JP2001047203A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for steel, with which a cast slab having good quality can be obtd. without clogging an immersion nozzle when the extra-low carbon steel, etc., is cast. SOLUTION: Respective pairs of static magnetic field molten steel fluid control devices, in which inert gas is blown into the molten steel at 1.0-2.5 Nl/ton of molten steel, of flowing quantity and the opposited set interposing the mold is made to one pair at the outside of two long sides in the mold, are disposed, each one pair, near a meniscus (magnetic field intensity B1), near spouting holes in the immersion nozzle (magnetic field intensity B2) and near the lower end of the mold (magnetic field intensity B3). The magnetic field intensities B1, B2 and B3 are acted as the electromagnetic force so as to satisfy the formula: B:B2:B3=0-1:1:1-3 with the B2 as the reference. Wherein, 500 (Gauss)<=B2<=1000 (Gauss) and the magnetic field impressing direction of B2 is made to the reverse direction to the B1 and the B3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting ultra-low carbon steel which can obtain a slab having good surface quality without clogging of a nozzle during casting.

[0002]

2. Description of the Related Art Ultra-low carbon steel is used for steel sheets which are required to have few surface defects and to be excellent in formability, for example, automotive exterior steel sheets. In the ladle of ultra-low carbon steel that has been deoxidized with Al after decarburization under vacuum, Al in molten steel and FeO and MnO in slag, etc., until the subsequent continuous casting is completed. Reacts with a lower oxide of Al to easily generate an oxide of Al (Al ₂ O ₃ ).

This oxide of Al causes the immersion nozzle to close during continuous casting, and does not remove Al ₂ O from the molten steel in the tundish or mold during continuous casting. They remain as O ₃ -based nonmetallic inclusions and may cause surface defects of the steel sheet.

[0004] In order to prevent the clogging of the immersion nozzle, it is common practice to blow an inert gas into the molten steel between the molten steel outlet of the tundish and the discharge hole of the immersion nozzle. For example, in JP-A-2-37948,
A method has been proposed in which Ar gas is blown into molten steel passing through an immersion nozzle at a blowing flow rate according to a casting speed. In Japanese Patent Application Laid-Open No. Hei 9-192803, a moving magnetic field applied outside a mold is arranged so that a floating amount of an inert gas blown in molten steel in the mold is the same on both sides of an immersion nozzle. It has been proposed to control the flow of molten steel using an apparatus. In either method, the flow rate of the inert gas to be blown is as large as about 3 to 5 liters / ton of molten steel, which is effective in preventing the clogging of the immersion nozzle. However, in some cases, cellular defects occur in the surface layer of the slab. is there.

On the other hand, the flow of molten steel in the mold is damped for the purpose of preventing accumulation of nonmetallic inclusions and occurrence of bubble defects in the surface layer of the slab, and prevention of breakout during high-speed casting. Means are taken to do so. For example, Japanese Patent Application Laid-Open
According to Japanese Patent No. -10290, a static magnetic field applying device is arranged at three stages of an upper stage, a middle stage and a lower stage in a height direction of a mold, and the strength of the electromagnetic force of each stage is adjusted to control the flow of molten steel in the mold. Braking methods have been proposed.

However, in the method disclosed in Japanese Patent Application Laid-Open No. H11-10290, depending on the flow rate of the inert gas to be blown,
No consideration is given to the change in the flow of the molten steel in the vicinity of the meniscus. In other words, the strength of the static magnetic field acting on the flow of molten steel near the meniscus from the short side of the mold toward the direction of the immersion nozzle is strong, and at this time, an inert gas is blown at a large flow rate into the molten steel passing through the immersion nozzle And the flow of molten steel due to the bubbles of the inert gas is formed,
The flow of the molten steel in the vicinity of the meniscus may partially stagnate, or the molten steel surface may be wavy.

