JPS6327409B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an online cooling method for thick steel plates in direct quenching (DQ) of thick steel plates. (Problems to be solved by the prior art and the invention) When performing controlled cooling or direct quenching of thick steel plates, especially direct quenching, a high cooling capacity is required, so a large flow rate of cooling water is required. . As a method for carrying out the above-mentioned direct hardening, Japanese Patent Publication No. 47-46641, Japanese Patent Publication No. 56-49974, Japanese Patent Application Laid-Open No. 57-92141, etc. are known. In these cooling methods, the steel plate is restrained between a lower presser roller that also serves as a table roller for conveying the thick steel plate, and an upper presser roller that pairs with the lower roller, and the steel plate is cooled from a cooling nozzle installed between the rollers. The thick steel plate is cooled by the cooling water flow. By the way, since the equipment that implements the conventional cooling method described above is an online cooling equipment, the equipment above the thick steel plate, such as the presser foot upper roller and the cooling nozzle, must be raised and lowered by a large distance, or the thick steel plate must be moved up and down the line. It is necessary to provide a separate table roller with a mechanism that can be turned off, which increases the size of the equipment, making maintenance difficult and raising equipment costs. Therefore, in order to solve this problem, we developed a method for cooling thick steel plates without restraint.
The one shown in Publication No. 58-3916 has already been proposed. However, with the above cooling method, there is a large difference in the cooling capacity of the thick steel plate directly below the cooling nozzle and in other areas, so the temperature history of the thick steel plate will have large amplitudes, causing unevenness on the thick steel plate. Organization arises and becomes a problem. The present invention can prevent non-uniform structures from forming in thick steel plates, and can also reduce the size of cooling equipment.
The purpose of the present invention is to provide an online cooling method that allows easy maintenance and reduces equipment costs. (Means for Solving the Problems) In order to achieve the above object, the present invention is characterized in that in the primary cooling device 5, the thick steel plate 4 is restrained from above and below by press rollers 19, 9. The slit jet nozzles 20 and 21 are oriented toward the pass line 7 of the thick steel plate 4 from above and below and are inclined with respect to the vertical direction so as to move toward the front side in the threading direction toward the tip. The upper and lower surfaces of the thick steel plate 4 are cooled with a cooling water flow over the entire width direction of the steel plate 4 and at a flow rate of 2 m ³ /min・m or more to bring the surface temperature of the thick steel plate 4 to about 600°C or less, and then primary cooling is performed. With the device 5 and the secondary cooling device 6 separated by a pair of upper and lower draining rollers 28 and 9 that sandwich the thick steel plate 4, the upper surface of the thick steel plate 4 is cooled by the pipe nozzle 36 in the secondary cooling device 6. In addition, the lower surface is sprayed with a water volume density of 0.7 m 3 / ³ by a spray nozzle 46 arranged so that the cooling water hits the entire lower surface.
The point is that it is cooled by a cooling water flow of min・m ² or more. (Function) According to the present invention, the thick steel plate 4 immediately after hot rolling is carried into the primary cooling device 5 online. Then, in the primary cooling device 5, the thick steel plate 4
is restrained from above and below by presser rollers 19, 9, and the slit jet nozzles 20, 21 are used to flow cooling water over the entire length in the width direction of the thick steel plate 4 from the top and bottom surfaces at a flow rate of 2 m ³ /min・m or more each. , the upper and lower surfaces of the thick steel plate 4 are cooled, and the surface temperature of the thick steel plate 4 is set to about 600°C or less. After that, the primary cooling device 5 and the secondary cooling device 6
In the secondary cooling device 6, the upper surface of the thick steel plate 4 is supplied with cooling water by the pipe nozzle 36, and the lower surface is supplied with cooling water. The spray nozzle 46 is arranged so that it hits the lower front surface,
Each is cooled by a cooling water flow with a water density of 0.7 m ³ /min·m ² or more. (Embodiment) An embodiment of the present invention will be described below with reference to the drawings. Fig. 2 shows an outline of the equipment layout. 1 is a finishing mill, and in front of it in the sheet passing direction is an online cooling facility. 