JP2009131856A - Continuous casting method of steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To apply the optimum electric current corresponding to the variation of steel type or throughput. <P>SOLUTION: Electromagnetic coils, which has two teeth parts 5a, two exciting coils formed by a winding 5b on the outer peripheral parts of these teeth parts 5a and one exciting coil formed by a winding 5c on the whole peripheral part of two teeth parts 5a, are arranged on the outer periphery of each long sides 3b of a mold 3 with the same number and on whole outer periphery of the long side 3b with (2n+2) number (n is a natural number). When electromagnetically stirring a molten steel 2, polyphase alternative electric current is conducted in each exciting coil in all electromagnetic coils. When applying electromagnetic braking on the molten steel 2, direct electric current is conducted in the three exciting coils, and the steel is continuously casted. Depending on the steel composition and the supplying amount of the molten steel 2 poured into the mold 3, electromagnetic stirring caused by a moving magnetic field with the frequency of 1 Hz or more or electromagnetic braking caused by a static magnetic field with the magnetic field strength of 0.1 T or more is switched. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for continuously casting steel while controlling the flow of molten steel in a mold using an electromagnetic coil device that can be used for both electromagnetic braking and electromagnetic stirring.

  In general continuous casting of steel, as shown in FIG. 5, molten steel 2 is supplied into a mold 3 using an immersion nozzle 1 having two discharge holes 1a. In this continuous casting, the molten steel 2 exiting from the discharge port 1a of the immersion nozzle 1 collides with the short side 3a of the mold 3 as shown in FIG. 5, and then branches into an upward flow 2a and a downward flow 2b. Further, 2a becomes a horizontal flow toward the immersion nozzle 1 at the position of the molten steel surface (meniscus) in the mold. In addition, 4 in FIG. 5 shows powder.

  The flow control of the molten steel in the mold is extremely important in terms of operation and quality control of the slab. As a method for realizing the flow control of the molten steel, an electromagnetic brake that applies a braking force to the molten steel discharge flow into the mold. In addition, electromagnetic stirring is known in which molten steel in a mold is stirred by electromagnetic force.

  Among these, electromagnetic stirring has the effect of washing away bubble defects and inclusion defects trapped in the solidified shell by applying a turning force to the molten steel in the meniscus portion. The electromagnetic brake has an effect of suppressing remelting of the solidified shell in the mold by applying a braking force to the discharge flow from the immersion nozzle.

  However, when the amount of molten steel in the mold calculated by the cross-sectional area of the cast in the mold and the casting speed (hereinafter referred to as throughput) is large, when electromagnetic stirring is applied, the discharge flow from the immersion nozzle is caused by the stirring force. In the region where the accelerating is accelerated, the solidified shell is remelted and broken, and the possibility of breakout increases. In order to avoid this problem, conventionally, measures such as regulation of the casting speed have been taken, which has led to the inhibition of the efficiency of continuous casting.

  On the other hand, when the electromagnetic brake is applied when the throughput is small, there is too much braking force applied to the discharge flow from the immersion nozzle, the heat supply to the meniscus is insufficient, and inclusions that cause surface flaws that occur during rolling are not present. The possibility of being trapped by the solidified shell is increased. Therefore, when the throughput is small, measures such as not applying an electromagnetic brake have been taken. However, slabs with low throughput have absolutely insufficient heat supply to the meniscus, so the quality is low, and slab maintenance and application to high-grade grades are inevitably restricted. It was.

  In order to avoid the above problems, electromagnetic brakes are generally used at high speed casting with a high throughput, and electromagnetic stirring is used at low speed casting with a low throughput. Both of these electromagnetic brake devices and electromagnetic stirrers are realized by installing an electromagnetic coil device with an exciting coil wound around an iron core on the back of the mold. I have only.

In view of this, an electromagnetic coil device (hereinafter referred to as a combined coil device) that can be used for both functions of electromagnetic brake and electromagnetic stirring has been developed, and the applicant has proposed Patent Documents 1 and 2, for example.
JP 2005-349454 A JP 2007-007719 A

  The combined coil device of Patent Documents 1 and 2 selectively applies an electromagnetic brake or electromagnetic stirring to molten steel in a mold by supplying a direct current or an alternating current to an electromagnetic coil disposed on the outer periphery of the mold. Thus, the electromagnetic brake and the electromagnetic stirring can be used together.

