Nozzle System

Abstract

To disclose a technique which restricts removal of air bubbles entrained in molten metal inside a nozzle from the nozzle and suppresses occurrences of bare melts and short paths in a tundish, in order to enhance the cleanliness of molten metal when molten metal is supplied from a ladle to a tundish via a nozzle.SOLUTION: In a nozzle system for supplying molten metal from a ladle to a tundish via a nozzle, the relationships of the following formulas (1)-(4) are satisfied. 1.0≤D1 / D0≤1.4 ...(1) 0.015≤((D2 / 2)2-(D1 / 2)2) / (U1*D1) ...(2) 1.2≤V*D1 / (4Q)≤4.8 ...(3) D1+8.8Q / V≤D3≤D1+35.2Q / V ...(4)SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present application discloses a nozzle system for delivering molten metal from a ladle through a nozzle to a tundish. [Background technology]

[0002] In the continuous casting process of molten metal, a tundish is used as an intermediate vessel for supplying molten metal from a ladle to a mold. For example, in the case of continuous casting of steel, the tundish has multiple functions, such as (1) stabilizing the amount of molten steel supplied to the mold, (2) distributing molten steel to multiple molds, (3) acting as a buffer to continuously perform continuous casting using multiple ladles, and (4) removing nonmetallic inclusions. In particular, the function of (4) removing nonmetallic inclusions is extremely important when efficiently producing high-quality steel with high cleanliness.

[0003] Nonmetallic inclusions in molten steel are mainly derived from impurities in steel, such as oxides, nitrides, sulfides, and bubbles that are generated during the steelmaking process. It is known that if such nonmetallic inclusions remain in the final product, they can become the starting point of fracture due to stress concentration, for example, and reduce the material quality of the final product. In addition, in the steelmaking process itself, nonmetallic inclusions adhere and accumulate on the inner walls of refractory flow passages, causing narrowing and clogging of the flow passages, which not only hinders smooth manufacturing, but also can cause defects both on the surface and inside of the base material during processing such as casting, thereby reducing product yields and putting pressure on manufacturing costs. Therefore, in many cases, nonmetallic inclusions need to be removed from molten steel in a limited process from secondary refining, where the final adjustment of the molten steel components is made, to the casting mold.

[0004] In order to remove nonmetallic inclusions from molten steel, a method is generally adopted in which the nonmetallic inclusions are floated in the molten steel by utilizing the difference in specific gravity between the molten steel and the nonmetallic inclusions, and then collected in a floating layer of oxides called flux, but it is known that the floating speed in this case decreases as the nonmetallic inclusions become smaller, and the time required to collect them in the flux layer increases. Therefore, in order to ensure the time required for the nonmetallic inclusions to float in order to reduce the nonmetallic inclusions in molten steel, it is considered effective to extend the residence time of the nonmetallic inclusions in the tundish.

[0005] Generally, molten steel is supplied from a ladle to a tundish by using potential energy to flow down through a sliding nozzle with a flow rate control function and a long nozzle, which is a cylindrical refractory material with its lower end immersed in the molten steel in the tundish. However, it is not always easy to ensure the residence time of the molten steel in the tundish because the high-speed discharge flow from the long nozzle can collide with the bottom of the tundish, forming a short-circuit flow toward the mold called a short pass (see Figs. 3 and 6). A common solution to this problem is to provide a weir inside the tundish to divert the molten steel flow, but providing a refractory material inside the tundish increases the material cost, construction time, and workload, creates a space near the weir where there is almost no flow and does not contribute to floating and removal, and may induce a new flow that detours but flows toward the mold at high speed, so it does not necessarily help the inclusions to float up. In particular, small inclusions have little buoyancy and tend to follow the flow of molten steel, so the effect of detouring is likely to be limited to removing large inclusions.

[0006] In addition, in the steelmaking process, attention must be paid to the unintended increase of nonmetallic inclusions due to reoxidation of molten steel. In general, in order to avoid the difficulty of stable casting due to gas generation caused by a drop in the temperature of molten steel, molten steel to be subjected to continuous casting is deoxidized in the refining process, and the oxygen concentration is much lower than the soluble oxygen concentration, making it very susceptible to oxygen absorption. When air or low-grade oxides come into contact with molten steel, the molten steel absorbs oxygen, which combines with elements (such as Al and Si dissolved in the molten steel) that have a higher affinity for oxygen than the molten steel, resulting in the reoxidation phenomenon in which nonmetallic inclusions are generated. For this reason, in the molten steel ladle or tundish, it is necessary to reduce the oxygen concentration in the tundish by replacing the atmosphere with an inert gas, or to cover the surface of the molten steel with a low-reactivity flux that contains a small amount of low-grade oxides to isolate the molten steel from the outside air. However, when molten steel is supplied to a tundish through a long nozzle, the molten steel flow discharged from the nozzle as described above is so fast that the flux covering the molten steel surface near the long nozzle is pushed aside by the reverse upward flow generated by collision with the bottom of the tundish, and the molten steel surface is directly exposed to the outside air as bare molten metal, which may cause a re-oxidation phenomenon in which the molten steel absorbs oxygen from the atmosphere (see Figures 3 and 7).Alternatively, since slag with a high concentration of low-grade oxides such as FeO that has flowed out from the ladle is present near the long nozzle, the strong molten steel flow near the long nozzle may cause re-oxidation of the molten steel by the low-grade oxides in the slag.

[0007] Conventionally, various means for solving the above problems have been proposed. For example, Patent Document 1 discloses a continuous casting method using an injection tube in which the injection flow is isolated from the outside air by a refractory cylindrical body with an inner diameter of 300 mm or more. According to the technology disclosed in Patent Document 1, it is considered possible that the injection flow falling from the nozzle knocks the gas in the tube at the liquid surface in the tube, thereby introducing a large number of bubbles into the molten steel, and the large buoyancy of the bubbles reduces the injection flow rate, thereby 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 expected that they will rise and be removed at high speed due to the large buoyancy of the bubbles. On the other hand, in the above-mentioned injection tube, when the splashes of molten steel discharged from the ladle nozzle or the molten steel scattered on the bath surface adhere to the inner wall of the tube, the heat of the deposits is significantly removed due to the large diameter of the tube, and the deposits solidify and accumulate, easily leading to blockage. This is a major problem that makes it difficult to continue the operation of continuous casting. In addition, the injection tube must be filled with a large amount of inert gas to prevent the molten steel from absorbing oxygen and nitrogen from the gas phase, which results in higher operating costs than when a long nozzle is immersed in the molten steel.

