FR3013738A1 - MARTENSITIC STAINLESS STEEL, PIECE PRODUCED IN THIS STEEL AND METHOD OF MANUFACTURING THE SAME - Google Patents

Abstract

Martensitic stainless steel, characterized in that its composition is: - traces ≤ C ≤ 0.030%; - traces ≤ If ≤ 0.25%; - traces ≤ Mn ≤ 0.25%; - traces ≤ S ≤ 0.020%; - traces ≤ P ≤ 0.040%; - 8% ≤ Ni ≤ 14%; - 8% ≤ Cr ≤ 14%; - 1.5% ≤ Mo + W / 2 ≤ 3.0%; - 1.0% ≤ Al ≤ 2.0%; - 0.5% ≤ Ti ≤ 2.0%; - 2% ≤ Co ≤ 9%; - traces ≤ N ≤ 0.030%; - traces ≤ 0 ≤ 0.020%; the rest being iron and impurities resulting from the elaboration; and in that its martensitic transformation start temperature Ms calculated by the formula (1) Ms (° C) = 1302 - 28Si - 50Mn - 63Ni - 42Cr - 30Mo + 20Al - 12Co - 25Cu + 10 [Ti - 4 (C + N)] in which the contents of the various elements are expressed in percentages by weight, is greater than or equal to 50 ° C, preferably greater than or equal to 75 ° C. Part made of this steel, and its manufacturing process.

Description

The present invention relates to stainless steels with high tensile strength and toughness, intended in particular for the manufacture of aeronautical structural parts, in particular for landing gear. Structurally hardened martensitic stainless steels have been developed in order to meet the needs related, in particular, to this application. Traditionally, non-stainless steels of the type 40NiSiCrMo7, more usually 300M, have been used, containing, in particular, 0.40% of C, 1.80% of Ni, 0.85% of Cr and 0.40% of Mo. These are weight percentages, as will all the contents quoted in the text. After adequate heat treatments, this steel can have a tensile strength Rm of more than 1930 MPa and a toughness K1 of more than 55 MPa.m112. It would be advantageous to be able to have steels having, in addition to these mechanical properties, high properties of corrosion resistance. For this purpose, different shades have been developed, but none of them gives complete satisfaction. The grade described in US Pat. No. 3,556,776 and for which, typically, C 0.050%, Si 0.6%, Mn 0.5%, S 0.015%, Cr = 11.5-13.5%, Ni = 7-10%, Mo = 1.75-2.5%, Al = 0.5-1.5%, Ti 0.5%, Nb 0.75%, N 0.050%, has a level of mechanical strength too low, less than 1800 MPa. The grade described in US-B-7 901 519, for which, typically, C 0.020%, Cr = 11-12.5%, Ni = 9-11%, Mo = 1-2.5%, Al = 0.7-1.5%, Ti = 0.15-0.5%, Cu = 0.5-2.5%, W = 0.5-1.5%, B 0.0010%, a, it too, an insufficient Rm. The grade disclosed in US-A-5,855,844, for which, typically, C 0.030%, Si 0.75%, Mn 1%, S 0.020%, P 0.040%, Cr = 10-13%, Ni = 10.5- 11.6%, Mo = 0.25-1.5%, Al 0.25%, Ti = 1.5-1.8%, Cu 0.95%, Nb 0.3%, N 0.030%, B 0.010% also has insufficient Rm. The grade described in US-A-2003/0049153, for which, typically, C 0.030%, Si 0.5%, Mn 0.5%, S 0, 0025%, P 0, 0040%, Cr = 9. 13%, Ni = 7-9%, Mo = 3-6%, Al = 1-1.5%, Ti 1%, Co = 5-11%, Cu 0.75%, Nb 1% , N 0, 030%, 0 0, 020%, B 0, 0100%, could have the desired levels of mechanical properties, but would have insufficient corrosion resistance. It may also not be sufficiently fit to be made into massive pieces, because it has been developed for the manufacture of thin products. During heat treatments, it must undergo dissolution at a generally high temperature of 930 to 980 ° C. WO-A-2012/002208 discloses a steel of composition C 0.200%, Si 0.1%, Mn 0.1%, S 0.008%, P 0.030%, Cr = 9.5-14%, Ni = 7-14%, Mo = 0.5-3%, Al = 0.25-1%, Ti = 0.75-2.5%, Co 3.5%, Cu 0.1%, N 0.010%, 0 0.005%, would have good mechanical properties for the main of them which have been cited. But its ductility would be insufficient if we added more than 1% of Al. The dissolution is always carried out at a very high temperature, from 940 to 1050 ° C, for 1 / 2h to 3h, so as to be sufficiently complete without causing excessive grain growth. EP-A-1 896 624 discloses a steel of typical composition C 0.025%, Si 0.25%, Mn 3%, S 0.005%, P 0.020%, Cr = 9-13%, Ni = 8-14% Mo = 1.5-3%, Al = 1-2%, Ti = 0.5-1.5%, Co 2%, Cu 0.5%, W 1%, N 0.006%, 0.005%. It has the advantage of containing little or no Co which is an expensive element, and to tolerate solutions in solution at not very high temperatures (850-950 ° C), so with less energy expenditure and less risk of grain enlargement. But its toughness-tensile toughness compromise is not as favorable as would be desirable. The object of the invention is to provide a structurally hardened martensitic stainless steel having simultaneously tensile properties Rm and toughness K1 high, high corrosion resistance and excellent formability of parts. massive. For this purpose, the subject of the invention is a martensitic stainless steel, characterized in that its composition is, in weight percentages: - traces C 0.030%, preferably 0.010%; - traces Si 0.25%, preferably 0.10%; traces Mn 0.25%, preferably 0.10%; traces S 0.020%, preferably 0.005%; traces P 0.040%, preferably 0.020%; - 8% Ni 14%, preferably 11.3% Ni 12.5%; - 8% Cr 14%, preferably 8.5% Cr 10%; 1.5% Mo + W / 2 3.0%, preferably 1.5 Mo + W / 2.5%; - 1.0% Al 2.0%, preferably 1.0% Al 1.5%; 0.5% Ti 2.0%, preferably 1.10% Ti 1.55%; - 2% Co 9%, preferably 2.5% Co 6.5%; better between 2.50 and 3.50%; traces N 0.030%, preferably 0.0060%; traces 0.020%, preferably 0.0050%; the rest being iron and impurities resulting from the elaboration; and in that its martensitic transformation start temperature Ms calculated by the formula (1) Ms (° C) = 1302 - 28Si - 50Mn - 63Ni - 42Cr - 30Mo + 20AI - 12Co - 25Cu + 10 [Ti - 4 (C + N)] in which the contents of the various elements are expressed in percentages by weight, is greater than or equal to 50 ° C, preferably greater than or equal to 75 ° C.

