JPH10263762A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a cast slab having little non-metallic inclusion by making the surface flow speed of molten steel in a mold suitable in a continuous casting of the steel. SOLUTION: In this continuous casting method of the steel, a moving magnetic field generating device 10 divided into at least two parts at the right and left sides in the width direction of the mold 1 while centering an immersion nozzle 7 is arranged on the back surfaces of the long sides 2 of the mold so that the center position of the moving magnetic field generating device 10 in the casting direction becomes the lower end position of spouting hole 8 of the immersion nozzle and the lower end position of the mold. The moving magnetic field is impressed to the molten steel 4 with the moving magnetic field generating device 10, and the average surface flow speed of the molten steel in the mold at the position apart of 1/4 of the mold width from the mold center is controlled to <=0.1 m/sec. The magnetic poles 11 faced by interposing the mold long sides 2 are set to the back surfaces of the mold long sides 2 over the whole widths of the mold at the position containing a meniscus 5, and the static magnetic field is improved to the meniscus 5 with this magnetic poles.

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 steel capable of optimizing the surface flow velocity of molten steel in a mold by electromagnetic force and obtaining a slab with less nonmetallic inclusions.

[0002]

2. Description of the Related Art In continuous casting of steel, molten steel is discharged into a mold at a high speed through an immersion nozzle, so that a flow of molten steel is generated in the mold due to the discharge flow. It has a significant effect on the surface and internal properties of the piece. In particular, the mold surface (hereinafter referred to as "meniscus")
When the surface flow velocity is too high, or when a vertical vortex is generated in the meniscus, the mold powder is caught in the molten steel and becomes a major defect in the product, so the flow of the molten steel in the meniscus is regarded as important. Therefore, as an important issue for improving the quality of cast slabs, many methods of controlling the flow of meniscus using electromagnetic force have been proposed.

For example, Japanese Patent Application Laid-Open No. 2-284750 (hereinafter referred to as "prior art 1") discloses that a static magnetic field is applied by a one-stage or two-stage DC power supply over the entire width of a mold.
A method is disclosed in which braking is applied to the discharge flow from an immersion nozzle. According to the prior art 1, a uniform magnetic field always acts on the discharge flow even if the operating conditions fluctuate, and the magnetic field has a role as a reflector for the discharge flow, so that the molten steel flow can be controlled stably. And

Japanese Unexamined Patent Publication No. 64-2771 (hereinafter referred to as "prior art 2") discloses a method in which a moving magnetic field generator using a plurality of pairs of AC power supplies is arranged opposite to the back side of a long side of a mold, and molten steel is generated by a moving magnetic field. A method is disclosed in which a flow is generated, and a discharge flow is decelerated or accelerated by the molten steel flow to obtain a desired molten steel flow. According to Prior Art 2, since the moving direction and the magnetic field strength of the magnetic field can be individually controlled by a plurality of pairs of moving magnetic field generators,
It is stated that cast pieces with few nonmetallic inclusions can be obtained.

Japanese Patent Application Laid-Open No. Hei 5-177317 (hereinafter referred to as "prior art 3") discloses a method of applying a static magnetic field to a lower portion of a mold or directly below a mold and applying a moving magnetic field to a meniscus portion. It has been disclosed. According to the prior art 3, the descending flow of the discharge flow is decelerated by the static magnetic field to promote the floating of the nonmetallic inclusions, and the unidirectional rotating flow is given to the meniscus by the moving magnetic field, thereby cleaning the solidified shell. Therefore, it is possible to obtain a cast slab in which both the surface layer and the inside are clean.

[0006]

However, in the prior art 1, since the braking force acts on the entire molten steel in the mold, it is difficult to control the flow direction and the flow pattern of the molten steel. It cannot be effective.
In the case of a static magnetic field, an electromagnetic force acts in proportion to the flow velocity of the molten steel passing through the region where the magnetic field is applied. Therefore,
When a static magnetic field is applied, some of the flow velocity of the molten steel is reduced. Usually, a magnetic field strength of about 0.3 Tesla is used, but it is reported that at this magnetic field strength, there is only a deceleration effect of at most about 20% (for example, M. Wash
io et al .: La Revue de Metallurgie -CIT, (1993), 50
8). Therefore, in the casting mode in which the slab drawing speed was increased recently for the purpose of improving efficiency, the surface flow velocity of the molten steel in the mold was originally high, so this moderating effect was sufficient to reduce nonmetallic inclusions. Does not work.

