JP2011025282A - Equipment and method for cooling thick steel plate - Google Patents

Abstract

The present invention provides a cooling facility and a cooling method for manufacturing a thick steel plate that suppresses a partial increase in hardness on the surface of the thick steel plate and has no hardness unevenness.
A cooling facility installed in a hot rolling line for thick steel plates, headers 3a and 3b supplying cooling water to the upper and lower surfaces of the thick steel plate 9, and spray-like cooling provided so as to protrude from the headers Spray cooling equipment 1 having spray nozzles 4a and 4b for jetting water, and headers 5a and 5b arranged downstream of the steel sheet conveying direction following the spray cooling equipment and supplying cooling water to the upper and lower surfaces of the thick steel sheets; And a laminar cooling facility 2 provided with a laminar nozzle 6a, 6b that protrudes from the header and injects rod-shaped cooling water, and stays on the upper surface of the thick steel plate between the spray cooling facility and the laminar cooling facility. A cooling apparatus for thick steel plates, characterized in that a draining roll 7 is provided for blocking the cooling water.
[Selection] Figure 1

Description

  The present invention relates to a thick steel plate cooling facility and a cooling method.

In the process of manufacturing steel plates such as thick plates and thin plates by hot rolling, for example, in equipment as shown in FIG. 7, after hot rough rolling and finish rolling, the structure is controlled by water cooling or air cooling. Yes. When cooling to a relatively low temperature, for example, about 450 to 650 ° C. by water cooling, fine ferrite and bainite structure can be obtained and the strength of the steel plate can be secured, so the technology for cooling the steel plate with spray cooling water, laminar cooling water, etc. It is common.
In recent years, development of a technique for obtaining a high cooling rate, making the structure finer, and increasing the strength of the steel sheet has been active.

  For example, there is a technique of Patent Document 1 as a technique for cooling a hot steel sheet by supplying a large amount of rod-shaped laminar cooling water. This is one in which cooling water is jetted at high speed from nozzles installed on the upper and lower surfaces of a steel plate, and a very high cooling rate can be obtained and a product excellent in material characteristics can be manufactured.

JP 2002-239623 A

  However, when using the conventional technology, the cooling capacity is greatly different between the portion where the rod-shaped cooling water sprayed from the nozzle is directly supplied to the steel plate (hereinafter referred to as the direct-irradiation portion) and its peripheral portion. There is a problem in that local hardness unevenness occurs.

For example, in a thick steel plate having a thickness of 60 mm or more, in order to cool from a predetermined cooling start temperature to a cooling end temperature, it is necessary to set a long cooling time, so the transport speed is slow (for example, 0.5 m / s). )Must.
The slower the conveyance speed is, the longer the time for passing through the direct-irradiation part is, and the surface is rapidly cooled. For example, as shown in (a) of the cooling curve diagram of FIG. At this time, since the cooling rate at portions other than the direct-light portion is not so high, hardness unevenness as shown in the width direction surface hardness distribution diagram of FIG. 3 may occur as a result. In the figure, reference numeral 14 denotes a portion (direct irradiation portion) where the rod-shaped cooling water sprayed from the laminar nozzle is directly supplied to the steel plate. As a result, a hard portion is locally generated on the surface layer, resulting in a problem that ductility (elongation) at the time of processing a thick steel plate is reduced or delayed fracture occurs.

  In view of the above, in the present invention, when supplying cooling water to the upper and lower surfaces of a thick steel plate after hot rolling, a technique for producing a thick steel plate that suppresses a partial hardness increase on the surface of the thick steel plate and has no hardness unevenness. The purpose is to provide.

  In order to solve the above-described problems, in the present invention, the thick steel plate after hot rolling is first sprayed in the entire width direction to spray-like spray cooling, and the steel plate surface is preliminarily uniformly cooled and then rod-shaped. Accelerated cooling to the center of the thick steel plate with cooling water. That is, the present invention has the following features.

