JP2950152B2 - Continuous casting mold for slab - Google Patents

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 a surface crack at the center in the width direction of a slab.

[0002]

2. Description of the Related Art In a continuous casting mold, a mold provided with a slit groove for water cooling is often used on the outer surface side (the 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. By circulating the cooling water at a constant flow rate in such a slit groove, uniform cooling in the width direction is ensured. Usually, copper or a copper alloy having high thermal conductivity is used as the material of the mold wall of this type.

[0003] However, in continuous casting using a water-cooled mold as described above, a surface crack may occur at the center in the width direction of the slab due to a large amount of heat removed from the mold in the early stage of solidification in the mold [for example. "Effect of Heat Removal Rate on Uneven Solidification in Continuous Casting Mold" Iron and Steel, 67th year (1981) 9th
No., pp. 1508-1514]. As a countermeasure against this surface cracking, slow cooling of the slab has been conventionally aimed at, and molds having various cooling structures or cooling means have been proposed. These are:
They are classified as (1) to (3).

(1) An unevenness is provided on the inner surface of the mold to generate an air gap between the inner surface of the mold and the slab, thereby reducing the amount of heat removed from the mold (Japanese Patent Application Laid-Open Nos. 57-11735 and 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 amount of heat removed from the mold (see JP-A-63-160751, JP-A-4-59153, etc.).

(3) The amount of heat removed from the mold is reduced by changing the slit shape such as the depth of the slit groove or the flow rate of cooling water in the slit.

In the method (1), irregularities on the inner surface of the mold are worn during continuous casting, and it is difficult to maintain the effect of reducing the amount of heat removed from the mold.
Similarly, in the method (2), the ceramic material on the inner surface of the mold is worn. Method (3) is a method that reduces the heat removal of the entire mold, so when performing high-speed casting, problems such as breakout are likely to occur due to insufficient solidification shell thickness at the bottom of the mold. .

Therefore, in the method (3), it is intended to perform partial slow cooling. For example, Japanese Patent Application Laid-Open No. 3-47654 discloses a cooling structure in which the width of the cooling water passage at the upper part of the mold corresponding to the meniscus part is made larger than the width of the lower part, so that the cooling water flow velocity is slowed down. A mold is shown that allows the cooling to be uniform across the width. Japanese Patent Application Laid-Open No. 3-453 discloses a mold capable of slowly cooling corners, that is, both ends in the width direction, as a device that slowly cools a part of the width direction without slowly cooling the entire mold. I have.

An example of a quality problem occurring in the slab in the case of the conventional cooling structure mold as described above will be described below.

The inventors of the present invention have conducted continuous casting of medium carbon steel ([C] = 0.16%) at a casting speed of 2.0 m / min using a conventional continuous casting mold for slab having cooling slit grooves shown in FIG. 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 performed, and a study was performed on a relationship between a thickness distribution of a solidified shell in the mold and occurrence of a slab crack.

FIG. 7 is a horizontal sectional view of a long side of the conventional mold, that is, a part of the mold wall in the width direction. This mold is
20 m depth 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 5 mm 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, the entire length in the thickness direction and 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.

FIG. 8 is a diagram showing an example of the initial solidified shell thickness in the width direction and a site where a surface crack occurs in a horizontal cross section of the slab 4. As shown in FIG. 8, the shell thickness in the vicinity of A, that is, the central portion in the width direction is thicker than the shell thickness at both ends in the width direction. It was confirmed that the shell thickness was small.

In order to investigate this phenomenon further, 5 mm below 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 interval between the secondary arms of the dendrite and the width direction, together with the factor of the cooling rate.
Here, the cooling rate R (° C./sec) was calculated according to the following equation.

L = 710R-0.39 As shown in FIG. 9, it was found that the cooling rate was higher at the center of the slab in the width direction than at both ends.

Conventionally, when continuously casting medium carbon steel having a C content of 0.10 to 0.16 Wt%, it is known that longitudinal cracks are concentrated at the center in the width direction. This can be explained by the above findings.

