JP2012206159A - Continuous casting method for steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for steel which suppresses temperature deviation in the width direction of a slab and overcooling of its surface and which achieves both prevention of internal cracks and prevention of surface cracks of the slab.SOLUTION: In the continuous casting method for steel using a continuous casting machine 12 having a mold 10 and a secondary cooling zone 11, the slab 13 containing by mass, ≥1.0% Si or ≥10% Cr or ≥0.5% C is cooled by cooling nozzles of the secondary cooling zone 11. CaO/SiOof powder supplied into the mold 10 is set to 0.90 to 1.45. Out of the secondary cooling zone 11, in a cooling range R1 of up to 0.6 m in the casting direction from directly below the mold 10 and in a cooling range R2 of from 0.6 to 1.2 m in the casting direction from directly below the mold 10, the water quantity densities P1, P2 of the cooling water sprayed from the cooling nozzles to the slab 13 satisfy the conditions of 150 L/m/min≤P1≤280 L/m/min, and 300 L/m/min≤P2≤700 L/m/min.

Description

The present invention relates to a continuous casting method of steel for preventing cracking of a cast piece.

In a continuous casting machine (hereinafter also referred to as a continuous casting machine), it is necessary to avoid internal cracking caused by bulging and surface cracking caused by supercooling in order to produce a high quality slab.
First, an internal crack caused by bulging will be described.
Bulging is a phenomenon in which when a slab is manufactured by a continuous casting machine, the slab swells between adjacent rolls in the casting direction (area where the slab is not supported) due to the static pressure of the liquid phase inside the slab. It is. This bulging may cause a crack (internal crack) in the surface layer of the solidified part of the slab.
In order to prevent this cracking, by increasing the amount of spray water (also called cooling water) from the cooling nozzle, the surface temperature of the slab is lowered to ensure the thickness of the solidified shell, and the collision of spray water In general, excessive bulging is suppressed by using pressure or the like.
The above-described situation in which cracking occurs due to bulging has been confirmed by a phenomenon that internal cracking occurs at a portion where the surface temperature of the slab is high (cracking at a high temperature portion).

Next, surface cracking due to supercooling will be described.
If the amount of spray water is excessively increased for the purpose of suppressing bulging, the surface temperature of the slab is extremely lowered and the ductility of the slab is reduced. For this reason, in the continuous casting machine, at the bent back portion where the slab is distorted, the surface in the width direction central portion of the slab excluding both ends in the width direction (regions from both ends in the width direction to 100 mm inside) Cracks may occur.
This situation has been confirmed by the phenomenon that surface cracks occur at the part where the surface temperature of the slab is low (cracking in the low temperature part).
In addition, in this invention, it is not made into the subject to prevent the edge part crack which generate | occur | produces in the width direction both ends of an above-described slab. This is because end cracks can be prevented by a known method such as reducing the amount of spray water sprayed on this portion.

However, in order to achieve both the prevention of internal cracks (caused by bulging) and surface cracks (caused by supercooling), there are the following problems.
In general, the increase in the amount of spray water for the purpose of suppressing bulging may lead to a state in which the temperature distribution in the width direction of the slab becomes non-uniform, i.e., a state in which a high temperature portion and a low temperature portion are mixed in the width direction of the slab. is there. For this reason, in order to avoid internal cracks and surface cracks at the same time, the surface temperature of the slab (temperature deviation in the width direction) can be adjusted within the optimum temperature range by adjusting the water volume distribution in the width direction of the slab. Therefore, it was necessary to keep the temperature in a temperature range where no internal cracks occur and in a temperature range where surface cracks in the low temperature portion do not occur.
In addition, when the slab passes through the bent back portion in a state where the surface temperature of the slab is lower than the embrittlement temperature, surface cracks are generated in the slab.

In particular, slabs of steel types containing Si: 1.0 mass% or more (for example, electromagnetic steel such as non-oriented electrical steel sheets), Cr: 10 mass% or more, or C: 0.5 mass% or more are susceptible to cracking. And one or both of internal cracks and surface cracks occur at a high frequency.
According to the investigation by the present inventors, in order to avoid surface cracks and internal cracks of the slab at the bent back portion, the temperature of the slab when passing through the bent back portion is set to 600 to 900 ° C. (cast It is necessary to keep the temperature deviation in the width direction of the piece within a narrow temperature range of 300 ° C. or less.
In response to this problem, for example, Patent Document 1 proposes a cooling method in which a temperature deviation is reduced by installing an auxiliary cooling nozzle at a specific portion in the width direction of a slab and cooling a high temperature portion. .

JP 2008-55454 A

However, although the method of Patent Document 1 has a certain effect, there is a concern that the installed auxiliary cooling nozzle may not be able to cool the high-temperature portion when the position of the high-temperature portion fluctuates when changing the cast steel type or the casting width. . In addition, since it is necessary to repeatedly operate and stop the auxiliary cooling nozzle and easily cause nozzle clogging, there is a concern about stable cooling of the slab.

The present invention was made in view of such circumstances, suppresses the temperature deviation in the width direction of the slab and the supercooling of the surface of the slab, and achieves both prevention of internal cracking and surface cracking of the slab, An object of the present invention is to provide a continuous casting method of steel capable of producing a high quality slab.

The continuous casting method of steel according to the present invention that meets the above-mentioned object uses a continuous casting machine having a mold and a secondary cooling zone disposed downstream of the mold, and 1.0 mass of Si from the mold. % Or more, or slab containing 10 mass% or more of Cr, or 0.5 mass% or more of C is drawn out, and the slab is cooled by a number of cooling nozzles provided in the secondary cooling zone. In the casting method,
The mass% of the ratio CaO / SiO ₂ with respect to SiO ₂ of CaO in the powder supplied into said mold and 0.90 to 1.45,
Among the secondary cooling zones, a cooling range R1 from directly below the mold to 0.6 m in the casting direction and a cooling range R2 from 0.6 m to 1.2 m in the casting direction from directly below the mold. The water density P1 and P2 of the cooling water sprayed from the cooling nozzle to the slab satisfy the following conditions.
150 liters / m ² / min ≦ P1 ≦ 280 liters / m ² / min 300 liters / m ² / min ≦ P2 ≦ 700 liters / m ² / min

In the continuous casting method of steel according to the present invention, it is preferable to use an air-water nozzle as the cooling nozzle that blows cooling water at least in the cooling range R2.
In the continuous casting method of steel according to the present invention, the secondary cooling zone is provided with a number of rolls that sandwich the slab drawn from the mold from the thickness direction and convey the slab in the casting direction. It is preferable to use a split roll that is split at a position excluding both ends in the width direction of the slab for part or all of the roll.