The proportion of delta ferrite in its microstructure is preferably less than or equal to 1%. The invention also relates to a method for manufacturing a martensitic stainless steel part, characterized in that: - a steel semi-finished product having the aforementioned composition is prepared by one of the following methods: liquid steel having the aforementioned composition, and from this liquid steel, is poured and solidified an ingot and is converted into a semi-product by at least one heat transformation; * metallurgically powdered semi-finished sintered product made of a steel having the aforementioned composition; the solution is completely dissolved in the austenitic region at a temperature of between 800 and 940 ° C .; quenching of the semi-finished product is carried out to a final quenching temperature of less than or equal to -60 ° C., preferably less than or equal to -75 ° C .; aging is carried out between 450 and 600 ° C. for 4 to 32 hours. Between the solidification of the cast and solidified ingot and the dissolution of the half-product, it is possible to homogenize the ingot or semi-finished product at 1200-1300 ° C. for at least 24 hours. Between quenching and aging, it is possible to carry out a cold transformation of the semi-finished product. The quenching can be carried out in two stages, in two different quenching media. The first quenching step is carried out in water. The liquid steel can be prepared by dual vacuum melting treatment, the second vacuum treatment being an ESR or VAR remelting treatment.

The invention also relates to a martensitic stainless steel part, characterized in that it was prepared by the above method. It can be a piece of aeronautical structure. As will be understood, the invention consists in providing a grade of martensitic stainless steel which, after having undergone suitable thermomechanical treatments which, combined with said shade, are also an element of the invention, present both properties of tensile strength, toughness and ductility which make it suitable for its use for the manufacture of massive parts such as landing gear, as well as an excellent resistance to corrosion compared to the shades already used for this purpose. effect. The steels of the invention have a martensitic structure which is obtained: by a complete dissolution in the austenitic domain, thus carried out beyond the temperature Ac3 of the steel concerned; for the grade under consideration, this dissolution temperature is from 800 to 940 ° C .; the dissolution is carried out for a period of 30 minutes to 3 hours; a temperature of the order of 850 ° C combined with a duration of the order of 1 h 30 min are generally adequate for both obtaining complete dissolution and a moderate grain size; a grain that is too coarse would be detrimental to the properties of resilience, stress corrosion and ductility; - Then quenching, preferably carried out from a temperature close to the solution temperature, said quench being extended to a cryogenic temperature, namely -60 ° C or lower, preferably up to - 75 ° C or lower, typically down to -80 ° C. The holding time in the cryogenic medium must be sufficient for the cooling at the chosen temperature and the desired transformations affect the steel piece in all its volume. This time therefore strongly depends on the mass and dimensions of the treated part, and is, of course, even higher than, for example, the treated part is thick. Different quenching media can be used: air, water, oil, gas, polymer, liquid nitrogen, dry ice (non-limiting list), and quenching is not necessarily carried out with a very high cooling rate.

It is conceivable to use successively two different quenching media, the first medium bringing the steel for example to an intermediate temperature, and the second medium then bringing the steel to -60 ° C or lower. For the most massive rooms, water is a prime tempering medium because it ensures that the heart of the room is cooled quickly enough. The quenching start temperature is preferably the temperature at which dissolution took place, to ensure that no hardenable metallurgical transformations occur between quenching and quenching and that could be adversely affected. to affect the final mechanical properties of the product If the quenching is interrupted for a certain time below Ms and above the martensite transformation end temperature Mf, the interruption must be short in order to avoid the risk of blocking the transformation when the quenching will be resumed. Another possibility would be to stop the quenching above Ms and then resume it until the cryogenic temperature. A possible advantage of such interruptions is that they make it possible to avoid the need to immediately use a cryogenic quenching medium, and therefore to avoid having a very high initial cooling rate which could lead to the appearance of taps. (superficial cracking), or cracks inside the semi-product that could be due to differential martensitic transformation phenomena between the surface and the still hot core of the semi-finished product if it is relatively thick.

