Abstract:
A retract mandrel mill, comprising a mandrel mill and an extractor, the mandrel mill including a mandrel bar and being configured to roll a tube blank into which the mandrel bar is inserted, the extractor being configured to extract the mandrel bar from the tube blank that has been rolled in the mandrel mill, wherein the distance between the mandrel mill and the extractor is adjustable, and a method for rolling a tube blank by using the retract mandrel mill. As the result of the adjustable distance, even when a tube blank which is shorter than usual is rolled, there is no need to add an extra extension to the length of the tube blank to be elongation-rolled, and the wear of the mandrel bar can be suppressed so that the tube blank can be rolled efficiently and with high yields.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a retract mandrel mill which does not need an extra extension to be added to the length of a tube blank that is subjected to an elongation-rolling process when producing a shorter-length product than usual, and which can suppress the wear of the mandrel bar. The present invention also relates to a method for rolling a tube blank by using the aforementioned retract mandrel mill. 
         [0002]    In the present description, a retract mandrel mill is an elongation-rolling apparatus that includes a mandrel mill and an extractor. As such an extractor, a sizing mill can also be used. When a typical extractor is used, a reducing-rolling mill is used to finish the outer diameter of the tube blank extracted by the extractor into a predetermined size. When a sizing mill is used, the tube blank is extracted and its outer diameter is finished into a predetermined size by the sizing mill. 
       BACKGROUND ART 
       [0003]    In the past, a retract mandrel mill has been used to produce a seamless tube by a rolling process. Among prior arts, instances of using a retract mandrel mill are included in Patent Literatures 1 to 4. 
       [Configuration of Retract Mandrel Mill] 
       [0004]      FIG. 1  is a configuration diagram of a conventional retract mandrel mill. As shown in  FIG. 1 , the retract mandrel mill includes a mandrel mill  10  which is a main rolling machine, and an extractor  20  which has the function of extracting a mandrel bar. In  FIG. 1 , the direction in which a tube blank  30  is rolled is indicated by an arrow A direction (hereafter, simply referred to as “rolling direction”). 
         [0005]    The mandrel mill  10  includes a mandrel bar  11  and a plurality of rolls  12 . A restraint mechanism  13  is provided at the end area on the entrance side (upstream in the rolling direction) of the mandrel mill  10 . The mandrel bar  11  during a rolling operation advances in the rolling direction while being retained by the restraint mechanism  13 , and retreats by the action of the restraint mechanism  13  after the rolling is completed. 
         [0006]    The extractor  20  is disposed at the exit side (downstream in the rolling direction) of the mandrel mill  10  in series with the mandrel mill  10 . The extractor  20  includes a plurality of rolls  22  in a housing  21 . 
       [Method for Rolling Tube Blank] 
       [0007]    When the tube blank  30  is rolled as the starting material for a seamless tube, the tube blank  30  is inserted with the mandrel bar  11  in the mandrel mill  10  and is rolled by means of the mandrel bar  11  and rolls  12 . The mandrel bar  11  advances together with the tube blank when the tube blank  30  is rolled, and retreats to an initial position by the action of the restraint mechanism  13  after the rolling has ended. 
         [0008]    Since the tube blank  30  rolled by the mandrel mill  10  is forced by the rolls  22  of the extractor  20  to advance in the rolling direction and the mandrel bar  11  is subject to the force exerted by the restraint mechanism  13  in the direction opposite to the advancing direction of the tube blank, the tube blank  30  can be separated from the mandrel bar  11 . This operation is called as stripping. 
         [0009]    In order to prohibit the mandrel bar  11  from intruding into the extractor  20 , it is necessary to arrange that the distance between the mandrel mill  10  and the extractor  20  is no less than an amount that is obtained by an expression: (speed of mandrel bar)×(rolling time in the final roll of the mandrel mill). Since the rolling time in the final roll of the mandrel mill is proportionate to the length of the tube blank to be rolled in the mandrel mill, the distance between the mandrel mill  10  and the extractor  20  is proportionate to the speed of the mandrel bar and the length of the tube blank to be rolled in the mandrel mill. 
