JPH09192802A - Method for continuously casting extra-low carbon steel slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the rate of occurrence of surface defect of a thin steel sheet by specifying the flow velocity of molten steel poured into a mold from an immersion nozzle. SOLUTION: Flow velocity of the molten steel at the position near the short side of the mold at 1/4 width of the mold is kept in the range of -0.07-0.05m/sec. The molten steel stream from the short side of the mold toward the immersion nozzle is shown by the positive. The flow velocity is controlled by using a linearly movable magnetic field type electromagnetic stirring device. At the time of keeping the flow velocity of the molten steel to near zero at -0.07-0.05m/sec, spouting stream of the molten steel from the immersion nozzle does not transmit the moving magnetic field by the linearly movable magnetic field type electromagnetic stirring device but sufficiently reduced in the velocity by the magnetic field. The molten steel is dispersed in the direction orthogonal to the moving direction of the magnetic field. Since the flow velocity of the molten steel just below the molten steel surface becomes sufficiently small, vertical eddy flow is not caused and the generation of dripping of mold powder is prevented. The sufficient heat supply is obtained on the molten steel surface by the spouting stream of the molten steel and the growth of claw defect on a solidified shell near a slab corner is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for slabs made of extremely low carbon steel.

[0002]

2. Description of the Related Art With respect to thin steel sheets used in many applications such as steel sheets for automobile exteriors, steel sheets for cans, and steel sheets for household appliances, improvement in workability is strongly required year after year. On the other hand, the annealing performed on the steel sheet after cold rolling has been rapidly changed from conventional batch annealing to continuous annealing. Due to such a situation, the steel for steel sheets has a carbon content of 0.0
From low carbon steel of 1 to 0.1 wt.%, The carbon content is 0.01
It is being converted to ultra low carbon steel with wt.% or less.

When such a molten steel of ultra-low carbon steel is continuously cast into a slab and a thin steel plate is manufactured using the cast slab as a raw material, a steel plate caused by non-metallic inclusions contained in the slab. It is known that surface defects occur more frequently than thin steel sheets made of low carbon steel slabs.

One of the characteristic surface flaws is a "blister" -shaped flaw called "blister flaw". The cause of this "blister flaw" is that when molten steel is continuously cast into a slab, alumina is trapped under the surface layer of the solidified shell, and during continuous annealing after cold rolling, the solid solution in the steel around the alumina is dissolved. It is said that hydrogen is condensed and vaporized and expands.

The ultra-low carbon steel has a high dissolved oxygen concentration in the molten steel during refining in order to reduce the carbon content in the steel to a low level of 0.01 wt. . Therefore, the amount of deoxidation by aluminum after the completion of the decarburization reaction is large, so that the amount of alumina suspended in the steel is larger than that in the low carbon steel, so that “blister defects” are likely to occur.

[0006] Another one of the characteristic surface flaws is a linear flaw called "sliver flaw". The cause of "sliver flaws" is that, when continuously casting molten steel consisting of extremely low carbon steel into a slab, droplets of mold powder and deoxidation are generated at the claws at the tip of the solidification shell at the molten metal surface position in the mold. It is said that this is because the alumina of the product is captured. The ultra-low carbon steel has a higher solidification temperature than the low-carbon steel, and the above-mentioned claws easily grow, so that “sliver flaw” is likely to occur.

As a means for reducing "blister defects",
A method of recovering alumina after aluminum deoxidation by slag on the pan, "Materials and Process" Vol.5 (1992) -21
As disclosed in No. 1, a method for improving the floating rate of alumina in molten steel in a mold by making the molten steel discharge hole angle of the immersion nozzle shallower (hereinafter referred to as prior art 1
It is known that by performing such a method, the frequency of occurrence of "blister defects" has been drastically reduced, and it has become comparable to low carbon steel.

As a means for reducing "sliver flaws", as disclosed in "Materials and Processes" Vol. 4 (1992) -253, the flow velocity of molten steel immediately below the molten metal surface near the short side of the mold is Proper range shown in FIG. 1, namely 0.25 to 0.33
A method of maintaining m / sec (hereinafter referred to as prior art 2) is known. The method of Prior Art 2 will be described below.

