar injection plug 421 to the immersion nozzle 420. Also, the preferred embodiment of the high-purity Ar gas is the same as the first aspect.

[0058] In addition, as a method of flowing high-purity Ar gas from the upper fixing plate 431 of the sliding gate 430 and indirectly flowing high-purity Ar gas into the flow path of the immersion nozzle 420, for example, as shown in FIG. 4, Ar gas is blown from the Ar injection plug 431p, passes through the slit 431q (gas storage space, thickness 7 mm), and high-purity Ar gas is blown into the flow path of the nozzle 400 through the through-hole 431r formed in the inner surface of the upper solid plate 431, thereby indirectly flowing high-purity Ar gas into the flow path of the immersion nozzle 420.

[0059] The configuration of the through-hole 431r is not particularly limited as long as it allows Ar gas to flow through the nozzle 400. Preferably, 10 to 15 through-holes are provided in 1 to 3 stages in the height direction in the circumferential direction of the inner surface of the upper solid plate 431. In FIG. 4, 12 through-holes are provided in 2 stages in the circumferential direction.

[0060] The flow rate of the Ar gas in the second aspect may be the same as that in the first aspect. Here, the flow rate of the Ar gas blown from the upper solid plate 431 in the second aspect can adopt the flow rate of the Ar gas blown from the upper nozzle in the first aspect.

[0061] [Third Aspect] The third aspect of the present invention is the first aspect or the second aspect, further comprising disposing a zirconia graphite-based refractory containing 50 to 85 mass% of ZrO2, 1 to 25 mass% of CaO, and 8 to 40 mass% of C as a chemical composition on at least the inner surface of the flow path of the nozzle, connecting one electrode to the nozzle, and dipping the other electrode into the molten steel in the tundish to form an energized circuit between the nozzle and the molten steel passing through the inside thereof, and setting the absolute value of the average current density in the nozzle to 1 to 50 mA / cm 2 and performing continuous casting while energizing so that the polarity of the nozzle becomes positive. This is a method for continuous casting of steel.

[0062] As described above, in the third aspect, in addition to the configuration of the first aspect or the second aspect, continuous casting is further performed while energizing between the nozzle and the molten steel.

[0063] The molten steel flowing in the nozzle is exposed to contamination such as oxides that deviate due to the erosion of the refractory on the inner surface of the flow path, outside air that penetrates the refractory and reaches the inner surface of the nozzle, and the oxygen potential that increases due to the temperature drop near the refractory wall surface. These contaminations act in a direction that impairs the effect of the blown high-purity Ar gas because they reduce the surface tension of the molten steel. Therefore, in the third aspect, a zirconia graphite-based refractory having oxygen ion permeability is disposed on the inner surface of the immersion nozzle, and by energizing so that the polarity of the nozzle becomes positive, oxygen ions are transferred from the inner surface of the nozzle to the outer surface to reduce the above-mentioned contamination.

[0064] <Zirconia graphite-based refractory> In the third aspect, a zirconia graphite-based refractory is disposed on at least the inner surface of the flow path of the nozzle, i.e., the inner surface of the immersion nozzle. Conventionally, it was considered that the coexistence with conductive carbon such as graphite was not possible to exhibit the deoxidation effect of zirconia. However, according to the research of the present inventors, it has been confirmed that decarburization occurs on the operating surface of the refractory in contact with the molten steel, and zirconia acts as a single substance to exhibit a deoxidation effect.

[0065] The zirconia graphite refractory is suitable for at least the inner surface of the flow path of the nozzle, preferably for the entire inner surface of the nozzle.

[0066] The zirconia graphite refractory contains, as its chemical composition, 50 to 85 mass% of ZrO2, 1 to 25 mass% of CaO, and 8 to 40 mass% of C. The concentration (content) of ZrO2 is 50 to 85 mass% because if it is less than 50 mass%, the corrosion resistance as a refractory is lost, and if it exceeds 85 mass%, it becomes vulnerable to thermal shock. The concentration of CaO is 1 to 25 mass% because if it is less than 1 mass%, the zirconia becomes unstable and the durability of the refractory decreases, and if it exceeds 25 mass%, significant low melting occurs when reacting with alumina as an inclusion and excessive melting loss occurs. Note that for the stabilization of zirconia crystals, MgO or Y2O3 may be used in the same composition range instead of CaO. The concentration of C is 8 to 40 mass% because if it is less than 8 mass%, the electrical resistance increases and stable energization becomes difficult, and if it exceeds 40 mass%, the corrosion resistance of the refractory decreases.

