JP2001321901A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for continuously casting a steel with which center segregation can stably be reduced over the whole width and the whole length of a cast slab in an inexpensive facility. SOLUTION: In the continuous casting method for casting a slab having a rectangle in the cross sectional shape by using an immersion nozzle having two spouting holes, in which the direction of spouting flow is respective short wall side directions at both sides of the width direction of a mold, the shape of these spouting holes is rectangle and the dimension of the spouting hole and the inner diameter of the immersion nozzle satisfy the conditions in the following formulas (A) and (B). W/H<=0.6...(A), (W×H)/(D×D)>=0.8...(B), wherein, W: the horizontal width (mm) of the spouting hole, H: the vertical length (mm) of the spouting hole, D: the inner diameter (mm) of the immersion nozzle.

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 to obtain a slab with less center segregation.

[0002]

2. Description of the Related Art Internal defects called center segregation may occur at the center of the thickness of a slab obtained by continuous casting of steel. This defect, C, in the final solidification part of the slab
The segregated components such as S, P, and Mn appear in a concentrated manner. The defects in the cast slab cause a reduction in toughness of a thick plate as a product and a cause of hydrogen-induced cracking of a large-diameter steel pipe manufactured by bending and welding a thick plate. The mechanism of generation of this center segregation is considered as follows. Solidification progresses,
Segregated components are concentrated between dendrite trees, which are one of the solidified structures, and this concentrated molten steel flows out of the dendrite trees due to shrinkage of the slab during solidification or blistering of the slab called bulging, and the final solidified part It flows toward the solidification completion point and solidifies as it is to form a thickened zone. This concentrated zone is the center segregation. As measures to prevent center segregation, it is effective to prevent the movement of the concentrated molten steel remaining between the dendrite trees and to prevent local accumulation of the concentrated molten steel. For example, Japanese Patent Application Laid-Open No. 9-57410 proposes a method in which a slab including an unsolidified portion is bulged, and an amount corresponding to the bulging amount is reduced upstream of the final solidified portion in the casting direction. In this method, since the short sides of both ends of the slab after solidification are not reduced, the rolling force by the pair of reduction rolls effectively acts on the reduction of the slab of the unsolidified portion. Therefore, the concentrated molten steel can be discharged to the upstream side in the casting direction, and reduction of center segregation can be expected.

However, even with this method, it is sometimes difficult to stably reduce center segregation over the entire width of the slab. The rolling effect of the slab including the unsolidified portion is obtained by accurately reducing the final solidified portion of the slab. However, the position of the final solidified portion tends to change in the width direction of the slab. When a commonly used immersion nozzle having two discharge holes is used, the molten steel having a relatively high temperature easily flows to both sides of the slab, so that the tip shape of the final solidified portion tends to have a so-called W-shaped profile. . On the other hand, when the slab is reduced by the pair of reduction rolls, the reduction is performed in a linear shape perpendicular to the casting direction. Therefore, it is difficult to simultaneously reduce the final solidified portion of the entire width of the slab using the reduction roll pair, and it is difficult to stably reduce the center segregation over the entire width of the slab.

On the other hand, Japanese Patent Application Laid-Open No. Hei 5-237621 discloses that the flow of a molten metal continuously supplied into a mold is controlled by an electromagnetic force, and the tip shape of a final solidified portion inside the slab is changed in the slab width direction. There has been proposed a method of rolling down a slab while keeping the shape uniform. But with this method,
In order to attach a heavy object such as an electromagnetic force generator to the mold,
The mold and the mold vibrator become excessive equipment.

Japanese Patent Application Laid-Open No. 9-47552 discloses that a plurality of discharge holes are provided on the side of a submerged nozzle in which the direction of the discharge flow is the short side direction on both sides in the width direction of the mold. A method has been proposed in which a plurality of lower discharge holes are provided at the bottom of the immersion nozzle, and the slab of the unsolidified portion is reduced. This method does not require excessive equipment such as the above-described electromagnetic force generator.

However, in this method, since a plurality of discharge holes are provided, each of the discharge holes becomes small, so that the discharge holes are liable to be clogged with an oxide of Al in molten steel during casting. In extreme cases, continuation of casting becomes difficult. Further, since some of the discharge holes are clogged during casting, the flow velocity distribution of the discharge flow in the width direction of the slab tends to be non-uniform. In such a case, it is difficult to obtain a uniform tip end shape of the final solidified portion in the slab width direction, so that a slab having a small center segregation over the entire width and the entire length of the slab cannot be stably obtained.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous casting method for steel in which center segregation can be stably reduced over the entire width and length of a slab with inexpensive equipment.

