JP3399627B2 - Flow control method of molten steel in mold by DC magnetic field - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel, in which a DC magnetic field is applied to a mold to control the flow of molten steel in the mold. The present invention relates to a flow control method for inner molten steel. [0002] Conventionally, for example, in the field of continuous casting of steel, it has been known that the flow of molten steel in a mold greatly affects the operability of continuous casting and the quality of slab. That is, the flow of molten steel injected into the mold from the injection nozzle brings slag-based or deoxidized oxide-based inclusions interposed in the molten steel deep below the strand pool. Inclusions are more likely to be trapped in the solidified shell and the slab defects occur as the depth of the molten steel increases, so it is desirable that the depth of penetration of the downward flow of the molten steel be as small as possible. On the other hand, on the molten steel surface, when the molten steel flow velocity at the meniscus is high, as in the case of high-speed casting, the powder on the molten steel surface is caught in the molten steel, and in addition to the increase in inclusions in the molten steel, the molten steel surface Since the level fluctuation becomes large and the solidified shell cannot be generated stably,
As the quality of the slab decreases, the operability of casting decreases. [0004] When the flow velocity at the meniscus is low, as in the case of low-speed casting, deckles are formed on the surface of molten steel, which hinders the casting operation, and inclusions and air bubbles are trapped in the solidified shell, resulting in cleaning. Therefore, it is inevitable that the quality of the slab is reduced as well as the operability of the casting. From such a viewpoint, in both the high-speed casting and the low-speed casting, there is a demand for controlling the flow pattern of the molten steel injected from the injection nozzle into the casting mold to a preferable state and to keep the flow pattern constant. Since it is difficult to control the flow pattern of the molten steel in the mold by adjusting the shape and depth of the nozzle, a method of controlling the flow of the molten steel in the mold using a DC magnetic field has conventionally been used. Proposed. For example,
No. 20349 relates to a method for controlling the flow of molten steel in a mold using a DC magnetic field. In this method, a direct current magnetic field is applied to a part of the main flow path of the molten steel s discharged from the immersion nozzle a, thereby decelerating the main flow sm of the molten steel, and controlling the descending flow entering deep into the strand pool p. , The main flow is divided into small flows so, st, etc., with the aim of stirring the molten steel inside the pool p. However, in this method, since a DC magnetic field is formed in a part of the width direction of the mold m (FIG. 5), the discharge flow from the immersion nozzle a may bypass the brake zone. That is, a flow (downward flow) si from the weak brake position to the lower part of the pool p is generated, and the inclusions are removed from the pool p.
In addition to being brought deep, this phenomenon is not stable, so that the flow in the mold becomes unstable, and there has been a problem that the stirring above the pool p is not stable. For this reason, it could not be a technique for improving cast slab quality. [0007] Also, Japanese Patent Application Laid-Open No. 2-284750 discloses that
In this method, a DC magnetic field is applied to the entire area in the mold width direction.This technology can brake the flow below the brake zone, but applies a DC magnetic field to the place where braking is desired, and also brakes the meniscus flow velocity. Figure 6
As shown in (a), (b) and FIG. 7, a DC magnetic field had to be applied to the entire mold m, or a two-stage DC magnetic field had to be applied. Further, although a method of applying a DC magnetic field below the discharge hole of the immersion nozzle is also disclosed in this publication, the controllability of the flow in the pool is increased by the solidified shell thickness of the portion to which the DC magnetic field is applied as described later. Although different, there is no statement on that point,
It was an unstable technology. The reason for this will be described by taking as an example a case where a DC magnetic field is applied below a position where the main flow of molten steel from the immersion nozzle a collides with the short sides ma and mb of the mold m (FIG. 8).
In this case, after the discharge flow from the immersion nozzle c collides with the short side ma, the descending flow crosses the DC magnetic field zone. At that time,
Since a current is induced from one short side ma to the other short side mb in the pool, a Lorentz force acts on the bulk pool to suppress the inflow of the discharge flow downward. However, in the vicinity of the short sides ma and mb, the current is divided into those flowing in the molten steel and those flowing in the solidified shell. If most of the current flows in the molten steel, the direction of the current changes greatly in three dimensions. This means that the direction in which the Lorentz force acts changes three-dimensionally, and the brake of the downflow becomes weak near the short sides ma and mb. As a result, as shown in FIG. 8B, the descending flow of the molten steel becomes large and non-uniform in the vicinity of the short sides ma and mb where the braking force is weak.
