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DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to the field of tubular body rolling systems. More precisely, the invention relates to a multi-stand rolling mill, preferably but not exclusively intended for rolling lines in which the thickness-finishing rolling mill is of oblique, hot pilger mill or plug mill type.

PRIOR ART

Systems for making pierced bodies (or tubular bodies), such as for example seamless tubes, are known. In particular, some rolling systems include a thickness-finishing rolling mill (i.e. with oblique rolls) for rolling on tool. EXPANDER rolling mills, in which the pierced body is rolled between two motorized rolls, the axes of which are normally inclined by 60° with respect to the rolling axis and in which the rolling occurs on a bit inserted in the pierced item, are an example of rolling mills of this type. A piercer, in which the body, subjected to the external action of two oblique rolls, is pierced by a bit, is typically included upstream of the EXPANDER machine. Another machine named REELER is included downstream of the EXPANDER machine for smoothing the thickness of the pierced part let out from the EXPANDER. The REELER machine reels by means two oblique rolls and an internal tool. Downstream of the REELER machine there is typically a furnace, e.g. of the fossil fuel type, in which the pierced body is maintained at a predetermined temperature before crossing a SIZER rolling mill, comprising a relatively limited number of mill stands, typically no more than six, the number of which may be variable according to the cases.

In a system of the type described above, there are at least three stations (piercer, EXPANDER and REELER) of the "oblique" type which intervene on the pierced body in substantially helicoidal manner. This aspect is particularly critical in terms of rolled product quality because this type of rolling inevitably produces dimensional and/or shape defects. Another disadvantage of the system described above is in the presence of the furnace arranged between the REELER machine and the SIZER machine. Indeed, such furnace has a considerable impact on the overall size of the system. Furthermore, the heating furnace is critical also for other reasons, such as the management of tube accumulations, the problem of excessive decarburization and/or oxidation in case of prolonged stop in the furnace, the weight loss generated by the formation of scale in the furnace. As a whole, these aspects also impact on the design/construction/management costs of the system, and consequently on manufacturing costs.

According to the above, the need arises to have a new technical solution which allows overcoming, or at least limit, the drawbacks indicated above. Therefore, it is a main task of the present invention to provide a rolling mill which allows solving said drawbacks. In the scope of this task, it is a first object of the present invention to provide a rolling mill to be used downstream of a thickness-finishing rolling mill of the oblique type (e.g. EXPANDER) in a pierced body rolling system. It is another object of the present invention to provide a rolling mill which allows improving the quality of the pierced body processed in said rolling system. It is a not last object of the present invention to provide a rolling mill which is reliable and easy to make at competitive costs.

EP2878389 discloses a tube rolling mill with mandrel consisting of two mill stands arranged downstream of the piercing mill stands. In the first group of mill stands, the tube is rolled with the mandrel inside it and the second group of the mill stands is arranged downstream of the first group of mill stands and has the function of extracting the mandrel without performing tube rolling functions. In this disclosure, no sizing operation is included at the end of the rolling mill and, since the tube is subjected to a single rolling on mandrel in the rolling mill, the surface quality of the tube remains insufficient during the completion of the passage through the rolling mill.

SUMMARY

The present invention relates to a rolling system for tubular bodies, wherein a first rolling mill comprises a supporting structure, which defines a housing for mill stands arranged in sequence along a rolling axis, said structure comprising

- a first section for rolling on mandrel comprising a first plurality of mill stands arranged in sequence along the rolling axis starting from an inlet section of said structure;

- a second section for rolling without an internal tool without mandrel, arranged downstream of the first section, with respect to the advancement direction of the tubular bodies, comprising a second plurality of rolling mill stands arranged in sequence along the rolling axis; mandrel-supporting means for supporting said mandrel, wherein

- a second rolling mill of the thickness-finishing type is included arranged upstream of the first rolling mill, and wherein the first section is adapted to reel the thickness of the previously finished tubular bodies and the second section is adapted to size the outer diameter of the previously reeled tubular bodies.

