JPH10211560A - Method for continuously casting billet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for billet which prevents the entrapment, etc., of powder near a meniscus, improves the ratio of equi-axed crystal and reduces the invading depth of inclusion, blow hole, etc. SOLUTION: (1) In the continuous casting method for casting the billet by using a bottomless immersed nozzle 1, the nozzle 1 gradually increasing the inner cross sectional area toward the outlet, is dipped into molten steel, and the molten steel in the nozzle 1 is circled and the molten steel in a mold is circled in the reverse direction of the circulation in the nozzle 1. (2) In the continuous casting method for casting the billet by using the bottomless nozzle 1, the nozzle 1 gradually increasing the inner cross sectional area toward the outlet is dipped into the molten steel and the molten steel in the nozzle 1 is circled to give the molten steel in the mold an electromagnetic brake.

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 a billet in which occurrence of slime and internal defects is suppressed.

[0002]

2. Description of the Related Art In the conventional continuous casting, particularly in the continuous casting of billets, casting is performed using an open-end cylindrical injection nozzle having no bottom surface (hereinafter abbreviated as "straight nozzle"). . On the other hand, in the case of continuous casting of a slab, a nozzle having a bottom surface and a pair of discharge ports on the nozzle side surface (hereinafter, abbreviated as “two-hole nozzle”) is used. In the case of continuous casting of billets, since the distance between the nozzle and the mold wall is small, a straight nozzle is used because a high-speed discharge flow directly hits the solidified shell when a two-hole nozzle is used.

[0003] In the case of continuous casting using a straight nozzle, since the molten steel flow is mainly a downward flow, inclusions and air bubbles are easily trapped inside the slab. In addition, since heat supply to the vicinity of the molten steel surface (meniscus) in the mold is reduced, there is a problem that a solidification phase occurs on the surface of the molten steel and surface defects occur. For this purpose, usually, a temperature near the meniscus is ensured by applying an electromagnetic field to molten steel in a mold to give a swirling flow to the molten steel. The electromagnetic stirring of the molten steel in the mold also has the effect of increasing the equiaxed crystal ratio of the billet and improving the internal quality of the slab. At present, in the case of continuous casting using a straight nozzle, it can be said that electromagnetic stirring in the mold is essential.

However, when the electromagnetic stirring is performed, the level of the molten metal near the immersion nozzle is reduced while the area near the mold wall rises, so that powder is trapped near the meniscus and the inflow of molten powder is hindered. There were problems such as the occurrence of internal defects.

[0005] In order to alleviate such problems inherent in the straight nozzle, research results have been published that have been analyzed fundamentally only with respect to the nozzle shape and the flow state of molten steel (Yokotani et al .:
Materials and Processes, Vol.9 (1996) No.4, P773). According to the results of this study, using a straight nozzle whose inner cross-sectional area gradually increases toward the outlet, turning the molten steel flow inside the nozzle forms a flow along the nozzle inner wall, and the molten steel near the meniscus is cast in the mold Flow towards the center.

It is suggested that the use of such research results can suppress defects occurring in billets continuously cast using a straight nozzle with electromagnetic stirring.
However, even if a nozzle having the above-mentioned shape is used, the flow in the descending direction still remains quite strong, so that the use of the above-mentioned nozzle alone cannot prevent defects, and what kind of improvement can be effectively combined with such a case, etc. Need to consider.

[0007]

SUMMARY OF THE INVENTION The present invention provides a method for securing a meniscus temperature while using a straight nozzle, preventing powder entrainment in the vicinity of the meniscus and increasing the equiaxed crystal ratio, and in addition to the above, It is an object of the present invention to provide a method for reducing the penetration depth of bubbles and the like.

[0008]

Means for Solving the Problems The present inventors conducted thought experiments in order to solve the above-mentioned problems, devised several methods, and conducted experiments on them. As a result, the following items could be confirmed. .

(A) When the molten steel flow in the straight nozzle whose inner cross-sectional area increases gradually toward the outlet, the meniscus molten steel near the molten metal surface flows toward the center of the mold. If the molten steel in the mold is appropriately swirled in this state, a centrifugal force is generated in the vicinity of the molten metal surface toward the mold, and the flow toward the center of the mold is canceled, and the meniscus flow remains in only the swirling direction. As a result, a state in which the molten powder runs short in the vicinity of the mold is avoided, and a flow can be given to the meniscus, and various defects can be prevented.

(B) When electromagnetic braking is applied to the descending flow in the mold immediately below the nozzle, the descending flow in the mold is rectified, and the intrusion of inclusions and bubbles into the slab is prevented. in this case,
Even if the swirling of the molten steel in the nozzle is not sufficiently performed, it is possible to sufficiently provide a flow to the meniscus.

The present invention has been completed through repeated field experiments based on the above matters, and is characterized by the following continuous casting method.

