JP2000237858A - Continuous casting method - Google Patents

Abstract

(57) Abstract: The present invention relates billet when secondary cooling by air-water mist, with the same air-water mist nozzles, that decreases once A less than ₃ transformation point temperature of the slab surface Provided is a continuous casting method of steel capable of cooling and cooling so as not to lower the temperature below the A ₃ transformation point. A from the mixing portion of the water and air within the nozzle to the tip of the nozzle length and 100 to 300 mm, between the up correction position of the slab, the temperature of the slab surface once below A ₃ transformation point In the case where the temperature is reduced to a value higher than the A ₃ transformation point, the cooling condition until the temperature becomes lower than the A ₃ transformation point is determined by setting the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure to 5%. ~ 1
5. Mixing ratio of water W and air A at normal temperature and normal pressure when the water density is 0.03-0.09 liter / cm ² · min and the temperature of the slab surface is not lowered below the A ₃ transformation point. A / W
(Volume ratio) 50-200, water density 0.005-
0.015 liter / cm ² · min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel in which a slab drawn from a mold is secondarily cooled by steam-water mist.

[0002]

2. Description of the Related Art In recent years, low alloy steels containing alloying elements such as Nb, V, Ni, and Cu have been widely used for thick plate products and the like due to demands on mechanical properties. When casting these low alloy steels using a curved or vertical bending type continuous casting machine, cracks called horizontal cracks and horizontal cracks are likely to occur on the slab surface. When a slab is straightened, since the straightening stress acting on the slab surface exceeds the limit stress inherent in these low alloy steels, lateral cracks and lateral cracks occur on the slab surface.

The reason why low alloy steel slabs containing alloying elements such as Nb, V, Ni, and Cu tend to cause lateral cracks and the like will be further described as follows. That is, the hot ductility of the cast slab of these low alloy steels indicates that the solidification structure of the cast slab is γ.
Temperature near the A ₃ transformation point to transformation from phase to α-phase, i.e., at a temperature range of about 600 to 850 ° C., significantly reduced. Furthermore, in these low-alloy steel slabs, AlN, NbC, and the like are likely to precipitate at the γ-phase grain boundaries in the secondary cooling process after being drawn from the mold. The γ-phase grain boundaries on which AlN, NbC, etc. are precipitated are easily cracked. Therefore, when a low-alloy steel slab is corrected in the above temperature range, the γ-phase grain boundary is easily cracked by the correction stress, and thus a lateral crack or a lateral crack is generated on the surface of the slab.

[0004] Therefore, when correcting the slab of low alloy steel, the temperature near the above A ₃ transformation point, than the so-called brittle temperature range, there is a further attempt to correct the cast piece at low temperature region However, since AlN and NbC are already precipitated at the γ phase grain boundary as described above, the susceptibility of the slab to cracking is high, and simply correcting in a temperature region lower than the embrittlement temperature region results in lateral cracking. Cannot be prevented.

In order to prevent the solidified structure on the surface of the slab from transforming from the γ phase to the α phase before the slab is pulled out of the mold and corrected, the temperature of the slab surface is set higher than the A ₃ transformation point. There is an attempt to correct the slab in a temperature range higher than the embrittlement temperature range. However, also in this case, horizontal cracks and the like are likely to occur. Since the γ phase of the solidified structure on the slab surface is not transformed, the crystal grains of the γ phase are large. Therefore, the slab surface is easily cracked by the correction stress.

In Japanese Patent Application Laid-Open No. 9-47854, a slab immediately after being extracted from a mold is strongly cooled using a steam-water mist nozzle, and the temperature of the slab surface is once reduced to about 850 ° C. below the A ₃ transformation point. After cooling to a temperature and then straightening the slab, raise the temperature of the slab surface to 900 ° C. above the A ₃ transformation point.
A method of recuperating to a degree is disclosed.

In this method, the solidified structure on the surface of the slab is transformed from the γ phase to the α phase, and then transformed to the γ phase by reheating. Therefore, crystal grains of the γ-phase of the slab surface when the slab is corrected is reduced. In addition, since pieces cast at a high temperature of at least A ₃ transformation point is corrected, transverse cracks on the cast slab surface, etc. Is less likely to occur.

