Continuous casting device and continuous casting method

Abstract

Provided is a nozzle for continuous casting, a continuous casting apparatus, a tundish, and a continuous casting method with which it is possible to suppress the entrainment of slag, suppress the occurrence of problems in operation, and inhibit the deterioration of the yield of molten steel.　A nozzle 1 for continuous casting for injecting molten steel from a ladle into a tundish 2 comprises: an immersion part 3 immersed in molten steel in the tundish 2; a bottom part opening part 5 formed, at the tip part of the immersion part 3 on the side towards the bottom part 4 of the tundish 2, so as to discharge molten steel along the direction of the center axis of the immersion part 3; and at least one side surface opening part 6 formed on the side surface of the immersion part 3. The side surface opening part 6 is configured so as to point downward within a range of a predetermined angle from the horizontal plane when the horizontal plane is deemed to be 0°, and the total area S of the opening area of the bottom part opening part 5 and the opening areas of the side surface opening parts 6 is 0.02 m2 to 0.15 m2 inclusive.

Description

[Technical Field]

[0001] The present invention relates to a continuous casting nozzle, a continuous casting apparatus, a tundish, and a continuous casting method. [Background technology]

[0002] High-strength steel sheets used in applications such as ultra-low-carbon steel sheets for automotive exterior panels, low-carbon aluminum-killed steel sheets for low-carbon tinplate steel sheets for cans, hot-rolled steel sheets for steel pipes, and automotive structural components require high formability and excellent surface quality. Surface and internal defects, including nonmetallic inclusions (slag) in steel, present a significant challenge to achieving these high formability and excellent surface quality. Therefore, when high formability and excellent surface quality are required for the final product, it is necessary to remove slag from the molten steel used in the production of high-strength steel to improve its cleanliness. Conventional methods for removing slag include installing a weir in the tundish to create an upward flow of molten steel or using an electromagnetic brake to brake the discharge flow from the submerged entry nozzle in the continuous casting mold. However, these methods have difficulty in adequately removing slag from the molten steel, potentially resulting in slag contamination in the final high-strength steel sheet (thin steel sheet). Therefore, technologies for further slag removal have been investigated.

[0003] For example, in the tundish described in Patent Document 1, a recess is formed in the area where the poured flow of molten steel falls. The depth of the recess is set so that slag entrainment does not occur when the poured flow of molten steel collides with the bottom of the recess and the flow direction is reversed. This is said to prevent slag entrainment in the tundish and improve the floating ability of slag in the tundish.

[0004] Patent Document 2 describes a molten metal pouring method that effectively floats slag in a tundish by generating an upward flow of molten steel in the tundish. Specifically, the tip of a long nozzle that pours molten steel into the tundish is blocked, and a horizontal hole that penetrates the outer periphery of the long nozzle in the thickness direction is formed. Then, by discharging molten steel from the horizontal hole, an upward flow of molten steel is generated in the tundish, thereby effectively floating slag in the tundish.

[0005] Patent Document 3 describes a long nozzle with a closed tip, similar to the long nozzle of Patent Document 2. The outer circumferential surface of the long nozzle is formed with a discharge port that penetrates the plate in the thickness direction and faces upward relative to the horizontal plane. It is said that by discharging molten steel upward from the discharge port, the residence time of the molten metal in the tundish can be extended and the floating separation of slag can be promoted.

[0006] Patent Document 4 describes a long nozzle with a closed tip, similar to the long nozzle of Patent Document 2. The long nozzle has an opening formed on its outer periphery that penetrates the plate in the thickness direction and discharges molten steel upward at a discharge angle of 50° to 60° relative to the horizontal. This arrangement is said to ensure sufficient time for the slag in the molten steel to float up in the tundish. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 04-344854 [Patent Document 2] Japanese Patent Application Publication No. 59-183960 [Patent Document 3] Japanese Patent Application Publication No. 05-200507 [Patent Document 4] Japanese Patent Application Publication No. 07-155911 Summary of the Invention [Problem to be solved by the invention]

[0008] As described in Patent Document 1, when a recess is formed in the bottom of a tundish, the amount of molten steel remaining in the tundish increases after continuous casting. This leads to a decrease in the yield of molten steel. Therefore, as an actual production process, there is still room for improvement in terms of yield and manufacturing costs.