  A first invention is a cooling facility installed in a hot rolling line for a thick steel plate, a header for supplying cooling water to the upper and lower surfaces of the thick steel plate, and sprayed cooling water provided protruding from the header. A spray cooling facility provided with a spray nozzle for spraying, a header that is arranged downstream of the spray cooling device in the direction of conveying the steel plate and that supplies cooling water to the upper and lower surfaces of the thick steel plate, and is provided so as to protrude from the header A laminar cooling facility provided with a laminar nozzle for injecting rod-shaped cooling water, and a draining roll for damming the cooling water staying on the upper surface of the thick steel plate provided between the spray cooling facility and the laminar cooling facility. This is a characteristic cooling equipment for thick steel plates.

The second invention is characterized in that the spray cooling system of steel plate conveyance direction length of 0.5~2m, 0.4~1.2m ³ the water density of the spray cooling water supplied to the steel plate upper and lower surfaces from spray nozzles / m ² · min, the inner diameter of the laminar nozzle of the laminar cooling equipment is 3 to 8 mm, and the water density of the rod-shaped cooling water supplied from the laminar nozzle to the upper and lower surfaces of the thick steel plate is 0.6 to 4.0 m ³ / m. It is the cooling equipment of the thick steel plate as described in 1st invention characterized by setting it as ² * min.

  According to a third aspect of the present invention, the area of the direct-irradiation part where the sprayed cooling water is directly supplied from the spray nozzle to the surface of the thick steel plate in the spray cooling facility is 70% or more on the upper surface side of the thick steel plate and 75% on the lower surface side. The cooling apparatus for thick steel plates according to the first or second invention, wherein the direct-irradiation part on the lower surface side of the thick steel plate is continuous in the width direction of the thick steel plate.

4th invention is the spray nozzle of the spray cooling equipment over the length of 0.5-2m in the thick steel plate conveyance direction on the surface of the thick steel plate after the hot rolling of the conveyance speed of 0.1-0.6m / s. Spray cooling water is sprayed from the temperature above the Ar ₃ transformation point temperature to 600 ° C. at an average cooling rate of 40 to 120 ° C./s on the surface layer of the thick steel plate, and then the rod cooling water from the laminar nozzle of the laminar cooling equipment Is cooled to the martensite transformation start temperature at an average cooling rate of 60 to 160 ° C./s on the surface layer of the thick steel plate, thereby cooling the thick steel plate.

  In the fifth aspect of the invention, the area of the direct irradiation part in which sprayed cooling water is directly supplied to the surface of the thick steel plate in the spray cooling facility is 70% or more on the upper surface side of the thick steel plate and 75% or more on the lower surface side. And the sprayed cooling water is sprayed from the spray nozzle of the spray cooling equipment so that the direct-irradiation portion on the lower surface side of the thick steel plate is continuous in the width direction of the thick steel plate. It is a cooling method.

  In addition, the "average cooling rate of the thick steel plate surface layer" in the present invention refers to the average cooling rate at a position 1 mm deep from the thick steel plate surface layer.

  By using the cooling equipment and cooling method of the thick steel plate of the present invention, it is possible to suppress a partial hardness increase on the surface of the thick steel plate and to obtain a uniform hardness in the width direction. Steel sheets can be manufactured.

It is a side view which shows the outline | summary of the cooling equipment which concerns on one embodiment of this invention. It is a figure which shows an example of the arrangement | sequence of the upper surface cooling spray nozzle of this invention, and the injection range. It is a figure which shows the width direction surface hardness distribution of the thick steel plate manufactured with the conventional installation. It is a figure which shows the width direction surface hardness distribution of the thick steel plate manufactured with the installation of this invention. It is a figure which shows the comparison of the continuous cooling curve when it cools with the installation of this invention, and the conventional installation. It is a side view of the laminar cooling equipment provided with the partition concerning one embodiment of the present invention. It is a figure explaining the outline of a thick plate rolling line.

  Hereinafter, an example of an embodiment of a cooling facility of the present invention in a thick plate rolling process will be described with reference to the drawings.