If the flow of molten steel near the meniscus stagnates, nonmetallic inclusions tend to accumulate on the surface layer of the slab, and the temperature of the molten steel in the stagnant portion decreases, so that molten slag (molten mold powder) Flow may be non-uniform, which may lead to cracks on the slab surface. Also, when the molten steel surface is wavy, molten slag and unmelted mold powder are caught in the molten steel in the mold, and powder defects are likely to be generated on the surface layer of the slab.

[0008]

As described above, if the flow rate of the inert gas is not appropriate, the clogging of the immersion nozzle cannot be prevented. Powder defects, bubble defects, and the like are likely to occur in the portion. Also, in the method of applying the electromagnetic force of the upper, middle and lower three static magnetic fields to the molten steel in the mold to brake the flow of the molten steel, the flow rate of the inert gas and the magnetic field strength of the static magnetic field are appropriately adjusted. Otherwise, accumulation of non-metallic inclusions and powder defects on the surface layer of the slab,
Further, cracks and the like easily occur on the surface of the slab.

According to the present invention, when casting ultra-low carbon steel or the like, a stable casting operation can be performed without clogging of the immersion nozzle, and a powder in which mold powder is wrapped around the surface layer of a slab. Slab of good quality with no porosity defects and no porosity defects trapping air bubbles of inert gas, no accumulation of non-metallic inclusions such as Al ₂ O _3, and no cracks on the slab surface It is an object of the present invention to provide a continuous casting method for steel capable of obtaining the following.

[0010]

The gist of the present invention resides in a continuous casting method characterized by the following (a) to (d).

(A) A method for continuous casting of molten steel using a mold for slabs of rectangular cross section and an immersion nozzle.

(B) Between the molten steel outlet of the tundish and the discharge hole of the immersion nozzle, an inert gas is introduced into the molten steel.
Blow at a flow rate of 0 to 2.5 Nl / ton of molten steel.

(C) A static magnetic field molten steel flow braking device having a pair of sets opposite to each other with the mold interposed outside the two long sides of the mold is provided near the meniscus with the immersion nozzle interposed therebetween, in both short side directions of the mold. Each pair is provided in the vicinity of the discharge flow immediately after flowing out of each discharge hole of the immersion nozzle (hereinafter, referred to as the vicinity of the discharge hole) and near the lower end of the mold with an extension of the immersion nozzle interposed therebetween.

(D) The magnetic field strengths of the molten steel flow braking devices in the vicinity of the meniscus, in the vicinity of the discharge hole, and in the vicinity of the lower end of the mold are B1, B2, and B3, respectively, and these magnetic field strengths B1, B2, and B3 are B2 The electromagnetic force is applied so as to satisfy the following equation (A) with reference to

B1: B2: B3 = 0 to 1: 1: 1 to 3 (A) Here, 500 (Gauss) ≦ B2 ≦ 1000 (Ga
uss) B2 magnetic field application direction: opposite direction to B1 and B3 The present inventors described the above-mentioned object of the present invention as follows:
Based on the knowledge of the above, and by taking the following measures. In the method of the present invention, the number of discharge holes is two,
An immersion nozzle is used in which each discharge flow is directed to the short side of the mold. The angle of the discharge hole is not particularly limited.
A general downward angle of 30 to 45 ° may be used.

The flow of the molten steel in the vicinity of the meniscus is a flow from the short side of the mold toward the immersion nozzle due to the influence of the discharge flow. However, when the inert gas is blown into the molten steel passing through the immersion nozzle, it is necessary to consider the flow of the molten steel near the meniscus formed by the inert gas. That is, a part of the bubble of the inert gas blown into the molten steel passing through the immersion nozzle immediately floats in the molten steel from near the discharge hole. The floating bubbles form a flow of molten steel from the periphery of the immersion nozzle toward the short side of the mold near the meniscus.

