WO2017047058A1

WO2017047058A1 - Continuous casting method for slab casting piece

Info

Publication number: WO2017047058A1
Application number: PCT/JP2016/004124
Authority: WO
Inventors: 章敏松井; 陽一伊藤; 三木　祐司; 智紘田中; 満園　将行; 亮祐千代原; 錦織　正規
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2015-09-16
Filing date: 2016-09-12
Publication date: 2017-03-23
Also published as: TW201716162A; JPWO2017047058A1; CN108025354A; EP3332889A1; TWI599417B; BR112018004704A2; EP3332889A4; KR20180039686A; JP6115690B1; BR112018004704B1; KR102088117B1; CN108025354B; EP3332889B1

Abstract

Provided is a continuous casting method that can manufacture a high quality slab casting piece. In the continuous casting method, a submerged nozzle is disposed within a casting mold for continuous casting, and molten steel is supplied to the submerged nozzle for casting. The submerged nozzle has a pair of discharge holes disposed symmetrically with respect to the vertical axis thereof. The submersion depth is 180 mm or greater and less than 300 mm, the molten steel discharge angle is in a range of 15 - 35°, and the ratio A/P for blown-in inert gas flow rate A and molten steel throughput P is in a range of 2.0 - 3.5 NL/ton. The discharge direction of the submerged nozzle is inclined within the range of Equation (1) with respect to a reference plane parallel with a long side surface of the casting mold and passing through the vertical axis center of the submerged nozzle. θ − 6 ≤ α ≤ θ + 10 … (1) In Equation (1), α is the angle of inclination to the reference plane, and θ is an angle defined by the following Equation (2). tan θ = (D/2)/(W/2) … (2) In equation (2), D is the thickness of the slab casting piece, and W is the width of the slab casting piece.

Description

Continuous casting method of slab slab

The present invention relates to a continuous casting method for producing a high quality slab slab with less non-metallic inclusions on the surface layer of the slab slab by controlling the molten steel flow in the mold.

In recent years, quality requirements for high-grade steel products such as steel plates for automobile outer plates and steel plates for cans have become stricter, and there is a demand for higher quality from the slab cast stage, that is, from the continuous casting stage. One of the qualities required for a slab slab is that there are few non-metallic inclusions (hereinafter also simply referred to as “inclusions”) on the surface layer of the slab slab.

The inclusions captured on the surface layer of the slab slab include the following.
(A) A deoxidation product produced in a deoxidation step of molten steel with aluminum or the like and suspended in the molten steel.
(B) Fine oxide contained in argon gas bubbles blown into molten steel with a tundish or immersion nozzle.
(C) Suspension in which mold powder added on the molten steel surface in the mold is entrained in the molten steel.

Since all the inclusions become surface defects at the steel product stage, it is important to reduce the amount of inclusions trapped in the surface layer of the slab slab. Further, when the argon gas bubbles trapped in the surface layer of the slab slab are opened in the surface layer of the slab slab, the inside of the bubbles is oxidized in a heating furnace or the like, and the oxidized portion becomes a surface defect.

Conventionally, in order to prevent deoxidation products, argon gas bubbles, and mold powder in molten steel from being trapped by the solidified shell and causing defects in steel products, the flow of molten steel is properly controlled in a continuous casting mold. A method for achieving the state has been proposed.

For example, in Patent Document 1, in a continuous casting method using an immersion nozzle having a pair of discharge holes arranged symmetrically with respect to the vertical axis of the immersion nozzle, the immersion nozzle has an immersion depth and a mold width of 1/4. Disclosed is a technology that regulates the maximum flow velocity of the discharge flow and the discharge angle of the discharge holes, and adjusts the angle formed by the discharge direction of the discharge flow from the immersion nozzle and the long side of the mold to 3 to 7 ° on the horizontal plane. Has been. The purpose of adjusting the angle formed by the discharge direction of the discharge flow and the long side of the mold to be 3 to 7 ° is to bring at least a part of the discharge flow into contact with the solidified shell on the long side of the mold, thereby This is to attenuate the flow velocity.

Japanese Patent No. 4285345 Japanese Patent No. 5742992

However, the above prior art has the following problems.
That is, with the recent stricter quality of steel plates for automobile outer panels, defects caused by entrapment of fine bubbles and mold powder that have not been a problem until now are becoming a problem. It is not possible to sufficiently meet such strict quality requirements. In particular, alloyed hot-dip galvanized steel sheets are heated after hot-dip plating to diffuse the iron component of the base steel sheet into the galvanized layer, and the surface layer properties of the base steel sheet greatly contribute to the quality of the alloyed hot-dip galvanized layer. Affect. That is, if there is a bubble or mold powder defect on the surface layer of the base steel plate, even if it is a small defect, unevenness occurs in the thickness of the plating layer, which appears as a streak defect on the surface, It can no longer be used for demanding applications such as steel sheets for industrial use.

Patent Document 1 attempts to control the flow of molten steel in a mold by adjusting the immersion depth and shape of the immersion nozzle discharge hole and the discharge direction from the immersion nozzle. Generally, an inert gas such as argon or nitrogen is blown from the tundish outflow hole to the immersion nozzle discharge hole in order to suppress the adhesion of inclusions (mainly Al ₂ O ₃ ) to the inner wall of the immersion nozzle. It is. The influence of such gas blowing on the behavior of the discharge jet is very large, and the molten steel throughput naturally has a great influence on the behavior of the discharge jet. The flow control effect obtained cannot be increased simply by adjusting the design items of the immersion nozzle without controlling the gas injection amount and the molten steel throughput within an appropriate range. Is not considered applicable.

The present invention has been made in view of the above problems, and the purpose thereof is to set the discharge position of molten steel from the immersion nozzle, the shape of the immersion nozzle, the molten steel throughput, the flow rate of the blown inert gas, and the like within an appropriate range. It is to provide a continuous casting method capable of producing a high-quality slab slab by controlling the molten steel flow in the mold by adjusting the discharge direction of the immersion nozzle to an appropriate direction after controlling . Furthermore, by providing a proper immersion nozzle discharge direction for various in-mold electromagnetic flow control technologies, and providing a continuous casting method capable of producing a high-quality slab slab with fewer inclusions. is there.

The features of the present invention for solving such problems are as follows.
[1] A continuous casting method in which an immersion nozzle is disposed in a continuous casting mold, molten steel is supplied to the immersion nozzle, and the molten steel is cast. The immersion nozzle is disposed symmetrically with respect to the vertical axis. The immersion depth of the immersion nozzle (distance from the molten steel surface in the mold to the upper end of the discharge hole) is 180 mm or more and less than 300 mm, and the molten steel discharge angle is downward from the horizontal direction of the discharge hole. The ratio A / P between the flow rate A (NL / min) of the inert gas blown between the tundish outflow hole and the discharge hole within the range of 15 to 35 ° and the molten steel throughput P (ton / min) is 2 In the range of 0.0 to 3.5 NL / ton, the discharge direction of the immersion nozzle is within the range of the expression (1) with respect to a reference plane passing through the vertical axis center of the immersion nozzle and parallel to the long side of the mold. Inclined slab cast slabs Production method.

θ−6 ≦ α ≦ θ + 10 (1)
However, in the formula (1), α is an inclination angle (°) with respect to the reference surface in the discharge direction of the immersion nozzle when the continuous casting mold is viewed from above.
θ is an angle (acute angle) formed by a straight line from the vertical axis center of the immersion nozzle to the contact point between the long side of the mold and the short side of the mold and the reference plane when the continuous casting mold is viewed from above. And an angle (°) defined by the following equation (2).

tan θ = (D / 2) / (W / 2) (2)
However, in Formula (2), D is the thickness (mm) of the slab slab continuously cast,
W is the width (mm) of the continuously cast slab slab.
[2] The α is measured during continuous casting or after completion of the mold width change during continuous casting, and when the α does not satisfy the equation (1), the discharge direction of the immersion nozzle is set so as to satisfy the equation (1). The continuous casting method of a slab slab according to [1] to be changed.
[3] A pair of upper magnetic poles and a pair of lower magnetic poles opposed to each other across the mold long side are disposed on the back side of the mold long side, and the position of the maximum value of the DC static magnetic field applied from the upper magnetic pole is The discharge hole is positioned between the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole, and the molten steel flow is braked by applying a DC static magnetic field between the upper magnetic pole and the lower magnetic pole, When the strength of the DC static magnetic field applied from the upper magnetic pole is 1500 Gs or more and less than 2500 Gs (Gauss; 1 Gs = 10 ⁻⁴ T), the discharge direction of the immersion nozzle is expressed by the formula (1) with respect to the reference plane. Instead, it is tilted within the range of the formula (3), and when the DC static magnetic field strength is 2500 Gs or more and less than 3500 Gs, it is tilted within the range of the formula (4) instead of the formula (1). Continuous casting method for slab slabs.

θ ≦ α ≦ θ + 5 (3)
θ + 6 ≦ α ≦ θ + 10 (4)
[4] When the α is measured during continuous casting or after completion of mold width change during continuous casting, and the strength of the DC static magnetic field is 1500 Gs or more and less than 2500 Gs, and the α does not satisfy the formula (3) When the discharge direction of the submerged nozzle is changed so as to satisfy the expression (3), and the strength of the DC static magnetic field is 2500 Gs or more and less than 3500 Gs, and the α does not satisfy the expression (4), 4) The continuous casting method of a slab cast according to [3], wherein the discharge direction of the immersion nozzle is changed so as to satisfy the formula.
[5] A linear moving magnetic field generator having a moving direction of the magnetic field in the mold width direction is provided on the back side of the long side of the mold, and the short side of the mold is used to apply a braking force to the molten steel flow discharged from the immersion nozzle. The flow control is performed by applying a moving magnetic field from the immersion nozzle side to the mold short side in order to apply a moving magnetic field from the immersion nozzle side to the immersion nozzle side, or to give an acceleration force to the molten steel flow. The continuous casting method of a slab slab according to [1], wherein the discharge direction of the slab is inclined with respect to the reference plane within the range of the formula (5) instead of the formula (1).

θ + 2 ≦ α ≦ θ + 7 (5)
[6] The α is measured during continuous casting or after completion of the mold width change during continuous casting, and when the α does not satisfy the equation (5), the discharge direction of the immersion nozzle so as to satisfy the equation (5) The continuous casting method for a slab cast according to [5].
[7] A pair of magnetic poles opposed to each other across the mold long side is disposed on the back surface of the mold long side, and an AC moving magnetic field is applied from the magnetic pole to give horizontal swirling stirring to the molten steel, The ratio X / W (Gs / G) of the AC moving magnetic field strength X (Gs) and the width W (mm) of the continuously cast slab slab after setting the AC moving magnetic field strength within the range of 300 to 1000 Gs. mm) is 0.30 or more and less than 0.45, the discharge direction of the immersion nozzle is directed toward the upstream side of the swirl flow formed by the AC moving magnetic field with respect to the reference plane, (1) When the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55, the equation (7) is substituted for (1) instead of the equation (6). The continuous casting method of a slab cast according to [1], wherein the casting is inclined within a range.