2. The hot levelers 3 are arranged in the above order, and the thick steel plate 4 is passed through each of the above facilities. Note that, as shown by the imaginary line in FIG. 2, the cooling equipment 2 may be arranged in front of the hot leveler 3 in the sheet passing direction. The cooling method using the cooling equipment 2 is a one-way cooling method in which the thick steel plate 4 is conveyed and sequentially cooled from the front end toward the rear end in the conveyance direction. The reason why such a cooling method is adopted is that, when considering the cooling capacity required for direct quenching, the above method requires the least amount of water. For example, a cooling method using oscillation threading, in which a thick steel plate is cooled while reciprocating, requires a cooling area that is longer than the length of the thick steel plate. Further, even in a cooling method using unidirectional threading in which the front surface of a thick steel plate is simultaneously cooled, a cooling area that is longer than the length of the thick steel plate is required. Note that depending on the length of the thick steel plate 4 (in the case of a short size), oscillation cooling can be performed in the secondary cooling device after the primary cooling device described below has passed through. As shown in FIGS. 1 and 3, the cooling equipment 2
, a primary cooling device 5 and a secondary cooling device 6 are arranged in parallel in the sheet passing direction in the above order, and a thick steel plate 4 is placed below the pass line 7 of each cooling device 5, 6. A large number of table rollers 9 for conveyance are arranged in parallel in the sheet passing direction. The primary cooling device 5 is a device that strongly cools the thick steel plate 4 while restraining it. Lit jet nozzle 20, 21, upper and lower cooling water discharge gutter 2
6, 27, a lower draining roller 28, a protector guide 30, and the like. The main fixed frame 11 is located above the pass line 7, and the frame 11 is provided with an elevating frame 12 which can be raised and lowered by means of an elevating screw jack 13. The jack 13 is driven by an electric motor via a transmission mechanism 14. A plurality of guide shafts 15 are installed below the lifting frame 12, and the lowering lifting frame 1 is attached to these guide shafts 15.
6 can be raised and lowered freely, and each guide shaft 15
Bombs 17 and 18 are installed in the upper and lower ends of the lower elevating frame 16 to urge the lower elevating frame 16 downward or upward, respectively, so that the lower elevating frame 16 is in a floating state. The presser foot upper roller 19 is provided on the front side of the lower elevating frame 16, and is vertically opposed to one table roller 9 serving as a presser foot lower roller.
By lowering the vertical elevating frames 12 and 16, the thick steel plate 4 is releasably restrained in cooperation with the table roller 9. The upper slit jet nozzle 20 is provided at the center of the lower elevating frame 16 in the sheet passing direction, and the lower slit jet nozzle 21 is arranged between the table rollers 9 and is vertically connected to the upper slit jet nozzle 20. The upper and lower surfaces of the thick steel plate 4 are cooled by the cooling water flow. The opening at the tip of each of the slit jet nozzles 20, 21 has a horizontally elongated shape that spans at least the entire length of the thick steel plate 4 in the width direction, so that the thick steel plate 4 is not locally cooled in the width direction. Each of the slit jet nozzles 20, 21 is oriented toward the pass line 7 from above and below, but is inclined with respect to the vertical direction so as to move toward the front side in the sheet passing direction toward the tip. This is done to prevent the cooling water from the upper slit jet nozzle 20 from flowing to the rear side of the nozzle 20 on the thick steel plate 4, thereby increasing the strength of the slit jet nozzle 20, 21. Before cooling,
The thick steel plate 4 is slowly cooled by the cooling water flowing from the nozzle 20 toward the rear in the sheet passing direction.
In order to prevent the material from deteriorating due to a drop in temperature, the slit jet nozzles 20 and 21 are oriented perpendicularly to the pass line 7, so that the cooling water splashes a lot after hitting the thick steel plate 4. This is to prevent the cooling efficiency from becoming low. The upper slit jet nozzle 20 is connected to an upper cooling water main pipe 23 via an upper cooling water pipe 22, and the lower slit jet nozzle 21 is connected to a lower cooling water pipe 24. The upper slit jet nozzle 20 can be freely changed in position between a non-cooling working position shown in FIG. 1 and a cooling working position below by raising and lowering the vertical elevating frames 12 and 16. A ball joint 25 is interposed in the upper cooling water pipe 22 in order to accommodate the raising and lowering of the upper slit jet nozzle 20. The upper cooling water discharge gutter 26 is disposed in front of the upper slit jet nozzle 20 of the lower elevating frame 16 in the sheet passing direction, and quickly discharges the cooling water flowing on the upper surface of the thick steel plate 4. The lower cooling water discharge gutter 27 is arranged on the front side of the lower slit jet nozzle 21 in the sheet passing direction, and quickly discharges the cooling water flowing on the lower surface side of the thick steel plate 4. The upper draining roller 28 is provided at the rear of the lower elevating frame 16, vertically faces one table roller 9 serving as the lower draining roller, and releasably holds the thick steel plate 4, thereby removing the slit jet. The cooling water from the nozzles 20 and 21 is prevented from flowing out to the secondary cooling device 6 side, and the slit jet nozzles 20 and 2 are cooled by the secondary cooling device 6.