Therefore, when the inert gas is blown into the molten steel passing through the immersion nozzle or the like and when the electromagnetic force of the static magnetic field is applied to the molten steel near the meniscus, the flow rate and the magnetic field strength of the blown inert gas are reduced. At the same time, it is necessary to set the range of appropriate conditions. In other words, without considering the flow rate of the inert gas, when applying the electromagnetic force of the static magnetic field to brake the flow of the molten steel from the short side of the mold to the immersion nozzle, the molten steel formed by the inert gas In addition to the damping effect of the flow, the flow of molten steel from the short side of the mold toward the immersion nozzle may be excessively damped. If the flow of molten steel from the short side of the mold to the immersion nozzle is excessively damped, the effect of washing out bubbles and oxides adhering to the surface of the solidified shell on the molten steel side is reduced, and cellular defects on the surface layer of the slab are reduced. Or accumulate non-metallic inclusions.

In the method of the present invention, an inert gas is blown into the molten steel at a flow rate of 1.0 to 2.5 Nl / ton of molten steel between the molten steel outlet of the tundish and the discharge hole of the immersion nozzle. , The upper, middle,
The respective magnetic field strengths of the molten steel flow braking device of the static magnetic field arranged at the lower stage are made to act so as to satisfy the above-mentioned expression (A). Thereby, the flow of the molten steel from the short side of the mold near the meniscus toward the immersion nozzle can be appropriately braked. For this reason, it is possible to prevent the occurrence of bubble defects and powder defects in the surface layer of the slab, the accumulation of nonmetallic inclusions, and the occurrence of cracks on the slab surface.

In the method of the present invention, the lower limit of the flow rate of the inert gas blown into the molten steel is set to 1.0 Nl / ton of molten steel, so that clogging of the immersion nozzle can be prevented.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram schematically showing an example of an apparatus for carrying out the method of the present invention. The molten steel 12
Upper nozzle refractory 2, which is an injection port from the tundish 1 to the mold, a sliding plate 3 for controlling the flow rate of molten steel
(Shown in the case of a three-layer plate in FIG. 1), immersion nozzle 4
(Indicating a normally used two-hole nozzle), and is injected into the mold from the discharge hole 5.

FIG. 1 shows an example in which a sliding plate is used for controlling the flow rate of molten steel, but a rod type stopper may be used. Inert gas is preferably blown into molten steel from an inert gas injection port 10 formed of a porous refractory, a steel thin tube, or the like disposed in the upper nozzle refractory 2, the sliding plate 3, or the immersion nozzle 4. FIG. 1 shows an example of blowing from the sliding plate 3-1.

FIG. 2 is a view showing a horizontal section at each position in FIG. FIGS. 2A, 2B, and 2C schematically show horizontal cross sections taken along lines A1-A2, B1-B2, and C1-C2 in FIG. 1, respectively. Outside the long side of the mold, a molten steel flow braking device 9 of a static magnetic field having a pair of pairs facing each other across the mold is provided in the vicinity of the meniscus 9-1 across the immersion nozzle, in both short side directions of the mold. A pair of 9-2 is disposed near each discharge hole of the opening immersion nozzle, and a pair of 9-3 is disposed near the lower end of the mold with an extension of the immersion nozzle interposed therebetween.

FIG. 3 is a diagram schematically showing the flow of molten steel in a mold. After colliding with the solidified shell 11 on the short side, the discharge flows 6, 6 rise, and the descending flows 7-1, 7-1 and 7-2, 7-
Branch to 2. After the ascending flow reaches the molten steel surface, the molten steel flows 7-3, 7-3 from the short side of the mold toward the immersion nozzle.
Becomes On the other hand, a part of the bubble of the injected inert gas immediately floats in the molten steel from the vicinity of the discharge hole, and the floating bubble causes the molten steel surface to move from the immersion nozzle toward the short side of the mold from the immersion nozzle. Streams 8, 8 are formed. Also, the flow of molten steel from the short side of the mold toward the immersion nozzle 7-
3, 7-3 is in front of the immersion nozzle, sneaks into the molten steel,
Form a circulating flow. These flows of molten steel are formed on the left and right sides of the immersion nozzle.

