Injection method of molten steel into tundish

Abstract

To provide an injection method of molten steel into a tundish capable of suppressing oxidization of the molten steel in the tundish.SOLUTION: Provided is an injection method of molten steel into a tundish in which a ladle 60 is lowered toward a tundish 1, and, at timing in which an entrainment index B indicated by the following expression (1) becomes less than 1.05, injection of the molten steel into the tundish 1 from the inside of the ladle 60 is started. v=Q / (2πrh) Expression (2)SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for injecting molten steel into a tundish, in which the molten steel is injected into the tundish.

Background Art

[0002] Patent Document 1 discloses a sealing method for suppressing oxidation of molten steel in a tundish when the upper part of the tundish is open. In this sealing method, when the sealing member disposed at the upper part of the tundish is opened, an inert gas is blown into the sealing space at a flow rate of 20 Nm 3 / hr or more so that the entrainment index of the atmosphere entrained in the tundish is less than 15.56.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The inventors of the present application have conducted research aiming to further enhance the effect of suppressing oxidation of molten steel in the tundish. And the inventors of the present application have found that the wind flowing over the tundish affects the atmosphere entrained in the tundish. This wind is, for example, the wind blowing inside a building. Also, this wind is, for example, the wind from a blower. The blower is, for example, for blowing air so that TD (tundish) powder, roasted rice husks, etc. introduced into the tundish do not fly towards the operator.

[0005] An object of the present invention is to provide a method for injecting molten steel into a tundish that can suppress oxidation of the molten steel in the tundish even under the influence of the wind flowing over the tundish.

Means for Solving the Problems

[0006] The present invention relates to a method for injecting molten steel into a tundish, wherein a seal member is disposed around the molten steel injected from a ladle into the tundish, the ladle is lowered toward the tundish, and the injection of the molten steel from the ladle into the tundish is started at a timing when the entrainment index B represented by the following formula (1) becomes less than 1.05.

Number

Advantages of the Invention

[0007] According to the present invention, the ladle is lowered toward the tundish, and the pouring of molten steel from the ladle into the tundish is started at the timing when the entrainment index B represented by the above formula (1) becomes less than 1.05. By starting the pouring of molten steel at such a timing, even when a gap is generated between the upper end of the seal member and the lower end of the ladle, the oxygen concentration in the tundish can be made less than 1.0%. Thereby, oxidation of the molten steel in the tundish during pouring can be suppressed. Further, the pouring of molten steel can be started in a state where a gap is generated between the upper end of the seal member and the lower surface of the ladle. Thereby, the flow of the molten steel poured from the ladle into the tundish can be visually confirmed.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0009] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

[0010] (Configuration of tundish and seal member) The method for injecting molten steel into a tundish (molten steel injection method) according to an embodiment of the present invention is a method for injecting molten steel into a tundish. Here, in continuous casting of steel, so-called "continuous casting" is performed in which molten steel in a plurality of ladles is continuously cast. In this operation, molten steel in the ladle is injected into the tundish, and when the ladle becomes empty or its weight becomes less than a predetermined weight, it is replaced with the next ladle, and the molten steel in the next ladle is repeatedly injected into the tundish.

[0011] As shown in FIG. 1, which is a cross-sectional view of the tundish 1 and the seal member 20, the seal member 20 is disposed above the tundish 1. The seal member 20 is disposed around the molten steel injected from the ladle 60 into the tundish 1. FIG. 1 shows a state in which molten steel is being injected from the ladle 60 into the tundish 1 with the upper opening 20a of the seal member 20 open. The state where the upper opening 20a of the seal member 20 is open means a state in which at least a part of the upper opening 20a of the seal member 20 is not blocked. The seal member 20 is disposed above the tundish 1 even when molten steel is not being injected from the ladle 60 into the tundish 1, for example, when the ladle 60 is being replaced.

[0012] As shown in FIG. 1, the tundish 1 has a tundish body 2 and a tundish cover 3. The tundish cover 3 covers the upper opening of the tundish body 2. An injection chamber 10 and a strand chamber (not shown) are formed in the tundish 1. The injection chamber 10 and the strand chamber are partitioned by a weir or the like. Molten steel is injected into the injection chamber 10 from the ladle 60. The strand chamber distributes the molten steel in the tundish 1 to each mold, for example.