This prevents the cooling water from No. 1 from causing disturbance. Note that the presser foot upper roller 19 and drainer upper roller 28 are driven by an electric motor 29, and the positions of each roller 19 and 28 shown by the imaginary lines in FIG. 1 are the restraint or clamping positions, and the positions shown by the solid lines in FIG. The position shown is the release position, and in the restraint or clamping position, each roller 19, 28 is pressed against the thick steel plate 4 with a predetermined pressing force. The protector guide 30 is provided at the front part of the lower elevating frame 16, and is used to move the presser foot upper roller 19 etc. to the thick steel plate 4.
This prevents the thick steel plate 4 from getting caught in the rollers easily. In addition, vertical elevating frame 12, 16 bullet machine 17, 1
8, presser foot upper roller 19, upper slit jet nozzle 20, upper cooling water discharge gutter 26, drainer upper roller 2
8. The elevating body 31 is constituted by the protector guide 30 and the like. In addition, in response to deformation in the vertical direction of the thick steel plate 4, such as warping, the elevating body 31 moves up and down minutely due to the deformation of the bombs 17 and 18. The secondary cooling device 6 cools the thick steel plate 4 in an unrestricted state, and is made up of a front cooling device 33 on the front stage side and a rear cooling device 34 on the rear stage side. Each cooling device 33, 34 includes a main fixed frame 35, a pipe nozzle header 37 having a pipe nozzle 36, a spray header 43 having a full cone spray nozzle 46, and the like. The main fixed frame 35 is located above the pass line 7, and the frame 35 is equipped with a pipe nozzle header 37 having a large number of pipe nozzles 36 as shown in FIG. The upper surface of the steel plate 4 is cooled. As shown in FIG. 5, the pipe nozzles 36 are arranged in a staggered manner, and the arrangement pitch a in the width direction and the arrangement pitch b in the threading direction are both about 100 mm or less. A and b are considered dense. Note that the pipe nozzle header 37 is of a box type as shown in FIG. 4, as shown in FIGS. 6 and 7.
In the illustrated hairpin pipe nozzle header, nozzle 3
This is because the interval cannot be narrowed. The pipe nozzle header 37 is connected to a main pipe 42 via an upper water supply pipe 40 and an upper main branch pipe 41. The spray header 43 is arranged in the left-right direction between adjacent table rollers 9 and is connected to a lower main pipe 45 via a lower water supply pipe 44. In each spray header 43, full cone spray nozzles 46 are arranged in a staggered manner as shown in FIG.
It is made to contact the lower surface in a predetermined cooling range 47, so that the air-cooled recuperation region of the thick steel plate 4 near the table roller 9 is suppressed as much as possible. Note that it is also possible to increase the arrangement density of the spray nozzles 46 as long as space permits. The distance between the spray nozzle 46 and the thick steel plate 4 during the cooling operation is generally about 50 mm. In addition, in FIG. 8, the spray nozzles 46 indicated by ● are oriented perpendicularly to the pass line 7, and the spray nozzles 46 indicated by ○ are oriented in the vertical direction so as to move forward or backward in the sheet passing direction as they move toward the tip. The angle of inclination is approximately 17 degrees. In addition, if the nozzle arrangement is considered, it is also possible to use a different type of spray nozzle instead of the full cone type. Next, the outline of cooling of the thick steel plate 4 by the cooling equipment 2 will be explained. First, before the cooling operation, the elevating body 31 of the primary cooling device 5 in the cooling equipment 2 is in the raised position as shown in FIGS. 1 and 3. After or before the high-temperature thick steel plate 4 is carried into the primary cooling device 5, the elevating body 31 is lowered, and the presser foot upper roller 19 and the table roller 9 move the thick steel plate 4 is set to be restrained, and in this state, the upper and lower surfaces of the thick steel plate 4 are successively exposed in the width direction of the thick steel plate 4 from the upper and lower slit jet nozzles 20 and 21 from the front end to the rear end in the threading direction. The thick steel plate 4 is strongly cooled by the cooling water flow over its entire length, and the surface temperature of the thick steel plate 4 is greatly reduced. By the way, the slit jet nozzles 20, 21
In the above cooling method, the thick steel plate 4 can be strongly cooled uniformly in the width direction and the threading direction, so it is possible to prevent the temperature history of the thick steel plate 4 from repeating large amplitudes, resulting in an uneven structure in the thick steel plate 4. In other words, it is possible to prevent material unevenness from occurring. The thick steel plate 4 is cooled by the presser foot upper roller 19.