FIG. 7 is a schematic view showing an example of a thick plate rolling line used for carrying out the present invention.
The slab extracted from the heating furnace is subjected to rough rolling and finish rolling by a rolling mill to a predetermined finishing temperature and finishing plate thickness, and then conveyed to an accelerated cooling facility online. It is suitable for the shape of the thick steel plate after cooling to perform accelerated cooling after adjusting the shape of the thick steel plate through a pre-leveler before cooling.

FIG. 1 is a side view showing an outline of the cooling equipment in one embodiment of the present invention, and FIG. 2 is a view in which the spraying state from the spray cooling nozzle is developed from the top.
In the accelerated cooling facility, the spray cooling facility 1 and the laminar cooling facility 2 are arranged in order toward the downstream side in the thick steel plate conveyance direction. The spray cooling facility 1 and the laminar cooling facility 2 are arranged in two zones and twelve zones, for example, with the table roll 8 as one zone, and supply cooling water to the upper and lower surfaces of the thick steel plate 9.
The steel plate is cooled to a predetermined temperature by adjusting the cooling water injection amount and the cooling zone to be used. Further, a draining roll 7 is provided at a position facing the table roll 8 between at least the spray cooling facility 1 and the laminar cooling facility 2 to dampen the water staying on the upper surface of the thick steel plate.

  The spray cooling facility 1 includes an upper surface spray cooling header 3a and a lower surface spray cooling header 3b. In these headers, spray nozzles 4a and 4b for injecting spray-like cooling water are arranged at a constant pitch in the width direction. A plurality of nozzle rows are arranged in the conveying direction of the thick steel plate. When there is only one nozzle row, the cooling capacity increases at the portion where the injection area of each nozzle overlaps in the width direction, and the cooling strength in the width direction may be increased and uneven cooling may occur, If two or more nozzle rows are arranged and the nozzles are arranged in a staggered manner, the strength of cooling in each nozzle row can be offset and more uniform cooling can be achieved.

  As a nozzle type, for example, as shown in FIG. 2, a square type nozzle having a square injection region is used. However, the nozzle that can be used is not limited to this type, and if the distribution of the cooling water amount can be generally smoothed. For example, an oval type nozzle in which the injection region is an ellipse may be used.

  The laminar cooling facility 2 includes an upper surface laminar cooling header 5a and a lower surface laminar cooling header 5b, and laminar nozzles (circular tube nozzles) 6a and 6b for injecting rod-shaped cooling water to these headers at a constant pitch in the width direction. Are arranged in a plurality of rows in the conveying direction of the thick steel plate.

  Here, the rod-shaped cooling water in the present invention is cooling water injected in a state of being pressurized to some extent from a circular (including elliptical or polygonal) nozzle outlet, and is cooled from the nozzle outlet. The water jet velocity is 6 m / s or more, and it refers to cooling water of continuous and straight water flow in which the cross section of the water flow jetted from the nozzle outlet is maintained in a substantially circular shape. In other words, it is different from a free fall flow from a circular tube laminar nozzle or a sprayed state such as a spray.

Next, the spray cooling equipment 1 of the present invention will be described in detail.
In the spray cooling facility 1, spray-like cooling water is sprayed from the spray nozzles 4 a and 4 b and is supplied to the upper and lower surfaces of the steel sheet with a substantially uniform water amount distribution in a cooling zone sandwiched between the draining roll 7 and the table roll 8. . The length of the zone for supplying the spray cooling water is preferably 0.5 to 2.0 m. This corresponds to 0.5 to 2 times the distance between general table rolls.

  If the spray cooling equipment is unnecessarily long, the equipment costs will be increased, and the laminar cooling equipment will be installed further downstream in the transport direction.Therefore, a steel plate that only performs laminar cooling with a high water density without spray cooling is used. The cooling start at the time of cooling is delayed, and the cooling start temperature is lowered, which is not preferable. Therefore, the length of the spray cooling equipment (the length of the zone for supplying the spray cooling water) is 2.0 m at the longest.

