JPH0810905A - Continuous casting mold for slab - Google Patents

Abstract

PURPOSE:To improve the center segregation and to reduce the surface cracking by forming the whole length or the meniscus part of a mold at the center part in the width direction having shallower depth, narrower width and larger interval than those at both end parts in slit grooves in the mold wall in the width direction. CONSTITUTION:In the continuous casting mold for slab having plural slit grooves for cooling at the outer surface side of the mold wall, the depth (y) of the slit grooves 6 at the center part of the outer surface side of the mold wall 2 in the width direction of the mold 1 is shallower than the depth (x) of the slit grooves 5 at both end parts. Further, the width of the slit grooves arranged in the whole length or the meniscus part of the mold wall at the center part in the width direction is narrower than that at both end parts. Further, the interval between the slit grooves arranged in the whole length or the meniscus part of the mold wall at the center part in the width direction is larger than that at both end parts. By this method, the center segregation is improved and the surface cracking can be reduced by slow cooling effect at the center part in the width direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting mold for preventing surface cracks in the widthwise central portion of a slab.

[0002]

2. Description of the Related Art A continuous casting mold is often used in which a water-cooling slit groove is provided on the outer surface side (the surface side not in contact with molten steel) inside the mold wall. On the long side of the continuous casting mold for slabs, that is, on the mold wall in the width direction, the slit grooves have the same depth and width, and are provided in parallel at equal intervals in the width direction. The uniform cooling property in the width direction is ensured by causing the cooling water to flow through such slit grooves at a constant flow rate. Usually, copper or copper alloy having high thermal conductivity is used for the material of this type of mold wall.

However, in continuous casting using a water-cooled mold as described above, surface heat cracking may occur at the widthwise center of the slab due to the large amount of heat removed from the mold at the initial stage of solidification in the mold [eg, "Effect of heat removal rate on non-uniform solidification in continuous casting mold" Iron and Steel, 67th (1981) No. 9
No., P. 1508-1514]. As a measure against this surface cracking, a mold having various cooling structures or cooling means has been conventionally proposed for gentle cooling of a cast piece. These are the following
It is classified as (1) to (3).

(1) Providing irregularities on the inner surface of the mold to create an air gap between the inner surface of the mold and the slab to reduce the amount of heat removed from the mold (JP-A-57-11735, JP-A-57-11735). 61-
18049, JP-A-2-70358, etc.).

(2) A ceramic chip is lined on the inner surface of the mold or a ceramic material is applied to reduce the heat removal amount from the mold (see Japanese Patent Laid-Open Nos. 63-160751 and 4-59153). (3) The amount of heat removed from the mold is reduced by changing the slit shape such as the slit groove depth or the cooling water flow velocity in the slit.

In the method (1), the irregularities on the inner surface of the mold are worn during continuous casting, and it is difficult to maintain the effect of reducing the heat removal amount of the mold.
Similarly, in the method (2), the ceramic material on the inner surface of the mold is worn. Since the method of (3) is a means to reduce the heat removal amount of the entire mold, when performing high speed casting, troubles such as breakout are likely to occur due to insufficient solidification shell thickness at the lower end of the mold. .

Therefore, in the method (3), it is aimed to carry out gentle cooling partially. For example, in Japanese Unexamined Patent Publication No. 3-47654, the cooling water passage width in the upper part of the mold, which corresponds to the meniscus portion, is made larger than that in the lower portion, and the cooling water flow velocity is slowed down, whereby A mold is shown that allows for uniform cooling across the width. Further, JP-A-3-453 discloses a mold capable of gently cooling a corner portion, that is, both end portions in the width direction, as a part of the mold in which the width direction is gradually cooled without slowly cooling the entire mold. There is.

An example of the quality problem that occurs in the slab in the case of the conventional cooling structure mold will be described below.

The present inventors used a continuous casting mold for slabs having a conventional cooling slit groove shown in FIG. 7 to continuously produce medium carbon steel ([C] = 0.16%) at a casting speed of 2.0 m / min. When casting and manufacturing a slab having a width of 1600 mm and a thickness of 200 mm, an S (sulfur) addition test in a mold was conducted, and an investigation was conducted on the relationship between the solidified shell thickness distribution in the mold and the occurrence of slab cracks.

