Patent Publication Number: US-9884355-B2

Title: Manufacturing method of seamless metal pipe, mandrel mill, and auxiliary tool

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a manufacturing method of a seamless metal pipe, a mandrel mill, and an auxiliary tool, and particularly, a manufacturing method of a seamless metal pipe using a mandrel mill, the mandrel mill, and an auxiliary tool used in the manufacturing method of the seamless metal pipe. 
     Priority is claimed on Japanese Patent Application No. 2012-163437, filed on Jul. 24, 2012, the content of which is incorporated herein by reference. 
     RELATED ART 
     In a manufacturing method of a seamless metal pipe using a mandrel mill, first, a heated round billet is pierced by a piercing mill, and thus, a hollow shell is manufactured. A mandrel bar is inserted into the manufactured hollow shell. The hollow shell into which the mandrel bar is inserted is elongated by a mandrel mill. At this time, each stand of the mandrel mill performs thickness reduction on the hollow shell. Accordingly, an outer diameter and a thickness of the hollow shell are changed by the elongating. The elongated hollow shell is heated as needed, and is reduction-rolled by a sizing mill or a stretch reducing mill. According to the above-described processes, a seamless metal pipe is manufactured. 
     In the elongating, a plurality of (for example, 10 to 20) mandrel bars are used every time one lot of the hollow shells having a specific size (outer diameter and thickness) is manufactured. Accordingly, if a plurality of sizes in the manufactured seamless metal pipe are present, the stock quantity of the mandrel bars is significantly increased. The costs of the mandrel bars are increased as the stock quantity is increased. 
     Patent Document 1 and Patent Document 2 suggest arts having objects for decreasing the costs of the mandrel bars. 
     In Patent Document 1, from a used mandrel bar, a rolling portion of a front half portion is cut, and a support portion of a rear half portion remains. Moreover, the front half portion is replaced by a new front half portion. At this time, a short joining material is disposed between the front half portion and the support portion, and the front half portion, the short joining material, and the support portion are integrated by friction welding. Accordingly, Patent Document 1 discloses that the mandrel bar can be reused. 
     Similar to Patent Document 1, also in Patent Document 2, the mandrel bar is divided into a rolling portion which comes into contact with a shell, and a holding portion which does not come into contact with the shell. The rolling portion is connected to the holding portion by a screw. Also in this case, Patent Document 2 discloses that the costs of the mandrel bars can be suppressed since only the rolling portion can be repaired and replaced. 
     CITATION LIST 
     Patent Document 
     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H04-344805 
     [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H10-249411 
     SUMMARY OF THE INVENTION 
     However, in Patent Documents 1 and 2, it is considered that the length of the rolling portion is constant. This is because each stand of the mandrel mill performs thickness reduction and the rolling portion requires a length corresponding to at least a distance from a head stand of the mandrel mill to an end stand. Accordingly, even when the manufacturing cost of the holding portion can be decreased by reusing the holding portion (support portion), the manufacturing cost of the rolling portion is not decreased. The rolling portion is formed of a material having improved strength, heat crack resistance and wear resistance than the holding portion, and the material is more expensive than a material used in the holding portion. That is, the manufacturing cost of the mandrel bar is dependent on the rolling portion. 
     Problems to be Solved by the Invention 
     An object of the present invention is to provide a manufacturing method of a seamless metal pipe, a mandrel mill, and an auxiliary tool capable of suppressing the cost of a mandrel bar required for elongating. 
     Means for Solving the Problem 
     In order to solve the above-described problems, the present invention adopts the following measures. 
     (1) According to a first aspect of the present invention, a manufacturing method of a seamless metal pipe which manufactures a seamless metal pipe from a hollow shell using a mandrel mill having a preceding-stage stand group including a plurality of stands arranged from a head along a pass line and a succeeding-stage stand group including a plurality of stands arranged behind the preceding-stage stand group, the manufacturing method includes: preparing a plurality of mandrel bars in which lengths of work portions which come into contact with the hollow shell during elongating are different from one another; selecting a mandrel bar including a work portion having a length corresponding to the number of stands used in the thickness reduction, among the plurality of mandrel bars; inserting the mandrel bar selected in the selecting into the hollow shell; and performing the elongating on the hollow shell into which the mandrel bar is inserted. In the manufacturing method of a seamless metal pipe, in the elongating, outer diameter reduction is performed by one of the preceding-stage stand group and the succeeding-stage stand group and the thickness reduction is performed by the other of the preceding-stage stand group and the succeeding-stage stand group, or the thickness reduction is performed by both of the preceding-stage stand group and the succeeding-stage stand group. 
     (2) In the aspect according to the above (1), the manufacturing method may further include mounting a rod shaped auxiliary tool, which includes a holding portion capable of holding a rear end portion of the mandrel bar at a tip, on a rear end of the mandrel bar; and moving a holding device forward while holding a rear end of the auxiliary tool by the holding device. 
     (3) In the aspect according to the above (2), the manufacturing method may further include supporting the mandrel bar during a forward movement by a support roll disposed between the plurality of stands and the holding device by lifting the support roll; and adjusting a height of the support roll by lifting and lowering the support roll based on a forward movement distance of the auxiliary tool when an outer diameter of the auxiliary tool is different from an outer diameter of the mandrel bar. 
     (4) In the aspect according to the above (3), in the adjusting, when the outer diameter of the auxiliary tool is larger than the outer diameter of the mandrel bar, the support roll may be lowered before the auxiliary tool passes through the support roll. 
     (5) In the aspect according to the above (1) or (2), in the elongating, the outer diameter reduction may be performed by the preceding-stage stand group, and total lengths of the plurality of mandrel bars are the same as one another. 
     (6) According to a second aspect of the present invention, a mandrel mill includes: a plurality of stands which are arranged along a pass line; and a retaining system which includes a rod shaped auxiliary tool which is disposed at an inlet side of a head stand among the plurality of stands and includes a holding portion capable of holding a rear end portion of a mandrel bar at a front end, a holding device capable of holding a rear end of the auxiliary tool, and a drive device which moves the holding device forward along the pass line. 
     (7) According to a third aspect of the present invention, an auxiliary tool which is used in a retaining system including a holding device capable of holding a rear end of a mandrel bar and a drive device which moves the holding device forward, the auxiliary tool includes: a rod shaped main body; a holding portion which is disposed on a front end of the main body and is capable of holding the rear end of the mandrel bar; and a mounting portion which is disposed on a rear end of the main body and has a shape capable of being held by the holding device. 
     Effects of the Invention 
     According to the aspects, it is possible to suppress a cost of a mandrel bar required for elongating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a function block diagram showing a manufacturing equipment of a seamless metal pipe. 
         FIG. 2  is a schematic diagram showing a main portion of a piercing mill in  FIG. 1 . 
         FIG. 3  is a function block diagram showing a mandrel mill in  FIG. 1 . 
         FIG. 4  is a side diagram of a rolling mill body of the mandrel mill in  FIG. 3 . 
         FIG. 5  is a front diagram of a stand in  FIG. 4 , and is a cross-sectional diagram taken along line A-A of  FIG. 4 . 
         FIG. 6  is a front diagram of a stand different from  FIG. 5 , and is a cross-sectional diagram taken along line B-B of  FIG. 4 . 
         FIG. 7  is a schematic diagram showing elongating of a hollow shell by the mandrel mill. 
         FIG. 8  is a vertical cross-sectional diagram of a retaining system in  FIG. 3 . 
         FIG. 9  is a front diagram of a support member in  FIG. 8 . 
         FIG. 10A  is a plan diagram of a holding member and a mandrel bar of the retaining system. 
         FIG. 10B  is a vertical cross-sectional diagram of the holding member and the mandrel bar shown in  FIG. 10A . 
         FIG. 10C  is a plan diagram showing a state where the mandrel bar is mounted on the holding member of  FIG. 10A . 