However, the solidification structure of the cast slab as proposed in the above-mentioned Japanese Patent Application Laid-Open No. 9-47854 is changed from the γ phase to the α phase.
In continuous casting machine having a air-water mist nozzles for temporarily transformed into phase, when casting with low crack susceptibility common carbon steel, for cooling so as not to lower than A ₃ transformation point temperature of the slab surface In this case, vertical cracks may occur instead of horizontal cracks on the slab surface. Using such steam-water mist nozzles, when the temperature of the slab surface so as not to lower than A ₃ transformation point, because the water content distribution in the width direction of the slab is not uniform, the temperature of the slab surface It becomes uneven in the width direction. Therefore, even if the steel has low cracking susceptibility, longitudinal cracks are likely to occur on the surface of the slab. Reason for the temperature of the slab surface not lower than A ₃ transformation point, recently many used are hot-rolling equipment in which is disposed downstream of the continuous casting machine as, as much as possible supply slab of high temperature, steel This is to save energy during product manufacturing.

[0009]

As described above, the temperature of the slab surface is temporarily reduced to A ₃ as used in the prior art.
Cooled to a temperature below state point, before correcting the subsequently cast piece, with the air-water mist nozzles for causing recuperation temperature of the slab surface to A ₃ transformation point or above the temperature, Ya transverse cracks in low alloy steels Although effective in preventing the occurrence of lateral cracks, it has the following problems. That is, in the case of using such an air / water mist nozzle, it is difficult to reduce the amount of cooling water with the same air / water mist nozzle, and even if the amount is reduced, the water amount distribution in the width direction of the slab is reduced. Becomes uneven. Therefore, using the same air-water mist nozzle as described above, when casting a general carbon steel having a low crack sensitivity and trying to obtain a slab in a high temperature state as much as possible, vertical cracks occur on the slab surface. There are cases. This is because when the cooling water amount is reduced, the water amount distribution in the width direction of the slab becomes non-uniform.

According to the present invention, when the cast slab drawn out of the mold is subjected to secondary cooling by steam-water mist, the same steam-water mist nozzle is used to reduce the temperature of the cast slab surface once to below the A ₃ transformation point. lowering the temperature, then for the sole purpose of providing cooling and a ₃ continuous casting method of the secondary cooling is available steels, such as not to lower the temperature below the transformation point, such as to recuperation or more a ₃ transformation point.

[0011]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a method for cooling a slab drawn from a mold by a steam-water mist after secondary cooling from a mixed portion of water and air in the nozzle to a tip of the nozzle. and 100~300mm a is, between the mold exit to a correction position of the slab, after lowering once a less than ₃ transformation point temperature of the slab surface, in the case of recuperation or more a ₃ transformation point , The cooling condition to below the A ₃ transformation point, the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure is 5
-15, water density 0.03-0.09 liter / cm ² ·
And the temperature of the slab surface is not lowered below the A ₃ transformation point, the mixing ratio A of water W and air A at normal temperature and normal pressure is
/ W (volume ratio) 50 to 200, water volume density 0.00
It is a method of continuously casting steel at a rate of 5 to 0.015 liter / cm ² · minute.

The continuous casting machine to which the method of the present invention is directed is:
This is a curved or vertical bending type continuous casting machine in which a cast curved slab is straightened. Also, Nb, V, Ni, C
Low alloy steel with high crack susceptibility containing alloy elements such as u and carbon steel with low crack susceptibility without the above alloy elements are cast by the same continuous casting machine using the same steam-water mist nozzle. It is intended for a continuous casting machine that operates as described below.

The present inventor has solved the above-mentioned problems of the conventional air / water mist nozzle as follows. That is, in the method of the present invention, an air / water mist nozzle having a length from the mixing portion of water and air to the tip of the nozzle of 100 to 300 mm is used. Furthermore, by properly selecting the mixing ratio A / W of water W and the air A, cooling using the same air-water mist nozzles, once to reduce the temperature below A ₃ transformation point temperature of the slab surface even by increasing the amount of water, on the contrary, even if reduced amount of cooling water to the temperature of the slab surface not lowered to a temperature lower than a ₃ transformation point before correcting slab, in any case, cast A uniform water amount distribution in the width direction of the slab and a uniform temperature distribution on the slab surface can be obtained. Therefore, it is possible to prevent the occurrence of lateral cracks in the slab of low alloy steel, and to prevent the occurrence of vertical cracks and the like in the slab of carbon steel using the same steam mist nozzle, and to remove the high-temperature slab. Obtainable.