θ-3 ≦ α ≦ θ (6)
θ-6 ≦ α ≦ θ-4 (7)
[8] The α is measured during continuous casting or after completion of mold width change during continuous casting, and the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45, and the α is ( 6) When the expression is not satisfied, the discharge direction of the immersion nozzle is changed so as to satisfy the expression (6), and the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55. The continuous casting method of a slab slab according to [7], wherein the discharge direction of the immersion nozzle is changed so as to satisfy the expression (7) when α does not satisfy the expression (7).
[9] A pair of upper magnetic poles and a pair of lower magnetic poles opposed to each other across the mold long side are arranged on the back surface of the mold long side, and the position of the maximum value of the DC static magnetic field applied from the upper magnetic pole is The discharge hole is positioned between the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole, the DC static magnetic field and the AC moving magnetic field are superimposed and applied from the upper magnetic pole, and applied from the upper magnetic pole The swirling flow of the molten steel rotating in the horizontal direction is formed on the molten steel surface in the mold by the AC moving magnetic field, and the molten steel flow is braked by the DC static magnetic field applied from the upper magnetic pole and applied from the lower magnetic pole. and damping the molten steel flow by a DC static magnetic field, the AC traveling magnetic field strength of 500 ~ 900Gs; range, the intensity of the DC static magnetic field applied from the upper pole of the (Gaussian 1Gs = 10 -4 ^T) In the range of 2000-3300 Gs, the strength of the DC static magnetic field applied from the lower magnetic pole is set in the range of 3000-4500 Gs, and the strength of the AC moving magnetic field X (Gs) is continuously cast. The ratio X / W (Gs / mm) to the width W (mm) is controlled to be 0.30 or more and less than 0.55, and the discharge direction of the immersion nozzle is set to the AC moving magnetic field with respect to the reference plane. The continuous casting method of a slab slab according to [1], wherein the slab slab is inclined toward the upstream side of the swirling flow of the molten steel formed by the step.
[10] When the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45, the discharge direction of the immersion nozzle is changed to the formula (1) with respect to the reference plane, and the following (8) When the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55, the discharge direction of the immersion nozzle is set to the expression (1) with respect to the reference plane. Instead, the continuous casting method for a slab slab according to [9], wherein the slab casting is inclined within the range of the following expression (9).

θ-2 ≦ α ≦ θ + 5 (8)
θ-5 ≦ α ≦ θ + 2 (9)
[11] The α is measured during continuous casting or after completion of the mold width change during continuous casting, and the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45, where the α When the expression (8) is not satisfied, the discharge direction of the immersion nozzle is changed so as to satisfy the expression (8), and the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55. When the α does not satisfy the equation (9), the discharge direction of the immersion nozzle is changed so as to satisfy the equation (9) [10].

According to the present invention, by controlling the immersion nozzle shape, molten steel throughput, flow rate of blown inert gas, etc. within an appropriate range, and adjusting the discharge direction of the immersion nozzle to an appropriate direction, the molten steel in the mold It is possible to produce a high-quality slab slab whose flow is appropriately controlled. Furthermore, it is possible to produce a high quality slab slab with fewer inclusions by controlling the discharge nozzle discharge direction to be appropriate for various in-mold electromagnetic flow control conditions.

It is the schematic sectional drawing which looked at an example of the mold for continuous casting from the mold long side. It is a schematic sectional drawing which shows the state of the discharge flow from the immersion nozzle in the casting mold for continuous casting. It is the schematic which looked at a mode that casting by inclining the discharge direction of an immersion nozzle from the perpendicular | vertical upper direction. It is a schematic sectional drawing of the casting mold for another casting different from FIG. It is the schematic sectional drawing which looked at Drawing 4 from the perpendicular upper part. FIG. 5 is a schematic cross-sectional view of a continuous casting mold different from FIGS. 1 and 4. It is the schematic sectional drawing which looked at FIG. 6 from the perpendicular | vertical upper direction. FIG. 7 is a schematic cross-sectional view of a continuous casting mold different from FIGS. 1, 4, and 6. It is the schematic sectional drawing which looked at Drawing 8 from the perpendicular upper part. It is a figure which shows typically the molten steel flow rate condition in a casting_mold | template in the casting conditions which made the discharge direction perpendicular | vertical to the casting_mold | template short side. It is the schematic which looked at the mode of casting by inclining a discharge direction from the perpendicular upper direction. FIG. 9 is a schematic cross-sectional view of a continuous casting mold different from those in FIGS. 1, 4, 6, and 8. FIG. 13 is a schematic cross-sectional view of a portion of the upper magnetic pole in FIG. 12. FIG. 13 is a schematic cross-sectional view of the lower magnetic pole part of FIG. 12. It is a figure which shows typically the molten steel flow rate condition in a casting_mold | template in the casting conditions which made the discharge direction from an immersion nozzle perpendicular | vertical to a casting_mold | template short side. It is the schematic which looked at a mode that it inclines and casts the discharge direction of an immersion nozzle to the upstream of a swirl | vortex flow from the perpendicular upper direction. It is a flowchart which shows an example of the process which changes the inclination angle (alpha) of a discharge direction. FIG. 18 is a flowchart showing an example different from FIG. 17. It is a flowchart which shows an example different from FIG.17 and FIG.18. FIG. 20 is a flowchart showing an example different from FIGS. FIG. 21 is a flowchart showing an example different from FIGS. 17 to 20. FIG. 22 is a flowchart showing an example of processing for properly using each of the processing in FIGS. 17 to 21 depending on casting conditions. FIG. 6 is a diagram showing the relationship between “α-θ” and “product defect index” in Invention Examples 19 to 38, with a static magnetic field strength of 2500 (Gs) as a boundary. FIG. 4 is a diagram showing the relationship between “α-θ” and “product defect index” in Inventive Examples 39 to 49. FIG. 6 is a diagram showing the relationship between “α-θ” and “product defect index” in inventive examples 50 to 71, with the ratio X / W value of 0.45 as a boundary. FIG. 10 is a diagram showing the relationship between “α-θ” and “product defect index” in Invention Examples 83 to 106, with the ratio X / W value of 0.45 as a boundary.

Hereinafter, the present invention will be described in detail through embodiments of the invention. FIG. 1 shows an example of a continuous casting mold 20 of a slab continuous casting machine to which the continuous casting method according to this embodiment can be applied, and is a cross-sectional view of the continuous casting mold 20 viewed from the long side of the mold. . A continuous casting mold 20 for a slab slab is configured by combining a pair of opposed mold long sides 2 and a pair of opposed mold short sides 3. An immersion nozzle 4 is arranged in a mold internal space surrounded by a pair of mold long sides and a pair of mold short sides, and molten steel 8 is injected into the mold internal space from a discharge hole 5 of the immersion nozzle 4. Continuous casting is performed. The discharge holes 5 are arranged on the side wall of the immersion nozzle 4 symmetrically with respect to the vertical axis of the immersion nozzle 4. The molten steel 8 injected from the discharge hole 5 is discharged in the direction of the left and right mold short sides 3 as a discharge flow. The molten steel 8 injected into the mold inner space comes into contact with the mold long side 2 and the mold short side 3 and is cooled, and a solidified shell 9 is formed on the contact surface between the mold long side 2 and the mold short side 3. A slab slab is produced by continuously drawing the slab slab with the solidified shell 9 as an outer shell and the inside thereof into an unsolidified molten steel 8 downward. At that time, mold powder (not shown) that functions as a lubricant, a heat retaining agent, an antioxidant, and the like is added to the molten steel surface 10 in the mold. In order to prevent inclusions from adhering to the inner surface of the immersion nozzle 4, argon gas, nitrogen gas, or the like is blown between the tundish outflow hole (not shown) and the discharge hole 5 of the immersion nozzle 4.

Fig. 1 shows flow behavior in a mold under casting conditions. Numerical calculation, actual 1/1 water model device, actual machine using low melting point alloy (Bi-Pb-Sn-Cd alloy; melting point about 70 ° C) It confirmed repeatedly by the flow rate measurement by the test casting apparatus of 1/4 size. At this time, focusing on the flow behavior of the discharge flow 11, an attempt was made to quantify the conditions for optimizing the flow in the mold.

FIG. 2 is a schematic cross-sectional view showing a state of a discharge flow from an immersion nozzle in a continuous casting mold. Conditions that the immersion depth of the immersion nozzle 4 (distance from the molten steel surface in the mold to the upper end of the discharge hole) is 180 mm or more and less than 300 mm, and the discharge angle of the molten steel from the horizontal direction of the discharge hole 5 is in the range of 15 to 35 °. , The ratio A / P of the flow rate A (NL / min) of the inert gas blown from the tundish outflow hole to the discharge hole 5 of the immersion nozzle 4 and the molten steel throughput P (ton / min) is 2.0 to When controlled within the range of 3.5 (NL / ton), as shown in FIG. 2A, the discharge flow 11 tends to float toward the meniscus after the discharge flow 11 has reached a relatively short mold side. I found out that That is, it has been found that the discharge flow 11 exhibits a rising behavior (lift-up) by blowing an inert gas into the discharge hole 5 of the immersion nozzle 4. From this, stable control of the flow behavior of the discharge flow 11 is possible by appropriately balancing the A / P, which is the ratio of the inert gas flow rate to the molten steel throughput, the immersion depth of the immersion nozzle 4 and the discharge angle. I found out. Moreover, as shown in FIG.2 (b), when there are too many inert gas flow rates A with respect to the molten steel throughput P, the discharge flow 11 floats early by the lift-up effect. It was found that such early rise of the discharge flow 11 promotes fluctuations in the meniscus level and makes it easier to entrain the mold powder. On the other hand, as shown in FIG. 2 (c), when the inert gas flow rate A is too small for the molten steel throughput P, the discharge 11 is difficult to float and directly collides with the mold short side 3, and there is a fear of breakout, It has also been found that the molten steel supply to the meniscus side becomes unstable and heat is not applied to the meniscus, which may cause defective melting of the mold powder. These findings are that the casting width is 1000 to 2000 (mm), the area per discharge hole is 4000 to 10000 (mm ² ), and the molten steel throughput is 3.0 to 8.0 (ton / min). Therefore, the flow behavior of the discharge flow 11 can be stably controlled under such casting conditions in this range.

Next, the present inventors investigated paying attention to the discharge direction of the discharge flow 11. As a result, when the lift-up behavior of the discharge flow 11 is appropriately controlled as described above and the discharge direction is controlled within an appropriate angle range toward the mold corner, an appropriate flow velocity can be obtained on the meniscus surface. It has been found that the trapping of bubbles, fine inclusions, powder, and the like, which become surface defect factors, into the solidified shell 9 can be suppressed, thereby reducing surface defects.

FIG. 3 is a schematic view of a state in which casting is performed by tilting the discharge direction of the immersion nozzle as viewed from above. As shown in FIG. 3, the immersion nozzle 4 passes through the center of the vertical axis of the immersion nozzle 4 and goes to the contact point (mold corner portion) between the mold long side 2 and the mold short side 3 and parallel to the mold long side surface. When the angle (acute angle) formed with the reference plane passing through the center of the vertical axis is θ (°), θ is the thickness D (mm) of the cast slab cast and the width W (mm of the cast slab cast ) And is defined by the following equation (2) (hereinafter, the angle θ is also referred to as “diagonal angle θ”). Thus, diagonal direction angle (theta) changes according to the cross-sectional dimension of a slab slab.

tan θ = (D / 2) / (W / 2) (2)
In addition, as shown in FIG. 3, when casting with the discharge direction of the submerged nozzle 4 inclined, the inclination angle of the submerged nozzle 4 with respect to the reference surface when the continuous casting mold 20 is viewed from vertically above is α (°).

The present inventors appropriately control the flow behavior of the discharge flow 11 and determine the inclination angle α with respect to the reference plane within the range of the following formula (1) to obtain a high-quality slab slab with few surface defects. I found out that That is, by controlling the inclination angle α within the range of the expression (1), it is possible to obtain an appropriate flow velocity that can eliminate the surface defect factor.

θ−6 ≦ α ≦ θ + 10 (1)
When the inclination angle α is smaller than θ-6, an appropriate flow rate cannot be obtained. Further, when the inclination angle α is larger than θ + 10, the discharge flow 11 directly collides with the long-side solidified shell 9, so that the risk of breakout increases.