         FIG. 10D  is a vertical cross-sectional diagram of the holding member and the mandrel bar shown in  FIG. 10C . 
         FIG. 11  is a schematic diagram of the rolling mill body shown in  FIG. 3  and an extracting mill. 
         FIG. 12  is a schematic diagram showing “entire thickness reduction” in the mandrel mill. 
         FIG. 13  is a schematic diagram showing “partial outer diameter reduction” in the mandrel mill. 
         FIG. 14  is a flowchart showing a manufacturing process of a seamless metal pipe according to the present embodiment. 
         FIG. 15  is a side diagram of the mandrel bar. 
         FIG. 16  is a schematic diagram showing the state of the mandrel bar during the entire thickness reduction. 
         FIG. 17  is a schematic diagram showing the state of the mandrel bar during the partial outer diameter reduction. 
         FIG. 18  is a schematic diagram showing the state of the mandrel bar in a case where outer diameter reduction is performed by a succeeding-stage stand group of the mandrel mill. 
         FIG. 19  is a schematic diagram showing the elongating in the mandrel mill when an auxiliary tool is used. 
         FIG. 20  is a vertical cross-sectional diagram of the auxiliary tool in  FIG. 19 . 
         FIG. 21  is a front diagram of the auxiliary tool of  FIG. 20 , and is a cross-sectional diagram taken along line C-C of  FIG. 20 . 
         FIG. 22  is a plan diagram of the auxiliary tool of  FIG. 20 . 
         FIG. 23  is a diagram showing a modification of the auxiliary tool of  FIG. 20 , and is a vertical cross-sectional diagram of the auxiliary tool having a plurality of grooves. 
         FIG. 24  is a plan diagram of the auxiliary tool. 
         FIG. 25  is a schematic diagram showing the elongating in the mandrel mill when the auxiliary tool and a support roll are used. 
         FIG. 26  is a flowchart showing an operation of a control device in  FIG. 25 . 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same reference numerals are assigned to the same portions or the corresponding portions in the drawings and descriptions thereof are not repeated here. 
     According to a manufacturing method of a seamless metal pipe according to the present embodiment, a seamless metal pipe is manufactured from a hollow shell using a mandrel mill having a preceding-stage stand group including a plurality of stands arranged from a head along a pass line and a succeeding-stage stand group including a plurality of stands arranged behind the preceding-stage stand group. The manufacturing method of a seamless metal pipe includes: preparing a plurality of mandrel bars in which lengths of work portions coming into contact with the hollow shell during elongating are different from one another; selecting a mandrel bar including a work portion having a length corresponding to the number of stands used in thickness reduction, among the plurality of mandrel bars; inserting the mandrel bar selected in the selecting into the hollow shell; and performing the elongating on the hollow shell into which the mandrel bar is inserted. Moreover, in the elongating, outer diameter reduction is performed by one of the preceding-stage stand group and the succeeding-stage stand group and the thickness reduction is performed by the other of the preceding-stage stand group and the succeeding-stage stand group, or the thickness reduction is performed by both of the preceding-stage stand group and the succeeding-stage stand group. 
     In the present embodiment, the mandrel mill does not only perform thickness reduction by all stands, but also performs outer diameter reduction by any one of the preceding-stage stand group and the succeeding-stage stand group. Here, elongating is referred to as “partial outer diameter reduction”, in which the outer diameter reduction is performed by any one of the preceding-stage stand group and the succeeding-stage stand group and the thickness reduction is performed by the other. Moreover, elongating is referred to as “entire thickness reduction”, in which the thickness reduction is performed by both of the preceding-stage stand group and the succeeding-stage stand group. 
     When the mandrel mill performs the partial outer diameter reduction, a work portion is not required in the stand in which the outer diameter reduction is performed. This is because an inner surface of the hollow shell is not needed to come into contact with the work portion in the outer diameter reduction. Accordingly, compared to when the entire thickness reduction is performed, when the partial outer diameter reduction is performed, the work portion may be shortened by a length corresponding to the number of the stands of the stand group in which the outer diameter reduction is performed. 
     In other words, when the partial outer diameter reduction is performed, the length corresponding to the number of the stands in which the thickness reduction is performed is enough for the length of the work portion. 
     Accordingly, in the present embodiment, a plurality of mandrel bars in which lengths of the work portions are different from one another are prepared in advance, and a mandrel bar, which includes a work portion having a length corresponding to the number of the stands used in the thickness reduction among the plurality of stands of the mandrel mill, is used. 
     In the above-described manufacturing method, unlike the related art, the length of the work portion does not need to be constant, and thus, a mandrel bar having a shorter work portion than that of the related art can be prepared. Accordingly, the cost of the mandrel bar can be suppressed. 
     Preferably, the above-described manufacturing method further includes mounting a rod shaped auxiliary tool, which includes a holding portion capable of holding a rear end portion of the mandrel bar at a tip, on a rear end of the mandrel bar; and moving a holding device forward while holding a rear end of the auxiliary tool by the holding device. 
     In this case, the auxiliary tool is used, and thus, the length of the mandrel bar can be shortened. Accordingly, a stock space of the mandrel bars can be suppressed, and thus, the costs of the mandrel bars can be also suppressed. 
     Preferably, the above-described manufacturing method further includes supporting the mandrel bar during a forward movement by the support roll by lifting the support roll which is disposed between the plurality of stands and the holding device and can be lifted and lowered; and adjusting a height of the support roll by lifting and lowering the support roll based on a forward movement distance of the auxiliary tool when an outer diameter of the auxiliary tool and an outer diameter of the mandrel bar are different from each other. 
     In this case, even when the outer diameter of the auxiliary tool and the outer diameter of the mandrel bar are different from each other, the height of the support roll can be appropriately adjusted by the auxiliary tool. 
     Preferably, in the adjusting, when the outer diameter of the auxiliary tool is larger than the outer diameter of the mandrel bar, the support roll is lowered before the auxiliary tool passes through the support roll. 
     In this case, collision between the auxiliary tool and the support roll can be suppressed. 
     In the above-described manufacturing method, in the elongating, when the outer diameter reduction is performed by the preceding-stage stand group, total lengths of the plurality of mandrel bars may be the same as one another. 
     When the outer diameter reduction is performed (that is, the partial outer diameter reduction is performed) in the preceding-stage stand group, the thickness reduction is performed in the succeeding-stage stand group. In this case, since the final stand of the succeeding-stage stand group performs the thickness reduction, total lengths of the plurality of mandrel bars used in the elongating are the same as one another. In this case, in the mandrel bar, a length of an extension portion which does not come into contact with the hollow shell HS during the elongating is also changed in the portions other than the work portion. Specifically, the extension portion is lengthened as the work portion is shortened. Since costs of the material and the machining of the work portion are more expensive than those of the extension portion, also in this case, the cost of the mandrel bar can be suppressed. 
     A mandrel mill according to the present embodiment is used in the above-described manufacturing method of a seamless metal pipe. The mandrel mill includes a plurality of stands and a retaining system. The plurality of stands are arranged along a pass line and include a plurality of rolls. The retaining system is disposed at an inlet side of a head stand among the plurality of stands and moves the mandrel bar forward during the elongating. The retaining system includes an auxiliary tool, a holding device, and a drive device. The auxiliary tool includes a holding portion capable of holding a rear end portion of a mandrel bar at a front end. The holding device can hold a rear end of the auxiliary tool. The drive device moves the holding device forward along the pass line. 
     The mandrel mill according to the present embodiment includes the auxiliary tool. Accordingly, the total length of the mandrel bar can be shortened. As a result, the cost of the mandrel bar can be decreased. 
     The auxiliary tool according to the present embodiment is used in a retaining system including a holding device capable of holding the rear end portion of the mandrel bar and a drive device moving the holding device forward. 