The temperature of the slab surface in the method of the present invention is
For example, it is a temperature that can be measured by a radiation thermometer, and means a temperature from the surface of the slab to just below the skin. Further, the temperature of the slab surface can also be obtained by calculation by solidification heat transfer analysis. That is, if conditions such as the type of steel, the size of the slab, the casting speed, and the secondary cooling condition of the slab are determined, the temperature of the slab surface according to the distance from the molten steel meniscus can be calculated. Further, by appropriately selecting the surface heat transfer coefficient, the temperature of the slab surface obtained by this calculation is in good agreement with the actually measured temperature. Since horizontal cracks and vertical cracks of the slab occur on the slab surface and immediately below the skin, cracking of the slab can be suppressed by controlling the temperature of the slab surface.

[0015]

FIG. 1 is a view conceptually showing an example of the structure of a steam-water mist nozzle used in the method of the present invention. The air / water mist nozzle 1 is provided with a water / air mixing section 2-1 or 2
-2 and nozzle tip 3 for spraying steam mist onto slab
And a pipe 5 for sending water to the mixing section 2-1 or 2-2 (a pipe to this pipe is not shown), and a pipe 6 for sending air (the pipe to this pipe is not shown). Be composed. In addition, a nozzle tip 7 is provided at the tip of the nozzle tip 3.
Is provided.

The length from the mixing portion of water and air to the tip of the nozzle specified by the method of the present invention is, as shown in FIG. 1A, the mixing portion 2-1 for mixing water and air. The shape is
If it is larger than the other portions, the length is h-1 from the boundary between the mixing section 2-1 and the nozzle tip 3 to the tip 4 of the nozzle tip. When the mixing section 2-2 that mixes water and air is substantially the same size as the nozzle tip 3 as in the example shown in FIG. 1B, the pipe 5 for sending water or the pipe 6 for sending air is used. The length h from the nozzle tip side end of the tube closer to the nozzle tip 3 to the nozzle tip 4
-2.

As the nozzle tip, a nozzle tip for a commonly used air / water mist nozzle can be used. In addition, the method of arranging the air / water mist nozzle with respect to the surface of the slab to be cooled, for example, the interval to be arranged in the width direction of the slab, the interval to be arranged in the casting direction and the range of the arrangement position, the distance between the slab surface and the nozzle tip The distance and the like may be determined according to the structure such as the roll arrangement of the continuous casting machine, the maximum width of the slab, the type of the nozzle tip, the maximum casting speed, and the like.

For example, under the casting conditions of a maximum slab width of 2300 mm and a maximum casting speed of 2 m / min, 200 to 300 m from the mold exit to a point 2 to 3 m ahead in the casting direction.
The distance between the cooling surface of the slab and the tip of the nozzle is set to 80 to 2 at a position between the rolls corresponding to a position having an interval of about mm.
It is preferable to arrange the steam-water mist nozzle at an interval of 120 to 300 mm in the slab width direction. By arranging the air-water mist nozzles to the extent it is possible to reduce the temperature of the once A less than ₃ transformation point temperature of the slab surface and, downstream in the casting direction than the position of these gas-water mist nozzles the amount of cooling water of the secondary cooling conditions sides, by reducing of the cooling water of the steam-water mist nozzle, before the slab is corrected, thereby recuperation temperature of the slab surface than a ₃ transformation point Can also.

The distance between the cooling surface of the slab and the tip of the nozzle is 80
If the diameter is less than mm, a nozzle for widening the air-water mist is required, and at this time, the water amount distribution tends to be non-uniform. On the other hand, if this distance exceeds 200 mm, the water amount distribution of the air-water mist of the adjacent nozzle tends to overlap, and the water amount distribution tends to be non-uniform. Nozzle spacing in the slab width direction is 1
If it is less than 20 mm, the number of nozzles increases, and equipment costs and maintenance costs increase. Also, this interval is 300m
If it exceeds m, the water distribution tends to be uneven in the slab width direction.

In the method of the present invention, a steam-water mist nozzle having a length of 100 to 300 mm from the mixing portion of water and air to the tip of the nozzle is used. The reason will be described below. FIG. 2 is a diagram showing the influence of the length from the water / air mixing portion to the tip of the nozzle on the width direction uniformity (%) of the water amount distribution. The width direction uniformity (%) of the water amount distribution was investigated as follows. That is, three air / water mist nozzles are installed at a height of 150 mm above the water tank at intervals of 150 mm in the width direction, and the same air / water mist nozzle is used to obtain 0.008 liter / cm ² · min. 0.05 liter /
Spray water-mist at a water density of cm ² · min. The mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure when the water volume density is 0.008 liter / cm ² · min is 10,
The mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure when the water volume density is 0.05 liter / cm ² · min is 1
00.

