JP6515388B2 - Upper nozzle for continuous casting - Google Patents

Description

  The present invention relates to an upper nozzle for continuous casting provided in a molten steel injection hole from a molten steel container including a tundish to a mold.

  In continuous casting of steel, molten steel contained in a ladle is first transferred to a tundish, which is an intermediate molten steel container, and then poured into a mold through a dipping nozzle connected to a molten steel injection hole at the bottom of the tundish, Form a continuous cast slab.

  When the molten steel melted in a smelting furnace such as a converter is delivered to a ladle, and further secondary refining is performed, non-metallic inclusions as a deoxidized product are formed in the molten steel, and the float separation can not be completed. Some are suspended in molten steel. In addition, during transfer from ladle to tundish and during residence in tundish, molten metal contacts air to form oxides and nitrides, non-metallic inclusions increase, and molten steel in ladle The slag at the top or at the top of the tundish is suspended in the molten steel and remains in the molten steel as nonmetallic inclusions. If these non-metallic inclusions are injected into the mold while remaining in the molten steel, some of them may not be floated and separated in the mold and may be taken into the slab, and these become defects in the steel and cause cracking of the steel. It may be the starting point of breakage or cracking. Therefore, in order to produce highly clean steel, it is necessary to efficiently remove these inclusions from the molten steel.

  In the injection hole for injecting the molten steel from the tundish into the mold, it is widely practiced to blow an inert gas into the immersion nozzle at the lower part of the injection hole through the upper nozzle at the upper part of the injection hole and the sliding plate provided in the middle of the injection hole. There is. For example, Patent Document 1 discloses a method of providing an upper nozzle with a gas injection portion made of porous brick and injecting argon gas or nitrogen gas. It is for the purpose of preventing nozzle clogging resulting from the adhesion of non-metallic inclusions such as alumina in the injection hole from the upper nozzle to the immersion nozzle. The inert gas blown into the injection hole is bubbled and enters the mold together with the molten steel. When bubbles are floated in the mold, inclusions adhere to the bubbles, and if bubbles float and separate on the meniscus, both bubbles and inclusions can be removed from the molten steel. On the other hand, since a part of the air bubbles contained in the molten steel in the mold is captured by the solidified shell, it becomes a pinhole defect in the cast slab and degrades the quality of the steel material.

  There is known a method of removing non-metallic inclusions by blowing inert gas into the molten steel in the tundish and trapping non-metallic inclusions in the molten steel in bubbles of the inert gas in the tundish. . In Patent Document 2, in the vicinity of the bottom of a tundish, a flow passage with a small cross section of molten steel having irregularities in the flow direction is provided at a position between the dropping position of the molten steel from the ladle and the immersion nozzle. A method has been proposed in which an inert gas is blown into molten steel from the inner convex portion. Patent Document 3 discloses a method in which a plurality of gas injection ports are arranged in the width direction of the bottom of a tundish and gas is blown into molten steel in the tundish. In these methods, even if nonmetallic inclusions can be removed by the inert gas in the tundish, recontamination is caused by air and slag in the process of molten steel passing from the inert gas blowing position to the injection hole. In order to have an inclusion removal effect on all the molten steel flowing in a wide tundish flow passage corresponding to the entire tundish longitudinal cross section, a larger amount of inert gas blowing is required. An increase in the flow rate of the inert gas is not preferable because it causes contamination of inclusions floating on the surface of the tundish metal and contamination sources of molten steel such as slag and promotes recontamination of the molten steel.