This phenomenon is not stable, and the flow of molten steel in the mold becomes unstable. SUMMARY OF THE INVENTION The present invention relates to a method for continuously casting steel by applying a DC magnetic field having a substantially uniform magnetic flux density distribution in a width direction of a mold in a thickness direction to continuously flow molten steel in the mold. A method of controlling the flow of molten steel in a mold by a DC magnetic field that can improve the quality of the obtained slab by stabilizing the flow pattern of the molten steel in the mold with the aim of optimizing the position where a DC magnetic field is applied when controlling I will provide a. According to the present invention, a DC magnetic field having a substantially uniform magnetic flux density distribution in the width direction of a mold is applied in the thickness direction of the mold, and the flow of molten steel injected into the mold is controlled. In the method for controlling the flow of molten steel in a mold by a DC magnetic field to be controlled, the DC magnetic field is applied to a part of a region where the thickness of a solidified shell generated in the mold is 17 to 40 mm, and the molten steel flow traverses this DC magnetic field zone. in Rukoto the return path of the induced current is formed on the solidified shell when, in a molten steel pool
Is a method for controlling the flow of molten steel in a mold by a direct current magnetic field, wherein a flow of molten steel is controlled by inducing a current in one direction . According to the present invention, the solidification shell thickness is 17 to 4 in order to control the flow of molten steel in the mold during continuous casting of steel.
A DC magnetic field having a uniform magnetic flux distribution in the width direction is applied in the thickness direction to a part of the region having a thickness of 0 mm. In this way, the direction in which the current flows in the vicinity of the short side is prevented from changing three-dimensionally, and a current in one direction is induced in the molten steel pool, thereby forming a return path of the induced current in the solidified shell. As a result, the flow of the molten steel in the mold can be efficiently and effectively made into a plug flow, and the quality of the obtained slab can be improved and stabilized. The position where the DC magnetic field is applied in the present invention is as follows.
It means the upper end position of the electromagnetic coil. In addition, the present invention is a continuous casting method using an injection nozzle having a rectangular mold as in the case of continuous casting of a slab-shaped cast piece, in which a discharge port is inclined downward toward the short side of the mold. It is more suitable to apply to. The present inventors have conducted various experiments on optimum conditions for applying a DC magnetic field to a mold during continuous casting of steel to efficiently and effectively plug flow the molten steel in the mold to stably produce a solidified shell. As a result of repeated studies, it was found that the quality of the slab also changes when the position where the DC magnetic field is applied is changed. In order to make this finding more reliable, the present inventors used a half-size mercury model experimental machine equipped with a DC magnetic field generator to reduce the solidified shell thickness. An experiment was conducted in which the change was replaced with a change in the thickness of the container corresponding to the solidified shell, mercury was supplied into the container, and a change in the mercury flow rate distribution near the container side wall was performed. FIG.
Shows the relationship between the vessel thickness obtained in this experiment and the velocity index near the inner wall of the vessel, and also shows a plug flow line (ideal, completely uniform downward flow). Here, the flow velocity index in the vicinity of the side wall is a value obtained by dividing the descending flow velocity by (discharge amount from nozzle / horizontal sectional area of the pool). Indicates that a flow line is obtained. Than this,
There is a close relationship between the vessel thickness and the flow velocity index near the container side wall, and when the container thickness is small, the flow velocity index near the side wall is large, but by setting the container thickness to a certain thickness or more, the side wall It can be seen that the flow velocity index in the vicinity can be approximated to the plug flow. In addition, when the current induced in the pool was measured for this phenomenon, the direction of the current changed three-dimensionally near the side wall when the container thickness was thin, but when the container thickness was more than a certain thickness, change of the direction of the current at the side wall near the not observed, in the pool, it was found that one direction of the current from one short side toward the other short side is induced. Next, an experiment was conducted by changing the position where a DC magnetic field was applied in actual continuous casting while changing the position at which the DC magnetic field was applied. There is a relationship as shown in FIG. From this, it was found that when the position where the DC magnetic field was applied was set to a position where the thickness of the solidified shell was 17 mm or more, slab defects were reduced. However, when a DC magnetic field is applied at a position where the thickness of the generated solidified shell is 40 mm or more, the solidified shell advances more than 1/3 of the total thickness of the slab, and the DC magnetic field controls the flow of molten steel in the mold. As a result, it does not exert a remarkable effect and loses its significance. The present invention has been obtained based on these findings. In the case of controlling the flow of molten steel by applying a DC magnetic field in a continuous casting method, the position at which the DC magnetic field is applied is determined by the thickness of the solidified shell. Are specified in a part of the area of 17 to 40 mm. An embodiment in which the present invention is applied to a continuous casting method of molten steel will be described below together with an example of an apparatus. In FIG. 3, reference numeral 1 denotes a mold, and a dipping nozzle 3 for injecting molten steel s from a tundish 2 is provided at a central portion in the mold. This immersion nozzle is of a closed bottom type, and a pair of discharge ports 3a, 3 inclined at about 45 ° downward at the lower end toward the short side 1b of the mold.