The trapping of mold powder droplets below the surface layer of the slab, which is the cause of the occurrence of "sliver flaws", is often near both ends in the width direction of the slab (hereinafter referred to as slab corners). In fact, the length of the claw at the tip of the solidified shell also tends to be the longest at the slab corner. FIG. 1 shows the relationship between the molten steel flow velocity just below the molten metal surface in the vicinity of the short side of the mold and the surface defect occurrence rate of the cold rolled thin steel sheet coil. As shown in FIG. 1, the molten steel flow velocity just below the molten metal surface in the vicinity of the short side of the mold has an appropriate range (0.25 to 0.33 m / sec) that minimizes the surface defect occurrence rate of the coil.

If the molten steel flow velocity just below the molten metal surface in the vicinity of the short side of the mold is too slow or too fast than the above-mentioned appropriate range,
The surface defect occurrence rate of the coil is increased. That is, in the region where the molten steel flow velocity is too slow, as described above, the claw at the tip of the solidification shell grows in the slab corner portion, the droplets of the mold powder are captured, and the subsurface inclusions are generated. "Blemish" is likely to occur. On the other hand, in the region where the molten steel flow velocity is too fast, the droplets of the mold powder are entrained by the molten steel flow just below the molten metal surface and are transferred to the inside of the strand, and the thickness of the solidified shell is 20 to 30 mm from the tip of the solidified shell. Over a wide range up to the depth position in the corresponding strand casting direction, it adheres to the solidification interface of the molten steel and becomes a subsurface inclusion.

Therefore, by maintaining the molten steel flow velocity just below the molten metal surface near the short side of the mold within the above-mentioned range of 0.25 to 0.33 m / sec, the heat supply of the molten steel near the short side of the mold is sufficiently performed. As a result, the growth of the claws at the tip of the solidified shell at the slab corner is suppressed, and as a result, the surface defect occurrence rate of the ultra-low carbon steel sheet steel coil is reduced from a few percent to about 1%.
Can be reduced to

[0012]

As described above, the molten steel flow velocity immediately below the molten metal surface in the vicinity of the short side of the mold is 0.25 to 0.
By maintaining the rate within the range of 33 m / sec, it became possible to reduce the surface defect occurrence rate of the thin steel sheet coil made of ultra-low carbon steel from the conventional several% to about 1%.
However, when this is compared with a thin steel plate coil made of low carbon steel, its surface defect occurrence rate is about twice, which is still at a high level. On the other hand, compared to the time when Prior Art 2 was developed, the ratio of ultra-low carbon steel to the thin steel sheet is now about doubled, so in order to keep the manufacturing yield of the thin steel sheet high. It has become necessary to further improve the production yield of thin steel sheets made of ultra-low carbon steel, that is, to further reduce the surface defect occurrence rate of thin steel sheets made of ultra-low carbon steel.

Therefore, an object of the present invention is to solve the above-mentioned problems, to further reduce the surface defect occurrence rate of a thin steel sheet made of ultra-low carbon steel, and to improve the production yield thereof. It is to provide a continuous casting method of a slab consisting of.

[0014]

The inventors of the present invention can solve the above-mentioned problems, further reduce the surface defect occurrence rate of a thin steel sheet made of an ultra-low carbon steel, and improve the production yield thereof. We have conducted intensive research to develop a continuous casting method for slabs made of ultra-low carbon steel.

In the prior art 2, as described above,
Focusing only on the fact that the flow of molten steel just below the molten metal surface near the short side of the mold affects the growth of the claws at the tip of the solidification shell, the flow rate of molten steel immediately below the molten metal surface near the short side of the mold is adjusted to an appropriate range. By controlling, the inclusions under the surface of the slab are reduced. Therefore, the inventors of the present invention, the elements of the molten steel flow in the mold that influence the inclusions of the subsurface inclusions of the slab,
We proceeded with the research, considering that there is other than the molten steel flow velocity just below the molten metal surface near the short side of the mold.

As a result, not only in the vicinity of the short side of the mold but also in the position near the short side of the mold of the mold width 1/4, the molten steel flow velocity just below the molten metal surface is cold when the slab continuously cast by the molten steel is used as the material. It was found that it has a great influence on the surface defect occurrence rate of the rolled thin steel sheet.

The present invention has been made on the basis of the above findings, and the invention according to claim 1 is an immersion method in which a molten steel made of an ultra-low carbon steel is immersed in the molten steel in the lower part of the molten steel. In a method of continuously casting a slab by injecting it into a mold through a nozzle and then continuously withdrawing it from the mold, a short mold having a mold width of 1/4 of a molten steel injected into the mold from the immersion nozzle. The flow velocity at a position near the edge represents the molten steel flow from the mold short side toward the immersion nozzle as a positive value, and when the molten steel flow from the immersion nozzle toward the mold short side as a negative value, − 0.07
It is characterized in that it is maintained within the range of m / sec to 0.05 m / sec.