     The auxiliary tool includes a rod shaped main body, a holding portion, and a mounting portion. The holding portion is disposed on a front end of the main body and holds the rear end portion of the mandrel bar. The mounting portion is disposed on a rear end of the main body and has a shape which can be held by the holding device. 
     The auxiliary tool according to the present embodiment can be disposed between the mandrel bar and the retaining system during the elongating. Accordingly, the length of the mandrel bar can be shortened, and the cost of the mandrel bar can be decreased. 
     Hereinafter, the present embodiment will be described in detail. 
     [Manufacturing Equipment of Seamless Metal Pipe] 
       FIG. 1  is a block diagram showing an outline of manufacturing equipment of a seamless metal pipe according to the present embodiment. In the manufacturing equipment of a seamless metal pipe, the seamless metal pipe is manufactured by a so-called Mannesmann mandrel mill method. With reference to  FIG. 1 , the manufacturing equipment of the present embodiment includes a heating furnace  1 , a piercing mill  2 , and a mandrel mill  3 . Each transport device  10  is disposed among the heating furnace  1 , the piercing mill  2 , and the mandrel mill  3 . For example, each transport device  10  includes a plurality of transport rollers and transports a round billet or a hollow shell. 
     [Heating Furnace  1  and Piercing Mill  2 ] 
     The heating furnace  1  accommodates a solid round billet which is a material of the seamless metal pipe, and heats the billet. As shown in  FIG. 2 , the piercing mill  2  includes a pair of inclined rolls  21  and a plug  22 . The plug  22  is disposed between the pair of inclined rolls  21  and on a pass line (rolling axis) PL. In the piercing mill  2 , by both inclined rolls  21 , a round billet BL interposed between both inclined rolls  21  is pushed into the plug  22  while being rotated around the circumferential direction, the round billet BL is pierced, and a hollow shell HS is manufactured. 
     [Mandrel Mill  3 ] 
     In the mandrel mill  3 , a mandrel bar is inserted into the hollow shell HS, and the hollow shell HS into which the mandrel bar is inserted is elongated by a rolling mill body. After the mandrel bar is extracted from the hollow shell HS which is elongated by the mandrel mill  3 , the hollow shell is transported to a reduction mill (not shown). For example, the reduction mill is a sizing mill or a stretch reducing mill. The reduction mill performs reduction rolling on the hollow shell HS and manufactures the seamless metal pipe. 
       FIG. 3  is a block diagram showing a configuration of the mandrel mill  3 . With reference to  FIG. 3 , the mandrel mill  3  includes a retaining system  31 , a rolling mill body  32 , and an extracting mill  33 . The retaining system  31 , the rolling mill body  32 , and the extracting mill  33  are arranged in a line. The retaining system  31  inserts the mandrel bar into the hollow shell HS before the rolling mill body  32  performs the elongating on the hollow shell HS, or extracts the mandrel bar from the hollow shell HS after the elongating. The rolling mill body  32  performs the elongating on the hollow shell HS. The extracting mill  33  is used for extracting the mandrel bar from the hollow shell HS after the elongating. Hereinafter, each facility will be described. 
     [Rolling Mill Body  32 ] 
       FIG. 4  is a side diagram of the rolling mill body  32  of the mandrel mill  3 . With reference to  FIG. 4 , the rolling mill body  32  includes a plurality of stands ST 1  to STm (m is a natural number) which are arranged in a line along the pass line PL. The total number m of the stands is not particularly limited. For example, the total number m of the stands is 4 to 8. 
       FIGS. 5 and 6  are cross-sectional diagrams of a stand STi (i=2 to m) and a stand STi−1. With reference to  FIGS. 5 and 6 , in the present example, each of the stands ST 1  to STm includes three rolls RO which are disposed at positions of 120° to one another around the pass line PL. Each roll RO includes a groove GR in which a cross-sectional shape is formed in an arcuate shape when viewed from the cross-section including the central axis, and a hole die PA is formed by the grooves GR of three rolls RO. 
     As shown in  FIGS. 5 and 6 , when viewed along the pass line PL, three rolls RO included in the succeeding-stage stand STi (i=2 to m) are disposed to be deviated by 60° around the pass line PL from three rolls RO included in the preceding-stage stand STi−1. 
     Three rolls RO of each of the stands ST 1  to STm are driven to be rotated by three motors (not shown). 
     In the cross-sectional area of the hole die PA formed of three rolls RO in each stand ST, the cross-sectional area of the hole die of the succeeding-stage stand is smaller than that of the preceding-stage stand. 
     As shown in  FIG. 7 , the hollow shell HS, into which the mandrel bar  40  is inserted, is elongated through the stand ST 1  to STm along the pass line PL, and an outer diameter and a thickness of the hollow shell HS are changed. 
     In the rolling mill body  32  shown in  FIGS. 4 to 7 , each stand STi includes three rolls RO. However, the number of rolls is not limited to three. Each stand STi may include a plurality of rolls RO. The number of the rolls may be two or four. More specifically, the stand STi includes n (n is a natural number of two or more) rolls disposed around the pass line PL, and the n rolls of the succeeding stage are disposed to be deviated by 180°/n around the pass line PL from n rolls included in the stand STi−1 of the preceding stage. 
     [Retaining System  31 ] 
       FIG. 8  is a vertical cross-sectional diagram of the retaining system  31 . The retaining system  31  moves the mandrel bar  40  forward while holding the rear end portion of the mandrel bar  40 , and inserts the mandrel bar  40  into the hollow shell HS. In addition, the retaining system  31  moves the hollow shell HS, into which the mandrel bar  40  is inserted, forward along the pass line PL during the elongating. 
     With reference to  FIG. 8 , the retaining system  31  includes a drive source  311  including a motor and a reducing gear, a drive wheel  312 , a driven wheel  313 , a chain  314 , a plurality of support members  315 , and a holding member  316 . 
     The drive source  311  rotates the drive wheel  312  in a forward direction (clockwise direction in  FIG. 8 ) and a backward direction (counterclockwise direction in  FIG. 8 ). The driven wheel  313  is disposed to be apart from the drive wheel  312  at the front side of the drive wheel  312 . The chain  314  is suspended over the drive wheel  312  and the driven wheel  313  and forms an endless track. The drive source  311 , the drive wheel  312 , the driven wheel  313 , and the chain  314  configure a drive device which moves the mandrel bar  40  forward or rearward by a reference distance Dref. 
     The plurality of support members  315  are arranged on the outer surface of the chain  314  in a line.  FIG. 9  is a front diagram of the support member  315 . In addition, a two-dot chain line in  FIG. 9  indicates the mandrel bar  40 . The support member  315  includes an inverted triangular groove  317 . A width of the groove  317  is gradually decreased from the upper end of the support member  315  toward the lower end. The plurality of support members  315  support the mandrel bar  40  so that the axis of the mandrel bar  40  continuously coincides with the pass line PL while the retaining system  31  moves the mandrel bar  40  forward. 
       FIGS. 10A and 10B  are a plan diagram and a vertical cross-sectional diagram of the holding member  316  and the mandrel bar  40 .  FIGS. 10C and 10D  are a plan diagram and a vertical cross-sectional diagram of the holding member  316  which holds the rear end of the mandrel bar  40 . 
     With reference to  FIGS. 8, 10A, and 10B , the holding member  316  is fixed onto the upper surface of the chain  314 . The holding member  316  moves forward or rearward (refer to  FIG. 8 ) by the reference distance Dref (between a start position Pstart and an end position Pend) by operating (rotating) the chain  314 . 
     With reference to  FIGS. 10A and 10B , the holding member  316  includes a groove  319  and a hook  318 . The groove  319  is formed on the upper surface of the holding member  316  and extends to be perpendicular to an axial direction of the mandrel bar  40 . The hook  318  is formed further forward than the groove  319  and includes an upward convex shape. 
