EP2310546B1 - Hardened martensitic steel having a low cobalt content, process for manufacturing a part from this steel, and part thus obtained - Google Patents

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 type of 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 and nitrurable, 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 maraging steels, whose elastic limit is significantly closer to their maximum tensile strength, which have a satisfactory strength up to 350-400 ° C, and which still offer good toughness for 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 has been proposed in US Patent 5,393,488 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 5,393,388 , 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.

Steels as proposed in WO-2006/114499 and US-A-5,393,488 allow to obtain a good resilience but for some applications it may be insufficient.

For these same applications it is also required to obtain a very high tensile strength (Rm).

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 higher resilience while having a high mechanical strength. 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 = 0,18 - 0,30%
• Co = 1,5 - 4 %, de préférence 2 - 3%
• 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

le reste étant du fer et des impuretés inévitables résultant de l'élaboration.For this purpose, the invention relates to a steel characterized in that its composition is, in percentages by weight:

• C = 0.18 - 0.30%
• Co = 1.5 - 4%, preferably 2 - 3%
• 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 between -80 ° C and 100 ° C or lower but not below -110 ° C, to convert all the austenite to martensite , the temperature being lower than 150 ° C. or more measured Ms, at least one of said treatments lasting between 4h and 50h and preferably between 4h and 10h.

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, or carburizing, or carbonitriding, can be performed during an aging cycle.

Nitriding can be carried out between 475 and 600 ° C.

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 lower but still significant Co content, between 1.5 and 4%. The contents of the other most commonly present significant alloying elements are only slightly modified, but certain levels of impurities must be carefully controlled.

Co is an expensive element whose content has been significantly reduced compared with the prior art, without, however, eliminating it or bringing it to a very low level. The steel according to the invention generally contains relatively few expensive addition elements, apart from nickel, the content of which, however, is not increased with respect to the prior art. But, it is necessary to take special care during development, to limit the nitrogen content to 20 ppm at most 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.

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.

The invention will be better understood on reading the description which follows, given with reference to the figure 1 annexed, which shows, for samples of various compositions, their tensile strength Rm and toughness Kv.

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 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 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 considered as impurities to be avoided, and the sum Ti + Zr / 2 must be ≤ 150 ppm.

The possible addition of rare earths, at the end of the elaboration, can also contribute to fix a fraction of N, besides the S and O. In this case, it must be ensured that the residual content of rare earths in free form remains less than or equal to 100 ppm, and preferably less than or equal to 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.

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. The carbide M ₂ C is metastable with respect 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 the half of the content in W 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 exceeded.

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 state. Its content must be between 0.20 and 0.30%, preferably 0.20-0.25%. The surface layer of the parts may be enriched in C by carburizing or carbonitriding if a very high surface hardness is required in the envisaged applications.

Cobalt slightly raises the ductile / brittle transition temperature, which is not favorable, particularly in compositions with low nickel contents, whereas, contrary to what can be observed in other steels, cobalt does not obviously raise the transformation point Ms 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.

As has been said, 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 lower Co content, between 1.5 and 4%, better between 2 and 3%. The contents of the other most commonly present significant alloying elements are only slightly modified, but certain impurity contents must be carefully controlled, especially the contents of Ti, Zr and N which affect the toughness.

In the context of an exploration of the effect of Co on the mechanical properties of this type of steel (Rm and Kv), it has been unexpectedly demonstrated that the adjustment of the concentration of this element makes it possible to to obtain the best compromise Resilience / Rm. This highlighting is illustrated on the figure 1 on which we observe that a population of points Rm / Kv is distributed around a polynomial curve of order 3 with an inflection for contents in Co between 1.5 and 4% of Co. A resilience of the order of 30 joules or more and an Rm greater than or equal to 2140 MPa are obtained simultaneously in this range of Co content.

