BACKGROUND OF THE INVENTION The present invention relates to a coiling temperature apparatus for a hot rolling line and a method of controlling the apparatus, and more particularly, to a coiling temperature control system which can suitably make a coiling temperature to coincide with a target temperature through simple and easy calculation. As one of priorart methods of performing coiling temperature control, there is disclosed, for example, in JP-A-8-66713 a method of setting a cooling pattern on the basis of information about a strip according to a predetermined strip processing speed pattern prior to start of cooling operation and then performing coiling temperature control according to the processing rate or speed of the roiling material being rolled. JP-A-2000-167615 also discloses a coiling temperature control method of dividing a strip in its longitudinal direction under control of a rolling material cooling device, predicting a temperature of each of the divided strip divisions as a material cooling unit, and making the predicted temperature to coincide with a target temperature. Also disclosed in JP-A-2000-167615 are a coiling temperature control method and apparatus which can determine a quantity of cooling water on a real time basis by receiving a change in the temperature, of the roilling material or a change in the inlet side temperature of a transportation table, and which can reduce the influence of an external disturbance by operating a valve according to the determined cooi ing water quantity. JP-A-2007-118027 discloses a method of dividing a strip in its longitudinal direction, introducing a control code according to a header open/close pattern, and finding a control code for each of the strip divisions. A technique similar to the above technique is also disclosed an JP-A-2007-181870 and JP-A-2007-237285. In these techniques, however, no consideration is paid to efficient selection of suitable one of many cooling patterns (cooling header combinations) as a very large number of combinations or to reduction of a quantity, of calculation necessary for the selection. These techniques disadvantageously involve reduction in the control accuracy of a coiling temperature or an enormous amount of calculation time required for the determination of the cooling pattern. In JP-A-8-66713, good control. can be obtained in a stationary section having a constant speed, but the strip speed of the rolling material is generally low when the strip is paid ,out from mill. Thereafter, the threading speed of the strip is gradually increased, and the increa)sing speed of the strip is enhanced after start of the strip to be coiled around a down coiler. After the threading speed reaches a constant level, the speed again becomes low directly before the strip fed out from mill rolls. In such a transition section that the strip speed varies, the speed is not associated directly with the coiling temperature pattern. For this reason, the technique JP-A-8-66713 for changing the cooling pattern by detecting the speed has a Iproblem that the accuracy of coiling temperature control is low. JP-A-8-66713 also fails to disclose a specific technique for determining the coiling temperature pajttern on the basis of the speed pattern. Thus, no donsideration is paid even to reduction of an amount of [calculation required for the coiling temperature patterjn determining technique. In the control miethod disclosed in JP-A-2000-167615, the division unit [for predicting the temperature of the rolling material depends on the size of a cooling device. Thusi the control method has a problem that the division accuracy becomes lower than a required value. In the technique disclosed in JP-A-2007-118027, the introduction of the control code enables determination of the cooliilig pattern through simple calculation. However, JP-A-2007-118027 fails to disclose a method of determining a time step for differential calculation to have a suitable value. Since coiling temperatures are calculated for all sections of the strip divided in is longitudinal direction to find the header pattern, this disadvantageously still requires a very large amount of calculation. SUMMARY OF THE INVENTION In view of the above problems in the prior arts, it is therefore an object of the present invention to provide a coiling temperature control apparatus and method whiqh can efficiently select suitable one of cooling patterns as a very large number of combinations to accurajtely control a coiling temperature, and can reduce a calculation time required for calculation of the cooling patterns In accordance with an aspect ot the present invention, the object can be attainedby providing a coiling temperature control apparatus which includes a time step determining means (or portion) for receiving a strip speed pattern and' calculating a time step for cooling model calculation to attain a required calculation accuracy in preset calculaton of coiling temperature control, and al includes a calculation section determining means (or portion) for determining target one of sections for calculation of a control code by estimating a coiling temperature from a strip speed pattern. The coilingtemperature control apparatus also includes a control code interpolating means (or portion) for determining a control code for another section having a control code not: directly calculated still on the basis of the control code of the target section. The time step determining