Patent Publication Number: US-11033942-B2

Title: Rolling mill exit side temperature control system

Description:
FIELD 
     The present disclosure relates to a rolling mill exit side (delivery side) temperature control system. In particular, the present disclosure relates to a hot rolling mill exit side temperature control system. 
     BACKGROUND 
     In a hot rolling line, controlling the temperature of a material to be rolled to maintain a target temperature on the exit side of a hot rolling mill is an important matter to ensure excellent quality of the material to be rolled. 
     A hot rolling mill exit side temperature control system disclosed in, for example, JP H10-277627 A (PTL 1) is well-known. The hot rolling mill includes a plurality of rolling stands rolling the material to be rolled, and a cooling spray system to spray cooling water onto the material to be rolled in the spaces between the rolling stands. Typically, the cooling spray system includes a spray nozzle at a downstream end of a cooling water passage, an openable spray valve upstream of the spray nozzle, and a butterfly valve upstream of the spray valve that can adjust a flow rate per unit time. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP H10-277627 A 
     SUMMARY 
     Technical Problem 
       FIG. 7  is a timing chart to explain the conventional temperature control by the above-described cooling spray system. Time t 1  is the timing when the material to be rolled reaches the rolling mill. Time t 2  is the timing of the cooling instruction for discharging cooling water. At the time t 2 , the butterfly valve is open (line  82 ), and the spray valve is switched from a closed state (OFF) to an open state (ON) (line  81 ). In other words, the spray valve on the nozzle side is opened at the same timing as the cooling instruction. At this time, in addition to an instructed amount of cooling water, cooling water remaining in the cooling water passage between the butterfly valve and the spray valve is also discharged. Accordingly, a larger amount than the instructed amount of cooling water gets on the material to be rolled, and the material to be rolled is rapidly cooled. As a result, the accuracy of the temperature control deteriorates, which increases temperature variation of the material to be rolled and influences the accuracy of thickness control. 
     The present disclosure is made to solve the above-described issues, and an object of the present disclosure is to provide a rolling mill exit side temperature control system that makes it possible to suppress rapid cooling of the material to be rolled to improve accuracy of the temperature control, and to improve accuracy of thickness control. 
     Solution to Problem 
     To achieve the above-described purpose, according to the present disclosure, a rolling mill exit side temperature control system including a plurality of rolling stands rolling a material to be rolled, includes a cooling apparatus provided in at least one of a plurality of spaces between the rolling stands, and a cooling apparatus control unit controlling the cooling apparatus. The cooling apparatus includes a spray nozzle spraying coolant onto the material to be rolled, a coolant passage supplying the coolant to the spray nozzle, a first valve provided in the coolant passage upstream of the spray nozzle and being changeable to an open state or a closed state, a first valve control unit controlling the first valve to the open state or the closed state, a second valve provided in the coolant passage upstream of the first valve and being changeable in valve opening, a flow rate detector detecting a flow rate of the coolant flowing through the coolant passage upstream of the second valve, and a second valve control unit controlling the valve opening of the second valve to cause a flow rate actual value detected by the flow rate detector to coincide with a flow rate target value. The cooling apparatus control unit includes a remaining coolant discharging section controlling the first valve to the open state and the second valve to the closed state by setting the flow rate target value to zero before the material to be rolled reaches the rolling mill, and a flow rate target value setting section setting the flow rate target value to a value corresponding to a target temperature of the material to be rolled on the entry side and the exit side of the rolling mill after the control by the remaining coolant discharging section. 
     Advantageous Effects of Invention 
     According to the present disclosure, before the material to be rolled next reaches the rolling mill, the first valve is controlled to the open state and the second valve is controlled to the closed state, which makes it possible to discharge the coolant remaining in the coolant passage downstream of the second valve, at a timing when the coolant will not get on the material to be rolled. Thereafter, the flow rate target value corresponding to the target temperature of the material to be rolled is set, and the instructed amount of the coolant is sprayed onto the material to be rolled. As a result, according to the present disclosure, it is possible to suppress rapid cooling of the material to be rolled to improve accuracy of temperature control, and to improve accuracy of thickness control. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram to explain a configuration of an exit side temperature control system according to a first embodiment of the present disclosure. 