  On the other hand, if the application of spray cooling is limited to an ultra-low speed conveying material, for example, one that is conveyed at about 0.1 m / s, the length of the spray cooling equipment may be short. As shown in FIG. 1, when the spray cooling equipment is installed in one zone and the equipment cost is reduced as much as possible by reducing the table roll pitch, the length of the spray cooling equipment is about 0.5 m.

Therefore, the length of the spray cooling facility is preferably in the range of 0.5 to 2.0 m.
Actually, since cooling control is often performed with the distance between table rolls (about 1 m) as one zone, it is only necessary to provide one or two spray cooling zones, and an ultra-low speed conveying material can be obtained by setting it to about half of one zone. Can be expected to suppress the unevenness of hardness.

  For example, a 60 mm-thick steel plate is transported at a speed of 0.25 m / s on a transport line having a distance between table rollers of 1 m (the length of one zone is 1 m), and the average cooling rate of the surface layer is 50 ° C. from a surface temperature of 800 ° C. Consider the case of spray cooling at / s.

  If the spray cooling facility is set to 2 zones, the cooling time is 8 seconds, so the steel sheet surface is spray cooled to 400 ° C. During this time, the bainite transformation and the ferrite transformation start, so the surface layer structure does not become a full martensite structure and is softer than when the technique of Patent Document 1 is used. As shown in FIG. 2, if the nozzles are arranged so that the distribution of the cooling water amount is substantially uniform in the zone, the surface structure becomes substantially uniform, and hardness unevenness does not occur, so that a steel sheet with good ductility is manufactured. be able to.

  When the spray cooling facility is set to one zone, the cooling time is 4 seconds, so the steel sheet surface is spray cooled to 600 ° C. In laminar cooling performed after spray cooling, the amount of water is suppressed, and the average cooling rate of the surface layer is set to 160 ° C./s or less to cool to the martensite transformation start temperature. In spray cooling, the cooling force is almost uniformly lowered to 600 ° C, and the driving force for starting bainite and ferrite transformation is stored. Further, since no hardness unevenness occurs, a steel sheet with good ductility can be manufactured.

In spray cooling, the average cooling rate of the surface layer of the thick steel plate as shown in FIGS. 5B and 5C from a temperature higher than the Ar ₃ transformation temperature to at least 600 ° C., preferably about 400 to 500 ° C. Is cooled to 40 to 120 ° C./s. This is because if the average cooling rate is less than 40 ° C./s, not only the surface of the steel plate but also the inside of the plate thickness is slowly cooled together, so that the strength of the thick steel plate may not be ensured. On the other hand, when the average cooling rate exceeds 120 ° C./s, the cooling rate is too high, so that spots where the surface hardness exceeds the allowable upper limit are sporadically seen in the same manner as the direct-irradiation part of laminar cooling.

In order to obtain the cooling rate, the water density is 0.4 to 1.2 m ³ / m ² · min.

  The conveying speed of the thick steel plate in the spray cooling facility is 0.1 to 0.6 m / s. This is because if the conveyance speed is less than 0.1 m / s, the setting accuracy of the conveyance speed is poor, and the cooling control accuracy is deteriorated. On the other hand, the conveyance speed may be set to exceed 0.6 m / s, but since the time for passing through the direct-irradiation portion of the laminar cooling is short and there is no large temperature drop during this time, hardness unevenness in the width direction does not occur originally. Therefore, the present invention exhibits the effect of suppressing the hardness unevenness when the conveying speed of the thick steel plate is 0.6 m / s or less.

Further, in the spray cooling facility, unevenness in hardness is less likely to occur by performing average cooling as much as possible. The area of the direct-irradiation part where the spray-like cooling water is directly supplied to the surface of the thick steel plate is 70% or more on the upper surface side of the thick steel plate and 75% or more on the lower surface side in the spray cooling facility. That is, in FIG. 2, the ratio of the area of the injection region 12 to the area sandwiched between the two table roll axes 13 is 70% or more on the upper surface side and 75% or more on the lower surface side.
Here, on the upper surface side of the thick steel plate, the cooling water after spray cooling becomes the water on the thick steel plate, so the cooling area is almost 100%. Therefore, a gap may be provided in the injection region from each nozzle so that the water is discharged from both ends in the thickness direction of the steel plate through the gap, and the area of the direct portion that directly supplies the cooling water is 70% or more. I just need it.

