Patent Publication Number: US-2016230246-A1

Title: Wire rod cooling device and wire rod cooling method

Description:
TECHNICAL FIELD 
     The present invention relates to a wire rod cooling apparatus and a wire rod cooling method in which a wire rod formed by hot rolling of a steel piece is wound in a shape of non-concentric rings and then the wire rod being transferred on a conveyor is subjected to controlled cooling. 
     This application claims the priority of Patent Application No. 2013-224279 filed with Japan on Oct. 29, 2013, the entire contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     A wire rod formed by hot rolling of a steel piece is cooled to approximately 800 to 900° C. by a water cooling means, and is then wound in a shape of non-concentric rings by a laying head (wire rod winder). The wound non-concentric ring-shaped wire rod (hereinafter, occasionally referred to as simply a “ring-shaped wire rod”) is transferred on a conveyor provided on the exit side of the laying head, and is further cooled by a cooling apparatus while being transferred. The water cooling and the cooling after winding are called controlled cooling, and are an important process that determines the structure, mechanical properties, and surface conditions of the wire rod. 
     As a conventional cooling apparatus, there is a Stelmor cooling apparatus, for example. In the cooling apparatus, a slit nozzle is provided below a roller conveyor or a chain conveyor over the entire area in the width direction of the conveyor (the direction orthogonal to the transfer direction in a planar view; hereinafter, referred to as simply a “width direction”), and coolant is sprayed from the slit nozzle toward the ring-shaped wire rod; thereby, the cooling of the ring-shaped wire rod is performed. 
     In both end portions in the width direction of the ring-shaped wire rod that is being transferred on the conveyor, there are many portions where parts of the wire rod overlap, and the wire rod is dense (hereinafter, occasionally referred to as a “dense-in-width-direction portion”). On the other hand, in the central portion in the width direction of the ring-shaped wire rod, the wire rod is sparse as compared to the dense-in-width-direction portion (hereinafter, occasionally referred to as a “sparse-in-width-direction portion”). Both portions are at the same temperature immediately after winding, but when the ring-shaped wire rod is cooled while being transferred under conditions where a uniform amount of coolant is sprayed in the width direction, a temperature difference occurs gradually between the dense-in-width-direction portion and the sparse-in-width-direction portion of the ring-shaped wire rod being transferred, because it is less easy for coolant to pass through the dense-in-width-direction portion. 
     To make the quality of the entire wire rod uniform, it is necessary to reduce the temperature unevenness of the entire wire rod, and therefore it is necessary to perform cooling taking into consideration the sparseness and denseness in the width direction of the ring-shaped wire rod during the cooling of the ring-shaped wire rod. In contrast, in a Stelmor cooling apparatus in which a slit nozzle is provided over the entire area in the width direction of the conveyor, it is possible only to spray coolant uniformly to the dense-in-width-direction portion and the sparse-in-width-direction portion, and cooling proceeds in the presence of a temperature difference between the dense-in-width-direction portion and the sparse-in-width-direction portion. In this case, the temperature unevenness of the entire wire rod cannot be reduced. 
     As a cooling apparatus taking into consideration the sparseness and denseness in the width direction of the ring-shaped wire rod, there is a cooling apparatus described in Patent Literature 1, for example. In the cooling apparatus, a slit nozzle is provided only in both end portions in the width direction of the ring-shaped wire rod, that is, in the dense-in-width-direction portion, not over the entire area in the width direction. Thereby, the dense-in-width-direction portion, which is at a relatively high temperature as compared to the sparse-in-width-direction portion, can be intensely cooled, and the temperature unevenness of the entire wire rod can be reduced. 
     There is also a cooling apparatus described in Patent Literature 2 as another cooling apparatus. In the cooling apparatus, a guide that causes the ring-shaped wire rod to meander is provided on a roller conveyor, and the position of the individual ring is shifted to change the position where the dense-in-width-direction portion is formed; consequently, the density of the portion that has been the previous dense-in-width-direction portion is reduced. Thereby, the temperature difference between the dense-in-width-direction portion and the sparse-in-width-direction portion of the ring-shaped wire rod is reduced, and the temperature unevenness of the entire wire rod can be reduced. 
     However, the ring-shaped wire rod being transferred receives the influence of speed differences resulting from a speed variation of the rolling mill, a winding speed variation of the laying head, a speed variation of the conveyor, and the like, vibration during transfer, etc., and is not uniform in the spacing in the transfer direction between rings of the ring-shaped wire rod (hereinafter, referred to as a “ring pitch”) and the ring diameter in many cases. Hence, in the ring-shaped wire rod, a dense portion and a sparse portion occur not only in the width direction but also in the transfer direction. 
     For example, as shown in  FIG. 1 , in a portion TD where the ring pitch is narrow, the distance between adjacent rings is short and the wire rod M is dense (hereinafter, occasionally referred to as a “dense-in-transfer-direction portion”). On the other hand, in a portion TS where the ring pitch is wide, the distance between adjacent rings is long and the wire rod M is sparse as compared to the dense-in-transfer-direction portion TD (hereinafter, occasionally referred to as a “sparse-in-transfer-direction portion”). When transfer is continued under these conditions, the temperature of the dense-in-transfer-direction portion TD becomes higher than the temperature of the sparse-in-transfer-direction portion TS, because of the situation where the dense-in-transfer-direction portion is less easy for coolant to pass through and is cooled less easily. 