The water tank is a water tank having a length of 300 mm and a width of 500 mm in the width direction and a depth of 500 mm. Partition plates are placed at intervals of 20 mm in the width direction, and the spray time is 1 minute.
Investigate the amount of water stored in the area between each partition. At this time, of the amount of water stored in the portion between the partition plates,
The ratio of the minimum amount of water to the maximum amount of water was evaluated as the widthwise uniformity (%) of the water amount distribution of the steam mist nozzle. Generally, the uniformity (%) in the width direction of the water amount distribution in the case of the uniform water amount distribution in the width direction is 80.
% Or more. The water density of 0.008 liter / cm ² · min is equivalent to weak cooling such that the temperature of the slab surface does not drop below the A ₃ transformation point until the slab correction position. Further, the water density of 0.05 liter / cm ² · min. Refers to the cooling when the slab surface can be strongly cooled and the temperature of the slab surface can be rapidly cooled to a temperature lower than the A ₃ transformation point. Is equivalent to

As shown in FIG. 2, when the length from the mixing portion of water and air to the tip of the nozzle is less than 100 mm,
The uniformity (%) in the width direction of the water amount distribution when spraying at a water amount density of 0.05 liter / cm ² · min is 80% or more, and a uniform water amount distribution is obtained, but 0.008 liter / cm ² · min When the spraying is performed at the water density, the uniformity (%) in the width direction of the water amount distribution is smaller than 80%, and the uniformity of the water amount distribution is poor.
On the other hand, when the length from the mixed portion of water and air to the tip of the nozzle is 100 mm or more, the water volume density is uniform at both 0.05 and 0.008 l / cm ² · minute. . More desirably, it is 150 mm or more.
However, if the length from the water-air mixing section to the tip of the nozzle exceeds 300 mm, the steam-water mist nozzle becomes longer, the entire nozzle equipment becomes longer, and it protrudes from the roll equipment that supports the slab. Disadvantageous on equipment. Further, the droplets of the steam-water mist are coarsened, and the water amount distribution tends to be non-uniform. Therefore, the length from the mixing portion of water and air to the tip of the nozzle is set to 100 to 300 mm. 150
~ 300 is more desirable.

In the method of the present invention, when the temperature of the slab surface is once reduced to a temperature lower than the A ₃ transformation point, the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure is used.
5-15, water density 0.03-0.09 liter / cm
² minutes. Fig. 3 shows the uniformity (%) in the width direction of the water distribution.
Mixing Ratio A / W of Water W and Air A at Normal Temperature and Normal Pressure
It is a figure which shows the influence of (volume ratio) and water amount density. The length from the water / air mixing section to the tip of the nozzle is 150m
The air / water mist nozzles having a width of 150 m were installed at intervals of 150 mm to change the mixing ratio A / W (volume ratio) of the water W and the air A and the water density, and the width direction uniformity (%) of the water distribution. )
Was investigated in the manner described above.

As shown as a hatched area 1 in FIG.
If the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure is 5 to 15 and the water density is 0.03 to 0.09 liter / cm ² · min, the width of the water distribution The directional uniformity (%) becomes 80% or more, and a substantially uniform water amount distribution is obtained. Mixing ratio A / W (volume ratio) of water W and air A is 5
Even when the water amount density is less than 0.03 liter / cm ² · min, the width direction uniformity (%) of the water amount distribution is 80
%. Even if the mixing ratio A / W (volume ratio) of water W and air A is 5 to 15, if the water density exceeds 0.09 liter / cm ² · minute, the water density becomes too large, until a position of correcting the strip, can not recuperator to more than a ₃ transformation point temperature of the cast slab surface is transformed into γ phase. Further, even if the water density is 0.03 to 0.09 liter / cm ² · minute,
If the mixing ratio A / W (volume ratio) of water W and air A is less than 5, the width direction uniformity (%) of the water amount distribution may be less than 80%, and the mixing ratio A / W (volume) Is 1)
If it exceeds 5, the effect on the width direction uniformity (%) of the water amount distribution is saturated. Thus, when lowered to temporarily A less than ₃ transformation point temperature of the slab surface, a mixing ratio A / W of water W and the air A in the normal temperature and pressure (volume) 5 to 1
5. The water density is 0.03 to 0.09 liter / cm ² · min.