Since the cooling is performed under restraint by the table rollers 9 and the table rollers 9, the cooling area can be made stable, uniform cooling is possible, and thermal deformation of the thick steel plate 4 that occurs during water cooling is extremely small. The thick steel plate 4 is then conveyed by the table rollers 9 and further advances to the front and rear cooling devices 33 and 34 of the secondary cooling device 6, where its upper and lower surfaces are cooled. 4 are sandwiched between the draining upper roller 28 and the table roller 9 at the rear end of the primary cooling device 5, so that the cooling water from the primary cooling device 5 is cooled by the secondary cooling device 6. This prevents disturbance from being applied to the Thus, each cooling device 33 of the secondary cooling device 6,
At 34, the upper and lower surfaces of the thick steel plate 4 are sequentially cooled by the cooling water flow from the pipe nozzle 36 and the full cone spray nozzle 46 from the front end toward the rear end in the sheet passing direction. Next, before intense cooling by the slit jet nozzle in the primary cooling device, the thick steel plate 4 is inserted into the nozzle 2.
An experiment was conducted under the following conditions in order to investigate the adverse effects of slow cooling by cooling water flowing from 0.21 to the rear side in the sheet passing direction.

[Table] Size of thick steel plate: Thickness 22mm, width 3200mm, length
10800mm Cooling stop temperature: 200℃ or less Water volume of slit jet nozzle: Top and bottom each 3.5m ³ /min・m Water volume density of pipe nozzle: 1.5m ³ /min・m Water density of ² spray nozzles: 2.0m ³ /min・m ² Tempering temperature: 650℃ The relationship between the cooling start temperature when a thick steel plate is cooled by cooling water from a slit jet nozzle, tensile strength and 0.2% yield strength was investigated through the above experiment. The results shown in Figure 9 were obtained. If we look at Figure 9, we can see that if the temperature of the thick steel plate decreases before strong cooling by the slit jet nozzle, it will lead to material deterioration, and this shows the effectiveness of tilting the slit jet nozzle. I understand. By the way, cooling by the slit jet nozzle has an important meaning in terms of the material quality of thick steel plates. In other words, when cooling with a pipe nozzle in a secondary cooling system, there is a large difference between the cooling capacity directly below the nozzle and the cooling capacity elsewhere, so depending on the operating conditions and equipment conditions, a non-uniform structure may occur on the thick steel plate. will occur. Furthermore, the magnitude of the cooling capacity also affects whether or not a predetermined material of the thick steel plate can be obtained. Therefore, an experiment was conducted under the following conditions. Steel type: A Thickness of steel plate: 25mm Cooling start temperature: 850℃ or higher Cooling stop temperature: 150℃ or lower Water volume of upper and lower slit jet nozzles: 1 to 4 m ³ /min・m Water volume density of pipe nozzle: 1.5 m ³ /min・m ² Spray nozzle water density: 2.0m ³ /min・m ² pipe nozzle pitch: 75 mm in width direction, threading direction
150mm staggered arrangement Spray nozzle pitch: 100mm in width direction, threading direction
150 mm plover array Tempering temperature: 650°C Steel plate conveyance speed: 0.4 to 1.2 m/s The steel plate conveyance speed was changed in order to change the temperature on the inlet side of the secondary cooling device. Figure 10 shows the experimental results in a graph, showing the amount of cooling water in the slit jet nozzle,
