CA2930140C - 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 corrosion.

traction and tenacity, intended in particular for the manufacture of parts of structure aeronautics, in particular for landing gear.
Martensitic age-hardening stainless steels have been developed with the aim of meeting the needs linked, in particular, to this application.
Traditionally, non-stainless steels of the 40NiSiCrMo7 type are used, say more usually 300M, and containing, in particular, 0.40% C, 1.80% Ni, 0.85%
Cr and 0.40% Mo. These are weight percentages, as will be all contents cited in the text. After adequate heat treatments, this steel can present a tensile strength Rm of more than 1930 MPa and a toughness K1, of over 55 MPa.m1/2. It would be advantageous to be able to have steels having, in more of these mechanical properties, high corrosion resistance properties.
For this purpose, different shades have been developed, but none of them give full satisfaction.
The grade described in document US-A-3,556,776 and for which typically, C 5 0.050%, Si 5 0.6%, Mn 5 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 5 0.5%, Nb 5 0.75%, N 5 0.050%, exhibits a level of mechanical strength too low, less than 1800 MPa.
The grade 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 5 0.0010%, also has insufficient Rm.
The grade described in US-A-5,855,844, for which, typically, C 5 0.030%, Si 5 0.75%, Mn 5 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 5 0.95%, Nb 5 0.3%, N 5 0.030%, B 5 0.010% also has an insufficient Rm.
The grade described in document US-A-2003/0049153, for which, typically, C 5 0.030%, Si 5 0.5%, Mn 5 0.5%, S 5 0.0025%, P 5 0.0040%, Cr = 9-13%, Ni = 7-9%, Mo = 3-6%, Al = 1-1.5%, Ti 5 1%, Co = 5-11%, Cu 50.75%, Nb 5 1%, N
5 0.030%, 0 5 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 suitable to be formed into massive pieces, because she was developed for the manufacture of thin products. During the treatments thermal, it must be put into solution at a temperature generally high, from 930 to 980 C.
Document WO-A-2012/002208 describes a steel of typical composition C 5 0.200%, Si 5 0.1%, Mn 5 0.1%, S 5 0.008%, P 5 0.030%, Cr = 9.5-14%, Ni = 7-14%, MB =
0.5-3%, Al = 0.25-1%, Ti = 0.75-2.5%, Co 5 3.5%, Cu 5 0.1%, N 5 0.010%, 0 5 0.005%, exhibit good mechanical properties with respect to the main of between those who have been cited. But its ductility would be insufficient if we added more than 1% Al.
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 cause a excessive grain coarsening.
Document EP-A-1 896 624 describes a steel of typical composition C 5 0.025%, Si 5 0.25%, Mn 5 3%, S 5 0.005%, P 5 0.020%, Cr = 9-13%, Ni = 8-14%, Mo = 1.5-

3%, Al = 1-2%, Ti = 0.5-1.5%, Co 5 2%, Cu 5 0.5%, W 5 1%, N 5 0.006%, 0 5 0.005%. He has the advantage of containing little or no Co which is an expensive element, and of tolerate put into solution at not very high temperatures (850-950 C), therefore with a less energy expenditure and less risk of grain enlargement. But his tensile strength-toughness trade-off is not as favorable as this who would be desirable.
The object of the invention is to provide a martensitic stainless steel with structural hardening simultaneously exhibiting strength properties to the high tensile Rm and toughness K1, high corrosion resistance and an excellent aptitude for shaping into massive parts.
To this end, the subject of the invention is a martensitic stainless steel, characterized in that its composition is, in weight percentages:
- traces 5 C 5 0.030%, preferably 5 0.010%;
- traces 5 Si 5 0.25%, preferably 5 0.10%;
- traces 5 Mn 5 0.25%, preferably 5 0.10%;
- traces 5 S 5 0.020%, preferably 5 0.005%;
- traces 5 P 5 0.040%, preferably 5 0.020%;
- 8% 5 Ni 5 14%, preferably 11.3% 5 Ni 5 12.5%;
- 8% 5 Cr 5 14%, preferably 8.5% 5 Cr 5 10%;
- 1.5% 5 Mo +W/2 53.0%, preferably 1.5 5 Mo +W/2 5 2.5 /0;
- 1.0% 5 Al 5 2.0%, preferably 1.0% 5 Al 5 1.5%;
- 0.5% 5 Ti 5 2.0%, preferably 1.10% 5 Ti 5 1.55%;
- 2% 5 Co 5 9%, preferably 2.5% 5 Co 5 6.5%; better between 2.50 and 3.50%;