[0009] When starting to pour molten steel from a ladle into a tundish using the long nozzles described in Patent Documents 2 to 4, molten steel with a large momentum and high molten steel static pressure falls onto the tip of the long nozzle. The impact force generated when the falling molten steel collides with the tip can damage or break the long nozzle, potentially resulting in fragments of the long nozzle being mixed into the molten steel in the tundish. Furthermore, the long nozzle is held at the bottom of the ladle by a gripping mechanism. Therefore, if the impact force exceeds the gripping force holding the long nozzle, a gap may form between the long nozzle and the bottom of the ladle, potentially resulting in molten steel leakage. Thus, the use of the long nozzles described in Patent Documents 2 to 4 can cause operational problems, and there is still room for improvement before they can be put into practical use.

[0010] Furthermore, the long nozzles described in Patent Documents 2 to 4 discharge molten steel upward relative to a horizontal plane. Therefore, the upward discharge flow of molten steel stirs the molten steel and slag floating on the surface of the molten steel in the tundish, which may result in the slag being entrained in the molten steel. If slag entrainment occurs, there is a risk that the slag may become mixed into the final product, a high-tensile steel plate.

[0011] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a continuous casting nozzle, a continuous casting apparatus, a tundish, and a continuous casting method that can suppress slag entrainment, the occurrence of operational problems, and the deterioration of molten steel yield. [Means for solving the problem]

[0012] The means for solving the above problems are as follows. [1] A continuous casting nozzle for pouring molten steel from a ladle into a tundish, the nozzle comprising: an immersion section immersed in the molten steel in the tundish; a bottom opening formed at the tip of the immersion section on the bottom side of the tundish so as to discharge molten steel along the central axis of the immersion section; and at least one side opening formed on a side of the immersion section, wherein the side opening is configured to face downward within a predetermined angle range from a horizontal plane when the horizontal plane is defined as 0°, and the total area S of the opening area of ​​the bottom opening and the opening area of ​​the side opening is 0.02 m. 2 More than 0.15m 2 A continuous casting nozzle as follows: [2] The continuous casting nozzle according to [1], wherein the predetermined angle ranges from 40° to 70° from the horizontal plane. [3] The continuous casting nozzle according to [1], wherein the submerged portion is formed in a cylindrical shape, and the side openings are formed in pairs at equal intervals in the circumferential direction of the submerged portion. [4] A continuous casting apparatus equipped with the continuous casting nozzle according to any one of [1] to [3], further comprising a weir provided at a projected position of the bottom opening at the bottom of the tundish in the height direction of the tundish, the weir having a wall portion surrounding the projected position at a predetermined distance from the projected position, and a eaves portion provided at the upper end of the wall portion and protruding inward of the weir. [5] A tundish into which molten steel is poured through the continuous casting nozzle described in any one of [1] to [3], comprising a weir provided at the bottom in the height direction at a projected position of the bottom opening at the bottom, the weir comprising a wall portion surrounding the projected position at a predetermined distance from the projected position, and a eaves portion provided at the upper end of the wall portion and protruding inward of the weir. [6] A continuous casting method for producing steel using the continuous casting nozzle according to any one of [1] to [3]. [7] In the continuous casting method according to [6], the amount of molten steel poured per unit opening area Q / S, calculated by dividing the amount of molten steel poured from the ladle to the tundish by the total area S, is 20 (tons / min) / m 2 More than 750(ton / min) / m 2 A continuous casting method as follows: [Effects of the Invention]

[0013] According to the present invention, it is possible to suppress slag entrainment, to suppress the occurrence of operational problems, and to suppress a decrease in the yield of molten steel. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a diagram showing a nozzle according to an embodiment of the present invention and a part of a continuous casting device to which the nozzle is applied. [Figure 2] FIG. 1 is a diagram showing all the nozzles used in Example 1. [Figure 3] 1 is a diagram showing the results of measuring the surface flow velocity of invention examples 1 to 3 and comparative example 1. FIG. [Figure 4] FIG. 1 is a diagram showing the relationship between the total area and the number density of inclusions in a slab. [Figure 5] FIG. 1 is a diagram showing the relationship between the flow rate of molten steel per unit opening area and the number density of inclusions in a slab. [Figure 6] FIG. 1 is a diagram showing the number density of surface cracks in a slab produced using a nozzle according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be specifically described below through embodiments of the present invention (hereinafter referred to as the present embodiment). The present embodiment described below shows a preferred example of the present invention, and the present invention is not limited to this example.