As described above, by using the continuous casting method according to the present embodiment, a high-quality slab slab can be manufactured without using an expensive electromagnetic fluid coil equipment. In the present embodiment, the slab cast having a thickness in the range of 220 to 300 (mm) is targeted, and the continuous casting method according to the present embodiment can be applied to any slab having a thickness in the range.

Next, the present inventors investigated the inclination angle α with respect to the optimum reference plane when using various in-mold electromagnetic flow control methods.

FIG. 4 is a schematic cross-sectional view of a continuous casting mold different from that described above. FIG. 5 is a schematic cross-sectional view of FIG. 4 viewed from vertically above. The continuous casting mold 30 for the slab slab further includes a pair of upper magnetic poles opposed to the continuous casting mold 20 shown in FIG. 6 and a pair of lower magnetic poles 7 are provided. Each of the upper magnetic pole 6 and the lower magnetic pole 7 is provided with a DC static magnetic field generating coil 13 for applying a DC static magnetic field, as shown in FIG. The DC static magnetic field generating coil 13 is arranged over the width of the slab slab to be cast or over it.

The molten steel flow on the molten steel surface 10 in the mold is braked (decelerated) by the DC static magnetic field applied from the DC static magnetic field generating coil 13 of the upper magnetic pole 6. Similarly, the molten steel flow that attempts to pass downward through the position of the DC static magnetic field generating coil 13 in the discharge flow 11 by the DC static magnetic field applied from the DC static magnetic field generating coil 13 of the lower magnetic pole 7 is braked ( Decelerated). The discharge hole 5 of the immersion nozzle 4 is disposed between the position of the maximum value of the DC static magnetic field applied from the upper magnetic pole 6 and the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole 7. .

When such electromagnetic flow control is performed, the molten steel flow is damped by a static magnetic field, so that the entrainment of mold powder is suppressed. On the other hand, in order to suppress trapping of bubbles and fine inclusions in the solidified shell 9, it is necessary to impart an appropriate flow rate to the molten steel flow. As a result of intensive studies, the present inventors have found that there is an appropriate inclination angle α in the ejection direction in accordance with the strength of the DC static magnetic field in the upper magnetic pole 6. That is, when the strength of the DC static magnetic field applied from the upper magnetic pole 6 is 1500 Gs or more and less than 2500 Gs (Gauss; 1 Gs = 10 ⁻⁴ T), the discharge direction of the immersion nozzle 4 passes through the vertical axis center of the immersion nozzle 4. When the strength of the DC static magnetic field applied from the upper magnetic pole 6 is 2500 Gs or more and less than 3500 Gs with respect to the reference plane parallel to the mold long side surface, the range of the equation (4) It was found that a good quality slab slab can be obtained by inclining inside. If the strength of the dc static magnetic field is relatively strong at 2500 Gs or more and less than 3500 Gs, increasing the tilt angle α as shown in equation (4) can give the molten steel flow a flow rate suitable for braking by the static magnetic field. is there.

θ ≦ α ≦ θ + 5 (3)
θ + 6 ≦ α ≦ θ + 10 (4)
FIG. 6 is a schematic cross-sectional view of a continuous casting mold different from that described above. FIG. 7 is a schematic cross-sectional view of FIG. 6 viewed from vertically above. The continuous casting mold 40 for slab slabs is a pair of linear molds opposed to the continuous casting mold 20 shown in FIG. A moving magnetic field generator 42 is provided. The moving direction of the magnetic field of the linear moving magnetic field generator 42 is the mold width direction. In the case of controlling the flow in the mold by applying a braking force to the discharge flow 11 discharged from the immersion nozzle 4 installed in the mold, as shown in FIG. In the case where the moving magnetic field is applied in the direction toward the surface and the flow in the mold is controlled by applying the acceleration force to the discharge flow 11, the direction from the immersion nozzle 4 toward the mold short side 3 as shown in FIG. A moving magnetic field is applied to. Such a flow control method is characterized in that a braking force or an acceleration force can be applied to the discharge flow 11 so that the discharge flow 11 can always be controlled in an appropriate state. Since the flow is symmetric, it has been found that a region with a small flow velocity occurs in the part where the left and right flow interfere. Also in this case, it was found that trapping of bubbles and fine inclusions in the solidified shell 9 can be suppressed by controlling the inclination angle α with respect to the reference surface in the discharge direction of the immersion nozzle 4 to an appropriate value. As a result of various experiments and investigations, in the case of the flow control method in the mold in which such a moving magnetic field is applied, the discharge direction of the immersion nozzle 4 passes through the center of the vertical axis of the immersion nozzle and is parallel to the long side surface of the mold. It was found that a good quality slab slab can be obtained by inclining within the range of the formula (5) with respect to the reference surface.

θ + 2 ≦ α ≦ θ + 7 (5)
That is, it is considered that an appropriate flow rate can be imparted to the molten steel flow by directing the immersion nozzle discharge direction at an inclination angle of the expression (5) with respect to the flow in the mold that tends to be symmetrical.

FIG. 8 is a schematic cross-sectional view of a continuous casting mold different from that described above. FIG. 9 is a schematic cross-sectional view of FIG. 8 viewed from vertically above. The continuous casting mold 50 is further provided with a pair of magnetic poles 52 opposed to the continuous casting mold 20 shown in FIG. . The magnetic pole 52 is provided with an AC moving magnetic field generating coil 12 for applying an AC moving magnetic field as shown in FIG. The magnetic pole 52 controls the flow of the molten steel flow by applying an AC moving magnetic field whose magnetic field is moved in the mold width direction and applying a horizontal swirling flow to the molten steel 8 in the mold. In such a flow control method, the flow in the mold will be described by taking as an example a case where the discharge direction from the immersion nozzle 4 is discharged symmetrically in the direction of the mold short side 3.

FIG. 10 is a diagram schematically showing the flow rate of molten steel in the mold under casting conditions in which the discharge direction is perpendicular to the mold short side. In FIG. 10, reference numeral 15 denotes a swirl flow that is formed by an AC moving magnetic field and rotates in one direction in the clockwise or counterclockwise direction in the molten steel surface 10 in the mold and the vicinity thereof. After colliding with the solidified shell 9 on the side 3 side, it flows upward in the vertical direction, and then flows in the molten steel surface 10 in the mold from the short side of the mold toward the immersion nozzle, 17 is a low flow velocity region, 18 is The vortex flow 19 generated by the collision of the swirling flow 15 and the reverse flow 16 is a downward flow that flows downward in the vertical direction after the discharge flow 11 collides with the solidified shell on the short side of the mold. In FIG. 10, the low flow velocity region 17 and the vortex 18 are shown only on one mold short side, but they also occur on the opposite mold short side. As shown in FIG. 10, in the vicinity of the mold short side 3 in the width direction of the mold long side 2, which is the downstream region of the swirling flow 15 by the AC moving magnetic field, a low flow rate region 17 in which the molten steel flow rate becomes small is generated. confirmed. From the investigation result of the defect distribution of the steel product cast by the continuous casting machine under such casting conditions, it is confirmed that the region where the defect of the steel product exists coincides with the generation position of the low flow velocity region 17.

Further, as a result of various considerations, the inventors have found that the reversal caused by the swirling flow 15 caused by the AC moving magnetic field and the discharge flow 11 colliding with the short-side solidified shell 9 in the vicinity of the mold short side 3. It has been found that a low flow velocity region 17 is generated by collision / interference with the flow 16, and bubbles or inclusions are trapped in the low flow velocity region 17 and eventually become defects in the steel product. Moreover, the swirl | vortex flow 15 and the reversal flow 16 collided, and the vortex | eddy_current 18 generate | occur | produced, the mold powder was caught by this eddy current, and it caught by the solidification shell 9, and also discovered the possibility of becoming a defect of steel products. The present inventors have intensively studied these solutions. As described above, the defect of the steel product caused by the inclusions, bubbles, and mold powder is caused by the low flow velocity region 17 caused by the collision / interference between the swirling flow 15 caused by the AC moving magnetic field and the reverse flow 16 of the discharge flow 11. And the generation of the vortex 18. Even in the flow control method in which an alternating moving magnetic field is applied to apply a horizontal swirling flow 15, collision / interference between the swirling flow 15 and the reversing flow 16 can be prevented by inclining the discharge direction of the immersion nozzle 4 from the reference plane. I found that it can be avoided.

FIG. 11 is a schematic view of a state where casting is performed while the discharge direction is inclined as viewed from above. As shown in FIG. 11, it was clarified from various flow investigation experiments that it is important to incline the discharge direction of the immersion nozzle 4 toward the upstream side of the swirling flow 15 by the AC moving magnetic field. . Furthermore, the present inventors proceeded with the flow investigation experiment to set the AC moving magnetic field strength within the range of 300 to 1000 Gs, and then the AC moving magnetic field strength X (Gs) and the width of the slab slab continuously cast. When the ratio X / W (Gs / mm) to W (mm) is 0.30 or more and less than 0.45, the discharge direction of the immersion nozzle 4 passes through the center of the vertical axis of the immersion nozzle 4 and the long side of the mold The slab is continuously cast with the strength X (Gs) of the AC moving magnetic field inclined in the range of the equation (6) toward the upstream side of the swirling flow formed by the AC moving magnetic field with respect to the reference plane parallel to When the ratio X / W (Gs / mm) to the width W (mm) of the slab is 0.45 or more and less than 0.55, a good quality slab slab is obtained by inclining within the range of the formula (7). I found out that

θ-3 ≦ α ≦ θ (6)
θ-6 ≦ α ≦ θ-4 (7)
When a DC static magnetic field is applied to the molten steel 8 shown in FIG. 4 or when a braking force or acceleration force is applied to the discharge flow 11 shown in FIG. 6, the molten steel flow in the mold is basically symmetrical. Therefore, it is preferable to greatly incline the discharge direction of the immersion nozzle 4 toward the mold long side 2 in order to provide an appropriate swirl flow rate. On the other hand, when the swirl stirring is applied by the electromagnetic flow control shown in FIG. 8, it is sufficient to tilt the reversal flow 16 and the swirl flow 15 so that the collision / interference can be suppressed. As shown in the equation (7), it is preferable to reduce the inclination angle α. If the inclination angle α is too large, the flow rate of the molten steel in the vicinity of the solidified shell 9 becomes too high, and there is a concern that the growth of the solidified shell 9 is hindered, causing an operation inhibition such as a breakout. It is not preferable.

FIG. 12 is a schematic cross-sectional view of a continuous casting mold 1 different from the above, wherein reference numeral 1 is a continuous casting mold, 2 is a mold long side, 3 is a mold short side, 4 is an immersion nozzle, and 5 is an immersion nozzle. , 6 is an upper magnetic pole, 7 is a lower magnetic pole, 8 is molten steel, 9 is a solidified shell, 10 is a molten steel surface in the mold, and 11 is a discharge flow from an immersion nozzle.

The continuous casting mold 1 is configured by combining a pair of opposed mold long sides 2 and a pair of opposed mold short sides 3, and the pair of mold long sides 2 and the pair of mold short sides 3 are combined. The immersion nozzle 4 is disposed in the enclosed mold inner space, and the molten steel 8 is injected from the discharge hole 5 of the immersion nozzle 4 into the mold inner space, and the molten steel 8 is continuously cast. The discharge holes 5 are arranged symmetrically on a straight line passing through the center of the immersion nozzle 4 on the side wall of the immersion nozzle 4, and the molten steel 8 injected from the discharge hole 5 becomes a discharge flow 11 and the left and right molds are short. It is discharged in the direction of side 3. The molten steel 8 injected into the mold inner space is cooled by contacting with the mold long side 2 and the mold short side 3, and a solidified shell 9 is formed on the contact surface between the mold long side 2 and the mold short side 3. A slab having a shell 9 as an outer shell and an inside of the unsolidified molten steel 8 is continuously drawn downward to produce a slab slab. At that time, mold powder (not shown) that functions as a lubricant, a heat retaining agent, an antioxidant, and the like is added to the molten steel surface 10 in the mold.