BRIEF DESCRIPTION OF THE FIGURES

Objects and advantages of the present invention will be apparent from the following detailed description of an example of embodiment of the same and from the accompanying drawings exclusively provided by way of non-limiting example, in which:

- Figure 1 shows a side view of a rolling mill according to the present invention;

- Figure 2 is a plan view of the rolling mill in Figure 1 ;

- Figures 3 and 4 are a side view and a front view of rolling mill stands of a rolling mill according to the present invention, respectively;

- Figures 5 and 6 are a side view and a front view of a mandrel-supporting mill stand of a rolling mill according to the present invention, respectively;

- Figure 7 shows a diagrammatic view of a rolling mill according to the present invention;

- Figure 8 is a section view taken along plane VIII-VIII indicated in Figure 1 .

The same reference numbers and letters in the figures refer to the same elements or components.

DETAILED DESCRIPTION With reference to the mentioned figures, the present invention relates to a multi- stand rolling mill 1 for pierced bodies (hereinafter also known as tubular bodies). The rolling mill 1 according to the invention may be advantageously used in a rolling system comprising an oblique type thickness-finishing rolling mill, such as for example an EXPANDER rolling mill, an ASSEL, DIESCHER or PLANETARY rolling mill, as main station or machine. The rolling mill 1 according to the invention may also be used in a system which includes a HOT PILGER MILL or PLUG MILL type rolling mill to define the thickness of the pierced items. The rolling mill types described above are well known to a person skilled in the art and for this reason are not described in detail. In this regard, in the reference technical sector, an example of rolling mill of the planetary type is the one known under the trademark KRM™ made by KOCKS. An example of PLUG MILL type rolling mill is the one supplied also under the name AUTOMATIC MILL by AETNA STANDARD. An example of DIESCHER type rolling mill is the one provided under the trademark ACCU ROLL™ by AETNA STANDARD.

The list of rolling mills provided above is by way of example only and is not to be considered as exhaustive in any manner. In particular, the rolling mill according to the invention may be used also in other systems in which an improved surface finish and a better dimensional quality with respect to that obtained with the traditional solutions are required. Such requirement is met by using the rolling mill according to the invention.

In general, the rolling mill according to the invention may be advantageously used instead of the REELER and SIZER machines normally included in the rolling system indicated above.

Figure 1 is a side view of a rolling mill 1 according to the invention which comprises a supporting structure 3 which defines a housing 4 for accommodating a plurality of mill stands arranged in sequence according to a rolling axis 100 (also named rolling direction 100). Such housing 4 defines a first rolling section 41 and a second rolling section 42. The latter is defined downstream of the first sector 41 with respect to the advancement direction of the pierced bodies along the roll 100. In the first section 41 , a first plurality of rolling mill stands 10A, 10B (hereinafter also indicated with the expression first mill stands 10A, 10B) is arranged in succession along the rolling axis 100 starting from an inlet section 4A of housing 4. In particular, the mill stands 10A, 10B implement rolling on mandrel 7. As described in greater detail below, when the latter is inserted in the pierced body immediately downstream of the inlet section 4A, with respect to the rolling axis 100. In particular, mandrel 7 is inserted so that an end portion 7A protrudes with respect to the head of the pierced body.

A second plurality of rolling mill stands 15A, 15, 15B (hereinafter also indicated with the expression second mill stands 15A, 15, 15B) for rolling without an internal tool, i.e. for rolling without using a mandrel inside the tubular body, is arranged in the second section 42. Such second mill stands 15A, 15, 15B, are thus arranged immediately downstream of the first mill stands 10A, 10B with respect to the advancement direction (indicated by the arrow with reference numeral 2 in Figure 1 ) of the pierced bodies along the rolling axis 100. In other words, the second mill stands 15A, 15, 15B are arranged immediately in succession with the first mill stands 10A, 10B. Therefore, as a whole, the rolling mill 1 appears as a single machine, defined in the scope of the same structure 3, which implements two rolling processes in series: the first on mandrel, the second without an internal tool inside the pierced body.

