CA2930140A1 - Martensitic stainless steel, part made of said steel and method for manufacturing same - Google Patents

Abstract

The invention relates to martensitic stainless steel, characterised by having the following composition: traces = C = 0.030%; traces = Si = 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 = O = 0.020%; the balance being iron and production impurities; and in that the martensitic transformation starting 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 weight percentages, is no lower than 50 °C, preferably no lower than 75 °C. The invention also relates to a part made of said steel and to the method for manufacturing same.

Description

WO 2015/07526

2 Martensitic stainless steel, part made of this steel and its method of manufacturing The present invention relates to stainless steels with high resistance to traction and toughness, intended in particular for the manufacture of structure aeronautics, 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 have been used, says more usually 300M, and containing, in particular, 0.40% of C, 1.80% of Ni, 0.85% of Cr and 0.40% Mo. These are weight percentages, as will all contents cited in the text. After adequate heat treatments, this steel can present a tensile strength Rm of over 1930 MPa and a toughness K1 of more than 55 MPa.m1 / 2. It would be advantageous to have available steel with more of these mechanical properties, high properties of corrosion resistance.
For this reason, different shades have been developed, but none of them gives satisfaction.
The shade described in US-A-3,556,776 and for which typically, C 5 0.050%, Si 0.6%, Mn 0.5%, S 5 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 5 0.050%, has a level of mechanical strength too low, less than 1800 MPa.
The shade described in US-B-7 901 519, for which, typically, C 5 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%, also has insufficient Rm.
The shade described in US-A-5 855 844, for which, typically, C 5 0.030%, Si 0.75%, Mn 1%, S 5 0.020%, P 5 0.040%, Cr = 10-13%, Ni =
10,5-11.6%, Mo = 0.25-1.5%, Al 5 0.25%, Ti = 1.5-1.8%, Cu 0.95%, Nb 5 0.3%, N 5 0.030%
B, 0.010% also has insufficient Rm.
The shade described in the document US-A-2003/0049153, for which, typically, C 5 0.030%, Si 0.5%, Mn 0.5%, S 0.500%, P 0.500%, Cr = 9-13%, Ni = 7-9%, Mo = 3-6%, Al = 1-1.5%, Ti 1%, Co = 5-11%, Cu 50.75%, Nb 5 1%, N
0, 030%, 0 0, 020%, B 5 0, 0100%, could present the levels of properties desired mechanical properties, but would have insufficient corrosion resistance.
She could also not be sufficiently fit to be put in the form of massive pieces, because she has been developed for the manufacture of thin products. During the treatments thermal, it must be dissolved at a temperature generally high, from 930 to 980 C.
WO-A-2012/002208 discloses a steel of typical composition C 5 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 5 0.010%, O 5 0.005%
have good mechanical properties with regard to the main of they have been cited. But its ductility would be insufficient if there added more than 1% of Al.
The dissolution is always carried out at a very high temperature, 940 to 1050 C, for 1 / 2h to 3h, so as to be sufficiently complete without lead a excessive grain growth.
EP-A-1 896 624 discloses a steel of typical composition C 5 0.025%, If 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 5 0.006%, 0.005%. he at the advantage of containing little or no Co which is an expensive element, and tolerate put in solution at temperatures not very high (850-950 C), so with a less energy expenditure and lower risk of grain growth. But his compromise toughness-toughness is not as favorable as this who would desirable.
The object of the invention is to propose a martensitic stainless steel structural hardening simultaneously exhibiting resistance properties to the Rm tensile and K1 toughness, high, high corrosion resistance and an excellent ability to form massive pieces.
For this purpose, the subject of the invention is a martensitic stainless steel, characterized in that its composition is, in percentages by weight:
traces C 5 0.030%, preferably 0.010%;
traces 5 Si 5 0.25%, preferably 0.10%;
traces 5 Mn 5 0.25%, preferably 0.10%;
traces 5 S 0.020%, preferably 0.005%;
trace amounts 0.040%, preferably 0.020%;
8% or 14%, preferably 11.3% or 12.5%;
- 8% 5 Cr 5 14%, preferably 8.5% 5 Cr 5 10%;
1.5% Mo + W / 2 53.0%, preferably 1.5 Mo + W / 2.5 / 0;
1.0% Al 2 2.0%, preferably 1.0% Al 1.5%;
0.5% Ti 2.0%, preferably 1.10% Ti 1.55%;
- 2% Co 5 9%, preferably 2.5% Co 5 6.5%; better between 2.50 and 3.50%;