     The mandrel bar  40  has a rod shape and a cross-section shape perpendicular to the axis is a circle. The mandrel bar  40  includes a neck  410  and a flange  420  at the rear end. The neck  410  has a rod shape in which the cross section perpendicular to the axis is a circle, and an outer diameter of the neck  410  is smaller than an outer diameter of a main body portion of the mandrel bar  40 . The flange  420  is disposed at the rear end of the neck  410 . The flange  420  is formed in a disk shape and has a larger outer diameter than that of the neck  410 . 
     A width of the groove  319  is approximately the same as or slight larger than a width of the flange  420 . In addition, a bottom surface of the groove  319  is curved to be concave in an arc shape. A concave portion  320  to which the neck  410  is fitted is formed on the upper surface of the hook  318 . 
     As shown in  FIGS. 10C and 10D , the flange  420  is fitted to the groove  319  of the holding member  316 . Accordingly, the holding member  316  holds the mandrel bar  40 . The holding member  316  moves forward by the reference distance Dref shown in  FIG. 8  while holding the rear end portion (neck  410  and flange  420 ) of the mandrel bar  40  disposed in the hollow shell HS during the elongating by the rolling mill body  32 . At this time, the drive device (the drive source  311 , the drive wheel  312 , the driven wheel  313 , and the chain  314 ) of the retaining system  31  moves the holding member  316  forward by the reference distance Dref. In this way, the retaining system  31  controls the forward speed of the mandrel bar  40  during the elongating by the rolling mill body  32 . In addition, the retaining system  31  inserts the mandrel bar  40  into the hollow shell HS before the elongating is performed. Moreover, the retaining system  31  moves the holding member  316  rearward after the elongating is performed, and extracts the mandrel bar  40  from the elongated hollow shell HS. 
     The retaining system  31  moves the holding member  316  forward or rearward by the drive device which forms an endless track by the chain  314 . However, the drive device of the retaining system  31  may include other configurations. For example, the drive device of the retaining system  31  may include a rack and pinion and thus, move the holding member  316  forward or rearward. In addition, the drive device may include an electric or hydraulic cylinder, mount the holding member  316  on the tip of the cylinder, and thus, move the holding member  316  forward or rearward. 
     [Extracting Mill  33 ] 
     With reference to  FIG. 11 , the extracting mill  33  includes a plurality of stands SA 1  to SAr (r is a natural number) which are arranged in a line along the pass line PL. Each of the stands SA 1  to SAr includes a plurality of rolls which are disposed at equal intervals around the pass line PL. The number of the rolls in each of the stands SA 1  to SAn may be two, three, or four. For example, the total number r of the stands of the extracting mill  33  is 2 to 4. 
     The extracting mill  33  bites the tip portion of the hollow shell HS when the hollow shell HS is elongated by the rolling mill body  32 , and performs slight reduction rolling on the tip portion. When the tip portion of the hollow shell HS is reduction-rolled by the extracting mill  33 , the retaining system  31  reversely rotates the drive wheel  312  and moves the holding member  316  rearward. Accordingly, the mandrel bar  40  is extracted from the hollow shell HS to the rear side. In brief, the extracting mill  33  is equipment for extracting the mandrel bar  40 . 
     In the present embodiment, the extracting mill  33  is used to extract the mandrel bar  40 . However, instead of the extracting mill  33 , a reduction mill such as a sizing mill or a stretch reducing mill may be disposed. Similar to the extracting mill  33 , the reduction mill also performs the reduction rolling on the hollow shell. Accordingly, similar to the case where the extracting mill  33  is used, the mandrel bar  40  can be extracted from the hollow shell HS. 
     [Manufacturing Process of Seamless Metal Pipe] 
     In a manufacturing method of a seamless metal pipe according to the present embodiment, the number of the stands used for thickness reduction in the rolling mill body  32  of the mandrel mill  3  is changed according to the steel grade and the elongation ratio of the seamless metal pipe. 
     For example, when a hollow shell formed of a steel grade having a high rolling force such as high alloy is elongated or when the elongation ratio of the seamless metal pipe is high, as shown in  FIG. 12 , the thickness reduction is performed by all stands ST 1  to STm of the mandrel mill  3 . Here, the “thickness reduction” means that the hollow shell HS is reduced while the inner surface of the hollow shell HS comes into contact with the outer surface of the mandrel bar  40  when the hollow shell HS comes into contact with the rolls RO in the stand STi and is reduced. In this case, the hollow shell HS is interposed between the rolls RO and the mandrel bar  40  and is elongated, and thus, the thickness of the hollow shell is changed. Since the thickness reduction is performed by all stands ST 1  to STm, this case is adopted when a seamless metal pipe having a high rolling force is manufactured and when a seamless metal pipe having a high elongation ratio is manufactured. Hereinafter, the elongating shown in  FIG. 12  is referred to as “entire thickness reduction”. 
     On the other hand, when a hollow shell formed of a steel grade having a low rolling force such as common steel is elongated, or when the elongation ratio of a seamless metal pipe is low, among the stands ST 1  to STm of the mandrel mill  3 , it is sufficient if a portion of the plurality of stands ST performs the thickness reduction. Accordingly, in this case, as shown in  FIG. 13 , instead of the thickness reduction, outer diameter reduction is performed in a stand group (hereinafter, referred to as a preceding-stage stand group FST) including the plurality of stands ST 1  to STj (j is a natural number, j&lt;m) which are continuously arranged from the head among the plurality of stands ST 1  to STm, and the thickness reduction is performed in a stand group (hereinafter, referred to a succeeding-stage stand group RST) including the stands STj+1 to STm. Here, the “outer diameter reduction” means that the hollow shell HS is reduced while the inner surface of the hollow shell HS does not come into contact with the outer surface of the mandrel bar  40  when the hollow shell HS comes into contact with the rolls RO in the stands STi (i=1 to j) and is reduced. In other words, in the preceding-stage stand group FST, reduction rolling is performed. Hereinafter, this elongating is referred to as “partial outer diameter reduction”. 
     In the partial outer diameter reduction, the diameter of the hollow shell HS manufactured by the piercing mill  2  can be further decreased. Accordingly, for example, the outer diameter reduction is performed on the hollow shell, which should be rolled to a predetermined outer diameter by the piercing mill  2  in the related art, by the preceding-stage stand group FST, and thus, a predetermined outer diameter can be achieved. Therefore, the outer diameter of the hollow shell, which is to be finished by the piercing mill  2 , can be larger than that of the related art. In this case, the frequency for exchanging the inclined roll  21  of the piercing mill  2  according to the outer diameter dimension of the hollow shell to be manufactured can be decreased. This is because the size which is to be reduced by the piercing mill  2  can be replaced by the preceding-stage stand group FST. Accordingly, by performing the partial outer diameter reduction, the frequency for exchanging the roll can be decreased, and a degree of freedom in rolling schedules of the piercing mill  2  and the mandrel mill  3  can be increased. In other words, in the manufacturing process of the seamless metal pipe of the present embodiment, operation ratios of the piercing mill  2  and the mandrel mill  3  can be increased, and as a result, production efficiency can be increased. 
     When the partial outer diameter reduction is performed, the outer diameter of the hollow shell HS manufactured by the piercing mill  2  can be more uniformly adjusted by the preceding-stage group FST. Accordingly, the dimension accuracy of the seamless metal pipe can be further increased. 
     In the present embodiment, the stands ST 1  to STm of the mandrel mill  3  are classified into the preceding-stage stand group FST and the succeeding-stage stand group RST as needed, and the “entire thickness reduction” or the “partial outer diameter reduction” is performed. Hereinafter, a manufacturing process will be described in detail. 
       FIG. 14  is a flowchart of the manufacturing method of the seamless metal pipe according to the present embodiment. With reference to  FIG. 14 , first, a roll distance Droll (a distance from the center of the pass line PL to the groove GR of the roll RO) of each of the stands ST 1  to STm of the mandrel mill  3  is set according to the steel grade and the size of a seamless metal pipe to be manufactured (Step S 1 ). 
