EP2164998B1 - Hardened martensitic steel having a low or zero content of cobalt, process for manufacturing a part from this steel, and part thus obtained - Google Patents

Abstract

Steel, characterized in that the composition thereof is, in percentages by weight: C=0.20-0.30% Co=trace levels-1% Cr=2-5% Al=1-2% Mo+W/2=1-4% V=trace levels-0.3% Nb=trace levels-0.1% B=trace levels-30 ppm Ni=11-16% with Ni≧7+3.5 Al Si=trace levels-1.0% Mn=trace levels-2.0% Ca=trace levels-20 ppm rare earths=trace levels-100 ppm if N≰10 ppm, Ti+Zr/2=trace levels-100 ppm with Ti+Zr/2≰10 N if 10 ppm<N≰20 ppm, Ti+Zr/2=trace levels-150 ppm O=trace levels-50 ppm N=trace levels-20 ppm S=trace levels-20 ppm Cu=trace levels-1% P=trace levels-200 ppm the remainder being iron and inevitable impurities resulting from the production operation. Method of producing a component from this steel and a component obtained in this manner.

Description

The invention relates to a martensitic steel hardened by a duplex system, that is to say by a precipitation of intermetallic compounds and carbides obtained by means of a suitable steel composition and heat aging treatment.

• une très haute résistance mécanique, mais en même temps une ténacité et une ductilité élevées, autrement dit une faible sensibilité à la rupture fragile ; cette très haute résistance subsiste à chaud, jusqu'à des températures de l'ordre de 400°C ;
• de bonnes propriétés en fatigue, ce qui implique notamment l'absence d'inclusions nocives telles que des nitrures et des oxydes ; cette caractéristique doit être obtenue par une composition appropriée et des conditions d'élaboration du métal liquide soignées.

This steel offers:

• a very high mechanical strength, but at the same time a high tenacity and ductility, ie a low sensitivity to brittle fracture; this very high resistance remains hot, up to temperatures of the order of 400 ° C;
• good fatigue properties, which notably implies the absence of harmful inclusions such as nitrides and oxides; this characteristic must be obtained by a suitable composition and conditions of preparation of the liquid metal treated.

In addition, it is cementable, nitrurable or carbonitrurable, so as to harden its surface to give it good resistance to abrasion and lubricated friction.

The possible applications of this steel concern all areas of mechanics where structural or transmission parts are required which must combine very high loads, under dynamic loads and in the presence of induced or surrounding heating. Non-exhaustive examples include transmission shafts, gear shafts, bearing shafts, etc.

The demand for excellent mechanical resistance to hot prevents the use in some applications of carbon steels or so-called "low alloyed" steels whose resistance deteriorates from 200 ° C. In addition, the toughness of these steels is generally no longer satisfactory when treated for mechanical strength levels greater than 2000 MPa, and, in general, their "true" elastic limit is much lower than their resistance. maximum measured at the tensile test: the elastic limit is therefore a dimensioning criterion which becomes penalizing in this case. We can then use the steels maraging, whose elastic limit is significantly closer to their maximum value of tensile strength, which have satisfactory strength up to 350-400 ° C, and which still offer good toughness for the very high levels of mechanical strength. But these maraging steels contain quite consistently high levels of nickel, cobalt and molybdenum, all of which are expensive and subject to significant changes in their rating in the commodity market. They also contain titanium, used for its strong contribution to secondary hardening, but which is mainly involved in lowering the fatigue strength of maraging steels due to TiN nitride, which it is almost impossible to avoid training during the making of steels even contains only a few tenths of a percent.

It was proposed in the document US Patent 5,393,388 a secondary hardening steel composition without addition of titanium, aimed at improving the heat resistance and especially to improve the fatigue properties, ductility and toughness. This composition has the disadvantage of requiring a high Co content (8 to 16%), which makes the steel very expensive. (NB: in the present text, all the contents of the various elements are expressed in% by weight).

In the document WO-2006/114499 , a hardened martensitic steel composition and an optimized heat treatment suite adapted to this composition have been proposed which, with respect to the prior art represented by US Patent 5,393,488 , had the advantage of requiring a lower cobalt content of between 5 and 7%. By adjusting the contents of the other elements and the parameters of the heat treatments accordingly, it has been possible to obtain parts offering a very satisfactory set of mechanical properties, in particular for aeronautical applications. These include, in particular, a cold tensile strength of between 2200 MPa and 2350 MPa, ductility and resilience at least equal to those of the best high-strength steels, and hot (400 ° C.) tensile strength. of the order of 1800 MPa, as well as optimal fatigue properties.

This steel is said to be "duplex-hardening" because its hardening is achieved by simultaneous hardening precipitation of intermetallic compounds and M ₂ C carbides.

However, this steel still contains relatively large amounts of cobalt. As this element is in any case expensive and its price is likely to undergo significant fluctuations on the raw materials market, it would be important to find ways to reduce its presence even more significantly, particularly in materials intended for more common mechanical applications. than aeronautical applications.

The object of the invention is to provide a usable steel, in particular, for manufacturing mechanical parts such as transmission shafts, or structural elements, having a further improved mechanical resistance to heat but also fatigue properties and fragility. always adapted to these uses. This steel should also have a lower production cost than the best performing steels currently known for these uses, thanks, in particular, to a significantly lower cobalt content.