According to the second aspect of the invention, in order to maintain the flow rate of the molten steel injected from the immersion nozzle into the mold within the range defined in the first aspect, the width direction is provided outside the mold in the width direction. It is characterized in that the flow velocity of the molten steel is controlled by using the linear moving magnetic field type electromagnetic stirrer provided in the above.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION A slab continuous casting machine having the specifications shown in Table 1 is used, and a molten steel made of an extremely low carbon steel having the chemical composition shown in Table 2 is continuously cast into a slab. The midpoint between the mold short side and the mold width direction, that is, the position near the mold short side of the mold width 1/4 (hereinafter, 1 /
The molten steel flow velocity immediately below the molten metal surface at 4 width positions) was measured, and the relationship between this and the surface defect occurrence rate of the cold-rolled thin steel sheet using the slab continuously cast by the molten steel as a raw material was investigated.

[0020]

[Table 1]

[0021]

[Table 2]

The molten steel flow velocity just below the molten metal surface at the 1/4 width position was measured by the method shown in FIG. That is, from the dipping nozzle 2 arranged at the center of the mold 1, a dipping rod 3 made of molybdenum-zirconia-based cermet having a length of 410 mm and a diameter of 20 mm is placed at a quarter width position near one of the mold short sides 1a. Was attached so as to be rotatably supported in the width direction of the mold 1 with the lower end portion being immersed in the molten steel in the mold 1 with the vicinity of the upper end as a fulcrum. The distance from the lower end of the immersion rod 3 to the molten metal surface 4, that is, the immersion depth D of the immersion rod 3 in the molten steel.
Is about 100 mm.

When the immersion rod 3 is immersed in the molten steel in the mold 1 in this way, the immersion portion of the immersion rod 3 is rotated around the fulcrum near the upper end by the molten steel flow immediately below the molten metal surface, and the immersion rod is rotated. It stops when the gravity acting on 3 and the force due to the molten steel flow just below the surface of the molten metal are balanced. At this time, the angle θ formed by the axial direction of the immersion rod 3 and the vertical direction is measured, and the molten steel flow velocity immediately below the molten metal surface is calculated by the balance calculation of the force acting on the immersion rod 3 and the molten steel flow immediately below the molten metal surface. You can ask.

By the above-mentioned method, the molten steel flow velocity immediately below the molten metal surface was measured under various casting conditions, and the relationship with the surface defect occurrence rate of the cold-rolled thin steel sheet made of the slab continuously cast by the molten steel was measured. It was investigated and shown in FIG.

In FIG. 3, the horizontal axis is the molten steel flow velocity just below the molten metal surface at the 1/4 width position, and the flow from the mold short side 1a to the immersion nozzle 2 is in the positive direction, and from the immersion nozzle 2 to the mold short side 1a. The flow was defined as a negative direction, the flow velocity in the positive direction was represented by a positive number, and the flow velocity in the negative direction was represented by a negative number.

As is apparent from FIG. 3, the molten steel flow velocity just below the molten metal surface at the 1/4 width position exceeds about 0.05 m / sec in the positive direction,
If it exceeds about 0.07 m / sec in the negative direction, the occurrence rate of surface defects increases.

FIG. 4 is a diagram showing the flow state of the molten steel in the mold 1 when the molten steel flow immediately below the molten metal surface at the 1/4 width position flows in the positive direction. As shown in FIG. 4, when the molten steel flow immediately below the molten metal surface at the 1/4 width position flows in the positive direction, the molten steel discharge flow from the immersion nozzle 2 is, as shown by the arrow, a short side 1a of the mold.
The molten steel flow that has collided with the above and branched in the upward and downward directions and branched in the upward direction flows immediately below the molten metal surface 4, reaches the position of the immersion nozzle 2, and then again flows into the molten steel discharge flow from the immersion nozzle 2. It is considered that they merge to form a circulation flow.

The velocity of the molten steel discharge flow from the immersion nozzle 2 is 1
.About.2 m / sec, and this speed is determined by the amount of molten steel discharged from the immersion nozzle 2 per unit time. Since such a circulating flow is generated on both sides from the immersion nozzle 2 toward both the short sides 1a, 1a of the mold, in the molten metal surface in the vicinity of the immersion nozzle 2, it is directly downstream from the both short sides 1a, 1a of the mold. Will meet you. Since the molten steel flow velocity immediately below the molten metal surface is not constant in time but fluctuates, if there is a difference in velocity immediately downstream of both molten metal surfaces from the mold short sides 1a, 1a, in the vicinity of the immersion nozzle 2. As a result of the generation of the vertical vortex 5, there is a possibility that the molding powder below the surface of the molten metal may be caught.

