JP2016182612A - Continuous casting method blowing inert gas from upper porous refractory and lower porous refractory - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a casting method capable of reducing inclusions captured by a slab in continuous casting for steel.SOLUTION: In continuous casting method blowing an inert gas for steel, equations: 0.07 m≤D≤0.15 m, 0.16 m≤D≤0.3 m, 0.2 m≤L≤0.45 m, (R+R/2)/(D/2)≤4.25(L+L)/D+0.2, (R+R/2)/(D/2)≥0.26(L+L/2)/D+1, 0.3≤(L+L/2)/D≤6.3 and 1.5≤(R+R/2)/(D/2)≤4.0 are satisfied (where, the top edge inside diameter of a nozzle is D; the outside diameter of the nozzle is D; a distance from the top edge of an upper porous refractory to the top edge of a lower porous refractory is L; a distance from the top edge of the lower porous refractory to the lower edge of the lower porous refractory is L; the outside dimension of a nozzle height is L; a distance from the center of the nozzle to the upper porous refractory is R; and the thickness of the upper porous refractory in a radial direction is R).SELECTED DRAWING: Figure 4

Description

  The present invention relates to a continuous casting method in which an inert gas is blown from, for example, an upper porous refractory and a lower porous refractory.

  Conventionally, in continuous casting, casting is performed by casting molten steel poured from a ladle into a tundish into a mold through a slide nozzle from a nozzle provided in the tundish. In this continuous casting, an inert gas is blown from a porous refractory provided at the bottom of the tundish. There exist some which are shown to patent documents 1-6 as a technique regarding continuous casting.

In Patent Document 1, for the purpose of floating inclusions in the tundish while preventing bubbles from entering the mold, a gas blowing portion is provided in the upper nozzle, and Ar gas is blown from that portion.
In Patent Document 2, Ar blowing parts are separately embedded in the upper and lower parts of the nozzle for the purpose of preventing the nozzle from being clogged and detecting an abnormal gas blowing.

In Patent Documents 3 to 6, Ar blowing portions are separately embedded in the upper and lower portions of the nozzle for the purpose of preventing the nozzle from being clogged.
In Patent Document 7, Ar gas is blown from one or more of an upper nozzle, a stopper, a plate, an intermediate nozzle, and an immersion nozzle for the purpose of preventing the nozzle from being clogged.

Japanese Patent Laid-Open No. 10-118751 JP 2012-45584 A JP 2011-110561 A JP 2009-66603 A JP 2007-237244 A JP 2007-90423 A JP-A-4-319055

  Although Patent Document 1 describes that Ar gas is blown from the upper nozzle, the configuration of the upper nozzle and the conditions for blowing Ar gas are not described in detail. There is a risk of hot water being produced and reoxidation of the molten steel. If the amount of Ar gas blown is reduced to prevent reoxidation of the molten steel due to the bare metal, the inclusions in the molten steel may not be sufficiently lifted.

  In Patent Documents 2 to 6, although it is described that Ar blowing portions are provided at the upper and lower portions of the nozzle, the configuration of the Ar blowing portion, the conditions for blowing Ar gas, and the like are not described in detail. Therefore, most of Ar gas may be introduced into the mold, and the molten metal surface level tends to become unstable. If the amount of Ar gas blown is reduced, the inclusions in the molten steel may not be sufficiently levitated.

In Patent Document 7, since Ar gas is blown from a plurality of upper nozzles, stoppers, plates, intermediate nozzles, and immersion nozzles, it is difficult to control Ar gas, and it is difficult to stably float molten steel inclusions. is there.
Therefore, in view of the above problems, the present invention provides a continuous casting method in which an inert gas is blown from an upper porous refractory and a lower porous refractory that can reduce inclusions captured by a slab in continuous casting. For the purpose.

In order to achieve the above object, the present invention has taken the following measures.
That is, the technical means for solving the problems in the present invention, when casting the molten steel poured into the tundish from the ladle from the nozzle provided at the bottom of the tundish into the mold through the slide nozzle, A continuous casting method in which an inert gas is blown from a ring-shaped upper porous refractory provided at the upper part of the nozzle and an inert gas is blown from a ring-shaped lower porous refractory provided at the lower part of the nozzle, The upper end inner diameter D _{1 of} the nozzle, the outer diameter D _{2 of the} nozzle, the distance L ₁ from the upper end of the upper porous refractory to the upper end of the lower porous refractory, the distance R ₁ from the center of the nozzle to the upper porous refractory, the upper porous radial thickness R ₂ of _refractory, the distance L ₂ from the upper end of the lower porous refractory to the lower end of the lower porous _refractory, when the external dimension height L of the nozzle, the formula (1) Satisfies Expression (7), molten steel throughput Q _s per _strand, the gas flow rate of the inert gas blown from the top porous refractory Q _g2, the inert gas blown from below the porous refractory gas flow rate Q _g1, lower gas average velocity U _g1, upper gas average speed _{U g2,} average speed _{U s} of the molten steel passing through the nozzle, and satisfies equation (8) to formula (11).

0.07m ≦ D ₁ ≦ 0.15m (1)
0.16m ≦ D ₂ ≦ 0.3m (2)
0.2m ≦ L ≦ 0.45m (3)
_{(R 1 + R 2/2} ) / (D 1 /2)≦4.25(L 1 + L 2/2) / D 1 +0.2 ··· (4)
_{(R 1 + R 2/2} ) / (D 1 /2)≧0.26(L 1 + L 2/2) / D 1 +1 ··· (5)
0.3 ≦ (L ₁ + L ₂ ) / D ₁ ≦ 6.3 (6)
_{1.5 ≦ (R 1 + R 2} /2) / (D 1 /2)≦4.0 ··· (7)
0.78 ≦ Q _s [ton / min / str] ≦ 1.02 (8)
Q _g1 + Q _g2 ≧ 1500 {tan (Q _s -0.91)} ³ +4.2 ・ ・ ・ (9)
_{Q gu = (Q g1 + Q} g2) tanh [870Re -1.45 {70 (4U g2 / U s) 0.5 ((R 1 + R 2/2) / (D 1/2)) 5 +370 (4U g1 / ^{_{U s) -0.3 ((L 1}} + L 2/2) / D 1) -0.7}] ≦ 4 ··· (10)
Q _g1 + Q _g2 -Q _gu ≦ 2 (11)

  According to the present invention, inclusions captured by a slab in continuous casting can be reduced.