FIG. 7 is a horizontal cross-sectional view of a part of the mold wall in the long side of the conventional mold, that is, in the width direction. This mold is
20m deep over the entire width of the mold wall 2 in the width direction of the mold 1
The slit grooves 3 having a width of m and a width of 5 mm are provided at equal intervals of 8 mm. The slit structure on the short side of the mold, that is, on the thickness direction and the entire length in the casting direction (the length direction of the mold) is basically the same. The results of continuous casting using this mold are shown in FIGS. 8 and 9.

FIG. 8 is a view showing an example of the initial solidified shell thickness in the width direction and the site where surface cracks occur in the horizontal cross section of the slab 4. As shown in FIG. 8, the shell thickness in the vicinity of A, that is, the center portion in the width direction is thicker than the shell thickness at both ends in the width direction, and the shell thickness at the center portion in the width direction where surface cracks occur is locally It was confirmed that the shell thickness was thin.

In order to investigate this phenomenon further, 5 mm under the epidermis at three positions A, B and C of the slab shown in FIG.
Then, the dendrite secondary arm interval L (μm) was measured.

FIG. 9 is a diagram showing the relationship between the secondary dendrite arm spacing and the width direction, with the factor of the cooling rate added.
Here, the cooling rate R (° C./sec) was calculated by the following formula.

L = 710R ^{-0.39 As} shown in FIG. 9, it was found that the cooling rate was higher in the widthwise central portion of the ^cast slab than in both end portions.

It has been conventionally known that when continuously casting a medium carbon steel having a C content of 0.10 to 0.16 Wt%, vertical cracks are concentrated in the center portion in the width direction. This can be explained by the above survey results.

That is, in the center portion in the width direction of the slab having a high cooling rate, the solidification shell surface is exfoliated from the wall surface of the mold because the solidification shrinkage and the shrinkage accompanying the transformation from δ to γ are large, and an air gap is generated. The molten powder locally flows into the generated air gap. In this air gap, the gap decreases due to the bulging phenomenon in which the solidified shell surface is pushed back to the inner surface of the mold by the static pressure of molten steel. Heat transfer resistance between the solidified shell and the inner surface of the mold increases due to the inclusion of powder and powder, and the amount of heat removed from the surface of the solidified shell is greatly reduced, resulting in a delay in solidification and a local decrease in shell thickness. However, the balance between the strength of the solidified shell and the static pressure of molten steel or the contraction pressure is lost, causing surface cracks.

The reason why the cooling rate becomes large in the widthwise central portion of the slab is that the high-temperature molten steel flow supplied from the dipping nozzle at the upper part of the mold is usually present at the widthwise both ends, so that the cooling rate at both ends is high. It is conceivable that, in the widthwise central part, the cooling rate increases because the contact state between the solidified shell and the mold wall surface is improved due to bulging.

Recent research has also revealed that the initially solidified shell having a non-uniform thickness in the width direction generated in the mold is carried over to the end of solidification, which adversely affects the center segregation of the slab.

This is because the solidification progress in the widthwise central portion in the mold is larger than that at both ends, so that there is a non-uniform distribution of the widthwise shell thickness such that the shell thickness is large at the central portion and the shell thickness is small at both ends. This is because the shell thickness is sufficiently thick in the secondary cooling zone from the mold outlet side, and the progress of solidification is rate-controlled by the heat transfer resistance of the shell. That is, the completion time of solidification becomes earlier in the central portion in the width direction, and the concentrated molten steel at the final stage of solidification moves to both ends where the completion time of solidification is delayed, thereby deteriorating the center segregation.

[0020]

An object of the present invention is to reduce the surface cracks and improve the center segregation by making the cooling rate of the widthwise central portion of the slab smaller than that at the widthwise both ends. The object is to provide a continuous casting mold that can be used.

[0021]

The summary of the present invention is as follows.
It is in the continuous casting mold of (1) to (3).

(1) A continuous casting mold for slabs having a plurality of slit grooves for cooling on the outer surface side of the mold wall, wherein the depth of the slit grooves provided in the mold wall in the width direction is A continuous casting mold characterized in that the entire length of the center part or the meniscus part is shallower than both ends.

(2) In the continuous casting mold for slabs according to (1) above, the width of the slit groove provided in the widthwise mold wall is such that the center of the widthwise direction is the entire length of the mold or both ends of the meniscus. Continuous casting mold characterized by being narrower than.

(3) In the continuous casting mold for a slab according to (1) above, the gap between the slit grooves provided in the widthwise mold wall is such that the center of the widthwise direction is the entire length of the mold or both ends of the meniscus. Continuous casting mold characterized by being larger than the part.

Here, the "central portion" is the width of the long side of the inner surface of the mold, that is, a range of (1/5) or more and (1/3) or less of the inner surface length in the width direction, and "both ends" Is preferably the rest.