- traces 5 N 5 0.030%, preferably 5 0.0060%;
- traces 5 0 5 0.020%, preferably 5 0.0050%;
the remainder being iron and impurities resulting from the elaboration;
and in that its martensitic transformation onset 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.
Preferably 1.05% 5 Al 5 2.0%, and more preferably 1.05% 5 Al 5 1.5%.
The proportion of delta ferrite in its microstructure is preferably lower or equal to 1%.
The invention also relates to a method of manufacturing a part in steel martensitic stainless steel, characterized in that:
- a steel semi-finished product having the aforementioned composition is prepared by one from following processes:
* a liquid steel having the aforementioned composition is prepared, and from this liquid steel, we cast and solidify an ingot and transform it into a semi-finished by at least one hot transformation;
* a sintered steel semi-finished product is prepared by powder metallurgy having the aforementioned composition;
- the semi-finished product is completely dissolved in the field austenitic, at a temperature between 800 and 940 C;
- the semi-finished product is quenched to a final temperature of quench 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 the semi-finished product 1300 C for at least 24 hours.
Between quenching and ageing, a transformation can be carried out at cold from semi-finished product.
Quenching can be done in two steps, in two quenching media different.
The first quenching step is carried out in water.

4 Liquid steel can be prepared by double melting treatment under empty, the second vacuum processing being an ESR or VAR reflow processing.
The invention also relates to a stainless steel part martensitic, characterized in that it has been prepared by the above process.
It may be a part of an aeronautical structure.
As will have been understood, the invention consists in proposing a grade of steel martensitic stainless steel which, after being treated thermomechanical suitable which, combined with said nuance, are also an element of the invention, introduced to properties of tensile strength, toughness and ductility which makes it adapted to its use for the manufacture of massive parts such as trains landing, as well as excellent corrosion resistance compared to to the nuances 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, therefore realized above the Ac3 temperature of the steel concerned; for the grade considered, this solution temperature is from 800 to 940 C; dissolving is carried out for a period of 30 minutes to 3 hours; a temperature of about 850 C
combined with a duration of the order of 1 h 30 min are generally adequate for both get a complete dissolution and moderate grain growth; a bit too much rude would be detrimental to the properties of resilience, corrosion under stress and ductility;
- then quenching, preferably carried out from a temperature close to the solution temperature, said quenching being prolonged up to a temperature cryogenic, i.e. -60 C or lower, preferably up to -75 C or higher low, typically down to -80 C.
The duration of maintenance in the cryogenic medium must be sufficient so that the cooling to the chosen temperature and the desired transformations affect the piece of steel in all its volume. This duration therefore strongly depends on the mass and of the dimensions of the part treated, and is, of course, all the higher by example, the treated part is thick. Different quenching media can be used:
air, water, oil, gas, polymer, liquid nitrogen, dry ice (list not limiting), and the quenching is not necessarily carried out with a very high cooling rate.
high.
It is possible to consider successively using 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 quenching medium because it allows to ensure that the core of the part is cooled sufficiently quickly. The temperature of beginning of tempering is, preferably, the temperature at which the setting took place solution, for guarantee that between solution treatment and quenching there is no transformations metallurgical conditions which are difficult to control and which could adversely affect the