FIG. 13 is a schematic cross-sectional view of the upper magnetic pole portion of FIG. FIG. 14 is a schematic cross-sectional view of the lower magnetic pole portion of FIG. In the continuous casting mold 1 used in this embodiment, a pair of upper magnetic poles 6 and a pair of lower magnetic poles 7 are arranged on the back surface of the mold long side 2 with the mold long side 2 interposed therebetween. As shown in FIG. 13, the upper magnetic pole 6 is provided with an AC moving magnetic field generating coil 12 for applying an AC moving magnetic field and a DC static magnetic field generating coil 13 for applying a DC static magnetic field. As shown in FIG. 14, a DC static magnetic field generating coil 14 for applying a DC static magnetic field is installed. The AC moving magnetic field generating coil 12, the DC static magnetic field generating coil 13, and the DC static magnetic field generating coil 14 are disposed over the width of the slab slab to be cast or more.

The AC moving magnetic field applied from the AC moving magnetic field generating coil 12 of the upper magnetic pole 6 forms a swirling flow of the molten steel 8 in which the molten steel in the mold rotates in the horizontal direction, and from the DC static magnetic field generating coil 13 of the upper magnetic pole 6. The molten steel flow on the molten steel surface 10 in the mold is braked (decelerated) by the applied DC static magnetic field. Similarly, the molten steel flow that attempts to pass downward through the position of the DC static magnetic field generating coil 14 in the discharge flow 11 by the DC static magnetic field applied from the DC static magnetic field generating coil 14 of the lower magnetic pole 7 is braked ( Decelerated). The discharge hole 5 of the immersion nozzle 4 is disposed between the position of the maximum value of the DC static magnetic field applied from the upper magnetic pole 6 and the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole 7. .

FIG. 15 is a diagram schematically showing the molten steel flow rate in the mold under casting conditions in which the discharge direction from the immersion nozzle is perpendicular to the mold short side. The inventors use a continuous casting mold 1 shown in FIG. 12, applying a DC static magnetic field and an AC moving magnetic field superimposed on the upper magnetic pole 6, applying a DC static magnetic field to the lower magnetic pole 7, and an immersion nozzle. The state of flow in the mold under the casting conditions in which the discharge direction from the four discharge holes 5 was parallel to the long side surface of the mold and the discharge flow 11 was perpendicular to the short side 3 of the mold was investigated.

The flow velocity distribution in the mold under this casting condition is obtained by numerical calculation and flow velocity measurement by a test casting apparatus of 1/4 size of actual machine using a low melting point alloy (Bi-Pb-Sn-Cd alloy; melting point about 70 ° C). Confirmed repeatedly. As a result, as shown in FIG. 15, a low flow velocity region 17 in which the molten steel flow velocity becomes small is generated in the vicinity of the mold short side 3 in the width direction of the mold long side 2, which is the downstream region of the swirling flow 15 by the AC moving magnetic field. Confirmed to do. In FIG. 15, reference numeral 15 denotes a swirling flow that is formed by an AC moving magnetic field and rotates in one direction clockwise or counterclockwise in the molten steel surface 10 in the mold and the vicinity thereof, and 16 is a discharge flow 11 that is short of the mold. After colliding with the solidified shell on the side, it flows vertically upward, and then on the molten steel surface 10 in the mold, it flows in the reverse direction from the short side of the mold toward the immersion nozzle, 17 is a low flow velocity region, and 18 is a swirl. A vortex flow 19 generated by the collision of the flow 15 and the reversal flow 16 is a downward flow that flows downward in the vertical direction after the discharge flow 11 collides with the solidified shell on the short side of the mold. In FIG. 15, the low flow velocity region 17 and the vortex 18 are shown only on one mold short side, but they also occur on the opposite mold short side.

In addition, from the investigation results of the defect distribution of the steel product cast by the actual continuous casting machine under such casting conditions, the region where the defect of the steel product exists coincides with the generation position of the low flow velocity region 17 described above. Have confirmed.

As a result of various considerations, the present inventors have found that a swirling flow 15 caused by an AC moving magnetic field and a reversal flow 16 after a short-side solidified shell collision of the discharge flow 11 from the discharge hole 5 collide in the vicinity of the mold short side. -It has been found that a low flow velocity region 17 is generated due to the interference, bubbles and inclusions are trapped in the low flow velocity region 17 and eventually become a defect of the steel product. Moreover, the swirl | vortex flow 15 and the reversal flow 16 collided, and the vortex | eddy_current 18 generate | occur | produced, the mold powder was drawn in by this vortex | eddy_current 18, and it caught the solidification shell, and also discovered the possibility of becoming a defect of steel products.

The present inventors diligently studied about these solutions. As described above, defects in steel products caused by inclusions, bubbles, and mold powder trapping are caused by collision / interference between the swirling flow 15 caused by the AC moving magnetic field and the reverse flow 16 of the discharge flow 11 from the discharge hole 5. This is due to the accompanying generation of the low flow velocity region 17 and the vortex 18. Therefore, the present inventors examined a method for avoiding the collision / interference between the swirling flow 15 and the reversing flow 16, and as a result, the discharge direction of the discharge flow 11 from the immersion nozzle 4 is perpendicular to the mold short side 3. I found out that it was shifted from any direction.

Usually, the discharge hole 5 of the immersion nozzle 4 is arranged in a direction parallel to the long side surface of the mold so that the molten steel 8 is discharged toward the short side surface of the mold. It is devised to keep. However, under the condition that the swirling flow 15 is applied to the molten steel 8 in the mold by the AC moving magnetic field as in the present invention, the axial symmetry about the immersion nozzle 4 is taken into consideration rather than the left-right symmetry in the mold. Therefore, it is considered that it is preferable in principle to determine the discharge direction of the discharge hole 5 of the immersion nozzle 4.

FIG. 16 is a schematic view of a state in which casting is performed by inclining the discharge direction of the immersion nozzle toward the upstream side of the swirling flow, as viewed from above. When the discharge direction from the immersion nozzle 4 was examined under various conditions using numerical analysis and a test casting apparatus using a low melting point alloy, the AC moving magnetic field strength of the upper magnetic pole 6 was in the range of 500 to 900 (Gs), and the upper part The strength of the DC static magnetic field of the magnetic pole 6 is in the range of 2000 to 3300 (Gs), the strength of the DC static magnetic field of the lower magnetic pole 7 is in the range of 3000 to 4500 (Gs), and the AC moving magnetic field strength X (Gs of the upper magnetic pole 6 is ) And the ratio X / W (Gs / mm) of the width W (mm) of the continuously cast slab slab is controlled to be 0.30 or more and less than 0.55, as shown in FIG. By tilting the discharge direction toward the upstream side of the swirling flow 15 formed by the AC moving magnetic field, collision / interference between the swirling flow 15 and the reversing flow 16 can be avoided, and the generation of the low flow velocity region 17 and the vortex flow 18 can be avoided. Desirable molten steel flow in mold Found that can be obtained.

When the ratio X / W is less than 0.30, the flow velocity of the swirling flow 15 is too small with respect to the width of the slab slab, so that a sufficient effect can be obtained even if the discharge direction of the immersion nozzle 4 is inclined. Can not. Further, when the ratio X / W is 0.55 or more, a sufficient flow velocity of the swirl flow 15 can be ensured with respect to the width of the slab slab, and adverse effects such as generation of a low flow velocity region 17 due to interference with the reverse flow 16 are obtained. Therefore, the effect of tilting the discharge direction of the immersion nozzle 4 is considered to be small. Moreover, it is not preferable to increase the ratio X / W excessively because the swirling flow 15 becomes too strong and the mold powder is entrained and defects due to the encapsulated mold powder may occur. Further, if the discharge direction is inclined when the flow velocity of the swirling flow 15 is sufficiently high, the swirling flow 15 becomes too strong and the thickness of the solidified shell 9 becomes thin, which may be a factor that hinders operational stability such as breakout. It is not preferable because it is also possible.

Furthermore, the present inventors have conducted intensive studies and experiments, paying attention to the ratio X / W between the strength X (Gs) of the AC moving magnetic field and the width W (mm) of the slab slab to be cast. I found a relationship.

When the ratio X / W (Gs / mm) between the AC moving magnetic field strength X and the width W of the slab is 0.30 or more and less than 0.45, the long side surface of the mold passes through the center of the vertical axis of the immersion nozzle 4 It was found that the inclination angle α in the discharge direction with respect to the reference plane parallel to is preferably in the range of the following equation (8). This is because the AC moving magnetic field strength X is relatively small with respect to the width W of the slab slab. It is thought that it is necessary to avoid collision and interference.

θ-2 ≦ α ≦ θ + 5 (8)
In addition, when the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55, the inclination angle α in the discharge direction with respect to a reference plane passing through the center of the vertical axis of the immersion nozzle 4 and parallel to the mold long side surface. It was also found that it is preferable to set the value within the range of the following formula (9). Since the AC moving magnetic field strength X is relatively large with respect to the width W of the slab slab, the swirling flow 15 and the reversing flow on the downstream side of the swirling flow 15 are set by tilting the discharge direction by setting the inclination angle α to be small. This is because, as described above, it is possible to eliminate the risk of an operational stability impediment such as breakout as described above while avoiding collision / interference with 16.

θ-5 ≦ α ≦ θ + 2 (9)
Further, the inclination angle α in the discharge direction of the immersion nozzle 4 may be set during continuous casting using an apparatus capable of changing the inclination angle α in the discharge direction in addition to setting before the start of continuous casting. Using this apparatus, each time the casting conditions are changed, such as changing the mold width during continuous casting, α and θ are sequentially measured, and α expressed by the formula (1) or (3) to (9) A more preferable effect can be obtained by changing the discharge direction of the submerged nozzle 4 so that the inclination angle α satisfies the magnitude relationship between and θ.

FIG. 17 is a flowchart showing an example of processing for changing the inclination angle α in the ejection direction. A process of changing the inclination angle α in the discharge direction of the immersion nozzle 4 will be described with reference to FIG. In the processing shown in FIG. 17, the immersion depth of the immersion nozzle 4 is 180 mm or more and less than 300 mm at every predetermined time, the molten steel discharge angle from the horizontal direction of the discharge hole is 15 to 35 °, the tundish outflow hole Whether the ratio A / P of the flow rate A (NL / min) of the inert gas blown between the gas and the discharge hole to the discharge hole and the molten steel throughput P (ton / min) is in the range of 2.0 to 3.5 NL / ton The process is started when the condition is satisfied. Further, the processing can be performed using, for example, an apparatus (hereinafter referred to as “angle adjustment apparatus”) provided with a drive mechanism that can change the ejection direction as disclosed in Patent Document 2. In the present embodiment, it is assumed that the angle adjusting device is a main body that changes the inclination angle α in the ejection direction.

In the process shown in FIG. 17, the angle adjusting device determines whether or not the continuous casting device is in steady casting (step S101). In the present embodiment, the term “steady casting” means that the casting is being performed and the casting speed is not changed, but the mold width is not changed, but the flow rate of the inert gas is changed. It means not a state but a state where the immersion depth of the immersion nozzle 4 is not changed.