  That is, there are many opportunities for the contamination of the molten steel in the tundish, and it is most effective as an inclusion removal method to remove the inclusions immediately before the molten steel flows into the injection hole of the mold. Patent Document 4 discloses, as a method for preventing the inflow of inert gas and inclusions into the injection hole as much as possible, a method of properly separating the blowing position of the inert gas from the inner surface of the injection hole of the tundish. . In FIG. 4 of the same document, the distance from the center of the upper nozzle to the gas injection position is X, the inner surface radius of the upper nozzle is R, and the distance ratio X / R at the limit where bubbles are not caught in the slab The relationship with the molten steel flow velocity v is clarified as equation (1). In the case described in the same document, the gas blowing zone is made of alumina graphite or a porous plug, etc., and installed so as to surround the injection hole connected to the immersion nozzle, and the inert gas blown in is a bubble in the molten metal of the tundish. To rise. At that time, by colliding with the fine inclusions in the molten metal flowing into the injection hole, the fine inclusions can be taken into the bubbles and the inclusions including the fine inclusions can be removed.

JP, 2009-66603, A JP-A-8-117939 Japanese Patent Application Laid-Open No. 2002-11555 Japanese Patent Application Laid-Open No. 10-34299

  According to the invention described in Patent Document 4, continuous casting is performed by blowing inert gas at a position close to the molten steel injection hole in a tundish to remove inclusions and minimize inflow of inert gas from the injection hole. It became possible. On the other hand, in the method described in Patent Document 4, the distance from the injection hole is still large. The present invention makes it possible to carry out the blowing of the inert gas from a position closer to the injection hole while suppressing the inflow of the inert gas to the molten steel in the mold, and the molten steel recontamination after the removal of inclusions by the inert gas It is an object of the present invention to provide an upper nozzle suitable for a continuous casting method which further reduces the chance of

That is, the place made into the summary of the present invention is as follows.
(1) An upper nozzle for continuous casting provided in a molten steel injection hole from a molten steel container to a mold,
At the top of the upper nozzle, a plurality of gas injection holes are installed on the circumference centered on the center of the injection hole,
The relationship between the total cross-sectional area A (m ² ) of the gas injection holes and the volume V _g (m ³ ) of the flow path through which the inert gas in the upper nozzle flows is characterized by satisfying the following equation (1) Top nozzle for continuous casting.
V _g / A ≧ 50 (m) (1)

In the present invention, the upper nozzle provided in the injection hole for continuous casting is provided with a gas injection hole, the total cross sectional area A (m ² ) of the gas injection hole, and the volume V _{g of the} flow path through which the inert gas in the upper nozzle flows. Let the relationship with (m ³ ) be the upper nozzle that satisfies the above equation (1). By performing continuous casting using this upper nozzle, it is possible to reduce the inclusions flowing into the mold while suppressing the inflow of inert gas into the molten steel in the mold, and to suppress the wave of the tundish surface. .

It is a fragmentary sectional view which shows upper nozzle vicinity used by the water model experiment of Experiment 1. FIG. It is a figure which shows the relationship between the gas blowing hole arrangement | positioning distance x in experiment 1, the through-put W, the blowing gas flow rate, and the bubble ratio inhaled by the casting_mold | template. It is a fragmentary view showing the upper nozzle neighborhood used by the offline experiment of experiment 2, (A) is a fragmentary sectional view, (B) is a BB arrow line view, (C) is a CC arrow line sectional view . It is a diagram showing a relationship between V _g / A and an argon gas flow rate in Experiment 2.

  When inert gas is blown into the molten steel from the bottom of the tundish, as a refractory of the gas blowing portion, a porous plug or a refractory provided with a fine blowing nozzle is used. Bubbling is performed such that the inert gas blown into the molten steel from the gas blowing portion becomes fine bubbles as quickly as possible. Also in the case described in Patent Document 4, the gas blowing zone is made of alumina graphite or a porous plug.

  The bubbles rising in the molten steel rise at a rising speed determined by the bubble diameter and the like. In the case described in Patent Document 4, since the gas is blown at a position very close to the injection hole at the bottom of the tundish to generate air bubbles, when the inflow rate of molten steel into the injection hole is high, the air bubbles to rise are molten steel It is taken into the flow and flows into the mold from the injection hole. Therefore, as described in Patent Document 4, it is necessary to separate the gas injection position from the center of the injection hole sufficiently.

