JPH10109150A - Secondary cooling device for cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the surface crack on a continuously cast slab of low alloy steel or 9% Ni steel by using a secondary cooling device in a top zone just below a mold with a mist cooling to both of the upper and the lower surfaces and making a cooling water quantity optimum control range and an air quantity optimum control range into the prescribed ranges. SOLUTION: In the cast slab secondary cooling device arranged in the top zone of an apparatus for producing the continuously cast slab of a steel, the cast slab 10 drawn out from the mold is forcedly cooled with each of spray nozzles 16, 18 to the upper and the lower parts of the top zone 14 during guiding through the guide constituted with rolls 12. In such a case, the mist spray devices to both of the upper and the lower surfaces having 300-600l/min/m<2> cooling water quantity optimun control range to the one side surface and 3-18 Nm<3> /min/m<2> air quantity optimum control range to the one side surface, is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slab secondary cooling device used for continuous casting of steel, and more particularly to a slab 2 for preventing surface cracks such as lateral cracks and lateral cracks in a slab.
The following relates to a cooling device.

[0002]

2. Description of the Related Art In recent years, the necessity of improving the orthogonality of continuous cast slabs has been increasing from the viewpoint of reducing the production cost of steel products. There is a problem of surface cracks called lateral cracks or lateral cracks. Hereinafter, it is simply called a surface crack.

[0003] In particular, recently, N
Small amount of various alloying elements such as b, V, Ni, Cu (~ 1.0%)
Although the production of low-alloy steel containing steel has been increasing, the frequency of surface cracks in continuous cast slabs has increased with the addition of these alloying elements. The rate remains at a standstill.

On the other hand, low-temperature steels represented by 9% Ni steel
(Natural liquefied gas tanks, etc.) are required to be manufactured by the continuous casting method, which has a low manufacturing cost. However, since severe surface cracking occurs, it is necessary to rely on the steel ingot (ingot) method for manufacturing. It is in.

[0005] The cause of the occurrence of these surface cracks is that the surface temperature of the slab decreases during the secondary cooling in continuous casting, and γ →
It is known that the temperature is close to the α transformation temperature (approximately 750 to 850 ° C.), at which time mechanical stress such as slab bending and slab correction is applied.

Therefore, a method of avoiding the slab surface temperature in the slab bending portion or the slab correction portion on the lower or higher temperature side than the above-described region where the hot ductility is reduced (hereinafter referred to as the embrittlement temperature region) is usually employed. Has been adopted. However, it is difficult to eliminate surface cracks only by avoiding the brittle temperature range of the slab temperature.Therefore, there has been a technology that focuses on the slab surface layer structure and selects the heat history to avoid surface cracks. Some are disclosed.

For example, Japanese Patent Publication No. 58-3790 discloses that the upper surface of the secondary cooling zone is forcibly cooled to reduce the surface temperature of the slab once to 650-700.
A method is disclosed in which a γ → α transformation is carried out by cooling to 750 ° C., followed by a gradual recuperation to avoid a slab surface temperature in a slab correction section below the brittle temperature range.

Japanese Unexamined Patent Publication (Kokai) No. 63-112058 discloses a method for preventing surface cracks by improving the surface structure by repeating transformation of austenite and ferrite in a secondary cooling zone. .

However, in low alloy steels, which have been increasing in recent years, the temperature in the bent portion and the straightening portion is low because the brittle temperature range shifts to the low temperature side and the cooling characteristics change due to the scale change accompanying the addition of alloying elements. Therefore, if these methods, which are based on the premise of avoiding the low-temperature side, are applied to low-alloy steel, the surface cracks are worsened.

Further, Japanese Patent Application Laid-Open Nos.
Japanese Patent No. 224055 discloses a heat history limited to both corners of a slab, but the slab surface temperature is 750 to 90 immediately below the mold.
After cooling to 0 ° C to improve the surface layer structure (there is a description of the refinement of γ grains), a method of cooling the slab surface temperature at the slab bending section and slab correction section to 800 ° C or higher has been proposed. It has been disclosed. However, in the case of low-alloy steel, it is necessary to improve the slab surface structure not only in the slab corner but also in the entire slab width.