It is important to avoid having an unnecessarily high concentration of Co which is not only very expensive but also damages resilience. It is known that Co degrades the transition of resilience of pure Fe ( page 52-54 Materials Science and Technology January 1994 Vol. 10 ). Indeed, as has been said the presence of Co increases the ductile / brittle transition temperature. Furthermore, a Co content greater than 1.5% Co is useful for improving the structural hardening by precipitation of M ₂ C carbide and thus significantly increasing Rm. Moreover, the inventors have surprisingly found, after several tests, that a Co content between about 1.5 and 4%, more preferably between 2 and 3%, significantly improves the mechanical strength virtually without degrading the resilience, compared to a very low Co content (<1%) whose composition would, moreover, be identical.

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 that 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 between 100 and 140 ° 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 from one 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 ° C., preferably lower than the actual Ms -200 ° C., in order to ensure a full martensitic transformation of the steel. The end-of-cooling temperature must therefore be lower than the measured temperature Mf of martensitic transformation end of the steel. For the most enriched C and Ni compositions in particular, a cryogenic treatment may be applied immediately following cooling to room temperature from the solution temperature. 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 useful to look for cryogenic temperatures below -110 ° C because the thermal agitation of the structure becomes insufficient to produce the martensitic transformation. In general, it is preferable that the Ms value of the steel is between 100 and 140 ° C if a cryogenic cycle is applied, and greater than or equal to 140 ° C in the absence of this cryogenic cycle. As already applied for martensitic steels cured by a duplex system and as already known from WO-2006/114499 the duration of the cryogenic cycle, if necessary, is between 4 and 50 hours, preferably 4 to 16 hours, and more preferably 4 to 10 hours. It is possible to practice several cryogenic cycles, the essential being that at least one of them has the aforementioned characteristics.

Specifically, 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. we have 1.5% Al and 12-14% Ni. These conditions favor the presence of NiAl which increases the tensile strength Rm, which we also note that it is not too deteriorated by the low Co content if the other conditions of the invention are met. The elastic limit R _p0,2 is influenced in the same way as Rm.

Compared to the known steels of US-A-5,393,488 , where a very high presence of reversion austenite is sought for high ductility and toughness, steels of the class of the invention prefer the presence of B2 hardening phases, especially NiAl, 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 grain size during forging or other hot processing at a content not exceeding 0.1%. 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 its concentration difficult to control in vacuum production 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 most, because of too great variations in its concentration in the same product will interfere with the repeatability of the properties.

Silicon is known to have a solid solution hardening effect of ferrite and, like cobalt, to decrease the solubility of certain elements or phases in ferrite. However, as we have seen, the steel of the invention comprises only relatively little cobalt, and it can do without silicon, especially since, in addition, 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, like P and S, must be controlled within the following limits: S = traces - 20ppm, preferably traces - 10ppm, better traces - 5ppm, and P = traces - 200ppm, preferably traces - 100ppm, 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 of the tested samples Field of the Invention A (ref.) B (ref.) VS D E F BOY WUT H I J K (ref.) VS% 0.233 0.239 0.22 0.23 0.24 0.21 0.24 0.22 0.18 0.23 0.21 Yes% 0.082 0.031 0,029 0.033 0.041 0,045 0.053 0,036 0,065 0.30 0,052 mn% 0,026 0.033 0,032 0, 035 0,028 0,035 0,039 0.041 0.38 0,052 0,061 S ppm 1 4 7 4 6 7 3 8 10 5 4 P ppm 54 30 29 31 30 25 15 28 80 45 29 Or% 13,43 12.67 13,31 12.42 12,30 14,11 12.99 12.70 15,10 11.25 12.91 Cr% 2.76 3.38 2.99 3.05 3.21 3.19 2.95 3.25 3.17 3.17 2.89 Mo% 1.44 1.52 1.61 1.52 1.49 1.46 1.47 1.51 1.48 1.55 1.46 al% 0.962 1.50 1.45 1.50 1.60 1.54 1.46 1.49 1.53 1.48 1.39 Co% 10.25 6.18 3.93 3.50 3.02 2.98 2.56 2.30 2.02 1.72 0.5 Cu% 0.014 0,011 <0.010 0,011 0,010 <0.010 0,025 0.35 0,052 0,061 0,032 Ti% <0.02 <0.020 <0.020 <0.020 <0.020 <0.020 0,025 <0.02 <0.02 <0.02 <0.02 No.% <0.05 <0.005 0 <0.005 0 0,050 <0.005 0 0,015 <0.005 <0.005 <0.005 <0.005 <0.005 B ppm <10 <5 <5 <5 <5 <5 <5 28 15 <5 <5 Ca pm <50 <50 <50 <50 <50 60 <50 <50 <50 <50 <50 N ppm <3 13 4 7 5 10 6 3 6 <3 <3 O ppm <3 3.4 4 3 10 15 <3 12 <3 20 6 V% <0.01 0,245 0.251 <0.010 0.248 0.243 <0, 010 0.115 0.292 0.241 <0.010

