Abstract:
A rolling mill ( 10 ) for processing seamless tubes comprises a plurality of rolling units (U) that are sequentially arranged along a rolling axis (L), each of which is provided with at least three removable working rolls ( 12 ), which are each equipped with a groove ( 12   a ) for accommodating the tube to be worked; the rolling mill comprises a mandrel suitable to be fitted in the tube cavity during the process; the tube is worked by being sequentially passed, with the mandrel ( 22 ) fitted in the tube cavity, within the grooves ( 12   a ) of the rolls ( 12 ) of the rolling units (U) such that the outer diameter of the tube (T) is reduced; in order to have low capital and production costs, each rolling unit (U) has an unchanged nominal diameter

Description:
[0001]    This is a continuation of International Application No. PCT/IT2006/000438, filed Jun. 12, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The object of the present invention is a retained mandrel rolling mill for seamless tubes. 
         [0003]    Continuous rolling mills for seamless tubes are known to provide a plurality of rolling units, commonly called the stands, which are sequentially arranged along a rolling axis; each rolling unit is provided with three removable working rolls, which are each equipped with a groove for accommodating the tube to be processed; the three rolls are rotatably driven about axes of rotation that are coplanar to each other and laying on a plane orthogonal to the rolling axis; a mandrel is further provided, which is suitable to be fitted within the tube cavity upon operation. During the processing, the tube passes through the rotating rolls of the several rolling units with the mandrel being fitted within the tube cavity; the feeding occurs by means of friction between the tube and rolls, and the geometry of the grooves of the rolls of the several rolling units is such as to exert a reducing action on the outer diameter of the tube, and thus a consequent reduction in the tube thickness. 
         [0004]    In this type of rolling mills, the rolls are always employed in the same rolling unit and are processed on their outer surface when they are excessively worn, by removing material from the roll surface by means of turning. 
         [0005]    This turning is carried out such that the roll has the same work profile with a smaller nominal diameter. By “roll work profile” is meant that part of the roll which is in contact with the tube. By “nominal diameter” is meant twice the distance between the rolling axis and the roll axis. 
         [0006]    When the least operating diameter has been reached, the roll is eliminated and replaced with a new roll. 
         [0007]    This type of rolling mills, however, has considerable drawbacks. 
         [0008]    First, the rolls are oversized in order to be turned and used several times, because the roll diameter is reduced at each turning. Due to this oversizing, rolling mills are provided which are large-sized, and thus bulky and expensive. In addition, a considerable energy consumption is involved due to the large moving masses. 
         [0009]    Secondly, as the stands are oversized, the operations of dismounting and subsequent re-mounting the rolls are also onerous. 
         [0010]    Finally, because the diameter of the roll decreases at each turning, the position of each roll must be adjusted in the rolling unit such that the work profiles of the rolls are in the same work position as before turning, in order to provide the same reduction in the tube diameter. Suitable adjusting members or shims are employed for this adjustment, by carrying out operations that, however, are time-consuming and thus increase the process cost. 
         [0011]    The object of the present invention is to overcome the above-mentioned drawbacks. 
         [0012]    This object is achieved by means of a rolling mill for processing seamless tubes, comprising a plurality of rolling units that are sequentially arranged along a rolling axis, each of which is provided with at least three removable working rolls, which are each provided with a groove for accommodating the tube to be worked, which are rotatably driven about axes of rotation co-planar to each other and laying in a plane orthogonal to the rolling axis, and comprising a mandrel suitable to be fitted in the tube cavity during the process, the tube being worked by sequentially passing, with the mandrel being fitted in the tube cavity, within the grooves of the rolls of the rolling units, such that the outer diameter of the tube is reduced, characterized in that the rolls of the rolling units have the same nominal diameter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    In order to better understand the invention, the description of an exemplary, non-limiting embodiment thereof is given below, as shown in the annexed drawings, in which: 
           [0014]      FIG. 1  shows a schematic diagram, on a transversal plane, of a retained mandrel rolling mill for seamless tubes according to the invention; 
           [0015]      FIG. 2  shows a detail of  FIG. 1 ; 
           [0016]      FIG. 3  shows a perspective, schematic view of series of roll sets of the rolling mill of  FIG. 1 ; 
           [0017]      FIG. 4  shows a schematic illustration of the profiles of the sequential rolls of the rolling mill of  FIG. 1 ; 
           [0018]      FIG. 5  shows, in partial longitudinal section, an exemplary structure of a roll mounted on an axis thereof. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The rolling mill illustrated in  FIG. 1 , which is generally designated with  10 , is shown in the known, general configuration thereof comprising a plurality of rolling units U, which are sequentially arranged aligned relative to each other along a rolling axis and fitted in a fixed structure S, which is suitable to connect them to each other in a rigid manner. 