In the prior art 2, since the discharge flow velocity is controlled by the molten steel flow induced by the periodic movement of the magnetic field, the average flow velocity can be easily controlled, but the effect of reducing the fluctuation value of the molten steel flow velocity is achieved. Is insufficient, and depending on the conditions, entrapment of the mold powder may occur, and the stability of flow control is lacking.

In the prior art 3, the meniscus flow velocity of the molten steel is not always optimally controlled. Conversely, the flow velocity of the molten steel flow of the meniscus is accelerated by the rotational flow, and the entrainment of the mold powder may be promoted. Lack of stability.

As described above, it cannot be said that the conventional method of controlling the flow of molten steel in a mold using a magnetic field is sufficiently effective in reducing nonmetallic inclusions.

The present invention has been made in view of the above circumstances, and an object of the present invention is to use a moving magnetic field and a static magnetic field in combination, and to optimize the surface flow velocity of molten steel in a mold by utilizing the excellent characteristics of both. It is intended to provide a continuous casting method of steel capable of obtaining a slab with reduced nonmetallic inclusions.

[0011]

The method for continuously casting steel according to the present invention comprises a moving magnetic field generator divided into at least two parts on the left and right sides in a mold width direction around an immersion nozzle. It is arranged on the back of the long side of the mold so that the center position is in the range between the lower end position of the discharge hole of the immersion nozzle and the lower end position of the mold. The average surface flow velocity of the molten steel in the mold at a position 1/4 of the width is 0.1 m / sec.
c or less, and the magnetic poles opposed to each other across the long side of the mold are arranged on the back of the long side of the mold over the entire width of the mold including the meniscus, and a static magnetic field is applied to the meniscus by the magnetic poles. Is what you do.

The present inventors have examined the relationship between the flow of molten steel in a mold and the quality of a slab by various measured values and model experiments in a continuous casting machine, and the quality of the slab was determined by the surface velocity and the surface velocity of the molten steel in the mold. It was found that the relationship with the fluctuation range was large. Details will be described below.

First, the relationship between the surface flow rate of molten steel in a mold and the incidence of mold powder defects in a thin steel sheet product was investigated. A position 1/4 of the width of the mold from the center of the mold
In the "1/4 mold width position"), the surface flow velocity of the molten steel in the mold is measured in slab units, and the maximum flow velocity of the surface velocities measured in slab units is defined as the surface flow velocity representative of the slab.
Then, the slab was rolled into a thin steel sheet product, and the occurrence rate of mold powder defects in the product was measured, and the relationship between the occurrence rate of the defects and the above-mentioned surface flow velocity was investigated. FIG. 4 shows the result. The horizontal axis in FIG. 4 shows the surface flow from the short side of the mold to the center of the mold and the surface flow from the center of the mold to the short side of the mold separately, as shown in FIG. In addition, when the maximum surface flow velocity at the 1/4 mold width position is substantially zero, the occurrence rate of mold powder defects is minimized, and the maximum flow velocity at the 1/4 mold width position is 0.1m / sec
In the case of c or less, the defect occurrence index was 0.1 or less, and it was found that the defect occurrence rate was stably low.

The reason for this was confirmed by a model experiment using a low melting point alloy. When the maximum surface flow velocity at the 1/4 mold width position was 0.1 m / sec or less, the generation of vortices in the meniscus and Without the appearance of strong surface currents,
It was found that the flow state was such that mold powder was not easily entrained.

