JP4987545B2 - Secondary cooling device for continuous casting machine and its secondary cooling method - Google Patents

Description

The present invention relates to a secondary cooling device of a continuous casting machine for cooling a slab manufactured by a continuous casting mold and a secondary cooling method thereof.

A continuous casting machine continuously cools a slab semi-solidified in a water-cooled mold using a spray nozzle disposed between adjacent rolls in a secondary cooling zone (secondary cooling device) following the mold. This is a facility that completes the solidification of the slab by the end of the secondary cooling zone (hereinafter also referred to as the machine end). If the solidification of the slab does not finish by the end of this machine, the solidified shell is pushed up by the static pressure of the molten steel acting inside the slab, causing cracks inside the slab, and the secondary cooling zone The casting speed could not be increased without increasing the cooling capacity. Here, the relationship between the cooling capacity of the secondary cooling zone and the casting speed will be described. By strengthening the cooling capacity of the secondary cooling zone, the amount of heat removal in the thickness direction of the slab increases, and the thickness of the solidified shell increases. The growth rate becomes faster, and the solidification completion point moves to the upstream side (mold side) than before, so that the casting speed can be increased.
Therefore, in the steel industry, various researches for enhancing the cooling capacity of the slab in the secondary cooling zone of the continuous casting machine are underway in order to cope with high production.
The evaluation of the cooling capacity between the plurality of rolls arranged in the secondary cooling zone is expressed by the following formulas (1) to (3), where Q (kcal / hour) is the amount of heat removed from the slab. be able to.
Q = α × A × ΔT (1)
Where α is a heat transfer coefficient (kcal / m ² / hour / ° C.), A is a cooling area (m ² ), ΔT is {slab surface temperature (Ts: ° C.)} − {Cooling water temperature (Tw : ° C.)}.
Α is represented by α1 which is spray cooling ability (cooling ability by cooling medium) and α2 which is dripping water cooling ability (cooling ability by dripping water).
α1 (spray cooling capacity) ∝ f (Ts, W1, Pc) (2)
Here, W1 represents the water density (L / m ² / min) of the spray (cooling medium), and Pc represents the back pressure (kgf / cm ² ) of the spray (in the ejection part of the cooling medium).
α2 (dripping water cooling capacity) ∝ f (Ts, W2) (3)
Here, W2 represents a drooping water density (L / m ² / min).

As can be seen from the above formulas (1) and (2), it is considered effective to increase the spray water density and increase α1 in order to increase the heat removal amount Q.
However, when cooling water is sprayed (sprayed) on the surface of a slab having a high temperature of 600 ° C. or higher, there is a problem that a steam film is formed on the surface of the slab and the surface of the slab cannot be directly cooled with cooling water.
Therefore, as a method for increasing the heat removal amount Q, various secondary cooling techniques shown below have been proposed.
For example, Patent Document 1 uses a collision pressure variable cooling means having a high pressure nozzle in at least one of a plurality of cooling means arranged along a slab pass line downstream of a mold of a continuous casting machine. In addition, an apparatus and a method for improving the casting speed by causing the high pressure spray water to collide with the surface of the slab to cool the slab are disclosed.
Further, in Patent Document 2, when the secondary cooling is performed using the cooling nozzles arranged between the rolls constituting the secondary cooling zone, the cooling capacity is made efficient by defining the shape of the water amount distribution of the cooling water. A well-enhanced method is disclosed.

A secondary cooling method using the roll itself has also been proposed.
For example, in Patent Document 3, the secondary cooling water ejected from the cooling nozzles arranged between the rolls is formed in the grooves on the front side of the plurality of rolls. An apparatus is disclosed that improves the cooling efficiency of the roll by flowing through the inside and prevents an accident of roll rotation failure due to thermal deformation.
Patent Document 4 discloses a device that cools the roll facing surface side of a slab by flowing cooling water into a fixed shaft of a roll and ejecting the cooling water from a cooling water ejection hole provided in a rotating sleeve of the roll. And a method are disclosed.

JP 2004-167521 A JP 2003-136205 A Japanese Utility Model Publication No. 62-169750 JP-A-2-255256

However, the conventional method still has the following problems to be solved.
As in Patent Document 1, when high-pressure spray water collides with a high-temperature slab surface, a vapor film generated on the slab surface during cooling is destroyed at a higher temperature than when high-pressure spray water is not used. Thereby, since the water and the slab surface are in direct contact with each other to improve the cooling capacity of the slab, the casting speed of the slab can be improved.
However, according to the present inventors, when the high pressure spray water (spray back pressure: 5 kgf / cm ² or more, that is, 0.49 MPa or more) is made to collide with the slab surface, the surface temperature of the slab at which the vapor film is destroyed. As shown in FIG. 7, it was found that the cooling capacity is not stable. Therefore, even when cooling is stably performed during steady casting, it is considered that unsteady overcooling occurs due to the above-described variation in cooling capacity, and the slab temperature is lowered to a temperature range where reheating is not possible. Therefore, recuperation cannot be ensured and there is a risk that the surface crack of the slab will occur at the bent back portion of the continuous casting machine.

FIG. 7 is an explanatory diagram in which the vertical axis represents the heat transfer coefficient index and the horizontal axis represents the spray back pressure. The heat transfer coefficient index is a ratio when the heat transfer coefficient is 1 when the surface temperature of the slab is 700 ° C. and the spray back pressure is 0.10 MPa (1 kgf / cm ² ). Further, the spray back pressure is a pressure at an ejection portion of the cooling water sprayed (ejected) from the cooling nozzle. The heat transfer coefficient at each spray back pressure is calculated by heating the cast piece (dummy) in which the thermocouple is embedded to 1000 ° C. or more and then spraying cooling water onto the cast piece from the cooling nozzle. It is determined from the cooling rate of the slab at around 700 ° C. when it is cooled to about ° C. Here, the water density of the cooling water is fixed at 2000 liters / m ² / min, and the spray back pressure is changed in the range of 0.10 to 0.79 MPa (1 to 8 kgf / cm ² ).
Further, in Patent Document 1, for the purpose of adjusting the cooling capacity according to the casting conditions of the slab, the collision pressure of the cooling water is adjusted by inverter control of the motor speed of the plunger pump. However, there is also an economic problem because it requires a great deal of cost to install or maintain the plunger pump.

Patent Document 2 is a technique for improving the cooling capacity A of the slab by improving the cooling area A by the cooling water (see the above formula (1)). Can be improved to some extent. However, when the cooling area is expanded with the aim of further improving the cooling capacity, a part of the cooling water is applied to the roll, so there is a limit to improving the cooling capacity without changing the equipment specifications.
And since the method of patent document 3 is improving the cooling efficiency of a roll by flowing cooling water in the ditch | groove formed in the roll, it is thought that the lifetime of a roll can be extended, but positively Therefore, the slab is not cooled, and further improvement in the cooling rate of the slab cannot be expected. For this reason, it is considered that the casting speed of the slab cannot be further improved.