traces 5 N 5 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 different elements are expressed in percentages weight, is greater than or equal to 50 C, preferably greater than or equal to 75 C.
Preferably 1.05% Al 2.0%, and preferably 1.05% Al 1.5%.
The proportion of delta ferrite in its microstructure is preferably lower or equal to 1%.
The subject of the invention is also a process for manufacturing a part made of steel martensitic stainless, characterized in that:
a steel semi-finished product having the aforementioned composition is prepared by one of of the following processes:
a liquid steel having the above-mentioned composition is prepared, and from this liquid steel, one sinks and solidifies an ingot and transforms it into a half-produced by at least one hot transformation;
* metallurgically prepared powders a sintered half-product made of a steel having the aforesaid composition;
a complete solution of the semi-product in the field is carried out austenitic at a temperature between 800 and 940 C;
the quenching of the semi-finished product is carried out to a final temperature of temper 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 solid ingot and the dissolution of the half-product, homogenization of the ingot or semi-finished product can be achieved.

1300 C for at least 24 hours.
Between quenching and aging, a transformation can be achieved cold semi-finished product.
The quenching can be carried out in two stages, in two quenching media different.
The first quenching step is carried out in water.

4 The liquid steel can be prepared by a double melting treatment under empty, the second vacuum treatment being an ESR or VAR remelting treatment.
The invention also relates to a stainless steel part martensitic characterized in that it has been prepared by the above method.
It can be a piece of aeronautical structure.
As will be understood, the invention consists in proposing a steel grade martensitic stainless which after undergoing treatments thermomechanical which, in combination with said grade, are also part of the invention, introduced to both properties of tensile strength, toughness and ductility that make it adapted to its use for the manufacture of massive parts such as trains landing, as well as excellent corrosion resistance over in shades already used for this purpose.
The steels of the invention have a martensitic structure which is obtained:
- by a complete solution in the austenitic domain, so conducted above the Ac3 temperature of the steel concerned; for the shade considered, this solution temperature is from 800 to 940 C; the solution is realized for a period of 30 minutes to 3 hours; a temperature of the order of 850 C
combined with duration of the order of 1 h 30 min are generally adequate for both get a complete dissolution and moderate grain growth; a grain too much 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 quenching being extended to a temperature cryogenic, ie -60 C or lower, preferably up to -75 C or higher low, typically up to -80 C.
The duration of maintenance in the cryogenic medium must be sufficient for the cooling to the chosen temperature and the desired transformations affect the piece of steel in all its volume. This duration therefore depends strongly on the mass and dimensions of the treated part, and is, of course, all the higher by for example, the treated part is thick. Different quenching media can to be used:
air, water, oil, gas, polymer, liquid nitrogen, dry ice (no limiting), and the quenching is not necessarily performed with a very fast cooling rate high.
It is possible 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 parts most massive, water is a prime tempering medium because it allows to ensure that the the heart of the room is cooled quickly enough. The temperature of beginning of quenching is preferably the temperature at which the setting took place solution, for to guarantee that between the dissolution and quenching it does not occur transformations difficult to control metallurgical affect the

5 final mechanical properties of the product If quenching is interrupted for a period of time below Ms and above above the end of martensitic transformation temperature Mf, the interruption must be short to avoid the risk of blocking the transformation when tempering will be resumed.
Another possibility would be to interrupt the quenching above Ms and the then resume until the cryogenic temperature.
One possible advantage of such interruptions is that they prevent of must immediately use a cryogenic quenching medium, so avoid to have a very high initial cooling rate which could lead to the appearance of cracking (superficial cracking), or cracks inside the half product that could be due to martensitic transformation phenomena differential between the surface and the still warm heart of the half-product if it is relatively thick.
But in practice it is better to do the quenching in one step, for more of convenience and not to risk undesirable metallurgical effects on the microstructure of the steel, because a quenching in two steps is often difficult to master the final temperature of the first stage and the homogeneity of its effects in the treated room.
The transition to cryogenic temperature can be done in a solid medium, gaseous or liquid depending on the available treatment technology. To get a completely martensitic structure the beginning of the transformation martensitic at cooling, Ms, must be mastered. This Ms point depends on the composition of the alloy and is calculated according to Equation (1):
(1) Ms (C) = 1302-285i - 50Mn - 63Ni - 42Cr - 30Mo + 20AI - 12Co - 25Cu +
10 [Ti - 4 (C + N)]
in which the contents of the different elements are expressed in percentages 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 met, the steel has residual quenching austenite which is detrimental to properties mechanical properties, in particular the breaking strength.