         FIG. 2  is a timing chart to explain the temperature control of the system. 
         FIG. 3  is a flowchart of a control routine executed by a cooling apparatus control unit  30  according to the first embodiment of the present disclosure. 
         FIG. 4  is a conceptual diagram to explain a configuration of an exit side temperature control system according to a second embodiment of the present disclosure. 
         FIG. 5  is a flowchart of a control routine executed by a cooling apparatus control unit  60  according to the second embodiment of the present disclosure. 
         FIG. 6  is a diagram illustrating a hardware configuration example of a processing circuit included in the cooling apparatus control unit  30  or  60 . 
         FIG. 7  is a timing chart to explain conventional temperature control by a cooling spray system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure are described in detail below with reference to the drawings. Note that components common to the drawings are denoted by the same reference numerals and overlapped description of the components is omitted. 
     First Embodiment 
     &lt;Entire Configuration&gt; 
       FIG. 1  is a conceptual diagram to explain a configuration of an exit side temperature control system according to a first embodiment of the present disclosure.  FIG. 1  illustrates a part of a hot rolling line. The hot rolling line includes a rolling mill  10 . The rolling mill  10  is, for example, a hot rolling mill. The hot rolling mill is, for example, a roughing mill or a finishing mill. In the following description, it is assumed that the rolling mill  10  is a finishing mill as an example. 
     The rolling mill  10  includes a plurality of rolling stands rolling a material to be rolled  2 . Some of n rolling stands arranged in tandem are illustrated in  FIG. 1  (n&gt;1, n is natural number). More specifically, a first rolling stand  11  disposed at the most upstream, an n−1-th rolling stand  13 , and an n-th rolling stand  14  disposed at the most downstream are illustrated. 
     &lt;Cooling Apparatus&gt; 
     A cooling apparatus is provided in at least one of a plurality of spaces between the rolling stands. The cooling apparatus is a cooling spray system to spray coolant toward the material to be rolled  2 . In  FIG. 1 , a cooling apparatus  20  provided in a space between the n−1-th rolling stand  13  and the n-th rolling stand  14  is illustrated. 
     The cooling apparatus  20  includes spray nozzles  21  (upper spray nozzle  21   a  and lower spray nozzle  21   b ), a coolant passage  22 , first valves  23  (upper spray valve  23   a  and lower spray valve  23   b ), a first valve control unit  24 , a second valve  25 , a flow rate detector  26 , and a second valve control unit  27 . 
     The upper spray nozzle  21   a  is a spray nozzle to spray the coolant onto an upper surface of the material to be rolled  2 . The lower spray nozzle  21   b  is a spray nozzle to spray the coolant onto a lower surface of the material to be rolled  2 . In the following description, in a case where it is unnecessary to distinguish the upper spray nozzle  21   a  and the lower spray nozzle  21   b  from each other, the spray nozzles are simply referred to as the spray nozzles  21 . The spray nozzles  21  are connected to respective downstream ends of the coolant passage  22 . The spray nozzles  21  are disposed between the n−1-th rolling stand  13  and the n-th rolling stand  14 . 
     The coolant passage  22  is a pipe to supply the coolant to the spray nozzles  21 . Examples of the coolant include cooling water, cooling oil, and other solutions. 
     The upper spray valve  23   a  is provided on the coolant passage  22  upstream of the upper spray nozzle  21   a , and is changeable to an open state or a closed state. The lower spray valve  23   b  is provided on the coolant passage  22  upstream of the lower spray nozzle  21   b , and is changeable to an open state or a closed state. In the following description, in a case where it is unnecessary to distinguish the upper spray valve  23   a  and the lower spray valve  23   b  from each other, the valves are simply referred to as the first valves  23 . 
     The first valve control unit  24  controls the open/closed states of the first valves  23 . More specifically, the first valve control unit  24  controls the first valves  23  to the open state in response to an ON signal from the cooling apparatus control unit  30 , and controls the first valves  23  to the closed state in response to an OFF signal from the cooling apparatus control unit  30 . 
