[Type of Document] SPECIFICATION [Title of the Invention] METHOD FOR COOLING HOT-ROLLED STEEL SHEET [Technical Field] [0001] The present invention relates to a method for cooling a hot-rolled steel sheet in which a hot-rolled steel sheet hot-rolled using a finishing mill is cooled. [Background Art] [0002] For example, a hot-rolled steel sheet used in cars, industrial machines and the like is generally manufactured through a rough-rolling process and a finish-rolling process. FIG. 21 is a view schematically illustrating a method for manufacturing a hot-rolled steel sheet of the related art. In the process for manufacturing a hot-rolled steel sheet, first, a slab S obtained by continuously casting molten steel having an adjusted predetermined composition is rolled using a roughing mill 201, and then, fiirthermore, hot-rolled using a finishing mill 203 constituted by a plurality of rolling stands 202a to 202d, thereby forming a hot-rolled steel sheet H having a predetermined thickness. In addition, the hot-rolled steel sheet H is cooled using cooling water supplied from a cooling apparatus 211, and then coiled into a coil shape using a coiling apparatus 212. [0003] The cooling apparatus 211 is generally a facility for carrying out so-called laminar cooling on the hot-rolled steel sheet H transported from the finishing mill 203. The cooling apparatus 211 sprays the cooling water on the top surface of the hot-rolled steel sheet H moving on a nm-out table from the top in the vertical direction in a water jet form through a cooling nozzle, and, simultaneously, sprays the cooling water on the - 1 - bottom surface of the hot-rolled steel sheet H through a pipe laminar in a water jet form, thereby cooling the hot-rolled steel sheet H. [0004] In addition, for example. Patent Document 1 discloses a technique of the related art which reduces the difference in surface temperature between the top and bottom surfaces of a thick steel sheet, thereby preventing the shape of the steel sheet from becoming defective. According to the technique disclosed in Patent Document 1, the water volume ratio of cooling water supplied to the top surface and the bottom surface of the steel sheet is adjusted based on the difference in surface temperature obtained by simultaneously measuring the surface temperatures of the top surface and the bottom surface of the steel sheet using a thermometer when the steel sheet is cooled using a cooling apparatus. [0005] In addition, for example. Patent Document 2 discloses a technique that cools a rolled material between two adjacent stands in a finishing mill using a sprayer, thereby beginning and completing the y-a transformation of the rolled material so as to prevent sheet-threading performance between the stands from deteriorating. [0006] In addition, for example. Patent Document 3 discloses a technique that measures the steepness at the tip of a steel sheet using a steepness meter installed on the exit side of a mill, and prevents the steel sheet from being perforated by adjusting the flow rate of cooling water to be different in the width direction based on the measured steepness. [0007] Furthermore, for example. Patent Document 4 discloses a technique that aims to solve a wave-shaped sheet thickness distribution in the sheet width direction of a hot-rolled steel sheet and to make uniform the sheet thickness in the sheet width direction, and controls the difference between the maximum heat transmissibiUty and the minimum heat transmissibiUty in the sheet width direction of the hot-rolled steel sheet to be in a range of predetermined values. [0008] Here, there are cases in which the hot-rolled steel sheet H manufactured using the manufacturing method illustrated in FIG. 21 forms a wave shape in the rolling direction (the arrow direction in FIG. 22) on transportation rolls 220 in the run-out table (hereinafter sometimes referred to as "ROT") in the cooling apparatus 211 as illustrated in FIG. 22. In this case, the top surface and the bottom surface of the hotrolled steel sheet H are not uniformly cooled. That is, there was a problem in that, due to cooling deviation caused by the wave shape of the hot-rolled steel sheet H, it became impossible to uniformly cool the steel sheet in the rolling direction. [0009] Therefore, for example, Patent Document 5 discloses a technique that, in a steel sheet formed into a wave shape in the rolling direction, makes uniform the cooling capabilities of top portion cooling and bottom portion cooling so as to minimize the influence of the distance between soaked water on the top portion of the steel sheet and a table roller at the bottom portion in order to uniformly cool the steel sheet. [Prior Art Document] [Patent Document] [0010] [Patent Document 1] Japanese Unexamined Patent Application, First - 3 - Publication No. 2005-74463 [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H05-337505 [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2005-271052 [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2003-48003 [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. H06-328117 [Summary of the Invention] [Problem that the Invention is to solve] [0011] However, in the cooling method of Patent Document 1, a case of a hot-rolled steel sheet having a wave shape in the rolling direction is not taken into consideration. In the hot-rolled steel sheet H having a wave shape described above, there are cases in which the bottom portion of the wave shape locally comes into contact with the transportation rolls 220 as illustrated in FIG. 22. In addition, there are cases in which the hot-rolled steel sheet H locally comes into contact with aprons (not illustrated in FIG. 22) provided as supports in order to prevent the hot-rolled steel sheet H from dropping between the transportation rolls 220 at the bottom portion of the wave shape. In the wave-shaped hot-rolled steel sheet H, the portions that locally come into contact with the transportation rolls 220 or the aprons become more easily cooled than other portions due to heat dissipation by contact. Therefore, there was a problem in that the hot-rolled steel sheet H was ununiformly cooled. That is, in Patent Document 1, the fact that the wave shape of the hot-rolled steel sheet causes the hot-rolled steel sheet to - 4 - locally come into contact with the transportation rolls or the aprons and the contact portions becomes easily cooled due to heat dissipation by contact is not taken into consideration. Therefore, there are cases in which it is impossible to uniformly cool a hot-rolled steel sheet having a wave shape formed as described above. [0012] In addition, the technique described in Patent Document 2 is to make (soft) ultra low carbon steel having a relatively low hardness undergo y-a transformation between stands in a finishing mill, and does not aim at uniform cooling. In addition, the invention of Patent Document 2 does not relate to cooling in a case in which a rolled material has a wave shape in the rolling direction or a rolled material is a steel material that is so-called high tensile strength steel having a tensile strength (TS) of 800 MPa or more, and therefore there is a concern that uniform cooling may not be possible in a case in which a rolled material is a hot-rolled steel sheet having a wave shape or a steel material having a relatively high hardness. [0013] In addition, in the cooling method of Patent Document 3, the steepness of the steel sheet in the width direction is measured, and the flow rate of cooling water is adjusted in portions having a high steepness. However, when the flow rate of cooling water in the sheet width direction of the steel sheet is changed, it becomes diflFicult to make uniform the temperature of the steel sheet in the sheet width direction. Furthermore, Patent Document 3 also does not take a hot-rolled steel sheet having a wave shape in the rolling direction into consideration, and there are cases in which it is not possible to uniformly cool a hot-rolled steel sheet as described above. [0014] In addition, the cooling of Patent Document 4 is the cooling of a hot-rolled 5 - steel sheet immediately before roll bites in the finishing mill, and therefore it is not possible to apply the cooling to a hot-rolled steel sheet which has undergone finishrolling so as to have a predetermined thickness. Furthermore, Patent Document 4 also does not take a hot-rolled steel sheet having a wave shape in the rolling direction into consideration, and there are cases in which it is not possible to uniformly cool a hot-rolled steel sheet in the rolling direction as described above. [0015] In addition, in the cooling method of Patent Document 5, the cooling capability of the top portion cooling includes not only cooling by the cooling water supplied to the steel sheet from a top portion water supply nozzle but also cooling by the soaked water in the top portion of the steel sheet. Since the soaked water is influenced by the steepness of the wave shape formed in the steel sheet or the sheetthreading speed of the steel sheet, strictly, it is not possible to specify the cooling capability of the steel sheet by the soaked water. Thus, it is difficult to accurately control the cooling capability of the top portion cooling. Therefore, it is also difficult to make the cooling capabilities of the top portion cooling and the bottom portion cooling equivalence. Furthermore, the patent document describes an example of a method for determining the cooling capabilities when the cooling capabilities of the top portion cooling and the bottom portion cooling are made uniform, but does not disclose ordinary determination methods. Therefore, in the cooling method of Patent Document 5, there are cases in which it is not possible to uniformly cool a hot-rolled steel