MALE ELEMENT FOR A SEALED THREADED TUBULAR CONNECTION Male tubular elements of threaded connections are generally obtained by machining tubes which are tightened in jaws acting on the outer surface of the tubes. For a variety of reasons, it may be necessary to machine the inner surface of the male tubular element close to the front surface of the free end. As an example, for the efficiency of a metal-metal sealing surface close to a free end it is desirable to provide a defined constant thickness beneath the surface around the circumference. Alternatively, when the front surface of the free end is to act as an axial abutment, the transverse cross sec¬ tion of that front surface must be well defined in order to determine the maximum makeup torque which that section can take. Other circumstances can also arise. The inner surface of the tubes carries imperfections, both macrogeometric (variations in diameter, eccentricity with respect to the outer surface) and microgeometric (surface quality) . API specification 5CT from the American Petroleum Institute (API) for casing and tubing prescribes the fol¬ lowing tolerances: • for the external diameter, -0.50% to +1%; • for the thickness, +12.5%; • for the mass, -3.5% to +6.5%. In respect of the internal diameter, the API indicates that the tolerances are governed by those for the external diameter and the mass. Thus, they are much wider than those for the external diameter. Regarding the eccentricity of the internal surface with respect to the external surface, this is included in the tolerance for the thickness. Conventionally, the internal surface is machined over an axial length of the order of 20 mm and this machined surface is connected to the internal surface of the remain- der of the tube via a tapered chamfer with a small inclina¬ tion: see, for example, the VAM catalogue published by the Applicant, under the NEW VAM heading. When the thickness of the tube is insufficient to al- low effective machining of the internal surface, prior to machining, a reduction swaging of the end is performed (also known as tapering) . Such known measures may prove to be insufficient in some cases, for example when the sealing surface is not adjacent to the free end or in the immediate vicinity thereof or, and in particular, when the male element is a tubular element constituting a threaded connection intended to undergo diametrical expansion and to be sealed after expansion. More particularly, we are concerned with threaded tu¬ bular connections intended to undergo diametrical expansion in the plastic deformation region using an expansion tool with a diameter that is greater than the internal diameter of the connection, which is axially displaced therein, as described, for example, in WO-A-02/01102. It has been shown that the transition between the non- machined zone and the machined zone, with a larger diame¬ ter, of the internal surface of the tube is detrimental to the action of the expansion mandrel and has a negative effect on the quality of contact, in particular the contact pressure, between the sealing surfaces of the male and female elements after expansion. The same disadvantages are caused by irregularities in the internal surface of the male element in its non- machined portion, whether it is due to eccentricity with respect to the external surface, circularity problems or irregularities in the axial direction, or surface imperfec¬ tions (seams, scratches, roughness, etc) . The invention aims to overcome the above disadvan- tages. The invention concerns a male tubular element formed in an end region of a tube the external surface and inter¬ nal surface of which have substantially circular transverse cross sections with a substantially constant diameter but which may not be perfectly concentric, the male tubular element comprising a first tapered male threading and at least one first sealing surface, which cooperates by makeup with a female tubular element comprising a second female tapered threading and at least one second sealing surface to form a sealed threaded tubular connection in which- said sealing surfaces are in mutual sealing contact metal to metal, the internal surface of said male tubular element having a first zone adjacent to the free end of the male element, with a circular cross section and concentric with the external surface of the tube. The invention provides in particular that the internal surface of said male tubular element comprises a second zone which follows the first zone axially, with a circular cross section which is concentric with the external surface of the tube, having a larger radius than that of said first zone, and which is larger than that of the internal surface of the remainder of the length of the tube. The term "sealing contact" as used here means contact between two surfaces which are strongly pressed against each other to produce a metal-on-metal seal, particularly against gas. Optional characteristics of the invention, which may be complementary or substitutional, are defined below: • The radius of the second zone is less than or equal to half the maximum internal diameter of the tube, resulting from the tolerances defined above. • The radius of the second zone is close to half the maximum internal diameter of the tube. • The second zone extends axially on the side oppo¬ site to the free end, at least to the end of the