EP2521626A1

EP2521626A1 - Tube rolling plant

Info

Publication number: EP2521626A1
Application number: EP10706747A
Authority: EP
Inventors: Paolo Marin; Vincenzo Palma; Guido Emilio Zanella; Jacopo Grassino
Original assignee: SMS Innse SpA
Current assignee: SMS Group SpA
Priority date: 2010-01-05
Filing date: 2010-01-05
Publication date: 2012-11-14
Anticipated expiration: 2030-01-05
Also published as: CN102802824A; BR112012016664A2; EA201200813A1; EP2521626B1; MX2012007817A; JP2013516325A; AR079829A1; WO2011083367A1; JP5615938B2; US20130074563A1; SA111320071B1

Abstract

The invention relates to a plant for performing the rolling of seamless tubes, typically with a large diameter. The plant comprises: a furnace for heating billets produced by means of continuous casting; a piercing mill for piercing longitudinally the billets so as to obtain a pierced blank; an expanding-elongating mill for expanding the diameter of the pierced blank and for elongating the pierced blank so as to obtain a semi-finished tube; a continuous main rolling mill of the type comprising stands with two or more rolls, in which the radial position of the rolls is adjustable, for performing rolling of a tube on a retained mandrel; a fixed-roll extracting-reducing mill positioned downstream of the main rolling mill and in series therewith, the extracting-reducing mill being designed to extract the tube from the mandrel and to define a predetermined value for the diameter of the tube; a sizing mill for defining the diameter of the finished tube, the sizing mill being of the type in which the radial position of the rolls is adjustable; and a cooling bed. The plant according to the invention also comprises a by-pass line for feeding the pierced blank leaving the piercing mill directly to the main rolling mill, thereby avoiding the expanding-elongating mill.

Description

Title: "Tube rolling plant" DESCRIPTION

The present invention relates to a plant for the production of seamless tubes, in particular for the production of large-diameter seamless tubes. The invention also relates to a method for achieving said production. The expression "large diameter" is understood here and below as meaning diameters of between 457.2 mm and 711.2 mm (i.e. between 18" and 28").

The production of small-thickness large-diameter tubes is performed at present by means of deformation of metal sheets, thereby obtaining longitudinally welded tubes. This tube production technology, although widely used, is not without drawbacks. Firstly, tubes with only a relatively small wall thickness may be obtained; the metal sheets from which the tubes are obtained may have a maximum thickness of the order of 30 to 35 mm. A further disadvantage of welded tubes compared to seamless tubes is that the former have a smaller mechanical strength and corrosion resistance, in particular along the weld.

As an alternative to welded tubes it is also possible to produce seamless tubes. A type of rolling mill called "pilger mill" is used in a known manner for the production of large-diameter seamless tubes. This rolling mill uses grooved rolls with a variable groove depth along their circumference. The central section of the roll is therefore cam-shaped, i.e. not circular. Processing of the tube in this rolling mill requires continuous displacement of the blank backwards and forwards along the rolling axis.

Although being at present the only machines used for the industrial production of large-diameter seamless tubes, pilger mills have a number of drawbacks.

Firstly, it is a fairly slow machine; for example, the typical production output of such a machine is about 12 to 15 tubes per hour, compared to the 60 or more tubes produced by normal continuous rolling mills.

Moreover, the pierced blank used in the pilger mill cannot be obtained from an ordinary continuous casting billet. In fact, because of its specific characteristics, a pilger milling results in considerable elongation of the pierced blank. This elongation of the pierced blank must necessarily be compensated by a substantial reduction in the diameter. In view of these technological constraints, the production of large-diameter tubes necessarily also requires large diameters of the pierced starting blanks, which consequently cannot be obtained from ordinary continuous casting billets. In fact, the maximum diameter of standard billets nowadays is no more than 500 to 550 mm and is therefore insufficient. Larger-diameter billets could be obtained from continuous casting plants designed with specific dimensions. The quantity of large-diameter billets normally required by the market does not justify, however, the huge investment needed for the construction of such a plant.