Moreover, the method of patent document 4 aims at the suppression of the internal crack of a slab, and the improvement of cooling efficiency by cooling also the site | part which cannot apply cooling water only by the nozzle for cooling by ejecting cooling water from a roll. be able to. However, the continuous casting machine needs to suppress the surface crack of the slab due to overcooling and the internal crack of the slab due to insufficient cooling by changing the amount of cooling water according to the casting speed. However, the control of the amount of cooling water jetted according to the fluctuation of the casting speed, that is, the control of the surface temperature of the slab is not taken into consideration, and it is considered that the above-described quality problem occurs when the actual machine is introduced.
Furthermore, the cooling water that has flowed into the fixed shaft of the roll is ejected from the cooling water jet holes provided in the rotating sleeve of the roll, but the cooling water is not filled unless the inside of the fixed shaft is completely filled with cooling water. It cannot be uniformly ejected from the ejection holes. For this reason, since the hydraulic power distribution of the cooling water becomes non-uniform in the width direction of the slab, there may be a possibility that the supercooling is partially generated in the slab. Also, even if the fixed shaft can be filled with cooling water, it is necessary to change the water pressure in the fixed shaft according to the casting speed. In this case, it is necessary to introduce a compressor or the like to increase the water pressure. There are economic problems with the introduction of this technology.

The present invention has been made in view of such circumstances, and when cooling a slab manufactured with a mold, the cooling capacity of the slab is improved and the slab temperature is stabilized without performing a large-scale facility modification. It is an object of the present invention to provide a secondary cooling device for a continuous casting machine and a secondary cooling method for the continuous casting machine that can be controlled in a controlled manner and can improve the casting speed while avoiding surface cracks of the slab.

A secondary cooling device for a continuous casting machine according to the first invention that meets the above object is disposed on the front side and the back side of a slab drawn from a continuous casting mold with an interval in the drawing direction. Vertically having a roll that sandwiches and conveys a piece from the thickness direction, and a plurality of cooling nozzles that are arranged in the width direction of the cast piece between the front-side roll and the back-side roll and cool the cast piece. In the secondary cooling device of the bending type or the bending type continuous casting machine,
Part or all of the range in which the total value d of the thicknesses of the solidified shells formed on the front and back sides of the slab is 0.25 <d / D <0.55 with respect to the thickness D of the slab A strong cooling range is provided , and one or a plurality of water passing portions are provided in the width direction of the slab with respect to the roll disposed at least on the front side of the slab in the strong cooling range, and sprayed from the cooling nozzle. While allowing the cooling medium to pass through the water passing portion of the roll, the density of the cooling medium sprayed from the cooling nozzle installed in the strong cooling range is 800 liters / m ² / min to 2000 liters / m ² / The water density of the cooling medium sprayed from the cooling nozzle installed in the range of 0.55 ≦ d / D <0.75 is 50 liters / m ² / minute or more and 700 liters / m ² / cold the cast piece as a minute or less To.

In the secondary cooling device of the continuous casting machine according to the first aspect of the present invention, the water flow portion of the roll and the cast piece arranged in the strong cooling range are the width of the cast piece of the water flow portion of the roll. When the total width in the width direction positioned inside is Lt and the width of the slab is w, the condition of 0.25 ≦ Lt / w <0.5 is satisfied, and adjacent to the drawing direction of the slab. It is preferable that the positions of the water passing portions of the matching rolls are alternately shifted in the width direction of the slab.
In the secondary cooling device of the continuous casting machine according to the first aspect of the present invention, the formation position of the water flow portion to each of the rolls is arranged upstream of each of the rolls adjacent in the drawing direction of the slab. Further, the width of the water flow portion of the roll is Lsa, the width of the water flow portion of the roll disposed on the downstream side is Lsb, and each water flow portion positioned at the closest distance is the width of the slab. When the interval between the center positions in the width direction of each water flow portion in the direction is L, it is preferable that the conditions of L / Lsa ≧ 1 and L / Lsb ≧ 1 are satisfied.
In the secondary cooling device of the continuous casting machine according to the first aspect of the present invention, it is preferable that the back pressure at the ejection portion of the cooling medium ejected from the cooling nozzle can be controlled to be less than 0.49 MPa.

In the secondary cooling method for a continuous casting machine according to the second aspect of the invention, the slab is strongly cooled using the secondary cooling device for the continuous casting machine according to the first aspect.

The secondary cooling device of the continuous casting machine according to claim 1 and the secondary cooling method of the continuous casting machine according to claim 5, wherein the slab is subjected to strong cooling without bending the slab as a strong cooling range. Time for recuperation can be secured, and the thickness of the solidified shell to be formed is defined as a relatively thin range. There is no concern about the surface cracking of the slab at the bending part), the cooling efficiency can be increased, and the distance that can be cooled can be sufficiently secured, so that the casting speed of the slab can be improved.
Moreover, since the roll in which the water flow part was formed is used in a strong cooling range, the cooling medium (dripping water) which has flowed down only from the axial direction both sides of the conventional roll is allowed to pass through the roll water flow part. It can be washed away. Thereby, since the cooling medium can be temporarily accumulated above the contact portion between the roll and the slab, the cooling area of the slab can be made wider than before, and the cooling capacity by the cooling medium can be improved. it can. Note that the slab can be uniformly cooled in the width direction also at the site where the water flow portion is formed and at the periphery thereof.
And since the water quantity density of the cooling medium injected by the cooling nozzle is controlled within a predetermined range, the casting speed of the slab can be improved while increasing the cooling capacity.
Therefore, it is possible to improve the cooling capacity of the slab without making a large-scale equipment modification, and to stably control the slab temperature, and to improve the casting speed while avoiding the surface crack of the slab, Products with good quality can be manufactured with good productivity.