In the embodiment shown in the figures, two mill stands 10A, 10B are accommodated in the first section 41 , while four mill stands 15A, 15, 15B are accommodated in the second section 42 for rolling without an internal tool. In all cases, the number of mill stands of the first section 41 may be greater than two. The number of mill stands 15 of the second section 42 may also be greater than or in all cases different from four, providing they are greater than two.

According to the invention, the rolling mill 1 is provided with first mandrel- supporting means and preferably with second mandrel-supporting means arranged respectively upstream and downstream of the first mill stands 10A, 10B, again with respect to the advancement direction 2 of the pierced bodies. The mandrel-supporting means are preferably arranged in the first section 41 and have the function of supporting mandrel 7 when the mandrel enters into the rolling mill without being sustained and guided by the pierced item itself and when the pierced body is pulled out from the mandrel itself to be rolled without an internal tool in the second section 42. Preferably, the first mandrel-supporting means comprise a first mandrel-supporting mill stand 1 12 arranged between the inlet section 4A of housing 4 and the first plurality of mill stands 10A, 10B. Preferably, the second mandrel-supporting means are also defined by a second mandrel-supporting mill stand 1 13 arranged between the first mill stands 10A, 10B of the first section 41 and the second rolling mill stands 15A,15,15B of the second section 42, instead. Figures 5 and 6 are views related to a possible embodiment of a mandrel- supporting stand which can be used in the rolling mill 1 according to the invention. Such embodiment is known in itself and already used in all rolling mills operating on mandrel by implementing the retained mandrel technique.

With reference to the above, it is worth noting that from the operative point of view, the first mandrel rolling section 41 and the second section 42 for rolling without an internal tool may replace a REELER machine and a SIZER machine, respectively. In other words, the first section 41 can perform the thickness reeling function, while the second section 42 can perform the sizing of the pierced body taking it to the required outer diameter. Therefore, the rolling mill 1 according to the invention may be advantageously used instead of these two machines (REELER-SIZER), allowing eliminating the heating furnace included in traditional systems between the machines themselves at the same time. In particular, according to another advantageous aspect, it is worth noting that the first section 41 allows a reeling of the thickness of the "longitudinaf and not of the "helicoidaf type, as in the traditional REELER machines. This possibility translates into a reduction of the number of defects with respect to the case of helicoidal reeling, which can be achieved by means of a traditional type REELER machine.

Figures 3 and 4 relate to a possible embodiment of the first mill stand 10A,10B and/or of the second mill stands 15A, 15, 15B of the rolling mill 1 . Four-roll mill stands are included in the illustrated example. However, the possibility of using three-roll mill stands falls within the scope of the present invention. In either case, for both sections 41 , 42 of the rolling mill 1 , each rolling mill stand is mechanically equal to the adjacent one. In particular, each mill stand is rotated by 180° with respect to the adjacent one about a vertical axis passing through the rolling axis 100. Therefore, in this regard, the first mill stand 15A of the second section 42 will be rotated by 180° about said vertical axis with respect to the last mill stand 10B of the first section 41 . The words "first" and "last" are considered with respect to the advancement direction 2 of the pierced body in the rolling mill 1 .

In all cases, the rolls of the rolling mill stands 10A,10B,15A,15,15B are actuated by means of drive shafts (not shown in the figures) connected, in turn, to motors 40. In this regard, each drive shaft may be actuated independently from the other drive shafts, i.e. by means of a functionally independent motor. Alternatively and according to known principles, in the case of three-roll mill stands, a mechanical transmission may be included inside the mill stand such that all the rolls of a mill stand are actuated by means of a single motor. In the case of four-roll mill stands, two adjacent rolls must be actuated by means of the corresponding drive shafts controlled by means of a same motor.