     Furthermore, since the ring pitch is not uniform, the way that parts of the wire rod overlap is irregular. For example, as shown in  FIG. 1 , it is found that, on a straight line L along the transfer direction T, there is no regularity in the way that parts of the wire rod M overlap. Hence, when the temperature of the ring-shaped wire rod M that passes through an arbitrary point P on the straight line L is measured, the temperature measured at the point P varies with time, and also the temperature variation is in a state of no regularity. Such a phenomenon has occurred also in portions other than on the straight line L. Therefore, the temperature of the ring-shaped wire rod M in which sparseness and denseness have occurred in the transfer direction T is in a state of being complexly distributed. 
     As a cooling apparatus taking into consideration also the sparseness and denseness in the transfer direction of the ring-shaped wire rod, there is a cooling apparatus described in Patent Literature 3, for example. The cooling apparatus uses a density detector to detect the linear density (sparseness and denseness) of the ring-shaped wire rod in the width direction on a time-series basis, and controls the gas flow rate of cooling gas jetted from a nozzle that is divided into a plurality of blocks in the width direction on a time-series basis. Thereby, the temperature differences in the transfer direction and the width direction of the ring-shaped wire rod can be reduced. 
     As a cooling apparatus in which the direct temperature of the ring-shaped wire rod is measured, there is a cooling apparatus described in Patent Literature 4, for example. The cooling apparatus uses a scanning radiation thermometer to measure the temperature of the ring-shaped wire rod, with division into zones in an arbitrary direction with respect to the transfer direction, and adjusts the temperature and amount of coolant sprayed to the ring-shaped wire rod in accordance with the measured temperature distribution of the entire ring-shaped wire rod. Thereby, the temperature difference between portions of the ring-shaped wire rod can be reduced. 
     PRIOR ART LITERATURE(S) 
     Patent Literature(s) 
     
         
         [Patent Literature 1] JP 2003-166021A 
         [Patent Literature 2] JP-UM H7-3810A 
         [Patent Literature 3] JP S62-274030A 
         [Patent Literature 4] JP 2002-39865A 
       
    
     SUMMARY OF THE INVENTION 
     Problem(s) to be Solved by the Invention 
     Here, in the ring-shaped wire rod before being cooled by the cooling apparatus, in general, the temperature is high in the portion where the wire rod is dense, and the temperature is low in the portion where the wire rod is sparse. However, from the results of extensive studies by the inventor, it has been found that, depending on various factors, there are also a portion where the wire rod is sparse and yet the temperature is high and a portion where the wire rod is dense and yet the temperature is low. That is, it has been found that, in the ring-shaped wire rod, there is not necessarily a certain correlation between the sparseness and denseness and the temperature. 
     However, in the cooling apparatuses described in Patent Literatures 1 and 2, although the sparseness and denseness in the width direction of the ring-shaped wire rod are taken into consideration, the sparseness and denseness in the transfer direction are not taken into consideration, and the adjustment of the amount of coolant in accordance with the temperature condition of the ring-shaped wire rod has not been made, either. That is, it has been impossible to cool the ring-shaped wire rod with adaptation to complexly distributed temperature conditions. 
     In the cooling apparatus described in Patent Literature 3, although the sparseness and denseness in the transfer direction are taken into consideration, a means for measuring the temperature of the ring-shaped wire rod being transferred is not provided, and therefore it has been impossible to adjust the amount of coolant in accordance with the temperature condition of the ring-shaped wire rod. In the case where the amount of coolant is thus controlled only on the basis of the result of detection of the sparseness and denseness of the ring-shaped wire rod, even when the temperature of the wire rod is different between some portions, the amounts of coolant to the portions are the same as long as the density is the same. In this case, since portions at different temperatures are cooled by the same amount of coolant, the ring-shaped wire rod cannot be cooled uniformly. 
     In the cooling apparatus described in Patent Literature 4, although a scanning radiation thermometer that measures the temperature of the ring-shaped wire rod is provided, a means for detecting the sparseness and denseness of the ring-shaped wire rod is not provided, and it has been impossible to adjust the amount of coolant in accordance with the sparseness and denseness condition of the ring-shaped wire rod. In the case where the amount of coolant is thus controlled only on the basis of the result of measurement of the temperature of the ring-shaped wire rod, even when the density of the wire rod is different between some portions, the amounts of coolant to the portions are the same as long as the temperature is the same. However, since the temperature decreases less easily in the portion where the wire rod is dense than in the portion where the wire rod is sparse, the temperatures of these portions are different temperatures during cooling, even when they are the same temperature during temperature measurement. In this case, since portions at different temperatures are cooled by the same amount of coolant, the ring-shaped wire rod cannot be cooled uniformly. 
     In conventional cooling apparatuses, since the spraying of coolant to the wire rod is performed by a slit nozzle, the coolant jetted from the slit hits the wire rod uniformly, and it has been impossible to cool a specified portion of the ring-shaped wire rod selectively. Furthermore, since coolant has been sent by a fan and the amount of coolant has been controlled by the rate of rotation of the fan and the degree of opening of the suction port, it has been difficult to control the amount of coolant to the specified portion in accordance with the quick change. Particularly in Patent Literature 3, as mentioned above, although the amount of coolant is controlled on a block basis, it is impossible to cool only a specified portion selectively, and the control is less good in fineness. In Patent Literature 4, although the amount of coolant is controlled on a zone basis, the zone basis does not make it possible to selectively cool only a specified portion that is a more local portion, and the control is less good in fineness likewise. Hence, in the cooling apparatuses described in Patent Literatures 3 and 4, it has been difficult to cope with the quick change of the specified portion. 