In particular, the secondary cooling device of the continuous casting machine according to claim 2 stipulates the relationship between the width of the water passing portion of the roll disposed in the strong cooling range and the width of the slab, and thus corresponds to the water passing portion. It is possible to obtain an effect of improving the cooling capacity of the slab while suppressing the slab surface from being bulged due to the deformation of the slab in the portion and preventing the quality of the slab from being lowered.
Since the secondary cooling device of the continuous casting machine of Claim 3 prescribes | regulates the position of the water flow part formed in each roll, it can perform uniform cooling over the width direction of a slab, Generation of the supercooled portion can be suppressed.
Since the secondary cooling device of the continuous casting machine according to claim 4 regulates the back pressure of the cooling medium injected from the cooling nozzle, the cooling of the slab due to the destruction of the vapor film formed on the surface of the slab The variation in ability can be further suppressed.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
Here, FIG. 1 is an explanatory view of a secondary cooling device and a secondary cooling method of a continuous casting machine according to an embodiment of the present invention, FIG. 2 is an explanatory view of the continuous casting machine, and FIG. 3 is a cooling nozzle. Explanatory diagram showing the relationship between the water density of sprayed cooling water and the heat transfer coefficient index, FIG. 4 (A) is an explanatory diagram of the roll span of the secondary cooling device, and (B) is the roll span and average for strong cooling FIG. 5 is a partial front view of a roll used in a secondary cooling device of a continuous casting machine according to an embodiment of the present invention, and FIGS. 6 (A) and 6 (B) are diagrams illustrating a relationship with a heat removal amount index. FIG. 4 is a partially omitted front view and a partially omitted side view of the secondary cooling device of the continuous casting machine, respectively.

As shown in FIGS. 1 and 2, a secondary cooling device (hereinafter also simply referred to as a secondary cooling device) 10 of a continuous casting machine according to an embodiment of the present invention is an apparatus disposed in a continuous casting machine 11. In the drawing direction (also referred to as casting direction or longitudinal direction), respectively, on the front side 15 and the back side 16 of a cast piece 14 (for example, slab) drawn from a continuous casting mold (hereinafter also simply referred to as mold) 13 The width of the slab 14 is disposed between the rolls 17 (also referred to as casting rolls) that are arranged with a gap and sandwich and convey (support) the slab 14 from the thickness direction, and between the front-side roll 17 and the back-side roll 17. It is an apparatus having a large number of cooling nozzles 18 arranged side by side to cool the slab 14. This will be described in detail below.

The continuous casting machine 11 is a slab vertical bending type continuous casting machine.
The continuous casting machine 11 includes a tundish 19 for storing molten steel, a continuous casting mold 13 for solidifying molten steel supplied from the tundish 19 via an immersion nozzle 20, and a front side 15 and a back side 16 by the mold 13. And a secondary cooling device (also referred to as a secondary cooling zone) 10 that cools the slab 14 in which the solidified shells 21 and 22 are formed while being continuously conveyed to the downstream side. In this secondary cooling device 10, the slab 14 is cooled and conveyed downstream while being bent, and the slab bent by a straightening device (not shown) arranged on the downstream side of the secondary cooling device 10. 14 is bent back and straightened (i.e., corrected). The slab 14 to be manufactured has, for example, a width of about 700 to 2200 mm and a thickness of about 150 to 300 mm.
As the continuous casting machine, a curved bending type continuous casting machine in which the bending amount of the slab is smaller (the curvature radius is large) than that of the vertical bending type continuous casting machine can also be used.

The roll 17 of the secondary cooling device 10 is a so-called split roll that is divided in the width direction of the slab 14 and connected by a roll shaft that does not contact the slab 14, but has a diameter larger than that of the split roll. An integrated roll may be used without being divided in the width direction of the slab.
Each cooling nozzle 18 sprays cooling water as a cooling medium against the slab 14, but it may be one that sprays air or a mixture of air into this cooling water. Although they have the same configuration, they may have different configurations.

Here, the arrangement positions of the cooling nozzles 18 adjacent to each other in the drawing direction of the slab 14 are alternately shifted in the width direction of the slab 14 in the width direction of the slab 14 (in a zigzag manner). Although it arrange | positions, it is good also as the same over the drawing direction of the slab 14 partially or entirely.
Moreover, the pitch of the cooling nozzles 18 arranged in the drawing direction of the slab 14 is set according to the width of the cast slab 14 to be cast, and is due to overcooling of the end in the width direction of the slab 14 that is easier to cool than other parts. In order to prevent the surface crack of the slab 14, it is preferable to set within the range of 200 mm or more and 400 mm or less, for example. In the present embodiment, the interval between the cooling nozzles 18 adjacent in the width direction of the slab 14 and the interval between the cooling nozzles 18 adjacent in the drawing direction of the slab 14 are made the same. Either one or both may be different.
Then, the distance between the cooling nozzle 18 and the surface of the slab 14 can be changed according to the diameter of the roll 17, so that the cooling area of the slab 14 is wider without a part of the cooling water coming into contact with the roll 17. It is preferable in terms of securing the range. For example, the distance may be about the radius of the roll 17 (for example, about 80 to 200 mm).

As described above, the continuous casting machine is roughly divided into a vertical bending type continuous casting machine and a curved bending type continuous casting machine, and a slab continuously cast with a mold is cooled by a secondary cooling device. However, it is manufactured by bending back at the bending back part. At this time, in the vertical bending die continuous casting machine, the slab bent at the bending portion is bent back by the bending return portion.
In this continuous casting process, in order to stably produce a high quality slab, the surface temperature of the slab at the bent part and the bent back part where the slab is distorted is set to the embrittlement temperature range (upper limit: approximately It is necessary to ensure a temperature exceeding the range of 650 ° C. to 700 ° C. to avoid surface cracking of the slab. Therefore, it can be seen that if strong cooling of the slab is performed at the bent portion or the bent back portion, there is a concern that surface cracks may occur in the slab due to a decrease in surface temperature, which is not preferable for strong cooling.

Therefore, the following three ranges were examined as areas for strong cooling of the slab.
(I) From lower end of mold to bent part (0.10 <d / D ≦ 0.25)
(Ii) Immediately after the bent portion to before the bent back portion (0.25 <d / D <0.55)
(Iii) From the bent back part to the downstream end of the continuous casting machine (d / D ≧ 0.75)
D / D is a ratio when the total value of the thicknesses d1 and d2 of the solidified shells 21 and 22 formed on the front side 15 and the back side 16 of the slab 14 is d and the thickness of the slab 14 is D. It is.
Here, the growth transition of the thickness of the solidified shell formed on the slab was estimated by an unsteady solidification analysis model described later.