  When such a dual-purpose coil device is used, it is thought that flow control can be given to the molten steel in the mold even in the region where the above-mentioned operational and quality problems occur, but it responds to changes in steel type and throughput during continuous casting. None of the disclosed optimal current application methods has been disclosed.

  The problem to be solved by the present invention is that there is an optimum current application method corresponding to changes in steel type and throughput in continuous casting using a combined coil device that enables both functions of electromagnetic brake and electromagnetic stirring. There was no disclosure.

The steel continuous casting method of the present invention is
In continuous casting using a combined coil device, it applies the optimal current corresponding to changes in steel type and throughput,
Two teeth, two excitation coils with windings around each of the teeth, and one excitation coil with windings around the two teeth An electromagnetic coil having the same number on each long side and (2n + 2) total (n is a natural number) on the outer periphery of the mold long side,
When electromagnetically stirring the molten steel in the mold, a multiphase alternating current is applied to each exciting coil in all the electromagnetic coils,
When applying an electromagnetic brake to molten steel in a mold, a method of continuously casting steel by passing a direct current through the three exciting coils,
The main feature is to switch between electromagnetic stirring by a moving magnetic field with a frequency of 1 Hz or more, or electromagnetic braking by a static magnetic field with a magnetic field strength of 0.1 T or more, depending on the composition of the molten steel to be supplied to the mold and the amount of hot water.

More specifically,
The component carbon concentration of the molten steel supplied to the mold is
In the case of 0.005% or less by mass%,
If the amount of hot water supplied to the mold is less than 5.0 ton / min, apply the electromagnetic stirring to the molten steel in the mold,
When the amount of hot water supplied into the mold is 5.0 ton / min or more, the electromagnetic brake is applied to the molten steel in the mold.

The component carbon concentration of the molten steel supplied to the mold is
In the case of mass% exceeding 0.005% and 0.07% or less,
When the amount of hot water supplied to the mold is less than 4.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold,
When the amount of hot water supplied into the mold is 4.0 ton / min or more, the electromagnetic brake is applied to the molten steel in the mold.

Furthermore, the component carbon concentration of the molten steel supplied to the mold is
In the case of mass% exceeding 0.07% and 0.16% or less,
When the amount of hot water supplied to the mold is less than 3.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold.
When the amount of hot water supplied into the mold is 3.0 ton / min or more, the electromagnetic brake is applied to the molten steel in the mold.

Furthermore, the component carbon concentration of the molten steel supplied to the mold is
In the case of mass% exceeding 0.16% and 1.0% or less,
When the amount of hot water supplied to the mold is less than 4.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold,
When the amount of hot water supplied into the mold is 4.0 ton / min or more, the electromagnetic brake is applied to the molten steel in the mold.

  In the present invention, when the index (-) indicated by the product of the amount of molten steel supplied to the mold (ton / min) and the molten steel superheat degree (° C) in the tundish exceeds 50, an electromagnetic brake is applied. When the index is 50 or less, it is desirable not to actuate the electromagnetic brake or to actuate electromagnetic stirring from the viewpoint of improving the quality and avoiding operational troubles associated with the fracture of the solidified shell.

  In the present invention, electromagnetic stirring and electromagnetic braking are arbitrarily switched according to the throughput conditions during casting, and optimum molten steel flow control is performed for the conditions, so that a continuous cast slab with good quality can be stabilized even under unsteady conditions. Can be manufactured.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to FIGS. 1 to 4 together with the process from the idea of the present invention to the solution of the problem.

  The inventors examined factors related to switching between electromagnetic stirring and electromagnetic braking when the combined coil device proposed in Patent Document 2 was used in continuous casting of steel. As a result, we found that it is possible to sort by the carbon concentration, which is the basic component of steel, which largely controls the steel quality, and the throughput, which is one of the factors of the casting speed that currently uses electromagnetic brake and electromagnetic stirring. .