     The present invention has been made in view of the circumstances mentioned above, and an object of the present invention is to perform cooling taking into consideration the sparseness and denseness in the transfer direction which occur in a wire rod that is wound in a shape of non-concentric rings after hot rolling and is being transferred, and thus reduce the temperature unevenness of the entire wire rod. 
     Means for Solving the Problem(s) 
     The present invention for solving the above problems is a wire rod cooling apparatus that cools a wire rod wound in a ring shape by a wire rod winder while transferring the wire rod on a conveyor, the wire rod cooling apparatus including: a plurality of jet nozzles that are arranged along a width direction of the conveyor and jet coolant toward the wire rod; an imaging device that is provided on a transfer-line upstream side of a jet nozzle row composed of the plurality of jet nozzles and captures an image of the wire rod being transferred; and a control unit that extracts sparseness and denseness information and temperature information of the wire rod from the captured image. The control unit is configured to control a flow rate of coolant jetted from the jet nozzles individually for each jet nozzle on the basis of the sparseness and denseness information and the temperature information of the wire rod in conformity with a timing when a specified portion corresponding to the information arrives at the jet nozzle. 
     According to the present invention, the flow rate of coolant jetted from the jet nozzles can be controlled individually for each nozzle on the basis of the sparseness and denseness information and the temperature information of the wire rod which are extracted from the image of the wire rod being transferred on the conveyor. Thus, the specified portion of the wire rod being transferred can be selectively cooled, and the amount of coolant can be controlled in accordance with the quick change of the specified portion. That is, fine control of the amount of coolant which has been unable to be achieved by conventional cooling apparatuses becomes possible. In addition, since the flow rate of coolant is controlled on the basis of both the sparseness and denseness information and the temperature information of the wire rod, the flow rate of coolant can be controlled appropriately even for portions where the wire rod is sparse and yet the temperature is high and portions where the wire rod is dense and yet the temperature is low, for which appropriate control has so far been unable to be made, not to mention portions where the wire rod is dense and the temperature is high and portions where the wire rod is sparse and the temperature is low. Thereby, cooling in which the temperature difference of the wire rod being transferred is reduced can be performed, and the temperature unevenness of the entire wire rod can be reduced. Consequently, the quality of the entire wire rod can be made uniform. 
     Pressure may be applied to coolant to be jetted from the jet nozzle. A shut-off valve that shuts off jetting of coolant from the jet nozzle may be provided. 
     A plurality of the jet nozzle rows may be provided along a transfer line. In this case, a plurality of the imaging devices may be provided along the transfer line, and the jet nozzle row may be provided between the imaging devices. The jet nozzles may be provided so as to avoid a case where jet nozzles of the jet nozzle rows exist together on a straight line along the transfer line. Furthermore, a slit nozzle that jets coolant toward the wire rod may be provided separately from the jet nozzle. 
     The conveyor may be a roller conveyor, a part of the roller conveyor may be formed of a disc roller including a plurality of discs, and the jet nozzle may be provided between the discs. The jet nozzle row may be provided between rollers of the roller conveyor. A slit nozzle that jets coolant toward the wire rod may be provided in, out of spaces between rollers of the roller conveyor, a space between rollers in which the jet nozzle row is not provided. 
     Another aspect of the present invention is a wire rod cooling method using a wire rod cooling apparatus that cools, on a conveyor, a wire rod wound in a ring shape by a wire rod winder, the method including capturing an image of the wire rod being transferred on the transfer-line upstream side of a jet nozzle row composed of a plurality of jet nozzles that are arranged along the width direction of the conveyor and jet coolant toward the wire rod, extracting sparseness and denseness information and temperature information of the wire rod from the captured image, and then controlling the flow rate of coolant jetted from the jet nozzles individually for each jet nozzle on the basis of the sparseness and denseness information and the temperature information of the wire rod in conformity with the timing at which a specified portion corresponding to the information arrives at the jet nozzle. 
     Pressure may be applied to coolant to be jetted from the jet nozzle. A shut-off valve that shuts off jetting of coolant from the jet nozzle may be provided, and control of coolant jetted from the jet nozzle may be performed by controlling opening and closing of the shut-off valve. 
     A plurality of the jet nozzle rows may be provided along a transfer line and coolant may be jetted from each jet nozzle. In this case, coolant may be jetted from each jet nozzle in a first jet nozzle row toward the wire rod on the basis of sparseness and denseness information and temperature information of the wire rod, then an image of the cooled wire rod may be captured again and the sparseness and denseness information and the temperature information of the wire rod may be updated on the basis of the captured image, and then a flow rate of coolant jetted from each jet nozzle in a second jet nozzle row may be controlled on the basis of the updated information in conformity with a timing when a specified portion corresponding to the information arrives at the jet nozzle. The jet nozzles may be provided so as to avoid a case where jet nozzles of the jet nozzle rows exist together on a straight line along the transfer line and coolant may be jetted from each jet nozzle toward the wire rod. Furthermore, a slit nozzle that jets coolant toward the wire rod may be provided separately from the jet nozzle and coolant may be jetted from the slit nozzle toward the wire rod. 
     The conveyor may be a roller conveyor, a part of the roller conveyor may be formed of a disc roller including a plurality of discs, the jet nozzle may be provided between the discs, and coolant may be jetted from each jet nozzle toward the wire rod. The jet nozzle row may be provided between rollers of the roller conveyor, and coolant may be jetted from each jet nozzle toward the wire rod. A slit nozzle that jets coolant toward the wire rod may be provided in, out of spaces between rollers of the roller conveyor, a space between rollers in which the jet nozzle row is not provided, and coolant may be jetted from the plurality of jet nozzles and the slit nozzle toward the wire rod. 