First, when performing strong cooling of the slab in the range of (i), the slab temperature is restored to a temperature exceeding the embrittlement temperature range by the bent part after strong cooling in order to prevent surface cracking of the slab. It needs to be heated. For this reason, the distance in which strong cooling is possible becomes too short, and the effect of improving the casting speed of the slab cannot be expected due to insufficient cooling.
In addition, when strong cooling of the slab is performed within the range of (iii), the solidified shell grows and becomes too thick as compared with the other (i) and (ii). Even in the case of decreasing the cooling rate, the cooling efficiency is low due to the thermal resistance caused by the thickness of the solidified shell, and the effect of improving the casting speed of the slab cannot be expected.
On the other hand, when performing strong cooling of the slab in the range of (ii), since the slab is strongly cooled immediately after the bending portion, there is no fear of surface cracking of the slab, and the thickness of the solidified shell to be formed is also Since it is relatively thin, the cooling efficiency can be increased, and a sufficient cooling distance can be secured, and an improvement in the casting speed of the slab can be expected.
From the above, the range in which the slab is strongly cooled, that is, the strong cooling range is (ii) from immediately after the bent portion to before the bent portion (0.25 <d / D <0.55, preferably 0 .26 ≦ d / D ≦ 0.54).
In addition, the region between the above (ii) and (iii), that is, the region immediately before the bend back portion is removed from the region to be examined in advance, that is, 0.55 ≦ d / D <0.75. However, if the slab is strongly cooled in the region, the surface temperature of the slab is bent back in the embrittlement temperature region, which causes a surface crack, which is not preferable.

Here, the amount density of the cooling water sprayed from the cooling nozzle in the above-described strong cooling range will be described.
As shown in the above equation (2), the heat transfer coefficient of the cooling water is expressed as a function of the water density. However, when the water density of the cooling water is increased, the cooling capacity of the slab is reduced by the interference between the water. Since the improvement is hindered, it is estimated that there is a limit to the improvement of the cooling capacity.
Therefore, when the researchers investigated the relationship between the water density and the heat transfer coefficient α1, the cooling water quantity density was 2000 L / m ² / min (liter / m ² / min, below) as shown in FIG. Similarly, it was found that the heat transfer coefficient hardly changed at the above. Therefore, when the cooling efficiency by cooling water is considered, it turns out that it is necessary to make water volume density 2000L / m < ² > / min into an upper limit.
In addition, the data obtained from the laboratory cooling test shown in FIG. 3 and the unsteady solidification heat transfer analysis model were used to estimate the influence of the water density on the surface temperature of the slab. As a result, it was found that when the water density is less than 500 L / m ² / min, the surface temperature does not decrease to the surface temperature range of the slab until the cooling capacity can be improved and the casting speed can be improved.

From the above, considering that the cooling efficiency is improved and the casting speed is increased, the water density of the cooling water is 500 L / m ² / min or more and 2000 L / m ² / min or less (preferably, the lower limit is 800 L). / M ² / min, and further it is necessary to control within the range of 1000 L / m ² / min) (the water density of the cooling water outside the strong cooling range is usually 50 L / m ² / min or more and 700 L / m) ² / min or less).
The unsteady solidification heat transfer analysis model described above uses, for example, a general method shown in Iron and Steel, 60th (1974) No. 7, page 1023. At this time, by applying the following items and matching the slab surface temperature transition with the model analysis value in the actual machine, the slab casting speed and surface temperature transition when various operations and equipment conditions change As well as the impact on quality.
(A) For the influence of the cooling water volume density and spray back pressure on the cooling capacity, and the cooling capacity of the dripping water, the regression formula of the cooling capacity obtained in the laboratory cooling test is incorporated into the unsteady solidification heat transfer analysis model.
(B) A thermometer was attached to the slab of actual machine operation, and the surface temperature of the slab was actually measured.
(C) By multiplying the regression formula of the cooling capacity obtained in the laboratory cooling test by a coefficient, the surface temperature of the slab in the actual machine operation and the analysis value of the model were made to coincide.

The regression equation of the cooling capacity described above is obtained, for example, by the method shown below in which FIG. 3 is obtained.
FIG. 3 is an explanatory diagram in which the vertical axis represents the heat transfer coefficient index and the horizontal axis represents the cooling water quantity density. The heat transfer coefficient index is a ratio when the heat transfer coefficient is 1 when the surface temperature of the slab is 700 ° C. and the water density of the cooling water is 1000 L / m ² / min.
In FIG. 3, the experimental value (■) and the regression equation (solid line) are described. The experimental value is a heat transfer coefficient calculated from the laboratory cooling test, and the regression equation is the experiment. This regression equation was calculated from the values, and this regression equation was incorporated into the above-described unsteady solidification heat transfer analysis model to calculate the temperature.
The heat transfer coefficient at each water density is calculated by heating a cast slab (dummy) embedded with a thermocouple to 1000 ° C. or higher and then ejecting cooling water from the cooling nozzle onto this slab by 400 ° C. It is obtained from the cooling rate of the slab at around 700 ° C. when cooled to the extent. Here, the water density of the cooling water is changed in the range of 100 to 3000 L / m ² / min.

Moreover, it is preferable that the back pressure (hereinafter also referred to as spray back pressure) at the cooling water jetting portion jetted from the cooling nozzle 18 is less than 0.49 MPa (5 kgf / cm ² ) (outside the strong cooling range). The back pressure of the cooling water is usually about 0.04 MPa or more and 0.25 MPa or less).
As a method of cooling the slab, it is common to use a spray type cooling nozzle. However, when cooling water is sprayed on a slab of a high temperature of 600 ° C. or higher, it is applied to the surface of the slab. There was a problem that a steam film was formed and the slab surface could not be directly cooled with cooling water. Therefore, as a method for breaking the vapor film and increasing the heat transfer coefficient α1 by the cooling water, there is a method for increasing the spray back pressure (Pc) and increasing the collision energy E (∝Pc) (the above equation (2) )reference). Thereby, the vapor film is destroyed in a state where the surface temperature of the slab is higher, and the cooling water directly contacts the slab surface, so that the cooling capacity can be increased.

However, when the investigators investigated the effect of spray back pressure on the heat transfer coefficient α1, as shown in FIG. 7, the higher the spray back pressure, the better the cooling capacity, but the cooling capacity varies. I understood.
As described above, the cooling capacity can be improved by increasing the collision energy E and breaking the vapor film at a higher temperature. However, the slab surface at high temperature is likely to generate scale (the thickness of the vapor film differs depending on whether or not the scale is generated), and the vapor film thickness on the cooling surface when cooled with cooling water is localized at each cooling timing. Because the temperature at which the vapor film is destroyed changes, the cooling capacity is considered to vary greatly.
It has also been found that this variation in cooling capacity can be reduced by lowering the spray back pressure and lowering the collision energy E. This is because the vapor film is destroyed at a lower temperature (temperature at which the vapor film becomes thinner) because the collision energy becomes smaller, and therefore it is less susceptible to fluctuations in the scale generation state and the vapor film thickness. This is probably because of this.