[0016] The continuous casting nozzle (hereinafter referred to as the nozzle) according to this embodiment is configured to maintain slag floating on the surface of the molten steel, while stirring the molten steel in the tundish with the flow of molten steel discharged from the nozzle, thereby promoting the floating of the slag in the molten steel. FIG. 1 shows a nozzle according to this embodiment and a portion of a continuous casting apparatus to which the nozzle is applied. The nozzle 1 shown in FIG. 1 is a nozzle sometimes referred to as a long nozzle, and is attached to a ladle (not shown). It is configured to pour the molten steel received in the ladle into the tundish 2. The tundish 2 temporarily stores the molten steel poured through the nozzle 1 and floats and removes the slag in the molten steel. The nozzle 1 extends in a direction substantially parallel to the height of the tundish 2. The nozzle 1 is formed into a cylindrical shape from a refractory material, and the inner diameter of its tip is set to approximately 220 mm. The inner and outer circumferential surfaces of the nozzle 1 are preferably coated with an antioxidant or the like.

[0017] The tip of the nozzle 1 forms a submerged section 3 that is submerged in molten steel stored in a tundish 2. A bottom opening 5 that discharges molten steel along the central axis of the nozzle 1 is formed on the bottom 4 side of the tundish 2 in the submerged section 3. In other words, the nozzle 1 is open or communicates along the central axis. The opening diameter of the bottom opening 5 shown in FIG. 1 is set to be approximately the same as the inner diameter of the submerged section 3. This is to prevent the molten steel from colliding with the inner wall surface of the nozzle 1 when starting to pour the molten steel into the tundish 2 and to ensure that the molten steel is poured into the tundish 2.

[0018] At least one side opening 6 is formed through the side surface of the submerged section 3 in the thickness direction. In this embodiment, two side openings 6 are formed at equal intervals in the circumferential direction of the submerged section 3. When the horizontal plane is defined as 0°, the side openings 6 are preferably formed downward at an angle ranging from 40° to 70° from the horizontal plane, and more preferably at an angle of approximately 60° from the horizontal plane. This is because, if the angle is less than 40°, the flow of molten steel discharged from the side opening 6 increases the flow velocity near the surface of the molten steel accumulated in the tundish 2 (hereinafter referred to as the surface flow velocity). This may cause slag floating on the upper surface of the molten steel to be stirred into the molten steel, resulting in slag entrainment. Furthermore, if the angle exceeds 70°, the flow of molten steel discharged from the side opening 6 collides with the bottom 4 of the tundish 2, generating a reverse flow in which the flow direction is reversed. The reverse flow flows toward the surface of the molten steel, thereby increasing the surface flow velocity. This is to avoid the possibility that slag may be entrained. The angle range in which the side opening 6 is formed can be rephrased as the range of the discharge angle of the molten steel discharged from the side opening 6.

[0019] By forming the side openings 6 within the above-mentioned angle range, slag entrainment can be suppressed even under high-throughput conditions, enabling the production of steel with high cleanliness. The high-throughput condition means that the amount of molten steel injected from the nozzle 1 into the tundish 2 is approximately the maximum injection amount determined by design. Furthermore, by increasing the number of side openings 6 formed in the nozzle 1, the discharge flow of molten steel discharged from each side opening 6 can be reduced, thereby reducing the surface flow velocity within the tundish 2.