On the other hand, a γ-phase solidified steel typified by 9% Ni steel has a severe cracking susceptibility to thermal stress, so that a method of improving the surface layer structure is not suitable. For example, Japanese Patent Publication No. Hei 5-4169 discloses that, during secondary cooling of low-temperature steel containing 5 to 10% of Ni, the cooling rate of the slab surface is set to 20 in the region where the slab surface temperature is 1150 to 950 ° C. A method of lowering the temperature to ° C / min or less is disclosed.

[0012] This method itself is a gamma-phase solidified steel (9% Ni steel).
It is presumed that it is a necessary condition for casting steel, but since the amount of cooling water must be reduced, problems such as seizure due to powder accumulation, increase in heat load on the machine, and increase in susceptibility to internal slab cracks are inevitable. However, it is not advisable to apply it to low alloy steels with many casting opportunities.

[0013]

As described above, several secondary cooling systems for preventing the slab surface cracks have been proposed so far, and are effective for ordinary steel containing no alloying element. However, casting of low-alloy steel containing Nb, V, Ni, Cu, etc., γ-phase solidified steel (9% Ni steel), etc. still has the drawbacks as described above, No effective solution has yet been found.

The optimum thermal history differs between the low alloy steel and the gamma phase solidified steel such as 9% Ni steel, and it is difficult to cast these steel types in one secondary cooling pattern. In fact, the prior art also shows contents suggesting the proper use of cooling patterns depending on the type of steel.

On the other hand, the secondary cooling method for preventing the slab surface cracks differs depending on the type of steel, but it is desirable that one secondary cooling device corresponds to different secondary cooling patterns from the viewpoint of equipment cost reduction. However, in the prior art,
A measure for coping with each optimum pattern with one secondary cooling device has not been completed.

Here, the object of the present invention is to provide Nb, V, Ni, Cu
It is an object of the present invention to provide a slab secondary cooling device that can be commonly used in casting of low alloy steel containing, etc., γ phase solidified steel (9% Ni steel), and the like.

More specifically, the object of the present invention is to
Increasing surface crack susceptibility, Nb, V, Ni, Cu
It is an object of the present invention to provide a slab secondary cooling device for preventing surface cracks of low alloy steel containing various alloying elements and gamma phase solidified steel (9% Ni steel).

Further, a specific object of the present invention is to provide a low-alloy steel capable of improving the surface structure of a slab and realizing a heat history enabling high-temperature bending and straightening of a slab, and a γ-phase solidified steel ( (9% Ni steel), to provide a secondary cooling device for slabs used in continuous casting, which can reduce the thermal stress by gentle cooling of the upper part and realize the heat history enabling high-temperature bending and straightening of slabs. is there.

[0019]

DISCLOSURE OF THE INVENTION The present inventors have developed an invention relating to a secondary cooling method for improving the surface layer structure of a cast slab and enabling high-temperature bending and straightening of a slab for the purpose of preventing surface cracks of low alloy steel. It is proposed as Japanese Patent Application No. 8-36488.

The present invention is an improvement and development of the invention disclosed in the above-mentioned prior application, and is improved so as to be applicable to a γ-phase solidified steel such as 9% Ni steel. . First, the concept of the secondary cooling pattern for preventing surface cracking of low alloy steel and the concept of the secondary cooling of gamma phase solidified steel (9% Ni steel) will be described.

FIG. 1 shows low alloy steel and gamma phase solidified steel (9%
The cooling history for preventing surface cracking of Ni steel) is shown together with the cooling history of the conventional method. Cooling curve A is an optimal secondary cooling pattern for low alloy steel. This cooling pattern has the following three features.

When the thickness of the solidified shell is 10 mm or more and 15 mm or less, the primary cooling is completed, and the secondary cooling is started. The surface temperature of the entire slab is once reduced to a range of 600 ° C. or more and ₃ points or less of Ar (minimum temperature Tm immediately below the mold) within at most 2 minutes after leaving the mold. Secondary cooling is performed so that both the surface temperature of the slab in the bent portion and the surface temperature of the slab in the straightening portion (indicated as Tb and Tu, respectively) are 850 ° C. or more.

By the effect of the above, an unclear structure of γ grain boundaries is generated in the surface layer of the slab as shown in FIG. 2 (b). this is,
This is because, once the temperature is lower than the Ar ₃ point, the grain boundary ferrite precipitates at the grain boundary portion irrespective of the parent phase crystal orientation, so that it grows isotropically and becomes a granular form. The surface layer structure of a slab having a surface crack in an actual casting apparatus is almost always a structure with a sharp γ grain boundary as shown in FIG. 2 (a) and a high crack susceptibility. Often occurs along. By obscuring this γ grain boundary,
Crack susceptibility is significantly reduced.