  The present invention brings bubbles into the injection flow even if the position of the gas injection hole is made closer to the center of the injection hole by increasing the gas flow rate in molten steel of the gas flowing out of the gas injection hole at the bottom of the tundish. It was born out of an idea that it could be prevented. When a straight tubular nozzle is used as the gas injection nozzle (i.e., when the divergent Laval nozzle is not used), the maximum flow velocity flowing out of the nozzle is limited to the speed of sound even if the injection gas pressure is increased. Therefore, the source pressure of the gas to be blown is increased, and the gas is blown into the molten steel at a sound velocity or a flow velocity close thereto, to evaluate the reduction degree of the bubble mixing into the injection hole.

  The water model experiment conducted as Experiment 1 will be described.

  Water is placed in a 2-cubic meter cubic water tank simulating a tundish, and the outlet at the bottom simulates the injection hole 5 from the tundish 7 to the mold, the shape of which is shown in FIG. The inner diameter of the injection hole 5 is 90 mmφ, and as shown in FIG. 1, the upper end of the injection hole has a circular arc shape in cross section, and the diameter of the portion connected to the tundish bottom 8 is 142 mm. Adjust the outflow water volume by adjusting the position of the stopper 6. In the upper nozzle 1, 20 gas injection holes 2 with a diameter of 0.3 mm are arranged at equal intervals on a circumference centered on the center of the injection hole 5. The distance obtained by subtracting the injection hole radius (45 mm) from the radius of the gas injection hole arrangement circumference is “gas injection hole arrangement distance x (mm)”, and the value of the gas injection hole arrangement distance x in the range of x = 10 to 70 mm Five different types of upper nozzles 1 were prepared. Argon gas was supplied to the gas introduction port for introducing the gas into the gas injection hole, the source pressure of the argon gas was changed to three types, and the gas injection amount from the gas injection hole 2 was changed. According to the results of the gas blowing amount, it was found that the gas flow velocity flowing out of the gas blowing hole 2 into water was respectively the sound velocity, 3/4 of the sound velocity, and 1/2 of the sound velocity. When the flow velocity of the gas was at the speed of sound, the gas flow rate was 28.3 NL / min, which was 330 m / sec in terms of the flow velocity. All parts in the water tank were made of transparent resin.

  While performing gas injection from the gas injection holes 2, water was made to flow out from the injection holes 5 at a predetermined water flow rate, while water was supplied to the water tank, and the water level in the water tank was kept constant. The behavior of the gas blown in this state is photographed by a camera, and the area of the air bubble sucked into the mold via the injection hole 5 and the air bubble floating on the surface of the tundish water surface is measured from the photographed image, and sucked into the mold The ratio of air bubbles was calculated.

About each upper nozzle 1 which changed gas blowing hole arrangement distance x, the blowing gas flow rate was changed into three types, and the water flow rate to the injection hole 5 was changed in each level. The behavior of the blown gas was photographed with a camera under each condition of x. The bubble ratio sucked into the injection hole 5 was measured by measuring the area of the bubble sucked into the injection hole 5 and the bubble floating on the tundish surface from the photographed image. As the water flow rate to the injection hole 5 increases, the bubble ratio sucked into the injection hole 5 increases. Then, the water flow rate (throughput W) at which the bubble ratio sucked into the injection hole 5 becomes 10% was determined from the experiment. In FIG. 2, the horizontal axis represents the gas injection hole arrangement distance x (mm), and the vertical axis represents the throughput W (m ³ / hr), and the throughput W in which the bubble ratio sucked into the injection hole 5 is 10% was plotted. Stratification is performed by the gas flow rate from the gas injection holes 2.