Therefore, the data obtained from the laboratory cooling test shown in FIG. 7 and the unsteady heat transfer analysis model described above were used to estimate the effect of spray back pressure on the slab surface temperature.
As a result, it was found that the surface temperature of the slab may be lowered to a temperature range where surface cracks occur in the range of variation when the spray back pressure is set to 0.49 MPa. Therefore, it is preferable to make the spray back pressure less than 0.49 MPa (preferably 0.44 MPa, more preferably 0.4 MPa) in consideration of improvement of spray cooling ability and stabilization of cooling ability.
Further, from the above model, when the spray back pressure is less than 0.01 MPa (0.1 kgf / cm ² ), the surface temperature is up to the surface temperature range of the slab until the cooling speed can be improved and the casting speed can be improved. The lower limit of the spray back pressure is preferably 0.01 MPa (preferably 0.20 MPa, more preferably 0.29 MPa).

Next, with respect to the strong cooling range described above, from the viewpoint of the roll span (distance from the axial center of a roll to the axial center of the downstream roll adjacent in the drawing direction of the slab), FIG. ) And will be described in more detail.
In order to improve the casting speed of the slab by strong cooling, the surface temperature of the slab is lowered, the thickness of the vapor film formed on the surface of the slab is reduced to destroy the steam film, and the cooling water is cast into the slab. Must be in direct contact with the surface. However, in the continuous casting process, the slab is cooled with cooling water for a short time (for example, 5 seconds when the casting speed is 1.3 m / min). There is a limit, and when aiming to improve the casting speed by strong cooling, it is necessary to secure a roll span for strong cooling to some extent.
The researchers used the data obtained from the laboratory cooling test shown in FIGS. 3 and 7 and the above-mentioned unsteady heat transfer analysis model, and ingot the thermocouple into the slab surface during actual operation. By measuring the temperature transition, the relationship between the number of roll spans subjected to strong cooling with cooling water and the average heat removal (A · α · ΔT) between them was investigated.

The results and analysis conditions are shown in FIGS. 4 (A) and 4 (B). FIG. 4B is an explanatory diagram in which the vertical axis represents the average heat removal amount index and the horizontal axis represents the number of roll spans for strong cooling. The average heat removal amount index is a ratio when the average heat removal amount is 1 when the roll span shown in FIG. .
The casting speed is 1.3 m / min, the slab width is 1200 mm, the slab thickness is 250 mm, the cooling water density is 2000 L / m ² / min, and the spray back pressure is 0.39 MPa (4 kgf / min. cm ² ).
Moreover, the average heat removal amount index also shows the result of using a roll in which a water passage portion to be described later is used. The roll in which the water passage portion is not formed (only spray cooling) and the water passage portion are formed. The results of both rolls (cooling with spray and dripping water) are shown respectively. In addition, the water flow part formed in multiple in the width direction of slab is 200 mm in width, the total width of all the water flow parts is 400 mm (200 mm × 2 locations), and the depth is 5 mm.

As apparent from FIG. 4 (B), when a roll having no water passage portion is used, if the strong cooling is performed within the range of 1 to 3 roll span, the surface temperature of the slab can be sufficiently lowered. The average heat removal index was hardly changed. However, it was confirmed that by strongly cooling the range of 4 roll spans or more, the average heat removal rate index was drastically improved and the cooling capacity was improved.
Moreover, when the roll in which the water flow part was formed is used, as shown in FIG. 1, the cooling area between rolls can be expanded rather than the case where the roll in which the water flow part is not formed is used. For this reason, as shown in FIG. 4B, if the range of 1 to 3 roll span is strongly cooled, the average heat removal index tends to increase slightly, and the range of 4 roll span or more is strongly cooled. It was confirmed that the drooping water cooling capacity when using a roll with a water passage was obtained from a laboratory cooling test, and using the unsteady solidification analysis model described above, The amount of heat removal when the part was formed was estimated).
From the above, it can be seen that the strong cooling range is preferably at least 4 roll spans (a range of about 1000 mm to 1600 mm in distance) (may be the entire range).

As described above, it has been found that the cooling capacity can be further increased by using a roll having a water passage portion in the strong cooling range. Therefore, the evaluation of the cooling capacity between the rolls will be described.
The evaluation of the cooling capacity between the rolls can be expressed by the above-described formulas (1) to (3) as the heat removal amount Q (kcal / hour).
Here, as shown in FIG. 1, it turns out that the cooling area A expands from A1 to A2 by providing a water flow part in a roll.
Further, from the above equation (3), the relationship of α2∝W2 (dripping water density) is clear. Here, by using a roll in which a water passing portion is formed in a part or all of the range of 0.25 <d / D <0.55 where spray cooling is performed with a high amount of water, it falls from the upstream side. Since the cooling water dripping water can be allowed to flow downstream while passing the water portion, dripping water is temporarily accumulated above the contact portion between the roll and the slab, and W2 further increases. Thereby, it turns out that the drooping water cooling capability (alpha) 2 rises and the cooling capability of a slab rises.

Next, the rolls 17a and 17b in which the water flow parts included in the plurality of rolls 17 are formed will be described.
As shown in FIGS. 5, 6 </ b> A, and 6 </ b> B, the roll 17 a has two (one or three) Lsa widths (length in the axial direction of the roll 17 a) in the width direction of the slab 14. The above may also be provided) and water flow portions (also referred to as slits) 23 and 24. Further, the roll 17b has two water passage portions (also referred to as slits) 25 whose width (the length in the axial direction of the roll 17b) is Lsb in the width direction of the slab 14, and may be one or three or more. 26 is provided.
The rolls 17a and 17b are arranged at least on the front side of the slab 14 in the strong cooling range.
In addition, the water flow portion is, for example, a groove (cut) formed by digging the outer peripheral portion of the roll, or a bearing portion of a small-diameter split roll that is generally adopted as an equipment measure for internal cracking of a slab, It means a part having a gap with the slab surface without directly contacting the slab. In addition, as a small diameter division | segmentation roll, there exists what divided into 2 (or 3 divisions) in the width direction of slab, for example.

In addition, when the part which the slab 14 and the rolls 17a and 17b do not contact directly generate | occur | produces, there exists a concern of the quality deterioration of the slab 14. FIG. Therefore, the present researchers changed the total width (total length of slits) Lt (= ΣLsa, ΣLsb) of the plurality of water flow portions Lsa and Lsb formed within the width of the slab, and the total width of the water flow portions The actual machine was investigated for the effect of slab quality on slab quality. The width of the slab is assumed to be w.
As a result, when Lt / w ≧ 0.5, the surface of the manufactured slab was observed, and it was confirmed that “dullness” corresponding to the depth of cut of the water passage portion occurred on the slab surface. It was done. In this way, when a slab with a bulge formed on the surface of the slab is rolled in a subsequent process, it may cause wrinkles on the surface of the plate after rolling. In consideration, it was found that Lt / w <0.5 was necessary.
In addition, when the total width of the water flow portions formed on the roll is set to Lt / w <0.25 under the arrangement conditions of the water flow portions described later, the total width of the water flow portions becomes too narrow, and the width of the slab The frequency that a part of the direction is drooped and cooled by water decreases, and the effect of improving the cooling capacity cannot be expected.
From the above, the total width Lt of the water flow portion is in the range of 0.25 ≦ Lt / w <0.5 (preferably, lower limit is 0.3, further 0.35, upper limit is 0.48). It is preferable to do.

