Patent Publication Number: US-2022220619-A1

Title: Method for pickling steel plate and pickling apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method for pickling a steel plate and a pickling apparatus. 
     BACKGROUND ART 
     In pickling of a steel plate, it is known that the pickling speed can be increased by adjusting the concentration of ferric ion (Fe 3+ ) contained in the acid solution, and methods for adjusting the Fe 3+  in the acid solution have been proposed. 
     For instance, Patent Document 1 discloses performing aeration of the acid solution to oxidize ferrous ion (Fe 2+ ) which is generated in the acid solution during pickling and increase the concentration of Fe 3+  contained in the acid solution, in order to maintain the concentration of Fe 3+  in the acid solution within a predetermined range. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP4186131B 
     SUMMARY 
     Problems to be Solved 
     Meanwhile, in a case where a gaseous oxidant (air or oxygen, for instance) is used to adjust the concentration of iron ion in the acid solution, the oxidation reaction of iron ion (Fe 2+  to Fe 3+ ) is relatively slow, as the rate of dissolution of the gaseous oxidant to the acid solution is limited. Thus, when the steel plate to be pickled is switched to a pickling-resistant member (steel plate that requires a longer time to be pickled), it is necessary to reduce the line speed compared to the previous speed. Furthermore, even when the concentration of Fe 3+  in the acid solution is to be increased by adjusting the supply amount of the gaseous oxidant or the like, it takes a long time to increase the concentration of Fe 3+ , and thus it is not possible to increase the line speed much during a period after the line speed is reduced as described above and until the concentration of Fe 3+  in the acid solution increases. Thus, the production efficiency of the steel plate may deteriorate. 
     In view of the above, an object of at least one embodiment of the present invention is to provide a method for pickling a steel plate capable of improving the production efficiency of the steel plate. 
     Solution to the Problems 
     According to at least one embodiment of the present invention, a method for pickling a steel plate having a first steel plate portion and a second steel plate portion which is connected to a tail end of the first steel plate portion and which requires a longer time for pickling than the first steel plate portion when pickled under the same condition, includes: a step of pickling the steel plate by immersing the steel plate in an acid solution in at least one pickling tank while conveying the steel plate; a step of circulating the acid solution, through a circulation line connected to any of the at least one pickling tank, between the pickling tank and an oxidizing device disposed in the circulation line; a step of oxidizing Fe 2+  in the acid solution to Fe 3+  by the oxidizing device using a gaseous oxidant; and a feeding start step of, upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, starting feeding of a liquid oxidant for oxidizing Fe 2+  in the acid solution to Fe 3+  to any of the at least one pickling tank or to the circulation line. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, provided is a method of pickling a steel plate capable of improving the production efficiency of the steel plate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a schematic diagram of a pickling facility according to an embodiment. 
         FIG. 1B  is a schematic diagram of a pickling facility according to an embodiment. 
         FIG. 1C  is a schematic diagram of a pickling facility according to an embodiment. 
         FIG. 2  is a schematic configuration diagram of a pickling facility according to an embodiment. 
         FIG. 3  is a schematic configuration diagram of a pickling facility according to an embodiment. 
         FIG. 4  is a schematic configuration diagram of a pickling facility according to an embodiment. 
         FIG. 5  is a graph showing a time-series change of the concentration of Fe 3+  and the line speed, etc. in the pickling method according to an embodiment. 
         FIG. 6  is a graph showing a time-series change of the concentration of Fe 3+  and the line speed, etc. in the pickling method according to an embodiment. 
         FIG. 7  is a graph showing a time-series change of the concentration of Fe 3+  and the line speed, etc. in the pickling method according to an embodiment. 
         FIG. 8  is a graph showing a time-series change of the concentration of Fe 3+  and the line speed, etc. in the pickling method according to an embodiment. 
         FIG. 9  is a block diagram illustrating the line speed control according to an embodiment. 
         FIG. 10  is a flowchart illustrating the control of concentration of Fe ion according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
     (Configuration of Pickling Apparatus) 
       FIGS. 1A to 4  are each a schematic diagram of a pickling facility to which a pickling method according to some embodiments is to be applied. The pickling apparatus  1  depicted in  FIGS. 1A to 4  is a pickling apparatus for pickling a steel plate  2  by using an acid solution  3 . 
     As depicted in  FIGS. 1A to 1C , the pickling apparatus  1  includes a pickling tank  12  for storing an acid solution  3 , and a conveyance roll  16  (conveyance part  10 ) for continuously conveying a steel plate  2  having a plate shape immersed in the acid solution  3 . The acid solution  3  is a pickling liquid for dissolving and removing the scale (oxide layer) formed on the surface of the steel plate  2 . For instance, the acid solution  3  is a liquid containing acid such as hydrochloric acid, sulfuric acid, nitric acid, or hydrofluoric acid. The conveyance roll  16  is configured to apply tension to the steel plate  2  and convey the steel plate  2  while the steel plate  2  is immersed in the acid solution in the pickling tank. A plurality of conveyance rolls  16  may be provided and configured to be driven by a motor  17  (see  FIG. 10 ). 
     The pickling apparatus  1  depicted in  FIGS. 2 to 4  is a pickling apparatus  1  which includes a plurality of pickling tanks  12  ( 12 A to  12 C) arranged in series in the conveyance direction of the steel plate  2 . The plurality of pickling tanks  12  ( 12 A to  12 C) are partitioned by partition walls. 
     The plurality of pickling tanks  12  ( 12 A to  12 C) have respective conveyance rolls  16  (conveyance parts  10 ), and the conveyance rolls  16  convey the steel plate  2  while the steel plate  2  is immersed in the acid solution  3  in the plurality of pickling tanks  12 . 
     In the pickling apparatus  1  depicted in  FIGS. 2 to 4 , the acid solution  3  for pickling the steel plate  2  is supplied to the pickling tank  12 C at the most downstream side, via an acid-solution supply part  18 . Furthermore, the acid solution  3  overflown from the pickling tanks  12  ( 12 A to  12 C) is conveyed to a pickling tank at the upstream side, over the partition wall between the pickling tanks  12 . The pickling tank  12 A at the most upstream side has an acid-solution discharge part  19  for discharging the acid solution  3 . 
     The pickling apparatus  1  includes a circulation line  21 , connected to the pickling tank  12 , for circulating the acid solution  3  in the pickling tank  12 , and an oxidizing device  20  disposed in the circulation line  21 . The circulation line  21  includes an extract line  22  for extracting the acid solution  3  from the pickling tank  12  and introducing the acid solution  3  to the oxidizing device  20 , and a return line  24  for returning the acid solution  3  from the oxidizing device  20  to the pickling tank  12 . 
     The oxidizing device  20  is configured to oxidize Fe 2+  in the acid solution  3  to Fe 3+  by using a gaseous oxidant. Although not illustrated, the oxidizing device  20  may include an airtight tank, and a gas supply part for supplying the gaseous oxidant to the airtight tank. The oxidizing device  20  may be configured such that the concentration of Fe 3+  in the acid solution inside the oxidizing device  20  is adjustable by adjusting the partial pressure of the gaseous oxidant inside the oxidizing device  20 . 