6 After dissolving and tempering extended to temperature cryogenic target, the final mechanical properties are obtained at the end a aging between 450 and 600 C lasting from 4 to 32 hours. The hardening obtained is ensured by the formation of NiAl and Ni3Ti intermetallic precipitates size nanoscale. During aging, reversion austenite can occur train and contribute to the toughness of steel. This aging can, possibly, to be interrupted using a water quench to improve toughness.
The final structure, for applications envisaged in a privileged way, especially in aeronautics, must be free of delta ferrite which degrades the mechanical properties. A maximum of 1% delta ferrite is tolerable. The composition steel according to the invention is chosen, precisely, to avoid as much as possible that delta ferrite subsists at the end of the treatments performed at the time of work of process according to the invention. From this point of view, it is very preferable, for ensure this lack of delta ferrite subsistence, as the Cr eq / Ni eq ratio of steel, that is to say the ratio of the weighted sum of the contents of the main elements alphagenic as Cr (chromium equivalent) and the weighted sum of the grades of the principal gammagens such as Ni (nickel equivalent), 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 the ingots which can be detrimental to the mechanical properties, especially when the mechanical stress is in the cross direction, and the contents in inclusions Oxides and nitrides should be minimized as much as possible. In this effect, a mode preferred preparation of steels according to the invention is a double fusion elaboration Vacuum Induction Melting (VIM) then casting of ingot steel to obtain an electrode, which is then treated by arc reflow under vacuum (Vacuum Arc Remelting, VAR) or by remelting under a slag electrically (Electroslag Remelting, ESR). Vacuum processing makes it possible to avoid the oxidation of Al and Ti by the air, therefore excessive formation of inclusions oxidized, and also allow some of the nitrogen and dissolved oxygen to be removed. We can obtain high fatigue life times.
After obtaining the solidified ingot, the hot transformations are carried out (rolling, forging, stamping ...) which put it in the form of a half product (bar, flat, block, forged or stamped piece ...) to give it dimensions less close of its final dimensions. These hot transformations are everything simply those

7 which are usual for the semi-finished products of general compositions comparable those of the invention, both with regard to deformations and temperatures treatment.
Preferably, a homogenization treatment of the ingot or of half-product at a temperature of 1200 to 1300 C for at least 24 hours for limit the segregation of the various elements present and thus more easily obtaining targeted mechanical properties. However, homogenization does not generally, preferably, not during the last hot shaping operations or after these, in order to keep more certainly an acceptable grain size on the products, according to their future use.
The half-product then undergoes, according to the invention, a heat treatment consisting in:
- A dissolution between 800 and 940 C practiced, as it is classic, for a time sufficient to dissolve the precipitates present in the entire semi-finished product and therefore closely depends on the dimensions of said half-product, followed by quenching to a temperature of -60 ° C or lower, preferably -75 ° C or lower, said starting quench, of preferably at a temperature close to the dissolution temperature, and can be carried out in two stages separated by a stay at a intermediate temperature (eg ambient, or a temperature between the beginning and the end of martensitic transformation, or a temperature above the start temperature of the transformation martensitic);
- Then, possibly, cold forming of the semi-finished product ;
- Then aging between 450 and 600 C for 4 to 32 hours allowing to balance the properties of strength, toughness and ductility according to the criteria following:
= The maximum resistance reached decreases when the temperature of Aging is growing, but vice versa ductility and toughness grow;
= The aging time required to cause hardening given increases when the aging temperature decreases;
= At each temperature level, the resistance goes through a maximum for a definite period, which is called hardening peak;

