DESCRIPTION

The present invention relates to a seamless tube production plant, 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 mainly by means of deformation of metal sheets, thereby obtaining longitudinally welded tubes. This rube 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.

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 semi-finished blank backwards and forwards along the rolling axis.

Although being at present the only machine suitable for the production of large- diameter seamless tubes, the pilger mill is not without drawbacks.

Firstly, it is a fairly slow machine; it should be noted that 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 hollow semi-finished blank used in the pilger mill cannot be obtained from an ordinary continuously cast billet. In fact, because of its specific characteristics, a pilger mill requires considerable elongation of the hollow semifinished blank. Said elongation of the hollow semi-finished blank must necessarily be offset by a substantial reduction in the diameter. In view of these technological constraints, the production of large-diameter tubes necessarily also requires large diameters as regards the hollow semi-finished blanks initially used, which blanks consequently cannot be obtained from ordinary continuously cast billets. In fact, the maximum diameter of standard billets is not greater than 450÷500 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, however, does not justify the huge investment needed for the construction of such a plant.

Therefore, the hollow semi-finished blanks used for rolling in a pilger mill must have diameters of up to 1000 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 about 30% more than a billet. Moreover, the quality of the steel in an ingot is inferior to that of the steel in a continuously cast billet. In fact, an ingot has characteristics which are not very uniform and the waste associated with this production method, namely the sprue, penalises significantly the manufacturing costs.

The waste associated with the 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 yield.

A major problem associated with the use of the pilger mill is, however, the poor quality of the finished tube. The type of processing described above is such that the walls of the finished tube are somewhat irregular. This characteristic of the tubes obtained by means of a pilger mill traditionally has not been viewed as being a problem. Nowadays, however, with the much higher quality standards which can be widely 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.

The production of large-diameter seamless tubes with a large wall-thickness in continuous rolling mills would theoretically be possible, but is in practice absolutely disadvantageous. A modern continuous rolling plant manages to produce tubes with a maximum diameter of about 457.2 mm (18 inches). Most of the tubes required by the market fall within this maximum diameter range. The production of large-diameter tubes, which may have a diameter of up to 28" and more, would therefore require the construction of a special continuous rolling mill with dimensions considerably greater than normal. Such a continuous rolling mill, in addition to posing technological problems which are difficult to solve and requiring equipment which is complex to produce, would require an extremely high initial outlay. It must be pointed out, however, that the quantity of large- diameter tubes required annually by the market cannot justify the huge investment which would be necessary in order to construct such a continuous rolling plant. It should also be considered that a larger-size rolling mill which may allow the production of 28" tubes would in fact not be competitive with a conventional-size plant during the production of small and medium-size tubes. The larger-size rolling mill would in fact be inadequate for ensuring the optimum quality which is now commonly available in the production of such tubes.

In the past (from the 1920s to the 1940s) another technology was also used for the production of large-diameter seamless tubes. This technology was based on a machine known as an "expander". An expander basically allows deformation of a tubular semi-finished 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 semi-finished blank. The percentage increase in diameter, or expansion, typically obtained with an expander may be reckoned as having a maximum value of 60%. The maximum expansion which can be obtained by the expander depends, however, on the wall thicknesses of the incoming tubular semi-finished blank. In an expander, in fact, as can be seen in the accompanying Figure 3, the rod which supports the nose 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 (i.e. instability resulting from axial 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 nose, result in low percentage expansion values.

Moreover, large diameter expansion values with large wall thicknesses result in increasing irregularity inside the tube leaving the expander. Such irregularities are difficult to eliminate using the reeler, since the smoothing action of the reeler decreases as the wall thickness increases.

For these reasons, during the production of large-diameter tubes with large wall thicknesses, it is often required to perform several successive passes in the expander so as to keep, during each of the passes, the loads acting on the nose within the limit values and lessen the internal irregularities. This results in high production costs, as well as considerable oxide formation on both the inner and outer surfaces of the tube.

This technology has not been very successful on account of the considerable number of drawbacks associated with it. Firstly the production of tubes was performed using tubular semi-finished blanks which were also, in practice, finished tubes. In view of the typical expansion ratios of expanders, in order to obtain a finished tube with a diameter of 28", it was necessary to use initially a semi-finished blank with a diameter of 18". At the time when expanders were widely used, the 18"-diameter tubes were obtained by means of the said pilger mill. 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, even 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 a comparable quality.