         [0010]    In a conventional retract mandrel mill, the distance between the mandrel mill  10  and the extractor  20  is set according to the maximum length of the tube blank  30  to be rolled in the mandrel mill. Both of the mandrel mill  10  and the extractor  20  are fixedly disposed so that the distance between the mandrel mill  10  and the extractor  20  is not adjustable. 
         [0011]      FIG. 2  is a diagram to illustrate a state where a tube blank, which is shorter than the distance between the mandrel mill and the extractor, is rolled in a conventional retract mandrel mill.  FIG. 2A  shows a state of rolling procedure at a mandrel mill,  FIG. 2B  shows a state where stripping is performed by using an extract fork,  FIG. 2C  shows a state where the tube blank after being rolled in the mandrel mill is moved by the mandrel bar, and  FIG. 2D  shows a state where the overlap between the mandrel bar and the tube blank is reduced. 
         [0012]    When a tube blank  30  which is shorter than the distance between the mandrel mill and the extractor is rolled in a conventional retract mandrel mill, that is, a retract mandrel mill in which the distance between the mandrel mill  10  and the extractor  20  is not adjustable, the front end of the tube blank  30  does not reach the extractor  20  after the rolling in the mandrel mill  10  has ended as shown in  FIG. 2A . 
         [0013]    In such a case, to make the tube blank  30  reach the extractor  20 , and also to extract the mandrel bar  11  from the tube blank  30  (to perform stripping), the following three methods are applied. 
         [0014]    (1) Regardless of the length required as a product, the tube blank  30  is produced with an extra length such that the length of the tube blank  30  after being rolled in the mandrel mill  10  is longer than the distance between the mandrel mill  10  and the extractor  20 . Then, the excess part of the tube blank  30  is cut off in a subsequent step after the mandrel bar  11  is extracted from the tube blank  30  with the extractor. 
         [0015]    However, in the method of (1) described above, since it is necessary to produce a tube blank having a length longer than the length needed for a product, there occurs a decrease in the yield of starting material and an excessive energy consumption. 
         [0016]    (2) As shown in  FIG. 2B , the mandrel bar  11  is forced to retreat while the tube blank  30  is prohibited from moving in the direction opposite to the rolling direction by using the extract fork  14 , thereby performing stripping. Thereafter, the tube blank  30  is conveyed to the extractor  20  by conveyor rolls  15 . 
         [0017]    (3) As shown in  FIG. 2C  the tube blank  30  after rolling is conveyed by the mandrel bar  11  until when its front end comes into contact with a roll  22  on the entrance side of the extractor  20 . Thereafter, the mandrel bar  11  is retreated while the tube blank  30  is rolled by the extractor  20 , thereby performing stripping. 
         [0018]    In the methods of (2) and (3) described above, it takes time for moving the extract fork  14  from a retreat position to a predetermined position, and for moving the tube blank  30  with the mandrel bar  11 . Moreover, the temperature of the tube blank  30  becomes lower while moving. Such a temperature drop causes a thermal contraction of the tube blank  30  so that the stripping becomes difficult to be performed when the overlap (overlapped portion between the tube blank  30  and the mandrel bar  11 ) is long. In particular, when the tube blank  30  is made of a material that exhibits a large thermal contraction as temperature decreases (for example, an alloy steel with a Cr content of not less than 10% by mass), the stripping may become impossible. 
         [0019]    Therefore, as shown in  FIG. 2D  it is necessary to shorten the overlap during or after rolling. As a method of shortening the overlap, there is a method of reducing the moving speed of the mandrel bar  11  during rolling to be lower than the moving speed of the tube blank  30 . However, reducing the moving speed of the mandrel bar  11  results in an increase in speed difference between the mandrel bar  11  and the tube blank  30  and there arises a problem such that the mandrel bar  11  is more liable to be damaged due to friction with the tube blank  30  during rolling in the mandrel mill  10 . 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: Japanese Patent Application Publication No. 7-214110 
         Patent Literature 2: Japanese Patent Application Publication No. 8-117816 
         Patent Literature 3: Japanese Patent Application Publication No. 8-300013 