[0026]

The continuous casting mold of the present invention is for preventing the deterioration of the slab quality due to the large cooling rate of the central portion in the width direction as shown in FIGS. 8 and 9. For this purpose, the meniscus portion in the mold is not entirely cooled in the width direction, but in the mold length or the meniscus portion, in the center portion of the mold width direction, the cooling ability is lower than both ends. A mold having a different slit structure is required. FIG. 1 shows an example of the first continuous casting mold of the present invention.

FIG. 1 is a horizontal cross-sectional view showing the main part of a continuous casting mold in which the depth of the slit groove in the mold wall at the long side of the mold, that is, in the center in the width direction, is shallower over the entire length of the mold than at both ends thereof. Is. In addition, a part of the slit groove is not shown. The depth y of the slit groove 6 at the center on the outer surface side of the mold wall 2 in the width direction of the mold 1 is shallower than the depth x of the slit grooves 5 at both ends.

The ratio of y to the slit depth x is 30 to 80.
%, And the inner surface of the mold and the deep slit groove 5
The amount impinges distance d ₁ between the (groove bottom) of the preferably set to 10-15 mm, the distance d ₂ between the abutment of the mold inner surface and a shallow slit grooves 6, which shallow slit depth (x-
y).

The slit groove width b at the center is equal to the slit groove width a at both ends, and the interval c ₂ between the slit grooves at the center is equal to the interval c ₁ between the slit grooves at both ends. Desirable slit groove widths and intervals between the slit grooves are in the ranges of 5 to 10 mm and 5 to 20 mm, respectively.

Two types of structures can be selected for the slit depth in the casting direction (the length direction of the mold). One is that the slit depths x and y are constant over the entire length in the casting direction from the upper end to the lower end of the mold as described above. The other one is that, in the casting direction, the depth y of the central slit groove 6 is made shallower than the depth x of the slit grooves 5 at both ends only at the molten steel meniscus portion in the mold, The depth x of the slit grooves 5 at both ends is set to be the same as the depth x in the lower part of the mold apart from the part. The meniscus portion varies somewhat depending on the casting conditions and casting conditions, but it is within a range of about 100 mm from the expected molten steel surface in the mold.

The width w ₁ of the central portion in the width direction for reducing the depth of the slit groove is not less than (1/5) the width of the long side of the inner surface of the mold, that is, the inner surface length W in the width direction, and the upper limit of this w ₁ Is W (1
/ 3) is desirable.

As for the slit grooves on the short side of the mold, that is, in the thickness direction, the slit groove depth and the interval between the slit grooves may be constant as in the conventional mold.

In the mold of the present invention shown in FIG. 1, the slit groove depths are extremely different at the portions corresponding to the boundaries between the widthwise central portion and the widthwise both ends, which is not preferable for achieving the desired slow cooling. At this boundary portion, a structure may be adopted in which the depth of the slit groove is gradually reduced toward the central portion in the width direction.

With such a structure, by making the depth of the slit groove at the central portion shallow, the cooling capacity at the central portion in the width direction is lowered, and the slow cooling of this portion is achieved. By appropriately reducing the depth of the slit groove in the central portion, the difference in cooling rate in the width direction shown in FIG. 9 is eliminated, so that the uneven solidification in the width direction is prevented and the slab corresponding to the central portion in the mold width direction is prevented. The number of cracks on the surface is reduced. Furthermore, the deterioration of central segregation at the end of solidification is also eliminated.

FIG. 2 is a horizontal sectional view showing an essential part of the second continuous casting mold of the present invention. Also in FIG. 2, a part of the slit groove is not shown. This continuous casting mold has a structure in which the width of the slit groove provided in the mold wall in the width direction is narrower over the entire length of the mold at the center portion in the width direction than at both ends. That is, in the relationship between the slit groove width b at the center and the slit groove width a at both ends shown in FIG. 2, the condition of a> b is satisfied. Desirable slit groove widths a and b are in the range of 5 to 10 mm and 3 to 6 mm, respectively. In this case, it is desirable that the ratio of b to a be 30 to 60%.

As in the case of FIG. 1, the desirable range of the length of the widthwise central portion for narrowing the slit groove width is the width on the long side of the inner surface of the mold, that is, (1/5) of the inner surface length in the width direction. ~
(1/3).

