Steel product made of the steel and its manufacturing method The invention relates to the field of said steels "maraging", used in particular for applications requiring one or more of the following properties: excellent mechanical properties (very good fatigue resistance, high tensile strength and high fracture), processing simplicity thermal and dimensional stability, easy welding and good formability. These maraging steels are characterized by a martensite structure, which can be aged in order to obtain, after an aging of steel, a phase precipitation hardening intermetallic favorable for obtaining high mechanical properties. Conventionally known maraging steel type M 250, typical composition (in weight percent, as will all the compositions given in the following) Ni = 18% = 9% Co, Ti = 0.45% (grade said X2NiCoMo18-9-5), the remainder usually being iron and impurities resulting from the smelting. They are used to perform, especially parts with high properties, suitable for use as well as massive parts (trees engines centrifuge blades ...) and as high performance precision parts: springs for watches, blades for foils, automatic transmission belts elements type CVT (Continuous variable transmission), in particular to cars or vehicles in general, or machine tools and other rotating machinery ... in the case of Also known, from EP-B1 -1339880, maraging steels whose composition is: Ni = 12 to 24.5%; Mo = 2.5%; Co = 4.17%; Al≤ 0.15%; Ti≤ 0.1%; N≤ 30 ppm; Si≤ 0.1%; Mn≤ 0.1%; C≤ 50 ppm; S≤ 10 ppm; P ≤ 50 ppm; H≤ 3 ppm; O≤ 10 ppm; the balance being Fe and impurities resulting from the smelting, with Ni + Mo between 20 and 27%; Co% x% Mo between 50 and 200; Ti x% N% ≤ 2.10 "4Which are produced by methods including treatment remelting VIM type vacuum (Vacuum Induction Melting) and / or VAR (Vacuum Argon Remelting), optionally coupled to a reflow treatment by electro slag ESR (Electroslag Remelting). These steels are then processed hot and cold for to obtain strips of small thickness (1 mm or less for example). After formatting the piece, one proceeds to a processing precipitation hardening that gives the part its mechanical properties heart; a subsequent surface treatment can impart to the piece increased surface properties regarding solicitation fatigue, static friction, dynamic friction ... Compared to previously known maraging steels containing about 18% Ni, 9% Co, 5% Mo, 0.5% Ti, and a bit of AI, these steels EP-B1 1,339,880 are characterized by holding in greater fatigue, associated with a inclusions controlled cleanliness and a higher mechanical strength, higher than 2000 MPa (in the aged condition). In more detail: the mechanical properties of cold rolling are referred Ar> 2.5%, Rp 0 . 2 <1140 MPa, Rm <1480 MPa; on aged condition, R 0 2 > 1770 MPa, Rm> 1800 MPa; finally nitrided state properties are referred Ar> 1%; Rm> 1930 MPa .. It is also known from EP-B1 2180073 maraging steels with high fatigue strength and tensile strength for use in CVT belts, composition: C≤ 100 ppm; Si≤ 0.1%; Mn≤ 0.1%; P ≤ 0.01%; S≤ 50 ppm; Ni = 17 -22%; Cr = 0.1 -4.0%; MB = 3.0-7.0%; Co = 10.0 to 20.0%; Ti≤ 0.1%; Al≤ O, 05%; N≤ 300 ppm; O≤ 50 ppm; 0 20.0%, of préférence≥ 21, 0%, 22.0% mieux≥; and: Ni + Co + Mo> 29%; of préférence≥ 41, 0%. The Al content is between traces and 4.0%. Al is not necessarily added. It can reduce its presence at that of a residual element, resulting in the amount it was eventually necessary to add to the initial deoxidation of the liquid metal at the beginning of the development, or make an addition AI volunteer for specific properties. Moderate cost, it increases the resilience and can participate in the hardening by forming intermetallic compounds. Also, it limits oxidation during the preparation of the liquid metal and transformations of solidified metal. But avoid to form Al nitrides and oxides big containing Al not to degrade the fatigue strength and toughness. Preferably there remains more than 0, The Ti content is between traces and 0, 1%. By avoiding a significant presence of Ti (other maraging steels require) it is intended to prevent Ti nitride formation during solidification of liquid metal, which deteriorate the fatigue strength of the final product. In the invention, the desired structural hardening is obtained by other means. The content of N is imposed at a low level, namely not more than 0.0050% (50 ppm) to avoid the formation of nitrides as far as possible. If the content is between traces and 2.0%, preferably between 0.04% and 2.0%. It can be used for deoxidation of the liquid metal during the preparation, but be careful to avoid keeping large oxides containing Si (like other oxidants) in the final solidified steel, hence the upper limit 2.0%. If stabilizes martensite, increases the solubility of certain elements and thus homogenizes the structure. It also improves the elastic limit. The Mn content, non-compulsory element, is between traces and 4.0% or between 0.2 and 4%. Mn improves the mechanical properties and hot work opportunities. Mn is itself an effective deoxidizer, and can act on this in synergy with Si. However, it must limit its content to 4% to avoid forming large precipitates, brittle phases, or to lowest point phase fusion. Finally, it is austenite and may hinder the martensitic transformation when present in excessive amounts. The C content is between traces and 0.03%. The idea is to form the soft martensite because the carbon martensite is fragile and does not allow the formatting necessary to the product. We also want to avoid the formation of carbides which degrade the mechanical properties. The S content is between traces and 0.0020%, preferably between traces and 0.0010%, not to form sulphides which degrade the fatigue if they were present in number and size. In addition, the S weakens the grain boundaries by segregating them, hence the possible formation of cracks when the steel stress. So avoid the presence of dissolved