• C = 0,20 - 0,30%
• Co = traces - 1 %
• Cr = 2 - 5%
• Al = 1 - 2%
• Mo + W/2 = 1 - 4%
• V = traces - 0,3%
• Nb = traces - 0,1%
• B = traces - 30 ppm
• Ni = 11 - 16% avec Ni ≥ 7+3,5 Al
• Si = traces - 1,0%
• Mn = traces - 2,0%
• Ca = traces - 20 ppm
• Terres rares = traces - 100 ppm
• si N ≤ 10 ppm, Ti + Zr/2 = traces -100 ppm avec Ti + Zr/2 ≤ 10 N
• si 10 ppm < N ≤ 20 ppm, Ti + Zr/2 = traces - 150 ppm
• O = traces - 50 ppm
• N = traces - 20 ppm
• S = traces - 20 ppm
• Cu = traces - 1%
• P = traces - 200 ppm

For this purpose, the invention relates to a steel characterized in that its composition is, in percentages by weight:

• C = 0.20 - 0.30%
• Co = traces - 1%
• Cr = 2 - 5%
• Al = 1 - 2%
• Mo + W / 2 = 1 - 4%
• V = traces - 0.3%
• Nb = traces - 0,1%
• B = traces - 30 ppm
• Ni = 11 - 16% with Ni ≥ 7 + 3.5 Al
• If = traces - 1.0%
• Mn = traces - 2.0%
• Ca = traces - 20 ppm
• Rare earth = traces - 100 ppm
• if N ≤ 10 ppm, Ti + Zr / 2 = traces -100 ppm with Ti + Zr / 2 ≤ 10 N
• if 10 ppm <N ≤ 20 ppm, Ti + Zr / 2 = traces - 150 ppm
• O = traces - 50 ppm
• N = traces - 20 ppm
• S = traces - 20 ppm
• Cu = traces - 1%
• P = traces - 200 ppm

the rest being iron and unavoidable impurities resulting from the elaboration.

It preferably contains C = 0.20 - 0.25%.

It preferably contains Cr = 2 - 4%.

It preferably contains Al = 1 - 1.6%, better 1.4 - 1.6%.

It preferably contains Mo ≥ 1%.

It preferably contains Mo + W / 2 = 1 - 2%.

It preferably contains V = 0.2 - 0.3%.

It preferably contains Ni = 12-14%, with Ni ≥ 7 + 3.5 Al.

It preferably contains Nb = traces - 0.05%.

It preferably contains Si = traces - 0.25%, better traces - 0.10%.

It preferably contains O = traces - 10 ppm.

It preferably contains N = traces - 10 ppm.

It preferably contains S = traces - 10 ppm, better traces - 5 ppm.

It preferably contains P = traces - 100 ppm.

Its measured martensitic transformation temperature Ms is preferably greater than or equal to 100 ° C.

Its martensitic transformation temperature Ms measured may be greater than or equal to 140 ° C.

• la préparation d'un acier ayant la composition précédente ;
• au moins une opération de mise en forme de cet acier ;
• un revenu d'adoucissement à 600-675°C pendant 4 à 20h suivi d'un refroidissement à l'air ;
• une mise en solution à 900-1000°C pendant au moins 1h, suivie par un refroidissement à l'huile ou à l'air suffisamment rapide pour éviter la précipitation de carbures intergranulaires dans la matrice d'austénite ;
• un vieillissement de durcissement à 475-600°C, de préférence de 490-525°C pendant 5-20h.

The invention also relates to a method for manufacturing a steel part, characterized in that it comprises the following steps preceding the completion of the part giving it its final shape:

• the preparation of a steel having the preceding composition;
• at least one shaping operation of this steel;
• softening income at 600-675 ° C for 4 to 20 hours followed by cooling in the air;
• solution at 900-1000 ° C for at least 1 hour, followed by cooling with oil or air fast enough to avoid the precipitation of intergranular carbides in the austenite matrix;
• curing aging at 475-600 ° C, preferably 490-525 ° C for 5-20h.

It further preferably comprises a cryogenic treatment at -50 ° C or lower, preferably at -80 ° C or lower, to convert all the austenite to martensite, the temperature being lower by 150 ° C or more to Measured ms, at least one of said treatments lasting at least 4 hours and at most 50 hours.

It also preferably comprises a softening treatment of the rough quenching martensite carried out at 150-250 ° C for 4-16h, followed by cooling with still air.

The part also preferably undergoes carburizing or nitriding or carbonitriding.

Nitriding can be performed during an aging cycle.

Preferably it is then carried out between 490 and 525 ° C for 5 to 100h.

Said nitriding or carburising or carbonitriding can be carried out during a thermal cycle prior to or simultaneously with said dissolution.

The invention also relates to a mechanical part or component for structural element, characterized in that it is manufactured according to the preceding method.

It may be in particular a motor transmission shaft, or a motor suspension device or a landing gear element or a gearbox element or a bearing axis.

As will be understood, the invention is based first of all on a steel composition which differs from the prior art represented by WO-2006/114499 in particular by a very low content of Co, not exceeding 1%, and can be typically limited to traces inevitably resulting from the elaboration. The contents of the other most commonly present significant alloying elements are only slightly modified, but certain levels of impurities must be carefully controlled.

This possibility of completely dispensing with the usual addition of cobalt in martensitic steels of the class of those of the invention is a particularly surprising result. The steel according to the invention therefore no longer contains significant quantities of expensive additive elements, apart from nickel, the content of which, however, is not increased with respect to the prior art. It is only Special care must be taken when preparing to limit the nitrogen content to not more than 20 ppm to avoid as much as possible the formation of aluminum nitrides. The maximum levels of titanium and zirconium must also be limited accordingly to prevent nitrides from forming with residual nitrogen.