[0067] The refractory can be arranged on the inner surface of the nozzle by, for example, the method described in JP-A-2005-199339.

[0068] <Energization method> In the third aspect, one electrode is connected to the nozzle, and the other electrode is immersed in the molten steel in the tundish to form an energization circuit between the nozzle and the molten steel passing through it, and the absolute value of the average current density in the nozzle is set to 1 to 50 mA / cm 2 and energization is performed so that the polarity of the nozzle becomes positive. The energization may be direct current or alternating current.

[0069] Here, the "average current density in the nozzle" means the current density obtained by dividing the average current value flowing between the nozzle and the molten steel when a voltage is applied by the total area of the inner and outer surfaces of the nozzle in contact with the molten steel (energization area).

[0070] In the third aspect, the absolute value of the average current density is set to 1 to 50 mA / cm 2 because if it is less than 1 mA / cm 2 , the energization effects such as the deoxidation of molten steel due to the movement of oxygen ions and the reaction wetting at the refractory / molten steel interface are lost. Conversely, if it is greater than 50 mA / cm 2 , practical difficulties such as the cable becoming too thick due to an excessive current value will surface.

[0071] Also, in the third aspect, energization is performed such that the polarity of the nozzle is positive. Zirconia-based refractories are known as solid electrolytes having oxygen ion permeability, and a deoxidation effect can be obtained by energizing with a positive polarity.

[0072] (AC pulse energization) In the third aspect, when energizing with an AC pulse-shaped current waveform, it is preferable to perform casting while energizing as follows. That is, in addition to the above configuration, energization is further performed with an AC pulse-shaped current waveform in which the polarity of the nozzle alternates between positive and negative, the period of the current waveform is 0.5 ms to 20 ms, and the average current density × energization time during the period when the polarity of the nozzle is positive is greater than the average current density × energization time during the period when the polarity of the nozzle is negative. Thus, it is preferable to perform casting while energizing such that the polarity of the nozzle is positive.

[0073] Here, the "average current density during the period when the polarity of the nozzle is negative" means the current density obtained by dividing the average current value flowing between the nozzle and the molten steel during the period when the polarity of the nozzle is negative, i.e., the cathode, by the total area of the inner and outer surfaces of the immersion nozzle in contact with the molten steel (energization area), and can also be referred to as the negative electrode effective current density. The "average current density × energization time during the period when the polarity of the immersion nozzle is negative" is the product of the average current density (negative electrode effective current density) during the period when the polarity of the immersion nozzle is negative and the energization time (negative electrode energization time) during the period when the polarity of the immersion nozzle is negative. Similarly, the "average current density during the period when the polarity of the immersion nozzle is positive" means the current density obtained by dividing the average current value flowing between the nozzle and the molten steel during the period when the polarity of the nozzle is positive, i.e., the anode, by the total area (current-carrying area) of the inner and outer surfaces of the immersion nozzle in contact with the molten steel, and can also be referred to as the positive electrode effective current density. The "average current density during the period when the polarity of the immersion nozzle is positive × energization time" is the product of the average current density (positive electrode effective current density) during the period when the polarity of the nozzle is positive and the energization time (positive electrode energization time) during the period when the polarity of the nozzle is positive. "The polarity of the nozzle becomes positive" means that when energization is performed using an alternating current pulse-shaped waveform, the average current density × energization time during the period when the polarity of the nozzle is positive is greater than the average current density × energization time during the period when the polarity of the nozzle is negative. When energization is performed with direct current, it means that energization is performed such that the polarity of the nozzle is positive.