S, through careful selection of raw materials and / or deep desulfurization. The precise maximum tolerable content is adjusted depending on the intended applications, in a known manner (in the limit of 0.002% or less, as mentioned above). The P content is between traces and 0.005%, preferably less, to limit the possibility that it segregates at the grain boundaries, such as S. B can be present only in trace amounts, but an addition is preferred, up 0.01%. This element promotes refinement of structures and reduces the grain size. It is good for the mechanical properties, but do not add too much not to lose ductility. The H content is limited to 5 ppm to avoid problems of hydrogen embrittlement. The fact of achieving one or treatment under vacuum for the preparation of the liquid metal and to avoid subsequent contamination of the liquid metal by the ambient humidity, slag or any added materials generally allows not exceed this level. O tolerable content depend, strictly speaking, applications envisaged for the final product, as with tolerable levels of S, B and other residual elements that can form precipitates. But it sets the maximum tolerable content of 25 ppm, which results from the processes used when developing the liquid metal. The goal is not to have in the final product of oxides which we could not control the composition, distribution and size. The Cr content is between traces and 5.0%. His presence is not mandatory, and must be limited to not lower Ms and risk degrading the quality of the inclusion population. But it increases the resistance to oxidation and nitriding aid, so that one can sometimes have reason to add voluntarily. However, it is generally preferred not to add Cr and leave this only trace resulting from the smelting. Such traces are usually considered to be contents of less than 0.10%, see e.g. EP-B1 1339880, cited in the introduction, which states that 0.06% are such "traces". Can reduce the level of Cr to even lower values, by a very careful choice of raw materials. The Cu content is between traces and 2.0%. The addition, if any, should be limited because Cu is austenite. But Cu involved in hardening and improves oxidation resistance, hence the potential interest of this addition. The W content is between traces and 4.0%. It is not mandatory and can be added mainly when you want to impose a Co content or also in MB, low enough, as we have seen above. The Zr content is between traces and 4.0%. May wish to add this item to do to contribute to the deoxidation and training nitrides purposes. Ca and Mg may be found in the metal due to the wear of the refractories developed in the form of oxides or sulphides. It is also desired voluntarily add these elements to contribute to the deoxidation. The final content should be limited to 0.1% for each, in order to avoid the formation of oxides, it could not control the size and distribution. Nb, Ta and V can be added up to 4.0% each, to supply a relatively low Co and / or Mo, as we have seen. The not mentioned elements are, at most, present only as impurities resulting from the production and are not added intentionally. Compared to maraging steels of EP-B1 1,339,880, so it tends to move to levels of Co and / or Mo higher than those at the preferred time. Regarding the inclusion population, the criterion to be met, according to the invention is that this inclusion population observed by image analysis on a polished surface of 650 mm 2 if the steel is in the form of a part or of a sheet hot transformed, and 800 mm 2 if the steel is in the form of a cold-rolled sheet, does not include non-metallic inclusions of greater diameter equivalent to 10 μηι, preferably no size inclusions greater than 8 μηι, and, in the case of a metal sheet processed hot, does not contain more than four, non-metallic inclusions of equivalent diameter of 5 to 10 μηι 100 mm 2 of the hot processed sheet, the observation being carried out by analysis ofimage on a polished surface of 650 mm 2. Non-metallic inclusions considered are the oxides, sulphides and nitrides. The oxide population is controlled mainly by the choice of development methods (controlled deoxidation of the liquid metal, after which care is taken to remove the best possible large inclusions, including the use of reflow methods of the electrode initially cast from the molten metal). The population of sulphides is controlled by the imposition of very low S contents, requiring careful selection of raw materials and / or desulfurization of the liquid metal. The population of nitrides is controlled by the imposition of the N low levels or very low, for example through the use of reduced pressures during the development of the liquid metal and reflow of the electrode, and the restriction of metal Ti content. The steels according to the invention are prepared, for example, by the following routes. Steel, first developed in the liquid state to adjust the composition of the essential elements, then cast as electrodes reflow. These electrodes are: Or - Revised once under vacuum (VAR process, Vacuum Arc Remelting known in itself), or electro slag (ESR method, Electro Slag Remelting, known in itself) to form, after casting and solidification of the liquid steel, ingots, billets or slabs; Or - Revised several times under vacuum (VAR) or electroslag (ESR) to form, after casting and solidification, ingots or slabs. This produces either a single reflow, a reflow multiple, e.g. VAR + VAR or ESR + VAR. These allow refuse to purify the metal and to improve the quality of the solidification by reducing segregation and refining its solidification structure. In particular, the ESR remelting can effectively lower the sulfur content, and VAR remelting can effectively reduce nitrogen levels. The ingots or slabs are then hot-rolled after reheating between 1050 and 1300 ° C, typically around 1200 ° C to obtain sheets or hot-rolled strip a few millimeters in thickness, e.g., about 1, 5-5 