It shows how the material and strength of the thick steel plate are affected by the surface temperature of the thick steel plate on the outlet side of the draining roll. Note that the surface temperature was estimated from a cooling curve obtained by cooling a thick steel plate in which thermocouples were embedded in advance. According to Figure 10, in order to secure the cooling capacity necessary to ensure the strength of the thick steel plate, the amount of cooling water for the slit jet nozzle is required to be at least 2 m ³ /min・m, and to prevent material unevenness. In order to achieve this, it is clear that the surface temperature of the thick steel plate must be 600℃ or less on the exit side of the draining roll. Incidentally, regarding the occurrence of material unevenness, the arrangement pitch of the pipe nozzles of the secondary cooling device also has an important influence. In other words, when the arrangement pitch of the pipe nozzles becomes coarse, it becomes necessary to increase the amount of cooling water for the slit jet nozzles, and it becomes necessary to provide the slit jet nozzles in multiple stages, resulting in a huge total amount of cooling water. Become. Also, if the arrangement pitch of the pipe nozzle is made dense,
Although this is the opposite of the above, the number of pipe nozzles becomes enormous, which poses a problem in equipment manufacturing. Therefore, it is preferable that the arrangement pitch of the pipe nozzles is as shown in FIG. Next, in order to investigate the relationship between the cooling stop temperature and the material unevenness in the thickness direction of the steel plate, an experiment was conducted under the following conditions.

[Table] Thick steel plate size: 30mm thick Cooling start temperature: 850℃ or higher Cooling stop temperature: Room temperature to 400℃ Water volume of upper and lower slit jet nozzles: 3m ³ /min・m Water volume density of pipe nozzle: 1.5m ³ /min・m ² spray nozzle water density: 2.0m ³ /min・m ² pipe nozzle pitch: 75 mm in width direction, threading direction
150mm staggered arrangement Spray nozzle pitch: 100mm in width direction, threading direction
150mm plover arrangement Steel plate conveyance speed: 0.3m/s No tempering In addition, in order to change the cooling stop temperature, the second
The cooling time of the secondary cooling device was changed. Figure 11 shows the above experimental results, and it can be seen that in order to obtain a Vickers hardness close to the Vickers hardness of a thick steel plate when the cooling stop temperature is room temperature, the cooling stop temperature must be 200°C or less. Next, an experiment was conducted under the following conditions to investigate the relationship between the water density of the cooling water in the secondary cooling device and the tensile strength of the thick steel plate. Steel type: A Thick steel plate size: 25mm thick Cooling start temperature: 850℃ or higher Cooling stop temperature: 150℃ or lower Water volume of upper and lower slit jet nozzles: 3m ³ /min・m Average water flow density of secondary cooling device: 0.5~1.2m ³ /min・m ² pipe nozzle pitch: 75mm in width direction, threading direction
150mm Chidori spray nozzle pitch: 100mm in width direction, 150mm in threading direction
mm staggered arrangement Steel plate conveyance speed: 0.6 m/s Tempering temperature: 650°C Figure 12 shows the above experimental results. It can be seen that the density needs to be 0.7m ³ /min·m ² or more. Next, results under typical operating conditions using the apparatus of the first example of the present invention will be shown. The conditions are as follows. Steel type: A Steel plate size: 25mm thick Cooling start temperature: 900℃ Cooling stop temperature: Room temperature Primary cooling device Water volume of upper and lower slit jet nozzles:
3m ³ /min・m Secondary cooling device Pipe nozzle water flow density: 1.5m ³ /min・m ^2Spray nozzle water flow density: 2.0m ³ /min・m ^2The cooling time in the primary cooling device is approximately 1.25
second, and that of the secondary cooling device is about 30 seconds. FIG. 13 shows the above experimental results as a cooling curve, which represents the relationship between the temperature of the thick steel plate and the cooling time. According to FIG. 13, the temperature amplitude of the thick steel plate is small, and as a result, a Vickers hardness of approximately 370 and a predetermined strength can be obtained even after direct quenching, and no non-uniform structure has occurred in the thick steel plate. Further, under the cooling conditions of this example, the shape of the steel plate is also good. The mechanical properties of the thick steel plate obtained through experiments are shown below.

