THICK STEEL PLATE MANUFACTURING DEVICE AND MANUFACTURING METHOD

Abstract

 It is an object to provide a facility and a method for manufacturing a steel plate which perform uniform cooling in a cooling step by uniformizing scale formed on a surface of the steel plate in a descaling step, to manufacture the steel plate having excellent shape.
A facility for manufacturing a steel plate includes a hot rolling apparatus, a shape correcting apparatus, a descaling apparatus and an accelerated cooling apparatus which are disposed in this order from an upstream side in a conveyance direction, wherein an energy density E of cooling water that is jetted towards a surface of a steel plate by the descaling apparatus is greater than or equal to 0.10 J/mm².

Description

Technical Field

[0001] The present invention relates to a method and a facility for manufacturing a steel plate that performs hot rolling, shape correction and controlled cooling thereto.

Background Art

[0002] In recent years, controlled cooling has been increasingly applied as a process for manufacturing steel plates. However, in general, for example, the shapes and the surface properties of hot steel plates are not necessarily same. Therefore, temperature irregularities tend to occur in the steel plates when they are being cooled. When the steel plates after being cooled are strained, are subjected to residual stress, or have material non-uniformity, for example, material defects or operational troubles occur.

[0003] Accordingly, Patent Literature 1 discloses a method in which descaling is performed at at least one of a location just in front of and a location just behind a finish rolling final path, then, hot shape correction is performed, then, descaling is performed, and forced cooling is performed. Patent Literature 2 discloses a method in which, after performing finish rolling and hot shape correction, descaling is performed and, then, controlled cooling is performed. Patent Literature 3 discloses a method in which, just before performing controlled cooling, descaling is performed while controlling collision pressure of cooling water.

Citation List

Patent Literature

[0004] 

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 9-57327

Patent Literature 2: Japanese Patent No. 3796133

Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2010-247228

Summary of Invention

Technical Problem

[0005] However, when steel plates are actually manufactured by the methods described in the aforementioned Patent Literature 1 and Patent Literature 2, scale is not completely removed off by the descaling. Rather, the descaling causes scale non-uniformity to occur, as a result thereof, uniform cooling cannot be performed during the controlled cooling. In order to prevent scale non-uniformity in the method described in Patent Literature 3, a high collision pressure is required. Therefore, scale non-uniformity occurs at a low collision pressure, as a result of which uniform cooling cannot be performed during the controlled cooling.

[0006] In particular, in recent years, it has been difficult to achieve the level of material uniformity required of steel plates. It has become impossible to ignore bad influence on, in particular, material uniformity in a width direction of the steel plates by non-uniformity of cooling velocities during the controlled cooling, caused by such scale non-uniformity mentioned above.

[0007] The present invention has been carried out as a result of focusing on the unsolved problems of the aforementioned related arts. It is an object of the present invention to provide a facility and a method for manufacturing a steel plate which performs uniform cooling during the cooling step by uniformizing scale formed on a surface of the steel plate in the descaling step, to manufacture the steel plate having excellent shape.

Solution to Problem

[0008]  Inventors diligently studies force that causes removing of scale by cooling water, to find that if the energy density of cooling water jetted to a steel plate from a descaling apparatus is greater than or equal to 0.10 J/mm² in performing descaling after hot shape correction, the thickness of the scale formed on a surface of a product is uniform. As a result, it has been found that, when the steel plate passes an accelerated cooling apparatus, it is possible to perform uniform cooling almost without any variations in surface temperature at locations of the steel plate in a width direction and for the steel plate to have excellent shape.

[0009] The gist of the present invention is as follows.

[1] A facility for manufacturing a steel plate including a hot rolling apparatus, a shape correcting apparatus, a descaling apparatus and an accelerated cooling apparatus which are disposed in this order from an upstream side in a conveyance direction, wherein an energy density E of cooling water that is jetted towards a surface of a steel plate by the descaling apparatus is greater than or equal to 0.10 J/mm².

[2] The facility for manufacturing a steel plate according to [1], wherein, a transport velocity V [m/s] from the descaling apparatus to the accelerated cooling apparatus and a steel plate temperature T [K] before cooling, and a distance L [m] from the descaling apparatus to the accelerated cooling apparatus satisfy the formula: L ≤ V × 5 × 10^-9 × exp (25000/T).

[3] The facility for manufacturing a steel plate according to [2], wherein the descaling apparatus and the accelerated cooling apparatus are disposed such that the distance L from the descaling apparatus to the accelerated cooling apparatus is less than or equal to 12 m.

[4] The facility for manufacturing a steel plate according to any one of [1] to [3], wherein a jetting distance H from an injection nozzle of the descaling apparatus to the surface of the steel plate is more than or equal to 40 mm and less than or equal to 200 mm.

[5] The facility for manufacturing a steel plate according to any one of [1] to [4], wherein the accelerated cooling apparatus includes a header that supplies cooling water to an upper surface of the steel plate, cooling water injection nozzles that are suspended from the header and that jet rod-like cooling water, and a partition wall that is set between the steel plate and the header, wherein the partition wall is provided with a plurality of water supply ports into which lower ends of the cooling water injection nozzles are inserted and a plurality of water drainage ports that drain the cooling water supplied to the upper surface of the steel plate to locations above the partition wall.

[6] A method for manufacturing a steel plate including a hot rolling step, a correcting step, and an accelerated cooling step which are performed in this order to manufacture the steel plate, the method further including a descaling step of jetting cooling water having an energy density E greater than or equal to 0.10 J/mm² to a surface of the steel plate between the correcting step and the cooling step.

[7] The method for manufacturing a steel plate according to [6], wherein a time t [s] from completion of the descaling step to starting of the accelerated cooling step satisfies the formula: t ≤ 5 × 10^-9 × exp(25000/T), where T: steel plate temperature (K) before cooling.

Advantageous Effects of Invention

[0010] According to the present invention, it is possible to perform uniform cooling during the accelerated cooling step and to manufacture the steel plate having excellent shape, by uniformizing scale formed on the surface of the steel plate in the descaling step.

Brief Description of Drawings

[0011] 

[Fig. 1] Fig. 1 is a schematic view of an exemplary steel plate rolling line.

[Fig. 2] Fig. 2 is a graph showing relationship between energy density of cooling water that is jetted and thickness of scale that is formed on a surface of a steel plate product in a descaling apparatus.

[Fig. 3] Fig. 3 illustrates the relationship between fluid velocity and jetting distance of an injection nozzle in the descaling apparatus.