5 final mechanical properties of the product If quenching is interrupted for a certain time below Ms and above above the martensitic transformation end temperature Mf, the interruption must be short to avoid the risk of blocking the transformation when quenching will be resumed.
Another possibility would be to interrupt quenching above Ms and the then resume to cryogenic temperature.
A possible advantage of such interruptions is that they avoid of having to immediately use a cryogenic quenching medium, therefore avoiding to have a very high cooling rate from the outset which could lead to the appearance cracks (surface cracks), or cracks inside the half-product which could be due to martensitic transformation phenomena differential between the surface and the still hot core of the semi-finished product if the latter is relatively thick.
But in practice it is preferable to carry out the quenching in one step, for more convenience and to avoid the risk of undesirable metallurgical effects on the microstructure of the steel, since a two-step hardening is often difficult to master as to the final temperature of the first stage and the homogeneity of its effects in the treated part.
The transition to cryogenic temperature can take place in a solid medium, gaseous or liquid depending on the treatment technology available. To to get a fully martensitic structure the beginning of the transformation martensitic cooling, Ms, must be mastered. This point Ms depends on the composition alloy and is calculated according to Equation (1):
(1) Ms (C) = 1302- 285i - 50Mn - 63Ni - 42Cr - 30Mo + 20Al - 12Co - 25Cu +
10[Ti - 4(C+N)]
in which the contents of the various 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 filled, the steel has residual austenite from quenching which is detrimental to properties mechanical, in particular the resistance to rupture.

6 After solution treatment and prolonged quenching to the temperature targeted cryogenic, the final mechanical properties are obtained after of one 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 such as NiAl and Ni3Ti size nanometric. During ageing, reversion austenite can occur train and contribute to the toughness of the steel. This aging may eventually be interrupted using water quenching to improve toughness.
The final structure, for the applications envisaged in a privileged way, particularly 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 of delta ferrite remains at the end of the treatments carried out during the work of process according to the invention. From this point of view, it is very preferable, for ensure this absence of delta ferrite subsistence, that the Cr eq / Ni eq ratio of steel, that is the ratio between the weighted sum of the contents of the main elements alphagens as Cr (equivalent chromium) and the weighted sum of the contents of the main gammagenic elements such as Ni (equivalent nickel), either 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 mechanical properties, particularly when the mechanical stress is in the transverse direction, and the contents of inclusions oxides and nitrides should be minimized as much as possible. In this effect, a mode privileged preparation of steels according to the invention is a double elaboration by fusion under vacuum with induction melting (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 conductive (Electroslag Remelting, ESR). Vacuum-packed preparations avoid the oxidations of Al and Ti by air, therefore the excessive formation of inclusions oxidized, and also eliminate some of the nitrogen and dissolved oxygen. We can so achieve high fatigue life.
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 part...) to give it dimensions less close of its final dimensions. These hot transformations are everything just those

7 which are usual for the covered semi-finished products of general compositions comparable to those of the invention, both in terms of deformations and temperatures treatment.
Preferably, a homogenization treatment of the ingot or of semi-finished 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 ensure more easily obtaining targeted mechanical properties. However, homogenization did, generally, of preferably not take place during the last hot forming operations or after these, in order to more assuredly maintain an acceptable grain size on the products, depending on their future use.
The semi-finished product then undergoes, according to the invention, a heat treatment consistent in:
- A solution treatment 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 which is therefore closely dependent on the dimensions said semi-finished product, followed by quenching to a temperature of -60 C or lower, preferably -75 C or lower, said quenching beginning, from preferably, at a temperature close to the solution temperature, and can be carried out in two stages separated by a stay at a intermediate temperature (for example ambient, or a temperature between the beginning and the end of martensitic transformation, or a temperature higher than the start temperature of the transformation martensitic);
- Then, possibly, cold shaping 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 increases, but reciprocally ductility and toughness grow;
= The aging time required to cause hardening given increases when the aging temperature decreases;
= At each temperature level, the resistance passes through a maximum for a determined duration, which is called hardening peak;