But in practice it is preferable to perform quenching in one step, for more convenience and to avoid undesirable metallurgical effects on the microstructure of the steel, because a quenching in two steps is often difficult to control as to at the final temperature of the first stage and the homogeneity of its effects in the treated part.

The transition to the cryogenic temperature can be done in a solid, gaseous or liquid medium depending on the available treatment technology. In order to obtain a completely martensitic structure, the beginning of the martensitic transformation on cooling, Ms, must be mastered. This point Ms depends on the composition of the alloy and is calculated according to Equation (1): (1) Ms (° C) = 1302 - 28Si - 50Mn - 63Ni - 42Cr - 30Mo + 20AI - 12Co - 25Cu + 10 [Ti - 4 (C + N)] in which the contents of the various elements are expressed in percentages by weight. In the context of the invention, Ms is necessarily greater than or equal to 50 ° C and preferably greater than or equal to 75 ° C. If this condition is not fulfilled, the steel has residual quenching austenite which is detrimental to the mechanical properties, in particular the breaking strength. After dissolution and prolonged quenching to the target cryogenic temperature, the final mechanical properties are obtained after aging between 450 and 600 ° C for a period of 4 to 32 hours. The hardening obtained is ensured by the formation of intermetallic precipitates NiAI and Ni3Ti type of nanometric size. During aging, reversion austenite can form and contribute to steel toughness. This aging can, optionally, be interrupted using a water quench to improve toughness.

The final structure, for the applications envisaged in a privileged way, in particular in the aeronautics, must be free of delta ferrite which degrades the mechanical properties. A maximum of 1% delta ferrite is tolerable. The composition of the steel according to the invention is chosen, precisely, to avoid as much as possible delta ferrite remains at the end of the treatments performed during the implementation of the method according to the invention. From this point of view, it is very preferable, in order to ensure this absence of delta ferrite subsistence, that the ratio Cr eq / Ni eq of steel, ie the ratio between the weighted sum of the contents of the main alphagenic elements such as Cr (chromium equivalent) and the weighted sum of the contents of the main gammagenic elements such as Ni (nickel equivalent), that is less than or equal to 1.05, with: Cr eq = Cr + 2 Si + Mo + 1.5 Ti + 5.5 Al + 0.6 W Ni eq = 2 Ni + 0.5 Mn + 30 C + 25 N + Co + 0.3 Cu The solidification of the shades of the invention must be controlled to limit the segregation of ingots which can be detrimental to the mechanical properties, especially when the mechanical stress is in the cross direction, and the inclusions content of oxides and nitrides should be minimized as much as possible. For this purpose, a preferred mode of preparation of steels according to the invention is a double vacuum melting with induction melting (Vacuum Induction Melting, VIM) and then casting of the ingot steel to obtain an electrode , which is then treated by vacuum arc remelting (Vacuum Arc Remelting, VAR) or by remelting under an electroconductive slag (Electroslag Remelting, ESR). Developments under vacuum prevent oxidation of Al and Ti by air, thus the excessive formation of oxidized inclusions, and also allow to eliminate a portion of nitrogen and dissolved oxygen. It is thus possible to obtain high fatigue life times. After obtaining the solidified ingot, the hot transformations (rolling, forging, stamping, etc.) are carried out and put into the form of a semi-finished product (bar, flat, block, forged or stamped part, etc.). to give it dimensions at least close to its final dimensions. These hot transformations are simply those which are customary for the semi-finished products of general compositions comparable to those of the invention, both with regard to deformations and treatment temperatures.

Preferably, a homogenization treatment of the ingot or semi-finished product is also carried out at a temperature of 1200 to 1300 ° C. for at least 24 hours to limit the segregation of the various elements present and thus more easily to obtain the properties mechanical devices. However, homogenization is generally not usually carried out during the last hot forming operations or after these, in order to more certainly preserve an acceptable grain size on the products, depending on their future use. The half-product then undergoes, according to the invention, a heat treatment consisting of: A dissolution between 800 and 940 ° C practiced, as is conventional, for a time sufficient to dissolve the precipitates present in the entirety of the half -produced and therefore closely dependent on the dimensions of said half-product, followed by quenching to a temperature of -60 ° C or lower, preferably -75 ° C or lower, said quenching beginner, preferably, to a temperature close to the dissolution temperature, and can be carried out in two steps separated by a stay at an intermediate temperature (for example ambient, or a temperature between the beginning and the end of martensitic transformation, or a higher temperature at the start temperature of the martensitic transformation); Then, possibly, cold forming of the semi-finished product; Then aging between 450 and 600 ° C for 4 to 32 hours to balance the properties of strength, toughness and ductility according to the following criteria: - The maximum resistance reached decreases when the aging temperature increases, but vice versa the ductility and the toughness grow; The aging time necessary to cause a given hardening increases as the aging temperature decreases; - At each temperature level, the resistance passes through a maximum for a determined period, which is called "curing peak"; - For each target level of resistance, which can be achieved by several pairs of time-temperature aging variables, there is only one such torque that gives the best compromise strength / ductility to steel; these optimum conditions correspond to an early survival of the structure, and are obtained when going beyond the peak of hardening; those skilled in the art can determine experimentally what is the optimal torque by means of reflections and routine tests. The alloying elements of the steel according to the invention are present in the quantities indicated for the reasons which will be exhibited. As mentioned, percentages are percentages by weight. The C content is at most 0.030% (300 ppm), preferably at most 0.010% (100 ppm). In practice, it is generally only present as a residual element resulting from the melting of the raw materials and the preparation, without a voluntary addition being made. It could form Cr carbides M23C6 type and thus penalize the corrosion resistance by capturing Cr which is thus no longer available to ensure the stainless steel is satisfactorily. It could also associate with Ti to form carbides and carbonitrides harmful to the fatigue behavior, and the consumption of Ti in these forms would decrease the amount of hardening intermetallic formed. The Si content is at most 0.25%, preferably at most 0.10% to better ensure the good compromise between Rm and K1C sought. Typically it is only a residual element not added voluntarily. It tends to lower Ms (see equation (1)) and to weaken the steel, hence its undesirable nature in larger quantities than has been said. The Mn content is at most 0.25%, preferably at most 0.10%. Typically it is only a residual element not added voluntarily. It tends to lower Ms (see equation (1)). It could possibly be used as a partial substitution of the Ni to avoid the presence of delta ferrite and contribute to the presence of reversion austenite during hardening aging. But the ease with which it evaporates during the vacuum treatment makes it difficult to control and leads to fouling of dust extraction devices fumes furnaces. Therefore, a significant presence of Mn in the steels of the invention is not recommended. The S content is at most 0.020% (200 ppm), preferably at most 0.005% (50 ppm), to better ensure the good compromise between Rm and K1C sought. Here again it is present in the residual state and, if necessary, its content must be controlled by a careful choice of raw materials and / or a metallurgical desulphurization treatment during the melting and composition adjustment step. steel. It reduces toughness by segregation at grain boundaries, and forms sulphides damaging to mechanical properties.