It is a general view of a continuous casting apparatus. It is a periphery figure of the bottom part of a tundish. It is a figure which shows the plane of a nozzle, a side surface, and a bottom face. It is a figure which shows the plane of the nozzle in a modification, a side surface, and a bottom face. It is a cross-sectional side view of a nozzle. It is explanatory drawing explaining the relationship between the outer diameter of a nozzle, and the thickness of the upper end of a nozzle. _{(L 1 + L 2/2} ) / D 1, is a diagram showing the relationship between _{(R 1 + R 2/2} ) / (D 1/2). It is explanatory drawing explaining the naked hot water in the hot_water | molten_metal surface of the molten steel in a tundish. It is explanatory drawing explaining the hot_water | molten_metal surface fluctuation | variation in a casting_mold | template. It is a figure which shows the relationship between molten steel throughput and Ar gas flow rate. It is a figure which shows the relationship between the measured value and calculated value of an air levitation rate. It is a schematic plan view of the tundish used in the water model. It is a figure which shows the method of collect | recovering flow beads with a water model. It is a figure which shows the method of collect | recovering bubbles with a water model. It is the figure which put together the water flow rate and the particle | grain outflow rate.

Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings.
The continuous casting method of the present invention is, for example, a method for continuously casting molten steel after the secondary refining treatment. In addition, casting of molten steel is not limited to molten steel after secondary refining.
FIG. 1 is an overall view of a continuous casting apparatus that performs continuous casting. First, the structure of the continuous casting apparatus will be described.

  As shown in FIG. 1, the continuous casting apparatus 1 is formed by a tundish 4 into which the molten steel 3 in the ladle 2 is poured, a mold 5 for casting the molten steel 3 in the tundish 4, and the mold 5. A support roll 7 that supports the slab 6 is provided. The tundish 4 includes an iron skin 10 and a refractory 11 provided inside the iron skin 10. The shape of the slab 6 cast by the continuous casting apparatus 1 is not limited, and may be a slab, bloom, billet or the like. The shape of the tundish 4 and the number of strands are not limited.

  A nozzle 20 for supplying the molten steel 3 in the tundish 4 to the mold 5 is provided at the bottom 15 of the tundish 4 in FIG. A slide nozzle (slide valve) 21 is provided below the nozzle 20. The slide valve 21 is provided between the upper plate 22 provided on the iron skin 10 side of the tundish 4, the lower plate 23 provided on the lower side of the upper plate 22, and the upper plate 22 and the lower plate 23. And a slidable slide plate 24. An immersion nozzle 25 is provided below the lower plate 23. In the slide valve 21, the supply of the molten steel 3 from the tundish 4 (nozzle 20) to the mold 5 (immersion nozzle 25) is stopped by stopping the slide plate 24 at a predetermined position, and the slide plate 24 is moved from the predetermined position. By doing so, the molten steel 3 can be supplied from the tundish 4 to the mold 5. The flow rate of the molten steel 3 supplied to the mold 5 can be adjusted by the amount of movement of the slide plate 24.

An upper porous refractory 30 is provided on the upper portion of the nozzle 20. A lower porous refractory 31 is provided below the nozzle 20. The upper porous refractory 30 and the lower porous refractory 31 are formed of a refractory or the like, and are for injecting an inert gas such as Ar gas.
In such a continuous casting apparatus 1, the molten steel 3 is poured from the ladle 2 toward the tundish 4, and the molten steel 3 in the tundish 4 is cast from the nozzle 20 to the mold 5 via the slide nozzle 21 and the immersion nozzle 25. The molten steel 3 can be cast by inserting. When casting the molten steel 3, the inert gas is blown from the upper porous refractory 30 and the inert gas is blown from the lower porous refractory 31, thereby promoting the floating of inclusions in the molten steel.

In order to sufficiently float the molten steel in the tundish 4, that is, to reduce inclusions trapped by the slab 6 in continuous casting, the nozzle 20, the upper porous refractory 30, and the lower porous refractory 31 are provided. The structure is important. The structure of the nozzle 20, the upper porous refractory 30, and the lower porous refractory 31 will be described.
As shown in FIGS. 3A and 3B, the nozzle (nozzle body) 20 is formed of a cylindrical refractory. The refractory material of the nozzle 20 is made of a material that does not pass an inert gas such as Ar gas.

  The nozzle 20 is fixed to the bottom 15 of the tundish 4 by being installed at a predetermined place when castable (irregular refractory) is poured into the bottom 15 of the tundish 4. In the nozzle (nozzle body) 20, the diameter (radius) of the nozzle 20 is constant in the section from the lower end of the nozzle 20 to the midway portion. Further, in the nozzle (nozzle body) 20, the diameter (radius) of the nozzle (nozzle body) 20 is gradually increased from the middle part to the upper end. An upper installation part 32 for installing a ring-shaped upper porous refractory 30 is provided at the upper part of the nozzle (nozzle body) 20, and a ring-shaped lower porous refractory 31 is provided at the lower part of the nozzle (nozzle body) 20. A lower installation part 33 to be installed is provided. Inside the nozzle 20, a gas supply path (not shown) for supplying an inert gas to the upper porous refractory 30 and the lower porous refractory 31 is provided.