         Patent Literature 4: Japanese Patent Application Publication No. 2001-205323 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0024]    It is an object of the present invention to provide a retract mandrel mill which does not require to prepare the tube blank to be subjected to the elongation-rolling, in a length in excess of what is needed as a product when producing a shorter-length product than usual, and which can suppress the wear of the mandrel bar. It is another object of the present invention to provide a method for rolling a tube blank by using the retract mandrel mill of the present invention. 
       Solution to Problem 
       [0025]    The summary of the present invention is as follows. 
         [0026]    (1) A retract mandrel mill, comprising a mandrel mill and an extractor, the mandrel mill including a mandrel bar and being configured to roll a tube blank into which the mandrel bar is inserted, the extractor being configured to extract the mandrel bar from the tube blank that completes the rolling in the mandrel mill, wherein the distance between the mandrel mill and the extractor is adjustable. 
         [0027]    (2) A method for rolling a tube blank, wherein the retract mandrel mill according to the above-described (1) is used. 
         [0028]    (3) The retract mandrel mill according to the above-described (1), or the method for rolling a tube blank according to the above-described (2), wherein the tube blank is made of a steel containing not less than 10% of Cr by mass. 
       Advantageous Effects of Invention 
       [0029]    Since the distance between the mandrel mill and the extractor can be adjusted by using the retract mandrel mill of the present invention, the front end of the tube blank  30  can reach the extractor  20  after the completion of the rolling in the mandrel mill  10  even when a tube blank which is shorter than usual is rolled. This eliminates the need to add an extra extension to the length of the tube blank to be subjected to an elongation-rolling process, and the wear of the mandrel bar can be suppressed. According to the retract mandrel mill of the present invention and the method for rolling a tube blank of the present invention, therefore, it is possible to roll a tube blank efficiently and with high yields. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0030]      FIG. 1  is a configuration diagram of a conventional retract mandrel mill. 
           [0031]      FIG. 2  is a diagram to illustrate a state where a tube blank, which is shorter than the distance between a mandrel mill and an extractor, is rolled in a conventional retract mandrel mill, wherein  FIG. 2A  shows a state of rolling procedure at the mandrel mill,  FIG. 2B  shows a state where stripping is performed by using an extract fork,  FIG. 2C  shows a state where the tube blank after the rolling in the mandrel mill is moved by a mandrel bar, and  FIG. 2D  shows a state where the overlap between the mandrel bar and the tube blank is reduced. 
           [0032]      FIG. 3  is a configuration diagram of a retract mandrel mill of the present invention, in which  FIG. 3A  shows a case where a tube blank of a normal length is rolled, and  FIG. 3B  shows a case where a tube blank, which is shorter than the distance between the mandrel mill and the extractor in  FIG. 3A , is rolled. 
           [0033]      FIG. 4  is a diagram to illustrate one example of the method for moving the extractor. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]      FIG. 3  is a configuration diagram of a retract mandrel mill of the present invention.  FIG. 3A  shows a case where a tube blank of a normal length is rolled, and  FIG. 3B  shows a case where a tube blank, which is shorter than the distance between the mandrel mill and the extractor in  FIG. 3A , is rolled. The retract mandrel mill shown in  FIG. 3  has the same configuration as that shown in the above-described  FIG. 1 , and substantially same parts are given the same reference symbols, excepting that the extractor is movable in parallel with the rolling direction (in the direction indicated by the arrow A). 
         [0035]    In the present embodiment, as shown in  FIG. 3 , the extractor  20  is provided with wheels  23  beneath a housing  21 , and is movable over a rail  24  of the floor surface in parallel with the rolling direction of the moving tube blank  30 . Thereby, the distance between the mandrel mill  10  and the extractor  20  is changeable. 
         [0036]      FIG. 4  is a diagram to illustrate one example of the method of moving the extractor. 
         [0037]    For example, as a method of moving the extractor  20 , listed are following four methods. 
         [0038]    (1) As shown in  FIG. 4 , a motor  27  for driving the rolls  22  and the wheels  23  is installed on a pedestal of the housing  21  so as to move with the extractor  20 . 