These steels have a plastic gap (gap between breaking strength R _m and resistance to elongation R _p0,2 ) intermediate between those carbon steels and maragings. For the latter, the difference is very small, providing a high elastic limit, but a quick break as soon as it is crossed. The steels of the invention have, from this point of view, properties that can be adjusted by the proportion of hardening phases and / or carbon.

The steel of the invention can be machined in the quenched state, with tools adapted to a hardness of 45HRC. It is intermediate between the maragings (rough machining quench since they have a mild low carbon martensite) and carbon steels that must be machined essentially in the annealed state.

In the steels of the class of those of the invention, a "duplex" curing is carried out, that is to say obtained jointly by intermetallics of β-NiAl type and carbides of M ₂ C type, in the presence of reversion austenite formed / stabilized by diffusion-enriched nickel enrichment during curing aging, which gives ductility to the structure by forming a sandwich structure (a few% of stable and ductile austenite between the slats of hardened martensite).

It is necessary to avoid forming nitrides, Ti, Zr and Al in particular, which are weakening: they deteriorate the tenacity and fatigue resistance. Since these nitrides can precipitate from 1 to a few ppm of N in the presence of Ti, Zr and / or Al, and the conventional elaboration means make it difficult to achieve less than 5 ppm of N, the steel of the invention respects the following rules.

In principle, any addition of Ti (maximum allowed: 100 ppm) is limited, and N is limited as much as possible. According to the invention, the N content should not exceed 20 ppm and more preferably 10 ppm, and the Ti content should not exceed 10 times the N content.

Nevertheless, a proportionate addition of titanium at the end of elaboration in a vacuum furnace is conceivable with a view to fixing the residual nitrogen and thus avoiding the harmful precipitation of the nitride AlN. As it is necessary, however, to avoid the formation of nitride TiN in the liquid phase, since it becomes coarse (from 5 to 10 μm or more), the addition of titanium can be practiced only for a residual maximum nitrogen content of 10 ppm in the liquid metal, and still not exceed 10 times this residual value of nitrogen. For example, for a final content of 8 ppm of N at the end of processing, the limiting content of the optional addition of titanium is 80 ppm.

• Ti + Zr/2 doit toujours être ≤ 100 ppm ;
• et que Ti + Zr/2 doit être ≤ 10 N.

It is possible to replace partially or totally Ti by Zr, these two elements behaving quite similarly. Their atomic masses being in a ratio of 2, if Zr is added in addition to or in place of the Ti it is necessary to reason on the sum Ti + Zr / 2 and to say that, at the same time as N ≤ 10 ppm,

• Ti + Zr / 2 should always be ≤ 100 ppm;
• and that Ti + Zr / 2 should be ≤ 10 N.

In the case where the N content is greater than 10 ppm and less than or equal to 20 ppm, Ti and Zr are to be regarded as impurities to be avoided, and the sum Ti + Zr / 2 must not exceed 150 ppm.

The possible addition of rare earths, at the end of the process, can also help to fix a fraction of N, besides the S and O. In this case, it must be ensured that the residual rare earth content remains below 100. ppm, and preferably less than 50 ppm, because these elements weaken the steel when they are present beyond these values. It is believed that rare earth (eg La) oxynitrides are less harmful than Ti or Al nitrides because of their globular form which would make them less likely to constitute fatigue fracture primers. It is nevertheless advantageous to leave as few of these inclusions as possible in the steel, thanks to the classic techniques of careful elaboration.

Calcium treatment may be practiced to complete the deoxidation / desulfurization of the liquid metal. This treatment is preferably conducted with the possible additions of Ti, Zr or rare earths.

The M ₂ C carbide of Cr, Mo, W and V containing very little Fe is preferred for its hardening and non-embrittling properties. M ₂ C carbide is metastable with regard to equilibrium carbides M ₇ C ₃ and / or M ₆ C and / or M ₂₃ C ₆ . It is stabilized by Mo and W. The sum of the Mo content and half of the W content must be at least 1%. However, it is not necessary to exceed Mo + W / 2 = 4% in order not to deteriorate the forgeability (or the heat deformability in general) and not to form intermetallics of the μ phase of type Fe ₇ Mo ₆ , which is the one of the essential hardening phases of conventional maraging steels but is not desired in the steel of the invention. Preferably, Mo + W / 2 is between 1 and 2%. It is also to prevent the formation of non-hardening Ti carbides which may weaken the grain boundaries that a 100 ppm imperative limitation of the Ti content of the steels according to the invention is required.

Cr and V are elements that activate the formation of "metastable" carbides.

V also forms carbides of MC type, stable up to the dissolution temperatures, which "block" the grain boundaries and limit the magnification of grains during heat treatments at high temperatures. V = 0.3% must not be exceeded in order not to fix too much C in carbides of V, during the dissolution cycle, to the detriment of the M ₂ C carbide of Cr, Mo, W, V which is sought precipitation during the subsequent aging cycle. Preferably, the V content is between 0.2 and 0.3%.

The presence of Cr (at least 2%) makes it possible to reduce the level of carbides of V and to increase the level of M ₂ C. It is not necessary to exceed 5% in order not to favor the formation of stable carbides, in particular M ₂₃ C ₆ . Preferably, no more than 4% of Cr is used to better ensure the absence of M ₂₃ C ₆ and not to lower the temperature of the beginning of martensitic transformation too much.

The presence of C favors the appearance of M ₂ C with respect to the μ phase. But an excessive content causes segregations, a lowering of Ms and causes difficulties in manufacturing on an industrial scale: sensitivity to the taps (superficial cracking during rapid cooling), difficult machinability of martensite too hard to l quenching condition ... Its content must be between 0.20 and 0.30%, preferably 0.20-0.25% so as not to give the part too hard a hardness which could require machining in the annealed state. The surface layer of the parts can be enriched in C by cementation, nitriding or carbonitriding if a very high surface hardness is required in the intended applications.