[Fig. 4] Fig. 4 is a side view of a cooling apparatus according to an embodiment of the present invention.

[Fig. 5] Fig. 5 is a side view of another cooling apparatus according to an embodiment of the present invention.

[Fig. 6] Fig. 6 illustrates an exemplary arrangement of nozzles in a partition wall according to an embodiment of the present invention.

[Fig. 7] Fig. 7 illustrates flow of drainage cooling water above the partition wall.

[Fig. 8] Fig. 8 illustrates another flow of drainage cooling water above the partition wall.

[Fig. 9] Fig. 9 illustrates temperature distribution in a width direction of a steel plate in a related art.

[Fig. 10] Fig. 10 illustrates flow of cooling water in an accelerated cooling apparatus.

[Fig. 11] Fig. 11 illustrates non-interference with drainage cooling water above the partition wall of the accelerated cooling apparatus.

Description of Embodiments

[0012] Embodiments of the present invention are hereinafter described with reference to the drawings. Here, the case in which the present invention is applied to cooling of a steel plate in a steel plate rolling process is described as an example.

[0013] Fig. 1 is a schematic view of an exemplary steel plate rolling line used for carrying out the present invention. A rolling apparatus 3 performs rough rolling and finish rolling on a slab taken away from a heating furnace 2, and the slab is rolled into a steel plate 1 having a predetermined plate thickness. Then, after scale formed on a surface of the steel plate 1 is removed by a descaling apparatus 4, the steel plate 1 is transported on-line to an accelerated cooling apparatus 6. Here, it is appropriate to perform accelerated cooling after correcting the shape of the steel plate to its proper shape by a first shape correcting apparatus 5 from the viewpoint of the shape of the steel plate after the cooling. In the accelerated cooling apparatus 6, the steel plate is cooled up to a predetermined temperature by cooling water that is jetted from an upper surface cooling facility and a lower surface cooling facility. Then, if necessary, the shape of the steel plate is corrected by a second shape correcting apparatus 7.

[0014] The descaling apparatus 4 is a apparatus that removes scale formed on a surface of the steel plate 1. In the descaling apparatus 4, after the rolling, a plurality of injection nozzles are caused to face the surface of the steel plate 1 whose distorted shape has been corrected by the first shape correcting apparatus 5, and cooling water is jetted from these nozzles.

[0015] The inventors have found out that, depending upon descaling conditions, scale is not sufficiently removed off, and that, rather, this example facilitates increasing scale non-uniformity. As a result of diligently studying the conditions under which the scale is sufficiently removed off, it has been made clear that, when descaling is performed after correcting the shape, as shown in Fig. 2, if energy density E of the cooling water jetted to the surface of the steel plate 1 from the injection nozzles of the descaling apparatus 4 is set at a value that is greater than or equal to 0.10 J/mm², the thickness of scale that is formed again thereafter becomes uniform at a value less than or equal to 5 µm. Because, it is thought to be the scale is uniformly completely removed off once by descaling, and then scale is uniformly and thinly formed again.

[0016] In the present invention, scale formed on the surface of the steel plate 1 is removed by descaling with the energy density E of the cooling water being greater than or equal to 0.10 J/mm². Then, the accelerated cooling apparatus 6 performs accelerated cooling on the steel plate 1. In the present invention, since the thickness of the scale becomes small and uniform by descaling, when the steel plate passes the accelerated cooling apparatus, it is possible to perform uniform cooling almost without any variations in surface temperature at locations on the steel plate in a width direction, and for the steel plate to have excellent shape.

[0017] The reasons are as follows. In an existing rolling facility, when a descaling apparatus removes scale after shape correction, the scale may be partly removed off. When the scale is partly removed off, the scale is not uniformly removed off, as a result of which variations in the distribution of thickness of the scale of approximately 10 to 50 µm occur. In this case, it is difficult to uniformly cool the steel plate at the accelerated cooling apparatus. That is, when, in the existing rolling facility, accelerated cooling is performed on the steel plate in which variations in the distribution of thickness of the scale has occurred, variations in surface temperature at locations on the steel plate in the width direction are large, as a result of which uniform cooling cannot be performed. As a result, the shape of the steel plate is affected.

[0018] Therefore, by performing descaling by the descaling apparatus 4 with the energy density E of the cooling water being greater than or equal to 0.10 J/mm², the variations in the distribution of thickness of the scale are eliminated. Consequently, when the accelerated cooling apparatus 6 has cooled the steel plate 1, it is possible to perform uniform cooling almost without any variations in surface temperature at locations on the steel plate in the width direction. As a result, it is possible to manufacture the steel plate 1 having excellent shape. In the case of the present invention, even if the collision pressure is low, it is possible to, by adjusting transport velocity, perform descaling that is similar to descaling that is performed when the collision pressure is high.

[0019] Here, the phrase "energy density E (J/mm²) of cooling water that is jetted to the steel plate" refers to a measure of the capability of removing scale by descaling, and is defined by the following Formula (1):


where Q: descaling water jetting flow rate [m³/s], d: spray jet thickness [mm] of flat nozzle, W: spray jet width [mm] of flat nozzle, ρ: fluid density [kg/m³], v: fluid velocity [m/s] during collision at steel plate, and t: collision time [s] (t = d/1000/V, transport velocity V [m/s]).

[0020] However, since it is not necessarily easy to measure the fluid velocity v during collision at the steel plate, it takes a lot of trouble to determine the exact value of the energy density E defined by Formula (1).

[0021] Accordingly, as a result of further studies, the inventors have found that the formula "water flow amount density × jetting pressure × collision time" may be used as a simple definition of the energy density E (J/mm²) of cooling water that is jetted to the steel plate. Here, "water flow rate (m³/mm²·min)" is a value calculated by using the formula "jetting flow rate of cooling water ÷ cooling water collision area". The jetting pressure (N/m² (= MPa)) is defined by ejection pressure of cooling water. The collision time (s) is a value that is calculated by "cooling water collision thickness ÷ transport velocity of steel plate". The relationship between the energy density of cooling water that is calculated on the basis of this simple definition and the thickness of scale that is formed on the surface of a product becomes the same as that shown in Fig. 2, as a result of which the larger the energy density of cooling water, the smaller the thickness of the scale. That is, if the energy density E is less than 0.01 J/mm², the variations in the thickness of scale on the steel plate becomes large. Therefore, the steel plate cannot be uniformly cooled, as a result of which a steel plate having an excellent shape may not be produced. In contrast, if the energy density E is greater than or equal to 0.10 J/mm², such a problem can be avoided. Therefore, in the present invention, the energy density E of cooling water is preferably greater than or equal to 0.10 J/mm² and is more preferably greater than or equal to 0.15 J/mm².