That is, at the center in the width direction of the slab having a high cooling rate, since the solidification shrinkage and the shrinkage accompanying the transformation from δ to γ are large, the surface of the solidified shell is separated from the mold wall surface to generate an air gap. 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 again due to the molten steel static pressure.However, once the air gap has been generated or the molten powder has flowed in locally, the air gap The heat transfer resistance between the solidified shell and the inner surface of the mold is increased by the presence of powder and powder, and the amount of heat removed from the solidified shell surface is greatly reduced. In addition, the balance between the strength of the solidified shell and the static pressure or contraction pressure of molten steel is lost, and surface cracks occur.

The reason why the cooling rate is increased at the center in the width direction of the slab is that the high-temperature molten steel flow supplied from the immersion nozzle at the upper part of the mold usually exists at both ends in the width direction. It can be considered that the cooling rate is increased because the contact state between the solidified shell and the mold wall surface is improved by bulging at the center in the width direction.

Recent investigations have also shown that an initially solidified shell of non-uniform thickness in the width direction created in the mold is carried over to the end of solidification, adversely affecting slab center segregation. This is because the solidification progress in the center in the width direction is larger than that in the both ends in the mold, so the shell thickness is large at the center and the shell thickness is small at both ends. In the secondary cooling zone from the outlet side, the shell thickness is sufficiently large, and the progress of solidification is limited by the heat transfer resistance of the shell, so that the unevenly solidified shell thickness is not eliminated. That is, the solidification completion time is advanced in the width direction central portion, and the concentrated molten steel at the end of solidification moves to both ends where the solidification completion time is delayed, thereby deteriorating the center segregation.

[0020]

SUMMARY OF THE INVENTION It is an object of the present invention to reduce surface cracks and improve center segregation by making the cooling rate at the center in the width direction of the slab smaller than at both ends in the width direction. To provide a continuous casting mold that can be used.

[0021]

The gist of the present invention is as follows.
(1) to (4) in the continuous casting mold.

(1) A continuous casting mold for a slab having a plurality of slit grooves for cooling on an outer surface side of a mold wall,
A continuous casting mold in which the width of a slit groove provided in a mold wall in the width direction is narrower than both ends at the entire length of the mold at the center in the width direction or at the meniscus portion.

(2) The width of the slit groove at the center in the width direction
And depth in the entire mold or meniscus,
30 to 60% of the width of the slit grooves at both ends in the width direction.
And the depth of the slit grooves at both ends in the width direction is 30 to
The continuous casting mold according to the above (1), which is 80%.

(3) The width of the slit groove at the center in the width direction
Distance between the mold and the slit groove
In the part, the width of the slit groove at both ends in the width direction is 30 to
60%, and between the slit grooves at both ends in the width direction.
Continuous as described in (1) above, which is 120 to 200% of the interval.
Casting mold.

(4) The width of the slit groove at the center in the width direction,
The depth and the gap between the slit grooves are
In the scass part, the width of the slit groove at both ends in the width direction is
30 to 60%, the depth of the slit grooves at both ends in the width direction
30-80% of the height, and at the both ends in the width direction
(1) which is 120 to 200% of the interval between the cut grooves.
The continuous casting mold as described.

Here, the "central portion" is defined as the width of the long side of the inner surface of the mold, that is, the range of (１ ／) to (以下) of the length of the inner surface in the width direction. Is desirably the remainder.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A continuous casting mold according to the present invention is for preventing deterioration of slab quality due to a high cooling rate in the center in the width direction as shown in FIGS. It is. For this purpose, rather than gradually cooling the meniscus part in the mold in the width direction as a whole, in the entire mold or meniscus part, the cooling capacity is lower at the center in the mold width direction than at both ends. A mold having a simple slit structure is required. FIG. 1 shows the slit groove of the present invention.
An example of a continuous casting mold when only the depth is changed is shown.