[0074] The reasons for performing energization using an alternating current pulse energization waveform are as follows. When zirconia graphite-based refractories are energized at the positive electrode, the deoxidation effect of the molten steel can be obtained. However, if energization is performed at the positive electrode for a long time, the reaction of the following formula (1) proceeds, and there is a problem that graphite is oxidized and CO gas is generated. C + O 2- → CO + 2e - ···(1)

[0075] Since the oxidation of graphite leads to the collapse of the refractory structure, it should be avoided. Furthermore, CO gas oxidizes aluminum in the molten steel by the reaction of the following formula (2) to generate alumina. Since alumina causes molten steel contamination, this should also be avoided. 2Al + 3CO → Al2O3 + 3C ···(2)

[0076] Therefore, the inventors have devised a method of suppressing the progress of the reaction of formula (1) by shortening the time during which the zirconia graphite refractory is maintained at the positive polarity by using an alternating current pulse energization waveform. The alternating current pulse energization waveform is such that the average current density × energization time during the period when the polarity of the nozzle is positive is greater than the average current density × energization time during the period when the polarity of the nozzle is negative, thereby making the polarity of the nozzle positive. This utilizes the fact that the CO gas generation reaction involves many elementary reaction processes and is a chemical reaction that takes time, shortening the time the refractory is maintained at the positive electrode to prevent CO gas generation.

[0077] The reason why the period of the alternating current pulse energization waveform is 0.5 ms to 20 ms is that if the period is shorter than 0.5 ms, various phenomena associated with energization will not be fully manifested, and the deoxidation effect will decrease. Also, if the period is longer than 20 ms, CO gas will be generated by the reaction of formula (1) during the positive electrode energization period. To suppress the CO gas generation reaction of formula (1), it is more preferable that the anode energization time in the alternating current pulse energization waveform is 15 ms or less.

[0078] [Continuous casting apparatus] Next, the continuous casting apparatus used to implement the continuous casting method of the present invention will be described. FIG. 7 is a diagram schematically showing a continuous casting apparatus 50 which is an example of the continuous casting apparatus used to implement the continuous casting method of the present invention. However, the continuous casting apparatus used to implement the continuous casting method of the present invention is not limited to this. For example, in FIG. 7, a form using a nozzle including a sliding gate is shown, but the form of the nozzle is not limited to this. Also, in FIG. 7, a form of energizing the continuous casting apparatus is shown, but a form without energization may also be used.

[0079] As shown in FIG. 7, a tundish 4 that houses the molten steel 2 poured from the ladle 1 is provided with an upper nozzle 3 at the bottom, and below this upper nozzle 3, a sliding gate 5 as a flow control mechanism and a cylindrical immersion nozzle 6 are successively provided in series.

[0080] Furthermore, in order to form an energization circuit between the immersion nozzle 6 and the molten steel 2, one electrode 7 is connected to the immersion nozzle 6, and the other electrode (hereinafter also referred to as the "counter electrode") 8 that serves as the counter electrode of the electrode 7 is immersed in the molten steel 2 in the tundish 4, and is connected to the power supply device 10 by wiring 9a and 9b, respectively. Both the electrode 7 and the counter electrode 8 are made of a graphite refractory having conductivity. Further, the immersion nozzle 6 to which the electrode 7 is connected is electrically insulated from the tundish 4 by the insulating refractory 11, and the counter electrode 8 immersed in the molten steel 2 is insulated from the tundish 4 by the insulating refractory 12 that supports it. The insulating refractories 11 and 12 are both alumina refractories that do not contain carbon.

[0081] Continuous casting is carried out as follows using such a continuous casting apparatus 50. First, the molten steel 2 supplied from the ladle 1 to the tundish 4 passes through the upper nozzle 3, the sliding gate 5, and the immersion nozzle 6, and is then injected into the mold 14 from the nozzle discharge hole 13 of the immersion nozzle 6. At this time, the flow rate of the molten steel passing through the inside of the immersion nozzle 6 is adjusted according to the opening degree of the sliding gate 5. Further, by driving the power supply device 10, energization is performed between the immersion nozzle 6 and the molten steel 2 under predetermined conditions via the electrode 7 and the counter electrode 8. Then, the molten steel 2 supplied to the mold 14 forms a solidified shell 15 from the contact portion with the mold 14 due to the heat extraction action from the mold 14 while being shielded from the atmosphere by the mold powder 17 sprayed on the molten steel surface, and is drawn downward to become a slab 16.

Example

[0082] Hereinafter, the present invention will be further described based on examples. However, the present invention is not limited thereto.