[0008] Patent Document 2 discloses a nozzle with a flow control section in the body to prevent exposure of bare molten metal due to an upward flow that reverses at the bottom of the tundish. However, the technology disclosed in Patent Document 2 requires additional measures to prevent the occurrence of short passes, which increases manufacturing costs. In addition, the increased weight of the nozzle places a heavy load on the device that holds the nozzle, which makes it impossible to support the nozzle during casting, and there is a risk that molten steel will spray out from the joint of the long nozzle.

[0009] Patent Document 3 discloses a continuous casting method and apparatus that form a gas space for an inert gas inside an injection nozzle, entrain the inert gas in the injection flow on the steel bath surface inside the nozzle, and stir the injection flow in a stirring box installed on the bottom surface of a tundish to promote the aggregation of inclusions and bubbles in the molten steel. In the technique disclosed in Patent Document 3, the injection nozzle of the continuous casting apparatus has a sufficiently large inner diameter of the discharge hole, so that the gas space is formed stably. On the other hand, the technique disclosed in Patent Document 3 is characterized in that the inner diameter of the injection nozzle is set to a predetermined value or less in order to prevent all bubbles entrained by the injection flow from re-floating into the injection nozzle and the stirring action in the stirring box from being fully exerted. However, the bubbles that leave the injection nozzle and rise to the surface of the molten steel in the tundish can cause re-oxidation as described above. There is also a problem that the stirring box is expensive and requires a lot of work to install. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 6575355 [Patent Document 2] Special Publication No. 2020-530813 [Patent Document 3] JP 2011-235339 A Summary of the Invention [Problem to be solved by the invention]

[0011] The present application discloses a technology that can increase the cleanliness of molten metal by suppressing the occurrence of bare metal and short passes in a tundish when molten metal is supplied from a ladle to a tundish through a nozzle. [Means for solving the problem]

[0012] As one of the means for solving the above problems, the present application provides: 1. A nozzle system for delivering molten metal from a ladle through a nozzle to a tundish, comprising: the nozzle is a cylindrical single-hole nozzle having an inlet on the ladle side, which is an upstream side, and an outlet on the tundish side, which is a downstream side, and extending downward from the upstream side to the downstream side; The outlet of the nozzle is located below the liquid level of the molten metal in the tundish and above the bottom surface of the tundish, the nozzle comprises a long nozzle section including the outlet, a collector nozzle section disposed upstream of the long nozzle section, and a connection section between the long nozzle section and the collector nozzle section, an inert gas is supplied from the outside to the inside of the nozzle, and a gas phase region containing the inert gas and a secondary meniscus of the molten metal exist inside the long nozzle portion; The relationships of the following formulas (1) to (4) are satisfied. Nozzle System Disclose.

[0013] 1.0≦D1 / D0≦1.4 … (1) 0.015≦((D2 / 2) 2 -(D1 / 2) 2 ) / (U1 D1) …(2) 1.2≦V·D1 / (4Q)≦4.8 …(3) D1+8.8Q / V≦D3≦D1+35.2Q / V…(4)

[0014] Where: D0 is the inner diameter (cm) of the collector nozzle portion at the connection portion, D1 is the minimum inner diameter of the long nozzle section upstream of the secondary meniscus (cm); D2 is the inner diameter of the long nozzle portion at the secondary meniscus (cm), D3 is the inner diameter of the long nozzle portion at the outlet (cm), U1 is the average downward flow velocity (cm / s) of the molten metal at the portion of the inner diameter D1, Q is the flow rate (cm) of the molten metal supplied to the nozzle. 3 / s), V is the volume (cm) of the long nozzle section from the secondary meniscus to the outlet 3 ).

[0015] In the nozzle system of the present disclosure, ΔH defined by the following formula (5) may be within the range of −10 cm to +50 cm.

[0016] ΔH = (P T -P L ) / (ρ·g) …(5)

[0017] Where: P T is the atmospheric pressure in the tundish, P L is the pressure of the gas phase region in the long nozzle section, ρ is the density of the molten metal; g is the acceleration due to gravity.

[0018] In the nozzle system of the present disclosure, The flow rate Q is 8000 cm 3 / s or more, The molten metal may have an average residence time τ in the tundish of 240 seconds or more.

[0019] In the nozzle system of the present disclosure, the tundish may not have a dam therein. Effect of the Invention

[0020] According to the technology disclosed herein, when molten metal is supplied from a ladle to a tundish through a nozzle, a gas phase region and a secondary meniscus of the molten metal are formed in the nozzle, and the downward flow rate of the molten metal is easily reduced and non-metallic inclusions are easily captured due to the gas phase entrainment phenomenon (formation of a bubble plume) when the falling flow of the molten metal collides with the secondary meniscus. In addition, by satisfying a predetermined relationship, large bubbles that form a bubble plume are easily collected in the nozzle, the molten metal surface in the tundish around the nozzle is kept calm, and reoxidation is easily prevented. As a result, it is possible to suppress the occurrence of bare metal and short passes in the tundish, and the cleanliness of the molten metal can be improved. [Brief description of the drawings]

[0021] [Figure 1] 1 shows a schematic diagram of an example of the positional relationship between a ladle, a nozzle, and a tundish. [Diagram 2] 2 shows a schematic diagram of an example of a cross-sectional shape of a nozzle. [Diagram 3] 1 illustrates a schematic diagram of a problem in the prior art. [Figure 4] 2 shows a schematic configuration of a nozzle system. [Diagram 5] 2 shows a schematic configuration of a nozzle system. [Figure 6] 2 shows a schematic configuration of a nozzle system. [Figure 7] 2 shows a schematic configuration of a nozzle system. [Figure 8] 2 shows a schematic cross-sectional shape of a nozzle. [Figure 9] 2 shows a schematic cross-sectional shape of a nozzle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] 1.Nozzle System 1 and 2 show schematic configurations of a nozzle system 100 according to an embodiment. As shown in FIG. 1, the nozzle system 100 is a system for supplying molten metal 105 from a ladle 101 to a tundish 102 through a nozzle 110. As shown in FIG. 2, the nozzle 110 is a cylindrical single-hole nozzle having an inlet 110a on the ladle 101 side, which is the upstream side, and an outlet 110b on the tundish 102 side, which is the downstream side, and extending downward from the upstream side to the downstream side. The outlet 110b of the nozzle 110 is located below a liquid level 105a of the molten metal 105 in the tundish 102 and above a bottom surface 102a of the tundish 102. The nozzle 110 also includes a long nozzle section 111 including the outlet 110b, a collector nozzle section 112 disposed upstream of the long nozzle section 111, and a connection section 113 between the long nozzle section 111 and the collector nozzle section 112. In the nozzle system 100, an inert gas is supplied from the outside to the inside of the nozzle 110, and a gas phase region 111a containing the inert gas and a secondary meniscus 111b of the molten metal 105 exist inside the long nozzle section 111. One of the features of the nozzle system 100 is that, on the premise of having the above-mentioned configuration, the relationships of the following formulas (1) to (4) are further satisfied.