     Effect(s) of the Invention 
     Cooling taking into consideration both the sparseness and denseness and the temperature of a non-concentric ring-shaped wire rod being transferred can be performed, and uniform cooling in which the temperature unevenness of the entire wire rod is reduced can be performed. Consequently, the mechanical properties and the surface conditions provided by controlled cooling can be made more uniform than conventional ones, and additional heat treatment can be omitted that is occasionally performed in the next or subsequent process in order to solve the yield reduction due to defects of them and solve the defects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         FIG. 1  is a schematic plan view of a ring-shaped wire rod. 
         FIG. 2  is a schematic side view of a wire rod cooling apparatus according to an embodiment of the present invention. 
         FIG. 3  is a schematic plan view of a wire rod cooling apparatus according to an embodiment of the present invention. 
         FIG. 4  is a cross-sectional view taken along A-A in  FIG. 3 . 
         FIG. 5  is an illustration diagram showing a manner of the ON/OFF control of each jet nozzle. 
         FIG. 6  is a schematic plan view showing an example of the case where a plurality of jet nozzle rows are provided. 
         FIG. 7  is a schematic plan view showing an example of the case where a plurality of jet nozzle rows are provided. 
         FIG. 8  is a schematic side view showing a nozzle configuration in the case where a plurality of jet nozzle rows are provided. 
         FIG. 9  is a schematic side view showing another nozzle configuration in the case where a plurality of jet nozzle rows are provided. 
         FIG. 10  is a schematic side view showing an arrangement example of thermo-cameras in the case where a plurality of jet nozzle rows are provided. 
         FIG. 11  is a schematic side view showing an arrangement example of thermo-cameras in the case where a plurality of jet nozzle rows are provided. 
         FIG. 12  is a schematic plan view in the case where the jet nozzle is provided between rollers. 
         FIG. 13  is a cross-sectional view taken along A-A in the case where each jet nozzle is provided with a flow regulating valve. 
         FIG. 14  shows an example of the control flow of the coolant jetted from the jet nozzle. 
         FIG. 15  is a graph of the amount of coolant with respect to the sparseness and denseness information and the temperature information. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     Hereinbelow, an embodiment of the present invention is described based on a wire rod cooling apparatus  1  that cools a non-concentric ring-shaped wire rod M wound by a laying head  2  (a wire rod winder). In the embodiment, the speed of transfer of the ring-shaped wire rod M by a roller conveyor  3  provided on the exit side of the laying head  2  is constant. In the embodiment, compressed air is used as the coolant that cools the ring-shaped wire rod M. Air sent by a fan is used as the coolant of the Stelmor cooling as conventional technology. In this specification and the drawings, components having substantially the same function and configuration are marked with the same reference numerals, and a repeated description is omitted. 
     As shown in  FIG. 2  and  FIG. 3 , the wire rod cooling apparatus  1  includes a slit nozzle  4  between rollers  3   a  of the roller conveyor  3 . Each slit nozzle  4  is disposed such that the slit (not illustrated) at the tip of the nozzle faces up. Each slit nozzle  4  is a nozzle in which an opening with a circular cross-sectional shape is formed on the upper surface of a hollow circular pipe, for example, and air at a constant flow rate is jetted from each slit nozzle  4  and the jetted air hits the ring-shaped wire rod M on the roller conveyor  3 ; thereby, the ring-shaped wire rod M is cooled. 
     As shown in  FIG. 3 , the slit nozzle  4  is categorized into some types; there are a slit nozzle  4   a  provided over the entire area in the width direction W between rollers  3   a  and slit nozzles  4   b  and  4   c  provided in both end portions in the width direction W between rollers  3   a.  The lengths in the width direction of the slit nozzles  4   b  and  4   c  provided in both end portions in the width direction are different from each other. The slit nozzle  4   a  provided over the entire area in the width direction cools the dense-in-width-direction portion WD and the sparse-in-width-direction portion WS of the ring-shaped wire rod M uniformly, and the slit nozzles  4   b  and  4   c  provided in both end portions in the width direction W cool only the dense-in-width-direction portion WD of the ring-shaped wire rod M intensively. By providing such a plurality of types of slit nozzles  4 , cooling taking into consideration the sparseness and denseness in the width direction W of the ring-shaped wire rod M can be performed. 
     As shown in  FIG. 3 , basically a circular cylindrical member is used as rollers  3   a  constituting the roller conveyer  3 , but one roller  3   a  is formed by attaching a plurality of circular plate-like discs  5  to a rotating shaft  6  provided along the width direction W. In the following description, this roller  3   a  may be referred to as a “disc roller  7 .” 
     As shown in  FIG. 3  and  FIG. 4 , a jet nozzle  8  that jets compressed air toward the ring-shaped wire rod M being transferred is provided between discs  5  and  5  of the disc roller  7 . The jet nozzles  8  are arranged on a straight line along the width direction W (hereinafter, the row of the plurality of jet nozzles  8  arranged in the width direction W is referred to as a “jet nozzle row  9 ”). Each jet nozzle  8  is connected to a header pipe  10 , and the header pipe  10  is connected to a compressor  12  via a compressed air supply path  11 . Each jet nozzle  8  is provided with a shut-off valve  13  that shuts off the jetting of compressed air. Each jet nozzle  8  may be a cut pipe, or may be one using a nozzle tip. To make it possible to cool a specified portion S of the ring-shaped wire rod M described later with reliability, each jet nozzle  8  is provided so that the spread of compressed air has a diameter of, for example, 5 mm to 20 mm, which is near the diameter of the wire rod, in the position where the jetted compressed air hits the wire rod. In other words, each jet nozzle  8  can control compressed air individually for each specified portion S. 