In the continuous casting method for steel according to the present invention, the distance from the position immediately below the mold to the position of 0.65D from the position immediately below the mold is D, where D is the distance from the position immediately below the mold to the bent back portion of the continuous casting machine. A surface temperature difference in the width direction of the slab in a region excluding a range of up to 100 mm from both ends in the width direction to the center portion of the slab by measuring a surface temperature of the slab by installing a thermometer inside The amount of cooling water sprayed from the cooling nozzle to the slab in the whole or a part of the range downstream from the cooling range R2 and upstream from the position of 0.4D on the condition that the temperature exceeds 300 ° C. The density should be reduced.

In the continuous casting method of steel according to the present invention, the ratio CaO / SiO ₂ of CaO to SiO ₂ in the powder supplied into the mold is 0.90 or more and 1.45 or less, and the casting direction starts from directly below the mold. In the cooling range R1 up to 0.6 m, the water density P1 of the cooling water sprayed from the cooling nozzle to the slab is adjusted, and in the cooling range R2 from 0.6 m to 1.2 m in the casting direction from directly below the mold. The cooling capacity index depending on the presence or absence of powder adhering to the slab in the cooling range R1 by increasing the water density P2 of the cooling water sprayed from the cooling nozzle to the slab than the water density P1 in the cooling range R1. Can be reduced, and the positive peeling of the powder can be prevented, and the peeling of the powder from the slab can be completed in the cooling range R2.
Therefore, Si: 1.0% by mass or more (for example, electromagnetic steel such as non-oriented electrical steel sheet), Cr: 10% by mass or more, or C: a steel type containing 0.5% by mass or more is highly susceptible to cracking. When casting a slab, the temperature deviation in the width direction of the slab due to the presence or absence of powder adhesion and the overcooling of the surface of the slab can be suppressed, so both internal crack prevention and surface crack prevention of the slab can be achieved. Can produce high quality slabs.

Further, when an air nozzle is used as the cooling nozzle, the spread of the cooling water in the casting direction can be widened. As a result, it is possible to ensure a long time during which the powder adhered to the surface of the slab can be separated (cooling time of the slab with cooling water) and to increase the average collision energy of the cooling water to the slab in the cooling range R2. The powder can be peeled from the slab more stably, and the crack occurrence rate of the slab can be further reduced.

And when using a split roll for the roll arrange | positioned in a secondary cooling zone, since the diameter of a roll can be made small compared with a normal roll, the installation space | interval of the roll adjacent to a casting direction can be narrowed. Thereby, since the amount of bulging of the slab can be suppressed, the restriction on the water density of the cooling water can be relaxed, and the adjustment of the water density for the purpose of adjusting the average temperature and temperature deviation in the width direction of the slab becomes easy.

Furthermore, the surface temperature of the slab is measured by installing a thermometer within the range from the position of 0.4D to the position of 0.65D immediately below the mold, and the surface temperature difference in the width direction of the slab is 300 ° C. If the water density of the cooling water sprayed from the cooling nozzle to the slab is reduced in the range downstream from the cooling range R2 and upstream from the position of 0.4D on the condition that it has been exceeded, Effective for reducing temperature deviation. This is because by reducing the water density, the surface temperature of the slab is raised by recuperation, and by this recuperation, the above-described temperature deviation exceeding 300 ° C. is eliminated, and in the temperature range where the temperature dependence is weak. This is because the cooling of the piece can be resumed. Note that recuperation refers to a reduction in the amount of heat removed from the slab surface due to a decrease in the amount of cooling water sprayed from the cooling nozzle, and the amount of heating on the slab surface becomes relatively significant. It means that the temperature rises.

It is explanatory drawing which shows the relationship between the cooling capacity index | exponent of a slab by the presence or absence of powder adhesion, and the distance from a casting_mold | template. It is explanatory drawing which shows the relationship between the cooling capacity index of the other slab by the presence or absence of powder adhesion, and the distance from a mold directly. It is explanatory drawing of the continuous casting machine to which the continuous casting method of steel which concerns on one embodiment of this invention is applied. It is explanatory drawing which shows the relationship between the pressure measurement position of a nozzle for cooling, and a spray pressure index | exponent. It is explanatory drawing which shows the relationship between the heat transfer coefficient of slab, and surface temperature.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
First, the background to the present invention will be described.
The continuous casting machine has a mold and a secondary cooling zone arranged in the casting direction from directly under the mold, and a plurality of cooling nozzles for cooling the slab are arranged in the secondary cooling zone. ing. Each cooling nozzle is divided into cooling zones in the range of 1 to 4 m across the casting direction so that the cooling rate of the slab can be controlled at each position in the casting direction. The amount of cooling water (spray water) sprayed from the nozzle to the slab is adjusted.
In addition, powder for the purpose of preventing seizure between the mold and the solidified shell of the slab is generally put on the mold surface. It has been generally said that when this powder is cooled in the secondary cooling zone in a state of adhering to the slab surface, the cooling capacity of the slab decreases. This is based on the fact that a powder having a lower thermal conductivity than the slab becomes a thermal resistance and inhibits heat removal from the slab itself.

Therefore, the present inventors investigated the influence of the presence or absence of powder adhesion on the cooling ability of the slab by a laboratory test.
First, in the laboratory test, after the steel material in which the thermocouple was embedded was heated to 1200 ° C. or higher, this was cooled with a cooling nozzle, and the influence of the cooling rate of the steel material depending on the presence or absence of powder adhesion was investigated. Subsequently, using the results of this laboratory test and the heat transfer analysis model, the effect of the presence or absence of powder adhesion on the cooling capacity of the slab was investigated. The heat transfer analysis model used in the present invention used a general method shown in, for example, iron and steel, Vol. 60 (1974), p. 1023.