In the case of FIG. 2, the intervals between the slit grooves are equal, and the depths of the slit grooves are the same in the central portion and both end portions, but like the third mold of the present invention (FIG. 3) described later. In addition, the gap between the slit grooves at the center in the width direction may be made larger than that at both ends. The depth of the slit groove may be shallow in the central portion as in FIG. Further, the shallowed portion may be the one that stops only in the meniscus portion as described above. Further, the slit groove width may be narrowed only in the meniscus portion.

FIG. 3 is a horizontal sectional view showing an essential part of the third continuous casting mold of the present invention. Similarly, illustration of a part of the slit groove is omitted. This continuous casting mold has a structure in which the interval between the slit grooves provided in the mold wall in the width direction is larger than the both ends at the widthwise central part over the entire length of the mold. That is, unlike the first mold of the present invention shown in FIG. 1, the distance between the slit grooves is not constant, and the distance between the slit grooves at both ends c ₁ and the distance between the central slit grooves c ₂ are In the relationship, the condition of c ₂ > c ₁ is satisfied. Desirable gaps c ₁ and c ₂ between the slit grooves are 5 to 20 mm and 10 to 25 mm, respectively. In this case, the ratio of c _{2 to} c ₁ is 120 to 200
It is desirable to set it as%.

As in the case of FIGS. 1 and 2, the desirable range of the length of the central portion in the width direction for increasing the spacing between the slit grooves is the width of the inner side of the mold on the long side, that is, the length of the inner surface in the width direction. (1/5) to (1/3).

In the case of FIG. 3, the depth and width of the slit groove are equal in the central portion and both ends, but the depth of the slit groove in the central portion is made smaller as in the first mold of the present invention shown in FIG. You may.

The slit groove width is the second width of the present invention shown in FIG.
It may be narrowed in the central part like the mold of. Further, the gap between the slit grooves may be increased only in the meniscus portion.

Also in the case of the second and third continuous casting molds of the present invention, the slit groove on the short side of the mold, that is, in the thickness direction may be the same as that of the conventional mold.

In the second and third continuous casting molds of the present invention, by adopting the above-mentioned structure, the cooling ability in the widthwise central portion can be lowered and the slab can be gently cooled in the widthwise central portion.

Therefore, the same effects as in the case of the continuous casting mold of the first invention described above can be obtained.

As described above, in the continuous casting mold of the present invention,
It is also possible to arbitrarily combine the slit structures of the first, second and third molds.

The continuous casting mold of the present invention can be applied not only to continuous casting of medium carbon steel having a high crack susceptibility but also to casting of low carbon steel or high carbon steel.

[0047]

[Example] For medium carbon steel and low carbon steel having the chemical compositions shown in Table 1, a vertical slab continuous casting machine (the number of strands 2) having a bending radius of 10 m was used, and the width was 1200 mm and the thickness was 200.
The casting speed of the mm slab was changed in the range of 1.0 to 2.4 m / min. However, in the present invention example the casting speed is 2.4 m / m
Only in.

[0048]

[Table 1]

The molds used are the mold of the conventional structure shown in FIG. 7 and the mold of the present invention having the structure shown in FIG. FIG. 4 is a horizontal sectional view of a part of the mold wall in the long side of the mold of the present invention, that is, in the width direction. In addition, a part of the slit groove is not shown.

The mold of FIG. 4 has basically the same structure as that of FIG. The slit groove width at both widthwise end portions is 5 mm, the depth thereof is 20 mm, and the slit groove width at the widthwise center portion is 5 mm, and the depth thereof is 8 mm. The slit groove intervals were all fixed at 8 mm, and the widthwise length of the central portion where the groove depth was changed was 300 mm. This corresponds to 1/4 of the total width of 1200 mm on the inner side of the long side of the mold. The mold wall thickness of the mold of the present invention is 35 mm, the distance between the inner surface of the mold and the abutment of the slit groove is at the central portion.
20mm, 15mm at both ends.

In the conventional mold shown in FIG. 7, the mold wall thickness is 35 mm, and the distance between the inner surface of the mold and the abutment of the slit groove is 15 mm at the center and at both ends.

The total length of the mold, that is, the length in the casting direction is 800 mm, and the structure of the slit grooves is the same from the upper end to the lower end of the mold.

In each of the slit grooves on the short side of the mold, the groove interval is 13 mm, and the distance between the inner surface of the mold and the abutment of the groove is 15 mm.
mm and the groove width is 5 mm, which are the same. The other casting conditions are as follows.