means sets the time step of the cooling model calculation at a maximum value in such a range as to obtain a required accuracy according to the strip speed. This can avoid such a situation that the time step becomes excessively large and a calculation time therefor becomes too large, or' that the time step becomes excessively small and the predicted accuracy of the,coiling temperature is reduced. Since the calculation section determining means sets the number of sections for calculation of the coiling temperature at a necessary minimum value with use of a change in the strip speed as an index, the number of calculation times necessary for the coiling temperature can also be decreased. As a result, the coiling temperature can be controlled based on preset control with a less amount of calculation while avoiding reduction of a coiling temperature accuracy in the longitudinal direction of the strip. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an arrangement ot a coiling temperature control apparatus in a hot rolling system in accordance with an embodiment; FIG. 2 shows a structure of a speed pattern table; FIG. 3 shows a structure of a target coiling temperature table; FIG. 4 shows a structure of a cooling header priority table; FIG. 5 shows an example of allocating control codes and header patterns; FIG- 6 shows a flow chart showing processing operation of a time step'determiining means; FIG. 7 shows a flow chart showing processing operation of a calculation section determining means; FIG. 8 shows a flow chart showing processing operation of a control code determining means; FIG. 9 shows a flow chart showing strip temperature estimating operation; FIG. 10 shows tables for explaining a process of determining control codes; FIG. 11 shows a' flow chart showing processing operation of a control code interpolating means; FIG. 12 is a diagram for explaining a distribution of a control code calculating method in an entire strip; FIG. 13 shows control code calculated results; FIG. 14 shows processed results of a control code smoothing means; FIG. 15 is a fl6w chart showing processing operation of a header pattern converting means; FIG. 16 is a flow chart showing processing operation of a time step determining means in an embodiment 2; FIG. 17 shows a structure of the speed pattern table; FIG. 18 is a flow chart showing processing operation of a calculation section determining means m embodiment 3; FIG. 19 is a flow chart showing processing operation of a control code interpolating means in the embodiment 3, and FIG. 2 0 shows a, diagram for explaining a user interface means in an embodiment 4. DESCRIPTION OF THE EMBODIMENTS In accordance with an embodiment or the present invention, in the control of the coiling temperature of a strip after hot-rolled, an accurate coiling temperature can be obtained even at any site of the strip in its longitudinal direction. As a result, the composition quality of the strip can be improved and at the same time, the coiling strip can have a nearly flat shape. Explanation will be made in connection with a plurality of embodiment, by referring to drawings. Embodiment 1: FIG. 1 shows an embodiment of a coiling temperature control system in accordance with an embodiment. A coiling temperature control apparatus 100 receives various types of signals from a control target 150 and outputs a control signal to the control target 150. Explanation will first Joe made as to the arrangement of the control target 150. In this embodiment, the control target 150 is a facility for controlling the coiling temperature of a strip being hot-rolled. A strip 151 of a temperature of about 900°C rolled between mill rolls 157 of a rolling mill 152 is cooled by a coiling/cooling device 153 and then coiled around a down coiler 154. Since about 7 mill rolls are used for continiuous rolling in tandem rolling, the illustrated mill rolls 157 correspond to a final stand. Though one stand reciprocation rolling may be used as in a steckel mill,, the present invention can be applied to both of The above cases. The coiling/cooling device 153 includes an upper cooler 158 for cooling the strip 151 with water from its upper side and a lower cooler 159 for cooling the strip 151 with water .from its lower side. Each of the coolers includes a plurality of banks 161 having several cooling headers 160 for ejecting water as combinations. In the present embodiment, explanation will be made in connection with an example when an operational command to each cooling header 160 is "OPEN" or "CLOSE". A mill output (br delivery)-side thermometer 155 measures a temperature of a strip directly after rolled by the rolling mill 152, and a coiling temperature thermometer 156 measures a temperature of the strip directly before coiled around the down coiler 154. The purpose of coiling temperature control is to make a temperature measured by the. coiling temperature thermometer 156 to coincide with a target temperature. The target temperature may be set to be constant at various sites in the coil longitudinal direction or may be set at different values at different sites in the leading end of the strip. An arrangement of the coiling temperature control apparatus 100 will next be explained. The coiling temperature control apparatus 100 includes a preset control means (or portion) 110 which, before the strip 151 is cooled by the coiling/cooling device 153, calculates a control code corresponding to an open/close