The P content is at most 0.040% (400 ppm), preferably at most 0.020% (200 ppm) to better ensure the good compromise between Rm and K1C sought. This is again a residual element that tends to segregate at the grain boundaries and therefore decreases toughness.

The Ni content is between 8 and 14%, preferably between 11.3 and 12.5%. It is a gamma element, and it must be at a sufficiently high level to avoid the stabilization of the delta ferrite during dissolution and homogenization operations. But it must also be maintained at a low enough level to ensure a complete martensitic transformation during quenching since it has a strong tendency to lower Ms according to equation (1). On the other hand, it contributes to the hardening of the steel during precipitation aging of the NiAl and Ni3Ti hardening phases which give the steels of the invention their level of mechanical strength. It also has the function of forming reversion austenite during aging, which precipitates finely between the martensite slats and provides ductility and toughness to the steels of the invention. The Cr content is between 8 and 14%, preferably between 8.5 and 10%. It is the main element that provides resistance to corrosion, which justifies the lower limit of 8%. But its content must be limited to 14% so that it does not contribute to the stabilization of delta ferrite and does not cause Ms, calculated according to equation (1), to pass below 50 ° C. The content of Mo + W / 2 is between 1.5 and 3.0%, preferably between 1.5 and 2.5%. Mo participates in the corrosion resistance and is likely to form a hardening phase Fe7Mo6. Optionally, at least part of the Mo can be replaced by W. It is well known that in steels, these two elements are functionally often comparable, and that, at equal weight percent, W is twice as effective as Mo. Al content is between 1.0 and 2.0%, preferably between 1 and 1.5%. During aging, it forms the hardening phase NiAI. The AI is usually known to degrade ductility, but this disadvantage is negated by the possibility offered by the invention to achieve dissolution at relatively low temperatures. The Ti content is between 0.5 and 2.0%, preferably between 1.10 and 1.55%. He also participates in hardening during aging by forming the Ni3Ti phase. It also makes it possible to fix C and N in the form of carbides and carbonitrides of Ti and thus to avoid the harmful effects of C. However, as has been said, these carbides and carbonitrides are harmful to the fatigue behavior, and can not afford to train too much. The contents of C, N and Ti must therefore be kept within the prescribed limits. The content of Co is between 2 and 9%, preferably between 2.50 and 6.5%, better between 2.50 and 3.50%. It makes it possible to stabilize the austenite at the homogenization and dissolution temperatures, and thus to avoid the formation of delta ferrite. It participates in the hardening by its presence in solid solution and also in that it promotes the precipitation of phases NiAI and Ni3Ti. It can be added as a substitution for Ni to raise the temperature Ms and ensure that it is above 50 ° C. Compared to the steel described in EP-A-1 896 624 where Co must be at most 2%, the aim here is to use Co to contribute significantly to the curing, this in combination with the other elements present and the heat treatments required. The target preferential content of 2.50-3.50% represents the best compromise between the cost of steel and its performance. N should be at most 0.030% (300 ppm), preferably at most 0.0060% (60 ppm) to better ensure the right compromise between Rm and K1C sought. Nitrogen is not intentionally added to the liquid metal, and the vacuum treatments which are generally performed during the preparation process make it possible to protect the liquid steel against atmospheric nitrogen uptakes, or even to remove part of the dissolved nitrogen. N is unfavorable to the ductility of the steel and forms angular Ti nitrides which are likely to be sites of crack initiation during fatigue stresses. O must be at most 0.020% (200 ppm), preferably 0.0050% (50 ppm) to better ensure the right compromise between Rm and K1C sought. It is also unfavorable to ductility, and the oxidized inclusions it forms are also potential sites for crack initiation in fatigue. The O content should be chosen according to the usual criteria for the skilled person, according to the precise mechanical characteristics required for the final product. In general, the mechanical properties of the steel of the invention are adversely affected by the inclusions of oxides and nitrides. The use of production processes to minimize their presence in the final steel (VIM, ESR, VAR) is preferred especially for this reason. The other elements present in the steel of the invention are iron and the impurities resulting from the preparation. It should be understood that the ranges given as preferential for each element are independent of each other, that is to say that the composition of the steel may be in these preferred ranges for certain elements only.