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.

The reference steel B corresponds to a steel according to WO-A-2006/114 499, it is distinguished from A by a lower Co content and a higher Al content.

Steels C to J are in accordance with the invention in all respects, in particular by their Co content, which is significantly lower than that of steel. B, but which nevertheless remains substantially higher than a simple residual content and is obtained by a deliberate addition during the preparation.

Steel C is distinguished from Reference Steel B primarily by a lower Co content.

Steel D differs from C by a slightly lower Co content for a lower Ni content, and by the absence of V, which is present only in trace amounts.

Steel E is distinguished from D by a Co content even lower than that of D and by a V content at a level comparable to steel C.

Steel F is distinguished from C, D, E mainly by a slightly higher Ni content, its Co content being comparable to that of E steel.

Steel G differs from steels C to F by an even lower Co content and has no V.

Steel H is distinguished from Steel G by a further steepening of the Co content and a significantly higher boron content.

Steel I is distinguished from Steel H by further lowering of Co content, and lower C content with higher Ni content.

Steel J is the one whose composition has the lowest Co content, while corresponding to a voluntary addition and which remains in accordance with the invention. It also has the lowest Ni content and contains V.

The reference steel K has a low Co content and below the minimum required by the invention. It is comparable on the other points to steels according to the invention without V and B and very low N.

These samples were forged from 200kg ingots of 75 x 35mm flat elements 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 flat elements whose granular structure is refined by these successive operations.

In addition, the samples were softened at a temperature of at least 600 ° C. 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. It will be noted that the softening income does not contribute to obtaining the final mechanical characteristics.

• une mise en solution à 900°C pendant 1h puis un refroidissement par trempe à l'huile ;
• de manière connue en soi et tel que déjà appliqué pour des aciers martensitiques durcis par un système duplex comme par exemple l'acier de WO-A-2006/114499 : un traitement cryogénique à - 80°C pendant 8h pour les échantillons A, B, C, E, G, I ,J et K ; les échantillons D et H ont subi un traitement cryogénique à - 90°C pendant 7h et l'échantillon F un traitement à - 100°C pendant 6h ;
• un revenu de détente de 16h à 200°C ;
• un vieillissement de durcissement à 500°C pendant 10h puis un refroidissement à l'air.

After the forging, the samples underwent:

• solubilization at 900 ° C for 1 h and then cooling by quenching with oil;
• in a manner known per se and as already applied for martensitic steels hardened by a duplex system such as for example the steel of WO-2006/114499 cryogenic treatment at -80 ° C. for 8 hours for samples A, B, C, E, G, I, J and K; samples D and H were cryogenically treated at -90 ° C for 7h and sample F treated at -100 ° C for 6h;
• a relaxation income of 16h at 200 ° C;
• curing aging at 500 ° C for 10h and then cooling in air.

