Submerged entry nozzle

Abstract

The present invention provides a technology for maintaining, in a mold, an ideal fluidized state which was assumed at the time of design, by preventing blockage of a nozzle. Provided is a submerged entry nozzle through which, when continuous casting of steel is carried out, molten steel is supplied from a molten steel storage container into a mold of a continuous casting machine that carries out the continuous casting, wherein: an end of a nozzle body of the submerged entry nozzle which is submerged in the molten steel in the mold is closed; an upper portion and a lower portion of a part of the nozzle body which is submerged in the molten steel are each provided with a pair of discharge ports that are symmetrical with respect to the central axis; an inner bottom of the nozzle body has an inclination which continues to a lower end of each of the discharge ports in the lower part; and a downward inclination angle θ1 with respect to the horizontal direction is within the range of not less than 10° but not more than 45°.

Description

Submerged Entry Nozzle

[0001] The present invention relates to a submerged entry nozzle for injecting molten steel into a mold during continuous casting of steel.

[0002] During continuous casting of steel, a submerged entry nozzle is immersed in the molten steel in a mold and the molten steel is poured in. The flow of molten steel in the mold is split into an upward flow that rises along the inner wall of the mold and a downward flow that descends along the inner wall of the mold when the flow of molten steel discharged from the pair of outlets on the left and right of the submerged entry nozzle hits the inner wall on the short side of the mold.

[0003] In this case, especially when the discharge flow velocity is high, uneven flow velocity distribution may occur at the top and bottom of the discharge port. This may cause the left and right flow rates to be unbalanced in the upward and downward flows, or may result in localized strong discharge flows, resulting in large flow fluctuations. Such fluctuations can lead to poor formation of a solidified shell and defects caused by the entrapment of bubbles and inclusions in the solidified shell.

[0004] In order to solve such problems, it is believed that continuous casting can be achieved without defects due to bubbles or inclusions by slowing down the flow of molten steel in the mold and forming a uniform flow. In line with this idea, a four-hole submerged entry nozzle (four-hole nozzle) with two tiers of molten steel outlets in the upper and lower directions has been proposed, for example, in the following patent documents:

[0005] Patent Document 1 discloses a nozzle in which the area of ​​the upper outlet is larger than that of the lower outlet in order to reduce the maximum downward flow velocity as much as possible, and a continuous casting method using the nozzle.

[0006] International Publication No. 2010 / 109887

[0007] Although the technology described in Patent Document 1 was successful in reducing the downward flow velocity, in actual casting, a large amount of alumina is sometimes present in the molten steel. In such cases, if the molten steel has poor cleanliness, alumina adheres to the inside of the submerged entry nozzle, impairing the flow intended at the time of nozzle design, resulting in so-called nozzle clogging. This problem is particularly serious with a four-hole submerged entry nozzle, which has a larger number of outlets and a relatively smaller cross-sectional area than a typical two-hole submerged entry nozzle.

[0008] The present invention has been made to solve the above problems, and aims to provide a technology that prevents nozzle clogging and thereby maintains the ideal flow conditions within the mold that were envisioned at the time of design.

[0009] In order to solve the above problems, the inventors performed flow calculations related to adhesion to the submerged entry nozzle and investigated a shape that would reduce adhesion, and as a result, arrived at the present invention.

[0010] That is, the gist of the present invention is as follows. [1] An immersion nozzle for supplying molten steel from a molten steel storage vessel into a mold of a continuous casting machine for continuous casting of steel, wherein the end of the immersion nozzle body on the side immersed in the molten steel in the mold is closed, and the upper and lower sections of the nozzle body immersed in the molten steel have a pair of outlets each symmetrical about a central axis, and the bottom of the inner bore of the nozzle body is inclined continuously to the lower end of the outlet of the lower section, with the downward inclination angle θ1 relative to the horizontal being in the range of 10° to 45°. [2] The immersion nozzle described in [1] above, wherein the upper end of the outlet of the lower section changes in inclination angle toward the outlet so as to narrow the outlet of the lower section, and the inclination angle θ2 of the innermost part of the upper end relative to the horizontal is in the range of 1.3 to 1.5 times the inclination angle θ1 of the bottom of the inner bore. [3] The submerged entry nozzle according to the above [1] or [2], wherein the opening area of ​​the outlet of the lower stage section is 1.0 to 1.6 times the opening area of ​​the outlet of the upper stage section. [4] The submerged entry nozzle according to any one of the above [1] to [3], wherein in the flow path inside the submerged entry nozzle, the ratio r / R of the inner diameter r from the upper end of the outlet of the upper stage section to the inner diameter R from the upper end of the submerged entry nozzle to the upper end of the outlet of the upper stage section is 0.6 or more and less than 1.0.