[0013] The seal member 20 has a seal box 21 and three refractory seal materials 23, 24, and 25. The seal box 21 is disposed on the tundish lid 3. The seal box 21 is a box-shaped member that penetrates from the upper end to the lower end. The seal box 21 is made of, for example, iron or stainless steel.

[0014] The three refractory seal materials 23, 24, and 25 are disposed on the seal box 21. The refractory seal materials 23, 24, and 25 are stacked from bottom to top in the order of the refractory seal material 23, the refractory seal material 24, and the refractory seal material 25. Through holes communicating with the inside of the seal box 21 are formed in the refractory seal materials 23, 24, and 25, respectively. The refractory seal materials 23, 24, and 25 may have the same configuration or different configurations. The refractory seal materials 23, 24, and 25 are made of, for example, ceramic fibers containing aluminum oxide, silica, and the like. Note that the number of the refractory seal materials 23, 24, and 25 is not limited to three, and may be two or less or four or more.

[0015] The space inside the seal box 21 communicates with the pouring chamber 10 of the tundish 1 and the through holes of the refractory seal materials 23, 24, and 25. In the present embodiment, the shape of the opening 20a at the upper end of the seal member 20 is circular, but may be rectangular or the like.

[0016] A "seal space 50" is formed by the seal member 20, the tundish 1, and the like. In FIG. 1, the "seal space 50" is colored.

[0017] As shown in FIG. 1, the seal member 20 and the tundish 1 are disposed from the opening 20a at the upper end of the seal member 20 to the bath surface Ms of the molten steel M in the tundish 1. The "seal space 50" is a space surrounded by the seal member 20 and the tundish 1 from the opening 20a at the upper end of the seal member 20 to the bath surface Ms of the molten steel M in the tundish 1, or a space surrounded by the seal member 20.

[0018] Specifically, when the molten steel bath surface Ms in the tundish 1 is above the lower end of the seal member 20 or at the same height as the lower end of the seal member 20, the "seal space 50" is the space surrounded by the seal member 20 from the opening 20a at the upper end of the seal member 20 to the molten steel bath surface Ms in the tundish 1. On the other hand, when the molten steel bath surface Ms in the tundish 1 is below the lower end of the seal member 20, the "seal space 50" is the space surrounded by the seal member 20 and the tundish 1 from the opening 20a at the upper end of the seal member 20 to the molten steel bath surface Ms in the tundish 1.

[0019] As shown in FIG. 2, which is a cross-sectional view of the tundish 1 and the seal member 20, the seal member 20 may have a seal pipe 121 instead of the seal box 21. The seal pipe 121 has a cylindrical portion 121a and a flange 121b.

[0020] The cylindrical portion 121a is a cylindrical member. The cylindrical portion 121a may be, for example, cylindrical or rectangular tubular. The flange 121b extends outward from the upper end of the cylindrical portion 121a. The flange 121b is supported by a support member 22 disposed on the tundish lid 3. The seal pipe 121 is made of, for example, a refractory such as aluminum oxide.

[0021] In the case of a modified example in which the seal member 20 has a seal pipe 121, the "seal space 50" is the space surrounded by the seal pipe 121 and the refractory seal materials 23, 24, 25 and the molten steel bath surface Ms inside the seal pipe 121.

[0022] Returning to FIG. 1, the "seal space 50" is a space that communicates with the outside through the opening 20a at the upper end of the seal member 20 when the opening 20a at the upper end of the seal member 20 is open. The outside is outside the seal member 20 and also outside the tundish 1. When the opening 20a at the upper end of the seal member 20 is open, there is a risk that air will be drawn into the "seal space 50" from the opening 20a at the upper end of the seal member 20.

[0023] Here, one or a plurality of gas discharge holes 41a are arranged in the seal space 50. The gas discharge hole 41a is an opening at the tip of the gas discharge pipe 41. The gas discharge hole 41a faces downward. The gas discharge hole 41a faces the bath surface Ms of the molten steel M.