[0023] 1.0≦D1 / D0≦1.4 … (1) 0.015≦((D2 / 2) 2 -(D1 / 2) 2 ) / (U1 D1) …(2) 1.2≦V·D1 / (4Q)≦4.8 …(3) D1+8.8Q / V≦D3≦D1+35.2Q / V…(4)

[0024] Where: D0 is the inner diameter (cm) of the collector nozzle portion 112 at the connection portion 113, D1 is the minimum inner diameter (cm) of the long nozzle portion 111 upstream of the secondary meniscus 111b, D2 is the inner diameter (cm) of the long nozzle portion 111 at the secondary meniscus 111b, D3 is the inner diameter (cm) of the long nozzle portion 111 at the outlet 110b, U1 is the average downward flow velocity (cm / s) of the molten metal 105 at the portion of the inner diameter D1, Q is the flow rate (cm ) of the molten metal 105 supplied to the nozzle 110 3 / s), V is the volume (cm) of the long nozzle portion 111 from the secondary meniscus 111b to the outlet 110b 3 ).

[0025] 1.1 Ladle The ladle 101 is a vessel that supplies the molten metal 105 to the tundish 102. As shown in FIGS. 1 and 2, in the nozzle system 100, the ladle 101 has a bottom surface 101a and a side wall 101b, and holds the molten metal 105. The ladle 101 may further have a lid (not shown). The ladle 101 may have any shape and material that can hold the molten metal 105. The ladle 101 may also be configured to have an outlet 101ax in a part of the bottom surface 101a, from which the molten metal 105 can flow out. The outlet 101ax may be provided with an opening / closing mechanism for controlling the amount of the molten metal 105 that flows out. The outlet 101ax of the ladle 101 may be directly or indirectly connected to a nozzle 110. The connection form between the ladle 101 and the nozzle 110 is not particularly limited, and they may be connected, for example, by fitting. The ladle 101 and the nozzle 110 may be connected via some intermediate member.

[0026] 1.2 Tundish The tundish 102 is a vessel to which the molten metal 105 is supplied from the ladle 101. As shown in FIGS. 1 and 2, the tundish 102 has a bottom surface 102a and a side wall 102b, and holds the molten metal 105 supplied from the ladle 101. The tundish 102 may further have a lid 102c. The tundish 102 may have a shape and a material capable of holding the molten metal 105. As shown in FIG. 1, a flow outlet 102ax may be provided in a part of the bottom surface 102a of the tundish 102, and may be configured so that the molten metal 105 can flow out from the flow outlet 102ax to another vessel (e.g., a mold). The flow outlet 102ax may be provided with an opening / closing mechanism for controlling the outflow amount of the molten metal 105. A nozzle 120 may be directly or indirectly connected to the flow outlet 102ax of the tundish 102. The manner of connection between the tundish 102 and the nozzle 120 is not particularly limited, and for example, they can be connected by fitting. The tundish 102 and the nozzle 120 may be connected via some intermediate member.

[0027] As shown in FIG. 2, a floating layer 106 containing flux may be present on the liquid surface 105a of the molten metal 105 supplied to the tundish 102. Any known flux may be used as the flux. By covering the liquid surface 105a of the molten metal 105 with the flux in this manner, the molten metal 105 can be isolated from the outside air. In addition, the nonmetallic inclusions in the molten metal 105 can be collected by the flux. As described later, the nozzle system 100 can suppress the reverse upward flow (see FIGS. 3 and 7) caused by the molten metal 105 flowing out of the nozzle 110 colliding with the bottom surface 102a of the tundish 102 to a small extent, so that the liquid surface 105a of the tundish 102 is not easily disturbed, and the flux is not easily pushed aside or cut off due to the disturbance of the liquid surface 105a, so that the problem of reoxidation due to bare metal is not easily caused.

[0028] 1, the tundish 102 may not have a dam inside. In the nozzle system 100 of the present disclosure, a secondary meniscus 111b is formed inside the long nozzle portion 111, and a phenomenon in which a gas phase is entrained when the falling flow of the molten metal 105 collides with the secondary meniscus 111b (formation of a bubble plume) tends to reduce the downward flow rate of the molten metal 105 and tends to suppress a short pass of the molten metal 105 in the tundish 102 (see FIGS. 3 and 6). Therefore, it is easy to remove nonmetallic inclusions and the like contained in the molten metal 105 without providing a dam inside the tundish 102.

[0029] 1.3 Nozzle 1 and 2, the nozzle 110 flows the molten metal 105 from the ladle 101 to the tundish 102. The nozzle 110 has an inlet 110a on the ladle 101 side, which is the upstream side, and an outlet 110b on the tundish 102 side, which is the downstream side. The nozzle 110 is a cylindrical nozzle with a single hole 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.

[0030] As shown in Figures 1 and 2, the outlet 110b of the nozzle 110 is located below the liquid level 105a of the molten metal 105 in the tundish 102 and above the bottom surface 102a of the tundish 102. That is, the downstream tip of the nozzle 110 is immersed in the molten metal 105 inside the tundish 102. As shown in Figure 2, in the nozzle system 100, a distance h1 may be provided between the liquid level 105a of the molten metal 105 in the tundish 102 and the outlet 110b of the nozzle 110, and a distance h2 may be provided between the outlet 110b of the nozzle 110 and the bottom surface 102a of the tundish 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 200 mm or more and 900 mm or less, the ratio h1 / h2 of h1 to h2 may be 0.2 or more and 1.0 or less, and the ratio D3 / h2 of the nozzle inner diameter D3 at the outlet 110b to h2 may be 1.0 or more and 3.0 or less. Adjusting h1 and h2 makes it easier to suppress the escape of generated bubbles from the nozzle 110 by the mechanism described below.

[0031] 2, the nozzle 110 includes at least a long nozzle section 111 including an outlet 110b, a collector nozzle section 112 disposed upstream of the long nozzle section 111, and a connection section 113 between the long nozzle section 111 and the collector nozzle section 112. The nozzle 110 is installed independently of the tundish 102 and does not need to be fixed to the tundish 102. In this respect, it can be said that the configuration of an injection pipe installed and fixed to the lid of a tundish is different from that of the nozzle referred to in the present application.

[0032] 1.3.1 Long nozzle section 2, the upstream end of the long nozzle section 111 is connected to the downstream end of the collector nozzle section 112 at a connection section 113. The long nozzle section 111 also has an outlet 110b at its downstream end. The long nozzle section 111 extends downward from the upstream side to the downstream side.