(1) A continuous casting method for casting a billet using a bottomless immersion nozzle, wherein the nozzle whose inner cross-sectional area near the outlet gradually increases toward the outlet is immersed in molten steel. A continuous casting method in which the molten steel is swirled and the molten steel in the mold is swirled in a direction opposite to that of the nozzle (referred to as [Invention 1]).

(2) A continuous casting method for casting a billet using a bottomless immersion nozzle, wherein the nozzle whose inner cross-sectional area near the outlet gradually increases toward the outlet is immersed in molten steel, and A continuous casting method in which molten steel is swirled and electromagnetic braking is applied to molten steel in a mold immediately below a nozzle (referred to as [Invention 2]).

In the above, the term "nozzle without a bottom" refers to a straight nozzle used exclusively for billet casting. "Swirl the molten steel in the nozzle" refers to swirling the molten steel falling by an electromagnetic coil attached around the nozzle to make a spiral movement. In [Invention 2], "providing electromagnetic braking to molten steel in a mold" refers to applying a brake mainly to a downward flow component of molten steel dropped from a nozzle.

[0015]

Next, the reason why the present invention is limited as described above will be described.

1. Nozzle shape ([Invention 1] and [Invention 2]) The immersion nozzle is used to ensure the quality of the billet such as inclusion of inclusions, and the straight nozzle is used to maintain the casting efficiency while maintaining the casting efficiency. This is for improving the surface properties.

The shape of the straight nozzle is such that the inner cross-sectional area near the outlet of the straight nozzle is gradually increased. The degree of the gradual increase is desirably increased by 25 cm or more per meter in the radius of the nozzle. The upper limit is not particularly limited, but is naturally limited by a shape that fits inside the mold.

When the internal cross-sectional area near the outlet is gradually increased, not only a lower component but also a component in the horizontal direction, that is, the vertical direction of the casting wall is generated in the flow of the molten steel, and a temperature drop near the meniscus can be prevented. In addition, in combination with the effect of stirring in the mold described later, an effect of giving a flow to the meniscus is exerted, and powder entrainment is prevented.

It is desirable that the inner wall near the nozzle outlet is axially symmetric, the inner diameter gradually increases gradually, and the inner wall in the vertical section near the outlet is particularly parabolic.

By changing the shape of the nozzle and the intensity of the electromagnetic stirring for turning the molten steel in the mold, it is possible to arbitrarily adjust the flow state in the mold.

2. Giving Swirling Force to Molten Steel in Nozzle ([Invention 1] and [Invention 2]) The swirling force normally turns the molten steel in the nozzle by applying a rotating magnetic field to the molten steel flow in the nozzle. It is not preferable to change the number of revolutions of the rotating magnetic field because it affects the penetration depth of the magnetic field. Since the rotating magnetic field is applied through the refractory nozzle, it is not necessary to have a low frequency on the order of several Hz as in the case of electromagnetic stirring in the mold, which must be shielded. For example, it is possible to use a commercial power supply frequency, and it is possible to reduce equipment costs.

The number of rotations of the rotating magnetic field is preferably in the range of about 5 to 10 rotations per second, and if it is slower than this, no flow is formed along the nozzle wall. In addition, providing a rotation speed of 10 rotations or more per second involves difficulties such as restrictions on the device capacity.

As described above, instead of applying the turning force by the electromagnetic force, the turning force may be applied by forming a spiral groove inside the nozzle.

3. Application of swirl flow to molten steel in mold ([Invention 1]) The application of swirl flow to molten steel in the mold is performed by an electromagnetic stirrer. This electromagnetic stirrer may be the same as the one usually used, and is provided with a 5
A magnetic field having a frequency of about 6 Hz is applied.

As a result, the maximum rotation speed of the molten steel in the mold is about three rotations per second. The applied current, which is the magnetomotive force, is at least 20
0A, maximum 500A, average 300A are applied.

If the direction of the rotation by the electromagnetic stirring in the mold is the same as the direction of the rotation in the nozzle, trapping of unslagged powder on the slab surface becomes a problem.

4. Electromagnetic braking on the downward flow of molten steel in the mold ([Invention 2]) Electromagnetic braking on the downward flow of molten steel in the mold is performed by applying a static magnetic field. The static magnetic field is generated by a pair of magnetic poles sandwiching the mold. The cross section of the magnetic pole should be about the inner diameter of the mold.
If the magnetic flux density generated in the mold by this electromagnetic braking device is 0.2 to 0.5 T, the penetration depth of inclusions, bubbles and the like can be greatly reduced. Further, a static magnetic field larger than this can be used as long as it can be used industrially.

Conventionally, in order to secure the equiaxed crystal ratio, electromagnetic stirring was performed to cut the dendrites to generate equiaxed nuclei. In the present invention as well, since the flow rate of molten steel is generated in the solidified shell to promote the generation of equiaxed crystals, the equiaxed crystal ratio is assured as compared with the prior art.