[0024] An inert gas is blown into the seal space 50 from the gas discharge hole 41a. When there are a plurality of gas discharge holes 41a, the flow rates of the inert gas blown into the seal space 50 from each of the gas discharge holes 41a may be the same as each other or may be different from each other.

[0025] An inert gas is blown into the seal space 50 at a flow rate of Q (Nm 3 / s). When an inert gas is blown into the seal space 50 from one gas discharge hole 41a, an inert gas with a total flow rate of Q (Nm 3 / s) is blown into the seal space 50 from one gas discharge hole 41a. When an inert gas is blown into the seal space 50 from two gas discharge holes 41a, an inert gas with a total flow rate of Q (Nm 3 / s) is blown into the seal space 50 from two gas discharge holes 41a. When an inert gas is blown into the seal space 50 from three or more gas discharge holes 41a, an inert gas with a total flow rate of Q (Nm 3 / s) is blown into the seal space 50 from three or more gas discharge holes 41a.

[0026] A ladle nozzle 62 is provided at the lower part of the ladle 60. The ladle nozzle 62 has an upper plate fixed to the lower surface of the ladle 60 and a lower plate provided below this upper plate and sliding horizontally by driving of a hydraulic cylinder or the like. By sliding the lower plate to open the ladle nozzle 62, the molten steel in the ladle 60 can be discharged.

[0027] A heat shield plate 61 is attached to the lower part of the ladle 60. The heat shield plate 61 is disposed below the ladle nozzle 62. When pouring molten steel from the ladle 60 into the tundish 1, the ladle 60 is lowered toward the tundish 1. By bringing the heat shield plate 61 into contact with the opening 20a at the upper end of the seal member 20, molten steel can be poured from the ladle 60 into the tundish 1 with the opening 20a at the upper end of the seal member 20 closed.

[0028] As shown in FIG. 1, pouring of molten steel from the ladle 60 into the tundish 1 is started with a gap provided between the seal member 20 and the heat shield plate 61 of the ladle 60. At this time, the opening 20a at the upper end of the seal member 20 is open. Thereafter, while the pouring of molten steel from the ladle 60 is continued, the ladle 60 is lowered toward the tundish 1, and the opening 20a at the upper end of the seal member 20 is closed by the heat shield plate 61 of the ladle 60. In this state, the pouring of molten steel from the ladle 60 is continued.

[0029] When starting to pour molten steel from the ladle 60 into the tundish 1, it is necessary to check the opening of the ladle 60 (ladle nozzle 62). Therefore, with a gap provided between the seal member 20 and the heat shield plate 61 of the ladle 60, the flow of molten steel poured from the ladle 60 into the tundish 1 is visually confirmed. At this time, the opening 20a at the upper end of the seal member 20 is open. At this time, air is drawn into the tundish 1 from the gap between the seal member 20 and the heat shield plate 61 in a form entrained by the flow of the molten steel (air entrainment). As a result, the sealing performance in the tundish 1 deteriorates, and the molten steel M in the tundish 1 is oxidized. In order to melt high-purity steel, it is necessary to suppress reoxidation of the molten steel in the tundish 1 during continuous casting.

[0030] Therefore, in the method for pouring molten steel according to the present embodiment, the ladle 60 is lowered toward the tundish 1, and pouring of molten steel from the ladle 60 into the tundish 1 is started at a timing when the entrainment index B represented by the following formula (1) becomes less than 1.05. The entrainment index B is represented by the following formula (1).

Number

[0031] Here, v is represented by the following formula (2). v = (Q / 2πrh) ··· Formula (2)

[0032] An image diagram of the lower part of the ladle 60 and the upper part of the tundish 1 is shown in FIG. 3. In FIG. 3, the arrow 81 indicates the flow of the inert gas blown into the seal space 50 from the gas discharge hole 41a arranged in the seal space 50. The flow rate of the inert gas blown into the seal space 50 is Q (Nm 3 / s). r is the radius (m) of the opening 20a at the upper end of the seal member 20. When the shape of the opening 20a is not circular, r is the radius (m) corresponding to the equivalent circle of the opening 20a with the same area. h is the vertical distance (m) from the upper end of the seal space 50 to the lower surface of the ladle 60 (the lower surface of the heat shield 61). The arrow 82 indicates the flow of the atmosphere (wind) flowing above the opening 20a. The wind speed of the atmosphere (wind) flowing above the opening 20a is u (m / s). The wind speed u is positive in the direction flowing toward the opening 20a. The measurement position 83 of the wind speed u is above the opening 20a at the upper end of the seal space 50 by a vertical distance h and is 5r away from the center of the opening 20a in the radial direction of the opening 20a.