If the angle adjusting device determines that the continuous casting device is not in a steady casting (step S101: No), the processing shown in FIG. When the continuous casting apparatus is in a steady casting state (step S101: Yes), the angle adjusting device measures the inclination angles α and θ, and does the inclination angle α satisfy the expression (1)? It is determined whether or not (step S102). When it is determined that the inclination angle α satisfies the expression (1) (step S102: Yes), the angle adjusting device ends the processing illustrated in FIG. On the other hand, when it is determined that the inclination angle α does not satisfy the expression (1) (step S102: No), the angle adjustment device resets the inclination angle α that satisfies the expression (1) (step S103). The angle adjusting device changes the discharge direction of the immersion nozzle 4 so as to obtain the reset inclination angle α (step S104), and ends the process shown in FIG.

As described above, the angle adjusting device can change the discharge direction of the submerged nozzle 4 by the process shown in FIG. 17 so that the inclination angle α satisfies the expression (1) every predetermined time. Thereby, even if the inclination angle α does not satisfy the expression (1) for some reason, the discharge direction of the immersion nozzle 4 is adjusted so that the inclination angle α satisfies the expression (1) by the angle adjusting device. Since it is changed, it can prevent that the slab cast containing an inclusion continues being manufactured.

FIG. 18 is a flowchart showing an example different from the above. Another process for changing the inclination angle α in the discharge direction of the immersion nozzle 4 will be described with reference to FIG. The process shown in FIG. 18 includes a pair of upper magnetic pole 6 and a pair of lower magnetic pole 7 opposed to the back surface of the mold long side 2 across the mold long side in addition to the processing start conditions shown in FIG. Is disposed between the position of the maximum value of the DC static magnetic field applied from the upper magnetic pole 6 and the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole 7, and the strength of the static magnetic field Is started when it is 1500 Gs or more and less than 3500 Gs.

In the process shown in FIG. 18, the angle adjusting device determines whether or not the continuous casting device is in steady casting (step S201). When the angle adjusting device determines that the continuous casting device is not in the steady casting (step S201: No), the processing shown in FIG. 18 is terminated. The angle adjusting device determines whether the strength of the DC static magnetic field is within a range of 1500 ≦ Gs <2500 when the continuous casting apparatus is in a steady casting (step S201: Yes) (step S201: Yes). S202). When the angle adjusting device determines that the intensity of the DC static magnetic field is within the range of 1500 ≦ Gs <2500 (step S202: Yes), the inclination angle α and θ are measured, and the inclination angle α is (3). It is determined whether or not the expression is satisfied (step S203).

When the angle adjustment device determines that the inclination angle α satisfies the expression (3) (step S203: Yes), the process illustrated in FIG. On the other hand, when it is determined that the inclination angle α does not satisfy the expression (3) (step S203: No), the angle adjustment device resets the inclination angle α that satisfies the expression (3) (step S204). The angle adjusting device changes the discharge direction of the immersion nozzle 4 so as to have the reset inclination angle α (step S205), and ends the process shown in FIG.

On the other hand, when the angle adjusting device determines that the strength of the DC static magnetic field is not in the range of 1500 ≦ Gs <2500 but in the range of 2500 ≦ Gs <3500 (step S202: No), the inclination angles α and θ To determine whether the inclination angle α satisfies the equation (4) (step S206). When it is determined that the inclination angle α satisfies the expression (4) (step S206: Yes), the angle adjusting device ends the process illustrated in FIG. On the other hand, when it is determined that the inclination angle α does not satisfy the expression (4) (step S206: No), the angle adjusting device resets the inclination angle α that satisfies the expression (4) (step S207). The angle adjusting device changes the discharge direction of the immersion nozzle 4 so as to have the reset inclination angle α (step S208), and the process shown in FIG. 18 is ended.

FIG. 19 is a flowchart showing an example different from the above. Another process for changing the inclination angle α in the discharge direction of the immersion nozzle 4 will be described with reference to FIG. The process shown in FIG. 19 is started when a linear type moving magnetic field generator 42 in which the magnetic field movement direction is the mold width direction is provided in addition to the process start conditions shown in FIG.

In the process shown in FIG. 19, the angle adjusting device determines whether or not the continuous casting device is in steady casting (step S301). When the angle adjusting device determines that the continuous casting device is not in a steady casting state (step S301: No), the processing shown in FIG. When the continuous casting apparatus is in the steady casting (step S301: Yes), the angle adjusting device measures the inclination angles α and θ, and whether the inclination angle α satisfies the equation (5). It is determined whether or not (step S302). When it is determined that the inclination angle α satisfies the expression (5) (step S302: Yes), the angle adjusting device ends the process illustrated in FIG. On the other hand, when it is determined that the inclination angle α does not satisfy the expression (5) (step S302: No), the angle adjustment device resets the inclination angle α that satisfies the expression (5) (step S303). The angle adjusting device changes the discharge direction of the immersion nozzle 4 so as to have the reset inclination angle α (step S304), and ends the process shown in FIG.

FIG. 20 is a flowchart showing an example different from the above. Another process for changing the inclination angle α in the discharge direction of the immersion nozzle 4 will be described with reference to FIG. The process shown in FIG. 20 is provided with a pair of magnetic poles 52 on the back surface of the mold long side 2 in addition to the process start conditions shown in FIG. 17, and the AC movement with a strength within the range of 300 to 1000 Gs from the magnetic poles. The process is started when a magnetic field is applied and the ratio X / W between the strength of the AC moving magnetic field and the width of the slab cast is not less than 0.30 and less than 0.55.

In the process shown in FIG. 20, the angle adjusting device determines whether or not the continuous casting device is in a steady casting (step S401). If the angle adjusting device determines that the continuous casting device is not in a steady casting state (step S401: No), the processing shown in FIG. When the continuous casting apparatus is in steady casting (step S401: Yes), the angle adjusting apparatus determines whether the ratio X / W is 0.30 or more and less than 0.45 (step S401: Yes). Step S402). When the angle adjustment device determines that the ratio X / W is not less than 0.30 and less than 0.45 (step S402: Yes), the angle adjustment device measures the inclination angles α and θ, and the inclination angle α is expressed by equation (6). Is satisfied (step S403).

When the angle adjusting device determines that the inclination angle α satisfies the expression (6) (step S403: Yes), the process illustrated in FIG. On the other hand, when the angle adjusting device determines that the inclination angle α does not satisfy the expression (6) (step S403: No), the angle adjustment apparatus resets the inclination angle α that satisfies the expression (6) (step S404). The angle adjusting device changes the discharge direction of the immersion nozzle 4 so as to have the reset inclination angle α (step S405), and ends the process shown in FIG.

On the other hand, when the angle adjustment device determines that the ratio X / W is not 0.30 or more and less than 0.45 but 0.45 or more and less than 0.55 (step S402: No), the inclination angles α and θ Is measured to determine whether the inclination angle α satisfies the expression (7) (step S406).

When the angle adjusting device determines that the inclination angle α satisfies the expression (7) (step S406: Yes), the process illustrated in FIG. On the other hand, when it is determined that the inclination angle α does not satisfy the expression (7) (step S406: No), the angle adjusting device resets the inclination angle α that satisfies the expression (7) (step S407). The angle adjusting device changes the discharge direction of the immersion nozzle 4 so as to have the reset inclination angle α (step S408), and ends the process shown in FIG.

FIG. 21 is a flowchart showing an example different from the above. Another process for changing the inclination angle α in the discharge direction of the immersion nozzle 4 will be described with reference to FIG. In addition to the processing start conditions shown in FIG. 17, the process shown in FIG. 21 further includes a pair of upper magnetic poles 6 and a pair of lower magnetic poles 7 that are opposed to each other across the mold long side on the back side of the mold long side. The discharge hole is disposed between the position of the maximum value of the DC static magnetic field applied from the upper magnetic pole 6 and the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole 7, and 500 to 500 to An AC moving magnetic field having a strength in the range of 900 Gs and a DC static magnetic field having a strength in the range of 2000 to 3300 Gs are superimposed and applied, and a DC static magnetic field having a strength in the range of 3000 to 4500 Gs is applied from the lower magnetic pole 7. And started when the ratio X / W between the strength of the AC moving magnetic field and the width of the slab slab is not less than 0.30 and less than 0.55.

21. In the process shown in FIG. 21, the angle adjusting device determines whether or not the continuous casting device is in steady casting (step S501). When the angle adjusting device determines that the continuous casting device is not in a steady casting (step S501: No), the processing shown in FIG. 21 is terminated. The angle adjusting device determines whether the ratio X / W is equal to or greater than 0.30 and less than 0.45 when the continuous casting device is in the steady casting (step S501: Yes) ( Step S502). When the angle adjustment device determines that the ratio X / W is not less than 0.30 and less than 0.45 (step S502: Yes), the angle adjustment device measures the inclination angles α and θ, and the inclination angle α is expressed by equation (8). Is determined (step S503).

When the angle adjusting device determines that the inclination angle α satisfies the equation (8) (step S503: Yes), the process illustrated in FIG. On the other hand, when it is determined that the inclination angle α does not satisfy the expression (8) (step S503: No), the angle adjustment device resets the inclination angle α that satisfies the expression (8) (step S504). The angle adjusting device changes the discharge direction of the immersion nozzle 4 so as to obtain the reset inclination angle α (step S505), and ends the process shown in FIG.

On the other hand, when the angle adjustment device determines that the ratio X / W is not 0.30 or more and less than 0.45 but 0.45 or more and less than 0.55 (step S502: No), the inclination angles α and θ To determine whether the inclination angle α satisfies the expression (9) (step S506).

When the angle adjusting device determines that the inclination angle α satisfies the expression (9) (step S506: Yes), the process illustrated in FIG. On the other hand, when it is determined that the inclination angle α does not satisfy the expression (9) (step S506: No), the angle adjustment device resets the inclination angle α that satisfies the expression (9) (step S507). The angle adjusting device changes the discharge direction of the immersion nozzle 4 so as to have the reset inclination angle α (step S508), and ends the processing shown in FIG.

FIG. 22 is a flowchart showing an example of processing for properly using each of the processing in FIGS. 17 to 21 depending on casting conditions. In the example shown in FIG. 22, the angle adjusting device confirms the casting conditions at predetermined time intervals, and uses the processes described in FIGS. 17 to 21 depending on the casting conditions. The angle adjusting device has an immersion nozzle 4 with an immersion depth of 180 mm or more and less than 300 mm, a discharge angle of 15 to 35 ° from the horizontal direction of the discharge hole, and an inert gas blown between the tundish outlet hole and the discharge hole. When the ratio A / P between the flow rate A (NL / min) and the molten steel throughput P (ton / min) is within the range of 2.0 to 3.5 NL / ton, it is determined that only the condition A is satisfied The process of changing the inclination angle α in the ejection direction shown in FIG. 17 is executed.

Further, in addition to the condition A, the angle adjusting device includes a pair of upper magnetic pole 6 and a pair of lower magnetic poles 7 that are opposed to each other with the mold long side on the back of the mold long side, and is applied from the upper magnetic pole 6. Condition B in which an ejection hole is arranged between the position of the maximum value of the DC static magnetic field and the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole 7, and the strength of the static magnetic field is 1500 Gs or more and less than 3500 Gs. When it is determined that the condition is satisfied, the process of changing the inclination angle α in the ejection direction shown in FIG. 18 is executed.

When the angle adjustment device determines that the condition C that the linear movement magnetic field generation device 42 in which the magnetic field movement direction is the mold width direction is provided in addition to the condition A is shown in FIG. A process of changing the inclination angle α in the discharge direction shown is executed.

In addition to the condition A, the angle adjusting device includes a pair of magnetic poles 52 on the back surface of the long side, and an AC moving magnetic field having a strength within the range of 300 to 1000 Gs is applied from the magnetic poles 52, and the strength of the AC moving magnetic field and the slab When it is determined that the condition D that the ratio X / W to the width of the slab is 0.30 or more and less than 0.55 is satisfied, the process of changing the inclination angle α in the discharge direction shown in FIG. 20 is executed. To do.