FIG. 1 is a horizontal cross-sectional view showing a main part of a continuous casting mold in which the depth of a slit groove in the long side of the mold, that is, the center of the width direction, in the mold wall is shallower than both ends thereof over the entire length of the mold. It is. Note that illustration of a part of the slit groove is omitted. 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 smaller than the depth x of the slit grooves 5 at both ends.

The ratio of y to slit depth x is 30 to 80.
%, The inner surface of the mold and the deep slit groove 5
It is desirable that the distance d1 from the end of the groove (groove bottom) be 10 to 15 mm, and the distance d2 between the inner surface of the mold and the end of the shallow slit groove 6 be determined by the amount (x-
y).

The slit groove width b at the center is equal to the slit groove width a at both ends, and the interval c2 between the slits at the center is equal to the interval c1 between the slit grooves at both ends. Desirable slit groove widths and intervals between the slit grooves are in the range of 5 to 10 mm and 5 to 20 mm, respectively.

With respect to the slit depth in the casting direction (the length direction of the mold), two types of structures can be selected. 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 is to limit the depth y of the slit 6 at the center part to the depth x of the slit groove 5 at both ends in the casting direction to a depth smaller than the depth x of the slit groove 5 at only the molten steel meniscus part in the mold, At the lower part of the mold away from the part, the depth x of the slit groove 5 at both ends is the same. The meniscus varies slightly during casting or depending on casting conditions, but is within a range of about 100 mm from the expected molten steel surface in the mold.

The length w1 of the central portion in the width direction for reducing the depth of the slit groove is equal to or more 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. It is desirable to set it to (１ ／) of W.

As for the slit groove on the short side of the mold, that is, in the thickness direction, both 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 depth of the slit groove is extremely different at a portion corresponding to the boundary between the center in the width direction and both ends in the width direction. Alternatively, a structure in which the depth of the slit groove is gradually reduced at the boundary portion toward the center in the width direction may be adopted.

By reducing the depth of the slit at the center in such a structure, the cooling capacity at the center in the width direction is reduced, and slow cooling of this portion is achieved. If the depth of the slit groove in the central part is appropriately made shallow, the difference in cooling rate in the width direction shown in FIG. 9 is eliminated, so that uneven solidification in the width direction is prevented, and the slab corresponding to the central part in the mold width direction is prevented. Cracks occurring on the surface are reduced. Further, deterioration of center segregation in the final stage of solidification is also eliminated.

FIG. 2 shows only the width of the slit groove of the present invention.
Is a horizontal sectional view showing that the main part of the continuous casting mold of the case of further. 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 at the center in the width direction than at both ends over the entire length of the mold. 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, the ratio of b to a is preferably 30 to 60%.

The desirable range of the length of the central portion in the width direction for reducing the width of the slit groove is, as in the case of FIG. 1, the long side width of the inner surface of the mold, that is, (（) of the inner surface length in the width direction. ~
(１ ／).

In the case of Figure 2, the spacing between the slit grooves are equally spaced, and is equal in depth also the central portion and both end portions of the slit groove, varying only the spacing between the slit grooves of the present invention to be described later
As in the case of a further mold (FIG. 3), the interval between the slit grooves at the center in the width direction may be larger than that at both ends.
The depth of the slit groove may be shallow at the center as in FIG. Further, the portion to be shallowed may be limited to only the meniscus portion as described above. Further, the slit groove width may be narrowed only in the meniscus portion.

FIG. 3 shows only the distance between the slit grooves of the present invention.
It is a horizontal sectional view showing an important section of a continuous casting mold when changing . Similarly, illustration of a part of the slit groove is omitted. This continuous casting mold has a structure in which a gap between slit grooves provided in a mold wall in the width direction is larger than the both ends at the center in the width direction over the entire length of the mold. That is, in relation to the interval c1 between the slit grooves at both ends and the interval c2 between the slit grooves at the center, c2> c
It satisfies condition 1. Desirable intervals c1 and c2 between the slit grooves are in the range of 5 to 20 mm and 10 to 25 mm, respectively. In this case, the ratio of c2 to c1 is 120-20.
It is desirable to be 0%.

