JP3149821B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a casting slab having a high quality when the casting speed is low at the beginning or end of casting and the temperature of the molten metal is reduced. It relates to a continuous casting method to be obtained.

[0002]

2. Description of the Related Art A continuous casting method of metal is a method of injecting molten metal (hereinafter sometimes referred to as "molten steel") into a mold using an immersion nozzle and continuously solidifying to obtain a cast piece. It is. The immersion nozzle is provided at the center of the mold,
In a slab with a rectangular horizontal section to obtain a metal plate,
In order to uniformly inject the molten metal in the longitudinal direction of the mold, the discharge holes of the immersion nozzle are provided on the left and right in a direction parallel to the long side of the cross section of the mold.

When the casting speed has to be reduced, such as at the beginning or end of continuous casting, the amount of molten metal injected per unit time (this is called "throughput", Since (cross-sectional area) × (drawing speed of slab) × (specific gravity) decreases, the amount of heat supplied into the mold decreases, and the temperature decreases. Therefore, non-metallic inclusions or bubbles of the inert gas blown to prevent nozzle clogging are less likely to float, are trapped inside the molten metal, and cause internal defects of the slab. In addition, when the temperature in the mold decreases, the solidified shell falls down more frequently at the meniscus, and the ability to capture inclusions of the powder decreases, and the powder is more likely to be trapped by the solidified shell, and the subcutaneous surface defects of the slab are reduced. It causes the occurrence.

[0004] When an internal defect exists in a cast slab, for example, in the case of an H-section steel for building, a double crack occurs, and when a subcutaneous subsurface defect exists, a sliver flaw or a stubble when a cold-rolled steel sheet is formed. It causes skin defects such as flaws and pinholes. All of these drawbacks must be excluded from the product, thus reducing product yield.

[0005] To solve this problem, "The Iron and Steel Institute of Japan Lecture Papers" Materials and Processes "132nd Autumn Lecture Meeting, High Temperature Processes, Social Steel Engineering," Improvement of Ultra-low Coal Bottom Quality "
1996, Vol. 9, No. 4, pp. 841 ”describes a method of adding exothermic material to powder or activating molten steel in a mold by using an electromagnetic stirring device in the mold at the start of casting. Proposed.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to obtain a good slab which does not cause internal defects or surface defects even in a case where the throughput is reduced in the initial and final stages of casting in continuous casting of metal. It is an object of the present invention to provide a continuous casting method.

[0007]

Means for Solving the Problems The present inventor has studied the flow control of molten steel in a mold by a magnetic field, and different poles are opposed to the upper, middle and lower outer surfaces of both side walls on the long side of the mold. By using a casting device equipped with magnets (electromagnets or permanent magnets) and setting the static magnetic field of each stage appropriately, it was found that the throughput did not cause a decrease in the temperature of the molten metal in the mold at least, and completed the present invention. did. The gist of the present invention resides in the following continuous casting method.

A continuous casting method in which a static magnetic field is applied to an upper stage U, a middle stage M and a lower stage L shown below on the outer side of the long side 1A of the mold 1 to solidify while injecting a molten metal. In the middle and lower stages, the direction of applying the magnetic field is opposite to that of the upper stage, and a continuous casting method in which the magnetic field intensity is applied in a range of 1.0 to 2.0 times the upper stage (see FIG. 1).

Upper part: upper part including meniscus 7 and not including discharge flow path 15B from immersion nozzle 2, middle part: intermediate part including discharge flow path from immersion nozzle, lower part: discharge flow path from immersion nozzle Not the lower part.

Here, the “discharge flow path” is a flow path until the molten metal discharged from the immersion nozzle collides with the side wall of the short side of the mold when no static magnetic field is applied (see FIG. 5).

[0011] The term "early casting stage or final casting stage" means a state in which the casting speed must be reduced (the throughput is 3.0 ton / min or less).

[0012]

FIG. 1 is a perspective view of an apparatus used in a continuous casting method according to the present invention. FIG. 2 is a cross-sectional view of FIG.
FIG. 2 is a longitudinal sectional view of the cross section indicated by the mark as viewed from the short side of the mold.

In both figures, a U-shaped iron core 3B as viewed from the casting direction is disposed in an upper stage U, a middle stage M and a lower stage L on the outer surfaces of both side walls of the long side 1A of the mold 1, and a parallel portion 3C of the iron core is formed. , And a DC power supply 5 is connected via a variable resistor 4 as shown in FIG. A magnetic field is formed in the direction of the short side 1B of the casting mold by reversing the polarity of the magnetic poles 3D facing each other with the short side of the slab 1 interposed therebetween. By changing the magnitude or direction of the current supplied to each coil 3A, the magnetic field strength or magnetic field distribution can be changed.