     The second valve  25  is a butterfly valve that is provided on the coolant passage  22  upstream of the first valves  23  and a valve opening thereof is changeable. Amount and pressure of the coolant are adjusted according to the valve opening. 
     The flow rate detector  26  is a flow transducer that detects a flow rate per unit time of the coolant flowing through the coolant passage  22  upstream of the second valve  25 . 
     The second valve control unit  27  controls the valve opening of the second valve  25  such that a flow rate actual value detected by the flow rate detector  26  coincides with a flow rate target value (closed loop control). The flow rate target value is provided through the cooling apparatus control unit  30 . The second valve control unit  27  changes the valve opening of the second valve  25  based on a difference between the flow rate actual value and the flow rate target value. For example, in a case where the flow rate target value is set to zero, the valve opening is controlled so as to be fully closed (opening 0%). 
     &lt;Cooling Apparatus Control Unit&gt; 
     The system illustrated in  FIG. 1  includes the cooling apparatus control unit  30  that controls the cooling apparatus  20 . The cooling apparatus control unit  30  is used to cool the temperature of the material to be rolled  2  on the exit side of the rolling mill  10  to a target temperature. A tracking apparatus  3 , a host computer  4 , a rolling mill entry side temperature sensor  5 , a rolling mill exit side temperature sensor  6  are connected to the input side of the cooling apparatus control unit  30 . The first valve control unit  24  and the second valve control unit  27  are connected to the output side of the cooling apparatus control unit  30 . The cooling apparatus control unit  30  sequentially receives signals from the tracking apparatus  3 , the host computer  4 , the rolling mill entry side temperature sensor  5 , and the rolling mill exit side temperature sensor  6 . 
     The tracking apparatus  3  outputs tracking information including a head end position and speed of the material to be rolled  2 . 
     The host computer  4  outputs an entry side temperature target value that is the target temperature of the material to be rolled  2  on the entry side of the rolling mill  10 , an exit side temperature target value that is the target temperature of the material to be rolled  2  on the exit side of the rolling mill  10 , a speed pattern, a specification of the material to be rolled  2 , and the like. 
     The rolling mill entry side temperature sensor  5  is provided on the entry side of the rolling mill  10  (upstream of the first rolling stand  11 ), and outputs the surface temperature of the passing material to be rolled  2 . In the case of the finishing mill, the finisher entry temperature (FET) is detected. 
     The rolling mill exit side temperature sensor  6  is provided on the exit side of the rolling mill  10  (downstream of n-th rolling stand  14 ), and outputs the surface temperature of the passing material to be rolled  2 . In the case of the finishing mill, the finisher delivery temperature (FDT) is detected. 
     The cooling apparatus control unit  30  includes a remaining coolant discharging section  31  and a flow rate target value setting section  32 . 
     The remaining coolant discharging section  31  controls the first valves  23  to the open state, and controls the second valve  25  to the closed state by setting the flow rate target value to zero, before the material to be rolled  2  reaches the rolling mill  10 . More specifically, when the remaining coolant discharging section  31  provides the ON signal to the first valve control unit  24 , the first valves  23  are controlled to the open state. Further, the remaining coolant discharging section  31  sets the flow rate target value to be provided to the second valve control unit  27 , to zero. As a result, the valve opening of the second valve  25  is controlled to the fully-closed state so as to bring the flow rate actual value close to zero, by the closed loop control. 
     The flow rate target value setting section  32  sets the flow rate target value to a value corresponding to the target temperature of the material to be rolled  2  on the entry side and the exit side of the rolling mill  10 , after the control by the remaining coolant discharging section  31 . When the flow rate target value is changed from zero to a predetermined flow rate target value (&gt;0), the valve opening of the second valve  25  is increased from zero to the opening corresponding to the predetermined flow rate target value, by the closed loop control. 