[0033] 2, the long nozzle portion 111 has a minimum inner diameter D1 upstream of the secondary meniscus 111b, an inner diameter D2 at the secondary meniscus 111b, and an inner diameter D3 at the outlet 110b downstream of the secondary meniscus 111b. The specific values ​​of the inner diameters D1 to D3 are not particularly limited as long as the relationships described below are satisfied.

[0034] 2, it is preferable that the long nozzle section 111 does not have a tapered portion whose inner diameter decreases from the upstream side to the downstream side between the portion having the minimum inner diameter D1 and the outlet 110b, and in particular, it is preferable that the long nozzle section 111 does not have a tapered portion whose inner diameter decreases from the upstream side to the downstream side between the portion where the secondary meniscus 111b is located and the outlet 110b. If a tapered portion exists downstream of the secondary meniscus 111b, the downward flow speed of the molten metal 105 increases in that portion, and there is a risk that the effect of the nozzle system 100 will be slightly reduced.

[0035] On the other hand, as shown in FIG. 2, the long nozzle section 111 may have an expanded diameter section 111c whose inner diameter increases from the upstream side to the downstream side. The expansion rate of the expanded diameter section 111c is not particularly limited. In the expanded diameter section 111c, the nozzle inner diameter may expand linearly (expands monotonically) from the upstream side to the downstream side, or may expand in a curved manner. In addition, in the expanded diameter section 111c, the nozzle inner diameter may expand continuously from the upstream side to the downstream side, or may expand intermittently. In addition, the ratio of the length of the expanded diameter section 111c to the entire length of the long nozzle section 111 is not particularly limited. In the long nozzle section 111, the nozzle inner diameter at the lower end (where the expansion ends) of the expanded diameter section 111c and the nozzle inner diameter D3 at the outlet 110b may be substantially the same or different from each other. In addition, the long nozzle section 111 may have a plurality of expanded diameter sections.

[0036] 1.3.2 Collector nozzle section As shown in Fig. 2, the upstream end of the collector nozzle section 112 is directly or indirectly connected to the ladle 101. In addition, the downstream end of the collector nozzle section 112 is connected to the upstream end of the long nozzle section 111 at a connection section 113. As shown in Fig. 2, the collector nozzle section 112 has an inner diameter D0 at the connection section 113. The collector nozzle section 112 may be configured by appropriately selecting from conventionally known collector nozzles so as to satisfy the relationship described below.

[0037] 1.3.3 Connections 2, the long nozzle part 111 and the collector nozzle part 112 are connected at the connection part 113. The structure of the connection part 113 may be the same as that of a conventional one. For example, the connection part 113 may be formed by fitting the upper end part of the long nozzle part 111 and the lower end part of the collector nozzle part 112 together. As described later, an inert gas supply mechanism may be provided at or near the connection part 113 so that an inert gas is supplied from the outside to the inside of the nozzle 110.

[0038] 1.4.Other configurations In addition to the configuration described above, the nozzle system 100 may have the following mechanisms, for example.

[0039] 1.4.1 Inert gas supply mechanism As shown in FIG. 2, in the nozzle system 100, an inert gas is supplied from the outside to the inside of the nozzle 110, and a gas phase region 111a containing the inert gas and a secondary meniscus 111b of the molten metal 105 are formed inside the long nozzle portion 111. The method or means for supplying the inert gas from the outside to the inside of the nozzle 110 is not particularly limited. For example, as shown in FIG. 2, the nozzle system 100 may have a gas supply mechanism 115 for supplying the inert gas to the inside of the nozzle 110. When the molten metal 105 is supplied from the ladle 101 to the tundish 102 through the nozzle 110, outside air may be taken into the inside of the nozzle 110 by an ejector from the connection between the ladle 101 and the nozzle 110 or the connection between the nozzle portions 113, etc. In addition to this, the nozzle system 100 may employ a mechanism 115 for intentionally supplying the inert gas to the inside of the nozzle 110. As described later, in the nozzle system 100, a predetermined relationship is satisfied during the supply of the molten metal 105, and the gas phase region 111a is maintained inside the long nozzle portion 111, and the secondary meniscus 111b of the molten metal 105 is formed. When the descending flow of the molten metal 105 collides with the secondary meniscus 111b, gas is entrained from the gas phase region 111a into the molten metal 105, and a group of bubbles of various sizes may be generated in the molten metal 105. The nozzle system 100 has a mechanism 115 for supplying an inert gas to the inside of the nozzle 110, which makes it easier to maintain the gas phase region 111a inside the nozzle 110. As an example of the inert gas, Ar can be used. The amount of the inert gas supplied to the nozzle 110 is not particularly limited. The pressure in the gas phase region 111a is also not particularly limited, and may be appropriately adjusted so that the relationship described later is satisfied.

[0040] The specific form of the mechanism 115 for supplying the inert gas to the inside of the nozzle 110 is not limited. For example, the mechanism 115 can be configured by connecting an inert gas supply source (such as a container filled with high-pressure inert gas) to the nozzle 110 with a pipe or the like. Alternatively, the inert gas supply source may be connected to a position upstream of the nozzle 110 (such as the ladle 101) so that the molten metal 105 to which the inert gas has been supplied flows into the nozzle 110. However, when the inert gas supply source is connected to the nozzle 110, it is easier to control the amount of inert gas blown into the nozzle 110. For example, the position to which the inert gas is supplied by the mechanism 115 may be the connection portion 113 between the long nozzle portion 111 and the collector nozzle portion 112 or in the vicinity thereof, and specifically, may be within a range of 100 mm from the connection portion 113. If it is within this range, the mechanism 115 can be easily installed in the nozzle 110. Furthermore, by supplying the inert gas within this range, the gas phase region 111a described above can be formed inside the nozzle 110 more easily.

[0041] 1.4.2 Flow rate adjustment mechanism As shown in Fig. 2, in the nozzle system 100, a flow rate adjustment mechanism 114 may be provided upstream of the collector nozzle portion 112. A specific example of the flow rate adjustment mechanism 114 is a sliding gate as shown in Fig. 2. In the sliding gate, at least one slide plate 114a having a flow port is slid in a direction intersecting the flow direction of the molten metal 105, thereby changing the flow path diameter. Alternatively, the flow rate adjustment mechanism 114 may be an opening / closing mechanism other than a sliding gate. Since the form of the flow rate adjustment mechanism 114 is publicly known, further description will be omitted here.

[0042] 1.5 Equations (1)~(4) In the nozzle system 100, it is important that the relationships of the formulas (1) to (4) are satisfied.