Therefore, the pierced blanks used for rolling in a pilger mill must have diameters of up to about 950 mm and consequently must be obtained from ingots which have sufficiently large diameters. The person skilled in the art knows that an ingot, for technological and production-related reasons, costs up to 30% more than a billet. Moreover, the quality of an ingot is inferior to that of a continuous casting billet. In fact, an ingot does no have very uniform characteristics and the waste associated with this production method, namely the sprue or riser, penalises significantly the manufacturing costs.

The waste associated with the tail end of the tube rolled in a pilger mill is also considerable. This rolling process in fact produces a typical "bell", i.e. an end part of the tube which cannot be rolled and which must inevitably be cut off and discarded. Considering, therefore, the starting material and the type of process, the pilger mill rolling method has overall a relatively low output.

A major problem associated with the use of the pilger mill is moreover the poor quality of the finished tube. The type of working process described above and the geometrical form of the incoming pierced blank are such that the walls of the finished tube are somewhat irregular. This characteristic of the tubes obtained by means of a pilger mill conventionally has not been regarded as a problem. Nowadays, however, with the much higher quality standards which can be achieved with continuous rolling mills, this characteristic is increasingly being regarded as a defect, in particular in view of the high cost of the product.

In the past another technology for the production of large-diameter seamless tubes has also been used. This technology is based on a machine known as an "expander". An expander basically allows deformation of a tubular blank so as to obtain a finished tube with a larger diameter, smaller wall thickness and length substantially the same as that of the tubular blank. The percentage increase in diameter, or expansion, typically obtained with an expander may be reckoned as having a value of up to 60%. The maximum expansion which can be obtained by the expander, however, depends on the wall thicknesses of the incoming tubular blank.

Typically, during processing with an expander, the metric weight of the incoming blank and of the outgoing product remains substantially unvaried. For this reason, in order to obtain large outgoing thicknesses, it would be necessary to start with incoming thicknesses so large that they would be difficult to achieve in practice. Moreover, even if it were possible to achieve these thicknesses for the blank entering the expander, the typical helical scoring present on the inside of the outgoing blank would be very marked and therefore unacceptable.

In an expander, in fact, the rod which supports the plug inside the tube operates under compression. It is known that this stressed condition places a limit on the maximum load, this limit being fairly low in order to prevent the rod being affected by buckling resulting from the compressive stress and to ensure correct set-up of the machine and precise control of the process. For this reason, the large wall thicknesses, responsible for high compressive loads on the plug, require low percentage expansion values.

Moreover, high expansion of the diameter with large wall thicknesses results in increasing unevenness inside the tube leaving the expander. This unevenness, in the form of helical scoring, can only be eliminated with difficulty by the subsequent machining operations.

This technology has not been very successful on account of the considerable number of drawbacks associated with it. First of all, the production of tubes was performed using tubular blanks which were also, in practice, finished tubes. In view of the typical expansion ratios of the expanders, in order to obtain a finished tube with a diameter of 28", it was necessary to use initially a blank with a diameter of 18". At the time when expanders were widely used, the 18"-diameter tubes were obtained by means of the already mentioned pilger mill since retained-mandrel rolling mills were not yet available for such diameters. Obviously the poor wall quality of the starting tubes directly affected the quality of the finished tubes. The expander processing step certainly could not improve the quality and, on the contrary, also introduced further defects. This was one of the reasons why this technology was in fact abandoned in favour of higher-capacity pilger mills able to produce directly in a single pass tubes with the desired diameter and of comparable quality.

A further disadvantage of the technology associated with an expander consisted in the fact that the tubular blanks had to be heated in a special furnace before processing. This heating stage always proved to be somewhat critical. The temperature of the tubes, in fact, had to be increased from the room temperature typical of warehouses to the 1200 to 1250° C required for working. This heating operation therefore increased considerably the amount of time and the costs involved. In particular, in order to achieve a temperature which was as uniform as possible on the tube and sufficiently high to allow optimum working thereof, the heating stage had to be prolonged, in particular in the case of large-thickness blanks. The longer the heating stage, the greater the production of oxides occurring inside the tube. These oxides then had to be removed in order to improve the workability of the tube, reduce the internal defects and ensure a minimum quality of the finished product. Removal of the oxides is, still nowadays, a fairly complex operation and involves the use of a saline solution. It is therefore a critical operation in particular from the point of view of environmental safety.