sheet. [0016] The present invention has been made in consideration of the above problems, and an object of the present invention is to uniformly cool a hot-rolled steel sheet hotrolled using a finishing mill. [Means for Solving the Problems] [0017] The present invention employs the following means for solving the problems and achieving the relevant object. That is, (1) According to an aspect of the present invention, a method for cooling a hot-rolled steel sheet is provided in which a hot-rolled steel sheet hot-rolled using a finishing mill is cooled in a cooling section provided on a sheet-threading path, including a target ratio-setting process in which a top and bottom heat transfer coefficient ratio XI, at which a temperature standard deviation Y becomes a minimum value Ymin, is set as a target ratio Xt based on correlation data indicating a correlation between a top and bottom heat transfer coefficient ratio X, which is a ratio of heat transfer coefficients of top and bottom surfaces of the hot-rolled steel sheet, and the temperature standard deviation Y during or after cooling of the hot-rolled steel sheet, which have been experimentally obtained in advance \inder conditions in which steepness and sheet-threading speed of the hot-rolled steel sheet are set to constant values; and a cooling control process in which at least one of an amount of heat dissipated fi"om a top surface by cooling and an amount of heat dissipated fi-om a bottom surface by cooling of the hot-rolled steel sheet in the cooling section is controlled so that the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet in the cooling section matches the target ratio Xt. [0018] (2) In the method for cooling a hot-rolled steel sheet according to the above (1), in the target ratio-setting process, a top and bottom heat transfer coefficient ratio X - 7 - at which the temperature standard deviation Y converges in a range of the minimum value Ymin to the minimum value Ymin+10°C may be set as the target ratio Xt based on the correlation data. [0019] (3) In the method for cooling a hot-rolled steel sheet according to the above (1) or (2), the correlation data may be prepared respectively for a plurality of conditions in which values of the steepness and the sheet-threading speed are different, and, in the target ratio-setting process, the target ratio Xt may be set based on correlation data matching actually measured values of the steepness and the sheetthreading speed among the plurality of correlation data. [0020] (4) hi the method for cooling a hot-rolled steel sheet according to the above (3), the correlation data may be data indicating the correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y using a regression formula. [0021] (5) In the method for cooling a hot-rolled steel sheet according to the above (4), the regression formula may be a formula derived using linear regression. [0022] (6) In the method for cooling a hot-rolled steel sheet according to the above (3), the correlation data may be data indicating the correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y using a table. [0023] (7) The method for cooling a hot-rolled steel sheet according to the above (1) - 8 - or (2) may further include a temperature-measuring process in which a temperature of the hot-rolled steel sheet is measured in chronological order on a downstream side of the cooling section; an average temperature value-computing process in which a chronological average value of the temperature is computed based on a measurement result of the temperature; and an amount of heat dissipated by cooling-adjusting process in which a total value of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet in the cooling section is adjusted so that the chronological average value of the temperature matches a predetermined target temperature. [0024] (8) The method for cooling a hot-rolled steel sheet according to the above (1) or (2) may further include a temperature-measuring process in which a temperature of the hot-rolled steel sheet is measured in chronological order on a downstream side of the cooling section; a changing speed-measuring process in which a changing speed of the hot-rolled steel sheet in a vertical direction is measured in chronological order at a same place as a temperature measurement place of the hot-rolled steel sheet on the downstream side of the cooling section; a control direction-determining process in which, when an upward side of the vertical direction of the hot-rolled steel sheet is set as positive, in an area with a positive changing speed, in a case in which a temperature of the hot-rolled steel sheet is lower than an average temperature in a range of one or more cycles of a wave shape of the hot-rolled steel sheet, at least one of a direction in which the amount of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as a control direction, in a case in which the temperature of the hot-rolled steel sheet is higher than the average temperature, at least one of a direction - 9 - in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction, and, in an area with a negative changing speed, in a case in which the temperature of the hot-rolled steel sheet is lower than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction, in a case in which the temperature of the hot-rolled steel sheet is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as the control direction; and an amovmt of heat dissipated by coolingadjusting process in which at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet in the cooling section is adjusted based on the control direction determined in the control direction-determining process. [0025] (9) In the method for cooling a hot-rolled steel sheet according to the above (8), the cooling section may be divided into a plurality of divided cooling sections in a sheet-threading direction of the hot-rolled steel sheet, the temperature and changing speed of the hot-rolled steel sheet may be measured in chronological order at each of borders of the divided cooling sections in the temperature-measimng process and the changing speed-measuring process; increase and decrease directions of the amounts of heat dissipated by cooling from the top and bottom surfaces of the hot-rolled steel sheet may be determined for the respective divided cooling sections based on - 10 - measurement results of the temperature and changing speeds of the hot-rolled steel sheet at the respective borders of the divided cooling sections in the control directiondetermining process; and feedback control or feedforward control may be carried out in order to adjust at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet at each of the divided cooling sections based on the control direction determined for each of the divided cooling sections in the amount of heat dissipated by cooling-adjusting process. [0026] (10) The method for cooling a hot-rolled steel sheet according to the above (9) may frirther include a measuring process in which the steepness or the sheet-threading speed of the hot-rolled steel sheet is measured at each of the borders of the divided cooling sections; and an amount of heat dissipated by cooling-correcting process in which at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet is corrected at each of the divided cooling sections based on measurement results of the steepness or the sheet-threading speeds. [0027] (11) The method for cooling a hot-rolled steel sheet according to the above (1) or (2) may frirther include a post-cooling process in which the hot-rolled steel sheet is further cooled in order to make the temperature standard deviation of the hot-rolled steel sheet fall into a permissible range on a downstream side of the cooling section. [0028] (12) In the method for cooling a hot-rolled steel sheet according to the above (1) or (2), the sheet-threading speed of the hot-rolled steel sheet in the cooling section - 11 - may be set in a range of 550 m/min to a mechanical limit speed. [0029] (13) In the method for cooling a hot-rolled steel sheet according to the above (1) or (2), a tensile strength of the hot-rolled steel sheet may be 800 MPa or more. [0030] (14) In the method for cooling a hot-rolled steel sheet according to the above (12), the finishing mill may be constituted by a plurality of rolling stands, and a supplementary cooling process in which the hot-rolled steel sheet is supplementarily cooled between the plurality of rolling stands may be further provided. [0031] (15) In the method for cooling a hot-rolled steel sheet according to the above (1) or (2), a top side cooling apparatus having a plurality of headers that ejects cooling water to a top surface of the hot-rolled steel sheet and a bottom side cooling apparatus having a plurality of headers that ejects cooling water to a bottom surface of the hotrolled steel sheet may be provided in the cooling section, and the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated fi"om the bottom surface by cooling may be adjusted by carrying out on-oflf control of the respective headers. [0032] (16) In the method for cooling a hot-rolled steel sheet according to the above (1) or (2), a top side cooling apparatus having a plurality of headers that ejects cooling water to a top surface of the hot-rolled steel sheet and a bottom side cooling apparatus having a plurality of headers that ejects cooling water to a bottom surface of the hotrolled steel sheet may be provided in the cooling section, and the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the - 12 - bottom surface by cooling may be adjusted by controlling at least one of sprayed water density, pressure and water temperature of each of the headers. [0033] (17) In the method for cooling a hot-rolled steel sheet according to the above (1) or (2), cooling