Casting temperature: degree of superheating of molten steel in the tundish
20 to 25 ° C Mold cooling water condition: Water is passed so that the overall heat transfer coefficient of the mold at both ends in the width direction is 1.2 × 10 ⁴ W / m ² · K. Secondary cooling condition: Both conventional mold and mold of the present invention , Specific water volume 1.
For slabs cast under the conditions of 8 liters / kg of steel or more, surface crack occurrence rate and center segregation degree were compared and investigated. The results are shown in FIGS. 5 and 6.

FIG. 5 is a diagram showing the influence of the casting speed on the surface crack occurrence rate in the case of medium carbon steel. The surface crack occurrence rate is a value obtained by dividing the total crack length generated in the slab (only one surface on one side) by the cast length. As is clear from FIG. 5, the surface crack occurrence rate is greatly reduced in the mold of the present invention.

FIG. 6 is a diagram showing the state of center segregation in the one-width direction of the slab in the case of low carbon steel. The degree of central segregation on the vertical axis is the value obtained by dividing the carbon concentration in the central segregated portion by the representative component of the slab (Table 1). From FIG. 6, it can be seen that both ends having a high degree of segregation in the conventional mold are also improved in the mold of the present invention.

These results were brought about by eliminating the non-uniform cooling rate distribution in the mold width direction and reducing the non-uniform solidification due to the slow cooling effect of the center part in the width direction by the mold of the present invention. is there.

[0058]

EFFECTS OF THE INVENTION According to the continuous casting mold of the present invention, it is possible to prevent uneven solidification by slow cooling of the central portion in the width direction, and thus to improve central segregation which is a quality problem in carbon steel in general. It is possible to reduce surface cracks that occur in steel types with high cracking sensitivity such as medium carbon steel.

[Brief description of drawings]

FIG. 1 is a horizontal cross-sectional view showing a main part of a continuous casting mold of the present invention in which a depth of a slit groove in a mold wall at a central portion in a width direction is shallower than both ends thereof.

FIG. 2 shows that the width of the slit groove in the mold wall at the center in the width direction is
It is a horizontal sectional view showing the important section of the present invention continuous casting mold narrower than the both ends.

FIG. 3 is a horizontal cross-sectional view showing a main part of the continuous casting mold of the present invention in which the interval between slit grooves in the mold wall at the center in the width direction is larger than both ends thereof.

FIG. 4 is a horizontal cross-sectional view of a part of the mold wall in the width direction of the mold of the present invention used in Examples.

FIG. 5 is a diagram showing the influence of the casting speed on the surface crack occurrence rate in the case of medium carbon steel.

FIG. 6 is a diagram showing a situation of center segregation in a width direction of a slab in the case of low carbon steel.

FIG. 7 is a horizontal cross-sectional view of a part of the mold wall in the width direction of the conventional mold.

FIG. 8 is a diagram showing an example of an initial solidified shell thickness in a slab width direction and a site where surface cracks occur in the case of a conventional mold.

FIG. 9 is a diagram showing a relationship between a dendrite secondary arm interval and a slab width direction in the case of a conventional mold.

[Explanation of symbols]

1: Mold, 2: Mold wall in width direction, 3: Slit groove of conventional mold, 4: Slab, 5: Slit groove at both ends,
6: central slit groove, a: slit groove width at both ends, b: central slit groove width, c ₁ : interval between slit grooves at both ends, c ₂ : interval between central slit grooves,
d ₁ : Distance between inner surface of mold and end of slit groove at both ends, d ₂ : Distance between inner surface of mold and end of slit groove at center, x: Depth of slit groove at both ends, y: Central part Slit groove depth of, W: mold width (length on the inner surface side),
w ₁ : length in the widthwise central part

Claims (3)

[Claims] 1. A continuous casting mold for a slab having a plurality of slit grooves for cooling on the outer surface side of the mold wall, wherein the depth of the slit groove provided in the mold wall in the width direction is the center in the width direction. Continuous casting mold characterized in that it is shallower than both ends at the entire length of the mold or at the meniscus. 2. A continuous casting mold for a slab having a plurality of slit grooves for cooling on the outer surface side of the mold wall, wherein the width of the slit groove provided in the mold wall in the width direction is the central portion in the width direction. Continuous casting mold characterized in that the entire length of the mold or the meniscus is narrower than both ends. 3. A continuous casting mold for a slab having a plurality of slit grooves for cooling on the outer surface side of the mold wall, wherein the slit grooves provided in the mold wall in the width direction have a center in the width direction. A continuous casting mold characterized in that the entire length of the mold is larger or the meniscus is larger than both ends.