  On the other hand, since the cooling water after supply falls downward on the lower surface side of the thick steel plate, it is cooled only at the portion where the cooling water is directly supplied. To be. In addition, since there is also a part where the table roll is shaded, it is quite difficult to make the area for directly supplying the sprayed cooling water 100%, and 75% or more is sufficient. Furthermore, if there is a portion where cooling water is not supplied in the width direction, the cooling is uneven in the width direction and hardness unevenness occurs. Therefore, at least in the nozzle row, the areas of the direct portions of adjacent nozzles are overlapped to some extent in the width direction. It is preferable to make it continuous.

  On the other hand, in the laminar cooling facility 2, rod-shaped cooling water is supplied to the upper and lower surfaces of the steel sheet from a circular tube nozzle having an inner diameter of 3 to 8 mm. If the inner diameter is less than 3 mm, nozzle clogging is likely to occur, and if the inner diameter exceeds 8 mm, the cooling water injection speed is reduced and the cooling capacity is reduced, so the inner diameter of the circular tube nozzle is set to 3 to 8 mm.

  In laminar cooling, the average cooling rate is set to 60 to 160 ° C./s, and cooling is performed to the martensitic transformation start temperature. If the average cooling rate is less than 60 ° C / s, the cooling rate may be low and the strength may not be secured. If the average cooling rate exceeds 160 ° C / s, the cooling rate is too high. This is because there is a possibility of forming a structure, and spots where the surface hardness exceeds the allowable upper limit are sporadically seen.

In order to obtain the above cooling rate, the water density is 0.6 to 4.0 m ³ / m ² · min.
In this paper, the average cooling rate at the surface of the thick steel plate is defined as the average cooling rate from the start to the end of water cooling at a depth of 1 mm from the surface of the thick steel plate. What is necessary is just to calculate and calculate from the results of temperature in Japan. By the way, since there is a scale layer and decarburization layer on the outermost surface of the thick steel plate, the hardness at a depth of 1 mm, which is slightly inside, is often quality controlled, so the cooling rate at this position is a typical value It is preferable to evaluate as.

  The combination of the spray cooling facility 1 and the laminar cooling facility 2 described above can suppress the occurrence of local hardness unevenness even when the thick steel plate is cooled while being conveyed at a low speed.

  By the way, as shown in FIG. 6, the laminar cooling facility 2 includes a partition wall 21 having a large number of through-holes installed horizontally across the steel plate width direction between the upper surface laminar cooling header 5 a and the thick steel plate 9. It is preferable.

  A large number of through holes are formed in the partition wall 21 in a grid pattern, the upper laminar nozzles 6a are inserted in a staggered manner into the predetermined through holes, and the lower ends of the through holes through which the upper laminar nozzles 6a are inserted. The opening is a water supply port 22. In addition, the lower end opening of the through hole through which the upper laminar nozzle 6 a is not inserted is a drain port 23. Then, as shown in FIG. 6, the cooling water 24 supplied from the upper laminar nozzle 6 a through the water supply port 22 cools the upper surface of the thick steel plate 9 to become high temperature drainage 25, and the partition wall through the drainage port 23. 21 flows upward.

  Here, when the partition wall 21 is not provided, the cooling water 24 supplied to the upper surface of the thick steel plate flows and drains in the width direction on the upper surface of the thick steel plate. This hinders the cooling water 24 from the upper laminar nozzle 6a from reaching the upper surface of the thick steel plate, and the cooling capacity in the vicinity of the end of the plate width decreases.

  On the other hand, when the partition wall 21 is present, the drainage 25 after cooling is quickly removed from the upper surface of the thick steel plate to above the partition wall 21, so that the cooling water 24 ejected from the upper laminar nozzle 6a sequentially contacts the thick steel plate. Sufficient cooling capacity can be obtained. In particular, since the through holes are provided to the water supply port 22 and the drainage port 23 in a function-sharing manner, the flow of cooling water and cooling drainage becomes smooth.