8 = For each target level of resistance, which can be achieved by many couples of time-temperature aging variables, there is only one such a combination that gives the best resistance / ductility compromise to steel;
these optimal conditions correspond to an onset of overgrowth of the structure, and are obtained when going beyond the peak of hardening;
the skilled person can determine experimentally what is the couple optimal with the help of reflections and routine tests.
The alloying elements of the steel according to the invention are present in the quantities indicated for the reasons that will be exposed. As we said, the 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 in the state of residual element resulting from the merging of raw materials and the elaboration, without any adding voluntary action. It could form Cr carbides of type M23C6 and penalize thus the resistance to corrosion by capturing Cr which is thus no longer available for to ensure the stainless character of the steel satisfactorily. He could as well associate with Ti to form carbides and carbonitrides harmful to the held in fatigue, 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% for better ensure the right compromise between Rm and K1C sought. Typically he is only one residual element not added voluntarily. It tends to lower Ms (see equation (1)) and weaken steel, hence its undesirable nature in more important that this which 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 in substitution partial Ni for avoid the presence of delta ferrite and contribute to the presence of austenite reversion when aging curing. But the ease with which he evaporates during the vacuum treatments makes it difficult to control and leads to a fouling dust extraction devices for furnace fumes. We do not advocate a significant presence of Mn in the steels of the invention.
The content of S is at most 0.020% (200 ppm), preferably at most 0.005%
(50 ppm), to better ensure the right compromise between Rm and K1C sought. The still he is present in the residual state and, if necessary, its content must be controlled by a choice

9 careful raw materials and / or a metallurgical treatment of desulfurization during of the step of melting and adjusting the composition of the steel. It reduces the tenacity by segregation at the grain boundaries, and form harmful sulphides for properties mechanical.
The P content is at most 0.040% (400 ppm), preferably at most 0.020%
(200 ppm) to better ensure the right compromise between Rm and K1C sought. he this is still a residual element that tends to segregate at the grain boundaries and, therefore, decreases the tenacity.
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 delta ferrite during solution and homogenization. But it must also be maintained at a sufficient level low for ensure a complete martensitic transformation during the quenching strongly tendency to lower Ms according to equation (1). On the other hand, he participates in hardening of steel during aging by precipitation of NiAl hardening phases and Ni3Ti which give the steels of the invention their level of mechanical strength. He has also for function of forming reversion austenite during aging, which finely precipitates between the slats of martensite and provides their ductility and tenacity to steels the invention.
The Cr content is between 8 and 14%, preferably between 8.5 and 10%.
he is the main element that provides resistance to corrosion, which justifies the limit lower by 8%. But we must limit its content to 14% so that it does not contribute not to stabilization of delta ferrite and that it does not pass Ms, calculated according to the equation (1), 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 corrosion resistance and is likely to form a phase hardening Fe7Mo6. However, adding an excessive amount of MB may lead to the formation of a phase ji. Fe6Mo7 and thus decrease the amount of MB
available to limit corrosion. Optionally, at least part of the MB can be replaced by W.
is well known that in steels, these two elements are functionally often comparable, and that, at equal percentage weight, W is twice as effective than Mo.
The Al content is between 1.0 and 2.0%, preferably between 1.05 and 2.0%, better between 1.0 and 1.5%, optimally between 1.05 and 1.5%. During the aging it forms the NiAl hardening phase. The IA is usually known as degrade the ductility, but this disadvantage is negated by the possibility offered by the invention of perform 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 sentence Ni 3 Ti. It also makes it possible to fix C and N in the form of carbides and carbonitrides of Ti and to avoid the harmful effects of C. However, as has been said, these carbides and carbonitrides are harmful to fatigue behavior, and we can not afford to train Too much. The contents of C, N and Ti must therefore be maintained in the prescribed limits.