Here, the structure of the continuous casting machine which analyzed, casting conditions, and cooling conditions are shown below.
・ Pitch of rolls from directly under the mold of continuous casting machine to 1.2m in casting direction: 200mm
・ Distance from directly under mold to bent back part: 16m
Casting conditions: casting speed 1.3 m / min, casting width (slab width) 1300 mm, casting thickness (slab thickness) 250 mm
Cooling condition (No. 1): The water density of cooling water sprayed from the cooling nozzle to the slab in the range from just below the mold to 2.0 m in the casting direction is 450 liter / m ² / min (hereinafter referred to as L / m ² / min).
・ Cooling condition (2): From right under the mold to 0.6 m in the casting direction, the density of the cooling water sprayed from the cooling nozzle to the slab is 280 liters / m ² / min. In the casting direction, the water density of cooling water sprayed from the cooling nozzle to the slab in the range from 0.6 m to 2.0 m was 450 liters / m ² / min.

The analysis result of the above-described cooling condition (part 1) is shown in FIG. 1, and the analysis result of the cooling condition (part 2) is shown in FIG.
The horizontal axis in FIGS. 1 and 2 indicates the distance (m) in the casting direction from directly under the mold. In FIGS. 1 and 2, the distance in the casting direction is 2.0 m (slab surface temperature of about 900 ° C.) immediately below the mold (the value of the horizontal axis is 0.0 m: the surface temperature of the slab is about 600 ° C.). ) Is shown.
The vertical axis in FIGS. 1 and 2 indicates that when the slab is cooled on the premise that powder adheres, the heat transfer coefficient of the slab whose distance in the casting direction is equivalent to 1.2 m is 1, and the powder is Figure shows values obtained by indexing (cooling capacity index) each heat transfer coefficient of the slab with no adhesion (solid line in FIGS. 1 and 2) and with adhesion (dotted line in FIGS. 1 and 2) Show.

First, the knowledge obtained from FIG. 1 is shown below.
-Cooling ability decreased with the progress of casting (increase in the horizontal axis) regardless of the presence or absence of powder adhesion.
-Although there is a magnitude relationship of cooling ability depending on the presence or absence of powder adhesion, the magnitude relationship may differ depending on the position of the horizontal axis (position where the surface temperature of the slab is different).
In the vicinity of the mold (for example, at a position of 0.2 m), the cooling ability index was about 30% larger when no powder was adhered than when powder was adhered.
-When the position of 1.2 m was exceeded in the casting direction from directly under the mold (for example, the position of 1.4 m), the cooling capacity index was smaller than 20% when the powder was not adhered, compared with the powder adhered.

-In the range of 0.6 to 1.2 m in the casting direction from directly under the mold (that is, in the vicinity of the intersection A in FIG. 1), the cooling capacity index without powder adhesion with respect to powder adhesion is There was a tendency not to differ greatly within a range not exceeding 20%.
From the above, it has been found that the presence or absence of powder on the surface of the slab causes the surface temperature of the slab to fluctuate locally.
Here, when the powder peels from the surface of the slab, it is unlikely that it is peeled off at once, and it is considered that the powder peels off gradually. For this reason, it is considered that the difference in the cooling capacity index described above causes a large temperature deviation in the width direction of the slab.

For example, when powder peeling starts at a position exceeding 1.2 m (for example, 1.5 m position) in the casting direction from directly below the mold, as shown in FIG. The cooling capacity is higher than that.
For this reason, it turns out that the surface temperature of the slab of the adhesion part of powder falls locally, and causes the temperature deviation of the width direction of a slab. In the study by the present inventors, it was considered that the temperature deviation of the slab is likely to exceed 300 ° C. at the bent back part of the continuous casting machine.
It is estimated that such a temperature deviation causes an internal crack due to a high temperature and a surface crack due to a decrease in the surface temperature of the slab.

Furthermore, in order to eliminate the above temperature deviation, considering the case where powder peeling is completed within 0.2 m in the casting direction from directly under the mold, the slab that causes surface cracking because of its high cooling capacity It turns out that it is easy to invite overcooling.
In this case, according to the study by the present inventors, the surface temperature of the slab is 600 at the bent back portion of the continuous casting machine even if the slab is cooled with a minimum amount of water for the purpose of preventing bulging of the slab. As a result, the surface crack of the slab was considered to occur at the bent back part.

From the above, the present inventors have a water volume density P1 in which the cooling capacity index difference due to the presence or absence of powder adhesion is small and the powder does not actively peel off in the cooling range R1 from directly under the mold to 0.6 m in the casting direction. In the cooling range R2 from 0.6 m to 1.2 m in the casting direction from just below the mold, the water density P2 rises above the water density P1 and from 1.2 to the casting direction from directly below the mold, It was conceived that the secondary cooling of the slab with the temperature deviation in the width direction of the slab suppressed can be realized by substantially realizing the separation completion.
In the study by the present inventors, the intersection A in FIG. 1 depends on the casting speed (for example, about 0.6 to 3.0 m / min) and the length of the continuous casting machine (distance from directly under the mold to the machine end). Although it changes to some extent, it does not change greatly, and according to the continuous casting method of steel of the present invention, the effect of suppressing cracking of the slab can be enjoyed.

Subsequently, the knowledge obtained from FIG. 2 is shown below.
-Cooling ability decreased with the progress of casting (increase in the horizontal axis) regardless of the presence or absence of powder adhesion.
-Although there is a magnitude relationship of cooling ability depending on the presence or absence of powder adhesion, the magnitude relationship may differ depending on the position of the horizontal axis (position where the surface temperature of the slab is different).
-Near the mold (for example, at a position of 0.2 m), the cooling ability tended not to change greatly regardless of the presence or absence of powder adhesion. In addition, without powder adhesion, the cooling capacity index is about 10% larger than with powder adhesion.
In this case, as compared with FIG. 1, it can be seen that the difference in cooling power index due to the presence or absence of powder adhesion in the vicinity of the mold is small.

-When the position of 1.2 m was exceeded in the casting direction from directly under the mold (for example, the position of 1.4 m), the cooling capacity index was smaller than 20% when the powder was not adhered, compared with the powder adhered.
-In the range of 0.6 to 1.2 m from directly under the mold to the casting direction, the cooling power index without powder adhesion tended to not vary greatly with and without powder adhesion regardless of the presence or absence of powder adhesion.
From the above, the cooling condition is changed at a position of about 0.6 m in the casting direction from directly under the mold, that is, the water density P1 is lowered from directly under the mold to 0.6 m in the casting direction, and the cooling capacity by the presence or absence of powder adhesion Cooling is performed so as to reduce the index difference, and in the range from 0.6 m to 1.2 m in the casting direction from directly below the mold, the water amount density P2 is increased above the water amount density P1 described above. As a result, powder separation is substantially completed from directly under the mold to 1.2 m in the casting direction, temperature deviation in the width direction of the slab is suppressed, and overcooling of the surface of the slab is suppressed. The inventors have come up with the idea that both internal crack prevention and surface crack prevention can be achieved.