A further disadvantage of the technology associated with the expander consisted in the fact that the tubular semi-finished 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 processing. This heating operation therefore increased considerably the amount of time and 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 processing thereof, the heating stage had to be prolonged, in particular in the case of large- thickness semi-finished 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 processability of the tube, reduce the internal defects and ensure a minimum quality of the finished product. Removal of the oxides is today still 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 further during the production of tubes made of high-alloy steels, for example steels with a chromium content of 10% or more. These steels must be processed at temperatures upon leaving the furnace lower than those of ordinary steel, i.e. temperatures which may be even 80°C lower. These relatively low temperatures, together with the mechanical characteristics typical of these steels, result in a reduced deformability of the material and, therefore, as regards the expander, increase the stresses acting on the nose for the same percentage expansion. Moreover, high-alloy steel tubes are commonly required by the market to have medium to large wall thicknesses, thereby further increasing the processing difficulties associated with the expander.

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, a task of the present invention is to provide a plant for the production of - seamless tubes with a large diameter and, potentially, a large wall thickness.

Furthermore, a task of the present invention is to provide a plant with which it is possible to obtain finished tubes of greater quality compared to those currently available on the market.

Finally, a task of the present invention is to provide a plant which is able to produce also seamless tubes with a medium diameter or medium-to-small diameter, i.e. of between 273.05 mm and 508 mm (10 ³ / ₄ " to 20").

The abovementioned object and tasks are achieved by a plant as claimed in Claim 1 and by a method according to Claim 2.

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 figures, 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 used in the plant 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 piercer for piercing longitudinally the billets.

- A main continuous rolling mill of the type comprising stands with two or more rolls, where the radial position of the rolls is adjustable, for performing retained-mandrel rolling of a semi-finished tube.

A fixed-roll extraction/reducing mill positioned downstream of the main rolling mill and in-line therewith, the extraction/reducing mill being designed to extract the semi-finished tube from the mandrel and define a predetermined value for the diameter of the semi-finished tube.

Means for measuring the wall thickness of the semi-finished tube. The main rolling mill is able to adjust the radial position of the rolls on the basis of measurement of the wall thickness of the tube leaving the extraction/reducing mill.

A first intermediate furnace for heating the semi-finished tube to a temperature suitable for processing by means of hot plastic deformation.

An expander able to expand the diameter of the semi-finished tube to a predetermined value which is close to the value of the finished tube.

- A reeler suitable for ensuring a uniform thickness of the wall and improving the surface finish inside the semi-finished tube.

A second intermediate furnace for heating the semi-finished tube to a temperature suitable for processing by means of hot plastic deformation.

A first sizing mill for defining the diameter of the finished tube. The first sizing mill is of the type with three or more rolls, where the radial position of the rolls is adjustable, and is positioned downstream of the second intermediate tube furnace. Said sizing mill also comprises means for measuring the temperature of the incoming tube and means for measuring the diameter of the outgoing tube. The first sizing mill is suitable for adjusting the radial position of the rolls on the basis of measurement of the temperature of the tube entering the first sizing mill and on the basis of measurement of the diameter of the tube leaving the first sizing mill.

A cooling bed.

Moreover, the plant according to the invention comprises, downstream of the extraction/reducing mill, a bifurcation generating a second production line which extends from the main production line. The second production line comprises:

A second sizing mill of the type in which the radial position of the rolls is adjustable. This second sizing mill is positioned downstream of the extraction/reducing mill and off-line with respect thereto. Said sizing mill comprises means for measuring the temperature of the incoming tube and means for measuring the diameter of the outgoing tube. The second sizing mill is suitable for adjusting the radial position of the rolls on the basis of measurement of the temperature of the tube entering the second sizing mill and on the basis of measurement of the diameter of the tube leaving the second sizing mill.

A cooling bed.

The components of the plant according to the invention, although being arranged in a novel layout, are known per se.

The least known component of the line is the expander, a diagram of which is shown in the accompanying Figure 3. The expander, denoted in its entirety by 10, comprises a pair of conical discs 12 rotating about respective axes. The expander 10 also comprises a conical nose 14 connected to a rod 16. The semi-finished tube 20 is rotated about its axis and pushed against the conical nose 14 in the direction of the arrow f shown in Figure 3. As can be seen from the diagram in Figure 3, the action of the conical discs 12 results in a reduction in the wall thickness and a simultaneous increase in the diameter of the tube being machined.

A rapid description of the further components of the plant is provided below, in accordance with an embodiment thereof.