     The flow rate target value setting section  32  executes feedforward control. In a case where the entry side temperature target value, the exit side temperature target value, and a flow rate reference value of the coolant corresponding to the speed pattern are determined and the entry side temperature actual value detected by the rolling mill entry side temperature sensor  5  is higher than the entry side temperature target value, the flow rate target value setting section  32  sets the flow rate target value to a value larger than the flow rate reference value according to a difference therebetween. In contrast, in a case where the entry side temperature actual value is lower than the entry side temperature target value, the flow rate target value setting section  32  sets the flow rate target value to a value lower than the flow rate reference value according to the difference, by the feedforward control. 
     Further, the flow rate target value setting section  32  starts execution of feedback control at a time when the material to be rolled  2  reaches the rolling mill exit side temperature sensor  6 . The flow rate target value setting section  32  corrects the flow rate target value based on a difference between the exit side temperature actual value and the exit side temperature target value such that the exit side temperature actual value detected by the rolling mill exit side temperature sensor  6  coincides with the exit side temperature target value (PI control). 
     &lt;Timing Chart&gt; 
       FIG. 2  is a timing chart to explain temperature control of the system according to the first embodiment of the present disclosure. Time t 0  is the timing before the material to be rolled  2  reaches the rolling mill  10 . Time t 1  is the timing when the material to be rolled  2  reaches the rolling mill  10 . Time t 2  is the timing of the cooling instruction. 
     At the time t 0 , the first valves  23  are controlled to the open state (line  71 ). In addition, at the time t 0 , the flow rate target value is set to zero (line  73 ). When the flow rate target value is set to zero, the valve opening of the second valve  25  is controlled to the fully-closed state so as to bring the flow rate actual value close to zero, by the closed loop control (line  72 ). In other words, before the material to be rolled  2  reaches the rolling mill  10 , the first valves  23  are controlled to the open state and the second valve  25  is controlled to the fully-closed state, and the coolant remaining in the coolant passage  22  downstream of the second valve  25  is discharged from the spray nozzles  21 . Since the coolant is discharged before the time t 1 , the coolant does not get on the material to be rolled  2 . 
     At the time t 1 , the material to be rolled  2  reaches the entry side of the rolling mill  10  (line  70 ). At the time t 2 , a new flow rate target value (&gt;0) is set by the flow rate target value setting section  32  (line  73 ). Thereafter, the valve opening of the second valve  25  is controlled to a predetermined opening by the closed loop control based on the set flow rate target value, and the coolant amount corresponding to the flow rate target value is sprayed. 
     &lt;Flowchart&gt; 
       FIG. 3  is a flowchart of a control routine executed by the cooling apparatus control unit  30  to realize the above-described operation. 
     First, in step S 100 , the cooling apparatus control unit  30  determines whether the head end position of the material to be rolled  2  has reached the entry side of the rolling mill  10 , based on the tracking information. In a case where it is determined to be before reaching, the processing of step S 110  is executed next. In a case where it is determined to be after reaching, passage of the material to be rolled  2  is waited for. 
     In step S 110 , the first valves  23  are controlled to the open state. More specifically, the remaining coolant discharging section  31  provides ON signal to the first valve control unit  24 . The first valve control unit  24  receives the ON signal to control the first valves  23  to the open state. Note that, as a condition, spraying of the coolant onto the preceding material to be rolled  2  is already completed. In other words, a tail end of the preceding material to be rolled  2  has already passed through the rolling mill  10  (spray range by cooling apparatus  20 ). 
     Next, in step S 120 , the second valve  25  is controlled to the closed state by setting the flow rate target value to zero. More specifically, the remaining coolant discharging section  31  sets the flow rate target value of the second valve control unit  27  to zero. The second valve control unit  27  controls, by the closed loop control, the valve opening of the second valve  25  to the fully-closed state such that the flow rate actual value becomes zero. As a result of the processing in step S 110  and step S 120 , the coolant remaining in the coolant passage  22  downstream of the second valve  25  is discharged from the spray nozzles  21 . 
     Next, in step S 130 , the flow rate target value (&gt;0) corresponding to the target temperature of the material to be rolled  2  is set. More specifically, the flow rate target value setting section  32  sets the flow rate target value to a value corresponding to the target temperature of the material to be rolled  2  on the entry side and the exit side of the rolling mill  10 , after execution of the processing in step S 120 . As a result, the valve opening of the second valve  25  is controlled to the predetermined opening by the closed loop control based on the set flow rate target value, and the coolant amount corresponding to the flow rate target value is sprayed. 