     In pickling of a steel plate, it is known that the pickling speed can be increased by adjusting the concentration of ferric ion (Fe 3+ ) contained in the acid solution. That is, it is known that the concentration ratio of iron ion (Fe 2+ , Fe 3+ ) in the acid solution and the pickling time has a predetermined relationship, and the pickling speed increases (that is, the pickling times becomes shorter) when the concentration of Fe 3+  in the acid solution is increased to some extent. Accordingly, by adjusting the concentration of Fe 3+  in the acid solution appropriately with the oxidizing device  20 , it is possible to pickle a steel plate efficiently. 
     The gaseous oxidant used in the oxidizing device  20  may contain air, oxygen, or ozone, for instance. 
     In a pickling apparatus including the plurality of pickling tanks  12  ( 12 A to  12 C), a circulation line  21  connected to one of the plurality of pickling tanks  12  may be provided, and the oxidizing device  20  may be disposed in the circulation line  21 . In the illustrative embodiment depicted in  FIGS. 2 and 4 , a circulation line  21  (including an extract line  22  and a return line  24 ) is connected to the pickling tank  12 C at the most downstream side of the plurality of pickling tanks  12  ( 12 A to  12 C), and the oxidizing device  20  is disposed in the circulation line  21 . In the illustrative embodiment depicted in  FIG. 4 , the return line  24  includes return lines  24 A to  24 C respectively connected to the plurality of pickling tanks  12 A to  12 C. 
     Alternatively, in the pickling apparatus including the plurality of pickling tanks  12  ( 12 A to  12 C), circulation lines  21  respectively connected to two or more of the plurality of pickling tanks  12  may be provided, and the oxidizing device  20  may be disposed in each of the circulation lines  21 . In the illustrative embodiment depicted in  FIG. 3 , circulation lines  21 A to  21 C (including extract lines  22 A to  22 C and return lines  24 A to  24 C) are provided so as to correspond to the plurality of pickling tanks  12  ( 12 A to  12 C) respectively, and oxidizing devices  20 A to  20 C are disposed in the circulation lines  21 A to  21 C, respectively. 
     Furthermore, in the illustrative embodiment depicted in  FIGS. 2 to 4 , the acid solution  3  from the oxidizing device  20  is supplied to the pickling tank  12 C at the most downstream side. 
     The pickling tank  12  at the downstream side may perform, in addition to dissolution of scale on the surface of the steel plate  2 , dissolution of the base material surface of the steel plate  2 . In a case where the base material of the steel plate  2  is dissolved by the acid solution as described above, Fe 3+  in the acid solution is consumed. Thus, by supplying the acid solution  3  whose Fe 3+  concentration is increased by the oxidizing device  20  to the downstream pickling tank of the plurality of pickling tanks  12  (e.g., the pickling tank  12 C at the downstream side), it is possible to pickle the steel plate  2  effectively. 
     The pickling apparatus  1  further includes, in one of the at least one pickling tank  12  or in the circulation line  21 , a liquid oxidant feeding part  30  capable of feeding a liquid oxidant for oxidizing the Fe 2+  in the acid solution  3  to Fe 3+ . The liquid oxidant feeding part  30  includes a liquid oxidant tank  32  for storing a liquid oxidant, a liquid oxidant feeding line  34  for feeding the liquid oxidant from the liquid oxidant tank  32 , and a liquid oxidant pump  33  disposed in the liquid oxidant feeding line  34  for pressurizing the liquid oxidant. 
     The liquid oxidant is not particularly limited, and any liquid having the capacity to oxidize iron ion (Fe 2+ ) may be used as the liquid oxidant. The liquid oxidant may include, for instance, at least one of hydrogen peroxide solution, hypochlorous acid, ammonium peroxydisulfate (ammonium persulfate), or potassium permanganate solution. 
     The liquid oxidant feeding line  34  is connected to the pickling tank  12  or the circulation line  21  (including the oxidizing device  20  disposed in the circulation line  21 ), and is configured to feed the liquid oxidant from the liquid oxidant tank  32  to the pickling tank  12  or the circulation line  21  (including the oxidizing device  20  disposed in the circulation line  21 ). 
     In the illustrative embodiment depicted in  FIGS. 1A and 2 to 4 , the liquid oxidant feeding line  34  includes a first feeding line  36  connected to the pickling tank  12  and configured to feed the liquid oxidant to the pickling tank  12 . The first feeding line  36  has a valve  37  disposed therein, for adjusting the supply amount of the liquid oxidant to the pickling tank  12  via the first feeding line  36 . Furthermore, in the illustrative embodiment depicted in  FIGS. 3 and 4 , the liquid oxidant feeding line  34  includes first feeding lines  36 A to  36 C connected to the pickling tanks  12 A to  12 C respectively, and configured to feed the liquid oxidant to the pickling tanks  12 A to  12 C respectively. The first feeding lines  36 A to  36 C have valves  37 A to  37 C disposed therein, respectively, for adjusting the supply amount of the liquid oxidant to the pickling tanks  12 A to  12 C via the first feeding lines  36 A to  36 C, respectively. 
     In the illustrative embodiment depicted in  FIGS. 1B, 3, and 4 , the liquid oxidant feeding line  34  includes a second feeding line  38  connected to the return line  24  (circulation line  21 ) between the oxidizing device  20  and the pickling tank  12 , and configured to feed the liquid oxidant to the return line  24 . The second feeding line  38  has a valve  39  disposed therein, for adjusting the supply amount of the liquid oxidant to the return line  24  via the second feeding line  38 . Furthermore, in the illustrative embodiment depicted in  FIGS. 3 and 4 , the liquid oxidant feeding line  34  includes second feeding lines  38 A to  38 C connected to the return lines  24 A to  24 C respectively, and configured to feed the liquid oxidant to the return lines  24 A to  24 C respectively. The second feeding lines  38 A to  38 C have valves  39 A to  39 C disposed therein, respectively, for adjusting the supply amount of the liquid oxidant to the return lines  24 A to  24 C via the second feeding lines  38 A to  38 C, respectively. 
     In the illustrative embodiment depicted in  FIGS. 1C, 3 and 4 , the liquid oxidant feeding line  34  includes a third feeding line  40  connected to the oxidizing device  20  in the circulation line  21  (circulation line  21 ), and configured to feed the liquid oxidant to the oxidizing device  20 . The third feeding line  40  has a valve  41  disposed therein, for adjusting the supply amount of the liquid oxidant to the oxidizing device  20  via the third feeding line  40 . Furthermore, in the illustrative embodiment depicted in  FIG. 3 , the liquid oxidant feeding line  34  includes third feeding lines  40 A to  40 C connected to the oxidizing devices  20 A to  20 C respectively, and configured to feed the liquid oxidant to the oxidizing devices  20 A to  20 C respectively. The third feeding lines  40 A to  40 C have valves  41 A to  41 C disposed therein, respectively, for adjusting the supply amount of the liquid oxidant to the oxidizing devices  20 A to  20 C via the third feeding lines  40 A to  40 C, respectively. 
     The pickling apparatus  1  may include a controller  100  for adjusting the concentration of Fe 3+  in the acid solution inside the pickling tanks  12  ( 12 A to  12 C) or the conveyance speed (line speed) of the steel plate  2 . The specific configuration of the controller  100  will be described later. 
     The controller  100  may include a processor, a memory (RAM), an auxiliary storage part, and an interface, for instance. The controller  100  is configured to receive signals from the above various measurement instruments via the interface. The processor is configured to process the accordingly received signals. Furthermore, the processor is configured to process programs expanded in the memory. 
     The content of process by the controller  100  may be implemented as programs to be executed by the processor, and stored in the auxiliary storage part. When the programs are executed, the programs are expanded in the memory. The processor is configured to read out the programs from the memory, and execute the orders contained in the programs. 