Further, in addition to the condition A, the angle adjusting device includes a pair of upper magnetic pole 6 and a pair of lower magnetic poles 7 which are opposed to each other with the long side of the mold interposed therebetween, and is applied from the upper magnetic pole 6. The discharge hole is arranged between the position of the maximum value of the DC static magnetic field and the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole 7, and the AC movement with a strength within the range of 500 to 900 Gs from the upper magnetic pole 6. A magnetic field and a DC static magnetic field having a strength in the range of 2000 to 3300 Gs are applied in a superimposed manner, and a DC static magnetic field having a strength in the range of 3000 to 4500 Gs is applied from the bottom magnetic pole 7, and the strength of the AC moving magnetic field and the slab When it is determined that the condition E that the ratio X / W to the width of the slab is 0.30 or more and less than 0.55 is satisfied, the process of changing the inclination angle α in the discharge direction shown in FIG. 21 is executed. To do.

As described above, in the present embodiment, the angle adjustment device executes the process of changing the inclination angle α corresponding to the casting condition at every predetermined time, and the inclination angle α corresponds to the casting condition (1). When it is determined that the expressions (3) to (9) are not satisfied, the discharge direction of the immersion nozzle 4 is changed so as to satisfy the expression. Thus, even if the inclination angle α does not satisfy the equations (3) to (9) for some reason, the discharge direction of the immersion nozzle 4 is changed by the angle adjusting device so as to satisfy the equation. Therefore, it can prevent that the slab slab containing an inclusion continues being manufactured.

As described above, by using the continuous casting method according to the present embodiment, the strength of the DC static magnetic field and the strength of the AC moving magnetic field applied from the upper magnetic pole and the lower magnetic pole are determined within the optimum range. Because the discharge direction from the immersion nozzle is tilted to the upstream side of the swirling flow due to the AC moving magnetic field, the low flow velocity region and vortex flow that are generated due to the collision and interference between the swirling flow and the reversing flow are avoided, and intervening High quality slab slabs with few objects can be manufactured.

Using a slab continuous casting machine having the continuous casting mold 20 shown in FIG. 1, a test for casting about 300 (ton) of aluminum killed molten steel was performed. The slab slab to be cast has a thickness of 250 (mm), a width of 1000 to 2000 (mm), and a molten steel injection flow rate of 3.0 to 8.0 (ton / min). The molten steel discharge angle of the discharge hole of the used two-hole immersion nozzle (the angle when the horizontal is zero) is 15 (°) downward, and the immersion depth of the immersion nozzle (from the molten steel surface in the mold to the upper end of the discharge hole) ) Is 180 (mm) or more and less than 300 (mm). The shape of the discharge hole of the immersion nozzle is a square having a side length of 80 (mm), and the inner diameter of the immersion nozzle is 80 (mm). Moreover, argon gas was used as the inert gas blown from the immersion nozzle. The discharge direction of the discharge flow from the immersion nozzle was made into two types: a direction parallel to the mold long side (direction perpendicular to the mold short side) and a direction inclined with respect to the reference surface.

The cast slab slab is sequentially subjected to hot rolling, cold rolling, and alloyed hot dip galvanizing treatment, and the surface defects of this hot galvanized steel sheet are continuously measured using an on-line surface defect meter. Measured and discriminated steelmaking defects (defects caused by inclusions in the slab cast) by the appearance of defects, SEM analysis, ICP analysis, etc., and 100 (m) length of the alloyed hot-dip galvanized steel sheet The number of defects per unit (called “product defect index”) was evaluated. Table 1 shows the examination results of casting conditions and product defect index in Examples 1 to 18 of the present invention and Comparative Examples 1 to 18.

Table 1 shows the diagonal direction angle θ calculated by the thickness D of the cast slab and the width W of the slab, and the inclination angle α of the discharge flow at the time of casting. The diagonal direction angle θ was calculated by rounding off the second decimal place. The value of “α−θ” is shown by rounding off the first decimal place.
In Examples 1 to 18 of the present invention, the ratio A / P between the molten steel throughput and the flow rate of argon gas was set to be in the range of 2.0 to 3.5 (NL / ton), and then “α-θ” It was found that the product defect index was lowered to 0.26 to 0.29 (pieces / 100 m) by setting the value in the range of −6 to 10 (°). That is, it was found that by setting the inclination angle α (°) within the range of “θ−6” or more and “θ + 10” or less, an appropriate flow rate can be imparted to the molten steel flow and the product defect index can be lowered.

In contrast, in Comparative Examples 1 to 6, since the discharge direction was not inclined, the product defect index was as high as 0.61 to 0.68 (pieces / 100 m). In Comparative Examples 7 to 9, the inclination angle α was made smaller than those of Examples 1 to 18, and in this case, the product defect index was as high as 0.63 to 0.67 (pieces / 100 m). It is presumed that this is because the proper flow velocity could not be imparted to the molten steel flow because the inclination angle α was small.

In Comparative Examples 10 to 12, the inclination angle α is larger than that of Examples 1 to 18 of the present invention. In this case, although the product defect index was as low as 0.27 to 0.29 (pieces / 100m), the results of the slab cross-sectional investigation showed that the shell growth thickness in the mold was slightly thin, and the breakout There was a possibility of hindering operational stability.

In Comparative Examples 13 to 18, the ratio A / P between the molten steel throughput P and the argon gas flow rate A is outside the range of 2.0 to 3.5 (NL / ton). In this case, the product defect index is 0.61. It became high to 0.68 (pieces / 100m). This is presumed to be the result that the effect of tilting the discharge direction could not be exhibited because the discharge jet flow could not be controlled.

Although not described in this embodiment, the same effect as described in this embodiment can be obtained even when the thickness of the cast slab slab is in the range of 220 to 300 (mm). Separately confirmed. It has also been confirmed that the same tendency can be obtained when the molten steel discharge angle of the immersion nozzle is in the range of 15 to 35 (°). In addition, the shape of the discharge hole of the immersion nozzle and the inner diameter of the immersion nozzle are not limited only to the conditions described in the present embodiment, and any one can be used as long as it is within the range that can be assumed by those skilled in the art. It does n’t matter.

Using a slab continuous casting machine having a continuous casting mold 30 having an upper magnetic pole 6 and a lower magnetic pole 7 shown in FIG. 4, a test for casting about 300 (ton) of aluminum killed molten steel was performed. The cast slab slab has a thickness of 250 (mm), a width of 1800 (mm), and a molten steel injection flow rate of 5.0 to 8.0 (ton / min). The discharge angle of the discharge hole of the used 2-hole immersion nozzle is downward 15 (°), and the immersion depth of the immersion nozzle (distance from the molten steel surface in the mold to the upper end of the discharge hole) is 180 (mm) or more (300). mm). The shape of the discharge hole of the immersion nozzle is a square having a side length of 80 (mm), and the inner diameter of the immersion nozzle is 80 (mm). Argon gas was used as an inert gas blown from the immersion nozzle. The ratio A / P between the molten steel throughput P and the argon gas flow rate A was set to be in the range of 2.0 to 3.5 (NL / ton).

The discharge direction of the discharge flow from the immersion nozzle was made into two types: a direction parallel to the mold long side (a direction perpendicular to the mold short side) and a direction inclined with respect to the reference surface.

The cast slab slab is successively subjected to hot rolling, cold rolling, and alloyed hot dip galvanizing, and the surface defects of this hot galvanized steel sheet are continuously measured using an on-line surface defect meter. Among them, the defect appearance, SEM analysis, ICP analysis, etc. are used to discriminate steelmaking defects (defects caused by inclusions in the slab cast), and the alloyed hot-dip galvanized steel sheet is 100 (m) long. The number of defects per product (called “product defect index”) was evaluated. Table 2 shows the examination results of casting conditions and product defect index in Examples 19 to 38 of the present invention and Comparative Examples 19 to 32. Also in Table 2, the diagonal direction angle θ was calculated by rounding off the second decimal place. The value of “α−θ” is shown by rounding off the first decimal place. FIG. 23 is a diagram showing the relationship between “α-θ” and “product defect index” in Invention Examples 19 to 38, with the static magnetic field strength of 2500 (Gs) as a boundary.

In Examples 19 to 28 of the present invention, the DC static magnetic field strength is set to 1500 (Gs) or more and less than 2500 (Gs). As shown in FIG. 23, in this case, the product defect index is lowered to 0.21 to 0.24 (pieces / 100 m) by setting “α-θ” within the range of 0 to 5 (°). I understood it. That is, when the DC static magnetic field intensity is set to 1500 (Gs) or more and less than 2500 (Gs), it is found that the inclination angle α (°) is preferably set in the range of “θ” to “θ + 5”. .

In Examples 29 to 38 of the present invention, the DC static magnetic field strength is set to 2500 (Gs) or more and less than 3500 (Gs). In this case, it was found that the product defect index was lowered to 0.21 to 0.24 (pieces / 100 m) by setting “α-θ” within the range of 6 to 10 (°). That is, when the DC static magnetic field strength is 2500 (Gs) or more and less than 3500 (Gs), it was found that the inclination angle α (°) is preferably in the range of “θ + 6” or more and “θ + 10” or less. .

In Comparative Examples 19 to 23 and Comparative Examples 26 to 30, the inclination angle α is made smaller than “θ-6”. In this case, the product defect index was as high as 0.51 to 0.54 (pieces / 100 m). In Comparative Examples 24 to 25 and Comparative Examples 31 to 32, the inclination angle α is larger than “θ + 10”. Although the product defect index in this case was as low as 0.22 to 0.25 (pieces / 100 m), the results of the slab cross-sectional investigation showed that the shell growth thickness in the mold was slightly thin, and the breakout There was a possibility of hindering operational stability.

Although not described in the present embodiment, the slab cast cast has a thickness of 220 to 300 (mm), a casting width of 1000 to 2000 (mm), and a molten steel throughput of 3.0 to 8.0. It has been separately confirmed that the same effects as those described in this example can be obtained. It has also been confirmed that the same tendency can be obtained when the molten steel discharge angle of the immersion nozzle is in the range of 15 to 35 (°). In addition, the shape of the discharge hole of the immersion nozzle and the inner diameter of the immersion nozzle are not limited only to the conditions described in the present embodiment, and any one can be used as long as it is within the range that can be assumed by those skilled in the art. It does n’t matter.

Using a slab continuous casting machine having a continuous casting mold 40 provided with a pair of linear type moving magnetic field generators 42 as shown in FIG. 6, a test for casting about 300 (ton) of molten aluminum killed steel was performed. The thickness of the cast slab slab is 250 (mm), the width is 1600 (mm), and the molten steel injection flow rate is 5.0 to 6.0 (ton / min). The discharge angle of the discharge hole of the used two-hole immersion nozzle is 25 (°) downward, and the immersion depth of the immersion nozzle (distance from the molten steel surface in the mold to the upper end of the discharge hole) is 180 (mm) or more and 300 ( mm). The shape of the discharge hole of the immersion nozzle is a square having a side length of 70 (mm), and the inner diameter of the immersion nozzle is 70 (mm). Argon gas was used as an inert gas blown from the immersion nozzle. The ratio A / P between the molten steel throughput P and the argon gas flow rate A was set to be in the range of 2.0 to 3.5 (NL / ton). Casting was performed under the condition that an AC moving magnetic field was applied from the magnetic pole and the molten steel discharge flow was damped.