     &lt;Effects&gt; 
     As described above, according to the routine illustrated in  FIG. 3 , before the material to be rolled  2  next reaches the rolling mill  10 , the first valves  23  are controlled to the open state and the second valve  25  is controlled to the closed state, and the coolant remaining in the coolant passage  22  downstream of the second valve  25  can be discharged at the timing when the coolant does not get on the material to be rolled  2 . Thereafter, the flow rate target value corresponding to the target temperature of the material to be rolled  2  is set, and the instructed amount of the coolant is sprayed onto the material to be rolled  2 . Therefore, according to the system of the present embodiment, it is possible to reduce disturbance, to suppress rapid cooling of the material to be rolled  2 , and to control the rolling mill exit side temperature to the target temperature. Further, since the rapid cooling of the material to be rolled  2  is suppressed, it is possible to improve accuracy of thickness control. Moreover, since the rapid cooling of the material to be rolled  2  is suppressed, it is possible to stabilize passability. 
     &lt;Modifications&gt; 
     Incidentally, in the system according to the first embodiment, the cooling apparatus  20  may be disposed in any of the spaces between the rolling stands. Further, the rolling mill  10  may be a roughing mill. Moreover, although two sets of the spray nozzles and the spray valves are illustrated in  FIG. 1 , the number of sets of the spray nozzles and the spray valves may be one or three or more. Note that these modifications are also applicable to the second embodiment. 
     Second Embodiment 
     &lt;Entire Configuration&gt; 
     Next, a second embodiment of the present disclosure is described with reference to  FIG. 4  and  FIG. 5 . A system according to the present embodiment is realized by causing a cooling apparatus control unit  60  to execute a routine of  FIG. 5  described later in a configuration illustrated in  FIG. 4 . 
     In the above-described first embodiment, the cooling apparatus control unit  30  that controls one cooling apparatus  20  has been described. The cooling apparatus, however, is typically disposed in a plurality of spaces between the rolling stands. Accordingly, in the second embodiment, the cooling apparatus control unit  60  that controls a plurality of cooling apparatuses is described. 
       FIG. 4  is a conceptual diagram to explain a configuration of an exit side temperature control system according to the second embodiment of the present disclosure. The system illustrated in  FIG. 4  includes a downstream cooling apparatus  40 , an upstream cooling apparatus  50 , and the cooling apparatus control unit  60 , in place of the cooling apparatus  20  and the cooling apparatus control unit  30  illustrated in  FIG. 1 . Description of configurations equivalent to the configurations in  FIG. 1  is simplified or omitted. 
     &lt;Plurality of Cooling Apparatuses&gt; 
     The downstream cooling apparatus  40  is provided in at least one of the plurality of spaces between the rolling stands. In the example illustrated in  FIG. 4 , the downstream cooling apparatus  40  is provided in the space between the n−1-th rolling stand  13  and the n-th rolling stand  14 . 
     The downstream cooling apparatus  40  includes spray nozzles  41  (upper spray nozzle  41   a  and lower spray nozzle  41   b ), a coolant passage  42 , first valves  43  (upper spray valve  43   a  and lower spray valve  43   b ), a first valve control unit  44 , a second valve  45 , a flow rate detector  46 , and a second valve control unit  47 . Configurations of these components are similar to the configurations of the respective components included in the cooling apparatus  20  described in the first embodiment. 
     The upstream cooling apparatus  50  is provided in any of the plurality of spaces between the rolling stands upstream of the downstream cooling apparatus  40 . In the example illustrated in  FIG. 4 , the upstream cooling apparatus  50  is provided in a space between the n−2-th rolling stand  12  and the n−1-th rolling stand  13 . 