Co delays the restoration of dislocations and, therefore, slows the mechanisms of hot survivability in martensite. It was thought that this made it possible to maintain high tensile strength at high temperature. But on the other hand, it was suspected that, since the Co promotes the formation of the aforementioned μ phase which is the one that hardens the maraging steels of the prior art to Fe-Ni-Co-Mo, its massive presence contributed to reducing the quantity of Mo and / or W available to form M ₂ C carbides which contribute to the hardening according to the mechanism that is to be promoted.

On the other hand, the cobalt somewhat raises the ductile / brittle transition temperature, which is not favorable, particularly in compositions with low nickel contents, whereas, contrary to what could be found in cobalt does not clearly show the transformation point Ms of the compositions of the invention and therefore has no obvious interest either in this respect.

The content of Co (5 to 7%) proposed in the steels of WO-2006/114499 , in combination with the contents of the other elements, resulted from the search for a compromise between these various advantages and disadvantages.

However, the inventors have found that, contrary to the prejudices in force among metallurgists who are specialists in the field of the invention, the presence of cobalt was not essential to obtain, in particular, a high mechanical strength in maraging steels. duplex hardening. Its absence may even have the advantage of offering a better compromise between the tensile strength Rm and the toughness Kv. But it must go hand in hand with tight tolerances on the contents of certain impurities, and preferably with an adjustment of the contents of certain elements ensuring a sufficiently high measured temperature Ms.

Ni and Al are bonded in the invention, where Ni must be ≥ 7 + 3.5 Al. These are the two essential elements which participate in a good part of aging hardening, thanks to the precipitation of the nanometric intermetallic phase of type B2 (NiAl for example). It is this phase which gives a large part of the mechanical strength when hot, up to about 400 ° C. Nickel is also the element which reduces brittleness by cleavage because it lowers the ductile / brittle transition temperature of martensites. If Al is too high relative to Ni, the martensitic matrix is too strongly depleted of nickel as a result of the precipitation of the NiAl curing precipitate during aging. This is detrimental to the tenacity and ductility criteria, since the lowering of the nickel content in the martensitic phase leads to the raising of its ductile / brittle transition temperature, and therefore to its embrittlement at temperatures close to ambient. In addition, nickel promotes the formation of reversion austenite and / or stabilizes the residual austenite fraction (possibly present) during the aging cycle. These mechanisms favor the ductility and tenacity criteria, but also the structural stability of the steel. If the aged matrix is too depleted of nickel, these virtuous mechanisms are diminished or inhibited: there is no longer any potential for reversion austenite. On the other hand, if there is too much Ni, the rate of NiAl-type hardening phase is excessively reduced by exaggerating the degree of reversion austenite in which Al remains largely in solution.

At the end of quenching, residual austenite (<3%) must not be used, and you must end up with an essentially martensitic structure. For this purpose, it is necessary to adjust the conditions of quenching, in particular the temperature of end of cooling, and also the composition of the steel. The latter determines the martensitic transformation start temperature Ms which, according to the invention, should preferably remain equal to or greater than 140 ° C. if no cryogenic cycle is used, and should preferably be equal to or greater than 100 ° C. if we practice a cryogenic cycle.

Ms is usually calculated according to the classic formula of the literature: Ms = 550 - 350 x C% - 40 x Mn% - 17 x Cr% - 10 x Mo% - 17 x Ni% - 8 x W% - 35 x V% - 10 x Cu% - 10 x Co% + 30 x Al% C. However, experience shows that this formula is only very approximate, in particular because the effects of Co and Al are highly variable type of steel to another. To know whether a steel is or not according to the invention, it is therefore necessary to rely on measurements of the actual temperature Ms, made for example by dilatometry as is conventional. Ni content is one of Ms.'s possible adjustment variables

The end-of-cooling temperature after quenching must be less than the actual Ms -150.degree. C., preferably less than the actual Ms -200.degree. to ensure a full martensitic transformation of steel. For the most enriched C and Ni compositions in particular, this end-of-cooling temperature can be obtained as a result of a cryogenic treatment applied immediately following cooling to ambient temperature from the solution temperature. It is also possible to apply the cryogenic treatment not from ambient temperature, but after isothermal quenching ending at a temperature slightly greater than Ms, preferably between Ms and Ms + 50 ° C. The overall rate of cooling should be as high as possible to avoid the mechanisms of stabilization of the carbon-rich residual austenite. However, it is not always very useful to look for cryogenic temperatures below -100 ° C because the thermal agitation of the structure may become insufficient to produce the martensitic transformation. In general, it is preferable that the Ms value of the steel is greater than or equal to 100 ° C if a cryogenic cycle is applied, and greater than or equal to 140 ° C in the absence of this cryogenic cycle. The duration of the cryogenic cycle, if necessary, is between 4 and 50 hours, preferably from 4 to 16 hours, and more preferably from 4 to 8 hours. It is possible to practice several cryogenic cycles, the essential being that at least one of them has the aforementioned characteristics.

We must have Al = 1-2%, preferably 1-1.6%, better 1.4-1.6%, and Ni = 11-16%, with Ni ≥ 7 + 3.5 Al. 1.5% Al and 12-14% Ni. These conditions favor the presence of NiAl which increases the tensile strength R _m , which we also note that it is not too deteriorated by the absence of Co if the other conditions of the invention are combined. The elastic limit R _p0,2 is influenced in the same way as R _m .