  Furthermore, since the tip of the upper laminar nozzle is inserted into the through hole of the partition wall 21, the drainage 25 flowing in the width direction above the partition wall 21 does not interfere with the cooling water 24 ejected from the upper laminar nozzle 6a. Uniform cooling can be performed in the width direction. Therefore, not only the local hardness increase 14 as shown in FIG. 3 is suppressed, but also the temperature distribution in the width direction becomes uniform, and an extremely uniform intensity distribution can be obtained in the entire width direction.

  Hereinafter, as an embodiment of the present invention, a case of cooling a steel plate for shipbuilding having a yield stress of 355 MPa class and an upper limit of surface Vickers hardness of 350 in a plate rolling process will be described with reference to the drawings.

  In the thick plate rolling equipment schematically shown in FIG. 7, the slab extracted from the heating furnace is formed by a rolling mill, subjected to tenter rolling, then rough rolled, and further subjected to finish rolling to a plate thickness of 60 mm, The plate width was 4.5 m. The steel sheet surface temperature measured immediately after finish rolling, that is, the finish temperature was 820 ° C. Thereafter, accelerated cooling was performed in an accelerated cooling facility through a hot leveler. Cooling was performed from a cooling start temperature of 800 ° C. to a cooling end temperature of 400 ° C. (a value obtained by measuring the temperature after reheating on the exit side of the accelerated cooling facility). The conveyance speed at this time was 0.2 m / s.

As an example of the present invention, the cooling equipment described in the embodiment shown in FIG. 1 was used. In the cooling facility of the present invention, sprayed cooling water was supplied from the spray nozzle to the upper and lower surfaces of the steel sheet in the two upstream zones, with the zone between the table rolls (distance being 1 m) as one zone. Thereafter, rod-shaped cooling water was supplied from the laminar nozzle to the upper and lower surfaces of the steel sheet in 10 zones. In the spray cooling zone, 0.6 m ³ / m ² · min on the upper surface of the steel plate, 0.9 m ³ / m ² · min on the lower surface of the steel plate, and in the laminar cooling zone, 2.0 m ³ / m ² · min on the upper surface of the steel plate Cooling water was supplied to the lower surface at a water density of 3.0 m ³ / m ² · min.

As shown in FIG. 2, the spray nozzle used was a square type nozzle having a square spray area, and two nozzles were arranged in the longitudinal direction in a zone having a distance of 1 m between table rolls. Cooling water was supplied to an area of 400 mm in the conveyance direction and 400 mm in the width direction with one nozzle, and at this time, the injection areas from adjacent nozzles were overlapped to some extent in the width direction.
In addition, the area of the direct irradiation part to which spray-like cooling water is directly supplied to the surface of the thick steel plate was 80% on both the upper and lower surfaces of the thick steel plate in the spray cooling facility.

  The laminar cooling nozzle had an inner diameter of 5 mm, an outer diameter of 9 mm, a length of 170 mm, and a cooling water injection speed of 8.9 m / s. The nozzle pitch in the steel sheet width direction was 50 mm, and 10 rows of nozzles were arranged in the longitudinal direction in a zone with a distance between table rolls of 1 m.

  The surface temperature immediately after spray cooling in 2 zones was (calculated) 600 ° C. Since the time for passing through the spray cooling zone was 5 s, the average cooling rate of the surface layer was 40 ° C./s.

  As a result, the surface hardness distribution (Vickers hardness at a depth of 1 mm from the surface) of the surface layer of the thick steel plate measured using a Vickers hardness tester with a load of 10 kg is almost uniform as shown in FIG. 280.

  Further, when the laminar cooling facility shown in FIG. 6 was provided with a partition wall, the same cooling was performed. As a result, the surface hardness distribution in the width direction became substantially uniform as shown in FIG. In addition, the intensity distribution in the width direction was extremely uniform.