[0043] 1.5.1 Formula (1) As shown in the above formula (1), the nozzle system 100 satisfies the relationship 1.0≦D1 / D0≦1.4. That is, the ratio D1 / D0 of the minimum inner diameter D1 (cm) of the long nozzle part 111 to the inner diameter D0 (cm) of the collector nozzle part 112 at the connection part 113 is 1.0 or more and 1.4 or less. If this ratio is too small, a step may be generated at the connection part 113 between the long nozzle part 111 and the collector nozzle part 112 or in the vicinity thereof due to the diameter being reduced from the upstream side to the downstream side, which may cause leakage of the molten metal 105 or abnormal wear of the refractory material. On the other hand, if this ratio is too large, the molten metal 105 is likely to stick to the inner wall of the long nozzle part 111 due to the effect of heat radiation from the side surface of the long nozzle part 111. In addition, there is a risk that the weight of the long nozzle part 111 will increase unnecessarily.

[0044] 1.5.2 Formula (2) As shown in the above formula (2), in the nozzle system 100, the minimum inner diameter D1 (cm) of the long nozzle portion 111 located upstream of the secondary meniscus 111b and the inner diameter D2 (cm) of the secondary meniscus 111b inside the long nozzle portion 111 are 0.015≦((D2 / 2) 2 -(D1 / 2) 2 ) / (U1·D1). If the lower limit of formula (2) is not satisfied, the area of ​​secondary meniscus 111b becomes insufficient, and the phenomenon of gas phase entrainment (formation of a bubble plume) when the falling flow of molten metal 105 collides with secondary meniscus 111b becomes unstable. On the other hand, the upper limit of formula (2) is not particularly limited. Taking into consideration ease of handling and costs, D1 and D2 may be set within a range in which the diameter of long nozzle portion 111 does not increase unnecessarily.

[0045] The meaning of formula (2) will be described in more detail. As a result of the inventor's search and research, it was found that one of the factors governing the amount of bubbles generated by the entrainment of the gas phase in the secondary meniscus 111b is the surface area per unit time of the downward flow colliding with the secondary meniscus 111b. Therefore, U1·π·D1 was introduced as a simple index expressing the surface area per unit time of the downward flow. Here, D1 is the minimum inner diameter (cm) at the upper part of the long nozzle portion 111 located upstream of the secondary meniscus 111b, and U1 is the average downward flow velocity (cm / s) of the molten metal 105 at the minimum inner diameter D1 of the long nozzle portion. Next, since the area of ​​the secondary meniscus 111b is, for example, the area of ​​a doughnut-shaped region obtained by subtracting the cross-sectional area of ​​the downward flow from the area of ​​a circle determined by the diameter D2, the cross-sectional area of ​​the downward flow can be simply expressed by representing it by the cross-sectional area of ​​the long nozzle portion 111 at the minimum inner diameter D1, as follows: π((D2 / 2) 2 -(D1 / 2) 2 ) Equation (2) is obtained by dividing the area on the left by the index U1·π·D1, which corresponds to the surface area of ​​the downward flow.

[0046] The position of the secondary meniscus 111b can be calculated, for example, from ΔH in equation (5) described later. That is, the difference ΔH between the height position of the liquid surface 105a in the tundish 102 and the height position of the secondary meniscus 111b can be calculated from equation (5), and the position of the secondary meniscus 111b can be specified based on this. Also, the average downward flow velocity U1 can be calculated, for example, from the flow rate Q (cm 3 / s) is the cross-sectional area of ​​the flow passage at the part with the smallest inner diameter D1, π (D1 / 2) 2 This can be found by dividing by .

[0047] 1.5.3 Equation (3) As shown in the above formula (3), in the nozzle system 100, the minimum inner diameter D1 (cm) of the upper part of the long nozzle section 111 and the flow rate Q (cm 3 / s) and the volume V (cm 3) satisfy the relationship 1.2≦V·D1 / (4Q)≦4.8. If the lower limit of formula (3) is not reached, the bubble plume is unlikely to exert its desired effect. This will be described in detail later. On the other hand, if the upper limit of formula (3) is exceeded, the diameter and length of the long nozzle portion 111 will unnecessarily increase, which may lead to difficulties in handling and increased costs.

[0048] The meaning of formula (3) will be described in more detail. As described above, as a result of the search and research of the present inventor, the index U1·π·D1 is shown to be an index that has a dominant effect on the amount of bubbles generated by the entrainment of the gas phase in the secondary meniscus 111b. Many bubbles generated by the entrainment of the gas phase generate a bubble plume. In order to keep the bubble plume inside the long nozzle section 111 and increase the probability of collision between the bubbles and non-metallic inclusions, the ratio V / (U1·π·D1) between the volume from the secondary meniscus 111b inside the long nozzle section 111 to the outlet 110b at the lower end of the long nozzle section 111, that is, the retained volume V of the bubble plume, and U1·π·D1, which is an index representing the amount of bubbles, is introduced. As a result of experimentally searching for an appropriate range of this ratio, it was found that good results are obtained when the ratio is 1.2 or more and 4.8 or less. The ratio may be 1.5 or more, or may be 4.5 or less, or 4.0 or less. Q=U1·π·(D1 / 2) 2 Based on this relationship, by transforming and rearranging V / (U1 π D1), we obtain the above equation (3).

[0049] 1.5.4 Equation (4) As shown in the above formula (4), in the nozzle system 100, the flow rate Q (cm) of the molten metal 105 is determined by the inner diameter D3 (cm) of the outlet 110b at the lower end of the long nozzle section 111, the minimum inner diameter D1 (cm) of the upper part of the long nozzle section 111 located upstream of the secondary meniscus 111b, and 3 / s) and the volume V (cm 3) satisfy the relationship D1+8.8Q / V≦D3≦D1+35.2Q / V. If D3 is below the lower limit of formula (4), it may be difficult to collect air bubbles. This will be described in detail later. On the other hand, if D3 exceeds the upper limit of formula (4), the diameter and length of the long nozzle portion 111 may be unnecessarily increased, which may cause difficulty in handling and increase costs.

[0050] The meaning of formula (4) will be described in more detail. As a result of the inventor's search and research, it was found that the smaller the value of the above-mentioned index V / (U1·π·D1), the larger the range in which the bubbles detached from the bubble plume spread horizontally as they try to leak out. In this regard, for example, when V is small, the amount of bubbles that can be held inside the long nozzle portion 111 is small and the bubbles are likely to leak out from the outlet 110b, so it is preferable to make it easier to collect the bubbles by increasing the inner diameter D3 of the outlet. According to the inventor's new findings, for a new index D1+D1·(α / (V / (U1·π·D1))) expressed using the above-mentioned index V / (U1·π·D1) and D1 that can be an index of the diameter of the downward flow of the molten metal 105, if the outlet diameter D3 is such that the value of α is within the range of 2.2 to 8.8, the bubbles that make up the bubble plume are easily retained inside the long nozzle portion 111. That is, when the relationship D1+D1·(2.2 / (V / (U1·π·D1)))≦D3≦D1+D1·(8.8 / (V / (U1·π·D1))) is satisfied, it is easy to suppress leakage of air bubbles from the long nozzle portion 111. Add Q=U1·π·(D1 / 2) to this formula. 2 By substituting the above relationship and transforming it, the relationship in equation (4) can be obtained.