         [0020]    Each rolling unit U comprises a support framework  11 , three working rolls  12  being mounted therein. 
         [0021]    Particularly, as shown in  FIG. 1  and in detail in  FIG. 2 , each roll  12  is pivotally mounted on a fork support  13 , which is, in turn, carried by a lever  14  that is pivoted on a pivot  15 . The pivots  15  are rigidly connected to each other by the support framework  11 . As may be seen, the three axes X of the three rolls lay on a same plane and cross such as to form an equilateral triangle. This plane is orthogonal to the rolling axis. 
         [0022]    Each roll  12  is rotatably driven by a motor  16  of its own, via a shaft  17  that is connected to the motor by means of gears. 
         [0023]    A hydraulic cylinder  18  acts on each fork support  13 , which cylinder has the function of adjusting the position of the roll  12  and maintaining the latter in this position by counteracting the contrary forces that are generated during the rolling process. 
         [0024]    A device  19  acts on one of the hydraulic cylinders  18 , which provides to move the cylinder away from its position by rotatably moving the same about a pivot  20  in the event that the support framework  11  requires to be removed from the rolling mill for servicing. 
         [0025]    The series of three-roll sets  12  of the several sequential rolling units U that are designated with U 1 , U 2 , U 3 , U 4 , U 5  is illustrated in a perspective view in  FIG. 3 . 
         [0026]    The profile of the grooves  12   a  of the working rolls  12  is illustrated in  FIG. 4 . As may be seen, the grooves  12   a  of the rolls  12  have a profile similar to an arc of circle with radiuses R 5 ,R 4 ,R 3 ,R 2 ,R 1  that gradually decrease from the last rolling unit U 5  to the first rolling unit U 1 , respectively. This decrease of the groove radiuses, however, is such that the nominal diameter “D” of the rolls  12 , which is twice the distance between the rolling axis L and the roll axis X, is the same, i.e. it is unchanged from a rolling unit to the next one. The rolling direction is indicated by the arrow A. 
         [0027]    A possible embodiment of the rolls  12  is shown in  FIG. 5 . The roll  12  can be substantially made in two parts  12 . 1  and  12 . 2 . The roll part  12 . 1  is made as one piece with a shaft  21  integral with the shaft  17 , which transmits the motion to the roll  12 . The roll part  12 . 2  is ring-shaped and the groove  12   a  of the roll is formed on the outer surface thereof. This roll part  12 . 2  is made integral to the roll part  12 . 1  by means of keying, hot shrinkage or preferably by means of bonding. 
         [0028]    The rolling mill  10  also comprises a mandrel  22 , shown in  FIG. 3 , which is movable along the rolling axis L and driven via known means. 
         [0029]    The operation of the rolling mill  10  described and illustrated herein is as follows. 
         [0030]    As shown in  FIG. 3 , a tube T is advanced through the series of three-roll sets  12  that are rotatably driven by the motors  16  via the drive shafts  17 . The mandrel  22  fitted within the tube T is also simultaneously advanced at a lower speed than the tube advancement speed. 