Hereinafter, a continuous casting method of steel according to an embodiment of the present invention will be described with reference to FIGS.
As shown in FIG. 3, when performing continuous casting, a continuous casting machine (hereinafter also referred to as a continuous casting machine) 12 having a mold 10 and a secondary cooling zone 11 disposed on the downstream side of the mold 10 is provided. Use.
The type of the continuous casting machine may be either a curved type continuous casting machine or a vertical bending type continuous casting machine. In particular, when the present invention is applied to a vertical bending type continuous casting machine, a greater effect can be expected. This is because the curved type continuous caster tends to cause cracks on the slab surface only at the bent back part of the continuous caster, whereas the vertical bend type continuous caster has the bending part and the bent back part of the continuous caster. This is because the surface cracks of the slab are likely to occur on both sides, and the vertical bending type continuous casting machine has a higher rate of cracking of the slab even under the same cooling conditions.

Molten steel is supplied into the mold 10, the slab 13 is pulled out from below the mold, and the slab 13 is cooled by a number of cooling nozzles (not shown) provided in the secondary cooling zone 11.
The steel type of the slab 13 is prone to cracking, and Si is 1.0 mass% or more (upper limit is 4 mass%, for example), or Cr is 10 mass% or more (upper limit is 20 mass%, for example). Or 0.5% by mass or more (the upper limit is, for example, 1.2% by mass). Here, a steel type (for example, an electromagnetic steel such as a non-oriented electrical steel sheet) having a Si content of 1.0% by mass or more is a steel type in which internal cracks and surface cracks are remarkable, but Si is 2.8% by mass or more. Cracking becomes more prominent when it becomes an electromagnetic steel, and more prominently when it becomes a high-grade electromagnetic steel of 3.5% by mass or more. Further, even in a steel type having Cr of 10% by mass or more, cracking becomes more prominent when the Cr is stainless steel having 13% by mass or more. And even in a steel type in which C is 0.5% by mass or more, cracking becomes more prominent when C is 0.8% by mass or more.

In addition to the steel types, cracks become prominent as the casting width increases.
For example, in electromagnetic steel (Si is 2.8% by mass or more, and further 3.5% by mass or more), a slab having a casting width of 1000 mm and a crack occurrence rate of a 1200 mm slab are compared with a 1200 mm slab. , Cracking rate is high. For this reason, the effect of this invention becomes more remarkable by making casting width 1000 mm or more, Furthermore, 1200 mm or more. The upper limit of the casting width is generally about 2000 mm.
From the above, a remarkable effect can be exhibited by applying the present invention to a steel type and size having a high crack occurrence rate as described above.

When manufacturing the above-described steel type cast slab 13, as described above, it is necessary to substantially complete the peeling of the powder from the surface of the slab 13 from just below the mold to 1.2 m in the casting direction.
Therefore, in the secondary cooling zone 11, a cooling range R1 from directly below the mold 10 to 0.6 m in the casting direction, and a cooling range R2 from 0.6 m to 1.2 m in the casting direction from directly below the mold 10. Then, the water volume density (hereinafter also referred to as spray water density) P1 and P2 of the cooling water sprayed from the cooling nozzle to the slab 13 (directly hitting the slab 13) is adjusted so as to satisfy the following conditions: To do.
150 L / m ² / min ≦ P1 ≦ 280 L / m ² / min 300 L / m ² / min ≦ P2 ≦ 700 L / m ² / min

The above-described cooling water volume densities P1 and P2 (L / m ² / min) are defined as follows.
{Water density of cooling water (L / m ² / min)} = {Q (L / min)} / {A (m ² )}
Here, Q (L / min) is in the range from 0.6 m to 1.2 m in the casting direction from directly under the mold, or from 0.6 m to 1.2 m in the casting direction from directly under the mold. It is the amount of cooling water per unit time (minute) sprayed (directly hit) on the side or lower surface side.
A (m ² ) is actually sprayed with cooling water in the range from 0.6 m to 1.2 m from directly under the mold to the casting direction, or from 0.6 m to 1.2 m from directly under the mold to the casting direction. The surface area of the slab (directly hit), that is, 0.6 m × cast width (m).

In the cooling range R1, when the slab is cooled by setting the spray water amount density P1 to 300 L / m ² / min or more, the cooling capacity index difference due to the presence or absence of powder adhesion becomes large, which causes a temperature deviation. Therefore, it is necessary to set the spray water density P1 in the cooling range R1 to be small so that the difference in the cooling capacity index due to the presence or absence of powder directly under the mold is reduced. It was m ² / minute less than 280L / ^{m 2} / min.
On the other hand, when the spray water density P1 in the cooling range R1 is less than 150 L / m ² / min, the cooling capacity of the spray itself is small, and internal cracking due to bulging due to high temperature is expected to occur. The lower limit was 150 L / m ² / min.
From the above, the spray water density P1 in the cooling range R1 150L / ^{m 2} / min or more (preferably, 180L / ^{m 2} / min or more, further 210L / ^{m 2} / min or more), 280L ^{/ m} 2 ^/ Less than a minute.

Further, in the cooling range R2, when the slab is cooled with the spray water density P2 exceeding 700 L / m ² / min, it is expected that the spray cooling ability itself becomes large and cracking due to supercooling occurs.
On the other hand, when the spray water density P2 in the cooling range R2 is less than 300 L / m ² / min, it is expected that the powder adheres to the slab even at a position downstream of 1.2 m in the casting direction from directly below the mold. It is expected that a temperature deviation caused by the powder will occur.
From the above, the spray water density P2 in the cooling range R2 is 300 L / m ² / min or more (preferably 400 L / m ² / min or more, more preferably 450 L / m ² / min or more), 700 L / m ² / min. min or less (preferably, 650L / ^{m 2} / min or less, more 600L / ^{m 2} / min or less) was. In addition, this water amount density P2 is larger (about twice) than the conventional water amount density.