Tests were carried out on samples from ingots castings having the compositions set forth in Table 1. The compositions of samples A to E correspond to reference steels: A, D and E are in accordance with the teaching of EP-A A-1 896 624. B and C are two examples of reference which make it possible to highlight the interest of imposing Ms according to the invention. The compositions of samples 1 to 16 correspond to steels according to the invention. Samples A, B, C and 1 to 5 are from 6 kg ingots, and the other samples are from 150 kg ingots. The 6 kg ingots were initially developed for a first validation of the concept of the invention, and their encouraging properties led to continuing the experiments with castings of 150 kg to confirm and refine the definition of the invention. The 6 kg ingots also made it possible to carry out tensile tests directly, whereas it was necessary to form the 150 kg ingots to extract thereafter the samples on which the measurements of the parameters governing the tenacity were carried out.

C% Si% Mn% S% P% Ni% Cr% Mo% Al% A 0.0031 0.031 <0.010 0.0005 <0.0050 12.41 9.80 2.03 1.38 B <0.0020 0.024 <0.010 0.0004 <0.0050 12.38 9.75 2.04 1.38 C <0.0020 0.028 <0.010 0.0005 <0.0050 12.41 9.68 2.03 1.36 D 0 , 0020 0.057 <0.010 0.0012 <0.0050 12.24 10.03 2.01 1.47 E 0.0042 0.087 <0.010 0.0001 <0.0050 12.48 9.97 2.05 1.42 1 <0.0020 0.030 <0.010 0.0004 <0.0050 12.41 9.70 2.03 1.38 2 0.0039 0.022 <0.010 0.0006 <0.0050 12.01 9.66 2.02 1.40 3 0.0022 0.035 <0.010 0.0004 <0.0050 11.47 9.74 2.03 1.33 4 <0.0020 0.026 <0.010 0.0004 <0.0050 10.52 9.71 2.05 1.39 5 0.0026 0.035 <0.010 0.0005 <0.0050 9.52 9.80 2.06 1.37 6 0.0059 0.046 <0.010 0.0013 <0.0050 11.86 10 , 04 2.01 1.25 7 0.0049 0.046 <0.010 0.0015 <0.0050 11.33 10.18 2.00 1.23 8 0.0018 0.023 <0.010 0.0016 <0.0050 10, 32 10.15 2.01 1.33 9 0.0130 0.029 <0.010 0.0014 <0.0050 11.47 10.14 1.99 1.32 10 0.0018 0.041 <0.010 0.0013 <0.0050 12.21 9.12 2.05 1.31 11 0.0020 0.036 <0.010 0.0016 <0.0050 11.26 9, 16 2.00 1.35 12 0.0030 0.063 <0.010 0.0001 <0.0050 12.43 8.98 2.08 1.38 13 0.0023 0.061 <0.010 0.0001 <0.0050 11.75 9.40 2.06 1.39 14 0.0048 0.022 <0.010 0.0003 <0.0050 11.82 9.60 2.03 1.09 15 0.0052 0.024 <0.010 0.0004 <0.0050 11 , 77 9.39 2.01 1.72 16 0.0049 0.024 <0.010 0.0004 <0.0050 11.15 9.55 2.00 1.05 Ti% Co% N% 0% Fe Ms (° C ) following equation (1) A 1,18 <0,010 0,0029 0,0008 rest 85 B 1,17 6,11 0,0021 0,0009 rest 16 C 1,16 9,11 0,0006 0,0009 remainder - 19 D 1.15 <0.010 0.0024 0.0010 rest 88 E 1.17 <0.010 0.0022 0.0013 remainder 72 1 1.18 3.06 0.0015 0.0015 remainder 53 2 1.18 3, 07 0.0017 0.0019 remainder 81 3 1.17 6.13 0.0014 0.0012 remainder 73 4 1.18 6.11 0.0015 0.0010 remainder 135 1.18 6.14 0.0004 0, 0013 remains 193 6 1.20 3.19 0.0009 0.0010 remainder 69 7 1.23 6.02 0.0008 0.0006 remainder 63 8 1.17 6.22 0.0021 0.0026 remain 128 9 1 , 16 5.00 0.0009 0.0007 Remain 70 1.22 3.09 0.0016 0.0006 Remain 88 11 1.17 6.20 0.0019 0.0008 Remainder 111 12 1.23 3.12 0 , 0039 0.0009 remains 79 13 1.21 3.09 0.0029 0.0 005 remainder 106 14 1.45 3.06 0.0044 0.0003 remainder 91 0.94 3.03 0.0036 0.0013 remainder 112 16 1.45 4.08 0.0016 0.0010 remainder 124 Table 1: Compositions of the test samples, with their Ms calculated according to equation (1) The 6 kg ingots (A, B, C 1 to 5) were prepared by vacuum treatment of the liquid metal before casting. They were homogenized at 1250 ° C. for 48 hours. They were then spun after heating to 940 ° C to be formed into bars of diameter 22 mm. Table 2 shows which treatments these rods were then subjected to, and what were their main ultimate mechanical properties measured in the long direction: tensile strength Rm, 0.2% yield strength Rp0.2, elongation at break A , necking at break Z, Vickers hardness. The small size of the spun samples did not allow the extraction of specimens that would have had the dimensions necessary to carry out the toughness tests. Ech. Heat treatment Aging Rm Rp0,2 AZ Hardness (Hv) (MPa) (MPa) (%) (%) Application Tempering temperature Temperature (° C) Time (h) in (° C) solution (° C / h) A 850 / 1,5 -80 510 16 1868 1758 11 48 548 B 850 / 1,5 -80 510 16 Unrealized tests 216 (too much austenite in the structure) C 850 / 1,5 -80 510 16 146 1,850 / 1.5 -80 510 16 1826 1678 11 48 546 2 850 / 1.5 -80 510 16 1947 1797 11 49 577 3 850 / 1.5 -80 510 16 1910 1794 11 50 574 4 850 / 1.5 - 80 510 16 1966 1872 11 49 590 5 850 / 1.5 -80 510 16 1977 1893 8 25 583 Table 2: Treatment conditions and mechanical properties of samples from ingots of 6 kg It should be noted that the excessive presence of austenite in the structure resulted, for the reference samples B and C, by a very low hardness, which was an indication of a poor tensile strength and certainly insufficient compared to the requirements of the invention. It was therefore considered unnecessary to carry out other mechanical tests on these samples. These samples had compositions which, in terms of the individual contents of each element, were in accordance with the requirements of the invention, but which, taken together, provided a martensitic transformation temperature Ms which was too low (below 50 ° C.). Quenching, performed under the experimental conditions, which correspond to what is usually practiced industrially, did not allow to obtain a sufficiently martensitic structure in the case of these samples. This shows that the condition placed on Ms is important to consider in the context of the invention. Concerning the 150 kg ingots (D, E, 6 to 16), they were evacuated, cast and then also vacuum-melted by the VAR process to give 200 mm diameter ingots. They were then homogenized at 1250 ° C. for 48 hours, then forged at this temperature into semi-finished products of octagonal section of 110 mm, then, after reheating to 940 ° C., again forged, this time in 80x40 section bars. mm. Table 3 shows the conditions under which the heat treatments that followed and the mechanical properties measured in the long direction were taken on the samples. Compared with the tests in Table 2, no hardness measurements were made which would have duplicated the Rm measurements, and resilience (Kv measurement) and toughness (K1C measurement) tests were performed. ).