     (Steel Plate to be Pickled) 
     In the pickling apparatus  1  according to some embodiments, a pickling process is performed on a steel plate  2  including the first steel plate portion  2   a  and the second steel plate portion  2   b  (see  FIGS. 1A to 1C ). The second steel plate portion  2   b  is connected to the tail end of the first steel plate portion  2   a  via the first connection portion  4  formed by welding or the like. The second steel plate portion  2   b  is a steel plate of a kind which requires a longer time be pickled than the first steel plate portion  2   a  when pickled under the same condition. 
     The steel plate  2  may include the third steel plate portion  2   c  in addition to the first steel plate portion  2   a  and the second steel plate portion  2   b  (see  FIGS. 1A to 1C ). The third steel plate portion  2   c  is connected to the tail end of the second steel plate portion  2   b  via the second connection portion  5  formed by welding or the like. The third steel plate portion  2   c  is a steel plate of a kind which requires a shorter time to be pickled than the second steel plate portion  2   b  when pickled under the same condition. 
     Furthermore, a steel which has a relatively high content of Si requires a relatively longer time of pickling. The second steel plate portion  2   b  may be a steel (e.g., a high-strength steel material) having a relatively high content of Si. 
     (Pickling Method) 
     Next, a method of pickling the steel plate  2  according to some embodiments will be described. 
     First, with reference to  FIG. 5 , an outline of the pickling method according to some embodiments will be described.  FIG. 5  is a graph showing a time-series change of the concentration of Fe 3+  in the acid solution  3  and the conveyance speed (line speed) of the steel plate  2 , etc. in the pickling method according to an embodiment.  FIG. 5  also shows the time-series changes ( 202 ,  203 ,  212 ,  213 ) of the concentration of Fe 3+  in the acid solution and the conveyance speed of the steel plate, etc. according to a conventional and typical pickling method. 
     In some embodiments, the steel plate  2  is pickled while the steel plate  2  is conveyed by the conveyance part  10  and the steel plate  2  is immersed in the acid solution  3  inside the pickling tank  12 . In the example illustrated in  FIG. 5 , pickling of the steel plate  2  is performed from the time before time t 0  to the time after time tl, and the first steel plate portion  2   a  of the steel plate  2  is conveyed continuously into the pickling tank  12  until time t 0 . At time t 0 , the first connection portion  4  (tip end portion of the second steel plate portion  2   b  ) connecting the first steel plate portion  2   a  and the second steel plate portion  2   b  reaches the pickling tank  12 , and the pickling is switched from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  . After time t 0 , the second steel plate portion  2   b  of the steel plate  2  is conveyed into the pickling tank  12 . Furthermore, after the first connection portion  4  reaches the pickling tank  12  and the pickling switches to pickling of the second steel plate portion  2   b  at time t 0 , a part of the first steel plate portion  2   a  continues to be pickled inside the pickling tank  12 , until the first connection portion  4  (tail end portion of the first steel plate portion  2   a  ) is discharged from the pickling tank  12 . 
     While the first steel plate portion  2   a  is pickled in the pickling tank  12  (until time t 0 ), the acid solution  3  is circulated between the pickling tank  12  and the oxidizing device  20  disposed in the circulation line  21 , via the circulation line  21  connected to the pickling tank  12 . Furthermore, the oxidizing device  20  oxidizes Fe 2+  in the acid solution  3  to Fe 3+  by using a gaseous oxidant. Accordingly, the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  is maintained at the concentration suitable for pickling of the first steel plate portion  2   a.    
     Upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  , at time t 0 , the liquid oxidant feeding part  30  starts feeding the liquid oxidant to at least one of the pickling tank  12  or to the circulation line  21 . The valve (valve  37 ,  39 , or  41 ) disposed in the liquid oxidant feeding line  34  is opened, and the liquid oxidant stored in the liquid oxidant tank  32  is fed to the pickling tank  12  or to the circulation line  21  via the liquid oxidant feeding line  34 . Accordingly, the concentration  201  (see  FIG. 5 ) of Fe 3+  in the pickling tank  12  increases quickly and considerably after time t 0 . 
     As depicted in  FIG. 5 , at time t 0 , upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  , the conveyance speed  211  (see  FIG. 5 ) of the steel plate  2  may be reduced. 
     In a case where a gaseous oxidant (air or oxygen, for instance) is used instead of a liquid oxidant to adjust the concentration of iron ion in the acid solution, the oxidation reaction of iron ion (Fe 2+  to Fe 3+ ) is relatively slow, as the rate of dissolution of the gaseous oxidant to the air solution is limited. Thus, when the steel plate to be pickled is switched from the first steel plate portion  2   a  to the second steel plate portion  2   b  which requires a longer time to be pickled at time t 0  shown in  FIG. 5 , it is necessary to reduce the line speed (conveyance speed of the steel plate  2 ) than the previous speed (see the Fe 3+  concentration  220  and the line speed  212  in  FIG. 5 ). Furthermore, even if the concentration of Fe 3+  in the acid solution  3  is to be increased by adjusting the supply amount of the gaseous oxidant or the like as indicated by the Fe 3+  concentration  203  in  FIG. 5 , it takes a long time to increase the concentration of Fe 3+ , and thus it is difficult to increase the line speed much during a period after the line speed is reduced as described above and until the concentration of Fe 3+  in the acid solution  3  increases (see the line speed  213  in  FIG. 5 ). Thus, the production efficiency of the steel plate  2  may deteriorate. 
     In contrast, in a case where the liquid oxidant is used, the oxidant is dissolved in a solution, and thus the oxidation reaction of iron ion in the acid solution  3  proceeds more quickly compared to a case in which a gaseous oxidant is used. Thus, it is easier to increase the concentration of Fe 3+  in the acid solution  3  quickly. In this regard, according to the above embodiment, upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  (time t 0  in  FIG. 5 ), the liquid oxidant is supplied to the pickling tank  12  or to the circulation line  21 . Thus, upon switching to pickling of the second steel plate portion  2   b  (pickling-resistant member) which requires a longer time to be pickled under the same condition, it is possible to quickly increase Fe 3+  in the acid solution  3  in the pickling tank  12 . Accordingly, it is possible to maintain the conveyance speed (line speed) of the steel plate  2  at a high speed even when the kind of steel to be pickled is switched, and thus it is possible to improve the production efficiency of the steel plate  2 . 
     Next, with reference to  FIGS. 6 to 8 , the pickling method according to some embodiments will be described more specifically. 
       FIGS. 6 and 7  are each a graph showing time-series changes of the concentration of Fe 3+  in the acid solution  3  and the conveyance speed (line speed) of the steel plate  2 , etc. in the pickling method according to an embodiment.  FIG. 6  is, like the case of  FIG. 5 , a graph illustrating pickling of the steel plate  2  by the pickling method according to an embodiment, including the timing of switch from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  .  FIG. 7  is a graph illustrating pickling of the steel plate  2  by the pickling method according to an embodiment, including the timing of switch from pickling of the second steel plate portion  2   b  to pickling of the third steel plate portion  2   c.    
     In the embodiment depicted in  FIG. 6 , the steel plate  2  is pickled similarly to the case illustrated in  FIG. 5 . At time t 10 , the first connection portion  4  connecting the first steel plate portion  2   a  and the second steel plate portion  2   b  (tip end portion of the second steel plate portion  2   b  ) reaches the pickling tank  12 , and the pickling is switched from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b.    