Switching between the water density P1 and the water density P2 can be performed at a position around 0.6 m in the casting direction from directly below the mold.
Here, when the water density is switched at a position upstream of 0.4 m in the casting direction from directly under the mold and the spray water density is increased, the cooling capacity index difference due to the presence or absence of powder adhesion is large. Deviations are expected to occur.
On the other hand, when the water density is switched to a position downstream of 0.8 m in the casting direction from directly below the mold, the difference in the cooling capacity index between the absence of powder and the adhesion is small. However, since the time for peeling off the powder by increasing the water density is shortened, it becomes difficult to peel off the powder from directly under the mold to 1.2 m in the casting direction, which may cause temperature deviation due to the powder. is there.
Therefore, it is preferable to set the switching position of the water density P1 and the water density P2 within the range from 0.4 m to 0.8 m in the casting direction from directly under the mold.

However, according to the regulation of only the water density described above, there is a case where the powder does not peel from the surface of the slab in the range from just below the mold to 1.2 m in the casting direction. Therefore, in order to prevent this, it is necessary to adjust the peelability of the powder, and the ratio CaO / SiO ₂ of CaO to SiO ₂ in CaO in the powder, that is, (CaO (mass%) / SiO ₂ (mass%). )) Between 0.90 and 1.45.
The change releasability of powder by the ratio CaO / SiO ₂ values, due to the formation temperature of the product produced by the ratio CaO / SiO ₂ FeO and CaO and SiO ₂ react changes.

That is, when the ratio CaO / SiO ₂ is less than 0.90, a low melting point product is generated at around 1200 ° C., and a lot of grain boundary oxides are generated at the interface between the slab and the powder, so that the powder is difficult to peel off. Become.
On the other hand, when the ratio CaO / SiO ₂ exceeds 1.45, it is presumed that the powder is difficult to peel due to the same phenomenon as when the ratio CaO / SiO ₂ is less than 0.90.
The lower limit of the ratio CaO / SiO ₂ is preferably 0.92, more preferably 0.95, and the upper limit is preferably 1.40, more preferably 1.35.

Although the water nozzle which sprays only cooling water on the surface of a slab is used for the cooling nozzle, it is preferable to use the air-water nozzle which air and cooling water are mixed and sprayed. In addition, although an air-water nozzle can be attached and used for all the secondary cooling zones 11, you may attach and use only for the part which sprays cooling water in cooling range R2.
Here, the features of the air-water nozzle will be described with reference to FIG. In addition, the horizontal axis of FIG. 4 is a pressure measurement position in the width direction (casting direction) of the spray water (relative position between the case where the water nozzle is used and the case where the water nozzle is used), and the vertical axis is the cooling water. The collision pressure is indexed with the maximum pressure at the air-water nozzle as 1.

As is clear from FIG. 4, the air-water nozzle (solid line in FIG. 4) has a wider range of spray water than the water nozzle (dotted line in FIG. 4), and cooling over a wide range is possible. For this reason, in the air-water nozzle, it is possible to ensure a long powder peeling time (cooling time by spray water) and to increase the average collision energy in the cooling range R2.
It should be noted that the securing of the powder peeling time and the increase in the average collision energy both contribute to stable powder peeling, so that the rate of occurrence of cracks in the slab can be reduced.

The air / water ratio of this air / water nozzle was expressed as “(air / water ratio) = (air volume) ÷ (water volume)” in units of volume (Nm ³ : normal cubic meter).
Here, if the air-water ratio is too low, it is not possible to secure a long time for peeling the powder, so 4.4 or more (or 10 or more) is preferable.
On the other hand, when the air / water ratio is too high, the collision pressure of the spray water becomes too low, and it becomes difficult to obtain the collision pressure necessary for peeling of the powder. For this reason, 20.5 or less is good, and 18.5 or less is even better in order to ensure stable peeling.

The secondary cooling zone 11 is provided with a large number of support rolls that sandwich the slab 13 drawn from the mold 10 in the thickness direction and transport it in the casting direction. In addition, it is preferable to use a split roll.
A division | segmentation roll is a roll divided | segmented in the position except the width direction both ends of a slab, and can suppress a bulging without using spray water. Specifically, it is a roll in which a plurality of rolls (for example, two) having a diameter of 250 to 400 mm are connected in series by providing a gap with a bearing portion (diameter portion smaller than the diameter of the roll).

Thus, since the gap is provided by the bearing portion, it is possible to suppress the roll deformation due to the heat effect of the slab. As a result, the roll diameter can be reduced compared to the support roll (single roll without bearings) of the conventional continuous casting machine, so that the installation interval between adjacent rolls in the casting direction should be narrowed compared to the conventional support roll. Can do.
Therefore, it becomes possible to suppress the bulging amount of the slab, and since the restriction of the water density by the spray water for the purpose of suppressing bulging (lower limit setting) can be relaxed, the average temperature and temperature deviation of the slab width direction can be reduced. It becomes easy to adjust the water density for the purpose of adjustment. That is, the degree of freedom in utilizing the recuperation of the slab is improved, and the amount of cooling water can be significantly reduced.

Further, as a method of suppressing the temperature deviation in the width direction of the slab, a recuperation phenomenon on the surface of the slab can be used. Hereinafter, the recuperation phenomenon will be described.
The surface temperature of the slab is determined by the heat removal of the slab surface by spray water and the heating of the slab surface by the internal molten steel in the slab where the solidified shell (low temperature) and the internal molten steel (high temperature) exist. It is determined. Here, when the amount of heat removed from the surface of the slab decreases due to a decrease in the amount of spray water or the like, and the amount of heating on the surface of the slab becomes relatively significant, the surface temperature of the slab increases. This is called recuperation.

In general, when a slab having a surface temperature of 500 to 900 ° C. is cooled by spray water, the temperature of the slab is lowered and its cooling ability (heat transfer coefficient) is improved as shown in FIG. Or not). When the surface temperature of the slab becomes 800 ° C. or higher, the temperature dependency of the cooling capacity of the slab surface is weakened, and the cooling capacity becomes substantially constant.
In other words, if you want to reduce the temperature deviation of the slab, reduce the spray water density, increase the surface temperature of the slab by recuperation, eliminate the temperature deviation of the slab, and weakly depend on temperature. It is better to restart the slab cooling in the temperature range.
Thus, since the recuperation phenomenon is effective in reducing the temperature deviation of the slab, the following configuration is preferable.