[0051] 1.6 Equation (5) In the nozzle system 100, ΔH defined by the following formula (5) may be in the range of −10 cm to +50 cm. As shown in FIG. 2, ΔH corresponds to the difference in height between the secondary meniscus 111b inside the long nozzle portion 111 and the liquid surface 105a of the molten metal 105 in the tundish 102.

[0052] ΔH = (P T -PL ) / (ρ·g) …(5)

[0053] Where: P T is the atmospheric pressure in the tundish 102, P L is the pressure of the gas phase region 111a in the long nozzle portion 111, ρ is the density of the molten metal 105; g is the acceleration due to gravity.

[0054] In the nozzle system 100, if the inside of the long nozzle section 111 is pressurized so that ΔH falls below -10 cm, the volume V of the long nozzle section 111 from the secondary meniscus 111b to the outlet 110b will decrease, which may be detrimental to the achievement of the action effect of the bubble plume and the collection of bubbles. On the other hand, if ΔH is increased to exceed 50 cm, the pressure of the gas phase region 111a inside the long nozzle section 111 will decrease, making it difficult to maintain airtightness and likely to cause the intake of outside air. ΔH may be 0 cm or more or 20 cm or less.

[0055] The atmospheric pressure P inside the tundish 102 required for the calculation of equation (5) T It is possible to measure the pressure P of the gas phase region 111a in the long nozzle portion 111 by a pressure gauge installed in a pipe leading to the inside of the tundish 102. L Similarly, a flow path leading to the gas phase region 111a inside the long nozzle portion 111 can be drilled in the refractory material and measured by a pressure gauge installed in a pipe connected to the flow path. Alternatively, the flow rate of the inert gas supplied to the inside of the long nozzle portion 111 and the pressure P L The relationship between the supply flow rate of the inert gas and P L It is acceptable to estimate the value of ΔH in the equation (5). When calculating using the equation (5), all units should be SI units. The unit of ΔH obtained is m, but this can be converted to cm.

[0056] 1.7 Other Terms As long as the above configurations and relationships are satisfied, there are no other limitations on the conditions of the nozzle system 100. An example of the other conditions is given below.

[0057] 1.7.1 Molten metal flow rate and mean residence time in the tundish The flow rate Q of the molten metal 105 supplied to the nozzle 110 is, for example, 8000 cm 3 / s or more, and the average residence time τ of the molten metal 105 in the tundish 102 may be, for example, 240 seconds or more. The effect of reducing the flow velocity of the molten metal 105 in the nozzle system 100 is particularly useful in high-productivity continuous casting in which a large flow rate of the molten metal 105 passes through the tundish. Specifically, when the flow rate Q of the molten metal 105 is 8000 cm 3 / s or more. Furthermore, if the flow rate is 18,000 cm 3 / s or more is even more useful. The upper limit of the flow rate is not particularly limited, but is preferably 40,000 cm 3 Continuous casting exceeding 1 / s is not practical because other processes cannot keep up with such a high production rate. On the other hand, if the average residence time τ in the tundish 102 is too short, it may be difficult to obtain a sufficient cleaning effect for the molten metal 105. It is considered that, by making the average residence time τ 240 seconds or more, inclusions of, for example, about 50 to 100 microns can be easily removed by floating. Although there is no particular upper limit for the average residence time, it is not practical for the average residence time to exceed 1500 seconds during steady operation of continuous casting. If the average residence time is too long, the temperature drop of the molten metal 105 in the tundish 102 becomes too large. The average residence time τ is determined by the capacity V of the tundish 102. τ (cm 3 ) and the flow rate of molten metal from the nozzle Q (cm 3 / s) τ It can be calculated as / Q.

[0058] 1.7.2 Weir in the tundish As described above, in the nozzle system 100, there is no need to provide a dam having a function of controlling the flow of the molten metal 105 inside the tundish 102. According to the nozzle system 100, it is possible to ensure the residence time of the molten metal 105 inside the tundish 102 without using a dam. On the contrary, providing a dam in the tundish 102 would result in a problem of forming a stagnation area called a dead zone, reducing the actual tundish capacity, and in the problem of increased costs.

[0059] 1.7.3 Height Dimensions There is no particular limitation on the height dimension of the nozzle 110. This dimension is determined by the distance between the collector nozzle part 112 on the ladle 101 side and the liquid surface 105a of the tundish 102, and the immersion depth of the long nozzle part 111, and is often about 1 m to 2.5 m.

[0060] 1.7.4 Type of molten metal In the nozzle system 100, there is no particular limitation on the type of molten metal 105 supplied from the ladle 101 to the tundish 102 through the nozzle 110. In particular, when the molten metal 105 is molten steel, the nozzle system 100 is expected to be highly effective. The type of molten steel is not particularly limited.

[0061] 1.8 Actions and Effects As described above, in the nozzle system 100, the relationships of the above formulas (1) to (4) are satisfied, so that it is possible to simultaneously perform the function of forming a bubble plume in the molten metal 105 downstream of the secondary meniscus 111b inside the long nozzle section 111 and the function of recovering large bubbles that constitute the bubble plume inside the long nozzle section 111. This makes it possible to simultaneously capture and remove nonmetallic inclusions, reduce the long nozzle discharge flow rate, and prevent re-oxidation on the molten metal surface around the long nozzle. As a result, problems such as bare metal and short pass problems as shown in FIG. 3 are unlikely to occur, and the cleanliness of the molten metal 105 is likely to be improved.

[0062] 2. Continuous casting method of molten metal The technology of the present disclosure also has an aspect as a method for continuously casting molten metal. The method for continuously casting molten metal of the present disclosure is characterized by using the nozzle system 100 of the present disclosure. The method for continuously casting molten metal of the present disclosure may include, for example, blowing an inert gas into the inside of the nozzle 110 so that the relationships shown in the above formulas (1) to (4) are satisfied. Details of the mechanism for blowing the inert gas into the inside of the nozzle 110 are as described above. The continuous casting method of the present disclosure may include, for example, using the nozzle system 100, supplying molten metal 105 from the ladle 101 to the tundish 102 through the nozzle 110, supplying molten metal 105 from the tundish 102 to a mold (not shown) through the nozzle 120, and continuously withdrawing a cast piece from the mold. In the method for continuously casting molten metal of the present disclosure, general continuous casting conditions may be adopted, except that the nozzle system 100 is adopted. Alternatively, as described above, the flow rate Q and the average residence time τ may be adjusted to be equal to or greater than a predetermined value.