The above-described three-step region is determined based on a normal casting state in which no static magnetic field is applied.

FIG. 5 is a vertical sectional view at the center of the short side of the mold, schematically showing the flow of the molten metal in the ordinary continuous casting mold. As shown by the dashed line in the figure, the flow path until the discharge flow 15 proceeds obliquely downward and collides with the side wall on the short side of the mold is defined as a “discharge flow path 15B”. Set at a position that includes at least a part of the road. The upper stage U is a region that does not include the discharge flow path, is above the discharge flow path, and includes the meniscus 7 of the molten metal 6 in the mold, that is, a region that includes the meniscus flow 17. The lower stage L does not include the discharge flow path,
It is a lower region.

FIG. 3 is a longitudinal sectional view of the center of the short side of the mold showing an example of the flow of the molten metal in the mold when the casting speed is reduced by applying a static magnetic field to each stage. In the figure, the left side shows a situation where casting is performed by a normal method when the casting speed is reduced, and the right side shows a situation where casting is performed by the method of the present invention.

In the continuous casting method of the present invention, when the casting speed is decreased at the beginning or end of casting, as shown in FIG. 3, the secondary descent suppression flow is made equal to or larger than the secondary descent suppression flow. It is intended to prevent the temperature of the molten metal in the mold from lowering and to reduce internal defects and surface defects of the slab.

(1) Magnetic field applied to the middle and lower stages: By applying the magnetic field to the middle and lower stages in a direction opposite to that of the upper stage, there is a region where the magnetic flux density becomes zero between the upper and middle stages. The effect decreases and the secondary updraft increases. As a result, even when the magnetic field strengths of all stages are equal, the secondary ascending flow is stronger than the secondary descending flow, and it becomes difficult for the high-temperature molten metal to enter the lower portion of the slab, so that the The temperature of the molten metal can be prevented from lowering. In addition, by making the middle and lower magnetic field application directions the same direction,
The effect of suppressing the secondary descending flow can be enhanced, and the decrease in the temperature of the molten metal in the mold can be reduced even when the throughput decreases. However, if the magnetic field application directions of the middle stage and the lower stage are opposite, there is a region where the magnetic flux density becomes zero between the middle stage and the lower stage, and the secondary downflow cannot be suppressed.

The reason why the magnetic field intensity applied to the middle and lower stages is equal to or higher than that of the upper stage (but up to twice) is that the suppression of the secondary downflow is made larger than that of the secondary upflow, This is because the molten metal is stirred inside to prevent a decrease in the temperature of the molten metal in the mold. The middle and lower magnetic field strengths may be the same or different. If the magnetic field strength applied to the middle and lower stages is less than 1 times that of the upper stage, the magnetic strength of the middle and lower stages is lower than that of the upper stage, so that the secondary descending flow is not suppressed, the secondary rising flow becomes smaller, and the molten metal to the meniscus Supply stagnates, the temperature of the meniscus decreases, and the amount of inclusions in the slab increases. When the magnetic field strength applied to the middle and lower stages exceeds twice that of the upper stage,
The secondary downward flow is suppressed, the secondary upward flow becomes large, and the slab surface subcutaneous inclusions due to the fluctuation of the molten metal level increase. Note that the magnetic field strengths of the middle and lower stages do not have to be equal.

(2) Magnetic field applied to the upper stage: The strength of the magnetic field applied to the upper stage is made equal to or smaller than the magnetic field intensity to the middle and lower stages because the secondary descending flow is less than the suppressing force of the secondary upward flow. This is to prevent the molten metal of high temperature from entering the lower part of the slab and to prevent the temperature of the molten metal in the mold from dropping. Since the suppressing force of the secondary descending flow is increased, the secondary ascending flow is increased, and the meniscus level is fluctuated. Therefore, the magnitude of the magnetic field strength needs to be set in consideration of the size of the slab. For example, using a mold whose inner wall dimensions are 1600 mm long, 270 mm short, and 900 mm high,
When casting low carbon steel at a casting speed of 1.3 m / min, fluctuations in the meniscus level cannot be reduced if the magnetic field strength is less than 1000 gauss, and if it exceeds 2,000 gauss, the secondary upward flow is excessively suppressed. Therefore, the supply of heat to the meniscus was insufficient, and the ability of the powder to trap inclusions due to a decrease in temperature was reduced.

