KR20040059083A - A funnel mold for thin slab continuous casting - Google Patents

Abstract

PURPOSE: A funnel type mold for thin slab continuous casting is provided to prevent surface defects of casting slab and extend life cycle of the mold by differently varying width change of funnel in a casting direction at upper part of the mold and lower part of the mold. CONSTITUTION: The funnel type mold(20) for thin slab continuous casting comprises a funnel part(9) which is depressed in an arc sectional shape in width center on an inner surface of long side part(3a) in a casting slab thickness direction so that a lower end of submerged nozzle is positioned in the funnel part without interference, and which is converted into a plane shape as it goes in a casting direction; a steep slope part(8) comprising first gradient (α) inclined to the center of width as corner at left and right sides of an upper part of the funnel part is being proceeded in a casting direction so that contraction amount of solidification layer at an upper part of the long side part is rapidly lowered so as to rapidly reduce an upper width of the funnel part; and a slow slope part(7) comprising second gradient (β) inclined to the center of width as corner at left and right sides of an intermediate part of the funnel part corresponding to a point at which the steep slope part is terminated is being proceeded in the casting direction so that contraction amount of the solidification layer at a position that is the same as or lower than an intermediate part of the long side part is slowly reduced so as to slowly reduce width of an intermediate part of the funnel part or a part lower than the intermediate part.

Description

A funnel mold for thin slab continuous casting {A FUNNEL MOLD FOR THIN SLAB CONTINUOUS CASTING}

The present invention relates to a funnel mold used to continuously cast a thin slab of thin plate thickness, and more particularly, a molten steel shrinkage larger than a short side by changing the funnel width change in the casting direction differently in the mold upper part and the mold lower part. By compensating for the reduction of shrinkage in the long side of the mold, the air gap between the mold short side and the molten steel solidification layer is eliminated to prevent surface defects of the slab, and the friction between the short side and the molten steel solidification layer is reduced. The present invention relates to a funnel mold for thin slab continuous casting, which can extend its service life.

In general, the process of continuously casting a thin slab thin plate thickness as shown in FIG. The mold (3) consisting of long and short sides is introduced into the mold (3) through the immersion nozzle (2) from the tundish (1) filled with molten steel while a dummy bar (4) is inserted at the bottom of the mold (3). When it is solidified in the dummy bar (4), the molten steel withdraws the derby bar (4) to the lower portion while cooling the cast by the cooling fluid injected from the injection nozzle (5), casting is started, Casting (3) As the initial solidification layer is formed on the surface of the casting is made continuously.

Recently, after thin casting (50 ~ 70mm) thin slab slabs compared to the existing slab thickness (200 ~ 250mm), it is possible to reduce the load and capacity of the rolling mill in the rolling process, and to continuously reduce thin slabs Casting process is adopted.

As shown in Figs. 2 and 3 (a), the mold 3 used in the thin slab continuous process has a narrow interval between the long side portions 3a for determining the thickness of the slab slab, and the thickness of the short side portions 3b. Is composed of a thin rectangular cross section, which faces each other so that sufficient space can be secured in a portion where the lower end of the immersion nozzle 2 for supplying molten steel from the tundish 1 to the mold 3 is disposed. The long side part 3a is comprised by funnel shape.

That is, the funnel-shaped mold 3 forms a funnel portion 9 in which the width center portion is recessed in the thickness direction of the long side portion 3a so that the lower end of the immersion nozzle 2 can be disposed without interference. The lower end of the funnel (9) is the shape of the funnel (9) at a point about 1/3 to 1/4 of the total length of the mold (3) measured from the lower end, which is the outlet of the mold (3) to the mold end After disappearing to change to a straight line coinciding with the flat portion of the long side portion 3a or to leave a little funnel shape, the thin slab slab of the rectangular shape is produced by the roll of the secondary cooling stand.

Here, as described above, forming a 1/3 to 1/4 of the entire length of the mold 3 in the width center portion of the mold 3 into a constant shape with a funnel shaped portion 9 or a straight line in the lower portion of the mold is at the beginning of casting. When the casting is started by inserting the dummy bar 4 into the mold bar, the casting work may be started without leakage of the molten steel only when the dummy bar 4 has a constant shape at the exit side of the lower end of the mold 3 over which the dummy bar 4 is spread. to be.