     Furthermore, similarly to the case illustrated in  FIG. 5 , while the first steel plate portion  2   a  is pickled in the pickling tank  12  (until time t 10 ), the acid solution  3  is circulated between the oxidizing device  20  disposed in the circulation line  21  and the pickling tank  12 , via the circulation line  21  connected to the pickling tank  12 . Furthermore, the oxidizing device  20  oxidizes the Fe 2+  in the acid solution  3  to Fe 3+  by using a gaseous oxidant. Accordingly, the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  is maintained at the concentration (C t10 ) suitable for pickling of the first steel plate portion  2   a.    
     Furthermore, similarly to the case illustrated in  FIG. 5 , upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  , at time t 10 , the liquid oxidant feeding part  30  starts feeding the liquid oxidant to at least one of the pickling tank  12  or to the circulation line  21 . Accordingly, as shown in  FIG. 6 , the concentration of Fe 3+  in the pickling tank  12  increases quickly and considerably from C t10  to Ct 11 , between time t 10  and time t 11 . 
     In some embodiments, feeding of the liquid oxidant is started within a period in which the first connection portion  4  connecting the first steel plate portion  2   a  and the second steel plate portion  2   b  exists inside the pickling tank  12 , for instance, as shown in  FIG. 6 , at the time (time t 10 ) when the first connection portion  4  reaches the pickling tank  12 . 
     In the example shown in  FIG. 6 , at time t 10 , the feeding amount of the liquid oxidant is increased from zero to qtio. Accordingly, it is possible to increase the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  quickly, after starting pickling of the second steel plate portion  2   b  . Thus, it is easier to maintain the conveyance speed of the steel plate  2  at a high speed after starting pickling of the second steel plate portion  2   b  being a pickling-resistant member, and thus it is possible to improve the production efficiency of the steel plate  2  effectively. 
     In the illustrative embodiment depicted in  FIG. 6 , upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  , the conveyance speed (line speed) of the steel plate  2  is reduced from V 0  to Vt 10  at time t 10 . 
     By feeding the liquid oxidant to the acid solution  3  at time t 10  in  FIG. 6 , it is possible to increase the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  relatively quickly, but it takes some time until the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  reaches the target value Ct (in the case illustrated in  FIG. 6 , the time from time t 10  to time t 11 ). With this regard, in the method according to the above described embodiment, upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  , the conveyance speed of the steel plate  2  is reduced at time t 10 , and thus it is possible to appropriately pickle the second steel plate portion  2   b  being a pickling-resistant member, by reducing the conveyance speed of the steel plate  2  after starting pickling of the second steel plate portion  2   b  being a pickling-resistant member and before the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  increases sufficiently. Accordingly, it is possible to suppress deterioration of the product quality. 
     In the embodiment illustrated in  FIG. 6 , the line speed is reduced to Vt 10  at time t 10 , and the line speed is increased to Vt 11  at time t 11 . 
     That is, upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  , feeding of the liquid oxidant is started at time t 10  and the conveyance speed of the steel plate is reduced, and then the conveyance speed of the steel plate is increased at time t 11 . Accordingly, it is possible to increase the conveyance speed of the steel plate  2  in accordance with an increase in Fe 3+  during pickling, and thereby it is possible to maintain the conveyance speed of the steel plate  2  at a high speed during pickling of the second steel plate portion  2   b  (pickling-resistant member). Thus, it is possible to improve the production efficiency of the steel plate. 
     In the embodiment illustrated in  FIG. 6 , upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  , Fe 3+  derived from oxidation reaction (Fe 3+  derived from the oxidizing device) using the gaseous oxidant by the oxidizing device  20  from time t 10  to time t 11 , and the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  is increased. More specifically, the supply amount of the gaseous oxidant to the acid solution  3  by the oxidizing device  20  is increased to raise the concentration of Fe 3+  in the acid solution  3  in the oxidizing device  20  from etio to etii, and the circulation flow rate of the acid solution  3  between the oxidizing device  20  and the pickling tank  12  via the circulation line  21  is increased from r 0  to rt 10 , thereby increasing the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12 . 
     Furthermore, in the illustrative embodiment depicted in  FIG. 6 , at time tii (that is, during pickling of the steel plate  2  in the pickling tank  12 ), supply of the liquid oxidant to the pickling tank  12  or the circulation line  21  is stopped. More specifically, Fe 3+  derived from the oxidizing device is increased from time t 10  to increase the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12 , and at time t 11 , when it is possible to maintain the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  by supply of Fe 3+  from the oxidizing device  20 , supply of the liquid oxidant to the pickling tank  12  or to the circulation line  21  is stopped. Herein, at this time, the circulation amount of the acid solution  3  between the oxidizing device  20  and the pickling tank  12  may be reduced to the extent such that it is possible to maintain the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  (in  FIG. 6 , the circulation amount is reduced to r t11 ). 
     As described above, upon switching from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion, Fe 3+  derived from oxidation reaction using the gaseous oxidant by the oxidizing device  20  is increased (from time t 10  to time t 11 ), and the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  is increased to raise the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  sufficiently, and thereby it is possible to stop feeding of the liquid oxidant, which is relatively expensive (time t 11 ). Accordingly, it is possible maintain the conveyance speed of the steel plate  2  and improve the production efficiency of the steel plate  2 , while suppressing an increase in the cost for pickling the steel plate  2 . 
     In some embodiments, supply of the liquid oxidant to the pickling tank  12  or to the circulation line  21  by the liquid oxidant feeding part  30  may be stopped when the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  reaches the target value Ct. Alternatively, supply of the above described liquid oxidant may be stopped before the tail end of the second steel plate portion  2   b  is discharged from the pickling tank  12 . As described above. supply of the liquid oxidant is stopped during pickling of the steel plate  2 , and thus the liquid oxidant is supplied to the pickling tank  12  or the circulation line  21  for a relatively short period of time. Accordingly, it is possible maintain the conveyance speed of the steel plate  2  and improve the production efficiency of the steel plate  2 , while suppressing an increase in the cost for pickling the steel plate  2  by suppressing the usage amount of the liquid oxidant, which is relatively expensive. 
     In the embodiment illustrated in  FIG. 7 , the steel plate  2  including the second steel plate portion  2   b  and the third steel plate portion  2   c  connected to the second steel plate portion  2   b  via the second connection portion  5  is pickled. Until time t 21 , the second steel plate portion  2   b  of the steel plate  2  is pickled inside the pickling tank  12 . At time t 21 , the second connection portion  5  connecting the second steel plate portion  2   b  and the third steel plate portion  2   c  (tip end portion of the third steel plate portion  2   c  ) reaches the pickling tank  12 , and the pickling is switched from pickling of the second steel plate portion  2   b  to pickling of the third steel plate portion  2   c  . After time t 21 , the third steel plate portion  2   c  of the steel plate  2  is conveyed into the pickling tank  12 , and pickled. Also after the second connection portion  5  reaches the pickling tank  12  and the pickling switches to pickling of the third steel plate portion  2   c  at time t 21 , a part of the second steel plate portion  2   b  continues to be pickled inside the pickling tank  12 , until the second connection portion  5  (tail end portion of the second steel plate portion  2   b  ) is discharged from the pickling tank  12 . In the time range shown in  FIG. 7 , the supply amount of the liquid oxidant by the liquid oxidant feeding part  30  is zero. 