Preferably, the first mill stands 10A, 10B of the first section 41 are adjustable mill stands, this expression meaning mill stands in which the radial position of the rolls may be adjusted with respect to the rolling axis 100. Preferably, at least one rolling mill with adjustable rollers is included also in the second section 42. According to a possible variant, all the first mill stands 10A, 10B and the second mill stands 15A, 15, 15B have adjustable rolls. Adjustment systems of the type known in itself, e.g. of the eccentric type, may be used in order to adjust the radial position of the rolls. Such systems are actuated by control devices integral with the structure of the rolling mill.

Furthermore, the first mill stands 10A,10B and/or the second mill stands 15A, 15, 15B of the rolling mill 1 preferably comprise removable rolls. This word indicates that the rolls may be removed from the mill stand without removing and/or opening the mill stand itself. Alternatively, fixed rolls may be used which require the opening of the mill stands if their replacement or repair is needed.

According to a further aspect, the rolling mill 1 comprises an aligning and locking system of the mill stands in housing 4. In a preferred embodiment, shown in the figures, the mill stands 10A, 10B, 1 12, 1 13 accommodated in the first section 41 and those accommodated in the second section 42 are provided with at least one pair of floating pins 71 located on opposite sides of the mill stand and on a horizontal plane 300 passing through the rolling axis 100. The alignment system comprises at least one first pair of hydraulic cylinders 81 and a second pair of hydraulic cylinders 82 fixed to structure 3 at an outlet section 4B of housing 4 and of the inlet section 4A of housing 4, respectively. The cylinders 81 of the first pair act on a corresponding floating pin 71 of the last mill stand 15B of the second section 41 , while the cylinders 81 of the second pair act on a floating pin of the first mill stand housed in the first section 41 , i.e. the mandrel-supporting mill stand 1 12. The cylinders 81 of the first pair push the floating pins 71 of the mill stand 15B from the second section 42 so that they are inserted in the corresponding female housings defined in the adjacent mill stand 15 pushing at the same time the floating pins already present in such housings. Such action is repeated to the first mandrel-supporting mill stand 1 12 adjacent to the inlet section 4A of housing 4. In particular, the pins 71 A of the first mandrel-supporting stand 1 12 are inserted in corresponding housings in structure 3. In this manner, all the mill stands are constrained and axially locked. Cylinders 82 are fixed to structure 3 at the inlet section 4A and act on the floating pins 7A of the mandrel-supporting stand 1 12 so as to push them in opposite sense, i.e. towards the outlet section 4B. In this manner, the floating pins 71 , 71 A are released from the housings in which they were previously engaged so as to allow the successive removal of one or more mill stands of the rolling mill 1 .

According to a preferred embodiment, the mill stands 1 12, 1 OA, 10B, 1 13, 15A, 15, 15B of the rolling mill 1 are provided with a further pair of fixed thrust points 73, 73A arranged on opposite sides of the mill stand, at a substantially vertical plane 302 passing through the rolling axis 100. In this configuration, the rolling mill 1 comprises further thrust cylinders 91 fixed to structure 3 as the outlet section 4B of housing 4. The cylinders 91 act on thrust points 73, 73A arranged on the vertical plane 302 in manner similar to that described above for the cylinders which act on the floating pins arranged on the horizontal plane. The actions exchanged between the cylinders 91 and the mill stands 1 12, 10A, 10B, 1 13, 15A, 15, 15B are of compression only.