In the method of the present invention, when the temperature of the slab surface is not lowered to a temperature lower than the A ₃ transformation point, the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure is used.
And the water density is 0.005 to 0.015 liter / cm ² · min. As shown as a shaded region 2 in FIG. 3, the mixing ratio A /
W (volume ratio) 50 to 200, water density 0.005
In the case of ０．０１0.015 liter / cm ² · minute, the uniformity (%) in the width direction of the water amount distribution becomes 80% or more, and a substantially uniform water amount distribution can be obtained. Even if the mixing ratio A / W (volume ratio) of water W and air A is 50 to 200, the water density is 0.00
In the case of less than 5 l / cm ² · min, may widthwise uniformity of water distribution (%) is less than 80%, the water amount density is more than ² · min 0.015 l / cm, water in addition to effects on widthwise uniformity of distribution (%) is saturated, the temperature of the cast slab surface is reduced to a temperature lower than a ₃ transformation point. Water density is 0.005 to 0.015 liter /
cm ² · min, mixing ratio A / W of water W and air A
When the (volume ratio) is less than 50, the width direction uniformity (%) of the water amount distribution may be less than 80%, and the mixing ratio A /
If W exceeds 200, the width direction uniformity of water amount distribution (%)
Effect saturates. Therefore, when the temperature of the slab surface is not reduced to a temperature lower than the A ₃ transformation point, the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure is used.
And the water density is 0.005 to 0.015 liter / cm ² · min.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
m using a continuous casting machine with a thickness of 235 m
m, a slab having a width of 2300 mm, a speed of 0.75 to 1.1 m
/ Min. The steam-water mist nozzle is placed at a position between the rolls corresponding to a position at an interval of about 200 to 250 mm between the mold outlet and 1.2 m ahead in the casting direction, at a distance of about 150 mm from the cast slab. The slabs were arranged at intervals of about 300 mm in the width direction. The test was conducted by changing the length from the water / air mixing portion of the air / water mist nozzle to the tip of the nozzle, the volume ratio of the mixing ratio of water and air at normal temperature and normal pressure, and the water density. Table 1 shows the steels used. Steel A
Is a low-alloy steel containing Cu, Ni and Nb, and is a steel that is liable to cause lateral cracks particularly when straightening a slab. Steel B
Is a common carbon steel and has low cracking susceptibility to side cracks and side cracks. However, if the cooling in the width direction of the slab surface is not uniform, vertical cracks may occur.

[0027]

[Table 1]

In each casting test, steel B was cast after steel A. At this time, in the casting of steel A, the temperature of the slab surface is once cooled to a temperature of about 850 ° C. below the A ₃ transformation point,
At the position after the slab at about 850 ° C., the amount of cooling water under the secondary cooling condition is reduced, the temperature of the slab surface is re-heated, and the temperature of the slab surface at the time of straightening is raised to 900 degrees above the A ₃ transformation point. ° C
Degree. Subsequently, in the casting of steel B, the temperature of the slab surface from the mold exit to the straightening position is set to 90 degrees or more at the A ₃ transformation point or higher.
While maintaining the temperature at 0 ° C. or higher, the slab was straightened. The temperature of the slab surface was measured with a radiation thermometer immediately below the mold outlet and near the correction position, and it was confirmed that the temperature was in the above range.

The surface of the obtained slab was visually observed to examine the occurrence of cracks on the slab surface. Table 2 shows the test conditions and test results.

[0030]

[Table 2]

Test No. of the present invention example 1 and No. In No. 2, the length from the mixing portion of water and air satisfying the conditions defined in the present invention to the tip of the nozzle is 150 mm, the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure, that is, steel In the case of A, the range is 10 to 12, and in the case of steel B, 125 to 1
In the range of 50, and also for the water density, ie steel A,
0.05 to 0.08 liter / cm ² · min.
The test was performed under the conditions of 008 to 0.010 liter / cm ² · minute. As described above, the temperature of the surface of the slab of steel A was cooled to about 850 ° C. below the A ₃ transformation point, and then reheated to 900 ° C. or higher above the A ₃ transformation point to correct the slab. Steel B
9 of the temperature of the cast slab surface is more than A ₃ transformation point to correct position
While maintaining the temperature at 00 ° C. or higher, the slab was straightened. In the obtained slabs of steel A and steel B, no crack was observed on the slab surface.