Hereinafter, the results of investigations conducted by the inventors will be described.
An ultra-low carbon steel, low carbon steel, subperitectic steel with a vertical bend type continuous casting machine capable of producing slabs with a width of 1625mm and a thickness of 270mm and a carbon concentration of 0.002 to 0.73% by mass High-carbon steel was cast with varying throughput.

FIG. 1 shows a combined coil device used for casting and typical dimensions thereof.
Reference numeral 5 in FIG. 1 denotes two dual-purpose coil devices continuously arranged on the long side 3b side of the mold 3, and each of the two tooth portions 5a is provided with a winding 5b, and the two tooth portions 5a The winding 5c is given to the outer side, and it unites. In addition, 5d shows the yoke part which made the upper end the same height as the meniscus, and 6 shows the backup plate installed in the outer side of the casting_mold | template 3. FIG.

  According to the experiments by the inventors, in the combined coil device shown in FIG. 1, in the case of electromagnetic stirring, a minimum stirring force can be obtained by applying a moving magnetic field having a frequency of 1 Hz or more as an alternating current. There was found.

  In the case of an electromagnetic brake, by applying a static magnetic field so that the maximum magnetic field strength at the center in the mold is 0.1 T (Tesla) or more as a direct current, the minimum braking force against the discharge flow from the immersion nozzle is obtained. It turns out that you can get.

The steel continuous casting method of the present invention is based on the above knowledge,
Two teeth, two excitation coils with windings around each of the teeth, and one excitation coil with windings around the two teeth An electromagnetic coil having the same number on each long side and (2n + 2) total (n is a natural number) on the outer periphery of the mold long side,
When electromagnetically stirring the molten steel in the mold, a multiphase alternating current is applied to each exciting coil in all the electromagnetic coils,
When applying an electromagnetic brake to molten steel in a mold, a method of continuously casting steel by passing a direct current through the three exciting coils,
The main feature is to switch between electromagnetic stirring by a moving magnetic field with a frequency of 1 Hz or more, or electromagnetic braking by a static magnetic field with a magnetic field strength of 0.1 T or more, depending on the composition of molten steel and the amount of hot water supplied to the mold. .

  The inventors have used the combined coil device 5 shown in FIG. 1 with the following specifications, and according to a more specific application mode in the method of the present invention, depending on the component carbon concentration and the throughput amount, The presence or absence of breakout occurrence was investigated by changing the application. The result is shown in FIG.

(Specifications of the combined coil device)
Electromagnetic force at the center of the mold: 3000 Gauss
Frequency: 4.0Hz
Current applied to the coil: 90 turns of 1000A (90000AT)
AC current phase: 120 ° phase 3-phase AC

From FIG. 2, the specific conditions in the method of the present invention were found.
(When the component carbon concentration of molten steel supplied to the mold is 0.005 mass% or less)
In this case, the solidification temperature of the molten steel is high, the δ phase appears during solidification, and the strength of the solidified shell in the mold is high. Therefore, at a high throughput of less than 5.0 ton / min, There was no breakage and no breakout occurred.

  Therefore, it is possible to apply electromagnetic stirring in a high throughput range of less than 5.0 ton / min. When the amount of hot water supplied into the mold is less than 5.0 ton / min, electromagnetic stirring is applied to the molten steel in the mold. In the case of 5.0 ton / min or more, the electromagnetic brake may be applied to the molten steel in the mold.

(When the component carbon concentration of the molten steel supplied to the mold exceeds 0.005 mass% and is 0.07 mass% or less)
In this case, the high-temperature strength is reduced as compared with steel having a component carbon concentration of molten steel of 0.005% by mass. However, since non-uniform solidification due to the peritectic reaction is not involved, electromagnetic stirring can be performed in a wider through-pit region as compared with steel having a component carbon concentration of 0.07 to 0.20 mass%.

  According to the investigation by the inventors, at a throughput of less than 4.0 ton / min, breakage of the solidified shell did not occur even when electromagnetic stirring was performed, and no breakout occurred.

  Therefore, when the amount of hot water supplied to the mold is less than 4.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold, and when it is 4.0 ton / min or more, the electromagnetic brake is applied to the molten steel in the mold. What is necessary is just to apply in order to act.