[0022] Next, the inventors studied the fluid velocity v of cooling water that is jetted from the injection nozzles of the descaling apparatus 4. As a result of the study, it has been found that the relationship between the fluid velocity v and the jetting distance is as shown in Fig. 3. The fluid velocity, which is indicated along the vertical axis, is determined by solving the equation of motion that considers buoyancy and air resistance. Until that the cooling water reaches the steel plate, the fluid velocity v of cooling water becomes lower than that at a time when the cooling water was jetted. Therefore, the smaller the jetting distance is, the larger the fluid velocity v is during collision at the steel plate, so that a high energy density can be obtained. From Fig. 3, since, in particular, when the jetting distance H exceeds 200 mm, attenuation becomes large, a jetting distance H preferably is less than or equal to 200 mm.

[0023] As the jetting distance is reduced, for example, jetting flow rate and jetting pressure for obtaining a predetermined energy density can be reduced. Therefore, it is possible to reduce pumping power of the descaling apparatus 4. In the embodiment according to the present invention in Fig. 1, the steel plate 1 of which shape has been corrected by the first shape correcting apparatus 5 moves into the descaling apparatus 4. Therefore, it is possible to bring the injection nozzles of the descaling apparatus 4 close to the surface of the steel plate 1. However, considering contact between the steel plate 1 and the injection nozzles, the lower limit of the jetting distance is preferably greater than or equal to 40 mm. From the above, in the present invention, the jetting distance H is preferably from 40 mm to 200 mm.

[0024] In the descaling apparatus 4, the jetting pressure of cooling water is preferably greater than or equal to 10 MPa, and, more preferably, greater than or equal to 15 MPa. It is advantageous because this makes it possible to set the energy density of cooling water to a value that is greater than or equal to 0.10 J/m² without excessively reducing the transport velocity. The upper limit of the jetting pressure is not particularly limited to a certain value. However, if the jetting pressure is increased, energy that is consumed by a pump that supplies high-pressure water becomes a tremendous amount. Therefore, the jetting pressure is preferably less than or equal to 50 MPa.

[0025] In general, the growth of scale on the surface of the steel plate 1 that affects the stability of cooling of the steel plate 1 by the accelerated cooling apparatus 6 is regulated by diffusion control, and is known to be represented by the following Formula (2):


where ξ: scale thickness, a: constant, Q: activation energy, R: constant, T: steel plate temperature [K] before cooling, t: time.

[0026] Therefore, considering the growth of scale after removing the scale by the descaling apparatus 4, simulation tests for the growth of the scale for various temperatures and times were carried out, the constants in the aforementioned Formula (2) were experimentally derived, and the scale thickness and cooling stability were diligently studied. As a result thereof, it has been found that the cooling is stable when the scale thickness is less than or equal to 15 µm, that the cooling is more stable when the scale thickness is less than or equal to 10 µm, and that the cooling is very stable when the scale thickness is less than or equal to 5 µm.

[0027] When the scale thickness is less than or equal to 15 µm, it is possible to derive the following Formula (3) on the basis of the aforementioned Formula (2). That is, when the time t [s] from after the end of removal of the scale on the steel plate 1 by the descaling apparatus 4 to the starting of the cooling of the steel plate 1 by the accelerated cooling apparatus 6 satisfies the following Formula (3), the cooling by the accelerated cooling apparatus 6 becomes stable:


where T: temperature [K] of steel plate before cooling.

[0028] When the scale thickness is less than or equal to 10 µm, it is possible to derive the following Formula (4) on the basis of the aforementioned Formula (2). That is, when the time t [s] from after the end of removal of the scale on the steel plate 1 by the descaling apparatus 4 to the starting of the cooling of the steel plate 1 by the accelerated cooling apparatus 6 satisfies the following Formula (4), the cooling by the accelerated cooling apparatus 6 becomes more stable:

[0029] Further, when the scale thickness is less than or equal to 5 µm, it is possible to derive the following Formula (5) on the basis of the aforementioned Formula (2). That is, when the time t [s] from after the end of removal of the scale on the steel plate 1 by the descaling apparatus 4 to the starting of the cooling of the steel plate 1 by the accelerated cooling apparatus 6 satisfies the following Formula (5), the cooling by the accelerated cooling apparatus 6 becomes very stable:

[0030] A distance L from an exit side of the descaling apparatus 4 to an entrance side of the accelerated cooling apparatus 6 is set with respect to the transport velocity V of the steel plate 1 and the time t (time from the end of the step of the descaling apparatus 4 to the starting of the step of the accelerated cooling apparatus 6) so as to satisfy the following Formula (6):


where L: distance (m) from descaling apparatus 4 to accelerated cooling apparatus 6, V: transport velocity (m/s) of steel plate 1, t: time (s).

[0031] From the aforementioned Formulas (6) and (3), it is possible to derive the following Formula (7). In the present invention, it is more preferable that Formula (7) be satisfied:

[0032] From the aforementioned Formulas (6) and (4), it is possible to derive the following Formula (8). In the present invention, it is even more preferable that Formula (8) be satisfied:

[0033] Further, from the aforementioned Formulas (6) and (5), it is possible to derive the following Formula (9). In the present invention, it is preferable that Formula (9) be satisfied:

[0034] From the aforementioned Formulas (7) to (9), if, for example, the temperature of the steel plate 1 before cooling by the accelerated cooling apparatus 6 is 820°C, and the transport velocity of the steel plate 1 is from 0.28 to 2.50 m/s, the cooling is stable when the distance L from the descaling apparatus 4 to the accelerated cooling apparatus 6 is from 12 m to 107 m, is more stable when the distance L is from 5 m to 47 m, and is very stable when the distance L is from 1.3 m to 12 m.

[0035] Accordingly, if the distance L from the descaling apparatus 4 to the accelerated cooling apparatus 6 is less than or equal to 12 m, even if the transport velocity V of the steel plate 1 is low (for example, V = 0.28 m/s), the cooling is stable, and, in contrast, if the transport velocity V of the steel plate 1 is high (for example, V = 2.50 m/s), the cooling is very stable. Therefore, this is preferable. It is more preferable that the distance L from the descaling apparatus 4 to the accelerated cooling apparatus 6 be less than or equal to 5 m.