Heat Treatment Aging Rm Rp0.2 AZ Kv K1C (MPa) (MPa) (%) (%) (J) (MPa.m1 / 2) Setting Temperature Temperature Solution Time (° C) (h) (° C / h) quenching (° C) D 850 / 1,5 -80 480 16 1952 1825 10 47 7 43 D 850 / 1,5 -80 490 16 1900 1696 10 48 9 46 D 850 / 1,5 -80 510 16 1829 1733 11 53 12 49 D 850 / 1,5 -80 530 16 1701 1593 13 58 25 66 E 850 / 1,5 -80 490 16 1872 1712 12 47 10 46 E 850 / 1,5 -80 510 16 1845 1685 13 53 11 58 E 900 / 1,5 -80 510 16 1885 1761 12 48 7 56 6 850 / 1.5 -80 490 16 1892 1748 13 53 15 67 6 850 / 1.5 -80 510 16 1814 1675 14 58 22 90 6 850 / 1.5 -80 530 16 1692 1563 16 59 32 115 7 850 / 1.5 -80 480 16 1888 1659 12 45 10 52 7 850 / 1.5 -80 490 16 1897 1755 13 53 19 63 7 850 / 1,5 -80 510 16 1809 1660 14 58 20 79 7 850 / 1.5 -80 530 16 1682 1521 16 61 31 125 8 850 / 1.5 -80 490 16 2078 1970 10 42 5 31 8 850 / 1.5 -80 510 16 2021 1952 10 51 6 40 8 850 / 1.5 -80 530 16 1820 1753 11 50 12 63 9 850 / 1.5 -80 490 16 1920 1768 12 52 16 56 9 850/1, 5 -80 510 16 1868 1719 13 53 17 68 9 850 / 1.5 -80 530 16 1721 1585 15 59 28 104 10 850 / 1.5 -80 490 16 1957 1803 13 57 15 59 10 850 / 1.5 -80 500 16 1949 1822 13 54 13 63 10 850 / 1.5 -80 510 16 1917 1787 13 59 18 65 10 850 / 1.5 -80 530 16 1785 1675 14 60 22 84 10 850 / 1.5 -80 490 4 1968 1839 11 43 10 46 10 850 / 1.5 -80 510 4 1969 1878 11 49 10 52 10 850 / 1.5 -80 530 4 1943 1812 12 53 10 58 11 850 / 1.5 -80 490 16 2014 1933 9 51 14 43 11 850 / 1.5 -80 500 16 2040 1940 12 53 7 45 11 850 / 1,5 -80 510 16 2004 1920 10 50 12 49 11 850 / 1.5 -80 530 16 1800 1727 12 54 27 69 11 850 / 1.5 -80 490 4 2011 1883 11 42 4 41 11 850 / 1.5 -80 510 4 2019 1934 10 46 7 38 11 850 / 1.5 -80 530 4 1983 1889 12 54 7 45 12 850 / 1.5 -80 490 16 1989 1840 13 55 14 52 12 850 / 1.5 -80 510 16 1953 1822 13 57 13 57 12 850 / 1.5 -80 530 16 1835 1721 13 61 16 74 13 850 / 1.5 -80 490 16 2028 1897 12 55 11 47 13 850 / 1,5 -80 510 16 1982 1870 12 56 15 55 13 850 / 1,5 -80 530 16 1851 1752 14 60 21 60 14 850 / 1,5 -80 490 16 1991 1833 9 41 9 53 14 850/1 , 5 -80 510 16 1943 1831 9 38 8 60 14 850 / 1.5 -8 0 530 16 1818 1698 11 48 16 72 14 875 / 1.5 -80 490 16 1984 1838 12 52 8 54 14 875 / 1.5 -80 510 16 1940 1815 12 55 7 55 14 875 / 1.5 -80 530 16 1819 1707 14 57 15 67 15 850 / 1.5 -80 490 16 2045 1917 12 54 6 40 15 850 / 1.5 -80 510 16 1995 1883 12 55 7 48 15 850 / 1.5 -80 530 16 1856 1757 13 60 9 62 16 850 / 1.5 -80 490 16 2000 1875 11 49 8 44 16 850 / 1.5 -80 510 16 1953 1856 12 53 7 49 16 850 / 1.5 -80 530 16 1841 1758 13 58 8 61 Table 3: Treatment conditions and mechanical properties of samples from ingots of 150 kg The properties of the different samples can be commented as follows.