First, assuming that the distance from the position immediately below the mold 10 to the bending back portion 14 of the continuous casting machine 12 is D, one thermometer is within the range from the position immediately below the mold 10 to the position 0.4D to the position 0.65D. (It may be two or more).
Conditions for avoiding cracking of the slab 13, that is, the surface temperature difference in the width direction of the slab (= maximum temperature−minimum temperature) 300 excluding the range of 100 mm from both ends of the slab in the width direction toward the center. In order to ensure the temperature within ° C, a temperature deviation (surface temperature distribution) is measured in the vicinity of the bent-back part 14, and when it is predicted that the temperature deviation of the bent-back part 14 exceeds the target value (300 ° C), the cooling condition It is good to change.
Here, since it is necessary to secure time for the surface of the slab to reheat, a thermometer is installed on the upstream side of the bent-back portion 14, that is, at a position of 0.65D or less.

Moreover, the above steel types, that is, steel types containing Si: 1.0% by mass or more, or Cr: 10% by mass or more, or C: 0.5% by mass or more have high cracking susceptibility, and internal cracking due to bulging occurs. It is easy to generate.
These steel grades are curved portions where bulging is likely to occur (especially, the first half of the curved portion located approximately 0.4D from directly under the mold, that is, the region where the molten steel has a high static pressure and the solidified shell is relatively thin). There is a tendency for temperature deviation to occur in the width direction. For this reason, a thermometer is installed after the place where the temperature deviation occurs, that is, at a position where it becomes 0.4 D or more.
From the above, the position of the thermometer is set within the range of 0.4D to 0.65D (preferably, the lower limit is 0.45D and the upper limit is 0.6D).

Then, the surface temperature of the slab 13 is measured with the thermometer described above, and on the condition that the surface temperature difference in the width direction excluding both ends in the width direction of the slab 13 exceeds 300 ° C., the downstream of the cooling range R2. And the water density of the cooling water sprayed from the cooling nozzle to the slab 13 in the whole or part of the range upstream from the position of 0.4D is smaller than the current (currently sprayed) water density (for example, , The current water density exceeds 0 and decreases by about 20% or less). The water density is adjusted with all or some of the cooling nozzles arranged in the above range.
As described above, the water density of the spray water was determined in the range from the position immediately below the mold to 1.2 m in the casting direction (cooling range R1 and cooling range R2) for the purpose of peeling and removing the powder. Therefore, when a temperature deviation to be corrected is detected by the thermometer described above, it is necessary to adjust the cooling by the spray water on the downstream side of the cooling range R2.
For this reason, it is preferable to reduce the spray water density in the whole or part of the range from directly below the mold to the casting direction downstream from 1.2 m and upstream from the position of 0.4D.

In addition, since a distance D is about 10 to 20 m in a general continuous casting machine (a curved continuous casting machine or a vertical bending type continuous casting machine), the position where 0.4D is about 4 to 8 m from directly below the mold. is there. For this reason, a thermometer will be installed in the casting direction from just under a casting_mold | template from the position of 1.2 m in the downstream by 2.8 m or more. Therefore, the position where the spray water density is reduced does not overlap with the cooling ranges R1 and R2 from just below the mold to 1.2 m in the casting direction.
Here, the surface temperature of the slab is reheated by reducing the water density and the temperature deviation in the width direction of the slab is suppressed. However, it is necessary to consider the occurrence of bulging due to the reduction of the water density.
However, according to the knowledge of the present inventors, when cooling is performed so that the temperature deviation at the installation position of the thermometer becomes remarkable (over 300 ° C.), at least a part of the slab width direction is the thickness of the solidified shell. Therefore, bulging is unlikely to occur, and by adjusting the reduction amount of the water amount density, it is possible to suppress remarkable occurrence of bulging and recover heat (reduction in water amount density).

As shown above, the mass ratio CaO / SiO ₂ of CaO to SiO ₂ in the powder supplied into the mold is 0.90 or more and 1.45 or less, and from just below the mold to 0.6 m in the casting direction. In the cooling range R1, the cooling capacity index difference due to the presence or absence of powder adhesion is small, and cooling is performed at a water density P1 at which the powder does not actively peel off, and from 0.6 m to 1.2 m in the casting direction from directly below the mold. In the cooling range R2, the water density P2 is made higher than the water density P1, and the powder peeling is substantially completed from just below the mold to 1.2 m in the casting direction.
Thereby, the temperature deviation in the width direction of the slab 13 and the supercooling of the surface are suppressed, and both the prevention of the internal crack and the prevention of the surface crack of the slab are achieved, and a high quality slab can be manufactured.

Next, examples carried out for confirming the effects of the present invention will be described.
Here, the continuous casting machine shown in FIG. 3 is used, and the mass ratio (CaO / SiO ₂ ) of CaO to SiO ₂ in the powder and the cooling range R1 from just below the mold to 0.6 m in the casting direction. , And the case where the spray water density P1 and P2 in the cooling range R2 from 0.6 m to 1.2 m are changed in the casting direction from directly below the mold, respectively, the surface temperature deviation of the slab and cracking of the slab The impact on incidence was investigated.
In addition, a camera capable of observing the surface of the slab is installed at a position 1.2 m in the casting direction from directly under the mold, and the state of powder adhesion (visual observation) when the spray water density P1, P2 is changed and the slab The relationship of temperature deviation was also investigated. In the continuous casting machine, a thermometer was installed at a position of 8 m (0.5 D) in the casting direction from directly under the mold, and the surface temperature of the slab was measured.

Here, experimental conditions are shown below.
<Continuous casting machine>
Roll type: All rolls are composed of a single support roll (no split rolls). Pitch of rolls: 200mm from just below the mold to 1.2m in the casting direction.
Cooling nozzle type: From just below the mold to 1.2 m in the casting direction, the distance D from the air-water nozzle mold to the bent back part D: 16 m

<Casting conditions>
Casting speed: 1.3 m / min, casting width: 1000-1300 mm, casting thickness: 250 mm, steel type: 1.0-3.5 mass% Si-containing steel <cooling conditions>
Spray water density in the cooling range R1 P1: 100 L / m ² / min, 150 L / m ² / min, 280 L / m ² / min, 300 L / m ² / min Spray water density in the cooling range R2 P2: 250 L / m ² / min, 300 L / m ² / min, 700 L / m ² / min, 750 L / m ² / min

Then, the consumption of powder supplied into the mold (supply amount), was 0.1 kg / ^{m 2} or more 0.7 kg / ^{m 2} or less. As the powder, for example, a powder having a viscosity of 0.6 to 1.0 poise at 1350 ° C. was used. In addition, the ratio CaO / SiO ₂ of the powder, 0.85,0.90,0.95,1.45,1.50, was.
The consumption amount Pw (kg / m ² ) of this powder is defined as follows.
{Powder consumption Pw (kg / m ² )}
= {Amount of powder put into meniscus during casting time (kg)}
/ {Casting speed (m / min) × {slab width (m) + slab thickness (m)} × 2 × casting time (min)}
Here, the casting time means, for example, a time for casting one charged molten steel of about 150 to 350 tons or a time for casting a plurality of charged molten steels.