[0020] The total area S (m 2 ) is 0.02m, as will be explained in more detail later. 2 More than 0.15m 2 It is preferable to set the total area S(m 2 ) is 0.02m2 If the total area S (m 2 ) is 0.15m 2 If the flow rate exceeds 1 / 2, the upward flow velocity in the tundish 2, that is, the flow velocity of the molten steel that acts to stir the molten steel in the tundish 2, decreases. This makes it difficult for the slag in the molten steel to float up, so this is to avoid this.

[0021] In addition, the molten steel flow rate Q / S per unit opening area is 20 (tons / min) / m 2 ) or more 750((ton / min) / m 2 ) or less. The molten steel flow rate Q (tons / min) is the flow rate of molten steel injected into the tundish 2 through the nozzle 1. The molten steel flow rate Q / S per unit opening area is calculated based on the total area S (m 2 The molten steel flow rate Q (tons / min) is divided by the opening area. The molten steel flow rate Q / S ((tons / min) / m 2 ) is 20 ((ton / min) / m 2 ), the flow rate of the molten steel discharged from the bottom opening 5 and the side opening 6 increases. This is undesirable because the surface flow rate increases accordingly, which may cause slag entrainment. The flow rate of molten steel per unit opening area Q / S ((tons / min) / m 2 If the value of (a) exceeds 750, the upward flow velocity in the tundish 2, i.e., the flow velocity of the molten steel that acts to stir the molten steel in the tundish 2, decreases. This makes it difficult for the slag in the molten steel to float up, which is undesirable.

[0022] A weir 7 is provided at the projection position P of the nozzle 1 on the bottom 4 of the tundish 2 in the height direction, i.e., below the nozzle 1 in the height direction. The weir 7 is configured to receive the molten steel discharge flow from the nozzle 1 to generate a reverse flow and to cause the reverse flow and the discharge flow to collide with each other. This reduces the kinetic energy of the reverse flow. Furthermore, the weir 7 causes the discharge flow and the reverse flow to collide with each other, resulting in a fluid flow of molten steel with lower kinetic energy than the reverse flow, which stirs the molten steel in the tundish 2 and promotes the floating of slag in the molten steel.

[0023] As shown in FIG. 1 , the weir 7 is formed in a box shape or a cylindrical shape with a bottom. The weir 7 includes a wall portion 8 formed to surround the projection position P of the nozzle 1 at a predetermined distance from the projection position P, and an eave portion 9 provided at the upper end of the wall portion 8 and protruding inward of the weir 7. The wall portion 8 may be formed to continuously surround the entire circumference of the projection position P, or may be formed at regular intervals around the projection position P. The distance between the projection position P and the wall portion 8 and the height of the wall portion 8 can be determined in advance by experiments, simulations, or the like. Optimizing the shape and dimensions of the weir 7 can laminarize the flow of molten steel in the tundish 2 and promote the floating of inclusions.

[0024] The eaves 9 extend almost horizontally from the top of the wall 8 toward the inside of the weir 7. If the eaves 9 are too short, they will not be able to prevent the upward flow of molten steel from the side of the tundish, causing a disturbance to the molten steel surface around the nozzle 1 and possibly entraining slag floating on the surface of the tundish 2. On the other hand, if the eaves 9 are too long, there is a concern that the eaves 9 may break off due to a reverse flow during operation. The appropriate length of the eaves 9 in the horizontal direction can be determined in advance by experiments, simulations, etc.

[0025] (Actions and Effects) The operation and effects of this embodiment will be described. Molten steel in the ladle flows into the nozzle 1. Because the nozzle 1 of this embodiment is connected along the central axis, the molten steel is poured into the tundish 2 through the bottom opening 5 and the side opening 6 without remaining in the submerged section 3. This makes it difficult for a large static pressure of molten steel to act on the nozzle 1 due to the accumulation of molten steel in the nozzle 1. Alternatively, it is possible to prevent the accumulation of molten steel in the nozzle 1 from causing a large static pressure of molten steel to act on the nozzle 1. Therefore, this embodiment can prevent operational problems, such as damage or breakage of the nozzle 1 caused by the large static pressure of molten steel acting on the nozzle 1, and contamination of molten steel with fragments of the nozzle 1. It can also prevent a decrease in the yield of molten steel caused by such problems.