FIG. 5 is a view showing the flow state of molten steel in the mold 1 when the molten steel flow immediately below the molten metal surface at the 1/4 width position flows in the negative direction. As shown in FIG. 5, when the molten steel flow just below the molten metal surface at the 1/4 width position flows in the negative direction, the deceleration force against the molten steel discharge flow by the linear moving magnetic field type electromagnetic stirrer, which will be described later, is too strong. A part of the molten steel discharge flow from the surface immediately rises toward the molten metal surface 4, and the immersion nozzle 2
Therefore, it is considered that the flow just below the surface of the molten metal is formed toward both short sides 1a of the mold. When such a molten steel flow just below the molten metal surface occurs, the molten steel discharge flow from the dipping nozzle 2 becomes
It collides with 1a and interferes with the flow reflected toward the molten metal surface 4,
As a result of the generation of the vertical vortex 5 and the wave of the molten metal surface, there is a possibility that the molding powder below the molten metal surface may be caught.

Therefore, in the present invention, as shown in FIG. 3, the flow velocity of the molten steel injected into the mold from the dipping nozzle at a position close to the mold short side of the mold width 1/4 is calculated from the mold short side. When the molten steel flow toward the immersion nozzle is positive and the molten steel flow from the immersion nozzle toward the short side of the mold is negative, it is maintained within the range of -0.07 m / sec to 0.05 m / sec. I chose

In actual casting, in order to maintain the molten steel flow velocity just below the molten metal surface at the 1/4 width position within the appropriate range, the casting speed with respect to the slab cross-sectional area, the shape of the immersion nozzle, the tundish to the mold. By appropriately setting the conditions such as the amount of argon gas blown into the furnace, and using a linear moving magnetic field type electromagnetic stirrer, the molten steel flow velocity just below the molten metal surface at the 1/4 width position should be within the above range. Control. This linear moving magnetic field type electromagnetic stirrer was also used in the casting for investigating the relationship between the molten steel flow velocity just below the molten metal surface and the surface defect occurrence rate at the 1/4 width position.

FIG. 6 is a schematic cross-sectional view in the width direction of the mold showing a state in which a braking force is applied to the molten steel discharge flow from the immersion nozzle by the linear moving magnetic field generated by the linear moving magnetic field type electromagnetic stirrer. 7 is a schematic plan view thereof.
As shown in FIGS. 6 and 7, along the long sides 1b, 1b of the mold 1, linear moving magnetic field type electromagnetic stirring coils 6a, 6b, 6a ', 6
b'is provided so that the generated magnetic field moves parallel to the width direction of the slab and horizontally from both short sides 1a, 1a of the mold 1 toward the immersion nozzle 2. As a result, the direction of the molten steel discharge flow from the immersion nozzle 2 and the direction of the magnetic field face each other, so that a braking force acts on the molten steel discharge flow by the electromagnetic force. Table 3 shows the specifications of the linear moving magnetic field type electromagnetic stirrer.

[0033]

[Table 3]

FIG. 8 is a diagram showing the relationship between the current value of the linear moving magnetic field type electromagnetic stirrer and the flow velocity of the molten steel flow immediately below the molten metal surface at the 1/4 width position in the mold. The molten steel flow velocity immediately below the molten metal surface at the 1/4 width position was measured using the above-mentioned dipping rod 3 made of molybdenum-zirconia-based cermet. As is apparent from FIG. 8, when the applied current to the linear moving magnetic field type electromagnetic stirrer is increased from 0 to 2160 A, 1
The molten steel flow velocity just below the molten steel surface at the / 4 width position decreased monotonically,
A molten steel flow rate corresponding to the applied current can be obtained.

Under the casting conditions of this example, in order to maintain the molten steel flow velocity immediately below the molten metal surface at the 1/4 width position within the range of -0.07 m / sec to 0.05 m / sec, a linear moving magnetic field was used. Type electromagnetic stirrer, about 1100A to about 140
It can be seen that a current of 0 A should be applied.