  In FIG. 2, the plot of ■ is the case where the gas flow velocity from the gas injection hole 2 is 1/2 of the speed of sound, and the bubbles drawn into the injection hole 5 in a better range on the lower right side than the line connecting this plot. The ratio is less than 10%. The plot of ◆ is the case where the gas flow velocity from the gas injection hole 2 is 3⁄4 of the speed of sound, and the good range on the same throughput W compared to the above also spreads to the left side, ie, the gas injection hole arrangement distance x becomes shorter ing. The plot of ▲ is the result when the gas flow velocity from the gas injection hole 2 is the speed of sound, and the throughput W in which the bubble ratio sucked into the injection hole 5 is 10% or less is the same as the throughput W It has spread to the left. From FIG. 2, it will be understood that if the throughput W is the same, the gas blowing hole arrangement distance x can be reduced as the gas flow velocity from the gas blowing hole 2 increases. Therefore, it is possible to keep the introduction of air bubbles into the injection hole 5 small while providing the gas injection hole 2 at a position closer to the injection hole 5. Also, if the gas injection hole arrangement distance x of the upper nozzle 1 is the same, the faster the gas flow velocity from the gas injection hole 2, the smaller the ratio of air bubbles sucked into the injection hole 5, and the water throughput W is increased. It can be said that you can do it.

  The above-described effect of increasing the gas flow velocity from the gas injection hole 2 to the sound velocity level can be described as follows. That is, as the gas flow rate from the gas injection hole 2 is increased, a gas jet is formed in the liquid from the outlet of the gas injection hole 2 and the length of the gas jet increases as the gas flow rate increases. Since the gas flow velocity is high in the gas jet, the longer the gas jet, the harder it is for the gas to be washed away by the water injection flow, and it becomes possible to reduce the gas injection hole arrangement distance x.

  Based on the water model test results of Experiment 1 described above, tests were conducted to increase the gas flow rate from the gas injection holes 2 in a tundish containing actual molten steel. The arrangement of the gas injection holes 2 of the upper nozzle 1 is the same as in Experiment 1. When the source pressure of the inert gas supplied to the upper nozzle 1 was increased to increase the gas flow rate, the amount of bubbles rising in the molten steel in the tundish became excessive and the rippling of the tundish surface became intense . This in turn encourages the inclusion of slag and floated inclusions on the surface of the tundish surface. When an inert gas at normal temperature is blown into the molten steel, the volume of the molten steel expands by about six times as the temperature of the gas rises due to the molten steel. The temperature of the gas spouted from the gas injection hole 2 at the speed of sound is lower than the temperature of the molten steel and is heated in the molten steel to raise the gas temperature and increase the gas volume, so the bubble volume becomes excessive. . In addition, rapid expansion of the gas in the molten steel has the disadvantage of impeding the momentum of the inert gas blown in.

  If the gas temperature is sufficiently raised before blowing out from the gas injection hole 2, even if the gas is ejected at the same speed of sound, expansion of the gas after ejection can be suppressed, and the amount of injected gas (NL / min ) Will be reduced. Since the bubbles are ejected at the speed of sound, it should be the same as in the case where the low temperature gas is blown out from the gas blowing hole in that air bubbles are not caught in the injection flow. However, providing a dedicated gas heater to raise the insufflation gas temperature complicates the installation and is undesirable.

  Therefore, the off-line experiment conducted as Experiment 2 will be described.

At the time of pouring the molten steel from the tundish into the mold through the pouring hole, the upper nozzle 1 located at the top of the pouring hole is sufficiently heated. Therefore, if the gas residence time in the upper nozzle 1 is increased, the gas is heated during the residence, and a high temperature gas should be ejected from the gas blowing hole 2. In Experiment 2, as shown in FIG. 3, the volume V _g of the flow path 3 through which the inert gas in the upper nozzle 1 flows is increased to prepare one having various values as the volume V _g (m ³ ). . The volume of the flow path 3 in the upper nozzle 1 was adjusted by inserting a ring-shaped material which disappears at the time of firing into the refractory and changing its thickness (FIG. 3). Therefore, the shape of the flow path 3 of the created upper nozzle 1 has a shape in which two cylinders are combined, and an inert gas is blown into the ring-shaped cavity in the inside to form an upper through hole (blowing hole 2) It was a structure that escapes to. However, if the volume V _g of the flow path 3 in the upper nozzle 1 is the same, it is considered that the same gas heating effect can be obtained regardless of the shape of the flow path 3.