The furnaces, both that for billets and those for semi-finished tubes, are furnaces conventionally used in the sector and well known to the person skilled in the art. The billet piercer is a standard conical-roll piercer comprising two inclined-axis rolls which act on the outer surface of the billet and a nose which is inserted in the middle of the billet along the hole.

The main rolling mill, which is of the type with stands having three 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.

According to one embodiment, the main rolling mill comprises four rolling stands arranged in succession. This solution constitutes a particularly convenient adaptation of the rolling mill with three adjustable rolls. The rolling mills of this type in fact usually comprise five or more stands.

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

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

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

The rolling machine or "reeler" comprises a pair of oppositely arranged rolls which deform the wall of the tube, rolling the outside and inside thereof.

The invention also relates to a method for rolling 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 semifinished blank; rolling the pierced semi-finished blank in a main rolling mill so as to obtain a semi-finished tube, the main rolling mill being of the continuous retained-mandrel type comprising stands with three or more adjustable rolls;

extracting the semi-finished tube from the mandrel;

- defining a predetermined value for the diameter of the semi-finished tube; measuring the wall thickness of the semi-finished tube;

adjusting the radial position of the rolls of the main rolling mill on the basis of measurement of the wall thickness of the semi-finished tube;

expanding, in an expander, the diameter of the semi-finished tube to a predetermined value which is close to the value of the finished tube;

rolling the wall of the semi-finished tube so as to obtain a uniform thickness and improve the surface finish thereof;

measuring the temperature of the semi-finished tube;

defining the diameter of the finished tube in a first sizing mill of the type with adjustable rolls;

measuring the diameter of the outgoing tube;

adjusting the radial position of the rolls of the first sizing mill on the basis of measurement of the temperature of the tube entering the first sizing mill and on the basis of measurement of the diameter of the tube leaving the sizing mill;

- cooling the finished tube.

In accordance with an embodiment of the invention, the method also comprises, after the step of adjusting the radial position of the rolls of the main rolling mill, the step of heating the semi-finished tube to a temperature suitable for processing by means of hot plastic deformation.

In accordance with an embodiment of the invention, the method also comprises, after the step of rolling the wall of the semi-finished tube, the step of heating the semi-finished tube to a temperature suitable for processing by means of hot plastic deformation.

The additional steps of heating the semi-finished tubes may be necessary in the case where, during processing, the tubes cool excessively in order to ensure a good processability. This is probable, for example, in the case of tubes with a small wall thickness. As described above, the plant and the method according to the invention envisage the use of billets obtained from continuous casting. 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 steel used in the billet is of a more homogeneous, more controlled and, generally, better quality. Furthermore, the cost of billets is about 30% less than the cost of ingots.

A major 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 from the available warehouse stock. In the configuration of the plant according to the invention, instead, the expander follows directly the extraction/reducing mill. This thus avoids the need for the step of cooling, prior to warehouse storage, the tubes in order to feed the expander according to the prior art. In this way the semifinished tube reaches the expander already hot and may require, at the most, a slight increase in temperature, this requiring a small amount of thermal energy and time. Since the tube is not required to remain for a long time in the furnace in order to heat it from room temperature to the processing temperature, the problem of oxide formation inside the tube is avoided. Moreover, processing on the piercer results in a substantial increase in the internal temperature of the tube, due to the friction and energy released during breakage of the material in the form of heat. This therefore results in two substantial advantages: the material inside the tube remains exposed to the atmosphere for a minimum period of time and the temperature inside the tube, which is the most difficult to increase inside the furnace, is even greater than the external temperature. In addition to the substantial reduction in energy and processing 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, resulting in significant savings from the point of view of financial investment, space, operating costs and maintenance.

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 very limited formation of oxides achieved by means of direct processing on the expander results in a distinctly superior processability of the material and therefore a better final quality. Finally, with elimination of the pilger rolling process, an improved surface quality of the semi- finished tube - and therefore finished tube - is achieved.

In terms of safety of the environment and the operators, the very limited formation of oxides significantly reduces to a minimum the problems associated with removal thereof 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 allows not only the production of large-diameter tubes, i.e. with a diameter of between 18" and 28", for which the main production line is intended; the plant according to the invention, by making use of the second production line, also allows the production of medium-diameter tubes, i.e. with a diameter of between 10¾" and 20". The production of medium-diameter tubes in the plant according to the invention is extremely competitive in terms of quality of the finished tube and allows the overall productivity of the plant to be increased. The production of large-diameter tubes represents, in fact, only a relatively small share of the market and combining it with the production of medium-diameter tubes is able 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 above 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.