[0026] The molten steel that has flowed into the nozzle 1 is dispersed and discharged from the bottom opening 5 and the side opening 6. The discharge angle of the molten steel from each opening 5, 6 is set within the above-mentioned range, so that an increase in the surface flow velocity due to the discharge flow of the molten steel can be suppressed. In addition, the total area S (m 2 ) and molten steel flow rate per unit opening area ((ton / min) / m 2 ) is set within the above-mentioned range, it is possible to prevent the flow velocity of the discharge flow from becoming too high or too low. In other words, it is possible to maintain the agitated state of the molten steel in the tundish 2 while suppressing an increase in the surface flow velocity of the molten steel in the tundish 2 caused by the discharge flow. As a result, it is possible to suppress slag entrainment and produce steel with high cleanliness. The method of producing steel using the above-mentioned nozzle 1 corresponds to the continuous casting method of this embodiment.

[0027] More specifically, in this embodiment, the molten steel discharged from the bottom opening 5 and the side opening 6 collides with the bottom and wall 8 of the weir 7, generating a reverse flow. However, because molten steel is continuously discharged from the nozzle 1, the molten steel discharged from the nozzle 1 continuously collides with the reverse flow. In this way, the discharge flow of molten steel from the nozzle 1 and the reverse flow oppose each other, reducing the kinetic energy of the reverse flow. Furthermore, the reverse flow, which still has high kinetic energy, rises along the wall of the weir 7, but its flow direction is changed to a nearly horizontal direction by the overhang 9, where it collides with the discharge flow described above, reducing its kinetic energy. Based on this principle, the kinetic energy of the molten steel overflowing from the weir 7 is reduced, and the tundish 2 is filled with molten steel flowing with such low kinetic energy.

[0028] Therefore, the molten steel in the tundish 2 maintains a fluid state, and as a result, the molten steel in the tundish 2 is in a moderately stirred state. As a result, the floating of slag in the molten steel can be promoted. Furthermore, because the kinetic energy of the molten steel is reduced as described above, an increase in the surface flow velocity due to the fluid flow of the molten steel in the tundish 2 can be suppressed. As a result, slag entrainment can be suppressed. This allows steel with high cleanliness to be produced.

[0029] The present invention is not limited to the above-described embodiment. In the above-described embodiment, two side openings 6 are formed at equal intervals in the immersion section 3, but instead, one or three or more side openings may be formed. For example, when three or more side openings are formed, the total area S (m 2) within the above-mentioned range, the opening area of ​​each side opening becomes smaller than when there are two side openings, and the flow rate of the molten steel discharged from each side opening increases. However, because the molten steel is dispersed and discharged from each side opening, the increase in the surface flow rate can be suppressed, and slag entrainment can be suppressed. Furthermore, the molten steel discharged from each side opening can stir the molten steel in the tundish, promoting the floating of slag, as in the above-mentioned embodiment. In other words, the same functions and effects as those of the above-mentioned embodiment can be obtained. [Example]

[0030] An example conducted to confirm the effect of the nozzle according to the embodiment of the present invention will be described.

[0031] (Example 1) FIG. 2 is a diagram showing the nozzles used in Example 1. As shown in FIG. 2, in Example 1, a side opening was formed in the immersion part of a conventionally known nozzle at an angle of about 40° downward relative to the horizontal plane. Two side openings were formed in the immersion part, equidistantly spaced in the circumferential direction. The total area S (m 2 ) is 0.02m 2 More than 0.15m 2 The following ranges were set: The distance in the height direction between the bottom end of the side opening and the bottom end of the nozzle was set to 30 mm.

[0032] The tundish was 1000 mm deep, and the vertical distance between the surface of the molten steel stored in the tundish and the bottom end of the nozzle was set to 300 mm. The nozzle described above was used to measure the surface flow velocity of the molten steel in the tundish when the molten steel was poured into the tundish at rates of 8 ton / min, 10 ton / min, and 12 ton / min. A weir with the same structure as weir 7 shown in Figure 1 was installed at the bottom of the tundish.