     As shown in  FIG. 2 , a thermo-camera  14  as an imaging device that images the ring-shaped wire rod M on the roller conveyor is provided on the transfer-line upstream side of the jet nozzle row  9 . The thermo-camera  14  images the entire width and a prescribed range in the transfer direction T of the ring-shaped wire rod M. The prescribed range refers to a range in which an image that allows the sparseness and denseness condition and the temperature condition of the ring-shaped wire rod M described later to be identified can be captured. The thermo-camera  14  is installed at such a height as to be free from the adverse effects of the temperature of the ring-shaped wire rod M being transferred. 
     As shown in  FIG. 2  and  FIG. 4 , the wire rod cooling apparatus  1  includes a control unit  15  that controls the operation of the shut-off valve  13  of each jet nozzle  8  and the compressor  12 . The control unit  15  has a function of, from the image of the ring-shaped wire rod captured by the thermo-camera  14 , identifying a portion where the temperature is high (a high temperature portion) and a portion where the temperature is low relative to the high temperature portion (a low temperature portion), and a portion where the wire rod is dense and a portion where the wire rod is sparse. Furthermore, the control unit  15  has a function of specifying a portion of the ring-shaped wire rod M where cooling is needed on the basis of the sparseness and denseness information and the temperature information of the ring-shaped wire rod M and selecting a jet nozzle  8  that can jet coolant toward the specified portion out of the jet nozzle row  9 . The control unit  15  has also a function of calculating the timing when the specified portion S of the ring-shaped wire rod M passes above the selected jet nozzle  8  on the basis of the position of image capture, the position of the selected jet nozzle  8 , and the speed of the roller conveyor. 
     The wire rod cooling apparatus  1  is configured in the above manner. Next, a method for cooling the ring-shaped wire rod M using the wire rod cooling apparatus  1  is described. 
     First, as shown in  FIG. 2 , the ring-shaped wire rod M wound by the laying head  2  is transferred while being cooled by the slit nozzle  4  between rollers  3   a.  An image of the ring-shaped wire rod M that has entered the imaging range A of the thermo-camera  14  is captured. 
     In the captured image, a portion where the temperature of the ring-shaped wire rod M is higher than the upper limit of the permissible temperature (the high temperature portion) and a portion where the temperature of the ring-shaped wire rod M is lower than the upper limit of the permissible temperature (the low temperature portion), and a portion where the wire rod M is dense (the dense portion) and a portion where the wire rod M is sparse (the sparse portion) are identified by the control unit  15 . On the basis of the sparseness and denseness information and the temperature information of the ring-shaped wire rod M, it is specified which portion of the ring-shaped wire rod M is to be cooled, in accordance with a prescribed determination criterion. In the embodiment, the determination criterion is that a portion that is the dense portion and at the same time the high temperature portion and a portion that is the sparse portion and yet the high temperature portion of the ring-shaped wire rod M are taken as the cooling object, and that a portion that is the dense portion and at the same time the low temperature portion and a portion that is the sparse portion and at the same time the low temperature portion are not taken as the cooling object. The upper limit of the permissible temperature is determined as appropriate in accordance with the length and position of the transfer line, the installation position and cooling capacity of the slit nozzle  4 , the installation position and cooling capacity of the jet nozzle row  9 , etc. 
     The “dense portion” refers to a portion where two or more parts of the wire rod overlap and a portion where the distance between adjacent rings is short, for example. At this time, even the dense-in-width-direction portion WD of the ring-shaped wire rod M may not be the “dense portion” when the distance between rings is long; on the other hand, even the sparse-in-width-direction portion WS may be the “dense portion” when the distance between rings is short. That is, in the embodiment, the sparseness and denseness of the ring-shaped wire rod M are identified while not only the sparseness and denseness in the width direction W but also the sparseness and denseness in the transfer direction T are taken into consideration. 
     Then, a jet nozzle  8  that can cool the portion specified as the cooling object (hereinafter, referred to as a “specified portion S”) is selected out of the jet nozzle row  9 . Subsequently, the timing when the specified portion S passes above the selected jet nozzle  8  is calculated on the basis of the position of image capture, the position of the selected jet nozzle  8 , and the transfer speed. 
     The opening and closing operation of each jet nozzle  8  will now be described with reference to  FIG. 5 .  FIG. 5  is schematic plan views for describing the opening and closing operation of each jet nozzle  8  to the ring-shaped wire rod M; each jet nozzle  8  is actually located below the ring-shaped wire rod M as shown in  FIG. 2 , but in  FIG. 5  an illustration manner in which each jet nozzle  8  is located above the ring-shaped wire rod M is employed in order to enhance the visibility of the opening and closing state of the nozzle. The specified portions S shown in  FIG. 5  are the illustration of parts of the plurality of specified portions existing. 
     As shown in  FIG. 5( a ) , even when the downstream end of the ring-shaped wire rod M has passed through the jet nozzle row  9 , the shut-off valve  13  ( FIG. 4 ) of each jet nozzle  8  is closed because the specified portion S has not passed. That is, compressed air is not jetted from each jet nozzle  8 . The wire rod M passing through the jet nozzle row  9  shown in  FIG. 5( a )  is the sparse portion, and the cooling of this portion is performed by the slit nozzle  4  provided between rollers  3   a.    