b is provided. On the outer periphery of the long side 1a of the mold below the immersion nozzle 3, an electromagnetic coil 4 for controlling the flow pattern of the molten steel in the mold by applying a DC magnetic field to the mold is provided. A DC magnetic field generator 6 comprising a U-shaped iron core 5 having a web portion on one short side is disposed so as to be vertically adjustable. In the present invention, the position where the electromagnetic coil 4 of the DC magnetic field generator 6 is disposed (the upper end of the electromagnetic coil is located at a part of the region where the thickness of the solidified shell of molten steel in the mold is 17 to 40 mm) It is necessary to confirm the position of this solidified shell thickness region. However, this position can be confirmed by actual measurement (for example, determining whether it is a sulfur point in a mold, The change in the solidification shell thickness in the casting length direction can be determined in advance by a method based on measurement.If this arrangement condition is not satisfied, the flow pattern of molten steel in the mold 1 during continuous casting cannot be effectively controlled, The formation of a solidified shell of molten steel in a mold becomes unstable, the operability of casting is reduced, and the quality of slabs is reduced. This is a case where the present invention is applied to a continuous casting method using a continuous casting apparatus having such a configuration: Casting conditions Cast slab: slab having a cross section (horizontal cross section) having a width of 1120 mm and a thickness of 254 mm Slab material: Low-carbon aluminum killed steel Mold: Vertical part: 2.4 m Casting temperature: 1560-1580 ° C Casting speed: 1.5 m / min Molten steel surface level: 100 mm downward from the top of the mold Top of the discharge hole of the immersion nozzle Position: A position 300 mm downward from the upper end of the mold In this experimental example, the thickness distribution of the generated solidified shell in the vertical direction on the short side of the mold was determined from the above casting conditions, and the upper end of the electromagnetic coil in the DC magnetic field generator was solidified. The shell was positioned at a position where the shell thickness was 17 mm, and continuous casting was performed while applying a DC magnetic field having a magnetic flux density of 3000 Gauss in the thickness direction of the mold. Magnetic coil height 200mm
Was used. The results are shown in FIG. 4 together with a conventional example in which no DC magnetic field is applied and a comparative example in which a DC magnetic field is applied at a position outside the scope of the present invention. As shown in this figure, in the embodiment of the present invention, the flow of the molten steel in the mold can be stabilized (uniform) in each stage as compared with the conventional example and the comparative example. According to the present invention, in order to control the flow of molten steel in a mold during continuous casting of steel, a DC magnetic field applied to the mold is applied to a part of the region having a solidified shell thickness of 17 to 40 mm. In the molten steel pool, a unidirectional current is induced, and a solidified shell forms a return path for the induced current, so that the flow of the molten steel in the mold can be efficiently and effectively plug-flowed, and the resulting slab is obtained. Quality can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing a relationship between a flow velocity index near a vessel and a vessel thickness to which a DC magnetic field is applied obtained from a model experiment using mercury and a vessel thickness. FIG. FIG. 3 is an explanatory view showing the relationship between the thickness of a solidified shell in a mold to which a DC magnetic field is applied and a slab defect index. FIG. 3 (a) is a schematic longitudinal sectional view showing an example of a continuous casting facility for carrying out the present invention. FIG. 4 (b) is a schematic cross-sectional view taken along the line Aa-Ab of FIG. 4 (a). FIG. 4 (a) is a flow (downflow, upflow) of molten steel in a mold to which a DC magnetic field is applied in the present invention. ) Explanatory diagram showing the pattern, (b) is an explanatory diagram showing the flow pattern of the molten steel in the conventional mold to which no DC magnetic field is applied, and (c) is a flow pattern of the molten steel in the mold to which a DC magnetic field is applied (comparative example). FIG. 5 is an example of a conventionally known continuous casting facility for applying a DC magnetic field to a mold. FIG. FIGS. 6A and 6B show another example of a conventionally known continuous casting facility for applying a DC magnetic field to a mold, in which FIG. 6A is a cross-sectional view and FIG. FIG. 7 is an explanatory longitudinal sectional view showing another example of continuous casting equipment for applying a DC magnetic field to a conventionally known mold. FIG. 8 (a) shows a conventional mold in which a DC magnetic field is applied;
FIG. 3 is an explanatory longitudinal sectional view showing an example of a continuous casting facility. (B)
(A) Explanatory drawing which shows the example of the flow pattern of the molten steel in a mold in the continuous casting equipment example of a figure. [Description of Signs] 1 Mold 1a Long side 1b Short side 2 Tundish 3 Immersion nozzle 3a, 3b Discharge hole of injection nozzle 4 Electromagnetic coil 5 Iron core 6 DC magnetic field generator 7 DC power supply s Molten steel s Solidified shell sm Main stream of molten steel so , St Split flow si Downflow a Immersion nozzle ad Magnetic pole c Electromagnetic coil f Iron core m Mold ma, mb (mold) Short side p Pool po Mainstream collision position of molten steel

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-271035 (JP, A) JP-A-2-117756 (JP, A) JP-A-4-344858 (JP, A) JP-A-5- 96349 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/18 B22D 11/04 311 B22D 11/115

Claims (1)

(57) [Claim 1] A DC magnetic field having a substantially uniform magnetic flux density distribution in the width direction of the mold is applied in the thickness direction of the mold to control the flow of molten steel injected into the mold. In the flow control method of molten steel in a mold by a DC magnetic field, the DC magnetic field is added to a part of a region where the thickness of a solidified shell generated in the mold is 17 to 40 mm, and when the molten steel flow crosses the DC magnetic field zone, A return path for the induced current is formed in the solidified shell.
A method for controlling the flow of molten steel in a mold by a DC magnetic field, wherein a current in one direction is induced in the molten steel pool to control the flow of the molten steel.