Even when the slab drawing speed is high,
In order to realize such a slow surface velocity of molten steel in a mold by a magnetic field, as described above, the method of applying a static magnetic field to a discharge flow or a surface flow requires 0.3 Tesla, which is currently applied industrially. A magnetic field strength of the order is insufficient, and it is necessary to apply a static magnetic field that greatly exceeds this, for example, 1 tesla or more. However, this is difficult in terms of equipment, and even if possible, the capital investment amount is high. In the present invention, the moving magnetic field is arranged on the back of the long side of the mold such that the center position of the moving magnetic field generator in the casting direction is in the range between the lower end position of the discharge hole of the immersion nozzle and the lower end position of the mold. Made it possible. That is, by applying the moving magnetic field to the molten steel in a range where the molten steel is discharged from the immersion nozzle into the mold, in addition to the braking action of the discharge flow that the magnetic field has in principle, the molten steel flow induced by the moving magnetic field can be generated. By slowing the discharge flow in opposition to the discharge flow from the immersion nozzle, for example, with an industrially applicable moving magnetic field generator having a maximum magnetic field strength of 0.3 Tesla or less, the surface velocity of the molten steel in the mold can be set to an arbitrary value. Speed, e.g.
This is because it can be reduced to 1 m / min or less.
The reason why the moving magnetic field generator is divided into at least two on the left and right sides in the mold width direction around the immersion nozzle is to reverse the moving direction of the moving magnetic field on the left and right sides of the mold width.

However, when the surface flow velocity is controlled by the moving magnetic field in the actual machine, even if the surface flow velocity at the 1/4 mold width position is controlled to 0.1 m / sec or less, the fluctuation of the surface flow velocity will be described later. 0.1m /
Since a surface flow exceeding sec may occur, the entire range of the continuous casting process over several hours may be 0.1 m /
It is difficult to control it in seconds or less. Therefore, in the present application, the average surface flow velocity of the molten steel in the mold at the 1/4 mold width position is controlled to 0.1 m / sec or less by the moving magnetic field. The average surface flow velocity is an average value of the surface flow velocity of the molten steel in the mold measured for 3 minutes or more.

By the way, even when the average surface flow velocity at the 1/4 mold width position is 0.1 m / sec or less, several cases can be considered for the flow pattern of the molten steel in the mold as shown in FIG. FIG. 5 schematically shows the flow pattern of the molten steel in the mold roughly classified into three patterns A to C based on the results of studies by the inventors.

For example, in the case of pattern A, the discharge flow from the immersion nozzle reaches and collides with the solidified shell on the short side of the mold, and then rises to the meniscus along the solidified shell on the short side of the mold. Furthermore, the surface flow is such that the meniscus is separated into a flow flowing from the short side of the mold toward the center of the mold and a flow descending downward from the point of collision with the solidified shell on the short side of the mold. The flow is in one direction from the side to the center of the mold, but when the speed of the discharge flow itself is low, the surface flow velocity is also low, and the average surface flow velocity at the 1/4 mold width position is 0.1 m / sec or less.

In the case of pattern B, the discharge flow from the immersion nozzle does not reach the solidified shell on the short side of the mold due to the floating of the Ar gas bubbles or the influence of the application of a magnetic field to the discharge flow. In a flow pattern that is dispersed between the solidified shell on the short side and forms an ascending flow and a descending flow, the meniscus moves from the mold short side to the mold center at the center of the mold as shown in FIG. Flow from the center of the mold to the short side of the mold on the short side of the mold, and the flow from the center of the mold to the short side of the mold is partially formed. The average surface flow velocity tends to be 0.1 m / sec or less.

In the case of the pattern C, the coarse A
Ascending flow appears near the immersion nozzle due to the floating of the r gas bubbles, and the flow from the center of the mold toward the short side of the mold becomes the main flow in the meniscus. When this flow is weak, a flow from the short side of the mold toward the center of the mold coexists, and both collide between the immersion nozzle and the short side of the mold. Also in this case, due to the collision of these flows, the average surface flow velocity at the 1/4 mold width position may be 0.1 m / sec or less.