The properties of the samples (tensile strength Rm in long direction, elastic limit Rp _0.2 , elongation A5d, necking Z, resilience KV, grain size ASTM) are reported in Table 2. They are here measured at room temperature normal. Table 2: Properties of the tested samples Co Rm (Mpa) Rp _0.2 (Mpa) A5d (%) Z (%) KV (J) ASTM grain AT 10.25 2176 1956 11.2 58 25/27 8 Ref. B 6.18 2316 2135 9.5 49 20/24 8 VS 3.93 2220 2030 10.1 52 29 7 D 3.50 2208 2011 10.3 55 31 9 Inv. E 3.02 2200 1998 10.3 55 30 8 F 2.98 2140 1935 10.9 61 32 7 BOY WUT 2.56 2188 1975 10.7 60 31 7 H 2.30 2150 1945 10.6 61 33 8 I 2.02 2185 1970 10.4 59 31 7 J 1.72 2170 1943 10.4 60 33 8 Ref. K 0.5 2085 1891 11.1 62 34 7

It is seen that the samples according to the invention C to J have tensile properties that are comparable to A and B but also a significantly improved resilience due to the significant lowering of the content of Co.

On the other hand, the inventors note after several tests that a Co content of between approximately 1.5 and 4% significantly improves the mechanical strength, practically without degrading the resilience compared to the reference sample K at 0.5% Co Sample K at less than 1.5% Co maintains resilience as good, but with reduced tensile strength.

It has been unexpectedly demonstrated that the concentration of Co according to the invention makes it possible to obtain the best compromise Resilience / Rm. This highlighting is illustrated on the figure 1 on which we observe that a population of points Rm / Kv is distributed around a polynomial curve of order 3 having an inflection between 1.5 and 4% of Co. A resilience of the order of 30 joules or more and an Rm greater than or equal to 2140 MPa are obtained in this range of contents of Co.

The desired resilience / resilience tradeoff could be further refined through a modification of the aging conditions, but the adjustment of the Co content remains the key parameter that must be used to achieve this compromise.

The hardening provided by the increase of Al, with the high Ni, to form the hardening phase NiAl, is not proportional to the concentration of Al, and exceed a value of 2% in Al does not bring gain significant on the tensile strength.

The additions of Nb and B of the samples D and H respectively are not necessary to obtain the high mechanical strengths targeted primarily 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 or at -90 ° C for 7h or at -100 ° C for 6h, then stress relief at 200 ° C for 8h (on the tensile test pieces) or 16h (on the test specimens of resilience), then aging to 500 ° C for 12h 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 are compatible with the desired properties.

• 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 entre -80°C et -100°C ou plus bas mais pas en dessous de -110°C, pour transformer toute l'austénite en martensite, la température étant inférieure de 150°C ou davantage à Ms, préférentiellement inférieure d'environ 200°C à Ms, un au moins desdits traitements cryogéniques durant au moins 4h et au plus 50h et de préférence entre 4h et 10h ; 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 ; la durée du traitement cryogénique dépendant notamment de la massivité de la pièce à traiter ;
• 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 à 490-525°C pendant 5-20h; un vieillissement en dessous de 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 is as follows:

• 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 between -80 ° C and -100 ° C or below but not below -110 ° C, to convert all the austenite to martensite, temperature being less than 150 ° C or more to Ms, preferably less than 200 ° C to Ms, at least one of said cryogenic treatments for at least 4h and at most 50h and preferably between 4h and 10h; for compositions having, in particular, a relatively low Ni content which leads to a relatively high Ms temperature, this cryogenic treatment is less useful; the duration of the cryogenic treatment depending in particular on the massiveness of the workpiece;
• 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 at 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 treatments for hot and / or cold forming can be carried out 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 greater than 2150 MPa must be cold, combined with higher values of resilience than better high-strength steels, and hot (400 ° C) 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.

The carburizing, or the nitriding, or the carbonitriding, can, possibly, be carried out during the heat treatments of aging or dissolution in solution, instead of being carried out during a separate step. In particular, nitriding can be carried out between 475 and 500 ° C during an aging cycle.