Note that the number of water flow portions formed in one roll is preferably two or more because the cooling capacity is improved by increasing the amount of drooping water by positively pouring the dripping water downstream. On the other hand, although the upper limit is not particularly provided, it can be set within a range that satisfies the conditions of the dimension and arrangement position of the water passage shown below.
Here, when the widths Lsa and Lsb of the water flow portions 23 to 26 formed on the rolls 17a and 17b are too narrow, dripping water is less likely to flow down due to the condensing pressure loss. For this reason, in order to promote the flow of dripping water, it is necessary to make the widths Lsa and Lsb of the water flow portions 23 to 26 each 10 mm or more, while the upper limit is due to the quality problem of the slab. It must be less than half the width w of 14.
Moreover, when the depth Dp of the water flow parts 23-26 is less than 2 mm from the drooping water cooling ability investigation result by a laboratory test, it turns out that a cooling capacity cannot be ensured, and needs to be 2 mm or more. On the other hand, considering the strength of the rolls 17a and 17b, the depth Dp of the water flow portions 23 to 26 needs to be 10 mm or less.

The formation positions of the water flow portions 23 to 26 to the rolls 17 a and 17 b are the same as the positions of the water flow portions 23 to 26 of the rolls 17 a and 17 b adjacent in the drawing direction of the slab 14 in the width direction of the slab 14. It is preferable to shift alternately. Specifically, among the rolls 17a and 17b adjacent to each other in the drawing direction of the slab 14, the width of the water passing portions 23 and 24 of the roll 17a disposed on the upstream side is Lsa, and the roll is disposed on the downstream side. The width of the water flow parts 25 and 26 of 17b is set to Lsb, and each water flow part (water flow part with the shortest distance) 23 and 26 located at the shortest distance is each water flow part in the width direction of the slab 14 When the interval between the center positions in the width direction of 23 and 26 is L, it is preferable that the conditions of L / Lsa ≧ 1 and L / Lsb ≧ 1 are satisfied.
As described above, in order to improve the quality of the slab, it is necessary to suppress Lt / w to less than 0.5. Therefore, the range in which dripping water can be cooled for each roll span is as follows. Will be less than half of the width w (less than 0.5 w). For this reason, if the formation positions of the water flow portions in the width direction of the slab are all the same position in the casting direction, cooling of only the portion where the water flow portions are formed proceeds, supercooling occurs, and the surface temperature of the slab It is conceivable that the temperature drops to a temperature range where recuperation is impossible.

As a countermeasure, L / Lsa ≧ 1 and L / Lsb ≧ 1 shown in FIG. 5 and FIG. 6, the formation position of the water passing portion is shifted for each roll, and the position cooled by dripping water is changed, It is necessary to perform uniform cooling in the width direction.
In addition, although the upper limit of L / Lsa and L / Lsb is not provided, in order to cool a slab uniformly over the width direction, it is preferable to set it as the value which can arrange | position a water flow part in the width | variety of a slab.
Usually, the continuous casting machine controls the surface crack of the slab due to overcooling or the internal crack of the slab due to insufficient cooling by changing the amount of spray water according to the casting speed of the slab.
As described above, the present invention uses dripping water generated after spraying cooling water in the strong cooling range of the slab, and the amount of spray water is changed according to the casting speed. By changing the amount of water, stable casting can be performed without causing overcooling or undercooling. In addition, the cooling of the slab with dripping water is such that the water staying between the rolls flows through the water-passing part to the downstream side by free-falling, and at the site where the water-flowing part is introduced and its surroundings, The piece can be cooled uniformly over the width direction.

Then, the secondary cooling method of the continuous casting machine which concerns on one embodiment of this invention is demonstrated, referring the secondary cooling device 10 of an above described continuous casting machine.
First, the ratio (d / D) between the total value d of the thicknesses d1 and d2 of the solidified shells 21 and 22 formed on the front side 15 and the back side 16 of the slab 14 and the thickness D of the slab 14 is 0.25 < A part or all of the range where d / D <0.55, that is, 4 roll spans or more, is set as the strong cooling range, and rolls 17a and 17b provided with water passage portions 23 to 26 are arranged in this range.
Further, the cooling nozzle 18 disposed in this strong cooling range has a cooling water volume density in the range of 500 L / m ² / min to 2000 L / m ² / min and a spray back pressure of less than 0.49 MPa. It is possible to control with.
Using this secondary cooling device 10, the slab 14 is drawn at a casting speed of, for example, about 1.3 m / min to 1.6 m / min, and immediately after the bent portion to before the bent back portion, the slab 14 is drawn. After the surface of the steel is strongly cooled, slow cooling is performed to ensure recuperation of the slab. Thereby, the cooling capacity of the slab 14 can be improved, the surface temperature of the slab 14 can be stably controlled, and the casting speed can be improved while avoiding the surface crack of the slab 14.

Next, examples carried out for confirming the effects of the present invention will be described.
In this example, as described above, the present inventors constructed an unsteady solidification analysis model for estimating the surface temperature transition of the slab and the growth transition of the solidified shell thickness, and the casting speed when various conditions were changed. The effect on surface cracks was estimated by computer analysis (simulation). The equipment conditions and casting conditions of the continuous casting machine used in the computer analysis are shown below.
<Conditions for continuous casting machine>
・ Captain of continuous casting machine (from the meniscus position of the mold to the downstream end position of the continuous casting machine, that is, the downstream end position of the casting roll): 30 m
・ Pitch of cooling nozzles arranged in the drawing direction of the slab: 300 mm
・ Distance between cooling nozzle and slab surface: 125 mm
<Casting conditions>
・ Cast width: 1250mm
-Thickness of slab: 250mm

As described above, if the surface temperature of the slab cannot ensure a temperature exceeding 700 ° C. in the bending part and the bending back part of the continuous casting machine, surface cracks occur in the slab. The surface crack was evaluated under these temperature conditions.
This surface crack of the slab indicates the situation when the slab passes through the bent part or the bent part, and “×” indicates that the surface temperature of the slab is steadily lower than 700 ° C. "△" indicates that unsteady overcooling (700 ° C or lower) occurs within the range of variation in cooling capacity, and there is a concern that surface cracks may be scattered in the slab. “◯” indicates that the surface temperature of the slab constantly exceeds 700 ° C., and there is no occurrence of surface cracks.