pattern of each cooling header 160 for each of sections of the strip longitudinal direction equally divided by a suitable length. The coiling temperature control apparatus 100 also includes a dynamic control means (or portion) 120 for receiving achievements or measurements including the measured temperature of the coiling temperature thermometer 156 on a real time basis and changing the control code when the strip 151 is being cooled in the coiling/cooling device 153. The coiling temperature control apparatus 100 also includes a header pattern converting means (or portion) 130 for converting the control code to an open/close pattern of each cooling header 160. A set of the open/close patterns of the cooling headers 160 will be referred to as a header pattern, hereinafter. The preset control means 110 .includes a control code calculating means (or portion; 117 which receives information from a speed pattern table 111, a target coiling temperatune table 112, and a cooling header priority table 113,, and which calculates a header pattern through calculation using a strip temperature estimating model 114. The preset control means 110 further includes a time step determining means (or portion) 115 which receives the speed pattern of the strip 151 from thej speed pattern table 111 and determines the time step of the coiling temperature estimating calculation of the control code calculating means 117, and also includes a calculation section determining means (or portion) 116 which determines a target section for a control code to be calculated according to the result of the time step determining means 115. The preset control means 110 includes a control code interpolating means (or portion) 118 which determines the control code of the section not calculated on the basis of an output from the control code calculating means 117, and also includes a control code smoothing means (or portion) 119 which finely corrects the control code with regard co the calculated result of the control code of each finally-obtained section in such a manner as to smooth the output of the header pattern with time. The dynamic control means 120 includes a coiling temperature deviation correcting means (or portion) 121 for correcting a deviation of the detected temperature of the coiling temperature thermometer 2 56 from the target temperature, a mill output-side temperature deviation correcting means (or portion) 122 for correcting a deviation of the detected temperature of the mill output-side thermometer 155 from, a mill output side temperature set upon the preset control calculation, and a speed deviation correcting means (or portion) 123 for calculating the threading speed of the strip 151 from the rotational speeds of the mill rolls 157 and the down coiler 154 and correcting a deviation of the calculated result from the strip speed set during the preset control calculation. FIG. 2 is a structure of the speed pattern table 111, showing speed patterns when the rolling mill 152 is a tandem mill as an example. Recorded to be stratified in the illustrated table are a steel type, a strip thickness, a strip width, a speed (initial speed) when the leading end of the strip 151 is paid out from the mill rolls 157, an acceleration (first acceleration) until the leading end of the strip 151 is later coiled around the down coiler 154, an acceleration (second acceleration) until the leading end reaches a maximum speed, a maximum speed, a deceleration when the speed is deceleraited from the maximum speed to a final speed, and the final speed, with respect to different'strip types. The speeds are expressed in the unit m/min (mpm). The time step determining means 115, the calculation section determining means 116, and the control code calculating means 117 judge the steel type, thickness and width of the coil, and extract a corresponding speed patte,rn from the speed pattern table 111. For example, when the steel type is SUS3 04, the strip thickness is between 3.0 and 4.0 mm and the strip width is 1200 mm; 670 mpm, 2 mpm/s, 12 mpm/s, 1000 mpm, 6mpm, and 900 mpm are extracted as the initial speed, the first 'acceleration the second acceleration, the maximum speed, the deceleration, and the final speed, respectively, FIG. 3 shows a,structure of the target coiling temperature table 112, Shown ,i n FIG. i is an example when target temperatu.res are stratified according to the types ($teel types) of strips. The control code calculating,means 117 determines the steel type of the corresponding coil and extracts a corresponding target temperature from the target coiling temperature table 112. FIG. 4 shows a structure of the cooling header priority table ll3. Explanation will be made in connection with an example when the total number of headers is 100. Priorities 1 to 100 are applied to theheaders in an order of opening of 100 headers, More specifically, recorded in the table are a steel type, strip thickness, header division (upper or lower header), and priority of cooling headers to be preferentially opened. The priority 14 determined according to the temperature drop pattern of the strip 15l, a cooling efficiency, an allowable temperature difference between the surface of the strip and the interior thereof, and so on. For example, when the strip 151 is thin, a temperature difference between the surface of the strip and the interior thereof is very small, headers close to the output (or delivery) side of the mill rolls 157 