Moreover, the crack incidence rate of the slab was defined as follows.
(Crack occurrence rate of slab) = (Number of slabs with cracks) / (Total number of slabs) × 100 (%)
(Total number of slabs) = {total casting length (m)} / {10 (m / piece)}
The total number of slabs is represented by the above formula because the number of slabs is counted in units of slabs cut at intervals of 10 m after casting.
The number of slabs in which cracks occurred was counted as a slab in which cracks occurred when one or both of surface cracks and internal cracks occurred at one location.

First, Table 1 shows the results when the powder ratio CaO / SiO ₂ was set to 0.85 and 1.50, respectively. In Table 1, the upper part shows the temperature deviation of the slab, and the lower part shows the crack occurrence rate of the slab (the same applies to Table 2 shown below).
This temperature deviation is “maximum temperature (° C.)” − “Minimum temperature (° C.)” in the width direction of the slab excluding both ends in the width direction of the slab (a region of 100 mm from both ends in the width direction of the slab to the center). Defined in
In addition, the crack occurrence rate of the slab is best when it is less than 1% (良好), when it is 1% or more and less than 3% is good (、 ３), when it is 3% or more and less than 5% is slightly defective (△), when it is 5% or more. Defective product (x) was used (the same applies to Tables 2 and 3 below).

First, the result of setting the spray water density P1 to 100 L / m ² / min will be described.
When the spray water density P2 is 250 L / m ² / min, 300 L / m ² / min, or 700 L / m ² / min, the powder adheres to the surface of the slab when observed with a camera, and the temperature exceeds 300 ° C. Deviation occurred (cracking rate: 5% or more).
When the spray water density P2 was 750 L / m ² / min, cracks caused by supercooling and internal cracks caused by high temperature occurred (crack generation rate: 5% or more).

Next, the result of setting the spray water density P1 to 150 L / m ² / min will be described.
When the spray water density P2 is 250 L / m ² / min, 300 L / m ² / min, or 700 L / m ² / min, the powder adheres to the surface of the slab when observed with a camera, and the temperature exceeds 300 ° C. Deviation occurred (cracking rate: 5% or more).
When the spray water density P2 was 750 L / m ² / min, cracks caused by supercooling and internal cracks caused by high temperature occurred (crack generation rate: 5% or more).

And the result of having set spray water quantity density P1 to 280L / m < ² > / min is demonstrated.
When the spray water density P2 is 250 L / m ² / min, 300 L / m ² / min, or 700 L / m ² / min, the powder adheres to the surface of the slab when observed with a camera, and the temperature exceeds 300 ° C. Deviation occurred (cracking rate: 5% or more).
When the spray water density P2 was 750 L / m ² / min, cracks caused by supercooling and internal cracks caused by high temperature occurred (crack generation rate: 5% or more).

Finally, the result of setting the spray water density P1 to 300 L / m ² / min will be described.
When the spray water density P2 was 250 L / m ² / min, powder was adhered to the surface of the slab and a temperature deviation exceeding 300 ° C. occurred (crack generation rate: 5% or more).
When the spray water density P2 is 300 L / m ² / min, 700 L / m ² / min, on average, the temperature deviation becomes 200 ° C. or less and the crack generation rate improves, but the temperature deviation exceeding 300 ° C. Spattered (breakage rate: 5% or more).
When the spray water density P2 was 750 L / m ² / min, cracks caused by supercooling and internal cracks caused by high temperature occurred (crack generation rate: 5% or more).

Next, Table 2 shows the results when the powder ratio CaO / SiO ₂ was set to 0.90, 0.95, and 1.45, respectively.

First, the result of setting the spray water density P1 to 100 L / m ² / min will be described.
When the spray water density P2 is 250 L / m ² / min, in the observation by the camera, powder is sparsely adhered to the surface of the slab, and a temperature deviation exceeding 300 ° C. occurs (crack generation rate: 5% or more) ).
When the spray water density P2 is 300 L / m ² / min, 700 L / m ² / min, on average, the temperature deviation is 200 ° C. or less, but internal cracks have occurred due to high temperature (cracking rate: 5 %more than).
When the spray water density P2 was 750 L / m ² / min, cracks caused by supercooling and internal cracks caused by high temperature occurred (crack generation rate: 5% or more).

Next, the result of setting the spray water density P1 to 150 L / m ² / min will be described.
When the spray water density P2 is 250 L / m ² / min, in the observation by the camera, powder is sparsely adhered to the surface of the slab, and a temperature deviation exceeding 300 ° C. occurs (crack generation rate: 5% or more) ).
When the spray water density P2 is 300 L / m ² / min, 700 L / m ² / min, a good powder peeling state is observed by the camera, the temperature deviation is small, and cracking due to supercooling or cracking due to high temperature occurs. The crack generation rate could be reduced (crack generation rate: 1% or more and less than 3%).
When the spray water density P2 was 750 L / m ² / min, cracking due to supercooling occurred (cracking rate: 5% or more).

And the result of having set spray water quantity density P1 to 280L / m < ² > / min is demonstrated.
When the spray water density P2 is 250 L / m ² / min, in the observation by the camera, powder is sparsely adhered to the surface of the slab, and a temperature deviation exceeding 300 ° C. occurs (crack generation rate: 5% or more) ).
When the spray water density P2 is 300 L / m ² / min, 700 L / m ² / min, a good powder peeling state is observed by the camera, the temperature deviation is small, and cracking due to supercooling or cracking due to high temperature occurs. The crack generation rate could be reduced (crack generation rate: 1% or more and less than 3%).
When the spray water density P2 was 750 L / m ² / min, cracking due to supercooling occurred (cracking rate: 5% or more).