10 The Co content is between 2 and 9%, preferably between 2.50 and 6.5%, better between 2.50 and 3.50%. It helps to stabilize the austenite with temperatures of homogenisation and dissolution, and thus to avoid the formation of ferrite delta. he contributes to the hardening by its presence in solid solution and also in that he favors the precipitation of NiAl and Ni3Ti phases. It can be added as a substitution to Ni so as to raise the temperature Ms and ensure that it is above 50 C.
to steel described in EP-A-1 896 624 where Co must be at most 2%, the goal is here to use Co for contribute significantly to hardening, this in combination with the other present elements and heat treatments required. Content preferential aim of 2.50-3.50% represents the best compromise between the cost of steel and its performance.
N must not exceed 0.030% (300 ppm), preferably not more than 0.0060% (60 ppm) to better ensure the right compromise between Rm and K1C sought. We do not add voluntarily nitrogen to the liquid metal, and the vacuum treatments that are generally used during processing to protect steel liquid against atmospheric nitrogen recovery, or even to remove some of the nitrogen dissolved. N is unfavorable to the ductility of steel and forms angular Ti nitrides which are likely to be sites of crack initiation during solicitations in tired.
0 must be at most 0.020% (200 ppm), preferably 0.0050% (50 ppm) for better to ensure the good compromise between Rm and K1C sought. He is also unfavorable ductility, and the oxidized inclusions it forms are also potentially sites crack initiation in fatigue. The content of 0 should be chosen according to criteria common to those skilled in the art, depending on the mechanical characteristics accurate required for the final product.
In general, the mechanical properties of the steel of the invention are adversely affected by inclusions of oxides and nitrides.
The use of

11 development processes to minimize their presence in the final steel (VIM, ESR, VAR) is particularly preferred for this reason.
The other elements present in the steel of the invention are iron and the impurities resulting from the elaboration.
It should be understood that the ranges given as preferential for each element are independent of each other, that is, the composition of steel may be in these preferential ranges only for certain elements only.
Tests were carried out on samples from ingots casting having the compositions set forth in Table 1. The compositions of samples A
to E correspond to reference steels: A, D and E comply with teaching EP-A-1 896 624. B and C are two reference examples which make it possible to bring into value 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 of 6 kg ingots, and the other samples are from ingots of 150 kg. Ingots 6 kg were first developed for a first validation of the concept of the invention, and their encouraging properties have led to 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 produce directly tests of traction, while it was necessary to form the ingots of 150 kg to extract then the samples on which the measurements of the parameters governing the tenacity have been made.
0% If% Mn% S% P%
Neither% 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 F
0.0130 0.064 <0.010 0.0011 <0.0050 8.21 10.63 4.99 1.19 1 <0.0020 0.030 <0.010 0.0004 <0.0050 12.41 9.70 2.03 1.38 0.0039 0.022 <0.010 0.0006 <0.0050 12.01 9.66 2.02 1.40 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 0.0026 0.035 <0.010 0.0005 <0.0050 9.52 9.80 2.06 1.37

12 6 0.0059 0.046 <0.010 0.0013 <0.0050 11.86 10.04 2.01 1.25 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 0.0130 0.029 <0.010 0.0014 <0.0050 11.47 10.14 1.99 1.32 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 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) according to equation (1) At 1.18 <0.010 0.0029 0.0008 remain 85 B 1,17 6,11 0,0021 0,0009 remain 16 C 1.16 9.11 0.0006 0.0009 remains -19 D 1.15 <0.010 0.0024 0.0010 remain 88 E 1.17 <0.010 0.0022 0.0013 remains 72 F 0.051 8.22 0.0018 0.0008 remains 210 1 1,18 3,06 0,0015 0,0015 remaining 53 2 1,18 3,07 0,0017 0.0019 left 81 3 1,17 6,13 0,0014 0,0012 remaining 73 4 1,18 6,11 0,0015 0,0010 remain 135 5 1,18 6,14 0,0004 0,0013 remaining 193 6 1,20 3,19 0,0009 0,0010 left 69 7 1.23 6.02 0.0008 0.0006 remains 63 8 1,17 6,22 0,0021 0,0026 remain 128 9 1,16 5,00 0,0009 0,0007 remaining 70 10 1.22 3.09 0.0016 0.0006 remains 88 11 1.17 6.20 0.0019 0.0008 remaining 111 12 1.23 3.12 0.0039 0.0009 remaining 79 13 1.21 3.09 0.0029 0.0005 remains 106 14 1.45 3.06 0.0044 0.0003 remaining 91