     In some embodiments, as depicted in  FIG. 7 , upon switching from pickling of the second steel plate portion  2   b  to pickling of the third steel plate portion  2   c  , one of the supply amount of the gaseous oxidant to the acid solution  3  by the oxidizing device  20  or the circulation flow rate of the acid solution  3  between the oxidizing device  20  and the pickling tank  12  is reduced, and the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  is reduced. In the embodiment illustrated in  FIG. 7 , upon switching from pickling of the second steel plate portion  2   b  to pickling of the third steel plate portion  2   c  , the supply amount of the gaseous oxidant to the acid solution  3  by the oxidizing device  20  is reduced from time t 20  before time t 21  when the second connection portion  5  arrives at the pickling tank  12  to reduce the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  from e t20  to r t21 , and the circulation flow rate of the acid solution  3  is reduced from r t20a  to r t20b , thereby reducing the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  from C t20  to C t21 . 
     As described above, upon switching from pickling of the second steel plate portion  2   b  to pickling of the third steel plate portion  2   c  , Fe 3+  derived from the oxidizing device  20  is reduced to reduce the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12 , and thereby it is possible to suppress excessive pickling of the third steel plate portion  2   c  which requires a shorter period of time to be pickled under the same condition. Thus, it is possible to reduce pickling loss of the steel plate  2  and improve the yield ratio, thereby improving the production efficiency of the steel plate  2 . 
     In some embodiments, as depicted in  FIG. 7  for instance, upon switching from pickling of the second steel plate portion  2   b  to pickling of the third steel plate portion  2   c  , the conveyance speed of the steel plate  2  is increased. In the example depicted in  FIG. 7 , the line speed is reduced from V t20  to V t21a  from time t 20  to time t 21  when the second connection portion  5  arrives at the pickling tank  12 , and the conveyance speed of the steel plate  2  is increased to V t21b  at time t 21  when the second connection portion  5  arrives at the pickling tank  12 . 
     The third steel plate portion  2   c  requires a shorter period of time to be pickled than the second steel plate portion  2   b  under the same condition, and thus it is possible to pickle the third steel plate portion  2   c  sufficiently even when the conveyance speed of the steel plate  2  is increased upon switching to pickling of the third steel plate portion  2   c  . According to the embodiment described above, upon switching from pickling of the second steel plate portion  2   b  to the third steel plate portion  2   c  , the conveyance speed of the steel plate  2  is increased, and thereby it is possible to maintain the conveyance speed of the steel plate  2  at a high speed while pickling the third steel plate portion  2   c  sufficiently. Thus, it is possible to improve the production efficiency of the steel plate  2 . 
       FIG. 8  is a graph showing time-series changes of the concentration of Fe 3+  in the acid solution and the conveyance speed (line speed) of the steel plate  2 , etc. in the pickling method according to an embodiment.  FIG. 8  is a graph according to a pickling method for the pickling apparatus  1  including a plurality of pickling tanks  12  A to  12 C and configured such that the acid solution  3  is supplied to the plurality of pickling tanks  12  A to  12 C from the oxidizing device  20 , and such that the liquid oxidant is supplied from the liquid oxidant feeding part  30 . 
     In the embodiment depicted in  FIG. 8 , the steel plate  2  is pickled, and at time t 40 , the first connection portion  4  (tip end portion of the second steel plate portion  2   b  ) connecting the first steel plate portion  2   a  and the second steel plate portion  2   b  reaches the pickling tank  12 A (pickling tank # 1 ) positioned at the most upstream side of the plurality of pickling tanks  12 , and pickling is switched from pickling of the first steel plate portion  2   a  to pickling of the second steel plate portion  2   b  . Subsequently, the first connection portion  4  proceeds downstream, and reaches the pickling tank  12 B (pickling tank # 2 ) at time t 41  and the pickling tank  12 C (pickling tank # 3 ; the most downstream pickling tank  12 ) at time t 42  sequentially. 
     At the timing (time t 40 , t 41 , t 42 ) when the first connection portion  4  reaches the respective pickling tanks  12  ( 12 A to  12 C), feeding of the liquid oxidant to the respective pickling tanks  12  ( 12 A to  12 C) or to the circulation lines  21  ( 21 A to  21 C) connected to the pickling tanks  12  is started in series. This is illustrated in the graph of the supply flow rate of the liquid oxidant in  FIG. 8 . Accordingly, the concentration of Fe 3+  in the acid solution  3  in the respective pickling tanks  12 A to  12 C is increased quickly. Therefore, for the pickling apparatus  1  including the plurality of pickling tanks  12  ( 12 A to  12 C), it is possible to maintain the conveyance speed (line speed) of the steel plate  2  at a high speed even when the kind of steel to be pickled is switched, and thus it is possible to improve the production efficiency of the steel plate  2 . 
     As shown in  FIG. 8 , at the timing (time t 40 , t 41 , t 42 ) when the first connection portion  4  reaches the respective pickling tanks  12  ( 12 A to  12 C), the circulation flow rate of the acid solution  3  between the respective pickling tanks  12 A to  12 C and the oxidizing devices  20  ( 20 A to  20 ) is increased. Accordingly, it is possible to maintain the concentration of Fe 3+  in the acid solution  3  in the respective pickling tanks  12 A to  12 C appropriately. Furthermore, for this reason, it is possible to stop feeding of the liquid oxidant to the respective pickling tanks  12  ( 12 A to  12 C) or the circulation lines  21  ( 21 A to  21 C) connected to the pickling tanks  12 . 
     In the illustrative embodiment depicted in  FIG. 8 , the line speed is changed at each of the following timings: when the first connection portion  4  enters the pickling tank  12 A (time t 40 ), when feeding of the liquid oxidant to the pickling tank  12 B is started (time t 41 ), when the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12 B reaches a predetermined value (time t 43 ), and when the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12 C reaches a predetermined value (time t 44 ). For instance, by changing the line speed appropriately at the above timings, it is possible to maintain the line speed of at a high speed even when the kind of steel to be pickled is switched, and thus it is possible to improve the production efficiency of the steel plate  2 . 
     In some embodiments, the controller  100  is configured to control the line speed and the timing to change the line speed. 
       FIG. 9  is a block diagram illustrating the line speed control by a controller  100  according to an embodiment. As depicted in  FIG. 9 , the controller  100  includes a pickling speed evaluation part  102 , a target line speed calculation part  104 , and a line speed control part  106 . 
     The pickling speed evaluation part  102  is configured to receive signals that indicate operation information, position of the welding portion (the first connection portion  4  or the second connection portion  5 ) in the conveyance direction, concentration of Fe ion (concentration of Fe 2+  or concentration of Fe 3+ ) in the acid solution  3  in the pickling tank  12 , and sensing information of components of the acid solution  3  in the pickling tank  12  or the like. The operation information includes the kind of steel of the steel plate  2  to be pickled and the operation conditions of the pickling apparatus  1  (temperature, pressure, and the like). The pickling speed evaluation part  102  evaluates the pickling speed of the steel plate  2  on the basis of the received signals. 
     The target line speed calculation part  104  calculates the target line speed by the conveyance part  10 , on the basis of the evaluation result of the pickling speed by the pickling speed evaluation part  102 . The line speed control part  106  controls the conveyance part  10  to achieve the calculated target line speed. For instance, the line speed control part  106  calculates an electric current command value for a motor  17  (motor which drives the conveyance roll  16 ) for obtaining the calculated target line speed, and sends the electric current command value to the motor. 