[0011] The submerged entry nozzle of the present invention is expected to be effective in preventing adhesion of alumina-based inclusions to the interior of the submerged entry nozzle and to achieve the ideal flow envisaged at the time of design without impairing it. The submerged entry nozzle of the present invention is suitable for use in a continuous steel casting method.

[0012] Fig. 1 is a longitudinal sectional view of an immersion nozzle according to one embodiment of the present invention, Fig. 2 is a longitudinal sectional view of a conventional immersion nozzle having a pool at its bottom, Fig. 3 is a longitudinal sectional view of an immersion nozzle according to another embodiment of the present invention.

[0013] Hereinafter, embodiments of the present invention will be described in detail. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical idea of ​​the present invention, and are not intended to limit the configuration to the following. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0014] Figure 1 is a longitudinal cross-sectional view showing the tip shape of a multi-hole submerged entry nozzle according to one embodiment of the present invention. During continuous steel casting, molten steel is poured into a mold by immersing such a submerged entry nozzle. In this embodiment, the nozzle has a total of four outlets 1 and 2, two pairs above and below, with the central axis CL as the axis of symmetry. This is a so-called four-hole submerged entry nozzle. The inner side of the outlets 1 and 2 is also referred to as the inlet, and the outer side is also referred to as the outlet.

[0015] In this embodiment, the nozzle has a mountain-shaped portion 3 at the bottom of its inner hole. The slope of the mountain-shaped portion 3 extends continuously to the lower end of the lower-stage outlet 2. The angle of the slope, measured from the central axis CL toward the outlet of the lower-stage outlet 2, is an inclination angle θ1 in the range of 10° to 45° from the horizontal plane. The reason for this is explained below.

[0016] Generally, the adhesion of alumina to the submerged entry nozzle (SEN) is focused on how to improve the cleanliness of the molten steel. However, the inventors have found that turbulence generated in the nozzle increases the frequency with which alumina particles approach the nozzle wall, promoting the adhesion of alumina particles to the nozzle inner surface, which is the main cause of nozzle clogging.

[0017] However, it is difficult to clarify the degree of turbulence experimentally, so we evaluated the local turbulence using numerical calculations.

[0018] <Analysis 1> The general-purpose thermal fluid solution STAR-CCM+ was used for the analysis. A turbulence model was used for the calculation model to evaluate turbulence, and the k-ε model, a Reynolds-averaged model, was used. In this model, k represents turbulence energy, a variable that abstractly represents the magnitude of flow turbulence. The upper inner diameter R of the submerged entry nozzle was 90 mm, and the lower inner diameter r was 75 mm, forming a stepped shape. The calculations were performed with the pressure near the outlet of the nozzle set to 0, and a steady-state analysis was performed under the inflow condition of 7 t / min of molten steel flowing from the top of the nozzle.