     According to the setting of Step S 1 , it is determined whether or not the mandrel mill  3  performs the entire thickness reduction or the partial outer diameter reduction. In addition, according to the setting of Step S 1 , when the partial outer diameter reduction is performed, the stands ST 1  to STj which are included in the preceding-stage stand group FST are determined. In brief, the total number j of the stands included in the preceding-stage stand group FST can be changed according to the setting of Step S 1 . For example, the total number j of the stands included in the preceding-stage stand group FST is determined based on the steel grade and/or the size (outer diameter and thickness) of the manufactured seamless metal pipe. 
     For example, the roll distance Droll of each stand STi is determined in advance in accordance with the steel grade and the size (outer diameter and thickness) of the manufactured seamless metal pipe. In addition, the roll distance Droll is in association with the steel grade and the size of the seamless metal pipe and is recorded in a storage device (HDD or memory) of a computer (not shown). By reading the value of the roll distance Droll corresponding to the steel grade and the size of the manufactured seamless metal pipe from the computer, the roll distance Droll of each of the stands ST 1  to STm is adjusted to the value of the roll distance Droll to be set. 
     In addition, the used mandrel bar is selected according to the size (outer diameter dimension and thickness dimension) of the seamless metal pipe to be manufactured (Step S 2 ). In the present embodiment, a plurality of mandrel bars having outer diameters different from one another are prepared in advance according to the size of the seamless metal pipe. In Step S 2 , a mandrel bar having an appropriate outer diameter is selected from the mandrel bars. 
     Subsequently, a round billet is heated in the heating furnace  1  (Step S 3 ). The round billet may be manufactured by continuous casting, or may be manufactured by rolling an ingot or a slab. The heated round billet is pierced by the piercing mill  2 , and thus, the hollow shell HS is manufactured (Step S 4 ). 
     Subsequently, the mandrel bar  40  selected in Step S 2  is inserted into the hollow shell HS (Step S 5 ). In the present embodiment, the retaining system  31  inserts the mandrel bar  40  into the hollow shell HS. 
     Subsequently, the hollow shell HS is elongated by the mandrel mill  3  (Step S 6 ). The mandrel mill  3  performs the entire thickness reduction or the partial outer diameter reduction on the hollow shell HS according to the setting of the roll distance Droll in Step S 1 . After the elongating is performed by the mandrel mill  3 , the hollow shell HS is reduction-rolled by the sizing mill or the stretch reducing mill, and thus, the seamless metal pipe is manufactured (Step S 7 ). 
     According to the above-described processes, in the manufacturing method of the seamless metal pipe of the present embodiment, the entire thickness reduction or the partial outer diameter reduction is performed by the mandrel mill  3  according to the steel grade and the size of the manufactured seamless metal pipe. Accordingly, with respect to the seamless metal pipe formed of a steel grade having a high rolling force and the seamless metal pipe having a high elongation ratio, the entire thickness reduction is performed, and the rolling can be performed by the mandrel mill  3 . In addition, with respect to the seamless metal pipe formed of a steel grade having a low rolling force and the seamless metal pipe having a low elongation ratio, the partial outer diameter reduction is performed, frequency of the roll exchange in the piercing mill  2  and the rolling mill body  32  of the mandrel mill  3  is decreased, and the degree of freedom of the rolling schedule can be increased. Accordingly, the operating ratios of the piercing mill  2  and the mandrel mill  3  are increased, and production efficiency can be increased. 
     As described above, the mandrel mill  3  performs the “entire thickness reduction” and the “partial outer diameter reduction”. Accordingly, the number of the stands performing the thickness reduction in the rolling mill body  32  of the mandrel mill  3  is changed according to the steel grade and the size of the hollow shell HS. Therefore, in the present embodiment, the mandrel bar  40  is selected according to the number of the stands performing the thickness reduction. 
       FIG. 15  is a side diagram of the mandrel bar  40 . With reference to  FIG. 15 , the mandrel bar  40  includes a work portion  401  and an extension portion  402 . The work portion  401  and the extension portion  402  are manufactured of a separate material, and are connected to be coaxial with each other. For example, threading is performed on the rear end of the work portion  401  and the front end of the extension portion  402 , the rear end and the front end are fastened, and thus, the work portion and the extension portion are connected to each other. The work portion  401  and the extension portion  402  may be connected to each other by screws, may be connected to each other by welding, and may be connected to each other by other methods. 
     The work portion  401  is disposed on the front portion of the mandrel bar  40 . The work portion  401  comes into contact with the inner surface of the hollow shell HS when the elongating is performed. That is, the work portion  401  is a portion which is used for the thickness reduction in the mandrel bar  40 . Since the work portion  401  easily receives heat from the hollow shell HS and easily receives compressive stress in the thickness direction and tensile stress in the axial direction, wear and crack easily occur in the work portion  401 . Therefore, an expensive material having improved high temperature strength, heat crack resistance, and wear resistance represented by an alloy tool steel (SKD) of JIS standard is used for the work portion  401 . In addition, accuracy in the thickness of the seamless metal pipe is dependent on the shape (outer diameter accuracy) of the work portion  401 , and cleanliness of the inner surface of the seamless metal pipe is dependent on the cleanliness of the outer surface of the work portion  401 . Accordingly, the work portion  401  requires a material having improved mechanical characteristics, high outer diameter accuracy, and high outer surface cleanliness. Accordingly, the manufacturing cost of the work portion  401  is high. 
     The extension portion  402  is mounted on the rear end of the work portion  401  to be coaxial with the work portion  401 . The neck  410  and the flange  420  are formed on the rear end portion of the extension portion  402 . The extension portion  402  does not come into contact with the inner surface of the hollow shell HS during the elongating. Accordingly, compared to the work portion  401 , the extension portion  402  does not require high mechanical characteristics (strength, heat crack resistance, and wear resistance), outer diameter accuracy, and outer surface cleanliness. Accordingly, the extension portion  402  can use a cheaper material than the work portion  401 , and thus, the manufacturing cost can be suppressed. In addition, the outer diameter of the extension portion  402  may be smaller than the outer diameter of the work portion  401 , and in this case, the manufacturing cost can be further suppressed. 
     As described above, in the mandrel mill  3 , either the entire thickness reduction or the partial outer diameter reduction is performed. In the case of the partial outer diameter reduction, the number j of the stands included in the preceding-stage stand group FST may be different according to the steel grade and the size of the manufactured seamless metal pipe. That is, in the mandrel mill  3 , the total number of the stands ST performing the thickness reduction may be different according to the steel grade and the size of the seamless metal pipe. 
     Accordingly, in the present embodiment, the plurality of mandrel bars  40  including the work portions  401  having different lengths are prepared according to the number of the stands performing the thickness reduction. As described above, in Step S 2  of  FIG. 14 , when the mandrel bar  40  is selected, a plurality of kinds of mandrel bars  40  having outer diameters according to the size of the manufactured seamless metal pipe are selected. 
     Here, the number of the stands performing the thickness reduction is determined by the setting of the roll distance Droll of Step S 1 . Accordingly, among the selected plurality of kinds of mandrel bars  40 , the mandrel bar  40  including the work portion  401  having the length corresponding to the number of the stands performing the thickness reduction is determined as the used mandrel bar  40  (Step S 2 ). 
     For example, as shown in  FIG. 16 , when the holding member  316  of the retaining system  31  moves forward to the end position Pend on the chain  314  in the case where the entire thickness reduction is performed, the mandrel bar  40  including the work portion  401  having at least the same length as a distance from an inlet position P 1 in of the head stand ST 1  of the rolling mill body  32  to an outlet position Pmout of the final stand STm is selected. In this case, the thickness reduction can be performed using the work portion  401  in each of the stands ST 1  to STm. In addition, in this case, the extension portion  402  may have at least the same length as the distance from the end position Pend to the inlet position P 1 in. 