  The upper porous refractory 30 and the lower porous refractory 31 are formed of a porous material through which an inert gas passes. The position of the upper end of the upper porous refractory 30 is substantially the same position as the upper end of the nozzle 20, and the upper porous refractory 30 can blow an inert gas toward the molten steel of the tundish. The side surface and the lower surface of the upper porous refractory 30 except the upper end (upper surface) are surrounded by the refractory of the nozzle 20. The position of the lower end of the lower porous refractory 31 is substantially the same position as the position of the lower end of the nozzle 20. Further, the position of the inner side (inner wall surface) of the lower porous refractory 31 is the same position as the inner wall surface 35 that forms the opening of the nozzle 20, and the lower porous refractory 31 passes the inert gas through the opening of the nozzle 20. Can be blown toward. Note that the vertical position of the lower porous refractory 31 is not limited to the lower end of the nozzle 20, and may be positioned at the center of the inner wall surface 35 of the nozzle 20, for example, as in the modification shown in FIG. 3B. Other positions may also be used. The radial position of the upper porous refractory 30 may be the outer edge of the upper surface of the nozzle 20, but may be positioned at the center of the upper surface in the radial direction, or may be at another position.

Next, each dimension of the nozzle 20 etc. will be described.
Figure 3A, 3B, as shown in FIG. 4, the upper end inner diameter _{D 1} of the nozzle satisfies the equation (1).
0.07m ≦ D ₁ ≦ 0.15m (1)
The upper inner diameter D ₁ of the nozzle, a inner diameter at the upper end of the nozzle, the size of the inner wall surface at the upper end of the nozzle. Since the nozzle 20 is connected to the slide nozzle 21 and the immersion nozzle 25, the inner diameter thereof needs to be slightly larger than or equal to the slide nozzle 21 and the immersion nozzle 25. The internal diameter of the general slide nozzle 21 is 0.05 to 0.09 m.

When the upper end inner diameter D ₁ of the nozzle is less than 0.07 m, clogging the nozzle is likely to occur due to non-metallic inclusions for the upper end inner diameter of the nozzle is too small. Since the upper limit of the inner diameter of the slide plate is 0.09 m, when the lower end inner diameter of the nozzle is 0.09 m, which is the same as that of the slide nozzle 21, it is difficult to manufacture a nozzle having an upper end inner diameter D ₁ exceeding 0.15 m. Note that the opening of the nozzle needs to be gradually increased from the lower end to the upper end in order to pull out the metal core during manufacture and to rectify the flow in the nozzle.

Outer diameter _{D 2} of the nozzle, which satisfies Expression (2).
0.16m ≦ D ₂ ≦ 0.3m (2)
The outer diameter D ₂ of the nozzle, a outer diameter of the nozzle, which is the diameter of the portion is constant in diameter. When the outer diameter D ₂ of the nozzle is 0.16m which is the lower limit, as shown in FIG. 5, the upper end of the nozzle except for the length of the upper porous refractory 30 thickness t (the upper set portion 32 thickness t) is 0.025 m. If the outer diameter D ₂ of the nozzle is less than 0.16 m, lack of thickness of the upper end of the nozzle can not be ensured over 0.025 m (referred to fabrication difficulties), a nozzle (non-porous refractory) during casting Is likely to occur. In addition, since the thickness of the upper porous refractory needs to be 0.02 m or more in production, when the lower limit value (0.07 m) of the upper end inner diameter D ₁ of the nozzle is also combined, the outer diameter D ₂ of the upper nozzle The minimum value is 0.16 m.

Now, when the outer diameter D ₂ of the nozzle exceeds the 0.3 m, workability is deteriorated. To apply the nozzle to the tundish, a kneaded refractory such as mortar is applied to the outside of the nozzle and fitted into the bottom hole of the tundish while turning. When the outer diameter D ₂ of the nozzle is more than 0.3 m, on which it becomes difficult fit while rotating the nozzle, installing the stability of the nozzle is deteriorated.
The distance L (the outer height of the nozzle) from the upper end of the upper porous refractory 30 to the lower end of the lower porous refractory 31 satisfies the formula (3).
0.2m ≦ L ≦ 0.45m (3)
The distance L is a vertical distance from the upper surface of the upper porous refractory 30 to the lower surface of the lower porous refractory 31. Hereinafter, the distance L is referred to as height.

  When the height L is less than 0.2 m, since the ground contact area of the tundish refractory and the nozzle becomes small, the fixing becomes unstable and the construction is difficult. Further, when the tundish refractory and the ground contact surface of the nozzle become small, there is a possibility that steel leakage occurs between the nozzle and the tundish refractory. As shown in FIG. 4, the upper limit of the height L is determined by the total value of the thickness of the refractory and the thickness of the iron skin at the bottom of the tundish. The total value is generally 0.35 to 0.45 m, and if it exceeds 0.45 m, it cannot be manufactured, that is, it is difficult to manufacture.

Distance R ₁ from the center of the nozzle to the upper porous refractory 30 (referred to as horizontal distance R ₁ ), radial thickness R _{2 of} the upper porous refractory 30 (referred to as upper thickness R ₂ ), upper end inner diameter D _{1 of} the nozzle, nozzle outer diameter D ₂ of the _upper porous refractory first vertical distance L ₁ from 30 top to the upper surface of the lower porous refractory _31, the distance L ₂ from the upper end of the lower porous refractory 31 to the lower end of the lower porous refractory 31 (Second vertical distance) satisfies Expression (4).

_{(R 1 + R 2/2} ) / (D 1 /2)≦4.25(L 1 + L 2/2) / D 1 +0.2 ··· (4)
_{(R 1 + R 2/2} ) / (D 1 /2)≧0.26(L 1 + L 2/2) / D 1 +1 ··· (5)
6, the horizontal axis _{(L 1 + L 2/2} ) / D 1, the vertical axis _{(R 1 + R 2/2} ) / (D 1/2), (4) and (5 ) And the like are plotted. In the formula (4) and (5), the horizontal distance R _1, indicated above thickness R ₂ and upper inner diameter D ₁ of the nozzle _{[(R 1 + R 2/} 2) / (D 1/2)] is It indicates the horizontal position of the upper porous refractory 30, a first vertical distance L _1, the second indicated by the vertical distance L ₂ and the upper end inner diameter _{D 1 [(L 1 + L} 2/2) / D 1] is The vertical position of the lower porous refractory 31 is shown. Therefore, when the _{[(R 1 + R 2/} 2) / (D 1/2)] is large, showing that the upper porous refractory 30 goes outward away from the center of the nozzle, [(L ₁ + L ₂ / 2) / D ₁ ] indicates that the lower porous refractory 31 is separated from the upper porous refractory 30.