15 0.94 3.03 0.0036 0.0013 remains 112

16 1.45 4.08 0.0016 0.0010 remains 124 Table 1: Compositions of the test samples, with their calculated Ms according to equation (1) The 6 kg ingots (A, B, C 1 to 5) were prepared by vacuum treatment of liquid metal before casting. They were homogenized at 1250 C during 48h. they have then spun after heating to 940 C to form bars of diameter 22 mm. Table 2 indicates which treatments these bars then underwent, and what were their main final mechanical properties measured in long :
tensile strength Rm, yield strength 0.2% Rp0.2, lengthening to break A, necking at break Z, Vickers hardness. The small size of spun samples did not allow to extract test tubes that would have the dimensions required to carry out toughness tests.
Ech. Heat treatment Aging Rm RP0,2 AZ
Hardness Setting Temperature Temperature Duration (MPa) (MPa) (%) (%) (Hv) quenched (C) (h) solution (C) (C / h) A 850 / 1.5 -80 510 16 1868 1758 11 48 548 B 850 / 1.5 -80 510 16 Unrealized tests 216 C 850 / 1.5 -80 510 16 (too much austenite in the 146 structure) 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 the samples from the 6 kg ingots It should be noted that the excessive presence of austenite in the structure translated for reference samples B and C, by a very low hardness, which was the index 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, as far as grades individual components of each element, complied with the requirements of the invention but which, taken together, provided a transformation temperature Martensitic Ms too low (less than 50 C). Quenching, performed under the conditions of experimentation, which correspond to what is usually practiced industrially, did not allow to obtain a structure sufficiently martensitic in the case of these samples. This shows that the condition on Ms is important to consider in the context of the invention.
Concerning ingots of 150 kg (D, E, 6 to 16), they were developed under empty, cast, then also remelted under vacuum by the VAR process to give ingots of diameter 200 mm. They were then homogenized at 1250 ° C. for 48 hours.
then forged at this temperature into half-products of octagonal section of 110 mm, then, after reheating to 940 C, again forged, this time in bars of section 80x40 mm. Table 3 sets out the conditions under which the treatments thermal conditions that followed, and the mechanical properties measured in the long on the samples. Compared with the tests in Table 2, no measure of the hardness that would have duplicated the Rm measurements, and we realized tests resilience (Kv measurement) and toughness (K1C measurement).
Heat treatment Aging Rm R130,2 AZ
Kv K1C
Set Temperature Temperature Duration (MPa) (MPa) (%) (%) (J) (MPa.m1 / 2) solution of (c) (h) (C / h) quenching ( vs) 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 E 850 / 1,5 -80 490 16 1872 1712 12 47 10 46 E 850 / 1.5 -80 510 E 900 / 1,5 -80 510 16 1885 1761 12 48 7 56 F 930 / 1.5 -80 540 4 1949 1809 14 52 8 50 F 930 / 1,5 -80 510 4 1908 1756 12 48 14 52 6,850 / 1.5 -80 490 16 1892 1748 13 53 15 67 6,850 / 1.5 -80 510 16 1814 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 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 10 850 / 1.5 -80 500 16 1949 10 850 / 1.5 -80 510 16 1917 1787 13 59 18 65 10 850 / 1.5 -80 530 16 1785 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 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 12,850 / 1.5 -80 530 16 1835 1721 13 61 16 74 13,850 / 1.5 -80 490 16 2028 13,850 / 1,5 -80 510 16 1982 1870 12 56 15 55 13 850 / 1.5 -80 530 16 1851 14,850 / 1.5 -80 490 16 1991 14,850 / 1.5 -80 510 16 1943 14,850 / 1,5 -80 530 16 1818 1698 11 48 16 72 14,875 / 1.5 -80 490 16 1984 14,875 / 1.5 -80 510 16 1940 14,875 / 1.5 -80 530 16 1819 15,850 / 1.5 -80,490 15 850 / 1.5 -80 510 15,850 / 1.5 -80,530 16,850 / 1.5 -80,490 16,850 / 1.5 -80,510 16,850 / 1.5 -80,530 Table 3: Treatment conditions and mechanical properties of the samples from 150 kg ingots The properties of the different samples can be commented as follows.
Reference samples A, D and E correspond to steels with a content of Co low or zero described in EP-A-1 896 624. Compared to the steels of the invention, we see that their Rm is relatively small.
Reference samples B and C have an MS of less than 50 C, so too weak to conform to the invention. This explains the excessive presence austenite residual that prevents a sufficient Rm, resulting in a low hardness.
Reference sample F shows that a high Mo content and a Ti content is too low compared to the requirements of the invention lead to obtaining mechanical properties that are only at the level of those of others samples reference.
Sample 1 is in accordance with the invention, but has a Ms less than the optimum 75 C and more. Its Rm is therefore relatively weak and will not be suitable for all conceivable applications. We can say the same thing, but in a lesser extent, sample 3.
Sample 2, on the other hand, has an optimum MS, and its Rm of 1947 MPa is excellent.
Samples 4 and 5, at high Ms because of their significant substitution of Or 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 who has also about 3% of Co. Similarly for sample 7 which has a content of Co about 6%, but not as good as sample 4 because of its higher low Ms.

