Molten steel supply system and method for continuous casting of steel

Abstract

To provide a system for supplying molten steel, in which, when molten steel is supplied from one vessel to another vessel through a nozzle, bubbles introduced into the molten steel in the nozzle can be prevented from leaving the nozzle, and the generation of bare hot water in the other vessel can be suppressed.SOLUTION: In a system of supplying molten steel 105 from a first vessel 101 to a second vessel 102 through a nozzle 110, the nozzle has an inlet 110a on the upstream side of the first vessel and an outlet 110b on the downstream side of the second vessel, and extends downward from the upstream side to the downstream side. The outlet of the nozzle is located below the liquid level of the molten steel supplied to the second vessel and above a bottom surface 102a of the second vessel. In the supply system, a flow rate Q (ton / min) of molten steel flowing inside the nozzle, the minimum inner diameter of the nozzle D1 (mm) upstream from the outlet, and the inner diameter of the nozzle D2(mm) at the outlet satisfy a prescribed relational expression.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] This application discloses a molten steel supply system and a continuous steel casting method.

Background Art

[0002] In the continuous steel casting process, a tundish is used as an intermediate vessel for supplying molten steel from a molten steel ladle to a mold. The tundish has multiple functions, such as (1) a function of stabilizing the supply amount of molten steel to the mold, (2) a function of distributing molten steel to multiple molds, (3) a buffer function for continuously performing continuous casting using multiple molten steel ladles, and (4) a function of removing non-metallic inclusions. In particular, when efficiently producing high-grade steel materials with high cleanliness, the (4) function of removing non-metallic inclusions becomes extremely important.

[0003] Non-metallic inclusions in molten steel mainly originate from oxides, nitrides, sulfides, and impurities in steel such as bubbles generated during the steelmaking process. When such non-metallic inclusions remain in the final product, for example, they are known to become the starting point of fracture due to stress concentration and deteriorate the material quality of the final product. Also, in the steelmaking process itself, non-metallic inclusions adhere and accumulate on the inner wall of the refractory material flow path, causing narrowing or blockage of the flow path, which not only hinders smooth production but also can generate defects in both the surface and interior of the base material during processing such as casting, resulting in factors that increase manufacturing costs, such as reducing the product yield. Therefore, in many cases, it is necessary to remove non-metallic inclusions from the molten steel in the limited processes from secondary refining where the final adjustment of the molten steel components is performed to the mold.

[0004] In order to remove non-metallic inclusions from molten steel, generally, a method is adopted in which the non-metallic inclusions are floated in the molten steel by utilizing the specific gravity difference between the molten steel and the non-metallic inclusions, and then recovered in a floating layer of oxides called flux. However, it is known that the floating speed at this time decreases as the size of the non-metallic inclusions becomes smaller, and the time required to recover in the flux layer becomes longer. Therefore, in order to reduce the non-metallic inclusions in the molten steel, it is considered effective to increase the residence time of the non-metallic inclusions in the tundish in order to ensure the time required for the non-metallic inclusions to float.

[0005] Generally, the supply of molten steel from the ladle to the tundish is carried out by flowing it down by utilizing the potential energy through a sliding nozzle having a flow rate adjustment function and a long nozzle which is a cylindrical refractory used by immersing the lower end in the molten steel in the tundish. However, since a high-speed discharge flow from the long nozzle may collide with the bottom of the tundish to form a short-circuit flow called a short path leading to the mold (see Fig. 6), it is not always easy to ensure the residence time of the molten steel in the tundish. A general countermeasure against this problem is a method of deflecting the molten steel flow by providing a weir inside the tundish. However, constructing a refractory inside the tundish causes an increase in material cost, construction time, and work load. In addition, a space where there is almost no flow near the weir and which does not contribute to floating and removing is generated, and a new flow that goes toward the mold at high speed while detouring can be induced, so it does not necessarily assist the floating of the inclusions. In particular, small inclusions have a small buoyancy and are easily followed by the flow of molten steel, so the effect of detouring is likely to be limited to the removal of large inclusions.