On the other hand, the surface defects generated in the cast slab continuously cast thin slab using the funnel mold 3, as shown in Fig. 3 (a) which is a cross-sectional view of the mold 3 cut perpendicular to the casting direction. , The slab formed while the molten steel supplied into the mold 3 solidifies, and the widthwise length of the long side portion 3a of the mold 3 is relative to the short side portion 3b of the mold 3. Since it is long, the shrinkage amount of the coagulation layer formed in the long side part 3a becomes larger than the shrinkage amount of the coagulation layer formed in the short side part 3b.

Accordingly, in order to make different shrinkages generated in the long and short sides 3a and 3b equal to each other, as shown in FIGS. 3 (a) (b) (c), the short sides of the mold 3 ( 3b) is formed to be inclined toward the width center side to compensate for the shrinkage. If the shrinkage is not compensated, an air gap G is generated between the short side portion 3b of the mold and the solidification layer, and the molten steel inside the solidification layer. If the iron static pressure cannot be overcome, the tensile stress is concentrated on the non-uniform coagulation layer, causing defects on the surface of the cast steel. Accordingly, in order to control the surface defects of the cast steel, the amount of shrinkage of the solidified layer formed at the long side portion 3a of the mold 3 is properly compensated by the inclination formed at the short side portion of the mold 3 to give an air gap G. ) Need to prevent formation.

And, the amount of shrinkage of the solidification layer in the mold (3) depends on the degree of heat flux to lose heat from the mold (3), the heat flux in the mold (3) is shown in Figure 5, The heat flux decreases toward the bottom of the mold 3 which is the largest in the mold part, which is the smallest heat resistance, and away from the water surface in the casting direction. As a result, the amount of shrinkage of the slab during casting is also reduced from the upper portion of the mold 3 to the lower portion.

The continuous casting process of thin slabs having a thin plate thickness has a casting speed of 4-7 m / min, which is relatively faster than that of a slab having a relatively thick plate thickness, which is 1 to 2 m / min. Since the heat flux of the mold 3 increases, the amount of shrinkage of the initial solidification layer is also increased directly below the surface of the mold when the solidification starts.

4 is a graph showing the correlation between the amount of shrinkage and the mold inclination according to the distance from the hot water surface. As shown in FIG. 4, in the hot water surface above the mold 3 inclined toward the width center side as the short side portion 3b proceeds in the casting direction, FIG. Since the mold slope of the short side portion is small (the change in width of the slab is small) compared with the large shrinkage amount of the molten steel solidified layer in the long side portion 3a, the molten steel shrinkage amount in the long side portion 3a cannot be sufficiently compensated. An air gap G occurred between the short side portion 3a of 3) and the molten steel solidification layer, which caused a defect in the surface of the cast steel.

In the lower part of the mold 3, the mold slope of the short side portion 3b is large (the change in width of the slab is large) compared with the large shrinkage amount of the molten steel solidified layer in the long side portion 3a and the inner surface of the mold short side portion 3a. Since friction increases between the molten steel solidified layers, abrasion is caused between the mold short edge 3b and the molten steel solidified layer, thereby causing a problem of lowering the service life of the mold 3.

European Patent EP0149734 does not compensate for the shrinkage of the mold long side, but when the funnel mold is used, it connects the mold short side with the parallel surface where the funnel shape does not remain in the part connected to the mold short side. In order to prevent the coagulation layer between the part and the long side part to be torn, European Patent EP0230886 and US Pat. No. 4,472,1151 make a parallel surface on the central part of the mold long side having the funnel to prevent the solidification layer from bending. This is to prevent the bursting of the coagulation layer that occurs in the.

In addition, European Patent EP0611619 is characterized in that the funnel shape remains in the lower part of the mold and the funnel shape remaining on the slab is removed by the roll in the lower part of the mold. In order to compensate the shrinkage of the coagulation layer in the mold by reducing the change in the funnel width to the point where the remaining funnel remains, the actual shrinkage in the mold is large at the upper part of the mold and small at the bottom, so There was a problem that can not compensate.

Accordingly, the present invention has been proposed to solve the conventional problems as described above, and its object is to change the funnel width in the casting direction differently in the mold upper part and the mold lower part so that the long side part having a larger molten steel shrinkage than the short side part. Compensated to reduce the shrinkage in the mold, thereby eliminating air gaps between the mold short edge and the molten steel solidification layer to prevent surface defects in the cast steel, and to reduce the friction between the short edge and the molten steel solidification layer to extend the service life of the mold. It is to provide a funnel mold for thin slab continuous casting.