8 = For each target resistance level, which can be reached by many pairs of aging time-temperature variables, there is only one such torque which gives the best strength/ductility compromise to the steel;
these optimal conditions correspond to the onset of overaging of the structure, and are obtained when going beyond the hardening peak;
a person skilled in the art can determine experimentally what is the torque optimum through routine reflection and testing.
The steel alloying elements according to the invention are present in the quantities indicated for the reasons which will be explained. As we said, the percentages are percentages by weight.
The C content is at most 0.030% (300 ppm), preferably at most 0.010%
(100ppm). In practice it is generally present only in the state of residual element resulting from the fusion of raw materials and the development, without a addition voluntary is done. It could form M23C6 type Cr carbides and penalize thus the resistance to corrosion by capturing Cr which is thus no longer available for ensure the stainless character of the steel in a satisfactory manner. He could also combine 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 intermetallics formed.
The Si content is at most 0.25%, preferably at most 0.10% for better ensure the good compromise between Rm and K1C sought. Typically there is only one residual element not added voluntarily. It tends to lower Ms (see equation (1)) and embrittle the steel, hence its undesirability in larger quantities important than 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 Mrs (see equation (1)). It could possibly be used as a substitute partial Ni for avoid the presence of delta ferrite and contribute to the presence of austenite of reversion during hardening aging. But the ease with which it evaporates during the vacuum treatments makes it difficult to control and leads to a fouling of furnace smoke dust removal devices. We therefore do not recommend a significant presence of Mn in the steels of the invention.
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. The still he is present as a residual and, if necessary, its content must be controlled by choice

9 careful treatment of raw materials and/or metallurgical treatment of desulfurization during of the melting stage and adjustment of the composition of the steel. It reduces the tenacity by segregation at grain boundaries, and forms sulfides that are harmful to properties mechanical.
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. He this is it still a residual element which tends to segregate at the grain boundaries and, so decrease the tenacity.
The Ni content is between 8 and 14%, preferably between 11.3 and 12.5%.
It is a gammagenic element, and it must be at a high enough level to avoid the stabilization of delta ferrite during solution treatment and of homogenization. But it must also be maintained at a level sufficiently low for ensure complete martensitic transformation during quenching since it strongly tendency to lower Ms according to equation (1). On the other hand, he participates in hardening of steel during precipitation aging of hardening NiAl 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 martensite laths and provides their ductility and toughness to the steels of the invention.
The Cr content is between 8 and 14%, preferably between 8.5 and 10%.
He is the primary element that provides corrosion resistance, which justify the limit 8% lower. But we must limit its content to 14% so that it does not contribute not at the stabilization of the delta ferrite and that it does not pass Ms, calculated according to the equation (1), below 50 C.
The Mo+W/2 content is between 1.5 and 3.0%, preferably between 1.5 and 2.5%. Mo contributes to corrosion resistance and is likely to form a phase hardener Fe7Mo6. However, adding an excessive amount of MB may drive to the formation of a ji phase. Fe6Mo7 and thus reduce the amount of Mo available to limit corrosion. Optionally, we can replace at least part of the Mo by of the W. He is well known that in steels these two elements are functionally often comparable, and that, for an equal mass percentage, W is twice as efficient 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 hardening NiAl phase. AI usually has a reputation for degrade the ductility, but this disadvantage is canceled by the possibility offered by the invention of carry out the solution treatment at relatively low temperatures.
The Ti content is between 0.5 and 2.0%, preferably between 1.10 and 1.55%. It also participates in hardening during aging by forming the sentence Ni3Ti. It also allows to fix C and N in the form of carbides and carbonitrides of Ti and to avoid the harmful effects of C. However, as we have said, these carbides and carbonitrides are detrimental to fatigue resistance, and we cannot afford to form in too large a quantity. The C, N and Ti contents must therefore be kept 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 stabilizes austenite at temperatures homogenization and dissolution, and therefore to avoid the formation of delta ferrite. He participates in hardening by its presence in solid solution and also in this that he favors the precipitation of the NiAl and Ni3Ti phases. It can be added as a substitute to Ni so as to raise the Ms temperature and ensure that it is above 50 C. Compared to steel described in EP-A-1 896 624 where Co must be at most 2%, the goal here is to use Co for contribute significantly to the hardening, this in combination with the others elements present and the heat treatments required. Content preferential aim of 2.50-3.50% represents the best compromise between the cost of steel and its performance.
N should be no more than 0.030% (300 ppm), preferably no more than 0.0060% (60 ppm) to better ensure the good compromise between Rm and K1C sought. We do not add voluntarily from nitrogen to the liquid metal, and the vacuum treatments which are generally practiced during the elaboration allow to protect the steel cash against absorption of atmospheric nitrogen, or even to remove part of the nitrogen dissolved. N is unfavorable to the ductility of steel and forms angular Ti nitrides which are likely to be crack initiation sites during stresses in fatigue.
0 should be at most 0.020% (200 ppm), preferably 0.0050% (50 ppm) for better ensure the good compromise between Rm and K1C sought. He too is unfavorable to ductility, and the oxidized inclusions it forms are also potentially sites fatigue crack initiation. The 0 content should be chosen according to the criteria usual for those skilled in the art, depending on the mechanical characteristics precise required for the final product.
In general, the mechanical properties of the steel of the invention are adversely affected by oxide and nitride inclusions.
The use of