Further, due to both effects, the slab quickly recovers heat, and a slab surface temperature of 850 ° C. at the bent portion and the straightening portion can be easily obtained. This is shell thickness 10mm or more
This is because secondary cooling is started at an early stage of 15 mm or less, and strong cooling is ended at an early stage within 2 minutes after leaving the mold, so that the recuperation capability of the slab is sufficiently ensured.

On the other hand, if a shell thickness of 10 to 15 mm is secured, no operating trouble such as breakout occurs. Bending part,
The reason why it is necessary to avoid the slab surface temperature on the high temperature side in the straightening section is as follows.

FIG. 3 is a diagram showing the high-temperature ductility of Ni-containing steel, which has a high frequency of occurrence of slab surface cracking in an actual production line, together with its steel composition. In the figure, the “%” amount on the graph represents the Ni content. In this example, the embrittlement temperature range moves to a lower temperature side than that of ordinary steel by containing several percent of alloys such as Ni. (The embrittlement temperature range of ordinary steel is 750 to 850 ° C, It is 600-850 ° C for Ni-containing steel). For this reason, it is necessary to avoid the high temperature side of the embrittlement region in low alloy steel.

Further, when a small amount of Ni is contained, a layer called a sub-scale tends to adhere to the surface of the slab in some places, and uneven cooling is likely to occur in a portion with and without a sub-scale. In the method of avoiding the slab surface temperature at a lower temperature than the embrittlement temperature range in the bent portion and the straightening portion, this cooling non-uniformity is promoted. From this point, it is necessary to avoid the high temperature side.

The cooling curve F in FIG. 1 shows a gamma-phase solidified steel (9% Ni
This is the optimal secondary cooling pattern for steel). The feature of this cooling pattern is that the surface temperature of the slab is gradually lowered as much as possible below the mold, and the temperature of the bent portion and the straightening portion is set to 900 ° C. or more. Figure 4 shows the high-temperature ductility of 9% Ni steel. At 850 ° C or lower, the embrittlement phenomenon is remarkable.
(Tb) and the temperature of the straightening section (Tu) need to be 900 ° C or more.

In the case of the cooling curve B shown in FIG. 1, the temperature is gradually lowered from immediately below the mold, and therefore, the cooling curve A is more advantageous than the cooling curve A for increasing the temperature of the bent portion and the correction portion. In this case, the surface structure of the slab becomes a clear structure of γ grain boundaries,
This is unavoidable for this type of steel because the effect of temperature is overwhelmingly large.

However, the cooling curve B is also a secondary cooling pattern in which the surface temperature of the slab is gradually lowered from immediately below the mold. Is less than 900 ° C., and in the case of γ-phase solidified steel (9% Ni steel), the brittle temperature range cannot be avoided on the high temperature side.

The cooling curves CE are the cooling curves of the conventional method. Cooling curve C is a cooling pattern in which intense cooling is performed immediately below the mold and then reheated, as in the present invention. However, secondary cooling is started when the shell thickness is 15 mm or more, and the recuperation ability of the slab is ensured. This is difficult. Cooling curve D performs strong cooling immediately below the mold, and a cooling pattern that avoids the lower side of the slab surface temperature at the bending portion and straightening portion. Cooling curve E performs stronger cooling than that of D, and transforms the surface structure to bainite transformation. The cooling curves D and E tend to cause uneven cooling, and are not suitable for low alloy steel and 9% Ni steel.

Next, the reason why the optimum secondary cooling pattern differs depending on the type of steel will be described below. FIGS. 5 (a) to 5 (c) show the phase diagram showing the gamma grain formation mechanism, the low alloy steel and the gamma solidified steel, respectively.
(9% Ni steel) and a schematic view of a solidification form of the same.

As shown in FIG. 5 (b), ordinary low-alloy steel has a solidification form of δ phase solidification, and as can be seen from FIG. 5 (a), it is transformed into γ phase after complete solidification in δ phase. The γ grain boundary and the final solidification position are irrelevant, and impurities such as P and S do not segregate much at the γ grain boundary. Therefore, crack susceptibility to thermal stress is not so high, and thermal history to improve surface texture
(Cooling directly below the mold) is most suitable for lowering the susceptibility of the slab to surface cracking.