[0006] In the steelmaking process, attention must also be paid to the unintentional increase in non-metallic inclusions due to the reoxidation of molten steel. Generally, from the perspective of avoiding difficulties in stable casting caused by gas generation accompanying the temperature drop of molten steel, etc., the molten steel used for continuous casting has been subjected to deoxidation treatment in the refining process and has an oxygen concentration far lower than the soluble oxygen concentration, being in a state where it is very easy to absorb oxygen. When air or lower oxides come into contact with the molten steel, the molten steel absorbs oxygen, and a reoxidation phenomenon occurs in which non-metallic inclusions are formed by combining with elements having a higher affinity for oxygen than the molten steel (such as Al and Si dissolved in the molten steel). Therefore, in the ladle and tundish, it is necessary to make the inside of the tundish have a low oxygen concentration by replacing the atmosphere with an inert gas, or to shield the molten steel from the outside air by coating the surface of the molten steel with a low-reactivity flux having a low content of lower oxides. However, when supplying molten steel to the tundish through a long nozzle, since the molten steel flow discharged from the nozzle is very fast as described above, the flux covering the surface of the molten steel near the long nozzle is pushed back by the reverse upward flow generated by the impact on the bottom of the tundish, and the surface of the molten steel is directly exposed to the outside air as bare liquid, and a reoxidation phenomenon in which the molten steel absorbs oxygen in the atmosphere may occur (see Fig. 6). Also, when there is a connection defect (misalignment) at the fitting part of the nozzle, or when the sealing performance of the flow path is impaired due to defective molding or wear of the refractory, etc., outside air can be introduced into the nozzle by an ejector (see Fig. 6). When the outside air introduced into the nozzle follows the flow of the molten steel and infiltrates to the tundish, a strong upward flow is formed around the long nozzle, making the generation of bare liquid more prominent.

[0007] The above problems can occur in any process of supplying molten steel from one container to another, and various means for solving the problems have been proposed. For example, Patent Document 1 discloses a continuous casting method using an injection tube that blocks the injection flow from the outside air with a cylindrical body having a refractory property with an inner diameter of 300 mm or more. According to the technique disclosed in Patent Document 1, the injection flow that has fallen from the nozzle hits the gas inside the tube at the liquid level inside the tube, introducing a large number of bubbles into the molten steel. It is considered possible to reduce the injection flow rate by the large buoyancy of the bubbles, generating a gentle upward flow in the tundish. In addition, since solid inclusions have poor wettability with molten steel and easily adhere to bubbles, it is also expected that they will float and be removed at high speed by the large buoyancy of the bubbles. On the other hand, in the injection tube as described above, in order to avoid the molten steel absorbing oxygen or nitrogen from the gas phase, it is necessary to fill the inside of the tube with a large amount of inert gas, which results in a large operating cost compared to the case of immersing the nozzle in the molten steel. Further, when splashes generated by the droplets of molten steel discharged from the nozzle or the impact on the bath surface adhere to the inner wall of the tube, the heat extraction of the adherent is remarkable due to the large pipe diameter, and the adherent is likely to solidify and accumulate, leading to blockage. Furthermore, when the bubbles introduced into the molten steel escape outside the injection tube, there is a risk that the bubbles will reach the molten steel surface in the tundish, causing reoxidation of the molten steel. Therefore, it is difficult to reduce the diameter of the tube, and the reduction of the refractory cost is limited.

[0008] Patent Document 2 discloses a nozzle provided with a flow control portion in the body portion in order to prevent the exposure of the bare bath due to the upward flow reversed at the bottom of the tundish. However, in the technique disclosed in Patent Document 2, it is necessary to take separate measures against the occurrence of a short path, increasing the manufacturing cost. In addition, since the weight of the nozzle increases, the load on the device for gripping the nozzle is large, and there is a risk that the nozzle cannot be supported during casting.

[0009] Patent Document 3 discloses a continuous casting method and a continuous casting apparatus for promoting the aggregation of inclusions and bubbles in molten steel by forming a gas space of an inert gas inside an injection nozzle, entraining the inert gas into the injection flow at the molten steel surface in the nozzle, and stirring the injection flow in a stirring box installed at the bottom of the tundish. In the technique disclosed in Patent Document 3, since the inner diameter of the discharge hole of the injection nozzle of the continuous casting apparatus is sufficiently large, a gas space is stably formed. On the other hand, in the technique disclosed in Patent Document 3, in order to avoid the situation where all the bubbles entrained by the injection flow re-float into the injection nozzle and the stirring action in the stirring box is not sufficiently exerted, the inner diameter of the injection nozzle is set to be equal to or less than a predetermined value. However, the bubbles that have left the injection nozzle and floated on the molten steel surface in the tundish can cause reoxidation as described above. In addition, the phase state of the gas-liquid two-phase flow in the nozzle depends on the flow velocity and dispersion state of each phase, and is generally represented by the cross-sectional void fraction. Therefore, depending on the amount of molten steel injected, the entrainment of the inert gas may take an undesirable form, and there is a risk of causing the outflow and reoxidation of inclusions into the mold by disturbing the flow in the tundish.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present application discloses a technique capable of suppressing the detachment of bubbles introduced into the molten steel in the nozzle from the nozzle when supplying molten steel from one container to another container through the nozzle, and suppressing the generation of bare molten metal in the other container.