[0033] In FIG. 3, A is the surface area of the side surface of the cylinder with the opening 20a as the lower end and the lower surface of the heat shield 61 as the upper end, and is represented by 2πrh. The v shown in the above formula (2) is the wind speed (m / s) of the inert gas discharged from the surface of the cylinder with the surface area A to the outside. In FIG. 3, v is indicated by the arrow 84. v is positive in the direction discharged from the surface of the cylinder with the surface area A to the outside.

[0034] Here, the air flowing over the opening 20a is the air blowing inside the building or the air from the blower. The blower blows air, for example, to prevent TD powder, roasted rice husks, etc. introduced into the tundish 1 from flying towards the operator. The air flowing over the opening 20a at the upper end of the seal member 20 is shown in FIGS. 4A and 4B. In FIGS. 4A and 4B, the air flow is indicated by an arrow 85. The measurement position 83 of the wind speed u is on the circumference of a circle 71 with a radius of 5r centered on the center of the opening 20a.

[0035] As shown in FIG. 4A, the blower usually blows air directly upward above the opening 20a. In this case, as shown in FIG. 4A, the wind speed u is the maximum value of the wind speed of the air blowing in a direction perpendicular to the tangent 72 of the circle 71. On the other hand, as shown in FIG. 4B, the air blowing inside the building may blow in a direction deviating from directly above the opening 20a. In this case, as shown in FIG. 4B, the wind speed u is the maximum value of the vector component parallel to the direction perpendicular to the tangent 72 of the circle 71 among the wind speeds of the air blowing inside the building. In the case of the air blowing inside the building, before the start of pouring the molten steel M (for example, 10 minutes before), the wind speed is measured in advance at a plurality of points on the circumference of the circle 71, and the point where the maximum wind speed is measured is set as the measurement position 83 of the wind speed u.

[0036] The entrainment index B represented by the above formula (1) is an index for evaluating the "atmospheric entrainment property". The "atmospheric entrainment property" is the amount of air entrained into the tundish 1. As the "atmospheric entrainment property" increases, the amount of air entrained into the tundish 1 increases, so the oxygen concentration in the tundish 1 increases. Therefore, the "atmospheric entrainment property" can be evaluated by the oxygen concentration. The larger the entrainment index B, the easier it is to entrain air, and the higher the oxygen concentration in the tundish 1.

[0037] Here, as disclosed in JP-A-4-327352, if the oxygen concentration in the tundish 1 is less than 1.0%, reoxidation of the molten steel in the tundish 1 can be suppressed.

[0038] As described above, the ladle 60 is lowered toward the tundish 1, and the pouring of the molten steel from the ladle 60 into the tundish 1 is started at a timing when the entrainment index B represented by the following formula (1) is less than 1.05. By starting the pouring of the molten steel at such a timing, even when a gap is formed between the upper end of the seal member 20 and the lower end of the ladle 60, the oxygen concentration in the tundish 1 can be made less than 1.0%. Thereby, oxidation of the molten steel in the tundish 1 during pouring of the molten steel can be suppressed.

[0039] Further, the pouring of the molten steel can be started in a state where a gap is formed between the upper end of the seal member 20 and the lower surface of the ladle 60. Thereby, the flow of the molten steel poured from the ladle 60 into the tundish 1 can be visually confirmed.

[0040] (Water model experiment) Next, the water model experiment will be described. In the present embodiment, the mathematical formula of the above entrainment index B is determined by the water model experiment.

[0041] In the water model experiment, water is used instead of the molten steel M. Water and the molten steel M have substantially the same kinematic viscosity and similar flow characteristics. Therefore, when water is used instead of the molten steel M, the same flow as when the molten steel M is used can be obtained.