On the other hand, gamma phase solidified steel (9% Ni steel)
As shown in (a) and FIG. 5 (c), since the final solidification position coincides with the γ grain boundary, segregation of impurities such as P and S occurs at the γ grain boundary, and the grain boundary is extremely fragile. Very high crack sensitivity to stress. Therefore, in this case, a cooling pattern in which strong cooling is performed directly below the mold is disadvantageous, and a cooling curve F that directs cooling as slowly as possible from directly below the mold is optimal.

The present inventors have determined that the optimum 2 according to the above steel type
In order to obtain the next cooling pattern, it has been found that the equipment specifications of the secondary cooling device in the top zone located immediately below the mold are extremely important, as described below.

In order to obtain the optimum cooling curve A for preventing surface cracking of low alloy steel, the following conditions are required as the cooling capacity of the secondary cooling device in the top zone immediately below the mold. Mist cooling is applied to both the top and bottom for uniform cooling. The cooling water optimum control range and one side ^{300 ~600 L / min / m 2} . The optimum air amount control range is ³ to 18 Nm ³ / min / m ² on one side.

FIG. 6 shows the result of performing the upper strong cooling pattern by variously changing the number of strong cooling zones immediately below the mold. Here, one zone is strongly cooled only in the top zone, two zones are strongly cooled in the top zone and the second zone next thereto, and three zones are strongly cooled in the top zone, the second zone, and the third zone. This is the case. There are few strong cooling zones just below the mold,
It is easier to avoid the high temperature side of the embrittlement zone by ending the strong cooling early.Especially, if the strong cooling is limited only to the top zone immediately below the mold, the embrittlement zone is stably shifted to the high temperature side in the bent part and straightening part. This can be avoided, and the cooling pattern of the cooling curve A in FIG. 1 can be realized. Generally, the mold length is 0.7-0.9m,
Since the vertical part of the vertical bending type continuous casting machine is 2.5 to 3.0 m, the length of the top zone for strong cooling should be about 0.8 to 1.0 m to avoid the embrittlement zone on the high temperature side at the bent part It is.

In the case of a curved continuous caster, the length of the top zone is 1.0 to 1.
It may be about 5m. When the strong cooling zone is limited to the top zone, the secondary cooling air amount, water amount and slab surface layer structure,
FIG. 7 shows the relationship between surface cracks. Cooling water range 300 per side
~ 600 L / min / m ² , air volume range ³ ~ 18Nm ³ / min / m ² on one side
As a result, it is possible to obtain a structure in which the γ grain boundaries are indistinct, and to avoid the bending portion temperature and the correction temperature on the high temperature side of the embrittlement region, and it is possible to effectively prevent surface cracking. is there.

If the cooling water amount is smaller than the above range, a desired surface layer structure cannot be obtained, and the structure of the γ grain boundary becomes clear and highly sensitive to cracking. If the amount of cooling water is larger than the above range, uneven cooling occurs, and cracks due to thermal stress tend to occur.

When the air amount range is smaller than the range,
Non-uniform cooling occurs and promotes cracking due to thermal stress.
Making the air amount range larger than the range requires a considerably large equipment technology and is not practical as a top zone for continuous casting.

By providing the above three points as the constituent conditions of the secondary cooling device in the top zone, it is possible to improve the surface structure and avoid the surface temperature on the high temperature side (obtain the cooling curve A) in the bent portion and the straightened portion. Yes, surface cracks of low alloy steel can be eliminated.

On the other hand, in order to obtain the optimum cooling curve F for preventing surface cracking of γ-phase solidified steel (9% Ni steel), the following conditions are required as equipment specifications of the secondary cooling device in the top zone immediately below the mold. is there. The optimum control range of the cooling water amount is 20 to 10 on one side.
And 0 L / min / m ^2.

FIG. 8 shows the relationship between the slab bending portion temperature and the correction temperature by variously changing the amount of cooling water in the top zone.
(Refer to the cooling curve F in Fig. 1 for the cooling pattern after the top zone.
As conditions are as much as possible to obtain, is a the amount of water adjusted), the cooling water amount ranges With 20~100 L / min / m ^2, cast pieces bend temperature (Tb) is above 900 ° C., brittle The formation temperature range can be avoided on the high temperature side, and the cooling pattern of the cooling curve F in FIG. 1 can be realized. At that time, the occurrence of lateral cracks can also be prevented.