Compared to the known steels of US-A-5,393,388 , where a high presence of reversion austenite is sought to obtain high ductility and toughness, the steels of the class of the invention prefer the presence of the hardening phases B2, in particular NiAl, in order to obtain a high mechanical strength when hot. Compliance with the conditions on Ni and Al that have been given ensures a sufficient potential content of reversion austenite to maintain ductility and toughness suitable for the intended applications.

It is possible to add B, but not more than 30ppm not to degrade the properties of the steel.

It is also possible to add Nb to control the grain size during forging or other hot processing, at a content not exceeding 0.1%, preferably not exceeding 0.05% for avoid segregations that may be excessive. The steel according to the invention therefore accepts raw materials that can contain significant residual contents in Nb.

A characteristic of the steels of the class of the invention is also the possibility of replacing at least a portion of Mo by W. At equivalent atomic fraction, W segregates less at solidification than Mo and provides an increase in mechanical strength when hot. It has the disadvantage of being expensive and we can optimize this cost by associating it with Mo. As has been said, Mo + W / 2 must be between 1 and 4%, preferably between 1 and 2% . It is preferred to maintain a minimum content of 1% Mo to limit the cost of steel, especially as the high temperature withstand is not a priority objective of the steel of the invention.

Cu can be up to 1%. It is likely to participate in the hardening with the help of its epsilon phase, and the presence of Ni makes it possible to limit its harmful effects, in particular the appearance of superficial cracks during the forging of the pieces, which one observes during additions of copper in steels not containing nickel. But its presence is not essential and it may be present only in the state of residual traces, resulting from the pollution of raw materials.

Manganese is not a priori useful for obtaining the properties of steel, but it has no recognized adverse effect; in addition, its low vapor pressure at the temperatures of the liquid steel makes it difficult to control its concentration in vacuum and vacuum remelting: its content may vary depending on the radial and axial location in a remelted ingot. As it is often present in the raw materials, and for the reasons above, its content will preferably be at most 0.25%, and in any case limited to 2% at the most because of too great variations of its concentration in a same product will interfere with the repeatability of the properties.

Silicon is known to have a hardening effect in solid solution of ferrite and, like cobalt, to decrease the solubility of some elements or certain phases in ferrite. Nevertheless, the steel of the invention requires a significant addition of cobalt, and the same is true of the addition of silicon, especially since, moreover, silicon generally promotes the precipitation of intermetallic phases. harmful in complex steels (Laves phase, silicides ...). Its content will be limited to 1%, preferably less than 0.25% and still more preferably less than 0.1%.

In general, the elements that can segregate at the grain boundaries and weaken them, such as P and S, must be controlled within the following limits: S = traces - 20ppm, preferably traces - 10ppm, better traces - 5ppm, and P = traces - 200ppm, preferably -100ppm traces, better traces-50ppm.

Ca can be used as a deoxidizer and as a sulfur sensor, finding it in the end (≤ 20ppm). Similarly, rare earth residues may ultimately remain (≤100ppm) following a refining treatment of the liquid metal where they would have been used to capture O, S and / or N. The use of Ca and rare earths for these effects not being mandatory, these elements may be present only in the form of traces in the steels of the invention.

The acceptable oxygen content is 50 ppm maximum, preferably 10 ppm maximum.

By way of examples, steel samples have been tested whose compositions (in percentages by weight) are reported in Table 1: Table 1: Composition and Measured Temperatures of the Samples Tested A (ref.) B (ref.) C (ref.) D (ref.) E (ref.) F (ref.) G (ref.) H (invention) VS% 0.233 0.247 0.239 0.244 0.247 0.19 0.22 0.21 Yes% 0.082 0.031 0.031 0,037 0,030 0.05 0.04 0.05 mn% 0,026 0,030 0.033 0.033 0,030 0.02 <0.03 0.04 S ppm 1.0 7.3 3.8 6.1 6.7 7 7 6 P ppm 54 <30 <30 <30 <30 28 <50 29 Or% 13,43 13,31 12.67 12.71 13.08 13,00 14.70 12.95 Cr% 2.76 3.08 3.38 3.38 3.29 3.66 3.19 3.17 Mo% 1.44 1.53 1.52 1.53 1.53 1.50 1.67 1.50 al% 0.962 1.01 1.50 1.50 1.49 1.56 1.68 1.54 Co% 10.25 10.35 6.18 6.24 6.33 6.00 <0.10 <0.10 Cu% 0.014 <0.010 0,011 0.012 0,011 <0.030 <0.020 <0.030 Ti% <0.020 <0.020 <0.020 <0.020 <0.020 <0.005 0,022 <0.005 No.% <0.0050 <0.0050 <0.0050 <0.0050 0,054 <0.005 <0.010 <0.005 B ppm <10 <5 <5 29 <5 <5 <5 <5 Ca ppm <50 <50 <50 <50 <50 <10 <10 <10 N ppm <3 13 13 12 14 3 28 <3 O ppm <3 4.8 3.4 4.4 7.7 <3 7.5 <3 V% <0.010 0.252 0,245 0,254 0.253 0.006 0.208 0,250 Ms measured ° C - 188 176 140 141 186 90 187

The Co content <0.10% of samples G and H corresponds to the usual precision limit of the analysis of this element. In both cases, no voluntary addition of Co was performed.

The elements not mentioned in the table are present at most in the state of traces resulting from the elaboration.

Reference steel A corresponds to a steel according to US-A-5,393,488 , thus having a high Co content.

Reference steel B corresponds to a steel comparable to steel A, to which V was added without modifying the content of Co.

Reference steel C corresponds to a steel according to WO-2006/114499 especially in that, with respect to steels A and B, its Al content has been increased and its Co content decreased.