The cast slab slab is successively subjected to hot rolling, cold rolling, and alloyed hot dip galvanizing, and the surface defects of this hot galvanized steel sheet are continuously measured using an on-line surface defect meter. Among these, the defect appearance per 100 m length of the alloyed hot-dip galvanized steel sheet is determined by the appearance of defects, SEM analysis, ICP analysis, etc., to determine steelmaking defects (defects caused by inclusions in the slab slab). It was evaluated by the number (referred to as “product defect index”). Table 3 shows the results of the investigation of the casting conditions and product defect index of Examples 30 to 49 of the present invention. Also in Table 3, the diagonal direction angle θ was calculated by rounding off the second decimal place. The value of “α−θ” is shown by rounding off the first decimal place. FIG. 24 is a diagram showing the relationship between “α-θ” and “Product Defect Index” in Inventive Examples 39-49.

Although not shown in Table 3, the product defect index is 0.51 to 0.56 (pieces / piece) when the immersion nozzle discharge hole is not inclined and is directed to the short side by the same flow control method. It is confirmed that the height becomes 100 m).

In Invention Examples 39 to 44, “α−θ” is set in the range of 2 to 7 (°). As shown in FIG. 24, in this case, the product defect index was as low as 0.22 to 0.24 (pieces / 100 m). That is, when a linear type moving magnetic field generator is provided and the flow is controlled while braking the molten steel flow, it is preferable that the inclination angle α (°) is in the range of “θ + 2” or more and “θ + 7” or less. all right.

On the other hand, in Examples 45 to 47 of the present invention, “α-θ” is set in the range of −6 to 0 (°). In this case, the product defect index was slightly higher, 0.27 to 0.29 (pieces / 100 m). This is presumably due to the fact that the flow velocity was slightly applied to the molten steel flow, but these were also sufficiently good quality slabs. In Examples 48 to 49 of the present invention, “α−θ” is set in the range of 8 to 10 (°). In this case, the product defect index was slightly high, 0.27 to 0.28 (pieces / 100 m). This is presumably due to the fact that the flow velocity is slightly increased in the molten steel flow and the entrainment of the mold powder is promoted, but these were also sufficiently good quality slabs. In addition, when the cross section of the slab was examined for these slabs, a portion where the shell growth thickness in the mold was slightly thin was found, but it did not hinder the operation stability.

Although not described in this example, it was confirmed that the same tendency was observed when the molten steel discharge flow from the immersion nozzle was accelerated. Further, in the present example, the thickness of the cast slab slab is in the range of 220 to 300 (mm), the casting width is 1000 to 2000 (mm), and the molten steel throughput is in the range of 3.0 to 8.0. It has also been confirmed that the same effect as that obtained can be obtained. It has also been confirmed that the same tendency can be obtained when the molten steel discharge angle of the immersion nozzle is in the range of 15 to 35 (°). In addition, the shape of the discharge hole of the immersion nozzle and the inner diameter of the immersion nozzle are not limited only to the conditions described in the present embodiment, and any one can be used as long as it is within the range that can be assumed by those skilled in the art. It does n’t matter.

Using a slab continuous casting machine having a continuous casting mold 50 provided with a pair of magnetic poles 52 as shown in FIG. 8, a test for casting about 300 (ton) of molten aluminum killed steel was performed. The cast slab slab has a thickness of 260 (mm), a width of 1600 (mm), and a molten steel injection flow rate of 5.0 to 6.0 (ton / min). The discharge angle of the discharge hole of the used two-hole immersion nozzle is 25 (°) downward, and the immersion depth of the immersion nozzle (distance from the molten steel surface in the mold to the upper end of the discharge hole) is 180 (mm) or more and 300 ( mm). The shape of the discharge hole of the immersion nozzle is a square having a side length of 70 (mm), and the inner diameter of the immersion nozzle is 70 (mm). Argon gas was used as an inert gas blown from the immersion nozzle. The ratio A / P between the molten steel throughput P and the argon gas flow rate A was set to be in the range of 2.0 to 3.5 (NL / ton). The AC moving magnetic field strength is in the range of 300 to 1000 (Gs), the ratio X / W (Gs / mm) between the AC moving magnetic field strength X and the width W of the cast slab slab, and the discharge from the immersion nozzle Casting was carried out by changing the flow inclination angle α.

The cast slab slab is successively subjected to hot rolling, cold rolling, and alloyed hot dip galvanizing, and the surface defects of this hot galvanized steel sheet are continuously measured using an on-line surface defect meter. Among them, the defect appearance, SEM analysis, ICP analysis, etc. are used to discriminate steelmaking defects (defects caused by inclusions in the slab cast), and the alloyed hot-dip galvanized steel sheet is 100 (m) long. The number of defects per product (called “product defect index”) was evaluated. Table 4 shows the results of investigating the casting conditions and product defect index of Examples 50 to 71 of the present invention. Also in Table 4, the diagonal direction angle θ was calculated by rounding off the second decimal place. The value of “α−θ” is shown by rounding off the first decimal place. In Comparative Examples 33 to 37, the level in which the discharge hole is directed downstream of the swivel is also described. FIG. 25 is a diagram showing the relationship between “α−θ” and “product defect index” in inventive examples 50 to 71, with the ratio X / W value of 0.45 as a boundary.

Although not described in Table 4, the product defect index is 0.45 to 0.51 (pieces / 100 m) when the immersion nozzle discharge hole is directed to the short side without being inclined by the same flow control method. ).

In the inventive examples 50 to 60, the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45. As shown in FIG. 25, in this case, by setting “α-θ” within a range of −3 to 0 (°), the product defect index is particularly low, 0.18 to 0.20 (pieces / 100 m). I understood it. That is, when the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45, the inclination angle α (°) may be in the range of “θ−3” or more and “θ” or less. It turned out to be preferable.

On the other hand, in Invention Examples 61 to 71, the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55. In this case, it was found that by setting “α-θ” within the range of −6 to −4 (°), the product defect index was particularly low, 0.18 to 0.20 (pieces / 100 m). That is, when the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55, the inclination angle α (°) is set within the range of “θ-6” or more and “θ-4” or less. It turned out to be preferable. This is presumed to be a result of good control of the molten steel flow by setting an appropriate inclination angle according to the value of X / W. Other examples of the present invention were able to obtain sufficiently good quality slabs.

On the other hand, as shown in Comparative Examples 33 to 37, when the discharge flow was directed downstream of the swirl, the product defect index was remarkably high, 0.65 to 0.68 (pieces / 100 m). This is presumed to be a result of promoting collision / interference between the swirling flow and the reverse flow.

Although not described in the present embodiment, the slab cast cast has a thickness of 220 to 300 (mm), a casting width of 1000 to 2000 (mm), and a molten steel throughput of 3.0 to 8.0. It has also been confirmed separately that the same effects as those described in this example can be obtained. It has also been confirmed that the same tendency can be obtained when the molten steel discharge angle of the immersion nozzle is in the range of 15 to 35 (°). In addition, the shape of the discharge hole of the immersion nozzle and the inner diameter of the immersion nozzle are not limited only to the conditions described in the present embodiment, and any one can be used as long as it is within the range that can be assumed by those skilled in the art. It does n’t matter.

A test for casting about 300 (ton) of molten aluminum killed steel using a slab continuous casting machine having a continuous casting mold 1 having a pair of upper magnetic poles 6 and a pair of lower magnetic poles 7 as shown in FIG. Went. The cast slab slab has a thickness of 260 (mm), a width of 1000 to 1900 (mm), and a molten steel injection flow rate of 4.0 to 7.5 (ton / min). The discharge angle of the discharge hole of the two-hole immersion nozzle used (the angle when the horizontal is zero) is 25 ° downward, and the immersion nozzle immersion depth (distance from the molten steel surface in the mold to the upper end of the discharge hole) Is 180 (mm) or more and less than 300 (mm). The shape of the discharge hole of the immersion nozzle is a square having a side length of 80 (mm), and the inner diameter of the immersion nozzle is 80 (mm). Argon gas was used as the inert gas blown from the immersion nozzle. The ratio A / P between the molten steel throughput P and the argon gas flow rate A was set to be in the range of 2.0 to 3.5 (NL / ton).

The discharge direction of the discharge flow from the immersion nozzle is 3 in the upstream direction of the swirl flow formed by the AC moving magnetic field, the downstream side of the swirl flow, and the direction parallel to the mold long side (direction perpendicular to the mold short side). Kind. The inclination angle α was also changed when the discharge direction of the discharge flow was inclined to the upstream side of the swirl flow and the downstream side of the swirl flow. Further, casting was performed by changing the strengths of the AC moving magnetic field applied from the upper magnetic pole, the DC static magnetic field applied from the upper magnetic pole, and the DC static magnetic field applied from the lower magnetic pole.

The cast slab slab is successively subjected to hot rolling, cold rolling, and alloyed hot dip galvanizing, and the surface defects of this hot galvanized steel sheet are continuously measured using an on-line surface defect meter. Among them, the defect appearance, SEM analysis, ICP analysis, etc. are used to discriminate steelmaking defects (defects caused by inclusions in the slab cast), and the alloyed hot-dip galvanized steel sheet is 100 (m) long. The number of defects per product (called “product defect index”) was evaluated. Table 5 shows the examination results of the casting conditions and the product defect index in the inventive examples 72 to 79 and the comparative examples 38 to 46. In Table 5, the diagonal direction angle θ and the inclination angle α were calculated by rounding off the second decimal place.

As shown in Table 5, the strength of the AC moving magnetic field applied from the upper magnetic pole is 500 to 900 (Gs), the strength of the DC static magnetic field applied from the upper magnetic pole is 2000 to 3300 (Gs), and applied from the lower magnetic pole. Continuous casting was performed while controlling the strength of the DC static magnetic field within a range of 3000 to 4500 (Gs). Although not described in Table 5, if the strength of the DC static magnetic field applied from the upper magnetic pole and the lower magnetic pole is outside these ranges, confirm that the product defect index generally increases. Yes. Although not shown in Table 5, the product defect index is high even when the ratio X / W between the strength X of the AC magnetic field of the upper magnetic pole and the width W of the slab slab is less than 0.30. I have confirmed that.

Table 5 shows the diagonal direction angle θ calculated by the thickness D of the cast slab and the width W of the slab, and the inclination angle α of the discharge flow at the time of casting. The diagonal direction angle θ was calculated by rounding off the second decimal place.

In the present invention examples 72 to 79, the discharge flow from the immersion nozzle is inclined toward the upstream side of the swirl flow formed by the AC moving magnetic field. In this case, the product defect index was as low as 0.12 to 0.25 (pieces / 100 m), which was a favorable result.

On the other hand, in Comparative Examples 38 to 41, the discharge direction is not inclined. In this case, the product defect index was 0.35 to 0.42 (pieces / 100 m), which was higher than the inventive examples 72 to 79. In Comparative Examples 42 to 43, the discharge direction of the immersion nozzle is inclined toward the downstream side of the swirling flow formed by the AC moving magnetic field. In this case, the product defect index was 0.55 to 0.58 (pieces / 100 m), and a significant deterioration was confirmed. This is presumed to be a result of promoting collision / interference between the swirl flow and the reversal flow on the downstream side of the swirl flow.

In Examples 80 to 82 of the present invention, the ratio X / W (Gs / mm) between the strength X of the AC magnetic field of the upper magnetic pole and the width W of the slab slab is 0.55 or more. In this case, the product defect index was 0.30 to 0.32 (pieces / 100 m), which was slightly higher than those of Examples 72 to 79 of the present invention. This is considered to be due to the fact that the AC moving magnetic field strength X is too strong for the width W and the molten steel flow in the mold becomes somewhat unstable.

Although not described in this embodiment, the same effect as described in this embodiment can be obtained even when the thickness of the cast slab slab is in the range of 220 to 300 (mm). Separately confirmed. Further, the shape of the discharge hole of the immersion nozzle and the inner diameter of the immersion nozzle are not limited only to the conditions described in the present embodiment, and any one can be used as long as it is within the range that can be assumed by those skilled in the art. It does n’t matter.