[0036] Further, considering that, in general, the transport velocity V of a large portion of the steel plate 1 that requires controlled cooling is greater than or equal to 0.5 m/s, it is further preferable that the distance L be less than or equal to 2.5 m, which is a condition that allows the cooling to be very stable at this transport velocity V.

[0037] Here, the case in which the temperature of the steel plate 1 before the cooling by the accelerated cooling apparatus 6 is 820°C is described. Similarly, in the case in which the temperature of the steel plate 1 before the cooling by the accelerated cooling apparatus 6 is other than 820°C, it is possible for the cooling to be stable when the distance L from the descaling apparatus 4 to the accelerated cooling apparatus 6 is preferably less than or equal to 12 m, is more preferably less than or equal to 5 m, and is even more preferably less than or equal to 2.5 m. This is because, when the temperature of the steel plate 1 before the cooling by the accelerated cooling apparatus 6 is lower than 820°C, the value at the right side of each of the aforementioned Formulas (7), (8), and (9) becomes greater than that when T = 820°C, so that, when T = 820°C, as long as the distance L from the descaling apparatus 4 to the accelerated cooling apparatus 6 is one that is suitably set, each of the aforementioned Formulas (7), (8), and (9) is necessarily satisfied. In contrast, when the temperature of the steel plate 1 before the cooling by the accelerated cooling apparatus 6 is higher than 820°C, it is possible to likewise satisfy each of the aforementioned Formulas (7), (8), and (9) by appropriately adjusting the transport velocity V of the steel plate 1 to a low value.

[0038] Next, as shown in Fig. 4, the accelerated cooling apparatus 6 according to the present invention includes an upper header 11 that supplies cooling water to an upper surface of the steel plate 1, cooling water injection nozzles 13 that are suspended from the upper header 11 and that jet rod-like cooling water, and a partition wall 15 that is set between the steel plate 1 and the upper header 11. It is preferable that the partition wall 15 be provided with a plurality of water supply ports 16 into which lower ends of the cooling water injection nozzles 13 are inserted and a plurality of water drainage ports 17 for draining the cooling water supplied to the upper surface of the steel plate 1 to locations above the partition wall 15.

[0039] More specifically, the upper surface cooling facility includes the upper header 11 that supplies cooling water to the upper surface of the steel plate 1, the cooling water injection nozzles 13 that are suspended from the upper header 11, and the partition wall 15 that is provided horizontally in the width direction of the steel plate between the upper header 11 and the steel plate 1 and that includes a plurality of through holes (the water supply ports 16 and the water drainage ports 17). Each cooling water injection nozzle 13 is a circular tube nozzle 13 that jets the rod-like cooling water. An end of each nozzle 13 is inserted into its corresponding through hole (water supply port 16) in the partition wall 15, and is set above a lower end portion of the partition wall 15. In order to prevent each cooling water injection nozzle 13 from sucking foreign material at a bottom portion of the interior of the upper header 11 and from becoming clogged, an upper end of each injection nozzle 13 preferably penetrates the upper header 11 so as to protrude into the interior of the upper header 11.

[0040] Here, the term "rod-like cooling water " in the present invention refers to cooling water which is jetted while pressure of a certain amount is applied from circular nozzle jetting openings (which may also refer to elliptical or polygonal nozzle jetting openings); which has a cooling-water jetting speed from each nozzle jetting opening of 6 m/s or greater, preferably 8 m/s or greater; and whose flow is a continuous and linear flow in which the shape of a cross section of the flow of water jetted from each nozzle jetting opening is maintained in a substantially circular shape. That is, the rod-like cooling water differs from cooling water that freely falls downward from a circular tube laminar nozzle, and cooling water that is jetted in liquid drops like a spray.

[0041] The end of each cooling water injection nozzle 13 is inserted into its corresponding through hole and is set above the lower end portion of the partition wall 15 to prevent, by the partition wall 15, the cooling water injection nozzles 13 from becoming damaged even when the steel plate whose end has been warped upward has moved in. This makes it possible to perform the cooling for a long period of time with the cooling water injection nozzles 13 being set in a good state. Therefore, it is possible to prevent temperature irregularities of the steel plate without, for example, repairing the facility.

[0042] Since the end of each circular tube nozzle 13 is inserted in its corresponding through hole, as shown in Fig. 11, the end of each circular tube nozzle 13 does not interfere with the flow in a width direction of drainage water 19 that flows along an upper surface of the partition wall 15 and that is indicated by a dotted arrow. Therefore, it is possible for the cooling water jetted from the cooling water injection nozzles 13 to equally reach the upper surface of the steel plate regardless of the locations in the width direction, and uniformly cool the steel plate in the width direction.

[0043] An example of the partition wall 15 is described. As shown in Fig. 6, the partition wall 15 is provided with a plurality of through holes having a diameter of 10 mm. These through holes are provided in a grid pattern at a pitch of 80 mm in the width direction of the steel plate and at a pitch of 80 mm in the conveyance direction. The cooling water injection nozzles 13 each having an outside diameter of 8 mm, an inside diameter of 3 mm, and a length of 140 mm are inserted in the corresponding water supply ports 16. The cooling water injection nozzles 13 are arranged in a staggered pattern form, and the through holes in which the cooling water injection nozzles 13 are not inserted are the water drainage ports 17 for the cooling water. In this way, the plurality of through holes in the partition wall 15 of the accelerated cooling apparatus according to the present invention include substantially the same number of water supply ports 16 and water drainage ports 17, and are assigned different roles and functions.

[0044] At this time, the total cross sectional area of the drainage water outlets 17 is sufficiently larger than the total cross sectional area of the inside diameters of the cooling water injection nozzles 13, which are the circular tube nozzles 13, and is approximately 11 times the total cross sectional area of the inside diameters of the circular tube nozzles 13. As shown in Fig. 4, the cooling water supplied to the upper surface of the steel plate fills a portion between the surface of the steel plate and the partition wall 15, and is guided to locations above the partition wall 15 and quickly drained via the water drainage ports 17. Fig. 7 is a front view for describing the flow of drainage cooling water above the partition wall near an end portion of the partition wall in a width direction of the steel plate. The water drainage direction of each water drainage port 17 is an upward direction, which is opposite to the cooling water jetting direction. The drainage cooling water that has flowed out of the partition wall 15 to locations above the partition wall 15 changes its direction towards an outer side in the width direction of the steel plate, flows through a drainage water flow path, provided between the upper header 11 and the partition wall 15, and is drained.