     Subsequently, as shown in  FIG. 5( b ) , the ring-shaped wire rod M is further transferred to the downstream side of the transfer line, and the specified portion S of the ring-shaped wire rod M arrives at the jet nozzle row  9 . In conformity with this timing, the shut-off valves  13  ( FIG. 4 ) of jet nozzles  8  selected out of the jet nozzle row  9  are opened, and compressed air is jetted toward the specified portion S. 
     The jet nozzles  8  selected in  FIG. 5( b )  are those in the positions corresponding to the dense-in-width-direction portion WD of the ring-shaped wire rod M, but some jet nozzles  8   a  in the lower portion of  FIG. 5( b )  are closed. These portions are portions where parts of the wire rod do not overlap and the ring pitch is wide, that is, the sparse portion. The cooling of these portions is performed by the slit nozzle  4 . 
     After that, as shown in  FIG. 5( c ) , the ring-shaped wire rod M is further transferred to the downstream side of the transfer line, and with the change of the position of the specified portion S passing through the jet nozzle row  9 , the opening and closing state of each jet nozzle  8  is changed accordingly. Thereby, only the specified portion S passing through the jet nozzle row  9  is selectively cooled. Such opening and closing operation of the jet nozzle  8  is performed on the entire ring-shaped wire rod transferred; thus, the cooling of the ring-shaped wire rod M finishes. 
     As above, by the embodiment, the opening and closing of each jet nozzle  8  can be controlled on the basis of the sparseness and denseness information and the temperature information of the ring-shaped wire rod M obtained by the thermo-camera  14 . Thereby, the specified portion S of the ring-shaped wire rod M can be selectively cooled; thus, it is possible to cool only the portion where cooling is needed. In addition, since compressed air to which pressure is applied is used as the coolant that cools the ring-shaped wire rod M and the shut-off valve  13  is used for the control of the compressed air, the jetting and shutoff of compressed air from the jet nozzle  8  can be controlled quickly, and the amount of coolant can be finely controlled in accordance with the quick change of the specified portion S. Hence, the temperature unevenness of the entire wire rod can be reduced. Consequently, the quality of the entire wire rod can be made uniform, and eventually the costs spent to remove defective quality portions can be reduced and a reduction in sales due to the downgrading of the quality grade can be avoided. 
     Hereinabove, a preferred embodiment of the present invention is described, but the present invention is not limited to these examples. It is clear that one skilled in the art may arrive at various alterations or modifications within the technical idea described in the scope of claims; such alterations and modifications should be seen as within the technical scope of the present invention as a matter of course. 
     For example, although in the above embodiment air is used as the coolant that cools the wire rod M, the type of the coolant is not limited thereto. In order to enhance the cooling effect by a short time of coolant spraying, a coolant in which gas and liquid are mixed together into a mist form may be jetted from each jet nozzle  8 . Also a coolant cooled beforehand may be used. When either of these coolants is used, it is preferable that pressure be applied to the coolant in order to quickly control the jetting and shutoff of the jet nozzle  8  as mentioned above. 
     Although in the above embodiment the thermo-camera  14  is used as the imaging device that captures the image of the ring-shaped wire rod M, the imaging device is not limited thereto. For example, a video camera may be used. In this case, when the captured image is converted to a black-and-white image, the high temperature portion of the ring-shaped wire rod M is shown as a portion of high brightness in the image and on the other hand the low temperature portion is shown as a portion of low brightness in the image; therefore, the sparseness and denseness information and the temperature information of the ring-shaped wire rod M can be extracted. The imaging device may be also a device that captures still images, not limited to a device that captures moving images. The “image” in this specification includes moving images and still images. 
     Although in the above embodiment the flow rate of coolant jetted from each jet nozzle  8  is controlled on the basis of the sparseness and denseness information and the temperature information of the ring-shaped wire rod M, also the flow rate of coolant jetted from the slit nozzle  4  may be controlled, in addition to the control of each jet nozzle  8 . 
     Although in the above embodiment each jet nozzle  8  is provided so that coolant is sprayed from below the ring-shaped wire rod M, the jet nozzle  8  may be provided so that coolant is sprayed from above the ring-shaped wire rod M. 
     Although in the above embodiment the roller conveyor  3  is used as the conveyor that transfers the ring-shaped wire rod M, a chain conveyor may be used, for example. The type of the conveyor is not particularly limited to the extent that each jet nozzle  8  can be provided so that coolant can be sprayed to the ring-shaped wire rod M. Although in the above embodiment the jet nozzles  8  are provided on a straight line along the width direction W of the roller conveyor  3 , they do not need to be provided strictly on a straight line to the extent that the jet nozzles  8  are provided along the width direction W so that the cooling of the portions in the width direction W of the ring-shaped wire rod M can be performed by sharing among the jet nozzles  8 . However, when the jet nozzles  8  are provided on a straight line, the control of the coolant jetting timing etc. of the jet nozzles  8  can be performed easily. 
     Although in the above embodiment one jet nozzle row  9  is configured by forming one of the rollers  3   a  constituting the roller conveyor  3  as the disc roller  7  and providing a plurality of jet nozzles  8  between discs  5  and  5 , the number of jet nozzle rows  9  is not limited to one. For example, as shown in  FIG. 6 , a plurality of jet nozzle rows  9  may be provided at certain intervals. When a larger number of jet nozzle rows  9  are provided, cooling in accordance with the temperature of the ring-shaped wire rod M can be performed in a larger number of places of the transfer line, and therefore the temperature unevenness of the entire wire rod can be reduced more. 