In the method of the present invention, the upward flow after the discharge flow collides with the short side of the slab is generated by the electromagnetic force applied by using the molten steel flow braking device 9-1 of the static magnetic field disposed on the upper stage of the mold. The flow 7-3 of the molten steel from the short side of the mold formed upon reaching the surface of the molten steel toward the immersion nozzle is damped. The flow velocity of the discharge flows 6, 6 is braked by the electromagnetic force applied by using the molten steel flow braking device 9-2 of the static magnetic field arranged in the middle stage of the mold. The downward flow 7-2, 7-2 after the discharge flow collides with the short side of the slab is braked by the electromagnetic force applied by using the molten steel flow braking device 9-3 of the static magnetic field arranged at the lower stage of the mold. You.

In the method of the present invention, an inert gas is blown into the molten steel at a flow rate of 1.0 to 2.5 Nl / ton of molten steel from the molten steel outlet of the tundish to the discharge hole of the immersion nozzle. If it exceeds 2.5, the surface of the mold in the mold is boiled (the surface of the mold foams due to the generation of inert gas),
Molten slag and unmelted mold powder are easily rolled into molten steel. If it is less than 1.0, the effect of preventing blockage of the immersion nozzle is small.

On the other hand, when the static magnetic field electromagnetic force is not applied, if it exceeds 4.0 Nl / ton of molten steel, cellular defects are likely to occur in the surface layer of the slab. In addition, if it is less than 1.0 N liter / molten steel ton, nonmetallic inclusions tend to accumulate in the surface layer of the slab. Thus, by applying the electromagnetic force of the static magnetic field, it is possible to widen the range of the flow rate of the appropriate inert gas to be blown.

In the method of the present invention, the magnetic field strengths B1, B2, and B3 of the upper, middle, and lower static magnetic fluid flow braking devices are determined based on B2 with magnetic field strengths satisfying the above-described equation (A). Ratio. The magnetic field strength of the static magnetic field referred to in the method of the present invention means the magnetic field strength at the center position of the coil width at the height position of each coil of each molten steel flow braking device, and at the position of the center of the thickness of the mold. I do.

The magnetic field strength B2 acting on the molten steel near the discharge hole of the immersion nozzle is set to 500 to 1000 Gauss.
The magnetic field strength in this range is most effective for braking the flow velocity of the discharge flow, and for equalizing the flow direction and flow velocity of the left and right discharge flows across the immersion nozzle. If it exceeds 1000 Gauss, the flow velocity of the discharge flow becomes low, and the flow velocity of the upward flow after colliding with the short side of the slab is too low. Therefore, the speed of the flow of the molten steel from the short side of the mold toward the immersion nozzle is also too low. For this reason, the effect of cleaning the solidified shell is reduced, so that bubble defects are generated in the surface layer of the slab, and nonmetallic inclusions and the like are easily accumulated. If it is less than 500 Gauss, the effect of braking the discharge flow velocity is small. Therefore, the magnetic field strength B2 is set to 500 to 1000 Gauss.

The magnetic field strength B1 acting on the molten steel in the vicinity of the meniscus has an index of 0 to 1 when the magnetic field strength B2 is 1.

As described above, the flow of the molten steel near the meniscus formed by the bubbles of the inert gas blown into the molten steel has the same effect as the magnetic field intensity applied to the molten steel near the meniscus.

Therefore, in the case of a mold having a wide width of, for example, about 1800 mm or more, the flow velocity of the molten steel flowing from the short side of the mold toward the immersion nozzle is low. At this time, the index of the magnetic field strength B1 is set to 0 or near 0 because there is a flow of the molten steel formed by the bubbles of the inert gas as described above. This is because the flow of the molten steel from the short side of the mold toward the immersion nozzle is not excessively damped.

In the case of a narrow mold of, for example, about 1600 mm or less, the flow velocity of the molten steel flowing from the short side of the mold toward the immersion nozzle is high. Therefore, even if there is a flow of the molten steel formed by the bubbles of the inert gas as described above, the index of the magnetic field strength B1 is set to 1 or less and set to be close to 1. This is for strongly damping the flow of the molten steel from the short side of the mold toward the immersion nozzle.