[0045] In the example shown in Fig. 8, each water drainage port 17 is inclined in the width direction of the steel plate such that the water drainage direction is an oblique direction in a widthwise outer-side direction so as to be set towards the outer side in the width direction of the steel plate. By this, the flow of the drainage water 19 above the partition wall 15 in the width direction of the steel plate becomes smooth, and the draining of water is accelerated. Therefore, this is desirable.

[0046] Here, when, as shown in Fig. 9, each water drainage port and its corresponding water supply port are set in the same through hole, it becomes difficult for the cooling water to flow out of the partition wall 15 to locations above the partition wall 15 after the cooling water has collided with the steel plate. As a result, the cooling water flows through a portion between the steel plate 1 and the partition wall 15 towards an end portion in a width direction of the steel plate. This causes the flow rate of the drainage cooling water between the steel plate 1 and the partition wall 15 to become large with decreasing distance to the end portion in the width direction of the plate. Therefore, with decreasing distance to the end portion in the width direction of the plate, this interferes with force for causing jetted cooling water 18 to reach the steel plate by penetrating through a retained water membrane.

[0047] For a steel sheet, a width thereof is 2 m at most, so that its influence is limited. However, in particular, when a steel plate has a plate width that is greater than or equal to 3 m, its influence cannot be ignored. Therefore, cooling of an end portion in a width direction of a steel plate becomes weak, in which case, the temperature distribution in the width direction of the steel plate becomes non-uniform.

[0048] In contrast, as shown in Fig. 10, the accelerated cooling apparatus according to the present invention is formed such that the water supply ports 16 and the water drainage ports 17 are separately formed and such that there is a division of roles into supplying water and draining water. Therefore, the cooling drainage water passes through the water drainage ports 17 in the partition wall 15 and smoothly flows to locations above the partition wall 15. Consequently, since the drainage water after the cooling is quickly removed from the upper surface of the steel plate, cooling water that is subsequently supplied can easily penetrate a retained water membrane, as a result of which sufficient cooling capability can be provided. The temperature distribution in the width direction of the steel plate in this case becomes uniform, as a result of which a uniform temperature distribution can be obtained in the width direction.

[0049] Incidentally, if the total cross sectional area of the water drainage ports 17 is greater than or equal to 1.5 times the total cross sectional area of the inside diameters of the circular tube nozzles 13, the cooling water can be quickly drained. This can be achieved by, for example, forming holes that are larger than the outside diameter of the circular tube nozzles 13 in the partition wall 15 and setting the number of water drainage ports so as to be equal to or greater than the number of water supply ports.

[0050] If the total cross sectional area of the water drainage ports 17 is less than 1.5 times the total cross sectional area of the inside diameters of the circular tube nozzles 13, the flow resistance at each water drainage port becomes high, and, thus, it becomes difficult for retained water to be drained. As a result, the amount of cooling water that can penetrate a retained water membrane and reach the surface of the steel plate is considerably reduced, as a result of which the cooling capability is reduced. Therefore, this is not preferable. It is more preferable for the total cross sectional area of the water drainage ports 17 to be greater than or equal to 4 times the total cross sectional area of the inside diameters of the circular tube nozzles 13. On the other hand, when there are too many water drainage ports or the cross sectional diameter of each water drainage port is too large, the rigidity of the partition wall 15 is reduced, as a result of which it tends to become damaged when the steel plate collides therewith. Therefore, it is desirable that the ratio between the total cross sectional area of the water drainage ports and the total cross sectional area of the inside diameters of the circular tube nozzles 13 be in the range of from 1.5 to 20.

[0051] It is desirable for a gap between an outer peripheral surface of each circular tube nozzle 13 inserted in its corresponding water supply port 16 in the partition wall 15 and an inner surface defining its corresponding water supply port 16 be less than or equal to 3 mm in size. If the gap is large, the influence of flow that accompanies the cooling water that is jetted from each circular tube nozzle 13 causes the drainage cooling water drained to an upper surface of the partition wall 15 to be introduced into the gap between each water supply port 16 and the outer peripheral surface of its corresponding circular tube nozzle 13, as a result of which the cooling water is supplied again to locations above the steel plate. Therefore, cooling efficiency is reduced. In order to prevent this, it is more desirable that the outside diameter of each circular tube nozzle 13 be substantially the same as the size of its corresponding water supply port 16. However, considering working accuracy and mounting errors, a gap of up to 3 mm at which the influence is essentially small is allowed. More desirably, the size is less than or equal to 2 mm.

[0052] Further, in order to allow the cooling water to penetrate the retained water membrane and reach the steel plate, the inside diameter of each circular tube nozzle 13, the length of each circular tube nozzle, the jetting velocity of the cooling water, and the nozzle distance also need to be optimal values.

[0053] That is, the inside diameter of each nozzle is suitably from 3 to 8 mm. If the inside diameter is less than 3 mm, batches of water jetted from the nozzles become thin, as a result of which they lose force. In contrast, if the diameter of each nozzle exceeds 8 mm, the flow velocity is reduced, as a result of which force for causing the cooling water to penetrate the retained water membrane is reduced.

[0054] The length of each circular tube nozzle 13 is suitably from 120 to 240 mm. Here, the phrase "the length of each circular tube nozzle 13" refers to a length from an inlet in the upper end of each nozzle that penetrates by a certain amount into the interior of the header to a lower end of each nozzle inserted in the corresponding water supply port in the partition wall. If the length of each circular tube nozzle 13 is less than 120 mm, the distance between a lower surface of the header and the upper surface of the partition wall becomes too small (for example, if the thickness of the header is 20 mm, a protruding amount of the upper end of each nozzle into the interior of the header is 20 mm, and an insertion amount of the lower end of each nozzle in the partition wall is 10 mm, the distance becomes less than 70 mm). Therefore, water drainage space above the partition wall becomes small, as a result of which the drainage cooling water cannot be smoothly drained. In contrast, if the length of each circular tube nozzle 13 is greater than 240 mm, pressure loss in each circular tube nozzle 13 becomes large, as a result of which the force for causing the cooling water to penetrate the retained water membrane is reduced.