[0033] (Example 2) In Example 2, the surface flow velocity of molten steel in the tundish was measured in the same manner as in Example 1, except that the side opening was formed at an angle of 60° downward relative to the horizontal plane, as shown in Figure 2.

[0034] (Example 3) In Example 3, the surface flow velocity of molten steel in the tundish was measured in the same manner as in Example 1, except that the side opening was formed at an angle of 70° downward relative to the horizontal plane, as shown in Figure 2.

[0035] (Comparative Example 1) In Comparative Example 1, as shown in FIG. 2, the surface flow velocity of molten steel in the tundish was measured in the same manner as in Invention Example 1, except that a conventionally known nozzle was used.

[0036] The measurement results of the maximum surface flow velocity (hereinafter simply referred to as the surface flow velocity) for Examples 1 to 3 and Comparative Example 1 are summarized in Figure 3. As shown in Figure 3, in Example 1, when the injection rate of molten steel was 8 ton / min, the surface flow velocity was lower than that of the Comparative Example. When the injection rate of molten steel was 10 ton / min and when the injection rate of molten steel was 12 ton / min, the surface flow velocity was also lower than that of the Comparative Example. This is thought to be because the flow velocity of molten steel discharged from the bottom opening could be reduced by the amount of molten steel discharged from the side opening, thereby suppressing an increase in the surface flow velocity due to the reverse flow. It is also thought to be because molten steel was discharged from the side opening downward relative to the horizontal plane.

[0037] In Example 3, as shown in Fig. 3, when the injection rate of molten steel was 8 ton / min, the surface flow velocity was lower than in the comparative example and Example 1, but higher than in Example 2. Similarly, when the injection rate of molten steel was 10 ton / min and when the injection rate of molten steel was 12 ton / min, the surface flow velocity was lower than in the comparative example and Example 1, but higher than in Example 2. This is thought to be because in Example 3, the molten steel was discharged more downward from the side opening than in Example 1, and therefore the flow velocity of the reverse flow was higher than in Example 1.

[0038] As shown in Fig. 3, in Example 2, when the injection rate of molten steel was 8 ton / min, the surface flow velocity was lower than in Comparative Example, Example 1, and Example 3. Similarly, when the injection rate of molten steel was 10 ton / min and when the injection rate of molten steel was 12 ton / min, the surface flow velocity was lower than in Comparative Example, Example 1, and Example 3. This is thought to be because in Example 2, the discharge flows from the side opening and the bottom opening could be made to oppose the reverse flow more effectively than in Examples 1 and 3, thereby suppressing the increase in the surface flow velocity caused by the reverse flow.

[0039] 3, when the angle is less than 40° or more than 70°, the surface flow velocity increases, which may cause slag entrainment. Therefore, in the present invention, it was found that the angle is preferably set to 40° or more and 70° or less, and more preferably to about 60°. [Example]

[0040] Next, the total area S (m 2 ) and the number density (number / m) of inclusions (slag) in the slab produced by the continuous casting machine using the nozzle. 3 ) was investigated. Specifically, a conventionally known nozzle was first prepared, and a side opening was formed in the immersed portion of the nozzle at an angle of 60° downward relative to the horizontal plane. Two side openings were formed in the immersed portion at equal intervals in the circumferential direction. The opening area of ​​the bottom opening was kept constant, and the opening diameter of the side opening was changed to change the opening area of ​​the side openings, and the total area S (m 2 ) was changed to suppress the large static pressure of molten steel acting on the lower end of the nozzle in the height direction.

[0041] The vertical distance between the bottom of the side opening and the bottom of the nozzle was set to 30 mm. The tundish was 1000 mm deep, and the vertical distance between the surface of the molten steel stored in the tundish and the bottom of the nozzle was set to 300 mm. A weir constructed similarly to weir 7 shown in Figure 1 was installed at the bottom of the tundish.

[0042] Then, molten steel was poured into the tundish using the nozzle described above. Continuous casting was carried out in this manner to produce a slab. In the slab, inclusions with a particle size of 10 μm or more were counted, and the number density (number / m 3 ) was calculated.