mm thick. The hot-rolled products having this thickness may in some cases be used either hot rolling raw state or in a controlled recrystallized state. Recrystallization can optionally already take place sufficiently at the hot rolling and the stay of the hot rolled strip in coil form, but if it would be insufficient to obtain the microstructure and / or the desired mechanical properties, annealing recrystallization may then be performed to adjust this recrystallization. In the latter case, the skilled person knows how to set the recrystallization annealing parameters (temperature, time ...) to adjust the microstructure (eg grain size) and the mechanical properties. It is typically after hot rolling and recrystallization possible (or even other types of heat treatment), the tensile strengths Rm of 1010 MPa or more, a Young's modulus of 130 GPa and a minimum uniform elongation Ar 2% or more. These hot-rolled strip, and optionally thermally treated, may also not be intended to be used directly, and it is then necessary to reduce their thickness by cold shaping for the envisaged applications. In this case, they are pickled, and then cold-rolled, with one or more intermediate annealing operations between the different passes of the cold rolling, and optionally one (or more) annealing (s) Final (final) aging, recrystallization, or other types of heat treatments, or suitable surface treatments according to the intended application (which will be detailed later), to obtain rolled strip thickness cold less than 2 mm, preferably less than or equal to 1 mm, and for example 0.4 mm or 0.2 mm thick. The recrystallization treatment of the cold rolled strip is preferably carried out at a thickness such that the cold-rolled strip has, at this time, a working rate greater than 30%, more preferably greater than 40%. It provides the band with a grain size of ASTM 8 (average grain diameter of less than 20 μηι) or finer, more preferably ASTM 10 (average grain diameter of less than 10 μηι) or finer (ASTM E1 12, where it is recalled that the figure giving the grain size is even higher than the grain is fine). The annealing treatment for obtaining a fine grain is performed in a protective atmosphere by suitably adjusting the temperature and time parameters. These parameters depend on the particular conditions under which the heat treatment, and the art know how to determine these parameters in each particular case. The preferred requirements on the grain size of the cold rolled products, optionally heat-treated, are moreover also, preferably, valuable product for use in the hot-rolled state, possibly after being heat treated. To improve the flatness of the strip, and, if necessary, to complete the martensitic transformation, the tape can also be subjected to a light final cold rolling (skin pass or temper rolling) with a reduction ratio of between 1 and 10%. Typically, the curing treatment (aging) of the cold-rolled strip are preferably made to a thickness such that the rolled strip is, at this time, a working rate greater than 30%, more preferably greater than 40%. then can be cut in the band a workpiece, and shaping the piece, for example by folding, welding ..., and perform thereon a hardening treatment consisting of a holding between 400 and 600 ° C for 30 min to 5 h. (Preferably preferably to 420-550 ° C for 30 minutes to 2 hours, for example 420 ° C for 30 min or 480 ° C for 2h) Hot rolled products are also susceptible to a hardening treatment, typical of maraging steel with the classically expected benefits of such treatment on their mechanical properties. The cold-rolled, and optionally heat-treated according to the invention have a tensile strength Rm of at least 2270 MPa, a conventional yield strength Rp 0.2 of at least 2250 MPa and a uniform elongation of Ar at least 2%. These characteristics can moreover also, optimally, be achieved on hot-rolled heat-treated appropriately, for example if there has been a significant recrystallization during hot-forming operation or during a subsequent treatment with led to such recrystallization. The products can then be hardened surface (nitriding, carburizing, carbonitriding, blasting ...) to increase their fatigue performance. Typically, an equivalent load, the products thus obtained can have superior fatigue life times of 10 000 to 50 000 cycles to those of conventional tints, for a period equal life, withstand increased stress of more than 50 MPa . The extreme hardness HV0.1 surface may be typically at least 1050 without methods and conditions other than nitriding methods and standard conditions are needed. Experiments were conducted on six samples of a steel according to the invention and on reference samples, whose compositions are summarized in Table 1, expressed in% by weight, or ppm for certain elements. The rest is iron and impurities resulting from production and are at a level considered unimportant for the properties. Ech. Co. Al Mo ni Ti N of Si Mn is CSPBH 2 0 (ppm) (ppm) ppm) Inv. 1 18,20 7,00 17,68 0,083 0,007 19 0,040 0,077 0,002 0,0006 0,0023 0,0005 0,2 5 Inv. 2 18.14 4.96 19.82 0.04 <0.005 28 0.050 0.088 0.003 0.0002 0.0029 <0.0002 <0.2 21 7.07 15.78 18.24 lnv.3 <0.005 0.007 34 0.049 0.088 0.003 <0, 0002 0.0030 0.0004 <0.2 6 lnv.4 18.12 6.96 16.05 0.026 0.007 18 0.040 0.195 0.002 0.0003 0.0024 0.0002 <0.2 5 lnv.5 18.15 6, 96 20.02 0.018 0.007 24 0.050 0.100 0.003 0.0005 0.0030 0.0003 <0.2 7 lnv.6 18.24 5.21 18.14 0.030 0.007 22 0.070 0.120 0.002 0.0004 0.0021 0.0004 <0.2 6 lnv.7 18.3 4.98 20.12 0.02 <0.005 15 0.08 0.600 0.002 0.0003 0.0027 0.0002 <0.2 7 Réf. 1 18,18 5,30 16,41 0,023 <0.005 8 0,068 0,071 0,007 0,0004 0,0022 <0,0002 <0,2 < 5 Réf. 2 18,33 4.98 8.99 0.116 0.473 < 5 0.047 <0.005 0.006 0.0004 0.0030 <0.0002 <0,2 <5 Table 1: Compositions of the samples tested 5 elaborations steels according to the invention and reference steels, in which were collected the samples of Table 1 were performed according to the following scheme, so as to obtain strips of 0.4 mm thickness. For Ref.1 and 2 if lnv.1, 5 and 7, we developed the steel in the ladle, and then applied the VIM processes, and VAR. For lnv.2 0, 3, 4 and 6, has been developed the steel in the ladle and then applied the VIM method, then the ESR method. Then the VAR or ESR ingots were transformed into slabs by blooming with a thickness reduction on the thickness between 200 and 100 mm (typically 160 mm). 