[0019] Under the above analysis conditions, the inclination angle θ1 of the angled portion 3 (shown in Figure 1 ) relative to the horizontal plane, i.e., the angle of the lower outlet port 2, was varied from 5° to 50°. For the evaluation, a conventional example, a SEN with a flat pool 4 at the bottom of the inner bore (shown in Figure 2 ), was also analyzed and used as a benchmark for the degree of turbulence. Table 1 shows the analysis results, showing the relationship between the maximum turbulence energy within the SEN and the inclination angle θ1 of the angled portion 3 at the bottom. Compared to Treatment No. B1, where the bottom of the SEN inner bore is conventional, the maximum turbulence energy tends to be reduced by providing an SEN inner bore with an angled bottom. This effect is particularly significant when the inclination angle θ1 is 10° or greater downward. This is due to the smoother flow of molten steel compared to the conventional bottom. However, the effect is almost unchanged even when the inclination angle θ1 exceeds 45°. On the other hand, if the lower-stage outlet 2 is tilted downward at an angle greater than 45°, the molten steel in the mold will penetrate too deeply in the casting direction, which will be detrimental to the floating and separation of inclusions. Furthermore, the negative pressure at the upper part of the outlet side of the outlet may suck in mold powder, significantly impairing the durability of the nozzle. Therefore, the inclination angle θ1 of the mountain-shaped portion 3 is set to a range of 10° to 45°. Preferably, the inclination angle θ1 is set to a range of 20° to 45°.

[0020]

[0021] <Analysis 2> Calculations were also performed under similar conditions and models, with two inclination angles added to the upper part of the outlet port 2 of the lower stage, as shown in Figure 3, and the angle θ2 of the inclined portion 5 on the inner hole side relative to the horizontal plane was changed. Basically, it is preferable that the angle θ2 on the inner hole side be larger than θ1.

[0022] The calculation results are shown in Table 2. When θ2 was 1.3 times or more than θ1, a further reduction in turbulence energy was observed. This is due to the effect of mitigating turbulence by dispersing the flow rate and lowering the maximum Reynolds number by guiding the streamlines from the top of the nozzle to the outlet 2 in the lower stage to the upper side of the outlet. However, this effect did not change even when θ2 exceeded 1.5 times θ1. On the other hand, if the angle θ2 on the inner bore side is made too large, the upper end of the outlet 2 in the lower stage will approach the lower end of the outlet 1 in the upper stage, which may reduce the effect of providing a step in the inner bore, as described below.

[0023]

[0024] <Area Ratio of Upper and Lower Discharge Ports> In this embodiment, it is preferable that the cross-sectional area of ​​the lower discharge port is 1.0 to 1.6 times the cross-sectional area of ​​the upper discharge port 1. The reason for this will be explained below.

[0025] In general, in a multi-hole submerged entry nozzle equipped with upper and lower discharge ports, attention is focused on how to obtain the effect of attenuating the discharge flow rate to reduce defects in the cast slab.

[0026] However, the inventors have found that the flow of molten steel tends to be biased toward the lower discharge port 2 due to the influence of gravity. As a result, the pressure at the bottom of the inner bore of the submerged entry nozzle increases, making it easier for a stagnation area to form, and negative pressure is also likely to form near the discharge port. The inventors have found that these two factors induce a reaction between inclusions in the molten steel and the refractory of the submerged entry nozzle, causing adhesion of the inclusions to the submerged entry nozzle and melting damage to the refractory of the nozzle, making stable operation difficult.

[0027] First, in an SEN having an upper outlet 1 and a lower outlet 2, the opening area of ​​the lower outlet 2 is set to be equal to or larger than the opening area of ​​the upper outlet 1, thereby rectifying the flow between the upper and lower outlets and reducing the stagnation area formed at the bottom of the SEN bore. The size of the stagnation area is determined by the balance between the outlet areas of the upper and lower outlets and by changes in the inner diameter of the nozzle body around the outlets, which are factors that determine the flow of molten steel. In addition, these factors also affect the "continuity" of the molten steel flow field, making it difficult to predict the impact of each factor individually.

[0028] Therefore, in order to control the formation of stagnation areas caused by local high pressure or negative pressure areas by balancing the sizes of the upper and lower outlets, we evaluated the effect of the ratio of the outlet areas of the upper and lower tiers on stagnation areas using numerical calculations.

[0029] Furthermore, it was thought that the stagnation area at the bottom of the nozzle bore could be controlled by forcibly directing part of the molten steel flow to the upper outlet by causing part of the molten steel flow to collide with the refractory material located between the upper and lower outlets. Therefore, the effect of changes in the inner diameter of the nozzle body around the outlet on the stagnation area was investigated by performing numerical calculations.