Means for Solving the Problems

[0012] One means for solving the above problems is that the present application provides a system for supplying molten steel from a first container to a second container via a nozzle, wherein the nozzle has an inlet on the side of the first container which is the upstream side, an outlet on the side of the second container which is the downstream side, and extends downward from the upstream side to the downstream side, wherein the outlet of the nozzle is located below the liquid level of the molten steel supplied to the second container and above the bottom surface of the second container, and the flow rate Q (ton / min) of the molten steel flowing inside the nozzle, the minimum nozzle inner diameter D1 (mm) upstream of the outlet, and the nozzle inner diameter D2 (mm) at the outlet satisfy the following relationships (1) to (3). Molten steel supply system is disclosed.

[0013]

Number

[0014] In the system of the present disclosure, the nozzle may have a diameter-expanded portion between the inlet and the outlet, and the nozzle inner diameter may expand from the upstream side to the downstream side in the diameter-expanded portion.

[0015] The system of the present disclosure may have a blowing mechanism for blowing an inert gas into the nozzle.

[0016] In the system of the present disclosure, the nozzle may have a first nozzle portion and a second nozzle portion, the first nozzle portion may be provided upstream of the second nozzle portion, the first nozzle portion and the second nozzle portion may be connected to each other, and the second nozzle portion may include the outlet.​ The second nozzle portion may include a long nozzle portion.

[0017] In the system of the present disclosure, The first nozzle portion may have a flow rate adjusting mechanism.

[0018] The system of the present disclosure may have a blowing mechanism for blowing an inert gas into the inside of the nozzle, The position where the inert gas is blown by the blowing mechanism may be within a range of 100 mm from the connection portion between the first nozzle portion and the second nozzle portion.

[0019] In the system of the present disclosure, The first container may be a molten steel ladle, and the second container may be a tundish.

[0020] As one means for solving the above problems, the present application provides A method for continuous casting of steel using the system of the present disclosure, Blowing an inert gas into the inside of the nozzle so that the pressure of the gas inside the nozzle is 0.9 atm or more and 1.1 atm or less, A method for continuous casting of steel including is disclosed.

Advantages of the Invention

[0021] According to the technology of the present disclosure, when supplying molten steel from one container to another container through a nozzle, it is possible to suppress the bubbles introduced into the molten steel in the nozzle from detaching from the nozzle, and it is possible to suppress the generation of bare molten metal in the other container.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

DETAILED DESCRIPTION OF THE INVENTION

[0023] 1. Molten Steel Supply System An example of the configuration of the molten steel supply system is shown in FIGS. 1 to 3. As shown in FIG. 1, the molten steel supply system 100 is a system that supplies molten steel 105 from a first container 101 to a second container 102 via a nozzle 110. As shown in FIGS. 2 and 3, in the system 100, the nozzle 110 has an inlet 110a on the side of the first container 101 which is the upstream side, an outlet 110b on the side of the second container 102 which is the downstream side, and extends downward from the upstream side to the downstream side. Further, the outlet 110b of the nozzle 110 is located below the liquid level 105a of the molten steel 105 supplied to the second container 102 and above the bottom surface 102a of the second container 102. In the system 100, the flow rate Q (ton / min) of the molten steel 105 flowing through the inside of the nozzle 110, the minimum nozzle inner diameter D1 (mm) on the upstream side of the outlet 110b, and the nozzle inner diameter D2 (mm) at the outlet 110b satisfy the following relationships (1) to (3).

[0024]

Number

[0025] 1.1 First Container The first container 101 is a container that serves as a source for supplying the molten steel 105 to the second container 102. As shown in FIGS. 1 to 3, in the system 100, the first container 101 has a bottom surface 101a and side walls 101b, and holds the molten steel 105. The first container 101 may further have a lid (not shown). The first container 101 may be made of a shape and material capable of holding the molten steel 105. Also, an outlet 101ax may be provided in a part of the bottom surface 101a of the first container 101, and it may be configured such that the molten steel 105 can flow out therefrom. An opening and closing mechanism for controlling the outflow amount of the molten steel 105 may be provided at the outlet 101ax. A nozzle 110 may be directly or indirectly connected to the outlet 101ax of the first container 101. The connection form between the first container 101 and the nozzle 110 is not particularly limited, and for example, they can be connected by fitting. The first container 101 and the nozzle 110 may be connected via some intermediate member.

[0026] The first container 101 may be any container capable of holding the molten steel 105, and various forms are assumed. In particular, when the first container 101 is a ladle, the effects of applying the technology of the present disclosure become more prominent.