According to the invention, shown in Figures 2, 7 and 8, the rolling mill 1 according to the invention is preferably provided with a rolling mill stand changing system for inserting and later removing the mill stands 1 12, 1 0A, 10B, 1 13, 15A, 15, 15B of the two sections 41 and 42. In this regard, Figure 8 is a cross section which allows observing the conformation of the structure 3 of the rolling mill 1 in which housing 4 is defined between a support plane 101 , on which the mill stands 1 12, 10A, 10B, 1 13, 15A, 15, 15B rest, and a reference plane 102 substantially parallel to the support plane and spaced by a height greater than the height of the mill stands. The mill stand changing system comprises a first platform 601 at a loading side 3A and a second platform 602 at an unloading side 3B. The first platform 601 is configured to carry spare mill stands (generically indicated by reference numeral 9), while the second is configured to transport the mill stands to be changed. The system further comprises a transversal translation device 650 which operates on one or more of the spare mill stands 9 located on the first platform 601 to push them against the corresponding mill stands to be changed (accommodated in housing 4) until the latter are shifted (according to the direction 105 perpendicular to the rolling axis 100) on the second platform 602. The two platforms 601 , 602 are actuated by means of appropriate feeding means 670, according to a direction parallel to the rolling axis, as shown by the arrows with reference numeral 8 shown in Figure 2.

With reference to Figure 7, according to another aspect of the invention, the rolling mill 1 is provided with retaining means of mandrel 7 (not shown) to withhold it during the step of rolling of the pierced body in the first section 41 . In particular, the retaining means may be configured to hold the mandrel stopped during the step of rolling, or alternatively to allow the advancement up to a predetermined position. Indeed, in order to ensure the correct rolling conditions, when the head of the pierced body reaches the central plane 200 of the last mill stand 10B of the first section 41 , the end 7A of mandrel 7 must protrude with respect to the plane itself. When, instead, the tail of the pierced body reaches the central plane 200, the end 7A of the mandrel must be at the inlet section (plane 205 in Figure 7) of the first mill stand 15A of the second section 42. Therefore, the retaining means are configured to allow mandrel 7 to advance so as to ensure the rolling conditions shown above. In this regard, it is worth noting that in the rolling mill 1 according to the invention the rolling speed is particularly contained (in the order of 0,1 -0,3 m/s). Therefore, in order to contain the heating of the mandrel during the rolling of the first section, pierced rolled bodies are normally made upstream of the rolling mill 1 , of single length, i.e. in the order of 6-15 m.

The present invention therefore also relates to a rolling system comprising at least one thickness-finishing rolling mill and a further rolling mill having the particularities described above. As mentioned above, the rolling mill 1 according to the invention is configured for performing rolling on mandrel and a rolling without an internal tool on pierced bodies, the thickness of which has already been defined by means of a thickness-finishing rolling mill. The latter may be of the oblique type, e.g. of the EXPANDER, ASSEL, DIESCHER or PLANETARY type. Alternatively, the finishing rolling mill may be of the PLUG MILL or of the HOT PILGER MILL type.

The rolling process by means of the rolling mill according to the invention will now be illustrated with reference to the diagram in Figure 7. The rolling mill preferably comprises a station 6 for cooling and lubricating mandrel 7 used in the first section 41 of the rolling mill 1 . At the end of the rolling in the first section 41 , mandrel 7 is retracted parallel to the rolling axis 100 until it is arranged in a reference position indicated with reference A in Figure 7. Successively, mandrel 7 is displaced transversally in direction of the arrow 105 up to a second predetermined position, indicated by reference B, inside station 6. In such position, mandrel 7 is cooled. Successively, mandrel 7 is made to advance parallel to the rolling axis 100 up to a third predetermined position, indicated by C, in which it is lubricated. After having been completely lubricated, mandrel 7 is taken back to the second predetermined position B and then to the first predetermined position A. From this position, mandrel 7 is made to advance along the rolling direction to be inserted in a pierced body intended for the rolling mill 1 . For this purpose, the pierced body is arranged in the rolling axis 100 starting from a side opposite to the one in which station 6 indicated above is placed.

The rolling mill 1 according to the invention allows fulfilling the predetermined tasks and objects. In particular, the configuration of the rolling mill allows containing the size and cost of the system, as well as to eliminate the use of a heating furnace. At the same time, the rolling mill according to the invention allows obtaining an end product with fewer defects than that which can currently be obtained in the traditional systems comprising, for example, a finishing rolling mill of the oblique type.