in the coohng section may be carried out at a temperature of the hotrolled steel sheet in a range of 600°C or higher. [Effect of the Invention] [0034] As a result of thorough investigation of the correlation between the top and bottom heat transfer coefficient ratio X, which is a ratio of heat transfer coefficients of top and bottom surfaces of the hot-rolled steel sheet, and the temperature standard deviation Y during or after cooling of the hot-rolled steel sheet under conditions in which the steepness and the sheet-threading speed of the hot-rolled steel sheet are set to constant values, the present inventors found that the temperature standard deviation Y can be minimized (that is, the hot-rolled steel sheet can be uniformly cooled) by controlling the top and bottom heat transfer coefficient ratio X to a specific value. Therefore, according to the present invention, since a top and bottom heat transfer coefficient ratio XI, at which the temperature standard deviation Y becomes a minimum value Ymin, is set as the target ratio Xt based on the correlation data of the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y of the hot-rolled steel sheet, which have been experimentally obtained in advance, and at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet is controlled so that the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet in the cooling section matches the target ratio Xt, it is possible to - 13 - # uniformly cool the hot-rolled steel sheet which has been hot-rolled using a finishing mill so as to have a wave shape. [Brief Description of the Drawing] [0035] FIG. 1 is an explanatory view illustrating a hot rolling facility 1 for realizing a method for cooling a hot-rolled steel sheet in an embodiment of the present invention. FIG. 2 is an explanatory view illustrating an outline of a configuration of a cooling apparatus 14 provided in the hot rolling facility 1. FIG. 3 is a graph illustrating a correlation between a top and bottom heat transfer coefficient ratio X and a temperature standard deviation Y which have been obtained under conditions in which steepness and sheet-threading speed of a hot-rolled steel sheet H are set to constant values. FIG. 4 is an explanatory view illustrating a method for searching a minimum point (minimum value Ymin) of the temperature standard deviation Y from the correlation illustrated in FIG. 3. FIG. 5 is a graph illustrating a relationship between temperature change and steepness of the hot-rolled steel sheet H during cooling in ROT of a typical strip in an ordinary operation, in which the top graph indicates the temperature change with respect to a distance fi-om a coil tip or a time at which a coil passes a fixed point, and the bottom graph indicates the steepness with respect to the distance from the coil tip or the time at which the coil passes the fixed point. FIG. 6 is a graph illustrating the relationship between the temperature change and steepness of the hot-rolled steel sheet H during cooling in ROT of the typical strip in the ordinary operation. FIG. 7 is a graph illustrating the relationship between the temperature change - 14 - # and steepness of the hot-rolled steel sheet H when an amount of heat dissipated from the top surface by cooling is decreased and an amount of heat dissipated from the bottom surface by cooling is increased in a case m which the temperature of the hotrolled steel sheet H becomes low with respect to an average temperature of the hotrolled steel sheet H in an area of a positive changing speed of the hot-rolled steel sheet H and the temperature of the hot-rolled steel sheet H becomes high in an area of a negative changing speed. Meanwhile, the steepness of a wave shape of the hot-rolled steel sheet H refers to a value obtained by dividing an amplitude of the wave shape by a length of a cycle in a rolling direction. FIG. 8 is a graph illustrating the relationship between the temperature change and steepness of the hot-rolled steel sheet H when the amount of heat dissipated from the top surface by cooling is increased and the amovmt of heat dissipated from the bottom surface by cooling is decreased in a case in which the temperature of the hotrolled steel sheet H becomes low with respect to the average temperature of the hotrolled steel sheet H in the area of a positive changing speed of the hot-rolled steel sheet H and the temperature of the hot-rolled steel sheet H becomes high in the area of a negative changing speed. FIG. 9 is a graph illustrating the correlation between the steepness and the temperature standard deviation Y of the hot-rolled steel sheet H which have been obtained under conditions in which the top and bottom heat transfer coefficient ratio X and the sheet-threading speed are set to constant values. FIG. 10 is a graph illustrating the correlations between the top and bottom heat transfer coefficient ratios X and the temperature standard deviations Y which have been obtained respectively under a plurality of conditions in which the values of the steepness are different (wherein the sheet-threading speed is constant). - 15 n^ FIG. 11 is a graph illustrating the correlation between the sheet-threading speed and temperature standard deviation Y of the hot-rolled steel sheet H which have been obtained under conditions in which the top and bottom heat transfer coefficient ratio X and the steepness are set to constant values. FIG 12 is a graph illustrating the correlation between the top and bottom heat transfer coefficient ratios X and the temperature standard deviations Y which have been obtained respectively under a plurality of conditions in which the values of the sheet-threading speed are different (wherein the steepness is constant). FIG 13 is an explanatory view illustrating the details of a periphery of the cooling apparatus 14 in the hot rolling facility 1. FIG. 14 is an explanatory view illustrating a modified example of the cooling apparatus 14. FIG. 15 is an explanatory view illustrating a shape of the temperature standard deviation of the hot-rolled steel sheet H formed in a sheet width direction. FIG. 16 is an explanatory view illustrating a hot rolling facility 2 for realizing a method for cooling the hot-rolled steel sheet H in another embodiment. FIG 17 is an explanatory view illustrating an outline of a configuration of a cooling apparatus 114 provided in the hot rolling facility 2. FIG. 18 A is an explanatory view illustrating a shape in which a bottom point of the hot-rolled steel sheet H comes into contact with a transportation roll 132. FIG. 18B is an explanatory view illustrating a shape in which the bottom point of the hot-rolled steel sheet H comes into contact with the transportation roll 132 and an apron 133. FIG. 19A is a graph illustrating a change of the temperature of the hot-rolled steel sheet H over time in a case in which the sheet-threading speed of the hot-rolled - 16 - •^H steel sheet H is slow. FIG. 19B is a graph illustrating a change of the temperature of the hot-rolled steel sheet H over time in a case in which the sheet-threading speed of the hot-rolled steel sheet H is high. FIG. 20 is an explanatory view of a finishing mill 113 that can carry out interstand cooling. FIG. 21 is an explanatory view illustrating a method for manufacturing the hot-rolled steel sheet H of the related art. FIG. 22 is an explanatory view illustrating a method for cooling the hot-rolled steel sheet H of the related art. [Embodiment of the Invention] [0036] Hereinafter, as an embodiment of the present invention, a method for cooling a hot-rolled steel sheet which is intended to cool a hot-rolled steel sheet used in, for example, cars and industrial machines will be described with reference to the accompanying drawings. [0037] FIG. 1 schematically illustrates an example of a hot rolling facility 1 for realizing the method for cooling a hot-rolled steel sheet in the present embodiment. The hot rolling facility 1 is a facility aimed to sandwich the top and bottom of a heated slab S using rolls, continuously roll the slab to make the slab as thin as a minimum of 1 mm, and coil the slab. The hot rolling facility 1 has a heating furnace 11 for heating the slab S, a width-direction mill 16 that rolls the slab S heated in the heating furnace 11 in a width direction, a roughing mill 12 that rolls the slab S rolled in the width direction from the - 17 - # vertical direction so as to produce a rough bar, a finishing mill 13 that further continuously hot-finishing-roUs the rough bar to a predetermined thickness, a cooling apparatus 14 that cools the hot-rolled steel sheet H hot-finishing-rolled using the finishing mill 13 using cooling water, and a coiling apparatus 15 that coils the hotrolled steel sheet H cooled using the cooling apparatus 14 into a coil shape. [0038] The heating furnace 11 is provided with a side burner, an axial burner and a roof burner that heat the slab S brought fi-om the outside through a charging hole by blowing flame. The slab S brought into the heating furnace 11 is sequentially heated in respective heating areas formed in respective zones, and, furthermore, a heatretention treatment for enabling transportation at an optimal temperature is carried out by uniformly heating the slab S using the roof burner in a soaking area formed in a final zone. When a heating treatment in the heating furnace 11 completely ends, the slab S is transported to the outside of the heating furnace 11, and moved into a rolling process by the roughing mill 12. [0039] The roughing mill 12 passes the transported slab S through gaps between columnar rotary rolls provided across a plurality of stands. For example, the roughing mill 12 hot-rolls the slab S only using work rolls 12a provided at the top and bottom of a first stand so as to form a rough