Finally, the result of setting the spray water density P1 to 300 L / m ² / min will be described.
When the spray water density P2 is 250 L / m ² / min, in the observation by the camera, powder is sparsely adhered to the surface of the slab, and a temperature deviation exceeding 300 ° C. occurs (crack generation rate: 5% or more) ).
When the spray water density P2 is 300 L / m ² / min, 700 L / m ² / min, on average, the temperature deviation becomes 150 ° C. or less and the crack occurrence rate improves, but the temperature deviation exceeding 300 ° C. Spattered (breakage rate: 3% or more and less than 5% or 5% or more depending on the amount of powder consumed).
When the spray water density P2 was 750 L / m ² / min, cracking due to supercooling occurred (cracking rate: 5% or more).

From the above, in order to suppress the temperature drop and temperature deviation caused by the powder, the ratio of the mass% of CaO in the powder to SiO ₂ is 0.90 ≦ (CaO / SiO ₂ ) ≦ 1.45, and the cooling range the water flow rate P1 of the cooling water in the R1 and 150L / ^{m 2} / min or more 280L / ^{m 2} / min or less, the water density P2 of the cooling water in the cooling range R2 300L / ^{m 2} / min or more 700L ^{/ m} 2 / Therefore, it was confirmed that the crack generation rate of the slab can be made 1% or more and less than 3%.
Furthermore, when a split roll is applied as a roll used in a continuous casting machine from directly under the mold to a position of at least 0.4D in the casting direction, internal cracks due to bulging are reduced, and the spray water density is reduced. Since recuperation can be used, it was estimated that the crack occurrence rate (crack occurrence rate of 1% or more and less than 3%) in Table 2 can be reduced by about 1%. This result was estimated from the effect of reducing temperature deviation and the effect of recuperation.

Next, the effect of the presence or absence of a thermometer on the continuous casting machine on the crack occurrence rate of the slab, and the effect on the crack occurrence rate of the slab when the water density is changed when a thermometer is installed are shown. The results are shown in Table 3.

In addition, Example 1 of Table 3 is the result when casting was performed without installing a thermometer in the continuous casting machine, and Example 4 was installed at a position of 8 m in the casting direction from directly under the mold. And when the temperature deviation of the slab measured with the thermometer exceeds 300 degreeC, the result at the time of casting by changing the spray water quantity density upstream from the position of 0.4D is shown.
In Examples 2, 3, 5, and 6 in Table 3, the thermometers are 5.6 m (0.35 D), 6.4 m (0.40 D), and 10.4 m in the casting direction from directly under the mold, respectively. (0.65D) The estimation result when installed at a position of 11.2 m (0.70 D) is shown. Note that the rate of occurrence of slab cracking is estimated by creating a model that estimates the temperature history of the slab in the casting direction and the temperature distribution (temperature deviation) in the width direction of the slab using the actual measurement data of Example 4. went.

From Table 3, the thermometer was installed in the range of 0.4D or more and 0.65D or less, the surface temperature of the slab was measured, and the surface temperature difference in the width direction of the slab exceeded 300 ° C. In the range downstream from the cooling range and upstream from the position of 0.4D, the density of the spray water sprayed on the slab is reduced from the density of the coolant water sprayed on the slab at the above-mentioned conditions. It was confirmed that the crack generation rate of the piece could be improved to less than 1% (Examples 3 to 5 in Table 3).
In addition, about the steel grade, the result substantially equivalent to Si containing steel was obtained also about 10 mass% or more Cr containing steel and 0.5 mass% or more C containing steel.
From the above, by using the steel continuous casting method of the present invention, the temperature deviation in the width direction of the slab and the overcooling of the surface of the slab are suppressed, and the internal crack prevention and surface of the slab are suppressed. It was confirmed that good quality slabs could be manufactured while preventing cracking.

The present invention has been described above with reference to the embodiments. However, the present invention is not limited to the configurations described in the above-described embodiments, and the matters described in the claims are not limited. Other embodiments and modifications conceivable within the scope are also included. For example, a case where the steel continuous casting method of the present invention is configured by combining some or all of the above-described embodiments and modifications is also included in the scope of the right of the present invention.

10: mold, 11: secondary cooling zone, 12: continuous casting machine, 13: slab, 14: bent back part

Claims (4)

Using a continuous casting machine having a mold and a secondary cooling zone disposed on the downstream side of the mold, 1.0 mass% or more of Si, 10 mass% or more of Cr, or 0 of C is extracted from the mold. In a continuous casting method of steel in which a slab containing 5% by mass or more is drawn and the slab is cooled by a number of cooling nozzles provided in the secondary cooling zone,
The mass% of the ratio CaO / SiO ₂ with respect to SiO ₂ of CaO in the powder supplied into said mold and 0.90 to 1.45,
Among the secondary cooling zones, a cooling range R1 from directly below the mold to 0.6 m in the casting direction and a cooling range R2 from 0.6 m to 1.2 m in the casting direction from directly below the mold. A continuous casting method of steel, wherein water density P1 and P2 of cooling water sprayed from the cooling nozzle to the slab satisfy the following conditions.
150 liters / m ² / min ≦ P1 ≦ 280 liters / m ² / min 300 liters / m ² / min ≦ P2 ≦ 700 liters / m ² / min The continuous casting method for steel according to claim 1, wherein an air-water nozzle is used as the cooling nozzle for blowing cooling water at least in the cooling range R2. In the continuous casting method of steel according to claim 1 or 2, in the secondary cooling zone, a large number of rolls are disposed that sandwich the slab drawn from the mold from the thickness direction and convey it in the casting direction, A steel continuous casting method characterized in that a split roll divided at a position excluding both ends in the width direction of the slab is used for a part or all of the multiple rolls. The steel continuous casting method according to any one of claims 1 to 3, wherein D is a distance from a position immediately below the mold to a bending return portion of the continuous casting machine, and a position of 0.4D from directly below the mold. The surface temperature of the slab is measured by installing a thermometer within the range from the position of 0.65D to the position of 0.65D, and the region in the range excluding the range up to 100 mm from both ends in the width direction toward the center portion is measured. On the condition that the surface temperature difference in the width direction of the slab has exceeded 300 ° C., the cooling nozzle is disposed in the whole or part of the range downstream from the cooling range R2 and upstream from the position of 0.4D. A continuous casting method for steel, characterized in that the density of cooling water sprayed on a slab is reduced.

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