The problems mentioned above in connection with the production of standard steel tubes are exacerbated even more during the production of tubes made of high-alloy steels, for example steels with a chromium content of 10% or more. The mechanical characteristics typical of these steels result in a reduced deformability of the material and, therefore, as regards the expander, increase the compressive stresses acting on the plug during operations involving a high degree of expansion. Moreover, high-alloy steel tubes are commonly required by the market in medium- to-large wall thicknesses, thereby further increasing the working difficulties associated with the expander.

The production of large-diameter seamless tubes could also be performed by means of a continuous rolling mill of the type commonly used for medium-diameter tubes. In this type of machine, the tube is rolled by passing it through a series of rolling stands (or stations) each comprising two or more rolls, usually three rolls. The rolling stands are normally five or more in number and the position of the rolls is adjustable in the radial direction. This type of working operation requires a mandrel arranged inside the tube so as to be able to oppose the radial thrust exerted by the rolls during rolling. In order to exert this opposing action, the mandrel must be extremely rigid in the radial direction. Moreover, in order to ensure a high quality finish on the inner surface of the tube, the mandrel must have an outer surface which is as smooth as possible. Because of this requirement, it would be extremely difficult to manufacture mandrels consisting of several parts joined together. The joining zone is in fact necessarily characterized by an irregular surface. Moreover, this zone would be too delicate to withstand adequately the radial rolling pressure. It is known, in this sector, to use a retained mandrel: the mandrel is axially constrained and is retained so as to advance at a controlled speed. This solution has a major drawback. The single section of the mandrel in fact, although being braked, is advanced axially along the rolling mill and is thus engaged in succession, under maximum deformation conditions, within all the rolling stations. Inside the rolling stations, the mandrel is subject to high thermal and mechanical stresses due to the deformation energy and the friction produced by the sliding contact of the tube material. The passage through more than one rolling station therefore causes a significant increase in the mandrel temperature, thereby resulting in the need to provide several mandrels which are identical to each other such that each one of them may be suitably cooled at the end of rolling and then lubricated for the next rolling cycle.

In addition to this, it must be considered that the individual mandrel must be made of a particularly high-quality material in order to withstand the stresses typically arising during rolling. Obviously the outlay required for a mandrel depends on its dimensions. The typical lengths of retained mandrels are in fact such that the manufacture of an entire set of large-diameter (i.e. more than 20") mandrels required for conventional continuous rolling is disadvantageous from the point of view of costs.

The object of the present invention is therefore to overcome at least partly the drawbacks mentioned above with reference to the prior art.

In particular, an aim of the present invention is to provide a plant and a method for the production of large-diameter seamless tubes.

Moreover, an aim of the present invention is to provide a plant and a method for the production of tubes with a wide range of wall thicknesses (from small to large). Furthermore, an aim of the present invention is to provide a plant and a method for the production of tubes made of different types of steel, including both carbon steel and high-alloy steel.

Furthermore, an aim of the present invention is to provide a plant and a method by means of which it is possible to obtain finished tubes of superior quality compared to those currently available on the market.

Finally, an aim of the present invention is to provide a plant and a method which are able to produce also medium-diameter seamless tubes, i.e. those with a diameter of between 339.7 mm and 508 mm (13.3/8" to 20").

The abovementioned object and aims are achieved by a plant as claimed in Claim 1 and by a method according to Claim 11.

The characteristic features and further advantages of the invention will emerge from the description, provided hereinbelow, of a number of examples of embodiment, provided by way of a non-limiting example, with reference to the accompanying drawings in which:

- Figure 1 shows a block diagram representing a plant according to the prior art;

- Figure 2 shows block diagram representing a plant according to the invention;

- Figure 3 shows schematically the detail of an expander-elongator used in the plant according to the invention;

- Figure 4 shows schematically the continuous main rolling mill used in the plant according to the invention;

- Figure 5 shows schematically, in the form of a thickness/diameter diagram, the various types of tubes which can be produced according to the invention.