[0055] It is necessary for the jetting velocity of the cooling water from each nozzle to be greater than or equal to 6 m/s, and, preferably, greater than or equal to 8 m/s. This is because, if the jetting velocity is less than 6 m/s, the force for causing the cooling water to penetrate the retained water membrane becomes extremely weak. It is preferable that the jetting velocity be greater than or equal to 8 m/s because it is possible to achieve higher cooling capability. In addition, it is desirable that the distance from the lower end of each cooling water injection nozzle 13 for upper surface cooling to the surface of the steel plate 1 be from 30 to 120 mm. If the distance is less than 30 mm, the frequency with which the steel plate 1 collides with the partition wall 15 is extremely high, as a result of which it becomes difficult to maintain facilities. If the distance exceeds 120 mm, the force for causing the cooling water to penetrate the retained water membrane becomes extremely small.

[0056] In cooling the upper surface of the steel plate, in order to prevent the cooling water from spreading in a longitudinal direction of the steel plate, draining rollers 20 may be set in front of and behind the upper header 11. This causes a cooling zone length to become constant, and facilitates temperature control. Here, the flow of cooling water in the conveyance direction of the steel plate is intercepted by the draining rollers 20. However, the cooling water tends to be retained near the draining rollers 20.

[0057] Therefore, as shown in Fig. 5, it is preferable that the cooling water injection nozzles in a most upstream side row in the conveyance direction of the steel plate among the rows of the circular tube nozzles 13 that are arranged side by side in the width direction of the steel plate be tilted by 15 to 60 degrees upstream in the conveyance direction of the steel plate, and the cooling water injection nozzles in a most downstream side row in the conveyance direction of the steel plate among the rows of the circular tube nozzles 13 that are arranged side by side in the width direction of the steel plate be tilted by 15 to 60 degrees downstream in the conveyance direction of the steel plate. This makes it possible to also supply the cooling water to locations close to the draining rollers 20, and increase cooling efficiency without the cooling water being retained near the draining rollers 20. Therefore, this is desirable.

[0058] The distance between the lower surface of the upper header 11 and the upper surface of the partition wall 15 is set such that the cross sectional area of a flow path in the width direction of the steel plate in a space surrounded by the lower surface of the header and the upper surface of the partition wall becomes greater than or equal to 1.5 times the total cross sectional area of the inside diameters of the cooling water injection nozzles, and is, for example, approximately greater than or equal to 100 mm. If the cross sectional area of the flow path in the width direction of the steel plate is not greater than or equal to 1.5 times the total cross sectional area of the inside diameters of the cooling water injection nozzles, the drainage cooling water drained to the upper surface of the partition wall 15 from each water drainage port 17 in the partition wall cannot be drained smoothly in the width direction of the steel plate.

[0059] In the accelerated cooling apparatus according to the present invention, the range of water flow rate at which the greatest effect is provided is a range of values greater than or equal to 1.5 m³/m²·min. If the water flow rate is lower therethan, the retained water membrane does not become so thick that, even if a publicly known technology for cooling a steel plate by causing rod-like cooling water to fall freely is applied, there are cases in which temperature irregularities in a width direction do not become large. In contrast, even when the water flow rate is greater than 4.0 m³/m²·min, the use of the technology according to the present invention is effective. However, since there are practical problems such as an increase in facility costs, the most practical water flow rate is from 1.5 to 4.0 m³/m²·min.

[0060] The application of the cooling technology according to the present invention is particularly effective when draining rollers are disposed in front of and behind the cooling header. However, it is also possible to apply the cooling technology according to the present invention when draining rollers are not provided. For example, when the header is relatively long in a longitudinal direction (when its length is approximately 2 to 4 m), it is also possible to apply the cooling technology according to the present invention to cooling facilities that prevent water leakage to a zone where water cooling is not performed by jetting water spray, used for purging, at locations in front of and behind the header.

[0061] In the present invention, a cooling apparatus at a side of the lower surface of the steel plate is not particularly limited to certain apparatus. In each of the embodiments shown in Figs. 4 and 5, a cooling lower header 12 including circular tube nozzles 14 as with the cooling apparatus at the upper surface side is shown as an example. In cooling the side of the lower surface of the steel plate, jetted cooling water natural falls after colliding with the steel plate. Therefore, a partition wall 15 for draining cooling water in the width direction of the steel plate need not be provided as it is for cooling the side of the upper surface of the steel plate. In addition, a publicly known technology for supplying, for example, membranous cooling water or spray cooling water may be used.

[0062] As mentioned above, in a steel plate manufacturing facility according to the present invention, when the energy density E for jetting towards the surface of the steel plate 1 from the injection nozzles of the descaling apparatus 4 is greater than or equal to 0.10 J/mm², scale that is formed on the steel plate 1 can be made uniform, and uniform cooling can be performed at the accelerated cooling apparatus 6. As a result, it is possible to produce steel plates 1 having excellent shapes.

[0063] By correcting the shape of the steel plate 1 by the first shape correcting apparatus 5, it is possible to bring the injection nozzles of the descaling apparatus 4 closer to the surface of the steel plate 1.

[0064] If the jetting distance H (distance between each injection nozzle of the descaling apparatus 4 and the surface of the steel plate 1 is set from 40 mm to 200 mm, descaling capability is increased. Since it is sufficient for, for example, the jetting flow rate and the jetting pressure for obtaining a predetermined energy density E to be small, it is possible to reduce pumping power of the descaling apparatus 4.

[0065] When the distance L from the descaling apparatus 4 to the accelerated cooling apparatus 6 satisfies L ≤ V × 5 × 10^-9 × exp (25000/T), it is possible to stabilize the cooling of the steel plate 1 by the accelerated cooling apparatus 6.

[0066] Further, as shown in Fig. 4, the accelerated cooling apparatus 6 according to the present invention is such that the cooling water supplied from the upper-portion cooling water injection nozzles 13 via the water supply ports 16 cool the upper surface of the steel plate 1 and becomes drainage hot water, and flows in the width direction of the steel plate 1 from locations above the partition wall 15 with the water drainage ports 17 in which the upper-portion cooling water injection nozzles 13 are not inserted being water drainage paths. The drainage water after the cooling is quickly removed from the steel plate 1. Therefore, when portions of the cooling water flowing from the upper-portion cooling water injection nozzles 13 via the water supply ports 16 successively contact the steel plate 1, it is possible to provide sufficient cooling power that is the same in the width direction.