As described above, even if the average surface flow velocity at the 1/4 mold width position is 0.1 m / sec or less,
The fluctuation range of the surface flow velocity has different aspects depending on the flow pattern of the molten steel in the mold. That is, taking the case of the pattern C as an example, the collision position between the flow from the mold center side to the mold short side and the flow from the mold short side to the mold center side is
Depending on the balance of the strengths of both flows, 1/4
In the surface flow at the mold width position, a flow toward the short side of the mold and a flow toward the center of the mold alternately appear, and the fluctuation width of the surface flow velocity increases. The fluctuation range of the surface flow velocity described in the present application is a differential value of the surface flow velocity or a difference between the maximum flow velocity and the minimum flow velocity of the surface flow velocity within a unit time.

FIG. 6 shows the relationship between the fluctuation width of the surface flow velocity and the occurrence rate of mold powder defects in a thin steel sheet product when the average surface flow velocity at the 1/4 mold width position is controlled to 0.1 m / sec or less. It shows the result of investigating the relationship. As shown in the figure, the fluctuation range of the surface velocity is reduced to 0.15 m / sec.
It can be seen that when the value is c or less, the defect occurrence rate is low. still,
FIG. 6 shows that the fluctuation range of the surface flow velocity is 1 minute as a unit time.
It is the difference between the maximum flow rate and the minimum flow rate within a unit time,
In the calculation, the flow from the short side of the mold toward the center of the mold was given a "positive" sign, and the flow in the opposite direction was given a "negative" sign.

This is because even when the average surface flow velocity at the 1/4 mold width position is 0.1 m / sec or less, when the fluctuation width of the surface flow velocity is large, the meniscus is not calmed down and the strong flow from the left and right Is mixed, and a vortex is generated in the meniscus or an instantaneous flow having a flow velocity of more than 0.1 m / sec is generated, so that entrainment of the mold powder occurs.

In the present invention, the average flow velocity of the meniscus at the 1/4 mold width position is controlled to 0.1 m / sec or less by controlling the discharge flow velocity by the moving magnetic field, and further, a static magnetic field is applied to the entire meniscus. Therefore, the magnetic braking force due to the static magnetic field acts on the surface flow according to the surface flow velocity, the fluctuation width of the surface flow velocity is suppressed small, and the occurrence of vortices and instantaneous high-speed flow on the meniscus is prevented. Can be.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the drawings.
FIG. 1 is a schematic view of a front section of a mold section of a continuous casting machine having a rectangular cast slab to which the present invention is applied, FIG. 2 is a section taken along line XX of FIG. 1, and FIG. 1 shows a schematic view of a Y section.

In the figure, a tundish 16 is disposed above a mold 1 composed of opposed mold long sides 2 and opposed mold short sides 3 provided inside the mold long sides 2. A sliding nozzle composed of a fixed plate 17 and a sliding plate 18 is arranged at the bottom of the tundish 16, and a rectifying nozzle 19 and a dipping nozzle 7 are arranged in order on the lower surface side of the sliding plate 18, and 16 to mold 1
Outflow holes 21 are formed. The molten steel 4 injected into the tundish 16 from a ladle (not shown) is provided at the lower part of the immersion nozzle 7 through the outflow hole 21 and is discharged from the discharge hole 8 immersed in the molten steel 4 in the mold 1. The discharge stream 9 is injected into the mold 1 toward the short side 3 of the mold. Then, the molten steel 4 is cooled in the mold 1 to form a solidified shell 6, and is drawn out below the mold 1 to become a cast slab.

An Ar gas introduction pipe 22 is connected to the fixed board 17. Ar gas introduced from the Ar gas introduction pipe 22 is
It is blown into the outflow hole 21 from the porous brick 20 provided on the fixed board 17. The blown Ar gas flows into the mold 1 through the discharge hole 8 through the immersion nozzle 7 together with the molten steel 4, and floats on the meniscus 5 through the molten steel 4 in the mold 1,
It penetrates the mold powder 15 added on the meniscus 5 and reaches the atmosphere.