  Therefore, equation (4) shows the relationship between the horizontal position of the upper porous refractory 30 and the vertical position of the lower porous refractory 31. This equation (4) is used to prevent the inert gas blown from the nozzles (the upper porous refractory 30 and the lower porous refractory 31) from boiling in the tundish and not generating bare hot water in the tundish. This is an equation that defines the boundary conditions of the positions of the object 30 and the lower porous refractory 31. As shown in FIG. 7, when the formula (4) is not satisfied, a large amount of air bubbles caused by the inert gas floats in the tundish, and the slag on the hot water surface in the tundish moves to the surroundings to produce naked hot water. Reoxidation of molten steel occurs. As a result, non-metallic inclusions in the slab increase, and the steel slab (slab) may be broken when it is drawn.

  Equation (5) also shows the relationship between the horizontal position of the upper porous refractory 30 and the vertical position of the lower porous refractory 31. This equation (5) is an equation that defines the boundary conditions of the positions of the upper porous refractory 30 and the lower porous refractory 31 so that the inert gas blown from the nozzle does not cause a fluctuation in the molten metal surface by boiling in the mold. It is. As shown in FIG. 8, when the formula (5) is not satisfied, a large amount of inert gas bubbles rise in the mold, resulting in large fluctuations in the molten metal surface, which makes casting difficult.

Further, the first vertical distance L ₁ and the second vertical distance L ₂ satisfy Expression (6).
_{0.3 ≦ (L 1 + L 2} /2) / D 1 ≦ 6.3 ··· (6)
When the first vertical distance L ₁ is less than 0.04 m, because the lower porous refractory 31 is too close to the upper porous refractory 30, in the nozzle, the upper portion from the lower porous refractory 31 is easily deficient. If the second vertical distance _{L 2} is less than 0.02 m, it is difficult to manufacture the lower porous refractory 31. Therefore, the minimum value of L _{1 +} L _2/2 is 0.05 m, the maximum value of the upper inner diameter D ₁ of the nozzle is 0.15 m, the _{(L 1 + L 2/2} ) / D 1 The minimum value is 0.3. The upper limit of the first vertical distance _{L 1} is 0.43 m, the maximum value from the lower limit of the second vertical distance _{L 2} is 0.02 m _(L 1 + L _2/2) is 0.44 m. Further, since the minimum value of the upper inner diameter D ₁ of the nozzle is 0.07 m, a _{(L 1 + L 2/2} ) / D maximum 6.3 _1.

_{1.5 ≦ (R 1 + R 2} /2) / (D 1 /2)≦4.0 ··· (7)
Further, the horizontal distance R ₁ , the upper thickness R ₂ , and the upper end inner diameter D _{1 of the} nozzle satisfy Expression (7).
When the horizontal distance R ₁ −the upper end inner diameter D _{1/2 of the} nozzle is less than 0.025 m, the refractory of the nozzle (non-porous portion) is likely to be lost. When the upper thickness _{R 2} is less than 0.02 m, the fabrication of the upper porous refractory 30 becomes difficult. Therefore, the maximum value of the upper inner diameter _{D 1} of the nozzle 0.15 m, if the minimum value of the above thickness _{R 2} is 0.02 m, the formula _{(7) (R 1 + R} 2/2) / (D 1 / 2) indicates a minimum value of 1.5. The maximum value from that the minimum value of the upper thickness _{R 2} is 0.02m maximum outer diameter _{D 2} of the nozzle is in _{0.3m, (R 1 + R 2} /2) becomes 0.14m . _Here, since the minimum value of _D 1/2 is 0.035 m, the maximum value is 4.0 _{(R 1 + R 2/2} ) / (D 1/2). When neither of formula (6) nor formula (7) satisfies the maximum value, it is difficult to manufacture a porous refractory.

As described above, according to the nozzle, the upper porous refractory 30 and the lower porous refractory 31, it is possible to blow an appropriate inert gas toward the tundish or the mold by satisfying the formulas (1) to (7). Thereby, inclusions captured by the slab can be reduced.
0.07m ≦ D ₁ ≦ 0.15m (1)
0.16m ≦ D ₂ ≦ 0.3m (2)
0.2m ≦ L ≦ 0.45m (3)
_{(R 1 + R 2/2} ) / (D 1 /2)≦4.25(L 1 + L 2/2) / D 1 +0.2 ··· (4)
_{(R 1 + R 2/2} ) / (D 1 /2)≧0.26(L 1 + L 2/2) / D 1 +1 ··· (5)
0.3 ≦ (L ₁ + L ₂ ) / D ₁ ≦ 6.3 (6)
_{1.5 ≦ (R 1 + R 2} /2) / (D 1 /2)≦4.0 ··· (7)
Now, the inventor has also verified the continuous casting method in order to reduce inclusions captured by the slab. As a result, molten steel throughput Q _s per strand (molten steel throughput Q _s ), inert gas flow rate Q _g2 (upper gas flow rate Q _g2 ) blown from the upper porous refractory 30, and inert gas flow rate blown from the lower porous refractory 31 Q _g1 (lower gas flow rate Q _g1 ) desirably satisfies the expressions (8) to (11).

FIG. 9 is a graph showing the molten steel throughput Q _s and the inert gas flow rate (upper gas flow rate Q _g2 , lower gas flow rate Q _g1 ) shown in equations (8) to (11). Q _gu is the rising flow rate of the inert gas. , U _g1 is the lower gas average speed, U _g2 is the upper gas average speed, and U _s is the average speed of the molten steel passing through the upper nozzle. Further, the inert gas may be other than Ar gas, but for the convenience of explanation, the explanation will be continued assuming that the inert gas is Ar gas.