     The upstream cooling apparatus  50  includes spray nozzles  51  (upper spray nozzle  51   a  and lower spray nozzle  51   b ), a coolant passage  52 , first valves  53  (upper spray valve  53   a  and lower spray valve  53   b ), a first valve control unit  54 , a second valve  55 , a flow rate detector  56 , and a second valve control unit  57 . The spray nozzles  51  are provided in the space between the n−2-th rolling stand  12  and the n−1-th rolling stand  13 . Configurations of other components are similar to the configurations of the components included in the cooling apparatus  20  described in the first embodiment. 
     &lt;Cooling Apparatus Control Unit&gt; 
     The system illustrated in  FIG. 4  includes the cooling apparatus control unit  60  that controls the downstream cooling apparatus  40  and the upstream cooling apparatus  50 . The tracking apparatus  3 , the host computer  4 , the rolling mill entry side temperature sensor  5 , and the rolling mill exit side temperature sensor  6  are connected to the input side of the cooling apparatus control unit  60 . The first valve control unit  44  and the second valve control unit  47  of the downstream cooling apparatus  40  and the first valve control unit  54  and the second valve control unit  57  of the upstream cooling apparatus  50  are connected to the output side of the cooling apparatus control unit  60 . The cooling apparatus control unit  60  sequentially receives signals from the tracking apparatus  3 , the host computer  4 , the rolling mill entry side temperature sensor  5 , and the rolling mill exit side temperature sensor  6 . 
     The cooling apparatus control unit  60  includes a remaining coolant discharging section  61  and a flow rate target value setting section  62 . 
     The remaining coolant discharging section  61  controls the first valves  43  and  53  to the open state, and controls the second valves  45  and  55  to the closed state by setting the flow rate target value to zero, in the downstream cooling apparatus  40  and the upstream cooling apparatus  50 , before the material to be rolled  2  reaches the rolling mill  10 . More specifically, when the remaining coolant discharging section  61  provides ON signal to the first valve control units  44  and  54 , the first valves  43  and  53  are controlled to the open state. Further, the remaining coolant discharging section  61  sets the flow rate target value to be provided to the second valve control units  47  and  57 , to zero. As a result, the valve opening of each of the second valves  45  and  55  is controlled to the fully-closed state so as to bring the flow rate actual value close to zero, by the closed loop control. 
     The flow rate target value setting section  62  sets the flow rate target value of the downstream cooling apparatus  40  to a value corresponding to the target temperature of the material to be rolled  2  on the exit side of the rolling mill  10 , after the control by the remaining coolant discharging section  61 . When the flow rate target value is changed from zero to a predetermined flow rate target value (&gt;0), the valve opening of the second valve  45  is increased from zero to the opening corresponding to the predetermined flow rate target value, by the closed loop control. 
     In a case where cooling capacity of the downstream cooling apparatus  40  is not saturated, the flow rate target value setting section  62  sets the flow rate target value of the upstream cooling apparatus  50  to zero. 
     In contrast, in a case where the cooling capacity of the downstream cooling apparatus  40  is saturated, the flow rate target value setting section  62  sets the flow rate target value of the upstream cooling apparatus  50  to a value corresponding to the target temperature of the material to be rolled on the entry side and the exit side of the rolling mill  10 . More specifically, as the flow rate target value of the upstream cooling apparatus  50 , an amount of coolant that is insufficient with the cooling capacity (maximum coolant amount) of the downstream cooling apparatus  40  is set. When the flow rate target value is changed from zero to the predetermined flow rate target value (&gt;0), the valve opening of the second valve  55  is increased from zero to the opening corresponding to the predetermined flow rate target value, by the closed loop control. 
     Note that, as with the flow rate target value setting section  32  described in the first embodiment, the flow rate target value setting section  62  executes feedforward control and feedback control. 
     &lt;Flowchart&gt; 
       FIG. 5  is a flowchart of a control routine executed by the cooling apparatus control unit  60  to realize the above-described operation. 
     First, in step S 200 , the cooling apparatus control unit  60  determines whether the head end position of the material to be rolled  2  has reached the entry side of the rolling mill  10 , based on the tracking information. In a case where it is determined to be before reaching, processing in step S 210  is next executed. In a case where it is determined to be after reaching, passage of the material to be rolled  2  is waited for. 