[0027] 1.2 The second container The second container 102 is a container that serves as the destination for the supply of molten steel 105 from the first container 101. As shown in FIGS. 1 to 3, the second container 102 has a bottom surface 102a and side walls 102b, and holds the molten steel 105 supplied from the first container 101. The second container 102 may further have a lid 102c. The second container 102 may be made of a shape and material capable of holding the molten steel 105. As shown in FIG. 1, an outlet 102ax may be provided in a part of the bottom surface 102a of the second container 102, and the molten steel 105 may be configured to flow out therefrom to another container (for example, a mold). An opening and closing mechanism for controlling the outflow amount of the molten steel 105 may be provided at the outlet 102ax. A nozzle 120 may be directly or indirectly connected to the outlet 102ax of the second container 102. The connection form between the second container 102 and the nozzle 120 is not particularly limited, and for example, they can be connected by fitting. The second container 102 and the nozzle 120 may be connected via some intermediate member.

[0028] The second container 102 may be any container capable of holding the molten steel 105, and various forms are envisaged. The second container 102 may be, for example, a tundish, a ladle, a mold (in this case, the first container 101 may be a tundish), or a furnace for adjusting the components of the molten steel 105. In particular, when the second container 102 is a tundish, the effects of applying the technology of the present disclosure become more prominent.

[0029] As shown in FIG. 3, a floating layer 106 containing a flux may exist on the liquid surface 105a of the molten steel 105 supplied to the second container 102. As the flux, a known flux may be adopted. By covering the liquid surface 105a of the molten steel 105 with the flux in this way, the molten steel 105 can be shielded from the outside air. Further, non-metallic inclusions in the molten steel 105 can be recovered by the flux. Incidentally, as will be described later, according to the system 100, the reverse upward flow caused by the molten steel 105 flowing out from the nozzle 110 and colliding with the bottom surface 102a of the second container 102 can be greatly suppressed, the liquid surface 105a of the molten steel 105 is difficult to be disturbed, and it is difficult for the flux to be pushed back or interrupted due to the disturbance of the liquid surface 105a, so the problem of reoxidation due to bare molten metal is less likely to occur.

[0030] 1.3 Nozzle The nozzle 110 allows the molten steel 105 to flow from the first container 101 to the second container 102. That is, the nozzle 110 has an inlet 110a on the side of the first container 101 which is the upstream side, and an outlet 110b on the side of the second container 102 which is the downstream side. The nozzle 110 extends downward from the upstream side to the downstream side. The nozzle 110 may be, for example, a cylindrical body (a cylindrical single-hole nozzle) that extends downward from the upstream side to the downstream side. Specifically, the nozzle 110 may be a cylindrical body having a central axis in the vertical direction. As shown in FIG. 2, the nozzle 110 has a minimum nozzle inner diameter D1 on the upstream side of the outlet 110b, and a nozzle inner diameter D2 at the outlet 110b. Incidentally, in the present application, the "nozzle inner diameter" refers to the inner diameter of the nozzle portion below the flow rate adjusting mechanism (such as a sliding gate or other flow rate adjusting mechanism 114) for adjusting the flow rate of the molten steel 105 when there is a mechanism for adjusting the flow rate of the molten steel 105, and when there is no such flow rate adjusting mechanism 114, it refers to the inner diameter of the nozzle portion between the first container 101 and the second container 102. The flow path in the mechanism 114 is not included in the "nozzle inner diameter".

[0031] As shown in FIG. 3, the outlet 110b of the nozzle 110 is located below the liquid surface 105a of the molten steel 105 supplied to the second container 102 and above the bottom surface 102a of the second container 102. That is, the nozzle 110 has its downstream tip immersed in the molten steel 105 of the second container 102. As shown in FIG. 3, the system 100 may have a distance h1 between the liquid surface 105a of the molten steel 105 in the second container 102 and the outlet 110b of the nozzle 110, and may have a distance h2 between the outlet 110b of the nozzle 110 and the bottom surface 102a of the second container 102. The specific values of h1 and h2 and the relationship between h1 and h2 are not particularly limited. For example, h1 may be 100 mm or more and 500 mm or less, h2 may be 300 mm or more and 900 mm or less, the ratio h1 / h2 of h1 to h2 may be 0.2 or more and 0.7 or less, and the ratio D2 / h2 of the nozzle inner diameter D2 (see FIG. 2) at the outlet 110b to h2 may be 1.0 or more and 3.0 or less. By adjusting h1 and h2, the detachment of the bubbles 105x generated by the mechanism described later from the nozzle 110 and the like are more easily suppressed.