[0083] Continuous casting was carried out using the continuous casting apparatus shown in FIG. 7 except for the configuration of the nozzle. The composition of the refractory arranged on the inner surface of the nozzle is shown in Table 1. Others in Table 1 are compounds and unavoidable impurities contained in trace amounts.

[0084] The casting tests on the continuous casting machine were carried out under the conditions that the mold thickness was 0.25 m, the mold width was 1.0 - 1.6 m, and the casting speed excluding the unsteady part was 1.0 - 1.5 m / min. In the examples (reference examples) and comparative examples where Ar was blown in, the Ar blowing flow rate was 2 NL / min from the immersion nozzle and 10 NL / min from the upper nozzle or the fixed plate on the sliding gate (in the case of the integrated immersion nozzle shown in Fig. 3, 2 NL / min from the lower blowing part and 10 NL / min from the upper part corresponding to the upper nozzle).

[0085] The steel grades cast were aluminum-killed low-carbon steels with a C concentration of 0.02 - 0.07 mass%, a Si concentration of 0.01 - 0.20 mass%, a Mn concentration of 0.3 - 0.6 mass%, and a sol.-Al concentration of 0.02 - 0.05 mass%. The amount of cast steel per immersion nozzle was 390 - 1050 tons. In the examples (reference examples) and comparative examples, the same molten steel was cast in the same amount in two strands for comparison, and the results of multiple casting tests with various combinations of conditions were averaged for evaluation.

[0086] Also, the immersion nozzles used in these examples (reference examples) and comparative examples all had two 80 mm square discharge holes drilled opposite each other on the side near the bottom of the immersion nozzle, and the operation was carried out with an immersion depth of 250 mm. In the examples (reference examples) and comparative examples, approximate values calculated by ignoring the areas of the openings of the discharge holes and the side walls (top, bottom, left, and right) of the discharge holes were used for the inner area and the energized area of the immersion nozzle.

[0087] The following provides further explanations for each of the examples (reference examples) and comparative examples. Here, the nozzle clogging index in Table 1 quantifies the degree of clogging with respect to the diameter of the immersion nozzle. Specifically, it was quantified as follows. First, the used product of the immersion nozzle through which molten steel had continuously passed was recovered, and the average adhesion thickness of the entire inner surface was calculated from its longitudinal section. Using the result, the average adhesion rate was determined. Since the test was conducted multiple times, the average value of the average adhesion rates obtained from multiple tests was then determined. Based on the obtained average value, with the value of Comparative Example r set to 100, it was indexed and shown as a ratio to Comparative Example r. In the following examples (reference examples) and comparative examples, those with a nozzle clogging index of 40 or less were evaluated as having good results.

[0088]

Table 1

[0089] (Reference Examples a - d) Reference Examples a - d are examples where casting was performed by blowing high - purity Ar gas from the nozzle while adjusting the flow rate from the tundish to the mold using a stopper. Reference Example a uses a nozzle with a step at the joint of the inner surface of the flow path. The cross - section of the nozzle used in Reference Example a is shown in Figure 1. Reference Example b uses a nozzle without a step at the joint of the inner surface of the flow path. The cross - section of the nozzle used in Reference Example b is shown in Figure 2. Reference Examples c and d use an integral immersion nozzle without a joint on the inner surface of the nozzle flow path. The cross - section of the nozzle used in Reference Examples c and d is shown in Figure 3.

[0090] Here, in Reference Example a, there is a step of 2.5 mm (diameter difference of 5 mm) at both the mating part (joint) between the upper nozzle and the collector nozzle and the mating part (joint) between the collector nozzle and the immersion nozzle. The nozzle of Reference Example b has no step at these joints. Reference Examples c and d use an integral immersion nozzle without a joint. Also, Reference Example d uses Ar gas with a lower purity compared to Reference Examples a - c.

[0091] From Table 1, all of Reference Examples a - d had a low nozzle clogging index and showed good results. Comparing the results of Reference Examples a to c, Reference Example a using a nozzle with a step at the joint had a smaller nozzle clogging prevention effect due to the influence of outside air suction from the joint compared to Reference Example b using a nozzle without a step at the joint. Also, compared to Reference Example c using an integrated immersion nozzle without a joint, although the influence was small, Reference Example b had a smaller nozzle clogging prevention effect due to the influence of outside air suction from the joint. From this, it can be seen that the nozzle clogging prevention effect becomes smaller due to the influence of outside air suction from the joint, and furthermore, it can be seen that the nozzle clogging prevention effect becomes even smaller when there is a step at the joint. Also, comparing the results of Reference Examples c and d, it can be seen that the higher the purity of the Ar gas, the higher the nozzle clogging prevention effect. The above effects are also supported by the results of Comparative Example q described later.