     In some embodiments, the controller  100  may obtain information on the position of the first connection portion  4  in the conveyance direction, and determine the timing to reduce the line speed on the basis of the information. 
     In this case, the timing to reduce the conveyance speed of the steel plate  2  is determined on the basis of the information on the position of the first connection portion  4  in the conveyance direction, and thus, for instance, it is possible to reduce the conveyance speed of the steel plate  2  at an appropriate timing in accordance with the timing to start pickling of the second steel plate portion  2   b  (that is, the timing when the second steel plate portion  2   b  reaches the pickling tank  12 ). Accordingly, it is possible to pickle the second steel plate portion  2   b  appropriately, and suppress deterioration of the product quality. 
     In some embodiments, the timing to start supply of the liquid oxidant may be determined on the basis of the information of the position of the first connection portion  4  in the conveyance direction. The supply start timing of the liquid oxidant may be determined in relation to the timing to reduce the conveyance speed of the steel plate  2 . 
     In the above described embodiment, the timing to start supplying the liquid oxidant is determined on the basis of the information on the position of the first connection portion  4  in the conveyance direction, and thus, for instance, it is possible to start feeding of the liquid oxidant at an appropriate timing in accordance with the timing to start pickling of the second steel plate portion  2   b  (that is, the timing when the second steel plate portion  2   b  reaches the pickling tank  12 ). Thus, upon switching to pickling of the second steel plate portion  12   b  , it is possible to increase Fe 3+  in the acid solution  3  in the pickling tank  12  at an appropriately timing, and thus it is easier to maintain the conveyance speed of the steel plate  2  at a high speed. Thus, it is possible to improve the production efficiency of the steel plate  2 . 
     In some embodiments, the controller  100  may be configured to adjust the concentration of Fe ion in the acid solution  3  in the pickling tank  12 . 
     The concentration of Fe ion in the acid solution  3  in the pickling tank  12  may be adjusted according to the procedure shown in the flowchart of  FIG. 10 , for instance.  FIG. 10  is a flowchart illustrating the control of concentration of Fe ion according to an embodiment. 
     As shown in the flow chart of  FIG. 10 , for instance, the mass balance at the pickling tanks  12  and the oxidizing devices  20  is calculated on the basis of the target concentration of Fe ion (target concentration of Fe 2+  and Fe 3+  ion) at the pickling tanks  12  and the oxidizing devices  20  and the operation conditions of the oxidizing devices  20  (step S 1 ). The operation conditions of the oxidizing devices  20  include, for instance, the supply amount of the gaseous oxidant (oxygen) by the oxidizing device  20 , the concentration of the gaseous oxidant, the bubbling gas flow rate, the temperature, the pressure, or the like. 
     Next, on the basis of the mass balance calculated in step S 1 , the feeding flow rate of fresh acid solution (hydrochloric acid or the like) to the pickling tank  12 , the circulation flow rate of the acid solution  3  between the oxidizing device  20  and the pickling tank  12 , and the supply amount flow rate and the supply time of the liquid oxidant by the liquid oxidant feeding part  30  are set (step S 40 ). 
     Next, the concentration of Fe 3+  and the concentration of Fe 2+  in the acid solution  3  in the pickling tank  12  are measured (detected) (step S 6 ), and it is determined whether the concentrations match the target values (step S 8 ). In a case where the measurement value and the target value of the concentration of Fe ion do not match in step S 8  (No in step S 8 ), the set values of the feeding flow rate of fresh acid solution (hydrochloric acid or the like) to the pickling tank  12 , the circulation flow rate of the acid solution  3  between the oxidizing device  20  and the pickling tank  12 , and the supply amount flow rate and the supply time of the liquid oxidant are changed (step S 10 ), and the procedure returns to step S 6 . On the other hand, in a case where the measurement value and the target value of the concentration of Fe ion match in step S 8  (Yes in step S 8 ), the set values of the feeding flow rate of fresh acid solution (hydrochloric acid or the like) to the pickling tank  12 , the circulation flow rate of the acid solution  3  between the oxidizing device  20  and the pickling tank  12 , and the supply amount flow rate and the supply time of the liquid oxidant are maintained, and the procedure is completed. 
     In some embodiments, as described with reference to  FIG. 10  for instance, the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  may be detected, and the supply amount of the liquid oxidant may be determined on the basis of the difference between the detected concentration of Fe 3+  and the target concentration of Fe 3+  in the acid solution  3  in the pickling tanks  12  for pickling of the second steel plate portion  2   b.    
     In this case, the supply amount of the liquid oxidant is determined on the basis of the difference between the measurement value and the target concentration of Fe 3+  in the acid solution in the pickling tank  12 , and thus, by supplying the liquid oxidant on the basis of the supply amount determined accordingly, it is possible to increase Fe 3+  in the acid solution  3  in the pickling tank  12 , and maintain the conveyance speed of the steel plate  2  at a high speed. Thus, it is possible to improve the production efficiency of the steel plate  2 . 
     Furthermore, in some embodiments, during pickling of the second steel plate portion  2   b  and after stopping supply of the liquid oxidant to the pickling tank  12  or the circulation line  21 , at least one of the supply amount of the gaseous oxidant or the circulation flow rate of the acid solution  3  between the oxidizing device  20  and the pickling tank  12  may be adjusted to maintain the concentration of Fe 3+  in the acid solution in the pickling tank  12  within a predetermined range including the target concentration of Fe 3+  in the acid solution  3  in the pickling tank  12  for pickling of the second steel plate portion  2   b.    
     In this case, by adjusting the supply amount of gaseous oxidant by the oxidizing device  20  or the circulation flow rate of the acid solution  3  between the oxidizing device  20  and the pickling tank  12  during pickling of the second steel plate portion  2   b  and after stopping supply of the liquid oxidant, the concentration of Fe 3+  in the acid solution in the pickling tank  12  is maintained in the above described predetermined range. Thus, it is possible to maintain the Fe 3+  in the acid solution  3  in the pickling tank  12  appropriately after stopping supply of the liquid oxidant to maintain the conveyance speed of the steel plate  2  at a high speed, and improve the production efficiency of the steel plate  2 . Furthermore, a gaseous oxidant, which is relatively inexpensive, is used to adjust the concentration of Fe 3+  in the acid solution  3  in the pickling tank  12 , and thus it is possible to suppress a cost increase. 
     Hereinafter, a method for pickling a steel plate and a pickling facility according to some embodiments will be described briefly. 
     (1) According to at least one embodiment of the present invention, a method for pickling a steel plate having a first steel plate portion and a second steel plate portion which is connected to a tail end of the first steel plate portion and which requires a longer time for pickling than the first steel plate portion when pickled under the same condition, includes: a step of pickling the steel plate by immersing the steel plate in an acid solution in at least one pickling tank while conveying the steel plate; a step of circulating the acid solution, through a circulation line connected to any of the at least one pickling tank, between the pickling tank and an oxidizing device disposed in the circulation line; a step of oxidizing Fe 3+  in the acid solution to Fe 3+  by the oxidizing device using a gaseous oxidant; and a feeding start step of, upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, starting feeding of a liquid oxidant for oxidizing Fe 3+  in the acid solution to Fe 3+  to any of the at least one pickling tank or to the circulation line. 