The reference steel D has undergone a C addition of B.

The reference steel E has undergone a Nb addition to C.

The reference steel F differs from C mainly by the absence of a significant addition of V, compensated by a lower C content, and a higher purity of residual elements.

The reference steel G is distinguished from F by a very low content of Co which would be in accordance with the invention, the presence of V at a level comparable to that of C, D and E, and a higher Ni content, but which, taken in isolation, would nonetheless conform to the invention. But its contents in Ti and N are slightly higher than the invention tolerates. Experience also shows that its measured temperature Ms is substantially too low compared to the requirements of the invention, the relatively high Ni content is not compensated by relatively low levels of Cr, Mo, Al and V.

The steel H is in accordance with the invention in all respects, in particular its very low Co content and its high N and Ti purity. Also, its O content is very low. Finally, its measured temperature Ms is entirely in accordance with the invention.

These samples were forged from 200kg ingots into 75x35mm dishes under the following conditions. A homogenization treatment of at least 16 hours at 1250 ° C. is followed by a first forging operation intended to split the coarse structures of the ingots; square section semifinished products of 75 x 75 mm were then forged after re-heating to 1180 ° C .; finally, each half-product was placed in an oven at 950 ° C, and was forged at this temperature in the form of 75 x 35 mm dishes whose granular structure is refined by these successive operations.

In addition, the samples were softened at a temperature of at least 600 ° C. Experience shows that it is necessary to obtain a complete recrystallization of the steel during the dissolution which will follow. In this case, this softening income was carried out at 650 ° C. for 8 hours and followed by cooling in air. Thanks to this, the raw products of thermomechanical transformations can undergo without particular problems the finishing operations (straightening, peeling, machining ...) giving the piece its final form.

• une mise en solution à 935°C pendant 1h puis un refroidissement par trempe à l'huile ;
• un traitement cryogénique à -80°C pendant 8h ; spécifiquement pour l'échantillon H, on y a ajouté un autre traitement cryogénique à -120°C pendant 2h ;
• un revenu de détente de 16h à 200°C ;
• un vieillissement de durcissement à 500°C pendant 12h puis un refroidissement à l'air.

After forging and softening income, the samples underwent:

• dissolution at 935 ° C for 1 h and then cooling by quenching with oil;
• cryogenic treatment at -80 ° C for 8h; specifically for sample H, another cryogenic treatment was added at -120 ° C for 2h;
• a relaxation income of 16h at 200 ° C;
• curing aging at 500 ° C for 12 hours and then cooling in air.

The properties of the samples (tensile strength R _m in long direction, elastic limit R _p0,2 , elongation A5d, necking Z, resilience KV, toughness K1 c, size of grain ASTM) are reported in Table 2. They are here measured at normal room temperature. Table 2: Properties of the tested samples R _m (Mpa) R _p0.2 (Mpa) A5d (%) Z (%) KV (J) K1c (MPa Vm) ASTM grain AT 2075 1915 11.5 59 26/30 57 8 B 2115 1963 11.3 60 27/27 57.1 8 VS 2274 1982 10.6 54 23/24 43.5 8 D 2286 1970 10.9 56 20/23 44.3 8 E 2270 1961 10.3 52 21/24 46.6 9 F 2060 1904 10.4 59 21/23 59 7 BOY WUT 2149 1715 10.2 52 28/28 - 7 H 2077 1866 10.9 62 34/35 70.4 7

It can be seen that the reference samples C, D and E have a tensile strength that is much greater than that of the reference samples A and B. The elastic limit is at least of the same order of magnitude. In return for this increase in tensile strength, the properties of ductility (necking and elongation at break), toughness and resilience are lowered, in the case of heat treatments described and applied. The desired resistance / toughness compromise can be adjusted by changing the aging conditions.

Reference sample B shows that the mere addition of V to steel A gives only an improvement in certain properties, and in proportions that are often less important than in the case of steels with reduced or no Co content. C to H.

In particular, the increase of Al in steels C to H, combined with the maintenance of a high Ni content, renders the NiAl hardening phase more present. and is an essential factor in improving the tensile strength or keeping it at a suitably high value.

The additions of B and Nb of the samples D and E respectively are not necessary to obtain the high mechanical strengths primarily targeted in the steels of the class of the invention. However, the addition of Nb makes it possible to refine the grain size, described by the conventional ASTM index (the highest ASTM values corresponding to the finest grains).

After the softening recovery at 650 ° C. for 8 hours and cooling in air, a solution at 935 ° C. for 1 hour followed by an oil cooling and then a cryogenic treatment at -80 ° C. for 8h, then un-tensioning at 200 ° C for 8h (on the tensile test specimens) or 16h (on the test specimens of resilience to facilitate the machining of the V-notch of the Charpy test tube; low temperature has the effect of softening a few units HRC (raw quenching structure), then aging at 500 ° C for 12 hours followed by cooling in air, have obtained in long direction at 20 ° C an excellent compromise between tensile strength, ductility and resilience.

Complementary experiments show that in the mean direction resilience values remain acceptable. At 400 ° C., the tensile strength remains very high, and relatively low Co contents as in samples C to F or, according to the invention, almost or completely negligible, as in samples G and H, are compatible with with the resolution of these aspects of the problems posed.

Sample G shows that the large decrease, up to the total removal, of cobalt, can still allow to maintain a high tensile strength. The ductility properties, surprisingly, are also improved. The elastic limit is, however, quite substantially deteriorated in the case of the sample G, in relation to a larger amount of austenite dispersed in the structure, due to the high Ni content of this sample. This contributes to an excessive lowering of the measured Ms which is not compensated by adjustments of the contents of the other elements.