5 Then the slabs were hot-rolled to a thickness 3.5 mm, after reheating to 1300 ° C. The metal was then pickled and was cold rolled to a thickness of 0.4 mm. Annealing or austenitizing of dissolution, recrystallization annealing and overaging annealing were performed respectively between 800 and 1000 ° C for 15 to 60 min, and then between 350 and 600 ° C for 30min 240min at 0, and between 420 and 510 ° C for 30 to 90 min. Table 2 shows the inclusionnaires densities of each sample of Table 1, after the hot rolling. Have been measured first in accordance with DIN 50602-M standard, then regions counting by image analysis by optical microscopy on areas of 650 mm 2 of samples of the hot rolled strip of 3.5 mm thickness. Then these densities have been reduced to an area of 100 mm 2sample. It should be understood that by "diameter" inclusions, it means "equivalent diameter" is to say that an inclusion would be of circular cross section and have the same surface as the observed inclusion, if -ci has a more complex section than a simple circle. In addition, the optical image analysis only distinguishing color contrasts and not differentiating the composition of inclusions, inclusions referred to as "TiN", in the table below, are colored orange contrast inclusions experience of skilled worker (which can be verified ex post by scanning electron microscopy). The inclusions rated "oxides" are the inclusions of gray contrast optical microscopy (these inclusions are actually oxides, or, in lesser proportions, sulphide or mixed sulphide-oxide inclusions). These analyzes are customary for the skilled and supplemented, in this case, by analysis using electron microscopy to automated scanning. Table 2: Density of the various samples inclusionnaires rolled tested, reduced to a particle density to 100 mm 2 These results show that in the case of the samples according to the invention was low inclusionnaires densities and a complete absence of relatively large size inclusions, namely greater than 10 μηι. Such large inclusions are also absent from the reference sample 1, but the number of small oxide inclusions (5 to 10 μηι in diameter) is significantly higher than in the case of the samples according to the invention, which is unfavorable mechanical properties and does not correspond to the optimum density inclusions referred. As to the reference sample 2, it has a too high density nitrides including nitrides of size greater than 10 μηι. This characteristic is, in itself, fatal to achieve the objective of Table 3 shows the maximum size of inclusions observed on 650 mm surfaces 2 of hot rolled previous samples. Table 3: Maximum size of inclusions observed on 650 mm samples of two of the various laminate samples tested hot Samples according to the invention therefore contain only small diameter inclusions, and in very low numbers. In particular there is no Ti nitride, which is linked in particular to the absence of Ti and very low content of N. The reference sample 1 is in the same case, although the density of inclusions and the diameter of its larger inclusions are somewhat higher than in the case of the invention. As for the reference sample 2, it was confirmed that nitrides (mainly Ti) are predominant and present as little ductile inclusions and often oversized for the goals of an excellent fatigue strength can be required. Observations by light and electron microscopy also showed that the presence of inclusions of a variety of equivalent diameter of less than 5 μηι was low, particularly for lnv.3 sample which is also one that contains less oxides 5-10 μηι. The skilled person knows that the cold transformation will affect the size of inclusions and downside by breaking down eventually, but can not in any way increase the proportions. Finally, inclusionnaires populations were more precisely characterized by counting and automated analysis in a scanning electron microscope with field emission gun (SEM FEG) on surfaces 200 to 2 mm 2 (for respective magnifications x300, x1000, x10 000), and their estimated areal densities. The grades according to the invention, as well as the reference shade 1, have the advantage, due to their intrinsic chemical composition, do not form nitrides during elaborations, and enable to control the size and nature of inclusions residual, particularly oxides. With careful control of the raw materials and production processes implemented that provide low levels of residual elements N, S and P, and through deoxidation of the liquid metal, the oxides formed particles are limited sizes below 1 0 μηι, preferably less than 8 μηι, and adequate compositions obtained by the described development of ranges. Forming a privileged manner oxides based on aluminum, for example mixed oxides The proportion by volume fraction of these families of oxides based on aluminum or container is significantly greater than 30%, whereby: - pure alumina oxides are distinguished by Al contents above 35% and less than 65% O (contents measured by energy dispersive spectroscopy EDX); - containing magnesium oxides are distinguished by EDX analysis by the presence of Mg more than 1 .5%, of Al more than 1 0% of O and more than 60%, the latter family may contain smaller proportions of Ca and / or Si, or be associated with sulphides small sizes. Important mechanical properties of various samples (except fatigue properties, which will be seen below) are summarized in Tables 4, 5 and 6, Table 4 for sample hot rolled stage, Table 5 for samples cold-rolled