Therefore, from the condition of the casting mold width,
The magnetic field strength B1 is an index 0 to 1. In each case,
The flow rate of the molten steel flowing from the short side of the mold toward the immersion nozzle at a position from the short side of the mold to one-fourth of the long side of the mold and from the long side of the mold to one-half of the thickness of the short side is about 30 cm / sec. It is desirable that

The magnetic field strength B3 acting on the molten steel near the lower end of the mold with the extension line of the immersion nozzle interposed therebetween has exponents 1-3 when the magnetic field strength B2 is 1. The action of the magnetic field strength B3 is to suppress the downward flow of the molten steel,
This is for increasing the flow of molten steel relatively above the mold. Thereby, the floating of the oxide in the molten steel can be promoted, and the temperature of the molten steel surface can be maintained at a high temperature. Index 1
If it is less than the above, the effect of suppressing the downward flow of the molten steel is small. On the other hand, when the index exceeds 3, the downward flow of the molten steel is excessively damped, so that drift is apt to occur in the mold, and the thickness of the solidified shell in the long side direction of the mold becomes uneven, and Cracks occur and non-metallic inclusions tend to accumulate locally on the slab surface layer. Therefore, the magnetic field strength B3 is set to an index of 1 to 3.

In the method of the present invention, the flow rate of the inert gas to be blown and the magnetic field strength of the static magnetic field are simultaneously set within appropriate ranges, so that the thickness of the surface layer of the slab from the slab surface to 10 mm in the thickness direction is 50 μm or more. It is desirable to select casting conditions such that the number of non-metallic inclusions is 20 or less per kg of steel. By setting the number to 20 or less, it is possible to effectively prevent the occurrence of surface defects of a steel plate using the cast slab as a raw material.

In the method of the present invention, the throughput of molten steel in the immersion nozzle (the amount of molten steel passing per unit time) is 6.5 t.
It is particularly effective if applied within a range of about on / min.

[0037]

EXAMPLE A slab having a cross-sectional shape of 270 mm in thickness and 1500 mm in width was cast using a vertical bending type continuous casting machine. The steel used was a very low carbon steel with a carbon content of 0.004% by weight and was cast at a speed of 1.5 m / min. The throughput of the immersion nozzle at this time is about 4.3 ton / min. In each test, three heats were continuously cast. The amount of molten steel in each heat is about 250 tons.

As the apparatus for controlling the flow rate of molten steel from the tundish to the mold, a three-layer sliding plate shown in FIG. 1 was used. Further, an inert gas was blown from a steel thin tube arranged on the uppermost plate of the three-layer sliding plate. Use a thin tube with an inner diameter of 1 mm as a refractory.
The one embedded individually was used. The blowing flow rate of the inert gas was in the range of 0 to 5 Nl / ton of molten steel. The immersion nozzle was made of alumina graphite, had two normal discharge holes, and had a downward angle of 30 °.

On the outside of the long side of the mold, a pair of pairs of opposing sets across the mold are provided with a flow brake device for molten steel of a static magnetic field. And one pair near the lower end of the mold with the extension line of the immersion nozzle interposed therebetween. The test was performed by changing the magnetic field strengths B1, B2, and B3 of the molten steel flow braking devices of the upper, middle, and lower static magnetic fields.

During casting, using a Karman eddy current meter, the mold was positioned at a position １ ／ of the short side of the mold to the long side of the mold and １ ／ of the thickness of the long side to the short side of the mold. The flow velocity of the molten steel from the short side toward the immersion nozzle was measured at any time. In addition, it was observed whether or not the immersion nozzle was clogged. In addition, one slab (10 m length) cast in a steady state was taken from each of the six consecutive heats for each test.

First, the number of powdery defects generated on the surface layer of the cast slab was visually inspected. Next, over the entire surface on the long side of the slab, 3 m
After m-cutting, the number of foamed defects having a diameter of 1 mm or more was examined by visual observation. Furthermore, the occurrence of cracks on the slab surface was visually observed.