1 is a schematic view showing a typical thin slab continuous casting process,

Figure 2 is a perspective view showing a conventional mold used in the thin slab continuous casting process,

Figure 3 (a) (b) (c) is a cross-sectional view showing a conventional mold, longitudinal cross-sectional view and a state cast state cast surface defects,

4 is a graph showing the relationship between the amount of shrinkage of the molten steel solidified layer and the mold inclination of the conventional mold long side portion;

5 is a graph showing a change in heat flux according to the casting direction distance from the hot water surface,

6 is a cross-sectional view showing the long and long ends of the funnel mold;

Figure 7 (a) (b) is a front view and a longitudinal sectional view of one long side portion constituting the funnel mold for continuous casting according to the present invention,

8 is a perspective view showing a funnel mold for continuous casting according to the present invention;

9 is a state diagram showing the curvature formed in the funnel portion of the funnel type mold,

10 is a graph showing the amount of shrinkage, the length of funnel excess and mold inclination of the long side solidified layer with distance from the hot water surface,

11 is a graph showing the present invention and the conventional surface defect index.

Explanation of symbols on the main parts of the drawings

1 ...... Tundish 2 ...... Immersion nozzle

3a ..... short side 3b ..... short side

4 ...... Dummy Bar 7 ......

8 ...... Slope incline 9 ...... funnel

As a technical configuration for achieving the above object, the present invention,

In the mold consisting of long and short sides to receive molten steel from the tundish stored in the molten steel through the immersion nozzle to continuously cast the thin slab on the rectangular cross section,

A funnel which is formed in an arc-shaped cross section at the center of the inner surface width of the long side portion in the cast piece thickness direction so that the lower end of the immersion nozzle is positioned without interference, and is converted into a planar shape toward the casting direction;

Rapid inclined portion having a first inclination inclined toward the center of the funnel as the left and right sides of the upper portion of the funnel proceeds in the casting direction so as to rapidly reduce the amount of shrinkage of the solidified layer on the long side portion ;

The left and right sides of the middle portion of the funnel corresponding to the point where the steep slope is terminated so as to lower the amount of shrinkage of the solidified layer less than the middle portion of the long side portion has a second inclination which is inclined toward the width center in the casting direction. By providing a funnel-type mold for continuous slab casting, characterized in that it comprises a;

Hereinafter, the present invention will be described in more detail.

Figure 7 (a) (b) is a front view and a longitudinal sectional view of one long side portion constituting the continuous casting funnel mold according to the present invention, Figure 8 is a perspective view showing a continuous casting funnel mold according to the present invention. .

7 and 8 of the present invention, as shown in Figures 7 and 8, long and short sides (3a) (3b) to form a space on the rectangular cross-section filled with molten steel supplied through the immersion nozzle from the tundish stored molten steel As a continuous casting of a thin slab having a thin plate thickness consisting of a), such a mold 20 is composed of a funnel 9, a steep inclined portion 8 and a mild inclined portion (7).

That is, the funnel 9 is recessed in an arc-shaped cross section so that the lower end of the immersion nozzle 2 immersed in the molten steel pool is located without interference from the central portion of the long side portion 3a, and the funnel 9 The upper end of the arc-shaped length is converted into a straight line toward the lower portion in the casting direction, and coincides with the straight inner surface of the long side portion 3a near the lower end of the long side portion 3a.

Here, the depth of the funnel portion 9 measured in the cast piece thickness direction at the long side of the funnel portion 9 is inclined at a constant inclination in the casting direction from the upper inlet end to the lower outlet end, so that the horizontal side of the long side part 3a. Is matched to one inner face.

In addition, the steep portion 8 is a funnel so as to sharply lower the amount of shrinkage of the solidified layer significantly higher than the shrinkage of the short side portion (3b) at the top of the long side portion (3a) corresponding to the upper surface of the hot water flow rate The upper left and right edges of (9) are provided with a first inclination α inclined rapidly toward the center of the width as the casting direction progresses in the casting direction, thereby rapidly reducing the width size formed on the funnel portion to the lower side.