As a result, when the casting speed is low, the temperature of the molten metal in the mold and the temperature of the meniscus are suppressed from lowering, the floating of inclusions and bubbles is promoted, and the stable absorption of nonmetallic inclusions by the molten powder is achieved. Noh can be secured.

[0022]

EXAMPLE A continuous slab casting machine as shown in Fig. 1 equipped with a water-cooled copper mold having inner wall dimensions of a long side width of 1600 mm, a short side width of 270 mm and a height of 900 mm was used.
A casting test was performed on low-carbon aluminum killed steel at a ton / minute and a stable time of 6.7 ton / minute. As shown in Table 1, a static magnetic field was applied to the upper, middle, and lower stages as shown in Table 1 during the initial stage and the final stage of casting, and a magnetic field of 2,200 gauss was applied to the upper and lower stages, and 3,000 gauss was applied to the middle stage, when stable.

[0023]

[Table 1]

FIGS. 4A and 4B are diagrams showing a magnetic field line distribution and a magnetic field intensity distribution when a static magnetic field is applied to the upper, middle, and lower stages. FIG. 4A shows a case where a magnetic field in a range defined by the method of the present invention is applied. (b) is a diagram showing the case of a magnetic field applied when the casting speed is stable.

In Charge Nos. 1 to 8 of the invention examples, the magnetic field application direction was changed in the upper and lower stages in the middle and lower stages, and the magnetic field intensity in the upper stage was changed within the range of 1000 to 2000 Gauss.
The strength of the applied magnetic field to the middle and lower stages is 1.0 times higher than the upper stage.
A magnetic field in a range up to 2.0 times was applied.

In Charge No. 9 of Comparative Example 1, the applied magnetic field strength to the middle and lower stages was 0.67 times that of the upper stage, and that of Charge No. 10
Then it was 2.3 times. Charge No. 11 and No. 1 of Comparative Example 2
In No. 2, 500 Gauss or 2500 Gauss (magnetic field strength outside the range defined in the present invention) was applied to the upper stage. Charge of Comparative Example 3
In No. 13, the magnetic field application directions of all stages were equal. In the case of Charge No. 14, casting was performed without adjusting the magnetic field application in each stage at the beginning and end of casting.

The temperature rise index of the meniscus was determined by measuring the temperature of the molten metal with a thermocouple embedded in a mold,
4 was set as a reference value 1 and shown as a relative value.

The level stability index was measured by an eddy current level meter, and the relative value was shown with charge No. 14 as the reference value 1.

The inclusion index in the slab is measured by a microscope.
The number of inclusions having a size of 20 μm or more was measured, and charge No. 14 was set as a reference value 1 and indicated as a relative value.

The subcutaneous inclusion index is 3 mm from the surface of the slab.
The number of inclusions up to this point was measured, and charge No. 14 was set as a reference value 1 and indicated as a relative value.

The coil product quality index is determined by visually observing defects such as sliver flaws, scab flaws, and pinholes on the surface of the steel sheet, dividing the weight of defective products by the total weight, and setting the charge No. 14 to a reference value of 1.
And the relative values are shown. The larger the value, the higher the incidence of defective products.

The results are shown in Table 1.

In Charge Nos. 1 to 8 of the invention examples, the meniscus temperature rise index was 1.05 to 1.11, which was higher than 1.00, the temperature drop was small, and the inclusion index in the slab was 0.77 to 0.90, and the number of inclusions decreased. . In addition, the level stability index is 0.63 to 0.94, which is lower than 1.00, and the level fluctuation is suppressed, and the subcutaneous inclusion index is 0.
It has fallen to 63-0.92, and subepidermal inclusions have decreased. As a result, the product defect rate has been reduced.

On the other hand, Charge No. 9 of Comparative Example 1
Since the applied magnetic field intensity in the upper stage was higher than that in the middle and lower stages, the secondary ascending flow was excessively suppressed. As a result, the meniscus temperature rise index became 0.90 or less than 1.00, and the inclusion index in the slab deteriorated to 1.43. As a result, the product defect rate was 1.20
And increased. Charge No. 10, since the upper magnetic field application strength is lower than the middle and lower stages, the secondary ascending flow is excessive, the meniscus temperature rise index is larger than 1.13 and 1.00, and the inclusion index in the slab is 0.86. However, the surface stability index becomes larger than 1.60 and 1.00, and the subcutaneous inclusion index becomes
1.40 and worse. As a result, the product defect rate increased to 1.17.