[0063] 3. Supplementary Information The above-mentioned various indices and their appropriate ranges were explored using an experimental apparatus proposed by the present inventor in Patent No. 6750533. This experimental apparatus faithfully reproduces the effects of gravity, inertial force, viscous force, and surface tension acting on a molten metal flow, and can accurately reproduce gas-liquid two-phase flow phenomena, including the buoyancy and rising speed of bubbles. EXAMPLES

[0064] The present invention will be further described below with reference to 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 of the invention and achieves its object.

[0065] Table 1 below shows the conditions for each of the examples and comparative examples. Table 2 below shows the compositions of the slabs used in the examples and comparative examples. The "steel cleanliness index" in Table 1 is the total amount of oxides in samples of 5 mm square x 50 mm length taken from six locations, namely, 1 / 4 thickness at the width center, 1 / 4 width 1 / 4 thickness, 3 / 4 width 1 / 4 thickness, 3 / 4 thickness at the width center, 1 / 4 width 3 / 4 thickness, and 3 / 4 width 3 / 4 thickness, and is indexed with the value of Comparative Example D shown later set at 10. The smaller the index, the higher the cleanliness of the molten steel.

[0066] [Table 1]

[0067] [Table 2]

[0068] Example A in Table 1 is an example in which a nozzle as shown in FIG. 8A is used in a nozzle system as shown in FIG. 4. As shown in FIG. 4, in Example A, a long nozzle is attached to a ladle via a collector nozzle located below the flow rate adjustment mechanism on the lower surface of the ladle, and molten metal is poured from the ladle to a tundish. The inner wall of the long nozzle has a flow passage for blowing Ar as an inert gas, and Ar is blown into the inside of the nozzle from here. The blown Ar flows into the long nozzle at a pressure P L The gas phase region was formed. The atmospheric pressure in the tundish P T and the gas phase pressure in the long nozzle P L Based on the above equation (5), the difference ΔH between the liquid level in the tundish and the secondary meniscus height in the long nozzle was calculated.

[0069] As shown in FIG. 4, in Example A, the downward flow in the long nozzle entrains the gas phase in the secondary meniscus, and a bubble plume consisting of a large number of bubbles is formed in the molten steel at the bottom of the long nozzle. Most of the bubbles constituting the bubble plume are large bubbles with a diameter of about 5 mm to 10 mm, which rise to the surface in the long nozzle and are collected. Small bubbles with a diameter of about 1 mm to 2 mm are dispersed outside the bubble plume, and many of them are dispersed outside the long nozzle and rise gently without disturbing the molten metal surface in the tundish. Nonmetallic inclusions contained in the downward flow in the long nozzle are captured by the bubbles constituting the bubble plume, and then float to the molten metal surface of the tundish on the gentle upward flow formed by the small bubbles, where they can be removed. The large bubbles constituting the bubble plume exert a braking action by clinging to the downward flow in the long nozzle, and the downward flow velocity at the bottom end of the bubble plume is calculated by multiplying the injection flow rate Q by the secondary meniscus height cross-sectional area S2 in the long nozzle = π·(D2 / 2) 2 The value divided by and the injection flow rate Q is the long nozzle outlet area S3 = π(D3 / 2) 2 The downward flow velocity can be effectively damped to a value between the value obtained by dividing the downward flow velocity by ...

[0070] In Example A, when molten metal is supplied from the ladle to the tundish through the nozzle by the above-mentioned mechanism, a gas phase region and a secondary meniscus of the molten metal are formed in the nozzle, and the downward flow rate of the molten metal is reduced and non-metallic inclusions are captured by the gas phase entrainment phenomenon (formation of a bubble plume) when the falling flow of the molten metal collides with the secondary meniscus. It is also considered that the large bubbles forming the bubble plume are easily captured in the nozzle, the molten metal surface in the tundish around the nozzle is kept calm, and reoxidation of the molten steel is prevented. As a result, Example A was able to obtain an excellent value of "5" for the cleanliness index of the steel.

[0071] Example B in Table 1 is an example in which a nozzle as shown in Figure 8B is used in a nozzle system as shown in Figure 4. It is considered that in Example B, as in Example A, nonmetallic inclusions are efficiently floated and removed while preventing reoxidation in the tundish. As a result, Example B was able to obtain an excellent value of "5" for the cleanliness index of the steel.

[0072] Example C in Table 1 is an example in which a nozzle as shown in Fig. 8C is used in a nozzle system as shown in Fig. 4. It is considered that in Example C, as in Example A, nonmetallic inclusions are efficiently floated and removed while preventing reoxidation in the tundish. As a result, Example C was able to obtain an excellent value of "5" for the cleanliness index of the steel.

[0073] Example A-2 in Table 1 is an example in which the supply flow rate Q of molten steel is changed in Example A. It is considered that in Example A-2, as in Example A, nonmetallic inclusions are efficiently floated and removed while preventing reoxidation in the tundish. As a result, Example A-2 cannot obtain an excellent value of "4" for the cleanliness index of steel. However, Example A-2 has a small molten steel flow rate, and it can be said that the adoption of the system of the present disclosure is excessive in terms of effect. For example, in Comparative Example F-2 in which the long nozzle of Example A-2 is replaced with a normal long nozzle (FIG. 9F), the cleanliness index of steel is also a small value, and the application of the system of the present disclosure is not necessarily required. Details of Comparative Example F-2 will be described later.

[0074] Example B-2 in Table 1 is an example in which the supply flow rate Q of molten steel was changed in Example B. In Example B-2, as in Example B, it is considered that the nonmetallic inclusions were efficiently floated and removed while preventing reoxidation in the tundish. However, because the average residence time τ of the molten steel in the tundish was short, the effect of floating and removing the nonmetallic inclusions was slightly impaired. As a result, the cleanliness index of the steel was "6", which was 1 point higher than Example B.

[0075] Example C-2 in Table 1 is an example in which a dam (see FIG. 7) was installed inside the tundish in Example C. In Example C-2, as in Example C, it is believed that nonmetallic inclusions were efficiently floated and removed while preventing reoxidation in the tundish. However, in Example C-2, the dam installed inside the tundish reduced the actual tundish capacity and residence time in the tundish, slightly affecting the floating of nonmetallic inclusions. As a result, the cleanliness index of the steel was "6", one point higher than in Example C without a dam.

[0076] In the above examples, the flow rate of Ar injected into the nozzle was a small flow rate of 0.5 to 2.0 NL / min during steady operation. This is because most of the large bubbles that make up the bubble plume were collected, so there were few bubbles discharged from the gas phase region in the long nozzle to the outside of the system, and there was little need to replenish Ar in the gas phase region.