     On the other hand, as shown in  FIG. 17 , when the partial outer diameter reduction is performed and stands ST 1  and ST 2  correspond to the preceding-stage stand group FST, the thickness reduction is performed in the stands ST 3  to STm. Accordingly, the work portion  401  may have at least the length corresponding to the number of the stands ST 3  to STm, more specifically, the same length as a distance from an inlet position P 3  in of the stand ST 3  to the outlet position Pmout of the final stand STm. Moreover, the extension portion  402  may have at least the same length as the distance from the end position Pend to the inlet position P 3  in of the third stand ST 3 . 
     The work portion  401  when the partial outer diameter reduction is performed may be shorter than the work portion  401  when the entire thickness reduction is performed. This is because the number of the stands by which the thickness reduction is performed in the partial outer diameter reduction is smaller than the number of the stands by which the thickness reduction is performed in the entire thickness reduction. In addition, also understood from  FIG. 17 , in the partial outer diameter reduction, the work portion  401  of the mandrel bar  40  can be shortened as the number of the stands included in the preceding-stage stand group FST is increased. 
     As described above, in the present embodiment, the plurality of mandrel bars  40  including the work portions  401  having lengths different from one another are prepared in advance. The length of the work portion  401  of each mandrel bar  40  is determined in advance according to the number of the stands performing the thickness reduction. In addition, in Step S 2  of the manufacturing process shown in  FIG. 14 , the mandrel bar  40  including the work portion  401  having the length corresponding to the number of the stands by which the thickness reduction is performed is selected. 
     As described above, the plurality of mandrel bars  40  are used every time one lot of seamless metal pipe is manufactured. Accordingly, if the plurality of sizes in the manufactured seamless metal pipe are present, a stock quantity of the mandrel bars  40  required for the elongating is significantly increased. In the present embodiment, the length of the work portion  401  of the mandrel bar  40  used in the partial outer diameter reduction can be shorter than that of the case of the entire thickness reduction. Since the work portion  401  can use the shorter mandrel bar, costs of the mandrel bars  40  required for stocking can be suppressed. 
     In the present embodiment, the partial outer diameter reduction is performed in the preceding-stage stand group FST. Accordingly, the mandrel bars  40  having the work portions  401  having lengths different from one another are included in the prepared plurality of mandrel bars  40 . However, the total lengths of the plurality of mandrel bars  40  are the same as one another. As shown in  FIGS. 16 and 17 , this is because the final stand STm performs the thickness reduction in both of the entire thickness reduction and the partial outer diameter reduction. Accordingly, when the work portion  401  is short, the extension portion  402  is lengthened. 
     In the above-described example, the outer diameter reduction is performed by the preceding-stage stand group FST in the partial outer diameter reduction. However, as shown in  FIG. 18 , the outer diameter reduction may be performed by the succeeding-stage stand group RST (STm-1 and STm). In this case, the work portion  401  of the mandrel bar  40  may have a length equal to at least a distance from an inlet position P 1 in of the head stand ST 1  to an outlet position Pm-2out of the final stand STm-2 of the preceding-stage stand group FST. Moreover, the extension portion  402  of the mandrel bar  40  may have a length equal to at least a distance from the end position Pend to the inlet position P 1 in of the head stand ST 1 . Accordingly, the length equal to the distance from the end position Pend to the outlet position Pm-2out of the stand STm-2 is enough for the total length of the mandrel bar  40 . This is because the mandrel bar  40  does not need to be inserted into the hollow shell HS subjected to the outer diameter reduction in the succeeding-stage stand group RST (Stand STm-1 and STm) in which the outer diameter reduction is performed. 
     In the partial outer diameter reduction, when the outer diameter reduction is performed by the succeeding-stage stand group RST, the final stand (the stand STm-2 in  FIG. 18 ) in which the thickness reduction is performed is changed according to the steel grade, the size, and the like of the manufactured seamless metal pipe. In this case, the total length of the mandrel bar  40  is also changed according to the position of the final stand in which the thickness reduction is performed. More specifically, the total length of the mandrel bar  40  is also shortened as the number of the stands in which the thickness reduction is performed is decreased. Accordingly, when the outer diameter reduction is performed by the succeeding-stage stand group RST, the mandrel bar  40  required for stock can be further shortened. 
     However, as described above, when the outer diameter reduction is performed by the preceding-stage stand group FST, the diameter of the hollow shell HS manufactured by the piercing mill  2  is further decreased by the preceding-stage stand group FST, and thus, the thickness reduction can be performed by the succeeding-stage stand group RST. Accordingly, compared to when the outer diameter reduction is performed by the succeeding-stage stand group RST, when the outer diameter reduction is performed by the preceding-stage stand group FST, the degree of freedom in rolling schedules of the piercing mill  2  and the mandrel mill  3  is increased, and frequency for exchanging the rolls can be suppressed. Accordingly, when the outer diameter reduction is performed by the preceding-stage stand group FST, the operating ratio of the manufacturing line is increased, and the production efficiency is increased. 
     [Second Embodiment] 
     As described above, in the elongating by the mandrel mill  3 , the plurality of mandrel bars  40  are prepared and stocked. The manufacturing cost of the mandrel bar  40  is increased as the mandrel bar  40  is lengthened. In addition, a wider stock space is required as the mandrel bar  40  is lengthened. It is preferable that the stock space be decreased if necessary. 
       FIG. 19  is a vertical cross-sectional diagram of the mandrel mill  3  according to the present embodiment. With reference to  FIG. 19 , compared to the mandrel mill  3  of the first embodiment, the mandrel mill  3  further includes an auxiliary tool  50 . 
     [Auxiliary Tool  50 ] 
       FIG. 20  is a vertical cross-sectional diagram of the auxiliary tool  50  in  FIG. 19 ,  FIG. 21  is a cross-sectional diagram when viewed from line C-C of  FIG. 20 , and  FIG. 22  is a plan diagram. With reference to  FIGS. 20 to 22 , the auxiliary tool  50  includes a main body portion  51 , a holding portion  52 , and a mounting portion  53 . 
     The main body portion  51  has a rod shape, and preferably, the cross-sectional shape of the main body portion is a circle. The material of the main body portion  51  is not particularly limited, and preferably, is metal. 
     The holding portion  52  is disposed at the front end of the main body portion  51 . The holding portion  52  is fitted to the flange  420  and the neck  410  of the rear end of the mandrel bar  40 . That is, the auxiliary tool  50  is mounted on the mandrel bar  40  to be coaxial with the mandrel bar  40  by the holding portion  52 . 
     The holding portion  52  includes a groove  521  and a hook portion  522 . The hook portion  522  is formed at an interval with a front end surface  511  in the front of the front end surface  511  of the main body portion  51 . In the present example, a groove  523  fitted to the neck  410  is formed on the upper surface of the hook portion  522 . 
     The groove  521  is formed between the hook portion  522  and the front end surface  511 , and extends in a transverse direction of the auxiliary tool  50 . More specifically, the groove  521  extends in an arcuate shape or an arc shape in the circumferential direction of the auxiliary tool  50 . The width of the groove  521  is slightly larger than the width of the flange  420 . The groove  521  is fitted to the flange  420 . 
     The holding portion  52  is held to the rear end portion of the mandrel bar  40  by the groove  521  and the hook portion  522 . 
     The mounting portion  53  has a shape which can be held by the holding member  316  of the retaining system  31 . Preferably, the mounting portion  53  has the same shape as the rear end portion of the mandrel bar  40 . The mounting portion  53  includes a neck  531  and a flange  532 . The neck  531  and the flange  532  have the same shapes as the neck  410  and the flange  420  of the mandrel bar  40 . The mounting portion  53  is fitted to the holding member  316  of the retaining system  31 . Accordingly, the auxiliary tool  50  is fixed to the holding member  316 . 