     In step S 210 , the first valves  43  and  53  of the respective cooling apparatuses are controlled to the open state. More specifically, the remaining coolant discharging section  61  provides ON signal to the first valve control units  44  and  54 . The first valve control units  44  and  54  receive the ON signal to each control the first valves  43  and  53  to the open state. Note that, as a condition, spraying of the coolant onto the preceding material to be rolled  2  is already completed. In other words, a tail end of the preceding material to be rolled  2  has already passed through the rolling mill  10  (spray range by downstream cooling apparatus  40 ). 
     Next, in step S 220 , the second valves  45  and  55  are controlled to the closed state by setting the flow rate target values of the respective cooling apparatuses to zero. More specifically, the remaining coolant discharging section  61  sets the flow rate target values of the respective second valve control units  47  and  57  to zero. The second valve control units  47  and  57  each control, by the closed loop control, the valve opening of the second valves  45  and  55  to the fully-closed state such that the flow rate actual values become zero. As a result of the processing in step S 210  and step S 220 , the coolant remaining in the coolant passages  42  and  52  downstream of the second valves  45  and  55  is discharged from the spray nozzles  41  and  51 . 
     Next, in step S 230 , the flow rate target value of the downstream cooling apparatus  40  is set to a value (&gt;0) corresponding to the target temperature of the material to be rolled  2 . More specifically, the flow rate target value setting section  62  sets the flow rate target value of the downstream cooling apparatus  40  to a value corresponding to the target temperature of the material to be rolled  2  on the entry side and the exit side of the rolling mill  10 , after execution of the processing in step S 220 . As a result, the valve opening of the second valve  45  is controlled to the predetermined opening by the closed loop control based on the set flow rate target value, and the coolant amount corresponding to the flow rate target value of the downstream cooling apparatus  40  is sprayed. 
     Next, in step S 240 , the cooling apparatus control unit  60  determines whether the cooling capacity of the downstream cooling apparatus  40  is saturated. In a case where it is determined to be a saturated state, it is not possible to cool the material to be rolled  2  to the target temperature only by the spray of the coolant from the downstream cooling apparatus  40 . Therefore, it is necessary to spray the coolant also from the upstream cooling apparatus  50 . Thus, processing in step S 250  is executed. 
     In step S 250 , the flow rate target value of the upstream cooling apparatus  50  is set to a value (&gt;0) corresponding to the target temperature of the material to be rolled  2 . More specifically, as the flow rate target value of the upstream cooling apparatus  50 , an amount of coolant that is insufficient with the cooling capacity of the downstream cooling apparatus  40  is set. As a result, the valve opening of the second valve  55  is controlled to the predetermined opening by the closed loop control based on the set flow rate target value, and the coolant amount corresponding to the flow rate target value of the upstream cooling apparatus  50  is sprayed. 
     In contrast, in a case where it is determined not to be the saturated state in step S 240 , the necessary amount of coolant is sprayed only by the downstream cooling apparatus  40 . Accordingly, the flow rate target value of the upstream cooling apparatus  50  is set to zero (step S 260 ). 
     &lt;Effects&gt; 
     As described above, according to the routine illustrated in  FIG. 5 , it is possible to compensate the shortage of the cooling capacity of the downstream cooling apparatus  40 , by the spray of the coolant from the upstream cooling apparatus  50  in order to cool the material to be rolled  2  to the exit side temperature target value. According to the system of the present embodiment, before the material  2  to be rolled next reaches the rolling mill  10 , the first valves  43  and  53  are controlled to the open state and the second valves  45  and  55  are controlled to the closed state, and the coolant remaining in the coolant passages  42  and  52  downstream of the respective second valves  45  and  55  can be discharged at the timing when the coolant does not get on the material to be rolled  2 . Thereafter, the flow rate target value corresponding to the target temperature of the material to be rolled  2  is set, and the instructed amount of the coolant is sprayed onto the material to be rolled  2 . Therefore, as with the above-described first embodiment, it is possible to suppress rapid cooling of the material to be rolled  2 , and to control the rolling mill exit side temperature to the target temperature. Further, since the rapid cooling of the material to be rolled  2  is suppressed, it is possible to improve accuracy of thickness control. Moreover, since the rapid cooling of the material to be rolled  2  is suppressed, it is possible to stabilize passability. 