The other end of the load cell 14 is located at the center of the thickness of the mold 1 and at the left and right 1/4 mold width positions in the width direction of the mold 1.
The refractory rod 13 connected to the refractory rod 13 is immersed in the meniscus 5 and arranged, and the force acting on the refractory rod 13 due to the surface flow of the molten steel 4 is measured by the load cell 14 to determine the speed of the surface flow of the meniscus 5. The direction is measured.

On the back side of the long side 2 of the mold, a moving magnetic field generator 10 in which the moving direction of the magnetic field is the width direction of the mold 1 is located at the center of the moving magnetic field generator 10 in the casting direction with the lower end position of the discharge hole 8. As a range from the lower end position of the mold 1, the mold 1 is disposed to face the mold long side 2. The moving magnetic field generator 10 is divided into left and right parts in the width direction of the mold 1 around the immersion nozzle 7, and is not shown so that the moving direction of the magnetic field is opposite in the left and right directions in the width direction of the mold 1. Connected to AC power supply. The moving direction of the magnetic field generated from the left and right moving magnetic field generators 10 is changed from the short side 3 of the mold to the long side 2 of the mold.
, The discharge flow 9 is decelerated, and conversely, the moving direction of the left and right magnetic fields is changed from the center of the mold long side 2 to the mold short side 3.
The discharge flow 9 is accelerated by being set to the side. The maximum magnetic field strength of the moving magnetic field generator 10 is 0.2 Tesla or more.
An industrially used one of about 0.3 Tesla may be used.

The surface velocity of the meniscus 5 is adjusted by the refractory rod 13.
After the measurement, the intensity of the moving magnetic field and the moving direction of the magnetic field are adjusted so that the average surface flow velocity is 0.1 m / sec or less, and then the adjustment values are fixed. The strength of the moving magnetic field is adjusted by changing the current or voltage applied to the moving magnetic field generator 10, and the moving direction of the moving magnetic field is changed by changing the connection of the AC power supply applied to the moving magnetic field generator 10.

If the AC power supply applied to the moving magnetic field generator 10 is separately arranged in the left and right magnetic field generators 10, the strength and the moving direction of the moving magnetic field can be independently controlled on the left and right sides, and the flow control of the molten steel in the mold can be performed. Is preferred because it becomes even easier. Further, the moving magnetic field generator 10 does not need to be opposed to each other with the mold long side 2 interposed therebetween, and the discharge flow 9 can be controlled only by disposing the moving magnetic field generator 10 on the back surface of the mold long side 2. However, when it is arranged only on the back surface on one side, it is necessary to arrange the moving magnetic field generator 10 having a higher magnetic field strength than when it is arranged on both sides in order to secure the electromagnetic force acting on the molten steel.

Further, on the back of the mold long side 2 over the entire width of the mold 1 at the position including the meniscus 5, the magnetic poles 11 facing the mold long side 2 are arranged. A coil 12 is wound around the magnetic pole 11, and a DC power supply (not shown) is applied to the coil 12 to apply a static magnetic field through the long side 2 of the mold to the molten steel 4 near the meniscus 5 at the magnetic pole 11. The application of the static magnetic field causes the average surface flow velocity of the meniscus 5 to be 0.1 m / sec by the moving magnetic field.
It is after the following control. The static magnetic field may have a magnetic field intensity of about 0.2 Tesla to about 0.3 Tesla, which is generally used in industry. If the magnetic field strength is 0.2 Tesla or more, the fluctuation width of the surface flow velocity can be 0.15 m / sec or less.

Incidentally, the Ar gas blowing position into the immersion nozzle 7 is not limited to the above, and in the case of the main body and upper nozzle of the immersion nozzle 7 or a stopper type opening / closing device,
Even the tip of the stopper does not hinder the implementation of the present invention. The means for measuring the surface flow velocity of the meniscus 5 is not limited to the above, and a measuring device using an electromagnetic force does not hinder the implementation of the present invention at all.

[0034]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention using a continuous casting machine having the structure shown in FIG. 1 will be described below.