Further, the casting speed of the slab was evaluated at the maximum casting speed at which the solidification completion point of the slab could be kept in the continuous casting machine under each condition. In this evaluation, when the casting speed is 1.3 m / min as a reference, the obtained increase in the maximum casting speed is 0%, “x”, and the case where it exceeds 0% and less than 15% is “△”, The case of 15% or more was designated as “◯”.
In addition, the overall evaluation is “X” when any one of the above two evaluation items of the surface crack and the maximum casting speed is “×”, and any one of the evaluation items is “△”. In this case, “△” is indicated, and when both evaluation items are “◯”, “◯” is indicated. Further, in the comprehensive evaluation, the condition in which the maximum casting speed condition can be obtained in this example is “◎”.
Here, the overall evaluation “◯” means that there is no problem in terms of both product quality and productivity, and “△” is inferior to “○”. Means that there is no problem.

First, the results of various changes in the strong cooling range of the slab and evaluation of surface cracks and maximum casting speed will be described with reference to Table 1.
Here, Examples 1 to 3 are results in which the strong cooling range is within the appropriate range of the present invention (0.25 <d / D <0.55), and Comparative Examples 1 to 5 have the strong cooling range within the appropriate range. This is the result of deviating. In particular, Comparative Example 1 used a roll in which a water passage portion was not formed, and the slab was cooled only by spraying cooling water from a cooling nozzle. On the other hand, Examples 1-3 and Comparative Examples 2-5 used the roll in which the water flow part was formed, and performed cooling of the slab by spraying of cooling water and dripping water. The water flow section has a depth Dp of 5 mm, widths Lsa and Lsb of 200 mm, total width Lt of 400 mm (200 mm × 2 locations), and the positions of the water flow sections of rolls adjacent in the drawing direction of the slab. Are alternately shifted in the width direction of the slab (L / Lsa, L / Lsb = 1). The water density of the cooling water in the cooling nozzle was fixed at 2000 L / m ² / min, and the spray back pressure was fixed at 0.39 MPa (4 kgf / cm ² ).

As in Comparative Examples 1 and 2 shown in Table 1, when the slab is strongly cooled in the range of 1 to 2.5 m below the meniscus position, that is, below the meniscus position (the thickness of the solidified shell is 13 to 30 mm), Although the speed increases, it is considered that surface cracks occur in the slab because the surface temperature at the time of passing through the bent portion cannot secure a temperature exceeding 700 ° C. In Comparative Examples 1 and 2, although the maximum casting speed is different, this is due to the expansion of the cooling area due to the presence or absence of the formation of a water flow portion with respect to the roll.
Comparative Example 3 is the result of strong cooling of the slab in the vicinity of the bent part. The casting speed increases, but the surface temperature when passing through the bent part cannot ensure a temperature exceeding 700 ° C. It is thought that surface cracking occurs.
Comparative Example 4 is the result when strong cooling is performed in the vicinity of the bent back portion, that is, in the range of 9 to 14.3 m below the meniscus position (the thickness of the solidified shell is 70 to 88 mm), and when passing through the bent back portion It is considered that surface cracking occurs because the surface temperature of the steel cannot secure a temperature exceeding 700 ° C.
Further, even in Comparative Example 5 in which the end point of the strong cooling range is slightly upstream from Comparative Example 4, it is considered that the slab cannot secure sufficient reheat after strong cooling and surface cracking occurs.

From the above, when controlling the casting speed by strong cooling while suppressing surface cracking, strong cooling is started from the bent portion (2.5 m below the meniscus position), and the bent back portion (at the meniscus position). It was found that the surface temperature of the slab needs to be reheated to a temperature at which the surface can be avoided at the bent back portion by performing gentle cooling before 14 m below.
On the other hand, as shown in Examples 1 to 3, by strongly cooling the slab after passing through the bending portion and by performing slow cooling from the front of the bending return portion, the casting speed is suppressed while suppressing surface cracking. I found that I could rise. Also, although the effect of improving the casting speed is different between the starting point of strong cooling and the strong cooling range, the former is the difference in thermal resistance due to the thickness of the solidified shell, and the latter is the heat inside the slab by increasing the cooling range. It is a difference due to taking away.

Next, the results of various changes in the water density of the cooling water and evaluation of surface cracks and maximum casting speed will be described with reference to Table 2.
In Example 3, Reference Example , and Comparative Example 6, the strong cooling range is within the appropriate range of the present invention (0.25 <d / D <0.55), and a roll is formed as a roll. It is the result. Here, Example 3, Reference Example is a result of the water density of the cooling water was 5 00L / ^{m 2} / min or more 2000L / ^{m 2} / min or less under cooling nozzle, Comparative Example 6, this it is the result of the case, which was below the range. In addition, the shape and arrangement | positioning position of a water flow part, and the spray back pressure of the cooling nozzle were made into the conditions similar to Table 1. FIG.

As in Comparative Example 6 shown in Table 2, when the water density was low, it was found that sufficient cooling capacity for lowering the surface temperature of the slab could not be obtained, and the casting speed did not increase.
On the other hand, Example 3 and Reference Example are the results of cooling the slab by setting the water density to 2000 L / m ² / min and 500 L / m ² / min, respectively. By optimizing the water density, the slab It was found that the casting speed can be increased while suppressing surface cracking by stably lowering the surface temperature of the steel sheet, performing slow cooling before the bent back portion, and ensuring recuperation of the slab.

Then, the shape of the water flow part formed in a roll is changed variously, and the result of having evaluated the surface crack and the maximum casting speed is demonstrated, referring Table 3. FIG.
In Examples 3 and 5 to 8, the strong cooling range and the water density of the cooling water in the cooling nozzle are both within the proper range of the present invention, and the result is that a roll having a water passage portion is used. It is. The spray back pressure of the cooling nozzle was fixed at 0.39 MPa.
Table 3 shows conditions and results obtained by fixing the depth Dp of the water passage portion to 5 mm and changing the number and positions of the water passage portions, respectively.

When forming a water flow portion on the roll, the points to be considered are suppression of “drip” generated on the surface of the slab and uniform cooling in the width direction of the slab.
Therefore, it is necessary to perform uniform cooling in the width direction by shifting the position of the water flow portion for each roll so that Lt / w is less than 0.5 and L / Lsa ≧ 1 and L / Lsb ≧ 1. .
In addition, when Lt / w <0.25, it is conceivable that a certain part in the width direction is cooled with dripping water less frequently, and an effect of improving the cooling capacity cannot be expected, and 0.25 ≦ Lt / w <0. The range of .5 is effective for increasing the casting speed.