[0008]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a method of forming a rectangular cross section by using an immersion nozzle having two discharge holes in which the direction of the discharge flow is the short side direction on each side of the mold width direction. A continuous casting method for casting pieces, wherein the shape of the discharge holes is rectangular, and the dimensions of the discharge holes and the inner diameter of the immersion nozzle satisfy the following conditions (A) and (B): The present invention is directed to a continuous casting method for steel in which a slab is cast by using the method and the unsolidified region of the slab is continuously reduced.

W / H ≦ 0.6 (A) (W × H) / (D × D) ≧ 0.8 (B) where W: width of discharge hole (mm) H : Vertical length of discharge hole (mm) D: Inner diameter of immersion nozzle (mm) FIG. 1 is a diagram schematically showing the shape of the immersion nozzle. The symbol W shown in the figure indicates the width of the rectangular discharge hole, and the symbol H
Indicates its vertical length. Symbol D indicates the inside diameter of the portion through which molten steel passes inside the immersion nozzle.

In the case of casting a slab having a rectangular cross-section using a normal immersion nozzle having two discharge holes in which the direction of the discharge flow is the direction of each short side on both sides of the mold width direction,
The reason that the shape of the final solidified portion tends to be the shape of the W-shaped profile is as follows.

FIG. 2 is a diagram schematically showing a flow velocity distribution of a discharge flow from a rectangular discharge hole. Only the vicinity of one ejection hole is shown. The size of the discharge hole 2 is the width W;
0 (mm), vertical length H; 60 to 200 (mm), inner diameter D; 60 to 100 (mm). When using the immersion nozzle 1, a three-dimensional incompressible Navier The results obtained by analyzing the flow velocity distribution of the discharge flow from the Stokes equation and the continuous equation are shown.

FIG. 2A shows an example of an analysis result of a non-uniform discharge flow velocity distribution when a normal immersion nozzle having two discharge holes is used, and FIG. 2B shows a result of the method of the present invention. An example of an analysis result of a flow velocity distribution of a uniform discharge flow when using a prescribed immersion nozzle will be described. Each arrow in the figure indicates a velocity vector 3 of the discharge flow.

When a normal immersion nozzle having two discharge holes is used, as shown in FIG. 2A, the flow velocity of the discharge flow from the lower part of the discharge hole is high, and the discharge is performed at the upper part of the discharge hole. Rather, it is understood that a small amount is sucked. Such uneven distribution of the discharge flow velocity is caused by
This is because the pressure loss of the discharge flow is small at the outlet of the discharge hole. Since the flow velocity distribution of the discharge flow is not uniform, molten steel having a relatively high temperature tends to flow on both sides of the cast slab, and the shape of the final solidified portion tends to have a W-shaped profile. By the way, if the sectional area of the discharge hole is simply increased, the pressure loss of the discharge flow is further reduced, and the flow velocity distribution of the discharge flow is more likely to be uneven.

Therefore, in the method of the present invention, two discharge holes are provided, the shape of the discharge hole is rectangular, and the size of the discharge hole and the inner diameter of the immersion nozzle are determined by the above-mentioned formulas (A) and (B). Casting is performed using an immersion nozzle that satisfies the conditions of the formula. The shape of the discharge hole is made flat so as to satisfy the expression (A), and the cross section of the portion of the immersion nozzle where the molten steel passes from above to below so as to satisfy the expression (B). When the cross-sectional area is increased within an appropriate range, a uniform discharge flow velocity distribution is obtained as shown in FIG. This is because the pressure loss of the discharge flow increases at the outlet of the discharge hole. Since the flow velocity distribution of the discharge flow becomes uniform, the molten steel having a relatively high temperature tends to flow uniformly in the width direction of the slab, and the tip shape of the final solidified portion becomes uniform in the width direction of the slab.

Further, in the method of the present invention, the aforementioned (A)
The unsolidified region of the cast slab is continuously reduced by using an immersion nozzle that satisfies the conditions of the equations (B) and (B). Since the shape of the tip of the final solidified part is uniformly aligned in the slab width direction, the segregation component near the final solidified part was concentrated by continuously reducing the unsolidified area of the slab. The molten steel can be effectively discharged to the upstream side in the casting direction over the entire width of the slab.

Further, in the method of the present invention, since the immersion nozzle having two discharge holes is used, the cross-sectional area of the discharge hole can be relatively large as compared with the immersion nozzle having a plurality of discharge holes. Therefore, clogging of the immersion nozzle hardly occurs during casting. Since the discharge hole is not easily clogged, the flow velocity distribution from the discharge hole can be secured uniformly during prolonged casting, and the molten steel in which the segregated component near the final solidification part is concentrated over the entire length of the slab is effective upstream in the casting direction. Can be discharged.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The use of an immersion nozzle satisfying the above-described equations (A) and (B) will result in a more uniform distribution of the flow velocity of the discharge flow from the discharge holes. .