bar. Next, the rough bar which has passed through the work rolls 12a is further continuously rolled using a plurality of fourfold mills 12b constituted by a work roll and a back-up roll. As a result, when the rough rolling process ends, the rough bar is rolled into a thickness of approximately 30 mm to 60 mm, and transported to the finishing mill 13. [0040] - li # The finishing mill 13 fmishing-rolls the rough bar transported from the roughing mill 12 until the thickness becomes approximately several millimeters. The finishing mill 13 passes the rough bar through gaps between top and bottom finish rolling rolls 13a linearly arranged across 6 to 7 stands so as to gradually reduce the rough bar. The hot-rolled steel sheet H finishing-rolled using the finishing mill 13 is transported to the cooling apparatus 14 using the transportation rolls 32 described below. [0041] The cooling apparatus 14 is a facility for carrying out so-called laminar cooling on the hot-rolled steel sheet H transported from the finishing mill 13. As illustrated in FIG. 2, the cooling apparatus 14 has a top side cooling apparatus 14a that sprays cooling water from cooling holes 31 on the top side to the top surface of the hot-rolled steel sheet H moving on the transportation rolls 32 in a run-out table, and a bottom side cooling apparatus 14b that sprays cooling water from cooling holes 31 on the bottom side to the bottom surface of the hot-rolled steel sheet H. A plurality of the cooling holes 31 is provided in the top side cooling apparatus 14a and the bottom side cooling apparatus 14b respectively. In addition, a cooling header (not illustrated) is cormected to the cooling hole 31. The number of the cooling holes 31 determines the cooling capabilities of the top side cooling apparatus 14a and the bottom side cooling apparatus 14b. Meanwhile, the cooling apparatus 14 may be constituted by at least one of a top and bottom split laminar, a pipe laminar, spray cooling and the like. In addition, a section in which the hot-rolled steel sheet H is cooled using the cooling apparatus 14 corresponds to a cooling section in the present invention. [0042] - 19 - # The coiling apparatus 15 coils the hot-rolled steel sheet H cooled using the cooling apparatus 14 at a predetermined coiling temperature as illustrated in FIG. 1. The hot-rolled steel sheet H coiled into a coil shape using the coiling apparatus 15 is transported to the outside of the hot rolling facility 1. [0043] Next, the method for cooling a hot-rolled steel sheet of the present embodiment, which is realized using the hot rolling facility 1 constituted as described above, will be described. Meanwhile, in the following description, a wave shape having a surface height (wave height) changing in the rolling direction is formed in the hot-rolled steel sheet H hot-rolled using the finishing mill 13 as illustrated in FIG. 17. In addition, in the following description, the influence of soaked water remaining on the hot-rolled steel sheet H will be ignored when cooling the hot-rolled steel sheet H. Actually, as a result of investigation by the inventors, it was found that the soaked water remaining on the hot-rolled steel sheet H has little influence. [0044] The method for cooling a hot-rolled steel sheet of the present embodiment has two processes of a target ratio-setting process and a cooling control process. The details will be described below, and, in the target ratio-setting process, a top and bottom heat transfer coefficient ratio XI, at which a temperature standard deviation Y becomes a minimum value Ymin, is set as a target ratio Xt based on correlation data indicating a correlation between a top and bottom heat transfer coefficient ratio X, which is a ratio of heat transfer coefficients of the top and bottom surfaces of the hot-rolled steel sheet H, and the temperature standard deviation Y during or after cooling of the hot-rolled steel sheet H, which have been experimentally - 20 - # obtained in advance under conditions in which the steepness and the sheet-threading speed of the hot-rolled steel sheet H are set to constant values. In addition, in the cooling control process, at least one of an amount of heat dissipated from the top surface by cooling and an amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is controlled so that the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet H in the cooling section (a section in which the hot-rolled steel sheet H is cooled using the cooling apparatus 14) matches the target ratio Xt. [0045] The correlation data used in the target ratio-setting process is experimentally obtained in advance using the hot rolling facility 1 before actual operation (before the hot-rolled steel sheet H is actually manufactured as a product). Hereinafter, a method for obtaining the correlation data used in the target ratio-setting process will be described in detail. First, before cooling the hot-rolled steel sheet H in the cooling apparatus 14, the cooling capability (top side cooling capability) of the top side cooling apparatus 14a and the cooling capability (bottom side cooling capability) of the bottom side cooling apparatus 14b of the cooling apparatus 14 are adjusted respectively in advance. The top side cooling capability and the bottom side cooling capability are adjusted using the heat transfer coeJSicient of the top surface of the hot-rolled steel sheet H, which is cooled using the top side cooling apparatus 14a, and the heat transfer coefficient of the bottom surface of the hot-rolled steel sheet H, which is cooled using the bottom side cooling apparatus 14b. [0046] Here, a method for computing the heat transfer coefficients of the top surface - 21 - 0 and bottom surface of the hot-rolled steel sheet H will be described. The heat transfer coefficient refers to a value obtained by dividing the amoimt of heat dissipated from unit area by cooling (heat energy) per unit time by the temperature difference between an article to which heat is transferred and a heat medium (heat transfer coefficient=amount of heat dissipated by cooling/temperature difference). The temperature difference herein refers to the difference between the temperature of the hot-rolled steel sheet H, which is measured using a thermometer on an entry side of the cooling apparatus 14, and the temperature of cooling water used in the cooling apparatus 14. In addition, the amount of heat dissipated by cooling refers to a value obtained by respectively multiplying the temperature difference, specific heat and mass of the hot-rolled steel sheet H (amount of heat dissipated by cooling=temperature dififerencexspecific heatxmass). That is, the amount of heat dissipated by cooling is an amount of heat dissipated by cooling of the hot-rolled steel sheet H in the cooling apparatus 14, and a value obtained by multiplying the difference between the temperatures of the hot-rolled steel sheet H respectively measured using the entry-side thermometer and an exit-side thermometer in the cooling apparatus 14, the specific heat of the hot-rolled steel sheet H and the mass of the hot-rolled steel sheet H cooled using the cooling apparatus 14 respectively. [0047] As described above, the computed heat transfer coefficient of the hot-rolled steel sheet H is classified into the heat transfer coefficient of the top surface and the heat transfer coefficient of the bottom surface of the hot-rolled steel sheet H. The heat transfer coefficients of the top surface and the bottom surface are computed using a ratio that is obtained in advance, for example, in the following manner. - 22 - j l ^ That is, the heat transfer coefficient of the hot-rolled steel sheet H in a case in which the hot-rolled steel sheet H is cooled only using the top side cooling apparatus 14a and the heat transfer coefficient of the hot-rolled steel sheet H in a case in which the hot-rolled steel sheet H is cooled only using the bottom side cooling apparatus 14b are measured. At this time, the amount of cooling water from the top side cooling apparatus 14a and the amount of cooling water from the bottom side cooling apparatus 14b are set to be equal. The inverse number of the ratio between the measured heat transfer coefficient in a case in which the top side cooling apparatus 14a is used and the heat transfer coefficient in a case in which the bottom side cooling apparatus 14b is used becomes a top and bottom ratio of the amount of cooling water of the top side cooling apparatus 14a and the amount of cooling water of the bottom side cooling apparatus 14b in a case in which a top and bottom heat transfer coefficient ratio X, which will be described below, is set to "1". In addition, the above-mentioned ratio of the heat transfer coefficients of the top surface and the bottom surface of the hot-rolled steel sheet H (top and bottom heat transfer coefficient ratio X) is computed by multiplying the amount of cooling water of the top side cooling apparatus 14a or the amount of cooling water of the bottom side cooling apparatus 14b when cooling the hot-rolled steel sheet H by the top and bottom ratio of the amounts of cooling water obtained in the above manner. In addition, in the above description, the heat transfer coefficients of the hotrolled steel sheet H cooled only using the top side cooling apparatus 14a and only using the bottom side cooling apparatus 14b are used, but the heat transfer coefficient of the hot-rolled steel sheet H cooled using both the top side cooling apparatus 14a and the bottom side cooling apparatus 14b may be used. That is, the heat transfer - 23 - coefficients of the hot-rolled steel sheet H in a case in which the amounts of cooling water of the top side cooling apparatus 14a and the bottom side cooling apparatus 14b are changed are measured, and the ratio of the heat