If the cooling water amount is smaller than the above range, cracking can be prevented, but roll bending occurs due to heat load,
If the cooling water amount is larger than the range, lateral cracks occur on the slab surface.

When casting with such a low water content,
In the top zone of the cooling grid structure, powder is likely to accumulate and cause troubles such as breakout due to burn-in. Therefore, the structure of the top zone should be a roll structure. Note that mist spraying is not always a necessary condition, since cooling is not likely to be uneven in such a low water amount range.

From the above-mentioned findings, in order to obtain the two cooling patterns of the cooling curves A and F in FIG. 1 without any trouble, the cooling water amount control range of the secondary cooling device in the top zone is set at 20.
^{~100 L / min / m 2,} 300 ~600 L / min / m must be controlled very different range of ^2. However, when such different control ranges are performed by one cooling water piping system,
Poor controllability may cause a fluctuation in the amount of cooling water, which may adversely affect slab quality.

Therefore, the present inventors, as shown in FIG. 9 and FIG. 10, respectively, have one cooling system of a large flow cooling water piping system in the top zone, a large cooling water piping system and a small cooling water piping system. For two cooling systems of the piping system, casting tests were performed in the respective optimal water flow control ranges. As a result, controllability was greatly improved, and there was no fluctuation in the amount of cooling water, and good surface quality could be obtained with both the low alloy steel and the 9% Ni steel. Here, the present invention has been completed based on the above findings, and is as follows.

(1) A slab secondary cooling device in a top zone for continuous casting for producing a continuous casting slab of steel, comprising a top and bottom double-sided mist spraying device, wherein the mist spraying device is 300 L on one side. / min / m ² or more and 600 L / min / m ² or less for the optimal control range of cooling water, and 3Nm ³ / min / m ² or more for one side 18 Nm ³ / min / m ² or less Characteristic slab secondary cooling device.

(2) A slab secondary cooling apparatus in a continuous casting top zone for producing a steel continuous casting slab, wherein one side of the slab is 300 L / min / m ² or more and 600 L / min / m ² or less. cooling system and one surface 20L / min / m ² or more with a cooling water optimum control range
Equipped with two types of cooling systems with a cooling water amount optimal control range of 100 L / min / m ² or less, the large flow cooling system side is top-bottom double-sided mist cooling, and single-sided ³ Nm ³ / min / m ² or more 18Nm
A slab secondary cooling device characterized by having an air amount optimal control range of ³ / min / m ² or less.

[0050]

Embodiments of the present invention will be described below. FIGS. 11 (a), (b) and (c) show the slab 2 according to the present invention.
It is an explanatory view of an embodiment in which the next cooling device is provided in the top zone, FIG.
11 (b) shows the case of the cooling grid structure, and FIG.
(c) shows the case where large flow cooling systems 16 and 18 and small flow cooling system spray nozzles 17 and 19 are provided, respectively.

In FIG. 11 (a), a slab 10 drawn from a mold (not shown) is sprayed at the top and bottom of a top zone 14 while being guided through a guide composed of rolls 12, respectively. It is strongly cooled by. The spray nozzles 16 and 18 are respectively connected to the top and bottom sides in FIG.

In general, the top zone 14 is located two meters below the mold.
m means a region for one segment of a roller apron or a green grid. In particular, in the case of the present invention, a region in which sufficient recuperation is performed before reaching a bending region is sufficient. Generally, the distance between the spray nozzles on the top and bottom sides may be between the regions sandwiching one guide roll.
Fig. 11 (b) shows the cooling grid 20 instead of the guide roll.
This is an example in which the structure is provided, and the other structures are the same as those in the case of FIG. 11 (a).

Here, in the case of cooling by one cooling system, as shown in FIG. 9, mist cooling is performed by a large-flow cooling water system in all directions. Fig. 10 for two cooling systems
The cooling by the large flow cooling water system and the cooling by the small flow cooling water system are selectively used depending on the type of steel.

The secondary cooling device in the continuous casting top zone according to the present invention is 300 L / min / m ² or more and 600 L / min / m ² or more.
Since it has the following cooling water supply capacity, it is possible to cool the slab surface temperature to 600 ° C. or more and less than the Ar ₃ points only by cooling the top zone. Further, 3Nm ³ / min / m ² or more 18 Nm ³ / min /
Since a mist spray device having an air supply capacity of m ² or less is provided, there is no uneven cooling caused by strong cooling. Thereby, in the surface layer portion of the slab, an unclear structure of the γ grain boundary having low crack susceptibility is uniformly generated in the width direction.