0.78 ≦ Q _s [ton / min / str] ≦ 1.02 (8)
Q _g1 + Q _g2 ≧ 1500 {tan (Q _s -0.91)} 3 + 4.2 ・ ・ ・ (9)
_{Q gu = (Q g1 + Q} g2) tanh [870Re -1.45 {70 (4U g2 / U s) 0.5 ((R 1 + R 2/2) / (D 1/2)) 5 +370 (4U g1 / ^{_{U s) -0.3 ((L 1}} + L 2/2) / D 1) -0.7}] ≦ 4 ··· (10)
Q _g1 + Q _g2 -Q _gu ≦ 2 (11)
The lower limit (minimum value) of the molten steel throughput Q _s per strand is set to 0.7 [ton / min / str] or more as shown in the equation (8), and the upper limit value of the molten steel throughput Q _s per strand ( The maximum value is preferably 1.02 [ton / min / str] or less as shown in the equation (8). However, equation (9) is outside the applicable range when Q _s <0.77 [ton / min / str]. By setting the molten steel throughput Q _s to the range shown in the equation (8), it is possible to form bare hot water on the surface of the tundish by the inert gas blown from the nozzle and to suppress large fluctuations in the hot water level in the mold. .

Separation of inclusions can be promoted by setting the lower limit value of the total value (Q _g1 + Q _g2 ) of the upper gas flow rate Q _g2 and the lower gas flow rate Q _g1 to satisfy the equation (9). In addition to equation (9), by satisfying the above gas flow rate Q _g2 and lower gas flow rate Q _g1 Hitoshigashiki (10), it is possible to bare hot water suppression tundish molten metal surface. Furthermore, when the upper gas flow rate _Qg2 and the lower gas flow rate _Qg1 satisfy the equation (11), it is possible to suppress large fluctuations in the molten metal surface in the mold.

Expressions (8) to (11) are values obtained by experiments such as a water model.
Next, the experiment of the water model and the derivation of the equations (8) to (11) will be described in detail.
The inventor made a nozzle of the same size as the actual machine and conducted a 1/1 water model experiment. In this water model experiment, the gas levitation rate (Q _gu / (Q _g1 + Q _g2 ) when gas was blown by the water model was verified.Table 1 summarizes each parameter in the water model experiment. For convenience of explanation, the gas levitation rate in the water model experiment is called the air levitation rate.

In the water model experiment, the flow rate of air blown from the upper porous refractory 30 (upper gas flow rate Q _g2 ), the flow rate of air blown from the lower porous refractory 31 (lower gas flow rate Q _g1 ), and the flow rate of floating air (floating flow rate Q _gu). ) Was actually measured, and using these, the actual measured value of the air floating rate (Q _gu / (Q _g1 + Q _g2 )) in the tundish was obtained.
In the water model experiment, the upper gas average velocity U _g2 is obtained from the upper gas flow rate Q _g2 , the horizontal distance R ₁ , and the upper thickness R ₂ , and the lower gas flow rate Q _g1 , the lower end inner diameter D ₃ of the nozzle, and the second vertical distance. It was determined under gas average velocity _{U g1} by L _2. Further, the molten steel throughput Q _s and the average speed U _{s of the} molten steel passing through the upper nozzle were determined from the upper end inner diameter D ₁ of the nozzle. Then, the upper gas average speed U _g2 , the lower gas average speed U _g1 , the number of lay nozzles, the first vertical distance L ₁ , the second vertical distance L ₂ , the horizontal distance R ₁ , the upper thickness R ₂ , and the upper end inner diameter D _{1 of the} nozzle Using this, the calculated value of the air floating rate (Q _gu / (Q _g1 + Q _g2 )) in the tundish was obtained. FIG. 10 shows measured values and calculated values of the air levitation rate in the tundish. As shown in FIG. 10, since the actually measured value and the calculated value are substantially the same, the air levitation rate was organized based on the actually measured value of the water model experiment. The air floating rate can be expressed by the formula (a). However, Re = U _s D ₁ / ν _L.

_{Q gu / (Q g1 + Q} g2) = tanh [870Re -1.45 {70 (U g2 / U s) 0.5 ((R 1 + R 2/2) / (D 1/2)) 5 +370 (U g1 ^{_{/ U s) -0.3 ((L}} 1 + L 2/2) / D 1) -0.7}] ··· (a)
Next, conversion from a water model to an actual machine will be described.
In the conversion from the water model to the actual machine, the air flow rate in the water model is approximated by the corrected Fr number and converted to the Ar flow rate, and the expansion due to the temperature expansion and the liquid static pressure is also taken into account according to Boyle's law. The corrected Fr number (Fr _m ) can be expressed by equation b.

Fr _m = ρ _g Q _g ² / (ρ _L gX ⁵ ) (b)
Where ρ _g : gas density [kg / m ³ ], Q _g : gas flow rate [L / min], ρ _L : liquid density [kg / m ³ ], g: gravitational acceleration [m / s ² ], X: The representative length [m]. The formula (b) is a formula shown in, for example, “M. Iguchi and H. Tokunaga: Met. Mat. Trans. 33B (2002) pp. 695-702”.

Here, when the actual machine is represented by subscript j, the water model is represented by subscript k, and the number of corrected Fr is equal, equation (c) is obtained.
Q _gj = Q _gk {(ρ _Lj ρ _gk L ⁵ ) / (ρ _Lk ρ _gj L ⁵ )} ^0.5 (c)
However, ρ _Lj (molten steel) = 7000 [kg / m ³ ], ρ _Lk (water) = 1000 [kg / m ³ ], ρ _gj (Ar) = 0.48 [kg / m ³ ], ρ _gk (air ) = 1.16 [kg / m ³ ]. ρ _Lk , ρ _gj , ρ _gk are `` Liquid Thermophysical Properties Collection: The Japan Society of Mechanical Engineers (1991) Maruzen '', ρ <sub>Lj</sub> is `` Yoshida et al .: Iron and Steel, Vol. 87 (2001) pp. 529-535 '' It is shown. Therefore, when these values are substituted, Q _gj = 17Q _gk .