     &lt;Modifications&gt; 
     Incidentally, in the above-described system according to the second embodiment, the arrangement of the downstream cooling apparatus  40  and the upstream cooling apparatus  50  is not limited to the example illustrated in  FIG. 5 . It is sufficient to install the upstream cooling apparatus  50  upstream of the downstream cooling apparatus  40 . Further, three or more cooling apparatus may be provided. 
     &lt;Hardware Configuration Example&gt; 
       FIG. 6  is a diagram illustrating a hardware configuration example of a processing circuit included in the cooling apparatus control unit  30  or  60 . Parts in the cooling apparatus control unit  30  or  60  correspond to a part of the functions, and each of the functions is realized by the processing circuit. For example, the processing circuit includes at least one processor  91  and at least one memory  92 . For example, the processing circuit includes at least one dedicated hardware  93 . 
     When the processing circuit includes the processor  91  and the memory  92 , each of the functions is realized by software, firmware, or a combination of software and firmware. At least one of the software and the firmware is described as a program. At least one of the software and the firmware is held by the memory  92 . The processor  91  reads and executes the program stored in the memory  92 , thereby realizing each of the functions. The processor  91  is also referred to as a CPU (Central Processing Unit), a central processing device, a processing device, a computing device, a microprocessor, a microcomputer, or a DSP. Examples of the memory  92  include nonvolatile or volatile semiconductor memory such as a RAM, a ROM, a flash memory, an EPROM, or an EEPROM, a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, or a DVD and the like. 
     In a case where the processing circuit includes the dedicated hardware  93 , the processing circuit is, for example, a single circuit, a composite circuit, a programmed processor, a parallelly-programmed processor, an ASIC, an FPGA, or a combination thereof. For example, each of the functions may be realized by the processing circuit. For example, each of the functions may be collectively realized by the processing circuit. 
     Furthermore, a part of the functions may be realized by the dedicated hardware  93 , and the other part may be realized by software or firmware. 
     In this way, the processing circuit realizes each of the functions by the hardware  93 , the software, the firmware, or a combination thereof. Note that the above-described hardware configuration example is also applicable to each of the first valve control units  24 ,  44 , and  54  and the second valve control units  27 ,  47 , and  57 . 
     REFERENCE SIGNS LIST 
     
         
           2  Material to be rolled 
           3  Tracking apparatus 
           4  Host computer 
           5  Rolling mill entry side temperature sensor 
           6  Rolling mill exit side temperature sensor 
           10  Rolling mill 
           11 ,  12 ,  13 ,  14  Rolling stand 
           20  Cooling apparatus 
           21  Spray nozzle 
           21   a  Upper spray nozzle 
           21   b  Lower spray nozzle 
           22  Coolant passage 
           23  First valve 
           23   a  Upper spray valve 
           23   b  Lower spray valve 
           24  First valve control unit 
           25  Second valve 
           26  Flow rate detector 
           27  Second valve control unit 
           30  Cooling apparatus control unit 
           31  Remaining coolant discharging section 
           32  Flow rate target value setting section 
           40  Downstream cooling apparatus 
           41  Spray nozzle 
           41   a  Upper spray nozzle 
           41   b  Lower spray nozzle 
           42  Coolant passage 
           43  First valve 
           43   a  Upper spray valve 
           43   b  Lower spray valve 
           44  First valve control unit 
           45  Second valve 
           46  Flow rate detector 
           47  Second valve control unit 
           50  Upstream cooling apparatus 
           51  Spray nozzle 
           51   a  Upper spray nozzle 
           51   b  Lower spray nozzle 
           52  Coolant passage 
           53  First valve 
           53   a  Upper spray valve 
           53   b  Lower spray valve 
           54  First valve control unit 
           55  Second valve 
           56  Flow rate detector 
           57  Second valve control unit 
           60  Cooling apparatus control unit 
           61  Remaining coolant discharging section 
           62  Flow rate target value setting section 
           91  Processor 
           92  Memory 
           93  Hardware