[0042] Also, nitrogen gas was used as the inert gas. The density of nitrogen gas is smaller than the density of other inert gases. Therefore, when nitrogen gas is blown into the seal space 50, it is more difficult to expel the atmosphere in the seal space 50 than when other inert gases are blown into the seal space 50. Therefore, if oxidation of the molten steel M can be suppressed using nitrogen gas, it is considered that oxidation of the molten steel M can also be suppressed when other inert gases are used.

[0043] The apparatus used in the water model experiment is 1 / 3 the size of the apparatus (see FIGS. 1 and 2) used when casting steel. In the water model experiment, the number of gas discharge holes 41a was set to "8".

[0044] In the water model experiment, the radius of the water injection nozzle was set to 0.01 m (0.04 m in terms of the actual machine). Also, the water injection flow rate was 0.0014 m 3 / s (0.022 m 3 / s in terms of the actual machine), and the flow rate Q of nitrogen gas blown from the gas discharge hole 41a into the seal space 50 was set to 0.0015 - 0.0060 Nm 3 / s (0.023 - 0.093 Nm 3 / s in terms of the actual machine). Also, the volume of the seal space 50 was 0.045 m 3 (1.22 m 3 in terms of the actual machine), the radius r of the opening 20a at the upper end of the seal member 20 was 0.075 m (2.25 m in terms of the actual machine), the vertical distance h from the height of the upper end of the seal space 50 to the lower end of the ladle 60 (the lower surface of the heat shield 61) was 0.017 - 0.17 m (0.050 - 0.50 m in terms of the actual machine), and the wind speed u of the atmosphere flowing above the opening 20a was 0.0 - 2.1 m / s (0.0 - 3.2 m / s in terms of the actual machine).

[0045] Here, by using the Froude number as the similarity rule, the water injection flow rate into the injection chamber 10, the flow rate Q of nitrogen gas, and the wind speed u of the atmosphere (wind) flowing above the opening 20a were set to be equivalent to the actual machine values. The Froude number is defined by Equation (3).

Equation

[0046] Here, Fr is the Froude number, v is the representative velocity (m / s), L is the representative length (m), and g is the gravitational acceleration (m / s 2 ²).

[0047] In this experiment, using the following Equation (4), the flow rate Q of nitrogen gas, the water injection flow rate into the injection chamber 10, and the wind speed u of the atmosphere (wind) flowing above the opening 20a were set to be equivalent to the actual machine values.

Equation

[0048] Here, v 1is the wind speed (m / s) of the inert gas discharged to the outside from the side surface of the cylinder with surface area A, or the pouring rate of the molten steel M, or the wind speed (m / s) of the atmosphere (wind) flowing over the opening 20a. L 1 is the radius (m) of the opening 20a at the upper end of the seal member 20 in the actual machine.

[0049] v 2 is the wind speed (m / s) of the inert gas discharged to the outside from the side surface of the cylinder with surface area A, or the pouring rate, or the wind speed (m / s) of the atmosphere (wind) flowing over the opening 20a in the water model experiment. L 2 is the radius (m) of the opening 20a at the upper end of the seal member 20 in the water model experiment.

[0050] Water was poured into the pouring chamber 10 of the tundish 1 until the volume of the seal space 50 reached 0.045 m 3 After that, the opening 20a at the upper end of the seal member 20 was closed with a lid. Next, an inert gas was blown into the seal space 50 from the gas discharge hole 41a so that the oxygen concentration in the seal space 50 was less than 0.01 vol%, which is the lower limit of measurement of the measuring instrument (hereinafter, the unit of the oxygen concentration in the seal space 50 may be indicated as "%"). The oxygen concentration was measured at a height 130 mm below the opening 20a (a height 390 mm below the opening 20a in terms of the actual machine) in the seal space 50 and at a position where the inert gas blown out from the gas discharge hole 41a did not directly hit.