having the strip 151 of a high temperature are preferentially opened from the viewpoint of the cooling efficiency. When the strip 151 is thick, the temperature difference between the strip surface and interior is suppressed within an allowable value range utilizing the recuperation of air cooling. To this end, priorities are applied to the headers in such a manner that the opened headers are not opened as continually as possible. In this way, the priorities are stratified and determjined according to the steel type or the strip thickness. Some steel types require middle temperature sustenance for a constant time, in which case, header priority order is applied so as to provide former half cooling, middle temperature sustenance, and latter half cooling. The cooling headers are controlled so that only ones of the cooling headers capable of providing a target coiling temperature are opened. Numbers are applied to the banks and the cooling headers in an increasing order of a distance from the mill rolls 157. For example, (1, 1) denotes a first coo.Ling header of a first bank. In the drawing, when the steel type is SUS304, the strip thickness is between 2.0 and 3.0 mm, and the cooling header division is an upper header,- the headers are preferentially opened in an order of (1, 1), (1, 2), (1, 3), (L, 4), (I, 5), (2, ,1.) (20, 4), and (20, 5) . This means that the headers are preferentially opened in an increasing order of the distance from the output side of the mill rolls 15'?. When the steel type is SUS304, the steel thickness is between 5.0 and 6.0 mm, the cooling header division is an upper header; the headers are preferentially opened in an order of (1, I), (1, 4), (2, 1), (:?, 4), (3, l), (3, 4),..., (20, 3), and (20, 5). In other words, since the strip 151 is thiick, priorities are applied to the headers so that opened headers are not continually. Although the priority order of the upper headers is set to be the same as the priority order of the lower headers in the present embodiment, different priorities may be applied to the upper and lower headers. In the present Invention, the header pattern is expressed by a control code. FIG. 5 shows a correlation between the control code and cooling header open/close patterns. A control code 0 means 'fully opened', and a control code 100 means 'fully closed'. In this manner, control codes are given so that a header open/close patterni having an opened cooling headers of only a priority 1 is denoted by reference numeral 99, a header open/close pattern having two opened cooling headers of priorities l and 2 are denoted by numeral 98, and so on. That is, a control code indicative of a state when all the cooling headers are opened, is assumed to be denoted by 0, whereas a control code indicative o|f a state when all the cooling headers are closed, is assumed to be denoted by 100 (a total number of upper and lower cooling headers). For example, when the steel type is SUS304, the strip thickness is between 2.0 and 3.0 mm, and the cooling header division is an upper header; a control code indicative of a state when the header of a priority (1, 1) is opened, is denoted jby 99; a control code indicative of a state when the headers of priorities (1, 1) and (1, 2) are opened, is denoted by 98; a control code indicative Ojf a state when the headers of priorities (1, 1), (1, 2), and {1, 3), is denoted by 97, and so on according to the priorities of the headers. In this manner, a last control code indicative of a state whein all the headers are opened, is denoted by 0, FIG. 6 shows an algorithm when the time step determining means 115 exelcutes. In a step S6-1, the time step determining means receives a maximum speed from the speed pattern table 111, In a step S6-2, the time step determining means calculates a standard time step TS from the maximum speed and a header pitch in the longitudinal direction of the strip according to an equation (1). (Equation Removed) Wherein, Lh denotes a header pitch, and Vmax denotes a strip maximum speed. In a step S6-3, the time step determining-means calculates a time step Tc for a differential calculation according to an equation (2) from the standard time step TS. (Equation Removed) Wherein, α1 denotes a constant which is set at 1 or a value smaller than 1 to secure the accuracy of coiling temperature calculation. When it is desired to shorten a calculation time by allowing somewhat accuracy reduction, it is' considered to set αl at a value not smaller than 1. The strip maximum speed is generally determined by the thickneiss of the strip, the strip width, and the steel type. In the example of FIG. 2, when the steel type is SUS304, the strip thickness is between 2.0 and 3.0 mm, and the strip width is 900 mm; the maximum speed is 1100 mpm (which will be referred to as the case 1, hereinafter). When the steel type is SUS3 04, the strip thickness is 12.0 or larger, and the strip width is 1200 mm,- the maximum speed is 400 mpm (which will be referred to as the case 2. hereinafter) . When αl = l and Lh=300 mm in the equations (1) and (2), TC=0. 