[Table] (Effects of the Invention) As detailed above, according to the present invention, in the primary cooling device, the slit jet nozzle that directs the pass line of the thick steel plate from above and below is used to cool the steel plate in the width direction of the thick steel plate. Entire length and water volume 2m ³ /min・m
The above cooling water flow cools the upper and lower surfaces of the thick steel plate,
Since the surface temperature of the thick steel plate is kept at about 600°C or less, the temperature history of the thick steel plate does not repeat large amplitudes, and no uneven structure occurs in the thick steel plate. In addition to cooling as described above, in the secondary cooling device, the upper surface of the thick steel plate is cooled by a pipe nozzle,
A spray nozzle is placed on the bottom surface so that the cooling water hits the entire surface of the bottom surface, each with a water density of 0.7 m ³ /
Since cooling is performed with a cooling water flow of min m ² or more, it is possible to secure the strength required for thick steel plates. Furthermore, since the slit jet nozzle is tilted in the vertical direction so that it moves toward the front side in the sheet passing direction as it approaches the tip, the thick steel plate is cooled by the slit jet nozzle. Due to the gradual cooling, the material does not deteriorate, and the cooling water does not splash significantly after hitting the thick steel plate, resulting in low cooling efficiency. Furthermore, during cooling by the primary cooling device, the thick steel plate is restrained from above and below by press rollers, so cooling is stable and thermal deformation of the thick steel plate can be prevented. or,
The primary cooling device and the secondary cooling device are separated by drain rollers that sandwich the thick steel plate from above and below, and the secondary cooling device cools the thick steel plate. The cooling water of the equipment is
It is possible to prevent disturbance from being caused to cooling by the secondary cooling device. Furthermore, in the primary cooling device,
The thick steel plate is cooled in a restrained state, but
In the secondary cooling system, the thick steel plate is cooled in an unrestrained state, and the entire cooling equipment is not designed to cool the thick steel plate in a restrained state.
Cooling equipment can be made smaller, its maintenance can be performed easily, and equipment costs can be reduced. The present invention has the above advantages and is of great practical benefit.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, and FIG. 1 is a longitudinal sectional view of the cooling equipment, FIG. 2 is an explanatory diagram showing the equipment layout, and FIG. 3 is a sectional view taken along the line - Figure 4 is a front view of the pipe nozzle header, Figure 5
The figure is an explanatory diagram showing the arrangement of pipe nozzles, Fig. 6 is a front view of the hairpin pipe nozzle header, Fig. 7 is a sectional view taken along the - line in Fig. 6, and Fig. 8 is the arrangement of full cone spray nozzles, etc. 9 to 13 are graphs showing experimental results. 2... Online cooling equipment, 5, 6... Primary/secondary cooling device, 7... Pass line, 9... Table roller, 19... Presser foot upper roller, 20, 21... Upper/lower slit jet nozzle, 2 ...Drainer upper roller,
36...Pipe nozzle, 46...Full cone spray nozzle.

Claims (1)

[Claims] 1. While conveying the thick steel plate 4 immediately after hot rolling online, it is passed through the primary cooling device 5 and the secondary cooling device 6 in the above order, and the thick steel plate 4 is conveyed from the front end in the sheet passing direction. In an online cooling method for a thick steel plate in which the thick steel plate is sequentially cooled toward the rear end, in the primary cooling device 5, the pass line 7 of the thick steel plate 4 is
The slit jet nozzles 20 and 21 are oriented from above and below and are inclined with respect to the vertical direction so as to move toward the front side in the sheet passing direction toward the tip.
Over the entire length of the thick steel plate 4 in the width direction and with a water volume of 2 m ³ /
The upper and lower surfaces of the thick steel plate 4 are cooled with a cooling water flow of min・m or more to bring the surface temperature of the thick steel plate 4 to about 600°C or less, and then the primary cooling device 5 and the secondary cooling device 6 are turned on. In the secondary cooling device 6, the thick steel plate 4 is partitioned by a pair of upper and lower draining rollers 28, 9 that sandwich the thick steel plate 4, and the pipe nozzle 36 sprays the upper surface of the thick steel plate 4, and the lower surface is sprayed with cooling water over the entire lower surface. With the spray nozzle 46 arranged so as to hit the
An online cooling method for thick steel plates characterized by cooling with a cooling water flow having a water flow density of 0.7m ³ /min・m ² or more.