[0067] As a result of the studies carried out by the inventors, it has been found that temperature irregularities in the width direction of the steel plate subjected to accelerated cooling are approximately 40°C when descaling such as that according to the present invention is not performed. On the other hand, it has been found that temperature irregularities in the width direction of the steel plate subjected to accelerated cooling are reduced to approximately 10°C after performing descaling by using the descaling apparatus 4 according to the present invention with the energy density of cooling water being greater than or equal to 0.10 J/mm². Further, it has been found that, after performing the descaling by using the descaling apparatus 4, temperature irregularities in the width direction of the steel plate subjected to accelerated cooling by using the accelerated cooling apparatus 6 shown in Fig. 4 are reduced to approximately 4°C. Temperature irregularities of the steel plate in the width direction were calculated from the results of measurement of a steel plate surface temperature distribution after the accelerated cooling by a scanning-type thermometer.

[0068] As in the present invention, any distortion that has occurred during rolling is corrected by the first shape correcting apparatus 5, and descaling of the steel plate 1 is performed by the descaling apparatus 4, to stabilize controllability of the cooling. Therefore, the steel plate 1 whose shape is to be corrected by the second shape correcting apparatus 7 originally has high flatness and the temperature of the steel plate 1 is uniform. Consequently, correction reaction force of the second shape correcting apparatus 7 need not be very high. In addition, the distance between the accelerated cooling apparatus 6 and the second shape correcting apparatus 7 may be longer than a longest length of the steel plate 1 that is produced in a rolling manufacturing line. By this, for example, reverse correction may often by performed by the second shape correcting apparatus 7. Therefore, it is possible to expect the effect of preventing troubles, such as the steel plate 1 transported in the opposite direction jumping up at a transport roller and colliding with the accelerated cooling apparatus 6, and the effect of eliminating slight temperature deviations that have occurred during the cooling by the accelerated cooling apparatus 6 to achieve uniform temperature and preventing the occurrence of warping caused by temperature deviations after the correction.

EXAMPLE 1

[0069] After causing a steel plate 1 having a plate thickness of 30 mm and a width of 3500 mm and rolled by the rolling apparatus 3 to pass through the first shape correcting apparatus 5 and the descaling apparatus 4, controlled cooling from 820°C to 420°C was performed. Here, the conditions causing the cooling to stabilize are, when calculated from the aforementioned Formulas (3), (4), and (5): time t from after the end of removal of scale on the steel plate 1 by the descaling apparatus 4 to the starting of the cooling of the steel plate 1 by the accelerated cooling apparatus 6 is desirably less than or equal to 42 s, more desirably, less than or equal to 19 s, and, even more desirably, less than or equal to 5 s.

[0070]  In the descaling apparatus 4, the jetting pressure of each nozzle = 17.7 MPa, the jetting flow rate per nozzle = 50 L/min (= 8.3 × 10^-4 m³/s), the jetting distance (distance between each injection nozzle of the descaling apparatus 4 and surface of the steel plate plate 1) = 130 mm, the nozzle jetting angle = 32°, the nozzle attack angle = 15°, jetting regions of adjacent nozzles are provided side by side in one row in a width direction so as to lap over to a certain extent, the spray jet thickness = 3 mm, and the spray jet width = 77 mm. Here, the energy density of cooling water is a value defined by "the aforementioned water flow rate × jetting pressure × collision time". The collision time (s) is the time during which descaling water is jetted to the surface of the steel plate, and is obtained by dividing the spray jet thickness by the transport velocity.

[0071] The accelerated cooling apparatus 6 was formed into a facility provided with flow paths allowing the cooling water supplied to the upper surface of the steel plate to flow to locations above the partition wall as shown in Fig. 4, and to be drained from a side in the width direction of the steel plate as shown in Fig. 7. In the partition wall, holes, each having a diameter of 12 mm, were formed in a grid pattern, and, as shown in Fig. 6, the upper-portion cooling water injection nozzles were inserted in the corresponding water supply ports arranged in a staggered pattern form, and the remaining holes were used as water drainage ports. The distance between the lower surface of the upper header and the upper surface of the partition wall was 100 mm.

[0072] Each upper-portion cooling water injection nozzle of the accelerated cooling apparatus 6 having an inside diameter of 5 mm, an outside diameter of 9 mm, and a length of 170 mm, was such that the upper end of each nozzle was caused to protrude into the interior of the header. The jetting velocity of rod-like cooling water was 8.9 m/s. With the nozzle pitch in the width direction of the steel plate being 50 mm, and the nozzles were arranged side by side in 10 rows in the longitudinal direction in a zone in which the distance between table rollers was 1 m. The water flow rate at the upper surface was 2.1 m³/m²·min. The lower end of each nozzle for cooling the upper surface was set so as to be at an intermediate position between the upper and lower surfaces of the partition wall having a plate thickness of 25 mm. The distance from the lower end of each nozzle for cooling the upper surface to the surface of the steel plate was 80 mm.

[0073] As shown in Fig. 4, as regards the lower surface cooling facility, a cooling facility that was the same as the upper surface cooling facility except that a partition wall was not included was used, and the water flow rate and the jetting velocity of the rod-like cooling water were 1.5 times those in the upper surface cooling facility.

[0074] As shown in Table 1, the distance L from the descaling apparatus 4 to the accelerated cooling apparatus 6, the transport velocity V of the steel plate, and the time t from the descaling apparatus 4 to the accelerated cooling apparatus 6 were variously changed.

[0075] The shape of the steel plate was evaluated on the basis of an additional correction rate (%). More specifically, if warp of the entire length of the steel plate and/or warp in the entire width of the steel plate were within a standard value determined on the basis of product standards corresponding to those of the steel plate, the result was "pass", whereas if they exceeded the standard value, the result was "member to be subjected to additional correction", and the additional correction rate was calculated by the formula "(number of members to be subjected to additional corrections)/(all members)) × 100". [Table 1]