state, before the aging annealing causing precipitation hardening, and the table 6 corresponding to the laminate in cold condition and aged. The properties were measured in both the longitudinal direction relative to the rolling direction of the strip and in the transverse direction perpendicular to said rolling direction. These properties are the Young's modulus E (for hot-rolled samples), the Vickers hardness HV 1 (for cold rolled samples, unaged and aged), the conventional yield strength Rp0.2 (expressed in MPa), the maximum tensile strength Rm (in MPa), elongation at break, elongation distributed Ar (expressed in% and measured from samples of length L 0 equal to 5.65 times the square root of the initial section S 0 ) and the total elongation at. Table 4: Mechanical properties of the samples in the hot-rolled state Ech. HV 1 Rpo,2 (MPa) Rm (MPa A (%) Inv.1 Sens long 385 1358 1378 5,0 394 1417 1451 Travers Sens Sens dragon lnv.2 3.1 370 1269 1294 5.6 372 1336 1368 Travers Sens Sens dragon lnv.3 3.3 390 1348 1369 4.8 392 1407 1438 Travers Sens Sens 3.0 lnv.4 Long 391 1354 1375 4.5 Sens dragon lnv.5 lnv.6 389 1360 1382 4.8 376 1260 1304 5.1 Sens dragon dragon Sens lnv.7 374 1276 1318 4.9 Ref.1 Sens long 370 1226 1 198 5,7 Transverse 373 1272 1284 5.2 Réf.2 long Sens 315 987 1021 January 1, 4 Transverse 318 1022 1058 8.7 Table 5: Mechanical properties of the samples cold rolled condition Ech RPO HV 1, 2 (MPa) Rm (MPa A (%) 2329 2353 669 lnv.1 dragon Sens 2.7 671 2349 2373 Travers Sens Sens 2.0 lnv.2 Long 663 2251 2269 2.7 660 2284 2299 Travers Sens Sens dragon lnv.3 2.3 658 2258 2277 2.0 667 2309 2330 Travers Sens Sens 2.0 lnv.4 dragon lnv.5 660 2260 2278 2.6 676 2332 2360 2.1 Sens dragon dragon lnv.6 Sens Sens lnv.7 636 2174 2204 2.8 666 2276 2302 2 dragon 3 Réf.1 Longitudinal direction 621 2101 2132 2.6 Transverse direction 2.5 624 2139 2159 Réf.2 Longitudinal direction 536 1 823 1 850 4.9 Transverse 539 1845 1880 4.8 Table 6: Mechanical properties of the samples cold rolled and aged condition As expected, those mechanical properties that are most important for preferred embodiments of the invention in the case of cold rolled products are much more favorable after aging than they are after simple rolling, and aging also can greatly reduce the differences between the elastic limits, tensile strengths and elongations at break in the longitudinal direction and in transverse direction recorded on the cold-rolled state. Note also that the example does not lnv.6 after aging, tensile strength and yield strength which correspond to the aims pursued at this stage. This is probably attributed to the fact that its Mo content is relatively low and that its Co content does not compensate for this weakness to obtain optimum properties after aging. It would compare with the example lnv.2 for which the Mo content is relatively low, but where significantly higher Co content than lnv.6 provides that compensation. The lnv.6 example must nevertheless be regarded as part of the invention, the fact that its properties in hot rolled condition comply with the requirements of the corresponding variant of the invention. It was also noted that for thermal treatment solution-and a little different aging that exemplified for the establishment of the table 6, the hardness obtained varied relatively little. For sample lnv.1 treated at 850 ° C for 30 min and then at 450 ° C for 2 h, the resulting hardness is 699 HV 1. For lnv.2 treated sample at 850 ° C for 30 min and then at 500 ° C for 1 h, the obtained hardness is 642 HV 1. For sample lnv.3 treated at 850 ° C for 30 min and then at 450 ° C for 4 h, the resulting hardness is 678 HV 1. The aging conditions were explored on cold rolled products for their optimization (after annealing standardization or solution annealing). A solution annealing at 850 ° C for 30 minutes under argon was previously implemented on every nuance (among several conditions explored between 800 ° and 1000 ° C under argon) providing access to an austenitic structure and homogeneous . Then different pairs (time and temperature) have been tested on these annealed materials to clarify aging conditions suitable for intended applications. Then according to the explored aging conditions, under argon between 350 ° and 600 ° C, the optimum in terms of hardness have been clarified. The lnv.1 shade preferred aging conditions between 450 ° and 550 ° C for periods of 30 min to 5 hours, to reach Hv hardness in vieili state of more than 675. It reaches 730 Hv for aging at 500 ° C for 1 h. The lnv.2 shade can reach higher surface hardness levels 600 Hv for aging to less than 550 ° C, for periods of 30 minutes to 5 hours, preferably 500 ° C for 1 h to reach Hv 630-640; Lnv.3 the shade can achieve higher surface hardness levels of Hv 650 for aging at temperatures over 550 ° C for 30 minutes to 5 hours. For aging conditions between 450 ° and 550 ° C for 30 minutes to 3 hours, the lnv.4 shades and lnv.5 accessed at higher levels of hardness 650 Hv with treatment times of less than 2 h. For example, there was obtained a hardness of 660 Hv to lnv.4, and 676 Hv for lnv.5, for treatments under Ar at 480 ° C for 3 h. Similarly lnv.6 lnv.7 shades and have respective hardnesses of 636 Hv and 666 with aging conditions of 3 hours at 480 ° C. It is notable here that these shades of the invention are compatible with aging conditions that allow increased mechanical characteristics, but they have in addition put facilities in place in terms of processes that associated costs. Indeed the stability of the resulting properties, including hardness, allows the implementation of annealing of short duration (typically 30 minutes) to obtain the same or similar properties to those obtained by treatment with a duration of 4 to 5 hours a classic shades. These qualities of simplicity and economy of heat treatments are particularly advantageous for these nuances of the invention. These new shades according to