• une résistance à la traction qui demeure élevée, et pourrait être, en cas de besoin, encore améliorée par une augmentation de la teneur en C qui favoriserait le durcissement par trempe et formation de carbures secondaires ; une résistance à la traction de l'ordre de 2300 MPa serait ainsi accessible pour une teneur en C de 0,25% environ ;
• une limite élastique sensiblement améliorée par rapport à l'échantillon G;
• et surtout des propriétés de ductilité remarquables, supérieures à celles de tous les échantillons de référence, permettant de réaliser un bon compromis entre résistance à la traction et ténacité, cette caractéristique étant très importante dans le cadre des applications privilégiées envisagées pour l'acier de l'invention.

On the other hand, in the case of sample H, which conforms in every respect to the composition according to the invention, and whose temperature Ms is sufficiently high, we obtain:

• a tensile strength which remains high, and could, if necessary, be further improved by an increase in the C content which would favor hardening by quenching and formation of secondary carbides; a tensile strength of the order of 2300 MPa would thus be accessible for a C content of about 0.25%;
• a substantially improved elastic limit with respect to the sample G;
• and above all remarkable ductility properties, superior to those of all the reference samples, making it possible to achieve a good compromise between tensile strength and toughness, this characteristic being very important in the context of the preferred applications envisaged for the steel of the 'invention.

The contents of N and Ti a little too high in the sample G compared to the requirements of the invention, and also its slightly higher oxygen content, also contribute in part to its poorer performance than that of the sample. H. Another factor to consider for this sample G is an S content which is not particularly low, and which tends to degrade toughness if it is not offset by other characteristics that would be favorable to this property. Finally, as has been said, this sample G has a fairly high Ni content (although remaining within the range of the invention), which lowers Ms and thus promotes the maintenance of a possibly too high residual austenite level. , even after the cryogenic treatment more particularly pushed (to -80 ° C and then -120 ° C) which was undergone by c and sample.

On the other hand, the sample H according to the invention, which has been cryogenically treated only at -80 ° C., but which has a judiciously adjusted Ni content, minimum impurity contents in all points of view and a temperature Ms measured high enough, responds very well to the problems posed.

• un revenu d'adoucissement à 600-675°C pendant 4 à 20h suivi d'un refroidissement à l'air ;
• une mise en solution à 900-1000°C pendant au moins 1h, suivie par un refroidissement à l'huile ou à l'air suffisamment rapide pour éviter la précipitation de carbures intergranulaires dans la matrice d'austénite ;
• si nécessaire, un traitement cryogénique à -50°C ou plus bas, de préférence à -80°C ou plus bas, pour transformer toute l'austénite en martensite, la température étant inférieure de 150°C ou davantage à Ms, préférentiellement inférieure d'environ 200°C, un au moins desdits traitements cryogéniques durant au moins 4h et au plus 50h ; pour les compositions ayant, notamment, une teneur en Ni relativement basse qui conduit à une température Ms relativement élevée, ce traitement cryogénique est moins utile ;
• optionnellement un traitement d'adoucissement de la martensite brute de trempe effectué à 150-250°C pendant 4-16h, suivi par un refroidissement l'air calme ;
• un vieillissement de durcissement à 475-600°C, de préférence de 490-525°C pendant 5-20h; un vieillissement en dessous d e 490°C n'est pas toujours recommandé car le carbure métastable M₃C pourrait encore être présent et apporterait une fragilité à la structure; les vieillissements au-delà de 525°C peuvent provoquer une perte de résistance mécanique par vieillissement, sans gain notable de ténacité ou de ductilité.

In general, an optimized heat treatment mode of the steel according to the invention for finally obtaining a part having the desired properties is, after forming the blank of the part and before the completion. giving the piece its final form:

• softening income at 600-675 ° C for 4 to 20 hours followed by cooling in the air;
• solution at 900-1000 ° C for at least 1 hour, followed by cooling with oil or air fast enough to avoid the precipitation of intergranular carbides in the austenite matrix;
• if necessary, cryogenic treatment at -50 ° C or lower, preferably at -80 ° C or lower, to convert all the austenite to martensite, the temperature being lower by 150 ° C or more to Ms, preferably lower about 200 ° C, at least one of said cryogenic treatments for at least 4h and at most 50h; for compositions having, in particular, a relatively low Ni content which leads to a relatively high Ms temperature, this cryogenic treatment is less useful;
• optionally softening treatment of the rough quenching martensite carried out at 150-250 ° C for 4-16h, followed by cooling the still air;
• curing aging at 475-600 ° C, preferably 490-525 ° C for 5-20h; aging below 490 ° C is not always recommended because the M ₃ C metastable carbide could still be present and would bring fragility to the structure; Aging beyond 525 ° C can cause a loss of mechanical strength by aging, without significant gain in toughness or ductility.

In the examples which have been described, the shaping operations of the steel following its casting and preceding the softening income and the other heat treatments have consisted of forging. But other types of thermomechanical heat-forming treatments may be performed in addition to or in place of this forging, depending on the type of end product that is desired (stamped parts, bars, semi-finished products. ..). In particular, mention may be made of one or more laminates, a stamping, a stamping, and a combination of several such treatments.

The preferred applications of the steel according to the invention are the endurance parts for mechanics and structural elements, for which a tensile strength of between 2000 MPa and 2350 MPa or more must be cold, combined with values ductility and resilience at least equivalent to those of the best high strength and hot (400 ° C) steels a tensile strength of the order of 1800 MPa, as well as optimal fatigue properties.