[0032] As shown in FIGS. 2 and 3, the nozzle 110 may have a diameter-expanded portion 110c between the inlet 110a and the outlet 110b, and in the diameter-expanded portion 110c, the nozzle inner diameter may expand from the upstream side to the downstream side. The rate of diameter expansion in the diameter-expanded portion 110c is not particularly limited. In the diameter-expanded portion 110c, the nozzle inner diameter may expand linearly (expand with monotonic increase) from the upstream side to the downstream side, or may expand curvilinearly. Also, in the diameter-expanded portion 110c, the nozzle inner diameter may expand continuously or intermittently from the upstream side to the downstream side. Also, the ratio of the length of the diameter-expanded portion 110c to the overall length of the nozzle 110 is not particularly limited. As shown in FIGS. 2 and 3, in the nozzle 110, the nozzle inner diameter at the lower end of the diameter-expanded portion 110c and the nozzle inner diameter D2 at the outlet 110b may be substantially the same or may be different from each other.

[0033] As shown in FIGS. 2 and 3, the nozzle 110 may have a first nozzle portion 111 and a second nozzle portion 112. The first nozzle portion 111 may be provided upstream of the second nozzle portion 112. The first nozzle portion 111 and the second nozzle portion 112 may be connected to each other. The second nozzle portion 112 may include an outlet 110b. The second nozzle portion 112 may include a long nozzle portion. That is, the nozzle 110 may be one in which a first nozzle and a second nozzle having different shapes from each other are connected via a connection portion 113. The connection form between the first nozzle and the second nozzle in this case is not particularly limited. For example, they may be connected by fitting. Note that the "long nozzle portion" refers to a portion constituted by a long nozzle. The long nozzle portion may have a shape that satisfies the above relations (1) to (3). For example, the long nozzle portion may have the above-described enlarged diameter portion 110c. As shown in FIGS. 2 and 3, the nozzle portion is installed independently of the second container 102 and does not need to be fixed to the second container 102. In this regard, the configuration of the injection pipe installed and fixed to the lid of the second container is clearly different from that of the nozzle portion (nozzle) referred to in the present application.

[0034] As shown in FIGS. 2 and 3, when the nozzle 110 has a first nozzle portion 111 and a second nozzle portion 112, the first nozzle portion 111 may have a flow rate adjustment mechanism 114. As a specific example of the flow rate adjustment mechanism 114, for example, a sliding gate as shown in FIGS. 2 and 3 can be mentioned. That is, the first nozzle portion 111 may be a portion constituted by a sliding nozzle. In the sliding gate, at least one slide plate 114a having a flow passage opening is slid in a direction intersecting the flow direction of the molten steel 105, so that the flow path can change. Alternatively, the flow rate adjustment mechanism 114 may be an opening / closing mechanism other than the sliding gate. Since the form of the flow rate adjustment mechanism 114 in the nozzle 110 itself is well-known, further description thereof is omitted here. Note that, as described above, the flow path in the flow rate adjustment mechanism 114 is not included in the nozzle inner diameter referred to in relations (1) and (2).

[0035] 1.4 Other mechanisms In addition to each of the above configurations, the molten steel supply system 100 may include other mechanisms. For example, as shown in FIGS. 2 and 3, the system 100 may have a gas injection mechanism 116 that injects an inert gas into the nozzle 110. As described above in the background art, when supplying molten steel from one container to another through a nozzle, outside air may be taken into the nozzle by an ejector from a fitting portion of the nozzle or the like. However, in the system 100, separately from this, a mechanism 116 for intentionally injecting an inert gas into the nozzle 110 can be adopted. As will be described later, in the system 100 of the present disclosure, by satisfying the above relationships (1) to (3) during the supply of molten steel 105, a gas space 115 can be maintained inside the nozzle 110, and at the liquid surface 105a of the molten steel 105 inside the nozzle 110, gas is entrained from the gas space 115 into the molten steel 105 along with the flow of the molten steel 105, and various bubble groups of different sizes can be generated in the molten steel 105. By having the mechanism 116 for injecting an inert gas into the nozzle 110 in the system 100, it becomes easier to maintain the above gas space 115 inside the nozzle 110. Examples of the inert gas include Ar. The pressure of the inert gas in the gas space 115 is not particularly limited, but may be, for example, 0.9 atm or more and 1.1 atm or less.

[0036] The specific form of the mechanism 116 for injecting an inert gas into the nozzle 110 is not limited. For example, the injection mechanism 116 can be configured by connecting an inert gas supply source (such as a container filled with high-pressure inert gas) and the nozzle 110 with piping or the like. Alternatively, the inert gas supply source may be connected to a position upstream of the nozzle 110 (for example, the first container 101) so that the molten steel 105 into which the inert gas has been injected flows into the nozzle 110. However, when the inert gas supply source and the nozzle 110 are connected, it becomes easier to control the amount of inert gas injected into the nozzle 110.