0164s in the case l and Tc=C.045s in che case 2. Hence, when compared with the case when TC is fixed at 0.0164, the case 2 can reduce an amount of calculation for strip temperature prediction to about 1/3 while avoiding reduction of the calculation accuracy. Conversely when compared with the case when ATC is fixed at 0.045, the case 1 can prevent reduction of the calculation accuracy in the case '2. In this way, in accordance with the present invention, when the calculation step necessary for securing the calculation accuracy is made different for different strip speeds, the coiling temperature cpntrol apparatus can be optimized with the reflected strip speed and can minimize an amount of calpulation while avoiding reduction of temperature calculation accuracy. In some arrangetnent of the coiling/cooling device, there is a case where the header pitch is not uniform due to interferende with the table roller or the like. In this case, the minimum header pitch may be used as Lh for determiming the calculation accuracy. Or an average header pitch or the like may be considered to be used as a representative value, FIG. 7 shows algorithm when the calculation section determining meang 116 executes. The calculation section determining means 1116, in addition to a fixed number of sectiions at the leading end of the strip, sets a section under the mill rolls 157 as a calculation target at timing corresponding to speed pattern change point of the strip. In the leading and trailing ends of the strip, for the purpose of increasing a strip coiling or take-up property to the down coiler 154 of the strip 151, the target coiling temperature is set, in some cases, at a high level according to the site. To this end, in a step S7-1, the Calculation section determining means 116 sets a fixed number of sections from the leading end of the strip as calculation targets. The fixed number may be determined based on a range when the target temperature is different from the stationary portion of the strip. The fixed number is usually set in a range covering several tens of meters from the strip leading end. In a step S7-2, ,the calculation section determining means 116 caldulates a first acceleration end section number and set's it as a calculation target, The first acceleration end section number SLla is calculated according to an, equation (3) (Equation Removed) Where, Lmd denotes a distance from the mill rolls 157 to the down coiler 154, and Seclen denotes a section length. In a step S7-3, the calculation section determining means 116 calculates a first acceleration end section number and sets it as a calculation target. The control code smoothing means 1l9 calculates a second acceleration end section number SL2a according to equations (4) and (5). (Equation Removed) Where, V1a deno|tes a first acceleration end speed, Accl denotes a first acceleration, Acc2 denotes a second acceleration, and Vmax denotes a maximum speed. In a step S7-4, the calculation section determining means 116 calculates a deceleration start section number and sets it as a calculation target. The calculation section determining means 116 calculates a deceleration start section number SLds according to an equation ;(6) , (Equation Removed) Where, Striplen denotes a strip length, Vf denotes an end speed, Dcc denotes a deceleration, and dccmargin denotes a margin indicative of how long in front the deceleration of. the strip 151 is completed before the pay out of the trailing end of the strip 151 In a step S7-5, the calculation section determining means 116 calculates a deceleration end section number and sets it as a calculation target. The calculation section determining means 116 calculates a deceleratiorl end section number SLde according to an equation (7). (Equation Removed) In a step S7-6, the calculation section determining means 116 sets a fixed number of sections from the trailing end of the strip, and sets them as calculation targets. The fixed number may be determined based on a range of a portion of the strip having a temperature different from the stationary portion of the strip, as in the case of the leading end of the strip in step S7-1, and usually may be set in a portion of the strip covering several tens of meters from the leading end. Through the aforementioned calculation, each of sections defined for the full length of the strip 151 has been extracted as a section as the calculation target: of the control code. When section numbers as the adjacent calculation targets are largely different from each other as a result of execution of the algorithm of FIG. 7, a section in the vicinity of an intermediate between the adjacent sections may be included as a calculation, target section to increase accuracy. FIG. 8 shows algorithm when the control code calculating means 117 exebutes. With respect to a section determined as a calculation target by the calculation section determining means 116, the control code calculating means 117 calculates a header pattern providing a tax-get coiling temperature in the format of control code through calculation using the strip temperature estimating model 114. In the present embodiment, an example when a control code is calculated by linear inverse interpolation is shown. In a step S8-1, first, the control code calculating means 117 defines such two control codes nL, nH as to sandwich a solution control code in. sections defined as the calculation target of the strip 151. Since a solution is present between the fully-opened and the fully-closed states of the cooling headers in this example, nL=0 and nH=100 are set equally. An increase of the control code causes simple decrease of the number of opened cooling headers. Thus, when nlTtarget, the control code calculating means 117 sets nO newly at nH because a I solution is present between no and nL. Conversely, when TcO