threading opposite to the free end. • The second zone extends axially to the end of the male tubular element which is not free. • The male threading is separated from the free end by a lip and the first zone extends axially up to the lip or the first threads. • The second zone is connected to the first zone via an essentially tapered surface. • Said essentially tapered surface connecting to the first zone has an apex half angle of 15° or less. • The second zone is connected to the internal sur¬ face of the remainder of the tube via an essen¬ tially tapered surface. • Said essentially tapered surface connecting to the internal surface of the remainder oE the tube has an apex half angle of 15° or less. • The radius of the first zone is less than half the nominal value for the diameter of the inter¬ nal surface of the tube. • At least one sealing surface is disposed on a lip disposed between the male threading and the free end. • At least one sealing surface is disposed beyond the male threading on the side opposite to the free end. • The front surface of the free end is an axial abutment surface for said female tubular element. • The surface quality of the second zone is better than that of the internal surface over the re- mainder of the length of the tube. The invention also concerns a threaded tubular connec¬ tion comprising a male tubular element as defined above and a female tubular element cooperating with the male tubular element, and a method for producing said tubular element in which the first zone is obtained by a first shaping and the second zone is obtained by a second shaping. The method of the invention may comprise some or all of the following features: • The second shaping is carried out after the first shaping. • The second zone is obtained by removing material. • The second zone is obtained by machining, • Over a fraction of the length of said end region, prior to producing said first zone, the tube un- dergoes a reduction swaging operation to progres¬ sively decrease the external and internal diame¬ ters up to the free end. • Said essentially tapered surface connecting the second zone to the first zone is formed by the reduction swaging operation. • The reduction swaging operation is carried out prior to the first and second shapings. The invention also provides a method for producing a sealed tubular connection in which an initial tubular connection as defined above undergoes diametrical expansion in the plastic deformation region using an expansion tool with a radius which is greater than the internal radius of said second zone of the male element, which is axially displaced in the connection. Advantageously, in the latter method: • The second zone extends to an axial distance from the free end of the male element that is at least equal to the total length of the connection. • The second zone extends to an axial distance from the free end of the male element such that the expansion tool expands the second zone when it axially passes under said sealing surface dis¬ posed beyond the male threading. The characteristics and advantages of the invention are described in more detail in the following description, made with reference to the accompanying drawings in which identical or analogous elements are designated by the same reference numerals in all of the figures. Figure 1 is a partial half cross-section of a male tu¬ bular element of the prior art. Figure 2 is an analogous view to Figure 1 relating to a male tubular element of the invention. Figure 3 is a partial half cross-section of a male tu- bular element of the invention. Figure 4 is a partial half cross-section showing a male tubular element of the invention and the adjacent zone of the tube of which it forms a part. Figure 5 is a partial half cross-section of an initial tubular connection formed by a male tubular element of the invention and a female tubular element. The known male tubular element 1 shown in Figure 1, which approximates to a figure of revolution about an axis A which is that of the external surface of the tube, com¬ prises a male tapered threading 2 and a male lip 3 extend¬ ing between the threading 2 and the free end 4 of the element. The threading 2 and the lip 3 are intended to cooperate in known manner with a tapered female threading and a female recess in a female tubular element (not shown) to produce a connection by making up the two elements and producing a seal between the internal space 5 of the re¬ sulting threaded tubular connection and the external medium 6, at least one sealing surface 7 being provided on the lip 3 to come into sealing contact with a corresponding surface of the female recess, either directly after makeup if the sealing surfaces interfere radially, for example taper on taper, or after diametrical expansion as described in US-A-6 712 401 if the tubular connection is intended to be expanded before being brought into service. The threading 2 and the lip 3 are produced by machin¬ ing after a reduction swaging operation applied to the end region of the tube intended to form the element 1 to give the internal surface 8 and the external surface 9 of the tube a tapered shape in this region. In this known tubular element, the radially internal surface 10 of the lip 3 is obtained by a first shaping, for example machining, after the reduction swaging operation. This surface 10 is coaxial with the