[0030] <Analysis 3> First, the immersion nozzle was designed to be straight with an inner diameter R of 150 mm, and four-hole immersion nozzles No. 1 to 5, each with an upper and lower outlet with openings of the shapes shown in Table 3, were subjected to numerical calculations. The analysis was performed using the general-purpose thermal fluid analysis solution STAR-CCM+, and the steady-state total pressure distribution was determined and evaluated under conditions where the pressure near the outlet side of the outlet was set to 0 and the maximum flow velocity inside the nozzle was 3.0 m / s. In Table 3, "vertical" represents the vertical direction, and "horizontal" represents the horizontal direction.

[0031]

[0032] Here, the cross-sectional area of ​​the upper outlet port 1 is SU, and the cross-sectional area of ​​the lower outlet port 2 is SL. As SL / SU increases, i.e., as the cross-sectional area of ​​the lower outlet port 2 increases relative to the cross-sectional area of ​​the upper outlet port 1, the maximum pressure tends to decrease, and it is thought that stagnation due to high pressure is eliminated. In particular, when SL / SU is 1.0 or more, a significant pressure reduction effect was obtained. Preferably, SL / SU is greater than 1.0.

[0033] Furthermore, as SL / SU increases, the minimum pressure near the discharge port decreases, and in particular, negative pressure occurs at values ​​exceeding 1.6. Inclusions in the molten steel tend to collect in negative pressure areas, which, like the retention area, is thought to induce a reaction between the inclusions in the molten steel and the refractory of the submerged entry nozzle, resulting in adhesion of the inclusions to the submerged entry nozzle and melting damage to the nozzle refractory. Therefore, it is preferable to set SL / SU to 1.6 or less.

[0034] <Analysis 4> Next, for an immersion nozzle with an SL / SU ratio of 1.0, the relationship between the ratio r / R (where R is the inner diameter up to the top end of the upper outlet port and r is the inner diameter from the top end of the upper outlet port to the bottom of the lower end of the immersion nozzle) and the maximum pressure in the flow path inside the nozzle was analyzed. The analysis results are shown in Table 4. Here, the maximum pressure inside the nozzle when r / R is 1.0 was normalized to 1.0.

[0035]

[0036] The results in Table 4 reveal that there is an optimum range for the inner diameter ratio r / R. The normalized maximum pressure is smallest when the inner diameter ratio is approximately 0.7, and increases both when the ratio is smaller and larger. In particular, when r / R is 0.5, the normalized maximum pressure exceeds 1.0. This is thought to be because the proportion of the area where the molten steel flow hits the refractory between the upper and lower discharge ports increases, creating a new high-pressure zone and a dangerous area for stagnation. Therefore, the inner diameter ratio r / R is preferably 0.6 or more and less than 1.0. This allows the normalized maximum pressure to be kept below 1.0. More preferably, r / R is 0.9 or less.

[0037] <Continuous Casting of Steel> Furthermore, when actually continuously casting steel using an immersion nozzle, an inert gas such as Ar gas can be mixed into the molten steel through the upper nozzle during casting. This allows the molten steel to be subjected to the buoyancy of gas bubbles, which can mitigate the formation of a high-pressure zone at the bottom 3 of the immersion nozzle.

[0038] However, if the amount of inert gas mixed in is excessive, the flow will tend to rise to the meniscus in the mold where the steel is cast after it leaves the submerged entry nozzle, causing large fluctuations in the molten metal surface, which will hinder operation. Therefore, there is a range of appropriate amounts of gas to be injected.

[0039] The present invention has been configured as described above. The feasibility and effects of the present invention will be further explained below with reference to examples.

[0040] (Example 1) Casting was performed using a vertical bending continuous casting machine with the nozzles of the invention shown in FIG. 1 and the conventional example shown in FIG. 2 under the conditions listed in Table 5. To assess nozzle clogging, a used nozzle was cut and the degree of deposits inside the nozzle on the lower interior side was measured as an index. This index was assigned a value of 1 for complete clogging and 0 for no clogging at all. Evaluation was performed using the nozzle for continuous casting of ultra-low carbon steel, with a casting time of 240 minutes. The results in Table 5 demonstrate that good results were obtained with the nozzles within the range specified in the present invention, which were used as invention examples. Furthermore, when θ1 was greater than 45°, the negative pressure above the discharge port caused mold powder to be sucked in, significantly reducing the durability of the nozzle; therefore, the upper limit was set at 45°.