The seamless tube rolling plant according to the invention comprises the following components:

- a furnace for heating billets produced by means of continuous casting;

- a piercing mill for piercing longitudinally the billets so as to obtain a pierced blank;

- an expanding-elongating mill for expanding the diameter of the pierced blank and for elongating the pierced blank so as to obtain a semi-finished tube;

- a continuous main rolling mill of the type comprising stands with two or more rolls, in which the radial position of the rolls is adjustable, for performing rolling of a tube on a retained mandrel;

a fixed-roll extracting-reducing mill positioned downstream of the main rolling mill and in series therewith, the extracting-reducing mill being designed to extract the tube from the mandrel and to define a predetermined value for the diameter of the tube;

a sizing mill for defining the diameter of the finished tube, the sizing mill being of the type in which the radial position of the rolls is adjustable; and a cooling bed.

Moreover, the plant according to the invention comprises a by-pass line for feeding, where possible, the pierced blank leaving the piercing mill directly to the main rolling mill, thereby avoiding the expanding-elongating mill.

The components of the plant, according to a number of embodiments thereof, are described below. This description is, in places, somewhat brief since some of the components of the plant are known per se to the person skilled in the art, even though arranged and used in different ways.

The billet furnace is a furnace conventionally used in the sector and well known to the person skilled in the art.

The billet piercing mill (or piercer) may be a standard conical-roll piercer comprising two inclined-axis rolls which act on the outer surface of the billet and a plug which is inserted in the middle of the billet along the hole.

From a conceptual point of view, the expanding-elongating mill (or expander- elongator) is a machine quite similar to the piercer. For this reason, in accordance with certain embodiments of the invention, the piercer and the expander-elongator may be the same machine preset in two different configurations, as described in greater detail below.

The attached Figure 3 shows a diagram of an inclined-axis conical-roll mill in the configuration where it is designed to perform the function of an expanding- elongating mill. The mill, denoted in its entirety by 10, comprises a pair of rolls 12 of variable conicity rotating about respective axes. The rotation axes of the rolls 12 are inclined relative to each other. The expander 10 also comprises an ogive-shaped plug 14 connected to a rod 16. The rod 16 may be arranged as shown in the attached Figure 3 so that it is subject to compression during rolling. Alternatively, the rod 16 may advantageously be arranged on the opposite side of the ogive-shaped plug 14 so as to be subject to a pulling force. The pierced blank 20 is rotated about its axis and pushed against the ogive-shaped plug 14 in the direction of the arrow f shown in Figure 3. As can be seen from the diagram in Figure 3, the combined configuration of the rolls 12 and the ogive-shaped plug 14 define a travel path along which the material of the pierced blank must flow. The travel movement along this path causes the desired deformation consisting of an expansion-elongation.

In particular, in accordance with certain embodiments, the profiles of the rolls 12 and the ogive-shaped plug 14 are defined so that a part of the travel path causes expansion of the diameter and elongation of the tube and the remainder of the travel path instead results in the desired expansion of the tube diameter. Obviously, a reduction in the thickness of the tube wall is also obtained along the entire travel path.

For example, according to one embodiment, the profiles of the rolls 12 and the ogive-shaped plug 14 are defined so that the first approximately two thirds of the travel path cause simultaneously reduction in the wall thickness, expansion of the diameter and elongation of the tube. The remaining approximately one third of the travel path instead causes reduction in the wall thickness and the remaining desired expansion of the tube diameter.

According to one embodiment, the expanding-elongating mill causes an expansion of the diameter equivalent to about 35% and a tube elongation by a factor of about 1.7.

In accordance with certain embodiments, the rolling mill shown in Figure 3 is designed to be rapidly reconfigured so as to perform alternately the function of a piercing mill and the function of an expanding-elongating mill. In particular, the transition from one configuration to the other may be achieved by means of a different orientation of the axes of the rolls 12 and by means of a different form of the ogive-shaped plug 14. In this case, the pierced blank leaving the machine configured as a piercing mill is processed again by the same machine reconfigured to act as an expanding- elongating mill. Only after the second pass the semi-finished tube is fed to the main rolling mill.

The use of such a machine which can be reconfigured, although complicated and per se somewhat costly, may in any case be advantageous compared to the use of two different machines of the conventional type.