11 production processes aimed at minimizing their presence in the final steel (VIM, ESR, VAR) is preferred for this reason in particular.
The other elements present in the steel of the invention are iron and the impurities resulting from processing.
It should be understood that the ranges given as preferential for each element are independent of each other, i.e. the composition of steel may only be within these preferential ranges for certain elements only.
Tests were carried out on samples from ingot castings having the compositions set forth in Table 1. The compositions of the A-samples to E correspond to reference steels: A, D and E comply with teaching of EP-A-1 896 624. B and C are two examples of reference 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 taken from ingots of 150 kg. ingots of 6 kg were initially developed for a first validation of the concept of the invention, and their encouraging properties have led to pursue the experiments with 150 kg castings to confirm and refine the definition of the invention. The 6 kg ingots also made it possible to carry out direct trials of traction, while it was necessary to form the 150 kg ingots to extract then the samples on which the measurements of the parameters governing the tenacity have been carried out.
0% 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 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 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 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) A 1.18 <0.010 0.0029 0.0008 remainder 85 B 1.17 6.11 0.0021 0.0009 remainder 16 C 1.16 9.11 0.0006 0.0009 remainder -19 D 1.15 <0.010 0.0024 0.0010 remainder 88 E 1.17 <0.010 0.0022 0.0013 remainder 72 F 0.051 8.22 0.0018 0.0008 remainder 210 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 5 1.18 6.14 0.0004 0.0013 remainder 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 remainder 128 9 1.16 5.00 0.0009 0.0007 remainder 70 10 1.22 3.09 0.0016 0.0006 remainder 88 11 1.17 6.20 0.0019 0.0008 remainder 111 12 1.23 3.12 0.0039 0.0009 remainder 79 13 1.21 3.09 0.0029 0.0005 remainder 106 14 1.45 3.06 0.0044 0.0003 remainder 91