Further, since the strong cooling for improving the surface layer structure can be completed only in the top zone, the cooling pattern of the slab reheating can be shifted to the slab slab in the bent portion and the straightening portion thereafter. It is possible to avoid surface temperatures on the hot side of the embrittlement zone.

As a result, the cooling pattern of the cooling curve A in FIG. 1 is obtained, and cracks on the slab surface of the low alloy steel are eliminated.

Further, the secondary cooling device in the continuous casting top zone according to the present invention is 300 L / min / m ² or more and 600 L / min / m ² or more.
Two cooling of a cooling system having a L / min / m ² or less cooling water optimum control range, and 20L / min / m ² or more 100 L / min / m ² cooling system having the following cooling water optimum control range Because of the system, it is necessary to obtain a stable cooling pattern for preventing surface cracking of γ-phase solidified steel (9% Ni steel), in addition to the effect of preventing the slab surface cracking of low alloy steel as described above. Can be.

That is, since the cooling water has an optimum control range of 20 L / min / m ² or more and 100 L / min / m ² or less, the cooling curve F in FIG.
As shown in (1), a pattern in which the surface temperature of the slab is gradually lowered from immediately below the mold is possible. With such a cooling effect, the slab surface temperature in the bent portion and the straightened portion can be made 900 ° C. or higher, and the slab surface crack of γ-phase solidified steel (9% Ni steel) is eliminated.

When the top zone structure is a roll structure, troubles such as breakout due to image sticking do not occur. Furthermore, the slab secondary cooling device according to the present invention can be applied to ordinary steel containing no alloying element, and can eliminate slab surface cracks.

[0060]

EXAMPLE A slab-shaped cast piece was continuously cast using a vertical bending type continuous casting machine. Table 1 shows the specifications of the continuous casting machine and the casting conditions, and Table 2 shows the cooling conditions. Table 2
As shown in Fig. 7, the secondary cooling conditions in the top zone were variously changed, and the correlation with the temperature history, the surface structure, and the surface crack was investigated. Temperature history is higher than corners where surface cracks occur frequently
It was measured at a position of 100 mm from below the mold with a bite type thermocouple.

Table 3 shows the chemical components of the cast steel types. Among the low-alloy steels, N-containing N steel with high surface cracking susceptibility was selected as the target steel type, and 9% Ni steel was selected as the γ-phase solidified steel.

[0062]

[Table 1]

[0063]

[Table 2]

[0064]

[Table 3]

Table 4 shows the thermal history, the surface layer structure of the slab, and the state of occurrence of surface cracks in Examples of the present invention, and Table 5 shows the thermal history,
The slab surface layer structure and the state of occurrence of surface cracks are shown together.

As can be seen from these results, in Comparative Examples 1 and 2, although the cooling water amount was within the range of the present invention,
It is a casting example of steel type A using water spray. In this case, cooling unevenness occurred, and the intended surface structure was obtained only partially, and the embrittlement area could only be partially avoided on the high-temperature side. Lateral cracks occurred.

In Comparative Examples 3 and 4, mist spray was used.
Although the amount of air was also within the range of the present invention, it is a casting example of steel type A in which the amount of cooling water was smaller than the range of the present invention.
In this case, the minimum temperature Tm immediately below the mold did not drop below the Ar ₃ point, and a clear and sensitive surface layer structure of the γ grain boundary was formed. As a result, a slight lateral crack was generated at a position 80 mm from the edge.

Comparative Example 5 is a casting example of steel type A using mist spray and having an air amount within the range of the present invention but a cooling water amount larger than the range of the present invention. In this case, since Tm became a temperature lower than 600 ° C., a bainite structure was formed in the surface layer, and lateral cracks having a depth of about 20 mm considered to be caused by thermal stress occurred on the entire top and bottom.

In Comparative Example 6, although the amount of cooling water was within the range of the present invention, the steel type A in which the amount of air was larger than the range of the present invention was used.
It is an example of casting. In this case, too, cooling was not uniform, and the desired surface layer structure was only partially obtained, and the embrittlement area could only be partially avoided on the high-temperature side. Lateral cracks occurred.