The main rolling mill, which is of the type with mill-stands having two or more adjustable rolls and a retained mandrel, may be for example of the type described in international patent application PCT/EP99/01402 filed in the name of Demag Italimpianti S.p.A. and published under number WO 99/47284. Preferably, the main rolling mill according to the invention comprises stands with three rolls.

According to one embodiment of the invention, the main rolling mill comprises four rolling stands arranged in succession. This solution constitutes a particularly convenient adaptation of conventional rolling mills comprising two or more adjustable rolls. These rolling mills in fact usually comprise five or more rolling stands arranged in succession.

The feedback controls as to the position of the rolls in the main rolling mill, based on the tube thickness, and in the sizing mill, based on the tube diameter and temperature, may advantageously be of the type described in patent application IT MI2009A001085 filed by the same applicant on 19 June 2009.

In accordance with certain embodiments of the plant according to the invention, the main rolling mill is characterized in that it uses a slow mandrel. In the present description, the term "slow mandrel" is understood as meaning a mandrel which is retained so that none of its sections is subject to the action of two successive rolling stations. More particularly, with reference also to the attached Figure 4, the following equation is applicable:

V_m < d/T,

where V_m is the speed of the mandrel 32; d is the minimum interaxial distance between two successive rolling stands 34; and Ti is the rolling time. Also applicable is the equation: where L_t is the length of the tube 20 and V_t is the axial speed of the tube 20 along the rolling mill 30.

From the above it can be understood that the mandrel 32, required for operation of the main rolling mill 30 used in the plant according to the invention, may be relatively short. The minimum length required will in fact be equal to the overall interaxial distance D (i.e. the distance between the first and last rolling station) increased by the displacement S_m which the mandrel 32 performs during the rolling time: S_m=V_mTi . The above equations also give the following value: S_m < d.

Considering the embodiment of the main rolling mill 30 according to the invention schematically shown in Figure 4, the overall interaxial distance D is fairly short because the rolling mill comprises a small number of rolling stands 34, in the specific case only four stands. Moreover the extremely low speed of the mandrel V_m also allows a small displacement S_m of the mandrel 32. Considering the average values typically assumed by the variables indicated above, the minimum length of the mandrel 32, equivalent to D + S_m, will be between about 5 and 6 metres. This length is such that the mandrel 32 may be manufactured at a relatively low cost, despite the large cross-sections required for the production of tubes with a diameter of up to 28 inches.

Moreover, since each individual section of the mandrel is subject to the action of only one rolling stand, the overall amount of heating of the mandrel during the process is limited. Due to this, it is possible to manufacture the mandrel using materials which are less expensive than those used for conventional faster mandrels, without any negative consequences.

Moreover, as can be noted from the attached Figure 4, the three interaxial distances separating the four rolling stands 34 are not all the same. The first interaxial distance d, which separates the first stand from the second stand, and the third interaxial distance d, which separates the third stand from the fourth stand, are substantially the same. However, the second interaxial distance, which separates the second stand from the third stand, is greater than the other two distances. A mini support stand 36 for the mandrel 32 is in fact positioned between the second rolling stand and third rolling stand since otherwise be the mandrel would extend cantilever-like along the rolling mill 30. It is assumed, as in Figure 4, that the second interaxial distance is greater by a distance j than the other two distances; each of the sections of the mandrel 32, during the entire rolling process, travels at the most along a section having a length S_m < d. In connection with the second interaxial distance it is therefore possible to identify a section of the mandrel 32 with a length at least equal to j which does not undergo any rolling either by the second stand or by the third stand. This section of length j is therefore available for performing a joint 33 between two sections 32' and 32" of the mandrel 32. With reference to the example considered above, the two sections 32' and 32" of the mandrel 32 would each have a length of between about 2.5 and 3 metres. With these lengths, it is possible to simplify drastically the manufacturing and management of the mandrel 32, even in the case of the large diameters considered here (greater than 24 inches).

Moreover, using a built-up type mandrel, it is possible if necessary to replace only the worn portion. In contrast, when using conventional non-built-up mandrels, the entire mandrel must be replaced even if it is subject to only localised wear. This possibility offered by the built-up mandrel reduces significantly the operating costs of the rolling mill.