In addition, the mild inclined portion 7 of the funnel portion 9 corresponding to the point where the steep slope 8 is terminated so as to gently lower the amount of shrinkage of the solidified layer in the middle of the long side portion 3a where the heat flux gradually decreases. As the left and right edges of the middle portion progress in the casting direction, the second inclination degree β is gently inclined toward the center portion of the width so as to gently reduce the width formed at the lower portion of the funnel 9 to the lower side.

Here, the steep slope 8, which causes the width change of the funnel portion 9 on the mold 3, is formed to be 1/5 to 1/3 of the entire length of the mold, and the funnel portion under the mold 3 is formed. The mildly inclined portion 7 causing the width change of (9) is preferably formed to be 1/3 to 1/2 of the total length of the mold.

In addition, the first inclination degree α of the steep inclination part 8 should always be larger than the second inclination degree β of the mild inclination part 7. For this purpose, the first inclination degree α is 1 to 20. It is preferable that the second inclination degree β of the mildly inclined portion 7 is 0.5 to 10 degrees.

In addition, the inflection point at which the mildly inclined portion 7 starts is most suitable for 1/4 to 1/3 of the total length of the mold in which the change in heat flux starts to decrease.

On the other hand, as shown by the dotted line in Fig. 7 (a), the steep slope 8 is formed convex toward the width center side so that the width change in the upper portion of the funnel (7) can be more rapidly generated toward the center of the slab It is formed in a straight line, the mildly inclined portion 7 is formed convexly or linearly formed on the side of the slab edge opposite the width center so that the width change in the lower portion of the funnel portion 9 can be generated more gently toward the center of the slab.

As shown in Fig. 4, in the mold 20, which is composed of long and short sides 3a and 3b, the funnel 9 is formed in the center of the inner surface of the long sides 3a facing each other. When casting, the heat flux is the largest in the hot water surface, and rapidly decreases toward the bottom of the mold 20, the molten steel solidified layer formed in the long side portion 3a of the mold 20 also shrinks directly under the hot water surface according to the heat flux, As it goes down, the shrinkage decreases. In this case, in the upper part of the mold 20, the air gap G is formed between the short side portion 3b and the solidification layer because the molten steel shrinkage of the long side portion 3a is greater than the inclination amount of the short side portion 3b having a constant slope. In the lower part of the mold, the inclination of the short side portion 3b is greater than the shrinkage of the long side portion 3a, so that the frictional force increases between the short side portion 3a and the solidification layer, causing wear of the short side portion 3b and the mold. .

Therefore, in order to reduce the air gap G generated in the upper part of the mold 20 during casting, a large shrinkage amount and a short side portion of the long side portion 3a are used by using the excess length formed in the funnel portion 9 having an arc cross section. The difference between the small shrinkage amounts of (3b) can be compensated.

That is, the excess length (L) of the funnel (9) is a funnel on the inner surface of the long side portion (3a) with respect to the inner surface arc length (L2) of the funnel (9) The funnel length L1 measured by the shortest straight distance between the left and right edges of the part 9 is calculated (L2-L1), and the surplus length L is formed on the arc-shaped cross section of the funnel part 9 by the bottom of the mold. If it changes to, it changes in the same straight line as the inner surface of the long side part 3a, and it gradually decreases toward a casting direction.

In addition, the phenomenon that the excess length of the funnel portion 9 decreases rapidly in the mold portion in the steep slope 7 and rapidly reduces the casting area in the cross section of the upper side of the mold, thereby passing molten steel therethrough. The solidification layer of the steel sheet is forcibly stretched in the length and width direction, and this serves as a pretext for pushing the molten steel solidification layer of the long side portion 3a toward the short side portion 3b.

On the other hand, in the middle of the mold 20 or less, the excess length of the funnel portion 9 decreases gently by the mildly inclined portion 7, so that the casting area in the upper cross section of the mold is smooth. Since it reduces, the elongation rate which the solidified layer of molten steel which extends through this reduces in elongation in a slab and the width direction can reduce the friction between the short side part 3b and the molten steel solidified layer.

As shown in FIG. 5, since the heat flux in the mold during the continuous slab casting is large at the mold bath surface and decreases toward the bottom of the mold, the long side shrinkage of the mold can be expressed by the following square root equation.

<Equation 1>

Where S is the molten steel shrinkage (mm), k is the solidification constant (usually 2-3 in thin slabs), and t is the casting time (minutes) from the mold bath surface.