         [0039]    (2) A motor (not shown) for driving the rolls  22  is provided separately from the extractor  20 , and the driving shaft of the motor (not shown) is connected with the driving shafts of the rolls  22  and the wheels  23  with a universal joint. 
         [0040]    (3) A motor (not shown) for driving the rolls  22  is provided separately from the extractor  20  so that power transmission from the driving shaft of the motor (not shown) to the driving shafts of the rolls  22  and wheels  23 , which are provided at each position to which the extractor  20  moves, is performed by switching gears with a clutch. 
         [0041]    (4) A driving apparatus similar to the restraint mechanism  13  of the mandrel bar  11  is installed on the exit side (downstream side) of the extractor  20  to move the extractor  20  by the same driving method as that for moving the mandrel bar  11 . In this case, the driving of the rolls  22  is performed by using any of the methods of (1) to (3) described above. 
         [0042]    In the methods of moving the extractor  20  described above, each of the methods of (1) to (3) described above is a method of transferring the rotational force of the motor to the rolls  22  and the wheels  23 , and the method of (4) is a method of moving the main body of the extractor  20  in the advancing or retreating direction in parallel with the rolling direction. 
         [0043]    According to the moving method shown in  FIG. 4 , the housing  21  is provided with anchors  25  on each side thereof with respect to the rolling direction. While the extractor  20  is in operation, the anchors  25  are inserted into insertion ports  26  provided on the floor. And when the extractor  20  is moved, the anchors  25  are pulled out from the insertion ports  26 . Inserting the anchors  25  into the insertion ports  26  can prevent the extractor  20  from being moved by the thrust force during rolling the tube blank  30 . 
         [0044]    When a tube blank of a regular length is rolled by using the retract mandrel mill of the present embodiment, the distance between the mandrel mill  10  and the extractor  20  is set at a predetermined spacing as shown in  FIG. 3A  (for example, in a similar manner to the case shown in  FIG. 1  described above). When the tube blank  30  is being rolled, since the mandrel bar  11  can be brought close to the extractor  20  while it is moved at a regular speed, that is, with the speed difference between the tube blank and the mandrel bar being reduced, the damage to the mandrel bar  11  due to friction with the tube blank  30  is incurred in the least. 
         [0045]    When a tube blank  30  which is shorter than the distance between the mandrel mill  10  and the extractor  20  and is set at a predetermined spacing is rolled, the extractor  20  is moved in the direction to approach the exit side of the mandrel mill  10  so that the distance between the mandrel mill  10  and the extractor  20  is shortened as shown in  FIG. 3B . 
         [0046]    As a result, when rolling a short tube blank  30 , the mandrel bar  11  can be moved at a regular speed as in the case where a tube blank of regular length is rolled, thereby reducing the speed difference between the tube blank and the mandrel bar so that the damage to the mandrel bar  11  due to friction with the tube blank  30  can be controlled in the least. Moreover, since the rolling of the tube blank  30  in the extractor  20  has started when the rolling of the tube blank  30  in the mandrel mill  10  completes, the mandrel bar  11  can be extracted from the tube blank  30  without any problem, and there is no need of preparing the tube blank  30  to be subjected to an elongation-rolling process to have a length in excess of what is needed as a product. 
       EXAMPLES 
       [0047]    To confirm advantageous effects of the present invention, a rolling testing of tube blanks was conducted as described below. 
       1. TESTING METHOD 
       [0048]    A conventional retract mandrel mill was used as Comparative Example, in which no adjustment of the distance between the mandrel mill and the extractor was performed. The retract mandrel mill of Comparative Example was designed to be able to roll a tube blank having a length of 25 m after rolling. The traveling speeds of tube blank at the entrance and exit of the mandrel mill were set at values shown in Table 1. In this case, time required for rolling a single tube blank was 8.33 sec (rolling length 25 m divided by tube blank exit speed 3.0 m/sec). 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Tube blank speed (m/sec) 
                   