     In the case where a plurality of jet nozzle rows  9  are provided, the jet nozzles  8  may be provided so as to avoid the case where jet nozzles of the jet nozzle rows  9  exist together on a straight line L along the transfer direction T, as shown in  FIG. 7 . For example, on the straight line L along the transfer direction T shown in  FIG. 7 , there is provided a jet nozzle  8  of an upstream-side jet nozzle row  16 , and there is provided no jet nozzle  8  of a downstream-side jet nozzle row  17 . By thus arranging the jet nozzles  8 , in a case where a specified portion S of the ring-shaped wire rod M exists between jet nozzles of the upstream-side jet nozzle row  16 , the specified portion S can be cooled by a jet nozzle  8  of the downstream-side jet nozzle row  17 . That is, the uncoolable ranges in the jet nozzle rows  9  can be mutually covered, and therefore the specified portion S of the ring-shaped wire rod M can be cooled with reliability. 
     In the case where a plurality of jet nozzle rows  9  are provided side by side as shown in  FIG. 7 , the header pipe  10  connected to the jet nozzle  8  may be provided for each jet nozzle row  9  to connect the header pipes  10  and the jet nozzles  8  together, as shown in  FIG. 8 . It is also possible to connect branched portions of one header pipe  10  to the jet nozzles  8  of the jet nozzle rows  9 , as shown in  FIG. 9 . 
     In the case where a plurality of jet nozzle rows  9  are provided, the number of imaging devices  14  that image the ring-shaped wire rod M may be the same as the number of jet nozzle rows  9 , or may be smaller than the number of jet nozzle rows  9 . In the case where the numbers of imaging devices  14  and jet nozzle rows  9  are the same, as shown in  FIG. 10  the imaging device  14  may be provided individually on the upstream side of each jet nozzle row  9 . In this case, an image of the ring-shaped wire rod M cooled by the upstream-side jet nozzle row  16  (a first jet nozzle row) may be captured again by the downstream-side imaging device  14 , and thus the sparseness and denseness information and the temperature information of the ring-shaped wire rod M can be updated. The ring-shaped wire rod M can be cooled by the downstream-side jet nozzle row  17  (a second jet nozzle row) on the basis of the sparseness and denseness information and the temperature information of the ring-shaped wire rod M updated. By thus controlling the flow rate of coolant of each jet nozzle  8  on the basis of the updated information, the temperature unevenness of the entire wire rod can be reduced more. 
     On the other hand, in the case where the number of imaging devices  14  is smaller than the number of jet nozzle rows  9 , it is not possible to provide the imaging device  14  for each jet nozzle row  9  as shown in  FIG. 11 . However, even in this case, the coolant jetting condition of each jet nozzle  8  can be controlled on the basis of the sparseness and denseness information and the temperature information of the ring-shaped wire rod M obtained from one imaging device  14 . In the example shown in  FIG. 11 , the temperature condition at the time point when the ring-shaped wire rod M has arrived at the downstream-side jet nozzle row  17  is predicted on the basis of the sparseness and denseness information and the temperature information of the ring-shaped wire rod M obtained from the imaging device  14 , and each jet nozzle  8  of the downstream-side jet nozzle row  17  is controlled on the basis of the predicted temperature condition. 
     The jet nozzles  8  may be provided between rollers  3   a  of the roller conveyor  3 , as shown in  FIG. 12 . By providing the jet nozzles  8  in place of the slit nozzle  4  like those provided in conventional wire rod cooling apparatuses, cooling in accordance with the sparseness and denseness condition and the temperature condition of the ring-shaped wire rod M can be performed. Thereby, the temperature unevenness of the entire wire rod can be reduced. When the number of jet nozzle rows  9  provided between rollers  3   a  is increased, the temperature unevenness of the entire wire rod can be reduced more. It is possible to provide the jet nozzle  8  in place of all the slit nozzles  4 , but when the jet nozzle  8  is provided in combination with a conventional wire rod cooling apparatus, the costs of plant and equipment investment etc. can be suppressed. 
     Although in the above embodiment each jet nozzle  8  is provided with the shut-off valve  13  to on/off-control the coolant jetting of each jet nozzle  8 , each jet nozzle  8  may be provided with a flow regulating valve  18  to control the flow rate of coolant jetted from each jet nozzle  8 , as shown in  FIG. 13 . That is, the specified portion S of the ring-shaped wire rod M can be selectively cooled by controlling the flow rate of each jet nozzle  8  on the basis of the sparseness and denseness information and the temperature information of the ring-shaped wire rod M obtained by the thermo-camera  14 . A pressure gauge (not illustrated) and a speed indicator (not illustrated) may be provided on the downstream side of the flow regulating valve  18 . Thereby, the flow rate of coolant can be calculated from the pressure-flow rate characteristics of each jet nozzle  8  or the relationship between the cross-sectional area of the nozzle and the flow velocity. When the calculated flow rate has a difference with the desired set value, control may be made so as to adjust the degree of opening of the flow regulating valve  18 . A means for measuring the temperature of coolant may be provided to control the flow rate of coolant in accordance with the temperature of coolant. 
     The method for controlling coolant from the jet nozzle  8  is not limited to the above embodiment, and various control methods may be used to the extent that they are control based on the sparseness and denseness information and the temperature information of the ring-shaped wire rod M.  FIG. 14  shows an example of the control flow of the coolant from the jet nozzle  8 . 