[0037] As shown in FIGS. 2 and 3, when the nozzle 110 has the first nozzle portion 111 and the second nozzle portion 112, the position where the inert gas is blown by the above-described blowing mechanism 116 may be within a range of 100 mm from the connection portion 113 between the first nozzle portion 111 and the second nozzle portion 112. If it is within this range, it is easy to install the above-described blowing mechanism 116 on the nozzle 110. Further, by blowing the inert gas within this range, it becomes easier to form the above-described gas space 115 inside the nozzle 110.

[0038] 1.5 Relationships (1) to (3) In the molten steel supply system 100, the flow rate Q (ton / min) of the molten steel 105 flowing inside the nozzle 110 as described above, the minimum nozzle inner diameter D1 (mm) upstream of the outlet 110b, and the nozzle inner diameter D2 (mm) at the outlet 110b need to satisfy the above relationships (1) to (3).

[0039] In the above relationship (3), the flow rate Q is 1.0 ton / min or more and 15.0 ton / min or less. The flow rate Q may be 1.5 ton / min or more, 2.0 ton / min or more, 2.5 ton / min or more, or 3.0 ton / min or more, and may be 14.0 ton / min or less, 13.0 ton / min or less, 12.0 ton / min or less, 11.0 ton / min or less, or 10.0 ton / min or less. From the viewpoint of being particularly often adopted in actual operation and obtaining particularly high effects, the flow rate Q may be 7.0 ton / min or more and 12.0 ton / min or less.

[0040] In the above relationship (2), the minimum nozzle diameter D1 is 30 mm or more and 140 mm or less. The minimum nozzle diameter D1 may be 35 mm or more, 40 mm or more, 45 mm or more, or 50 mm or more, and may be 130 mm or less, 120 mm or less, 110 mm or less, or 100 mm or less. From the viewpoint of being particularly often adopted in actual operation and obtaining particularly high effects, the minimum nozzle diameter D1 may be 70 mm or more and 120 mm or less.

[0041] In the molten steel supply system 100, on the premise that the above relationships (2) and (3) are satisfied, and further, when the above relationship (1) is satisfied, a predetermined effect described later is exhibited. In the system 100, the flow rate Q may be adjusted according to the nozzle diameters D1 and D2 so that the above relationship (1) is satisfied, or an optimal nozzle shape (nozzle diameters D1 and D2) may be selected according to the target flow rate Q.

[0042] 1.6 Actions and effects As shown in FIG. 3, in the system 100, by expanding the inner diameter of the nozzle 110 on the outlet 110b side of the nozzle 110, a clear gas phase (gas space 115) is formed in the nozzle 110 while introducing an inert gas from the outside through the gas injection mechanism 116 if necessary. At the liquid level 105a in the nozzle 110, the gas space 115 is involved in the injection flow as a bubble group 105x in the sub-millimeter to millimeter order. Among these, the larger the bubble diameter, the greater the lifting force acting in the horizontal direction. Due to the buoyancy of the bubble 105x, the downward flow velocity of the injection flow, particularly the flow in the center of the nozzle 110 with a high flow velocity, is decelerated, and the flow field is easily made uniform. When the above relationships (1) to (3) are satisfied, the generation and maintenance of the above gas space 115 and bubble group 105x are easy, and the bubbles 105x flowing out from the nozzle 110 are easily recovered into the nozzle 110. The bubbles 105x recovered into the nozzle 110 function as the above gas space 115 or remain as bubbles 105x in the nozzle 110. In this way, in the system 100, the detachment of the bubbles 105x from the inside of the nozzle 110 is suppressed, so that the deceleration effect of the above injection flow is maintained, and the reverse upward flow after the collision with the bottom surface 102a of the second container 102 is also reduced. As a result, it is difficult for a situation (the problem of reoxidation due to bare molten metal) in which the bubbles 105x push back the flux floating on the liquid surface 105a of the molten steel 105 in the second container 102 and the molten steel 105 is exposed to the outside air to occur. Furthermore, since the discharge flow velocity from the nozzle 110 is made uniform, the molten steel flow in the second container 102 becomes closer to plug flow toward the outlet, so that the formation of a short path in the second container 102 is prevented, and the risk of the flux and inclusions being brought into the next process is reduced. As a secondary effect, the bubble group 105x pounded at the liquid level 105a in the nozzle 110 is confined in the nozzle 110, and the gas phase ratio in the nozzle 110 becomes high. Therefore, the inclusions can be efficiently floated and removed by the adhesion of the bubbles 105x to the intervening part.