Reference samples A, D and E correspond to the low or zero Co content steels described in EP-A-1 896 624. Compared to the steels of the invention, we see that their Rm is relatively low. The reference samples B and C have an MS of less than 50 ° C, therefore too low to be in accordance with the invention. This explains the excessive presence of residual austenite which prevents a sufficient Rm, translated by a low hardness. Sample 1 is in accordance with the invention, but has a Ms less than the optimum of 75 ° C and more. Its Rm is therefore relatively weak and will not be suitable for all conceivable applications. The same can be said, but to a lesser extent, of sample 3.

Sample 2 has, on the contrary, an optimum Ms, and its Rm of 1947 MPa is excellent. Samples 4 and 5, high Ms because of their significant substitution of Ni by Co, have an excellent Rm of 1966 and 1977 MPa respectively. Sample 6 has an MS that is not optimal compared to sample 2 which also has about 3% of C. Similarly for sample 7 which has a Co content of about 6%, but a lower Rm than sample 4 due to its very low Ms. La Rm in sample 8 is due to its high Ms combined with a Co content of about 6%. The sample 9 to 5% Co has a Ms less than the optimum and its Rm is relatively limited. This shows that a relatively high content of Co is not sufficient to ensure a high Rm in the context of the invention. Samples 10 and 12 show the best compromise between Rm and K1C. In fact, their compositions are in accordance with the preferred contents on all the elements.

Sample 11 has a high Ms and a high Rm. The equilibrium between Rm and K1C is better than for sample 8 because of a better balance between the contents of Ni and Cr. The comparison between the samples 13, 14 and 15 demonstrates the advantageous effect of the partial substitution of Al with Ti: the sample 14 is the one that has the best compromise between Rm and K1C. It should also be noted that these samples have a Cr content (9.4-9.6%) higher than that (about 9%) of the samples 10 and 12. The sample 16 has a high Ms. Its Rm is equivalent to that of sample 12 but its K1C is less favorable because of a slightly higher Cr content. Figure 1 shows the results of Table 3 in terms of trade-off between Rm and K1C for samples from ingots of 150 kg, these being the only ones for which toughness was measured. Overall, K1C decreases when Rm increases, and the steels according to the invention have a better compromise between these two properties than the reference steels D and E whose compositions are relatively close to the invention except for the content of Co. For reference samples, an Rm of 1701 MPa corresponds to a tenacity of 66 MPA.m112. This steel is therefore not at all suitable for the intended uses envisaged because of its insufficient Rm. The maximum Rm of the reference samples is 1952 MPa, which would be correct for these uses, but the corresponding tenacity is only 43 MPa.m112, which would be very insufficient. The best compromise strength / toughness are obtained for Rm from 1845 to 1900 MPa, which correspond to toughness of the order of 46 to 56 MPA.m112. These mechanical properties taken as a whole are therefore not as favorable as for carbon steels of type 300M. With regard to the samples according to the invention, we see in FIG. 1 that a very good compromise between Rm and K1C is generally obtained for Rm of the order of 1950 MPa, which correspond to K1Cs of the order from 46 to 63 MPa.m112, most often greater than 50 MPa.m112. The orders of magnitude of the corresponding properties of the 300M steels are therefore found. It is also seen that if a reduction in Rm was acceptable, the tenacity would be increased in large proportions, and vice versa. The steels according to the invention therefore provide the user with a great flexibility in the choice of their properties, which can be modulated by the composition, the heat treatments and the final aging chosen in the context that has been mentioned. As regards the ductility, the values of A% and Z% of the samples according to the invention are very comparable to those obtained on 300M steels. The invention therefore provides no degradation with respect to the 300M from this point of view. On some of these same samples from ingots of 150 kg (samples D, 6 to 8 and 10 to 16), salt spray corrosion tests were also carried out in a 50 g / l aqueous solution. NaCl at 35 ° C. They had all previously been subjected to the same heat treatment solution at 850 ° C for 1 h 30 min, tempering at -80 ° C and aging at 510 ° C for 16 h. None of these samples showed signs of corrosion after 200 hours of exposure.