The desirable range of the length of the central portion in the width direction for increasing the interval between the slit grooves is the width of the long side of the inner surface of the mold, that is, the length of the inner surface in the width direction, as in FIGS. (１ ／) to (１ ／).

[0041] In the case of FIG. 3, the depth and width of the slit grooves is equal between the central portion and both end portions, slits of the present invention shown in FIG. 1
The depth of the central slit groove may be reduced as in a mold in which only the width of the groove is changed .

The slit groove width, Sri of the invention shown in FIG. 2
It may be narrowed at the center like a mold when only the gap between the cut grooves is changed . Further, the interval between the slit grooves may be increased only in the meniscus portion.

When only the width of the slit groove of the present invention is changed,
Even if the slip and the continuous casting mold in the case of only the change spacing between the slit grooves, the shorter side of the mold, i.e. the slit grooves in the thickness direction may be to the the same as the conventional mold.

When only the width of the slit groove of the present invention is changed,
In the continuous casting mold in which only the gap between the joint and the slit groove is changed , the cooling capacity of the central portion in the width direction is reduced by adopting the above structure, and the slow cooling can be applied to the central portion in the width direction of the slab. .

Therefore, each of the aforementioned slit grooves
The same effect as in the case of the continuous casting mold of the invention in the case where only the depth is changed can be obtained.

As described above, in the continuous casting mold of the present invention,
When changing only the depth of the slit groove, the width of the slit groove
Only the gap and only the gap between the slit grooves
In the case of further modification, the slit structure of the mold may be arbitrarily combined. For example, the width of the slit groove
When combining depth and depth, the width of the center
The width and the depth of the lit groove are the slits at both ends in the width direction.
30-60% of the width of the groove, and at both ends in the width direction
It is desirable that the depth is 30 to 80% of the depth of the slit groove.
No. Also, the width of the slit groove and the interval between the slit grooves
When combining, the slit groove at the center in the width direction
The gap between the width and the slit groove is
30 to 60% of the width of the slot, and both ends in the width direction
120% to 200% of the interval between the slit grooves
Is desirable. In addition, the width, depth and
When combining the gaps between the slot
30% to 60% of the width of the slit groove of the
30 to 80% of the depth of the slit groove at the end, and
120-200 of the interval between the slit grooves at both ends in the width direction
% Is desirable.

The continuous casting mold of the present invention is sufficiently applicable not only to continuous casting of medium carbon steel having high crack sensitivity but also to casting of low carbon steel or high carbon steel.

[0048]

EXAMPLE For a medium-carbon steel and a low-carbon steel having the chemical compositions shown in Table 1, using a vertical slab continuous casting machine (number of strands 2) having a curvature radius of 10 m, a width of 1200 mm and a thickness of 200 mm.
The casting speed of the slab of mm 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.

[0049]

[Table 1]

The molds used are a mold having a conventional structure shown in FIG. 7 and a mold of the present invention having a structure shown in FIG. FIG. 4 is a horizontal sectional view of a long side of the mold of the present invention, that is, a part of the mold wall in the width direction. Note that illustration of a part of the slit groove is omitted.

The mold of FIG. 4 has basically the same structure as that of FIG. The width of the slit groove at both ends in the width direction is 5 mm, the depth is 20 mm, and the width of the slit groove at the center in the width direction is 5 mm, and the depth is 8 mm. The slit groove intervals were all constant 8 mm, and the width-wise length of the central portion where the groove depth was changed was 300 mm. This is equivalent 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, and the distance between the inner surface of the mold and the end of the slit groove is at the center.
20mm, 15mm at both ends.