[0051] Next, using a blower, air was blown at a predetermined wind speed over the opening 20a at the upper end of the seal member 20. Next, the lid that had closed the opening 20a at the upper end of the seal member 20 was removed, and the opening 20a at the upper end of the seal member 20 was opened. After that, water was poured from a predetermined pouring height, and the change in the oxygen concentration (%) in the seal space 50 was measured. Table 1 shows the experimental conditions of the water model experiment, the converted values obtained by converting these to the actual machine, and the oxygen concentration (water model experiment oxygen concentration) in the water model experiment.

[0052]

Table 1

[0053] (Calculation of Oxygen Concentration in the Actual Machine) Next, the oxygen concentration in the seal space 50 was calculated using the actual machine. Primary refining using a converter and secondary refining using a ladle furnace (LF) were carried out in the usual manner of those skilled in the art. Thereafter, high-carbon steel was cast using a bloom continuous casting machine in the usual manner of those skilled in the art. Argon gas was used as the inert gas.

[0054] During casting, when the opening 20a at the upper end of the seal member 20 was opened, the transition of the oxygen concentration (%) in the seal space 50 was measured. The measurement of the oxygen concentration was carried out at a position 150 mm below the opening 20a at the lower end of the seal member 20.

[0055] The vertical distance h from the upper end of the seal space 50 to the lower surface of the ladle 60 (the lower surface of the heat shield 61) decreases with time. Therefore, the operating conditions and oxygen concentration when the pouring of molten steel into the tundish 1, where the influence of air entrainment is the greatest, were measured. The oxygen concentration in the actual machine (actual machine oxygen concentration) is shown in Experiment Nos. 2, 10, and 23 in Table 1.

[0056] (Calculation of Oxygen Concentration in Terms of the Actual Machine) The oxygen concentration in the water model experiment (water model experiment oxygen concentration) is different from the oxygen concentration in the seal space 50 of the actual machine (actual machine oxygen concentration). This is for the following reasons.

[0057] In the case of the actual machine, since high-temperature molten steel is poured from the ladle into the tundish 1, the temperature in the tundish 1 is high. When an inert gas is blown into this, the temperature of the inert gas rises and the inert gas expands in the tundish 1. Therefore, it is difficult for air to be entrained into the tundish 1. On the other hand, in the water model experiment, since water is used instead of molten steel, the temperature in the tundish 1 does not become as high as in the actual machine. When an inert gas is blown into this, the inert gas does not expand. Therefore, in the water model experiment, air is more likely to be entrained into the tundish 1 than in the case of the actual machine.

[0058] As described above, the oxygen concentration in the water model experiment is higher than that in the actual machine. Also, the oxygen measurement location in the actual machine is installed on the side of the opening 20a where it is easier to entrain air compared to the water model experiment. Therefore, it shows a value different from the oxygen measurement value in the water model experiment. Thus, it was decided to estimate the actual machine oxygen concentration from the water model experiment oxygen concentration and obtain the estimated value of the actual machine oxygen concentration (oxygen concentration converted to the actual machine).

[0059] Using the oxygen concentration in the water model experiment of experiment numbers 2 and 10 in Table 1 and the oxygen concentration in the actual machine of experiment numbers 2 and 10 in Table 1, the relationship between the oxygen concentration in the water model experiment and the oxygen concentration in the actual machine is as shown in Fig. 5. From Fig. 5, a linear equation passing through the origin is obtained. Using this linear equation, by interpolation, the oxygen concentration in the water model experiment of experiment numbers 1 to 22 was converted to the oxygen concentration converted to the actual machine (%). The oxygen concentration converted to the actual machine is shown in Table 1.

[0060] Using the oxygen concentration converted to the actual machine of experiment numbers 1 to 22, the entrainment index B was calculated. The relationship between the entrainment index B, the oxygen concentration converted to the actual machine, and the oxygen concentration in the actual machine is shown in Fig. 6.

[0061] Here, when the flow rate Q of the inert gas is small, it takes time to replace the inside of the tundish 1 with the inert gas when the casting starts or when air is entrained. Also, when the flow rate Q of the inert gas is small, the oxygen concentration inside the tundish 1 increases. As a result, reoxidation of the molten steel occurs due to the remaining oxygen. On the other hand, when the flow rate Q of the inert gas is large, the scattering of the molten steel inside the tundish 1 is promoted during casting, so the amount of metal adhering to the seal member 20 increases and the operability deteriorates.