The steels according to the invention therefore do not see their degraded salt spray corrosion results with respect to the reference steel D which does not contain Co. Stress corrosion tests have also been carried out in an aqueous medium with 3.5% NaCl at 23 ° C, on samples E and 10, subjected to solution at 850 ° C for 1 h 30 min, quenched at -80 ° C and aged at 510 ° C for 16 hours. h. K1C fracture toughness and time to failure were measured for loads equal to 75% K1C. In both cases, the samples withstood for more than 500 hours before rupture. This is a good result, and the invention therefore does not degrade the resistance to stress corrosion with respect to reference steels without Co. The steels according to the invention can therefore be substituted mechanically satisfactorily for 300M steels. , in addition to the fact that they exhibit salt spray corrosion resistance and stress corrosion resistance which are quite favorable, because comparable to those of stainless steels by which it could be envisaged to replace the 300M. It should be understood that throughout this description, the solidified "ingot" which is cast from the liquid metal may have any shape capable of leading, after the various deformations, to a final product having the shape and dimensions desired for its use. . In particular, casting in a conventional mold with a bottom and fixed side walls is only one of the possible ways to proceed, and the various continuous casting processes in a bottomless mold with fixed or movable walls can be used to achieve the solidification of the "ingot". An alternative solution to that which has just been described is to carry out the series of heat treatments on a semi-finished product not of an ingot transformed by hot rolling, forging, stamping or other, but on a sintered half-product manufactured by metallurgy of powders, to which it would be possible to directly confer a form, possibly complex, and dimensions very close to those of the final piece. The powder used is a metal powder which has the composition of the steel according to the invention. In his case, homogenization of the sintered semi-product is not necessary. However, the manufacturing process may include prior to sintering itself, as is conventional for the skilled person, a pre-sintering step performed under less severe conditions than sintering in terms of temperature and / or duration. In general, the sintering process is conducted as one skilled in the art would do by using his usual knowledge.

Claims (10)

CLAIMS1.- Martensitic stainless steel, characterized in that its composition is, in percentages by weight: - traces C 0.030%, preferably 0.010%; - traces Si 0.25%, preferably 0.10%; traces Mn 0.25%, preferably 0.10%; traces S 0.020%, preferably 0.005%; traces P 0.040%, preferably 0.020%; - 8% Ni 14%, preferably 11.3% Ni 12.5%; - 8% Cr 14%, preferably 8.5% Cr 10%; 1.5% Mo + W / 2 3.0%, preferably 1.5 Mo + W / 2.5%; - 1.0% Al 2.0%, preferably 1.0% Al 1.5%; 0.5% Ti 2.0%, preferably 1.10% Ti 1.55%; - 2% Co 9%, preferably 2.5% Co 6.5%; better between 2.50 and 3.50%; traces N 0.030%, preferably 0.0060%; traces O 0.020%, preferably 0.0050%; the rest being iron and impurities resulting from the elaboration; and in that its martensitic transformation start temperature Ms calculated by the formula (1) Ms (° C) = 1302 - 28Si - 50Mn - 63Ni - 42Cr - 30Mo + 20AI - 12Co - 25Cu + 10 [Ti - 4 (C + N)] in which the contents of the various elements are expressed in percentages by weight, is greater than or equal to 50 ° C, preferably greater than or equal to 75 ° C. 2.- martensitic stainless steel according to claim 1, characterized in that the proportion of delta ferrite in its microstructure is less than or equal to 1%. 3. A method for manufacturing a martensitic stainless steel part, characterized in that: - a steel semi-finished product having the composition according to claim 1 is prepared by one of the following processes: * a liquid steel is prepared having the composition of claim 1, and from this liquid steel is poured and solidified an ingot and converted into a semi-product by at least one heat transformation; a semi-finished sintered product made of a steel having the composition according to claim 1 is prepared by powder metallurgy; the semi-finished product in the austenitic range is brought into complete solution at a temperature of between 800 and 940 ° C. ; quenching of the semi-finished product is carried out to a final quenching temperature of less than or equal to -60 ° C., preferably less than or equal to -75 ° C .; aging is carried out between 450 and 600 ° C. for 4 to 32 hours. 4.- Method according to claim 3, characterized in that sinks and solidifies an ingot, and in that, between the solidification of the ingot and the dissolution of the semi-finished product, homogenization of the ingot or half is carried out -produced at 1200-1300 ° C for at least 24 hours. 5. A process according to claim 3 or 4, characterized in that between quenching and aging, a cold transformation of the semi-finished product is carried out. 6. A process according to one of claims 3 to 5, characterized in that the quenching is carried out in two stages, in two different quenching media. 7. A process according to claim 6, characterized in that the first quenching step is carried out in water. 8.- Method according to one of claims 3 to 6, characterized in that the liquid steel is prepared by a double vacuum melting treatment, the second vacuum treatment being a remelting treatment ESR or VAR. 9. Martensitic stainless steel part, characterized in that it was prepared by the method according to one of claims 3 to 8. 10. Martensitic stainless steel part according to claim 9, characterized in that it is a piece of aeronautical structure.