[0043] 2. Method for continuous casting of steel The technology of the present disclosure also has aspects as a method for continuous casting of steel. That is, the method for continuous casting of steel of the present disclosure is characterized by using the above-described system 100 of the present disclosure. Here, the method for continuous casting of steel of the present disclosure may include blowing an inert gas into the nozzle 110 such that the pressure of the gas inside the nozzle 110 is 0.9 atm or more and 1.1 atm or less. Details of the mechanism for blowing the inert gas into the nozzle 110 are as described above. In the continuous casting method of the present disclosure, for example, the first container 101 may be a molten steel ladle and the second container 102 may be a tundish. In this case, the method for continuous casting of steel includes supplying the molten steel 105 from the molten steel ladle to the tundish via the nozzle 110, supplying the molten steel 105 from the tundish to a mold (not shown) via the nozzle 120, and continuously withdrawing a slab from the mold. In the method for continuous casting of steel of the present disclosure, general continuous casting conditions may be employed except that the above-described system 100 is adopted.

[0044] Incidentally, the pressure of the gas inside the nozzle 110 can be measured by various methods. For example, a member having a pressure measurement space is connected to a part of the nozzle 110, and the gas space 115 inside the nozzle 110 and the space of the member are communicated, so that the pressure of the space of the member is made substantially the same as the gas space 115 inside the nozzle 110, and then the pressure of the space of the member is measured, whereby the pressure of the gas inside the nozzle 110 can be measured.

Examples

[0045] Hereinafter, the present invention will be further described while showing examples, but the present invention is not limited to the following examples. The present invention can adopt various conditions as long as it does not deviate from the gist and can achieve its purpose.

[0046] Generally, it is known that observing a system containing molten steel is extremely difficult because the object to be observed is very hot and opaque, and there is the presence of dust, etc. Therefore, usually, visualization is attempted by reproducing the flow and heat transfer by numerical fluid dynamics. However, when there is a free interface such as in a gas-liquid two-phase flow, there is a density deviation between different phases, and the shape of the interface changes greatly over time. Therefore, an enormous amount of time is required to accurately calculate its behavior. Thus, the inventor utilized the fact that the Fr number, which is the ratio of the inertial force to the buoyant force and is represented by the following formula (4), and the We number, which is the ratio of the surface tension to the inertial force and is represented by the following formula (5), generally match the molten steel system in a 1 / 2 scale water model at room temperature, and decided to investigate the behavior of the gas phase in the molten steel.

[0047]

Number

[0048] Fig. 4 schematically shows the configuration of the apparatus used in the water model experiment. To facilitate observation, the nozzle and the water receiving container were made of acrylic material, and water with a predetermined flow rate was continuously supplied from above via a pump and a mass flow controller. Also, the water receiving container is provided with drain outlets at the four corners of the bottom surface, and each drain outlet is equipped with a throttling mechanism. While adjusting the drainage volume according to the flow rate to maintain the water level in the water receiving container at 1000 mm, the tip of the outlet side of the nozzle was immersed by 100 mm. After confirming that the water level in the water receiving container was stable, the gas inlet was temporarily opened until the water surface inside the nozzle and the water surface of the water receiving container were generally the same, thereby forming a gas space inside the nozzle.

[0049] 1. Observation and evaluation of bubbles The water supply amount (convertible to the flow rate Q of the molten steel) and the nozzle shape (the minimum nozzle inner diameter D1 and the nozzle inner diameter D2 at the outlet) were changed, and the bubbles were observed and evaluated respectively. For each condition, as described above, a gas space was formed inside the nozzle, and bubbles were generated downward from the liquid surface inside the nozzle due to entrainment by the water flow, and at least a part of the bubbles flowed out below the outlet of the nozzle. Whether the bubbles flowing out of the nozzle under each condition were recovered inside the nozzle (whether the generated bubbles detached from the nozzle and did not float on the liquid surface of the water receiving container outside the nozzle but returned to the inside of the nozzle again) was visually confirmed for 10 minutes. Those in which almost all the bubbles were recovered (the disappearance of the gas space was less than 50% by volume) were marked as "○", those with a small amount of bubble detachment (the gas space disappeared by 50% to less than 80% by volume) were marked as "△", and those in which most of the bubbles detached from the nozzle and the gas space almost completely disappeared (the gas space disappeared by 80% or more) were evaluated as "×". The results are shown in Table 1 and Figure 5. Incidentally, the throughput in Table 1 is a value of the molten steel system converted so that the flow at the nozzle inlet satisfies the Fr number approximation. That is, the value obtained by converting the water supply amount in the water model experiment to the flow rate Q of the molten steel in the molten steel system is shown.