FIG. 3 is a diagram showing the influence of the shape of the discharge hole of the immersion nozzle on the flow velocity distribution of the discharge flow. The horizontal axis represents the ratio W of the width W of the rectangular discharge hole to the inner diameter D of the immersion nozzle.
/ D. The vertical axis represents the maximum flow velocity Umax of the discharge flow when the pressure loss of the discharge flow at the outlet of the discharge hole is considered, and the average when the discharge flow is formed with a uniform flow distribution over the entire discharge hole. Ratio Uma divided by discharge flow rate Um
x / Um. It is a figure which shows the influence of the shape of a discharge hole on the flow velocity distribution of the discharge flow on the conditions which make the value of (W * H) the same as the value of (D * D). The result of analyzing the flow velocity distribution of the discharge flow using the same analysis method as described above is shown.

FIG. 2 shows that Umax / Um becomes a constant minimum value of approximately 1.6 by setting W / D to 0.6 or less. That is, (W × H) = (D ×
Under the condition D), by setting the W / D to 0.6 or less, the maximum flow velocity Umax of the discharge flow can be reduced, and the flow velocity distribution of the discharge flow can be made more uniform.

Since the inner diameter D of the immersion nozzle is a constant value to be determined according to the rectangular cross section size of the slab and the casting speed, the condition of (W × H) = (D × D) is defined as Corresponds to a constant case. Therefore, reducing the W / D to 0.6 or less means reducing the lateral width W of the discharge hole while keeping the sectional area of the discharge hole constant.
That is, the shape of the rectangular discharge hole is made flat. By making the shape of the discharge hole flat, the pressure loss at the outlet of the discharge hole increases, so that the flow velocity distribution of the discharge flow becomes uniform.

It is desirable that W / D be 0.1 or more.
If less than 0.1, the discharge hole is too flat, the discharge flow from the upper part of the discharge hole affects the meniscus portion of the molten steel,
It becomes easy to get the mold powder into the molten steel.

FIG. 4 is a diagram showing the influence of the shape of the discharge hole of the immersion nozzle on the flow velocity distribution of the discharge flow. FIG. 4 is a diagram illustrating the effect of the ejection hole shape when the value of W / D is 0.6, analyzed by the same analysis method as used in FIG. 3. (W × H) / (D
XD) is set to 0.8 or more, Umax / U obtained by dividing the maximum flow rate Umax of the discharge flow by the maximum flow rate Umaxo of the discharge flow when the shape of the discharge hole is a square having a side length D.
maxo is a substantially constant minimum value of 0.42, and it can be seen that the flow velocity distribution of the discharge flow can be made more uniform.

As described above, the inner diameter D of the immersion nozzle is a constant value to be determined according to the rectangular slab cross-sectional size and the casting speed, and (W × H) / (D × D) is set to 0.8 or more. This means that the sectional area of the discharge hole is increased. By setting the value of W / D to 0.6 and making the shape of the discharge hole flat and increasing the cross-sectional area of the discharge hole, the pressure loss at the outlet of the discharge hole is secured, and the average flow velocity of the discharge flow is reduced. The flow velocity distribution of the discharge flow becomes uniform.

(W × H) / (D × D) is desirably 2.0 or less. If it exceeds 2.0, the cross-sectional area of the discharge hole becomes too large, and the discharge flow from the upper portion of the discharge hole affects the meniscus portion of the molten steel, so that the mold powder is easily entangled in the molten steel.

From FIGS. 3 and 4, an immersion nozzle that satisfies the conditions of the above-described expressions (A) and (B), that is, an immersion nozzle in which the cross-sectional area of the discharge hole is large and its shape is flat is used. This indicates that the maximum flow velocity Umax of the discharge flow can be reduced, and the flow velocity distribution of the discharge flow becomes more uniform. Since the flow velocity distribution of the discharge flow becomes uniform, the shape of the tip of the final solidified portion becomes uniform in the slab width direction. Since the unsolidified region of the slab, which is the final solidified portion of such a shape, is continuously reduced, the molten steel in which the segregated component near the final solidified portion is concentrated is effectively applied upstream in the casting direction over the entire width of the slab. Can be exhausted. Therefore, center segregation can be stably reduced.

[0026]

EXAMPLE Using a vertical bending type continuous casting machine with a vertical length of 3.0 m, a medium carbon steel, which is a steel for a thick plate having a C content of 0.15 to 0.20 mass%, was 235 mm thick and 235 mm wide. 1800mm
Was cast into a slab. In each test, molten steel of about 60 t per heat was cast.