         [0031]    By passing through the grooves  12   a  of the rolls  12  having gradually decreasing radiuses, the tube T with the mandrel  22  fitted therein, determines a reduction in the diameter and thickness of the tube T. 
         [0032]    When the rolls  12  are worn, the work profile of their groove  12   a  is turn such as to maintain the same nominal diameter. By removing material from the roll  12 , the new profile will be such that a wider passage section than the previous profile is created through the rolling unit U. As the diameter of the tube T decreases in the rolling mill by passing from a rolling unit to the next one, the thus-processed rolls  12  are placed in the preceding rolling unit relative to the rolling direction A; for example, if the roll  12  belonged to the rolling unit U 5 , it is transferred to the rolling unit U 4  after turning, etc. When the rolls  12  of the first rolling unit U 1  have finished their operating life, they are eliminated. 
         [0033]    The rolling mill  12  as described above and illustrated according to the invention has considerable advantages over the rolling mills described in the preamble, in which the rolls are always used in a same rolling unit. 
         [0034]    First, in the rolling mill  10 , the new rolls do not require to be oversized, unlike the rolls of the rolling mills described in the preamble. This is because the rolls have constant diameters in the rolling mill  10 , and it is the radius of the roll groove that changes, as may be seen in  FIG. 4 . Due to the above, the rolling mill  10  has a lower size relative to the rolling mills cited in the introductory part, thus being less bulky, less expensive, less energy-consuming, and providing for an easy assembly/disassembly of the rolls. 
         [0035]    The adjustment of the rolls&#39; position in the rolling mill  10  is minimal, as one does not require to compensate changes in the roll diameter. A much shorter time is thus taken for such adjustment, and accordingly less labour and management costs are required as compared with the rolling mills mentioned in the preamble. 
         [0036]    In a rolling mill with rolls having a variable nominal diameter, the rolling mill requires to be adjusted each time that re-turned rolls are mounted thereon. This is because a different rotation speed corresponds to each nominal diameter of a roll. Furthermore, at the beginning of the rolling operations, a further adjustment is required, which can imply that a part of the production will be of low quality until the rolling mill has reached the optimum operating conditions. With the rolls always having the same nominal diameter, the adjustment of the rolling mill is much simplified and the fine-tuning step is unnecessary, thereby the entire production has the same quality level. 
         [0037]    It should be added that the rolling mill  10  is also advantageous when more bores have to be worked (by “bore” is meant the diameter of the tube exiting the rolling mill). In this case, for a certain bore, the new roll can start working in the last rolling unit U 5  (outlet of the tube) and end working in the first rolling unit U 1  (inlet of the tube); subsequently, for a larger bore, the same roll can be processed such as to be employed in the last rolling unit U 5  to the first rolling unit U 1 . Thereby, the use of the roll is optimized, as it is used as much as possible and the incidence on the production cost is minimized. 
         [0038]    It may also be envisaged, in the rolling mill  10 , that the roll profile is re-turned one or two times with the roll being maintained in the same rolling unit. Thereby, the nominal diameter would undergo a minimum variation that can be compensated with an equally minimum adjustment. 
         [0039]    By providing the roll  12  as in  FIG. 5 , the waste is advantageously reduced when the roll has come to the end of its life cycle. In fact, the ring-shaped part  12 . 2  that comes in contact with the tube T can be removed from the part  12 . 1  when it is completely worn, and can be replaced by a new, ring-shaped part  12 . 2 . Thereby, one avoids having to replace the entire roll, and can replace only one part thereof. 
         [0040]    Variants and/or additions can be provided to what has been described and illustrated above. 
         [0041]    The several members of the rolling mill  10  can be changed in structure, operation and shape. 
         [0042]    The number of rolling units can be different from that illustrated herein, according to the requirements. 
         [0043]    More than three rolls may be also provided for each rolling unit. 
         [0044]    The shape of the roll and the profile of its groove may also be changed from what has been illustrated above. 
         [0045]    Each roll can be made as one piece. Making the roll in two pieces, as in  FIG. 5 , is however particularly advantageous.