[0077] Comparative Example D in Table 1 is an example in which a long nozzle as shown in FIG. 9D is used in a nozzle system as shown in FIG. 5. In Comparative Example D, it is believed that the value of formula (3) became too small mainly due to an insufficient volume V, and as a result, the effect of the bubbles in capturing nonmetallic inclusions was not fully achieved. In addition, it is believed that the outlet diameter D3 was too small, so that the large bubbles that make up the bubble plume were not fully captured, disturbing the tundish molten surface and causing re-oxidation of the molten steel. Due to these adverse effects, the cleanliness index of the steel was determined to be "10."

[0078] The condition of Comparative Example D is shown in Fig. 5. Fig. 5 shows a situation in which large bubbles constituting a bubble plume rise to the surface of the molten metal in the tundish around the long nozzle and disturb the molten metal surface. In addition, most of the bubble plume is outside the long nozzle, and the effect of the bubbles in capturing nonmetallic inclusions inside the long nozzle is small.

[0079] In Comparative Example D, the flow rate of Ar injected into the nozzle reached 10 NL / min during steady operation. This indicates that many of the bubbles that make up the bubble plume were not captured and were discharged outside the long nozzle system, and it was necessary to replenish a large amount of Ar to maintain the gas phase region.

[0080] Comparative Example E in Table 1 is an example in which a long nozzle as shown in FIG. 9E was used in a nozzle system as shown in FIG. 6. In Comparative Example E, gas such as Ar was not blown into the long nozzle, and the long nozzle joint surface was highly airtight, so the inside of the long nozzle was filled with gas, with the presence of a gas phase being negligible. At this time, as shown in FIG. 6, it is considered that a high-speed short-circuiting flow crawled along the bottom of the tundish, causing a problem that the residence time of molten steel in the tundish became extremely short. As a result, the cleanliness index of the steel in Comparative Example E was "9", and sufficient cleanliness was not obtained.

[0081] Comparative Example F in Table 1 is an example in which a long nozzle as shown in FIG. 9F was used in a nozzle system as shown in FIG. 7. In Comparative Example F, although 1.0 NL / min of Ar was blown into the nozzle, a stable gas phase region was not formed inside the long nozzle. On the other hand, in Comparative Example F, a weir was installed in the tundish, so that a short-circuit flow like that in Comparative Example E did not occur. However, the high-speed downward flow was reversed at the bottom of the tundish, and the resulting upward flow caused a rise on the molten steel surface in the tundish, which broke the coating of the flux on the molten steel surface, and this is thought to have caused reoxidation of the molten steel. As a result, the cleanliness index of the steel in Comparative Example F was "8", which was not sufficient cleanliness.

[0082] Comparative Example F-2 in Table 1 is a case where the flow rate Q of molten steel is changed in Comparative Example F. In Comparative Example F-2, although 1.0 NL / min of Ar was blown into the nozzle, a stable gas phase region was not formed inside the long-long nozzle. On the other hand, in Comparative Example F-2, a weir was installed in the tundish, so it is considered that a short-circuit flow like that in Comparative Example E did not occur. In Comparative Example F-2, the flow rate Q of molten steel is considerably smaller than that in Comparative Example F, so the occurrence of high-speed flow in the tundish is suppressed, and reoxidation due to the rise of the molten steel surface in the tundish like that in Comparative Example F is avoided. In addition, in Comparative Example F-2, the average residence time τ of molten steel in the tundish is sufficiently large. As a result, the cleanliness index of the steel in Comparative Example F-2 was "6", and a high cleanliness was obtained. In other words, it can be said that a significant decrease in productivity is required to obtain a steel with a high cleanliness in Comparative Example F-2. [Explanation of symbols]

[0083] 100 Nozzle System 101 Ladle 101a Bottom 101b side wall 102 Tundish 102a Bottom 102b side wall 102c lid 105 Molten Metal 110 Nozzle 110a Inlet 110b Outlet 111 Long nozzle part 111a Gas phase region 111b Secondary meniscus 111c Expanded section 112 Collector nozzle part 113 Connection 114 Flow control mechanism (sliding gate) 114a Slide plate 115 Gas supply mechanism 120 Nozzle

Claims

1. 1. A nozzle system for delivering molten metal from a ladle through a nozzle to a tundish, comprising: the nozzle is a cylindrical single-hole nozzle having an inlet on the ladle side, which is an upstream side, and an outlet on the tundish side, which is a downstream side, and extending downward from the upstream side to the downstream side; The outlet of the nozzle is located below the liquid level of the molten metal in the tundish and above the bottom surface of the tundish, the nozzle comprises a long nozzle section including the outlet, a collector nozzle section disposed upstream of the long nozzle section, and a connection section between the long nozzle section and the collector nozzle section, an inert gas is supplied from the outside to the inside of the nozzle, and a gas phase region containing the inert gas and a secondary meniscus of the molten metal exist inside the long nozzle portion; The relationships of the following formulas (1) to (4) are satisfied. Nozzle system. 1.0≦D １ / D ０ ≦1.4 …(1) 0.015≦((D ２ / 2) ２ -(D １ / 2) ２ ) / (U １ ・D １ ) …(2) 1.2≦V・D １ / (4Q)≦4.8 …(3) D １ +8.8Q / V≦D ３ ≦D １ +35.2Q / V …(4) Where: D ０ is the inner diameter (cm) of the collector nozzle portion at the connection portion, D １ is the minimum inner diameter (cm) of the long nozzle section upstream of the secondary meniscus, D ２ is the inner diameter (cm) of the long nozzle portion at the secondary meniscus, D ３ is the inner diameter (cm) of the long nozzle portion at the outlet, U １ is the inner diameter D １ is the average downward flow velocity (cm / s) of the molten metal in the portion Q is the flow rate (cm) of the molten metal supplied to the nozzle ３ / s), V is the volume (cm) of the long nozzle portion from the secondary meniscus to the outlet. ３ ).

2. ΔH defined by the following formula (5) is within the range of −10 cm to +50 cm. The nozzle system of claim 1 . ΔH=(P Ｔ -P Ｌ ) / (ρ・g) …(5) Where: P Ｔ is the atmospheric pressure in the tundish, P Ｌ is the pressure of the gas phase region in the long nozzle section, ρ is the density of the molten metal; g is the acceleration due to gravity.

3. The flow rate Q is 8000 cm ３ / s or more, The average residence time τ of the molten metal in the tundish is 240 seconds or more.

3. A nozzle system according to claim 1 or 2.

4. The tundish has no internal dam. A nozzle system according to any one of claims 1 to 3.

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