In the mold having the conventional structure shown in FIG. 7, the wall thickness of the mold is also 35 mm, and the distance between the inner surface of the mold and the end of the slit groove is the same of 15 mm at the center and 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 end of the groove is 15 mm.
mm and the groove width were the same at 5 mm. Other casting conditions are as follows.

Casting temperature: degree of superheat of molten steel in tundish
Cooling water conditions of the mold at 20 to 25 ° C: Secondary water cooling conditions so that the overall heat transfer coefficient of the mold at both ends in the width direction is 1.2 × 104 W / m 2 · K. Surface cracking rate and center segregation were compared and investigated for slabs cast at a water volume of 1.8 liters / kg over steel.
The results are shown in FIGS.

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

FIG. 6 is a view showing the state of center segregation in the one-width direction of the slab in the case of low carbon steel. The degree of center segregation on the vertical axis is a value obtained by dividing the carbon concentration in the center 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 obtained by eliminating the uneven cooling rate distribution in the mold width direction and reducing the uneven solidification by the slow cooling effect of the central part in the width direction by the mold of the present invention. is there.

[0059]

According to the continuous casting mold of the present invention, non-uniform solidification can be prevented by slow cooling at the center in the width direction. In addition, it is possible to reduce surface cracks occurring in steel types having high crack susceptibility such as medium carbon steel.

[Brief description of the drawings]

FIG. 1 shows an example in which only the depth of a slit groove is changed.
It is a horizontal sectional view of an important section of a continuous casting mold.

FIG. 2 shows an example in which only the width of a slit groove is changed.
It is a horizontal sectional view of an important section of a continuous casting mold.

FIG. 3 shows an example in which only the interval between slit grooves is changed.
It is a horizontal sectional view of the important section of the continuous casting mold shown.

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

FIG. 5 is a diagram showing the effect of casting speed on the rate of surface cracking in the case of medium carbon steel.

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

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

FIG. 8 is a view showing an example of an initial solidified shell thickness in a slab width direction and a site where a surface crack occurs in a case of a conventional mold.

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

[Description of Signs] 1: Mold, 2: Mold wall in width direction, 3: Slit groove of conventional mold, 4: Slab, 5: Slit groove at both ends, 6: Slit groove at center, a: Slit at both ends Slit groove width, b: slit groove width at the center, c1: interval between slit grooves at both ends, c2: interval between slit grooves at the center, d1: contact between the inner surface of the mold and the end of the slit grooves at both ends Distance, d2: distance between inner surface of mold and end of slit groove at center, x: depth of slit groove at both ends, y: depth of slit groove at center, W: width of mold (length of inner surface) , W1: length in the center in the width direction

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-57-206555 (JP, A) Japanese Utility Model Application 2-1536 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/04 314

Claims (4)

(57) [Claims] 1. A continuous casting mold for a slab having a plurality of slit grooves for cooling on an outer surface side of a mold wall, wherein a width of a slit groove provided in a width direction of the mold wall is a central portion in a width direction. A continuous casting mold, characterized in that the entire length of the mold or the meniscus portion is narrower than both ends. 2. The width and depth of a slit groove at the center in the width direction.
However, in the full length of the mold or the meniscus,
30 to 60% of the width of the slit grooves at both ends, and
30 to 80% of the depth of the slit grooves at both ends in the width direction
The continuous casting mold according to claim 1, wherein: 3. The width and width of a slit groove at a central portion in the width direction.
The gap between the lit grooves is limited to the entire length of the mold or the meniscus.
30% to 60% of the width of the slit grooves at both ends in the width direction
And the distance between the slit grooves at both ends in the width direction.
2. The method according to claim 1, wherein the amount is 120 to 200%.
On the continuous casting mold. 4. The width and depth of the slit groove at the center in the width direction.
Distance between the mold and the slit groove
In the part, the width of the slit groove at both ends in the width direction is 30 to
60%, which is 3% of the depth of the slit grooves at both ends in the width direction.
0 to 80%, and slit grooves at both ends in the width direction
120-200% of the interval between
The continuous casting mold according to claim 1.