FIG. 9 shows a linear moving magnetic field type electromagnetic stirrer for controlling the flow velocity of the molten steel flow immediately below the molten metal surface at a 1/4 width position in the mold within the range of -0.07 m / sec to 0.05 m / sec. It is a figure which shows the flowing state of the molten steel in a mold at the time of doing. As shown in FIG. 9, the molten steel flow velocity immediately below the molten metal surface at the 1/4 width position was changed from -0.07 m / sec to 0.05 m.
When maintained at a value close to 0 per second, the molten steel discharge flow from the immersion nozzle does not pass through the moving magnetic field generated by the linear moving magnetic field type electromagnetic stirrer, and is sufficiently decelerated by the magnetic field while moving in the moving direction of the magnetic field. Disperse in the orthogonal direction.

In such a state where the molten steel flow velocity just below the molten metal surface at the 1/4 width position is realized, the molten steel flow velocity immediately below the molten metal surface becomes sufficiently small, and therefore, as described with reference to FIGS. 4 and 5. The generation of mold powder droplets due to the generation of the vertical vortex 5 is prevented, and the molten steel discharge flow decelerated and dispersed by the linear moving magnetic field supplies sufficient heat of molten steel to the molten metal surface, resulting in the vicinity of the slab corner. It is also believed that the growth of the nails of the coagulated shell is prevented.

[0038]

EXAMPLES The present invention will now be described with reference to examples. Using the slab continuous casting machine having the specifications shown in Table 1, molten steel made of ultra-low carbon steel having the chemical composition shown in Table 2 was continuously cast into the slab. During continuous casting, the linear moving magnetic field type electromagnetic stirrer shown in FIGS. 6 and 7 was used to control the molten steel flow velocity immediately below the molten metal surface at the 1/4 width position so as to be maintained within the range of the present invention.

In FIG. 10, the casting speed is 2.0 to 2.4 m /
The applied current value for each slab width to the linear moving magnetic field type electromagnetic stirrer at the time of minute is shown in comparison with the case of the conventional method of the prior art 2. In the drawings, white circles represent applied current values in the method of the present invention, and black circles represent applied current values in the conventional method. In the case of the method of the present invention, the applied current value is ¼
The molten steel flow velocity just below the surface of the molten metal at the width position was controlled to be maintained within the range of -0.07 m / sec to 0.05 m / sec. The molten steel flow velocity just below the molten metal surface at 0.25 to 0.3
It was controlled so as to be maintained within the range of 3 m / sec.

In FIG. 11, the casting speed is 2.0 to 2.4 m /
The molten steel flow velocity just below the molten metal surface for each slab width at the 1/4 width position in the method of the present invention and the conventional method is shown in minutes. As is clear from FIG. 11, in the case of the conventional method, in the range where the slab width is narrow, in order to maintain the molten steel flow velocity just below the molten metal surface near the mold short side at an appropriate value, the current of the linear moving magnetic field type electromagnetic stirring device is increased. Since the value is relatively small, the molten steel flow velocity just below the molten metal surface at the 1/4 width position exceeds 0.07 m / sec. Further, in the wide slab width range, the current value of the linear moving magnetic field type electromagnetic stirrer is relatively large in order to maintain the molten steel flow velocity just below the molten metal surface near the mold short side at an appropriate value.
The molten steel flow immediately below the molten metal surface at the / 4 width position is flowing in the negative direction, that is, in the direction from the immersion nozzle to the short side of the mold, and its flow rate exceeds 0.05 m / sec.

On the other hand, in the case of the method of the present invention, 1 /
The flow velocity of the molten steel flow immediately below the molten metal surface at the 4-width position is maintained within the appropriate range of -0.07 m / sec to 0.05 m / sec.

FIG. 12 is a diagram showing the cold rolling coil surface defect occurrence rate for each slab width by the method of the present invention and the conventional method of the prior art 2. As is clear from FIG. 12, 1 /
The cold rolling coil surface defect occurrence rate for each slab width by the slab cast by the conventional method in which the flow velocity of the molten steel flow immediately below the molten metal surface at the 4 width position is outside the range of the present invention is about 0.2 to.
It showed a high value of 0.8%. On the other hand, according to the method of the present invention, the cold rolling coil surface defect occurrence rate for each slab width by the slab cast by maintaining the flow velocity of the molten steel flow immediately below the molten metal surface at the 1/4 width position within the range of the present invention is , Was extremely low over the entire slab width.

FIG. 13 shows the method according to the present invention and the conventional method.
It is a figure which shows the cold rolling coil surface defect generation rate of all slab width averages. As is clear from FIG. 13, in the case of the method of the present invention, the surface defect occurrence rate of the cold rolling coil is reduced to about 1/4 of that of the conventional method, and the production yield of the cold rolling coil made of ultra low carbon steel is reduced. Was able to be greatly improved.