One reduction roll pair is arranged at a position 16 m from the meniscus, and the thickness is 20 to 30 m at the center of the width of the slab.
The slab that had been bulged by about m was pressed down to the original thickness of the slab. The casting speed was about 1 m / min, and the amount of molten steel passing through the immersion nozzle was about 3.2 t / min.

The immersion nozzle used in the test was a cylindrical immersion nozzle made of alumina graphite and having a bottom. The outer diameter was 130 mm and the inner diameter was 90 mm. The discharge flow was two downward 15 ° discharge holes directed toward the short side of the mold, and the shape of the discharge holes was rectangular. The size of the rectangle was varied and tested. In each test, a cross section sample of a 200 mm long slab in the casting direction was taken. 200m from the center and short side of the slab sample
A portion 3 m apart, including the center of the thickness, was cut from 11 locations in the thickness direction of the slab using a drill blade with a diameter of 3 mm, and the C content was analyzed. The state of center segregation of the slab was evaluated by the ratio C / C ₀ obtained by dividing the C value of the C content by the ladle value C ₀ . Table 1 shows each test condition and each test result.

[0029]

[Table 1] Test No. In the case of 1, within the range defined by the present invention,
An immersion nozzle having W / H of 0.6 and (W × H) / (D × D) of 1.0 was used. Test No. In No. 2, W / H is 0.6 and (W × H) / (D) within the range defined by the present invention.
× D) was used for the immersion nozzle of 0.8. Test N
o. In No. 3, the conditions specified in the present invention were removed, and an immersion nozzle having W / H of 1.0 and (W × H) / (D × D) of 1.0 was used.

Test No. 1 and No. 1 In No. 2, the occurrence of center segregation at the part 200 mm away from the center of the width and the short side of the slab is almost the same and mild, and the value of C / C ₀ at the center of the slab thickness is It was about 1.05, which was a favorable state of center segregation. It is considered that by increasing the cross-sectional area of the discharge hole and making the shape of the discharge hole flat, the flow velocity distribution of the discharge flow was made uniform, and the center segregation was improved by the effect.

Test No. In No. 3, the value of C / C ₀ at the center of the thickness at the center of the width of the slab was almost 1.05, which was good, but the center of the slab at a distance of 200 mm from the short side was good. Has a maximum C / C ₀ value of 1.4, and significant center segregation has occurred.

FIG. FIG. 6 is a diagram showing a state of center segregation in the case of No. 3; The cross-sectional area of the discharge hole was measured using Test No. 1
However, the shape of the discharge holes was not flat but square. Therefore, it is presumed that the flow velocity distribution of the discharge flow becomes non-uniform, and the shape of the final solidified portion has a W-shaped profile. The thickness of the unsolidified portion at the rolling position of the portion of the slab 200 mm away from the short side was thicker than the center of the width, and significant center segregation occurred in the portion of the slab. It is.

[0033]

According to the method of the present invention, center segregation can be stably reduced over the entire width and the entire length of the slab with inexpensive equipment, and a slab of good internal quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing the shape of a submerged nozzle.

FIG. 2 is a diagram schematically illustrating a flow velocity distribution of a discharge flow from a rectangular discharge hole.

FIG. 3 is a diagram showing the influence of the shape of the discharge hole of the immersion nozzle on the flow velocity distribution of the discharge flow.

FIG. 4 is a diagram showing the influence of the shape of the discharge hole of the immersion nozzle on the flow velocity distribution of the discharge flow.

FIG. FIG. 6 is a diagram showing a state of center segregation in the case of No. 3;

[Explanation of symbols]

1: Immersion nozzle 2: Discharge hole 3:
Speed vector W: horizontal width of discharge hole H: vertical length of discharge hole D:
Inner diameter of immersion nozzle

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B22D 11/128 350 B22D 11/128 350A 41/50 520 41/50 520

Claims (1)

[Claims] 1. A continuous casting method for casting a slab having a rectangular cross section using an immersion nozzle having two discharge holes in which the direction of a discharge flow is the direction of each short side on both sides in the mold width direction. Casting a slab using an immersion nozzle in which the shape of these discharge holes is rectangular, and the dimensions of the discharge holes and the inner diameter of the immersion nozzle satisfy the following conditions (A) and (B); A continuous casting method for steel, wherein an unsolidified region of a slab is continuously reduced. W / H ≦ 0.6 (A) (W × H) / (D × D) ≧ 0.8 (B) where W: width of discharge hole (mm) H: discharge hole Vertical length (mm) D: Inner diameter of immersion nozzle (mm)