[0062] As shown in the above formula (2), the larger the flow rate Q of the inert gas or the smaller the surface area A of the side surface of the cylinder represented by 2πrh, the larger the wind speed v of the inert gas and the more the entrainment of air is suppressed. Therefore, in the entrainment index B, the wind speed v of the inert gas is placed in the denominator. Also, the larger the wind speed u of the air flowing over the opening 20a, the more the entrainment of air is promoted. Therefore, in the entrainment index B, the wind speed u of the air flowing over the opening 20a is placed in the numerator.

[0063] The entrainment index B = b(u c + a) d / v e is set, and by the least squares approximation, the constant a, the coefficient b, the exponents c, d, and e are varied to determine the constant a, the coefficient b, the exponents c, d, and e such that the coefficient of determination of the approximate straight line with the actual machine-converted oxygen concentration is closest to 1. The equation is the above equation (1).

[0064] As described above, if the oxygen concentration in the tundish 1 is less than 1.0%, reoxidation of the molten steel in the tundish 1 can be suppressed. From FIG. 6, the value of the entrainment index B when the approximate straight line between the entrainment index B and the actual machine-converted oxygen concentration intersects with the oxygen concentration of 1.0% is 1.05. Therefore, if the entrainment index B is less than 1.05, the oxygen concentration in the tundish 1 can be made less than 1.0%.

[0065] (Concept of injection start timing) The weight of the molten steel in the tundish 1 and the operation timing are shown in FIG. 7. As shown in FIG. 7, while the ladle 60 is being replaced, the weight of the molten steel in the tundish 1 decreases. When the replacement of the ladle 60 is completed and the injection of the molten steel is started, the weight of the molten steel in the tundish 1 increases and eventually becomes constant.

[0066] Before the replacement of the ladle 60 is started, the opening 20a at the upper end of the seal member 20 is closed by the heat shield 61. Therefore, there is no gap between the seal member 20 and the heat shield 61 of the ladle 60. When the replacement of the ladle 60 is started, since the heat shield 61 separates from the opening 20a at the upper end of the seal member 20, a gap is formed between the seal member 20 and the heat shield 61 of the ladle 60. While the ladle 60 is being replaced, the opening 20a at the upper end of the seal member 20 is closed by a lid (not shown). Then, at a timing earlier than the completion of the replacement of the ladle 60, the lid is removed, creating a gap between the seal member 20 and the heat shield 61 of the ladle 60. In this state, the replacement of the ladle 60 is completed and the pouring of molten steel is started. That is, at the start of the pouring of molten steel, there is a gap between the seal member 20 and the heat shield 61 of the ladle 60. After that, while the pouring of molten steel continues, the opening 20a at the upper end of the seal member 20 is closed by the heat shield 61. As a result, the gap between the seal member 20 and the heat shield 61 of the ladle 60 disappears. Entrainment of air occurs during the period from the start of the pouring of molten steel until the opening 20a at the upper end of the seal member 20 is closed by the heat shield 61.

[0067] The influence of the gap during the pouring of molten steel on the oxygen concentration in the tundish 1 will be described with reference to FIG. 8. Here, the gap refers to the gap between the upper end of the seal member 20 and the lower end (heat shield 61) of the ladle 60. FIG. 8 shows the vertical distance h and the time change of the oxygen concentration under Conditions 1 to 3. Timing (1) is the timing when the pouring of molten steel is started from the ladle 60 into the tundish 1. Timing (2) is the timing when the opening 20a at the upper end of the seal member 20 is closed by the heat shield 61 of the ladle 60.

[0068] Condition 1 in FIG. 8 is the case where the pouring of molten steel is started when the entrainment index B is 1.05 or more. For example, when the vertical distance h is 300 mm, the pouring of molten steel M is started. When the pouring of molten steel is started from the ladle 60 into the tundish 1 at Timing (1), the oxygen concentration in the seal space 50 increases and exceeds the target oxygen concentration of 1.0%.