[0044]

As described above, according to the present invention,
When continuously casting molten steel made of ultra-low carbon steel into a slab, the flow velocity of the molten steel injected into the mold from the dipping nozzle at a position close to the mold short side of the mold width 1/4 is measured from the mold short side to the dipping nozzle. Positively represent the molten steel flow toward
When the molten steel flow from the dipping nozzle toward the short side of the mold is expressed as a negative value, it was controlled so as to be maintained within the range of -0.07 m / sec to 0.05 m / sec, so that the mold powder liquid Droplets are prevented from being taken into the steel and generating inclusions under the surface of the slab, which significantly reduces the surface defect occurrence rate of the thin steel sheet and improves the manufacturing yield thereof. It has a useful effect.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a molten steel flow velocity just below a molten metal surface near a short side of a mold and a surface defect occurrence rate of a cold rolling coil according to a conventional technique.

FIG. 2 is a diagram showing a method for measuring a molten steel flow velocity just below a molten metal surface at a 1/4 width position in a mold.

FIG. 3 is a diagram showing a relationship between a molten steel flow rate immediately below a molten metal surface at a 1/4 width position in a mold and a surface defect occurrence rate of a cold rolling coil.

FIG. 4 is a diagram showing a molten steel flow state in the mold when the molten steel flow just below the molten metal surface at a 1/4 width position in the mold flows in a positive direction.

FIG. 5 is a diagram showing a molten steel flow state in the mold when the molten steel flow just below the molten metal surface at a quarter width position in the mold flows in a negative direction.

FIG. 6 is a schematic cross-sectional view in the width direction of the mold showing a state in which a braking force is applied to the molten steel discharge flow from the immersion nozzle by the linear moving magnetic field generated by the linear moving magnetic field type electromagnetic stirring device.

FIG. 7 is a schematic plan view of FIG.

FIG. 8 is a diagram showing a relationship between a current value of a linear moving magnetic field type electromagnetic stirrer and a flow velocity of a molten steel flow just below a molten metal surface at a 1/4 width position in a mold.

FIG. 9 shows the flow velocity of the molten steel flow just below the molten metal surface at the 1/4 width position in the mold by the linear moving magnetic field type electromagnetic stirring device.
It is a figure which shows the flow state of the molten steel in a mold when controlling in the range of 0.07 m / sec to 0.05 m / sec.

FIG. 10 is a diagram showing applied current values of a linear moving magnetic field type electromagnetic stirrer for each slab width according to the method of the present invention and the conventional method.

FIG. 11 is a molten steel directly below the molten metal surface at a ¼ width position in the mold according to the method of the present invention and the conventional method when the linear moving magnetic field type electromagnetic stirrer is operated by the current value shown in FIG. 10 to perform casting. It is a figure which shows a flow velocity.

FIG. 12 is a diagram showing a cold rolling coil surface defect occurrence rate for each slab width according to the method of the present invention and the conventional method.

FIG. 13 is a diagram showing the average defect rate of cold rolling coil surface defects over the entire slab width according to the method of the present invention and the conventional method.

[Explanation of symbols]

 1 Mold 1a Mold short side 1b Mold long side 2 Immersion nozzle 3 Immersion rod 4 Melt surface 5 Vertical vortex 6 Linear moving magnetic field type electromagnetic stirring coil

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masayuki Nakata, 1-2 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Makoto Suzuki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Date Inside the Steel Pipe Co., Ltd. (72) Inventor Toshio Ishii 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A slab is continuously cast by injecting molten steel made of ultra-low carbon steel into a mold through a dipping nozzle whose lower portion is immersed in the molten steel in the mold, and continuously withdrawing from the mold. In the method, the flow velocity of the molten steel injected into the mold from the immersion nozzle at a position close to the mold short side of the mold width ¼, the molten steel flow from the mold short side toward the immersion nozzle is positive. When the molten steel flow from the dipping nozzle toward the short side of the mold is expressed as a negative value, it is expressed as −0.07 m / sec to 0.
A continuous casting method for ultra-low carbon steel slabs, characterized in that the ultra-low carbon steel slab is maintained within a range of 05 m / sec. 2. A linear moving magnetic field type electromagnetic wave provided outside the mold in the width direction thereof in order to maintain the flow velocity of the molten steel injected from the immersion nozzle into the mold within the range defined in claim 1. A continuous casting method for ultra-low carbon steel slabs, wherein a flow rate of the molten steel is controlled by using a stirrer.