The solution employed here consisting of a slow built-up type mandrel, together with the overall smaller dimensions described above, thus make it possible to provide a main rolling mill for large-diameter tubes which is extremely competitive on the market.

The fixed-roll extracting-reducing mill has the function of extracting the semifinished tube from the mandrel and of reducing the diameter of the semi-fmished tube to a predetermined value which is close to the desired value of the finished tube.

According to one embodiment of the plant, the extracting-reducing mill may be replaced by a combination of machines which together are designed to perform a similar function. For example, the extracting-reducing mill may be replaced by the combination consisting of an extracting mill, specifically intended to extract the tube from the mandrel, and a reducing mill, designed to define a predetermined diameter of the semi-finished tube.

In accordance with certain embodiments of the invention, the plant also comprises, downstream of the extracting-reducing mill, means for measuring the wall thickness of the semi-finished tube. In these embodiments, the main rolling mill is able to adjust the radial position of the rolls depending on the measurement of the wall thickness of the tube leaving the extracting-reducing mill.

In accordance with certain embodiments of the invention, the sizing mill comprises means for measuring the temperature of the incoming tube and means for measuring the diameter of the outgoing finished tube. In these embodiments, the sizing mill is able to adjust the radial position of the rolls depending on the measurement of the temperature of the incoming tube and the measurement of the diameter of the outgoing finished tube.

The invention also relates to a method for performing the rolling of seamless tubes. The method according to the invention comprises the following steps:

heating a billet produced by means of continuous casting;

longitudinally piercing the heated billet so as to obtain a pierced blank;

expanding and elongating the pierced blank so as to increase its diameter and length and reduce its thickness;

rolling the semi-finished tube in a main rolling mill so as to obtain a tube, the main rolling mill being of the continuous retained-mandrel type comprising stands with two or more adjustable rolls;

extracting the tube from the mandrel;

defining a predetermined value for the diameter of the finished tube in a sizing mill of the type comprising adjustable rolls;

cooling the finished tube.

In accordance with other embodiments, the method also comprises the steps of: measuring the wall thickness of the tube after extraction from the mandrel; and

adjusting the radial position of the rolls of the main rolling mill depending on the measurement of the wall thickness of the tube.

In accordance with other embodiments, the method also comprises the steps of: measuring the temperature of the tube entering the sizing mill;

measuring the diameter of the tube leaving the sizing mill; and

adjusting the radial position of the rolls of the sizing mill depending on the measurement of the temperature of the incoming tube and the measurement of the diameter of the outgoing tube.

In accordance with certain modes for implementing the method, the step of longitudinally piercing the billet is performed by means of a machine which can be reconfigured. According to these modes of implementation, the method also comprises, following the step of longitudinally piercing the billet to obtain a pierced blank, the step of reconfiguring the machine so that it is adapted for expanding and elongating the pierced blank so as to increase its diameter and length, while reducing its thickness.

Some of the advantages arising from the plant and the method for the production of tubes according to the invention will be described below.

Figure 4 shows schematically, in the form of a thickness/diameter diagram, the various types of tubes which can be produced by means of the plant according to the invention. In particular, three classes of tube have been defined in this diagram. A first class is that included in the area denoted by A, representing tubes with a small wall thickness and a medium-to-small diameters. A second class is that included in the area denoted by B, representing tubes with large diameters and any wall thickness. A third class is that included in the area denoted by C, representing tubes with medium-to-large wall thicknesses and medium-to-small diameters.

The C class of tubes is the only class which may be produced with by-passing of the expanding-elongating mill and using only the single continuous main rolling mill with two or more adjustable rolls for performing rolling on a retained mandrel. As can be seen, therefore, owing to the addition of the expanding-elongating mill to the plant, it is possible to broaden considerably the range of tube types which may be produced by the plant. In particular, the class of tubes denoted by A may not be obtained by means of the main rolling mill alone because it requires a significant reduction in the wall thickness with regard to the pierced blank leaving the piercer. On the other hand, the class of tubes denoted by B may not be obtained by means of the main rolling mill alone because it requires a significant expansion of the diameter of the pierced blank leaving the piercer.

