DESCRIPTION COOLING APPARATUS OF HOT STEEL PLATE, COOLING METHOD OF HOT STEEL PLATE, AND PROGRAM BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for controlled cooling of hot steel plate obtained by hot rolling while horizontally conveying the plate constrained by constraining rolls, more particularly relates to a cooling apparatus of hot steel plate enabling continuous wide range control of the cooling ability, a cooling method of hot steel plate, and a program. 2. Description of the Related Art In order to improve the mechanical properties, workability, and weldability of steel plate, for example the general practice has been to acceleratedly cool a steel material in a high temperature state immediately after hot rolling while conveying the plate on a rolling line so as to give a predetermined cooling history to the steel material. The required cooling ability differs according to the type, purpose, etc. of the steel material. Development of a cooling apparatus enabling selection of the range of control of the cooling ability with a good precision and in a broad range is demanded. As a cooling apparatus able to control the cooling ability in a broad range, there is a cooling apparatus using two-fluid (air and water) nozzles. However, two-fluid nozzles have complex nozzle structures, so easily become clogged, therefore the production cost and maintenance cost of the apparatus become high. Further, pressure control of the air and/or water is complex and it is difficult to maintain the air/water ratio constant. The cooling ability changes according to this air/water ratio. In this way, the above-described cooling apparatus has the problem that sophisticated control and maintenance of equipment are necessary in order to accurately control the cooling ability. On the other hand, when using spray nozzles, the cooling ability can be controlled by adjusting the nozzle water amounts, but if the nozzle load pressures become small, it becomes impossible to secure a variety of spray patterns, therefore the range of control of the cooling ability becomes narrower in comparison with the case of using two-fluid nozzles. Further, as a method of controlling the cooling ability, Japanese Patent Publication (A) No. 10-216821 shows a method of dividing the cooling apparatus into a plurality of cooling blocks in a transfer direction of the steel plate and controlling the supply of cooling water to each cooling block to turn on/off in units of individual cooling blocks or units of pluralities of cooling blocks. In this case, however, in a cooling block where the supply of the cooling water is turned on, the cooling rate near the steel material surface instantaneously becomes very large, therefore the hardness near the surface rises and, according to the type of the steel material, the required elongation of the steel material can no longer be secured. Further, Japanese Patent Publication (A) No. 10-291019 shows a method of controlling the cooling ability, in a cooling apparatus cooling steel plate by running cooling water along its longitudinal direction, by moving the point where the cooling water contacts the steel plate along the longitudinal direction of the steel plate so as to change a contact length of the cooling water and the steel plate. However, this is a method of spraying a gas into a space between the steel plate and the cooling water to move the contact point, therefore, since a gas has a smaller density in comparison with the water, a very large flow rate is needed, so the running cost becomes high. As a method of controlling the cooling ability of steel shapes, Japanese Patent Publication (A) No. 7-157826 shows a method of controlling the cooling performance over a broad range by adjusting the spray pitch of cooling water from cooling water nozzles aligned in the steel material conveyance direction, but in this case as well, a pitch adjustment mechanism of the cooling water nozzles becomes necessary, therefore there is a problem that the production cost and maintenance cost of the cooling apparatus become high. SUMMARY OF THE INVENTION The present invention was made to solve the above problems, relates to an apparatus for controlled cooling hot steel plate while constraining and conveying the plate by constraining rolls horizontally, and has as an object thereof to propose an inexpensive cooling apparatus of hot steel plate, a cooling method of the hot steel plate, and a program enabling continuous control of the cooling ability over a broad range. A cooling apparatus of the present invention is a cooling apparatus of hot steel plate provided with a plurality of pairs of constraining rolls for constraining and conveying hot steel plate horizontally and spraying the top and bottom surfaces of the hot steel plate between adjoining pairs of constraining rolls with cooling water from respective pluralities of lines of spray nozzles so as to cool the hot steel plate, said cooling apparatus of hot steel plate characterized by having lines of gentle cooling spray nozzles each having a small cooling water impact pressure integrated value, defined as the value of the n power of the cooling water impact pressure integrated between a pair of constraining rolls in the conveyance direction, and lines of strong cooling spray nozzles each having a large cooling water impact pressure integrated value and by making the maximum cooling water impact pressure integrated value of said lines of gentle cooling spray nozzles and the minimum cooling water impact pressure integrated value of said lines of strong cooling spray nozzles equal and connecting the fluctuation regions of cooling water impact pressure integrated values of the two types of lines of spray nozzles, where, 0.05