transfer coefficients of the top surface and the bottom surface of the hot-rolled steel sheet H may be computed using the ratio of the heat transfer coefficients. [0048] As described above, the heat transfer coefficients of the hot-rolled steel sheet H are computed, and the heat transfer coefficients of the top surface and the bottom surface of the hot-rolled steel sheet H are computed based on the above ratio of the heat transfer coefficients of the top surface and the bottom surface of the hot-rolled steel sheet H (top and bottom heat transfer coefficient ratio X). [0049] In addition, the cooling capabilities of the top side cooling apparatus 14a and the bottom side cooling apparatus 14b are adjusted respectively using the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet H based on FIG. 3. The horizontal axis of FIG. 3 indicates a ratio of an average heat transfer coefficient of the top surface to an average heat transfer coefficient of the bottom surface of the hotrolled steel sheet H (that is, equivalent to the top and bottom heat transfer coefficient ratio X), and the vertical axis indicates a standard deviation of temperature between the maximum temperature and the minimum temperature of the hot-rolled steel sheet H in the rolling direction (temperature standard deviation Y). In addition, FIG. 3 shows data (correlation data) indicating the correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y which are obtained by actually measuring the temperature standard deviation Y of the cooled hot-rolled steel sheet H while changing the top and - 24 - J ^ bottom heat transfer coefficient ratio X of the hot-rolled steel sheet H by adjusting the cooling capabilities of the top side cooling apparatus 14a and the bottom side cooling apparatus 14b under conditions in which the steepness of the wave shape of the hotrolled steel sheet H and the sheet-threading speed of the hot-rolled steel sheet H are set to constant values. With reference to FIG. 3, it was found that the correlation between the temperature standard deviation Y and the top and bottom heat transfer coefficient ratio X becomes a V-shaped relationship in which the temperature standard deviation Y becomes the minimum value Ymin when the top and bottom heat transfer coefficient ratio X is " 1 ". Meanwhile, the steepness of the wave shape of the hot-rolled steel sheet H refers to a value obtained by dividing the amplitude of the wave shape by the length of a cycle in the rolling direction. FIG. 3 is correlation data between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y which are obtained under conditions in which the steepness of the hot-rolled steel sheet H is set to 2% and the sheet-threading speed is set to 600 m/min (10 m/sec). The temperature standard deviation Y may be measured during the cooling of the hot-rolled steel sheet H, or may be measured after the cooling. In addition, in FIG. 3, the target cooling temperature of the hot-rolled steel sheet H is a temperature of 600°C or higher, for example, 800°C. [0050] In the target ratio-setting process, the top and bottom heat transfer coefficient ratio XI, at which the temperature standard deviation Y becomes the minimum value Ymin, is set as the target ratio Xt based on the correlation data experimentally obtained in advance as described above. The correlation data may be prepared in a form of - 25 - ^ data (table data) that indicate the correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y using a table (table form), or may be prepared in a form of data that indicate the correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y using a mathematical formula (for example, regression formula). [0051] For example, in a case in which the correlation data are prepared in a form of data indicating the correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y using a regression formula, since the V-shaped line illustrated in FIG. 3 is drawn to be almost linear on both sides of the bottom portion, the regression formula may be derived by linearly regressing the line. When the data is considered to be a linear distribution, the number of times of confirmation using test materials or the number of times of correction for estimating calculation can be small. [0052] Therefore, the minimum value Ymin of the temperature standard deviation Y is searched using a variety of methods, for example, a binary method, a golden section method and random search which are generally known search algorithms. The top and bottom heat transfer coefficient ratio XI at which the temperature standard deviation Y of the hot-rolled steel sheet H becomes the minimum value Ymin is derived in the above manner based on the correlation data illustrated in FIG. 3. In addition, here, the regression formulae of the temperature standard deviations Y of the hot-rolled steel sheet H in the rolling direction with respect to the top and bottom heat transfer coefficient ratio X may be obtained respectively on both sides of an equal point above and below the average heat transfer coefficient. 26 - ^^g# [0053] Here, a method for searching the minimum value Ymin of the temperature standard deviation Y of the hot-rolled steel sheet H using the above-described binary method will be described. [0054] FIG. 4 illustrates a standard case in which mutually different regression lines are obtained on both sides of the minimum value Ymin of the temperature standard deviation Y As illustrated in FIG. 4, first, temperature standard deviations Ya, Yb and Yc actually measured at a point, b point and c point which is in the center between the a point and the b point are extracted respectively. Meanwhile, the center between the a point and the b point indicates the c point at which a value between the top and bottom heat transfer coefficient ratio Xa at the a point and the top and bottom heat transfer coefficient ratio Xb at the b point is present, and this shall apply below. In addition, to which of Ya and Yb is the temperature standard deviation Yc closer is determined. In the embodiment, Yc is closer to Ya. Next, a temperature standard deviation Yd at a d point between the a point and the c point is extracted. In addition, to which of Ya and Yc is the temperature standard deviation Yd closer is determined. In the embodiment, Yd is closer to Yc. Next, a temperature standard deviation Ye at an e point between the c point and the d point is extracted. In addition, to which of Yc and Yd is the temperature standard deviation Ye closer is determined. In the embodiment. Ye is closer to Yd. The above computation is repeated, and a minimum point f (minimum value Ymin) of the temperature standard deviation Y of the hot-rolled steel sheet H is specified. Meanwhile, in order to specify the practical minimum point f, the above computation needs to be carried out, for example, five times. In addition, the - 27 - 0 minimum point f may be specified by dividing the range of the top and bottom heat transfer coefficient ratio X of a search target into 10 sections, and carrying out the above computation in each of the sections. [0055] In addition, the top and bottom heat transfer coefficient ratio X may be corrected using the so-called Newton's method. In this case, a partial diflFerence between the top and bottom heat transfer coefficient ratio X with respect to the actual value of the temperature standard deviation Y and the top and bottom heat transfer coefficient ratio X at which the temperature standard deviation Y becomes zero is obtained using the above-described regression formula, and the top and bottom heat transfer coefficient ratio X when cooling the hot-rolled steel sheet H may be amended using the partial difference. [0056] The top and bottom heat transfer coefficient ratio Xi at which the temperature standard deviation Y of the hot-rolled steel sheet H becomes the minimum value Ymin (Xf in FIG. 4) is derived as described above. In addition, for the relationship between the temperature standard deviation Y and the top and bottom heat transfer coefficient ratio X, which forms a V shape, it is easy to divide the graph into two sides, and obtain regression functions respectively using the method of least squares. [0057] In addition, when FIG. 3 is referenced, the top and bottom heat transfer coefficient ratio XI at which the temperature standard deviation Y of the hot-rolled steel sheet H becomes the minimum value Ymin is " 1 " . Therefore, in a case in which the correlation data as illustrated in FIG. 3 is obtained, the target ratio Xt is set to " 1 " in the target ratio-setting process during an actual operation in order to minimize the - 28 0 temperature standard deviation Y, that is, in order to uniformly cool the hot-rolled steel sheet H. In addition, in the cooling control process, at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is controlled so that the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet H in the cooling section matches the target ratio Xt (that is "1"). Specifically, in order to match the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet H in the cooling section to the target ratio Xt (that is "1"), the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H may be equaled by, for example, adjusting the cooling capability of the top side cooling apparatus 14a and the cooling capability of the bottom side cooling apparatus 14b to be equal. Table 1 describes the correlation data illustrated in FIG. 3 (that is, the correlation between the top and bottom heat transfer coeflTicient ratio X and the temperature standard deviation Y), values obtained by subtracting the respective temperature standard deviations Y by the minimum value Ymin (=2.3°C) (the differences of the standard deviations from the minimum value), and the evaluation of the respective temperature standard deviations Y. In the top and bottom heat transfer coefficient ratio X in Table 1, the numerator is the heat transfer coefficient of the hot-rolled steel sheet H on the top surface, and the denominator is the heat transfer coefficient of the hot-rolled steel sheet H on the bottom surface. In addition, in the evaluation in Table 1 (the evaluation of the conditions of the top and bottom heat transfer coefficient ratio X), the condition - 29 - p under which the temperature standard deviation Y becomes the minimum value Ymin is considered as "A", the condition under which the difference of the standard deviation from the minimum value becomes 10°C or less, that is, the operation becomes preferable as described below is considered as "B", and the condition under which the computation is heuristically carried out in order to obtain the abovedescribed regression formula is considered as"C". In addition, when Table 1 is referenced, the top and bottom heat transfer coefficient ratio XI at which the evaluation becomes "A", that is, the temperature standard deviation Y of the hot-rolled steel sheet H becomes the minimum value Ymin is " 1 ". 