external surface of the tube and has a radius Rl which is greater than the internal radius RO of the remainder of the tube, the portions with radii RO and Rl being connected via a chamfer 11 facing the free end of the tube. It has been shown that this configuration affects the behaviour of the connection during passage of the expansion tool along the chamfer 11 and as a result, damages the seal of the connection after expansion. The same effects are caused by variations in the radius RO both in the longitu- dinal direction and in the circumferential direction, and by the eccentricity of the raw internal surface of the element with respect to the machined external surface, the axis with reference numeral A being that of the external surface of the tube, which is held in the jaws of the lathe used to machine the element 1. These disadvantages are eliminated by the second shap¬ ing step of the method of the invention, thanks to which, after machining threading 2 and lip 3 and the first shaping of the surface 10, element 1 has, over at least a fraction of its length adjacent to the lip 3, a cylindrical internal surface 20 with a radius R2 which is greater than the radius Rl of the internal surface 10 of the lip 3, the surfaces 10 and 20 then being connected via a chamfer 21 which faces in a direction opposite to the free end 4 of the element. Further, the surface 20, preferably formed by lathe machining, has a precisely constant radius R.2 both in the longitudinal direction and in the circumferential direction, and is concentric with the machined external surface of the element 1. Other methods for producing the surface 20 may be car¬ ried out: chemical machining, electrolytic machining by electro-erosion or any other means, material removal (la¬ ser, ion beam etching etc) or without removing material, by hot or cold deformation, or by using a shaping piece. As can be seen in Figure 3, the shaped surface 20 preferably extends over the entire length L of the threaded element with the exception of the lip 3, i.e. axially up to the junction between the machined portion 12 of the exter- nal surface of the tube 13 and the remaining raw portion 14 of this external surface. In the case in which the male element 1 has, in addi¬ tion to or instead of the sealing surface 7, a sealing surface 15 located beyond the threading 2 with respect to its free end 4, it is advantageous, as shown in Figure 4, for the shaped surface 20 to extend to a distance Ll from the free end 4 which is slightly greater than the length L of the element 1, so that the expansion tool expands the surface 20 when it axially passes the sealing surface 15. Figure 5 shows a threaded -tubular connection obtained by making up the male element 1 of Figure 3 into a suitable female element 30 that is known per se. EXAMPLE A male tubular element is produced in accordance with Figure 3 from a tube with a nominal diameter of 193.68 mm (external diameter) and a linear mass of 44.2 kg/m, produc¬ ing a nominal internal diameter of 174.63 mm (minimum diameter and maximum diameter according to API of 172.24 mm and 177.47 mm respectively) . This element has a sealing surface 7 on the lip 3 (internal sealing surface) and a sealing surface 15 opposite to the free end 4 with respect to the threading 2 (external sealing surface) . It also has the following numerical characteristics: Radius Rl 87.315 mm Radius R2 88.025 mm (API maximum semi-diameter = 88.735 mm) inclination of chamfer 21 5° inclination of chamfer 22 6° length Ll 101 mm The male element obtained is made up into a suitable female element and an expansion tool with a diameter of 194 mm is passed into the resulting connection. The internal sealing surface then has two annular zones of effective contact with the corresponding surface of the female ele¬ ment, and the external sealing surface has an effective annular contact zone with the corresponding surface of the female element. Computer simulation (finished elements) is used to determine the axial length of the effective contact zone and the distribution of contact pressure for each of these three contact zones, which allows the integral of the contact pressure to be calculated over the length of the zone, which is the most significant measure of contact quality. COMPARATIVE EXAMPLE The procedure of the Example is followed, omitting the shaping step. After expansion, contact zones analogous to those of the example are obtained for which the same calcu- lations are carried out, based on a radius RO equal to half the minimal internal diameter. The results are given in the table below, as a per¬ centage of the integral of the contact pressure with re¬ spect to the internal sealing zone 1, or to the external sealing surface, in the comparative example. Reference should be made to WO-A-03/60370 for a de¬ scription of the two internal sealing zones. TABLE

It can be seen that the invention provides a consider¬ able improvement in contact pressures after expansion, in particular as regards the two contact zones of the internal seal. The invention is applicable to threaded tubular con¬ nections of various configurations, in particular those described in US-A-β 712 401, WO-A-03/60370 and WO-A-04/003416, in International patent application PCT/EP2004/002196 filed on 4 March 2004 and in French patent applications 0314037, 0314038 and 0314039 filed on 28 November 2003.