The cross section of the slab has a thickness of 250 mm and a width of 160
An extremely low carbon Al killed steel was cast at a slab continuous casting machine of 0 mm at a slab drawing speed of 2.2 m / min. On the back side of the long side of the mold, the moving magnetic field generator divided into two parts on the left and right sides of the immersion nozzle in the width direction of the mold, the center position in the casting direction of the moving magnetic field generator is set 150 mm below the lower end of the immersion nozzle discharge hole. Placed as a position. With this moving magnetic field generator, a moving magnetic field of 0.2 Tesla at the maximum can be applied to the discharge flow.

Further, a magnetic pole is arranged on the back side of the long side of the mold so that the center of the magnetic pole in the casting direction is the meniscus position.
A static magnetic field of 0.2 Tesla was constantly applied from this magnetic pole. Then, 9 Nl / min of Ar gas was blown into the immersion nozzle.

Under these casting conditions, the moving direction of the moving magnetic field is from the short side of the mold to the center of the mold, the strength of the moving magnetic field is 0.1 Tesla (magnetic field strength 50%), and 1 / (1/4) of the left and right of the mold width.
Average surface flow velocity at 4 mold width position is 0.1m / sec
Therefore, the moving direction of the moving magnetic field was from the shorter side of the mold to the center of the mold, and the strength of the moving magnetic field was cast at a constant value of 0.1 Tesla.

During casting, the average surface flow velocity of the meniscus is between 0.
It was controlled to 1 m / sec or less, and the fluctuation width of the surface flow velocity hardly occurred.

On the other hand, as a comparative example, casting was performed under the same casting conditions without applying a static magnetic field to the meniscus and only controlling the moving magnetic field. In this case, the average surface velocity is 0.1 m
/ Sec or less, but 0.3 m / sec
A slight surface flow was observed, and a back flow sometimes occurred, and the time change of the surface flow velocity was severe.

According to the slab inclusion index obtained by inspecting the cross section of the obtained slab and statistically processing the number and size distribution of inclusions by exponential function approximation, the embodiment of the present invention It was about 1/3 of the comparative example. In addition, when these slabs were rolled into sheet products, and the sheet products were subjected to ultrasonic flaw testing to investigate defects caused by inclusions, the defect occurrence rate in the comparative example was 1.3%. , 0.5% in the embodiment of the present invention.
And it was extremely good.

[0041]

According to the present invention, the moving magnetic field and the static magnetic field are used in combination, and the surface flow velocity of the molten steel in the mold can be always optimized by utilizing the excellent characteristics of both. Pieces can be manufactured stably.

[Brief description of the drawings]

FIG. 1 is a schematic view of a front section of a mold portion of a continuous casting machine having a rectangular cast slab to which the present invention is applied.

FIG. 2 is a schematic view of a section taken along line XX of FIG.

FIG. 3 is a schematic diagram of a YY cross section of FIG. 1;

FIG. 4 is a diagram showing the relationship between the surface flow velocity of molten steel in a mold and the incidence of mold powder defects in a thin steel sheet product.

FIG. 5 is a view schematically showing a flow pattern of molten steel in a mold roughly divided into three patterns A to C.

FIG. 6 is a graph showing the relationship between the fluctuation width of the surface flow velocity and the occurrence rate of mold powder defects in a thin steel sheet product.

[Explanation of symbols]

 Reference Signs List 1 mold 2 mold long side 3 mold short side 4 molten steel 5 meniscus 6 solidified shell 7 immersion nozzle 8 discharge hole 9 discharge flow 10 moving magnetic field generator 11 magnetic pole 12 coil 13 refractory rod 14 load cell 15 mold powder 16 tundish

Claims (1)

[Claims] 1. A moving magnetic field generator divided into at least two parts on the left and right sides in a mold width direction around an immersion nozzle. The moving magnetic field is applied to the molten steel by this moving magnetic field generator, and the average of the molten steel in the mold at a position 1/4 of the width of the mold from the center of the mold is arranged. 0.1 m /
In addition to controlling the length of the mold to less than or equal to sec, the opposite magnetic poles sandwiching the long side of the mold are arranged on the back side of the long side of the mold over the entire width of the mold including the meniscus, and a static magnetic field is applied to the meniscus by this magnetic pole. Steel continuous casting method.