[0029]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

FIG. 1 is a view showing a vertical cross section near a mold of a continuous casting apparatus for a billet provided with a straight nozzle having an inner diameter gradually increasing, an electromagnetic coil for imparting swirling flow to molten steel in the nozzle and the mold, and an electromagnetic braking coil in the mold. It is.

In the continuous casting machine used in the embodiment, the mold had a cross section of 200 mm in inner diameter, and the billet was drawn out in a curved shape (not shown). The immersion nozzle is made of alumina graphite and the outer diameter is 90 mm and the inner diameter is 50 mm except for the outlet.
Straight pipe.

FIG. 2 is a longitudinal sectional view of the nozzle used in the embodiment. As shown in the figure, only the inner diameter of the vicinity of the outlet of the nozzle was gradually increased, and the sectional area was gradually increased toward the outlet.

A current of 0 to 10 A was passed as a magnetomotive force at a frequency of 50 Hz using a three-part coil closely attached to the periphery of the nozzle in order to impart a swirl to the molten steel in the nozzle.

In the electromagnetic stirring for imparting a swirl to the molten steel in the mold, a current having a frequency of 6 Hz and a current of 0 to 400 A was applied as a magnetomotive force by three divided coils arranged around the mold.

Electromagnetic braking by a downflow static magnetic field is performed by placing a mold between two coaxial coils and arranging the axis of the mold so as to be orthogonal to the molten steel downflow. Performed by generating a magnetic flux density of .4T.

The mounting position of each coil is as shown in FIG.

Table 1 shows the chemical composition of medium carbon steel cast by the above apparatus.

[0038]

[Table 1]

Table 2 shows the operating conditions of the continuous casting performed.

Table 3 shows the conditions for applying the swirling flow in the nozzle and the mold and the conditions for applying the electromagnetic braking in the mold.

[0041]

[Table 2]

[0042]

[Table 3]

For each test number, the occurrence rate of norokami and the occurrence rate of internal defects of the cast slab were examined. Here, norokami occurrence rate = (number of norokami occurrence billets / total number of billets) × 100 (%), and internal defect occurrence rate = (number of internal defective product tubes / total number of product tubes) × 100
(%).

Table 4 is a list showing the results of these investigations. In Table 4, the indication of [right] or [left] indicates that the molten steel in the nozzle or the mold was turned right or left when the nozzle and the mold were viewed from above.

[0045]

[Table 4]

FIG. 3 shows the occurrence rate of the norokami and the internal defect rate for each test number. In Test No. 1 as a comparative example, although both internal defects and norokami were small, the equiaxed crystal ratio was small, and defects during processing became a problem. Test No. 2 which is a comparative example is the current operating condition, but there are many norokami,
There were also many internal defects. Test No. 3, which is a comparative example, was a case in which swirling in the nozzle was given. Although there was little sloshing, internal defects frequently occurred due to the rise of the molten metal surface due to the swirling flow in the nozzle.

On the other hand, in Test No. 4 of the present invention, the swirling flow in the nozzle and the electromagnetic stirring in the mold were canceled out to obtain a stable molten metal surface shape, and there were few norogami and internal defects. In Test No. 5 of the present invention, the incidence of internal defects is reduced due to a decrease in the intrusion of inclusions and the like into the inside, and the shape of the molten metal surface is stabilized by the rectification effect of the static magnetic field, and norogami is reduced. did.

In Test No. 6 of the comparative example, when the electromagnetic stirring in the mold and the swirling flow in the nozzle were turned in the same direction, internal defects occurred frequently.

[0049]

According to the present invention, it has become possible to improve the equiaxed crystal ratio, prevent norokami and internal defects, and improve the quality of cast slab while ensuring casting efficiency using a straight nozzle.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a nozzle and a mold used in an example.

FIG. 2 is a longitudinal sectional view showing an example of the nozzle of the present invention.

FIG. 3 is a diagram showing a defect occurrence rate according to an example.

[Explanation of symbols]

 1: Nozzle 2: Electromagnetic coil for applying swirl flow in nozzle 3: Electromagnetic coil for imparting swirl flow in mold 4: Electromagnetic braking coil in mold 5: Mold 6: Solidified shell

Claims (2)

[Claims] 1. A continuous casting method for casting a billet using an immersion nozzle having no bottom, wherein a nozzle whose inner cross-sectional area near an outlet gradually increases toward an outlet is immersed in molten steel, and a molten steel in the nozzle is provided. , And the molten steel in the mold is swirled in the opposite direction to the inside of the nozzle. 2. A continuous casting method for casting a billet using an immersion nozzle having no bottom, wherein a nozzle whose inner cross-sectional area near an outlet gradually increases toward the outlet is immersed in molten steel, and a molten steel in the nozzle is provided. And a method for continuously casting a billet, wherein electromagnetic steel is applied to molten steel in a mold immediately below a nozzle.