30 - "% [0058] [Table 1] Top and bottom heat transfer coefficient ratio X 1.6/1.0 1.2/1.0 1.1/1.0 1.0/1.0 1.0/1.1 1.0/1.2 1.0/1.6 Temperature standard deviation Y(°C) 33.2 14.6 8.5 2.3 6.1 9.8 28.7 Difference of standard deviation from minimum value (°C) 30.9 12.3 6.2 0.0 3.8 7.5 26.4 Evaluation C C B A B B C - 31 ^ [0059] Meanwhile, when the temperature standard deviation Y of the hot-rolled steel sheet H converges at least in a range of the minimum value Ymin to the minimum value Ymin+10°C, it can be said that the variations in yield stress, tensile strength and the like are suppressed within the manufacturing permissible ranges, and the hot-rolled steel sheet H can be imiformly cooled. That is, in the target ratio-setting process, the top and bottom heat transfer ratio X at which the temperatin'e standard deviation Y converges in a range of the minimum value Ymin to the minimum value Ymin+10°C may be set as the target ratio Xt based on the correlation data experimentally obtained in advance. Meanwhile, since there is a variety of noise in the temperatvire measurement of the hot-rolled steel sheet H, there are cases in which the minimum value Ymin of the temperature standard deviation Y of the hot-rolled steel sheet H is not strictly zero. Therefore, the manufacturing permissible range is set to a range in which the temperature standard deviation Y of the hot-rolled steel sheet H is the minimum value Ymin to the minimum value Ymin+10°C in order to remove the influence of the noise. [0060] In order to converge the temperature standard deviation Y in a range of the minimum value Ymin to the minimum value Ymin+10°C, in FIG. 3 or 4, it is necessary to pull the straight line in the horizontal axis direction from a point in the vertical axis at which the temperature standard deviation Y becomes the minimum value Ymin+10°C, obtain two intersections between the straight line and two regression lines on both sides of the V-shaped curve, and set the target ratio Xt from the top and bottom heat transfer coefficient ratio X between the two intersections. Meanwhile, in Table 1, the temperature standard deviation Y can be converged in a range of the minimum - 32 - ^B value Ymin to the minimum value Ymin+10°C by setting the top and bottom heat transfer coefficient ratio X wdth an evaluation of "B" as the target ratio Xt. [0061] In addition, in order to match the top and bottom heat transfer coefficient ratio X to the target ratio Xt, it is easiest to operate the sprayed cooling water density of at least one of the top side cooling apparatus 14a and the bottom side cooling apparatus 14b. Therefore, for example, in FIGS. 3 and 4, the values in the horizontal axis are replaced by the top and bottom sprayed water density ratio, and the regression formula of the temperature standard deviation Y of the hot-rolled steel sheet H with respect to the top and bottom ratio of the sprayed water density may be obtained on both sides of an equal point above and below the average heat transfer coefficient. Here, the equal point above and below the average heat transfer coefficient does not necessarily become an equal point above and below the sprayed cooling water density, and therefore the regression formula may be obtained by carrying out tests slightly widely. [0062] In addition, during an actual operation, there is a possibility that the value of at least one of the steepness and the sheet-threading speed may change due to a change in the manufacturing conditions. When at least one of the steepness and the sheetthreading speed is changed, the correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y changes. Therefore, the correlation data are prepared for each of a plurality of conditions having different values of the steepness and the sheet-threading speed, and, in the target ratio-setting process, the target ratio Xt may be set based on a correlation data in accordance with actually measured values of the steepness and the sheet-threading speed during the actual operation of the plurality of correlation data. Thereby, it becomes possible to - 33 - carry out uniform cooling suitable for the manufacturing conditions during the actual operation. [0063] Here, as a result of thorough studies regarding the adjustment of the cooling capabilities of the top side cooling apparatus 14a and the bottom side cooling apparatus 14b (control of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H) in order to uniformly cool the hot-rolled steel sheet H, the inventors further obtained the following findings. [0064] As a result of repeating thorough studies regarding the characteristics of the temperature standard deviation Y generated by cooling in a state in which a wave shape of the hot-rolled steel sheet H is generated, the inventors clarified the following fact. [0065] Generally, during an actual operation, it is necessary to maintain the quality of the hot-rolled steel sheet H by controlling the temperature of the hot-rolled steel sheet H at a predetermined target temperature (a temperature suitable for coiling) when coiling the hot-rolled steel sheet H using the coiling apparatus 15. Therefore, a temperature-measuring process in which the temperature of the hot-rolled steel sheet H on the dovmstream side of the cooling section (that is, the cooling apparatus 14) is measured in chronological order, an average temperature value-computing process in which a chronological average value of the temperature is computed based on the measurement result of the temperature so that the chronological average value of the temperature matches a predetermined target temperature, and an - 34 - # amount of heat dissipated by cooling-adjusting process in which the total value of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is adjusted may be newly added to the above-described target ratiosetting process and cooling control process. In order to realize the new processes, a thermometer 40 which is disposed between the cooling apparatus 14 and the coiling apparatus 15 as illustrated in FIG. 13 and measures the temperature of the hot-rolled steel sheet H can be used. [0066] In the temperature-measuring process, with respect to the hot-rolled steel sheet H transported from the cooling apparatus 14 to the coiling apparatus 15, the temperatures at locations set in the rolling direction of the hot-rolled steel sheet H are measured at certain time intervals (sampling intervals) using the thermometer 40, and chronological data of the temperature measurement results are obtained. Meanwhile, the temperature measurement area using the thermometer 40 includes all the area of the hot-rolled steel sheet H in the width direction. In addition, when the sheetthreading speed (transportation speed) of the hot-rolled steel sheet H is multiplied at the sampling times of the respective temperature measurement results, the locations of the hot-rolled steel sheet H in the rolling direction, at which the respective temperature measurement results have been obtained, can be computed. That is, when the sampling times of the respective temperature measurement results are multiplied by the sheet-threading speed, it becomes possible to link the chronological data of the temperature measurement results to the locations in the rolling direction. [0067] In the average temperature value-computing process, a chronological average - 35 - 0 value of the temperature measurement results is computed using the chronological data of the temperature measurement results. Specifically, each time when a certain number of the temperature measurement results are obtained, the average value of the certain number of the temperature measurement results may be computed. In addition, in the amount of heat dissipated by cooling-adjusting process, the total value of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is adjusted so that the chronological average value of the temperature measurement results computed as described above matches a predetermined target temperature. Here, it is necessary to adjust the total value of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling while achieving a control target that matches the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet H in the cooling section to the target ratio Xt. Specifically, when adjusting the total value of the amoimt of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling, the on-off control of cooling headers connected to the cooling apparatus 14 may be carried out on a theoretical value obtained in advance using an experiment theoretical formula represented by, for example, Mitsuzuka's formula based on a learned value set to correct the error with an