  On the other hand, as a comparative example, cooling was performed using a conventional laminar facility described in Patent Document 1. Hardness increased at the portion that passed directly under the nozzle, and the surface hardness (Vickers hardness at a depth of 1 mm from the surface) distribution in the width direction was as shown in FIG. Since the C and alloy components were changed so as not to exceed the allowable upper limit of hardness, the manufacturing cost increased.

DESCRIPTION OF SYMBOLS 1 Spray cooling equipment 2 Laminar cooling equipment 3 Spray cooling header 3a Upper surface spray cooling header 3b Lower surface spray cooling header 4 Spray nozzle 4a Upper spray nozzle 4b Lower spray nozzle 5 Laminar cooling header 5a Upper surface laminar cooling header 5b Lower surface laminar cooling header 6 Laminar nozzle 6a Upper laminar nozzle 6b Lower laminar nozzle 7 Draining roll 8 Table roll 9 Thick steel plate 10 Stagnant water 11 Nozzle 12 Injection region 13 Table roll axis 14 Laminar nozzle direct irradiation position 21 Bulkhead 22 Water supply port 23 Drainage port 24 Injection cooling water 25 Drained water

Claims (5)

  A cooling facility installed in a hot rolling line for a thick steel plate, comprising: a header that supplies cooling water to the upper and lower surfaces of the thick steel plate; and a spray nozzle that protrudes from the header and injects spray-like cooling water. Spray cooling equipment provided, a header that is arranged downstream of the spray cooling equipment in the steel sheet conveyance direction, supplies cooling water to the upper and lower surfaces of the thick steel plate, and protrudes from the header to inject rod-shaped cooling water A laminar cooling facility comprising a laminar nozzle, and a draining roll for damming cooling water staying on the upper surface of the thick steel plate is provided between the spray cooling facility and the laminar cooling facility. Cooling equipment. The length of the spray cooling equipment in the conveying direction of the thick steel plate is 0.5 to ² m, and the amount of spray cooling water supplied from the spray nozzle to the upper and lower surfaces of the thick steel plate is 0.4 to 1.2 m ³ / m ² · min. The inner diameter of the laminar nozzle of the laminar cooling equipment is 3 to 8 mm, and the amount of rod-like cooling water supplied from the laminar nozzle to the upper and lower surfaces of the thick steel plate is 0.6 to 4.0 m ³ / m ² · min. The thick steel plate cooling equipment according to claim 1.   The area of the direct irradiation part where the sprayed cooling water is directly supplied from the spray nozzle to the surface of the thick steel plate in the spray cooling facility is 70% or more on the upper surface side of the thick steel plate, 75% or more on the lower surface side, and The equipment for cooling a thick steel plate according to claim 1 or 2, wherein the direct-irradiation part on the lower surface side of the thick steel plate is continuous in the width direction of the thick steel plate. Spray cooling water is sprayed from the spray nozzle of the spray cooling equipment over a length of 0.5 to 2 m in the thick steel plate transport direction on the surface of the thick steel plate after hot rolling at a transport speed of 0.1 to 0.6 m / s. Then, the temperature from Ar ₃ transformation point temperature or higher to 600 ° C. is cooled at an average cooling rate of 40 to 120 ° C./s on the surface layer of the thick steel plate, and then rod-like cooling water is injected from the laminar nozzle of the laminar cooling equipment. A method for cooling a thick steel plate, characterized by cooling to a martensite transformation start temperature at an average cooling rate of 60 to 160 ° C./s for the surface layer.   The area of the direct irradiation part where the sprayed cooling water is directly supplied to the surface of the thick steel plate in the spray cooling facility is 70% or more on the upper surface side of the thick steel plate, 75% or more on the lower surface side, and the lower surface side of the thick steel plate 5. The method for cooling a thick steel plate according to claim 4, wherein spray cooling water is sprayed from a spray nozzle of the spray cooling facility so that the direct-irradiation portion of the spray cooling device is continuous in the width direction of the thick steel plate.

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