[0043] Figure 4 shows the total area S (m 2 ) and the number density of inclusions in the slab (number / m 3 ) is a graph showing the relationship between the total area S (m 2 ) is 0.02m 2 If less than the total area S (m 2 ) is 0.02m 2 The number density (number / m 3 ) has increased. This is because the total area S(m 2 ) is small, the flow velocity of the molten steel discharged from the nozzle increases, which causes the surface flow velocity to increase and causes slag entrainment.

[0044] Also, the total area S (m 2 ) is 0.15m 2 If it exceeds the total area S (m 2 ) is 0.15m 2 The number density (number / m 3 ) has increased. This is because the total area S(m 2 ) is large, the flow velocity of the molten steel discharged from the nozzle decreases, which causes the stirring of the molten steel by the fluid flow in the tundish 2 to be suppressed. As a result, it is thought that the slag becomes difficult to float up and remains in the molten steel.

[0045] Therefore, as shown in Figure 4, 0.02m2 More than 0.15m 2 The total area S (m 2 ) to determine the number density (number / m 3 ) can be significantly reduced. [Example]

[0046] Molten steel flow rate per unit opening area Q / S ((ton / min) / m 2 The relationship between the area of ​​the bottom opening and the number density of inclusions in the slab was investigated. First, a conventional nozzle was prepared, and a side opening was formed in the submerged part of the nozzle at an angle of 60° downward with respect to the horizontal plane. Two side openings were formed in the submerged part at equal intervals in the circumferential direction. The opening area of ​​the bottom opening was kept constant, and the opening area of ​​the side opening was changed to change the total area S (m 2 ) was changed, which changed the amount of molten steel poured.

[0047] The vertical distance between the bottom of the side opening and the bottom of the nozzle was set to 30 mm. The tundish was 1000 mm deep, and the vertical distance between the surface of the molten steel stored in the tundish and the bottom of the nozzle was set to 300 mm. A weir constructed similarly to weir 7 shown in Figure 1 was installed at the bottom of the tundish.

[0048] Then, molten steel was poured into the tundish using the nozzle described above. Continuous casting was carried out in this manner to produce a slab. In the slab, inclusions with a particle size of 10 μm or more were counted, and the number density (number / m 3 ) was calculated.

[0049] Figure 5 shows the molten steel flow rate per unit opening area Q / S ((ton / min) / m 2 ) and the number density of inclusions in the slab. As shown in Figure 5, the molten steel flow rate per unit opening area Q / S ((tons / min) / m 2 ) is 20 ((ton / min) / m 2 ) less than 20 ((ton / min) / m 2) or more, the number density of inclusions (number / m 3 ) increased. This is due to the increase in the molten steel flow rate per unit opening area Q / S ((ton / min) / m 2 ) is small, the flow velocity of the molten steel discharged from the nozzle increases, which is presumably the reason for the high surface flow velocity and the entrainment of slag.

[0050] Molten steel flow rate per unit opening area Q / S ((ton / min) / m 2 ) is 750 ((ton / min) / m 2 ) exceeding 750 ((ton / min) / m 2 ) or less, the number density of inclusions (number / m 3 ) increased. This is due to the increase in the molten steel flow rate per unit opening area Q / S ((ton / min) / m 2 ), the flow rate of the molten steel discharged from the nozzle decreases. This causes the stirring of the molten steel in tundish 2 by the fluid flow to be suppressed. It is presumed that this makes it difficult for the slag to float up, and the slag remains in the molten steel.

[0051] Therefore, as shown in Figure 5, 50 (ton / min) / m 2 ) or more 750((ton / min) / m 2 ) within the range below, the flow rate of molten steel per unit opening area Q / S ((ton / min) / m 2 ) is set. By doing so, the number density of inclusions in the slab (number / m 3 ) can be significantly reduced. [Example]

[0052] The number density (number / m) of surface cracks in a slab produced using the nozzle according to the embodiment of the present invention 2 ) was confirmed. The test conditions in Example 4 are summarized in Table 1. The conventional example shown in Table 1 is an example in which continuous casting was carried out using a conventionally known long nozzle. In Example 4, molten steel was used that had been subjected to oxygen blowing in a converter and RH vacuum degassing treatment. The number density (number / m) of surface cracks in the slab mentioned above was also confirmed.2 ) means the number of cracks per unit area on the slab surface.