Regarding the number of powdery defects or bubble defects, the number of occurrences for each slab of each heat was investigated.
The average of the values was defined as the number of powdery defects or bubble defects generated in the test, and was classified into the following indices 1, 2 and 3 from the number of generated defects.

That is, the index 1 of the powder defect index or the bubble defect index indicates that these defects are not generated, or even if they are generated, they are minor and do not require maintenance. The index 2 means that these defects are generated to such an extent that maintenance is required, and if hot rolling is performed on untreated slabs, the surface defects are slightly generated in the product. An index of 3 indicates that these defects are remarkably generated, and if hot rolling is performed without taking care of the slab, surface defects are generated to such an extent that the product is scrapped.

Regarding the accumulation state of the nonmetallic inclusions, the thickness of the slab was 10 mm and the width was 90 mm from the slab surface at each position of １ ／, 、 ３ and ／ of the width of the long side of the slab in the slab cross section. A sample of 120 mm (about 0.85 kg) was collected in the casting direction, nonmetallic inclusions were extracted by an electrolytic method (slime extraction method), and the number of nonmetallic inclusions having a size of 50 μm or more was investigated. The number of non-metallic inclusions per kg of slab was converted to the number of non-metallic inclusions to evaluate the state of non-metallic inclusions. Table 1 shows test conditions and test results.

[0045]

[Table 1]

Test No. of the present invention example 1 to No. In 4,
Ar gas is blown into molten steel at a flow rate within the range defined by the method of the present invention, and the electromagnetic force of the static magnetic field within the range defined by the method of the present invention is applied to melt the molten steel in the mold. Braked the flow. There is no clogging of the immersion nozzle during casting, no powdery defects or bubble defects on the surface layer of the slab, and the amount of nonmetallic inclusions is 14-19 /
kg and no accumulation of non-metallic inclusions occurred.
Furthermore, no cracks occurred on the slab surface. These test Nos. 1 to No. In 4, the flow rate of molten steel in the mold is 30
３５35 cm / sec.

Test No. of Comparative Example In No. 5, no Ar gas was blown in, but the electromagnetic force of the static magnetic field was applied in all three stages, and the magnetic field strength of the static magnetic field was within the range defined by the method of the present invention. There was no problem with the quality of the slab surface layer.
Since the r gas was not blown, clogging of the immersion nozzle occurred in the latter half of the casting.

Test No. of Comparative Example In 6, the Ar gas is
4.65 N at a flow rate exceeding the range specified by the method of the present invention
It was blown with liter / ton of molten steel, and the electromagnetic force of the static magnetic field was applied to all three stages, and the test was performed as the static magnetic field strength within the range defined by the method of the present invention. Boiling occurred on the surface of the molten metal in the mold, so that a remarkable powder defect having an index of 3 occurred.

Test No. of Comparative Example In 7, the Ar gas is
Blowing was performed at a flow rate within the range specified by the method of the present invention at 1.16 Nl / ton of molten steel, but the electromagnetic force of the static magnetic field was not applied. A phenomenon was observed in which the surface of the molten steel near the meniscus sometimes waved due to the flow of the molten steel from the short side of the mold toward the immersion nozzle. Therefore, a remarkable powder defect having an index of 3 occurred in the surface layer of the slab. In addition, because the electromagnetic force of the lower static magnetic field was not applied, the temperature of the molten steel in the vicinity of the meniscus was lowered, so that the floating of oxides in the molten steel was hindered, and the amount of nonmetallic inclusions on the surface layer of the slab was 80 pieces. / Kg, and the slab quality was poor.

Test No. of Comparative Example 8 to No. In 11,
Ar gas was blown at a flow rate within the range specified by the method of the present invention at 1.16 Nl / ton of molten steel, and the magnetic field strength of the static magnetic field was tested by removing the conditions specified by the method of the present invention.

Test No. 1 in which an electromagnetic force of a static magnetic field was applied only to the middle stage. In No. 8, since the electromagnetic force of the lower static magnetic field was not applied, the temperature of the molten steel near the meniscus decreased,
As a result, the floating of the oxide in the molten steel was hindered, and the amount of nonmetallic inclusions in the surface layer of the slab was poor at 75 pieces / kg.