The gas injection holes 2 at the upper part of the upper nozzle are arranged concentrically 20 in diameter with a diameter of 0.3 mm. The total cross sectional area of the gas injection holes 2 is A (m ² ). An argon gas was supplied at a source pressure of 0.7 MPa, and a test was conducted to release the argon gas from the gas injection hole 2 into the atmosphere at the speed of sound. V _g / A was adopted as an evaluation index. The unit is m. In the experiment, it was prepared those of V _g / A = 15~191m.

First, with the upper nozzle 1 kept at normal temperature, argon gas was supplied at a source pressure of 0.7 MPa, and the relationship between V _g / A and argon gas flow rate (NL / min) was measured. As a result, regardless of the value of V _g / A, the argon gas flow rate was constant at 28.3NL / min.

Next, the upper nozzle 1 was heated off-line with a burner, and the thermocouple measurement temperature set in the upper nozzle was maintained at 1000 ° C. Similarly, argon gas was supplied at a source pressure of 0.7 MPa, and the relationship between V _g / A (m) and argon gas flow rate (NL / min) was measured. The results are shown in FIG. As apparent from FIG. 4, the argon gas flow rate decreases as V _g / A increases.

Supplied argon gas, as the volume V _g of the flow path 3 of the upper nozzle 1 is large, the residence time of the upper nozzle 1 is increased. In the experiment in which the upper nozzle 1 was heated to 1000 ° C., the gas temperature at the time of reaching the gas injection hole 2 increases as the residence time of the gas becomes longer. And since the source pressure of the gas is sufficiently high, the gas is ejected from the gas injection hole 2 at the speed of sound. Assuming that the gas temperature is T (K), the speed of sound is proportional to √T, and the actual volume of the gas flowing out of the gas injection hole 2 is proportional to √T. On the other hand, since the capacity in terms of NL is proportional to 1 / T of the actual capacity, as a result, the higher the gas temperature, the lower the gas flow rate (NL / min). In FIG. 4, in the result in the case where the upper nozzle 1 is heated to 1000 ° C., the phenomenon that the gas flow rate decreases as V _g / A increases can be explained in this way.

If the source pressure of the supply gas to the upper nozzle 1 is sufficiently high, the flow velocity of the discharged gas reaches the speed of sound regardless of whether the temperature of the gas blown from the gas blowing hole 2 is low or high. On the other hand, the higher the gas temperature blown, the lower the gas flow rate (NL / min). In addition, since the gas temperature is raised when discharging from the gas injection hole 2, the further expansion after the discharge is reduced. As a result, when the upper nozzle 1 having a large V _g / A is used while ejecting the gas at the same sound velocity, the runaway of the tundish surface due to the argon bubbles is reduced, and the gas to the in-mold molten steel It can be expected that the inflow can be further reduced.

Then, from FIG. 4, where the gas flow rate higher the V _g / A (NL / min) is lowered, in particular V _g / A gas flow rate (NL / min due to enlargement V _g / A until 50m It can be seen that the decrease is remarkable and that the influence of the expansion is reduced when it becomes 50 m or more. Therefore, in order to fully enjoy the effect of expanding V _g / A, it is preferable to the V _g / A more than 50 m. By setting V _g / A to 50 m or more, the temperature of the gas discharged from the gas injection hole 2 can be sufficiently raised, and it is considered that the expansion of the gas can be sufficiently suppressed. The V _g / A larger effect, since the V _g / A is up to about 80m remains dependent on V _g / A somewhat, it is most preferable to set the V _g / A more than 80m. The upper limit of this V _g / A is not particularly limited, but considering that the expansion effect is saturated, it is considered to be practically sufficient up to about 250 m.