[0050]

Table 1

[0051] As shown in Table 1 and Figure 5, when the flow rate Q (ton / min), the minimum nozzle inner diameter D1 (mm), and the nozzle inner diameter D2 (mm) at the outlet satisfy the following relationships (1) to (3), almost all the bubbles flowing out of the nozzle were recovered inside the nozzle. On the other hand, when the following relationship (1) is not satisfied, the bubbles entrained at the gas-liquid interface inside the nozzle immediately after closing the gas inlet are carried along in the discharge flow, and the gas space exists inside the nozzle for only a short time, and almost all the gas is discharged outside the nozzle in several seconds to several tens of seconds. That is, when the following relationships (1) to (3) are satisfied, it is considered that the arrival of bubbles at the molten steel liquid surface outside the nozzle can be suppressed, and the problem of reoxidation of the molten steel by the bare bath can be suppressed.

[0052]

Number

[0053] 2. Evaluation of flow velocity In order to confirm the effect of suppressing the occurrence of short paths due to the generation of the above-mentioned bubbles, for each condition, the flow velocity was measured with an electromagnetic flowmeter at a point on the nozzle central axis 300 mm from the water surface. As a result, when the above relationship (1) was satisfied, the discharge flow velocity on the central axis decreased more clearly than when the above relationship (1) was not satisfied. That is, when relationships (1) to (3) are satisfied, an effect of suppressing short paths can be expected, an effect of suppressing reverse upward flow can be expected, and an effect of preventing reoxidation of molten steel by bare molten metal can be expected.

Explanation of symbols

[0054] 100 Molten steel supply system 101 First container 101a Bottom surface 101b Side wall 102 Second container 102a Bottom surface 102b Side wall 102c Lid 105 Molten steel 105x Bubbles (bubble group) 110 Nozzle 110a Inlet 110b Outlet 110c Diameter-expanded part 111 First nozzle part 112 Second nozzle part 113 Connection part 114 Flow rate adjustment mechanism (sliding gate) 114a Slide plate 115 Gas space 116 Gas injection mechanism 120 Nozzle

Claims

1. A system for supplying molten steel from a first container to a second container via a nozzle, wherein the nozzle has an inlet on the side of the first container, which is the upstream side, an outlet on the side of the second container, which is the downstream side, and extends downward from the upstream side to the downstream side, wherein the outlet of the nozzle is located below the liquid level of the molten steel supplied to the second container and above the bottom surface of the second container, wherein the distance h1 between the liquid level of the molten steel in the second container and the outlet of the nozzle is 100 mm or more and 500 mm or less, the distance h2 between the outlet of the nozzle and the bottom surface of the second container is 300 mm or more and 900 mm or less, and the ratio h1 / h2 of h1 to h2 is 0.2 or more and 0.7 or less, The flow rate Q (ton / min) of the molten steel flowing inside the nozzle, and the minimum nozzle inner diameter D １ (mm) on the upstream side of the outlet, and the nozzle inner diameter D at the outlet ２ (mm) satisfy the following relationships (1) to (3): A molten steel supply system. 【Number 1】

2. wherein the nozzle has a diameter-expanded portion between the inlet and the outlet, and in the diameter-expanded portion, the inner diameter of the nozzle expands from the upstream side to the downstream side, The system according to claim 1.

3. having a blowing mechanism for blowing an inert gas into the nozzle, The system according to claim 1 or 2.

4. wherein the nozzle has a first nozzle portion and a second nozzle portion, wherein the first nozzle portion is provided on the upstream side of the second nozzle portion, wherein the first nozzle portion and the second nozzle portion are connected to each other, wherein the second nozzle portion includes the outlet, wherein the second nozzle portion includes a long nozzle portion, The system according to any one of claims 1 to 3.

5. wherein the first nozzle portion has a flow rate adjusting mechanism, The system according to claim 4.

6. having a blowing mechanism for blowing an inert gas into the nozzle, wherein the position where the inert gas is blown by the blowing mechanism is within a range of 100 mm from the connection portion between the first nozzle portion and the second nozzle portion, The system according to claim 4 or 5.

7. wherein the first container is a molten steel ladle and the second container is a tundish, The system according to any one of claims 1 to 6.

8. A method for continuous casting of steel using the system according to any one of claims 1 to 7, wherein an inert gas is blown into the nozzle so that the pressure of the gas inside the nozzle is 0.9 atm or more and 1.1 atm or less, A method for continuous casting of steel including this.

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