the invention have mechanical properties (hardness, yield strength, tensile strength ...) increased relative to the references Ref.1 and Ref.2 under optimized aging conditions, which, combined with a population improved inclusions, allow access to properties also increased during dynamic loading, fatigue, for example. To this end samples of the invention and the sample ref. 1 were tested for fatigue, in the non-nitrided aged state, a hydraulic equipment INSTRON frequency 25 Hz, in biasing at R = 0.1, on cold rolled material (thickness less than 5 mm) treated at 850 ° C for 30 min and then at 450 ° C for 2 h under Ar. the aging conditions are not optimized for all grades, but it possible to compare shades therebetween in a same condition of aging). The results of these stress tests are presented in Figure 1. It shows the number of cycles the imposed stress level at which a break was observed on various samples. It was also reported for each sample of results From 50 000 cycles the examples according to the invention lnv.1, lnv.2 and lnv.3 support endurance stresses greater than the reference 1, and imposed stress, endurance examples according to the invention is increased. Reference 2 was not tested because given its content nitrides he was assured that his fatigue tests would give inferior results compared to those of other samples. Finally, the improved properties of these grades according to the invention have also been explored after treatment of compression of the surface. In this case, these materials in thin thickness fatigue stress, nitriding treatments are traditionally implemented before biasing, to delay the surface crack initiation. This compression formatting can also be performed by shot peening in a known manner. Thus, various conditions of nitriding were tested for the grades according to the invention, including treatment between 420 and 510 ° C from 30 to 90 min gas atmosphere (NH 3 or under an atmosphere of NH 3 cracked) and ion nitriding. Nitrided layers are formed by the diffusion of nitrogen in the various matrices, being features thicknesses of a few tens of μηι, evaluated by chemical etching or profile by glow discharge spectrometry (SDL). They may contain precipitates distributed uniformly purposes, which can be taken, if useful, adjusting the chemical nature by adjusting the steel composition and the conditions of nitriding. In the range of temperatures and durations explored the nuances of the invention reveal increased surface properties after nitriding, when compared to the references 1 and 2. Table 7 shows as an example of hardness levels evaluated micro indentation on identical terms. Three cases of nitriding treatments were tested in addition to the varying conditions of time and temperature: - Gas nitriding under NH 3 (NG1 tests); - Gas nitriding under NH 3 cracked (NG2 tests); - Plasma nitriding (NI tests). Table 7: Hardness Hv0.1 evaluated by micro indentation end surface under different conditions of nitriding. It should be understood that the measurement uncertainty of such tests are important when, as is the case here, the tests are carried out on samples with rough surfaces and local footprints. We must therefore interpret the results in Table 7 in terms of the overall trend. Thus, in addition to having a high-performance matrix fatigue, the samples according to the invention in particular exhibit a better mechanical strength on the surface and increased resistance to friction, due to an increase in the surface hardness after nitriding compared to samples reference 1 and 2. Thus, gains of over 50 Hv0.1 compared to references 1 and 2 can be easily anticipated for reduced nitriding times. extreme hardness area can thus reach more than 1050 Hv0.1 on the nuances of the invention under conditions of adequate nitriding and not particularly demanding. The minimum and maximum values ​​of hardness achieved by end surface according to the different conditions of nitriding for Ref.1 and samples 2 and lnv.1 to 3 are shown in Figure 3. Finally, it was verified that these important hardness accessible surface are accompanied by the presence of residual stress (measured by X-ray diffraction) that allow to the compression surface to the matrix and thus delay the crack initiation surface. Figure 2 is a qualitative assessment of residual stresses surface compression rolled sheet nitrided for Ref.1 samples Ref.2, lnv.1, lnv.2 and lnv.3. For each sample is plotted the minimum and maximum values of compressive stresses evaluated for all the tests carried out by varying processes, temperatures and times of nitriding. Evaluation of residual stresses in the nitrided layers was carried out sample surface with the following parameters: Chi varying from 0 to 51 °, measurement interval 1 s sin 2(Psi) of 0 to 0.6 in increments of 0.1. The values ​​obtained under these conditions for determining in accordance with changes in the position of the line of Fe the stress level in the material surface. It is seen that the residual stress of the samples according to the invention are not degraded significantly as compared with those observed on the reference samples. It is also easier through the use of the invention to control the composition of the nitrided layer and, therefore, to adapt to the specific needs of the intended use for future products. If this seems preferable, and the one can easily avoid presence in extreme phases surface commonly called "combination of layers" within the nitrided layers. These phases result from iron nitrides precipitation type Fe 4 N, Fe 2 N, Fe 2 N 1 . x ... In general, steels according to the invention under different conditions nitriding of use and economic performance increased compared to the reference steels within the prior art, since the nitriding layer is easily controllable: - for its composition, in particular concerning the existence or not of combination of layers; - to its thickness; - For hardness levels available in conditions of nitriding data; - the distribution in the layer thickness of the nitrogen content, of the precipitated phases, their nature and their distribution, as well as levels of hardness, residual stresses; - for ease and economic conditions of industrial implementation of the nitriding, to equal properties obtained from reference shades, the nitriding is carried out by plasma or especially by gaseous process. preferred of the invention of applications, including: - for products derived from hot-rolled semi-finished products placed or hot forming, turbine shafts or transmission parts in general; - for products from sheet or cold-rolled strips, elements of transmission belts of vehicles or rotating machines such as machine tools, particularly belts for automatic transmissions CVT type of motor vehicles. CLAIMS 1 .