Test No. of Comparative Example In No. 3, the test was conducted at a length of 80 mm under a condition that the length from the mixed portion of water and air specified in the present invention to the tip of the nozzle deviated from the lower limit. The mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure and the water density were tested within a range satisfying the conditions specified in the present invention. No cracks occurred in the slab of steel A, but significant vertical cracks occurred in the slab of steel B. It was necessary to clean the surface of the slab and then hot-roll it, which was a certain degree of vertical cracking. Since the length from the water / air mixing section to the tip of the nozzle is short, the water density is particularly low
When the amount was as small as 1 liter / cm ² · minute, the uniformity of the air-water mist in the width direction became poor. Therefore, only in the case of steel B,
The water distribution in the width direction of the slab became uneven, and vertical cracks occurred in the slab.

Test No. of Comparative Example In No. 4, the length from the water-air mixing section defined in the present invention to the tip of the nozzle is 1
The test was conducted at 50 mm. For steel A, the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure and the water mass density were tested within a range satisfying the conditions specified in the present invention. Then, these were tested under conditions outside the lower limit of the conditions defined in the present invention. No cracks occurred in the slab of steel A, but significant vertical cracks occurred in the slab of steel B. In the test of steel B, the mixing ratio A / W (volume ratio) of water W and air A was as small as 30, and the water density was 0.002.
Since it was as small as 1 liter / cm ² · minute, the uniformity of the water distribution in the width direction of the slab was poor, and the slab had vertical cracks.

Test No. of Comparative Example In No. 5, the length from the water / air mixing section defined in the present invention to the tip of the nozzle is 1
The test was conducted at 50 mm. For steel B, the mixing ratio A / W (volume ratio) of water W and air A at normal temperature and normal pressure and the water mass density were tested within a range satisfying the conditions specified in the present invention. Then, these were tested under conditions outside the lower limit of the conditions defined in the present invention. No cracks occurred in the slab of steel B, but significant lateral cracks partially occurred in the slab of the slab of steel A. It was necessary to clean the slab surface and then perform hot rolling, which was a certain degree of lateral cracking. In the test of steel A, the mixing ratio A / W (volume ratio) of water W and air A was as small as 3 and the water density was as small as 0.02 liter / cm ² · min. uniformity of water distribution is deteriorated in, variation was observed in the temperature of the slab surface at just under the mold exit, the portion of the slab surface not lowered to a temperature lower than a ₃ transformation point was present. Therefore, lateral cracks partially occurred on the slab surface.

[0035]

According to the method of the present invention, when the slab drawn from the mold is secondarily cooled by steam-water mist,
Using the same steam mist nozzle, the temperature of the slab surface,
Cooling or A ₃ once lowering below the A ₃ transformation point
Cooling that does not lower the temperature below the transformation point is possible. Thus, using the same air / water mist nozzle, Nb,
It is possible to prevent the occurrence of horizontal cracks in the slab of low alloy steel containing alloy elements such as V, Ni, and Cu and vertical cracks in carbon steel and the like.

[Brief description of the drawings]

FIG. 1 is a view conceptually showing an example of the structure of a gas-water mist nozzle of the method of the present invention.

FIG. 2 is a diagram showing the influence of the length from the mixing section to the tip of the nozzle on the widthwise uniformity (%) of the water amount distribution.

FIG. 3 is a diagram showing the influence of the mixed volume ratio of air to water and the water density at normal temperature and normal pressure on the width direction uniformity (%) of the water distribution.

[Explanation of symbols]

1: Steam-water mist nozzle 2-1 2-2: Mixing section 3: Nozzle tip 4: Tip of nozzle tip 5: Pipe for feeding water 6: Pipe for sending air 7: Nozzle tip h-1, h-2 : Length from the mixing section to the tip of the nozzle

Claims (1)

[Claims] When the slab drawn from the mold is subjected to secondary cooling by steam-water mist, the length from the water / air mixing portion in the nozzle to the tip of the nozzle is 100 to 300 mm,
In between the mold exit to a correction position of the slab, after lowering once A less than ₃ transformation point temperature of the slab surface, in the case of recuperation or more A ₃ transformation point, to less than A ₃ transformation point The cooling condition is a mixing ratio A / of water W and air A at normal temperature and normal pressure.
W (volume ratio) is 5 to 15, and the water density is 0.03 to 0.
09 l / cm ² · min and, if not to lower the temperature of the slab surface to below A ₃ transformation point, the water W in the normal temperature and pressure
The mixing ratio A / W (volume ratio) of
A continuous casting method for steel, wherein the water density is 0.005 to 0.015 liter / cm ² · minute.