As described above, the plant and the method according to the invention envisage the use of continuous casting billets. These billets offer, compared to the ingots conventionally used for the production of large-diameter tubes, a number of significant advantages. First of all, the billet steel is of a more uniform, more controlled and, generally, superior quality. Furthermore, the cost of billets is about 30% less than the cost of ingots.

A main advantage, resulting from the plant and method according to the invention, is the significant reduction of the production costs. As mentioned in the introduction, the prior art involved the expander being fed with finished tubes stored in warehouses. In the plant configuration according to the invention, on the other hand, the initial blank is obtained immediately before from a billet. Since the tubes are not required to remain for a long time in the furnace in order to heat it from room temperature to the working temperature, the problem of oxide formation inside the tube is avoided. Moreover, working on the piercer results in a substantial increase in the internal temperature of the blank, due to the friction and energy released in the form of heat during breakage of the material. This therefore results in two substantial advantages: the material inside the blank remains exposed to the atmosphere for a minimum period of time and the temperature inside the blank, which is the most difficult to increase inside the furnace, is even greater than the outside temperature. In addition to the substantial reduction in energy and working time, there is also the lower cost of the billet compared to an ingot, as already mentioned above.

A further reduction in the costs arises from the total elimination of intermediate storage of the tubes, resulting in significant savings from the point of view of investment, space, operating costs and maintenance.

Finally, the elongation operations performed in the main rolling mill are of a limited nature and therefore the amount of tube waste (front end and tail end) is minimal compared to that which occurs in other machines, such as the pilger mill. The high yield of the material therefore reduces production costs. Incidentally, the limited nature of the elongation operations also means that the stresses are very small, such that the apparatus is subject to less wear.

Compared to the prior art, the plant and method according to the invention are also able to offer substantial advantages in terms of the quality of the finished tube. The superior quality of billet steel compared to that of an ingot has already been mentioned above. Moreover, the much smaller formation of oxides achieved with the working process according to the invention results in a distinctly superior workability of the material and therefore a better final quality. Finally, with elimination of the pilger rolling process, an improved surface quality of the semifinished blank - and therefore of the finished tube - is achieved, along with much smaller dimensional tolerances.

Moreover, since in the plant according to the invention the expanding-elongating mill is positioned at the start of the production line, the subsequent machining operations manage to reduce substantially the problems associated with the use of this machine. In particular, the main rolling mill is able to smooth out the helical scoring which typically is present on the inner wall of the tube at the end of the expander working operation. Owing to this characteristic feature of the invention, it is possible to obtain tubes of distinctly superior quality compared to those obtained using conventional technology. Special studies carried out by the applicant have defined the internal quality of the tubes produced according to the invention as being "very high". Similar studies carried out on tubes of the same type, but produced using a pilger mill or a conventional expanding mill, have defined the quality of these tubes as being "medium to low".

It should also be mentioned that the studies conducted by the applicant have clearly highlighted the superior concentricity of the wall thickness (i.e. the uniformity thereof along the circumference of the tube) obtainable with the plant according to the invention compared to plants of the known type, in particular the pilger mill and conventional expander.

Considering, for example, the large and extra-large wall thicknesses, the percentage tolerances in terms of the concentricity are about half of those obtained with a pilger mill and slightly more than half those obtained with a conventional expander. This qualitative advantage diminishes slightly with a reduction in the wall thickness, but remains at percentage tolerance values significantly lower than those of the prior art. In terms of safety for the environment and for the operators, the very limited formation of oxides reduces to a minimum the problems associated with eliminating the oxides and the consequent use of a saline solution.

Finally, the plant according to the invention is characterized also by a certain flexibility. In fact, this plant according to the invention allows not only the production of large-diameter tubes, i.e. with a diameter of between 18" and 28", but also, with by-passing of the expanding-elongating mill, the production of medium- diameter tubes, i.e. with a diameter of between 13.3/8" and 20", and also of large- thickness tubes. The production is extremely competitive in terms of quality of the finished tube and allows the overall productivity of the plant to be increased. Large- diameter tubes represent, in fact, only a relatively small share of the market and combining it with the production of medium-diameter tubes enable to speed up significantly amortization of the entire plant and the return obtained from the corresponding investment made.