Further, the Ar gas flow rate Q _gj is corrected according to Boyle's law. The static pressure at a molten steel depth of 0.8 [m] of the actual machine located at the upper end of the nozzle is 0.16 [MPa], and the static pressure at a water depth of 0.8 m in the water model is 0.11 [MPa]. Assuming that the gas volume is V [m ³ ], the pressure is P [Pa], and the temperature is T [K], V _j / V _k = (P _k T _j ) / (P _j T _k )become. The subscript j in the above formula is an actual machine, and the subscript k is a water model. Since V _j = 4.3 V _k , the actual gas flow rate Q _gj is given by equation (d).

Q _gj = 4Q _gk (d)
Then, when the formula (a) is converted into an actual machine by the formula (d) and the gas flow rate (Q _gj = Q _gu ) of Ar gas floating on the tundish is expressed, the formula (e) is obtained.
_{Q gu = (Q g1 + Q} g2) tanh [870Re - 1.45 {70 (4U g2 / U s) 0.5 ((R 1 + R 2/2) / (D 1/2)) 5 +370 (4U g1 / ^{_{U s) -0.3 ((L 1}} + L 2/2) / D 1) -0.7} ··· (e)
Expression (e) is the same expression except for the gas flow rate condition (signs indicating the upper limit value and the upper limit value) shown in Expression (10). Thus, by converting the water model into the actual machine, the flow rate Q _{gu of} Ar gas that rises toward the tundish in the actual machine can be obtained.

Here, the upper limit value of the gas flow rate Q _gu (Ar gas flow rate Q _gu ) of Ar gas floating on the tundish is verified. In actual operation with the actual machine, flux and heat insulating material are added to the surface of the tundish. As described above, when Ar gas is blown, when the bare metal (molten steel surface) continues to be exposed, the molten steel surface is oxidized or flux is easily caught in the molten steel. An experiment was performed using an actual machine, and an experiment was performed to determine the relationship between the state in which naked hot water continues to be exposed (continuously visible) and the upper limit value of Ar gas. Table 2 summarizes the actual machine conditions in the actual machine. Table 3 summarizes the relationship between whether or not naked hot water continued to be seen and the Ar gas flow rate Q _gu .

As shown in Table 3, when the Ar gas flow rate Q _gu is set to 4 NL / min or less, naked hot water cannot be seen in the tundish. That is, exposure of naked hot water can be suppressed by satisfying the above-described formula (10).
Next, to verify the gas flow rate Q _g of Ar gas toward the molten steel of the mold. In an actual machine, when the amount of Ar gas that floats in the mold increases, the fluctuation of the molten metal level becomes large, and the control of the molten metal surface level sensor becomes impossible and casting becomes difficult. In the actual machine, the relationship between the Ar gas flowing into the mold and the abnormality of the molten metal level sensor was investigated, and the upper limit threshold of the Ar flow rate introduced into the mold was verified. Incidentally, Ar gas flow rate _{Q g} towards the mold _can be determined by _{Q g = Q g1 + Q g2} -Q gu. Table 4 summarizes the actual machine conditions of the actual machine. Table 5 summarizes the whether it has detected the melt surface level sensor abnormality by water level changes, the relationship between the Ar gas flow rate Q _g towards the mold.

As shown in Table 5, when the Ar gas flow rate Q _g below 2 NL / min, it is possible to prevent abnormal hot water plane variation in the mold. That is, by satisfying the above-described formula (11), the molten metal surface fluctuation in the mold can be suppressed.
Next, a water model experiment was performed to verify the influence of the distribution of the Ar gas flow rate Q _gu of Ar gas toward the tundish and the Ar gas flow rate Q _g of Ar gas toward the mold on the inclusion separation. Before examining the influence of the distribution of the gas flow rate Q _gu to the tundish and the gas flow rate Q _g to the mold, a water model experiment was conducted on the difference in the outflow rate of inclusions by the nozzles defined in the present invention.

  In the water model experiment, the experiment was conducted with a 1/3 model in which the actual machine was similarly reduced to 1/3. The water model tundish was a T-type tundish shown in FIG. The number of strands was 4 strands, and the water depth was 0.27 m. As shown in FIG. 11, the T-type tundish 4 of the actual machine includes an injection chamber 16 for injecting the molten steel 3 in the ladle 2, a distribution chamber 17 for distributing the molten steel 3 to each mold 5, and an injection chamber 16 and distribution. A partition wall 18 that partitions the chamber 17 is provided. A plurality of runners (not shown) through which the molten steel 3 flows are formed in the partition wall 18, and the molten steel 3 that has entered the injection chamber 16 passes through the runner and flows into the distribution chamber 17.

In the water model, as shown in FIG. 12, 100 g of flow beads having an average particle diameter of 173 μm were supplied as inclusions to the injection chamber. The range of the particle size of the flow beads is 100 to 220 μm, and the flow beads are particles having the same terminal velocity as the alumina inclusions having a particle size of 68 to 78 μm in an actual machine. The specific gravity of the alumina inclusions is 3.8 g / cm ³ . In the water model, casting was performed by flowing water containing flow beads from the tundish into the mold. The particle outflow rate of the flow beads that flowed out below the mold in 30 minutes from the start of casting (particle outflow rate = flowed weight of flow beads [g] / 100 [g]) was determined.

  Table 6 summarizes the nozzle shapes (nozzle 1 and nozzle 2) in the water model experiment (1/3 model) described above. Table 7 summarizes the particle outflow rate (inclusion outflow rate) of the flow beads when the nozzle 1 and the nozzle 2 are used.