Comparative Example 7 is an example in which a steel type B was cast by cooling in a two-cooling system, but the amount of cooling water was larger than the range of the present invention. In this case, lateral cracks occurred on the entire surface of the slab on both the top and bottom, which were considered to be caused by thermal stress.

On the other hand, in Examples 1 to 8 of the present invention,
For steel type A, reduction of crack susceptibility of the slab surface layer structure and avoidance of the high temperature side of the embrittlement zone in the bent portion and straightening portion are both achieved. In this case, it was possible to obtain a slab of good quality free from surface cracks and compatible with avoiding the high temperature side of the embrittlement zone.

[0072]

[Table 4]

[0073]

[Table 5]

[0074]

According to the present invention, the surface cracks of the continuously cast slab of low alloy steel and 9% Ni steel have been eliminated. This means that for low alloy steels, it is possible to reduce both the susceptibility of the slab to cracking in the surface layer structure (structure with unclear γ grain boundaries) and to avoid the high temperature side of the embrittlement temperature range in the bent and straightened parts, and 9% For Ni steel, this was achieved because it was possible to both reduce the thermal stress in the upper part of the continuous casting machine and avoid the high-temperature side of the brittle temperature range in the bending and straightening sections. As a result, the slab is turned into a north calf,
No maintenance on the surface and improvement of the straightness ratio of continuous cast slabs have been achieved.
This has greatly contributed to the reduction of manufacturing costs.

[Brief description of the drawings]

FIG. 1 is a comparison diagram of a slab cooling pattern obtained by a secondary cooling device of the present invention and a conventional slab cooling pattern.

FIG. 2 is a conceptual diagram of a slab surface structure;
(a) is a structure that is apparently highly sensitive to cracking of the γ grain boundary,
FIG. 2 (b) shows a structure in which the susceptibility of the γ grain boundary to indistinct cracking is low.

FIG. 3 is a graph showing the high temperature ductility of a low alloy steel.

FIG. 4 is a graph showing the hot ductility of 9% Ni steel.

FIGS. 5 (a) to 5 (c) are a phase diagram of a γ-particle generation mechanism and schematic diagrams of solidification forms of a δ-phase solidified steel and a γ-phase solidified steel, respectively.

FIG. 6 is a graph showing a relationship between the number of cooling zones, a bent portion temperature, and a correction temperature in the pattern of FIG. 1-A.

FIG. 7 is a diagram showing the relationship between the amount of secondary cooling air, the amount of water, the slab surface layer structure, and surface cracks when cooling a low alloy steel to a strong cooling zone and a top zone.

FIG. 8 is a graph showing a relationship between a top zone cooling water amount, a slab bending portion temperature, and a correction temperature in cooling a γ-phase solidified steel.

FIG. 9 is a conceptual diagram of a cooling system in a case where one cooling system of a large flow cooling water piping system is provided in a top zone.

FIG. 10 is a conceptual diagram of a cooling system in a case where a cooling water piping system having a large flow rate and a cooling water piping system having a small flow rate are provided in a top zone.

FIGS. 11 (a), (b), and (c) are explanatory views of a spray nozzle installation mode when a top zone has a roll structure and a cooling grid structure, respectively. FIG. It is explanatory drawing which shows the structure at the time of two systems.

Claims (2)

[Claims] 1. A slab secondary cooling device provided in a top zone of a device for producing a continuously cast slab of steel, comprising a top and bottom double-sided mist spraying device, wherein the mist spraying device is 300 L / min / side on one side. m ² or more 600 L / min / m ² with the following amounts of cooling water optimum control range, one side 3 Nm ³ / min / m ² or more 18 Nm ³ / min /
A slab secondary cooling device comprising an air amount optimal control range of m ² or less. 2. A slab secondary cooling apparatus provided in a top zone of an apparatus for producing a continuously cast slab of steel, comprising:
0 L / min / m ² or more and 600 L / min / m ² or less, a large flow cooling system with an optimal control range, and 20 L / min / m ² or more on one side 100
Equipped with two types of cooling systems having a cooling water amount optimal control range of L / min / m ² or less, the large flow cooling system side is top-bottom double-sided mist cooling, and single-sided 3Nm ³ / min / m ² or more 18Nm
A slab secondary cooling apparatus characterized by having an air amount optimal control range of ³ / min / m ² or less.