(When the component carbon concentration of the molten steel supplied to the mold is more than 0.07% by mass and 0.16% by mass or less)
In this case, the solidified shell grows non-uniformly with the peritectic reaction, which is caused by the separation of the mold from the solidified shell due to the initial non-uniformity and further progress of the solidification delay. Accordingly, the thinned portion of the solidified shell becomes conspicuous, and the throughput range in which electromagnetic stirring can be applied is significantly narrowed. In a high throughput region, it is necessary to suppress the discharge flow rate by applying an electromagnetic brake.

  According to the investigation by the inventors, in the case of a throughput of less than 3.0 ton / min, even if electromagnetic stirring was performed, the solidified shell was not broken and no breakout occurred.

  Therefore, when the amount of hot water supplied into the mold is less than 3.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold, and when it is 3.0 ton / min or more, the electromagnetic brake is applied to the molten steel in the mold. What is necessary is just to apply in order to act.

(When the component carbon concentration of the molten steel supplied to the mold exceeds 0.16% by mass and is 1.0% by mass or less)
Also in this case, the high-temperature strength is reduced as compared with steel having a component carbon concentration of 0.005% by mass, as in the case where the component carbon concentration of the molten steel exceeds 0.005% by mass and is 0.07% by mass or less. However, since non-uniform solidification due to the peritectic reaction is not involved, electromagnetic stirring can be performed in a wider throughput region than steel having a component carbon concentration of 0.07 to 0.20 mass%.

  Therefore, when the amount of hot water supplied to the mold is less than 4.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold, and when it is 4.0 ton / min or more, the electromagnetic brake is applied to the molten steel in the mold. What is necessary is just to apply in order to act.

The inventors further examined factors relating to the relationship with the degree of superheated molten steel in the tundish.
As a result, even at high throughput, it was found that when the molten steel superheat degree in the tundish is low, the surface quality of a product such as a coil is lowered by applying an electromagnetic brake.

  From this, the inventors found that the relationship between the surface quality of the product and the application method of electromagnetic flow control is arranged by an index indicated by the product of the throughput and the molten steel superheat degree, and the relationship between FIG. 3 and FIG. Got.

  FIG. 3 shows a result of continuous casting of molten steel having a component carbon concentration of 0.01 to 0.10% by mass at a casting speed of 1.6 m / min or more while applying an electromagnetic brake. As can be seen from FIG. 3, when the electromagnetic brake is applied when the index is 50 or less, the surface defect occurrence rate of the product is deteriorated as compared with the case where the electromagnetic brake is applied when the index exceeds 50.

  This is because when the index is 50 or less, the molten steel temperature at the meniscus position is lowered, and the trapping of inclusions has progressed. Therefore, when the index is 50 or less, the quality is improved by not applying an electromagnetic brake that impedes heat supply to the meniscus by the molten steel flow.

  On the other hand, FIG. 4 shows the result of continuous casting of molten steel having a component carbon concentration of 0.01 to 0.11% by mass at a throughput of 3.8 to 6.1 ton / min without applying an electromagnetic brake. Yes (casting width is 1140-1625 mm).

  From FIG. 4, in the region where the index exceeds 50, when the electromagnetic brake is not applied, the breakout occurrence index becomes larger and breakout is likely to occur compared to the region where the index is 50 or less. I understand.

  Therefore, in the region where the index exceeds 50, by applying an electromagnetic brake, it is possible to prevent the solidified shell from being broken by the discharge flow from the immersion nozzle that leads to breakout, and to avoid the operation index associated with the fracture of the solidified shell. However, it is desirable to improve the surface quality of the product.

  In the region where the index is 50 or less, it is desirable to improve the surface quality of the product by not operating the electromagnetic brake or by applying electromagnetic stirring.

From the results of FIGS. 3 and 4 above, in the region where the index is 50 or less, the electromagnetic brake is not applied or electromagnetic stirring is applied to improve the surface quality of the product, while the index is 50 It has been found that it is desirable to avoid the operational trouble associated with the fracture of the solidified shell by applying an electromagnetic brake in the region exceeding.

  It goes without saying that the present invention is not limited to the above-described examples, and the embodiments may be appropriately changed within the scope of the technical idea described in each claim.

  For example, the AC current may not be three-phase, but may be more as long as the current phase difference is 90 degrees to 120 degrees.