15 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 test samples, with their calculated Ms according to equation (1) The 6 kg ingots (A, B, C 1 to 5) were produced by vacuum treatment of liquid metal before casting. They were homogenized at 1250 C for 48h. They have then spun after heating to 940 C to be formed into bars of diameter 22mm. Table 2 indicates what treatments these bars then underwent, and what were their main final mechanical properties measured in the long direction :
tensile strength Rm, yield strength at 0.2% Rp0.2, elongation at fracture A, necking at fracture Z, Vickers hardness. The reduced size of spun samples did not make it possible to extract from it specimens which would have had the dimensions required to perform toughness tests.
Scale Heat treatment Aging Rm RP0.2 AZ
Hardness Setting Temperature Temperature Duration (MPa) (MPa) (%) (%) (Hv) quenching ( 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 Tests not carried out 216 C 850/1.5 -80 510 16 (too much austenite in the 146 structure) 1850/1.5 -80 510 16 1826 1678 11 48 546 2,850/1.5 -80,510 16 1,947 1,797 11 49,577 3850/1.5 -80 510 16 1910 1794 11 50 574 4,850/1.5 -80,510 16 1966 1,872 11 49,590 5,850/1.5 -80,510 16 1977 1893 8 25,583 Table 2: Processing conditions and mechanical properties of samples from 6 kg ingots It will be noted that the excessive presence of austenite in the structure is translated, for the reference samples B and C, by a very low hardness, which was the index of a mediocre and certainly insufficient tensile strength compared to the requirements of the invention. It was therefore deemed unnecessary to carry out further mechanical tests.
on these samples. These samples had compositions which, in terms of grades individual elements of each element, complied with the requirements of the invention, but which, taken together, provided a transformation temperature martensitic Ms too low (below 50 C). Hardening, carried out under the conditions of experimentation, which correspond to what is usually practiced industrially, did not make it possible to obtain a sufficiently martensitic in the case of these samples. This shows that the condition on Ms is important to consider within the scope of the invention.
Concerning the 150 kg ingots (D, E, 6 to 16), they were produced under empty, cast, then also remelted under vacuum by the VAR process to give ingots 200mm diameter. They were then homogenized at 1250 C for 48 h, then forged at this temperature into semi-finished products with an octagonal section of 110 mm, then, after reheating at 940 C, again forged, this time into bars of section 80x40 mm. Table 3 shows the conditions under which the tests were carried out.
treatments thermal conditions that followed, and the mechanical properties measured in the long direction on the samples. Compared to the tests in Table 2, no measurement of hardness which would have duplicated the measurements of Rm, and we realized tests resilience (measurement of Kv) and toughness (measurement of K1C).
Heat treatment Aging Rm R130.2 AZ
Kv K1C
Heating Temperature Duration (MPa) (MPa) (%) (%) (J) (MPa.m1/2) solution of (c)(h) ( C/h) hardening ( 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 6850/1.5 -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 7850/1.5 -80 480 16 1888 1659 12 45 10 52 7850/1.5 -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 8850/1.5 -80 530 16 1820 1753 11 50 12 63 9850/1.5 -80 490 16 1920 1768 12 52 16 56 9850/1.5 -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: Processing conditions and mechanical properties of samples from 150 kg ingots The properties of the different samples can be commented on as follows.
Reference samples A, D and E correspond to steels with a Co low or zero described in EP-A-1 896 624. Compared to steels of the invention, we see that their Rm is relatively low.
Reference samples B and C have a Ms of less than 50 C, so too low to be in accordance with the invention. This explains the excessive presence austenite residual which prevents obtaining a sufficient Rm, translated by a weak hardness.
Reference sample F shows that too high a Mo content and a too low Ti content compared to the requirements of the invention lead to obtaining mechanical properties that are only at the level of those of other samples reference.
Sample 1 is in accordance with the invention, but has an Ms lower than the optimum 75 C and more. Its Rm is therefore relatively low and will not be suitable for all possible applications. We can say the same thing, but in a lesser extent, of sample 3.
Sample 2 has, on the contrary, an Ms conforming to the optimum, and its Rm of 1947 MPa is excellent.
Samples 4 and 5, at high Ms due to their significant substitution of the Neither by Co, have an excellent Rm of 1966 and 1977 MPa respectively.
Sample 6 has a Ms that is not optimal compared to sample 2 who has also about 3% of Co. The same for sample 7 which has a content of Co of about 6%, but a lower Rm than sample 4 due to its higher bass 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 Ms below optimum and its Rm is relatively limited. This clearly 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 are those which present the best compromise between Rm and K1C. In fact, their compositions conform to the levels preferential on 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 due to a better balance 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 the one who has the best compromise between Rm and K1C. Note also that these samples have a Cr content (9.4-9.6%) higher than that (about 9%) of 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 due to a Cr content little more strong.
Figure 1 translates the results of Table 3 in terms of compromise between Rm and K1C for samples from 150 kg ingots, 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 that reference steels D and E whose compositions are relatively close to the invention except on the Co content.
For the reference samples, an Rm of 1701 MPa corresponds to a tenacity of 66 MPA.m1/2. This steel would therefore not be at all suitable for uses privileged considered because of its very insufficient Rm. The maximum Rm of reference samples is 1952 MPa, which would be correct for said uses, but the corresponding toughness 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 tenacities of the order of 46 to 56 MPA.m1/2.
Those mechanical properties taken as a whole are therefore not as favorable than for type 300M carbon steels.
As regards the samples according to the invention, it is seen in FIG. 1 that a very good compromise between Rm and K1C is generally obtained for Rm of the order of