TABLE 1

Item	Descaling before controlled cooling	Jetting Height (mm)	Water amount per nozzle (m3/s)	Water flow rate (m3/mm2s)	Energy density (J/mm2)	Distance from descaling device to accelerated cooling device (m)	Transport velocity (m/s)	Collision time (s)	Time from descaling to controlled cooling (s)	Additional correction rate (%)	Collision pressure (MPa)
Example 1 of present invention	Yes	130	8.3 × 10-4	3.6 × 10-6	0.60	5	0.32	9.4 × 10-3	16	5	1.65
Example 2 of present invention	Yes	130	8.3 × 10-4	3.6 × 10-6	0.32	5	0.6	5.0 ×10-3	8	4	1.65
Example 3 of present invention	Yes	130	8.3×10-4	3.6 × 10-6	0.10	5	1.9	1.6 × 10-3	3	2	1.65
Example 4 of present invention	Yes	130	8.3 × 10-4	3.6 × 10-6	0.60	14	0.32	9.4 × 10-3	44	12	1.65
Example 5 of present invention	Yes	130	3.8 × 10-4	1.7 × 10-6	0.19	5	0.32	9.4 × 10-3	16	5	0.63
Comparative Example 1	No	-	-	-	-	-	-	-	-	40	-
Comparative Example 2	Yes	130	4.2 ×10-4	1.8 × 10-6	0.08	5	0.6	5.0 × 10-3	8	70	0.59
Comparative Example 3	Yes	130	8.3×10-4	3.6 × 10-6	0.09	6	2.1	1.4 ×10-3	3	65	1.65

[0076] In Examples 1 to 5 of the present invention in Table 1, since the energy densities were greater than or equal to 0.10 J/mm², the additional correction rates, resulting from unsuitable shapes, were low, so that good results were obtained. This is thought to be because, when the cooling was performed by the accelerated cooling apparatus 6, the cooling was uniformly performed almost without any variations in the surface temperature at locations in the width direction, and flatness thought to result from the temperature distribution of the steel plate was excellent, as a result of which the additional correction rates, resulting from improper shapes, were reduced. In Examples 1 to 5, scale was removed, so that surface properties were good. The surface properties were evaluated by determining the existence/absence of scale from image processing performed by using an image of the surface of the steel plate cooled to room temperature, the image processing making use of the difference between the color tones of a portion where the scale remained and a portion where the scale had been removed.

[0077] In particular, in each of the Examples 1 to 3 of the present invention in which the distance from the descaling apparatus 4 to the accelerated cooling apparatus 6 was 5 m, the time t from after the end of the removal of scale on the steel plate 1 by the descaling apparatus 4 to the starting of the cooling of the steel plate 1 by the accelerated cooling apparatus 6 was less than or equal to 19 s, corresponding to a condition in which the cooling by the accelerated cooling apparatus 6 stabilized, regardless of the transport velocity V of the steel plate. Therefore, the additional correction rate was good at a value less than or equal to 5%.

[0078] In Example 5 of the present invention, good results were obtained by setting the energy density within the range of the present invention without a high collision pressure (1.0 MPa), such as those in Patent Literature 1 and Patent Literature 2, being required.

[0079] On the other hand, in Comparative Example 1 in which the cooling by the accelerated cooling apparatus 6 was performed without removing scale by the descaling apparatus 4, the flatness thought to result from the temperature distribution of the steel plate became poor, and the additional correction rate was 40%.

[0080] In Comparative Example 2, with the setting conditions based on the descaling apparatus 4 being water pressure = 9 MPa and jetting flow rate per nozzle = 25 L/min (= 4.2 × 10^-4 m³/s), and with the other conditions being the same as those in Example 2 of the present invention, the energy density was set at 0.08 J/mm². In Comparative Example 2, by partly removing the scale, the temperature distribution in the width direction of the steel plate became poor and, thus, the flatness of the steel plate also became poor. Therefore, the additional correction rate was 70%.

[0081] In Comparative Example 3, regardless of the fact that the collision pressure was within a high collision pressure range, such as those in Patent Literature 1 and Patent Literature 2, since the energy density was outside the range according to the present invention, partly removing the scale caused the temperature distribution in the width direction of the steel plate to become poor and, thus, the flatness of the steel plate to also become poor. Therefore, the additional correction rate was 65%.

Reference Signs List

[0082] 

1

steel plate

2

heating furnace

3

rolling apparatus

4

descaling apparatus

5

first shape correcting apparatus

6

accelerated cooling apparatus

7

second shape correcting apparatus

11

upper header

12

lower header

13

upper cooling water injection nozzle (circular tube nozzle)

14

lower cooling water injection nozzle (circular tube nozzle)

15

partition wall

16

water supply port

17

water drainage port

18

jetting cooling water

19

drainage water

20

draining roller

21

draining roller

Claims

1. A facility for manufacturing a steel plate comprising:

a hot rolling apparatus, a shape correcting apparatus, a descaling apparatus and an accelerated cooling apparatus which are disposed in this order from an upstream side in a conveyance direction,

wherein an energy density E of cooling water that is jetted towards a surface of a steel plate by the descaling apparatus is greater than or equal to 0.10 J/mm².

2. The facility for manufacturing a steel plate according to Claim 1, wherein, a transport velocity V [m/s] from the descaling apparatus to the accelerated cooling apparatus and a steel plate temperature T [K] before cooling, and a distance L [m] from the descaling apparatus to the accelerated cooling apparatus satisfy the formula:

3. The facility for manufacturing a steel plate according to Claim 2, wherein the descaling apparatus and the accelerated cooling apparatus are disposed such that the distance L from the descaling apparatus to the accelerated cooling apparatus is less than or equal to 12 m.

4.  The facility for manufacturing a steel plate according to any one of Claims 1 to 3, wherein a jetting distance H from an injection nozzle of the descaling apparatus to the surface of the steel plate is more than or equal to 40 mm and less than or equal to 200 mm.

5. The facility for manufacturing a steel plate according to any one of Claims 1 to 4, wherein the accelerated cooling apparatus includes:

a header that supplies cooling water to an upper surface of the steel plate,

cooling water injection nozzles that are suspended from the header and that jet rod-like cooling water, and

a partition wall that is set between the steel plate and the header,

wherein the partition wall is provided with a plurality of water supply ports into which lower ends of the cooling water injection nozzles are inserted and a plurality of water drainage ports that drain the cooling water supplied to the upper surface of the steel plate to locations above the partition wall.

6. A method for manufacturing a steel plate comprising a hot rolling step, a hot correcting step, and an accelerated cooling step which are performed in this order to manufacture the steel plate, the method further comprising:

a descaling step of jetting cooling water having an energy density E greater than or equal to 0.10 J/mm² to a surface of the steel plate between the hot correcting step and the cooling step.

7. The method for manufacturing a steel plate according to Claim 6, wherein a time t [s] from completion of the descaling step to starting of the accelerated cooling step satisfies the formula:


where T: steel plate temperature (K) before cooling.

Drawing