Test No. 2 was conducted in which the electromagnetic force of the static magnetic field of the upper stage and the middle stage was applied, and the magnetic field strength of the static magnetic field of the upper stage was twice that of the middle stage. 9, the meniscus flow rate of molten steel is 12 cm
/ Sec, and the effect of cleaning the solidified shell in the mold was reduced. As a result, a remarkable porosity defect having an index of 3 was generated, and the amount of nonmetallic inclusions in the surface layer portion of the slab was as poor as 85 / kg.

In Test No. 2, the electromagnetic force of the static magnetic field of the middle stage and the lower stage was applied, and the magnetic field strength of the static magnetic field of the lower stage was four times that of the middle stage. In No. 10, cracks occurred on the slab surface. This is because uneven flow occurs in the flow of molten steel in the mold, and the thickness of the solidified shell in the width direction of the mold becomes uneven. In addition, the amount of nonmetallic inclusions was as low as 40 pieces / kg because nonmetallic inclusions were likely to partially accumulate in the surface layer of the slab.

The upper, middle, and lower three-stage static magnetic field electromagnetic force is applied, and the magnetic field strength of the upper static magnetic field is set to twice that of the middle magnetic field, and the magnetic field strength of the lower static magnetic field is set to four times that of the middle magnetic field. Test No. In Test No. 11, Test No. Although the flow velocity of the molten steel in the vicinity of the meniscus increased to 20 cm / sec as compared with 9, the drift of the molten steel flow in the mold was more likely to occur, and significant porosity defects having an index of 3 were found in the stagnation portion of the molten steel flow. At the same time, the number of nonmetallic inclusions in the surface layer of the slab was as poor as 45 / kg.

[0055]

By applying the method of the present invention, when casting ultra-low carbon steel or the like, a stable casting operation can be performed without clogging of the immersion nozzle. A good quality slab without powder entrainment, bubble defects or cracks, and without accumulation of non-metallic inclusions such as Al ₂ O ₃ can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing an example of an apparatus for performing a method of the present invention.

FIG. 2 is a view showing a horizontal section at each position in FIG. 1;

FIG. 3 is a diagram schematically showing the flow of molten steel in a mold.

[Explanation of symbols]

1: Tundish 2: Upper nozzle refractory 3: Sliding plate 4: Immersion nozzle 5: Discharge hole 6: Discharge flow 7-1: Upflow 7-2: Downflow 7-3: From the short side of the mold to the immersion nozzle Flow of molten steel going 8: Flow of molten steel going from the immersion nozzle to the short side of the mold 9: Flow damping device for molten steel 10: Injection port of inert gas 11: Solidified shell 12: Molten steel

Claims (1)

[Claims] 1. A method for continuously casting molten steel using a mold for slabs of rectangular cross section and an immersion nozzle, wherein an inert gas is introduced into the molten steel between a molten steel outlet of a tundish and a discharge hole of the immersion nozzle. The molten steel flow braking device having a static magnetic field, which is blown at a flow rate of 0.0 to 2.5 Nl / molten steel ton and has a pair of sets opposed to each other with the mold interposed outside the two long sides of the mold, is provided with an immersion nozzle. Each pair is provided in the vicinity of the meniscus, in the vicinity of the discharge flow immediately after flowing out of each discharge hole of the immersion nozzle that opens in both short side directions of the mold, and near the lower end of the mold with the extension of the immersion nozzle in between, The magnetic field strengths of the molten steel flow braking devices near the meniscus, near the discharge flow, and near the lower end of the mold are B1, B2, and B3, respectively, and these magnetic field strengths B1, B2, and B3
However, a continuous casting method of steel characterized in that an electromagnetic force is applied so as to satisfy the following expression (A) based on B2. B1: B2: B3 = 0 to 1: 1: 1 to 3 (A) Here, 500 (Gauss) ≦ B2 ≦ 1000 (Ga
uss) Magnetic field application direction of B2: opposite direction to B1 and B3