18 1950 MPa, which correspond to K1C of the order of 46 to 63 MPa.m1/2, the highest often greater than 50 MPa.m1/2. We therefore fall back on the orders of magnitude of the properties corresponding to 300M steels We also see that if a decrease in Rm was acceptable, the tenacity would be increased significantly, and vice versa. Steels according to the invention therefore provide the user with great flexibility in the choice of their properties, which can be modulated by the composition, the heat treatments and the final aging chosen within the framework that has been cited.
Concerning the 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 compared to the 300M of this point of sight.
On some of these same samples from castings of 150 kg ingots (samples D, 6 to 8 and 10 to 16), we also carried out tests of corrosion salt spray, in an aqueous solution containing 50 g/l of NaCl at 35 C. They all had, previously been subjected to the same solution heat treatment at 850 VS
for 1 h 30 min, quenching at -80 C and aging at 510 C
for 4 p.m.
None of these samples showed signs of corrosion after 200 h of exposure.
The steels according to the invention therefore do not see their corrosion results in the fog salt degraded compared to the reference steel D which does not contain Co.
Stress corrosion tests were also carried out in an environment solution at 3.5% NaCl at 23 C, on samples E and 10, subjected to in solution at 850 C for 1 h 30 min, quenched at -80 C and aged at 510 C
for 16 h. K1C toughness in air and times to failure were measured for fillers equal to 75% of K1C. In both cases, the samples resisted for more than 500 h before failure. This is a good result, and the invention therefore does not degrade not holding on to corrosion under stress compared to reference steels without Co.
The steels according to the invention can therefore replace themselves mechanically satisfactory to 300M type steels, with the added fact that they present from salt spray and corrosion resistance performance under constraint which are quite favorable, as they are comparable to those of stainless steels by which one could consider replacing the 300M.
It should be understood that throughout this description, the solidified ingot who is cast from the liquid metal can have any shape capable of drive, after various deformations, to a final product having the shape and dimensions desired for its use. In particular, casting in a conventional ingot mold fitted with of a background and

19 fixed sidewalls is only one possible way to do this, and the different continuous casting processes in a bottomless ingot mold with fixed walls or mobiles can be used to achieve ingot solidification.
An alternative solution to that which has just been described is to carry out the Following heat treatments on a semi-finished product not originating from an ingot transformed 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 form, possibly complex, and dimensions very close to those of the room final. The powder used is a metal powder which has the composition steel according to the invention. In his case, a homogenization of the sintered semi-finished product is not necessary. But the manufacturing process may include prior to the sintering strictly speaking, as is conventional for those skilled in the art, a step of pre-sintering carried out under less severe conditions than sintering in terms of temperature and/or time. Generally speaking, the sintering process is leads as the person skilled in the art would do using his usual knowledge.