As shown in Table 7, the particle outflow rate in nozzle 1 and nozzle 2 is constant, and the particle outflow rate (inclusion outflow) relative to the air floating rate in the tundish (Q _gu / (Q _g1 + Q _g2 )) It was found that there was almost no influence of the rate). Then, the nozzle used in the water model was fixed to “Nozzle 1”, and an experiment was conducted on the particle outflow rate while changing the water flow rate, the upper gas flow rate Q _g2 , and the lower gas flow rate Q _g1 . Table 8 summarizes the experimental results. FIG. 14 is a graph of the water flow rate and the particle outflow rate in the experimental results.

  As shown in FIG. 14, the particle outflow rate increases as the water flow rate increases. Here, the water flow rate in the water model and the disconnection rate at the time of wire drawing of the steel material when the operation was actually performed at a flow rate corresponding to the water flow rate were investigated. Specifically, in a conventional tundish, the casting speed is changed and cast, and a steel material (C = 0.72 to 0.83% by mass) formed from a cast slab after being cast is drawn to have an outer diameter. The disconnection rate when a 0.12 mm wire was manufactured was examined. Table 9 summarizes the particle outflow rate in the water model and the disconnection rate at the time of steel wire drawing.

As shown in Table 9, when the particle outflow rate in the water model is larger than 6%, the disconnection rate is rapidly increased. In addition, a steel type is not limited to the said steel type. That is, since inclusions that cause defects can be reduced, the present invention can be applied to other steel types and other products (wires, thin plates).
Therefore, the upper limit value (threshold value) of the particle outflow rate in the water model described above is 6%, and the water flow rate and air flow rate (Q _g1 + Q _g2 ) when the particle outflow rate is 6% are obtained from the curve shown in FIG. It was. In addition, the water flow rate was converted to the actual machine throughput, and the air flow rate was converted to the actual Ar gas flow rate. Table 10 summarizes the water flow rate, air flow rate (Q _g1 + Q _g2 ), molten steel throughput, and Ar gas flow rate when the particle outflow rate is 6%.

The molten steel throughput and Ar gas flow rate when the particle outflow rate shown in Table 10 was 6% were arranged. As a result, the relationship between the molten steel throughput and the Ar gas flow rate when the particle outflow rate is 6% is expressed by equation (f).
Q _g1 + Q _g2 = 1500 {tan (Q _s -0.91)} ³ +4.2 ・ ・ ・ (f)
Formula (f) is the same formula except for the inequality sign shown in Formula (9) (the inequality sign indicating a particle outflow rate of 6% or less). Therefore, inclusions can be reduced by satisfying the molten steel throughput and the Ar gas flow rate in the formula (9).

In addition, conversion of the water flow rate to molten steel throughput was performed by the fluid number approximation. The water flow rate to the molten steel throughput will be described.
The Froude number Fr can be expressed by the formula (g).
Fr = (U _L ² / Xg) ^0.5 ... (g)
However, _UL : Average velocity in water tundish [m / s], X: Representative length [m], g: Gravitational acceleration [m / s ² ].

Since the flow rate Q _L [m ³ / s] is expressed by Q _L = UX ² , formula (h) is obtained by rearranging formula (g).
Fr = Q _L / (X ⁵ g) ^0.5・ ・ ・ (h)
When the water flow rate of the water model is the same as the actual scale, Q _Lk [L / min], the water model flow rate of 1/3 scale is Q _Ln [L / min], and the representative length is X / 3. Assuming that the Fr number is equal between the actual machine and the water model, the water flow rate of the water model is expressed by equation (i).

Q _Ln = 0.0642Q _Lk・ ・ ・ (i)
Here, _assuming that the molten steel throughput Q _s (t / min) in the actual machine is 7 and the specific gravity of the molten steel is 7, the molten steel throughput Q _s = 7Q _LK / 1000, so the molten steel throughput Q _s is expressed by the equation (j). .
Q _s = 7Q _Ln /64.2 (j)
The conversion of the air flow rate to the Ar gas flow rate is the same as the conversion in the water model described above. However, in the experiment, since the behavior of bubbles entering the tundish and the molten steel is converted into the actual machine, it corresponds to half of the depth of the molten steel in the tundish of the actual machine or half of the gas penetration depth in the mold. Converted to .4m. In this case, the static pressure of the actual machine is 0.13 MPa, and the static pressure of 0.14 m, which is half the water depth of the 1/3 water model, is 0.1 MPa. According to Boyle-Charles' law, V _j = 5.0 V _k and the actual gas flow rate Q _gj is given by equation (k).

Q _gj = 12.8Q _gk (k)
As described above, if the molten steel throughput Q _s and the inert gas flow rate (upper gas flow rate Q _g2 , lower gas flow rate Q _g1 ) satisfy Expressions (8) to (11), inclusions supplemented by the slab are reduced. Can do.
Tables 11 and 12 show examples in which casting was performed by the continuous casting method of the present invention and comparative examples in which continuous casting was performed by a method different from the present invention. In addition, an Example and a comparative example are the results by a water model. In this water model, it was performed on the same scale (1/1) as the actual machine, or on the 1/3 scale with respect to the actual machine. Moreover, the implementation conditions in the examples and comparative examples are the same as in the water model described above.

In Examples and Comparative Examples, evaluation was performed on five items of molten steel reoxidation, molten metal level abnormality, nozzle clogging, manufacturability, and workability. The table shows good “◯” and bad “×” in each item. When all items were satisfactory, “◯” indicating good was indicated in the column for comprehensive evaluation.
The defect in molten steel reoxidation indicates, for example, a state in which naked hot water is continuously appearing in the tundish, and there is a possibility that the molten steel is reoxidized by the naked hot water. A defect due to an abnormal molten metal level indicates that the molten metal surface of the molten steel in the mold becomes unstable due to a large amount of Ar gas bubbles, making operation difficult. Nozzle clogging indicates that clogging has occurred due to clogging due to adhesion of non-metallic inclusions in the nozzle or solidification of molten steel (steel), for example. The defect in manufacturability indicates, for example, that the problem of durability of the refractory and the dimensional relationship do not hold, and the manufacturability is difficult. The poor workability indicates that, for example, installation of a nozzle or the like is unstable and construction is difficult. Good “◯” in each item indicates that no defect occurs.