actual operation achievement. Alternatively, the on-off of the cooling headers may be feedback-controlled or feedforward-controlled based on the temperature actually measured using the thermometer 40. [0068] - 36 - 0 Next, the cooling control of ROT of the related art will be described using data obtained from the above-described thermometer 40 and a shape meter 41 that measures the wave shape of the hot-rolled steel sheet H which is disposed between the cooling apparatus 14 and the coiling apparatus 15 as illustrated in FIG. 13. Meanwhile, the shape meter 41 measures the shape of the same measurement location (hereinafter this measurement location will be sometimes referred to as a fixed point) as the thermometer 40 set on the hot-rolled steel sheet H. Here, the shape refers to the steepness obtained through the line integration of the heights or changing components of pitches of the wave using the movement amount of the hot-rolled steel sheet H in the sheet-threading direction as the changing amount of the hot-rolled steel sheet H in the height direction observed in a measurement at the fixed point. In addition, at the same time, the changing amount per unit time, that is, the changing speed is also obtained. Furthermore, similarly to the temperature measurement area, the shape measurement area includes all the areas of the hot-rolled steel sheet H in the width direction. Similarly to the temperature measurement results, when the sampling times of the respective measurement results (steepness, changing speed and the like) are multiplied by the sheet-threading speed, it becomes possible to link the chronological data of the respective measurement results to the locations in the rolling direction. FIG. 5 illustrates the relationship between the temperature change and steepness of the hot-rolled steel sheet H during cooling in ROT of a typical strip in an ordinary operation. The top and bottom heat transfer coefficient ratio X of the hotrolled steel sheet H in FIG. 5 is 1.2:1, and the top side cooling capability is superior to the bottom side cooling capability. The top graph in FIG. 5 indicates the temperature change with respect to the distance from a coil tip or a time at which a coil passes the - 37 - ^ fixed point, and the bottom graph in FIG. 5 indicates the steepness with respect to the distance from the coil tip or the time at which the coil passes the fixed point. The area A in FIG. 5 is an area before the strip tip portion illustrated in FIG. 13 is bit in a coiler of the coiling apparatus 15 (since there is no tension, the shape is defective in this area). The area B in FIG. 5 is an area after the strip tip portion is bit in the coiler (the area in which the wave shape is changed to be flat by the influence of unit tension). There is a demand for improving a large temperature change (that is, the temperature standard deviation Y) occurring in the area A in which the shape of the hot-rolled steel sheet H is not flat. [0069] Therefore, the inventors carried out thorough tests for the purpose of controlling the increase in the temperature standard deviation Y in ROT, and, consequently, obtained the following findings. [0070] Similarly to FIG. 5, FIG. 6 illustrates the temperature-changing component with respect to the steepness of the same shape during cooling in ROT of the typical strip in the ordinary operation. The temperature-changing component is a residual error obtained by subtracting the actual steel sheet temperature by the chronological average of the temperature (hereinafter sometimes referred to as "average temperature"). For example, the average temperature may be the average of the temperature of a range that is a cycle or more of the wave shape of the hot-rolled steel sheet H. Meanwhile, the average temperature is, in principle, the average of the temperature of a range of the unit cycle. In addition, it is confirmed from operation data that there is no large difference between the average temperature of a range of a - 38 - 0 cycle and the average temperature of a range of two or more cycles. Therefore, the average temperature simply needs to be computed from a range of at least a cycle of the wave shape. The upper limit of the range of the wave shape of the hot-rolled steel sheet H is not particularly limited; however, a sufficiently accurate average temperature can be obtained when the range is preferably set to 5 cycles. In addition, even when the average temperature is computed not from a range of the unit cycle but from a range of 2 to 5 cycles, a permissible average temperature can be obtained. [0071] Here, when the upward side of the vertical direction (the direction that intersects the top and bottom surfaces of the hot-rolled steel sheet H) of the hot-rolled steel sheet H is set as positive, in an area with a positive changing speed measured at the fixed point, in a case in which the temperature (the temperature measured at the fixed point) of the hot-rolled steel sheet H is lower than the average temperature of a range of one or more cycles of the wave shape of the hot-rolled steel sheet H, at least one of a direction in which the amovmt of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as a control direction, and, in a case in which the temperature of the hot-rolled steel sheet H is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction. In addition, in an area with a negative changing speed measured at the fixed point, in a case in which the temperature of the hot-rolled steel sheet H is lower than the average temperature, at least one of a direction in which the amount of heat - 39 - dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction, and, in a case in which the temperature of the hot-rolled steel sheet H is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as the control direction. In addition, it was found that, when at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is adjusted based on the control direction determined as described above, as illustrated in FIG. 7, the temperature change occurring in the area A in which the shape of the hotrolled steel sheet H is not flat can be reduced compared with FIG. 6. [0072] A case in which an opposite operation to the above case is carried out will be described below. In an area with a positive changing speed measured at the fixed point, in a case in which the temperature of the hot-rolled steel sheet H is lower than the average temperature of the hot-rolled steel sheet H, at least one of a direction in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction, and, in a case in which the temperature of the hot-rolled steel sheet H is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as the control direction. - 40 - ^ In addition, in an area with a negative changing speed measured at the fixed point, in a case in which the temperature of the hot-rolled steel sheet H is lower than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as the control direction, and, in a case in which the temperature of the hot-rolled steel sheet H is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction. In addition, it was found that, when at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is adjusted based on the control direction determined as described above, as illustrated in FIG. 8, the temperature change occurring in the area A in which the shape of the hotrolled steel sheet H is not flat enlarges compared with FIG. 6. Meanwhile, in the examples described herein, an assumption does not apply in which the cooling end temperature may be changed. [0073] Use of the above relationship clarifies which cooling capability of the top side cooling apparatus 14a and the bottom side cooling apparatus 14b in the cooling apparatus 14 needs to be adjusted in order to reduce the temperature change, that is, the temperature standard deviation Y. Meanwhile, the above relationship is summarized in Table 2. [0074] - 41 - o [Table 2] Changing speed Temperature Amount of heat dissipated by cooling Top surface side Bottom surface side Positive Low Decrease Increase High Increase Decrease Negative Low Increase Decrease High Decrease Increase - 42 - 9imMMim0iiiimimM»mw0fm¥immmmmm pi»«i!ii!»vHmm25°C B:25>CT>10 A: 10>CT - 72 - |>^!iiaW25°C B: 25>CT>10 A: 10>CT - 75 - ipSi|(^S!P»JSfMJfipiP!P!! ¥mm90itfmm?i. mmmmmimm^^^^^'m mm'^^vmrn^^mmm^ ewp^TO'^BapiPiiggwwJwwjBj^jP^ .fl^ [0132] As described in Table 6, in a case in which the sheet-threading speed was 500 m/min or less, even when the inter-stand cooling was carried out, the CT temperature change amount was not sufficiently reduced (higher than 25°C), and the hot-rolled steel sheet was not sufficiently uniformly cooled. On the other hand, in a case in which the sheet-threading speed was 500 m/min or more, it was found that the CT temperature change amount was suppressed to 25°C or less, and the hot-rolled steel sheet was uniformly cooled. [0133] In addition, in cases in which the inter-stand cooling was carried out (that is, the cases described in Table 6), the CT temperature change amount was suppressed even in the hot-rolled steel sheets having a relatively high hardness (tensile strength 800 MPa). That is, it was found that it became possible to uniformly cool all steel materials, particularly, steel materials having a high hardness by setting the sheetthreading speed during the cooling of the hot-rolled steel sheet to 550 m/min or more, and, additionally, carrying out the inter-stand cooling in a finishing mill, [hidustrial Applicability] [0134] The present invention is usefiil when cooling a hot-rolled steel sheet which has been hot-rolled using a finishing mill so as to have a wave shape having a surface height changing in the rolling direction. [Description of Reference Numerals and Signs] [0135] 1,2: HOT ROLLING FACILITY 11,111: HEATING