The steel according to the invention also has the advantage of being cementable, nitrurable and carbonitrurable. The parts that use it can therefore be given high abrasion resistance without affecting its core properties. This is particularly advantageous in the intended applications that have been cited. Other surface treatments, such as mechanical treatments that limit the initiation of fatigue cracking from superficial defects, are conceivable. Shot peening is an example of such treatment.

If nitriding is carried out, this can be carried out during the aging cycle, preferably at a temperature of 490 to 525 ° C and for a period of time ranging from 5 to 100 hours, the longest ages causing progressive structural softening and, as a result, a progressive decrease in the maximum tensile strength.

Another possibility is to perform carburizing, nitriding or carbonitriding during a thermal cycle prior to or simultaneously with the dissolution, the steel substrate of the invention retaining in this case all its potential mechanical properties.

Claims (25)

1. Steel characterised in that its composition is, in percentages by weight:

   - C = 0.20 - 0.30%

   - Co = traces - 1%

   - Cr =2-5%

   - Al = 1 - 2%

   - Mo + W/2 = 1 - 4%

   - V = traces - 0.3%

   - Nb = traces - 0.1%

   - B = traces - 30 ppm

   - Ni = 11 - 16% with Ni ≥ 7 + 3.5 Al

   - Si = traces - 1.0%

   - Mn = traces - 2.0%

   - Ca = traces - 20 ppm

   - Rare earths = traces - 100 ppm

   - if N ≤ 10 ppm, Ti + Zr/2 = traces - 100 ppm with Ti + Zr/2 ≤ 10 N

   - if 10 ppm < N ≤ 20 ppm, Ti Zr/2 = traces - 150 ppm

   - O = traces - 50 ppm

   - N = traces - 20 ppm

   - S = traces - 20 ppm

   - Cu = traces - 1%

   - P = traces - 200 ppm

   the remainder being iron and unavoidable impurities resulting from the melting.
2. Steel according to claim 1, characterised in that it contains C = 0.20 - 0.25%.
3. Steel according to claim 1 or 2, characterised in that it contains Cr = 2 - 4%.
4. Steel according to one of the claims 1 to 3, characterised in that it contains Al = 1 - 1.6%, preferably 1.4 - 1.6%.
5. Steel according to one of the claims 1 to 4, characterised in that it contains Mo ≥ 1%.
6. Steel according to one of the claims 1 to 5, characterised in that it contains Mo + W/2 = 1 - 2%.
7. Steel according to one of the claims 1 to 6, characterised in that it contains V = 0.2 - 0.3%.
8. Steel according to one of the claims 1 to 7, characterised in that it contains Ni = 12 - 14% with Ni ≥ 7 + 3.5 Al.
9. Steel according to one of the claims 1 to 8, characterised in that it contains Nb = traces - 0.05%.
10. Steel according to one of the claims 1 to 9, characterised in that it contains Si = traces - 0.25%, preferably traces - 0.10%.
11. Steel according to one of the claims 1 to 10, characterised in that it contains O = traces - 10 ppm.
12. Steel according to one of the claims 1 to 11, characterised in that it contains N = traces - 10 ppm.
13. Steel according to one of the claims 1 to 12, characterised in that it contains S = traces - 10 ppm, preferably traces - 5 ppm.
14. Steel according to one of the claims 1 to 13, characterised in that it contains P = traces - 100 ppm.
15. Steel according to one of the claims 1 to 14, characterised in that its measured martensitic transformation temperature Ms is greater than or equal to 100°C.
16. Steel according to claim 15, characterised in that its measured martensitic transformation temperature Ms is greater than or equal to 140°C.
17. Method for producing a martensitic steel part, characterised in that it comprises the following steps preceding finishing of the part which endows it with its definitive shape:

    - the preparation of a steel having the composition according to one of the claims 1 to 16;

    - at least one operation for shaping this steel;

    - tempering at 600 - 675°C for 4 to 20 h followed by cooling in air;

    - solution heat treatment at 900 - 1000°C for at least 1 h, followed by sufficiently rapid cooling in oil or in air to avoid the precipitation of intergranular carbides in the austenite matrix;

    - age hardening at 475 - 600°C, preferably from 490 - 525°C for 5 - 20 h;
18. Method for producing a steel part according to claim 17, characterised in that it comprises furthermore a cryogenic treatment at - 50°C or lower, preferably at - 80°C or lower, before the age hardening step in order to transform all the austenite into martensite, the temperature being less by 150°C or more than the measured Ms, at least one of said treatments lasting at least 4 h and at most 50 h.
19. Method for producing a steel part according to one of the claims 17 or 18, characterised in that it comprises furthermore a softening treatment of the crude martensite resulting from the quenching effected at 150 - 250°C for 4 - 16 h, followed by cooling in still air.
20. Method for producing a steel part according to one of the claims 17 to 19, characterised in that the part is subjected likewise to a carburising or a nitriding or a carbonitriding.
21. Method for producing a steel part according to claim 20, characterised in that the nitriding is effected during an age hardening cycle.
22. Method for producing a steel part according to claim 21, characterised in that the nitriding is effected before 490 and 525°C for 5 to 100 h.
23. Method for producing a steel part according to one of the claims 20 to 22, characterised in that said nitriding or carburising is effected during a thermal cycle prior to or simultaneously with said solution heat treatment.
24. Mechanical part or part for a structural element, characterised in that it is produced according to the method of one of the claims 17 to 23.
25. Mechanical part according to claim 24, characterised in that it concerns an engine transmission shaft, or an engine suspension device or a landing gear element or a gearbox element or a bearing axle.