In the examples, since all of the formulas (1) to (11) are satisfied, the particle outflow rate can be reduced to 6.0% or less, and all the items described above are good. On the other hand, in Comparative Examples 1 and 2, the molten steel throughput (converted to the actual machine) is less than 0.78 [ton / min / str] or exceeds 01.02 [ton / min / str] and satisfies the formula (8). As a result, the particle flow rate exceeded 6%. In Comparative Examples 12 and 13, since the upper inner diameter _{D 1} of the nozzle or exceed 0.15 m, and less than 0.07m does not satisfy the equation (1), molten metal surface level error, workability, items of nozzle clogging Became defective.

In Comparative Example 10, 11, 14, or the outer diameter _{D 2} exceeds 0.3m nozzles, for a less than 0.16m does not satisfy the equation (2), workability, manufacturing of items defective became. In Comparative Examples 9, 11, and 19, the height L was less than 0.2 m or exceeded 0.45 m, and the formula (3) was not satisfied, so the items of workability and manufacturability were poor.
In Comparative Example 15,16,19, or a _{(L 1 + L 2/2} ) / D 1 is less than 0.3, greater than 6.3, because it does not satisfy the equation (6), manufacture of the item Became defective. In Comparative Example 17 and 18, because it does not meet the or a _{(R 1 + R 2/2} ) / (D 1/2) is less than 1.5, greater than 4.0, Equation (7), manufacturability Item became defective. In Comparative Example 20, since the formula (5) was not satisfied, the item of the hot water surface level abnormality was defective. In Comparative Examples 21 and 22, since Equation (4) was not satisfied, the particle outflow rate exceeded 6%, or the item of reoxidation of molten steel was defective.

In Comparative Examples 3, 6, 8, 21, and 22, the gas flow rate Q _{gu of} Ar gas did not satisfy the formula (10), so the item of molten steel reoxidation became defective. Since Comparative Examples 4, 7, 8, 12, and 20 did not satisfy the formula (11), the item of the hot water surface level abnormality was defective. Since the comparative examples 2 and 5-7 did not satisfy | fill Formula (9), the particle | grain outflow rate exceeded 6%.
As described above, according to the present invention, by appropriately setting the nozzle shape and the gas flow rate in continuous casting, etc., while ensuring manufacturability and workability, molten steel reoxidation, abnormal surface level, nozzle clogging, etc. The inclusions captured by the slab can be reduced without being generated.

  It should be noted that matters not explicitly disclosed in the embodiment disclosed this time, such as operating conditions and operating conditions, various parameters, dimensions, weights, volumes, and the like of a component, deviate from the range normally practiced by those skilled in the art. However, matters that can be easily assumed by those skilled in the art are employed.

DESCRIPTION OF SYMBOLS 1 Continuous casting apparatus 2 Ladle 3 Molten steel 4 Tundish 5 Mold 6 Cast piece 7 Support roll 10 Iron skin 11 Refractory 15 Bottom 20 Nozzle 21 Slide nozzle (slide valve)
22 Upper plate 23 Lower plate 24 Slide plate 25 Immersion nozzle 30 Upper porous refractory 31 Lower porous refractory 32 Upper installation part 33 Lower installation part 35 Inner wall surface

Claims (1)

When the molten steel poured into the tundish from the ladle is cast from the nozzle provided at the bottom of the tundish into the mold via the slide nozzle, the ring-shaped upper porous refractory provided at the top of the nozzle is used. A continuous casting method for blowing inert gas and blowing inert gas from a ring-shaped lower porous refractory provided at the lower part of the nozzle,
The upper end inner diameter D _{1 of} the nozzle, the outer diameter D _{2 of the} nozzle, the distance L ₁ from the upper end of the upper porous refractory to the upper end of the lower porous refractory, the distance R ₁ from the center of the nozzle to the upper porous refractory, the upper porous When the thickness R ₂ of the refractory in the radial direction, the distance L ₂ from the upper end of the lower porous refractory to the lower end of the lower porous refractory, and the outer height L of the nozzle, the formulas (1) to (7) are expressed. Meet,
Molten steel throughput per strand Q _s , gas flow rate Q _{g2 of} inert gas blown from the upper porous refractory, gas flow rate Q _{g1 of} inert gas blown from the lower porous refractory, lower gas average velocity U _g1 , upper gas average velocity U _g2, average speed U _s of the molten steel passing through the nozzle, equation (8) to (11) the method continuous casting of blowing an inert gas from the porous refractory and lower porous refractory upper and satisfies the.
0.07m ≦ D ₁ ≦ 0.15m (1)
0.16m ≦ D ₂ ≦ 0.3m (2)
0.2m ≦ L ≦ 0.45m (3)
_{(R 1 + R 2/2} ) / (D 1 /2)≦4.25(L 1 + L 2/2) / D 1 +0.2 ··· (4)
_{(R 1 + R 2/2} ) / (D 1 /2)≧0.26(L 1 + L 2/2) / D 1 +1 ··· (5)
0.3 ≦ (L ₁ + L ₂ ) / D ₁ ≦ 6.3 (6)
_{1.5 ≦ (R 1 + R 2} /2) / (D 1 /2)≦4.0 ··· (7)
0.78 ≦ Q _s [ton / min / str] ≦ 1.02 (8)
Q _g1 + Q _g2 ≧ 1500 {tan (Q _s -0.91)} ³ +4.2 ・ ・ ・ (9)
_{Q gu = (Q g1 + Q} g2) tanh [870Re -1.45 {70 (4U g2 / U s) 0.5 ((R 1 + R 2/2) / (D 1/2)) 5 +370 (4U g1 / ^{_{U s) -0.3 ((L 1}} + L 2/2) / D 1) -0.7}] ≦ 4 ··· (10)
Q _g1 + Q _g2 -Q _gu ≦ 2 (11)