[0041]

[0042] (Example 2) Under the conditions of A1 in Example 1, a nozzle was used in which two inclination angles were imparted to the upper part of the lowest outlet port as shown in Figure 3, and the angle θ2 on the inner bore side was varied, and casting was performed at the levels shown in Table 6. The results in Table 6 show that even better results were obtained for nozzles within the specified range. Furthermore, no change in the degree of nozzle clogging was observed even when θ2 exceeded 1.5 times θ1.

[0043]

[0044] Example 3: Casting was performed using a submerged nozzle with a vertical bending continuous casting machine, using the nozzle configuration and casting method described in Table 7. The angle of inclination θ1 of the angled portion 3 was 20°. As an index of operational stability in Table 3, an eddy current displacement sensor was installed directly above the molten metal surface at the center of the thickness, located ¼ of the casting width W away from the short side toward the center of the width. The eddy current displacement sensor was used to measure changes in the molten metal surface level over time. The magnitude of the molten metal surface level fluctuation for each process was indexed, with the magnitude of the molten metal surface level fluctuation for Process No. D1 being set at 100. The average value for the first and second halves of casting was used for evaluation as an index showing operational stability.

[0045]

[0046] From the results in Table 7, all of the inventive examples obtained better results than the reference examples. Ａｒ When comparing samples with the same inner diameter ratio r / R, Process Nos. D2 and D4, in which the inner diameter ratio r / R was set within an appropriate range, showed better results than Process Nos. D1 and D3, in which the inner diameter ratio r / R was 1.0. When comparing samples with the same inner diameter ratio r / R, the Ar gas volume ratio Q blown from the upper nozzle Ａｒ Processes D3 and D4, in which / M was set in an appropriate range, achieved better results than Processes D1 and D2, respectively. Among them, Process D4 showed the lowest average molten metal level fluctuation index and demonstrated high operational stability.

[0047] In this specification, the unit of mass, "t", is metric ton = 10 ３ It means kg.

[0048] 1 (Upper stage) outlet 2 (Lower stage) outlet 3 (Bottom) mountain-shaped portion 4 (Bottom) pool 5 (Lower stage outlet upper end inner hole side) inclined portion CL central axis R (to the upper end of the upper stage outlet) inner diameter r (from the upper end of the upper stage outlet to the bottom of the lower end of the submerged nozzle)

Claims

1. An immersion nozzle for supplying molten steel from a molten steel storage vessel into the mold of a continuous casting machine that performs continuous casting of steel, wherein the end of the nozzle body on the side that is immersed in the molten steel in the mold is closed, and the nozzle body has a pair of discharge ports at each of the upper and lower sections of the part that is immersed in the molten steel, with the central axis as an axis of symmetry, and the bottom of the inner bore of the nozzle body has a slope that continues to the lower end of the discharge port at the lower section, and the downward inclination angle θ1 with respect to the horizontal is within the range of 10° to 45°.

2. The submerged nozzle according to claim 1, wherein the upper end of the outlet of the lower section has an inclination angle that changes so as to narrow the outlet of the lower section toward the outlet, and the inclination angle θ2 of the innermost upper end relative to the horizontal direction is in the range of 1.3 to 1.5 times the inclination angle θ1 of the bottom of the inner hole.

3. The submerged nozzle according to claim 1 or 2, wherein the opening area of ​​the discharge port in the lower section is in the range of 1.0 to 1.6 times the opening area of ​​the discharge port in the upper section.

4. The submerged entry nozzle according to any one of claims 1 to 3, wherein in a flow path inside the submerged entry nozzle, the ratio r / R of the inner diameter r from the upper end of the upper stage outlet to the bottom of the inner bore of the nozzle body, to the inner diameter R from the upper end of the submerged entry nozzle to the upper end of the upper stage outlet, is in the range of 0.6 or more and less than 1.0.

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