The charge No. 11 has an upper magnetic field applied strength of 5
Since the value was 00 gauss, which is smaller than the range defined by the present invention, the secondary ascending flow became large, the molten metal surface stability index became larger than 1.40 and 1.00, and the subcutaneous inclusion index worsened to 1.30. As a result, the defect rate of products increased to 1.06.

The charge No. 12 has an upper magnetic field applied strength of 2
500 gauss, which greatly deviates from the range defined by the present invention,
As a result of excessively suppressing the secondary upward flow, the meniscus temperature increase index was 0.98, which was smaller than 1.00, and the inclusion index in the slab was deteriorated to 1.08. As a result, the product defect rate increased to 1.05.

In charge No. 13, the meniscus temperature rise index was 0.90 because the magnetic field application directions of all stages were equal.
It became smaller than 1.00, and the inclusion index in the slab deteriorated to 1.36. As a result, the number of defective products increased to 1.20. This is because there is no region where the magnetic flux density becomes zero between the upper stage and the middle stage, so that the secondary ascending flow is suppressed, and the temperature of the meniscus portion decreases.

[0038]

According to the method of the present invention, when the casting speed in the initial stage or the final stage of continuous casting is lowered, the magnetic field applying devices arranged in the upper, middle and lower stages are used.
By adjusting the respective magnetic field application intensities, it is possible to prevent the temperature of the molten metal in the mold from lowering, to calm down the meniscus surface, and to reduce the occurrence of surface defects and internal defects in the slab. The present invention can be applied not only to casting of ordinary steel, but also to continuous casting of non-ferrous metals such as stainless steel and copper.

[Brief description of the drawings]

FIG. 1 is a perspective view of an apparatus used for a continuous casting method of the present invention.

FIG. 2 is a longitudinal sectional view of the section indicated by XX in FIG. 1 as viewed from the short side of the mold.

FIG. 3 is a vertical cross-sectional view of a center of a short side of a mold showing an example of a flow of a molten metal in the mold when a static magnetic field is applied to each stage to reduce a casting speed.

FIG. 4 is a diagram showing a magnetic field line distribution and a magnetic field intensity distribution when a static magnetic field is applied to an upper stage, a middle stage, and a lower stage, wherein (a) shows a case where a magnetic field in a range defined by the method of the present invention is applied, FIG. 4 is a diagram showing a case of a magnetic field applied when the casting speed is stable.

FIG. 5 is a longitudinal sectional view schematically showing the flow of a molten metal in a normal continuous casting mold at the center of the short side of the mold.

[Explanation of symbols]

 1: Mold 1A: Mold long side wall 1B: Mold short side wall 2: Immersion nozzle 3: Electromagnet 3A: Coil 3B: Iron core 4: Variable resistor 5: DC power supply 6: Molten metal 7: Meniscus 8: Solidified shell 9: Slab 10: Solid powder 11: Melting powder 12: Upper magnetic pole center line 13: Middle magnetic pole center line 14: Lower magnetic pole center line 15: Discharge flow 15A: Discharge suppression flow 16: Secondary rising flow 16A: Secondary rising suppression flow 17: Meniscus flow 18: Secondary descending flow 18A: Secondary descending suppressing flow

Continuation of the front page (56) References JP-A-8-10917 (JP, A) JP-A-1-289550 (JP, A) JP-A-2-117756 (JP, A) JP-A-11-10295 (JP) JP-A-9-174216 (JP, A) JP-A-11-10290 (JP, A) JP-A-6-344080 (JP, A) JP-A-5-154621 (JP, A) 3-275256 (JP, A) JP-A-4-84650 (JP, A) JP 11-502466 (JP, A) JP 10-505792 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/115 B22D 11/04 311 B22D 11/10

Claims (1)

(57) [Claims] 1. A continuous casting method in which a static magnetic field is applied to the upper, middle, and lower stages shown below on the outside of the long side of a mold to solidify while injecting molten metal. Is a continuous casting method characterized by applying a magnetic field in the direction opposite to that of the upper stage and applying a magnetic field intensity in a range of 1.0 to 2.0 times the upper stage. Upper part: upper part including meniscus and not including discharge flow path from immersion nozzle, middle part: middle part including discharge flow path from immersion nozzle, lower part: lower part not including discharge flow path from immersion nozzle, where The discharge flow path is a flow path until the molten metal discharged from the immersion nozzle when no static magnetic field is applied hits the side wall of the short side of the mold.