- Steel characterized in that its composition is, in percentages by weight: - 10.0% ≤ Ni≤ 24.5%, preferably 12.0% Ni≤ ≤ 24.5%; - 1, 0% ≤ Mo≤ 12.0%, preferably 2.5% ≤ 9.0% Mo≤; - 1 ,0%≤ Co≤ 25,0% ; - 20.0% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C≤ 29.0%, de préférence 22.0% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C≤ 29.0% mieux 22.5% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C ≤ 29.0%; - Co + Mo≥ 20.0%; preferably Co + Mo≥ 21, 0%; better Mo≥ Co + 22.0%; - Ni + Co + Mo≥ 29%; preferably Ni + Co + Mo≥ 41, 0%; - traces≤ Al≤ 4.0%, preferably ≤ 0.01% Al≤ 1, 0%; - traces≤Ti≤ 0,1 % ; - traces≤ N≤ 0,0050% ; - traces≤ Si≤ 2.0%; preferably 0.04% ≤ 2.0% Si≤; - traces≤ Mn≤ 4,0% ; - traces≤ C≤ 0,03% ; - traces≤ S≤ 0.0020%, preferably 0.0010% traces≤ S≤; - traces≤ P≤ 0,005% ; - traces≤ B≤ 0,01 % ; - traces≤ H≤ 0,0005% ; - traces≤ O≤ 0,0025% ; - traces≤ Cr≤ 5,0% ; - traces≤ Cu≤ 2,0% ; - traces≤ W≤ 4,0% ; - traces≤ Zr≤ 4,0% ; - traces≤Ca≤ 0,1 % ; - traces≤ Mg≤ 0,1 % ; - traces≤ Nb≤ 4,0% ; - traces≤ V≤ 4,0% ; - traces≤ Ta≤ 4,0% ; the rest being iron and impurities resulting from preparation; and in that the inclusion population observed by image analysis on a polished surface of 650 mm 2 if the steel is in the form of a part converted to hot or hot-rolled sheet, and 800 mm 2 if the steel is in the form of a cold-rolled sheet, does not include non-metallic inclusions of greater diameter equivalent to 10 μηι, preferably contains no non-metallic inclusions of greater diameter equivalent to 8 μηι and, in the case of a hot rolled sheet, contains not more than four non-metallic inclusions of equivalent diameter of 5 to 10 μηι 100 mm 2 , the observation being performed by image analysis on a surface polished 650 mm 2 . 2. - A steel according to claim 1, characterized in that 18.0% ≤ Ni + Mo ≤ 27,0%. 3. - A steel according to claim 1 or 2, characterized in that Cr is present only in trace resulting from the smelting 4. - A steel according to claim 1 or 2, characterized in that traces ≤ Cr < 0,10%. 5. - A method of making a steel product, characterized in that: - a remelting electrode is prepared by a steel whose composition is in conformity with one of claims 1 to 4; - a reflow is performed to this electrode by a method in single or multiple reflow to obtain a consolidated electrode; - it performs at least one melt processing of the electrode remelted, at a temperature between 1050 and 1300 ° C, to obtain a sheet turned hot or hot-band transform; - and optionally is performed a heat treatment on said sheet turned hot or said hot band transform. 6. - A method according to claim 5, characterized in that said sheet or strip transformed to hot, optionally heat treated, has a strength greater traction than or equal to 1010 MPa, a Young's modulus greater than or equal to 130 GPa and a uniform elongation greater than or equal to 2%. 7. - Method according to one of claims 5 or 6, characterized in that said sheet or said transformed hot band transform hot and optionally heat-treated is then cold rolled in one or more passes to obtain a sheet or strip having a thickness less than or equal to 2 mm, preferably less than or equal to 1 mm. 8. - Process according to claim 7, characterized in that the sheet or strip undergoes at least one heat treatment between cold rolling passes and / or after the last cold rolling pass. 9. - A method according to claim 7 or 8, characterized in that the combined cold rolling rate of the various cuts is at least 30%, preferably at least 40%. 10. - Method according to one of Claims 5 to 9, characterized in that said sheet or hot-rolled strip or cold and optionally heat-treated has a tensile strength greater traction than or equal to 2270 MPa, a yield greater than conventional or equal to 2250 MPa and a uniform elongation greater than or equal to 2%. January 1. - Method according to one of claims 45 a 10, characterized in that said sheet or hot-rolled strip or cold and optionally heat-treated is cut, and then optionally shaped. 12. - Method according to claim 1 1, characterized in that said sheet or hot-rolled strip or cold optionally heat-treated, cut and, optionally, shaping, is subjected to a hardening treatment between 420 and 550 ° C for 30 minutes to 2 hours, preferably at 450-550 ° C for 1 to 2 h. 13. - Method according to claim 12, characterized in that said sheet or hot-rolled strip or cold, and optionally heat-treated, cut and, optionally, shaping, is subjected, after its hardening, a surface treatment for the improved resistance to dynamic loads. 14.- Method according to claim 13, characterized in that said surface treatment is a carburizing gas or a nitriding or an ion nitriding or carbonitriding, or a shot peening. 15. - Method according to one of claims 5 to 14, characterized in that the grain size of the sheet or hot rolled strip may be heat-treated, or sheet, or cold rolled strip may be heat-treated is 8 ASTM or finer, preferably 10 ASTM or finer. 