     With reference to  FIG. 19 , the holding portion  52  of the auxiliary tool  50  holds the rear end portion (neck  410  and flange  420 ) of the mandrel bar  40 , and is fixed to and detached from the mandrel bar  40 . In addition, the mounting portion  53  of the auxiliary tool  50  is fitted to the holding member  316 , and is fixed to and detached from the holding member  316 . 
     In brief, the auxiliary tool  50  supplements the length of the mandrel bar  40 . The auxiliary tool  50  plays the same role as the extension portion  402 , and extends the extension portion  402 . Accordingly, the total length of the mandrel bar  40  prepared in advance can be shortened. 
     Preferably, even when the plurality of mandrel bars  40  have outer diameters different from one another, the shapes of the rear end portions (necks  410  and flanges  420 ) are the same as one another. In this, the holding portion  52  of the auxiliary tool  50  can hold the mandrel bar  40  having various sizes (outer diameters). Accordingly, the auxiliary tool  50  can be used in common by the plurality of mandrel bars  40  which have different sizes. Therefore, the total length of the plurality of mandrel bars  40  can be shortened. 
     The manufacturing process of the seamless metal pipe of the present embodiment is as follows. With reference to  FIG. 14 , in Step S 5 , the auxiliary tool  50  is mounted on the holding member  316  of the retaining system  31 . Thereafter, the mandrel bar  40  selected in Step S 2  is mounted on the auxiliary tool  50 . According to the processes, the auxiliary tool  50  is mounted on the rear end portion of the mandrel bar  40 . The retaining system  31  inserts the mandrel bar  40 , on which the auxiliary tool  50  is mounted, into the hollow shell HS. Other operations are the same as those of the first embodiment. In addition, after the auxiliary tool  50  is mounted on the mandrel bar  40 , the auxiliary tool  50  may be mounted on the holding member  316 . 
     In the present embodiment, only one kind of auxiliary tool  50  may be prepared, and a plurality of kinds of auxiliary tools  50  having outer diameters different from one another may be prepared. When the plurality of kinds of auxiliary tools  50  are prepared, in Step S 2  of  FIG. 14 , an optimal mandrel bar  40  and auxiliary tool  50  are selected. 
     In addition, in the present embodiment, the holding portion  52  includes one groove  521 . However, as shown in  FIGS. 23 and 24 , the holding portion  52  may include a plurality of grooves having sizes different from one another. In this case, for example, the holding portion  52  includes the plurality of grooves which are arranged in a line in the axial direction. The groove is small as it approaches the hook portion  522 . In this case, the holding portion  52  can hold the plurality of mandrel bars  40  having different sizes from one another in the rear end portion. The plurality of grooves are formed corresponding to each rear end portion of the plurality of mandrel bars having sizes different from one another. Accordingly, the holding portion  52  can even hold the mandrel bars having different sizes from one another in the rear end portion. 
     Moreover, the configuration of the holding portion  52  is not limited to  FIGS. 20 to 22 . For example, the holding portion  52  includes an openable and closable arm, and the mandrel bar  40  may be held by interposing the rear end portion of the mandrel bar  40  between arms by opening and closing the arms. Also in this case, one auxiliary tool  50  can hold the plurality of mandrel bars  40  having outer diameters different from one another. The holding portion  52  may have the same configuration as the holding member  316 . 
     [Third Embodiment] 
     When the auxiliary tool  50  is applied to the plurality of mandrel bars  40  having sizes different from one another, the outer diameter of the auxiliary tool  50  may be different from the outer diameter of the mandrel bar  40 . Also in this case, it is preferable that the elongating is appropriately performed. 
     With reference to  FIG. 25 , compared to the second embodiment, the mandrel mill  3  according to the present embodiment further includes a control device  70 . 
     The control device  70  controls lifting and lowering of a plurality of support rolls SR 1  to SRk (k is a natural number). 
     The support rolls SR 1  to SRk are arranged along the pass line between the retaining system  31  and the rolling mill body  32 . For example, each of the support rolls may be a roll having a flat outer circumferential surface, and may be a V roll which has a groove having a triangular cross-sectional shape in the circumferential direction of the outer circumferential surface. 
     The support rolls SR 1  to SRk are lifted and lowered up and down by lifting devices DR 1  to DRk. For example, each of the lifting devices DR 1  to DRk is a hydraulic cylinder, an electric cylinder, or the like. In  FIG. 25 , one lifting device DR is disposed in each support roll SR. However, one lifting device DR may be disposed in the plurality of support rolls SR. 
     The control device  70  controls the lifting devices DR 1  to DRk, and lifts and lowers the support rolls SR 1  to SRk. The retaining system  31  and the rolling mill body  32  are apart from each other. Accordingly, the mandrel bar  40  may be curved downward between the retaining system  31  and the rolling mill body  32 . This curvature influences the stable transport of the mandrel bar during the rolling and dimension accuracy of the hollow shell HS after the elongating. Accordingly, the support rolls SR 1  to SRk are lifted according to the positions of the mandrel bar  40  during the elongating, and the mandrel bar  40  is supported on the pass line PL. 
     However, as described above, when the auxiliary tool  50  is used in common, the outer diameter of the auxiliary tool  50  may be different from the outer diameter of the mandrel bar  40 . In this case, the lower end position of the mandrel bar  40  during the elongating is different from the lower end position of the auxiliary tool  50 . If the height of the support roll SR is maintained while being matched to the height of the lower end position of the mandrel bar  40 , a gap may occur between the support roll SR and the auxiliary tool  50 , or the auxiliary tool  50  may collide with the support roll SR. 
     Accordingly, the control device  70  adjusts the height of the support roll according to the movement distance (forward movement distance) of the auxiliary tool  50  during the elongating. Specifically, when the outer diameter of the auxiliary tool  50  is larger than the outer diameter of the mandrel bar  40 , the control device controls the lifting device DRq and lowers the support roll SRq before the auxiliary tool  50  passes through the support roll SRq (q is a natural number of 1 to k). At this time, the control device  70  may determine a lowering amount based on a difference value between the outer diameter of the auxiliary tool  50  and the outer diameter of the mandrel bar  40 . In this case, the control device can lower the support roll SRq to an extent that the support roll SRq comes into contact with the lower end of the auxiliary tool  50  after the lowering. 
     On the other hand, when the outer diameter of the auxiliary tool  50  is smaller than the outer diameter of the mandrel bar  40 , the control device controls the lifting device DRq and lifts the support roll SRq after the auxiliary tool  50  passes through the support roll SRq. At this time, the control device  70  may determine the lifting amount based on the difference value between the outer diameter of the auxiliary tool  50  and the outer diameter of the mandrel bar  40 . In this case, the control device can lift the support roll SRq to an extent that the support roll SRq comes into contact with the lower end of the auxiliary tool  50  after the lifting. 
     As described above, the control device  70  lifts and lowers the support roll SRq and adjusts the height of the support roll SRq according to the movement distance of the auxiliary tool  50 . Accordingly, collision of the auxiliary tool  50  with respect to the support roll SR can be suppressed. Moreover, preferably, considering the outer diameter difference between the auxiliary tool  50  and the mandrel bar  40 , the control device  70  lifts and lowers the support roll SRq. In this case, the auxiliary tool  50  can be supported by the support roll SRq. 
     The details of the manufacturing process of the present embodiment are as follows. 
     The operations of Step S 1  to S 7  in  FIG. 14  are also performed in the present embodiment. The control device  70  performs an operation shown in  FIG. 26  during the elongating of Step S 6 . 
     First, the control device  70  reads the outer diameter of the auxiliary tool  50  and the outer diameter of the mandrel bar  40 , and compares the outer diameters (Step S 601 ). At this time, the control device  70  obtains the difference value between the outer diameter of the auxiliary tool  50  and the outer diameter of the mandrel bar  40 . Subsequently, the control device determines the height of the support roll SRq when the auxiliary tool  50  passes through the support roll SRq (Step S 602 ). Every time the mandrel bar  40  and the auxiliary tool  50  are combined with each other, the control device  70  may manage the height of the support roll SRq on a table in advance and store the table in memory. 