FURNACE - 76 - 12,112: ROUGHING MILL 12a, 112a: WORK ROLL 12b, 112b: FOURFOLD MILL 13, 113: FINISHING MILL 13a, 113a: FINISH ROLLING ROLL 14,114: COOLING APPARATUS 14a, 114a: TOP SIDE COOLING APPARATUS 14b, 114b: BOTTOM SIDE COOLING APPARATUS 15,115: COILING APPARATUS 16, 116: WIDTH-DIRECTION MILL 31, 131: COOLING HOLE 32, 132: TRANSPORTATION ROLL 40: THERMOMETER 41: SHAPE METER H: HOT-ROLLED STEEL SHEET S: SLAB Zl, Z2: DIVIDED COOLING SECTION - 77 ^ [Type of Document] CLAIMS [Claim 1] A method for cooling a hot-rolled steel sheet in which a hot-rolled steel sheet hot-rolled using a finishing mill is cooled in a cooling section provided on a sheetthreading path, the method comprising: a target ratio-setting process in which a top and bottom heat transfer coefficient ratio XI, at which a temperatxire standard deviation Y becomes a minimum value Ymin, is set as a target ratio Xt based on correlation data indicating a correlation between a top and bottom heat transfer coefficient ratio X, which is a ratio of heat transfer coefficients of top and bottom surfaces of the hot-rolled steel sheet, and the temperature standard deviation Y during or after cooling of the hot-rolled steel sheet, which have been experimentally obtained in advance under conditions in which steepness and sheet-threading speed of the hot-rolled steel sheet are set to constant values; and a cooling control process in which at least one of an amount of heat dissipated firom a top surface by cooling and an amount of heat dissipated from a bottom surface by cooling of the hot-rolled steel sheet in the cooling section is controlled so that the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet in the cooling section matches the target ratio Xt. [Claim 2] The method for cooling a hot-rolled steel sheet according to Claim 1, wherein, in the target ratio-setting process, a top and bottom heat transfer coefficient ratio X at which the temperature standard deviation Y converges in a range of the minimiim value Ymin to the minimum value Ymin+10°C is set as the target ratio Xt based on the correlation data. - 78 # [Claim 3] The method for cooling a hot-rolled steel sheet according to Claim 1 or 2, wherein the correlation data are prepared respectively for a plurality of conditions in which values of the steepness and the sheet-threading speed are different, and, in the target ratio-setting process, the target ratio Xt is set based on correlation data matching actually measured values of the steepness and the sheet-threading speed of the plurality of correlation data. [Claim 4] The method for cooling a hot-rolled steel sheet according to Claim 3, wherein the correlation data are data indicating a correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y using a regression formula. [Claim 5] The method for cooling a hot-rolled steel sheet according to Claim 4, wherein the regression formula is a formula derived using linear regression. [Claim 6] The method for cooling a hot-rolled steel sheet according to Claim 3, wherein the correlation data are data indicating a correlation between the top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y using a table. [Claim 7] The method for cooling a hot-rolled steel sheet according to Claim 1 or 2, the method further comprising: a temperature-measuring process in which a temperature of the hot-rolled - 79 - steel sheet is measured in chronological order on a downstream side of the cooling section; an average temperature value-computing process in which a chronological average value of the temperature is computed based on a measurement result of the temperature; and an amount of heat dissipated by cooling-adjusting process in which a total value of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet in the cooling section is adjusted so that the chronological average value of the temperature matches a predetermined target temperature. [Claim 8] The method for cooling a hot-rolled steel sheet according to Claim 1 or 2, the method further comprising: a temperature-measuring process in which a temperature of the hot-rolled steel sheet is measured in chronological order on a downstream side of the cooling section; a changing speed-measuring process in which a changing speed of the hotrolled steel sheet in a vertical direction is measured in chronological order at a same place as a temperature measurement place of the hot-rolled steel sheet on the downstream side of the cooling section; a control direction-determining process in which, when an upward side of the vertical direction of the hot-rolled steel sheet is set as positive, in an area with a positive changing speed, in a case in which a temperature of the hot-rolled steel sheet is lower than an average temperature in a range of one or more cycles of a wave shape of the hot-rolled steel sheet, at least one of a direction in which the amount of heat - 80 - ^fe dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as a control direction, in a case in which the temperature of the hot-rolled steel sheet is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction, in an area with a negative changing speed, in a case in which the temperature of the hot-rolled steel sheet is lower than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction, and, in a case in which the temperature of the hot-rolled steel sheet is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as the control direction; and an amount of heat dissipated by cooling-adjusting process in which at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet in the cooling section is adjusted based on the control direction determined in the control direction-determining process. [Claim 9] The method for cooling a hot-rolled steel sheet according to Claim 8, wherein the cooling section is divided into a plurality of divided cooling - 81 - sections in a sheet-threading direction of the hot-rolled steel sheet, the temperature and changing speed of the hot-rolled steel sheet are measured in chronological order at each of borders of the divided cooling sections in the temperature-measuring process and the changing speed-measuring process; increase and decrease directions of the amounts of heat dissipated by cooling from the top and bottom surfaces of the hot-rolled steel sheet are determined for the respective divided cooling sections based on measurement results of the temperature and changing speeds of the hot-rolled steel sheet at the respective borders of the divided cooling sections in the control direction-determining process; and feedback control or feedforward control is carried out in order to adjust at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet at each of the divided cooling sections based on the control direction determined for each of the divided cooling sections in the amount of heat dissipated by coolingadjusting process. [Claim 10] The method for cooling a hot-rolled steel sheet according to Claim 9, the method further comprising: a measuring process in which the steepness or sheet-threading speed of the hot-rolled steel sheet is measured at each of the borders of the divided cooling sections; and an amount of heat dissipated by cooling-correcting process in which at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet is corrected at each of the divided cooling sections based on measurement results of the - 82 - 4§ steepness or the sheet-threading speeds. [Claim 11] The method for cooling a hot-rolled steel sheet according to Claim 1 or 2, the method further comprising: a post-cooling process in which the hot-rolled steel sheet is flirther cooled on a downstream side of the cooling section in order to make the temperature standard deviation of the hot-rolled steel sheet fall into a permissible range. [Claim 12] The method for cooling a hot-rolled steel sheet according to Claim 1 or 2, wherein the sheet-threading speed of the hot-rolled steel sheet in the cooling section is set in a range of 550 m/min to a mechanical limit speed. [Claim 13] The method for cooling a hot-rolled steel sheet according to Claim 12, wherein a tensile strength of the hot-rolled steel sheet is 800 MPa or more. [Claim 14] The method for cooling a hot-rolled steel sheet according to Claim 12, wherein the finishing mill is constituted by a plurality of rolling stands, and a supplementary cooling process in which the hot-rolled steel sheet is supplementarily cooled between the plurality of rolling stands is further provided. [Claim 15] The method for cooling a hot-rolled steel sheet according to Claim 1 or 2, wherein a top side cooling apparatus having a plurality of headers that ejects cooling water to a top surface of the hot-rolled steel sheet and a bottom side cooling apparatus having a plurality of headers that ejects cooling water to a bottom surface of the hot-rolled steel sheet are provided in the cooling section, and - 83 - c the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling are adjusted by carrying out onofif control of the respective headers. [Claim 16] The method for cooling a hot-rolled steel sheet according to Claim 1 or 2, wherein a top side cooling apparatus having, a plurality of headers that ejects cooling water to a top surface of the hot-rolled steel sheet and a bottom side cooling apparatus having a plurality of headers that ejects cooling water to a bottom surface of the hot-rolled steel sheet are provided in the cooling section, and the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling are adjusted by controlling at least one of sprayed water density, pressure and water temperature of each of the headers. [Claim 17] The method for cooling a hot-rolled steel sheet according to Claim 1 or 2, wherein cooling in the cooling section is carried out at a^mperature of the hot-rolled steel sheet in a range of 600°C or higher.