16. - steel product that has undergone a melt processing and having been optionally treated thermally, characterized in that its composition is, in percentages by weight: - 10.0% ≤ Ni≤ 24.5%, preferably 12.0% Ni≤ ≤ 24.5%; - 1, 0% ≤ Mo≤ 12.0%, preferably 2.5% ≤ 9.0% Mo≤; - 1 ,0%≤ Co≤ 25,0% ; - 20.0% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C≤ 29.0%, de préférence 22.0% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C≤ 29.0% mieux 22.5% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C ≤ 29.0%; - Co + Mo≥ 20.0%; preferably Co + Mo≥ 21, 0%; better Mo≥ Co + 22.0%; - Ni + Co + Mo≥ 29%; preferably Ni + Co + Mo≥ 41, 0%; - traces≤ Al≤ 4.0%, preferably ≤ 0.01% Al≤ 1, 0%; - traces≤Ti≤ 0,1 % ; - traces≤ N≤ 0,0050% ; - traces≤ Si≤ 2.0%; preferably 0.04% ≤ 2.0% Si≤; - traces≤ Mn≤ 4,0% ; - traces≤ C≤ 0,03% ; - traces≤ S≤ 0.0020%, preferably 0.0010% traces≤ S≤; - traces≤ P≤ 0,005% ; - traces≤ B≤ 0,01 % ; - traces≤ H≤ 0,0005% ; - traces≤ O≤ 0,0025% ; - traces≤ Cr≤ 5,0% ; - traces≤ Cu≤ 2,0% ; - traces≤ W≤ 4,0% ; - traces≤ Zr≤ 4,0% ; - traces≤Ca≤ 0,1 % ; - traces≤ Mg≤ 0,1 % ; - traces≤ Nb≤ 4,0% ; - traces≤ V≤ 4,0% ; - traces≤ Ta≤ 4,0% ; the rest being iron and impurities resulting from preparation; and in that the inclusion population observed by image analysis on a polished surface of 650 mm 2 if the steel is in the form as a piece transformed warm or hot rolled sheet, does not contain non-metallic inclusions of greater diameter equivalent to 10 μηι, preferably no inclusion larger than 8 μηι, and, in the case of a hot rolled sheet, contains not more than four, inclusions non- metallic diameter equivalent of 5 to 10 μηι 100 mm 2 , the observation being performed by image analysis of a polished surface of 650 mm 2 . 17. A steel product that has undergone a melt processing and possibly having been heat treated according to claim 16, characterized in that 18.0% ≤ Ni + Mo≤ 27.0%. 18. - steel product that has undergone a melt processing and possibly having been heat treated according to claim 16 or 17, characterized in that Cr is present only in trace resulting from the smelting. 19. - steel product that has undergone a melt processing and possibly having been heat treated according to claim 16 or 17, characterized in that traces≤ Cr <0.10%. 20. - steel product that has undergone a melt processing according to one of claims 17 to 19 and optionally having been heat-treated, characterized in that it has a tensile strength greater traction than or equal to 1010 MPa, a modulus 'Young greater than or equal to 130 GPa and a uniform elongation greater than or equal to 2%. 21. - sheet or cold-rolled strip of steel, characterized in that its composition is, in percentages by weight: - 10.0% ≤ Ni≤ 24.5%, preferably 12.0% Ni≤ ≤ 24.5%; - 1, 0% ≤ Mo≤ 12.0%, preferably 2.5% ≤ 9.0% Mo≤; - 1 ,0%≤ Co≤ 25,0% ; - 20.0% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C≤ 29.0%, de préférence 22.0% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C≤ 29.0% mieux 22.5% ≤ Mo + Co + Si + Mn + Cu + W + Nb + V + Zr + Ta + Cr + C ≤ 29.0%; - Co + Mo≥ 20.0%; preferably Co + Mo≥ 21, 0%; better Mo≥ Co + 22.0%; - Ni + Co + Mo≥ 29%; preferably Ni + Co + Mo≥ 41, 0%; - traces≤ Al≤ 4.0%, preferably ≤ 0.01% Al≤ 1, 0%; - traces≤Ti≤ 0,1 % ; - traces≤ N≤ 0,0050% ; - traces≤ Si≤ 2.0%; preferably 0.04% ≤ 2.0% Si≤; - traces≤ Mn≤ 4,0% ; - traces≤ C≤ 0,03% ; - traces≤ S≤ 0.0020%, preferably 0.0010% traces≤ S≤; - traces≤ P≤ 0,005% ; - traces≤ B≤ 0,01 % ; - traces≤ H≤ 0,0005% ; - traces≤ O≤ 0,0025% ; - traces≤ Cr≤ 5,0% ; - traces≤ Cu≤ 2,0% ; - traces≤ W≤ 4,0% ; - traces≤ Zr≤ 4,0% ; - traces≤Ca≤ 0,1 % ; - traces≤ Mg≤ 0,1 % ; - traces≤ Nb≤ 4,0% ; - traces≤ V≤ 4,0% ; - traces≤ Ta≤ 4,0% ; the rest being iron and impurities resulting from preparation; and in that the inclusion population observed by image analysis on a polished surface of 800 mm 2 , does not include non-metallic inclusions of greater diameter equivalent to 10 μηι, preferably no inclusion larger than 8 μηι. 22.- sheet or cold-rolled strip according to claim 21, characterized in that 18.0% ≤ Ni + Mo≤ 27.0%. 23. - sheet or cold-rolled strip according to claim 21 or 22, characterized in that Cr is present only in trace resulting from the smelting. 24. - sheet or rolled strip according to claim 21 or 22, characterized in that traces≤Cr <0.10%. 25. - sheet or cold-rolled strip according to one of claims 21 to 24, characterized in that it has undergone at least a heat treatment after cold rolling. 26. - sheet or hot-rolled strip or cold according to one of claims 16 to 25, characterized in that said sheet or cold-rolled strip and optionally heat-treated has a tensile strength greater traction than or equal to 2270 MPa, an upper conventional elastic limit or equal to 2250 MPa and a uniform elongation greater than or equal to 2%. 27. - steel product, characterized in that it is derived from a sheet or hot-rolled strip or cold according to one of claims 16 to 26, optionally shaped, and in that it has been a surface treatment to improve its resistance to dynamic loads. 28-. Product according to claim 27, characterized in that said surface treatment is selected from carburization, a gas nitriding, ion nitriding, carbonitriding, shot peening. 29-. turbine shaft or transmission part transformed hot, characterized in that said shaft or said part comprises at least one element made from a hot-processed product according to one of claims 16 to 20 or 27. 30-. transmission belt, characterized in that it comprises at least one element made from a metal sheet or cold-rolled strip or a product according to one of claims 21 to 28. 31. - The power transmission belt according to claim 30, characterized in that it is a CVT type transmission belt for a motor vehicle. 32. - steel product that has undergone a melt processing and possibly having been heat treated according to one of claims 16 to 20, or sheet or cold-rolled strip and optionally heat-treated according to one of claims 21 to 28, characterized in that the grain size of the product or of the sheet or strip is of ASTM 8 or finer, preferably 10 or finer ASTM.