               
             
          
           
               
                 Mandrel mill 
                 Mandrel mill 
                 Mandrel bar speed 
               
               
                 entrance 
                 exit 
                 (m/sec) 
               
               
                   
               
               
                 1.2 
                 3.0 
                 1.0 
               
               
                   
               
             
          
         
       
     
         [0049]    To that end, when the position of the mandrel bar is controlled such that the front end of the mandrel bar be located immediately under the final rolls at the start of the rolling in these rolls of the mandrel mill, the front end of the mandrel bar moves toward the entrance side of the extractor by 8.33 m when the rolling of the tube blank in the mandrel mill completes (mandrel bar speed 1.0 msec times rolling time in final rolls 8.33 sec). 
         [0050]    Thus, in the retract mandrel mill of Comparative Example, the distance between the mandrel mill and the extractor was set at 8.4 m such that the front end of the mandrel bar would not intrude into the extractor. 
         [0051]    In contrast to the retract mandrel mill of Comparative Example, the retract mandrel mill of Inventive Example of the present invention was configured such that the extractor was movable in parallel with the rolling direction. In the retract mandrel mill of Inventive Example of the present invention, it was arranged such that the distance between the mandrel mill and the extractor was changeable by 3.0 m at the maximum. Specifically, it was arranged such that the distance had a standard value of 8.4 m, and was changeable up to 5.4 m which was 3.0 m shorter than the standard value. Excepting those described above, the length of tube blank that can be rolled, and the speed of the tube blank were the same as those of Comparative Example. 
         [0052]    Three anchors were provided at a spacing of 2 m on each side of the housing of the extractor along the rolling direction. Since the force of the extractor to pull the tube blank was about 10 tons, each anchor was designed to be able to bear a thrust force of 2 tons. 
         [0053]    In the present examples, the tube blank was made of a plain steel (C: 0.2% by mass) and an alloy steel (C: 0.2% by mass, Cr: 13% by mass). Further, the rolled size of the tube blank was an outer diameter of 245 mm and a wall thickness of 14 mm. 
         [0054]    Regarding the length of the tube blank, supposing that the lengths of product steel tubes be 6 m, 12 m, 18 m, and 24 m, the lengths of tube blanks after rolling were set to be 6.5 m, 12.5 m, 18.5 m, and 25 m. 
       2. TESTING RESULTS 
     2-1 Plain Steel 
     Comparative Example 
       [0055]    When the plain steel was used for the tube blank, the retract mandrel mill of Comparative Example was able to roll any of the tube blanks having lengths of 6.5 m, 12.5 m, 18.5 m, and 25 m. When rolling tube blanks of 12.5 m, 18.5 m, and 25 m which were longer than the distance (8.4 m) between the mandrel mill and the extractor, the speed of the mandrel bar was set at 1.0 m/s as listed in Table 1. 
         [0056]    When rolling a tube blank of 6.5 m which was shorter than the distance between the mandrel mill and the extractor, the speed of the mandrel bar was set at less than 1.0 m/s. In this case, the stripping of the tube blank was conducted by advancing the mandrel bar after the rolling in the mandrel mill to make the tube blank intrude into the extractor, or by using an extract fork. This was because setting the speed of the mandrel bar at 1.0 m/sec would result in an excessive overlap between the mandrel bar and the tube blank thereby making the stripping difficult. 
       Inventive Example of the Present Invention 