     In a case where the liquid oxidant is used, an oxidant is dissolved in a solution, and thus the oxidation reaction of iron ion during pickling proceeds more quickly compared to a case in which a gaseous oxidant is used. Thus, it is easier to increase the concentration of Fe 3+  in the acid solution quickly. In this regard, according to the above method (1), upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, the liquid oxidant is supplied to the pickling tank or to the circulation line. Thus, when switching to pickling of the second steel plate portion (pickling-resistant member) which requires a longer time to be pickled under the same condition, it is possible to quickly increase Fe 3+  in the acid solution in the pickling tank. Accordingly, it is possible to maintain the conveyance speed (line speed) of the steel plate at a high speed even when the kind of steel to be pickled is switched, and thus it is possible to improve the production efficiency of the steel plate. 
     (2) In some embodiments, in the above method (1), feeding of the liquid oxidant to the at least one pickling tank or the circulation line is started within a period of time during which a first connection portion being a connection portion connecting the first steel plate portion and the second steel plate portion exists in the at least one pickling tank. 
     According to the above method (2), feeding of the liquid oxidant is started within a period of time during which the first connection portion being a connection portion connecting the first steel plate portion and the second steel plate portion exists in the pickling tank, and thus it is possible to increase the concentration of Fe 3+  in the acid solution quickly after starting pickling of the second steel plate portion. Thus, it is easier to maintain the conveyance speed of the steel plate at a high speed after starting pickling of the second steel plate portion being a pickling-resistant member, and thus it is possible to improve the production efficiency of the steel plate effectively. 
     (3) In some embodiments, in the above method (1) or (2), the pickling method further includes a speed reduction step of, upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, reducing a conveyance speed of the steel plate. 
     By feeding the liquid oxidant to the acid solution, it is possible to increase the concentration of Fe 3+  in the acid solution relatively quickly, but it takes some time until the concentration of Fe 3+  in the acid solution reaches the target value. With this regard, according to the above method (3), upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, the conveyance speed of the steel plate is reduced, and thus it is possible to appropriately pickle the second steel plate portion by reducing the conveyance speed of the steel plate after starting pickling of the second steel plate portion and before the concentration of Fe 3+  in the acid solution in the pickling tank increases sufficiently. Accordingly, it is possible to suppress deterioration of the product quality. 
     (4) In some embodiments, in the above method (3), the pickling method further includes: a step of obtaining information on a position, in the conveyance direction, of a first connection portion being a connection portion connecting the first steel plate portion and the second steel plate portion; and a step of deciding a timing to reduce the conveyance speed of the steel plate on the basis of the information. 
     According to the above method (4), the timing to reduce the conveyance speed of the steel plate is determined on the basis of the information on the position of the first connection portion in the conveyance direction, and thus, for instance, it is possible to reduce the conveyance speed of the steel plate at an appropriate timing in accordance with the timing to start pickling of the second steel plate portion (that is, the timing when the second steel plate portion reaches the pickling tank). Accordingly, it is possible to pickle the second steel plate portion appropriately, and suppress deterioration of the product quality. 
     (5) In some embodiments, in the above method (3) or (4), the pickling method further includes: a step of, after the feeding start step and the speed reduction step, increasing the conveyance speed of the steel plate. 
     According to the above method (5), upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, feeding of the liquid oxidant to the acid solution is started and the conveyance speed of the steel plate is reduced, and then the conveyance speed of the steel plate is increased. Accordingly, it is possible to increase the conveyance speed of the steel plate in accordance with an increase in Fe 3+  during pickling, and thereby it is possible to maintain the conveyance speed of the steel plate at a high speed during pickling of the second steel plate portion (pickling-resistant member). Thus, it is possible to improve the production efficiency of the steel plate. 
     (6) In some embodiments, in any one of the above methods (1) to (5), the pickling method further includes a step of, upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, increasing at least one of a supply amount of the gaseous oxidant to the acid solution by the oxidizing device or a circulation flow rate of the acid solution between the oxidizing device and the at least one pickling tank to increase a concentration of Fe 3+  in the acid solution in the pickling tank. 
     According to the above method (6), upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, Fe 3+  derived from oxidation reaction using the gaseous oxidant by the oxidizing device (Fe 3+  derived from the oxidizing device) is increased to raise the concentration of Fe 3+  in the acidic solution in the pickling tank. Thus, when the concentration of Fe 3+  in the acid solution in the pickling tank is sufficiently high, it is possible to stop feeding of the liquid oxidant, which is relatively expensive. Accordingly, it is possible maintain the conveyance speed of the steel plate at a high speed and improve the production efficiency of the steel plate, while suppressing an increase in the cost for pickling the steel plate. 
     (7) In some embodiments, in any one of the above methods (1) to (6), the pickling method further includes: a step of, during pickling of the steel plate in the at least one pickling tank, stopping supply of the liquid oxidant to the at least one pickling tank or the circulation line. 
     According to the above method (7), supply of the liquid oxidant is stopped during pickling of the steel plate, and thus it is possible to supply the liquid oxidant to the pickling tank or the circulation line for a relatively short period of time. Accordingly, it is possible maintain the conveyance speed of the steel plate at a high speed and improve the production efficiency of the steel plate, while suppressing an increase in the cost for pickling the steel plate by suppressing the usage amount of the liquid oxidant, which is relatively expensive. 
     (8) In some embodiments, in the above method (7), the steel plate includes a third steel plate portion which is connected to a tail end of the second steel plate portion and which requires a shorter time to be pickled than the second steel plate portion when pickled under the same condition, and the pickling method further includes a step of, upon switching from pickling of the second steel plate portion to pickling of the third steel plate portion, reducing at least one of a supply amount of the gaseous oxidant to the acid solution by the oxidizing device or a circulation flow rate of the acid solution between the oxidizing device and the at least one pickling tank to reduce a concentration of Fe 3+  in the acid solution in the pickling tank. 
     According to the above method (8), upon switching from pickling of the second steel plate portion to pickling of the third steel plate portion, Fe 3+  derived from the oxidizing device is reduced to lower the concentration of Fe 3+  in the acid solution in the pickling tank, and thereby it is possible to suppress excessive pickling of the third steel plate portion which requires a shorter period of time to be pickled under the same condition. Thus, it is possible to reduce pickling loss of the steel plate and improve the yield ratio, thereby improving the production efficiency of the steel plate. 
     (9) In some embodiments, the above method (8) further includes a step of, upon switching from pickling of the second steel plate portion to pickling of the third steel plate portion, increasing a conveyance speed of the steel plate. 
     The third steel plate portion requires a shorter period of time to be pickled than the second steel plate portion under the same condition, and thus it is possible to pickle the third steel plate portion sufficiently even when the conveyance speed of the steel plate is increased upon switching to the third steel plate portion. According to the above method (9), upon switching from pickling of the second steel plate portion to the third steel plate portion, the conveyance speed of the steel plate is increased, and thereby it is possible to maintain the conveyance speed of the steel plate at a high speed while pickling the third steel plate portion sufficiently. Thus, it is possible to improve the production efficiency of the steel plate. 
     (10) In some embodiments, in any one of the above methods (1) to (9), the at least one pickling tank includes a plurality of pickling tanks arranged along a conveyance direction of the steel plate. The pickling method includes a step of transferring the acid solution in the pickling tank positioned at a downstream side in the conveyance direction to the pickling tank positioned at an upstream side in the conveyance direction. The feeding start step includes feeding the liquid oxidant to at least one of the plurality of pickling tanks or to the circulation line connected to the at least one of the plurality of pickling tanks. 