17 The very high Rm of Sample 8 is due to its high Ms combined with a Co content about 6%.
Sample 9 at 5% Co has a Ms less than optimum and its Rm is relatively limited. This shows that a relatively high content of Co is not sufficient to ensure a high Rm within the scope of the invention.
Samples 10 and 12 have the best compromise between Rm and K1C. In fact, their compositions are consistent with the contents preferential all the elements.
Sample 11 has a high Ms and a high Rm. The balance between Rm and K1C is better than for sample 8 because of better balancing between Ni and Cr contents.
The comparison between samples 13, 14 and 15 highlights the effect advantage of the partial substitution of Al by Ti: the sample 14 is whoever has the better 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.
Sample 16 has a high Ms. Its Rm is equivalent to that of sample 12 but its K1C is less favorable because of a Cr content little more strong.
Figure 1 shows the results of Table 3 in terms of trade-off between Rm and K1C for samples from 150 kg ingots, these being the only ones for which toughness was measured. Overall, K1C decreases as Rm increases, and steels according to the invention have a better compromise between these two properties that reference steels D and E whose compositions are relatively close to the invention except on the content of Co.
For reference samples, an Rm of 1701 MPa corresponds to one toughness of 66 MPA.m1 / 2. This steel would therefore not be suitable for uses preferred because of its very inadequate RM. The maximum Rm of reference samples is 1952 MPa, which would be correct for said uses, but the corresponding tenacity is only 43 MPa.m1 / 2, which would be very insufficient.
The best resistance / toughness compromises are obtained for Rm from 1845 to MPa, to which correspond the tenacity of the order of 46 to 56 MPA.m1 / 2.
These mechanical properties taken as a whole are therefore not as favorable that for type 300M carbon steels.
With regard to the samples according to the invention, we see in FIG.
that a very good trade-off between Rm and K1C is usually obtained for Rm of the order of

18 1950 MPa, which correspond to K1Cs of the order of 46 to 63 MPa.m1 / 2, the most often greater than 50 MPa.m1 / 2. We therefore fall back on the orders of magnitude of properties corresponding 300M steels We also see that if a decrease in Rm was acceptable, the tenacity would be increased in large proportions, and vice versa. Steels according to the invention thus provide the user with great flexibility in the choice of of their properties, which can be modulated by the composition, the heat treatments and the final aging chosen in the framework that has been cited.
With regard to ductility, the values of A% and Z% of the samples according to the invention are very comparable to those obtained on the steels of type 300M.
The invention therefore provides no degradation with respect to the 300M of this point of view.
On some of these same samples from ingots of 150 kg (samples D, 6 to 8 and 10 to 16), tests of corrosion at salt spray, in a 50 g / l aqueous solution of 35 C NaCl.
had all, previously, subjected to the same 850 heat treatment solution VS
for 1 h 30 min, tempering at -80 ° C. and aging at 510 ° C.
for 16 hours.
None of these samples showed signs of corrosion after 200 h exposure.
The steels according to the invention therefore do not see their corrosion results.
in fog saline degraded compared to the reference steel D which does not contain Co.
Stress corrosion tests have also been carried out in a medium aqueous solution containing 3.5% NaCl at 23 ° C on samples E and in solution at 850 ° C. for 1 h 30 min, tempered at -80 ° C. and aged at 510 ° C.
during 16 h. The K1C toughness in the air and the times before failure were measured for loads equal to 75% of K1C. In both cases, the samples withstood more than 500 h before the break. This is a good result, and the invention does not degrade so not holding at stress corrosion compared to reference steels without Co.
The steels according to the invention can therefore be substituted mechanically 300M steels, in addition to the fact that they present of the corrosion resistance performance in salt spray and corrosion under constraints which are quite favorable, since they are comparable to those stainless steels which could be considered to replace the 300M.
It must be understood that throughout this description, the solidified ingot who is poured from the liquid metal may have any shape likely to drive, after various deformations, to a final product having the shape and dimensions desired for its use. In particular, the casting in a conventional mold with of a background and