[0092] (Examples e, f) Examples e and f are examples in which casting was performed by blowing high-purity Ar gas from a nozzle while adjusting the flow rate from the tundish to the mold using a sliding gate (S / G). Fig. 4 shows the cross-section of the nozzle used in Examples e and f.

[0093] Here, in Example e, Ar gas was flowed on the outer periphery of the sliding surface of the sliding gate, and in Example f, N2 gas was flowed on the outer periphery of the sliding surface of the sliding gate to keep the oxygen concentration in the outer peripheral portion of the sliding surface at 5% or less.

[0094] From Table 1, both Examples e and f had a low nozzle clogging index and good results. Also, the effect of blowing an inert gas on the outer periphery of the sliding surface of the sliding gate showed no difference in effect whether it was Ar gas or N2 gas. This is considered to be due to the fact that the influence of nitrogen on the surface tension of the molten steel is smaller than the influence of oxygen. Also, as is clear from the comparison with Comparative Example o described later, if an inert gas is not flowed on the outer peripheral portion of the sliding surface, outside air is sucked from the sliding surface and the nozzle clogging prevention effect becomes very small.

[0095] (Reference Example g, Example h, Reference Example i) Reference Example g, Example h, and Reference Example i are examples in which casting is performed by passing a direct current between the molten steel and the nozzle. Reference Examples g and i are those obtained by further applying direct current to the conditions of Reference Example d, and Example h is those obtained by further applying direct current to the conditions of Example e. Here, Reference Examples g and i are tested with the polarity of the nozzle reversed.

[0096] From Table 1, all of Reference Example g, Example h, and Reference Example i had a low nozzle clogging index, indicating good results. In Reference Example g and Example h, the nozzle clogging index is lower than that of Reference Example d and Example e, respectively, and it was confirmed that the effect of preventing immersion nozzle clogging is improved by the electrochemical deoxidation action due to energization. On the other hand, since the polarity of the nozzle in Reference Example i is negative, the electrochemical deoxidation action is not exerted, and instead, oxygen ions are supplied to the molten steel side. As a result, the nozzle clogging prevention effect is inferior to that of Reference Example d without energization. However, since casting is performed while blowing high-purity Ar gas into the nozzle, the effect of preventing nozzle clogging by blowing high-purity Ar gas is sufficiently exerted. This can be confirmed from the results of Comparative Example q without blowing high-purity Ar gas.

[0097] (Reference Example j, Example k, Example m) Reference Example j, Example k, and Example m are examples in which casting is performed by passing an alternating current pulse-shaped energization waveform between the molten steel and the nozzle. Reference Example j is an example in which the energization conditions of Reference Example g are changed to an alternating current pulse, Example k is an example in which the energization conditions of Example h are changed to an alternating current pulse, and Example m is an example in which the inert gas flowing on the outer periphery of the sliding gate in Example k is changed to N2 gas and the energization conditions are reset. Here, the energization waveform of Reference Example j is shown in FIG. 8, and the energization waveform of Example m is shown in FIG. 9.

[0098] From Table 1, in Reference Example j, Example k, and Example m, the nozzle clogging index was very low, and the results were excellent. This is because the average current density × energization time during the period when the polarity of the nozzle is positive is greater than the average current density × energization time during the period when the polarity of the nozzle is negative. By performing casting while energizing so that the polarity of the nozzle becomes positive, it is considered that the effect of suppressing the CO gas generation reaction while maintaining the electrochemical deoxidation action was exerted. From this, it is considered that Reference Example j is more effective than Reference Example g, and Examples k and m are more effective than Example h.

[0099] (Comparative Examples n to r) Comparative Example n is an example in which low-purity Ar gas was used instead of the Ar gas in Example k, and a nozzle containing 30 mass% CaO in the refractory was used. From Table 1, Comparative Example n appears to be an extremely excellent example with a nozzle clogging index of 0. However, since the refractory on the inner surface of the nozzle was eroded and severely worn, it was not suitable for practical use. In addition, there is a risk of introducing large inclusions formed by the combination of the eroded refractory and alumina inclusions into the slab, resulting in quality defects.