And the radius R of the funnel part 9 can be calculated | required by following formula (2) considering only 1/4 of the long side part 3a of a mold, as shown in FIG.

<Equation 2>

R = (a ² + b ² ) / 4b

Where R is the radius of the funnel, a is the funnel width, and b is the funnel depth.

<Example>

Thin slab slab width is 1000mm, the depth of funnel thickness at the bottom of the funnel is 30mm, the width of the funnel is 640mm, the shape of the funnel ends at 600mm from the bath surface, and the first inclination degree (α) is 15 ° from the bath surface to 200mm. Forming a steep inclined portion 8, and then using a mold having a mildly inclined portion 7 having a second inclination degree β of 5 degrees. The circumferential speed is 5 m / min, the length of the mold is 1 m, Applying 2.77, the solidification shrinkage of the long side portion of the mold is 9.6 mm, and the shrinkage corresponding to 1/2 of the long side portion is 4.8 mm.

In this case, as shown in Fig. 10, since the sum by the amount of mold inclination and the amount of excess length change, which is the shape change of the funnel part, is almost equal to the amount of solidification shrinkage of the slab, the air gap between the short side portion 3b of the mold and the molten steel solidification layer. It is possible to reduce the occurrence of (G) and to significantly reduce defects occurring on the surface of the cast steel as shown in FIG. 11.

According to the present invention as described above, the depth of the slab of the funnel measured in the slab thickness direction is constantly reduced in the casting direction, while the width of the funnel portion in the mold is rapidly reduced by the steep slope, The size of the funnel width in the stiffness is reduced gently by the mildly inclined portion, thereby reducing the shrinkage rate of the molten steel solidified layer in the upper part of the mold with high heat flux, and reducing the friction between the short side and the molten solidified layer in the lower part of the mold. It is possible to produce a thin slab cast of excellent quality while improving the service life of the mold as compared with the prior art.

While the invention has been shown and described with respect to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit or scope of the invention as set forth in the claims below. I would like to clarify that knowledge is easy to know.

Claims (6)

In the mold consisting of long and short sides (3a) (3b) to receive molten steel from the tundish (1) stored in the molten steel through the immersion nozzle (2) to continuously cast the thin slab on the rectangular cross-section, The funnel part 9 is formed in an arc-shaped cross section at the center of the inner surface width of the long side portion 3a in the thickness direction of the cast piece so that the lower end of the immersion nozzle 2 is positioned without interference, and is converted into a planar shape toward the casting direction. ; The funnel having a first inclination (α) inclined toward the center of the width while the left and right sides of the upper end of the funnel (9) proceeds in the casting direction so as to sharply lower the amount of shrinkage of the solidified layer on the long side (3a) A steep slope 8 for rapidly decreasing the upper width of the portion 9; The middle and left and right edges of the middle portion of the funnel portion 9 corresponding to the point where the steep slopes 8 are terminated so as to lower the amount of shrinkage of the solidification layer below the middle portion of the long side portion 3a proceed in the casting direction. Thin slanted portion (7) gently having a second inclination degree (β) inclined toward the center of the width while gently reducing the middle or less width of the funnel (9); Funnel type for thin slab continuous casting . The method of claim 1, The steep slope 8 is formed with a length of 1/5 to 1/3 with respect to the entire mold length, and the mild slope 7 is formed with a length of 1/3 to 1/2 with respect to the entire mold length. A funnel mold for thin slab continuous casting. The method of claim 1, The first inclination degree (α) of the steep slope (8) is always formed larger than the second slope (β) of the mild inclined portion (7) funnel mold for continuous slab casting. The method according to claim 1 or 3, The first inclination degree (α) of the steep slope (8) is 1 to 20 °, the second inclination (β) of the gentle slope 7 is formed for thin slab continuous casting, characterized in that formed in 0.5 to 10 ° Funnel molds. The method of claim 1, The inflexion point at which the mild inclined portion (7) starts is 1/4 to 1/3 of the total length of the mold in which the change in heat flux begins to decrease, funnel type for thin slab continuous casting. The method of claim 1, The steep slopes 8 are formed in a convex form or in a straight line on the width center side, and the mildly inclined portion 7 is formed in a convex form on the side of the slab edge which is the opposite half of the width, or a straight slab. Crude funnel mold.