19 fixed sidewalls is only one of the possible ways to proceed, and the different continuous casting processes in a bottomless mold with fixed walls or mobile can be used to effect the solidification of the ingot.
An alternative solution to the one just described is to realize the Following heat treatments on a semi-finished product derived from an ingot processed hot by rolling, forging, stamping or other, but on a sintered semi-finished product made by powder metallurgy, to which it would therefore be possible to confer directly a shape, possibly complex, and dimensions very close to those of the room final. The powder used is a metal powder that has the composition steel according to the invention. In his case, a homogenization of the sintered semi-product is not necessary. But the manufacturing process can include beforehand the sintering proper, as is conventional for those skilled in the art, a step of pre-sintering performed under conditions less severe than sintering in terms of temperature and / or duration. In general, the sintering process is pipe as the person skilled in the art would do by using his usual knowledge.

Claims (12)

20 1.- martensitic stainless steel, characterized in that its composition is, in weight percentages:
traces <= C <= 0.030%, preferably <= 0.010%;
- traces <= If <= 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 <= 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 <= 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 formula (1) Ms (° C) = 1302 - 28Si - 50Mn - 63Ni - 42Cr - 30Mo + 20AI - 12Co -25Cu + 10 [Ti -4 (C + N)]
in which the contents of the different elements are expressed in percentages weight, is greater than or equal to 50 ° C, preferably greater than or equal to equal to 75 ° C. 2. Martensitic stainless steel according to claim 1, characterized in that than 1.05% <= Al <= 2.0%, and preferably 1.05% <= Al <=
1.5%. 3. Martensitic stainless steel according to claim 1 or 2, characterized in this that Creq / Nieq <= 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 4. Martensitic stainless steel according to one of claims 1 to 3, characterized in that the proportion of delta ferrite in its microstructure is lower than or equal to 1%. 5.- Method for manufacturing a martensitic stainless steel part, characterized in that a steel semi-finished product having the composition according to one of the Claims 1 to 4 by one of the following methods:
a liquid steel having the composition according to one of the 1 to 4, and from this liquid steel, one sinks and solidifies an ingot and converts it into a half-product by at least one hot transformation;
* metallurgically prepared powders a sintered half-product made of a steel having the composition according to one of claims 1 to 4;
a complete solution of the semi-product in the field is carried out austenitic at a temperature between 800 and 940 ° C;
the quenching of the semi-finished product is carried out to a final temperature of temper 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. 6. A process according to claim 5, characterized in that it flows and solidifies a ingot, and in that, between the solidification of the ingot and the dissolution in solution the semi-finished product, the ingot or semi-finished product is homogenized at 1200.degree.
1300 ° C during the less than 24h. 7. Process according to claim 5 or 6, characterized in that between quenching and aging, a cold transformation of the semi-finished product is carried out. 8. Method according to one of claims 5 to 7, characterized in that the temper is carried out in two stages, in two different quenching media. 9. A process according to claim 8, characterized in that the first step of quenching is carried out in the water. 10.- Method according to one of claims 5 to 9, characterized in that prepare liquid steel by a double vacuum melting treatment, the second treatment under empty being an ESR or VAR remelting treatment. 11.- martensitic stainless steel part, characterized in that it has been prepared by the process according to one of claims 5 to 10. 12.- martensitic stainless steel part according to claim 11, characterized in that it is a piece of aeronautical structure.