[0069] Condition 2 in Fig. 8 is the case where the pouring of molten steel starts when the entrainment index B is less than 1.05. In Condition 2, when the vertical distance h is 0 mm, the pouring of molten steel starts. That is, the pouring of molten steel starts at the timing when the opening 20a at the upper end of the seal member 20 is closed by the heat shield plate 61. That is, timing (1) and timing (2) are almost simultaneous. When the pouring of molten steel from the ladle 60 into the tundish 1 starts, the oxygen concentration in the seal space 50 does not exceed the target oxygen concentration of 1.0%, but since there is no gap between the upper end of the seal member 20 and the lower surface of the ladle 60, the flow of molten steel poured from the ladle 60 into the tundish 1 cannot be visually confirmed.

[0070] Condition 3 in Fig. 8 is the case where the pouring of molten steel starts when the entrainment index B is less than 1.05. For example, when the vertical distance h is 50 mm, the pouring of molten steel starts (timing (1)). When the pouring of molten steel from the ladle 60 into the tundish 1 starts, the oxygen concentration in the seal space 50 slightly increases but does not exceed the target oxygen concentration of 1.0%. And when the pouring of molten steel starts, the vertical distance h is 50 mm and there is a gap between the upper end of the seal member 20 and the lower surface of the ladle 60, so the flow of molten steel poured from the ladle 60 into the tundish 1 can be visually confirmed.

[0071] (Effect) As described above, according to the method for injecting molten steel according to this embodiment, the ladle 60 is lowered toward the tundish 1, and the injection of molten steel from the ladle 60 into the tundish 1 is started at the timing when the entrainment index B represented by the above formula (1) becomes less than 1.05. By starting the injection of molten steel at such a timing, even when a gap is generated between the upper end of the seal member 20 and the lower end of the ladle 60, the oxygen concentration in the tundish 1 can be made less than 1.0%. Thereby, oxidation of the molten steel in the tundish 1 during the injection of molten steel can be suppressed. Further, the injection of molten steel can be started in a state where a gap is generated between the upper end of the seal member 20 and the lower surface of the ladle 60. Thereby, the flow of the molten steel injected from the ladle 60 into the tundish 1 can be visually confirmed.

[0072] As described above, the embodiments of the present invention have been described, but these are merely examples, and do not particularly limit the present invention. The specific configuration and the like can be appropriately changed in design. Further, the actions and effects described in the embodiments of the invention merely list the most suitable actions and effects resulting from the present invention, and the actions and effects according to the present invention are not limited to those described in the embodiments of the present invention.

[0073] For example, the steel type to which the present invention is applied is not limited. Further, the present invention can be applied to any of the casting of slabs, blooms, and billets.

Description of reference numerals

[0074] 1 Tundish 2 Tundish body 3 Tundish lid 10 Injection chamber 20 Seal member 20a Opening 21 Seal box 22 Support material 23, 24, 25 Refractory seal material 41 Gas discharge pipe 41a Gas discharge hole 50 Seal space 60 Ladle 61 Heat insulation plate 62 Pot-taking nozzle 71 Circle 72 Tangent line 81, 82, 84, 85 Arrow 83 Measurement position 121 Seal pipe

Claims

1. A method for injecting molten steel into a tundish, the method comprising injecting molten steel into the tundish from a ladle, wherein a seal member is disposed around the molten steel injected into the tundish from the ladle, lowering the ladle toward the tundish, and starting the injection of the molten steel from the ladle into the tundish at a timing when an entrainment index B represented by the following formula (1) becomes less than 1.

05. A method for injecting molten steel into a tundish, characterized by the above. 【Number 1】 v = Q / (2πrh) ··· Formula (2) Here, when the space surrounded by the seal member and the tundish, or the space surrounded by the seal member, from the opening at the upper end of the seal member to the molten steel bath surface in the tundish, is defined as the seal space, Q is the flow rate of the inert gas (Nm 3 / s) blown into the seal space from the gas discharge holes arranged in the seal space, r is the radius (m) of the opening at the upper end of the seal member, or the radius (m) corresponding to the equivalent circle with the same area as the opening at the upper end of the seal member, h is the vertical distance (m) from the upper end of the seal space to the lower surface of the ladle, and u is the wind speed (m / s) of the atmosphere flowing over the opening at the upper end of the seal member.

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