Claims (24)

20 1.- Martensitic stainless steel, characterized in that its composition is, in weight percentages:
- 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%;
- 1.05% <= Al <= 2.0%;
- 0.5% <= Ti <= 2.0%;
- 2% <= Co <= 9%;
- traces <= N <= 0.030%;
- traces <= O <= 0.020%;
the remainder being iron and impurities resulting from the elaboration;
and in that its martensitic transformation onset temperature Ms calculated by the formula (1) Ms (°C) = 1302 - 28Si - 50Mn - 63Ni - 42Cr - 30Mo + 20A1 - 12Co -25Cu + 10[Ti - 4(C+N)]
in which the contents of the various elements are expressed in weight percentages, is greater than or equal to 50°C, preferably greater than or equal to 75°C, in 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;
and in that the proportion of delta ferrite in its microstructure is less than or equal to 1%. 2.- martensitic stainless steel according to claim 1, characterized in that what traces <
C <= 0.010%. 3.- martensitic stainless steel according to claim 1 or 2, characterized in that traces <= Si <= 0.10%. 4.- martensitic stainless steel according to one of claims 1 to 3, characterized in that only traces < Mn < 0.10%. 5.- martensitic stainless steel according to one of claims 1 to 4, characterized in that that traces < S < 0.005%. 6.- martensitic stainless steel according to one of claims 1 to 5, characterized in that only traces < P < 0.020%. 7.- martensitic stainless steel according to one of claims 1 to 6, characterized in that that 11.3% < Ni < 12.5%. 8.- martensitic stainless steel according to one of claims 1 to 7, characterized in that that 8.5% < Cr < 10%. 9.- martensitic stainless steel according to one of claims 1 to 8, characterized in that that 1.5 < MB + W/2 < 2.5%. 10.- martensitic stainless steel according to one of claims 1 to 9, characterized in that 1.05% < Al < 1.5%. 11.- martensitic stainless steel according to one of claims 1 to 10, characterized in that 1.10% < Ti < 1.55%. 12.- martensitic stainless steel according to one of claims 1 to 11, characterized in that 2.5% < Co < 6.5%. 13.- martensitic stainless steel according to claim 12, characterized in that 2.50% < Co < 3.50%. 14.- martensitic stainless steel according to one of claims 1 to 13, characterized in what traces < N < 0.0060%. 15.- martensitic stainless steel according to one of claims 1 to 14, characterized in what traces < 0 < 0.0050%. 16.- Manufacturing process of a martensitic stainless steel part, characterized in that :
- a steel semi-finished product having the composition according to one of the claims 1 to 15 by one of the following methods:
* a liquid steel is prepared having the composition according to one of the claims 1 to 15, and from this liquid steel, we cast and solidify an ingot and we turns it into a semi-finished product by at least one hot transformation;

* a sintered steel semi-finished product is prepared by powder metallurgy having the composition according to one of claims 1 to 15;
- the semi-finished product is completely dissolved in the field austenitic, at a temperature of between 800 and 940° C.;
- the semi-finished product is quenched to a final temperature of quench less than or equal to -60°C;
- aging is carried out between 450 and 600° C. for 4 to 32 hours. 17.- Manufacturing process of a martensitic stainless steel part according to claim 16, characterized in that said quenching of the half-produced up to a final quenching temperature less than or equal to -75°C 18.- Process according to claim 16 or 17, characterized in that one pours and solidifies a ingot, and in that, between the solidification of the ingot and the dissolving semi-finished product, performs homogenization of the ingot or semi-finished product at 1200-1300°C
for at least 24 hours. 19.- Method according to one of claims 16 to 18, characterized in that between tempering and aging, a cold transformation of the semi-finished product is carried out. 20.- Method according to one of claims 16 to 19, characterized in that the temper is carried out in two stages, in two different quenching media. 21.- Method according to claim 20, characterized in that the first quenching step is performed in water. 22.- Method according to one of claims 16 to 21, characterized in that prepared liquid steel through double vacuum melting treatment, the second vacuum treatment being an ESR or VAR reflow treatment. 23.- Martensitic stainless steel part, characterized in that it has been prepared by the method according to one of claims 16 to 22. 24.- Martensitic stainless steel part according to claim 23, characterized in that that it is an aeronautical structural part.