     First, as shown in  FIG. 2 , an image of the ring-shaped wire rod M that has entered the imaging range A of the thermo-camera  14  is captured (step S 1  of  FIG. 14 ). The captured image is outputted to the control unit  15 , and the sparseness and denseness information D and the temperature information T of the ring-shaped wire rod M are obtained in the control unit  15  (step S 2  of  FIG. 14 ). These steps S 1  and S 2  are similar to the above embodiment, and a detailed description is omitted. 
     On the other hand, a sparseness and denseness standard Ds and a temperature standard Ts serving as the standard of the amount of coolant are set beforehand (step S 3  of  FIG. 14 ). The sparseness and denseness standard Ds and the temperature standard Ts are set on the basis of the quality (strength), thickness, alloy components, transfer speed, etc. of the ring-shaped wire rod M, for example. 
     Subsequently, the flow rate of coolant jetted from the jet nozzle  8  is calculated on the basis of the sparseness and denseness information D and the temperature information T obtained in step S 2  and the sparseness and denseness standard Ds and the temperature standard Is obtained in step S 3 . A specific method for calculating the flow rate of coolant is described later. A jet nozzle  8  that can cool the specified portion S is selected out of the jet nozzle row  9 , and the timing of jetting coolant from the jet nozzle  8  is calculated on the basis of the position of image capture, the position of the selected jet nozzle  8 , and the transfer speed (step S 4  of  FIG. 14 ). 
     The method for calculating the amount of coolant in this step S 4  may have variations. For example, the sparseness and denseness information D and the temperature information T obtained in step S 2  may be corrected on the basis of the sparseness and denseness standard Ds and the temperature standard Ts obtained in step S 3 , and the amount of coolant may be calculated using linear programming. Specifically, as shown in  FIG. 15 , a graph of the amount of coolant with respect to the sparseness and denseness information D and the temperature information T is found beforehand. Then, the sparseness and denseness information D and the temperature information T after correction are applied to the graph; thus, the amount of coolant is derived. The amount of coolant in  FIG. 15  may be the flow rate of coolant jetted from each jet nozzle  8 , or may be the number of jet nozzle rows  9  in the case where a plurality of jet nozzle rows  9  are provided, for example. 
     Alternatively, in the case where, for example, the graph shown in  FIG. 15  is a graph in which the sparseness and denseness standard Ds and the temperature standard Ts obtained in step S 3  beforehand are taken into consideration, the amount of coolant may be derived by directly applying the sparseness and denseness information D and the temperature information T obtained in step S 2  to the graph. 
     After that, the shut-off valve  13  and the flow regulating valve  18  are controlled on the basis of the amount of coolant and the jetting timing calculated in step S 4 , and coolant at an appropriate flow rate is jetted from the jet nozzle  8  to the specified portion S at an appropriate timing. Thus, the cooling of the ring-shaped wire rod M is performed. 
     In the case where the flow rate of coolant jetted from the jet nozzle  8  is controlled on the basis of both the sparseness and denseness information D and the temperature information T of the ring-shaped wire rod M as in the embodiment, it is necessary to construct a control logic completely different from the control logics in the conventional methods described in Patent Literatures 3 and 4 etc., for example. Therefore, this method for controlling the flow rate of coolant is not one in which conventional control methods are simply combined. 
     Since the flow rate of coolant is controlled on the basis of both the sparseness and denseness information D and the temperature information T of the ring-shaped wire rod M, the flow rate of coolant can be controlled appropriately even for portions where the ring-shaped wire rod M is sparse and yet the temperature is high and portions where the ring-shaped wire rod M is dense and yet the temperature is low, for which appropriate control has so far been unable to be made, not to mention portions where the ring-shaped wire rod M is dense and the temperature is high and portions where the ring-shaped wire rod M is sparse and the temperature is low. 
     In addition, similar effects to the embodiment described above can be enjoyed, that is, the specified portion of the ring-shaped wire rod M can be selectively cooled, and the amount of coolant can be finely controlled in accordance with the quick change of the specified portion S. Thus, coolant can be jetted appropriately even to a portion where the cooling rate is locally low, such as a portion where parts of the ring-shaped wire rod M overlap or a portion where parts of the ring-shaped wire rod M are stuck together, that is, a portion where the strength may be reduced locally. Consequently, the temperature unevenness of the entire wire rod can be reduced, and the quality of the entire wire rod can be made uniform. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to the cooling of a wire rod wound by a laying head. 
     REFERENCE SIGNS LIST 
       1  wire rod cooling apparatus 
       2  laying head (wire rod winder) 
       3  roller conveyer 
       3   a  roller 
       4  slit nozzle 
       4   a  slit nozzle 
       4   b  slit nozzle 
       4   c  slit nozzle 
       5  disc 
       6  rotating shaft 
       7  disc roller 
       8  jet nozzle 
       9  jet nozzle row 
       10  header pipe 
       11  compressed air supply path 
       12  compressor 
       13  shut-off valve 
       14  thermo-camera (imaging device) 
       15  control unit 
       16  upstream-side jet nozzle row 
       17  downstream-side jet nozzle row 
       18  flow regulating valve 
     A imaging range 
     P arbitrary point 
     S specified portion 
     T transfer direction 
     TD dense-in-transfer-direction portion 
     TS sparse-in-transfer-direction portion 
     M wire rod 
     W width direction 
     WD dense-in-width-direction portion 
     WS sparse-in-width-direction portion 
     L straight line along the transfer direction