     The control device  70  confirms the movement start of the mandrel bar  40  and the auxiliary tool  50  (Step S 603 ). For example, when the forward movement of the holding member  316  starts in the elongating, the retaining system  31  notifies the control device  70  accordingly. The control device  70  receives the notification and recognizes the movement start of the auxiliary tool  50  and the like (Step S 603 ). 
     The control device  70  lifts the support roll SRq every time the mandrel bar  40  passes through the support roll SRq (Step S 604 ). At this time, the control device  70  determines the lifting amount of the support roll SRq according to the size (outer diameter) of the mandrel bar  40 . 
     According to the above-described operations, the mandrel bar  40  during the elongating is supported by the support rolls SR 1  to SRk. 
     Subsequently, the control device  70  reads the reviewed results of Step S 601  (Step S 605 ). When the outer diameter of the auxiliary tool  50  is the same as the outer diameter of the mandrel bar  40 , it is not necessary to adjust the height of the support roll SRq. Accordingly, the control device  70  maintains the height of the support roll SRq as it is until the elongating of one hollow shell HS ends. 
     On the other hand, when the outer diameter of the auxiliary tool  50  is larger than the outer diameter of the mandrel bar  40 , the control device  70  performs the lowering processing of the support roll (Step S 610 ). Specifically, the control device  70  checks the present movement amount of the auxiliary tool  50  (Step S 611 ). For example, the control device  70  receives the notification of the movement amount of the holding member  316  for each predetermined time from the retaining system  31 , and recognizes the movement amount (forward movement distance from the start position Pstart) of the auxiliary tool  50 . 
     When the auxiliary tool  50  reaches near a predetermined distance of the support roll SR 1  (YES in Step S 612 ), the control device  70  lowers the support roll SR 1  based on the movement amount of the auxiliary tool  50  checked in Step S 611 . At this time, the control device  70  may lower the support roll SR 1  so that the support roll is separated from the auxiliary tool  50 . In addition, the control device  70  may lower the support roll SR 1  so that the support roll SR 1  comes into contact with the auxiliary tool  50  based on an outer diameter difference between the auxiliary tool  50  and the mandrel bar  40 . 
     After the support roll SR 1  is lowered, an increment of the counter q is performed (Step S 615 ) and it is returned to Step S 611 . Until the counter q exceeds k (YES in Step S 614 ), that is, operations S 611  to S 613  are performed on each of the support rolls SR 1  to SRk. 
     According to the above-described operations, when the outer diameter of the auxiliary tool  50  is larger than the outer diameter of the mandrel bar  40 , the control device  70  lowers the support roll SRq. Accordingly, it is possible to suppress collision of the auxiliary tool  50  with respect to the support roll SRq. 
     Returning to Step S 605 , when the outer diameter of the auxiliary tool  50  is smaller than the outer diameter of the mandrel bar  40 , the lifting processing of the support roll is performed (Step S 620 ). The control device  70  checks the present movement amount (forward movement distance) of the auxiliary tool  50  for each predetermined time (Step S 621 ). 
     The control device  70  lifts the support roll SR 1  by a predetermined amount based on the movement amount of the auxiliary tool  50  checked in Step S 621  when the auxiliary tool  50  passes through a predetermined distance of the support roll SR 1  (YES in Step S 622 ). At this time, the control device  70  lifts the support roll SR 1  by a predetermined amount so that the support roll SR 1  comes into contact with the auxiliary tool  50  based on the outer diameter difference between the auxiliary tool  50  and the mandrel bar  40 . 
     Thereafter, similar to the lowering processing of the support roll S 610 , operations Step S 621  to S 623  are performed on each of the support rolls SR 1  to SRk (Steps S 624  and S 625 ). 
     According to the above-described operations, when the outer diameter of the auxiliary tool  50  is smaller than the outer diameter of the mandrel bar  40 , the control device  70  lifts the support roll SRq by a predetermined amount and causes the support roll SRq to come into contact with the auxiliary tool  50 . The auxiliary tool  50  can move forward without being curved downward. 
     In the above-described example, the control device  70  performs the lowering processing S 610  of the support roll and the lifting processing S 620  of the support roll. However, the control device  70  may perform only the lowering processing  5610  of the support roll. In addition, the control device  70  may lower the support roll SRq by a constant amount regardless of the outer diameter of the auxiliary tool  50  in the lowering processing  5610  of the support roll. In this case, at least the collision of the auxiliary tool  50  with respect to the support roll SRq can be suppressed, and more appropriate elongating can be performed. 
     In the above-described embodiment, the processing of Steps S 611  to S 613  is performed on each of the support rolls SR 1  to SRk. However, a plurality of support rolls SR may be lowered at once. Moreover, all support rolls SR 1  to SRk may be lowered at once. 
     In the above-described embodiment, the plurality of support rolls SR 1  to SRk are disposed between the retaining system  31  and the head stand ST 1  of the rolling mill body  32 . However, one or more support rolls may be disposed. 
     In the above, the present embodiments are described. However, the present embodiments are not limited to the above-described embodiments. 
     In the third embodiment, the support rolls SR 1  to SRk are disposed. However, in the first and second embodiments, the support rolls SR 1  to SRk may be not present. 
     In the above-described embodiments, the mandrel bar  40  is inserted into the hollow shell HS by the retaining system  31 . However, the mandrel bar  40  may be inserted into the hollow shell HS according to other methods. For example, the mandrel bar  40  may be inserted into the hollow shell HS by an inserter which is a device differing from the retaining system  31 . 
     The holding member  316  of the retaining system  31  is not limited to the above-described configuration. For example, the holding member  316  may include a plurality of arms which can be opened and closed. In this case, the holding member  316  may hold the mandrel bar  40  by interposing the rear end portion of the mandrel bar  40  between the arms. 
     In the above-described embodiments, the rear end portion of the mandrel bar  40  includes the neck  410  and the flange  420 . However, the shape of the rear end portion of the mandrel bar  40  is not limited to this. In brief, the shape of the rear end portion of the mandrel bar  40  is not particularly limited if the rear end portion has a shape which can hold the holding member  316  and the holding portion  52  of the auxiliary tool  50 . 
     In the above, the embodiments of the present invention are described. However, the above-described embodiments are only exemplary examples of the present invention. Accordingly, the present invention is not limited to only the above-described embodiments, and the above-described embodiments can be appropriately modified within a scope which does not depart from the gist of the invention. For example, in the above-described embodiments, the mandrel mill includes the preceding-stage stand group and the succeeding-stage stand group performing the outer diameter reduction or the thickness reduction, and performs the elongating on the hollow shell. However, the mandrel mill may include a stand which does not perform the outer diameter reduction and the thickness reduction. That is, the stand used in the preceding-stage stand group and the succeeding-stage stand group may be appropriately selected from the stands of the mandrel mill if necessary. 
     INDUSTRIAL APPLICABILITY 
     It is possible to provide a manufacturing method of a seamless metal pipe, a mandrel mill, and an auxiliary tool capable of suppressing a cost of a mandrel bar required for elongating. 
     BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS 
       2 : piercing mill 
       3 : mandrel mill 
       31 : retaining system 
       32 : rolling mill body 
       40 : mandrel bar 
       50 : auxiliary tool 
       52 : holding portion 
       53 : mounting portion 
       311 : drive source 
       312 : drive wheel 
       313 : driven wheel 
       314 : chain 
       316 : holding member 
     HS: hollow shell 
     ST 1  to STm: stand 
     FST: preceding-stage stand group 
     RST: succeeding-stage stand group 
     SR 1  to SRk: support roll