As in Example 5, a test for casting about 300 (ton) of aluminum killed molten steel was performed using a slab continuous casting machine having a continuous casting mold 1 having two upper and lower magnetic poles as shown in FIG. It was. The slab slab to be cast has a thickness of 260 (mm), a width of 1600 to 1700 (mm), and a molten steel injection flow rate of 6.0 to 7.0 (ton / min). The discharge angle of the discharge hole of the used two-hole immersion nozzle is 25 (°) downward, and the immersion depth of the immersion nozzle (distance from the molten steel surface in the mold to the upper end of the discharge hole) is 180 (mm) or more and 300 ( mm). The shape of the discharge hole of the immersion nozzle is a square having a side length of 80 (mm), and the inner diameter of the immersion nozzle is 80 (mm). Argon gas was used as an inert gas blown from the immersion nozzle.

In Example 6, the discharge direction from the immersion nozzle was inclined to the upstream side of the swirling flow formed by the AC moving magnetic field. The strength of the AC moving magnetic field applied to the upper magnetic pole is 500 to 900 (Gs), the strength of the DC static magnetic field applied to the upper magnetic pole is 2000 to 3300 (Gs), and the strength of the DC static magnetic field applied to the lower magnetic pole is 3000. The ratio X / W (Gs / mm) between the AC moving magnetic field strength X and the width W of the cast slab slab and the inclination angle α of the discharge flow from the submerged nozzle are within the range of 4500 (Gs). The casting was carried out while changing.

The cast slab slab is successively subjected to hot rolling, cold rolling, and alloyed hot dip galvanizing, and the surface defects of this hot galvanized steel sheet are continuously measured using an on-line surface defect meter. Among them, the defect appearance, SEM analysis, ICP analysis, etc. are used to discriminate steelmaking defects (defects caused by inclusions in the slab cast), and the alloyed hot-dip galvanized steel sheet is 100 (m) long. The number of defects per product (called “product defect index”) was evaluated. Table 6 shows the results of investigating the casting conditions and product defect index in Examples 80 to 103 of the present invention. In Table 6, the value of the diagonal direction angle θ is calculated by rounding off the first decimal place, and the value of “α−θ” is calculated by rounding off the first decimal place. FIG. 26 is a diagram showing the relationship between “α−θ” and “product defect index” in Invention Examples 83 to 106, with the ratio X / W value of 0.45 as a boundary. .

In Invention Examples 83 to 94, the ratio X / W (Gs / mm) is set to 0.30 or more and less than 0.45. As shown in FIG. 26, in this case, by setting “α-θ” within the range of −2 ° to 5 °, the product defect index is particularly low at 0.13 to 0.15 (pieces / 100 m). I found out that That is, when the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45, the inclination angle α (°) may be set within the range of “θ−2” or more and “θ + 5” or less. It turned out to be preferable.

On the other hand, Examples 95 to 106 of the present invention are conditions where the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55, and at this time, “α-θ” is −5 to 2 (°) Within the range, the product defect index was found to be particularly low, 0.13 to 0.15 (pieces / 100 m). That is, when the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55, the inclination angle α (°) is preferably in the range of “θ−5” or more and “θ + 2” or less. I understood it.

In Table 6, only the thickness and width of the slab cast are within a certain range. For example, the thickness of the slab cast is within a range of 220 to 300 (mm). It has been confirmed that there is an equivalent effect within the range of 1000 to 2000 (mm).

DESCRIPTION OF SYMBOLS 1 Mold for continuous casting 2 Mold long side 3 Mold short side 4 Immersion nozzle 5 Discharge hole 6 Upper magnetic pole 7 Lower magnetic pole 8 Molten steel 9 Solidified shell 10 Molten steel surface in mold 11 Discharge flow 12 AC moving magnetic field generation coil 13 DC static magnetic field generation Coil 14 DC static magnetic field generating coil 15 Swirling flow 16 Reversing flow 17 Low flow velocity region 18 Vortex flow 19 Downflow 20 Continuous casting mold 30 Continuous casting mold 40 Continuous casting mold 42 Linear type moving magnetic field generator 50 Continuous casting mold 52 Magnetic pole

Claims

Place an immersion nozzle in the continuous casting mold,
A continuous casting method for supplying molten steel to the immersion nozzle and casting the molten steel,
The immersion nozzle has a pair of discharge holes arranged symmetrically with respect to the vertical axis, and the immersion nozzle has an immersion depth (distance from the molten steel surface in the mold to the upper end of the discharge hole) of 180 mm or more and 300 mm. Less than, the molten steel discharge angle from the horizontal direction of the discharge hole in the range of 15 to 35 °, the flow rate A (NL / min) of the inert gas blown between the tundish outflow hole and the discharge hole, and the molten steel throughput The ratio A / P to P (ton / min) is in the range of 2.0 to 3.5 NL / ton, and the discharge direction of the immersion nozzle is parallel to the long side of the mold through the center of the vertical axis of the immersion nozzle. A continuous casting method of a slab slab that is inclined within the range of the expression (1) with respect to a simple reference plane.
θ−6 ≦ α ≦ θ + 10 (1)
However, in the formula (1), α is an inclination angle (°) with respect to the reference surface in the discharge direction of the immersion nozzle when the continuous casting mold is viewed from above.
θ is an angle (acute angle) formed by a straight line from the vertical axis center of the immersion nozzle to the contact point between the long side of the mold and the short side of the mold and the reference plane when the continuous casting mold is viewed from above. And an angle (°) defined by the following equation (2).
tan θ = (D / 2) / (W / 2) (2)
However, in Formula (2), D is the thickness (mm) of the slab slab continuously cast,
W is the width (mm) of the continuously cast slab slab.
Measure the α during continuous casting or after completion of mold width change during continuous casting,
The continuous casting method of a slab slab according to claim 1, wherein the discharge direction of the immersion nozzle is changed so as to satisfy the expression (1) when the α does not satisfy the expression (1).
On the back surface of the mold long side, a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the mold long side are arranged,
The discharge hole is positioned between the position of the maximum value of the DC static magnetic field applied from the upper magnetic pole and the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole, and the upper magnetic pole and the lower magnetic pole In order to brake the molten steel flow by applying a DC static magnetic field at
When the strength of the DC static magnetic field applied from the upper magnetic pole is 1500 Gs or more and less than 2500 Gs (Gauss; 1 Gs = 10 −4 T), the discharge direction of the immersion nozzle is expressed by the formula (1) with respect to the reference plane. Instead, it is tilted within the range of the formula (3), and when the strength of the DC static magnetic field is 2500 Gs or more and less than 3500 Gs, it is tilted within the range of the formula (4) instead of the formula (1). Continuous casting method for slab slabs.
θ ≦ α ≦ θ + 5 (3)
θ + 6 ≦ α ≦ θ + 10 (4)
Measure the α after completion of mold width change during continuous casting or during continuous casting,
When the intensity of the DC static magnetic field is 1500 Gs or more and less than 2500 Gs, and the α does not satisfy the equation (3), the discharge direction of the immersion nozzle is changed so as to satisfy the equation (3),
The discharge direction of the submerged nozzle is changed to satisfy the formula (4) when the strength of the DC static magnetic field is 2500 Gs or more and less than 3500 Gs and the α does not satisfy the formula (4). The continuous casting method of the slab slab as described.
Provided on the back side of the long side of the mold is a linear type moving magnetic field generator in which the moving direction of the magnetic field is the mold width direction,
Applying a moving magnetic field from the short side of the mold toward the immersion nozzle to give a braking force to the molten steel flow discharged from the immersion nozzle, or from the immersion nozzle side to give an acceleration force to the molten steel flow The flow control is performed by applying a moving magnetic field toward the side,
The continuous casting method of a slab cast piece according to claim 1, wherein the discharge direction of the immersion nozzle is inclined with respect to the reference plane within the range of the formula (5) instead of the formula (1).
θ + 2 ≦ α ≦ θ + 7 (5)
Measure the α after completion of mold width change during continuous casting or during continuous casting,
The continuous casting method of a slab slab according to claim 5, wherein the discharge direction of the immersion nozzle is changed so as to satisfy the expression (5) when the α does not satisfy the expression (5).
On the back surface of the mold long side, a pair of magnetic poles facing each other with the mold long side interposed therebetween are arranged,
By applying an AC moving magnetic field from the magnetic pole to give horizontal swirl stirring to the molten steel,
A ratio X / W (Gs) between the strength X (Gs) of the AC moving magnetic field and the width W (mm) of the continuously cast slab slab after setting the strength of the AC moving magnetic field within the range of 300 to 1000 Gs. / Mm) is 0.30 or more and less than 0.45, the discharge direction of the immersion nozzle is directed toward the upstream side of the swirl flow formed by the AC moving magnetic field with respect to the reference plane (1 When the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55, the formula (7) is substituted for the formula (1) instead of the formula (6). The continuous casting method of a slab slab according to claim 1, wherein the slab slab is inclined within the range.
θ-3 ≦ α ≦ θ (6)
θ-6 ≦ α ≦ θ-4 (7)
Measure the α after completion of mold width change during continuous casting or during continuous casting,
When the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45, and the α does not satisfy the expression (6), the discharge direction of the immersion nozzle is set so as to satisfy the expression (6). change,
When the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55 and the α does not satisfy the equation (7), the submerged nozzle is discharged so as to satisfy the equation (7). The continuous casting method for a slab cast according to claim 7, wherein the direction is changed.
On the back surface of the mold long side, a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the mold long side are arranged,
The discharge hole is positioned between the position of the maximum value of the DC static magnetic field applied from the upper magnetic pole and the position of the maximum value of the DC static magnetic field applied from the lower magnetic pole,
A DC static magnetic field and an AC moving magnetic field are applied in a superimposed manner from the upper magnetic pole, and a swirling flow of the molten steel rotating in the horizontal direction is formed on the molten steel surface in the mold by the AC moving magnetic field applied from the upper magnetic pole, The molten steel flow is braked by a DC static magnetic field applied from the upper magnetic pole, and the molten steel flow is braked by a DC static magnetic field applied from the lower magnetic pole,
The AC moving magnetic field is applied from the lower magnetic pole within the range of 500 to 900 Gs (Gauss; 1 Gs = 10 −4 T), and the DC static magnetic field applied from the upper magnetic pole is within the range of 2000 to 3300 Gs. The ratio X / W (Gs) of the strength of the alternating-current moving magnetic field X (Gs) and the width W (mm) of the continuously cast slab slab is set within the range of 3000 to 4500 Gs. / Mm) to 0.30 or more and less than 0.55, and
The continuous casting method of a slab slab according to claim 1, wherein the discharge direction of the immersion nozzle is inclined toward the upstream side of the swirling flow of the molten steel formed by the AC moving magnetic field with respect to the reference surface.
When the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45, the discharge direction of the immersion nozzle is changed to the following formula (8) instead of the formula (1) with respect to the reference plane. Tilt within range,
In the case where the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55, the discharge direction of the immersion nozzle is changed to the following formula (9) instead of the formula (1) with respect to the reference plane. The method for continuously casting a slab slab according to claim 9, wherein the slab slab is inclined within a range.
θ-2 ≦ α ≦ θ + 5 (8)
θ-5 ≦ α ≦ θ + 2 (9)
Measure the α after completion of mold width change during continuous casting or during continuous casting,
When the ratio X / W (Gs / mm) is 0.30 or more and less than 0.45, and the α does not satisfy the equation (8), the submerged nozzle is discharged so as to satisfy the equation (8). Change direction,
When the ratio X / W (Gs / mm) is 0.45 or more and less than 0.55 and the α does not satisfy the equation (9), the submerged nozzle is discharged so as to satisfy the equation (9). The continuous casting method of a slab cast according to claim 10, wherein the direction is changed.