       [0057]    The retract mandrel mill of Inventive Example of the present invention was able to roll any of the tube blanks having lengths of 6.5 m, 12.5 m, 18.5 m, and 25 m as well with the distance between the mandrel mill and the extractor being kept at 8.4 m, by adjusting the speed of the mandrel bar as in Comparative Example. 
         [0058]    When rolling a tube blank having a length of 6.5 m by using the retract mandrel mill of Inventive Example of the present invention, the rolling was successfully performed with the speed of the mandrel bar being set at 1.0 m/sec by shortening the distance between the mandrel mill and the extractor to 5.4 m. In this case, since the speed difference between the tube blank and the mandrel bar was small, the damage to the mandrel mill bar due to the friction with the tube blank was much less than in the case where the distance between the mandrel mill and the extractor was kept at 8.4 m. 
       2-2. Alloy Steel 
     Comparative Example 
       [0059]    When the alloy steel was used for the tube blank, the retract mandrel mill of Comparative Example was able to roll the tube blanks of 12.5 m, 18.5 m, and 25 m under the same condition as in the case of the plain steel. 
         [0060]    However, it was unable to roll the tube blank having a length of 6.5 m. This was because the thermal contraction rate of the tube blank was large, and when advancing the mandrel bar after the rolling in the mandrel mill, or while moving the extract fork to a predetermined position, the tube blank contracted thereby disabling the stripping. 
       Inventive Example of the Present Invention 
       [0061]    Meanwhile, the retract mandrel mill of Inventive Example of the present invention was able to roll any of the tube blanks having lengths of 6.5 m, 12.5 m, 18.5 m, and 25 m. 
         [0062]    It was possible to roll the tube blanks having lengths of 12.5 m, 18.5 m, and 25 m with the distance between the mandrel mill and the extractor being kept at 8.4 m, under the same condition as that of Comparative Example. It was possible to roll the tube blank having a length of 6.5 m with the speed of the mandrel bar being set at 1.0 msec by shortening the distance between the mandrel mill and the extractor to 5.4 m. 
       3. CONCLUSION AND SUPPLEMENT 
       [0063]    It is seen from the results of the above-described examples that according to the present invention, it is possible to produce tube blanks having a length in the range of 6.5 m to 25 m by a single retract mandrel mill even if the tube blank is made of a material having a large thermal contraction rate. 
         [0064]    Further, supplementing about the setting of the distance between the mandrel mill and the extractor, in the above-described examples, the retract mandrel mill was set up such that the distance between the mandrel mill and the extractor had a standard value of 8.4 m, to enable the rolling of a tube blank having a length of 25 m after rolling. 
         [0065]    Similarly, to enable the rolling of a tube blank having a length of 18 m after rolling, the standard value for the distance between the mandrel mill and the extractor was set at 6.0 m (mandrel bar speed 1.0 msec times (rolling length 18 m divided by tube blank exit speed 3.0 m/sec)). 
         [0066]    Further, to enable the rolling of a tube blank having a length of 32 m after rolling, the standard value for the distance between the mandrel mill and the extractor was set at 10.7 m (mandrel bar speed 1.0 m/sec times (rolling length 32 m divided by tube blank exit speed 3.0 msec) equals or nearly equals 10.67 m). 
       INDUSTRIAL APPLICABILITY 
       [0067]    The present invention is applicable to the rolling of tube blanks, such as the production of seamless tubes through the application of the Mannesmann process, and the like. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10 : Mandrel mill,  11 : Mandrel bar,  12 : Roll, 
           13 : Restraint mechanism,  14 : Extract fork,  15 : Conveyor roll, 
           20 : Extractor,  21 : Housing,  22 : Roll,  23 : Wheel, 
           24 : Rail,  25 : Anchor,  26 : Insertion port, 
           27 : Motor,  30 : Tube blank