The invention was applied in the continuous casting of steel slabs. The continuous casting apparatus casts slab slabs 230 mm thick and 1600 mm wide by injecting molten steel into the mold from the injection hole at the bottom of the tundish using a wedge-shaped 30-ton wedge-shaped tundish. A tundish stopper 6 is used to control the injection amount. The shape of the upper nozzle 1 disposed at the upper portion of the injection hole 5 is the same as in Experiments 1 and 2, the inner diameter of the injection hole 5 is 90 mmφ, and as shown in FIG. The diameter of the portion connected to the tundish bottom 8 is 142 mm. In the upper nozzle 1, 20 gas injection holes 2 with a diameter of 0.3 mm are arranged at equal intervals on a circumference centered on the injection hole center. The total cross-sectional area A of the gas injection holes is A = 1.4 × 10 ⁻⁶ m ² in all cases. The flow path 3 through which the inert gas in the upper nozzle 1 flows is formed on the slit surrounding the injection hole 5 as shown in FIG. 3, and the volume V _g of the flow path 3 is adjusted by adjusting the width of the slit. It was adjusted. The volumes _Vg and _Vg / A of the flow path 3, the arrangement distance x of the gas injection holes, and the throughput W of the molten steel are as shown in Table 1 below. Argon gas was supplied at a source pressure of 0.5 MPa. Steel composition is mass%, C: 0.05%, Si: 0.01%, Mn: 0.1%, P: 0.01%, S: 0.005%, molten steel in tundish at the start of casting The temperature is 1590 ° C.

  The cell density of the slab was evaluated by measuring the pinhole defect density of the slab. Samples of 50 mm in the thickness direction, 100 mm in the width direction, and 10 mm in the casting direction are cut out from positions of 1⁄4 and 3⁄4 width of both long side surfaces of cast slab, and X-rays are transmitted to transmit pinholes The density was measured.

  Regarding the inclusion density in the slab, the sample cut out for measuring the above-mentioned pinhole defect density was observed with a microscope, and the density of inclusions present in the steel was measured.

  A flux containing Sr (strontium) is disposed on the surface of the Tundish bath. If the flux is suspended in the molten steel due to the hot surface runaway when the inert gas blown from the upper nozzle 1 floats and separates on the tundish surface, Sr is carried to the molten steel in the mold, so the Sr content in the slab By evaluating, the tundish hot water runaway situation resulting from inert gas blowing was evaluated.

Comparative Example 1 shown in Table 1 uses the conventional upper nozzle 1 with a gas injection hole arrangement distance x = 15 mm, _Vg / A = 31 m, and a gas source pressure of 0.5 MPa and a gas at a speed of sound from the gas injection hole 2 It is spouted and casting is performed at molten steel throughput W = 40 m ³ / hr. Therefore, the cell density, inclusion density, and Sr content of Comparative Example 1 are all 1, and the numerical values are indexed, and Table 1 shows the results of Examples and Comparative Examples. The bubble density index made 0.5 or less the suitable range. The inclusion index was 0.9 or less, which shows a slight improvement over Comparative Example 1, as a preferred range. Sr concentration index made 1.5 or less the suitable range (permissible range).

  Table 1 shows the results. The values outside the scope of the present invention are underlined.

Examples 1 to 4 are examples of the present invention, and all satisfy the requirements of the upper nozzle shape according to the present invention. Example 1 was within the scope of the present invention at V _g / A = 92 m, and the cell density index was 0.4. The inclusion index is 0.9, and although the bubble density index is greatly reduced, the inclusion index is also slightly reduced, and Ar injected by continuous casting using the upper nozzle according to the present invention It is considered that the rate of drawing into the mold was reduced to provide a pinhole reduction effect. On the other hand, since the ratio of Ar rising in the tundish increased while the ratio of Ar drawn into the mold decreased, the position of the gas injection hole of the upper nozzle was separated from the conventional upper nozzle (Comparative Example 1) Nevertheless, it was possible to obtain the intended result of the present invention that the inclusion index decreases. Since the Sr concentration index in this Example 1 was 1.5, as compared with Comparative Example 1, the runaway of the surface of the molten steel in the tundish was large, but as shown by the decrease in the inclusion index, this An increase in the Sr concentration index of degree is said to be acceptable.