     According to the above method (10), with the pickling apparatus including the plurality of pickling tanks, upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, the liquid oxidant is supplied to any of the pickling tanks or to the circulation line connected to any of the pickling tanks. Thus, when switching to pickling of the second steel plate portion (pickling-resistant member) which requires a longer time to be pickled under the same condition, it is possible to quickly increase the concentration of Fe 3+  in the acid solution in the pickling tank. Accordingly, it is possible to maintain the conveyance speed of the steel plate at a high speed even when the kind of steel to be pickled is switched, and thus it is possible to improve the production efficiency of the steel plate. 
     (11) In some embodiments, in the above method (10), feeding of the liquid oxidant to the plurality of pickling tanks or the circulation line connected to the pickling tanks is started sequentially in an order of passing of a first connection portion being a connection portion connecting the first steel plate portion and the second plate portion. 
     According to the above method (11), feeding of the liquid oxidant to the plurality of pickling tanks or the circulation line connected to the pickling tanks is started sequentially in an order of passing of a first connection portion being a connection portion connecting the first steel plate portion and the second plate portion. Thus, it is possible to increase the concentration of Fe 3+  in the acid solution in the plurality of pickling tanks quickly, and thus it is easier to maintain the conveyance speed of the steel plate at a high speed after switching to pickling of the second steel plate portion. Thus, it is possible to improve the production efficiency of the steel plate effectively. 
     (12) In some embodiments, in any one of the above methods (1) to (11), the method further includes: a step of obtaining information on a position of the first connection portion in the conveyance direction; and a step of deciding a timing to start supply of the liquid oxidant on the basis of the information. 
     According to the above method (12), the timing to start supplying the liquid oxidant is determined on the basis of the information on the position of the first connection portion in the conveyance direction, and thus, for instance, it is possible to start feeding of the liquid oxidant at an appropriate timing in accordance with the timing to start pickling of the second steel plate portion (that is, the timing when the second steel plate portion reaches the pickling tank). Thus, upon switching to pickling of the second steel plate portion, it is possible to increase the concentration of Fe 3+  in the acid solution in the pickling tank at an appropriately timing, and thus it is easier to maintain the conveyance speed of the steel plate at a high speed. Thus, it is possible to improve the production efficiency of the steel plate. 
     (13) In some embodiments, in any one of the above methods (1) to (12), the method further includes: a step of detecting a concentration of Fe 3+  in the acid solution in the pickling tank; and a step of deciding a supply amount of the liquid oxidant on the basis of a difference between the detected concentration of Fe 3+  and a target concentration of Fe 3+  in the acid solution in the pickling tank for pickling of the second steel plate portion. 
     According to the above method (13), the concentration of Fe 3+  in the pickling tank is detected, and the supply amount of the liquid oxidant is determined on the basis of the difference between the detected concentration of Fe 3+  and the target concentration of Fe 3+  in the acid solution in the pickling tanks for pickling of the second steel plate portion. Therefore, by supplying the liquid oxidant on the basis of the supply amount determined accordingly, it is possible to increase Fe 3+  in the acid solution in the pickling tank, and maintain the conveyance speed of the steel plate at a high speed. Thus, it is possible to improve the production efficiency of the steel plate. 
     (14) In some embodiments, in any one of the above methods (1) to (13), the method further includes a step of, during pickling of the second steel plate portion and after stopping supply of the liquid oxidant to the at least one pickling tank or to the circulation line, adjusting at least one of a supply amount of the gaseous oxidant or a circulation flow rate of the acid solution between the oxidizing device and the at least one pickling tank so as to maintain a concentration of Fe 3+  in the acid solution in the pickling tank within a predetermined range including a target concentration of Fe 3+  in the acid solution in the pickling tank for pickling of the second steel plate portion. 
     According to the above method (14), by adjusting at least one of the supply amount of gaseous oxidant in the oxidizing device or the circulation flow rate of the acid solution between the oxidizing device and the pickling tank during pickling of the second steel plate portion and after stopping supply of the liquid oxidant, the concentration of Fe 3+  in the acid solution in the pickling tank is maintained in the above described predetermined range. Thus, it is possible to maintain the concentration of Fe 3+  in the acid solution in the pickling tank appropriately after stopping supply of the liquid oxidant and maintain the conveyance speed of the steel plate at a high speed, and improve the production efficiency of the steel plate. Furthermore, a gaseous oxidant, which is relatively inexpensive, is used to adjust the concentration of Fe 3+  in the acid solution in the pickling tank, and thus it is possible to suppress a cost increase. 
     (15) According to at least one embodiment of the present invention, a pickling apparatus for pickling a steel plate having a first steel plate portion and a second steel plate portion which is connected to a tail end of the first steel plate portion and which requires a longer time for pickling than the first steel plate portion when pickled under the same condition, includes: at least one pickling tank storing an acid solution; a conveyance part configured to convey the steel plate while immersing the steel plate in the acid solution in the at least one pickling tank; a circulation line for circulating the acid solution inside any of the at least one pickling tank, the circulation line being connected to the at least one pickling tank; an oxidizing device disposed in the circulation line and configured to oxidize Fe 3+  in the acid solution to Fe 3+  by using a gaseous oxidant; and a liquid oxidant feeding part capable of feeding a liquid oxidant for oxidizing Fe 2+  in the acid solution to Fe 3+  to any one of the at least one pickling tank or to the circulation line. 
     According to the above configuration (15), upon switching from pickling of the first steel plate portion to pickling of the second steel plate portion, the liquid oxidant is supplied to the pickling tank or the circulation line. Thus, when switching to pickling of the second steel plate portion (pickling-resistant member) which requires a longer time to be pickled under the same condition, it is possible to quickly increase the concentration of Fe 3+  in the acid solution in the pickling tank. Accordingly, it is possible to maintain the conveyance speed (line speed) of the steel plate at a high speed even when the kind of steel to be pickled is switched, and thus it is possible to improve the production efficiency of the steel plate. 
     Embodiments of the present invention were described in detail above, but the present invention is not limited thereto, and various amendments and modifications may be implemented. 
     Further, in the present specification, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. 
     For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function. Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved. 
     On the other hand, an expression such as “comprise”, “include”, “have”, “contain” and “constitute” are not intended to be exclusive of other components. 
     DESCRIPTION OF REFERENCE NUMERAL 
     
         
           1  Pickling apparatus 
           2  Steel plate 
           2   a  First steel plate portion 
           2   b  Second steel plate portion 
           2   c  Third steel plate portion 
           3  Acid solution 
           4  First connection portion 
           5  Second connection portion 
           10  Conveyance part 
           12 ,  12 A to  12 C Pickling tank 
           16  Conveyance roll 
           17  Motor 
           18  Acid-solution supply part 
           19  Acid-solution discharge part 
           20 ,  20 A to  20 C Oxidizing device 
           21 ,  21 Ato  21 C Circulation line 
           22 ,  22 A to  22 C Extract line 
           24 ,  24 A to  24 C Return line 
           30  Liquid oxidant feeding part 
           32  Liquid oxidant tank 
           33  Liquid oxidant pump 
           34  Liquid oxidant feeding line 
           36 ,  36 A to  36 C First feeding line 
           37 ,  37 A to  37 C Valve 
           38 ,  38 A to  38 C Second feeding line 
           39 ,  39 A to  39 C Valve 
           40 ,  40 A to  40 C Third feeding line 
           41 ,  41 A to  41 C Valve 
           100  Controller 
           102  Pickling speed evaluation part 
           104  Target line speed calculation part 
           106  Line speed control part