  In the above example, the case where the component carbon concentration of the molten steel exceeds 0.73% by mass is not investigated, but if the component carbon concentration exceeds 0.5% by mass, the liquid phase → γ single phase in all regions Therefore, the solidification characteristics of the shell are estimated to be almost the same.

  The present invention described above can be applied to any type of continuous casting such as a curved type and a vertical type as long as it is continuous casting. Moreover, it can be applied not only to continuous casting of slabs but also to continuous casting of blooms.

It is a figure explaining the shape of the combined coil used for this invention method, (a) is a horizontal sectional view, (b) is a vertical sectional view. (A) (b) is the figure which showed the result of having investigated the presence or absence of breakout generation | occurrence | production, changing a carbon concentration and a throughput, electromagnetic stirring, and the application of an electromagnetic brake using a combined coil apparatus. It is the figure which showed the relationship between the parameter | index shown by a throughput x molten steel superheat degree, and the surface defect occurrence index of a product. It is the figure which showed the relationship between the parameter | index shown by a throughput x molten steel superheat degree, and a breakout generation | occurrence | production index. It is a longitudinal cross-sectional view which shows typically the flow state of the molten steel in a mold in a general continuous casting method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Immersion nozzle 2 Molten steel 3 Mold 3a Short side 3b Long side 5 Combined coil apparatus 5a Teeth part 5b Inner winding 5c Outer winding 5d Yoke part

Claims (6)

Two teeth, two excitation coils with windings around each of the teeth, and one excitation coil with windings around the two teeth An electromagnetic coil having the same number on each long side and (2n + 2) total (n is a natural number) on the outer periphery of the mold long side,
When electromagnetically stirring the molten steel in the mold, a multiphase alternating current is applied to each exciting coil in all the electromagnetic coils,
When applying an electromagnetic brake to molten steel in a mold, a method of continuously casting steel by passing a direct current through the three exciting coils,
A method for continuous casting of steel characterized by switching electromagnetic stirring by a moving magnetic field having a frequency of 1 Hz or more, or electromagnetic braking by a static magnetic field having a magnetic field strength of 0.1 T or more, depending on the composition of the molten steel to be supplied to the mold and the amount of hot water to be supplied. . The component carbon concentration of the molten steel supplied to the mold is
In the case of 0.005% or less by mass%,
If the amount of hot water supplied to the mold is less than 5.0 ton / min, apply the electromagnetic stirring to the molten steel in the mold,
2. The steel continuous casting method according to claim 1, wherein when the amount of hot water supplied into the mold is 5.0 ton / min or more, an electromagnetic brake is applied to the molten steel in the mold. The component carbon concentration of the molten steel supplied to the mold is
In the case of mass% exceeding 0.005% and 0.07% or less,
When the amount of hot water supplied to the mold is less than 4.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold,
2. The steel continuous casting method according to claim 1, wherein when the amount of hot water supplied into the mold is 4.0 ton / min or more, an electromagnetic brake is applied to the molten steel in the mold. The component carbon concentration of the molten steel supplied to the mold is
In the case of mass% exceeding 0.07% and 0.16% or less,
When the amount of hot water supplied to the mold is less than 3.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold.
2. The steel continuous casting method according to claim 1, wherein when the amount of hot water supplied into the mold is 3.0 ton / min or more, an electromagnetic brake is applied to the molten steel in the mold. The component carbon concentration of the molten steel supplied to the mold is
In the case of mass% exceeding 0.16% and 1.0% or less,
When the amount of hot water supplied to the mold is less than 4.0 ton / min, it is applied to cause electromagnetic stirring to the molten steel in the mold,
2. The steel continuous casting method according to claim 1, wherein when the amount of hot water supplied into the mold is 4.0 ton / min or more, an electromagnetic brake is applied to the molten steel in the mold.   When the index (−) indicated by the product of the amount of hot water supplied into the mold (ton / min) and the molten steel superheat degree (° C.) in the tundish exceeds 50, an electromagnetic brake is applied and the index is 50 6. The continuous casting method according to claim 1, wherein an electromagnetic brake is not applied or electromagnetic stirring is applied in the following cases.

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