Example 2 has V _g / A = 183 m, and the bubble density index is further improved as compared with Example 1. The inclusion index is also lower than that of Example 1, and it can be considered as a result that the heating effect of the blowing Ar is sufficiently exhibited. Furthermore, since the Sr concentration index is also lower than that of Example 1, there is a possibility that the effect of suppressing the runaway of the surface of the molten steel is also added.

Example 3 is an example in which V _g / A is 183 m, which is the same as Example 2, but the position of the gas injection holes is 20 mm close to that of Comparative Example 1. The molten steel throughput is also slightly larger than in Example 2, but the cell density index is still much lower than in Comparative Example 1, and the inclusion index is also lower. Therefore, the effect of the present invention is confirmed until the position of the gas injection hole is 20 mm. It can be said that On the other hand, Example 4 is an example where V _g / A is 183 m, which is the same as Examples 2 and 3, but the position of the gas injection holes is 70 mm, which is the same as Comparative Example 3. In this case, the bubble density index was significantly lowered to 0.01, but even in Comparative Example 3, since the bubble density index is low, the ratio of Ar drawn into the mold is originally considerably low when this distance is made far it is conceivable that. Instead, in Comparative Example 3, the inclusion index was noticeably deteriorated, and it is considered that as the Sr concentration index was high, the runaway of the molten steel surface in the tundish increased. In Example 4, the inclusion index is lower than that of Comparative Example 1, and it is considered that the effect of the present invention of blowing Ar into the tundish surface by suppressing runaway on the surface of the molten steel is exerted toward reducing inclusions.

  Comparative Examples 1 to 3 will be described.

Comparative Example 1 is an example using the conventional upper nozzle in which the gas injection hole arrangement distance x is 15 mm and V _g / A is deviated from the equation (1). As described above, the cell density and the like of the comparative example are set to 1.

The comparative example 2 is an example which used the upper nozzle which _Vg / A remove | deviated from (1) Formula, and made the position of the gas injection hole 45 mm same as Example 1,2. At 31 m at which V _g / A deviated from the equation (1), the bubble density index was slightly improved to 0.9, but it was not much different from the index of Comparative Example 1, and the inclusion index was not particularly changed.

Comparative Example 3 is an example in which V _g / A deviates from the equation (1), and the blow hole arrangement distance x = 70 mm and the position of the blow hole is far from the injection hole. Therefore, although the bubble density index was significantly low, the inclusion reduction effect by Ar injection was difficult to be exhibited. As the inert gas blown in expands after being blown in and most of it reaches the tundish hot water surface, the Sr concentration index is poor at 2.3 and there is also an influence of the runaway surface of the molten steel in the tundish. It is considered that the inclusion index became poor at 1.3.

Reference Signs List 1 upper nozzle 2 gas injection hole 3 flow path 4 gas inlet 5 injection hole 6 stopper 7 tundish 8 tundish bottom x gas injection hole arrangement distance

Claims (1)

An upper nozzle for continuous casting provided in a molten steel injection hole from a molten steel container to a mold,
At the top of the upper nozzle, a plurality of gas injection holes are installed on the circumference centered on the center of the injection hole,
The relationship between the total cross-sectional area A (m ² ) of the gas injection holes and the volume V _g (m ³ ) of the flow path through which the inert gas in the upper nozzle flows is characterized by satisfying the following equation (1) Top nozzle for continuous casting.
V _g / A ≧ 50 (m) (1)

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