[0100] Comparative Example o is an example in which an inert gas was not blown into the outer periphery of the sliding gate in Examples e and f. From Table 1, Comparative Example o had a high nozzle clogging index and inferior results compared to the examples. This is considered to be because although the purity of the Ar gas in Comparative Example o was high, the sliding surface of the sliding gate was exposed to the outside air, so the purity of the Ar gas blown into the nozzle decreased due to the influence of oxygen in the outside air sucked in from the sliding surface.

[0101] Comparative Example p is an example in which the purity of the Ar gas blown into the nozzle was further decreased compared to Comparative Example o. Therefore, as described in Table 1, the results were even worse than those of Comparative Example o.

[0102] Comparative Example q is an example in which low-purity Ar gas was used in Reference Examples c and d. Therefore, as described in Table 1, the nozzle clogging index was low and the results were inferior.

[0103] Comparative example r is an example in which Ar gas with a high oxygen concentration was used in comparative example p. As described in Table 1, the nozzle clogging index was low, resulting in inferior results. In addition, because the oxygen concentration was high, it affected the surface tension at the interface between the Ar gas phase and the molten steel, resulting in even more inferior results than comparative example p.

Industrial Applicability

[0104] According to the continuous casting method of steel of the present invention, by flowing high-purity Ar gas in the flow path of the immersion nozzle and further suppressing the intrusion of outside air into the nozzle, the adhesion of inclusions to the inner surface of the nozzle can be suppressed, and the clogging of the nozzle can be suppressed. Therefore, the continuous casting method of steel of the present invention is an extremely useful technique that can suppress the clogging of the nozzle and enable stable operation.

Explanation of Symbols

[0105] 1: Ladle 2: Molten steel 3: Upper nozzle 4: Tundish 5: Sliding gate 6: Immersion nozzle 7: Electrode 8: Counter electrode 9a: Cable 9b: Cable 10: Power supply device 11: Refractory for insulation 12: Refractory for insulation 13: Discharge port 14: Mold 15: Solidified shell 16: Cast slab, 17: Mold powder 50: Continuous casting apparatus 100, 200, 300, 400: Nozzle 110, 410: Upper nozzle 120, 420: Immersion nozzle 130: Collector nozzle 150: Stopper 430: Sliding gate 431: Upper fixing plate 432: Lower fixing plate 433: Intermediate plate

Claims

1. A continuous casting method of steel using a nozzle for injecting molten steel from a tundish into a mold, wherein the nozzle includes an upper nozzle, a sliding gate, and a submerged nozzle, and the sliding gate is disposed between the upper nozzle and the submerged nozzle, an Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, a dew point of -65°C or less, and a nitrogen concentration of 10 ppm or less is flowed through the flow path of the submerged nozzle, the flow rate of the molten steel injected into the mold is adjusted using the sliding gate, and an inert gas is flowed through the outer peripheral portion of the sliding surface of the sliding gate to make the oxygen concentration of the outer peripheral portion 5% or less, a zirconia graphite-based refractory containing 50 to 85 mass% of ZrO₂, 1 to 25 mass% of CaO, and 8 to 40 mass% of C as a chemical composition is disposed on at least the inner surface of the flow path of the submerged nozzle among the inner surfaces of the flow path of the nozzle, one electrode is connected to the nozzle, and the other electrode is immersed in the molten steel in the tundish to form an energization circuit between the nozzle and the molten steel passing through the inside thereof, the absolute value of the average current density in the nozzle is set to 1 to 50 mA / cm², and casting is performed while energizing so that the polarity of the nozzle becomes positive. A continuous casting method of steel.

2. The continuous casting method according to claim 1, wherein energization is performed with an alternating current pulse-shaped current waveform in which the polarity of the nozzle alternates between positive and negative, the period of the current waveform is 0.5 ms to 20 ms, and the average current density × energization time during the period when the polarity of the nozzle is positive is larger than the average current density × energization time during the period when the polarity of the nozzle is negative, and casting is performed while energizing so that the polarity of the nozzle becomes positive.

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