Claims (26)

1. A steel characterized in that its composition is, in weight percentages:

   - C = 0.18 - 0.30%

   - Co = 1.5 - 4%

   - 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% where 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 where 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 inevitable impurities resulting from the smelting.
2. The steel according to claim 1, characterized in that it contains between 2 and 3% of Co.
3. The steel according to claim 1 or 2, characterized in that it contains C = 0.20 - 0.25%.
4. The steel according to any of claims 1 to 3, characterized in that it contains Cr = 2 - 4%.
5. The steel according to any of claims 1 to 4, characterized in that it contains Al = 1 - 1.6%, preferably 1.4 - 1.6%.
6. The steel according to any of claims 1 to 5, characterized in that it contains Mo ≥ 1%.
7. The steel according to any of claims 1 to 6, characterized in that it contains Mo + W/2 = 1 - 2%.
8. The steel according to any of claims 1 to 7, characterized in that it contains V = 0.2 - 0.3%.
9. The steel according to any of claims 1 to 8, characterized in that it contains Ni = 12 - 14%, with Ni ≥ 7 + 3.5 Al.
10. The steel according to any of claims 1 to 9, characterized in that it contains Nb = traces - 0.05%.
11. The steel according to any of claims 1 to 10, characterized in that it contains Si = traces - 0.25%, preferably traces - 0.10%.
12. The steel according to any of claims 1 to 11, characterized in that it contains O = traces - 10 ppm.
13. The steel according to any of claims 1 to 12, characterized in that it contains N = traces - 10 ppm.
14. The steel according to any of claims 1 to 13, characterized in that it contains S = traces - 10 ppm, preferably traces - 5 ppm.
15. The steel according to any of claims 1 to 14, characterized in that it contains P = traces - 100 ppm.
16. The steel according to any of claims 1 to 15, characterized in that its measured martensitic transformation temperature Ms is greater than or equal to 100°C.
17. The steel according to claim 16, characterized in that its measured martensitic transformation temperature Ms is greater than or equal to 140°C.
18. A method for manufacturing a steel part, characterized in that it includes the following steps preceding the finishing of the part providing it with its definitive shape:

    - preparation of a steel having the composition according to any of claims 1 to 17;

    - at least one operation for shaping this steel;

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

    - solution heat treatment at 900-1,000°C for at least 1hr, followed by sufficiently fast cooling in oil or air in order to avoid precipitation of intergranular carbides in the austenite matrix;

    - age hardening at 475-600°C, preferably at 490-525°C for 5-20hrs.
19. A method for manufacturing a steel part according to claim 18, characterized in that it further includes a cryogenic treatment at -50°C or lower, preferably between -80°C and -100°C or lower but not below -110°C, in order to transform all the austenite into martensite, the temperature being less than the measured Ms by 150°C or more at, at least one of said treatments lasting between 4hrs and 50hrs and preferably between 4hrs and 10hrs.
20. The method for manufacturing a steel part according to any of claims 18 or 19, characterized in that it further includes a treatment for softening the crude quenched martensite carried out at 150-250°C for 4-16hrs, followed by quiet air cooling.
21. The method for manufacturing a steel part according to any of claims 18 to 20, characterized in that the part also undergoes carburization or nitridation or carbonitridation.
22. The method for manufacturing a steel part according to claim 21, characterized in that nitridation or carburization or carbonitridation is carried out during an ageing cycle.
23. The method for manufacturing a steel part according to claim 22, characterized in that nitridation is carried out between 475 et 600°C.
24. The method for manufacturing a steel part according to any of claims 21 to 23, characterized in that said nitridation or carburization or carbonitridation is carried out during a thermal cycle before or simultaneously with said solution heat treatment.
25. A mechanical part or part for a structural member, characterized in that it is manufactured by the method according to any of claims 18 to 24.
26. The mechanical part according to claim 25, characterized in that this is an engine transmission shaft, or an engine suspension device or a landing gear member or a gearbox member or a bearing axle.

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