As will be clear to the person skilled in the art, the plant and the method according to the invention overcome at least partly the drawbacks mentioned with reference to the prior art.

With regard to the embodiments of the plant and method for the production of large- diameter seamless tubes according to the invention, the person skilled in the art may, in order to satisfy specific requirements, make modifications to and/or replace elements described with equivalent elements, without thereby departing from the scope of the accompanying claims.

Claims

1. Plant for performing the rolling of seamless tubes, comprising:

a furnace for heating billets produced by means of continuous casting;

- a fixed-roll extracting-reducing mill positioned downstream of the main rolling mill and in series therewith, the extracting-reducing mill being designed to extract the tube from the mandrel and to define a predetermined value for the diameter of the tube;

- a sizing mill for defining the diameter of the finished tube, the sizing mill being of the type in which the radial position of the rolls is adjustable; and a cooling bed;

wherein the plant comprises a by-pass line for feeding the pierced blank leaving the piercing mill directly to the main rolling mill, thereby avoiding the expanding-elongating mill.

2. Plant according to claim 1, wherein the piercing mill comprises a pair of rolls rotating about respective axes inclined relative to each other, and an ogive- shaped plug.

3. Plant according to claim 1 or 2, wherein the expanding-elongating mill (10) comprises a pair of variable-conicity rolls (12) rotating about respective axes inclined relative to each other, and an ogive-shaped plug.

4. Plant according to any one of the preceding claims, wherein the piercing mill and the expanding-elongating mill are one and the same machine (10), this machine being designed to assume two different configurations defined by the orientation of the axes of the rolls (12) and by the form of the ogive-shaped plug (14).

5. Plant according to any one of the preceding claims, wherein the main rolling mill (30) comprises rolling stands (34) with three or more rolls.

6. Plant according to any one of the preceding claims, wherein the main rolling mill (30) comprises four rolling stands (34) arranged in succession.

7. Plant according to any one of the preceding claims, further comprising, downstream of the extracting-reducing mill, means for measuring the wall thickness of the semi-finished tube; the main rolling mill being designed to adjust the radial position of the rolls depending on the measurement of the wall thickness of the tube leaving the extracting-reducing mill.

8. Plant according to any one of the preceding claims, wherein the mandrel (32) of the main rolling mill (30) is retained so that none of its sections is subject to the action of two successive rolling stations (34).

9. Plant according to any one of the preceding claims, wherein the mandrel (32) of the main rolling mill (30) is composed of at least two sections (32', 32"); and wherein the joint (33) between two sections (32', 32") is not engaged under any rolling station (34) during rolling.

10. Plant according to any one of the preceding claims, wherein the sizing mill comprises means for measuring the temperature of the incoming tube and means for measuring the diameter of the outgoing finished tube; said sizing mill being designed to adjust the radial position of the rolls depending on the measurement of the temperature of the incoming tube and the measurement of the diameter of the outgoing finished tube.

11. Method for performing the rolling of seamless tubes, comprising the steps of:

- heating a billet produced by means of continuous casting;

- longitudinally piercing the heated billet so as to obtain a pierced blank;

- expanding and elongating the pierced blank so as to increase its diameter and length, while reducing its thickness;

- rolling the semi-finished tube in a main rolling mill of the retained mandrel type, with stands having two or more adjustable rolls, so as to obtain a tube; extracting the tube from the mandrel;

- defining a predetermined value for the diameter of the finished tube, in a sizing mill of the type comprising adjustable rolls; and cooling the finished tube.

12. Method according to the preceding claim, further comprising the steps of:

- measuring the wall thickness of the tube after extraction from the mandrel; and

- adjusting the radial position of the rolls of the main rolling mill depending on the measurement of the wall thickness of the tube.

13. Method according to claim 11 or 12, further comprising the steps of:

- measuring the temperature of the tube entering the sizing mill;

- measuring the diameter of the tube leaving the sizing mill; and

14. Method according to any one of claims 11 to 13, wherein the step of longitudinally piercing the billet so as to obtain a pierced blank is performed by means of a machine which can be reconfigured, and is followed by the step involving reconfiguration of the machine so as to adapt it to perform expansion and elongation of the pierced blank so as to increase its diameter and length, while reducing its thickness.