[0053] [Table 1]

[0054] FIG. 6 shows the number density (number / m) of surface cracks in a cast slab produced using a nozzle according to an embodiment of the present invention. 2 6 is a diagram showing the number density (number / m) of surface cracks in Examples 1 to 12 of the slab, compared to Conventional Example, Comparative Example 1, and Comparative Example 2. 2 ) was reduced. In Examples 1 to 6, the total area S and the molten steel flow rate Q / S per unit opening area were both set within the above-mentioned ranges, and a weir was provided in the tundish. Therefore, the number density (number / m) of surface cracks in the slab was lower than in Examples 7 to 12, the conventional example, and Comparative Examples 1 and 2. 2 ) appears to have decreased.

[0055] In Example 2, although the discharge angle from the side opening 6 is small, the injection amount Q of molten steel is small, so slag entrainment is unlikely to occur, and the number density (number / m 2 ) is thought to have been reduced. In Example 6, although the discharge angle from the side opening 6 is large, the discharge flow of molten steel is received by the weir. Therefore, slag entrainment is unlikely to occur, and the number density (number / m 2 ) appears to have decreased.

[0056] In Examples 7 and 8, the molten steel flow rate Q / S per unit opening area was set outside the above-mentioned range, and therefore the number density (number / m) of surface cracks in the slab was lower than in Examples 1 to 6. 2 ) is thought to have increased. In Examples 9 and 10, no weir is provided on the tundish, and the discharge angle from the side opening 6 is set outside the above-mentioned range of values. Therefore, the number density (number / m) of surface cracks in the slab is lower than in Examples 1 to 6. 2) is thought to have increased. In Examples 11 and 12, the molten steel flow rate Q / S per unit opening area is set outside the above-mentioned range, and no weir is provided on the tundish. Therefore, the number density (number / m) of surface cracks in the slab is lower than in Examples 1 to 6. 2 ) appears to have increased. In addition, in Example 11, the discharge angle from the side opening 6 is set within the above-mentioned range. Therefore, the number density (number / m) of surface cracks in the slab is lower than that in Example 12. 2 ) is thought to have decreased. [Explanation of symbols]

[0057] 1 nozzle 2 tundishes 3 Immersion section 4 Bottom of the tundish 5 Bottom opening of nozzle 6 Side opening of nozzle 7 Weir 8 Wall section 9 Eaves P projection position

Claims

1. A continuous casting apparatus equipped with a continuous casting nozzle for pouring molten steel from a ladle into a tundish, the continuous casting nozzle comprises an immersion section immersed in molten steel in the tundish, a bottom opening formed at a tip end of the immersion section on the bottom side of the tundish so as to discharge molten steel along a central axis direction of the immersion section, and at least one side opening formed on a side surface of the immersion section, the side opening is configured to face downward within a predetermined angle range from a horizontal plane, where the horizontal plane is defined as 0°; The total area S of the opening area of ​​the bottom opening and the opening area of ​​the side opening is 0.02 m ２ More than 0.15m ２ is as follows: the predetermined angle range is 40° or more and 70° or less from the horizontal plane, a weir provided at a projection position of the bottom opening on the bottom of the tundish in the height direction of the tundish, The weir is formed in a box-like or bottomed cylindrical shape in a continuous casting apparatus, and has a wall portion surrounding the projection position at a predetermined distance from the projection position, a bottom portion, and a eave portion provided at the upper end of the wall portion and protruding inward of the weir.

2. The immersion section is formed in a cylindrical shape, The continuous casting apparatus according to claim 1, wherein the side openings are two in number and are formed at equal intervals in the circumferential direction of the immersion section.

3. A continuous casting method for producing steel using the continuous casting apparatus according to claim 1 or 2.

4. The continuous casting method according to claim 3, The amount of molten steel poured per unit opening area Q / S, calculated by dividing the amount of molten steel poured from the ladle to the tundish by the total area S, is 20 (tons / min) / m ２ More than 750 (ton / min) / m ２ A continuous casting method as follows:

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