As in Example 5 shown in Table 3, when L / Lsa and L / Lsb = 0.5, the same part is continuously cooled by dripping water, so that the total width Lt of the water passing portion is It turns out that the improvement effect of casting speed is falling compared with Example 3 which is the same. Moreover, in this arrangement, the slab is supercooled, so that there is a concern that surface cracks may be scattered.
Moreover, as shown in Examples 6 and 7, when the total width Lt of the water passage portion is 300 mm (three water passage portions are formed in the width direction) (Lt / w <0.25), It can be seen that the effect of improving the cooling capacity is small because the total width becomes too narrow and the frequency in which a part of the slab in the width direction is drooped and cooled is reduced.
And as shown in Example 8, it was confirmed that a casting speed can be improved by optimizing the width | variety and arrangement | positioning of a water flow part.

In addition, the results of evaluating the surface cracking and the maximum casting speed by variously changing the spray back pressure of the cooling water will be described with reference to Table 4.
In Examples 8 to 10, the strong cooling range and the cooling water amount density of the cooling nozzle are both within the appropriate range of the present invention, and the result is that a roll having a water passage portion is used. . The water passage portion has a depth Dp of 5 mm, widths Lsa and Lsb of 300 mm, a total width Lt of 600 mm, and the position of the water passage portion of the roll adjacent in the drawing direction of the slab in the width direction of the slab. They are alternately shifted over (L / Lsa, L / Lsb = 1). Moreover, the water density of the cooling water of the cooling nozzle was fixed at 2000 L / m ² / min.

As in Example 10 shown in Table 4, when the spray back pressure and 0.59MPa (6kgf / cm ^2), the effect of improving the casting speed as compared to Example 8 is increased. However, as described above, in the spray back pressure region, it is confirmed from the laboratory test results that the variation in the cooling capacity is large, and there is a concern that surface cracks may occur, which is not preferable.
In addition, Example 9 is a result of setting the spray back pressure to 0.10 MPa (1 kgf / cm ² ) less than 0.49 MPa (5 kgf / cm ² ), but it is stable by optimizing the spray back pressure. It was confirmed that the casting speed could be improved.

As described above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the configuration described in the above embodiment, and the matters described in the scope of claims. Other embodiments and modifications conceivable within the scope are also included. For example, the case where the secondary cooling device and the secondary cooling method of the continuous casting machine of the present invention are configured by combining some or all of the above-described embodiments and modifications are also included in the scope of the present invention. .
In the above-described embodiment, the case where the roll having the water passage portion is disposed on the front side of the slab in the strong cooling range has been described. It may be further arranged on the back side.

It is explanatory drawing of the secondary cooling device and secondary cooling method of the continuous casting machine which concern on one embodiment of this invention. It is explanatory drawing of the continuous casting machine. It is explanatory drawing which shows the relationship between the water quantity density of the cooling water sprayed from the nozzle for cooling, and a heat transfer coefficient index | exponent. (A) is explanatory drawing of the roll span of a secondary cooling device, (B) is explanatory drawing which shows the relationship between the roll span which performs strong cooling, and an average heat removal amount index | exponent. It is a partial front view of the roll used for the secondary cooling device of the continuous casting machine which concerns on one embodiment of this invention. (A), (B) is a partially omitted front view and a partially omitted side view of a secondary cooling device of the continuous casting machine, respectively. It is explanatory drawing which shows the relationship between the spray back pressure in the ejection part of a cooling nozzle, and a heat transfer coefficient index | exponent.

Explanation of symbols

10: Secondary cooling device of continuous casting machine, 11: Continuous casting machine, 13: Continuous casting mold, 14: Cast slab, 15: Front side, 16: Back side, 17, 17a, 17b: Roll, 18: Cooling nozzle , 19: Tundish, 20: Immersion nozzle, 21, 22: Solidified shell, 23-26: Water flow part

Claims (5)

On the front side and the back side of the slab drawn from the continuous casting mold, the rolls are arranged with intervals in the drawing direction, and the slab is sandwiched and conveyed from the thickness direction between the rolls on the front side and on the back side. In a secondary cooling device of a continuous bending machine of a vertical bending type or a curved bending type, which is arranged between rolls in the width direction of the slab and has a plurality of cooling nozzles for cooling the slab.
Part or all of the range in which the total value d of the thicknesses of the solidified shells formed on the front and back sides of the slab is 0.25 <d / D <0.55 with respect to the thickness D of the slab A strong cooling range is provided , and one or a plurality of water passing portions are provided in the width direction of the slab with respect to the roll disposed at least on the front side of the slab in the strong cooling range, and sprayed from the cooling nozzle. While allowing the cooling medium to pass through the water passing portion of the roll, the density of the cooling medium sprayed from the cooling nozzle installed in the strong cooling range is 800 liters / m ² / min to 2000 liters / m ² / The water density of the cooling medium sprayed from the cooling nozzle installed in the range of 0.55 ≦ d / D <0.75 is 50 liters / m ² / minute or more and 700 liters / m ² / cold the cast piece as a minute or less Continuous casting machine of the secondary cooling device, characterized by. The secondary cooling device for a continuous casting machine according to claim 1, wherein the water flow portion of the roll and the slab disposed in the strong cooling range are within the width of the slab of the water flow portion of the roll. Where the total width in the width direction is Lt and the width of the slab is w, the condition of 0.25 ≦ Lt / w <0.5 is satisfied, and adjacent to the drawing direction of the slab A secondary cooling device for a continuous casting machine, wherein the position of the water passing portion of the roll is alternately shifted in the width direction of the slab. The secondary cooling device of the continuous casting machine according to claim 2, wherein the formation position of the water flow portion to each roll is arranged upstream of each roll adjacent to the drawing direction of the slab. The width of the water flow portion of the roll is Lsa, the width of the water flow portion of the roll disposed on the downstream side is Lsb, and each water flow portion positioned at the closest distance is the width direction of the slab. A secondary cooling device for a continuous casting machine, wherein the condition of L / Lsa ≧ 1 and L / Lsb ≧ 1 is satisfied, where L is the distance between the center positions in the width direction of the water flow portions in FIG. The secondary cooling device for a continuous casting machine according to any one of claims 1 to 3, wherein a back pressure at a jetting portion of the cooling medium jetted from the cooling nozzle is less than 0.49 MPa. A secondary cooling device for a continuous casting machine. A secondary cooling method for a continuous casting machine, wherein the slab is strongly cooled using the secondary cooling device for a continuous casting machine according to any one of claims 1 to 4.

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